CO.sub.2 Methane Separation by Low Temperature Distillation

Many hydrocarbon gas streams in addition to containing a substantial methane component also possess significant amounts of heavier hydrocarbons such as ethane, propane, butane, etc., as well as substantial amounts of acid gases such as COS, H.sub.2 S and, particularly, CO.sub.2. Frequently it is necessary to separate the acid gas, e.g., CO.sub.2 component, from the hydrocarbon component as well as separate the heavier hydrocarbon fractions from the methane. For example, in order for natural gas to be commercially acceptable, it must first meet stringent specifications with regard to heating value, hydrogen sulfide and carbon dioxide content.

There are currently a number of established technologies for the removal or partial separation of acid gases from hydrocarbon streams. Thus, it is known to use physical solvents which are selective toward the acid gas component and chemical solvents which will react with such components. Suitable examples of physical solvents include propylene carbonate, dimethylether and polyethylene glycol. Examples of suitable chemical solvents are aqueous solutions of potassium carbonate as well as a variety of amines, such as monoethanolamine, diethanolamine and the like.

Carbon dioxide flooding is a process which can be particularly useful in enhanced oil recovery processes. Carbon dioxide flooding can proceed through a variety of mechanisms, including:

(1) immiscible CO.sub.2 drive;

(2) miscible CO.sub.2 drive;

(3) hydrocarbon-CO.sub.2 miscible drive;

(4) solution gas drive;

(5) hydrocarbon vaporization; and

(6) multiple-contact dynamic miscible drive. In the L. M. Home et al, December 1974 publication in the Journal of Petroleum Technology, the CO.sub.2 properties believed to be important in causing oil displacement were stated to be the following:

(1) CO.sub.2 reduces oil viscosity;

(2) CO.sub.2 increases oil density;

(3) CO.sub.2 promotes swelling of oil;

(4) CO.sub.2 is highly soluble in water;

(5) CO.sub.2 in water has an acidic effect on limestone and carbonate rock and thus dissolves the rock;

(6) CO.sub.2 vaporizes and extracts portions of crude oil; and

(7) CO.sub.2 is transported chromatographically through porous rock.

Miscible flooding with CO.sub.2 has recently become increasingly popular, due both to its ability to recover valuable hydrocarbons from naturally occurring gas formations which were previously uneconomical to recover and also to the increased suitability for the recovery of petroleum deposits. Thus, the use of carbon dioxide for miscible flooding is gaining momentum; consequently, growing amounts of associated gas utilized in wells stimulated by this method must be processed, both in order to recover the extracted hydrocarbon content and also to efficiently recover and recycle the carbon dioxide for reinjection into the well. However, the effectiveness of CO.sub.2 is greatly reduced when contamined with impurities such as methane and nitrogen. Accordingly, an economical method for producing a rich CO.sub.2 stream substantially either reduced or free from these impurities has been a goal of the art.

A number of distillation techniques can be useful for the separation of CO.sub.2 -hydrocarbon streams, particularly at higher CO.sub.2 levels. One such attempt, the so-called Ryan-Holmes process, involves an initial separation of the methane component present from the CO.sub.2 and the heavier hydrocarbons. Since CO.sub.2 freezes at the temperatures usually encountered in a demethanizer, an alkane is fed in at the top of the column, thereby preventing freezing of the CO.sub.2. Subsequent distillation can separate CO.sub.2 from the hydrocarbon stream, and the various hydrocarbons themselves, but this leads to an expensive, involved process.

A combination distillation-membrane process has been set forth in U.S. Pat. No. 4,374,657 to Fluor, Inc.

In U.S. Pat. No. 4,417,449 a process for the separation of CO.sub.2 and sulfide gases from oil shale retorting, coal gasification, oxygen fireflooding, and CO.sub.2 miscible flood enhanced oil recovery of the off gases for various recycle processes, such as to a petroleum reservoir, is reported. The process separates the off gases into an essentially sulfur-free light BTU fuel gas, a heavy hydrocarbon stream, and a CO.sub.2 acid gas stream, with the CO.sub.2 stream being expanded in an auto-refrigeration step to provide the necessary process refrigeration.

