Which is the best process for production of acetic acid?

Acetic acid is a commodity chemical with the global demand of approximately 15 million tons per year with several applications in the chemical and food industry. The production of acetic acid can be widely categorized into chemical and fermentative routes, with the chemical route being the predominant one in the current industrial practice. In this chapter, we have reviewed the most recent developments in acetic acid production and applications over past two decades, including process intensification and catalysis by keeping the main emphasis on process sustainability. Acetic acid is used in several industrial sectors such as chemical, pharmaceutical, textile, polymer and paints, food and beverages. Furthermore, acetic acid has several applications in food industry and is traditionally known as vinegar. In addition, it is an acidulant, which is used to give a characteristic flavor profile to food. It can be used for microbial decontamination of meat and as a mild descaling agent in the food industry. More recently, acetic acid is reported to be used as an antimicrobial edible food coating agent. The diversified food culture has a significant demand in the development of such kind of innovation and acetic acid can be an efficient solution.

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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].

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].

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.

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3. Need for development of novel sustainable technologies

Looking at the ever-increasing threats of global warming and ever-increasing global demand of acetic leads to an urgent need to develop a novel technological approach and sustainable feedstock for the generation of acetic acid. Even though many processes and technological developments are reported recently, they fail to sustain the production cost to profit margins. The separation of acetic acid remains the key issue to overcome the economical and energy consumption barriers. The different operations such as distillation, evaporation, absorption, filtration crystallization and alkali neutralization are time and energy consuming. Even though these processes involve multiple steps, the ever-growing demand forces to follow this path. On the other hand, fermentation process is reliable but cannot match the scale of current demand. Thus, the development of a novel route for generation or process intensification in separation can drastically reduce the overall production cost of acetic acid. Utilization of CO and CO2 as feedstock generated from natural gas can offer long-term sustainability of acetic acid production. This technology offers high purity of acetic acid with eco-friendly production. Furthermore, membrane-based separation processes can provide efficient way to produce acetic acid. The pathways are discussed briefly.

3.1 CO and CO2 as valuable feedstock

Utilization of CO2 and syngas can offer sustainable alternatives to produce acetic acid. BP has announced the breakthrough process, wherein, acetic acid will be manufactured from syngas as a feedstock derived from natural gas. This will give an alternative to SaaBre process that produces acetic acid in three integrated steps. The production of acetic acid from syngas will avoid the purification of CO and purchase of methanol. Though the technology is not fully developed, it provides better alternatives in terms of sustainability. Similarly, acetic acid can be synthesized via reacting CO2 and H2 to give methanol followed by subsequent carbonylation step. This route gives liberty to utilize CO2 as value-added feedstock.

3.2 Membrane-based technologies

The membrane technology can offer the separation of liquid, vapour and gas selectively with controlled mass transfer rates. These processes are easy to operate and simple to design. The technology can offer development on energy intensification. Several types of processes are reported based on the pore size of the membrane for separation of different components. These are namely microfiltration, ultrafiltration and nanofiltration membrane. The operating pressure (varying from 1 to 20 bar) of the system varies according the pore size of the membrane are used. Reverse osmosis is another membrane technology with non-porous membrane. This process operates at pressure more than 20 bar. The membrane technologies collectively can be applied in downstream processing for separation of acetic acid in chemical process as well as fermentation processes. The combination of fermenter with acetic acid permeable membrane can help in separation of acetic acid to avoid the self-inhibition.

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4. Application of acetic acid in food industry

Direct applications of acetic acid are reported from ancient times. It was used as a medicine and food preservative. Over the period, applications of acetic have diversified as per the demands of modern life. Using different concentrations, it is utilized in food additives, food preservation, antimicrobial agent, acidulant, flavour and taste enhancer, edible packaging material, artificial food ripening agent, etc. Some of the applications such as acidulant and as acetification agents are described in detail here.

4.1 Acetification

Acetification is simply the bacterial oxidation of ethanol to produce acetic acid and water (Figure 5). The process is also termed as oxidative fermentation. The rate of the reaction in acetification mainly depends on the type of microorganism used to catalyse and the concentration of available oxygen in the media [13].

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There are different types of microorganisms that occur naturally in food and are responsible for the different natural processes such as acetification, alcoholism, proteolysis and enzymatic reactions, which alter the natural condition of the food. This bioprocess technology is studied and systematically utilized to improve the quality of food s in terms of texture, taste, mouthfeel, colour and prolonged shelf life. The overall concept has grown into generating different types of food and beverages produced in a cheap and sustainable way.

