Reflux ratio stripping column sections

In contrast, the stripping section of the column is quite stable since the operating line in that section is a good distance from the equilibrium curve. Lowering or raising reflux ratio by reasonably small amounts causes minor changes in the bottoms composition. 

The number of trays in each column section (rectifying or stripping) is determined by the required fractionation in that section at a given L/V ratio. This ratio, which is directly related to the reflux ratio, is usually limited by practical considerations such as economically acceptable reboiler and condenser duties. Also, the reflux ratio is limited by tray hydraulics considerations, discussed in Chapters 14 and 15. Column flooding can occur if the vapor or liquid velocities become excessive. The total column trays and feed location are set once the number of trays in each section is known. 

An intermediate product such as the upper side draw, which is mostly propane, may contain impurities from components both lighter and heavier than the main component. The upper side draw is at the same time the bottom product of the top column section and the top product of the second section. The fractionation in the upper section determines to what extent ethane is stripped off from the propane product, and the fractionation in the second section determines to what extent butane is removed from the propane product. Again, in this situation since the number of stages in each section and the reflux ratio are all fixed, the fractionation is fixed. The propane recovery and purity depend mostly on its flow rate and on the flow rates of the adjacent products above and below it. If the propane product contains too much ethane, its flow rate should be cut back and the overhead rate increased. If the propane product contains too much butane, its flow rate should be cut back and the lower side draw rate increased. Table 9.14 summarizes the purities of components in different products at different flow rates. The recoveries can also be calculated from Table 9.14. The dependence of the other products compositions on their rates may be analyzed in a similar manner. 

Eor the purpose of estimating column pressure drops, vapor and liquid flow rates are estimated from the reflux ratio. Calculations are performed at two points in the column one in the rectifying section and the other in the stripping section. 

In Table 1, the column entitled "reflux ratio" is the external reflux ratio -i.e., the ratio of liquid condensate returned as reflux to that kept as product. Having chosen this ratio and the concentration of stillage, feed and product, the ideal number of stages required in the rectifying and stripping sections, as well as the energy, are then determined. 

Total reflux of type 1 may be approached in a continuous distillation column by approaching total reflux conditions in both the rectifying and stripping sections. The designer approaches this type of operation of a continuous distillation column as the reflux ratio is increased indefinitely at a fixed feed rate and nonzero product rates. In the limit, this type of operation is recognized as total reflux of type 1, continuous distillation columns at total reflux. The necessary conditions for an equivalence to exist between columns at the operation conditions of total reflux of types 1 and 2 are presented in Sec. 10-2. Total reflux of types 1 and 2 are of significant interest because columns at these types of operation produce the best possible separations. 

Consider the case of minimum reflux ratio (infinite stages). As the amount of solvent is reduced, point M (equal to S + F) in Fig. 11.9 moves towards F, and P (equal to D + So) moves towards D. Point P" (equal to B - S) moves away from the equilibrium curve. The maximum distance that points M, P, and P" can be moved is determined by the slope of the tie lines. The minimum solvent ratio, which corresponds to the minimum reflux ratio, is reached when a tie line and an operating line coincide. A pinch point can occur either in the enriching or in the stripping section of the column, so it is necessary to seek the highest value of the minimum reflux ratio by trial and error. In this example, it occurs at the feed stage. The minimum reflux ratio is 0.58 and the corresponding minimum solvent ratio is 0.74. 

It is also useful to note that the generalized reflux in the rectifying section, R r, reduces to exactly the classical reflux ratio of r = IJD. However, the reflux ratio in the stripping section, is almost always expressed as a reboil ratio in classical column design as 5 = V B. In this case, the relationship between the classical parameter and our generalized one is given as R/ s = — 5 — 1. 

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