Analysis method of circulating cooling water serial water distribution

Abstract

The invention discloses a circulating cooling water serial water distribution analysis method, which comprises the following steps of: extracting inlet and outlet temperatures, flow and composition of cold / hot flow strands to determine heat absorbed / released by the cold / hot flow strands, and respectively combining all the cold / hot flow strands to obtain a cold / hot compound curve; extracting pinch point temperature difference, different grades of saturated steam and corresponding temperatures thereof, a cooling tower design return water temperature upper limit, and a maximum feasible flow of circulating water in a series water distribution system; determining the lower temperature limit of circulating water byproduct steam based on different grades of saturated steam and corresponding temperatures thereof, and determining the highest temperature of the byproduct steam which can be reached after circulating cooling water flows through each heat exchanger in series based on a cold-hot composite curve and a pinch point temperature difference; and determining the highest return water temperature when surplus heat of the circulating cooling water removal system returns to the cooling tower based on the cold and hot compound curve and the upper limit of the return water temperature of the cooling tower design, and determining the lowest return water temperature when surplus heat of the circulating cooling water removal system returns to the cooling tower based on the maximum feasible flow of circulating water in the series water distribution system.

Description

Technical Field

[0001] The invention relates to the technical field of petrochemical circulating cooling water, and in particular to an analysis method for serial water distribution of circulating cooling water. Background Art

[0002] Circulating cooling water is an important public utility in the petrochemical industry. Its water consumption accounts for about 70% of industrial water consumption. According to statistics, its fresh water replenishment accounts for about 35% of the enterprise's water consumption, second only to boiler replenishment. The circulating cooling water system is also a large power consumer, accounting for about 20% to 30% of the total electricity consumption of the enterprise. The renovation and operation optimization of the circulating cooling water system is an important part of water and electricity saving in petrochemical refining enterprises, and has far-reaching significance for improving the energy utilization level of the device. Circulating water series connection can not only achieve a significant reduction in industrial water consumption, but also plays an important role in reducing the operating load of cooling towers and water pumps and the cost of system construction. At present, the circulating cooling water series water distribution has problems such as complex process, unclear mechanism, unclear coupling relationship between circulating water flow and supply and return water temperature and cooling tower. It is difficult to make an accurate judgment on the feasibility of circulating cooling water series water distribution.

[0003] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention proposes an analysis method for circulating cooling water series distribution, which clearly and intuitively displays the matching relationship between the cold and hot flow streams of the system when circulating cooling water is distributed in series, reveals the changing rules of circulating water flow rate and outlet temperature, clarifies the feasibility of circulating cooling water series distribution, provides guidance for circulating cooling water distribution, and helps petrochemical refining enterprises save energy, save water, and reduce consumption.

[0005] The present invention is achieved through the following technical solutions:

[0006] An analysis method for circulating cooling water series distribution includes:

[0007] Step S100: extract the inlet and outlet temperatures, flow rates and composition of each hot stream that needs to be cooled by circulating cooling water in the circulating cooling water distribution system; extract the inlet and outlet temperatures, flow rates and composition of the cold stream, i.e., the circulating cooling water, in the circulating cooling water distribution system; determine the heat capacity flow rate of each hot stream and cold stream based on the stream flow rate, composition and heat capacity flow rate definition formula; determine the heat absorbed by the cold stream and the heat released by the hot stream based on the inlet and outlet temperatures and heat capacity flow rate of the cold stream and the inlet and outlet temperatures and heat capacity flow rate of the hot stream, and record them as heat loads; based on the inlet and outlet temperatures and heat loads, each stream is represented by a straight line segment of a temperature enthalpy line, the corresponding ordinates of its two end points represent the inlet and outlet temperatures, respectively, and the difference between the abscissas is the enthalpy difference to represent the heat load of the stream; combine all hot streams to obtain a thermal composite curve, the difference between the abscissas of each of its two end points represents the sum of the heat loads of all hot streams in the temperature range, and similarly draw a cold composite curve, i.e., a circulating water composite curve, and then represent the two together on the temperature enthalpy diagram;

[0008] Step S200: Extracting the pinch point temperature difference ΔT min , different grades of saturated steam and their corresponding temperatures, the upper limit of the design return water temperature of the cooling tower, and the maximum allowed circulating water flow rate on the cold flow side of each heat exchanger in the circulating cooling water distribution system, and then extract the minimum value, which is the maximum feasible circulating water flow rate of the circulating cooling water distribution system in series;

[0009] Step S300: Based on the hot composite curve, the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water flows through each heat exchanger in series, and the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow is determined. and the minimum flow rate of the corresponding by-product steam; based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, determine the lower limit of the temperature of the circulating water by-product steam and its corresponding maximum flow rate of by-product steam;

[0010] Step S400: Based on the heat composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, determine the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower and its corresponding minimum return water flow rate; based on the maximum feasible flow rate of circulating water, determine the maximum return water flow rate and its corresponding minimum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower

[0011] Step S500: Based on the heat composite curve and the cold composite curve, the feasibility of circulating cooling water series distribution is analyzed, wherein the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow is Greater than the lower limit of the temperature of the circulating water by-product steam The circulating cooling water distribution system can be connected in series and produce steam as a by-product; if the maximum return water temperature Greater than the minimum return water temperature The circulating cooling water distribution system can be connected in series, and the return water temperature does not exceed the design limit of the cooling tower.

[0012] In the analysis method of circulating cooling water series distribution, in step S100, the relationship between the heat load of the hot stream and the inlet and outlet temperatures, flow rate and composition is expressed as:

[0013]

[0014] and

[0015]

[0016] Among them, ΔQ h is the heat load of the hot stream, that is, the heat released by the hot stream; CP h is the heat capacity flow rate of the heat stream; is the inlet temperature of the hot stream; is the outlet temperature of the hot stream; m h，Ai Component A in the hot stream i The flow rate; Cp Ai Indicates component A i The specific heat capacity; n represents the number of components in the heat flow.

