Stamicarbon project.pdf

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

1. INTRODUCTION Urea (NH2CONH2) is produced at industrial scale by the reaction between ammonia and carbon dioxide at high pressure (13–30MPa) and high temperature (170–200 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 François, 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 Wöhler 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 Bosch–Meiser 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 180 oC . 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 180-210ºC 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 3


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 Stripping agent Reactor temperature,oC Reactor pressure,atm Molar NH3/CO2 ratio CO2 conversion,% NH3 conversion,% No. of high pressure vessels Recirculation stages NH3 consumption, t/t urea CO2 consumption, t/t urea Import steam,t/t urea Cooling water,t/t urea Electricity, kWh/t urea

Stamicarbon Snamprogetti Carbon dioxide Initially ammonia now switch to steam 183 188 140 156 2.95 3.3-3.6 60 64 36 41 4 5 1 2 0.566 0.566 0.733 0.735 0.920 0.950 70 75 15 21-23

TEC ACES process Carbon dioxide 190 175 4.0 6.8 34 5 2 0.568 0.735-0.740 0.80 80 15

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4. PROCESS DIAGRAM 4.1. Process block diagram Reactor vent

High pressure feed

Feed compressor

Reactor

Wet gas

Reactor products

CO2 compressor

High

Rectifying

pressure

Column

stripper

Discharge gas

Moisture Discharge Mixer

CO2 feed

NH3 feed

Dry gas

Feed NH3 pump

Mixed feed

Mixer

Vent gas Scrubber Recycle

Separator Effluent

Flash Liquid products

Urea Separator

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

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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 75 0C. 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 Rxn1>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 . 10


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 12


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 o

C, 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 price×Urea production rate×Urea composition-Ammonia price×Ammonia feed rate×Ammonia composition-CO2 price×CO2 feed rate×CO2 composition-Pump duty×Cost 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 reactor×Reactor Coolant duty-Cost of heating and cooling in rectifying column×Rectifying 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 18


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.

Effect of mole ratio on rate of urea synthesis chemical species 7.00E+04 6.00E+04 5.00E+04 Mass flow rate /kg/h

4.00E+04 unreacted CO2 3.00E+04

urea Unreacted ammonia

2.00E+04 1.00E+04 0.00E+00 1

2

3

4

5

Mole ratio of ammonia to CO2

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

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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.

Reactor Chemical species distribution at reactor temperature

1.1 1 0.9 0.8 0.7 Reactor 0.6 Chemical species Distribution 0.5 0.4

Urea ammonia Carbon dioxide

0.3

water

0.2 0.1 0 0

20

40

60

80

100

120

Reactor temperature/ºC

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

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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.

Profit Vs Price of Urea synthesis raw materials 4.2 4.1 4 Profit(hundred 3.9 thousand dollars) 3.8

Carbon dioxide Ammonia

3.7 3.6 3.5 0

0.2

0.4

0.6

0.8

1

1.2

Price of raw materials ($/kg)

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

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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/ C

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

o

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 40 oC 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 25


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) 357–371. 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. Fréjacques, M., “Theoretical Basis of the Industrial Synthesis of Urea,” Chim. Ind., 60, 2235 (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) 2671–2680. 5. Hamidipour, Mohsen; Mostoufi, Navid; Sotudeh-Gharebagh, Rahmat; “Modeling the synthesis section of an industrial urea plant”, Chemical Engineering Journal 106 (2005) 249–260 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. Chem.Biotechnol., 30, 85-101 (1980). 9. M. Dente, M. Rovaglio, G. Bozzano, A. Sogaro, “Gas-liquid reactor in the synthesis of urea”, Chem. Eng. Sci. 47 (1992) 2475–2480. 10. M. Dente, S. Pierucci, A. Sogaro, G. Carloni, E. Rigolli, “Simulation program for urea plants”, Comput. Chem. Eng. 21 (1988) 389–400. 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) 2662–2670. 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

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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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Stamicarbon project.pdf

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