Dextran Polymer Fractionation by Production Scale Chromatography and Ultrafiltration

Abstract

Reviews of gel permeation chromatography (GPC) and of ultrafiltration (UF) theory have been made. The aims of the project were to fractionate dextran, a polyglucose, on an existing chromatograph to achieve more optimum conditions of operation and to eliminate the silica dissolved from the chromatographic packing and present in the dextran products. Also to modify an existing mathematical model of the GPC process to take into account concentration effects on the chromatograph when fractionating dextran. The semi-continuous chromatograph (SCCR5) used for dextran fractionations consisted of ten stainless steel columns of 5.1 cm I.D. by 70 cm length, packed initially with Spherosil XOB 075 that has been later replaced with the XOB 030 grade in an attempt to improve the GPC fractionation. Ultrafiltration (UF) has been tried for removing silica from dextran solutions, and it was found that UF was not only useful in removing silica but can be also used for the fractionation and concentration of dextran solutions. The UF has been carried out on a 402A Amicon stirred cell system, on a DC2A Amicon hollow fibre cartridge system, a Patterson Candy International reverse osmosis system and an Amicon DC30 system. Ion exchange resins have been used for the complete removal of silica from dextran solutions. GPC, UF and ion exchange have been combined into a novel process that produces clinical dextran 40 from dextran hydrolysate with a satisfactory yield. The GPC process removed the very high molecular weight material. The UF process removed the very low molecular weight dextran and most of the silica and concentrated the final solution. Then this solution was passed through an ion exchange cartridge to produce a silica-free product. This process could offer a more satisfactory alternative to dextran fractionation using ethanol-water solutions, although the economics of the process have still to be established. A description of the effects of the operating conditions on the fractionation performance of the GPC on the SCCR5 have been discussed. The existing simulation program for GPC fractionations on the SCCR5 unit and, based on the linear exclusion model, has been modified to take into account the concentration and temperature changes that affect the fractionation. A good agreement between the computer simulations and experimental results has been achieved, but the model can be further improved.