Patent Description:
It may be desirable to operate a liquefaction plant using different a feed stream than originally planned. For example, it may be desirable to liquefy ethylene at a plant originally designed to liquefy ethane. There exists therefore, a need for hydrocarbon liquefaction plants that are capable of efficiently liquefying a variety of feed streams.

It is also desirable to provide such flexibility, while also enabling the simultaneous liquefaction of multiple feed streams, each having a different composition, temperature, and/or pressure (hereinafter "different feed properties"). Regardless of the nature of the feed streams, it is also desirable to liquefy the feed streams in a manner that enables each product to be stored in a low-pressure tank (typically less than <NUM> bara and preferably less than <NUM> bara) and with little or no product flash (preferably less than <NUM> mole % vapor).

One option for liquefying multiple feed streams, each having different feed properties, and storing each product in a low pressure product tank with minimum or no flash, would be to require the product streams to leave the main cryogenic heat exchanger (MCHE) at different temperatures. This option is undesirable because it would add complexity to the MCHE, including the addition of side-headers. Another option would be to have the product streams leave MCHE at the same temperature and sub-cool the least-volatile product stream beyond what is required for the storage. This option would require additional power or may lead to collapse of the product tank. In addition, the most volatile product may flash, leading to product loss or the need for re-liquefaction.

<CIT> discloses a process and apparatus for liquefying two feed streams, such as for example an oxygen stream and a nitrogen stream, by heat exchange with a refrigerant gas, such as for example nitrogen gas, circulating in a reverse-Brayton cycle. The two feed streams are introduced into the warm end of the heat exchanger, passed through separate circuits in the heat exchanger, and are withdrawn from the cold end of the heat exchanger as liquefied product streams at the same temperature. In order to account for the difference in normal boiling points of oxygen and nitrogen, the nitrogen feed stream is compressed to a higher feed pressure than the oxygen feed stream. In case it should be necessary to transfer either warm feed gas between the two feed streams or liquefied product between the two liquified product streams, interconnecting conduits with shut-off valves are provided interconnecting the two streams at both the warm and cold ends of the heat exchange system. In this arrangement, the need to compress one of the feed streams to a higher pressure again requires additional power.

Accordingly, there is a need for a hydrocarbon liquefaction plant and process that is capable of liquefying multiple different feed streams with minimal product flash, that is capable of adjusting to changes in the properties of the feed streams, and is simple, reliable, and relatively inexpensive to construct, maintain, and operate.

Described embodiments, as described below and as defined by the claims which follow, comprise improvements to systems used as part of a natural gas liquefaction process. The proposed hydrocarbon liquefaction process and system is capable of simultaneously handling multiple feed streams to liquefy such streams having different properties with minimum or no flash (simultaneous operation). The proposed MCHE has separate circuits for handling multiple feed streams. For example, a coil wound heat exchanger (CWHE) has separate circuits to handle different hydrocarbons such as ethane and ethylene. Different streams leave the cold end of the MCHE at substantially the same temperature (i.e., a temperature difference of no more than <NUM> degrees C). There are bypass lines connecting warm feeds with the liquefied products. The products are stored as saturated liquid in low-pressure tanks. The most volatile product (i.e., the product with the lowest normal boiling point) is sub-cooled sufficiently to suppress most of the flash, except what is required to get rid of more volatile impurities. Less volatile products (products with relatively high normal boiling points) are cooled to substantially the same temperature, then blended with warm or partially cooled feed streams (referred to as bypass streams) to maintain each product near its bubble point.

End flash and/or boil-off gas (BOG) can be compressed and recycled to the warm end of the MCHE as another way of controlling product temperature. Such recycling makes the cold end of the MCHE warmer. Recycling may also help maintain product purity or avoid producing end flash vapor product from the liquefaction system. This is particularly desirable when electric motors are used to drive compressors, because the motors have no fuel requirement that can be met by using end flash vapor.

In some embodiments, the product stream temperature of the MCHE may be selected to remove a light contaminant from one of the product streams, rather than cooling to bubble point at storage pressure. Such removal is accomplished by cooling to a warmer product temperature, then flashing the stream in question in its product tank or an end flash drum to remove the contaminant in the resulting vapor. In this case, other products can be warmed to the desired enthalpy by blending with warmer feed gas, while other more volatile products may be handled by recycling the resulting end flash.

