Process simulation and environmental problemsstudenti.di3.units.it/Progettazione di materi… ·...

Examples and ExercisesProcess simulation

Process and product design

Ammonia production: Haber Bosh process

R1F1

SPLITTER

MIXER

S3

S4

S7

S1

S2

S6

S5

REACTOR FLASH

FEED


Example: Ammonia productionHaber Process: N2 + 3 H2 2 NH3

Reaction conditions: T= 930 °F, P= 7350 psiFeed composition: 74% H2, 24.5% N2, 1.2% CH4, 0.3% Ar; T= 300°F; P = 500 psiFeed flow rate: 100 lb-mole/hrConversion 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.


Example: Ammonia productionGoal 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 1Goal 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

MXS-1

S-3

S-2RXTR

S-4

SEPA

S-5

S-6 SPLS-7


Example: Ammonia productionRun base case with the stream manipulatorRun 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 sensitivityOptimize 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


Ammonia: the real processFEED-MIX

S1

S14

S2 STAGE1

S3 INTCOOL

STAGE2

S5REACTMIX

S12

S6

PREHTR

S7

REACTS8

QUENCH

S9A

CONDNSER

S9HPFLASH

S10

S13

LPFLASH

PRODUCT

SPLIT

S11BLEED RECOMPR


Optimizing steam consumption for solvent recovery

Typical example of organic aqueous stream separationThe 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


The processSimulation details

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 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

TOWER1

FEED

STEAM1

TOP1

BOT1

TOWER2

STEAM2

TOP2

BOT2

MIXER

CONDIN

COND

CONDOUT

DECANT

VENT

MECL

WATER


Process Simulation goalRecovery 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.


Input specifications

STREAM ID S1 S2 S4 NAME

PHASE LIQUID VAPOR VAPOR FLUID WEIGHT FRACTIONS 1 CH2CL2 0.014 0 0 2 H2O 0.986 1 1 TOTAL RATE, LB/HR 100000.1767 10000.00 5000.002 TEMPERATURE, F 99.9999 381.8395 381.8395 PRESSURE, PSIA 24.7 200 200 ENTHALPY, MM BTU/HR 6.7263 15.1648 2.5956 MOLECULAR WEIGHT 18.2159 18.015 18.015 WEIGHT FRAC VAPOR 0 1 1 WEIGHT FRAC LIQUID 1 1 1


Thermodynamic validationPure component properties

Components Tc 0F Pc PSIA TNB 0F

A+ PRO II Lit. A+ PRO II Lit. A+ PRO II Lit. CH2Cl2 458.33 458.6 445.58 881.8295 881.757 881.83 103.55 103.64 103.59

H2O 705.64 705.56 705.65 3198.81 3208.12 3208.13 212 212 212

Δ Δ Δ Components Tc 0F Pc PSIA TNB

0F

CH2Cl2 ASPEN -12.75 0 0.04 PRO II -13.02 0.073 0.05 H2O

ASPEN 0.01 9.32 0 PRO II 0.09 0.01 0


Thermodynamic validationBinary LLE

Literature Aspen+ PRO II Solubility of 1 in 2 (mole %)

0.420 0.419 0.443

Solubility of 2 in 1 (mole %)

0.760 0.812 0.991


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


Sensitivity analysis 1SENSITIVITY TOWER 1

0

500

1000

1500

2000

2500

6000 8000 10000 12000 14000

STEAM FLOWRATE, LB/H

MET

HYLE

NE C

HLO

RIDE

CO

NCEN

TRAT

ION,

S-7

. ppm


Sensitivity analysis 2Independent 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


Sensitivity analysis 2SENSITIVITY TOWER 2

50

70

90

110

130

150

170

6000 8000 10000 12000 14000

STEAM FLOWRATE, LB/H

MET

HYLE

NE C

HLO

RIDE

CO

NCEN

TRAT

ION,

S-7

, ppm


Design specificationController

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.


Optimization resultsResults

savings in steam usage realized without any major process or equipment changes

Steam saving of 30000 $ per year (assuming $ 2.5/ 1000 lb steam)

STEAM lb/hr Tower I Tower II TOTAL Sensitivity I 10,650 5,000 15,650 lb/hr

Sensitivity II 10,000 6,200 16,200 lb/hr

Optimizer 12,659.71 2163.97 14,823.7 lb/hr


Cyclohexane productionReaction section

Benzene + 3 Hydrogen = Cyclo hexaneSeparation section

Recovery of cyclohexane

S5

S2

S15

S12

S6

S7

S8

S13

S9

S11

S10

S16

S14

S17

S19

S18

S20

S3

S4

S23

S21

S1

MIXER

PREHTR

REACTOR

FLASH

SPLIT1

SPLITREC

PUMP

COLUMN

TANK

COMPRESS

RECPUMP

MULT

MULT2

TANK2

TANK1

MULT

MULT1

FEEDPUMP


Cyclo hexane production processThe 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


Thermodynamic and data bankComponents Tc 0F Pc PSIA TNB

0F

A+ PRO II Lit. A+ PRO II Lit. A+ PRO II Lit.

