Patent Grant
10520249
The present invention is directed to a process and apparatus for processing a mixed hydrocarbon gas stream such as natural gas, refinery gas, and synthetic gas streams. More particularly, the present process and apparatus provide an enhanced and sharper separation of hydrocarbon mid-components (i.e., C3 and/or C4 hydrocarbons) from a mixed hydrocarbon gas stream. The present invention further provides the capability to direct a desired portion of the mid-components to either a gaseous product comprising the majority of C1, C2, hydrocarbons or a liquid product comprising the majority of C5 and C6+ hydrocarbons, or a chosen amount to both gas and liquid products in order to maximize profitability at any time.
Raw or mixed hydrocarbon gas consists primarily of methane (CH4) but also includes heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8), butanes (C4H10), pentanes (C5H12), higher molecular weight hydrocarbons (C6+), and other hydrocarbon species and non-hydrocarbons associated with the raw gas source.
The relative amount of the heavier gaseous hydrocarbons, or richness, can be expressed in terms of gallons per mcf (thousand cubic feet), abbreviated as GPM. In this embodiment, GPM includes all hydrocarbon components heavier than methane and represents the total volume, in liquid gallons, contained in one thousand cubic feet of a particular gas at standard conditions.
The raw gas must be purified to produce a gas product that meets the quality standards specified by a particular gas transmission pipeline (“sales gas”). Typically, one of the objectives of gas processing is to remove liquefiable hydrocarbons commonly referred to as hydrocarbon gas liquids (HGL) comprised of propane, butanes, pentanes and higher molecular weight C6+ hydrocarbons to meet the pipeline gas quality specification desired. A second objective is often then to remove further HGL (deeper cut, e.g., incremental C3+ and perhaps even ethane, C2) for economic gain. If the raw gas stream contains objectionable quantities of non-hydrocarbon compounds such as sulphur compounds or carbon dioxide, these are typically removed by pre-treatment processes not shown or discussed here. Similarly, excess water vapor in the raw gas stream is removed through dehydration.
A conventional approach to removing the HGL is to use the straight refrigeration process as described in the Gas Processors Suppliers Association Engineering Data Book, Chapter 16, 13th Edition. While there are several configurations known in the art for the straight refrigeration process,
The cold gas 117 is sent to a cold gas separator (cold separator 105), which is also referred to in the literature as a low temperature separator or LTS, where the condensed liquids 122 are separated from gas. The residual gas stream 118 from the cold separator 105 is returned to the gas-to-gas heat exchanger 102, and is warmed by the incoming raw gas stream 112. The warmed residual gas 119 is dry relative to the rich gas feed stream 110 and is often intended to be conveyed to the gas transmission pipeline for sale (dry sales gas 120).
Liquids 122 from the cold separator 105 are warmed in the gas-to-liquid exchanger 101 and the warmed liquids 124 are then expanded through adjustable valve 106 which functions to hold a constant liquid level in the cold separator 105 and reduce the liquid stream pressure (stream 125) before stream 125 enters the fractionation column or tower 108. Heat exchangers 101 and 102 reduce the energy requirement in gas chiller 103, by utilizing the energy already expended to chill the cold gas stream and transferring energy from warm to cold streams.
The fractionation tower 108 comprises a reboiler 156, which provides heat and generates vapors to drive the distillation or fractionation process. The fractionation tower 108 distills the co-absorbed light components (primarily C1, C2, and sometimes C3, and C4 hydrocarbons) from the liquid stream to meet the HGL quality specifications of a liquids transporter and downstream refinery or fractionation facility. The fractionation tower may further comprises a reflux condenser (not shown) to improve separation of the light hydrocarbon components from the heavier ends liquid stream.
However, the composition of the HGL from the conventional refrigeration process is somewhat inflexible as discussed later,
The overhead gas stream 126 from the fractionation tower 108 is then compressed in overhead compressor 109 to produce recycle gas 127. Recycle gas 127 is recycled by combining with the rich gas feed 110 and reprocessed. The bottom liquids product 130 contains the HGL extracted from the gas stream. In one alternative, the overhead compressor gas 109 can be added as gas 128 to residual gas 119 to produce dry sales gas 120.
