Journal of Industrial and Engineering Chemistry 16 (2010) 577–586

Optimization of ammonia production from urea in continuous process usingASPEN Plus and computational fluid dynamics study of the reactor used forhydrolysis process

J.N. Sahu a,b,*, V.S. Rama Krishna Chava a, Shadab Hussain a, A.V. Patwardhan c, B.C. Meikap a,d

a Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, P.O. Kharagpur Technology, West Bengal 721302, Indiab Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Pin 50603, Malaysiac Department of Chemical Engineering, Institute of Chemical Technology (ICT), Mumbai 400019, Indiad School of Chemical Engineering, University of KwaZulu-Natal, Faculty of Engineering, Howard College Campus, King George V. Avenue, Durban 4041, South Africa

A R T I C L E I N F O

Article history:

Received 1 November 2009

Accepted 12 January 2010

Keywords:

Hydrolysis of urea

Ammonia

Computational fluid dynamics (CFD)

Simulation

Modelling

Optimization

A B S T R A C T

The present study addresses the methods and means to safely produce relatively small amounts (i.e., up

to 50 kg/h) of ammonia. The optimization and simulation study conducted for continuous process and

effect of operation conditions like reaction temperature, initial feed concentration and pressure on

ammonia production carried out using ASPEN Plus. Also, a computational fluid dynamics (CFD) model

was proposed to simulate the hydrolysis of urea for synthesis of ammonia. A series of parametric studies

to investigate flow rates, thermal boundary conditions and reactor geometry was performed for

hydrolysis of urea and the optimized operating conditions and reactor geometry were obtained. Detailed

three-dimensional flow, heat and chemistry simulations of ammonia, carbon dioxide and ammonium

carbamate. The study demonstrates that simulation is a useful tool for diagnosing hydrolysis reactor

mixing pathologies and for identifying practical countermeasures that could improve process

performance.

� 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

journal homepage: www.e lsev ier .com/ locate / j iec

1. Introduction

Ammonia is an extremely important chemical that hasinnumerable uses in a wide range of areas, including processindustry, and utility uses [1–3]. Many industrial plants require thesupply of large quantities of ammonia, which frequently must betransported through and stored in populated areas. Importantusers among these are industrial furnaces, incinerators and electricpower generation industries [4,5]. All of these are faced with alowering of the amount of nitrogen oxides being discharged to theatmosphere in the combustion gases being emitted from theiroperations, as required by environmental regulations [6–12].Another important use is for the so-called ‘‘conditioning’’ of fluegas by which an improved collection and removal of particlesmatter (fly ash) is obtained [13–22]. But unfortunately, ammoniapresents significant danger to human health as a hazardouschemical. Its transportation, storage and handling triggers serious

* Corresponding author at: Department of Chemical Engineering, Faculty of

Engineering, University of Malaya, Kuala Lumpur, Pin 50603, Malaysia.

Tel.: +60 3 7967 5295; fax: +60 3 7967 5319.

E-mail addresses: jay_sahu@yahoo.co.in, Jnsahu@um.edu.my (J.N. Sahu).

1226-086X/$ – see front matter � 2010 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2010.03.016

safety and environmental regulatory requirements for riskmanagement plans, accident prevention programs, emergencyresponse plans and release analysis [23–25]. The present study isconcerned with the methods and means to safely producerelatively small amounts (i.e., up to 50 kg/h) of ammonia.

There are several chemical processes that are used tomanufacture ammonia. The three most prevalent methods includethe Haber–Bosch process, indirect electrochemical dissociation,and urea decomposition [26]. The Haber–Bosch process reactsgaseous hydrogen and nitrogen over a metal catalyst at hightemperatures (e.g., at 748 K) and pressures (e.g., at 20 MPa). Thisprocess is a proven large-scale industrial process; however, it usesharsh conditions and has not been proven technically oreconomically effective below the ton/hour range. The electro-chemical dissociation process has been proposed by some in thesemiconductor industry as an alternative to the Haber–Boschprocess for the generation of ammonia. This process also reacthydrogen and nitrogen. However, it is an indirect synthesis via amolten alkali-metal halide electrolyte with nitrogen introduced atthe cathode and hydrogen introduced at the anode. The electro-chemical dissociation process also operates at elevated tempera-tures (e.g., at 673 K) but at ambient pressure. While utilizing lessharsh operating conditions or parameters than the Haber–Bosch

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. Schematic of urea hydrolysis system used in thermal power plants.

