Periodic distillation methods are still largely applied to :-the distillation of crude oil, the redistillation of benzine (spirit, petrol, gasoline, naphtha, etc.) and kerosene fractions, the manufacture of lubricating oils, and of asphalts, paraffin wax, and petroleum coke. In nearly all of these cases continuous distillation methods may be, and often are, applied, but particularly in those cases where a residue of definite properties, e.g. lubricating oil or asphalt, is required, periodic methods afford easier, though more costly, operation.

Periodic oil distillation may be carried out (1) at ordinary pressures without the introduction of live steam; (2) at ordinary pressures with the introduction of live steam, or (3) with live steam under vacuum. The first is the method used for distilling certain crudes when large yields of benzine and kerosene are required, and when the residue is not required for cylinder oil manufacture. The second is the method in general use, and the third is as yet only employed occasionally, chiefly for the manufacture of lubricating oils. Distillation without steam and under increased pressure is also employed in certain "cracking" processes, the treatment of which lies outside the scope of this work.

When the distillation is carried out at ordinary pressures without the introduction of live steam, a certain amount of cracking or decomposition takes place, which is accompanied by an increased yield of the lower boiling fractions. The quality, however, of the distillates is thereby somewhat impaired, the colour is not so good, and a more intensive refining is necessary in order to produce good marketable products. The quality of the residue is also impaired from the point of view of manufacture of lubricating oils, the viscosity and flash-point both being reduced.

Distillation with live steam is the method usually employed, especially when decomposition of the oil is to be avoided, as in the case of lubricating oils and asphalts especially. The steam is usually admitted through perforated pipes placed near the bottom of the still. It assists not only in the agitation of the oil, thereby preventing long contact with the hot bottom of the still, but also in the actual distillation, in that the distillates come off at relatively lower temperatures than is the case when steam is not used. The decomposition of the higher boiling fractions is thus arrested, so that the nature of these fractions may be considerably varied by varying the quantity of steam. As a general rule, limited steam supply produces distillates of low viscosity and flash-point, of poor colour, and, in the case of paraffin wax, of low melting point and good crystallising quality ; ample steam supply produces distillates of high viscosity, high flash-point, and, in the case of wax, high melting point, but poor crystallising quality (owing partly to the more viscous nature of the oil associated with the wax in the distillate).

The difference between "dry" and "steam' distillates is well illustrated by the following: A certain Mexican crude was distilled in both ways. With steam, 0.9 per cent of light distillate of .848 sp. gr. was obtained, whereas without steam, 9.0 per cent benzine of sp. gr. .742 was obtained, and a further 10.8 per cent of distillate of sp. gr. .816. This is a particularly striking example, as the crude oil in question was evidently very liable to "crack."

Vacuum distillation of heavier fractions, in conjunction with ample use of steam, is sometimes employed for lubricating oil manufacture, very fine distillates of high viscosity, high flash-point, and good colour being so obtained. This method of distillation is, however, rarely employed for any other purpose.

As the plant and methods employed for dry distillation and steam distillation are very similar, the description of the latter method may be taken to apply to both. In the case of dry distillation the stills are often unlagged to allow of a certain amount of condensation and dropping back of distillate into the hot oil, but in the case of steam distillation the stills are always well insulated to prevent loss of heat.

"Periodic" Crude Oil Distillation. - As crude oils are so complex in character and contain components of such widely varying characters as, for example, light motor spirit and paraffin wax, efficient fractionating apparatus is not usually fitted to a periodic still which has to yield such a series of distillates from every run.

Modern periodic crude oil stills are usually cylindrical in shape (sometimes of egg-shaped or elliptical cross-section) of capacities up to 150 tons or more. The bottom is, when possible, composed of one plate. Large stills are often fitted with internal fire tubes like those of a Lancashire boiler. The still is provided with the following fittings (vide Fig. 129): - A cast steel draw-off pipe (1) fitted with internal and external valves; a steam spider line (2) perforated, lying just above the bottom of the still for the purpose of admitting live steam ; two or more manholes (3); a safety valve (4); gauge glasses (5); thermometer well (6); inlet pipe (7); dome and vapour pipe (8); and a pressure and vacuum gauge (9). The still is usually set on brickwork, so that the furnace gases after passing along the bottom may be returned along the sides, or diverted direct to the flue when the still contains small quantities of oil.

