acetwagn.a01醋酸

ACETIC ACID 1. Introduction Acetic acid [64-19-7], CH3COOH, is a corrosive organic acid having a sharp odor, burning taste, and pernicious blistering properties. It is found in ocean water, oilfield brines, rain, and at trace concentrations in many plant and animal liquids. It is central to all biological energy pathways. Fermentation of fruit and vegetable juices yields 2–12% acetic acid solutions, usually called vinegar (qv). Any sugar-containing sap or juice can be transformed by bacterial or fungal processes to dilute acetic acid. Theophrastos (272–287 BC) studied the utilization of acetic acid to make white lead and verdigris [52503-64-7]. Acetic acid was also well known to alche- mists of the Renaissance. Andreas Libavius (AD 1540–1600) distinguished the properties of vinegar from those of icelike (glacial) acetic acid obtained by dry dis- tillation of copper acetate or similar heavy metal acetates. Numerous attempts to prepare glacial acetic acid by distillation of vinegar proved to be in vain, however. Lavoisier believed he could distinguish acetic acid from acetous acid, the hypothetical acid of vinegar, which he thought was converted into acetic acid by oxidation. Following Lavoisier’s demise, Adet proved the essential identity of acetic acid and acetous acid, the latter being the monohydrate, and in 1847, Kolbe finally prepared acetic acid from the elements. Worldwide demand for acetic acid in 1999 was 2.8� 106 t (6.17� 109 lb). Estimated demand for 2003 is 3.1� 106 t (6.84� 109 lb) (1). Uses include the manufacture of vinyl acetate [108-05-4] and acetic anhydride [108-24-7]. Vinyl acetate is used to make latex emulsion resins for paints, adhesives, paper coat- ings, and textile finishing agents. Acetic anhydride is used in making cellulose acetate fibers, cigarette filter tow, and cellulosic plastics. 2. Physical Properties Acetic acid, fp 16.6358C (2), bp 117.878C at 101.3 kPa (3), is a clear, colorless liquid. Water is the chief impurity in acetic acid although other materials such as acetaldehyde, acetic anhydride, formic acid, biacetyl, methyl acetate, ethyl acetoacetate, iron, and mercury are also sometimes found. Water significantly lowers the freezing point of glacial acetic acid as do acetic anhydride and methyl acetate (4). The presence of acetaldehyde [75-07-0] or formic acid [64-18-6] is commonly revealed by permanganate tests; biacetyl [431-03-8] and iron are indi- cated by color. Ethyl acetoacetate [141-97-9]may cause slight color in acetic acid and is often mistaken for formic acid because it reduces mercuric chloride to calomel. Traces of mercury provoke catastrophic corrosion of aluminum metal, often employed in shipping the acid. The vapor density of acetic acid suggests a molecular weight much higher than the formula weight, 60.06. Indeed, the acid normally exists as a dimer (5), both in the vapor phase (6) and in solution (6). This vapor density anomaly has important consequences in engineering computations, particularly in distillations. Vol. 1 ACETIC ACID 115 Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved. Acetic acid containing <1% water is called glacial. It is hygroscopic and the freezing point is a convenient way to determine purity (8). Water is nearly always present in far greater quantities than any other impurity. Table 1 shows the freezing points for acetic acid–water mixtures. The Antoine equation for acetic acid has recently been revised (3) ln Pð Þ ¼ 15:19234þ �3654:622ð Þ=T þ �45:392ð Þ The pressure P is measured in kelopascal (kPa) and the temperature T in kelvin (K). The vapor pressure of pure acetic acid is tabulated in Table 2. Precise liquid density measurements are significant for determining the mass of tank car quan- tities of acid. Liquid density data (9) as a function of temperature are given in Table 3. Acetic acid forms a monohydrate containing �23% water; thus the density of acetic acid–water mixtures goes through a