Process Simulation (Chem Eng)

Process Simulation (CE2105) Aston University

1 Dr Amir Amiri

Coursework --------------------------------------------------------------------------------------------------------------

Important Notes: 1. The designated coursework enable you to demonstrate your skills in practical

utilisation of the commercial process simulators for process computations. Moreover, you show your competence in analysing the results.

2. This is a group work in which all members MUST evenly contribute. Peer assessment will be done to evaluate individual members’ contributions.

3. As the class test will be an individual assessment with similar elements to this work, your attempts for this commitment certainly equip you with the necessary skills to properly accomplish that part of the module’s assessment too.

4. The given problem is same for all groups. The contents that can make your work more distinguishable are, but not limited to, a good literature review, rigorous results, high quality interpretation, well managed and articulated report, etc. These are highly recommended as definitely make your work outstanding.

5. Critical thinking and interpretation of the results is required and highly acknowledged. The more professional/technical interpretation, the higher value. This is an open task that you can put in creativity and analysis skills. Few examples, but not all, can be commenting on: How well the process is simulated and if you see any problem how you can resolve/improve it? What are the assumptions used for simulation simplicity that might be risky for final results’ accuracy, why? Can these assumptions be avoided? If so how? How the simulation results improves your understandings about this case study? How can you use them to suggest process improvement strategies? Support your answers with examples and results.

6. You should submit your simulation and report files. Maximum page limits are given for some sections of the report, and are indicated with square brackets.

7. You are welcome to ask your questions by contacting Lecture/tutors. The response(s) to your question might be posted on the website (BB) to be accessible by all students.

8. Further guidance will be given in lecture/tutorial times or via the website updates.

Technical tasks and report preparation

Part A: Simulation Principle and VLE [Repot: 4 pages, Marks: 20]

(a) Which Fluid Package/Property Method can be suitable for this simulation? Justify your answer through Vapour Liquid Equilibrium (VLE) evaluation. Note: In order to make decision on which Fluid Package/Properly Method is suitable for this project, you may compare VLE data (such as xy, Txy and Pxy equilibrium data) attained using 3 to 4 Fluid Package/Property Method and judge which one(s) are more reliable. Moreover, you may compare the theoretical xy data (achieved by using Fluid Package/Property Method) with practical data for the same species and under same conditions (T, P). For practical data you may refer to the literature or search in Aspen data base for equilibrium data (i.e., NIST).

(b) Separation of the final products and other species is necessary. Conduct a VLE analysis and discuss if distillation process can be used for separation and if any processing difficulty, such as azeotrope formation, may occur.

Process Simulation (CE2105) Aston University

2 Dr Amir Amiri

Part B: Base case simulation [Repot: 8 pages; Marks: 45]

Develop an Aspen Plus simulation of the process as given in the Process description with the details given. Please use the same stream names as given in Figure 1. For your report, please provide the following titles:

(c) Simulation file with proper units/modules and without error/warning (14 marks) (d) Aspen Plus PFD printout: a neat arrangement of the flowsheet (2 marks) (e) Input Summary (1 marks). (f) Stream tables: Showing material stream, energy stream and composition

information; must be easy to read. (1 marks) (g) Brief simulation notes on:

1. Three problems encountered and how you solved those (3 marks). 2. Three modelling decisions you had to make (3 marks). 3. Three independent checks you performed to give you confidence that the

simulation results are correct, with evidence (6 marks). 4. Two technical discussion you would like to make about how the simulation

results help you to understand and interpret this process (10 marks). 5. A discussion of how your simulation might differ from reality and the top three

things you would do to improve the fidelity of the work (5 marks).

Part C: Extension study [Repot: 8 pages; Marks: 25]

Decide on ONE topic related to your simulation to investigate further and perform a detailed study of it. Some possible topics include:

• More detailed reactor modelling, including kinetics for the main reaction and accounting for the side reaction that forms biphenyl, with case studies or optimisation of the reactor size or operating conditions.

• Energy integration around the reactor and/or other parts of the process. • Study into the effect of the choice of property package on the simulation, for the

whole process and for selected individual units, including a comparison with any available data.

However, you are strongly encouraged to think of your own topics or interesting variations on the above. You need literature review and further reading for this. If you would like to do something different, please consult your lecturer about its suitability beforehand.

