Novel single stripper with side-draw to remove ammonia and sour gas simultaneously for coal-gasifica

Novel Single Stripper with Side-Draw to Remove Ammonia and Sour GasSimultaneously for Coal-Gasification Wastewater Treatment and the Industrial ImplementationDachun Feng,Zhenjiang Yu,Yun Chen,and Yu Qian*School of Chemical Engineering,South China Uni V ersity of Technology,Guangzhou 510640,P.R.ChinaA large amount of wastewater is produced in the Lurgi coal-gasification process with the complex compounds carbon dioxide,ammonia,phenol,etc.,which cause a serious environmental problem.Most of the known pretreatment techniques do not yield sufficient performance for phenol removal,which hampers the functions of the subsequent biochemical treatment process.In this paper,a novel stripper operated at elevated pressure is designed to improve the pretreatment process.In this technology,two noticeable improvements were established.First,the carbon dioxide and ammonia were removed simultaneously in a single striper where sour gas (mainly carbon dioxide)is removed from the tower top and the ammonia vapor is drawn from the side and recovered by partial condensation.Second,the ammonia is removed before the phenol recovery to reduce the pH value of the subsequent extraction units,so as the phenol removal performance of the extraction is greatly improved.To ensure the operational efficiency,some key operational parameters are analyzed and optimized though simulation.It is shown that when the top temperature is kept at 40°C and the weight ratio of the side draw to the feed is above 9%,the elevated pressures can ensure the removal efficiency of NH 3and carbon dioxide and the desired purified water as the bottom product of the unit is obtained.A real industrial application demonstrates the attractiveness of the new technique:it removes 99.9%CO 2and 99.6%ammonia,compared to known techniques which remove 66.5%and 94.4%,respectively.As a result,the pH value of the wastewater is reduced from above 9to below 7.This ensures that the phenol removal ratio is above 93%in the following extraction units.The operating cost is lower than that of known techniques,and the operation is simplified.IntroductionLurgi pressurized coal-gasification is one of the most widely used coal-gasification techniques for its advantages of low energy consumption,less oxygen consumption,and high ef-ficiency.1In that process,a large amount of highly polluted wastewater is produced,which contains phenols,carbon dioxide,hydrogen sulfide,ammonia,fatty acid,oils,etc.2These com-pounds in the wastewater fluctuate as shown in Table 1.The concentrations of phenols,ammonia,and the chemical oxygen demand (COD)in the wastewater is over 4000,8000,and 20000mg/L,respectively.The pH value of the wastewater is above 9.Phenols are toxic organic compounds listed as priority pollutants by the US EPA.3The high concentration of phenol makes biological treatment difficult.4Because they are harmful to the environment,these substances must be removed before the wastewater enters a subsequent biological treatment process.In the existing coal-gasification process,a wastewater treat-ment process is adopted that consists of the following operation steps:the acid gas removal,followed by phenol extraction with diisopropyl ether (DIPE),ammonia stripping,extracting solvent recovery (as shown in Figure 1),and finally biological treat-ment.5The acid gas including CO 2and H 2S,with a slight amount of steam and ammonia,is stripped overhead from the sour water stripper T1at the atmospheric pressure and 105°C.The bottom water stream of T1,whose pH value is about 10,is cooled and pumped to the top of extractor E1.Meanwhile,an extraction solvent DIPE is fed to the bottom of the countercur-rent extractor,E1,to remove phenol.The solvent is then separated and recovered with the distillation column T3,fromwhich the solvent is recycled to the previous extractor while the phenols stream is recovered.The raffinate stream enters ammonia stripper T2.About 2wt %dissolved DIPE in the raffinate is stripped and recovered from the top of T2.The side stream comprised of NH 3and H 2O from the T2is partially condensed with F2to obtain an NH 3-rich vapor.The treated bottom water of T2,which is the greatest proportion of the streams leaving the process,is sent to a consequent biochemical treatment process.A number of problems in this process have not been solved for