Word Count: 3 140

CHE 4049 F Project 2

University of Cape Town Department of Chemical Engineering

i | P a g e

Contents List of Figures .............................................................................................................................................. ii

List of Tables ............................................................................................................................................... ii

1 PFD, Mass balance, Utility table and Equipment list for the benzene extraction process ..... 1

2 Discussion of the thermodynamic models available in Aspen Plus ........................................... 6

2.1 EOS models ................................................................................................................................ 6

2.2 Activity Coefficient models ...................................................................................................... 6

2.3 Henrys Coefficients .................................................................................................................. 6

3 Justification of the thermodynamic property methods ................................................................ 7

4 Discussion of the Aspen model ..................................................................................................... 11

4.1 Description of Aspen models used to simulate the important units ................................ 11

4.1.1 Stream mixers ................................................................................................................... 11

4.1.2 F-splitter ............................................................................................................................ 11

4.1.3 Thermal and phase state changer (heaters and coolers) ............................................. 11

4.1.4 DSTWU columns ............................................................................................................ 11

4.1.5 RADFRAC columns ....................................................................................................... 11

4.1.6 Pump .................................................................................................................................. 11

4.1.7 Valve (valve 2) .................................................................................................................. 11

4.2 Method used to solve the simulation .................................................................................... 12

4.3 Solution method for section one ........................................................................................... 12

4.4 Solution method for section two ........................................................................................... 12

4.5 Solution method for section three ......................................................................................... 13

4.6 Assumptions and Improvements of the simulation ............................................................ 13

ii | P a g e

List of Figures

FIGURE 1:VLE DATA FOR BENZENE AND HEPTANE IN THE PRE- DISTILLATION COLUMN. THE

GRAPH COMPARES VARIOUS THERMODYNAMIC MODELS TO THE EXPERIMENTAL VLE

DATA ........................................................................................................................................... 8

FIGURE 2: VLE DATA FOR O-XYLENE AND NONANE IN THE HEAVY COLUMN. THE GRAPH

COMPARES VARIOUS THERMODYNAMIC MODELS TO THE EXPERIMENTAL VLE DATA . 8

FIGURE 3:VLE DATA FOR BENZENE AND HEXANE IN THE EXTRACTIVE DISTILLATION COLUMN.

THE GRAPH COMPARES VARIOUS THERMODYNAMIC MODELS TO THE EXPERIMENTAL

VLE DATA ................................................................................................................................. 9

FIGURE 4: VLE DATA FOR BENZENE AND SOLVENT IN THE SOLVENT EXTRACTION DISTILLATION

COLUMN. THE GRAPH COMPARES VARIOUS THERMODYNAMIC MODELS TO THE

EXPERIMENTAL VLE DATA .................................................................................................... 9

List of Tables

TABLE 1: ASPEN MASS BALANCE FOR THE BENZENE EXTRACTION PROCESS ..................................... 2

TABLE 2: ASPEN STREAM TABLE FOR BENZENE EXTRACTION (CONTINUED) .................................... 3

TABLE 3: HEAT DUTY AND UTILITY FLOW RATE FOR THE HEATER AND REBOILERS IN THE

BENZENE EXTRACTION PROCESS .............................................................................................. 4

TABLE 4: HEAT DUTIES AND UTILITY FLOW RATES FOR THE CONDENSERS AND COOLERS IN THE

BENZENE EXTRACTION PROCESS ........................................................................................... 4

TABLE5: EQUIPMENT LIST FOR THE BENZENE EXTRACTION PDF ..................................................... 5

1 | P a g e

1 PFD, Mass balance, Utility table and Equipment list for the benzene extraction process

100-PP-03

100-CO-02

100-HX-03

100-VE-02

100-PP-04

100-CO-03

100-HX-06

100-VE-03

100-PP-07

100-PP-01

100-CO-01

100-HX-02

100-VE-01

100-PP-02

Naphtha

Reformed Gasoline

100-HX-01

100-PP-05

100-CO-04

100-HX-08

100-VE-04

100-PP-06

100-HX-10

100-HX-11

100-HX-04

100-HX-05

100-VV-01

5

2

10CW

CWCW

CW

CW

CW

CW

MPS

Raffinate

Benzene

Gasoline

Heavy Aromatics

Title: PFD for benzene recovery unit Name: Wesley Neutt ( Nttwes001)Area: 100 Date: 16/03/2013

