Method for determining overall distribution of maximum heat transfer temperature difference of stream cold box

Abstract

The invention provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box, and the method comprises the steps: obtaining a change rule of the heat transfer temperature difference in the cold box and a relation between the maximum heat transfer temperature difference and a refrigerant on the basis of thermodynamic equilibrium and a multi-stream energy volume transfer rule; and then the optimal mixed refrigerant composition is determined according to cold box constraint and system operation requirements. According to the method, the heat transfer temperature difference distribution of the multi-stream cold box can be clearly and visually displayed, the relation between the key parameters and the heat transfer temperature difference of the cold box can be determined, the key parameter adjustment strategy for reducing the maximum heat transfer temperature difference can be determined, and therefore efficient operation of the cold box is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat integration, and in particular relates to a method for determining the overall distribution of maximum heat transfer temperature difference of a multi-stream cold box. Background Art

[0002] At present, multi-stream cold boxes are widely used in natural gas, light hydrocarbon and hydrogen production systems. The heat transfer temperature difference of the entire cold box is usually subject to an upper limit constraint. In the cold box, different cold and hot streams flow through different channels respectively, and their performance is related to the heat transfer temperature difference of the flow channels. Due to the mutual influence of multiple streams and multiple flow channels, it is difficult to quickly and accurately determine the heat transfer temperature difference distribution and maximum heat transfer temperature difference in the cold box based on the mechanism. In actual production, the temperature is only measured at key locations, which is difficult to accurately reflect the overall situation. The relationship between the maximum heat transfer temperature difference of the main cold box and the refrigerant composition is mainly based on a large number of simulations. The research cycle is long, and complex preparation and subsequent processing are required. In addition, the simulation is greatly affected by the software, and it is impossible to achieve rapid and accurate optimization of the mixed refrigerant.

[0003] The State Intellectual Property Office has published an invention patent with application number 202210773709.X and patent name "Natural Gas Heat Exchange Network Optimization Method Based on Thermodynamic Analysis and Point Adding Technology". The method includes the following steps: S1. Extract the hot and cold logistics of the heat exchange network of the natural gas processing plant to establish a heat exchange network model; S2. Based on the pinch point technology analysis, determine the minimum temperature difference, pinch point position and problems that violate the pinch point design principles of the heat exchange network model, and find defects in the utilization of cold and heat energy; S3. Based on thermodynamic analysis, the optimization potential of the heat exchange network model can be reflected by the heat transfer effectiveness based on the first law of thermodynamics. The relative size of the heat transferred by the heat exchanger can be reflected, and combined with the second law of thermodynamics. Efficiency can be evaluated from the energy quality of the heat exchanger. The two indicators evaluate the energy efficiency of the heat exchange process of the natural gas processing plant heat exchange network. S4. Combined with the pinch point technology heat exchange network criteria, within the scope of process limitations, the pinch point technology is used to reasonably match the cold and hot logistics and optimize the natural gas processing plant heat exchange network. This application should use the pinch point technology to reasonably match the cold and hot logistics, but it cannot determine the relationship between the maximum heat transfer temperature difference and the influence of the refrigerant, and cannot perform fast and accurate optimization of the refrigerant. Summary of the invention

[0004] The purpose of the present invention is to provide a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box, so as to overcome the above-mentioned technical problems existing in the prior art.

[0005] To this end, the technical solution provided by the present invention is as follows:

[0006] The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box includes the following steps:

[0007] S100: Obtain the relationship between the composition of the mixed refrigerant and the average heat capacity; obtain the relationship between the composition of the mixed refrigerant and the key heat flow temperature; obtain the relationship between the inlet and outlet temperatures of each stream of the cold box and the heat load of the cold box and the refrigerant outlet temperature;

[0008] S200: Obtain the relationship between the heat capacity flow rate and the heat load of each cold flow stream in the cold box; Obtain the relationship between the heat capacity flow rate and the heat load of each hot flow stream in the cold box;

[0009] Combining all hot streams to obtain a hot composite curve, combining all cold streams to obtain a cold composite curve, and representing the hot composite curve and the cold composite curve on a temperature-enthalpy diagram;

[0010] S300: Based on the hot composite curve and the cold composite curve obtained in S200, determine the distribution of the heat transfer temperature difference in the cold box along with the refrigerant temperature, and determine the maximum heat transfer temperature difference;

[0011] S400: Calculate the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions based on the energy balance of the cold box; based on the hot composite curve and the cold composite curve in S200 and the relationship between the mixed refrigerant composition and the average heat capacity in S100, obtain the influence curve of the composition of the mixed refrigerant and the outlet temperature change on the heat transfer temperature difference curve in the cold box to determine the change in the maximum heat transfer temperature difference and the optimal refrigerant composition.

