US2818454A

US2818454A - Dehydration of gaseous streams

Info

Publication number: US2818454A
Application number: US391511A
Authority: US
Inventors: Samuel A Wilson
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-11-12
Filing date: 1953-11-12
Publication date: 1957-12-31
Anticipated expiration: 1974-12-31

Description

Dec. 31, 1957 s. A. WILSON DEHYDRATION 0F GASEOUS STREAMS Filed NOV. 12, 1955 IN VEN 7'01? 5 ll. Wilson 5 fitter/Icy Unite St tes Patent DEHYDRATION 0F GASEOUS STREAMS Samuel A. Wilson, Tulsa, Okla.

Application November 12, 1953, Serial No. 391,511

1 Claim. (Cl. 260-676) This invention relates to improvements in the method of dehydrating natural gas streams, and more particularly, is an improvement in dehydrating natural gas in the general manner such as disclosed in United States Letters Patent No. 2,528,028, issued to Arthur F. Barry on October 31, 1950. This application is a continuation in part of applicants co-pending application, Serial No- 221,802, filed April 19, 1951, and now abandoned.

The above mentioned patent to Barry discloses a method of dehydrating natural gas wherein the entire gas stream, consisting of both liquid and vapor components, is initially passed through a heat exchanger. The gas stream is then directed through a knock out unit for the removal of free water from the gas stream. Subsequently, the remaining vapor and hydrocarbon condensates are passed through a liquid bath and then expanded to form hydrates and condense a portion of the hydrocarbon vapors. The hydrates and condensates are precipitated into the liquid bath, and the dehydrated gas is utilized to cool the heat exchanger prior to being transferred to a natural gas plant or the like. The liquid bath must be retained above hydrate formation temperature to melt the precipitated hydrates.

It is readily seen that when the gas stream is cooled previous to the passage thereof through the liquid bath, the heat transfer from the gas stream to the liquid bath will be minimized, usually necessitating an auxiliary heat supply for the liquid bath. Furthermore, cooling of the gas stream previous to the removal of the free water minimizes the reduction of the ultimate dew point of the gas, because the saturated gas cannot be cooled an appreciable degree without forming hydrates.

Another method of dehydration disclosed in the above mentioned patent consists of initially passing the gas stream through the liquid bath and removing the free water from the stream. Subsequently, the remaining stream is expanded as set forth above. In the event the incoming gas stream is at a relatively low pressure and temperature, a sufficient heat transfer is not obtained to retain the liquid bath at the desired temperature, thereby necessitating an auxiliary heat supply for the liquid bath.

The present invention contemplates an improvement of the methods of gas dehydration disclosed in patent No. 2,528,028, wherein the incoming gas stream from the well is initially passed through a liquid bath to assist in retaining the liquid bath at a temperature about hydrate formation. Subsequently the free Water and hydrocarbon condensates are removed from the stream and returned to the liquid bath to assure retention of the bath at a temperature about that of hydrate formation temperature. The remaining vapor components of the stream are then cooled through heat exchange to substantially hydrate formation temperature by returning dehydrated gas pro duced through use of the process. The cooled vapor components of the stream are subsequently expanded in a manner similar to the above mentioned patent. In returning the free water and hydrocarbon condensates to H Patented Dec. 31, 1957 2 the liquid bath, the bath will be retained at the desired temperature above hydrate formation temperature. By cooling only the vapor components of the gas stream, a greater cooling can be provided with a resulting lower dew point (the saturation temperature of a gas at a specified pressure) for the dehydrated gas.

In addition, the present invention contemplates an improvement over applicants co-pending application No. 221,802 wherein the lighter gas constituents of the stream discharged through the reduction valve that are not condensed are specially cooled prior to any discharge from the secondary stage separation vessel, so that they may be effectively used in a subsequent heat exchange function to cool the incoming gas stream prior to discharging into the secondary separating vessel back at a temperature slightly below hydrate formation temperature.

An important object of this invention is to provide an improved method of dehydrating natural gas streams and particularly natural gas streams of relatively low temperature.

