Acetic acid, production methanol carbonylation

The formation of C-C bonds is of key importance in organic synthesis. An important catalytic methodology for generating C-C bonds is provided by carbonylation. In the bulk chemicals arena this is used for the production of acetic acid by methanol carbonylation (Eqn. (9)) in the presence of rhodium- or, more recently, iridium-based catalysts (Maitlis et al, 1998). 

Backus, J., Fabiilli, M., Sanchez, D., and Wong, E. 2003. Acetic acid production via carbonylation of methanol Technical and economical feasibility study, Vol. I, Fugacitech, Inc., Ann Arbor, Michigan, April, 4 (online publication, http //www-personal.engin.umich.edu/ mfabiill/Report% 20rev06.doc). 

In 1970, the discovery of these new organo-soluble catalysts based on Rh, Pd, or Pt was generally considered unfeasible for industrial processes because of the prohibitive price of the metals involved. However, the hgh activity and productivity of these catalysts made possible production levels of 100000 ty 1 with only a few dozen kilograms of precious metals needed as inventory by each single plant. Thus, the amount of precious metal involved represents only a minor part of the investment and the manufacturing costs, i.e., the price of the metal was not an important factor in the production unit cost, provided that its usage occurred without any loss. In 1972 this hypothesis was confirmed by Monsanto and its commercialization of the important process to generate acetic acid by methanol carbonylation [16]. 

Acetaldehyde (ethanal) used to be the most important intermediate for the production of acetic acid. However, since 1970 the production of acetaldehyde lost importance as acetic acid production switched increasingly to methanol carbonylation. The reason for this development has to do with the different feedstock base for both processes. While the production of acetaldehyde starts from ethylene, the synthesis of acetic acid via methanol carbonylation uses the significantly cheaper synthesis gas (CO/H2) as feed, from which methanol is made first followed by a carbonylation step (see Section 6.15 for details). 

In this chapter, we discuss the basic mechanisms and other details of a few industrial carbonylation processes. Among them, in terms of scales of production, acetic acid by methanol carbonylation and acetic anhydride by methyl acetate carbonylation are the two most important (reactions 4.1.1 and 4.1.2). 

Figure 3 shows the production of acetaldehyde in the years 1969 through 1987 as well as an estimate of 1989—1995 production. The year 1969 was a peak year for acetaldehyde with a reported production of 748,000 t. Acetaldehyde production is linked with the demand for acetic acid, acetic anhydride, cellulose acetate, vinyl acetate resins, acetate esters, pentaerythritol, synthetic pyridine derivatives, terephthaHc acid, and peracetic acid. In 1976 acetic acid production represented 60% of the acetaldehyde demand. That demand has diminished as a result of the rising cost of ethylene as feedstock and methanol carbonylation as the preferred route to acetic acid (qv). 

About half of the wodd production comes from methanol carbonylation and about one-third from acetaldehyde oxidation. Another tenth of the wodd capacity can be attributed to butane—naphtha Hquid-phase oxidation. Appreciable quantities of acetic acid are recovered from reactions involving peracetic acid. Precise statistics on acetic acid production are compHcated by recycling of acid from cellulose acetate and poly(vinyl alcohol) production. Acetic acid that is by-product from peracetic acid [79-21-0] is normally designated as virgin acid, yet acid from hydrolysis of cellulose acetate or poly(vinyl acetate) is designated recycle acid. Indeterrninate quantities of acetic acid are coproduced with acetic anhydride from coal-based carbon monoxide and unknown amounts are bartered or exchanged between corporations as a device to lessen transport costs. 

Butane. Butane LPO has been a significant source for the commercial production of acetic acid and acetic anhydride for many years. At various times, plants have operated in the former USSR, Germany, Holland, the United States, and Canada. Only the Hoechst-Celanese Chemical Group, Inc. plants in Pampa, Texas, and Edmonton, Alberta, Canada, continue to operate. The Pampa plant, with a reported aimual production of 250,000 t/yr, represents about 15% of the 1994 installed U.S. capacity (212). Methanol carbonylation is now the dominant process for acetic acid production, but butane LPO in estabhshed plants remains competitive. 

The carbonylation of methanol is currently one of the major routes for acetic acid production. The basic liquid-phase process developed by BASF uses a cobalt catalyst at 250°C and a high pressure of about 70... 

A typical configuration for a methanol carbonylation plant is shown in Fig. 1. The feedstocks (MeOH and CO) are fed to the reactor vessel on a continuous basis. In the initial product separation step, the reaction mixture is passed from the reactor into a flash-tank where the pressure is reduced to induce vapourisation of most of the volatiles. The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel. The vapour from the flash-tank is directed into a distillation train which removes methyl iodide, water and heavier by-products (e.g. propionic acid) from the acetic acid product. 

