CHEMICAL ENGINEERING ASSESSMENT BRIEF 2025-26

Core Information

CENG0013 Design Project

Design of a process for the production of ethylbenzene

1. The Process

The majority of ethylbenzene (EB) processes produce EB for internal consumption within a coupled process that produces styrene monomer. We wish to design the process for the production of ethylbenzene. The production of EB takes place via the direct addition reaction between ethylene and benzene:

C6H6 + C2H4 −−→ C6H5C2H5 . (R1a)

The reaction between EB and ethylene to produce diethylbenzene (DEB) also takes place:

C6H5C2H5 + C2H4 −−→ C6H4(C2H5)2 . (R1b)

The competition between these two reactions limits the selectivity towards the desired EB prod-uct. The chemists have done a number of experiments in their lab. Using their data, they have come up with models for the single pass conversion of benzene, x, and the selectivity of EB to DEB production, S, as functions of the residence time, t, in seconds:

for t ∈ [5, 1000] s in both cases and ρ ∈ [1, 8] is the molar ratio of benzene to ethylene fed to the reactor. The chemists have determined that the ratio, ρ, has a significant effect on the selectivity.

Reaction R1a is an exothermic reaction. Therefore, the reaction takes place in the vapour phase. Since the moles of reactants are more than the moles of products, any reactor used will need to be run at high pressure (20 bar), according to Le Chatelier’s principle. The usually adopted temperature range for this reaction is T ∈ [350, 440] ◦C.

2. The design task

The process needs to be designed to meet a production requirement of (20+ d/10) mol s-1 of ethyl-benzene with molar purity 0.98. d is the last digit of your student ID number. Please state the value of d at the start of your submission. This ethylbenzene will be subsequently fed into a process (not considered here) for the production of styrene. To meet this requirement, separate feeds of benzene and ethylene are available. Both of the feed streams are available at T = 25 ◦C and P = 20 bar.

The benzene feed has a 2% by mole impurity of toluene (C6H5CH3 ). The chemists have deter-mined that this small amount of toluene also reacts with ethylene:

C6H5CH3 + 2 C2H4 −−→ C6H5C2H5 + C3H6 (R1c)

The toluene reacts completely in a few seconds (t < 5 s).

The design of the process flowsheet will be based on the application of the full Douglas approach, levels 1 through 5. You will be expected to identify and to model all the main processing units, along with sizing information, and their interconnections including possible heat exchanges.

3. Market information and cost models

The accountants have given you a detailed report of the market prices of reactants and products as well as models for estimating processing unit costs. Table 1 summarises the costs or values for particular streams. Table 2 presents the cost models for various types of units. The costs of the different utilities available, for heating, cooling, and electricity, are given in Table 3.

Table 1: Market prices for feeds and products.

Table 2: Cost models for processing units for estimating the total annualised cost, TAC, in mil-lions of €, of buying, installing, and maintaining the equipment.

Table 3: Cost data for utilities.

4. Things to do

Given the chemistry, the process requirements and the cost data and models above, develop a full process flowsheet for the production of ethylbenzene by following these steps. In attempting each of the following steps, fully itemise and justify all decisions taken. Marks allocated to each step are indicated in the right margin for a total of 100.

1. Apply levels 1 through 4 of the Douglas approach so as to develop a process flowsheet consisting of the main process units. In your report, summarise the key decisions you took, including the identification of design variables, with justification for these decisions. There is no need to include any discussion of elements of the Douglas hierarchy that do not apply to your case. Do not perform. any economic potential analysis at this point. Sketch the process structure obtained after applying all 4 levels of the hierarchy, labelling all input, output, and recycle streams; diagrams for each of levels 1-3 are not required. [20]

2. Implement a process and cost model based on the process identified in the previous step. Use one of GAMS (recommended), MATLAB, gPROMS, or a spreadsheet calculator (not recommended). The model should include costing information sufficient to calculate the economic potential for the process design obtained after level 4. Details on how to model the various potential processing units are given in the Appendices below. The model should be fully commented and must be included as an Appendix in your report, as text and not as a screenshot, and must also be submitted as a separate file in its native format to allow us to run your model. [20]

3. For the final flowsheet obtained with level 4 of the Douglas approach, use your model to investigate the behaviour of the economic potential in terms of the chosen design variable(s), considering annual hours of operation as indicated in the lecture notes for the type of process operation expected (level 1 of the Douglas hierarchy). Discuss how the design variables and the decisions you made in levels 1 to 4 impact on the economic potential. Plots of the economic potential with respect to the design variables will be required to support your discussion. [10]

4. Prepare a full stream table for the final process design from level 4 for your choice of design variable value(s). Fully justify your choice of values for the design variable(s) for this stream table. [5]

5. Use Aspen Plus to simulate the process identified in the first step. Use this simulation to

a) fill in a stream table for the process and

b) determine the heating and cooling duties of all of the units.

Discuss the possible reasons for and any implications of the differences you may observe between the stream tables from the previous step (step 4) and this step. If you were unable to prepare a stream table for either of the steps, discuss what you may have expected to see when comparing the stream tables. In your report, include a screenshot of the process flow diagram in Aspen and a screenshot of the stream table. The Aspen model must also be submitted as a separate file in Moodle. [10]

6. Applythe Pinch method, directly yourself as taught in the lectures and tutorial sessions, to design and cost a heat exchanger network for the process using the heating and cooling duties determined by the simulation. Should any reactors not be adiabatic and require either heating or cooling, the inlet and outlet temperatures for heat exchange should be considered to be ±5 ◦C around the operating temperature of the reactor. Use ∆Tmin = 20◦C for the pinch analysis and U = 1.5 kW m-2 ◦C-1 for the overall heat transfer coefficient for the design of any exchanger in the network.

Discuss the economic implications of the heat exchanger network on your previous conclusions in step 3 regarding the economic potential of the process. [30]

Note: Please see Appendix A for alternative heating and cooling loads in the case that you find that you are unable to answer question 5. However, if you were able to generate heating and cooling loads in Aspen, you must use those values.

7. Which decisions taken in levels 1 to 4 of the Douglas hierarchy had the most impact on the final design and why? Draw alternative process diagram(s) to illustrate your reasoning. [5]

5. Submission

The submission, via Moodle, will consist of three parts:

1. A report which answers the above questions, written using a word processor and submitted as a PDF document. This report must include your model (step 2) as an appendix in the form. of text (not screenshots) and also two screenshots of your Aspen simulation, one showing the full process flow diagram and one for the stream table. If you have used a spreadsheet calculator for the modelling in step 2, please insert two screenshots of the spreadsheet, one with values and one showing the equations, in the report as the appendix.

2. The model for level 4 from step 2, in your chosen modelling language, including the economic potential calculations for that level. The model should be submitted in the form. of the original file (e.g. as a .gms file for a GAMS model); a text version of the model must also be included in the report as an appendix, as noted above.

3. Your Aspen Plus simulation model should be included in the form. that Aspen can read. Also include, as noted above, screenshots of Aspen showing your process flow diagram and a screenshot of the stream table within your report.

Please note that parts 2 and 3 must be submitted as original files (e.g. .gms for a GAMS model) and not a PDF document. We must be able to load your model into the appropriate software to verify that it works.

All work must be your own and you must not discuss the project with anybody else. You may use the various tutorials to discuss doubts with others in your class.

The report should be a maximum of 20 pages. The content that is over the limit will not be marked and 5 percentage points will be deducted. The penalty will not take the mark below the pass mark. Assistive use of AI is permitted.