Method for preparing THPC based on sodium hypophosphite byproduct tail gas and preparation method of THPC-Urea

Abstract

The invention discloses a method for continuously preparing THPC, and particularly relates to a system and a method for continuously preparing tetrakis hydroxymethyl phosphonium chloride based on sodium hypophosphite byproduct tail gas and a urea initial condensation body of tetrakis hydroxymethyl phosphonium chloride. Pretreating the sodium hypophosphite byproduct tail gas to remove impurities; continuously introducing the purified tail gas, formaldehyde and hydrochloric acid into a system formed by connecting 4-6 reaction kettles in series according to a specific molar ratio, and carrying out multi-stage countercurrent reaction; the reaction endpoint is accurately regulated and controlled by monitoring the density and the formaldehyde concentration of the last-stage reaction liquid on line and feeding back and controlling feeding; the reaction completion liquid enters an evaporator with a silicon carbide heat exchange crack tube as a core for continuous concentration; and finally, carrying out online pH regulation and filtration to obtain a THPC product, and further carrying out continuous condensation with urea. According to the invention, the safe and high-valued utilization of the industrial tail gas with complex components is realized, the industrial problems of equipment corrosion, product quality fluctuation and the like are overcome through whole-process continuity and intelligent control, and the production efficiency and the product stability are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of green preparation of high-end fine chemicals and high-value utilization of industrial waste gas, and particularly to a system and method for achieving continuous, automated and safe production of THPC and THPC-Urea through multi-module integration, process intensification and intelligent control, using phosphine (PH3) tail gas, a complex component generated during the production of sodium hypophosphite, as raw material. Background Technology

[0002] THPC is the core of the proban flame retardant process. Its primary condensate, formed by condensation with urea, is used in cotton fabric finishing, imparting excellent durability, flame retardancy, a good hand feel, and physical and mechanical properties. Traditional THPC synthesis processes generally employ batch reactor reactions, as described in CN102167709A, where high-purity PH3 gas is passed into a mixture of formaldehyde and hydrochloric acid for reaction, followed by concentration and pH adjustment to obtain the product. This method presents significant safety risks (PH3 is highly toxic and explosive), low production efficiency, high energy consumption, and large batch-to-batch quality fluctuations. Although improved processes (such as CN101143878A) attempt to utilize sodium hypophosphite byproduct tail gas, these often employ tower or batch absorption, exhibiting poor tolerance to complex impurities in the tail gas (such as H2, N2, P2H4, AsH3), easily leading to product coloring, catalyst poisoning, or the generation of byproducts, and failing to address the issue of production continuity.

[0003] The bottlenecks of existing technologies can be summarized as follows: (1) Raw material utilization and environmental protection: High-purity PH3 is expensive; the conventional treatment method for sodium hypophosphite tail gas is direct combustion ("flare"), which not only wastes resources but also produces pollutants such as P2O5, which violates the principles of green chemistry.

[0004] (2) Process safety and continuity: Intermittent operation causes PH3 gas to accumulate in the reaction system, which increases the safety risk sharply; the unit operations such as reaction, evaporation, and pH adjustment are isolated and cannot form a continuous material flow, resulting in a low level of automation.

[0005] (3) Equipment corrosion and long-term operation: The THPC concentration process is in a high-temperature, high-concentration hydrochloric acid environment. Traditional metal or graphite evaporators are prone to corrosion, leakage and scaling, which become a key bottleneck restricting continuous production. As shown in Comparative Example 1, the graphite equipment fails after 88 hours.