100-RB-0117

100-RB-02

100-RB-03 117

100-RB-04

CW

Solvent

Make up

18

9

19

20

4

15

100-HX-09

100-HX-07

CW

CW

8

21

22

25

24

Purge 23

6

16

12

1314

31

2 | P a g e

Stream no. 1 2 3 4 5 6 7 8 9 10 11 12

Temperature (0C) 182 39.9 136 116 110 82 82.3 111 204 150 64 159

Pressure ( bar) 8 9 8 2 2 2 5 5 5 0.6 0.6 0.6

Vapor Frac 0.17 0 0 0.17 0 0 0 0 0 0.21 0 0

Mass Flow (kg/hr) 34250 25000 59250 59250 59250 19590 19590 10330 66940 66940 9195 57750

CLYCLOPE 856 0 856 856 856 856 856 856 0 0 0 0

1-PENTEN 1880 0 1880 1880 1880 1880 1880 1880 0 0 0 0

ISOPENTA 411 0 411 411 411 411 411 411 0 0 0 0

HEXANE 1920 2550 4470 4470 4470 4470 4470 4470 0 0 0 0

BENZENE 5000 5880 10880 10880 10880 10350 10350 1102 10780 10780 9195 1584

HEXADIEN 120 0 120 120 120 120 120 120 0 0 0 0

HEPTANE 1580 2630 4200 4200 4200 734 734 734 0 0 0 0

TOLUENE 13530 4050 17580 17580 17580 9.03 9.03 3.14 165 165 0 165

OCTANE 86 1750 1840 1840 1840 0.12 0.12 0.12 0 0 0 0

ETHYLB 6850 0 6850 6850 6850 0 0 0 0.08 0.08 0 0.08

STYRENE 17 0 17 17 17 0 0 0 0 0 0 0

NONANE 651 1550 2200 2200 2200 0 0 0 0 0 0 0

DECANE 940 2330 3270 3270 3270 0 0 0 0 0 0 0

UNDECANE 411 0 411 411 411 0 0 0 0 0 0 0

PENTANE 0 750 750 750 750 750 750 750 0 0 0 0

O-XYLENE 0 2230 2230 2230 2230 0 0 0 0 0 0 0

1ME2BET 0 900 900 900 900 0 0 0 0 0 0 0

N-BUTBEN 0 400 400 400 400 0 0 0 0 0 0 0

SOLVENT 0 0 0 0 0 0 0 0.07 56000 56000 0 56000

Mole Flow ( kmol/hr) 375 261 635 635 635 248 248 130 626 626 118 508

CLYCLOPE 12.2 0 12.2 12.2 12.2 12.2 12.2 12.2 0 0 0 0

1-PENTEN 26.9 0 26.9 26.9 26.9 26.9 26.9 26.9 0 0 0 0

ISOPENTA 5.7 0 5.7 5.7 5.7 5.7 5.7 5.7 0 0 0 0

HEXANE 22.3 29.6 51.8 51.8 51.8 51.8 51.8 51.8 0 0 0 0

BENZENE 64 75.2 139 139 139 133 133 14.1 138 138 118 20.3

HEXADIEN 1.46 0 1.46 1.46 1.46 1.46 1.46 1.46 0 0 0 0

HEPTANE 15.7 26.2 41.9 41.9 41.9 7.32 7.32 7.32 0 0 0 0

TOLUENE 147 44 191 191 191 0.1 0.1 0.03 1.79 1.79 0 1.79

OCTANE 0.75 15.3 16.1 16.1 16.1 0 0 0 0 0 0 0

ETHYLB 64.5 0 64.5 64.5 64.5 0 0 0 0 0 0 0

STYRENE 0.16 0 0.16 0.16 0.16 0 0 0 0 0 0 0

