1,4-Butanediol production process

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The mainly industrialized production routes for 1,4-butanediol are as follows: (1) Reimer method: 1,4-butanediol is produced in two steps via synthesis and hydrogenation using acetylene and formaldehyde as raw materials. This method is a traditional approach for producing 1,4-butanediol. The improved copper acetylene catalyst uses aluminum silicate as a carrier, and bismuth is added to suppress the polymerization reaction; this overcomes the shortcomings of the original process, resulting in uniform reaction temperatures, stable quality, and ensured safety. (2) Butadiene method: 1,4-butanediol is produced using butadiene as a raw material. The established production methods include the butadiene acetyloxylation method and the butadiene chlorination method, with the former being the most common. The diene acetyloxylation method was first industrialized in 1970 by Mitsubishi Chemical Corporation in Japan. It features a complex production process, high investment costs, expensive catalysts, and significant steam consumption during the hydrolysis step; however, the raw materials are readily available, the reaction exhibits high selectivity, and the ratio of 1,4-butanediol to furan can be easily adjusted. (3) Corail method: Using propylene oxide as the starting material, it is first catalytically isomerized into allyl alcohol; under the action of an organic phosphine ligand catalyst, a formylation reaction takes place to yield the main product, 4-hydroxybutyraldehyde. Subsequently, extraction, hydrogenation, and purification are carried out to obtain 1,4-butanediol. This reaction requires low investment, has a simple process, the by-products have high value for utilization, the rhodium-based catalysts can be reused and have a long lifespan. It yields a high amount of 1,4-butanediol, consumes little steam, and both the formylation and hydrogenation steps are liquid-phase reactions; thus, it is easy to adjust the process load, allowing the production volume of 1,4-butanediol to be adjusted according to market demand. (4) Anhydride esterification hydrogenation method: This process was developed by the British company Davy; by adjusting the process conditions, it is possible to change the ratios of 1,4-butanediol, γ-butyrolactone, and **furan. To maximize the production of 1,4-butanediol in industrial plants, it is possible to rely on the chemical equilibrium between 1,4-butanediol and gamma-butyrolactone; by recycling gamma-butyrolactone until it is exhausted, the production of 1,4-butanediol can be maximized. Its advantages include a high conversion rate of the ester, mild reaction conditions, low requirements for the material of the equipment, a low cost of the catalyst, a long lifespan, low investment and production costs, as well as a wide range of adjustment possibilities for the ratio of 1,4-butanediol to furan. (5) Butane-cyclohexanone-1,4-butanediol combination method: This process combines the gas-phase oxidation method for converting butane into cyclohexanone with the cyclohexanone hydrogenation technique; it still uses C4 fractions as raw materials, and the entire process includes cyclohexanone production, maleic acid hydrogenation, and 1,4-butanediol purification. This process requires only hydrogenation and purification to produce 1,4-butanediol; no esterification step is needed, which shortens the entire process, reduces the number of equipment units, and accordingly lowers investment and operation/maintenance costs. It also has relatively low requirements regarding the purity of maleic anhydride. In this process, the catalyst exhibits high selectivity and a long service life; there is no need to replace it, and the amount of by-products generated is low. It is almost possible to convert all maleic anhydride into 1,4-butanediol. By making slight adjustments to the process conditions during hydrogenation, recovery, and purification steps, furan and γ-butyrolactone can also be produced. 1 Main production technologies At present, there are mainly four industrial methods for producing BDO: (1) the Reppe method using acetylene and formaldehyde as raw materials ； (2) Butadiene acetyloxylation method using butadiene and acetic acid as raw materials ； (3) The allyl alcohol method based on the carbonylation of propylene oxide as the starting material ； (4) Anhydride method. Among them, the traditional Reppe method remains the main production method, accounting for over 80% of the world’s total production capacity. The production process for newly built plants mainly relies on the maleic anhydride method. 