Utilizing liquefied natural gas for cryogenic cooling of cracked gas from a light hydrocarbon catalytic cracker

Abstract

Novel processes are provided for the cooling of light hydrocarbon cracker effluent streams using Liquefied Natural Gas (LNG). The effluent streams are passed through LNG cold boxes where they are cooled by LNG. The cooling takes place as a result of direct or indirect heat transfer. As the LNG heats up from this process it can be fed into a light hydrocarbon cracker. The use of the present processes offers significant economic and environmental benefits over the typical industry practice of compression and cooling using C2 / C3 refrigerants.

Description

70205.0689WOU1 (T-12550-W001)UTILIZING LIQUEFIED NATURAL GAS FOR CRYOGENIC COOLING OF CRACKED GAS FROM A LIGHT HYDROCARBONCATALYTIC CRACKERCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application Serial No. 63 / 687,995 filed August 28, 2024, the complete disclosure of which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to a process for cooling the effluent gas from a light hydrocarbon catalytic cracker, such as a light hydrocarbon cracker. The present disclosure further relates to the use of liquefied natural gas to cool the effluent gas.BACKGROUND

[0003] Hydrogen provides clean, emissions free energy when used as a fuel source. However, hydrogen, while it exists atomically in vast numbers, exists scarcely as a gas. Thus, the reliable, economic, and sustainable production of hydrogen gas represents a crucial step in reducing net global carbon emissions.

[0004] One potential solution for the low carbon production of hydrogen gas for energy is light hydrocarbon cracking. This solution relies on cracking light hydrocarbons such as methane into hydrogen and liquid C2+ products in the presence of a catalyst. While this solution is promising, many challenges are inherent in the process of light hydrocarbon cracking. Present among these many challenges is the need to cool down cracked gas exiting the cracking reactor to temperatures in the range of - 100°F or even -200°F or below. Such cooling is needed to effectively separate and purify products from unreacted methane. Conventional methods to achieve this task require cryogenic cooling via C2 / C3 refrigerants coupled with compression in the range of up to 400-500 psig. In these conventional methods, the reactor effluent stream must also pass through several stages of compression and cooling to reach cryogenic conditions before being fed into a demethanizer for further separation. These factors impose a significant energy consumption burden for undergoing the necessary cooling process. Moreover, the use of C2 / C3 refrigerants and the necessary70205.0689WOU1 (T-12550-W001) equipment required to operate a closed loop refrigeration system raise the overall operating cost of such a system.

[0005] Thus, to provide a novel, economic, and more efficient process for cooling the light hydrocarbon cracking reactor effluent stream would be of great interest to the industry.SUMMARY

[0006] The present process economically and efficiently cools the effluent gas from a light hydrocarbon cracker. In one embodiment, the cracker is a methane catalytic cracker. In one embodiment, the present process for cooling the effluent stream from a methane catalytic cracking unit uses liquefied natural gas (LNG). The process comprises converting natural gas into LNG at a production site, transporting the LNG to receiving terminals, and transporting the LNG to light hydrocarbon cracking units located at the receiving terminals.

[0007] In one embodiment, the process comprises cooling the effluent stream from a light hydrocarbon cracking unit using LNG as a cooling medium. The process comprises pumping the LNG directly to an LNG cold box, cooling reactor effluent stream by passing the effluent stream through the LNG cold box to produce a cooled reactor effluent stream, and separating the cooled reactor effluent stream into a methane / hydrogen gas stream and a C2+ liquid stream. By using a LNG cold box to integrate the LNG into the light hydrocarbon cracking process, compression cooling is replaced with LNG cooling.

[0008] In one embodiment, the process for cooling the effluent stream from a light hydrocarbon cracking unit using LNG comprises using a column or contactor where the LNG as the cooling medium comes into direct contact with cracked gas effluent. In one embodiment, the LNG comes into indirect contact with the cracked gas effluent via a heat exchanger.

[0009] In one embodiment, after the LNG is used to cool the reactor effluent stream the LNG can be fed into the light hydrocarbon cracking reactor as feed.

[0010] In another embodiment, the effluent stream from the light hydrocarbon cracker is scrubbed prior to being cooled by the LNG.

[0011] Among other factors, the present processes permit the cooling of light hydrocarbon cracker effluent streams using LNG. The use of the present processes offers significant economic and environmental benefits, as well as significant energy savings.70205.0689WOU1 (T-12550-W001)BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 depicts the current practice of cryogenically cooling cracked gas from a light hydrocarbon cracking reactor by using C2 / C3 refrigerants.

[0013] FIG. 2 depicts an embodiment of cooling a light hydrocarbon cracking reactor effluent stream comprising pumping LNG to the light hydrocarbon cracking unit as coolant and eliminating the need for cryogenic cooling.

