Petrochemical Processes: Technical and Economic Characteristics

Institut français du pétrole publications Alain CHAUVEL Professeur École Nationale Supérieure du Pétrole et des Moteurs Deputy Director Economics and Information Division Institut Français du Pétrole Gilles LEFEBVRE Senior Engineer Institut Français du Pétrole Foreword by Pierre LEPRINCE Director Institut Français du Pétrole PETROCHEMICAL PROCESSES TECHNICAL AND ECONOMIC CHARACTERISTICS 2 MAJOR OXYGENATED CHLORINATED AND NITRATED DERIVATIVES 1989 ÉDITIONS TECHNIP 27 RUE GINOUX 75737 PARIS CEDEX 15 Publications in English Catalytic Cracking of Heavy Petroleum Fractions D DECROOCQ Methanol and Carbonylation J GAUTHIERLAFAYE R PERRON Principles of Turbulent Fired Heat G MONNOT international Symposium on Alcohol Fuels VIIth International Symposium Paris October 2023 1986 Applied Heterogeneous Catalysis Design Manufacture Use of Solid Catalysts J F LE PAGE Chemical Reactors P TRAMBOUZE H VAN LANDEGHEM J P WAUQUIER Translation of Procédés de pétrochimie Caractéristiques techniques et économiques Tome 2 Les grands intermédiaires oxygénés chlorés et nitrés A Chauvel G Lefebvre L Castex Éditions Technip Paris 1986 2nd Edition 1989 Éditions Technip Paris All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means electronic or mechanical including photocopying recording or any information storage and retrieval system without the prior written permission of the publisher ISBN 2710805618 édition complète ISBN 2710805634 tome 2 Printed in France by Imprimerie Nouvelle 45800 SaintJeandeBraye Since 1971 when the first edition of this book appeared the petrochemical industry has experienced the upheavals resulting from the two oil crises and the economic recession which has struck most nations to different degrees and at different times Accordingly the petrochemical industry has witnessed a rise in the prices of its raw materials as well as changes in its markets this led to the development of flexible steam crackers capable of treating in a single unit different raw materials which the operator selected according to prices and the needs of downstream units Finally the trend towards vertical integration among the oil and gas producing countries which manufactured finished products resulted in the construction of new facilities for the production of ethylene by ethane cracking and the production of methanol from associated gas especially in the Middle East Since the production of these plants outstrips domestic needs by a wide margin their products enjoying the benefit of a cheap raw material are liable to compete with the products of the industrial countries Thus in ammonia synthesis mixed oxide base catalysts allowed new progress towards operating conditions lower pressure approaching optimal thermodynamic conditions Catalytic systems of the same type with high weight productivity achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation Also worth mentioning is the vast development enjoyed as catalysts by artificial zeolites molecular sieves Their use as a precious metal support or as a substitute for conventional silicoaluminates led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes xylene isomerization toluene dismutation in benzene alkylation and even in the oxychlorination of ethane to vinyl chloride Foreword monomer in the manufacture of low density polyethylene tertiary butanol a byproduct of propylene oxide manufacture which is popular as a gasoline additive and 14butanediol and dimethylcyclohexane for the manufacture of specialty polyesters This broad review highlights the innovative dynamism of the petrochemical industry which despite the crisis has succeeded in improving its techniques to adapt them to the economic circumstances While this remark is optimistic the difficulties of the future must not be underestimated the shift of the centers of production to the oil producing countries the absorption of surplus production capacity market redistribution and the development of new products In this context investment decisions will require a sound knowledge of the technical and economic value of the available technologies I believe that this book which the authors have striven to make both complete and precise offers an outstanding guide for engineers in their technical and economic analyses of new petrochemical projects P LEPRINCE Director Institut Français du Pétrole Chapter 7 ETHYLENE AND PROPYLENE OXIDES 71 ETHYLENE OXIDE Ethylene oxide d²⁴ 0836971¹¹ bp 1013 107C which was synthesized for the first time by Wurtz in 1859 was only manufactured industrially in 1925 by Union Carbide It subsequently witnessed considerable development and US production which was 7000 tyear in 1930 exceeded 500000 tyear in 1984 Two processes were employed to manufacture ethylene oxide in the 1970s a Indirect oxidation of ethylene with chlorohydrin as an intermediate b Direct oxidation by air or oxygen 711 The ethylene chlorohydrin process This method is now rarely used to synthesize ethylene oxide but is still employed to manufacture propylene oxide The main reactions involved in the process are the following Cl₂ H₂O ClOH HCl CH₂CH₂ ClOH CH₂OHCH₂Cl HOCH₂CH₂Cl CaOH₂ 2CH₂CH₂ CaCl₂ 2H₂O Hypochlorous acid ClOH produced by the action of chlorine on water is added to ethylene The chlorohydrin obtained is then treated with lime to form ethylene oxide Despite a high molar yield in relation to ethylene 80 per cent and relatively low investment this process suffered from major drawbacks a Corrosion due to the use of chlorine incurring high maintenance costs b High operating costs with a large share due to the price of chlorine c The inevitable production of largely useless calcium chloride and to a lesser degree that of another byproduct 12ethylene dichloride 712 Direct oxidation processes Ethylene oxide was first manufactured industrially by this route in 1938 by Union Carbide which acquired the patents published by Lefort in 1931 concerning the synthesis of ethylene oxide by direct oxidation Two years later in 1940 this type of process accounted for 10 per cent of total installed capacity at the time in the United States and since 1973 the process is employed by nearly all plants in operation or planned throughout the world 7121 Theoretical considerations The main reactions involved are the following CH₂CH₂ 12O₂ CH₂CH₂ ΔHr298 105 kJmol CH₂CH₂ 3O₂ 2CO₂ 2H₂O ΔHr298 135 kJmol CH₂CH₂ 52O₂ 2CO₂ 2H₂O ΔHr298 1225 kJmol All these reactions and especially the latter two which correspond to the complete combustion of ethylene and of its oxide are highly exothermic and complete in the operating conditions of ethylene oxide synthesis To guide the transformation in the direction of the first reaction the operations require the presence of a metallic catalyst The catalyst is generally considered to act according to the following reaction mechanism 5M 5O₂ 5MO₂ 4MO₂ 4CH₂CH₂ 4CH₂CH₂ 4MO giving the overall reaction 5CH₂CH₂ 5O₂ 4CH₂CH₂ 2CO₂ 2H₂O This transformation presumes the adsorption of ethylene on the metal surface together with that of oxygen which is partly or completely dissociated into atoms It also provides a maximum theoretical molar yield of 80 per cent which may explain the apparent mediocrity of the performance achieved by commercial processes A Catalysts All present industrial catalyst systems are based on silver deposited on a slightly porous solid The most widely used support is αalumina but silicaalumina and carbonrundum can also be employed The specific surface area of the support its porosity and the pore size exert a considerable influence on the metal distribution at the surface and consequently on catalytic activity Several techniques are also available for fixing the silver either by impregnation from a solution or by deposition from a suspension An initiator usually consisting of alkaline