US3502547A - Separation and recovery of liquid fractions - Google Patents

Description

March 24, 1970 R. E. BRIDGEFORD 3,502,547

SEPARATION AND RECOVERY OF LIQUID FRACTIONS Filed April 27, 1966 PRIMARY OVERHEAD (PRO PANE) FEED (ALKYLATION EFFLUENT) VAPOR DOWNCOMER '-l SECONDARY OVERHEAD IS OBUTA N E) BOTTOMS (ALKYLATE) INVENTOR R. E. BRIDGEFORD "if A TTORNEYS United States Patent 3,502,547 SEPARATION AND RECOVERY OF LIQUID FRACTIONS Ronald E. Bridgeford, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed Apr. 27, 1966, Ser. No. 545,664 Int. Cl. B0111 3/22 U.S. Cl. 203-71 8 Claims ABSTRACT OF THE DISCLOSURE A feed stream comprising propane, isobutane and C alkylate is introduced into the top section of a single fractional distillation column having a top section and a bottom section separated by a solid, vapor impermeable plate. At least one downcomer, which serves as the only fluid passageway through said plate, extends downwardly into the liquid on a tray in the top portion of the bottom section to permit the passage of only liquid from the top section to the bottom section while preventing the passage of vapor from the bottom section to the top section. Each section is provided with means for reboiling the liquid contained therein. An overhead product stream containing propane is withdrawn from the top of the top section while an intermediate stream containing isobutane is withdrawn from the top of the bottom section. The bottom section can have a smaller diameter than the top section.

This invention relates to the separation and recovery of liquid fractions. In one of its aspects, it relates to an apparatus for recovery of a middle boiling hydrocarbon fraction comprising a fractional distillation tower containing a top section and -a bottom section, means for passing a mixture of hydrocarbons to the top portion of the top section of the column, means for removing gaseous overhead from the top section of the column, a solid plate separating the top section from the bottom section, a means to allow liquid only to pass from the top section to the bottom section, reboiler means in the lower portion of the top section, reboiler means in the lower portion of the bottom section, means for removing vapor from the upper portion of the bottom section, means for removing liquid from the lower portion of the bototm section. In another of its aspects, the invention relates to an apparatus for recovering a middle boiling hydrocarbon fraction as hereinbefore described wherein said means to allow liquid only to pass from the top section to the bottom section comprises a downcomer pipe through said solid plate in combination with a doughnut tray in the top portion of the bottom section. In a still further aspect, the invention relates to an apparatus for recovery of a middle boiling hydrocarbon fraction as hereinbefore described wherein the bottom section of the column is smaller in diameter than the top section.

The invention also relates to a process for recovering a middle boiling fraction from a feed stream comprising passing said stream to the upper portion of a fractional distillation zone containing an upper zone and a lower zone, the upper zone being separated from the lower zone by an impermeable plate adapted to pass liquid only from the upper zone to the lower zone, reboiling liquid in the upper zone, passing liquid at bubble point from the upper zone to the lower zone, reboiling liquid in the bottom zone, removing vapor from the upper portion of the bottom zone and removing liquid from the lower portion of the bottom zone.

In the alkylation of, for example, ethylene with isobutane, the eflluent from the reaction contains propane, isobutane and an alkylate, the isobutane boiling at a 3,502,547 Patented Mar. 24, 1970 temperature between that of the alkylate and the propane. It is desirable to separate the products so that the alkylate can be recovered and so that the isobutane can be recycled.

The three components of the efiluent can be separated in a two stripper device wherein the feed is passed to the first stripper and the lower boiling product, the propane, is removed overhead, and the isobutane and alkylate are removed as liquid and passed to the second stripper wherein the isobutane is removed overhead from the second stripper.

A less expensive method and apparatus for separating the effluent products is a single column stripper in which isobutane is removed as a vaporous side draw in a central portion of the column, propane is removed overhead and alkylate is removed as a liquid from the bottom portion of the column. Most prior art single strippers are rather large in that they require a reboiler in the bottom portion of the column to heat the entire column. Some prior art apparatus provide a reboiling means in a middle portion of the column.

