CN114410331B - A low-cost method for preparing spinnable mesophase pitch - Google Patents

Abstract

The present invention relates to a low-cost method for preparing spinnable mesophase pitch. Aromatic-rich heavy oil is used as feedstock oil, after preheating fresh feedstock oil and nitrogen are mixed, enter from the upper end of the variable temperature and variable diameter tubular reactor and quickly pass through the reactor to obtain a pitch-like intermediate product with a softening point of 40-160°C ; Then enter the hot low fraction tank, separate nitrogen and some light components from the upper part of the hot low fraction tank, the bottom product enters the wiped film evaporator to remove and recover the light components, and the heavy components obtained at the bottom of the evaporator enter to the mesophase storage tank, resulting in a high-quality spinnable mesophase pitch. After mixing and preheating the nitrogen gas recovered by the hot low fraction tank and the wiped film evaporator and the fractions of ≥ 200 °C in the light components, it enters the reactor from the variable diameter in the middle of the reactor for circulating reaction, so as to improve the raw material. Conversion of oil to produce mesophase pitch. The invention can produce high-quality mesophase pitch at low cost and on a large scale through a continuous fast and cyclic reaction mode and production system.

Description

A low-cost method for preparing spinnable mesophase pitch

technical field

The invention relates to a method for producing high-quality spinnable mesophase pitch at low cost and on a large scale by using an aromatic-rich heavy oil as a raw material through a continuous fast and cyclic reaction mode and a production system, and belongs to the technical field of high-quality carbon fiber preparation of advanced carbon materials.

Background technique

The preparation process of mesophase pitch-based carbon fiber mainly includes, using aromatic-rich heavy oil as raw material, through polycondensation to form mesophase pitch precursor, and then through a series of processes such as melt spinning, pre-oxidation, carbonization/graphitization, etc. to become high-performance mesophase pitch base carbon fiber. Compared with the properties of polyacrylonitrile-based carbon fibers, the most prominent properties of mesophase pitch-based carbon fibers are ultra-high modulus, ultra-high thermal conductivity and electrical conductivity, low thermal expansion coefficient (can achieve a negative thermal expansion coefficient), etc. It is suitable for important parts such as satellite antennas, rocket nozzles, aircraft high-rigidity panels, aircraft electromagnetic shielding protection systems, etc. It can also be used in robot arms, large industrial rollers, pressure vessels, electronic product thermal management, LED thermal management, etc. It is the development of high-end equipment An important strategic material for the manufacturing industry, at present, mesophase pitch-based carbon fiber is still a mid-to-high-end product. However, since the manufacture of mesophase pitch-based carbon fibers uses cheap heavy aromatics as raw materials, mesophase pitch-based carbon fibers have great potential for cost reduction. The large-scale and low-cost preparation of mesophase pitch will greatly promote the wide application of pitch-based carbon fibers in rail transit, automotive lightweight, IT manufacturing robots, building reinforcement, sports, civil and other fields.

The traditional preparation of mesophase pitch adopts a high-pressure reaction kettle, which requires a series of operations such as heating up - constant temperature - cooling - out of the kettle during the production process, and the batch production efficiency is very low. Although some researchers have prepared multiple reactors in series in multiple stages, the whole process is a batch production process, which not only takes a long time to produce, but also has high labor intensity, low output rate, high production cost and poor batch-to-batch stability. In addition, the reaction time of traditional preparation of mesophase pitch is mostly 6-18h, or even longer, which also greatly affects the large-scale production capacity. At the same time, traditional processes such as high-pressure polymerization and vacuum distillation are used to remove non-mesophase products (often light aromatic hydrocarbons that can be further converted into mesophase molecules) in the product, and the yield of the obtained spinnable mesophase pitch is mostly ≤40%, This results in a low conversion rate of raw material utilization. Chinese patent CN207996650U utilizes a tubular reactor and two parallel vacuum reactors to replace the traditional high-pressure-vacuum reactor device, and the temperature of the reactor device is relatively low 450°C. Although the patent specification does not give the reaction time, it can be reasonably The predicted reaction time is still several hours, which also greatly affects the production efficiency. And it does not involve the process flow of further cyclic conversion of light components. Chinese patent CN113773871A discloses a continuous cyclic polymerization and distillation reaction system for preparing mesophase pitch. It adopts the pipeline continuous line production mode and the circulating flow reaction mode, and the reaction time is controlled by adjusting the number of cycles to produce different mesophase pitches. Especially in the devolatilization stage of vacuum distillation, the material is circulated repeatedly under vacuum to improve the devolatilization effect. However, the problem of low conversion utilization rate in the raw oil is also not solved. Therefore, in order to develop a low-cost and engineered method and reaction system for the preparation of spinnable mesophase pitch, it is necessary to develop the above-mentioned three limiting factors, such as the inability of continuous production of the above-mentioned reactor, the long reaction time, and the low utilization and conversion rate of feedstock oil. In order to enable large-scale engineering continuous preparation, reduce production cost, improve production efficiency, and produce high-performance, high-purity mesophase pitch for spinning.