many years.Having the sour gas removed in stripper T1,the water contains a considerable amount of NH 3.This means that the pH value of the current extraction system is in the range of 9-10.5,which leads to poor phenol removal because a significant fraction of the phenol is ionized.6It has been found that when pH >8,the distribution coefficient for phenol extracted with the solvent begins to drop.Little phenol can be extracted when pH >12.It appears that a high pH value in the extraction system is the bottleneck of the existing process,which makes the final phenol and COD very high:the content of phenols is above 1000mg/L and the COD is above 6000mg/L.In order to meet the requirement for the inlet flow of the subsequent biochemical treatment process,the residual phenol*To whom correspondence should be addressed.E-mail:ceyuqian@.Phone:+86-20-87113046.Fax:+86-20-22236337.Table 1.Main Water Quality of the Coal-Gasification Wastewaterwater quality measured valuerange volatile phenol (mg/L)33402900-3900nonvolatile phenols (mg/L)18001600-3600ammonia (mg/L)85003000-9000carbon dioxide (mg/L)45004000-11000hydrogen sulfide (mg/L)8050-200fatty acid (mg/L)2000-3500COD (mg/L)2050020000-30000Ind.Eng.Chem.Res.2009,48,5816–5823581610.1021/ie9002987CCC:$40.75 2009American Chemical SocietyPublished on Web 05/05/2009D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987should be reduced to <400mg/L.To remove a desired amount of phenol,it could be a good idea to remove the sour gas and NH 3earlier so that the extraction is operated in an acceptable pH range and thus ensures phenol removal efficiency.Chevron developed a technique to a similar problem in wastewater treatment for oil refining processes.Coupled distil-lation columns are adopted.The first column removes most of the sour gas while the second removes NH 3.This process is applied to hydrocarbon oils processing for purifying wastewater containing ammonia and hydrogen sulfide.7,8The situation in the coal-gasification process,however,is not exactly the same because of the existence of a large amount of acidic gases,in which the main compounds are carbon dioxide and ammonia.There is a high concentration of ammonia in the wastewater and much OH -is produced though ammonia dissociation reaction.Consequently,more carbon dioxide and hydrogen sulfide are ionized in the bottom of the first distillation column.This means that the compositions of carbon dioxide,hydrogen sulfide,and ammonia to be removed are not entirely free to exist in the water.Making things more complicated,they also exist in the form of ions.As a result,the ionic interaction among carbon dioxide,hydrogen sulfide,and ammonia makes them difficult to differentiate.In addition,the existence of two columns in the Chevron technology is very costly.The technique is not very effective for acidic gas and ammonia removal from aqueous solution.A single stripper with a side-draw to remove ammonia and sour gas simultaneously is feasible and used in the oil refining industry for the removal of hydrogen sulfide and ammonia in wastewater simultanously.9,10On the other side,the main acid component is carbon dioxide in the coal-gasification wastewater process;the successful industrial application of such a processing technology has not been reported.To deal with the problems and improve the mentioned coal-gasification wastewater pretreatment process to meet the water inlet quality requirement to the subsequent biochemical treat-ment process,a great number of fundamental investigations have been conducted.Some of the authors proposed using methyl isobutyl ketone (MIBK)as a more promising extraction solvent for phenol removal from the coal-gasification wastewater,2and liquid -liquid equilibria for the ternary system of MIBK -water -phenol and the quaternary system of MIBK -water -phenol -hydroquinone were investigated,respectively.11,12A new phenol extraction technique was proposed,and the industrial experi-mental program was designed.2The bottleneck of the existing process was basically analyzed,and a conceptual design for a new process was discussed.13–15On the basis of these funda-mental studies,a new process of a complex stripper to remove sour gases and ammonia simultaneously in coal-gasification wastewater was proposed in this paper.The too high pH value of the inlet flow to the extraction was efficiently lowed for betterphenol removal.The proposed new process is finally