maximum between 77 and 80 wt % Table 1. Acetic Acid–Water Freezing Points Acetic acid, wt % Freezing point, 8C 100 16.635 99.95 16.50 99.70 16.06 99.60 15.84 99.2 15.12 98.8 14.49 98.4 13.86 98.0 13.25 97.6 12.66 97.2 12.09 96.8 11.48 96.4 10.83 96.0 10.17 Table 2. Acetic Acid Vapor Pressure Temperature, 8C Pressure, kPaa Temperature, 8C Pressure, kPaa 0 4.7 110 776.7 10 8.5 118.2 1013 20 15.7 130 1386.5 30 26.5 140 1841.1 40 45.3 150 2461.1 50 74.9 160 3160 60 117.7 170 4041 70 182.8 180 5091 80 269.4 190 6333 90 390.4 200 7813 100 555.3 210 9612 a To convert kPa to psi, multiply by 0.145. 116 ACETIC ACID Vol. 1 acid at 158C. When water is mixed with acetic acid at 15–188C, heat is given off. At greater acetic acid concentrations, heat is taken up. The measured heat of mixing is consistent with dimer formation in the pure acid. The monohydrate, sometimes called acetous acid, was formerly the main article of commerce. Data on solidification points of aqueous acetic acid mixtures have been tabulated, and the eutectic formation mapped (10). The aqueous eutectic temperature is about �26�C. A procedure for concentrating acetic acid by freezing, hampered by eutectic formation, has been sought for some time. The eutectic can be decom- posed through adding a substance to form a compound with acetic acid, eg, urea or potassium acetate. Glacial acetic acid can then be distilled. The densities of acetic acid–water mixtures at 158C are given in Table 4. A summary of the physical properties of glacial acetic acid is given in Table 5. Table 3. Density of Acetic Acid (Liquid) Temperature, 8C Density, kg/m3 20 1049.55 25 1043.92 30 1038.25 47 1019.19 67 996.46 87 973.42 107 949.90 127 925.60 147 900.27 167 873.56 187 845.04 197 829.88 207 814.07 217 797.44 Table 4. Density of Aqueous Acetic Acid Acetic acid, wt % Density, g/cm3 1 1.007 5 1.0067 10 1.0142 15 1.0214 20 1.0284 30 1.0412 40 1.0523 50 1.0615 60 1.0685 70 1.0733 80 1.0748 90 1.0713 95 1.0660 100 1.0550 Vol. 1 ACETIC ACID 117 3. Chemical Properties 3.1. Decomposition Reactions. Minute traces of acetic anhydride are formed when very dry acetic acid is distilled. Without a catalyst, equilibrium is reached after �7 h of boiling, but a trace of acid catalyst produces equilibrium in 20 min. At equilibrium, �4.2 mmol of anhydride is present per liter of acetic acid, even at temperatures as low as 808C (18). Thermolysis of acetic acid occurs at 4428C and 101.3 kPa (1 atm), leading by parallel pathways to methane [72-82-8] and carbon dioxide [124-38-9], and to ketene [463-51-4] and water (19). Both reactions have great industrial significance. Single pulse, shock tube decomposition of acetic acid in argon involves the same pair of homogeneous, molecular first-order reactions as thermolysis (20). Platinum on graphite catalyzes the decomposition at 500–800 K at low pressures (21). Ketene, methane, carbon oxides, and a variety of minor products are obtained. Photochemical decomposition yields methane and carbon dioxide and a number of free radicals, which have complicated pathways (22). Electron impact and gamma rays appear to generate these same products (23). Electron cyclotron resonance plasma made from acetic acid deposits a diamond [7782-40-3] film on suitable surfaces (24). The film, having a polycrystalline structure, is a useful electrical insulator (25) and widespread industrial exploitation of diamond films appears to be on the horizon (26). Table 5. Properties of Glacial Acetic Acid Property Value Reference freezing point, 8C 16.635 2 boiling point, 8C 117.87 5 density, g/mL at 208C 1.0495 9 refractive index, n25D 1.36965 11 heat of vaporization DHv, J/g a at bp 394.5 12 specific heat (vapor), J/(g�K)a at 1248C 5.029 12 critical temperature, K 592.71 3 critical pressure, MPab 4.53 3 enthalpy of formation, kJ/mola at 258C liquid �484:50 13 gas �432:25 13 normal entropy, J/(mol�K)a at 258C liquid 159.8 14 gas 282.5 14 liquid viscosity, mPa (¼cP) 208C 11.83 15 408C 8.18 15 surface tension, mN/m (¼dyn=cm) at 20.18C 27.57 16 flammability limits, vol % in air 4.0 to 16.0 16 autoignition temperature, 8C 465 flash point, 8C 17 closed cup 43 open cup 57 a To convert J to cal, divide by 4.184. b To convert MPa to psi, multiply by 145. 