Patent Grant
9291387
The present invention relates to improved processes for recovery of natural gas liquids from gas feed streams containing hydrocarbons, and in particular to recovery of propane and ethane from gas feed streams.
Natural gas contains various hydrocarbons, including methane, ethane and propane. Natural gas usually has a major proportion of methane and ethane, i.e. methane and ethane together typically comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases. In addition to natural gas, other gas streams containing hydrocarbons may contain a mixture of lighter and heavier hydrocarbons. For example, gas streams formed in the refining process can contain mixtures of hydrocarbons to be separated. Separation and recovery of these hydrocarbons can provide valuable products that may be used directly or as feedstocks for other processes. These hydrocarbons are typically recovered as natural gas liquids (NGL).
The present invention is primarily directed to recovery of C3+ components in gas streams containing hydrocarbons, and in particular to recovery of propane from these gas streams. A typical natural gas feed to be processed in accordance with the processes described below typically may contain, in approximate mole percent, 92.12% methane, 3.96% ethane and other C2 components, 1.05% propane and other C3 components, 0.15% iso-butane, 0.21% normal butane, 0.11% pentanes or heavier, and the balance made up primarily of nitrogen and carbon dioxide. Refinery gas streams may contain less methane and higher amounts of heavier hydrocarbons.
Recovery of natural gas liquids from a gas feed stream has been performed using various processes, such as cooling and refrigeration of gas, oil absorption, refrigerated oil absorption or through the use of multiple distillation towers. More recently, cryogenic expansion processes utilizing Joule-Thompson valves or turbo expanders have become preferred processes for recovery of NGL from natural gas.
In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high pressure liquids containing the desired components.
The high-pressure liquids may be expanded to a lower pressure and fractionated. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation column. In the distillation column volatile gases and lighter hydrocarbons are removed as overhead vapors and heavier hydrocarbon components exit as liquid product in the bottoms.
The feed gas is typically not totally condensed, and the vapor remaining from the partial condensation may be passed through a Joule-Thompson valve or a turbo expander to a lower pressure at which further liquids are condensed as a result of further cooling of the stream. The expanded stream is supplied as a feed stream to the distillation column.
A reflux stream is provided to the distillation column, typically a portion of partially condensed feed gas after cooling but prior to expansion. Various processes have used other sources for the reflux, such as a recycled stream of residue gas supplied under pressure.
While various improvements to the general cryogenic processes described above have been attempted, these improvements continue to use a turbo expander or Joule-Thompson valve to expand the feed stream to the distillation column. It would be desirable to have an improved process for enhanced recovery of NGLs from a natural gas feed stream.
The present invention relates to improved processes for recovery of NGLs from a feed gas stream. The process utilizes an open loop mixed refrigerant process to achieve the low temperatures necessary for high levels of NGL recovery. A single distillation column is utilized to separate heavier hydrocarbons from lighter components such as sales gas. The overhead stream from the distillation column is cooled to partially liquefy the overhead stream. The partially liquefied overhead stream is separated into a vapor stream comprising lighter hydrocarbons, such as sales gas, and a liquid component that serves as a mixed refrigerant. The mixed refrigerant provides process cooling and a portion of the mixed refrigerant is used as a reflux stream to enrich the distillation column with key components. With the gas in the distillation column enriched, the overhead stream of the distillation column condenses at warmer temperatures, and the distillation column runs at warmer temperatures than typically used for high recoveries of NGLs. The process achieves high recovery of desired NGL components without expanding the gas as in a Joule-Thompson valve or turbo expander based plant, and with only a single distillation column.
In one embodiment of the process of the present invention, C3+ hydrocarbons, and in particular propane, are recovered. Temperatures and pressures are maintained as required to achieve the desired recovery of C3+ hydrocarbons based upon the composition of the incoming feed stream. In this embodiment of the process, feed gas enters a main heat exchanger and is cooled. The cooled feed gas is fed to a distillation column, which in this embodiment functions as a deethanizer Cooling for the feed stream may be provided primarily by a warm refrigerant such as propane. The overhead stream from the distillation column enters the main heat exchanger and is cooled to the temperature required to produce the mixed refrigerant and to provide the desired NGL recovery from the system.
