A heat pump system for waste heat recovery from pulp black liquor and a pulp heating method

Abstract

This invention relates to the field of waste heat utilization technology, providing a heat pump system and pulp heating method for waste heat recovery from pulp black liquor. It addresses the problem that existing pulp black liquor waste heat recovery methods struggle to determine the lowest overall energy consumption heat extraction ratio and achieve stable heating. The invention includes a pulp heating method employing a heat pump system for pulp black liquor waste heat recovery. The pulp heating method includes the following steps executed periodically: S1, parameter acquisition; S2, heat calculation; S3, determination of a candidate heat extraction ratio set; S4, calculation of a comprehensive objective function; S5, load allocation; and S6, dynamic protection and recovery control. The pulp heating method proposed in this invention can simultaneously consider energy saving, heating stability, and a safe heat extraction boundary on the black liquor side during black liquor waste heat recovery. By selecting the candidate heat extraction ratio with the lowest comprehensive objective function value from the candidate heat extraction ratio set, dynamic load allocation can be achieved between heat pump heating, compensation branch heating, and bypass protection.

Description

Technical Field

[0001] This invention relates to the field of waste heat utilization technology, and in particular to a heat pump system and a pulp heating method for waste heat recovery from pulp black liquor. Background Technology

[0002] The pulp production process typically generates high-temperature waste heat media, among which high-temperature waste heat media, represented by pulp black liquor, have significant waste heat recovery value. Existing technologies already include solutions for waste heat recovery from pulp black liquor. For example, Chinese Patent Publication No. CN221975349U discloses a flash evaporation electrostatic precipitator system suitable for black liquor waste heat utilization. This system includes components such as a dilute black liquor supply tank, a flash evaporator, a generator, a condenser, an absorber, and an evaporator. By combining flash evaporation electrostatic precipitator with a lithium bromide absorption heat pump, the dilute black liquor undergoes multi-stage treatment to achieve full recovery of its thermal energy.

[0003] In actual pulp production, black liquor is not a typical clean heat source. The availability of waste heat is limited by factors such as the target temperature of subsequent processes, the concentration of the medium, the pressure difference of the heat exchanger, and the risk of scaling or clogging in the heat exchanger. If the goal is simply to increase the heat output to improve the waste heat utilization rate, it may result in an excessively low outlet temperature of the medium in the first process, affecting the stable operation of subsequent evaporation, concentration, or recovery processes. It may also exacerbate the risk of clogging and scaling due to increased medium concentration or increased pressure difference in the heat exchanger. Existing waste heat utilization systems typically focus more on multi-stage heat exchange or multi-stage utilization in the system structure, lacking a comparison of the overall energy consumption of the system. Summary of the Invention

[0004] Therefore, there is a need to provide a heat pump system and pulp heating method for waste heat recovery from pulp black liquor, in order to solve the technical problem that existing waste heat recovery methods for pulp black liquor are unable to determine the heat extraction ratio with the lowest overall energy consumption and achieve stable heating.

[0005] To achieve the above objectives, the inventors provide a pulp heating method employing a heat pump system for recovering waste heat from pulp black liquor. The heat pump system includes a waste heat-side heat exchange branch for receiving a first process medium, a waste heat-side bypass branch for allowing at least a portion of the first process medium to bypass the waste heat-side heat exchange branch, a heat pump circulation heat exchange branch for exchanging heat with the waste heat-side heat exchange branch, a heat-using-side heat exchange branch for receiving a second process medium and exchanging heat with the heat pump circulation heat exchange branch, a compensation branch for compensating the heat-using-side heat exchange branch, and a controller. The heat pump circulation heat exchange branch includes a variable frequency compressor. The pulp heating method includes periodically performing the following steps:

[0006] S1. Obtain the inlet temperature, outlet temperature, flow rate, concentration, specific heat capacity, allowable outlet temperature, and differential pressure change rate of the heat exchanger in the waste heat side heat exchange branch of the first process medium, as well as the inlet temperature, target temperature, flow rate, and specific heat capacity of the second process medium.

[0007] S2. Calculate the safe transferable heat based on the inlet temperature, flow rate, specific heat capacity and allowable outlet temperature of the first process medium, and calculate the required heat based on the inlet temperature, target temperature, flow rate and specific heat capacity of the second process medium.

[0008] S3. Based on the safe transferable heat, the required heat, the minimum stable operating frequency of the variable frequency compressor, the minimum circulation flow of the heat exchange branch on the heat use side, and the preset heat extraction ratio step size, determine the candidate heat extraction ratio set. The candidate heat extraction ratio in the candidate heat extraction ratio set is used to represent the proportion of the heating load undertaken by the heat pump circulation heat exchange branch to the required heat.

[0009] S4. Calculate the comprehensive objective function value for each candidate heat extraction ratio in the candidate heat extraction ratio set. The comprehensive objective function includes at least the compressor power of the heat pump cycle heat exchange branch, the equivalent energy consumption of the compensation branch, the energy consumption of the waste heat side heat exchange branch, and the energy consumption of the heat use side heat exchange branch.

[0010] S5. Select the candidate heat extraction ratio that meets the safety constraints and has the lowest comprehensive objective function value, determine the load distribution between the heat pump circulation heat exchange branch, the waste heat side bypass branch and the compensation branch, and adjust the frequency of the variable frequency compressor and the opening degree of at least one valve accordingly.

[0011] S6. When the outlet temperature of the first process medium is lower than the allowable outlet temperature, or the differential pressure change rate exceeds the first threshold, the waste heat side bypass ratio is increased at the first adjustment rate, the heat pump circulation heat exchange branch load is reduced at the second adjustment rate, and the compensation branch load is increased; when the outlet temperature of the first process medium is higher than the allowable outlet temperature and the differential pressure change rate is lower than the second threshold for several consecutive control cycles, the waste heat side bypass ratio is reduced at a recovery rate lower than the first adjustment rate, and the heat pump circulation heat exchange branch load is restored.

[0012] Furthermore, the allowable outlet temperature of the first process medium is determined as follows: Allowable outlet temperature of the first process medium = the maximum value among the minimum temperature of the subsequent process, the concentration correction temperature, and the differential pressure correction temperature + safety temperature margin.

