GB2107597A - Process and apparatus for fractionating close-boiling components of a multi-component system - Google Patents

Abstract

A process and apparatus for the fractionation of close-boiling components of a multi-component system comprising at least two fractionation columns 10, 12 in series having a plurality of equilibrium stages in which the vapor stream 24 from a downstream fractionation column 12 is compressed by a compressor 40 and passed into a lower portion of a preceding fractionation column 10 and a liquid bottom stream 18 from any one of said columns except the last is expanded by an orifice 42 sufficiently to convey the resulting liquid-vapor mixture to the upper portion of the next fractionation column 12. In a particularly preferred embodiment, the compressed overhead vapor stream 24 is passed in heat transfer relationship to a liquid stream 18 withdrawn from the preceding fractionation column 10 prior to introduction into the lower portion of such preceding fractionation column 10. <IMAGE>

Description

SPECIFICATION Process and apparatus for fractionating close-boiling components of a multi-component system This invention relates to the fractionation of close-boiling components, in particular, those developed in the production of heavy water, through use of an improved process and apparatus for the concentration, by fractionation, of a heavy water-containing liquid.

Dual temperature enrichment systems for producing concentrated deuterium oxide are disclosed in U.S. Patent No. 4,008,046. Various fractionation systems are known, as well, including the rectifying column apparatus in U.S. Patent No. 2,999,795.

In general, the fractionation of a 30% heavy water stream to a concentration of 99.7% deuterium oxide requires an excess of 300 equilibrium stages, when the fractionation is effected under reduced pressure, to improve the separation factor, which varies between 1.052 (at 1 30 F.) and 1.030 (at 195"F.) Usually, two to four fractionation columns are employed for this purpose, operating with respective condensers and reboilers at pre-select low pressures; hence they cannot function as separate sections of a single fractionation column in series in order to reduce the operating pressures of each section. Such an increase in operating pressure of such sections in series lowers the separation factor to the extent that an increase in reflux and number of equilibrium stages cancels any savings in equipment and utilities.

In the present invention there is provided a process and apparatus for the fractionation of close-boiling components of a multi-component system, said apparatus comprising at least two fractionation columns having a plurality of equilibrium stages in which the vapor from a downstream fractionation column is compressed and passed into a lower portion of a preceding fractionation column and a liquid bottom stream from any one of said columns except the last is expanded sufficiently to convey the resulting liquid-vapor mixture to the upper portion of the next fractionation column. This eliminates both the requirement for a pump to convey the liquid bottom stream to the next fractionation column and the requirement of a reboiler for the preceding fractionation column.In a particularly preferred embodiment, the compressed overhead vapor stream is passed in heat transfer relationship to a liquid stream withdrawn from the preceding fractionation column prior to introduction into the lower portion of such preceding fractionation column.

To facilitate the description of the present process and apparatus, reference is made to the fractionation of deuterium oxide-water system in the production of heavy water, shown in the drawing, although it should be recognised that such process and apparatus are applicable to close-boiling components of a variety of multi-component systems, such as xylene isomers, n-hexane/2-methyl pentane, and n-pentane/2-methyl butane. The term close-boiling components of a multi-component system is herein understood to define a component system wherein the difference between the boiling points of the components are less than 10'F.

Referring now to the present drawing, there are illustrated two fractionation columns, generally designed as 10 and 12, formed with a plurality of equilibrium stages (not shown). For illustrative purposes, these constitute the last two fractionation columns of a heavy water-concentrating plant, employing four fractionation columns for concentrating, to about 99.7%, a 30% heavy water stream.

Generally, such fractionation columns are operated at the same or similar temperatures and pressure in both the upper and lower portions thereof, i.e. a temperature and pressure in the upper portions of from 100 F. to 190 F. at 40 to 400mm Hg., and a temperature and pressure in the lower portion of from 180 F. to 220 F. at 400 to 850mm Hg. The latter column is operated at a lower pressure level. The fractionation column 10 is provided with a feed line 14, an overhead vapor line 16, a liquid effluent line 18 and lower vapor line 20.The fractionation column 12 is provided with feed line 22, an overhead vapor line 24, a liquid effluent line 26, and a reboiler 28 having an inlet line 30 and a vapor line 32 for introducing reboiled vapors into the lower portion of the fractionation column 12. The effluent line 26 is in fluid communication with the suction side of a pump 34 with the discharge side being in fluid flow communication with line 36.