Fig. 2. Effect of temperature on production of ammonia in CSTR.

Fig. 3. Effect of pressure on production of ammonia in CSTR.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586578

process, the electrochemical dissociation process has not beenproven above pilot scale production rates and has a high risk ofalkali metal contamination. Another concern with adopting thesetwo processes for generating ammonia it that the Haber–Bosch andelectrochemical distribution processes require large amounts ofhydrogen, which adds significantly to the risk of operating anammonia generation facility. An alternative approach to ammoniasupply suggested in the late eighties includes using urea feedstockto generate ammonia on site [27]. The method of urea to ammoniaconversion by hydrolysis process, urea is an ideal candidate for themanufacture of ammonia [3,23–27]. Urea is an environmentallysafe material used primarily as fertilizer. Urea is a non-toxicchemical compound and presents essentially no danger to theenvironment, animals, plants life and human beings. It is solidunder ambient temperatures and pressures. Consequently, ureacan be safely and inexpensively shipped in bulk and stored for longperiods of time until it is converted into ammonia. It will not leak,explode, be a source of toxic fumes, require pressurization,increase insurance premiums, require extensive safety programs,or be a concern to the plant, community and individuals who maybe aware of the transportation and/or storage dangers of ammonia.It has been determined that using urea thermal hydrolysis is thepreferred process for converting urea/water solution into a gaseousmixture containing ammonia, carbon dioxide and water vapor[28]. A typical urea hydrolysis process used in industry is shown inFig. 1.

The published information in literature about hydrolysis of ureafor production of ammonia is very little detailed and patented[4,5,27–37]. However, in our early laboratory study [38–44] givesclear overview regarding the equilibrium and kinetic study of ureahydrolysis for production of ammonia in batch and semi-batchreactors. Batch and semi-batch reactors were easy to use in thelaboratory study, but less convenient for industrial applications.Therefore, we decided to study more thoroughly the continuousprocess for optimization of operating variable using ASPEN Plus byusing all data achieved in early study. Also a modelled, usingcomputational fluid dynamics (CFD) approach to determine thelocal and instantaneous values of liquid velocity, temperature andreactant concentrations inside the reactor.

2. Reaction pathway

The hydrolysis of urea to ammonia is endothermic and proceedsrapidly above a temperature of approximately 373 K. The basicchemistry employed in the process is the reverse of that employedin industrial production of urea from ammonia and carbon dioxide

and employs two reaction steps as follows [45–48]:

NH2CONH2urea

þH2Owater

!�HeatNH2COONH4;

ammonium carbamate

DH1 ¼ �15:5 kJ=mol

(1)

The ammonium carbamate decomposed then to yield carbondioxide and ammonia gases:

NH2COONH4ammonium carbamate

!þHeat2NH3

ammoniaþ CO2

carbon dioxide;

DH2 ¼ þ177 kJ=mol

(2)

The first reaction in which urea hydrolyzes to form ammoniumcarbamate is mildly exothermic, while the second, in whichammonia and carbon dioxide are produced, is strongly endother-mic, with the result that the reaction to release ammonia andcarbon dioxide requires heat and quickly stops when the supply ofheat is withdrawn. Excess water promotes the hydrolysis reaction,the overall reaction for which is as follows:

xH2Oþ NH2CONH2!2NH3 þ CO2 þ ðx� 1ÞH2O;

DHoverall ¼ þ161:5 kJ=mol(3)

Fig. 4. 3D combine effect of pressure and temperature on production of ammonia in

CSTR.

Fig. 6. Effect of pressure on production of ammonia in PFR.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586 579

The completion of the reaction is favored by high temperature,stirring speed and high reaction pressure. The overall reaction isendothermic and the first reaction, i.e. urea to ammoniumcarbamate reaction is a slow reaction and the second reaction isvery fast and goes towards completion [47,48].