The capacity of such a still for crude oil varies, naturally, with the percentage distilled off. As a general rule it may be taken that 1 square metre of heating surface will give 11/2 tons of distillate per 24 hours, or

1 square foot will give about 1 barrel of distillate in the same time.

It is important that the vapour pipe be sufficiently large to allow of the vapours getting freely away.

Fig. 129.- Modern periodic crude oil still.

Such a still may be used for handling any grade of dry crude. In the case of heavy crudes, however, containing water in emulsion, difficulties will be met owing to foaming over. Such crudes are best handled by one of the types of continuous topping or dehydrating plants, described later (vide p. 345). The vapour pipe is often fitted with an oil arrester (Fig. 130), which is merely a cylindrical vessel fitted with perforated plates, rather like a water trap in a steam line, for the purpose of retaining any spray of crude oil carried over mechanically. Owing to the reduced velocity in the arrester, this spray settles down and is returned by a trapped pipe to the still. Such an oil arrester considerably improves the colour of the distillates obtained, and reduces to some extent the amount of redistillation necessary.

In distilling a crude oil the temperature is brought slowly up to about 130° C, and at this point live steam is usually admitted, in small quantities at first, the distillation being thereby much facilitated. By the use of live steam the distillates come off at temperatures lower by 50° C. or more than would otherwise be the case. The steam should be superheated to about the same temperature as that of the oil in the still, and the amount of steam used should be gradually increased as the distillation proceeds. When the higher lubricating or wax fractions are being given off, the rate of distillation is increased and more steam is given, so that the volume of water in the mixed oil and water condensate may be equal to that of the oil, and even, in the case of the last distillates taken off when reducing down to asphalt, three or four times greater.

Fig. 130. - Oil arrester.

The residue from such a steam distillation may be a liquid fuel, if the distillation has been stopped after removal of the kerosene fractions, or earlier; a steam refined cylinder stock if lubricating oils and wax have been distilled off; a waxy or asphaltic residue; or an asphalt to a particular specification, depending on the nature of the crude oil worked and the products desired.

In periodic working the distillation is often interrupted owing to the small volume of residue left, this being then pumped out to be transferred to other stills for completion of the process. This means a waste of time and heat, which can be obviated by continuous working methods. In the case of dry distillation the process is sometimes carried on further in special coking stills until the residue is completely coked, the coke being removed as soon as the temperature is low enough to allow men to enter the still. Such distillation to coke is very severe on still bottoms, so that an ordinary cylindrical still does not stand many runs before patching is necessary. In some refineries, therefore, cast-iron stills with hemispherical bottoms have been used for carrying out this distillation down to coke, the removal of the coke being sometimes facilitated by an iron anchor or grapnel placed in the still, which becomes embedded in the coke, which can be lifted out en bloc when the top of the still is removed.

The "cutting" or dividing of the fractions as they distil over into separate portions depends entirely on the crude oil worked and on the products required. The method can, therefore, only be indicated in a general way. The first runnings will probably be good enough to be put into light benzine distillate (for motor spirit). The laboratory distillation of the fractions will determine this in the first instance, and, after some experience, the specific gravity of the distillate will give a good working indication of the point at which a change must be made. The next distillates will go into benzine distillate for redistillation, as they will contain a percentage of heavier distillates which will eventually find their way, after redistillation, into kerosene. As soon as the flash-point or distillation test allows, the distillate will be run into kerosene distillate according to requirements. When the distillate becomes too highly coloured, or when it contains higher boiling fractions than are permissible in a kerosene (up to 300° C), then it may be run into a fraction for further redistillation, the distillate from which will go into kerosene and the residue to solar oil or liquid fuel. In practice the cuts are made according to the specific gravity of the distillate, the properties corresponding to various specific gravities having been previously determined in the laboratory. Thermometer control is also largely adopted, especially in continuous plants.

The reducing of lubricating oils and the manufacture of asphalt need not be here described, as the distillation is merely carried out in order to obtain a residue of properties conforming to certain requirements, with minor regard to the properties of the distillates. The ample supply and uniform distribution of the live steam plays an important part in such distillations.

Periodic stills are often fitted with some form of simple so-called dephlegmator (really an air-cooled condenser) the condensate from which is either separately cooled or returned to the still according to circumstances (Fig. 131). Such dephlegmators are usually simply cylindrical air-cooled vessels, in some cases fitted with baffle plates and perhaps with water coils so that the amount of distillate condensing therein may be varied.

