Examples and Exercises
Process simulation
Ammonia production: Haber Bosh process
S6
S7
SPLITTER
S4
S2
S3
S5
FEED
S1
MIXER
REACTOR
R1
Process and product design
FLASH
F1
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.
Process and product design
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:


Process and product design
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
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:




Process and product design
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
Ammonia: the real process
Process and product design
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



Process and product design
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
The process
Simulation details


calculation of the steam consumption
identification of the optimum conditions (sensitivity analysis)
Process Input






Process and product design
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 0.0140
of CH2Cl2 and 0.986 of H2O in mass fraction
Steam Streams injected to the two towers ( S2 & S4)
Steam saturate vapor at 200 psi flow rate of 10.000 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.
Methylene chloride recovery
Process and product design
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.
Process and product design
Input specifications
STREAM ID
PHASE
FLUID WEIGHT
FRACTIONS
1 CH2CL2
2 H2O
TOTAL RATE, LB/HR
TEMPERATURE, F
PRESSURE, PSIA
ENTHALPY, MM BTU/HR
MOLECULAR WEIGHT
WEIGHT FRAC VAPOR
WEIGHT FRAC LIQUID
Process and product design
S1
NAME
LIQUID
S2
S4
VAPOR
VAPOR
0.014
0.986
0
1
0
1
100000.1767
99.9999
24.7
6.7263
18.2159
0
1
10000.00
381.8395
200
15.1648
18.015
1
1
5000.002
381.8395
200
2.5956
18.015
1
1
Thermodynamic validation
Pure component properties
Components
0
Tc
A+
F
PRO II
Pc
Lit.
A+
0
TNB F
PSIA
PRO II
Lit.
A+
CH2Cl2
458.33
458.6
445.58
881.8295
881.757
881.83
H2O
705.64
705.56
705.65
3198.81
3208.12
3208.13
Components
Δ
Tc
Δ
0
F
Δ
0
TNB F
Pc PSIA
CH2Cl2
ASPEN
-12.75
0
0.04
PRO II
-13.02
0.073
0.05
ASPEN
0.01
9.32
0
PRO II
0.09
0.01
0
H2O
Process and product design
PRO II
Lit.
103.55
103.64
103.59
212
212
212
Thermodynamic validation
Binary LLE
Literature
Solubility of 1 in 2 0.420
(mole %)
Solubility of 2 in 1 0.760
(mole %)
Process and product design
Aspen+
0.419
PRO II
0.443
0.812
0.991
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
Process and product design
Sensitivity analysis 1
SENSITIVITY TOWER 1
METHYLENE CHLORIDE
CONCENTRATION, S-7. ppm
2500
2000
1500
1000
500
0
6000
8000
10000
12000
STEAM FLOWRATE, LB/H
Process and product design
14000
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
Process and product design
Sensitivity analysis 2
SENSITIVITY TOWER 2
METHYLENE CHLORIDE
CONCENTRATION, S-7, ppm
170
150
130
110
90
70
50
6000
8000
10000
12000
STEAM FLOWRATE, LB/H
Process and product design
14000
Design specification
Controller


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



Process and product design
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.
Optimization results
Results


savings in steam usage realized without any major process or
equipment changes
Steam saving of 30000 $ per year (assuming $ 2.5/ 1000 lb steam)
Sensitivity I
STEAM
Tower I
10,650
Sensitivity II
10,000
6,200
16,200 lb/hr
Optimizer
12,659.71
2163.97
14,823.7 lb/hr
Process and product design
lb/hr
Tower II
5,000
TOTAL
15,650 lb/hr
Cyclohexane production
Reaction section

Benzene + 3 Hydrogen = Cyclo hexane
Separation section

Recovery of cyclohexane
S11
S8
S1
FEEDPUMP
TANK2
S23
COMPRESS
S10
SPLIT1
S5
MULT2
S12
S18
S4
S9
S3
REACTOR
M ULT
MIXER
S7
COLUMN
FLASH
PREHTR
S2
S21
S17
M ULT
TANK
MULT1
PUMP
S13
S19
S20
TANK1
SPLITREC
S16
S15
S14
RECPUMP
Process and product design
Cyclo hexane production process
The production process
The objectives of the case study are the following:







