Nanofiltration for dextrose recovery from crystallization mother liquors: A feasibility study

A wide experimentation was carried out to test performances of industrial NF modules for dextrose (DX) recovery from crystallization mother liquors, coming from dextrose manufacturing by saccharification of starch hydrolizates. Real solutions were processed containing total Dry Substance (DS) in the range from 17 to 43 wt% and DX content close to 80-83%; Impurities being mainly maltose and other sugars with higher degree of polymerization. Aim of the work was to achieve at least a DX purity of 95% in correspondence with average transmembrane fluxes higher than 4 dm3/(hm2). Four commercial spiral wound modules with polyamide membranes were tested, manufactured by KOCH-Membrane Systems and by GE Power&Water (well-known as DESAL membrane products). Module performances were measured at temperatures from 30 to 50 °C, at pressures from 15 to 30 bar. Nanofiltration is confirmed as a feasible technique to get preferential permeation of dextrose. For all the modules tested many analogies were observed in their behavior as a function of operative conditions. In spite of the complexity of the solutions investigated, module performances can be easily described by using few simple quantities (DS wt%, DX purity, total flux, DX rejection and Impurities rejection). DX Purity in the Permeate is strictly dependent on the DX purity in the module feed and on the total volume flux. Results give a general indication about the experimentation protocol to be performed in order to obtain basic data useful both to test the process feasibility and to scale-up the process. Membranes tested showed different separation efficiencies; only the module GE-DL4040C1025 fitted the industrial requirements for the process application. Modules GE-DL can be used with confidence at 50°C and 30 bar, in a range of feed flow rates from 2300 to 3600 L/h, achieving DX purity in the Permeate higher than 97 %; correspondingly, the NF plant can be designed to work with a DS content in the feed stream from 22 wt% to 35 wt%, obtaining total fluxes from 16 to 6 dm3/(hm2) and a DS content in the Permeate from 11.5 wt% to 22.5 wt%, correspondingly.