[0014] FIG. 3 depicts an embodiment of the present process where the LNG is transported from off site to the light hydrocarbon cracking unit.DETAILED DESCRIPTION

[0015] Before the present processes utilizing LNG as a coolant are disclosed and described, it is to be understood that this disclosure is not limited to the particular structures, process steps, or materials disclosed herein, but is extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting. It must be noted that, as used in this specification, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a step" may include multiple steps, reference to "producing" or "products" of a reaction or treatment should not be taken to be all of the products of a reaction / treatment, and reference to "treating" may include reference to one or more of such treatment steps. As such, the step of treating can include multiple or repeated treatment of similar materials / streams to produce identified treatment products.

[0016] Numerical values with "about" or "approximately" include typical experimental variances and these two terms are used interchangeably. As used herein, the term "about" means within a statistically meaningful range of a value, such as a stated particle size, concentration range, time frame, molecular weight, temperature, or pH. Such a range can be within an order of magnitude, typically within 10%, and more typically within 5% of the indicated value or range. Sometimes, such a range can be within the experimental error typical of standard methods used for the measurement and / or determination of a given value or range. The allowable variation encompassed by the term "about" will depend upon the particular system under study, and can be readily appreciated by one of ordinary skill in the art. Whenever a range is recited within70205.0689WOU1 (T-12550-W001) this application, every whole number integer within the range is also contemplated as an embodiment of the invention.

[0017] The present processes relate to the cooling of light hydrocarbon cracker effluent streams using Liquefied Natural Gas (LNG).

[0018] The light hydrocarbon feed stream to the cracker is not particularly limited and may include, for example, Ci to Ce or Ci to C4 or Ci to C3 or Ci to C2 alkanes such as methane, ethane, or natural gas either pure or in any suitable mixture. In some embodiments, the light hydrocarbon feed stream may also contain minor amounts of other components including, for example, carbon dioxide, sulfur compounds such as H2S, water, nitrogen, and mixtures thereof. In some embodiments the light hydrocarbon feed stream may also include steam, superheated steam, an inert gas such as nitrogen, or any mixture thereof. In some embodiments, the light hydrocarbon feed stream to be employed may include any suitable composition such that the resulting product includes at least hydrogen.

[0019] In some embodiments, the light hydrocarbon feed stream comprises methane or natural gas such as, for example, a light hydrocarbon feed stream comprising greater than about 80%, or greater than about 90%, or greater than about 95%, or greater than about 99% methane. As used herein, natural gas comprises methane and potentially higher alkanes, carbon dioxide, nitrogen or other gases, and / or sulfide compounds such as hydrogen sulfide, and mixtures thereof. In illustrative embodiments, the light hydrocarbon feed stream may further contain a portion of the produced products that are recycled back to the light hydrocarbon feed stream along with unreacted methane.

[0020] Cracking of light hydrocarbon is one of the newer methods currently being evaluated as a potential low carbon solution to produce hydrogen. The method relies on cracking the light hydrocarbons into hydrogen and liquid C2+ products in the presence of a catalyst. In one embodiment, the effluent streams from the light hydrocarbon cracker are passed through LNG cold boxes where they are cooled by LNG. The cooling takes place as a result of direct or indirect heat transfer. As the LNG heats up from this process in one embodiment, it can be fed into a light hydrocarbon cracker as feed.

[0021] In one embodiment, the light hydrocarbon cracker is a methane catalytic cracker.

[0022] Turning now to the figures, of the drawing, FIG. 1 shows a diagram of a process of cooling effluent stream that is generally operated in the industry today. Generally, the reactor effluent streams go through several stages of compression and70205.0689WOU1 (T-12550-W001) cooling to reach cryogenic conditions before feeding into the demethanizer for further separation. This would enable recovery of C2 and lighter products present in the cracked gas. Generally, the stages of compression are of the range of from 400-500 psig, while the low temperatures required to achieve the cryogenic cooling are of the range of -100°F, or even -200°F. These low temperatures are achieved using C2 / C3 refrigerants in a closed loop refrigeration system. Following compression and cryogenic cooling the effluent stream is generally fed into a demethanizer for further separation of the effluent into C2 and lighter products present in the cracked gas.

[0023] The present process, however, does not depend on compression and refrigeration. Rather, it uses LNG as the cooling medium to lower cracked gas temperatures to the required range where C2 and heavier products can be easily separated from the H2 and methane gas. As a result, the present process substantially reduces energy costs (CAPEX and OPEX) associated with the LNG cooling when compared to the current practice of compression and cryogenic cooling. Additionally, by eliminating multi-stage compression energy consumption can be reduced by over 50%. Furthermore, by eliminating C2 / C3 refrigerants the necessary equipment required to operate a closed loop refrigeration system can also be eliminated. The feed to the demethanizer can also be substantially reduced since most of it is removed in the upstream two-phase separator. This will significantly reduce the CAPEX for the design and in installation of the demethanizer.

