QATAR UNIVERSITY. Graduate Studies. College of Engineering CHARACHTERIZATION AND TREATMENT OF SPENT CAUSTIC FROM AN ETHYLENE PLANT.

Transcription

1 QATAR UNIVERSITY Graduate Studies College of Engineering CHARACHTERIZATION AND TREATMENT OF SPENT CAUSTIC FROM AN ETHYLENE PLANT A Thesis in Environmental Engineering By Hasanat Mohammed Ramadan 2013 Hasanat Mohammed Ramadan Submitted in Partial Fulfillment Of the Requirements For the Degree of Master of Science in Environmental Engineering December 2013 I

2 The thesis of Hasanat Mohammed Ramadan was reviewed and approved by the following: We, the committee members listed below accept and approve the Thesis of the student named above. To the best of this committee s knowledge, the Thesis conforms the requirements of Qatar University, and we endorse this Thesis for examination. Name Signature Date Name Signature Date Name Signature Date II

3 ABSTRACT Spent caustic solution is generated in oil refineries and petrochemical plants as a result of scrubbing processes where hydrogen sulfide and carbon dioxide compounds are removed. Spent caustic is considered as one of the most difficult streams to handle by wastewater treatment plants. Selecting the best treatment technique is considered to be a critical task, in order to meet the discharge limit. The present work aims to find an effective treatment process to treat spent caustic solution produced from an ethylene plant. Neutralization and neutralization coupled with oxidation are the two processes studied. Two methods of oxidation are tested, classical oxidation by using H 2 O 2 alone and advanced oxidation by using Fenton s reagent. Applying neutralization alone or neutralization with oxidation was verified for the achievement of the required degree of treatment. For the neutralization alone, it was found that the highest chemical oxygen demand removal achieved was 88 % (1699 mg/l) at ph=1 while the sulfide removal was 99.8 % ( 9.9 mg/l). For classical oxidation using H 2 O 2, the best removal was achieved at ph=2.5. The COD % removal was 89 % with a COD value of 1630 mg/l. Furthermore, the sulfide removal reached a value of almost 100%. While for the advanced chemical oxidation using Fenton s process, the best result was obtained at ph=2.5. The COD % removal was 96 % with a COD value of 542 mg/l. This was achieved with hydrogen peroxide to ferrous sulfate ratio of 1: 7.5. The sulfide removal also reached a value of almost 100%. III

4 TABLE OF CONTENTS Abstract..... III List of Figures VII List of Tables.... VIII Abbreviation..... IX Acknowledgment.. X CHAPTER 1: 1. Introduction Spent Caustic Sources of Spent Caustic Solution from the Ethylene Plant Hot section Steam Cracker Quenched Tower Compression Section Gas Compressors Cold Section Treating (Caustic Tower) Fractionation Section Spent Caustic Management Background Reduction Reuse within the Process Recycle Outside the Facility Treatment and Disposal Spent Caustic Treatment Biological Treatment of Spent Caustic Thermal Treatment Chemical Treatment Classification of the Chemical Oxidation Processes Classical Chemical Oxidation. 22 IV

5 Advanced Oxidation Processes (AOPs) Objectives of Present Study CHAPTER 2 : 2. Research Methodology Spent Caustic Characteristics Total Suspended Solids and Total Dissolved Solids Measurements of Chemical Oxygen Demand (COD) Biological Oxygen Demand (BOD) Total Sulfide S 2, H 2 S, HS by Titration (sulfide above 1 39 mg/l) Determination of Total SulfideS 2, H 2 S, HS (sulfide 0 to μg/l) Free Soda and Complete Alkalinity Total Petroleum Hydrocarbons (TPH) Inorganic anions Heavy Metals Phenol Experimental Setup and Procedure Neutralization Neutralization Coupled with Oxidation.. 48 CHAPTER 3 : 3. Results and Discussion Neutralization Effect of ph on Sulfide Removal Effect of Temperature on Sulfide Removal Effect of ph on COD Removal Effect of ph on TDS Removal Neutralization Coupled with Oxidation: Classical Oxidation Effect of Hydrogen Peroxide Concentrations on COD Removal Effect of Hydrogen Peroxide Concentrations at Different ph V

6 on COD Removal Neutralization coupled with Oxidation: Advance oxidation Effect of ph in Fenton s Reagent on the Removal of COD Effect of Ferrous Sulfate Concentrations on COD Removal Effect of Hydrogen Peroxide to Ferrous Sulfate Ratio on COD Removal Effect of Hydrogen peroxide to COD ratio on COD Removal CHAPTER 4: 4. Conclusions and Recommendations CHAPTER 5: 5. Future Work References 80 VI

7 LISTS OF FIGURES Figure 1 : Ethylene process flow diagram (PFD)... 7 Figure 2 : Steam cracker diagram... 9 Figure 3 : Quenched tower... 9 Figure 4 : Gas compressors Figure 5 : Treating unit Figure 6 : Fractionation unit Figure 7: Waste management hierarchy Figure 8 : Treatment technologies according to COD contents Figure 9 : Oxidation potential Figure 10: Summarized of the treatment processes of ethylene spent caustic Figure 11 : Experimental schematic diagram Figure 12 : Hydrogen sulfide and ph dependent Figure 13: Neutralization of spent caustic Figure 14 : Sulfide % removal at different ph after neutralization Figure 15 : Sulfide % removal at different temperature Figure 16: COD removal % for ph=1,3,5 before neutralization Figure 17: COD removal % at different ph after neutralization Figure 18: Effect of ph on TDS removal Figure 19: Blank sample with a) 0.1 H 2 O 2 and b) 1 ml of H 2 O Figure 20 : COD removal % at different H 2 O 2 concentration Figure 21 : Sulfide % removal at different H 2 O 2 concentration Figure 22 : Effect of ph on the COD removal Figure 23 : Effect of ferrous sulfate concentration on COD % removal Figure 24 : Effect of hydrogen peroxide to ferrous sulfate ratio on % COD removal Figure 25 : Effect of hydrogen peroxide to COD ratio on % COD removal VII

8 LISTS OF TABLES Table 1: Spent caustic types and characteristics... 4 Table 2 : Physical properties of ethylene... 5 Table 3: WAO operational conditions Table 4: Classical chemical oxidation Table 4: Classical chemical oxidation Table 4: Cont Classical chemical oxidation Table 4: Cont Classical chemical oxidation Table 5: Advanced chemical oxidation Table 5: Cont Advanced chemical oxidation Table 5: Cont Advanced chemical oxidation Table 5: Cont Advanced chemical oxidation Table 6 : Spent caustic characteristics used in this experiment Table 7 : Effect of hydrogen peroxide concentrations on COD removal Table 8 : Analyses interfere with H 2 O Table 9: Effect of H 2 O 2 at different ph on COD removal Table 12: Effect of hydrogen peroxide to ferrous sulfate ratio on COD % removal Table 11: Effect of hydrogen peroxide to COD ratio on % COD removal VIII

9 ABBREVIATIONS NaOH Sodium hydroxide H 2 S hydrogen sulfide C 2 H 4 Ethylene C 2 H 6 Ethane HS - & S -2 Sulfide SO 4 Sulfate S 2- H 2 O 2 O 3 OH Sulfide Hydrogen peroxide Ozone Hydroxyle redical HO 2 Hydroperoxyl Fe +2 Fe +3 FeSO 4 /H 2 O 2 UV TiO 2 DMDS COD TOC BOD TDS TSS TPH WAO CWAO AOPs Ferrous Ferric Fenton s reagent Ultra violet Titanium peroxide Dimethyl disulfide Chemical oxygen demand Total organic carbon Biological Oxygen Demand Total Dissolved Solids Total Suspended Solids Total Petroleum Hydrocarbons Wet Air Oxidation catalytic wet air oxidation Advanced Oxidation Processes IX

10 ACKNOWLEDGMENTS My sincere thanks to my supervisors, Dr. Alaa Al Hawari and Professor Ibrahim abureesh, who have supported me throughout my thesis work and provided me with advice, guidance, and continuous encouragement throughout the work on this thesis. Special thanks to Dr. Ahmed Al Khatat, senior lab technician of chemical engineering department, who provided us with his time and knowledge in the lab measurements. For his sincere help I m deeply grateful. Also I wish to express my special thanks to Qatar Pertrochemical Company (QAPCO) namely Dr. Mabrouk and Mr. Mejali Al Kuwari, who helped me throughout the period of this research and for the supply of waste samples. My gratitude is for my loving parents, my husband, and my family for keeping me motivated during this period of research work. At the end, my regards to all people who supported me directly or indirectly during my graduates studies at Qatar University. X

11 CHAPTER 1: 1. INTRODUCTION : 1.1. Spent Caustic : Qatar developed and built a strong industrial sector by employing the latest technological innovations in their production processes [1]. This eventually will create a positive and highly beneficial impact on the national economy. Qatar depends on two main sources of water to meet its need which are conventional water sources that come from groundwater and non-conventional water sources that come from desalination of seawater and recycling of treated wastewater [1]. Qatar is working to achieve a zero liquid discharge by The aim of this project is not to allow any discharge of treated wastewater into water bodies but to reuse and recycle the produced water. The project is directed by the Ministry of Energy and Industry with the Ministry of Environment involving various industries in Qatar [2]. The zero discharge can be mainly achieved by enhancing the quality of treated waste water where it could be recycled and reused in irrigation or in operations and production processes. In Qatar several industrial companies such as Qatar Chemical Company (Q-Chem), Qatar Petrochemical Company (QAPCO) and Qatar Petroleum (QP) generate liquid waste that is known as spent caustic solution. Spent caustic is an industrial waste solution that consists of sodium hydroxide, water, and other pollutants. 1

12 Sodium hydroxide (NaOH) solutions are used in many industries to wash out acid gases such as hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) from different hydrocarbon streams [3]. Once these gases react with the majority of NaOH, a waste solution known as spent caustic will be produced [4]. Spent caustics are the most difficult class of liquid industrial waste to handle and to dispose due to the high concentration of pollutants in it [5]. According to the Environmental Protection Agency EPA under the Resource Conservation and Recovery Act RCRA used to classify the wastes as hazardous that create potential harmful impact to the human health and to the environment [6]. The classification of waste depends on the specific characteristics of the waste itself. The waste is considered to be hazardous if it exhibits one or more of the four characteristics [6, 7]: 1) Ignitability (D001) 2) Corrosivity (D002) 3) Reactivity (D003) 4) Toxicity (D004 - D043) Spent caustic could be classified as D003 hazardous waste due to the reactive sulfide it contains [8]. Also, spent caustic is the highly corrosive due to the high ph value. Recent environmental regulations have a great impact on the spent caustic treatment method design since previous usual disposal methods are becoming legally prohibited [8]. Without treatment, spent caustic stream may cause environmental problems because of their alkalinity (ph>12), salinity (sodium of 5-12% wt) and high sulfide S -2 levels exceeding 2-3 wt% [9-11]. However, sulfide can be 2

