Acrylic Acid

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ACRYLIC ACID

PROCESS

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Introduction:

The majority of plants manufacture acrylic acid by the catalytic oxidation of propylene. This compound is an organic compound with the formula

CH2=CHCO2H which is used in several industrial applications and it attracts a

wide range of industries for painting, chemical fibres and adhesives. However, the highest growing use of acrylic acid is in the production of superabsorbent polymers. In addition, more than 65% of acrylic acid is used to produce Acrylate esters (Glauser, 2007).

Acrylic acid can seriously damage the environment when released in industrial effluents due to its high toxicity (A. Eftaxias a, 2001). The toxicity can be reduced by combination of mechanisms e.g. biodegradation, oxidation, and volatilisation.

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Nature of the reaction:

Production:

The propylene market price is closely tied to the prices of crude oil due to the use of crude oil derivatives such as naphtha via steam cracking, and vacuum gas oil via fluid catalytic cracking as a feedstock for making propylene. Alternative feedstock for making acrylic acid have been considered, that might appeal in the long term e.g. glycerol (https://www.shokubai.co.jp, n.d.).

Nearly all companies produce acrylic acid via the two-stage oxidation of the propylene.

Propylene is first oxidised to acrolein and then acrolein oxidised to acrylic acid. Please see below:

Acrolein: Acrylic Acid: O H O H C O H C₃ ₆ ₂   ₃ ₄  ₂ 2 4 3 2 4 3 2 1 O H C O O H C    

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Propylene is fed to the reactor either from cracking of naphtha or from a storage tank. Steam is then added to provide stability for the exothermic heat of the process. Therefore, a feed mixture of air, steam and propylene is fed to the reactor. After the production of acrolein, it is essential to cool the products to below 100ºC to avoid further oxidation. This is achieved by introducing the product[s] to a quenching tower where the reaction is rapidly quenched. The product[s] can be cooled down to 40o _{C, however}

to avoid further oxidation temperature should not exceed 3100_C. To be more efficient, an additional recovery of acrylic acid and the by-product, acetic acid, can take place in an absorber.

The liquid stream, stream [S-8], leaving the absorber contains dilute aqueous acid, and is connected to the liquid-liquid extraction section in the extraction tower to purify the acid. The organic phase leaving the extraction tower, stream[S-10], contains all of the acrylic acid and essentially there is no water or other components which is fed to tower [distillation tower].

Final purification takes place in a distillation tower to produce 99% purified acrylic acid as the bottom product and acetic acid is produced as the top product. Acrylic acid is then cooled and collected. If the acrylic acid temperature exceeds 90°C, spontaneous polymerisation would occur. Several heating and cooling constraints are important in order to avoid explosive conditions and runaway reactions.

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Table 1:

Code Name S Stream V Valve E-10 Fluid pump Figure 1

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Catalysts:

The homogeneous catalysts are avoided due to their toxic contamination. Most companies prefer heterogeneous catalysts based in noble and oxide-metals (Chemie and deHaën, 2003). One of the disadvantages of using noble metals can be the cost factor; noble metals significantly increase the process cost. Oxides metals such as titanium and nickel are found to be, in some cases, highly unstable (Shende and Levec, 2000). In this process catalysts are being used in two oxidation processes:

Stage 1: propylene to acrolein Stage 2: acrolein to acrylic acid.

Wide range of catalyst can be used in stage one e.g. Iron, Cobalt. However heavy oxides e.g. tellurium and arsenic oxides are used for second stage. (Krik-Othmer, 1978)

By-Products:

Some typical side reactions are given below which occur as a result of oxidation of reactant and product:

In Reactor:

Acrolein to produce carbon dioxide and acetic Acid:

O H CO O O H C₃ ₄ 3 ₂ 2 ₂ 2 7 _ _{ } _  2 2 4 2 2 4 3 2 3 CO O H C O O H C     O H CO O H C₃ ₆ ₂ 3 ₂ 3 ₂ 2 9     

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Acetic acid uses varied, it is one of the most important reagents. It commonly used as a solvent for many industrial processes.

Materials:

There are several columns, exchangers, pumps, vessels, and other miscellaneous equipment in the extraction column. Heat Exchangers are essential for this reaction. The cost depends on the size and the required type. Reactor and distillation tower consist of many small tubes, carrying a heat transfer medium. Since acrylic acid and acetic acid have boiling points of 141.8 ° C and 117 ° C respectively. Distillations for this separation process must take place under vacuum.

Discussion and Modification:

Deionised water can be added into stream, [S-5], to slightly cool down the product before it reaches the quenching tower. Deionised water also can be used in the absorber to prevent the reaction from over-heating. Moreover, in a wide range of chemical processes, molten salt is fed into the reactor to cool down the temperature inside the reactor. However, this is disadvantageous as it can be an additional cost for the process.

Since acrylic acid has a trend to polymerise, hydroquinone can be injected throughout the system to make the reaction more efficient. The waste gas from the absorber can be recycled to stream, [S-6]. If the operation would take place at temperatures between 180 and 200° C, this would be

relatively more convenient since the feed stream temperature is usually between 180-190° C. Hence, no additional heating/cooling source is necessary to maintain the above mentioned temperature. This in turn, would increase the profitability and reduce the cost[s] involved.

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Health & Safety Precautions & Potential

Hazards:

Acrylic acid and acetic acid are extremely hazardous and they can cause long term injuries. They are a strong irritant to the skin, eyes, and mucous membranes in humans, inhalation has been observed to produce nose and eye irritation, lungs internal bleeding, and degenerative changes in the liver and kidneys. The liquid may cause blindness if splashed into the eye (Technology Transfer Network - Air Toxics Web Site, 2000).

Eye protection, non-vented and designed specifically to protect against chemical splash goggles should be worn (www.basf.com, n.d.). Hand protection such as chemical resistant gloves, protective arm sleeves, aprons, full body coveralls, boots e.g. steel toe safety shoes, and head coverings are essential to wear. The material should be resistant to acrylic acid e.g. Butyl rubber of 0.4 to 0.6 mm thickness (www.basf.com, n.d.). Hence it is essential for all workers to follow all the above health and safety guidelines.

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References:

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A. Eftaxias a, J. F. (2001). Applied Catalysis B: Environmental 33 (2001) 175– 190. Kinetic modelling of catalytic wet air oxidation of, p. 175–190.

- Fluka Chemie and Riedel-deHaën, 2003. Scientific Research. Madrid, p. 45-46.

- Kirk-Othmer, 1978. Encyclopedia of Chemical Technology, A-Alkanolamines (Volume 1). Volume 1 Edition. Wiley-Interscience. p. 339

- Sittig, M, 2002. Handbook of Toxic and Hazardous Chemicals and Carcinogens, 4th ed. Vol 1 A-H Norwich, NY: Noyes Publications, p. 2243

- Acrylic Acid | Technology Transfer Network Air Toxics Web site | US EPA. 2014. Acrylic Acid. [ONLINE] Available

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Wiley, New York, 1986.

- Perry, R. H. and D. Green, eds., Perry’s Chemical Engineering Handbook (6th ed.),

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- Roscoe, Henry (1891). A treatise on chemistry, Volume 3, Part 3. London: Macmillan & Co. p. 165.

- Sittig, M, 1985. Handbook of Toxic and Hazardous Chemicals and Carcinogens. 2nd ed. Park Ridge, NJ: Noyes Publications( Noyes Data Corporation), p 257-258,

- Sittig, M, 2002. Handbook of Toxic and Hazardous Chemicals and Carcinogens,4th ed.Vol 1 A-H Norwich, NY: Noyes Publications, p. 2243

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