Vibrating polymeric microsieves: Antifouling strategies for microfiltration

Abstract

Constant flux performance in time is achieved with polyethersulfone (PES) polymeric microsieves when filtering protein solutions, skimmed milk and white beer in combination with backpulsing. Such microsieves are fabricated by phase separation micromolding (PSμM) and possess pores around 2 μm. The filtration of bovine serum albumin (BSA) solutions at neutral pH results in constant flux when backpulsing. The constant flux performance is related to the ability of polymeric microsieves to flex during permeate pressure pulsing. Their flexibility allows pressure pulse transmission to the feed and, therefore, almost no flow reversal occurs. The membrane motion affects the hydrodynamics in the feed channel and disturbs the polarization layer and the cake deposited. Reference experiments with stiff Si_xN_y-based microsieves, nuclepore and macroporous microfiltration membranes show different behavior: the permeate pressure pulse hardly translates into the feed channel. Backpulsing for these membranes is less effect as anti-fouling strategy. Backpulsing of polymeric microsieves also allows stable flux operation for other complex feeds like skimmed milk and white Belgian beer.

Introduction

Ceramic and polymeric membranes are nowadays used for the filtration of beer and dairy foods, replacing conventional Kieselguhr filtration or pasteurization. Crossflow microfiltration is currently used to remove bacteria from raw milk, separate casein from whey, and recover serum proteins from cheese whey, for instance [1], [2], [3], [4]. In the brewing industry, current industrial crossflow microfiltration applications concern the clarification of rough beer to eliminate yeast and colloids responsible for haze, and sterile filtration of clarified beer.

Milk is a very complex fluid, mainly constituted by water and with a solid content around 12.6%. For the recovery of the milk protein fraction, skimmed milk is normally used as feed. Skimmed milk contains <0.5% (w/v) fat, 3.3% (w/v) proteins, 4.6–5.0% (w/v) lactose and minerals like calcium, sodium and potassium [4]. Fouling in skimmed milk filtration is normally associated to the protein fraction, formed by casein micelles and whey proteins like β-lactoglobulins, α-lactalbumins, immunoglobulins, bovine serum albumin (BSA), and others. Precipitation of calcium phosphates can also result in flux reduction. Currently used strategies to reduce fouling in milk filtration involve the use of backpulsing, acoustic or ultrasonic waves, turbulence promoters, vibrating and rotating disk modules and air sparging. A recent review of Brans et al. [3] has covered the state-of-the-art of these techniques and their possible disadvantages in milk fractionation. The differences between such strategies are their principle of operation, efficiency and costs. Turbulence promoters, vibration and rotating disk modules increase the shear rate close to the membrane, while backpulsing removes the cake layer by reversing the flow through the membrane. Air sparging reduces concentration polarization by mixing. Industrial implementation of these methods is very dependant on their efficiency (this at the same time depends on the feed and process parameters) and feasibility in terms of costs, upscaling and energy consumption.

Beer is a multi component feed, containing polysaccharides, higher dextrins and glucans, proteins and alcohols. The key membrane foulants in beer are proteins, carbohydrates (β-glucan, pentosan, etc.) and starch molecules or yeast cells. Crossflow microfiltration intends to replace conventional dead-end filtration with diatomaceous earth, for example. The main components of beer to be removed during clarification are yeast cells and haze (polyphenols, proteins, carbohydrates and metal ions) [5]. To maximize the removal of chill haze, beer is normally filtered at 0 °C. This causes a flux limitation due to the increased solution viscosity. A number of mechanisms can be used to improve beer flux, including backpulsing or backflushing [6], [7], flow pulsation, and oscillatory flow on the feed [8]. Together with backpulsing, the latter two are based on disturbing concentration polarization and the cake layer.

Today, commercially relevant fluxes in beer filtration are between 10 and 100 kg h⁻¹ m⁻² [6]. Microsieve membranes could very well supply the requirements needed for this application, because of their high flux and selectivity [9]. Currently, silicon nitride (Si_xN_y) microsieves are being introduced in dairy and brewing industry as a breakthrough technology. In spite of their advantages, there is still a trade-off associated with the high fluxes that such membranes can deliver: fouling. We have reported in previous work that fouling by proteins, for instance, can be reduced by several strategies such as backpulsing or antifouling coatings [10], [11].

A new approach is being developed as an alternative to silicon nitride microsieves: polymeric microsieves. Microfiltration with novel polymeric microsieves represents a new technology with a viable potential for industrial implementation. These membranes exhibit similar selectivity/permeability characteristics to inorganic microsieves, but at much lower production costs. Due to the versatility of the fabrication process different pore sizes and shapes can be obtained [12], [13], [14], [15]. Microsieves with pore diameters ranging from 5 μm (or higher) to approx. 0.5 μm are currently available [15]. Polymeric microsieves can deliver large product volumes operating at very low pressures, together with selective separations due to their precise pore size and shape.

Since polymeric microsieves are currently an emerging technology and no information about their performance in real filtrations is available yet, the outcome of the investigations in this article will serve as guidance for future applications. The main goal of this work is the evaluation of the filtration performance and fouling behavior of PES microsieves, using model protein solutions like BSA and real complex beverages like skimmed milk and beer. Polymeric microsieves with pore diameters of 2 μm have been selected as membranes for the fouling tests. Strategies like air sparging or backpulsing, which were extensively studied in previous research with Si_xN_y microsieves [10], [11], have also been applied in order to enhance permeation. The backpulsing performance of polymeric microsieves in aqueous solutions will also be compared to other systems, like track-etched membranes and depth-filters.