Low-hydrogen-hydrocarbon-ratio cold box transformation device for Oleflex process and process method of low-hydrogen-hydrocarbon-ratio cold box transformation device

Abstract

The invention discloses an Oleflex process low hydrogen-hydrocarbon ratio cold box transformation device and a process method thereof, and belongs to the technical field of alkane dehydrogenation cryogenic separation processes. According to the device, on the premise that an original cold box unit and a high-low pressure expansion unit are kept unchanged, a first-stage heat exchanger and a third-stage cold box unit are additionally arranged in front of and behind an original system, and an independent MR compressor unit is matched. An upstream raw material is pre-cooled by newly adding a first-stage heat exchanger, so that the load of an original cold box is reduced; and the third-stage cold box unit is used for deeply cooling and separating the gas phase separated by the original cold box, so that residual hydrocarbons are effectively recovered, and the hydrogen-hydrocarbon ratio is obviously reduced from 0.5 to 0.1, thereby greatly reducing the load of the gas entering the expansion machine and the overall energy consumption of the system. Flow optimization is achieved only through external connection, the modification cost is low, the device flow is simple, operation and maintenance are convenient and fast, and the economic benefits of the whole alkane dehydrogenation device can be effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of cryogenic separation technology for alkane dehydrogenation, specifically relating to a device for modifying a cold box with a low hydrogen-to-hydrogen ratio in the Oleflex process and its process method. Background Technology

[0002] The Oleflex process is an alkane catalytic dehydrogenation technology, a chemical process specifically designed to convert low-carbon alkanes (such as propane and isobutane) into corresponding olefins (such as propylene and isobutene).

[0003] With the decline in olefin product prices and the increase in production costs, many cryogenic separation cold box units in China using the Oleflex process with high hydrogen-to-hydrogen ratios are facing pressure to reduce costs and increase efficiency. Therefore, corresponding modifications to the existing Oleflex process are necessary to achieve better economic benefits. Currently, in the cold box units operating in China for alkane dehydrogenation, the high hydrogen-to-hydrogen ratio is the main reason for the high energy consumption. Therefore, reducing the hydrogen-to-hydrogen ratio is the key technical focus for the modification of such alkane dehydrogenation units. However, the original cold box equipment can no longer meet the process requirements for low hydrogen-to-hydrogen ratio parameters, making the technical modification of the original cold box urgent. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of the high hydrogen-to-hydrogen ratio parameter in the existing Oleflex process technology, and to provide a device and process method for modifying the Oleflex process cold box with a low hydrogen-to-hydrogen ratio.

[0005] The specific technical solution adopted in this invention is as follows:

[0006] In a first aspect, the present invention provides a low-hydrogen-to-hydrocarbon ratio cold box retrofit device for the Oleflex process, comprising a newly added primary heat exchanger, a tertiary cold box unit, and an MR compressor unit; the tertiary cold box unit is internally provided with a tertiary heat exchanger, a tertiary separator, and a refrigerant distribution tank; the MR compressor unit is internally provided with an MR compressor, a primary oil separator, a secondary oil separator, a final-stage water cooler, a final-stage separator, an inlet buffer tank, and a refrigerant recovery tank;

[0007] The newly added primary heat exchanger is installed before the original cold box unit to pre-cool the upstream alkane feedstock entering the original cold box unit; the original cold box unit includes a combined cold feed heat exchanger, a feed heat exchanger, a secondary heat exchanger, a flash tank, a primary separator, a secondary separator, and a high-pressure expander outlet separator.

[0008] The tertiary cold box unit is located after the original cold box unit; the A-channel inlet of the tertiary heat exchanger receives the gas phase from the top of the secondary separator and after heat exchange in the secondary heat exchanger; the A-channel outlet of the tertiary heat exchanger is connected to the tertiary separator, and the liquid phase outlet at the bottom of the tertiary separator is connected to the flash tank via a pipeline; the gas phase outlet at the top of the tertiary separator is connected to the E-channel inlet of the tertiary heat exchanger; the E-channel outlet of the tertiary heat exchanger is connected to the inlet of the high-pressure expander in the original high and low pressure expander unit via a pipeline; the outlet of the high-pressure expander is connected to the outlet separator of the high-pressure expander via a pipeline; the inlet of the low-pressure expander in the original high and low pressure expander unit is connected to the cold source outlet of the secondary heat exchanger via a pipeline, and the outlet of the low-pressure expander flows back to the cold source inlet of the secondary heat exchanger via a pipeline;

