Method for Using a Solid-Tolerant Heat Exchanger in Cryogenic Gas Treatment Processes

Abstract

Methods and systems for removing contaminants, such as water and/or carbon dioxide, from a gas stream, such as a natural gas stream or a flue gas stream. One or more solid-tolerant heat exchangers are employed to chill the gas stream to a temperature at which the contaminants solidify. The solidified contaminants may then be separated and removed from the gas stream. In one or more aspects, the one or more solid-tolerant heat exchangers may include a scraped heat exchanger.

Claims

1. A method for removing water and carbon dioxide from a feed gas stream containing water and carbon dioxide, the method comprising: producing a first treated gas stream by feeding the feed gas stream to a first solid-tolerant heat exchanger, the first solid-tolerant heat exchanger chilling the feed gas stream to a first temperature, wherein the first temperature is i) below which at least a portion of the water contained in the feed gas stream freezes out of the feed gas stream, and ii) above which at least a portion of the carbon dioxide contained in the feed gas freezes out of the feed gas as a pure phase; and producing a second treated gas stream by feeding the first treated gas stream to a second solid-tolerant heat exchanger, the second solid-tolerant heat exchanger chilling the first treated gas stream to a second temperature, wherein the second temperature is below which at least a portion of the carbon dioxide contained in the feed gas stream freezes out of the first treated gas stream as a pure phase.

2. The method of claim 1, further comprising: further chilling the second treated gas stream to at least partially liquefy the second treated gas stream.

3. The method of claim 1, wherein the second treated gas stream comprises primarily methane on a molar basis.

4. The method of claim 1, wherein the first and second solid-tolerant heat exchangers are constructed using different metallurgies.

5. The method of claim 1, wherein the first solid-tolerant heat exchanger is chilled using a refrigerant which is fed to the second solid-tolerant heat exchanger, and after exiting the second solid heat exchanger, at least a portion of the refrigerant is fed to the first solid-tolerant heat exchanger.

6. The method of claim 1, further comprising: prior to feeding the feed gas stream to the first solid-tolerant heat exchanger, feeding the feed gas stream to a first non-freezing heat exchanger in which the feed gas stream is cooled using a coolant at an ambient temperature.

7. The method of claim 6, further comprising: after feeding the feed gas stream through the first non-freezing heat exchanger and before feeding the feed gas stream to the first solid-tolerant heat exchanger, feeding the feed gas stream to a second non-freezing heat exchanger to cool to a temperature at which liquid water is condensed from the feed gas stream and removing at least a portion of the condensed liquid water.

8. The method of claim 7, wherein a first refrigerant is used in the second non-freezing heat exchanger, a second refrigerant is used in the first solid-tolerant heat exchanger, and a third refrigerant is used in the second solid-tolerant heat exchanger.

9. The method of claim 8, wherein a portion of the first refrigerant is used to cool the second refrigerant, and wherein a portion of the second refrigerant is used to cool the third refrigerant.

10. The method of claim 8, wherein: the first refrigerant comprises primarily propane; the second refrigerant comprises primarily propane; and the third refrigerant comprises primarily ethane, primarily ethylene, primarily methane, or primarily nitrogen, or primarily any combination thereof.

11. The method of claim 7, wherein a non-ambient temperature refrigerant is directed in a repeating circuit in which the non-ambient temperature refrigerant cools the feed gas stream in the second solid-tolerant heat exchanger, cools the feed gas stream in the first solid-tolerant heat exchanger, cools the feed gas stream in the second non-freezing heat exchanger, and is cooled in a heat sink.

12. The method of claim 1, wherein one or both of the solid-tolerant heat exchangers comprises a scraped heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below.

[0011] FIG. 1 is a schematic diagram of a process according to an aspect of the disclosure.

[0012] FIG. 2 is a schematic diagram of a process according to another aspect of the disclosure.

[0013] FIG. 3 is a schematic diagram of a process according to further aspects of the disclosure.

[0014] FIG. 4 is a schematic diagram of a process according to still another aspect of the disclosure.

[0015] FIG. 5 is a schematic diagram of a process according to yet another of the disclosure.

[0016] It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure.

DETAILED DESCRIPTION

[0017] For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. It will be apparent to those skilled in the relevant art that some features that are not relevant to the present disclosure may not be shown in the drawings for the sake of clarity.

