Stamicarbon project.pdf

Simulation and Optimization of Total Recycle Stamicarbon CO2 Stripping Urea Synthesis Process

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1. INTRODUCTION Urea (NH2CONH2) is produced at industrial scale by the reaction between ammonia and carbon

dioxide at high pressure (1330MPa) and high temperature (170200 0C) . There are different

types of processes to produce urea in the commercial units. These processes are typically called

once through, partial recycle and total recycle . In the total recycle process, which is employed

widely, all the ammonia leaving the synthesis section is recycled to the reactor and the overall

conversion of ammonia to urea reaches 99% . Stamicarbon and Snamprogetti processes are the

most common examples of such process . Since urea has became almost the most widely used

fertilizer and its production is important in the petrochemical industry, there has been many

attempts to model and simulate the reactor of urea production as the heart of the process . In the

present work the entire urea synthesis section based on the of stamicarbon process (including

urea reactor, stripper, scrubber , rectifying column and flash separator) is modelled. Urea

production consists of reaction between ammonia and carbon dioxide react to form urea and

water .The urea synthesis is considered to occur in heterogeneous phase. In stamicarbon process

compressed carbon dioxide feed passes through the stripper along which ammonia and carbon

dioxide are stripped off from the liquid phase to the gas phase . The gas flow from the rectifying

column which carries the stripped off ammonia and carbon dioxide is mixed with pumped

ammonia feed and gas flow from srubber and on further heating , compression and cooling is fed

to the reactor. The liquid mixture in the reactor overflows into the stripper. The gas phase exiting

the reactor contains free ammonia and carbon dioxide as well as inert gas and is discharged into

the scrubber. In the scrubbing part, remaining gases are scrubbed with effluent from flash

separator. This stream is an aqueous solution of unreacted carbon dioxide and originates from

flash separation of urea from liquid outlet of CO2 stripper.


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2. HISTORICAL BACKGROUND

Urea was first noticed by Hermann Boerhaave in the early 18th century from evaporates of urine.

In 1773, Hilaire Rouelle obtained crystals containing urea from human urine by evaporating it

and treating it with alcohol in successive filtrations. This method was aided by Carl Wilhelm

Scheele's discovery that urine treated by concentrated nitric acid precipitated crystals. Antoine

Franois, comte de Fourcroy and Louis Nicolas Vauquelin discovered in 1799 that the nitrated

crystals were identical to Rouelle's substance and invented the term "urea." Berzelius made

further improvements to its purification and finally William Prout, in 1817, succeeded in

obtaining and determining the chemical composition of the pure substance. In the evolved

procedure, urea was precipitated as urea nitrate by adding strong nitric acid to urine. To purify

the resulting crystals, they were dissolved in boiling water with charcoal and filtered. After

cooling, pure crystals of urea nitrate form. To reconstitute the urea from the nitrate, the crystals

are dissolved in warm water, and barium carbonate added. The water is then evaporated and

anhydrous alcohol added to extract the urea. This solution is drained off and allowed to

evaporate resulting in pure urea. In 1828, the German chemist Friedrich Whler obtained urea

artificially by treating silver cyanate with ammonium chloride.This was the first time an organic

compound was artificially synthesized from inorganic starting materials, without the

involvement of living organisms.The basic process for urea synthesis, developed in 1922, is

called the BoschMeiser urea process after its discoverers. Commercial production started in

1922 Germany, 1932 USA and 1935 UK . The stripping concept developed in 1966 by

Stamicarbon in The Netherlands improved heat recovery and reuse in the synthesis process. The

stripping concept proved to be such a major advance that competitors such as Snamprogetti

now Saipem (Italy), the former Montedison (Italy), Toyo Engineering Corporation (Japan) and

Urea Casale (Switzerland) all developed their own versions of it. Today effectively all new urea

plants use the principle, and many total recycle urea plants have been converted to stripping

processes. No radical alternative to it has been proposed; the main thrust of technological

development today, in response to industry demands for ever larger individual plants, is directed

at reconfiguring and reorientating major items in the plant to reduce their size and the overall

height of the plant, as well as at meeting ever more challenging environmental performance

targets.


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3. PROCESS DESCRIPTION 3.1.Urea synthesis

Two reactions are involved in the manufacture of urea. First ammonium carbamate is formed

under pressure of 14 MPa by reaction between carbon dioxide and ammonia at 180oC .

2NH3+ CO2 H2NCOONH4 H= -155MJ/kg.mol.

