A pre-heat device and method for cooking in pulp production

Abstract

This invention discloses a preheating device and method for pulp production, applicable to long-distance pulp production lines. The device includes a distributed hybrid heat pump unit with dual compressors and three condensers, a waste heat collection loop, a refrigerant distribution and control component, an energy storage bidirectional scheduling component, and a return flow distribution component. The first and second heat pump units correspond to the front and rear sections of the pipeline, respectively, with their condensers serving as front and rear section heat-traced condensers, forming a three-stage heat exchange system in conjunction with an independent intermediate section condenser. The compressor operation is controlled by both the intermediate medium and pulp temperature. Excess refrigerant is diverted to the intermediate section for supplemental heating, and the intermediate section refrigerant is returned to the two units via a pressure adaptive valve group. This device replaces electric heat tracing, solving problems such as large temperature drops in long pipelines, insufficient heat exchange at high flow rates, and low waste heat utilization, achieving full waste heat recovery and reducing energy consumption in the production line.

Description

Technical Field

[0001] This invention belongs to the field of waste heat recovery and energy-saving control technology in pulp production, specifically relating to a preheating device and method for cooking in pulp production. Background Technology

[0002] In modern pulp and paper production processes, wood chips are softened and mechanically ground in a wood chip cooking unit to form semi-finished pulp. This semi-finished pulp needs to be transported through long-distance pipelines to the downstream pulping unit for deep cooking reaction, completing the pulp refining process. Currently, the industry's insulation and heat tracing methods for long-distance pulp conveying pipelines are extremely limited, generally relying solely on passive heat insulation achieved by installing an insulation layer on the outer wall of the pipeline. Some production lines are equipped with electric heating wire structures to compensate for temperature drop.

[0003] This traditional process has significant technical defects: First, the pulp conveying pipelines are generally 200m to 500m long, with long conveying distances and large heat dissipation areas. Passive insulation cannot effectively suppress temperature drops over long distances, resulting in large temperature fluctuations throughout the pulp conveying process. Second, the flow rate fluctuates frequently during pulp production. Under high flow rate conditions, the pulp stays in the heating zone at the rear of the pipeline for heat exchange for a very short time. The traditional single-point heating method at the rear end has insufficient heat exchange time. In addition, the factory recovers mostly low-grade waste heat with limited temperature rise capacity, which easily leads to insufficient preheating of the pulp and low inlet temperature. This directly causes an imbalance in the cooking conditions inside the pulping unit, affecting the stability of pulp quality. Third, electric heating consumes extremely high energy, resulting in high production and maintenance costs for the factory.

[0004] Meanwhile, paper mills generate a large amount of recoverable waste heat during production. Combustion exhaust gas and flash steam from wood chip cooking units, as well as process exhaust steam and secondary steam from pulping units, all contain significant amounts of low-grade heat energy. Currently, most of this waste heat is directly discharged or simply used to heat hot water in the production workshop. Due to limitations in hot water usage, a large amount of surplus hot water and waste heat is ultimately lost and wasted, resulting in extremely low energy utilization and a serious energy loss problem.

[0005] Existing waste heat recovery heat pumps are mostly centralized single-unit heating or single-source single-point independent heating structures, which cannot adapt to the special working conditions of paper mills with dual waste heat sources distributed layout, ultra-long pipeline segmented temperature drop, large fluctuations in pulp flow rate, and priority pressure stabilization in downstream processes. They cannot achieve multi-source waste heat coordinated scheduling, segmented precise heat tracing, and peak and valley filling of waste heat fluctuations. There is no dedicated preheating system or production method to address the above technical pain points. Summary of the Invention

[0006] In view of the above problems, the present invention aims to solve the technical problems of large temperature drop during long-distance pulp transportation, insufficient preheating of high-flow-rate pulp, low utilization rate of low-grade waste heat, and waste of waste heat resources in the prior art. The preheating device and method for pulp production in this application realizes distributed recovery of multi-source waste heat, segmented precise heating, intelligent scheduling of surplus heat, and peak shaving and valley filling of waste heat fluctuations. It prioritizes ensuring the stability of the downstream pulping process temperature, significantly reduces production energy consumption, and improves pulp production quality and energy utilization.

[0007] To achieve the above objectives, the inventors have provided a preheating device for pulp production, including a pulp conveying pipeline, a distributed hybrid heat pump unit, a mid-section refrigerant heating component, a waste heat collection circuit, and a refrigerant distribution and control component.

[0008] The pulp conveying pipeline connects the wood chip cooking and grinding equipment with the pulping device. The distributed hybrid heat pump unit adopts a dual-compressor + three-condenser hybrid coupling architecture, including a first heat pump unit and a second heat pump unit. Both units are equipped with independent waste heat evaporators, variable frequency compressors, and closed refrigerant circulation loops. The waste heat collection loop is equipped with an intermediate heat exchange medium to collect low-grade waste heat from the wood chip cooking device and the pulping device, and heats the low-temperature refrigerant in the waste heat evaporator, causing the refrigerant to absorb heat and vaporize. The variable frequency compressor is used to compress the vaporized refrigerant to generate high-temperature, high-pressure gaseous refrigerant. The first heat pump unit and the second heat pump unit are equipped with front-end and rear-end heat tracing condensers, respectively, corresponding to the front and rear sections of the pipeline. The intermediate refrigerant heating components constitute the intermediate-end heat tracing condenser, and the three form the front and middle sections. The system features a three-section zoned heat exchange structure in the rear section. Both units have refrigerant proportional distribution valves installed at the high-pressure output ends of their compressors. These valves have two independent output circuits: one connects to the corresponding front or rear section heat-tracing condenser for local independent heat tracing, and the other connects to the middle section heat-tracing condenser for centralized supplementary heating of surplus refrigerant. The refrigerant distribution and control component collects pulp temperature and flow rate signals within the pipeline, following a local load priority control logic. It prioritizes meeting the local heat tracing needs of the front and rear sections of the pipeline, then diverts surplus high-pressure refrigerant to the middle section heat-tracing condenser for supplementary heating. This constitutes a hybrid structure with independent operation of both units, segmented and zoned heat exchange, and interconnected scheduling of surplus refrigerant for waste heat preheating.

[0009] Furthermore, it also includes a high-pressure refrigerant storage tank and a two-way refrigerant switching valve assembly; the high-pressure refrigerant storage tank is connected to the first heat pump unit, the second heat pump unit, and the intermediate heat-tracing condenser through high-pressure refrigerant pipelines; the two-way refrigerant switching valve assembly is installed between the high-pressure refrigerant storage tank and the second heat pump unit; when the first heat pump unit and / or the second heat pump unit produces surplus high-pressure high-temperature refrigerant, it can be channeled into the high-pressure refrigerant storage tank to achieve temporary storage of high-pressure gaseous refrigerant heat; the high-pressure refrigerant storage tank can release energy and replenish heat to the intermediate heat-tracing condenser in one direction, and can also replenish high-pressure refrigerant to the second heat pump unit in the opposite direction through the two-way refrigerant switching valve assembly, realizing peak shaving and valley filling of system waste heat and two-way heat scheduling when the downstream heat source is insufficient.

[0010] Furthermore, the refrigerant distribution and control component has a built-in back-end process priority control logic; it collects the inlet pulp temperature of the pulping unit and the waste heat supply load of the second heat pump unit in real time; when the inlet pulp temperature of the pulping unit is lower than the set threshold and the second heat pump unit is affected by waste heat fluctuations, resulting in insufficient heat supply, it prioritizes triggering the bidirectional refrigerant switching valve group to conduct, and the high-pressure refrigerant energy storage tank supplements heat to the second heat pump unit and the back-end heat tracing circuit, so as to prioritize ensuring the stability of the inlet pulp temperature of the pulping unit.

