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Unformatted text preview: Homework 3 Problem 2 Aspen Energy Analyzer™
20 points possible due Friday September 16, 2011 Grader: Alan Problem Statement: Four streams are to be cooled or heated with a ∆Tmin = 10°C. Design the corresponding heat exchanger network (HEN) for maximum energy recovery (MER) on both the hot and cold sides of the pinch. Calculate the required duty for each exchanger, identify any heat loops, and determine the minimum number of heat exchangers. s (°C)
H1 300 100 1.2 H2 240 60 4.7 C1 20 120 1.5 C2 65 250 3.9 Method of Solution: Define the system in HX‐NetTM and use the program to identify the heat loop and determine the minimum number of heat exchangers. Solution: 1) Start the HX‐NetTM program. Typically from the START menu: All Programs/AspenTech/Process Modeling V7.2/Aspen Energy Analyzer 2) Open the Heat Integration Manager icon on the toolbar or by selecting Managers Heat Integration Manager… from the Menu bar. 3) The Heat Integration Manager window will appear. Select HI Case from the Heat Integration Manager and click Add. 4) The HI Case: Case 1 window will appear. Page 1 of 12 5) On the Process Streams tab, enter the streams to be heated and cooled. Also enter the inlet temperatures, outlet temperatures, and mCp values given in the problem statement. Warning: Make sure to use the correct units for mCp. The problem statement specifies kW/oC, which is equivalent to a kJ/s/oC. 6) Performing the heat integration by hand, it should be apparent that heating and cooling utilities are needed to meet the design specifications. HX‐Net performs this analysis as a default. Click on the Utility Streams tab. Page 2 of 12 7) Heating and cooling utilities are added on the Utility Streams tab. In the first empty cell in the Name column, select HP Steam from the drop‐down arrow that appears when the mouse is hovered over it. Notice that the status message for Hot has changed from Insufficient to Sufficient and the Utilities is already fully specified. 8) Add Cooling Water in the same manner. 9) Open the targets view by click the icon, , in the lower left hand corner of the HI‐Case: Case 1 window. 10) Notice that you can change the ΔTmin. With a ΔTmin of 10°C, check that the Energy Targets are 6 kW and 220.5 kW for Heating and Cooling, respectively. The minimum number of heat exchangers is calculated to be 5 with pinch temperatures of 240oC and 230oC. Page 3 of 12 11) The HEN can now be set‐up. Open the heat exchanger grid diagram by click on the icon, . A blank HEN will appear. Warning: The streams do not always appear on the diagram in the order that they were entered. The diagram order for this solution is Cooling Water, H1, H2, C2, C1, HP Steam. 12) Heat exchangers can be added through the Design Tools palette. Click the icon in the lower right‐hand corner to view the palette. Notice that the design palette icons also appear at the top of the HEN diagram. 13) To add a heat exchanger to a stream, hold the right mouse button on the Add Heat Exchanger icon, , drag‐and‐drop the bulls‐eye icon over a stream and release the right mouse button. 14) Next, hold the left mouse button on the red circle installed on stream and move to the connecting cold or hot stream. Release the mouse button when the four arrow icon is over the second stream. Page 4 of 12 To delete a heat exchanger, right click on one of the circles and select Delete. 15) The HEN diagram satisfied streams will have a solid line while unsatisfied streams will have a dashed line. 16) There are always multiple solutions to HEN diagrams and never just one correct answer. The goal is to minimize the number of heat exchangers and loops while following the basic guidelines listed below: a. Do not transfer heat across the pinch point; the pinch divides the heat exchanger network into two distinct regions. b. Do not use a hot utility below the pinch point. c. Do not use a cold utility above the pinch point. d. Heat transfer always takes place from a higher to a lower temperature. e. No process‐process heat exchanger should have an approach temperature less than the specified Tmin. f. Minimize the number of heat exchangers. g. Avoid loops in the heat integration system. 17) Below is one HEN solution to this problem containing at least one heat loop. Connections
H2 to Cooling Water 220.5 H1 to C2 168.0 H2 to C1 150.0 H2 to C2 475.8 H1 to C2 72.0 C2 to HP Steam 6.00 Page 5 of 12 18) Add all of the heat exchangers and duties shown in the table above. Do not tie any stream lines at this point. Start with the heater C2 to HP Steam and add the heat exchangers in order moving left to right along the diagram. The complete diagram should look like the one shown below: 19) To begin solving the HEN diagram, the streams need to be tied to each other. Tieing the stream connects the upstream or downstream temperature to the heat exchanger basically changing the degrees of freedom in order to solve the energy balance. Many times it is easier to tie the stream lines once all of the know information has been entered i.e. temperatures and/or duties. It is always best to tie lines across the diagram in one direction left to right or right to left. 20) Open the cooler that connects the cooling water stream to H2. Tie both sides of the hot stream and close the window. Enter a heat duty of 220.5 kW. Page 6 of 12 21) For HX: H1 to C2, tie all of the streams in and out. Enter a heat duty of 168 kW. 22) For HX: H2 to C1, tie both sides of the hot stream and the cold stream in. enter a duty of 150 kW. Page 7 of 12 23) For HX: H2 to C2, tie the hot stream out and both sides of the cold stream. 24) For HX: H1 to C2, tie all of the streams. Page 8 of 12 25) For the heater to C2, tie the cold stream out. When adding heating or cooling utilities to a HEN diagram, the following error may result. It is impossible to heat a cold stream to the temperature of the hot stream as a heat exchanger with infinite surface area would be required. This error is fixed by adjusting the utility inlet temperature to a reasonable value. Click back to the HI Case: Case 1 window and change the HP Steam inlet to 251oC under the Utility Streams tab. Page 9 of 12 26) The complete diagram should closely resemble the following 27) Loops can be found by opening the Topology view by click on the icon and click on the Loops tab. Or right click on the HEN and Selecting Show Loops and then the click on appropriate loop. Page 10 of 12 Results/Considerations: The HX‐NetTM results match the hand calculation results. With a ΔTmin of 10°C, the energy targets are 6 kW and 220.5 kW for Heating and Cooling, respectively. The minimum number of heat exchangers is calculated to be 5 with pinch temperatures of 240oC and 230oC. The rest of the results are found in the table and figure below: Connections
H2 to Cooling Water 220.5 H1 to C2 168.0 H2 to C1 150.0 H2 to C2 475.8 H1 to C2 72.0 C2 to HP Steam 6.00 Page 11 of 12 H1 300 °C 100 °C 240 °C 138.8 °C H2 240 °C 106.9 °C C 6 0 °C Heat Loop C1 C2 120 °C 250 °C 2 0 °C H 248.5 °C 230 °C 108 °C 6 5 °C Though, only one heat loop was present in this example, Aspen Energy AnalyzerTM has the ability to find multiple heats in a Heat Exchanger Network. Aspen Energy AnalyzerTM can also be used in conjunction with a process simulation file (i.e. HYSYSTM). Once loaded, Aspen Energy AnalyzerTM will model all coolers, heaters, heat exchangers, etc. in the simulation. Page 12 of 12 ...
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