Feedstock acetic acid

A simplified process flow diagram of the ethylene-based vinyl acetate process is shown in Figure 20 [25]. Acetic acid feedstock is vaporized in the presence of fresh ethylene feed and unreacted ethylene recycle gas in the acetic acid vaporizer. The stream is preheated and high purity oxygen is added with a special mixing nozzle. The quantity of reactants and oxygen are carefully controlled to ensure that the mixture is outside of the explosive limits. 

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

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). 

Although the rapid cost increases and shortages of petroleum-based feedstocks forecast a decade ago have yet to materialize, shift to natural gas or coal may become necessary in the new century. Under such conditions, it is possible that acrylate manufacture via acetylene, as described above, could again become attractive. It appears that condensation of formaldehyde with acetic acid might be preferred. A coal gasification complex readily provides all of the necessary intermediates for manufacture of acrylates (92). 

Anhydrous Acetic Acid. In the manufacture of acetic acid by direct oxidation of a petroleum-based feedstock, solvent extraction has been used to separate acetic acid [64-19-7] from the aqueous reaction Hquor containing significant quantities of formic and propionic acids. Isoamyl acetate [123-92-2] is used as solvent to extract nearly all the acetic acid, and some water, from the aqueous feed (236). The extract is then dehydrated by azeotropic distillation using isoamyl acetate as water entrainer (see DISTILLATION, AZEOTROPIC AND EXTRACTIVE). It is claimed that the extraction step in this process affords substantial savings in plant capital investment and operating cost (see Acetic acid and derivatives). A detailed description of various extraction processes is available (237). 

Liquid-phase oxidation of lower hydrocarbons has for many years been an important route to acetic acid [64-19-7]. In the United States, butane has been the preferred feedstock, whereas ia Europe naphtha has been used. Formic acid is a coproduct of such processes. Between 0.05 and 0.25 tons of formic acid are produced for every ton of acetic acid. The reaction product is a highly complex mixture, and a number of distillation steps are required to isolate the products and to recycle the iatermediates. The purification of the formic acid requires the use of a2eotropiag agents (24). Siace the early 1980s hydrocarbon oxidation routes to acetic acid have decliaed somewhat ia importance owiag to the development of the rhodium-cataly2ed route from CO and methanol (see Acetic acid). 

There are some chemicals that can be made economically from coal or coal-derived substances. Methanol and CO are used to make acetic anhydride and acetic acid. Methanol itself can be made from synthesis gas over a copper-2inc catalyst (see Feedstocks, coal chemicals). 

Production of maleic anhydride by oxidation of / -butane represents one of butane s largest markets. Butane and LPG are also used as feedstocks for ethylene production by thermal cracking. A relatively new use for butane of growing importance is isomerization to isobutane, followed by dehydrogenation to isobutylene for use in MTBE synthesis. Smaller chemical uses include production of acetic acid and by-products. Methyl ethyl ketone (MEK) is the principal by-product, though small amounts of formic, propionic, and butyric acid are also produced. / -Butane is also used as a solvent in Hquid—Hquid extraction of heavy oils in a deasphalting process. 

Other Markets. The use of methanol in the production of formaldehyde, MTBE, and acetic acid [64-19-7] accounts for approximately two-thirds of the worldwide demand for methanol. Methanol is used as feedstock for various other chemicals, such as dimethyl terephthalate (DMT)... 

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

Coumarone—Indene Kesins. These should be called polyindene resins (17) (see Hydrocarbon resins). They are derived from a close-cut fraction of a coke-oven naphtha free of tar acids and bases. This feedstock, distilling between 178 and 190°C and containing a minimum of 30% indene, is warmed to 35°C and polymeri2ed by a dding 0.7—0.8% of the phenol or acetic acid complex of boron trifluoride as catalyst. With the phenol complex, tar acids need not be completely removed and the yield is better. The reaction is exothermic and the temperature is kept below 120°C. When the reaction is complete, the catalyst is decomposed by using a hot concentrated solution of sodium carbonate. Unreacted naphtha is removed, first with Hve steam and then by vacuum distillation to leave an amber-colored resin. It is poured into trays, allowed to cool, and broken up for sale. 

Potential Use. Processes using butylenes as feedstocks have been developed for a group of industrial chemicals that are not currendy produced by these processes or are produced only on a relatively small scale. Such chemicals are isoprene [78-79-5] maleic anhydride [108-31-6] acetic acid [64-19-7] and until recendy, methyl methacrylate and methyl tert-huty ether. These processes are of interest because they may emerge as important processes with suitable improvements, changes in product values, or development of new markets. 

Acetaldehyde. Until the early 1970s, the maia use of iadustrial ethanol was for the production of acetaldehyde [75-07-0]. By 1977, the ethanol route to acetaldehyde had largely been phased out ia the United States as ethylene and ethane became the preferred feedstocks for acetaldehyde production (286—304). Acetaldehyde usage itself has also changed two primary derivatives of acetaldehyde, acetic acid, and butanol, are now produced from feedstocks other than acetaldehyde. Acetaldehyde is stiU produced from ethanol ia India. 

