[Ammonia absorption] Experiment conducted on [13 February 2013]

Transcription

1 [Ammonia absorption] Experiment conducted on [13 February 2013] Module title: EUB_5_968 Principles of reaction and separation Occurrence Number: [9] Course: [BEng (Hons) Petroleum Engineering] Prepared for: Dr Rim Saada Prepared by: [Arcanjo Malo Wacunzo] Student Number: [ ] [February/2013] London South Bank University Department of Urban Engineering Faculty of Engineering, Science and the Built Environment 103 Borough Road, London, SE1 0AA

2 CONTENTS Contents... 2 List of Appendices.2 Summary... 3 CHAPTER 1 INTRODUCTION... 4 BACKGROUND... 5 CHAPTER 2 EXPERIMENTAL EQUIPMENT... 5 INSTRUMENTS... 6 EXPERIMENTAL PROCEDURE... 7 CHAPTER 3 PROCEDURE AND OBSERVATIONS CHAPTER 4 RESULTS AMD CALCULATIONS CHAPTER 5 DISCUSSION CONCLUSION 17 REFERENCES LIST OF TABLES TABLE 1.1 RESULTS OBTAINED... 8,9,10 LIST OF FIGURES L o n d o n S o u t h b a n k U n i v e r s i t y Page 2

3 SUMMARY The experiment undertaken was the ammonia absorption using counter current system. This is a process used in the chemical industry and also within the petroleum industry whenever the engineers obtain impurities from the down hole. These impurities can be diverse undesired fluids or gas like H2S, CO2 and others. By and large, the main objective of the experiment was to calculate the Kga which is known as the overall mass-transfer coefficient. This coefficient would have been calculated based on the data acquired from a system constituted of ammonia, air and water. This system had a packed column, in addition to it the air flow rate was been changed whenever the system was been restated. In general, to proceed with the experiment in order to achieve the goals mentioned, a range of instruments composed a system such as rasching rings, pipette, and volumetric glass. Moreover, Air passing from a rotameter bubbles through a concentrated solution of ammonia to pick up ammonia gas, also a stop watch to control the time. Safety was not forgotten, equipment such as gloves and glasses were used due to the highly dangerous and volatile substances used. In general, a range of data were obtained and the flow rate of the air was been reduced from 14 to 4 overall. This resulted in a reduction of the volume of water absorbed and the duration of the test was also longer as the air flow rate was been decreased. This fact was noticed by the last test which nearly 30 minutes until the change of the parameter acid/basic. Overall, the data required to make the calculation of the mass-transfer coefficient were obtained in normal conditions, and it can be affirmed that the decrease of the air flow rate in a absorption system can result in a longer time to determine whether the based was totally transformed. These facts were relevant for our knowledge baggage, it was possible for the group to understand a process used in real life and be able to observe the colour that indicated an acid and an alkaline. L o n d o n S o u t h b a n k U n i v e r s i t y Page 3

4 CHAPTER 1 -- INTRODUCTION I.I Background The counter current System consists of two or more High Rate fluidized bed reactor vessels connected in series raw water flows from vessel to vessel being contacted with resin of a higher capacity in each subsequent reactor vessel. Fresh regenerated resin is fed to the final reactor vessel while loaded resin is removed from this vessel at the same rate and sent to the preceding vessel. (10) The counter current contacting of raw water with Resin Allows more selective competing anions to be removed with partially loaded resin in the first reactor vessel so that fresh resin added to the final reactor vessel will remove less selective target anions such as nitrate and bromide. This configuration can also be used to remove high levels of more selective anions. The more efficient sequential loading of Resin reduces the regeneration system size that would otherwise be required for challenging contaminant removal using a standard High Rate System, allowing significant reductions in waste volumes, salt consumption and operating costs. (10) Gas absorption is a mass transfer process in which a gas mixture is contacted with a liquid to specially absorb the components of the gas stream. This action is found in the chemical industry for the recovery of valuable products and cleaning of exhaust or vent streams. (4) In the oil and gas industry the ammonia is neutralizing the acid constituents of crude oil and for protection of equipment from corrosion. In addition to it, Ammonia is recycled in the mining industry for mining of metals. (3) Ammonia is a very valuable source of nitrogen that is essential for plant growth Agricultural industries are the major users of ammonia. (2) The NH3 is a usual factor that is used in the production of fertilisers such as the Urea. This has a great impact on various products which adds to its significance and importance in the chemical and petroleum industries. (1) Ammonia Absorption is a process in which a gas mixture of ammonia contacted with a solvent commonly water to dissolve on or more components of the gas and to provide a solution of them in the liquid. (1) L o n d o n S o u t h b a n k U n i v e r s i t y Page 4

