Since the 1959 development by Sourirajan of the first viable RO membrane, crossflow membrane technology has been applied to an ever increasing range of liquid separation processes and markets. In food and beverage applications, crossflow membrane technology is used to purify the liquids we drink, concentrate and reconstitute juices, dewater and purify dairy products, and to help keep our water supplies potable by treating wastes at all levels of production.
Over the years, the empirical study of the crossflow filtration process has led to several interrelated operational principles and engineering 'rules of thumb.' This paper discusses a few of these, supported by examples from actual food and beverage applications.
Crossflow Compared to Normal Flow Filtration
Most people think of filtration in the 'normal flow' mode; where the entire fluid volume passes through the filter media, producing two streams: a feed (influent) and filtration (effluent). In the crossflow process, a feed stream flows across a membrane, with only a portion passing through the membrane pores to produce a 'permeate.' As the solvent or carrier liquid passes through the membrane, dissolved, colloidal and suspended solids retained by the membrane are concentrated, producing a 'concentrate' stream. Thus, a single feed with two effluent streams defines crossflow filtration.
As with normal flow filtration, two performance characteristics are used to measure the crossflow process; permeate flux (work rate) and separation (rejection of solids).
The flux is determined by several factors, including the feed composition, temperature and viscosity, driving pressure, and inherent membrane structure and its overall condition when in operation.
Factors which affect the separation achieved include pore volume and size distribution, solids concentration at the membrane/feed stream interface, chemical interaction between the membrane material and feed stream, and the net driving pressure.
Determining Factors for Crossflow Membrane Performance
Many factors influence flux and separation, so the engineer must understand how those factors affect one another to optimize the design and operational parameters of the overall process. In crossflow membrane processes, the degree of solids concentration affects both the separation and flux of the system. The fluid dynamics of the system in turn, affect the degree of concentration possible. Thus, achieving the correct balance of fluid dynamics and fluid condition in the system ultimately determines the degree of success for an application.