earth or alkaline metals can be added to the catalyst but other metallic additions have also been recommended Certain halogenated organic derivatives such as dichloropropane may increase selectivity in trace amounts 10 ppm in the feed by reducing combustion side reactions Despite the variety of the catalyst systems proposed none of them offers an ethylene oxide molar selectivity better than 70 per cent with the liberation of heat at the rate of 500 kJmol of converted ethylene B Raw materials The degree of purity of the ethylene feed does not impose any particular constraints as long as the acetylene sulfur and carbonmonoxide contents do not exceed 2 ppm Each ethylene currently produced by steam cracking is perfectly satisfactory Air or oxygen can be used as the oxidizing agent but it appears that economic advantages can be procured by using pure oxygen thus avoiding an excessively high material loss in the offgases so that nearly all modern plants employ pure oxygen C Operating conditions The temperature must be kept between 260 and 290C to obtain the optimal yield This value must not be exceeded to avoid combustion reactions which are by far the most exothermic Temperatures higher by 20 to 40C are recorded at the catalyst surface Although thermodynamic calculations show that the pressure has no effect on conversion at the reaction temperatures operations are conducted at 1 to 3 106 Pa absolute to facilitate the subsequent absorption of ethylene oxide in water Yield per pass reaches a maximum with increased residence time but to maintain high selectivity this is limited to between 1 and 4 s in industrial plants The ethylene oxidation rate is proportional to the oxygen concentration This means that the airtoethylene ratio has a predominant influence on the conversion and yield For practical purposes however the optimal ethylene concentration is determined by the flammability limits of the mixtures with oxygen or air and by the olefin loss in the offgases Hence in ambient conditions the airethylene mixture exhibits an autoignition range between 2 and 286 per cent volume ethylene The lower limit is substantially the same with rising temperature while the upper limit is increased For mixtures of air and ethylene oxide the lower limit is 25 to 3 per cent volume oxide and the upper limit approaches 100 per cent To remain below 3 per cent volume in both cases it is necessary to use inert diluents The presence of carbon dioxide which may also result from the recycling of the products of combustion reactions can help to reduce the flammability contain 10 to 40 per cent ethylene This high olefin content produces a reaction mixture which is always above the upper flammability limit in the operating conditions The temperature is about 250 to 270C and the operating pressure about 12 106 Pa absolute Molar selectivity is as high as 72 per cent with a oncethrough conversion rate of 18 per cent and total yield is around 65 molar per cent The presence of a moderator ethylene dichloride added at the level of 5 ppm in the feed helps to achieve this performance The reactors are supplied with a mixture of ethylene oxygen and recycle gas acting as diluents The recirculation stream is dissolved in a potassium carbonate solution to reduce the carbon dioxide content In this process which features closedcircuit circulation of inert gases purge and hence ethylene losses 05 per cent of the amount introduced are minimal Fig 72 Ethylene oxide production by air oxidation Scientific Design process 7123 Economic data Table 71 summarizes the economic data concerning processes for manufacturing ethylene oxide employing oxygen and air TABLE 71 ETHYLENE OXIDE PRODUCTION BY DIRECT OXIDATION OF ETHYLENE ECONOMIC DATA France conditions mid1986 PRODUCTION CAPACITY 140000 tyear Oxidant Oxygen Air Typical technology ShellSD Scientific Design Battery limits investments 106 US 58 85 Consumption per ton of ethylene oxide Raw materials Ethylene t 088 096 Oxygen 995 t 115 Byproducts Ethylene glycol t 004 004 CO2 t 088 Utilities Steam t 15 45 Electricity kWh 510 860 Cooling water m3 250 270 Process water m3 15 15 Labor Operators per shift 6 7 TABLE 72 AVERAGE COMMERCIAL SPECIFICATIONS OF ETHYLENE OXIDE Characteristics Values Purity Wt min 999 Acidity acetic acid ppm max 20 Aldehydes acetaldehyde ppm max 10 Acetylenics ppm max None Carbon dioxide ppm max 100 Water ppm max 300 Nonvolatile residue g100 ml max 005 Color PtCo max 10 713 Uses and producers Table 72 gives the average commercial specifications of ethylene oxide Its main uses in 1984 are listed in Table 73 for Western Europe the United States and Japan Production capacities and consumption for these three geographic areas are also given TABLE 73 ETHYLENE OXIDE PRODUCTION AND CONSUMPTION IN 1984 Geographic areas Western Europe United States Japan Uses product Ethanolamines 10 8 6 Ethyleneglycol 45 61 58 Glycolic ethers 11 6 6 Surfactants nonionic 21 12 21 Miscellaneous 13 13 9 Total 100 100 100 Production 103 tyear 1515 2585 530 Capacity 103 tyear 1780 2950 620 Consumption 103 tyear 1505 2580 530 1 Acetal copolymer resins arylethanolamines choline ethylene chlorohydrin ethyleneglycols di tri tetra polyethyleneglycols hydroxyethyl cellulose hydroxyethyl starch polyetherpolyols 2 In 1984 the worldwide production capacity of ethylene oxide was 76 106 tyear and in 1986 77 106 tyear United States 28 Western Europe 18 Middle East 04 Canada 04 Eastern Europe 09 Japan 06 Latin America 03 Africa 05 72 PROPYLENE OXIDE Propylene oxide d420 08301 bp1013 35C mp 112C has witnessed considerable industrial development in the past fifteen years leading to its present level of world output exceeding 2300000 tyear This expansion is essentially connected with that of polyurethane foams which consume about 60 per cent of the propylene oxide produced and with that of polyester resins which use about 20 per cent Until 1969 the only method for producing propylene oxide was the chlorohydrin process using a technique similar to that used to synthesize ethylene oxide and most of the production units were converted ethylene oxide plants 721 The propylene chlorohydrin process Fig 73 This process is similar in principle to the ethylene oxide synthesis process The main reactions involved are the following C3H6 H2O ClOH HCl CH3CHCH2 ClOH CH3CHOHCH2Cl 90 per cent CH3CHClCH2OH 10 per cent ΔH298 225 kJmol for both reactions CH3CHOHCH2Cl NaOH CH3CHCH2 NaCl H2O Selectivity in relation to propylene is 94 molar per cent Byproducts formed are 12 dichloropropane 4 to 5 molar per cent and chlorinated diisopropyl ether CH3CHOCH3 CH2Cl CH2Cl 1 to 2 molar per cent The association of a chlorine plant makes the process more profitable The chlorine produced directly by electrolysis cells is added to the recycle chlorohydrin in which it dissolves before the injection of cooled water Chemical grade propylene containing 8 per cent volume of propane is added to the chlorohydrinchlorinewater mixture before it enters the reactor The reactor is an unpacked tower in which the hypochlorous acid addition reaction takes place around 40C Chlorine conversion is practically complete A gas phase and a liquid fraction are separated at the reactor outlet by passage through an absorption column The gas phase is recycled after a purge designed to remove the propane on which supplementary adsorption treatment is used to recover traces of entrained chlorinated compounds The aqueous solution which contains 4 to 5 per cent weight of chlorohydrin is sent to the dehydrochlorination reactor where it reacts with a basic solution from the electrolysis cells whose NaOH and NaCl content is 13 per cent weight for each of these two components The propylene oxide is stripped as it formed at the top of the hydrolyser Chlorohydrin conversion per pass is practically total 99 per cent and the propylene oxide