Wall Effect On Foam Drainage And Its Application In Foam Separation

Foam fractionation is a new fast-developing separation technology. It is an interesting topic in foam fractionation to enhance foam drainage and improve enrichment ratio. The wall effect on foam drainage and foam separation performances in foam separation has not been reported. In this paper, wall effect on foam drainage in foam fractionation was investigated. A wall drainage model was established for revealing the mechanism of wall effect in the enhancement of foam drainage. A foam fractionation column with wall internals was designed and its separation performance was investigated. Based on the hydrodynamics of rising foam, a model was established to determine foam separation performances using the column with wall internals. The column with wall internals was assessed for removing color from simulation crystal violet(CV) wastewater, real textile wastewater and recovering whey protein from whey wastewater.The results showed that the column wall had the performance of enhancing foam drainage. The increase of wall contact area in the foam phase significantly decreased the liquid holdup of the foam out of the column, intensified bubble coalescence and increased the surfactant concentration of the foam out of the column. The column wall had better performance in enhancing foam drainage at a lower surfactant concentration, air flow rate, liquid viscosity and a larger pore diameter of gas sparger. With increasing the wall internal number, the wall effect increased.The wall drainage model was obtained by the theory of Plateau channel and dimensionless drainage model. The theoretical model analysis showed that the increase of exterior Plateau channel number and transition from the channel-dominated drainage regime to node-dominate drainage regime contributed to wall effect in enhancing drainage of bovine serum albumin(BSA). The wall effect in enhancing drainage of sodium dodecyl sulfate(SDS) attributed to the transition of drainage regime.The effect of the wall internal on foam performance was investigated. The results showed that the column with wall internals had the higher enrichment ratio compared with the column without any wall internals. With decreasing surfactant concentration, volumetric gas velocity and increasing the pore diameter of the gas distributor, internal number, internal length, and pH deviating from the isoelectric point, the wall internal performance in improving enrichment ratio was intensified. Wall internals on the upper part of foam phase had the higher enrichment ratio than that on the lower part of foam phase for SDS. However, it was opposite for BSA.Based on the hydrodynamics of rising foam, a relationship between the wall area and the foam separation performance was obtained. The predicted enrichment values and recovery values were in good agreement with the experimental results. The theoretical model analysis showed that the increase of drainage velocity played a critical role in enhancing enrichment while bubble coalescence of BSA and a higher adsorption flux of SDS made slight contribution.The foam fractionation column with wall internals was assessed for removing color from simulation crystal violet wastewater, real textile wastewater and recovering whey protein from whey wastewater. The results showed that wall internals had good performance in the enrichment of target product in the actual systems. Under the suitable operations conditions, the enrichment ratio of CV was 16.5 by using the column with wall internals and it was 3.59 times higher than the column without wall internals. The two-stage foam fractionation technology with the column of wall effect and Fenton reagent were used in treatment of real dyeing wastewater. Under the suitable operations conditions, removal color efficiency reached 86.5% and removal CODcr efficiency reached 80%. The two-stage foam fractionation technology with the column of wall effect was used in the treatment of whey wastewater. Under the suitable operations conditions, the enrichment ratio of whey protein by using the column with wall internals was 12.5 and it was 1.65 times higher than the column without wall internals, 1.47 times higher than the inclined column.All above considered, the increase of wall area in foam phase had a significant influence on foam drainage in foam separation. Wall effect has play an important role in enhancing foam drainage and improving foam separation performaces. The column with wall internals can improve enrichment ratio effectively and has industrial application values.