By the use of one or more fractional condensers, much subsequent redistillation may be avoided, e.g. when the still is producing a heavy distillate towards the end of the kerosene cut, some gas oil may be eliminated which would otherwise require to be separated by redistillation from that particular fraction. As an example of the separation which may be expected with such simple fractional condensers, the following example may be given from actual practice, from a still fitted with two such dephlegmators of the type'. Fig. 131 C, with water cooling, in series.

Fig. 131. - Dephlegmators.

Three samples drawn simultaneously yielded the following results :1. Distillate from bottom of Dephlegmator I. Sp. gr. 0.840 Flash 80° C.

2. ,, ,, ,, „ ,, II. ,, ,, 0.816 „ 43 C.

3. ,, „ main condenser . ,, ,, 0.795 „ low.

The distillate from Dephlegmator I. was yellow in colour, while the other two were colourless. From this it is evident that the use of these simple fractional condensers avoids much redistillation. In the above case, for example, fraction 1 was run into kerosene-gas oil distillate for redistillation, fraction 2 was run into kerosene distillate, and fraction 3 into kerosene-benzine distillate for redistillation. Without these condensers the whole of the distillate would have been of bad colour and low flash and would have required redistillation in toto. Further examples will be found under "Continuous Distillation."

In place of the fractional condensers above described simple atmospheric condensers consisting merely of a series of horizontal or vertical pipes, each fitted with a drain from which the distillate is drawn off, are sometimes employed. As the vapours pass through the series of pipes, they condense, giving a series of distillates which become successively lighter and more volatile as the vapours pass along the condenser.

Various forms of condensers are used dependent to some extent on the nature of the crude oil to be distilled. Many crudes give off sulphuretted hydrogen during distillation, e.g. those of Mexico, Egypt, Persia, and those of certain American fields, and some crudes which contain chlorides give off hydrochloric acid. For use with such crudes condenser pipes of cast iron stand longer than those of wrought iron or steel.

Water is still generally adopted as the cooling medium, though this necessarily entails the loss of the heat given out by the vapours. To obviate this, crude oil-cooled condensers (heat exchangers or distillate preheaters) have been in many cases adopted. Details of these are given in the later chapters. For high boiling distillates such as paraffin wax and lubricating oils air-cooled condensers are often used, and in some modern plants direct contact spray condensers find application. The form of condenser still in most general use is the old type in which the vapours traverse a coil of pipes, arranged either as one continuous coil or as several coils in parallel, immersed in water. These pipes may be made of decreasing diameter downwards. The cooling surface provided should be ample, especially in the case of condensers for light distillates, in order to avoid loss of the more valuable lighter vapours by imperfect condensation. From one to three square feet of condensing surface per gallon of distillate per hour is the usual practice.

The quantity of water required for efficient cooling may be easily calculated if the temperature of the incoming vapour is known. For this purpose the specific heat of petroleum oils may be taken as roughly 0.5. The latent heats of petroleum oils are low, ranging between 60 and 80 (centigrade) calories. When the nature of the crude permits, economies may be effected by using condensers of the tubular type. In this form of condenser the water circulates through the tubes which are of mild steel. As the walls of these tubes are much thinner than those of the cast-iron tubes, the heat transmission is much better, though the liability to corrosion is greater and the life of the condenser is shorter. Such condensers, however, may be easily retubed.

A vertical condenser as depicted in Fig. 132, about 1 metre in diameter and 5 metres high with ] 50 x 3 cm. tubes, would have a cooling surface of about 80 square metres.

Simple forms of condensers or rather condensate coolers may be made of water-jacketed pipes, the ordinary types of pipes in general use in a refinery being available for the purpose.

In the case of high boiling fractions, lubricating oil, paraffin wax, etc., air-cooled condensers may be used. Such condensers are usually made of ordinary piping arranged either vertically or horizontally, the end of each pair of pipes being connected by bends. From each pair of pipes a separate distillate fraction may be drawn off.

The principle of direct contact is also sometimes made use of, jets of water being sprayed into cast-iron pipes or vessels containing the vapours to be condensed , the resulting mixture of oils and water being subsequently separated by settling. Such condensers are very simple in construction and are particularly useful in cases where the distillates contain acid vapours or sulphuretted hydrogen, as these are to some extent, if not completely, removed and washed out by the condensing water, the subsequent chemical treatment necessary being thereby somewhat cheapened.