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Basics of Process Simulation With Aspen HYSYS
Chapter 11
Basics of Process Simulation With Aspen HYSYS Nishanth Chemmangattuvalappil, Siewhui Chong University of Nottingham Malaysia Campus, Semenyih, Malaysia
In this chapter, a step-by-step guide is provided for the simulation of an integrated process flowsheet using Aspen HYSYS. The concept of simulation is based on sequential modular approach and follows the onion model for flowsheet synthesis (see Chapter 1 for details). The case study on n-octane production is used for illustration throughout the chapter.
11.1 EXAMPLE ON N-OCTANE PRODUCTION A simple example that involves the production of n-octane (C8H18) (Foo et al., 2005) is demonstrated, with detailed descriptions given in Example 1.1. The basic simulation setup involving registration of components, thermodynamic models, and reaction stoichiometry is to be carried out in the Basis Environments of Aspen HYSYS, while the modeling of reactor, separation, and recycle system are to be carried out in the Simulation Environments. The individual steps are discussed in the following subsections. Step 1: Basic Simulation Setup The first step in simulation using Aspen HYSYS is the definition of components. All components involved in the process are entered into the flowsheet by selecting from the component database, as shown in Fig. 11.1. Once the components are chosen, the appropriate thermodynamic model (this is known as “fluid package” in Aspen HYSYS terminoogy) for the system is chosen.1 Because this process involves the hydrocarbons at high pressure, “PengeRobinson” equation of state has been chosen as the fluid package, with steps shown in Fig. 11.2.
1. See Chapter 3 for guidelines on thermodynamic package section. Chemical Engineering Process Simulation. http://dx.doi.org/10.1016/B978-0-12-803782-9.00011-X Copyright © 2017 Elsevier Inc. All rights reserved.
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FIGURE 11.1 Defining components.
FIGURE 11.2 Selecting thermodynamic model (fluid package).
In the next step, the reactions involved in the process must be defined at the “Reaction” tab (Fig. 11.3). The first step is the selection of reactor model. In this example, the reaction is modeled as a “conversion reactor.” The conversion reactor model will treat the reactor as a stoichiometric problem and solve the mass balance based on the specified conversion (see steps in Fig. 11.3A). Once the reactor model is chosen, all components taking part in the reaction are selected accordingly and their stoichiometric coefficients are entered. The limiting component must be chosen along with the conversion. It is to be noted that, in Aspen HYSYS, the conversion must be specified in percentage (see Fig. 11.3B). Once all information is entered, a thermodynamic package must be assigned to this reaction to estimate the conditions after the reaction. In this example, the reaction is linked to the PengeRobinson equation of state model (see steps in Fig. 11.3C). These are the basic information required for creating the simulation flowsheet. Now we may proceed to the Simulation Environments to perform
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FIGURE 11.3 Steps in specifying reaction sets. (A) Select reactor type, (B) enter reaction details, (C) assign a fluid package, (D) enter the Simulation Environments.
modeling of the unit operations. To enter the Simulation Environments, we shall press the “Enter Simulation Environment” button on the Simulation Basis Manager (Fig. 11.3D). Step 2: Modeling of Reactor The Simulation Environments of Aspen HYSYS consist of main flowsheet, subflowsheet, and column subflowsheet environments. For the n-octane production example, only the main flowsheet is used. In this stage, the topology of the flowsheet must first be defined to identify the sequence of unit operations. Now, according to the onion model,2 the reactor system is simulated in the first step. It is necessary to define the reactions in a flowsheet before entering the Simulation Environment. At this stage, the type of reactor, information on conversion, 2. See Example 1.1 for details.