118 ACETIC ACID Vol. 1 3.2. Acid–Base Chemistry. Acetic acid dissociates in water, pKa ¼ 4:76 at 258C. It is a mild acid that can be used for analysis of bases too weak to detect in water (27). It readily neutralizes the ordinary hydroxides of the alkali metals and the alkaline earths to form the corresponding acetates. When the crude material pyroligneous acid is neutralized with limestone or magnesia the commercial acetate of lime or acetate of magnesia is obtained (8). Acetic acid accepts protons only from the strongest acids, such as nitric acid and sulfu- ric acid. Other acids exhibit very powerful, superacid properties in acetic acid solutions and are thus useful catalysts for esterifications of olefins and alcohols (28). Nitrations conducted in acetic acid solvent are effected because of the for- mation of the nitronium ion, NOþ2 . Hexamethylenetetramine [100-97-0] may be nitrated in acetic acid solvent to yield the explosive cyclotrimethylenetrinitra- mine [121-82-4], also known as cyclonit or RDX. 3.3. Acetylation Reactions. Alcohols may be acetylated without cata- lysts by using a large excess of acetic acid. CH3COOHþ ROH �! CH3COORþH2O The reaction rate is increased by using an entraining agent such as hexane, ben- zene, toluene, or cyclohexane, depending on the reactant alcohol, to remove the water formed. The concentration of water in the reaction medium can be mea- sured, either by means of the Karl-Fischer reagent, or automatically by specific conductance and used as a control of the rate. The specific electrical conductance of acetic acid containing small amounts of water is given in Table 6. Nearly all commercial acetylations are realized using acid catalysts. Cata- lytic acetylation of alcohols can be carried out using mineral acids, eg, perchloric acid [7601-90-3], phosphoric acid [7664-38-2], sulfuric acid [7664-93-9], benzene- sulfonic acid [98-11-3], or methanesulfonic acid [75-75-2], as the catalyst. Certain acid-reacting ion-exchange resins may also be used, but these tend to decompose in hot acetic acid. Mordenite [12445-20-4], a decationized Y-zeolite, is a useful acetylation catalyst (29) and aluminum chloride [7446-70-0], Al2Cl6, catalyzes n-butanol [71-36-3] acetylation (30). Table 6. Specific Conductance of Aqueous Acetic Acid Acetic acid, wt % Specific conductance k, S=cm� 107 100 0.060 99.9515 0.065 99.746 0.103 99.320 0.261 98.84 0.531 97.66 2.19 96.68 5.45 94.82 20.1 92.50 59.9 90.75 111 82.30 688 Vol. 1 ACETIC ACID 119 Olefins add anhydrous acetic acid to give esters, usually of secondary or ter- tiary alcohols: propylene [115-07-1] yields isopropyl acetate [108-21-4]; isobuty- lene [115-11-7] gives tert-butyl acetate [540-88-5]. Minute amounts of water inhibit the reaction. Unsaturated esters can be prepared by a combined oxidative esterification over a platinum group metal catalyst. For example, ethylene-air- acetic acid passed over a palladium–lithium acetate catalyst yields vinyl acetate. Acetylation of acetaldehyde to ethylidene diacetate [542-10-9], a precursor of vinyl acetate, has long been known (8), but the condensation of formaldehyde [50-00-0] and acetic acid vapors to furnish acrylic acid [97-10-7] is more recent (31). These reactions consume relatively more energy than other routes for man- ufacturing vinyl acetate or acrylic acid, and thus are not likely to be further developed. Vapor-phase methanol–methyl acetate oxidation using simultaneous condensation to yield methyl acrylate is still being developed (29). A vanadium– titania phosphate catalyst is employed in that process. 