PERP Program – New Report Alert October 2003
Nexant’s ChemSystems Process Evaluation/Research Planning program has published a new report, Acetic Acid (02/03-1). Acetic acid is a raw material for several key petrochemical intermediates and products including vinyl acetate monomer (VAM), acetate esters, cellulose acetate, acetic anhydride, monochloroacetic acid (MCA), etc., as well as a key solvent in the production of purified terephthalic acid (PTA). No other large volume industrial organic chemical can claim the varied feedstocks and production approaches that acetic acid can. As shown in the figure below, commercially employed feedstocks include: (1) natural gas based derivatives methanol and carbon monoxide, (2) ethylene and ethylene derivatives, (3) alkanes such as ethane, butane, and naphtha, (4) syngas derived from coal, and (5) renewable natural sources. All of these carbon sources are still in commercial use for acetic acid production; however the proportion that each of these feedstocks contributes to total acetic acid production has changed over time and will continue to do so, as seen in the next figure. Feedstock Choices and Process Routes for Acetic Acid Production Liquid Phase Oxidation Naphtha
Direct Vapor Phase
Liquid Phase Oxidation Acetaldehyde Direct Vapor Phase Natural Gas
Methanol Fuel Oil
Fermentation Carbohydrates Q303_4108.ppt
* Comparative process economics analyzed in this study
Acetic Acid Feedstock Change in the U.S. and W.E.
Capacity, Thousand Metric Tons
4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 1973
Before 1960, natural biosynthesis using sugars and alcohols and non-selective alkane activation using butane and naphtha were the major technologies employed for acetic acid production. In the 1960s, the Wacker 2-step ethylene oxidation process via acetaldehyde became a commercial process, while methanol carbonylation technology began to emerge as a serious contender and has become today’s dominant acetic acid technology. In the late 1990s, Showa Denko commercialized an improved ethylene oxidation process by using a one-step oxidation process instead of the old 2step oxidation as used in the Wacker process. During the last few years, taking advantage of the inexpensive ethane feedstock readily available in the Middle East, SABIC revived the alkane activation technology by catalytically oxidizing ethane to selectively and directly produce acetic acid. Finally, Celanese announced last year its intention to develop a novel high tech biocatalytic process for acetic acid manufacturing. It is interesting to note that the development of acetic acid technology began with fermentation, was followed by oxidation and then carbonylation processes, and is now headed back to oxidation and fermentation technologies. In the 1970s Monsanto developed the rhodium/iodide catalyst system for methanol carbonylation, supplanting the cobalt iodode catalyzed high pressure process pioneered by BASF in 1960. In 1986, ownership of the Monsanto technology was acquired by BP, which has further developed the process. This technology features acetic acid selectivity greater than 99 percent based on methanol.
CH3OH + CO
The rhodium catalyzed methanol carbonylation process is highly selective and operates under mild reaction pressure (around 500 psia). However, because of the high price of rhodium and an expensive and elaborate rhodium recovery section, new developments and other catalysts for methanol carbonylation are continually being investigated. Improvements to the original Monsanto/BP technology have been introduced by Celanese (AO Plus Process), BP (Cativa Process), and Chiyoda Corporation (Acetica Process). An overview of Monsanto/BP’s catalyst system in comparison with that of Celanese AO Plus, BP Cativa, and Chiyoda Acetica processes is given in the following table. Catalyst Systems For Methanol Carbonylation Company/Technology Monsanto/BP Celanese AO Plus BP Cativa Chiyoda Acetica
Central Catalyst Atom Rhodium Rhodium Iridium Rhodium
Cocatalyst (Promoter) CH3I/HI LiI/CH3I CH3I/Re or Ru CH3I/Immobilized Complex on solid support
Methanol carbonylation technology is presently the dominant acetic acid production technology accounting for over 65 percent of global capacity, and this share is growing because it generally affords the lowest cost acetic acid. The “world-scale” of acetic acid plant size using methanol carbonylation technology has also grown significantly from less than 50 thousand metric tons per year in the 1960s to greater than 1 billion metric tons per year currently. Showa Denko has developed a one step, vapor phase process for the production of acetic acid by direct oxidation of ethylene, commercialized in late 1997. Owing to relatively reduced capital outlays needed (no carbon monoxide production), the Showa Denko ethylene based process is claimed to be economical for relatively small 50,000 to 100,000 ton per year acetic acid plants. Acetic acid is produced with high selectivity from a mixture of ethylene and oxygen in the vapor phase at 160 to 210°C over supported palladium based catalyst. The major side reactions are the combustion of ethylene and the production of acetaldehyde.