3. Results and discussion

3.1. ASPEN Plus simulations

ASPEN Plus is widely accepted in the chemical industry as adesign tool because of its ability to simulate a variety of steady-state processes ranging from single unit operation to complexprocesses involving many units [49–53]. Consequently, ASPEN Plus

Fig. 5. Effect of temperature on production of ammonia in PFR.

was chosen as a framework for the development of a hydrolysisprocess simulation. Since there is no hydrolysis of urea modelprovided by ASPEN PLUS, we must develop our own using the toolsoffered by ASPEN Plus 15, 16. In addition to its conventional reactormodels, ASPEN Plus has the flexibility to allow the insertion ofFortran blocks and user kinetic subroutines into the simulation. Inthis work, it was optimized and simulation for a continuousprocess to produce 50 kg/h ammonia from urea. It determined thetype of reactor to be used as continuous stirred-tank reactor (CSTR)or plug flow reactor (PFR) and also it found out the processvariables optimization. The ammonia output from a reactordepends on four factors. They are (i) feed input, (ii) temperature,(iii) pressure, and (iv) reactor volume. It has two types of reactorsthat are generally used viz. CSTR and PFR. So for choosing the typeof reactor it need to check which reactor gives more ammoniaoutput under same conditions of feed flow, temperature, pressureand reactor volume. The simulations for the reactors CSTR and PFR

Fig. 7. 3D combine effect of pressure and temperature on production of ammonia in

PFR.

Table 1Optimization of production of ammonia from urea.

Initial feed

concentration (wt%)

Pressure (atm) Temperature (K) Feed rate (kg/h) Urea in feed

(kg/h)

Water in feed

(kg/h)

Ammonia

(kg/h)

CO2 (kg/h)