CYCLOHEX 537.17 536.5 536.88 591.75 590.78 591.02 177.29 177.33 177.30

BENZENE 552.02 553.0 552.22 709.96 714.22 710.39 176.16 176.8 176.62

METHANE -116.7 -116.7 -116.7 667.03 667.19 666.88 -258.68 -258.68 -258.74

Δ Tc 0F Δ Pc PSIA Δ TNB 0F Components

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


Vapor pressure of cyclohexane

VAPOR PRESSURE CYCLOHEXANE

70

75

80

85

90

95

100

0.00332 0.00333 0.00334 0.00335 0.00336 0.00337 0.00338

TEMPERATURE,1/K

VAPO

R P

RES

SUR

E,m

mH

g

Literature PRO II


Binary VLE cyclo hexane - benzeneThermodynamic model: RKS (or GE )

X-Y Plot for BENZENE and CYCLOHEX

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

Liquid Composition, Mole Fraction BENZENE

Vapo

r Com

posi

tion,

Mol

e Fr

actio

n B

ENZE

NE

x = yEquilibrium curve


Reaction sectionReaction is a catalytic hydrogenation with total conversion = 0.998Feed 1 Gas: T= 48°C - P= 22 atm – Rate 523 kgmol/hr

H2= 465 – N2= 15 – CH4= 43Feed 2 Benzene: T=40°C - P= 20.4 atm – Rate 144.4

M1

R1E1

S1

S2

S3 S4 S5

Stream NameStream DescriptionPhase

TemperaturePressureEnthalpyMolecular WeightMole Fraction VaporMole Fraction LiquidRate

Fluid Rates BENZENE CH H2 N2 METHANE

CATMM*KJ/HR

KG-MOL/HR

KG-MOL/HR

S5

Vapor

200.000015.00009.3944

56.69591.00000.0000

235.066

0.2888144.111232.666415.000043.0000

Results


Cyclohexane production

M1 E1

R1

F1

SP1

SP2

P1

C1

P2SC1

S1

S2

S3 S4 S5

S6

S7

S8

S9

S10

S11

S12

S13

S14

S15

S16

Stream NameStream Description

Phase

TemperaturePressure

Flowrate

Composition BENZENE METHANE N2 H2 CH

FPSIA

LB-MOL/HR

S14

Liquid

120.646200.000

236.221

0.0010.0010.0000.0000.998

S15

Liquid

-200.000200.000

235.002

0.0000.0010.0000.0000.999

S16

Vapor

386.000200.000

1.218

0.1410.0380.0000.0000.821

S5

Vapor

400.00015.000

1289.377

0.0000.1630.0450.3350.457

S6

Vapor

120.00020.000

951.918

0.0000.2210.0610.4540.265

S10

Liquid

120.00020.000

337.458

0.0010.0010.0000.0000.998

S12

Liquid

120.00020.000

101.237

0.0010.0010.0000.0000.998

S8

Vapor

120.00020.000

875.765

0.0000.2210.0610.4540.265


Separation sectionThe initial feed flow rate is 232 kg-mol/hr at a temperature of 200 °C and pressure of 15 atmInlet 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


Separation section

F1

2

3

4

5

6

7

8

9

10

11

12

13

14

1

15T1

CN1

S1

S3

S2

S4

S5

T= 200 CP= 15 atm

H2 = 30.0 kmol/hrN2 = 15.0CH4 = 43.0CYC6 = 144.2Bz = 0.2

T= 50 CP= 15 atm

N stages= 15 (feed at 8)P= 13.33 atmRef R= 1.2Vapor dist (1: vap; 15: liq)

Rate initial = 135 kmol/hr


Base case: resultsStream Name S1 S3 S2 S4 S5

Stream Description

Phase Vapor Vapor Liquid Vapor Liquid

Temperature, C 200 50 50 169.2375183 199.5368652

Pressure,ATM 15 15 15 13.32999992 13.32999992

Flowrate,kg-mol/hr 232.3999939 86.66875458 145.7312469 10.73137283 134.9998779

FLUID RATES, Kg-mol/hr

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

CYCLOHEX 144.2 141.7496 2.4504 6.9231 134.8266

COMPOSITION

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


Base Case resultsOnly 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.


Internal design specificationTo 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 kg-mol/hr which indicates that 99.5 % of cyclohexane is recovered.The second goal to attain high recovery of cyclohexane is reached.


Internal design specificationSTREAM ID S1 S2 S3 S4 S5

NAME

PHASE VAPOR LIQUID VAPOR VAPOR LIQUID

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

4 CYCLOHEX 144.2 141.7496 2.4504 0.7088 141.0409

5 BENZENE 0.2 0.1952 4.85E-03 4.20E-03 0.1909

TOTAL RATE, KG-MOL/HR 232.4 145.7312 86.6688 4.4994 141.2318

TEMPERATURE, C 200 50 50 102.2286 199.5356

PRESSURE, ATM 15 15 15 13.33 13.33

ENTHALPY, M*KCAL/HR 2.2402 0.2559 0.0917 0.0131 1.2584

MOLECULAR WEIGHT 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


Sensitivity Analysis 1The independent variable for this parametric study is the variation of the temperature in the flash.