[0006] (4) Quality control and stability: Batch reactions rely on manual sampling and offline analysis (such as formaldehyde titration) to determine the endpoint, which is severely lagging and makes it difficult to accurately control the reaction depth, resulting in large differences in product quality between batches. Therefore, developing a new system and method that can efficiently, safely, and cleanly convert complex industrial waste gas and achieve continuous and stable production of THPC and THPC-Urea is a technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of the existing technology and provide an integrated, automated, and intrinsically safe continuous preparation system and method. The core concept of this invention is to construct a seamless, continuous process consisting of "tail gas pretreatment - multi-stage series reaction - online intelligent monitoring - high-efficiency evaporation and concentration - precise pH control," and to use pressureless sintered silicon carbide (SiC) heat exchange tubes as the core evaporation unit, overcoming the challenge of long-term equipment operation under highly corrosive conditions, ultimately achieving the production of high-quality THPC and THPC-Urea from complex impurity tail gas as raw material. The continuous THPC preparation method of this invention includes the following steps: Raw material pretreatment and continuous feeding: The tail gas (containing PH3, H2, H2S, AsH3, P2H6, phosphorus slag, etc.) from the sodium hypophosphite production workshop is first treated by a pretreatment unit (such as a three-stage alkaline scrubbing tower and condenser) to remove most of the phosphorus slag and H2S. The purified tail gas after pressure stabilization and metering, formaldehyde solution (concentration 30-37%), and hydrochloric acid (concentration 30-36%) are mixed at a molar ratio of n(formaldehyde): n(hydrochloric acid): n(PH3) = (4.05-4.25): 1.0: 1.2-1.5 and continuously pumped into the reaction system.

[0008] Multi-stage series continuous reaction: The reaction system consists of 4 to 6 (preferably 5) mechanically stirred tank reactors (R1 to R5) connected in series. The reaction temperature of each reactor is controlled at 60-80℃ (preferably 65-75℃), and the system pressure is maintained at a slightly positive pressure (130-160 kPa) to ensure that PH3 is fully absorbed and formaldehyde volatilization is inhibited. Through the step-by-step reaction, PH3 is fully absorbed and converted. Online real-time monitoring and feedback control: An online density meter and an online near-infrared (NIR) spectrometer are integrated and installed on the main material outlet line of the final reactor (R5).

[0009] The material flow can adopt a fully counter-current, parallel flow, or mixed flow mode, preferably gas-liquid counter-current (taking 5 devices in series as an example, the exhaust gas enters through R1 and exits through R5; the liquid portion of formaldehyde solution and hydrochloric acid is added through R5 and overflows through R1). Parallel flow refers to the purified exhaust gas, formaldehyde solution, and hydrochloric acid entering the reactor together for reaction. Mixed flow refers to the purified exhaust gas, formaldehyde solution, and hydrochloric acid being mixed before entering the reactor together for reaction.

[0010] Density meter monitors the density of the reaction solution in real time. r The residual formaldehyde content ([CH2O]) was monitored in real time using an NIR spectrometer. r The [CH2O] signal is transmitted to the central control system (DCS). The system sets the reaction endpoint as follows: r = 1.285-1.295 g / cm 3 (25℃) and [CH2O]<0.5%. The DCS compares the monitored value with the set value in real time through an algorithm and dynamically adjusts the speed of the raw material feed pump to achieve precise closed-loop control of the reaction endpoint.

[0011] Continuous high-efficiency evaporation and concentration: The reaction liquid reaching the endpoint is continuously fed into a vertical falling film evaporator via a pressure differential-resistant transfer pump. Its core heat exchange element is a pressureless sintered silicon carbide (SiC) cracked tube. Operating conditions are: absolute pressure 1-20 kPa, evaporation temperature 65-80℃. Silicon carbide material's excellent resistance to hydrofluoric acid and hydrochloric acid corrosion, high thermal conductivity (≈100 W / m·K), and thermal shock resistance make it ideally suited for this demanding operating condition, enabling continuous and stable operation for thousands of hours without interruption.

[0012] The material is concentrated to a THPC content of 79-82%. Precision filtration and pH control: The concentrated hot material is continuously cooled and filtered through multi-stage filter cartridges to remove any trace amounts of coking matter or impurity particles that may be generated.

[0013] Subsequently, the clarified liquid continuously enters an online static mixer, where it is instantly mixed with liquid alkali (such as 30% NaOH solution) added through a precision metering pump. Through the linkage control of the pH online sensor and the metering pump, the pH value of the product is precisely stabilized within the target range of 3.8-4.2, resulting in a high-quality THPC product.