30 4 403 530 159 371 50.5 65.3

30 4 413 520 156 364 50.5 65.3

30 4 423 520 156 364 50.0 64.6

30 5 393 500 150 350 50.4 65.07

30 5 403 440 132 308 50.5 65.3

30 5 413 420 126 294 50.8 65.7

30 5 423 420 126 294 50.4 65.1

30 5 433 460 138 322 50.0 64.6

30 6 393 450 135 315 50.4 65.1

30 6 403 400 120 280 50.7 65.4

30 6 413 370 111 259 50.1 64.8

30 6 423 370 111 259 50.5 65.2

30 6 433 400 120 280 51.0 65.9

30 7 393 420 126 294 50.5 65.3

30 7 403 370 111 259 50.0 64.5

30 7 413 350 105 245 50.3 65.0

30 7 423 340 102 238 50.0 64.6

30 7 433 360 108 252 50.9 65.8

30 8 393 400 120 280 50.7 65.4

30 8 403 360 108 252 50.5 65.2

30 8 413 340 102 238 50.6 65.3

30 8 423 330 99 231 50.5 65.3

30 8 433 330 99 231 50.0 64.7

40 3 413 470 188 282 50.6 65.4

40 3 423 460 184 276 50.6 65.4

40 4 393 440 176 264 50.2 64.8

40 4 403 380 152 228 50.1 64.8

40 4 413 350 140 210 50.2 64.9

40 4 423 340 136 204 49.9 64.4

40 4 433 380 152 228 50.1 64.7

40 5 393 380 152 228 50.8 65.6

40 5 403 330 132 198 50.8 65.68

40 5 413 300 120 180 49.98 64.58

40 5 423 300 120 180 50.7 65.5

40 5 433 320 128 192 50.6 65.4

40 6 393 340 136 204 50.44 65.2

40 6 403 300 120 180 50.6 65.4

40 6 413 280 112 168 50.7 65.5

40 6 423 270 108 162 49.9 64.5

40 6 433 280 112 168 49.5 63.9

40 7 383 380 152 228 50.2 64.8

40 7 393 320 128 192 50.8 65.6

40 7 403 280 112 168 50.2 64.8

40 7 413 260 104 156 49.8 64.4

40 7 423 260 104 156 50.9 65.7

40 7 433 260 104 156 49.4 63.8

40 8 383 360 144 216 50.7 65.5

40 8 393 300 120 180 50.3 64.9

40 8 403 270 108 162 50.3 65.0

40 8 413 250 100 150 49.6 64.0

40 8 423 250 100 150 50.8 65.7

40 8 433 250 100 150 50.0 64.5

40 8 443 270 108 162 50.0 64.6

40 8 423 260 104 156 50.9 65.7

40 8 433 260 104 156 49.4 63.8

50 3 393 450 225 225 50.2 64.8

50 3 403 370 185 185 50.5 65.2

50 3 413 320 160 160 50.85 65.7

50 3 423 310 155 155 50.5 65.27

50 3 433 350 175 175 50.2 64.9

50 4 393 350 175 175 50.7 65.5

50 4 403 290 145 145 50.2 64.8

50 4 413 260 130 130 50.2 64.8

50 4 423 250 125 125 49.4 63.9

50 4 433 270 135 135 49.5 63.99

50 5 393 300 150 150 50.5 65.3

50 5 403 260 130 130 50.9 65.7

50 5 413 230 115 115 49.4 63.9

50 5 423 230 115 115 50.3 65.1

50 5 433 240 120 120 49.9 64.5

50 6 393 270 135 135 50.2 64.8

50 6 403 240 120 120 50.8 65.7

50 6 413 220 110 110 50.4 65.1

50 6 423 210 105 105 49.2 63.6

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586580

Table 1 (Continued )

Initial feed

concentration (wt%)

Pressure (atm) Temperature (K) Feed rate (kg/h) Urea in feed

(kg/h)

Water in feed

(kg/h)

Ammonia

(kg/h)

CO2 (kg/h)

50 6 433 220 110 110 49.6 64.1

50 7 393 250 125 125 49.8 64.3

50 7 403 220 110 110 49.5 63.9

50 7 413 210 105 105 50.3 65.0

50 7 423 200 100 100 49.0 63.4

50 7 433 210 105 105 50.0 64.6

50 7 443 230 115 115 50.7 65.5

50 8 383 290 145 145 50.5 65.2

50 8 393 240 120 120 50.1 64.7

50 8 403 220 110 110 51.0 65.9

50 8 413 200 100 100 49.5 64.0

50 8 423 200 100 100 50.5 65.3

50 8 433 200 100 100 49.6 64.0

50 8 443 220 110 110 51.2 66.1

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586 581

for calculating the output of ammonia varying the temperature,pressure, input feed rate and reactor volume in ASPEN Plus.

3.1.1. Continuous stirred-tank reactor

The single effect of temperature on production of ammoniastudy for CSTR is presented in Fig. 2. The ammonia output increasewith increase in temperature initially and reaches a maximumafter which it starts decreasing with increase in temperature. Thusit will have a point of maximum output for ammonia at a particulartemperature when other parameters (pressure, feed rate, reactorvolume) are kept constant. This maximum output will be attemperature around 423 K.

The effect of pressure on production of ammonia can be seenfrom Fig. 3 keeping temperature, feed flow rate and reactor volumeconstant. It observed that there is increase in output of ammoniawith pressure till some point and after this change in output willbecome very less with increase in pressure. After crossing thispoint it can consider that ammonia output does not vary withpressure.

The three dimensional response surfaces which were con-structed to show the most important two variables (reaction

Fig. 8. 3D geometry of reactor with meshing created in Gambit (front view).

temperature and pressure) on the production of ammonia atconstant reaction time and constant reactor volume is shown inFig. 4. A maximum production of ammonia achieved 64 kg/h at inletfeed concentration 50 wt% of urea and pressure 6 atm in the CSTR.

Fig. 9. (a) Plot of velocity vector colored by velocity magnitude along the wall of

reactor (m/s); (b) plot of velocity vector colored by velocity magnitude along the

impeller of reactor (m/s).

Fig. 10. Contours of static pressure (Pascal) along the impeller of reactor (front

view).

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586582

3.1.2. Plug flow reactor

It can be seen from Fig. 5 that the production of ammonia isfunction of temperatures. It increases exponentially with increasein temperatures. The ammonia output increases with increase intemperature initially and reaches a maximum after which it startsdecreasing with increase in temperature. Thus it will have a pointof maximum output for ammonia at a particular temperaturewhen other parameters (pressure, feed rate, reactor volume) arekept constant.

From Fig. 6 it can seen that the effect of pressure on productionof ammonia in PFR. The simulation run keeping temperature, feedflow rate and reactor volume constant. It was observed that there is

Fig. 11. Contours for temperature plots at different reaction

increase in output of ammonia with pressure till some point andafter this change in output will become very less with increase inpressure. After crossing this point it can consider that ammoniaoutput does not vary with pressure.