[0017] In the analysis method of circulating cooling water series distribution, in step S100, the relationship between the heat load of the cold stream and its inlet and outlet temperatures, flow rate and composition is expressed as:

[0018]

[0019] and

[0020] CP w =m w Cp w ,

[0021] Among them, ΔQ w The heat absorbed by the cold stream; CP w represents the heat capacity flow rate of the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; m w Indicates the cold flow rate; Cp w Represents the specific heat capacity of the cold stream.

[0022] In the analysis method of circulating cooling water series water distribution, in step S100, the steps of combining all hot streams to obtain a hot composite curve and combining all cold streams to obtain a cold composite curve include:

[0023] The temperature intervals are divided according to the inlet and outlet temperatures of each hot stream and the inlet and outlet temperatures of each cold stream; the hot streams and cold streams in each temperature interval are combined to obtain the corresponding composite hot streams and composite cold streams. The heat capacity flow rate and heat load of the composite hot stream and composite cold stream in each temperature interval are the sum of the heat capacity flow rates and heat loads of all hot streams and cold streams in the corresponding temperature interval, that is,

[0024]

[0025] Where ΔH k represents the heat load of the kth temperature zone; m is the number of streams in the kth temperature zone; CP m represents the heat capacity flow rate of the mth stream in the kth temperature zone; T k is the upper limit temperature of the kth temperature zone; T k+1 is the lower limit temperature of the kth temperature zone.

[0026] The composite hot flow temperature enthalpy curve and the composite cold flow temperature enthalpy curve in each temperature range are drawn on the temperature enthalpy diagram, and they are connected end to end to obtain the hot composite curve of the composite curve of multiple hot flow streams and the cold composite curve of the composite curve of multiple cold flow streams.

[0027] In the analysis method of circulating cooling water series distribution, in step S200, different saturated grades of steam and their corresponding temperatures are as follows:

[0028] High-pressure steam: pressure 9.8~12.5MPaG, temperature 310~328℃,

[0029] Medium pressure steam: pressure 3.5~4.2MPaG, temperature 242~253℃,

[0030] Low-pressure steam: pressure 0.8~2.5MPaG, temperature 170~225℃,

[0031] Low pressure steam: 0.25~0.5MPaG, temperature 127~151℃,

[0032] Steam heating: 80~120℃.

[0033] In the analysis method of circulating cooling water series distribution, in step S200, each heat exchanger cold flow side has a circulating water flow upper limit, and the minimum value is m max , the maximum feasible flow rate of circulating water in the series water distribution system m max It is expressed as,

[0034] m max =min(m 1，max , m 2，max , ..., m n，max )

[0035] Among them, m n，max is the maximum circulating cooling water flow rate allowed for the nth heat exchanger;

[0036] Flow rate is m max The circulating cooling water flows through each heat exchanger in series and exchanges heat with the hot flow in sequence to achieve the lowest return water temperature. for:

[0037]

[0038] Where f is the heat exchanger action function; A total It represents the total heat transfer area of ​​each heat exchanger in the circulating water system; K represents the total heat transfer coefficient of the heat exchanger.

[0039] In the analysis method of circulating cooling water series water distribution, in step S300, based on the hot composite curve and the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water is determined to flow through each heat exchanger in series. After removing the excess heat of the hot flow, the upper limit of the temperature of the by-product steam and its corresponding minimum flow rate of the by-product steam include:

[0040] Shift the cold composite curve so that the horizontal coordinates corresponding to its upper and lower endpoints are equal to the horizontal coordinates corresponding to the upper and lower endpoints of the hot composite curve;

[0041] Taking the lowest endpoint of the cold composite curve as the fixed point, rotate the cold composite curve counterclockwise so that the minimum vertical distance between the cold composite curve and the hot composite curve is equal to the pinch point temperature difference ΔT min , extend the cold composite curve so that the abscissa of its highest endpoint is equal to the abscissa of the highest endpoint of the hot composite curve. At this time, the highest endpoint of the cold composite curve is the upper limit of the temperature of the by-product steam after the circulating cooling water flows through each heat exchanger in series to remove the excess heat of the hot flow.

[0042] The minimum flow rate of circulating water that can be produced as steam is determined based on the slope of the cold composite curve. The relationship between the slope and the minimum flow rate of circulating water that can produce steam as a byproduct is expressed as,

[0043]

[0044] In the analysis method of circulating cooling water series water distribution, in step S300, based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, the lower limit of the temperature of the circulating water byproduct steam is determined. The corresponding maximum flow rate of by-product steam includes:

[0045] Draw a vertical line from the highest end point of the superheat composite curve, which intersects with the horizontal line of the lowest temperature required to produce steam. The ordinate value corresponding to the intersection is the lower limit of the temperature of the by-product steam of the circulating water.

[0046] Take the intersection as the highest endpoint and connect the lowest endpoint of the original cold composite curve to construct a new cold composite curve. According to the slope of the new cold composite curve, determine the maximum flow rate of circulating water that can produce steam as a byproduct. The relationship between the slope and the maximum flow rate of circulating water that can produce steam as a byproduct is expressed as:

[0047]

[0048] In the analysis method of circulating cooling water series water distribution, in step S400, based on the thermal composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower is determined. And the corresponding minimum return flow rate includes:

[0049] Draw a vertical line at the highest end point of the superheat composite curve, which intersects with the horizontal line where the upper limit of the cooling tower design return water temperature is located. The ordinate corresponding to the intersection is the highest return water temperature of the circulating water returning to the cooling tower

[0050] The intersection is used as the highest endpoint to connect the lowest endpoint of the cold composite curve to construct a new cold composite curve. The minimum return water flow rate of circulating water to remove the surplus heat of the system and return to the cooling tower is determined based on the slope of the new cold composite curve. The relationship between the slope and the minimum return flow of circulating water is expressed as,

[0051]

[0052] In the analysis method of circulating cooling water series water distribution, in step S400, the maximum return water flow rate and the corresponding minimum return water temperature of the circulating cooling water when removing the system surplus heat and returning to the cooling tower are determined based on the maximum feasible flow rate of the circulating water. include:

[0053] The maximum feasible flow rate of circulating water in the series water distribution system m max The maximum return flow rate when the circulating cooling water removes excess heat from the system and returns to the cooling tower

[0054] Based on maximum return flow Determine the slope of the cold composite curve, and the relationship between the maximum return flow of circulating water and the slope is expressed as:

[0055]

[0056] Based on the slope, a cold composite curve of the circulating water in this state is drawn in the temperature-enthalpy diagram. The ordinate corresponding to the highest end point of the curve is the lowest return water temperature when the cooling water removes the excess heat of the system and returns to the cooling tower.

