A temperature control method for a wiped film evaporator for cellulose dissolution

Abstract

The present application belongs to the technical field of chemical fibers, and relates to a temperature control method for a wiped film evaporator for cellulose dissolution. First, the wiped film section in the wiped film evaporator is equally divided into four sections from top to bottom, and the structure parameters of the wiped film evaporator are obtained by using a four-section split heating mode. Then, the process parameters of each section of the wiped film section, the condensate water temperature at the outlet of the heating jacket of each section of the wiped film section, and the inner wall surface temperature and outer wall surface temperature of the inner cylinder are obtained, and the average heat flux density q of the outer wall surface of the inner cylinder is calculated. Then, the latent heat proportion of each section of the wiped film section is calculated based on the obtained parameters and the feed amount. The total heat required by each section is calculated, and the condensate water temperature at the outlet of the heating jacket of each section is calculated. Finally, the saturated steam pressure in the heating jacket of each section required is obtained by referring to a water saturated steam pressure table, and the steam temperature required to be introduced into each section is determined. The method can realize the heating temperature adjustment in the wiped film evaporator under different production capacity requirements.

Description

Technical Field

[0001] This invention belongs to the field of chemical fiber technology and relates to a temperature control method for a scraped film evaporator used for cellulose dissolution. Background Technology

[0002] A scraped film evaporator is a highly efficient evaporator that uses a scraper to apply material to the evaporation wall, where it is heated by steam within a jacket for evaporation and purification. In industrial production, due to its wide range of applicable materials and superior performance such as high heat transfer coefficient, it is now widely used in modern chemical, food, and pharmaceutical production.

[0003] Scraped film evaporators are widely designed for the efficient dissolution of cellulose, ensuring uniform dissolution of cellulose through precise control of the evaporation process. Figure 1 The diagram illustrates the heating process. In particular, the method of using NMMO aqueous solution as a solvent to dissolve cellulose to produce Lyocell fibers has attracted increasing attention.

[0004] When preparing Lyocell spinning dope using a scraped-film evaporator, solvent concentration and heating temperature are key factors affecting the cellulose dissolution process. When the solvent concentration is below 72%, water molecules occupy the sites where NMMO molecules could form hydrogen bonds with cellulose molecules, causing NMMO to lose its ability to dissolve cellulose. As the solvent concentration increases, the fiber dissolution rate accelerates. When the NMMO solution concentration reaches 87%, the fiber dissolves rapidly with almost no observed swelling. The fiber's solubility in NMMO solution increases with increasing solvent concentration. Heating reduces the viscosity of the cellulose solution. Simultaneously, the evaporation of water from the solvent increases the solvent concentration and accelerates molecular thermal motion, speeding up the penetration of NMMO small molecules into the plant fiber and promoting cellulose dissolution. Lower temperatures result in higher material viscosity, making heat transfer difficult and increasing energy consumption for water evaporation; however, excessively high heating temperatures lead to NMMO decomposition (at 120℃, NMMO exhibits a significant discoloration reaction, affecting recovery efficiency).

[0005] Therefore, precise control of the heating temperature of the scraped-film evaporator is crucial. Cellulose dissolution efficiency: Cellulose needs to dissolve in an aqueous solution of NMMO (N-methylmorpholine-N-oxide). Temperature directly affects the dissolution efficiency of cellulose. If the temperature is too low, cellulose cannot dissolve completely, leading to a decrease in fiber quality and affecting the strength and softness of the finished fiber. Conversely, if the temperature is too high, it may lead to excessive degradation of cellulose, affecting fiber performance. Spinning solution rheology: The rheology of the spinning solution directly affects fiber formation and quality. Temperature has a significant impact on the rheology of the spinning solution. Lower temperatures result in higher apparent viscosity, which affects the flowability and spinnability of the spinning solution. Therefore, precise control of the heating temperature is necessary to optimize the rheology of the spinning solution. Thermal stability: Lyocell fiber spinning solutions are heat-sensitive. High temperatures may cause NMMO decomposition, affecting the stability of the spinning solution and the quality of the fiber. The scraped-film evaporator can perform evaporation at a lower operating temperature, reducing the risk of thermal decomposition of the product. Solvent stability: NMMO is very sensitive to temperature, especially with long residence times. Higher temperatures lead to easier decomposition of NMMO, and prolonged exposure to high temperatures intensifies this decomposition reaction. Therefore, for solvent stability, it is desirable to use the lowest possible operating temperature. For safety reasons, experiments show that NMMO decomposition significantly increases when temperatures exceed 130°C. Therefore, to ensure production safety, the solution temperature must be controlled below 130°C. Regarding process efficiency and cost, high temperatures can accelerate the dissolution process, shorten residence time, and increase equipment capacity. However, high temperatures may also increase energy consumption and the burden of solvent recovery. Therefore, while ensuring product quality, appropriate temperature control strategies are necessary. In conclusion, precise control of the heating temperature of the scraped-film evaporator is crucial for ensuring the quality and production efficiency of the lyocell fiber spinning solution. It directly affects the cellulose dissolution efficiency, the rheological properties of the spinning solution, the thermal stability of the spinning solution, the stability of the solvent, equipment operating limitations, and production.