imple-mented in the industry successfully.Novel Complex Stripper with Side-Draws to Remove Ammonia and Sour Gases SimultaneouslyThe proposed process with a single stripper is shown in Figure 2.For the reason that the ammonia and carbon dioxide have different solubilities at different temperatures,they are separated in the stripper where the temperature is changed from 40to 160°C.The wastewater from the stabilization tanks is intro-duced into two parts:one heated by the feed-bottoms heat exchanger and fed in the middle of the sour water stripper,which is the heat feed,and the other cooled and fed to the top of the stripper.The sour gas is removed overhead.When the temper-ature in the stripper is properly controlled,the NH 3is ac-cumulated in the middle of the stripper and exists in the liquid phase with a high ammonia concentration and in the vapor phase with rich ammonia.The ammonia-rich gas is drawn from the middle side.Then,it is further purified through a three-step condensation.Refined ammonia of 99%concentration is obtained,while the condensate is recycled to the hot feed.When a large amount of acid gas and ammonia is removed from the wastewater,the pH value of the bottom stream is effectively reduced and helpful to an effective extraction.According to functions,the stripper consists of three sections:The section of CO 2distillation is from the top to the heat feed site,the section of CO 2stripping is from the heat feed to the side-draw,and the section of ammonia stripping is from the side-draw to the bottom.Under the conditions of pressure >0.5MPa and temperature <40°C,the relative volatility of carbon dioxide is higher than that of ammonia and the solubility of ammonia in the water is much higher than that of carbon dioxide.Therefore,ammonia is absorbed in the top section through the top feed wastewater which is cooled to the temperature below 40°C.Most of the ammonia is absorbed from the vapor phase to the aqueous phase and moves down in the stripper.The contents of CO 2and H 2S increase intheFigure 1.Flowchart of the existing coal-gasification wastewater treatment process:T1,sour water stripper;T2,ammonia stripper;T3,solvent recovery tower;E1,extractor;F1and F2,partial condensers;V1,solvent storagetank.Figure 2.Flowchart of the proposed process to strip ammonia and sour gas simultaneously in a single column.Ind.Eng.Chem.Res.,Vol.48,No.12,20095817D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987upstream vapor.As the results,most of CO 2and H 2S are stripped overhead.In the section of CO 2stripping,the top feed wastewater,which contains ammonia,a small amount of carbon dioxide,and hydrogen sulfide,interacts with the hot feed from the middle of the stripper.In this section,the carbon dioxide and ammonia are stripped by the rising hot vapor and then absorbed by the liquid from the top continuously.Because the temperature in the stripper increases along from the top to the bottom,carbon dioxide and ammonia decrease continuously.The ammonia is gathered in the middle section because of the liquid absorption.When the ammonia concentration equilibrium in the vapor and liquid is achieved,i.e.,the ammonia concentra-tion in the liquid phase is close to its solubility in the aqueous solution,the ammonia will move to the bottom of the stripper with the liquid flow,whereas the ammonia-rich vapor is drawn from the side.The partial pressure of the vapor phase is reduced and the initial gas -liquid equilibrium is broken.Consequently the ammonia is transferred from the liquid into the vapor rapidly.Preferably,the favorable condition that the ammonia can gather in the middle section is attained.In the section of the ammonia stripping,the ammonia is stripped by the hot steam from the bottom and drawn from the side constantly.Therefore,the ammonia transfer from the liquid into the vapor continuously.Finally,it removed from the side.In this way,the desired purified water is obtained in the bottom.The chemistry equations,the phase equilibrium model,and the distillation model of the stripping system are given below,respectively.Being validated by an existing pretreatment process,the models were used to design a new process for the removal of ammonia and main acidic components including CO 2and H 2S.Then,the process parameters,such as the temperature,pressure,and flow rate of the side-draw,were optimized.They are used as the basis of the process and equipment design