KR20080008984A - Integrated ngl recovery in the production of liquefied natural gas - Google Patents

Abstract

An integrated method for recovering natural gas liquid is provided to recover components heavier than methane from natural gas during the production of liquefied natural gas, and to avoid a need for expanding a feed material and compressing a scrubbing column, thereby simplifying the overall system. A method for producing liquefied natural gas and recovering components heavier than methane comprises the steps of: cooling a natural gas and introducing the cooled natural gas into a first distillation column(118); drawing a methane-enriched overhead stream(120) and heavier component-enriched bottom stream(134) from the first distillation column; cooling and condensing at least a part of the overhead stream to provide a condensed methane-enriched stream; separating the bottom stream in at least one additional distillation column to provide at least one product stream selected from a methane-containing residual vapor stream, ethane-enriched liquid stream, propane-enriched liquid stream and pentane-enriched stream; withdrawing any one of the product streams partially or totally as hydrocarbon; and introducing at least one reflux stream into the first distillation column. The reflux stream comprises: a liquefied methane-containing reflux stream and a non-recovered liquid hydrocarbon stream(138) pumped by the pressure inside the first distillation column; or a combined stream comprising the liquefied methane-containing reflux stream and the non-recovered liquid hydrocarbon stream pumped by the pressure inside the first distillation column. The liquefied methane-containing reflux stream is provided by any one method selected from: cooling and totally condensing the overhead vapor stream to form the condensed methane-enriched stream, and partially withdrawing the condensed methane-enriched stream to provide the liquefied methane-containing reflux stream; cooling and totally condensing the first overhead vapor stream to provide the liquefied methane-containing reflux stream; and cooling and totally condensing the overhead vapor stream to provide the condensed methane-enriched stream, and heating a part of the condensed methane-enriched stream to provide the liquefied methane-containing reflux stream.

Description

INTEGRATED NGL RECOVERY IN THE PRODUCTION OF LIQUEFIED NATURAL GAS}

The present invention relates to a method and apparatus for the liquefaction of natural gas and the recovery of components heavier than methane from natural gas.

The raw natural gas mainly contains methane and also contains many minor components, which may include water, hydrogen sulfide, carbon dioxide, mercury, nitrogen, and light hydrocarbons typically having 2 to 6 carbon atoms. Some of these components, such as water, hydrogen sulfide, carbon dioxide and mercury, are contaminants that are harmful to downstream stages, such as the production of liquefied natural gas (LNG) or natural gas treatment, and these contaminants must be removed upstream of these treatment stages. Hydrocarbons heavier than methane are typically condensed and recovered as natural gas liquids (NGL) and fractionated to produce valuable products.

The first step in the NGL recovery process uses a distillation column or scrub column to separate hydrocarbons heavier than methane from the pretreated natural gas feed to recover methane purified for liquefaction and NGL for separation and recovery. Create This process utilizes cooling, partial condensation and fractionation steps that require a significant amount of refrigeration. This refrigeration can be provided by the expansion of the pressurized natural gas feed and the vaporization of the resulting condensed hydrocarbons. Additional refrigeration is typically provided by external closed loop refrigeration using refrigeration agents such as propane and / or mixed refrigeration agents to liquefy methane in the main heat exchanger. Reflux to the NGL scrub column may utilize a portion of the partially liquefied natural gas from the main heat exchanger.

It is desirable to recover NGL from pressurized natural gas without significantly reducing the natural gas feed pressure. This allows natural gas products (eg pipeline gas or LNG) to be provided at or slightly below the feed pressure, thereby eliminating the need for feed and / or product recompression. It is also desirable to simplify the main heat exchanger design and eliminate the need for scrub column overhead compression when withdrawing some of the liquefied natural gas from the main heat exchanger for use as scrub column reflux. These requirements are met by the embodiments of the invention described below and defined by the claims that follow.

One embodiment of the invention relates to a method of liquefaction of natural gas and recovery of components heavier than methane from natural gas. This way

(a) cooling the natural gas feed to provide a cooled natural gas feed, and introducing the cooled natural gas feed into a first distillation column;

(b) withdrawing a methane enriched overhead vapor stream and a component enrichment bottoms heavier than methane from the first distillation column;

(c) cooling and condensing at least a portion of the overhead vapor stream to provide a condensed methane enriched stream;

(d) separating the bottom stream in one or more further distillation columns to remove one or more selected from the group consisting of methane-containing residual vapors, ethane-reinforced liquids, propane-reinforced liquids, butane-reinforced liquids, and pentane-reinforced liquids. Providing a product stream;

(e) withdrawing some or all of any of said one or more product streams as recovered hydrocarbons; And

(f) introducing one or more reflux streams into the first distillation column.

The at least one reflux stream is

(f1) liquefied methane containing reflux and unrecovered liquid hydrocarbons pumped to pressure in the first distillation column or

(f2) a combined stream comprising said liquefied methane containing reflux and said unrecovered liquid hydrocarbon pumped to pressure in a first distillation column.

The liquefied methane-containing reflux is

(1) cooling and condensing the overhead vapor stream to form the condensed methane enriched stream and withdrawing a portion of the condensed methane enriched stream to provide the liquefied methane-containing reflux stream,

(2) cooling and condensing a portion of the first overhead vapor stream to provide the liquefied methane-containing reflux stream, and

(3) cooling the top vapor stream and condensing it completely to form the condensed methane enriched stream and warming a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux stream. Can be provided by

The unrecovered liquid hydrocarbons include (i) a portion of ethane fortified liquids; (ii) a portion of propane reinforced liquids; (iii) a portion of butane enhanced liquids; (iv) a portion of pentane enhanced liquids; And (v) any or all of the residual vapor stream dissolved in a portion of the propane enhanced liquid stream and / or a portion of the butane enhanced liquid stream and / or a portion of the pentane enhanced liquid stream.

