Cryogenic system for removing acid gases from a hydrocarbon gas stream

A system for removing acid gases from a raw gas stream is provided. The system includes a cryogenic distillation tower. The cryogenic distillation tower has a controlled freezing zone that receives a cold liquid spray comprised primarily of methane. The tower receives and then separates the raw gas stream into an overhead methane gas stream and a substantially solid material comprised on carbon dioxide. The system includes a collector tray below the controlled freezing zone. The collector tray receives the substantially solid material as it is precipitated in the controlled freezing zone. The system also has a filter. The filter receives the substantially solid material and then separates it into a solid material comprised primarily of carbon dioxide, and a liquid material comprising methane. The solid material may be warmed as a liquid and sold, while the liquid material is returned to the cryogenic distillation tower.

BACKGROUND OF THE INVENTION

Field Of The Invention

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

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 carbon dioxide may be used to create artificial reservoir pressure for enhanced oil recovery operations.

To facilitate AGI, it is desirable to have a gas processing facility that separates the acid gas components from the hydrocarbon gases. However, for “highly sour” streams, that is, production streams containing greater than about 15% CO2and/or 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 overhead gas stream at moderate pressures (e.g., 300-600 pounds per square inch gauge (psig)). In addition, liquefied acid gas is generated as a “bottoms” product. Since the liquefied acid gas has a relatively high density, hydrostatic head can be beneficially used in an AGI well to assist in the injection process. In this respect, the acid gas may be recovered as a liquid at column pressure (e.g. 300-600 psia). 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.

Cryogenic gas processing has additional advantages. For example, a solvent is not required for the adsorption of carbon dioxide. In addition, methane recovery may be obtained in a single vessel (as opposed to the multi-vessel systems used in the Ryan-Holmes processes). Finally, depending on the refrigeration capacity, a tight H2S specification, e.g., down to or less than 4 ppm, may be met for the product gas.

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 as they fail onto 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 cold liquid CO2/H2S stream is generated at the bottom of the tower as the bottoms product.

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 in the distillation zone at 30° to 40° F.

The controlled freezing 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 into the rectification zone.

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 a part of the liquid returned to the column as the reflux. The liquid reflux is then injected as the cold spray into the rectification zone and the controlled freezing zone. In this respect, the process of generating cold liquid methane for reflux requires equipment ancillary to the CFZ tower. This equipment includes pipes, nozzles, compressors, separators, pumps, and expansion valves.

The methane produced in the upper rectification 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 specifications for higher purity 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 specifications may be met by increasing the quantity of liquid methane reflux. This, in turn, requires larger refrigeration equipment. 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 LNG specifications 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 has an intermediate controlled freezing zone. The controlled freezing zone, or spray section, receives a cold liquid spray comprised primarily of methane. The cold spray is preferably a liquid reflux generated from an overhead loop downstream of the distillation tower.

The cryogenic distillation tower is configured to receive a raw gas stream, and then separate the raw gas stream into (1) an overhead methane gas stream, and (2) a substantially solid material comprised of carbon dioxide.

The system also has refrigeration equipment downstream of the cryogenic distillation tower. The refrigeration equipment serves to cool the overhead methane stream and then return a portion of the overhead methane stream as reflux to the rectification zone in the cryogenic distillation tower. A portion of the liquid reflux may be sprayed in the controlled freezing zone to cause precipitation of solid carbon dioxide particles.

The system further comprises a collector tray. The collector tray is positioned below the controlled freezing zone for receiving the solid CO2particles as they are precipitated in the controlled freezing zone. Preferably, the collector tray has an inclined base to direct precipitate into a central downcomer. The downcomer, in turn, may optionally include a mechanical translation device such as an auger to move a slurry that includes the solid CO2material out of the cryogenic distillation tower and towards a CO2recovery facility.

The CO2recovery facility is preferably comprised of a plurality of filters. Thus, the system includes at least a first filter for receiving the slurry. The slurry is separated into a frozen or solid material (referred to as a “filter cake”) and a liquid material (referred to as a “filtrate”). The solid material is comprised primarily of carbon dioxide, while the liquid material comprises methane. The liquid material may also comprise smaller amounts of carbon dioxide, hydrogen sulfide, mercury and heavy hydrocarbons. It should be understood that as used herein, the slurry is referred to as include a solid material and a liquid material, but may further include a gaseous material or other non-solid material. The liquid material portion of the slurry may be separated therefrom for further processing. The processing of the non-solid material may convert liquids into gases and/or solids, which may subsequently be used for various purposes, such as reinjection to the recovery facility. However, for ease of reference, the non-solid portion of the slurry, once separated from the slurry, may be referred to herein as the liquid material regardless of the state of the material.

The system further includes a liquid return line. The liquid return line returns at least a portion of the liquid material from the CO2recovery facility to the cryogenic distillation tower. There, further processing of the methane and any acid gas components entrained therein takes place.

The cryogenic distillation tower preferably includes an upper rectification zone above the controlled freezing zone. The tower may further have a lower distillation zone below the controlled freezing zone. In the latter instance, the cryogenic distillation tower is preferably configured to receive the raw gas stream into the lower distillation zone. Moreover, the tower receives the liquid material from the liquid return line into the lower distillation zone. Further processing of the methane and trace acid gas components takes place in the lower distillation zone. There, the methane vaporizes in the warm lower distillation zone, travels upward through the controlled freezing zone and upper rectification zone, and merges with the overhead methane stream. The carbon dioxide components will mostly vaporize in the lower distillation zone, move upward into the controlled freezing zone, and precipitate back down on the collector tray. The CO2components are then transported to the CO2recovery facility with the slurry.

When the tower includes a lower distillation zone, acid gases will fall out of the relatively warm lower distillation zone as a bottoms liquid stream. The bottoms liquid stream may comprise ethane, propane, butane, hydrogen sulfide, or combinations thereof, in substantially liquid phase. Carbon dioxide may also be present.

