Production of acrylonitrile using propylene Ammonoxidation

 

·    History

Acrylonitrile attained a certain importance in Germany during the second world war owing to its use as a raw material for the polyacrylonitrile fiber produced by the former I.G. Farbenindustrie and as an additive in the production of Buna N. It was originally synthesized from ethylene oxide and hydrocyanic acid and later, more economically, from acetylene and hydrocyanic acid

The production af acrylonitrile by the direct reaction of propylene, ammonia, and oxygen in accordance with the equation : 

CH3=CH-CH3 + NH3 + 3/2 O2  C3H3N + 3H2O

H2C = CH - CN

was described as early as 1949 in a patent application by Allied Chemical and Dye Corp., New York (USA) 111. A few ?ears later, the Distillers Company, Edinburgh (UK), published a process for the reaction of acrolein with ammonia and oxygen, the acrolein being obtained by the air oxidation of propylener21. In this case, the same reactants as are used in the Allied Chemical and Dye Corp. process are converted into acrylonitrile by a two-step reaction. However, the yields obtained in both cases were low, so that the processes could not yet be used economically.

 

WHY PROPYLENE AMMONOXIDATION METHOD?

 

At the beginning of 1959, Standard Oil of Ohio (SOHIO), Cleveland (Ohio, USA) published a process in which acrylonitrile is produced from the same raw materials, i.e. propylene, ammonia, and oxygen, in a conversion of about 50% 131. A new feature of this process was the use of a bismuth/molybdenum catalyst with silica as a porous support. It thus became feasible to use this synthesis on an industrial scale, and experience gained over the next five years showed that it is economically far superior to all other processes. Fifteen plants using the SOHIO process are already in operation, and after completion of all plants currently under construction, it is expected that this process is used for 75 % of the acrylonitrile production of the western world.. Since ammonia is a cheap industrial product, and propylene is formed in the cracking of nearly all petroleum products, acrylonitrile has become a cheap organic base chemical. This has led to the development of many new applications, as is illustrated e.g. by the recent increase in production of copolymers of acrylonitrile. It is probable that in addition to polymerization and copolymerization, other reactions of acrylonitrile, leading to products of low molecular weight, will also become important.

Watch this video for plant design and greater understanding :

 

 

LAB EXPERIMENT AND CATALYST USE 

 The fixed-bed reactors each had one reaction tube with an internal diameter of 25 mm; a thermocouple shield tube (outside diameter 6 mm) with adjustable temperature measuring position was inserted axially from the top. The depth of the catalyst bed was 160 cm. The reaction tube was surrounded by a salt-bath jacket, in which the salt melt was circulated by means of an airlift pump driven by compressed air. The reaction gas leaving the reactor was washed with dilute sulfuric acid, whereupon an aqueous solution of acrylonitrile was obtained.

Primary attributes of Acrylonitrile Catalyst technology include:

High AN yields

High co-product HCN yield

High propylene conversion

New catalyst developments compatible with all  AN plants

Stable performance and operational flexibility

PILOT PLANT EXPERIMENTS

The synthesis was carried out in a tube furnace containing 60 tubes 2 m long and having an internal diameter of 25 mm. The heat of the reaction was removed

by a salt bath. The catalyst was arranged in two layers. The gas mixture first passed through a 60 cm bed of a corundum catalyst having a particle size of 3 to 4 mm and a coating thickness of about 0.05 mm, and then through a 100 cm bed having a particle size of 4 to 5 mm and a coating thickness of 0.3 mm The residence time in the contact zone (based on the empty tube) was about 4 sec. The highest temperature measured in the reaction zone was 480 "C. The temperature of the salt bath used to dissipate the heat of reaction was 455 "C. 1.5 mole of oxygen (as air) and 3 moles of water vapor were used per mole of propylene (95 %) and per mole of ammonia. The gas mixture formed in the reactor was washed at about 80°C with an aqueous ammonium hydrogen sulfate solution to remove ammonia.

The water-soluble components were absorbed in an excess of water. The organic components were concentrated from the aque- [6] Austrian Pat. 228188 (1962); 236357 (1964). 644 ous solution by distillation, and were determined mainly by gas chromatography. The exhausi gases from the water wash were deterrnined by gas analysis. The overall reactions shown in Table 4 were deduced from the analytical results. Some of these reactions are coupled with one another. For example, one molecule of propylene gives one molecule of acetonitrile and one molecule of COz. Table 4 also shows that about 20% each of propylene and ammonia pass through the reaction zone unchanged.

The proportions of the various reaction products in relation to the starting materials propylene, ammonia, and oxygen are shown graphically below.

USES

Homopolymers of polyacrylonitrile have been used as fibers in hot gas filtration systems, outdoor awnings, sails for yachts, and fiber-reinforced concrete. Copolymers containing polyacrylonitrile are often used as fibers to make knitted clothing like socks and sweaters, as well as outdoor products like tents and similar items. If the label of a piece of clothing says "acrylic", then it is made out of some copolymer of polyacrylonitrile. It was made into the spun fiber at DuPont in 1942 and marketed under the name of Orlon. 

Acrylonitrile is commonly employed as a comonomer with styrene, e.g. acrylonitrile, styrene and acrylate plastics. Labelling of items of clothing with acrylic (see acrylic fiber) means the polymer consists of at least 85% acrylonitrile as the monomer. A typical comonomer is vinyl acetate, which can be solution-spun readily to obtain fibers that soften enough to allow penetration by dyes. 

The advantages of the use of these acrylics are that they are :

1. Low-cost compared to natural fiber

2. They offer better sunlight resistance 

3. Have superior resistance to attack by moths.

Acrylics modified with halogen-containing comonomers are classified as modacrylics, which by definition contain more than PAN percentages between 35-85%. Incorporation of halogen groups increases the flame resistance of the fiber, which makes modacrylics suitable for the use in sleepwear, tents and blankets.However, the disadvantage of these products is that they are costly and can shrink after drying.

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