I have now discovered that the apparatus for separating these components can be substantially reduced in cost and that the cost of separating the components with steam supplied to the column can be substantially reduced if a vapor impermeable plate is provided in a single fractionation column between an upper section and a lower section, liquid in the upper section is reboiled above the plate, liquid in the lower section is reboiled, liquid in the upper section is passed to the lower section, and a process stream of intermediate volatility is removed as a gaseous stream from an area below the vapor impermeable plate.

By various aspects of this invention, one or more of the following or other objects can be obtained.

It is an object of this invention to provide a process and apparatus for separating a hydrocarbon stream from a mixture of hydrocarbons wherein the desired stream boils intermediate the other components of the feed.

It is a further object of this invention to provide an improved apparatus for separating a plurality of hydrocarbon components wherein the size and cost of the equip ment is substantially reduced.

It is -a still further object of this invention to provide an improved distillation process and apparatus for the separation of hydrocarbon mixture wherein the heating costs are substantially reduced.

Other aspects, objects, and the several advantages of the invention are apparent to one skilled in the art from a study of this disclosure, the drawing and the appended claims.

According to the invention there is provided a method and means by which a middle boiling liquid fraction is separated from a mixture containing higher and lower boiling point fractions. The mixture is passed to the upper portion of a two section fractional distillation unit wherein the two sections are separated by a vapor-impermeable plate. Liquid in the bottom portion of the upper section is reboiled and liquid in the lower portion of the bottom section is reboiled. The middle boiling fraction is removed as a vapor beneath the vapor impermeable plate and there is provided a means to pass liquid at bubble point from the bottom portion of the top section to the top portion of the bottom section. According to one embodiment of the invention, the means for passing the liquid from the top section to the bottom section comprises a downcomer pipe which extends through the vapor impermeable plate and extends into a doughnut-type tray wherein the liquid level in the doughnut tray is above the lowest portion of the downcomer pipe. In another embodiment, a hydrocarbon-containing mixture is separated.

The invention will now be described with reference to the drawing which shows an embodiment of the invention.

Referring now to the drawing, a liquid hydrocarbon stream such as the efiiuent from an isobutane-ethylene alkylation process is fed by line 1 to the upper portion of an upper section 2 of a fractional distillation column. Vaporous overhead, containing principally propane, is removed through line 17 while the liquid in process passes across a number of plates 4 and 5 and through downcomers 6 and 7 to bottom plate 8 which separates the upper section 2 from the lower section 3 of the fractional distillation unit. Plates 4 and 5 are standard fractional distillation unit components which are adapted to pass vapor upwardly through them and are adapted to allow liquid to overflow to a downcomer which feeds a lower portion of the unit. Plate 8 is a solid, vapor impermeable plate in the preferred embodiment of the invention. The plate, in other words, will not allow vapor or liquid to pass therethrough. A downcomer pipe 9 is provided to allow liquid to pass over the upper end thereof, and downwardly through plate 8 into the lower section 3 of the unit. Liquid is removed by line 10, passed to side reboiler 11, and the vapor-liquid mixture through line 12 and back into the upper section 2, thus providing stripping vapor. Thus, downcomer pipe 9 supplies liquid from the upper section 2 to the lower section 3 at the mixtures boiling point. In lower section 3, fractionation plate 13 has weirs 14 at the outer edge thereof to maintain a liquid level on plate 13 above the level of the lowest portion of downcomer pipe 9. Thus, liquid is maintained in downcomer pipe 9 so that vapor cannot pass upwardly through pipe 9. Isobutane is removed as a vaporous product from the upper portion of bottom section 3 through line 15. A reboiler 18 is provided in the bottom portion of bottom section 3 to supply the requisite heat and provide stripping-vapors to the bottom section 3 of the unit. Liquid alkylate is removed through line 19.

While both section 2 and section 3 have been shown with 2 and 3 fractionation plates each (4, 5 and 13, 16, 17) of the so-called split-cross fiow or doughnut type, a plurality of plates of similar or differing types, or of other vapor-liquid contacting devices can be used as desired.

By the use of the separator plate 8, it has been discovered that the diameter in the bottom section 3 can be somewhat different from that of upper section 2, in the particular embodiment herein it can be smaller in diameter. Further, it has also been discovered that the diameter of upper section 2 is smaller than that which would be required if a single column without plate 8 had been used.