In one arrangement, the cryogenic distillation tower does not include a lower distillation zone. In this instance, the raw gas stream is injected into the distillation tower in the controlled freezing zone. In addition, the liquid return line merges at least a portion of the liquid material with the raw gas stream before the raw gas stream is injected into the cryogenic distillation tower, or simultaneously therewith. The distillation tower will not have a bottoms stream for capturing hydrogen sulfide; instead, hydrogen sulfide and trace elements of methane and carbon dioxide are captured within the CO2recovery facility through second and, optionally, third and fourth filters. Hydrogen sulfide and the trace elements of methane and carbon dioxide are released from the filters as cold liquid filtrate. The filtrate is subsequently processed in a distillation tower so that a recovery methane stream is separated from the acid gases. The recovery methane stream is merged with the overhead methane stream for sale as a commercial product.

In either embodiment, a heat exchanger is optionally provided at the end of the CO2recovery facility. The heat exchanger is configured to warm substantially solid material taken at least partially from a final-stage filter cake to produce a substantially pure carbon dioxide stream, in liquid phase. The substantially solid material is warmed by using, for example, the raw gas stream as a heat source.

A method for removing acid gases from a raw gas stream using an acid gas removal system is also provided herein. The raw gas stream comprises methane, carbon dioxide and, most likely, other components such as ethane and hydrogen sulfide.

In one embodiment, the method first includes providing a cryogenic distillation tower. The tower has a controlled freezing zone that receives a cold liquid spray comprised primarily of methane. The tower further has a collector tray below the controlled freezing zone.

The method also includes injecting the raw gas stream into the cryogenic distillation tower. In one arrangement, the raw gas stream is injected into the distillation tower in a lower distillation zone below the controlled freezing zone. In another arrangement, the raw gas stream is injected into the distillation tower in the controlled freezing zone itself. Preferably, the raw gas stream has been substantially dehydrated before it is injected into the distillation tower.

The method further includes chilling the raw gas stream. Chilling the raw gas stream causes carbon dioxide within the raw gas stream to precipitate upon the collector tray as a substantially solid material and become a slurry thereon. At the same time, the pressure in the distillation tower is lower than a feed stream, causing methane within the raw gas stream to flash. The methane travels through a rectification zone above the controlled freezing zone and exits the cryogenic distillation tower as an overhead methane stream.

The method also includes passing the overhead methane stream through a refrigeration system downstream of the cryogenic distillation tower. The refrigeration system cools at least a portion of the overhead methane stream to a liquid. The method additionally includes returning a portion of the cooled overhead methane stream to the cryogenic distillation tower as liquid reflux. A portion of the liquid reflux, in turn, may serve as the cold liquid spray.

Also as part of the method, the substantially solid material is removed from the cryogenic distillation tower. In one aspect, removal of the substantially solid material is accomplished through use of a mechanical translation device such as a screw conveyor or auger. The auger may reside within a downcomer of the collector tray as indicated above. The auger cuts through the substantially solid material, or slurry, translating it out of the distillation tower and towards solid CO2processing equipment. It is preferred that the collector tray operates at a temperature of, for example, about −70° F. to −80° F. This is at or slightly below the freezing point of the CO2component.

The method further includes separating the substantially solid material into a substantially solid filter cake and a substantially liquid filtrate. The filter cake is comprised primarily of carbon dioxide, while the filtrate comprises methane and residual carbon dioxide. The filtrate may include other components such as heavy hydrocarbons and even light aromatics.

The separating step may be accomplished by passing the substantially solid material or slurry through a first filter. This produces a first filter cake comprised primarily of solid carbon dioxide, and a first filtrate comprising methane and carbon dioxide, in liquid phase. The first filter may be, for example, a porous media or a centrifuge.

The separating step may further comprise rinsing the first filter cake using a cold carbon dioxide stream, mixing the first filter cake to produce a first solid-liquid slurry, and delivering the first solid-liquid slurry to a second filter. The second filter produces a second filter cake comprised primarily of solid carbon dioxide, and a second filtrate comprising methane and carbon dioxide, again in liquid phase.

While a single separation step may be sufficient in some implementations, additional CO2removal may be undertaken. For example, the separating step may further comprise rinsing the second filter cake using the cold carbon dioxide stream, mixing the second filter cake to produce a solid-liquid slurry, and delivering the solid-liquid slurry to yet a third filter. This produces a third filter cake comprised primarily of solid carbon dioxide, and a third filtrate comprising yet a smaller amount of methane and carbon dioxide, again in liquid phase.

The method also includes returning at least a portion of the second liquid material to the cryogenic distillation tower. In one aspect, the second liquid material is directed back to the lower distillation zone. In another aspect, the second liquid material is merged with the raw gas stream and is re-injected into the tower in the controlled freezing zone.

In one embodiment of the method, the first filtrate and the second filtrate are combined. The combined fluid from the filtrates forms the liquid filtrate that is returned to the cryozenic distillation tower. In this instance, the combined liquid filtrate is preferably injected into the lower distillation zone.

In another embodiment of the method, only the first filtrate is returned to the distillation tower as the liquid filtrate. In this instance, the first filtrate may be returned back to the controlled freezing zone. The distillation tower preferably will not have a lower distillation zone. The second and, optionally, third (or subsequent) filtrates are delivered to a separate, downstream distillation tower where residual acid gases are finally separated from methane. In this instance, a recovery methane stream is obtained that is merged with the overhead methane stream of the cryogenic distillation tower for sale.

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 40%.

The methane may be present along with trace elements of other 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 include acid gases such as CO2and H2S.

The initial fluid stream10may be at a post-production pressure of approximately 600 pounds per square inch (psi) or lower. 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° F. 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 initial fluid stream10into a chilled fluid stream12. Preferably, the temperature of the chilled fluid stream12is around −40° F. 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 zone106, an intermediate controlled freezing zone, or “spray section”108, and an upper distillation or “rectification” zone110. In the tower arrangement ofFIG. 1, the chilled fluid stream12is introduced into the distillation tower100at the controlled freezing zone108. However, the chilled fluid stream12may alternatively be introduced near the top of the lower distillation zone106.

It is noted in the arrangement ofFIG. 1that the lower distillation zone106, the intermediate spray section108, the upper rectification zone110, and all the components are housed within a single vessel. 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 distillation tower110may optionally be split into two separate pressure vessels (not shown). For example, the lower distillation zone106and the controlled freezing zone108may be located in one vessel, while the upper rectification zone110is in another vessel. External piping would then be used to interconnect the two vessels.