1.1 Reppe method The Reppe method is a gas-liquid-solid three-phase reaction system. There are two processes: the traditional Reppe method and the modified Reppe method. Due to the drawbacks of the traditional Reppe process, such as high acetylene partial pressure, susceptibility to **, high investment costs for equipment, low space-time yield, rapid catalyst deactivation, tendency to form acetylene aggregates, and low production capacity, modern BDO production facilities abroad all use the improved Reppe process; only China’s BDO production plants still employ the traditional Reppe method. 　　The modified Reppe process typically uses a slurry bed or suspension bed configuration, with the catalyst present in the reaction liquid medium in a slurry or suspended state; there are roughly two process flows that have been industrialized to date. First is the slurry bed process used by BASF and DuPont ； The other is the slurry bed process used by GAF Company. The main difference between these two processes lies in the way the catalyst is separated from the product. In the BASF and DuPont processes, the catalyst and product are separated within the reactor, whereas in the GAF process, separation takes place outside the reactor. 　　The representative GAF process uses a highly active catalyst and a slurry bed reactor, which improves the removal of reaction heat; the mass fraction of formaldehyde in the feed is increased to 37%, with a conversion rate of 98% based on formaldehyde and a selectivity of 95%, while the reaction pressure is reduced to below 0.2 MPa. The hydrogenation of butynediol is carried out in two stages; the first stage of hydrogenation uses a stirred-tank reactor, with Raney nickel as the catalyst, and the reaction pressure ranges from 1.4 to 2.5 MPa. The material after the first stage of hydrogenation is subjected to a second stage of hydrogenation after the catalyst is separated. The second-stage hydrogenation is carried out in a gas-solid-liquid three-phase trickle-bed reactor using a nickel-based catalyst, at a reaction temperature of 100–150°C and a hydrogen pressure of 10–20 MPa. The improved acetylenediol hydrogenation process increased the yield and selectivity of butanediol. The overall conversion of the two segments of hydrogenated butyrdiol was 100%, with a butanediol selectivity of 95%. 　　The production cost of this method mainly depends on the source of the raw material, acetylene. Its process is simple, the technology is mature, and the raw materials are readily available. The disadvantages are that acetylene gas is flammable and explosive under high pressure, the catalyst has low efficiency, and the equipment investment cost is high. 2 Acetoxylation of butadiene The acetoxylation of butadiene method was developed by Mitsubishi Kasei in Japan in 1970; a butanediol production facility with a capacity of 15 kt/a was built in Yokkaichi, Japan, in 1982. After several expansions, the current production capacity is nearly 40 kt/a. 　　The method consists of three steps. First, acetoxylation of butadiene, acetic acid, and oxygen was carried out at 70°C and 6.8 MPa using Pd-Te/C as a catalyst; the acetic acid was distilled off from the reaction products, yielding 1,4-diacetoxy-2-butene (1,4-DAB) in a yield of over 90%, all in one step ； Then, 1,4-DAB is subjected to catalytic hydrogenation in two stages using Pd/C as a catalyst to yield 1,4-diacetoxybutane ； Finally, BDO was synthesized by hydrolysis at 60 °C and atmospheric pressure using a sulfonic acid-type cation exchange resin as a catalyst, with an overall yield of 99%. Under appropriate conditions, the semi-hydrolyzed product 1-acetoxy-4-hydroxybutane can be deacetylated and cyclized to yield high-purity THF. The separated acetic acid can be reused. By changing the ion exchange resin catalyst, the production ratio of BDO to THF can be artificially controlled. 　　This method uses readily available raw materials, features a safe process with no environmental pollution. High-value THF does not need to be obtained through the dehydration of BDO, thus enabling effective savings in raw materials and energy, while also allowing for arbitrary adjustment of the ratio between the products BDO and THF. However, the entire process is lengthy, requires large investment, and has high steam consumption. 1.3 Allyl alcohol method The acrolein carbonylation method was developed by the Japanese company Kolon, but it has not been put into operation due to a shortage of propylene oxide in Japan. American company Arco Chemical obtained inexpensive propylene oxide feedstock by using the Halcon co-oxidation process; it acquired this technology from Kolon Company in 1988 and built a 34 kt/year BDO plant in Texas, USA. By the end of 1997, Arco Company increased the capacity of its plant to 55 kt/year by removing bottlenecks. 　　This method uses allyl alcohol as the main raw material to produce BDO through processes such as acylation and hydrogenation. Allyl alcohol can be produced by methods such as the hydrolysis of chloropropene, the isomerization of propylene oxide, or the oxidation of propylene. 　　