Nanofiltration of CIP waters from iodine X-ray contrast media production: process design and modelling

Abstract

CIP (cleaning in place) waters from X-ray contrast material production cause considerable disposal costs as for discharge of iodine-containing effluents; a legal limit of only 1 ppm. AOX has to be met. Therefore, CIP waters which typically contain more than 1000 ppm are currently incinerated, thus requiring a huge amount of energy. A two-stage nanofiltration process consisting of an enrichment and a purification step was designed for separating contrast agents out of rinsing waters. Required areas for both stages were exemplarily calculated. Rejection was greater than 99% at transmembrane pressures of 20–40 bar and resulting fluxes of up to 200 L/(m 2h). Besides designing an economically feasible nanofiltration process which yields a permeate with concentrations below the legal limit and a retentate of much less volume for incineration, the aim of this project was to further application of mathematical model descriptions of NF transport phenomena and to find an appropriate design equation. It could be shown that in this case concentration polarisation or gel layer formation were of minor importance. As adsorption appeared to cause the principal resistance to both solvent and solute flux, parameters of an extended solution-diffusion model were fitted in such a way that they account for this physical effect.