Process and product design
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
Thermodynamic and data bank
Components
0
Tc
A+
PRO II
CYCLOHEX
537.17
536.5
BENZENE
552.02
METHANE
-116.7
Components
F
Pc
Lit.
A+
0
PSIA
PRO II
TNB F
Lit.
A+
PRO II
536.88
591.75
590.78
591.02
177.29
177.33
177.30
553.0
552.22
709.96
714.22
710.39
176.16
176.8
176.62
-116.7
-116.7
667.03
667.19
666.88
-258.68
-258.68
-258.74
Δ Tc
0
Δ Pc PSIA
F
0
Δ T NB F
PRO II
ASPEN
PRO II
ASPEN
PRO II
ASPEN
CYCLOHEX
0.34
0.29
0.247
0.73
0.034
0
BENZENE
0.21
0.2
0.383
0.435
0.18
0.46
METHANE
0.02
0
0.145
0.145
-0.06
.06
Process and product design
Lit.
Vapor pressure of cyclohexane
VAPOR PRESSURE CYCLOHEXANE
Literature
PRO II
VAPOR PRESSURE,mmHg
100
95
90
85
80
75
70
0.00332 0.00333 0.00334 0.00335 0.00336 0.00337 0.00338
TEM PERATURE,1/K
Process and product design
Binary VLE cyclo hexane - benzene
Thermodynamic model: RKS (or GE )
X-Y Plot for BENZENE and CYCLOHEX
1.2
Vapor Composition, Mole Fraction BENZENE
1
0.8
x=y
Equilibrium curve
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
Liquid Composition, Mole Fraction BENZENE
Process and product design
1
1.2
Reaction section
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 144.4
S1
S3
S4
S5
S2
M1
E1
R1
Stream Nam e
Stream D escriptio n
Phase
Process and product design
S5
Vapor
Tem perature
Press ure
Enth alpy
Mole cular W eig ht
Mole Fraction Vapor
Mole Fraction Liqu id
R ate
C
ATM
M*KJ/HR
Fluid R ates
BENZENE
CH
H2
N2
METHANE
KG-MOL/HR
KG-MOL/HR
200.0000
15.0000
9.3944
56.6959
1.0000
0.0000
235.066
0.2888
144.1112
32.6664
15.0000
43.0000
Results
Cyclohexane production
Process and product design
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
Process and product design
Separation section
S4
1
CN1
2
3
4
N stages= 15
(feed at 8)
P= 13.33 atm
Ref R= 1.2
Vapor dist (1: vap; 15: liq)
5
6
T= 200 C
P= 15 atm
S1
H2 = 30.0 kmol/hr
N2 = 15.0
CH4 = 43.0
CYC6 = 144.2
Bz = 0.2
Process and product design
7
S3
T= 50 C
P= 15 atm
8
9
10
F1
11
S2
12
13
14
T1
15
S5
Rate initial = 135 kmol/hr
Base case: results
Stream Name
S1
S3
S2
S4
S5
Liquid
Vapor
Liquid
Stream Description
Phase
Vapor
Vapor
Temperature, C
200
50
50
169.2375183
199.5368652
Pressure,ATM
15
15
15
13.32999992
13.32999992
232.3999939
86.66875458
145.7312469
10.73137283
134.9998779
HYDROGEN
30
0.3855
29.6145
0.3855
1.27E-11
NITROGEN
15
0.3108
14.6892
0.3108
2.37E-11
METHANE
43
3.0901
39.9099
3.0901
2.53E-08
144.2
141.7496
2.4504
6.9231
134.8266
HYDROGEN
0.129087776
0.341697156
0.002645517
0.035925929
9.40E-14
NITROGEN
0.064543888
0.169486165
0.002133012
0.028966138
1.76E-13
METHANE
0.185025826
0.460488141
0.021203889
0.287947208
1.87E-10
CYCLOHEX
0.620481908
0.028272673
0.972678483
0.645144701
0.998714685
BENZENE
0.000860585
5.59E-05
0.001339122
0.002016034
0.001285313
Flowrate,kg-mol/hr
FLUID RATES, Kgmol/hr
CYCLOHEX
COMPOSITION
Process and product design
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 kmol/hr in Stream 4 is not reached.
Process and product design
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 141.04 kgmol/hr which indicates that 99.5 % of cyclohexane is
recovered.
The second goal to attain high recovery of cyclohexane is
reached.
Process and product design
Internal design specification
STREAM ID
S1
S2
S3
S4
S5
LIQUID
VAPOR
VAPOR
LIQUID
NAME
PHASE
VAPOR
FLUID RATES, KG-MOL/HR
1 HYDROGEN
30
0.3855
29.6145
0.3855
3.35E-11
2 NITROGEN
15
0.3108
14.6892
0.3108
6.40E-11
3 METHANE
43
3.0901
39.9099
3.0901
7.24E-08
144.2
141.7496
2.4504
0.7088