As an example of the apparatus which would be required without plate 8, a tower having a diameter of 18' would be required for the particular feed rate, composition and desired separation. With the use of plate 8, the diameter of the top section 2 can be reduced to 15 and the diameter of the bottom section can be reduced from 18' to 13 /2. The top section 2 would require ten trays and the bottom section 3 would require fifty trays. Obviously, the smaller diameter unit is less expensive because it requires less material and is easier to construct, transport and erect.

An additional advantage is obtained by employing the smaller diameter unit. The additional advantage is that a lower heating cost is required to separate the constituents of feed stream 1. The cost of providing stripping vapors for the column is roughly in proportion to the square of the diameters. Thus, the cost of reboiling the upper section of a column without plate 8 would be proportional to 18 and of reboiling the upper portion of the column of the invention would be proportional to 15 Thus, the ratio of reboiling cost of the prior art column to the column of the invention for the upper section is 1.44, and similarly, the cost of reboiling the bottom section of a prior art column as compared with the bottom section of the column of this invention is 18 /135 or 1.78.

Thus, it can be seen that the cost of reboiling the column to separate the components by the invention is substantially reduced from that of the prior art.

The column also has advantages over the use of two independent strippers. The advantages would be that in the invention there is no need for a bottom head for the first stripper, no need for a top head for the second stripper, no need for pumps and piping for the bottom flow from the first stripper to the second stripper with additional utility lines and insulation with the accompanying installation and labor costs in erecting two columns rather than one. Further, the foundation and site space for the second column is eliminated by the invention.

As an example of the use of this method and apparatus, in the separation of the hydrocarbon eflluent stream from a reaction step wherein ethylene and isobutane are alkylated by a hydrogen chloride-promoted aluminum chloride complex catalyst, a two section fractionation column is employed ot process approximately 4954 barrels per hour of feed stream. This columns upper section is 15 feet in diameter and contains 10 fractionating trays while the lower section is 13.5 feet in diameter and contains 50 fractionating trays.

The columns feed stream is comprised of 182 barrels per hour of propane and lighter, 3971 barrels per hour of butanes, predominately isobutane, and 801 barrels per hour of pentane and heavier alkylate, predominately isohexanes. Operating at p.s.i.g., overhead pressure, the

overhead vapor stream at 166 F. is removed to further separation steps as desired at a rate of 1799 barrels per hour and contains isobutane plus essentially all of the above-mentioned propane and lighter components. The sidedraw vapor stream at approximately F. and 163 p.s.i.g. is removed at a rate of 2561 barrels per hour, comprising predominately isobutane, and is recycled directly to the reaction step. The bottoms liquid product at approximately 320 F. and 173 p.s.i.g. is removed at a rate of 594 barrels per hour, comprising predominately isohexanes alkylate product.

While the invention has been particularly described with reference to the separation of isobutane from propane and a heavier alkylate, it is obvious that the invention is also applicable to separating other volatile materials.

Further, the invention can be used for any feed mixture to separate and recover a low boiling, high boiling and intermediate boiling component.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, drawing and the appended claims to the invention, the essence of which is that there has been provided a process and apparatus for separating a middle boiling liquid from a mixture of liquids wherein the mixture is fed to the top portion of a two section fractional distillation unit which has a vapor impermeable plate separating the top section from the bottom section, and a means to pass liquid at bubble point from the bottom portion of the top section to the top portion of the bottom section, and means for removing vaporous product from the top portion of the bottom section of the fractional distillation unit.

I claim:

1. A separation system for the recovery of a middle boiling fraction from a feed mixture containing said middle boiling fraction and higher and lower boiling fractrons, comprising (a) a single fractional distillation column containing (1) a top section, and (2) a bottom section, said bottom section having a smaller diameter than said top section;

(b) means for passing said feed mixture into said top section;

(0) a solid vapor impermeable plate separating said top section from said bottom section and having as the only fluid passageway through said plate at least one downcomer;

(d) a tray positioned as the top tray in the top portion of said bottom section and spaced below said plate to form a vapor space therebetween, said tray having weir means for maintaining a liquid level thereon, said at least one downcomer extending downwardly into the liquid retained on said tray so that said at least one downcomer passes only liquid from the bottom portion of said top section to the top portion of said bottom section while vapor is prevented from passing upwardly from said bottom section to said top section;

