Colloids and Surfaces A: Physicochemical and Engineering Aspects
Enhancing foam drainage using inclined foam channels of different angles for recovering the protein from whey wastewater
Graphical abstract
Highlights
► A theoretical framework was developed to analyze the foam flow in the inclined channel. ► The effect of the tilt angle was theoretically and experimentally studied. ► The optimal angle of 45° for enhancing foam drainage was achieved. ► Foam fractionation of the whey protein with the inclined channel of 45° was studied.
Introduction
Soybean is an important economic crop which is abundant in proteins of high nutrition value such as isolated soy protein (ISP) and whey soy protein (WSP) [1]. In the industrial ISP production, a large amount of the whey wastewater is yielded at a rate of 20 t/1 t ISP. The wastewater contains plenty of useful substances such as proteins [2], soyasaponin [3], isoflavone [4] and oligosaccharides [5]. They will be wasted and pollute the environment if the wastewater is discharged without any treatment. How to recover and reuse these substances certainly becomes an urgent issue in the industry of ISP production and environmental protection. In the current work, the recovery of the protein from the whey wastewater will be investigated.
Strategies for recovering the protein from the whey wastewater mainly include ultrafiltration [6] and foam fractionation [2]. Ultrafiltration needs more costs than foam fractionation, and the protein flux is readily limited by concentration polarization. Foam fractionation has low equipment cost, simple operation and free pollution [7]. More importantly, the results of the protein recovery using foam fractionation are satisfactory [2], [8]. Thus foam fractionation is more suitable in the protein recovery from the whey wastewater.
There are two essential factors in determining the separation performances of foam fractionation: interfacial adsorption and foam drainage [9], [10]. Particularly for the latter, it is a significantly attractive issue in physics and chemical engineering, and abundant references have reported its mechanisms and methodologies [11], [12], [13], [14], [15]. Foam drainage, primarily due to gravity, reduces the net liquid flux for a given bubble surface area flux, and this is what contributes to the enrichment. The enhancement of foam drainage is generally recognized in foam fractionation as an effective method to improve enrichment ratio.
In a column of rising foam, the interstitial liquid has to flow down through the intricate network formed by numerous Plateau borders before it returns the bulk liquid. The network results in high resistance for the liquid reflux. The foam will have a high liquid holdup when the falling liquid flux is in equilibrium with the rising one. Then how to allow the interstitial liquid to drain quickly out of the foam becomes the essence of enhancing foam drainage. It is generally considered that the effective enhancement of foam drainage is attributed to the development of novel foam fractionation equipment [8], [16], [17], [18]. Dickinson et al. [17] designed a novel column with parallel inclined channels which were tilted 20° from the vertical direction. The interstitial liquid drained quickly onto the channel walls and formed a thin liquid film, as presented in Fig. 1, when the foam was rising along the inclined channels. The drained liquid quickly returned the bulk liquid phase along the thin film, and thus foam drainage was enhanced. Their investigations indicated that the enrichment ratio of CTAB increased by 4 times using the novel column compared to that using a conventional column. However, only one angle of the inclined channels was investigated in their paper. Jiang et al. [8] found that the tilt angle had a significant effect on the separation performances in the protein recovery from the whey wastewater. However, the hydrodynamics of the rising foam in the inclined channel and the effect of the tilt angle on foam drainage were not elucidated in theoretically.
At present, most foam fractionation columns are cylindrical in the lab-scale experiments. If the cylindrical columns are scaled up for industrial production, the cost for them will be not low when the separation capacity reaches several hundreds or even thousands of tons per day. However, the cost will substantially decrease when the cylindrical columns are replaced by the cubic ones, because the cubic columns are readily obtainable by dividing a large cubic tank. So a column with square cross section will be adopted in the current work.
In this paper, the enhancement of foam drainage using the inclined foam channel (IFC) will be investigated for effective recovery of the protein from the whey wastewater. The effect of the tilt angle on foam drainage will be experimentally examined, and theoretical explanations will be specifically provided. Subsequently, the foam fractionation performances obtained with the IFC at the optimal tilt angle and the conventional column will be compared under various conditions. It is aimed at achieving more comprehensive knowledge of the role of the IFC in improving the enrichment ratio.
Section snippets
Materials
The whey wastewater was provided by Yu Xin Soy Protein Industry Co. Ltd., Shandong, China, and its physicochemical properties are described in Table 1. In the present work, all the solutions used were obtained by diluting the wastewater. Ethanol 95% (Tianjin Fengchuan Chemical Reagent Factory, China), phosphoric acid 85% (Tianjin Beifang Fine Chemical Co. Ltd., China), coomassie blue G-250 (Beijing Dingguo Biotechnology Co. Ltd., China) and bovine serum albumin (BSA) (Tianjin Lianxing
Effect of the tilt angle on the superficial drainage velocity of the static foam
Foam drainage in the static foam was firstly investigated in the section for elucidating the effect of the tilt angle. The results are presented in Fig. 3. The superficial drainage velocity was proportional to εn, where n is a dimensionless number [15]. At the beginning, the static foam had a high liquid holdup, so the drainage velocity was high and in turn this resulted in large decrease in the liquid holdup. The drainage velocity substantially decreased with decreasing the liquid holdup to a
Conclusion
The enhancement of foam drainage using the IFC of different tilt angles was investigated experimentally and theoretically. The interstitial liquid quickly released to the IFC wall and then returned the liquid phase along it. That was the reason why the IFC enhanced foam drainage. The theoretical analysis presents that the velocity for the liquid reflux increased with increasing the tilt angle from 0°to 45°and then decreased. At the tilt angle more than 60°, the drained liquid hardly returned
Acknowledgment
This work was financially supported by the Natural Science Foundation of Hebei, China (No. B2011202056).
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