A method for preparing difluoromethane

Abstract

The present invention discloses a method for preparing difluoromethane, comprising the following steps: (a) loading a catalyst; (b) fluorination reaction; (c) raw material circulation reaction; (d) impurity removal; and (e) collection. The present invention is easy to control, convenient for loading and replacing the catalyst, low in impurity content, ensuring the purity of difluoromethane, low in pollution, easy to separate products and continuous production.

Description

Technical Field

[0001] The invention belongs to the technical field of difluoromethane production, and in particular relates to a method for preparing difluoromethane. Background Art

[0002] Difluoromethane, molecular formula CH2F2, commonly known as HFC-32 or R-32, is an important component of mixed R410a and R407c and an excellent substitute for Freon.

[0003] At present, the main methods for synthesizing difluoromethane include hydrogenolysis dechlorination, liquid phase fluorination and gas phase fluorination. The catalyst used in the hydrogenolysis dechlorination method is the precious metal Pd, which has high cost, low conversion rate, and the catalyst is easily deactivated by carbonization, accompanied by loss; the production process of the liquid phase fluorination method is difficult to be continuous, and the catalyst and hydrofluoric acid cause serious corrosion to the equipment; the gas phase fluorination method is greatly affected by the performance of the catalyst and has a high impurity content, resulting in low purity of CH2F2; the catalyst in the above-mentioned synthesis methods is inconvenient to load and replace, and the waste catalyst is difficult to recycle and reuse, which will cause serious environmental pollution after discharge. Summary of the invention

[0004] The present invention aims to solve the above-mentioned technical problems existing in the prior art and provide a method for preparing difluoromethane, which is easy to control, convenient for catalyst loading and replacement, low in impurity content, high in difluoromethane purity, low in pollution, easy to separate products and continuous production.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A method for preparing difluoromethane, characterized in that it comprises the following steps:

[0007] (a) Catalyst loading:

[0008] ①Put the catalyst and additive into the stuffing tube, and then install the air inlet end on the open end of the stuffing tube;

[0009] ② Screw the packing tube onto the lower support plate to fix the packing tube onto the lower support plate, and then cover the air inlet end with the limiting hole on the upper pressing plate to limit the upper pressing plate onto the packing tube to obtain the reaction assembly;

[0010] ③Install the reaction assembly into the fluorination reactor until the lower support plate contacts the fixed ring in the cylinder, and the reaction assembly is supported by the fixed ring;

[0011] ④ Install the cylinder cover on the cylinder body until the pressure rod on the bottom of the cylinder cover presses the upper pressure plate, so that the reaction component is limited and fixed in the fluorination reactor;

[0012] ⑤Connect the pipelines between the reactors;

[0013] ⑥ Fluorination of the catalyst;

[0014] (b) Fluorination reaction:

[0015] ① Preheat hydrogen fluoride and dichloromethane and introduce them into the fluorination reactor, mix them in the upper mixing zone, enter the packing pipe through the air inlet end, and carry out fluorination reaction under the action of the catalyst;

[0016] ② The reacted mixture flows out through the micropores at the lower end of the packing tube and is mixed again in the lower mixing zone;

[0017] (c) Raw material circulation reaction: The mixed product is discharged from the bottom of the fluorination reactor and sent to the gas-liquid separation tower. After flowing through the spiral conduit, gas-liquid separation occurs. The gas phase mixture is discharged through the L-shaped air guide pipe, and the liquid phase mixture remains in the gas-liquid separation tower, is discharged from the bottom of the tower, and is sent back to the fluorination reactor to continue to participate in the reaction;

[0018] (d) Impurity removal:

[0019] ① The gas phase mixture discharged from the gas-liquid separation tower passes through the water washing tower and the alkali washing tower in sequence to remove HCl and HF;

[0020] ② The gas phase mixture discharged from the alkali washing tower enters the drying tower for dehydration;

[0021] (e) Collection: The dehydrated difluoromethane is compressed and liquefied by a compressor and then stored in a receiving tank.

