Sequential Liquid Solid Desiccation of Supercritical and Nearcritical CO.sub. 2

This invention relates to a process for drying moist CO.sub.2 at conditions above or near the critical point (87.9.degree. F. and 1071 psia). More particularly, the present invention relates to an efficient process for drying supercritical or nearcritical CO.sub.2 to an extent materially reducing its corrosivity without the severe loss of desiccant and contamination of CO.sub.2 which happens in drying CO.sub.2 with a glycol such as triethylene glycol at conditions above or near the critical point of CO.sub.2.

Numerous desiccants and systems for drying gases are described in U.S. patents such as the following: U.S. Pat. No. 2,812,830 describes a system using triethylene glycol (TEG) and recycling partially spent TEG while removing most of the water with dry TEG. U.S. Pat. No. 3,390,511 describes a gas drying system using aqueous electrolyte desiccant adsorbed on water soluble carrier pellets such as sodium chloride pellets. U.S. Pat. No. 3,609,942 describes the drying of wet hydrocarbon gases at pressures of from about 500 to 2000 psig with ethylene glycol or other glycols. U.S. Pat. No. 3,750,369 describes a system for regenerating spent liquid desiccants such as a glycol or hygroscopic solution of electrolytes such as lithium, calcium or other chloride salts. U.S. Pat. No. 3,885,926 describes an aqueous desiccant solution of calcium chloride and magnesium nitrate which is said to be particularly suitable for dehydrating hydrocarbon gases. U.S. Pat. No. 4,235,289 describes a system for producing supercritical CO.sub.2 from a subterranean reservoir and drying it with TEG to provide dried supercritical CO.sub.2. U.S. Pat. No. 4,344,486 describes an oil recovery process for producing CO.sub.2 containing contaminants such as hydrocarbons and hydrogen sulfides from an underground reservior, burning the mixture to form a concentrated carbon dioxide stream, compressing and dehydrating that stream with a molecular sieve or ethylene glycol water-removing arrangement and injecting the treated CO.sub.2.

Such patents contain no suggestion of any significant problem due to a glycol such as TEG remaining dissolved in the dehydrated CO.sub.2.

An article in Oil and Gas Journal, Nov. 8, 1971, page 53, describes preliminary plans for pipelining CO.sub.2 to the Sacroc unit of the Wasson field for use in oil recovery. The article describes the desirability of maintaining a pressure greater than about 1400 psig to keep the CO.sub.2 supercritical and indicates that a maximum of 50 parts per million of water in the dehydrated CO.sub.2 would adequately prevent corrosion of the pipeline. It also indicates that, in pilot tests, drying with TEG appeared satisfactory but "little experience exists in large scale dehydration of CO.sub.2 produced from natural gas wells". An Oil and Gas Journal article (Jan. 3, 1983, page 92) relating to CO.sub.2 injection for enhanced oil recovery, indicates that the gas should be dehydrated to a water dew point of about 10.degree. F. below the minimum ambient temperature and that TEG offers the best combination of ease in operation and economics since dew points obtained with it are normally within the range required for CO.sub.2 handling systems. An Oil and Gas Journal article (Mar. 21, 1983, page 160) indicates that "Corrosion by CO.sub.2 can be prevented in gas transmission lines by dehydrating the gas to less than 60% relative humidity for the coldest portion of the pipeline." and also that oil-soluble amine film-forming inhibitors can be used in CO.sub.2 -pipelines.

In summary, the prior art teachings and beliefs indicate (1) the desirability of pipelining CO.sub.2 as a supercritical single phase fluid, (2) the desirability of drying such CO.sub.2 to a non-corrosive moisture content, and (3) an expectation that triethylene glycol is the best dessicant for such a drying operation.

The present invention relates to a process for drying water-containing CO.sub.2, at conditions above or near its critical point, to a substantially noncorrosive water content, while maintaining high drying efficiency and minimizing desiccant loss and product contamination due to desiccant solubility. The supercritical or nearcritical CO.sub.2 is flowed into and out of contact with, first, an aqueous liquid desiccant solution of lithium and/or calcium halide and, second, a permeable mass of water-insoluble solid desiccant.

FIG. 1 illustrates a plot of comparative solubilities of triethylene glycol in pure CO.sub.2 and various CO.sub.2 -rich mixtures with increasing temperature and at various pressures.

FIG. 2 is a schematic illustration of a CO.sub.2 -drying test apparatus suitable for use in the present invention.