In either embodiment, the temperature of the lower distillation zone106is higher than the feed temperature of the chilled fluid stream12. The temperature of the lower distillation zone106is 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 (such as ethane or propane), this will increase the boiling temperature of the mixture. These factors become design considerations for the operating temperatures within the distillation tower100.

In the lower distillation zone106, 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 lower distillation zone106to facilitate removal of CO2and heat transfer.

Referring again toFIG. 1, the chilled fluid stream12may be introduced into the column100near the top of the lower distillation zone106. Alternatively, it may be desirable to introduce the feed stream12into the intermediate spray or controlled freezing zone108above the melt tray130. The point of injection of the chilled fluid stream12is a design issue dictated primarily by the composition of the initial fluid stream10.

It may be preferable to inject the chilled fluid stream12directly into the lower distillation zone106through a two-phase flashbox type device (or vapor distributor)124in the column100. The use of a flashbox124serves to partially 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 significant liquid slugging or frequent process upsets are anticipated, the chilled fluid stream12may need to be partially separated in a vessel173prior to feeding the column100. In this case, the chilled feed stream12may be separated in a two phase vessel173. 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. Liquid13is discharged from the two phase vessel173. The liquid13is directed into the column100through the distributor124. The liquid13can 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 lower distillation zone106. 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 lower distillation zone106, cause methane to break out of solution. The resulting vapor carries the methane and any entrained carbon dioxide molecules that have boiled off.

The vapor further proceeds upward through chimneys131of the melt tray130(seen inFIG. 2B) and 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 CO2Stated another way, CO2will freeze and then precipitate or “snow” back onto the melt tray130. The solid CO2then melts and gravitationally flows in liquid form down the melt tray130and through the lower distillation zone106there below.

As will be discussed more fully below, the spray section108is an intermediate freezing zone of the cryogenic distillation tower100. With the alternate configuration in which the chilled fluid stream12is separated in vessel173prior to entering the tower100, a part of the separated liquid/solid slurry13is introduced into the tower100immediately above 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 lower distillation zone106in liquid form for further purification. The melt tray130collects and warms the solid-liquid mixture from the controlled freezing zone108in a pool of liquid. The melt tray130is designed to release vapor flow back to the controlled freezing zone108, to provide adequate heat transfer to melt the solid CO2, and to facilitate liquid/slurry drainage to the lower distillation or lower distillation zone106of 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 controlled freezing zone108. This serves to increase heat transfer from the vapors passing up from the lower distillation zone106of 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 lower distillation zone106there 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 controlled freezing zone108.

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 controlled freezing zone108. 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 controlled freezing zone108can 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 oil, 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(seen inFIG. 2C). Fluids collected in the downcomer138are delivered to a transfer line135. Flow through the transfer line135may be controlled by a control valve137and a level controller “LC” (seen inFIG. 1). Fluids are returned to the lower distillation zone106via 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 lower distillation zone106, 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 lower distillation zone106. 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 lower distillation zone106. As noted, a plurality of stripping trays126is provided in the lower distillation zone106below 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 tray126to 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 downcotners129to help prevent vapor from 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 lower distillation zone106. The extent of distillation depends on the number of trays126in the lower distillation zone106. in the upper part of the lower distillation zone106, 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 lower distillation zone106is a matter of design choice based on the composition of the raw gas stream10. However, only a few levels of stripping trays126need be typically 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 represent 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 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 lower distillation zone106to travel upwards more efficiently to the melt tray130and through the chimneys131.

In one aspect, the trays (such as trays126or426) may 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 corrosive 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 distillation tower100. In this respect, the jet tabs422may be oriented to push liquid along the wall of the tower100, thereby preventing solids accumulation along the wall of the tower100and ensuring good vapor-liquid contact.

In any of the tray arrangements, as the down-flowing liquid hits the stripping trays126, separation of materials 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 lower distillation zone106. The liquid is then moved out of the cryogenic distillation tower100in an exit line as a bottoms fluid stream22.

Upon exiting the distillation tower100, the bottoms fluid stream22enters a reboiler160. InFIG. 1, the reboiler160is a kettle-type vessel 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 cool bottoms fluid stream22from the distillation tower100while pre-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 bottoms fluid stream22. The warm fluids in heater line25exist in one aspect at 30° F. 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, most of the reboiled vapor stream is introduced at the bottom of the column through line27, above the bottom liquid level and at or below the last stripping tray126. As the reboiled vapor passes upward 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 vapor stream from line27reaches the corrugated melt tray130, the temperature may drop 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 −70° F. The vapor still has enough enthalpy to melt the solids CO2as it comes in contact with 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 lower distillation zone106of the distillation tower100, gas moves up through the lower distillation zone106, through the chimneys131in the melt tray130, and into the controlled freezing zone108. The controlled freezing zone108defines an open chamber having a plurality of spray nozzles122. As the vapor moves upward through the controlled freezing zone108, the temperature of the vapor becomes much colder. The vapor is contacted by liquid methane coming from the spray nozzles122. This liquid methane is much colder than the upwardly-moving vapor, having been chilled by an external refrigeration unit170. In one arrangement, the liquid methane exits 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 tower100.

As the methane vapors move further up the cryogenic distillation tower100, they leave the controlled freezing zone108and enter the upper rectification zone110. 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 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 about −135° F. to −145° F. This serves to at least partially liquefy the overhead methane stream14. The refrigerated methane stream14is then moved to a reflux condenser or separation chamber172.

The 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 tower100. A pump19may be used to move the liquid reflux18back into the tower100. Alternatively, the separation chamber172is mounted above the tower100to provide a gravity feed of the liquid reflux18. The liquid reflux18will include any carbon dioxide that escaped from the upper rectification zone110. However, most of the liquid reflux18is methane, typically 95% or more, with nitrogen (if present in the initial fluid stream10) and traces of 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. 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 again toFIG. 1, the liquid reflux18is returned into the upper distillation or rectification zone110. The liquid reflux18is then gravitationally carried through one or more mass transfer devices116in the upper rectification zone110. In one embodiment, the mass transfer devices116are rectification trays that provide a cascading series of weirs118and downcomers119, similar to trays126described above. In lieu of trays, random or structured packing may also be employed.