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FIGURE 11.3 cont’d
kinetics, equilibrium, etc., (depend on the available information) have to be specified as well. Although there are different types of reactor models available in Aspen HYSYS such as continuous stirred tank reactor and plug flow reactor, a conversion model is chosen for this example. The kinetic reaction models require the information on the reaction kinetics. For the n-octane production example, we shall utilize the “conversion reactor” model. It is to be noted that the conversion reactor model can only perform mass and energy calculations based on the stoichiometry. Detailed steps to draw the flowsheet on the process flow diagram (PFD) are shown in Fig. 11.4, where the conversion reactor consists of two inlet (Streams 1 and Q-101) and two outlet streams (Streams 2 and 3). Note that Streams 1, 2, and 3 are the actual process streams that consist of material (termed as Material Stream in HYSYS), while Stream Q-101 is actually virtual stream that is used for performing heat balances.
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FIGURE 11.4 Setting up feed.
After the connections are made, the conditions need to be specified in the incoming stream according to Table 11.1. It is advisable to conduct a degree of freedom analysis before designing the equipment to make sure that there are enough process parameters for the design of equipment and the variables are not overspecified. To do a degree of freedom analysis, we list all the variables involved in the process units. These variables can be operating conditions, such as temperature and pressure, flowrates, and compositions. Once sufficient conditions, which are molar flowrates, temperature, and pressure in this case, are entered, the incoming stream is completely defined.
TABLE 11.1 Feed Condition Component
Flowrate (kmol/h)
Condition
Nitrogen, N2
0.1
Ethylene, C2H4
20
T ¼ 93 C P ¼ 20 psia
n-Butane, C4H10
0.5
i-Butane, C4H10
10
n-Octane, C8H18
0
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FIGURE 11.5 Setting up reactor connection.
In the next step, the reactor model must be completely specified. We may enter the pressure drop and also define the reaction set to completely define the reaction (see detailed steps in Fig. 11.5). In this example, there is only one reaction set that is chosen. Because the reaction is conducted at isothermal mode, the outlet stream temperature should be selected and set to the same temperature as the incoming stream. We may notice that the energy stream now indicates a heat flow to maintain the reactor at isothermal conditions. In case of adiabatic reactors, no energy stream must be connected to the reactor. At this stage, we may notice that the reactor model is converged because all variables have been specified. Aspen HYSYS has been set to solve once the necessary data are sufficient. A convenient way of displaying the simulation results is via the Workbook. Fig. 11.6 shows the detailed steps in displaying molar flowrates of all components on the Workbook. One may also insert the Workbook Table within the PFD. This is illustrated with Fig. 11.7, where material and energy streams are displayed (one may also choose to display the stream compositions). Note that the energy stream in the converged reactor model has a negative value, which indicates that energy must be removed to maintain the isothermal conditions. This indicates an exothermic reaction in the reactor. In the final step, the basic mass balance calculations can be performed to see that the outlet composition from the reactor is in agreement with the stoichiometry. However, in more complex flowsheets, it may not be easily verified through hand calculations.
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FIGURE 11.6 Setting up Workbook in Aspen HYSYS.
FIGURE 11.7
Adding Workbook Table to process flow diagram in Aspen HYSYS.
Step 3: Separation Units In this process, the only separation system is a distillation column. The preliminary design of the distillation column can be done using the “shortcut distillation” model in HYSYS. This model is based on the Fenskee UnderwoodeGilliland model, which is useful for conducting the preliminary design of a distillation column. The parameters obtained from shortcut distillation model can be used as initial estimates in the rigorous distillation model, which performs stage-by-stage calculations. To continue in building
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FIGURE 11.8 Adding shortcut distillation model to the process flow diagram.
the topology of the flowsheet, one may refer to Fig. 11.8 for the alternative steps in connecting the material and energy streams. Fig. 11.8 also shows that the column is set to operate with partial condenser, where the column top stream exists in vapor form. The following design parameters are defined for this separation unit (Fig. 11.9): l l l l l
Condenser type: partial condenser Light key and mole fraction: ethylene (0.0015 in bottom stream) Heavy key and mole fraction: n-octane (0.28 in distillate stream) Column pressure: 25 psia in condenser Pressure drop: 10 psia
FIGURE 11.9 Changing the type of condenser.
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FIGURE 11.10 Specifications for the shortcut distillation model.