[0006] In the preparation of Lyocell spinning solutions, a scraped-film evaporator with a staged heating design is crucial for optimizing evaporation efficiency and ensuring solution quality. This design effectively adapts to the changes in the physicochemical properties of the material from top to bottom during evaporation. However, traditional methods of adjusting the heating temperature when changing the capacity of evaporators have shortcomings: operators need to repeatedly adjust parameters such as feed temperature, heating temperature of each stage, and flow rate through staged temperature trials, and observe the equipment's operating status and product quality to judge the adjustment effect. This method relies on experience and is time-consuming and labor-intensive, leading to extended production cycles and low production efficiency.

[0007] CN221788163U discloses a thin-film evaporator for easy material preheating, belonging to the technical field of thin-film evaporator technology. The evaporator includes: a frame; a boiler for producing steam; an evaporator body fixed to the frame; a steam pipe for connecting the evaporator body to the boiler; a preheating component mounted on the frame for material preheating; and a material pipe mounted on the evaporator body and connected to the preheating component. In this thin-film evaporator for easy material preheating, material enters the heat exchange tubes through a distribution plate, which evenly distributes the material. High-temperature flue gas generated during boiler operation enters the heat exchange chamber, thus preheating the material. After preheating, the material is collected by a gathering plate and finally enters the evaporator through the material pipe. Because flue gas is used for preheating, no additional heating equipment is needed, effectively utilizing high-temperature energy and reducing costs. This patent utilizes high-temperature flue gas to preheat materials, eliminating the need for additional heating equipment, effectively utilizing high-temperature energy and reducing costs. The material enters the heat exchange tubes through the distribution plate, achieving uniform preheating. While this patent excels in preheating, there may be room for improvement in terms of material evaporation efficiency and energy consumption. For example, a segmented preheating method could be used to further optimize energy consumption control.

[0008] CN219879109U relates to the field of chemical equipment technology, specifically a thin-film evaporator with a built-in condenser. It includes an evaporator cylinder with a hollow rotating shaft extending through its bottom at the cylinder's axis. A refrigerant inlet and outlet pipe are located inside the hollow rotating shaft, and heat exchange tubes are located outside the shaft. The bottom and top ends of the heat exchange tubes are fixedly connected to the hollow rotating shaft and respectively to the refrigerant inlet and outlet pipes. A first tube box connected to the refrigerant inlet pipe is fixedly located at the bottom end of the hollow rotating shaft, and a second tube box connected to the refrigerant outlet pipe is fixedly located at the top end. Both the first and second tube boxes are fixedly connected to rotary joints. This invention is simple to install and easy to operate. By rotating the heat exchange tubes synchronously with the hollow rotating shaft, the speed and intensity of turbulent pulsation near the heat exchange tubes are increased, making the surface temperature field of the heat exchange tubes more uniform. This enhances heat transfer while preventing scaling on the outer wall of the heat exchange tubes. The shortcoming of this invention is that while it provides a concept for enhancing heat transfer, it fails to incorporate a corresponding method for temperature control.

[0009] CN206473835U discloses an electrically heated heat pipe jacketed scraped film evaporator, including an evaporator body heating device and a stirring device. The heating device includes a heat pipe jacket and an electric heating rod. The heat pipe jacket is fitted onto the outer wall of the evaporator body, and the bottom surface of the heat pipe has a groove. The electric heating rod is placed in the groove of the heat pipe to provide heat to the heat pipe. Compared with the prior art, this patent uses the vapor-liquid phase change of the working liquid inside the heat pipe for heat transfer, resulting in a large heating capacity, uniform heating, flexible temperature control of the electric heating rod, and rapid heating, greatly enhancing the evaporation efficiency. It also has a short residence time, enabling continuous operation. While the patent mentions heat transfer through the phase change of the internal working liquid, it does not specifically explain how this is efficiently combined with the heat transfer of the electric heating rod. If the electric heating rod is only placed in the groove, there may be insufficient heat conduction efficiency, resulting in uneven heat transfer to the entire heat pipe jacket. The ring-shaped design of the electric heating rod, although matching the groove, means that local damage (such as a broken section of the heating wire) may cause the entire heating device to fail, leading to high maintenance costs.