[0009] The inlet of channel B of the three-stage heat exchanger is connected to the gas phase outlet at the top of the final stage separator via a pipeline; the outlet of channel B of the three-stage heat exchanger is connected to the refrigerant distribution tank; the gas phase outlet at the top and the liquid phase outlet at the bottom of the refrigerant distribution tank are respectively connected to the two inlets of channel C of the three-stage heat exchanger; the outlet of channel C of the three-stage heat exchanger is connected to the inlet buffer tank via a pipeline.

[0010] The outlet at the top of the inlet buffer tank in the MR compressor unit is connected to the inlet of the MR compressor. The outlet of the MR compressor passes through a pipeline in sequence through a primary oil separator, a secondary oil separator, and a final water cooler before connecting to the final separator. The liquid phase outlets at the bottom of the final separator and the inlet buffer tank are both connected to the inlet of the refrigerant recovery tank through pipelines. The outlet of the refrigerant recovery tank flows back to the inlet of the inlet buffer tank through a pipeline.

[0011] Preferably, the inlet of channel A of the newly added primary heat exchanger receives alkane feedstock from upstream, and the outlet of channel A is connected to the heat source inlet of the cold combined feed heat exchanger in the original cold box unit; the inlet of channel B of the newly added primary heat exchanger is connected to the cold source outlet of the cold combined feed heat exchanger, and the feedstock is discharged from the outlet of channel B after receiving the combined feedstock; the inlet of channel C of the newly added primary heat exchanger is connected to the cold source outlet of the cold combined feed heat exchanger, and the dry gas product is discharged from the outlet of channel C.

[0012] Preferably, the gas phase outlet at the top of the flash tank is connected to the cold box via a pipeline through the feed heat exchanger.

[0013] Preferably, the D-channel inlet of the three-stage heat exchanger receives propane feedstock from the original cold box unit; the D-channel outlet of the three-stage heat exchanger is connected to the cold box via a pipeline.

[0014] Furthermore, a flow regulating valve is installed on the pipeline between the D-channel inlet of the three-stage heat exchanger and the propane feedstock outlet of the original cold box unit.

[0015] Preferably, the MR compressor unit is also equipped with a refrigerant dryer, a refrigerant storage and distribution cylinder, and a refrigerant vaporizer; the refrigerant outlet from the refrigerant storage and distribution cylinder passes through the refrigerant vaporizer and the refrigerant dryer in sequence and then connects to the inlet of the inlet buffer tank to provide supplementary refrigerant.

[0016] Preferably, a liquid level regulating valve is provided on the pipeline connecting the bottom liquid phase outlet of the three-stage separator and the flash tank.

[0017] Preferably, the MR compressor is a screw compressor or a centrifugal compressor.

[0018] Preferably, the three-stage heat exchanger is equipped with shut-off valves at both the B-channel inlet and the C-channel outlet.

[0019] Secondly, the present invention provides a process method for modifying a cold box device using the Oleflex process with a low hydrogen-to-hydrogen ratio as described in the first aspect. The newly added first-stage heat exchanger receives room-temperature alkane feedstock from upstream and cools the room-temperature alkane feedstock to -15℃ to -30℃ before it enters the original cold box unit. The heat exchanger in the original cold box unit cools the reaction products from -15℃ to -30℃ to -50℃ to -80℃. The third-stage heat exchanger in the third-stage cold box can cool the reaction products from -50℃ to -80℃ to -100℃ to -130℃. The energy consumption of the MR compressor unit is controlled between 1000KW and 4000KW.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] The modification device provided by this invention, while fully retaining the original cold box equipment, successfully overcomes the key problems of high system energy consumption caused by the high hydrogen-to-hydrogen ratio in the existing Oleflex process by adding a set of high-efficiency external devices and optimizing and integrating the process flow.