In the next step, the specifications of final product stream are added (mole fractions of the key component), and the pressure of condenser and reboiler is defined as shown in Fig. 11.10. After entering the given specifications, it can be noticed that the minimum reflux ratio is automatically calculated. In this case it is 0.007. So, an actual reflux ratio must be chosen to estimate the number of trays required to obtain the desired separation. In this case, we use an actual reflux ratio of 1. Once this information is entered, the column simulation converges. The calculated number of trays and column conditions can be seen on the tab “Performance” as shown in Fig. 11.11. We next move on to display the simulation results using the Workbook Table, as shown in Fig. 11.12. Step 4: Recycle System (Materials) Example 1.1 shows that the n-octane case contains both material and heat recycle streams. In this step, the convergence of these recycle streams is illustrated. Fig. 11.12 indicates that the distillate stream (Stream 4) contains some unconverted raw material that can be recycled to the reactor. Hence, we need to insert a recycle loop into the simulation to model the material recycle stream, which involves a purge stream unit, compressor, and cooler. Purging unit is necessary to avoid the trapping of materials inside the system. In this case, it is assumed that 10% of the distillate is purged. The compressor and a cooler are then used to adjust the pressure and temperature of the recycle stream to match those of the reactor. Specifications for these units are given in Table 11.2. To model the purge unit, a stream splitting model (called the “Tee” in Aspen HYSYS) is introduced in PFD. Detailed steps to connect the Tee model (to distillation top stream) and to provide its model specifications are shown in
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FIGURE 11.11 Results of shortcut distillation column design.
FIGURE 11.12 Simulation results.
Fig. 11.13. Because 10% of the distillate is purged, the flow ratio for recycle stream is set to 0.9. The distillate is collected at a pressure of 15 psia, whereas the reactor operates at 20 psia. To match the pressure of the distillate steam to that of reactor, a compressor is added to raise its pressure to 20 psia. Detailed steps to connect the compressor model and to provide its specifications are shown in Fig. 11.14.
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TABLE 11.2 Specifications for Units in the Material Recycle System Equipment
Specifications
Purge unit
Flow ratio for recycle stream: 0.9
Compressor
Outlet P: 22 psia
Cooler
Outlet T: 93 C Delta P: 2 psi
FIGURE 11.13 Adding a Tee for purge stream.
FIGURE 11.14 Adjusting the stream pressure.
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FIGURE 11.15 Adjusting the stream temperature.
Compression will increase the temperature of streams and in this case, the temperature of recycle stream after compression is raised to 99 C, which is higher than the reactor operating temperature. A cooler unit is added to reduce the temperature to the reactor temperature of 93 C. Fig. 11.15 shows the detailed steps to connect the cooler model and to provide its specifications. Because both pressure and temperature of the recycle stream are now adjusted to match those of the reactor, we can connect the recycle stream to the reactor. To converge this material recycle stream, we can make use of the “Recycle” unit. The latter facilitates the convergence of a recycle loop following the “tear stream” concept.3 Detailed steps to converge the recycle stream to the reactor with the Recycle unit are shown in Figs. 11.16e11.19. Note that the Recycle unit shows a yellow outline when the recycle stream is first connected to the reactor. This means that some parameters are not converged after 20 rounds of iteration (default setting in Aspen HYSYS). Hence more iteration is needed to ensure all parameters are converged completely (by pressing the “Continue” button in its Connections page). The simulation results are also displayed in the Workbook Table in Fig. 11.20. After the material recycle system is converged, we next proceed to converge the energy recycle stream. This will be done using the “tear stream” concept, i.e., without the use of the Recycle unit. Specifications for heat exchanger and heater in the energy recycle system are given in Table 11.3. 3. The Recycle unit in Aspen HYSYS performs the tear-stream calculation (see Chapter 4 for details) to converge the recycle stream automatically.
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FIGURE 11.16 Adding the Recycle unit.
FIGURE 11.17 Break the connection of fresh feed to the reactor.
