US2327643A - Fractional distillation - Google Patents

Description

Aug. 24, 1943. G, s, HOUGHLAND 2,327,643

FRAGTIONAL DISTILLATION Filed Aug. 16, 1941 v ATTORNEY Eatentec Aug. 24, 1943 UNITED STATES PATE NT OFFICE FRAcTIoNAL DISTILLA'rroN Glen S..Houghland, New York, N. Y., asslgnor to The M. W. Kellogg Company, Jersey City, N. J., a corporation of Delaware Application August 16, 1941, Serial No. 407,137

7 Claims.

. vide an economical method of reboiling and reiluxing a fractionating column.

Another object of the invention is to reduce the cost of operating a, fractionating column at a high reflux ratio, so as to accomplish inexpensively the separation of closely boiling constituents. Other objects of the invention will appear during the course of the description hereinafter given.

It is contemplated by the invention to withdraw liquid from a fractionating column, at some point below the feed inlet, and to reduce the withdrawn liquid to a pressure at which it will have a boiling point lower than the condensing temperature of the vapors issuing from the top of the column. The liquid is then indirectly contacted with the overhead vapors, and in vaporizing, cools and condenses them. Thereafter it is recompressed toits original pressure and returned to the lower part of the column, at the higher temperature to which it is raised by thev compression, so supplying heat for reboiling the column.

A more detailed description of a preferred embodiment of the invention will now be given by Way of illustration and with reference to the accompanylng drawing, which is a diagrammatic ilow sheet. It is to be understood that the embodiment of the invention shown and described l is exemplary only.

Referring now to the drawing, material to be fractionated is introduced at any suitable pressure and temperature through line I into a fractionating column 2, and is subjected to conventional fractionation therein. The overhead prod- 'uct of the fractionation is withdrawn from the from separator 'l through line 8 and disposed of as desired while the condensate is withdrawn through line I0 and picked up by pump 9. A quantity sufficient for reiluxing purposes is returned to the top of the column through line II while the remainder, constituting the liquid overhead product of the distillation, is disposed of as desired through line I0.

The net production of bottoms product from the distillation operation is withdrawn from the base of column 2 through line 3 and disposed of as desired. An addiitonal quantity of bottoms product, which is subject to continuous recycling to the column as hereinafter described, is also withdrawn through line 3 and diverted through line I2 to a valve I3 through which it is expanded and partially vaporized at a lower pressure, such that upon being subsequently passed through line I4 into indirect contact with the column overhead vapors in heat exchanger 5 it will be completely vaporized, and will extract heat from the column overhead vapors to bring about condensation thereof. Immediately upon being reduced temperature of the vapor's to be condensed in 5.

in pressure, sulcient of the bottoms product material will vaporize to eiect cooling of the whole.

quantity down to the boiling point which it has at the reduced pressure, and as previously noted this boiling point must be below the condensation The bottoms product material vaporized in heat exchanger 5 passes through line I5 to a compressor IB wherein it is raised to its original pressure and wherein it simultaneously undergoes a substantial rise in temperature. ed compressed vapors travel through line I'I and are re-introduced into the base of column 2.

They supply heat to the material in the base of the column for reboiling purposes and subsequently undergo condensation, eventually being withdrawn again as previously described. Inl the event that the quantity of material required to be so re-circulated for condensing purposes in ex-j changer 5 is less than the amount needed for reboiling purposes after recompression, I may supply thev deilciency by means of a conventional reboiler such as a. steam heated coil-30 located in the base 'of column 2. If on the other hand the` quantity of material re-ci'rculated is desired to suiice for reboiling purposes and this amount is insufficient for condensing the reflux and product, the deficiency is made up by means of the auxiliary condenser 5a, to which any suitable extraneous refrigerant may be supplied. Otherwise condenser 5a is unnecessary and may be omitted.

The procedure described is most advanta- The heatgeously applied to distillation processes which operate `at pressures abovev atmospheric, but it may be applied if desired to atmospheric or even vacuum distillations. 'I'he utility of the invention is particularly great in connection with the separation of binary mixtures, the components of which differ only slightly in their boiling points. In the separation of normal from iso-butane for example, the column top temperature may difier by as little as 24 F. from the bottom temperature, and a relatively high reilux ratio and a large amount of reboiler heat are necessary. Under such circumstances it is necessary to reduce the pressure of the bottom product re-circulatedthrough line I2 relatively little in order to make possible the vaporization thereof in contact with the overhead vapors, and a correspondingly small compression ratio is necessary to return the yaporized material to the base of the column. It is characteristic of narrow boiling range fractionation that large amounts of heat must .be added andy withdrawn at closely adjacent temperature levels. Ordinarily, of course, the generation of heat by gas compression is not particularly economical, but under the circumstances outlined, in which the compression serves the dual purpose of providing heat and refrigeration, the arrangement is a very economical one.

Under other circumstances I may employ my invention in connection with a two-stage distil- -lation process involving re-distillation of the vbottoms product from a primary fractionatng column. A suitable arrangement for practicing this embodiment is shown in the drawing, and comprises a line I8 for transferring the bottoms product of a distillation column 2 to a secondary column I9 which is operated in a conventional manner to deliver a final bottom product through line 2I. Reboiling of column I9 is accomplished by means of steam heated coil 20, and overhead vapors are withdrawn through line 22 for condensation in condenser 23. The condensate is collected in an accumulator 24 and reflux is returned through line 25, pump 26 and line 21 to the top of column I9. The net production of overhead product from column I9 is withdrawn through line 28, and an additional quantity which is to be constantly recirculated as hereinafter described is passed through line 29 into line I2 for reduction in pressure through valve I3, in exactly the same manner as has been explained with reference to the material re-circulated from line 3 through line I2. The material returned through line 29 for cooling and reboiling purposes will serve equally as effectively as the original bottoms from column 2, of whichV it is a derivative. If desired, both the bottoms from column 2 and the overhead material from column I9 may be re-circulated and admixed in any desired proportions for joint service in cooling the overhead and reboiling the bottom of column 2. The re-circulation of overhead materialfrom I9 is not so advantageous as the recirculation of bottoms from 2, because it involves the refractionation of the material re-circulated.

