TABLE OF CONTENTS The twoand four-stream examples used so far have been relatively simple situations. We will now introduce a third example which is based closely on a real process a small-scale fractional distillation of an organic mixture and is an existing plant, rather than a new design. In some respects this is similar to a smaller-scale version of the crude distillation unit described in Section 9.2. This will allow us to see stream data extraction at work in a practical situation.
The crude feed is supplied at ambient temperature and fed to a distillation column at atmospheric pressure, where it is split into three fractions: light oil, middle oil and residue. Naturally, the feed has to be heated up to the operating temperature of the column and in this case it must also be partially vaporised, as there is no separate reboiler; some flashing also occurs as the hot liquid enters the column. The light oil comes off the top as vapour (overheads), is condensed and the majority is recycled to provide the top reflux; the remainder is cooled and a small amount of water is removed in a gravity separator. The various products are cooled to different levels, depending on their viscosity. The crude feed passes through two heat exchangers, and is heated by the overheads and middle oil, before entering a furnace which brings it up to its final feed temperature. All other heating and cooling duties are performed...