[0022] Further, in step (c), the mixed product discharged from the bottom of the fluorination reactor is fed into the extraction tower, and dichloromethane is added to the liquid distributor from the liquid adding pipe as an extractant, sprayed down through the nozzle, and contacted with the mixed product in the packing layer for extraction, the temperature is controlled at -20°C to -10°C, and the pressure is controlled at 0.3-0.6Mpa, so as to ensure sufficient contact area and contact time between the mixed product and the extractant, thereby improving the extraction effect; after the extractant is added, the mixture is allowed to stand for 3-5 minutes, the solution is separated into layers, and the hydrogen fluoride layer is returned to the extraction tower. The mixed layer of difluoromethane and the extractant is sent to the fluorination reactor, and the mixed layer of difluoromethane and the extractant is sent to the distillation tower for pressure distillation. The separated difluoromethane is discharged from the top of the tower and enters the gas-liquid separation tower. The extractant layer is discharged from the bottom of the tower and sent back to the fluorination reactor to separate difluoromethane from dichloromethane, monochloromonofluoromethane and hydrogen fluoride, thereby improving the purity of difluoromethane. Dichloromethane, monochloromonofluoromethane and hydrogen fluoride are returned to the fluorination reactor as raw materials to continue to participate in the reaction, thereby improving the utilization rate and reducing the additional distillation operation of separating the extractant from the impurities.

[0023] Furthermore, in step (b), during the reaction process, oxygen is introduced into the fluorination reactor, and the molar ratio of oxygen to dichloromethane is 1%-2.5%. Oxygen can inhibit or prevent carbon accumulation on the catalyst surface, slow down catalyst deactivation, and extend the service life of the catalyst, thereby improving the space-time yield of the catalyst.

[0024] Furthermore, in step (c), a spiral conduit, an L-shaped air duct and an arc-shaped baffle are provided in the gas-liquid separation tower, the spiral conduit is wound around the outside of the L-shaped air duct, the arc-shaped baffle is located below the air inlet of the L-shaped air duct, the spiral conduit is communicated with the air inlet of the gas-liquid separation tower, and the L-shaped air duct is communicated with the air outlet of the gas-liquid separation tower. After the material flows through the spiral conduit, a high-speed spiral airflow is generated, and centrifugal force is used to separate the liquid phase and the gas phase, and then collides with the arc-shaped baffle to perform secondary gas-liquid separation. The separation efficiency is high and the effect is good, which solves the problems of low separation efficiency and incomplete separation in the prior art of gas-liquid separation using gravity.

[0025] Furthermore, in step (b), the reaction temperature is controlled at 230-280° C. Under the reaction temperature condition, the catalyst can have a higher initial activity, slow down its deactivation, and the conversion rate of CH2Cl2 and the selectivity of CH2F2 are also higher; the reaction pressure is controlled at 0.15-0.3Mpa. Under the reaction pressure condition, the contact time between the reaction materials and the catalyst can be increased, which is beneficial to improving the conversion rate and selectivity of the reaction, and the decomposition temperature of CH2Cl2 is increased, the decomposition of CH2Cl2 and the carbon deposition of the catalyst are inhibited, and the life of the catalyst is extended. The molar ratio of hydrogen fluoride to dichloromethane is 8:1-12:1, and the excess hydrogen fluoride is beneficial to the conversion of CH2Cl2 and the generation of CH2F2, and the deactivation rate of the catalyst is reduced, and its service life is extended.

[0026] Furthermore, the catalyst used in step (a) is Cr2O3, and the auxiliary agent used is one of CaO, MgO, Fe, and Al. After the alkaline earth metal is added, the catalyst surface becomes alkaline, which can prevent the hydrogen chloride reaction on the catalyst surface. The water generated during the reaction is greatly reduced, the corrosion to the packing tube and the fluorination reactor is reduced, and the service life is extended.

[0027] Furthermore, in step (a), the fluorination reactor includes a cylinder and a cylinder cover, the cylinder cover is arranged on the cylinder, a reaction assembly is arranged in the cylinder, the reaction assembly divides the space in the cylinder into an upper mixing zone, a reaction zone and a lower mixing zone, the reaction assembly includes an upper pressure plate, a filling tube and a lower support plate, the filling tube is arranged between the upper pressure plate and the lower support plate, one end of the filling tube is provided with an air inlet end, the other end of the filling tube is provided with a microhole, and a catalyst is installed in the filling tube.

[0028] Furthermore, in step (a), the cylinder cover is threadedly connected to the cylinder body, a pressure rod is provided on the bottom surface of the cylinder cover, and a fixing ring is provided in the cylinder body. The fixing ring cooperates with the pressure rod to clamp the reaction component, which, on the one hand, realizes a sealed connection between the cylinder body and the cylinder cover, and on the other hand, realizes the limited fixation of the reaction component, which cannot move freely, thereby ensuring the installation stability of the reaction component, fixing it in one step, and convenient operation.