This highly exothermic reaction is followed by an endothermic decomposition of the

ammonium carbamate.

H2NCOONH4 H2NCONH2 + H2O H = +42MJ/kg.mol.

Both are equilibrium reactions. The formation reaction goes to virtual completion under

usual reaction conditions; the decomposition reaction is less complete. Rate of carbamate

decomposition reaction increases with temperature. It is slow at temperature less than 150oC

at stoichiometric proportion of ammonia to carbon dioxide and quite rapid at 210oC.

Therefore optimum temperature for urea synthesis is 180-210C with a retention time of 0.3

to 1 hr. At high temperature corrosion rate is high. The preferred pressure for urea synthesis

process is 140-250 atm. The stoichiometric ratio of NH3/CO2 conversion to urea is 2:1 but

excess ammonia above the stoichimetric ratio favors the rate of reaction. So optimum mole

ratio of NH3 to CO2 is 3.1-4.1 is preferred for urea synthesis. The presence of water

decreases conversion to urea. So the feedstock is made moisture free before charged to urea

reactor. Carbamate is highly corrosive and its corrosiveness can be minimized using small

amount of oxygen.

At now urea synthesis process are of three following types:

(a) Once through process: The once through process is simplest and least expensive

(both capital and operating cost) among the three processes. It is least flexible and cannot be

operated unless some provision is made to utilize large amount of unconverted ammonia and

off-gas.The unchanged ammonia is converted to ammonium compounds like ammonium

nitrate but this proves to be expensive and markets for secondary products are problematical.

(b) Partial recycle process: In this process part of the off gas is recycled back to the

reactor. The amount of ammonia is reduced to 15% to that of once through that must be


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used in other processes. Investment cost is somewhat lower than the total recycle

process, this advantage apparently does not compensate the inflexibility arising from the

necessity to operate a co-product plant with mutual interdependency problems. However

it finds application in UAN co-product plants.

(c) Total recycle process: In this process all unconverted NH3 and CO2 is recycled back to

the reactor (99% conversion) and therefore no nitrogen co-product is necessary.It is most

flexible urea process as it depends only NH3 and CO2 supply but incurs investment and

operating cost. Basically Total recycle process are of five types namely hot-gas mixture

recycle, separated gas recycle, slurry recycle, carbamate solution recycle and stripping.

Among them stripping process is the modern process developed by various urea process

licencors Stamicarbon, Snamprogetti and ACES .

3.2. Comparison of three types of total recycle stripping urea process

Table 01: Comparison of three types of total recycle stripping urea process

Process parameters Stamicarbon Snamprogetti TEC ACES process Stripping agent Carbon dioxide Initially ammonia

now switch to steam Carbon dioxide

Reactor temperature,oC 183 188 190 Reactor pressure,atm 140 156 175 Molar NH3/CO2 ratio 2.95 3.3-3.6 4.0 CO2 conversion,% 60 64 6.8 NH3 conversion,% 36 41 34 No. of high pressure vessels 4 5 5 Recirculation stages 1 2 2 NH3 consumption, t/t urea 0.566 0.566 0.568 CO2 consumption, t/t urea 0.733 0.735 0.735-0.740 Import steam,t/t urea 0.920 0.950 0.80 Cooling water,t/t urea 70 75 80 Electricity, kWh/t urea 15 21-23 15


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4. PROCESS DIAGRAM 4.1. Process block diagram

Reactor vent

High pressure feed

Wet gas

Reactor products

Discharge gas

Moisture

CO2 feed Dry gas

NH3 feed Mixed feed

Vent gas

Recycle

Separator Effluent

Liquid products Urea

Figure 01: Process Block diagram for Total recycle Stamicarbon CO2 stripping urea synthesis process

CO2 compressor

Discharge

Mixer

Reactor

Scrubber

Feed compressor

Feed

Mixer

Flash

Separator

NH3 pump

High

pressure

stripper

Rectifying

Column


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4.2.Process flow diagram

Figure 02: Process Flow diagram for Total recycle Stamicarbon CO2 stripping urea synthesis process


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5. SIMULATION 5.1 Software package

Simulation refers to the application of computational models to the study and prediction of

physical events or the behavior of engineered systems. With the depth of its intellectual

development and its wide range of applications, computer simulation has emerged as a

powerful tool, one that promises to revolutionize the way engineering and science are

conducted in the twenty-first century. In engineering, simulation is important because

description of system behavior by experimentation might not be feasible due o inaccessible

inputs and outputs, experiment may be too dangerous or too costly, experimental behavior

might be obscured with disturbances. Simulation-based engineering science provides the

scientific and mathematical basis for the simulation of engineered systems. It facilitates the

engineers to be better able to predict and optimize systems.

