Patent Description:
<CIT> relates to a process for liquefying natural gas in conjunction with processing natural gas to recover natural gas liquids (NGL). In the process, the natural gas stream to be liquefied is taken from one of the streams in the NGL recovery plant and cooled under pressure to condense it. A distillation stream is withdrawn from the NGL recovery plant to provide some of the cooling required to condense the natural gas stream. The condensed natural gas stream is expanded to an intermediate pressure and supplied to a mid-column feed point on a distillation column. The bottom product from this distillation column preferentially contains the majority of any hydrocarbons heavier than methane that would otherwise reduce the purity of the liquefied natural gas, and is routed to the NGL recovery plant so that these heavier hydrocarbons can be recovered in the NGL product. The overhead vapor from the distillation column is cooled and condensed, and a portion of the condensed stream is supplied to a top feed point on the distillation column to serve as reflux. A second portion of the condensed stream is expanded to low pressure to form the liquefied natural gas stream.

<CIT> discloses a process for treating a crude containing natural gas comprising supplying the crude to a stabilization unit to obtain a gaseous stream and crude oil; supplying a compressed, gaseous stream at a low temperature to the bottom of a first column; partly condensing the first gaseous overhead stream, returning the liquid phase to the first column and supplying the methane-rich stream to a liquefaction plant; supplying an expanded bottom stream at a low temperature to the top of a second column; removing from the top of the second column a second gaseous overhead stream, and removing from the bottom of the second column a liquid bottom stream; vaporizing part of the bottom stream and introducing the vapour into the bottom of the second column; and introducing the remainder of the bottom stream into a crude oil stream at an appropriate point in or upstream of the stabilization unit.

<CIT> deals with the processing of gas streams containing hydrocarbons and other gases of similar volatility to recover high yields of components such as ethane, propane, and heavier hydrocarbons therefrom by expanding said gas stream in at least two stages through turboexpanders, each stage cooling the gas stream and producing energy in the form of horsepower used to drive a recompressor unit or other mechanical apparatus. Condensed liquids are collected intermediate the expansion stages. Preferably, a supplemental gas cooling stage and condensed liquid recovery occurs following each expansion stage.

In <CIT> a low-pressure olefins recovery process and plant are described. The feed gas is compressed and distilled at a primary distillation pressure. The overhead stream is chilled at a pressure less than <NUM>/cm<<NUM> >(<NUM> psia) to partially condense the overheads. The primary distillation tower is refluxed with at least a portion of the condensate. The overhead vapor is further chilled and partially condensed and the condensate is fed to a demethanizer. The remaining vapor is cooled in a cold section and the resultant liquid is phase-separated and expanded to provide refrigeration for the cold section. The expanded vapor from the cold section is recycled to the process gas compressor. The bottoms streams from the primary distillation zone and the demethanizer are fractionated into respective streams consisting essentially of ethylene <NUM> , ethane <NUM> , propylene <NUM> , propane <NUM> , C4's <NUM> , and C5+ <NUM>.

<CIT> discloses a gas-liquid mixture is introduced into a phase separation zone wherein there is effected a separation between a gas phase and its equilibrium liquid; the gas phase is required to perform a maximum of expansion work and then passed in indirect heat exchange with the gas-liquid mixture; the desired cooling of the gas-liquid mixture not completely supplied by the gas phase is supplied by a portion of the equilibrium liquid with the remaining portion of the equilibrium liquid being passed to convenient locations for further processing.

<CIT> discloses a processing of gas streams containing hydrocarbons and other gases of similar volatility to recover high yields of components such as ethane, propane, and heavier hydrocarbons therefrom by cooling said gas stream under pressure to form a liquid portion, and expanding the liquid portion to a pressure lower than feed pressure whereby a part of the liquid portion vaporizes to cool the remaining part of the liquid portion is improved by pre-cooling the liquid portion prior to flash expansion. In one embodiment this is accomplished by dividing the remaining part of the liquid portion into a first and second stream, directing the first liquid stream into heat exchange relation with the liquid portion of the feed stream prior to expansion to warm the first liquid stream and pre-cool the liquid portion prior to expansion. Both first and second liquid streams are then supplied to a fractionating column, the second stream being supplied to the fractionating column at a point thereon higher than the first stream. Several other methods of pre-cooling the liquid portion are also described.