[0024] Referring to FIG. 2, when the light hydrocarbon feed 1 is fed to the cracker 2, the hydrocarbons are cracked into hydrogen and liquid C2+ products. After the cracked gas has exited the reactor 2, it is typically quenched 3 to lower the temperature to a manageable range. The process of quenching involves contact with water / or any high boiling solvent and effectively drops the temperature of gas mixture to the range of about 100°F. The gas then undergoes a single stage of compression to increase the pressure. In one embodiment, the effluent gas is sent through an amine scrubber to remove most / all of the CO2 (not shown). Ce+ products can also be removed 4.

[0025] In one embodiment the scrubbed cracked gas is then fed to a cold box 5 where it exchanges heat with the LNG. Enough LNG is used to target the cracked gas temperature leaving the cold box to drop to its dew point range. This ensures that the majority of C2 and heavier gases condense while most of the methane and all the H2 will remain in a vapor state, the hydrogen and methane mixture can then be separated via membrane separation, PSA purification, or combination of both methods 7. In another embodiment the lighter gases can be removed / separated from the heavier ones by direct contact with LNG in a70205.0689WOU1 (T-12550-W001)Liquid / Gas contactor, the overhead stream 6 would be fed to a membrane / PSA 7 for H2 purification / separation and recovered as hydrogen product 8. The bottoms stream comprising of some methane and light paraffins can be fed to the demethanizer 7. The unconverted methane 9 recovered in the demethanizer 7 can then be recycled to the reactor 2. The bottoms product from the demethanizer 10 comprising of light paraffins can also be recycled to the reactor 2.

[0026] The present process allows for the use of H2 at any location where there is a direct need for H2 as a fuel source or in a refinery where H2 is needed for hydroprocessing applications such as hydrocracking / hydrotreating. The availability or proximity of LNG represents one of the key requirements in the implementation of this process.

[0027] In one embodiment of the present process, as shown in FIG. 3, natural gas is produced in a production site and subsequently converted to LNG. The LNG can then be shipped over long distances, for example across oceans, to another site near end users with LNG receiving terminals. By building a light hydrocarbon cracking unit near the LNG receiving terminals, the LNG can be pumped to the cracking unit at very low temperature (e.g., -260°F), permitting its use as coolant to cool the reactor effluent stream. Such cooling helps separate products from unconverted methane.

[0028] In one embodiment, as the LNG warms up, it can become the feed to the light hydrocarbon cracking unit. This ability to feed the warmed LNG into the cracking unit comprises an extraordinary benefit of the present processes. As enumerated previously, utilizing LNG to cool the reactor effluent stream provides significant economic and environmental benefits. As the LNG cools the reactor effluent stream it inevitably heats up either through direct or indirect heat exchange. Ultimately the LNG will heat to a temperature where it is no longer capable of cooling the reactor effluent stream. At this time, the heated LNG can be fed into the cracking unit. This avoids the difficulty of transporting, changing out, or otherwise disposing of the LNG while simultaneously increasing the feed of the light hydrocarbon cracking unit.

[0029] In one embodiment, an extremely cold LNG stream can be fed directly to a LNG cold box as coolant to cool the reactor effluent stream down to the -100°F range, or even -200°F range. The cooling can take place as the result of either direct or indirect heat exchange. This produces a cooled reactor effluent stream containing C2+ liquid and methane plus hydrogen gases, which can then be readily separated in a hydrogen separation unit via cryogenic separation or a flash drum into a methane / TL gas stream as well as a C2+70205.0689WOU1 (T-12550-W001) liquid stream as demethanizer feed. LNG can warm up in the cold box becoming a gas stream which can be fed into the light hydrocarbon cracking reactor as feed.

[0030] One embodiment of the present process comprises integrating LNG into the light hydrocarbon cracking process. In this embodiment the cracked gas is partially compressed and then sent through an LNG cold box for heat exchange. This embodiment differentiates itself from current industry practice by cooling the cracked gas using LNG instead of C2 / C3 refrigerant.

[0031] In one embodiment a column or contactor is used where the LNG comes into direct contact with the cracked gas. In this embodiment, most of the H2 and methane is absorbed, while C2 and heavier components are sent in the feed to the demethanizer. In a further embodiment, a demethanizer can be such as column for direct contact of LNG with cracked gas where cracked gas is directly fed into a lower portion of the demethanizer and LNG is fed into the overhead condenser as reflux liquid.