As fluids from liquid reflux stream18move downward through the rectification trays116, additional methane vaporizes out of the upper rectification zone110. The methane gases rejoin the overhead methane stream14to become part of the gas product stream16. However, the remaining liquid phase of liquid reflux18falls onto a collector tray140. As it does so, the liquid reflux stream18unavoidably will pick up a small percentage of hydrocarbon and residual acid gases moving upward from the controlled freezing zone108. The liquid mixture of methane and carbon dioxide is collected at a 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 controlled freezing zone108. 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 upper rectification zone110, 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 a 4 ppm specification. As fluid moves through the mass transfer devices116in the upper rectification zone110, 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 lower distillation zone106and ultimately exits the cryogenic distillation tower100as part of the liquefied acid gas bottoms stream22.

In cryogenic distillation tower100, the liquid captured at collector tray140is drawn out of the upper rectification zone110as 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 fluid18. The liquid stream20is directed into a spray header collection drum174. The purpose of the spray header collection drum174is to provide surge capacity for a pump176. Upon exiting the spray header collection 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 controlled freezing zone108and 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 controlled freezing zone108, through the chimneys in the collector tray140, and through the mass transfer devices116in the upper rectification zone110. The methane leaves the distillation tower100as the overhead methane stream14and ultimately becomes part of the 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 controlled freezing zone108, 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 lower distillation zone106, along with liquid CO2from the chilled raw gas 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 upper rectification zone110.

It is desirable to have chilled liquid contacting as much of the gas that is moving up the tower100as possible. If vapor bypasses the spray stream21emanating from the nozzles122, higher levels of CO2could reach the upper rectification zone110of the tower100. To improve the efficiency of gas/liquid contact in the controlled freezing zone108, 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 controlled freezing zone108.

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 6Btherein are incorporated herein by reference for teachings of the nozzle configurations. The nozzles seek to ensure 360° coverage within the controlled freezing zone108and 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 entering the upper rectification zone110. Otherwise, these particles would re-mix with methane and 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 gas comprised of a commercial methane product16. The gas product16is sent down a pipeline for sale. The gas product16preferably meets a pipeline CO2specification of 1 to 4 mol. percent, as well as a 4 ppm or less H2S specification, assuming sufficient reflux is generated. At the same time, acid gases and, if present, heavy hydrocarbons, are removed through bottoms fluid stream22.

It is observed that an inherent inefficiency exists in freezing the acid gas components into a solid in the controlled freezing zone108, then melting them into a liquid bottom stream22in the lower distillation zone106, and then separating the CO2from any entrained natural gases using a bottom reboiler160. A considerable amount of energy is consumed in connection with freezing the CO2. This energy is at least partially wasted as the solid components in the controlled freezing zone108melt and then re-mix in the lower distillation zone106with any H2S and other heavy hydrocarbons in the liquid phase.

Because relatively pure CO2is more desirable for acid gas injection or disposal, an acid gas enrichment process or other purification method is desired to separate frozen CO2. This separation should take place at the bottom of the controlled freezing zone108or at the top of the lower distillation zone106. Thus, instead of melting the CO2(and remixing with any liquid H2S and heavy hydrocarbon components) and gravitationally dropping the liquid-phase components through the lower distillation zone106, it is proposed herein to replace the melt tray130with a collector tray. The collector tray will receive precipitates from the controlled freezing zone108in the form of a solid-liquid slurry. The solid-liquids slurry will be collected on the collector tray and removed from the cryogenic distillation tower for separate processing.

FIG. 6Aprovides a plan view of a collector tray610, in one embodiment.FIG. 6Bprovides a cross-sectional view of the collector tray610, taken across line B-B ofFIG. 6A.FIG. 6Cshows a cross-sectional view of the collector tray610, taken across line C-C. The collector tray610will be described with reference to these three drawings together.

First, the collector tray610includes a base620. The base620may be a substantially planar body, or may have undulations to increase surface area. In either respect, the base620is preferably tilted inwardly along opposite sides so that fluids landing on the base620will gravitationally drain towards a central downcomer630.

In the view ofFIG. 6C, a sump or “downcomer”630is more clearly seen central to the collector tray130. The collector tray base620slopes inwardly towards the downcomer630to deliver the solid-liquid slurry. The base620may be sloped in any manner to facilitate gravitational solid and liquid draw-off.

As with the melt tray130ofFIG. 2A, the collector tray610ofFIGS. 6A, 6B and 6Chas a plurality of chimneys622,624(or “risers”). The chimneys622,624provide improved vapor distribution, allowing fluids in the gas phase to travel upward from the lower distillation zone106and into the intermediate controlled freezing zone108. This also contributes to better heat/mass transfer in the controlled freezing zone108.

The chimneys622,624may be of any profile. For instance, the chimneys622,624may be round, rectangular, or any other shape that allows vapor to pass through the collector tray610. The chimneys622,624may also be narrow and extend upward into the controlled freezing zone108. This enables a beneficial pressure drop to distribute the vapor evenly as it rises into the freezing zone108.

The top openings of the chimneys622,624are preferably covered with hats or caps626. The caps626minimize the chance that solids dropping from the controlled freezing zone108will bypass the collector tray610and travel into the lower distillation zone706.

Along with the base620, the downcomer630is preferably inclined. An incline arrangement for a downcomer630′ is demonstrated in the side view ofFIG. 6D.FIG. 6Dis a cross-sectional view of the collector tray610ofFIG. 6A, in an alternate embodiment. The view is taken across line B-B ofFIG. 6A.

The collector tray610is designed to be incorporated into a cryogenic distillation tower as part of a system for removing acid gases from a raw gas stream. The collector tray610is configured to receive solid and liquid particles falling from the controlled freezing zone of a cryogenic distillation tower. The collector tray610is further configured to transport slurry made up of the solid and liquid particles out of the tower and to a CO2recovery facility.

FIG. 7is a schematic diagram showing a gas processing facility700for removing acid gases from a hydrocarbon gas stream in accordance with the present invention, in one embodiment. The hydrocarbon gas stream originates from hydrocarbon production activities that take place in a reservoir development area, or “field.” The field may be any location where compressible hydrocarbons are produced. The field may be onshore, near shore or offshore. The field may 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 components.

