Supercritical CO₂ assisted strategy for acetic acid elimination from industrial cellulose acetate–water mixtures

Highlights

• Critical analysis of the major steps involved in the cellulose acetate industrial process.

• Possible improvements using supercritical CO₂ based sub-processes.

• Supercritical antisolvent extraction of the acetic dope is the most attractive alternative.

• Micro- and nanoparticles, or filaments, of cellulose acetate can be directly produced.

• Acetic acid residue of 23 ppm in the supercritical CO₂ treated cellulose acetate.

Abstract

A critical analysis of the major steps involved in the cellulose acetate industrial process is performed, with the aim of proposing possible improvements using supercritical CO₂ based sub-processes. Once highlighted the main weakness of the traditional process, related to the (i) fine modulation of the acetylation reaction to obtain 2.5 acetate, (ii) acetic acid removal from the acetic dope, and (iii) treatment of the diluted acetic acid–water solution, the most attractive alternative resulted the adoption of a supercritical antisolvent extraction (SAE) performed on the acetic dope. Operating in this way, the problems related to the use of large quantities of water to remove acetic acid from the acetic dope are resolved, since it will be directly extracted by supercritical CO₂. Micro- and nanoparticles, or filaments, of cellulose acetate can be produced. Finally, an acetic acid residue of 23 ppm, in the supercritical CO₂ treated cellulose acetate, confirmed the success of this alternative process configuration.

Introduction

The production of cellulose derivatives is one of the major industrial business worldwide, due to the starting material properties, such as abundance, low cost and biodegradability, and to the lower environmental impact when compared to polymers obtained from fossil sources (Zhang, 2007). Among the main cellulose derivatives, cellulose acetate (CA), prepared by acetylating cellulose, is widely used in various fields, to form membranes, films, fibers, biodegradable plastics, filters, etc. (Fischer et al., 2008; Cardea and De Marco, 2020).

Cotton linters are frequently used as starting material for the production of CA; but several authors proposed CA synthesis, production and characterization, starting from different sources, such as bacterial cellulose (Barud et al., 2008), recycled newspaper (Filho et al., 2008), sugarcane bagasse (Candido et al., 2017; Shaikh et al., 2009), rice straw (Fan et al., 2013), rice husk (Das et al., 2014), landscaping waste (Cao et al., 2018), and coconut shells (Amaral et al., 2019).

CA preparation procedure is in all cases substantially the same. Cellulose-based material is put in contact with diluted acetic acid to swell the cellulose fibers. This material, after the addition of a polymerization starter (sulfuric acid, as a rule), and in presence of acetic anhydride, is put in contact with concentrated acetic acid that, by acetylation, transforms cellulose in cellulose acetate. Acetylation evolves to diacetate and triacetate; however, it is difficult to stop the reaction at the desired polymerization level, since it continues until a complete acetylation is obtained, with the formation of triacetate. The most frequently desired product is at about 2.5 acetylation, since it shows the most useful mechanical properties. The arrest of reaction is not simple and can be obtained adding very large quantities of water. Operating in this way, the limiting reactant (acetic acid) is extremely diluted and polymerization is interrupted. An alternative is to allow the completion of polymerization to triacetate and, then, a saponification step is added to partly reduce the polymerization down to about 2.5 diacetate. The material produced (called acetic dope) requires several washing steps with water, to progressively eliminate acetic acid. The further step is to eliminate the remaining low acetic acid concentration from water: this part of the process is also problematic.

Fig. 1 reports a schematic representation of the major steps involved in the industrial process. Other details about the description of the process are available in the technical literature.

The classical CA production process presents relevant limits from a technological, environmental and energetic point of view, mainly due to the use of large quantities of water that, contaminated with acetic acid, require purification. Indeed, acetic acid, even at very low concentrations, if discharged in free waters, will contribute to the eutrophication of water basins (Zhang et al., 2016).

Supercritical CO₂ (SC−CO₂) assisted processes have been proposed as a green alternative to the production of several materials, such as micro- and nanoparticles (Athamneh et al., 2019; Baldino et al., 2019a,b,c; Reverchon et al., 2002), liposomes (Santo et al., 2014; Trucillo et al., 2020; William et al., 2020), membranes (Baldino et al., 2019a,b,c; Sizov et al., 2018), and aerogels (Baldino et al., 2019a,b,c; Smirnova and Gurikov, 2018). SC−CO₂ has also been used for the extraction of high added-value compounds from vegetable matter (Baldino et al., 2017; Knez et al., 2019), and of organic solvents from polymeric solutions, to produce co-precipitates (Prosapio et al., 2015). Generally speaking, the key point that assures the effectiveness of these last processes is the chemical affinity between SC−CO₂ and the compound, or the family of compounds, to be extracted. Many solvents are largely soluble in SC−CO₂ (Gupta and Shim, 2006), and also acetic acid is known to have a “large solubility” in SC−CO₂ operating at the opportune process conditions (De Marco and Reverchon, 2011; Jónasson et al., 1998).

Therefore, this work is aimed at the analysis of the CA industrial process, looking at its major critical points, in the view of possible improvements obtainable using supercritical CO₂ based sub-processes. To the best of our knowledge, this is the first time that the adoption of an alternative supercritical process configuration is considered, with the aim of maximizing the process or post-process performance, and to eliminate acetic acid contamination from water. Experimental trials will be performed to verify the practical feasibility of the new sub-process configuration proposed.