Low Temperature Method For Removing Condensable Components From Hydrocarbon Gas

Abstract

A low temperature method is provided for removing condensable components from an inlet hydrocarbon gas, such as natural gas having a high proportion of methane. The system includes a fractionating column to remove condensed components and a novel reflux step which is arranged to condense additional condensable components from the gas after passing the gas through the fractionating column and to return the additional condensed components back to the fractionating column as reflux. The system utilizes expansion means in the form of a gas turbine which is connected such that the energy generated by the turbine is used to operate a compressor to recompress the gas after removal of certain of the condensable components therefrom.

Description

This invention relates to a novel method and system for removing condensable components from inlet hydrocarbon gas, such as gas having a high proportion of methane, for example. More particularly, this invention relates to a low temperature fractionating method utilizing a novel reflux system and method.

In the past, there has been need for a method and system of recovering condensable components from natural gas or the like, as for example gas having a high proportion of methane.

Certain prior art methods and systems have been developed for recovering condensable components from such gas, such as systems particularly for receiving primarily propane, for example. However, certain of these processes do not provide optimum product separation or operating flexibility, as with respect to various operating conditions, for example. Moreover, certain of these processes are not entirely suitable for ethane recovery in particular, as opposed to propane, for example.

Examples of such other systems include those described in U.S. Pat. Nos. 3,292,380; 3,359,743; 2,608,070 and 2,494,126 and in the article entitled "Low Temperature Gas Processing Operations" by W. A. Halliburton, Jr., published in the 1968 issue of the Proceedings of the Forty-Seventh Annual Convention of Natural Gas Processors Association Technical Papers which papers were presented Mar. 19 - 21, 1968 at New Orleans, La.

It is therefore an object of this invention to provide an improved low temperature method and system for removing condensable components, such as ethane and the like from hydrocarbon gas, such as natural gas having a high proportion of methane.

Briefly stated, the method of this invention contemplates cooling the inlet gas to below about 0.degree. F. Thereafter the cooled gas is expanded through expansion means, such as a turbine, to produce a gas-condensate mixture having a temperature below about -50.degree. F. The cooled mixture is thereafter fractionated to remove condensable components therefrom. The fractionated gas is then flowed through a heat exchanger to thereby cool the fractionated gas. The fractionated cooled gas is then expanded again followed by flowing the expanded fractionated gas through the heat exchanger as the cooling medium. The fractionated gas is then recompressed by the energy output of the expansion means.

In certain embodiments, the method may include carrying out the fractionation step in a fractionating column, and withdrawing liquid from an intermediate point in the column and thereafter heating the withdrawn liquid to expel methane therefrom, after which the heated liquid is returned to the column. This may sometimes be referred to as the side reboiler step.

Briefly stated, the system of the invention includes means for expanding and thereby cooling the inlet gas passed therethrough, and gas compressor means driven by the energy output of the expansion means, for compressing gas passed therethrough. It also includes means for passing inlet gas through the expansion means to produce a gas-condensate mixture. A low temperature fractionation column is included for fractionating the gas-condensate mixture. It also includes means for delivering the gas-condensate mixture from the expansion means to the column, and cooling means for cooling the fractionated gas after withdrawal from the column, to thereby condensate additional components from the fractionated gas. Means are provided for passing the cooled fractionated gas after removal of the condensed components through the expansion means to thereby expand the cooled fractionated gas. Means are also included for flowing the expanded fractionated gas through the cooling means as the cooling medium therefor, and means for passing the expanded fractionated gas through the compressor means after passage thereof through the cooling means. The expansion means may include a two stage turbine expander for carrying out the two expansion steps discussed above. In certain embodiments, a side reboiler may be provided for the fractionating column for reheating liquid drawn from an intermediate point in the column.

Certain embodiments of the apparatus may also include means for removing the condensed additional components from the fractionating gas and reflowing such components back to the column as reflux. In certain instances the aforesaid two-stage turbine may be referred to as first and second expansion means. Certain embodiments may also include second cooling means for cooling the inlet gas before passage thereof through the expansion means and including means for flowing the expanded fractionated gas through the second cooling means as the cooling medium therefor. In certain instances the aforesaid side reboiler may be connected for passage of inlet gas therethrough as the heating medium. Means may also be provided for controlling outlet pressure from the fractionation column and for controlling outlet pressure from the second stage expansion means. In addition, the system preferably includes a dry bed desiccant system arranged for dehydrating an inlet gas to a low water dew point, such as -150.degree. F.