Using allyl alcohol and syngas as raw materials, an aromatic hydrocarbon as solvent, Rh6(CO)16 and triphenylphosphine solution as catalysts, the reaction was carried out at 50–80°C and a pressure of 0.05–0.5 MPa (for H2 and CO) for 6 hours; after water extraction, a 4-hydroxybutyraldehyde solution with a pH of 10 was obtained. Further, in the presence of a Raney nickel slurry catalyst, hydrogenation is carried out at 80–120°C and under appropriate pressure to produce BDO, along with by-products such as n-propanol and 2-methyl-1,3-propanediol. 　　This process is simple, pollution-free, and requires low investment in construction; even thousand-ton-scale plants remain competitive, with by-products having high value for utilization. Rhodium-based catalysts can be reused, have a long service life, achieve high BDO yields, require low steam consumption, are easy to adjust for process changes, and allow production to be adjusted according to market demands for BDO. The economics of this process depend mainly on the cost of allyl alcohol. 1.4 Maleic Anhydride Method Since the 1980s, with the development of technologies for oxidizing n-butane to produce maleic anhydride, the production cost of this compound has decreased, and the process of using maleic anhydride as a raw material to manufacture BDO has gained attention. The maleic anhydride method can be divided into two approaches: direct hydrogenation and esterification hydrogenation. 1.4.1 Direct hydrogenation of maleic anhydride The process for producing BDO through the direct hydrogenation of maleic anhydride was developed in the 1970s by Mitsubishi Oil Chemical and Mitsubishi Kasei in Japan. Hydrogenation can be carried out through liquid-phase or gas-phase processes, with liquid-phase hydrogenation being the most commonly used method. 　　The Mitsubishi process is carried out in two steps. In the first step, maleic anhydride is subjected to liquid-phase catalytic hydrogenation to produce GBL and THF; the catalyst used is Ni-Re, the reaction temperature is 260°C, and the reaction pressure is 12 MPa. The total yield of THF and GBL, based on maleic anhydride, is 90%. By controlling the process conditions, the ratio of GBL to THF can be adjusted from 10:1 to 1:3. The second step is to catalytically hydrogenate GBL to BDO using a Cu–Cr catalyst with K2O as a co-catalyst; the reaction is carried out at 200°C and 10.0 MPa for 4 hours, resulting in a GBL conversion rate of 85%–90% and a selectivity of over 99%. 　　The maleic anhydride hydrogenation method can simultaneously produce THF, GBL, and BDO. Depending on the process conditions, the composition of the product also varies. Typically, the main products are THF and GBL, with a small amount of BDO produced as a by-product. 1.4.2 Maleic anhydride esterification hydrogenation This process [4] was developed by the British company Davy McKee and consists of three main steps. 　　(1) Anhydride esterification. Maleic acid monoethyl ester is first formed through an esterification reaction between maleic anhydride and excess ethanol. The mono-esterification reaction is carried out at 0.1 MPa and 50–80°C without the need for a catalyst, yielding 99% yield. Diethyl ester is further subjected to a diesterification reaction with ethanol to produce diethyl maleate. In this step, a solid acid ion exchange resin is used as a catalyst; the reaction temperature is 100–130°C, the pressure is 0.1 MPa, and the yield of the diester can reach 99%. Excess ethanol and water are removed from the reactor through distillation, and the resulting diethyl maleate gas is further distilled and purified to remove unreacted monoethyl ester, which is then recycled back to the reactor. The by-products, which account for 1% of the total amount of the product, are burned away. The excess ethanol recovered from the reaction section is dehydrated through distillation and then mixed with the ethanol recycled from the product distillation before being returned to the esterification reactor. 　　(2) Gas-phase hydrogenation of diethyl maleate. The hydrogenation of diethyl maleate to diethyl succinate is the key step in the entire process. Copper chromite treated by reduction is used as a catalyst, with Ba or Mn added to stabilize it. The hydrogenation reaction is carried out at a reaction temperature of 170–190°C and a reaction pressure of 4.0–4.5 MPa to produce diethyl succinate, which is then hydrolyzed into BDO, with GBL and THF as by-products. The conversion rate of diethyl succinate was 99.5%, and the selectivity for BDO was 79.3%. By adjusting the process conditions, the generation ratios of BDO, THF, and GBL can be changed. Low temperatures (170–175°C) and high pressures (less than 4 MPa) are favorable for the formation of BDO, while high temperatures (above 190°C) and low pressures (less than 3 MPa) are conducive to the formation of GBL and THF. The product structure can be flexibly adjusted according to market changes. To maximize BDO production in industrial plants, it is possible to take advantage of the chemical equilibrium between BDO and GBL by recycling GBL until it is exhausted, thereby achieving the highest possible BDO yield. The product is concentrated and recovered from the hydrogen stream, and then sent to a distiller via a pressurizer for cyclic distillation. A hydrogen source is ensured by adding hydrogen to the circulation system. 　　