Acetification of different food categories using acetic acid bacteria (AAB) has led to the production of several food products [14]. AAB are naturally found on fruits, flowers, and plants, which naturally react and convert carbohydrate sugars into organic acids in the presence of oxygen. The same concept is biotechnologically utilized to prepare a diverse variety of food and beverages.

4.2 Flavouring agent

Different parts of the world have utilized the acetification process to generate a variety of foods and beverages. The famous Lambic beer is produced from malted barley, aged dry hops and unmalted wheat. The different AAB and yeast are responsible for the generation of this beer, which is matured for over a period of 3 years. The typical acidic flavour of the beer is achieved with the help AAB together with lactic acid. The sparkling water is another famous example, which gives typical acidic and fruity flavour via fermentation of water and natural sucrose. Water kefir is one of the examples of such type. Kombucha is another type of beverage produced by oxidative fermentation. It is prepared from Kombucha (tea fungus), water and sugar. Similarly, Cocoa is fermented from cocoa beans with the help of AAB and yeast, which is used as raw material for chocolate production.

4.3 Acidulant

Acidulants are essential ingredients or additives that are generally used to improve the taste of food and make it sharper. There are naturally occurring acidulants such as acetic acid, citric acid, malic acid, fumaric acid, lactic acid, tartaric acid, succinic acid, phosphoric acid etc. having different taste profiles. Many fruits such as orange, lemon, apples, tomatoes and yogurt contain natural acids with the most common example being citric acid. Citric acid comes with lemon flavour, acetic acid with strong familiar vinegar flavour, tartaric acid gives sharp taste and lactic acid comes with a smooth taste [10]. Apart from taste enhancement, acidulants also act as a food preservative. The choice of the acidulant is usually made based on its characteristic flavour and the physical state and solubility. Some food formulations require solid acidulants. In general, inorganic acids such as sulphuric acid, phosphoric acid, monosodium orthophosphate and diphosphates are used as dry acidulants in controlled concentrations. The composition of the acidulants is based on their selection and different concentrations calculated by total titratable acid. Acetic acid is mainly used in the form of vinegar with the pungent smell. As it appears in the liquid state, it is used as a preservative in pickles. It is also used in the manufacture of cheese to improve the shelf life period, good mouthfeel and taste [15].

4.4 Edible packing

Acidulants are also used as food coating, which may be edible or non-edible to prevent food from contamination with the surrounding environment, to protect it from bacterial infection and to improve the shelf life of the food. These films are easily biodegradable. The water-soluble non-edible coating is used for the packing of food [16]. The edible coating is used for breath freshening agent, in drug delivery and as flavour. Acetic acid is used in edible films to enhance sour flavour. Various compositions of acetic acid are used to develop antimicrobial food coating to stop the outgrowth of bacterial and fungal cells. It is also used in meat coating and preservation of meat products. The chitosan-based edible food coatings along with aqueous acetic acid are used to enhance anti-listerial activity.

4.5 Antibacterial agent

Acetic acid is commonly used in medicine since ancient times. The low concentrations (3%) of acetic acid can be used as a local antiseptic against various microorganisms. Acetic acid is always considered as an alternative. It can be utilized as in vitro antimicrobial agent combined with other antiseptics. Acetic acid covers the wide range of spectrum with Gram-positive as well as Gram-negative bacteria.

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5. Conclusion

Acetic acid has remained one of the key chemical molecules associated with human life. It is one of the main building blocks for developing several chemical entities. Cavita process shares a major part of the production to meet the global demand. The process utilizes methanol as a raw material, which is obtained from biogas. Though the process utilizes bio-derived feedstock, it utilizes high energy and manpower with multiple separation steps. The innovative and simple technologies for separation of acetic acid can improve the overall process. The other well-known process, that is, fermentative route, is slow and commercially unsuitable to meet the global demand. The fermentation process is globally followed to generate the food-grade acetic acid commonly known as vinegar. The demand for acetic acid will always keep growing, which necessitates the development of an eco-friendly process. Utilization of CO2 and syngas may offer excellent alternatives as a sustainable feedstock to develop innovative technologies to develop commercial processes. This offers development of 100% bio-derived feedstock process. Further, the modern food industry has come up with different innovative applications of the acetic acid in food preservations and improved quality of food.

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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