ASPEN HYSYS is a commercially available process simulator for process analysis. It is a

powerful engineering simulation tool that has been uniquely created with respect to the

program architecture, interface design, engineering capabilities and interactive operation. It

contains a rigorous thermodynamic and physical property database and provides

comprehensive built-in process models, offering a convenient and time saving means for

chemical process studies, including system modeling, integration and optimization. The

original purpose of this software is for supporting the chemical engineering of crude oil

refineries. Process components of the simulation were implemented in ASPEN HYSYS using

standard, built-in unit operation modules and functions including all the components and

functions contained in the process.


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5.2. Methodology

The simulation is carried out using UNIQUAC fluid package. It is an ideal model fit for

inorganic and organic reactive system. Prior to simulation ammonia, carbon dioxide, urea, water

and nitrogen are selected as simulation model components. The basis for simulation is selected

taking into account production rate of urea 1000kgmol/hr. To maintain such production rate of

urea 860kmol/hr of carbon dioxide at 1000C and 115kPa is compressed to 253.9kPa such that its

temperature rises to 185 0C and then fed to a high pressure stripper at the bottom where it strips

off unreacted ammonia and carbon dioxide present with urea solution in liquid reaction products

fed to the top of stripper column. The stripped gas passes to a rectifying column where the gas is

dried and water content is reduced to nil. The dry gas from rectifying column is passed to a

mixer where it is mixed with incoming ammonia feed of 1672 kmol/hr at -2100C which is

pumped to raise its pressure from 50.66kPa to 101.3 kPa. Meanwhile these two streams are

mixed with a recycle stream from scrubber and the combined feed undergoes heating,

compression and cooling before entering the reactor such that its temperature is maintained at

1800C and pressure is kept at 506.6 kPa . The mole ratio of ammonia to carbon dioxide in the

combined feed is maintained at 4.44. In the reactor fractional conversion of CO2 is specified at

0.80 and cooling energy recovery is set up to keep reactor temperature at 750C. The reactor vent

gas then passes to the scrubber where it is scrubbed with aqueous solution of unreacted CO2 that

come from the flash separator through cooling. The liquid reaction product is fed to the high

pressure stripper from where the rich urea product is preheated to 1500C in order to be flashed in

a flash separator where remaining carbon dioxide is removed and urea solution is concentrated

to produce fairly concentrated urea solution. In both the scrubber and rectifying column inert

nitrogen, water vapor and slight amount of carbon dioxide is vented out. The simulation

maximize urea production through complete recycle of ammonia and almost total reuse of

carbon dioxide.


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5.3. Procedure

1. Beginning the simulation: Select New Case from the File Menu>Press the New Case button.

The Simulation Basis Manager will appear. The next step is to create a Fluid Package.

2. Creating a Fluid Package: Press the Add button and the property view for new Fluid Package

appear>For Property Pkg select UNIQUAC by scrolling down the available property package

and then click on it.

3.Selecting components: Click View in Fluid Package . Then component list view will

appear>Type ammonia in Match box > Press Add Pure; the component will appear in Selected

Components list. Repeat for other components CO2 , urea, water and nitrogen.

4.Selecting physical coefficient: Move to Binary Coeffs>Click on UNIFAC VLE>Press

Unknowns only button. The unknown coefficients will appear.>Close the Fluid Package

5. Creating the reaction: Press Reactions tab in Simulation Basis Manager>Press Add

Rxn>Select Conversion type>Press Add Reaction tab. The Conversion Reaction Rxn-1 will

appear> Click on Add comp>Select ammonia from the Drop-Down arrow. Repeat for other

components. In the Stoichiometric Coeff corresponding to Ammonia type -2> Type -1 for

CO2>Type 1 for Urea >Type 1 for H2O> Balance Error will be noticeable 0.0>Move to the

Basis tab>Choose CO2 for Base Component>Leave Rxn Phase Overall>Type 80 for Co

>Close the Conversion Reaction property view > Press the Add Set button in the Reaction Sets

group> In the Active List for the cell called from the Drop-Down arrow select Rxn-

1>Press Close>Click on Set-1 in the Reaction sets on the Reactions tab>Press the Add to FP

button, the Add Set-1 view will appear>Press The Add Set to Fluid Package button.