[0010] CN203886211U discloses a rotary scraped film evaporator, which has the advantages of using centrifugal force to evenly spread the material along the circumference of the cylinder wall, thus multiplying the evaporation area; employing a multi-stage heating method to separate the material; and using a guide scraper to guide the material downwards by gravity, achieving multi-stage heating and evaporation. While this patent uses a multi-stage heating method, improving heating efficiency and control flexibility, it does not propose a method for calculating the heating temperature of each stage, which to some extent limits the accuracy and optimization potential of the temperature control system.

[0011] In the production of lyocell spinning solutions, temperature control is a key factor in ensuring product quality and improving energy efficiency. Furthermore, for multi-stage heating systems, the ability to accurately calculate and control the temperature at each stage is crucial. Therefore, researching a temperature control method for scraped-film evaporators used for cellulose dissolution to address the problems existing in current technologies is of great significance. Summary of the Invention

[0012] The purpose of this invention is to solve the problems existing in the prior art and provide a temperature control method for a scraped film evaporator used for cellulose dissolution.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0014] A temperature control method for a scraped-film evaporator used for cellulose dissolution, specifically a method for controlling the temperature of the scraped-film evaporator after changing the feed rate, includes the following steps:

[0015] (1) Divide the scraped film section in the scraped film evaporator into four equal sections from top to bottom, which are respectively called the first scraped film section, the second scraped film section, the third scraped film section and the fourth scraped film section. The structural parameters of the scraped film evaporator are obtained by using the four-section separate heating method, including: the characteristic length of each section in the scraped film section. The inner cylinder diameter D of the scraping section and the diameter d of the feed inlet are three structural parameters, each in meters (m).

[0016] (2) Obtain the process parameters of each section in the scraping section, including: the specific heat capacity of the material in each section, the inlet material temperature of each section, and the outlet material temperature of each section.

[0017] (3) Obtain the condensate temperature at the outlet of the heating jacket of each section of the scraped film evaporator. , inner cylinder wall surface temperature and the temperature of the inner and outer walls of the cylinder Then, the average heat flux density q on the outer wall surface of the inner cylinder is calculated, in W / m². 2 ;

[0018] (4) Based on the parameters obtained in steps (1) to (3) and the feed rate, calculate the latent heat ratio of each section in the scraping section;

[0019] (5) Since the latent heat ratio of each section remains unchanged when only the feed rate is changed to adjust the production capacity (because the feed rate G will be canceled out and the latent heat ratio is not affected by G), after changing the feed rate to adjust the production capacity, based on the latent heat ratio of each section in the scraping section obtained in step (4), the total heat required for each section is calculated, and then the condensate temperature of the heating jacket outlet of each section is calculated.

[0020] (6) Based on the condensate temperature of the heating jacket outlet of each section in the scraping film section calculated in step (5), the required saturated vapor pressure in the heating jacket of each section is obtained by looking up the saturated vapor pressure table of water, and then the required steam temperature of each section is determined.

[0021] As a preferred technical solution:

[0022] As described above, in a method for temperature control of a scraped-film evaporator for cellulose dissolution, the specific heat capacities of the materials in the first to fourth scraped-film sections in step (2) are respectively... , , , Unit: J / (kg·℃); Inlet material temperatures are respectively , , , The unit is °C; the outlet material temperatures are respectively , , , , in °C.

[0023] The above describes a temperature control method for a scraped-film evaporator used for cellulose dissolution. and It was obtained through testing. and They are based on and Estimated through linear fitting;

[0024] The specific heat capacities of the materials in the first to fourth scraping sections are respectively , , , This invention addresses the evaporation and dissolution process of cellulose pulp in a scraped-film evaporator. In the scraped-film evaporator, the cellulose pulp undergoes a swelling and dissolution process from top to bottom, ultimately forming a spinning solution. During this process, the proportions of the various components of the cellulose pulp continuously change, and its specific heat capacity also changes. The scraped-film evaporator is a pressure vessel operated under vacuum; currently, it is not possible to open holes in the device to sample material for specific heat capacity testing. Only the specific heat capacity of the inlet and outlet materials can be measured. , The two sets of data were obtained by linear interpolation using the specific heat capacities of the imported and exported materials, i.e.:

[0025] ;

[0026] ;

[0027] , , , , , , and All data were obtained online using temperature sensors installed on the scraped film evaporator.

[0028] As described above, in a method for temperature control of a scraped-film evaporator for cellulose dissolution, step (3) It is detected by a temperature sensor installed at the condensate drain outlet. It is detected by a temperature sensor embedded in the inner cylinder, and the unit is °C.

