Method for continuously preparing trimethylolpropane phosphite

Abstract

The invention discloses a method for continuously preparing trimethylolpropane phosphite, which comprises the following steps of: metering and conveying a trimethylolpropane solution containing a catalyst and phosphorus trichloride serving as raw materials into a single-stage or multi-stage series falling film reactor respectively through a pump for continuous reaction, and arranging a jacket outside the reactor for temperature control, solvent steam and generated hydrogen chloride gas in the reaction process are discharged from the upper part of the reactor, and after a certain retention time, reaction completion liquid flows out from the bottom of the reactor to obtain the product trimethylolpropane phosphite. The continuous preparation of the trimethylolpropane phosphite ester is realized through the falling film reactor, the synchronous removal of the solvent and the byproduct hydrogen chloride is realized while the reaction is completed, the reaction efficiency is high, the system is simple, the cost is low, the yield is high, the intrinsic safety is high, and industrial production is easy to realize.

Description

Technical Field

[0001] The present invention relates to the technical field of organic synthesis, and in particular to a method for continuously preparing trimethylolpropane phosphite. Background Art

[0002] Trimethylolpropane phosphite is an organophosphorus compound containing a cage structure. Due to its own cage structure, it is more stable than other phosphite compounds and can be stored stably at low temperatures.

[0003] As an important chemical intermediate, trimethylolpropane phosphite can be used in many fields such as pesticide synthesis, polymer synthesis, coordination chemistry, catalysts, and flame retardant material preparation. Currently, the industrial preparation routes of trimethylolpropane phosphite are divided into two types: transesterification method and phosphorus trichloride method. The starting material is trimethylolpropane, specifically including:

[0004] ① Transesterification: Patent US3155703 discloses a transesterification process for preparing trimethylolpropane phosphite. Using trimethylolpropane and triethyl phosphite as raw materials, triethylamine as a catalyst, the reaction is heated to 100°C to initiate, followed by distillation of ethanol. After an 8-hour reaction time, the temperature is raised to 130°C, and after removal of the by-product ethanol, the product yield reaches 90%. Patent CN103467520A proposes a transesterification method using triethyl phosphite as a composite organic acid catalyst to prepare trimethylolpropane phosphite. An orthogonal test concluded that temperature is the most significant factor affecting reaction yield. Patent CN101608401A proposes a transesterification method using trimethyl phosphite under an acidic catalyst, with a reaction temperature of 70-80°C and a reaction time of 3-4 hours. Patent CN101792466A proposes an ester exchange reaction between trimethylolpropane and triethyl phosphite under the catalysis of ionic liquid [Bmim]BF4, with a reaction temperature of 78.4-100°C and a reaction time of 1.5-5.5h. After post-treatment, the product yield reaches 98% and the purity is 99.27%.

[0005] ② Phosphorus trichloride method: This method was first reported by Wadsworth in 1962 (Journal of the American Chemical Society 1962, 84, 610-617). During the reaction, trimethylolpropane is added to PCl3 in batches, while nitrogen is continuously introduced to drive out the by-product hydrogen chloride. The product yield after purification is only 72%. Because the method of removing hydrogen chloride in this reaction consumes a large amount of nitrogen, and a large amount of hydrogen chloride escapes during the addition of solid materials, the reaction is difficult to control. Subsequent US3189633 reported the use of an acid binding agent to absorb the by-product hydrogen chloride, but the yield was only 53%. Patent CN109593103A has increased the yield to a maximum of 90.5% and a purity of 98.6% by optimizing and improving the feed ratio and the control of hydrogen chloride removal conditions. Although this method has lower raw material costs, the reaction yield is still lower than that of the transesterification method, mainly because intermolecular cross-esterification byproducts are easily generated during the reaction.

[0006] Furthermore, the applicant discovered during experiments that the byproduct hydrogen chloride generated by the reaction of trimethylolpropane with PCl3 must be promptly removed from the reaction system, and the reaction heat must also be removed quickly to avoid excessively high absolute temperatures. Otherwise, cross-esterification byproducts are more likely to form, further reducing the yield. In a kettle reaction, the greater the feed volume, the lower the efficiency of removing both the reaction heat and hydrogen chloride, making byproducts more likely to form and the yield lower.

