Current development status of the separation and cracking of C5 fractions at home and abroad

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Current development status of the separation and cracking of C5 fractions at home and abroad. Different countries around the world apply C5 fractions in various depths and ways. Generally, it can be divided into two approaches: the first category involves the use of mixed C5, including in the production of synthetic petroleum resins, hydrogenated gasoline, as a feedstock for cracking after hydrogenation, in aromatization processes, and as a fuel. The second category involves separate utilization after separation; among these, the separation and recovery of isoprene, cyclopentadiene, (dicyclopentadiene), and mesitylene, which have high concentrations, are key to the utilization of mixed C5 fractions, with isoprene being the core component in this process. Industrially, thermal dimerization is generally used to separate cyclopentadiene, followed by solvent extraction distillation or azeotropic distillation to separate isoprene. Common methods include dimethylformamide extraction, hexanenitrile extraction, N-methylpyrrolidone extraction, and azeotropic distillation. (1) Dimethylformamide extraction method. This method, also known as the GPI method, was developed by the Japanese company Ruion in 1971, resulting in the construction of a production facility with an annual capacity of 60,000 tons. The GPI method uses naphtha as the feedstock for cracking the C5 fraction, with dimethylformamide as the solvent. The advantage of this method is that dimethylformamide has a high solubility for isoprene, good selectivity, requires a low amount, and results in low operating costs. The solvent does not cause corrosion to the equipment, and ordinary carbon steel can be used throughout the entire process ； (2) Hexanenitrile extraction method. This method is one of the most widely used techniques for separating the C5 fraction abroad. It was developed by ESSO in the United States, Atlantic Richfield Company, and Nippon Synthetic Rubber Company; the basic principles are the same and the technologies are similar, with only differences in the process layout. The advantage of this method is that hexanenitrile is a by-product of the oxidation of propylene to acrylonitrile; it is readily available, inexpensive, and causes little corrosion to equipment. Due to the low viscosity of hexanenitrile, the plate efficiency of the extractive distillation column is high ； (3) N-Methylpyrrolidine, same method. This method was first developed by BASF; it features the use of N-methylpyrrolidinum chloride for pre-washing in order to remove cyclopentadiene, 1,3-pentadiene, and dibutyne. Its process is simple, and the solvents used are non-toxic. Furthermore, since the thermal distillation method is not used to remove glutaraldehyde, the yield of isoprene is higher ； (4) Azeotropic distillation method. This method was developed by the American company Goodyear. The separation principle relies on the fact that isoprene and n-pentane can form a binary azeotrope; first, the component in the C5 fraction with a lower boiling point than that of the azeotic mixture is distilled off from this azeotrope. Then, n-pentane present in the C5 fraction (or some additional n-pentane) is used to form an azeotrope with isoprene at a boiling point of 33.6°C. The composition of this azeotrope is typically 73% isoprene and 27% n-pentane. This process is simple and has low energy consumption; due to the low relative volatility, 118 trays are required for the azeotropic distillation, with a reflux ratio of 100:1. This method is applicable to C5 hydrocarbon fractions containing a high amount of n-pentane, or in cases where the presence of n-pentane has no effect on the processing of isoprene. In 2002, United States ethylene production reached 18.7 million tons. Since gaseous hydrocarbons such as ethane and propane account for 75% of the feedstock used in cracking, only a small amount of C5 fraction is produced as a by-product; the total amount of C5 generated through cracking was estimated at 1.1 million tons. Assuming that C5 constitutes 15% of this total, the separation efficiency exceeds 70%. Currently, the United States has 7 concentration units and 3 refining units, whose products are mainly used to produce polyisoprene rubber and styrene-isoprene block copolymers (SIC). Exxon Corporation produces butyl rubber using isoprene as a raw material; it currently has two production facilities that have been operating at full capacity since 1985. Japan’s ethylene production in 2002 was 6.1 million tons, of which 95% was produced using naphtha as a raw material; the amount of C5 cracked was large, roughly on par with that in the United States, at about 1.2 million tons. In 2002, Japan’s production of isoprene was 76,000 tons, which was less than half of the available resources; in other words, the utilization rate of C5 fractions was 50%, lower than that in the United States. However, Japan makes full use of all the components obtained after separation. Not only are three types of dienes utilized, but many new uses have also been developed for the large amounts of monoenes and the small quantities of alkynes present. The market sales of these polymers have been growing steadily, with demand consistently exceeding supply over the years. Western Europe’s ethylene production in 2002 was 15.6 million tons, of which 83% came from liquid hydrocarbon feedstocks. The amount of C5 used in cracking was higher than the total amount used in the United States and Japan combined, estimated at over 2.6 million tons per year. However, these fractions are spread across more than a dozen locations, not as concentrated as in the United States and Japan; as a result, the efficiency of separation is relatively low. Our country began research on the comprehensive utilization of cracked C5 fractions in the early 1970s. A large amount of operational data was collected regarding the separation of cracked C5 fractions; on this basis, mathematical models for separation were developed, and simulations of extraction columns and processes were carried out. After years of effort, a reasonable process was established and put into operation, thereby accumulating the necessary data for scaling up the project. In recent years, new progress has been made in the research on the application of C5 fractions. For example, by hydrogenating the cracking fractions under appropriate conditions – with a space velocity of less than 55 h-1, a pressure of 2.8 MPa, a hydrogen-to-oil ratio of 15–20, and an inlet temperature of 50–70°C – hydrogenated products with an octane number (MON) exceeding 80 can be obtained, making them excellent gasoline blending components. In the production of petroleum resins, Daqing Petrochemical Complex uses catalytic polymerization with catalysts to produce light-colored C5 petroleum resins with a softening point above 90, a color grade of four, an active content of over 60%, and high transparency. In terms of analytical testing, PEC400/6201 (80–100 mesh) ion exchange columns were used to separate components such as cyclopentadiene and dicyclopentadiene from the C5 fraction, resulting in the development of a viable and practical method for raw material analysis. Since our country has failed to establish industrial facilities for C5 separation, this has restricted the development of comprehensive utilization after separation, made it difficult to advance research and development to the industrial testing stage, and prevented the production of products with higher added value. As a result, the vast majority of C5 fraction resources have not been utilized effectively and are used as fuel instead. At present, the output of C5 fraction in our country exceeds 1 million tons per year; it is necessary to make comprehensive use of this resource. This will not only help reduce the production costs of ethylene but also enable the production of many high-value-added products.