molar yield as high as 96 per cent The remaining solution is a brine that is recycled to the electrolysis unit The integration of such an installation eliminates all the problems formerly raised by the use of lime to hydrolyse the chlorohydrin Each ton of propylene oxide produced was formerly accompanied by the production of about 40 tons of a solution containing 5 to 6 per cent weight of calcium chloride The hydrolyser effluent is then purified after having been rid of the different chlorinated byproducts This operation is performed in a series of three distillation columns a heavy end separation column 25 trays a light end separation column 15 trays and a third column to adjust the product to specifications 35 trays The final product is propylene oxide with a purity of 999 per cent weight 722 Electrochemical processes The use of electrochemistry to convert propylene to the oxide was researched in particular by Bayer and Kellogg In this method propylene is injected in the neighborhood of the anode of a sodium chloride electrolysis cell with a mercury cathode see Section 11252B The hypochlorous acid formed with the chlorine liberated at the anode is added to the propylene The chlorohydrin obtained is hydrolysed at the cathode by the caustic produced by the action of water on the amalgam Propylene oxide is separated from the mixture by stripping while the sodium chloride is returned to the electrolysis 724 Oxidation processes using peroxide compounds 7241 Action mechanism of these compounds The difficulties encountered in obtaining propylene oxide by direct oxidation with high yields and acceptable purity led to the search for more selective means of oxygen input and fixation This led to the use of hydroperoxides ROOH and peracids RCOOH which yielded excellent performance For the manufacture of the epoxide however the use of a coreactant in amounts close to stoichiometry causes the simultaneous production of a coproduct alcohol or acid whose tonnage is necessarily high according to the following reaction CH3CHCH2 ROOH or RCOOH CH3CHCH2 ROH or RCOH These techniques were developed independently by Halcon International and Atlantic Richfield Co ARCO who then cooperated to develop the Oxirane now ARCO Chemical process The reaction takes place by oxygen in the liquid phase in several series of agitated reactors each series laid out in parallel in the same horizontal shell Propylene oxide production by oxidation with peroxide compounds Diacol process 7243 Techniques employing peracids and hydrogen peroxide The two main peracids proposed are peracetic acid and perpropionic acid A The Daicel process The only process in use today is the one developed by Daicel Chemical Industries in Japan with a 12000 tyear plant at Ohtake It operates in two steps The first consists of the production of peracetic acid by the oxidation with oxygen of acetaldehyde in solution in ethyl acetate at room temperature and between 25 and 4 10 6 Pa absolute in the presence of an acidic catalyst The peracid formed is concentrated to about 30 per cent weight In the second step Fig 75 the propylene peracetic acid and a solution of 10 to 15 per cent weight of acetic acid in ethyl acetate containing a stabilizer are introduced continuously into three reactors mounted in series Epoxidation takes place between 50 and 80 C and 09 to 12 10 6 Pa absolute For a residence time of 2 to 3 h oncethrough conversion of peracetic acid is 97 to 98 per cent and the molar yield of propylene oxide is 90 to 92 per cent The reaction products are distilled 35 trays at between 015 and 05 10 6 Pa absolute A mixture of propylene and its oxide is collected at the top and a cut at the bottom consisting chiefly of ethyl acetate and acetic acid The distillate is condensed by cooling and compression and sent to a column 15 trays operating between 12 and 15 10 6 Pa absolute which produces propylene at the top recycled to the first reactor after separating the propane 70 trays if necessary The crude propylene oxide obtained at the bottom is then subjected to light end separation 30 to 35 trays and then heavy end separation 50 trays to meet commercial specifications Its purity can also be improved by extractive distillation The ethyl acetateacetic acid cut and the bottoms from the propylene oxide heavy end separation column are sent to two final distillations 20 to 25 trays each where ethyl acetate and acetic acid are separated in succession The acetate is recycled to the peracetic acid synthesis reactor and the acid is marketed B Other industrial techniques In the Propylox process developed in Belgium peracetic acid is obtained by the action of hydrogen peroxide on acetic at about 40 C in the presence of catalytic traces of sulfuric acid The water formed in the reaction is removed by stripping or azeotropic distillation with ethyl acetate The use of perpropionic acid as an epoxidation agent for propylene has been proposed by BayerDegussa Interox Carbochimique Laporte Sohray and Ugine Kuhlmann The perpropionic acid is produced by the oxidation of propionic acid with hydrogen peroxide in the presence of sulfuric acid The propylene is epoxidized between 05 and 14 10 6 Pa absolute at about 60 to 80 C in the BayerDegussa process which operates in the presence of benzene and at 100 C in the Interox process which uses 12dichloropropane as a solvent Among the other processes using peracids Asahi Chemical employs perisobutyric acid Metallgesellschaft employs perbenzolic acid and Mitsubish perparatoluic acid obtained by the oxidation of paratoluic aldehyde itself produced by the carbonylation of toluene In this case the paratoluic acid byproduct can be oxidized subsequently to terephthalic acid Direct epoxidation by hydrogen peroxide has sparked considerable research IFP Naphtachimie PCUK Produits Chimiques Ugine Kuhlmann Shell Union Carbide and several catalyst systems have been proposed compounds of molybdenum tungsten arsenic but although the propylene oxide selectivities are as high as 85 to 95 molar per cent the conversions never exceed 50 per cent making the process uneconomic large volume of reactors cost of recycling high price of hydrogen peroxide Remark Among other methods under development are the following a Catalytic decomposition of propylene glycol hydroxyacetate obtained by the acetoxylation of propylene ChemSystems process b Enzymatic conversion of Dglucose to Dfructose and propylene oxide Cetus process or directly of propylene in the presence of methane monooxygenase Exxon process TABLE 74 PROPYLENE OXIDE PRODUCTION ECONOMIC DATA France conditions mid1986 PRODUCTION CAPACITY 100000 tyear Process intermediate chemical compound Chlorohydrin tbutyl hydroperoxide Typical technology Integrated electrolysis ARCO Chemical11 Battery limits investments 106 US 88 102 Consumption per ton of propylene oxide Raw materials Propylene t 088 090 Sodium chloride t 015 235 Isobutane t 100 Oxygen t Byproducts Dichloropropane t 011 Chloroether kg 25 tbutyl alcohol t 245 Miscellaneous acetone etc t 025 Utilities Steam t 90 95 Electricity kWh 4500 400 Fuels 106 kJ 115 400 Cooling water m3 250 380 Process water m3 50 Chemicals and catalysts US 30 25 Labor Operators per shift 20 12 Monoethylene glycol which is more routinely called glycol OHCH2CH2OH d²⁰ 11154 bp1013 197C is the principal application for ethylene oxide from which it is obtained by hydration Despite many developments under way designed to produce it directly from ethylene or synthesis gas this method is practically the only one employed industrially at the present time Fig 76 Ethylene glycol production by ethylene oxide hydration The old process by which sodium bicarbonate was used to hydrolyse the chlorohydrin produced by the action of hypochlorous acid on ethylene has been abandoned CH2CH2 HClO HOCH2CH2Cl HOCH2CH2Cl NaHCO3 H2O HOCH2CH2OH CO2 H2O NaCl The