James Kewley, M.A. F.I.C. Memrer Of Council Of The Institution Of Petroleum Technologists.

Chapter XXIX. Introduction

From time immemorial petroleum products, in the first instances crude oils and natural bitumens, have been employed in the service of man, but the real development of the industry, which has now assumed such large proportions, dates from the application of distillation methods to the working up of crude oils, and distillation, in one form or another, still affords the most practical method of splitting up crudes into their commercial products. Although many and great modifications of the original process have been developed, it can hardly be claimed that really accurate or intensive fractionation as yet finds any extensive application in the petroleum industry. This is not surprising, seeing that (1) the industry is now only in its infancy, only a limited number of products, generally boiling over considerable ranges of temperature, being in demand ; and that (2) crude oils are very complex mixtures of hydrocarbons, the boiling points of which, in many cases, lie close together, so that the extraction of any one of them by distillation alone becomes an exceedingly difficult and quite impracticable operation.

Crude petroleums are found in many countries, occurring in various geological formations, the most prolific deposits being generally of tertiary age. A discussion of their origin is beyond the scope of this volume. They differ widely in character, ranging from very light, slightly coloured, naturally filtered oils such as have been found in Western Canada, Russia, and elsewhere, to very heavy asphaltic oils containing little or no volatile fractions, such as occur in Mexico, and even to natural asphalts such as those of Trinidad and Venezuela and the natural waxes or ozokerites. Some of the light crude oils of Sumatra contain as much as 40 per cent of benzine and 50 per cent of kerosene, whereas many heavy crudes contain no benzine or kerosene fractions whatever.

Crude oils differ not only in the relative proportions of the various commercial products which they contain, but also in the chemical composition of these products. Certain crudes, e.g. those of the Eastern fields of the United States, are composed primarily of hydrocarbons of the paraffin series; certain Russian crudes and others are composed largely of hydrocarbons of the naphthene or alicyclic series ; certain others consist of paraffins and naphthenes in varying proportions in admixture with hydrocarbons of the aromatic series, those of

East Borneo being conspicuous in this respect. Crude oils show much variation also in respect of the content of sulphur compounds. These are generally regarded as impurities, but, in the higher boiling fractions and residues especially, they must be regarded as intrinsic components of the crude. While the chemical composition of the lighter components of many crudes has been fairly well worked out, it is safe to say that little is so far known of the chemical nature of the components of the higher boiling fractions, and practically nothing of that of the residues and of the heavy asphaltic crudes.

Of recent years fractional distillation has been to some extent supplemented by fractional condensation, a process in which the requisite fractions are distilled off from the crude oil en bloc, and separated during the condensing process. This method possesses certain technical advantages, the chief being the smaller and more compact plant required for any given throughput of crude oil and the lower operating costs. It finds its chief application so far in the distillation or topping of crude oils containing only small percentages of volatile fractions.

Further, of recent years, owing to the steadily increasing demand for light petroleum fractions, systems of destructive distillation (so-called "cracking processes ") have been introduced. This method of treatment is, however, beyond the scope of this work.

The distillation of petroleum may be conveniently considered under two main headings, viz. periodic and continuous methods. Periodic methods are the older and have been largely replaced in modern refineries by continuous methods, which offer many advantages, although for certain purposes, e.g. the manufacture of special spirits of narrow boiling point ranges, of lubricating oil and of asphalts, the periodic methods are still largely in vogue. There is little doubt, however, that in course of time, except for such operations as distilling down to coke, continuous methods will eventually be almost entirely adopted. As the early forms of distilling apparatus were naturally crude and inefficient, and are now merely of historical interest, little need be said of them.

In consequence of the great differences in character displayed by various crude oils, different methods of treatment are called for, but as this work deals with the processes of distillation involved, rather than with their application to various types of oil, detailed descriptions of methods of working up of different crudes would be out of place. A few remarks on this subject, however, may not be inappropriate.

Crude oils may be roughly subdivided into three classes, viz. asphalt base oils, paraffin (wax) base oils, and mixed base oils, each of which may be divided into light and heavy crudes. Several types of crude may occur in the same field, but at different geological levels. In general, however, any one field produces a particular type of crude, e.g. the crudes of the famous Russian fields on the west shore of the Caspian Sea are of the light asphalt base type, containing little or no paraffin wax ; those of Burmah are of the light paraffin wax type ; while those of Mexico belong chiefly to the heavy asphalt type.

The method of working up of crude oils depends to a large extent on their nature, and on the nature of the commercial products which they will yield. Thus certain crudes are used direct as liquid fuel in their natural state, others require the removal of a small percentage of light fractions, in order to raise the flash-point of the residue to liquid fuel standard, while others must be subjected to a fairly complete distillation and subsequent redistillations, in order to separate off the relatively small proportion of residues they contain, and split up the distillates into their commercial products.

A method of working up of an asphalt-base crude oil is set forth in the diagram.

Diagram illustrating method of working up of an asphalt-base crude oil.

The working up of paraffin wax base crude oils is more complicated, owing to the filtration and working methods involved in the separation and purification of the wax.

For diagrams illustrating the working up of various typical crudes reference may be made to Engler-Hofer, Das Erdol, vol. iii. ; Bacon and Ham'or, The American Petroleum Industry, vol. ii., etc.

References

For general information on the petroleum industry reference may be made to: Abraham : Asphalts and Allied Substances.

Bacon and Hamor : The American Petroleum Industry.

Campbell : Petroleum Refining.

Engler-Hofer : Das Erdol.

Redwood : A Treatise on Petroleum.