Thread Content
This post was last edited by I am kind on 2016-8-14 at 16:39. Global Chemicals GMT 1 Introduction: The C5 fraction is a by-product obtained during the production of ethylene in refineries, catalytic cracking units, and heavy hydrocarbon cracking facilities. It consists of more than 30 components with similar boiling points, and represents a valuable raw material that can be used to produce a range of high-value chemical products. This helps to reduce the cost of producing ethylene and enhances the economic efficiency and competitiveness of enterprises. In the comprehensive utilization of C5 fractions, the most valuable components are isoprene, mesitylene, and (dicyclopentadiene); these three account for 40%-55% of the C5 fraction obtained from cracking. Isoprene is one of the main products, making up 15%-25% of the C5 fraction. Its primary uses include the production of isoprene rubber, butyl rubber, and SIS thermoplastic elastomers, as well as intermediates for pharmaceuticals and pesticides. It is also used in the synthesis of lubricant additives, rubber vulcanizing agents, and catalysts, offering great prospects for further development. 2 Properties of Isoprene Isoprene (2-methylbutadiene), also known as 2-methyl-1,3-butadiene, has the molecular formula C5H8 and a molecular weight of 68.12. At room temperature, isoprene is a colorless, volatile, and irritating oily liquid that is insoluble in water but soluble in ethanol, ether, and propane. It forms explosive mixtures with air, with an explosion limit of >1.6%. Due to the presence of conjugated double bonds, isoprene has reactive chemical properties; it readily undergoes homopolymerization and copolymerization reactions, and can react with many substances to form new compounds. 3 Methods of producing isoprene Isoprene is one of the most widely used and highest-volume components in C5 fractions; it is typically obtained by extracting it from mixed C5 fractions derived from the pyrolysis of heavy liquid hydrocarbons. The main production methods include three types: the synthesis method, the dehydrogenation method, and the extraction method, as shown in Figure 1. 3.1 Synthesis methods (1) Aldehyde method: Synthesized from isobutylene and formaldehyde, in a one-step or two-step process. The enal one-step method involves the gas-phase catalytic synthesis of isobutylene and formaldehyde in a single step. The butene-formaldehyde two-step process involves the synthesis of 4,4-dimethyl-1,3-dioxolane (DMD) through a Prins reaction between isobutylene and formaldehyde in the presence of an acidic catalyst; in the second step, DMD is decomposed to yield isoprene, formaldehyde, and water. This method was developed in Japan and industrialized there, with Russia also achieving industrialization. The disadvantages of this method are: long process, high cost, low yield, and poor selectivity. ②The isobutylene-formaldehyde one-step and two-step processes have long procedures and complex by-products. The one-step synthesis of isoprene from isobutylene and formaldehyde is very attractive, and extensive research has been carried out in Japan and the former Soviet Union. This method was recommended as Russian technology at the China-Russia technology exchange meeting. (2) Propyne method: The Italian company Anicschi uses natural gas as a raw material to produce acetylene, which is then combined with propyne to synthesize methylbutanol, and further dehydration yields isoprene. Its reaction occurs in three steps. The reagents used in this method are non-corrosive, and the yield is high. However, due to the high cost of raw materials and the great danger associated with acetylene, it is generally not used. (3) Turpentine pyrolysis method: There are experimental units in China, but no reports of successful results have been published. The American company Goodsey once used the propylene dimerization method, but it has stopped production now. 3.2 Dehydrogenation method: Isopentane or isoprene is used as a raw material for dehydrogenation to produce isoprene. It is produced by the American company Guthrie, the Dutch company Sieloe, and Russia. The process for producing isoprene from isopentene dehydration is as follows: The production of crude isoprene involves 3 steps: dehydration, adsorption, and distillation. To produce polymeric-grade products, crude isoprene must also be purified by extraction. The process is shown in Figures 5 and 6. 3.3 Extraction method: The extractants used include acetonitrile, dimethylformamide, N-methylpyrrolidone, and N-formylmorpholine, among others. Countries such as Japan, the United States, the Netherlands, and Germany all use different extractants. Acetonitrile is used to produce isoprene; its raw materials are readily available and inexpensive, it causes little corrosion to carbon steel, it has a low viscosity and a low boiling point, and the production process is short. Therefore, the acetonitrile method is primarily used in industry. The isoprene obtained by this method has a purity of up to 98%, with a yield of over 90%. Another method uses dimethylformamide, with low energy consumption; the purity of isoprene reaches 99.5% and the yield is 95%. Of particular interest is the ARCO extraction process. This process is a typical extraction process that has been industrialized and for which a license has been obtained. Starting from the C5 fraction of the olefin plant, this process consists of four parts: decyclopentadiene removal, extractive distillation, solvent regeneration, and product distillation. The simplified flowchart of this process is shown in Figure 7. (1) DMF method: Also known as the GPI method, it was first developed successfully by the Japanese company Ruion. The main process flow is as follows: The C5 feedstock obtained from naphtha cracking comes into full contact with a solvent in a tower; pentane and pentene evaporate from the top of the tower, while the dienes and solvent proceed to the first