Vapor split

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... 

It was known long ago that a discharge in water vapor splits water into OH and H. The properties of OH in the zone of electric discharge have been discussed i n a great number of papers. The first works in which hydroxyl was detected in the zone of discharge in water vapor by means of its absorption spectrum were those of Oldenberg23 and by Kondratiev and Ziskin.17... 

In this calculation, T and P are known, but the liquid-phase mole fractions ( ci), the vapor-phase mole fractions (j l). and the liquid-to-vapor split (L/ V) are unknowns. 

Similarly for the side rectifying column, a vapor split ratio... 

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 divided-wall column has many degrees of freedom at the steady-state design stage. The number of stages in the four different sections of the column, the locations of the feed and sidestream withdrawal points, and the location of the wall are seven of the parameters that must be specified and are all fixed by the physical equipment at the time of construction. They cannot be changed during operation. The location of the wall fixes how the vapor splits between the two sides of the wall, so the vapor split is not adjustable during operation for control purposes. 

Many papers discuss the steady-state design issues and propose heuristic and rigorous design optimization methods. Design of a divided-wall column is more difficult than a simple conventional column because of the interaction between the many design optimization variables number of total trays, number of trays in the wall section, feed and sidestream locations, vapor split, and liquid split. In addition, the purity (or impurity) specifications of the three product streams must be satisfied simultaneously. 

A comparison of Figure 12.1 (the RadFrac design) and Figure 12.14 (the MultiFrac Petlyuk design) shows that they are almost the same with only slight differences in the reboiler duties, reflux ratios, liquid splits, and vapor splits. 

After the compressor in the stripper vapor line, the vapor line splits into two lines, one going to the prefractionator and the other going to the sidestream side of the wall. Control valves were inserted in both lines. A ratio control system is used to keep the vapor split constant. The total vapor from the stripper is determined by the compressor (to hold pressure in the stripper) and is measured. This flow signal is sent to a multiplier, whose other input is the desired ratio of vapor to the prefractionator to total vapor (the vapor split ratio). The output of the multiplier is the set point signal of a flow controller that controls the flow of vapor to the prefractionator by changing the position of the control valve in the line. 

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