(3) Separation and purification of BDO and THF. The main components in the mixed product are separated using simple distillation, and then further distillation is carried out to refine THF and BDO into products. Ethanol is recovered and recycled to the esterification process, while GBL and a small amount of unconverted diethyl succinate in the product are recycled to the hydrogenation section. 　　Building on this, the British company Davy Mckee developed a maleic anhydride esterification process using methanol as the esterifying agent. The advantage of this process is that it makes the separation of methanol and water after esterification easier ； It increases the volatility of diethyl maleate, thereby widening the operating range for vapor-phase hydrogenation ； The esterification conversion rate of methanol is as high as 99.5%, so there is no need for the purification of diethyl maleate, nor is there a requirement for recycling unreacted maleic anhydride or monomethyl ester; only pure methanol needs to be recycled ； Thus, **the process was simplified, resulting in the total project cost being 15% lower than that of the previous approach. 　　The maleic anhydride esterification and hydrogenation process is characterized by a wide range of available raw materials, a simple process, low capital investment. It enables the simultaneous production of BDO, THF, and GBL in adjustable proportions. The catalyst used has high selectivity, resulting in few by-products, and no precious metal catalysts are required; it is therefore a relatively advanced industrial method at present. 1.2 Economic Evaluation and Development Trends of Various Processes Table 1 presents a technical and economic comparison of 4 methods for producing BDO (estimates are based on the Gulf region in the United States in 1992); the production capacity of each facility is 45 kt/year. 　　As can be seen from Table 1, the investment cost is lowest using the Davy method, at 6** million dollars ； The Reppe method has the highest cost, at 114.8 million dollars, while the acetoxylation of butadiene and the allyl alcohol methods have similar investment costs. 　　When selecting a production process, it is necessary to consider the sensitivity of the raw materials used in that process to fluctuations in their market prices. Between 1991 and 1992, as the price of butadiene dropped to historic lows, the butadiene acetyloxylation method became the lowest-cost process for producing BDO and THF. Compared to the widely used Reppe method, lower levels of butadiene reduce the production cost of this process by 36.3 cents/kg compared to the former. However, the technical route using butadiene as a raw material involves many steps and is quite challenging; to date, only Mitsubishi Chemical Corporation in Japan has adopted this method. 　　At that time, the production process with the highest cost was the Davy process using maleic anhydride as a raw material. If maleic anhydride is used as the starting material, the cost of raw materials alone is 1.5 to 1.6 times that of the Reppe method (based on the price of maleic anhydride at that time). In recent years, as the raw materials used for succinic anhydride production have gradually shifted to n-butane, the production process for succinic anhydride has also evolved from a fixed-bed oxidation process to a fluidized-bed oxidation process, thereby reducing the production costs associated with this method. It has been reported [5] that the American companies Momsanto and Du Pont collaborated on a pilot-scale study for the oxidation of n-butane using a moving-bed process to produce maleic anhydride; the yield of maleic anhydride in this process was about 30%–40% higher than that achieved with a fluidized-bed process, while costs were reduced by 10%–20%. With the improvements in the process for producing maleic anhydride using n-butane as a starting material, the price of maleic anhydride will further decrease. Additionally, since the esterification method for maleic anhydride requires less investment and generates additional revenue through co-production, this method is highly competitive. The Reppe process is mature, the raw materials are readily available, the product quality is stable, and the cost is low ； The Arco process is pollution-free and features advanced technology; it is particularly suitable for enterprises that produce propylene oxide, making it one of the methods with good prospects for development. 　　