5 Image 1: A counter current process system by MIEX. (Source Ref.10) Falling film Theory statuses that the mass transfer process happens alongside an interface, in which each phase is in the form of a stagnant layer of fluid of specified thickness. This layer of fluid is called film and it is adjacent to the interface. In the film, mass transfer takes place by steady-state one-dimensional molecular diffusion in the direction normal to the interface. (3) The penetration theory proposes that the time of contact of a fluid to mass transfer is little. Consequently the concentration of the dissolved gas might be considered to be equal to the bulk liquid concentration when the element is reaches the surface. The residence time at the phase interface is the same for all elements. Mass transfer takes place by unsteady molecular diffusion in the various elements of the liquid surface. (3) L o n d o n S o u t h b a n k U n i v e r s i t y Page 5

6 The procedure taking place in the absorption column can also be visualised on the basis of Two-Film theory. This happens in a way that an imaginary interface exists at the border between the gas and water. With this the Gas and liquid films similarly exist at either side of the interface and they are respectively bounded at the other end by the bulk gas and bulk liquid. The concept clarifies that mass transfer will only occur at the interface when there is a driving force on the gas film. The aims of this experiment are to define the overall Mass Transfer coefficient of the (ammonia, air, water) mixture at different air flow rates. This is a combined coefficient of the fluid in the turbulent section, the transition section and the laminar film as well as the coefficient of the mass transfer in the liquid. A further objective of the experiment was to demonstrate why the gradient obtained from the experimental curve did not match the literature value. This was verified by calculating the Reynolds s number as explained above. Based on the theory investigated the formula used to calculate the mass transfer was: K g a B FA 5.0E E E E E E+00 Mass Transfer Coefficient vs Air Flow Rate Mass Transfer Coefficie nt vs Air Flow Rate This information investigated shows that as the air flow rate increased the mass transfer coefficient increased which shows a direct proportionality between them. L o n d o n S o u t h b a n k U n i v e r s i t y Page 6

7 CHAPTER 2 - EXPERIMENTAL EQUIPMENT 2 - Diagram of the ammonia absorption system Counter Current Water rotamete r Packed rings Air rotameter 20ml H2SO4 5ml H2SO4 H2O Air Acid H2O + NH3 Excess H2O NH3 + H2H NH3 L o n d o n S o u t h b a n k U n i v e r s i t y Page 7

8 Figure 2 Photograph taken from the logbook sketch illustrating the system L o n d o n S o u t h b a n k U n i v e r s i t y Page 8

9 EXPERIMENTAL PROCEDURE Firstly, The water flow rate was set at 1L/min, and the water was allowed to flow for 10 minutes to allow the packing to be fully wetted Then bottle one and two were filled up with 5ml and 20ml of sulphuric acid respectively before being diluted with distilled water up to an equal mark on both bottles 6 drops of Phenolphthalein indicator was then added to the two solutions, Air flow was set to 14L/min and the air was allowed to bubble through the Ammonia supply bottle. The air then went up through the column into the safety bottle of sulphuric acid. The stop watch was initiated as the three-way valve was turned on allowing the air to go through the absorption train (Bottle one & two). When the contents of bottle one changed colour to pink, the test was stopped by closing the three-way valve and recording the time taken using the stop watch. Then the water containing the NH 3 was collected over the same period. the two bottles of acid were mixed together and were them used in back titration using 0.01M solution of sodium Hydroxide (NaOH) After that, 25ml sample of the water collected was also titrated using 0.01M solution of Sulphuric acid ( H SO ) 2 4 The experiment was repeated for different air flow typically (14,10, 8, 4 L/min) The results were tabulated in the tables shown in results & calculations section. L o n d o n S o u t h b a n k U n i v e r s i t y Page 9

