Hydrogen Sulfide Removal from Waste Air by Oxidation reaction with Sodium Hypochlorite in CSTR

Transcription

1 he9 The Forth PSU Engineering onference 8-9 December 5 ydrogen Sulfide Removal from Waste Air by Oxidation reaction with Sodium ypochlorite in STR harun Bunyakan* Juntima hungsiriporn Junya Intamanee azardous Pollutant Research Group, Department of hemical Engineering, Faculty of Engineering, Prince of Songkla University,at Yai, Songkhla charun.b@psu.ac.th * Abstract An additional product from wastewater treatment plant is odorous air. ydrogen sulfide ( S) releasing from the wastewater causes the problems with odors. The treatment of S in air stream was established for effective odor control from source. The kinetics of the oxidation of S with was studied. Kinetics parameters including reaction orders and the rate constant were determined. The results indicate that the order of reaction with respect to S and were 1. and, respectively. The rate law obtained from this work was used to simulate the treatment of wasted air using STR. The simulation results reveal that the space time of STR is strongly influenced by the initial concentration of S in the reactor. Another word, the higher S concentration the faster reaction rate is achieved. Thus less space time is required to reach the specific conversion than those required for lower S concentration. Moreover, from the simulation results, we can conclude that the θ value of is suitable for treating S in STR with a space of -3 minute. Finally, the simulated results indicating that the treatment of air contained S using simple STR is possible. Keywords: S,, oxidation, STR 1. Introduction Air quality problems commonly encountered in production operation were surveyed, highlighting hydrogen sulfide ( S) as odorous gases of most concern. The legislation-controlling odor at wastewater plants tends to more concentrate in the future. The S gas is known as sewer gas because it is produced by the decay of waste material. The S gas has a strong odor at low levels, colorless, and occurs naturally in environment. S gas can be formed and released whenever waste contains sulfur is broken down by bacteria. A number of chemical means have been characterized in terms of the prevention of S formation and release to atmospheres. The S gas that causes the problems with odors and corrosion can be treated with oxidation reaction as commonly used in removing odor. Sodium hypochlorite is a strong oxidizing agent that was normally used for disinfections and control malodorous in wastewater or emission air. The sodium hypochlorite is produced when chlorine gas is dissolved in a sodium hydroxide solution. The stability of sodium hypochlorite solution depends on the hypochorite concentration, the storage temperature, the length of storage (time), the impurity of the solution, and exposure to light. Decomposition of hypochlorite over time can affect the feed rate and dosage. By using, S acid odor can be efficiently absorbed and oxidized as the following oxidation reaction [3]. S + 4 Na O 4 + 4Nal The reaction between S and oxidizing agent is almost instantaneous (assuming sufficient oxidizing agent is present). This reaction converts the sulfide to - sulfate ( SO 4 ) ion. The goal of this research is to develop a treatment system with oxidation reaction for treating S in waste air. Thus, to accomplish our goal, the understanding of the reaction kinetics of S oxidation by applied oxidant is crucial. The aim of this work is to determine the kinetics of S oxidation by sodium hypochlorite (). The study was focused on the determination of rate law for S oxidation by. The kinetics knowledge obtained from this work can be used to design continuous stirred tank reactor (STR) for treating air contained S at room temperature.. Experimental Methods.1 Experimental procedure The kinetic studies for S oxidation by, under excess concentration of, were investigated in a constant batch reactor. Two liters of wastewater with a known concentration of S was placed in the reactor. An initial liquid sample (before the addition of oxidants) was collected and analyzed for initial S concentration. To start reaction, excess amount of, commercial grade solution of 1% sodium hypochlorite, was added into reactor to give he-54