For pipeline transportation, it may be most economical to transport hydrocarbon liquid at ambient temperature and high pressure because it is easier to address requirements for wall thickness of the pipe without the need to insulate the entire length of the pipeline. For storage, it may be better for hydrocarbon liquid to be at or near atmospheric pressure to economically resolve the insulation and wall thickness requirements.

The invention relates generally to hydrocarbon processing, wherein a fluid circuit that incorporates components to prepare an incoming liquid ethane stream for storage. These components include a distilling unit embodied as a plurality of vessels to separate the incoming liquid ethane stream into a liquid for storage. The fluid circuit also includes a demethanizer column that is in position downstream of the vessels.

The invention configure the vessels to permit a position for the demethanizer column in the back or "tail" end of the fluid circuit. The vessels can reduce the amount of flash gas processed by the demethanizer column. In turn, compression requirements are lower in order maintain pressure of the flash gas and boil-off gas that the embodiments combine together for processing at the demethanizer column. This boil-off gas can originate from storage of the final, liquid ethane product. In this way, horsepower requirements for the embodiments will compare favorably to other processes that may utilize, for example, one or more demethanizer columns at the "front" end of the fluid circuit.

The embodiments can also be configured to recover other hydrocarbons from the incoming ethane stream. These other hydrocarbons are particularly useful as fuel gas and/or as raw materials for use in various petrochemical applications. In this way, the embodiments may avoid unnecessary loss of products from the feed stream, effectively adding value and/or optimizing profitability of the liquefaction process.

The embodiments may find use in many different types of processing facilities. These facilities may be found onshore and/or offshore. In one application, the embodiments can incorporate into and/or as part of processing facilities that reside on land, typically on (or near) shore. These processing facilities can process the feedstock from production facilitates found both onshore and offshore. Offshore production facilitates use pipelines to transport feedstock extracted from gas fields and/or gas-laden oil-rich fields, often from deep sea wells, to the processing facilitates. For liquefying processes, the processing facility can turn the feedstock to liquid using suitably configured refrigeration equipment or "trains. " In other applications, the embodiments can incorporate into production facilities on board a ship (or like floating vessel).

Reference is now made briefly to the accompanying drawings, in which:.

Where applicable like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated. The embodiments disclosed herein may include elements that appear in one or more of the several views or in combinations of the several views. Moreover, methods are exemplary only and may be modified by, for example, reordering, adding, removing, and/or altering the individual stages.

The discussion below contemplates embodiments that are useful to process liquid hydrocarbons for storage. The embodiments herein feature improvements that can reduce the overall size and, in turn, the overall investment necessary for commercial processing of ethane. Large operations that process quantities of liquid ethane in excess of <NUM> m3 (<NUM>,<NUM> barrels) per day may benefit in particular because the embodiments can use components that are substantially smaller than similar components, even when such similar components are "split" to more easily fabricate and ship to the installation site or facility. Other embodiments are contemplated with the scope of the disclosed subject matter.

<FIG> illustrates a schematic diagram of an exemplary embodiment of a processing system <NUM> (also "system <NUM>") for use to process hydrocarbon streams. The system <NUM> can receive a feedstock <NUM> from a source <NUM>. The feedstock <NUM> can comprise liquid with a composition that is predominantly ethane, although the system <NUM> may be useful for other compositions as well. In one implementation, incoming feedstock <NUM> may comprise ethane liquid with a first concentration of methane of approximately <NUM> % or less. The system <NUM> can have a fluid circuit <NUM> to process incoming feedstock <NUM> to form one or more products (e.g., a first product <NUM> and a second product <NUM>). The products <NUM>, <NUM> can exit the system <NUM> to a storage facility <NUM>, a pipeline <NUM>, and/or other collateral process equipment. In operation, the fluid circuit <NUM> is configured so that the first product <NUM> meet specifications for storage, e.g., at the storage facility <NUM>. These specifications may require a second concentration of methane that is lower than the first concentration of incoming feedstock <NUM>. In one example, the second concentration of methane in the first product <NUM> for may be approximately <NUM> % or less.

