Process Simulation (Chem Eng)

Technical tasks and report preparation:

Part A: Simulation Principle and VLE

Part B: Base case simulation

Part C: Extension study

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.

Essa Alshayji

Essa Alshayji

Essa Alshayji

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).

Essa Alshayji

Essa Alshayji

Essa Alshayji

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 OR G_Your Group Number x [Example: G_100 x]

– 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.

Essa Alshayji

Essa Alshayji

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-1

00

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.

Essa Alshayji

Essa Alshayji

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

Essa Alshayji

Essa Alshayji

Process Simulation (CE2105) Aston University

6
Dr Amir Amiri

Figure 1: Process Flowsheet

TN
K

-1
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TO
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1

P
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S
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S
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S
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S
7

E
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hp
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fg

cw

V
-1

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S
9

S
11

S
10

V
LV

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V
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01

S
14

S
12

E
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03

S
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lp
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S
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LV

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S
26

S
21

TE
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V
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S
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K
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S

20

TE
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S
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S
22

S
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M
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T-
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Essa Alshayji

Essa Alshayji

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”.

Essa Alshayji

Essa Alshayji

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.

Essa Alshayji

Essa Alshayji