Patent Application
20200088465
The present invention relates to processes and apparatuses for the extraction of helium. In particular, the invention relates to the separation of helium from a natural gas stream comprising methane, nitrogen, and helium using cryogenic distillation.
Helium exists in many natural gas deposits worldwide, but there is a growing interest in efficiently recovering helium from natural gas deposits with low concentrations of helium, e.g. below 2000 ppmv. Recovery of helium from natural gas at these low levels has long been considered uneconomical. Helium recovery from natural gas occurs normally as a by-product of liquefied natural gas (LNG) production or nitrogen rejection. In both cases methane is condensed and the lighter helium is easily recovered as a gas. The present invention relates to the case in which the natural gas stream does not require liquefaction or nitrogen rejection. In this case, the gas may still contain significant nitrogen, but not enough to prevent the natural gas from being used in a pipeline or gas turbine.
Helium extraction from natural gas is known. Gottier (U.S. Pat. No. 5,011,521) teaches helium extraction using a stripping column to enrich the helium concentration above the feed gas composition. Helium enrichment is limited to the action of the stripping column, in the example given as roughly one order of magnitude, from 0.44% to 5.16% helium. The aim of enriching helium in the overhead stream is to reduce the flow to the helium purifier by increasing the helium molar fraction. No additional means to enrich the helium in the stream leaving the top of the stripping column prior to entering the purifier are disclosed.
Gottier also discloses the use of a dense fluid expander (DFE) to recover energy from expanding a higher pressure stream to a lower pressure to feed a distillation column. Operating the distillation column at a higher pressure incurs higher capital costs due to the difficulty of effecting a separation at high pressure and the complexity of supplying reboiler duty to the distillation column. The difficult separation results in a higher reboiler duty for a given helium recovery, which causes a higher vapor flow rate. The higher vapor flow rate coupled with unfavorable surface tension and vapor-liquid density ratio leads to larger column diameters. To avoid these disadvantages, the feed pressure is reduced prior to entering the distillation column.
Oeflke (US2014/0137599) teaches an additional separation to further enrich the helium content of the overhead stream from the stripping column. The overhead stream is cooled and reduced in pressure to form a helium-rich vapor stream and a helium-depleted liquid stream. The helium-depleted liquid stream, which still contains some helium, is pumped and combined with the helium-depleted natural gas from the bottom of the stripping column. The helium not recovered from the helium-depleted liquid stream reduces overall recovery by 0.4% according to the example given. Furthermore, the pressure of the helium-rich vapor stream is reduced from 550 psia to 100 psia in the example which may require recompression to enter the downstream helium purification step.
Mitchell et al (U.S. Pat. No. 4,758,258) teach a multistage separation for recovery of helium from natural gas along with separation of ethane, propane, and heavier hydrocarbons from the bulk methane. It is similar to Oeflke in two respects. First, the refrigeration for the final separation of helium and nitrogen from methane is achieved by reducing the pressure of the feed to the separator to produce a crude helium stream. Second, the helium contained in the liquid stream from the separator is not recovered, reducing the overall helium recovery.
Agrawal (U.S. Pat. No. 5,167,125) teaches a process where light gases, such as helium, are removed by partially condensing the overhead vapor from a distillation column. The liquid stream formed provides reflux to the distillation column and the helium-enriched vapor stream can be further purified.
In order to minimize the power required in helium extraction processes described in the prior art, intermediate streams that contain small but significant amounts of helium are rejected to the helium-depleted natural gas product, lowering overall helium recovery. There is a need for achieving the highest possible overall helium recovery by recovering helium from intermediate streams in a power-efficient manner.
This invention relates to a multi-step process to extract helium from a natural gas stream optimized for high helium recovery and low power consumption. First, contaminants are removed as needed, for example CO2 by amine absorption, water and heavy hydrocarbons by temperature swing adsorption, and/or mercury by adsorption on activated carbon. Next helium is extracted using a cryogenic distillation column system. The helium content in the column overhead stream is enhanced with a condenser to recover nitrogen and methane, both increasing methane recovery and reducing the flow rate to downstream helium purification. The crude helium stream passes to a cryogenic partial condensation process to further increase the helium concentration before hydrogen is removed by catalytic combustion. Final purification is by pressure swing adsorption (PSA), from which the tail gas is recompressed, dried and recycled. The pure helium product from the PSA can then be liquefied for transport and sale.
