EP1904800A1

EP1904800A1 - Ngl recovery methods and configurations

Info

Publication number: EP1904800A1
Application number: EP06734542A
Authority: EP
Inventors: John Mak; Richard B. Nielsen; Curt Graham
Original assignee: Fluor Technologies Corp
Current assignee: Fluor Technologies Corp
Priority date: 2005-07-07
Filing date: 2006-02-07
Publication date: 2008-04-02
Also published as: US20100011810A1; EA014452B1; AU2006269696A1; CA2614414A1; MX2007015603A; WO2007008254A1; AU2006269696B2; EA200800270A1; PE20070408A1; CA2614414C

Abstract

Contemplated plants and methods for NGL recovery from feed gases having a carbon dioxide content equal or greater than about 2% employ temperature control configurations that allow high-level and flexible recovery of ethane and heavier components while avoiding freezing of the carbon dioxide in the process. Where the feed gas has a significant fraction of C3+ components and moderate carbon dioxide content, a single column configuration with an intermediate reflux condenser may be used, while two-column configurations may be used for feed gases with high carbon dioxide content and relatively low C3+ component concentration.

Description

NGL RECOVERY METHODS AND CONFIGURATIONS

This application claims priority to our copending U.S. provisional patent application with the serial number 60/697458, which was filed July 7, 2005. Field of The Invention The field of the invention is gas processing, and especially gas processing for ethane recovery and/or propane recovery from various natural gas sources.

Background of The Invention

Expansion of gas is often used as a source of refrigeration in various processes for recovery of hydrocarbon liquids from feed gases, and particularly for recovery of ethane and propane from high pressure feed gas. However, and especially where feed gas pressure is relatively low or contains significant quantities of propane and/or heavier components, extra refrigeration (e.g., propane refrigeration) may also be required.

In most known expander-based plants for natural gas liquids (NGL) recovery, the feed gas is cooled and partially condensed, typically by heat exchange with demethanizer overhead vapor, side reboilers, and/or external propane refrigeration. The so formed liquid portion (that contains less volatile components) is then separated, while the vapor portion is usually split in two portions. One portion is then chilled and fed to the upper section of a demethanizer, while the other portion is letdown in pressure in a turbo-expander and fed to the mid section. These configurations are often used for feed gas with low CO₂ (e.g., less than 2%) and relatively high C₃+ (e.g., greater than 5%) content, and are generally not practical nor economically viable for feed gas with high CO₂ content (greater than 2%) and low C₃ + content (less than 2%, and more typically less than 1%).

However, the residue gas from the fractionation column in commonly known plants often still contains significant amounts of ethane and propane that could be further recovered if chilled to an even lower temperature, or subjected to another rectification stage. Lower temperature is often achieved using a higher expansion ratio across the turbo-expander, which lowers the column pressure and temperature. However, even with significantly lowered temperatures in commonly known plants, high ethane recovery in excess of 90% is often not achievable due to CO₂ freezing in the demethanizer. Moreover, such operation is also typically economically not justifiable due to the high capital cost of the compression equipment and energy costs. Thus, based on most known configurations, ethane recovery is typically limited to the 70% to 80% range due to CO₂ freezing problems and economic constraints.

Exemplary NGL recovery plants with a turbo-expander, feed gas chiller, separators, and a refluxed fractionation column are described, for example, in U.S. Pat. No. 4,854,955 to Campbell et al. Here, a configuration is employed for moderate ethane recovery with turbo- expansion, in which the demethanizer column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from feed gas chilling. Such an additional cooling step condenses most of the NGL components (especially propane and heavier) from the column overhead gas, which is later recovered in a separator, and returned to the column as reflux. Unfortunately, while high propane recovery can be achieved with such a process, ethane recovery is often moderate (typically at about 70% to 80%). In another example, as taught in U.S. Pat. No. 6,453,698 by Jain et al., the inventors describe a configuration that withdraws an intermediate vapor stream from the column followed by compression, chilling with cold residue gas to produce a lean vapor that is further cooled with the residue gas and eventually condensed to generate a lean reflux to the column. Although high ethane recovery can be achieved with such a process, the cost of additional equipment such as additional compressors, separators, and heat exchangers is usually difficult to justify.

Further examples include two-column configurations, with the first column acting as a reflux absorber while the second column is operated as a demethanizer or deethanizer. Such plants generally employ an additional fractionation column that receives multiple vapor streams in distinct positions, which allows the column to produce top reflux to the reflux absorber. For example, U.S. Pat. Nos. 5,953,935 to Sorensen describes the use of overhead vapor from the second column to chill the overhead gas from the first column generating a lean reflux. Similarly, as disclosed in U.S. Pat. No. 5,771,712 to Campell, the overhead liquid from the first distillation column is employed as a lean reflux to the second column.

