OPERATING AT HIGH AMBIENT TEMPERATURES

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application takes priority to Indian provisional patent application no. 201741002107 entitled "System for the liquefaction of nitrogen operating at high ambient temperatures" filed on 19-01-2017.

FIELD OF THE INVENTION

[0002] The disclosure relates generally to refrigerating a feed gas in a multistage refrigeration system and in particular to method, system and refrigerant compositions for the liquefaction of feed gas at high ambient temperatures in a multi- staged air-cooled liquefaction system.

DESCRIPTION OF THE RELATED ART

[0003] The Kapitza process is used to mass produce liquid air products in large commercial air separation plants. Smaller quantities of liquid air products may be produced using Stirling cycle liquefiers. Kapitza and Stirling processes can be used to liquefy feed gas such as nitrogen but both of these are limited by the use of moving components at temperatures much lower than the ambient temperature to produce the refrigeration. The use of moving parts makes these plants very expensive and complex. Cryogenic mixed refrigerant processes may be used to produce liquefied feed gas such as liquefied natural gas and nitrogen below the ambient temperature without using any moving parts. The efficiency of liquefaction plants increases with an increase in the cooling capacity or the liquid production rate. There is a need for small liquid nitrogen plants with a production capacity of only a few L/h, typically from less than 1 L/h to about 10 L/h, which could be used by research organizations, hospitals, veterinary clinics etc. for storing biological materials such as semen, frozen embryos, tissues, and cultures at low temperature. Because of the smaller efficiency of small compressors and motors, a number of innovations need to be made to make small nitrogen liquefaction plants efficient.

[0004] European patent application EP1092930A1 discloses a nitrogen liquefaction process which combines auto refrigeration with one or more closed-loop multicomponent refrigeration systems. US patent US6308531B2 discloses a system comprising two integrated refrigeration systems wherein one of the systems utilizes one or more vaporizing refrigerant cycles to provide refrigeration below about -40° C and utilizes a gas expander cycle to provide refrigeration below about -100° C. A process for liquefaction of a nitrogen stream produced by a cryogenic air separation unit by employing cold liquid natural gas (LNG) as refrigerant is disclosed in US granted patent US 5139547 A. US granted patent 5454226 A relates to a process for liquefying a gas by means of a refrigerating cycle comprising a so-called "warm" expansion turbine and a so-called "cold" expansion turbine fed respectively at a first temperature and at a second temperature below the first temperature.

[0005] Liquefaction uses the principle of refrigeration, i.e., absorb heat from a low- temperature source and reject heat to the ambient. Liquefiers that operate on G-M, Stirling cycle refrigerators/liquefiers reject heat to the ambient using water supplied by a separate cold water source (water chiller) which increases cost, energy, and physical space. This is particularly evident in tropical and sub-tropical countries where the ambient temperatures are usually high. Air-cooling is possible but this method is less efficient than water-cooling. There is a need, therefore, for an improvement in the art for a high-efficiency air-cooled small liquefier system for nitrogen and other gases. SUMMARY OF THE INVENTION

[0006] In various embodiments, the invention relates to a method of refrigerating a feed gas. A multistage refrigeration system including at least a first stage circulating a first refrigerant composition and a second stage circulating a second refrigerant composition is provided. Each stage includes at least a compressor for compressing the refrigerant composition to a high pressure refrigerant composition, a condenser for cooling the high pressure refrigerant composition, a throttle for throttling the high pressure refrigerant composition to a low pressure refrigerant composition, and a heat exchanger for evaporating the low pressure refrigerant composition. The first and second stages are thermally coupled at a first heat exchanger (HX-3) and a second heat exchanger (HX-6). The multistage refrigeration system is operated to cool the feed gas in the first stage and in the second stage. The first refrigerant composition may include at least one of ethane, ethylene, propane, n-butane, isobutene, n-pentane, isopentane, R134a, R1234yf, R1234ze, R1234ze(Z), R744, R125, R32, R152a, R23, R116, R245ca, R236ea,or R227ea. The second refrigerant composition may include at least one of nitrogen, neon, helium, ethane, ethylene, propane, propylene, isobutene, n-butane, R14, R23, R116, R218, or R1234yf. The feed gas may be nitrogen, argon, helium, hydrogen, neon, carbon monoxide, methane, ethane, propane, fluorine, nitrous oxide, refrigerants R14, R23, Rl 16, natural gas, or mixtures thereof.

