US2018778A - Method and apparatus for vacuum distillation of liquids - Google Patents

Description

Oct. 29, 1935. K, EBNER 2,018,778

METHOD AND APPARATUS FOR VACUUM DISTILLATI ON OF LIQUIDS Filed June 14, 1952 Patented Oct. 29, 1935 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR VACUUM DISTILLATION 0F LIQUIDS Application June 14, 1932, Serial No. 617,172

Germany June 25, 1931 3 Claims. This invention relates to a method or and apparatus for distillingwith or without the simultaneous performance of chemical reactions-substances that are liquid at distillation temperatures.

If substances that are either liquid, or behave as liquids, as distillation temperature are to be distilled, provision shouldpreferably be made to ensure intimate intermixture of the material under treatment. Such procedure accelerates the distillation and enables a uniform temperature to be maintained, as far as is possible, in all portions of the contents of the still, thereby minimizing the possibility of local overheating that -may give rise to undesirable decomposition, espe-.

cially at the high temperatures necessarily em ployed in the distillation, for example, of mineral oils or in the expulsion of fatty acids from oils, fats or the like. The mixing is effected, for example, by passing steam, or gases such as hydrogen, carbon dioxide and the like, through the contents of the still. These vapours and gases have already been employed to keep the contents of the still in continual motion, for example by lifting and finely distributing the material under treatment, in the vapour space of the still, by means of steam, on so called gas-lift principle. At the same time, the admission of gases or vapours lowers the partial pressure of the distillation vapours, and thereby lowers the distillation temperature. Consequently, owing to the lower working temperature, greater protection is accorded to the material to be distilled -especially during vacuum distillation and better distillation products are obtained. a

On the other hand, the admission of steam, hydrogen or the like prevents the distillation from being performed under a hgh vacuum. It is true that, for example in the distillation of mineral oils or fatty acids, the distillation vapours could be collected separately and condensed, without further precautions at temperatures at which their vapour pressure is as low as 1 mm. of mercury, or even lessUOn the other hand, however, the condensation of the steam is dependent on the temperature of the available cooling water, so that, even when the conditions in respect of cooling water are favourable,

and eflicient condensers are employed, the absolute pressure in the distillation plant cannot be less than 40-30 mm. of mercury gauge. If, during the distillation process, very large quantities of steam be admitted in order to obtaina very low partial pressure of the distillation vapours, the. drawback isencountered that, in consequence of the presence of the steam, the distillate condenses with difliculty, or condensed steam is included, as water, in the distillate. Although, by the extensive admission of steam, it is possible to reduce the partial pressure of the distillate 5 vapour to a minimum, it is preferable, for this and economic reasons, to operate with a somewhat higher pressure and smaller amount of steam. It is true that the vacuumin the distillation plant can be increased by compressing, 10 before passing it to the steam condenser the steam that remains after the condensation of the distillate vapours. However, the initial and operating expenses of the compression plant very quickly imposes on the increase of the vacuum a 15 limit beyond which the process ceases to be economical. Similar conditions exist in the employ- -ment of gases for the circulation and the reduction of the partial pressure of the substance to be distilled.

Moreover, it is already known to distil mineral oils (for example) without using steam and the like, by distributing the material under treatment in the form of a thin layer in the vacuum chamber. The formation of distillate vapour during the distillation produces a further atomization of the product, so that a very large evaporative surface results. Since no steam or the like is admitted into the plant, the task of the air pump in maintaining the vacuum is restricted to the evacuation of any air that may leak into the apparatus, and of any other uncondensable gases that may be present or formed. Nevertheless, in carrying out this method in practice, difiiculties arise because the distribution of the charge in the vacuum chamber necessitates the employment of pumps for circulating the liquid, which pumps frequently give rise to working troubles in consequence of the high temperatures and the resulting heavy wear of the stuffing boxes. More 40 over-for example in the final stages of the fractional distillation of mineral oils-the viscosity of the residue is so high that the. circulation by pumping is rendered diflicult.