In yet another approach, as described in U.S. Pat. No. 6,363,744 to Finn et al., residue gas is recycled from the residue gas compressor, subsequently chilled with the column overhead vapor, and used as a lean reflux to the demethanizer. Alternatively, residue gas recycling as described in U.S. Pat No. 4,687,499 to Aghili, is commonly used to generate refrigeration and a methane rich lean reflux when high ethane recovery is required. While such plants improve ethane and propane recovery to at least some degree, they also require very low temperatures (-100°F or lower) in the demethanizer to ensure high ethane recovery. However, due to the very low temperatures, the methane content in the tray liquid is also very high, which invariably causes significant internal recycle of the methane component in the lower section. Consequently, such configurations are inefficient as they require high reboiler duties and refrigeration requirements. Moreover, due to the relatively low temperatures in the lower section, CO₂ freezing is frequently encountered, which presents a significant obstacle for continuous operation. Alternatively, CO2 concentration must be reduced in the feed gas to a tolerable limit, which typically adds significant expense.

Thus, numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas and other sources. However, all or almost all of them fail to achieve economic operation for high ethane recovery. Therefore, there is still a need to provide improved methods and configurations for flexible natural gas liquids recovery.

Summary of the Invention

The present invention is directed to configurations and methods for recovery of NGL from feed gases with a CO₂ content of about > 2%, and especially to those configurations in which the recovery of ethane, propane, and heavier components can be variable. Especially contemplated configurations are those that allow high recovery (i.e., greater than 80%) of ethane while preventing at the same time freezing of CO₂.

hi one aspect of the inventive subject matter, a method of operating a plant for NGL recovery from a feed gas includes a step of separating the feed gas in a refluxed column to thereby produce a residue gas, and using a portion of the compressed residue gas after cooling as a first reflux, hi a further step, a portion of the feed gas is expanded upstream of the column to thereby form a second reflux, and in yet another step, the temperature of the column is controlled by using a control circuit that controls a temperature of an expander inlet stream that is fed to the column after expansion and/or by using an intermediate reflux condenser disposed between an upper section and a lower section of the column that maintains temperature of the column above a temperature sufficient to prevent carbon dioxide freezing. In especially preferred methods, the feed gas comprises at least 2% CO₂, and the column is operated such that at least 90% of propane and heavier components, and variable amounts of ethane up to 90% are recovered from the feed gas. In some aspects of contemplated methods, the column temperature is controlled using an intermediate reflux condenser, and the upper section of the column generates a liquid intermediate product that is used to cool the feed gas. Where desirable, the feed gas is cooled and separated in a separator into a vapor portion and a liquid portion, wherein a portion of the vapor is further cooled and expanded to form the second reflux.

Alternatively, the column temperature is controlled using a control circuit, wherein the column is a demethanizer that produces a bottom product, which is preferably fed to a second column that is operated at a lower pressure. The second column will typically produce a NGL bottom product and a methane and ethane rich overhead product. In such methods, it is generally preferred to route at least part of the methane and ethane rich overhead product back to the demethanizer (after appropriate compression). Moreover, such methods will also typically include a step of splitting the feed gas into three streams, wherein the first stream is sub-cooled (below the bubble point temperature of the gas) to a first temperature before entering the column, wherein the second stream is cooled and expanded in a turbo expander before entering the column at a second temperature (typically a higher temperature than the first temperature), and wherein the third stream bypasses the feed exchanger and feeds the turbo expander suction for temperature control by the control circuit.

Therefore, in another aspect of the inventive subject matter, contemplated plants will include a column having an intermediate reflux condenser (located between a upper fractionation section and a lower rectification section of the column) that is configured to operate at a temperature of between about -20 °F and about -40 °F. The column is further configured to receive a first and a second reflux stream and to produce an overhead product. A conduit is preferably coupled to the column such that a liquid stream is fed from the upper section to the lower section via a heat exchanger that is configured such that the liquid stream is heated in the heat exchanger. Additionally, contemplated plants will include a recycle circuit that is configured to provide a portion of the overhead product as the first reflux stream to the column. In most cases, the column is configured to provide an ethane side product stream and a propane plus liquids bottom product stream. Most preferably, the heat exchanger in such plants is a feed gas exchanger that is configured to heat the liquid stream to a temperature that is suitable for at least partial removal of methane and ethane from C₃+ components in the lower section, and/or to cool the portion of the overhead product. A second heat exchanger may also be included that is configured to cool a vapor portion of a feed gas using refrigeration cold of the overhead product to thereby produce a cooled vapor portion. Where desirable, an expansion device may be added that is configured to reduce the temperature of the cooled vapor portion to thereby form the second reflux stream.

