US3028332A - Liquid recovery from an originally vaporous mixture - Google Patents

Description

A ril 3, 1962 LIQUID RECOVERY FROM AN ORIGINALLY VAPOROUS MIXTURE Filed June 15, 1959 H. FORBES ETAL INVENTORS:

GODFRIED J.VAN DEN BERG HENRY FORBES HUBRECHT VAN DER MAREL THEIR ATTORNEY Sfiltes Patent Patented Apr. 3, 1962 3,028,332 LIQUID RECOVERY FROM AN ORIGINALLY VAPOROUS MIXTURE Henry Forbes, Huhrecht van der Marc], and Godfried J. van den Berg, The Hague, Netherlands, assignors to Shell i] Company, New York, N.Y., a corporation of Delaware 7 Filed June 15, 1959, er. No. 820,517 Claims priority, application Netherlands Oct. 7, 1958 9 Claims. (Cl. 208-340) This invention provides an improvement in the process involving pressurization of an originally vaporous hydrocarbon mixture, containing normally gaseous components, to recover further liquid therefrom, which improvement permits a significant reduction in the total compression work required for the operation.

It is conventional practice in the processing of a vaporous hydrocarbon stream, containing normally gaseous components, to pressurize the stream to increase liquid recovery therefrom. The stream may be first cooled to bring about a condensation, following which the stream is separated into a gaseous portion and a liquid portion. The individual portions are then separately pressurized to substantially the same elevated pressure and then recombined for further processing. There is an economic advantage in separately pressurizing the two portions. Processing schemes of this general nature are used in refinery gas recovery and for gasoline stabilization in some instances. A process of this general type is described in Petroleum Refiner, page 232, September 1949.

It is an object of the invention to provide an improvement in the process of pressurizing an original vaporous hydrocarbon mixture and more particularly to provide an improved process reducing the amount of work required in pressurization. This and other objects will become more apparent in the following description of the process, taken in conjunction with the drawing which is a schematic representation of a preferred embodiment of the improved process.

It has now been discovered that in the processing of a vaporous hydrocarbon stream, containing normally gaseous components, to increase the liquid recovery therefrom, that it is advantageous to recycle a portion of the recovered liquid to the vaporous hydrocarbon stream undergoing treatment. In the improved process the stream is cooled to bring about a condensation, following which it is separated into a gaseous portion and a liquid portion and the individual portions are then separately pressurized to substantially the same elevated pressure. Following pressurization, light components are conjointly liberated from the two pressurized portions to obtain a liquid fraction. A portion of this liquid fraction is returned to the vaporous hydrocarbon stream at a point preceding its separation into the gaseous and liquid portions. The recycling of the portion of the recovered liquid in this manner will significantly reduce the total compression work required for the pressurization of the two separate portions.

The term pressurizing and the like is employed to mean raising the pressure to which the vaporous mixture is under in the initial stage of the process which original pressure may be atmospheric, superatmospheric, or subatmospheric. The recycled or returned portion of the recovered liquid may be a portion of the first liquid initially separated or a fraction from that liquid obtained say in a later distillation.

The conjoint liberation of the relatively light components from the pressurized gaseous and liquid portions may, for instance, be brought about by treating the two portions together (premix or otherwise) in a distillation column with the light components being removed over head. In this instance the gaseous portion may, if desired, be introduced into the distillation column on a tray which is higher than the one on which the liquid portion is introduced. It is, however, often advisable to mix the two pressurized portions and then to liberate the light components therefrom by subsequent cooling of the resultant mixture to promote condensation, after which the mixture is passed to a separator wherein it is separated into gas and liquid. The separated liquid is preferably further treated in a distillation to remove additional light components therefrom, yielding a heavy liquid fraction which may be combined with the original vaporous mixture, or it may be deemed more feasible to pass the heavy liquid fraction to another fractionation to obtain a still more heavier material for the recycling.