hydrogenation of nbutyl oxalate into ethylene glycol and nbutyl alcohol jointly developed by Union Carbide and Ube Industries Oxalate is obtained by oxidative carbonation of nbutanol on the liquid phase by using a palladium based catalyst and an accelerator nitric acid nbutyl nitrite The economic data available on the production of ethylene glycol by the hydration of ethylene oxide are listed in Table 77 Capacity tyear 100000 Battery limits investments 106 US 12 Consumption per ton of propylene glycol Raw materials Ethylene oxide t 081 Byproducts Diethylene glycol kg 100 Triethylene glycol kg 5 Utilities Steam t 53 Electricity kWh 30 Cooling water m3 520 Process water m3 5 Chemicals US 05 Labor Operators per shift 4 Uses for 1984 and figures for production capacities and consumption of ethylene glycol in Western Europe the United States and Japan are given in Table 79 Glycol type Monethylene glycol Diethylene glycol Triethylene glycol Grade Chemical Polymerization j20h 1115111156 1115111156 1117012000 11241126 Distillation range C 1932015 196200 242250 278300 Fig 77 Propylene glycol production by propylene oxide hydration 743 Economic data The main economic data concerning the process for manufacturing propylene glycol by hydration of propylene oxide are summarized in Table 710 TABLE 710 PROPYLENE GLYCOL PRODUCTION BY HYDRATION OF PROPYLENE OXIDE ECONOMIC DATA France conditions mid1986 Capacity tyear 30000 Battery limits investments 106 US 7 Consumption per ton of propylene oxide Raw materials Propylene oxide t 080 Byproducts Dipropylene glycol kg 30 Utilities Steam t 22 Electricity kWh 100 Cooling water m3 1300 Process water m3 1 Chemicals US 02 Labor Operators per shift 3 744 Uses and producers Table 711 gives the average commercial specifications of propylene glycol Table 712 summarizes the uses of this product as well as figures for production capacities and consumption in 1984 for Western Europe the United States and Japan TABLE 712 PROPYLENE GLYCOL PRODUCTION AND CONSUMPTION IN 1984 Geographic areas Western Europe United States Japan Uses product Unsaturated polyester resins 90 45 44 Food additives 9 20 Pharmaceuticals and personal care 12 13 Tobacco humectant 8 Cellophane 10 4 Functional fluids 6 6 Paints and coatings 6 6 Plasticizers 6 Miscellaneous 1 4 Total 100 100 100 Production 103 tyear 210 220 25 Capacity 103 tyear 2 290 430 75 Consumption 103 tyear 170 210 35 Chapter 8 ACETIC DERIVATIVES 81 ACETALDEHYDE This is currently the most widespread method for manufacturing acetaldehyde Initial research and development conducted by the Consortium für Electrochemische Industrie a Wacker Chemie affiliated organization culminated in 1956 in the development of an industrial process with two variants One of them proposed by Hoechst employs oxygen as the oxidant and the second examined by Wacker employs air The commercialization of these two alternatives by Aldehyd a joint venture led to the construction of the first industrial plant in 1960 8141 Principle In theory this transformation in the presence of palladium chloride and hydrochloric acid medium gives rise with ethylene to the formation of a z complex which through an intramolecular rearrangement with water yields a hydroxethyl palladium as an active intermediate ultimately yielding acetaldehyde and palladium metal The reaction mechanism is as follows CH2CH2 PdCl2 2HCl PdCl2 CH2CH2 Cl 2H HOCH2CH2PdCl2 2Cl 2H PdCl2CCH3 2Cl 2H OH CH3CHO 3H 4Cl Pd The excess acetylene entrains the acetaldehyde formed which is then condensed by cooling and then washed with water The aldehyde is purified by distillation Unreacted acetylene is recycled During the operation the catalytic ion Hg2 is reduced partly to Hg0 and then to metallic mercury This reduction can be prevented by adding Fe3 ions to the catalyst solution German process As a rule techniques of this type achieve acetylene oncethrough conversions of 50 to 60 per cent with molar yields above 95 per cent Among the suggested variants to this initial scheme are a Total conversion of ethylene in a single pass b Use of other catalyst systems oxides phosphates silicates tungstates etc of zinc copper iron and cadmium etc The Chisso process developed in Japan in the 1950s1960s represents one of the latest technologies for the hydration of acetylene It operates at around 70C and 025 106 Pa absolute in a vertical reactor An aqueous solution of catalyst mercury and iron salts and sulfuric acid flows downward in countercurrent to the gaseous acetylene entering at the bottom The heat liberated by the reaction is removed by vaporizing part of the medium The acetaldehyde entrained in the offgases is recovered by water scrubbing and returned to the reactor It is contained in the liquid product stream at the rate of 2 per cent weight with sulfuric acid 20 to 25 per cent weight and mercury and iron salts Most of the aldehyde 60 per cent is separated by flash By cooling and partial condensation of this stream the acetaldehyde concentration in the remaining gaseous fraction is raised to over 85 per cent weight This fraction is then recompressed at 025 106 Pa absolute and distilled The 40 per cent of products which have not been separated by flash are found with the catalyst solution It is recycled to the reactor after the catalyst system has been regenerated In this type of process oncethrough conversion of acetylene is 60 per cent and the molar yield nearly 90 per cent The main byproducts formed are acetic acid and crotonaldehyde by Celanese A plant with a production capacity of 90000 tyear built in 1946 at Bishop Texas was shut down in 1972 By this process the hydrocarbon feed is mixed with compressed air and recycle gas containing unreacted paraffins The recycle gas contains CO CO2 and N2 in the volumetric ratio 127 The mixture is preheated to 370C at 07 106 Pa absolute and oxidized at 450C After decomposition of the peroxides formed in a column containing a ceramic packing the hot gases leaving the oxidation reactor are quenched with a cooled aqueous solution of formaldehyde containing 12 to 14 per cent weight and then scrubbed with water to recover the oxygenated compounds Unreacted hydrocarbons are separated and recycled Fractionation and purification of the forty or so coproducts produced by this conversion are highly complex and involve simple azeotropic and extractive distillations As a rule the oxidation of 3 t of nbutane yields 1 t of acetaldehyde 1 t of formaldehyde 0586 t of methanol 0352 t of miscellaneous oxygenated solvents and 0118 t of acetone a Production of acetaldehyde CH2CH2 PdCl2 H2O CH3CHO Pd 2HCl b Oxidation of palladium by cupric chloride Pd 2CuCl2 PdCl2 2CuCl ΔH298 11 kJmol c Regeneration of cupric chloride with air or oxygen 2CuCl 2HCl 12O2 2CuCl2 H2O ΔH298 233 kJmol which represents the second step of the process The following overall exothermic reaction takes place CH2CH2 12O2 CH3CHO ΔH298 244 kJmol From the kinetic standpoint it can be shown that the rate of disappearance of ethylene V and hence of the production of acetaldehyde assumes the form V kPdCl2 HCl2 Consequently if the presence of palladium chloride proves favorable that of H and Cl ions exerts an inhibiting effect Accordingly a catalytic solution of cupric chloride containing chlorine below stoichiometry is more active than that corresponding to the theory in other words exhibiting a ClCu atomic ratio of 21 This encourages the formation of copper oxychloride a basic salt which acts by neutralization to reduce the concentration of H ions produced during the reaction Conversion generally takes place between 80 and 90C at between 02 and 5 106 Pa absolute and regeneration of the catalyst system in similar conditions so that both operations can be conducted jointly The catalyst solution contains 50 to 500 times more copper atoms than palladium and the ClCu atomic ratio ranges