Alternatively, in a further aspect of the inventive subject matter, contemplated plants will include a first column configured to receive a first and a second reflux stream and further configured to receive an expanded feed gas stream. A temperature control unit is typically thermally coupled to the first column and configured to control a temperature in the first column, and a heat exchanger is configured to cool a first and a second portion of the feed gas to thereby form the second reflux stream and a cooled second portion of the feed gas, respectively. A bypass valve is generally configured to control flow volume of a third portion of the feed gas to the cooled second portion of the feed gas, while a control system is provided to adjust the flow volume of the third portion of the feed gas as a function of the temperature in the first column.

Most preferably, a recycle circuit is added that is configured to provide a portion of a compressed overhead product of the first column back to the first column as the first reflux stream. A second column and an expansion device are preferably fluidly coupled with the recycle circuit, wherein the first column is configured to produce a bottom product, wherein the expansion device is configured to receive expanded first column bottom product, and wherein the second column is configured to produce a propane plus liquids bottom product and an ethane overhead product. In such plants, it is generally preferred that the expansion device is configured to reduce pressure of the first column bottom product by at least 50-400 psi. Therefore, the columns are typically operated at a pressure differential of between about 50 psi and about 400 psi, wherein the first column is typically operated at a pressure between about 450 - 700 psig, and the second column is typically operated at a pressure between about 300 - 450 psig. It is further preferred that a further recycle circuit provides at least a portion of the methane and ethane rich overhead product back to the first column. Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention.

Brief Description of the Drawings Figure 1 is a schematic of an exemplary ethane recovery plant with integral intermediate reflux condenser according to the inventive subject matter.

Figure 2 is a schematic of another exemplary ethane recovery plant with two-column configuration according to the inventive subject matter.

Figure 3 is a schematic of a further exemplary ethane recovery plant with two- column configuration according to the inventive subject matter.

Figure 4 is a graph depicting the mole fraction of methane and ethane in the column liquids of a plant according to the inventive subject matter.

Figure 5 is a graph depicting the mole fraction of methane and ethane in the column liquids of a typical known plant.

Figure 6 is a composite curve of the ethane recovery process according to the inventive subject matter.

Detailed Description

The inventors have discovered that flexible and high ethane and/or propane recovery (e.g., at least about 90% C₂ and at least about 99% C₃) can be achieved for a feed gas with relatively high CO₂ content (greater than 2%) using optimum temperature control in the separation column(s). In most configurations, optimum temperature control is achieved with an intermediate reflux condenser in a single-column configuration or by control of the expander discharge in a dual-column configuration. Such temperature control advantageously reduces, if not even entirely eliminates carbon dioxide freezing in the column. Most typically, the fractionator in contemplated configurations will receive at least two lean reflux streams.

In a particularly preferred configuration as illustrated in Figure 1, an exemplary plant includes a fractionation column 58 that is fluidly coupled to an intermediate reflux condenser 61 that is used to provide cooling to a lower rectification section. It should be appreciated that high ethane recovery (e.g., over 95%) is achieved in such configurations at least in part by recycling, cooling, and JT expanding a portion of the lean residue gas, turbo expansion of a vapor portion of the feed gas, and/or refrigeration at the intermediate reflux condenser and at the feed exchanger (the refrigerant streams can be either internally generated or externally supplied). As used herein in the following examples, the term "about" in conjunction with a numeral refers to a range of that numeral starting from 10% below the absolute of the numeral to 20% above the absolute of the numeral, inclusive. For example, the term "about - 100°F" refers to a range of -80°F to -120°F, and the term "about 1000 psig" refers to a range of 800 psig to 1200 psig.

A typical feed gas composition in mole percent is as follows: 2.2 % CO₂, 83.6% C₁,

4.9% C₂, 4.0% C₃, 3.1% C₄ and 2.2% C₅+. The feed gas stream 1, at about 110°F and about 1200 psig, is cooled in heat exchanger 51 with residue gas stream 17, separator liquid stream 8, stream 18 drawn from a chimney tray (typically above intermediate reflux condenser), and refrigerant stream 41 (optional). Feed gas is cooled to about -25°F to about -45°F forming a two phase stream 2 that is separated in the separator 52 into a vapor stream 3, and a liquid stream 4 that is further split into stream 5 and stream 6. It should be noted that the flow ratio (ratio of stream 5 to stream 4) is adjusted as necessary to provide an optimum internal reflux (i.e. stream 6) and stripping of the lighter components (C₁ , C₂ , and CO₂) in the liquids (i.e. stream 5) in the upper section of the column for a specific feed gas composition.