For a process in which three products are desired, one optional mode of operation is to recycle the flash gas of the most volatile product, produce the intermediate volatility product as saturate liquid (after a pressure reduction), and use a bypass for the least volatile product.

Described herein are methods for liquefying multiple feed streams of different composition by bypassing a warm feed to achieve a desired temperature and also the use of end flash recycle for more volatile products. Also disclosed is a flexible main exchanger with multiple feed circuits along with means (valves and pipes) for allocating the feed circuits to various different feed sources depending on the desired products.

According to a first aspect of the invention, there is provided a method for cooling and liquefying at least two feed streams in a coil-wound heat exchanger, the method comprising:.

Each of the at least two feed streams may comprise a hydrocarbon fluid.

Step (e) may comprise:
(e) withdrawing a first bypass stream from the first feed stream upstream from the warm end of the coil-wound heat exchanger.

The method may further comprise:
(g) phase separating the second cooled feed stream into a second flash vapor stream and the second product stream, the predetermined product stream temperature of the second product stream being lower than the withdrawal temperature of the second cooled feed stream.

The method may further comprise:
(j) warming the second flash vapor stream by indirect heat exchange against the first bypass stream.

The at least two feed streams further comprise a third feed stream having third normal bubble point that is lower than the first normal bubble point and higher than the second normal bubble point, the at least two cooled feed streams further comprising a third cooled feed stream, and the at least two product streams further comprising a third product stream.

In such a method, step (d) may further comprise providing the third product stream having a predetermined product stream temperature that is the same as the withdrawal temperature of the third cooled feed stream.

The method may further comprise:
(I) separating impurities from the second feed stream downstream from the second cooled feed stream in a phase separator to produce a second vapor stream containing the impurities and the second product stream.

The predetermined product stream temperature range for each of the at least two product streams may be <NUM> degrees C.

According to a second aspect of the invention, there is provided apparatus configured to carry out the method according to the first aspect, the apparatus comprising:.

The apparatus may further comprise:
a plurality of connecting conduits, each of the connecting conduits having a connecting valve thereon, the plurality of connecting conduits and connecting valves being operationally configured to selectively place the first feed stream conduit in fluid flow communication with more than one of the plurality of cooling conduits.

Exemplary embodiments will hereinafter be described in conjunction with the appended figures wherein like numerals denote like elements:.

The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the claimed invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the claimed invention. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the claimed invention.

Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features. In the figures, elements that are similar to those of other embodiments are represented by reference numerals increased by factors of <NUM>. For example, the MCHE <NUM> associated with the embodiment of <FIG> corresponds to the MCHE <NUM> associated with the embodiment of <FIG>. Such elements should be regarded as having the same function and features unless otherwise stated or depicted herein, and the discussion of such elements may therefore not be repeated for multiple embodiments.

In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

Directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing exemplary embodiments, and are not intended to limit the scope of the claimed invention. As used herein, the term "upstream" is intended to mean in a direction that is opposite the direction of flow of a fluid in a conduit from a point of reference. Similarly, the term "downstream" is intended to mean in a direction that is the same as the direction of flow of a fluid in a conduit from a point of reference.

The term "fluid flow communication," as used in the specification and claims, refers to the nature of connectivity between two or more components that enables liquids, vapors, and/or two-phase mixtures to be transported between the components in a controlled fashion (i.e., without leakage) either directly or indirectly. Coupling two or more components such that they are in fluid flow communication with each other can involve any suitable method known in the art, such as with the use of welds, flanged conduits, gaskets, and bolts. Two or more components may also be coupled together via other components of the system that may separate them, for example, valves, gates, or other devices that may selectively restrict or direct fluid flow.

The term "conduit," as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases. The term "circuit", as used in the specification and claims, refers to a path through which a fluid can flow in a contained manner and may comprise one or more connected conduits, as well as equipment that contains conduits, such as compressors and heat exchangers.

The term "natural gas", as used in the specification and claims, means a hydrocarbon gas mixture consisting primarily of methane.

