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When the column is set up and the streams connected are specified, the column will converge, giving the results of the first step of the acid gas cleaning process. To see the resulting H2S and CO2 compositions, as well as the H2S and CO2 loading in the Amine streams, select the “Performance” tab, and the “Acid Gas” ply on the left, as seen in Figure 12.
Figure 12: Results of the Acid Gas Cleaning process, as shown in the Absorber column results.
Separator After the Absorber has been set up, the rich amine stream is connected to a valve to flash the amine solution. The mixture is then split in the Separator, which can be set up by specifying an inlet stream (Rich Amine) and two outlet streams: Light Hydrocarbons and the liquid Rich Amine mixture, as seen in Figure 13.
Figure 13: The setup of a Separator in the Acid Gas Cleaning process.
Regenerator After the Separator, the liquid Rich Amine mixture gets sent through a heat exchanger and into the Regenerator. In this example, a Distillation Column Subflowsheet was used. To set up the Column, there is a Distillation Column Input Expert.
First, the inlet stream and output streams need to be defined, as well as energy streams for the condenser and reboiler, as seen in Figure 14. We have selected a Full Reflux Condenser as an example.
Figure 14: First page of the Setup for a Distillation Column with the Input Expert.
On page 2, you can configure the reboiler. In this example, we used a Once-through, regular HYSYS Reboiler. On page 3, you add the pressures in the Reboiler and in the Condenser, as well as any pressure drop across the equipment. On page 4, you have the option of inputting estimated temperatures for the Condenser and Reboiler, and on page 5 you have the option of setting a Vapor Rate and a Reflux Ratio for the Condenser.
Once the Input Expert is done, all the information necessary to converge the Regenerator will be defined. As with the Absorber, the Regenerator has Acid Gas-specific options under the Parameters tab for the type of column, tray packing, etc.
Valve and Pump In addition to the columns, a valve is necessary to drop the pressure of the Rich Amine stream coming out of the Absorber to flash separate it from lighter hydrocarbons in the Separator. Downstream, before the amine stream, is recycled into the Absorber. It is important to add a pump to make up the pressure difference so that the absorption in the column happens at a higher pressure.
Heat Exchangers Two heat exchangers should be added to the simulation for the Acid Gas Cleaning process, in addition to those already included in the column Unit Operations. One of them should be included after the pump to cool down the pressurized amine stream, and the other should be between the Separator and the Regenerator, to warm up the Low Pressure Rich Amine stream with energy left over from the Lean Amine coming out of the Regenerator.
Makeup Block In the Acid Gas Cleaning process there are losses of amines and water in the system with outgoing streams of Sweet Gas, Sour Gas, and other areas of the process. To make up for those losses, you can add a “Makeup Block”, which avoids convergence issues by calculating necessary makeup and purge streams to the system. The overall overview of the Makeup Block in the flowsheet can be seen in Figure 15. You only need to specify the inlet and outlet amine streams and the amine concentration and total flow of the outlet stream, which eliminates the need for adding spreadsheet operations.
Figure 15: On the left, the Makeup Block in the “Common” palette. Top right, the inputs (Amine Concentration, Total Flow) and connections (Inlet and Outlet Amine Streams, Amine Makeup, Water Makeup, and Purge Stream) necessary for the Makeup Block to converge. On the bottom right, the Makeup Block as it appears in the flowsheet, connected to the Amine input and output, Makeup streams, and purge stream.
Streams As these pieces of equipment are added, it’s necessary to add the streams connecting them as well. The most important
• Acid Gas into the Absorber
• Sweet Gas out of the Absorber
• Sour Gas out of the Regenerator
• Light Hydrocarbon out of the flash separation process
• Lean Amine between the Regenerator and the Absorber
- Gives up some heat in the Lean/Rich Exchanger
- Gets additional amines and water in the Makeup block
- Pressurized in the Pump
- Cooled in the Cooler
• Rich Amine between the Absorber and the Regenerator
- Flashed to a lower pressure through a Valve
- Separated from lighter hydrocarbons in the Separator
- Heated up before Regenerator Stream Setup After the unit operations are properly set up, some of the streams need to be characterized between the simulation converges. To set up a stream, the Temperature, Pressure, and Flow Rate need to be specified, as shown in Figure 16. In addition, the Composition of some streams needs to be specified. For the Acid Gas Cleaning process specifically, information about Heat Stable Salt content can be entered in the composition by clicking the Composition ply, then “Edit”, and entering the HSS Composition Wizard, shown in Figure 17.
Figure 16: Basic stream specifications.
Figure 17: The Heat Stable Salt Composition Wizard, where the amount of heat stable salts in the stream can be specified.
When all basic streams and columns are connected, the flowsheet should look like the one in Figure 18, which includes from left to right: an Absorber, Separator, and Regenerator. Two heat exchangers: Cooler and Lean/Rich Exchanger. A Booster Pump to increase the pressure before the Absorber and a Valve to decrease pressure significantly before the flash separation. Lastly, a Makeup Block with incoming Water and Amine Makeup streams and a purge stream.
Figure 18: A complete Acid Gas Cleaning Model in Aspen HYSYS V8.3.