Distillation control scheme design using

Distillation control scheme design using steady-state models... 

DISTILLATION CONTROL SCHEME DESIGN USING STEADY-STATE MODELS 203... 

For the case study in this work with Ntray = 30 and NC = 2, there are 1316 DAEs in 1324 variables (101 states). The remaining eight variables consist of the three continuous design variables for optimization (column diameter, surface areas of the reboiler and the condenser), and the five manipulated variables (reflux, distillate, cooling water, steam and bottoms flow rates), whose values are determined by the tuning parameters and the set-points of the control scheme used. 

In the previous chapter the procedure for the design of control schemes was discussed. The procedure was illustrated on a reactor with recycle. The selection of appropriate combinations of controlled and manipulated variables was relatively simple, since the interactions were limited. In this chapter the procedure will be applied to a distillation column. This is a unit operation with many interactions between the corrections that are made. Using a basic knowledge of the process dynamics, a basic control scheme is designed. Subsequently, two control schemes will be compared a basic control scheme based on material balance control and a control scheme based on "energy balance control. The distillation column can also be used to demonstrate the optimization of the control scheme. The principle is that the control scheme should be designed in such a way, that an objective function can be maximized. 

Figure 3.14 shows the temperature profiles of the optimum designs for three different kinetic cases ( eq)366 = 1,2, and 5. The temperature profiles of each case are similar with higher temperatures for higher values of (A eq)366 because of the higher optimum pressures. There are fairly significant temperature breaks around tray 4 for all cases, and these tray temperatures can be used in control schemes to infer bottoms purity. A similar break occurs at different tray numbers near the top of the column for each case. These could be used to infer distillate purity. For all kinetic cases, the temperatures show little change in the reactive zone. 

While split-ranging is common in industry it does have some limitations. Figure 5.31 shows another method of controlling pressure in a distillation column, often used when vapour production is intermittent. In the absence of sufficient vapour the scheme is designed to allow a noncondensible gas into the column. So, on increasing pressure, the controller will first begin to close valve A until it is fully closed. If the pressure does not fall sufficiently, it will then begin to open valve B. 

There are several issues which need to be addressed when designing the basic control for the column. The first is one of pairing. We will see that on our simple column that there are five PVs that we must control - pressme, reflux drum level, column base level, distillate composition and bottoms composition. We normally have available five MVs - distillate flow, bottoms flow, reflux flow, reboiler duty and condenser duty. We need therefore to decide which MV is going to be used to control which PV. Theoretically there are 5 , or 120, possible combinations. While many of these are nonsensical, a large number of feasible schemes are possible. 

The column at this stage of the control design wdl have two MVs remainmg for use to control the composition of both products. Which these variables are depends on the choice of level control configuration. If the material balance scheme is in place then, usually, distillate flow and reboil are available or, less usually, bottoms flow and reflux. If the energy balance scheme is in place then reboil and reflux are available. 

Figure 7.7 has been designated by Doukas and Luyben as the L scheme. Sidestream drawoff location is used to control the concentration of the lightest component in the sidestream product. The D scheme of Figure 7.8 provides manipulation of the distillate product flow from the prefractionator to control sidestream composition. 

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