The fluid circuit <NUM> can circulate fluids (e.g., gases and liquids). For clarity, these fluids are identified and discussed in connection with operations of the embodiments herein as a process stream <NUM>. At a high level, the embodiments may include a pre-cooling unit <NUM>, a distilling unit <NUM>, a mixing unit <NUM>, and a demethanizer unit <NUM>. In one implementation, the fluid circuit <NUM> can receive a return stream <NUM> that may originate from the storage facility <NUM>, although this disclosure is not limited only to that configuration. The fluid circuit <NUM> can also be configured to separately couple the separator unit <NUM> and the demethanizer unit <NUM>, as shown by the phantom line enumerated by the numeral <NUM>. This configuration mixes outlet products from each of the units <NUM>, <NUM> together to form the first product <NUM>. As also shown in <FIG>, the fluid circuit <NUM> may couple with certain collateral equipment, namely, a refrigeration unit <NUM> that couples with the fluid circuit <NUM>. Examples of the refrigeration unit <NUM> may circulate a refrigerant <NUM> to coolers and/or like devices that condition temperature of the process stream <NUM> at one or more of the units <NUM>, <NUM>, <NUM>, <NUM>.

Broadly, use of the distilling unit <NUM> permits the demethanizer unit <NUM> to be located at the end of the fluid circuit <NUM>. This position reduces the volume of incoming feedstock <NUM> that the demethanizer unit <NUM> processes during operation of the system <NUM>. Some embodiments only require the demethanizer unit <NUM> to process approximately <NUM> % of incoming feedstock <NUM>, with the distilling unit <NUM> (and or other units in the fluid circuit <NUM>) configured to process approximately <NUM> % of incoming feedstock <NUM>. In such embodiments, the demethanizer unit <NUM> receives and processes predominantly "flashed" gas (also, "vapor") that results from one or more of the other units <NUM>, <NUM>, <NUM>. This feature is useful to reduce costs of the system <NUM> because the size of the demethanizer unit <NUM> is much smaller when at the "tail" end of the system <NUM> than in other positions further upstream in the fluid circuit <NUM>. In one implementation, the demethanizer unit <NUM> has a diameter that is <NUM>,<NUM> (nine (<NUM>) feet) or less.

<FIG> illustrates an example of components to implement the processing system <NUM> to achieve the second concentration of methane in the first product <NUM>. The refrigeration unit <NUM> can be configured to disperse the refrigerant <NUM> as a first refrigerant <NUM> and a second refrigerant <NUM>. The refrigerants <NUM>, <NUM> can facilitate thermal transfer at coolers disposed throughout the fluid circuit <NUM>. In turn, the coolers can be configured to implement cooling in stages (also, "cooling stages") to reduce temperature of the process stream <NUM>. Compositions for the refrigerants <NUM>, <NUM> can include propylene and ethylene, respectively; however, other compositions may also pose as workable solutions to affect thermal transfer in the coolers. In the pre-cooling unit <NUM>, the first refrigerant <NUM> can circulate across one or more coolers (e.g., a first cooler <NUM>, a second cooler <NUM>, and a third cooler <NUM>). The second refrigerant <NUM> can regulate temperature at coolers at each of the separation unit <NUM> and the demethanizer unit <NUM>. For the present implementation, the units <NUM>, <NUM> can be configured to include one or more coolers (e.g., a fourth cooler <NUM>, a fifth cooler <NUM>, and a sixth cooler <NUM>, a seventh cooler <NUM>).