The terms "hydrocarbon gas" or "hydrocarbon fluid", as used in the specification and claims, means a gas/fluid comprising at least one hydrocarbon and for which hydrocarbons comprise at least <NUM>%, and more preferably at least <NUM>% of the overall composition of the gas/fluid.

The term "liquefaction", as used in the specification and claims, means cooling the fluid in question to a temperature at which at least <NUM> mole % of the fluid remains liquid when let down to a storage pressure of <NUM> bara or less. Similarly, the term "liquefier" refers to the equipment in which liquefaction takes place. In the context of the liquefaction processes disclosed herein, it is preferable that more than <NUM> mole % of the fluid remains liquid when let down to the storage pressure used by that process. Typical storage pressures are in the range of <NUM> to <NUM> bara. Feed streams are often supplied at a supercritical pressure and do not undergo a discrete phase transition during the cooling associated with liquefaction.

The term "sub-cooling", as used in the specification and claims, means that the fluid in question is further cooled (beyond what is necessary for liquefaction) so that, when let down to the storage pressure of the system, at least <NUM> mole % of the fluid remains liquid.

The terms "boiling point" and "boiling temperature" are used interchangeably in the specification and claims and are intended to be synonymous. Similarly, the terms "bubble point" and "bubble temperature" are also used interchangeably in the specification and claims and are intended to be synonymous. As is known in the art, the term "bubble point" is the temperature at which the first bubble of vapor appears in a liquid. The term "boiling point" is the temperature at which the vapor pressure of a liquid is equal to the pressure of the gas above it. The term "bubble point" is typically used in connection with a multi-component fluid in which at least two of the components have different boiling points. The terms "normal boiling point" and "normal bubble point", as used the specification and claims, mean the boiling point and bubble point, respectively, at a pressure of <NUM> atm.

Unless otherwise state herein, introducing a stream at a location is intended to mean introducing substantially all of the said stream at the location. All streams discussed in the specification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Further, each piece of equipment should be understood to have at least one inlet and at least one outlet.

The term "essentially water-free", as used in the specification and claims, means that any residual water in the stream in question is present at a sufficiently low concentration to prevent operational problems due to water freeze out in any stream downstream from, and in fluid flow communication with, the stream in question. Typically, this will mean less than <NUM> ppm water.

The term "substantially the same temperature," as used in the specification and claims in relation to temperature differences between cooled feed streams at the cold end of an MCHE, means that no cooled feed stream has a temperature difference of more than <NUM> degrees C (preferably, no more than <NUM> degrees C) from any other cooled feed stream.

As used herein, the term "compressor" in intended to mean a device having at least one compressor stage contained within a casing and that increases the pressure of a fluid stream.

Described embodiments provide an efficient process for the simultaneous liquefaction of multiple feed gas streams and are particularly applicable for the liquefaction of hydrocarbon gases. Possible hydrocarbon gasses include ethane, ethane-propane mix (E/P Mix), ethylene, propane, and natural gas.

As used in the specification and claims, a temperature range of X degrees is intended to mean a range of X degrees above and below the temperature at issue.

Referring to <FIG>, a hydrocarbon liquefaction system <NUM> using an SMR process is shown. It should be noted that any suitable refrigeration cycles could be used, such as propane-precooled mixed refrigerant (C3MR), dual mixed refrigerant (DMR), or reverse-Brayton, such as gaseous nitrogen recycle.

An essentially water-free first feed stream <NUM>, and one or more additional feed streams such as the second feed stream <NUM>, are cooled in a MCHE <NUM>. The first feed stream <NUM> may be combined with a first feed recycle stream <NUM> to form a combined first feed stream <NUM>. The combined first feed stream <NUM> may, optionally, be divided into a first MCHE feed stream <NUM> and a first feed bypass stream <NUM>. The first MCHE feed stream <NUM> is cooled and liquefied in the MCHE <NUM> to form a liquefied first product stream <NUM>. The first feed bypass stream <NUM> may be reduced in pressure in valve <NUM> to produce a reduced pressure first feed bypass stream <NUM>.