For example, when processing a relatively lean feed gas, the flow ratio (that is, stream

5 to stream 4) is reduced, resulting in an increased flow in stream 6 that is letdown in pressure via JT valve 54 forming a semi-lean reflux stream 7, which is fed to the upper rectification section of column 58. When operated with a relatively rich feed gas, the flow ratio is increased, resulting in an increased flow in stream 5. Stream 5 is letdown in pressure via JT valve 53 forming stream 8, which is used to provide cooling to the feed gas in exchanger 51. The heated stream 9, typically at about -10°, is then routed to the upper mid section just above the intermediate reflux condenser, stripping at least a portion of the lighter components, thereby reducing the reboiler duty in the column. It should be especially recognized that the fractionation column produces at least one liquid stream in the upper section that is heat exchanged with the feed gas, and that is fed to the lower rectification section. This stream then supplies at least a portion of feed gas chilling duty and stripping requirement by the reboiler, thereby advantageously improving removal of undesirable light components and preventing of CO₂ freezing.

Further feed gas chilling is achieved by chilling vapor stream 3 from the high- pressure separator 52 using sub-cooling, JT and turbo-expansion. Here, vapor stream 3 is split into two portions, stream 11 and stream 10. The first portion, stream 11, is expanded in a turbo-expander 55 forming an expanded stream 14, typically at about -95°F to about -115°F, which is introduced to the upper section of column. The second portion, stream 10, is cooled in heat exchanger 56 to stream 12 by overhead vapor stream 16 to typically about -120°F to about -14O°F, and further reduced in pressure and temperature via JT valve 57 forming a sub- cooled reflux stream 13, typically at about -125°F to about -145°F. The so formed stream 13 is then fed to the column as the second reflux stream. It should be appreciated that where relatively rich gas is processed, the vapor split of the vapor stream 3 will be at a flow ratio (i.e., stream 10 to stream 3) ranging from about 0.1 to about 0.3. Leaner gas processing typically will increase the ratio of stream 10 to stream 3. An increase of the compressed residue gas recycle flow (stream 42) generally requires a corresponding change in the flow ratio to maintain high ethane recovery.

To ensure high ethane recovery, absorber 58 also receives a first reflux stream 43 that is formed from cooling (e.g., via JT valve 70, and exchangers 56 and 51, via streams 15 and 19) a portion of the compressed vapor stream 42 from residue compressor 71. It should be noted that about 5% to about 60%, and more typically 10% to about 25% of the total residue gas flow is used as a recycle stream, preferably after the residue gas is cooled at high pressure using ambient cooler 72 (forming stream 31), feed gas exchanger 51, and/or reflux exchanger 56 to about -125°F to about -145°F. The recycle vapor is thus totally condensed and/or sub- cooled that is letdown in pressure in JT valve 70 to about 450 psig to about 700 psig to the column.

It should be particularly pointed out that the fractionation column is self-sufficient in heating requirement and typically does not require external heating for ethane recovery: Stripping of methane from the NGL stream 25 is achieved with vapor stream 7 in the upper section, stream 23 in the lower section, and finally with stream 42 at the bottom of the column. To supply bottom reboiler duty, stream 30 at about 4O°F is withdrawn from the bottom tray, pumped by pump 66 forming stream 32 that is heated in the feed exchanger 51 to about 85°F. The two phase stream 32' is then flashed at the bottom of the column forming the NGL product with suitable methane content (typically 0.02 to 0.6% by volume). Optionally, the high pressure residue gas compressor discharge (stream 38) can also be used to supplement the reboiler duty.

Residue gas from feed exchanger 51 at a temperature of about 80°F to about 100°F (stream 20) is compressed by expander compressor 55, forming residue gas stream 21 that is further compressed by residue gas compressor 71 forming stream 38 at about 1200 psig or other suitable pipeline or delivery pressure. The compressor discharge 38 is typically cooled by an ambient air cooler 72, and about 10% to 25% is diverted as stream 42 and recycled back to column 58 forming the first reflux stream 43 that is required for high ethane recovery (over 90%). The remaining portion of the discharge vapor forms pipeline sales gas stream 39. It should be noted that recycling of the compressed residue gas stream is typically not required when ethane recovery drops below 80%, as the chilling requirements can be satisfied with the feed gas turboexpansion alone.