Preferably, the recycled liquid portion is added to the vaporous hydrocarbon stream undergoing treatment at a point preceding the cooling of that stream, although it may be combined with the cooled stream before its separation into its vaporous and liquid portion and prior to their separate pressurizations.

The vaporous stream undergoing treatment may be the top product of a primary distillation operated at approximately atmospheric pressure. The feed to the primary distillation column may, for instance, be a hydrocarbon mixture relatively rich in light components (such as hydrogen and methane) which is therefore difiicult to condense; such hydrocarbon mixtures include the effluent from a reforming unit or from a hydrodesulfurization treatment.

In a preferred embodiment of the process the top product will be a gasoline range material containing lighter components from an atmospheric primary distillation. This top product prior to cooling is combined with a recycle heavy gasoline (naphtha), then cooled and passed to a separator maintained say at 1.1 atmospheres and around 45 C. There the liquid and gas phases separate and are separately removed, pressurized and subsequently recombined, passed to a second cooler to obtain further condensation, and then to a second separator maintained at say, 5 atmospheres and 40 C. A gaseous stream containing principally non-condensibles is removed from this latter separator. The liquid recovered in this second separator is moved to a secondary distillation (a dcbutanizer) where the butane and remaining lighter components are distilled overhead. This column may operate at around 12 atmospheres. The liquid gasoline product from the base of the debutanizer goes to a fractionation column where it is separated into a top product boiling below say 93 C. and a bottom product having the boiling range of approximately 93 C. to C. A portion of this bottom product is now recycled according to the invention to combine with the top product from the primary distillation column at a point preferably preceding its initial partial condensation. As a result of the recycle the compression work to be exerted on the liquid portion is admittedly slightly increased, but that on the gaseous portion is greatly reduced. Since from an econornic point of view it is much more attractive to pressurize a liquid instead of a gas, the practice of the process of the invention results in a considerable reduction of the total compression work required in the pressurization of the two portions.

When it is desired to change the cutting point in the primary column (for instance, when a change is necessary to the preparation of a product answering difierent specifications), the quantity of the recycled liquid fraction which is combined with the primary top product should be accordingly varied to hold the gas-compression work substantially constant, thus permitting the gas compressor to operate (with changes in the primary distillation cutting points) at a capacity approaching the optimum. For instance, if the cutting point in the primary column is varied to take overhead a larger proportion of heavier material, the amount of recycle required will be less. It is recommended that the amount of recycle material em ployed be such that the total quantity of debutanized gasoline removed from the secondary distillation column remains substantially constant with variations in the cutting point of the primary column. In the instance where the debutanized gasoline is passed to a subsequent fractionation where it is separated into a light gasoline and a heavy gasoline product, and a portion of the heavy gasoline is used as the recycle material, the quantity of the recycle and the quantity of heavy gasoline actually withdrawn as product should give a total that remains substantially constant at difierent primary cutting points. This practice will result, it will be seen, in the amount of debutanized gasoline being maintained at substantially constant volume.

The invention will nowbe further explained with reference to the accompanying drawing, which relates to the use of the process of the invention in working up a hydrocarbon oil having a relatively high content of normally gaseous components. The term gases as used herein includes vapors. The hydrocarbon stream is introduced through a line 10 to an atmospheric primary distillation column '12 which is operated with a cutting point at 165 C. to separate overhead a light fraction containing .gasoline and lighter components and a heavier bottom fraction composed of kerosene and gas oil. The heavy fraction is removed from the distillation column through a line 14. In the present instance circulating reflux is supplied to the column via a line 60 opening into the top of the column. Conventional refluxing may be-employed. The light overhead fraction is removed from the top of the column through a line 13 to a partial condenser 15 and then to an accumulator 16. The accumulator is operated under a pressure of 1.1 atmospheres absolute with a temperature of approximately 45 C. In the cooler 15 there occurs only a partial condensation, with the result that a partly gaseous and partly liquid product is collected in the accumulator. A liquid product is removed from the bottom of the accumulator through a line 17, while the gaseous product is led through a line 18 to a gas compressor 19. The liquid flowing in line 17 is compressed by a pump 21 to substantially the same pressure as the pressurized gas leaving the compressor 19 in line 20. The two pressurized streams combine in a line 22 which opens into a cooler 23. From the cooler the combined stream passes to an accumulator 2.4 which operates at a pressure of approximately atmospheres and a temperature of say 40 C. In the accumulator a partly gaseous and a partly liquid product is again collected. Complete condensation of the top product of the primary distillation column 12 is impossible even at this relatively high pressure of 5 atmospheres and relatively low temperature, because of the relatively high content of low boiling components in the feed to the primary distillation column 12. The uncondensed gases gathering in the head space of the accumulator 24 are discharged through a line 25 and a pressure, reducing valve 26 from, the