[0029] As described above, in a method for temperature control of a scraped-film evaporator for cellulose dissolution, step (3) is based on... and The formula for calculating q is:

[0030] ;

[0031] in, The total heat transfer coefficient between the condensate and the inner wall of the cylinder is expressed in W / (m²). 2 ·K), The calculation formula is:

[0032] ;

[0033] Where h is the heat transfer coefficient between the condensate and the outer wall of the inner cylinder, in W / (m²). 2 ·K); The thermal conductivity of the inner cylinder is expressed in W / (m·K). This indicates the thickness of the inner cylinder wall, in meters (m).

[0034] The formula for calculating h is:

[0035] ;

[0036] in, Acceleration due to gravity, unit m / s² 2 ; This refers to the density of condensate, in kg / m³. 3 ; η is the thermal conductivity of condensate, in W / (m·K); r is the latent heat of phase change, in kJ / kg; η is the viscosity of condensate, in Pa·s. The characteristic length of each segment in the scraping section, in meters.

[0037] As described above, the temperature control method for a scraped film evaporator used for cellulose dissolution involves a process where the material properties within the evaporator gradually change from top to bottom during actual production. These properties include specific heat capacity and viscosity. To adapt to these changes and improve evaporation and dissolution efficiency, scraped film evaporators generally employ a segmented steam heating process. Taking a scraped film evaporator with four scraped film sections as an example, the latent heat percentage of each section is calculated. The latent heat percentages of the first to fourth scraped film sections in step (4) are as follows:

[0038] First scraping section:

[0039] ;

[0040] Second scraping section:

[0041] ;

[0042] Third scraping section:

[0043] ;

[0044] Fourth scraping section:

[0045] ;

[0046] Among them, R1, R2, R3, and R4 are the latent heat percentages of the first to fourth scraping sections, respectively, in % %. , , and These represent the sensible heat transfer rates of the first to fourth scraped film sections, in W. , , and q1, q2, q3, and q4 represent the latent heat transfer of the first to fourth scraped film sections, in W; G is the feed rate, in kg / s; q1, q2, q3, and q4 are the average heat flux densities of the outer wall surface of the inner cylinder of the first to fourth scraped film sections, in W / m³. 2 .

[0047] As described above, the temperature control method for a scraped film evaporator for cellulose dissolution includes step (5) as follows: First, based on the latent heat ratio of the first to fourth scraped film sections obtained in step (4), the total heat required for each section is calculated. Then, the average heat flux density of the outer wall of the inner cylinder of each section is calculated according to formula (I). Finally, the condensate temperature at the outlet of the heating jacket of each section is calculated according to formula (II).

[0048] (I);

[0049] (II);

[0050] in, Total heat exchange, in W; For sensible heat transfer, Latent heat transfer, unit: W; The average heat flux density of the outer wall surface of the inner cylinder is expressed in W / m². The total heat transfer coefficient between the condensate and the inner wall of the cylinder is expressed in W / (m²). 2 ·K).

[0051] The temperature control method for a scraped-film evaporator used for cellulose dissolution, as described above, uses cellulose pulp composed of cellulose, NMMO, and water.

[0052] Since the latent heat ratio of each section remains unchanged when only the feed rate is changed to adjust the production capacity, the sensible heat transfer rate and sensible heat ratio inside the evaporator are determined, and the required heating temperature after changing the production capacity is calculated.

[0053] There are two main forms of heat exchange in a scraped film evaporator: sensible heat and heat exchange. and latent heat Sensible heat refers to the heat absorbed or released during a temperature change of a material without a phase change (i.e., the state of matter remains unchanged). Latent heat, on the other hand, refers to the heat absorbed or released during a phase change, i.e., when a substance changes from one phase to another, while the temperature remains constant during this process. In a scraped-film evaporator, latent heat exchange mainly occurs during the process of liquid turning into gas, such as the evaporation of water.

[0054] Sensible heat transfer capacity:

[0055] (III);

[0056] Latent heat transfer:

[0057] (Ⅳ);

[0058] In the formula, denoted as specific heat capacity; e is the proportion of water required to evaporate to reach the required production capacity relative to the mass of the slurry; m is the mass of the slurry.

[0059] As can be seen from equations (III) and (IV) above, when the inlet and outlet slurry temperatures remain unchanged and the e value is the same (the composition of the inlet and outlet slurry is consistent), the ratio between latent heat transfer and sensible heat transfer remains unchanged when the m value is changed to meet the production capacity requirements.

[0060] Based on the above general latent heat transfer formula (Ⅳ) and sensible heat transfer formula (Ⅲ), the calculation formulas for latent heat transfer and sensible heat transfer in the scraped film evaporator are derived.