[0011] Furthermore, the variable frequency compressor adopts a dual-factor linkage control mechanism of intermediate medium temperature and pulp temperature; the refrigerant distribution and control component includes a multi-point temperature detection unit, a PLC intelligent control unit, and a variable frequency drive unit. The multi-point temperature detection unit is respectively arranged in the intermediate medium pipeline of the waste heat collection loop and the front and rear sections of the pulp conveying pipeline; when the intermediate medium temperature reaches or exceeds the preset medium threshold, the variable frequency compressor is forcibly started, and the compressor working power is positively correlated with the intermediate medium temperature. The higher the intermediate medium temperature, the greater the compressor power, ensuring full absorption of low-grade waste heat in the intermediate medium; when the pulp temperature in the corresponding area reaches or falls below the preset pulp threshold, the variable frequency compressor is forcibly started, and the compressor working power is inversely correlated with the pulp temperature. The lower the pulp temperature, the greater the compressor power, prioritizing the basic preheating needs of the pulp; when the compressor operates according to the above dual-factor control mechanism, and the pulp temperature in the corresponding area reaches the process target value and the local heat tracing load is saturated, it is determined that the high-pressure refrigerant is surplus, and the refrigerant proportional distribution valve automatically opens the diversion loop to transport the surplus high-pressure refrigerant to the intermediate heat tracing condenser.

[0012] Furthermore, both the first and second heat pump units are compression-type high-temperature heat pump structures, and both are equipped with intermediate heat exchange loops and throttling valves. The waste heat evaporator of the first heat pump unit is connected to the exhaust gas and flash steam of the wood chip cooking device through the intermediate heat exchange loop, and the waste heat evaporator of the second heat pump unit is connected to the process exhaust steam and secondary steam of the pulp cooking device through the intermediate heat exchange loop. The intermediate heat exchange loop is filled with anti-corrosion heat exchange medium. The high-temperature and high-pressure gaseous refrigerant is diverted by the refrigerant proportional distribution valve and enters the local heat tracing condenser and the intermediate heat tracing condenser respectively. After heat exchange, the liquid refrigerant is cooled and depressurized by the throttling valve and then flows back to the waste heat evaporator to complete the closed-loop cycle.

[0013] Furthermore, both the front-end and rear-end heat tracing condensers adopt a shell-and-tube heat exchange structure. The gap between the shells forms a high-pressure refrigerant flow cavity, which is tightly wrapped around the outer wall of the pulp conveying pipeline. The outer side is covered with a heat insulation layer to achieve closed-loop directional heat tracing of the front and rear sections of the pipeline.

[0014] Furthermore, the mid-section refrigerant heating assembly includes a refrigerant manifold, multiple sets of distributed spiral heat exchange branch pipes, a flow control valve, and a thermal insulation layer. The refrigerant manifold is connected to the refrigerant proportional distribution valve outlet of the first heat pump unit and the outlet of the high-pressure refrigerant storage tank of the second heat pump unit. Multiple sets of distributed spiral heat exchange branch pipes are evenly spirally wound on the outer wall of the mid-section of the pulp conveying pipeline, and both ends of each branch pipe are connected in parallel to the refrigerant manifold. The flow control valve is independently set for each set of branch pipes and is used to adjust the refrigerant flow in each area of ​​the mid-section to form a continuous and uniform progressive heat replenishment area.

[0015] Furthermore, the refrigerant distribution and control component also includes a pipeline flow detection unit, which is installed in the main section of the pulp conveying pipeline to collect pulp flow rate and flow fluctuation parameters; the PLC intelligent control unit links various signals such as temperature, flow rate, and pressure to dynamically adjust the opening of the refrigerant proportional distribution valve, the flow rate of the refrigerant return distribution valve group, and the operating frequency of the heat pump compressor to achieve adaptive control under all operating conditions.

[0016] Furthermore, the refrigerant output end of the intermediate-section heat-tracing condenser is equipped with a refrigerant return distribution valve group, which is a pressure-adaptive distribution structure. Both the closed-loop refrigerant circulation loops of the first and second heat pump units are equipped with refrigerant pressure sensors. The PLC intelligent control unit collects the refrigerant pressure values ​​of the circulation loops of the two heat pump units in real time, and dynamically adjusts the return refrigerant flow rate through the refrigerant return distribution valve group. The low-pressure liquid refrigerant after heat exchange in the intermediate-section heat-tracing condenser is distributed to the throttling valve inlets of the first and second heat pump units according to the pressure difference ratio. When the refrigerant pressure in the circulation loop of a certain heat pump unit is lower than the preset pressure threshold, the return refrigerant is preferentially distributed to it to ensure the balance of refrigerant circulation between the two units and avoid the situation of excessive or insufficient refrigerant.

[0017] To solve the above-mentioned technical problems, this application also provides another technical solution: a preheating method for pulp production before cooking, which uses the preheating device for pulp production described in the above-mentioned technical solution, and includes the following steps:

[0018] S1. Wood chips are softened and ground by the wood chip cooking device to form pulp. The pulp is transported to the pulping device through the pulp conveying pipeline. The first heat pump unit and the second heat pump unit recover the exhaust gas and flash steam of the wood chip cooking device, the process exhaust steam and secondary steam waste heat of the pulping device through the intermediate heat exchange loop, respectively, to complete the collection and replacement of low-grade waste heat.

[0019] S2. Based on the dual-factor control mechanism of intermediate medium temperature and pulp temperature, the variable frequency compressor is started to compress the refrigerant vaporized in the evaporator into high-temperature and high-pressure gaseous refrigerant; the control system prioritizes the high-temperature and high-pressure refrigerant to enter their respective front-end and rear-end heat tracing condensers to complete the on-site heat tracing preheating of the front and rear sections of the pipeline.

[0020] The S3 and PLC intelligent control unit collects the pulp temperature and flow data in the pipeline in real time. After the pulp temperature in the front and rear sections reaches the process set value and the heat tracing load is saturated, it determines that there is excess high-pressure refrigerant and automatically adjusts the opening of the refrigerant proportional distribution valve to divert the excess high-pressure refrigerant to the middle section heat tracing condenser.

[0021] S4. Excess refrigerant is distributed to each group of spiral heat exchange branch pipes through the manifold of the middle section heat tracing condenser to uniformly supplement the heat in the middle section of the pipeline, forming a progressive preheating mode of front section, middle section and rear section.

