Improvement of microalgae filtration efficiency by adopting turbulent jet

Abstract

A turbulent-jet implemented hollow fiber membrane module was developed to improve the efficiency of filtration applied for harvesting/concentrating processes in a microalgae-based biorefinery. By manipulating the feed flow in the form of a turbulent jet, both the fouling on the membrane surface and the specific energy requirement were reduced significantly, compared to the conventional cross-flow filtration system using a typical hollow fiber membrane module. The highest shear stress was generated at the impinging point formed by the jet collision on the membrane surface in a radial direction, which was higher than the value obtained by maximizing the total feed flowrate and mean crossflow velocity in an axial direction. A wall jet created after the impingement drastically reduced the boundary layer, and this phenomenon contributed to the fouling reduction by increasing the local fluid velocity near the membrane surface. When comparing the contribution of the impinging jet and the wall jet in terms of fouling reduction, the wall jet was shown to have a greater impact. For an in-depth understanding, the jet-scouring phenomenon was modeled. Simple force balance on a sphere was coupled with the velocity profile of the wall jet, and the net angular moment was calculated to determine whether the sphere escapes from the pore or spins on the pore. The fouling removal area estimated from the model and that obtained through the experiment showed high consistency with an error of 1-5%, confirming that the model was precisely established. The removal area was proportional to the initial jet velocity, and the shape of the area became elliptical with a larger eccentricity as the impinging angle became smaller. The correlation between parameters involved in the filtration and the removal area was summarized in the form of an empirical formula, revealing the degree of contribution of each parameter to the fouling reduction phenomenon. By using the formula, the geometry of the perforated cylinder, a key factor for generating the turbulent jets in the hollow fiber membrane module, was designed. The newly designed module was applied to an actual harvesting process to compare the differences in permeate flux and biomass concentration with the conventional system. The mean permeate flux increased roughly three-fold over the entire concentration range, requiring only 35% of the specific energy. The final biomass concentration was 2.4 times higher, reaching 80 g/L. This is far above the minimum concentration (50 g/L) required for downstream hydrothermal liquefaction or hydrothermal pretreatment, which could not be achieved when using the conventional modules. The developed system enables omission of the secondary harvesting process (centrifugation, for example), which requires the highest operational and installation cost in the entire process, and therefore may reduce the capital expenditures as well as the operating expenses. This also can improve the overall economic feasibility of microalgae-based biorefinery.