Unsteady-state flux behaviour in relation to the presence of a gel layer

The unsteady-state flux behavior for silica and dextran in a stirred ultrafiltration cell was studied. The 2 compds. show clearly different flux behaviors. During the filtration of dextran, only a polarization layer is built up. For silica, a gel layer formation also occurs. As a result, the time to reach steady-state flux is 1 h. The osmotic pressure model provides a good description of the flux for the expts. with dextran. If mass-transfer coeffs. are used that are higher than those electrochem. measured, the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements under unsteady-state conditions as an indication for the presence of a gel layer is discussed. [on SciFinder (R)

Solute rejection in the presence of a deposited layer during ultrafiltration

During ultrafiltration deposited layers are often formed on the membrane surface. These layers not only reduce the volumetric flux through the membrane, but also may influence the rejection of other solutes in the feed. In the present paper we will show that besides an increase in the rejection, a decrease in rejection may also occur, which can completely alter the aimed selectivity of the separation process. The influence of deposited layers has been studied experimentally by two types of depositing components: silica sol and the protein BSA. In the presence of a relatively open silica deposit a strong drop in the rejection of PEG and dextran was found compared to the rejection on a clean membrane. For thick deposit layers the rejection even decreased to zero, thus resulting in a total permeation of a normally partially rejected solute. On the other hand an increase in PEG rejection occurred in the presence of a BSA deposit. Due to the compressibility of the protein deposit the highest rejections were measured at the highest pressures. The effects were the most pronounced at the isoelectric point of BSA. A model is presented to describe the underlying phenomena

Desalting a process cooling water using nanofiltration

The cooling water system of a chemical plant of Akzo Nobel is a partly open system. The site is located at the North Sea. The air in contact with the cooling water contains seawater droplets dissolving and increasing the chloride concentration. The cooling water contains chromate to protect the installation for corrosion. Nevertheless, the chloride concentration in the cooling water may not exceed 50 ppm. To maintain a constant chloride concentration level, a part of the cooling water should be drained and replaced by de-mineralized water. However, the drainage of chromate is limited by governmental regulations. This paper describes a systematic approach from problem definition to the membrane process design encompassing membrane module pre-selection, lab-scale proof of principle and on-site pilot demonstration, the latter to characterize the fouling and scaling potential of the feed solution. Finally, the paper discusses various membrane process design options