Reflux ratio side rectifiers

The optimization can be carried out using nonlinear optimization techniques such as SQP (see Chapter 3). The nonlinear optimization has the problems of local optima if techniques such as SQP are used for the optimization. Constraints need to be added to the optimization in order that a mass balance can be maintained and the product specifications achieved. The optimization of the side-rectifier and side-stripper in a capital-energy trade-off determines the distribution of plates, the reflux ratios in the main and sidestream columns and condition of the feed. If a partitioned side-rectifier (Figure ll.lOd) or partitioned side-stripper (Figure 11.lid) is to be used, then the ratio of the vapor flowrates on each side of the partition can be used to fix the location of the partition across the column. The partition is located such that the ratio of areas on each side of the partition is the same as the optimized ratio of vapor flowrates on each side of the partition. However, the vapor split for the side-rectifier will only follow this ratio if the pressure drop on each side of the partition is the... 

To redistribute the stages in the remaining sections, a shortcut simulation is used to find out the required number of trays, the feed tray location and the minimum reflux ratio for each column in the sequence. To make use of the existing column with the same number of trays (24 trays) iterations are required to adjust the sum of the rectifying sections in each column equal to 24 (number of trays in the main column). Finally, the sequence of the simple columns is merged into a complex column. The main column is not changed, but the side strippers and pump arounds need to be relocated or adjusted. 

With a normal equilibrium curve it is seen from Fig. 18.19 that at minimum reflux ratio the intersection of the q line and the equilibrium curve gives the concentrations of liquid and vapor at the feed plate (and at an infinite number of plates on either side of that plate). So an invariant zone forms at the bottom of the rectifying section and a second one at the top of the stripping sec tion. The two zones differ only in that the liquid-vapor ratio is LjV in one and LjV in the other. 

In the same way, for the side rectifier, aU refluxes can be written in terms of the bottommost reflux (/ a3) and the vapor split ratio, given in Equations 6.16 6.18... 

The two internal degrees of fi eedom of each column have been specified through the appropriate split ratio and a single, reference reflux. Both Rm in the side stripper and Rm in the side rectifier have been chosen as the respective reference refluxes since they lie the closest to the thermally coupled sections. This choice is of course arbitrary, and one may equivalently choose any CS to serve as the reference CS. 

Let us first treat the three internal flow variables required to be specified in the Petlyuk. Satisfying this requirement essentially means setting the reflux ratios in three CSs. Again it is useful to specify a reference reflux. For this purpose it is useful to specify the reflux ratio in either CSj or CSe, but we will choose to set / ai throughout this chapter. The two remaining flow variables are thus associated with the internal CSs (2 5), and therefore require that the reflux ratios in any two of these CSs be set. This is not an easy task, because refluxes can be positive or negative, and their magnitudes are not bound. Thus, just as with side-rectifiers and... 

On the other hand, when applied to multistage operations, the concept of F pertains to the introduction of both V and L (that is, to V + L) to the reject side of the membrane, where V and L originate from the adjacent membrane cells, that is, from the posterior and anterior cells. Thus, the so-called internal reflux ratio or recycle ratio L/V can be used to establish a value for V/F, from which a value for the permeate flux V" could in turn be calculated by the same methods of Chapter 3. The value of V" so determined is the uniform and constant permeate rate at each stage in the rectifying section. This would supersede the determination of the permeate flux V" based on the feedstream per se. 

In addition, there are four degrees of freedom that are adjustable during design and are also adjustable during operation of the column reflux flow rate (/ ), vapor boilup (V), sidestream flow rate (5), and the liquid split ratio (jSl = i-p/i-R)- The variable Lp is the liquid flow rate fed to the prefractionator side of the wall, and Lp is the total liquid leaving the bottom tray in the rectifying section. Of course, the rest of the liquid coming from the bottom of the rectification section is fed to the sidestream side of the column. Distillate and bottoms flow rates are used to maintain liquid levels in the reflux drum and column base, respectively. 

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