[0014] Furthermore, the THPC product can be continuously fed into a plug flow reactor or a stirred continuous reactor and mixed with a urea solution precisely measured at a molar ratio of n(THPC):n(urea) = 1:2. The mixture is then reacted at 80-90°C for 1-2 hours to continuously prepare THPC-Urea.

[0015] The system for implementing the above method comprises, in sequence: a tail gas pretreatment unit (including an alkaline scrubbing tower and a condenser); a reaction module consisting of 4-6 reactors in series, each equipped with a stirrer, a temperature control jacket, and a gas distributor; an online analyzer system (densitometer and NIR spectrometer) installed at the outlet of the final reactor; a silicon carbide heat exchange split-tube falling film evaporator; an online filtration and pH precision control unit (including an automatic backwash filter and a static mixer); and a central control unit (DCS) for receiving all online signals and controlling all power equipment.

[0016] The beneficial effects and inventiveness of this invention are reflected in: 1. Raw material innovation and environmental benefits: For the first time, the direct and high-value utilization of sodium hypophosphite by-product tail gas containing high impurities (phosphorus slag, H2S, AsH3) has been realized, transforming hazardous waste into high-value-added products and avoiding pollution caused by traditional incineration from the source, which is in line with the principles of green chemistry.

[0017] 2. Enhanced Process Safety and Intelligence: The "multi-stage series reaction" and "gas-liquid countercurrent" design prevents the accumulation of pH3 in the system. More importantly, the "dual-variable online real-time monitoring and feedback control of density and formaldehyde concentration" enables advanced, accurate judgment and closed-loop control of the reaction endpoint, fundamentally eliminating the problems of excessive pH3 or formaldehyde residue caused by delayed human judgment, thus achieving inherent process safety.

[0018] 3. Equipment Breakthrough and Long-Term Operation: The innovative use of pressureless sintered silicon carbide (SiC) cracked tubes as heat exchange elements in the evaporator, leveraging their excellent resistance to hydrochloric acid / hydrofluoric acid corrosion, high thermal conductivity (>100 W / m·K) and thermal shock resistance, successfully solved the industry problem of easy corrosion and short lifespan of traditional metal or graphite evaporators in high-temperature, high-concentration hydrogen chloride environments (Comparative Example 2 strongly proves this point), providing core equipment support for achieving continuous operation of the entire process.

[0019] 4. Product Quality and Economic Benefits: The entire process is continuous and automated, eliminating the batch variations inherent in intermittent operations. As shown in Example 1, the product yield is high (85%), quality is stable (density fluctuation σ < ±0.002), and the appearance is colorless and transparent. Production efficiency is several times higher than that of the intermittent method (Comparative Example 1), energy consumption and labor costs are significantly reduced, equipment lifespan is long, and the overall economic benefits are substantial. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] Example 1: Raw material: Sodium hypophosphite tail gas (composition: pH 328-32%, H2 60-65%, H2S ~3500ppm, AsH3~800ppm, N2 balance). After pretreatment, H2S < 80ppm, AsH3 < 100ppm.

[0022] The equipment consists of five 3000L series glass reactors (R1, R2, R3, R4, and R5), a silicon carbide split-tube falling film evaporator, and a complete online monitoring and DCS control system. The DCS system collects data every 30 seconds and performs PID calculations for adjustment.

[0023] Process parameters: Feed molar ratio: n(formaldehyde): n(hydrochloric acid): n(pH3) = 4.15:1.0:1.4; Reaction temperature: R1:75℃, R2:75℃, R3:75℃, R4:75℃, R5:79℃; System pressure: 140 kPa; Evaporation conditions: Absolute pressure 10 kPa, temperature 75℃; Endpoint control: ρ = 1.32 g / cm³ 3 [CH2O] = 0.2%; pH control: 4.0.