The helium-depleted liquid from the bottom of the distillation column system is used to provide refrigeration to the process. Multiple pressures are chosen for the refrigerant to optimize the cooling curves and thus the efficiency of heat transfer. Some of the helium-depleted liquid is pumped to minimise overall recompression power. All of the returning natural gas streams are recompressed to match the feed pressure if returning to a pipeline, or are recompressed to whatever pressure is required for utilization of the natural gas, e.g. combustion in a gas turbine.
The pressure of the distillation column system is selected to reduce the risk of poor separation resulting from operating at too high of a pressure. To mitigate the increased power demand, a dense fluid expander (DFE) can be used to generate power that can be used in the process by expanding the feed stream to column pressure. Expanding the fluid isentropically through a DFE also produces a lower temperature in the outlet stream than would be produced by expanding isenthalpically though a valve. Using a DFE saves power for an increased capital cost, and must be optimized accordingly. The process can also utilize an expander on one or more of the returning streams to reduce overall net power consumption and provide refrigeration to the process.
The present invention will hereinafter be described in conjunction with the appended figures wherein like numerals denote like elements:
The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention, as set forth in the appended claims.
The articles “a” or “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.
The term “and/or” placed between a first entity and a second entity includes any of the meanings of (1) only the first entity, (2) only the second entity, and (3) the first entity and the second entity. The term “and/or” placed between the last two entities of a list of 3 or more entities means at least one of the entities in the list including any specific combination of entities in this list. For example, “A, B and/or C” has the same meaning as “A and/or B and/or C” and comprises the following combinations of A, B and C: (1) only A, (2) only B, (3) only C, (4) A and B and not C, (5) A and C and not B, (6) B and C and not A, and (7) A and B and C.
The term “plurality” means “two or more than two.”
The adjective “any” means one, some, or all, indiscriminately of quantity.
The phrase “at least a portion” means “a portion or all.” The “at least a portion of a stream” has the same composition, with the same concentration of each of the species, as the stream from which it is derived.
As used herein, “first,” “second,” “third,” etc. are used to distinguish among a plurality of steps and/or features, and is not indicative of the total number, or relative position in time and/or space, unless expressly stated as such.
All composition values will be specified in mole percent.
The terms “depleted” or “lean” mean having a lesser mole percent concentration of the indicated component than the original stream from which it was formed. “Depleted” and “lean” do not mean that the stream is completely lacking the indicated component.
The terms “rich” or “enriched” mean having a greater mole percent concentration of the indicated component than the original stream from which it was formed.
“Downstream” and “upstream” refer to the intended flow direction of the process fluid transferred. If the intended flow direction of the process fluid is from the first device to the second device, the second device is downstream of the first device. In case of a recycle stream, downstream and upstream refer to the first pass of the process fluid.
The term “dense fluid expander,” abbreviated DFE, also known as a liquid expander, refers to equipment that extracts mechanical work from lowering the pressure of a dense fluid such as a liquid or a supercritical fluid, similar in function to an expander for gases. This expansion is best approximated as an isentropic process, as opposed to a valve which is best approximated as an isenthalpic process.
The term “indirect heat exchange” refers to the process of transferring sensible heat and/or latent heat between two or more fluids without the fluids in question coming into physical contact with one another. The heat may be transferred through the wall of a heat exchanger or with the use of an intermediate heat transfer fluid. The term “hot stream” refers to any stream that exits the heat exchanger at a lower temperature than it entered. Conversely, a “cold stream” is one that exits the heat exchanger at a higher temperature than it entered.
The term “distillation column” includes fractionating columns, rectifying columns, and stripping columns. The distillation column may refer to a single column or a plurality of columns in series or parallel, where the plurality can be any combination of the above column types. Each column may comprise one or more sections of trays and/or packing.
The term “reboiling” refers to partially vaporizing a liquid present in the distillation column, typically by indirect heat exchange against a warmer process stream. This produces a vapor that facilitates mass transfer within the distillation column. The liquid may originate in the bottoms liquid or an intermediate stage in the column. The heat duty for reboiling may be transferred in the distillation column using an in situ reboiler or externally in a heat exchanger dedicated for the purpose or part of a larger heat exchanger system. The vapor-liquid separation also may take place within the distillation column or within an external flash vessel.
