Flux enhancement by using helical baffles in ultrafiltration of suspended solids

The main reason for the flux decline during the initial period of all filtration processes is the usual phenomena of concentration polarization and fouling. After this stage follows the cake filtration process that allows the obtaining of the steady state flux. The solute accumulated on the membrane surface forms a high concentration gel layer, which increases the effective membrane thickness and so reduces its hydraulic permeability. Different techniques are used to reduce this formation and use of helical baffles inside the membrane element is one of such techniques. The selection of appropriate helical baffle is vital to get improved permeation flux with minimum pressure drop for cross-flow feed. The number of helices/unit length has a considerable influence on the selected helical baffle. All experiments have been conducted with an inorganic tubular ultrafiltration membrane for filtering a supernatant from activated sludge plant consisting of suspended and biological solids. The influence of the operational parameters is studied in this paper. Nevertheless, the feed temperature and the concentration were kept constant at the industrial values. We found 1 bar as an optimal pressure, above this pressure the permeation flux decreases, contrarily to several works, which observe a plateau after certain value of pressure. Progressive fouling can be limited by use of helical baffles in the filtration element operated at low pressures and the flocculation of particles is reduced. On the other hand, we have found that the influence of Reynolds number inside the membrane tube and the feed flow-rate are similar to other studies that used different helical baffles.

Fig. 10. Variation of ln Js with ln Uc, 1 bar. flow velocity with the influence of helical baffles leads to an increase of turbulence in the membrane tube and mass transfer coefficient, reduces the effect of concentration polarization and increases the permeation flux. Several authors, using different helical baffles, found similar results. Sebbane found slope values, close to that of the Harriot and Hamilton ones [20], in the order of 0.9 with the helical baffle; in addition they were less sensitive to the evolution of the fluid vein thickness. On the other hand, without helical baffle they become >1. Quemeneur and Schlumpf [19], Goldsmith [11], Cadanel and Bartoldi [5] also found, with different devices, slope values of the same order (a = 1, 1 – 1.5). Poyen et al. [18] found, with or without helical baffle, in ultrafiltration of an engine oil additive, slope values = 0.6, independent of membranes used and the hydraulic diameter. Gekas and Hallstrom [10], Aimar et al. [1] explained the difference in slope values since the diffusion coefficient, the viscosity and the density (D, µ and ρ) depend on the gel concentration and not on the bulk concentration. Indeed, it is necessary to note as Jaffrin et al. [14] found, that the D and µ affect the friction at the membrane wall or the velocity. The oily emulsion could have, as milk, a pseudo-plastic behavior. In addition, the influence of the shear should be as much stronger than the layer is thick.