*3.1. Case Study 1: Material Streams Recovery—Acetic Acid*

The global consumption of acetic acid (AA) as a chemical product in 2018 was estimated at over 16 <sup>×</sup> 10<sup>6</sup> t. BP Chemicals is the largest supplier of acetic acid. The overall acetic acid output is shared between the manufacturing of vinyl acetate monomer (VAM) at 33%, acetic anhydride at 18%, monochloroacetic acid (MCA) at 17%, acetate esters at 17%, and terephthalic acid at 17% [78]. The manufacturing of acetic acid by the carbonylation of methanol represents the largest part of the market, at about 75%. This share shows continuous growth. Besides methanol, the synthesis may start from dimethyl ether (DME) and later from materials leading to syngas, such as biogas and biomass. For this reason, acetic acid may be a 100% green product [79]. For example, if both acetic acid and ethylene are manufactured using methanol obtained from syngas, then the vinyl acetate is a bio-product, as well as the valuable acrylic paints produced from it. In this way, a variety of products issued from a syngas bio-refinery can replace traditional petrochemicals.

Diluted solutions of acetic acid in water are typical for side streams in several production processes including terephthalic acid synthesis, acetyl cellulose manufacture and biochemical processes. Since the concentration of AA in those streams is typically in the range of 10% to 40% (mass), it may be profitable to recover and recycle it as a solvent [80]. However, the separation processes are often expensive and energy-intensive; these processes include reactive distillation, extractive distillation, and membrane separation. Thus, the side streams are commonly diluted and eliminated in a wastewater treatment plant. This makes acetic acid one of the major water pollutants treated in the chemical industry, especially in petrochemical or fine chemical industries [81].

In Section 3.1.1, the exergy footprint of the AA waste stream is evaluated. The AA waste stream comes from a methanol carbonylation process, while VAM production is considered as a potential downstream process that may utilise AA. Section 3.1.2 evaluates the exergy profit of using the acetic acid waste stream as input for VAM production.