[0022] The device boasts a simple process design, requiring no complex modifications to existing core equipment. This not only minimizes retrofit investment costs but also translates to more convenient operation and significantly reduced daily operating and maintenance costs. Ultimately, this invention maximizes the benefits of cold box retrofitting with minimal economic and technical costs, effectively enhancing the economic performance and market competitiveness of the entire alkane dehydrogenation unit. Experiments have shown that this process can significantly reduce the hydrogen-to-hydrogen ratio in the original alkane dehydrogenation unit from 0.5 to 0.1. This fundamental improvement directly leads to a substantial reduction in system energy consumption, resulting in savings in operating costs. Attached Figure Description

[0023] Figure 1 This is an overall flow chart of the Oleflex process low-hydrogen-to-hydrogen ratio cold box modification device provided in this embodiment;

[0024] Figure 2 This is a schematic diagram showing the connection between the three-stage cold box unit and the MR compressor unit provided in this embodiment;

[0025] Figure 3 This is a schematic diagram of the original cold box unit provided in this embodiment;

[0026] In the diagram: 1. New primary heat exchanger; 2. Original cold box unit; 2-1 combined cold feed heat exchanger; 2-2 feed heat exchanger; 2-3 secondary heat exchanger; 2-4 flash tank; 2-5 primary separator; 2-6 secondary separator; 2-7 high-pressure expander outlet separator; 3. Tertiary cold box unit; 4. Original high and low pressure expander unit; 4-1 high-pressure expander; 4-2 low-pressure expander; 5. MR compressor unit; 6. Tertiary heat exchanger; 7. Tertiary separator; 8. Refrigerant distribution tank; 9. MR compressor; 10. Primary oil separator; 11. Secondary oil separator; 12. Final stage water cooler; 13. Final stage separator; 14. Inlet buffer tank; 15. Refrigerant recovery tank; 16. Refrigerant dryer; 17. Refrigerant storage and distribution cylinder; 18. Refrigerant vaporizer.

[0027] Interface 1 N01, Interface 2 N02, Interface 3 N03, Interface 4 N04, Interface 5 N05, Interface 6 N06, Interface 7 N07, Interface 9 N09, Interface 10 N10, Interface 11 N11, Interface 21 N21, Interface 22 N22, Interface 23 N23, Interface 24 N24, Interface 25 N25, Interface 26 N26, Interface 27 N27, Interface 31 N31, Interface 41 N41, Interface 42 N42. Detailed Implementation

[0028] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly, provided that there is no mutual conflict.

[0029] In the description of this invention, it should be understood that the terms "first" and "second" are used only for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first" and "second" may explicitly or implicitly include at least one of those features.

[0030] like Figure 1 As shown, this embodiment, as a preferred embodiment of the present invention, provides a cold box retrofit device for the Oleflex process with a low hydrogen-to-hydrogen ratio. The device includes a newly added primary heat exchanger 1, the original cold box unit 2, a tertiary cold box unit 3, the original high and low pressure expander unit 4, and an MR compressor unit 5.

[0031] It should be noted that this invention is a modification of the existing Oleflex process unit. While retaining the original cold box equipment, it maximizes the benefits of the cold box modification at minimal economic cost. The original cold box unit 2 conventionally includes a combined cold feed heat exchanger 2-1, a feed heat exchanger 2-2, a secondary heat exchanger 2-3, a flash tank 2-4, a primary separator 2-5, a secondary separator 2-6, a high-pressure expander outlet separator 2-7, a gas-liquid distributor, and a redistributor. The specific structure of the original cold box unit 2 used in this embodiment is as follows... Figure 3 As shown. This invention does not specifically limit these devices and their connection relationships within the original cold box unit 2. The Oleflex process apparatus is prior art known in the art, and its specific internal connection methods are clear to those skilled in the art.

[0032] The core of this invention lies in providing a new external modification device. This device, while completely preserving and utilizing the original cold box equipment and its internal processes, optimizes the overall process through external interface connection. Therefore, as long as the original cold box unit 2 can perform its basic cryogenic separation function, the applicability and scope of protection of this patented improved device are not affected regardless of the specific internal equipment connection method.

[0033] The original cold box unit 2 is an independent cold box structure, which is provided with the first interface N01, the second interface N02, the third interface N03, the fourth interface N04, the fifth interface N05, the sixth interface N06, the seventh interface N07, the ninth interface N09, the tenth interface N10 and the eleventh interface N11 for connecting the upstream and downstream.

