Cryogenic system for removing acid gases from a hydrocarbon gas stream, and method of removing acid gases

A system for removing acid gases from a raw gas stream the system includes a cryogenic distillation tower. The tower receives and separates the raw gas stream into an overhead methane stream and a bottom liquefied acid gas stream. Refrigeration equipment downstream of the cryogenic distillation tower cools the overhead methane stream and returns a portion of the overhead methane stream to the cryogenic distillation tower as liquid reflux. The system also may include a first molecular sieve bed upstream of the distillation tower and a second molecular sieve bed downstream of the distillation tower. The first molecular sieve bed adsorbs water while the second molecular sieve bed adsorbs additional acid gases from the cooled overhead methane stream.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of fluid separation. More specifically, the present invention relates to the separation of carbon dioxide and other acid gases from a hydrocarbon fluid stream.

2. Discussion of Technology

The production of hydrocarbons from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants such as hydrogen sulfide (H2S) and carbon dioxide (CO2). When H2S and CO2are produced as part of a hydrocarbon gas stream (such as methane or ethane), the gas stream is sometimes referred to as “sour gas.”

Sour gas is usually treated to remove CO2, H2S, and other contaminants before it is sent downstream for further processing or sale. The separation process creates an issue as to the disposal of the separated contaminants. In some cases, the concentrated acid gas (consisting primarily of H2S and CO2) is sent to a sulfur recovery unit (“SRU”). The SRU converts the H2S into benign elemental sulfur. However, in some areas (such as the Caspian Sea region), additional elemental sulfur production is undesirable because there is a limited market. Consequently, millions of tons of sulfur have been stored in large, above-ground blocks in some areas of the world, most notably Canada and Kazakhstan.

While the sulfur is stored on land, the carbon dioxide gas is oftentimes vented to the atmosphere. However, the practice of venting CO2is sometimes undesirable. One proposal to minimizing CO2emissions is a process called acid gas injection (“AGI”). AGI means that unwanted sour gases are re-injected into a subterranean formation under pressure and sequestered for potential later use. Alternatively, the sour gases are used to create artificial reservoir pressure for enhanced oil recovery operations.

For AGI, it is necessary to have a gas processing facility that separates out the acid gas components from the hydrocarbon gases. However, for “highly sour” streams, that is, production streams containing greater than about 15% CO2and H2S, it can be particularly challenging to design, construct, and operate a facility that can economically separate contaminants from the desired hydrocarbons. Many natural gas reservoirs contain relatively low percentages of hydrocarbons (less than 40%, for example) and high percentages of acid gases, principally carbon dioxide, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide and various mercaptans. In these instances, cryogenic gas processing may be beneficially employed.

Cryogenic gas processing is a distillation process sometimes used for gas separation. Cryogenic gas separation generates a cooled and liquefied gas stream at moderate pressures (e.g., 350-600 pounds per square inch gauge (psig)). In the case of cryogenic distillation of sour gas, liquefied acid gas is generated as a “bottoms” product. Since liquefied acid gas has a relatively high density, hydrostatic head can be beneficially used in an AGI well to assist in the injection process. This means that the energy required to pump the liquefied acid gas into the formation is lower than the energy required to compress low-pressure acid gases to reservoir pressure. Thus, cryogenic gas processing is particularly suitable for AGI.

Challenges also exist with respect to cryogenic distillation of sour gases. When CO2is present at concentrations greater than about 5 mol. percent in the gas to be processed, it will freeze out as a solid in a standard cryogenic distillation unit. The formation of CO2as a solid disrupts the cryogenic distillation process. To circumvent this problem, the assignee has previously designed various “Controlled Freeze Zone™” (CFZ™) processes. The CFZ™ process takes advantage of the propensity of carbon dioxide to form solid particles by allowing frozen CO2particles to form within an open portion of the distillation tower, and then capturing the particles on a melt tray. As a result, a clean methane stream (along with any nitrogen or helium present in the raw gas) is generated at the top of the tower, while a liquid CO2/H2S stream is generated at the bottom of the tower.

Certain aspects of the CFZ™ process and associated equipment are described in U.S. Pat. Nos. 4,533,372; 4,923,493; 5,062,270; 5,120,338; and 6,053,007.

As generally described in the above U.S. patents, the distillation tower, or column, used for cryogenic gas processing includes a lower distillation zone and an intermediate controlled freezing zone. Preferably, an upper rectification zone is also included. The column operates to create solid CO2particles by providing a portion of the column having a temperature range below the freezing point of carbon dioxide, but above the boiling temperature of methane at that pressure. More preferably, the controlled freezing zone is operated at a temperature and pressure that permits methane and other light hydrocarbon gases to vaporize, while causing CO2to form frozen (solid) particles.

As the gas feed stream moves up the column, frozen CO2particles break out of the feed stream and gravitationally descend from the controlled freezing zone onto a melt tray. There, the particles liquefy. A carbon dioxide-rich liquid stream then flows from the melt tray down to the lower distillation zone at the bottom of the column. The lower distillation zone is maintained at a temperature and pressure at which substantially no carbon dioxide solids are formed, but dissolved methane boils out. In one aspect, a bottom acid gas stream is created at 30° to 40° F.

The controlled freeze zone includes a cold liquid spray. This is a methane-enriched liquid stream known as “reflux.” As the vapor stream of light hydrocarbon gases and entrained sour gases moves upward through the column, the vapor stream encounters the liquid spray. The cold liquid spray aids in breaking out solid CO2particles while permitting methane gas to evaporate and flow upward in the column.

In the upper rectification zone, the methane (or overhead gas) is captured and piped away for sale or made available for fuel. In one aspect, the overhead methane stream is released at about −130° F. The overhead gas may be partially liquefied by additional cooling, and the liquid returned to the column as “reflux.” The reflux liquid is injected as the cold spray into the spray section of the controlled freezing zone.

The methane produced in the upper distillation zone meets most specifications for pipeline delivery. For example, the methane can meet a pipeline CO2specification of less than 2 mol. percent, as well as a 4 ppm H2S specification, if sufficient reflux is generated. However, more stringent CO2specifications for natural gas exist for applications such as helium recovery, cryogenic natural gas liquids recovery, conversion to liquid natural gas (LNG), and nitrogen rejection.

The more stringent CO2specifications may be met by increasing the quantity of liquid methane reflux. This, in turn, requires more and/or larger refrigeration equipment. In this respect, the process of generating cold liquid methane for the spray section requires equipment ancillary to the CFZ tower. This equipment includes pipes, nozzles, compressors, separators, pumps, and expansion valves. The more vigorously the operator wishes to remove CO2, the greater the refrigeration requirements become.

There is a need to reduce the refrigeration requirements of the CFZ process while still reducing the CO2content down to very low levels. There is also a need for a cryogenic gas separation system and accompanying processes that are augmented by other CO2removal techniques. Further, there is a need for a cryogenic gas separation process that is able to reduce the CO2and H2S content of the gas down to levels acceptable for downstream liquefaction processes without increasing refrigeration equipment capacity.

SUMMARY OF THE INVENTION

A system for removing acid gases from a raw gas stream is provided. In one embodiment, the system includes a cryogenic distillation tower. The distillation tower may have a lower distillation zone and an intermediate controlled freezing zone. The intermediate controlled freezing zone, or spray section, receives a cold liquid spray comprised primarily of methane. The cold spray is a liquid reflux generated from an overhead loop downstream of the distillation tower.

The distillation tower operates to remove acid gases. In this respect, the distillation tower receives and then separates the raw gas stream into an overhead methane stream and a bottom acid gas stream. The system also includes refrigeration equipment. Some of the refrigeration equipment may be used to pre-chill the feed gas, while the remainder of the refrigeration equipment resides downstream of the cryogenic distillation tower for cooling the overhead methane stream. A portion of the overhead methane stream is returned to the cryogenic distillation tower as the liquid reflux.

The distillation tower also includes a solid adsorbent bed downstream of the cryogenic distillation tower. The solid adsorbent bed operates to adsorb at least some remaining acid gases from the cooled overhead methane stream.

In the present system, the refrigeration equipment is designed to generate liquid reflux in a sufficient quantity to remove acid gases from the raw gas stream within the cryogenic distillation tower down to a first selected composition. This means that some percentage or amount of acid gas composition is intentionally released within the overhead methane stream. The overhead methane stream is delivered to the solid adsorbent bed downstream of the cryogenic distillation tower to remove remaining acid gases down to a second lower composition. Thus, the solid adsorbent bed downstream of the cryogenic distillation tower provides post-polishing of the methane stream. It further allows a reduction in refrigeration equipment or capacity that might otherwise need to be devoted to generating liquid reflux.

In one aspect, the acid gases comprise carbon dioxide. The quantity of the generated liquid reflux is sufficient to remove carbon dioxide from the raw gas stream down to a first composition of about 1 to 4 mol. percent. At the same time, the solid adsorbent bed downstream of the cryogenic distillation tower removes additional carbon dioxide from the overhead methane stream down to a second composition. For example, the second composition may be about 0.2 to 3.0 mol. percent.

In another aspect, the quantity of the generated liquid reflux is sufficient to remove carbon dioxide from the raw gas stream down to a first composition that meets pipeline specifications. At the same time, the solid adsorbent bed downstream of the cryogenic distillation tower removes carbon dioxide from the overhead methane stream down to a second composition that meets LNG specifications. For example, the solid adsorbent bed downstream of the cryogenic distillation tower may remove carbon dioxide from the overhead methane stream down to a composition less than about 100 parts per million (“ppm”) or, more preferably, down to a composition less than about 50 ppm.

In yet another aspect, the acid gases further comprise hydrogen sulfide. In this instance, the solid adsorbent bed downstream of the cryogenic distillation tower may remove hydrogen sulfide from the overhead methane stream down to a composition less than about 10 ppm or, more preferably, down to a composition less than about 4 ppm.

