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Depressuring with Aspen HYSYS® V8.0 1. Lesson Objective: Objective:
To construct a simpl e case using the Depressuring Analysis in Aspen HYSYS. HYSYS.
2. Prerequisites
Aspen HYSYS V8.0 V 8.0
3. Background Any process dealing with gasses has the potential for unsafe pressure buildup. This can happen because of instrument failure, loss of power, or an unforeseen heat source such as a fire. If pressure does build up, there must be a depressuring system in place to depressurize in a safe manner. The pressure is bled through a valve until it reaches a safe level. The excess gas can also be sent to a pressure vessel, but these vessels are also equipped with valves to prevent overpressure. Gas blowdown valves are common in oil wells. When the wells are not in use, the pressure can build up. A blowdown valve vents the excess gas to a flare, where the hydrocarbons can be burned be fore bei ng released into the atmosphere. The examples example s presented are are solely intended to illustrat ill ustrate e specific concepts and principles. They may may not reflect refle ct an industrial application or real si tuation.
4. Problem Statement and Solution Problem Statement A ni trogen stream that has has built bui lt up an excess pressure
Pressure: 2161 2161 psia
Temperature: 62.6 °F
Molar flow rate: 2.205 2.205 lbmole/hr lbmol e/hr
Use the Aspe n HYSYS HYSYS Depressuring Util Utility ity to de termine the behavior behavior of the gas in i n a depressuring depre ssuring process that takes 100 100 seconds. Assume an adiabatic case with no external heat source.
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Aspen HYSYS Solution 4.01.
Start Aspen HYSYS V8.0. Open a New case.
4.02.
The Component Lists window will be displayed. Click Add to create Component List-1 and add Nitrogen to the list.
4.03.
Click the Fluid Packages folder in the Navigation Pane, then click Add to create Basis-1. Select the Peng-Robinson property package.
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4.04.
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Move to the simulation environment by clicking on the Simulation button in the bottom right corner of the screen.
4.05.
Insert a Material Stream from the Model Palette.
4.06.
Double click on the stream. In the Worksheet tab, enter a Temperature of 62.6° F, a Pressure of 2161 psia, and a Molar Flow of 2.205 lbmole/hr. If necessary, change the Unit Set to Field in order to match the units being used. In the Worksheet | Composition frame, enter a Mole Frac of 1 for Nitrogen.
4.07.
In the Home tab of the Ribbon, click on Analysis | Depressuring.
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4.08.
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Click Add to create Depressuring-Dynamics-1, then click Edit to enter in values for the Depressuring utility.
4.09.
Select stream 1 for Inlets. HYSYS will automatically size the vessel. However, we will choose different values. Make sure the Vertical radio button is selected under Vessel Parameters, then delete the entry for Flat End Vessel Volume. Enter a Height of 5 ft and a Diameter of 0.8957 ft, and the rest of the values should be calculated. The page should resemble the image below.
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4.10.
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Click Heat Flux in the left side of the window, and change the Ambient Temperature to 62.33° F under Heat Loss Parameters.
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4.11.
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Notice that Unknown Vessel Metal Thickness is displayed in the status bar at the bottom of the window. To address this, select the Conduction radio button. In the table under Metal, enter a Thickness of 0.9843 inches. Also, under insulation enter a Thickness of 0 inches.
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4.12.
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Click on Valve Parameters on the left side of the window. Change the Vapour Flow Equation to General, 2
then ender a Cd of 0.7 and an Area of 4.907e-2 in . A Cd that is less than 1 signifies that the effe ctive orifi ce flow area is less than the physical area, which is a common occurrence.
4.13.
Click on Operating Conditions in the left side of the window. Under Operating Parameters, change the Time Step Size to 0.05 seconds and the Depressuring Time to 100 seconds.
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4.14.
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The Depressuring utility should be ready to calculate. Move to the Performance tab and select Strip Charts on the left side of the window. There is a premade chart named Depressuring-Dynamics-1-DL. However, this chart has over 20 variables, and will be difficult to read. We will create a new plot that only contains the most relevant information. Click on the Create Plot button to create DataLogger1, then click Add Variable… to select the variables for the plot.
4.15.
We are only concerned with the vapour flow, as we down not anticipate any liqui d. DepressuringDynamics-1 should be selected under Flowsheet. Under the Object column, select Vapour@TPL1 and Mass Flow under Variable. Also add Vapour@TPL1, Pressure and Vapour@TPL1, Temperature.
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4.16.
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In the Performance tab, change the Sampling Interval of DataLogger1 to 0.05 seconds, then click View Strip Chart… .
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4.17.
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Click Display in the DataLogger1 window to display the plot. Right click on the plot and sele ct Graph Control. In the Axes tab, check the box es marked Automatic Auto Scale in the Auto Scale section and Show All in the Axis Display section.
4.18.
Move to the Time Axis tab, and click on Set-up Logger. Change the Logger Size to 2003, and make sure the Sample Interval is 0.05 seconds.
4.19.
We are now ready to run the simulation. Close the Strip Chart Configuration window and return to the Depressuring-Dynamics-1 window. The sub-flowsheet will run in dynamics until the depressuring time is complete, and then the system will return to steady state. Click the Run button at the bottom of the Depressuring-Dynamics-1 window, and wait for the simulati on to complete.
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4.20.
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View DataLogger1 to see how the Vapour Mass Flow, Pressure, and Temperature behaved. The Time Axis can be adjusted by using the red triangle at the bottom of the chart.
4.21.
Additi onal information can be found in the Summary page of the Performance tab.
4.22.
In the Main Flowsheet, double click on Depressuring-Dynamics-1 (Flowsheet), and click the SubFlowsheet Environment… button. This will enter the depressuring sub-flowsheet.
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4.23.
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The sub-fl owsheet contains information for the vessel, as well as spreadsheets that can export data.
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5. Conclusion In thi s lesson we ex amined a basic case of an adiabatic depressuring. The adiabatic case describes a system such as an oi l well where excess pressure has built up but there is no external heat. Pressure bui ldup can also be the result of an accident such as a fire. In this case, the Operating Mode can be changed to Fire Mode, which allows you to specify a heat flux. The Use Spreadsheet option is also available, which all ows a user to edit the duty spreadsheet without the values being overwritten when the utility runs. Depressuring is important in assuring the safety of a process.
6. Copyright Copyright © 2012 by Aspen Technology, Inc. (“AspenTech”). All rights reserved. This work may not be
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