The present apparatus and process are described with reference to the figures. In this disclosure, a single reference number may be used to identify a process gas stream and the process gas transfer line that carries said process gas stream. Which feature the reference number refers to will be understood depending on the context.
For the purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.
The natural gas feed described in the present invention refers to a gas comprising hydrocarbons, usually originating underground in a geological formation. The natural gas is typically produced at a pressure ranging from about 1 to about 200 bar. All pressures referred to are absolute, not gauge. The pressure of the natural gas is preferably from about 10 to about 100 bar.
The methane content in natural gas typically ranges from about 50% to about 99%. All composition percentages referred to are in volume, or molar, basis, not weight basis.
The nitrogen content in natural gas typically ranges from about 1% to about 50%, or from about 10% to about 35%.
The helium content in natural gas typically ranges from about 0.01% to about 10%. Some embodiments of the present invention are directed to extracting helium from natural gas comprising from about 0.05% to about 1.0%, or from about 0.05% to about 0.2% helium.
The acid gases in stream 2 can be vented to the atmosphere or sent to sulfur removal as needed. There are several options for acid gas removal, including pressure swing adsorption, vacuum swing adsorption, or methanol absorption, which in the following examples presented herein is assumed to be an amine absorber regenerated with steam.
The contaminant-lean natural gas leaving A in stream 3 now contains an acceptably low level of acid gases, typically at a specification of less than about 100 ppmv. If an amine absorber is used, stream 3 will be saturated in water vapor that would solidify in the downstream cryogenic process. Stream 3 would therefore feed dehydration unit D which preferably comprises a temperature swing adsorber (TSA) and a mercury guard bed comprising activated carbon, both well-known in the art for water and mercury removal, respectively. The TSA removes water, CO2, and aromatics such as benzene, toluene, and xylene (collectively known as BTX). Specifications are set to prevent the formation of a solid phase in the cryogenic process; for example the water specification is often about 1 ppmv.
The impurities are adsorbed and then removed when the TSA is regenerated. Regeneration requires both heat, which can be provided by electrical heaters or process steam, and a process stream to carry the impurities out of the TSA, such as nitrogen or a portion of a helium-depleted stream from the helium extraction unit X. Depending on the pressure required to regenerate the TSA, that stream may be at least a portion of a low-pressure return stream 27 or the helium-depleted natural gas stream 29. Shown in
The natural gas feed stream 5 enters the helium extraction unit X, which is the subject of the present invention. The helium extraction unit is a cryogenic process that separates at least 99% of the helium, or at least 99.5% of the helium from the natural gas feed stream, which can contain from 0.05% to 1.0% helium by volume, or from 0.05% to 0.2% helium by volume, to create a crude helium stream, which can contain 4 to 20% helium by volume. The cryogenic process is designed for maximum efficiency at this high helium recovery rate that requires careful heat integration to reduce the overall power requirements of the process.
The low-pressure return stream 27 is recompressed in compressor R if needed to be returned to the pipeline, combusted in a gas turbine, or otherwise utilized as helium-depleted natural gas 29. The low pressure return stream may leave the extraction unit as one or more streams at different pressures. For an example, see streams 27 and 28 in
Crude helium stream 16 is sent to helium purification unit P to produce a pure helium stream 30 with a typical specification of 99.99% or 99.999%. Within the helium purification unit (and therefore not shown in
Alternate embodiments of the helium purification unit are well known in the art. Blackwell and Kalman (U.S. Pat. No. 3,599,438) describe helium purification in more detail, including the steps of hydrogen removal by catalytic oxidation, dehydration by adsorption, and helium enrichment by partial condensation. Blackwell and Kalman also show the recycle of the intermediate pressure helium stream (16). Kirk-Othmer Encyclopedia of Chemical Technology, “Cryogenic technology,” (2012) also describes alternative helium purification arrangements. For example,
The pure helium stream 30 can be sold as a gaseous product, but more commonly it is liquefied in helium liquefier L to produce a liquid helium stream 32 that can be transported long distances more efficiently. The liquefier also removes traces of neon if present. The liquefier may use liquid nitrogen for refrigeration at the warm end of the process, provided by a small nitrogen generator or imported by truck as liquid, or it may use any other refrigeration option known in the art. The cold end of the process typically uses recycled helium in a heat pump arrangement for refrigeration.