SUMMARY OF THE INVENTION

The invention mainly solves the process of low-cost and large-scale production of high-quality mesophase pitch for spinning. To this end, the aromatic-rich heavy oil is used as the feedstock oil, and the preheated fresh feedstock oil is mixed with nitrogen, and then enters from the upper end of the variable-temperature and variable-diameter tubular reactor and quickly passes through the reactor to obtain asphalt with a softening point of 40-160°C. Then enter into the hot low fraction tank, separate and recover nitrogen gas and some light components, the bottom product of the tank enters the wiped film evaporator to further remove and recover the light components, and the heavy components obtained at the bottom of the evaporator enter to the mesophase storage tank, resulting in a high-quality spinnable mesophase pitch. The nitrogen and light components recovered from the hot low fraction tank and the wiped film evaporator are mixed and preheated, and then enter the reactor from the variable diameter in the middle of the reactor for cyclic reaction, so as to improve the conversion utilization rate of raw oil. The invention can produce high-quality mesophase pitch at low cost and on a large scale through a continuous fast and cyclic reaction mode and production system.

In order to achieve the above object, a low-cost method for preparing spinnable mesophase pitch, with catalytic cracking tar, FCC oil slurry clarified oil, reduced fourth-line fraction, deasphalted oil of naphthenic vacuum residue, and de-QI refining One or more of coal tar and anthracene oil are raw materials, which specifically include the following technological process:

(1) After preheating the fresh raw oil and nitrogen, the upper part of the variable temperature and variable diameter tubular reactor continuously enters the reactor, the fresh raw oil stays for 1min to 30min, and the bottom of the reactor obtains asphalt with a softening point of 40 to 160°C intermediate product;

(2) The bitumen-like intermediate product then enters into a hot low fraction tank with a constant temperature of 350 to 430°C and a pressure of 0.01 to 0.1 MPa. The nitrogen and light components are separated from the upper part of the hot low fraction tank, and the bottom product enters the scraper. in film evaporators;

(3) The constant temperature of the wiped film evaporator is 350~430℃ and the pressure is 0.1~5.0KPa. After the material enters the wiped film evaporator, the light components are efficiently and quickly removed and separated from the upper part of the evaporator. The heavy components enter the mesophase storage tank, so as to obtain a spinnable mesophase pitch product with a mesophase content of ≥90% and a softening point of 220-350°C; the residence time in the wiped film evaporator is 1-60s;

(4) After the nitrogen and light components separated from the upper part of the hot low fraction tank and the light components separated from the upper part of the wiped film evaporator are mixed, the fractions with a temperature of ≥ 200 °C are distilled and separated, and separated from gaseous hydrocarbons by membrane separation. Nitrogen is used to obtain the light components of refining, and then the ≥200°C fraction and nitrogen are mixed and preheated according to the mass ratio of 0.25 to 10, and enter the reactor from the variable diameter in the middle of the variable temperature variable diameter tubular reactor for circulating reaction. Improve the conversion rate of raw oil to produce mesophase pitch.