from 14 to 181 In both cases the molar yield of acetaldehyde is 95 per cent The main byproducts are acetic and oxalic acids crotonaldehyde and various chlorinated organic compounds methyl and ethyl chlorides chloroethanol chloroacetaldehyde chloro crotonaldehyde etc To avoid the flammability range of the gas mixture 8 to 20 per cent volume of oxygen at low pressure 8 to 14 per cent at higher pressure the oxygen to ethylene ratio must be kept to a level below stoichiometry in other words with an oxygen content less than 8 per cent volume 8142 Industrial manufacture The process offers the following two variants a Single step in the presence of oxygen with recycle of unconverted reactants according to the Hoechst scheme b Two steps in the presence of air without recycle according to the WackerChemie scheme Both processes rely on the possibility as to whether or not acetaldehyde production and cupric chloride regeneration can be conducted simultaneously A WackerHoechst singlestep process Fig 83 In this version highpurity ethylene 998 per cent volume and oxygen 995 per cent volume mixed with dilution steam are introduced at different levels at the base of a titanium reactor more than 20 m high containing 10 to 15 perforated trays and holdup catalyst solution Conversion takes place at 03 to 05 106 Pa absolute at a temperature kept at around 120 to 130C by the vaporization of a fraction of the reaction medium especially water which removes the heat liberated by the oxidation of ethylene The streams leaving the top of the reactor pass through a separator where they are partly condensed The liquid phase recovered is recycled The remaining gases are sent to a quenching tower where their temperature is lowered from 125 to 50C and then to a water scrub column to recover the small amounts of acetaldehyde entrained by the offgases These gases rich in unconverted ethylene are recompressed and returned to the reaction zone A small part 1 per cent is purged to prevent the accumulation of inerts especially nitrogen in the recycle loop The dilute acetaldehyde 8 to 10 per cent weight obtained after quenching and waterscrubbing is concentrated and purified first by stripping of light products in a column with about 25 trays and then by the removal of heavy products and water in a second tower 20 trays Acetaldehyde in a purity of 997 to 999 per cent weight is recovered at the top A sidestream is drawn off to separate the crotonaldehyde byproduct as well as many other organic compounds The bottom consists chiefly of water acetic acid and heavier products A fraction of this is purged but most of it is recycled to the scrubbing stage The catalyst solution deteriorates to some degree during oxidation leading to the formation of copper oxalate A regenerator operating on the liquid phase from the separator around 170C in the presence of oxygen serves to decompose the products thus formed and to restore the catalysts activity Makeup hydrochloric acid 30 per cent weight is added in the reactor For better acid corrosion resistance the units are built of stainless steel or even titanium or provided with internal linings titanium elastomers etc In this type of process ethylene oncethrough conversion is about 25 to 30 per cent and the total yield is about 94 molar per cent in relation to the fresh feed and 90 molar per cent in relation to oxygen Fig 83 Acetaldehyde production from ethylene WackerHoechst singlestep process Fig 84 Acetaldehyde production from ethylene WackerHoechst twostep process B WackerHoechst twostep process Fig 84 This version offers the advantage of being able to operate with lower purity ethylene 95 per cent volume and air as oxidant However it requires larger capital expenditure The ethylene and catalyst solution are introduced simultaneously into a tubular titanium reactor operating at 110C and between 08 and 09 106 Pa absolute The effluent produced is flashed at atmospheric pressure Owing to the heat released by the reaction the acetaldehyde and water are contained in the vapor phase The liquid phase consisting essentially of catalyst is pumped at 1 106 Pa absolute into the cupric chloride regenerator This is also a tubular reactor operating at 100C in which compressed air is also injected After separation of the waste gases mainly nitrogen by flash the bulk of the regenerated catalyst solution is returned to the first conversion stage A small fraction is purged and heated to 160C to destroy the degradation products formed copper oxalate The gaseous mixture of acetaldehyde and steam obtained by flash is first concentrated to 60 to 90 per cent weight in a primary distillation column 10 trays The light and heavy compounds water acetic acid etc are removed in a series of two distillation columns containing about 25 and 20 trays respectively and the second column is provided with a side stream consisting mainly of chlorinated aldehydes In the twostep variant ethylene oncethrough conversion is between 97 and 98 per cent and the molar yield between 94 and 95 per cent Table 81 ACETALDEHYDE PRODUCTION ECONOMIC DATA France conditions mid86 PROCESS Acetylene hydration Ethylene oxidation TYPICAL TECHNOLOGY Chisso WackerHoechst BATTERY LIMITS INVESTMENTS 106 US 17 19 30 CONSUMPTION PER TON OF ACETALDEHYDE RAW MATERIALS Acetylene t 0620 Ethylene t 0675 0675 Oxygen Nm3 280 UTILITIES Steam t 2 12 12 Electricity kWh 350 100 500 Cooling water m3 250 200 220 Water at 12C m3 6 Process water m3 1 2 2 Treated water m3 Chemicals and catalysts US 20 7 7 Hydrochloric acid 100 C kg 10 50 Sulfuric acid kg 5 Labor Operators per shift 4 4 5 TABLE 83 ACETALDEHYDE PRODUCTION AND CONSUMPTION IN 1984 Geographic areas Western Europe United States Japan Uses product Acetic acid acetic anhydride 64 55 27 Peracetic acid 18 6 45 Ethyllactate 5 8 4 Pentaerythritol 8 9 Glyoxal 3 Crotonaldehyde 1 3 24 13butylene glycol 3 Miscellaneous1 1 15 Total 100 100 100 Sources product Acetylene 14 Ethanol 8 Ethylene 78 100 100 Total 100 100 100 Production 103 tyear 540 290 295 Capacity 103 tyear2 1030 330 675 Consumption 103 tyear 550 255 265 1 Chloral trichloroacetaldehyde lactic acid isobutylacetate 2 The worldwide production capacity of acetaleyde was nearly 31 106 tyear in 1984 and 1986 with the following distribution United States 035 Western Europe 105 Japan 075 Mexico 015 Eastern Europe 075 Asia and Far East 025 82 ACETIC ACID Apart from the production of vinegar for food uses the manufacture of dilute acetic acid by ethanol fermentation for industrial applications has practically disappeared and has been superseded chiefly by synthesis from hydrocarbons In addition to the distillation of wood which was originally widespread in the United States and is still practiced in certain European countries three main methods are available for manufacturing concentrated acetic acid d42 104922 mp 166C bp1013 1185C a Liquid phase oxidation of acetaldehyde b Direct or indirect oxidation of hydrocarbons in the liquid phase c Methanol carbonylation a process developed early but capable in its latest commercial version of supplanting the other technologies this already accounts for more than onethird of worldwide installed capacity 821 Acetic acid synthesis by liquid phase oxidation of acetaldehyde The catalytic oxidation of acetaldehyde in the liquid phase to acetic acid by air or oxygen is still widely applied and accounts for about 40 percent of installed worldwide production capacity 8211 Principle This conversion employs the following exothermic reaction CH3CHO 12O2 CH3COOH ΔH298 265 kJmol It is generally conducted in the liquid phase in the presence of manganese cobalt or copper salts acetates by a chain free radical mechanism involving the intermediate formation of peracetic acid This may either decompose to form acetaldehyde and oxygen or react with the components of the reaction medium to