[0007] In various embodiments, the invention relates to a method for liquefaction of a feed fluid in gaseous state. A multistage refrigeration system including at least a first stage circulating a first refrigerant composition and a second stage circulating a second refrigerant composition is provided. Each stage includes at least a compressor for compressing the refrigerant composition to a high pressure refrigerant composition, a condenser for cooling the high pressure refrigerant composition, a throttle for throttling the high pressure refrigerant composition to a low pressure refrigerant composition, and a heat exchanger for evaporating the low pressure refrigerant composition. The first and second stages are thermally coupled at a first heat exchanger (HX-3) and a second heat exchanger (HX-6). The multistage refrigeration system is operated to cool the feed fluid in the first stage and in the second stage. The feed fluid further throttled and passed through a phase separator for separating the liquefied feed fluid from the feed fluid in vapor state. The first refrigerant composition may include at least one of ethylene, propane, n-butane, isobutene, n-pentane, isopentane, R134a, R1234yf, R1234ze, R1234ze(Z), R744, R125, R32, R152a, R23, R116, R245ca, R236ea, or R227ea. The second refrigerant may include at least one of nitrogen, neon, helium, ethane, ethylene, propane, propylene, isobutene, n-butane, R14, R23, R116, R218, or R1234yf. The feed fluid may be nitrogen, argon, helium, hydrogen, neon, carbon monoxide, methane, ethane, propane, fluorine, nitrous oxide, refrigerants R14, R23, R116, natural gas, or mixtures thereof. In some embodiments, the feed fluid enters the first heat exchanger (HX-3) at a temperature of 30°C or greater and a pressure in the range of 5-50 bar. In some embodiments, the feed fluid is cooled to a temperature in the range of -20 °C to -35 °C in the first stage and to a temperature of -35 °C or lower in the second stage. In some embodiments, operating the multistage refrigeration system to cool the feed fluid comprises includes passing the feed fluid serially through at least the first heat exchanger (HX-3), a third heat exchanger (HX-4) and optionally a fourth heat exchanger (HX-5). In one embodiment, the third heat exchanger (HX-4) and fourth heat exchangers (HX-5) are integrated to form a single heat exchanger. In some embodiments, operating the multistage refrigeration system to cool the feed fluid further includes evaporating low pressure first refrigerant composition in the first heat exchanger (HX-3) to cool at least the feed fluid and high pressure second refrigerant composition; cooling the feed fluid and high pressure second refrigerant in the third heat exchanger (HX-4) using at least low pressure second refrigerant composition; evaporating low pressure second refrigerant in the third heat exchanger (HX-3) or the fourth heat exchanger (HX-5) to cool the feed fluid; and cooling high pressure first refrigerant composition in the second heat exchanger (HX-6) using low pressure second refrigerant provided at a temperature in the range of -20°C to -35°C. In some embodiments, the first refrigerant composition is throttled to a pressure in the range of 1-6 bar. In some embodiments, the second refrigerant composition is throttled to a pressure in the range of 1-6 bar. In some embodiments, the first refrigerant composition is compressed to a pressure in the range of 10-30 bar. In some embodiments, the second refrigerant composition is compressed to a pressure in the range of 15-30 bar. In some embodiments, operating the multistage refrigeration system to cool the feed fluid includes passing said low pressure second refrigerant composition through at least three heat exchangers (HX-4, HX-3, HX-6) before recirculating to compressor. In some embodiments, operating the multistage refrigeration system to cool the feed fluid includes passing said feed fluid in vapor state through at least one heat exchanger (HX-4, HX-3).In some embodiments, the high pressure refrigerant composition is partially or completely condensed in the condenser (HX-1, HX-2). In some embodiments, the low pressure refrigerant composition is partially or completely evaporated in the heat exchanger to cool the feed fluid. In some embodiments, the feed fluid is nitrogen. In some embodiments, the feed fluid liquefied is in the range of 0.5-10 L/h. In some embodiments, the feed fluid liquefaction rate is at least 0.7 L h for a feed fluid pressure of 10 bar.

