IMPROVEMENT OF TECHNOLOGY OF VINYL ACETATE PRODUCTION FROM ACETYLENE

ABSTRACT

The vapour-phase process of VA synthesis on an industrial scale was first carried out in Munich after the First World War [6-8]. To obtain VA excess acetylene and acetic acid vapours were passed at a temperature of 210-250°C over zinc acetate, used as a catalyst, applied to activated carbon. The conversion of acetic acid in this process was 15-20 per cent. The catalyst, over which a mixture of acetylene and acetic acid vapour was circulated, was placed in a shaft furnace. The catalyst was prepared by impregnation of activated clay of AK-5 grade with 5-10 % zinc acetate solution followed by heat treatment at 200-400°C in a nitrogen current and repeated impregnation with 20-30 % zinc acetate solution and drying. The results of studies on the influence of factors (temperature, catalyst loading, gas mixture flow rate, contact apparatus parameters) on the degree of conversion of reagents in the process of vapour-phase synthesis of HA in the presence of zinc (cadmium) acetate on activated keramzite are presented.

АННОТАЦИЯ

Паровозный процесс синтеза ВА в промышленных масштабах впервые был осуществлен в Мюнхене после Первой мировой войны [6-8]. Для получения ВА избыточные пары ацетилена и уксусной кислоты пропускали при температуре 210-250°C над ацетатом цинка, используемым в качестве катализатора, нанесенного на активированный уголь. Конверсия уксусной кислоты в этом процессе составляла 15-20%. Катализатор, над которым циркулировала смесь паров ацетилена и уксусной кислоты, помещался в шахтную печь. Катализатор готовился путем пропитки активированного керамзита марки АК-5 5-10 %-ним раствором ацетата цинка с последующей термообработкой при 200-400 °С в токе азота, повторной пропиткой 20-30 %-ним раствором ацетата цинка и сушкой. Представлены результаты исследований влияния факторов (температуры, загрузки катализатора, скорости потока газовой смеси, параметров контактного аппарата) на степень конверсии реагентов в процессе парофазного синтеза НА в присутствии ацетата цинка (кадмия) на активированном керамзит.

Keywords: acetylene, acetic acid, vinyl acetate, textural characteristics, acetaldehyde, refrigerator

Ключевые слова: ацетилен, уксусная кислота, винилацетат, текстурные характеристики, ацетальдегид, холодильник.

Introduction

Vinyl acetate (VA) is one of the most important monomers (component for production of industrial organic synthesis polymers). Vinyl acetate monomer is a substance with a wide range of applications. Polymers and copolymers based on VA have good adhesive, optical, electrical insulating and fibre-forming properties, so they are widely used not only in everyday life, but also in industry: technology, construction, medicine, etc. [1-2]

The main application of VA monomer is industrial. It is a component of poly-vinyl acetate, which is used for production of water-based paints and varnishes, various types of adhesives, impregnations, floor tiles, acrylic fibres, paper coverings and nonwovens. In addition, VA monomer is used to produce polyvinyl alcohol, a component raw material in the manufacture of pack King film and laminated glass. A small part of VA monomer is used to produce polymers based on ethylene vinyl acetate, barrier resins from ethylene vinyl alcohol and polyvinyl butyral.

According to Internet information [RUE "National Centre of Marketing and Price Conjuncture"] - the main producers of the world market of VA monomer are Celanese (USA), Lyondel Basell (USA), Dow Chemical (USA), DuPont (USA), INEOS (UK). [3-5]

The development trends of the global vinyl acetate market are easily predictable. The substance is widely used in industry, so the market growth, according to experts, will continue at a small rate, comparable to the growth of the world gross domestic product. Experts forecast an annual increase in the global market by an average of 5% per year, with higher growth rates expected in Asian countries, primarily in China.

The consumption of VA in the production of barrier resins from ethylene vinyl alcohol, polymers based on ethylene vinyl acetate and polyvinyl butyral will grow at rates higher than the world average. However, these markets account for a small share of global monomer consumption, so will have no impact on the overall figures.

Several large VA plants are planned to be built in the next few years. By 2010, new plants will appear in China, India, Iran, and Saudi Arabia. Three plants will be built in China: two with a capacity of 200,000 tonnes per year and one with a capacity of 300,000 tonnes. At the beginning of 2009. Saudi Arabia will build a plant with a capacity of 300,000 tonnes. Iran plans to build two plants with annual capacity of 150,000 and 140,000 tonnes. At the end of 2010, India will have two plants with annual capacity of 300,000 tonnes.