13 converted to elemental sulfur and/or sulfate that are preferred finishing product as it does not represent COD and maybe allowed to be discharged into the environment [14]. Moreover, spent caustics may contain toxic organo-sulfur compounds such as methanethiol and aromatic hydrocarbons like benzene [12, 13]. Spent caustics can be classified into many types depend upon the industry producing it and the source of fuel that fresh caustic wash. Table 1 summarizes the type of spent caustic and there characteristics. Usually refineries don t separate each type of spent caustic and they mix the three types and this is called the mixed refinery spent caustic [9]. 3

14 Table 1: Spent caustic types and characteristics Type of Sulfidic Cresylic Naphthenic Ref spent caustic Source Ethylene & LPG Gasoline Kerosene & Diesel [5] Content High Conc. of Sulfides & Mercaptans High Conc. of Phenols & Cresols High Conc. Of Polycyclic aliphatic organic compounds [5] Effect after neutralization Release gases Foaming and settling issue in the biological Oil layer & foaming [5] Chemical oxygen demand (COD) (ppm) Total organic carbon (TOC) (ppm) 5,000-90,000 50, , , ,000 [15] 20-3,000 10,000-24,000 24,000-60,000 [15] Sulfides (ppm) 2,000-52,000 < ,000 [15] Total phenol (ppm) ,900-1,000 14,000-19,000 [15] 1.2. Sources of Spent Caustic Solution from the Ethylene Plant: In this study spent caustic produced from an ethylene plant will be targeted. In order to understand where spent caustic will be produced it is important to illustrate the ethylene process. Ethylene is the chemical compound with the formula C 2 H 4, because it contains a carbon-carbon double bond. Ethylene is one of the simplest unsaturated 4

15 hydrocarbons. Ethylene is a colorless flammable gas with a sweet odor. The physical properties of ethylene are summarized in Table 2. Table 2 : Physical properties of ethylene [16] Property Value Structural formula IUPAC name Molecular weight Appearance Density Solubility in water Thermodynamic data Ethene g/mol Colorless gas kg/m 3 at 15 C, gas 3.5 mg/100 ml (17 C); Phase behavior Solid, liquid, gas Ethylene is an important building block in the petrochemical industry and its global production exceeds any other organic compound. It can undergo many types of reactions which lead to major chemical products. Ethylene is the raw material to produce a wide range of products such as, ethylene glycol, ethylene dichloride, polyvinyl chloride, styrene, and polyethylene which is the common plastic in our daily life. Ethane is preferred for ethylene production because the steam cracking of ethane consumes less energy than cracking heavier hydrocarbon. Ethylene process produce less emissions to the atmosphere because of that it is considered as an 5

16 environmental friendly process. The process includes a caustic tower which remove harmful gases such as NOx, mercaptans, H 2 S and CO 2. Figure 1 shows the process flow diagram of an ethylene plant. Ethylene is produced from ethane by a sequence of different processing steps which can be classified as [17]: Hot section: 1. Steam Cracker. 2. Quenched Tower. Compression section : 1. Gas Compressor. Cold section: 1. Treating. 2. Chilling and Fractionation. 6

17 Figure 1 : Ethylene process flow diagram (PFD) 7

18 Hot Section Steam Cracker The Ethane Rich Gas (ERG) supplied from Qatar petroleum refinery contains high ethane content (65%) and other impurities like methane, hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ).The ERG gas is treated to remove CO 2 and H 2 S by using Amine solution to achieve less than 100 ppm. The ERG gas, free of acid gases, is cooled and refrigerated to 100 o C to separate methane. Finally pure ethane is sent to cracker unit to produce ethylene. Figure 2 shows the furnace where the ethane gas is mixed with steam. Steam is added at controlled rates in order to increase the petrochemical yield and to minimize carbon deposits (coke) forming in the furnace and heated to about 850 o C. The ethane is partially converted to ethylene and other hydrocarbons. In this section Dimethyl disulfide (DMDS) is added in the furnace as a coating to minimize the coke formation. Also, it is used to prevent over cracking of the gas to maintain high conversion of ethane to ethylene. This material with high temperature will generate H 2 S and CO 2 that will be removed later using fresh caustic soda in the treating section. 8

19 Figure 2 : Steam cracker diagram Quenched Tower: Figure 3 shows the quench tower unit where the effluent (cracked gases coming from the furnace) is immediately quenched by direct contact with water. The temperature drop to about 30 C is necessary to stop the cracking reaction. This quench water is then recovered and re-used. Figure 3 : Quenched tower 9

20 Compression Section : Gas Compressors: Separation of hydrocarbons in distillation column requires the furnace effluent to be liquefied by increase the pressure of the gas then cool it down. The cracked gas from the quench tower is compressed in five stages multi-stage centrifugal compressor as shown in Figure 4. Figure 4 : Gas compressors Cold Section : Treating (Caustic Tower) : The cracked gas stream will contain impurities (acid gases) that need to be removed. These impurities include carbon dioxide, and hydrogen sulfide that are generated as a result of addition DMDS in the steam cracker unit. Treatment of the cracked gas to remove impurities occurs between the fourth stage and the fifth stage of the compression section and it is treated in a caustic soda washing tower. Figure 5 shows the caustic tower where in this tower, the gas stream is contacted with dilute sodium hydroxide as in the following reaction: 10

21 2NaOH (aq) + H 2 S (g) Na 2 S (aq) + 2H 2 O (1.1) 2NaOH (aq) + CO 2 (g) Na 2 CO 3 (aq) + H 2 O (1.2) The reason behind selecting the sodium hydroxide to remove the acid gases is its ability to remove the very small quantities of the acid gases. Then caustic tower overhead gas that is free from acid gases is treated in dryers to remove moisture and send to De-ethanizer to separate C3 and heavier components. Figure 5 : Treating unit Fractionation Section The cracked gas that is free from acid gases and moisture is cooled down gradually in the fractionation section. In this section there are four distillation columns as shown in Figure 6. The first column is the de-ethanizer that separates out heavy gases such as propane (C3) from the light hydrocarbons (ethylene, ethane and methane). 11

22 De-ethanizer overhead is compressed in the 5 th stage of compression and passed through acetylene reactor to remove acetylene. As the gas stream passes the catalyst the following reaction occurs: C 2 H 2 (g) + H 2 (g) C 2 H 4 (g) (1.3) Then gas stream is sent to refrigeration chilling to condense ethane. The gas is cooled and then it liquefies. The second distillation column is the de-propanizer column. The de-ethanizer bottom goes to the de-propanizer column to separates C3 hydrocarbon from C4. The third column is the de-methanizer. This column separates ethylene from the lighter components which are methane and hydrogen. The methane is then used as fuel gas. The fourth column is the C2-splitter that separates ethylene from the ethane. The ethylene stream is sent to ethylene storage and the bottoms stream which is ethane is sent back to the cracking furnace. 12

23 Figure 6 : Fractionation unit 13

24 1.3. Spent Caustic Management Background: Waste management is one of the major environmental concerns in the world that involve solids, liquids and gases [18]. Large quantities of waste cannot be eliminated but, their environmental impact can be reduced by making more sustainable use of the waste [18]. The waste hierarchy shown in Figure 7 shows that disposal is the least favorable option [20, 21]. Regarding spent caustic which is a liquid waste, disposal is not an option according to Environmental Protection Agency (EPA) regulations. Most favorable Reduce Reuse Recycle Treatment & Disposal Least favorable Figure 7: Waste management hierarchy The management of spent caustic should include: Reduction: Source reduction is the best practice when designing a spent caustic treating process. It is done by generating the least amount of spent caustic while 14

25 maintaining the desired efficiency of the process. This can be done by using maximum caustic strength or applying multistage caustic wash [22]. But in case of reduction spent caustic still will be produced Reuse within the Process: Reuse of spent caustic will result a decrease in the total amounts that it need to be disposed. According to Ahmad.W (2012), the reuse of spent caustics is suggested to be used for crude oil neutralization and in the biological wastewater treatment process to control the ph value [23]. The issue with the reusing spent caustic is that the concentration of sodium in the spent caustic is not steady and an appropriate amount is not easy to control [22]. Moreover phenols and napthenates are more suitable for reuse at controlled concentrations but sulfidic causes odor issues in this application [22] Recycle Outside the Facility: It has been implemented in many refineries in the US and Canada after applying additional treatment [24]. The valuable compounds from caustic such as sulfide, phenols and naphthenic acids can be removed and reused as a raw material in many industries such as pulp and paper, tannery, mining, wood preservatives and paint industries [22]. However, this approach may need appropriate cost analysis before proceeding Treatment and Disposal: Deep well injection is a previous practice used to dispose spent caustic but it is prohibited nowadays [4]. Recently many treatment methods can be applied and it 15

26 proofs to be effective. The treatments processes must guarantee the destruction of the contaminants in order to reach the discharge limit [22]. The removal of contaminants in liquid solution requires one or more of the available treatment methods. It is essential to decide on the most appropriate method according to the characteristics of the liquid solution, cost of the process and volume of the stream that need be treated [22] Spent Caustic Treatment: The treatment processes of spent caustic must guarantee the elimination of the pollutants in order to reach the authorized limit for discharge. The elimination of pollutants in aqueous solution may need one or various basic treatment techniques depending on the type of compounds and concentration in solution [25]. It is necessary to choose the most adequate method according to the characteristic of the effluent. Numerous efforts have been made to develop and to enhance the treatment of spent caustic. Treatment methods for spent caustic can be classified as biological, chemical and thermal processes [22]. According to Andreozzia,R. Caprioa, V. Insolab, and A. Marottac,R. (1999) the treatment process could be selected depending on COD concentration [26]. Figure 8 shows the relation between COD value and the appropriate treatment method [26]. Advanced oxidation processes (AOPs) is selected for COD less than 20 g/l while wet air oxidation (WAO) is implemented for COD values between 20 and 200 g/l higher than this value incineration is considered to be the best method [26]. 16

27 Incineration WAO AOPs COD g/l Figure 8 : Treatment technologies according to COD contents [26] Biological Treatment of Spent Caustic : As mentioned before spent caustic stream needs excessive treatment before discharge. Biological treatment is preferred due to the low cost and the low environmental impact [27]. The treatment should be done by two steps: pretreatment followed by biological treatment. The biological treatment can be an inexpensive disposal option; however there are several drawbacks if it is applied directly to without pretreatment. These drawbacks are: 1- Noxious odors: Sulfides and mercaptans are highly odors even at the ppb level. These compounds are considered very toxic and hazardous [28]. 2- It is not readily biodegradable: Often spent caustic contains a mixture of compounds that would limit the biodegradation in the biological treatment processes [29, 5]. 3- Foaming: Spent caustic has compounds that have foaming characteristics when aerated or agitated during the treatment [29, 5]. 17