[0057] Compared with the prior art, the present invention has the following advantages:

[0058] The present invention constructs a feasibility evaluation method for serial water distribution of circulating cooling water based on the temperature-enthalpy diagram, steam grade-temperature correlation data and cooling tower design parameters. The present disclosure can also be extended to analyze the coupling relationship between the circulating water flow and return water temperature of the serial water distribution system and the cooling tower, and identify the influence of the circulating water flow and return water temperature on the system energy consumption. 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

[0059] By reading the detailed description of the preferred specific embodiments below, various other advantages and benefits of the present invention will become clear to those of ordinary skill in the art. The drawings in the specification are only for the purpose of illustrating the preferred embodiments and are not considered to be limitations of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative work. Moreover, the same reference numerals are used to represent the same components throughout the drawings.

[0060] In the attached picture:

[0061] Figure 1 It is a schematic diagram of the steps of a feasibility analysis method for circulating cooling water series water distribution according to an embodiment of the present disclosure;

[0062] Figure 2 is a schematic diagram of temperature enthalpy of four hot streams according to an embodiment of the present disclosure;

[0063] Figure 3 is a schematic diagram of a thermal composite curve formed after four heat streams are combined according to an embodiment of the present disclosure;

[0064] Figure 4 are a hot composite curve and a cold composite curve of one embodiment of the present disclosure;

[0065] Figure 5 is a temperature enthalpy diagram of by-product steam of a series water distribution system according to an embodiment of the present disclosure;

[0066] Figure 6 is a temperature enthalpy diagram of a cooling water cycle of a series water distribution system according to an embodiment of the present disclosure;

[0067] Figure 7 It is a feasibility analysis diagram of a series water distribution system according to an embodiment of the present disclosure.

[0068] The present invention is further explained below in conjunction with the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0069] The specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the specific embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0070] It should be noted that certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component. This specification and claims do not use the difference in nouns as a way to distinguish components, but use the difference in the functions of the components as the criterion for distinction. As mentioned throughout the specification and claims, "including" or "comprising" is an open term, so it should be interpreted as "including but not limited to". The subsequent description of the specification is a preferred embodiment of the present invention, but the description is based on the general principles of the specification and is not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the attached claims.

[0071] To facilitate understanding of the embodiments of the present invention, further explanation will be given below by taking specific embodiments as examples in conjunction with the accompanying drawings, and each of the accompanying drawings does not constitute a limitation on the embodiments of the present invention.

[0072] like Figures 1 to 7 As shown, the present invention discloses an analysis method for circulating cooling water series distribution, comprising:

[0073] S100: extract the inlet and outlet temperatures, flow rates and composition of each hot stream that needs to be cooled by circulating cooling water in the circulating cooling water distribution system; extract the inlet and outlet temperatures, flow rates and composition of the cold stream, i.e., the circulating cooling water, in the circulating cooling water distribution system; determine the heat capacity flow rate of each hot stream and cold stream based on the stream flow rate, composition and heat capacity flow rate definition formula; determine the heat absorbed by the cold stream and the heat released by the hot stream based on the inlet and outlet temperatures and heat capacity flow rates of the cold stream and the inlet and outlet temperatures and heat capacity flow rates of the hot stream, and record them as heat loads; based on the inlet and outlet temperatures and heat loads, each stream is represented by a straight line segment of a temperature enthalpy line, the corresponding ordinates of the two end points represent the inlet and outlet temperatures, respectively, and the difference between the abscissas is the enthalpy difference to represent the heat load of the stream; combine all hot streams to obtain a thermal composite curve, the difference between the abscissas of each of the two end points represents the sum of the heat loads of all hot streams in the temperature range, and similarly draw a cold composite curve, i.e., a circulating water composite curve, and then represent the two together on the temperature enthalpy diagram;

[0074] S200: Extract pinch point temperature difference ΔT min , different grades of saturated steam and their corresponding temperatures, the upper limit of the design return water temperature of the cooling tower, and the maximum allowed circulating water flow rate on the cold flow side of each heat exchanger in the circulating cooling water distribution system, and then extract the minimum value, which is the maximum feasible circulating water flow rate of the circulating cooling water distribution system in series;

[0075] S300: Based on the hot composite curve, the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water flows through each heat exchanger in series, and the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow is determined. and the minimum flow rate of the corresponding by-product steam; based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, determine the lower limit of the temperature of the circulating water by-product steam and its corresponding maximum flow rate of by-product steam;

[0076] S400: Based on the heat composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, determine the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower and its corresponding minimum return water flow rate; based on the maximum feasible flow rate of circulating water, determine the maximum return water flow rate and its corresponding minimum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower

[0077] S500: Based on the heat composite curve and the cold composite curve, the feasibility of circulating cooling water series distribution is analyzed, wherein the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow Greater than the lower limit of the temperature of the circulating water by-product steam The circulating cooling water distribution system can be connected in series and produce steam as a by-product; if the maximum return water temperature Greater than the minimum return water temperature The circulating cooling water distribution system can be connected in series, and the return water temperature does not exceed the design limit of the cooling tower.

[0078] In a preferred embodiment of the analytical method for circulating cooling water series distribution, in step S100, the relationship between the heat load of the hot stream and the inlet and outlet temperatures, flow rate and composition is expressed as follows:

[0079]

[0080] and

[0081]

[0082] Among them, ΔQ h is the heat load of the hot stream, that is, the heat released by the hot stream; CP h is the heat capacity flow rate of the heat stream; is the inlet temperature of the hot stream; is the outlet temperature of the hot stream; m h，Ai Component A in the hot stream i The flow rate; Cp Ai Indicates component A i The specific heat capacity; n represents the number of components in the heat flow.