[0032] The following examples are given to illustrate the potential benefits of the present processes of using LNG for cooling. The scope of the present invention is not to be limited by the following examples in any way.Example 1

[0033] Preliminary simulation results show significant energy savings as compared to the practice of cooling as is generally practiced in the industry today. The procedure followed in this example comprised minor compression for CO2 removal in an amine absorber, followed by LNG cooling to condense the cracked gas to its dew point, flashing the vapor and pumping the liquid into a demethanizer. The results of this preliminary simulation show a compressor Energy Demand saving of ~60% (19.1 MW vs. 45.5 MW), Total Condenser Energy Demand similar to base case (—8.8 MMBTU / hr vs. 9 MMBTU / hr), Total Reboiler Energy Demand, similar to base case (~16 MMBTU / hr vs. 15.6) and Total LNG Cooling Energy Demand —87 MMBTU / hr.Example 2

[0034] Preliminary simulation results show significant energy savings as compared to the practice of cooling as is generally practiced in the industry today. The procedure followed in this example comprised compression followed by LNG cooling, then sending all the cooled gas to the demethanizer. The results of this preliminary simulation show a Compressor Energy Demand saving of ~20% (37.5 MW vs. 45.5 MW), Total Condenser70205.0689WOU1 (T-12550-W001)Energy Demand of — 12.0 MMBTU / hr, Total Reboiler Energy Demand of ~7.4 MMBTU / hr, and Total LNG Cooling Energy Demand —63 MMBTU / hr.

[0035] As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of’ or “consisting essentially of’ is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of’ or “consists of’ is intended as a transition meaning the exclusion of all but the recited elements except for only minor traces of impurities.

[0036] As those skilled in the art will appreciate, numerous modifications and variations of the present invention are possible considering these teachings, and all such are contemplated hereby. For example, in addition to the embodiments described herein, the present invention contemplates and claims those inventions resulting from the combination of features of the invention cited herein and those of the cited prior art references which complement the features of the present invention. Similarly, it will be appreciated that any described material, feature, or article may be used in combination with any other material, feature, or article, and such combinations are considered within the scope of this invention.

[0037] All of the publications cited in this disclosure are incorporated by reference herein in their entireties for all purposes.

Claims

70205.0689WOU1 (T-12550-W001)What is claimed is:

1. A process for cooling an effluent stream of a light hydrocarbon cracking unit using liquefied natural gas (LNG) comprising: a) producing natural gas and subsequently converting the natural gas into LNG at a production site; b) transporting the LNG to a light hydrocarbon cracking unit in a cryogenic temperature range with the cracking unit producing a cracking unit effluent stream; and c) using the LNG as coolant to cool the cracking unit effluent stream.

2. The process of claim 1, wherein the light hydrocarbon cracking unit cracks light hydrocarbons, comprising methane, ethane, propane, natural gas, or a mixture of these, into hydrogen and liquid C2+ products.

3. The process of claim 1 , wherein step c) further comprises separating C2 and heavier fractions from unconverted methane.

4. The process of claim 1 , wherein step c) comprises using direct contact cooling.

5. The process of claim 1 , wherein step c) comprises indirect contact cooling.

6. The process of claim 1, wherein following step c) the LNG is added as feed into the cracking unit to produce cracked effluent once the LNG has sufficiently warmed.

7. A process for cooling the effluent stream of a light hydrocarbon cracker using liquefied natural gas (LNG) comprising: a) feeding cold LNG to a LNG cold box; b) cooling a reactor effluent stream from the cracker to a temperature of about -100°F or less by passing the stream through the LNG cold box; c) separating the effluent stream into a methane / hydrogen gas stream and a C2+ liquid stream; and d) passing the C2+ liquid stream of step c) to a demethanizer.70205.0689WOU1 (T-12550-W001)8. The process of claim 7, wherein the methane / hydrogen gas stream of step c) is passed to a hydrogen separation unit to separate hydrogen from unconverted methane or other light hydrocarbons.

9. The process of claim 7, wherein the LNG that was fed to the cold box is added into the catalytic cracker following step d).

10. The process of claim 7, wherein the LNG is only fed into the cracker unit once it has warmed.

11. The process of claim 7, wherein the reactor effluent stream is partially compressed and then sent through the LNG cold box.

12. The process of claim 7, wherein the cooling in the LNG cold box is by direct or indirect heat exchange.

13. A process for cooling the effluent stream of a light hydrocarbon cracker with LNG, comprising passing LNG and cracked gas from the light hydrocarbon cracker to a column or contractor where the LNG comes into direct contact with the cracked gas.

14. The process of claim 13, wherein the cracked gas is partially compressed prior to being passed to the column or contactor.

15. The process of claim 14, wherein C2+products are separated from the cooled cracked gas product from the column or contactor and sent to a downstream cryogenic separation comprising a demethanizer for further separation.

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