At the distilling unit <NUM>, the fluid circuit <NUM> may include a separator <NUM> to form vapor, liquid, and mixed phase products. The separator <NUM> can generally be configured as a plurality of vessels (e.g., a first vessel <NUM>, a second vessel <NUM>, and a third vessel <NUM>). The fluid circuit <NUM> may also include a fourth vessel <NUM> that couples with a demethanizer column <NUM> at the demethanizer unit <NUM>. For operation, the components <NUM>, <NUM> may benefit from use of one or more peripheral components (e.g., a first peripheral component <NUM> and a second peripheral component <NUM>). Examples of these peripheral components <NUM>, <NUM> can include pumps, boilers, heaters, and like devices that can facilitate operation of the vessel <NUM> and/or the demethanizer <NUM>. In one implementation, the second peripheral component <NUM> may embody a boiler that couples with both the fourth vessel <NUM> and with the refrigeration unit <NUM> to condition temperature of the first refrigerant <NUM>.

The fluid circuit <NUM> may couple the vessels <NUM>, <NUM> with a flash drum <NUM> or like vessel. The flash drum <NUM> can couple with the storage facility <NUM> to provide the first product <NUM> for storage. The fluid circuit <NUM> may also include one or more throttling devices (e.g., a first throttling device <NUM>, a second throttling device <NUM>, and a third throttling device <NUM>). Examples of the throttling <NUM>, <NUM>, <NUM> can include valves (e.g., Joule-Thompson valves) and/or devices that are similarly situated to throttle the flow of a fluid stream. These devices may be interposed between components in the fluid circuit <NUM> as necessary to achieve certain changes in fluid parameters (e.g., temperature, pressure, etc.). As noted below, the device may provide an expansion stage and a cooling stage, where applicable, to reduce pressure and/or temperature of the process stream <NUM>.

<FIG> illustrates an example of a mixing unit <NUM> for use in the processing system <NUM> of <FIG> and <FIG>. This example can couple with the storage facility <NUM>, the separation unit <NUM>, and the demethanizer unit <NUM>. In one implementation, the mixing unit <NUM> may include a heat exchanger <NUM> that couples with a compression system <NUM>. Examples of the heat exchanger <NUM> can include crossflow devices of varying designs (e.g., spiral flow, counter-current flow, distributed flow, etc.), although other devices and designs that can effectively transfer thermal energy may also be desirable. The compression system can have one or more compressors (e.g., a first compressor <NUM> and a second compressor <NUM>) and one or more coolers (e.g., a first cooler <NUM> and a second cooler <NUM>).

Referring back to <FIG>, the fluid circuit <NUM> can direct the process stream <NUM> through the various components to generate the products <NUM>, <NUM>. The pre-cooling unit <NUM> can sub-cool the incoming feedstock <NUM> from a first temperature to a second temperature that is less than the first temperature. Incoming feedstock <NUM> may enter the device (at <NUM>) at ambient temperature that prevails at the system <NUM> and/or surrounding facility. The coolers <NUM>, <NUM>, <NUM> can effectively reduce temperature of incoming feedstock <NUM> by at least about <NUM>,<NUM> (<NUM> °F), with one example being configured to condition the process stream <NUM> to exit the cooling stages (at <NUM>) at approximately -<NUM> (°F). The fourth cooler <NUM> may provide a cooling stage to further reduce temperature of the liquefied ethane stream. This cooling stage can reduce temperature of the liquefied ethane stream by at least approximately -<NUM>,<NUM> (<NUM> °F), with one example of the fourth cooler <NUM> being configured so that the liquefied ethane stream exits this cooling stage (at <NUM>) at approximately -<NUM>,<NUM> (-<NUM> °F).