The liquefied methane containing reflux can be introduced to the top of the first distillation column. The unrecovered liquid hydrocarbons can be introduced to the top of the first distillation column. Alternatively, a combined stream comprising the liquefied methane containing reflux and the unrecovered liquid hydrocarbons can be introduced to the top of the first distillation column. Alternatively, the unrecovered liquid hydrocarbon stream can be introduced into this column at a position below the top of the first distillation column and above the position at which the cooled natural gas feed is introduced into this column.

Cooling and condensing at least a portion of the overhead vapor stream may be accomplished in the main heat exchanger by indirect heat exchange with the first vaporizable refrigerant that is provided by the reduced pressure of the first cooled multicomponent liquid refrigerant. A portion of the overhead vapor stream may be condensed in a heat exchanger separate from the main heat exchanger by indirect heat exchange with the vaporizable refrigerant stream provided by withdrawal and decompression of a portion of the first cooled multicomponent liquid refrigerant. Can be.

The first cooled multicomponent liquid refrigerant may be provided by cooling of a saturated multicomponent liquid refrigerant in a main heat exchanger, and warming a portion of the condensed methane enrichment stream to provide the liquefied methane containing reflux. The indirect heat exchange with a portion of the saturated multicomponent liquid refrigerant takes place in a heat exchanger separate from the main heat exchanger. Subcooling at least a portion of the condensed methane enriched stream to provide a pressurized liquefied natural gas product, the subcooling being a second vaporizable refrigerant provided by the depressurization of the second cooled multicomponent liquid refrigerant. Indirect heat exchange with the main heat exchanger. Cooling the natural gas feed to provide the cooled natural gas feed may be accomplished by indirect heat exchange with the methane enriched overhead vapor stream.

The unrecovered liquid hydrocarbons may contain hydrocarbons having more than about 50 mol% of three or more carbon atoms. Alternatively, the unrecovered liquid hydrocarbons can contain more than about 50 mole percent pentane. Alternatively, the unrecovered liquid hydrocarbons may comprise a portion of the propane enriched liquids and a portion of the butane enriched liquids. In this alternative, the unrecovered liquid hydrocarbons may comprise a portion of the ethane fortified liquids. The unrecovered liquid hydrocarbons may comprise a portion of the residual vapor stream comprising methane dissolved in a liquid comprising hydrocarbons heavier than methane. The molar flow rate of the unrecovered liquid hydrocarbon may be less than about 25% of the molar flow rate of the liquefied methane reflux.

Another embodiment of the present invention is a liquefaction of natural gas and recovery device of components heavier than methane from natural gas,

(a) a cooling system adapted to cool the natural gas feed to provide a cooled natural gas feed;

(b) a first distillation column adapted to separate the cooled natural gas feed into a methane enriched overhead vapor stream and a component enrichment bottoms stream heavier than methane;

(c) a main heat exchanger adapted to cool and condense at least a portion of said overhead vapor stream to provide a condensed methane enriched stream;

(d) at least one further adapted to separate the bottoms into at least one product stream selected from the group consisting of methane containing residual vapors, ethane-enhanced liquids, propane-enhanced liquids, butane-enhanced liquids, and pentane-enriched liquids. Distillation column;

(e) piping adapted to withdraw some or all of any of said one or more product streams as recovered hydrocarbons;

(f) piping adapted to introduce one or more reflux streams into the first distillation column, wherein the one or more reflux streams are (f1) liquefied methane containing reflux streams and unrecovered liquid hydrocarbon streams pumped to pressure in the first distillation column; f2) piping comprising a combined stream comprising the liquefied methane containing reflux and the unrecovered liquid hydrocarbons pumped to a pressure in a first distillation column; And

(g) an apparatus comprising piping and a pump or pumps adapted to transfer unrecovered liquid hydrocarbons from said at least one further distillation column to piping adapted to introduce at least one reflux stream into said first distillation column.

The liquefied methane containing reflux (1) cools and condenses the overhead vapor stream to form the condensed methane enriched stream, and withdraws a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux. (2) cooling and condensing a portion of the first overhead vapor stream to provide the liquefied methane-containing reflux stream, and (3) cooling and condensing the overhead vapor stream to condense the condensed methane enriched stream. And warming up a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux.

The apparatus may also include a heat exchanger separate from the main heat exchanger adapted to condense a portion of the overhead vapor stream from the first distillation column by indirect heat exchange with the vaporizable refrigerant stream. The main heat exchanger may be a winding coil heat exchanger and further cooled by subcooling the first bundle and the condensed methane enrichment stream adapted to cool and condense at least a portion of the overhead vapor stream to provide a condensed methane enrichment stream. And a second bundle adapted to provide a liquid product.

Further embodiments of the invention

(a) cooling the natural gas feed to provide a cooled natural gas feed, and introducing the cooled natural gas feed into a first distillation column;

(b) withdrawing a methane enriched overhead vapor stream and a bottom stream enriched in components heavier than methane from the first distillation column;

(c) cooling and condensing at least a portion of said overhead vapor stream in a main heat exchanger to provide a condensed methane enriched stream; And

(d) introducing a liquefied methane containing reflux into the first distillation column, wherein the liquefied methane containing reflux is

(1) dividing the overhead vapor stream into a first vapor portion and a second vapor portion and cooling and condensing the first vapor portion to provide the liquefied methane-containing reflux, and

(2) cooling and totally condensing the overhead vapor stream to form the condensed methane enrichment stream, dividing the condensed methane enrichment stream into first and second portions, and heating the first portion to warm up the first portion. And providing the liquefied methane-containing reflux using the warmed first portion.

It relates to a liquefaction method of natural gas comprising a.