[0018] According to aspects of the disclosure, a solid-tolerant heat exchanger is integrated into cryogenic cooling processes to accommodate gas that is minimally treated, i.e., still containing significant amounts of water and carbon dioxide (CO.sub.2). The solid-tolerant heat exchanger is a heat exchanger which is designed to maintain acceptable performance despite the formation of frozen solids at its operating temperatures. Conversely, a non-solid-tolerant heat exchanger is a heat exchanger which is not designed with the expectation of solids freezing out within ite.g., a conventional shell-and-tube or plate heat exchanger. The solid-tolerant heat exchanger may be a scraped heat exchanger, which include heat exchangers with scraped surfaces using simple mechanical scrapers (e.g., fixed blades sliding over surfaces) and/or using dynamic mechanical scrapers, such as the rotating blades found, for example, in U.S. Pat. No. 3,403,532, the disclosure of which is incorporated herein by reference. Other types of solid-tolerant heat exchangers include but are not limited to fluidized bed heat exchangers and reversing heat exchangers. Some solid-tolerant heat exchangers may employ low adhesion coatings or surface treatments to reduce the impact of solids formation on performance By using a solid-tolerant heat exchanger, solid forming components in the gas feed are simultaneously separated from the gas as it is cryogenically cooled.

[0019] FIG. 1 is a schematic diagram depicting a process 100 using one or more solid-tolerant heat exchangers according to an aspect of the disclosure. Process 100 may be used to capture CO.sub.2 from flue gas. Process 100 may be used to generate liquefied natural gas (LNG) from a feed gas stream 102 that is contaminated with CO.sub.2 and/or water. At its most basic design, the feed gas stream 102 is fed into a solid-tolerant heat exchanger, which in FIG. 1 is shown as a second scraped heat exchanger 104. While known scraped heat exchanger designs may be suitable for such use, it is probable that known designs (which have generally focused on liquid feeds and temperatures warmer than the deep cryogenic temperatures associated with LNG or CO.sub.2 freeze-out) would need to be modified for effective use with feed gas stream 102. Such modifications may include those disclosed in co-pending U.S. Patent Application Methods and Apparatus for Improving Multi-Plate Scraped Heat Exchangers, (2018EM332) filed on an even date herewith and incorporated by reference herein. Second scraped heat exchanger 104 facilitates the cooling of feed gas stream 102 using a fourth coolant 106. The fourth coolant 106 may be primarily ethane, primarily ethylene, primarily methane, primarily nitrogen, or may comprise any one of known types of single mixed refrigerants (SMR), i.e., a mixture of two or more refrigerant species chosen to provide a desired cooling energy versus temperature behavior. The feed gas stream 102 is cooled to a temperature sufficient for contaminants, such as water, CO.sub.2, potentially small amounts of other contaminants such as waxes, or the like, to solidify and separate from the feed gas stream as a solids stream 105. The feed gas stream exits the second scraped heat exchanger 104 as a cold treated fluid 108. The fourth coolant 106 is directed to a third refrigeration unit 110 that chills the fourth coolant before it is recycled to the second scraped heat exchanger.

[0020] FIG. 1 depicts further process steps that may be used to more efficiently treat the feed gas stream using a combination of non-freezing heat exchangers and solid-tolerant heat exchangers. A non-freezing heat exchanger is a heat exchanger which operates at temperatures above which any freeze-out of species occurs. For example, the feed gas stream 102 may be compressed using a compressor 112 and passed through a first non-freezing heat exchanger 114, which may use ambient air or water as a first refrigerant or coolant 116 to cool the compressed feed gas stream and remove liquid water 118 therefrom. The cooled compressed feed gas stream may then pass through a second non-freezing heat exchanger 120, which may use a second refrigerant or coolant 122, which may comprise primarily propane, to additionally cool the cooled compressed feed gas stream and remove liquid water 124 therefrom. The second coolant 122 may be directed to a first refrigeration unit 126 that cools the second coolant to be recycled to the second non-freezing heat exchanger 120. The additionally cooled compressed feed gas stream may pass through a solid-tolerant heat exchanger, which in FIG. 1 is shown as a first scraped heat exchanger 128. First scraped heat exchanger 128 may have a structure similar to the second scraped heat exchanger 104 previously described. First scraped heat exchanger 128 facilitates the cooling of the additionally cooled compressed feed gas stream using a third refrigerant or coolant 130. The third coolant 130 may be primarily propane. The additionally cooled compressed feed gas stream is cooled to a temperature sufficient for water to solidify and separate therefrom at 132. The third coolant 130 is directed to a second refrigeration unit 134 that chills the third coolant before it is recycled to the first scraped heat exchanger. The additionally cooled compressed feed gas stream exits the first scraped heat exchanger 128 and is passed through the second scraped heat exchanger 104 as previously described. The cold treated fluid 108 exiting the second scraped heat exchanger 104 may be expanded in an expander 136 to liquefy or further cool the cold treated fluid. A separator 138 may then separate the cold treated fluid into a cold gas stream 140 and a cold liquid stream 142, which in some aspects may comprise an LNG stream. The cold gas stream 140 may be vented, used or sold as a product, used as fuel to drive compressors or other machinery in process 100 or elsewhere, used at least in part as one of the second through the fourth coolants, or used at least in part as an additional coolant that cools one of the second through fourth coolants within the first through third refrigeration systems.