[0022] Unlike existing technologies, the preheating device for pulp production described above includes a pulp conveying pipeline, a distributed hybrid heat pump unit, a mid-section refrigerant heating component, a waste heat collection loop, and a refrigerant distribution and control component. The pulp conveying pipeline connects the wood chip cooking and grinding equipment with the pulping unit. The distributed hybrid heat pump unit adopts a dual-compressor + three-condenser hybrid coupling architecture, comprising a first heat pump unit and a second heat pump unit. Both units are equipped with independent waste heat evaporators, variable frequency compressors, and closed-loop refrigerant circulation loops. The condenser of the first heat pump unit is a front-end heat-traced condenser, and the condenser of the second heat pump unit is a rear-end heat-traced condenser. The mid-section refrigerant heating component constitutes the mid-section heat-traced condenser, forming a three-section zoned heat exchange structure. A refrigerant proportional distribution valve is installed at the high-pressure output end of both compressor units. This valve has two output loops corresponding to the front and rear heat-traced condensers and the mid-section heat-traced condenser of the unit, respectively. The waste heat collection loop is equipped with an intermediate heat exchange medium to collect the low-grade waste heat of the paper mill and heat the refrigerant. The refrigerant distribution and control component collects the pulp temperature and flow rate signals, and follows the local load priority logic to prioritize the local heat tracing needs of the upstream and downstream sections. The surplus high-pressure refrigerant is then diverted to the intermediate heat tracing condenser for heat supplementation, forming a hybrid structure with two machines working independently, segmented and zoned heat exchange, and surplus refrigerant interconnected and scheduled. Compared with existing technologies, this solution abandons the traditional single heat pump centralized heating, electric heat tracing, or single-end single-point heating modes. It solves the shortcomings of existing technologies, such as the inability to adapt to distributed dual waste heat sources before and after paper mills, uneven temperature drop in long-distance pipelines, insufficient heat exchange in high-flow-rate pulp, and low utilization rate of low-grade waste heat. Through dual-unit heat extraction from nearby locations, three-stage zoned heat exchange, and precise diversion design of surplus refrigerant, it significantly reduces waste heat transmission losses, realizes on-demand distribution of refrigerant heat, effectively compensates for the problems of large temperature drop in the middle section of long pipelines and insufficient heat exchange time under high-flow-rate conditions, and significantly improves the recovery and utilization rate of low-grade waste heat, ensuring stable temperature throughout the pulp transportation process. It replaces high-energy-consuming electric heat tracing and reduces the overall energy consumption of the production line.

[0023] 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

[0024] 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.

[0025] In the accompanying drawings of the instruction manual:

[0026] Figure 1This is a schematic diagram of the overall structure of the preheating device before cooking in pulp production as described in the specific implementation method.

[0027] Figure 2 This is a schematic diagram of the dual-compressor, three-condenser hybrid coupling architecture described in a specific implementation method;

[0028] Figure 3 This is a schematic diagram of the structure of the mid-section refrigerant heating assembly described in a specific embodiment;

[0029] Figure 4 A flowchart of the preheating method for pulp production described in the specific implementation embodiment;

[0030] The reference numerals used in the above figures are explained as follows:

[0031] 1. Preheating device before cooking; 2. Pulp conveying pipeline; 3. Wood chip cooking device; 4. Grinding device; 5. Pulping device;

[0032] 10. First heat pump unit; 11. Second heat pump unit;

[0033] 101. Front-end heat tracing condenser; 102. Rear-end heat tracing condenser; 103. Middle-end heat tracing condenser;

[0034] 104. High-pressure refrigerant storage tank; 105. Two-way refrigerant switching valve assembly; 106. Refrigerant return distribution valve assembly;

[0035] 111. Variable frequency compressor; 112. Expansion valve; 114. Evaporator; 115. Evaporator;

[0036] 1031. Refrigerant manifold; 1032. Flow control valve; 1033. Heat exchange branch pipe; Detailed Implementation

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Without further limitations, the use of terms such as “comprising,” “including,” “having,” or other similar open-ended 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.

[0043] 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.

[0044] 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.

[0045] 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 arrangement; it can be a direct connection or an indirect connection through an intermediate medium; it can be a relationship of two components combined together, an interaction relationship between two components, or a connection within two structures. Those skilled in the art to which this application pertains can understand the specific meaning of the above terms in the embodiments of this application according to the specific circumstances.

[0046] Please see Figure 1 The preheating device 1 for pulp production in this embodiment is suitable for long-distance pulp production lines in paper mills. The total length of the pulp conveying pipeline 2 in this production line is 200m to 500m. The pulp conveying pipeline 2 connects the grinding device 4 at the front end and the pulp cooking device 5 at the rear end. The wood chip cooking device 3 is installed at the front end of the grinding device 4. The original production line relied solely on passive insulation of the pipeline insulation layer and active heating with electric heating wires, which had core problems such as large temperature drop over long distances, short heat exchange time for high-flow-rate pulp, ineffective utilization of low-grade waste heat, high power consumption, and large fluctuations in pulp temperature at the pulp cooking inlet. The preheating device for pulp production in this embodiment completely replaces the original electric heating system by building a distributed hybrid heat pump waste heat preheating system. It is precisely adapted to the special production scenarios of paper mills with distributed dual waste heat sources (wood chip cooking exhaust gas / flash steam, pulp cooking process exhaust steam / secondary steam), ultra-long pipelines, frequent fluctuations in operating conditions, and high priority of downstream processes.

[0047] See also Figure 1 and Figure 2 In this embodiment, the overall structure of the preheating device 1 for pulp production includes a distributed hybrid heat pump unit with dual compressors and three condensers, and a waste heat collection intermediate medium heat exchange circuit (i.e., Figure 2The components include: a heat exchange loop, a refrigerant proportional distribution and control component, a high-pressure refrigerant energy storage bidirectional scheduling component, a three-section zoned heat exchange structure, a two-factor compressor control component, and a mid-section refrigerant pressure adaptive reflux component. The specific layout and connection relationships of each of these components are as follows: Figure 2 As shown, the dual-compressor + three-condenser distributed hybrid heat pump unit includes a first heat pump unit 10 and a second heat pump unit 11. The first heat pump unit 10 is located at the front of the pulp conveying pipeline 2 and close to the wood chip cooking device 3. The second heat pump unit 11 is located at the rear of the pulp conveying pipeline and close to the pulp cooking device 5. The first heat pump unit 10 and the second heat pump unit 11 are independent of each other, and their refrigerant circuits are interconnected and coupled. Both the first heat pump unit 10 and the second heat pump unit 11 adopt the mainstream industrial compression-type high-temperature heat pump structure. Both are equipped with four core components: a complete evaporator 114, a variable frequency compressor 111, and condensers (i.e., a front-end heat-traced condenser 101 and a rear-end heat-traced condenser 102) and a throttling valve 112. Furthermore, the condenser of the first heat pump unit is the front-end heat-traced condenser 101, and the condenser of the second heat pump unit is the rear-end heat-traced condenser 102. The first heat pump unit 10 and the second heat pump unit 11 each form an independent closed-loop refrigerant circulation circuit through the aforementioned components. The three-section zoned heat exchange structure is composed of the front-end heat-traced condenser 101, the rear-end heat-traced condenser 102, and the middle-section refrigerant heating component (i.e., the middle-section heat-traced condenser 103). The front-end heat-traced condenser and the rear-end heat-traced condenser serve as the condensers of the first heat pump unit and the second heat pump unit, respectively. The middle-section refrigerant heating component is an independently installed middle-section heat-traced condenser 103. The three components are respectively arranged in the front, rear, and middle sections of the pulp conveying pipeline, achieving full-area heat tracing without dead zones in the pulp conveying pipeline.

[0048] like Figure 1 As shown, the waste heat collection intermediate medium heat exchange circuit includes two independent corrosion-resistant intermediate heat exchange circuits. One corrosion-resistant intermediate heat exchange circuit connects the first heat pump unit 10 and the wood chip cooking device 3, and is used to transfer the waste heat from the combustion exhaust gas and flash steam generated by the wood chip cooking device to the first heat pump unit. The other corrosion-resistant intermediate heat exchange circuit connects the second heat pump unit 11 and the pulp cooking device 5, and is used to transfer the waste heat from the process exhaust steam and secondary steam generated by the pulp cooking device to the second heat pump unit. Both corrosion-resistant intermediate heat exchange circuits are filled with a corrosion-resistant heat exchange medium (such as propylene glycol aqueous solution). The corrosion-resistant heat exchange medium can isolate flue gas dust and acidic corrosive media, ensuring the long-term stable operation of the first and second heat pump units.

