Acetic acid: production, reactions and uses

acetic acid structure

Acetic acid is a corrosive and colorless liquid that has a pungent odor and is a vesicant of great danger. It is present in dilute solutions in several plant and animal systems.

Vinegar, an aqueous solution that contains approximately 4–12% acetic acid, has been in use for over 5000 years, and it is produced by the fermentation of wine.

The leading producers of acetic acid, which account for more than 80% of total worldwide, are located in Asia and North America. The global capacity of acetic acid production is above 12 million tons per annum.

Acetic acid is primarily used in the manufacture of acetic anhydride, vinyl acetate, and as a process solvent for terephthalic acid production.

Vinyl acetate is used to produce latex emulsion resins that are applied in paints, adhesives, paper coatings, and textile treatment. Acetic anhydride is used in the production of cellulose acetate textile fibers, cigarette filter tow, and cellulose plastics.

Table of Contents

1. Production of Acetic acid

The process of vinegar production is primarily based on the traditional method of fermentation.

The modern synthetic routes to acetic acid involve methanol carbonylation and liquid-phase oxidation of butane, naphtha, or acetaldehyde.

1.1. Carbonylation of Methanol

Methanol carbonylation has been the preferred and dominant method for the large-scale production of acetic acid for the past 35 years.

The conversion of methanol and carbon monoxide to acetic acid at high temperature and high pressure was first reported by BASF in 1913.

Subsequently, in 1941, REPPE at BASF demonstrated the efficacy of group VIII metal carbonyls as catalysts for carbonylation reactions, which led to the development of a high-pressure, high-temperature process with a cobalt iodide catalyst.

The primary objective of this research was to develop an acetic acid process that is not reliant on petroleum-based feedstocks.

The current superiority of the methanol carbonylation process over other routes to acetic acid is attributed to its favorable raw material and energy costs.

The synthesis gas raw material required for this process can be sourced from various sources such as natural gas to coal.

The cobalt-based carbonylation process was commercially launched by BASF in Ludwigshafen, Germany, in 1960, with an initial capacity of 3600 t/a that was later expanded to 45 000 t/a by 1981.

In the same year, Borden Chemical Co. initiated a 45 000 t/a acetic acid unit in Geismar, Louisiana, United States, based on the BASF technology.

This unit was expanded to 64 000 t/a by 1981 before it was shut down in 1982, but it was later reopened for one year in 1988 to meet acetic acid supply shortages in the United States.

In the late 1960s, Monsanto developed a low-pressure acetic acid process with a rhodium iodide-promoted catalyst system that demonstrated higher activity and selectivity than the cobalt-based process.

The commercial production of acetic acid using the Monsanto process began in 1970 at Texas City, Texas, with an initial plant capacity of 135 000 t/a that has since been expanded to 270 000 t/a since 1975. Operating conditions in the reactor are milder (3 MPa and 180°C) than in the BASF process.

Due to its superior performance, the Monsanto process became the preferred technology for basic level acetic acid units, and more than ten companies worldwide have licensed and operated this technology since the start-up of the Texas City plant by Monsanto.

Methanol can also be carbonylated at atmospheric pressure with yields of 99% and 90% with respect to methanol and carbon monoxide, respectively.

Production of acetic acid (Monsanto process)
Figure 1: Production of acetic acid (Monsanto process) a) Reactor; b) Flasher; c) Light-ends column; d) Dehydration column; e) Heavy-ends column

1.2. Direct Oxidation of Saturated Hydrocarbons

Liquid-phase oxidation (LPO) of aliphatic hydrocarbons has been a widely adopted industrial process in the past. However, due to the emergence of carbonylation technology, the production of LPO has been significantly reduced by plants worldwide.

The availability of raw materials, such as n-butane and light naphtha, can influence the process used. For example, BP in the United Kingdom uses light naphtha, while Celanese in the United States and Canada employs butane to produce acetic acid.

1.2.1. Reaction Mechanism

The oxidation of hydrocarbons follows similar kinetics in both gas and liquid phases, particularly in slightly polar solvents. However, the reaction mechanism is complex and can be considered a radical chain reaction.