NONANE 5.08 12.1 17.2 17.2 17.2 0 0 0 0 0 0 0

DECANE 6.61 16.3 22.9 22.9 22.9 0 0 0 0 0 0 0

UNDECANE 2.63 0 2.63 2.63 2.63 0 0 0 0 0 0 0

PENTANE 0 10.4 10.4 10.4 10.4 10.4 10.4 10.4 0 0 0 0

O-XYLENE 0 21 21 21 21 0 0 0 0 0 0 0

1ME2BET 0 7.49 7.49 7.49 7.49 0 0 0 0 0 0 0

N-BUTBEN 0 2.98 2.98 2.98 2.98 0 0 0 0 0 0 0

SOLVENT 0 0 0 0 0 0 0 0 486 486 0 486

Aspen-Stream table for benzene extraction

Table 1: Aspen mass balance for the benzene extraction process

3 | P a g e

Stream no. 13 14 15 16 17 18 19 20 21 22 23 24 25

Temperature (0C) 159 170 170 170 142 143 148 209 45 40 159 40 40

Pressure bar 0.6 5 5 5 2 3 2.5 2.5 2 2 0.6 10.6 11.6

Vapor Frac 0 0 0 0 0 0 0 0 0 0 0 0 0

Mass Flow kg/hr 1250 1240 1270 1240 697 697 700 593 653 857 1250 808 744

1-PENTEN 0 0 0 0 0 0 0 0 856 0 0 0 0

ISOPENTA 0 0 0 0 0 0 0 0 1880 0 0 0 0

HEXANE 0 0 0 0 0 0 0 0 411 0 0 0 0

BENZENE 0 0 0 0 0.18 0.18 0.18 0 4470 0 0 0.18 0

HEXADIEN 1530 1530 0 1530 522 522 522 0 1100 9200 56.6 522 0

HEPTANE 0 0 0 0 0 0 0 0 120 0 0 0 0

TOLUENE 0 0 0 0 3470 3470 3470 0 734 0 0 3470 0

OCTANE 159 159 0 159 17570 17570 17570 0 3.14 0 5.9 17570 0

ETHYLB 0 0 0 0 1840 1840 1840 0 0.12 0 0 1840 0

STYRENE 0.08 0.08 0 0.08 6850 6850 6850 0 0 0 0 6850 0

NONANE 0 0 0 0 17 17 17 0.01 0 0 0 17 0.01

DECANE 0 0 0 0 2200 2200 1860 339 0 0 0 1860 339

UNDECANE 0 0 0 0 3270 3270 0 3270 0 0 0 0 3270

PENTANE 0 0 0 0 411 411 0 411 0 0 0 0 411

O-XYLENE 0 0 0 0 0 0 0 0 750 0 0 0 0

1ME2BET 0 0 0 0 2230 2230 2220 0.41 0 0 0 2220 0.41

N-BUTBEN 0 0 0 0 900 900 0 900 0 0 0 0 900

SOLVENT 0 0 0 0 400 400 0 400 0 0 0 0 400

Mole Flow kmol/hr 54000 54000 2000 56000 0 0 0 0 0.07 0 2000 0 0

CLYCLOPE 490 490 17.4 508 387 387 348 38.7 130 118 18.2 348 38.7

1-PENTEN 0 0 0 0 0 0 0 0 12.2 0 0 0 0

ISOPENTA 0 0 0 0 0 0 0 0 26.9 0 0 0 0

HEXANE 0 0 0 0 0 0 0 0 5.7 0 0 0 0

BENZENE 0 0 0 0 0 0 0 0 51.8 0 0 0 0

HEXADIEN 19.6 19.6 0 19.5 6.69 6.69 6.69 0 14.1 118 0.72 6.69 0

HEPTANE 0 0 0 0 0 0 0 0 1.46 0 0 0 0

TOLUENE 0 0 0 0 34.6 34.6 34.6 0 7.32 0 0 34.6 0

OCTANE 1.73 1.73 0 1.73 191 191 191 0 0.03 0 0.06 191 0