[0021] FIG. 2 is a schematic depicting a process 200 using one or more solid-tolerant heat exchangers according to further aspects of the disclosure. Process 200 is similar to process 100, and elements with previously described reference numbers will not be additionally described. In process 200, a first coolant 116 is used with first non-freezing heat exchanger 114 as previously described, and a single refrigerant circuit employs a second coolant 202 to cool, in order, the second scraped heat exchanger 104, first scraped heat exchanger 128, and second non-freezing heat exchanger 120. After exiting the second non-freezing heat exchanger 120, the second coolant 202 is cooled in a refrigeration unit 204 and then directed back to the second scraped heat exchanger 104. The second coolant 202 is a non-ambient temperature coolant, or in other words, the second coolant is not ambient air or ambient water. In an aspect, the second coolant 202 may comprise a hydrocarbon fluid, a halogenated hydrocarbon fluid, or ammonia.

[0022] FIG. 3 is a schematic depicting a process 300 using one or more solid-tolerant heat exchangers according to further aspects of the disclosure. Process 300 is similar to processes 100, and elements with previously described reference numbers will not be additionally described. In process 300 portions of each of the second and third coolants 122, 130 are used as a refrigeration source and/or heat sink to cool the third and fourth coolants in the second and third refrigeration units 134, 110, respectively. Specifically, a first slipstream 301 is taken from third coolant 130 downstream of the second refrigeration unit 134 and is used to cool fourth coolant 106 in the third refrigeration unit 110. The subsequently warmed first slipstream 302 is then removed from the third refrigeration unit and re-combined with the third coolant upstream of the third coolant inlet into the second refrigeration unit 134. Likewise, a second slipstream 303 is taken from second coolant 122 downstream of the first refrigeration unit 126 and is used to cool third coolant 130 in the second refrigeration unit 134. The subsequently warmed second slipstream 304 is then removed from the second refrigeration unit and re-combined with the second coolant upstream of the second coolant inlet into the first refrigeration unit 126.

[0023] As described herein, for example in the processes described herein and shown in FIGS. 1-3, use of multiple solid-tolerant heat exchangers, operating at different temperatures, may be beneficial for separating various contaminants (e.g., water, CO.sub.2, hydrogen sulfide (H.sub.2S)) from each other. This also allows optimization of the construction of the equipment. For example, the metallurgy of each heat exchanger may be optimized to address strength at the operating temperatures and modes of corrosion. As a non-limiting example, a heat exchanger designed to primarily precipitate and freeze-out water may be constructed from stainless steel SS316, which is very corrosion resistant, whereas a heat exchanger which will primarily precipitate and freeze-out CO.sub.2 may be constructed from stainless steel SS304, which is more fracture resistant at cryogenic temperatures, although not quite as corrosion resistant as SS316 if exposed to liquid water.

[0024] Although methods have been disclosed above for using solid-tolerant heat exchangers to remove solid forming species from gas feeds, methods integrating the refrigeration process with the solid-tolerant heat exchangers are less described, especially as applied to the generating of LNG or capturing of CO.sub.2 from flue gas.

[0025] According to disclosed aspects, a refrigerant flow may be split into two parallel streams. One stream is used to pre-cool the refrigerant prior to expanding (i.e., self-refrigeration) in a recuperative heat exchanger. The second stream is used as the heat sink for the process flow in a solid-tolerant heat exchanger. This approach directly solves the problem of incorporating a solid-tolerant heat exchanger. Typical cryogenic cooling cycles, especially for LNG generation, employ multi-stream heat exchangers where more than two streams are brought into thermal contact for heat transfer to maximize process efficiency. This is in contrast to typical two-stream heat exchangers used in the vast majority of heat transfer applications. Indeed, solid tolerant heat exchangers are only available for two-stream configurations and, therefore, cannot be directly implemented in traditional cryogenic cooling cycles.