The first vapor portion of the overhead vapor stream may be condensed in a heat exchanger separate from the main heat exchanger by indirect heat exchange with the vaporizable refrigerant stream. The heating of the first portion of the condensed methane enriched stream to provide the liquefied methane containing reflux can be made in a heat exchanger separate from the main heat exchanger. Subcooling at least a portion of the condensed methane enrichment stream to provide a pressurized liquefied natural gas product can be accomplished in the main heat exchanger by indirect heat exchange with the vaporizable refrigerant stream.

Additional embodiments involved

(a) a cooling system adapted to cool the natural gas feed to provide a cooled natural gas feed;

(b) a first distillation column adapted to separate the cooled natural gas feed into a methane enriched overhead vapor stream and a component enrichment bottoms stream heavier than methane;

(c) a main heat exchanger adapted to cool and condense at least a portion of said overhead vapor stream to provide a condensed methane enriched stream; And

(d) piping adapted to introduce liquefied methane containing reflux into the first distillation column, wherein the liquefied methane containing reflux is

(1) cooling and condensing a portion of the overhead vapor stream to provide the liquefied methane-containing reflux stream, and

(2) cooling the top vapor stream and condensing it completely to form the condensed methane enriched stream, and warming a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux stream. Piping provided by

It includes a liquefaction apparatus of natural gas comprising a.

The apparatus may comprise a heat exchanger separate from the main heat exchanger adapted to condense a portion of the overhead vapor stream from the first distillation column by indirect heat exchange with the vaporizable refrigerant stream. The apparatus may comprise a heat exchanger separate from the main heat exchanger adapted to warm a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux.

The main heat exchanger may be a winding coil heat exchanger. The main heat exchanger further cools at least a portion of the overhead vapor stream and at least a portion of the condensed methane enrichment stream adapted to provide cooling and condensation to provide the condensed methane enrichment stream to provide a subcooled liquid product. And a second bundle adapted to be applied.

According to the present invention, there is provided an improved integrated process for NGL recovery in LNG production that simplifies equipment configuration by eliminating the need for feed expansion and scrub column overhead compression. In addition, when the scrub column uses reflux that includes a scrub column top that condenses in the winding coil main heat exchanger, there is no need to split the warm bundle of the heat exchanger to partially condense the column top, No phase separator is needed to recover the liquid. In addition, there is no need to compress and condense the deethanizer overhead vapor to provide scrub column reflux.

In the embodiments described below, reflux to the scrub column is provided by various combinations of condensed scrub column overhead vapor and unrecovered liquid hydrocarbons from the NGL recovery system. In the present disclosure, the terms "recovery hydrocarbons" and "recovery hydrocarbons" are equivalent and refer to any hydrocarbons withdrawn from this integrated system as products exported from the integrated LNG production and NGL recovery systems. The recovered hydrocarbons may be sent off as one or more product streams enriched in any hydrocarbons in the natural gas feed. Exported streams may include, for example, any of ethane fortified, propane fortified, butane and isobutane fortified, pentane and isopentane fortified, and ethane fortified methane-ethane blends. LNG products can be thought of as recovered hydrocarbons. The terms "unrecovered liquid hydrocarbon" and "unrecovered liquid hydrocarbons" are equivalent and any liquid portion separated in the NGL recovery system that is not directly present in the product stream of recovered hydrocarbons from the integrated LNG production and NGL recovery system. Means. Unrecovered liquid hydrocarbons can be thought of as internal recycle streams in integrated LNG production and NGL recovery systems.

The terms " enhanced " and " enhanced " as applied for any stream withdrawn from the process mean that the withdrawn stream contains certain components at concentrations higher than the concentrations in the feed stream for the process. Reflux is defined as the flow introduced into this column at any location above the location where the feed is introduced into the distillation column, and the reflux includes one or more components already withdrawn from the column. Reflux is typically a liquid but may be a vapor-liquid mixture.

As used herein, the indefinite articles "a" and "an" mean one or more when applied to any feature of an embodiment of the invention described in the specification and claims. The use of "a" and "an" does not limit the meaning to a single feature unless a singular limitation is specifically described. Definite article “the” preceding singular or plural nouns or noun phrases refer to specific specific features or specific specific features, and may have the singular or plural implications depending on the context in which the article is used. The adjective "any" means one, some or all of them, regardless of how much. The term "and / or" located between a first entity and a second entity means one of (1) the first entity, (2) the second entity, and (3) the first entity and the second entity.

A first embodiment of the present invention is shown in the integrated LNG production and NGL recovery system shown in FIG. The pretreated pressurized natural gas feed of line 100 contains heavier hydrocarbons in the C2-C6 range, mainly with methane. Contaminants including water, CO ₂ , H ₂ S and mercury are removed in upstream pretreatment systems (not shown) by known methods. Feed gas is typically provided at a pressure of 600 to 900 psia and ambient temperature, and cooled to -20 ° F. to -35 ° F. in heat exchanger 110 to provide a cooled feed stream in line 112. do. Heat exchanger 110 may include multiple cooling steps by evaporation of propane at various pressures; Alternatively or additionally other cooling means may be used, which may for example be mixed vaporizable coolants in a single exchanger. This stream may be further cooled in an optional economizer heat exchanger 114 and introduced into the first distillation column or scrub column 118 via line 116.

Scrub column 118 separates the feed fed via line 116 into the bottom liquid product in hydrocarbon enriched line 134 that is heavier than methane and the top vapor product in methane enriched line 120. A portion of the bottom liquid may be withdrawn via line 130 and vaporized in reboiler 132 to provide a boil up to the scrub column. The reboiler can cool part of the stream 100 (not shown) to provide heat to vaporize the liquid in line 130. The scrub column may also have an intermediate reboiler (not shown) above the bottom of the column and below the location of the feed line 116, which may also be heated by a portion of the feed stream.