Certain embodiments may include a condensate separator prior to the first expansion step to remove condensate along with wax, after the inlet gas has initially been cooled, which condensate and wax material is directed to the demethanizer column.

Certain embodiments may also include CO.sub.2 removal means associated with the column for stripping CO.sub.2 from the gas as it passes through the column. In certain embodiments the heat for operation of the fractionating demethanizer column may be supplied by a portion of the recompressed gas which is passed in heat exchange relationship with the bottoms of the column to thereby heat the column.

Reference to the drawings will further explain the invention wherein:

FIG. 1A is the left hand portion and,

FIG. 1B is the right hand portion of a presently preferred system embodying the invention, in schematic form.

Referring now to the drawings, relatively high pressure hydrocarbon gas such as methane containing condensable components such as ethane and propane and heavier components is introduced through line 1 to a dry bed desiccant system 2 where the inlet gas is dehydrated to a relatively low water dew point, such as about -150.degree. F. Desiccant system 2 is preferably of the molecular sieve type or the like which is capable of achieving the desired low water dew point, and preferably of the type which may be regenerated in accordance with the conventional teachings, when required.

From desiccant system 2 the inlet gas flows through the primary gas-gas heat exchanger 3 where it is cooled to an appropriate temperature by exchange with residue gas, to remove any wax and/or heavy condensate which may be in the gas. From heat exchanger 3, the resulting condensate and gas flows through line 4 to condensate separator 5. Separator 5 is capable of phase separation of hydrocarbon condensate which is withdrawn through line 6 and flowed to an intermediate tray in stripping section 17 of a fractionating column, such as demethanizer 16.

The gas is flowed from separator 5 through line 7 to cold gas heat exchanger 8 where the inlet gas is further cooled to below 0.degree. F. by exchange with residue gas, as will be explained further hereinafter. Cooled gas leaves heat exchanger 8 through line 9 where it is directed to scrubber 10 where the liquid previously condensed by heat exchanger 8 is separated and removed through line 11 and directed to an upper tray 18 of stripping section 17 of demethanizer 16.

High pressure gas from scrubber 10 flows through line 12 to first stage 13 of expansion turbine 51 where the gas undergoes essentially constant entropy expansion. The work force developed in turbine 51 is used to drive a booster compressor 35 which is directly coupled thereto. It is to be noted that gas expander turbine 51 is a two stage expander and is equipped with inlet nozzles 14 for controlling the flow or pressure through the system up to that point.

The two expansion stages 13 and 27 of turbine 51 allow for separately expanding gas flowed in separate streams therethrough. Gas expansion turbines of this general type are sold by the Rotoflow Corporation of Los Angeles, Cal. The design and operation of a single turbine expander of this type is described in the article entitled "Turbo Expander Design and Operation" by Judson S. Swearingen at pages 124 - 127 of the aforesaid Proceedings of the Forty-Seventh Annual Convention of the Natural Gas Processors Association Technical Papers, as discussed above.

For purposes of convenience, turbine 51 may sometimes be referred to as the expansion means. At other times first stage 13 may be referred to as one expansion means and stage 27 as another expansion means. In this connection it is to be noted that in certain embodiments, one of the two stages may be substituted with a conventional expansion valve, for example, which would in that case be referred to as one of the expansion means or as being inclusive in the expansion means. Nevertheless, it is contemplated that at least one of the expansion means will include a turbine to drive a sales gas booster compressor such as compressor 35 attached to the rotor of the gas turbine included in the expansion means.

The exhaust gas from stage 13 of turbine 51 is then at an intermediate pressure and cold temperature, and flows through line 15 to demethanizer 16, where the ethane and heavier constituents of the gas are recovered in the liquid product and the methane is expelled. It is to be understood that demethanizer 16 is a conventional fractionating tower having an upper rectifying section 50 and a lower stripping section 17, with both sections having a plurality of vertically spaced liquid trays 18. It is to be further understood that demethanizer 16 has outlet means in the upper portion thereof for passing of non-condensed gas through line 20. In addition, demethanizer 16 is provided with an outlet at the bottom thereof connecting with line 29 leading to a product storage means (not shown). Demethanizer 16 is provided with a conventional type reboiler in the form of heat exchanger 19, for example. In this instance a hot heating liquid may be flowed through heat exchanger 19 in the direction of the arrows 60 and 61 to thereby heat the bottoms or liquid contained in the bottom of demethanizer 16 and in an enclosed loop indicated by the numeral 62, where the fluid is flowed in the direction of arrow 63 through the lower portion of stripping section 17.