It will be understood that the reboiler heat and reflux refrigeration provided by the invention are not necessarily utilized at the extreme bottom of the tower and in the external reflux condenser, respectively. It is frequently desirable to provide for reflux cooling at intermediate points of a tower above the feed inlet, and for reboiling at intermediate points below the feed inlet. In accordance with the invention, by suitable selecton of the point of withdrawal and the pressure to which the withdrawn material is expanded, reiluxing and reboiling at intermediate points may be easily achieved. For ex ample, I may withdraw liquid from a column at a point between the bottom and the feed inlet and after expansion thereof to a sufciently low pressure and temperature, pass the expanded material through a coil located in the column between the feed inlet and the top. In this` manner I may condense distillate vapors as reflux intermediate their traverse of the column. Thereafter the expanded and vaporized material may be returned to the column preferably at or adjacent its point of withdrawal, for reboiling y purposes.

In order to illustrate more fully the manner In which the invention may be practiced, an example will now be given. A mixture of ethane and ethylene may be fractionated by means of the panded to a pressure of 110 pounds per square inch absolute, at which part of it will vaporize and its temperature will fall to approximately 42 F. After abstracting heat from the ethylene vapors in a condenser such as 5 and recompression to 250 pounds absolute the ethane re-circulated would be at approximately 25 F. When re-introduced into the base of the column this ethane would produce a reboiling effect equivalent to the vaporization by direct heating of slightly more than an equivalent amount of ethane, because the return vapors would be superheated slightly (from 9 to 25 F.). It would be fortuitous, depending chiefly upon the reux ratio, the feed composition, and the conditions of the feed entering the column, if the reboiling and reiiuxing provided by any rate of ethane re-circulation in accordance with the invention happened to be exactly in balance forcirculation of 18,300 pounds per hour of ethane would provide about 2,340,000 B. t. u. per hour of reflux refrigeration and 3,180,000 B. t. u. per hour of reboiler heat. The ethane recompressor would consume approximately 239 shaft horsepower.

I claim:

1. In the fractional distillation of a volatile mixture of relatively narrow boiling range involving the cooling of distillate vapors by indirect heat exchange with a cooling medium to condense at least a portion of said vapors and the use of thus-produced condensate as reilux in the fractionating zone, the improvement which comprises withdrawing from the fractionating zone liquid which has been enriched by fractionation with the heaviest component of the feed, reducing thus-withdrawn liquid to a pressure at which its boiling point wil1 be below the condensation temperature of distillate `vapors to be condensed, using material thus reduced in pressure as cooling medium to accomplish condensation of distillate vapors and vthereby vaporizing material so used, compressing thus-produced vapors to the pressure existing in the "material to its original pressure and returning it to said zone.

5. In "the fractional distillation of a volatile mixture of relatively narrow boiling range involving the cooling of aistiuate vapors by indirect heat exchange with a cooling medium to condense at least a portionV of said vapors and the use of thus-produced condensate as reflux in the fractionating zone, the improvement which comprises usingas said cooling medium a material obtained by re-distillation of the less volatile product of said distillation, said material while so used being under a pressure low 3. In the' distillation of a volatile mixture of narrow boiling range in a. fractionating column to produce a bottoms product consistingsubstantially entirely of a single component of said mixture, the improvement which comprises withdrawing liquid from said column at a point below the feed inletand using said liquid ata pressure lower than that"maintained in said column as a cooling medium to cool by indirect heat transfer and condense distillate vapors while said liquid is under a pressure at which it has a boiling point below the'condensation temperature of said vapors, whereby said liquid is vaporized, compressing the thus-produced vapors and returning them to the column.

4. In the distillation of a volatile mixture of I narrow boiling range in a fractionating column to produce a bottoms products 'consisting suisstantially entirely of a single component of said mixture, the improvement which comprises withdrawing bottoms product liquid from said 'column and reducing the pressurethereon sutilcient to lower the temperature and boiling point thereof below the condensation temperature of the column overhead vapors, exchanging heat between bottoms product liquid at said reduced pressure and overhead vapors to bring about condensation of said vapors and vaporization of said liquid, compressing vapors thus produced and introducingftbe compressed vapors into the lower portion of said column to effect reboiling of said cohunn.

enough to permit vaporization thereof at the condensation temperature of said distillate vapors to be condensed, and introducing said material after said useinto the fractioating zone adjacent-the point of withdrawal of said less-volatile product.

6. In the distillation of a volatilemixture of narrow boiling range, the improvement which comprises withdrawing liquid from a fractionating zone, separating liquid thus withdrawn into light and heavy fractions, reducing the pressure on said vlight fraction to eiect partial vaporization and cooling thereof and using the material thus reduced in pressure as a cooling mediuin for. vapors produced in the fractionating zone to effectV condensation thereof,

thereafter increasing the pressure on the thus `used material and returning it to said frac- Y tionating zone.

t'1. A fractional distillation process for a vola.- tile liquid of relatively narrow boiling range which comprises withdrawing liquid from a fractonating'zone and reducing the pressure thereon to effect partial vaporization and cooling thereof, using thus cooled material to cool vapors -produced in the process and to `produce a condensate therefrom, thereafter increasing the.

pressure onthe thus used material and return.

ing it to said fractionating zone.

GLEN s. HoUGI-ILAND.

Images