6. Entering Simulation environment: Press the Enter Simulation Environment button on the

Simulation Basis Manager view . The PFD-Case(Main ) will appear. Click on File>Select

Save As>Type STAMICARBON UREA PROCESS SIMULATION>Click on Save> Click

on Tools> Press Variables> Set Unit to SI>Close.

7. Installing CO2 compressor: From the Object Pallete onto the PFD choose the

Compressor>Double Click the Compressor. In the Connections Page rename it CO2


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compressor>In the Inlet type CO2 feed> In the Outlet type Compressed CO2 feed> In the

Energy type Compressor duty.

8. Installing High pressure stripper: From the Object Pallete onto the PFD choose the

Absorber>Double Click the Absorber. In the Connections Page rename it High pressure

stripper>In the Top Stage Inlet type Reaction Products> In the Bottom Stage Inlet select

Compressed CO2 feed by scrolling Drop-Down arrow>In the Ovhd Vapour Outlet type Wet

gas> In the Bottom Liquid Outlet type Liquid outlet> Keep the Number of stages default

10>Press Next to page 2 > Enter 405.3kPa and 506.6kPa at the Top Stage Pressure and

Bottom Stage Pressure respectively > Press Next to page 3> Press Done.

9. Installing Rectifying column : From the Object Pallete onto the PFD choose the Component

Splitter>Double Click the Component Splitter. In the Connections Page rename it Rectifying

column>In the Inlet select Wet gas by scrolling down Drop-Down arrow> In the Overhead

Outlet type Dry gas> In the Bottoms Outlet type Moisture> In the Energy Streams type

Column duty> Move to Parameters page> Click on Equalize all stream pressures > Move to

Splits page > Set Ammonia split in Dry gas column 1> Set CO2 split in Dry gas column

0.97190 > Set Urea split in Dry gas column 1> Set H2O split in Dry gas column 0> Set

Nitrogen split in Dry gas column 1.

10. Installing Centrifugal pump: From the Object Pallete onto the PFD choose the

Pump>Double Click the Pump. In the Connections Page rename it Centrifugal pump>In the

Inlet type Ammonia feed> In the outlet type Pumped Ammonia feed> In the Energy type

Pump duty>Move to the Parameters page and specify the Adiabatic Efficiency of 75.

11. Installing Feed mixer: From the Object Pallete onto the PFD choose the Mixer>Double

Click the Mixer. In the Connections Page rename it Feed mixer>In the Inlet select Dry gas and

Pumped Ammonia feed by scrolling down Drop-Down arrow and then type Recycle> In the

outlet type Mixed feed.

12. Installing Feed heater : From the Object Pallete onto the PFD choose the Heater>Double

Click the Heater. In the Connections Page rename it Feed heater >In the Inlet select Mixed

feed by scrolling down Drop-Down arrow> In the Outlet type Hot feed> In the Energy type H

duty>Move to the Parameters page and specify Delta P 0 kPa .


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13. Installing Feed compressor: From the Object Pallete onto the PFD choose the

Compressor>Double Click the Compressor. In the Connections Page rename it Feed

compressor>In the Inlet type Hot feed> In the Outlet type feed> In the Energy type FCduty

14. Installing Feed cooler : From the Object Pallete onto the PFD choose the Cooler>Double

Click the Cooler. In the Connections Page rename it Feed Cooler >In the Inlet select feed by

scrolling down Drop-Down arrow> In the Outlet type Cooled feed> In the Energy type FeC

duty>Move to the Parameters page and specify Delta P 0 kPa .

15. Installing Balance: From the Object Pallete onto the PFD choose the Balance>Double Click

the Balance. In the Connections Page rename it Balance>In the Inlet Streams type Cooled

feed> In the Outlet streams type Combined feed> Move to the Parameters page and select

Balance Type as Mole.

16. Installing Reactor : From the Object pallete onto the PFD choose and click the General

Reactors > Double Click the Conversion Reactor. In the Connections Page rename it

Reactor>In the Inlets select Combined feed by scrolling down Drop-Down arrow> In the

Vapour Outlet type Reactor vent> In the Energy type Coolant> In the Liquid Outlet select

Reaction Products by scrolling down Drop-Down arrow>Move to the Reactions tab>Select

Set-1 for the reaction>Click Conversion (%) and set Co as default >Move to Parameters page

> In the Delta P type 0 kPa > Set Duty to Cooling.

17. Installing Vaporizer: From the Object Pallete onto the PFD choose the Heater>Double

Click the Heater. In the Connections Page rename it Vaporizer >In the Inlet select Liquid

outlet by scrolling down Drop-Down arrow> In the Outlet type Hot liquid> In the Energy

type V duty>Move to the Parameters page and specify Delta P 30 kPa .