[0029] Furthermore, in step (a), the lower support plate is provided with a mounting screw hole which is threadedly connected to the stuffing tube, and the upper pressure plate is provided with a limiting hole corresponding to the air inlet end, and the cross-sectional diameter of the limiting hole is smaller than the cross-sectional diameter of the stuffing tube. The reaction assembly adopts a split design, and the docking between the components is simple, the operation difficulty is low, and it can be installed and used at any time, which is flexible and convenient. Damaged parts can be replaced separately, thereby reducing maintenance costs.

[0030] Furthermore, in step (a), the air inlet end is screwed and fixed to the pipe mouth of the filling tube through a threaded joint, which is convenient for assembly and disassembly, can be used at any time, is flexible and convenient, and removing the air inlet end increases the operating space for loading the catalyst, and is also convenient for recovering the discarded catalyst for secondary utilization, reducing the difficulty of operation.

[0031] The present invention has the following beneficial effects due to the adoption of the above technical solution:

[0032] 1. The reaction component adopts a split design, with simple docking between components, low operation difficulty, ready for use, flexible and convenient, and damaged parts can be replaced separately, reducing maintenance costs; the packing tube and the air inlet end are detachable, which increases the operating space for loading catalysts, and is also convenient for recycling discarded catalysts for secondary use, avoiding environmental pollution caused by emissions, and reducing the difficulty of operation; the fluorination reactor adopts a split structure, which is convenient for internal cleaning and maintenance, and has a large operating space. The fixing ring in the cylinder body cooperates with the pressure rod on the bottom of the cylinder cover to clamp the reaction component, so that the reaction component is limited and fixed in the fluorination reactor, realizing the limited fixation of the reaction component, and cannot move freely, ensuring the installation stability of the reaction component, and at the same time limiting the upper support plate from detaching upward from the packing tube, improving the assembly stability of the reaction component, fixing in one step, and convenient operation.

[0033] 2. The mixed product enters the extraction tower, and dichloromethane is used as the extractant for extraction. The solution is divided into a hydrogen fluoride layer and a mixed layer of difluoromethane and the extractant. The hydrogen fluoride layer is sent back to the fluorination reactor to continue to participate in the reaction, and the mixed layer of difluoromethane and the extractant is sent to the distillation tower for pressure distillation. The separated difluoromethane is discharged from the top of the distillation tower and enters the gas-liquid separation tower. The extractant layer is discharged from the bottom of the tower and sent back to the fluorination reactor to separate difluoromethane from dichloromethane, monochloromonofluoromethane and hydrogen fluoride, thereby improving the purity of difluoromethane. Dichloromethane, monochloromonofluoromethane and hydrogen fluoride are returned to the fluorination reactor as raw materials to continue to participate in the reaction, thereby improving the utilization rate of raw materials, reducing the treatment amount of the above-mentioned three wastes, and reducing the additional cost of separating the extractant from impurities. Add distillation operation; in the gas-liquid separation tower, after difluoromethane flows through the spiral conduit, a high-speed spiral airflow is generated, and the liquid phase and the gas phase are separated by centrifugal force, and then collide with the arc baffle to perform secondary gas-liquid separation, with high separation efficiency and good effect, solving the problems of low separation efficiency and incomplete separation in the prior art of gas-liquid separation by gravity. The gas phase mixture is discharged through the L-shaped air guide pipe, and the liquid phase mixture remains in the gas-liquid separation tower, is discharged from the bottom of the tower, and is sent back to the fluorination reactor to continue to participate in the reaction, further reducing the content of dichloromethane, monochloromonofluoromethane and hydrogen fluoride in the crude product, thereby improving the purity of difluoromethane, and the separated liquid phase mixture continues to participate in the fluorination reaction, thereby improving the utilization rate of raw materials and reducing the treatment pressure of waste materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described below in conjunction with the accompanying drawings:

[0035] Figure 1 It is a structural schematic diagram of the reaction system in the present invention;

[0036] Figure 2 It is a structural schematic diagram of the fluorination reactor in the present invention;

[0037] Figure 3 It is a schematic diagram of the structure of the connection between the cylinder body and the cylinder cover in the present invention;

[0038] Figure 4 It is a structural schematic diagram of the connection between the upper pressing plate and the filling pipe in the present invention;

[0039] Figure 5 It is a schematic diagram of the structure of the reaction assembly in the present invention;

[0040] Figure 6 It is a schematic diagram of the internal structure of the cylinder in the present invention;

[0041] Figure 7 It is a schematic diagram of the structure of the connection of the water washing tower, the alkali washing tower and the drying tower in the present invention;

[0042] Figure 8It is a schematic structural diagram of the connection of the extraction tower, the distillation tower and the gas-liquid separation tower in the present invention.

