Production technologies and market for acrylic acid and esters

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1. Overview Acrylic acid is an important basic organic raw material, primarily used in the production of acrylates such as methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), and 2-ethylhexyl acrylate (2EHA). Other applications include superabsorbent resins (SAP), detergents, and water treatment agents. Acrylates are primarily manufactured into resins that are used as surface coatings, adhesives, sealants, binders for non-woven fabrics, and coatings for textile fibers; or they are synthesized into polyacrylates used as thickeners, dispersants, and rheology control agents. According to statistics, the global production capacity of acrylic acid in 2004 was 3.54 million tons per year, of which 1.335 million tons per year came from the Americas, accounting for 37.6%, 1.16 million tons per year from Asia, accounting for 32.9%, 0.965 million tons per year from Europe, accounting for 27.2%, and 80,000 tons per year from South Africa, accounting for 2.3%. In the same year, the consumption of acrylic acid was approximately 2.9 million tons; of this, 8% was used for producing MA, 8% for EA, 30% for BA, 5% for 2EHA, 33% for SAP, 9% for polyacrylates, and 7% for other purposes. In 2004, global acrylate consumption was approximately 2.85 million tons. With the development of industries such as construction, electronics, and automotive, the range of applications for acrylates continues to expand, and demand for SAP keeps rising. It is expected that the global demand for acrylics and esters will grow at a rate of over 4% between 2005 and 2015. Regionally, demand for acrylic acid in Europe is only slightly higher than the GDP growth rate, and the growth rate of demand in the United States will also decline. Asia, particularly China, will be the main driver of growth in demand for acrylic acid and its esters in the future, with an average annual growth rate of 8% to 10%. 2. Current status and progress of acrylic acid production technology. The industrial production of acrylic acid has gone through various process routes, including the Reppe method using acetylene and carbon monoxide as raw materials, the hydrolysis of acrylonitrile, and the oxidation of propylene. In 1995, BASF shut down the last Reppe process plant in Ludwigshafen, Germany, and in 1999, Ciba Specialty Chemicals discontinued its only acrylonitrile process plant in Bradford, UK, thereby making the oxidation of propylene the only method for producing acrylic acid today. The process for producing acrylic acid via the gas-phase oxidation of propylene was developed by UCC Company in 1969, utilizing patented technology from Sohio Company to build a production facility. In this process, propylene is first converted into acrolein under the action of a Mo-Bi oxide catalyst ； Subsequently, under the action of Mo-V oxides, acraldehyde is further oxidized to acrylic acid, and the product is purified by azeotropic distillation. A system of two series-connected fixed-bed reactors is typically used. Since the technical capabilities of Nippon Catalyst Company and Nippon Mitsubishi have surpassed those of the Sohio process, licensing for the entire set of technologies has been granted almost exclusively by these two companies since the 1980s. After more than thirty years of process development and industrial application, the technology for the two-stage fixed-bed gas-phase oxidation of propylene to acrylic acid has become highly mature. To further improve production efficiency and reduce production costs, the relevant manufacturers have improved this process and actively developed new processes that use inexpensive propane as a raw material for production. 2.1 Development of propylene oxidation technology The technical development trends in the two-step oxidation method for propylene include improving catalyst performance, optimizing reaction conditions, and developing new types of reactors. In 1959, Sohio Corporation was the first to develop a Mo-Bi-P composite catalyst for the oxidation of propylene to acrolein, establishing this as a research direction for catalysts used in acrolein production. Subsequently, companies such as LG Chem, Nippon Catalyst, and BASF began developing new catalysts by focusing on catalyst components and production processes, in order to continuously improve the propylene conversion rate and acrolein selectivity; the main components of these multi-component composite catalysts are Mo-Bi-Fe-Co(Ni). During the preparation of composite metal catalysts, LG