yield a mixed metallic complex of the acetaldehyde and the peracid Whereas the cobalt and manganese salts actually catalyze the oxidation the effect of copper acetate essentially involves the decomposition of this complex to acetic acid The reaction takes place in slightly different conditions depending on the type of oxidant employed They can be summarized as follows Oxidant air oxygen Temperature C 55 to 65 70 to 80 Pressure 106 Pa absolute3 02 to 1 015 to 03 Oncethrough conversion 91 to 92 96 to 98 Yield molar 93 to 94 95 to 96 The main byproducts are formaldehyde formic acid methyl and ethyl acetates acetone carbon dioxide etc and ethylene oxide and ethylene glycol diacetate if the acetaldehyde used derives from the conversion of ethylene Oxidation can also take place in the vapor phase in the presence of water and a palladium base catalyst system around 200 to 250C and between 02 and 1 106 Pa absolute with an acetaldehyde oncethrough conversion greater than 45 percent and an acid molar yield of 92 per cent 8212 Industrial manufacture Fig 85 Only liquid phase oxidation has hitherto been industrialized particularly by Hoechst who employs oxygen as the oxidizing agent Makeup acetaldehyde in a purity of 99 to 998 per cent weight as well as recycle aldehyde and acetic acid containing metallic salts used as catalysts in solution are introduced at the top of a reactor with countercurrent flow of air previously compressed and scrubbed with acetic acid The heat liberated by the reaction is removed by forced circulation of a fraction of the reaction medium through an external heat exchanger The operating conditions are designed to establish a steady state acetaldehyde concentration of about 5 per cent weight Figure 85 Acetic acid production by acetaldehyde oxidation The offgases which entrain small amounts of products and reactants are scrubbed with water and crude acetic acid and then discharged to the atmosphere The recovered condensates are recycled The liquid stream drawn off from the reactor is first rid of most of the peracetic acid it contains by heating in an inert atmosphere around 85 to 90C followed by distillation This fractionation operation is performed in a series of four columns to achieve the following a Separation of unconverted acetaldehyde in the distillate which is then recycled to the reaction section and an acetic acid rich fraction at the bottom used to scrub the air and offgases b Removal of byproduct methyl acetate at the top in a distillation column supplied with the effluent from air prescrubbing and yielding crude acetic acid at the bottom c Purification of the acetic acid obtained by separation of water in the distillate in the form of a heteroazeotrope with ethyl acetate bp1013 704C water content per cent weight 85 Two layers are formed by cooling and settling the upper organic fraction containing 97 per cent weight acetate is used as a reflux and the lower aqueous fraction containing 93 per cent water is sent to the final column A sidestream supplies glacial acetic acid in a purity of 998 per cent weight The bottom consisting chiefly of metallic acetates is recycled to the reactor This purification is carried out in the presence of potassium permanganate or sodium bichromate to remove traces of contaminants liable to color the final product d Recovery of the ethyl acetate contained in the aqueous phase by azeotropic distillation Given the high risk of corrosion most of the equipment is built of stainless steel or with an aluminum lining As in all conversions of this type which are autocatalytic the induction period is relatively long Catalysts are used to shorten it These catalysts are soluble salts of cobalt chromium vanadium or manganese usually acetates The oxidation rate rises with the number of carbon atoms in the hydrocarbon and with the extent to which the chain is linear Thus if it is 1 for ethane it is as high as 100 for propane 500 for nbutane and 1000 for npentane The oxidation temperature normally ranges from 130 to 200C preferably between 160 and 180C The pressure must be sufficient to maintain the medium in the liquid phase or between 45 and 55 106 Pa absolute in the case of nbutane and between 2 and 4 106 Pa absolute in the case of naphtha nbutane oxidation nbutane is converted to acetic acid by the following overall reaction C4H10 52O2 2CH3COOH H2O H298 990 kJmol Many byproducts are also obtained These can be classified in two main categories a Those with a boiling point lower than that of water ethyl acetate methyl ethyl ketone methyl vinylketone etc b Those with a boiling point higher than that of water formic propionic acrylic butyric and succinic acids dimethyl glyoxal acetyl acetone acetone butyrolactone etc Formic acid displays an unusual behavior however significantly complicating the scheme for the separation of the different products formed and for the purification of the acetic acid Formic acid which boils at a temperature approaching that of water bp1013 1007C forms an azeotrope with water with a boiling point higher than those of the pure components E1013 1072C water content per cent weight 226 Hence its separation requires azeotropic or extractive distillation This also applies to dimethyl glyoxal which even in very small amounts colors the acetic acid yellow and makes it unsuitable for certain applications This impurity can only be removed by specific treatment with hydroxylamine As a first approximation the yields of these different products expressed as a percentage of carbon are as follows Formic and propionic acids etc 4 Carbon dioxide and monoxide 17 Esters and ketones 12 Acetic acid 57 Total 100 The main promoters of this technology are Celanese whose first plant at Pampa Texas 230000 tyear dates from 1952 and Hüls whose Marl plant 40000 tyear is now shut down The Celanese process involves oxidation in the presence of cobalt or manganese salts and in the Hüls process conversion takes place without catalyst Other process licensors include Distillers Union Carbide etc This process developed by Distillers and industrialized by BP Chemicals in the United Kingdom at Sallend 190000 tyear and in the Soviet Union at Yerevan 35000 tyear by RhônePoulenc in France at Pont de Claix 35000 tyear and by Dainippon Chemical in Japan in a plant that is currently shut down 15000 tyear operates on a light gasoline naphtha whose upper cut point is generally 95C Oxidation takes place by air in reactors in the form of towers designed to achieve an effective mixture of the gas and liquid phases and to ensure good temperature control since the reaction is highly exothermic H 420 kJmol of oxygen converted The average operating conditions are as follows Temperature 180 to 185C Pressure 45 to 106 Pa absolute The oxygen to feed weight ratio must be at least 05 at the reactor inlet The heat of reaction is used to produce lowpressure steam In addition to acetic acid formic propionic butyric and succinic acids are mainly formed together with carbon dioxide carbon monoxide water and a number of heavy oxygenated compounds The remaining byproducts oxalic glutaric and adipic acids etc are recycled to the reactors The use of a cobalt base catalyst serves to increase the acetic acid yield at the expense in particular of formic acid and also of the total amount of upgraded acids By altering the operating conditions the respective output of the different acids can also be adjusted Hence in the RhônePoulenc plant production is limited in practice to formic acetic and propionic acids As a rule the reactor streams are treated as follows a The gaseous products are cooled to ambient temperature to condense most of the less volatile components and then below 0C to recover unconverted hydrocarbons A suitable heat exchange system combined with expansion turbines operating on the gas stream helps to avoid the use of a