(h) State what you are going to study and why. (4 marks). (i) Clearly outline your assumptions and methodology. (4 marks). (j) Present evidence of your work: modified PFD(s), Input Summary file(s) and

stream table(s); manual calculations; and similar as needed. (5 marks). (k) Present and discuss the results, including comments on their implications for the

process. Please draw on your knowledge gained in other units to help answer this question. (8 marks).

(l) Discuss the top two things you would do to improve the realism of your extension study. (4 marks).

Process Simulation (CE2105) Aston University

3 Dr Amir Amiri

Report Quality [Marks: 10]

Report structure and quality must be professional, written in technical and correct language. Please use a standard report format, proper fonts and titles/subtitles with an Executive Summary [0.5 pages] and a Conclusions and Recommendations section [1 page]. Equations, tables, graphs and pictures quality and consistency are important. Use Part A to Part C as title of each section, started from a new page, and each item (a - l OR 1 - 5) as subtitles.

All files generated, including Aspen Hysys/Aspen Plus simulations, spreadsheets and the final report document itself, should be submiited electronically. In the report, please very briefly describe the contents of each simulation and spreadsheet file.

Advice

Save your Aspen Hysys/Aspen Plus work often, and give the file a different (version) name when you complete a major step in the flowsheet. It is a good idea to save it just before linking up a recycle stream. All of the skills you need to complete this report have been covered in the tutorials and lectures.

Submission

One electronic submission for each group with proper file names

- Please use one of these formats for the report’s file name:

G_ Your Group Number.doc OR G_Your Group Number.docx [Example: G_100.docx]

- Please use one of these formats for the simulation’s file name:

If you use Aspen Plus: G_ Your Group Number.apw [Example: G_100.apw] If you use Aspen Hysys: G_ Your Group Number.hsc [example: G_100.hsc]

PROCESS DESCRIPTION STARTS FROM THE NEXT PAGE.

Process Simulation (CE2105) Aston University

4 Dr Amir Amiri

Process description

The preliminary process flow diagram (PFD) shown in Figure 1 represents a plant for the production of benzene (C6H6) from toluene (C7H8) by an exothermic reaction with hydrogen in the presence of a solid catalyst:

C7H8(g) + H2(g) Æ C6H6(g) + CH4(g) (1)

The proposed production rate is 65,000 t/y of 99.5 mol% pure benzene, based on 7920 hours of plant operation per year.

Fresh and recycled liquid toluene is pumped from tank TNK-100 and is combined with a high pressure hydrogen feed and a recycled gas stream in unit MIX-100. The combined feed stream S4 is vaporised using high pressure steam in exchanger E-100 and then heated further to 600°C in fired heater E-101 prior to being fed into reactor R-100. The feed enters the reactor at a pressure of 2500 kPa. The reactors is of the catalytic packed bed type, is operated adiabatically and is intended to achieve 75% conversion of toluene. The feed contains a large excess of hydrogen, which acts as a diluent to moderate the temperature rise in the reactor. A small flow of cold gas, stream S24, is used for further reactor temperature control. The reactor effluent is cooled and partly condensed in exchanger E-102 using cooling water and partial separation of this stream is achieved in high pressure (V-100) and low pressure (V- 101) flash drums. Part of the overhead vapour from V-100 is recycled via compressor K-100 to the reaction section. The liquid product from the low pressure flash drum is heated to near its bubble point in E-103 using low pressure steam and is then distilled in the benzene column.

The column produces a high purity benzene distillate, S18, an impure toluene bottoms stream, S19, which is recycled to the toluene storage tank, and a small non-condensable gas stream, S16, vented from the column’s reflux drum. This vent stream, the balance of vapour from V- 100 and all the vapour from V-101 are combined in MIX-101 and become fuel gas for use in the plant or elsewhere on the site. The benzene product from the column is cooled in exchanger E-104 using cooling water prior to being pumped to storage.

In the Aspen Hysys simulation, it is suggested that the benzene column be modelled by a combination of two units: X-100, a component splitter, and T-100, a shortcut distillation column. This is because the Aspen Hysys shortcut column model cannot produce both vapour and liquid overhead products. Hence a simple component splitter is used to remove all the H2 and CH4 in the feed S15 prior to its entry to the shortcut column. Please note that unit X-100 does not exist in reality – stream S16 should actually be produced from the top of column T- 100 along with the liquid distillate S18.