18. Installing Flash Separator : From the Object pallete onto the PFD choose the

Separator>Double Click the Separator. In the Connections Page rename it Flash Separator>In

the Inlets select Hot liquid by scrolling down Drop-Down arrow > In the Vapour Outlet type

Off gas> In the Liquid Outlet type Urea .


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19.Installing Effluent Cooler: From the Object Pallete onto the PFD choose the Cooler>Double

Click the Cooler. In the Connections Page rename it Effluent Cooler >In the Inlet select feed

by scrolling down Drop-Down arrow> In the Outlet type Cooled feed> In the Energy type

FeC duty>Move to the Parameters page and specify Delta P 0 kPa .

20. Installing Scrubber: From the Object Pallete onto the PFD choose the Component

Splitter>Double Click the Component Splitter. In the Connections Page rename it S>In the

Inlet select Wet gas by scrolling down Drop-Down arrow> In the Overhead Outlet type

Moisture> In the Bottoms Outlet type Dry gas> In the Energy Streams type Column duty>

Move to Parameters page> Click on Equalize all stream pressures > Move to Splits page >

Set Ammonia split in Moisture column 0> Set CO2 split in Moisture column 0.02810> Set

Urea split in Moisture column 0> Set H2O split in Moisture column 1> Set Nitrogen split in

Moisture column 0.

21. Installing Valve : From the Object pallete onto the PFD choose the Valve>Double Click the

Valve. In the Connections page rename it Valve>In the Inlet select Reactor vent by scrolling

down Drop-Down arrow> In the Outlet select Relief gas by scrolling down Drop-Down arrow>

Click on the Parameters page and specify Delta P 30kPa.

22 Installing Discharge Mixer: From the Object Pallete onto the PFD choose the

Mixer>Double Click the Mixer. In the Connections Page rename it Discharge mixer>In the

Inlet select Moisture and Vent gas by scrolling down Drop-Down arrow > In the outlet type

Reaction discharge.

23. Entering Process Specification: Press Workbook button on the button bar> In the CO2 feed

enter Temperature 100oC , Pressure 115kPa , Molar Flow Rate 860 kgmol/hr>Move to

Compositions tab>In the CO2 feed enter Comp Mole Frac(CO2) 0.9999 and Comp Mole

Frac(Nitrogen) 0.0001>Click Normalize>Click OK>Press Material Streams> In the

Compressed CO2 feed enter Temperature 1850C>In the Reactor vent enter Temperature

750C >In the Ammonia feed enter Temperature -210oC and Pressure 50.66kPa > In the

Pumped Ammonia feed Enter Temperature -2100C and Pressure 101.3kPa > In the

Combined feed enter Temperature 1800C , Pressure 506.6kPa and Molar Flow Rate set to


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5688kgmol/hr>Move to Compositions Tab>In the Combined feed enter Comp Mole

frac(Ammonia) 0.8162, Comp Mole frac(CO2) 0.1837 and Comp Mole frac(Nitrogen)

0.0001>Click Normalize>Click OK> Return to Material streams > Enter Temperaure for Hot

feed , Moisture, Dry gas, Hot liquid, Cooled feed and Liquid effluent as 180 oC, 75 oC, 150 oC, 150 oC,180 oC and 74.98oC respectively>In the feed enter Pressure 506.6kPa.

24. Converging High pressure stripper: Return to the PFD>Double Click High pressure

Stripper> Press Run to begin the calculation> The Converged box appear green and Simulation

is set to Solver Active mode and all Unit Ops, Material Streams and Energy Streams appear

green.


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6. OPTIMIZATION 6.1. Methodology

Optimization is carried out taking profit as objective function. The basis for optimization is 1 hr

of operation .Profit is defined as excess of Sales over Operating Cost . Sales price of Urea is

taken to be $1.1/kg and total Sales is computed from production rate of urea , urea composition

and Sales price of Urea. Operating Cost is divided into two types: raw material cost and energy

cost.Raw material cost is computed from total cost of ammonia feed and total cost of carbon

dioxide where ammonia feed cost is the product of ammonia feed rate , its composition and

price and similarly carbon dioxide feed cost is the product of carbon dioxide feed rate , its

composition and price. Price of ammonia is set at $ 0.2/kg while that of CO2 is $ 0.02/kg. The

energy cost is derived from the summation of the product of heat duties of different unit

operations with their respective costs. Below given the energy costs of different operations:

Cost of puming/kWh - $ 0.500

Cost of compression /kWh-$ 0.600

Cost of heating/Kwh-$ 0.7370

Cost of Cooling/Kwh =$0.4710

Cost of Cooling in reactor/kWh = $0.25

Cost of heating and cooling in rectifying Column =$0.30

Incorporating the cost factors the profit function comes to be:

Profit = Urea sales priceUrea production rateUrea composition-Ammonia priceAmmonia

feed rateAmmonia composition-CO2 priceCO2 feed rateCO2 composition-Pump dutyCost

of pumping-(Feed compressor Duty+CO2 compressor duty)Cost of compression-Cost of

heating(Heater duty+ Vaporizer duty)-Cost of Cooling(Feed cooler duty+Effluent cooler

duty)-Cost of cooling in reactorReactor Coolant duty-Cost of heating and cooling in rectifying

columnRectifying Column energy duty.

The material flow rate and heat duties are imported from Hysys. Two modifying variables are

selected for optimization. They are:

CO2 feed rate = 850 to 870kgmol/hr and Combined flow rate = 5680 to 5690 kgmol/hr.


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6.2. Procedure:

1. Click on Simulation on Menu bar

2. Press Optimizer

3. Configuration page appear

4. Press Spreadsheet tab

5. Fill up the specifications in spreadsheet

6. To import variable Right click on the mouse. A scroll list appear . Select Import

variable > Select Case (Main) from Flowsheet > Select desired Object >Select

desired Variable> Click OK

7. Define profit function.

8. Return to Simulation and Press Optimizer

9. Press Variables tab>Press Add > Select Object>Select Variables> Set Low Bound

and High Bound values.

10. Move to Parameters page> Set Maximum Iterations to 300> Set Tolerance to 1e6

11. Move to Functions tab> Select the Profit cell from scroll list > Click Maximize>

Press Start to begin the iteration. Then Proceed box appears green with Optimum

Found(Small Delta X) written.


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7. GRAPHICAL TREND ANALYSIS 1. Effect of reactor temperature on Urea Production: Urea production increases with reactor

temperature at a hig rate up to 40oC and then slower rate perceived between 40oC and 80oC

maximum production found at 110oC.

Figure 03: Effect of reactor temperature on Urea Production

2. Effect of reactor pressure on Urea production: Urea production increases exponentially

with reactor pressure with optimum found at 325kPa

Figure 04: Effect of reactor pressure on Urea Production


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3. Effect of reactor temperature on unconverted ammonia and CO2: With increase in reactor

temperature unconverted CO2 decreaseses exponentially attaining steady conversion at

100oC. On the contrary unconverted ammonia increases up to 20oC reaching minimum

conversion at 40oC before reaching steady state conversion at 110oC.The minimum

conversion at 40oC induces slower rate of urea production between 40oC and 80oC .

Figure 05 : Effect of reactor temperature on unconverted ammonia and CO2


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4. Effect of reactor pressure on unconverted ammonia and CO2 : With increase in reactor

presssure conversion of both ammonia and carbon dioxide increases with ammonia

converted faster to urea than CO2. The peak conversion for both chemical species is found at

325kPa. Onward conversion remains steady.

Figure 06: Effect of reactor pressure on unconverted ammonia and CO2

5. Effect of reactor temperature on reactor duty: Lesser energy consumption on reactor at

higher temperature.

Figure 07: Effect of reactor temperature on reactor duty


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6. Effect of fractional conversion of CO2 on fraction of non-reacted ammonia: It is evident

from the graph that ammonia conversion decreases with increase in CO2 conversion.

Figure 08: Effect of fractional conversion of CO2 on fraction of non-reacted ammonia


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7. Effect of mole ratio of NH3 to CO2 on rate of urea synthesis chemical species : Increasing

the mole ratio of NH3 to CO2 increases conversion of both ammonia and carbon dioxide

with carbon dioxide converted to urea faster than ammonia and optimum urea production

occurs at mole ratio 4.25.

Figure 09: Effect of mole ratio of NH3 to CO2 on rate of urea synthesis chemical species

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

7.00E+04

1 2 3 4 5

Mass flow rate/kg/h

Mole ratio of ammonia to CO2

Effect of mole ratio on rate of urea synthesis chemical species

unreacted CO2

urea

Unreacted ammonia


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8. Effect of reactor temperature on reactor chemical species distribution: Urea distribution in

reactor increases with reactor temperature while that of ammonia and carbon dioxide

decreases with temperature . Water distribution increases up to 40oC and remains steady

between 40oC and 70oC and then decreases due to vaporization. This shows that presence of

high amount of water between 40oC and 70oC slows down urea production.