Ethylene is produced in quantity using acetylene or propylene as feedstock to make a large number of products (Figure 7.2-3) such as acetaldehyde, acrylonitrile, acetic acid, and acetic anhydride. These are made generally from acetylene which is made from calcium carbide. 

Butane is primarily used as a fuel gas within the LPG mixture. Like ethane and propane, the main chemical use of butane is as feedstock for steam cracking units for olefin production. Dehydrogenation of n-butane to butenes and to butadiene is an important route for the production of synthetic rubber. n-Butane is also a starting material for acetic acid and maleic anhydride production (Chapter 6). 

Light naphtha containing hydrocarbons in the C5-C7 range is the preferred feedstock in Europe for producing acetic acid by oxidation. Similar to the catalytic oxidation of n-butane, the oxidation of light naphtha is performed at approximately the same temperature and pressure ranges (170-200°C and =50 atmospheres) in the presence of manganese acetate catalyst. The yield of acetic acid is approximately 40 wt%. 

Potential sources of carbon monoxide hazards include metal-refining processes, in which it is formed as a byproduct and used as a fuel (LEL 12.5%), and running vehicle engines (particularly petrol-driven) or gas-fired heaters in poorly ventilated confined spaces. It is also a feedstock in the manufacture of a variety of chemicals, e.g. methanol, acetic acid, phosgene and oxo-alcohols. 

Thus, while water is an indispensable ingredient for the organic cycle (Equations (2) and (3)), a high concentration of water causes the major loss of one of the feedstocks. Water is also made in situ from methanol and acetic acid together with methyl acetate. In addition to water, HI is the cause of by-product formation ... 

Numerous chemical intermediates are oxygen rich. Methanol, acetic acid and ethylene glycol show a O/C atomic ratio of 1, as does biomass. Other major chemicals intermediates show a lower O/C ratio, typically between 1/3 and 2/3. This holds for instance for propene and butene glycols, ethanol, (meth)acrylic acids, adipic acid and many others. The presence of some oxygen atoms is required to confer the desired physical and chemicals properties to the product. Selective and partial deoxygenation of biomass may represent an attractive and competitive route compared with the selective and partial oxidation of hydrocarbon feedstock. 

Animal fats and oilseeds (soybean, cottonseed, coconut, palm) are important feedstocks for plasticizers and cosmetics. Destructive distillation of biomass yields methanol ( wood alcohol ), acetic acid, tar and charcoal, and was the backbone of the chemical industry in former centuries. 

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. 

The overall reaction represents an all syn-gas route to acetic acid and acetic anhydride. At present, in Europe, syn-gas is being produced from natural gas, but eventually it will represent a route from coal, the future feedstock for syngas. 

Uses. About 35-40% of the methanol made is converted to formaldehyde. That s not because the embalming business is so good. Formaldehyde is a feedstock for amino and phenolic resins, which are used as adhesives in plywood, and in the automotive and appliance industry to make parts (all the agitators in washing machines used to be made out of phenolic resins). It is used as feedstock for hexamethylene tetramine, used in electronic plastics for pentaerythritol, used for making enamel coatings and for floor polish and inks for butanediol, a chemical intermediate and for acetic acid, which is widely used itself as a feedstock and solvent and warrants its own treatrnent later on. In the textile business, formaldehyde is used to make fire retardants, mildew resistant linens, and permanent press clothing. 

Methanol, CH3OH, the simplest alcohol, is made by reacting CO and H2 at high pressures over a catalyst. Methanol is a liquid at room temperature and is highly toxic. It is used to make formaldehyde, acetic acid, and other chemical intermediates. It is also used as a feedstock for MTBE (methyl tertiary butyl ether), a gasoline-blending component. 

It should be pointed out that the raw materials for VAM and its related polymers (i.e. ethylene and acetic acid) are produced from fossil resources, mainly crude oil. It is possible to completely substitute the feedstock for these raw materials and switch to ethanol, which can be produced from renewable resources like sugar cane, com, or preferably straw and other non-food parts of plants. Having that in mind, the whole production of PVAc, that nowadays is based on traditional fossil resources, could be switched to a renewable, sustainable and C02-neutral production process based on bioethanol, as shown in Fig. 3. If the vinyl acetate circle can be closed by the important steps of biodegradation or hydrolysis and biodegradation of vinyl ester-based polymers back to carbon dioxide, then a tmly sustainable material circle can be established. 

Acetic acid has been generated directly from synthesis gas (CO/H2) in up to 95 wt % selectivity and 97% carbon efficiency using a Ru-Co-I/Bu4PBr "melt" catalyst combination. The critical roles of each of the ruthenium, cobalt and iodide catalyst components in achieving maximum selectivity to HOAc have been identified. Ci Oxygenate formation is observed only in the presence of ruthenium carbonyls [Ru(C0)3l3] is here the dominant species. Controlled quantities of iodide ensure that initially formed MeOH is rapidly converted to the more reactive methyl iodide. Subsequent cobalt-catalyzed carbonylation to acetic acid may be preparatively attractive (>80% selectivity) relative to competing syntheses where the [00(00)4] concentration is optimized that is, where the Co/Ru ratio is >1, the syngas feedstock is rich in 00 and the initial iodide/cobalt ratios are close to unity. 

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