Appendix 2: Reaction information

The main reaction taking place in the packed bed catalytic reactor is:

C7H8(g) + H2(g) Æ C6H6(g) + CH4(g) (1)

For the particular catalyst in the reactor, the rate of reaction of toluene in kgmole/(m3.s) is given by

r1 = k1 . exp(–E1/(RT)) . CC7H8 . CH20.5 (2)

where k1 = 2.29×1011 (kgmole/m3)–0.5.s–1, E1 = 2.13×105 kJ/kgmole, and C is molar concentration in kgmole/m3. Note that this rate equation applies in the vapour phase only and is valid in the range 500–900°C.

Process Simulation (CE2105) Aston University

5 Dr Amir Amiri

An unwanted, reversible reaction also takes place in which the benzene product reacts further to form biphenyl (C12H10):

C6H6(g) ' ½C12H10(g) + ½H2(g) (3)

The rate of the benzene consumption via reaction (3) in kgmole/(m3.s) is given by

r2 = k2 . exp(–E2/(RT)) . CC6H62 – k3 . exp(–E3/(RT)) . CC12H10 . CH2 (4)

where k2 = 3.8×1014 (kgmole/m3)–1.s–1, E2 = 2.68×105 kJ/kgmole, k3 = 2.2×1015 (kgmole/m3)– 1.s–1 and E3 = 2.68×105 kJ/kgmole (also). This information also applies in the vapour phase for 500–900°C.

To get started in your study, consider the following reactor set-up:

• Reactor orientation: Vertical, with gas downflow • External heat transfer: None (adiabatic) • Reactor diameter: 2.2 m • Reactor height: 10 m • Pressure drop: 100 kPa

You can consider modifications to the reactor, e.g. changes to length and diameter, two packed bed stages with inter-stage heat transfer, addition of stream S24 at some position along the length of the reactor rather than with the main feed stream, different operating conditions within reason (inlet temperature, pressure), …

You may wish to explore how the reactor behaves in isolation; that is, for fixed streams S6 and S24 as found in the base case simulation, or you can link the reactor with the rest of the process and investigate the effect that reactor changes make on the whole flowsheet.

Further information

As this is an open-ended and self-selected problem, more information will likely be needed than appears here. You are encouraged to find the information yourself, but if you have trouble please contact your lecturer. However, any extra information provided by the lecturer may be shared with whole class via the unit web site

Process Simulation (CE2105) Aston University

6 Dr Amir Amiri

Figure 1: Process Flowsheet

TN K

-1 00

TO LF

E E

D

S 1

P -1

00

S 2

S 3

S 4

E -1

00S 5

R -1

00

S 6

S 8

S 7

E -1

02

hp s

ai r

E -1

01 fg

cw

V -1

00

S 9

S 11

S 10

V LV

-1 00

V -1

01

S 14

S 12

E -1

03

S 15

lp s

S 13

V LV

-1 01

S 26

S 21

TE E

-1 00

V LV

-1 02

S 25

K -1

00 S

20

TE E

-1 01

S 23

S 22

S 24

M IX

-1 00

H 2F

E E

D

T- 10

0 X

-1 00

S 17

S 19

S 18

E -1

04

B E

N ZE

N E

cw

S 16

FU E

LG A

S

M IX

-1 01

cw m ps

Process Simulation (CE2105) Aston University

7 Dr Amir Amiri

Appendix 1: Equipment and stream information for the base case simulation

Value Comment Property package

SRK SRK = Soave-Redlich-Kwong

Reaction See reaction (1) Toluene conversion: 75%

Enter as a “Conversion” reaction. Exothermic. You will need to use reaction kinetics after you successfully simulated a constant conversion case. This will be more rigorous model by using kinetics information in the simulation.

TOLFEED Temperature: 25°C Pressure: 190 kPa (abs) Toluene: 108.0 kgmol/h

Liquid toluene feed stream. Note boiling point of toluene at 1 atm is 111°C.

H2FEED Temperature: 25°C Pressure: 2550 kPa (abs) Hydrogen: 284.2 kgmol/h Methane: 14.9 kgmol/h

High pressure hydrogen feed stream with approx. 5% methane impurity.

TNK-100 Toluene storage tank. Enter as a “Tank” unit if you use Aspen Hysys or enter a “Separator” ( flash separator) if you use Aspen plus.

S1 Toluene vent stream. Zero flow expected in normal operation.

P-100 Outlet pressure: 2580 kPa (abs) Adiabatic efficiency: 75%

Toluene feed pump. Energy stream “P- 100DUTY”.