(e) means for removing from said fractional distillation column and from the system a primary overhead product stream comprising said lower boiling fraction by removing an overhead vaporous fraction from the top portion of said top section;

(f) means for removing from said fractional distillation column and from the system an intermediate vaporous product stream comprising said middle boiling fraction by removing a vaporous fraction from the vapor space above said tray in the top portion of said bottom section;

(g) means for removing from said fractional distillation column and from the system a bottom product stream comprising said higher boiling fraction by removing liquid from the bottom portion of said bottom section;

(h) reboiling means for heating liquid in the bottom portion of said top section to provide stripping vapor for said top section; and

(i) reboiling means for heating liquid in the bottom portion of said bottom section to provide stripping vapor for said bottom section.

2. Apparatus in accordance with claim 1 wherein said tray is a doughnut tray wherein the liquid level on said tray is above the lowest portion of said downcomer.

3. Apparatus in accordance with claim 1 wherein said at least one downcomer is a downcomer pipe.

4. Apparatus in accordance with claim 1 wherein said bottom section comprises a bottom portion of smaller diameter than said top section, and a diverging portion connecting the top of said bottom portion to the bottom of said top section, and wherein said means for removing an intermediate vaporous product stream is connected to said diverging portion.

5. A process for recovering the middle boiling fraction from a feed stream containing said fraction and higher and lower boiling fractions, comprising passing said feed stream to the upper portion of a single fractional distillation zone containing an upper zone and a lower zone, said lower zone having a smaller diameter than said upper zone, said upper zone and lower zone being separated by a solid, vapor impermeable plate having as the only fluid passageway through said plate at least one downcomer, the lower portion of said at least one downcomer extending into the liquid retained on a tray positioned as the top tray in the upper portion of said lower zone, passing only liquid through said at least one downcomer from said upper zone to said lower zone while maintaining the level of the liquid on said tray above said lower portion of said at least one downcomer to prevent the flow of vapor from said lower zone to said upper zone, reboiling liquid in said upper zone to provide stripping vapor for said upper zone, removing a vaporous overhead product stream from said fractional distillation zone and from the process by withdrawing a vaporous stream from the upper portion of said upper zone, heating liquid in said lower zone to provide stripping vapor for said lower zone, removing a bottom product stream from said fractional distillation zone and from the process by withdrawing liquid from the lower portion of said lower zone, and removing an intermediate vaporous product stream containing said middle boiling fraction from said fractional distillation zone and from the process by withdrawing a vaporous stream from the vapor space above said tray in the upper portion of said lower zone.

6. A process according to claim 5 wherein said feed stream comprises propane, isobutane, and a C alkylate, said middle boiling fraction comprising an isobutane fraction.

7. A process in accordance with claim 6 wherein said lower zone comprises a lower portion of smaller diameter than the diameter of said upper zone, and a diverging portion connecting the upper end of said lower portion and the lower end of said upper zone, said intermediate vaporous product stream being withdrawn from said diverging portion.

8. A process in accordance with claim 7 wherein each of said upper zone and said lower zone contains at least one liquid holding tray provided with overflow weirs.

References Cited UNITED STATES PATENTS 2,311,180 2/1943 Bogart et al 203 2,366,361 1/1945 Semon et al 20378 X 2,388,919 11/1945 Iverson 203-73 X 2,410,498 11/1946 Hepp 260-683.45 2,530,143 11/1950 Baker 260683.53 2,555,939 6/1951 Sherwin 26075 2,642,388 6/1953 Steitz et al 26073 X 2,738,964 3/1956 Binder et a1. 202158 X 2,852,582 9/1958 Stallings et a1. 260--683.53 2,855,448 10/1958 Goard 26068357 2,886,492 5/1959 Hanson et al. 20 375 X 3,357,901 12/1967 Otsuka et al. 20378 3,401,092 9/ 1968 Matta 203-1 OTHER REFERENCES American Petroleum Refining, 4th Edition (Bell) pp. 278-284 (1959).

NORMAN YUDKOFF, Primary Examiner F. E. DRUMMOND, Assistant Examiner Us. 01. X.R.

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