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FIGURE 11.18 Add the fresh feed and recycle stream to the reactor.
FIGURE 11.19 Final process flow diagram with recycle stream.
In earlier steps, it has been assumed that the fresh feed stream is available at 93 C (see Table 11.2). This assumption is now relaxed. A heater is added to raise the temperature of the fresh feed stream from 30 C. Detailed steps to do so are given in Fig. 11.21. The simulated results indicate that the heater requires a total heating duty of 131 MJ/h (indicated by energy stream Q-105); while 5.4 MJ/h of energy needs to be removed by the cooler (indicated by energy stream Q-104).
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FIGURE 11.20 Simulation results after material recycle.
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TABLE 11.3 Specifications for Units in the Energy Recycle System Equipment
Specifications
Heat exchanger
Delta P: 2 psi (tube side) Delta P: 2 psi (shell side)
Heater
Outlet T: 93 C Delta P: 2 psi
FIGURE 11.21 Set the inlet condition of feed.
Fig. 11.22 shows a temperatureeenthalpy plot4 for the streams undergoing heating and cooling in the heaters and cooler. As shown, the temperature profiles of the cooler (Q-104dthe material recycle stream) are higher than those of the heater (Q-105dfresh feed). Hence, energy released from the heater can be completely recovered to the cold stream. In other words, part of the heating requirement of the heater (5.4 MJ/h) is to be fulfilled by the cooling duty of the cooler, through a process-to-process heat exchanger. The remaining heating duty of the cold stream (QH ¼ 125.6 MJ/h) is to be supplied by the heater, as shown in Fig. 11.22.
4. This is the most basic form of heat transfer composite curves in process integration; see Linnhoff et al. (1982) or Smith (2016) for more details.
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FIGURE 11.22 Temperatureeenthalpy plot for heat recovery system.
FIGURE 11.23 Identification of heat load.
With the heating and cooling requirements identified, we can now move on to simulate the process-to-process heat exchanger in the original PFD (Fig. 11.23). The “heat exchanger” model is utilized and added to replace the cooler model. Because the recycle model is not utilized in this case, we shall create a tear stream5 for the energy recycle system, for the stream connecting the heat exchanger and heater. Detailed steps for doing so are shown in Fig. 11.24. Note that the flowsheet is unconverged at this stage. 5. See Chapters 1 and 4 for detailed discussion on the use of tear stream for recycle simulation.
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FIGURE 11.24 Delete cooler to bring heat exchanger.
FIGURE 11.25 Bring heat exchanger to make use of the process stream temperature.
We next proceed to provide the missing parameters to converge the flowsheet. These include the estimation of values for the tear stream. Because this stream is essentially the same fresh feed stream that enters the heat exchanger at the shell side, its condition should be very similar (except that
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FIGURE 11.26 Final process flow diagram.
FIGURE 11.27 Workbook table of final process flow diagram.
with different temperature). Once the tear stream is specified, the “open loop” flowsheet is converged (Fig. 11.25). In the final step, the tear stream is removed and the outlet stream from the heat exchanger is connected to the heater. A converged “close loop” flowsheet is resulted and is shown in Fig. 11.26. The material stream conditions are shown using the Workbook Table in Fig. 11.27.
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REFERENCES Foo, D.C.Y., Manan, Z.A., Selvan, M., McGuire, M.L., October, 2005. Integrate process simulation and process synthesis. Chemical Engineering Progress 101 (10), 25e29. Linnhoff, B., Townsend, D.W., Boland, D., Hewitt, G.F., Thomas, B.E.A., Guy, A.R., Marshall, R.H., 1982. A User Guide on Process Integration for the Efficient Use of Energy. IChemE, Rugby, UK. Smith, R., 2016. Chemical Process: Design and Integration, second ed. John Wiley and Sons, New York.
FURTHER READING Foo, D.C.Y., Manan, Z.A., Selvan, M., McGuire, M.L., October, 2005. Integrate process simulation and process synthesis. Chemical Engineering Progress 101 (10), 25e29.