ETHYLB 0 0 0 0 16.1 16.1 16.1 0 0 0 0 16.1 0

STYRENE 0 0 0 0 64.5 64.5 64.5 0 0 0 0 64.5 0

NONANE 0 0 0 0 0.16 0.16 0.16 0 0 0 0 0.16 0

DECANE 0 0 0 0 17.2 17.2 14.5 2.64 0 0 0 14.5 2.64

UNDECANE 0 0 0 0 22.9 22.9 0 22.9 0 0 0 0 22.9

PENTANE 0 0 0 0 2.63 2.63 0 2.63 0 0 0 0 2.63

O-XYLENE 0 0 0 0 0 0 0 0 10.4 0 0 0 0

1ME2BET 0 0 0 0 21 21 21 0 0 0 0 21 0

N-BUTBEN 0 0 0 0 7.49 7.49 0 7.49 0 0 0 0 7.49

SOLVENT 0 0 0 0 2.98 2.98 0 2.98 0 0 0 0 2.98

Aspen stream table for benzene extraction

Table 2: Aspen stream table for benzene extraction (continued)

4 | P a g e

Table 3: Heat duty and utility flow rate for the heater and reboilers in the benzene extraction process

Duty (kW) Utility Utility flow

(kg/hr)

Heaters

100-HX-05 518 MPS 932

Reboilers

100-RB-01 9590 MPS 17300

100-RB-02 5460 MPS 9830

100-RB-03 5000 MPS 9000

100-RB-04 14430 MPS 26000

Total heat duty 35000 Total MPS Flow 53200

Table 4: Heat duties and utility flow rates for the condensers and coolers in the benzene extraction process

Duty (kW) Utility Utility flow

(kg/hr)

Coolers

100-HX-01 -1040 CW 597

100-HX-04 -1320 CW 758

100-HX-07 -470 CW 270

100-HX-09 -110 CW 63.2

100-HX-10 -2180 CW 1250

100-HX-11 -625 CW 359

Condensers

100-HX-02 -9100 CW 5220

100-HX-03 -3220 CW 1850

100-HX-06 -14000 CW 8040

100-HX-08 -6220 CW 3570

Total cooling duty -38000

Total CW Flow 20100

5 | P a g e

Table5: Equipment list for the benzene extraction PDF

Equipment code Equipment details Equipment description

100-CO-01 Pre-distillation column 25-stages, vertical,2bar,82-142 oC,SS316

100-CO-02 Extractive distillation column 50-stages, vertical,5bar,110-200 oC,SS316

100-CO-03 Heavy-distillation column 30-stages, vertical,0.6bar,64-160 oC,SS316

100-CO-04 Solvent extraction distillation column 83-stages, vertical,2.5bar,150-200 oC,SS316

100-HX-01 Feed cooler Counter current, 109 oC, 2 bar, SS316

100-HX-02 Pre-distillation condenser Counter current, 82 oC, 2 bar, SS316

100-HX-03 Extractive distillation condenser Counter current, 110 oC, 5 bar, SS316