Production of vinyl acetate by vapour-phase process. The vapour-phase process of VA synthesis on an industrial scale was first carried out in Munich after the First World War [6-8]. To obtain VA excess acetylene and acetic acid vapours were passed at a temperature of 210-250°C over zinc acetate, used as a catalyst, applied to activated carbon. The conversion of acetic acid in this process was 15-20 per cent. The catalyst, over which a mixture of acetylene and acetic acid vapour was circulated, was placed in a shaft furnace.

The formed VA was condensed together with excess acetic acid and extracted by fractional distillation. The unreacted acetylene and acetic acid were returned to the cycle. However, shaft furnaces had a number of disadvantages, mainly with regard to poor temperature regulation in the furnaces, which led to sintering of the catalyst and decomposition of acetylene. Another disadvantaged was the low activity of the catalyst. In 1929 in Burghausen instead of shaft furnaces were used tubular reactors in which the heat of reaction was removed by circulation of liquid, for example, isophorone.

Acetylene used for the production of VA was washed first in a tower with water saturated with chlorine and then in a second tower with a 3% solution of caustic soda. After passing through carbide dryers to remove moisture, acetylene was fed into purifiers loaded with dry purifying mass consisting of diatomaceous earth impregnated with chromic acid, and from here (Fig. 6) through a blower (10) into an acetic acid evaporator (1), in which the temperature was maintained at 120° [9]. Acetylene at a temperature of 90 °, saturated with acetic acid, heated successively in the heater (2) to 120-140 °, in the heat exchanger (3) to 165-190 ° and, finally, in the electric heater (4) to 170-220 °. The gas was freed from acetylene condensation products in a coupler trap (5), loaded with used catalyst and located in front of the contact furnace. The acetylene-acetic acid mixture entered the contact furnace (6) and passed through a catalyst consisting of activated carbon with zinc acetate (20 % Zn) deposited on it, which was placed in 785 tubes 3.5 m high and 50 mm in diameter. The 5.5 m3 furnace contained 2100 kg of catalyst. The conversion of acetic acid reaches 30-40 %. Cooling is carried out by isophorone (with a boiling point of 213-214°), which enters the furnace at a temperature 4° below the furnace temperature and leaves it at the furnace temperature. The initial reaction temperature is 175° and the final temperature about 220°.

Figure 1. Apparatus scheme of vinyl acetate production by vapour-phase scheme

The contacted gas mixture passed from the contact furnace (6) through the heat exchanger (3) and preheater (2) to the first refrigerator (7a), where it was cooled to a temperature of 45-50°, and to the second refrigerator (7b), where it was cooled to a temperature of 15° [10-11] In the next refrigerator (8) the gas was cooled to 12° and recirculated through a trap (9) in which it was washed with acetic acid. Condensate from the second refrigerator, from the gas cooler (8) and the trap passed to the top of the column (11). Acetaldehyde and acetylene were distilled off, and the residue, consisting of 40 % VA, 0.6 % acetaldehyde and 59.2 % acetic acid, was distilled successively in three columns. The first column yields crude VA containing acetaldehyde; in the second column the aldehyde and other impurities are separated; at the top of the third column pure VA is obtained (boiling point 71° at 730 mm Hg). The residue from the third column, consisting of acetic acid, VA and acetaldehyde, containing impurities, and from the second column was distilled periodically on two other columns. Hydroquinone solution was added during distillation to prevent polymer formation. About 50 g of HA was obtained in 1 h per 1 L of catalyst. The yield of HA was ~92 % for acetylene and 96-98 % for acetic acid.

II.   Experimental part

The catalyst was prepared by impregnation of activated claydite of AK-5 grade with 5-10 % zinc acetate solution followed by heat treatment at 200-400 °C in a nitrogen current and repeated impregnation with 20-30 % zinc acetate solution and drying.

To increase the volume of microspores of the carrier - activated carbon, degassing - deducting of microspores by "wet" method was carried out.