Figure 10: Effect of reactor temperature on reactor chemical species distribution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100 120

ReactorChemical species

Distribution

Reactor temperature/C

Reactor Chemical species distribution at reactor temperature

Urea

ammonia

Carbon dioxide

water


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9. Effect of Separator temperature on rate of Off gas: Graph shows that Off gas kicks of

separation at 148oC and then increases constantly. It may be the fact that CO2 is partially

dissolved in urea solution and that its solubility decrease with increase in temperature with

all of CO2 start to leave liquid phase at 148oC and partly due to the fact that water vaporizes

making urea solution concentrated.

Figure 11: Effect of Separator temperature on rate of Off gas


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10. Effect of Price of urea synthesis raw materials on Profit: Both ammonia and CO2 high

pricing decreases profitability through enhancing operating cost but that of ammonia price

has higher affect on net return than that of CO2. So efficient use of ammonia helps to keep a

Urea plant profitable.

Figure 12: Effect of Price of urea synthesis raw materials on Profit

3.5

3.6

3.7

3.8

3.9

4

4.1

4.2

0 0.2 0.4 0.6 0.8 1 1.2

Profit(hundredthousanddollars)

Price of raw materials ($/kg)

Profit Vs Price of Urea synthesis raw materials

Carbon dioxide

Ammonia


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11. Effect of Sales price of Urea on Profit : Profit increases linearly with increase in Urea sales

price.

Figure 13: Effect of Sales price of Urea on Profit


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8. RESULTS AND DISCUSSION

Table 02: Comparison between Process Specifications And Simulation findings

Case Variables Process Specifications Simulation findings

Reactor Temperature/oC 180 110 (optimum)

Reactor Pressure/kPa 506.6 325 (optimum)

Mole ratio of NH3 to CO2 4.44 4.25 (optimum)

CO2 conversion(%) 80 80

NH3 conversion(%) 36 35.997

NH3 consumption t/t urea 0.566 0.536

The simulation performed for Stamicarbon urea synthesis brings some factors for consideration.

Evidently, urea production increases at high temperature though it slows down between 40oC and

80oC. From reactor species distribution graph it is found that water content in reactor is

considerably high that shifts equilibrium to left for urea formation reaction stated in section 3.1.

Within this temperature interval ammonia conversion reaches minimum resulting in high

percentage of ammonia left over and furthermore CO2 conversion is also steady in this

temperature interval . The matter can be explained by ammonium carbamate formation and

decomposition though carbamate is not considered in the simulation. Ammonium carbamate

formation is exothermic and reversible and its decomposition to urea is endothermic and also

reversible. High temperature increases the rate of both reactions and particularly favors the

decomposition of ammonium carbamate and so urea production increases. Meanwhile too high

temperature limits carbamate formation from ammonia and carbon dioxide which in turn slows

down urea production . Elevated reactor pressure also enhances urea production as high pressure

shifts equilibrium to right for urea synthesis that results in higher conversion of both CO2 and

ammonia with ammonia converted faster to urea than CO2.

Compared to process specification simulation provides optimum temperature and

pressure at low values. The mole ratio of ammonia to carbon dioxide found in simulation almost

matches that of process specification . CO2 conversion for simulation and process specification is


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same. Ammonia conversion found from simulation varies very little from process specification

but the ammonia consumption per ton of urea is lesser for simulation than process specifications.

On the other hand optimization shows the economic aspects of simulated urea

synthesis process. Though profit is maximized using optimization tool the rate of net return is

found to be 8.5% that shows high operating expenses eats up the remaining revenues. High

operating expenses is contributed to raw material cost and energy expenditure where energy

expenditure predominates raw material cost by factor of 104 . This considerably high energy

expenditure arises from heating , compression and cooling of reaction feed . High coolant duty in

the reactor also adds up energy expenditure. To increase profitability of urea synthesis process

some improvements can be made:

To decrease coolant duty the feed can be charged at optimum temperature and pressure

prescribed by simulation or raising the reactor temperature.

Installing a condenser which will condense the dry gas from rectifying column using

incoming ammonia feed followed by mixing of condensate with ammonia feed and

subsequently pumped and preheated to be charged to the reactor. This will replace the

compression and cooling costs for reaction feed and heating costs will be kept to low as

possible.