4. Manufacture Commercial production of acetic acid has been revolutionized in the decade 1978–1988. Butane–naphtha liquid-phase catalytic oxidation has declined pre- cipitously as methanol [67-56-1] or methyl acetate [79-20-9] carbonylation has become the technology of choice in the world market. Most commercial produc- tion of virgin synthetic acetic acid is based on methanol carbonylation (1). By- product acetic acid recovery in other hydrocarbon oxidations, eg, in xylene oxida- tion to terephthalic acid and propylene conversion to acrylic acid, has also grown. Production from synthesis gas is increasing and the development of alternative raw materials is under serious consideration following widespread dislocations in the cost of raw material (see CHEMURGY). Ethanol fermentation is still used in vinegar production. Research on fer- mentative routes to glacial acetic acid is also being pursued. Thermophilic, anae- robic microbial fermentations of carbohydrates can be realized at high rates, if practical schemes can be developed for removing acetic acid as fast as it is formed. Under usual conditions, �5% acid brings the anaerobic reactions to a halt, but continuous separation produces high yields at high production rates. Heat for the reaction is provided by the metabolic activity of the microorganisms. Fermentative condensation of CO2 is another possible route to acetic acid. Currently, almost all acetic acid produced commercially comes from acetal- dehyde oxidation, methanol or methyl acetate carbonylation, or light hydrocar- bon liquid-phase oxidation. Comparatively small amounts are generated by butane liquid-phase oxidation, direct ethanol oxidation, and synthesis gas. Large amounts of acetic acid are recycled industrially in the production of cellu- lose acetate, poly(vinyl alcohol), aspirin peracetic acid, and in a broad array of other proprietary processes. (These recycling processes are not regarded as pro- duction and are not discussed herein.) 4.1. Acetaldehyde Oxidation. Ethanol [64-17-5] is easily dehydroge- nated oxidatively to acetaldehyde (qv) using silver, brass, or bronze catalysts. Acetaldehyde can then be oxidized in the liquid phase in the presence of cobalt or manganese salts to yield acetic acid. Peracetic acid [79-21-0] formation is 120 ACETIC ACID Vol. 1 prevented by the transition metal catalysts (8). (Most transition metal salts decompose any peroxides that form, but manganese is uniquely effective.) Kinetic system models are useful for visualizing the industrial operation (32, 33). Stirred-tank and sparger reactor rates have been compared for this reaction and both are so high that they are negligible in the reaction’s mathematical description. Figure 1 is a typical flow sheet for acetaldehyde oxidation. The reactor is an upright vessel, fitted with baffles to redistribute and redirect the air bubbles. Oxygen is fully depleted by the time a bubble reaches the first baffle and bubbles above the first baffle serve mainly for liquid agitation. Such mechanical contact- ing decomposes transitory intermediates and stabilizes the reactor solution. Even though the oxidizer-reactor operates under mild pressure, sufficient alde- hyde boils away to require an off-gas scrubber. Oxidate is passed into a column operated under a positive nitrogen pressure, hence to an acetaldehyde recovery column where unreacted aldehyde is recycled. More importantly, many danger- ous peroxides are decomposed in this column, some into acetic acid, while traces of ethanol are esterified to ethyl acetate. Crude acid is taken off at the bottom and led to a column for stripping off the low boiling constituents other than aldehyde. Crude oxidate is passed to a still where any remaining unreacted acetalde- hyde and low boiling by-products, eg, methyl acetate and acetone [67-64-1], are removed as are CO, CO2, and N2. High concentration aqueous acetic acid is obtained. The main impurities are ethyl acetate, formaldehyde, and formic acid although sometimes traces of a powerful oxidizing agent, possibly diacetyl