and the industrial operation.Chemical Model and Phase Equilibrium Models of the Stripping SystemComposition of coal-gasification wastewater is very compli-cated.The carbon dioxide,ammonia,phenol,and hydrogen sulfide in the wastewater are weak electrolytes.Therefore,they form a system of NH 3-CO 2-H 2S -H 2O -PHOH.In consider-ation that the volatility of the phenol in this distillation process is relatively small,phenol is ignored.The system is thus simplified to a quaternary system of NH 3-CO 2-H 2S -H 2O.In a closed system,there are equilibria in this volatile weak electrolyte solution as follows:the dissociation equilibrium and chemical equilibrium in the liquid phase and the vapor -liquid equilibrium of the free molecular in the gas and liquid phase.There are also material and charge balances in the system.Therefore,the system models include dissociation equilibrium equations,phase equilibrium equations,material balance equa-tions,charge balance equations,and normalization equations.(a)Dissociation Equilibrium and Chemical Equilibrium in the Liquid Phase.NH 3+H 2O T NH 4++OH -(1)CO 2+H 2O T HCO 3-+H +(2)HCO 3-T CO 32-+H +(3)NH 3+HCO 3-T NH 2COO -+H 2O(4)H 2S T HS -+H+(5)HS -T S 2-+H +(6)H 2O T H ++OH -(7)NH 3+H 2S T HS -+NH 4+(8)NH 3+HS -T S 2-+H +(9)The dissociation equilibrium constant is expressed as follows:K i )m A γA m B γB m AB γAB(10)Temperature is an important factor to influence the dissocia-tion equilibrium constant.The relation of the equilibrium constant K with the temperature is expressed as follows:ln K )A +BT+C ln T +DT (11)where A ,B ,C ,and D can be considered to be constant in a certain range.(b)Vapor -Liquid Equilibrium of the Free Molecular.The equilibrium of free molecular in the gas and liquid phases obeys Henry’s law,which is described asy i φi P )x i γi He i(12)(c)Material Balance Equation of the Compositions.m T ,i )m i +m +i +m -i λ+i +λ-i(13)(d)Charge Balance Equation.m NH 4++m H3O +)2m CO 32-+m HCO 3-+m NH 2COO -+2m S 2-+m HS -+m OH -(14)(e)Normalization Equation.y CO 2+y NH 3+y H 2S +y H 2O )1(15)The models of this weak electrolyte system embody math-ematical model calculation of thermodynamic equilibrium.This is a key in process simulation.The species in the liquid phase of the coal-gasification wastewater include CO 2,NH 3,H 2O,H 2S,H +,OH -,HS -,S 2-,NH 4+,HCO 3-,CO 32-,etc.The thermo-dynamic model in the liquid phase is truly nonideal because of the existence of those ions.Therefore,it is vital to select the suitable liquid phase activity coefficient models in process simulation.Since the method for computing the activity coefficient was introduced by van Krevelen,16various in-depth research of the activity coefficient models has been reported for electrolytes.17,18For this type of systems,the electrolyte nonrandom two liquid (ELECNRTL)model is a widely recognized model for the activity coefficients.19By using binary and pair parameters of molecule -molecule interactions,molecule -ion interactions,and ion -ion interactions,the aqueous electrolyte system covering the entire range of electrolyte concentrations can be represented by the ELECNRTL model,as well as mixed solvent electrolyte systems.When the concentration of electrolyte decreases to zero,ELECNRTL is simplified to an ideal model.A complete form of the model can be found in literature.20,21The ELECNRTL model is conveniently embedded in the Aspen Plus software with many built-in electrolyte parameter packages.The ELEC-NRTL is taken as the basis of physical property methods for this stripping system in this work.Model of the StripperThe operation of the stripper is a multistage multicomponent separation process.In this process,there are two types of models5818Ind.Eng.Chem.Res.,Vol.48,No.12,2009D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987usually used in simulation and design:the equilibrium stage model and the nonequilibrium stage model.The nonequilibrium stage model is based on the mass transfer rate equation involving much more property and model parameters such as diffusion coefficient,boundary tension,viscosity,etc.Unfortunately,these parameters in this complex industrial case are difficult to obtain.Moreover,calculation of hydrodynamics and mass transforma-tion in the stripper involves more strongly nonlinear equations and nonideal fluidity cannot be correctly described,which makes the simulation for the weak volatile electrolyte system difficult.These reasons hold back application of the nonequilibrium stage model in the industry.22On the contrary,the equilibrium stage model based on Sorel theory is widely applied in simulation and design of distillation.The model assumes that every tray in the stripper is a theoretical tray,on which there is V -L phase equilibrium between the vapor and the liquid leaving the tray.The difference of an actual tray and a theoretical tray is associated with the efficiency of the actual tray.It satisfies the equation of MESH.