US2149943A - Fractionating method - Google Patents

Description

Patented Mar. 7, 1939 UNITED STATES FRAGTIONATING METHOD Johannes Hendrik Willem Rost van Tonningen,

The Hague, Netherlands, assignor to Shell Development Company,

San Francisco, Calif., a

corporation of Delaware Application December 23, 1935, Serial No. 55,736 In the Netherlands January 4, 1935 3 Claims.

This invention pertains to methods of fractional distillation and is more specifically con.- cerned with the treatment of fractions withdrawn from a fractionating column as side streams from 5 points intermediate its upper and lower ends.

The sharpness of separation of these fractions is determined by various factors within. the main columns, such as the amount of reflux, the length of the column, the number of plates, etc. and

cannot usually be controlled independently of these factors.

Since the boiling ranges of these fractions, even when determined by such relatively inaccurate method as the A. S. T. M. method, are often found separation effected is insufliciently sharp, it is customary to subject these fractions to a further treatment in secondary fractionating columns known as sideor steam-strippers, the open steam used in these columns serving to promote the separation of the lighter components.

It has, however, been found that even when using direct superheated steam as a stripping agent, a suiiicient amount of heat cannot usually be supplied without employing excessively high temperatures. According to known practice, particularly when fractionating relatively high-boiling hydrocarbon oils of the type of lubrication oils, the temperature gradient in such direct steam rectification of side streams decreases toward the bottom of the steam-strippers, since the heat necessary for the evaporation of the lighter components is abstracted from the liquid itself, and an effective separation of undesirable components from the bottom product becomes impossible.

It is the object of this invention to provide a method for separating a liquid mixture, such as mineral oils, into non-overlapping fractions with sharply defined boiling ranges.

It is another object of this invention to provide a simple and economical method for re-boiling liquid products of distillation.

These and other objects of the invention will be understood from the following description, taken with reference to the drawing, in which Figs. 1, 2 and 3 diagrammatically represent several preferred embodiments of the invention, the same numerals being used throughout to designate the same elements.

Referring to Fig. 1, a conventional fractionating column I, into which the feed is admitted through a line 2, is provided with the usual bubble trays 3, heating means shown in the form of a heating coil 4, and a condenser 5. A line 6 is used for the circulation of the reflux. Open steam may be supplied by means of line 3. A liquid side-stream is withdrawn from the column I by means of a line I and admitted to the top of a secondary column I I. Although only one secondary column is shown on the drawing, it is to overlap, which indicates that the degree of obvious that several side-streams may be withdrawn from the main column and treated in a. plurality of secondary columns. The secondary column is shown provided with bubble plates I2.

The liquid admitted through line I flows downwardly through the column II, and the heaviest fraction formed during secondary fractionation accumulates at its bottom. It is removed therefrom through line i5 and pump I6, and passed through a heating apparatus, such, for example, as a heat exchanger H, where it is heated, for instance, by means of the hot residue from the main column to a temperature above that in the conduit 7. Instead of the heat exchanger I'I, any other type of heater, such as a furnace, etc., may be used to supply any desired amount of additional heat to the liquid withdrawn through pipe l5. The heated liquid is then returned to the secondary column through pipe I8, at a point above the liquid level at the bottom of column I I. The lighter components are thereby flashed oil and separated from the mixture.

Since the amount of heat supplied to the material re-circulated through the reboiler I I can be sufiiciently great, it will be noted that the temperature within the column I I can be readily made to rise towards the bottom of this secondary column, which condition is the reverse of the one usually prevailing in so-called steamstrip-pers in which only an. insuflicient amount of heat, if any at all, can be supplied by the steam injected thereinto.

In view of the fact that during the process of recirculation described above a part of the residue formed in the secondary column is also continuously drawn off through pipe 2! as a final product, it is of importance to insure, as far as possible, that all of this final product is freed of the light components present therein and, therefore. actually passed through the reboiler IT.

For this purpose, the space at the bottom of the secondary column, wherein the secondary residue collects, is divided into two compartments A and B, by a vertical partition 22, extending above the liquid level, and spaced from the lowermost plate to permit vapor communication between the compartments; if desired, this partition is provided with an opening 23 below the liquid level. The liquid flowing down the plates of the column is delivered by the downspout I3 only to compartment A, which is also connected to the intake of pump I5. This pump is designed to handle greater quantities. of liquid than can be supplied through the downspout I3. As a result of this, the flow through the orifice '23 is in the direction from B to A, and only liquid which has already been recirculated through heater l1 and flashed in compartment B will be removed through line 2! from compartment B to storage.

