High pressure packed column for fractionation of low volatile and viscous mixtures. Conceptual design of glycerol acetates refinement with scCO2

FORTUNATTI MONTOYA, M.; F. A. SANCHEZ; HEGEL, P. E.; S. PEREDA

Cordoba

Conferencia; VI Iberoamerican Conference on Supercritical Fluids; 2023

Universidad Nacional de Córdoba

Glycerol esterification with carboxylic acids is a highly studied chemical route to add value torenewable byproducts in biorefineries. This is a stepwise reversible reaction, whoseintermediate products have higher value than the final fully esterified glycerol molecule(triglyceride). Therefore, high yield fractionation processes are needed to recover mono anddi-acyl glycerides. Like glycerol, both, low molecular weight, and fatty acids are todayavailable as low-cost reactants in biorefineries. Interestingly, the acetylation of glycerolcomprises the condensation reaction of two by-products from different-platform biorefineries:glycerol from oil-based and carbohydrate- and/or lignocellulosic-based biorefineries. Acylglycerides are typically non-volatile, thermolabile, and viscous compounds, features thathinder their separation and purification. Additionally, depending on the molecular weight ofthe carboxylic acid, intermediate products may even show azeotropic behaviour, likemonoacetin and diacetin.Supercritical CO2 fractionation is a convenient technology for processing these mixtures, itoperates at moderate temperatures and remarkably reduces the mass transfer resistancestypically found in viscous systems. In previous works, we demonstrated the technicalfeasibility of fractionating glycerol acetates using CO2 as a solvent. Moreover, we extended the GCA-EOS to accurately model, by group contribution, the phase equilibria of mixtures thatcan be found in the homologous family of acylglicerides.In this work, we carry out the conceptual design of a high-pressure fractionation column forthe rectification of intermediate products of glycerol acetylation (mono- and di-acetin) basedon GCA-EOS predictions. To challenge the model accuracy, we contrasted its predictions withthe results of experimental scCO2 fractionation of a commercial mixture of glycerol acetates,conducted at temperatures ranging from 301 K to 323 K and pressures between 100 bar and130 bar. Given the group contribution nature of GCA-EOS, the simulation tool developed inthis work is also valuable for designing separation processes of other surfactants.A proof of concept was conducted in a bench-scale high-pressure packed column (19 mm ID,total height 4 m) at 313 K and 95 bar, using a solvent-to-feed ratio (S/F) of 17.5 g CO2/g feed.Based on these findings, the height equivalent of theoretical plate (HETP) was determined fromthe number of effective equilibrium stages in the equipment and the total height of the packingmaterial. Both, the experimental results and the simulations show that a reflux is needed, as acountercurrent isothermal operation leads to a low recovery of the target compounds.Therefore, two design options are considered: (i) a countercurrent column with internal refluxusing a hot finger point, and (ii) a countercurrent column design with external reflux using ahigh-pressure pump. Finally, we use the GCA-EOS to design a feasible operating window tofractionate a commercial mixture of glycerol acetates (23.3 wt% triacetin, 48.7 wt% diacetin,and 28.0 wt% monoacetin) in two steps, using a single column of 11 stages, and fed at stage 4.According to our simulations, the more favourable operating conditions for monoacetinrefining are 306 K and 91 bar, with a reflux ratio of 1.6 and 65 kg CO2/kg of feed, while for diacetin the conditions are 310 K and 93 bar, with a reflux ratio of 8 and 45 kg CO2/kg feed.