Acetic acid technology is perhaps the most diverse of all major industrial organic chemicals. No other large volume chemical can claim the varied feedstocks and production approaches acetic acid can. However, methanol carbonylation has become the dominant acetic acid production technology, accounting for over 65% of global capacity.
The first production route for acetic acid was aerobic fermentation of ethanol. The ethanol is catalytically dehydrogenated or oxidised to acetaldehyde, which in turn is oxidised to acetic acid. While this technology is old, in 2001 Perkebunan Nusantara X built a 30,000 tonnes/year molasses-based acetic acid plant in Jakarta, Indonesia. In addition, Celanese announced that it was exploring a biocatalytic route to acetic acid in collaboration with Diversa.
In 1913, BASF discovered that methanol could be carbonylated to acetic acid. BASF started its first methanol carbonylation plant in 1960 using cobalt iodide as a catalyst. Synthesis took place at around 250oC and at pressures up to 10 000 psi.
In the 1970s, Monsanto developed the rhodium/iodide catalyst system for methanol carbonylation. In 1986, ownership of the Monsanto technology was acquired by BP Chemicals, which further developed the process. The rhodium-catalysed methanol carbonylation process is highly selective and operates under mild reaction pressure (around 500 psi).
In 1996, BP announced details of a new advance in methanol carbonylation technology for acetic acid and claimed significantly lower production costs. The Cativa process uses a catalyst system based on iridium, in conjunction with several novel promoters, such as rhenium, ruthenium and osmium.
The iridium catalyst system has a higher activity compared with the rhodium process, produces fewer byproducts, and is able to operate at reduced water levels (less than 5% for Cativa versus 14-15% with the Monsanto process). All of these factors combine to allow plants to increase their capacity at relatively low capital cost.
In the 1980s, Celanese developed its proprietary AO Plus (Acid Optimisation Plus) technology, greatly improving the Monsanto process. The AO Plus technology was achieved in part by increasing the rhodium catalyst stability by adding inorganic iodide (primarily lithium iodide) in high concentrations, permitting a dramatic reduction in water concentration (to roughly 4-5% water) in the reactor while maintaining a high carbonylation rate. This subsequently reduces the separation costs involved.
Process development in methanol carbonylation is still continuing. Chiyoda has recently developed an acetic acid process, Acetica, which uses a heterogeneous supported catalyst system and a bubble column reactor. It is reported that the supported catalyst system leads to high productivity, improved rhodium management, and produces an acetic acid yield of more than 99% from methanol.
The Acetica process can be operated at a low water content in the range 3-8 wt% of the reactor liquid. The reactor has a low hydrogen iodide concentration and subsequently a less corrosive environment. The use of the bubble column reactor eliminates the need for high pressure seals required with stirred tank reactors. This feature allows the use of low purity carbon monoxide since operating pressures can be increased (up to 900 psi) to maintain optimum carbon monoxide partial pressure.
The liquid phase oxidation of acetaldehyde (using air or oxygen) in the presence of manganese acetate, cobalt acetate, or copper acetate is still used, especially in Europe. This route to acetic acid production generally uses acetaldehyde as an intermediate via oxidation of ethylene (Wacker process).
Showa Denko has developed a one-step, vapour phase process for the production of acetic acid by direct oxidation of ethylene. Owing to relatively reduced capital outlays needed, the Showa Denko ethylene based process is claimed to be economical for 50 000-100 000 tonne/year acetic acid plants.
Showa Denko's process is based on a supported palladium based catalyst containing three components. The reaction takes place in a fixed bed reactor at 150-160oC. Selectivity to acetic acid is believed to be over 86%.
The oxidation of n-butane and light naphtha (which contains low boiling hydrocarbons, especially pentanes and hexanes) is carried out at 160-200oC. The oxidation can be carried out catalytically, usually in the presence of cobalt or manganese, or non-catalytically.
The principal products are acetic acid and methylethylketone. Other organic products, however, such as ethanol, methanol, formic, propionic and butyric acids are also produced. It is unlikely that any new acetic acid plants using non-selective alkane oxidation will be built in the future.
In 2001, Sabic announced its intention to build a 30,000 tonnes/year acetic acid semi-works plant based on a proprietary catalytic oxidation process. According to Sabic patents, ethane is oxidised with either pure oxygen or air at temperatures ranging from 150-450oC and at pressures ranging from 15-750 psi, to form acetic acid.
The new Sabic catalyst system, which is a calcined mixture of oxides of Mo, V, Nb and Pd, allows selectivities to acetic acid as high as 71%. Combining this technology with low cost ethane may result in production economics competitive with methanol carbonylation technology.
(Source: Extracts from Spoilt for choice, by Jeffrey Plotkin and Larry Song of Nexant, ECN 7 April 2003)