Aspen Plus Urea Synthesis Synthesis Loop Model Mode l
Contents 1Introduction...............................................................................1 2Components and Units of Measurement.......................................1 3Process Description....................................................................2 4Physical Properties.....................................................................3 5Chemical Reactions inetics........................................................5 !"imulation #pproach................................ #pproach................................................................... ................................... $ $"imulation Results......................................................................$ %Conclusions................................................................................& References.................................................................................1'
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1 Introduction This document describes the steady-state Aspen Plus® model of the high-pressure synthesis loop of a urea plant, with a capacity of about 1,100 metric tons of prilled urea per year. This simulation is based on the Stamicarbon ! " Stripping Process, which is a popular and fast growing process for manufacturing urea. The wor# demonstrates the capability of Aspen Plus to rigorously model the urea synthesis process. The modeling is complicated due to the formation of ammonium carbamate, an intermediate product for which a special property pac#age has to be de$eloped. This type of model is useful to analy%e the plant performance and to impro$e plant operation, including&
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'nergy sa$ing studies to impro$e economics of the plant.
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Studies of indi$idual pieces of e(uipment with a $iew o f increasing their throughput and)or impro$ing their performance.
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*lowsheet modification for better plant operation.
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+dentifying bottlenec#s.
• As a basis for optimi%ation study and for de$eloping on-line control system of the plant. hile this document describes the simulation of the Stamicarbon ! " stripping process, the accurate results obtained support the applicability of Aspen Plus and the data pac#age to other urea processes.
2 Components and Units of Measurement The table below lists the components modeled in the simulation.
Components Component Name ater Ammonia arbon io2ide 3rea Ammonium arbamate itrogen !2ygen
Component ID
Type
ormula
"! !" 34'A A46 " !"
!/ !/ !/ !/ !/ !/ !/
"! !" 5"! 7"!" " !"
A small amount of biuret 8 "9!": is produced during the synthesis. +n this wor#, the biuret production is not considered, but the component can easily be added if re(uired. The pure component properties of all the components e2cept ammonium carbamate can be retrie$ed from the Aspen Plus databan#s. Special efforts were made to incorporate pure component properties of ammonium carbamate in the simulation. A special contribution of this wor# is the de$elopment of a physical property model to describe the simultaneous physical-chemical e(uilibrium occurring in the urea-synthesis process. Aspen Plus Example Library Proprietary Information of AspenTech Unauthorized duplication or distribution strictly prohibited without prior written permission
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;etric units are used in this wor#, e2cept that temperature unit is o, pressure unit is #g)cm " 8abs:, enthalpy flow unit is mmcal)hr, and mole flow unit is #mol)day.
! Process Description A simulation flowsheet of the synthesis loop is shown in *igure 1. 1'0<*F4SAL!" "'01S9&I$;(A#TPE)%
05S
*01 FLA!"
401
S19
'0" '0
S1
A01 #&I''
;01 (I)E#
S01 '01 S0< SP1 S1"
A0" #T$I%
EP
i"ure 1# Simulation lo$sheet of Urea Synthesis Loop The high-pressure loop is operated at around 151 #g)cm " 8abs: and consists of the following #ey pieces of e(uipment&
%&uipment
Purpose
401
3rea 4eactor, where ammonium carbamate is dehydrated to urea
'01
.P. !" Stripper, where the bul# of the unreacted carbamate from the reactor effluent is decomposed by stripping with !" gas and with heat input.
'0"
.P. ondenser, where the gaseous !" and condense and react to form ammonium carbamate.
'0
.P Scrubber, where the recycled carbamate solution from the downstream lowpressure section is used to absorb unreacted gases from the reactor.
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The process is described as follows& •
*eed !" gas 8S0<: is fed to the ! " stripper, '01, to strip the urea solution coming from the reactor. +n the stripper, ammonium carbamate decomposes, liberating more and !" to be stripped out. eat is supplied on the shell side of tubes by condensing "=9 psig steam while the urea solution falls inside the tubes countercurrently down past the rising ! " stripping gas. The outlet li(uid solution from the stripper is rich in urea and goes to the downstream section for urea purification.
