Pressure-driven dynamic simulation

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... 

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. 

In a real plant and in a pressure-driven dynamic simulation, plumbing is important for stable operation of process. A few useful mles should be remembered when setting up the locations of pumps, compressors, and valves. 

The flowsheet is exported to Aspen Dynamics after all the parameters required for a dynamic simulation are specified (equipment sizes), and the flowsheet is pressure checked so that a realistic pressure driven dynamic simulation can be used. Reflux drums and column bases are sized to provide 5 min of holdup when half full. Pump heads and control valve pressure drops are specified to give reasonable plumbing. Typical design control valve pressure drops are 3 bar with the valve half open at design flowrates. 

In Chapter 9 we explored the steady-state designs of both the MTBE and the ETBE reactive distillation columns using Aspen Plus. In this chapter we export the files into Aspen Dynamics as pressure-driven dynamic simulations and then look at dynamics and control. The control structures evaluated on both systems are based on those developed in Chapter 12 for ternary systems with inerts. 

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 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. 

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. 

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... 

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. 

Without the need of any fitting parameter, molecular dynamics (MD) simulations play a unique role in studying FO phenomena because MD simulations can allow for computing water transportation, analyzing molecular interactions and structures, as well as quantitatively probing various properties at atomic and molecular scales. The current literature on the use of MD simulations to study osmotic pressure-driven water flow is quite limited, and more studies are needed. 

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. 

Molecular Dynamics Simulation Method Multiscale Modeling and Numerical Simulations Non-Continuous Approaches Pressure Driven Single Phase Gas Flows Spectral Methods... 

Nagayama et al. [57] carried out nonequilibrium molecular dynamic simulations to study the effect of interface wettability on the pressure driven flow of a Lennard-Jones fluid in a nanochannel. The velocity profile changed significantly depending on the wettability of the wall. The no-slip boundary condition breaks down for a hydrophobic wall. Siegel et al. [58] developed a two-dimensional computational model for fuel cells. 

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