[0049] like Figure 2 As shown ( Figure 2Only the specific structure of the second heat pump unit 11 is shown in the diagram (the first heat pump unit 10 is the same, so it is not shown again). The refrigerant proportional distribution control component includes two electric refrigerant proportional distribution valves 115, a multi-point temperature sensor, a pipeline flow sensor, a programmable logic controller (PLC), and a variable frequency drive unit. The two electric refrigerant proportional distribution valves 115 are respectively installed at the high-pressure output ends of the first and second heat pump units, i.e., the output ends of the variable frequency compressor 111. The multi-point temperature sensors are respectively installed at the front, middle, and rear sections of the pulp conveying pipeline 2 and at the pulp boiling inlet. The pipeline flow sensor is installed in the main section of the pulp conveying pipeline. The signal output ends of the multi-point temperature sensors and the pipeline flow sensor are connected to the signal input ends of the PLC. The signal output ends of the PLC are connected to the signal input ends of the variable frequency drive unit. The variable frequency drive unit is electrically connected to the variable frequency compressor of the first heat pump unit 10 and the variable frequency compressor 111 of the second heat pump unit 11, respectively, to achieve adaptive intelligent control under all operating conditions.

[0050] like Figure 2 As shown, the high-pressure refrigerant energy storage bidirectional dispatching component includes a high-pressure refrigerant energy storage tank 104 and a bidirectional refrigerant switching valve assembly 105. The high-pressure refrigerant energy storage tank 104 is a high-pressure pressure-bearing tank, which is connected to the refrigerant circuit of the first heat pump unit 10, the refrigerant circuit of the second heat pump unit 11, and the intermediate heat-tracing condenser 103 via pipelines. The bidirectional refrigerant switching valve assembly 105 is installed on the pipeline between the high-pressure refrigerant energy storage tank 104 and the second heat pump unit, and is used to control the refrigerant flow between the high-pressure refrigerant energy storage tank and the second heat pump unit. The mid-section refrigerant pressure adaptive reflux assembly includes an electrically operated three-way reflux distribution valve assembly 106 and two refrigerant pressure sensors. The electrically operated three-way reflux distribution valve assembly is installed at the refrigerant output end (low-pressure liquid refrigerant outlet) of the mid-section heated condenser 103. The two output ports of the electrically operated three-way reflux distribution valve assembly 106 are respectively connected to the throttle valve inlet of the first heat pump unit and the throttle valve inlet of the second heat pump unit through pipelines, forming a pressure adaptive reflux loop. The two refrigerant pressure sensors are respectively installed in the closed refrigerant circulation loop of the first heat pump unit and the closed refrigerant circulation loop of the second heat pump unit, and both refrigerant pressure sensors are set close to their respective throttle valve inlets. The signal output terminals of both refrigerant pressure sensors are connected to the signal input terminal of the programmable logic controller.

[0051] The core improvements of the preheating device for pulp production described in this embodiment are reflected in the following aspects, and the specific implementation methods and technical advantages of each improvement are as follows: The device abandons the traditional single heat pump centralized heating and single-end single-point heating mode, and adopts a dual-unit hybrid coupling structure of the first heat pump unit 10 and the second heat pump unit 11. The first heat pump unit 10 and the second heat pump unit 11 are both equipped with independent evaporators, variable frequency compressors, condensers and throttling valves, which can realize that the two compressors can work independently, without interfering with each other and each taking heat from nearby. The first heat pump unit 10 is dedicated to serving the front section of the pulp conveying pipeline. It collects the waste heat from the wood chip cooking device through a corresponding anti-corrosion intermediate heat exchange circuit, and then provides heat tracing for the pulp in the front section of the pulp conveying pipeline through its own front-end heat tracing condenser. The second heat pump unit 11 is dedicated to serving the rear section of the pulp conveying pipeline. It collects the waste heat from the pulp cooking device through a corresponding anti-corrosion intermediate heat exchange circuit, and then provides heat tracing for the pulp in the rear section of the pulp conveying pipeline through its own rear-end heat tracing condenser. The middle-section heat tracing condenser 103 is installed as an independent heat exchange component in the middle section of the pulp conveying pipeline. Together, the three form a hybrid working mode of "front-end local heating, rear-end local heating, and middle-section surplus heat supplementation". The first and second heat pump units can operate independently and output on demand. The surplus refrigerant generated by the two units can be shared through the refrigerant loop, breaking the limitations of traditional heat pumps that are single closed-loop and cannot be coordinated. This structural design solves the problems of the paper mill's dual waste heat sources being dispersed at the front and rear ends and uneven temperature drop in sections of ultra-long pipelines. The method of taking heat from the front and rear ends nearby greatly reduces waste heat transmission losses (loss rate ≤5%). The design of the two units working independently can adapt to the different waste heat fluctuation characteristics at the front and rear ends. The three-stage zoned heat exchange structure completely solves the technical pain points of insufficient heat exchange time for high flow rate pulp, overload of single-point heating load, and insufficient preheating.

[0052] The compression-type high-temperature heat pump structure of the first heat pump unit 10 and the second heat pump unit 11 completes the standard phase change cycle through the coordinated work of the evaporator 114, the variable frequency compressor 111, the condenser (i.e. the front-end heat-tracing condenser 101 or the rear-end heat-tracing condenser 102) and the throttle valve 112. The evaporator 114, as the core component for waste heat absorption, is connected to a low-temperature intermediate medium of 45-55°C from the heat exchange circuit of the waste heat collection intermediate medium. The liquid refrigerant flowing through the heat exchange pipe inside the evaporator 114 is continuously heated. After the liquid refrigerant fully absorbs heat, it undergoes an evaporation phase change to generate a low-temperature, low-pressure gaseous refrigerant, thereby completing the collection and transfer of low-grade waste heat. The variable frequency compressor 111, as the core energy-consuming and energy-improving component of the system, can forcefully compress the low-temperature, low-pressure gaseous refrigerant output from the evaporator 114, significantly increasing the pressure and temperature of the refrigerant, transforming it into a high-temperature, high-pressure gaseous refrigerant of 80–150°C, thus completing the leap from low-grade waste heat to high-grade thermal energy. The condenser includes a front-stage heat-traced condenser 101, a rear-stage heat-traced condenser 102, and a middle-stage heat-traced condenser 103. All three are heat-release heat exchange components. When the high-temperature, high-pressure gaseous refrigerant flows through each condenser, it exchanges heat with the outer wall of the pulp conveying pipeline, continuously releasing heat to preheat the pulp in the pipeline. After releasing heat, the refrigerant undergoes a condensation phase change, transforming into a high-pressure liquid refrigerant. This process can stably preheat the pulp in the pipeline to 60–120°C, meeting the temperature requirements of the pre-cooking process. As a pressure and temperature regulating component, the throttle valve 112 can quickly reduce the pressure and temperature of the high-pressure liquid refrigerant, so that the refrigerant returns to a low-temperature and low-pressure liquid state. Then the refrigerant flows back to the evaporator to participate in the next cycle, forming a continuous, stable, and closed-loop heat pump heat exchange cycle.

