Pressure-driven simulation

All of the dynamic simulations discussed in this book use pressure-driven flows. The alternative of using a flow-driven simulation is more simple, but not at all realistic of the actual situation in a real physical process. The plumbing in the real process has to be set up so that water flows downhill. Pumps, compressors, and valves must be used in the appropriate locations to make the hydraulics of the system operate. If valves are not designed with sufficient pressure drop under steady-state conditions, they may not be able to provide the required increase in flow even when wide open. So valve saturation must be included in the rigorous nonlinear dynamic simulation. It is much better to simulate a realistic system by using a pressure-driven simulation. 

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. 

In pressure driven simulation the flow rate is related with the pressure drop. Consequently, valves must be added for both inlet and outlet streams. For example, the inlet flow rate can be calculated from the pressure drop between feed supply and vessel. Similarly, the product flow rates can be calculated from the pressure drop between vessel and outlet lines. As before, controllers are added automatically for the basic inventory loops, as levels and pressures. 

Manage properly the pressure is a key aspect in dynamic simulation. As shown in Chapter 4, Aspen Dynamics offers two possibilities flow driven or pressure driven simulations. We selected the first possibility. We take care to specify all the items needed in dynamic simulation pressure change units (pumps, valves), mixers and heat... 

The Aspen Plus file is exported to Aspen Dynamics as a pressure-driven simulation after reflux-dram and base volumes are specified to provide 5 min of holdup when at 50%... 

Pressure-driven simulation in Aspen Dynamics is used with the top stage pressures of the Cl column set at 1.1 atm to allow for some pressure drop in the condenser and decanter. The top pressure of the C2 column is set to be at atmospheric pressure. The pressure drops inside both columns are automatically calculated in Aspen Dynamics to account for liquid... 

The Aspen Plus steady-state simulation in the last section is exported to the dynamic simulation of Aspen Dynamics. The tray sizing option in Aspen Plus is utilized to calculate the column diameter to be 0.3259 m and the tray spacing is 0.6096 m. Other equipment sizing recommended by Luyben is used here. The volume of the reboiler is sized to give 10 min holdup with 50% liquid level. The decanter is sized to be bigger to allow for two liquid phases to separate. The holdup time of 20 min is used in the dynamic simulation. Pressure-driven simulation in Aspen Dynamics is used with the top pressure of the azeotropic column controlled at 1.1 atm to allow for some pressure drop in the condenser and decanter to give the decanter at atmospheric pressure. The pressure drop inside the colunm is automatically calculated in Aspen Dynamics. Since the tray pressures in the colunms are different than the constant atmospheric pressure assumption used in steady-state simulation, the established base-case condition in Aspen Dynamics will be slightly different than Table 9.11. The final base-case steady-state condition used for control study can be seen in Table 9.15. 

Pressure-driven simulation in Aspen Dynamics is used in the control strategy development. Before converting the Aspen Plus simulation to Aspen Dynamics, sizing of all equipments is needed. The tray sizing tool in Aspen Plus is used to calculate the column diameters of both columns to be 0.78 m and 0.82 m for the first and the second column, respectively. Tray spacing and weir height of both columns are assumed to be 0.6096 m and 0.0508 m. 

Figure 3.59 Specifying pressure-driven dynamic simulation.

Under the Dynamics item, the vessel is specified to have CSTR-type dynamics with a diameter of 1 m and a length of 2 m, as shown in Figure 3.107. This corresponds to a residence time of 5 min for the given feed flowrate (0.1 kmol/s). The file is pressure-checked and exported as a pressure-driven dynamic simulation. 

The program in Aspen Plus is run and pressure-checked. It is then exported to Aspen Dynamics as a pressure-driven dynamic simulation as was done in Chapter 3 with CSTRs. The Aspen Dynamics file is opened, giving the window shown in Figure 6.37. The default control scheme has a pressure controller manipulating the valve in the reactor exit line. The simulation is run until all variables stop changing. 

Note that the flowsheet in Figure 6.77 shows a number of valves that are installed between the various units. These valves are not needed for a steady-state design. However, they must be used for a pressure-driven dynamic simulation to provide some... 