2 he9 the desired initial oxidant concentration. The reaction temperatures were maintained at 3±1 o, room temperature, for all runs. The initial conditions for all experiments are summarized in Table 1. All reactions were allowed to proceed for 6 min. During experiment, liquid samples were collected periodically, at every 5 minute in the first 15 minutes, and every 1 minutes for the rest of reaction time.. Analytical techniques S in liquid samples were analyzed using Iodometric Method [1]. The S concentration-time data were used to determine the kinetic parameters (i.e., order of reaction and rate constant) using the initial rate data and the differential method []. Table 1. The initial experimental condition Experimental Run. No. Reaction Temperature ( o ) Initial oncentration (mmol/l) S ± ± ± ± ± ± ± ±1 3. Results and Discussion 3.1 oncentration-time data The experimentation result for S oxidation by can be shown by concentration-time data at constant temperature, which typically depicted as Figure 1. S conc. (mmol/l) initial.83 mmol/l initial.497 mmol/l initial.39 mmol/l initial.9 mmol/l time (min) Figure 1. S concentration-time curves for various initial S concentrations at initial concentration of.77 mol/l (T= 3 ±1 o ) Figure 1 gives the S concentrations at any reaction time with respect to the variation of initial S concentrations in the range of.9.83 mmol/l and constant initial concentration at.77 mol/l. The curves show that the concentration of S is sharply decreased with approximately 8 to 87% by mole of initial concentration within the reaction time of 5 minutes for all controlled conditions. From reaction time of 5 minutes to the end of experiment (6 minutes), the rate of oxidation of S by is less with approximately S removal of only 3 to 1% by mole of initial concentration. Moreover, at the end of reaction, the concentration of S is nearly same with approximately.3 mmol/l for all runs. 3. Initial Oxidation Rate of S The initial oxidation rate of S for each experimental run was determined from concentrationtime data according to Eq. () d r, = () dt t= where r S, is the initial oxidation rate of S (mol L -1 sec -1 ), and is S concentration (mol L - S 1 ) in reactors at any reaction time t (sec). The initial rate for each reaction condition is listed in Table. Table. The initial oxidation rate of, r, at S, 3±1 o Experimental Run. No. Initial oncentration (mmol/l) S r x1 6 S, (mol L -1 s -1 ) The initial rate and initial concentration were used to determine the kinetic parameter (i.e., order of reaction and rate constant). 3.3 Rate law for S oxidation by The rate law for oxidation reaction of S by can be simply expressed by power law model as given by equation (1) α β r = k (1) where k is the rate constant ((mol L -1 ) 1-α-β sec -1 ), α and β are reaction orders with respect to S and, respectively whereas the S and are the concentration of S and, (mol L -1 ), respectively Order of reaction with respect to S At constant concentration, the initial he-55

3 he9 rate law for S oxidation by can be simply expressed as Eq. (). r = k () ' α, o, where k ' is the pseudo rate constant (= k ) β, ((mol L -1 ) 1-α sec -1 ), α is reaction orders with respect to S, S, is initial S concentration (mol L -1 ). The orders of reactions (α) were determined by fitting Eq. () with the experimental data at constant reaction temperature and constant concentration (Run No. 1-4). The initial rate law of Eq. () can be rewritten as Eq.(3). ' ln( r, ) ln, ln k = α + (3) To easy in the data plotting, all terms of Eq. (3) were multiplied by 1 as shown in as Eq. (4). Then the curve will appear in the + quadrant. ln( r S ) = α ln S, ln k (4), The plot of ln( r S, ) versus ln( ) S, was performed for α determination. Trend of the curve was fitted to straight-line equation and measured for slope and intercept. The plot according to Eq. (4) is shown in Figure. -ln[r,] ln[,] Figure. The plot of ln r ) versus (, ln( S, ) at constant initial concentration and T= 3 ±1 o It was found from the Figure that the curve fits well with the R of.99 and shown the relationship of ln( r ) = 1.54 ln( ) S. omparing with Eq. (4), the α value is 1.54 that can be approximated to the 1 st order reaction with respect to S concentration Order of reaction with respect to At constant S concentration, the initial rate law for S oxidation by can be simply expressed as Eq. (1). r = k (4) " β, o, where k " α is the pseudo rate constant (= k S, ) ((mol L -1 ) 1-α sec -1 ), β is reaction orders with respect to,, is initial concentration (mol L -1 ). The order of reaction (β) was determined from initial oxidation rate and initial concentration data, at constant initial S concentration, using the same procedure as discussed above. In this case, the initial rate of S can be expressed in term β as given by Eq. (5). and, ln( r S, ) = β ln, + ln " (5) k The β value was then determined by fitting Eq. (5) with the experimental data at constant reaction temperature and constant S concentration (Run No. 5-8). To easy in the data plotting, all terms of Eq. (5) were multiplied by 1 as shown in as Eq. (6). Then the curve will appear in the + quadrant. " ln( r, ) ln, ln k = β (6) The plot of ln( r S, ) versus ln( ), was performed for β determination. Trend of the curve was fitted to straight-line equation and measured for slope. The plot according to Eq. (6) is shown in Figure 3. -ln[-r, ] ln[] Figure 3. The plot of ln r ) versus (, ln ) at constant initial S (, concentration and at T= 3 ±1 o he-56

