Unformatted text preview: Homework 6 Problem 1 Aspen™ Distillation Column Processing and
Design Using the Full Design Column Method
20 points possible due Monday October 10, 2011 Grader: Alan Problem Statement: A bubble cap distillation column with a 9 inch tray spacing is to be used to separate a biphenyl/butadiene mixture. The feed is liquid at the bubble point with a flow rate of 100 lbmol/hr and the following composition: Component mol%
K‐value Propylene 21.58 1.464 Propane 18.17 1.375 1,3‐Butadiene 20.10
0.8599 Butane 23.12 0.7964 Pentane 17.03 0.4729 The column operates at a constant pressure of 400 psia. It is desired to obtain 97% propane (LK) recovery in the distillate and a 98% butadiene (HK) recovery in the bottoms. The distillate does not contain any pentane and all other components are distributed throughout the column. The distillate has a propylene concentration of 54.05%. Determine the distillate and bottoms concentrations and flow rates, number of trays, optimum feed location, reflux ratio, and the column diameter. Solution: 1) Open Aspen PlusTM and start a new case. 2) Add a distillation column (RadFrac) from the columns menu to the flowsheet. Page 1 of 11 3) Connect material streams to the feed and a vapor and liquid product stream to the unit. 4) Open the Data Browser by clicking on the icon on the toolbar. First, add the system components. Click on the Components section of the Data Browser. 5) Add the process components given in the problem statement. When the Selected Components List is complete, close the Component List View and Fluid Package windows. Page 2 of 11 6) Click on the Setup section and select SI units under the Specifications section. Then open the Report Options of the Setup menu. Under the Stream tab, change both the flow basis and the fraction basis to mole. 7) Open the Properties section by clicking on the folder. Click on Specifications. On the Global tab, under Base method, select the appropriate property package. Since the process streams are hydrocarbon mixtures, select the Peng‐Robinson equations of state. Page 3 of 11 Once Peng Robinson has been selected, the Properties Binary Interactions folder will turn red signifying that the required input is incomplete. Click the next button, , and Aspen PlusTM will automatically calculation all of the binary interactions for the system components. 8) Open the feed stream (1) input form to define the charge stream. Enter a feed pressure of 400 psi with a vapor fraction of 0 and a total flowrate of 100 lbmol/hr. Also, enter the mole fractions of each component given in the problem statement. Page 4 of 11 9) Open the distillation column input by clicking on the Blocks, B1, and Input folders. On the Configuration tab enter 32 stages, which was the number of stages previously determined by running a short‐cut column in Aspen PlusTM. Select a total condenser with a distillate flowrate of 39.4 lbmol/hr and a reflux ratio of 3.9. 10) Under the streams tab, enter a feed stage of 17, which was also determined using the short‐cut column. Page 5 of 11 11) This column runs at a constant pressure of 400 psi. Under the Pressure tab, enter 400 psi for both the stage 1 and stage 2 pressures. 12) The system has been fully defined. Press F5 or the run button, , to run the simulation. 13) Check the simulation results. As shown below, the simulation only produced a product with 52.9% propylene in the distillate while the propane recovery was only 93.5%. 94.6% of the butadiene was recovered in the bottoms product. Page 6 of 11 14) In order to achieve the desired products, design specifications need to be added to the simulation. Save the current file before moving on to the next step. 15) Open the column Design Specs folder by clicking on the Blocks, B1, and Design Specs folder. 16) Click the New… button to add a design spec and click OK to accept the default name. Page 7 of 11 17) Under the Specifications tab, select Mole Purity as the type and enter a target value of 0.5405. 18) Open the Components tab, select propylene by either double clicking the component name or click the component box. button to add propylene to the selected Page 8 of 11 19) Open the Feed/Product Streams tab and select stream 2 or the vapor stream as the appropriate product stream for this design spec. 20) The design spec input is complete and now the variable to be changed to meet the design spec must be entered. Click on the Vary folder, select new to add a variable, and click OK to accept the default name. 21) The specifications tab will automatically open, select reflux ratio from 1 to 25. Page 9 of 11 22) The system has again been fully defined. Press to reinitialize the simulation. This will purge all of the data from the last run. Press F5 to run the simulation. 23) Check the simulation results. With the addition of an optimization sequence the simulation now produces a 54.1% propylene product with 97% recovery of propane and 98% recovery of butadiene. Page 10 of 11 24) Check the convergence results under Blocks B1 Results to see the new reflux ratio value of 5.16, which was originally entered as 3.9. Page 11 of 11 ...
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- Distillation, Design Column Method