[0061] Sensible heat transfer :

[0062] ;

[0063] Latent heat transfer :

[0064] ;

[0065] Total heat exchange :

[0066] .

[0067] Heat calculation for water vapor condensation process inside the heating jacket:

[0068] Condensation can be considered the reverse process of evaporation. When gas molecules lose energy and recombine to form a liquid, this process is the opposite of evaporation, where liquid molecules gain energy and turn into a gas.

[0069] like Figure 2As shown, the heating jacket of a scraped film evaporator is a hollow cylindrical section installed on the outer wall of the evaporator. It typically uses saturated water vapor as a heat source to heat the liquid material inside the evaporator. When the saturated water vapor entering the evaporator jacket encounters the cold wall surface, it condenses to form a liquid film, releasing latent heat. (See figure.) To heat the condensate inside the jacket, heat reaches the outer wall of the evaporator via a liquid film. This heat transfer process can be achieved through... calculate.

[0070] Based on the assumption that the liquid film is in laminar flow and that heat transfer within it is only through conduction and not convection, the heat transfer coefficient h between the condensate and the outer wall of the inner cylinder can be calculated using the formula proposed by Nusselt for calculating the average surface heat transfer coefficient of pure water vapor laminar film condensation.

[0071] ;

[0072] In the formula: Ø is the heat flow rate through a certain area per unit time, in W; A is the heating area, in m². 2 ; r is the latent heat of phase change, in kJ / kg.

[0073] Heat flow Ø and Given a specific condition, the total heat capacity, including sensible and latent heat, can be determined. During condensation, there is a one-to-one correspondence between the pressure and temperature of saturated steam. This means that at a specific pressure, there is a specific condensation temperature, which can be obtained by looking up a saturated steam pressure table.

[0074] Beneficial effects:

[0075] (1) The present invention provides a method for temperature control of a scraped film evaporator for cellulose dissolution. By calculating the ratio of latent heat to sensible heat in the four sections of the scraped film section of the scraped film evaporator during the process of material evaporation and dissolution to form spinning solution, and using this ratio as a benchmark, the method calculates the required steam heating temperature for each section by precisely controlling the ratio of latent heat to sensible heat in the evaporator under the condition of changing feed rate (capacity), so as to adapt to the heating temperature adjustment under different capacity requirements.

[0076] (2) The temperature control method for a scraped-film evaporator for cellulose dissolution of the present invention firstly ensures that the evaporation process is in the optimal energy efficiency state under any production load; secondly, by finely adjusting the heat ratio in segments, the evaporator can always maintain high-efficiency heat transfer performance; and finally, while improving energy utilization efficiency, it can also ensure the stability of product quality. This control method based on thermodynamic principles fundamentally overcomes the lag of the gradual temperature adjustment method and significantly improves the accuracy and response speed of production control.

[0077] (3) The temperature control method of the scraped film evaporator for cellulose dissolution of the present invention improves evaporation efficiency and product quality by precisely controlling the ratio of latent heat transfer to sensible heat transfer, and also achieves energy efficiency optimization and environmental protection goals. The application of this method will bring new breakthroughs to the development of scraped film evaporator technology and provide more efficient, energy-saving and environmentally friendly temperature control solutions for related industries. Attached Figure Description

[0078] Figure 1 A schematic diagram of the heating process for cellulose dissolution in a thin-film evaporator;

[0079] Figure 2 This is a diagram illustrating the heat transfer process in a scraped film evaporator.

[0080] Figure 3 To verify the comparison between the calculated heating temperature and the measured heating temperature in Case 1;

[0081] Figure 4 To verify the comparison between the calculated heating temperature and the actual heating temperature in Case 2. Detailed Implementation

[0082] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0083] A temperature control method for a scraped-film evaporator for cellulose dissolution, using cellulose pulp as material, the cellulose pulp being composed of cellulose, NMMO, and water, includes the following steps:

[0084] (1) Divide the scraped film section in the scraped film evaporator into four equal sections from top to bottom, which are respectively called the first scraped film section, the second scraped film section, the third scraped film section and the fourth scraped film section. The structural parameters of the scraped film evaporator are obtained by using the four-section separate heating method, including: the characteristic length of each section in the scraped film section. The inner cylinder diameter D of the scraping section, and the feed inlet diameter d;

[0085] (2) Obtain the process parameters of each section in the scraping film section, including: the specific heat capacity of the material in the first to fourth scraping film sections, respectively. , , , The inlet material temperatures of the first to fourth scraping sections are respectively , , , The outlet material temperatures of the first to fourth scraping sections are respectively , , , ;

[0086] in, and It was obtained through testing. and They are based on and Estimated through linear fitting; , , , , , , and All data were obtained online using temperature sensors installed on the scraped film evaporator.