The bottom liquid of line 134 flows into the general NGL fractionation system 136. The NGL feed stream is typically depressurized (not shown) and separated in one or more further distillation columns, including any of demethane, deethane, depropane, debutane, and depentane groups to provide at least two Provide a hydrocarbon fraction. In the exemplary general NGL fractionation system of FIG. 1, three recovered hydrocarbons are withdrawn from the integrated LNG production and NGL recovery system as C2, C3 and C4 product streams representing ethane, propane, and butane and isobutane enriched streams, respectively. Are exported. Unrecovered liquid hydrocarbons are withdrawn from the NGL recovery system via line 138.

Methane enriched overhead vapor stream may be withdrawn from scrub column 118 via line 120 and warmed by indirect heat exchange with feed stream of line 112 in economizer heat exchanger 114. This warmed overhead vapor stream of line 122 is cooled, totally condensed and optionally subcooled in the passage 123 of the first or (lower) bundle of winding coil main heat exchanger 124 to To provide a condensed methane enriched stream. A first portion of the liquid of line 125 is withdrawn from line 125 downstream of passage 123 and pumped by pump 127 to provide a liquefied methane containing reflux. The liquefied methane containing reflux returns to the top of the scrub column 118 as a liquid reflux in combination with the unrecovered liquid hydrocarbons in line 138. Alternatively, liquefied methane-containing reflux from pump 127 may be introduced to the top of scrub column 118, and the unrecovered liquid hydrocarbons of line 138 may be below the top of the column and the cooled feed may be It may be introduced into the scrub column 118 at a separate location (not shown) above where it is introduced into the column via 116. In another alternative, liquefied methane containing reflux from pump 127 and unrecovered hydrocarbons in line 138 may be introduced to the top of scrub column 118 as separate streams (not shown).

Typically, depending on the composition of the feed of line 100, the molar flow rate of the unrecovered liquid hydrocarbons of line 138 is less than about 25% of the molar flow rate of the methane enriched stream of line 126. If the natural gas feed of line 100 does not contain a sufficient amount of components necessary to provide the unrecovered liquid hydrocarbon stream of line 138, the required components may be externally supplied from any suitable source.

The second portion of the condensed methane enrichment stream of line 125 is further cooled in passage 128 of the second or (top) cool bundle of winding coil main heat exchanger 124 to pass through line 129. Is extracted as the LNG product. LNG can be depressurized before and / or after subcooling in cold bundles if desired. If the LNG product is stored at high pressure (PLNG), there is no need for subcooling and no cold bundles. A portion of the LNG product in line 129 can be used as methane enriched reflux for scrub column 118 if desired, but this configuration would waste refrigeration by providing reflux at temperatures much lower than needed.

The temperature of the liquefied methane containing reflux withdrawn from main heat exchanger 124 via line 126 and pump 127 of FIG. 1 may be lower than actually needed based on the temperature of the top of scrub column 118. Can be. In order to match the temperature of the methane enriched reflux to the temperature of the top of the scrub column 118, the bundle of main heat exchanger 124 will have to be split to enable the withdrawal of the methane enriched reflux at the intermediate position. In addition, a phase separator will be required when the withdrawn stream is a mixed vapor-liquid stream. However, the thermodynamic inefficiency of providing reflux at temperatures lower than required in the embodiment of FIG. 1 is compensated for by eliminating the need to split the bundles of the main heat exchanger 124.

Refrigeration to the main heat exchanger 124 may be provided by any known refrigeration system used for the production of LNG. For example, as shown in FIG. 1, a single mixed refrigeration (MR) system may be used wherein a liquid coolant is provided via line 152 and a vapor coolant is provided via line 156. The steam in line 156 is condensed and cooled in the main heat exchanger 124 and expanded through the throttle valve 158 to provide first vaporizable refrigeration for the (upper) cold bundle of the exchanger and subsequently the (lower) bundle of the exchanger. Offer The liquid coolant 152 is cooled in the main heat exchanger 124 to produce the supercooled liquid coolant of the line 153, expanded through the throttle valve 154, and at a location near the cold end of the bundle of the main heat exchanger ( Top) in combination with a vaporizable refrigerant from the cold bundle. As an alternative to the throttle valve 154 and / or 158, as well as to the LNG product outlet valve, expansion may be accomplished by an isentropic dense fluid expander (hydro turbine).

The refrigerant stream is completely vaporized leaving the main heat exchanger 124 as refrigerant refrigerant via line 150. The mixed refrigerant vapor flows into the refrigeration system (not shown), where it is compressed, cooled by a number of steps of vaporizing propane, and separated to form liquid refrigerant 152 and lighter vapor refrigerant 156. ).

Any other refrigeration subsystem or system combination known in the art can be used to provide refrigeration for the main heat exchanger 124. For example, a pure fluid cascade and isentropic vapor expansion wells can be used as described in US Pat. No. 6,308,531, which is incorporated herein by reference.

In the embodiment of FIG. 1, the use of a portion of the condensed scrub column column top as methane enriched reflux via line 126 may be used to draw the bundle of main heat exchanger 124 to draw the methane enriched stream for use as reflux. Avoid dividing into two separate bundles. This also eliminates the potential need to separate the biphasic methane enrichment stream from the phase separator if the methane enrichment stream is a vapor-liquid mixture to use the liquid fraction as reflux and redistribute the vapor fraction for further condensation in the main heat exchanger. do. As described below, smaller phase separators may be needed at the start. The use of economizer heat exchanger 114 allows the top rectification of line 122 to enter main heat exchanger 124 at approximately the same temperature as the refrigerants of lines 152 and 156 typically produced by propane refrigeration. To ensure that.

Using unrecovered liquid hydrocarbons via line 138 as additional reflux for scrub column 118 eliminates the need to expand the column feed and recompress column column tops. To minimize power consumption, the natural gas feed pressure should be significantly higher than the critical pressure of methane. In addition, the scrub column must be operated below the critical pressure of the feed mixture to achieve separation. A common solution known in the art is to isotropically expand the scrub column feed and then recompress the overhead vapor product. Work from the isentropic expansion of the feed may be used to at least partially drive the overhead compressor or compressors. This solution is shown, for example, in US Pat. No. 4,065,267 and in FIG. 2 of Elliot, Qualls, Huang, Chen, Yao and Zhang, " Benefits of Integrating NGL Extraction and LNG Liquefaction Technology ", AIChE Spring Meeting, April 2005.