Chem added substances that are prone to sublimation at high temperatures such as urea, melamine, and methyl oxalate, thereby giving the resulting catalysts a larger pore volume and higher specific surface area, which enhances their catalytic activity. Using Mo12Bi1Fe1Co44K0.036 as the active catalyst component and adding 10% naphthalene, the desired catalyst was obtained through high-temperature calcination. Propylene and oxygen react at 320°C; when the propylene conversion rate is 98.45%, the acrolein selectivity is 79.16%, and the total yield of acrolein and acrylic acid reaches 92.78%. To improve the productivity of acrylic acid, there is a tendency to use high-concentration propylene as raw material and increase the feed rate, which allows for the direct production of acrylic acid at concentrations of 50 wt% to 80 wt%, thereby eliminating the need for adsorption equipment. However, the downside is that heat accumulation tends to occur in the catalyst bed, leading to hot spots; therefore, it is necessary to control the catalyst’s activity to prevent excessive reaction of the products, which could cause coking and deactivation of the catalyst. Nippon Catalyst Company has developed a multi-stage catalyst process that prevents the formation of hot spots. At least two types of catalysts with essentially the same composition but different calcination methods and particle sizes are installed in each fixed-bed reactor tube to create multiple catalyst layers, with the catalyst activity increasing gradually from the gas feed inlet side to the outlet side. The catalyst composition is Mo12W0.5Bi1.7Fe1.4Co7CsxSi1 (where x = 0.02 for the catalyst at the gas inlet and x = 0.01 for the catalyst at the outlet), with a feed consisting of 10 Vol% propylene, 16 Vol% O2, 10 Vol% water vapor, and 64 Vol% inert gas. When the propylene conversion rate reached 98.1 mol%, the selectivities for acrolein and acrylic acid were 96.2 mol%, and the overall yield was 94.4 mol%. In addition to using catalysts with different activities at various reaction stages, the proper use of heat transfer media is also very important for controlling the reaction temperature. BASF uses a spiral baffle with a thickness of 10 mm and a spacing of 730 mm to allow the heat transfer medium (50 wt% each of potassium nitrate and sodium nitrite) to flow upward along the outer wall of the reaction tube. The heat transfer medium is preheated to 335°C before being fed into the reactor; the temperature at the outlet rises by only 3–6°C, thereby maintaining uniform temperature within the reactor and ensuring even distribution of hot spots. When the one-pass conversion of propylene is 90 mol%, the selectivity for acrolein can reach 85 mol%. In catalysts for the oxidation of acraldehyde to acrylic acid, Nippon Catalyst Company has developed high-yield, highly stable catalysts using alkaline earth metals, alkali metals, and inert oxides as carriers. When the carrier was Mg1Si1.5Al0.1 (18.3 wt%), the catalytically active component was Mo12V5W1Cu2Sb0.5 (with a loading of 22 wt%), and acraldehyde, oxygen, water vapor, and an inert gas were used as feedstocks, an acraldehyde conversion rate of 99.2%, an acrylic acid selectivity of 96.0%, and a yield of 95.2% were achieved. After 8000 hours, the acraldehyde conversion rate remained at 99.2%, the acrylic acid selectivity was 95.8%, and the yield was 95.0%, demonstrating good catalyst stability. In terms of reactor improvements, Nippon Catalyst Company has developed a single fixed-bed shell-and-tube heat-exchange reactor that combines the two-step reactions of oxidizing propylene to acrolein and oxidizing acrolein to acrylic acid. By reducing one reactor and related equipment, production costs and investment expenses can be lowered. To overcome the drawbacks of traditional processes, such as the difficult temperature control due to the large amount of heat release in the propylene oxidation reactor, the need for low feed concentrations, and expensive and complex heat transfer equipment, DuPont/Atofina jointly developed a moving bed reactor process that features excellent solid mixing and good heat transfer properties. The catalyst particles are injected at one end of the lift tube reactor, transported at high speed together with the gaseous reactants to undergo reaction, and then discharged at the other end of the reactor, where the acraldehyde product is separated and the catalyst is regenerated and recycled. This reaction does not require air or oxygen; instead, propylene is oxidized through lattice oxygen on the surface of the catalyst. Due to the propylene concentration of over 20%, the short reaction contact time (about 1.2 s), and the catalyst being in its optimal oxidation state, the selectivity for acrolein is very high. At a moving bed temperature of 348°C and a feed composition of propylene/vapor/N2 = 21.1/9.5/69.3 (mol%), the propylene conversion in both the riser and the moving bed is greater than 60%, and the C3/C2 ratio is greater than 95%. 