In especially preferred aspects, the expander discharge is fed to the mid section of the column, at a location below the second reflux that effectively improves the fractionation efficiency over known configurations. With respect to the liquid portion from the feed gas separator, and especially when processing a rich gas, it is preferred that the liquid is split into two portions with one portion being letdown and fed to the column as a cold semi-lean reflux and the second portion being heated in the feed gas exchanger to thereby form a heated vapor that is used for removal of the light components (e.g., methane) in the upper section. Thus, it should be appreciated that in especially preferred aspects of the inventive subject matter, an intermediate reflux condenser coupled to the fractionator operation allows rectification of the vapors from the lower section with refrigeration cooling, subsequently improving stripping of methane and recovery of ethane in the lower section of the column.

The intermediate reflux condenser 61 preferably has a plurality of exchanger surfaces that are cooled with refrigerant stream 40 (e.g., internally generated or externally supplied with propane refrigeration, or with column overhead gas). Ascending vapor stream 83 from the lower section is cooled to about -2O°F to -4O°F in the intermediate reflux exchanger 61, thereby condensing most of the C₂ and heavier components, and a portion of the condensate (i.e. stream 80) is refluxed to the lower section via a downcomer for rectification and recovery of the propane and heavier components. The remaining portion is withdrawn as the ethane product stream 29. The vapor stream 82 ascending from the intermediate reflux exchanger 61 is redistributed in a chimney tray to the upper section of the column. Consequently, a C₂ plus NGL product is produced with a low CO₂ content and low energy consumption while eliminating CO₂ freezing in the column.

Moreover, it should be appreciated that an intermediate reflux condenser is especially advantageous, as it maintains the lower section of the column at temperatures at typically above -4O°F, thereby minimizing methane and maximizing ethane content in the mid to lower section of the column. Thus, where the fractionation column is optionally fluidly coupled to an intermediate condenser (integrated internally or externally), cooling to the rectification section is provided and fractionation efficiency is improved. In contrast, heretofore known plants typically operate the mid section at cryogenic temperatures (-100°F or lower), which increase the vapor liquid traffics and energy consumption (refrigeration and reboiler duties).

It should be especially appreciated that the intermediate reflux condenser significantly improves the fractionation efficiency relative to known processes, which is evident when methane and ethane compositions in the tray liquids are compared. Figures 4 and 5 depict the methane and ethane contents in the tray liquids starting from the top (here: Tray 1) to the bottom (here: Tray 28) of the column. Figure 4 is a plot for methane and ethane compositions in the tray liquids in configurations presently contemplated herein, wherein Figure 5 is the same plot for heretofore known configurations. For example, in configurations according to the inventive subject matter, the methane content in tray 8 has been reduced to about 0.1 mol fraction (Figure 4) as compared to about 0.6 mol fraction of previously known configurations. These composition curves show that contemplated configurations are about five times more effective in stripping methane content from C₂ plus NGL product. Additionally, the temperature profile of the upper column is significantly higher than that of all known configurations, which plays a significant role in eliminating the potential CO₂ freezing problems. Similarly, in terms of ethane recovery, presently contemplated configurations are also more efficient as can be taken from the plots. For example, ethane content in tray 9 is at about 0.5 mol fraction of contemplated configuration (Figure 4), as compared to 0.15 mol fraction of previously known configurations (Figure 5). These composition curves show that contemplated configurations are at least three times more effective than previously known configurations in the ethane recovery. Such comparative composition profiles are indicative of the more efficient stripping of methane and recovery of ethane with the intermediate reflux condenser of contemplated configurations. It is generally preferred that at least a portion of the residue gas compressor discharge is cooled and recycled to the column overhead as a first lean reflux to the column in the recovery of the ethane and heavier components when ethane recovery higher than 90% is desired. Furthermore, with respect to the vapor portions from the feed gas separator, it should be recognized that the reflux vapor portion is fed into an exchanger that is cooled and condensed by the column overhead vapor prior being used as reflux in the column. It is therefore preferred that the column overhead product may act as a refrigerant in at least one, and preferably at two additional heat exchangers, wherein the first column overhead product typically cools at least a portion the feed gas and/or separated vapor portion, and may also provide the second column reflux condensing duty. After heat exchange, the warmed gas may then be recompressed to residue gas pressure of which a portion is then recycled to the column as first lean reflux. Similarly, with respect to the high pressure separator liquid and the liquid withdrawn from the upper section it is generally preferred that such liquids are employed as a refrigerant to cool the feed gas stream before entering the column as column feed. Suitable columns may vary depending on the particular configurations, however, it is generally preferred that the column is a tray or packed bed type column.

