Spoilt for choice

07 April 2003 00:00  [Source: ICB]

Acetic acid production has more feedstock and production approaches than any other volume chemical. Jeffrey Plotkin and Larry Song look at the varying methods

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. Feedstocks include the natural gas-based derivatives methanol and carbon monoxide; ethylene and ethylene derivatives; alkanes, such as ethane, butane and naphtha; syngas derived from coal; and renewable natural sources.

Methanol carbonylation (MC) is now the dominant acetic acid production technology, accounting for over 65% of global capacity. This share is growing as MCis the preferred technology for most new plants. In general, the good conversion and selectivity achievable by MC technology, coupled with low cost methanol feedstock and economies of scale enjoyed by mega-sized plants, result in process economics that are tough to beat.

BP and Celanese are engaged in a battle for global leadership in the acetyls business area, each with about a 25% share of global acetic acid capacity; no other company has more than even a 5-6% share.

Renewable resource processes

The first production route for acetic acid was aerobic fermentation of ethanol. In India there are over 50 small volume acetic acid producers, with all but one of these companies using sugar cane-derived molasses fermentation as the source 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 tonne/year molasses-based acetic acid plant in Jakarta, Indonesia.

Interestingly, in 2001, Celanese announced that it is exploring a biocatalytic route to acetic acid in collaboration with Diversa. Diversa's expertise is in bioengineering robust biocatalysts by means of genetic manipulation. This effort must be taken seriously as Celanese is one of the world leaders in acetic acid production technologies via methanol carbonylation. Celanese has stated that a biotech route may allow smaller plants with the same or even better economics.

Natural gas-based routes: methanol carbonylation

In 1913, BASF discovered that methanol could be carbonylated to acetic acid. The reaction takes place according to the following equation: CH₃OH + CO - CH₃COOH

BASF started its first methanol carbonylation plant in 1960 using cobalt iodide (CoI2) as a catalyst. Synthesis took place at around 250¡C and at pressures up to 10 000 psi.

The selectivity to acetic acid from methanol and carbon monoxide was 90% and 70%, respectively. Two plants utilising this technology were built: one in Germany by BASF, and the other in the US by Borden Chemicals. The BASF plant still operates.

In the 1970s Monsanto developed the rhodium/iodide catalyst system for MC. In 1986, ownership of the Monsanto technology was acquired by BP Chemicals, which further developed the process. This technology features acetic acid selectivity greater than 99% based on methanol.

The rhodium-catalysed methanol carbonylation process is highly selective and operates under mild reaction pressure (around 500 psi). However, because of the high price of rhodium and an expensive and elaborate rhodium recovery section, new developments and other catalysts for MC are continually being investigated.

Two major improvements to the original Monsanto/BP technology have been introduced separately by Celanese - (AO Plus) and BP (Cativa process). In the original Monsanto/BP process a substantial quantity of the water in the reaction system is required to maintain good catalyst stability and reaction rates.

Because of the high water concentration in the reactor (14-15%), the separation of water from acetic acid is a major energy cost and unit capacity limitation. Considerable savings in operating and capital costs could be realised by operating at a low water concentration if a way could be found to compensate for the decrease in catalyst stability.

BP Cativa process

In 1996, BP announced details of a new advance in MC 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. BP claims that the Cativa process offers several advantages over the original Monsanto/BP technology.

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 addition, improved carbon monoxide efficiency is achieved and steam consumption is decreased owing to lower water levels. The Cativa process was first implemented at the Sterling Chemicals' plant in Texas City, Texas, US, in 1995 and has since been installed in several existing and new plants.

Celanese AO Plus

In 1978, Hoechst Celanese, now Celanese Chemical, was licensed to operate the Monsanto acetic acid process commercially in its Clear Lake, Texas, plant. Later, 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, above a level not usually thought to be effective as a catalyst stabiliser and promoter. The addition of lithium iodide, with methyl iodide, permits 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. In the Celanese AO Plus technology, this alteration to the catalyst composition allows reactor operation at low water leading to increased reactor productivity and purification capacity. With this proprietary technology, methanol carbonylation capacity in Celanese Clear Lake, Texas, has been increased to 1.2m tonne/year.

