ASPEN inlet streams

The product stream from the reactor (Bl) was flash cooled using a flash drum (B3) to separate vapors from the liquid phase. The flash models available in ASPEN-Plus determine the thermal and phase conditions of a mixture with one or more inlet streams. A separator (B6) was employed to separate C02 and steam. The resulting recycle streams no. 10 and no. 9 were sent to B13 and B7, respectively. The liquid stream (no. 14) from the flash drum was sent to a Pneumapress filter (B10), where it was separated into filter cake (stream no. 13) and filtrate (streamno. 8). This separation was done to facilitate heat extraction from the product stream for heat exchanger Bll. 

All ASPEN models allow solids to be in inlet streams. The user does not need to take special precautions. The solids are normally assumed to be non-distributing in the liquid and vapor phases. That is, the phase equilibria is unaffected by the solid phase. However, the system does allow for the case of solids distributing into other phases. Solids are taken into account in the energy balance around each equipment model. 

As shown in Figure 4.C.2, Aspen HYSYS simulation file is opened first. In VBA, Dim statement is used for declaration of variables, whereas Set statement is used for creating new objects. Then, values of feed flow rate, length and diameter of each reactor, and temperatures of inlet streams of both separators are transferred from cells C3 to Cll in Excel worksheet named DV to Aspen HYSYS , to Aspen HYSYS. To reduce computational time, remember to deactivate HYSYS solver before this transfer, and then activate it after transferring values of all decision variables to proceed with the calculations... 

In the Item 1 column that contains the values, the items for Hot Inlet Stream and Hot Outlet Stream are ICUST-IN and ICUST-EX, respectively, which correspond to the default utility, in this case, steam at 50 psi. To change to steam at 100 psi, right click on the appropriate cells and select Steam 100 PSI - IPE Utility from the pull-down menu that appears. Next, delete the Final Surface Area, previously computed, since it must be re-sized by Aspen IPE ... 

Aspen is used to construct the pump process flow sheet shown in Figure 2.71. Add pure water as the only component. Click on Physical Property in the toolbar, and select Stream-TA as the base method. Double click on the feed stream and specify inlet stream conditions (temperature, pressure, total flow rate, and composition). Double click on the pump icon and set the Discharge Pressure to 1200 psig and the pump efficiency to 0.1. Note that in the status bar, the message "Required Input Completed" means that the system is ready to be run. Click Run after the message run is successfully completed, click on results, and then click on block results. The process flow sheet and the stream table are shown in Figure 2.71. The brake power of the pump is shown in Figure 2.72. 

Aspen is capable of modeling chemical reactions. It can handle single and multiple reactions. Material balance can be done in the stoichiometric reactor, Rsto/c from Reactors in the model library. Click on Material Streams, and connect the inlet and product streams. Click on Components and choose the components involved. Peng-Robinson EOS is selected as the thermodynamic fluid package. Doubleclick on the conversion reaction block. Click on the Specification tab enter pressure as 1 atm and temperature as 25°C. Then click on the Reactions tab, click on New and enter the components involved in the reaction, stoichiometric coefficient, and fractional conversion as shown in Figure 3.13. Close the stoichiometric windows and then double click on the inlet stream, specify temperature, pressure, flow rate, and composition. Click Run and then generate the stream table as shown in Figure 3.14. 

The control structure shown in Figure 6.57 is installed on the flowsheet. The feed is flow-controlled. The outlet temperature is controlled by manipulating the coolant flowrate. Note that the OP signal is sent to both of the control valves on the coolant stream, opening and closing them simultaneously. The setup works in the simulations, but it is not what would be used in a real physical system. A pressure-driven simulation in Aspen Plus requires that valves be placed on both the inlet and outlet coolant streams. In a real system, the cooling water would be drawn from a supply header, which operates a fixed pressure. A single control valve would be used, either on the inlet or on the outlet, to manipulate the flowrate of coolant. 

Table 2.12 shows the table stream calculated with Aspen Plus for toluene fresh feed of 100 kmol/h, purge fraction of 0.06 and ratio of hydrogen/toluene in the inlet reactor mixture of 5. In these conditions, the gas recycle rate is about ten times the molar flow rate of the inputs. 

CO2 is absorbed into propylene carbonate in a packed column. The inlet gas stream is 20 mol% CO2 and 80 mol% methane. The gas stream flows at a rate of 2 m /s and the column operates at 60°C and 60.1 atm. The inlet solvent flow is 2000 kmol/h. Use Aspen HYSYS to determine the eoncentration of CO2 (mole%) in the exit gas stream, the column height (m) and the column diameter (m). 