[0013] Furthermore, the safe transferable heat is calculated as follows: Safe transferable heat = first medium mass flow rate × first medium specific heat capacity × (first medium inlet temperature - first process medium allowable outlet temperature) × concentration correction factor × scaling risk correction factor.

[0014] Furthermore, the required heat is calculated as follows: Required heat = Second medium mass flow rate × Second medium specific heat capacity × (Second medium target temperature - Second medium inlet temperature).

[0015] Furthermore, the candidate heat extraction ratio set is determined in the following manner:

[0016] The maximum heat extraction ratio is determined based on the ratio of the safe, transferable heat to the required heat.

[0017] The first minimum heat extraction ratio is determined based on the ratio of the minimum heat pump heating capacity corresponding to the lowest stable operating frequency of the variable frequency compressor to the required heat; the second minimum heat extraction ratio is determined based on the ratio of the minimum exchangeable heat capacity corresponding to the minimum circulation flow rate of the heat exchange branch on the heat use side to the required heat; the larger value between the first minimum heat extraction ratio and the second minimum heat extraction ratio is determined as the minimum heat extraction ratio.

[0018] Between the minimum heat extraction ratio and the maximum heat extraction ratio, multiple candidate heat extraction ratios are generated according to a preset heat extraction ratio step size to form the candidate heat extraction ratio set.

[0019] When the minimum heat extraction ratio is greater than the maximum heat extraction ratio, the candidate heat extraction ratio set is determined to be an empty set or the heat pump circulation heat exchange branch is switched to shutdown or minimum safe operation state, and the compensation branch undertakes at least part of the required heat.

[0020] Furthermore, the set of candidate heat extraction ratios is generated using a finite candidate enumeration method, with the number of candidate values ​​ranging from 5 to 25. When a certain candidate heat extraction ratio no longer meets the safety constraint, the calculation of a higher candidate value than that candidate heat extraction ratio is stopped.

[0021] Furthermore, the sensor sampling period is 1 to 10 seconds, the optimization update period of the candidate heat extraction ratio set is 10 to 60 seconds, and the differential pressure change rate is smoothed by the moving average or running average of the most recent 20 to 100 sampled values.

[0022] Furthermore, the first threshold is greater than the second threshold, and the recovery phase requires that the outlet temperature of the first process medium be higher than the allowable outlet temperature and that the differential pressure change rate be lower than the second threshold for 3 to 20 consecutive control cycles.

[0023] Furthermore, the compensation branch is one of a steam heating branch, a hot water heating branch, or an electric heating branch.

[0024] A heat pump system for recovering waste heat from pulp black liquor includes a waste heat-side heat exchange branch for receiving a first process medium, a waste heat-side bypass branch for allowing at least a portion of the first process medium to bypass the waste heat-side heat exchange branch, a heat pump circulation heat exchange branch for exchanging heat with the waste heat-side heat exchange branch, a heat-use-side heat exchange branch for receiving a second process medium and exchanging heat with the heat pump circulation heat exchange branch, a compensation branch for compensating the heat-use-side heat exchange branch, and a controller. It further includes:

[0025] The system comprises a first process medium inlet temperature sensor, a first process medium outlet temperature sensor, a first process medium flow meter, a first process medium concentration sensor, a waste heat side heat exchanger differential pressure sensor, a second process medium inlet temperature sensor, a second process medium outlet temperature sensor, a variable frequency compressor frequency feedback module, and a pump power acquisition module; the controller is electrically connected to the variable frequency compressor, the waste heat side bypass valve, and the compensation branch regulating valve, and is configured to execute the method described in any one of the above-mentioned inventors.

[0026] The above technical solution has the following advantages, unlike existing technologies:

[0027] This invention incorporates safe transferable heat, required heat, minimum stable operating frequency of the variable frequency compressor, minimum circulating flow rate on the heat-consuming side, and differential pressure change rate into the control process. This allows for simultaneous energy saving, heating stability, and safe heat extraction boundaries on the black liquor side during black liquor waste heat recovery. Compared to methods that only increase the heat pump load based on heat demand, this method avoids excessive extraction of black liquor heat, which could lead to excessively low outlet temperatures of the first process medium, thus reducing the impact on subsequent black liquor treatment, evaporation, or recovery processes. Furthermore, when the differential pressure change rate of the heat exchanger abnormally increases, the waste heat side bypass ratio can be increased in a timely manner, and the heat pump load can be reduced, thereby mitigating the risks of heat exchanger scaling, blockage, and abnormally widening pressure drop.

[0028] Furthermore, by selecting the candidate heat extraction ratio with the lowest comprehensive objective function value from the candidate heat extraction ratio set, dynamic load distribution can be achieved among heat pump heating, compensation branch heating, and bypass protection, thereby improving the overall energy efficiency of the system and reducing the frequent and large-scale operation of compressors, valves, and pump sets.

[0029] The above description of the invention is merely an overview of the technical solution of this application. In order to enable those skilled in the art to better understand the technical solution of this application and to implement it based on the description and drawings, and to make the above-mentioned objectives and other objectives, features and advantages of this application easier to understand, the following description is provided in conjunction with the specific embodiments and drawings of this application. Attached Figure Description

[0030] The accompanying drawings are only used to illustrate the principles, implementation methods, applications, features, and effects of specific embodiments of the present invention and other related contents, and should not be considered as limitations on this application.

[0031] In the accompanying drawings of the instruction manual:

[0032] Figure 1 This is a schematic diagram of the waste heat recovery heat pump system for pulp black liquor in Embodiment 2.