2.2 Development of propane oxidation processes In recent years, the development of new processes for producing acrylic acid using propane, which is inexpensive and readily available, has become a topic of interest. The process of oxidizing propane to acrylic acid can be divided into two methods: the dehydrogenation of propane and the direct oxidation of propane. 2.2.1 Propane Dehydrogenation Process The propane dehydrogenation process involves first dehydrogenating propane to produce propylene, which is then converted into acrylic acid using a conventional two-step oxidation process. A new oxidation-dehydrogenation process is employed in the production of propylene from propane, overcoming the drawbacks of traditional catalytic dehydrogenation processes such as excessively high reaction temperatures and easy carbon deposition on the active surface of the catalyst. With a propane conversion rate of 10% to 30%, the selectivity for propylene can reach 74%; therefore, BASF has actively developed this process. Propane dehydrogenation to propylene primarily uses multimetal oxide catalysts containing Mo, V, Te, etc. However, due to the longer reaction pathway and complex process, both the cash cost and the net production cost are high. Furthermore, in terms of utility services, recovering propane and removing light components from the recycled gas also require significant additional costs; therefore, it is only attractive when the price of propane is 1/3 or even lower than that of propylene. 2.2.2 Direct oxidation of propane process The key to the process of converting propane directly into acraldehyde or acrylic acid lies in developing effective catalysts that can selectively activate the strong C-H bonds in propane under suitable operating conditions. Research on catalysts focuses on three categories, namely modified vanadium phosphoxide (VPO), heteropoly acids and their salts, and multi-component mixed metal oxide catalysts. To date, the catalyst with the best performance that has been developed is the Mo-V-Te(Sb)-Nb-O catalyst for the ammoxidation of propane to acrylonitrile, which enables a one-pass yield of acrylic acid of around 50%. It is generally believed that Te can promote the progression of oxidation reactions, while Nb can prevent further oxidation of the desired products. The composition of a similar catalyst developed by Nippon East Asia Synthetic Chemical Company is Mo:Sb:V:Nb:K = 1.0:0.25:0.30:0.12:0.013, with solid particle sizes ranging from 16 to 30 mesh. A gas mixture with a volume composition of 4.4% propane, 7.0% oxygen, 26.3% nitrogen, and 62.3% water vapor was fed into a reactor at 400°C at an air velocity of 1600 h-1; when the propane conversion rate reached 35%, the selectivity for acrylic acid was 72.6%. VPO catalysts containing Ce and La can be prepared by microwave heating. 99% V2O5 was reduced in an alcohol solution for 6 hours, after which 85% phosphoric acid was added, followed by the addition of La(NO3)3, Ce(NO3)2, ((NH4)6Mo7O24)·4H2O, or Bi(NO3)3, to yield a black solid. The black solid was heated with microwaves at a frequency of 2450 MHz and an output power of 160 W for 2 minutes. At 450°C and atmospheric pressure, the feed rate was N2/O2/C3H8 = 17.5/3.2/5.1 ml/min, with saturated water vapor at 65°C added. The results showed that the catalyst with a composition of P:V:La:Ce = 1.1:1.0:0.04:0.04 achieved the highest propane conversion rate and acrylic acid selectivity, at 50.3% and 85.5%, respectively. Due to the highly exothermic nature of propane oxidation, fluidized bed or circulating fluidized bed reactors (CFBR) have been developed to remove the reaction heat promptly, thereby improving yield. CFBR can also prevent the catalyst from being carried away by reaction products as it moves from the top to the bottom of the reactor, thereby reducing losses. Nexant/Chemsystems conducted an economic analysis of the process for producing acrylic acid from propane, and the results showed that the investment cost for a propane processing plant with a capacity of 220,000 tons per year is approximately $270 million, whereas a similar-scale acrylic acid production plant requires only $190 million. This is because alkanes are difficult to activate, and the reactions generate more heat, necessitating larger heat exchangers and reactors. The company’s comparison of production costs for different processes also showed that the total production cost for the propane process was 953 dollars per ton, whereas it was 935 dollars per ton for the propylene process. Therefore, despite the lower price of propane, its economic viability must be improved by enhancing the selectivity of the catalyst to at least 60%–75% in order to be comparable to that of the propylene process. 