[0008] In various embodiments, the invention relates to a system for liquefaction of a feed fluid in gaseous state. The system includes a multistage refrigeration system made of at least a first stage for circulating a first refrigerant composition and a second stage for circulating a second refrigerant composition, each stage including at least: a compressor for compressing the refrigerant composition to a high pressure refrigerant composition, a condenser for cooling the high pressure refrigerant composition, a throttle for throttling the high pressure refrigerant composition to a low pressure refrigerant composition, and a heat exchanger for evaporating the low pressure refrigerant composition. The first and second stages are thermally coupled at a first heat exchanger (HX-3) and a second heat exchanger (HX-6). The system also includes a throttle for throttling the feed fluid and a phase separator for the separating liquefied feed fluid from the feed fluid in vapor state. The feed fluid may be nitrogen, argon, helium, hydrogen, neon, carbon monoxide, methane, ethane, propane, fluorine, nitrous oxide, refrigerants R14, R23, Rl 16, natural gas, or mixtures thereof. The first stage is configured to cool the feed fluid to a temperature in the range of -20 °C to -35 °C and the second stage is configured to cool the fluid to a temperature of -30 °C or lower in the second stage. The system may further include a third heat exchanger (HX-4) and optionally a fourth heat exchanger (HX-5). In some embodiments, the feed fluid is configured to serially pass through at least the first heat exchanger (HX-3), the third heat exchanger (HX-4) and optionally the fourth heat exchanger (HX-5). In one embodiment, the third heat exchanger (HX-4) and fourth heat exchangers (HX-5) are integrated to form a single heat exchanger. In some embodiments, the system is configured to: evaporate the low pressure first refrigerant composition in the first heat exchanger (HX-3) to cool at least the feed fluid and high pressure second refrigerant composition; cool the feed fluid and high pressure second refrigerant in the third heat exchanger (HX-4) using at least low pressure second refrigerant composition; evaporate low pressure second refrigerant in the third heat exchanger (HX-4) or fourth heat exchanger (HX-5) to cool the feed fluid; and cool high pressure first refrigerant composition in the second heat exchanger (HX-6) using low pressure second refrigerant provided at a temperature in the range of -20°C to - 35°C. In some embodiments, the first heat exchanger (HX-3) includes at least: a feed fluid inlet configured to receive the feed fluid; a first inlet configured to receive the low pressure first refrigerant from a first throttle (V-l); a second inlet configured to receive the high pressure second refrigerant composition from a second condenser (HX-1); a third inlet configured to receive the low pressure second refrigerant composition from the third heat exchanger (HX-4); or a vapor inlet configured to receive feed fluid vapor exiting the phase separator or the third heat exchanger (HX-4). In some embodiments, the third heat exchanger (HX-4) includes at least: a feed fluid inlet configured to receive the feed fluid from the first heat exchanger (HX-3); a first inlet configured to receive the high pressure second refrigerant composition from the first heat exchanger (HX-3); a second inlet configured to receive the low pressure second refrigerant composition from the fourth heat exchanger (HX-5); or a vapor inlet configured to receive the feed gas exiting the phase separator (124). In some embodiments, the fourth heat exchanger (HX- 5) includes at least: a feed fluid inlet configured to receive the feed fluid from the third heat exchanger (HX-4); or a first inlet configured to receive the second refrigerant composition from a second throttle (V-2). In some embodiments, the second heat exchanger (HX-6) comprises at least: a first inlet configured to receive the first refrigerant composition from a first condenser (HX-2); or a second inlet configured to receive the second refrigerant composition from the first heat exchanger (HX-3) or third heat exchanger (HX-4). In some embodiments, the capacity of the system of liquefy the feed fluid is in the range of 0.5-10 L h. In some embodiments, the feed gas liquefaction rate of the system is at least 0.7 L/h for a feed fluid pressure of 10 bar .In some embodiments, the one or more of the heat exchangers is a shell and tube heat exchanger, Giaque-Hampson type heat exchanger, or a Plate-Fin heat exchanger.In some embodiments, the feed fluid is nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0010] FIG. 1A illustrates a method for air-cooled liquefaction of a feed fluid in a gaseous state using a multistage refrigeration system.