[0024] Includes the following steps: Continuous preparation of THPC a) Raw material pretreatment and continuous feeding: The tail gas from sodium hypophosphite production is pretreated to remove phosphorus slag, H2S and AsH3 to obtain purified tail gas; the purified tail gas, formaldehyde aqueous solution and hydrochloric acid aqueous solution are continuously fed to the reaction system. b) Multi-stage series reaction: The material from step a) is subjected to a gas-liquid reaction in a reaction system consisting of the above 5 reactors connected in series, and the gas-liquid reaction is carried out in a countercurrent manner. The reaction temperature is 65-75℃ and the system pressure is maintained at 130-160kPa. The tail gas is introduced from R1 and discharged from R5; the liquid is added from R5 and overflows from R1. c) Online monitoring and intelligent control: Real-time online monitoring of the density and formaldehyde concentration of the material at the outlet of the final stage reactor, and feedback of the monitoring data to the control system to dynamically adjust the feed rate of the raw materials, so that the reaction endpoint is controlled at a density of 1.285-1.295 g / cm³. 3 (25℃) and formaldehyde concentration below 0.5%; d) Continuous evaporation and concentration: The reaction liquid obtained in step c) is continuously fed into an evaporator composed of silicon carbide heat exchange tubes. The absolute operating pressure is 10-20 kPa and the operating temperature is 70-80℃. Continuous evaporation and concentration are carried out under reduced pressure to obtain THPC concentrate. e) Online pH adjustment and filtration: The concentrated solution obtained in step d) is continuously added to the alkaline solution, and the pH is adjusted to 3.8-4.2 through online mixing and monitoring, and then continuously filtered to obtain the THPC product.

[0025] Results: After 1500 hours of continuous operation, the PH3 absorption rate was >90%, and the THPC yield (based on phosphorus) was 85%. The product was colorless and transparent, with excellent performance across all indicators. The evaporator showed no signs of corrosion or performance degradation.

[0026] Example 2: Continuous preparation of THPC-Urea Raw materials: THPC product (THPC content 80±1%, pH=4.0, colorless and transparent) obtained in Example 1 and industrial grade urea were used to prepare a 50% urea aqueous solution.

[0027] The apparatus consists of a 500L tubular continuous reactor with multiple static mixing units inside, and the reactor is equipped with a precise temperature control system.

[0028] Process parameters and flow: The THPC product obtained in Example 1 and a 50% urea aqueous solution were continuously and stably fed to the inlet of the tubular reactor via a precision metering pump at a molar ratio of n(THPC):n(urea) = 2.1:1.0. The residence time of the materials in the reactor was controlled at 70±5 minutes, and the reaction temperature was maintained at 75±2℃. The reaction system was kept in a closed state.

[0029] Results: After the reaction was completed, a colorless to slightly yellow transparent liquid was continuously collected from the reactor outlet, which was the tetrahydroxymethylphosphourea urea prepolymer (THPC-Urea) product. High-performance liquid chromatography (HPLC) analysis showed that the conversion rate of the key intermediates was greater than 98%, the product performance was stable, and it could be directly used for flame-retardant finishing of cotton fabrics. Furthermore, the flame-retardant properties of the finished fabrics remained durable after standard water washing tests.

[0030] Comparative Example 1 (Simulated Intermittent Process) The same exhaust gas source and pretreated as in Example 1 were used. Formaldehyde and hydrochloric acid were added in a single batch to a 2000L enamel-lined reactor. After heating to 75°C, the exhaust gas was introduced, and absorption was enhanced using a gas-liquid circulation pump. Formaldehyde was intermittently sampled and titrated to determine the endpoint. After the reaction, the material was transferred to a graphite-coated evaporator for concentration.

[0031] Results: Average batch cycle time was 11 hours, THPC yield was 76%. Leakage occurred in the graphite evaporator after the 8th batch. Product quality varied between batches (density fluctuation range ±0.010 g / cm³). 3 ).

[0032] Comparative Example 2 (Alternative Evaporation Equipment) The process is the same as in Example 1, except that the silicon carbide evaporator is replaced with a modified polytetrafluoroethylene (PTFE) coil evaporator with the same heat exchange area.