"Wet" - water - method of degassing. The carrier (claydite of AK-5 grade) was lowered into a heat-resistant glass with distilled water and boiled on an electric cooker for 30-60 minutes. The dirty water was decanted and the expanded clay was washed with several portions of distilled water. The expanded claydite was then filtered and dried under natural conditions.

Activated claydite of AK-5 grade in the amount of 300 g was immersed in 1200 ml of solution of the following composition (% weight):

• zinc acetate - 5.0
• acetic acid - 1.0
• distilled water - 94.0

Coal impregnation was carried out at a temperature of 75-85 °C (on a water bath) for 25 min. The liquid was decanted; the wet catalyst was dried for 35 minutes in a desiccator at a temperature of 105-110 °С. The catalyst after pre-impregnation was subjected to heat treatment at a temperature of 200-300 °C in a nitrogen current at a rate of 30 l/hour (volume rate 90 h-1) for 0.5 hours.

After heat treatment the catalyst was immersed in 1200 ml of 20 % zinc acetic acid solution containing 1 % acetic acid and the catalyst was kept in this solution on a boiling water bath for 60 minutes. The mass was then cooled in vivo, the liquid was decanted and dried in a desiccator at 120-150 °C for 5 hours. It was then loaded into a reactor and dried in a nitrogen current at 150-180 °C for 2 hours. The content of zinc acetate in the prepared catalyst was 45 % (Zn - 16 %).

The scheme of the unit for vinyl acetate production is shown in Fig. 2.

Figure 2. Principal scheme of vinyl acetate production:

1 - needle valve; 2 - rheometer; 3 - absorber with hydroxylamine; 4 - drying column; 5 - evaporator; 6 - bath; 7 - reactor; 8 - thermowell; 9 - transformer; 10 - voltmeter; 11 -block regulating the temperature in the reactor; 12 -air refrigerator; 13 -water-cooled refrigerator; 14 - traps with xylene; 13 - waste gas discharge line

The reactor is metal (7) with a diameter of 40 mm, height 380 mm with a catalyst grid-plate mounted in the lower part. The upper flange-cover is equipped with two connections: for supply of vapour-gas mixture (acetylene saturated with acetic acid vapour) and thermo-pocket (8). The lower flange-bottom is provided with a connection for reaction products outlet.

Around the reactor body (7) evenly distributed heating elements TEN-06 with a total capacity of 3.6 kW, which through a regulating transformer (9) connected to the network 220 V. In order to avoid overheating it is not allowed to supply voltage more than 220 V.

In order to preserve heat and ensure stable temperature, the reactor surface is insulated and covered. The temperature in the reactor was measured by thermocouple ТХК with output of signals on the temperature control unit (11), with recording of readings on the potentiometer KSP or on the indicating device.

Acetylene used from the cylinder was purified from acetone in an absorber with 10% hydroxylamine (3), acetylene was dried in a drying column with calcium chloride (4).

To carry out the synthesis, a certain amount of catalyst was loaded into the reactor. Heating of the catalyst was carried out in a nitrogen current. When the temperature reached 180 °C, the ventilator in the mixed gas line of the 007 shop was opened and the valve in the nitrogen line was closed. For saturation with acetic acid, the gas was passed through an acetic acid evaporator placed in a water bath with a temperature of ≈ 60-65 °C. Then the gas saturated with acetic acid entered the reactor, where the vinylation reaction took place on the catalyst at a temperature of 180-210°C. In the process of the reaction the parameters of the technological regime were measured: temperature regime, flow rates of the supplied components.

The gaseous reaction products were cooled first in an air cooler (12), then in a water-cooled (Liebich) cooler (13). Condensate and VA vapours were absorbed in absorbers with xylene (14) placed in an ice bath (3-8 °C). The condensed reaction products in xylene collected during the shift were analysed chromatographically for component composition. The incoming and outgoing gases were periodically analysed by gas chromatography.

At the end of the contacting cycle, which was determined by a significant decrease in VA yield, the spent catalyst was regenerated with air at 380-400°C until the residual content of CO₂ in the exhaust gas was less than 0.5 % vol.

From the quantitative and qualitative data of the condensate, the VA selectivity of the catalyst was calculated. According to the amount of acetic acid fed to the synthesis and the amount of obtained VA, the degree of VA conversion in one pass of raw materials - acetic acid and gas - was calculated.

From the gas flow rate and the volume of the loaded catalyst, the volumetric rate was calculated.