[0033] The reference numerals used in the above figures are explained as follows:

[0034] 1. Waste heat side heat exchange branch; 11. Waste heat side heat exchanger; 12. First process medium inlet temperature sensor; 13. First process medium outlet temperature sensor; 14. First process medium flow meter; 15. First process medium concentration sensor; 16. Waste heat side heat exchanger differential pressure sensor;

[0035] 2. Waste heat side bypass branch; 21. Waste heat side bypass valve;

[0036] 3. Heat pump circulation heat exchange branch; 31. Variable frequency compressor; 32. Variable frequency compressor frequency feedback module;

[0037] 4. Heat exchange branch on the hot side; 41. Heat exchanger on the hot side; 42. Circulating pump; 43. Second process medium inlet temperature sensor; 44. Second process medium outlet temperature sensor; 45. Pump power acquisition module;

[0038] 5. Compensation branch; 51. Compensation branch regulating valve;

[0039] 6. Controller; 61. Alarm module; 62. Host computer; 63. Data storage module. Detailed Implementation

[0040] To illustrate the possible application scenarios, technical principles, implementable specific solutions, and achievable objectives and effects of this application in detail, the following description, in conjunction with the listed specific embodiments and accompanying drawings, provides a detailed explanation. The embodiments described herein are merely illustrative of the technical solutions of this application and are therefore intended to limit the scope of protection of this application.

[0041] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0042] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0043] In the description of this application, the term "and / or" is used to describe the logical relationship between objects, indicating that three relationships can exist. For example, A and / or B means: A exists, B exists, and A and B exist simultaneously. Additionally, the character " / " in this document generally indicates that the preceding and following objects have an "or" logical relationship.

[0044] In this application, terms such as “first” and “second” are used only to distinguish one entity or operation from another, and do not necessarily require or imply any actual quantity, hierarchy or order relationship between these entities or operations.

[0045] Unless otherwise specified, the use of terms such as “comprising,” “including,” “having,” or other similar expressions in this application is intended to cover non-exclusive inclusion, which does not exclude the presence of additional elements in a process, method, or product that includes the stated elements, such that a process, method, or product that includes a list of elements may include not only those defined elements but also other elements not expressly listed, or elements inherent to such a process, method, or product.

[0046] As understood in the Examination Guidelines, in this application, expressions such as "greater than," "less than," and "exceeding" are understood to exclude the stated number; expressions such as "above," "below," and "within" are understood to include the stated number. Furthermore, in the description of the embodiments in this application, "multiple" means two or more (including two), and similar expressions related to "multiple" are also understood in this way, such as "multiple groups" and "multiple times," unless otherwise explicitly specified.

[0047] In the description of the embodiments of this application, the space-related expressions used, such as "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "vertical," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," indicate the orientation or positional relationship based on the orientation or positional relationship shown in the specific embodiments or drawings. They are only for the purpose of describing the specific embodiments of this application or for the reader's understanding, and do not indicate or imply that the device or component referred to must have a specific position, a specific orientation, or be constructed or operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0048] Unless otherwise expressly specified or limited, the terms "installation," "connection," "linking," "fixing," and "setting," as used in the description of the embodiments of this application, should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral setting; it can be a mechanical connection, an electrical connection, or a communication connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal connection of two components or the interaction between two components. For those skilled in the art to which this application pertains, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0049] It should be noted that the first process medium in this invention can be black liquor generated during pulp production, or a process waste heat medium with similar characteristics of high scaling risk and high concentration variation; the second process medium can be pulp to be heated, process hot water, or other process media that require heating. The following embodiments use pulp black liquor as the first process medium and pulp to be heated as the second process medium for illustration, but this does not constitute a limitation on the scope of protection of this invention.

[0050] The first process medium described in this invention can be black liquor, dilute black liquor, concentrated black liquor, or other process media containing recoverable heat and subject to scaling, concentration changes, or subsequent process temperature limitations, generated during pulp production. The second process medium can be pulp to be heated, process water, washing liquid, pre-bleaching pulp, or other papermaking process media requiring temperature increases. The following embodiments use pulp black liquor as the first process medium and pulp to be heated as the second process medium for illustration.

[0051] Example 1: This example provides a pulp heating method, which is implemented using a heat pump system for recovering waste heat from pulp black liquor. The heat pump system includes a waste heat-side heat exchange branch, a waste heat-side bypass branch, a heat pump circulation heat exchange branch, a heat-using-side heat exchange branch, a compensation branch, and a controller. Specifically, the waste heat-side heat exchange branch receives pulp black liquor as the first process medium and transfers heat from the pulp black liquor to the heat pump circulation heat exchange branch; the waste heat-side bypass branch allows at least a portion of the pulp black liquor to bypass the waste heat-side heat exchange branch; the heat pump circulation heat exchange branch upgrades the recovered low-grade heat to a level usable for the second process medium; the heat-using-side heat exchange branch receives the pulp to be heated and exchanges heat with the heat pump circulation heat exchange branch; and the compensation branch compensates for insufficient heat pump output or limited waste heat on the waste heat side when heating the heat-using-side heat exchange branch is insufficient.

[0052] The pulp heating method in this embodiment does not simply take the maximum waste heat recovery as the control target. Instead, it determines the load distribution between the heat pump circulation heat exchange branch and the compensation branch on the basis of ensuring the black liquor outlet temperature, black liquor concentration adaptability, heat exchanger pressure difference change and subsequent process stability. This avoids excessive heat extraction, which could lead to excessively low black liquor temperature, poor fluidity, aggravated heat exchanger scaling, or affect subsequent evaporation, combustion, concentration and other processes.

[0053] The pulp heating method includes periodically performing the following steps:

[0054] S1. Parameter Acquisition: Acquire the inlet temperature, outlet temperature, flow rate, concentration, specific heat capacity, allowable outlet temperature, and differential pressure change rate of the heat exchanger in the waste heat side heat exchange branch of the first process medium, as well as the inlet temperature, target temperature, flow rate, and specific heat capacity of the second process medium.

[0055] The specific heat capacity of the first process medium can be pre-input by the operator or obtained by the controller from a table lookup based on the black liquor concentration, temperature range, and a preset physical property database. The specific heat capacity of the second process medium can be determined by adjusting the slurry concentration, moisture content, and temperature range. The sensor sampling period can be 1 to 10 seconds, for example, 5 seconds; the optimization update period for the candidate heat extraction ratio set can be 10 to 60 seconds, for example, 30 seconds. The heat exchanger differential pressure change rate is preferably smoothed using a moving average or running average of the most recent 20 to 100 sampled values. For example, the smoothed differential pressure change rate can be calculated using the most recent 60 sampled values ​​to reduce the impact of instantaneous fluctuations on the control results.

