Mixed C4 separation technology

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?? Mixed C4 usually contains components such as butadiene, isobutylene, 1-butene, 2-butene, isobutane, n-butane, etc. Among them, the first 3 components have similar boiling points and are relatively reactive chemically, requiring special methods for separation, while the last 3 components can be separated using ordinary distillation. ?? (a) Separation of butadiene. Using extractive distillation, the separation methods vary depending on the solvent used; the main ones are the acetonitrile method (ACN method), the dimethylformamide method (DMF method), and the N-methylpyrrolidone method (NMP method). Production facilities for the 3 commonly used separation processes abroad are currently available in China. For the introduced technologies, domestic manufacturers have carried out multiple technical upgrades. Jilin Petrochemical Industry Company adopted Japanese JSR production technology to separate polymer-grade butadiene from acetonitrile through two-stage extractive distillation and weight removal purification; initially, the energy consumption was high, but after renovations in 1986, it has now reached the level of JSR Corporation ； Lanzhou Petrochemical Company used a propionitrile-based method of its own design to build China’s first industrial facility for producing butadiene. However, due to outdated technology and high energy consumption, the facility was completely renovated twice, in 1988 and 1996. As a result of these renovations, the butadiene yield increased from 94% to 97%, while the product quality improved to 99.6%-99.8%. The butadiene content in the raffinate dropped from 0.8% to below 40×10-6, and the ACN content fell below 1×10-6. The consumption of circulating water and steam was reduced by 57% and 32%, respectively ； The acetonitrile unit at Beijing Yanshan Petrochemical Company was also upgraded technically in 1986, primarily by adding a alkyne extraction and distillation system, as well as implementing some energy-saving measures. Our country has also made numerous improvements to the imported DMF process technology. Since the commissioning of the plant, the Synthetic Rubber Plant of Beijing Yanshan Petrochemical Company has carried out more than 100 modifications to the existing production processes. By upgrading the extraction distillation column system, the C4 feed evaporator process, the circulation extraction system of the first distillation tower, and the solvent purification system, the plant has optimized its processes and improved process control. Several other DMF units in the country have also been modified and upgraded according to their respective characteristics. Daqing Petrochemical Company and Yangzi Petrochemical Company added several sieve holes to the two-stage trays, creating float-valve/sieve-hole composite trays which increased the open area ratio; they also changed the bottom gap of the liquid drop pipes in each tower to 40–60 mm. Qilu Petrochemical Company also carried out modifications, increasing the tray spacing to boost the production capacity of the second extraction tower. To keep up with the development of production, Qilu Petrochemical Company built a second DMF process unit, designing the diameter of the double extraction tower at 0.6 meters. China has 2 sets of NMP production facilities, both of which utilize technology from the German company BASF. Polymer-grade butadiene is produced using NMP with a water content of 5%-8% as the extractant, through a process that combines two stages of extractive distillation (counter-current washing) with two stages of conventional distillation. The outstanding feature of the NMP solvent is its high hydrolytic stability and thermal stability; all equipment can be manufactured from carbon steel without suffering from corrosion. Due to its short production history, our country is currently constantly working to digest, absorb, improve, and enhance it. ?? (b) Separation of isobutylene. In the C4 fraction, since the boiling points of isobutylene and n-butylene differ by only 0.6°C and their relative volatilities differ by only 0.022, it is difficult to separate them using conventional physical methods. However, because the chemical reactivity of isobutylene is second only to that of butadiene, its chemical reactivity is generally utilized in industry for separation purposes. Currently, the main industrial methods for separating isobutylene include sulfuric acid extraction, adsorption separation, resin hydration and dehydration, and methyl tert-butyl ether cracking. The sulfuric acid extraction method was the first to be used in industry for the production of isobutylene; it relies on the difference in reaction rates between n-butylene and isobutylene with sulfuric acid to achieve their separation. The adsorption separation method is a process technology that utilizes the difference in adsorption capacity of n-butylene and isobutylene on molecular sieves to produce isobutylene. The process technology for producing isobutylene via resin dehydration involves the catalytic hydration of isobutylene using cation exchange resins to produce tert-butanol, which is then dehydrated through a catalyst bed of strongly acidic ion exchange resins to yield high-purity isobutylene. The research institute of Lanzhou Chemical Industry Company in our country began researching the resin-based process in the 1960s, and industrial production was established in 1973 at Shanghai Gaoqiao Chemical Plant and Tianjin Petrochemical Plant No. 2. The methyl tert-butyl ether cracking method is an industrial process that takes place under liquid-phase conditions, using large-pore strongly acidic ion exchange resins as catalysts to enable a selective reaction between C4 fractions containing isobutylene and methanol to produce methyl tert-butyl ether (MTBE); the conversion rate of isobutylene exceeds 99.99%. Subsequently, MTBE is cracked to regenerate isobutylene. Compared with other methods, this technology features no corrosion to equipment, no environmental pollution, a rational industrial process, mild operating conditions, low energy consumption, high product purity, and great flexibility in plant scale. It can be used to produce MTBE or isobutylene according to market demand; since its successful development, it has remained one of the main methods for producing isobutylene both domestically and internationally. ?? (c) Separation of 1-butene. China’s first 1-butene plant was introduced by Qilu Petrochemical Company from the Japanese company Ruion, with a capacity of 50,000 tons per year. After years of digestion and absorption, the entire set of equipment has now been localized in China. The process route employed by the unit is as follows: the unreacted C4 (i.e., the residue remaining after removing butadiene and isobutylene from the mixed C4) is subjected to azeotropic distillation in an isobutane removal tower, where isobutane, water, and some light components are removed from the top of the tower ； The C4 from the bottom of the isobutane removal tower is precisely fractionated in a 1-butene distillation column, yielding high-purity 1-butene product with a purity of over 99% (by mass) at the tower top. To ensure the quality of 1-butene, requirements are also placed on the butadiene impurities in the raw materials. 1 ； The boiling points of 3-butadiene and 1-butene differ by only 1.85°C, and the majority of the butadiene in the feed material ends up in the 1-butene product. Therefore, deep extraction distillation is employed to remove butadiene thoroughly in a single step, so that the butadiene content in the raffinate C4 does not exceed 40×10-6. This avoids the need for selective hydrogenation of the liquid phase to remove the remaining butadiene, after the stripping C4 obtained from conventional extractive distillation of butadiene has had isobutylene removed using an MI''BE unit, thereby simplifying the process.