C2H4 + O2
Under the reaction conditions employed, based on patent data, per-pass selectivities to acetic acid, acetaldehyde (recycled), and carbon dioxide of 85.5 percent, 8.9 percent, and 5.2 percent, respectively, can be achieved. Showa Denko has developed an environmentally friendly, energy saving process by combining the extraction and the distillation operation in which by-product water is efficiently separated from acetic acid. SABIC has developed a new process for producing acetic acid via catalytic gas phase oxidation of ethane. An acetic acid production semi-works plant is set to have an initial capacity of 30,000 metric tons per year and is slated to start up in the second quarter of 2004. According to SABIC’s patents, ethane is oxidized with either pure oxygen (i.e., ethane rich) or air (i.e., ethane lean), at temperatures ranging from 150°C to 450°C and at pressures ranging from 1 to 50 bar, to form acetic acid according to the following stoichiometry: C2H6 + 1.5 O2
CH3COOH + H2O
Undesired by-products of CO, CO2, and ethylene (largely lost on recycle) can also be formed. The new SABIC catalyst system is a calcined mixture of oxides of Mo, V, Nb, and Pd. When an ethane-oxygen system is employed, the selectivity to acetic acid can reach as high as 67 percent at per pass conversions of 13.6 percent and 100 percent of ethane and oxygen, respectively. Alternatively, when an ethane-air system is employed, the selectivity to acetic acid is slightly lower at 60 percent but at a much higher per pass conversion of ethane of 49.6 percent and again at 100 percent conversion of oxygen. The report presents and compares cost of production estimates for acetic acid at U.S. Gulf Coast and Middle East locations by the following processes: Methanol carbonylation - Monsanto/BP - Celanese AO Plus - BP Cativa - Chiyoda Acetica Ethylene direct oxidation (Showa Denko)
Ethane direct oxidation (SABIC) - Oxygen-based with recycle - Air-based without recycle An analysis of the total capital employed per unit of acetic acid produced favors the newer, megascale methanol carbonylation processes. This is an important factor in the overall economics. The largest market for acetic acid is in the manufacture of vinyl acetate, which is subsequently used in polyvinyl acetate and polyvinyl alcohol. Major end uses are in the emulsion paint, adhesive, and coating sectors. The second largest market is as a solvent in the manufacture of purified terephthalic acid. In this high temperature oxidation process, some of the acetic acid solvent is also oxidized, leading to the replacement requirement. The remaining acetic acid market is about evenly divided between acetate esters, acetic anhydride, and collected other uses. Acetate esters are oxygenated solvents with applications in pharmaceuticals, printing inks, coatings, and adhesives. Acetic anhydride is used principally in cellulose acetate for cigarette filters. The commercial portion of the report presents global acetic acid demand by end use and by five world regions. Global acetic acid capacity aggregated by producer is backed up with detailed producer capacity lists for the United States, Western Europe, East Asia, Japan, Central and South America, and rest of world. BP and Celanese are engaged in a battle for global leadership in the acetyls business area, each with about a 25 percent share of global acetic acid capacity. The next tier of producers are Eastman, Millennium, Acetex, and Daicel, each with about 5 percent of global acetic acid capacity. Global and regional acetic acid supply/demand balances are forecast through 2010.
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