Basics of Process Simulation With Aspen HYSYS Nishanth Chemmangattuvalappil, Siewhui Chong University of Nottingham Malaysia Campus, Semenyih, Malaysia
In this chapter, a step-by-step guide is provided for the simulation of an integrated process flowsheet using Aspen HYSYS. The concept of simulation is based on sequential modular approach and follows the onion model for flowsheet synthesis (see Chapter 1 for details). The case study on n-octane production is used for illustration throughout the chapter.
11.1 EXAMPLE ON N-OCTANE PRODUCTION A simple example that involves the production of n-octane (C8H18) (Foo et al., 2005) is demonstrated, with detailed descriptions given in Example 1.1. The basic simulation setup involving registration of components, thermodynamic models, and reaction stoichiometry is to be carried out in the Basis Environments of Aspen HYSYS, while the modeling of reactor, separation, and recycle system are to be carried out in the Simulation Environments. The individual steps are discussed in the following subsections. Step 1: Basic Simulation Setup The first step in simulation using Aspen HYSYS is the definition of components. All components involved in the process are entered into the flowsheet by selecting from the component database, as shown in Fig. 11.1. Once the components are chosen, the appropriate thermodynamic model (this is known as “fluid package” in Aspen HYSYS terminoogy) for the system is chosen.1 Because this process involves the hydrocarbons at high pressure, “PengeRobinson” equation of state has been chosen as the fluid package, with steps shown in Fig. 11.2.
1. See Chapter 3 for guidelines on thermodynamic package section. Chemical Engineering Process Simulation. http://dx.doi.org/10.1016/B978-0-12-803782-9.00011-X Copyright © 2017 Elsevier Inc. All rights reserved.
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FIGURE 11.1 Defining components.
FIGURE 11.2 Selecting thermodynamic model (fluid package).
In the next step, the reactions involved in the process must be defined at the “Reaction” tab (Fig. 11.3). The first step is the selection of reactor model. In this example, the reaction is modeled as a “conversion reactor.” The conversion reactor model will treat the reactor as a stoichiometric problem and solve the mass balance based on the specified conversion (see steps in Fig. 11.3A). Once the reactor model is chosen, all components taking part in the reaction are selected accordingly and their stoichiometric coefficients are entered. The limiting component must be chosen along with the conversion. It is to be noted that, in Aspen HYSYS, the conversion must be specified in percentage (see Fig. 11.3B). Once all information is entered, a thermodynamic package must be assigned to this reaction to estimate the conditions after the reaction. In this example, the reaction is linked to the PengeRobinson equation of state model (see steps in Fig. 11.3C). These are the basic information required for creating the simulation flowsheet. Now we may proceed to the Simulation Environments to perform
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FIGURE 11.3 Steps in specifying reaction sets. (A) Select reactor type, (B) enter reaction details, (C) assign a fluid package, (D) enter the Simulation Environments.
modeling of the unit operations. To enter the Simulation Environments, we shall press the “Enter Simulation Environment” button on the Simulation Basis Manager (Fig. 11.3D). Step 2: Modeling of Reactor The Simulation Environments of Aspen HYSYS consist of main flowsheet, subflowsheet, and column subflowsheet environments. For the n-octane production example, only the main flowsheet is used. In this stage, the topology of the flowsheet must first be defined to identify the sequence of unit operations. Now, according to the onion model,2 the reactor system is simulated in the first step. It is necessary to define the reactions in a flowsheet before entering the Simulation Environment. At this stage, the type of reactor, information on conversion, 2. See Example 1.1 for details.