28 4- High chemical oxygen demand (COD): This can cause high load to the biological process [29, 5]. 5- PH swings: spent caustic is highly alkaline solution with a ph value that can reach up to 14 [28] Thermal Treatment: 1) Wet Air Oxidation (WAO) Another conventional method is the wet air oxidation (WAO), which is a high pressure treatment at elevated temperature. Here the oxidation agent is the oxygen present in the air, which is introduced into the spent caustic as steam. This reaction can accomplish either mineralization of organics into CO 2 and H 2 O or destroy complex molecules into simpler molecules that is easier to degrade [30, 31]. The process is very expensive, and due to severe reaction conditions, safety is a main concern. Although several tests with low pressure has been conducted without remarkable success [32]. WAO can be classified into three types based on the temperature implemented to achieve the oxidation. Table 3 shows the three types of WAO that can be applied to the different kinds of spent caustic. Using appropriate catalysts for WAO process minimize the severity of reaction conditions and simply destroy refractory pollutants resulting in reducing capital and operational cost [33]. The operating cost of catalytic wet air oxidation (CWAO) is around half the noncatalytic WAO. However, an additional step is required to remove the metal ions from the treated effluent that would result in increasing operational costs [34]. Catalysts allow overcoming the drawbacks of the WAO; however the discovery of 18

29 low cost and stable catalysts remains the major weaknesses of CWAO for wide applications [35]. Table 3: WAO operational conditions [5] Type of WAO Temperature ( C) Pressure (psig) Kind of spent caustic Low to 100 sulfides in spent caustic temperature Mid temperature to 600 complete treatment of sulfides and mercaptans, cresylic acids and naphthenic acids High temperature to 1100 complete treatment of sulfides and mercaptans, cresylic acids and naphthenic acids 2) Incineration: Incineration is a process used to convert solid, liquid or gas at elevated concentration of pollutants into more stable states at higher temperatures [36]. The economic aspect presents the disadvantage of this process because it requires high energy cost. In addition toxic emissions that results from this process are high [22]. 19

30 Chemical Treatment: 1) Neutralization Followed by Air Stripping: Aeration depends on two fundamental principles: equilibrium conditions and mass transfer considerations [36]. Equilibrium conditions will identify the limits of the gas transfer process. Aeration is an efficient method for H 2 S gas removal. The function of aeration is not particularly to oxygenate the water, but it is to strip the dissolved gas (H 2 S) out of the water by changing the equilibrium conditions of the water and thus drive the dissolved gas out [36]. Neutralization converts the spent caustic components into their original elements, such as hydrogen sulfide (H 2 S), mercaptan sulfur (RSH), phenol and naphthenic acid. But it requires stripping and additional managing of volatile gases [22]. This technology is a widely understood method and the simplest and cheapest for the removal of volatile compounds. However, the effluent stream has elevated COD concentrations because a major part of the organic component is unaffected by the stripping process [32]. 2) Chemical Oxidation Chemical oxidation is a method that involves the transfer of one or more electrons from an electron donor (reductant) to an electron acceptor (oxidant), which has a higher affinity for electrons. The result of electron transfer is a chemical change of the oxidant and the reductant [37]. Oxidation technologies are established to decompose refractory molecules into simpler molecules that can be further treated by other methods. The mechanism of oxidation works by transferring electrons from the contaminants to the oxidant. The contaminant is 20

31 Chemical oxidant oxidized and the oxidant that accepts the electron is reduced [38]. In natural waters, chemical oxidation processes also take place due to the presence of microorganisms that work as natural oxidants [37]. Oxidation reactions generate chemical species with an odd number of valence electrons known as radicals. These tend to be highly unstable therefore, highly reactive because one of their electrons is unpaired. The reactions that produce radicals tend to be followed by chain reactions between the radical, oxidants and other reactants until stable oxidation products are formed. The ability of an oxidant to initiate chemical reactions is measured in terms of oxidative power of an oxidant [39]. The oxidation potentials are presented in terms of the electro-potential. The electropotential based on half-cell reactions [38]. Figure 9 shows the potentials of the most commonly used oxidizers [40, 41]. Bromine Oxygen Chlorine Chlorine Dioxide Permanganate Hydrogen Peroxide Ozone persulfate Sulfate Radical Hydroxyl Radical Oxidation potential (volts) Figure 9 : Oxidation potential [1, 2] 21

32 1.5. Classification of the Chemical Oxidation Processes Chemical oxidation can be classified into two categories that are described below in details: 1. Classical chemical oxidation 2. Advanced Oxidation Processes (AOPs) Classical Chemical Oxidation: Classical chemical oxidation is a direct chemical oxidation process that is achieved by the addition of an oxidation agent to the contaminated aqueous solution to oxidize it. The most common chemical oxidants are chlorine (Cl 2 ), chlorine dioxide (ClO 2 ), oxygen (O 2 ), persulfate, permanganate (KMnO 4 ), ozone (O 3 ), and hydrogen peroxide (H 2 O 2 ). Moreover, advantages and disadvantages of each oxidant are summarized in the Table 4. Table 4: Classical chemical oxidation Chemical Advantages Disadvantages Ref. Oxidant Chlorine Strong oxidant o Generate halogenated [42, Strong disinfectant generate persistent deposit DBPs o Possibly contribute to 43] cheap oxidant odor and taste issues very simple to injected o Require high dosage of into the system chlorine used wildly in the past o Create a carcinogenic Available in gaseous organochloride by form, as Cl 2 ; or as products aqueous solution, sodium hypochlorite NaOCl; or as a solid, calcium hypochlorite, Ca(OCl) 2. 22

33 Table 5: Classical chemical oxidation (cont.) Chlorine Strong oxidant dioxide Strong disinfectant Gaseous that is very soluble in water. ph values in the range of 3.5 to 5.5 are preferred Used efficiently to taste and odor for specific types Does not generate halogenated DBPs Does not react with ammonia Oxygen Easy to feed in the system Does not generate halogenated by-products. Low operation costs Persulfate It is much more stable and it does not react quickly by nature. High oxidation potential applicable to wide range of organics It can be catalyzed by heat, ultraviolet light, high ph, hydrogen peroxide, and transition metals highly reactive at ph <3, but it is also highly reactive at ph > 10 Fewer mass transfer and mass transport limitations o Produces chlorite as an inorganic DBP o It might create unwanted odors o hard to maintain a persistent disinfection o Difficult to handle and transport because it is unstable at high concentrations and can explode if it exposed to heat, light. o Quite weak oxidant for the majority of water treatment system. o Oxygen require certain operation conditions and it is not working under normal temperature and pressure o It is requires large investments in installations. o New technology and few studies are tested in the field. o Might degrade soft metal o Undesirable long lasting sulfate (SO -2 4 ) Byproducts. [42] [42, 43] [44, 46-48] 23

34 Table 6: Classical chemical oxidation (cont.) Chemical Oxidant Permanga nate Advantages Disadvantages Ref. Easy to feed in the system Does not generate halogenated DBPs Efficient for certain types of odor and taste Applicable over wide ph range. widely available Cheap oxidant Easily transport and handling. Less health and safety problem. (no gas/heat production) Ozone It is a very powerful oxidizing agent Recommended ph =7.5 or higher Applicable to wide range of organics. Very strong disinfectant Efficient for taste and odor Does not generate halogenated DBPs except in bromide-rich waters May used to support in the coagulation and flocculation o Produces manganese dioxide by product that should be removed o Can result a pink water color if dosage not controlled o Limited disinfection capabilities o Reduction in the oxidant efficiency due to the reaction of the molecules other than the desired contaminants. This lead to increase the need for oxidant requirements. o Low oxidation potential. o Not effective for a wide range of contaminants. o Does not create a persistent disinfectant residual o Quietly costly o generate bromate in bromide-rich waters o Ozone can react with a variety of contaminants, but it also can react with many other molecules. o Process efficiency is dependent on gas liquid mass transfer, which is quite difficult to maintain due to the low solubility of ozone in the aqueous solutions. This Cause non uniform distribution through the complete substance. o Lack of studies on large scale operation [42-44, 48] [42, 44, 45, 47, 48] 24

35 Table 7: Classical chemical oxidation (cont.) Chemical Oxidant Hydrogen Peroxide Advantages Disadvantages Ref. One of the cheapest oxidizers It has high oxidizing potential It is water-soluble. It does not produce toxins or color byproducts Applicable over a wide range of organic contaminants Can be combined with ozone or UV to increase the efficiency It can store, operate and transport safely. Available and relatively cheap. o The oxidation process doesn t produce by product o Not effective for complex materials o Mass transfer limitations between the hydrogen peroxide with the organics o Over-oxidation reaction could happen. So it should be used in controlled manner. o large residence time should be provided for H 2 O 2 in the waste stream because of the low solubility [43-45, 48] From Table 4 it can be seen that chemical oxidants offer a diversity of benefits in the treatment process. Some are simple and others are more difficult. The classical chemical oxidation is a multipurpose process that is implemented in many applications either to treat the wastewater or to improve the quality of water. Each oxidant has specific advantages that need to be evaluated before employing [41] Advanced Oxidation Processes (AOPs) Advanced oxidation processes are considered as promising methods for the treatment of spent caustic. The mechanism of AOPs is the process of forming sufficient quantity of highly reactive Hydroxyl radicals (HO ) at near ambient temperature and pressure. Once it is generated, it can attack the complex chemical contaminants in water and oxidize most of them [49].When AOPs are applied in 25

36 controlled conditions, they can reduce the concentration of contaminants from hundreds of ppm to less than 5 ppb and therefore bringing the COD and TOC to discharge limits [50]. In some cases these AOPs must be complemented with other treatment techniques in order to achieve the final treatment level. This leads to a more complex process and to an increase in the treatment cost [51]. A large number of methods are classified under the AOPs. The generation of the Hydroxyl radicals are achieved by the use of one or more strong oxidants (H 2 O 2, O 2, and O 3 ) and/or catalysts (titanium dioxide, transition metal ions ) and/or energy sources (ultraviolet radiation) [49]. Advanced oxidation processes have several advantages which are [52, 53]: Fast reaction rates Simple and easy to implement. Possible to reduce toxicity and possibly to complete mineralization of organic pollutants to CO 2 and H 2 O without generating sludge. Treatment of various organic compounds at the same time. While these processes have disadvantages which are [52, 53]: Some processes might be high in capital cost Need high controlled conditions Some applications require quenching of excess peroxide is required. 26