In the present invention, the temperature-variable and diameter-variable tubular reactor has a structure of upper and lower sections, the length ratio of the upper and lower sections is 1:1 to 2:1, the diameter of the upper section is larger than that of the lower section, and the diameter of the upper and lower sections is larger than that of the lower section. The ratio is at least 1.5:1, not more than 5:1, and the length-diameter ratio of the upper and lower sections is at least 50 respectively; as a specific solution, the inner diameter of the upper section of the reactor is 20-50mm, and the inner diameter of the lower section is 10-30mm; The total length of the reactor is 5-20m; the upper and lower sections are rigidly connected (welded) or integrally formed.

In the present invention, the temperature of the upper section of the temperature-variable and variable-diameter tubular reactor is set at 550 to 700° C., and the temperature of the lower section is set to be 500 to 550 ° C; ～30min, mixing and preheating the light components and nitrogen according to the mass ratio of 0.25～10, entering the reactor from the variable diameter of the variable temperature variable diameter tubular reactor for cyclic reaction, and staying for 5s～20min.

In the present invention, the wiped film evaporator adopts a vertical structure, which consists of an upper driving part and a lower evaporation and concentration part. The driving part consists of a motor and a reducer, and the reducer is a gear reducer or a pulley reducer. ; The evaporation and concentration part is composed of distributor, gas-liquid separator, evaporation cylinder, cylinder jacket and rotating scraper; the upper and lower shaft seals use double-sided mechanical seals to ensure the tightness of the equipment; the scraper is fixed type and rotor type. The preheated feed liquid is added along the tangential direction from the upper part of the wiped film evaporator, and distributed to the heating wall surface of the evaporation cylinder through the distributor. Due to the effect of gravity, centrifugal force and the scraping action of the rotating scraper, the material liquid forms a uniform downward spiral film on the inner wall of the cylinder in the evaporation section, and is adjusted by the heating medium such as molten salt or heat transfer oil in the jacket of the cylinder during this process. The temperature of the cylinder wall is maintained, and the material is kept at the set temperature through the heat transfer of the cylinder wall, the surface material evaporates rapidly, the concentrated solution is discharged at the bottom, and the evaporated secondary steam is recovered through the upper outlet to the external low-temperature sub-tank.

According to the difference in the reaction severity required for the formation of the mesophase structure between the raw oil molecules and the fractions with a temperature of ≥ 200 ° C, the present invention adopts a variable temperature and variable diameter tubular reactor to carry out a staged reaction, that is, the molecular size of the fresh raw oil is smaller. Small, more and longer side chains, which are quite different from the molecular structure of the mesophase, and require harsh reaction conditions for conversion; while the light component molecules of the refining are obtained through thermal conversion reactions, so the side chains are shorter and the molecules are shorter. The size is also closer to that of the mesophase molecules, so relatively mild reaction conditions should be used to promote their conversion into mesophase molecules, while avoiding the “excessive polycondensation” caused by the sharp rise in the softening point of the mesophase pitch, or even impossible. melt spinning. On the other hand, the conversion of feedstock oil molecules into planar polycyclic aromatic hydrocarbon macromolecules requires a thermal cracking reaction with side chain scission to induce free radicals, followed by a condensation reaction to increase the size of the aromatic ring.