•
+n the adiabatic urea reactor, 401, an a(ueous solution of and !" 8much in the form of ammonium carbamate: and $apors flow upward through = stages of reactor $olume to minimi%e bac#-mi2ing and pro$ide enough residence time for urea formation. 4emaining gases condense and carbamate decomposes in the reactor to pro$ide h eat for the slightly endothermic reaction o f carbamate to urea. The urea solution 8S07: o$erflows from the top of the reactor bac# to the Stripper '01, while the unreacted gases 8S09: pass out the top of the reactor.
•
These unreacted gases pass to the Scrubber, '0, where recycled carbamate solution from the '$aporation)4ecirculation section 8S1: is passed o$er the top of a pac#ed bed and fills the tube side section of $ertical tubes. The gases rise up through the tubes and pass up through the pac#ed section before lea$ing the top of the $essel. The contact with the carbamate solution absorbs the unreacted and !", while the inert gases of ! ", ", and others 8S19: $ent out from the top. !n the bottom, recirculated cooling water flowing inside tubes remo$es the heat o f absorption from the carbamate solution. arbamate solution 8S1<: o$erflows out of the $essel.
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The solution 8S1<: together with the top $apor stream from the Stripper 8S0=: is fed to the arbamate ondenser, '0", through the use of an e>ector, where the ammonia feed 8S01: ser$es as the pumping fluid. Ammonium carbamate forms in this condenser. The $apor-li(uid mi2ture 8S0: falls through tubes and the heat of reaction is remo$ ed by generation of 90 psig steam on the shell side. The mi2ture goes to the bottom of the reactor for urea production.
•
The bottom stream 8S0?: from the Stripper '01 is sent to the downstream section to reco$er urea. The recycled stream after reco$ering the urea 8S1: is bac# to the .P. Scrubber to complete the loop.
' Physical Properties The model for the thermodynamic properties of the -!"-"!-34'A-A46-"-!" system is based upon the S4-P!@A4 model within Aspen Plus 8Soa$e, 1?<" Penelou2 and 4au%y, 1?=" Schwart%entruber and 4enon, 1?=?:. The mode l uses an e(uation of state and is thus suitable for the high-pressure, high-temperature conditions of urea synthesis. *urther, the model contains e2tensions that enable an accurate description of the phase and chemical e(uilibria, the density and the other thermodynamic properties 8e.g., enthalpy: of this system. e chose our approach to the modeling of the thermodynamic properties after studying the pre$ious modeling attempts in the literature and analy%ing the a$ailable data. *rB>ac(ues 81?5=:, Cawasumi 81?9", 1?9 and 1?95: and @em#owit% 81?=0: de$eloped chemical and thermodynamic models by postulating reactions for urea formation and $arious simplified assumptions for the phase non-ideality. The simplifying assumptions do not permit an accurate and general model for the chemicalthermodynamic properties. 6ernadis et al. 81?=?: and +sla et al. 81??: de$eloped impro$ed theoretical models by including ionic species and describing the nonideality of the li(uid phase by a modified 3+D3A model. e belie$e that under the high temperatures 8170 to "00°: and the relati$ely low water concentrations of urea synthesis, the e2tent of ioni%ation will be small. *urther, modern e(uations of state such as the S4-P!@A4 model are well suited to the description of the thermodynamic properties of nonideal systems at high pressures and temperatures. Thus we ha$e chosen to use the S4-P!@A4 model as the physical-property option. Aspen Plus Example Library Proprietary Information of AspenTech Unauthorized duplication or distribution strictly prohibited without prior written permission
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e$elopment of a data pac#age for this system is difficult since most of the data are only a$ailable as combined physical and chemical e(uilibria. isassociation pressure data are a$ailable for ammonium carbamate 8Eanac#e, 1?0:, which is the p ressure at a specified temperature where ammonia and carbon dio2ide are in e(uilibrium with the condensed-phase ammonium carbamate. ata are also a$ailable for the e(uilibrium con$ersion of defined mi2tures of -!"-"! to urea 8Cawasumi, 1?9", 1?9 and 1?95 +noue, 1?<":. *inally, bubble pressures ha$e been measured for defined mi2tures of -!"-"! at chemical e(uilibrium. The Aspen Plus data analysis capabilities 84S and ATA-*+T: ha$e been used to obtain a simultaneous good fit of these $aried and comple2 sets of data. Forlo$s#ii and Cucherya$yi 81?
i"ure 2# %&uili(rium Con)ersion of C*2 to Urea at 1+, C Comparison of Aspen Plus- %*S to Correlation of .orlo)s/ii and 0ucherya)yi 13,4
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*igure presents a comparison between the present AspenTech model and the bubble pressure data measured by @em#owit% 81?<1, 1?<" and 1?<<:. The model pro$ides an accurate description of the data and, in particular, pro$ides an accurate description of the minimum in bubble pressure, which is necessary for an effecti$e description of the urea synthesis process.