The fluid circuit <NUM> can direct the liquefied ethane stream to the first throttling device <NUM>. In one implementation, this device can be configured to reduce pressure of the liquefied ethane stream <NUM> from a first pressure to a second pressure that is less than the first pressure. The first pressure may correspond with the super critical pressure for incoming feedstock <NUM>. For liquid ethane, this super critical pressure may be approximately <NUM>,<NUM> Pa (<NUM> psig) or greater. The expansion stage can reduce pressure by at least approximately <NUM> Pa (<NUM> psig). In one example, the first expansion unit <NUM> being configured so that the liquefied ethane stream exits this expansion stage (at <NUM>) at approximately <NUM>,<NUM> Pa (<NUM> psig). Expansion across the first throttling unit <NUM> may also provide a cooling stage to further lower the temperature of the process stream <NUM>, e.g., to approximately -<NUM> (-<NUM> °F).

The fluid circuit <NUM> can process the liquefied ethane stream at the reduced pressure and reduced temperature to obtain the first product <NUM>. In use, the first product <NUM> will meet the methane concentration and other specifications for storage. Examples of these processes can form a top product and a bottom product at each of the vessels <NUM>, <NUM>, <NUM>. The top product can be in vapor form. The bottom product can be in liquid form and/or mixed-phase form (e.g., a combination of liquid and vapor), often depending on temperature and/or pressure of the resulting fluid. In one implementation, the fluid circuit <NUM> can be configured to direct a mixed-phase bottom product from the first vessel <NUM> to the second vessel <NUM>. The second throttling unit <NUM> can provide an expansion stage (and a cooling stage) to reduce pressure and temperature and produce a mixed-phase product between the vessels <NUM>, <NUM>. For example, the mixed-phase product can exit the expansion/cooling stage (at <NUM>) at approximately <NUM> Pa (<NUM> psig) and approximately -<NUM> (-<NUM> °F) prior to entry into the second vessel <NUM>.

The fluid circuit <NUM> can be configured to combine the vapor top products from the vessels <NUM>, <NUM> upstream of the fifth cooler <NUM>. In use, the fifth cooler <NUM> can provide a cooling stage so that the combined mixed phase product exits the cooling stage (at <NUM>) at approximately -<NUM> (-<NUM> °F) prior to entry into the third vessel <NUM>. The fluid circuit <NUM> can also combine the bottom product from the vessels <NUM>, <NUM>, either in liquid form and/or mixed-phase form, as the process stream <NUM>. The sixth cooler <NUM> can provide a cooling stage so that the combined mixed phase bottom product exits the cooling stage (at <NUM>) at approximately -<NUM> (-<NUM> °F) and approximately <NUM> Pa (<NUM> psig).

The fluid circuit <NUM> can direct the combined liquid bottom product to the flash drum <NUM> at a reduced temperature and pressure. The flash drum <NUM> can form a liquid product and a vapor product. The fluid circuit <NUM> can direct the liquid product to the storage facility <NUM> or elsewhere as desired.

As best shown in <FIG>, the fluid circuit <NUM> can direct the vapor product from the flash drum <NUM> through the heat exchanger <NUM>. Downstream of the heat exchanger <NUM>, the fluid circuit <NUM> can combine the vapor product from the flash drum <NUM> with incoming return stream <NUM>, often the boil-off vapor that forms at the storage facility <NUM>. The compressors <NUM>, <NUM> and the coolers <NUM>, <NUM> can condition temperature and pressure of the combined vapor stream upstream of the heat exchanger <NUM>. The conditioned vapor flows onto the demethanizer column <NUM> via the heat exchanger <NUM>.

Referring back to <FIG>, processes at the demethanizer column <NUM> can form a top product and a bottom product, typically in vapor phase and liquid (or mixed) phase, respectively. In one implementation, the bottom product exits the demethanizer column <NUM> to the third throttling device <NUM>. The third throttling device <NUM> can provide an expansion stage to reduce pressure (and temperature) of this bottom product between the second vessel <NUM> and the demethanizer column <NUM>. For example, the bottom product can enter the expansion stage (at <NUM>) at approximately <NUM>,<NUM> Pa (<NUM> psig) and approximately <NUM> (<NUM> °F) and exit the expansion stage (at <NUM>) at approximately <NUM> Pa (<NUM> psig) and approximately -<NUM> (-<NUM> °F) prior to entry into the second vessel <NUM>.

