1. Examples and Exercises
Process simulation

2. Ammonia production: Haber Bosh process

3. Example: Ammonia production
Haber Process: N2 + 3 H2  2 NH3
Reaction conditions: T= 930 °F, P= 7350 psi
Feed composition: 74% H2, 24.5% N2, 1.2% CH4, 0.3% Ar; T= 300°F; P = 500 psi
Feed flow rate: 100 lb-mole/hr
Conversion per pass: 65% of N2
Thermodynamic model: Equation of state (Peng Robinson)
Reaction products are refrigerated to separate 75% mole of NH3 product per pass (no pressure drop). The product is pure NH3 at -20 °F (i.e. all other compositions in bottom product is zero).
The remaining product is recycled back after purge (10%) i.e. no gases in the product stream. T=300°F.

4. Example: Ammonia production
Goal 1: adjust the purge flow rate so that the stream to the reactor contains 0.11 CH4(mole fraction)
Goal 2: calculate all streams for two values of initial molar flow rate of methane (2 and 3 lbmole/hr). Prepare a graph with % CH4 in carica  spurgo con vincolo 1
Goal 3: maximize the value of the product:
Equal to the difference between the flow rate of NH3 in S-5 and S-7.
Subject to constraint: total flow rate to the reactor less than 240 lbmol/hr

5. Example: Ammonia production
Run base case with the stream manipulator
Run the base case with the flash
Flash is substituted to SM (T= -20°F; P=500 psi)
Perform sensitivity analysis on the methane concentration vs recycle ratio (purge from .92 to .98)
Set a design specification to keep methane mole frac. = 0.11 at the inlet of the reaction (take a suggestion from sensitivity)
Save a final flowsheet with the desired value of purge and without any design spec or sensitivity
Optimize the problem with the following Objective function
FOB = Flow rate of NH3 in product – Flow rate NH3 in purge
Constraint: the total flow rate to the reactor be less than 240 lb mol/hr
Suggestions:
Reinitialize the simulation, particularly after sensitivity analysis
If needed modify tolerances for convergence
If convergence is slow, increase the number of iterations for Wegstein method
Force the tear stream to be S-3

6. Ammonia: the real process

7. Optimizing steam consumption for solvent recovery
Typical example of organic aqueous stream separation
The final goal is to comply the emission specification ( in this case 150 ppm) by reducing the amount of steam injections to a minimum
reducing the operation cost
addressing the environmental restrictions.
Goal
recovery of methylene chloride from waste
Use of a liq-liq decanter for separating organic rich phase
Lower organic content to less than 150 ppm in water stream

8. The process Simulation details Process Input
calculation of the steam consumption
identification of the optimum conditions (sensitivity analysis)
Process Input
Feed stream containing an aqueous stream of Methylene chloride (S1): total flow rate of 100,000 lb/hr, Temperature at 100 0F, pressure of 24.7 psia and components in the feed stream are of CH2Cl2 and of H2O in mass fraction
Steam Streams injected to the two towers ( S2 & S4)
Steam saturate vapor at 200 psi flow rate of lb/hr is fed to the primary tower of.
Steam (S2) at a flow rate of 5000 lb/hr at the secondary tower.
The top product of both towers (primary & secondary) or vapor overheads are collected in the mixer and condensed at a Temperature of 75 0F and pressure of 14.7 psi.
The remaining stream of the process is carried off to a decanter to separate the condensate into a methylene–chloride-rich stream and a water-rich stream.

9. Methylene chloride recovery

10. Process Simulation goal
Recovery of methylene chloride in waste water;
Sensitivity analysis 1 to obtain methylene chloride concentration of 150 ppm in the bottom of the secondary tower (Stream 7 in Figure 2) by regulating the flowrate of the steam inlet to the primary tower;
Simulate sensitivity analysis 2 to obtain a methylene chloride concentration of 150 ppm in the bottom of the second tower in (Stream 7) by regulating the flow rate of the steam inlet to the secondary tower;
Optimize the total stream injections in the process.

