Process Design and Simulation Project: Esterification of Succinic Acid
Succinic acid (butanedioic acid, C4H6O4) is one of a new class of materials that is made from renewable resources instead of petroleum. One useful product that can be made from succinic acid (SA) is its diethyl ester, diethyl succinate (DES). Diethyl succinate has found commercial use as an environmentally friendly paint stripper and solvent and as an intermediate for further reactions of succinic acid.
Formation of DES proceeds according to the following stoichiometry:
C4H6O4 + 2 C2H5OH = (C2H5)2C4H4O4 + 2 H2O
SA ethanol DES
Unfortunately, this reaction is equilibrium limited; in other words, it does not go to completion. This difficulty makes formation of the ester an interesting challenge economically; this challenge is made greater because succinic acid is available only in aqueous solution and the water present drives the reaction backwards according to LeChatelier's Principle.
You are to design a process to make DES using AspenPlus simulation software. The process is to consist of no more than two reactors (RCSTR), and no more than three separator blocks (SEP2 in Aspen) to separate out unwanted or desired reactants and products. You will specify the above reaction in the Reactions block in AspenPlus with an equilibrium constant of 0.71. See the Reactor Block Tutorial to enter the reaction data and set up the RCSTR. The reactor blocks in Aspen use the properties of reactants and products and the data you enter to calculate the stream composition leaving the reactor. For the separators, you are to specify that each separator removes 95% of any one particular component you choose from the stream entering the separator without removing any other components.
The feed stream to your process is a 30 wt% aqueous solution of succinic acid in water at 1.0 atm and 70o flowing at a rate of 2000 kg/hr. All process units are run at this temperature and pressure. Specify the Uniquac equation of state as Property Method in the Global Properties Specifications in AspenPlus.
Your goal in the design is to produce a stream of pure diethyl succinate (DES) at the greatest "profit." For our purposes, "profit" is the value of the product DES minus the cost of the reactants. Assume the following values/costs for products and reactants: DES, $0.90/kg; SA, $0.50/kg; ethanol, $0.30/kg. Any unreacted SA or ethanol that leaves the process unreacted is considered waste and has zero value.
You are to use the process simulators to find a process configuration and flow rate of ethanol that give the highest possible "profit." You may add ethanol at any point in the process, and you may arrange the reactors and separators in any configuration you wish. You also have the option of splitting and combining streams as you see fit. Please note that there are many solutions to this problem, and it may not be possible to find a single "best" solution. This is often true in engineering design situations.
A final note: the separators we are using in this problem are gross simplifications of the actual separation processes required for this process. You will gain expertise in designing and analyzing such separation units in later courses. Nevertheless, this exercise provides you with a real-world problem of configuring a process that gives the best economic outcome, something chemical engineers do all the time.
Requirements for the project:
1. You must simulate at least two different process configurations for ester production (you should investigate more to get an idea of an optimum configuration). For each configuration, you must vary the ethanol flow rate to find the maximum profit for that configuration. For the optimum flow rate in each configuration, you must print out the flowsheet and stream table.
2. You are to prepare a report on your simulation. The report should not exceed two pages in length excluding the flow diagrams and stream tables you printed from 1) above. In the report, you must provide a Table that gives a brief description of each case and the calculated profit or loss for that case. The report must give a brief description of the approach you took to solving the problem, and a discussion of how you came to your "best" process configuration and why you chose it.
3. On an additional page, provide any hand calculations you did that support your Aspen simulation. (Conversion of units, stoichiometric calculations, etc...)
4. You are to work in groups of up to 2 individuals on this project.
ASPEN Reactions and RCSTR Reactor Block Tutorial:
When using ASPENPlus you will need to specify a reaction and set up the RCSTR reactor from the Reactor menu. Please use the following procedure:
1) After you have entered your components and your Property Method as Uniquac under the Properties Specifications menu, click the Next (blue N→) button.
2) This will bring you to the first stream input box. Before you enter anything in for your streams, on the menu on the left scroll down and click on the + next to the Reactions folder.
3) On the subfolders, click on the Reactions folder.
4) On the Object manager on the right, click on "New".
5) Enter in a reaction ID (or you can just use the default R-1 that is there), and then on the Select Type scroll down menu choose “Powerlaw”.
6) On the Stoichiometry tab click New. This will take you to the window that specifies reactants and products. Click on the reactant Component box, and then click the arrow to get the dropdown menu with your components.
7) Enter in the reactant components and their stoichiometric coefficients (negative for reactants, positive for products), and then enter in the product components and their coefficients.
8) On the top right, under Reaction type choose Equilibrium. Then click Close to close the window.
9) Click on the Equilibrium tab in the top middle to get to the Equilibrium page.
10) The reaction you entered should appear in the first box, do not change it!
11) Set the reacting phase to Liquid; leave temperature approach as 0, and click on the option to Compute Keq from Built-in Expression. Set the Keq basis as “mole fraction.”
12) Enter A as 0.71 and leave the rest of the variables as 0 or blank
13) Click the Next button to go back to entering stream information.
14) When you get to the Reactor (RCSTR) block(s), you will enter pressure (1 atm), temperature (70oC), and valid phases as Liquid-only.
15) Specify the Reactor volume as 5000 L
16) In the Reactions window, highlight your reaction (R-1) under Available reaction sets and click on the single arrow so that the reaction is now listed under the Selected reaction sets.
If you use more than one reactor, repeat steps 14-16 for each reactor.