For instance, the oxidation of butane occurs via initiation, oxidation, propagation, and decomposition steps. The initiation, particularly with catalysts, significantly affects the induction period and propagation likely involve radicals abstracting hydrogen from a secondary carbon atom of butane, followed by reaction with oxygen to produce hydroperoxides.

These intermediates decompose to yield acetic acid, a process accelerated by catalysts, agitation, and high temperature. Although not essential for LPO, metal catalysts can affect product distribution, induction period, and operating temperature.

The first step of oxidation involves the abstraction of a secondary hydrogen atom to form alkyl radicals, which are rapidly converted by oxygen in the solvent to sec-butylperoxy radicals.

Some interpretations suggest that oxygen reacts directly with one or two alkane molecules to form radicals.

1.2.2. Industrial Operation

Air or oxygen-enriched air can serve as the oxidant, while multivalent metal ions, such as Mn, Co, Ni, and Cr, are used as catalysts in the LPO process but some processes are noncatalytic.

The reaction conditions involve a temperature range of 150-200°C and a reaction pressures of 5.6 MPa. The reaction solvent comprises acetic acid, intermediates, water, and dissolved hydrocarbons. Water concentration must be controled beacause it is critical to the process’s success.

production of acetic acid by Oxidation of n-butane in the liquid phase (Chemische Werke H€uls process)
Figure 2: Oxidation of n-butane in the liquid phase (Chemische Werke H€uls process) a) Reactor; b) Air cooler; c) Collector; d) Separation vessel; e) Pressure column; f) Distillation column

1.3. Acetaldehyde Process

Oxidation of acetaldehyde represents a major process for making acetic acid. The oxidation of acetaldehyde to acetic acid proceeds through a free-radical chain which produces peracetic acid as an intermediate.

oxidation of acetaldehyde to acetic acid

Peracetic acid reacts with acetaldehyde to generate acetaldehyde monoperacetate. The acetaldehyde monoperacetate decomposes efficiently to acetic acid by a hydride shift in a Baeyer–Villiger reaction.

The methyl migration leads to the byproduct methyl formate.

The alkyl migration becomes more pronounced with higher aldehydes, particularly aldehydes having a branch at the α-position. Chain termination occurs primarily through bimolecular reactions of acetylperoxy radicals via an intermediate tetroxide.

Uncatalyzed oxidation is efficient as long as the conversion of acetaldehyde is low and there is a significant concentration of aldehyde in the solvent. This keeps the steady state concentration of acetyl peroxy radicals low and favors the Baeyer–Villiger reaction.

2. Chemical reactions of Acetic acid

Many useful products are made from acetic acid.

Acetic acid forms acetate esters when reacting with olefins or alcohols.

Acetamide is produced by thermal decomposition of ammonium acetate.

Phosphorous trichloride or thionyl chloride reacts with Acetic acid to make acetyl chloride.

Acetic acid serves as an important feedstock that contribute to the production of various industrial chemicals.

An exemple of this process is the conversion of acetic acid to vinyl acetate in the presence of ethylene and oxygen.

Acetic acid is employed in the production of acetic anhydride through ketene formation, which further undergoes reaction with acetic acid to produce the anhydride.

Also, acetic acid is used in the manufacture of chloroacetic acid using chlorine as a reactant.

3. Uses of Acetic acid

Acetic acid has a wide range of applications spanning several industries. Notably, it is utilized in organic synthesis, polymer production, cosmetics, food processing, pharmaceuticals, and detergents.

Over 65% of global acetic acid production is directed towards the production of polymers derived from vinyl acetate or cellulose. A significant portion of poly(vinyl acetate) serves as a component in paints, coatings, as well as in the manufacture of poly(vinyl alcohol) and plastics.

Cellulose acetate is a derivative of acetic acid employed for the production of acetate fibers.

Moreover, acetic acid and its esters are commonly used as solvents for various applications. These versatile properties make acetic acid an essential and valuable resource across several industries.

References