Accordingly, the invention comprises a process for the regeneration of a carbon dioxide gaseous stream further containing a significant acid gas component, such as H.sub.2 S and COS, as well as a variety of carbonaceous, e.g., petroleum-based compounds. The process comprises: compressing and cooling the CO.sub.2 feed stream, typically containing CO.sub.2, acid gases, light fuel gases, and heavy hydrocarbons, e.g., petroleum products from a subsurface well, to a suitable elevated temperature e.g., 70.degree. to 90.degree. F., and superatmospheric, e.g., 450 to 550 psig, pressure; substantially removing any acid gases, i.e., sour gases, present, e.g., by liquid phase oxidation, from the CO.sub.2 feed stream; preferably further cooling the feed stream by contacting with a suitable cool product stream, e.g., a pressurized, liquid CO.sub.2 stream in a suitable heat exchange zone; drying the CO.sub.2 feed stream, e.g., by conventional low temperature procedures to about 0-2 ppm water; separating the CO.sub.2 feed stream in a single distillation zone which is primarily externally refrigerated into a (1) light fuel gas stream, (2) a liquid, substantially CO.sub.2 -containing stream, and (3) a heavy hydrocarbon stream; the external refrigeration, preferably comprising about 60-90%, most preferably about 75%, occurring by the utilization of a distillation intercooler unit positioned above the CO.sub.2 containing sidedraw stream withdrawal; cooling the light fuel gas stream e.g., preparing it for use as a suitable product, e.g., by contacting in a suitably refrigerated overhead condenser unit and removing a desired light fuel gas fraction; pressurizing and transporting the substantially CO.sub.2 -containing liquid stream, e.g., a sidedraw stream, preferably substantially stripped of non CO.sub.2 component in an adjacent stripping zone, into a suitable heat exchange zone, i.e., for heating by contacting the entering CO.sub.2 feed stream before exiting as product, e.g., recycling the stream to underground petroleum formations; separating the heavy hydrocarbon stream into a lighter hydrocarbon gas stream which, preferably, is combined with the light fuel gas product stream, and a heavier hydrocarbon stream, e.g., by a suitable distillation stage.

FIG. 1 discloses a simplified outline of the preferred embodiment of the invention.

FIG. 2 discloses a range of calculated CO.sub.2 product purity streams depending upon whether or not a side stripping unit is utilized in the primary distillation zone.

One major utilization of carbon dioxide miscible flooding is the recovery of liquid hydrocarbons such as petroleum in underground formations which are not naturally producing due to high viscosities, surface tensions, and/or a lack of adequate pressure present in the petroleum formations. Carbon dioxide miscible flooding enhanced oil recovery involves no deliberate combustion, but rather a high pressurization of the subsurface formation with CO.sub.2 so as to dissolve the oil in the CO.sub.2 and withdraw a single phase, CO.sub.2 -petroleum containing fluid toward a surface well.

In FIG. 1 a simplified outline of the preferred embodiment of the regeneration of the carbon dioxide utilized in a miscible flooding process or the like has been set forth. A feed gas stream 10, typically passing from a crude separation unit or the like, comprises a CO.sub.2 -hydrocarbon stream having a CO.sub.2 content ranging from about 10 to 95 mole %, preferably, from about 60 to 90 mole %, at about 100.degree. F. and 5-10 psig. The stream is compressed in feed gas compressor unit 12, preferably to an intermediate pressure level, e.g., 30 to 60 psig, with the hydrocarbon condensate being recovered in the decanters following the aftercoolers, flashed and returned to the crude oil separator for recovery (not shown). If, as usually is the case, the feed stream contains H.sub.2 S, COS, mercapatans, or the like, some or all of these components can be removed in gas cleaning unit 14 by suitable procedures, e.g., direct liquid phase oxidation of H.sub.2 S to sulfur, and the like. After one or, preferably two additional compression steps and suitable amounts of aftercooling, the feed stream 16 preferably next passes into a feed-product heat interchanger 18, where the feed gas is cooled to 80.degree. F., preferably by contacting with a cooled, substantially liquid CO.sub.2 product stream 20 maintained at a temperature of about 50 to 60.degree. F. Hydrocarbon and water condensates are again separated from the feed stream following cooling, with the hydrocarbons being decanted, flashed, and passed back to the crude oil separator. The feed gas stream is then dehydrated to about 0-2 ppm H.sub.2 O, preferably less than about 1 ppm H.sub.2 O , in dryer unit 22, which is typically a solid desiccant dryer, or the like. After dehydration, the feed gas stream is then passed through line 24 to the primary distillation unit 26, which is a low-temperature multi-product column in which a cold fuel gas overhead stream 28, typically comprised of about 64 mole % CH.sub.4, 18-22% CO.sub.2, 5% propane, 6% butane, and the remaining various other hydrocarbons and inerts, is taken off and passes through condensor 30 at a pressure of about 400 to 600 psig and -60.degree. to -70.degree. F.; a liquid product stream 32, typically comprised of at least 90 mole % CO.sub.2 with minor fractions of methane, ethane and the like, at a pressure of about 400-600 psig and 20.degree.-30.degree. F., passes from the column as a sidedraw stream, in a manner similar to that described in U.S. Pat. No. 4,417,449. In the most preferred embodiment, a substantial improvement in CO.sub.2 purity is obtained by further stripping the exiting sidedraw liquid product stream 32, as is further indicated in FIG. 2 and accompanying Table I. Surprisingly, a 95 mole % CO.sub.2 product stream can be obtained using the side stripper unit 34 with feeds as low as 57 mole % CO, whereas only about 75% is obtainable without using the side stripper unit. Sidedraw stream 32 is so processed by entering CO.sub.2 stripping unit 34, typically a small, suitably packed column known to those in the art, whereupon the incoming liquid stream 32 is stripped by the vapors generated in the stripper reboiler unit 36, with the exiting vapors from the stripper in line 38 re-entering the primary distillation unit 26 at a suitable tray positioned above the sidedraw stream 32 location. The vapor for primary distillation unit 26 is provided by reboiler unit 40.