Based on this, in step (1) of the present invention, by adopting a variable temperature and variable diameter tubular reactor, it is realized that both the above-mentioned fresh raw materials and "refined" raw materials can be converted under suitable thermal polycondensation reaction conditions, thereby improving the raw material conversion rate and ensuring Quality of mesophase pitch. Specifically, the temperature in the upper part of the reactor is higher than the conventional temperature range (400-500°C) for preparing mesophase pitch, which is 550-700°C, which not only promotes the cracking reaction but also speeds up the reaction rate, laying a foundation for improving production efficiency. ; The inner diameter of the upper pipe diameter of the reactor is 20-50mm. This diameter range not only ensures the processing capacity of raw materials, but also ensures that the reaction process has a good effect of mass transfer and heat transfer. The lower part of the reactor adopts a relatively moderate reaction temperature of 500-550 °C, but it is still higher than the conventional temperature for preparing mesophase pitch. The lower reaction temperature is conducive to the further condensation reaction of thermal cracking product molecules and promotes the formation of fused ring aromatic hydrocarbon planar molecules; The inner diameter of the lower pipe diameter is also smaller than that of the upper pipe diameter, which is 10-30mm. When the "refining" components and the products of the upper part of the reactor are in smaller pipe diameters, the faster the material flow rate, the greater the degree of material turbulence, the shorter the residence time and the longer the transmission. The better the quality effect, the further conversion to the mesophase molecule can be promoted, and the excessive carbonization reaction can be effectively avoided, especially the occurrence of coking on the tube wall. The total length of the reactor can be designed to be 5~20m, the length of the larger part of the upper pipe diameter accounts for 1/2~2/3 of the total length, the fresh material stays for 1min~30min, and the fraction of light components ≥200℃ stays for 5s ~20min, the reaction time is greatly reduced compared with the conventional process reaction time (several hours to tens of hours). In addition, the thermal polycondensation reaction decoupling of different raw materials is realized in the variable temperature and variable diameter reactor, thereby ensuring the continuous production of spinnable mesophase pitch products, thus forming an organic integral system with the subsequent recycling process of light components. In general, the significant reduction in reaction time, the improvement in the utilization rate of raw materials, and the overall compatibility of the reaction process ensure that high-quality mesophase pitch can be prepared at low cost and high production efficiency.

Steps (2) and (3) of the present invention are continuous two-stage distillation recovery and separation operations. The temperature of the hot low fraction tank used in the step (2) is 350-430° C. and the pressure is 0.01-0.1 MPa. After preliminary separation of nitrogen and some light components, the material can be effectively prevented from being under the high vacuum condition of the wiped film evaporator. Vapor entrainment occurs due to substantial volatilization. In step (3), the material enters the wiped film evaporator and is continuously and evenly distributed to the vertical heating surface of the cylinder through the distributor. While the material falls by gravity, it is forced to form a very thin turbulent flow on the heating surface by the rotating film scraping device. The vapor of the light components evaporated by heating enters the external condenser for condensation from the upper outlet, while the heavy components that are not evaporated are discharged from the bottom of the wiped film evaporator. The whole process time is usually several seconds to tens of seconds, which can effectively prevent the material from being decomposed by heating for a long time, which will affect the product quality and improve the evaporation efficiency.

Compared with the prior art, the beneficial effects of the present invention are at least:

(1) The present invention greatly improves the production efficiency and reduces the reaction time by developing a rapid polycondensation reaction mode in a high-temperature tubular reaction, and cooperating with an efficient and rapid light component removal operation;

(2) The technical strategy adopted in the present invention does not have strict requirements on the properties of raw materials. By using variable temperature and variable diameter tubular reactors to couple light components "refining", while ensuring the preparation of high-quality spinnable mesophase pitch, it overcomes the problem of raw materials. The problem of low conversion utilization.

(3) The present invention can produce high-quality mesophase pitch at low cost and on a large scale through the continuous fast and cyclic reaction mode and production system.

Description of drawings

Fig. 1 is a process flow diagram of a low-cost method for preparing spinnable mesophase pitch according to the present invention, wherein: 1- preheating furnace; 2- variable temperature and variable diameter tubular reactor; 3- thermal low fraction tank; 4 - wiped film evaporator; 5-cold low sub-tank; 6-cold low sub-tank; 7-membrane separator; 8-distillation column; 9- mesophase pitch product storage tank.

Detailed ways

The following describes a low-cost method for preparing spinnable mesophase pitch provided by the present invention in conjunction with the examples.