The main advantages of the AO Plus technology are increased productivity and lower utility and capital costs per pound of product. This is achieved, however, in a higher iodide environment, which could lead to increased corrosion problems and higher residual iodide in the final product.

High iodide concentration in acetic acid could lead to catalyst poisoning problems in some downstream applications, such as in vinyl acetate monomer manufacture. To overcome such problems, Celanese has developed the Silverguard process for the removal of low levels of iodide impurities from acetic acid.

In a patent issued in 1993, Celanese discloses the use of a silver metal ion exchange resin that removes iodide level to below 2 ppb, as opposed to 10 ppm normally achieved by conventional methods. Celanese discloses the use of polymeric resins co-ordinated with metal salts, which react with and precipitate halide impurities from halide contaminated liquids.

One particular advantage of this system is the ability effectively to remove the halide impurity in a single step, thus avoiding additional distillation and recovery steps. The preferred polymers are reported to have functional groups capable of forming coordination complexes with metal salts such as silver or mercury salts.

Chiyoda Acetica process

Process development in methanol carbonylation is still continuing. Chiyoda has recently developed an acetic acid process, Acetica, based on methanol carbonylation technology, which uses a heterogeneous supported catalyst system and a bubble column reactor. The technology was unveiled in May 1997 at a technology symposium in China.

The feedstocks are methanol and carbon monoxide, with methyl iodide as a promoter and a rhodium catalyst complexed to a polyvinylpyridine resin. 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.

Like the Celanese AO Plus and BP Cativa processes, the Acetica process can be operated at a low water content in the range 3-8 wt% of the reactor liquid. Unlike conventional homogeneous catalytic systems, surplus water is not required to keep the catalyst metal in solution. The reactor has a low hydrogen iodide concentration and subsequently a less corrosive environment.

Another feature of the process is the use of the bubble column reactor, which 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 absence of high pressure seals eliminates leakage concerns at this high pressure. Use of low purity carbon monoxide can lower feedstock costs and capital investment. Chiyoda has recently granted a licence for the Acetica process to Guizhou Crystal Organic Chemical Group in China.

The primary advantage of these latest advances in methanol carbonylation technology is that they permit tremendous increases in capacity compared with conventional MC technology (see chart ), with commensurate decrease in investment dollars per unit volume of acetic acid product.

Two-stage ethylene oxidation

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 (ie Wacker process).

Before the development of the Wacker process, acetylene was used to make acetaldehyde, but this approach suffered from the high cost of acetylene and the toxicity of the required mercury-based catalyst. The basic chemical reaction for the acetaldehyde oxidation to acetic acid is as follows: CH₃CHCO + 0.5O₂ - CH₃COOH

Conventional acetaldehyde oxidation processes involve air oxidation in an acetic acid solution containing 5-15% acetaldehyde in the presence of dissolved cobalt or manganese acetate, typically 0.1 wt%manganese acetate, at 50-70degC. If oxygen is used in place of air, the reaction temperature is raised slightly. The system is pressurised sufficiently to keep the acetaldehyde liquid. Acetic acid forms via a free radical mechanism with peracetic acid as an intermediate.

Since acetaldehyde is usually prepared by the oxidation of ethylene, and acetic acid is made by subsequent oxidation of acetaldehyde, combining both processes into one has always been conceptually attractive to chemists. Much developmental work has been undertaken over the years to produce a simpler single stage process for producing acetic acid directly from ethylene.

Direct ethylene oxidation

Showa Denko has developed a one-step, vapour phase process for the production of acetic acid by direct oxidation of ethylene. Although other producers have patented such processes, Showa Denko has commercialised its technology and started production in late 1997 at a 100 000 tonne/year plant in Japan. 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 can be summarised by the equation: CH₂ = CH₂ + O₂ - CH₃COOH

The reaction takes place in a fixed bed reactor at 150-160¡C. The gases fed to the reactor are ethylene, oxygen, steam and nitrogen, which is used as a diluent. The presence of steam is required to enhance the acetic acid selectivity. The reaction is highly exothermic and boiler feed water fed to the shell side of the reactor is converted into steam.

Selectivity to acetic acid is believed to be over 86%. Acetaldehyde and carbon dioxide are also formed. Acetaldehyde can be recycled to the oxidation reactor further increasing overall acetic acid yields.