Where the feed gas has a relatively high level of CO₂ (e.g., ≥3%) and is substantially depleted of C₃+ components (e.g., <1% C₃+), a two-column configuration may be employed. An exemplary two-column configuration is depicted in Figure 2. It should be noted that substantially the same configuration can be used for high pressure feed gas (e.g., 1200 psig and above) using a two-column configuration as shown in Figure 3, with the first column operating at high pressure (e.g., 450 psig to 700 psig), the second column operating at lower pressures (e.g., 300 psig to 450 psig), and with a methane rich vapor recycled from the second column to the first column with the use of a compressor. This enhanced configuration is more energy efficient when compared to known configurations with single column operating at lower pressure. The contemplated configuration requires less overall power (compression plus external refrigeration) for the comparable ethane recovery.

In addition, it should be appreciated that such high first column pressure is particularly advantageous for higher ethane recovery with lower compression horsepower than known processes, and the corresponding high column temperatures also helps avoid CO₂ freezing problems. It should be recognized that the fractionation column in such configurations is also fed by at least two reflux streams, wherein the first reflux stream is generated by JT expansion of a portion of the chilled compressed residue gas, and wherein the second reflux stream is generated by expansion of a chilled portion of the feed gas. The expander discharge temperature in such configurations is controlled (preferably using a bypass stream from the feed gas) to avoid CO₂ freezing in the column. Moreover, as the second fractionation column operates at a lower pressure than the first column (e.g., by 50 psi to 400 psi lower) and separates the first column bottoms into desirable products, the first column overhead vapor can advantageously be used for refrigeration in reflux condensation for the second column, thus eliminating the need for additional external refrigeration.

It should further be noted that the two column configurations contemplated herein can typically operate without a feed gas separator, due to the feed gas bypass that maintains the chilled gas in superheated state (i.e., without liquid formation), thus avoiding liquid dropout in the expander. Known configurations typically chill the feed gas to lower temperatures requiring a separator for removal of the liquids prior to the expander. It should also be appreciated that the feed gas inlet is split into two streams that are chilled separately and to different temperatures, which is particularly energy efficient as illustrated in Figure 6 by the close temperature approaches of the heat composite curves. In contrast, conventional configurations typically chill the feed gas to a common temperature which is not energy efficient when processing a lean feed gas (with less than 1% C₃ plus components).

A typical lean feed gas composition in mole percent for a two-column configuration is as follows: 0.58% N₂, 3.0 % CO₂, 89% C₁, 7.0% C₂, 0.6% C₃, and 0.07% C₄+. An exemplary two-column configuration typically includes a first fractionation column (demethanizer) that is fluidly coupled to a second fractionation column (deethanizer). Here, a feed gas bypass is used to control the demethanizer tray temperature to thereby avoid CO₂ freezing. It should be appreciated that the residue gas is once more used to provide refrigeration cold to the reflux condenser of the deethanizer, thereby eliminating the need for external propane refrigeration.

In Figure 2, feed gas stream 1, at about 4O°F and about 1250 psig, is split into three streams, 2, 3, and 4. Streams 2 and 3 are separately cooled by the residual gas stream 16 (thereby forming stream 20) in exchanger 51 to different temperatures. Stream 2 (typically at a flow rate of 10% to 30% of stream 1) is cooled, condensed, and subcooled to about -120°F forming stream 12 while stream 3 (at a flow rate of 70% to 90% of stream 1) is cooled to about -10°F forming stream 5. It should be noted that cooling is achieved by self- generated refrigerant (e.g., the demethanizer overhead gas stream 16, column side-draw streams 18 and stream 30), thereby eliminating external refrigeration requirement. The third portion, stream 4, (at a flow rate of about 0% to 5% of stream 1) bypasses exchanger 51 via control valve 60, and is combined with stream 5 forming stream 11, thus maintaining the suction temperature to expander 53 at a desired/predetermined level.

It is generally preferred that the expander suction temperature is controlled using a control unit (not shown) and feedback from temperature sensing elements located in the demethanizer trays. Most typically, the control unit is a microprocessor controlled device that controls operation of the control valve 60 in dependence of a temperature measurement in the first column. Alternatively, or additionally, the control unit may also receive temperature information from the expander outlet, sensors thermally coupled to streams 1, 2, 3, and/or 13, or other streams that directly or indirectly affect column temperature. In less preferred aspects, the control unit may also be replaced (at least temporarily) with manual operator intervention. Increasing the bypass flow of stream 4 will increase the expander discharge temperature, subsequently increasing tray temperatures, and thereby eliminate CO₂ freezing. Furthermore, the higher expander suction temperature has the side benefit of an increase in power output from the expander 55, thus reducing the overall energy consumption. The mixed stream 11 is preferably maintained in a superheated state. Thus, a feed gas separator (commonly used in known processes) is not required in the feed gas circuit. Stream 11 is then expanded via expander 55 to a pressure of about 510 psig, forming stream 14 at about -90°F that is fed to the mid section of demethanizer 58.