E-100 Outlet temperature: 225°C Pressure drop: 30 kPa

Reactor pre-heater. Vaporises toluene feed. Heated with high pressure steam. Use “Heater” unit. Energy stream “E-100DUTY”.

E-101 Outlet temperature: 600°C Pressure drop: 20 kPa

Reactor furnace. Heated by combustion of fuel gas and air, producing flue gases. Use “Heater” unit. Energy stream “E-101DUTY”.

R-100 Pressure drop: 100 kPa Benzene reactor. Vertical catalytic packed bed gas-phase reactor operating adiabatically. Note large excess of hydrogen supplied to reactor, far above stoichiometric requirement. You may use the “Conversion Reactor” model for simplicity. But you can use reaction kinetics data given in the appendix for a more rigorous simulation.

S24 This stream (recycled H2 / CH4, approx. 45°C, small flow) assists with reactor temperature control using a “cold shot” strategy.

S8 Fictitious reactor liquid product stream. Should always be zero flow.

E-102 Outlet temperature: 38°C Pressure drop: 10 kPa

Reactor effluent cooler. Cools the reactor product, most of the benzene and toluene condenses out. Cooled using cooling water. Use “Cooler” unit. Energy stream “E-102DUTY”.

V-100 High pressure flash vessel. Vertical vessel. Adiabatic, negligible pressure drop.

TEE-100 S20 flow ratio: 73% Gas recycle tee. Remaining flow (27%) goes to fuel gas line.

VLV-100 Outlet pressure: 290 kPa (abs) HP flash level control valve. Use “Valve” unit. V-101 Low pressure flash vessel. Vertical vessel.

Adiabatic, negligible pressure drop. E-103 Outlet temperature: 90°C

Pressure drop: 30 kPa Column pre-heater. Heats up the mostly benzene / toluene mixture to near its bubble point. Heated using low pressure steam. Use “Heater” unit. Energy stream “E-103DUTY”.

Process Simulation (CE2105) Aston University

8 Dr Amir Amiri

Value Comment VLV-101 Outlet pressure: 260 kPa (abs) LP flash pressure control valve. Use “Valve”

unit. X-100 All H2 and CH4 to S16

All C6H6 and C7H8 to S17 Use stream flash specifications. Use lowest feed pressure option.

Fictitious unit. Needed because shortcut column model used for benzene column (T-100) cannot handle a partial condenser with liquid distillate. Small flow (approx. 0.6% of feed) of light gases are removed prior to the shortcut column. Use “Component splitter” unit.

S16 Temperature: 113°C This light gas stream should be vented from the reflux drum of the benzene column. The temperature needs to be specified to assist in the X-100 flash calculations. It should be set the same as the distillate from the benzene column, but setting it manually is ok initially.

T-100 Top product phase: Liquid Light key (benzene) in bottoms:

3 mol% Heavy key (toluene) in distillate:

0.5 mol% Condenser pressure: 250 kPa (abs) Reboiler pressure: 280 kPa (abs) Use reflux ratio of 1.3 × minimum reflux ratio

Benzene column. Sieve tray distillation column. Tray efficiency about 60%. Produces 99.5 mol% pure benzene product as liquid distillate. Bottoms is essentially toluene to be recycled. As noted in X-100, the shortcut column model cannot handle a partial condenser with both liquid and vapour distillates. Use “Shortcut column” unit. Energy streams “CONDUTY” for condenser, “REBDUTY” for reboiler. If you are using Aspen Plus, try a rigours column template.

E-104 Outlet temperature: 38°C Pressure drop: 20 kPa

Benzene cooler. Cools product prior to storage. Uses cooling water. Use “Cooler” unit. Energy stream “E-104DUTY”.

K-100 Outlet pressure: 2550 kPa (abs) Adiabatic efficiency: 75%

Recycle gas compressor. Returns H2 and CH4 rich gas back to reaction section of the plant. Energy stream “K-100DUTY”.

TEE-101 S24 flow ratio: 5% Reactor temperature control flow splitter. See S24 in this table. Remaining flow (95%) gets mixed with main reactor feed and undergoes preheating.

VLV-102 Outlet pressure: 260 kPa (abs) HP flash pressure control valve. Use “Valve” unit.

BENZENE Main product stream ready to be sent to storage.

FUELGAS Fuel gas by-product composed mostly of hydrogen and methane. May be burnt to provide energy, or possibly reprocessed to recover H2 to recycle to process.