system. The liquid separating out in the accumulator 24 is removed via line 27 and is forced under the pressure of a pump 28 to a central section of a secondary distillation column 29 where it is separated into a butane-free gasoline and an overhead fraction containing butane and lower boiling components. The secondary distillation column operates under a pressure of approximately 12 atmospheres. The heat needed for the column is supplied by a reboiler 30. The butane-free gasoline bottom product is removed from the column in a line 31 to a fractionation column 32 where it is separated into a light gasoline discharge overhead through a line 33 and into a heavy gasoline (naphtha) which is withdrawn through a line 34. A reboiler 35 supplies the heat required for the operation of this column.

The top product of the second distillation column 29 is withdrawn through a line 37, cooled in a cooler 38 and collected in an accumulator 39. The accumulator is operated under a pressure of 12 atmospheres absolute and at a temperature of approximately 45 C. A portion of the liquid collecting in the accumulator is returned via a line 40 as reflux to the top of the second distillation column. The rest of the liquid is removed from the accumulator in a line 41 and forced under the pressure of a pump 42 to the central section of a distillation column 43. In the latter column the material is separated into C; hydrocarbons on the one hand and lower boiling components on the other. The C fractions are discharged from the column through a bottom line 45 and the top product is removed via an overhead line 46, to a cooler 47 and an accumulator 48. In the accumulator a liquid substantially consisting of propane separates which is withdrawn through a line 49 and partially returned through a line 50 as reflux to the distillation column 43. The pressure in the distillation column 43 and the accumulator 48 is approximately 24 atmospheres absolute. The temperature in the accumulator is about 45 C.

The uncondensed gases gathering in the accumulators 39 and 48 are discharged through the lines 52 and 53, respectively, which are provided with the necessary reducing valves.

According to the present invention, a portion of the naphtha fraction withdrawn through the line 34 is recycled via a line 55 to the top product line 13 at a point preceding the cooler 15. The recycled naphtha may be mixed with the top product beyond the cooler 15 at a point preerably combined with the top product stream before the cooler.

EXAMPLE A crude oil is separated by distillation into a fraction boiling below 350 C. and a fraction boiling above 350 C. The former fraction is subjected to a hydrodesulfurization treatment in which a cobalt oxide-molybdenum oxide-alumina catalyst is used. After cooling the reaction product is subjected to an expansion in stages to separate the bulk of the dissolved gases and vapors. The liquid finally obtained (486 tons per 1000 tons of crude oil), in which small quantities of light components such as hydrogen, hydrogen sulfide and normally gaseous hydrocarbons are still dissolved, is then separated by distillation into a number of fractions with the use of the plant shown in the above-described accompanying drawing.

In the primary column 12 (for which circulating reflux is exclusively used again) a bottom product boiling above C. is obtained at about atmospheric pressure (1.1 atm. abs); the feed components boiling below this temperature pass overhead as primary top product (184.15 tons per 1000 tons of crude oil) and are led through the line 13 and the cooler 15, where they are cooled to 45 C., into the accumulator 16 where the pressure is 1.1 atm. abs.