refrigeration machine b The liquid products are treated in a complex series of distillation columns about 13 Light and heavy ends are first taken off Most of the water is removed by azeotropic distillation in the presence of isopropyl ether A new operation of this type using toluene then serves to isolate the formic acid which is purified Pure acetic and propionic acids are separated and purified by simple distillation However hydrogenation treatment of the propionic acid before its final purification is necessary to remove all traces of unsaturated compounds Depending on each specific case the succinic acid can be isolated from the residual products by crystallization To resist corrosion by organic acids the reaction section of the installation is built of a stainless steel stabilized with titanium or niobium Cr 18 per cent Ni 145 per cent Mo 25 to 3 per cent Ti 03 to 05 per cent or Nb 06 to 08 per cent and a large part of the separation section is built of copper Fig 87 Acetic acid production by oxidation of nbutenes Bayer process Chapter 8 Acetic derivatives Development work has also been undertaken to manufacture acetic acid from propylene operating in the gas phase in the presence of metallic oxide base catalysts U W Mo Ti etc and steam between 250 and 400C at atmospheric pressure with molar yields not exceeding 50 to 70 per cent However the ideal outfit for the synthesis of acetic acid remains the nbutenes despite the few industrial achievements in this area Conversion is carried out directly in the vapor or liquid phase or indirectly with the intermediate formation of acetates A Direct oxidation of nbutenes The overall reaction is as follows nC4H8 2O2 2C4H6COOH ΔH298 985 kJ mol of butenes This can be achieved on a mixture of isomers considering the thermodynamic equilibrium existing between 1butene and 2butenes which can replenish the system permanently with the more rapidly converted substance It appears that acetaldehyde and acetone are the ideal intermediates for the oxidation of nbutenes This is preferably conducted in the gas phase in the presence of excess oxygen oxygen to nbutenes molar ratio of 10 to 25 to achieve maximum oncethrough conversion 60 to 90 per cent while maintaining high selectivity 50 to 70 molar per cent The steam and oxygen exert complementary influences in this case Conversion takes place between 250 and 300C at between 15 and 3 106 Pa absolute in the presence of vanadium oxide base catalysts promoted by various other metals Ti Zn Al etc Many byproducts are formed including acids formic maleic propionic acrylic etc aldehydes formaldehyde acetaldehyde etc alcohols isopropanol butanol etc ketones acetone methyl ethyl ketone etc esters carbon monoxide carbon dioxide etc As a rule the scheme of an industrial installation comprises two main sections a The first features the introduction of the preheated nbutenes feed into an air oxidation reactor operating under pressure in fixed or fluidized catalyst beds and designed to ensure the removal of the heat liberated during the conversion by the production of steam b The second involves the removal of vent residual gases by flash followed by the separation of the different components of the reactor effluent in a relatively complex sequence of simple and azeotropic distillations which may include solvent extraction stages This type of process has been developed in particular by Chemische Werke Hüls etc but the lack of selectivity of the operation which is even greater than in the oxidation of nbutane has compromised its attainment of the industrial stage B Indirect oxidation of nbutenes Fig 87 To overcome the drawbacks of the direct conversion of nbutenes Bayer has developed an indirect oxidation process that helps to improve the overall selectivity of the operation and which consists in passing through the intermediate formation of secondary butyl acetate Acetic acid is manufactured in two steps according to the following reaction mechanism Fig 86 Acetic acid production by naphtha oxidation Distillers process CH₃ CH₃COOH CH₃CH₂CHOC₃H₇ CH₃OH CO CH₃COOH ΔH298 137 kJ mol This process operates in the aqueous phase at 250C and 65 10⁶ Pa absolute in the presence of cobalt iodide as catalyst Acetic acid production from methanol Monsanto process Carbonylation The methanol is carbonylated in the liquid phase in a stirred tank reactor The temperature of the reaction medium is kept constant by the vaporization of a fraction of the reactants and products thus removing the heat generated by the reaction This gas phase consisting chiefly of carbon monoxide nitrogen carbon dioxide methanol and methyl iodide is cooled and then scrubbed with methanol to recover most of the stripped raw materials The liquid effluent leaving the reactor is flashed The gases containing acetic acid moisture methyl iodide and formic and propionic acids etc are sent to the purification section The liquid fraction chiefly formed of the catalyst complex methyl acetate and acetic acid is cooled and sent to the reaction zone Product separation and purification the first distillation column is designed to produce a cut enriched with acetic acid by the removal of the lighter and heavier components methyl iodide methyl acetate etc This cut is then dehydrated by heteroazeotropic distillation The aqueous fraction recovered at the top is refractionated to remove excess water The heavy stream is treated in a finishing column which produces glacial acetic acid in the distillate while the residual acetic acid at the bottom is also recovered in a complementary fractionation that separates the heavy products such as propionic acid These highalloy steel columns each have between 35 and 45 actual trays Other industrial methods for manufacturing acetic acid Among the developing technologies likely to reach the commercial stage is the direct production of acetic acid from a synthesis gas H2CO 1 by the following reaction 2CO 2H2 CH3COOH The conversion developed in particular by Union Carbide takes place with a yield of about 70 molar per cent in the presence of a supported rhodium base catalyst at a temperature between 250 and 350C and 10 to 30 106 Pa absolute with a carbon monoxide oncethrough conversion close to 20 per cent It should be pointed out that the carbonization of wood between 100 and 150C leads to the formation of four large varieties of products charcoal an acid liquor tars and incondensable gases The second after supplementary fractionation and purification is used to produce various chemical compounds particularly acetic acid ACETIC ACID PRODUCTION ECONOMIC DATA France conditions mid36 PRODUCTION CAPACITY 80000 tyear Process Acetaldehyde oxidation nbutane oxidation Naphtha oxidation Methanol carbonylation Typical technology Hoechst Celanese Distillers BASF Monsanto Battery limits investments 106 US 17 35 56 53 28 Consumption per ton of acetic acid Raw materials Acetaldehyde t 077 nbutane t 108 naphtha t 095 Methanol t 061 054 Carbon monoxide t 080 051 Byproducts Methyl acetate kg 35 Formic acid kg 50 290 Propionic acid kg 35 10 Butyric acid kg 7 Succinic acid kg 15 Acetonemethyl acetate kg 270 Methyl ethyl ketone ethyl acetate kg 460 Utilities Steam t 36 80 55 30 20 Electricity kWh 290 1550 1530 350 180 Cooling water m3h 260 450 490 185 140 Process water m3 5 25 20 Chemicals and catalysts US 5 35 8 7 11 Labor Operators per shift 3 5 5 4 3 TABLE 86 ACETIC ACID PRODUCTION AND CONSUMPTION IN 1984 Geographic areas Western Europe United States Japan Uses product Acetic anhydride Cellulose acetate Esters 1 Monochloroacetic acid Terephthalic acid diethylterephthalate Vinyl acetate Textiles Miscellaneous 2 Total 100 Sources product Acetaldehyde Butane and naphtha Ethanol Methanol Miscellaneous 3 Total Production 103 tyear Capacity 103 tyear 4 Consumption 103 tyear 1 Amyl benzyl butyl ethyl 2ethylhexyl methyl and propyl