Alkane oxidation - non-selective butane or naphtha oxidation

The oxidation of n-butane and light naphtha (which contains low boiling hydrocarbons, especially pentanes and hexanes) is carried out at 160-200¡C. 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. The product ratio can be varied somewhat to obtain more of a desired product.

Liquid phase catalytic oxidation of n-butane was introduced by Celanese in 1952 in a large plant located near Pampa, Texas. This plant is still in operation. A non-catalytic process for the oxidation of light naphtha (in the C4-C8 carbon range) was developed by British Distillers in the UK.

Oxidation of naphtha can be effected at lower temperature and pressures than for butanes. The product composition from naphtha oxidation, however, is far more complex and acetic acid separation more expensive. Process selection, therefore, depends on feedstock availability and suitable outlets for the co-products formed.

Butane gives a higher yield of acetic acid than the higher paraffins, with the major advantages of naphtha feedstock being its lower initial cost and ease of oxidation. Process economics are heavily influenced by co-product valuation.

Today, there are three plants using the naphtha oxidation process to make acetic acid: BP Chemicals in Hull, UK, Daicel Chemical in Otake, Japan, and a state complex in Armenia. It is unlikely that any new acetic acid plants using non-selective alkane oxidation will be built in the future.

Selective catalytic ethane oxidation

Work by Union Carbide conducted in the 1980s revealed good selectivity to acetic acid from the catalytic oxidation of mixtures of ethane and ethylene (Ethoxene process). A key characteristic of this approach is, that in addition to acetic acid, a lot of ethylene is produced as a co-product.

Union Carbide piloted this process in the 1980s, but eventually dropped development. One of the drawbacks of this route is the limited market opportunity, since acetic acid and ethylene, in specific ratios, must find outlets.

In 2001, Sabic announced its intention to build a 30 000 tonne/year acetic acid semi-works plant based on a proprietary catalytic oxidation process. Sabic has not disclosed the detailed process design of this novel acetic acid production technology.

However, according to Sabic patents, ethane is oxidised with either pure oxygen or air at temperatures ranging from 150-450¡C and at pressures ranging from 15-750 psi, to form acetic acid according to the following stoichiometry: C₂H₆ + 1.50₂  - CH₃COOH + H₂0

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 MC technology.

Coal-based process

In a twist on MCtechnology, the process can be adapted for preparing acetic anhydride via methyl acetate carbonylation. There are two methyl acetate carbonylation systems in commercial operation: one by Eastman Chemical in Kingsport, Tennessee, and the other by BP Chemicals in Hull.

The reaction chemistries of these two processes are broadly similar. However, in the case of the Eastman plant recycle acetic acid is produced in an adjacent facility where acetic anhydride is used to convert cellulose to cellulose acetate and acetic acid.

Also unique to the Eastman plant is the fact that the methanol and carbon monoxide are produced within the complex from synthesis gas produced from coal. This process starts with coal, which is partially oxidised in gasifiers to produce synthesis gas with a H2 to CO ratio of about 1:1. A portion of the syngas is separated into pure CO and pure hydrogen. The pure CO is fed to the methyl acetate carbonylation step while the pure hydrogen is combined with the other portion of the synthesis gas and converted to methanol.

Part of the methanol is used to scrub H2S from the coal gasification step. The remainder of the methanol is combined with acetic acid to make methyl acetate. The methyl acetate is carbonylated to give acetic anhydride. The acetic anhydride is used to produce cellulose acetate in another process, and the resulting acetic acid is recycled to the esterification section.

The methyl acetate carbonylation step of the process is catalysed by rhodium. This approach also allows co-production of both acetic acid and acetic anhydride. Reacting acetic anhydride with methanol affords methyl acetate, feedstock for the anhydride, and acetic acid.

Clearly, acetic acid production technology has a rich and diverse history with important advances still being made. Future acetic acid process developments may include direct syngas routes, direct methane carbonylation and technologies based on low cost raffinate-2.

Nexant Chem Systems' upcoming PERP report on acetic acid analyses these various technologies, compares the production economics of existing and developing routes, and presents a market outlook for acetic acid.

Jeffrey Plotkin is director of Nexant's Chem Systems PERP programme (jplotkin@nexant.com). Larry Song is a senior consultant at Nexant (ylsong@nexant.com)

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