;easured and alculated 6ubble Pressures in the -!" System omparison of ASP' P@3S '!S to ata of @em#owit% et al. 81?<2: 00 "00G "90 : r a"00 b 8 e r u s s e190 r P e l b b u100 6
1=0G
170G
150G
90
0 0.79
0.<0
0.<9
0.=0
0.=9
0.?0
0.?9
;ole *raction in *eed
i"ure !# Measured and Calculated 5u((le Pressures in the N6! C*2 System Comparison of Aspen Plus- %*S to Data of Lem/o$it7 et al8 1+94 The AspenTech model pro$ides an accurate description of the phase and chemical e (uilibria related to urea synthesis. +t also accurately describes the other properties needed for reliable simulations, namely enthalpies and densities.
: Chemical ;eactions 0inetics There are two main reactions that ta#e place in the urea synthesis process&
¬ → A46
81:
" H !"
8":
A46 ¬→
34'A H "!
The first reaction, which ta#es place in the li(uid phase, con$erts a mmonia and carbon dio2ide into ammonium carbamate. This reaction is highly e2othermic and fast. hemical e(uilibrium is readily reached under the operating conditions in the reactor. The second reaction also ta#es place in the li(uid phase and is endothermic. +ts rate is slow and e(uilibrium is usually not reached in the reactor. A user subroutine, 3S34A.*, was de$eloped to include the reaction #inetics of both reactions. 3S34A.* is used in the reactor simulations. 6oth forward and re$erse reactions were considered. The Aspen Plus Example Library Proprietary Information of AspenTech Unauthorized duplication or distribution strictly prohibited without prior written permission
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#inetics of 4eaction 1 has been set to be rapid so that e(uilibrium is effecti$ely reached. @imited literature data were used for the #inetics of 4eaction ". The reaction #inetics has been formulated to approach the e(uilibrium composition for large residence times. The e(uilibrium has been described in terms of the fugacity coefficients since an e(uation of state is used as the thermodynamic model. The e(uilibrium constant for 4eaction 1, in terms of mole fractions, is written as follows&
− (G 0 K 1 = e2p
CARB
− "G 0 − G 0 " ) P " φ " φ 0 φ RT P NH
CO
NH
CO "
CARB
here, T P 2 4 P0
-
Temperature Pressure ;ole fraction $ector Fas constant 4eference pressure 8I 1 atmosphere:
Gi
-
+deal-gas Fibbs *ree energy of component i at T, P0
φi
-
*ugacity coefficient of component i at T, P, 2
0
The e(uilibrium constant for 4eaction 1 in terms of mole fractions is as follows& K 1
x CARB =
"
x NH x CO "
Similar e(uilibrium e(uations can be written for 4eaction ". The rates for 4eactions 1 and ", in units of #mol)s) m , are as follows&
Rate1
= k x NH
Rate"
x x = k xCARB − UREA H K
"
1
xCO "
+
−
xCARB K 1
"
"
"O
The two rate e2pressions ha$e been formulated so that they will necessarily reach e(uilibrium at large residence times. The rate constant for 4eaction 1 8C1: is set to a large $alue so that this reaction is essentially at e(uilibrium. The rate constant for 4eaction " determines the urea con$ersion in the reactor. !nly scant information is a$ailable to determine C" and it is usually best to ad>ust its $alue to fit plant data. A reasonable appro2imation for C" is the following&
k "
= 19. J 10
=
e
100.J107 − RT
L
) v
here 4I=15. and /@ is the molar $olume of the li(uid.
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< Simulation Approach The !" Stripper, '01, is of the fall ing film type, which was appro2imated by a 4A*4A 8multistage distillation: bloc# with 10 stages. eat is supplied to the stages "-? to simulate the heat transfer from the tubes. The urea solution, falling down on tube walls, is stripped off $olatile by the entering ! " gas 8S0<:. !n each stage the model considers the e(uilibrium of carbamate in the li(uid as well as the /@' of the mi2ture. ote that the #inetics of carbamate formation is large enough to ensure that chemical e(uilibrium for the carbamate reaction is reached in each stage of the 4A*4A bloc#. The 3rea 4eactor 401, <.9" ft in diameter and ?9 ft in length, is modeled with an 4P@3F bloc#. The #inetics is pro$ided by the user subroutine 3S34A in the 4P@3F bloc#. +n the reactor, the e2othermic ca rbamate reaction and the endothermic urea formation reaction are ta#ing place. The reactor is designed such that its $olume is big enough for the desired urea production.