In Fig. 7 shows the three dimensional response surfaces, thecombined effect of reaction temperature and pressure onproduction of ammonia at constant reaction time and inlet feedconcentration. A maximum production of ammonia achieved80 kg/h at inlet feed concentration 50 wt% of urea and pressure6 atm in the PFR.

3.1.3. Comparison of reactors performance

From the above study, it observe that for same reactionconditions and same feed rate, the output of ammonia is slightlymore in the case of PFR than in CSTR. But there are certain problemsthat are associated with use of PFR. Handling of Plug Flow reactor isdifficult. Also there is the chance of solid deposition (ammoniumcarbamate) which will cause problems to the flow in case of PFR.But this does not happen in case of CSTR as there is continuousagitation. Therefore to avoid the above said problems it decided touse CSTR for the hydrolysis process even though yield is slightlymore in case of PFR. Now it needs to find out optimization thereaction conditions and feed rate that should be given for obtaining50 kg/h ammonia as product in CSTR. There may be different sets ofconditions possible. It needs to consider the cost as well as theproblems that arise in each case.

3.1.4. Optimization production of ammonia

Table 1 gives the temperature and feed rate required in apressure range of 3–8 atm to give the desired output of 50 kg/hammonia production at various concentrations of urea in water. Itobserved as the concentration of urea is increased there is large

times (a) 1.5 min, (b) 2.5 min, (c) 5 min, and (d) 7 min.

Fig. 12. Contours for production of ammonium carbamate plots at different reaction times (a) 3 min, (b) 12 min, and (c) 30 min.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586 583

decrease in the amount of water in the solution required and also asmall amount of decrease in case of urea also. With decrease in thequantity of water the heat requirement will also be lessenedgreatly along with the requirement of urea which is desirable. Butit cannot go beyond 50% concentration as the problem of solubilityof urea in water arises. So it will fix our concentration to be 50%urea by weight. In each case it can see that the optimumtemperature is 413–423 K as this is the range where the minimumamount of urea is required to get the desired output. Also theoptimum pressure will be between 5 atm and 6 atm. From theASPEN Plus simulations it finds out that the volume of the reactorrequired to carry out the process must be 20 l.

3.2. CFD simulations

A computational technology that enables us to study thedynamics of things that flow is CFD. Using CFD, we can build acomputational model that represents a system or device that wewant to study. Then apply the fluid flow physics and chemistry tothis virtual prototype, and the software will output a prediction ofthe fluid dynamics and related physical phenomena. Therefore,CFD is a sophisticated computationally based design and analysistechnique. CFD software gives us the power to simulate flows ofgases and liquids, heat and mass transfer, moving bodies,multiphase physics, chemical reaction, fluid-structure interactionand acoustics through computer modeling [54–57].

In the present work, the system investigated consists of a flatbottom stirred cylindrical reactor (diameter, T and liquid height, H

both equal to 132 mm) with impeller having two sets of blades on.

The shaft (diameter is 5 mm) of the impeller is concentric with thereactor axis and extends to a distance of 10 mm from the bottom ofthe reactor. A standard Rushton turbine (diameter, D = T/3 = 100 mm) is used in all the simulations. The impeller off-bottom clearance is (C = 10 mm) measured from the agitator mid-plane.

A commercial grid-generation tool (GAMBIT 2.0 of Fluent Inc.,USA) is used to model the geometry and to generate the body-fittedgrids. It is very important to use an adequate number ofcomputational cells while numerically solving the governingequations over the solution domain. With the available dimensionsof the reactor three-dimensional geometry is created. For meshing,tetrahedral meshes are used for reactor. Based on our previousexperience and some preliminary numerical experiments, about300 computational cells are used for the simulations. As the wholestudy is based on the reaction in the reactor, following is a gambitdiagram of a 3-D reactor, which has impellor with two sets ofblades on it as shown in Fig. 8. Using Fluent the created geometryby Gambit can be read and simulation is done. For analysis of theresults the contour plots and vector plots are analyzed. Residualplot is observed continuously during simulation. Accuracy andconvergence are very important during simulation. Current studyinvolves two things: one is to create three-dimensional geometryin Gambit along with the meshing, and second is to know thegeneral procedure to simulate this reactor. Analysis of the result isalso important to know the flow field and pressure distribution.