The present invention is, at least in part, premised on a discovery that a significant problem exists in a large scale use of the previously suggested procedures for drying supercritical or nearcritical CO.sub.2 to a substantially noncorrosive degree. The desiccant previously considered to be the most cost effective and versatile, triethylene glycol, has been found to exhibit an undesirably high solubility which varies widely with relatively small variations in pressure and temperature. Aqueous liquid desiccant solutions of lithium and/or calcium halides (such as the chlorides) have a suitably low solubility in supercritical and nearcritical CO.sub.2. They also have a high water capacity, but they have a drying efficiency which is incapable of removing water from said CO.sub.2 to the low levels required for corrosion protection.

Applicant has discovered that it is possible to achieve a high capacity for water removal, small desiccant loss and a high drying efficiency if a two-step desiccant system is used in which an aqueous electrolyte desiccant solution is followed by a bed of water insoluble desiccant. Unexpectedly, this procedure provides a synergistic result. The volume of supercritical or nearcritical CO.sub.2 desiccated to a selected noncorrosive water content in a single cycle with the two-step system is greater than the sum of the volumes which can be dried to the same extent by separate single cycles through the liquid or solid desiccants of the two cycle system.

Particularly attractive sources of CO.sub.2 for use in oil recovery operations comprise subterranean reservoirs such as those in the McElmo Dome and Doe Canyon fields. In a particularly attractive CO.sub.2 producing procedure, the CO.sub.2 is produced as a supercritical single-phase fluid. This plus the desirability of supercritical pipelining provides a desirability for dehydrating the CO.sub.2 at conditions above but near the critical point. In such operations, changes over several orders of magnitude can occur in the solubility of triethylene glycol in the CO.sub.2 being dehydrated.

Equation of state calculations were performed to establish correlation patterns of TEG losses in CO.sub.2 drying facilities. FIG. 1 shows a graph of the calculated solubility of TEG in CO.sub.2 as a function of temperature along isobars close to the critical pressure of 1071 psia with variations in temperatures near the critical temperature of 87.9.degree. F. At pressures close to the critical pressure, the solubility reaches a minimum value as temperature increases to about the critical temperature, after which it starts increasing with increasing temperature. With increasing pressure the minimum of the curves shift to higher temperature and the sudden drop becomes less pronounced. An addition of an inert diluent such as nitrogen to the CO.sub.2 results in a decrease in the solubility of TEG over the range of near critical conditions while at the same time shifting the minimum of the curves to lower temperatures.

FIG. 2 shows a system and procedure for determining the suitability of a particular two-stage CO.sub.2 drying system for drying supercritical or nearcritical CO.sub.2 to a selected moisture content at selected conditions. A stream of the CO.sub.2 to be tested is supplied from a CO.sub.2 source (not shown) to compressor 1 and is pressurized to the selected pressure. The pressurized CO.sub.2 is fed through surge tank 2, through a conduit provided with relief valve 3, gauges 4 and 5, and back-pressure regulator 6, to provide a stream at the selected pressure. The stream is metered through needle valve 7 and flowed into water sparger 8 where the CO.sub.2 is substantially saturated with water.

The moist stream is then fed into the drying sparger 9 where it flows into and out of contact with a selected aqueous desiccant, such as a solution of lithium or calcium chloride, for removing the bulk of the water. The partially dried stream of CO.sub.2 is then fed into drying column 10. In column 10 the CO.sub.2 stream is flowed into an and out of contact with a permeable bed of solid desiccant, such as aluminum oxide, and is substantially completely dried.

The dried stream is flowed through a conduit which is provided with relief valve 11 and gauge 12 and leads into a needle valve 13. The valve 13 is maintained at the temperature selected for the experiment, and releases a stream of the dried CO.sub.2 at substantially atmospheric pressure into a series of cold traps 14 maintained at a temperature, such as about -70.degree. C., sufficient to freeze and hold substantially all of the moisture in the stream. The volume of the treated CO.sub.2 is measured by means of Rotameter 15 and wet test meter 17.