37 The selection of a certain advanced oxidation process depends on the application. The selection could be based on the type of compounds to be removed, treatment objectives, concentrations, site considerations, and cost. However, it has been observed that none of the methods can be used individually in treatment applications due to substantially lower energy efficiencies and higher costs of operation and usually a combination of different AOPs has been found to be more efficient for the treatment [54, 55]. The main processes found in literature for producing these radicals are summarized in Table 5. It can be note that UV system has major drawbacks such as mass transfer limitation, turbidity that can inhibit UV light diffusion, and some compounds (nitrate) can absorb UV light. All of these will result lowering process efficiency. Also, ozone with hydrogen peroxide system is like UV with hydrogen peroxide system. However, this system is less affected by feed characteristics. One method can be used to improve contaminants removal is the implementation of ozone, hydrogen peroxide, and ultraviolet radiation system (O 3 /H 2 O 2 /UV). However, in this case the cost of treatment system will be huge because of the usage of the two oxidants. This system is recommended when wastewater pollutants weakly absorb UV radiation light. 27

38 Table 8: Advanced chemical oxidation Chemical oxidant UV/O 3 UV /H 2 O 2 Brief description Advantages Disadvantages Ref. When low pressure UV light is applied to ozonated water hydroxyl radicals are generated. Destruction of organic compounds occurs by hydroxyl radical reactions coupled with direct photolysis and oxidation by molecular ozone. H 2 O 2 is injected and mixed followed by a reactor that is equipped with UV light. During this process, UV is used to cleave the O-O bond in hydrogen peroxide and generate the hydroxyl radical 1. Supplemented disinfectant 2. More effective than O 3 or UV alone. 3. More efficient generating OH radical than H 2 O 2 & UV for equal oxidant concentrations. 1. UV decompose H 2 O 2 to produce (2OH) free radicals, 2. No sludge generation, 3. UV/H 2 O 2 process is efficient in mineralizing organic pollutants 4. No potential for promate formation 5. No off gas treatment requires. 6. Not limited by mass transfer relative to O 3 processes. 1. Energy and cost intensive process. 2. Potential for bromate formation but it can be controlled through adjustment of ph. 3. Turbidity can interfere with UV light. 4. Ozone diffusion can result in mass transfer limitations. 5. May require ozone off gas treatment. 1. it cannot utilize solar light as the source of UV light 2. H 2 O 2 has poor UV absorption characteristics because of that special reactor designed for UV is required. 3. Turbidity can interfere with UV light penetrating 4. Less stoichometric efficient in generating OH radical than O 3 /H 2 O 2 process. 5. Interference compounds like nitrate can absorb UV light [57-60] [57, 60] 28

39 Table 9: Advanced chemical oxidation (Cont d) Chemical oxidant UV/ TiO 2 Perozone (O 3 + H 2 O 2 Brief description Advantages Disadvantages Ref. a titanium peroxide is a semiconductor absorbs UV light and causing to generate hydroxyl radicals. Once O 3 and H 2 O 2 are at once applied, they react to form hydroxyl radicals. 1. Chemical stability of TiO 2 in aqueous media and high potential to produce radicals. 2. Easy availability and low price. 3. Possible use of solar irradiation. 4. TiO 2 is a cheap, readily available material 5. TiO 2 is capable for oxidation of a wide range of organic compounds 6. No potential for bromate formation 7. Can be performed at high UV wavelength than other UV oxidation processes. 8. No off gas treatment required. 1. Peroxone process is much rapid than using O 3 or H 2 O 2 alone. 2. It is extremely efficient to treat complex compounds 3. H 2 O 2 is stable in acidic medium 1. impossible to achieve uniform irradiation of the entire catalyst surface 2. Pretreatment is essential to avoid fouling of the TiO 2 catalyst. 3. If TiO 2 is added as slurry then a separation step is required. 4. Need more study to determine the optimum TiO 2 dose 5. Reaction efficiency is highly depending on ph because of that close monitoring and control is required. 6. Require onsite storage or regeneration method. 7. No full scale exists. 1. Greatly dangerous and should carefully handle when it is used and stored. 2. Not very efficient when it is used to oxidize iron and manganese. 3. Potential to produce byproducts such as aldehydes, ketones, peroxides. 4. May require treatment of excess H 2 O May require gas treatment. [57, 59,60] [58, 59] 29

40 Table 10: Advanced chemical oxidation (Cont d) Chemical oxidant Fenton Oxidation H 2 O 2 and ferrous iron (Fe(II)) Brief description Advantages Disadvantages Ref. Fenton s reagent is powerful oxidant for organic contaminants.it is a mixture of ferrous iron (catalyst) and hydrogen peroxide (oxidizing agent). 1. Fenton may lead to complete destruction of the contaminants under ideal conditions to nontoxic compounds. 2. the iron used can be removed from the solution 3. The generation of OH cause to a rapid reaction to many contaminants. 4. High oxidation potential that can target complex organic compounds. 5. Treatment of both organic and inorganic substances under laboratory conditions as well as real effluents 6. Can oxidize wide range of contaminants. 7. Iron and hydrogen peroxide are cheap and safe. 8. Hydrogen peroxide easy to storage and to handle 1. Need to reduce the ph, followed by neutralization. 2. Hazards associated with using H 2 O 2 3. Hydroxyl radical with high concentration might react with the other species 4. The reaction is exothermic and might cause an increase in the temperature but it can be controlled. [56, 57,59] 30

41 Table 11: Advanced chemical oxidation (Cont d) Chemical oxidant Ultrasound systems Brief description Advantages Disadvantages Ref Ultrasound waves are introduced to the wastewater as compression and expansion cycles. Micro-bubbles are produced.these compression cycles will collapse the micro-bubbles create extremely high temperature and pressure. These conditions are capable of breaking water molecular producing hydroxyl radicals. Usually ozone or hydrogen peroxide is used along ultrasound to promote hydroxyl radical s generation which enhances pollutants removal. Higher ultrasound frequency will provide shorter time for the microbubble to collapse resulting in lower possibility of hydroxyl radicals to recombine which result in higher generation rate of hydroxyl radicals. The main disadvantages are no commercial plant using this system has been built yet and the amount of oxidant either ozone or hydrogen peroxide required to increase hydroxyl radical is large which increases the cost of operations [61, 62] 31

42 The treatment of spent caustic by application of Fenton s method is highly recommended. As seen in table 4 Fenton s reaction has several advantages that make this method to be widely implemented in the treatment processes. It has high efficiency and its ability to treat various contaminants and can lead to complete destruction of contaminants [36]. There are many researches that were done to treat refinery spent caustic. It is very difficult to treat with conventional wastewater processes because of that it has been incinerated. On the other hand, ethylene spent caustic is highly diluted than refinery that make it possible to treat and dispose it in a save manner. Ethylene spent caustic solutions are disposed of through wet air oxidation. However, the major problem is the exothermal reaction that needs to control the heat buildup in the process. Also, it is a very expensive process for the treatment. There are several researches that study the treatment of ethylene spent caustic by Fenton s method. Sheu and Weng, (2001) studied a new method of treatment of spent caustic from a naphtha cracking plant by neutralization followed by oxidation with Fenton s reagent. Spent caustic contains high H 2 S concentration and some mercaptans, phenols and oil. Over 90% of dissolved H 2 S were converted to gas by neutralization at ph=5 and T = 70 o C. The remaining residual sulfides were oxidized to less than 0.1 mg/l by Fenton s reagent. The total COD removal of spent caustic is over 99.5% and the final COD value of the effluent can be lower than 100 mg/l. As a result, the spent caustic treatment becomes economical and effective [11]. Moreover, Nunez, et all (2009) studied electro-fenton process. The efficiency of the Electro-Fenton process was investigated as the COD reduction in pure phenol and sulphide solutions and real 32

43 spent caustic samples. Approximately 97% COD removal was achieved for sulphide treatment, as the sulphide was highly affected by both the ph reduction and the oxidation by Fenton s reagent. In the real spent caustic sample, 93% COD reduction was obtained. The process designed includes a ph reduction unit followed by an Electro-Fenton s reactor. Its advantages regarding safety and costs make it a process that has to be considered in petroleum refineries [32]. Nowadays, real treatment plant by Fenton s method exists. The treatment by Fenton s is done by a company called FMC Foret. They modified Fenton s reaction to treat spent caustic and they named the process as Oxidation with Hydrogen Peroxide (OHP) [62]. However, there are differences between this method and the Fenton s reaction. The first difference is the catalyst used in FMC Foret, the catalyst used is ferrous salt without specifying the type of salt. The second difference is the operational conditions; Fenton s reaction operates at ambient temperature and pressure while FMC Foret operates at mild conditions [62]. Spent caustic is first pumped to an acidification tank to adjust the ph value to 3-5 so Fenton s reaction can take place [11]. After that, the feed is pumped to raise the pressure to bar. The pressurized spent caustic is then fed into a heat exchanger to raise the temperature to C [62]. Then the reactor effluent is send to a heat exchanger to cool the product. The effluent is then sent to neutralization tank where the ph is adjusted to a value around 7. As a result of neutralization, the ferric ion generated in the reaction will precipitate [61]. Finally the treated effluent is decanted then sent to biological treatment for post treatment. The main advantage of this process is the ability of treating influents with different organic content and some inorganic contaminants such as sulfides 33

44 and mercaptans. Also, COD removal can reach up to 95 % as well as the process is easy to install with low capital cost [62]. 34

45 1.6. Objectives: The main objectives of this thesis is to characterize and treat spent caustic produced from ethylene plants. The treatment process targeted a COD value less than 1000 mg/l and a sulfide concentration of 2 mg/l. These values were chosen since these are the limits that should enter the biological process which proceeds the chemical process. Two treatment processes will be studied: 1. Neutralization: neutralization will release all carbonates as carbon dioxide and all sulfides as hydrogen sulfide this in return will result in the COD reduction. 2. Neutralization coupled with oxidation: oxidation will further reduce the contaminant s concentration. Neutralization is applied before the oxidation because of high concentration of acid gases (H 2 S and CO 2 ) that would react with ferric ion causing a loss of iron catalyst For the neutralization coupled with oxidation two methods will be tested: 1. Classical oxidation by using hydrogen peroxide alone. 2. Advanced oxidation by Fenton s reagent. The effect of different parameters on the treatment process will be investigated namely, ph value, temperature, oxidants and catalyst concentration. The effect of these parameters on COD and sulfide removal will be measured. Figure 10 summarizes the treatment processes of ethylene plant spent caustic that were included in this study. 35

46 Figure 10: Summarized of the treatment processes of ethylene spent caustic 36

47 CHAPTER 2: 2. RESEARCH METHODOLOGY 2.1. Spent Caustic Characteristics A spent caustic sample was obtained from Qatar Petrochemical Company (QAPCO). The ph value and conductivity were initially measured using a ph/ conductivity meter (WTW/Germany). The ph and conductivity values were and ms /cm, respectively Total Suspended Solids and Total Dissolved Solids: Total dissolved solids (TDS), and total suspended solids (TSS) were determined according to standard method (ASTM D5907) [63]. To measure the total solids a weighted beaker was filled with 100 ml of spent caustic sample. The sample was placed inside an oven (Heraeus) at 100 and left until the sample was completely dry. The weight of dried sample was measured, subtracting the empty weight of the beaker it was found that the TS were equal to (85400 mg/l). To measure the TDS the sample was filtered using a 0.45 μm whatman filter paper. The filter was then dried at 100 until completely dry. The weight of the dried sample was measured it was found that TDS were equal to (77230 mg/l). TSS was measured by subtracting TDS from TS. It was found that TSS was equal to (8170 mg/l). All samples were done in triplicates and the mean values were reported. It was observed that most of the solids in the spent caustic sample were dissolved solids were they made 90 % of the TS. 37