[0083] In a preferred embodiment of the analytical method for circulating cooling water series distribution, in step S100, the relationship between the heat load of the cold stream and its inlet and outlet temperatures, flow rate and composition is expressed as:

[0084]

[0085] and

[0086] CP w =m w Cp w ,

[0087] Among them, ΔQ w The heat absorbed by the cold stream; CP w represents the heat capacity flow rate of the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; m w Indicates the cold flow rate; Cp w Represents the specific heat capacity of the cold stream.

[0088] In a preferred embodiment of the analysis method of circulating cooling water series distribution, in step S100, the step of combining all hot streams to obtain a hot composite curve and combining all cold streams to obtain a cold composite curve comprises:

[0089] The temperature intervals are divided according to the inlet and outlet temperatures of each hot stream and the inlet and outlet temperatures of each cold stream; the hot streams and cold streams in each temperature interval are combined to obtain the corresponding composite hot streams and composite cold streams. The heat capacity flow rate and heat load of the composite hot stream and composite cold stream in each temperature interval are the sum of the heat capacity flow rates and heat loads of all hot streams and cold streams in the corresponding temperature interval, that is,

[0090]

[0091] Where ΔH k represents the heat load of the kth temperature zone; m is the number of streams in the kth temperature zone; CP m represents the heat capacity flow rate of the mth stream in the kth temperature zone; T k is the upper limit temperature of the kth temperature zone; T k+1 is the lower limit temperature of the kth temperature zone.

[0092] The composite hot flow temperature enthalpy curve and the composite cold flow temperature enthalpy curve in each temperature range are drawn on the temperature enthalpy diagram, and they are connected end to end to obtain the hot composite curve of the composite curve of multiple hot flow streams and the cold composite curve of the composite curve of multiple cold flow streams.

[0093] In a preferred embodiment of the analytical method for circulating cooling water series distribution, in step S200, different saturated grades of steam and their corresponding temperatures are as follows:

[0094] High-pressure steam: pressure 9.8~12.5MPaG, temperature 310~328℃,

[0095] Medium pressure steam: pressure 3.5~4.2MPaG, temperature 242~253℃,

[0096] Low-pressure steam: pressure 0.8~2.5MPaG, temperature 170~225℃,

[0097] Low pressure steam: 0.25~0.5MPaG, temperature 127~151℃,

[0098] Steam heating: 80~120℃.

[0099] In a preferred embodiment of the analytical method for circulating cooling water series distribution, in step S200, each heat exchanger cold flow side has a circulating water flow upper limit, and the minimum value is m max , the maximum feasible flow rate of circulating water in the series water distribution system m maxIt is expressed as,

[0100] m max =min(m 1，max , m 2，max , .., m n，max )

[0101] Among them, m n，max is the maximum circulating cooling water flow rate allowed for the nth heat exchanger;

[0102] Flow rate is m max The circulating cooling water flows through each heat exchanger in series and exchanges heat with the hot flow in sequence to achieve the lowest return water temperature. for:

[0103]

[0104] Where f is the heat exchanger action function; A total It represents the total heat transfer area of ​​each heat exchanger in the circulating water system; K represents the total heat transfer coefficient of the heat exchanger.

[0105] In a preferred embodiment of the analysis method of circulating cooling water series water distribution, in step S300, based on the hot composite curve and the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water is determined to flow through each heat exchanger in series. After removing the excess heat of the hot flow, the upper limit of the temperature of the by-product steam and its corresponding minimum flow rate of the by-product steam include:

[0106] Shift the cold composite curve so that the horizontal coordinates corresponding to its upper and lower endpoints are equal to the horizontal coordinates corresponding to the upper and lower endpoints of the hot composite curve;

[0107] Taking the lowest endpoint of the cold composite curve as the fixed point, rotate the cold composite curve counterclockwise so that the minimum vertical distance between the cold composite curve and the hot composite curve is equal to the pinch point temperature difference ΔT min , extend the cold composite curve so that the abscissa of its highest endpoint is equal to the abscissa of the highest endpoint of the hot composite curve. At this time, the highest endpoint of the cold composite curve is the upper limit of the temperature of the by-product steam after the circulating cooling water flows through each heat exchanger in series to remove the excess heat of the hot flow.

[0108] The minimum flow rate of circulating water that can be produced as steam is determined based on the slope of the cold composite curve. The relationship between the slope and the minimum flow rate of circulating water that can produce steam as a byproduct is expressed as,

[0109]

[0110] In a preferred embodiment of the analytical method for circulating cooling water series water distribution, in step S300, based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, the lower limit of the temperature of the circulating water byproduct steam is determined. The corresponding maximum flow rate of by-product steam includes:

[0111] Draw a vertical line from the highest end point of the superheat composite curve, which intersects with the horizontal line of the lowest temperature required to produce steam. The ordinate value corresponding to the intersection is the lower limit of the temperature of the by-product steam of the circulating water.

[0112] Take the intersection as the highest endpoint and connect the lowest endpoint of the original cold composite curve to construct a new cold composite curve. According to the slope of the new cold composite curve, determine the maximum flow rate of circulating water that can produce steam as a byproduct. The relationship between the slope and the maximum flow rate of circulating water that can produce steam as a byproduct is expressed as:

[0113]

[0114] In a preferred embodiment of the analysis method of circulating cooling water series water distribution, in step S400, based on the thermal composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower is determined. And the corresponding minimum return flow rate includes:

[0115] Draw a vertical line at the highest end point of the superheat composite curve, which intersects with the horizontal line where the upper limit of the cooling tower design return water temperature is located. The ordinate corresponding to the intersection is the highest return water temperature of the circulating water returning to the cooling tower

[0116] The intersection is used as the highest endpoint to connect the lowest endpoint of the cold composite curve to construct a new cold composite curve. The minimum return water flow rate of circulating water to remove the surplus heat of the system and return to the cooling tower is determined based on the slope of the new cold composite curve. The relationship between the slope and the minimum return flow of circulating water is expressed as,

[0117]

[0118] In a preferred embodiment of the analysis method of circulating cooling water series water distribution, in step S400, the maximum return water flow rate and the corresponding minimum return water temperature when the circulating cooling water removes the system surplus heat and returns to the cooling tower are determined based on the maximum feasible flow rate of the circulating water. include:

[0119] The maximum feasible flow rate of circulating water in the series water distribution system m max The maximum return flow rate when the circulating cooling water removes excess heat from the system and returns to the cooling tower

[0120] Based on maximum return flow Determine the slope of the cold composite curve, and the relationship between the maximum return flow of circulating water and the slope is expressed as:

[0121]

[0122] Based on the slope, a cold composite curve of the circulating water in this state is drawn in the temperature-enthalpy diagram. The ordinate corresponding to the highest end point of the curve is the lowest return water temperature when the cooling water removes the excess heat of the system and returns to the cooling tower.