[0053] The refrigerant proportional distribution control component achieves precise refrigerant distribution through the electric refrigerant proportional distribution valve 115. The high-pressure refrigerant output terminals of the variable frequency compressors of the first heat pump unit 10 and the second heat pump unit 11 are respectively equipped with high-precision electric refrigerant proportional distribution valves, each with two independent output circuits, and both strictly adhere to the local load priority distribution logic. The programmable logic controller (PLC) collects real-time pulp temperature and flow data from the upstream and downstream sections of the pulp conveying pipeline. Based on this data, it prioritizes controlling the electric refrigerant proportional distribution valve to supply high-pressure refrigerant to the corresponding upstream or downstream heat tracing condenser, ensuring sufficient local heat tracing load and offsetting pipeline foundation heat dissipation. When the local pulp temperature reaches the process-set threshold (60–120°C) and the local heat tracing load is saturated, the PLC automatically adjusts the opening of the electric refrigerant proportional distribution valve, converging and transporting the excess high-pressure, high-temperature refrigerant to the mid-section heat tracing condenser, achieving centralized utilization of excess heat. This refrigerant distribution method enables precise on-demand distribution of refrigerant heat, avoiding heat waste. While ensuring stable process temperatures in the upstream and downstream sections of the pulp conveying pipeline, it maximizes the recovery and utilization of surplus waste heat capacity, solving the problems of traditional heat pumps being unable to distribute heat on demand and having low waste heat utilization rates. It can also adapt to the working characteristics of gradual heating of low-grade waste heat.

[0054] The high-pressure refrigerant energy storage tank in the bidirectional high-pressure refrigerant energy storage component is connected in parallel via pipelines to the refrigerant output terminals of the first and second heat pump units and the inlet of the intermediate-section heat-tracing condenser. It employs a direct heat storage mode using high-temperature, high-pressure gaseous refrigerant. This mode differs from the secondary heat exchange mode of traditional water-based heat storage, avoiding the drawback of response lag. During peak production periods, when the waste heat output from the wood chip cooking and pulping units is high, and the excess refrigerant generated by the first and / or second heat pump units exceeds the heating demand of the intermediate-section heat-tracing condenser, the excess high-pressure refrigerant is directly stored in the high-pressure refrigerant energy storage tank for temporary heat storage. During periods of low waste heat, the high-grade refrigerant stored in the high-pressure refrigerant energy storage tank can be directly released to supplement the heat of the intermediate-section heat-tracing condenser, achieving peak shaving and valley filling of waste heat. This structural design effectively offsets the operational defects of intermittent and fluctuating waste heat output in paper mills, solves the problems of excessive waste heat during peak periods and insufficient heating during off-peak periods, and requires no additional energy consumption, thus greatly improving the adaptability of the equipment to operating conditions and the utilization rate of waste heat.

[0055] The bidirectional refrigerant switching valve assembly establishes a bidirectional system dispatch loop between the high-pressure refrigerant storage tank and the second heat pump unit. The high-pressure refrigerant storage tank can not only release energy to replenish heat to the mid-section heat-tracing condenser through a unidirectional pipeline, but also connect to the downstream heat-tracing loop of the second heat pump unit via the bidirectional refrigerant switching valve assembly. When waste heat fluctuations occur in the downstream boiling unit or the heating capacity of the second heat pump unit is insufficient, the high-pressure refrigerant storage tank can output high-pressure refrigerant in the reverse direction through the bidirectional refrigerant switching valve assembly to replenish the second heat pump unit and the downstream heat-tracing condenser, achieving bidirectional dispatch of heat across time periods and regions. This bidirectional dispatch structure breaks the limitation of traditional energy storage devices that can only store heat and cannot replenish heat in reverse, realizing dynamic heat exchange within the system, greatly improving the device's resistance to fluctuations, and providing a hardware foundation for downstream process pressure stabilization.

[0056] The programmable logic controller (PLC) incorporates a dedicated back-end process priority control logic, which takes precedence over the mid-stage heat replenishment control logic and the front-end load regulation control logic. The PLC monitors the pulp temperature at the inlet of the pulping unit (target value 60–120°C, threshold fluctuation ±1°C) and the waste heat load of the second heat pump unit 24 / 7. When the pulp temperature at the inlet of the pulping unit falls below the set threshold and a heat supply shortfall occurs at the back end, the PLC immediately locks the back-end heat supply priority, preferentially triggering the bidirectional refrigerant switching valve group to open. Simultaneously, it suspends the diversion of excess refrigerant to the mid-stage heat tracing condenser, prioritizing the supply of heat to the back end from the high-pressure refrigerant storage tank, thus forcibly stabilizing the pulp temperature at the inlet of the pulping unit. This control logic strictly ensures the stability of the core pulping process, avoiding imbalances in cooking conditions and pulp quality fluctuations (final pulp temperature fluctuation ≤ ±1°C) caused by waste heat fluctuations and insufficient heat supply, fully meeting the core production needs of the paper mill.

[0057] The dual-factor compressor control component controls the operation of the variable frequency compressor through the linkage of intermediate medium temperature and pulp temperature. The multi-point temperature sensor includes a medium temperature sensor and a pulp temperature sensor. The medium temperature sensor is installed on the intermediate medium pipeline of the waste heat collection loop corresponding to the first heat pump unit and the second heat pump unit, respectively. The pulp temperature sensor is installed at the front and rear sections of the pulp conveying pipeline, respectively. The variable frequency drive unit is linked with the programmable logic controller, and the variable frequency drive unit supports continuous power adjustment from 0 to 100%. When the medium temperature sensor detects an intermediate medium temperature ≥ 45℃, regardless of whether the pulp temperature meets the standard, the programmable logic controller (PLC) will control the corresponding variable frequency compressor to force start via the variable frequency drive unit, preventing waste due to unabsorbed residual heat. Furthermore, the operating power of the variable frequency compressor is positively correlated with the intermediate medium temperature; the higher the intermediate medium temperature, the greater the power of the variable frequency compressor (e.g., 30% power at 45℃, 70% power at 60℃, and 100% power at 75℃), ensuring that low-grade residual heat in the intermediate medium is fully extracted. When the pulp temperature sensor detects a pulp temperature ≤ 50℃ in the corresponding area, regardless of whether the intermediate medium temperature meets the standard, the PLC will control the corresponding variable frequency compressor to start via the variable frequency drive unit. The variable frequency compressor is forced to start, and its operating power is inversely related to the pulp temperature. The lower the pulp temperature, the greater the power of the variable frequency compressor (e.g., 50% power at 50℃, 80% power at 40℃, and 100% power at 30℃), prioritizing the basic preheating needs of the pulp. When the variable frequency compressor operates according to the above dual-factor control mechanism, and the pulp temperature in the corresponding area reaches the process target value (60~120℃), and the temperature difference between the inlet and outlet of the local heat tracing condenser is ≤5℃ (determining local load saturation), the programmable logic controller directly determines that "high-pressure refrigerant is surplus" and automatically controls the electric refrigerant proportional distribution valve to open the diversion circuit, delivering the surplus high-pressure refrigerant to the intermediate heat tracing condenser. This dual-factor control mechanism completely solves the industry pain point that "refrigerant pressure / temperature alone cannot determine the surplus of waste heat," achieving precise matching between waste heat absorption and pulp preheating needs. It avoids the waste heat caused by excessively high intermediate medium temperature but failure of the variable frequency compressor to start, while ensuring emergency heat replenishment when the pulp temperature is too low, thus achieving a balance between the two objectives. At the same time, the variable frequency compressor power is dynamically adjustable, which can avoid energy waste and improve the energy efficiency ratio of the device to 3.5~5.0.