The liquid and the vapor are pumped, as shown in the drawing, to the accumulator 23 where the pressure is 5 atm. abs. The temperature in this vessel is 40 C. The liquid collecting in the accumulator is removed and then separated in the secondary distillation column 29 at an elevated pressure into butane-free gasoline on the one hand and butane+lighter components on the other. The top product is obtained with the use of a conventional reflux system, as indicated in the drawing. The pressure and temperature, in the reflux accumulator 39 are 12 atm. abs. and 45 C. respectively.

The partly liquefied top product of the column 29 is separated in the column 43 into a bottom product consisting of butanes, and into a more volatile top product. The top product is again obtained with the use of a conventional reflux system, the pressure and the temperature in the reflux accumulator 48 being 24 atm. abs. and 45 C. respectively. The condensed top product consists substantially of propane and is withdrawn through the line 49.

Gases not condensed in the reflux accumulators 24, 39 and 48 are discharged through the lines 25, 52 and 53. The bottom product of the column 29 is separated in the column 32 into a top product boiling below 93 C. and a bottom product boiling between 93 C. and 165 C. which is withdrawn through the line 34. Part of this bottom product (27.2 tons per 1000 tons of crude oil) is now recycled according to the invention through the line 55, as a result of which the work to be done by the gas compressor 19 is considerably decreased. This is shown in the table.

Table Top Product Top Product Quantities of Gases and Primary Primary Vapors to be Compressed Distillation Distillation by Compressor 19 Column 12 Column 12 Cutting Point In 185 C. 165 C. 185 C. 165 C. 165 C.

Distillation In Column 12 Recycling 1 None None 27.2

Quantitiesz 1 In tons per 1000 tons of crude oil.

The second data column of the table shows the quantity of top product supplied by the primary distillation column 12 at the said final boiling point of 165 C. The last two data columns of the table show the quantities of gas from this top product which are to be compressed by the compressor 19, without and with recycling through the line 55 of a quantity of the bottom product from the column 32 (27.2 tons per 1000 tons of crude oil). For the purpose of comparison the first data column of the table shows the quantity of the top product from the primary distillation column 12 formed when the cutting point in this distillation column is 185 C.; the third column shows the quantities of gas which are to be compressed in this case by' the compressor 19.

When the bottom product of column 32 is not recycled at a cutting point of 165 C. nor at a cutting point of 185 C., the quantity of gas to be compressed by the compressor 19 is approximately 37.1 tons per 1000 tons of crude oil in the first instance and approximately 33.7 tons in the second. In the first case the load on the compressor 19 is 9.9% greater than in the second. When the recycling is, however, effected at a primary cutting point of C., the quantity of gas to be compressed is not more than about 34.7 tons per 1000 tons of crude oil, i.e., a quantity only slightly greater than the quantity to be compressed at a cutting point of C.; the diflerence in compressor load at the two final boiling points is now not more than 2.5%. Recycling thus makes it possible to use the same (relatively small) compressor even though the cutting point in the primary column is altered, thereby improving the operational economy and also the flexibility of the plant.

In this case the recycled quantityof the bottom product of the fractionation column 32 is equal to the difference between the quantity of the bottom product (of column 32) at a primary distillation cutting point of 185' C. (145.4 tons per 1000 tons of crude oil) and the quantity of bottom product at a primary cutting point of 165 C. (118.2 tons per 1000 tons of crude oil), there being no recycling in the latter case. In other words, care is taken to ensure that the total quantity of bottom product (viz. the quantity to be recycled plus the quantity immediately withdrawn) is substantially constant at the different primary cutting points.

We claim as our invention:

1. In the processing of a vaporous hydrocarbon stream containing normally gaseous components to increase the liquid recovery therefrom, wherein the stream is cooled to effect a condensation, following which the stream is separated into a gaseous portion and a liquid portion and the individual portions are separately pressurized to substantially the same elevated pressure, the improvement comprising conjointly liberating light components from the two pressurized portions to obtain therefrom as one new portion a liquid fraction recovered by fractional distillation and returning a portion of said liquid fraction to the hydrocarbon stream at a point preceding its separation into the gaseous and liquid portions, thereby significantly reducing the total compression work required for the pressurizations of the two separated portions.