[0087] (3) Obtain the condensate temperature at the outlet of the heating jacket of each section of the scraped film evaporator. , inner cylinder wall surface temperature Inner cylinder outer wall surface temperature Then, the average heat flux density q on the outer wall of the inner cylinder can be calculated using the following formula:

[0088] ;

[0089] in, It is detected by a temperature sensor installed at the condensate drain outlet. It is detected by a temperature sensor embedded in the inner cylinder; The coefficient of heat transfer between the condensate and the inner wall of the cylinder is given. The calculation formula is:

[0090] ;

[0091] Where h is the heat transfer coefficient between the condensate and the outer wall of the inner cylinder. The thermal conductivity of the inner cylinder is... Indicates the thickness of the inner cylinder wall;

[0092] The formula for calculating h is:

[0093] ;

[0094] in, It is the acceleration due to gravity. The density of condensate. The thermal conductivity of condensate is For latent heat of phase transition, This refers to the viscosity of the condensate.

[0095] (4) Based on the parameters obtained in steps (1) to (3) and the feed rate, calculate the latent heat ratio of each section in the scraping section;

[0096] The latent heat percentages of the first to fourth scraping sections are as follows:

[0097] First scraping section:

[0098] ;

[0099] Second scraping section:

[0100] ;

[0101] Third scraping section:

[0102] ;

[0103] Fourth scraping section:

[0104] ;

[0105] Wherein, R1, R2, R3, and R4 are the latent heat percentages of the first to fourth scraping sections, respectively. , , and These represent the sensible heat transfer rates of the first to fourth scraped film sections, respectively. , , and q1, q2, q3 and q4 are the latent heat transfer rates of the first to fourth scraped film sections, respectively; G is the feed rate; and q1, q2, q3 and q4 are the average heat flux densities of the outer wall surface of the inner cylinder of the first to fourth scraped film sections, respectively.

[0106] (5) After changing the feed rate, based on the latent heat ratio of the first to fourth scraping sections obtained in step (4), then according to the formula Calculate the average heat flux density q on the outer wall surface of each section of the inner cylinder, and finally use the formula... Calculate the condensate temperature at the outlet of each section of the heating jacket;

[0107] in, Total calories, For sensible heat transfer, For latent heat exchange, The average heat flux density of the outer wall surface of the inner cylinder. The total heat transfer coefficient k is the heat transfer coefficient between the condensate and the inner wall of the cylinder. The total heat transfer coefficient k is only slightly affected by the change in the feed rate, so this effect is negligible here.

[0108] (6) Based on the condensate temperature of the heating jacket outlet of each section in the scraping film section calculated in step (5), the required saturated vapor pressure in the heating jacket of each section is obtained by looking up the saturated vapor pressure table of water, and then the required steam temperature of each section is determined.

[0109] The following specific embodiments illustrate a method for temperature control of a scraped-film evaporator for cellulose dissolution according to the present invention, taking the production of 50,000 tons / year of Lyocell spinning solution as an example:

[0110] (1) Calculate the latent heat transfer ratio under an annual production capacity of 50,000 tons; the following are the process data:

[0111] Inlet components: cellulose / NMMO / water (11.7wt% / 67.19wt% / 21.11wt%); Outlet components: cellulose / NMMO / water (12.90wt% / 76.21wt% / 10.89wt%); Rotation speed: 90 rpm; Inlet feed temperature: 87℃; Vacuum degree: 4.5 kPa.

[0112] It is 30 W / (m·K); =0.02m; G = 13.32kg / s; D = 2.2m; L = 2.5m; d = 0.2m.

[0113] The data in Table 1 below are from: Heat Transfer, 5th Edition, edited by Tao Wenquan, Appendix 9 and Appendix 10.

[0114] Table 1: Physical properties of condensate in the heating jacket at each stage

[0115]

[0116] Table 2: Physical property parameters of slurry at each stage

[0117] stage <![CDATA[Density / kg / m 3 > Specific heat capacity / J / (kg·℃) First paragraph 1153 2500 Second paragraph 1160 2600 Third paragraph 1168 2700 Fourth paragraph 1175 2800

[0118] Table 3: Inlet and Outlet Material Temperatures

[0119]

[0120] Table 4: Parameters of a scraped film evaporator with an annual production capacity of 50,000 tons

[0121]

[0122] Based on the data in Tables 1 to 4, the latent heat percentage for each stage was calculated using the formulas for calculating the latent heat percentage for each stage. The results are shown in Table 5.