[0058] The middle-section refrigerant pressure adaptive reflux distribution component achieves the dynamic balanced distribution of the reflux refrigerant of the middle-section heat tracing condenser through an electric three-way reflux distribution valve train. The response time of the electric three-way reflux distribution valve train is ≤0.5 s, and the flow regulation range is 0 - 100%; the measurement ranges of the two refrigerant pressure sensors are 0.1 - 1.0 MPa, the accuracy is ±0.01 MPa, and the set pressure balance threshold is 0.3 - 0.5 MPa. After the high-temperature and high-pressure refrigerant in the middle-section heat tracing condenser exchanges heat with the pulp, it condenses into a low-pressure liquid refrigerant, and this low-pressure liquid refrigerant enters the shunt link through the electric three-way reflux distribution valve train; the programmable logic controller collects the refrigerant pressure values of the two heat pump unit circulation loops in real time (i.e., the first heat pump loop pressure P1 and the second heat pump loop pressure P2). When |P1 - P2| ≤ 0.05 MPa (pressure balance), the electric three-way reflux distribution valve train distributes the reflux refrigerant in a 50:50 ratio; when P1 < P2 - 0.05 MPa (refrigerant shortage in the first heat pump loop), the electric three-way reflux distribution valve train automatically increases the refrigerant flow rate to the first heat pump unit until the difference between P1 and P2 is ≤ 0.05 MPa; when P2 < P1 - 0.05 MPa (refrigerant shortage in the second heat pump loop), the electric three-way reflux distribution valve train automatically increases the refrigerant flow rate to the second heat pump unit until the pressure is balanced; when the pressure of a certain loop is lower than 0.3 MPa (low liquid level alarm threshold), the electric three-way reflux distribution valve train preferentially distributes more than 80% of the reflux refrigerant to this loop to quickly replenish the refrigerant and avoid the damage of the variable frequency compressor due to idling. This kind of pressure adaptive reflux distribution mechanism realizes the dynamic balanced distribution of the reflux refrigerant of the middle-section heat tracing condenser, completely solves the problem of "liquid shortage / overflow of a certain heat pump unit caused by uneven refrigerant reflux", ensures the independence and stability of the closed-loop cycles of the first heat pump unit and the second heat pump unit, and avoids the influence on the heat pump energy efficiency ratio due to unbalanced refrigerant distribution; moreover, the adaptive adjustment response speed is fast, which can adapt to the working conditions of waste heat fluctuation and pulp flow fluctuation, and makes the continuous operation stability of the device ≥99%.

[0059] The middle-section refrigerant heating component is a dedicated designed manifold + multiple groups of parallel wound branch pipes + independent shunt regulating valves + distributed spiral wound heat exchange structure, as Figure 4As shown, its core components include a refrigerant manifold 1031, multiple sets of distributed spiral heat exchange branch pipes 1033, an independent diversion regulating valve 1032, and an outer insulation and protection layer. The refrigerant manifold serves as the main confluence channel, connecting to the surplus refrigerant outlet of the electric refrigerant proportional distribution valve of the first heat pump unit, the surplus refrigerant outlet of the electric refrigerant proportional distribution valve of the second heat pump unit, and the energy release outlet of the high-pressure refrigerant storage tank, thus achieving centralized collection and distribution of surplus refrigerant from multiple sources. Multiple sets of distributed spiral heat exchange branch pipes (≥10 sets, adjusted according to the length of the pulp conveying pipeline) are evenly and equidistantly spirally wound on the outer wall of the middle section of the pulp conveying pipeline, with a winding spacing of 5-10cm. All distributed spiral heat exchange branch pipes are arranged in parallel to ensure no heat exchange dead zones and no flow deviation. Each distributed spiral heat exchange branch pipe is matched with a high-precision independent flow control valve, which can individually adjust the refrigerant flow and start / stop status of the corresponding distributed spiral heat exchange branch pipe. An outer insulation and protective layer is wrapped around the outside of the distributed spiral heat exchange branch pipes, with a thickness of ≥50mm, which can eliminate heat loss during the refrigerant heat exchange process. After multiple sources of surplus high-pressure refrigerant are collected in the refrigerant manifold, the flow rate is precisely distributed through independent diversion regulating valves, and evenly delivered to each set of distributed spiral heat exchange branch pipes. The distributed spiral winding structure can significantly increase the heat exchange contact area (3-5 times higher than the contact area of ​​traditional point heating) and the heat exchange stroke, providing continuous, uniform, and gradual heat replenishment for the pulp in the middle section of the pulp conveying pipeline. When the pulp flow rate increases instantaneously, the programmable logic controller can synchronously increase the refrigerant flow rate of each set of distributed spiral heat exchange branch pipes through the independent diversion regulating valves, extending the effective heat exchange time and compensating for the defects of short pulp residence time and insufficient preheating under high flow rate conditions. This type of mid-section refrigerant heating component solves the problems of uneven local point heating, short heat exchange stroke, and poor adaptability to high flow rates in traditional systems, achieving uniform heat replenishment across the entire mid-section over a long distance, keeping the temperature drop in the middle section of the pulp within 1°C, perfectly adapting to the gradual heating characteristics of low-grade waste heat, and significantly improving the overall preheating efficiency.

[0060] like Figure 4 As shown, the complete working cycle of the preheating device before cooking in pulp production in this embodiment is as follows:

[0061] S1. Wood chips are softened and ground in a wood chip cooking device to form pulp. The pulp is transported to a pulping device through a pulp conveying pipeline. The first heat pump unit and the second heat pump unit recover the waste heat from the wood chip cooking device and the pulping device through an intermediate heat exchange circuit, respectively.

[0062] S2. The mechanism starts the variable frequency compressor, compressing the refrigerant vaporized in the evaporator into a high-temperature, high-pressure gaseous refrigerant; the control system prioritizes the high-temperature, high-pressure refrigerant to enter its corresponding front-end and rear-end heat-tracing condensers, completing the on-site heat tracing preheating of the pipeline front and rear sections.

[0063] S3. When it is determined that there is excess high-pressure refrigerant, the opening of the refrigerant proportional distribution valve is automatically adjusted to divert the excess high-pressure refrigerant to the intermediate heat tracing condenser.

[0064] S4. Excess refrigerant is evenly heated in the middle section of the pipeline through the middle section heat condenser, forming a step-by-step preheating mode of the front, middle and rear sections.

[0065] S5. When the peak value of the system waste heat output is too large and the surplus refrigerant exceeds the heating demand of the middle section, the excess high-pressure high-temperature refrigerant is stored in the high-pressure refrigerant storage tank to complete the heat storage.

[0066] S6. The low-pressure liquid refrigerant after heat exchange in the middle section heat tracing condenser is distributed back through the refrigerant return distribution valve group according to the pressure difference ratio of the two heat pump units' circulation loops, ensuring the refrigerant circulation balance of the two units.

[0067] S7. Real-time monitoring of the pulp temperature at the inlet of the pulp cooking unit and the waste heat supply status of the second heat pump unit. When the waste heat of the pulp cooking unit fluctuates or the heating capacity of the second heat pump unit is insufficient, resulting in a low pulp temperature at the back end, the back end process priority logic is activated, the bidirectional refrigerant switching valve group is opened, and the high-pressure refrigerant is supplied from the high-pressure refrigerant storage tank to the second heat pump unit and the downstream heat tracing condenser in reverse to prioritize stabilizing the pulp temperature at the inlet of the pulp cooking unit.

[0068] S8. When the instantaneous flow rate of pulp is too high and the temperature drop rate of the middle section pipeline is accelerated, the control system links the first heat pump unit and the second heat pump unit to simultaneously supplement the heat to the middle section heat-tracing condenser with the excess refrigerant and the high-pressure refrigerant energy tank. The long-distance distributed winding heat exchange structure is used to extend the effective heat exchange time and make up for the deficiency of insufficient heat exchange time under high flow rate conditions.