Computations are carried out for constant impeller rotationalspeed of 1200 rpm. For the given operating condition, the flows inthe stirred vessel are in a turbulent region. Wall functions are used

Fig. 13. Contours for production of ammonia plots at different reaction times (a) 2 min, (b) 4 min, (c) 10 min, (d) 12 min, (e) 16 min, (f) 20 min, and (g) 30 min.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586584

to specify the wall boundary conditions. The top surface of theliquid pool is assumed to be flat and is modeled as symmetry (zeronormal velocity and zero shear stress). All the computations arecarried out until the desired convergence criteria are satisfied

(residues less than 10�6). Fig. 9(a) shows the velocity magnitudeplot indicted along the rector wall, where Fig. 9(b) shows thevelocity magnitude plot indicated along the impeller. Fig. 9(a) and(b) shows the velocity magnitude plot indicated by the velocity

Fig. 14. Contours for production of carbon dioxide plots at different reaction times (a) 3 min, (b) 8 min, and (c) 10 min.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586 585

vectors and it is observed that the turbulent flow which goes insideour batch reactor. This turbulence only ensures the proper mixingand the uniformity of temperature and reactants throughout thereactor. This distribution of the absolute pressure contour can alsobe seen from Fig. 10. Colors indicated in this figure have the samesignificance as that of Fig. 9 (b). This model also indicates that atthe operating conditions with an assured almost perfect mixing.

The model was developed through simulation helps to answer alot of questions about the dependency of the reaction on itsparameters. Although the assumptions in the model tilt it towardsthe ideal side, it still able to predict the nature of the reaction andthe changes, in accordance to the variation it brings about in thereaction parameters. The simulation was performed for differentreaction times, keeping other parameters constant, as temperature423 K, initial feed concentration equals to 50 wt% of urea andstirring speed equals to 1200 rpm. The effect of temperature atdifferent reaction time can be seen from Fig. 11. The model predictsthe contours temperature plots performance of the reactor moreaccurately, see also Fig. 11. It is evident from Fig. 12 that, thepercent conversion of ammonium carbamate increases withincrease in reaction time. Fig. 13 shows the contours for ammoniai.e. the initial molar concentration of urea and its concentrationafter reaction time at constant reaction temperature with constantstirring speed, 1200 rpm, and initial feed concentration 50 wt% tourea. Also from Fig. 14, it is clear that with the increase the reactiontime the production of carbon dioxide increases. From the figure, ithas been a very clear idea about the dependence of the reaction ontime. As the contours are based on region wise concentrations,therefore in order to get the numerical value of concentration, itassumed it to be averaged over the entire area.

4. Conclusions

A model was developed for the hydrolysis of urea for productionof ammonia using the ASPEN Plus simulator. To provide such ahydrolysis model, several ASPEN Plus unit operation blocks werecombined and, where necessary, kinetic expressions and hydro-dynamic model were developed using data and models from theearly study. It found out the output of ammonia varyingtemperature, pressure, initial feed flow rate at various concentra-tion of urea solution with the help of Aspen Plus. In both CSTR andPFR, it observed that the output of ammonia will increase withincrease in temperature up to a certain point and then decreaseswhen the other parameters (pressure, feed flow, reactor volume)are kept constant. Also the output increases with increase inpressure but after some point the increase is very small. It wasobserved that for same reaction conditions and same reactorvolume, the output of ammonia is slightly more in case of PFR thanin CSTR. But it end up using CSTR in design as the reactions isheterogeneous and deposition of solids may occur in PFR. But thisdeposition does not take place in CSTR as there is continuousagitation.

Also a computational fluid dynamics model was used tosimulate the hydrolysis of urea for synthesis of ammonia. Thestudy capable of modeling fluid flow, heat transfer, and chemicalreactions in complex reactor geometries. Detailed parametricstudies were performed to improve the urea conversion in thepresence of double impeller and thermal conditions. A detailedfluid flow, heat transfer, and chemical reactions simulation ofammonia was also performed. The simulations revealed variousdetails about mixing non-idealities inside the reactor chamber.

J.N. Sahu et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 577–586586

This study demonstrates that CFD simulation is a useful tool fordiagnosing hydrolysis reactor mixing pathologies and for identify-ing practical countermeasures that could improve reactor perfor-mance.

Acknowledgements

The authors would like to gratefully acknowledge the financialsupport to the National Thermal Power Corporation (NTPC), NewDelhi, India, has extended towards them for this work.

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