[0033] Result: Due to the extremely poor thermal conductivity of PTFE (~0.25 W / m·K) and low evaporation efficiency, the temperature must be increased to above 85℃ to achieve the same concentration ratio, resulting in slight coking and discoloration of the product. After 400 hours of operation, the product failed due to pipe creep rupture.

[0034] Experimental Data Comparison Table

[0035] The comparison between the embodiments and comparative examples fully demonstrates the inventiveness, novelty, and practicality of the present invention. The system and method provided by the present invention successfully solve the long-standing industry problems of safety, environmental protection, efficiency, and equipment reliability, providing a brand-new technical path for the green, efficient, and high-quality production of THPC and its derivatives.

[0036] Comparative Example 3 (Simulated Intermittent Condensation Process) The THPC product and urea solution were from the same source as in Example 2. In a 1000L enamel batch reactor, THPC was first added, followed by urea solution in batches with stirring. The temperature was slowly raised to 75°C and maintained for 2 hours.

[0037] Results: The production cycle for a single batch was approximately 3.5 hours. The product color was slightly darker than that of the continuous production product in Example 2, and there were observable fluctuations in viscosity between different batches (±5%), indicating batch-to-batch differences in the degree of condensation reaction.

Claims

1. A method for continuous preparation of THPC, characterized in that, Includes the following steps: a) Raw material pretreatment and continuous feeding: The tail gas from sodium hypophosphite production is pretreated to obtain purified tail gas; the purified tail gas, formaldehyde aqueous solution and hydrochloric acid aqueous solution are continuously fed into the reaction system for reaction; b) Online monitoring and intelligent control: Real-time online monitoring of the density and formaldehyde concentration of the effluent from the reaction system, ensuring the reaction endpoint is controlled at a density of 1.285-1.295 g / cm³. 3 (25℃), and the formaldehyde concentration is below 0.5%; c) Continuous evaporation and concentration: The reaction solution obtained in step b) is continuously fed into an evaporator and continuously evaporated and concentrated under reduced pressure to obtain THPC concentrate; d) Online pH adjustment and filtration: The concentrated solution obtained in step c) is continuously added to the alkaline solution, and the pH is adjusted to 3.8-4.2 through online mixing and monitoring, and then continuously filtered to obtain the THPC product.

2. The method according to claim 1, characterized in that, In the pretreated purified exhaust gas described in step a), the phosphorus slag content is less than 10 ppm, the H2S content is less than 100 ppm, and the AsH3 content is less than 100 ppm.

3. The method according to claim 1, characterized in that, In step a), the molar ratio of n(formaldehyde):n(hydrochloric acid):n(PH3) is (4.05-4.25):1.0:(1.2-1.5).

4. The method according to claim 1, characterized in that, The reaction system described in step b) consists of 4 to 6 reactors connected in series, in which gas-liquid reactions take place; the material flow direction is selected from full countercurrent, cocurrent or mixed flow modes.

5. The method according to claim 1, characterized in that, The reaction temperature is 60-85℃, and the system pressure is maintained at 130-180 KPa.

6. The method according to claim 1, characterized in that, The reaction system consists of five mechanically stirred tank reactors connected in series. The material flow adopts a full countercurrent mode, that is, a gas-liquid countercurrent mode. The purified exhaust gas is introduced from the first reactor and discharged from the fifth reactor; the liquid is added from the fifth reactor and overflows from the first reactor.

7. The method according to claim 6, characterized in that, The evaporator is a vertical falling film evaporator with an operating absolute pressure of 1-20 kPa and an operating temperature of 65-80℃.

8. The method according to claim 7, characterized in that, The heat exchange components in the vertical falling film evaporator are pressureless sintered silicon carbide cracked tubes.

9. The method according to claim 1, characterized in that, Concentrated to a THPC content of 79-82%.

10. A method for preparing THPC-Urea, characterized in that, The THPC product obtained by any one of the methods described in claims 1-9 is continuously fed into a reactor with a urea solution, and a condensation reaction is carried out at 70-90°C to obtain THPC-Urea.

Images