48 Chemical Oxygen Demand (COD): Chemical oxygen demand was measured by HACH Program [64]. This method determines COD depending on the quantity of consumed oxygen by certain impurities in water based on the reduction of dichromate solution. Three main solutions were prepared for this test. The stock COD standard solution was prepared by dissolving g potassium hydrogen phosphate in 400 ml distilled water which was then diluted to 500 ml to get 1000 mg/l COD. Standard solutions with known concentrations of 100, 300, 500, 700, 900, 1000 mg/l COD, were then prepared. The second solution is the digestion solution which was prepared by adding 10.2 g potassium dichromate (K 2 Cr 2 O 7 ) standard grade, 167 ml H 2 SO 4 and 33.3 g powdered mercuric sulfate (HgSO 4 ) to 500 ml of distilled water, dilute and complete to 1 liter. The third solution is the catalyst solution that was prepared by adding 5.5 g of powdered silver sulphate (AgSO 4 ) to 1 kg of concentrated sulfuric acid (H 2 SO 4 ). After that, 2.5 ml of samples, standards and blank were added to the cultured tubes. Next 1.5 ml of the digestion solution and 3.5 ml of the catalyst solution were added to the cultured tubes. The tubes were caped tightly and shacked to mix the layers. Then tubes were placed in (Heraeus) oven at 150 for 2 hours. After that, the samples were allowed to cool for about 20 minutes until reaching room temperature. Blank and standard tubes were placed in the (Varian) UV- spectrophotometer to calibrate the device before measuring COD values. 38

49 Biological Oxygen Demand (BOD): Biological oxygen demand was measured by standard method (SM 5210 B) [65]. The BOD test measures the dissolved oxygen consumed by microbial life while assimilating and oxidizing the organic matter present. A sample has brought to the desired temperature 20 C and mixed very well to homogenize the sample. 94 ml of sample has poured into sample bottle. To inhibit nitrification three drops of Ally Thiourea or ATH were added because BOD measurement shouldn t include the oxygen consumption by nitrifying bacteria. Clean magnetic stirring rod was added in the bottle. 3-4 drops of 45% potassium hydroxide solution was added to seal gasket. This will absorb the CO 2. After that, the seal gasket was inserted in the neck of the bottle and the device was switched. The Oxidirect has an optional auto start function that enables it to start. BOD bottle was placed in position into the bottle rack.the incubation period for the sample is 5 days at 20 C. After five days the BOD value was found around 431 mg/l Total Sulfides (S 2, H 2 S, HS ) (sulfide above 1 mg/l) Total sulfides were measured by titration method [66]. The titration method is applicable to the measurement of total and dissolved sulfides for water in concentrations above 1 mg/l [66]. Sulfide is reacted with an excess of iodine in acid solution, and the remaining iodine is then determined by titration with sodium thiosulfate. Approximately 0.5 ml of a sample was taken into 100 ml conical flask and diluted with dematerialized water until 100 ml. Then 10 ml of 0.010N iodine was added 39

50 and mixed.without delay 10 ml of concentrated hydrochloric acid HCl was added and the sample was shacked vigorously. Immediately sample was titrated with 0.010N N 2 S 2 O 3, till faint yellow appear.then starch was added as an indicator and titration was continued until the blue color disappeared and became like light-straw color. A blank of approximately 100 ml dematerialized water was prepared and carry it through the same procedure as the sample. Then calculate total sulfide using equation 2.4. S 2 (mg/l) = (1000/ml of sample) (ml blank ml sample) (2.1) Determination of Total Sulfide (S 2, H 2 S, HS ) (sulfide 0 to 800 μg/l): The Methylene blue method by HACH Program is used to measure total sulfide (0 to 800 μg/l) [64]. 25 ml of sample was measured and filled in the sample cell. Also, a blank was prepared by measure 25 ml of dematerialized water into second sample cell. Then 1 ml of sulfide reagent (1) was added to each cell and swirl to mix. After that, 1 ml of sulfide reagent (2) was added to each cell. Immediately swirl to mix. The blank was placed and zero key was pressed. Then the prepared sample was placed in the cell holder. The light shield was closed then result is displayed Free Soda and Complete Alkalinity: To determine the quantity of available free soda and complete alkalinity in solution by manual titration was performed. This method is applicable for very low free soda to 0.1% and complete alkalinity as low as 0.1 % [64]. 40

51 A flask was cleaned by distilled water and dried with clean paper. The weight of empty flask was measured using Aeadam sensitive balance. With clean pipette 10 ml of sample was taken and weighted. Then 100 ml distilled water and three drops of phenolphthalein were added. After that, titrate it against 1N sulfuric acid till the pink color of solution changes to colorless. Then three drops of methyl orange was added to the colorless solution in the flask. Titration was continued against 1N sulfuric acid till yellowish colored solution changes to orange color. Calculate the Free soda equation (2.2): Free soda in % wt = 4 (V 1(6.8 ml) V 2 (9 ml)) p(10.19) (2.2) Complete alkalinity % wt = 4 (V 1(6.8)+V 2 (9)) p (10.19) (2.3) Where: P: is the weight of sample V 1: is the volume of H 2 SO 4 used in titration after addition of phenolphthalein V 2: volume of H 2 SO 4 used in titration after addition of methyl orange Calculate the total alkalinity by equation Total Petroleum Hydrocarbons (TPH) Total petroleum hydrocarbon (TPH) is a measure of concentrations mineral oil, hydrocarbon oil, extractable hydrocarbons and oil and grease. TPH was measured by standard method (ASTM D7678) [67]. This method covers the range between 0.5 to 1000 mg/l. 500 ml of the sample was taken in flask and sulfuric acid drops 41

52 were added to the sample until the ph value reached 2. Then 10 ml of solvent cyclohexane and magnetic stirrer bar were added to the sample. The sample was left in the magenitic stirrer to around an hour. After that, the mixture was transferred to a separation funnel and allowed to separate. The aqueous layer was run through the separation funnel and the upper oil layer was left in the funnel. For the calibration (1000, 2000, 3000, 4000 mg/l) sample by weight was prepared a 1.0, 2.0, g Tetradecane into a 100 ml volumetric flask then filled to the mark with cyclohexane.then the flask was shacked to get well mix of liquid. The CaF 2 cell was installed and carried out scan for blanks, standards and samples. Using Spectrum Beer s software, the concentration of the total petroleum hydrocarbon (TPH) in the water samples is calculated. The software uses the absorption data of the samples and standards obtained by FTIR analysis to measure the concentration of TPH. First, the standards concentrations (1000, 2000, 3000, 4000 ppm) are measured and a calibration curve is generated. The calibration curve is then used by the software to quantify and measure the TPH content in the tested water sample Inorganic Anions Inorganic anions were measured by standard method (EPA 300.1) [68].This method covers the determination of inorganic anions such as bromide (Br), nitrite (NO 2 ), nitrate (NO 3 ), fluoride (F - ), sulfate (SO 4 ), chloride (Cl - ), ortho-phosphate (H 3 PO 4 ). Dilution of the sample was done by taking 1 ml of spent caustic sample in 500 ml flask and filling it with distilled water to the mark. Then a small volume 42

53 of sample was inserted in the special tube for the device. Then the sample was injected into a Metrohm 850 professional ion chromatograph after calibration. The sample was analyzed and results were collected from computer Heavy Metals Heavy metals were measured by standard method (EPA 200.7) [69]. Inductively Coupled Plasma (ICP) device is used to analyze the major, minor, and trace elements in sample. It need relatively low sample size <10 ml for analysis and measures heavy metals up to 10 ppb or lower. This was done by taking 1 ml of sample in 100ml flask and fills it with distilled water 100 ml. I Cap 6000 series Thermo Scientific device was used.the plasma was ignited and an appropraite incident power with minimum reflected power was selected. The instrument was allowed to become thermally stable before beginning which took around min. The samples were inserted in the auto sampler rack and the work sheet for samples were filled Phenol Phenols were measured by standard method (EPA 420.1) [70]. In this method phenolic materials react with 4-aminoantipyrine in the presence of potassium ferricyanide at a ph of 10 to form a stable reddish-brown colored. The amount of color formed is a function of the concentration of phenolic material. Also, distillation is necessary to be done to eliminate any interference materials. About 500 ml of the sample was measured into a beaker. 5 ml of copper sulfate was added to the sample and ph of the sample was lowered to approximately 4 by 43

54 using phosphoric acid solution 85% H 3 PO 4. This is done to inhibit the biological degradation. Around 450 ml of distilled sample was collected then the distillation was stopped, and 50 ml of water was added to the flask and resume distillation until 500 ml have been collected. Standards were prepared by Dilution of 10 ml stock phenol solution to 1 liter with distilled then series of standards were (0, 50, 100, 200, 500, and 1000) were done in 100 ml volumetric flasks. Buffer solution (NH 4 Cl in NH 4 OH) was added to 100 ml of distillate and standards until the ph of the sample and standards should be 10 ± 0.2. Then 2.0 ml of Aminoantipyrine solution was added and mix Also 2.0 ml of potassium ferricyanide solution was added and mix. After 15 minutes read absorbance using spectrophotometer, at 510 nm. The main characteristics of the ethylene plant spent caustic are represented in Table 6. Average values for three samples are recorded. The characteristics of spent caustic solution produced from ethylene plants are different than those for spent caustic solution produced from any other petrochemical industry. The impact of these different characteristics on the several treatment processes will be studied. Sheu and Weng (2001) found that the ph value for a spent caustic solution produced from an ethylene plant was around which is close to the values measured in this study. However, phenols concentration was 300 mg/l which is very high compared to the analyzed sample where phenols concentration was almost zero. Phenol concentration was found 0.1mg/l which is less than discharge limit which is 0.5mg/l [72]. In addition Forbess et al. (2009) found that COD concentration was mg/l which is close to the analyzed sample in this study where it was mg/l. Moreover, It was found that sulfate concentration 44

55 was 507mg/l and the sulfide concentration was 4990 mg/l [15]. While for the analyzed sample the sulfate concentration was 8200 mg/l and the sulfide concentration was 5200 mg/l. Non of the previous studies studied the treatment process of spent caustic produced from ethylene plants. The different characteristics of such spent caustic solution would highly affect the treatment process. Table 12 : Spent caustic characteristics used in this experiment Parameters Unit Average Value COD mg/l BOD mg/l 431 ph 13.5 Conductivity ms/cm Oil & grease mg/l Sulfide mg/l 5200 TS mg/l TDS mg/l TSS mg/l 8170 Sulfate mg/l Chloride mg/l Phenol mg/l 0.1 Heavy metals mg/l traces Free soda wt % Total alkalinity wt %