In the conventional refrigeration process, the “mid-components”, which are defined herein as C3 and C4 hydrocarbons, are extracted from the rich gas feed as liquid product and are therefore found in the HGL product. However, there may be times when market demand and product pricing does not economically support the extraction of the propane and butane from the raw gas, and, therefore, rejection of these constituents from the HGL product to the gas product is desired. In other words, there is more value for these mid-components to remain in the gas stream rather than be recovered as HGL product, provided gas transmission pipeline specifications are met.
There may also be times where the extraction of the heavier hydrocarbons is desired close to the source of the raw gas, and the extraction of the mid-components is desired at an alternate downstream gas processing plant, generally distant from the source and which feedstock often comprises an aggregation of several residue gas streams, to achieve economies of scale and close proximity to consumer markets.
To maintain these mid-components in the residue gas steam in a conventional refrigeration process, conventional practice has been to: adjust the temperature in gas chiller 103 to a higher temperature so as to reduce the amount of these constituents condensed and separated in cold separator 105 as liquid stream 122; and/or adjust the operating conditions in the fractionation tower 108 and reboiler 156 in order to reject the co-absorbed mid components into stream 126 and recycle stream 127 back into the rich gas feed stream 110; and in very rich raw gas feed streams, conventional practice is to re-direct the fractionation overhead gas stream 127 from combining with the raw gas feed stream 110, to combining with the residue gas stream 119, via stream 128, to form dry Sales Gas 120.
However, the above practices to maintain mid-components in the dry sales gas stream 120 can result in low recoveries of the constituents that are desirable in the HGL product stream 130. In other words, economic value is lost because a portion of the heavier hydrocarbons such as C6+, pentanes, and sometimes butanes remain in the dry sales gas stream 120. Therefore, there is a need in the industry for a process and apparatus that is capable of customizing the amount of propane and butane retained in a sales gas product from a raw gas stream without compromising the recovery of valuable heavy hydrocarbons such as C5 and C6+ components (HGL products) from the raw gas stream.
The present invention is directed to a process and apparatus for purifying a mixed hydrocarbon gas stream by removing hydrocarbon gas liquids (HGL) therefrom to produce a gas product that meets the sales gas quality standards intended. The present invention may be used to purify a variety of gases such as natural gas, refinery gas, and synthetic gas streams. In particular, the present invention is directed to a process and apparatus with the flexibility to direct mid-component hydrocarbons (i.e., C3 and C4 hydrocarbons) contained in a mixed hydrocarbon gas stream (C1 to C6+) to produce a gas product comprising the majority of C1 and C2 components and having a desired proportion of mid-components (i.e., C3 and C4 hydrocarbons) and a liquid product comprising the majority of C5 and C6+ hydrocarbons and having a desired proportion of mid-components. In other words, the present invention provides the versatility to produce either: a C3+ rich HGL stream, with a very high percentage recovery of the C4+ components; or a high recovery C4+ HGL stream; or strictly a very high recovery C5+ HGL stream; with all other components remaining in the sales gas stream.
In one aspect, the present invention provides a process and apparatus that enhances hydrocarbon gas liquids (HGL) recovery from mixed hydrocarbon gas streams when compared to a conventional refrigeration process. More particularly, a process and apparatus is provided which includes the addition of an absorber between a gas/gas heat exchanger and a gas chiller of a conventional refrigeration process. Cold separator liquids are pumped to the top of the absorber as a rectifying solution. The enhancement results in very high recovery of heavy hydrocarbons such as C5+ components from the gas stream, with the potential to direct a desired proportion of mid-components to the gas product or the HGL product. Furthermore, the enhancement often results in lower utility loads for very rich gas streams as compared to the conventional refrigeration process.