Example 1:

Taking catalytic cracking tar as raw material, the vertical variable temperature and variable diameter tubular reactor used has a total length of 5m, and consists of an upper section with a tube diameter (inner diameter) of 20mm and a tube length of 2.5m, and a tube diameter (inner diameter) of 10mm and a tube length of 2.5m. The lower section consists of two parts, wherein the temperature of the upper section is set to 550 °C, and the temperature of the lower section is set to 500 °C;

The present embodiment provides a low-cost method for preparing spinnable mesophase pitch, including the following specific process flow:

The catalytic cracking tar and nitrogen are mixed and preheated to 300 ℃ in a mass ratio of 9.0, and rapidly heated to 550 ℃ near the inlet of the reactor, and then enter into the variable temperature variable diameter tubular reactor from the upper part of the reactor for the reaction, in the reactor The total residence time is 30min; in addition, the nitrogen and light components separated in the subsequent process (mainly from the hot low fraction tank and the upper part of the wiped film evaporator) are subjected to distillation and membrane separation operations to obtain ≥200 °C fraction and nitrogen, And preheated to 320 ℃ according to the mass ratio of the two at 9.0, and rapidly heated to 500 ℃ near the variable diameter inlet in the middle of the reactor, then entered the reactor and stayed for 10 minutes; the bottom of the reactor got a softening point of 40 ℃ The pitch-like intermediate product;

The bitumen-like intermediate product immediately enters into the hot low fraction tank with a constant temperature of 350°C and a pressure of 0.01 MPa. The nitrogen gas and some light components are separated from the upper part of the hot low fraction tank, and the bottom product enters the wiped film evaporator;

The constant temperature of the wiped film evaporator is set to 350°C, the pressure is 0.1KPa, and the residence time is 5s. The mesophase asphalt product flows into the product storage tank at the bottom of the wiped film reactor, so as to obtain a mesophase content of 90% and a softening point of 250 ℃ spinnable mesophase pitch products, the conversion utilization rate of raw oil to produce mesophase pitch can reach 80%.

Example 2:

Using FCC slurry clarified oil as raw material, the vertical variable temperature and variable diameter tubular reactor used has a total length of 10m, and consists of an upper section with a tube diameter (inner diameter) of 30mm and a tube length of 7.0m, and a tube diameter (inner diameter) of 15mm and a tube length of 7.0m. The lower section of 3m consists of two parts, wherein the temperature of the upper section is set to 600°C, and the temperature of the lower section is set to 520°C.

The present embodiment provides a low-cost method for preparing spinnable mesophase pitch, including the following specific process flow:

The catalytic tar and nitrogen are mixed and preheated to 300°C in a mass ratio of 7, and rapidly heated to 600°C near the inlet of the reactor, and then enter the reactor from the upper part of the reactor for reaction, and the total residence time in the reactor is 10min; In addition, the nitrogen and light components separated in the subsequent process (mainly from the hot low fraction tank and the upper part of the wiped film evaporator) are subjected to distillation and membrane separation operations to obtain ≥200 °C fraction and nitrogen, and the mass ratio of the two is carried out. Mixing and preheating to 320 °C for 7.0, and rapidly heating to 520 °C near the variable diameter inlet in the middle of the reactor, then entering the reactor and staying for 5 minutes; the asphalt-like intermediate product with a softening point of 70 °C can be obtained at the bottom of the reactor ;

The bitumen-like intermediate product immediately enters into the hot low fraction tank with a constant temperature of 380°C and a pressure of 0.05MPa, and the nitrogen gas and some light components are separated from the upper part of the hot low fraction tank, and the bottom product enters the wiped film evaporator;

The constant temperature of the wiped film evaporator is set to 380°C, the pressure is 0.5KPa, and the residence time is 7s. The mesophase asphalt product flows into the product storage tank at the bottom of the wiped film reactor to obtain a mesophase content of 95% and a softening point of 240 ℃ spinnable mesophase pitch product, the conversion utilization rate of raw oil to produce mesophase pitch can reach 75%.

Example 3:

Taking the minus four-line fraction as the raw material, the vertical variable temperature and variable diameter tubular reactor used has a total length of 13m, and consists of an upper section with a pipe diameter (inner diameter) of 35mm and a pipe length of 8m, and a pipe diameter (inner diameter) of 17mm and a pipe length of 5m. The lower section consists of two parts, wherein the temperature of the upper section is set to 630°C, and the temperature of the lower section is set to 530°C.