[0012] In S100, the relationship between the mixed refrigerant composition and the average heat capacity is as follows:

[0013]

[0014] in, is the average heat capacity of the mixed refrigerant; x Ai is the molar fraction of the mixed refrigerant Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states respectively; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

[0015] In S100, the relationship between the mixed refrigerant composition and the critical heat flow temperature is as follows:

[0016]

[0017] in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and is the phase change enthalpy and phase change temperature of component Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states, respectively.

[0018] In S100, the relationship between the inlet and outlet temperatures of the hot and cold streams of the cold box and the heat load and outlet temperature of the cold box are as follows:

[0019] Hot flow stocks:

[0020] Cold flow stocks:

[0021] in, and are the high pressure mixed refrigerant outlet temperature and the low pressure mixed refrigerant inlet temperature respectively; and are the molar flow rates of hot stream i and cold stream j respectively; and denote the constant-pressure heat capacity of hot stream i and cold stream j respectively; is the molar flow rate of high-pressure mixed refrigerant; is the molar flow rate of low-pressure mixed refrigerant; Q h is the heat load of the main cold box hot stream; η HEx is the heat exchange efficiency of the main cold box; T i in , T i out are the inlet and outlet temperatures of hot stream i, respectively; are the inlet and outlet temperatures of cold stream j, respectively; is the constant pressure specific heat capacity of the mixed refrigerant.

[0022] In S200, the relationship between the heat capacity flow rate and the heat load of each cold stream in the cold box is as follows:

[0023]

[0024]

[0025] Where ΔQ c The heat absorbed by the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; CP c is the heat capacity flow rate of the cold stream; is the flow rate of component Ai in the cold stream; is the constant pressure specific heat capacity of component Ai; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

[0026] In S200, the relationship between the heat capacity flow rate and the heat load of each hot stream in the cold box is as follows:

[0027]

[0028]

[0029] Among them, ΔQ h The heat released by the hot stream; is the termination temperature of the hot stream; is the initial temperature of the hot stream; CP h is the heat capacity flow rate of the heat stream; is the flow rate of component Ai in the hot stream; is the constant-pressure specific heat capacity of component Ai.

[0030] The steps of combining all the heat streams to obtain a thermal composite curve described in S200 are as follows:

[0031] Step 1) dividing the temperature interval according to the initial temperature and target temperature of each heat stream;

[0032] Step 2) merging the streams in each temperature interval to obtain the corresponding composite heat flow;

[0033] Step 3) Draw the temperature enthalpy curve of the composite heat flow in each temperature interval, and connect them end to end to obtain a composite curve of multiple heat flows, which is the thermal composite curve.

[0034] The specific process of determining the maximum heat transfer temperature difference in S300 is:

[0035] Step 1) translating the cold composite curve so that the horizontal coordinates of its upper and lower end points are equal to the horizontal coordinates of the upper and lower end points of the hot composite curve;

[0036] Step 2) Draw a vertical line at each inflection point on the supercooling and heating composite curve, and the difference between the vertical line and the ordinate of the intersection of the two composite curves is the heat transfer temperature difference corresponding to the inflection point;

[0037] Step 3) Take the temperature of the corresponding thermal composite curve as the horizontal coordinate and the heat transfer temperature difference as the vertical coordinate, and connect the heat transfer temperature differences corresponding to each inflection point to draw the heat transfer temperature difference curve under different heat flow temperatures.

[0038] In S400, the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions is calculated based on the cold box energy balance as shown in the formula:

[0039]

[0040] in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and is the phase change enthalpy and phase change temperature of component Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states, respectively.