[0056] The allowable outlet temperature of the first process medium is determined as follows: Allowable outlet temperature of the first process medium = max(minimum temperature of subsequent processes, concentration correction temperature, differential pressure correction temperature) + safety temperature margin.

[0057] The minimum temperature for subsequent processes refers to the lowest permissible temperature at which black liquor enters subsequent evaporation, concentration, combustion, or conveying processes; the concentration correction temperature refers to the lowest safe flow temperature determined based on the black liquor concentration; the differential pressure correction temperature refers to the lowest safe heat exchange temperature determined based on the differential pressure or its trend in the waste heat side heat exchanger; the safe temperature margin is 1°C to 10°C, preferably 3°C to 5°C. When the black liquor concentration increases or the rate of change in differential pressure increases, the concentration correction temperature or differential pressure correction temperature increases accordingly, thereby increasing the permissible outlet temperature of the first process medium and reducing the heat extracted by the heat pump from the black liquor side.

[0058] S2. Heat Calculation: Calculate the safe transferable heat based on the inlet temperature, flow rate, specific heat capacity, and allowable outlet temperature of the first process medium. Specifically, calculate using the following formula: Safe Transferable Heat = Mass Flow Rate of First Medium × Specific Heat Capacity of First Medium × (Inlet Temperature of First Medium - Allowable Outlet Temperature of First Process Medium) × Concentration Correction Factor × Scaling Risk Correction Factor.

[0059] The concentration correction factor and the scaling risk correction factor are coefficients between 0 and 1. When the black liquor concentration is within the normal range and the rate of change of the heat exchanger differential pressure is low, the concentration correction factor and the scaling risk correction factor can approach 1. When the black liquor concentration increases, the viscosity increases, or the rate of change of the differential pressure increases, the concentration correction factor and the scaling risk correction factor decrease to reduce the safe transferable heat that can be used for heat pump extraction. If the inlet temperature of the first medium is less than or equal to the allowable outlet temperature of the first process medium, the safe transferable heat can be set to 0, indicating that it is not advisable to continue extracting heat from the first process medium in the current cycle.

[0060] The required heat is calculated based on the inlet temperature, target temperature, flow rate, and specific heat capacity of the second process medium. Specifically, the calculation is as follows: Required heat = Second medium mass flow rate × Second medium specific heat capacity × (Second medium target temperature - Second medium inlet temperature).

[0061] When the target temperature of the second medium is less than or equal to the inlet temperature of the second medium, the required heat can be set to 0, indicating that heating is not required or only the temperature needs to be maintained.

[0062] S3. Determination of candidate heat extraction ratio set: The candidate heat extraction ratio is used to represent the proportion of heating load undertaken by the heat pump circulation heat exchange branch to the required heat.

[0063] The maximum heat extraction ratio is determined based on the ratio of safe, transferable heat to demanded heat: Maximum heat extraction ratio = min(safe, transferable heat / demanded heat, 1).

[0064] When the heat demand is 0, the heat pump circulation heat exchange branch enters a shutdown, standby, or low-load maintenance state. When the heat demand is greater than 0, the controller continues to determine the minimum heat extraction ratio.

[0065] Specifically, the first minimum heat extraction ratio is determined based on the minimum heat pump heating capacity corresponding to the lowest stable operating frequency of the variable frequency compressor: First minimum heat extraction ratio = minimum heat pump heating capacity / required heat.

[0066] Meanwhile, the second minimum heat extraction ratio is determined based on the minimum exchangeable heat corresponding to the minimum circulation flow rate of the heat exchange branch on the heat-using side: Second minimum heat extraction ratio = minimum exchangeable heat / demanded heat.

[0067] Then take: minimum heat extraction ratio = max(first minimum heat extraction ratio, second minimum heat extraction ratio).

[0068] When the minimum heat extraction ratio is greater than the maximum heat extraction ratio, it indicates that under the current safe heat extraction capacity and heat demand conditions of black liquor, the heat pump circulation heat exchange branch cannot stably bear the effective load within a safe range. In this case, the candidate heat extraction ratio set is determined to be an empty set, causing the heat pump circulation heat exchange branch to shut down or switch to the minimum safe operating state, and the compensation branch to bear at least part of the demanded heat, preferably all or most of the demanded heat.

[0069] When the minimum heat extraction ratio is less than or equal to the maximum heat extraction ratio, multiple candidate heat extraction ratios are generated between the minimum and maximum heat extraction ratios according to a preset heat extraction ratio step size ΔR, forming a candidate heat extraction ratio set {R1, R2, ..., Rn}. Here, ΔR can be from 0.02 to 0.10, for example, 0.05; the number of candidate values ​​can be from 5 to 25. To reduce the computational burden, this embodiment uses a finite candidate enumeration method to generate candidate values. Furthermore, when calculating sequentially from low to high heat extraction ratios, if a certain candidate heat extraction ratio has caused the outlet temperature of the first process medium to be lower than the allowable outlet temperature, the differential pressure change rate of the waste heat side heat exchanger to exceed the safety threshold, or the variable frequency compressor or circulating pump to exceed the allowable operating range, then the calculation of candidate values ​​higher than that candidate heat extraction ratio is stopped.

[0070] S4. Calculation of the comprehensive objective function: For each candidate heat extraction ratio Ri in the candidate heat extraction ratio set, calculate its corresponding comprehensive objective function value J(Ri). The comprehensive objective function includes at least the compressor power of the heat pump cycle heat exchange branch, the equivalent energy consumption of the compensation branch, the energy consumption of the waste heat side heat exchange branch, and the energy consumption of the heat use side heat exchange branch.

[0071] The overall objective function is expressed as: J(Ri) = α1Php(Ri) + α2Eaux(Ri) + α3Pp1(Ri) + α4Pp2(Ri) + α5Psafe(Ri).