Thus, in one aspect, a process is provided for separating a mixed or raw gas feed such as natural gas, refinery gas and synthetic gas, the gas feed containing methane, C2 components, C3 components, C4 components and heavier hydrocarbon components (C5+), into a dry gas product containing a portion of C3 and C4 components, comprising:
In one embodiment, the process includes cooling the gas feed prior to scrubbing to reduce energy consumption required for cooling the lighter ends gas stream. In another embodiment, the process further comprises flash distillation and/or fractionating the heavier ends liquid stream to form a hydrocarbon gas liquid product and an overhead gas stream. In one embodiment, the overhead gas stream is compressed and the compressed gas stream is combined with the mixed or raw gas feed. In one embodiment, the hydrocarbon gas liquid product comprises the majority of C5+ hydrocarbon components from the mixed or raw gas feed.
In another aspect, an apparatus is provided for separating a mixed or raw gas feed such as natural gas, refinery gas and synthetic gas, the gas feed containing methane, C2 components, C3 components, C4 components and heavier hydrocarbon components (C5+) into a dry gas product containing a portion of C3 and C4 components, comprising:
In one embodiment, the apparatus further comprises at least one second cooling device for cooling the gas feed prior to sending it to the absorber. In another embodiment, the apparatus further comprises a feed pump for pumping the condensed liquids to the absorber to assist in scrubbing heavier hydrocarbon components from the gas feed. In another embodiment, the apparatus further comprises a flash separator and/or a fractionation tower for receiving the heavier ends liquid stream from the absorber and fractionating the heavier ends liquid stream to form hydrocarbon gas liquids and an overhead gas stream, In another embodiment, the apparatus further comprises an overhead gas compressor to increase the pressure of the overhead gas stream to form a recycle gas stream for reprocessing. In one embodiment, the recycle gas stream is added to the mixed or raw gas feed prior to further processing.
In another aspect, an improved apparatus is provided for separating a mixed or raw gas feed including natural gas, refinery gas, and synthetic gas, the gas feed containing methane, C2 components, C3 components, C4 components and heavier hydrocarbon components (C5+), into a dry gas product containing a portion of C3 and C4 components, said apparatus comprising in series at least one heat exchanger, a gas chiller, a cold separator and a gas/liquid fractionator, said improvement comprising:
Other features will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific embodiments, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Referring to the drawings wherein like reference numerals indicate similar parts throughout the several views, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the following figures. It is understood that the drawings provided herein are for illustration purposes only and are not necessarily drawn to scale.
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
The purpose of gas processing such as natural gas processing is to convert raw natural gas into sales gas and HGL which can be delivered to end user markets, In other words, gas processing conditions gas to commercial specifications, e.g., hydrocarbon dew point (HCDP), water content, heating value, and other qualities as specified by a particular gas transmission or distribution company. Further, gas processing allows for the recovery of higher value liquefied products such as C2, C3, C4, and C5+ hydrocarbons, processed by one or more fractionation steps to meet commercial specifications such as hydrocarbon component content, vapor pressure, and density. As used herein “rich gas” means a gas which contains heavier hydrocarbons and is typically between temperatures of 30° F. and 120° F., and is typically provided at pressures between 200 Psig and 1000 Psig.
However, unlike in the conventional refrigeration process shown in
The cold gas 217 is then fed to a cold gas separator 205, where the condensed liquids 222 are separated from the gas stream. The dry residue gas stream 218 that is produced in cold gas separator 205 is directed to gas/gas heat exchanger 202 and is used to cool gas stream 212. It is understood that heat exchanger 202 aids in reducing the energy requirement in gas chiller 203, and elevates the temperature of the sales gas stream 220 for further processing or transmission. The warmed sales gas 220 is dry relative to the rich gas feed stream 210, and is often intended to be conveyed to the gas transmission pipeline for sale (dry sales gas 220). It is understood, however, that warming the sales gas stream may not be necessary if the sales gas stream is intended to go to further cryogenic processing, for example, ethane production or liquefied natural gas (LNG) production.
