Influence of operating parameters on the separation of colorants from sugar house juices by ultra- and nano-filtration

Abstract

In this work, the separation of colorants from sugar house juices by ultrafiltration/nano-filtration (UF/NF) was examined on the laboratory scale as a possible additional color elimination method to increase the efficiency of the traditional sucrose evaporating crystallization. The filtration experiments were performed on three different polymer membranes, two of them with pore sizes in the ultrafiltration (UF) range with nominal cut-offs: 6-8 kg/mol and 15-20 kg/mol, and the third membrane with a pore size of 0.5 kg/mol being about on the border of UF and nano-filtration (NF). As a model solution the mother liquor of the 2 nd crystallization stage (B sugar green run-off syrup) was used. The flux and the quality (color, dry substance and sucrose content) of the permeate was measured as a function of filtration temperature, transmembrane pressure and cross-flow velocity. The results were evaluated by statistical methods. The highest color retention (ca. 78%) was found on the lowest pore size (0.5 kg/mol cut-off) membrane at 30 bar transmembrane pressure, but the flux (14 L/[m 2 · h]) was only about half of that found on membrane of 6-8 kg/mol cut-off, with which ca. 50% color retention was obtained at a much lower transmembrane pressure (4 bar). Moreover, a small retention of sucrose (M S ∼342 g/mol) was detected on the 0.5 kg/mol membrane, showing that this pore size is the lowest limit to avoid sucrose retention.