In one arrangement, at least one solid adsorbent bed downstream of the cryogenic distillation tower is configured to replace at least one solid adsorbent bed upstream of the cryogenic distillation tower in a regeneration cycle.

A method for removing acid gases from a raw gas stream is also provided herein. In one aspect the method includes providing a cryogenic distillation tower. The tower has a lower distillation zone and an intermediate controlled freezing zone that receives a cold liquid spray comprised primarily of methane. The method also includes receiving the raw gas stream into the cryogenic distillation tower.

After receiving the raw gas stream, the method includes separating the raw gas stream in the cryogenic distillation tower into an overhead methane stream and a bottom acid gas stream. The method then includes passing the overhead methane stream through a refrigeration system downstream of the cryogenic distillation tower. The refrigeration system cools the overhead methane stream.

The method further includes returning a first portion of the cooled overhead methane stream to the cryogenic distillation tower as liquid reflux. The liquid reflux serves as the cold liquid spray within the tower. The method also includes passing a second portion of the cooled overhead methane stream through a solid adsorbent bed downstream of the cryogenic distillation tower. This serves to remove additional acid gases and to generate a polished gas stream. Preferably, the solid adsorbent bed is a molecular sieve bed.

The method also includes designing the refrigeration system to produce liquid reflux at a sufficient quantity as to remove acid gases from the raw gas stream within the cryogenic distillation tower down to a first selected composition, and deliver the overhead methane stream to the solid adsorbent bed downstream of the cryogenic distillation tower to remove additional acid gases down to a second lower composition.

In one aspect, the acid gases comprise carbon dioxide. In this instance, the quantity of the generated liquid reflux is sufficient to remove carbon dioxide from the raw gas stream down to a first composition of about 1 to 4 mol. percent. In addition, the solid adsorbent bed downstream of the cryogenic distillation tower removes additional carbon dioxide from the overhead methane stream down to a second composition of about 0.2 to 3.0 mol. percent.

In one arrangement, the quantity of the generated liquid reflux is sufficient to remove carbon dioxide from the raw gas stream down to a first composition that meets pipeline specifications. For example, the pipeline specifications may require a carbon dioxide content that is less than about 3.0 mol. percent. The solid adsorbent bed downstream of the cryogenic distillation tower then removes carbon dioxide from the overhead methane stream down to a second composition that meets LNG specifications. For example, the LNG specifications may require a carbon dioxide content that is less than about 100 ppm or even about 50 ppm.

In another embodiment of the method, the acid gases further comprise hydrogen sulfide. In this instance, the solid adsorbent bed downstream of the cryogenic distillation tower may remove hydrogen sulfide from the overhead methane stream down to a composition less than about 10 ppm. Preferably, the solid adsorbent bed downstream of the cryogenic distillation tower is a molecular sieve bed, and the molecular sieve bed removes hydrogen sulfide from the overhead methane stream down to a composition less than about 4 ppm.

In one aspect, the method further includes the step of at least partially dehydrating the raw gas stream by passing the raw gas stream through a solid adsorbent bed upstream of the cryogenic distillation tower. Preferably, the solid adsorbent bed upstream of the cryogenic distillation tower is also a molecular sieve bed. The method may then further include replacing the molecular sieve bed upstream of the cryogenic distillation tower when the molecular sieve bed upstream of the cryogenic distillation tower becomes substantially saturated with water. When the molecular sieve bed upstream of the cryogenic distillation tower becomes substantially saturated with water, the molecular sieve bed upstream of the cryogenic distillation tower is replaced with a molecular sieve bed that was previously used to adsorb carbon dioxide downstream of the cryogenic distillation tower.

The method may further include regenerating the molecular sieve bed upstream of the cryogenic distillation tower after the molecular sieve bed upstream of the cryogenic distillation tower becomes substantially saturated with water. Regenerating the molecular sieve bed upstream of the cryogenic distillation tower may involve either thermal swing adsorption or pressure swing adsorption. When the molecular sieve bed downstream of the cryogenic distillation tower becomes substantially saturated with carbon dioxide, the molecular sieve bed may be replaced with a regenerated bed previously in service for dehydration upstream of the cryogenic distillation tower.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Definitions

As used herein, the term “hydrocarbon” refers to an organic compound that includes primarily, if not exclusively, the elements hydrogen and carbon. Hydrocarbons generally fall into two classes: aliphatic, or straight chain hydrocarbons, and cyclic, or closed ring hydrocarbons, including cyclic terpenes. Examples of hydrocarbon-containing materials include any form of natural gas, oil, coal, and bitumen that can be used as a fuel or upgraded into a fuel.

As used herein, the term “hydrocarbon fluids” refers to a hydrocarbon or mixtures of hydrocarbons that are gases or liquids. For example, hydrocarbon fluids may include a hydrocarbon or mixtures of hydrocarbons that are gases or liquids at formation conditions, at processing conditions or at ambient conditions (15° C. and 1 atm pressure). Hydrocarbon fluids may include, for example, oil, natural gas, coal bed methane, shale oil, pyrolysis oil, pyrolysis gas, a pyrolysis product of coal, and other hydrocarbons that are in a gaseous or liquid state.

The term “mass transfer device” refers to any object that receives fluids to be contacted, and passes those fluids to other objects, such as through gravitational flow. One non-limiting example is a tray for stripping out certain fluids. A grid packing is another example.

As used herein, the term “fluid” refers to gases, liquids, and combinations of gases and liquids, as well as to combinations of gases and solids, and combinations of liquids and solids.

As used herein, the term “condensable hydrocarbons” means those hydrocarbons that condense at about 15° C. and one atmosphere absolute pressure. Condensable hydrocarbons may include, for example, a mixture of hydrocarbons having carbon numbers greater than 4.

As used herein, the term “closed loop refrigeration system” means any refrigeration system wherein an external working fluid such as propane or ethylene is used as a coolant to chill an overhead methane stream. This is in contrast to an “open loop refrigeration system” wherein a portion of the overhead methane stream itself is used as the working fluid.

As used herein, the term “subsurface” refers to geologic strata occurring below the earth's surface.

Description of Specific Embodiments

FIG. 1presents a schematic view of a cryogenic distillation tower100as may be used in connection with the present inventions, in one embodiment. The cryogenic distillation tower100may be interchangeably referred to herein as a “cryogenic distillation tower,” a “column,” a “CFZ column,” or a “splitter tower.”

The cryogenic distillation tower100ofFIG. 1receives an initial fluid stream10. The fluid stream10is comprised primarily of production gases. Typically, the fluid stream represents a dried gas stream from a wellhead (not shown), and contains about 65% to about 95% methane. However, the fluid stream10may contain a lower percentage of methane, such as about 30% to 65%, or even 20% to 30%.

The methane may be present along with trace elements of other light hydrocarbon gases such as ethane. In addition, trace amounts of helium and nitrogen may be present. In the present application, the fluid stream10will also include certain contaminants. These are acid gases such as CO2and H2S.

The initial fluid stream10may be at a post-production pressure of approximately 600 pounds per square inch (psi). In some instances, the pressure of the initial fluid stream10may be up to about 750 psi or even 1,000 psi.

The fluid stream10is typically chilled before entering the distillation tower100. A heat exchanger150, such as a shell-and-tube exchanger, is provided for the initial fluid stream10. A refrigeration unit (not shown) provides cooling fluid (such as liquid propane) to heat exchanger150to bring the temperature of the initial fluid stream10down to about −30° to −40° F. The chilled fluid stream may then be moved through an expansion device152. The expansion device152may be, for example, a Joule-Thompson (“J-T”) valve.

The expansion device152serves as an expander to obtain additional cooling of the fluid stream10. Preferably, partial liquefaction of the fluid stream10is also created. A Joule-Thompson (or “J-T”) valve is preferred for gas feed streams that are prone to forming solids. The expansion device152is preferably mounted close to the cryogenic distillation tower100to minimize heat loss in the feed piping.

As an alternative to a J-T valve, the expander device152may be a turbo-expander. A turbo-expander provides greater cooling and creates a source of shaft work for processes like the refrigeration unit mentioned above. The refrigeration unit is part of the heat exchanger150. In this manner, the operator may minimize the overall energy requirements for the distillation process. However, the turbo-expander may not handle frozen particles as well as the J-T valve.

In either instance, the heat exchanger150and the expander device152convert the raw gas in the initial fluid stream10into a chilled fluid stream12. Preferably, the temperature of the chilled fluid stream12is around −40° to −70° F. In one aspect, the cryogenic distillation tower100is operated at a pressure of about 550 psi, and the chilled fluid stream12is at approximately −62° F. At these conditions, the chilled fluid stream12is in a substantially liquid phase, although some vapor phase may inevitably be entrained into the chilled fluid stream12. Most likely, no solids formation has arisen from the presence of CO2.

The cryogenic distillation tower100is divided into three primary sections. These are a lower distillation zone, or “stripping section”106, an intermediate controlled freezing zone, or “spray section”108, and an upper distillation section, or “rectification section”110. In the tower arrangement ofFIG. 1, the chilled fluid stream12is introduced into the distillation tower100the spray section108. However, the chilled fluid stream12may alternatively be introduced into near the top of the stripping section106.

It is noted in the arrangement ofFIG. 1that the stripping section106, the intermediate spray section108, the rectification section108, and all the components are housed within a single vessel100. However, for offshore applications in which height of the tower100and motion considerations may need to be considered, or for remote locations in which transportation limitations are an issue, the tower100may optionally be split into two separate pressure vessels (not shown). For example, the stripping section106and the spray section108may be located in one vessel, while the rectification section108is in another section. External piping would then be used to interconnect the two vessels.