The cooled overhead stream from the distillation column is combined with an overhead stream from a reflux drum and separated in a distillation column overhead drum. The overhead vapor from the distillation column overhead drum is sales gas (i.e. methane, ethane and inert gases) and the liquid bottoms are the mixed refrigerant. The mixed refrigerant is enriched in C2 and lighter components as compared to the feed gas. The sales gas is fed through the main heat exchanger where it is warmed. The temperature of the mixed refrigerant is reduced to a temperature cold enough to facilitate the necessary heat transfer in the main heat exchanger. The temperature of the refrigerant is lowered by reducing the refrigerant pressure across a control valve. The mixed refrigerant is fed to the main heat exchanger where it is evaporated and super heated as it passes through the main heat exchanger.
After passing through the main heat exchanger, the mixed refrigerant is compressed. Preferably, the compressor discharge pressure is greater than the distillation column pressure so no reflux pump is necessary. The compressed gas passes through the main heat exchanger, where it is partially condensed. The partially condensed mixed refrigerant is routed to a reflux drum. The bottom liquid from the reflux drum is used as a reflux stream for the distillation column. The vapors from the reflux drum are combined with the distillation column over head stream exiting the main heat exchanger and the combined stream is routed to the distillation column overhead drum. In this embodiment, the process of the invention can achieve over 99 percent recovery of propane from the feed gas.
In another embodiment of the process, the feed gas is treated as described above and a portion of the mixed refrigerant is removed from the plant following compression and cooling. The portion of the mixed refrigerant removed from the plant is fed to a C2 recovery unit to recover the ethane in the mixed refrigerant. Removal of a portion of the mixed refrigerant stream after it has passed through the main heat exchanger and been compressed and cooled has minimal effect on the process provided that enough C2 components remain in the system to provide the required refrigeration. In some embodiments, as much as 95 percent of the mixed refrigerant stream may be removed for C2 recovery. The removed stream may be used as a feed stream in an ethylene cracking unit.
In another embodiment of the process, an absorber column is used to separate the distillation column overhead stream. The overhead stream from the absorber is sales gas, and the bottoms are the mixed refrigerant.
In yet another embodiment of the invention, only one separator drum is used. In this embodiment of the invention, the compressed, cooled mixed refrigerant is returned to the distillation column as a reflux stream.
The process described above may be modified to achieve separation of hydrocarbons in any manner desired. For example, the plant may be operated such that the distillation column separates C4+ hydrocarbons, primarily butane, from C3 and lighter hydrocarbons. In another embodiment of the invention, the plant may be operated to recover both ethane and propane. In this embodiment of the invention, the distillation column is used as a demethanizer, and the plant pressures and temperatures are adjusted accordingly. In this embodiment, the bottoms from the distillation tower contain primarily the C2+ components, while the overhead stream contains primarily methane and inert gases. In this embodiment, recovery of as much as 55 percent of the C2+ components in the feed gas can be obtained.
Among the advantages of the process is that the reflux to the distillation column is enriched, for example in ethane, reducing loss of propane from the distillation column. The reflux also increases the mole fraction of lighter hydrocarbons, such as ethane, in the distillation column making it easier to condense the overhead stream. This process uses the liquid condensed in the distillation column overhead twice, once as a low temperature refrigerant and the second time as a reflux stream for the distillation column. Other advantages of the processes of the present invention will be apparent to those skilled in the art based upon the detailed description of preferred embodiments provided below.
The present invention relates to improved processes for recovery of natural gas liquids (NGL) from gas feed streams containing hydrocarbons, such as natural gas or gas streams from petroleum processing. The process of the present invention runs at approximately constant pressures with no intentional reduction in gas pressures through the plant. The process uses a single distillation column to separate lighter hydrocarbons and heavier hydrocarbons. An open loop mixed refrigerant provides process cooling to achieve the temperatures required for high recovery of NGL gases. The mixed refrigerant is comprised of a mixture of the lighter and heavier hydrocarbons in the feed gas, and is generally enriched in the lighter hydrocarbons as compared to the feed gas.
The open loop mixed refrigerant is also used to provide an enriched reflux stream to the distillation column, which allows the distillation column to operate at higher temperatures and enhances the recovery of NGLs. The overhead stream from the distillation column is cooled to partially liquefy the overhead stream. The partially liquefied overhead stream is separated into a vapor stream comprising lighter hydrocarbons, such as sales gas, and a liquid component that serves as a mixed refrigerant.
The process of the present invention may be used to obtain the desired separation of hydrocarbons in a mixed feed gas stream. In one embodiment, the process of the present application may be used to obtain high levels of propane recovery. Recovery of as much as 99 percent or more of the propane in the feed case may be recovered in the process. The process can also be operated in a manner to recover significant amounts of ethane with the propane or reject most of the ethane with the sales gas. Alternatively, the process can be operated to recover a high percentage of C4+ components of the feed stream and discharge C3 and lighter components.