Further, when the gas is returned to near inlet pressure by compressor 35, the product gas at the discharge of compressor 35 will contain adequate heat to reboil demethanizer 16 to thereby effect a fuel saving, as well as supplying a portion of the cooling of the gas before it returns to the pipeline for distribution purposes. Hence, in a preferred embodiment of the invention a portion of the gas discharged from compressor 35 is directed through a conduit (not shown) to the hot fluid side of reboiler 19, where it is heat exchanged with the liquid bottoms of demethanizer 16 to supply reboiled heat to demethanizer 16. The diverted gas stream is thereafter rejoined with the compressor discharge before going to the product gas cooler through an appropriate conduit (not shown).

Passage of the inlet gas through stage 13 of gas expansion turbine 51 allows the fractionation operation to be carried out in demethanizer 16 at temperatures below about -50.degree. F. It is to be further understood that demethanizer 16 is a low temperature intermediate pressure fractionation column which contains a suitable means, such as conventional trays 18, for intimately contacting the vapor rising from reboiler 19 and down flowing condensate. The term "low temperature" fractionating column will generally be referred to as a column wherein the temperature of the outlet gas from the upper portion thereof is below about -50.degree. F.

The outlet or overhead fractionated gas flows from demethanizer 16 through line 20 to demethanizer reflux condenser 21, where the fractionated gas is cooled by exchange with residue gas as will be described hereinafter. The cooled fractionated gas and condensate flow from condenser 21 through line 22 into reflux separator 23. Reflux separator 23 is arranged to separate out the condensed fluids which are flowed through line 24 to pump 25 which then pumps the separated fluid over the rectifying section 50 of demethanizer 16, which serves to condense the ethane and heavier constituents of the feed gas.

The residue gas flows from the top of separator 23 through line 26 to second stage 27 of gas expansion turbine 51, through inlet nozzles 28. Nozzles 28 are used to control the pressure on the demethanizer 16 by opening and closing in response to a pressure sensor (not shown) in the upper portion of rectifying section 50 of demethanizer 16. The second expansion step through stage 27 is operated to produce the required reflux to remove the ethane and heavier constituents of the gas portion of the feed inlet gas. As stated above, demethanizer 16 is equipped with a reboiler, indicated at 19 which serves to vaporize the lighter fractions from the liquified petroleum gas which is removed through line 29 to storage.

Demethanizer 16 is also equipped with one or more side reboilers, such as side reboiler 30 which is a means for providing additional heat to demethanizer 16 without disturbing the over-all heat balance. Refrigeration is scavenged by heating the condensate collected at a relatively high point in the stripping section 17, which is passed through line 31 to reboiler 30 and back to demethanizer 16 through line 32. In reboiler 30, the withdrawn liquid is heat exchanged with a portion of inlet gas supplied through line 38. After passage of the diverted inlet gas through side reboiler 30, it is directed back to the gas inlet stream through line 39 to line 9, thus achieving a colder feed temperature for the inlet gas. The liquid withdrawn from demethanizer 16 through line 31 is heated inside reboiler 30 to remove methane from the condensate. As stated above, the heated methane and condensate is returned to the column through line 32.

The gas which has been directed to second stage 27 of turbine 51 is expanded and thereby cooled and is thereafter passed through line 33, through heat exchangers 21, 8, and 3 successively and thereafter through line 34 into booster compressor 35, which in the embodiment shown is driven by both stages 13 and 27 of turbine 51 by direct coupling thereto as shown. Compressor 35 compresses the gas to the desired discharge pressure in line 36, which is preferably below the pressure in the upper portion of demethanizer 16. Generally, the operating pressure of demethanizer 16 will be 20 to 200 psi higher than gas outlet pressure in outlet 36. Back pressure valve 37 is provided in line 36 to control the outlet pressure from second stage 27 of turbine 51. The pressure rise across compressor 35 will be such to satisfy the energy generated by the gas quantity and pressure differential across both stages 13 and 27 of turbine 51.

When the system is processing gas which must be returned to near the inlet pressure, the product gas may be compressed independently by other compressor means, with turbine 51 compression contributing to the total compression, while at the same time serving the purposes of feed gas expansion and cooling.