The liquefied first product stream <NUM> is withdrawn from the MCHE <NUM> and reduced in pressure though valve <NUM> to produce a two-phase first product stream <NUM>. The two-phase first product stream <NUM> may be combined with the reduced pressure first feed bypass stream <NUM>, resulting in a combined two-phase first product stream <NUM>. The combined two-phase first product stream <NUM> is fed to a first end flash drum <NUM>, in which the combined two-phase first product stream <NUM> is separated into a first end flash drum vapor stream <NUM> and a first end flash drum liquid stream <NUM>. The first end flash drum vapor stream <NUM> may contain impurities.

The first end flash drum liquid stream <NUM> is further reduced in pressure through valve <NUM>, resulting in a reduced pressure first end flash drum liquid stream <NUM>, which is fed to a first storage tank <NUM>. A final first liquid product stream <NUM> is extracted from the lower end of the first storage tank <NUM>, and is the final product of the first feed stream <NUM>. The system <NUM> is operated to deliver the first liquid product stream <NUM> at temperature that is within a predetermined product temperature range, which is preferably a range of <NUM> degrees C (i.e., <NUM> degrees above or below a set point temperature) and, more preferably, a range of <NUM> degrees C.

A first storage tank vapor stream <NUM> may be extracted from an upper end of the first storage tank <NUM> is compressed in a compressor <NUM> to create a compressed storage tank first product vapor stream <NUM>, which is cooled to ambient temperature in aftercooler <NUM> to create the first feed recycle stream <NUM>.

Optionally, a portion of either of the vapor streams (first end flash drum vapor stream <NUM> or first storage tank vapor stream <NUM>) may also be used as fuel elsewhere in the plant. The compressor <NUM> may have multiple stages with intercoolers, with fuel withdrawn between stages (not shown).

A second feed stream <NUM> is divided into the second MCHE feed stream <NUM> and second feed bypass stream <NUM>. The second MCHE feed stream <NUM> is cooled and liquefied in the MCHE <NUM> to form a liquefied second product stream <NUM>. The second feed bypass stream <NUM> is reduced in pressure in valve <NUM> to produce a reduced pressure second feed bypass stream <NUM>. The liquefied second product stream <NUM> is withdrawn from the MCHE <NUM>, reduced in pressure though valve <NUM>, resulting in a two-phase second product stream <NUM>. The two-phase second product stream <NUM> is combined with the reduced pressure second feed bypass stream <NUM> to form a combined two-phase second product stream <NUM>, which is fed into to a second end flash drum <NUM>. The second end flash drum <NUM> separates the combined two-phase second product stream <NUM> into a second end flash drum vapor stream <NUM> and a second end flash drum liquid stream <NUM>. The second end flash drum vapor stream <NUM> may contain impurities. The second end flash drum liquid stream <NUM> may be stored in a product tank (not shown).

It should be noted that, depending upon operational conditions, either of the bypass streams (the first feed bypass stream <NUM> and the second feed bypass stream <NUM>) may have a zero flow.

In this embodiment, the system <NUM> provides two ways to control the product temperature for each feed stream, by adjusting the amount of fluid flowing through the bypass line associated with that stream and adjusting the amount of recycling flash vapor associated with that stream. For example, increasing the fraction of the combined first feed stream <NUM> that flows through the first feed bypass stream <NUM> results in the combined two-phase first product stream <NUM> becoming warmer (assuming all other process variables remain constant). Conversely, increasing the flow rate of the first feed recycle stream <NUM> will result in the cold end of the MCHE <NUM> being warmer for all streams leaving the cold end of the MCHE <NUM> (including the liquefied first product stream <NUM> and the liquefied second product stream <NUM>, or any other liquefied product stream). Although <FIG> only shows two feed circuits and two product streams, any number of feed circuits and product streams may be utilized. Further, <FIG> shows the refrigeration system including the compression system. The compression system is part of the systems <NUM>, <NUM> of <FIG>, but is omitted in the figures in order to simplify the drawings.

The system <NUM> provides the ability for flexible, multi-feed stream operation. For example, the MCHE <NUM> could be operated so that the feed stream having the lowest boiling point is supplied to its storage tank at the bubble point temperature for that feed stream. The liquefied product stream associated with each other feed stream (with a higher boiling point) is warmed by its bypass stream to prevent excessive sub-cooling. Operating the system <NUM> in this way is particularly useful if feed streams for feeds having relatively high boiling points also have contaminants that require warmer operating temperatures for removal. For example, the second end flash drum vapor stream <NUM> could be used to remove contaminants from the combined two-phase second product stream <NUM>.