A plant for performing some embodiments of the process of the present invention is shown schematically in
Feed gas is fed through line (12) to main heat exchanger (10). The feed gas may be natural gas, refinery gas or other gas stream requiring separation. The feed gas is typically filtered and dehydrated prior to being fed into the plant to prevent freezing in the NGL unit. The feed gas is typically fed to the main heat exchanger at a temperature between about 110° F. and 130° F. and at a pressure between about 100 psia and 450 psia. The feed gas is cooled and partially liquefied in the main heat exchanger (10) by making heat exchange contact with cooler process streams and with a refrigerant which may be fed to the main heat exchanger through line (15) in an amount necessary to provide additional cooling necessary for the process. A warm refrigerant such as propane may be used to provide the necessary cooling for the feed gas. The feed gas is cooled in the main heat exchanger to a temperature between about 0° F. and −40° F.
The cool feed gas (12) exits the main heat exchanger (10) and enters the distillation column (20) through feed line (13). The distillation column operates at a pressure slightly below the pressure of the feed gas, typically at a pressure of between about 5 psi and 10 psi less than the pressure of the feed gas. In the distillation column, heavier hydrocarbons, such as for example propane and other C3+ components, are separated from the lighter hydrocarbons, such as ethane, methane and other gases. The heavier hydrocarbon components exit in the liquid bottoms from the distillation column through line (16), while the lighter components exit through vapor overhead line (14). Preferably, the bottoms stream (16) exits the distillation column at a temperature of between about 150° F. and 300° F., and the overhead stream (14) exits the distillation column at a temperature of between about −10° F. and −80° F.
The bottoms stream (16) from the distillation column is split, with a product stream (18) and a recycle stream (22) directed to a reboiler (30) which receives heat input (Q). Optionally, the product stream (18) may be cooled in a cooler to a temperature between about 60° F. and 130° F. The product stream (18) is highly enriched in the heavier hydrocarbons in the feed gas stream. In the embodiment shown in
The distillation column overhead stream (14) passes through main heat exchanger (10), where it is cooled by heat exchange contact with process gases to partially liquefy the stream. The distillation column overhead stream exits the main heat exchanger through line (19) and is cooled sufficiently to produce the mixed refrigerant as described below. Preferably, the distillation column overhead stream is cooled to between about −30° F. and −130° F. in the main heat exchanger.
In the embodiment of the process shown in
In the embodiment shown in
The sales gas flows through the main heat exchanger (10) through line (42) and is warmed. In a typical plant, the sales gas exits the deethanizer overhead separator at a temperature of between about −40° F. and −120° F. and a pressure of between about 85 psia and 435 psia, and exits the main heat exchanger at a temperature of between about 100° F. and 120° F. The sales gas is sent for further processing through line (43).
The mixed refrigerant flows through the distillation column overhead separator bottoms line (34). The temperature of the mixed refrigerant may be lowered by reducing the pressure of the refrigerant across control valve (65). The temperature of the mixed refrigerant is reduced to a temperature cold enough to provide the necessary cooling in the main heat exchanger (10). The mixed refrigerant is fed to the main heat exchanger through line (35). The temperature of the mixed refrigerant entering the main heat exchanger is typically between about −60° F. to −175° F. Where the control valve (65) is used to reduce the temperature of the mixed refrigerant, the temperature is typically reduced by between about 20° F. to 50° F. and the pressure is reduced by between about 90 psi to 250 psi. The mixed refrigerant is evaporated and superheated as it passes through the main heat exchanger (10) and exits through line (35a). The temperature of the mixed refrigerant exiting the main heat exchanger is between about 80° F. and 100° F.
After exiting the main heat exchanger, the mixed refrigerant is fed to ethane compressor (80). The mixed refrigerant is compressed to a pressure about 15 psi to 25 psi greater than the operating pressure of the distillation column at a temperature of between about 230° F. to 350° F. By compressing the mixed refrigerant to a pressure greater than the distillation column pressure, there is no need for a reflux pump. The compressed mixed refrigerant flows through line (36) to cooler (90) where it is cooled to a temperature of between about 70° F. and 130° F. Optionally, cooler (90) may be omitted and the compressed mixed refrigerant may flow directly to main heat exchanger (10) as described below. The compressed mixed refrigerant then flows through line (38) through the main heat exchanger (10) where it is further cooled and partially liquefied. The mixed refrigerant is cooled in the main heat exchanger to a temperature of between about 15° F. to −70° F. The partially liquefied mixed refrigerant is introduced through line (39) to the reflux separator (40). As described previously, in the embodiment of
The open loop mixed refrigerant used as reflux enriches the distillation column with gas phase components. With the gas in the distillation column enriched, the overhead stream of the column condenses at warmer temperatures, and the distillation column runs at warmer temperatures than normally required for high recovery of NGLs.