The lower vapor line 20 of the fractionation column 10 is in fluid flow communication with the overhead vapor line 24 of the fractionation column 10 via a heat exchanger 36 and the discharge side of a compressor 40 having a compression ratio of from 1.5 to 10.

The liquid effluent line 18 is provided with a restriction orifice 42 at a height "d" from the lower portion of line 18 and is in fluid communication via the heat exchanger 38 with the feed line 22 of the fractionation column 12.

In operation, a partially concentrated heavy water stream, e.g. a stream containing 90% deuterium oxide is introduced by line 14 into the fractionation column 10 and is passed in counter-current contacting relationship with a vapor introduced by line 20 into the fractionation column 10. A liquid bottom stream in line 18 is withdrawn and passed through the restriction orifice 42 wherein a pressure reduction of at least about 5 psi causes about 3% of the liquid to vaporize, and this is sufficient to lift the resulting liquid-vapor mixture to the top portion of column 12, thereby obviating the need for a pump to effect transmission of such stream.To ensure that the vapor portion of such liquid-vapor mixture is maintained, the liquid-vapor mixture is passed through heat exchanger 38 in heat transfer relationship to the vapor stream in line 24, after compression in compressor 40, such vapor stream being superheated.

The orifice 42 is positioned at a height "d" of at least 5 feet from the lower portion of the outlet line 18 and sized for self-venting. A level of liquid is automatically maintained in line 18. Thus, as the liquid level in line 18 drops below the elevation of the orifice 42, a portion of the liquid is caused to vaporize before reaching the orifice 42 and thereby restrict the flow through the orifice 42. Therefore, a liquid level will be automatically maintained in the outlet line 18 of the fractionation column 10 to meet the designed pressure drop requirement of the system.The elimination of a pump to pass the liquid bottom stream from the fractionation column 10 to fractionation column 12 lowers the elevation of the fractionation column 10, thereby eliminating the liquid surge requirements in the bottom of the fractionation column 10 with a concomitant inventory savings in heavy water.

The vapor-liquid stream, containing 98% deuterium oxide, in line 22 is passed in counter-current contacting relationship in fractionation column 12 to reboiled vapor in line 32 to form a concentrated heavy water product (99.7% deuterium oxide) withdrawn via line 26 by pump 34 and passed by line 36 to storage facilities (not shown).

In accordance with the present invention, the compressor 40 overcomes a pressure build-up thereby to maintain low operating pressures for the fractionation columns 10 and 12 with a resulting savings in steam and cooling water that substantially exceeds the cost of compressor power, i.e. the elimination of a condensor for the overhead in line 24 of fractionation column 12 which would be revaporized in a reboiler for fractionation column 10.

EXAMPLE Operation of the process and apparatus is described in the following example.

Sixteen thousand, seven hundred (16,700) pounds per hour of a heavy water stream of 90% deuterium oxide in line 14 is introduced into fractionating column 10 operated at a temperature and pressure in its lower portion of 190 1 F. and 425 mm Hg., respectively. Fif- teen thousand, two hundred (15,200) pounds per hour of an overhead stream at a temperature and pressure of 126 F. and 100mm Hg.

is withdrawn by line 24 from fractionating column 12 and is compressed in compressor 40, whereby the compressed overhead stream is heated to 425 F. prior to passage through heat exchanger 38 for introduction into the fractionating column 10 as the vapor stream in line 20. A bottom stream in line 18 of 15,400 pounds per hour of a deuterium oxide content of 98% is passed through the restriction orifice 42 and thence through heat exchanger 38 prior to introduction into fractionating column 12 as the vapor-liquid mixture in line 22. A heavy water stream of a concentration of 99.7 percent deuterium oxide is withdrawn at the rate of 213 pounds per hour by line 36.