Another embodiment of the present invention is illustrated in FIG. 2. In this embodiment, a portion of the scrub column overhead vapor of line 120 is withdrawn via line 220 and condensed in heat exchanger 200 to produce a reflux stream containing liquefied methane, which is a In combination with the recovered liquid hydrocarbon, it is introduced as a combined stream via line 221 to the top of scrub column 118. Liquefied methane containing reflux from heat exchanger 200 may be pumped if desired.

Alternatively, liquefied methane containing reflux from heat exchanger 200 may be introduced to the top of scrub column 118 and the unrecovered liquid hydrocarbons of line 138 may be below the top of the column and the cooled feed may be line ( It may be introduced into the scrub column 118 at a separate location (not shown) above where it is introduced into the column via 116. In another alternative, the liquefied methane containing reflux from heat exchanger 200 and the unrecovered liquid hydrocarbon of line 138 may be introduced to the top of scrub column 118 as separate streams (not shown).

Refrigeration for the main heat exchanger 124 is provided in the manner as described above with reference to FIG. 1 to provide the liquid refrigerant 152 and the vapor refrigerant 156. Refrigeration to the heat exchanger 200 draws a portion of the mixed liquid refrigerant in line 153 via line 252, depressurizes the pressure of the refrigerant through throttle valve 254, and decompresses the refrigerant By introducing into a heat exchanger. The mixed vaporization refrigerant from heat exchanger 200 is combined with the mixed vaporization refrigerant from main heat exchanger 124 to provide the vaporization refrigerant in line 150. Alternatively, the coolant in line 252 is withdrawn from line 152 prior to main heat exchanger 124, expanded to intermediate pressures or pressures, vaporized in heat exchanger 200, and appropriate step position or At the locations it may be returned to the mixed refrigerant compressor (not shown). All other process features of FIG. 2 are the same as described above with reference to FIG. 1.

In an alternative variant of the process described above with reference to FIG. 2, a situation may arise where it is desirable to export all hydrocarbons recovered at the bottom from scrub column 118 and fractionated in the NGL fractionation system. In this case, the flow rate of the unrecovered hydrocarbons in line 138 will be zero, and the scrub column 118 is condensed in the heat exchanger 200 by the portion of the top rectification of the scrub column 118 of the line 220. The reflux of line 221 provided will be used.

An alternative embodiment of the invention is illustrated in FIG. 3. In this embodiment, the liquefied methane containing reflux from pump 127 is transferred to heat exchanger 300 by indirect heat exchange with a portion of the mixed refrigerant coolant withdrawn from line 152 via line 352. Warms up. In this case, the combined reflux is close to its optimum temperature when introduced into the scrub column 118. The cooled refrigerant from heat exchanger 300 flows through line 302 and is combined with the refrigerant in line 153 prior to throttle valve 154.

Alternatively, condensed methane enriched reflux from heat exchanger 300 may be introduced to the top of scrub column 118 and the unrecovered hydrocarbons of line 138 may be below the top of the column and the cooled feed may be The scrub column 118 may be introduced at a location (not shown) above where it is introduced into the column via 116. In another alternative, the liquefied methane containing reflux from heat exchanger 300 and the unrecovered liquid hydrocarbon of line 138 may be introduced to the top of scrub column 118 as separate streams (not shown). All other process features of FIG. 3 are the same as described above with reference to FIG. 1.

In an alternative variant of the process described above with reference to FIG. 3, a situation may arise where it is desirable to export all hydrocarbons recovered at the bottom from scrub column 118 and fractionated in the NGL fractionation system. In this case, the flow rate of the unrecovered hydrocarbons of the line 138 will be zero, and the scrub column 118 heats up the portion provided by the fully condensed overhead pump 127 from the scrub column 118. The reflux provided by heating in the exchanger 300 will be used.

4 shows an optional arrangement that can be used to return the condensed methane enriched stream of line 126 to scrub column 118. The condensed methane enriched stream of line 126 is depressurized to its boiling point via throttle valve 426, introduced into drum 427 to maintain some steam inventory, and pumped to scrub column pressure by pump 127. do. A portion of the pumped stream is recycled to the drum 427 through the valve 428 to maintain the liquid level in the drum, and the remaining portion flows through the optional valve 429 to the scrub column 118. During plant initiation, excess steam is evacuated (not shown) from the top of the drum 427 and flared or compressed and recovered. Since the condensed methane enrichment stream of line 126 is a very small fraction of the total LNG stream and there is no net vapor flow during normal operation, drum 127 is supplied with a main heat exchanger to provide reflux liquid to the scrub column. It is much smaller than the reflux drum typically used in conventional plants to separate the partially condensed methane enriched stream withdrawn from the plant.

Throttle valve 426 and drum 427 detect the liquid in line 126 (e.g., using a thermocouple) and vapor from main heat exchanger 124 in the initiating situation (which is usually a supercooled liquid in operation) or Two-phase flow can be avoided by diverting to other existing drums such as helium recovery or fuel gas flash drums or simply by burning. In another alternative, the system utilizes a pump 127 of a type that can tolerate two-phase flow in off-design conditions such as cryogenic gear or screw pumps or centrifugal pumps with high performance inducers. This can be simplified.

