Name of Tool/Strategy: Aspen Plus Chemical Process Modeling
Course Information
Course Name: ECH 158A, Process Design and Economics
Enrollment: 144
Brief Description of Course: Senior design experience in process and product creation and design with multiple realistic constraints. Cost accounting and capital investment estimation. Profitability analysis and techniques.
Description of Tool/Strategy Implementation
Learning how to model chemical processes using the Aspen Plus process modeling software is a key skill that chemical engineering students acquire during the ECH 158 (Chemical Engineering Process Design) sequence at UC Davis. They come into the series with limited knowledge of the software package and have mainly negative viewpoints towards it based on limited experience in previous coursework. In order to re-introduce students to the software in ECH 158A (Process Design and Economics), the first course in the chemical engineering senior design sequence, students were given access to a series of Aspen Plus tutorials posted on the Canvas course shell in addition to an in class demonstration on how to use the software. Three Aspen Plus problems were then given to the students on consecutive homework assignments. In these assignments, students were gradually exposed to more open ended questions, derived from one of the previous years’ senior design projects, dealing with optimizing the amount of reactant fed to a chemical process in order to meet a desired amount of product per year.
The first Aspen problem was a guided design of a chlorobenzene production plant model (see problem statement in the Appendix). The purpose of this problem was to allow students to hone their Aspen skills before attempting to approach more open-ended problems. In this assignment, the students followed a tutorial, presented in class and made available electronically, in order to set up a complete plant flowsheet for the chlorobenzene process. They then ran a number of analyses using the sensitivity analysis, design specification, and calculator block tools, following other electronic Aspen Plus tutorials made available by the instructor.
On the second Aspen Plus homework problem, the students were tasked with designing the upstream part of a methyl acetate production process (see problem statement in the Appendix). On an earlier homework assignment, the students were asked to complete an input/output (I/O) analysis of the methyl acetate production process (see problem statement in the Appendix). Since the students had not yet taken the Unit Operations and Separations class (the second course in the chemical engineering senior design sequence, ECH 158B), the reactor model was set up for them in a file that they could download. The students were responsible for defining the raw materials and manipulating their temperature and pressure to the reaction conditions prior to sending the feeds to the reactor. They then were required to determine the amount of raw materials needed to meet a plant capacity specification. They accomplished this using the Aspen Plus design specification tool.
On the third Aspen homework problem, the students were tasked with completing the model of the methyl acetate production process, building off of their flowsheet from the previous assignment and completing the downstream part of the process (see problem statement in the Appendix). As in the previous assignment, since the students had not yet taken the Unit Operations and Separations class, the distillation column models were provided to them. The students had to learn how to properly use the given models, as well as how to configure the recycle loops. Finally, they had to again determine, with the downstream part of the process defined, the required amount of raw materials needed in order to meet a capacity requirement. It is difficult to define recycle loops in Aspen Plus and have the software converge upon a solution. Therefore, the students learned how to configure the recycle loops such that Aspen had the degrees of freedom necessary to converge upon an optimal solution.
Evaluation
The goal of these assignments was to develop the students’ Aspen Plus process modeling skills in the context of a real design project, while also allowing them to gain confidence with the software as they moved from more to less guided assignments. Overall, the implementation of this strategy for introducing chemical process modeling using Aspen Plus was successful. Students commented that they appreciated the posted tutorials and the in-class demonstration. Many students also commented that they felt much more comfortable with Aspen Plus after these assignments. The students will again use Aspen Plus for their Plant Design Project course (the third course in the chemical engineering senior design sequence at UC Davis, ECH 158C) as well as for smaller assignments in their Unit Operations and Separations class. Given this experience with Aspen Plus in the introductory process design course, I expect the students to be better prepared to use this software in both courses.
Appendix
Chlorobenzene Plant Guided Assignment (1st Aspen Plus Homework Problem)
In class we developed a model for a chlorobenzene production plant (shown below). The following assignment will use this model in order to carry out some additional advanced analyses.
- Replicate the chlorobenzene model (shown above). Instructions and the necessary parameters can be found in the “Chlorobenzene Plant Tutorial” located under the “Aspen Tutorials” folder on Canvas. When you have successfully replicated the model, print out your stream summary. Report the concentration of monochlorobenzene and dichlorobenzene (mol%) in their respective product streams.
- Run the two sensitivity analyses described below.
- Investigate how fluctuations in the chlorine feed stream impact the chlorobenzene product streams. Vary the flowrate of the chlorine feed between 400 and 600 kmol/hr (increment=20). Print out your sensitivity analysis results curve. Report (either using the plot or the table), what range of flowrates examined in this analysis result in chlorobenzene product purities that meet our design specifications (> 95 mol% monochlorobenzene and > 80 mol% dichlorobenzene in their respective product streams).
- Investigate how fluctuations in the fraction conversion of HCl in the WASHING unit impact the chlorobenzene product streams. Vary the conversion from 0 to 1 (increment=0.05). Note that fractional conversion is abbreviated CONV and is a “Block-Var” type. Print out your sensitivity analysis results curve. Report (either using the plot or the table), what range of conversions examined in this analysis result in chlorobenzene product purities that meet our design specifications (> 95 mol% monochlorobenzene and > 80 mol% dichlorobenzene in their respective product streams).
- Run simulations (separately) to achieve the two design specifications below:
- You want to produce at least 10 kmol/hr of dichlorobenzene product. What is the feed rate of the benzene component needed to achieve this output? Print out your stream summary. Report the purity of both the monochlorobenene and dichlorobenzene product streams and comment on if these product streams meet the specification.