56 2.2. Experimental Setup and Procedure Neutralization: Spent caustic sample 2- Burette 3- Mixer 4- Thermometer 5- ph meter 6- Hot plate Figure 11 : Experimental schematic diagram Figure 11 shows the experimental setup. Temperature and ph were continuously measured using (WTW/Germany). In order to ensure homogenous condition in the reactor the solution was continuously mixed at 200 rpm. The degree of stirring was kept mild as any excessive stirring lead to excessive foaming as a result of presence of some of polycyclic aliphatic organic compounds and some phenols and cresols [5]. The acidification reaction temperature was maintained by a thermal hot plate. Also, a mercury thermometer was placed within the reactor to monitor the temperature increase throughout the reaction. The selection of the chemicals used for the neutralization of an acid or base is almost as important as 46

57 the design of the neutralization system. There are many considerations ranging from health and safety to cost and convenience of operation. Strong acid sulfuric acid is almost universally used for neutralization reactions that it is used in this step to lower the ph from 1 to 8. Sulfuric acid is used because of its availability at concentrations ranging from 0% to 98% and it is the least expensive acid to use. It is easier and safer to use than HCl or HNO 3 [72]. The experiments were conducted in a 250 ml round bottom flask filled with 150 ml of the spent caustic sample. The flask is made of pyrex glass equipped with two 25 ml burettes, one filled with 98% wt concentrated sulphuric acid (panreac) and the other filled with 5.0 M sodium hydroxide solution. Sulphuric acid was added to each sample to maintain different ph values (1, 3, 5, 7, and 8). At each ph value temperature should maintain 30, 60, and 90. Samples were collected from the reactor for COD, sulfide and TDS analysis. After that, for the ph of 1, 3, & 5 the samples were immediately neutralized to a ph of 7 using sodium hydroxide since the suitable condition for to the biological process is around ph = 7 to 9 [73]. After adjusting the ph, samples were taken for COD, TDS and sulfide analysis. Total dissolved solids (TDS) measurements were done by filtering and evaporating spent caustic solution and measuring the mass of residues left. Also to check electrical conductivity was measured by conductivity meter. TDS is directly related to the concentration of dissolved ionized solids to create and conduct an electrical current [74]. 47

58 Neutralization Coupled with Oxidation: The Fenton oxidation technology works at ambient conditions however with the exothermic neutralization reaction the reactor temperature maintained about 65 by hot plate. This is also the reaction temperature; observed in the full-scale neutralization unit of ethylene plant.a mercury thermometer was also placed within the reactor to monitor the temperature increase throughout the reaction. Batch time was fixed at 60 minutes. The batch time was selected given that almost 90% of the COD removal occurs within the first ten minutes of the reaction [75]. The flask is equipped with three 25 ml burettes.the first burette is filled with sulfuric acid to adjust the ph of the samples to 1.5, 2.5, and 3.5. The second burette is filled with Fenton reagent 30 wt% lab grade hydrogen peroxide (panreac) for the oxidant regent. Ferrous sulfate catalyst is added before adding the oxidant reagent as solid and enough mixing is applied. And the third burette is for the 5.0 M sodium hydroxide to neutralize the sample after oxidation. The sample is neutralized to a ph of 7 to 9 since the suitable condition for to the biological process is around this ph value [74]. After adjusting the ph samples were taken for COD and sulfide analysis. 48

59 CHAPTER 3: 3. RESULTS AND DISCUSSION 3.1. Neutralization Effect of ph on Sulfide Removal: The removal of H 2 S by neutralization is defined by Henry s Law which is commonly associated with dilute solutions. It relates the concentration of gas in water to the partial pressure of the gas above the liquid [76]. For H 2 S removal it is necessary to verify the quantity exists in the water which is a function of ph and temperature. According to Arrhenius's, an acid is a substance that dissociates in aqueous solution, releasing the hydrogen ion H+ [77]. The equilibrium constant for this dissociation reaction is known as a dissociation constant pk a [78]. Hydrogen sulfide exists in equilibrium in three different forms H 2 S, HS -, S -2 as shown in reactions (3.1) and (3.2): H 2 S HS - + H + pk a =7.1 (3.1) HS - S -2 + H + pk a =14 (3.2) Acidification plays a significant role to prepare the spent caustic feed for further treatment. Contribution of acidification in treatment comes from releasing acidic compounds (total sulfide) that are captured by alkaline compounds (spent caustic) [5]. As shown in Figure 12 hydrogen sulfide varies with varying the water ph. At a ph equal to 7.0 approximately 50% of the total sulfide is H 2 S while 50% is 49

60 present as HS -. At a ph equal to 8.0, only 10% of total sulfide is present in the H 2 S form while the remaining 90% is HS - [77]. Figure 12 : Hydrogen sulfide and ph dependent In this experiment adjusting the ph of the spent caustic sample to 7, an excessive foaming was formed as shown in Figure 13-a. The foaming is due to the presence of containing some of polycyclic aliphatic organic compounds and some phenols & cresols [5]. Also, an extreme color change was observed as shown in Figure 13- b and very strong odor as rotten eggs was noticed because of the presence of sulfide compounds in the spent caustic sample. 50

61 ph=1 ph=3 ph=5 ph=7 ph=8 a) Foaming of spent caustic formed after addition of acid (b): color changed for neutralized Spent caustic at different ph values Figure 13: Neutralization of spent caustic Starting the experiments with a reaction temperature of 60 o C since this temperature is observed in the full-scale neutralization unit of ethylene plant. As shown in Figure 14 the best removal percentage of sulfide was at a ph value equal to 1 where 99.8 % of sulfides were removed from solution. Whereas the least removal percentage was At ph= 8. The amount of sulfide remaining in the treated solution was 9.9 mg/l at ph=1 and 63.5 mg/l at ph=8. As the ph increased the removal percentage of sulfides decreased. This is due to the fact that sulfides are mainly in the form of H 2 S gas at low ph values as shown in Figure 12. At a ph value equal to 7 approximately 50% of the total sulfide is H 2 S while 50% is present as HS -. Similar results were obtained by Weng and Sheu (2001) where they found that by in the neutralization of spent caustic, the reduction in sulphide concentration is significant when reducing the ph to around 4 to 5 [11]. Decreasing the ph below 4 has a little effect on the reduction of sulphide concentration. 51

62 Sulfide removal % ph=1 ph=3 ph=5 ph=7 ph=8 ph Figure 14 : Sulfide % removal at different ph after neutralization Effect of Temperature on Sulfide Removal: After the effect of ph on the sulfide removal has been studied the effect of temperature on the removal should be also investigated. From Figure 14 the best removal was found at ph value of 1 because of that the effect of temperature at this ph value is selected to be tested. Three temperatures values have been selected to be performed in these experiments which are 30, 60, and 90 o C. When the neutralization was carried out at ph=1, the best removal achieved at temperature of 60 with percentage removal reached to as shown in Figure 15. While at temperature of 30 and 90 the removal of sulfide was around 97.8% and 98.5 % which are lower than the removal at

63 Sulfide % Removal T=30 C T=60 C T=90 C Temperature Figure 15 : Sulfide % removal at different temperature Effect of ph on COD Removal : In these experiments the effect of neutralization reaction on the removal of COD from the spent caustic is shown in Figure 16 and 17. At the beginning the ph of spent caustic samples were dropped to 1, 3, 5, 7, and 8 at temperature of 60 o C by using concentrated sulfuric acid. After that, samples 1, 3, and 5 were neutralized again by using sodium hydroxide to ph 7 to 8. While for samples 7 and 8 the ph is left the same. This step is done due biological discharge limit. Figure 16 shows that the removal of COD reached around 60% for the samples 1, 3, and 5. While Figure 17 shows the removal of COD after the neutralization to ph=7-8 for ph=1, 3 and 5 reach around 80 to 88%. The data indicate that the average total COD removal by neutralization was about 85% at ph between 1 to 5. From the results, the increases on COD percentage removal were due to the hydrogen sulfide 53

64 COD % removal COD % removal removal from spent caustic solution. The remaining compounds in the treated solution are mainly hydrocarbons compounds [5] ph=1 ph=3 ph=5 PH Figure 16: COD removal % for ph=1,3,5 before neutralization T= 60 C T=60 C ph=1 ph=3 ph=5 ph=7 ph=8 ph Figure 17: COD removal % at different ph after neutralization 54

65 TDS % removal (mg/l) Effect of ph on TDS Removal: The expected impact on TDS from addition of various chemical dosages could be straight forward such as neutralization with acid or base to more complexes [79]. Addition of chemical will almost always increase TDS while the biological treatment alone will lower the TDS [79]. Figure 18 shows the effect of ph on TDS % removal after addition of acid to spent caustic samples to ph 1, 3, 5, 7 and 8. The results show that the change in the ph as a function of chemical addition has a very slight effect on TDS ph ph=1 ph=3 ph=5 ph=7 ph=8 ph Figure 18: Effect of ph on TDS removal 3.2. Neutralization Coupled with Oxidation: Classical Oxidation Oxidation by hydrogen peroxide is a complex reaction controlled by different variables, such as ph value, temperature, peroxide concentration and reaction 55

66 time [38]. These variables control the rate of the reaction, the consumption of hydrogen peroxide and the end products formed [80]. Hydrogen peroxide is a multipurpose oxidant for many systems. It can be applied to the system directly or with a catalyst. Incase catalyst is employed then this process is known as an advanced oxidation process Effect of Hydrogen Peroxide Concentrations on COD Removal: The effect of hydrogen peroxide H 2 O 2 concentration on COD removal in a spent caustic solution was studied. Different hydrogen peroxide concentrations were added to the spent caustic samples at a ph value of 2.5 and 60 minute reaction time. Treated samples were neutralized to a ph of 7 by 5.0M sodium hydroxide solution. The samples were then decanted and analyzed for COD. It was observed that after the addition of H 2 O 2 to both spent caustic samples and the blank samples that an increase in the COD value more than the original sample occurred as indicated in Table 7. For the blank sample, the device could read the COD reading for 0.1 dosage of H 2 O 2. However, it is out of range when the dosed increase to 1ml and 5ml. Furthermore, for the spent caustic sample, the COD value became more than the initial value. Table 13 : Effect of hydrogen peroxide concentrations on COD removal Volume H 2 O 2 Blank Spent caustic (ml) (ml) COD (mg/l) COD (mg/l) Over the range Over the range