Alternatively, the MCHE <NUM> could be operated at the bubble point temperature of the highest boiling feed or an intermediate temperature between the highest-boiling feed and the lowest-boiling feed. The latter method of operating would result in a significant flash vapor stream, such the first storage tank vapor stream <NUM>, at the storage tank of a lowest-boiling feed. The first storage tank vapor stream <NUM> can be used in other parts of the plant or compressed and recycled to the warm end of the MCHE <NUM> to avoid producing net vapor export stream, as described before and shown on <FIG>.

In this MCHE <NUM>, at least a portion of, and preferably all of the refrigeration is provided by vaporizing at least a portion of sub-cooled refrigerant streams after pressure reduction across reducing valves.

As noted above, any suitable refrigeration cycle could be used to provide the refrigeration to the MCHE <NUM>. In this exemplary embodiment, a low-pressure gaseous mixed refrigerant (MR) stream <NUM> is withdrawn from the bottom of the shell-side of the MCHE <NUM> and is compressed in a compressor <NUM> to form a high pressure gaseous MR stream <NUM>, which is at a pressure of less than <NUM> bar. The high pressure gaseous MR stream <NUM> is cooled in an aftercooler <NUM> to a temperature at or near ambient temperature to form a high-pressure two-phase MR stream <NUM>.

The high-pressure two-phase MR stream <NUM> is separated in a phase separator <NUM> into a high-pressure liquid MR stream <NUM> and a high-pressure vapor MR stream <NUM>. The high-pressure liquid MR stream <NUM> is cooled in the warm bundle of the MCHE <NUM> to form a cooled high-pressure liquid MR stream <NUM> which is then reduced in pressure across a valve <NUM> to form a reduced pressure liquid MR stream <NUM>. The reduced pressure liquid MR stream <NUM> is then introduced to the shell side of the MCHE <NUM> between the warm and cold bundles to provide refrigeration for the pre-cooling and liquefaction step.

The high-pressure vapor MR stream <NUM> is cooled and liquefied in the warm and cold bundles of the MCHE <NUM> to produce a liquefied MR stream <NUM>. The liquefied MR stream <NUM> is reduced in pressure across a valve <NUM> to produce a reduced pressure liquid MR stream <NUM>, which is introduced into the shell side of the MCHE <NUM> at the cold end of the MCHE <NUM> to provide refrigeration in the sub-cooling step.

In this exemplary embodiment, the compressor <NUM> typically has two stages with an intercooler <NUM>. A medium pressure MR stream <NUM> is withdrawn after the first compressor stage and is cooled in the intercooler <NUM> to produce a cooled medium pressure MR stream <NUM>. The cooled medium pressure MR stream <NUM> then flows through a phase separator <NUM> and is separated into a medium pressure vapor MR stream <NUM> and a medium pressure liquid MR stream <NUM>. The pressure of the medium pressure liquid MR stream <NUM> is then increased by pump <NUM> before being combined with the high pressure gaseous MR stream <NUM>.

<FIG> and <FIG> and <FIG> are block diagrams showing further multi-feed liquefaction systems. In order to simplify these diagrams, only the MCHE, and feed streams, product streams, storage tanks, bypass conduits, recycle conduits, and associated valves are shown. It should be understood that these systems include compression subsystems and circuits for the refrigerant, as shown in <FIG>, for example. In <FIG> and <FIG> and <FIG>, valves that are at least partially open (such as valve 588a in <FIG>) have white fill are filled and valves that are closed have black fill (such as valve 588b in <FIG>).