[0041] The beneficial effects of the present invention are:

[0042] Based on thermodynamic equilibrium and multi-stream energy volume transfer law, the present invention obtains the change law of heat transfer temperature difference in the cold box and the relationship between the maximum heat transfer temperature difference and the refrigerant composition, and then determines the optimal mixed refrigerant composition according to the cold box constraints and system operation requirements. The optimization of the mixed refrigerant composition is of great significance for improving the efficiency of the cold box and reducing production energy consumption.

[0043] The present invention can analyze and determine the influence of other parameters on the cold box and energy consumption when the mixed refrigerant is composed. In actual production, combined with the automatic monitoring and control system, real-time adjustment of production operations can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the heat exchange process of the main cold box provided by an embodiment of the present invention;

[0045] Figure 2 is a schematic diagram of three heat streams provided by an embodiment of the present invention;

[0046] Figure 3 is a schematic diagram of a composite curve formed by merging three heat flow streams provided by an embodiment of the present invention;

[0047] Figure 4 is a temperature-enthalpy diagram provided by an embodiment of the present invention;

[0048] Figure 5 It is a heat transfer temperature difference distribution diagram in a cold box provided by an embodiment of the present invention;

[0049] Figure 6 It is a relationship diagram between the refrigerant composition of the present invention and the heat transfer temperature difference distribution in the cold box. DETAILED DESCRIPTION

[0050] The following describes the implementation of the present invention through specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0051] The exemplary embodiments of the present invention are now described with reference to the accompanying drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely and to fully convey the scope of the present invention to those skilled in the art. The terms in the exemplary embodiments shown in the accompanying drawings are not intended to limit the present invention.

[0052] Unless otherwise specified, the terms (including technical terms) used herein have the commonly understood meanings to those skilled in the art. In addition, it is understood that the terms defined in commonly used dictionaries should be understood to have the same meanings as those in the context of the relevant fields, and should not be understood as idealized or overly formal meanings.

[0053] Example 1

[0054] This embodiment provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box, comprising the following steps:

[0055] S100: Obtain the relationship between the composition of the mixed refrigerant and the average heat capacity; obtain the relationship between the composition of the mixed refrigerant and the key heat flow temperature; obtain the relationship between the inlet and outlet temperatures of each stream of the cold box and the heat load of the cold box and the refrigerant outlet temperature; wherein the key heat flow temperature refers to the temperature required by production;

[0056] S200: Obtain the relationship between the heat capacity flow rate and the heat load of each cold flow stream in the cold box; Obtain the relationship between the heat capacity flow rate and the heat load of each hot flow stream in the cold box;

[0057] Combining all hot streams to obtain a hot composite curve, combining all cold streams to obtain a cold composite curve, and representing the hot composite curve and the cold composite curve on a temperature-enthalpy diagram;

[0058] S300: Based on the hot composite curve and the cold composite curve obtained in S200, determine the distribution of the heat transfer temperature difference in the cold box along with the refrigerant temperature, and determine the maximum heat transfer temperature difference;

[0059] S400: Calculate the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions based on the energy balance of the cold box; based on the hot composite curve and the cold composite curve in S200 and the relationship between the mixed refrigerant composition and the average heat capacity in S100, obtain the influence curve of the composition of the mixed refrigerant and the outlet temperature change on the heat transfer temperature difference curve in the cold box to determine the change in the maximum heat transfer temperature difference and the optimal refrigerant composition.

[0060] The present invention first extracts the self-circulation parameters of the mixed refrigerant streams of a multi-stream cold box and the physical and chemical properties of each component, and determines the relationship between the inlet and outlet temperatures of the cold box streams, the cold box heat load, the outlet temperature of the mixed refrigerant and the composition according to the cold box material balance, heat balance and thermodynamic properties of the mixed refrigerant; then, the initial and final temperatures, flow rates and compositions of the hot and cold streams of the multi-stream cold box are extracted, the amount of cold absorbed / released is calculated, recorded as the cold load, and a cold-heat composite curve is drawn; secondly, based on the cold-heat composite curve, the relationship between the heat transfer temperature difference of the cold box and the refrigerant temperature is determined, and the maximum heat transfer temperature difference is identified; finally, based on the influence of the mixed refrigerant composition and its main cold box temperature on the heat transfer temperature difference of the cold box, a composition-outlet temperature-heat transfer temperature difference diagram is constructed to determine the optimal composition of the refrigerant and the maximum heat transfer temperature difference change.