[0123] In one embodiment, Figure 1 As shown, a feasibility analysis method for circulating cooling water series distribution includes the following steps:

[0124] S100: Extract the inlet and outlet temperature, flow rate and composition of each hot stream that needs to be cooled by circulating cooling water in the circulating cooling water distribution system; extract the inlet and outlet temperature, flow rate and composition of the cold stream (i.e., circulating cooling water) in the system. Determine the heat capacity flow rate of each hot stream and cold stream based on the stream flow rate, composition and heat capacity flow rate definition formula; determine the heat absorbed by the cold stream / released by the hot stream based on the inlet and outlet temperature and heat capacity flow rate of the cold / hot stream, and record it as the heat load; Figure 2 As shown in , according to the inlet and outlet temperatures and heat load, each stream is represented by a straight line segment (temperature enthalpy line), the vertical coordinates of the two end points respectively represent the inlet and outlet temperatures, and the difference between the horizontal coordinates (i.e., enthalpy difference) represents the heat load of the stream; Figure 3 As shown in the figure, all the hot streams are combined to obtain the thermal composite curve. The difference between the horizontal coordinates of each of the two end points represents the sum of the heat loads of all the hot streams in the temperature range. Similarly, the cold composite curve (i.e., the circulating water composite curve) is drawn, and then the two are expressed together on the temperature enthalpy diagram, as shown in the figure. Figure 4 As shown;

[0125] In this step, the relationship between the heat load of the hot stream and the inlet and outlet temperatures, flow rate and composition is expressed as:

[0126]

[0127] and

[0128]

[0129] Among them, ΔQ h is the heat load of the hot stream, that is, the heat released by the hot stream; CP h is the heat capacity flow rate of the heat stream; is the inlet temperature of the hot stream; is the outlet temperature of the hot stream; m h，AiComponent A in the hot stream i The flow rate; Cp Ai Indicates component A i The specific heat capacity; n represents the number of components in the heat flow.

[0130] The relationship between the heat load of the cold stream (circulating cooling water) and its inlet and outlet temperatures, flow rate and composition is expressed as:

[0131]

[0132] and

[0133] CP w =m w Cp w

[0134] Among them, ΔQ w The heat absorbed by the cold stream; CP w represents the heat capacity flow rate of the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; m w Indicates the cold flow rate; Cp w Represents the specific heat capacity of the cold stream.

[0135] Combine all hot streams / cold streams to obtain the hot composite curve / cold composite curve:

[0136] 1) Divide the temperature range according to the inlet temperature and outlet temperature of each hot stream / cold stream;

[0137] 2) The corresponding composite hot / cold stream can be obtained by combining the hot / cold streams in each temperature interval. The heat capacity flow rate and heat load of the composite hot / cold stream in each temperature interval are the sum of the heat capacity flow rates and heat loads of all hot / cold streams in the corresponding temperature interval, that is,

[0138]

[0139] Where ΔH k represents the heat load of the kth temperature zone; m is the number of streams in the kth temperature zone; CP m represents the heat capacity flow rate of the mth stream in the kth temperature zone; T k is the upper limit temperature of the kth temperature zone; T k+1 is the lower limit temperature of the kth temperature zone.

[0140] 3) Draw the temperature enthalpy curves of the composite hot flow / cold flow in each temperature range on the temperature enthalpy diagram, and connect them end to end to obtain the composite curves of multiple hot flow streams / cold flow streams, which are called hot composite curves / cold composite curves. Note: The cold flow streams mentioned in this article are all circulating cooling water, and the cold composite curve is the circulating water composite curve.

[0141] S200: Extract pinch point temperature difference ΔT min ; Extract different grades of saturated steam and their corresponding temperatures; extract the upper limit of the design return water temperature of the cooling tower; extract the maximum allowed circulating water flow rate on the cold flow side of each heat exchanger in the system, and then extract the minimum value, which is the maximum feasible circulating water flow rate in the series water distribution system;

[0142] In this step, the saturated steam grade is related to the actual industrial demand. Here we extract the commonly used saturated steam grades. The different saturated steam grades and their corresponding temperatures are as follows:

[0143] High-pressure steam: pressure 9.8~12.5MPaG, temperature 310~328℃

[0144] Medium pressure steam: pressure 3.5~4.2MPaG, temperature 242~253℃

[0145] Low-pressure steam: pressure 0.8~2.5MPaG, temperature 170~225℃

[0146] Low pressure steam: 0.25~0.5MPaG, temperature 127~151℃

[0147] Steam heating: 80~120℃

[0148] Due to design restrictions, each heat exchanger has an upper limit on the circulating water flow rate on the cold flow side, and the minimum value is m max , m max This is the maximum feasible flow rate of circulating water in the series water distribution system, which can be expressed as,

[0149] m max =min(m 1，max , m 2，max , .., m n，max )

[0150] Among them, m n，max is the maximum circulating cooling water flow rate allowed for the nth heat exchanger;

[0151] Flow rate is m max The circulating cooling water flows through each heat exchanger in series, and the final temperature after heat exchange with the hot flow can be expressed as:

[0152]

[0153] Where f is the heat exchanger action function; A total It represents the total heat transfer area of ​​each heat exchanger in the circulating water system; K represents the total heat transfer coefficient of the heat exchanger.

