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

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

Coimbra

Encuentro; segundo Encuentro Ibérico de Fluidos Supercríticos; 2022

The supercritical CO2 (scCO2) fractionation can be a technically and economically feasible technology 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 the fractionation of glycerol acetates, a mixture of triacetin, diacetin and monoacetin obtained by the acetylation of glycerol [1-4]. This chemical reaction is an interesting route for glycerol valorization, and it has been explored in the literature by different authors [5]. The isolation of the three compounds obtained in the reaction of acetylation is a key step in the process to get value-added products in actual biorefineries [4,5]. Previous works shows GCA-EOS model predicts 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 is a 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; 19 mm 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 higher solvent to feed ratio and loading conditions closer to the flooding point to decrease the height equivalent of theoretical stage [6]. Moreover, simulations of the unit show low recovery of the target compounds in a countercurrent isothermal operation. Thus, two design options are considered: i. a countercurrent column with internal reflux using a hot finger point, and ii. a countercurrent 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 in two separation steps. In the first part, monoacetin can be obtained in the raffinate, while triacetin and diacetin can be separated in a second step. GCA-EOS modeling shows a commercial glycerol acetates mixture (23.3% g/g triacetín, 48.7% g/g diacetín y 28.0%g/g monoacetin) can be fractionated in two steps using a single column of 11 stages, fed at stage 4, and using a hot finger in the top of the column and high solvent consumption (S/F=80) to obtain 95 wt.% monoacetin at high recovery (95%). On the other hand, triacetin and diacetin can only be fractionated with an acceptable recovery of diacetin (83 %) using external reflux after 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 is carried out. The more favorable operating conditions to raffinate monoacetin are 306 K and 91 bar, using a reflux ratio of 1.6 and 65 kg CO2/kg of feed, and diacetin are 310 K and 93 bar, using a reflux ratio of 8 and 45 kg CO2/kg Feed.