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FIGURE 11.3 cont’d
kinetics, equilibrium, etc., (depend on the available information) have to be specified as well. Although there are different types of reactor models available in Aspen HYSYS such as continuous stirred tank reactor and plug flow reactor, a conversion model is chosen for this example. The kinetic reaction models require the information on the reaction kinetics. For the n-octane production example, we shall utilize the “conversion reactor” model. It is to be noted that the conversion reactor model can only perform mass and energy calculations based on the stoichiometry. Detailed steps to draw the flowsheet on the process flow diagram (PFD) are shown in Fig. 11.4, where the conversion reactor consists of two inlet (Streams 1 and Q-101) and two outlet streams (Streams 2 and 3). Note that Streams 1, 2, and 3 are the actual process streams that consist of material (termed as Material Stream in HYSYS), while Stream Q-101 is actually virtual stream that is used for performing heat balances.
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FIGURE 11.4 Setting up feed.
After the connections are made, the conditions need to be specified in the incoming stream according to Table 11.1. It is advisable to conduct a degree of freedom analysis before designing the equipment to make sure that there are enough process parameters for the design of equipment and the variables are not overspecified. To do a degree of freedom analysis, we list all the variables involved in the process units. These variables can be operating conditions, such as temperature and pressure, flowrates, and compositions. Once sufficient conditions, which are molar flowrates, temperature, and pressure in this case, are entered, the incoming stream is completely defined.
TABLE 11.1 Feed Condition Component
Flowrate (kmol/h)
Condition
Nitrogen, N2
0.1
Ethylene, C2H4
20
T ¼ 93 C P ¼ 20 psia
n-Butane, C4H10
0.5
i-Butane, C4H10
10
n-Octane, C8H18
0
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FIGURE 11.5 Setting up reactor connection.
In the next step, the reactor model must be completely specified. We may enter the pressure drop and also define the reaction set to completely define the reaction (see detailed steps in Fig. 11.5). In this example, there is only one reaction set that is chosen. Because the reaction is conducted at isothermal mode, the outlet stream temperature should be selected and set to the same temperature as the incoming stream. We may notice that the energy stream now indicates a heat flow to maintain the reactor at isothermal conditions. In case of adiabatic reactors, no energy stream must be connected to the reactor. At this stage, we may notice that the reactor model is converged because all variables have been specified. Aspen HYSYS has been set to solve once the necessary data are sufficient. A convenient way of displaying the simulation results is via the Workbook. Fig. 11.6 shows the detailed steps in displaying molar flowrates of all components on the Workbook. One may also insert the Workbook Table within the PFD. This is illustrated with Fig. 11.7, where material and energy streams are displayed (one may also choose to display the stream compositions). Note that the energy stream in the converged reactor model has a negative value, which indicates that energy must be removed to maintain the isothermal conditions. This indicates an exothermic reaction in the reactor. In the final step, the basic mass balance calculations can be performed to see that the outlet composition from the reactor is in agreement with the stoichiometry. However, in more complex flowsheets, it may not be easily verified through hand calculations.
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FIGURE 11.6 Setting up Workbook in Aspen HYSYS.
FIGURE 11.7
Adding Workbook Table to process flow diagram in Aspen HYSYS.
Step 3: Separation Units In this process, the only separation system is a distillation column. The preliminary design of the distillation column can be done using the “shortcut distillation” model in HYSYS. This model is based on the Fenskee UnderwoodeGilliland model, which is useful for conducting the preliminary design of a distillation column. The parameters obtained from shortcut distillation model can be used as initial estimates in the rigorous distillation model, which performs stage-by-stage calculations. To continue in building
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FIGURE 11.8 Adding shortcut distillation model to the process flow diagram.
the topology of the flowsheet, one may refer to Fig. 11.8 for the alternative steps in connecting the material and energy streams. Fig. 11.8 also shows that the column is set to operate with partial condenser, where the column top stream exists in vapor form. The following design parameters are defined for this separation unit (Fig. 11.9): l l l l l
Condenser type: partial condenser Light key and mole fraction: ethylene (0.0015 in bottom stream) Heavy key and mole fraction: n-octane (0.28 in distillate stream) Column pressure: 25 psia in condenser Pressure drop: 10 psia
FIGURE 11.9 Changing the type of condenser.
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FIGURE 11.10 Specifications for the shortcut distillation model.