According to the present invention, the distillation is performed under a very high vacuum, without the employment of steam or of circulating' devices provided with moving parts According to the invention, the material to be distilled is kept in circulation in the still by providing the latter with an internal upcast pipe which dips into the liquid and through which there is admitted a substance-for example in the distillaticn of mineral oils, one of the fractions of low boiling point-which condenses completely,

at the temperature of the available cooling water, and therefore possesses a vapour pressure of somewhere below l0for example, 3 mm. or lessof mercury. Another factor on which the selection of this substance depends is that it should be capable of fairly sudden vaporization on entering the upcast pipe, so that its vapours lift the charge material in the pipe. On issuing from the upper end of the pipe the material is brought into a more or less finely divided condition. Thus the distillation is performed whilst the material to be distilled is dispersed in the vapour space of the still. A deflector, facilitating the dispersion and atomization may be provided, in known manner, above the upper mouth of the upcast pipe. In addition, the vapour space of the still may be provided with devices which retard the descent of the charge material through 'said space and-also in known manner-which help to facilitate distillation.

According to the invention, chemical action on the charge material may also be combined with the distillation. For example, with a view to decolorizing and refining the products, substances such as hydrogen peroxide, formaldehyde and the like, may be admitted into the still with the liquid effecting the circulation. If, in this operation, water is produced by the chemical reaction, the amount formed is nevertheless too small to have any adverse influence on the evacuation of the apparatus or on the condensation of the gases passing from the still. Therefore, even the presence of small quantities of steam does not prevent the distillation from being performed, in accordance with the invention, under-to a certain extent-any convenient high vacuum and without any substantial extra outlay for the production and maintenance of the vacuum in the plant. In addition the present; invention provides, for the first time, a satisfactory solution of the problem of maintaining the charge material in circulation without impeding the process of distillation.

In order more clearly to understand the nature of the invention, reference is made to the accompanying drawing, which illustrates diagrammatically and by way of example, two embodiments of apparatus suitable for carrying out the invention.

In the embodiment illustrated in Fig. 1, l is a vacuum still, into which the substance to be distilled is introduced through the branch 2. The vessel I is provided with a circulation device, consisting of a cylindrical pipe 3, flared at top and bottom, and into the lower end of which extends a tube 4 provided with a nozzle. Above the pipe 3 is mounted a deflector 5, below which can be arranged means, such as cups or baflle plates, over which the liquid to be treated is conducted in order to increase its superficial area still further. The still I may be provided with internal heating means, such as tubes 6, or it may also be heated from the outside. If, for example, the still be charged with mineral oil, heated to 300 0., from which (for example) the fractions of lower boiling point have already been distilled off, and fractions of higher boiling point are to be distilled at that temperature, then a light oil, vaporizing at 200 C., is introduced by way of the tube 4. This oil is delivered, by means of a pump 1 and the pipe 8, into the heater 9 and is heated to such an extent that a considerable portion of it vaporizes on expanding in the nozzle III. The heating and vaporizing of the light oil may, however, be effected solely by mixing'it with the oil contained in the still I. The vaporized light oil ascends in the pipe 3 and, on the air-lift principle, carries mineral oil from the still up with it, thereby producing a circulation of the said 011. Between the deflector 5 and the surface of the liquid in I, the liquid introduced through the tube 4 and a portion of the charge material detach themselves, in the form of vapour, from the mineral-oil circulation. If the vapours passing out of the still by way of the pipe ll be passed through several-e. g. 3 or 4-condensers in succession, it will be possible to condense the distillate vapours separately from the light-oil vapours employed as the conveying medium. For example, two different fractions of the hydrocarbons distilled from the mineral oil will be condensed in the first two condensers, whereas the vapours of the circulatory liquid will be condensed only in the subsequent condenser or condensers. Of course, the condensation temperatures in the several condensers must be suitably adjusted by arranging the cooling conditions accordingly. The liquid employed for producing the circulation can therefore be completely recovered and introduced into the still again. Consequently, owing to theexclusion of the steam hitherto required for the agitation and the reduction of the partial pressure, the air pump of the plant has now merely to evacuate the amounts of gas that have leaked in and are uncondensable, and therefore an absolute pressure of a few mm. of

. mercury-or even lesscan be maintained in the still. The air pump may be assisted by inserting an injector in front of it-which may also be located in front of or between the condenserswhich, however, will be operated (of course) by a medium more easily condensable than steam, in

order to maintain the optimum vacuum in the apparatus.