Condensed liquids 222 produced in cold separator 205 are then pumped via feed pump 251 and returned to the top of absorber 250 as liquid stream 223. The counter-current flow of cooled gas stream 213 and liquid stream 223 allow the two streams to contact one another in the absorber 250 and, thus, provides multiple stages of contact to alter the composition of both the gas stream produced (gas stream 216) and the liquid stream 224. In particular, liquid stream 224 will contain fewer light hydrocarbons (e.g., C1 and C2 hydrocarbons) than in stream 223 and additional heavy hydrocarbons (e.g., C5+ hydrocarbons), which have been removed (scrubbed) from the cooled gas stream 213. Hence, the heavy hydrocarbons are scrubbed from the cooled gas stream 213 and the light hydrocarbons are stripped from light liquid stream 223. Thus, the absorber 250, gas chiller 203 and cold separator 205, in combination, operate as a rectifier column, reducing the light end components and increasing the heavier components in the absorber 250 bottom liquid product stream 224, thereby providing sharp separation between light key components and heavy key components.
The liquid stream 224 is removed from the bottom of absorber 250 and flash expanded through expansion (adjustable) valve 206 to the operating pressure of fractionation tower 208. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. The expanded stream 225 leaving expansion valve 206 is sent to fractionating tower 208 to distill the light ends (e.g. methane, ethane and propane) from the liquid, resulting in heavy liquid product 230 and light ends gaseous stream 226. The fractionation tower 208 comprises a reboiler 256, which provides heat and generates vapors to drive the distillation or fractionation process. The fractionation tower pressure typically ranges between 80 Psig to 350 Psig and reboiler temperatures range may from 140° F. to 320° F. The fractionation tower provides the versatility and ability to reject the desired proportion of mid-components from the heavy liquid product stream 225, back into the raw gas as the overhead gas stream 226. The selected operating pressure and temperature is dependent in part on what portions of mid-components are to be rejected, and the composition of the heavy liquid stream. In one embodiment, the fractionation tower 208 may further comprises a reflux condenser (not shown) to improve separation of the mid-components from the heavier ends liquid stream.
The overhead gas stream 226 from the fractionation tower 208 is then compressed in overhead compressor 209 to produce recycle gas 227. Recycle gas 227 is recycled by combining with the rich gas feed 210 and reprocessed. The bottom heavy liquids product 230 contains the HGL extracted from the gas stream.
In comparison to the conventional refrigeration process shown in
In one embodiment, the conventional refrigeration process equipment shown in
The cold gas 317 is then fed to a cold gas separator 305, where the condensed liquids 322 are separated from the gas stream. The dry residue gas stream 318 that is produced in cold gas separator 305 is directed to gas/gas heat exchanger 302 and is used to cool gas stream 312. It is understood that heat exchangers 301 and 302 aid in reducing the energy requirement in gas chiller 303, and elevate the temperature of the sales gas stream 320 for further processing or transmission. The warmed sales gas 320 is dry relative to the rich gas feed stream 310, and is often intended to be conveyed to the gas transmission pipeline for sale (dry sales gas 320). It is understood, however, that warming the sales gas stream may not be necessary if the sales gas stream is intended to go to further cryogenic processing, for example, ethane production or liquefied natural gas (LNG) production.
When block valve 353 is in the closed position, condensed liquids 322 produced in cold separator 305 are pumped via feed pump 351 and returned to the top of absorber 350 as liquid stream 323. The counter-current flow of combined cool gas stream 331 and liquid stream 323 allow the two streams to contact one another in the absorber 350 and, thus, provides multiple stages of contact to alter the composition of both the gas stream produced (gas stream 316) and the liquid stream 332. In particular, liquid stream 332 will contain fewer light hydrocarbons (e.g., C1 and C2 hydrocarbons) than in stream 323 and more heavy hydrocarbons (e.g., C5+ hydrocarbons), which have been removed (scrubbed) from the combined cool gas stream 331. Hence, the heavy hydrocarbons are scrubbed from the combined cool gas stream 331 and the light hydrocarbons are stripped from the liquid. Thus, in this embodiment, the absorber 350, gas chiller 303 and cold separator 305, in combination, also operate as a rectifier column, reducing the light end components and increasing the heavier components in the absorber 350 bottom liquid product stream 332, thereby providing sharp separation between light key components and heavy key components.
