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Unformatted text preview: Homework 5 Problem 3 Aspen HYSYS™ Ammonia Absorption
20 points possible due Friday September 30, 2011 Grader: Alan Problem Statement: An absorption process requires that a contaminated air stream be contacted with water at atmospheric pressure to remove the ammonia contaminant. Both streams are at 25 °C and 1 atm. The gas stream enters the absorber with a molar flowrate of air at 4.75 kgmole/h and ammonia at 0.25 kgmole/h. The molar flowrate of the water stream is constant at 10 kgmole/h. It is desired to obtain an ammonia concentration of 0.1 mol percent in the exit air stream. Determine the minimum number of stages required to achive this concentration. Method of Solution: Use the Gas Absorption Unit in HYSYSTM to simulate the process which will adequately purify the gas feed mixture. Solution: 1) Open the HYSYSTM program and start a New Case. 2) The Simulation Basis Manager window will open. Click the Fluid Pkgs tab and click the Add… button. 3) Select the appropriate property package. Since the liquid product will be contaminated water, select the Sour SRK property package, to more accurately define the properties of the components involved in this simulation. 4) Click View next to the Component List Selection box. Page 1 of 9 5) Add the process components. When the Selected Components List is complete, close the Component List View and Fluid Package windows. 6) Click Enter Simulation Environment. 7) Click on the Absorber icon on the Object Palette, and click again in the PFD workspace to place the column in the PFD. 8) Double‐click on the column you just placed on the PFD to open the Input Expert window. Input Experts ease the information input sequence by prompting you to update the unit systematically. The initial page allows you to rename your absorber, specify the connecting material streams and # of stages. For this problem, specify a Top Stage Inlet, Bottom Stage Inlet, Overhead Vapour Outlet, Bottoms Liquid Outlet, and an estimate of 10 Stages. The Next button should become available. Page 2 of 9 9) Click Next. Now, it is required to specify the Top Stage and Bottom Stage Pressures. The system is at atmospheric pressure. Enter 1 atm for both values. 10) Click Next. The third and final window asks for temperature estimates. Temperature estimates are not needed for convergence, but help to speed to the calculation process. Click Done. 11) The Column window should appear. Close the column window. 12) Open the Liquid feed stream and enter the specified temperature and pressure. Page 3 of 9 13) Click on the Compositions section. Change the basis to Mole Flow by selecting the appropriate radio button. Now, enter a value of 10 kgmole/h for H2O and press OK. 14) Next, define the gas feed stream. Enter the temperature, and molar flowrate. Enter the corresponding molar flow rates for air and ammonia. Page 4 of 9 15) Double click the absorber and click the run button at the bottom. The Absorber Unit should converge. If it does not, click Run until it does. 16) Notice that the status bar has turned green and displays “Converged”, indicating that the column has converged. Click on the Performance tab to view the results. Page 5 of 9 17) The Exit Gas has an ammonia composition of 0.0001 (.01 mole percent). This is better than the process specifications, but because money is typically an issue inengineering and fewer stages correlate to a cheaper absorber, the number of stages required to meet the specification needs to be calculated. 18) Click back to the Design tab. Change the Number of Stages to 1. Click Run. 19) Press Ctrl+D to open the Databook. On the Variables tab click Insert. Select Exit Gas from the Object menu, Master Comp Mole Frac from the Variable menu, and Ammonia from the Variable Specifics menu. Click OK. 20) Click on the Data Recorder tab. To start a new recorder, click Add. In the Data Recorder Data Selection box, click the Exit Gas check box. Page 6 of 9 21) A one stage Absorber is already loaded as the default value, click the Record button to store the value. In the New Solved pop‐up window, change the name for the new state to ‘1 Stage.’ 22) Minimize the DataBook window by pressing . 23) Double‐click on the Absorber column and change the number of stages to 2, which will reset the P1 pressure. Re‐enter 1 atm. 24) Click Run. 25) Open the DataBook window (Ctrl‐D). Press Record. Change the name to ‘2 Stage.’ Minimize the DataBook and record values for absorbers with 3‐
10 stages. 26) Once all scenarios have been recorded. In the Available Display box, click View. Page 7 of 9 27) If the Graph radio button is not selected, do so now. Right‐click on the graph area. In the Graph Control window, click on the Axes tab. Select ‘Solved State,’ and change the Label to ‘Number of Stages’ and click in the check box to activate the label. Close the Graph Control window. 28) The created graph should relate the ammonia mole fraction in the Exit Gas to the number of stages in the absorber 29) An ammonia concentration of 0.1 mole percent can be obtained from an absorber with approximately 5‐6 stages. Page 8 of 9 Results/Considerations: For comparison, a summary of the HYSYSTM results is displayed below. Number of Stages Ammonia Exit Gas Mole% 1 2 3 4 5 6 7 8 9 10 1.62 0.71 0.36 0.20 0.12 0.07 0.04 0.03 0.02 0.01 Notice that HYSYSTM cannot compute an exact value of 0.1 mole% ammonia because only integers can be used to define the number of stages. So as not risk an ammonia concentration that is too high, 6 stages should be specified. Page 9 of 9 ...
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