[0007] The transesterification method produces less heat and is more temperature-controllable, but its raw material costs are higher than the phosphorus trichloride method. Therefore, developing a more inherently safe, efficient, and high-yield continuous phosphorus trichloride method for preparing trimethylolpropane phosphite is more competitive in the market. Furthermore, the continuous process is more stable and easier to scale up for industrial production. Summary of the Invention

[0008] In view of the above technical problems existing in the prior art, the purpose of this application is to provide a method for continuously preparing trimethylolpropane phosphite to improve the reaction efficiency, product purity and separation yield.

[0009] The technical solution adopted in the present invention is as follows:

[0010] A method for continuously preparing trimethylolpropane phosphite comprises the following steps: a trimethylolpropane solution containing a catalyst and phosphorus trichloride as raw materials are metered and delivered by pumps into a single-stage or multi-stage falling film reactor connected in series for continuous reaction; a heat exchange jacket is provided on the outside of the falling film reactor for passing a heat exchange fluid to control the temperature; during the reaction, solvent vapor and generated hydrogen chloride gas are discharged from an upper gas phase outlet of the falling film reactor; after a certain residence time, a reaction-completed liquid flows out from the bottom of the falling film reactor to obtain the product trimethylolpropane phosphite;

[0011] The molar flow ratio of the raw material trimethylolpropane, the catalyst and phosphorus trichloride is 1:0.001-0.005:1.01-1.20; the catalyst is selected from at least one of tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphite, butyl bromide, butyl iodide and pentane iodide.

[0012] Furthermore, the trimethylolpropane solution is molten trimethylolpropane, or a mixture of trimethylolpropane and an organic solvent; the organic solvent is at least one of petroleum ether, n-hexane, cyclohexane, chlorobenzene, dichloroethane, and dichloromethane, and the mass of the organic solvent is 0.5 to 5 times the mass of the trimethylolpropane.

[0013] Furthermore, a rotating scraper is provided in the falling film reactor, and the thickness of the liquid film in the falling film reactor is controlled to be no more than 10 mm by the gap between the internal rotating scraper and the inner wall of the reactor. The pressure in the falling film reactor is a negative pressure of -0.095 to -0.05 MPa.

[0014] Furthermore, when only a single-stage falling film reactor is provided, the trimethylolpropane phosphite product is discharged from the bottom liquid phase outlet of the falling film reactor, the molar flow ratio of the raw material trimethylolpropane to phosphorus trichloride is 1:1.15-1.2, the reaction temperature is controlled at 30-95°C, preferably 60-70°C, the reaction pressure is -0.05 to -0.07 MPa, and the reaction residence time is 0.1-5 min, preferably 0.8-2 min.

[0015] Furthermore, when two stages of falling film reactors are connected in series, the molar flow ratio of the raw materials trimethylolpropane to phosphorus trichloride is 1:1.05-1.08, the reaction temperature in the first stage falling film reactor is -10 to -5°C, the reaction pressure is -0.050 to -0.055 MPa, and the reaction residence time is 4 to 5 minutes; the reaction temperature in the second stage falling film reactor is 65 to 70°C, the reaction pressure is -0.080 to -0.095 MPa, and the reaction residence time is 2 to 2.5 minutes. The trimethylolpropane solution is a mixture of trimethylolpropane and an organic solvent, and the mass of the organic solvent is 3 to 5 times the mass of the trimethylolpropane.

[0016] Furthermore, when three or more falling film reactors are connected in series, the molar flow ratio of the raw materials trimethylolpropane and phosphorus trichloride is 1:1.01-1.05, the trimethylolpropane solution is a mixture of trimethylolpropane and an organic solvent, the mass of the organic solvent is 0.5-2 times the mass of the trimethylolpropane, and along the direction of flow of the reaction materials, the reaction temperature in the latter falling film reactor is higher than or equal to the reaction temperature in the previous falling film reactor; wherein, the reaction is divided into three stages according to the reaction temperature range:

[0017] The first stage reaction temperature is 10-35°C, the reaction pressure is -0.085 to -0.095 MPa, and the reaction residence time is 1-2 minutes;

[0018] The second stage reaction temperature is 55-70°C, the reaction pressure is -0.075 to -0.085 MPa, and the reaction residence time is 1-5 minutes;

[0019] The reaction temperature of the third stage is 75-90°C, the reaction pressure is -0.065 to -0.09 MPa, and the reaction residence time is 2-8 minutes.