(a)Material Balance Equation.L j -1x i ,j -1-(V j +G j )y ij -(L j +U j )x ij +V j +1y i ,j +1)-F j z ij(16)(b)Equilibrium Equation.y ij )k ij x ij(17)(c)Summary Equation.∑xij)1or∑yij)1(18)(d)Heat Balance Equation.L j -1h j -1-(V j +G j )H j -(L j +U j )h j +V j +1H j +1)-F j H Fj +Q j(19)In an actual tray,the liquid and vapor do not reach phase equilibrium because there is not enough time for vapor and liquid contacting.Therefore,the Murphree efficiency is adopted to revise the difference between an actual condition and an ideal condition.However,different components have differences in efficiency in the distillation.According to Hoogendoor et al.,23the Murphree efficiency of NH 3is between 0.65and 0.85,that for CO 2is 0.01-0.05,and that for H 2S is 0.15-0.4,respectively.In this work,Murphree efficiencies of the components in the stripper have been computationally estimated from a similar system in the petroleum industry.For the reason that efficiency can be greatly affected by temperature,pressure,composition,throughput of flow,mass transfer coefficient,etc.,the results listed in Table 2vary with the industrial cases.The industrial case discussed in this work is very complicated.It is difficult to acquire a set of Mruphree efficiencies exactly.Taking it cautiously,a set of conservative data is chosen in the simulation,that is 0.61,0.05,and 0.07for NH 3,CO 2,and H 2S,respectively.The models and approach used in simulation were validated using Aspen Plus to simulate stripper T1that is in Figure 1,and the results are listed in Table 3.It has been found that the simulation results for the operation parameters are consistent with the actual industrial data.The results also show that the difference between simulation and the actual industrial data isacceptable for the key components,CO 2and NH 3,in treated wastewater.Therefore,the models and approach of simulation are adopted in the new process system to provide a guideline for operation design and optimization.Design,Simulation,and Optimization of the Novel StripperThe new process is designed to remove NH 3,CO 2,and H 2S simultaneously in the stripper.According to the demand,the quality is defined that H 2O is less than 3%(mass),NH 3less than 1.5%(mass)in the top of the stripper,while NH 3less than 100mg/L in the bottom of stripper.Process Parameter Definition and Optimization.Due to the fact there is a vast amount of compositions in the coal-gasification wastewater and the complexity of the stripper with side-draw,the performance of the stripper is affected by a number of operation conditions,such as temperature and pressure in the stripper,feed ratio,side-draw tray location,and the rate of side-draw,etc.In addition,according to the previous industrial practice,coal dust existed in the wastewater ac-cumulated and blocked in the packing stripper in a short operation period.Therefore,the “complete”packings were not considered.Before discussing the parameter above,the process parameters are defined as below:the number of total trays is 57,the cold feed temperature is 35°C,the cold feed/hot feed ration is 0.25,the top pressure is 0.6MPa,and the side draw/feed mass ratio is 13%.Through simulation and optimization,they are designed and optimized further.Temperature.The top temperature of the stripper is related to the fraction of NH 3and H 2O in the stream overhead.When the temperature is controlled low,the mass fractions of NH 3and H 2O are very low.If the stream overhead contains a lot of NH 3and H 2O,then it can lead to crystallization of ammonium salt and make the pipeline obstructed.Thus,it is necessary to make the temperature lower in order to reduce the problem of crystallization.In Figure 3,the relationship can be found between the top temperature of the stripper and the mass fractions of NH 3and H 2O.With the increase of temperature,the mass fractions of NH 3and H 2O increase observably.In the industry,it