In either embodiment, the temperature of the stripping section106is higher than the feed temperature of the chilled fluid stream12. The temperature of the stripping section106is designed to be well above the boiling point of the methane in the chilled fluid stream12at the operating pressure of the column100. In this manner, methane is preferentially stripped from the heavier hydrocarbon and liquid acid gas components. Of course, those of ordinary skill in the art will understand that the liquid within the distillation tower100is a mixture, meaning that the liquid will “boil” at some intermediate temperature between pure methane and pure CO2. Further, in the event that there are heavier hydrocarbons present in the mixture, this will increase the boiling temperature of the mixture. These factors become design considerations for the operating temperatures within the distillation tower100.

In the stripping section106, the CO2and any other liquid-phase fluids gravitationally fall towards the bottom of the cryogenic distillation tower100. At the same time, methane and other vapor-phase fluids break out and rise upwards towards the top of the tower100. This separation is accomplished primarily through the density differential between the gas and liquid phases. However, the separation process is optionally aided by internal components within the distillation tower100. As described below, these include a melt tray130, a plurality of advantageously-configured mass transfer devices126, and an optional heater line25. Side reboilers (not shown) may likewise be added to the stripping section106to facilitate removal of CO2and heat transfer.

Referring again toFIG. 1, the chilled fluid stream12may be introduced into the column100near the top of the stripping section106. Alternatively, it may be desirable to introduce the feed stream12into the intermediate spray section108above the melt tray130. The point of injection of the chilled fluid stream12is a design issue dictated by the composition of the initial fluid stream10.

Where the temperature of the fluid stream10is high enough (such as greater than −70° C.) such that solids are not expected, it may be preferable to inject the chilled fluid stream12directly into the stripping section106through a two-phase flashbox type device (or vapor distributor)124in the column100. The use of a flashbox124serves to separate the two-phase vapor-liquid mixture in the chilled fluid stream12. The flashbox124may be slotted such that the two-phase fluid impinges against baffles in the flashbox124.

If solids are anticipated due to a low inlet temperature, the chilled fluid stream12may need to be partially separated in a vessel173prior to feeding the column100as described above. In this case, the chilled feed stream12may be separated in a two phase vessel173to minimize the possibility of solids plugging the inlet line and internal components of the column100. Gas vapor leaves the two phase vessel173through a vessel inlet line11, where it enters the column100through an inlet distributor121. The gas then travels upward through the column100. A liquid/solid slurry13is discharged from the two phase vessel173. The liquid/solid slurry is directed into the column100through the vapor distributor124and to the melt tray130. The liquid/solid slurry13can be fed to the column100by gravity or by a pump175.

In either arrangement, that is, with or without the two phase vessel173, the chilled fluid stream12(or11) enters the column100. The liquid component leaves the flashbox124and travels down a collection of stripping trays126within the stripping section106. The stripping trays126include a series of downcomers129and weirs128. These are described more fully below in connection withFIG. 3. The stripping trays126, in combination with the warmer temperature in the stripping section106, cause methane and other light gases to break out of solution. The resulting vapor carries the methane and light gases along with carbon dioxide molecules that have boiled off.

The vapor further proceeds upward through risers131of the melt tray130and into the freeze zone108. The melt tray risers131act as a vapor distributor for uniform distribution through the freeze zone108. The vapor will then contact cold liquid from spray headers120to “freeze out” the CO2. Stated another way, CO2will freeze and then “snow” back onto the melt tray130. The solid CO2then melts and gravitationally flows in liquid form down the melt tray130and through the stripping section106there below.

As will be discussed more fully below, the spray section108is an intermediate freeze zone of the cryogenic distillation tower100. With the alternate configuration in which the chilled fluid stream12is separated in vessel173prior to entering the tower100, the separated liquid/solid slurry13is introduced into the tower100on to the melt tray130. Thus, a liquid-solid mixture of sour gas and heavier hydrocarbon components will flow from the distributor121, with solids and liquids falling down onto the melt tray130.

The melt tray130is configured to gravitationally receive liquid and solid materials, primarily CO2and H2S, from the intermediate spray section108. The melt tray130serves to warm the liquid and solid materials and direct them downward through the stripping section106in liquid form for further purification. The melt tray130collects and warms the solid-liquid mixture from the intermediate spray section108in a pool of liquid. The melt tray130is designed to release vapor flow back to the intermediate spray section108, to provide adequate heat transfer to melt the solid CO2, and to facilitate liquid/slurry drainage to the lower distillation or stripping section106of the column100below the melt tray130.

FIG. 2Aprovides a plan view of the melt tray130, in one embodiment.FIG. 2Bprovides a cross-sectional view of the melt tray130, taken across line B-B ofFIG. 2A.FIG. 2Cshows a cross-sectional view of the melt tray130, taken across line C-C. The melt tray130will be described with reference to these three drawings collectively.

First, the melt tray130includes a base134. The base134may be a substantially planar body. However, in the preferred embodiment shown inFIGS. 2A, 2B and 2C, the base134employs a substantially non-planar profile. The non-planar configuration provides an increased surface area for contacting liquids and solids landing on the melt tray130from the intermediate spray section108. This serves to increase heat transfer from the vapors passing up from the stripping section106of the column100to the liquids and thawing solids. In one aspect, the base134is corrugated. In another aspect, the base134is substantially sinusoidal. This aspect of the tray design is shown inFIG. 2B. It is understood that other non-planar geometries may alternatively be used to increase the heat transfer area of the melt tray130.

The melt tray base134is preferably inclined. The incline is demonstrated in the side view ofFIG. 2C. Although most solids should be melted, the incline serves to ensure that any unmelted solids in the liquid mixture drain off of the melt tray130and into the stripping section106there below.

In the view ofFIG. 2C, a sump or “downcomer”138is seen central to the melt tray130. The melt tray base134slopes inwardly towards the downcomer138to deliver the solid-liquid mixture. The base134may be sloped in any manner to facilitate gravitational liquid draw-off.

As described in U.S. Pat. No. 4,533,372, the melt tray was referred to as a “chimney tray.” This was due to the presence of a single venting chimney. The chimney provided an opening through which vapors may move upward through the chimney tray. However, the presence of a single chimney meant that all gases moving upward through the chimney tray had to egress through the single opening. On the other hand, in the melt tray130ofFIGS. 2A, 2B and 2C, a plurality of chimneys131(or “risers”) is provided. The use of multiple chimneys131provides improved vapor distribution. This contributes to better heat/mass transfer in the intermediate spray section108.

The chimneys131may be of any profile. For instance, the chimneys131may be round, rectangular, or any other shape that allows vapor to pass through the melt tray130. The chimneys131may also be narrow and extend upwards into the intermediate spray section108. This enables a beneficial pressure drop to distribute the vapor evenly as it rises into the CFZ intermediate spray section108. The chimneys131are preferably located on peaks of the corrugated base134to provide additional heat transfer area.

The top openings of the chimneys131are preferably covered with hats or caps132. This minimizes the chance that solids dropping from the spray section108can avoid falling onto the melt tray130. InFIGS. 2A, 2B and 2C, caps132are seen above each of the chimneys131.

The melt tray130may also be designed with bubble caps. The bubble caps define convex indentations in the base134rising from underneath the melt tray130. The bubble caps further increase surface area in the melt tray130to provide additional heat transfer to the CO2-rich liquid. With this design, a suitable liquid draw off, such as an increased incline angle, should be provided to insure that liquid is directed to the stripping trays126below.

Referring again toFIG. 1, the melt tray130may also be designed with an external liquid transfer system. The transfer system serves to ensure that all liquid is substantially free of solids and that sufficient heat transfer has been provided. The transfer system first includes a draw-off nozzle136. In one embodiment, the draw-off nozzle136resides within the draw-off sump, or downcomer138. Fluids collected in downcomer138are delivered to a transfer line135. Flow through transfer line135is controlled by a control valve137and a level controller “LC” (seen inFIG. 1). Fluids are returned to the stripping section106via the transfer line135. If the liquid level is too high, the control valve137opens; if the level is too low, the control valve137closes. If the operator chooses not to employ the transfer system in the stripping section106, then the control valve137is closed and fluids are directed immediately to the mass transfer devices, or “stripping trays”126below the melt tray130for stripping via an overflow downcomer139.

Whether or not an external transfer system is used, solid CO2is warmed on the melt tray130and converted to a CO2-rich liquid. The melt tray130is heated from below by vapors from the stripping section106. Supplemental heat may optionally be added to the melt tray130or just above the melt tray base134by various means such as heater line25. The heater line25utilizes thermal energy already available from a bottom reboiler160to facilitate thawing of the solids.

The CO2-rich liquid is drawn off from the melt tray130under liquid level control and gravitationally introduced to the stripping section106. As noted, a plurality of stripping trays126are provided in the stripping section106below the melt tray130. The stripping trays126are preferably in a substantially parallel relation, one above the other. Each of the stripping trays126may optionally be positioned at a very slight incline, with a weir such that a liquid level is maintained on the tray. Fluids gravitationally flow along each tray, over the weir, and then flow down onto the next tray via a downcomer.

The stripping trays126may be in a variety of arrangements. The stripping trays126may be arranged in generally horizontal relation to form a sinusoidal, cascading liquid flow. However, it is preferred that the stripping trays126be arranged to create a cascading liquid flow that is divided by separate stripping trays substantially along the same horizontal plane. This is shown in the arrangement ofFIG. 3, where the liquid flow is split at least once so that liquid falls into two opposing downcomers129.