The future development trends of BDO production technology include the following aspects. 　　(1) Changes in production capacity. As many European and American manufacturers are optimistic about the Asian market, they are investing in Asia or establishing factories there through joint ventures. With the commissioning of new BDO production facilities in East Asia in 2005, the region’s BDO production capacity will rise to 22% of the world’s total production capacity, while the shares held by the United States and Western Europe will all decline. 　　(2) Changes in production methods. Apart from the Linde-oil method (low-pressure Reppe process) and the technological upgrades of existing acetylene-based BDO plants, it is unlikely that new BDO plants using acetylene as a raw material will be built in Europe and the United States over the next 10 years. The production route for BDO will gradually shift from the Reppe process to methods that use butane/cyclohexanone, butadiene, and propylene oxide as raw materials. 　　(3) Joint production of upstream and downstream products. To ensure and maintain their profit margins, manufacturers are moving toward highly integrated production of upstream and downstream products in the BDO model, as well as establishing branches around the world (globalization). For example, BASF has BDO production facilities in the United States, Western Europe, and Japan. It also produces a variety of derivatives including PBT, thermoplastic polyurethanes, and polyvinylpyrrolidone, with most of these products being manufactured at the same sites, which gives it good market competitiveness. 3 Technological Development Trends Overseas, technological development is progressing in two directions: one is to improve and refine existing processes ； Another is to develop new processes. The development of new processes focuses mainly on the production technology using butane/cycloanhydride as raw materials, and significant progress has been made so far. This technology includes the Geminox process jointly developed by BP and Lurgi, the Sisas process, and the combined process of BASF and Kvaerner. 3.1 Linde/Oilgong process improved by the Peppe method This process [6–8] is a technology successfully developed through collaboration between the German company Linde and the South Korean company Oilgong, now known as SK Company, and it has the following characteristics. 　　Uses low-pressure operation, featuring a safe, modern slurry-type catalyst with a long service life ； Throughout the entire process, the partial pressure of acetylene never exceeds 0.14 MPa; low-pressure operation ensures the safety of the process ； The composition of the feed product can be adjusted flexibly; the methanol content in the formalin aqueous solution feed can range from 2% to 10% (by mass), and formalin can be of various grades. Acetylene feed does not require dilution with an inert gas, and very little energy is consumed in the compression of recycled acetylene ； Each reactor is equipped with a dedicated filtration system, allowing the catalyst to be easily separated from the reaction materials within the reactor ； Compared with the traditional Reppe method, the total operating and investment costs are reduced by 10% to 20%. 　　The Linde/oil-gas process consists of two operational steps: in the first step, butyrdiol is synthesized under low pressure via acetylation using bismuth oxide and active copper oxide suspended catalysts supported on Mg-SiO2 carriers, at temperatures of 80–90°C and pressures of 0.12–0.13 MPa, using acetylene and aqueous formaldehyde. After separation in the reactor, the catalyst and reaction materials remain inside the reactor, while the liquid phase stream proceeds to the acetylenediol purification section; the purified acetylenediol is then sent to the hydrogenation section. The purification unit is equipped with recovery devices for formaldehyde and acetylene. The second step is two-stage hydrogenation. The first stage of hydrogenation is carried out in a slurry reactor. Under conditions of 2.0–2.5 MPa and 60–70°C, with palladium supported on alumina as the catalyst, 1,4-butyne diol undergoes catalytic hydrogenation to yield BDO along with a small amount of 1,4-butene Diol. After being filtered by the cross-filter, the liquid material is sent to the second hydrogenation section. The second hydrogenation reactor is a fixed-bed trickle reactor, operating at a temperature of 120–150°C and a pressure of 2.0–2.5 MPa; the catalyst used is nickel supported on SiO2, and after hydrogenation, BDO is produced with high selectivity after a certain residence time. The raw material for the second hydrogenation reaction is a crude BDO solution containing small amounts of impurities; this solution is then purified through multiple stages of distillation and thin-film evaporation to refine the crude BDO to meet the product specifications. The BDO selectivity on the pilot plant can reach 94%–99%, with a yield of 94%–96% (by mass). If necessary, fatty alcohols and useful by-products can be recovered at the top of the first column in the distillation section. 　　