A portion of inlet gas, usually on the order of 10 to 25 percent thereof, may be directed through line 38, as explained above, for cooling side reboiler 30 by exchange with tray liquid from demethanizer 16. The pre-cooled partial inlet gas discharges from re-boiler 30 through line 39 to combine with line 9 gas flowing into scrubber 10. The hydrocarbon condensate collected in scrubber 10 undergoes expansion and flash vaporization at valve 40 in flowing through line 11 to an appropriate location in stripping section 17 of demethanizer 16.

Similarly, the liquid from condensate separator 5 likewise undergoes expansion and flash vaporization at valve 41 and is delivered through line 6 to a likewise appropriate lower tray in stripping section 17, as shown.

Any wax which may be present in the inlet gas is retained with the liquid condensate in separator 5 and thereafter fed to an appropriate tray of demethanizer 16, which tray is set at a level where the temperature is sufficiently high to prevent the wax from precipitating and causing a problem, and thus permitting the wax to be withdrawn from the bottom of demethanizer 16 with the condensed ethane and other components. The wax in the inlet gas is thereby maintained at a temperature warm enough to prevent precipitation and is removed from the system in the liquid condensate. The initial cooling step carried out by heat exchanger 3 is an important aspect of this portion of the system for removing wax. Thus, the system of this invention permits removal of the wax in separator 5 at a higher temperature than with other systems.

Other features of the system include valves 42 and 43 connected to control flow respectively through expansion sections 13 and 27 or to by-pass the same when desired. Valves 42 and 43 allow operation of the system at reduced LPG recovery when expansion turbine 51 is out of service for any reason. In addition, valve 44 in line 33 allows for changing between cooling for the purpose of making reflux in condensor 21 to cooling inlet gas in exchangers 8 and 3, to thereby effect better product recovery under certain loads and operating conditions.

In certain installations, it may be desirable to utilize a single stage expansion turbine in the place of the two stage turbine 51. In such instances, other expansion means could be substituted for one of the expansion stages, as for example a conventional expansion valve, in which event the expansion valve could be placed at the second stage and substitute for stage 27 of turbine 51 under certain operating conditions.

In certain embodiments it may be desirable to extract a vapor stream from demethanizer 16 several trays above re-boiler 19 for the purpose of treating the stream with molecular sieves or other suitable means for removal of CO.sub.2. This would be especially advantageous in systems where the inlet gas contains a relatively high CO.sub.2 content. Conveniently, such CO.sub.2 removal means may be in the form of a CO.sub.2 absorption column 70 connected to line 71 which is arranged to draw gas vapors from demethanizer 16 for passage through column 70 where CO.sub.2 is removed therefrom by conventional means. Thereafter, the vapor which has been stripped of CO.sub.2 is returned to demethanizer 16 through line 72, compressor 73 and line 74.

In operation of the aforesaid system and with typical operating conditions, inlet gas at line 1 may be at a pressure of about 900 psig and may contain approximately 94 percent methane, 2.5 percent ethane and 2 percent propane and heavier constituents and 1.5 percent combined CO.sub.2 and nitrogen. The combined inlet gas streams enter condensate separator 5 at about 0.degree. F. Gas enter scrubber 10 at about -50.degree. F. with a slight pressure reduction so that the first stage 13 of expander turbine 51 is driven at about inlet gas pressure. In flowing through stage 13 of turbine 51, the line 12 gas pressure is reduced to about 400 psig and about -110.degree. F. temperature. For substantially 60 percent ethane recovery from demethanizer 16 through line 29, the bottom temperature of demethanizer 16 is operated at a bottom temperature of about 100.degree. F. Overhead gas from demethanizer 16 flows through line 20 to reflux condenser 21 where it is cooled to about -140.degree. F. and into reflux separator 23 where the reflux is separated from the residue gas and pumped by pump 25 into the upper section of demethanizer 16, as shown.

Residue gas flows through line 26 to second stage 27 of expander turbine 51. In flowing through second stage 27, gas pressure is reduced to about 300 psi and temperature to about -150.degree. F. Residue gas in line 33 flows through condenser 21 where it is exchanged to make reflux and thereafter through gas heat exchanges 8 and 3 where it is exchanged against inlet gas, and thereafter flowed into booster compressor 35 for delivery from the system through line 36 at about 360 psig, for example. The pressure under such operating conditions in the upper portion of demethanizer 16 or at the gas outlet therefrom would be on the order of approximately 400 psig.