Another object of this invention is to provide an improved method of dehydrating natural gas streams utilizing only the heat of the gas to provide the necessary heat transfer.

Another important object of this invention is to provide an improved method of dehydrating natural gas streams wherein the lighter constituents of the gas which are reduced and expanded are passed over the cold zone of the separating vessel without being subjected to the vapors of the hot liquid bath in order to prevent any rise in temperature thereof.

Another object of this invention is to provide an improved method of dehydrating natural gas streams wherein the free condensates in the stream are removed prior to cooling of the stream, thereby providing a low dew point for the dehydrated gas. 7

A further object of this inventionis to provide an efficient and economical method of dehydrating natural gas streams.

Other objects and advantages of this invention will be evident from the following detailed description, read in conjunction with the accompanying drawings, which illustrate my invention.

In the drawings:

The figure is a diagrammatic view of the necessary apparatus for practising the improved method of gas dehydration.

Referring to the drawings in detail, reference character 2 designates a suitable flow line leading from a high pres-.

sure distillate well for the transportation of natural gas therefrom. This gas consists of a mixture of hydrocarbon and water in both the liquid and vapor phases, and is usually at a relatively high temperature and pressure condition, although the condition of the gas will vary in the various localities.

The gas stream flowing through the conduit 2 is initially passed through a coil 4 disposed in the lower portion of a horizontally disposed vessel 6. The vessel 6 comprises,

the second stage separator or dehydration tank of the apparatus utilized in practising my invention. A liquid bath 8 consisting of a mixture of water and hydrocarbon condensates or distillate is maintained in the vessel 6 at a predetermined level 10. The coil 4 is preferably completely immersed in the liquid bath 8 to provide a transfer of heat from the gas stream to the bath 8 and facilitate the maintenance of the bath above hydrate formation temperature corresponding to the pressure condition ex isting in the vessel 6. The temperature of the gas stream is only slightly reduced by the heat transfer to the bath 8;

therefore, the temperature of the gas stream remains above hydrate formation temperature as the gas is dis-.

charged from the coil 4. However, the amount of free water in the gas stream is increased for purposes as will be hereinafter set forth.

A conduit 12 conveys the gas discharging from the coil 4 into the upper portion of another horizontally disposed vessel 14. The vessel 14 comprises the first stage separator or knock out unit and is provided to remove the free water and liquid hydrocarbon components from the gas stream. As the gas stream enters the upper portion of the vessel 14, the free water and liquid hydrocarbon components, being heavier than the gas, fall by gravity into the lower portion of the vessel 14 and accumulate to form a mixture 16. The mixture 16 is periodically withdrawn from the vessel 14 through a conduit 13 and directed into the liquid bath 3 provided in the vessel 6. It will be apparent that the mixture 16 will be at a relatively high temperature, corresponding essentially to the initial temperature of the gas stream. Therefore, the bath 8 may be conveniently maintained at a temperature well above hydrate formation temperature by the injection of the mixture 16 therein. A suitable control valve 20 is interposed in the conduit 18 to control the flow of the mixture 16 into the vessel 6 and hence the temperature of the liquid bath 8.

The vapor components of the gas stream, remaining after the removal of the free water and liquid hydrocarbon components, are discharged from the upper portion of the vessel 14 through a conduit 22 into the tube side (not shown) of a heat exchanger unit 24. The gas stream is cooled during passage through the heat exchanger 2 t by the ultimately dehydrated gas, as will be hereinafter set forth, to substantially hydrate formation temperature. In this manner, the ultimate dew point of the gas stream may be reduced to a minimum as will hereinafter appear. It has been found in actual practice that the gas stream can be slightly super-cooled during passage through the exchanger 24, Without materially afiecting the efi'iciency of the heat exchanger 24 or subsequent flow of the gas stream.