[0034] In the device provided by this invention, a new primary heat exchanger 1 is installed before the original cold box unit 2 to pre-cool the upstream alkane feedstock entering the original cold box unit 2. By adding a primary heat exchanger 1, the upstream feedstock is pre-cooled by the system's internal cold source before entering the original cold box unit 2, which has the highest energy consumption, directly reducing the heat exchange load of the original cold box. Specifically:

[0035] The A-channel inlet of the newly added primary heat exchanger 1 receives alkane feedstock from upstream. The A-channel outlet is connected via a pipeline to the heat source inlet of the cold combined feed heat exchanger 2-1 on the original cold box unit 2 through the first interface N01. The B-channel inlet of the newly added primary heat exchanger 1 is connected to the cold source outlet of the cold combined feed heat exchanger 2-1 through the second interface N02, receiving the combined feedstock and exiting from the B-channel outlet. The C-channel inlet of the newly added primary heat exchanger 1 is connected to the cold source outlet of the cold combined feed heat exchanger 2-1 through the third interface N03, receiving dry gas product and exiting from the C-channel outlet.

[0036] In the device provided by this invention, a three-stage cold box unit 3 is installed after the original cold box unit 2. The three-stage cold box unit 3 contains a three-stage heat exchanger 6, a three-stage separator 7, and a refrigerant distribution tank 8. The MR compressor unit 5 contains an MR compressor 9, a first-stage oil separator 10, a second-stage oil separator 11, a final-stage water cooler 12, a final-stage separator 13, an inlet buffer tank 14, and a refrigerant recovery tank 15. Through the three-stage cold box unit 3, the gas phase initially separated in the original cold box unit 2 undergoes further deep cooling and separation, condensing and recovering a large amount of residual liquefiable hydrocarbon components. This improves the overall yield of hydrocarbon products and results in higher hydrogen purity and a lower flow rate in the gas entering the expander, fundamentally reducing the expander's power consumption.

[0037] like Figure 2 As shown, the three-stage cold box unit 3 is an independent cold box structure, which has twenty-first interface N21, twenty-second interface N22, twenty-third interface N23, twenty-fourth interface N24, twenty-fifth interface N25, twenty-sixth interface N26, and twenty-seventh interface N27 for connecting upstream and downstream. The MR compressor unit 5 has forty-first interface N41 and forty-second interface N42. Details are as follows:

[0038] The A-channel inlet of the tertiary heat exchanger 6 is connected to the eleventh interface N11 of the original cold box unit 2 via the twenty-fourth interface N24. The A-channel inlet of the tertiary heat exchanger 6 receives the gas phase from the top of the secondary separator 2-6, which has undergone heat exchange in the secondary heat exchanger 2-3. The A-channel outlet of the tertiary heat exchanger 6 is connected to the tertiary separator 7. The liquid phase outlet at the bottom of the tertiary separator 7 is connected to the flash tank 2-4 via a pipeline through the twenty-third interface N23 and the tenth interface N10. A level regulating valve is installed on the pipeline between the twenty-third interface N23 and the tenth interface N10. The gas phase outlet at the top of the flash tank 2-4, after heat exchange in the feed heat exchanger 2-2, is connected to the cold box via the sixth interface N06.

[0039] The top gas phase outlet of the third-stage separator 7 is connected to the E-channel inlet of the third-stage heat exchanger 6. The E-channel outlet of the third-stage heat exchanger 6 is connected via a pipeline sequentially through the 25th interface N25 and the 31st interface N31 of the original high- and low-pressure expander unit 4. The E-channel outlet of the third-stage heat exchanger 6 is connected via a pipeline to the inlet of the high-pressure expander 4-1 in the original high- and low-pressure expander unit 4. The outlet of the high-pressure expander 4-1 is connected via a pipeline to the high-pressure expander outlet separator 2-7. The inlet of the low-pressure expander 4-2 in the original high- and low-pressure expander unit 4 is connected via a pipeline to the cold source outlet of the second-stage heat exchanger 2-3. The outlet of the low-pressure expander 4-2 flows back to the cold source inlet of the second-stage heat exchanger 2-3 via a pipeline.