A smaller, heavier hydrocarbon bottoms stream 42, typically comprising about 42 mole % hexane, 22% isopentane, 27% pentane, 8% butane, along with various minor products is removed by pressure let-down from the bottom of distillation column 26. In sharp contrast to the expensive auto-refrigeration system required in the aforementioned U.S. Pat. No. 4,417,449, since in this system much of the condenser duty is required for the liquefaction of the CO.sub.2 component, a surprising improvement has been realized through the utilization of a distillation intercooler unit 44. The unit, when appropriately positioned near the top of the column above the CO.sub.2 sidedraw tray, as would be apparent to one skilled in the art, substantially reduces the use of the expensive refrigerant in line 29 otherwise required in the primary distillation unit condenser 30 positioned at the top of the column treating the light fuel gas overhead stream 28. The vapors from a suitable upper tray in the column 26 are withdrawn through line 46 and cooled in the intercooler 44 by a suitable, e.g., -10.degree. F., refrigerant, with the vapor and liquid exchanger effluent returning to the column via line 48. In such a manner the expensive refrigeration requirement is substantially shifted from the more expensive condenser unit 30 to the intercooler 44, thereby providing a substantial economic benefit. That is, the distillation column now employs a dual overhead "knockback" condenser, as well as a side stream refrigerated condenser which is adapted to operate to shift a very substantial amount, e.g., about 60 to 90%, usually about 75%, of the condensing load to conventional refrigeration at about -10.degree. F., with only the remaining, e.g., 25% being at a substantially lower, e.g., -75.degree. F., and more expensive temperature level.

The fuel gas distillate stream 28, which is typically 76-80 mole % hydrocarbon and at a temperature of -60.degree. to -70.degree. F., is preferably passed to a heavies column condenser unit 50 and utilized as a coolant in the condenser for the column overhead stream 52. After heating, the gas stream is depressurized to a desired pressure, e.g., a suitable delivery pressure to customers, such as 200 psig, and mixed with the cooled overhead from the heavies column 52, typically comprising mostly propane and butanes, thereby increasing the heating value of the resulting gas stream to a desired heating value, e.g., 800 BTU/SCF. Preferably, a substantial amount of the gas stream can be consumed within the process as fuel for the refrigeration and compressor gas engines.

The substantially liquid CO.sub.2 sidedraw stream 32 after leaving the distillation unit and side stripper passes through line 54 to pump 56, preferably a CO.sub.2 booster pump, in order to obtain an adequate NPSH for the following high pressure CO.sub.2 pump 58. The pressurized CO.sub.2 stream is then heated, preferably in feed product heat interchanger 18 by incoming feed stream 16, and the exiting CO.sub.2 product stream typically leaves the process at about 800 to 3500 psig and about 30.degree.-60.degree. F. through line 20, where it is preferably mixed with fresh CO.sub.2 before being reinjected into the petroleum-bearing subsurface, or utilized in another manner.

The bottoms stream 42 of the primary distillation unit 26 typically comprises a propane fraction along with lesser amounts of other heavier components present in the feed stream, together with a small amount of CO.sub.2 and ethane. The stream is passed to a suitable separation unit, e.g., a small heavies distillation column 60, which, typically, is a small packed tower with an overhead condenser unit 50 cooled by the primary distillation unit overhead stream 28, and heated by a suitable reboiler unit 62. The small bottoms stream 64 from the column, typically comprising about 42 mole % hexane, 27% pentane, 22% isopentane and 8% butane, is preferably cooled and pressured back to a crude oil separator (not shown), while the lighter components are preferably taken overhead as a vapor distillate 52 and combined with the fuel gas stream 28 (as noted above) so as to increase the heating value of the exit gas to a desired content.

In the preferred embodiment of the invention, the liquid CO.sub.2 sidedraw stream 32 is substantially stripped of its remaining small methane content by passing the stream through a suitable stripping zone, e.g., CO.sub.2 stripping unit 34. The stripping enables the process to operate with a higher CO.sub.2 purity, thereby greatly expanding the range of allowable CO.sub.2 concentrations in feed gas streams. This is in contrast to prior art systems, particularly the one disclosed in U.S. Pat. No. 4,417,449, which are by necessity severely limited by their ability to process CO.sub.2 streams containing a minimum of about 90% CO.sub.2.

It is to be noted that, although the preferred embodiment of the invention has been set forth, various modifications can be made without deviating from the scope of the invention, such as the use of recycle streams and the like and, accordingly, the invention is not to be limited except as so defined in the appended claims.