peroxide, are present. If the acetaldehyde contains ethylene oxide, then ethylene glycol diacetate is present as an impurity. Formic acid can be entrained using hexane or heptane, ethyl acetate, or a similar azeotroping agent. Often the total contaminant mass is low enough to permit destruction by chemical oxida- tion. The oxidizing agent, such as sodium dichromate, is fed down the finishing column as a concentrated solution. Potassium permanganate solution is also effective, but it often clogs the plates of the distillation tower. Final purification is effected by distillation giving high purity acid. Some designs add ethyl acetate to entrain water and formic acid overhead in the finish- ing column. The acid product is removed as a sidestream. Potassium permanga- nate has been employed to oxidize formaldehyde and formic acid because the finished acid must pass a permanganate test. The quantity of water in the che- mical oxidizer solution is important for regulating the corrosion rate of the finish- ing column: Acetic acid having a purity of 99.90–100% corrodes stainless steel SS-316 or SS-320. Lowering the acid concentration to 99.75–99.80% with dis- tilled water in the permanganate solution diminishes the corrosion rate drama- tically. Residues containing manganese acetate or chromium acetate are washed with a two-phase mixture of water–butyl acetate or water–toluene. The organic solvent removes high boiling materials, tars and residual acid, and the metallic acetates remain in the water layer (34). Alternative purification treatments have been explored but have no indus- trial application. Nitric acid or sodium nitrate causes the oxidation of formic acid and formaldehyde, but provokes serious corrosion problems. Schemes have been devised to reduce rather than oxidize the impurities; eg, injecting a current of Vol. 1 ACETIC ACID 121 F ig . 1 . A ty p ic a l a ce ta ld eh y d e ox id a ti on fl ow sh ee t. 122 hydrogen and passing the acid over a metallic catalyst such as nickel or copper turnings. Since reduction occurs at 110–1208C, the reaction can be run in the final column. The risk of an explosion from the hydrogen passing through the col- umn and venting at the top probably discourages the use of this treatment. Cer- tain simple salts, eg, FeSO4 or MnSO4, may be introduced in the same way as the permanganate or dichromate discussed earlier. These serve to eliminate most of the quality-damaging impurities. Conversion of acetaldehyde is typically >90% and the selectivity to acetic acid is higher than 95%. Stainless steel must be used in constructing the plant. This established process and most of the engineering is well understood. The problems that exist are related to more extensively automating control of the system, notably at start-up and shutdown, although even these matters have been largely solved. This route is the most reliable of acetic acid processes. 4.2. Methanol Carbonylation. Several processes were patented in the 1920s for adding carbon monoxide to methanol to produce acetic acid (35). The earliest reaction systems used phosphoric acid at 300–4008C under high CO pressures. Copper phosphate, hydrated tungstic oxide, iodides, and other mater- ials were tried as catalysts or promoters. Nickel iodide proved to be particularly valuable. At that time, only gold and graphite were recognized as adequate to resist temperatures of 300–3208C and pressures of 20 MPa (2900 psi). In 1945–1946, when German work was disclosed by capture of the Central Research Files at Badische Anilin, a virtually complete plant design became pub- lic. Although this high pressure methanol carbonylation system suffered, many of the difficulties experienced in earlier processes, eg, loss of iodine, corrosive conditions, and dangerously high pressures, new alloys such as Hastelloy C permitted the containment of nearly all the practical problems. Experimental and pilot-plant units were operated successfully and,