[0123] Table 5: Proportion of Latent Heat Share in Each Stage

[0124] stage Latent heat percentage First paragraph 98.013 Second paragraph 97.802 Third paragraph 93.201 Fourth paragraph 92.912

[0125] (2) Verification of Case 1;

[0126] like Figure 3 As shown, under the same equipment conditions, with the feed temperature, feed composition, rotation speed, and vacuum degree remaining constant, the heating temperature for an annual production capacity of 28,000 tons is estimated using the latent heat ratio in (1), and compared with the actual heating temperature. Table 6 shows the measurement and calculation data in the actual production process, in which... and For measurement data, k is calculated data, and q is a value derived from the latent heat ratio in (1).

[0127] At this capacity, the imported components are: cellulose / NMMO / water (11.7wt% / 67.19wt% / 21.11wt%); the exported components are: cellulose / NMMO / water (12.90wt% / 76.21wt% / 10.89wt%); the rotation speed is 67 rpm; the inlet feed temperature is 87℃; and the vacuum degree is 4.5 kPa. It is 30 W / (m·K); =0.02m; G = 7.46kg / s; D = 2.2m; L = 2.5m; d = 0.2m.

[0128] Table 6: Parameters of the scraped film evaporator with an annual capacity of 28,000 tons

[0129]

[0130] Table 7: Calculation Results of Heating Temperature

[0131] stage Measured heating temperature (°C) Calculated heating temperature (°C) deviation Phase 1 102.2 108.1 5.8% Phase Two 125.1 121.7 -2.7% Phase Three 122.4 118.0 -3.6% Phase 4 133.3 132.2 -0.8%

[0132] (3) Verify Case 2;

[0133] like Figure 4 As shown, under the same equipment conditions, keeping the feed temperature, feed composition, rotation speed, and vacuum level constant, the heating temperature for an annual production capacity of 36,000 tons was calculated using this method and compared with the actual heating temperature. Table 8 shows the measurement and calculation data from the actual production process. and For measurement data, k is calculated data, and q is a value derived from the latent heat ratio in (1).

[0134] At this capacity, the imported components are: cellulose / NMMO / water (11.7wt% / 67.19wt% / 21.11wt%); the exported components are: cellulose / NMMO / water (12.90wt% / 76.21wt% / 10.89wt%); the rotation speed is 73 rpm; the inlet feed temperature is 87℃; and the vacuum degree is 4.5 kPa. It is 30 W / (m·K); =0.02m; G = 9.59kg / s; D = 2.2m; L = 2.5m; d = 0.2m.

[0135] Table 8: Parameters of the scraped film evaporator with an annual capacity of 36,000 tons

[0136]

[0137] Table 9: Calculation Results of Heating Temperature

[0138] stage Measured heating temperature (°C) Calculated heating temperature (°C) deviation Phase 1 110.2 114.7 4.1% Phase Two 124.5 130.1 4.5% Phase Three 132.4 121.1 -8.5% Phase 4 130.2 135.8 4.3%

[0139] The verification results show that the predicted heating temperatures at each stage are within 10% of the actual measured temperatures, demonstrating good accuracy in predicting the required heating temperatures when changing production capacity. The reason for this error is that when adjusting the feed rate to control production capacity, the flow field morphology within the scraped film evaporator also changes accordingly. For example, the liquid film thickness on the wall decreases with a decrease in feed rate, and the average residence time decreases with an increase in feed rate. An increase in liquid film thickness means that heat needs to pass through a thicker layer of liquid to be transferred to the flowing liquid, which increases thermal resistance. As thermal resistance increases, heat transfer efficiency decreases. Simultaneously, an increase in liquid film thickness leads to a steeper temperature gradient within the liquid film, meaning the temperature change becomes more concentrated. These two parameters determine the rate and time of heat absorption by the material, and are the main reasons for the discrepancy between the calculated and actual heating temperatures.

[0140] In actual production, this method can be used to predict the heating temperature and make fine adjustments to achieve the desired production efficiency and product quality of the thin-film evaporator, while saving on start-up and commissioning costs. The method is summarized as follows: Determine the latent heat percentage for each stage. Given the feed rate, inlet and outlet material temperatures, and model dimensions, the heat flux density q can be determined using the latent heat percentage formula, and thus the heating temperature can be determined.

[0141] Simultaneously, the calculated predicted heating temperature can be combined with simulation software to simulate and verify the predicted heating temperature under different production capacities without actually running the equipment. This allows operators to have a more accurate expectation before actual adjustments, reducing trial and error and improving adjustment efficiency, moving from the traditional step-by-step temperature testing method towards intelligent control. Furthermore, by integrating sensors and automated control systems, the temperature control algorithm of this invention can be incorporated into the system while monitoring the evaporator's operating status in real time, including key parameters such as temperature, pressure, and flow rate. This allows for rapid response to changes in production capacity, significantly reducing manual intervention and improving the accuracy and speed of adjustments.