Similar to the previous configuration of Figure 1, the demethanizer column is refluxed with two reflux streams. The first reflux (stream 23) is generated by chilling a portion of the residue gas compressor discharge in exchanger 51 to about -120°F, and then by expansion in JT valve 70. The second reflux (stream 13) is produced by chilling by a portion of the feed gas to about -120°F and then by JT expansion in valve 57.

The demethanizer column is reboiled with heat content from feed gas streams 2 and/or 3, and residue gas streams 38 and/or 42, thereby controlling the methane content in the bottom product of demethanizer 58 at about 2 wt% or less. An upper side draw stream 18 at about -5 °F, and a lower side draw stream 30 at about 20°F, coupled with the bottom reboiler (65, heated by stream 38, which then forms stream 31) supply the demethanizer column reboiler duties. Heated upper and lower side draw streams 9 and 23 are returned to the column. The demethanizer produces an overhead vapor stream 16 at about -125°F and about 510 psig, and a bottom stream 24 at about 5O°F and about 515 psig. The overhead vapor is first used to supply cooling in exchanger 51 and then in the deethanizer reflux condenser 62. With this arrangement, it should be appreciated that the NGL plant is self sufficient in refrigeration, thus significantly reducing capital and operating costs.

The residue gas stream 32 from exchanger 62 is compressed by compressor 53 driven by expander 55 forming stream 21 at about 45°F and about 600 psig, which is further compressed by residue gas compressor 59, forming stream 38 at about 1260 psig and about 150°F. At least a portion of the high pressure residue gas is used to supply the demethanizer reboiler duty {supra). Furthermore, at least a portion of the chilled residue gas, stream 42, is recycled, cooled to form stream 19 and JT expanded to the demethanizer, while the remaining portion of the residue gas 39 is delivered to a gas pipeline or downstream processing facility.

The demethanized bottom product is letdown in pressure to about 350 to 450 psig, let down in pressure via valve 67 forming stream 26 that is fed to deethanizer 61. The deethanizer overhead product 27 is condensed in exchanger 62 using refrigeration from residue gas stream 20. The so obtained two phase stream 28 is separated in reflux drum 63, producing a deethanizer reflux stream 30 that is pumped to the column via pump 64 as stream 31, and an ethane product stream 29. The deethanizer is reboiled in reboiler 61 with external heat, producing a propane plus NGL product 25.

It may also be desirable to reduce the CO₂ content in the NGL product even further to the maximum extent that is economically feasible. Among other advantages, a reduced CO₂ content has significant economic benefits, including lower transmission cost, reduced treating requirements, and/or CO₂ emissions. Figure 3 shows an exemplary configuration for the removal of CO₂ from the ethane product using a plant configuration substantially as described above for Figure 2. In the configuration of Figure 3, a compressor 77 is used to compress the CO₂ rich stream from the deethanizer overhead to the demethanizer, while producing an ethane liquid that is even lower in CO₂ content. Although economically attractive, it should be noted that the additional heat from the compressor discharge must be removed. Most typically, this can be achieved using increased residue gas recycle volume. Therefore, it should be appreciated that preferred configurations will avoid CO₂ freezing commonly encountered in conventional processes. Viewed from another perspective, contemplated configurations may also be used to remove CO₂ from the NGL to low levels without impacting ethane recovery. Operation of the components of the process of Figure 3 is similar to the configuration of Figure 2, and with respect to the components and numbering, the same considerations as described for Figure 2 above apply.

With respect to suitable feed gas streams, it is contemplated that various feed gas streams are appropriate, and especially suitable feed gas streams typically include various hydrocarbons of different molecular weight. With respect to the molecular weight of contemplated hydrocarbons, it is generally preferred that the feed gas stream predominantly includes C₁-C₆ hydrocarbons, with Cl components being the dominant component. Suitable feed gas streams may additionally comprise acid gases {e.g., carbon dioxide, hydrogen sulfide) and other gaseous components {e.g., hydrogen). Consequently, particularly preferred feed gas streams are processed and unprocessed natural gas and natural gas liquids.

With respect to the C₂ recovery, it is contemplated that configurations according to the inventive subject matter provide at least 90%, more typically at least 92%, and most typically at least 95% recovery, while it is contemplated that C₃ recovery will be at least 90%, more typically at least 98%, and most typically at least 99%. Further aspects and considerations related to this application are presented in our International patent applications with the publication numbers WO 2005/045338 and WO 03/100334 , both of which are incorporated by reference herein.