If desired, at least a portion of the acid gas stream 2′ and/or the tail gas 31′ can be mixed with the helium-depleted natural gas 29 prior to recompression or at an interstage in the recompression. This can be advantageous if the helium-depleted natural gas was designed for a given mass flow rate, such as for a gas turbine. If there is a small amount of H2S present in the acid gas stream, then recompression and dilution may avoid the complications of venting H2S-containing CO2, or the expense of oxidizing the H2S, or the cost of a tall vent stack. Similarly, recompression of stream 31′ can avoid the added cost of oxidizing the remaining methane in the tail gas stream if needed prior to venting.
The distillation column 103 separates the helium from the column feed stream 7, which leaves the top of the column as helium-enriched overhead vapor 8. The distillation column requires a reboiler, which is shown in
The distillation column system is shown in
The helium-enriched overhead vapor 8 is then partially condensed in heat exchanger 105. The partially condensed overhead 13 enters overhead separator 106. This overhead separator 106 may be a simple flash vessel or a distillation column with multiple stages. The overhead from 106 is the crude helium vapor stream 14. The crude helium vapor stream 14 provides refrigeration by traveling through both heat exchangers 105 and 101 before leaving the helium extraction unit as stream 16. This crude helium vapor 14 is now at a high enough concentration, typically 4% to 20% by volume, to enter a helium purifier, shown as unit P in
The recycle liquid stream 17 exits the bottom of overhead separator 106 and is returned to the distillation column 103. Although stream 17 appears in the same location in the flow sheet as would a reflux stream for a conventional distillation process, the recycle liquid in the present invention is unsuitable for providing reflux for two reasons. First, the flow of stream 17 is small compared to the flow in the column feed 7, unlike a reflux stream that must have a liquid flow rate high enough to wash the vapor flowing up the column. Because the recycle liquid stream 17 has a relatively small flow rate, it does not affect the separation and is only returned to the distillation column to recover the helium contained in stream 17. Second, the distillation column 103 operates as a stripping column with the column feed entering at the top stage. The recycle liquid 17 can enter at the top stage or any lower stage, so it does not have the opportunity to wash the vapor leaving the top stage.
The pressure in the overhead separator 106 must be kept as close as possible to the pressure of the distillation column system such that the liquid head pressure in stream 17 is sufficient to overcome the pressure drop and flow into the distillation column 103. This lowers the overall power consumption of the process because the crude helium vapor stream from the overhead separator 106 thus requires no recompression. Note that the higher pressure in the overhead separator results in more helium being trapped in the stream 17, but this liquid-phase helium is recovered by recycling stream 17 back to the distillation column 103.
The helium-depleted bottoms liquid 12 may be split into at least two streams, each of which provides cooling at a different pressure and so different temperature. Stream 12 may be split into up to as many streams as one more than the number of stages of compression available in the recompressor R. This is because each stage of compression can accept one stream at its suction pressure, and one additional stream may bypass R if it is at the same pressure as the outlet of R. In the embodiment shown in
After stream 22 is warmed in heat exchanger 101, the resulting warmed second helium-depleted bottoms fraction 25 may be expanded in expander 109, if desired, which both cools the stream and generates power. The resulting expanded second helium-depleted bottoms fraction 26 can be returned to heat exchanger 101 to provide more cooling, then be combined with stream 20, and finally exit the heat exchanger as low-pressure return stream 27. Stream 27 is then recompressed in return compressor R. Stream 24 exits the heat exchanger 101 as medium-pressure return stream 28, which can be recompressed by feeding an interstage of return compressor R. Depending on the pressure required in the final helium-depleted natural gas product, pump 108 could increase the pressure of stream 23 to a high enough level that no further compression is needed.