The gas processing facility700utilizes a collector tray such as the collector tray610ofFIG. 6A. It can be seen inFIG. 7that the collector tray610is incorporated into a cryogenic distillation tower705. The distillation tower705has an intermediate controlled freezing zone708. The controlled freezing zone708, or spray section, receives a cold liquid spray comprised primarily of methane.

The cold spray is preferably a liquid reflux generated from an overhead loop714downstream of the distillation tower705. The overhead loop714includes refrigeration equipment within a heat exchanger170that serves to cool the overhead methane stream14and then return a portion of the overhead methane stream14to the cryogenic distillation tower705as liquid reflux18. The liquid reflux18is sprayed within the controlled freezing zone708through spray headers120to cause precipitation of solid carbon dioxide particles. As illustrated inFIG. 7, the liquid reflux18is delivered to an upper rectification section710, which will be discussed further below, before being sprayed through the spray headers120. Other implementations, may draw some or all of the liquid reflux18directly to the spray headers120or to the spray header collection drum174.

As with tower100ofFIG. 1, the cryogenic distillation tower705is configured to receive an initial fluid stream10comprised of acid gases. The initial fluid stream10contains methane, carbon dioxide and, possibly, trace amounts of ethane, nitrogen, helium and hydrogen sulfide. The initial fluid stream10preferably undergoes some degree of dehydration before being injected into the distillation tower705. Dehydration may be accomplished by passing the initial fluid stream through a glycol dehydration process. (A dehydration system is not shown inFIG. 7.)

In addition, the initial fluid stream10is preferably chilled before entering the distillation tower705. A heat exchanger150, such as a shell-and-tube exchanger, is provided for chilling 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° F. to −40° F. The initial fluid stream10may then be moved through an expansion device152such as a Joule-Thompson (“J-T”) valve. The result is a chilled raw gas stream712. Preferably, the temperature of the chilled raw gas stream712is around −40° F. to −70° F.

It is noted that in the gas processing facility700, the raw gas stream712is received into the distillation tower705below the controlled freezing zone708. More specifically, the raw gas stream712is injected into a lower distillation zone706below the controlled freezing zone708. However, it is understood that the raw gas stream712may be directed through a two-phase vessel such as vessel173shown inFIG. 1. This generates a split stream comprised primarily of methane vapor (injected into the controlled freezing zone708) and liquid acid gases and, possibly, heavy hydrocarbons (injected into the lower distillation zone706.) The two-phase vessel173minimizes the possibility of solids plugging the inlet line and internal components of the distillation tower705.

In one aspect, the cryogenic distillation tower712is operated at a pressure of about 550 psi in the controlled freezing zone708, and the chilled raw gas stream712is at approximately −62° F. At these conditions, the raw gas stream712is in a substantially liquid phase, although some vapor phase may inevitably be entrained into the chilled gas stream712. Most likely, no solids formation has arisen from the presence of CO2.

The cryogenic distillation tower705also includes an upper rectification zone710. The upper rectification zone710resides above the controlled freezing zone708. As discussed above in connection with the cryogenic distillation tower100ofFIG. 1, the distillation zone710serves to further separate methane vapor from any entrained carbon dioxide molecules. The distillation zone710releases an overhead methane gas stream14. It also distributes a portion of fluid into liquid stream20which is passed through spray header collection drum174, then to pressure booster176, and then injected back into the tower705through spray headers120.

As noted, the gas processing facility700further comprises a collector tray610. The collector tray610is positioned below the controlled freezing zone708for receiving substantially solid material as it is precipitated from the controlled freezing zone708. It is preferred that the collector tray610operate at a temperature of for example, about −70° F. to −80° F. This is at or slightly below the freezing point of the CO2. A slurry is thus generated at the collector tray610.

Preferably, the collector tray610has an inclined base (shown at620inFIG. 6C) to direct slurry into a central downcomer (shown at630inFIG. 6C). The downcomer630, in turn, may optionally include a mechanical translation device such as an auger (shown at640inFIG. 6B) within the downcomer. The auger640serves to mechanically move a slurry that includes the solid CO2material out of the cryogenic distillation tower705and towards a CO2recovery facility740.

A slurry exit line741is provided in the gas processing facility700. The slurry exit line741moves slurry from the distillation tower705to the CO2recovery facility740. in this way, carbon dioxide is substantially removed from the distillation tower705before it drops into the lower distillation zone706. The slurry may be moved gravitationally. Alternatively or in addition, the slurry may be translated with the aid of the auger640. Alternatively still, a portion of the cold liquid reflux18may be directed from a side wall of the distillation tower705into the collector tray610to urge the slurry from the collector tray and out of the distillation tower705.

There are several potential advantages to extracting CO2in a solid state without allowing the solid to melt and exit as part of the bottoms fluid stream722. First, when done at the proper temperature and pressure, the process of crystallizing carbon dioxide into a solid state typically produces a substantially pure solid material. While some trace amounts of methane, hydrogen sulfide and heavy hydrocarbons may be entrained in the solids as part of the slurry, separation of solid CO2allows for a substantially pure CO2product. Stripping of light products such as methane or other acid products such as H2S is not necessarily required as such products will fall as liquids into the bottoms fluid stream722.

Also, because a substantial portion of the CO2within the raw gas stream712is recovered as a pure solid, the amount of CO2in the bottoms fluid stream will be reduced. This, in turn, lowers the demands on downstream processes such as acid gas enrichment and sulfur recovery units (not shown). In addition, valuable heavy hydrocarbons such as ethane or propane may be more easily recovered from the bottoms fluid stream722as the CO2content is substantially reduced.

In addition, removing a substantial portion of the CO2within the raw gas stream712as a side-draw (at slurry exit line741) may reduce the vapor and liquid loads in the distillation tower705. This, in turn, allows for lower capacities in the reboiler160and condenser, that is, the separation chamber172and reduced refrigeration requirements. More importantly, extraction of solid CO2may allow for a smaller diameter tower705for an equivalent feed capacity. The size reduction is generally proportional to the amount of solid CO2extracted from the tower705.