The reflux to the distillation column also reduces losses of heavier hydrocarbons from the column. For example, in processes for recovery of propane, the reflux increases the mole fraction of ethane in the distillation column, which makes it easier to condense the overhead stream. The process uses the liquid condensed in the distillation column overhead drum twice, once as a low temperature refrigerant and the second time as a reflux stream for the distillation column.
In another embodiment of the invention shown in
In another embodiment of the invention, the NGL recovery unit can recover significant amounts of ethane with the propane. In this embodiment of the process, the distillation column is a demethanizer, and the overhead stream contains primarily methane and inert gases, while the column bottoms contain ethane, propane and heavier components.
In another embodiment of the process, the deethanizer overhead drum may be replaced by an absorber. As shown in
In yet another embodiment of the invention shown in
Examples of specific embodiments of the process of the process of the present invention are described below. These examples are provided to further describe the processes of the present invention and they are not intended to limit the full scope of the invention in any way.
In the following examples, operation of the processing plant shown in
As can be seen in Table 2, the product stream (18) from the bottom of the distillation column is highly enriched in C3+ components, while the sales gas stream (43) contains almost entirely C2 and lighter hydrocarbons and gases. Approximately 99.6% of the propane in the feed gas is recovered in the product stream. The mixed refrigerant is comprised primarily of methane and ethane, but contains more propane than the sales gas.
In this example, operating parameters are provided for the processing plant shown in
As can be seen in Table 4, the product stream (18) from the bottom of the distillation column is highly enriched in C3+ components, while the sales gas stream (43) contains almost entirely C2 and lighter hydrocarbons and gases, in particular hydrogen. This stream could be used to feed a membrane unit or PSA to upgrade this stream to useful hydrogen. Approximately 97.2% of the propane in the feed gas is recovered in the product stream. The mixed refrigerant is comprised primarily of methane and ethane, but contains more propane than the sales gas.
In this example, operating parameters are provided for the processing plant shown in
As can be seen in Table 6, in this embodiment, the product stream (18) from the bottom of the distillation column is highly enriched in C4+ components, while the sales gas stream (43) contains almost entirely C3 and lighter hydrocarbons and gases. Approximately 99.7% of the C4+ components in the feed gas is recovered in the product stream. The mixed refrigerant is comprised primarily of C3 and lighter components, but contains more butane than the sales gas.
In this example, operating parameters are provided for the processing plant shown in
As can be seen in Table 8, in this embodiment, the product stream (18) from the bottom of the distillation column is highly enriched in C3+ components, while the sales gas stream (43) contains almost entirely C2 and lighter hydrocarbons and gases. The mixed refrigerant is comprised primarily of C2 and lighter components, but contains more propane than the sales gas.
In this example, operating parameters are provided for the processing plant shown in
As can be seen in Table 10, in this embodiment, the product stream (18) from the bottom of the distillation column is highly enriched in C3+ components, while the sales gas stream (43) contains almost entirely C2 and lighter hydrocarbons and gases. The mixed refrigerant is comprised primarily of C2 and lighter components, but contains more propane than the sales gas.
In this example, operating parameters are provided for the processing plant shown in
As can be seen in Table 12, in this embodiment, the product stream (18) from the bottom of the distillation column is highly enriched in C3+ components, while the sales gas stream (43) contains almost entirely C2 and lighter hydrocarbons and gases. The mixed refrigerant is comprised primarily of C2 and lighter components, but contains more propane than the sales gas.
While specific embodiments of the present invention have been described above, one skilled in the art will recognize that numerous variations or changes may be made to the process described above without departing from the scope of the invention as recited in the appended claims. Accordingly, the foregoing description of preferred embodiments is intended to describe the invention in an exemplary, rather than a limiting, sense.
This application is a divisional of U.S. application Ser. No. 13/493,267, filed Jun. 12, 2012, which is a divisional of U.S. application Ser. No. 12/121,880 filed May 16, 2008.
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| Number | Date | Country | |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 13493267 | Jun 2012 | US |
| Child | 13858585 | US | |
| Parent | 12121880 | May 2008 | US |
| Child | 13493267 | US |