4 he9 It was found from the Figure 3 that the curve fits well with the R of.9 and shown the relationship of ln( r ) =.8ln( ) S. omparing with Eq. (6), the β value is.8 that can be approximated to the th order reaction with respect to concentration. The small effect of concentration on S oxidation rate may attribute to the excess in amount of, which intently used in our investigation in order to obtain the oxidation rate law for oxidation of S under excess concentration. This rate law will be effectively applied for air pollution treatment in real environment since not only S will be presented. To completely remove S from wasted air, the excess amount of is definitely required The rate constant, k Once the orders of reaction are known, the rate constant is calculated from the initial rate as expressed by equation (7). r, o k = (7) α β,, The calculated k values for each run are listed in Table 3. Table 3. The rate constant for oxidation of S with at 3±1 o Experimental Run. No. Initial oncentration (mmol/l) k x1 3 (mol L -1 s -1 ) S From Table 3, if the highest (4.74) and lowest (.38) value of k are excluded, the rest are approximately the same. The k value average from 6 experimental runs was.34 (mol L -1 ) - sec -1 with the standard deviation of.67. Thus, we can say that the calculated k are approximately the same for all experimental runs. This indicates that the orders of reaction (α and β) and the rate constant ( k ) are reliable. onsequently, the rate law for oxidation of S with under excess concentration of at 3 o can be expressed as Eq. (8). r. 34 = (8) where is the reaction rate S in mol L -1 r S sec -1 and the and are the concentration of S S and, (mol L -1 ), respectively. 4. Simulate S removal from wasted air using continuous stirred tank reactor (STR) Application of rate law for the oxidation of S by (Eq. (8)) is demonstrated through an example. onsider the wasted air contains S at level of.1- % by mole. The wasted air is treated using STR contained solution of. The reaction is completely mixed and took place in liquid phase at reaction temperature of 3 o. The diagram of STR for treating wasted air is depicted in Figure 4. S ontaminated air (i.e. from wasted water) Solution Blower F S, S, lean air F S = mol/s = mol/dm S 3 Air distributor Figure 4. Schematic diagram of STR for treating S contaminated air The set of STR mainly comprises of reactor unit with an agitator, feed line, and overflow line. The oxidizing solution was first fed into the reactor. The S contaminated air is fed to reactor and completely mixed with oxidizing solution. Thus the reaction takes place in liquid phase. For constant flow of wasted air in and out of reactor, the efficiency of S removal, within the STR, is defined as Eq. (8) S, in, out Removal Efficiency = (8),,, in where in and out are S concentrations at reactor inlet and outlet, respectively. Eq. (8), however, is essentially S conversion in STR which given by Eq. (9)., onversion = 1 (9) where S, and S are S concentrations at the reactor inlet and outlet, respectively. The conversion or removal efficiency in this case, depends on reactor volume, molar feed rate, and rate of reaction as given by Eq. (1) [4]. F, V = (1) r he-57