3 Current Status and Progress of Esterification Technology Common acrylate varieties include general acrylates such as MA, EA, n-BA, and 2EHA, which are typically produced by the esterification of crude acrylic acid with corresponding alcohols. The esterification reaction uses acidic catalysts, such as sulfuric acid, p-toluenesulfonic acid, or strongly acidic cation exchange resins. Special acrylates such as hydroxyethyl acrylate and 2-hydroxypropyl acrylate are synthesized by reacting acryic acid with ethylene oxide and propylene oxide respectively. MA, EA, and n-BA are typically produced on dedicated production lines using a continuous process, while other esters used in relatively smaller quantities are manufactured via a batch process; one production line can be used to produce multiple types of esters. The production process for acrylates was essentially matured by the 1970s. Since acrylic acid and its esters polymerize easily, a polymerization inhibitor is generally added in the esterification reaction. To increase the conversion rate of the reaction, an excess of reactant (alcohol or acrylic acid) is usually used; therefore, the aim of improving the esterification process is primarily to enhance the efficiency of separating and recovering the product from the catalyst. 4 Progress in the Development of Acrylic Acid and Esters Technologies in China Since 1984, when Beijing Dongfang Chemical Factory introduced the two-step oxidation technology for acrylic acid along with the corresponding equipment from Nippon Catalyst, thereby establishing China’s first large-scale production facility for acrylic acid and esters, domestic research institutions have been working to assimilate this imported technology. Through the construction of new facilities and technical upgrades to existing ones, they have strived to develop processes and technologies with independent intellectual property rights, and have made certain progress in the development of catalysts. Shanghai Huayi Acrylic Company has completed the research and development of \"domestically developed production process software for 50,000 t/a of acrylic acid,\" establishing an acrylic acid production technology with independent intellectual property rights; it has also built an industrial-scale testing facility capable of producing 6,000 t/a of acrylic acid. In 2003, a expansion project to increase the production capacity of acrylic acid to 30,000 t/a was carried out; it was completed and put into operation in July 2004, using the LY-a series of acrylic acid oxidation catalysts developed by the Lanzhou Petrochemical Research Institute of China National Petroleum Corporation. The LY-a series catalysts have been tested in a 6,000 t/a plant for 7,000 hours continuously, with a propylene conversion rate of ≥98%, a total yield of acrolein + acrylic acid of ≥92 mol%, and an acrylic acid yield of ≥87 mol%. It is characterized by high catalyst activity and selectivity, as well as a high space velocity; due to the low reaction temperature and bed resistance, the catalyst has a long service life. The 30,000 t/a acrylic acid oxidation reactor used in the facilities of Shanghai Huayi Company was manufactured by Xi’an Aerospace Huawei Pressure Vessel Equipment Manufacturing Co., Ltd.; it is currently the fixed-bed reactor with the highest production capacity among acrylic acid reaction units in China. This reactor was designed and manufactured using domestic dual-pump technology, putting an end to the reliance on imported reactors for acrylic acid production. The new esterification catalytic system developed by Shanghai Huayi Company uses a mixed catalyst of methanesulfonic acid and p-toluenesulfonic acid, which causes less corrosion to the equipment. A small amount of this catalyst is sufficient to produce butyl acrylate at high yields under the same or even more favorable reaction conditions, thereby increasing the plant’s production capacity from the originally designed 30,000 t/a to 65,000 t/a. On this basis, the factory has also developed new production technologies for MA, BA, and hydroxypropyl/ethyl acrylate on its own, which have passed the expert evaluation organized by the Shanghai Municipal Economic Commission. Currently, the production capacities of MA and BA have been increased to 30,000 t/a and 70,000 t/a respectively. 