Upgrading grape pomace contained ethanol into hexanoic acid, fuel additives and a sticky polyhydroxyalkanoate: an effective alternative to ethanol distillation

Abstract

The management of grape pomace (GP), the main winery solid residue, is presently supported by government subsidies, promoting the energetically expensive recovery of ethanol by distillation. This work proposes and assesses a novel sustainable alternative GP valorisation strategy: chain elongation fermentation. Besides, the proof-of-concept of a multipurpose cascading scheme is presented based on experimental data for each step on the laboratory scale. The new cascading biorefinery scheme includes: (1) the ethanol upgrade into highly concentrated (ca. 900 g L⁻¹) n-hexanoic acid (C6) by anaerobic acidogenic fermentation and a simple downstream; the exploitation of the obtained C6 as (2) a reagent for obtaining an ester-alcohol mixture as well as (3) a substrate for the production of medium chain length polyhydroxylalkanoates (mcl-PHAs), and (4) the complementary biomethanization of the solids leftovers from the acidogenic step. Specifically, the identified fermentation conditions (pH 7, 37 °C and P > P_atm) allowed obtaining the highest C6 titer (22 g L⁻¹) and productivity (6.2 g L⁻¹ d⁻¹) ever achieved from non pre-treated biowaste and without the need of either exogenus ethanol or methanization inhibitor or expensive in-line extraction methods. Such titer, allowed employing a cheap easy-direct C6 downstream leading to 54% C6 recovery (87% purity) at potentially competitive overall costs. Although a preliminary assessment showed that this partial valorisation could be economically sustainable in itself for GP-management, the exploitation of the highly concentrated GP-derived C6 was demonstrated for the first time. Heterogenous catalytic hydrogenation (180 °C and 115 bar), with pre-reduced commercial catalyst Re/C 5 wt%, allowed the conversion of the obtained C6 into a mixture of 1-hexanol and hexyl-hexanoate (molar yield of 75%), which represents a promising blendstock for both diesel and biodiesel fuels. On the other hand, a fed-batch culture system, carried out on a bench-top bioreactor, allowed obtaining 60% PHA content with a yield of 0.30 g g⁻¹. The recovered sticky bioelastomer was mainly composed of 3-hydroxyhexanoate (90%). Complementarily, ca. 200 N-L kgVS⁻¹ of biomethane was obtained from GP leftover solids.