[0026] The split refrigerant approach enables a relatively efficient process despite the limitations of two-stream solid-tolerant heat exchangers. This directly permits adoption of solid-tolerant heat exchangers in cryogenic cooling processes for LNG and CO.sub.2 capture. FIG. 4 schematically illustrates a modified Poly Refrigerated Integrated Cycle Operation (PRICO) system 400 for LNG production. The traditional PRICO cycle combines a first heat exchanger and a second heat exchanger with a single cold refrigerant stream. As such, the combined heat exchanger is both a recuperative heat exchanger (precooling of the warmer refrigerant stream) and a process cooling heat exchanger (for the process stream). According to disclosed aspects, system 400 modifies and adapts the traditional PRICO cycle for solid-tolerant heat exchanger use by splitting the refrigerant stream 402 using a splitter 404. Splitting the refrigerant stream 402 separates the function of the combined heat exchanger so that a recuperative heat exchanger 406 and a process cooling heat exchanger 408 can be provided separately. Thus, solid-tolerant heat exchanger technology can be readily integrated into a cryogenic cycle, such as the PRICO cycle, for LNG production. Specifically, the refrigerant stream 402 is split by splitter 404 into a first refrigerant stream 410 and a second refrigerant stream 412. The first refrigerant stream 410 passes through the recuperative heat exchanger 406, which may be a non-solid tolerant heat exchanger, and is warmed therein to produce a warmed first refrigerant stream 411. The second refrigerant stream 412 passes through the process cooling heat exchanger 408, in which it cools or chills a process stream 414 and freeze-out solid forming species, such as CO.sub.2 or water. These solids may be rejected as stream 438, and may be further processed as desired. The process stream may be a natural gas stream or other type of gas stream. The cooled process stream 416 may be subject to further processing, such as with an expander element 418, to liquefy or further cool the cooled process stream and produce an LNG stream 420. This expansion cooling may generate further amounts of solids. The process cooling heat exchanger is a solid-tolerant heat exchanger, such as a scraped heat exchanger as previously discussed. The warmed second refrigerant stream 422 is combined with the warmed first refrigerant stream 411, and the combined warmed refrigerant stream 424 is compressed in a first compressor 426. The compressed refrigerant stream 428 is cooled in a first cooler 430. The first cooler 430 preferably uses an ambient cool fluid, such as air or water taken from or in thermal contact with an ambient source, to cool the compressed refrigerant stream. The compressed refrigerant stream may be further compressed and cooled in a second compressor 432 and a second cooler 434, respectively. After the refrigerant stream has been sufficiently compressed and cooled, it passes through the recuperative heat exchanger 406, which it is cooled by the first refrigerant stream 410 and expanded in an expander 436 to reduce its temperature prior to repeating the refrigeration circuit.

[0027] FIG. 5 schematically illustrates a modified Poly Refrigerated Integrated Cycle Operation (PRICO) system 500 adapted for cryogenic carbon dioxide capture. Elements having common function with similar elements to system 400 may be identified with similar reference numbers and may not be further described. System 500 uses a multi-stream recuperative heat exchanger where both the decarbonized flue gas stream as well as one of the refrigerant split streams are used as heat sinks to precool the refrigerant prior to expansion. Specifically, a refrigerant stream 502 is separated in a separation vessel 504 into first and second portions 504a, 504b. Streams 504a and 504b may reflect single-phase vapor streams and liquid streams respectively. Although not necessary, use of single-phase feeds (as opposed to multi-phase feeds) into heat exchangers can simplify design and help ensure against maldistribution issues. Second portion 504b is split into an intermediate stream 504c and a second refrigerant stream 512. The first portion 504a is combined with the intermediate stream 504c to form a first refrigerant stream 510. The first refrigerant stream 510 passes through a multi-stream recuperative heat exchanger 506, which may be a non-solid tolerant heat exchanger, and is warmed therein to produce a warmed first refrigerant stream 511. The second refrigerant stream 512 passes through a process cooling heat exchanger 508, in which it cools or chills a process stream 514 to produce a cooled process stream 516. During the cooling or chilling process in the process cooling heat exchanger, carbon dioxide and/or other contaminants in the process stream solidify and are thereby separated from the process stream. The solids stream is shown as stream 524. The solid contaminants are removed from the process cooling heat exchanger. The process stream may be a flue gas stream or other type of gas stream. The cooled process stream 516, which has now been depleted of solid forming components, may then be directed to the recuperative heat exchanger 506. The process cooling heat exchanger 508 is a solid-tolerant heat exchanger, such as a scraped heat exchanger as previously discussed. The warmed second refrigerant stream 522 is combined with the warmed first refrigerant stream 511, and the combined warmed refrigerant stream is compressed and cooled, as described with respect to system 400. The refrigerant stream then passes through the recuperative heat exchanger 506, which it is cooled by the first refrigerant stream 510 and by the cooled process stream 516, and then expanded in an expander 526 to reduce its temperature prior to repeating the refrigeration circuit. After exiting the recuperative heat exchanger 506, the warmed process stream 528 may, if flue gas, be exhausted to the atmosphere now that is depleted in CO.sub.2. The rejected solids in stream 524 may be purified, converted to liquid or gas, and/or be sold, stored, re-injected into a geologic formation, or the like.

