Control and Simulation - Chemical Engineering

ICA - HYSYS

Objective

In this assessment, you will demonstrate your ability to construct and analyse an Aspen HYSYS flowsheet for a specific problem statement. Most of the assessment will be based on concepts you have practised during taught sessions, but you may be required to demonstrate that you can apply your learning in an unfamiliar setting. Use some of the Analysis Tasks to demonstrate your ability to build Case Studies.

Section 1: Simplified Preliminary PFD
This submission should contain a clear, simplified preliminary Process Flow Diagram depicting the process you will be simulating in Aspen HYSYS. It should satisfy the following requirements:

The diagram must be in either MS Word or MS Visio.
Your font type and size should be appropriate and visible.
The diagram should be on a single A4 page with landscape orientation.
You may present the units either as blocks or as process units. All directional arrows must be visible. Include relevant stream details (e.g. flows, conditions).

Section 2: HYSYS Simulation
This should be one HYSYS file containing all your simulation work. It should satisfy the following requirements:

Provide suitable descriptive titles for all material and energy streams.
Position items on the flowsheet in a clear manner.
Close all parameter windows and Case Study tabs prior to submission.
Reset your parameters from values set during Case Studies prior to submission.

Section 3: Analysis Report
This report should contain a comprehensive analysis of the performance of the simulation. It must satisfy the following requirements:

Your report should be in MS Word. You may use Arial, Times New Roman or Calibri font, with font size 12 and 1.15 line spacing. Use ‘Normal' margins (2.54 cm all round). The report should be structured in a professional manner.
Any graphical representations should be presented in an appropriate format (i.e. screengrabs are not acceptable).
Do not include any appendices (if included, they will not contribute to your mark).

Problem Statement
You are to simulate a simplified process to produce acetic anhydride from acetone. The transformation proceeds in two steps via the formation of ketene: CH3COCH3(g)→CH2CO(g)+CH4(g) Rxn 1 CH2CO(l)+CH3COOH(l)→(CH3CO)2O(l) Rxn 2

Rxn 1 can be modelled as a kinetic reaction with the following rate constant (based on the molar concentration in kmol/m3 of acetone in the vapour-phase): k1=8.2×1014exp[-67999calmol⁄RT]s-1

The only information available regarding Rxn 2 is the expected conversion of ketene in the liquid phase, which is 100%.

Area 1: Acetone Cracker
10000 kg/h of pure acetone at 1 atm and 30 °C is mixed with the acetone recycle from Area 4 (see below). This acetone enters an adiabatic plug flow reactor at 1.6 atm and 750 °C, where Rxn 1 occurs with an acetone conversion of 20%. The tube diameter of the reactor is fixed at 1.0 m, whereas the tube length can be varied.

Area 2: Simplified Quench Reactor
The product stream from Area 1 is mixed with the acetic acid recycle from Area 4 (see below) and a make-up stream of fresh acetic acid, available at 1 atm and 30 °C. This stream enters an isothermal reactor operating at 1 atm and 60 °C.

Area 3: Fuel Gas Recovery
The vapour outlet from Area 2 is flashed at 1 atm and 40 °C. The vapour outlet of the flash drum is contacted with fresh acetic acid (available at 1 atm and 30 °C) in a 7-stage absorber operating at 1 atm. The absorber vapour outlet can be used as fuel gas if the acetone content is less than 1% on a molar basis.

Area 4: Recycle
The liquid outlet streams in Area 3 are mixed and passed through a distillation column operating at 1 atm. A mole fraction of 99.9% is required for both the Light Key (the main component leaving at the top of the column) and Heavy Key (the main component leaving at the bottom of the column) in their respective outlet streams. The Light Key exits the column as a vapour.

99% of the overheads from this distillation column are recycled back to Area 1, while 99% of the bottoms are recycled back to Area 2.

Design Tasks
Present the information detailed above in a simplified preliminary Process Flow Diagram (completion requirements above). Include temperature- and pressure-changing equipment where appropriate.

Construct the process flowsheet in HYSYS (completion requirements above), using Wilson as your property package. When constructing the flowsheet, take the following design constraints into consideration:

There is a 25 kPa pressure drop across each heater/cooler.
There is a 50 kPa pressure drop across each reactor.
Given the number of Adjust blocks required, you may not be able to run them all together at first. You may find the following points useful when constructing the flowsheet.

Area 1: Acetone Cracker
Starting with an estimate of 3.0 m, adjust the length of the plug flow reactor so that the actual conversion achieved is 20%. You may wish to do this before continuing to Area 2.

Area 2: Simplified Quench Reactor
The mole ratio of acetic acid to ketene entering the reactor must be at least 7:1. Start by setting the flow rate of the make-up stream to zero - you will need to adjust this value later on to achieve the required ratio.

Area 3: Fuel Gas Recovery
Starting with an estimate of 10000 kg/h, adjust the flow rate of the acetic acid entering the absorber so that the vapour outlet of the column can be used as fuel gas. You have not set up an absorber in HYSYS before, but in principle it is similar to the set-up process for a distillation column.

Area 4: Recycle
Model the distillation column as a short-cut column. The external reflux ratio should be set in the range of 1.1 - 1.5 times the minimum reflux ratio.

Analysis Tasks
You are required to carry out a comprehensive analysis of the performance of your simulation (completion requirements above). You are expected to relate your discussion to the process you have simulated and the theory behind your findings. You should also use this section to demonstrate your ability to use the Case Study feature in HYSYS.

During your analysis, you should justify the design decisions made during the simulation process and answer the questions below (you are not required to use Case Studies for all questions). You are encouraged to investigate aspects of the process not listed here, and marks are available for relevant additional discussion.

Area 1: Acetone Cracker
How does the conversion in the Acetone Cracker vary with reactor length? Explain this trend.
How does the temperature in the Acetone Cracker vary with reactor length? Explain this trend.
Area 2: Simplified Quench Reactor
Does the actual conversion in the Quench Reactor match the expected conversion? Explain the reasons for your answer.
How does the temperature of the Quench Reactor affect the actual conversion of ketene to acetic anhydride?
How does the mole ratio of acetic acid to ketene affect the actual conversion in the Quench Reactor?
Discuss the potential for improving the performance of the Quench Reactor model in your simulation.
Area 3: Fuel Gas Recovery
Are there any contaminants in the fuel gas stream that might hinder its use in other processes?
How would you increase the mole fraction of methane in the fuel gas stream?
Area 4: Recycle
What are the characteristic parameters of the distillation column?
What effect do the recycle ratios have on the system performance?