[0020] Furthermore, when the number of stages of the multi-stage series falling film reactor is 3 or 4 stages in series, the three-stage reaction corresponds to the reactions of the first, second and third or higher falling film reactors, respectively. The reaction temperatures of the three-stage reaction are 30-35°C, 65-70°C and 85-90°C, respectively. The residence times of the three-stage reaction are 1-1.5 min, 1-1.5 min and 2-2.5 min, respectively. The pressure of the third-stage reaction is -0.08 to -0.09 MPa.

[0021] The phosphorus trichloride is added stepwise in two parts. The first part is added from the inlet of the first-stage falling film reactor, and the second part is added from the inlet of the second-stage or third-stage falling film reactor. The amount of the first part of phosphorus trichloride is 40-80% of the total amount of phosphorus trichloride, preferably 45-55%.

[0022] Furthermore, when the number of stages of the multi-stage falling film reactor is 5 to 10 in series, the reaction temperatures of the three-stage reaction are 10 to 15° C., 55 to 60° C., and 75 to 80° C., respectively; the residence times of the three-stage reaction are 1 to 1.5 min, 5 to 6 min, and 8 to 10 min, respectively; and the pressure of the third-stage reaction is -0.065 to -0.075 MPa;

[0023] The phosphorus trichloride is added stepwise in n portions, the first portion is added from the inlet of the first-stage falling film reactor, and the remaining portions are added from the inlets of several intermediate falling film reactors; wherein n is an integer of 2-5, and the amount of phosphorus trichloride in each portion is (0.8-1.2) / n*100% of the total amount of phosphorus trichloride.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) The present invention utilizes a falling film reactor to continuously prepare trimethylolpropane phosphite, which has the following advantages: ① Due to the thin liquid layer thickness of the falling film reactor, the heat transfer efficiency is high and the temperature is easier to control. In addition, the hydrogen chloride gas produced by the reaction in the reactor under negative pressure conditions can leave the reaction system more quickly, effectively reducing cross-esterification by-products, accelerating the reaction in the positive direction, and improving the reaction efficiency. At the same time, the use of a catalyst can also reduce intermolecular cross-esterification by-products and improve the reaction yield. ② The material in the falling film reactor flows from top to bottom, which consumes less power and reduces material backmixing, which is also beneficial to reducing intermolecular cross-esterification side reactions and improving the yield.

[0026] (2) In the process for preparing trimethylolpropane phosphite of the present invention, phosphorus trichloride reacts with trimethylolpropane immediately after contact, converting it into a high-boiling-point substance, and the reaction and conversion continue to obtain the target product trimethylolpropane phosphite. In addition, while completing the continuous reaction, the solvent and the by-product hydrogen chloride are simultaneously removed. The process is simple and does not require additional desolventizing and degassing operations.

[0027] (3) In the process method of the present invention, the continuous flow reaction has a small liquid holding volume. By replacing the intermittent tank reaction with the continuous flow reaction, the liquid holding volume is small, the thermal safety risk is reduced, the process is inherently safer, and it is suitable for industrial production.

[0028] (4) The process of the present invention greatly improves the yield of the reaction by selecting a suitable catalyst, and is conducive to further improving the purity of the product.

[0029] (5) The process of the present invention further controls the parameters such as the feeding method of raw materials, reaction temperature and pressure in steps, further improving the reaction yield and product purity, which is conducive to the realization of industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The present invention is a schematic diagram of the process for continuously preparing trimethylolpropane phosphite. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0032] The process diagram of the continuous preparation of trimethylolpropane phosphite of the present invention is shown in FIG. Figure 1 .

[0033] Example 1:

[0034] Molten trimethylolpropane, catalyst tris(trimethylsilyl)borate, and phosphorus trichloride are respectively delivered into a single-stage falling film reactor by metering pumps at a molar flow ratio of 1:0.001:1.15 for reaction. The reaction temperature of the falling film reactor is controlled at 65° C. by circulating heat transfer oil in an external heat exchange jacket. The reaction residence time is 1 minute, the reaction pressure is -0.06±0.01 MPa, and the liquid film thickness is controlled at approximately 3 to 5 mm. Hydrogen chloride gas generated by the reaction is discharged from the upper gas phase outlet of the falling film reactor to a tail gas absorption tower for absorption. The reaction product flows out of the falling film reactor outlet with a yield of 94% and a purity of 97.3%.

[0035] Example 2:

[0036] Catalyst butyl bromide was added to a mixture of trimethylolpropane and petroleum ether (mass ratio 1:5) to obtain a raw material solution containing the catalyst.