[0072] Where Php(Ri) is the power of the variable frequency compressor in the heat pump cycle heat exchange branch at the candidate heat extraction ratio Ri; Eaux(Ri) is the equivalent energy consumption of the compensation branch; Pp1(Ri) is the energy consumption of the waste heat side heat exchange branch; Pp2(Ri) is the energy consumption of the heat use side heat exchange branch; Psafe(Ri) is the safety penalty term; α1 to α5 are weighting coefficients. The safety penalty term can be 0 when the candidate heat extraction ratio meets all safety constraints, and positive when it is close to the black liquor allowable outlet temperature, the differential pressure change rate is close to the threshold, or the compressor is close to the boundary condition, so that the controller tends to select the operating point with a larger safety margin.

[0073] The heat load borne by the heat pump circulation heat exchange branch is: Ri × demanded heat load; the heat load borne by the compensation branch is: (1 - Ri) × demanded heat load.

[0074] When the compensation branch is a steam-heated branch, Eaux(Ri) can be calculated based on steam unit consumption or steam-converted electricity consumption; when the compensation branch is a hot water-heated branch, Eaux(Ri) is calculated based on the energy consumption of the hot water circulating pump and the equivalent energy consumption of the heat source; when the compensation branch is an electric heating branch, Eaux(Ri) is directly calculated based on the electric heating power. The energy consumption Pp1(Ri) of the waste heat side heat exchange branch is estimated based on the black liquor flow rate entering the waste heat side heat exchange branch, the pipeline resistance, and the pump efficiency; the energy consumption Pp2(Ri) of the heat-using side heat exchange branch is estimated based on the heat-using side circulation flow rate and the pump efficiency.

[0075] S5. Load Allocation: After calculating the heat extraction ratios for each candidate, select the candidate heat extraction ratio R with the lowest comprehensive objective function value from those that meet safety constraints. Determine the load allocation among the heat pump circulation heat exchange branch, the waste heat side bypass branch, and the compensation branch based on R. Specifically, the heat pump circulation heat exchange branch bears a heating load of R × required heat, and the compensation branch bears a heating load of (1-R) ​​× required heat; the waste heat side heat exchange branch receives a corresponding proportion of the first process medium according to the heat extraction needs of the heat pump, and the remaining portion of the first process medium bypasses the waste heat side heat exchange branch through the waste heat side bypass branch.

[0076] Adjust the frequency of the variable frequency compressor, the valve opening of the bypass branch to the waste heat side, and the valve opening of the compensation branch according to the determined load distribution. Specifically, when R increases, increase the frequency of the variable frequency compressor, increase the flow rate of the waste heat side heat exchange branch, and decrease the valve opening of the compensation branch; when R decreases, decrease the frequency of the variable frequency compressor, increase the valve opening of the bypass branch to the waste heat side, and increase the valve opening of the compensation branch.

[0077] S6. Dynamic Protection and Recovery Control: When the outlet temperature of the first process medium is lower than the allowable outlet temperature, or the differential pressure change rate exceeds the first threshold, it indicates that the current heat extraction on the waste heat side is too strong or that the heat exchanger is prone to scaling or blockage. At this time, the bypass ratio on the waste heat side is increased at the first adjustment rate, while the load on the heat pump circulation heat exchange branch is reduced and the load on the compensation branch is increased at the second adjustment rate to ensure that the second process medium can still reach the target temperature. The first adjustment rate can increase the bypass ratio by 2% to 15% per control cycle, for example, 5%; the second adjustment rate can reduce the heat pump load by 2% to 10% per control cycle, for example, 3%.

[0078] When the outlet temperature of the first process medium is higher than the allowable outlet temperature and the differential pressure change rate is lower than the second threshold for several consecutive control cycles, the system enters the recovery phase. The first threshold is greater than the second threshold to form hysteresis control and avoid frequent system switching. The number of consecutive control cycles can be 3 to 20, for example, 8. During the recovery phase, the controller gradually reduces the waste heat bypass ratio and restores the heat pump circulation heat exchange branch load at a recovery rate lower than the first adjustment rate. The recovery rate can reduce the bypass ratio by 0.5% to 5% per control cycle, for example, 1%. Thus, the system can quickly protect itself when there is a risk of low temperature or increased differential pressure, and slowly restore heat extraction after the risk is eliminated, avoiding the heat exchanger from re-entering a high-risk state due to excessively rapid recovery.

[0079] As a specific operational example, when the black liquor inlet temperature is 85℃, the allowable outlet temperature is 68℃, the black liquor mass flow rate is 10kg / s, the black liquor specific heat capacity is 3.6kJ / (kg·℃), the concentration correction factor is 0.90, and the scaling risk correction factor is 0.85, the safe transferable heat can be calculated to be approximately: 0×3.6×(85-68)×0.90×0.85=468.18kW.

[0080] When the inlet temperature of the second process medium is 45℃, the target temperature is 70℃, the mass flow rate is 5kg / s, and the specific heat capacity is 4.0kJ / (kg·℃), the required heat is: 5×4.0×(70-45)=500kW.

[0081] At this point, the maximum heat extraction ratio is approximately 0.936. If the minimum heat pump heating capacity corresponding to the lowest stable operating frequency of the variable frequency compressor is 100kW, then the first minimum heat extraction ratio is 0.20; if the minimum heat exchangeable capacity corresponding to the minimum circulation flow rate on the heat-using side is 75kW, then the second minimum heat extraction ratio is 0.15; therefore, the minimum heat extraction ratio is 0.20. Candidate heat extraction ratios are generated in steps of 0.05 between 0.20 and 0.936, and the compressor power, equivalent energy consumption of the compensation branch, and pump delivery energy consumption are calculated for each candidate heat extraction ratio. The candidate heat extraction ratio with the lowest comprehensive objective function value and that satisfies the safety constraints is then selected.

[0082] Through the above method, this embodiment can balance energy saving, safety, and process continuity in the waste heat recovery process of pulp black liquor. On the one hand, it avoids pursuing the maximum waste heat utilization rate, which could lead to excessively low black liquor outlet temperature and increased risk of scaling and blockage. On the other hand, through finite candidate enumeration and comprehensive objective function selection, it achieves a better load distribution between the heat pump circulation heat exchange branch and the compensation branch, thereby reducing the total equivalent energy consumption of the system.

