Deliverable 2.7 - Report on biosolvents
The main goal of this task is to produce and isolate pure bio-solvents (namely ethyl esters of volatile fatty acids) from the effluent of an anaerobic acidogenic fermentation, fed with organic waste, namely the organic fraction of municipal solid waste and the sludge from urban wastewater treatment. Such a work is divided into two sub-tasks:
i) the recovery of volatile fatty acids through solid phase extraction (Subtask 2.3.1 into the proposal, performed by UNIBO) and
ii) their successive conversion into the respective ethyl esters (Subtask 2.3.2, performed by IRSA).
A continuos solid phase extraction process in an up-flow thermostated packed column has been developed for the recovery and separation of short chain length carboxylic acids (also mentioned as volatile fatty acids) from the anaerobic acidogenic effluent. A 130 cm column was filled by 160 g of the anion exchange Lewatit A365 resin, which was employed as the active process solid phase to chemically adsorb the acids by ionic interactions. The overall acid concentration in the experimental matrix was about 25 g/L, this representing about 62% of the organic soluble fraction. Unsoluble total solids were separated by conventional protocols (centrifugation / filtration) since they would have negatively affected the extraction process by inducing clogging problems and/or lowering the amount of resin active sites. The anaerobic acidogenic effluent pH was 5.4: thus, after the solid separation step, the whole process scheme included an operation unit dedicated to lower the effluent pH, since optimal conditions for the extraction of acids were observed in the occurrence of a pH of about 1.5. The pre-treatment was conducted in a column packed with the Lewatit S2568H resin, by a cation-exchange extraction approach. In agreement with preliminary experimental evidences, the acidified liquid phase was fed to the anion exchange solid phase extraction step by a flow rate of 40 ml/min for about 80 min, until the resin adsorption capacity was exhausted (i.e., after 11 dimensionless retention times). Then, acids were desorbed by using both alkaline and acid ethanol as the extraction solvent in a batch process carried out by alcohol recirculation. More than 90% of the acidic fraction occurring in the initial experimental matrix were recovered. Final overall acid concentration was about 34 g/L.
Then, specific and tailored procedures were optimized on the abovementioned alkaline and acid ethanolic solutions of volatile fatty acids, to obtain from them the ethyl esters in a pure form: an overall amount of about 400 mL have been eventually isolated.
Besides the use of conventional sulfuric acid, a new process for obtaining ethyl esters of volatile fatty acids with ethanol was also investigated by using aluminium chloride hexahydrate as a catalyst. This approach was initially optimised on pure acids and then applied on ethanolic solutions obtained from UNIBO. Besides the good activity, since equilibrium composition was achievable already after 3-4 hours at 343 K, differently from conventional mineral acids, aluminium chloride also induced a separation of phases with a convenient distribution of components. In fact, ethyl esters were easily recovered into the upper layer, >99%wt, together with most of unreacted acid and ethanol, well separated by a bottom phase, in which there were the co-formed water and most of catalyst, >97.8%wt. Such a process intensification of reaction and separation was deeply investigated and operational conditions were optimised. Then, for the specific case of production of ethyl-acetate, a new configuration of unit-operations was designed, simulated through Aspen Plus V9® and compared with the present industrial process based on sulfuric acid catalysis. The overall production and purification of ethyl acetate was found economically competitive, less energy demanding, with over 50% of energy saved, and a potentially zero-waste process, by resulting definitively in a cleaner production.