Referring again toFIG. 7, the carbon dioxide-based slurry is transported through the slurry exit line741to the CO2recovery facility740. The CO2recovery facility740includes a first filter742. Preferably, the first filter742represents a porous media that catches a substantial portion of the solid material from the slurry. The first filter742may be, for example, wire mesh. Alternatively, the filter742may be a polyester or other synthetic porous material. The filter742may alternatively be a centrifugal separator, a hydrocyclone, one or more belt filters, one or more filter presses, or combinations thereof.

The liquid portion of the slurry is known as a “filtrate.” The filtrate passes through the first filter742and is delivered to a liquid line744. The filtrate comprises primarily CH4, but may also include CO2and H2S. The liquid line744delivers the filtrate to a liquid return line760. The liquid return line760returns the filtrate to the lower distillation zone706of the cryogenic distillation tower705. The CH4is vaporized and becomes part of the overhead methane stream14. The H2S and any heavy hydrocarbon components are dropped out of the tower705as liquids in the bottoms fluid stream722.

It is noted that the liquid line744may also contain heavy hydrocarbons, particularly measurable components of ethane and propane. These components may be recovered by sending the filtrate744through a process similar to a conventional natural gas liquids (“NGL”) train (not shown).

The first filter742captures the solid portion of the slurry, known as a “filter cake.” The filter cake comprises primarily carbon dioxide. The solid filter cake is delivered along a first solid material line746. The filter cake may be carried from the first filter742through the first solid material line746by means of a screw-conveyor, by Hildebrandt extractors, or by other means known in the art.

From there, the filter cake may be warmed so that it enters the liquid phase. In one aspect, the solid carbon dioxide from first solid material line746is warmed in a heat exchanger772. The heat exchanger772may, for example, use heat from the initial fluid stream10for melting the carbon dioxide. This beneficially cools the initial fluid stream10before it enters the heat exchanger150. At the same time, the warmed liquid CO2is delivered as substantially pure carbon dioxide liquid through CO2fluid line786.

In lieu of delivering the frozen carbon dioxide (or filter cake) in solid material line746directly to a heat exchanger772, the operator may choose to carry the frozen carbon dioxide through additional filtering. In the gas processing facility700, the CO2recovery facility740may include a rinsing vessel748. In the rinsing vessel748, cold liquid CO2is sprayed onto the frozen carbon dioxide. This has the effect of creating a new slurry, with any residual methane and hydrogen sulfide being rinsed away from the solid filter cake as a liquid.

The cold carbon dioxide used as the rinsing agent is delivered through CO2delivery line784. The cold CO2used as the rinsing agent is preferably drawn from an outlet778for the heat exchanger772. A cold CO2line is shown at780.

Referring again to the rinsing vessel748, preferably, the slurry is mixed in the rinsing vessel748. A stirring apparatus747may be provided in the rinsing vessel748. The stirring apparatus747may be, for example, a set of blades that rotate through the solid material to create surface area. Creating surface area exposes the solid material to the cold liquid CO2from deliver line784. This, in turn, helps to rinse the residual methane and hydrogen sulfide from the solid.

The new slurry is carried from the rinsing vessel748through slurry line750. The new slurry is delivered to a second filter, noted inFIG. 7as752. The second filter752captures the solid portion of the new slurry. The solid portion again comprises primarily carbon dioxide. The solid portion represents a second filter cake, and is delivered along a second solid material line756. From there, the second filter cake may be warmed so that it enters the liquid phase.

It is noted that the rinsing and filtration steps are shown taking place in separate vessels, e.g., a rinsing vessel748and a filtration vessel752. However, the operator may choose to combine the rinsing and filtration of solid material in a single vessel.

The liquid portion of the new slurry, known as a second filtrate, passes through the second filter752and is delivered to a liquid line754. The second filtrate comprises CH4and, possibly, H2S and heavy hydrocarbons. The liquid line754delivers the liquid portion of the slurry to the liquid return line760. Thus, the liquid representing the second filtrate754merges with the liquid representing the first filtrate754before being injected into the distillation tower705through liquid return line760. The CH4is vaporized and becomes part of the overhead methane stream14. The H2S and C2+ compounds are dropped out of the tower705as liquids in the bottoms fluid stream722. Should either the first filtrate744or the second filtrate754contain any melted CO2, the melted CO2will evaporate into the controlled freezing zone708and ultimately precipitate back onto the collector tray610as frozen material.

Optionally, the first744and/or second754filtrate may be carried through the liquid return line760to a small, peripheral distillation column (not shown) for further purification.

The operator may choose to early the substantially pure, solid CO2in line756directly to the heat exchanger772. Alternatively, additional separation of impurities may take place. Box770inFIG. 7depicts one or more additional rinsing and filtration stages for the solid CO2in line756. The number of rinse and filtration steps is dependent on the desired purity of the CO2product. Line782is shown delivering cold CO2as a rinsing agent. A third (or subsequent) filtrate774is released from the additional rinsing and filtration stage770. A third (or subsequent) solid CO2(or filter cake) is delivered through line776to the heat exchanger772. A final CO2product line is seen at line786. The liquid CO2product may be used for acid gas injection, or may be delivered for sale as a high-purity product. A customer may, for example, use the liquid CO2product for enhanced oil recovery or for other purposes.

The gas processing facility700ofFIG. 7is ideally used in conditions where the chilled raw gas stream712has a high CO2content, such as greater than approximately 30%. In this condition, significant refrigeration may be required to freeze all CO2from the raw gas stream712. Therefore, it is believed to be more energy efficient to inject the raw gas stream712in the lower distillation zone706below the controlled freezing zone808and below the collector tray610. Any CO2that stays in liquid form and drops out of the distillation tower705with the bottoms feed stream722will be recovered through the reboiler160and re-injected into the lower distillation zone706.

In the illustrative gas processing system700, the collector tray610and corresponding slurry exit line741are positioned well above the raw gas injection point. The operator may choose to raise the point at which the raw gas stream12enters the cryogenic distillation tower705. It is believed that raising the injection point will increase the amount of fluid from the raw gas stream712that is recovered on the collector tray610as solid. This is more advantageous where the raw gas stream712has a lower CO2content, such as about 10 to 30 mol. percent.