The system of <NUM> <FIG> & <FIG> the MCHE <NUM> includes two cooling circuits 583a, 583b. In <FIG>, the system <NUM> is configured (not in accordance with the present invention) to liquefy a single feed stream 500a of natural gas. The feed stream 500a is fed through both of the hydrocarbon cooling circuits 583a, 583b. The natural gas exits the cold end of the MCHE <NUM> at temperature designed to result in the liquefied natural gas being at or near its bubble point in its storage tank 534a when stored at a pressure of less than <NUM> bara. No bypass or flash recycle is desirable under these operating conditions. Accordingly, valve 588b is closed to prevent backflow into the second feed stream 500b. Valve <NUM> is closed to prevent any flow through the bypass circuit <NUM> for the second feed stream 500b. Valve <NUM> is closed to prevent and flash gas from the storage tank 534a from being recycled. Optionally, valve 504b is closed to prevent LNG from entering the second storage tank 534b. Valves <NUM>, <NUM> for connecting conduits are open to allow fluid from the first feed stream 500a to flow through both hydrocarbon cooling circuits 583a, 583b.

In <FIG>, the same system <NUM> is shown, but instead of processing only natural gas, the system <NUM> is operationally configured (in accordance with the present invention) to process both natural gas (through feed line 500a) and propane (through feed line 500b). The system <NUM> is configured so that the natural gas and propane exit the MCHE <NUM> at substantially the same temperature, with the exit temperature resulting in the liquefied natural gas being at or near its bubble point in its storage tank 534a when stored at a pressure of less than <NUM> bara. Under these operating conditions, natural gas flows through one hydrocarbon cooling circuit 583a and propane flows through the other hydrocarbon cooling circuit 583b. Valves <NUM>, <NUM> on the connecting conduits are closed to prevent mixing of the natural gas and propane. Valves 504a, 504b are open to enable liquefied natural gas and liquefied propane to flow from the cold end of the MCHE <NUM> into separate storage tanks 534a, 534b.

In order to enable the propane to be stored at or near its bubble point in its storage tank 534b at a pressure of no more than <NUM> bara, a bypass portion of the propane is directed to a bypass circuit <NUM> and a feed portion of the propane stream flows through the hydrocarbon cooling circuit 583b, then the bypass portion is recombined with the feed portion of the propane stream downstream from the cold end of the MCHE <NUM> and before the propane enters the storage tank 534b. A bypass valve <NUM> is at least partially open to allow flow through the bypass circuit <NUM>. The amount of the propane feed stream that is directed to the bypass circuit <NUM> is selected to sufficiently warm propane exiting the cold end of the MCHE <NUM> to a temperature that is at or near the bubble point when stored in the storage tank 534b at a pressure of no more than <NUM> bara. Optionally, a portion of any flash gas from the first storage tank 534a could be compressed, cooled, and mixed with the natural gas feed 500a upstream from the MCHE <NUM>.

The operational configurations shown in <FIG> and <FIG> and described above enable the system <NUM> to easily adapt to changes in feed stream composition. In the operational configuration of <FIG>, the system <NUM> is capable of simultaneously liquefying both natural gas and propane, without the complexity and cost associated with cooling tube side streams to different temperatures in the MCHE <NUM>, and while avoiding the risks associated with storing sub-cooled propane at low pressure. The bypass circuit <NUM> also increases efficiency by reducing the refrigeration load on the cooling circuit 583b through which propane flows. Simply by changing the position of valves, the system <NUM> is capable of switching from processing simultaneous natural gas and propane feeds (<FIG>) to processing only natural gas (<FIG>) without a significant reduction in efficiency.

<FIG> also shows an optional end flash heat exchange, in which an end flash stream <NUM> from storage tank 534a is warmed in a heat exchanger <NUM> against a portion <NUM> of the natural gas feed stream 500a to produce a warmed end flash stream <NUM>. The portion <NUM> of the natural gas feed stream 500a is at least partially liquefied in the heat exchanger <NUM> to form an at least partially liquefied stream <NUM>, which is sent to tank 534a. Valves <NUM> and <NUM> are shown as being open in <FIG> to allow flow through the heat exchanger <NUM>. In an alternative embodiment, a portion of the refrigerant stream, such as <NUM> or <NUM> or <NUM> (see <FIG>) could be cooled against the end flash stream <NUM> in heat exchanger <NUM> instead of the portion <NUM> of the natural gas feed stream 500a. Alternatively, the end flash stream <NUM> may be obtained from an end flash drum instead of the storage tank 534a.