[0040] The inlet of channel B of the three-stage heat exchanger 6 is connected to the gas phase outlet at the top of the final stage separator 13 via the twenty-sixth port N26, the forty-second port N42 on the MR compressor unit 5, and the port N42 on the top of the final stage separator 13. The outlet of channel B of the three-stage heat exchanger 6 is connected to the refrigerant distribution tank 8. The gas phase outlet at the top and the liquid phase outlet at the bottom of the refrigerant distribution tank 8 are connected to the two inlets of channel C of the three-stage heat exchanger 6, respectively. The outlet of channel C of the three-stage heat exchanger 6 is connected to the inlet buffer tank 14 via the twenty-seventh port N27, the forty-first port N41 on the MR compressor unit 5, and the port N27 on the top of the final stage separator 5. Preferably, a shut-off valve is provided at both the inlet of channel B and the outlet of channel C of the three-stage heat exchanger 6.

[0041] The inlet of the D channel of the three-stage heat exchanger 6 is connected to the original cold box unit 2 via the twenty-second interface N22 and the ninth interface N09, receiving propane feedstock from the original cold box unit 2. A flow regulating valve is installed on the pipeline between the twenty-second interface N22 and the ninth interface N09. The pipeline at the outlet of the D channel of the three-stage heat exchanger 6 is connected to the cold box via the twenty-first interface N21.

[0042] In the device provided by this invention, the outlet at the top of the inlet buffer tank 14 in the MR compressor unit 5 is connected to the inlet of the MR compressor 9. The outlet of the MR compressor 9 passes through a pipeline sequentially through a primary oil separator 10, a secondary oil separator 11, and a final-stage water cooler 12 before connecting to the final-stage separator 13. The liquid phase outlets at the bottom of both the final-stage separator 13 and the inlet buffer tank 14 are connected to the inlet of the refrigerant recovery tank 15 via pipelines. The outlet of the refrigerant recovery tank 15 flows back to the inlet of the inlet buffer tank 14 via a pipeline.

[0043] Preferably, the MR compressor unit 5 provided in this embodiment also includes a refrigerant dryer 16, a refrigerant storage and distribution cylinder 17, and a refrigerant vaporizer 18. The refrigerant outlet from the refrigerant storage and distribution cylinder 17 passes sequentially through the refrigerant vaporizer 18 and the refrigerant dryer 16 before connecting to the inlet of the inlet buffer tank 14 to provide supplementary refrigerant. It should be noted that the MR compressor 9 in this invention can be a screw compressor or a centrifugal compressor, and those skilled in the art can choose according to the actual situation.

[0044] Additionally, it should be noted that in this invention, the fourth interface N04 on the original cold box unit 2 serves as the inlet for replenishing fresh raw materials. The fifth interface N05 serves as the outlet for liquid products from the original cold box unit 2, connected to the cold box via a pipeline. The seventh interface N07 serves as the outlet for replenishing dry gas products from the original cold box unit 2, connected to the cold box via a pipeline.

[0045] This embodiment also provides a process method using the above-described modified device, as follows:

[0046] (1) Alkane feedstock process

[0047] The upstream alkane feedstock first enters channel A of the newly added primary heat exchanger 1, where it is initially pre-cooled. The pre-cooled feedstock then enters the original cold box unit 2, where it undergoes conventional cooling via the combined cold feed heat exchanger 2-1, feed heat exchanger 2-2, and secondary heat exchanger 2-3, followed by conventional gas-liquid separation via primary separator 2-5 and secondary separator 2-6. The gas phase exiting from the top of secondary separator 2-6 enters channel 6A of the tertiary heat exchanger in the tertiary cold box unit 3, where it is deeply cooled by the mixed refrigerant.

[0048] The cooled gas-liquid mixture enters the three-stage separator 7 for final separation: the bottom liquid phase returns to the flash tank 2-4 in the original cold box unit 2, flowing into the main process to ensure full hydrocarbon recovery. The top gas phase, typically high-purity hydrogen, enters the E channel of the three-stage heat exchanger 6 for reheating, then drives the high-pressure expander 4-1 to perform refrigeration. Finally, after separation by the high-pressure expander outlet separator 2-7, it is sent out of the cold box as a hydrogen-rich gas product or recycled hydrogen. Simultaneously, propane feedstock drawn from the original cold box unit 2 enters the D channel of the three-stage heat exchanger 6, is cooled, and then sent out as additional refrigeration for users or products.