[0154] S300: Based on the hot and cold composite curves of S100 and the pinch point temperature difference ΔT extracted by S200 minBy adjusting the circulating cooling water flow rate, the circulating cooling water flows through each heat exchanger in series, and the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow and the corresponding minimum flow rate of the by-product steam are determined: Figure 5 As shown in the figure, the cold composite curve is translated so that the horizontal coordinates corresponding to the upper and lower ends of the cold composite curve are equal to the horizontal coordinates corresponding to the upper and lower ends of the hot composite curve; with the lowest end point of the cold composite curve as the fixed point, the cold composite curve is rotated counterclockwise so that the minimum vertical distance between the cold composite curve and the hot composite curve is equal to the pinch point temperature difference ΔT min , extend the cold composite curve so that the abscissa of its highest endpoint is equal to the abscissa of the highest endpoint of the hot composite curve. At this time, the highest endpoint of the cold composite curve is the upper limit of the temperature of the by-product steam after the circulating cooling water flows through each heat exchanger in series and removes the excess heat of the hot flow, which is recorded as The minimum flow rate of circulating water that can be produced as steam can be determined based on the slope of the cold composite curve, which is recorded as The relationship between the slope and the minimum flow rate of circulating water that can produce steam as a byproduct can be expressed as,

[0155] Based on the hot composite curve and cold composite curve of S100 and the different grades of saturated steam extracted from S200 and their corresponding temperatures, the lower limit of the temperature of the circulating water by-product steam and its corresponding maximum flow rate of the by-product steam are determined: a vertical line is drawn from the highest end point of the superheated composite curve, which intersects with the horizontal line of the lowest temperature required to generate steam. The ordinate value corresponding to the intersection is the lower limit of the temperature of the circulating water by-product steam, recorded as The intersection is used as the highest endpoint to connect the lowest endpoint of the original cold composite curve to construct a new cold composite curve. The maximum flow rate of circulating water that can be produced as steam can be determined based on the slope of the cold composite curve, which is recorded as The relationship between the slope and the maximum flow rate of circulating water that can be produced as steam can be expressed as:

[0156] S400: Based on the thermal composite curve and cold composite curve of S100 and the upper limit of the cooling tower design return water temperature extracted by S200, determine the maximum return water temperature and the corresponding minimum return water flow rate when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower: Figure 6 As shown, the highest end point of the superheat composite curve is a vertical line, which intersects with the horizontal line where the upper limit of the design return water temperature of the cooling tower is located. The ordinate corresponding to the intersection is the highest return water temperature of the circulating water returning to the cooling tower, which is recorded as The intersection is used as the highest endpoint to connect the lowest endpoint of the cold composite curve to construct a new cold composite curve. Based on the slope of the cold composite curve, the minimum return water flow rate of circulating water to remove the excess heat of the system and return to the cooling tower can be determined, which is recorded as The relationship between the slope and the minimum return flow of circulating water can be expressed as:

[0157] Based on the maximum feasible flow rate of circulating water in the series water distribution system extracted by S200, the maximum return water flow rate and the corresponding minimum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower are determined: the maximum feasible flow rate of circulating water in the series water distribution system m max It is the maximum return flow rate of circulating cooling water when it removes excess heat from the system and returns to the cooling tower, which can be recorded as The slope of the cold composite curve can be determined based on the maximum return flow rate. The relationship between the maximum return flow rate of circulating water and the slope can be expressed as: Based on the slope, a cold composite curve of the circulating water in this state is drawn in the temperature-enthalpy diagram. The ordinate corresponding to the highest end point of the curve is the lowest return water temperature when the cooling water removes the excess heat of the system and returns to the cooling tower, which is recorded as

[0158] S500: Analyze the feasibility of circulating cooling water series distribution based on the cold and hot composite curves of S300 and S400.

[0159] In this step, based on the cold and hot composite curves of S300 and S400, if Greater than This means that the water distribution system can be connected in series and produce steam as a by-product, such as Figure 5 As shown; if S400 Greater than This means that the water distribution system can be connected in series and the return water temperature does not exceed the design limit of the cooling tower. Figure 6 If both conditions are met, then Figure 7 shown.

[0160] Next, an ethylbenzene unit is analyzed by using the existing method and the scheme disclosed in the present invention to further illustrate the technical effect of the scheme.

[0161] In step S100, according to the relationship between the inlet and outlet temperatures, flow rates and compositions of the cold and hot streams, the heat absorbed by the cold stream / released by the hot stream in the circulating water cooling water system is calculated as shown in Table 1. All hot streams are combined to obtain a thermal composite curve, the difference between the abscissas of each of its two end points represents the sum of the heat loads of all hot streams in the temperature range, and the cold composite curve (i.e., the circulating water composite curve) is similarly drawn, and then both are represented together on the temperature enthalpy diagram;

[0162] Table 1

[0163]

[0164] In step S200, the pinch point temperature difference ΔT min 10℃; the upper limit of the design return water temperature of the cooling tower is 50℃; the maximum feasible flow rate of circulating water in the series water distribution system is 22,000 kg / hour;

[0165] In step S300, based on the hot composite curve and the cold composite curve of S100 and the pinch point temperature difference ΔT extracted in S200 min By adjusting the circulating cooling water flow rate, the upper temperature limit of the circulating cooling water flowing through each heat exchanger in series is determined to be 100°C. The corresponding minimum flow rate of by-product steam is 60750kg / h by using the relationship between the inlet and outlet temperatures, flow rates and composition of the cold stream. Based on the thermal composite curve and cold composite curve of S100 and the different grades of saturated steam extracted by S200 and their corresponding temperatures, the lower temperature limit of the circulating water by-product steam is determined to be 80°C. By using the relationship between the inlet and outlet temperatures, flow rates and composition of the cold stream, the corresponding maximum flow rate of by-product steam is 84116 kg / hour. Note: The temperature of the steam currently generated is relatively low, and the by-product steam can be converted into by-product hot water as needed.

[0166] In step S400, based on the thermal composite curve and cold composite curve of S100 and the upper limit of the design return water temperature of the cooling tower extracted in S200 of 50°C, it is determined that the maximum return water temperature of the circulating cooling water when removing the surplus heat of the system and returning to the cooling tower is 50°C, and the corresponding minimum return water flow rate is 198819 kg / hour through the relationship between the inlet and outlet temperatures, flow rates and composition of the cold flow stock; the maximum feasible flow rate of circulating water in the series water distribution system extracted based on S200 is 220,000 kg / hour, and the corresponding minimum return water temperature of 47°C is calculated through the relationship between the inlet and outlet temperatures, flow rates and composition of the cold flow stock.