Fig. 2 represents a somewhat modified embodiment of the distilling apparatus of the present invention which is suitable for use in cases where mineral oil, for example, is distilled in two stills, such as at 200 C. in still I2 and at 300 C. in still l3. The stills may be of known types and heated, for example, by the furnaces I20 and I30. However, since it is hardly possible, in existing plants,

. to make the stills sufliciently tight for an absolute pressure of 2-3 mm. of mercury gauge, the present invention provides the still I3 with a superstructure M. The two chambers I3a and Ma are separated by the partition I4b so that the oil circulation proceeds through the upcast pipe 28 and the down pipe 29. With this arrangement the same pressure-for example 30-50 mm. of mercurycan be maintained in the still I3 as in the still I2, and the distillation-in the vapour'space I4a can, nevertheless, proceed under any convenient higher vacuum, such as 1 mm. of mercury, or even less. The distillate vapour obtained in the still I2 is passed to the condensers I5, I6, I! where the corresponding fractions are condensed. The vapours from the still I3 and the chamber I4 pass to the condensers I8, I9, and 20. These are'operated in such a manner that the vapours formed in the still l3 and the vessel I4 are substantially condensed in the condensers I8 and I9, whilst the light oil, or the like, employed for the circulation, is recovered in the condenser 20. However, it is likewise possible to condense, in 20, also a portion of the distillate originating from the contents, of the still I3. In such case the whole or part of the condensate from 20 can be returned to the still I3. For example, a portion of this condensate is passed through the pipe 2| into a vessel and subjected to further treatment, if necessary, the remainder being returned to the circulator 28 of the still I 3 by way-of the pipes 22 and 24, the superheater 25, the pipe .26 and nozzle 21, by means of a pump 23. By the circulator 28, the contents of the still 13 are circulated through the vessel I 4, which is under a lower absolute pressure than the still I3 itself. Such of the charge material as is not vaporized in I 4 returns into the heated still l3 by way of the pipe 29. The vacuum in Ha is suitably maintained by a steam exhauster 30, the operative steam of which passes into the condenser 3|, where it is condensed, the uncondensable gases and vapours drawn out of the apparatus by the exhauster being completely cooled at the same time. The pipe 32 leads to an air pump which evacuates the condenser 3|. The gases and vapours from the still l2 that are uncondensed in I 5, l8 and I! can be passed into the same, or a second, condenser, this measure being, of course,

adapted to include the water vapours when the distillation in still I2 is conducted with steam and not by the method according to the invention. The oil to be treated is supplied to the still 12, either continuously or in measured intermittent quantities, through the pipe 33. When the desired fractions have been distilled off in still l2 the oil is transferred-continuously or in batches to the still l3 through the pipe 34, which is provided withshut-off or regulating devices. The

residue in the still I3 is evacuated in known manner.

The method of the presentinvention may, of course, be applied to plants equipped with more than two stills.

I' claim:

1. Process for the distillation of a liquid which comprises heating the same in a heating chamber to a temperature below boiling, temperature,

conveying the heated liquid from the heating chamber into a vaporizing chamber by the airlift principle by injecting thereinto a second liquid which vaporizes at the temperature of the heated liquid but exists in the liquid state at atmospheric temperatures under low absolute pressures, maintaining in said vaporizing chamber asubatmospheric pressure at which a portion of the heated liquid and said second liquid vaporize,

withdrawing vapors from the vaporizing chaml ber and subjecting them to condensation; and returning the unvaporized liquid from the vaporizing chamber to the heating chamber.

2. Process ,for the distillation of a mineral oil which comprises heating the same in a heating 1o chamber to a temperature below boiling temperature, conveying the heated oil from the heating chamber into a vaporizing chamber by the airlift principle by injecting thereinto a liquid hydrocarbon which vaporizes at the temperature of the heated oil but exists in the liquid state at atmospheric temperatures under low absolute pressure, maintaining in said vaporizing chamber a subatmospheric pressure at which a portion of the heated oil and said liquid hydrocarbon vanporize, withdrawing vapors from the vaporizing chamber and'subjecting them to condensation,

and returning the unvaporized mineral oil from the vaporizing chamber to the heating chamber.

3. Apparatus for vacuum distillation of liquids which comprises a heating chamber, means for heating the same, a separate vaporizing chamber positioned above the heating chamber, an upcast .pipe extending from near the bottom of the heating chamber into and terminating near the top of the vaporizing chamber, a down-cast pipe leading from the bottom of the vaporizing chamber into and terminating near the bottom of the heating chamber,'said'chambers communicating with each other through said pipes only, v

means for withdrawing vapor from the vaporizing chamber, condensers for these vapors, a vacuum pump connected to the last condenser, a nozzle at the bottom of the up-cast pipe in the heating chamber, and means for conveying fiuid tosaid 40 nozzle.

KARL EBNER.

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