It is to be understood that the foregoing is given as a typical operating situation and operating perameters can, of course, be varied to cope with the various conditions which may exist with respect to the inlet gas which is to be processed and the outlet gas which is to be produced, as well as the type of liquid petroleum gas which is to be removed from the inlet gas.

Certain operating ranges have been established for the various operating conditions and they are recited hereinafter for purposes of illustration and not as limiting factors on the invention. For example, typical operating conditions may contemplate inlet gas pressures in the range of 600 to 1,100 psi. Further, the inlet gas may contain from 90 to 95 percent methane, 2 to 5 percent ethane, 1 to 3 percent propane and heavier constituents and 1 to 2 percent combined CO.sub.2 and nitrogen, all of which would be considered typical conditions for the inlet gas. The combined inlet gas streams entering scrubber 10 may be on the order of -50.degree. to -70.degree. F. Further, the first stage expansion of the gas in stage 13 may reduce the pressure to 350 psi to 450 psi and from about -90.degree. F. to about -120.degree. F., for example. For recovery of from 50 to 80 percent of the ethane in the inlet gas, demethanizer 16 would be operated at temperature ranges from 60.degree. to 150.degree. F. In typical operating conditions, the reflux condensor 21 may cool the fractionated gas passed therethrough to a temperature range of -110.degree. to -150.degree. F., for example. During the flow of gas through second stage 27 of expander turbine 15, the gas pressure may be reduced to the range of about 200 psi to 300 psi, for example, and the residue gas pressure on line 36 may vary from 300 psig to 400 psig. Again, it is to be understood that the foregoing ranges are for purposes of illustrating a typical installation under the conditions noted. However, it is to be understood that these ranges are not to be considered as limitations upon the invention which will admit to many variations as will be obvious to those skilled in the art, in view of the teachings herein.

Applicant's novel method of refluxing allow better product separation and provides more flexibility in operating conditions. It is a system and method which is particularly suitable for a high ethane recovery from the gas where the ethane recovery in this system may be as high as 40% or more above that for other systems.

Claims

What is claimed is:

1. In a method for removing condensable components from a hydrocarbon gas, the combination of steps comprising:

heat exchanging a turbine expanded fractionally distilled gas stream, a cooled separated inlet gas stream, and an inlet gas stream with turbine expanded fractionally distilled gas, thereby condensing successively higher boiling components from said turbine expanded fractionally distilled gas stream, cooled separated gas stream and inlet gas stream;

separating said condensed components from said streams;

flowing said separated condensed components separately to said fractional distillation column;

withdrawing a gaseous non-condensed portion from the cooled separated inlet gas stream;

turbine expanding the withdrawn gas for flow to the column;

the turbine expander of the respective streams being mechanically connected; and

removing liquid components from said fractional distillation column to storage.

2. In a method of removing ethane and heavier components from an inlet hydrocarbon gas comprised principally of methane, the combination of steps comprising:

turbine expanding said gas to produce a gas-condensate mixture having a temperature below about -50.degree. F.;

fractionally distilling said cooled gas condensate mixture to remove ethane and heavier components therefrom by passing said mixture through a low temperature fractionating column having vertically spaced trays;

withdrawing condensed ethane and heavier components from a lower portion of said column;

flowing the withdrawn condensed ethane and heavier components to liquid storage;

withdrawing fractionally distilled non-condensed gas from a top portion of said column;

flowing the fractionally distilled non-condensed gas through a heat exchanger to thereby cool said fractionally distilled gas;

flowing additional condensed components from said cooled fractionally distilled gas and flowing said additional condensed components back to said column as reflux;

turbine expanding said cooled fractionally distilled non-condensed gas a second time, the turbine expanders of said first and second expanding steps being mechanically connected;

flowing twice turbine expanded fractionally distilled non-condensed gas through said heat exchanger as the cooling medium therefor;

flowing said twice turbine expanded fractionally distilled non-condensed gas through a second and third heat exchanger as the cooling medium therefor to cool the inlet gas to produce a first gas-condensate mixture and to further cool the non-condensed gas separated from the first gas-condensate mixture to produce a second gas-condensate mixture, the non-condensed gas of which is fed to the first turbine and the separated condensates are fed separately to the column as reflux; and

recompressing said twice turbine expanded fractionally distilled non-condensed gas after passage through said heat exchangers.