Lower the mole ratio of ammonia to carbon dioxide to optimum mole ratio 4.25 to favor

more production of urea that will enhance profitability.

The liquid outlet from stripper can be heated to a lower temperature than 150 oC sothat

energy expenditure for flash separation can be reduced.


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9. CONCLUSION

Finally the simulation and subsequent optimization of Stamicarbon urea synthesis process will

be useful for energy saving studies to improve economics of the plant , study of individual

pieces of equipment with a view of improving their performance and last of all

troubleshooting. The close concordance of simulation results with process specifications

indicate that AspenHysys can be accurately applied to simulate other urea synthesis processes.

The treatise presented can be extended to develop kinetic and thermodynamic models for urea

synthesis along with on-line control system of the plant.


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10. REFERENCES 1. B. Claudel, E. Brousse, G. Shehadeh, Novel thermodynamics and kinetics investigation of

ammonium carbamate decomposition into urea and water, Thermochemica Acta 102

(1986) 357371.

2. Cline, J. E. , Manufacture of urea, a literature survey, Tennes-see Valley Authority

Research Section, p. 11; p. 26; p. 35; Wilson Dam, Alabama, Report No. 646, 1951.

3. Frjacques, M., Theoretical Basis of the Industrial Synthesis of Urea, Chim. Ind., 60, 22-

35 (1948).

4. H.A. Irazoqui, M.A. Isla, C.M. Genoud, Simulation of an urea synthesis reactor. 2.

Reactor model, Ind. Eng. Chem. Res. 32 (1993) 26712680.

5. Hamidipour, Mohsen; Mostoufi, Navid; Sotudeh-Gharebagh, Rahmat; Modeling the

synthesis section of an industrial urea plant, Chemical Engineering Journal 106 (2005)

249260

6. Inoue, S.; Kanai, K.; Otsuka, E., Equilibrium of Urea Synthesis. I. Bull. Chem. Soc.

Japan, 45, 13339-1345 (1972).

7. Kawasumi, S., Equilibrium of the CO2-NH3-Urea-H2O System Under High Temperature

and Pressure. II. Liquid-Vapor Equilibrium in the Loading Mole Ratio of 2NH3 to CO2,

Bull. Chem. Soc. Japan, 26, 218-227 (1953).

8. Lemkowitz, S.M.; van Erp J.C.; Rekers, D.M.; van den Berg, P.J., Phase Equilibria in the

Ammonia-Carbon Dioxide Systems at and Above Urea Synthesis Conditions, J. Appl.

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9. M. Dente, M. Rovaglio, G. Bozzano, A. Sogaro, Gas-liquid reactor in the synthesis of

urea, Chem. Eng. Sci. 47 (1992) 24752480.

10. M. Dente, S. Pierucci, A. Sogaro, G. Carloni, E. Rigolli, Simulation program for urea

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11. M.A. Isla, H.A. Irazoqui, C.M. Genoud, Simulation of an urea synthesis reactor.

1.Thermodynamic framework, Ind. Eng. Chem. Res. 32 (1993) 26622670.

12. M.J. Joncich, B.H. Solka, J.E. Bower, The Thermodynamic properties of ammonium

carbamate. An experiment in heterogeneous equilibrium, J. Chem. Educ. 44 (1967) 598

600.


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13. M.A. Satyro, Y. Li, R.K. Agarwal, O.J. Santollani, Modeling urea processes, A new

thermodynamic model and software integration paradigm, from The Virtual Materials

Group, Presented at the Chemical Engineers Resource Page,

http://www.virtualmaterials.com, 2003.

14. Otsuka, E. and K. Tanimoto, Conversion rate and reaction conditions in urea synthesis,

Journal of the Chem. Soc. of Japan, Ind. Chem. Section, 63, No. 2, 254-258, 1960.

(Translated)

15. Peters, Max M. and Timmerhaus, Klaus D.: Plant Design And Economics For Chemical

Engineers,4th edition,McGraw-Hill,Inc.,p 341-420(1958)

16. R.K. Agarwal, Y.-K. Li, O.J. Santollani, M.A. Satyro, Modeling of Urea Production

Processes, in: 52nd Canadian Chemical Engineering Conference, Vancouver, Canada,

2002.


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11. APPENDIX A. Workbook:Case(Main)-Material Streams B. Workbook:Case(Main)-Composition C. Workbook:Case(Main)-Energy Streams D. Workbook:Case(Main)-Unit Operations E. Optimizer Spreadsheet

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