Thus, specific embodiments and applications of NGL recovery have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

Claims

CLAIMSWhat is claimed is:

1. A method of operating a plant for NGL recovery from a feed gas, comprising:

separating the feed gas in a refluxed column to thereby produce a residue gas, and . using a portion of the residue gas after cooling as a first reflux;

expanding a portion of the feed gas upstream of the column to thereby form a second reflux; and

controlling temperature of ihe column by using at least one of (1) a control circuit that controls a temperature of an expander discharge stream that is fed to the column and (2) an intermediate reflux condenser disposed between an upper section and a lower section of the column that maintains temperature of the lower section above a temperature sufficient to prevent carbon dioxide freezing.

2. The method of claim 1 wherein the feed gas comprises at least 2% carbon dioxide, and wherein the column is operated such that at least 90% of ethane is recovered from the feed gas.

3. The method of claim 1 wherein the step of controlling temperature of the column is performed using the intermediate reflux condenser, and wherein the upper section of the column generates a liquid product that is used to cool the feed gas.

4. The method of claim 1 further comprising a step of cooling the feed gas and separating the cooled feed gas in a separator into a vapor portion and a liquid portion, wherein a portion of the vapor is further cooled and expanded to form the second reflux.

5. The method of claim 1 wherein step of controlling temperature of the column is performed using the control circuit, and wherein the column is a demethanizer that produces a bottom product.

6. The method of claim 5 wherein the bottom product is fed to a second column that is operated at a lower pressure, and that produces a natural gas liquids bottom product and an ethane overhead product.

7. The method of claim 6 wherein at least part of the ethane overhead product is compressed and routed back to the demethanizer.

8. The method of claim 1 further comprising a step of splitting the feed gas into three streams, wherein the first stream is cooled to a first temperature before entering the column, wherein the second stream is cooled and expanded in a turbo expander before entering the column at a second temperature, and wherein the third stream bypasses the feed exchanger and is used for temperature control by the control circuit.

9. A plant comprising:

a column having an intermediate reflux condenser that is configured to operate at a temperature of between about -20 °F and about -40 °F and that is located between a fractionation section and a rectification section of the column, and wherein the column is further configured to receive a first and a second reflux stream and to produce an overhead product;

a circuit coupled to the column such that a liquid stream is fed from the fractionation section to the rectification section via a heat exchanger that is configured such that the liquid stream is heated in the heat exchanger; and

a recycle circuit that is configured to provide a portion of the overhead product as the first reflux stream to the column.

10. The plant of claim 9 wherein the heat exchanger is a feed gas exchanger, and wherein the feed gas exchanger is configured to heat the liquid stream to a temperature that is suitable for at least partial removal of methane and ethane from C3+ components in the rectification section.

11. The plant of claim 10 wherein the feed gas exchanger is further configured to cool the portion of the overhead product.

12. The plant of claim 9 further comprising a second heat exchanger that is configured to cool a vapor portion of a feed gas using refrigeration cold of the overhead product to thereby produce a cooled vapor portion.

13. The plant of claim 12 further comprising an expansion device that is configured to reduce a temperature of the cooled vapor portion to thereby form the second reflux stream.

14. The plant of claim 9 wherein the column is further configured to provide an ethane product stream and a natural gas liquids product stream.

15. A plant comprising:

a first column configured to receive a first and a second reflux stream and further configured to receive an expanded feed gas stream;

a temperature control unit thermally coupled to the first column and configured to determine a temperature in the first column;

a heat exchanger that is configured to cool a first and a second portion of a feed gas to thereby form the second reflux stream and a cooled second portion of the feed gas, respectively;

a bypass valve configured to control flow volume of a third portion of the feed gas to the cooled second portion of the feed gas; and

a control system that is configured to adjust the flow volume of the third portion of the feed gas as a function of the temperature in the first column.

16. The plant of claim 15 further comprising a recycle circuit that is configured to provide a portion of an overhead product of the first column back to the first column as the first reflux stream.

17. The plant of claim 15 further comprising a second column and an expansion device, wherein the first column is configured to produce a bottom product, wherein the expansion device is configured to receive expanded bottom product, and wherein the second column is configured to produce a natural gas liquids bottom product and an ethane overhead product.

18. The plant of claim 17 wherein the expansion device is configured to reduce pressure of the first column bottom product by at least 50 psi.

19. The plant of claim 17 further comprising a circuit that provides at least a portion of the ethane overhead product back to the first column.

20. The plant of claim 15 further comprising a turbo expander that is configured to receive the cooled second portion and the third portion of the feed gas, and that is further configured to provide expanded feed gas to the first column.