is feasible to keep the top temperature at 40°C.While the stripper is operated at this temperature (see Figure 3),it can be found that the mass fraction of NH 3and H 2O are very low in stream overhead.The top temperature is adjusted by changing the flow rate of the cold feed.It has been proved practicable in industrial operation.Operation Pressure.Figure 4shows the changes of mass fraction of CO 2in the side stream,the mass fraction of NH 3in the top gas of stripper,and the reboiler duty when the pressure in the stripper changes.From the results,it can be found that the mass fraction of CO 2in the side stream and the mass fraction of NH 3in the top gas of the stripper will reduce with the increase of the pressure in the stripper.That is beneficial to improve-ment of the efficiency of distillation.In addition,the temperature in the stripper will increase as the pressure increases,so the NH 3in the wastewater is easy to remove.In the process,the pressure set of the stripper is 0.6MPa.Table 2.Murphree Value for Components in a Pressurized Environment component case 1case 2literature 23used in simulationNH 30.710.610.65-0.850.61CO 20.050.060.01-0.050.05H 2S0.080.070.15-0.40.07Table parison of the Simulation and the Actual Industrial Data for T1Tower in the Existing Processtemperature (°C)composition of the bottom product (mg/L)topbottomCO 2H 2S NH 3industrial 75-7999-1021683716700simulation 771011805766740Ind.Eng.Chem.Res.,Vol.48,No.12,20095819D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987Side Stream and Side-Draw Tray.The selection of the location of the side-draw is a significant variable in the stripper design and optimization,which could significantly affect the ammonia concentration in the side-draw.The process is calculated when the hot feed is at the 11th tray and there is not side stream,and the mass fraction of NH 3in each stage is shown in Figure 5.From the results,the mass fraction of NH 3in the vapor phase is very high from the 16th to the 45th tray,and the difference between them is small.The computation also shows that the concentration of NH 3in vapor is above 12%(mass)when side stream is not drawn,which is much higher than 2.5%(mass)in liquid.Therefore,the vapor is considered to be removed from the side-draw.Moreover,the vapor drawn from the side makes it easy for the further NH 3purification through a three-step condensation.Figure 6shows the relationship between side-draw tray and the mass fraction of NH 3in the top gas and in the bottom stream.The result indicates that the mass fraction of NH 3in the top gas decrease with the side-draw tray moving down to the next stage,this benefits the removal of NH 3from the top of the stripper.When the side-draw tray is too low,the mass fraction of NH 3in the bottom will increase.In the stripper,the temperature gradually rises from the top to the bottom.Therefore,if the stripping performance meets the process demands,moving the side-draw tray up is in favor of less heat in the rich ammonia vapor being drawn out;this will decrease the heat duty of the stripper.Overall,considering that the mass fraction of NH 3in the vapor phase fluctuates with other factors in the operation,such as disturbances of mass fraction of feed,three side-draw trays are set located at the 28th,30th,and 32nd trays,respectively.Thus,as one of them can be chosen as the most appropriate site by mass fraction of NH 3in the side-draw stream.To verify the sensitivity of the location selection,a comparison for three locations respectively is presented below.Side-Draw Rate.The side-draw rate is a key factor to the capacity of distillation and stream consumption.From Figure 7,the mass fraction of NH 3in the bottom stream decreases with the increase of the side-draw flow rate.If the side-draw flow rate to the feed rate is >9%(mass),the mass fraction of NH 3in the bottom stream is <100mg/L.However,with the increase of the side-draw rate,the steam consumption and the side-draw condensation amount increase,so the recycle of NH 3increases accordingly.It is not practical to increase the side-draw amount significantly,neither the mass ratio of side-draw to the feed greater than9%.Figure 3.NH 3and H 2O concentration in the tower top gas change with the tower toptemperature.Figure 4.Relation of the mass fraction of NH 3in the top stream,mass fraction of CO 2in NH 3-rich vapor,and reboiler heat duty with the toppressure.Figure 5.NH 