[0061] The method of the present invention can clearly and intuitively display the heat transfer temperature difference distribution of a multi-stream cold box, can determine the relationship between key parameters and the heat transfer temperature difference of the cold box, and can determine the key parameter adjustment strategy for reducing the maximum heat transfer temperature difference, thereby achieving efficient operation of the cold box.

[0062] Example 2

[0063] Based on Example 1, this example provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box. In S100, the relationship between the composition of the mixed refrigerant and the average heat capacity is as follows:

[0064]

[0065] in, is the average heat capacity of the mixed refrigerant; x Ai is the molar fraction of the mixed refrigerant Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states respectively; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

[0066] In S100, the relationship between the mixed refrigerant composition and the critical heat flow temperature is as follows:

[0067]

[0068] in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

[0069] In S100, the relationship between the inlet and outlet temperatures of the hot and cold streams of the cold box and the heat load and outlet temperature of the cold box are as follows:

[0070] Hot flow stocks:

[0071] Cold flow stocks:

[0072] in, and are the high pressure mixed refrigerant outlet temperature and the low pressure mixed refrigerant inlet temperature respectively; and are the molar flow rates of hot stream i and cold stream j respectively; and denote the constant-pressure heat capacity of hot stream i and cold stream j respectively; is the molar flow rate of high-pressure mixed refrigerant; is the molar flow rate of low-pressure mixed refrigerant; Q h is the heat load of the main cold box hot stream; η HEx is the heat exchange efficiency of the main cold box; T i in , T i out are the inlet and outlet temperatures of hot stream i, respectively; are the inlet and outlet temperatures of cold stream j, respectively; is the constant pressure specific heat capacity of the mixed refrigerant.

[0073] Example 3

[0074] Based on Example 1, this example provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box. In S200, the relationship between the heat capacity flow rate and the heat load of each cold stream in the cold box is as follows:

[0075]

[0076]

[0077] Where ΔQ c The heat absorbed by the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; CP c is the heat capacity flow rate of the cold stream; is the flow rate of component Ai in the cold stream; is the constant pressure specific heat capacity of component Ai; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

[0078] In S200, the relationship between the heat capacity flow rate and the heat load of each hot stream in the cold box is as follows:

[0079]

[0080]

[0081] Among them, ΔQ h The heat released by the hot stream; is the termination temperature of the hot stream; is the initial temperature of the hot stream; CP h is the heat capacity flow rate of the heat stream; is the flow rate of component Ai in the hot stream; is the constant-pressure specific heat capacity of component Ai.

[0082] Example 4

[0083] Based on Example 1, this example provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box. The step of combining all the heat streams to obtain a thermal composite curve in S200 is as follows:

[0084] Step 1) dividing the temperature interval according to the initial temperature and target temperature of each heat stream;

[0085] Step 2) merging the streams in each temperature interval to obtain the corresponding composite heat flow;

[0086] Step 3) Draw the temperature enthalpy curve of the composite heat flow in each temperature interval, and connect them end to end to obtain a composite curve of multiple heat flows, which is the thermal composite curve.

[0087] The temperature and enthalpy change of the composite heat flow are equivalent to those of multiple heat flows. Its CP is the sum of the CPs of all streams existing in the corresponding temperature interval, and its enthalpy change is the sum of the enthalpy changes of each stream, that is,

[0088]

[0089] Where ΔH k represents the enthalpy change in the kth temperature zone; i represents the heat flow in the kth temperature zone.

[0090] like Figure 2 As shown, the temperature range between the initial inlet temperature T1 and the outlet temperature (target temperature) T5 of the heat flow is divided into four temperature intervals, and the temperature enthalpy curve of the composite heat flow in each temperature interval is obtained. The thermal composite curve is obtained by connecting the beginning and the end, as shown in FIG. Figure 3 shown.