An exemplary NGL recovery system that can be used with embodiments of the present invention is illustrated in FIG. 5, which is a demethanizer 501, a deethane group 503, a depropane group 505, and a dealter in series operation. Four distillation columns containing butane group 507 are included. The bottom liquid from scrub column 118 via line 134 is cooled in heat exchanger 509 to approximately ambient temperature and flows to demethanizer column 501. The overhead vapor containing methane and some ethane can be withdrawn from the top of the demethanizer as recovered hydrocarbons via line 509 to be used as fuel or liquefied and reinjected as LNG product. The ethane and heavier hydrocarbon-enriched bottom liquid is withdrawn via line 511 and partially vaporized in heat exchanger 513, the boiling vapor is returned to the column via line 517 and the remaining stream is line 519 And the deethanizer column 503 via the valve 521.

The high purity ethane vapor is withdrawn from the column via line 523 and condensed in the overhead condenser 525. A portion of the condensed liquid is returned as reflux via line 527 and the other portion is withdrawn via line 529 as recovered hydrocarbons, typically containing high purity ethane containing greater than 98 mol% ethane. The bottom liquid from the deethanizer via line 531 is partially vaporized in heat exchanger 533, the boil water vapor is returned to the column via line 535, and the remaining flow is line 537 and valve ( 539) to depropaner column 505. High purity propane vapor is withdrawn from column via line 541 and condensed in top condenser 543. A portion of the condensed liquid is returned as reflux via line 545 and the other portion is withdrawn via line 547 as a recovered hydrocarbon comprising high purity propane, typically containing greater than 98 mol% propane.

The bottom liquid from the depropaner via line 549 is partially vaporized in heat exchanger 551, the boil water vapor is returned to the column via line 553, and the remaining flow is line 555 and valve ( 557 to debutane column 507. High purity butane (if present butobutane) vapor is withdrawn from column via line 559 and condensed in overhead condenser 561. Some of the condensed liquid is returned as reflux via line 563 and the other portion is a recovered hydrocarbon comprising high purity butane (isobutane, if present), typically containing more than 98 mol% butane and isobutane. Withdrawn via line 565. The bottom liquid from the debutane group is withdrawn via line 567 and partially vaporized in heat exchanger 569, the boiling vapor is returned to the column via line 571, and the remaining stream is pentane (if present). Pentane) and heavier hydrocarbons, withdrawn via line 573.

In this example, propane and butane liquids can be withdrawn as unrecovered liquid hydrocarbons via lines 575 and 577, respectively, and mixed in line 579. Mixed unrecovered liquid hydrocarbons are cooled to a temperature of vaporizable propane coolant in heat exchanger 581, pumped to scrub column pressure in pump 583, and in any of the embodiments of FIGS. 1, 2 and 3. The scrub column flows via line 138. Optionally, a portion of the ethane liquid from the deethane group can be withdrawn via line 585 as unrecovered liquid hydrocarbon and combined with unrecovered propane and / or butane in line 579. Optionally, a portion of the overhead vapor from line 509 from demethanizer 501 may be withdrawn via line 587 and absorbed in unrecovered liquid propane and / or butane in line 589. In this option, compression of the demethanizer overhead vapor is not necessary. In one alternative, all butanes from the debutane group are recovered via line 565 and are not withdrawn as unrecovered liquid hydrocarbons via line 577 at all. In another alternative, all propane from the depropane machine is recovered via line 547 and is not withdrawn at all as unrecovered liquid hydrocarbon via line 575. Generally, any of the dissolved column tops from the demethane group 501 and the condensed ethane, propane and butane column heads from the deethan group 503, the depropane group 505 and the debutane group 507, respectively. May be withdrawn in whole or in part as unrecovered liquid hydrocarbon, which is returned to scrub column 118 as long as the withdrawn hydrocarbon product requirements are met.

Depending on the specific hydrocarbon to be recovered, other NGL fractionation systems may be used. For example, the system may utilize a depentane group column to recover residual products containing high purity pentane and hydrocarbons heavier than pentane. Some of the pentane may be returned to the scrub column 118 as unrecovered hydrocarbons. In another alternative, no demethanizer is used and the deethane group is operated to withdraw the ethane liquid product in an intermediate stage and withdraw the mixture of methane and ethane vapor from the reflux drum as recovered hydrocarbon product. Some of this vapor can be withdrawn as unrecovered hydrocarbon product and dissolved in the unrecovered liquid hydrocarbon mixture as described above.

The following examples illustrate embodiments of the invention, but do not limit the embodiments of the invention to any of the specific details described therein.

Example

Process simulations were performed to illustrate the embodiment of FIG. 1. The pre-purified natural gas stream of line 100 has a flow rate of 100,000 lb / hr and a pressure of 960 psia, 1.9% helium, 5.8% nitrogen, 83.2% methane, 7.1% ethane, propane 2.3%, isobutane 0.4%, butane 0.6%, isopentane 0.1%, pentane 0.2%, and hexane 0.2%. The stream is cooled by three stages of propane cooling to −29 ° F., further cooled to −62.8 ° F. in the economizer heat exchanger, and fed to scrub column 118. The column is operated at an average pressure of 886 psia. At a flow rate of 104,770 lb mol / hr, the column top of line 120 is warmed from -73 ° F to -32 ° F against the feed in heat exchanger 114. Thus, the stream of line 122 is cooled and liquefied in the on-pass passage 123 of the main heat exchanger 124 to provide the condensed methane enriched stream of line 125. A portion of this liquid is drawn via line 126 at a flow rate of 10,943 lb mol / hr and -197.6 ° F. This flow is pumped at the pump 127 with scrub column pressure, because typically the liquid head is insufficient to overcome the pressure drop in the heat exchanger 124. The remainder of the liquid in line 125 is subcooled in passage 128 to withdraw from the cold bundle of the exchanger as the liquefied natural gas product of line 129 at a flow rate of 93,827 lb mol / hr and a temperature of -228.8 ° F. The product stream can be further processed to recover helium and then depressurized to storage pressure.