The cooled gas stream is conveyed from the heat exchanger 24 through a conduit 26 into one end 28 of the vessel 6 above the level of the liquid bath 8. A suitable pressure reducing valve 36) is interposed in the conduit 26 adjacent the vessel 6. The valve 30 provides for an expansion of the gas stream, resulting in a drastic pressure reduction and a substantial temperature reduction thereof. Hydrates are thereby formed, and a portion of the hydrocarbon components of the gas stream are condensed. The hydrates, condensates and vapors are discharged into the upper portion of the vessel 6,

whereupon the hydrates and condensates, being heavier than air, are precipitated downwardly by gravity into the liquid bath 8. The hydrates are melted by the relatively hot liquid bath 8 upon contact therewith to facilitate their subsequent removal from the vessel 6.

The present invention contemplates a specific improvement over prior similar processes relating to gas dehydration in that after the gas is passed through the reduction valve 30 and expanded in the secondary separation vessel 6, a certain portion of this gas is formed into hydrates which are melted in the liquid bath 3, however, the lighter constituents of the expanded gas that are not condensed are brought into contact with a vertical baffie member 31 disposed longitudinally at a point beyond the media point of the vessel 6 looking from the end 28 of the vessel at which is disposed the valve 30. The baffie member 31 assures that there is a reverse flow of the lighter dry gas vapors and upon such a reverse How, these gases are permitted to flow through a discharge line 32 extending longitudinally in the upper portion of the tank 6 and in spaced relation to the liquid level 10 of the liquid bath 8 in order. that these dehydrated lighter portions are cooled or remain in a substantially cooler state to be utilized in the heat exchanger 24 as will be hereinafter set forth. The discharge line 32 prevents the hot vapors rising from the liquid bath 8 from contacting the lighter dehydrated gases, and the avoidance of contact with the hot vapors of the bath 8 as well as maintaining these lighter constituents at a lower temperature, substantially to that of the pre-cooling temperature in the upper cold zone of the vessel 6, consequently providing a much more etfective heat exchange for cooling the oncoming gas stream discharging from conduit 22 into the heat exchanger 24. As has been previously mentioned, this temperature is maintained slightly below hydration temperature before discharging through the conduit 26 and through the pressure reduction valve 39 and expanded in the vessel 6. The effective maintenance of this predetermined temperature by utilizing the lighter dehydrated gas for discharge through the conduit 32 back to the heat exchange is a substantial impovement over the process disclosed in applicants prior application, or that of the Barry patent previously mentioned.

The distillate is removed from the vessel 6 through an outlet conduit 34 which extends upwardly in the vessel adjacent an endv 36 thereof in order to provide stratification of the water and distillate in the vessel 6. Distillate flowing from the conduit 34 is transferred to any suitable storage means (not shown) through a suitable control valve 38 interposed in the conduit 34 for controlling the flow and maintaining the level 1! of the bath 8 at a desired height. Water being heavier than distillate will settle in a suitable water leg or sump as provided on the lower side of the vessel 6 and the Water is withdrawn when desired through a drain line 42 which in turn is controlled through a suitable valve 44.

It will thus be apparent that the dry gas vapors, after expansion of the gas stream, remain in the upper cool zone of the vessel 6 and are discharged through the conduit 32. The flowing of the gas stream from the conduit 26 and end 28 of the vessel 6 is through substantially the entire length of the liquid level 10, thereby assuring complete precipitation of all hydrates formed upon expansion. The dehydrated gas will obviously be at a lower temperature and is directed into the outlet conduit 32 in the upper cooling zone of the vessel and hence to the heat exchanger 24 in order to cool 'by heat exchange the gas stream flowing through the tube side thereof as previously set forth. A by-pass (not shown) may be provided in the conduit 32 around the heat exchanger 24 to control the flow of dehydrated gas through the heat exchanger and thereby control the temperature of gas stream discharged into conduit 26. A conduit 48 conveys the dehydrated gas from the heat exchanger 24 to a suitable storage or natural gasoline plant or the like (not shown).

In summation, the gas stream is initially passed through a liquid bath contained in a second stage separator to facilitate maintenance of the bath above hydrate formation condition and to increase the free water content of the gas stream. The free water and liquid hydrocarbon components are then removed from the gas stream in the first stage separator and returned to the liquid bath to assure the retention of the temperature of the bath at the desired degree. Therefore, the temperature of the the liquid bath may be controlled as desired and without the necessity of an auxiliary heat supply, even though the initial temperature of the gas stream is relatively low.