[0011] FIG. IB illustrates a method of operation of a first or second stage refrigerator of the multistage refrigeration system.

[0012] FIG. 2A illustrates a system for multi-staged air-cooled liquefaction of a feed fluid in gaseous state.

[0013] FIG. 2B illustrates a system for multi-staged air-cooled liquefaction of a feed fluid in gaseous state.

[0014] FIG. 2C illustrates a system for multi-staged air-cooled liquefaction of a feed fluid in gaseous state.

[0015] FIG. 3 illustrates a state of the art system for liquefaction of a feed fluid in gaseous state.

[0016] Referring to the drawings, like numbers indicate like parts throughout the views. DETAILED DESCRIPTION

[0017] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0018] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0019] The invention in its various embodiments proposes a method for refrigerating a feed gas in a multistage air-cooled refrigeration system. In various embodiments, a method, system and refrigerant compositions for liquefaction of a feed fluid in gaseous state in a low capacity, multi-staged, air-cooled refrigeration system with a high efficiency is provided. The feed gas liquefied may be at high ambient temperatures. The method efficiently uses the available cold in the system and optimum refrigerant compositions to liquefy feed gas at high efficiency from feed gas supplied at high ambient temperatures.

[0020] In various embodiments, a method 1000 for refrigerating and liquefying a feed fluid in gaseous state in a multistage air-cooled refrigeration system is as illustrated in FIG. 1A. In step 1001, the feed gas enters a first refrigeration stage at a high ambient temperature typically 30°C or greater and a typical pressure of 5 to 50 bar, more typically 5 to 20 bar where it is cooled to a temperature in the range of -20°C to -35°C against a first refrigerant composition. In one embodiment, the ambient temperature may increase to 35°C or higher. In step 1002, the feed gas is further cooled in a second refrigeration stage to a temperature of -30°C or lower against a second refrigerant composition. In one embodiment, the feed gas is cooled to a temperature of -160°C or lower to liquefy it. The feed fluid is throttled to a lower pressure using an expansion valve or throttle in step 1003 and then passed through a phase separator in step 1004. The resulting liquefied feed gas and vapor phases are collected separately in step 1005. The liquefied feed gas may be stored or immediately used for applications. The collected vapor phase feed gas may be supplied back to the multistage refrigeration system for improving its efficiency and liquid production rate.

[0021] In some embodiments, the feed gas may include nitrogen, argon, helium, hydrogen, neon, carbon monoxide, methane, ethane, propane, fluorine, nitrous oxide, refrigerants R14, R23, R116, natural gas, or mixtures thereof. In some embodiments, the feed gas is nitrogen.

[0022] In various embodiments, the multistage system may include more than two stages. In some embodiments, the multistage system may include up to 10 stages. In one embodiment, the refrigerator stages may work on different thermodynamic processes and may use different refrigerants.

[0023] In some embodiments, the capacity of the system is in the range of 0.5- lOL/h. In some embodiments, the efficiency of liquefaction of the feed gas is at least 0.7L/h for a feed fluid pressure of 10 bar.