10 CHAPTER 2.1 OBSERVATIONS The Colum with the packed rasching rings was showing vapour. The indicator showed the purple colour whenever the parameter changed As soon as the three way valve is opened the ammonia/air mixture bubbles through the acid bottles The raching rings are randomly positioned to give maximum contact area The air rotameter was fluctuating at higher flow rates As the Air flow rate increased errors started occurring and less liable results were obtained. The volume of NaOH used for the back titration in the first test was unusually very high and the test was rerun and found to be normal. The air rotameter and water flow meter were not steady throughout the experiment. This could lead possibilities of minor errors in the readings obtained The Three-way valve was controlled manually. There are possibilities of discrepancies in the time recorded to shut the valve. L o n d o n S o u t h b a n k U n i v e r s i t y Page 10

11 CHAPTER 4 CALCULATIONS Graph 1 Results obtained in the experiment to use for the calculations. Duration of test Water flow (L/min) Air flow (L/min) H 2 SO 4 concentration used in absorption Initial H 2 SO 4 Volume used Back titre Volume of water in absorption (s) F W F A N ml ml ml Sample Calculations Sample calculation for the following tests tests in minute at air flow rate = 14, 10, 8, 4 L/m First the values are converted to SI units the flow rate from L/m to L/s as shown below: F = 14 L / m 14/60 = 0.23 L/s A F W F W F W = 10 L / m 10/60 = 0.16 L/s = 8 L / m 8/60 = 0.13 L/s = 4 L / m 4/60 = 0.06 L/s L o n d o n S o u t h b a n k U n i v e r s i t y Page 11

12 Calculations of the volume The volume of H SO 2 4 used in absorption = 25 ml and the volume of back titration = 19.3 ml Volume of H SO neutralized by 2 4 NH 3 = = 2.5 ml 2.5 ml of 0.01 N solution contain ( 2.5*0.01 ) mol = mol Calculating moles of NH 3 absorbed: Moles of NH3 Moles of NH 3 absorbed = Duration = mol / s NH Calculating 3 in air leaving ( y o ): y o = Moles of NH3 absorbed Air flow rate in L/s l mol NH 3 /L Calculating moles of NH 3 in 10 gm sample: mol Calculating NH 3 in water leaving ( x o ): x o = mol/l L o n d o n S o u t h b a n k U n i v e r s i t y Page 12

13 Calculating y i by mass balance as shown below: F A y i F A y o F W x o y i F W x 0 F F A A y 0 ( ) ( ) mol / L Now calculating the mass transfer coefficient equation: a k g using this N k a ( y) g mean Where: N F ( y y ) 0.23( ) Mol NH / s A i o And: 1 ( y) mean ( ytop ybottom) 2 3 to find y ytop and bottom y top ( y * -4 0 yi ) yo since Y i =0 when X i =0 Change of concentration of ammonia (NH 3 ) in air at the bottom of column is given by; y bottom ( y i y * o ) Y*= x phase y*= equilibrium value for ammonia concentration in the liquid y * o x o y bottom x10 3 L o n d o n S o u t h b a n k U n i v e r s i t y Page 13

14 And now calculating : (y) mean -6 3 ( y) mean 0.5( x10 ) So now calculating K g a : k a g N ( y) mean mol.s -1.m -2 Determination of the slope n from the Log Log graph of kga against FA Using the equation K g.a= B*F A n After log both side of the equations Log(K g.a) = logb + n logf A using the points on figure 2. Slope (n) = Duration of test Water Flowrate Air flowrate NH3 in Air absorption NH3 in Air Leaving NH3 in Water Leaving Mass Transfer Coefficient S L/s L/s Y i (mol/l) Y o (mol/l) X 0 (mol/l) K g a log F A logkg.a E E E E L o n d o n S o u t h b a n k U n i v e r s i t y Page 14

15 slop 2 flowrate February 27, 2012 [AMMONIA ABSORPTION] E E E-08 Series1 Linear (Series1) 0-5E kag Chart Title Series slop L o n d o n S o u t h b a n k U n i v e r s i t y Page 15