In one simulation conducted by the Applicant, the collector tray610and corresponding slurry exit line741were positioned at or slightly above the raw gas injection point. The raw gas stream712was simulated to have a composition of 70 mol. percent CO2and 30 mol. percent CH4. An initial gas temperature of 40° C. was assumed, with an injection flow rate of approximately 10,000 standard m3/hour. The cryogenic distillation tower705was simulated to operate at 450 psia.

In this simulation, approximately 93% of the feed CO2exited the cryogenic distillation tower as a solid. Very little fluid was left to travel down the distillation tower in liquid form. This, of course, produced a substantial reduction in volume for the bottoms fluid stream722and reduced the load requirements for the reboiler160by about 89%. The drawback to this approach is that more refrigeration is required in the heat exchanger150upstream of the distillation tower705to chill the initial fluid stream10. This is partially offset by a slight reduction in refrigeration required in the heat exchanger170downstream of the distillation tower705.

FIG. 8is a schematic diagram showing a gas processing facility800for removing acid gases from a gas stream in accordance with the present invention, in an alternate embodiment. The gas processing facility800is generally similar to the gas processing facility700. In this respect, the gas processing facility800also utilizes a collector tray such as the collector tray610ofFIG. 6A. The collector tray610is incorporated into a cryogenic distillation tower805. The distillation tower805again has an intermediate controlled freezing zone808. The controlled freezing zone808, or spray section, receives a cold liquid spray comprised primarily of methane.

As with tower705ofFIG. 1, the cryogenic distillation tower805is configured to receive an initial fluid stream10comprised of hydrocarbon and acid gases. The initial fluid stream10preferably undergoes some degree of dehydration before being injected into the distillation tower805. In addition, the initial fluid stream10is preferably chilled before entering the distillation tower805. A heat exchanger150, such as a shell-and-tube exchanger, is provided for chilling the initial fluid stream10. A refrigeration unit (not shown) provides cooling fluid (such as liquid propane) within the heat exchanger150to bring the temperature of the initial fluid stream10down to about −60° F. to −80° F. The initial fluid stream10may then be moved through an expansion device152such as a Joule-Thompson (“J-T”) valve. The result is a chilled raw gas stream812.

As noted above, in the gas processing facility700, the raw gas stream712is received below the controlled freezing zone708. More specifically, the raw gas stream712is injected into the lower distillation zone706. However, in the cryogenic distillation tower805, the distillation zone (706fromFIG. 7) has been removed, and the collector tray610now resides in the controlled freezing zone808. In addition, the raw gas stream812is injected into the controlled freezing zone808above the collector tray610. This is consistent with the simulation described above.

The purpose for moving the injection point for the chilled raw gas stream812up into the controlled freezing zone808is to obtain a higher solid CO2recovery. To effectuate this, the temperature of the chilled raw gas stream812is brought down to around −60° F. to −80° F. This is a lower temperature range than was imposed on the raw gas stream712inFIG. 7. As the raw gas stream812enters the tower805, it flashes and cools, precipitating the CO2in the controlled freezing zone808. Any vapor CO2will be cooled by the liquid CH4reflux descending from the spray headers120. his produces a solid that precipitates onto the collector tray610, forming a slurry.

As with gas processing facility700, the slurry is moved in gas processing facility800from the cryogenic distillation tower805, through a slurry exit line741, and to a CO2recovery system840. The CO2recovery system840may be the same as the CO2recovery system740ofFIG. 7. In this respect, a slurry comprised primarily of carbon dioxide is moved through a first filter742, and then optionally moved through one, two, or three stages of rinsing and filtration until a substantially pure CO2solid is obtained. The CO2solid is preferably warmed through the heat exchanger772and then released through outlet778as a liquid. Liquid CO2is released as a product through line786.

In the CO2recovery system840ofFIG. 8, the first filtrate744from first filter742is returned to the cryogenic distillation tower805. This is consistent with the operation of the CO2recovery system740ofFIG. 7. However, instead of returning the methane-rich first filtrate744through the liquid return line760and directly into the tower805, the first filtrate744is merged with the initial fluid stream10. In this way, the methane-rich first filtrate744may be re-chilled before injection into the controlled freezing zone808.

In the CO2recovery system840, the second filtrate754and subsequent filtrate(s)874are not merged with the liquid return line760; rather, the second filtrate754and the subsequent filtrate(s)874are merged together and delivered to a downstream distillation column892. The second filtrate754and the subsequent filtrate(s)874are comprised primarily of hydrogen sulfide, but may contain trace amounts of methane and carbon dioxide. In the reboiler892, the methane is released as a recovery methane stream894. The recovery methane stream894is merged with the methane sales product16and delivered to market as sales product898.

The reboiler892also releases a liquid896. The liquid896comprises primarily hydrogen sulfide with trace amounts of carbon dioxide. The H2S-rich liquid896is disposed of or taken through a sulfur recovery unit (not shown). As an alternative, the second filtrate754and the subsequent filtrate(s)874may be disposed of or taken through a sulfur recovery unit without going through the reboiler892. This is particularly applicable if the amount of CH4content does not warrant recovery, or require separation.

As can be seen fromFIGS. 7 and 8, different processing schemes may be utilized. The optimum arrangement will depend on a number of variables. These variable include the availability (or capacity) of refrigeration as generated within an overhead heat exchanger such as heat exchanger170, the desired purity of the CO2product786and, most importantly, the composition of the initial fluid stream10. Regardless of the selected flow scheme, the basic principle of distillation combined with solid removal and purification will apply.

In some situations, the initial fluid stream10may have a high concentration of hydrogen sulfide, such as greater than about 5 to 10 percent. It some implementations, such as when solid CO2recovery is desired, it may be undesirable to run a gas feed stream having a high H2S concentration through a cryogenic distillation tower as it is believed that high levels of H2S can solubilize CO2, thereby preventing solid formation in the controlled freezing zone. In this situation, natural gases with a high ratio of H2S to CO2may be fed to a. pre-treating column (not shown) for selective H2S removal prior to being introduced to the main distillation tower705or805. The separation can be achieved using H2S separation processes such as absorption by selective amines, redox processes, or adsorption. Thereafter, the gas stream may be dehydrated and refrigerated in accordance with the illustrative processing facilities700or800described above. Additionally or alternatively, other implementations may be insensitive to the state of the CO2recovery product and the H2S may be left in the initial fluid stream.