141.0409
0.2
0.1952
4.85E-03
4.20E-03
0.1909
232.4
145.7312
86.6688
4.4994
141.2318
200
50
50
102.2286
199.5356
15
15
15
13.33
13.33
2.2402
0.2559
0.0917
0.0131
1.2584
57.3255
82.3734
15.2082
26.4565
84.1548
MOLE FRAC VAPOR
1
0
1
1
0
MOLE FRAC LIQUID
0
1
0
0
1
4 CYCLOHEX
5 BENZENE
TOTAL RATE, KG-MOL/HR
TEMPERATURE, C
PRESSURE, ATM
ENTHALPY, M*KCAL/HR
MOLECULAR WEIGHT
Process and product design
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
Process and product design
Sensitivity analysis 1
SENSITIVTY FLASH TEMPERATURE
Kg-mol/H
FLOWRATE, Kg-mol/H
5.4
5.1
4.8
4.5
4.2
0
10
20
30
TEMPERATURE, 0C
Process and product design
40
50
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
Process and product design
Sensitivity analysis 2
SENSITIVTY FLASH PRESSURE
Kg-mol/H
FLOWRATE, Kg-mol/H
10
8
6
4
2
10
15
20
25
PRESSURE, ATM
Process and product design
30
35
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).
Process and product design
Design specification
Stream Name
S1
S3
S2
S4
S5
Vapor
Liquid
Vapor
Liquid
Stream Description
Phase
Vapor
Temperature, C
200
2.957733154
2.957733154
25.61639404
199.5356445
Pressure, ATM
15
15
15
0.99999994
13.32999992
232.3999939
83.74118805
148.6588135
5.300059795
143.3587494
HYDROGEN
0.129087776
0.354866177
0.001904261
0.053411685
6.42E-14
NITROGEN
0.064543888
0.175634116
0.001965511
0.055129658
1.56E-13
METHANE
0.185025826
0.465723932
0.02690541
0.754656792
2.39E-10
CYCLOHEX
0.620481908
0.003767717
0.967884004
0.135731429
0.99864918
BENZENE
0.000860585
8.07E-06
0.001340818
0.001070414
0.001350815
Flowrate,kg-mol/hr
Composition
Process and product design
The complete process
Stream manipulator  distillation column
S8
C1
S7
S9
SP1
S15
S6
1
S1
S3
2
S5
S4
S2
M1
E1
S11
F1
3
P2
4
R1
S14
5
S13
6
S10
S12
7
8
9
SP2
10
P1
11
12
Stream Name
Stream Description
S14
S15
S16
S5
S6
S10
S12
13
S8
14
Phase
Liquid
Vapor
Liquid
Vapor
Vapor
Liquid
Liquid
Vapor
Temperature
Pressure
F
PSIA
120.646
200.000
380.376
195.456
392.346
195.456
400.000
15.000
120.000
20.000
120.000
20.000
120.000
20.000
120.000
20.000
Flowrate
LB-MOL/HR
236.220
4.000
232.220
1289.011
951.554
337.457
101.237
875.430
0.001
0.001
0.000
0.000
0.998
0.002
0.058
0.005
0.016
0.919
0.001
0.000
0.000
0.000
0.999
0.000
0.163
0.044
0.336
0.457
0.000
0.220
0.060
0.455
0.265
0.001
0.001
0.000
0.000
0.998
0.001
0.001
0.000
0.000
0.998
0.000
0.220
0.060
0.455
0.265
Composition
BENZENE
METHANE
N2
H2
CH
Process and product design
T1
15
S16
Cyclohexane production process
Cyclohexane production
Process and product design
Process data
Components: Hydrogen, Nitrogen, methane, Cyclohexane,
benzene
Thermodynamics: Equation of State (Peng Robinson)
Reaction: Hydrogenation of benzene to cyclohexane with
conversion 0.998 at T= 400°F
C6 H 6  3H 2  C6 H12
Process and product design
Reaction section
Reaction hydrogenation with total conversion = 0.998; T= 400°F
Feed 1 Gas: T=120°C - P= 335 psi – Rate 823.43 lbmol/hr

H2= 802.73 – N2= 4.15 – CH4= 16.55 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
Process and product design
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




Process and product design
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
Excercise
Setup the Units, Description of the flowsheet, autosave off
Flowsheet
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:



Process and product design
Total flow rate in overhead product S15
Composition of Bottom product S16 of CyC6
Recovery of cyclo hexane in the separation section
= 5.1 lb mol /hr
= 0.996
=