[0083] Example 2: Figure 1As shown, this embodiment provides a heat pump system for recovering waste heat from pulp black liquor. This system can be used to execute the pulp heating method described in Embodiment 1. The heat pump system includes a waste heat side heat exchange branch 1, a waste heat side bypass branch 2, a heat pump circulation heat exchange branch 3, a heat use side heat exchange branch 4, a compensation branch 5, a detection unit, and a controller 6.

[0084] The waste heat side heat exchange branch 1 is used to receive the first process medium. The first process medium is black liquor from the pulp production process. A waste heat side heat exchanger 11 is provided on the waste heat side heat exchange branch 1. The first process medium flows through one side of the waste heat side heat exchanger 11, and the other side exchanges heat with the refrigerant or intermediate heat exchange medium in the heat pump circulation heat exchange branch 3.

[0085] Waste heat side bypass branch 2 is connected in parallel with waste heat side heat exchange branch 1. Waste heat side bypass branch 2 is used to allow at least a portion of the first process medium to bypass the waste heat side heat exchanger 11. Waste heat side bypass branch 2 is equipped with waste heat side bypass valve 21. By adjusting the opening of waste heat side bypass valve 21, the proportion of the first process medium entering waste heat side heat exchange branch 1 can be changed. When the black liquor outlet temperature is lower than the allowable outlet temperature or the differential pressure change rate of waste heat side heat exchanger 11 exceeds the threshold, the controller 6 increases the opening of waste heat side bypass valve 21, allowing more black liquor to bypass waste heat side heat exchanger 11, thereby reducing heat extraction and lowering the risk of blockage or scaling.

[0086] The heat pump cycle heat exchange branch 3 is used for heat exchange with the waste heat side heat exchange branch 1 and the heat use side heat exchange branch 4. The heat pump cycle heat exchange branch 3 includes a variable frequency compressor 31. The refrigerant in the waste heat side heat exchanger 11 is compressed by the variable frequency compressor 31 to form a high-temperature, high-pressure state, and releases heat to the second process medium in the heat use side heat exchanger 41. The refrigerant after releasing heat enters the waste heat side heat exchanger 11, absorbs waste heat from the first process medium, and then returns to the variable frequency compressor 31. The refrigerant can be CO2, ammonia, Freon, or other working fluids suitable for industrial heat pumps. Preferably, when a higher outlet water temperature or a higher pulp heating temperature is required, a CO2 transcritical heat pump cycle or other high-temperature heat pump cycles can be used.

[0087] The heat exchange branch 4 on the heat-using side is used to receive the second process medium. The second process medium can be pulp to be heated, process water, or other media that require temperature increase. The heat exchange branch 4 on the heat-using side is equipped with a heat exchanger 41 on the heat-using side, a circulating pump 42, an inlet temperature detection point, and an outlet temperature detection point. After passing through the heat exchanger 41 on the heat-using side, the second process medium is heated to the target temperature or close to the target temperature by the heat pump circulating heat exchange branch 3. The heat exchange branch 4 on the heat-using side is set with a minimum circulation flow limit to ensure stable flow within the heat exchanger and avoid local overheating or uneven heat exchange.

[0088] The compensation branch 5 is used to compensate for the heating of the heat exchange branch 4 on the heat-consuming side. The compensation branch 5 can be a steam compensation branch, a hot water compensation branch, or an electric heating branch. In one embodiment, the compensation branch 5 is a steam compensation branch, which includes a steam inlet pipe, a steam regulating valve, and a steam heat exchanger. The steam heat exchanger is installed on or in parallel with the heat exchange branch 4 on the heat-consuming side. When the heating load borne by the heat pump circulation heat exchange branch 3 is insufficient to bring the second process medium to the target temperature, the controller 6 increases the opening of the compensation branch regulating valve 51 to increase the compensation heating amount. In another embodiment, the compensation branch 5 is an electric heating branch, and the controller 6 adjusts the electric heating power according to the required compensation heat.

[0089] The detection unit includes a first process medium inlet temperature sensor 12, a first process medium outlet temperature sensor 13, a first process medium flow meter 14, a first process medium concentration sensor 15, a waste heat side heat exchanger differential pressure sensor 16, a second process medium inlet temperature sensor 43, a second process medium outlet temperature sensor 44, a variable frequency compressor frequency feedback module 32, and a pump power acquisition module 45.

[0090] The first process medium inlet temperature sensor 12 is installed at the inlet of the waste heat side heat exchange branch 1 or on the main pipeline before the first process medium is diverted, and is used to detect the inlet temperature of the first process medium. The first process medium outlet temperature sensor 13 is installed at the outlet of the waste heat side heat exchange branch 1 or on the main pipeline after the bypass branch and the heat exchange branch merge, and is used to detect the outlet temperature of the first process medium. The first process medium flow meter 14 is used to detect the flow rate entering the waste heat side heat exchange branch 1 or the first process medium main pipeline. The first process medium concentration sensor 15 is used to detect the black liquor concentration or to receive the concentration signal from the upstream concentration detection device. The waste heat side heat exchanger differential pressure sensor 16 collects the inlet pressure and outlet pressure of the waste heat side heat exchanger 11 to obtain the heat exchanger differential pressure and differential pressure change rate.

[0091] The second process medium inlet temperature sensor 43 is installed at the inlet of the heat exchange branch 4 on the heat-using side to detect the inlet temperature of the second process medium. The second process medium outlet temperature sensor 44 is installed at the outlet of the heat exchange branch 4 on the heat-using side to detect the actual outlet temperature of the second process medium. This temperature can be compared with the target temperature to perform closed-loop correction of the heat pump cycle heat exchange branch 3 and the compensation branch 5. The variable frequency compressor frequency feedback module 32 is used to provide feedback on the current operating frequency of the variable frequency compressor 31 to determine whether the compressor is between the minimum stable operating frequency and the maximum allowable operating frequency. The pump power acquisition module 45 is used to acquire the real-time power of the circulating pump 42 to provide data for the calculation of delivery energy consumption in the comprehensive objective function.