Claims

1. A temperature control method for a wiped film evaporator used for cellulose dissolution, characterized by The method comprises the following steps: (1) The wiped film evaporator is divided into four sections from top to bottom, respectively recorded as the first wiped film section, the second wiped film section, the third wiped film section and the fourth wiped film section, and the structure parameters of the wiped film evaporator are obtained by using four-section split heating, including: the characteristic length of each section in the wiped film section , the inner cylinder diameter D of the wiped film section, and the diameter d of the feed inlet; (2) obtaining the process parameters of each section of the wiped film section, including: specific heat capacity of the material in each section, inlet material temperature of each section, outlet material temperature of each section; (3) Obtain the condensate water temperature at the outlet of the heating jacket of each section of the wiped film evaporator , the inner wall surface temperature of the inner cylinder , and the outer wall surface temperature of the inner cylinder , and then calculate the average heat flux density q of the outer wall surface of the inner cylinder; (4) calculating the latent heat proportion of each section in the wiped film section according to the parameters obtained in steps (1)-(3) and the feed amount; (5) after changing the feed amount, the total heat required by each section is calculated based on the latent heat proportion of each section in the wiped film section obtained in step (4), and then the heating jacket outlet condensate temperature of each section is calculated; (6) according to the heating jacket outlet condensate temperature of each section in the wiped film section calculated in step (5), the saturated steam pressure required in each section is obtained by referring to the water saturated steam pressure table, and then the steam temperature required to be introduced into each section is determined.

2. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 1, characterized in that, The specific heat capacity of the material in the first to fourth film scraping sections in step (2) is respectively 、 、 、 , unit: J / (kg·℃); the inlet material temperature is respectively 、 、 、 , unit: ℃; the outlet material temperature is respectively 、 、 、 , unit: ℃; With are obtained by detection, With are based on With are obtained by linear fitting estimation; , , , , , , and are obtained by online detection with temperature sensors installed on the wiped film evaporator.

3. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 2, characterized in that, In step (3) is detected by a temperature sensor installed at the condensate water discharge outlet, is detected by a temperature sensor embedded in the inner cylinder, in units of °C.

4. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 3, characterized in that, In step (3) according to With The formula for calculating q is: （a）； wherein, is the average heat flux density of the outer wall surface of the inner cylinder, in W / m2; is the total heat transfer coefficient between the condensate and the inner wall surface of the inner cylinder, in W / (m 2 ·K), The calculation formula is: （b）； wherein h is a heat transfer coefficient between the condensed water and the outer wall surface of the inner cylinder, in units of W / (m 2 ·K); is a thermal conductivity of the inner cylinder, in units of W / (m·K); denotes a wall thickness of the inner cylinder, in units of m; The calculation formula of h is: （c）； wherein g is the gravitational acceleration, in m / s 2 ; p is the density of the condensate, in kg / m 3 ; k is the thermal conductivity of the condensate, in W / (m-K); r is the latent heat of phase change, in kJ / kg; and h is the viscosity of the condensate, in Pa-s; L is the characteristic length of each section of the wiped film section, in m.

5. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 4, characterized in that, The latent heat proportions of the first to fourth wiped film sections in step (4) are as follows: First wiped film section: （d）； Second wiped film section: （e）； Third wiped film section: （f）； Fourth wiped film section: （g）； Wherein, R1, R2, R3 and R4 are latent heat proportions of the first to fourth falling film sections, unit %; , , and are sensible heat exchange amounts of the first to fourth falling film sections, unit W; , , and are latent heat exchange amounts of the first to fourth falling film sections, unit W; G is the feed amount, unit kg / s; q1, q2, q3 and q4 are average heat flux densities of the outer wall surfaces of the inner cylinders in the first to fourth falling film sections, unit W / m 2 .

6. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 5, characterized in that, Step (5) is specifically: first, the total heat required by each section is calculated based on the latent heat proportion of the first to fourth wiped film sections obtained in step (4), then the average heat flux density of the outer wall surface of the inner cylinder in each section is calculated according to formula (h), and finally the heating jacket outlet condensate temperature of each section is calculated according to formula (a); （h）； wherein, Qtotal is the total heat exchange, in W; Qsensible is the sensible heat exchange, in W; Qlatent is the latent heat exchange, in W.

7. A temperature control method for a wiped film evaporator for cellulose dissolution according to claim 6, characterized in that, The material is cellulose pulp, which is composed of cellulose, NMMO and water.

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