5 he9 where F S, is molar flow rate of S to reactor, (mol sec -1 ) while (dimensionless) and r S (mol L -1 sec -1 ) are conversion and reaction rate of S. The F S, relates to a volumetric flow rate at reactor inlet and inlet S concentration as given by Eq. (1) F = v (11) S, o, where v is a feed volumetric flow rate (L sec -1 ). Substitution of Eq. (11) into Eq. (1), we have v, V = (1) r or in terms of space time, τ (sec), V S, τ = = v r r τ or = (13), The rate law is given by Eq. (8). r S r. 34 = (8) Where the concentration of S and are related to conversion as expressed by Eq. (14) and Eq. (15), respectively. S = S, (1 ) (14) = S,( θ ) (15), where θ = (16) S, Substitute Eq. (8) and Eq. (14) to Eq. (16) into Eq. (13), we have =, (1 )( θ ) τ V or τ = = v, (1 )( ) θ (17) From, Eq. (17), it clearly showed that the space time or volume of reactor required to obtain high conversion of S is strongly depended on the initial concentration of S, the initial concentration of and the volumetric flow rate of wasted air stream fed into the reactor. Eq. (17) can be used to determine the space timeτ. The calculated space time required to achieve 99% conversion of S in STR system at various θ and inlet S concentration are illustrated in Figure 5. From figure 5, it clearly shown that the space time is strongly influenced by the initial concentration of S in the reactor. This due to the oxidation rate of S is the first order reaction with respect to concentration of S. Another word, the higher S concentration the faster reaction rate is achieved. Thus less space time is required to reach the specific conversion than those required for lower concentration. Moreover, the results of simulation in figure 5 also pointed out that the θ value of is recommended for treating S for any initial S concentration. At θ greater than, with all initial S concentration, only slightly decrease in space time was observed. The typical space of -3 minutes was obtained at θ about. Space Time (min) ,=.1 mol/l, =.1 mol/l, = mol/l,/,,=.5 mol/l, =.5 mol/l Figure 5. alculated space time required to achieve 99% conversion of S in STR system operated at 3 o for various initial concentrations of S and. 5. onclusion The kinetics of the oxidation of S with was studied. Kinetics parameters including reaction orders and the rate constant were determined. The results indicate that the order of reaction with respect to S and were 1. and, respectively. The rate law obtained from this work was used to simulate the treatment of wasted air using STR. The simulation results reveal that the space time of STR is strongly influenced by the initial concentration of S in the reactor. Another word, the higher S concentration the faster reaction rate is achieved. Thus less space time is required to reach the specific conversion than those required for lower S concentration. Moreover, from the simulation results, we can conclude that the θ value of is suitable for treating S in STR with a space of -3 minute. he-58

6 he9 Acknowledgments This research was financially supported by Faculty of Engineering, (Grant for Research Group Development budget year 5). The Graduate School at Prince of Songkla University provided partial funding for the student. Other supports from Department of hemical Engineering and the Faculty of Engineering at Prince of Songkla University are gratefully acknowledged. References [1] APA, AWWA and WPF Standard Methods for the Experimentation of Water and Wastewater, 16th Ed [] Bunyakan,., Akuru, T., and hungsiriporn,j. 4. Kinetics of the Oxidation of Methyl Ethyl Ketone (MEK) by Potassium Permanganate. Regional Symposium on hemical Engineering December 4. Bangkok. Thailand. [3] Gao, L., Keener, T.., Zhuang, L., and Siddiqui, K. F. 1. A Technical and Economic omparisson of Biofiltration and Wet hemical Oxidation (Scrubbing) for Odor ontrol at Wastewater Treatment Plants. Environ Eng Policy. : 3-1 [4] Fogler,.S Elements of hemical Reaction Engineering. Prentice-all International, Inc. New Jersey, USA. he-59