FIG. 3provides a side view of a stripping tray126arrangement, in one embodiment. Each of the stripping trays126receives and collects fluids from above. Each stripping tray126preferably has a weir128that serves as a dam to enable the collection of a small pool of fluid on each of the stripping trays126. The buildup may be ½ to 1 inch, though any height may be employed. A waterfall effect is created by the weirs128as fluid falls from one tray126on to a next lower tray126. In one aspect, no incline is provided to the stripping trays126, but the waterfall effect is created through a higher weir128configuration. The fluid is contacted with upcoming vapor rich in lighter hydrocarbons that strip out the methane from the cross flowing liquid in this “contact area” of the trays126. The weirs128serve to dynamically seal the downcomers129to prevent vapor bypassing through the downcomers129and to further facilitate the breakout of hydrocarbon gases.

The percentage of methane in the liquid becomes increasingly small as the liquid moves downward through the stripping section106. The extent of distillation depends on the number of trays126in the stripping section106. In the upper part of the stripping section106, the methane content of the liquid may be as high as 25 mol percent, while at the bottom stripping tray the methane content may be as low as 0.04 mol percent. The methane content flashes out quickly along the stripping trays126(or other mass transfer devices). The number of mass transfer devices used in the stripping section106is a matter of design choice based on the composition of the raw gas stream10. However, only a few levels of stripping trays126need be utilized to remove methane to a desired level of 1% or less in the liquefied acid gas, for example.

Various individual stripping tray126configurations that facilitate methane breakout may be employed. The stripping tray126may simply be a panel with sieve holes or bubble caps. However, to provide further heat transfer to the fluid and to prevent unwanted blockage due to solids, so called “jet trays” may be employed below the melt tray. In lieu of trays, random or structured packing may also be employed.

FIG. 4Aprovides a plan view of an illustrative jet tray426, in one embodiment.FIG. 4Bprovides a cross-sectional view of a jet tab422from the jet tray426. As shown, each jet tray426has a body424, with a plurality of jet tabs422formed within the body424. Each jet tab422includes an inclined tab member428covering an opening425. Thus, a jet tray426has a plurality of small openings425.

In operation, one or more jet trays426may be located in the stripping106and/or rectification110sections of the tower100. The trays426may be arranged with multiple passes such as the pattern of stripping trays126inFIG. 3. However, any tray or packing arrangement may be utilized that facilitates the breakout of methane gas. Fluid cascades down upon each jet tray426. The fluids then flow along the body424. The fluid is then contacted with the vapor exiting the openings425. The tabs422are optimally oriented to move the fluid quickly and efficiently across the tray426. An adjoined downcomer (not shown) may optionally be provided to move the liquid to the subsequent tray426. The openings425also permit gas vapors released during the fluid movement process in the stripping section106to travel upwards more efficiently to the melt tray130and through the chimneys131.

In one aspect, the trays126or426may be fabricated from fouling-resistant materials, that is, materials that prevent solids-buildup. Fouling-resistant materials are utilized in some processing equipment to prevent the buildup of metal particles, polymers, salts, hydrates, catalyst fines, or other chemical solids compounds. In the case of the cryogenic distillation tower100, fouling resistant materials may be used in the trays126or426to limit sticking of CO2solids. For example, a Teflon™ coating may be applied to the surface of the trays126or426.

Alternatively, a physical design may be provided to ensure that the CO2does not start to build up in solid form along the inner diameter of the column100. In this respect, the jet tabs422may be oriented to push liquid along the wall of the column100, thereby preventing solids accumulation along the wall of the column100and ensuring good vapor-liquid contact.

In any of the tray arrangements, as the down-flowing liquid hits the stripping trays126, separation of components occurs. Methane gas breaks out of solution and moves upward in vapor form. The CO2, however, is cold enough and in high enough concentration that it remains in its liquid form and travels down to the bottom of the stripping section106. The liquid is then moved out of the cryogenic distillation tower100in an exit line as an exit fluid stream22.

Upon exiting the distillation tower100, the exit fluid stream22enters a reboiler160. InFIG. 1, the reboiler160is a kettle type that provides reboiled vapor to the bottom of the stripping trays. A reboiled vapor line is seen at27. In addition, reboiled vapor may be delivered through a heater line25to provide supplemental heat to the melt tray130. The supplemental heat is controlled through a valve165and temperature controller TC. Alternately, a heat exchanger, such as a thermosyphon heat exchanger (not shown) may be used for the initial fluid stream10to economize energy. In this respect, the liquids entering the reboiler160remain at a relatively low temperature, for example, about 30° to 40° F. By heat integrating with the initial fluid stream10, the operator may warm the cold exit fluid stream22from the distillation tower100while cooling the production fluid stream10. For this case, the fluid providing supplemental heat through line25is a mixed phase return from the reboiler160.

It is contemplated that under some conditions, the melt tray130may operate without heater line25. In these instances, the melt tray130may be designed with an internal heating feature such as an electric heater. However, it is preferred that a heat system be offered that employs the heat energy available in exit fluid stream22. The warm fluids in heater line25after exiting reboiler160exist in one aspect at 30° to 40° F., so they contain relative heat energy. Thus, inFIG. 1, vapor stream25is shown being directed to the melt tray130through a heating coil (not shown) on the melt tray130. The vapor stream25may alternatively be tied to the transfer line135.

In operation, the reboiled vapor stream is introduced at the bottom of the column through line27, above the bottom liquid level and below the last stripping tray126. As the reboiled vapor passes through each tray126, residual methane is stripped out of the liquid. This vapor cools off as it travels up the tower. By the time the stripping vapors of stream27reach the corrugated melt tray130, the temperature may have dropped to about −20° F. to 0° F. However, this remains quite warm compared to the melting solid on the melt tray130, which may be around −50° F. to −80° F. The vapor still has enough enthalpy to melt the solids CO2as it comes in contact with the melt tray130to warm the melt tray130.

Referring back to reboiler160, fluids in a bottom stream24that exit the reboiler160in liquid form may optionally pass through an expander valve162. The expander valve162reduces the pressure of the bottom liquid product, effectively providing a refrigeration effect. Thus, a chilled bottom stream26is provided. This also creates hydrostatic head. In this respect, the CO2-rich liquid exiting the reboiler160may be pumped downhole through one or more AGI wells (seen schematically at250inFIG. 1). In some situations, the liquid CO2may be pumped into a partially recovered oil reservoir as part of an enhanced oil recovery process. Thus, the CO2could be a miscible injectant. As an alternative, the CO2may be used as a miscible flood agent for enhanced oil recovery.

Referring again to the stripping section106of the column100, gas moves up through the stripping section106, through the chimneys131in the melt tray130, and into the intermediate spray section108. The spray section108is an open chamber having a plurality of spray nozzles122. As the vapor moves upward through the spray section108, the temperature of the vapor becomes much colder. The vapor is contacted by liquid methane coming from the spray nozzles122. This liquid methane is colder than the upwardly-moving vapor, having been chilled by an external refrigeration unit170. In one arrangement, the liquid methane exists from spray nozzles122at a temperature of approximately −120° F. to −130° F. However, as the liquid methane evaporates, it absorbs heat from its surroundings, thereby reducing the temperature of the upwardly-moving vapor. The vaporized methane also flows upward due to its reduced density (relative to liquid methane) and the pressure gradient within the column100.

As the methane vapors move further up the cryogenic distillation tower100, they leave the intermediate spray section108and enter the rectification section110. The vapors continue to move upward along with other light gases broken out from the original chilled fluid stream12. The combined hydrocarbon vapors move out of the top of the cryogenic distillation tower100, becoming an overhead methane stream14.

The hydrocarbon gas in the overhead methane stream14is moved into the external refrigeration unit170. In one aspect, the refrigeration unit170uses an ethylene refrigerant or other refrigerant capable of chilling the overhead methane stream14down to −135° to −45° F. This serves to at least partially liquefy the overhead methane stream14. The chilled methane stream14is then moved to a reflux condenser or separation chamber172.

Separation chamber172is used to separate gas16from liquid reflux18. The gas16represents the lighter hydrocarbon gases, primarily methane, from the original raw gas stream10. Nitrogen and helium may also be present. The methane gas16is, of course, the “product” ultimately sought to be captured and sold commercially, along with any ethane.

A portion of the overhead methane stream14exiting the refrigeration unit170remains condensed. This portion becomes liquid reflux18that is separated in the separation chamber172and returned to the column100. A pump19may be used to move the liquid reflux18back into the column100. Alternatively, the separation chamber172is mounted above the column100to provide a gravity feed of the liquid reflux18. The liquid reflux18will include any carbon dioxide that escaped from the upper rectification section110. However, most of the reflux liquid18is methane, typically 95% or more, with nitrogen (if present in the initial fluid stream10) and traces of carbon dioxide and hydrogen sulfide (also if present in the initial fluid stream10).

In one cooling arrangement, the overhead methane stream14is taken through an open-loop refrigeration system, such as the refrigeration system shown in and described in connection withFIG. 6. In this arrangement, the overhead methane stream14is taken through a cross-exchanger to chill a return portion of the overhead methane stream used as the liquid reflux18. Thereafter, the overhead methane stream14is pressurized to about 1,000 psi to 1,400 psi, and then cooled using ambient air and possibly an external propane refrigerant. The pressurized and chilled gas stream is then directed through an expander for further cooling. A turbo expander may be used to recover even more liquid as well as some shaft work. U.S. Pat. No. 6,053,007 entitled “Process For Separating a Multi-Component Gas Stream Containing at Least One Freezable Component,” describes the cooling of an overhead methane stream, and is incorporated herein in its entirety by reference.

It is understood here that the present inventions are not limited by the cooling method for the overhead methane stream14. It is also understood that the degree of cooling between refrigeration unit170and the initial refrigeration unit150may be varied. In some instances, it may be desirable to operate the refrigeration unit150at a higher temperature, but then be more aggressive with cooling the overhead methane stream14in the refrigeration unit170. Again, the present inventions are not limited to these types of design choices.