The .P. Scrubber is modeled using a 9-stage 4A*4A bloc#. eat is ta#en out from the bottom stage. Similar to the Stripper, '01, on each stage the model considers the e(uilibrium of carbamate in the li(uid as well as the /@' of the mi2tu re. The '0" .P. ! " ondenser is modeled with an 4ST!+ bloc#. A esign Specification paragraph is included to monitor the specified reactor outlet temperature of 1=o by ad>usting the e2tent of the carbamate reaction in '0". This simulation is based on a closed-loop flowsheet. The downstream section is appro2imated by using a S'P bloc# to lin# the Stripper bottom urea solution 8S0?: to the recycled carbamate solution 8S1:.
+ Simulation ;esults The Aspen Plus run was made using /ersion "07.9. Some of the results are shown below, and a simulation flowsheet with stream data is shown in *igure 5. The simulation results of this generic model are reasonable compared with similar plant operations.
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i"ure '# Simulation lo$sheet of Urea Synthesis Loop Cey Process Simulation results&
%&uipment ;,1 > ;eactor
%,1 > C*2 Stripper
%,2 ? C* 2 Condenser
%,! ? Scru((er
=aria(le eat duty Top temperature 3rea in e2it stream eat duty Top temperature 6ottom temperature Top stream 8S0=:, 6ottom stream 8S0?:, 3rea production eat duty '2it temperature eat duty Top temperature 6ottom temperature Top stream 8S19: 6ottom stream 8S1<:
=alue
Unit 0 1=.0 8spec.: 5",<9" 17.0 8spec.: 1=9.0 179.7 =,"? <7,<71 5",77 -1<.?" 17< 8spec.: -." 8spec.: =7.1 171.< 1707.1 ,?=7
mm#cal)hr #g)hr mm#cal)hr o o #g)hr #g)hr #g)hr mm#cal)hr o mm#cal)hr o o #g)hr #g)hr o
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3 Conclusions 1. This urea process model has been de$eloped using Aspen Plus /ersion "007.9. This is a rigorous closed-loop model for the plant while the reco$ery section is appro2imated by using a S'P model. The carbon dio2ide compression section is not included. *rom the results, it is shown that the S4P!@A4 property pac#age used for simulation is appropriate. ". *or further refinement of the model, the following upgrades should be made& a: The cooling water circuit for '0 and the low pressure steam circuit for '0" should be implemented. This implementation is useful for energy sa$ing studies. b: '0" is simulated in this wor# by a 4ST!+ model. owe$er, a 4P@3F model is more suitable for the simulation of this e(uipment. owe$er, to do so, detailed e(uipment data for '0" are needed. c:
The stripper '01 is a falling-film type e(uipment. +t embodies /@' e(uilibrium, mass transfer, reaction, and heat transfer. To rigorously simulate this e(uipment, rate-based calculations 84ateSep: should be used. A special subroutine will be needed to incorporate the mass transfer limitations of the falling-film. +n this simulation, the 4A*4A model was used with component efficiencies for , !" and "! as a wor# around.
. The accurate results obtained in the present simulation indicate that Aspen Plus and the physicalproperty data pac#age will pro$ide accurate simulations of other urea processes.
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;eferences K1L 6ernadis, ;. ar$oli, F. Santini, ;., M3rea--!"-"! /@' alculations 3sing an '2tended 3+D3A '(uation,N *luid Phase '(uilibria, 9, "0<-"1= 81?=?:. K"L *rB>ac(ues, ;., MTheoretical 6asis of the +ndustrial Synthesis of 3rea,N him. +nd., 70, ""-9 81?5=:. KL Forlo$s#ii, .;. Cucherya$yi, /.+., M'(uation for etermination of the '(uilibrium egree of !" on$ersion uring Synthesis of 3rea,N Oh. Pri#l. Chim., 9, "95=-"991 81?
1.
K7L Eanec#e, '., O. 'lectrochem., 7, 759 81?0:. K
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