- (Delete the 1st design spec) You have found that you are only allowed to produce 10 kmol/hr of water waste from your washing unit in order to meet environmental regulations. What is the feed rate of the benzene component needed to reduce your water waste to this level? Print out your stream summary. Report the purity of both the monochlorobenene and dichlorobenzene product streams and comment on if these product streams meet the specification.
- Before starting, be sure to delete the previous design specification. You decide that in addition to the current product specifications, you want to add an additional specification that your benzene product purity should be at least 74.8 mol% benzene. Use a calculator block to determine the range of benzene (mixed with water) feed flowrates necessary in order to meet this new design spec along with the previous design specs (write this range down as part of your solution). Be sure to keep the composition of the benzene feed stream at 95 mol% benzene and 5 mol% water. Also, print out your sensitivity analysis results curve. The curve should show the results of the analysis for all three product purities. Note that to complete this problem you will need to choose reasonable variables/ranges over which to carry out your sensitivity analysis.
Methyl Acetate Input/Output Analysis Problem
Aggie Chemical Company (ACC) is planning to build a new plant for the production of methyl acetate. Methyl acetate is to be synthesized by esterifying a carboxylic acid (acetic acid) with an alcohol (methanol) in the presence of a mineral catalyst such as sulfuric acid (see the reaction below). The esterification is highly selective. Typical reaction conditions include temperatures in the 150-275 F range and enough pressure to keep the reaction mixture all in the liquid phase. To assess the commercial viability of this project, we need you to conduct a preliminary economic evaluation of the process based on input/output analysis. You may assume that we can sell methyl acetate for a price of $0.55/lb, and acquire methanol for a price of $0.06/lb and acetic acid for $0.22/lb. The plant must output 200 million lb of 99.5 wt% methyl acetate per year where the other 0.05 wt% is mostly methanol. You may also assume that by using sulfuric acid as a catalyst that 99.5% of the acetic acid can be converted to methyl acetate. Further, you may assume that the cost of the catalyst is negligible as compared to the cost of the other input/output streams. The result of your preliminary economic evaluation should be a forecast of the profit (or loss) per year for this perspective plant. Be sure to include a diagram that shows all input/output streams with your solution.
Methyl Acetate Upstream Process Problem (2nd Aspen Plus Homework Problem)
Download the file Homework X Problem X.apw from the Homework Assignments folder on Canvas. You will find a reactor model already defined for you (R-001). Your assignment is to construct the upstream section of a methyl acetate production process. Your raw materials are pure acetic acid and pure methanol available at standard temperature and pressure (1 atm and 273 K). You need to prepare these raw material streams for the reaction by bringing them up to 100 psia and 275 °F. Define streams and blocks on the flowsheet that will simulate taking the raw materials from standard temperature and pressure to the reaction conditions. In the reactor, you want to make at least 200 million lbs/yr of methyl acetate. Based on the results of our I/O analysis from Homework 1, we will feed 1.86 lbs of acetic acid for every 1 lb of methanol fed. Using the Aspen Plus analysis features, determine the optimal (smallest) methanol and acetic acid feed rates to meet this capacity constraint. Please submit a screen shot of your process, a screenshot of the flow summary (showing key input/output streams), and state the results of your analysis (optimal feed flow rates). You may also submit any supporting figures to back up your analysis.
Methyl Acetate Complete Process Problem (3rd Aspen Plus Homework Problem)
You began to design the upstream part of a methyl acetate production process. In this assignment, you will continue work on the design of this process. Download the file Homework X Problem X.apw from the Homework Assignments folder on Canvas.
a) Run the simulation of the process. In case you change something by accident, the initial flows of acetic acid and methanol that are defined in this version of the flowsheet are 1.5996*108 lbs/yr and 8.6*107 lbs/yr respectively. There are 5 outlet streams (PRODUCT, OUT1, OUT2, OUT3, and OUT4). What is the flow and composition of each of these streams?
b) To complete the design, you need to determine what you would like to do with the four OUT streams. In the form of a bulleted list, describe if you will treat the stream as a waste stream or if it could be recycled to the process.
c) For any streams that can be recycled, model the recycle in Aspen. Be sure to make and T/P adjustments necessary. If you need to decrease the pressure of a fluid, you can model this using a valve. Setting recycle streams up in Aspen can be tricky. You can use a mixer and splitter in series, as shown below, to model “storage”. This may help alleviate potential convergence errors you may encounter. Once you have this working, your goal is to determine the amount of methanol and acetic acid you need to feed to the process in order to produce 200 million lbs/yr of the methyl acetate component of the product stream. Be sure to keep the ratio of acetic acid to methanol fed to the reactor at 1.86 lbs AA:1 lb MeOH. You will find that you will encounter many errors/warnings as you try to solve this problem. A few words of advice…first, not all warnings/errors are bad. Some may just be part of the design spec not being able to find solutions for particular values of your manipulated variable as it is trying to converge to a solution. What you are looking for is successful convergence to the design specification (see the Convergence folder on the left hand toolbar). Also, you can play with the tolerance of the design spec, as you did in homework 3, as well as with other convergence parameters (see the Options folder under Convergence).
Report the approximate amount of fresh methanol and acetic acid needed to achieve this design specification (200 millions lbs/yr methyl acetate in its product stream). Be sure to account for what you are sending “to storage” and what you are taking “from storage”. Also you must provide a screenshot of your flowsheet with the recycle included, a screenshot of your stream summary showing the flow and composition of your key input and output streams, and a screenshot of your design spec results.