11. Input specifications

12. Thermodynamic validation
Pure component properties

13. Thermodynamic validation
Binary LLE

14. Sensitivity analysis 1
Independent Variable: Steam flow rate injected to the primary tower
Dependent Variable : Methylene Chloride concentration in Bottom 2
Fixed Parameters: Steam flow rate injected to the secondary tower fixed at 5000 lb/hr and Feed conditions

15. Sensitivity analysis 1

16. Sensitivity analysis 2
Independent Variable: Steam feed rate inlet to the secondary tower
Dependent Variable: Methylene chloride concentration in Bottom 2
Fixed Parameters: Steam feed rate inlet to the primary tower fixed at 10,000 lb/hr, feed conditions

17. Sensitivity analysis 2

18. Design specification Controller Optimizer
set design specification on the concentration of methylene chloride in stream 7 is equal to 150 ppm
the variable is the flow rate (lb/hr) of steam injected to the primary and secondary tower (S2 & S4).
Optimizer
The sensitivity studies show that several solutions exist to give a methylene chloride concentration of 150 ppm at the bottom of the secondary tower.
The optimizer is introduced to determine among them the solution with the minimum consumption of energy.
The optimization procedure followed is to couple the controller with an optimizer to minimize the total steam consumption and the variable is the steam flow rate injected to the first and second tower.

19. Optimization results Results
savings in steam usage realized without any major process or equipment changes
Steam saving of $ per year (assuming $ 2.5/ 1000 lb steam)

20. Cyclohexane production
Reaction section
Benzene + 3 Hydrogen = Cyclo hexane
Separation section
Recovery of cyclohexane

21. Cyclo hexane production process
The production process
The objectives of the case study are the following:
Verification of the thermodynamic data and models
Simulate the base case of the reaction section
Simulate the base case of the entire process with a simple model for the distillation column
Simulate the base case of the separation section such that the bottom liquid product from the distillation column is equal to 135 kgmol/hr
Modify the process condition to obtain high recovery of cyclohexane (99.5 %) at the bottom product stream (S5)
Maintain a flow rate of 5.3 kmol/hr in the distillate stream (S4)
Simulate the complete process with a distillation column

22. Thermodynamic and data bank

23. Vapor pressure of cyclohexane

24. Binary VLE cyclo hexane - benzene
Thermodynamic model: RKS (or GE )

25. Reaction section Results
Reaction is a catalytic hydrogenation with total conversion = 0.998
Feed 1 Gas: T= 48°C - P= 22 atm – Rate 523 kgmol/hr
H2= 465 – N2= 15 – CH4= 43
Feed 2 Benzene: T=40°C - P= 20.4 atm – Rate
Results

26. Cyclohexane production

27. Separation section
The initial feed flow rate is 232 kg-mol/hr at a temperature of 200 °C and pressure of 15 atm
Inlet to a flash separator, operated at a Pressure of 15 atm and Temperature of 50 °C.
The vapor out of the flash is an output of the process,
The liquid product is fed in the middle tray of a distillation column with a reboiler and partial condenser.
The internal design specification of the process is the reflux ratio is equal to 1.2.
Thermodynamic: RKS or GE

28. Separation section N stages= 15 (feed at 8) P= 13.33 atm Ref R= 1.2
Vapor dist (1: vap; 15: liq)
T= 200 C
P= 15 atm
H2 = 30.0 kmol/hr
N2 = 15.0
CH4 = 43.0
CYC6 = 144.2
Bz = 0.2
T= 50 C
P= 15 atm
Rate initial = 135 kmol/hr

29. Base case: results Stream Name S1 S3 S2 S4 S5 Stream Description Phase
Vapor
Liquid
Temperature, C
200 50
Pressure,ATM 15
Flowrate,kg-mol/hr
FLUID RATES, Kg-mol/hr
HYDROGEN 30
0.3855
1.27E-11
NITROGEN
0.3108
2.37E-11
METHANE 43
3.0901
2.53E-08
CYCLOHEX
144.2
2.4504
6.9231
COMPOSITION
9.40E-14
1.76E-13
1.87E-10
BENZENE
5.59E-05

30. Base Case results Only 90 % of cyclohexane is recovered.
Concentrations of cyclohexane and benzene emitted in the vapor stream of the flash (S3) are within the TLV standard of NIOSH.
The third goal to maintain a minimum flow rate of 5.3 k-mol/hr in Stream 4 is not reached.