It is important when specifying the control valves to select the correct valid phase. If the stream is aU liquid, select Liquid-Only in the Valid Phases under Flash options on the Operation page tab of the valve block. If the stream is all vapor, select Vapor-Only. Some valves have both phases (particularly when the inlet is liquid at its bubble point temperature and pressure, which means flashing occurs when the pressure decreases as the fluid flows through the valve) and Vapor-Liquid should be selected. Numerical problems can occur in Aspen Dynamics if these valid phases are not correct. 

Finally, specify the makeup feed stream input to the second mixer to have a temperature of 35°C and pressure of 2 bar. To be an effective makeup stream, the flow rates of MDEA and H2O must equal the flow rates of those two components lost via the CO2 product and clean syngas streams. Although trivial to compute at the moment, once the recycle loop is closed, this number must be computed for each flow sheet convergence iteration in order to be able to physically achieve a steady state and prevent eventual dry-up of the solvent. Unlike ProMax, which has a Make-up/Blow-down block to handle this specific scenario. Aspen Plus has more generic tools that can be used instead. For example, a calculator block can be added that computes and sets the inlet composition and flow rate of the makeup feed stream for every flowsheet iteration. To do this, first configure the makeup stream to have a mole flow of H2O and MDEA of 1 kmol/h each. These are strictly dummy variables. However, it is critical that the Total Flow Rate is left blank. By specifying it in this manner, we are creating a specification in which the calculator block will be able to directly overwrite the dummy numbers of 1 kmol/h with its own calculations just before the second Mixer block executes. 

The comparison between pressure drop values calculated by hand calculation (50.4 kPa), Hysys (51.69 kPa), PRO/II (52.1 kPa), and Aspen (51.51 kPa) reveals that there is a slight deviation between hand calculations and software simulations. The discrepancy in the hand-calculated value is due to the assumption made by taking the inlet conditions in calculating Reynolds number, while the average of inlet and exit streams should be considered to have better results. 

Follow the same procedure shown in the previous example for constructing the process flow sheet using Aspen. Specify feed stream conditions. Double click on pipe segment and set the pipe rise to 15 m. The pump exit pressure should be defined as the pipe inlet pressure (506.3 kPa) to overcome the pressure drop in the pipeline. The Aspen process flow sheet and stream table are shown in Figure 2.55. The pump brake hp is shown in Figure 2.56. 

Open the Aspen user interface and then click on the Pressure Changes tab on the model library and select compressor. Click anywhere in the process flow sheet area. Click on Material Streams in the model library and connect inlet and exit stream lines. Click on the arrow on the left of the model library to cancel the insert mode. Click on Component on the toolbar, and select methane, ethane, COj, and Nj. 

Under Properties, Specifications, select the base property method. Since these components are liquids, NRTL thermodynamic package is the most convenient fluid package. Install CSTR reactor under Reactors in the model library, and connect inlet and exit streams. Specify the feed stream conditions and composition. Input the reactor specifications double click on the reactor block. The reactor Data Browser opens. Specify an adiabatic reactor and the reactor volume to 4433 liters the value obtained from hand calculations (Figure 5.11). Add the reactions to complete the specifications of the CSTR. Choose the Reactions block in the browser window and then click on Reactions. Click New on the window that appears. A new dialog box opens enter a reaction ID and specify the reaction as Power Law. Then click on Ok. The kinetic data are very important to make Aspen converge. Mainly specifying accurate units for pre-exponential factor A, is very important (see the k value in Figure 5.12). The value MUST be in SI units. 

The process flow sheet for a PFR in Aspen Plus is constructed in the same way as previous examples. In the data browser, specify the feed stream properties. Specify inlet reactions stoichiometry and parameters as shown in Figure 5.48 for the first reaction. For the thermodynamic data, Peng-Robinson is selected. The reactor is considered isothermal. The process flow sheet and stream property table are both shown in Figure 5.49. 

Press on the Columns tab in the Equipment Model Library and place a DSTWU column into the process flowsheet window. To create the inlet and exit streams, first click on the Material Streams button at the bottom left corner of the window. Red and blue arrows appear around the column. A red arrow signifies a stream that is required for a design specification blue arrows signify an optional stream. Connect the feed and product streams where Aspen indicates they are required (Figure 6.18). 

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