Table 117

Each still, or pair of stills, is fitted with a distillate-crude oil heat exchanger or distillate preheater. This is constructed somewhat like a steam-heated benzine still. The form is that of a normal still (Fig. 140) with inlet and outlet pipes a, b, dome and vapour pipe c, draw-off pipe D, and the usual fittings. Two or more nests of pipes are fitted with their ends opening into vapour boxes at the end of the still. The vapours enter at inlet pipe E, and after passing through the two nests of tubes emerge partially or wholly condensed at F. The condensate is led to a cooler and the vapours are further condensed in a separate condenser. The vapours given off from the distillate preheater are condensed in the usual way. Each of these distillate preheaters is thus virtually a crude oil still fired without any extra fuel consumption. The path of the crude oil through such a system is diagrammatically represented in Fig. 141 : a-f, crude oil stills ; G, crude oil residue heat exchanger; hij, distillate-crude oil preheaters (functioning as condensers for distillates from main stills and as preliminary crude oil stills); k-p, coolers for distillates from main stills; Q-s, coolers for distillates from distillate heat exchangers; t, cooler for distillate from crude oil residue heat exchanger.

The crude oil enters the system at the heat exchanger G, where it is heated by the residue flowing away from the last still ; it then flows through the distillate heat exchangers, where it is partly distilled by the heat of the vapours from the main stills, these being thereby partly condensed. It then flows on through the continuous stills in the usual way. The portions of the distillate vapours from the main stills which are condensed in the distillate heat exchangers are separately cooled, the uncondensed portions being separately condensed and cooled. Each of the five main stills in such a bench gives, therefore, two fractions, (a) the fraction condensed in the dis-

Fig. 140. - Distillate-crude oil heat exchanger or distillate preheater.

Fig. 141. - Path of crude oil through bench of stills fitted with distillate preheaters.

tillate heat exchanger, (b) the fraction which escapes condensation there, but which is condensed by water, and in addition each distillate heat exchanger gives a separate fraction. Such a bench, therefore, of 5 stills and 3 distillate heat exchangers would yield at least 13 distillates.

With such an arrangement a crude oil yielding over 70 per cent of distillates can be distilled with a fuel consumption (oil fuel) of below 1.5 per cent. The difference in character of the distillates which are condensed and which escape condensation in the distillate preheaters is shown by the following analyses: -

Dl in each case represents the vapour which passes on uncondensed, and D2, the portion condensed in the distillate preheater.