[0037] The raw material solution containing the catalyst and phosphorus trichloride are respectively transported by metering pumps into the first-stage falling film reactor for reaction. The molar flow ratio of trimethylolpropane, butyl bromide and phosphorus trichloride is controlled to be 1:0.002:1.05. The three feeds first enter a first-stage falling film reactor for reaction. The reaction temperature is controlled to be -10 to -5°C, the residence time is 5 minutes, the reaction pressure is a negative pressure of -0.050 to -0.055 MPa, and the liquid film thickness is controlled to be about 8 to 10 mm. The hydrogen chloride gas generated by the reaction is discharged from the upper gas phase outlet of the first-stage falling film reactor to an absorption tower for absorption. The reactants flowing out of the first-stage falling film reaction outlet continue to flow into a second-stage falling film reactor for reaction. The second-stage reaction temperature is 65 to 70°C, the residence time is 2 minutes, the reaction pressure is a negative pressure of -0.080 to -0.095 MPa, and the liquid film thickness is 4 to 6 mm. The hydrogen chloride gas and solvent vapor generated by the reaction are discharged from the upper gas phase outlet of the second-stage falling film reactor to a condensation and tail gas absorption system. The reaction product flows out of the second-stage falling film reactor outlet with a yield of 97.5% and a purity of 98.8%.

[0038] According to the experimental steps of Example 2, the catalyst butyl bromide was replaced by tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphite, butyl iodide or pentane iodide at the same molar flow rate, and the other experimental conditions remained unchanged. The final reaction results are shown in Table 1.

[0039] Table 1

[0040]

[0041]

[0042] In the experiment of sequence number 1 in Table 1 of Example 2, the same catalyst "tris(trimethylsilyl)borate" as in Example 1 was used. Comparison of the two experiments showed that: ① Example 1 was reacted in the absence of a solvent, and the raw material concentration in the reaction solution was higher, so the reaction rate was faster, but the reaction yield and purity needed to be further improved; ② Example 2 introduced a solvent into the reaction solution and carried out the temperature control reaction in steps. The experimental effect was significantly improved compared to Example 1, and the amount of phosphorus trichloride used was significantly reduced while further improving the reaction yield and purity.

[0043] Example 3:

[0044] A catalyst tris(trimethylsilyl)borate was added to a mixture of trimethylolpropane and chlorobenzene (mass ratio 1:0.5) to obtain a raw material solution containing the catalyst.

[0045] The raw material solution containing the catalyst and phosphorus trichloride are respectively transported by metering pumps into the first-stage falling film reactor for reaction. The molar flow ratio of trimethylolpropane, tris(trimethylsilyl)borate and phosphorus trichloride is controlled to be 1:0.005:0.515. First, the reaction is carried out in the first-stage falling film reactor. The reaction temperature is controlled to be 30-35°C, the residence time is 1min, the reaction pressure is a negative pressure of -0.088 to -0.095MPa, and the liquid film thickness is controlled to be about 6-8mm. The hydrogen chloride gas produced by the reaction is discharged from the upper gas phase outlet of the first-stage falling film reactor to the tail gas absorption system. The reactants flowing out of the first-stage falling film reaction outlet continue to flow into the second-stage falling film reactor. The inlet of the second-stage falling film reactor is simultaneously pumped into 0.515 molar equivalents of phosphorus trichloride to continue the reaction (i.e., the molar flow ratio of the phosphorus trichloride supplemented by the pumping is 0.515:1). The second-stage reaction temperature is 100-150°C. The reaction temperature of the falling film reactor is 65-70° C., the residence time is 1 min, the negative pressure is -0.078 to -0.085, the liquid film thickness is 4-6 mm, the hydrogen chloride gas and solvent vapor produced by the reaction are discharged from the upper gas phase outlet of the second-stage falling film reactor to the condensation and tail gas absorption system, the reactants flowing out of the outlet of the second-stage falling film reactor continue to flow into the third and fourth-stage falling film reactors in sequence for reaction, the reaction temperature of the third and fourth-stage falling film reactors are both 85-90° C., the residence time is both 1 min, the reaction pressure is both negative pressure of -0.08 to -0.09 MPa, the liquid film thickness is both 2-5 mm, the hydrogen chloride gas and solvent vapor produced by the reaction are discharged from the upper gas phase outlet of the falling film reactor to the condensation and tail gas absorption system, the reaction product flows out of the outlet of the fourth-stage falling film reactor, the yield is 99.1%, and the purity is 98.9%.