In the next step, the specifications of final product stream are added (mole fractions of the key component), and the pressure of condenser and reboiler is defined as shown in Fig. 11.10. After entering the given specifications, it can be noticed that the minimum reflux ratio is automatically calculated. In this case it is 0.007. So, an actual reflux ratio must be chosen to estimate the number of trays required to obtain the desired separation. In this case, we use an actual reflux ratio of 1. Once this information is entered, the column simulation converges. The calculated number of trays and column conditions can be seen on the tab “Performance” as shown in Fig. 11.11. We next move on to display the simulation results using the Workbook Table, as shown in Fig. 11.12. Step 4: Recycle System (Materials) Example 1.1 shows that the n-octane case contains both material and heat recycle streams. In this step, the convergence of these recycle streams is illustrated. Fig. 11.12 indicates that the distillate stream (Stream 4) contains some unconverted raw material that can be recycled to the reactor. Hence, we need to insert a recycle loop into the simulation to model the material recycle stream, which involves a purge stream unit, compressor, and cooler. Purging unit is necessary to avoid the trapping of materials inside the system. In this case, it is assumed that 10% of the distillate is purged. The compressor and a cooler are then used to adjust the pressure and temperature of the recycle stream to match those of the reactor. Specifications for these units are given in Table 11.2. To model the purge unit, a stream splitting model (called the “Tee” in Aspen HYSYS) is introduced in PFD. Detailed steps to connect the Tee model (to distillation top stream) and to provide its model specifications are shown in
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FIGURE 11.11 Results of shortcut distillation column design.
FIGURE 11.12 Simulation results.
Fig. 11.13. Because 10% of the distillate is purged, the flow ratio for recycle stream is set to 0.9. The distillate is collected at a pressure of 15 psia, whereas the reactor operates at 20 psia. To match the pressure of the distillate steam to that of reactor, a compressor is added to raise its pressure to 20 psia. Detailed steps to connect the compressor model and to provide its specifications are shown in Fig. 11.14.
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TABLE 11.2 Specifications for Units in the Material Recycle System Equipment
Specifications
Purge unit
Flow ratio for recycle stream: 0.9
Compressor
Outlet P: 22 psia
Cooler
Outlet T: 93 C Delta P: 2 psi
FIGURE 11.13 Adding a Tee for purge stream.
FIGURE 11.14 Adjusting the stream pressure.
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FIGURE 11.15 Adjusting the stream temperature.
Compression will increase the temperature of streams and in this case, the temperature of recycle stream after compression is raised to 99 C, which is higher than the reactor operating temperature. A cooler unit is added to reduce the temperature to the reactor temperature of 93 C. Fig. 11.15 shows the detailed steps to connect the cooler model and to provide its specifications. Because both pressure and temperature of the recycle stream are now adjusted to match those of the reactor, we can connect the recycle stream to the reactor. To converge this material recycle stream, we can make use of the “Recycle” unit. The latter facilitates the convergence of a recycle loop following the “tear stream” concept.3 Detailed steps to converge the recycle stream to the reactor with the Recycle unit are shown in Figs. 11.16e11.19. Note that the Recycle unit shows a yellow outline when the recycle stream is first connected to the reactor. This means that some parameters are not converged after 20 rounds of iteration (default setting in Aspen HYSYS). Hence more iteration is needed to ensure all parameters are converged completely (by pressing the “Continue” button in its Connections page). The simulation results are also displayed in the Workbook Table in Fig. 11.20. After the material recycle system is converged, we next proceed to converge the energy recycle stream. This will be done using the “tear stream” concept, i.e., without the use of the Recycle unit. Specifications for heat exchanger and heater in the energy recycle system are given in Table 11.3. 3. The Recycle unit in Aspen HYSYS performs the tear-stream calculation (see Chapter 4 for details) to converge the recycle stream automatically.
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FIGURE 11.16 Adding the Recycle unit.
FIGURE 11.17 Break the connection of fresh feed to the reactor.
