Pressure effects in dead-end filtration of skimmed milk: Analysis of cake properties.
Advancing the chemical engineering fundamentals
Filtration - I (T2-11a)
Keywords: milk, membrane, fouling, cake, compressibility
Ultra- and microfiltration are now widely used in the dairy industry as tools for the separation and the fractionation of specific solutes and proteins in milk [1]. For instance, the microfiltration of skimmed milk concentrates the "naturally aggregated" proteins of milk (the casein micelles) and eliminates a great part of the whey (lactose, minerals and small molecular weight proteins, i.e. mainly -lactoglobulin and -lactalbumin) in a single and fast operation. Both concentrate and permeate are of great interest for the dairy industry and are then used in different ways.
The performances, in terms of permeability and selectivity, of skimmed milk ultra- and microfiltration are mainly ruled by the formation of a cake layer at the membrane surface during the course of filtration. Understanding the properties of this cake layer and the mechanisms involved in its formation is of major importance for developing strategies to improve membrane-based processes in the dairy industry. Casein micelles are the main components of the fouling layer developing over the membrane. They are complex colloidal entities with a mean diameter of roughly 150 nm and are in dynamic equilibrium with the solvent phase of milk [2]. Recent studies, based on the variation of the cake resistance with milk physicochemical parameters, clearly show the direct relation between the casein micelles properties (charge, size...) and the degree of fouling (see [3] for instance). However, only a few works dealt with the effects of other important filtration parameters (temperature, pressure, time ...) on the cake properties, and consequently on the performances of the operation. More particularly, the impact of transmembrane pressure on the cake properties, as it is commonly done in colloids filtration, has surprisingly never been exhaustively analyzed until now [4].
The effect of transmembrane pressure on a variety of cake properties is deeply investigated in the present work. Filtrations were performed in dead-end mode with ultrafiltration membranes (PES Membrane of 100 kg.mol-1 MWCO, Novasep, France) at 50°C, i.e. the temperature commonly used in the dairy industry. The cakes formed upon filtrations performed at pressures from 1 to 4 bar were classically analyzed in terms of average specific resistance by following the permeate volume with time. The actual deposited mass, the cake dry matter content, as well as the absolute remaining cake resistance were also determined after filtration and rinsing of the cell.
In a general manner, the average specific resistances reported in this study are unexpectedly high compared to resistances calculated from classical models such as the familiar correlation from Karman-Cozeny. Moreover, a clear increase in these resistances with transmembrane pressure was also observed, which indicates a compressible cake layer. Additionally, variations of the average specific resistance with time during a same filtration run were also observed (particle rearrangement). All these observations are discussed based on the known characteristics of the casein micelle (polydispersity, micelle "softness", etc...) and the last refinements in the theory of filtration of soft colloids [5]. In addition, we show that changes in dry matter content and absolute cake resistance after filtration are also correlated with pressure and help to understand the involved mechanisms.
References:
[1] G. Brans, C.G.P.H. Schroen, R.G.M. van der Sman, R.M. Boom, Membrane fractionation of milk: state of the art and challenges, Journal of Membrane Science 243 (2004) 263-272.
[2] D.S. Horne, Casein micelle structure: Models and muddles, Current Opinion in Colloid & Interface Science 11 (2006) 148-153.
[3] M. Rabiller-Baudry, G. Gesan-Guiziou, D. Roldan-Calbo, S. Beaulieu, F. Michel, Limiting flux in skimmed milk ultrafiltration: impact of electrostatic repulsion due to casein micelles, Desalination 175 (2005) 49-59.
[4] J. Kromkamp, S. Rijnsent, R. Huttenhuis, K. Schroen, R. Boom, Differential analysis of deposition layers from micellar casein and milk fat globule suspensions onto ultrafiltration and microfiltration membranes, Journal of Food Engineering 80 (2007) 257-266.
[5] K.J. Hwang, C.L. Hsueh, Dynamic analysis of cake properties in microfiltration of soft colloids, Journal of Membrane Science 214 (2003) 259-273.
Presented Monday 17, 11:52 to 12:11, in session Filtration - I (T2-11a).