In the system <NUM> of <FIG>, <FIG> and <FIG>, the MCHE <NUM> includes four cooling circuits 683a, 683b, 683c, 683d. <FIG> shows a single feed mode (not in accordance with the present invention) where ethane is liquefied in the MCHE <NUM>. Valves 688b, 688c, 688d are closed to isolate unused feed circuits 600b, 600c, 600d. Similarly, valves 687b, 687c, 687d are also closed to isolate unused storage tanks 634b, 634c, 634d. Because only one hydrocarbon fluid is being processed, bypass valves 627a, 627b, 627c are closed, as well as the recycle valve <NUM>. At the cold end of the MCHE <NUM>, the ethane feed is preferably at a temperature that will result in the ethane being at its bubble point in the storage tank 634a. Optionally, the temperature at the cold end of the MCHE <NUM> could be set to result in vaporization of impurities through vent/flash stream <NUM>0a. Alternatively, in the event that the temperature at the cold end of the MCHE <NUM> was set to liquefy a more volatile product, such as ethylene, cooled ethane could be warmed by the bypass stream 622a (meaning that the bypass valve 627a would be at least partially open), in order to prevent excessive cooling of the ethane product, which may lead to a collapse of the storage tank 634a.

<FIG> shows this system <NUM>, operationally configured (in accordance with the present invention) to process two simultaneous feeds, in this case ethane (feed stream 600a) and ethylene (feed stream 600d). In this configuration, the ethane feed is being cooled in three of the cooling circuits 683a, 683b, 683c, meaning that connecting valves 686a, 686b, 686c are open. Cooled ethane from each of the cooling circuits 683a, 683b, 683c is then directed to a single product stream 613a. In <FIG>, one of the bypass circuits 622a is open, so that a portion of the warm ethane feed is mixed with cooled ethane downstream from the cold end of the MCHE <NUM>, which is intended to maintain the ethane product stream at a temperature at close to its bubble point in the storage tank 634a. In this exemplary embodiment, the system <NUM> is operationally configured to produce a temperature at the cold end of the MCHE <NUM> that is close to the bubble point of ethylene in the storage tank 634d to suppress flash. Under these operating conditions, there is no need to recycle ethylene.

Alternatively, the system <NUM> could be operationally configured to maintain a temperature at the cold end of the MCHE <NUM> that is warmer than ethylene's bubble point but colder than ethane's bubble point. In this case, a portion of the ethylene flash stream 611d is recycled (via recycle circuit <NUM>) to the feed stream 600c to avoid net flash export. This operational configuration could be desirable if electric motors are used to drive the compressors of system <NUM> and it is desirable to configure the system to be capable of processing more volatile feed streams that ethylene.

<FIG> shows operation (in accordance with the present invention) of the system <NUM> with three simultaneous feeds: ethane (feed stream 600a), ethylene (feed stream 600d), and an ethane/propane mixture (feed stream 600c). In this operational configuration, temperatures of both the ethane and ethane/propane mixture products are kept near bubble point in their respective storage tanks 634a, 634c using bypass circuits 622a, 622c. In this embodiment, at least some of the ethylene flash stream 611d is recycled via recycle circuit <NUM>. The temperature of the cooled feed streams at the cold end of the MCHE <NUM> is preferably between the bubble points of ethane and ethylene.

The following are examples provide data based on simulations of an SMR process similar to that shown in <FIG>. Cases using multiple feeds or producing LNG, are run in rating mode. They are designed to produce <NUM> MTPA of ethane product by using four feed circuits. Table <NUM> provides a list of the operating regimes and resulting production rates for a liquefaction plant able to liquefy ethane, ethane-propane mixture, ethylene, propane, and natural gas.

In Example <NUM>, only ethane is processed. This example is used to set the sizing of critical equipment, such as the MCHE <NUM> and refrigeration compressor C1. In this example, ethane enters the MCHE <NUM> at <NUM> degrees Celsius and <NUM> bar and is cooled to -<NUM> degrees Celsius. Feed and product rates and compositions are specified in Table <NUM> below.

The low-pressure gaseous MR stream <NUM> has a flow rate of <NUM> moles per hour. The MR has the composition shown in Table <NUM> and leaves the MCHE <NUM> at a temperature close to ambient temperature, for example, <NUM> degrees Celsius. The MR is compressed the compressor C1 from <NUM> bar to <NUM> bar, cooled by the high-pressure aftercooler <NUM> to <NUM> degrees Celsius, then separated in the phase separator <NUM> into the high-pressure vapor MR stream <NUM> and the high-pressure liquid MR stream <NUM>.