The scrub column bottoms are withdrawn via line 134 at a flow rate of 1862 lb mol / hr and sent to NGL fractionation system 136, which is a demethanizer, liquid that produces a methane-ethane mixture as a vapor top product. A series of distillations as shown in FIG. 5 comprising a deethane group that produces high purity ethane as a tower product, a depropane group that produces high purity propane as a liquid tower product, and a debutane group that produces high purity butane as a liquid tower product. It is a column. Ethane, propane and butane liquids have a purity of greater than 98 mol%. The methane and ethane mixture from the demethanizer is taken out as recovered hydrocarbons and used as fuel.

Unrecovered liquid propane and butane in lines 575 and 577 are combined in line 138, cooled in heat exchanger 581 to -32 ° F by propane refrigeration, and pumped to scrub column pressure in pump 583. . The unrecovered propane in line 575 is 50% of the overhead rectification of the depropane overhead line 541 and the unrecovered butane of butane of the line 577 is 60% of the overhead rectification of the debutane overhead column 559. . The combined unrecovered hydrocarbons of line 579 have a flow rate of 1116 lb mol / hr and a composition (mol%) of 39% propane, 60% butane and isobutane, and 1% heavier than butane. The pumped unrecovered liquid hydrocarbon is combined with the liquefied methane containing reflux from the pump 127 and the combined stream is introduced to the top of the scrub column 118.

1 is a schematic process flow diagram of an embodiment of the invention.

2 is a schematic process flow diagram of another embodiment of the present invention.

3 is a schematic process flow diagram of an alternative embodiment of the present invention.

4 is a schematic process flow diagram of an alternative method that may be used with any embodiment of the present invention.

5 is a schematic process flow diagram of an exemplary NGL fractionation system that may be used with any embodiment of the present invention.

Claims (28)