desorption tower. The dienes that evaporate from the top of this tower are sent to a distillation tower, where 1,3-pentadiene and cyclopentadiene are separated out at the bottom of the tower. The crude isoprene obtained from the top of the tower then passes through a second extraction tower, a second desorption tower, and a second distillation tower, ultimately yielding an isoprene product of 99.5% purity suitable for polymerization purposes. The solvent used in this method has high solubility for isoprene, good selectivity, requires a low amount, and results in low operating costs ； The solvent is non-corrosive to the equipment, and ordinary carbon steel can be used throughout the process ； It can simultaneously produce isopentadiene and dicyclopentadiene products of a certain purity as by-products. (2) ACN method: The ACN method is currently the most common approach used abroad for separating the C5 fraction. Its process consists of 3 steps: the first step is to separate cyclopentadiene ； The second step is for the extract to enter the desorption tower, where the desorbed dienes and alkynes are washed with water to remove the acetonitrile carried along; acetonitrile and water are then regenerated in a solvent recovery tower ； The third step is to separate isopropyl and isopropenyl acetylene along with 1,4-pentadiene from the top of the tower; the liquid at the bottom of the tower is then subjected to further distillation, and the isoprene product can be obtained from the top of the tower. The advantages of this method include an abundant supply and low cost of acetonitrile, low corrosivity to equipment, low solvent viscosity, high efficiency of the extraction tower, low operating temperatures, and the ease of resolving issues such as equipment blockage caused by material polymerization. However, the purity of the separated product is not high, and it can only meet the requirements of butyl rubber raw material specifications. To obtain a product of higher purity, further processing is required, which complicates the production process and increases costs. (3) NMP method: The NMP method was first developed by the German company BASF, and subsequent improvements were made. The improved process utilizes catalytic hydrogenation technology and eliminates the second extraction tower. The stream containing dienes is taken from the side stream of the extractive distillation column and fed into the distillation column ； Isoprene, mesoprene, and cyclopentadiene are obtained at the top of the distillation column, while the saturated solvent is returned to the bottom of the extractive distillation column ； The substrate enters the hydrogenation reactor, where it undergoes a hydrogenation reaction on a palladium-containing alumina-supported catalyst; as a result, mesopentadiene is converted into pentene and pentane, while cyclopentadiene is converted into cyclopentene and cyclopentane. The material after hydrogenation is returned to the top of the extraction distillation column, and the distillate from the column top consists of components such as pentene, pentane, cyclopentene, and cyclopentane. This method is characterized by a relatively simple process flow, low solvent toxicity, low energy consumption, and the ability to prevent the polymerization reaction of dienes. 3.4 Other preparation methods: (1) In addition to the solvent method mentioned above, there are also the dimethyl sulfoxide method developed by the French Institute of Petroleum Sciences, the N-formylmorpholine (NMF) method developed by the Italian company Snam, and the B-methoxypropionitrile method developed by the Japanese company Gas Chemical Company. However, these methods have poor overall performance, which is why they have not been widely adopted to date. (2) Utilizing the principle that isoprene and n-pentane can form a binary azeotrope, the American company Goodyear developed an azeotropic distillation method for separating isoprene. This method is characterized by a simple process flow and low energy consumption, and it solves various problems directly related to solvents, such as solvent loss, toxicity, recovery, decomposition, and environmental concerns associated with the use of solvents ； The inhibition of polymerization by isoprene, mesoprene, cyclopentadiene, alkynes, etc., under high temperatures (where the presence of a solvent keeps the system at higher temperatures) ； No explosion of concentrated alkynes occurred ； There is no issue of accumulation of harmful impurities in the solvent. The disadvantage is that the product obtained is an azeotrope of isoprene and n-pentane, which requires more trays and a higher reflux ratio, making it impossible to obtain isoprene with high purity. Further purification and regeneration are rather complicated and not very cost-effective. (3) The chemical adsorption method utilizes the reversible reaction between metal cations (Ag+ and Cu2+) and dienes to produce Ag (or Cu)-Π diene electron complexes; since these complexes are immiscible with organic substances, dienes can be separated from alkanes. The complexation reaction is a reversible reaction, and the diene in the complex can be recovered by changing the temperature or pressure. Chemical adsorption methods have advantages such as low energy consumption, good selectivity, simple equipment, reduced capital investment for equipment, and environmental friendliness, offering great potential for development; however, no industrial applications have been reported to date. A comparison of the production costs for the various methods of producing isoprene is shown in Table 1. As can be seen from Table 1, isoprene obtained through extractive distillation using the C5 fraction has the lowest cost and requires the least investment. Among extraction distillation methods, dimethylformamide is the best choice; however, extraction methods have certain limitations, and the concentration of the C5 fraction production has a significant impact on them