Which is the best process for production of acetic acid?

1. Introduction

The bridge between chemistry and the day-to-day human life is always growing wider and stronger, and acetic acid is one of the perfect examples. Acetic acid is a clear liquid with a pungent odour, sharp taste, melting point of 16.73°C and boils at 117.9°C. Acetic acid, traditionally known as &#;vinegar&#; is widely used as a food preservative, first discovered (c.  BC) when unattended grape juice turned into wine. A famous physician Hippocrates II (c. 420 BC) used acetic acid to clean the wounds [1]. With direct and indirect applications of acetic acid, it has diversified into several chemical sectors such as food, pharma, chemical, textile, polymer, medicinal, cosmetics etc. Since then, acetic acid is proven to be a multi-application chemical building block resulting in ever-increasing demand. The production of acetic acid is expected to reach 18 million ton with an average growth of 5% per year [2, 3].

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The overall routes for production and the applications of acetic acid are shown in Figure 1. Currently, the manufacturing demand is fulfilled via two main production routes, which are chemical and fermentative. Among the chemical manufacturing processes, the key processes are Cavita process (carbonylation of methanol), oxidation of aldehyde and oxidation of ethylene. The major players are BP chemicals and BASF, which follow carbonylation route. The major consumption of acetic acid mainly comes from the preparation of vinyl acetate monomer (VAM), acetic anhydride and C1-C4 acetates and it is used as a solvent in synthesis of terephthalic acid (PET). VAM is one of the main ingredients used in polymer industry with application as emulsifier, resins, as intermediate in surface coating agent, acrylic fiber and polymer wires. It is also used in textile industry to generate synthetic fibers as a result of condensation reaction. The other condensation reaction of acetic acid produces acetic anhydride used as typical acetylation agent, which is subsequently utilized to produce cellulose acetate, used in synthetic textiles and for silver-based photographic films. Most derived esters of acetic acid are ethyl acetate, n-butyl acetate, isobutyl acetate and propyl acetate, which are frequently used as solvents for inks, paints and coatings. Glacial acetic acid is an excellent polar protic solvent that is frequently used as a solvent for recrystallization to purify organic compounds. Several researchers are working on developing a sustainable process with the simple design to produce acetic acid that meets current demand. Several homogeneous as well as heterogeneous catalytic systems are reported for the production of acetic acid with carbonylation process [4].

Acetic acid produced via fermentation route is mainly utilized in the food industry in the form of vinegar. Use of vinegar is more diversified these days, with more innovative ways to adjust and suit the current lifestyle and food culture. The different concentrations of acetic acid are used to sharpen the taste of food with a longer shelf life period and as a food preservative. Some new applications have also come such as edible and non-edible antimicrobial coating [5, 6].

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This chapter reviews the current commercial processes for the synthesis of acetic acid to meet an ever-increasing global demand. The chapter also gives insight into the pros and cons associated with the process available and then how should we design a sustainable strategy to develop a simple commercial process. Further, the state of art to produce vinegar is discussed with exploitation as a multiapplication tool in the modern food industry.

Acetic Acid is a largely used industrial product, with a world demand of about 6 million tons per year. Most of the production processes are based on the carbonylation of methanol promoted by an iodine compound and catalysed by Rhodium catalyst (Monsanto process) or Iridium catalyst (Cativa process). Monsanto method was used intensively until when BP Chemicals introduced the Cativa process, which is a more efficient technology that significantly reduces the cost and produces a high quality acetic acid with very low impurity content.

Iridium-based catalyst is responsible for a series of major improvements on the carbonylation of methanol process. Being more stable allows to extend the previously limited operating conditions. For instance a highly concentrated methanol feed can be used (0.5% water) instead of a 10% water content in Monsanto. This greatly reduces the impact of the side reaction between water and carbon monoxide and consequently improves the selectivity. The overall impact is a less expensive downstream purification process of the acetic acid compared to the technology used in Monsanto process. To be more specific, the new configuration uses a compact two distillation columns configuration.The major units of a commercial scale Cativa methanol carbonylation plant are shown in the following figure.

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