Bai, X., Josserand, J., Jensen, H., Rossier, J.S., Girault, H.H., Finite element simulation of pinched pressure-driven flow injection in microchannels. Anal. Chem. 2002, 74(24), 6205-6215. 

The dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

We present results obtained with Aspen Dynamics. A dynamic simulation with this package can be conducted in two modes flow or pressure driven. 

In flow-driven simulation the flow rates and the pressure variations are decoupled. For example, the pressure and flow rate of an outlet stream are determined from the inlet conditions and block specifications. The pressure of the downstream block affects neither the outlet pressure nor the flow rate of the current unit. In other words, the flows are governed by the assumption of perfect control. This is often realistic, particularly for liquid flows. Therefore, in flow-driven mode the simulation of valves is not necessary. However, in some cases pressure and level controllers are added automatically with default tuning parameters. 

As illustration, we present the dynamic simulation of a flash in pressure driven mode. We consider a vertical cylindrical vessel of elliptical head shape with diameter of 1.5 m and height of 3 m. The liquid fraction is 0.5 and the residence time of 10 minutes. 

Thomas, A. et al. Pressure-driven Water Flow through Carbon Nanotubes Insights from Molecular Dynamics Simulation, Carnegie Mellon University, USA, Department of Mechanical Engineering 2009. 

Figure 1 Sideways view of a fluid-carbon nanotube system. The solid lines represent the nanotube. The dashedlines represent either minimum image boundaries (shock wave simulations) or initial locations of driving and driven membranes (pressure wave simulations). Fluid atoms are enclosed within...

Figure 3 Typical pressure wave simulation results for helium within a 19.9 A diameter nanotube locations of driving (solid line) and driven (dashed line) membranes. The coordinates of the driving membrane are displaced 90 A for the sake of easy visual comparison with the driven membrane coordinates.

All the pieces of a distillation column will be specified (column, control valves, and pumps) so that we can perform a dynamic simulation after the steady-state simulation is completed. If we were only interested in a steady-state simulation, pumps and control valves would not have to be included in the flowsheet. However, if we want the capability to do simultaneous design (steady-state and dynamic), these items must be included to permit a pressure-driven dynamic simulation. 

The flowsheet shown in Figure 10.15 does not show the plumbing required to run a realistic pressure-driven dynamic simulation. The key feature is that the pressure in the stripper must be greater than that in the main column so that vapor can flow from the top of the stripper back to the main column. Therefore in the simulation, a pump and a control valve are placed in the liquid sidestream. A control valve is also placed in the stripper overhead vapor line. All this plumbing is shown in Figure 10.16. In a real physical setup, it is usually possible to use elevation differences to provide the necessary differential pressure driving force to get the liquid to flow from the main column into the stripper at a higher pressure and avoid the use of a pump. 

The steady-state RadFrac model in Aspen Plus consisted of four-column sections one stripper, two parallel absorbers, and a rectifier. In reality, there is only one column, but these four fictitious vessels are used in the simulation to model the real physical equipment Before exporting the file into Aspen Dynamics, a number of important changes had to be made in order to obtain a pressure-driven dynamic simulation. Figure 12.21a gives the Aspen Dynamics process flow diagram with aU the real and fictitious elements shown. The lower part of Figure 12.21b shows the controller faceplates. Note that the two controllers with remote set points (RCl and RC2) are on cascade. 

Combined Pressure-Driven Fiow and Eiectroosmotic Fiow, Fig. 1 Simulation results of the velocity vector of EOF in a T-shaped microgeometry of uniform zeta potential. The parameters used in computation are cross section dimensions of the microchannels 100 X 100 pm, concentration of NaHCOs electrolyte 1 mM, zeta potential 70 mV, and applied electric field strength 120 V/cm... 

Combined Pressure-Driven Flow and Electroosmotic Flow, Fig. 3 Simulation results of the velocity vector and streamline of EOF in the microchannel shown in Fig. 2... 

Combined Pressure-Driven Flow and Electroosmotic Flow, Fig. 4 Simulation results of the time evolution of EOF velocity distributions due to finite reservoir size effects (a) r = 1 s, (b) r = 10 s, (c) r = 30 s, and (d) t = 100 s. The parameters used in computation are cross... 

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