in recent years, the main progress of methanol carbonylation technology for industrial production of acetic acid includes: Cativa process of BP company, Celanese low water content process developed by Celanese company, UOP/Chiyoda acetate process developed by UOP/Chiyoda, new process of producing acetic acid from synthesis gas of Haldor Topsoe by methanol/dimethyl ether, and evaporation process developed by Southwest China Chemical Research and Design Institute. Some of the above new technologies have been used for the improvement of industrial production devices, and some are being prepared for the construction or transformation of industrial devices.
1.1 BP company's Cativa process
In 1986, BP Chemical Company purchased methanol carbonylation technology to produce acetic acid from Monsanto, and the company has been seeking to improve this technology for many years. By 1996, it was finally announced that a new CATIVA acetic acid process based on methanol carbonylation had been successfully developed.
iridium is used as the main catalyst in CATIVA process, and some rhenium, ruthenium and osmium can be added as cocatalysts. The new catalyst was prepared by the reflux reaction of iridium carbonyl [Ir(CO)12], hydroiodic acid and acetic acid aqueous solution at 12℃.
compared with the traditional Monsanto /BP technology, Cativa process has the following advantages: because the price of iridium is obviously lower than that of rhodium, it is more competitive economically; The activity of iridium catalyst system is higher than that of rhodium catalyst system. Less reaction by-products; It can be operated under the condition of low water content (less than 8% in Cativa process and 14% ~ 15% in Monsanto process). If these technologies are used in the renovation of existing plants, the capacity of the plants can be increased with low investment. Moreover, the low water content also leads to the decrease of steam consumption and the improvement of CO conversion rate.
the process was industrialized in the Texas city plant of Sterling company at the end of 1995. The capacity of the plant has been increased from 28 thousand t/a to 34 thousand t/a after being transformed by new technology. Further capacity expansion is still in progress, and it is estimated that the capacity will reach 453,6 t/a after the completion of capacity expansion. In the third quarter of 1997, the capacity of the original BP/Samsung joint venture plant in Ulsan, South Korea was improved from 21 thousand t/a to 35 thousand t/a by this process. In addition, in 1998, BP's methanol carbonylation plant in Hull, England, was changed to Cativa process, and the production capacity increased by 1, t/a..
1.2 Celanese low water content process
In Monsanto process, a large amount of water must exist in the reaction system in order to make the catalyst have high enough activity and maintain sufficient stability. The existence of high mass fraction water (14% ~ 15%) in the reactor leads to the fractionation of water from acetic acid as the largest energy-consuming step, and also becomes the "bottleneck" for the expansion of plant capacity. If a way can be found to compensate the decrease of reaction speed and the stability of catalyst under the condition of low water content, then the operation with low water content will definitely bring about a great reduction of operation cost.
In 1978, Hoechst Company, now Celanese Chemical Company, built a large-scale acetic acid production plant with Monsanto process in Clear Lake, Texas. On this basis, the company successfully developed a new production process of Celanese low-water acetic acid in the early 198s. The core of acetic acid technology with low water content is to add high concentration of inorganic iodide (mainly lithium iodide) to rhodium-based catalyst to enhance the stability of the catalyst system. After adding lithium iodide and methyl iodide, the water content in the reactor can be greatly reduced while maintaining a high reaction rate, thus greatly reducing the separation cost of the new process. The change of catalyst composition in Celanese low-water acetic acid production process allows the reactor to operate at low water content and high methyl acetate reaction concentration, which increases the capacity of the reactor and purification system.
Celanese low-water acetic acid process is similar to the traditional Monsanto /BP process, and its main technical advantages are: increased plant capacity, reduced utility consumption and investment cost per unit product; The disadvantage is that the use of high concentration of iodized salt leads to increased corrosion and the amount of residual iodized salt in the product increases. The high content of iodized salt in the product may lead to catalyst poisoning in the production of downstream products of acetic acid, such as vinyl acetate monomer (VAM), so it must be removed.
in order to overcome the problem of high iodide concentration in acetic acid products, Celanese has developed the Silverguard process to separate trace iodide impurities from acetic acid. In this process, silver metal ion exchange resin is used to separate iodide impurities from acetic acid. After treatment, the mass fraction of iodide in acetic acid is lower than 2×1-9, which is far lower than the level of 1×1-6 in ordinary process. The company also announced another new technology to separate iodide from acetic acid, which uses polymer resin combined with metal salt to react with halide impurities from halide-containing liquid. The advantage of this method is that halide impurities can be separated effectively in one step, which can avoid adding distillation and recovery systems.
1.3 UOP/Chiyoda Acetica process
For liquid phase carbonylation reaction, the catalyst immobilized on solid carrier has some potential advantages, especially it is easy to be separated from the reaction mother liquor. The mechanism of heterogeneous carbonylation promoted by iodide is similar to that of homogeneous system. It has been reported that high reaction rate can be obtained with this supported catalyst under reaction conditions. BP has developed a heterogeneous catalyst system. In the new catalyst system, rhodium, nickel, cobalt or iridium are impregnated on an activated carbon carrier, and then the catalyst is reduced with hydrogen at 4℃ to obtain a catalyst with reactivity. Using this catalyst, the methanol conversion rate is 98.4% and the acetic acid selectivity is 58%. There are many kinds of polymer carriers with thermal stability, such as polyvinyl pyridine and polyvinylpyrrolidone (PVP) crosslinked polymer developed by Chiyoda Company. On this basis, the company developed a new production process of Acetica acetic acid.