In such an operation, for example, a stream of supercritical CO.sub.2 is saturated with water at 1250 psi and 115.degree. F., in the water sparger 8. The bulk of the water is then removed by flowing the stream into and out of contact with a known amount of an aqueous liquid electrolyte desiccant, e.g., a pumpable slurry of lithium chloride in water, in the drying sparger 9. The desiccation is completed by flowing the partially dried stream through a permeable bed of solid desiccant, e.g., aluminum oxide, in the drying column 10. Measurements of the amount and the water content of the treated CO.sub.2 are made by means of cold traps 14 and meters 15 and 16. From those measurements determinations are made of the amount of CO.sub.2 which was treated by the tested desiccants before the water content of the treated CO.sub.2 reached a selected level, e.g., 0.39 milligrams per liter of CO.sub.2 at room temperature and pressure. Then, to the extent required, the composition and concentration of the desiccant, and/or the residence time of the CO.sub.2 in contact with the desiccant can be adjusted in order to substantially optimize the amount of CO.sub.2 that can be treated before its water content exceeds a selected level.

The results of comparative tests of the water capacity of various electrolyte systems are listed in Table 1. These data show that, for example, at 115.degree. F. and 1250 psi, the amount of CO.sub.2 which was treated sequentially with LiCl slurry and solid Al.sub.2 O.sub.3 before its water content rose to 0.39 milligrams per liter of CO.sub.2 was 2520 liters (at room temperature and pressure) while the volume which could be similarly treated by the most effective one of the desiccant aqueous liquids, the lithium chloride slurry, was only 1980 liters. The volume which could be similarly treated with only the solid desiccant, aluminum oxide, was only 300 liters. Thus, in the present process, the use of the same amount of lithium chloride and aluminum oxide in a two-step procedure dried more than a total of only 2280 liters which could have been similarly dried by independent parallel treatments with the same amounts of the slurry and solid desiccants.

As known to those skilled in the art, numerous devices and arrangements are known for flowing a stream of gas into and out of contact with either liquid or solid desiccants. In addition, appropriate means for avoiding problems of maintenance, corrosion prevention, and reactivation of spent desiccants, are known. Substantially any of such devices and procedures can be utilized in the present process.

The present process can be used in conjunction with corrosion inhibitors, hydrate inhibitors, and the like. For example, where produced supercritical CO.sub.2 is to be flowed from producing wells to a central treating station, a hydrate inhibitor such as methanol can be added at the wellheads before the produced CO.sub.2 enters gathering lines through which it is flowed into a central processing station, for example, in a manner such as that described in U.S. Pat. No. 4,235,289. Such a hydrate inhibitor can be condensed out of the CO.sub.2 stream being treated by means of conventional procedures and devices.

One advantage of the present invention is the ease with which it can be adjusted to changing climatic conditions, pumping arrangements, and/or changing concentrations of fluids produced along with the CO.sub.2 from subterranean reservoirs, or the like. Depending upon factors such as the conditions at which the dried CO.sub.2 is injected into a pipeline, the conditions to which it will be subjected as it is transported into a use location, the number of pumping stations that will be involved, the expense and desirability of utilizing some corrosion inhibitor to maintain a selected same rate of corrosion while using less drying and thus leaving a higher water content, changes may be desirable from time to time in the maximum amount of water to be left in the dried CO.sub.2.

Aqueous liquid solutions and/or slurries of electrolytes which are suitable as desiccants in the present process are pumpable liquid solutions consisting essentially of water and a dissolved salt or a dissolved salt plus suspended particles of salt. The anhydrous form of the salt used should have a very low solubility in supercritical CO.sub.2. Particularly suitable salts for use in forming aqueous liquid desiccant solutions for use in the present process are lithium chloride, calcium chloride and their mixtures. Examples of other suitable salts include calcium bromide and lithium bromide.

A permeable mass of solid desiccant suitable for use in the present process can comprise substantially any permeable bed of a solid material which is substantially insoluble in supercritical and nearcritical CO.sub.2, tends to remain a permeable solid mass when saturated with water vapor, and is capable of removing enough water from partially dried supercritical or nearcritical CO.sub.2 to provide the selected low moisture content in the CO.sub.2. Particularly suitable solid desiccants are granules or other permeable porous masses of aluminum oxide. Examples of other suitable solid desiccants include silica gel and zeolites.

Suitable hydrate inhibitors for use in the present process comprise relatively volatile water-soluble compounds having vapor pressures near those of water, such as lower alcohols, ethers and the like. Methyl alcohol is a particularly suitable hydrate inhibitor. Examples of other suitable inhibitors include ethylene glycol.

Suitable corrosion inhibitors for use in conjunction with the present process comprise commercially available oil-soluble amine, film-forming inhibitors.