100-HX-04 Solvent extraction feed cooler Counter current, 150 oC, 0.6 bar, SS316

100-HX-05 Solvent recycle heater Counter current, 170 oC, 5 bar, SS316

100-HX-06 Solvent recycle cooler Counter current, 150 oC, 2.5 bar, SS316

100-HX-07 Raffinate cooler Counter current, 45 oC, 2 bar, SS316

100-HX-08 solvent extraction condenser Counter current, 64 oC, 0.6 bar, SS316

100-HX-09 Benzene product cooler Counter current, 45 oC, 11.6 bar, SS316

100-HX-10 Gasoline cooler Counter current, 40 oC, 5 bar, SS316

100-HX-11 Heavy aromatics cooler Counter current, 40 oC, 11.6 bar, SS316

100-PP-01 Pre-distillation reflux pump Centrifugal pump,2-3 bar, 82 oC, 4.17 KW, high strength alloy

100-PP-02 Pre-distillation bottoms pump Centrifugal pump,2-2.5 bar, 142 oC, 2.45 KW, high strength alloy

100-PP-03 Extractive distillation reflux pump Centrifugal pump,5-2 bar, 110 oC, high strength alloy

100-PP-04 Heavy distillation reflux pump Centrifugal pump,5 bar, 150 oC, high strength alloy

100-PP-05 Solvent extraction reflux pump Centrifugal pump,2 bar, 64 oC, high strength alloy

100-PP-06 Solvent recycle and purge pump Centrifugal pump,0.6-5 bar, 160 oC, 9.04 KW, high strength alloy

100-PP-07 Heavy aromatics pump Centrifugal pump,2.5-11 bar, 40 oC, high strength alloy

100-RB-01 Pre-distillation reboiler Kettle reboiler , 142 oC, 2 bar, SS316

100-RB-02 Extractive distillation reboiler Kettle reboiler , 200 oC, 5 bar, SS316

100-RB-03 Solvent extraction reboiler Kettle reboiler , 160 oC, 0.6 bar, SS316

100-RB-04 heavy distillation reboiler Kettle reboiler , 200 oC, 2.5 bar, SS316

6 | P a g e

2 Discussion of the thermodynamic models available in Aspen Plus

Thermodynamic models are a critical part of process simulations. The main two main categories

for the thermodynamic models are the equation of state models and the activity coefficient

models.

2.1 EOS models

There are five main equation of state models namely, The Ideal gas model, Hougen-Watson,

Lee-Kesler, Peng Robinson (PR) and Soave Redlich Kwong (SRK). All the models are

used for predicting the pressure, volume and temperature (P-V-T properties), behaviour of a

fluid.

The Ideal gas law is the simplest and least accurate model of all. The Ideal gas law makes use of

the compressibility factor to predict the P-V-T properties of a real gas. This compressibility

factor is different for each fluid. The more general EOS models are the Hougen-Watson and the

Lee-Kesler models. These models assume that the compressibility factor is a function of the

reduced temperature and pressure and a third factor. The Hougen-Watson model, assume that

the third parameter is the critical compressibility parameter while the Lee-Kesler model assumes

that the third parameter is the acentric factor which is a material property.

The PR and SRK models are known as the cubic equation of states. The models predict three

real values of the volume in the vapour-liquid region. Several modifications to extend the ranges

of the models are available. The PR and SRK models are mainly recommended for predicting

hydrocarbon and non-ideal systems accurately.

2.2 Activity Coefficient models

Activity coefficient models are used to predict thermodynamic data for highly non-ideal

solutions. For multi-component mixtures we use the Wilson, NRTL and UNIFAC activity

coefficient models.

The Wilson and NRTL models are known as the local compensation models. The local

compensation models assume that the ratio of the species interactions is the same as the mole

fractions. The Wilson model uses two parameters ij and ji for each binary pair and the NRTL

model makes use of three parameters , 12 and 21. These parameters account for the different

weighting of the mole fractions to account for the differences in size and interaction energies.

The UNIFAC model uses the idea of group contributions. This idea assumes that the volume

and surface area of a molecule can be accounted for by the different functional groups within the

model (Mller 2011).

2.3 Henrys Coefficients

The Henrys law constant accounts for the fugacity of a dilute gaseous species in a liquid. The

constant depends on the solute-solvent pair, temperature and pressure. The equation fails at

higher mole fractions of the mixture. Therefore, the henrys law constants are a hypothetical

fugacity of a solute species as a pure liquid extrapolated from its infinite dilution behaviour

(Sandler 2006).