Acetica acetic acid production process was successfully developed by Chiyoda and UOP, which used heterogeneous supported catalyst and bubble column reactor for methanol carbonylation. Using methanol and CO as raw materials, the rhodium-based catalyst supported by polyvinyl pyridine resin with methyl iodide promoter was used. It is said that heterogeneous catalysts can obtain high yield and improve the performance of rhodium-based catalysts, and the acetic acid yield is higher than 99% in terms of methanol. The synthetic reactor of this process can be operated under the condition of low water content (3% ~ 8%). The concentration of hydrogen iodide in the reactor is low, and the corrosion problem is small. Moreover, compared with the traditional process, another major feature of the new process is that the reactor uses a bubble column, which eliminates the sealing problem of the stirred tower reactor, and the operating pressure can be increased to 6.2Mpa. In addition, UOP has also developed a patented iodide separation technology, which can reduce the mass fraction of iodide in acetic acid products to 1× 1-9 ~ 2× 1-9.
in order to promote the application of this process in China, UOP/Chiyoda signed a * * * development agreement with Southwest Chemical Research and Design Institute, and carried out a scale-up verification test in Chengdu, Sichuan from 1998 to 1999, and all the indexes reached or exceeded the design value.
1.4 process of producing acetic acid from synthesis gas of Hald or TopSOE by methanol/dimethyl ether
The methanol-acetic acid co-production process of Hald or TopSOE is a brand-new acetic acid production technology. Methanol, the raw material of traditional carbonylation process to produce acetic acid, is generally purchased from outside. In order to cancel the need for external supply of methanol, Haldor Topsoe adopted the method of combining methanol synthesis with acetic acid production, and juxtaposed methanol production with CO production. The main disadvantage of this process is that the pressure of methanol synthesis is much higher than that of acetic acid synthesis. However, the combined production of methanol and dimethyl ether has basically overcome this defect. The process is divided into two steps: the first step is to produce methanol and dimethyl ether (DME) from synthesis gas; Step 2: Carbonylation of methanol and dimethyl ether to produce acetic acid.
in the presence of methanol synthesis and methanol dehydration catalysts, synthesis gas is converted into a mixture of methanol and dimethyl ether:
CO+2H2 = CH3OH
2CH3OH = The wat generated by that reaction of CH3OCH3+H2O
is convert into CO2 and H2 through the water-gas shift reaction:
H2O+co = CO2+H2
in order to reduce the amount of CO2 generated by the water-gas shift reaction, The feed gas of methanol/dimethyl ether reactor is operated at high V(H2)/V(CO) ratio (2∶1 to 3∶1), and the conversion rate is equivalent to that of traditional methanol synthesis process at 2.5 ~ 5. MPa. This synthesis pressure is equivalent to the pressure of acetic acid synthesis part. The material flow from the methanol/dimethyl ether reactor is cooled to separate methanol, dimethyl ether and water.
in the synthesis of acetic acid, dimethyl ether and methanol are catalytically carbonylation to produce acetic acid. In order to meet the demand for CO in carbonylation reaction, the raw material CO is kept in excess, generally, V(CO)/V (methanol+dimethyl ether) = 1 ~ 1.5 ∶ 1.
ch3oh+co = ch3cooh
ch3och3+2co+H2O = 2ch3cooh
carbonylation is carried out in liquid phase at 1 ~ 25℃ and higher pressure of 2.5 ~ 5. MPa.
1.5 Evaporation Process of Southwest Research and Design Institute of Chemical Industry
Southwest Research and Design Institute of Chemical Industry in China started the research and development of carbonylation to acetic acid in the 197s, and achieved a lot of research results, which finally formed the "Reaction Method of Acetic Acid Synthesis from Methanol in Low Pressure Liquid Phase" authorized by China National Intellectual Property Administration, a patent with independent intellectual property rights in China. This patent takes rhodium carbonyl complex as catalytic active substance, adopts reaction engineering and separation engineering technology different from BP rhodium catalyst technology, and increases the reaction depth by adding a second conversion reactor, reducing the water content in the reaction solution and cooperating with other reaction engineering methods, and at the same time, converts rhodium catalyst which is easy to decompose and precipitate into stable rhodium complex which can withstand heating evaporation. Therefore, this technology can adopt evaporation technology different from BP technology, which can greatly improve the acetic acid content in crude products and reduce the circulation of evaporator mother liquor.
The process of carbonyl synthesis of acetic acid developed by Southwest Research and Design Institute of Chemical Industry has the following characteristics:
A. High conversion rate and selectivity, less by-products, less discharge of three wastes and good product quality; Close to or reached the advanced level in the world;
b due to the adoption of evaporation process, the production capacity of the reactor is improved and the energy consumption is reduced;
C. The reaction conditions are mild. Although the catalyst is precious metal, its stability is enhanced, its life is long and its dosage is reduced.
D. The production cost is not higher than any other carbonyl synthesis production method;
e. the process flow is reasonable, easy to control and stable and reliable in operation.