In mid-1995, reports indicated that a German engineering company and a user whose name was not disclosed planned to build a plant with a capacity of 150 kt/a, while Nippon Oil Corporation decided to construct a plant with a capacity of 20 kt/a in 1996 [6], marking that this new process had reached an industrial scale. But so far, there have been no further reports. 3.2 New Process for Butane/Maleic Anhydride The Geminox process [10–11] is a new process for the direct synthesis of BDO from n-butane, which combines BP Chemical’s maleic anhydride process with Lurgi’s fatty acid hydrogenation process. According to BP officials, compared with other processes that use n-butane as a raw material, the Geminox method eliminates the need for steps such as anhydride dehydration, purification, and esterification. This reduces the number of main processing steps from 8 to 4, resulting in a 20% reduction in capital investment costs. Compared with a BDO production plant in the Gulf region of the United States that uses other technologies and has an annual capacity of 30 kt, it is estimated that operational costs can be reduced by 40%. Another feature is that THF and GBL can be produced by making slight adjustments to the process conditions in the hydrogenation, recovery, and purification steps. 　　The key to the process is the catalyst technology for the hydrogenation reaction. Maleic anhydride is produced through the catalytic oxidation of butane and air in a fluidized bed reactor. The material coming out of the maleic anhydride reactor undergoes liquid-phase hydrogenation with hydrogen in a fixed-bed reactor equipped with BP’s hydrogenation catalyst to produce BDO. According to BP, the catalyst used in this process exhibits high selectivity, a long service life, and low levels of by-product formation; the catalyst system ensures a BDO yield of over 94%. In May 1998, BP signed contracts with several companies to use this technology to build a facility at its base in Ohio [12], with completion expected in 2000. 　　Sisas Group, Europe’s second-largest succinic anhydride producer, has also developed proprietary technology for the direct hydrogenation of succinic anhydride, eliminating the intermediate step of esterification of succinic anhydride [13,14]. Its succinic anhydride production process is said to be the most advanced in the world, and this technology is now being used in Feluy’s 106 kt/a BDO facility under construction. The company will also develop a new set of processes by the end of 1999, using butane as a raw material for the production of GBL, THF, and BDO. This simplified process will help reduce costs associated with investment, raw materials, and utility expenses, and a plant using this technology is set to be built in the United States in 2000. 　　Furthermore, a new integrated process developed from BASF’s gas-phase oxidation of butane to maleic anhydride and Kvaerner’s esterification-hydrogenation technology is undergoing pilot testing at a demonstration plant under construction in Ludwigshafen, Germany [15]. This integrated process essentially reduces capital costs through the elimination of the distillation and purification steps for maleic anhydride. Cycloanhydride present in the solution can be esterified without the need for separation. During the hydrogenation process, hydrogen vaporizes the maleate in the solution, enabling its cyclic hydrogenation. Through an esterification intermediate, the Kvaerner process converts the acidic environment into a non-acidic one, thereby allowing the use of carbon steel equipment and enabling copper-based catalysts to be employed in the hydrogenation reactor. 3.3 Green Processes Eastman Chemical is developing a new process for producing BDO from butadiene [16,17]. This so-called “pollution-free green process” consists of 4 steps: first, butadiene is oxidized to 3,4-epoxy-1-butene (EPB) in the presence of a silver catalyst; EPB is then subjected to thermal rearrangement to produce 2,5-dihydrofuran, which is subsequently hydrogenated to yield THF; finally, THF is hydrolyzed to produce BDO. Eastman plans to build a 1,362 t/a EPB pilot plant at its Longview facility in Texas. It is said that this new process offers better economic efficiency compared to traditional technologies.