67 This could be due to the hydrogen peroxide interference with analytical procedures which caused the increase the COD values [81]. When potassium dichromate is added to the hydrogen peroxide in the solution that is acidified with sulphuric acid, a green color appears as shown in Figure 19- a. It is mainly due to the Cr +3 ions formed by the reduction of potassium dichromate as in equation 3.3 [82]. K 2 Cr 2 O 7 + 3H 2 O 2 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) H 2 O +3O 2 (3.3) Also, the ratio of the COD to the concentration of hydrogen peroxide is important. The reason behind that is the reaction of the dichromate ions and hydrogen peroxide in an acidified solution proceeds by a more complex reaction mechanism as well as a reaction equation (3.3). According to Kang, Cho and Hwang (1999) [82], peroxodichromic acid which is blue in color is formed as shown in Figure 19-b: H 2 Cr 2 O 7 +5H 2 O 2 H 2 Cr 2 O H 2 O (3.4) This compound immediately reacts with hydrogen peroxide and forms chromic oxide (Cr 2 O 3 ) as follows: [82] H 2 Cr 2 O H 2 O 2 Cr 2 O 3 + 9H 2 O + 8O 2 (3.5) 57

68 (a ) (b) Figure 19: Blank sample with a) 0.1 H 2 O 2 and b) 1 ml of H 2 O 2 Hydrogen peroxide has three properties which are an oxidizing agent or reducing agent or liberates oxygen that may cause it to interfere with analytical procedures. Table (8) below lists those specific analyses in which H 2 O 2 is known to interfere with the analytical test [81]. Table 14 : Analyses interfere with H 2 O 2 [81] Analysis Procedure Effect Biochemical Oxygen Demand (BOD) Chemical Oxygen Demand (COD) Oxygen Uptake Dichromate Digestion Reduces Value Increases Value Sulfide Methylene Blue Reduces Value Sulfide Iodine Titration Reduces Value Several methods are commonly used to remove interferences due to hydrogen peroxide residual. The selection of the most suitable method is based on the desired analytical method [81]. 58

69 Effect of Hydrogen Peroxide Concentrations at Different ph on COD and Sulfide Removal: In order to remove the interference of H 2 O 2 on COD, elevated ph and temperature was used [81]. Treatment of spent caustic sample with high temperature should be maintained around 65. However in the real plant the temperature of spent caustic stream is around 65 therefore, there is no additional heating is required. Then neutralize the treated samples to high ph to around 8-9. Then samples were allowed to settle before being tested for the COD this can be done by centrifuging the samples. The liquid is decanted for analysis of COD and sulfide test. This process may be further accelerated by addition of iron compounds [45]. As shown in the Table 9 the interference of H 2 O 2 with COD was minimum after applying high temperature (less than 65 ) and neutralize to ph value of 7-9. Table 15: Effect of H 2 O 2 at different ph on COD removal Volume H 2 O 2 H 2 O 2 Initial ph=3.5 ph=2.5 ph=1.5 (ml) (ml) mmole/l COD COD COD COD (mg/l) (mg/l) (mg/l) (mg/l)

70 As shown in Figure 20, at the beginning when the spent caustic samples were neutralized to ph 1.5, 2.5, 3.5 the COD removal reach around 87%, 84%, and 80 %. Then when H 2 O 2 is started to be implemented in the system, it can be noticed that COD percentage of removal starts to increase. The COD removal increases as the concentration of hydrogen peroxide increase due to the generation of hydroxyl radicals. COD % removal keeps on increasing until it reaches a maximum value and then it starts to decrease. The maximum COD % removal is around 89% at ph=2.5 and hydrogen peroxide concentration 0.1 ml. Above this concentration, COD % removal decreases as the concentration of hydrogen peroxide increases. COD decreases at high concentration of hydrogen peroxide because of the scavenging effect of hydrogen peroxide [76]. Hydrogen peroxide is the oxidant of Fenton s reaction. However, at high concentration of hydrogen peroxide, hydrogen peroxide tend to react with hydroxyl radical to produce a weaker radicals HO 2 as was shown in reaction (3.6). Reactions (3.6) and (3.7) will compete and at high concentration of hydrogen peroxide, scavenging of hydroxyl radical will dominate [83]. As a result, hydrogen peroxide is not fully utilized to generate hydroxyl radicals. This will decrease the efficiency of the process as more hydrogen peroxide is needed to achieve the desired COD removal. This reaction can also imply that for the same volume of hydrogen peroxide, higher COD removal can be achieved if the concentration of hydrogen peroxide is kept low [83]. OH + H 2 O 2 H 2 O + HO 2 (3.6) Fe +2 + H 2 O 2 Fe +3 + OH + OH (3.7) 60

71 COD % removal ph=1.5 ph=2.5 ph= H 2 O 2 mmole/l Figure 20 : COD removal % at different H 2 O 2 concentration H 2 O 2 can control sulfides in two ways destruction by oxidizing sulfide to elemental sulfur or sulfate ion and prevention by providing dissolved oxygen that inhibits the septic conditions that lead to biological sulfide formation. The reaction between sulfides and hydrogen peroxide depends greatly on the ph of the solution. At low ph values, sulfide exists primarily as molecular hydrogen sulfide, H 2 S which reacts on a 1:1 (w/w) basis with hydrogen peroxide to form elemental sulfur. This is the most efficient use of hydrogen peroxide. At neutral ph, H 2 S and HS coexist and hydrogen peroxide reacts at a 1.5:1 (w/w) ratio with the sulfides. At alkaline ph, it takes four times as much hydrogen peroxide to turn the S -2 ion into sulfate. The reactions that occur in each ph range are shown in equation ( ) [80]: Acid ph: H 2 S + H 2 O 2 Sº + 2H 2 O (3.8) 61

72 Sulfide % removal Neutral ph: H + + HS + H 2 O 2 Sº + 2H 2 O (3.9) HS + 4H 2 O 2 SO H 2 O + H + (3.10) Alkaline ph: S H 2 O 2 SO H 2 O (3.11) In order to use less chemical oxidant spent caustic is neutralized to become an acidic solution. Therefore, sulfide is converted to hydrogen sulfide and oxidant will complete the removal of sulfide. By using the H 2 O 2 the concentration of sulfide is very low because of that Methylene blue method by HACH program is used to measure total sulfide (0 to 800 μg/l). As shown in Figure 21 almost 100% removal of sulfide was achieved H 2 O 2 mmole/l ph=3.5 ph=2.5 ph=1.5 Figure 21 : Sulfide % removal at different H 2 O 2 concentration 62

73 3.3. Neutralization coupled with Oxidation : Advanced Oxidation Process using Fenton s Reagent Oxidation by H 2 O 2 alone is not effective for high concentrations of contaminants. [84]. Because of that transition metal, ozone or UV-light is implemented to accelerate the reaction rate of H 2 O 2 decomposition to form hydroxyl radicals. The oxidation capability of Fenton s reaction comes from the production of hydroxyl radical. Fenton s reagent is a reaction between ferrous ion with hydrogen peroxide resulting ferric ion, hydroxyl radical and hydroxyl anion according to the reaction (3.12 ) [85-87]. Fe +2 + H 2 O 2 Fe +3 + OH + OH (chain initiation) (3.12) This reaction is the chain initiation of Fenton s reaction.the ferrous ion function is to initiate and catalyze the decomposition of hydrogen peroxide that causes the generation of hydroxyl radicals. The generation of hydroxyl radicals follow complex chain reaction [88]. Once the ferric ion is formed, it reacts with the hydrogen peroxide resulting decomposition into water and oxygen. Also, ferrous ions and radicals are also formed according to the following reactions [89, 90]: Fe +3 + H 2 O 2 Fe OOH +2 + H + (3.13) Fe OOH +2 HO 2 + Fe +2 (3.14) Reactions (3.13) and (3.14) are known as Fenton-like reaction. Moreover, Fe OOH +2 is an intermediate that can decompose to produce HO 2 radical. HO 2 radical can oxidize contaminants; its oxidation potential is much less than hydroxyl radical. Also, several chain reactions occur in Fenton s reactions which are [90]: 63

74 Fe +2 + HO 2 Fe +3 + HO 2 (3.15) Fe +3 + HO 2 Fe +2 + O 2 + H + (3.16) OH + H 2 O 2 H 2 O + HO 2 (3.17) From reaction (3.17), H 2 O 2 can work as an OH scavenger or as an initiator as is seen in reaction (3.12). Finally hydroxyl radical reacts with ferrous ions forming hydroxyl anion and ferric ions. This is the termination step which is shown in the following reaction [27]: OH + Fe +2 OH + Fe +3 (chain termination) (3.18) The overall Fenton chemistry can be simplified by accounting for the dissociation water this is according to the following reaction [91] 2Fe +2 + H 2 O 2 + 2H + 2Fe H 2 O (3.19) Equation (3.19) shows that the presence of H + is necessary to decompose H 2 O 2. This gives an indication of the need for an acidic environment to generate maximum quantity of hydroxyl radicals that will oxidize the contaminants. Incase complete oxidation occurs; contaminants decompose into water, carbon dioxide and some non toxic inorganic salts [92]. Hydroxyl radicals oxidize the organic compounds (RH) resulting a production of organic radicals (R ), which are very reactive and can be further oxidized [93, 94] RH + OH H 2 O + R (chain propagation) (3.20) Where R refers to any organic contaminant R refers to organic free radicals This reaction is chain propagation initiating a radical chain oxidation which can be further oxidized [95, 96]: R + H 2 O 2 ROH + OH (3.21) 64

75 R + O 2 ROO (3.22) The organic free radicals produced in reaction (3.20) may then be oxidized, reduced, or demniralized according to the following reactions [97] R + Fe +3 oxidation R + + Fe +2 (oxidation) (3.23) R + Fe +2 R + Fe +3 (reduction) (3.24) 2R - R R (dimerization) (3.25) Fenton s reagent can accomplish both chemical oxidation as well as coagulation treatment. Adjusting the ph to a range of 5-7 will result in precipitation of dissolved ferric particle. Precipitated ferric particle solids will combine to form flocs that will help remove the dissolved solids. As a result, Fenton s method can achieve both chemical oxidation as well as coagulation treatment. Fenton s reagent is known to have different treatment functions depending on the H 2 O 2 /FeSO 4 ratio. The ratio can determine the degree of oxidation to coagulation. In Fenton s reaction chemical oxidation effect is dominant when the amount of hydrogen peroxide exceeds the amount of ferrous salt (ratio above 2).While, the coagulation effect is dominant when the two amounts are reversed (ratio below 1/5) [98]. The aim of implementing the chemical oxidation process is destruction of contaminants and not to physically separate the contaminants. Because of that, it is very significant to select the accurate ratio to guarantee destruction of contaminant [99]. Other important parameters in Fenton s reaction that must be studied are ph, temperature, ferrous salt concentration and reaction time [99-103]. From the overall reaction of Fenton s chemistry, the ph can affect the system effectiveness as acidic media is required for the reaction to occur. The best ph value falls in the 65