As a method of liquefaction of natural gas and recovery of components heavier than methane from natural gas, (a) cooling the natural gas feed to provide a cooled natural gas feed, and introducing the cooled natural gas feed into a first distillation column; (b) withdrawing a methane enriched overhead vapor stream and a component enrichment bottoms heavier than methane from the first distillation column; (c) cooling and condensing at least a portion of the overhead vapor stream to provide a condensed methane enriched stream; (d) separating the bottom stream in one or more further distillation columns to remove one or more selected from the group consisting of methane-containing residual vapors, ethane-reinforced liquids, propane-reinforced liquids, butane-reinforced liquids, and pentane-reinforced liquids. Providing a product stream; (e) withdrawing some or all of any of said one or more product streams as recovered hydrocarbons; And (f) introducing at least one reflux to the first distillation column, wherein the at least one reflux is (f1) liquefied methane containing reflux and unrecovered liquid hydrocarbons pumped to pressure in the first distillation column or (f2) a combined stream comprising said liquefied methane containing reflux and said unrecovered liquid hydrocarbons pumped to pressure in a first distillation column, The liquefied methane-containing reflux is (1) cooling and condensing the overhead vapor stream to form the condensed methane enriched stream and withdrawing a portion of the condensed methane enriched stream to provide the liquefied methane-containing reflux stream, (2) cooling and condensing a portion of the first overhead vapor stream to provide the liquefied methane-containing reflux stream, and (3) cooling the top vapor stream and condensing it completely to form the condensed methane enriched stream and warming a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux stream. Provided by steps How to include. The method of claim 1, wherein the unrecovered liquid hydrocarbons are (i) a portion of ethane-reinforced liquids, (ii) a portion of propane-reinforced liquids, (iii) a portion of butane fortified liquids, (iv) a portion of pentane enhanced liquids, and (v) some or all of the residual vapor stream dissolved in some of the propane-reinforced liquids and / or some of the butane-reinforced liquids and / or some of the pentane-enhanced liquids. A method comprising any of the above. The process of claim 1 wherein the liquefied methane containing reflux is introduced to the top of the first distillation column. The process of claim 1 wherein the unrecovered liquid hydrocarbon stream is introduced to the top of the first distillation column. The process of claim 1 wherein the combined stream comprising the liquefied methane containing reflux and the unrecovered liquid hydrocarbons is introduced to the top of the first distillation column. The process of claim 1, wherein the unrecovered liquid hydrocarbon stream is introduced into this column at a position below the top of the first distillation column and above a position at which the cooled natural gas feed is introduced into the column. The method of claim 1, wherein cooling and condensing at least a portion of the overhead vapor stream is performed in a main heat exchanger by indirect heat exchange with a first vaporizable refrigerant that is provided by a reduced pressure of the first cooled multicomponent liquid refrigerant. How it is done. 8. The method of claim 7, wherein a portion of the overhead vapor stream is separate from the main heat exchanger by indirect heat exchange with the vaporizable refrigerant stream provided by the withdrawal and decompression of the portion of the first cooled multicomponent liquid refrigeration agent. Condensation in the heat exchanger. 8. The liquid liquefied methane-containing reflux stream of claim 7 wherein the first cooled multicomponent liquid refrigeration agent is provided by cooling of a saturated multicomponent liquid refrigeration agent in a main heat exchanger and heats a portion of the condensed methane enrichment stream. Providing is in a heat exchanger separate from the main heat exchanger by indirect heat exchange with a portion of the saturated multicomponent liquid refrigerant. 8. The method of claim 7, comprising subcooling at least a portion of the condensed methane enriched stream to provide a pressurized liquefied natural gas product, the subcooling being performed at a reduced pressure of the second cooled multicomponent liquid refrigeration agent. Process in the main heat exchanger by indirect heat exchange with a second vaporizable refrigeration agent provided. The method of claim 1, wherein cooling the natural gas feed to provide the cooled natural gas feed is by indirect heat exchange with a methane enriched overhead vapor stream. The method of claim 1, wherein the unrecovered liquid hydrocarbons contain hydrocarbons having more than about 50 mol% of three or more carbon atoms. The method of claim 1, wherein the unrecovered liquid hydrocarbons contain greater than about 50 mole percent pentane. The method of claim 1, wherein the unrecovered liquid hydrocarbons comprise a portion of the propane enriched liquids and a portion of the butane enriched liquids. 15. The method of claim 14, wherein the unrecovered liquid hydrocarbons comprise a portion of the ethane-enhanced liquids. 15. The process of claim 14, wherein the unrecovered liquid hydrocarbons comprise a portion of the residual vapor stream comprising methane dissolved in a liquid comprising hydrocarbons heavier than methane. The process of claim 1 wherein the molar flow rate of the unrecovered liquid hydrocarbon is less than about 25% of the molar flow rate of the liquefied methane reflux stream of (f1). As a device for liquefaction of natural gas and recovery of components heavier than methane from natural gas, (a) a cooling system adapted to cool the natural gas feed to provide a cooled natural gas feed; (b) a first distillation column adapted to separate the cooled natural gas feed into a methane enriched overhead vapor stream and a component enrichment bottoms stream heavier than methane; (c) a main heat exchanger adapted to cool and condense at least a portion of said overhead vapor stream to provide a condensed methane enriched stream; (d) at least one further adapted to separate the bottoms into at least one product stream selected from the group consisting of methane containing residual vapors, ethane-enhanced liquids, propane-enhanced liquids, butane-enhanced liquids, and pentane-enriched liquids. Distillation column; (e) piping adapted to withdraw some or all of any of said one or more product streams as recovered hydrocarbons; (f) piping adapted to introduce one or more reflux streams into the first distillation column, wherein the one or more reflux streams is (f1) liquefied methane containing reflux and unrecovered liquid hydrocarbons pumped to pressure in the first distillation column or (f2) a combined stream comprising said liquefied methane containing reflux and said unrecovered liquid hydrocarbons pumped to pressure in a first distillation column, The liquefied methane-containing reflux is (1) cooling and condensing the overhead vapor stream to form the condensed methane enriched stream and withdrawing a portion of the condensed methane enriched stream to provide the liquefied methane-containing reflux stream, (2) cooling and condensing a portion of the first overhead vapor stream to provide the liquefied methane-containing reflux stream, and (3) cooling the top vapor stream and condensing it completely to form the condensed methane enriched stream and warming a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux stream. Piping provided by; And (g) piping and pumps or pumps adapted to convey unrecovered liquid hydrocarbons from said at least one further distillation column to piping adapted to introduce at least one reflux stream into said first distillation column. 21. The condensed coil of claim 20 wherein the main heat exchanger is a winding coil heat exchanger and the main heat exchanger is adapted to cool and condense at least a portion of the overhead vapor stream to provide a condensed methane enriched stream. And a second bundle adapted to further cool the methane enriched stream to provide a subcooled liquid product. (a) cooling the natural gas feed to provide a cooled natural gas feed, and introducing the cooled natural gas feed into a first distillation column; (b) withdrawing a methane enriched overhead vapor stream and a bottom stream enriched in components heavier than methane from the first distillation column; (c) cooling and condensing at least a portion of said overhead vapor stream in a main heat exchanger to provide a condensed methane enriched stream; And (d) introducing a liquefied methane containing reflux into the first distillation column, wherein the liquefied methane containing reflux is (1) dividing the overhead vapor stream into a first vapor portion and a second vapor portion and cooling and condensing the first vapor portion to provide the liquefied methane-containing reflux, and (2) cooling and totally condensing the overhead vapor stream to form the condensed methane enrichment stream, dividing the condensed methane enrichment stream into first and second portions, and heating the first portion to warm up the first portion. And providing the liquefied methane-containing reflux using the warmed first portion. Liquefaction method of natural gas comprising a. 21. The method of claim 20, wherein the first vapor portion of the overhead vapor stream is condensed in a heat exchanger separate from the main heat exchanger by indirect heat exchange with the vaporizable refrigerant stream. 21. The method of claim 20, wherein heating the first portion of the condensed methane enrichment stream to provide the liquefied methane containing reflux is in a heat exchanger separate from the main heat exchanger. 21. The method of claim 20, comprising subcooling at least a portion of the condensed methane enrichment stream to provide a pressurized liquefied natural gas product, wherein the subcooling is performed in a main heat exchanger by indirect heat exchange with a vaporizable refrigerant stream. How it is done. (a) a cooling system adapted to cool the natural gas feed to provide a cooled natural gas feed; (b) a first distillation column adapted to separate the cooled natural gas feed into a methane enriched overhead vapor stream and a component enrichment bottoms stream heavier than methane; (c) a main heat exchanger adapted to cool and condense at least a portion of said overhead vapor stream to provide a condensed methane enriched stream; And (d) piping adapted to introduce liquefied methane containing reflux into the first distillation column, wherein the liquefied methane containing reflux is (1) cooling and condensing a portion of the overhead vapor stream to provide the liquefied methane-containing reflux stream, and (2) cooling the top vapor stream and condensing it completely to form the condensed methane enriched stream, and warming a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux stream. Piping provided by Liquefaction apparatus of natural gas comprising a. 25. An apparatus according to claim 18 or 24 comprising a heat exchanger separate from the main heat exchanger adapted to condense a portion of said overhead vapor stream from the first distillation column by indirect heat exchange with a vaporizable refrigerant stream. . The apparatus of claim 24 comprising a heat exchanger separate from the main heat exchanger adapted to warm a portion of the condensed methane enriched stream to provide the liquefied methane containing reflux. The apparatus of claim 18 or 24, wherein the main heat exchanger is a winding coil heat exchanger. 25. The first bundle and the condensation of claim 24, wherein the main heat exchanger is a winding coil heat exchanger, wherein the main heat exchanger is adapted to cool and condense at least a portion of the overhead vapor stream to provide the condensed methane enriched stream. And a second bundle adapted to further cool at least a portion of the methane fortified stream to provide a subcooled liquid product.

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