2. A process in accordance with claim 1 wherein said returned portion comprises a further heavy fraction obtained from said liquid fraction.

3. A process in accordance with claim 1 wherein the returned portion of the liquid fraction is combined with the vaporous hydrocarbon stream at a point preceding the cooling of said stream.

4. A process in accordance with claim 1 wherein a first liberation of light components is effected by 1) mixing the two pressurized portions, (2) cooling the resulting mixture, and (3) thereafter separating said light components from the cooled mixture.

5. A process in accordance with claim 4 wherein the liquid separated from the light components in step (3) of that claim is subsequently distilled at a still more elevated pressure to separate further light components therefrom to provide a heavy liquid fraction, a portion of which is returned to the process at a point preceding the initial separation of the hydrocarbon stream.

6. A process in accordance with claim 5 wherein the heavy liquid fraction of that claim is subjected to a still further distillation to separate another heavy liquid fraction which is recycled in part to the process at a point preceding the initial separation of the hydrocarbon stream.

7. A process in accordance with claim 4 wherein the vaporous stream undergoing treatment is a reformed naphtha and the light component separated in step (3) of claim 4 are principally normally non-condensibles including hydrogen and the liquid fraction of step (3) is a gasoline fraction containing butane and wherein the liquid fraction is passed to a first distillation to separate the butane and light components therefrom and the resulting debutanized gasoline fraction is subjected to a further distillation to separate a light gasoline product overhead from a heavy gasoline product which is returned in part to the vaporous stream of reformed naphtha prior to its cooling and initial separation.

8. A process in accordance with claim 1 wherein the vaporous hydrocarbon stream is the top product of a primary distillation operating at approximately atmospheric pressure and wherein said liquid fraction of claim 1 is derived from a second distillation and a portion of that liquid fraction is the material returned to the top product of the primary distillation.

9. A process in accordance with claim 8 wherein, when the distillation cutting point in the primary distillation is changed, the quantity, of the liquid fraction combined with the top product is so regulated in amount that the total quantity of heavy components Withdrawn from the secondary distillation remains substantially constant.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Fetroleurn Refiner, vol. 28, No. 9, 1949, pages 213, 216, 217, 220, 22 1, 224, 225, 229, 232,233, 236, 237 and 240.

Petroleum Refiner, April 1950, pages 97 to 100.

Claims (1)

1. IN THE PROCESS OF A VAPOROUS HYDROCARBON STREAM CONTAINING NORMALLY GASEOUS COMPONENTS TO INCREASE THE LIQUID RECOVERY THEREFROM, WHEREIN THE STREAM IS COOLED TO EFFECT A CONDENSATION, FOLOWING WHICH THE STREAM IS SEPARATED INTO A GASEOUS PORTION AND A LIQUID PORTION AND THE INDIVIDUAL PORTIONS ARE SEPARATELY PRESSURIZED TO SUBSTANTIALLY THE SAME ELEVATED PRESSURE, THE IMPROVEMENT COMPRISING CONJOINTLY LIBERATING LIGHT COMPONENTS FROM THE TWO PRESSURIZED PORTIONS TO OBTAIN THEREFROM AS NEW PORTION A LIQUID FRACTION RECOVERED BY FRACTIONAL DISTILLATION AND RETURNING A PORTION OF SAID LIQUID FRACTION TO THE HYDROCARBON STREAM AT A POINT PRECEDING ITS SEPARATION INTO THE GASEOUS AND LIQUID PORTIONS, THEREBY SIGNIFICANTLY REDUCING THE TOTAL COMPRESSION WORK REQUIRED FOR THE PRESSURIZATIONS OF THE TWO SEPARATED PORTIONS.

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