[0028] In some embodiments, prior to entering heat exchanger 508, process stream 514 may be precooled to temperature close to but above 0 C. This enables moisture removal through liquid water condensation and reduction of the freeze-out load in heat exchanger 508.

[0029] The aspects disclosed in FIGS. 4 and 5 may be used to actively control the split ratio between refrigerant streams, e.g., between first and second refrigerant streams 410, 412 (FIG. 4) or between intermediate stream 504c and second refrigerant stream 512 (FIG. 5). This can be accomplished by incorporating actively controlled valves in these streams. Control of the split ratio may be performed to adjust the temperature of refrigerant stream 402 or 502, which in turn impacts the amount of solids frozen-out in heat exchanger 408 or 508.

[0030] The effectiveness of the disclosed aspects may be improved by recirculating process streams through the solid-tolerant heat exchanger. With reference to Error! Reference source not found., this configuration may include a pump that receives a fraction of LNG stream 420 to feed it back to an inlet of the process cooling heat exchanger 408. This increases the amount of liquid present in the heat exchanger with the intention to facilitate solid removal. Many solid tolerant heat exchangers can demonstrate effective solid removal with a liquid process fluid stream to help wash away solids and better entrain solids in a flowing slurry. As the proposed process stream is primarily gaseous, an increase in liquid fraction may increase solid removal effectiveness for a particular solid-tolerant heat exchanger.

[0031] Similarly, the recirculation of decarbonized gas can be recirculated to achieve high gas velocities in the solid-tolerant heat exchanger. This facilitates removal of solids formed on the heat exchanger wall by inducing shear stress through high gas velocities. That is, high gas velocities can help blow solids off the heat exchanger walls and entrain the removed solids in the gas flow. In some embodiments the solids may have been partially or fully dislodged from the walls via scrapers. In this particular configuration, recirculation is recommended to be controlled in a cyclical pattern. That is, high velocity gas recirculation is induced periodically based on the rate of solid accumulation and the effectiveness of solid removal. A pulsing recirculation pattern may be employed. Intermittent recirculation is preferred over continuous recirculation to minimize cooling requirements due to dilution of CO.sub.2 concentration in the gas stream.

[0032] Similarly, the entire process fluid stream, i.e., the full feed gas stream (LNG) and the full flue gas stream (carbon capture) may be pulsed to facilitate solid removal from the heat exchanger surface through intermittently inducing high shear stresses at the solid/process fluid interface. However, the overall process must be able to accommodate this approach with respect to stability.

[0033] While the disclosed aspects in FIGS. 4 and 5 may be suitable for many target applications, other cycle configuration may be considered for these applications. Cycles such as dual mixed refrigerant cycles, gas expander cycles as well as other single mixed refrigerant cycles may be modified, adapted and optimized for the effective integration of solid tolerant heat exchangers.

[0034] Disclosed aspects may be used in hydrocarbon management activities. As used herein, hydrocarbon management or managing hydrocarbons includes hydrocarbon extraction, hydrocarbon production, hydrocarbon exploration, identifying potential hydrocarbon resources, identifying well locations, determining well injection and/or extraction rates, identifying reservoir connectivity, acquiring, disposing of and/or abandoning hydrocarbon resources, reviewing prior hydrocarbon management decisions, and any other hydrocarbon-related acts or activities. The term hydrocarbon management is also used for the injection or storage of hydrocarbons or CO.sub.2, for example the sequestration of CO.sub.2, such as reservoir evaluation, development planning, and reservoir management. The disclosed methodologies and techniques may be used to produce hydrocarbons in a feed stream extracted from, for example, a subsurface region. Hydrocarbon extraction may be conducted to remove the feed stream from for example, the subsurface region, which may be accomplished by drilling a well using oil well drilling equipment. The equipment and techniques used to drill a well and/or extract the hydrocarbons are well known by those skilled in the relevant art. Other hydrocarbon extraction activities and, more generally, other hydrocarbon management activities, may be performed according to known principles.

[0035] As used herein, the terms approximately, about, substantially, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described are considered to be within the scope of the disclosure.

[0036] The articles the, a and an are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements.

[0037] It should be understood that numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.