Production technology of pentaerythritol

Let’s answer one question first regarding the domestic situation: Currently, the main methods used in China are the sodium method and the calcium method. The by-products of the sodium method are more useful, and the production process for this method is shorter; moreover, the equipment investment and maintenance costs are lower compared to the calcium method. Currently, for the two methods I’ve come across, both the yield and the specific consumption in the sodium method are lower than those in the calcium method. The major domestic manufacturers include: Yihua, Yuntianhua, Pengxin Chemical, Sanmu Chemical, Liyang Chemical, and so on. However, it’s hard to find books that provide specific introductions to this knowledge! One has to be truly involved in this field to do it well; haha, I’ve done it before!

(1) Continuous synthesis process. The production of pentaerythritol abroad has gradually shifted to continuous-process methods, including continuous synthesis, multi-effect continuous evaporation, continuous crystallization and separation, as well as continuous drying and packaging. The continuity of the entire production process has led to a significant increase in the level of automation of the equipment. 　Countries such as Japan, Italy, the Czech Republic, and Spain have conducted in-depth research on the continuous synthesis process, and most manufacturers use this production method. The continuous synthesis process in Japan and the Czech Republic involves a reaction system composed of about 6 series-connected reactors. Each reactor consists of a mixing section, a cooling section, and a reaction section; the length-to-diameter ratio of the reaction section is approximately 7. Formaldehyde and base are fed continuously into the mixing section of the first reactor, and then flow through each subsequent reactor in turn. Acetaldehyde is introduced into the mixing section of each reactor (with the amount of acetaldehyde added decreasing sequentially from one reactor to the next). The residence time of the reaction mixture in each reactor is between 5 and 30 minutes, while the residence time in the last reactor can range from 30 to 60 minutes. This process has been industrialized, with a yield of over 95%. 　The continuous synthesis process employed by the Italian company Poliali involves the reaction taking place in two reactors connected in series. Formaldehyde, acetaldehyde, and an aqueous solution of caustic soda, which have been cooled to 30°C, are fed into the first reactor. This reactor is a vertical vessel at atmospheric pressure, divided into four distinct sections, and is equipped with a vertical mixer featuring four layers of stirrers. Vertical baffle plates are located on the walls of the reactor, ensuring thorough mixing of the reaction mixture in all sections ； All the formalin solution and 60% acetaldehyde solution were added to the first section of the reactor, while the remaining 40% acetaldehyde solution was added evenly to the third and fourth sections respectively. Sodium hydroxide solution was added evenly to the first and second sections of the reactor respectively ； The solution that overflows from the first reactor continues to react in the second reactor, with residence times of 15 minutes and 42 minutes in the two reactors respectively ； The reaction mixture containing trace amounts of base flows out of the second reactor, and is neutralized to pH=6 using formic acid in a static mixer; the neutralized reaction mixture is then thoroughly mixed in a vertically arranged intermediate tank equipped with a stirrer. 　The continuous crystallization process can be divided into fully continuous and semi-continuous types. The fully continuous process involves continuously feeding a concentrate at around 90°C into a vacuum crystallizer maintained at around 30°C, with an average residence time of about 4 hours. The product obtained through this process has a fine particle size, requires a long time for separation and filtration, and has a high ash content. To overcome the drawbacks of fully continuous crystallizers, a two-stage continuous crystallization process was proposed, in which the concentrate at around 90°C is continuously fed successively to a first-stage crystallizer maintained at around 45°C and a second-stage crystallizer maintained at around 25°C. The particle size of the products obtained by this process is improved compared to the fully continuous process, but it remains suboptimal. To address the issues of fine particles and high ash content associated with the aforementioned two processes, Japan proposed a semi-continuous crystallization process that combines batch crystallization and continuous crystallization. The process involves first using batch crystallization to cool the concentrate at around 90°C in the first crystallizer to around 50°C, and then continuously feeding it into the second crystallizer, which is maintained at around 30°C. This process can yield pentaerythritol products with large particle sizes and high purity. 　