[0069] First, the waste heat collection and heat exchange stage begins. The low-grade exhaust gas and waste steam generated by the wood chip cooking device and pulp cooking device undergo heat exchange through the anti-corrosion intermediate heat exchange medium inside the waste heat collection loop. The anti-corrosion intermediate medium is stably heated to 45-55℃, forming a constant-temperature low-temperature heat source. Then, the anti-corrosion intermediate medium is connected to the evaporators of the first and second heat pump units to heat the low-temperature liquid refrigerant in the evaporators. After absorbing heat, the refrigerant vaporizes to generate low-temperature low-pressure gaseous refrigerant. Next, the dual-factor compressor start-up stage begins. The programmable logic controller (PLC) controls the corresponding variable frequency compressor to start at the set power according to the intermediate medium temperature detected by the medium temperature sensor (≥45℃ for forced start) and the pulp temperature detected by the pulp temperature sensor (≤50℃ for forced start). The variable frequency compressor compresses the low-temperature low-pressure gaseous refrigerant into a high-temperature high-pressure gaseous refrigerant of 80-150℃, completing the energy grade leap. Then, the local priority heat tracing stage begins. High-temperature and high-pressure gaseous refrigerant is preferentially delivered to the corresponding front-end or rear-end heat tracing condenser of the machine via an electric refrigerant proportional distribution valve. The shell-and-tube closed heat exchange structure can quickly compensate for the heat dissipation of the pulp conveying pipeline before and after the machine, ensuring that the basic temperature of the pulp in the front and rear sections is stable (temperature drop ≤1℃). The process then enters the surplus refrigerant diversion and reheating stage. The programmable logic controller (PLC) collects real-time data on pulp temperature and flow rate at each section of the pulp conveying pipeline. When the pulp temperature at the front and rear reaches the process-set threshold and the local heating load is saturated (temperature difference between refrigerant inlet and outlet ≤ 5℃), the PLC determines that there is surplus refrigerant and automatically adjusts the opening of the electric refrigerant proportional distribution valve to divert the surplus high-pressure refrigerant to the mid-section heating condenser. Multiple sets of distributed wound heat exchange branch pipes uniformly reheat the pulp in the mid-section of the pulp conveying pipeline, constructing a three-stage progressive preheating system (front, middle, and rear), extending the heat exchange stroke (equivalent to 2-3 times the pipeline length), and adapting to the preheating requirements of high-flow-rate pulp. When the waste heat output is large during peak production and the surplus refrigerant exceeds the heating demand of the mid-section heating condenser, the unit enters the energy storage and heat preservation peak-shaving stage. The excess high-pressure, high-temperature refrigerant is directly stored in the high-pressure refrigerant energy storage tank for temporary heat storage, achieving waste heat peak shaving and avoiding waste. After the refrigerant in the intermediate heat-tracing condenser exchanges heat with the pulp, it enters the intermediate heat exchange and reflux distribution stage. The refrigerant condenses into a low-pressure liquid refrigerant, which is then distributed back to the intermediate heat pump unit and the second heat pump unit according to the pressure difference ratio of the circulation loops (50:50 ratio when the pressure is balanced, and adaptive adjustment when there is an imbalance) through the electric three-way reflux distribution valve group. This ensures that the refrigerant circulation volume of the two units is balanced and avoids liquid shortage / overflow.Then, the system enters a bidirectional pressure stabilization phase. When the operating conditions of the pulping unit fluctuate, waste heat output is insufficient, the heating capacity of the second heat pump unit decreases, and the pulp inlet temperature is too low, the programmable logic controller triggers the back-end priority logic, opening the bidirectional refrigerant switching valve group. The high-pressure refrigerant storage tank supplies high-temperature, high-pressure refrigerant to the second heat pump unit and the downstream heat-tracing condenser in reverse, prioritizing the stabilization of the pulp inlet temperature. When the instantaneous flow rate of the pulp increases sharply and the temperature drop in the middle section accelerates, the surplus refrigerant from the first and second heat pump units, together with the high-pressure refrigerant storage tank, synchronously supplies heat to the middle-section heat-tracing condenser to compensate for the insufficient heat exchange time. Finally, the system enters a refrigerant closed-loop reflux phase. The low-pressure liquid refrigerant flowing back to the first and second heat pump units is cooled and depressurized by their respective throttling valves before returning to the evaporator to participate in the next cycle, forming a complete closed loop.

[0070] The preheating device for pulp production described in this embodiment, through its complete architecture design and closed-loop control throughout the entire process, completely solves the shortcomings of existing technologies and achieves significant technical effects: In terms of energy saving, the device achieves full recovery and zero waste of waste heat from paper mills, completely replacing traditional high-energy-consuming electric heating, reducing the overall energy consumption of the production line by more than 40%, and saving more than one million yuan in electricity costs annually; in terms of temperature control, the temperature drop of pulp throughout the 200m-500m long pipeline is controlled within 2-3℃ from the original 8-12℃, and the pulp temperature fluctuation at the cooking inlet is ≤±1℃, significantly improving temperature uniformity; in terms of operating condition adaptability, the device is perfectly adapted to pulp flow rates of 0.5-2m / s. 3 The system effectively solves the problem of insufficient pulp preheating under high flow rates, achieving a 100% pulp preheating qualification rate. Regarding system stability, through backend priority control logic, bidirectional energy storage scheduling, and pressure adaptive reflux distribution, the continuous operation stability of the device is ≥99%, with no faults such as liquid shortage, overflow, or compressor idling. In terms of versatility, the device boasts a high degree of automation and low maintenance costs, making it widely applicable to the upgrading and transformation of various pulping production lines, with broad market prospects.

[0071] 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 preheating device for pulp cooking in pulp production, characterized in that: This includes pulp conveying pipelines, distributed hybrid heat pump units, mid-section refrigerant heating components, waste heat collection loops, and refrigerant distribution and control components; The pulp conveying pipeline connects the wood chip cooking and grinding equipment with the pulping device; the distributed hybrid heat pump unit adopts a dual-compressor, three-condenser hybrid coupling architecture, including a first heat pump unit and a second heat pump unit, both of which are equipped with independent waste heat evaporators, variable frequency compressors, and closed-loop refrigerant circulation loops; the waste heat collection loop is equipped with an intermediate heat exchange medium to collect low-grade waste heat from the wood chip cooking device and the pulping device, and to heat the low-temperature refrigerant in the waste heat evaporator, causing the refrigerant to absorb heat and vaporize; the variable frequency compressor is used to compress the vaporized refrigerant to generate high-temperature, high-pressure gaseous refrigerant; the first heat pump unit and the second heat pump unit are respectively equipped with front and rear sections of the pipeline. The system includes a heat-tracing condenser and a downstream heat-tracing condenser. The intermediate refrigerant heating component is equipped with a intermediate heat-tracing condenser. Both the high-pressure output terminals of the compressors of the first and second heat pump units are equipped with refrigerant proportional distribution valves. These proportional distribution valves have two independent output circuits. One circuit connects to the corresponding upstream or downstream heat-tracing condenser to achieve local independent heat tracing, while the other circuit connects to the intermediate heat-tracing condenser to achieve centralized supplementary heating of excess refrigerant. The refrigerant distribution and control component collects the pulp temperature and flow rate signals within the pipeline and follows a local load priority control logic. It prioritizes meeting the local heat tracing needs of the upstream and downstream sections of the pipeline before diverting excess high-pressure refrigerant to the intermediate heat-tracing condenser for supplementary heating.