Independent Variable: Flash T 10.°C to 50 °CFixed Parameters: Feed Conditions & Column Conditions Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr


Sensitivity analysis 1SENSITIVTY FLASH TEMPERATURE

4.2

4.5

4.8

5.1

5.4

0 10 20 30 40 50

TEMPERATURE, 0C

FLO

WRA

TE, K

g-m

ol/H

Kg-mol/H


Sensitivity Analysis 2The independent variable for this parametric study is the variation of the pressure in the flash.

Independent Variable: Flash P 10 to 35 atmFixed Parameters: Feed Conditions & Column Conditions Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr


Sensitivity analysis 2SENSITIVTY FLASH PRESSURE

2

4

6

8

10

10 15 20 25 30 35

PRESSURE, ATM

FLO

WRA

TE, K

g-m

ol/H

Kg-mol/H


External design specificationSensitivity 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 reachableAn 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).


Design specificationStream Name S1 S3 S2 S4 S5

Stream Description

Phase Vapor Vapor Liquid Vapor Liquid

Temperature, C 200 2.957733154 2.957733154 25.61639404 199.5356445

Pressure, ATM 15 15 15 0.99999994 13.32999992

Flowrate,kg-mol/hr 232.3999939 83.74118805 148.6588135 5.300059795 143.3587494

Composition

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


The complete process Stream manipulator distillation column

M1 E1

R1

F1

SP1

SP2

P1

C1

P2

2

3

4

5

6

7

8

9

10

11

12

13

14

1

15T1

S1

S2

S3 S4 S5

S6

S7

S8

S9

S10

S11

S12

S13

S14

S15

S16


Phase

TemperaturePressure

Flowrate


FPSIA

LB-MOL/HR

S14

Liquid

120.646200.000

236.220

0.0010.0010.0000.0000.998

S15

Vapor

380.376195.456

4.000

0.0020.0580.0050.0160.919

S16

Liquid

392.346195.456

232.220

0.0010.0000.0000.0000.999

S5

Vapor

400.00015.000

1289.011

0.0000.1630.0440.3360.457

S6

Vapor

120.00020.000

951.554

0.0000.2200.0600.4550.265

S10

Liquid

120.00020.000

337.457

0.0010.0010.0000.0000.998

S12

Liquid

120.00020.000

101.237

0.0010.0010.0000.0000.998

S8

Vapor

120.00020.000

875.430

0.0000.2200.0600.4550.265

Cyclohexane production process


Cyclohexane production

M1 E1

R1

F1

SP1

SP2

P1

C1

P2SC1

S1

S2

S3 S4 S5

S6

S7

S8

S9

S10

S11

S12

S13

S14

S15

S16


Phase

TemperaturePressure

Flowrate


FPSIA

LB-MOL/HR

S14

Liquid

120.646200.000

236.221

0.0010.0010.0000.0000.998

S15

Liquid

-200.000200.000

235.002

0.0000.0010.0000.0000.999

S16

Vapor

386.000200.000

1.218

0.1410.0380.0000.0000.821

S5

Vapor

400.00015.000

1289.377

0.0000.1630.0450.3350.457

S6

Vapor

120.00020.000

951.918

0.0000.2210.0610.4540.265

S10

Liquid

120.00020.000

337.458

0.0010.0010.0000.0000.998

S12

Liquid

120.00020.000

101.237

0.0010.0010.0000.0000.998

S8

Vapor

120.00020.000

875.765

0.0000.2210.0610.4540.265


Process dataComponents: Hydrogen, Nitrogen, methane, Cyclohexane, benzeneThermodynamics: Equation of State (Peng Robinson)Reaction: Hydrogenation of benzene to cyclohexane with conversion 0.998 at T= 400°F

126266 3 HCHHC


Reaction sectionReaction hydrogenation with total conversion = 0.998; T= 400°FFeed 1 Gas: T=120°C - P= 335 psi – Rate 823.43 lbmol/hr

H2= 802.73 – N2= 4.15 – CH4= 16.55 lbmol/hrFeed 2 Benzene: T=104.1°F - P= 300 psi – Rate 256 lbmol/hr pure benzeneMixer: no specificationsHeater: process stream exit temperature = 300 °F


Separation and recycleFlash: temperature 120°F, pressure drop 5 psiVapor recycle:

Splitter: purge ratio (purged stream/feed to the splitter) = 0.08

Compressor: outlet pressure 382 psiLiquid recycle:

Splitter: recycle ratio (recycle stream/feed to the splitter) = 0.30

pump: pressure rise 5 psiSeparation 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


ExcerciseSetup the Units, Description of the flowsheet, autosave offFlowsheet Name streams and unitsComponentsThermodynamic specification

SRK and UNIFACData retrieval

Vapor pressure – Enthalpy of vaporization Verification of VLE for benzene – cyclo hexane

Reaction specificationBase case calculationInclude a stream property tableVerify:

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 =

Process simulation and environmental problemsstudenti.di3.units.it/Progettazione di materi… ·...

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