acetates glyceryl triacetate 2 Explosives grain fumigants herbicides metallic salts pharmaceuticals photographic and rubber chemicals 3 Byproducts spent acetic anhydride terephthalic acid coproduct 4 The worldwide production capacity of acetic acid was 44 106 tyear in 1984 and 46 106 tyear in 1986 831 Production of acetic anhydride from acetic acid only This operation comprises two steps a Pyrolysis of acetic acid to ketene b The action of the ketene obtained on the acetic acid 8311 Principle The following reactions are involved CH3COOH CH4 CO H2O ΔH298 147 kJmol CH3COOH CH2 CO CH3CO2O ΔH298 63 kJmol The first conversion which is highly endothermic must take place in the vapor phase at high temperature 700 to 800C and at reduced pressure 10 to 20 kPa The second conversion which is exothermic can be carried out in the absence of catalyst by absorption in acetic acid between 30 and 40C at reduced pressure 7 to 20 kPa 8312 Industrial manufacture Fig 89 An industrial facility has four main sections a Acetic acid pyrolysis b Ketene absorption c Acetic anhydride purification d Recovery of unconverted acetic acid Acetic anhydride production from acetic acid The main reactions involved are as follows CH3COCH3 CH2CO CH4 ΔH298 124 kJmol CH3COOH CH2CO CH3CO2O ΔH298 63 kJmol The first conversion takes place around 700 to 800C at atmospheric pressure in the vapor phase usually in the absence of catalyst Side reactions lead chiefly to the formation of coke favored by the presence of a nickel base material At 760C oncethrough nickel and facilitate the complete cracking of the reactants and products as well as the formation of coke it is preferable to use highchromium steels as the tube material or alloys of chromium 23 per centaluminum 15 per cent and silicon 15 per cent If not the coking process can also be slowed down by the addition of carbon disulfide to the feed The reactor effluents available at about 700C first receive an inline injection of ammonia to neutralize the catalyst They are then cooled rapidly to 0C in a series of heat exchangers The liquid obtained by condensation and containing about 35 per cent weight acetic acid is sent to the recovery section Ketene absorption takes place on the offgases with a countercurrent of acetic acid in two absorbers in series the first collecting 80 per cent of the available ketene A series of two scrubbers using cold acetic anhydride and acid then serves to recover the acetic acid entrained by these gases These units operate at around 0C and between 15 and 5 kPa absolute The liquid leaving the absorption stage contains more than 90 per cent weight acetic anhydride It is sent to the purification section Purification takes place by distillation in a series of three columns separating the following products in succession at the top crude acid 70 trays sent to the recovery stage glacial acetic acid 100 trays recycled to pyrolysis and absorption and acetic anhydride in a purity of over 99 per cent 10 trays The heavier components are collected at the bottom of the final fractionation The recovered acetic acid unconverted acetic acid is reconcentrated in two columns 45 and 55 trays The first removes excess water at the top in the form of a heteroazeotrope with pxylene for example The organic phase obtained by condensation and settling serves as a reflux and the aqueous phase is partly purged The second column removes the polymerization products at the bottom and produces glacial acetic acid at the top which is recycled Acetic anhydride production from acetone and acetic acid Industrial manufacture comprises four main steps The first is the pyrolysis of acetone in a tubular reactor As for the cracking of acetic acid it is preferable to use chromium steel 25 per cent tubes The use of conventional alloys may be feasible after passivation of the inner walls by a mixture of H2 and CO or by prior treatment with steam or by the injection of small amounts of sulfur compounds H2S CS2 etc The effluents leaving the furnace are quenched in two stages a By inline injection of a mixture consisting of fresh and recycled acid and acetic anhydride which lowers the temperature from 760 to 550C b By scrubbing in a packed tower using the same mixture lowering the temperature to about 150C The second stage the production of acetic anhydride takes place in two steps a 90 per cent of the potential anhydride is obtained by supplementary cooling to 70C and partial condensation of the gases leaving the quenching stage consisting chiefly of ketene acetic acid acetone and acetic anhydride b An additional 9 per cent is formed during the recovery of unconverted acetone entrained in the gases by absorption in a packed tower featuring countercurrent flow of acetic acid This is followed by recovery of the unconverted acid entrained in the offgases by scrubbing with water and reconcentration of the solution obtained by heteroazeotropic distillation 65 trays Separation and purification In the final step the components of the liquid phase leaving with the quenching stage are separated and purified at 70C in a series of three columns which yield the following products in succession at the top unconverted acetone 20 trays excess acetic acid 80 trays and acetic anhydride of more than 99 per cent purity 10 trays The effluents from the first two distillations are recycled The second operates only on twothirds of the stream drawn off from the first with the remainder used as a quenching fluid chemical compounds such as peracetic acid and acetaldehyde monoperacetate according to the following reactions CH3CHO O2 CH3COOOH CH3COOOH CH3CHO CH3COOCCH3 O OH CH3COOCCH3 H CH3CO2O H2O 2CH3COOH Certain side reactions also occur a Hydrolysis of acetic anhydride to the acid b Oxidation of acetic anhydride by peracetic acid to yield diacetyl peroxide CH3CO2O2 and acetic acid c Decomposition of this peroxide by water to produce peracetic and acetic acids Acetaldehyde monoperacetate normally decomposes to yield 97 per cent anhydride and water and 3 per cent acid However the hydrolysis takes place rapidly so that acetic anhydride is merely the forerunner of the acid in the oxidation of acetaldehyde The difficulty of the operation which takes place in the liquid phase with oxygen or air at moderate temperature 45 to 60C and low pressure 01 to 04 106 Pa absolute thus consists in preventing the hydrolysis of acetic anhydride from developing Indepandant of certain technical arrangements this can be achieved in the presence of diluents and catalysts The diluents are intended to reduce the hydrolysis rate and also lower the acetaldehyde concentration to limit the risks of explosion The most widely used diluents are acetic acid and ethyl acetate but aromatics benzene chlorinated compounds carbon tetrachloride chlorobenzene etc and ketone compounds cyclohexanone can also be used The catalyst systems are all based on the synergy effect existing between copper and cobalt acetates introduced in a weight ratio of 3 to 4 with less than 2 per cent weight for the first and 05 per cent weight for the second Depending on the technology employed oncethrough conversion varies from 30 to 80 per cent and molar yields from 50 to 80 per cent for acetic anhydride and 15 to 45 per cent for the acid The main byproducts are carbon dioxide methyl acetate acetone ethylidene diacetate and various heavy components whose recycling serves to improve the anhydride selectivity 8332 Industrial manufacture Two types of processes can be distinguished from the technological standpoint a The previous ones draw off the reaction products in the liquid phase They operate in the presence of oxygen with acetaldehyde contents of 30 to 40 per cent weight in the reaction medium Oncethrough conversion is 70 to 80 per cent and molar yields are 50 to 75 per cent for acetic anhydride and 20 to 45 per cent for the acid