[0046] Although Example 3 significantly reduces the amount of reaction solvent used compared to Example 2, it also achieves a good reaction effect due to the step-by-step feeding operation, that is, it further reduces the amount of phosphorus trichloride used and improves the reaction yield and purity.

[0047] Example 4:

[0048] A catalyst, bis(trimethylsilyl)phosphite, was added to a mixture of trimethylolpropane and dichloroethane (mass ratio 1:2) to obtain a raw material solution containing the catalyst.

[0049] The catalyst-containing raw material solution and phosphorus trichloride are separately delivered by metering pumps, and the reaction is carried out at a total molar flow ratio of trimethylolpropane, catalyst bis(trimethylsilyl)phosphite, and phosphorus trichloride of 1:0.003:1.03. The phosphorus trichloride is divided into five equal parts and fed into the first, third, fifth, seventh, and ninth falling film reactors in five batches. The trimethylolpropane solution is prepared by mixing trimethylolpropane and chlorobenzene in a mass ratio of 1:2.

[0050] The first stage falling film reactor controls the reaction temperature to be 10-15°C, the residence time to be 1min, the reaction pressure to be negative pressure -0.085-0.095MPa, the liquid film thickness to be controlled to be about 3-5mm, the hydrogen chloride gas produced by the reaction is discharged from the upper gas phase outlet of the first stage falling film reactor to the tail gas absorption system, the reactants flowing out of the first stage falling film reaction outlet continue to flow into the second to sixth stage falling film reactors in sequence, the reaction temperature of the second to sixth stage falling film reactors are all 55-60°C, the residence time is all 1min, the reaction pressure is all negative pressure -0.075-0.085MPa, the liquid film thickness is all 3-6mm, the reaction produces The hydrogen chloride gas and solvent vapor are discharged from the upper gas phase outlet of the falling film reactor to the condensation and tail gas absorption system, and the reactants flowing out of the outlet of the sixth-stage reactor continue to flow into the seventh to tenth-stage falling film reactors in sequence. The reaction temperatures of the seventh to tenth-stage falling film reactors are all 75-80°C, the residence time is all 2 minutes, the reaction pressure is all negative pressure of -0.065 to -0.075 MPa, the liquid film thickness is all 4 to 7 mm, and the hydrogen chloride gas and solvent vapor produced by the reaction are discharged from the upper gas phase outlet of the falling film reactor to the condensation and tail gas absorption system. The reaction product flows out of the outlet of the tenth-stage falling film reactor with a yield of 100% and a purity of 99.7%.

[0051] Comparative Example 1:

[0052] Compared with Example 2, the only difference is that the catalyst butyl bromide is not added during the reaction, and the other conditions remain unchanged. The final reaction yield is 90.5% and the purity is 97.8%.

[0053] Comparative Example 2:

[0054] Compared with Example 3, the only difference is that the operating pressures of the first and second falling film reactors are both at normal pressure, and the other conditions remain unchanged. The final reaction yield is 70% and the purity is 87.5%.

[0055] Comparative Example 3:

[0056] The experimental steps of Example 3 were repeated, except that "all phosphorus trichloride was fed into the first-stage reactor" and other conditions remained unchanged. The final reaction yield was 93% and the purity was 95.5%.

[0057] The contents described in this specification are merely an enumeration of implementation forms of the inventive concept, and the protection scope of the present invention should not be considered as being limited to the specific forms described in the embodiments.

Claims

1. A method for continuously preparing trimethylolpropane phosphite, characterized in that: The method comprises the following steps: feeding a catalyst-containing trimethylolpropane solution and phosphorus trichloride as raw materials into a single-stage or multi-stage falling film reactor in series by metering through a pump for continuous reaction; a heat exchange jacket for passing a heat exchange fluid is provided on the outside of the falling film reactor for temperature control; during the reaction, solvent vapor and generated hydrogen chloride gas are discharged from an upper gas phase outlet of the falling film reactor; after a certain residence time, a reaction-completed liquid flows out from the bottom of the falling film reactor to obtain a product, trimethylolpropane phosphite; The molar flow ratio of the raw material trimethylolpropane, the catalyst and phosphorus trichloride is 1: 0.001~0.005: 1.01~1.20; the catalyst is selected from at least one of tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphite, butyl bromide, butyl iodide and pentane iodide.