Heat exchangers 101 and 105 represent a heat exchanger system, which in various embodiments of the invention may be a single heat exchanger or be split into two or more heat exchangers in series or parallel. For instance, the heat exchanger 101 may be divided into two separate heat exchangers at the point the expanded second helium-depleted bottoms fraction 26 is returned to the exchanger and mixed with stream 20 as it returns from 105. It may also be that the duty required for the reboiler is provided by a separate heat exchanger either in parallel with 101 or at the cold end of 101, exchanging heat solely between stream 6 and stream 9 to simplify the operation of the distillation column system. In general, the more integrated the heat exchanger system is, the more efficient the heat exchange is between all of the desired streams. However, the heat exchanger is often divided, which sacrifices efficiency, because a small increase in overall power consumption allows an advantage such as simplified operation, a smaller heat exchanger system, a simpler design of the heat exchanger system, or the reduction of risk to the process.
Return compressor R can be a single compressor with one or more stages, with or without intercoolers between stages, or a plurality of compressors in series or parallel. In the series arrangement, stream 27 could enter the first of the compressors and stream 28 could enter a compressor further along the series. In a parallel arrangement, separate compressors could compress streams 27 and 28 to the desired final discharge pressure. The recompressed gas exits the helium extraction unit as helium-depleted natural gas stream 29, which can then be fed to a pipeline, combusted, or otherwise utilized. If waste streams 2′ and/or 31′ are to be recompressed and combined with the helium-depleted natural gas stream, they are also fed to R.
There are situations where recompression of the medium-pressure return stream may not be required. The pressures of the return streams 20, 22, and 24 must all be less than the pressure of feed stream 5 because the return streams must boil at a pressure lower than the feed stream condenses at to allow efficient operation of heat exchanger 101. If the desired pressure of stream 29 is less than the pressure of stream 5, then stream 24 may be pumped to a pressure equal to that of stream 29 and not need further compression. In that case, the medium-pressure return stream 28′ may instead bypass the return compressor and be mixed directly with stream 29.
Certain embodiments and features of the invention have been described using a set of numerical upper limits and a set of numerical lower limits. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, it should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. Similarly, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, and ranges from any upper limit may be combined with any other upper limit to recite a range not explicitly recited. Further, a range includes every point or individual value between its end points even though not explicitly recited. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited. All numerical values are “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.
Aspects of the present invention include:
A computer simulation of the process of
For purposes of Example 1, two changes were made to the process depicted in
As shown in Table 1, the helium extraction unit X produces a crude helium stream 16 with greater than 12% helium, rich enough to feed the helium purification unit P, while maintaining 99.9% recovery in the helium extraction unit. Recovery in unit X is defined as the helium contained in stream 16 leaving the unit divided by the helium contained in stream 5 entering the unit. This high recovery is possible because recycle liquid stream 17, which holds 6.8% of the helium contained in the helium-enriched condensed overhead stream 12, is returned to the distillation column. In known processes that further concentrate the distillation column overhead, that liquid-phase helium would be lost because the equivalent of stream 17 would be routed to the equivalent of helium-depleted natural gas stream 29. The 99.9% helium recovery in the helium extraction unit X allows an overall helium recovery of 99.6% due the small loss of helium in stream 31′, where the overall helium recovery is defined as the helium contained in pure helium stream 30 divided by the helium contained in raw natural gas stream 1.
This process provides flexibility over the crude helium stream 16 composition. The helium mole fraction of stream 16 can be increased by either increasing the flow rate or decreasing the pressure of the low pressure return stream 19. Either option results in a higher concentration of helium in stream 16 at the cost of an increased power requirement to compress stream 27.
If the waste stream from the helium purification process were to be vented as stream 31, an optimization that minimizes power would increase the flow rate of methane in stream 16 to avoid recompression in compressor R. The optimization would need to include the the value of methane in the vent 31 to balance the increase in stream 16.
A computer simulation of the process of
For purposes of Example 2, two changes were made to the process depicted in
Example 2 shares many of the same features as Example 1, such as high overall helium recovery, but differs in the nitrogen content of the feed. The lower nitrogen content in Example 2 results in higher temperatures in the distillation column 103, as shown by a stream 8 that is about 20° C. warmer than its counterpart in Example 1. Because the distillation column does not require as cold of a temperature, stream 19 does not need to be let down to as low of a pressure: 7.7 bar as opposed to 5.5 bar. Stream 19 operating at a higher pressure reduces the recompression duty, resulting in a lower net power of 7.75 MW compared to 9.01 MW in Example 1.
While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention.