[0043] In the figure: 1-cylinder; 2-cylinder cover; 3-pressure rod; 4-reaction assembly; 5-fixing ring; 6-upper mixing zone; 7-reaction zone; 8-lower mixing zone; 9-upper pressure plate; 10-filling pipe; 11-lower support plate; 12-installation screw hole; 13-inlet end; 14-threaded joint; 15-micropore; 16-limiting hole; 17-extraction tower; 18-liquid addition pipe; 19-liquid distributor; 20-filling layer ; 21-distillation tower; 22-gas-liquid separation tower; 23-spiral conduit; 24-L-type air duct; 25-arc baffle; 26-water washing tower; 27-alkali washing tower; 28-drying tower; 29-wire mesh demister; 30-drying layer; 31-support plate; 32-color-changing silica gel; 33-flow guide hole; 34-flow guide channel; 35-compressor; 36-receiving tank; 37-external threaded ring; 38-preheater. DETAILED DESCRIPTION

[0044] like Figures 1 to 8 The figure shows the reaction system used in the method for preparing difluoromethane in the present invention.

[0045] A method for preparing difluoromethane comprises the following steps:

[0046] (a) Catalyst loading:

[0047] The fluorination reactor comprises a cylinder 1 and a cylinder cover 2. The cylinder 1 is provided with an external threaded ring 37. The bottom of the cylinder cover 2 is provided with a threaded groove corresponding to the external threaded ring 37, so that the cylinder cover 2 is threadedly connected to the cylinder 1 to achieve a sealed connection between the cylinder 1 and the cylinder cover 2, with high reliability and convenient disassembly and assembly. A pressure rod 3 is provided on the bottom surface of the cylinder cover 2, and a reaction component 4 and a fixing ring 5 are provided in the cylinder body 1. The fixing ring 5 supports the reaction component 4. The reaction component 4 divides the space in the cylinder body 1 into an upper mixing zone 6, a reaction zone 7 and a lower mixing zone 8. The reaction component 4 includes an upper pressure plate 9, a stuffing tube 10 and a lower support plate 11. The stuffing tube 10 is arranged between the upper pressure plate 9 and the lower support plate 11. An air inlet terminal 13 is provided at the upper end of the stuffing tube 10. The air inlet terminal 13 is screwed and fixed to the pipe mouth of the stuffing tube 10 through a threaded joint 14. The air inlet terminal 13 is convenient to disassemble and assemble, can be used as needed, is flexible and convenient, and removing the air inlet terminal 13 increases the operating space for loading the catalyst, and is also convenient for recycling the discarded catalyst for secondary utilization, avoiding pollution to the environment caused by emissions, and reducing the difficulty of operation. The lower end of the stuffing tube 10 is provided with a micro hole 15, and the catalyst is loaded into the stuffing tube 10. The air inlet end 13 does not affect the entry of the material, and the micro hole 15 does not affect the outflow of the material. The micro hole 15 has a very small aperture, which can prevent the catalyst from leaking through the micro hole 15. The lower support plate 11 is provided with a mounting screw hole 12 that is screwed with the stuffing tube 10, and the upper pressure plate 9 is provided with a limit hole 16 corresponding to the air inlet end 13. The cross-sectional diameter of the limit hole 16 is smaller than the cross-sectional diameter of the stuffing tube 10. The reaction assembly 4 adopts a split design, and the docking between the components is simple, the operation difficulty is low, and it can be used as soon as it is installed, which is flexible and convenient. The damaged part can be replaced separately, which reduces the maintenance cost.

[0048] ① Put the catalyst and the auxiliary agent into the packing tube 10. The catalyst used is Cr2O3, and the auxiliary agent used is one of CaO, MgO, Fe, and Al. The molar ratio of Cr element to the metal element in the auxiliary agent is 1:0.1-0.5. After adding the alkaline earth metal, the catalyst surface is alkaline, which can prevent the hydrogen chloride reaction on the catalyst surface. The water generated during the reaction process is greatly reduced, which reduces the corrosion to the packing tube 10 and the fluorination reactor and prolongs the service life. Then, the air inlet end 13 is screwed to the upper open end of the packing tube 10 to prevent the catalyst and the auxiliary agent from escaping.