Returning toFIG. 1, the reflux liquid18is returned into the rectification section110. The reflux liquid18is then gravitationally carried through one or more mass transfer devices116in the rectification section110. In one embodiment, the mass transfer devices116are rectification trays that provide a cascading series of weirs118and downcomers119, similar to trays126described above.

As fluids from reflux liquid stream18move downward through the rectification trays116, additional methane vaporizes out of the rectification section110. The methane gases rejoin the overhead methane stream14to become part of the gas product stream16. However, the remaining liquid phase of reflux liquid18falls onto a collector tray140. As it does so, the reflux liquid stream18unavoidably will pick up a small percentage of hydrocarbon and residual acid gases moving upward from the spray section108. The liquid mixture of methane and carbon dioxide is collected at collector tray140.

The collector tray140preferably defines a substantially planar body for collecting liquids. However, as with melt tray130, collector tray140also has one, and preferably a plurality of chimneys for venting gases coming up from the spray section108. A chimney and cap arrangement such as that presented by components131and132inFIGS. 2B and 2Cmay be used. Chimneys141and caps142for collector tray140are shown in the enlarged view ofFIG. 5, discussed further below.

It is noted here that in the rectification section110, any H2S present has a preference towards being dissolved in the liquid versus being in the gas at the processing temperature. In this respect, the H2S has a comparatively low relative volatility. By contacting the remaining vapor with more liquid, the cryogenic distillation tower100drives the H2S concentration down to within the desired parts-per-million (ppm) limit, such as a 10 or even 4 ppm specification. As fluid moves through the mass transfer devices116in the rectification section110, the H2S contacts the liquid methane and is pulled out of the vapor phase and becomes a part of the liquid stream20. From there, the H2S moves in liquid form downward through the stripping section106and ultimately exits the cryogenic distillation tower100as part of the liquefied acid gas stream22.

In cryogenic distillation tower100, the liquid captured at collector tray140is drawn out of the rectification section110as a liquid stream20. The liquid stream20is comprised primarily of methane. In one aspect, the liquid stream20is comprised of about 93 mol. percent methane, 3% CO2, 0.5% H2S, and 3.5% N2, At this point, the liquid stream20is at about −125° F. to −130° F. This is only slightly warmer than the reflux fluid stream18. The liquid stream20is directed into a reflux drum174. The purpose of the reflux drum174is to provide surge capacity for a pump176. Upon exiting the reflux drum174, a spray stream21is created. Spray stream21is pressurized in a pump176for a second reintroduction into the cryogenic distillation tower100. In this instance, the spray stream21is pumped into the intermediate spray section108and emitted through nozzles122.

Some portion of the spray stream21, particularly the methane, vaporizes and evaporates upon exiting the nozzles122. From there, the methane rises through the intermediate spray section108, through the chimneys in the collector tray140, and through the mass transfer devices116in the rectification section110. The methane leaves the distillation tower100as the overhead methane stream14and ultimately become commercial product in gas stream16.

The spray stream21from the nozzles122also causes carbon dioxide to desublime from the gas phase. In this respect, some CO2momentarily enters the gas phase and moves upward with the methane. However, because of the cold temperature within the intermediate spray section108, the gaseous carbon dioxide quickly turns into a solid phase and begins to “snow.” This phenomenon is referred to as desublimation. In this way, some CO2never re-enters the liquid phase until it hits the melt tray130. This carbon dioxide “snows” upon the melt tray130, and melts into the liquid phase. From there, the CO2-rich liquid cascades down the mass transfer devices or trays126in the stripping section106, along with liquid CO2from the chilled fluid stream12as described above. At that point, any remaining methane from the spray stream21of the nozzles122should quickly break out into vapor. These vapors move upwards in the cryogenic distillation tower100and re-enter the rectification section110.

It is desirable to have chilled liquid contacting as much of the gas that is moving up the column as possible. If vapor bypasses the spray stream21emanating from the nozzles122, higher levels of CO2could reach the rectification section110of the tower100. To improve the efficiency of gas/liquid contact in the intermediate spray section108, a plurality of nozzles122having a designed configuration may be employed. Thus, rather than employing a single spray source at one or more levels in a reflux fluid stream21, several spray headers120optionally designed with multiple spray nozzles122may be used. Thus, the configuration of the spray nozzles122has an impact on the mass transfer taking place within the intermediate spray section108.

The assignee herein has previously proposed various nozzle arrangements in co-pending WO Pat. Publ. No. 2008/091316 having an international filing date of Nov. 20, 2007. That application andFIGS. 6A and 6Bare incorporated herein by reference for teachings of the nozzle configurations. The nozzles seek to ensure 360° and full radial coverage within the spray section108and provide good vapor-liquid contact and heat/mass transfer. This, in turn, more effectively chills any gaseous carbon dioxide moving upward through the cryogenic distillation tower100.

The use of multiple headers120and a corresponding overlapping nozzle122arrangement for complete coverage minimizes back-mixing as well. In this respect, complete coverage prevents the fine, low-mass CO2particles from moving back up the column and re-entering the rectification section110. These particles would then remix with methane and re-enter the overhead methane stream14, only to be recycled again.

It can be seen that the process of cycling vapors through the cryogenic distillation tower100ultimately produces a hydrocarbon product comprised of a commercial methane product16. The gas product16is sent down a pipeline for sale. The gas product stream16preferably meets a pipeline CO2specification of 1 to 4 mol. percent, as well as a 4 ppm H2S specification, if sufficient reflux is generated. At the same time, acid gases are removed through exit fluid stream22.

Should nitrogen be present in quantities of, for example, greater than 3 mol. percent, a separate nitrogen rejection process may be used. Pipeline specifications generally require a total inert gas composition of less than 3 mol. percent. One option for removing excessive nitrogen is to use a solid adsorbent bed (not shown). The solid adsorbent may be a zeolite material that forms a molecular sieve having a selected pore size. The molecular sieve is placed along the overhead methane stream to remove nitrogen from the overhead stream. Preferably, this occurs prior to chilling.

Once this molecular sieve is fully adsorbed with nitrogen, it may be regenerated using either pressure swing adsorption or thermal swing adsorption. The molecular sieve generally cannot be regenerated using water adsorption upstream of the cryogenic tower as the nitrogen will end up back in the column and, thus, is not eliminated from the system.

While the above system described in connection withFIG. 1is beneficial for producing a substantially acid-free pipeline gas product16, it is acknowledged that in some applications, supplemental refrigeration may be required. For example, LNG specifications can be as low as 50 ppm for CO2. Using the system described above, significant reflux refrigeration from external refrigeration unit170(or other refrigeration system) is required. In addition, additional refrigeration may be required prior to reflux injection from pump19or pump176. Still further, significant heat removal may be needed from the initial heat exchanger150to partially liquefy the feed stream10. These refrigerators demand capacity for liquid reflux generation and energy for horsepower.

As an alternative, a “polishing” process is proposed herein. The “polishing” is applied to the overhead methane stream14as a post-CFZ process. This means that the overhead methane stream14released from the cryogenic distillation tower100is further polished downstream of the tower100to meet, for example, liquefied natural gas (LNG) specifications without increasing the quantity of liquid reflux injected through the spray nozzles122.

FIG. 6is a schematic diagram showing a gas processing facility600for removing acid gases from a gas stream, in one embodiment. A production gas stream is shown at612. The gas stream612originates from hydrocarbon production activities that take place in a reservoir development area or “field”610.

It is understood that the field610may represent any location where compressible hydrocarbons are produced. The field610may be onshore, near shore or offshore. The field610may be operating from original reservoir pressure or may be undergoing enhanced recovery procedures. The systems and methods claimed herein are not limited to the type of field that is under development so long as it is producing compressible hydrocarbons contaminated with acid gas.

The production gas stream612may be directed through a dehydration process such as a glycol dehydration process. A dehydration vessel is shown schematically at620. As a result of passing the production gas stream612through the dehydration vessel620, a stream of removed water622is generated. The water622may be sent to a water treatment facility. Alternatively, the water622may be re-injected into a subsurface formation, indicated at block630. Alternatively still, the water may be treated and then injected back into the subsurface formation630associated with field610.

Also, as a result of passing the production gas stream612through the dehydration vessel620, a substantially dehydrated raw gas stream624is produced. The raw gas stream624preferably contains methane and, possibly, trace amounts of ethane, nitrogen, helium and other light gases.

The raw gas stream624may be passed through a pipeline, for example, from an offshore treating facility to an onshore treating facility. The gas may then be passed through a solid adsorbent bed625to further dehydrate the gas in the gas stream624. Removing water from the raw gas stream624helps ensure that neither hydrates nor ice form in the subsequent chilling or cryogenic facilities.

The solid adsorbent bed625may be a molecular sieve bed. A “molecular sieve” refers to a material containing tiny pores of a precise and substantially uniform size. In the present context, such sieves are used as an adsorbent for gases and/or liquids. Molecular sieves often consist of solid materials and not polymeric materials. Exemplary materials include alumino-silicate minerals, clays, porous glasses, micro-porous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water, can diffuse. Polar molecules (such as water molecules) that are small enough to pass into the pores are adsorbed, while slightly polarizable molecules (such as methane and nitrogen), as well as larger molecules (e.g., propane and butane) flow around the particles and crystallites, and are thus passed downstream. In the present case, the molecular sieve bed625adsorbs water molecules and allows light gases to pass through.

The raw gas stream624next passes through a refrigeration unit626. The refrigeration unit626chills the dehydrated raw gas stream624down to a temperature of about −30° F. to −40° F. The refrigeration unit626may be, for example, an ethylene or a propane refrigerator.