[0091] The steps for combining all cold streams to obtain a cold composite curve are the same as for the hot composite curve.

[0092] Example 5

[0093] Based on Example 1, this example provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box. The specific process of determining the maximum heat transfer temperature difference in S300 is as follows:

[0094] Step 1) translating the cold composite curve so that the horizontal coordinates of its upper and lower end points are equal to the horizontal coordinates of the upper and lower end points of the hot composite curve;

[0095] Step 2) Draw a vertical line at each inflection point on the supercooling and heating composite curve, and the difference between the vertical line and the ordinate of the intersection of the two composite curves is the heat transfer temperature difference corresponding to the inflection point; Figure 4 As shown;

[0096] Step 3) Take the temperature of the corresponding thermal composite curve as the horizontal axis and the heat transfer temperature difference as the vertical axis, and connect the heat transfer temperature differences corresponding to each inflection point to draw the heat transfer temperature difference curve under different heat flow temperatures. Figure 5 shown.

[0097] Example 6

[0098] Based on Example 1, this embodiment provides a method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box. In S400, the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions is calculated based on the cold box energy balance, as shown in the formula:

[0099]

[0100] in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and is the phase change enthalpy and phase change temperature of component Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states, respectively.

[0101] Figure 1 This is a simplified diagram of the heat exchange process of the main cold box, with 5 hot streams and 3 cold streams. When the target outlet temperature remains unchanged, the refrigerant flow rate is multivariately related to the refrigerant outlet temperature and components. Figure 6It is used to describe the relationship between the temperature of the hot stream and the heat transfer temperature difference of the cold box. The process variable is the sample data of the mixed refrigerant. The multi-stream cold box logistics data is shown in Table 1. The mixed refrigerant sample data is shown in Table 2.

[0102] Table 1 Multi-stream cold box logistics data

[0103]

[0104] Table 2 Mixed refrigerant sample data

[0105]

[0106] like Figure 6 As shown in the figure, when the refrigerant sample changes, the heat transfer temperature difference in different temperature intervals of the multi-cold box heat flow stream changes differently. When the temperature is lower than T (inside the rectangular frame), the heat transfer temperature difference tends to increase with the sample change, indicating that the sample change in this process is not conducive to reducing the heat transfer temperature difference in this temperature interval. When the temperature is higher than T (outside the rectangular frame), the heat transfer temperature difference tends to decrease with the sample change, indicating that the sample change in this process is beneficial. Due to the complex characteristics of the multi-cold box flow channel, it is necessary to consider the adjustment and optimization of the heat transfer temperature difference in different temperature intervals, combined with Figure 6 The process shown, the influence of the change of mixed refrigerant parameters on the heat transfer temperature difference in different temperature ranges of the cold box, and the established overall distribution method of the maximum heat transfer temperature difference of the cold box can realize the optimization of the heat transfer temperature difference of the multi-stream cold box.

[0107] The above examples are merely illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. All designs that are the same or similar to the present invention fall within the protection scope of the present invention.

Claims

1. A method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box, characterized in that: The following steps are involved: S100: Obtain the relationship between the composition of the mixed refrigerant and the average heat capacity; obtain the relationship between the composition of the mixed refrigerant and the key heat flow temperature; obtain the relationship between the inlet and outlet temperatures of each stream of the cold box and the heat load of the cold box and the refrigerant outlet temperature; S200: Obtain the relationship between the heat capacity flow rate and the heat load of each cold flow stream in the cold box; Obtain the relationship between the heat capacity flow rate and the heat load of each hot flow stream in the cold box; Combining all hot streams to obtain a hot composite curve, combining all cold streams to obtain a cold composite curve, and representing the hot composite curve and the cold composite curve on a temperature-enthalpy diagram; S300: Based on the hot composite curve and the cold composite curve obtained in S200, determine the distribution of the heat transfer temperature difference in the cold box along with the refrigerant temperature, and determine the maximum heat transfer temperature difference; S400: Calculate the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions based on the energy balance of the cold box; based on the hot composite curve and the cold composite curve in S200 and the relationship between the mixed refrigerant composition and the average heat capacity in S100, obtain the influence curve of the composition of the mixed refrigerant and the outlet temperature change on the heat transfer temperature difference curve in the cold box to determine the change in the maximum heat transfer temperature difference and the optimal refrigerant composition.

2. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S100, the relationship between the mixed refrigerant composition and the average heat capacity is as follows: in, is the average heat capacity of the mixed refrigerant; x Ai is the molar fraction of the mixed refrigerant Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states respectively; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

3. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S100, the relationship between the mixed refrigerant composition and the critical heat flow temperature is as follows: in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and is the phase change enthalpy and phase change temperature of component Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states, respectively.

4. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S100, the relationship between the inlet and outlet temperatures of the hot and cold streams of the cold box and the heat load and outlet temperature of the cold box are as follows: Hot flow stocks: Cold flow stocks: in, and are the high pressure mixed refrigerant outlet temperature and the low pressure mixed refrigerant inlet temperature respectively; and are the molar flow rates of hot stream i and cold stream j respectively; and denote the constant-pressure heat capacity of hot stream i and cold stream j respectively; is the molar flow rate of high-pressure mixed refrigerant; is the molar flow rate of low-pressure mixed refrigerant; Q h is the heat load of the main cold box hot stream; η HEx is the heat exchange efficiency of the main cold box; T i in 、T i out are the inlet and outlet temperatures of hot stream i, respectively; are the inlet and outlet temperatures of cold stream j, respectively; is the constant pressure specific heat capacity of the mixed refrigerant.

5. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S200, the relationship between the heat capacity flow rate and the heat load of each cold stream in the cold box is as follows: Where ΔQ c The heat absorbed by the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; CP c is the heat capacity flow rate of the cold stream; is the flow rate of component Ai in the cold stream; is the constant pressure specific heat capacity of component Ai; r Ai and are the phase change enthalpy and phase change temperature of component Ai.

6. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S200, the relationship between the heat capacity flow rate and the heat load of each hot stream in the cold box is as follows: Among them, ΔQ h The heat released by the hot stream; is the termination temperature of the hot stream; is the initial temperature of the hot stream; CP h is the heat capacity flow rate of the heat stream; is the flow rate of component Ai in the hot stream; is the constant-pressure specific heat capacity of component Ai.

7. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: The steps of combining all the heat streams to obtain a thermal composite curve described in S200 are as follows: Step 1) dividing the temperature interval according to the initial temperature and target temperature of each heat stream; Step 2) merging the streams in each temperature interval to obtain the corresponding composite heat flow; Step 3) Draw the temperature enthalpy curve of the composite heat flow in each temperature interval, and connect them end to end to obtain a composite curve of multiple heat flows, which is the thermal composite curve.

8. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: The specific process of determining the maximum heat transfer temperature difference in S300 is: Step 1) translating the cold composite curve so that the horizontal coordinates of its upper and lower end points are equal to the horizontal coordinates of the upper and lower end points of the hot composite curve; Step 2) Draw a vertical line at each inflection point on the supercooling and heating composite curve, and the difference between the vertical line and the ordinate of the intersection of the two composite curves is the heat transfer temperature difference corresponding to the inflection point; Step 3) Take the temperature of the corresponding thermal composite curve as the horizontal coordinate and the heat transfer temperature difference as the vertical coordinate, and connect the heat transfer temperature differences corresponding to each inflection point to draw the heat transfer temperature difference curve under different heat flow temperatures.

9. The method for determining the overall distribution of the maximum heat transfer temperature difference of a multi-stream cold box according to claim 1, characterized in that: In S400, the outlet temperature of the mixed refrigerant corresponding to different mixed refrigerant compositions is calculated based on the cold box energy balance as shown in the formula: in, are the low-pressure mixed refrigerant inlet temperature; Q hi and Q cj is the heat load of the hot stream i and the cold stream j in the main cold box; η HEx is the heat exchange efficiency of the main cold box; is the molar flow rate of low-pressure mixed refrigerant; r Ai and is the phase change enthalpy and phase change temperature of component Ai; T s and T t are the temperatures of the refrigerant entering and leaving the main cold box respectively; and are the heat capacity flow rates of component Ai in liquid and gaseous states, respectively.

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