[0049] ② Screw the filling tube 10 onto the lower support plate 11 to fix the filling tube 10 on the lower support plate 11, and then cover the limiting hole 16 on the upper pressure plate 9 with the air inlet end 13 to limit the upper pressure plate 9 on the filling tube 10 to obtain the reaction component 4. The assembly is simple and the operation difficulty is low. It replaces the integrated structure and can be replaced separately if damaged. It can be used at any time after installation, which is flexible and convenient.

[0050] ③ Load the reaction assembly 4 into the fluorination reactor until the lower support plate 11 contacts the fixing ring 5 in the cylinder 1, and support the reaction assembly 4 through the fixing ring 5 to achieve rapid positioning of the reaction assembly 4 in the fluorination reactor, with low operation difficulty.

[0051] ④ Install the cylinder cover 2 on the cylinder body 1 until the pressure rod 3 on the bottom surface of the cylinder cover 2 presses the upper pressure plate 9, and the fixing ring 5 cooperates with the pressure rod 3 to clamp the reaction component 4, so that the reaction component 4 is limited and fixed in the fluorination reactor, so that the reaction component 4 is limited and fixed, and cannot move freely, thereby ensuring the installation stability of the reaction component 4, and at the same time limiting the upper support plate from detaching upward from the filling tube 10, thereby improving the assembly stability of the reaction component 4, fixing it in one step, and convenient operation.

[0052] ⑤Connect the pipelines between the reactors.

[0053] ⑥ Fluorination of the catalyst: hydrogen fluoride and nitrogen are introduced into the fluorination reactor, and the temperature is raised to 120-150°C to preheat and fluorinate the catalyst. When it is detected that the hydrogen fluoride content at the air inlet and the air outlet of the fluorination reactor is basically the same, the ventilation is stopped; nitrogen protects the catalyst to prevent the strong heat release during the fluorination process from causing catalyst sintering.

[0054] (b) Fluorination reaction:

[0055] ① Hydrogen fluoride and dichloromethane are preheated in a preheater 38 and introduced into a fluorination reactor, mixed in an upper mixing zone 6, and enter a packing pipe 10 through an air inlet end 13. A fluorination reaction is carried out under the action of a catalyst, and the raw materials are divided into multiple small streams, thereby increasing the contact area between the materials and improving the reaction rate. The reaction temperature is controlled at 230-280°C. Under the reaction temperature condition, the catalyst can have a higher initial activity, slow down its deactivation, and the conversion rate of CH2Cl2 and the selectivity of CH2F2 are also higher; the reaction pressure is controlled at 0.15-0.3Mpa. Under the reaction pressure condition, the contact time between the reaction materials and the catalyst can be increased, which is beneficial to improving the conversion rate and selectivity of the reaction, and the decomposition temperature of CH2Cl2 is increased, the decomposition of CH2Cl2 and the carbon deposition of the catalyst are inhibited, and the life of the catalyst is extended. The molar ratio of hydrogen fluoride to dichloromethane is 8:1-12:1, and the excess hydrogen fluoride is beneficial to the conversion of CH2Cl2 and the generation of CH2F2, and the deactivation rate of the catalyst is reduced, and its service life is extended.

[0056] During the reaction, oxygen is introduced into the fluorination reactor, and the molar ratio of oxygen to dichloromethane is 1%-2.5%. Oxygen can inhibit or prevent carbon accumulation on the catalyst surface, slow down catalyst deactivation, and extend the service life of the catalyst, thereby improving the space-time yield of the catalyst.

[0057] ② The reacted mixture flows out through the micropores 15 at the lower end of the filling tube 10 and is mixed again in the lower mixing zone 8.

[0058] (c) Raw material circulation reaction: The extraction tower 17 is provided with a liquid adding pipe 18, a liquid distributor 19 and a packing layer 20. The liquid adding pipe 18 is connected to the liquid distributor 19. The packing layer 20 is located below the liquid distributor 19. The packing layer 20 is made of stainless steel corrugated wire mesh packing, which is a disc-shaped packing composed of a plurality of parallel and straight stainless steel corrugated mesh sheets.