When it is preferred to effect the recirculation of the liquid by gravity only, thereby dispensing with pump [5, the arrangement of Fig. 2 has been advantageously used. The primary column, similar to that of Fig. l, is only partially shown in Fig. 2. In this case, the liquid flowing down the plates of the secondary column I I is not allowed to run directly into the residue accumulating at the bottom of this column, but is conveyed by conduit I3 into a special deep tray 24, whence it flows by gravity through line 15 to heater I1, and is returned to the column by line l8. The inlet of pipe I8 is located considerably lower than the ofitake of pipe l5 to provide an effective hydraulic head.

It may sometimes be possible to dispense also with the tray 24, in which case there is no liquid communication between the lowermost bubble plate and the space below that plate, the liquid being withdrawn from the lower plate by means of line l5a and recirculated as above.

The methods and apparatus described above are concerned with the production of sharply separated fractions with initial boiling points strictly conforming to desired specifications. These fractions are obtained as the bottom product of the secondary fraotionating columns, after stripping ofi the lighter components.

These lighter components, however, often contain a certain percentage of the heavy components which are not condensed when the vapor fraction leaves the secondary column through line I9. If it is desired to free this vapor fraction entirely from the heavy components, it is necessary to subject it to dephlegmation. This may preferably be achieved by using asomewhatlonger secondary column consisting of two sections: a dephlegmating section above the entrance of the feed, provided with a reflux condenser, and a stripping section below the entrance of the feed, provided with a reboiler.

The apparatus in which such process may conveniently be conducted is shown in Fig. 3. The secondary column II is shown provided with a condenser 21, from which the condensate formed is delivered as reflux to column ll through line 28, and/or to storage through line 26, and/or to the main column through line 29.

Since it has been found preferable to effect the recirculation of the heaviest fraction formed in the column of Fig. 3 without auxiliary pumps, the space below the lowest plate of column II is divided into two compartments A and B by a solid partition 22, the level of the liquid filling the bottom of the secondary column being kept at a higher level in compartment A than in compartment '3. A downspout l3 conveys the liquid from the lowest plate to compartment A, and a line I5 is provided to convey the liquid from compartment A through a heater I1, and to reintroduce it through line l8 provided with valve means 30 adapted to maintain the liquid in compartment A at a higher level than that in compartment B into the compartment B of column I l at a point above the level of the liquid in compartment B, but below the level of the liquid in compartment A, so that the recirculation is effected by hydrostatic pressure only.

It is evident that open steam, admitted to the main column through line 8, and to the secondary columns through line Ill, may be used, if desired, to modify the distillation according to the present invention. The distillation may also be carried out under vacuum or at any desired pressure, it being preferable to operate the main and the secondary columns at the same absolute pressure to avoid difiiculties in transferring fractions from one column to another.

I claim as my invention:

1. In the process of fractionally distilling a liquid mixture by means of a fractionating column, the steps of withdrawing a portion of said mixture from the column intermediate its highest and lowest points, introducing the withdrawn portion into a secondary column, subjecting it therein to redistillation to produce a vapor fraction consisting of the most volatile components and a liquid fraction consisting of heavy components and containing a small amount of volatile components, maintaining two pools of liquid at the bottom of the secondary column, allowing the liquid descending through the secondary column to collect in the first pool, preventing any of the liquid collected in-the first pool from passing directly to the second pool, withdrawing from the secondary column a portion of the liquid collected in the first pool, heating the withdrawn portion to a temperature above that of the feed to the secondary column, returning the heated portion to the vapor space above the second pool, flashing off the volatile components contained in the heated portion, returning said components to the main fractionating column together with the vapor fraction formed in the secondary column, allowing the heavy components of the heated portion to collect in the second pool, and withdrawing a portion of the liquid in the second pool from the system. a

2. In the process of fractionally distilling a liquid mixture by means of a fractionating col umn, the steps of withdrawing a portion of said mixture from the column intermediate its highest and lowest points, introducing the withdrawn portion into a secondary column, subjecting it therein to redistillation to produce a vapor fraction consisting of the most volatile components and a liquid fraction consisting of heavy components and containing a small amount of vola-i tile components, maintaining two pools of liquid at the bottom of the secondary column, allowing the liquid descending through the secondary column to collect in the first pool, passing a liquid stream from the first pool to the second pool through a heating zone outside the secondary column, heating said stream to a temperature above that of the feed to the secondary column, discharging said stream into the vapor space above the second pool, flashing of! the volatile components contained in said heated stream, returning said components to the main fractionating column together with the vapor fraction formed in the secondary column, allowing the heavy components of the heated stream to collect in the second pool, maintaining adirect stream of liquid within the secondary column from the second pool to the first pool, and withdrawing a portion of the liquid in the second pool from the system.

' 3. In the process of claim 1, effecting the recirculation of the liquid material from the bottom of the secondary column through the heating step only by means of gravity flow.

JOHANNES HENDRIK WILLEM ROST VAN TONNINGEN.

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