[0092] The controller 6 can be a PLC, industrial computer, embedded controller, or control unit with data acquisition and processing capabilities. The controller 6 is electrically connected to the variable frequency compressor 31, the waste heat side bypass valve 21, and the compensation branch regulating valve 51. The controller 6 is also communicatively connected to the circulating pump 42, the steam regulating valve, the electric heating power regulating module, the alarm module 61, and the host computer 62.

[0093] The controller 6 stores a program for executing the method described in Embodiment 1. Specifically, the controller is configured to: receive first process medium parameters and second process medium parameters collected by the detection unit; generate a candidate heat extraction ratio set based on the allowable outlet temperature of the first process medium, the safe transferable heat, and the required heat; calculate the comprehensive objective function value for each candidate value in the candidate heat extraction ratio set; select the candidate heat extraction ratio that satisfies the safety constraints and has the lowest comprehensive objective function value; and output control commands based on the candidate heat extraction ratio to adjust the frequency of the variable frequency compressor 31, the opening degree of the waste heat side bypass valve 21, and the opening degree of the compensation branch regulating valve 51.

[0094] In a specific control process, controller 6 uses a 5-second sensor sampling period and a 30-second optimization update period. Within each sampling period, the controller updates temperature, flow rate, concentration, and differential pressure data; within each optimization update period, the controller performs a candidate heat extraction ratio enumeration and a comprehensive objective function calculation. When the outlet temperature of the first process medium is detected to be lower than the allowable outlet temperature, or the differential pressure change rate exceeds a first threshold, controller 6 does not wait for the next optimization update period but immediately enters the protection regulation logic, rapidly increasing the opening of the waste heat side bypass valve 21 and reducing the frequency of the variable frequency compressor 31, while simultaneously increasing the opening of the compensation branch regulating valve 51. Therefore, even if the black liquor state changes rapidly, the system can promptly protect the waste heat side heat exchanger and subsequent processes.

[0095] In one optional implementation, the controller 6 can also be configured with operating mode switching logic. The operating modes include normal optimization mode, bypass protection mode, compensation priority mode, and shutdown protection mode. In normal optimization mode, the controller 6 selects candidate heat extraction ratios according to a comprehensive objective function; in bypass protection mode, the controller 6 prioritizes ensuring that the outlet temperature of the first process medium and the rate of change of the heat exchanger differential pressure meet safety requirements; in compensation priority mode, the controller 6 assigns a larger proportion of load to the compensation branch to ensure the stability of the outlet temperature of the second process medium; in shutdown protection mode, the controller 6 shuts down or reduces the load on the heat pump circulation heat exchange branch 3, allowing the first process medium to pass through the waste heat side bypass branch 2, while the compensation branch 5 maintains the temperature of the second process medium.

[0096] In another optional embodiment, the heat pump system further includes a data storage module 63. The data storage module 63 records the inlet temperature, outlet temperature, concentration, differential pressure change rate, candidate heat extraction ratio, final selected heat extraction ratio, frequency of the variable frequency compressor 31, opening degree of the waste heat side bypass valve 21, opening degree of the compensation branch regulating valve 51, and outlet temperature of the second process medium. Using this data, statistical analysis can be performed on the black liquor scaling trend, heat pump operating efficiency, and compensation energy consumption, thereby enabling subsequent adjustments to the concentration correction coefficient, scaling risk correction coefficient, comprehensive objective function weight, and threshold.

[0097] The heat pump system described in this embodiment, by setting up a waste heat side bypass branch 2, a compensation branch 5, and a controller 6 based on a comprehensive objective function, enables the system not only to recover waste heat from pulp black liquor but also to dynamically adjust the heat extraction capacity according to the black liquor temperature, concentration, and heat exchanger differential pressure. Compared with simple fixed-flow heat extraction or simply adjusting the compressor frequency based on the outlet temperature of the second process medium, this embodiment reduces the risk of excessive heat extraction from the black liquor side, improves the operational stability of the heat exchanger, and ensures the pulp heating temperature through the compensation branch when the waste heat from the heat pump is insufficient.

[0098] It should be noted that the placement of the sensors, valves, heat exchangers, and controller 6 in the above embodiments is merely an example. In actual engineering, equivalent adjustments can be made based on the layout of the pulp production line, the form of the black liquor pipeline, the type of heat exchanger, and the form of the compensation heat source. For example, the waste heat side heat exchanger 11 can be a plate heat exchanger, a shell-and-tube heat exchanger, a spiral plate heat exchanger, or a scale-resistant heat exchanger; the waste heat side bypass valve 21 can be located on the inlet side, outlet side, or in the bypass pipeline of the heat exchanger; the compensation branch 5 can be located at the front end, rear end, or in a branch connected in parallel with the heat-using side heat exchanger 41. The above equivalent changes do not affect the core control concept of this invention, which involves load allocation through safe transferable heat, a set of candidate heat extraction ratios, and a comprehensive objective function.

[0099] Finally, it should be noted that although the above embodiments have been described in the text and drawings of this application, this should not limit the scope of patent protection of this application. Any technical solutions that are based on the essential concept of this application and utilize the content described in the text and drawings of this application, resulting in equivalent structural or procedural substitutions or modifications, as well as the direct or indirect application of the technical solutions of the above embodiments to other related technical fields, are all included within the scope of patent protection of this application.