An additional advantage to the removal of sulfur species upstream of the distillation tower is that sulfur removal may enable the production of a higher purity CO2product786from the recovery system740or840. In addition, a higher purity of C2+ products may be recovered from the bottoms fluid stream722. Of course, small amounts of H2S can be allowed to slip into the cryogenic distillation tower705,805, provided the phase behavior within the tower705,805allows solid CO2formation. Such small amounts of H2S will be recovered in the bottoms fluid stream22.

A method of removing acid gases from a raw gas stream is also provided herein.FIG. 9is a flow chart that presents a method900for removing acid gases from a raw gas stream using an acid gas removal system in accordance with the present inventions, in one embodiment. The raw gas stream comprises methane, carbon dioxide and, most likely, other components such as ethane and hydrogen sulfide.

The method900first includes providing a cryogenic distillation tower. This step is shown at Box905. The tower has a controlled freezing zone that receives a cold liquid spray comprised primarily of methane. The tower further has a collector tray below the controlled freezing zone.

The method900also includes injecting the raw gas stream into the cryogenic distillation tower. This is demonstrated at Box910. In one arrangement, the raw gas stream is injected into the distillation tower in a lower distillation zone below the controlled freezing zone. In another arrangement, the raw gas stream is injected into the distillation tower in the controlled freezing zone itself. Preferably, the raw gas stream has been substantially dehydrated before it is injected into the distillation tower.

The method900further includes chilling the raw gas stream. This is indicated at Box915ofFIG. 9. Chilling the raw gas stream causes carbon dioxide within the raw gas stream to precipitate upon the collector tray as substantially solid material. At the same time, the pressure in the distillation tower is lower than a feed stream, causing methane within the raw gas stream to substantially vaporize. The methane travels through an upper rectification zone above the controlled freezing zone, and then exits the cryogenic distillation tower as an overhead methane stream.

The method900also includes passing the overhead methane stream through a refrigeration system downstream of the cryogenic distillation tower. This is provided in Box920. The refrigeration system cools at least a portion of the overhead methane stream to a liquid.

The method900additionally includes returning a portion of the cooled overhead methane stream to the cryogenic distillation tower as liquid reflux. The liquid reflux, in turn, serves as the cold liquid spray. This is provided at Box925.

Also as part of method900, the solid material is substantially removed from the cryogenic distillation tower. This is shown at Box930. Preferably, removal of the substantially solid material is accomplished through gravitational flow. Alternatively, a mechanical translation device such as a screw conveyor or auger may be provided. The auger may reside within a downcomer of the collector tray as demonstrated inFIGS. 6A, 6B, 6C and 6D. The auger may alternatively be placed outside of the distillation tower to direct the substantially solid material to the CO2recovery system, in either instance, the auger cuts through the substantially solid material, translating it as a slurry out of the distillation tower and towards a CO2recovery system.

The method900further includes separating the carbon dioxide slurry into a solid material and a liquid material. This is shown at Box935. The first solid material is comprised primarily of carbon dioxide, while the liquid material comprises methane and residual carbon dioxide. The liquid material may include other components such as hydrogen sulfide, heavy hydrocarbons and even light aromatics.

The separating step of Box935may be accomplished by passing the slurry through a first filter. This produces a first filter cake comprised primarily of solid carbon dioxide, and a first filtrate comprising methane and carbon dioxide, in liquid phase. The first filter may be, for example, a porous media or a centrifuge.

The separating step of Box935may further comprise rinsing the first filter cake using a cold carbon dioxide stream, mixing the first filter cake to produce a first solid-liquid slurry, and delivering the first solid-liquid slurry to a second filter. The second filter produces a second filter cake comprised primarily of solid carbon dioxide, and a second filtrate comprising primarily methane but also carbon dioxide and hydrogen sulfide, again in liquid phase.

Additional CO2removal may be undertaken. For example, the separating step of Box935may further comprise rinsing the second filter cake using the cold carbon dioxide stream, mixing the second filter cake to produce a solid-liquids slurry, and delivering the solids-liquid slurry to yet a third filter. This produces a third filter cake comprised primarily of solid carbon dioxide, and a third filtrate comprising methane, hydrogen sulfide, and carbon dioxide, again in liquid phase.

The method900also includes returning at least a portion of the second liquid material to the cryogenic distillation tower. This is shown at Box940. In one aspect, the second liquid material is directed back to the lower distillation zone. In another aspect, the second liquid material is merged with the raw gas stream and is injected into the tower in the controlled freezing zone.

In one embodiment of the method900, the first filtrate and the second filtrate are combined. The combined fluid from the filtrates forms the liquid material that is returned to the cryogenic distillation tower. In this instance, the liquid material is preferably injected into the lower distillation zone.

In another embodiment of the method900, only the first filtrate is returned to the distillation tower. In this instance, the first filtrate may be returned back to the controlled freezing zone. The distillation tower preferably will not have a lower distillation zone; instead, the second and, optionally, third filtrates are delivered to a separate, downstream distillation tower where residual acid gases are finally separated from methane. In this instance, a recovery methane stream is obtained that is merged with the overhead methane stream of the cryogenic distillation tower for sale.

In yet another arrangement of the method900, the final filter cake is warmed. This is done regardless of whether there are one, two, three or more filtration stages. The final filter cake is a final-stage filter cake taken from the final filter, whatever that may be. This will, of course, include at least a portion of the first solid material. This warming step is shown in Box945ofFIG. 9.

Warming may be done, for example, by heat exchanging the solid carbon dioxide making up the filter cake with the raw gas stream. The result is a cold, pure carbon dioxide liquid. The pure carbon dioxide may be sold on the market or used for enhanced oil recovery operations. In addition, a portion of the cold carbon dioxide stream may be used as a rinse for creating the solid-liquid slurry as described above.

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 may also reduce the refrigeration requirements of cryogenic distillation towers while meeting LNG specifications for maximum allowable CO2.