By the mass of synthesised HA, volume and mass of loaded catalyst the catalyst productivity was calculated - HA removal from a unit volume (or mass) of catalyst for a unit of time (per hour). The duration of the catalyst run was estimated by a noticeable decrease in the conversion rate, VA selectivity and productivity.

On the prepared contact mass - zinc-acetate catalyst 45 % on activated claydite of mark AK-5 a series of trial syntheses of VA was carried out under the following conditions:

• molar ratio of components C₂H₂: CH₃COOH (7.29-11.48): 1
• acetylene volume rate 303 h-1
• temperature in the reaction zone 180-185 °C

The gas stream leaving the plant was not analysed, therefore the calculated process characteristics will be underestimated.

Process characteristics:

• conversion 9.8-14.81 %
• throughput 10.18-19.9 mg/g - hour
• process selectivity 71,07-94,37 %.

III.   Discussion of results and conclusions

A literature review on the vapour-phase synthesis of VA from acetic acid and acetylene has been carried out. The main methods of VA production in the gas phase are reduced to passing a mixture of acetylene with acetic acid vapour through a heated catalyst. Acetic acid or phosphoric acid salts of zinc, cadmium, silver, etc. can serve as catalysts. Silica gel, activated carbons, activated aluminium oxide are used as a carrier. The use of this or that catalyst is conditioned by the conditions of synthesis. In most of the schemes proposed by the authors, excess acetylene is used. Significant excess of acetylene (Akins stoichiometric ratios) contribute to almost complete dissociation of acetic acid dimer at temperatures of 170-180 ° C, ensure the course of the reaction towards the formation of VA, lead to high yields of spent components and high conversion of acetic acid. However, on the other hand, this reduces the productivity of the contact apparatus and complicates the separation of VA from the contacted mixture. At stoichiometric ratios of acetic acid and acetylene to ensure the decomposition of acetic acid dimer requires higher temperatures (260-290°C) with the use of zinc catalysts on activated aluminium oxide. Catalysts based on activated carbons at such high temperatures are quickly deactivated.

The results of studies on the influence of factors (temperature, catalyst loading, gas mixture flow rate, contact apparatus parameters) on the degree of conversion of reagent’s in the process of vapour-phase synthesis of HA in the presence of zinc (cadmium) acetate on activated carbon are presented.

The paper presents methods of preparation of contact mass, shows the possibility of re-regulating the activity of the catalyst and increasing the duration of its operation by a gradual step-by-step rise in the reaction temperature.

Several variants of technological schemes of VA production from acetylene and acetic acid by vapour-phase method are given, differing by design feature of contact apparatus, temperature regime of synthesis, volume rate. According to the given technologies the gas mixture from the contact apparatus was cooled, condensed. Acetaldehyde and acetylene were distilled from the reaction mixture, and the residue containing VA, acetic acid was distilled in several columns sequentially.

Preliminary acetylene should be purified from impurities by their oxidation (e.g. with chromium mixture), followed by absorption of oxidised impurities with alkali, and then dried with calcium chloride and phosphoric anhydride. Depending on the selected technology, different optimal degrees of con-version are achieved. The yield of the product is quite high. A review of the vapour-phase method for the synthesis of VA from acetylene and acetic acid shows significant advantages over the liquid-phase method. The vapour-phase method uses cheaper and non-poisonous catalysts with longer lifetime than the liquid-phase method. By the very essence of the process it is continuous and allows easy obtaining any ratio of components in the vapour-gas mixture, which makes it possible to regulate the process, to create virtually any excess of acetylene and work with a selected percentage of conversion, which is determined by both technical and economic considerations (practically applied ratio of acetic acid and acetylene lie within the limits of 1: 2 to 1: 10). Application of the vapour-phase method, however, requires the use of higher temperatures and a number of additional fire and explosion safety measures.

According to the results of the literature search, a series of trial experiments were carried out in the laboratory, which confirm the possibility of obtaining VA from acetylene and acetic acid in the vapour phase in the presence of zinc acetate catalyst on activated carbon. It should be noted that laboratory conditions of VA synthesis are not adequate to the real process in industry. Low conversions of initial reagent’s in one pass are obtained due to our studies without the use of recycling of raw materials. We have carried out an analytical literature and patent review on HA, the materials of which can be taken as a basis for making basic decisions.

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