For Example <NUM>, pretreated feed streams of ethane, ethylene, and ethane/propane mix enter the MCHE <NUM> unit at <NUM> degrees Celsius and <NUM> bar and are cooled to -<NUM> degrees Celsius. In this example, process flow is as shown in <FIG>. Feed and product rates and compositions are specified in Table <NUM> and Table <NUM>, respectively, below. Table <NUM> also show normal bubble points of mixtures.

The low-pressure gaseous MR stream <NUM> has a flow rate of <NUM> moles per hour. The MR has the composition shown in Table <NUM>, leaves the MCHE <NUM> at close to ambient temperature, for example, <NUM> degrees Celsius, is compressed in the MR Compressor C1 from <NUM> bar to <NUM> bar, and cooled by the high-pressure aftercooler <NUM> to <NUM> degrees Celsius. The rest of the process of Example <NUM> is identical to Example <NUM>.

For Examples <NUM>, pretreated natural gas feed stream enters the MCHE at <NUM> degrees Celsius and <NUM> bar. Example <NUM> used the configuration of <FIG>, but without the first feed stream 500b. The flow scheme includes an exchanger which cools a slipstream of hot natural gas feed against the cold end flash gas. The end flash gas and the vapor from the storage tank are recycled and mixed with the natural gas feed. The need to recycle may be necessary at facilities which use electric motors to power the refrigerant compressors, and thus do not have a need or have a reduced need for fuel gas. LNG is cooled to -<NUM> degrees Celsius. Examples <NUM> uses the feed rate and composition specified in Table <NUM> below and produce the product composition and feed rates shown in Table <NUM> below.

MR compositions for Examples <NUM> are shown below in Table <NUM>. For Example <NUM>, the low-pressure gaseous MR stream <NUM> has a flow rate of <NUM> moles per hour. The MR leaves the MCHE <NUM> at close to ambient temperature, for example, <NUM> degrees Celsius, is compressed from <NUM> bar to <NUM> bar, and cooled by the aftercooler <NUM> to <NUM> degrees Celsius.

Claim 1:
A method for cooling and liquefying at least two feed streams (<NUM>, <NUM>) in a heat exchanger (<NUM>), the method comprising:
(a) Introducing the at least two feed streams (<NUM>, <NUM>) into a warm end of the heat exchanger (<NUM>), the at least two feed streams comprising a first feed stream (<NUM>) having a first normal bubble point and a second feed stream (<NUM>) having a second normal bubble point that is lower than the first normal bubble point;
(b) cooling by indirect heat exchange in the heat exchanger (<NUM>) at least a first portion (<NUM>, <NUM>) of each of the first feed stream (<NUM>) and the second feed stream (<NUM>) against a refrigerant to form at least two cooled feed streams (<NUM>, <NUM>) comprising a first cooled feed stream (<NUM>) and a second cooled feed stream (<NUM>);
(c) withdrawing the at least two cooled feed streams (<NUM>, <NUM>) from a cold end of the heat exchanger (<NUM>) at substantially the same withdrawal temperature; and
(d) providing at least two product streams (<NUM>, <NUM>), each of the at least two product streams being downstream from and in fluid flow communication with one of the at least two cooled feed streams (<NUM>, <NUM>), each of the at least two product streams being maintained within a predetermined product stream temperature range of a predetermined product stream temperature, the at least two product streams comprising a first product stream (<NUM>) and a second product stream (<NUM>), the predetermined product stream temperature for the first product stream being the first predetermined product stream temperature and the predetermined product stream temperature of the second product stream being the second predetermined product stream temperature; and
(e) withdrawing a first bypass stream from the first feed stream upstream from the cold end of the heat exchanger;
characterized in that the heat exchanger is a coil-wound heat exchanger (<NUM>), and the method further comprises:
(f) forming the first product stream (<NUM>) by mixing the first cooled feed stream (<NUM>) with the first bypass stream (<NUM>), the first predetermined product stream temperature being warmer than the withdrawal temperature of the first cooled feed stream.