The present embodiment provides a low-cost method for preparing spinnable mesophase pitch, including the following specific process flow:

The catalytic tar and nitrogen were mixed and preheated to 300°C in a mass ratio of 2.0, and rapidly heated to 630°C near the reactor inlet, and then entered the reactor from the upper part of the reactor for reaction, and the total residence time in the reactor was 5min; In addition, the nitrogen and light components separated in the subsequent process (mainly from the hot low fraction tank and the upper part of the wiped film evaporator) are subjected to distillation and membrane separation operations to obtain ≥200 °C fraction and nitrogen, and the mass ratio of the two is carried out. Mixing and preheating to 320°C for 2.0, and rapidly heating to 530°C near the variable diameter inlet in the middle of the reactor, then entering the reactor and staying for 2min; the pitch-like intermediate product with a softening point of 120°C can be obtained at the bottom of the reactor ;

The bitumen-like intermediate product then enters into the hot low fraction tank with a constant temperature of 400°C and a pressure of 0.06MPa, and the nitrogen gas and some light components are separated from the upper part of the hot low fraction tank, and the bottom product enters the wiped film evaporator;

The constant temperature of the wiped film evaporator is set to 400°C, the pressure is 0.6KPa, and the residence time is 8s. The mesophase asphalt product flows into the product storage tank at the bottom of the wiped film reactor, so as to obtain a mesophase content of 100% and a softening point of 260 ℃ spinnable mesophase pitch products, the conversion utilization rate of raw oil to produce mesophase pitch can reach 72%.

Example 4:

Using anthracene oil as raw material, the vertical variable temperature and variable diameter tubular reactor used has a total length of 20m, consisting of an upper section with a tube diameter (inner diameter) of 45mm and a tube length of 13m, and a lower section with a tube diameter (inner diameter) of 25mm and a tube length of 7m. Partial composition, wherein the temperature of the upper section is set to 680°C, and the temperature of the lower section is set to 540°C.

The present embodiment provides a low-cost method for preparing spinnable mesophase pitch, including the following specific process flow:

The catalytic tar and nitrogen are mixed and preheated to 300°C at a mass ratio of 0.25, and rapidly heated to 680°C near the reactor inlet, and then enter the reactor from the upper part of the reactor for reaction, and the total residence time in the reactor is 1min; In addition, the nitrogen and light components separated in the subsequent process (mainly from the hot low fraction tank and the upper part of the wiped film evaporator) are subjected to distillation and membrane separation operations to obtain ≥200 °C fraction and nitrogen, and the mass ratio of the two is carried out. Mixing and preheating to 320°C for 0.25°C, and rapidly heating to 540°C near the variable diameter inlet in the middle of the reactor, then entering the reactor and staying for 5s; the pitch-like intermediate product with a softening point of 160°C can be obtained at the bottom of the reactor ;

The bitumen-like intermediate product immediately enters into the hot low fraction tank with a constant temperature of 430°C and a pressure of 0.1 MPa. The nitrogen gas and some light components are separated from the upper part of the hot low fraction tank, and the bottom product enters the wiped film evaporator;

The constant temperature of the wiped film evaporator is set to 430°C, the pressure is 5.0KPa, and the residence time is 3s. The mesophase asphalt product flows into the product storage tank at the bottom of the wiped film reactor to obtain a mesophase content of 100% and a softening point of 280 ℃ spinnable mesophase pitch products, the conversion utilization rate of raw oil to produce mesophase pitch can reach 70%.