The raw gas stream624is next preferably moved through an expansion device628. The expansion device628may be, for example, a Joule-Thompson (“J-T”) valve. The expansion device628serves as an expander to obtain further cooling of the raw gas stream624. The expansion device628further reduces the temperature of the dehydrated raw gas stream624down to, for example, about −70° F. to −80° F. Preferably, at least partial liquefaction of the gas stream624is also accomplished.

The chilled raw gas stream624next enters a cryogenic distillation tower640. The cryogenic distillation tower may be any tower that operates to distill methane from acid gases through a process that intentionally freezes CO2particles. The cryogenic distillation tower may be, for example, tower100ofFIG. 1. The chilled raw gas stream624enters the vessel at about 500 to 600 psig.

The distillation tower640includes a lower stripping section (seen as section106inFIG. 1). The distillation tower640also includes a controlled freezing zone (seen as section108inFIG. 1) where vapor is contacted with cold reflux liquid (primarily methane). This causes CO2to freeze out of the gas phase. The remaining methane and other gas phase components vaporize and rise upward towards an upper rectification section (seen as section110ofFIG. 1). There, residual H2S and CO2are further removed from the gas phase. In one aspect (not seen), a “split tower” is used in which the rectification section110is in a separate vessel from the stripping section106and the controlled freezing zone108.

As explained in connection withFIG. 1, the acid gases are removed from the distillation tower640as a bottom liquefied acid gas stream642. This liquid stream642may optionally be sent through a reboiler643where gas containing trace amounts of methane is redirected back into the tower as gas stream644. The remaining liquid comprised primarily of acid gases is released through acid gas line646. The liquefied acid gas in line646may be vaporized, depressured, and then sent to a sulfur recovery unit (not shown). Alternatively, the liquefied acid gas may be injected into a subsurface formation through one or more acid gas injection (AGI) wells as indicated by block650. In this instance, the acid gas in line646is preferably passed through a pressure booster648.

Methane is released from the distillation tower640as an overhead methane stream662. The overhead methane stream662will preferably comprise no more than about 2% carbon dioxide. At this percentage, the overhead methane stream662may be used as fuel gas or may be sold into certain markets as natural gas. However, in accordance with certain methods herein, it is desirable that the overhead methane stream662undergo further processing.

As indicated above, the overhead methane stream662is passed through an open loop refrigeration system. First, the overhead methane stream662is passed through a heat exchanger664. This serves to pre-cool a reflux stream680that is reintroduced into the cryogenic distillation tower640. The overhead methane stream662is then sent through a compressor666to increase its pressure.

Next, the pressurized methane stream662, heated by compression, is cooled. This may be done through, for example, passing the methane stream662through an aerial cooler668. A cool and pressurized methane stream is produced.

A part of the cooled and pressurized methane stream leaving the cooler668is split into a reflux stream680. The reflux stream680is further cooled in cross-exchanger664, then expanded through device682to generate liquid reflux stream684. The liquid reflux stream684enters the distillation tower640where it is used as reflux, and ultimately in the sprays to freeze out CO2and other acid gas particles from the dehydrated gas stream624. Sufficient reflux is generated such that a CO2or H2S specification is partially met at the top of the tower640. The liquid reflux stream684from expander682may be injected directly into the spray nozzles in the controlled freezing zone. More preferably, the liquid reflux stream684is first routed to the rectification section of the distillation tower640in order to further reduce the CO2concentration in the overhead gas. In either instance, the chilled reflux stream680preferably passes through an expansion valve or turboexpander682to generate liquid.

Returning to the overhead methane stream662, the overhead methane stream662may be passed through one or more expansion valves or turboexpanders. An expansion valve is shown at670inFIG. 6. The expansion valve or (turboexpander)670decreases the pressure of the gas stream662and cools it further in preparation of liquefaction to generate LNG. A cooled gas stream672is thus created.

It is understood that specifications for liquefied natural gas, or LNG, may require that the acid gas concentration be lower than 2 mol. percent. LNG-specifications typically permit no more than 50 ppm CO2and 4 ppm H2S. Accordingly, in accordance with the present methods, the pre-LNG stream672is sent to a solid adsorbent bed674as a post-processor for further acid gas removal. Preferably, the solid adsorbent bed674is a molecular sieve bed. Certain molecular sieves such as 4-Angstrom and 5-Angstrom beds can be used to remove CO2as well as H2O from a methane gas stream. Mercaptans may also be removed. The overhead methane stream672is at high pressure and relatively cool, which helps facilitate the adsorption of CO2on the solid adsorbent bed674.

The solid adsorbent bed674may be a molecular sieve bed that employs pressure swing adsorption. Pressure swing adsorption, or PSA, refers to a process wherein a contaminant is adsorbed onto a solid adsorbent, then removed from the adsorbent by decreasing the pressure. In the case of acid gas removal, CO2may be removed from a sour gas. In one aspect, the pressure swing adsorption bed is a rapid-cycle, pressure swing adsorption, or RCPSA, bed. An RCPSA vessel is particularly advantageous as it is compact relative to a standard PSA device. PSA's are used for a variety of gas separations, for example, for removal of impurities from hydrogen streams in refinery service. Such systems are manufactured, for example, by QuestAir of British Columbia, Canada.

The solid adsorbent used in a molecular sieve bed may be fabricated from any one of several materials. Preferably, zeolites are used. Zeolites are crystalline materials composed of silicon and aluminum. Alternatively, the post-processor may be a vessel that uses an adsorbent that relies on the rate at which certain species are adsorbed relative to other species. This is known as adsorptive kinetic separation (AKS). After CO2saturation, the zeolite or other bed material may be regenerated or cleaned. This may be done, for example, by lowering pressure on the bed and releasing acid gases, or by passing a hot gas through it. The hot gas may be, for example, a hot methane gas or heated nitrogen. The hot methane gas may be captured and then used as process fuel such as for operating refrigeration equipment, assuming that the H2S content is sufficiently low.

Molecular sieve beds are ideal for post-processing inasmuch as the overhead methane stream662is already quite clean. No additional pretreatment is required for reaching an LNG-specification gas stream672. Further, there is no need for a sacrificial layer of material in the packed bed.

In lieu of a molecular sieve bed, other types of post-processors (not shown) may also be employed in connection with the gas processing system600. For example, one or more vessels that applies chemical solvents to absorb acid gases may be used. Alternatively, the post-processor may be one or more vessels that apply physical solvents or ionic solvents to remove residual CO2. An advantage to these solvents is that they are not aqueous-based, so no further dehydration should be required of the overhead methane stream662. Of course, the addition of a solvent-based system as the post-processor will add significantly to the equipment count and is therefore not preferred.

It can be seen that the post-processor allows for optimization of a cryogenic acid gas removal system by trading reduced liquid reflux (that is, reducing refrigeration capacity and horsepower) with a downstream molecular sieve bed674or other post-processor to pick up additional CO2, as well as H2S. Thus, the overhead methane stream662may exceed a 50 ppm LNG specification for LNG, and a 4 ppm H2S specification for LNG, knowing that the downstream post-processor674, e.g., molecular sieve bed, will remove additional acid gas to bring the final gas product into compliance with specifications.

In whatever form, the post-processor generates a final sales gas stream676. The final sales gas stream is suitable for a downstream liquefaction process, ultimately resulting in the sale of liquefied natural gas, or LNG.

FIGS. 7A, 7B and 7Ctogether present a flow chart. The flow chart presents a method700for removing acid gases from a raw gas stream using an acid gas removal system in accordance with the present inventions, in one embodiment.

The method700first includes receiving a raw gas stream. This step is shown at Box705. The raw gas stream is received as a result of hydrocarbon producing activities. Stated another way, the raw gas stream has been produced from a subsurface reservoir and brought to the surface for processing.

In accordance with the method700, the raw gas stream contains compressible hydrocarbons. These primarily represent methane. However, trace amounts of ethane, nitrogen, helium and other light gases may also be present. The raw gas stream further contains acid gases. The acid gases may comprise carbon dioxide, hydrogen sulfide, mercaptans, or other contaminants. The acid gas content of the raw gas stream may represent, for example, anywhere from about 5 to 60 mol. percent of the total gas stream.

In its produced form, the raw gas stream most likely also contains water or brine. Therefore, as part of the method700the raw gas stream is taken through a dehydration process. In one aspect, the raw gas stream is optionally passed through a first molecular sieve bed. This step is shown at Box710.

In the present application, the molecular sieve bed adsorbs water molecules from the raw gas stream. Thus, the molecular sieve bed in step710acts as a desiccant. Molecular sieve beds for adsorbing water are available from various sources such as Texas Technologies, Inc. of Cedar Park, Tex. and Eurecat U.S. of Houston, Tex.

After dehydration, the raw gas stream may be passed through a refrigeration system. This step is shown at Box715. This may be done, for example, by moving the gas stream through a chiller such as refrigeration unit150ofFIG. 1. This may optionally be followed by passing the raw gas stream through an expander such as expansion device152. The expander provides further cooling of the raw gas stream.

In accordance with the method700, the raw gas stream is next scrubbed of acid gases. This is accomplished by passing the raw gas stream through a cryogenic distillation tower. The step of passing the raw gas stream through a cryogenic distillation tower is shown in Box720.

The cryogenic distillation tower may be any tower that operates to distill methane or other light hydrocarbon gases from acid gases through a process that involves intentionally freezing CO2particles. The cryogenic distillation tower may be, for example, tower100ofFIG. 1. The frozen CO2particles are preferably collected on a melt tray at the bottom of the spray section of the distillation tower. As the particles melt and turn to liquid, the liquid may cascade down through a series of trays or downcomers. This causes additional methane to break out of solution and to move upward through the tower as a vapor, as described above. The remaining CO2-enriched liquid is then captured as a bottom liquefied acid gas stream including any hydrogen sulfide.