[0024] In various embodiments, a method 2000 for operating one or more stages of the multistage refrigeration system used in refrigerating the feed fluid in gaseous state is as illustrated in FIG. IB. In one embodiment, the multistage includes at least a first stage and a second stage refrigeration. In step 2001, the refrigerant composition circulating in each stage of the multistage refrigeration system is compressed to obtain a high pressure refrigerant at elevated temperatures. In step 2002, the high pressure refrigerant composition is sent to an aftercooler or condenser unit for cooling the heat from compression. The refrigerant composition may be partially or fully condensed in the condenser unit. In step 2003, the high pressure refrigerant is additionally cooled in one or more heat exchangers associated with the multistage refrigerant system. In step 2004, the cooled high pressure refrigerant is throttled in an expansion valve or throttle to obtain a low pressure refrigerant. In step 2005, the refrigerant is evaporated in a heat exchanger associated with the refrigerant system. The evaporated refrigerant cools the feed fluid passing through the heat exchanger. In some embodiments, the refrigerant composition may be partially evaporated. In other embodiments, the refrigerant composition may be fully evaporated. In step 2006, the used refrigerant stream passes through one or more heat exchangers associated with the refrigerant system to provide additional cooling. In step 2007, the refrigerant composition is recirculated to the compressor for carrying out another cycle of refrigeration.

[0025] In some embodiments, the first refrigerant composition may include a pure refrigerant or mixed refrigerant selected from ethane, ethylene, propane, n-butane, isobutene, n-pentane, isopentane, R134a, R1234yf, R1234ze, R1234ze(Z), R744, R125, R32, R152a, R23, R116, R245ca, R236ea, or R227ea. In one embodiment, the first refrigerant composition includes a mixture of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 different refrigerants. In one embodiment, the first refrigerant includes propane, ethane and isobutane. In one embodiment, the normal boiling point of the first refrigerant composition is in the range of -60 to 30 °C. In one embodiment, the first refrigerant composition is compressed to a high pressure in the range of 10-30 bar. In another embodiment, the first refrigerant composition is throttled to a low pressure in the range 1-6 bar.

[0026] In some embodiments, the second refrigerant composition includes a multi component mixture. The multicomponent mixture may include at least one of nitrogen, neon, helium, ethane, ethylene, propane, propylene, isobutene, n-butane, R14, R23, R116, R218, or R1234yf. In one embodiment, the second refrigerant composition includes a mixture of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 different refrigerants. In one embodiment, the second refrigerant composition includes a mixture of nitrogen, methane, ethylene and isobutane. In one embodiment, some of the refrigerants in the second refrigerant composition have a boiling point lower than 150K while others have a boiling point above 150K. In other embodiments, some of the refrigerants in the second refrigerant composition have a boiling point lower than 100K while others have a boiling point above 150K. In one embodiment, the second refrigerant composition is compressed to a high pressure in the range of 15-30 bar. In another embodiment, the second refrigerant composition is throttled to a low pressure in the range 1-6 bar.

[0027] In various embodiments, a multistage system 3000 for refrigerating and liquefying a feed gas is provided as illustrated in FIG. 2A-C includes at least a first stage for circulating the first refrigerant composition and a second stage for circulating the second refrigerant composition. Each stage includes a compressor C-l, C-2 for compressing the refrigerant composition to a high pressure, a condenser HX-1, HX-2 for partially or completely condensing the compressed refrigerant compositions at elevated temperature, and a throttle or expansion valve V-l, V2 for throttling the refrigerant composition to a lower pressure.

[0028] The first stage and second stage are thermally coupled at a first heat exchanger HX-3 and a second heat exchanger HX-6. In the first heat exchanger HX-3, the low pressure first refrigerant composition 202 cools at least the feed fluid 101 and high pressure second refrigerant composition 302 entering the first heat exchanger HX-3 through separate inlets. In the second heat exchanger HX-6, the high pressure first refrigerant composition 204 exiting the condenser HX-2 is cooled using low pressure second refrigerant 301. In some embodiments, the second refrigerant leaves heat exchanger HX-3 and enters the heat exchanger HX-6 at a temperature of about -20 °C to -35 °C, or more typically about -25 °C to -30 °C, where it warms up to a temperature close to ambient temperature, providing cooling necessary to reduce the temperature of the high pressure refrigerant composition. The use of HX-6 as configured in the system allows the stream 204 to leave heat exchanger HX-6 at a much lower temperature of -10 to -30 °C, than when the heat exchanger HX-6 is not present. A lowering of temperature of stream leaving the heat exchanger HX-3 also results in lowering of temperatures of feed gas stream entering the valve V-2. Lower the temperature of first refrigerant stream entering valve V-l, higher is the flow rate of liquid feed gas stream 105. The use of an additional heat exchanger HX-6 can be attributed to an increased production of liquid nitrogen 106.