7 | P a g e

3 Justification of the thermodynamic property methods

From the available EOS and activity coefficient thermodynamic models available in Aspen Plus

the following models where considered.

For the EOS models the Peng-Robinson (PR), and Soave-Redlich-Kwong (SRK), equations

where considered. The PR and SRK models are superior to other EOS models for the following

reasons.

The models are more accurate in predicting VLE data for multicomponent mixtures.

The models are widely accepted for predicting hydrocarbon mixture VLE data.

The system under consideration deals with hydrocarbon multicomponent mixtures. Therefore,

PR and SRK where the only two EOS models considered for the system. (Ibrahim Ashour,

Nabeel Al-Rawahi, Amin Fatemi and Gholamreza Vakili-Nezhaad 2011)

From the available activity coefficient models the UNIFAC and NRTL activity coefficient

models where considered. The UNIFAC and NRTL models are superior to the other activity

coefficient models for the following reasons.

The models are more accurate in predicting VLE data for non-ideal, non-polar or polar

mixtures such as hydrocarbon mixture.

Since the system under consideration deals with hydrocarbon mixtures the UNIFAC and NRTL

models where considered for this system. (Sandler 2006)

Thermodynamic data for binary interactions between key components have been collected and

plotted for each column. The data shows plots of the experimental VLE data and the VLE data

predicted using the models above.

Figure 1 shows the data for the pre-distillation column. In this column the key components are

benzene and heptane. From the graph it is seen that all the models do not provide accurate

predictions of the experimental data over the whole mole fraction range. The PR model predicts

the data the most accurately but fails at the infinite dilution points. However, the other models

do predict the infinite dilution data but do not predict for the data well for the middle ranges.

Figure 2 shows the data for the heavy column. In this column the key components are o-xylene

and nonane. From the graph it is seen that all the models do not provide accurate predictions of

the experimental data over the whole mole fraction range.

Figure 3 shows the data for the extractive distillation column. In this column the key

components are benzene and hexane. From the graph it is seen that all the models, except the

SRK model predict the experimental data accurately for the whole mole fraction range. The SRK

model fails to predict the experimental data at the infinite dilution points.

Figure 4 shows the data for the solvent extraction model. In this column the key components are

benzene and 4-formylmorpholine. Form the graph it is seen that all the models predict the data

accurately.

8 | P a g e

Figure 1: VLE data for benzene and heptane in the pre- distillation column. The graph compares various thermodynamic models to the experimental VLE data

Figure 2: VLE data for o-xylene and nonane in the heavy column. The graph compares various thermodynamic models to the experimental VLE data

310

312

314

316

318

320

322

324

326

328

330

332

0 0.2 0.4 0.6 0.8 1

Tem

pe

ratu

re (

K)

Vapour and Liquid mole fractions

Experimental(L) Experimental(V) NRTL

PR UNIFAC SRK

416

417

418

419

420

421

422

423

424

425

0 0.2 0.4 0.6 0.8 1

Tem

pe

ratu

re (

K)

Liquid and vapour mole fractions

Experimental(V) Experimental(L) Unifac

SRK PR NRLT

9 | P a g e

Figure 3: VLE data for benzene and hexane in the extractive distillation column. The graph compares various thermodynamic models to the experimental VLE data

Figure 4: VLE data for benzene and solvent in the solvent extraction distillation column. The graph compares various thermodynamic models to the experimental VLE data

320

322

324

326

328

330

332

334

336

0 0.2 0.4 0.6 0.8 1

Tem

pe

ratu

r (K

)

Vapour and Liquid mole fraction

Experimental(V) Experimental Unifac

SRK PR NRTL

300

350

400

450

500

550

0 0.2 0.4 0.6 0.8 1

Tem

pe

ratu

r(K

)

Vapor and Liquid mole fraction

Experimental (V) Experimental (L) UNIFAC

SRK PR NRLT(V)

10 | P a g e

From the observations it is seen that different thermodynamic models work for different

systems. The system under investigation deals with very complex liquid mixtures. The extractive

distillation is used in this system to deal with benzene extraction from mixtures which form

azeotrops with benzene (Julka, Chiplunkar & O'Young 2009). From figures 1, 2 and 3 it is seen

that the key components form azeotrops at the infinite dilution points.