Acetic Acid Production via Low-Pressure, Nickel-Catalyzed Methanol Carbonylation... 

The major conventional processes for the production of acetic acid include the carbonylation of methanol (originally developed by Monsanto, and now carried out by several companies, such as Celanese-ACID OPTIMIZATION, BP-CATIVA, etc.), the liquid-phase oxidation of acetaldehyde, still carried out by a few companies, and the liquid-phase oxidation of n-butane and naphtha. More recent developments include the gas-phase oxidation of ethylene, developed by Showa Denko K.K., and the liquid-phase oxidation of butenes, developed by Wacker [2a],... 

The appeal of an acetic acid process, based on ethane oxidation, lies mostly in the absence of the need for the energy demanding step for syngas production. On the other hand, it has to compete not only with the well established methanol carbonylation (Section 4.2), but also with the current utilization of ethane in steam crackers for ethylene manufacture. In fact, ethane feedstock becomes attractive for acetic acid production if it is locally abundant and can be supplied at minimal cost, e.g., in a petrochemical complex close to a large gas field. The construction of a semi-commercial plant of 30 kt/a in the Persian Gulf region has been announced. 

Rhodium compounds and complexes are also commercially important catalysts. The hydroformylation of propene to butanal (a precursor of hfr(2-ethyUiexyl) phthalate, the PVC plasticizer) is catalyzed by hydridocarbonylrhodium(I) complexes. Iodo(carbonyl)rhodium(I) species catalyze the production of acetic acid from methanol. In the flne chemical industry, rhodium complexes with chiral ligands catalyze the production of L-DOPA, used in the treatment of Parkinson s disease. Rhodium(II) carboxylates are increasingly important as catalysts in the synthesis of cyclopropyl compounds from diazo compounds. Many of the products are used as synthetic, pyrethroid insecticides. Hexacyanorhodate(III) salts are used to dope silver halides in photographic emulsions to reduce grain size and improve gradation. 

Acetic acid is a key commodity building block [1], Its most important derivative, vinyl acetate monomer, is the largest and fastest growing outlet for acetic acid. It accounts for an estimated 40 % of the total global acetic acid consumption. The majority of the remaining worldwide acetic acid production is used to manufacture other acetate esters (i.e., cellulose acetates from acetic anhydride and ethyl, propyl, and butyl esters) and monoehloroacetic acid. Acetic acid is also used as a solvent in the manufacture of terephthalic acid [2] (cf. Section 2.8.1.2). Since Monsanto commercially introduced the rhodium- catalyzed carbonylation process Monsanto process ) in 1970, over 90 % of all new acetic acid capacity worldwide is produced by this process [2], Currently, more than 50 % of the annual world acetic acid capacity of 7 million metric tons is derived from the methanol carbonylation process [2]. The low-pressure reaction conditions, the high catalyst activity, and exceptional product selectivity are key factors for the success of this process in the acetic acid industry [13]. 

This is the simplest of all carbonylations and is best illustrated by the conversion of iodomethane to acetyl iodide, a reaction crucial to the success of the low pressure Monsanto process for acetic acid production from methanol (equation 1). 

A new CD process for the production of vinyl acetate from acetic acid, ethylene, and oxygen using a Pd-type catalyst at 338 20 K, 2-5 bar was disclosed. This illustrates the wide-ranging possibilities for the application of CD in a variety of processes for the chemical, petrochemical, and petroleum industry. The production of acetic acid from the carbonylation of dimethyl ether or methanol using RD and homogeneous catalyst was also patented. ... 

Acetic Acid Demand. The production capacity for acetic acid by each major process is illustrated in Table 4 [10]. Table 4 shows that methanol carbonylation has nearly replaced oxidation processes for acetic acid production in the United States. Carbonylation accounted for less than 20% of total capacity in 1978 and more than 75% in 1994. The same trend has occurred outside of the United States. In 1978, carbonylation was less than 10% of worldwide capacity and today amounts to about 50%. 

The commercial production of acetic acid by the carbonylation of methanol is carried out continuously in bubble-column reactors at 170 190°C and 20 0 atmospheres using a soluble rhodium catalyst with methyl iodide as a promoter. Tests in small stirred reactors showed that the reaction rate is proportional to the product of the rhodium and iodide concentrations but is independent of the methanol concentration and the partial pressure of carbon monoxide for partial pressures greater than about 2 atmospheres... 

This process still operates today, but is only viable due to the added value of the by-products and can not compete with modem methanol carbonylation for acetic acid production. There is continued interested even today in the oxidation of hydrocarbons, especially ethane, but these technologies are not competitive with modem methanol carbonylation for the generation of acetic acid. 

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