Conceptual design of a high-pressure packed column for the supercritical CO2 fractionation of glycerol acetates

FORTUNATTI MONTOYA, MARIANA; SANCHEZ, FRANCISCO; HEGEL, PABLO E.; PEREDA, SELVA

Coimbra

Congreso; segundo Encuentro Ibérico de Fluidos Supercríticos; 2022

Faculty of Science and Technology Universidade de Coimbra

The supercritical CO2 (scCO2) fractionation can be a technically and economically feasibletechnology for the separation of liquid products that find application in the chemical, food,and pharmaceutical sector. Particularly, previous works shows the scCO2 can be used for thefractionation of glycerol acetates, a mixture of triacetin, diacetin and monoacetin obtained bythe acetylation of glycerol [1-4]. This chemical reaction is an interesting route for glycerolvalorization, and it has been explored in the literature by different authors [5]. The isolation ofthe three compounds obtained in the reaction of acetylation is a key step in the process to getvalue-added products in actual biorefineries [4,5]. Previous works shows GCA-EOS modelpredicts accurately the phase equilibria between CO2 and glycerol acetates mixtures [1]. Also,modeling of scCO2 extractions of a commercial glycerol acetates mixture at temperatures between 301 K and 323 K and pressure between 100 bar and 130 bar shows that GCA-EOS isa useful tool to design and optimize the scale up separation process [2].A proof of concept carried out in a bench scale high-pressure packed column (3.5 m long; 19mm di and 373.1 kg/m3 packing density) at 313 K and 95 bar using a solvent to feed ratio(S/F) of 17.5 g CO2/g feed shows the technical feasibility of the fractionation process.However, the experimental results point out it is necessary to operate the column using highersolvent to feed ratio and loading conditions closer to the flooding point to decrease the heightequivalent of theoretical stage [6]. Moreover, simulations of the unit show low recovery of thetarget compounds in a countercurrent isothermal operation. Thus, two design options areconsidered: i. a countercurrent column with internal reflux using a hot finger point, and ii. acountercurrent column design using an external reflux using a high-pressure pump.The fractionation of the ternary mixture requires two columns, or a single column operated intwo separation steps. In the first part, monoacetin can be obtained in the raffinate, whiletriacetin and diacetin can be separated in a second step. GCA-EOS modeling shows acommercial glycerol acetates mixture (23.3% g/g triacetín, 48.7% g/g diacetín y 28.0%g/gmonoacetin) can be fractionated in two steps using a single column of 11 stages, fed at stage4, and using a hot finger in the top of the column and high solvent consumption (S/F=80) toobtain 95 wt.% monoacetin at high recovery (95%). On the other hand, triacetin and diacetincan only be fractionated with an acceptable recovery of diacetin (83 %) using external refluxafter recompression of the top product released from a separator at 323 K and 55 bar. Finally,to reduce solvent consumption on both columns, a thermodynamic sensitivity analysis iscarried out. The more favorable operating conditions to raffinate monoacetin are 306 K and91 bar, using a reflux ratio of 1.6 and 65 kg CO2/kg of feed, and diacetin are 310 K and 93bar, using a reflux ratio of 8 and 45 kg CO2/kg Feed.