The liquid stream 332 is removed from the bottom of absorber 350 and passes into Gas/Liquid heat exchanger 301, being warmed by the raw gas stream 314. Stream 324 exits Gas/Liquid heat exchanger 301 and is flash expanded through expansion (adjustable) valve 306 to the operating pressure of fractionation tower 308. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. The expanded stream 325 leaving expansion valve 306 is sent to fractionating tower 308 to distill the light ends (e.g. methane, ethane and propane) from the liquid, resulting in heavy liquid product 330 and light ends gaseous stream 326. The fractionation tower 308 comprises a reboiler 356, which provides heat and generates vapors to drive the distillation or fractionation process. In one embodiment, the fractionation tower 308 may further comprises a reflux condenser (not shown) to improve separation of the light ends (e.g. methane, ethane and propane) from the heavier ends liquid stream.
The overhead gas stream 326 from the fractionation tower 308 is then compressed in overhead compressor 309 to produce recycle gas 327. Recycle gas 327 is recycled by combining with the rich gas feed 310 and reprocessed. The bottom heavy liquids product 330 contains the HGL extracted from the gas stream.
Thus, in the embodiment shown in
A comparison of the composition of the sales gas produced from the conventional refrigeration process (
It can be seen from Table 1 that when using the present invention (Enhanced), much more propane and i-butane report to the sales gas and much less C5+ hydrocarbons (i.e., i-pentane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane and n-decane) are present therein when compared to the sales gas of the Prior Art (Conventional). Further, essentially no heavy hydrocarbons (C5+ components) were found in the sales gas. Hence, the enhancement of the present invention results in high recovery (removal) of heavy hydrocarbons (C5+ components) from the rich gas feed and more rejection of lighter hydrocarbons such as C3 and i-C4 to the sales gas product. Yet, both conventional and enhanced sales gas streams achieve the same HCDP.
Thus, under prescribed conditions, the present invention (Enhanced) produces a more valuable HGL stream per unit of volume than when using the Prior Art conventional refrigeration process. This can be seen more clearly in Table 2.
In particular, when targeting a sales gas HCDP of 23° F. at 800 Psig, it can be seen from Table 2 that the HGL stream produced using the Conventional process yields 293.3 bbl/d propane, 408.4 bbl/d of butanes and 240.7 bbl/d of pentanes+ (C5+). On the other hand, when using the Enhanced process of the present invention, only 38.7 bbl/d of propane are yielded in the HGL stream. This results in a marginally higher sales gas volume with slightly higher heating value due to more propane (C3) being directed to the sales gas stream, which may be favorable for specific economic conditions. Further, 267.8 bbl/d of pentanes+ (C5+) are yielded in the HGL stream (as compared to 240.7 bbl/d in Conventional), resulting in better recovery of heavy hydrocarbons in the HGL product.
Furthermore, Table 2 shows that when using the Enhanced process of the present invention the refrigeration compressor load decreases 27.8%, and the overhead compressor load decreases 22.6%, resulting in significantly lower utility requirements and slightly smaller equipment having lower capital costs, when comparing to the conventional refrigeration process (Prior Art). Further, the cold separator can be operated at a much higher temperature, i.e., 11.1° F. versus −4.0° F. with the conventional refrigeration process.
Table 3 below further illustrates the difference in intermediate liquid streams compositions generated in the conventional refrigeration of the prior art as compared to the present invention (Enhanced). Specifically, in the present invention, stream 222, which is the liquid stream produced in the cold separator 205, contains much less C5+ (heavy ends) as compared to stream 122 in the conventional refrigeration process. This is because the C5+ (heavy ends) have already been stripped out of the raw gas in the absorber 250, and, thus, a much lighter liquid product is produced in cold separator 205 to send to the absorber 250 as a scrubbing fluid.