2. The method for continuously preparing trimethylolpropane phosphite according to claim 1, wherein: The trimethylolpropane solution is molten trimethylolpropane, or a mixture of trimethylolpropane and an organic solvent; the organic solvent is at least one of petroleum ether, n-hexane, cyclohexane, chlorobenzene, dichloroethane, and dichloromethane, and the mass of the organic solvent is 0.5 to 5 times the mass of the trimethylolpropane.

3. The method for continuously preparing trimethylolpropane phosphite according to claim 1, wherein: A rotating scraper is provided in the falling film reactor, and the thickness of the liquid film in the falling film reactor is controlled to be no more than 10 mm by the gap between the internal rotating scraper and the inner wall of the reactor. The pressure in the falling film reactor is a negative pressure of -0.095 to -0.05 MPa.

4. The method for continuously preparing trimethylolpropane phosphite according to claim 1, wherein: When only a single-stage falling film reactor is provided, the trimethylolpropane phosphite product is discharged from the bottom liquid phase outlet of the falling film reactor, the molar flow ratio of the raw material trimethylolpropane to phosphorus trichloride is 1:1.15-1.2, the reaction temperature is controlled at 30-95°C, the reaction pressure is -0.05-0.07 MPa, and the reaction residence time is 0.1-5 min.

5. The method for continuously preparing trimethylolpropane phosphite according to claim 1, wherein: When two stages of falling film reactors are connected in series, the molar flow ratio of the raw materials trimethylolpropane and phosphorus trichloride is 1:1.05-1.08, the reaction temperature in the first stage falling film reactor is -10~-5°C, the reaction pressure is -0.050~-0.055MPa, and the reaction residence time is 4-5min; the reaction temperature in the second stage falling film reactor is 65~70°C, the reaction pressure is -0.080~-0.095MPa, and the reaction residence time is 2-2.5min.

6. The method for continuously preparing trimethylolpropane phosphite according to claim 1, wherein: When three or more falling film reactors are connected in series, the molar flow ratio of the raw material trimethylolpropane to phosphorus trichloride is 1:1.01-1.05, and along the flow direction of the reaction materials, the reaction temperature in the latter falling film reactor is higher than or equal to the reaction temperature in the former falling film reactor; wherein, the reaction is divided into three stages according to the reaction temperature range: The first stage reaction temperature is 10-35°C, the reaction pressure is -0.085~-0.095MPa, and the reaction residence time is 1-2min; The second stage reaction temperature is 55-70°C, the reaction pressure is -0.075~-0.085MPa, and the reaction residence time is 1-5min; The reaction temperature of the third stage is 75-90°C, the reaction pressure is -0.065~-0.09MPa, and the reaction residence time is 2-8min.

7. The method for continuously preparing trimethylolpropane phosphite according to claim 6, wherein: When the number of stages of the multi-stage series falling film reactor is 3 or 4 stages in series, the three-stage reaction corresponds to the reactions of the first, second and third or higher falling film reactors, respectively. The reaction temperatures of the three-stage reaction are 30-35°C, 65-70°C and 85-90°C, respectively. The residence times of the three-stage reaction are 1-1.5 min, 1-1.5 min and 2-2.5 min, respectively. The pressure of the third-stage reaction is -0.08 to -0.09 MPa. The phosphorus trichloride is added stepwise in two parts. The first part is added from the inlet of the first-stage falling film reactor, and the second part is added from the inlet of the second-stage or third-stage falling film reactor. The amount of the first part of phosphorus trichloride is 40-80% of the total amount of phosphorus trichloride.

8. The method for continuously preparing trimethylolpropane phosphite according to claim 6, wherein: When the number of stages of the multi-stage falling film reactor is 5 to 10, the reaction temperatures of the three stages are 10 to 15°C, 55 to 60°C, and 75 to 80°C, respectively; the residence times of the three stages are 1 to 1.5 min, 5 to 6 min, and 8 to 10 min, respectively; and the pressure of the third stage is -0.065 to -0.075 MPa. The phosphorus trichloride is added stepwise in n portions, the first portion is added from the inlet of the first-stage falling film reactor, and the remaining portions are added from the inlets of the intermediate falling film reactors; wherein n is an integer of 2-5, and the amount of phosphorus trichloride in each portion is (0.8-1.2) / n*100% of the total amount of phosphorus trichloride.

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