5 Supply and demand in domestic and international markets. In 2004, the world’s six major acrylic acid producers were BASF (750,000 t/a), Stohas (700,000 t/a), Dow Chemical (550,000 t/a), Nippon Catalyst (380,000 t/a), Arkema (290,000 t/a), and Mitsubishi Chemical (190,000 t/a). Their production capacities accounted for 21.2%, 19.8%, 15.5%, 10.7%, 8.2%, and 5.4% of the global total respectively. In developed countries, the growth of acrylics is primarily driven by the rapid expansion of SAP applications. In 2004, the global SAP production capacity was 1.204 million tons per year, while consumption was 1.078 million tons; 95% of this was used in personal care products, with 957,000 tons of acrylic acid being consumed. Consumption increased by 27% compared to 850,000 t/a in 2000. As the developed **SAP and acrylate markets are nearing saturation, demand for acrylates is expected to grow at a slower pace over the next 5 years, at around 2% to 2.2%. The growth of acrylate in the Asian region (especially in China) is mainly driven by the rapid development of infrastructure such as construction and transportation, which increases the demand for acrylate-based products such as architectural coatings and sealants. Acrylic acid in our country is primarily used to produce acrylates, accounting for about 80% of the total consumption of acrylic acid. Acrylates are mainly used in adhesives, coatings, chemical fibers, leather, and other fields. As of March 2005, China’s acrylic acid production capacity reached 374,000 tons per year (including 33,500 tons per year of high-purity acrylic acid). Of this, Beijing Dongfang Petrochemical Company had a production capacity of 75,000 tons per year, Sinopec Jihua Branch had 33,000 tons per year, Shanghai Huayi Acrylic Acid Company had 66,000 tons per year, Jiangsu Yulang Chemical Company had 40,000 tons per year, and Yangba Company had 160,000 tons per year. In 2004, China’s actual production was 170,000 tons. In 2004, the production capacity for acrylates was 230,000 tons per year, of which MA accounted for 45,000 tons per year, representing 19.6% of the total ； EA: 45,000 t/a, accounting for 19.6%, BA: 140,000 t/a, accounting for 60.8%. Despite the increase in capacity and production of acrylic acid and its esters, these still cannot meet demand; in 2004, China’s net imports of acrylic acid and its salts, as well as acrylic acid esters, were 112,000 tons and 210,000 tons respectively. With the further development of China’s economy, the demand for acrylates and their esters from industries such as construction, chemical fibers and textiles, sanitary materials, and automobiles will continue to rise. It is estimated that this demand will reach 1.16 million tons in 2010 and 1.7 million tons in 2015; in 2010, China’s consumption will account for approximately 31.4% of the world’s total consumption. Due to their optimism about the Chinese market, domestic and foreign companies are investing in the construction of new acrylic acid and ester production facilities or expanding existing ones. Investment projects such as Yangba Group, Shenyang Paraffin Chemical Co., Ltd., Formosa Plastics (Ningbo) Company, Shanghai Huayi Acrylic Company, and Beijing Dongfang Petrochemical Company are scheduled to come online between 2005 and 2007. Areas such as Guangrao in Shandong, Cangzhou in Hebei, and Luzhou in Sichuan are also planning acrylonitrile plants with capacities of 80,000 to 110,000 tons per year. With the construction of new facilities and capacity expansions, as well as the gradual maturity of the domestic market, it is estimated that by 2010 China’s acrylic acid production capacity will reach 630,000 tons per year, while the production capacity for acrylates will reach 750,000 tons per year. However, there will still be a certain gap compared to demand levels. At present, the production of acrylic acid in our country relies mainly on imported technologies; acrylic acid production technologies with independent intellectual property rights have also been put into industrial use. However, catalysts and manufacturing processes will continue to be improved, and new applications for acrylic acid and its esters need to be explored in order to produce more green products that can meet the increasingly stringent environmental regulations.