Experimental analysis of a gel layer in dead-end ultrafiltration of a silica suspension*

Unstirred dead-end ultrafiltration of a silica suspension was studied using a fully retentive membrane. A constant pressure filtration approach was used to analyze steady-state flux data. The effects of operating conditions such as applied pressure and bulk concentration on specific resistance, thickness, porosity and silica concentration of gel are reported. The specific resistance was obtained from the flux data while the Kozeny–Carman equation was used to determine the silica gel porosity for subsequent calculation of concentration of silica on a deposited layer. Further, experimental verification of time-invariant flux of earlier theoretical work is also presented. Gel concentrations and thickness estimated by utilizing the procedure described in this work were found to be within 1–12% of experimentally determined values.

A new analysis of a deposited gel layer for silica, which does not exert osmotic pressure, is presented. It was shown that there was a characteristic time for which flux was relatively constant, but after this time the concentration at the membrane surface increased, which led to gel formation. The steady-state rejection was found to be 100%. It was concluded that the steady-state flux was independent of change in applied pressure for our experiments. It appeared that deposit thickness, as a function of cumulative volume, is directly linked to convective flux. It was observed that pressure dependence of specific resistance led to an insignificant variation in the compressibility coefficient with bulk feed concentration.