The Industrial Production and Use of Acetic Acid

Acetic acid is a colorless organic acid found in the carboxylic family of organic compounds.  Its chemical formula is CH₃COOH, which can also be written as CH₃CO₂H or C₂H₄O₂. The laboratory name for acetic acid is ethanoic acid. In its pure state, acetic acid is clear and colorless, with a very sharp odor that is found in vinegar. The name acetic acid is derived from a Latin name acetum, which is basically the name for vinegar in Latin. This acid is categorized as a weak acid because it has a PH value of 4.76. Ethanoic acid does not dissociate fully into its individual ions when it is dissolved in an aqueous solution. Acetic acid has a lot of demand both in industries and domestically. The huge application for the acid makes its global demand to be high. This demand keeps on increasing with time.

Acetic acid has various chemical and physical properties. It is a colorless liquid, volatile organic compound, flammable at a temperature higher than 39°C, and explosive. It has a temperature range of 101.3⁰C which is almost equal to that of water. The melting point is 16.7⁰C and the boiling point is 118⁰C. The acid can absorb moisture, mixed with other organic alcohols, and reacts with bases and oxidizing products.

According to Jones (2000), the industrial demand for acetic acid globally stands at 6 million tonnes annually. This demand makes acetic gain importance among the industrial chemical products. The most effective and widely method used in the manufacture of acetic acid is where methanol is carbonylated under an iridium catalyst. This is an improvement from the traditional rhodium-based method. In the production of acetic acid, the key raw materials used are methanol, hydrogen iodide, and carbon monoxide. The iridium catalyst makes the rate of reaction faster than the rhodium-catalyzed reaction. However, the raw materials used in both methods are the same, and the steps followed are also similar. This method alone produces over 60% of the ethanol that is used in the world today (Jones, 2000).

The process of methanol carbonylation involves a reaction between carbon monoxide and methanol at a temperature of between 150⁰C and 200⁰C. Pressure of approximately 30-40 atmospheres is the most suitable for the process. A catalyst, which can either be iridium or rhodium, is added to the products to increase the rate of the reaction. The chemical equation for that reaction is shown below:

[Rh(CO)2I2]            O

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CO + CH₃OH                                      CH₃COH

As had earlier been indicated, the catalyst used in this reaction can either be rhodium or iridium even though in the equation rhodium is indicated. Rhodium was the first catalyst used. Therefore, the equation is written indicating it as the catalyst. The latter is the most preferred and recent development of the former method. It is more efficient and environmentally friendly. The rhodium-catalyzed process is called the Monsanto process whereas the iridium catalyzed process is referred to as the Cativa process (Jones, 2000).

In the Monsanto process, hydrogen iodide reacts with methanol to produce methanol iodide. The methanol iodide then oxidizes the rhodium catalysts to give rhodium-methyl complex, which undergoes a fast change with the addition of carbon monoxide to form a carbonyl group and later its acyl form (Jones, 2000).  The acyl form of the rhodium-methyl complex is liberated to produce the acetic acid plus hydrogen iodide (Jones, 2000). The description above consists of the major steps involved in the process of making ethanoic acid.

This process of making acetic acid takes place in a reactor tank which is stirred continually. The Liquid through a pressure valve is removed from the reactor tank which via an adiabatic flash tank enters. At the adiabatic flash tank, some components are removed as vapor at the top of the machine. The products removed are the light methyl iodide, acetic acid, methyl acetate, and some water components. Fast forward, they are poured into a distillation train for purification.

Acetic acid is widely used industrially and in homes. Firstly, acetic acid is converted into vinyl acetate, a monomer for acetic anhydride and ethenyl ethanoate, which are mainly used to produce cellulose ethanoate. These monomers are some of the components in adhesives and paints (Essential Chemistry Industry, n.d). This application consumes a large percentage of the total acetic acid produced around the world. It is considered a primary use of ethanoic acid as it uses over 60% of the global ethanoic acid produced.

Secondly, cellulose reacts with acetic acid to produce a compound called cellulose acetate. This compound is used in the manufacture of textiles and films (Shakashiri, 2008). The photographic films made under this process are made from the fibers of cellulose acetate.

Thirdly, the acid has found a lot of application in the medical field during the preparation of pharmaceuticals. Salicyclic acid is reacted with acetic acid in the manufacture of aspirin, also called acetylsalysalycylic acid (Shakashiri, 2008). Aspirin has widely been used as a pain reliever, and it does not have the side effect of loss of consciousness. It is also used to prevent clotting of the blood, and it is often given to patients with heart problems to prevent the death of the cardiac tissue. Dilute acetic at 4-18% is vinegar which is used in and outside the home mainly to help in cleaning. Vinegar is also used as a food additive and preservative.

Lastly, acetic acid can be used to produce esters. Solvents for coatings, paint, and ink are esters. The esters are reacted, in the presence of a catalyst, between alcohol and acetic acid. This is another application that consumes a substantial percentage of global acetic acid production (Essential Chemistry Industry, n.d). The acid can also be oxidized which leads to the production of polyester. Acetic acid also finds application in the production of explosives, perfumes, weed killers, and industrial chemicals (Shakashiri, 2008).

Acetic acid enjoys a good market in Japan, the United States, China, and Western Europe. Most of these countries are highly industrialized. They need such an important gas to drive forward their economic and dominance agendas. Western Europe is the leading importer of acetic acid followed by the Middle East. These countries have such a high demand for acid because most of them are leading manufacturers of most products used globally. Coincidentally these products like plastics, vinegar need an acetic acid product for their manufacture.

However, China is growing to be the global leader in the acetic acid trade. This is because two of the global leader factories in acetic acids are cementing their presence in China. These companies, BP and Celanese are taking care of over 50% of all the acetic acid used in the world. It is projected that a country’s demand for acetic acid increases at a rate of around 1.25 of its Gross Domestic Product. If these companies are continuing with their expansion work in China, it would, therefore, take a miracle to dislodge them, and therefore china from the top.

However, it is worth noting that acetic acid poses a risk to the environment. The acid has its vapor form and it is soluble in water. At the production site, when acetic acid vapors find their way into the atmosphere, they offer a great risk in helping deplete the ozone layer. This will increase the effects of global warming. On the other hand, as it dissolves in water and gets into the ground, it puts the life of most organisms that live on and in the ground in some danger. These included humans who are going to feel the impact of these fumes from the manufacturing sites and beyond. Sadly, there is no legislature to control the manufacture of acetic acid. This makes the manufacturers enjoy a free space to continue posing great danger to the environment.