Thus, liquid streams 224, 225, which are produced in the absorber 250 and fed to the fractionation tower 208, respectively, will contain higher volume flows of C4 (and C5+ heavier components) and much less C3 and lighter components when compared to liquid streams 122, 124 and 125 of the conventional art, which streams are produced in the cold separator 105, cooled in gas/liquid exchanger 101, and fed into fractionation tower 108, respectively.
Table 4 below shows the flexibility of the present invention using the same rich gas feed as in Example 1. In this example, the objective is to obtain the maximum liquids possible, including propane and butanes, while the refrigeration compressor load is limited to 1151 HP for both the Conventional and Enhanced process.
Under the prescribed conditions, the present invention (Enhanced) is capable of achieving a lower Cold Separator temperature and consequently recovering higher amounts of HGL than when using the conventional refrigeration process (Conventional). Once again, the sales gas composition of the Enhanced process contains a lesser amount of butanes (C4) and pentanes+ (C5+) than does the Conventional process. As well, the utility requirements for the Overhead compressor load and Fractionation Reboiler Duty are significantly less.
Table 5 further shows the flexibility of the Enhanced process of the present invention using the same rich gas feed as in Example 1 and Example 2 above. Example 3 reflects the ability to reject propane into the sales gas stream, yet with high butane recovery in the HGL stream. Example 4 reflects the ability to direct both propane and butane into the sales gas stream to produce a de-butanized condensate (C5+) product stream.
In Example 3, the results show a sharp separation between C3 (propane) recovery at 2.99% and i-C4 (i-butane) recovery at 86.32%. Example 4, under different operating conditions, shows a sharp distinction between n-C4 (n-butane) recovery of 4.94%, and iC5 (i-pentane) recovery of 92.02%.
Examples 1 to 4 demonstrate the flexibility of the present invention and enable one to respond to market conditions to maximize the profitability from a raw gas stream.
In Example 5, the process of the present invention is used (Enhanced refrigeration) to recover sales gas and HGL product. The cold separator is operated at −13° F. and 600 Psig and various gas feeds of varying richness were contemplated. The relative richness or amount of the heavier gaseous hydrocarbons, can be expressed in terms of gallons per mcf (thousand cubic feet), abbreviated as GPM. In this embodiment, GPM includes all hydrocarbon components heavier than methane and represents the total volume, in liquid gallons, contained in one thousand cubic feet of a particular gas at standard conditions.
Recovery of C6+ (heavy hydrocarbons), C5 (pentane) and C4 (butane) in the HGL product were determined and the results are shown in
Also significant is the fact that, when using the present invention (Enhanced) with any of the raw gas feed richness examined, essentially no propane is recovered in the HGL product. Thus, when using the present invention (Enhanced), essentially the total amount of propane can report to the sales gas stream under selected conditions. However, as demonstrated in Example 2, a significant amount of propane recovery is possible.
In Example 6, the cold separator was operated at 600 Psig and various operating temperatures of the cold separator were contemplated using a raw gas feed stream having a richness of 5 GPM employing both the conventional prior art process (Conventional) and the process of the present invention (Enhanced). The graph in
In Example 7, the cold separator was operated at −13° F. and various operating pressures between 200 and 1200 Psig of the cold separator were contemplated, using a raw gas feed stream having a richness of 5 GPM. In this example, it was desirable to have all of the C3 report to the sales gas stream and the majority of the C4 and C5 report to the HGL stream. Both the conventional prior art process (Conventional) and the process of the present invention (Enhanced) were investigated. The graph in
The present invention provides the potential to produce a sales gas stream meeting pipeline specifications at a higher pressure when compared to the Prior Art (Conventional). This may be an advantage when the gas transmission pipeline operates at pressures between 900 and 1200 Psig.
In Example 8, a gas feed stream having a richness of 5 GPM was used. The cold separator was operated at 600 Psi and various operating temperatures of the cold separator were tested using both the conventional prior art process (Conventional) and the process of the present invention (Enhanced). The butane recovery (%) versus refrigeration load (HP/MMscf) was determined. It can be seen in graph shown in
The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
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