From the data it can be seen that the UNIFAC model predicts the VLE data accurately for all

the columns except the heavy column. The UNIFAC model is recommended in literature as the

model of choice for prediction non-ideal aziotropic mixtures. Therefore, the UNIFAC model

was used in the Aspen simulation (Pienaar 2012).

11 | P a g e

4 Discussion of the Aspen model

4.1 Description of Aspen models used to simulate the important units

The Aspen simulation for benzene extraction made use of the following Aspen models from the

Aspen model library.

Stream mixers

F-splitter (stream splitter)

Thermal and phase state changer (heaters and coolers)

DSTWU columns

RADFRAC columns

Pump

Valve (valve 2)

4.1.1 Stream mixers

The stream mixer was used to mix the C5+ and Naptha feed streams at different pressures and

temperatures and the solvent recycle and make up streams. The mixer mixes the streams

adiabatically and isobarically.

4.1.2 F-splitter

The F-splitter was used to split the bottoms of the solvent extraction column. The F-splitter is

stream splitter based on the mass/mol flow of the stream. The splitter operates adiabatically and

isobarically.

4.1.3 Thermal and phase state changer (heaters and coolers)

The heaters and coolers are used to change the temperature of the streams. The heaters and

coolers operate isothermally and adiabatically.

4.1.4 DSTWU columns

The DSTWU columns was used for a first estimation of the number of plates, feed stage, min

reflux ratio and the actual reflux ratio for each of the columns at the specified split fractions for

the key components using the Winn-Underwood-Gilliland method

4.1.5 RADFRAC columns

The RADFRAC column was used to simulate the actual columns in the final simulation. The

column provides a more rigorous approach to solving the columns. The column can be used to

simulate simple and more complex distillation with multiple feeds and side streams. The column

assumes that equilibriums is achieved on each stage but can be used to model more complex

systems.

4.1.6 Pump

The pumps are used to increase the pressure of liquid streams between units. In the simulation

the pumps efficiency was set to 100 %. This means that the pump would operate isothermally.

4.1.7 Valve (valve 2)

The valve was used to decrease the pressure of the feed streams. The valves operate

isothermally.

12 | P a g e

4.2 Method used to solve the simulation

The system was solved in three sections. Section one was solved first using an iterative approach.

Once section one was completely solved section two was added onto section one and solved

iteratively while keeping all the specifications of section one constant. Section three was then

added onto section one and two and solved iteratively while keeping the specifications in section

one and two constant.

4.3 Solution method for section one

Section one consists of following units:

Heater

Pressure relief valve

Pre-distillation column.

In the pre-distillation column the aim was to separate benzene and heptane. The column

achieved 95% split of benzene to the distillate and an 82% split of heptane to the bottoms. The

DSTWU column was used to find a first estimation of the design variables for the pre-distillation

column. From the DSTWU column the first estimation for the reflux ratio, distillate to feed

ratio, number of theoretical stages and the feed stage was used in the RADFRAC column. The

column design variables, including the feed temperature and column pressure, were varied

independently while the other design variables remained constant until the desired specifications

were achieved.

The pre-distillation column achieved the split ratios at a pressure of 2 bar using 25 stages. This

column is relatively small column. However, the heat duties on the reboiler and condenser are

9500 kW and -9100 kW respectively. Therefore, although the column is small and operates at a

low pressure the duties on the column is large.