[0049] The cooling capacity of the newly added primary heat exchanger 1 comes from the combined low-temperature feed and dry gas products from the outlet of the combined feed heat exchanger 2-1 in the original cold box unit 2. Through cross heat exchange, the cooling capacity is efficiently recovered and utilized, and the upstream raw materials are pre-cooled.

[0050] (2) Mixed refrigerant MR refrigeration cycle process

[0051] The gaseous refrigerant mixture exits from the top of the final stage separator 13 and enters channel B of the tertiary heat exchanger 6. After being cooled, it partially liquefies and enters the refrigerant distribution tank 8 for gas-liquid separation. The gaseous refrigerant mixture at the top and the liquid refrigerant mixture at the bottom of the refrigerant distribution tank 8 enter the two inlets of channel C of the tertiary heat exchanger 6, respectively. After heat exchange in the tertiary heat exchanger 6, it flows back to the inlet buffer tank 14.

[0052] The gaseous mixed refrigerant is compressed by the MR compressor 9, and after being de-oiled by the first-stage oil separator 10 and the second-stage oil separator 11, it is cooled by the final-stage water cooler 12 and then enters the final-stage separator 13 to complete a closed-loop cycle.

[0053] The liquid-phase mixed refrigerant from the bottom of the final stage separator 13 and the inlet buffer tank 14 enters the refrigerant recovery tank 15 and can be pumped back to the inlet buffer tank 14 to maintain refrigerant balance.

[0054] Following the above process, the newly added primary heat exchanger 1 cools the upstream ambient-temperature alkane feedstock to -15℃ to -30℃ before it enters the original cold box unit 2. The heat exchanger in the original cold box unit 2 cools the reaction products from -15℃ to -30℃ to -50℃ to -80℃. The tertiary heat exchanger 6 in the tertiary cold box 3 can cool the reaction products from -50℃ to -80℃ to -100℃ to -130℃. The MR compressor unit 5 has its power consumption controlled between 1000KW and 4000KW. This device can reduce the hydrogen-to-hydrogen ratio from the original 0.5 to 0.1.

[0055] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. An Oleflex process low hydrogen hydrocarbon ratio cold box revamp, characterized in that, It comprises a newly added primary heat exchanger (1), a three-stage cold box unit (3) and an MR compressor set (5); the three-stage cold box unit (3) is internally provided with a three-stage heat exchanger (6), a three-stage separator (7) and a refrigerant distribution tank (8); the MR compressor set (5) is internally provided with an MR compressor (9), a primary oil separator (10), a secondary oil separator (11), a final-stage water cooler (12), a final-stage separator (13), an inlet buffer tank (14) and a refrigerant recovery tank (15); The newly added primary heat exchanger (1) is arranged before the original cold box unit (2) to precool the upstream alkane raw material entering the original cold box unit (2); the original cold box unit (2) comprises a cold combined feed heat exchanger (2-1), a feed heat exchanger (2-2), a secondary heat exchanger (2-3), a flash tank (2-4), a primary separator (2-5), a secondary separator (2-6) and a high-pressure expander outlet separator (2-7); The three-stage cold box unit (3) is arranged after the original cold box unit (2); the A channel inlet of the three-stage heat exchanger (6) receives the secondary separator gas phase from the top of the secondary separator (2-6) and passing through the secondary heat exchanger (2-3); the A channel outlet of the three-stage heat exchanger (6) is communicated with the three-stage separator (7), and the liquid phase outlet at the bottom of the three-stage separator (7) is communicated with the flash tank (2-4) through a pipeline; the gas phase outlet at the top of the three-stage separator (7) is connected with the E channel inlet of the three-stage heat exchanger (6); the E channel outlet of the three-stage heat exchanger (6) is connected with the inlet of the high-pressure expander (4-1) in the original high-low pressure expander set (4) through a pipeline; the outlet of the high-pressure expander (4-1) is connected with the high-pressure expander outlet separator (2-7) through a pipeline; the inlet of the low-pressure expander (4-2) in the original high-low pressure expander set (4) is connected with the cold source outlet of the secondary heat exchanger (2-3) through a pipeline, and the outlet of the low-pressure expander (4-2) is returned to the cold source inlet of the secondary heat exchanger (2-3) through a pipeline; The B channel inlet of the three-stage heat exchanger (6) is communicated with the gas phase outlet at the top of the final-stage separator (13) through a pipeline; the B channel outlet of the three-stage heat exchanger (6) is connected with the refrigerant distribution tank (8); the gas phase outlet at the top of the refrigerant distribution tank (8) and the liquid phase outlet at the bottom thereof are respectively connected with the two inlets of the C channel of the three-stage heat exchanger (6); the C channel outlet of the three-stage heat exchanger (6) is connected with the inlet buffer tank (14) through a pipeline; The outlet at the top of the inlet buffer tank (14) in the MR compressor set (5) is communicated with the inlet of the MR compressor (9), and the outlet of the MR compressor (9) is communicated with the final-stage separator (13) after sequentially passing through the primary oil separator (10), the secondary oil separator (11) and the final-stage water cooler (12) through pipelines; the liquid phase outlets at the bottoms of the final-stage separator (13) and the inlet buffer tank (14) are both connected with the inlet of the refrigerant recovery tank (15) through pipelines; the outlet of the refrigerant recovery tank (15) is returned to the inlet of the inlet buffer tank (14) through a pipeline.

2. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofitting apparatus of claim 1, wherein, The A passage inlet of the added primary heat exchanger (1) receives the alkane raw material from the upstream, and the A passage outlet is communicated with the heat source inlet of the cold combined feed heat exchanger (2-1) in the original cold box unit (2); the B passage inlet of the added primary heat exchanger (1) is communicated with the cold source outlet of the cold combined feed heat exchanger (2-1), and the combined feed is received from the B passage outlet; the C passage inlet of the added primary heat exchanger (1) is communicated with the cold source outlet of the cold combined feed heat exchanger (2-1), and the dry gas product is received from the C passage outlet.

3. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofitting apparatus of claim 1, wherein, The gas phase outlet at the top of the flash tank (2-4) is connected to the cold box through the feed heat exchanger (2-2) by a pipeline.

4. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofitting apparatus of claim 1, wherein, The D passage inlet of the tertiary heat exchanger (6) receives the propane raw material from the original cold box unit (2); the D passage outlet of the tertiary heat exchanger (6) is connected to the cold box by a pipeline.

5. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofitting apparatus of claim 4, wherein, A flow regulating valve is arranged on the pipeline between the D passage inlet of the tertiary heat exchanger (6) and the propane raw material outlet of the original cold box unit (2).

6. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofitting apparatus of claim 1, wherein, The MR compressor set (5) is further provided with a refrigerant dryer (16), a refrigerant storage cylinder (17) and a refrigerant gasifier (18); the refrigerant outlet from the refrigerant storage cylinder (17) is communicated with the inlet of the inlet buffer tank (14) after passing through the refrigerant gasifier (18) and the refrigerant dryer (16) in sequence, and provides the supplementary refrigerant.

7. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofit apparatus of claim 1, wherein, A liquid level regulating valve is arranged on the pipeline connecting the liquid phase outlet at the bottom of the tertiary separator (7) and the flash tank (2-4).

8. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofit apparatus of claim 1, wherein, The MR compressor (9) is a screw compressor or a centrifugal compressor.

9. The Oleflex process low hydrogen to hydrocarbon ratio cold box retrofit apparatus of claim 1, wherein, A cut-off valve is arranged at the B passage inlet and the C passage outlet of the tertiary heat exchanger (6).

10. A process for revamping a low-hydrocarbon-ratio cold box plant using the Oleflex process of any of claims 1 to 9, wherein the process comprises: introducing a feed stream comprising hydrogen and methane into the cold box; and recovering a product stream comprising ethylene and unreacted hydrogen and methane from the cold box. The added primary heat exchanger (1) receives the normal temperature alkane raw material from the upstream, and the normal temperature alkane raw material is cooled to-15℃~-30℃ and then enters the original cold box unit (2); the heat exchanger in the original cold box unit (2) cools the reaction product at-15℃~-30℃ to-50℃~-80℃; the tertiary heat exchanger (6) in the tertiary cold box (3) can cool the reaction product at-50℃~-80℃ to-100℃~-130℃; and the energy consumption power of the MR compressor set (5) is controlled to be between 1000KW and 4000KW.

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