3concentration profile in vapor along the stripper (no side-draw).Figure 6.Influence of the stripper side-draw location on the concentration of NH 3in the top gas and the bottom stream.5820Ind.Eng.Chem.Res.,Vol.48,No.12,2009D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987Finally,a stripper is designed with 2.2m diameter,53.8m height,and treatment capacity of 80t/h.Finally,57stages were chosen for a large design margin of 25%to deal with the high concentration of pollutants into the system during influent fluctuations in case of accidents,abnormal situation,and even for burden increase in the future.The hot feed site is the 11th stage,and the side-draw trays are set located at the 28th,30th,and 32nd trays,respectively.The top temperature is controlled to below 40°C,and the operating pressure is 0.6MPa,while the mass ratio of side-draw to the feed should be greater than 9%(mass).In order to ensure NH 3to be absorbed completely by the cold feed,the packings were used in the segment of the stripper between the cold feed and the hot feed,which is equivalent to around 10stages.There are 47stages under the hot feed.Industrial Operation and Results AnalysisThe new process has been implemented successfully,in which the side-draw is located at the 28th tray.The stripper is operated in following conditions:the cold feed temperature is between 35to 40°C,the cold feed/hot feed ration is 0.2to 0.4,the top pressure is 0.6MPa,and the side draw/feed mass ratio is 9%to 13%,respectively.The system runs in good condition for over 1year,and the operation performance is fine.It makes the subsequent biochemical treatment more effective and flexible to the fluctuation of the quality of the inlet wastewater.Temperature Profiles.Figure 8shows the simulated trays temperatures as compared to the industrial results.As shown in the figure,there is a good agreement between the simulation and the industrial results at the tower top and the trays below the hot feed (the 11th tray).Side-Draw.The simulation and industrial results of the mass fraction of CO 2in side stream are 0.5%and 0.9%,respectively,while those of the NH 3are 19.9%and 18.7%.This indicates that the mass fraction in NH 3-rich vapor is satisfactory for ammonia concentration and purification.The simulation results are consistent with the industrial results.Three different side-draw locations where the NH 3-rich vapor was drawn from were simulated separately according to the industrial operating conditions.The profile of mass fraction of NH 3in the side streams is shown in Figure 9.It shows that the influence of the side-draw location on the concentration of NH 3of the NH 3-rich vapor is insignificant.Therefore,it may be inferred that the impact of side-draw site switching in theindustrial operation on the stripping performance is relatively small.For further industrial verification,three side-draw tubes were fabricated at three locations at the 28th,30th,and 32nd plates,respectively.The industrial results are satisfactory and verify the design and simulation.Industrial Operation Performance.The industrial operation results are listed together with the simulation results in Table 4.It is shown that,in the top stream of the tower,the concentration of the sour gas is greater than 95%,while the mass fraction of NH 3is less than 1%and that of water is less than 3%.This is sufficient to satisfy the requirement of the subsequent process.In addition,at this low temperature,the crystallization of ammonium salts in the pipeline is avoidable for less ammonia and water vapor.Moreover,as for the quality of the treated water in the bottom,all of the NH 3,CO 2,and pH values meet the desired specifications.From Table 4,it is found that most of the CO 2is removed and the removal ratio of ammonia is greater than 99%,while pH value of the treated wastewater decreases to a range of 5-7.Therefore,it is good for extractionprocess.Figure 7.Influence of side draw rate on NH 3concentration in the bottom stream and reboiler heatduty.Figure parison of simulation and industrial measurement of the temperature profile along thestripper.Figure 9.Profiles of the concentration of NH 3in vapor for the three different side draw location.Ind.Eng.Chem.Res.,Vol.48,No.12,20095821D o w n l o a d e d b y C A L I S C O N S O R T I A C H I N A o n J u l y 8, 2009P u b l i s h e d o n M a y 5, 2009 o n h t t p ://p u b s .a c s .o r g | d o i : 10.1021/i e 9002987。