As a result of processing the raw gas stream through the cryogenic distillation tower, an overhead methane stream is produced. The overhead methane stream will preferably comprise no more than 5 mol. percent and, more preferably, no more than about 2 mol. percent carbon dioxide.

The method700next includes refrigerating the overhead methane stream. The step of refrigerating the overhead methane stream is shown in Box725ofFIG. 7A. The purpose of cooling the overhead methane stream is to at least partially liquefy the methane therein.

The overhead methane stream is cooled using a refrigeration system. The system may be, for example, an external, closed-loop refrigeration unit such as an ethylene or propane refrigeration unit170inFIG. 1. Another example of a closed loop refrigeration system is a series of propane and ethylene-cascaded chillers. Alternatively, the refrigeration system may be an open loop refrigeration system such as any of the open loop refrigeration systems shown in U.S. Pat. No. 6,053,007. In one aspect, an open loop refrigeration system is used wherein the overhead methane stream14takes heat from the returning liquid stream through a heat-exchanger, and is then compressed. The compressed gas is then cooled to near ambient temperatures using aerial coolers or, possibly, a seawater cooler. The gas may be further cooled to sub-ambient temperatures using a propane chiller, for example. An open loop refrigeration system is shown in and described in connection withFIG. 6.

The method700next includes separating out at least a portion of the overhead methane stream to generate liquid reflux. This step is provided in Box730ofFIG. 7B. The process of separating out part of the overhead methane stream is preferably done after the methane stream has been at least partially liquefied. In another aspect, the partially liquefied overhead methane stream (such as stream14) is carried through a separator (such as reflux condenser or separator172). The separator creates a gas stream16and a liquid stream18. Some or all of the liquid stream18is reintroduced into the tower100as the liquid reflux.

In connection with refrigerating the overhead methane stream, and as implied above, a refrigeration system is operated. The step of operating the refrigeration system is shown at Box735. One purpose for operating the refrigeration system is to generate a quantity of liquid reflux that will remove a desired percentage of the acid gases from the raw gas stream. Those of ordinary skill in the art will understand that a correlation exists between the quantity of the liquid reflux injected back into the cryogenic distillation tower and the efficiency with which carbon dioxide solids are precipitated out of the vapor stream within the cryogenic distillation tower. The more vigorously the liquid methane is refrigerated and the greater the quantity of liquid reflux18is generated, the more effective the cryogenic distillation tower will precipitate out carbon dioxide and other acid gases. However, refrigeration equipment is expensive, and the lower the temperature of the liquid reflux and the more liquid reflux generated, the more energy and equipment are required.

Under step735, the operator or process designer intentionally does not generate so much liquid reflux as to remove all of the acid gases. Stated another way, a quantity of liquid reflux is set to remove most but not all of the carbon dioxide. For example, the quantity of the liquid reflux may be set to bring the carbon dioxide content of the raw gas stream down to about 2 to 5 mol. percent. Alternatively, the temperature of the liquid reflux may be set to bring the carbon dioxide content down to pipeline specifications. It is understood that the temperature will be set by the operating pressure. More specifically, temperature is set based upon an optimization between operating pressure, the number of trays in the upper distillation section, or “rectification section,” and the amount of reflux liquid used. In any event, energy otherwise required of the refrigeration system to bring the overhead methane stream into LNG specifications is conserved.

In one embodiment of the method700, sufficient refrigeration is provided in step735to provide a quantity of liquid reflux that will remove carbon dioxide from the overhead methane stream down to a percentage of about 0.5 to 2.0 mol. percent. This represents a first selected composition. Thereafter, the post-processor removes carbon dioxide from the overhead methane stream down to a level of about 50 to 200 ppm. This represents a second selected composition.

In one potential embodiment, CFZ refrigeration is minimized by utilizing only enough liquid reflux (generated by either external refrigeration, or via the open-loop process, or from an LNG plant) to freeze the inlet concentration of CO2down to about 2% CO2at the top of the spray section. The post-processor then removes additional CO2below 2% to, for example, LNG requirements. Thus, the refrigeration step735provides a trade-off of refrigeration requirements with molecular sieve equipment during normal operations.

The method700next includes injecting at least a portion of the liquid reflux back into the cryogenic distillation tower. This step is shown at Box740ofFIG. 7B. The step740of injecting liquid reflux into the cryogenic distillation tower is presented, for example, through line21ofFIG. 1and line680ofFIG. 6. The purpose for injecting the liquid reflux back into the cryogenic distillation tower is to provide very cold fluid within the tower that causes carbon dioxide to precipitate out of methane vapor as a solid.

In one aspect, the separated liquid reflux18drops down through trays and ultimately becomes cold liquid spray21. In another embodiment, the portion of the overhead methane stream that becomes liquid reflux may be directly injected into the distillation tower such as through liquid line680from gas processing system600. In either instance, the liquid stream21or680preferably undergoes pressure boosting, such as through booster176ofFIG. 1.

In one arrangement, the liquid methane is re-injected into the column640at a temperature of, for example, about −120° F. to −140° F. However, the operating conditions under which an acid gas component of the raw gas stream will freeze out is a function of a number of variables. These include, for example, the raw gas stream composition, the chemical character of the freezable acid gas component, and the separation system's temperature and pressure. Beneficially, the solid forming conditions for any freezable component of the raw gas stream may be determined through the use of commercially available process simulation software.

Next, the portion of the overhead methane stream that is not separated out as liquid reflux is passed through a non-cryogenic post-processor. This step is shown in Box745. The purpose of passing the remaining overhead methane stream through the molecular sieve bed674is to further remove acid gases. The result is a final gas stream that has less than about 200 ppm carbon dioxide. More preferably, the CO2content is less than about 50 ppm.

In one aspect, the carbon dioxide content meets pipeline specifications. Alternatively, the carbon dioxide content meets liquefied natural gas (LNG) standards, that is, less than 50 ppm CO2and less than about 4 ppm H2S. The LNG may be imported from another country and offloaded in the United States at an import terminal.

The non-cryogenic post-processor may be one of a number of different types of vessels or acid gas removal systems for removing carbon dioxide from a methane gas stream. Preferably, the pre-processor is a molecular sieve bed. The molecular sieve bed may be, for example, a 4-Angstrom or a 5-Angstrom bed that is capable of removing carbon dioxide from a methane gas stream. However, the post-processor may alternatively be one or more vessels that employs chemical solvents to strip acid gas from the methane stream. Alternatively, the post-processor may be one or more vessels that contacts ionic liquids or physical solvents with the methane to remove residual carbon dioxide.

When a molecular sieve bed is used as the post-processor, it is preferably introduced to the acid gas removal system in a substantially clean state. During the course of operation the sieve bed becomes saturated with carbon dioxide. Of interest, in the present method700, the molecular sieve bed that has become saturated with carbon dioxide may serve as a second molecular sieve bed to replace the first molecular sieve bed from step710. This takes place when the first molecular sieve bed has become saturated with water. The step of replacing the first molecular sieve bed with the second molecular sieve bed is demonstrated in Box750.

It is noted that a molecular sieve bed will preferentially displace carbon dioxide by adsorbing water vapor. This is because water is more strongly adsorbed than CO2. Consequently, water vapor will displace or desorb CO2that has been previously adsorbed on a molecular sieve bed. This means that the second molecular sieve bed (saturated with adsorbed carbon dioxide) may be installed as the first molecular sieve bed for dehydration purposes, without undergoing a prior regeneration process. The second molecular sieve bed will now dehydrate the raw gas stream and pass displaced carbon dioxide into the cryogenic distillation tower. Since the amount of CO2released from the second molecular sieve bed into the tower is relatively small, the distillation tower can process it with little or no loss in capacity. Thus, an inherent synergy is created between molecular sieve beds used upstream and downstream with the method700.

It is understood that the first molecular sieve bed from step710that has now become saturated with water must undergo a dedicated regeneration process. The regeneration step is indicated at Box755inFIG. 7C. Regeneration of a hydrated sieve bed typically involves directing a heated gas through and across the molecular sieve bed. The heated gas may be methane gas from, for example, sales gas stream676, or heated nitrogen. Thereafter, the molecular sieve bed is cooled and set aside for later service. In the present method700, later service means CO2removal downstream of the distillation tower, e.g., a post-processor or post-polishing bed.

The method700may also include the step of replacing the second molecular sieve bed (used downstream of the cryogenic distillation tower) with a third molecular sieve bed. This step is shown at Box760. The third molecular sieve bed may be a new sieve bed. Alternatively, the third molecular sieve bed may be a bed that has been regenerated from a previous service, such as a bed regenerated from step755.

It is understood that the rate of saturation for a molecular sieve bed may be different when adsorbing water than when adsorbing acid gases such as CO2and H2S. The rate of saturation for any molecular sieve bed depends on such variables as the fluid flow rates, the respective concentrations of contaminants within the fluids, and the capacity for adsorbing the different contaminants. In the present context, this means that the first molecular sieve bed (which adsorbs primarily water in connection with step710), and the second molecular sieve bed (which adsorbs primarily CO2in connection with step740), may not become saturated at the same time. In general, a molecular sieve bed's capacity is substantially lower for H2S and CO2than for water vapor. However, there may be a greater quantity of water than CO2, for example. In any case, more than two molecular sieve beds are to be included in a rotation for continuous operation.

In one mode of operation, three molecular sieve beds are placed into service. One bed would be in dehydration service; another would be in post-polishing service; and a third bed would be undergoing regeneration. Regeneration would, of course, include heating, followed by cooling, followed by placement in standby mode. This process of using three molecular sieve beds advantageously compares with the known use of a minimum of four sieve beds wherein the dehydration and polishing services are kept separate.