31. Internal design specification
To obtain a high recovery of cyclohexane at the bottom product stream (S5) an additional design specification is established
Adjustment of the flow rate of cyclohexane at the bottom product stream (S5) with reference to the bottom of the flash (S2) be 0.995
The variable is the duty of heater in the condenser.
The other design specification is the reflux ratio be equal to 1:2
The variable is the duty of the heater in the reboiler.
The total flowrate in Stream 5 is equal to kg-mol/hr which indicates that 99.5 % of cyclohexane is recovered.
The second goal to attain high recovery of cyclohexane is reached.

32. Internal design specification
STREAM ID S1 S2 S3 S4 S5
NAME
PHASE
VAPOR
LIQUID
FLUID RATES, KG-MOL/HR
1 HYDROGEN 30
0.3855
3.35E-11
2 NITROGEN 15
0.3108
6.40E-11
3 METHANE 43
3.0901
7.24E-08
4 CYCLOHEX
144.2
2.4504
0.7088
5 BENZENE
0.2
0.1952
4.85E-03
4.20E-03
0.1909
TOTAL RATE, KG-MOL/HR
232.4
4.4994
TEMPERATURE, C
200 50
PRESSURE, ATM
13.33
ENTHALPY, M*KCAL/HR
2.2402
0.2559
0.0917
0.0131
1.2584
MOLECULAR WEIGHT
MOLE FRAC VAPOR 1
MOLE FRAC LIQUID

33. Sensitivity Analysis 1
The independent variable for this parametric study is the variation of the temperature in the flash.
Independent Variable: Flash T 10.°C to 50 °C
Fixed Parameters: Feed Conditions & Column Conditions
Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr

34. Sensitivity analysis 1

35. Sensitivity Analysis 2
The independent variable for this parametric study is the variation of the pressure in the flash.
Independent Variable: Flash P 10 to 35 atm
Fixed Parameters: Feed Conditions & Column Conditions
Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr

36. Sensitivity analysis 2

37. External design specification
Sensitivity analysis gave indications and feasibility of process modification (T is the best variable)
The target of of 5.3 kg-mol/hr in the distillate or Stream 4 of the process is reachable
An external design specification is established to this aim.
The flow rate of 5.3 kg-mol/hr in the distillate is reached with out affecting the product quality ( purity of cyclohexane recovered at the bottom product stream).

38. Design specification Stream Name S1 S3 S2 S4 S5 Stream Description
Phase
Vapor
Liquid
Temperature, C
200
Pressure, ATM 15
Flowrate,kg-mol/hr
Composition
HYDROGEN
6.42E-14
NITROGEN
1.56E-13
METHANE
2.39E-10
CYCLOHEX
BENZENE
8.07E-06

39. The complete process
Stream manipulator  distillation column

40. Cyclohexane production process

41. Cyclohexane production

42. Process data
Components: Hydrogen, Nitrogen, methane, Cyclohexane, benzene
Thermodynamics: Equation of State (Peng Robinson)
Reaction: Hydrogenation of benzene to cyclohexane with conversion at T= 400°F

43. Reaction section
Reaction hydrogenation with total conversion = 0.998; T= 400°F
Feed 1 Gas: T=120°C - P= 335 psi – Rate lbmol/hr
H2= – N2= – CH4= lbmol/hr
Feed 2 Benzene: T=104.1°F - P= 300 psi – Rate 256 lbmol/hr pure benzene
Mixer: no specifications
Heater: process stream exit temperature = 300 °F

44. Separation and recycle
Flash: temperature 120°F, pressure drop 5 psi
Vapor recycle:
Splitter: purge ratio (purged stream/feed to the splitter) = 0.08
Compressor: outlet pressure 382 psi
Liquid recycle:
Splitter: recycle ratio (recycle stream/feed to the splitter) = 0.30
pump: pressure rise 5 psi
Separation section
Pump: outlet pressure 200 psi
Separator (stream calculator): Overhead temperature = 120°F - Bottom temperature = 120 °F
Specifications recovery in overhead: H2=1, N2=1, CH4=0.8
Specifications recovery in bottom: Benzene=1, CyC6=1

45. Excercise Setup the Units, Description of the flowsheet, autosave off
Name streams and units
Components
Thermodynamic specification
SRK and UNIFAC
Data retrieval
Vapor pressure – Enthalpy of vaporization
Verification of VLE for benzene – cyclo hexane
Reaction specification
Base case calculation
Include a stream property table
Verify:
Total flow rate in overhead product S15 = 5.1 lb mol /hr
Composition of Bottom product S16 of CyC6 = 0.996
Recovery of cyclo hexane in the separation section =