2. The apparatus according to claim 1, characterized in that: It also includes a high-pressure refrigerant storage tank and a two-way refrigerant switching valve assembly; the high-pressure refrigerant storage tank is connected to the first heat pump unit, the second heat pump unit, and the intermediate heat-tracing condenser through high-pressure refrigerant pipelines; the two-way refrigerant switching valve assembly is installed between the high-pressure refrigerant storage tank and the second heat pump unit; when the first heat pump unit and / or the second heat pump unit produces surplus high-pressure high-temperature refrigerant, it can be channeled into the high-pressure refrigerant storage tank to achieve temporary storage of high-pressure gaseous refrigerant heat; the high-pressure refrigerant storage tank can release energy and supplement heat to the intermediate heat-tracing condenser in one direction, and can also supply high-pressure refrigerant to the second heat pump unit in the opposite direction through the two-way refrigerant switching valve assembly, so as to realize the peak shaving and valley filling of system waste heat and the two-way heat scheduling when the downstream heat source is insufficient.

3. The apparatus according to claim 2, characterized in that: The refrigerant distribution and control component has built-in back-end process priority control logic; it collects the inlet pulp temperature of the pulping unit and the waste heat supply load of the second heat pump unit in real time; when the inlet pulp temperature of the pulping unit is lower than the set threshold and the second heat pump unit is affected by waste heat fluctuations, resulting in insufficient heat supply, it prioritizes triggering the bidirectional refrigerant switching valve group to conduct, and the high-pressure refrigerant energy storage tank supplements heat to the second heat pump unit and the back-end heat tracing circuit, so as to prioritize ensuring the stability of the inlet pulp temperature of the pulping unit.

4. The apparatus according to claim 1, characterized in that: The variable frequency compressor adopts a dual-factor linkage control mechanism of intermediate medium temperature and pulp temperature. The refrigerant distribution and control component includes a multi-point temperature detection unit, a PLC intelligent control unit, and a variable frequency drive unit. The multi-point temperature detection unit is respectively arranged in the intermediate medium pipeline of the waste heat collection loop and the front and rear sections of the pulp conveying pipeline. When the intermediate medium temperature reaches or exceeds the preset medium threshold, the variable frequency compressor is forcibly started, and the compressor's working power is positively correlated with the intermediate medium temperature. The higher the intermediate medium temperature, the greater the compressor power, ensuring full absorption of low-grade waste heat in the intermediate medium. When the pulp temperature in the corresponding area reaches or falls below the preset pulp threshold, the variable frequency compressor is forcibly started, and the compressor's working power is inversely correlated with the pulp temperature. The lower the pulp temperature, the greater the compressor power, prioritizing the basic preheating needs of the pulp. When the compressor operates according to the above dual-factor control mechanism, and the pulp temperature in the corresponding area reaches the process target value and the local heat tracing load is saturated, it is determined that there is excess high-pressure refrigerant. The refrigerant proportional distribution valve automatically opens the diversion loop to transport the excess high-pressure refrigerant to the intermediate heat tracing condenser.

5. The apparatus according to claim 1, characterized in that: Both the first and second heat pump units are compression-type high-temperature heat pump structures, and are equipped with intermediate heat exchange loops and throttling valves. The waste heat evaporator of the first heat pump unit is connected to the exhaust gas and flash steam of the wood chip cooking device through the intermediate heat exchange loop, and the waste heat evaporator of the second heat pump unit is connected to the process exhaust steam and secondary steam of the pulp cooking device through the intermediate heat exchange loop. The intermediate heat exchange loop is filled with anti-corrosion heat exchange medium. The high-temperature and high-pressure gaseous refrigerant is diverted by the refrigerant proportional distribution valve and enters the local heat tracing condenser and the intermediate heat tracing condenser respectively. After heat exchange, the liquid refrigerant is cooled and depressurized by the throttling valve and then flows back to the waste heat evaporator to complete the closed-loop cycle.

6. The apparatus according to claim 5, characterized in that: Both the front-end and rear-end heat tracing condensers adopt a shell-and-tube heat exchange structure. The gap between the shells forms a high-pressure refrigerant flow cavity, which is tightly wrapped around the outer wall of the pulp conveying pipeline. The outer side is covered with a heat insulation layer to achieve closed-loop directional heat tracing of the front and rear sections of the pipeline.

7. The apparatus according to claim 1, characterized in that: The mid-section refrigerant heating assembly includes a refrigerant manifold, multiple sets of distributed spiral heat exchange branch pipes, a flow control valve, and an insulation layer. The refrigerant manifold is connected to the refrigerant proportional distribution valve outlet of the first heat pump unit and the outlet of the high-pressure refrigerant storage tank of the second heat pump unit. Multiple sets of distributed spiral heat exchange branch pipes are evenly spirally wound on the outer wall of the mid-section of the pulp conveying pipeline, and both ends of each branch pipe are connected in parallel to the refrigerant manifold. The flow control valve is independently set for each set of branch pipes and is used to adjust the refrigerant flow in each area of ​​the mid-section to form a continuous and uniform progressive heat replenishment area.

8. The apparatus according to claim 1, characterized in that: The refrigerant distribution and control component also includes a pipeline flow detection unit, which is installed in the main section of the pulp conveying pipeline to collect pulp flow rate and flow fluctuation parameters. The PLC intelligent control unit links various signals such as temperature, flow rate, and pressure to dynamically adjust the opening of the refrigerant proportional distribution valve, the flow rate of the refrigerant return distribution valve group, and the operating frequency of the heat pump compressor, so as to achieve adaptive control under all operating conditions.

9. The apparatus according to claim 1, characterized in that: The refrigerant output end of the intermediate heat tracing condenser is equipped with a refrigerant return distribution valve group, which is a pressure adaptive distribution structure. The closed refrigerant circulation loops of the first heat pump unit and the second heat pump unit are both equipped with refrigerant pressure sensors. The PLC intelligent control unit collects the refrigerant pressure values ​​of the circulation loops of the two heat pump units in real time, and dynamically adjusts the refrigerant return flow rate through the refrigerant return distribution valve group. The low-pressure liquid refrigerant after heat exchange in the middle section heat tracing condenser is distributed to the throttle valve inlet of the first heat pump unit and the second heat pump unit according to the pressure difference ratio. When the refrigerant pressure of the circulation loop of a certain heat pump unit is lower than the preset pressure threshold, the refrigerant return is preferentially distributed to it to ensure the refrigerant circulation volume of the two units is balanced and to avoid the situation of excessive or insufficient refrigerant.

10. A method for preheating before cooking in pulp production, characterized in that, Using the apparatus according to any one of claims 1 to 9, the steps include: S1. Wood chips are softened and ground in a wood chip cooking device to form pulp. The pulp is transported to a pulping device through a pulp conveying pipeline. The first heat pump unit and the second heat pump unit recover the waste heat from the wood chip cooking device and the pulping device through an intermediate heat exchange circuit, respectively. S2. The mechanism starts the variable frequency compressor, compressing the refrigerant vaporized in the evaporator into a high-temperature, high-pressure gaseous refrigerant; the control system prioritizes the high-temperature, high-pressure refrigerant to enter its corresponding front-end and rear-end heat-tracing condensers, completing the on-site heat tracing preheating of the pipeline front and rear sections. S3. When it is determined that there is excess high-pressure refrigerant, the opening of the refrigerant proportional distribution valve is automatically adjusted to divert the excess high-pressure refrigerant to the intermediate heat tracing condenser. S4. Excess refrigerant is used to uniformly heat the middle section of the pipeline through the mid-section heat condenser, forming a progressive preheating mode of front, middle and rear sections.

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