FIG. 7Cshows a step765of placing the regenerated first molecular sieve bed in waiting to replace the third molecular sieve bed. Replacement takes place when the third molecular sieve bed becomes saturated with carbon dioxide, or when some subsequent replacement sieve bed becomes saturated with carbon dioxide. Alternatively, replacement may be driven by saturation of the sorbent bed in desiccation service.

In one aspect, four molecular sieve beds are employed in a rotation. A first sieve bed is employed for CO2removal service for the overhead methane stream. A second sieve bed, which was previously in CO2removal service for the overhead gas, is now in dehydration service for the raw gas stream. The third sieve bed is being regenerated. This is typically done by heating the third bed with a purge gas. The purge gas is usually passed through the third bed in a counter-flow direction at a lower pressure. The fourth bed is in cool-down mode after having been in regeneration. Cooling is typically done by flowing a cool gas across the fourth bed. The fourth molecular sieve bed will be switched to CO2-removal service as soon as the first sieve bed is substantially saturated with carbon dioxide. In this way the rotation continues.

In connection with the method700for removing acid gases from a raw gas stream, the cycle times related to steps750and760may be set. The operator should have an understanding as to how long it takes the solid adsorbent bed upstream of the distillation tower to become hydrated, that is, saturated with water, and how long it takes the solid adsorbent bed downstream of the distillation tower to become saturated with CO2. The cycle times should be set to prevent a “breakthrough” of water or acid gases from the respective beds, even in the most extreme feed conditions. This means that some part of the respective beds may not be fully utilized for adsorption.

It is also noted that material such as zeolite that is used in solid adsorbent beds tends to become less active or less adsorptive over time. The adsorptive material will be more effective at the start of a run (“SOR”) than at the end of a run (“EOR”). This means that the solid adsorbent beds may need to be cycled more frequently as time goes on. Thus, in one aspect, cycle times related to steps750and760are progressively shortened.

Fixed cycle-time control is applicable whether the solid adsorbent bed (such as dehydration bed625) is a molecular sieve bed or a pressure swing adsorption bed. It is understood that for pressure swing adsorption processes, cycle times will be much shorter. For example, cycle times for pressure swing adsorption beds tend to be on the order of minutes or even seconds rather than hours.

It is preferred that the cycle times be controlled through a control system. Thus, in one aspect the gas processing system600includes a control system. The control system may, for example, control regeneration of the solid adsorbent bed625used for dehydration to ensure that measurable “breakthrough” of water does not occur during the dehydration process. The control may be done based on fixed time schedules. For example, the second bed described above may be in dehydration service for 12 hours, while the third bed undergoes regeneration for 6 hours. The fourth bed may undergo cooling for three hours, and then be placed in standby for three hours. The regenerated and cooled fourth bed is then put into service while the first bed is taken out of service and put into dehydration service.

In one aspect, the control system employs a “feed-forward” control scheme.FIG. 8Aprovides a schematic diagram of a control system800A that uses feed-forward control. InFIG. 8A, the solid adsorbent bed625from system600used for dehydration is seen. In addition, the partially dehydrated raw gas stream624is indicated. The raw gas stream624is being directed into the bed625for further dehydration. The raw gas stream624is ultimately processed in an acid gas removal system having a cryogenic distillation tower. An acid gas removal system is shown schematically at850receiving a further dehydrated acid gas stream820from the bed625.

The control system800A may first include a flow meter805. The flow meter805is placed along the raw gas stream624. The flow meter805measures the flow rate of the raw gas stream624into the solid adsorbent bed625. The control system800A also includes a measurement device810. The measurement device810is used to measure the concentration of water in the raw gas stream624. The measurement device810may be, for example, one where the change in dielectric constant of alumina is measured as a function of the moisture adsorbed thereon. Note that the measurement device810is optional. In this respect, the concentration of water vapor may be known indirectly. For example, if the water content upstream is controlled by cooling the gas, it may be assumed that the gas is saturated with water at the cooler's temperature.

Using the detected flow rate of the acid gas stream624and the measured or known water vapor concentration, the operator may multiply these values to determine the total quantity of contaminant, i.e., water, that has gone to the solid adsorbent bed625over a period of time. By comparing this quantity to a model calculation of the bed capacity (optionally taking its age into account), it is possible to determine a more optimal cycle time. This may be done automatically by using a distributed control unit811.

An alternative means of modifying the cycle is to provide a measurement device capable of on-line measurement of the water content near the effluent of the solid adsorbent bed625used for dehydration. A control system800B that uses such a measurement device is provided inFIG. 8B. The control system800B uses a direct feedback control.

FIG. 8Bprovides a schematic diagram of a control system800B that uses a direct feedback control. InFIG. 8B, the solid adsorbent bed625from system600used for dehydration is again seen. In addition, partially dehydrated raw gas stream624is indicated. The raw gas stream624is being directed into the bed625for further dehydration. The raw gas stream624is ultimately processed (after further dehydration using bed625) in the acid gas removal system850.

Unlike control system800A, the control system800B does not include a flow meter. However, the control system800B does include a measurement device810′. Instead of measuring the concentration of acid gases in the raw gas stream624, the measurement device810′ measures the concentration of water above the exit to the solid adsorbent bed625. The measurement device810′ samples effluent immediately inside the bed625or just above an outlet815. In this way, the adsorbent can be switched before any contaminant, i.e, water, passes the outlet815. Preferably, the measurement device810′ is placed about a foot above a bottom of the solid adsorbent bed625to avoid any leakage of water vapor into the treated gas.

The measurements taken by the measurement device810′ are sent to a control unit811. The control unit811comprises a central processing unit, or CPU. The CPU may be implemented as any type of processor including commercially available microprocessors from companies such as Intel, AMD, Motorola, Hitachi and NEC. The control unit811receives water measurements from the measurement device810′ and determines if the bed625is close to water breakthrough. If the bed625is close to full capacity, the control unit811sends the appropriate signal to end the dehydration service cycle for that bed. The next solid adsorbent bed (that is, a bed that has been in acid gas removal service) is then placed into dehydration service.

The control systems800A and800B are demonstrated in the context of measuring water concentrations in connection with the operation of a solid adsorbent bed used for dehydration. Such a solid adsorbent bed is used in the present context upstream of a cryogenic distillation tower. However, the control systems800A and800B may likewise be used in connection with the operation of a solid adsorbent bed used for removal of CO2from an overhead methane stream. Such a solid adsorbent bed is used in the present context downstream of the cryogenic distillation tower.

The use of molecular sieve equipment downstream from a cryogenic distillation tower is not only advantageous for reducing acid gas concentrations to meet LNG specifications during normal operations, but may also be utilized to initially remove CO2from the raw gas stream during “dirty” startup. Such a system is described more fully in connection with U.S. Pat. No. 5,062,270.FIG. 1of that patent and the accompanying textual description are referred to and incorporated herein by reference.

In operation, the raw gas stream is initially fed into a dehydration vessel (such as dehydration vessel620). Prior to entering the distillation tower, the gas stream is next passed through refrigeration equipment in order to generate a chilled raw gas stream. The refrigeration equipment may comprise a propane chiller that lowers the temperature of the raw gas stream to about −35° F. The refrigeration equipment may also include an inlet chiller that further reduces the temperature of the raw gas stream to about −40° F. to −65° F. The refrigeration equipment may also include an expansion device to provide still further cooling of the dehydrated gas. The expansion device may be a turbo expander, a hydraulic turbine or a Joule-Thompson valve. The expansion device further reduces the temperature of the raw gas stream to about −112° F. to −123° F.

The dehydrated and chilled gas stream is next fed into the distillation tower. The distillation tower operates to separate the raw gas stream into an overhead methane stream and a bottom acid gas stream, as discussed above in connection with either of towers100or640. The entire reflux plus sales stream is treated with the downstream molecular sieve bed.

The distillation column (such as tower640), including the rectification section and the cross-exchanger (such as cross-exchanger664) are cooled by initiating the refrigeration systems upstream of the column, but not making them so cold as to generate solid CO2on heat transfer surfaces within the chillers. During this time, sweetened methane gas is released from the column overhead and then flowed through the solid adsorbent bed downstream of the column in order to remove CO2. The gas is then recycled to the inlet of the column such that a progressively lower CO2content is achieved. Once the CO2content is low enough to meet initial pipeline specifications, the refrigeration can be adjusted to normal levels, as there will not be sufficient CO2in the gas to freeze out on the heat transfer surfaces within the heat exchangers.

It is noted that no gas goes to the sales pipeline (or LNG plant) while gas is being recycled through the column. Post-polishing removes just enough CO2so that inlet piping and internal components of the distillation tower do not accumulate solids during the latter stages of start-up. Stated another way, gas containing high levels of CO2is not initially exposed to cold surfaces within the distillation tower or the cross-exchanger where CO2may freeze and cause plugging. Once appropriate CO2concentration profiles are established, the sweetened gas can be processed through the refrigeration system and the molecular sieve bed downstream of the column, with a substantial portion of the overhead methane product going to sales or to an LNG process.

It is also noted that during start-up, a portion of the polished gas is delivered to the distillation tower as liquid reflux. The molecular sieve bed removes CO2to low levels so that the distillation tower will not freeze out significant CO2as the liquid reflux is sprayed down through the rectification section.

While it will be apparent that the inventions herein described are well calculated to achieve the benefits and advantages set forth above, it will be appreciated that the inventions are susceptible to modification, variation and change without departing from the spirit thereof. Improvements to the operation of an acid gas removal process using a controlled freezing zone are provided. The improvements provide a design for the removal of CO2down to very low levels in the product gas. The inventions herein also reduce the refrigeration requirements of cryogenic distillation towers while meeting LNG specifications for maximum allowable CO2