Claims (8)

1. a low-cost method for preparing spinnable mesophase pitch, with catalytic cracking tar, FCC oil slurry clarified oil, minus four-line fraction, deasphalted oil of naphthenic vacuum residue, de-QI refined coal tar, One or more in the anthracene oil is raw material oil, it is characterized in that, comprises the following steps: (1) After preheating the fresh raw material oil and nitrogen, enter the reactor continuously from the upper part of the variable-temperature and variable-diameter tubular reactor, the fresh raw material oil stays and reacts for 1min-30min, and the bottom of the reactor obtains a pitch-like substance with a softening point of 40-160°C. Intermediate product; the temperature-variable and variable-diameter tubular reactor has a structure of upper and lower sections, the length ratio of the upper and lower sections is 1:1 to 2:1, the diameter of the upper section is larger than that of the lower section, and a temperature-variable and variable-diameter tubular reaction The temperature of the upper section of the device is 550~700℃, and the temperature of the lower section is 500~550℃; (2) The bituminous intermediate product then enters into the hot low fraction tank, separates nitrogen and light components from the top of the thermal low fraction tank, and the bottom product enters the wiped film evaporator; the hot low fraction tank is kept at a constant temperature of 350 ～430℃, pressure is 0.01～0.1MPa; (3) After the material enters the wiped film evaporator, the light components are efficiently and quickly removed and separated from the upper part of the wiped film evaporator. 90% spinnable mesophase pitch product with a softening point of 220-350°C; the wiped-film evaporator has a constant temperature of 350-430°C, a pressure of 0.1-5.0KPa, and the residence time of the feed liquid is 1-60s. 2. A kind of low-cost method for preparing spinnable mesophase pitch according to claim 1, characterized in that: nitrogen and light components separated from the upper part of the hot low fraction tank, and light components separated from the upper part of the wiped film evaporator After the fractions are mixed, the fractions of ≥200°C are separated by distillation, and the nitrogen is separated from the gaseous hydrocarbons by membrane separation. The variable diameter in the middle of the reactor enters the reactor for cyclic reaction, and stays for 5s to 20min to improve the conversion rate of the raw material oil to produce mesophase pitch. 3. A kind of low-cost method for preparing spinnable mesophase pitch according to claim 1 or 2, characterized in that: the upper temperature and the pipe diameter of the temperature-variable and variable-diameter tubular reactor are greater than the lower temperature and the pipe diameter, The pipe diameter ratio of the upper and lower sections shall be at least 1.5:1 and shall not exceed 5:1, and the length-diameter ratio of the upper and lower sections shall be at least 50 respectively. 4. A low-cost method for preparing spinnable mesophase pitch according to claim 3, characterized in that: the inner diameter of the upper section of the reactor is 20-50 mm, and the inner diameter of the lower section of the reactor is 10-30 mm; the total length of the reactor The length is 5-20m, and the upper and lower sections are rigidly connected or integrally formed. 5. The low-cost method for preparing spinnable mesophase pitch according to claim 1, wherein the wiped-film evaporator adopts a vertical structure and consists of a driving part at the upper part and an evaporation and concentration part at the lower part, The driving part consists of a motor and a reducer, and the evaporation and concentration part consists of a distributor, a gas-liquid separator, an evaporation cylinder, a cylinder jacket and a rotating scraper; the upper and lower shaft seals use double-sided mechanical seals to ensure the equipment of sealing. 6. A low-cost method for preparing spinnable mesophase pitch according to claim 5, characterized in that: the reducer is a gear reducer or a pulley reducer; the scraper adopts a fixed type and a rotor type. . 7. A low-cost method for preparing spinnable mesophase pitch according to claim 5, characterized in that: the preheated feed liquid is added along the tangential direction from the upper part of the wiped-film evaporator, and distributed to the evaporation cylinder through the distributor The heating wall surface of the body; due to the effect of gravity, centrifugal force and the scraping of the rotating scraper, the material liquid forms a uniform downward spiral film on the inner wall of the cylinder of the evaporation section, and passes through the molten salt or heat transfer oil in the jacket of the cylinder during this process. The temperature of the cylinder wall is adjusted by heating the medium, and the heat transfer through the cylinder wall keeps the material at the set temperature, the surface material evaporates rapidly, the concentrated solution is discharged at the bottom, and the evaporated secondary steam passes through the upper outlet to the external cold sub-tank. recycling. 8 . The low-cost method for preparing spinnable mesophase pitch according to claim 1 , wherein the conversion utilization rate of raw material oil to produce mesophase pitch reaches at least 70%. 9 .

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