The gas stream remaining after the removal of the free water and liquid hydrocarbon condensates is cooled to substantially hydrate formation temperature by the ultimately dehydrated gas. The cooled gas is then expanded to reduce the pressure and temperature thereof and form hydrates as Well as condense a portion of the remaining hydrocarbon components.

-t will be readily seen that the maintaining of a lowtemperature for the dehydrated. and the lighter constituents of the gas vapors-by discharge through the outlet conduit 32 will provide a lower temperature for heat exchange, thereby maintaining a greater cooling of the gas stream immediately prior to expansion which in turn will provide a greater dehydration.

From the foregoing, it is apparent that the present invention provides an improved method of dehydrating natural gas, particularly adapted for the dehydration of gas streams having a relatively low temperature. The free water and liquid hydrocarbon components of the gas stream are utilized to melt hydrates intentionally formed by expansion of the gas stream, thereby negativing the necessity of an auxiliary heat supply. The non-dehydrated lighter constituents of the gas are cooled subsequent to the expansion of the gas stream, thereby maintaining a maximum low temperature on the gas which is discharged for return to the heat exchange in order to maintain a temperature slighly below hydration formation temperature prior to expansion and thus obtaining a maximum ultimate dehydration of the gas stream. A gas stream may therefore be dehydrated to a greater extent than has formerly been possible and the inherent properties of the gas are utilized the maximum degree. it is also apparent that the present invention provides an economical method of dehydrating natural gas existing at a high temperature and pressure.

It will thus be apparent that the present process provides a cooler gas in heat exchange thereby reducing the volume of heat exchange required and is particularly efiective on gas streams produced in hot wells.

The free water and liquid hydrocarbon components that were knocked out in the primary separation chamber 14, and hence discharged through the conduit 1% to the liquid bath 8 may be lay-passed through a suitable conduit (not shown) for entry into an inlet (not shown) in proximity of the end 36 of the separator 6, in order to come in contact with a weir type baffle member 49 thereby directing the free water from conduit 18 into the opposite end 36 of the vessel 6 as distinguished from the end 28. The by-passing of the knock-out liquids is preferably usually done in wells having a high parafiin content in the influent mixture in order to discharge it against the weir type baffle 49 disposed at the cooler end of the vessel 6, rather than discharging it into contact with the hot liquid bath 8 which would have a tendency to melt the paraffin and cause deposit of the parafin against the coils 4 disposed in the bath 8.

Changes may be made in the combination and arrangement of parts as heretofore set forth in the specification and shown in the drawings, it being understood that any modification in the precise embodiment of the invention may be made within the scope of the following claim without departing from the spirit of the invention.

1 claim:

A method of dehydrating a natural gas stream, consisting of passing the gas stream through a liquid bath to trans fer a portion of the heat of the stream to the liquid bath, removing the free water and hydrocarbon condensates from the stream, cooling the remaining vapor components of the stream to substantially hydrate formation temperature, expanding the cooled gas to form hydrates, precipitating the hydrates into the liquid bath, directing the separated free Water and hydrocarbon components directly into the liquid bath to retain the liquid bath above hydrate formation temperature for melting the hydrates, separating the water and hydrocarbon components of the liquid bath for separate removal, utilizing the dehydrated gas to cool the vapor components of the gas stream, directing the dehydrated gas through the cooling stage of the infiuent vapor components in a reverse direction thereto and simultaneously preventing contact therewith of the hot gases from the liquid bath in order to maintain the dehydrated gas at a temperature sufficient to cool the incoming vapor components of the stream.

References Cited in the file of this patent UNITED STATES PATENTS 2,356,407 Hutchinson Aug. 22, 1944 2,399,723 Crowther May 7, 1946 2,528,028 Barry Oct. 31, 1950 2,538,947 Ragatz Jan. 23, 1951 2,665,565 Parks Jan. 12, 1954