The fluid circuit <NUM> can be configured to recycle the top product from the demethanizer column <NUM>. The seventh cooler <NUM> may operate as an overhead condenser for the demethanizer column <NUM>. This overhead condenser can provide a cooling stage so that the top product exits the cooling stage (at <NUM>). The cooled top product enters the fourth vessel <NUM>, operating here as a reflux drum. In turn, the fourth vessel <NUM> can form a top product and a bottom product. The pump <NUM> can pump the liquid bottom product from the fourth vessel <NUM> back to the demethanizer column <NUM>. The top product can be predominantly methane vapor that exits the system <NUM> as the second product <NUM> via the heat exchanger <NUM> (<FIG>).

<FIG>, <FIG>, and <FIG> depict flow diagrams of a process <NUM> to prepare incoming liquid ethane (and, generally, feedstock <NUM>) for storage. In <FIG>, the process <NUM> can include, at stage <NUM>, distilling an incoming feedstock at a plurality of vessels to form a vapor and a liquid for storage. The process <NUM> can also include, at stage <NUM>, directing the vapor to a demethanizer column and, at stage <NUM>, circulating liquid from the demethanizer back to the plurality of vessels. As shown in <FIG>, the process <NUM> can also include, at stage <NUM>, cooling the incoming feedstock upstream of the plurality of vessels and, at stage <NUM>, throttling flow of the incoming feedstock upstream of the plurality of vessels.

Referring also to <FIG>, stage <NUM> in the process <NUM> can incorporate various stages to distill the incoming feedstock, as desired. In one implementation, these stages may include, at stage <NUM>, forming a first top product and a first bottom product from the incoming feedstock in a first vessel. The stages may also include, at stage <NUM>, directing the first bottom product and the liquid from the demethanizer column to a second vessel and, at stage <NUM>, separating the first bottom product into a second top product and a second bottom product in the second vessel. The stages may further include, at stage <NUM>, mixing the first top product with the second top product upstream of a third vessel, at stage <NUM>, cooling the first top product and the second top product upstream of the third vessel, and, at stage <NUM>, forming a third bottom product from the first top product and the second top product in the third vessel.

As used herein, an element or function recited in the singular and proceeded with the word "a" or "an" should be understood as not excluding plural said elements or functions, unless such exclusion is explicitly recited. Furthermore, references to "one embodiment" should not be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

Claim 1:
A liquefaction process for an incoming feedstock (<NUM>) comprising predominantly ethane liquid into a liquid that meets specification for liquid ethane, comprising:
distilling an incoming feedstock (<NUM>) at a plurality of vessels (<NUM>, <NUM>, <NUM>, <NUM>) to form a vapor and the liquid ethane for storage;
directing the vapor to a demethanizer column (<NUM>); and
circulating liquid from the demethanizer column (<NUM>) back to the plurality of vessels (<NUM>, <NUM>, <NUM>, <NUM>),
at the plurality of vessels (<NUM>, <NUM>, <NUM>, <NUM>):
forming a first top product and a first bottom product from the incoming feedstock (<NUM>);
separating the first bottom product into a second top product and a second bottom product; and
forming a third bottom product from the first top product and the second top product,
wherein the second bottom product and the third bottom product meets specification for liquid ethane;
wherein the plurality of vessels (<NUM>, <NUM>, <NUM>, <NUM>) comprises a flash drum (<NUM>), the process further comprising directing the second bottom product and the third bottom product to the flash drum (<NUM>), wherein the vapor and the liquid ethane originate from the flash drum (<NUM>), and
it further comprises mixing the vapor with a return stream (<NUM>) upstream of the demethanizer column (<NUM>), the return stream (<NUM>) comprising boil-off gases.