16 CAPTER 6 ANALISYS AND DISCUSSION The increase of air flow rate (8, 10, 12, 16 L/min) as shown in figure 2 in this experiment with constant water flow rate of (1 L/min) has resulted an increase of mass transfer coefficient, this indicates that the mass transfer coefficient is directly proportional to the air flow rates, as the air flow rates increases the mass transfer increases, this is means the air molecules entering the column increase there will be a better mixing of air and water. Which in return results increases mass transfer coefficient. The results obtained from the experiments indicates an increase in air flow rate leads to more transfer of ammonia which in turn leads to an increase in mass transfer coefficient. To accomplish the aims of experiment, which was to determine the overall mass-transfer coefficient (Kga), in a small packed column for the air/ammonia/ water system at several different air flow rates? In experiment Four different air flow rates of 14, 10, 8 and 4 L/min) were carried out with constant water flow rate of 1 L/min and 25 ml of 0.01N sulphuric acid. L o n d o n S o u t h b a n k U n i v e r s i t y Page 16

17 The packed Rasching rings were in the absorption column of up to 12 cm height to provide greater contact area between the air and water. Which in turn provides increased contact area between the air, which contains ammonia, and other side which contains no ammonia just water only, therefore increased mass transfer coefficient The 25ml volume of Sulphuric acid with of concentration of 0.01 N was recycled during the experiment for the ammonia absorption, the volume of H2SO4 used in absorption increases with the increases of air flow rate start with (1.5 and 2.9 ml) at air flow rate of 10 L/min and 14L/min respectively, during this experiment the volume of H2SO4 absorber was not part of parameter intended to determine, therefore will require other parameter to take account into. Adding 6 drops droplets of phenolthalein as an indicator, the time taken to change colour in the bottle containing diluted sulphuric acid decreases with the increase of air flow rate. This indicates that the higher the air flow rate the shorter the time taken for the acid to exhaust. The investigations undertaken during the research shows that the reduce of the air would definitely affect the results of the mass transfer coefficient. The counter-current is a very different process used in the industry compared to the rest of the currents used in the absorp[tion. L o n d o n S o u t h b a n k U n i v e r s i t y Page 17

18 CHAPTER 5-- CONCLUSION In conclusion, the experiment allowed the group to consider the key points. These points are: The counter current system is the most effective in this type of process were the air enters from the bottom and the water from the top The parameter change takes longer as the airflow rate decreases. Considering the results it can be affirmed that the mass transfer coefficient is proportional to the flow rate. L o n d o n S o u t h b a n k U n i v e r s i t y Page 18

19 References 1- Unit Operations of Chemical Engineering, 5th ed., W.L. McCabe & J.C. Smith, McGraw-Hill, ( accessed on ) 2- Chemical Engineering, Vol. 2. 4th ed., J.M. Coulson & J. F. Richardson, Pergamon, ( accessed on ) 3- Mass Transfer in Engineering Practice, A. L. Lydersen, Wiley ( accessed on ) 4- Separation Process Principles, 2nd ed., J.D. Seader & Ernest J. Henley, John Wiley & Sons ( accessed on ) 5- Physical chemistry: Chemical principles: the quest for insight; P.Atkins, L.Jones. 3rd edition 2005; Freeman. (Or earlier works by Atkins & Jones). ( accessed on ) 6- Chemical reactors: Introduction to chemical reaction engineering and kinetics; R.Missen, C.Mims, B.Saville; 1999; Wiley. ( accessed on ) 7- Catalysis: Catalytic chemistry; B.C.Gates; 1992; Wiley. ( accessed on ) 8- Concepts of modern catalysis and kinetics; I.Chorkendorff, W.Niemantsverdriet; 2003, Wiley. ( accessed on ) 9- ( accessed ) L o n d o n S o u t h b a n k U n i v e r s i t y Page 19

20 Nomenclature Symbols Description Units T test Seconds N mass transfer mol NH 3 s -1 F A Flow rate of the air L s -1 F W Flow rate of water L s -1 y i y o y * x o concentration of ammonia in inlet air concentration of ammonia in outlet air concentration of ammonia in air in equilibrium with the solution of ammonia Concentration of ammonia in water out mol NH 3 L -1 of air mol NH 3 L -1 of air mol NH 3 L -1 of water x i Concentration of ammonia in inlet water mol NH 3 L -1 of water k g a mass transfer coefficient, mol s -1 bar - L o n d o n S o u t h b a n k U n i v e r s i t y Page 20