Light ends fractionation

Fig. 9. Xylenes separation via Mitsubishi Gas—Chemical Co. HF-BF extraction—isomerization process (107). A = extractor B = decomposer C = separator D = isomerization reactor E = heavy ends tower F = raffinate tower G = separator H = light ends fractionator ...

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

The overhead products from light ends fractionators are frequently gases, or a mixture of gas and liquid, at normal atmospheric pressure and 100°F. (an average temperamre for products condensed by cooling water). To avoid the... 

This section covers the various steps used to recover and separate into useful products the desirable light ends fractions present in the large volumes of effluent gases produced by the various refinery processes. The recovery is usually accomplished by a combination of compression and absorption. The subsequent separation into useful fractions is invariably carried out by distillation, usually in combination with distillates in the gasoline boiling range which are recovered with the light ends fractions. 

The crude phthalic anhydride from tank (246) is heated as it passes through preheater (260) before it enters decomposer vessel (258). The bottoms from decomposer vessel (258) are sent to decomposer vessel (262) via conduit (259). Crude phthalic anhydride from decomposer vessel (262) is then pumped through cooler (266) to light ends fractionation colunrn (268) (i.e., first distillation tower) via pump (270). ... 

The vapor generated from decomposer vessels (258) and (262) is piped via steam-traced conduit (272) directly to the top portion of light ends fractionation coluttm or first distillation tower (268). ... 

Of course, at steady state, the water must eventually leave the tower, but not before a separate water phase is formed. The water phase accumulates on the tray decks, possibly as high as the tray weir. Indications of free water in a lights-ends fractionator are ... 

A light ends fractionation unit can be found in several locations within the refinery processing scheme. Many refineries will have several units in this service, their multiplicity being dictated by being keyed into various types of processing units. They are always found in the following places. 

As pointed out earlier, separation criteria are somewhat less stringent than in pure-component light ends fractionation. Establishing the design separation for a naphtha debutanizer, for example, might reduce to a study of how much butane can be left in the bottoms without exceeding vapor pressure specifications on the gasolme. If, however,... 

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