[0059] The mixed product is discharged from the bottom of the fluorination reactor and sent to the extraction tower 17. At the same time, dichloromethane is added to the liquid distributor 19 through the liquid adding pipe 18 as an extractant, sprayed down through the nozzle, and contacted with the mixed product in the packing layer 20 for extraction. The temperature is controlled at -20°C to -10°C, and the pressure is controlled at 0.3-0.6Mpa, which ensures sufficient contact area and contact time between the mixed product and the extractant, thereby improving the extraction effect. After the extractant is added, it is allowed to stand for 3-5 minutes, the solution is separated into layers, and the hydrogen fluoride layer is returned to the fluorination reactor. The difluoromethane and extractant are separated into layers. The mixed layer of the extractant is sent to the distillation tower 21 for pressure distillation, and the separated difluoromethane is discharged from the top of the distillation tower 21 and enters the gas-liquid separation tower 22. The extractant layer is discharged from the bottom of the distillation tower 21 and sent back to the fluorination reactor to separate difluoromethane from dichloromethane, monochloromonofluoromethane and hydrogen fluoride, thereby improving the purity of difluoromethane. Dichloromethane, monochloromonofluoromethane and hydrogen fluoride are returned to the fluorination reactor as raw materials to continue to participate in the reaction, thereby improving the utilization rate of the raw materials, reducing the treatment amount of the above-mentioned three wastes, and reducing the additional distillation operation of separating the extractant from the impurities.

[0060] A spiral duct 23, an L-shaped air duct 24 and an arc-shaped baffle 25 are provided in the gas-liquid separation tower 22. The spiral duct 23 is wound around the L-shaped air duct 24, and the arc-shaped baffle 25 is located below the air inlet of the L-shaped air duct 24. The spiral duct 23 is communicated with the air inlet of the gas-liquid separation tower 22, and the L-shaped air duct 24 is communicated with the air outlet of the gas-liquid separation tower 22.

[0061] In the gas-liquid separation tower 22, after difluoromethane flows through the spiral conduit 23, a high-speed spiral airflow is generated, and the liquid phase and gas phase are separated by centrifugal force, and then collide with the arc baffle 25 to perform secondary gas-liquid separation, with high separation efficiency and good effect, solving the problems of low separation efficiency and incomplete separation in the prior art of using gravity to separate gas and liquid. The gas phase mixture is discharged through the L-shaped air guide pipe 24, and the liquid phase mixture remains in the gas-liquid separation tower 22, is discharged from the bottom of the tower, and is sent back to the fluorination reactor to continue to participate in the reaction, further reducing the content of dichloromethane, monochloromonofluoromethane, and hydrogen fluoride in the crude product, thereby improving the purity of difluoromethane, and the separated liquid phase mixture continues to participate in the fluorination reaction, improving the utilization rate of raw materials and reducing the processing pressure of waste materials.

[0062] (d) Impurity removal:

[0063] ① The gas phase mixture discharged from the gas-liquid separation tower 22 passes through the water washing tower 26 and the alkali washing tower 27 in sequence to remove HCl and HF;

[0064] ② A wire mesh demister 29 and a drying layer 30 are provided in the drying tower 28. The wire mesh demister 29 is located above the drying layer 30. The drying layer 30 includes a support plate 31 and a color-changing silica gel 32. The color-changing silica gel 32 is arranged between the support plates 31. The color-changing silica gel 32 has a strong water absorption capacity. The support plates 31 fix and support the color-changing silica gel 32 to prevent the color-changing silica gel 32 from escaping freely due to the impact of airflow. Guide holes 33 are evenly distributed on the support plates 31, and guide channels 34 are formed between adjacent color-changing silica gels 32.

[0065] The gaseous mixture discharged from the alkali washing tower 27 enters the drying tower 28 for dehydration; in the drying tower 28, the gaseous mixture flows through the guide holes 33 of the lower support plate 31, enters the guide channel 34, is divided into multiple small streams, and the flow rate is reduced. It is constantly in contact with the color-changing silica gel 32, has sufficient contact area and contact time, and the moisture is effectively absorbed, thereby improving the purity of difluoromethane. The dehydrated gas flows out from the guide holes 33 of the upper support plate 31 and is then discharged from the top of the tower.

[0066] (e) Collection: The dehydrated difluoromethane is compressed and liquefied by the compressor 35 and then stored in the receiving tank 36.