Claims

1. A pulp heating method employing a heat pump system for recovering waste heat from pulp black liquor, the heat pump system comprising a waste heat-side heat exchange branch for receiving a first process medium, a waste heat-side bypass branch for allowing at least a portion of the first process medium to bypass the waste heat-side heat exchange branch, a heat pump circulation heat exchange branch for exchanging heat with the waste heat-side heat exchange branch, a heat-using-side heat exchange branch for receiving a second process medium and exchanging heat with the heat pump circulation heat exchange branch, a compensation branch for compensating for heating the heat-using-side heat exchange branch, and a controller, wherein the heat pump circulation heat exchange branch includes a variable frequency compressor; characterized in that... The pulp heating method includes periodically performing the following steps: S1. Obtain the inlet temperature, outlet temperature, flow rate, concentration, specific heat capacity, allowable outlet temperature, and differential pressure change rate of the heat exchanger in the waste heat side heat exchange branch of the first process medium, as well as the inlet temperature, target temperature, flow rate, and specific heat capacity of the second process medium. S2. Calculate the safe transferable heat based on the inlet temperature, flow rate, specific heat capacity and allowable outlet temperature of the first process medium, and calculate the required heat based on the inlet temperature, target temperature, flow rate and specific heat capacity of the second process medium. S3. Based on the safe transferable heat, the required heat, the minimum stable operating frequency of the variable frequency compressor, the minimum circulation flow of the heat exchange branch on the heat use side, and the preset heat extraction ratio step size, determine the candidate heat extraction ratio set. The candidate heat extraction ratio in the candidate heat extraction ratio set is used to represent the proportion of the heating load undertaken by the heat pump circulation heat exchange branch to the required heat. S4. Calculate the comprehensive objective function value for each candidate heat extraction ratio in the candidate heat extraction ratio set. The comprehensive objective function includes at least the compressor power of the heat pump cycle heat exchange branch, the equivalent energy consumption of the compensation branch, the energy consumption of the waste heat side heat exchange branch, and the energy consumption of the heat use side heat exchange branch. S5. Select the candidate heat extraction ratio that meets the safety constraints and has the lowest comprehensive objective function value, determine the load distribution between the heat pump circulation heat exchange branch, the waste heat side bypass branch and the compensation branch, and adjust the frequency of the variable frequency compressor and the opening degree of at least one valve accordingly. S6. When the outlet temperature of the first process medium is lower than the allowable outlet temperature, or the differential pressure change rate exceeds the first threshold, the waste heat side bypass ratio is increased at the first adjustment rate, the heat pump circulation heat exchange branch load is reduced at the second adjustment rate, and the compensation branch load is increased; when the outlet temperature of the first process medium is higher than the allowable outlet temperature and the differential pressure change rate is lower than the second threshold for several consecutive control cycles, the waste heat side bypass ratio is reduced at a recovery rate lower than the first adjustment rate, and the heat pump circulation heat exchange branch load is restored.

2. The heating method according to claim 1, characterized in that, The allowable outlet temperature of the first process medium is determined as follows: Allowable outlet temperature of the first process medium = the maximum value among the minimum temperature of the subsequent process, the concentration correction temperature, and the differential pressure correction temperature + safety temperature margin.

3. The heating method according to claim 1, characterized in that, The safe-to-transfer heat is calculated as follows: Safe-to-transfer heat = First medium mass flow rate × First medium specific heat capacity × (First medium inlet temperature - First process medium allowable outlet temperature) × Concentration correction factor × Scaling risk correction factor.

4. The heating method according to claim 1, characterized in that, The required heat is calculated as follows: Required heat = Second medium mass flow rate × Second medium specific heat capacity × (Second medium target temperature - Second medium inlet temperature).

5. The heating method according to claim 1, characterized in that, The candidate heat extraction ratio set is determined in the following manner: The maximum heat extraction ratio is determined based on the ratio of the safe, transferable heat to the required heat. The first minimum heat extraction ratio is determined based on the ratio of the minimum heat pump heating capacity corresponding to the lowest stable operating frequency of the variable frequency compressor to the required heat; the second minimum heat extraction ratio is determined based on the ratio of the minimum exchangeable heat capacity corresponding to the minimum circulation flow rate of the heat exchange branch on the heat use side to the required heat; the larger value between the first minimum heat extraction ratio and the second minimum heat extraction ratio is determined as the minimum heat extraction ratio. Between the minimum heat extraction ratio and the maximum heat extraction ratio, multiple candidate heat extraction ratios are generated according to a preset heat extraction ratio step size to form the candidate heat extraction ratio set. When the minimum heat extraction ratio is greater than the maximum heat extraction ratio, the candidate heat extraction ratio set is determined to be an empty set or the heat pump circulation heat exchange branch is switched to shutdown or minimum safe operation state, and the compensation branch undertakes at least part of the required heat.

6. The heating method according to claim 1, characterized in that: The set of candidate heat extraction ratios is generated using a finite candidate enumeration method, with the number of candidate values ​​ranging from 5 to 25. When a candidate heat extraction ratio no longer meets the safety constraints, the calculation of a higher candidate value than that candidate heat extraction ratio is stopped.

7. The heating method according to claim 1, characterized in that: The sensor sampling period is 1 to 10 seconds, the optimization update period of the candidate heat extraction ratio set is 10 to 60 seconds, and the differential pressure change rate is smoothed by the moving average or running average of the most recent 20 to 100 sampled values.

8. The heating method according to claim 1, characterized in that: The first threshold is greater than the second threshold. During the recovery phase, the outlet temperature of the first process medium is required to be higher than the allowable outlet temperature and the differential pressure change rate is lower than the second threshold for 3 to 20 consecutive control cycles.

9. The heating method according to claim 1, characterized in that: The compensation branch is one of a steam heating branch, a hot water heating branch, or an electric heating branch.

10. A heat pump system for waste heat recovery from pulp black liquor, comprising: A waste heat-side heat exchange branch for receiving a first process medium, a waste heat-side bypass branch for allowing at least a portion of the first process medium to bypass the waste heat-side heat exchange branch, a heat pump circulation heat exchange branch for exchanging heat with the waste heat-side heat exchange branch, a heat-using side heat exchange branch for receiving a second process medium and exchanging heat with the heat pump circulation heat exchange branch, a compensation branch for compensating the heat-using side heat exchange branch, and a controller, characterized in that it further includes: The system comprises a first process medium inlet temperature sensor, a first process medium outlet temperature sensor, a first process medium flow meter, a first process medium concentration sensor, a waste heat side heat exchanger differential pressure sensor, a second process medium inlet temperature sensor, a second process medium outlet temperature sensor, a variable frequency compressor frequency feedback module, and a pump power acquisition module; the controller is electrically connected to the variable frequency compressor, the waste heat side bypass valve, and the compensation branch regulating valve, and is configured to perform the method described in any one of claims 1 to 8.

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