[0167] In step S500, based on the cold and hot composite curves of S300 and S400, the feasibility of circulating cooling water series distribution is analyzed.

[0168] Based on S300 Right now Greater than This means that the water distribution system can be connected in series and produce steam (hot water) as a byproduct; based on S400 This means that the water distribution system can be connected in series and the return water temperature does not exceed the design limit of the cooling tower.

[0169] Currently, due to the complexity of the circulating cooling water series water distribution process, unclear mechanism, unclear coupling relationship between circulating water flow rate and supply and return water temperature and cooling tower, there are few reports on the analysis method of the feasibility of the series connection of the water distribution system. The feasibility analysis method of the series connection of the circulating water distribution system disclosed in the present invention is based on strict material balance and energy balance. By introducing the idea of ​​series connection of the water distribution system and using the temperature enthalpy diagram, it can intuitively reflect the coupling relationship between the heat flow and the circulating water in the water distribution system. At the same time, the series connection of the water distribution system can achieve a significant reduction in the water consumption and energy consumption of the circulating water system. In addition, it provides a certain direction for the development of the water distribution system from parallel connection to high-quality series connection.

[0170] In one embodiment, the inlet and outlet temperatures, flow rates and compositions of the cold / hot streams are extracted to determine the heat absorbed / released, recorded as heat load, and all the cold / hot streams are merged to obtain a cold / hot composite curve, which is represented on a temperature-enthalpy diagram; the pinch point temperature difference is extracted, saturated steam of different grades and their corresponding temperatures are extracted, the upper limit of the design return water temperature of the cooling tower is extracted, and the maximum feasible flow rate of the circulating water in the series water distribution system is extracted; based on the saturated steam of different grades and their corresponding temperatures, the lower limit of the temperature of the by-product steam of the circulating water is determined, and based on the cold-hot composite curve and the pinch point temperature difference, the maximum temperature of the by-product steam that can be reached by the circulating cooling water after flowing through each heat exchanger in series is determined; based on the cold-hot composite curve and the upper limit of the design return water temperature of the cooling tower, the maximum return water temperature of the circulating cooling water removing the surplus heat from the system and returning to the cooling tower is determined, and based on the maximum feasible flow rate of the circulating water in the series water distribution system, the minimum return water temperature of the circulating cooling water removing the surplus heat from the system and returning to the cooling tower is determined.

[0171] Although the embodiments of the present invention are described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields, and the above specific embodiments are only illustrative and instructive, rather than restrictive. A person of ordinary skill in the art can also make many forms under the guidance of this specification and without departing from the scope of protection of the claims of the present invention, all of which belong to the protection of the present invention.

Claims

1. An analytical method for circulating cooling water series distribution, characterized in that: include: Step S100: extracting the inlet and outlet temperatures, flow rates and compositions of each hot stream that needs to be cooled by circulating cooling water in the circulating cooling water distribution system; Extract the inlet and outlet temperatures, flow rates and composition of the cold stream, i.e., the circulating cooling water, in the circulating cooling water distribution system; determine the heat capacity flow rate of each hot stream and cold stream based on the stream flow rate, composition and heat capacity flow rate definition formula; determine the heat absorbed by the cold stream and the heat released by the hot stream based on the inlet and outlet temperatures and heat capacity flow rates of the cold stream and the inlet and outlet temperatures and heat capacity flow rates of the hot stream, and record them as heat loads; based on the inlet and outlet temperatures and heat loads, each stream is represented by a straight line segment of a temperature enthalpy line, the corresponding ordinates of its two end points represent the inlet and outlet temperatures, respectively, and the difference between the abscissas is the enthalpy difference to represent the heat load of the stream; combine all hot streams to obtain a thermal composite curve, the difference between the abscissas of each of its two end points represents the sum of the heat loads of all hot streams in the temperature range, and similarly draw a cold composite curve, i.e., a circulating water composite curve, and then represent the two together on the temperature enthalpy diagram; Step S200: Extracting the pinch point temperature difference ΔT min , different grades of saturated steam and their corresponding temperatures, the upper limit of the design return water temperature of the cooling tower, and the maximum allowed circulating water flow rate on the cold flow side of each heat exchanger in the circulating cooling water distribution system, and then extract the minimum value, which is the maximum feasible circulating water flow rate of the circulating cooling water distribution system in series; Step S300: Based on the hot composite curve, the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water flows through each heat exchanger in series, and the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow is determined. and the minimum flow rate of the corresponding by-product steam; based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, determine the lower limit of the temperature of the circulating water by-product steam and its corresponding maximum flow rate of by-product steam; Step S400: Based on the heat composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, determine the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower and its corresponding minimum return water flow rate; based on the maximum feasible flow rate of circulating water, determine the maximum return water flow rate and its corresponding minimum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower Step S500: Based on the heat composite curve and the cold composite curve, the feasibility of circulating cooling water series distribution is analyzed, wherein the upper limit of the temperature of the by-product steam after removing the excess heat of the hot flow is Greater than the lower limit of the temperature of the circulating water by-product steam The circulating cooling water distribution system can be connected in series and produce steam as a by-product; if the maximum return water temperature Greater than the minimum return water temperature The circulating cooling water distribution system can be connected in series, and the return water temperature does not exceed the design limit of the cooling tower.

2. The method for analyzing circulating cooling water series distribution according to claim 1 is characterized in that: Preferably, in step S100, the relationship between the heat load of the hot stream and the inlet and outlet temperatures, flow rate and composition is expressed as: and Among them, ΔQ h is the heat load of the hot stream, that is, the heat released by the hot stream; CP h is the heat capacity flow rate of the heat stream; is the inlet temperature of the hot stream; is the outlet temperature of the hot stream; m h，Ai Component A in the hot stream i The flow rate; Cp Ai Indicates component A i The specific heat capacity; n represents the number of components in the heat flow.

3. The method for analyzing circulating cooling water series distribution according to claim 1 is characterized in that: In step S100, the relationship between the heat load of the cold stream and its inlet and outlet temperatures, flow rate and composition is expressed as: and CP w =m w Cp w , Among them, ΔQ w The heat absorbed by the cold stream; CP w represents the heat capacity flow rate of the cold stream; is the outlet temperature of the cold stream; is the inlet temperature of the cold stream; m w Indicates the cold flow rate; Cp w Represents the specific heat capacity of the cold stream.