[0067] Example 1

[0068] (a) loading the catalyst: loading the catalyst and the auxiliary agent into the fluorination reactor, the catalyst used is Cr2O3, the auxiliary agent used is CaO, and the molar ratio of Cr element to Ca element is 1:0.1;

[0069] (b) Fluorination reaction: preheating hydrogen fluoride and dichloromethane and introducing them into a fluorination reactor for fluorination reaction, the reaction temperature is 230°C, the reaction pressure is controlled at 0.2 MPa, and the molar ratio of hydrogen fluoride to dichloromethane is 8:1;

[0070] (c) Raw material circulation reaction: The product is sent to the gas-liquid separation tower 22 for gas-liquid separation, the gas phase mixture is discharged through the L-shaped air guide pipe 24, and the liquid phase mixture is discharged from the bottom of the tower and sent back to the fluorination reactor to continue the reaction;

[0071] (d) Impurity removal: The gas phase mixture discharged from the gas-liquid separation tower 22 passes through the water washing tower 26, the alkali washing tower 27, and the drying tower 28 in sequence;

[0072] (e) Collection: The dehydrated product was collected and the purity of CH2F2 was analyzed by chromatography. The results are shown in Table 1.

[0073] Example 2

[0074] (a) loading the catalyst: loading the catalyst and the auxiliary agent into the fluorination reactor, the catalyst used is Cr2O3, the auxiliary agent used is CaO, and the molar ratio of Cr element to Ca element is 1:0.1;

[0075] (b) Fluorination reaction: preheating hydrogen fluoride and dichloromethane and introducing them into a fluorination reactor for fluorination reaction, the reaction temperature is 230°C, the reaction pressure is controlled at 0.2 MPa, and the molar ratio of hydrogen fluoride to dichloromethane is 8:1;

[0076] (c) Raw material circulation reaction: the product is sent to the extraction tower 17 for extraction. The temperature in the extraction tower 17 is -20°C and the pressure is 0.3 MPa. The solution is allowed to stand for stratification. The hydrogen fluoride layer is sent back to the fluorination reactor. The mixed layer of difluoromethane and the extractant is sent to the distillation tower 21 for pressure distillation. The separated difluoromethane is discharged from the top of the distillation tower 21 and enters the gas-liquid separation tower 22 for gas-liquid separation. The extractant layer is discharged from the bottom of the distillation tower 21 and sent back to the fluorination reactor. In the gas-liquid separation tower 22, the gas phase mixture is discharged through the L-shaped air guide pipe 24, and the liquid phase mixture is discharged from the bottom of the tower and sent back to the fluorination reactor for further reaction.

[0077] (d) Impurity removal: The gas phase mixture discharged from the gas-liquid separation tower 22 passes through the water washing tower 26, the alkali washing tower 27, and the drying tower 28 in sequence;

[0078] (e) Collection: The dehydrated product was collected and the purity of CH2F2 was analyzed by chromatography. The results are shown in Table 1.

[0079] Example 3

[0080] The operation was the same as in Example 2, except that the reaction temperature in the fluorination reactor was 220°C. The results are shown in Table 1.

[0081] Example 4

[0082] The operation was the same as in Example 2, except that the reaction temperature in the fluorination reactor was 280°C. The results are shown in Table 1.

[0083] Example 5

[0084] The operation was the same as in Example 2, except that the reaction temperature in the fluorination reactor was 300°C. The results are shown in Table 1.

[0085] Example 6

[0086] The operation was the same as that in Example 2, except that the auxiliary agent used was Fe, and the molar ratio of the Cr element in the catalyst to the Fe element in the auxiliary agent was 1:0.3. The results are shown in Table 1.

[0087] Example 7

[0088] The operation was the same as in Example 2, except that the molar ratio of hydrogen fluoride to dichloromethane was 10:1. The results are shown in Table 1.

[0089] Example 8

[0090] The operation was the same as in Example 2, except that the reaction pressure in the fluorination reactor was 0.3 MPa. The results are shown in Table 1.

[0091] Example 1 2 3 4 5 6 7 8 purity / % 91.2 95.1 93.8 96.2 92.3 95.9 97.0 96.1

[0092] Table 1

[0093] The above are only specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent replacements or modifications made based on the present invention to solve basically the same technical problems and achieve basically the same technical effects are all included in the protection scope of the present invention.