(2) Process for separating sodium formate and pentaerythritol by electrodialysis. Electrodialysis is a separation process that uses an electric field to allow ions in a solution to pass through selectively, thereby purifying the product. The electrodialysis unit consists of an anode chamber separated from a cathode chamber by alternately arranged cationic and anionic selectively permeable membranes, as well as a series of alternating concentration chambers and dilution chambers. A conductive fluid is added to the anode chamber and the cathode chamber and continuously circulated. After formaldehyde was removed by distillation, the concentrated solution was diluted with mother liquor to a pentaerythritol content of around 10%. The solution was heated to 50–80°C, and electrodialysis was carried out at a current density of 500–800 A/m2. The concentrated sodium formate solution was continuously discharged from the concentration chamber, while the purified pentaerythritol solution was continuously discharged from the other end of the dilution chamber. The purified pentaerythritol and sodium formate solution can be used, through evaporation, crystallization, and filtration, to obtain pentaerythritol with a sodium formate content of less than 0.01%, as well as high-purity sodium formate with a very low content of organic substances. 　(3) Production process for high-purity pentaerythritol. Abroad, there is a wide variety of pentaerythritol products available in different specifications, covering high-, medium-, and low-end grades. In addition to ordinary industrial pentaerythritol products, there are also products such as high-purity monopentaerythritol, dipentaerythritol, and tripentaerythritol. High-purity pentaerythritol accounts for a considerable proportion of these. The key points of the production process for high-purity pentaerythritol are as follows: ① A continuous synthesis process using a high concentration of formaldehyde is employed, with an actual formaldehyde/acetaldehyde ratio during the reaction of 10–15, thereby ensuring high selectivity for pentaerythritol ； ②An acid-conditioned pressurized hydrolysis process is employed to decompose the pentaerythritol acetals formed during the reaction ； ③By subjecting the crude pentaerythritol solution to decolorization using activated carbon, removing impurities such as formic acid sodium using ion exchange resins, and recrystallization to eliminate by-products such as poly-pentaerythritol, high-purity pentaerythritol with a purity of up to 98% can be obtained. 　(4) Process for extracting and recovering formic acid sodium using organic solvents. Abroad, the recovery of mother liquor mostly involves the use of organic solvents such as isobutanol to extract and recover formic acid sodium. The process involves: the preheated mother liquor coming into counter-current contact with an organic solvent in an extraction tank, whereby organic substances such as pentaerythritol are extracted. The extract phase is passed through a deionization tower to remove trace sodium ions, and then it enters a solvent evaporator where the organic solvent is recovered by evaporation. The extract phase discharged from the bottom of the evaporator is sent to incineration using a pump. The raffinate phase enters the sodium formate evaporator; the concentrated sodium formate solution is then sent to the crystallization system where it crystallizes under vacuum conditions. Subsequently, it is transferred to the separation system, and the sodium formate obtained through centrifugal separation is dried to yield the final product. The mother liquor is returned to the pentaerythritol crystallization unit. 　(5) Process for synthesizing pentaerythritol by catalytic hydrogenation. In this process, under very low alkali ratios (usually controlled at 0.2–0.3 kmol/kmol of acetaldehyde to minimize the occurrence of the Cannizzaro reaction), the condensation mixture whose main product is trimethylolacetaldehyde is subjected to flash distillation at 29.4–98.1 kPa (gauge pressure) to remove and recover formaldehyde. The formaldehyde-free condensation mixture is then subjected to catalytic hydrogenation using a nickel-based catalyst to yield pentaerythritol. In this method, the nickel-based catalyst can be used for a long time, and high-purity pentaerythritol can be easily obtained due to the extremely low content of formate in the condensation solution.

Thank you for the owner’s reply. I was wondering if currently, in China, the sodium and calcium processes are the main ones used, and are there no new processes in use? Do online sources say that these two methods have many shortcomings? Are the mixed-alkali method, the condensation-hydrogenation method, the continuous synthesis process, the electrodialysis method, and the organic solvent extraction method used on an industrial scale in China? That one is more mature. Thank you all!