[0029] The system may include one or more additional heat exchangers for efficient liquefaction of the feed gas. The one or more additional heat exchangers further cool the feed fluid exiting HX-3 before it is expanded in a valve V3 and sent to a phase separator PS for separation of the liquefied feed gas from vapor. In one embodiment, the system includes a third heat exchanger HX-4. In other embodiments, the system further includes a fourth heat exchanger HX-5. In one embodiment, the system does not include the heat exchanger HX-5 as a separate unit and its components are integrated within HX- 4 to function as a single heat exchanger. In some embodiments, the feed fluid serially passes through the first heat exchanger HX-3, the third heat exchanger HX-4 and the fourth heat exchanger HX-5 for liquefaction before it enters the valve V3 for expansion. In the third heat exchanger HX-4, the feed gas 102 and the high pressure second refrigerant 303 exiting the first heat exchanger HX-3 are cooled using the low pressure second refrigerant 305 as shown in FIG. 2A. In the third heat exchanger HX-3 or fourth heat exchanger HX-5, the low pressure second refrigerant 304 exiting the valve V-2 is evaporated to cool the feed fluid 103.

[0030] In some embodiments, the configuration can be further modified to improve the efficiency of the system. In one embodiments, the first heat exchanger HX-3 may include inlet for the low pressure second refrigerant 306 for providing additional cooling as shown in FIG. 2B. In other embodiments, the first heat exchanger HX-3 may include inlet for the vapor stream 108 exiting HX-4 for providing additional cooling or reducing the cooling load of 306 as shown in FIG. 2C. In some embodiments, the vapor stream 107, 108, 109 is recycled for liquefaction.

[0031] In some embodiments, the capacity of the system to liquefy the feed fluid is in the range of 0.5-10 L/h. In one embodiment, the liquefaction rate of the system is at least 0.7 L/h for feed gas at a pressure of 10 bar. In some embodiments, one or more heat exchangers in the system is a shell and tube heat exchanger, Giaque-Hampson type heat exchanger, a Plate-Fin heat exchanger, or any similar heat exchangers. In one embodiment, the first heat exchanger HX-3 as illustrated in FIG. 2A is a shell and tube or Giaque-Hampson type heat exchanger. In another embodiment, the first heat exchanger HX-3 as illustrated in FIG. 2B is a Plate-Fin or a similar heat exchanger configuration.

[0032] While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope. Therefore, the above description and the examples to follow should not be taken as limiting the scope of the invention which is defined by the appended claims.

[0033] EXAMPLES

[0034] Example 1: Liquefaction rate of Nitrogen [0035] The following example illustrates a system 3000 shown in FIG. 2A with heat exchanger HX-6 compared to a state of the art refrigeration system as shown in FIG. 3 lacking the heat exchanger HX-6 for liquefaction of nitrogen gas entering the system at an ambient temperature of 35°C and pressure of 10 bar. A mixture of propane and isobutane is used as first refrigerant. A mixture of nitrogen, methane, ethylene and propane is used as second refrigerant. The proposed system is compared with the existing system as shown in Table 1 and results shows considerable improvement of the proposed system for liquefaction of nitrogen gas. The efficient use of the additional heat exchanger HX-6 in the system results in an enhancement of the liquid nitrogen production rates by nearly 40% with only a very little increase in capital and operating cost of the plant.

Table 1: Results obtained with and without heat exchanger

[0036] Example 2 - Multicomponent mixtures for the second refrigerator stage

[0037] The following example illustrates the concentration range of components used in second refrigeration stage for the multistage refrigeration system 3000. The preferred composition of multicomponent mixture for the second refrigeration stage is given in Table 2 and 3. Table 2: Composition A of multicomponent mixtures for the second refrigeration stage

Table 3: Composition B of multicomponent mixtures for the second refrigeration stage