Claims

1. A method for preparing difluoromethane, characterized in that The steps include: (a) Catalyst loading: ①Put the catalyst and additive into the stuffing tube, and then install the air inlet end on the open end of the stuffing tube; ② Screw the packing tube onto the lower support plate to fix the packing tube onto the lower support plate, and then cover the air inlet end with the limiting hole on the upper pressing plate to limit the upper pressing plate onto the packing tube to obtain the reaction assembly; ③Install the reaction assembly into the fluorination reactor until the lower support plate contacts the fixed ring in the cylinder, and the reaction assembly is supported by the fixed ring; ④ Install the cylinder cover on the cylinder body until the pressure rod on the bottom of the cylinder cover presses the upper pressure plate, so that the reaction component is limited and fixed in the fluorination reactor; ⑤Connect the pipelines between the reactors; ⑥ Fluorination of the catalyst; (b) Fluorination reaction: ① Preheat hydrogen fluoride and dichloromethane and introduce them into the fluorination reactor, mix them in the upper mixing zone, enter the packing pipe through the air inlet end, and carry out fluorination reaction under the action of the catalyst; The reaction temperature is controlled at 230-280°C, the reaction pressure is controlled at 0.15-0.3Mpa, and the molar ratio of hydrogen fluoride to dichloromethane is 8:1-12:1; During the reaction, oxygen is introduced into the fluorination reactor, and the molar ratio of oxygen to dichloromethane is 1%-2.5%; ② The reacted mixture flows out through the micropores at the lower end of the packing tube and is mixed again in the lower mixing zone; (c) Raw material circulation reaction: the mixed product is discharged from the bottom of the fluorination reactor and sent to the extraction tower. At the same time, dichloromethane is added to the liquid distributor from the liquid adding pipe as an extractant, sprayed down through the nozzle, and contacted with the mixed product in the packing layer for extraction. The temperature is controlled at -20°C to -10°C, and the pressure is controlled at 0.3-0.6Mpa. After the extractant is added, it is allowed to stand for 3-5 minutes, and the solution is separated into layers. The hydrogen fluoride layer is sent back to the fluorination reactor, and the mixed layer of difluoromethane and the extractant is sent to the distillation tower for pressure distillation. The separated difluoromethane is discharged from the top of the tower and enters the gas-liquid separation tower. The extractant layer is discharged from the bottom of the tower and sent back to the fluorination reactor. In the gas-liquid separation tower, after difluoromethane flows through the spiral conduit, gas-liquid separation occurs, and the gas phase mixture is discharged through the L-shaped air guide pipe, while the liquid phase mixture remains in the gas-liquid separation tower, is discharged from the bottom of the tower, and is sent back to the fluorination reactor to continue the reaction; The gas-liquid separation tower is provided with a spiral conduit, an L-shaped air duct and an arc-shaped baffle, the spiral conduit is wound around the L-shaped air duct, the arc-shaped baffle is located below the air inlet of the L-shaped air duct, the spiral conduit is communicated with the air inlet of the gas-liquid separation tower, and the L-shaped air duct is communicated with the air outlet of the gas-liquid separation tower; (d) Impurity removal: ① The gas phase mixture discharged from the gas-liquid separation tower passes through the water washing tower and the alkali washing tower in sequence to remove HCl and HF; ② The gas phase mixture discharged from the alkali washing tower enters the drying tower for dehydration; (e) Collection: The dehydrated difluoromethane is compressed and liquefied by a compressor and then stored in a receiving tank.

2. The method for preparing difluoromethane according to claim 1, characterized in that: The catalyst used in step (a) is Cr2O3, and the auxiliary agent used is one of CaO, MgO, Fe, and Al.

3. The method for preparing difluoromethane according to claim 1, characterized in that: The fluorination reactor in step (a) comprises a cylinder and a cylinder cover, wherein the cylinder cover is arranged on the cylinder, a reaction assembly is arranged in the cylinder, and the reaction assembly divides the space in the cylinder into an upper mixing zone, a reaction zone and a lower mixing zone, the reaction assembly comprises an upper pressure plate, a stuffing tube and a lower support plate, the stuffing tube is arranged between the upper pressure plate and the lower support plate, an air inlet end is arranged at one end of the stuffing tube, a microhole is opened at the other end of the stuffing tube, and a catalyst is contained in the stuffing tube.

4. The method for preparing difluoromethane according to claim 1, characterized in that: In step (a), the cylinder cover is threadedly connected to the cylinder body, a pressure rod is provided on the bottom surface of the cylinder cover, and a fixing ring is provided in the cylinder body. The fixing ring cooperates with the pressure rod to clamp the reaction component.

5. The method for preparing difluoromethane according to claim 1, characterized in that: In step (a), the lower support plate is provided with a mounting screw hole that is threadedly matched with the stuffing tube, and the upper pressure plate is provided with a limiting hole corresponding to the air inlet end, and the cross-sectional diameter of the limiting hole is smaller than the cross-sectional diameter of the stuffing tube.

6. The method for preparing difluoromethane according to claim 1, characterized in that: In step (a), the air inlet end is screwed and fixed to the pipe opening of the stuffing pipe via a threaded joint.

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