4. The method for analyzing circulating cooling water series distribution according to claim 1 is characterized in that: In step S100, the steps of combining all hot streams to obtain a hot composite curve and combining all cold streams to obtain a cold composite curve include: Dividing the temperature intervals according to the inlet temperature and outlet temperature of each hot stream and the inlet temperature and outlet temperature of each cold stream; The hot stream and cold stream in each temperature interval are combined to obtain the corresponding composite hot stream and composite cold stream. The heat capacity flow rate and heat load of the composite hot stream and composite cold stream in each temperature interval are the sum of the heat capacity flow rate and heat load of all hot streams and cold streams in the corresponding temperature interval, that is, Where ΔH k represents the heat load of the kth temperature zone; m is the number of streams in the kth temperature zone; CP m represents the heat capacity flow rate of the mth stream in the kth temperature zone; T k is the upper limit temperature of the kth temperature zone; T k+1 is the lower limit temperature of the kth temperature zone; The composite hot flow temperature enthalpy curve and the composite cold flow temperature enthalpy curve in each temperature range are drawn on the temperature enthalpy diagram, and they are connected end to end to obtain the hot composite curve of the composite curve of multiple hot flow streams and the cold composite curve of the composite curve of multiple cold flow streams.

5. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S200, different saturated grades of steam and their corresponding temperatures are as follows: High-pressure steam: pressure 9.8~12.5MPaG, temperature 310~328℃, Medium pressure steam: pressure 3.5~4.2MPaG, temperature 242~253℃, Low-pressure steam: pressure 0.8~2.5MPaG, temperature 170~225℃, Low pressure steam: 0.25~0.5MPaG, temperature 127~151℃, Steam heating: 80~120℃.

6. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S200, each heat exchanger has an upper limit on the cold flow side of the circulating water flow, and the minimum value is m max , the maximum feasible flow rate of circulating water in the series water distribution system m max It is expressed as, m max =min(m 1，max ,m 2，max ,...,m n，max ), Among them, m n，max is the maximum circulating cooling water flow rate allowed for the nth heat exchanger; Flow rate is m max The circulating cooling water flows through each heat exchanger in series and exchanges heat with the hot flow in sequence to achieve the lowest return water temperature. for: Where f is the heat exchanger action function; A total It represents the total heat transfer area of ​​each heat exchanger in the circulating water system; K represents the total heat transfer coefficient of the heat exchanger.

7. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S300, based on the hot composite curve, the cold composite curve and the pinch point temperature difference ΔT min By adjusting the circulating cooling water flow rate, the circulating cooling water is determined to flow through each heat exchanger in series. After removing the excess heat of the hot flow, the upper limit of the temperature of the by-product steam and its corresponding minimum flow rate of the by-product steam include: Shift the cold composite curve so that the horizontal coordinates corresponding to its upper and lower endpoints are equal to the horizontal coordinates corresponding to the upper and lower endpoints of the hot composite curve; Taking the lowest endpoint of the cold composite curve as the fixed point, rotate the cold composite curve counterclockwise so that the minimum vertical distance between the cold composite curve and the hot composite curve is equal to the pinch point temperature difference ΔT min , extend the cold composite curve so that the abscissa of its highest endpoint is equal to the abscissa of the highest endpoint of the hot composite curve. At this time, the highest endpoint of the cold composite curve is the upper limit of the temperature of the by-product steam after the circulating cooling water flows through each heat exchanger in series to remove the excess heat of the hot flow. The minimum flow rate of circulating water that can be produced as steam is determined based on the slope of the cold composite curve. The relationship between the slope and the minimum flow rate of circulating water that can produce steam as a byproduct is expressed as, 8. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S300, based on the hot composite curve and the cold composite curve and the different grades of saturated steam and their corresponding temperatures, the lower limit of the temperature of the by-product steam of the circulating water is determined. The corresponding maximum flow rate of by-product steam includes: Draw a vertical line from the highest end point of the superheat composite curve, which intersects with the horizontal line of the lowest temperature required to produce steam. The ordinate value corresponding to the intersection is the lower limit of the temperature of the by-product steam of the circulating water. Take the intersection as the highest endpoint and connect the lowest endpoint of the original cold composite curve to construct a new cold composite curve. According to the slope of the new cold composite curve, determine the maximum flow rate of circulating water that can produce steam as a byproduct. The relationship between the slope and the maximum flow rate of circulating water that can produce steam as a byproduct is expressed as:

9. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S400, based on the heat composite curve and the cold composite curve and the upper limit of the designed return water temperature of the cooling tower, the maximum return water temperature when the circulating cooling water removes the surplus heat of the system and returns to the cooling tower is determined. And the corresponding minimum return flow rate includes: Draw a vertical line at the highest end point of the superheat composite curve, which intersects with the horizontal line where the upper limit of the cooling tower design return water temperature is located. The ordinate corresponding to the intersection is the highest return water temperature of the circulating water returning to the cooling tower The intersection is used as the highest endpoint to connect the lowest endpoint of the cold composite curve to construct a new cold composite curve. The minimum return water flow rate of circulating water to remove the surplus heat of the system and return to the cooling tower is determined based on the slope of the new cold composite curve. The relationship between the slope and the minimum return flow of circulating water is expressed as, 10. The method for analyzing circulating cooling water series distribution according to claim 1, characterized in that: In step S400, the maximum return water flow rate and the corresponding minimum return water temperature when the circulating cooling water removes the excess heat of the system and returns to the cooling tower are determined based on the maximum feasible flow rate of the circulating water. include: The maximum feasible flow rate of circulating water in the series water distribution system m max The maximum return flow rate when the circulating cooling water removes excess heat from the system and returns to the cooling tower Based on maximum return flow Determine the slope of the cold composite curve, and the relationship between the maximum return flow of circulating water and the slope is expressed as: Based on the slope, a cold composite curve of the circulating water in this state is drawn in the temperature-enthalpy diagram. The ordinate corresponding to the highest end point of the curve is the lowest return water temperature when the cooling water removes the excess heat of the system and returns to the cooling tower.

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