While the above discussion has been with reference to a preferred embodiment of this invention, it will be understood that a number of additional features are also within the scope of this invention. For example, the restrictive orifice may be replaced by a liquid levelling device; however, it should be noted that the use of a liquid levelling device increases the residence time, with a concomitant increase in liquid inventory. Additionally, the use of insulation about the transfer lines eliminates the requirement of the heat exchanger in the respective vapor-liquid lines. Additionally, if there are more than two fractionation columns in series operating at the same or similar pressures, a similar compressor and conduit assembly can be provided for such fractionation columns, as previously described. Thus, the vapor from any downstream fractionation column in series, can be compressed and passed into a lower portion of a preceding fractionation column in the same series; and a liquid bottom stream from any one of said columns in series except the last can be expanded sufficiently to convey the resulting liquid-vapor mixture to the upper portion of the next fractionation column.

Claims (12)

1. In a process for fractionating closeboiling components of a multi-component system wherein such multi-component system is sequentially passed through at least two fractionation columns operating in series at like pressures and wherein a higher boiling component of said multi-component system is withdrawn as a liquid from a last fractionation column and wherein the lower boiling component is separated as a vapor from a fractionation column, the improvement comprising: a) withdrawing a liquid bottom stream from one of said columns in series except the last of said columns and expanding said liquid bottom stream sufficient to convey the resulting liquid-vapor mixture to the upper portion of the next fractionation column; b) compressing a vapor stream withdrawn from an upper portion of any of said fractionation columns in series except the first; and c) introducing the compressed vapor stream of step b) into a lower portion of the fractionation column immediately preceding the frac tionation column from which said vapor stream was withdrawn and compressed.

2. A process as defined in Claim 1, wherein said compressed vapor stream of step b) is cooled prior to introduction into said preceding -fractionation column.

3. A process as defined in Claim 1 or 2, wherein the liquid withdrawn from said one column is expanded, prior to passage to said next column, in indirect heat transfer relationship with said compressed vapor stream.

4. A process as defined in Claims 1-3, wherein said expansion is effected at a liquid head of at least 5 feet.

5. A process as defined in Claims 1-4, wherein said multi-component close-boiling system is a deuterium oxide-water solution.

6. A process as defined in Claims 1-5, wherein said fractionation columns are operated at a top column temperature and pressure of 100 to 190 F. at 50 to 500mm Hg.

and at a bottom temperature and pressure of 180 to 220 F. at 400 to 850mm Hg.

7. In an apparatus for fractionating close boiling components of a multi-component system in a plurality of fractionation columns of which at least two of said columns are in series and of like operating pressures for purposes of the same fractionation wherein a liquid conduit is provided in said apparatus for passing a liquid bottom stream from a penultimate fractionation column to an upper portion of the last fractionation column and wherein a vapor conduit is provided for withdrawing a vapor stream from the upper portion of said last fractionation column and wherein a higher boiling component of said multi-component system is withdrawn as a liquid from said last fractionation column and wherein a lower boiling component of said multi-component system is withdrawn as a vapor, an improved apparatus which comprises: a compressor means for compressing said vapor stream; and conduit means for introducing said compressed vapor stream from any downstream column in said series into a preceding fractionation column in said series.

8. The apparatus as defined in Claim 7 further including a heat exchanger for said liquid conduit and conduit means for cooling said compressed vapor stream.

9. The apparatus as defined in Claim 7 or 8, wherein a restriction orifice is disposed in said liquid conduit with a head level of at least five feet.

10. The apparatus as defined in any one of Claims 7-9, wherein there are more than two fractionation columns, and further including a compressor means in a vapor conduit between said next to last fractionation column and a preceding fractionation column.

11. A process for fractionating close-boiling components of a multi-component system substantially as described herein with reference to the drawing.

12. An apparatus substantially as described herein with reference to the drawing.