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FIGURE 11.18 Add the fresh feed and recycle stream to the reactor.
FIGURE 11.19 Final process flow diagram with recycle stream.
In earlier steps, it has been assumed that the fresh feed stream is available at 93 C (see Table 11.2). This assumption is now relaxed. A heater is added to raise the temperature of the fresh feed stream from 30 C. Detailed steps to do so are given in Fig. 11.21. The simulated results indicate that the heater requires a total heating duty of 131 MJ/h (indicated by energy stream Q-105); while 5.4 MJ/h of energy needs to be removed by the cooler (indicated by energy stream Q-104).
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FIGURE 11.20 Simulation results after material recycle.
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TABLE 11.3 Specifications for Units in the Energy Recycle System Equipment
Specifications
Heat exchanger
Delta P: 2 psi (tube side) Delta P: 2 psi (shell side)
Heater
Outlet T: 93 C Delta P: 2 psi
FIGURE 11.21 Set the inlet condition of feed.
Fig. 11.22 shows a temperatureeenthalpy plot4 for the streams undergoing heating and cooling in the heaters and cooler. As shown, the temperature profiles of the cooler (Q-104dthe material recycle stream) are higher than those of the heater (Q-105dfresh feed). Hence, energy released from the heater can be completely recovered to the cold stream. In other words, part of the heating requirement of the heater (5.4 MJ/h) is to be fulfilled by the cooling duty of the cooler, through a process-to-process heat exchanger. The remaining heating duty of the cold stream (QH ¼ 125.6 MJ/h) is to be supplied by the heater, as shown in Fig. 11.22.
4. This is the most basic form of heat transfer composite curves in process integration; see Linnhoff et al. (1982) or Smith (2016) for more details.
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FIGURE 11.22 Temperatureeenthalpy plot for heat recovery system.
FIGURE 11.23 Identification of heat load.
With the heating and cooling requirements identified, we can now move on to simulate the process-to-process heat exchanger in the original PFD (Fig. 11.23). The “heat exchanger” model is utilized and added to replace the cooler model. Because the recycle model is not utilized in this case, we shall create a tear stream5 for the energy recycle system, for the stream connecting the heat exchanger and heater. Detailed steps for doing so are shown in Fig. 11.24. Note that the flowsheet is unconverged at this stage. 5. See Chapters 1 and 4 for detailed discussion on the use of tear stream for recycle simulation.
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FIGURE 11.24 Delete cooler to bring heat exchanger.
FIGURE 11.25 Bring heat exchanger to make use of the process stream temperature.
We next proceed to provide the missing parameters to converge the flowsheet. These include the estimation of values for the tear stream. Because this stream is essentially the same fresh feed stream that enters the heat exchanger at the shell side, its condition should be very similar (except that
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FIGURE 11.26 Final process flow diagram.
FIGURE 11.27 Workbook table of final process flow diagram.
with different temperature). Once the tear stream is specified, the “open loop” flowsheet is converged (Fig. 11.25). In the final step, the tear stream is removed and the outlet stream from the heat exchanger is connected to the heater. A converged “close loop” flowsheet is resulted and is shown in Fig. 11.26. The material stream conditions are shown using the Workbook Table in Fig. 11.27.
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REFERENCES Foo, D.C.Y., Manan, Z.A., Selvan, M., McGuire, M.L., October, 2005. Integrate process simulation and process synthesis. Chemical Engineering Progress 101 (10), 25e29. Linnhoff, B., Townsend, D.W., Boland, D., Hewitt, G.F., Thomas, B.E.A., Guy, A.R., Marshall, R.H., 1982. A User Guide on Process Integration for the Efficient Use of Energy. IChemE, Rugby, UK. Smith, R., 2016. Chemical Process: Design and Integration, second ed. John Wiley and Sons, New York.
FURTHER READING Foo, D.C.Y., Manan, Z.A., Selvan, M., McGuire, M.L., October, 2005. Integrate process simulation and process synthesis. Chemical Engineering Progress 101 (10), 25e29.