Beer Filtration
Cross-Flow Filtration
The filters described to this point are normal-flow (or dead-end) filtration. In these cases, the bulk flow of fluid was normal or perpendicular to the filter itself, leading to the buildup of a filter cake on the filter and an increase in resistance over time. Eventually, in these cases, the cake will provide enough resistance, that flow will stop or pressure drops across the filter will rise past the rupture point for the filter. Cross-flow filters (i.e., sometimes called tangential flow filters in the beer industry) operate in a fundamentally different manner (Figure 15.6).
Benefits in Cross-Flow Filtration
Cross-flow filtration have benefits over traditional filtration. Cross-flow filtration minimizes solids build-up on the membrane surface, delivers a higher permeation rate, and maintains stable throughput rates thus reducing need for filter replacement. Moreover, the quality of the filtration is constant over time because the fouling is reduced. Cross-flow filtration is a soft process because the filtering is made without any change of state of the filtered element, and never gets distorted. It also simplifies the steps in filtration and is environmentally-friendly.
Membrane Types
Today, only two types of membranes are used in cross-flow filtration of beer: ceramic and, the most popular of all, polymeric.
Ceramic
Ceramic membranes are extremely strong and are made of an aluminum/titanium) and as such, they are considerably more expensive. Though resistant to high temperatures and cleaning solutions, they are often accused of being brittle and not holding up well to temperature shock. Unlike large, hollow fiber membranes in singular modules, multiple ceramic rods are contained inside one large housing.
Polymeric
Polymeric filters are typically cheaper and swappable, they are lightweight, and they can be changed out in the filter housing for different applications, or when a filter becomes worn out and needs to be replaced.
Membrane Geometries
There are three common geometries for cross-flow filtration membranes. First, flat membrane sheets can be used, usually stacked parallel to each other. This is sometimes called a flat plate arrangement. This geometry is least susceptible to clogging, but it is the least efficient means of packing filter area into a given volume. The second geometry is a spiral wound geometry, uses two flat rectangular sheets, fused along the long sides and then wound in a spiral and placed in a long cylindrical housing.
Choosing a Cross-Flow Filtration System
The operation of a cross-flow system not only depends on the technology itself, but also the auxiliary systems (e.g., filtration tanks, CIPs) that have to be integrated into the process of filtration. Installation of a filtrate tank and transfer pump is advantageous as it allows the filtered beer to be pumped to a destination tank some distance from the filter, or pumped against a high head of a high-capacity tank, without slowing the system.t.
Sizing the Unit
One of the most basic requirements is to process a given volume of beer within a given number of hours. Means of potential systems are by membrane area and/or by output.
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