Distillation variable-pressure simulation

Similar kinds of constraints involve the reflux ratio in distillation, which must exceed the minimum value for the required separation. If the distillation tower pressure is adjusted, the minimum reflux ratio will change and the actual ratio must be maintained above the minimum value. Even when optimization is not performed, the decision variable values must be selected to avoid violating the inequality constraints. In some cases, the violations can be detected when examining the simulation results. In other cases, the imit subroutines are unable to solve the equations as, for example, when the reflux ratio is adjusted to a value below the minimum value for a specified split of the key components. 

It is important to have the correct set of variables specified as independent and dependent to meet the modeling objectives. For monitoring objectives observed conditions, including the aforementioned independent variables (FICs, TICs, etc.) and many of the "normally" (for simulation and optimization cases) dependent variables (FIs, TIs, etc.) are specified as independent, while numerous equipment performance parameters are specified as dependent. These equipment performance parameters include heat exchanger heat transfer coefficients, heterogeneous catalyst "activities" (representing the relative number of active sites), distillation column efficiencies, and similar parameters for compressors, gas and steam turbines, resistance-to-flow parameters (indicated by pressure drops), as well as many others. These equipment performance parameters are independent in simulation and optimization model executions. 

If the pressure is fixed, there are 5 degrees of freedom. A mass balance allows specily the flow of distillate (D) (benzene) and intermediate product (P) (toluene) to a narrow interval. Therefore, although the kriging metamodel depends on 5 variables two of them can be almost fixed a priori, which increase the robustness of the model. The simulator (Hysys.Plant) forced us to specify the liquid (L) and vapor (V) streams that are withdraw from second column and returned to the first one. As remaining specification we chose the reflux ratio (RR). Initial values for these last three degrees of freedom, a reasonable interval of values as well as the number of trays of each column the feed and products tray positions, can be estimated using a shortcut method [6]... 

Let s consider a simple one-feed two-product distillation column. When examined as stand-alone item with a fixed feed, its control is typically a 5x5 multivariable problem. Among the five controlled variables three are for the basic inventory pressure, level in reflux drum and reboiler. The two remaining are for quality eontrol purity of distillate and bottom products. The five manipulated variables are distillate flow D, bottom product B, boilup V or reboiler duty Qr, condenser duty and reflux flow L. The combination of controlled and manipulated variables may lead to several control structures. Here we present some typical situations useful for dynamic simulation. 

Several of the control loops in Figure 21.35 are provided for inventory control, in three level-control loops and two pressure-control loops. Note, however, that the pressure in V-100 is assumed to be constant and loop PC-1 is not simulated by HYSYS.Plant. In contrast, pressure control is crucial to maintain stable internal flows in the column. Finally, because the feed flow rate and temperature controllers are decoupled from the rest of the process, they are not included in the C R analysis. Consequently, the interactions to be analyzed involve the four valves V-7, V-9, V-10, and V-12, and four controlled variables Xdj, JC/u (mole fractions of benzene in the distillate, MCB in the bottoms, and HCl in the absorber overhead stream, respectively) and Tg, the recycle temperature. Note that to improve the dynamic performance, the temperature of tray 4 is controlled rather than the distillate benzene mole fraction. 

As the dynamic simulation results will show, the preferred control structure depends on the control objectives of the entire process. For example, when the distillate goes to a downstream unit and large variability in its flow rate is undesirable, the control structure should control pressure with condenser heat removal, control level with reflux, and maintain a constant RR. 

In the design or operation of a distillation column, a number of variables must be specified. For both design and simulation problems we usually specify column pressure (which sets the equilibrium data) feed conposition, flow rate and feed tenperature or feed enthalpy or feed quality and tenperature or enthalpy of the reflux liquid. The usual reflux condition set is a saturated liquid reflux. These variables are listed in Table 3-1. The other variables set depend upon the type of problem. 

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