4.4 Solution method for section two

Section two consisted of the following units:

Heavy column

Feed pump.

In the heavies column the aim was to separate o-xylene and nonane while recovering 99 wt% of

all C8 aromatics to the aromatic gasoline product stream. The DSTWU column was used to find

a first estimation of the design variables for the heavy column. The first estimations for the

reflux ratio, distillate to feed ratio, number of theoretical stages and the feed stage were used in

the RADFRAC column. The column design variables, including the feed temperature and

column pressure, were varied independently while the other design variables remained constant

until the desired specifications were achieved.

The column achieved the specification on the gasoline product stream at a pressure of 2.5 bar

using 83 stages. To achieve the specification on the gasoline product stream, 85% of the nonane

had to be recovered in the distillate. This column is a relatively large column and has high heat

duties on the reboiler and condenser of 14400 kW and -14000 kW respectively.

13 | P a g e

4.5 Solution method for section three

Section three consisted of the following units:

Extractive distillation column

Solvent extraction column with the recycle and purge.

In this system the aim was to remove the benzene form the feed to the extractive distillation

column while achieving a benzene product stream containing 99.9 wt%, benzene and a rafanate

stream lean in aromatics. The DSTWU column was used to find a first estimation of the design

variables for the columns. The first estimations for the reflux ratio, reboiler duty, number of

theoretical stages and the feed stage were used in the RADFRAC columns.

The deign specification feature in Aspen was used to solve for the solvent recycle rate to the

extractive distillation column by varying the split ratio to the purge stream. This design

specification was done to minimise the solvent and benzene purge rate. A second design

specification was performed to meet the benzene purity specification in the benzene product

stream by varying the duty in the extractive distillation column.

4.6 Assumptions and Improvements of the simulation

The specifications on all the product streams were met with the simulation. It was assumed that

the columns operated isobarically. This was done to simplify the simulation. However, a more

rigorous simulation could be done which optimises the duties of the columns by varying the

column pressure, feed stage temperatures and the column size. The sequencing of the column

could have also been arranged differently and more recycles with integrated heating systems

could have decreased the overall heat duty of the system.

14 | P a g e

5 List of References

1. Ibrahim Ashour, Nabeel Al-Rawahi, Amin Fatemi and Gholamreza Vakili-Nezhaad

2011, 'Applications of Equations of State', Department of Petroleum and Chemical

Engineering, University of Kashan, In Tech.

2. Julka, V, Chiplunkar, M & O'Young, L 2009, 'Selecting Entrainers for Azeotropic

Distillation', Chemical Engineering , Univ. of Massachusetts, ClearWaterBay Tec.,

Manchester.

3. Mller, K 2011, Chemical Engineering Thermodynamics 2, Capetown, Western Cape , South

Afica , viewed Monday March 2013,

.

4. Pienaar, C 2012, 'Evalluattiion off Enttraiiners ffor tthe', Masters, CHEMICAL

ENGINEERING, Stellenbosch University, Stellenbosch University, Stellenbosch.

5. Sandler, SI 2006, Chemical, Biochemical, and Engineering Thermodynamics, Fourth Edition edn,

John Wiley & Sons, Inc., United States of America.

15 | P a g e

Faculty of Engineering and the Built Environment

Department of Chemical Engineering

Chemical Engineering

Process design CHE4049F

Plagiarism Declaration

Name

Wesley Neutt

Student No. Nttwes001 Project no. Project 2

Date Completed

16/03/13

Date Handed-In 16/03/13

DECLARATION

1. I know that plagiarism is wrong. Plagiarism is to use anothers work and to pretend that it is ones own.

2. I have used the prescribed referencing system for citation and referencing. Each significant contribution to, and quotation in, this report from the work, or works, of other people has been attributed, and has been cited and referenced.

3. This report is my own work. 4. I have not allowed, and will not allow, anyone to copy my work with the

intention of passing it off as his or her own work.

Signature