Stripping vapor rate, increase

Too much feed preheat, therefore, can make it impossible to meet bottoms product specifications. Thai s why experienced operators hold a constant feed temperature. Unfortunately, for certain systems a very large increase in percent vaporized (and hence enthalpy) occurs for a small change in temperature. As an example, consider a tower with a feed composition of 2% propane, 72% butane, 24% pentane, and 2% hexane. A few degrees extra preheat in this tower s feed caused a large increase in the amount of butane vaporized. This led to a considerable drop in stripping vapor rate and a consequent increase in butane in the bottoms pentane product. 

The steady-state vapor and liquid rates are constant through the stripping and rectilying sections because equimolal overflow is assumed. However, these rates change through the reactive zone because of the exothermic reaction. The heat of reaction vaporizes some liquid on each tray in that section therefore, the vapor rate increases up through the reactive trays and the liquid rate decreases down through the reactive trays. 

This contradicts common sense that flooding is promoted by higher vapor rates. But I have stopped flooding in a crude distillation tower recently in a Kansas refinery by increasing the stripping steam rate. Also,... 

Hence by doubling the vapor rate on this particular tower we increased the amount of ethane stripped out from the feed from 80 percent to 98 percent, so we increased the amount stripped out 18 percent based on the amount of ethane in the feed. 

Dilute (1—3%), chloride-containing solutions of either HOCl, hypochlorite, or aqueous base, can be stripped in a column against a current of CI2, steam, and air at 95—100°C and the vapors condensed giving virtually chloride-free HOCl solutions of higher concentration in yields as high as 90% (122—124). Distillation of more concentrated solutions requires reduced pressure, lower temperature, and shorter residence times to offset the increased decomposition rates. 

Once in the column, volatile compounds vaporize and less-volatile liquids drop to the bottom of the column. Reflux begins and oil fractions condense and collect in various trays throughout the column. Heating within the column can influence the rate of reflux and holdup within the column. Steam is introduced into the column at the level of the lower trays and strips lighter compounds from the condensed fractions on these trays. As a result, the flash point of the fractions remaining on the lower trays increases. 

The 200°F stripper tower-top temperature is the dew point of the vapors leaving the top tray. Most of these vapors are steam, and that is why the tower-top temperature is so high. The high steam content of the overhead vapors causes a water stripper to behave in a strange way When the top reflux rate is increased, the tower-top temperature goes up, not down. This odd behavior is easily understood if we note that there is no liquid product made from the reflux drum. Therefore, the only way to increase the reflux rate, without losing the level in the reflux drum, is to increase the steam rate to the bottom of the stripper. The extra stripping steam drives up the tower-top temperature. 

Side columns are used, for instance, in the most important distillation processes worldwide, the fractionation of air (see Fig. 11.2-18) and the distillation of cmde oil (Meyers 1996). The atmospheric tower of oil refineries consists of a main column and four stripping side columns (Fig. 11.2-12). In this tower the crude oil is split into six fractions which are processed further in several subsequent columns. Oil refineries also have some other interesting features. Steam is fed into the bottom of the main column and most of the side columns. This causes a stripping effect and reduces the temperatures in the columns (steam distillation). The overhead fractions of all side columns are fed into the main colunrn thus increasing the vapor flow there. So-called pump arounds effect a partial condensation of the vapor in the main column and, in turn, a reduction of the vapor flow rates in the upper sections. 

Part of diesel fuel has to be recovered not in an atmospheric unit but in a vacuum one. This creates a double effect (1) in the feeding of a vacuum column the fraction of light components increases, which, due to their stripping influence, increases recovery of lubricant fractions or gas-oil and (2) the volatilities of components grow under vacuum, which separates diesel fuel from heavier products more sharply and deeply. Figure 8.34 shows the calculation dependence of yield of lubricant fractions in dry vacuum column on the fraction in its feeding of components with bubble temperatures below 360°C. However, increase of vapor flow rate in vacuum column requires application of more productive contact devices. 

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