76 range of 3 to 5 [101]. Once Fenton s reactions reach to completion, ferric particles in the treated effluent enable coagulation treatment easily by neutralizing the ph to 7. At this ferric ion converts into insoluble solid that precipitate and easily removed by sedimentation basin [98]. Finally, the treated effluent is sent to the biological treatment to achieve the desirable quality of treatment [100]. Ferrous sulfate concentration can contribute in Fenton s reaction in two main ways: oxidation as well as coagulation treatment. When the ferrous concentration increases, more coagulation treatment occurs [102]. Another parameter that affects the reaction efficiency is the temperature. Fenton s reaction is an exothermic reaction and any increase in the temperature will result of an increase of exothermic reaction rate. However, temperature increase causes decomposition of hydrogen peroxide that reduces the efficiency of the process [103]. The last parameter is the reaction time. It is essential to ensure enough residence time to allow oxidation to occur [101] Effect of ph in Fenton s Reagent on the Removal of COD The ph was adjusted to 1.5, 2.5, and 3.5, and 4.5 using 98wt% sulfuric acid. Ferrous sulfate catalyst 6.6 mmole/l was added to the spent caustic solution as solid and continuous mixing was applied to totally dissolve the catalyst. Hydrogen peroxide mmole was then dosed into the reactor. The temperature was controlled and maintained around minutes reaction time was fixed. Then the treated sample was neutralized to a ph value around 8-9 by a 5.0 M sodium hydroxide. A neutralized sample is then withdrawn and 66

77 COD % removal centrifuged in order to separate the iron floc from the treated liquid; the liquid is then decanted and analyzed for COD and sulfide. The ph value considered to be a major factor in Fenton s reaction and can influence the process efficiency [45]. It can be observed the overall reaction of Fenton s chemistry in equation (3.12), the amount of HO generated by the Fenton process is affected by the ph. Acidic media is required for the reaction to occur to produce the hydroxyl radicals as in equation (3.19). It was noticed that the sample get clear when applying the oxidant reagent alone, however applying the Fenton s reagent to sample result in a slight turbidity due to the presence of traces of iron catalyst. The results show that the optimum ph tested was found to be ph 2.5 with a maximum conversion about 95.6% as in Figure ph=1.5 ph=2.5 ph=3.5 ph=4.5 ph Figure 22 : Effect of ph on the COD removal 67

78 At operating ph >2.5 the decomposition rate decreases because of the decrease of the free iron species in the solution, probably due to the formation of Fe (II) complexes with the buffer inhibiting the formation of free radicals and also due to the precipitation of ferric oxyhydroxides [104, 105] which inhibit the regeneration of ferrous ions as in equation (3.26) and (3.27). Also, the oxidation potential of HO radical is known to decrease with an increase in the ph [106] Fe +2 + H 2 O 2 Fe +3 + HO + HO (3.26) Fe +2 + HO Fe +3 + HO (3.27) At lower ph (ph<2.5) the formation of (Fe(II) H 2 O) +2 occurs, which reacts more slowly with hydrogen peroxide, therefore, generates less amount of hydroxyl radicals that lead to reducing the degradation efficiency [107]. Moreover, the scavenging effect of hydroxyl radicals by hydrogen ions becomes important at a very low ph [108]. And the reaction of Fe +3 with hydrogen peroxide is inhibited [109] Effect of Ferrous Sulfate Concentrations on COD Removal The main objective of ferrous sulfate catalyst is to release hydroxyl radicals from hydrogen peroxide. Because of that it is essential to estimate the best ferrous sulfate concentration that will generate the highest amount of hydroxyl radicals. It was observed that after the addition of ferrous iron to the sample the color of the sample started changing while adjusting the ph value due to the reactions happening. It was also observed that a precipitate of iron was formed after quenching the reaction with sodium hydroxide. 68

79 COD % reemoval The effect of ferrous sulfate concentration on COD removal was studied. For all samples, 97mMol/l of hydrogen peroxide is fixed and ferrous sulfate added to spend caustic was varied (2.63, 1.32, 0.88, 0.66, and 0.13) mmole/l. As shown in Figure 23, COD % removal decreases as the concentration of ferrous sulfate decrease since ferrous ions will activate hydroxyl radicals. The maximum COD removal is around 87.5 % at ferrous sulfate concentration of 2.63 mmol/l and ph=2.5. The decrease in COD removal happens due to the scavenging effect of ferrous ion as shown in equation (3.28). Ferrous ions consume hydroxyl radicals to form HO 2 radicals that ends with several chain reactions as in equation ( ). This results in lowering the COD removal [27]. Fe +3 + HO 2 Fe +2 + O 2 + H + (3.28) FeSO 4 mmole Figure 23 : Effect of ferrous sulfate concentration on COD % removal ph=2.5 ph=1.5 69

80 Effect of Hydrogen Peroxide to Ferrous Sulfate Ratio on COD Removal It is critical to find the best ratio of hydrogen peroxide to ferrous sulfate that will generate the highest amount of hydroxyl radicals. Hydroxyl radicals oxidize contaminants in spent caustic which result in lower COD [45]. As COD removal increases, more hydroxyl radicals are being generated and vise versa [45]. Fenton s reagent is known to have different treatment functions, depending on the H 2 O 2 /FeSO 4 ratio. There are three kinds of treatment that can be achieved according to the ratio: 1. Physical separation as chemical coagulation: When the amount of Fe +2 employed exceeds that of H 2 O 2. Coagulation is dominant rather than oxidation which is undesirable since the aim is oxidation treatment [45]. Also, ferrous ions will compete with contaminants to react with hydroxyl radicals [98]. Ferrous ions will terminate hydroxyl radicals as shown in reaction (3.29). This reaction can convert ferrous ions from a catalyst to a reactant. This leads to lower hydroxyl radical s utilized for the oxidation of contaminants. OH + Fe +2 OH + Fe +3 (3.29) 2. Chemical oxidation (high ratio): When the amount of hydrogen peroxide is much higher than ferrous ions, oxidation treatment is dominant [98]. This case is desirable because the aim of applying Fenton s reaction is the oxidation of contaminants rather than physical separation. However, the issue with this ratio is competition of hydrogen peroxide and contaminants to react with hydroxyl radicals. Hydroxyl radicals tend to react with hydrogen peroxide instead of reacting with contaminants generating HO 2 as shown in reaction 70

81 (3.32). Moreover, HO 2 reacts with ferrous or ferric ions as shown in reactions (3.30) and (3.31). This will end with decreasing the removal of COD and loss in efficiency will occur. Fe +2 + HO 2 Fe +3 + HO 2 (3.30) Fe +3 + HO 2 Fe +2 + O 2 + H + (3.31) OH + H 2 O 2 H 2 O + HO 2 (3.32) 3. Chemical oxidation (Medium ratio of [Fe +2 ]/[H 2 O 2 ] 1): When the amount of hydrogen peroxide to ferrous ions falls in between the previous two extremes. At this range, hydroxyl radicals tend to react with contaminants instead hydrogen peroxide or ferrous as shown in reaction (3.33). At this range, ferrous ions react with hydrogen peroxide rather than reacting with hydroxyl radicals and maximum amount of hydroxyl radicals are generated [98]. RH + OH H 2 O + R (3.33) Table 16: Effect of hydrogen peroxide to ferrous sulfate ratio on COD % removal H 2 O 2 (ml) FeSO 4 (g) FeSO 4 (mmole) H 2 O 2 (mmole) H 2 O 2 /FeSO 4 Ratio ph=2.5 COD (mg/l) ph=1.5 COD (mg/l) The optimal ratios of chemicals in the Fenton process recommended in the literature are the ratios H 2 O 2 /catalyst from 5:1 to 40:1 [110]. 71

82 COD % removal In the experiments, the selected ratio was of hydrogen peroxide to ferrous sulfate (1.5:1), (3:1), (7:1), (15:1), and (30:1). As shown in Table ph=2.5 ph= H 2 O 2 /FeSO 4 Figure 24 : Effect of hydrogen peroxide to ferrous sulfate ratio on % COD removal Figure 24 shows the COD % removal for different hydrogen peroxide to ferrous sulfate ratios for ph=1.5 and 2.5. At the beginning of the run, concentration of ferrous ions in spent is high since it is all added at once while concentration of hydrogen peroxide is low since it is dosed to the system. As a result, ferrous ions consume hydroxyl radicals as in equation (3.29). The reaction proceeds, ferrous ions concentration drops as more ferrous ions react with hydrogen peroxide and hydrogen peroxide to ferrous ions ratio approaches optimum ratio. At this ratio chemical oxidation will take place. In this case hydroxyl radicals tends to react with contaminants as in equations ( ). At ph=2.5, the maximum COD 72

83 removal of 96.4% is achieved at hydrogen peroxide to ferrous sulfate ratio of (7:1) while the minimum COD removal is 94.3% at a ratio of (30:1). Furthermore, at ph=1.5 the maximum COD removal of 95.1% is achieved at hydrogen peroxide to ferrous sulfate ratio of (7:1) while the minimum COD removal is 92.6% at a ratio of (30:1) Effect of Hydrogen Peroxide to COD ratio on Percentage COD Removal The optimum ratio of hydrogen peroxide to ferrous sulfate ratio was found, hydrogen peroxide to spent caustic initial COD ratio is needs to be determined. This ratio is essential in Fenton s reaction because it verifies the amount of hydrogen peroxide that should be used to carry the treatment of spent caustic. The cost of treatment by Fenton s process mainly depends on the cost of chemicals. Chemicals used in Fenton s reaction are hydrogen peroxide, ferrous sulfate, sulfuric acid and sodium hydroxide. The cost of hydrogen peroxide is the most expensive. So the running cost mainly depends on hydrogen peroxide to COD ratio. Different hydrogen peroxide to COD ratios is tested to find out the required ratio to achieve the desired treatment degree. Table 1 show three experiments were done at ph=2.5 with different H 2 O 2 to COD ratio. Then the COD % removal was studied. 73

84 Table 17: Effect of hydrogen peroxide to COD ratio on % COD removal H 2 O 2 H 2 O 2 FeSO 4 FeSO 4 / Initial Final COD H 2 O 2 /COD (ml) (g/l) g/l H 2 O 2 COD COD % g/g Ratio (mg/l) (mg/l) removal From Figure 25 it can be seen that as the hydrogen peroxide to COD ratio increases, COD removal decrease. This is due to the competition of hydrogen peroxide and contaminants to react with hydroxyl radicals. Hydroxyl radicals react with hydrogen peroxide instead of reacting with contaminants as discussed previously. This will decrease the removal of COD. The maximum tested COD removal achieved was 96.4% with a ratio 1.23 H 2 O 2 /COD (g/g) of and the lowest tested COD removal was 83.9% at a ratio of 2.76 H 2 O 2 /COD (g/g). Maximum COD removal obtained is 542 mg/l which satisfy the desirable removal to proceed to the biological treatment. 74

85 COD % removal H2O2/COD (g/g) Figure 25 : Effect of hydrogen peroxide to COD ratio on % COD removal 75