Crossflow membranes are widely used in the production of milk and whey products, wine, vinegar, gelatin and fruit juices and are utilized for concentration, clarification and fractionation – each of which requires specific membrane polymers and pore sizes.
Explore this issueJune/July 2005
Proper cleaning and disinfection of crossflow systems must be tailored to the particular process feed stream contaminants and to the geometry and material composition of the membrane. Different membrane materials have varying levels of tolerance to pH, temperature and chemical exposure. The cleaning regime should also account for variations in water quality.
Sanitation regulations require that food plant operators clean membrane systems on a frequent basis – typically once per day – a potentially expensive and time-consuming process that can harm the membranes and diminish their life. Establishing an efficient and cost-effective cleaning regime is a critical aspect for all food and dairy applications.
The leading suppliers of membrane cleaning chemicals provide broad ranges of products. These fall into six basic categories: Alkaline compounds, mineral acid blends, surface active agents, enzymes, disinfectants and preservative solutions. As many as four chemical clean-in-place (CIP) steps are normally required to adequately clean and sanitize a membrane plant. Using the proper formulation of ingredients and operating conditions, this process can be accomplished economically with repeatable results.
The first step in establishing a cleaning protocol is to understand the requirements and limitations of the membrane in use. The materials of construction (including backing, feed spacer, permeate support, glues and epoxies), configuration and pore size are all factors in developing suitable cleaning regimes.
The most common membranes in use in the food industry are constructed of polyethersulfone (PES) and polyvinylidene fluoride (PVDF). These are durable polymers that can withstand pH levels from 2 to 13 and temperatures to 75ºC. They are partially hydrophilic making them suitable for most ultrafiltration (UF) and microfiltration (MF) food applications.
Nanofiltration (NF) and reverse osmosis (RO) membranes have PES substrates, but they also have a secondary thin-film composite membrane comprised of a polyamide or similar material with specific chemical limitations that must be considered.
Crossflow membranes are manufactured in three configurations, tubular, hollow fiber and spiral wound. In addition, membrane products vary from open pore-size MF membranes to tighter NF and RO products. In all cases, the primary design criteria for cleaning is the provision for adequate crossflow velocity to sweep the membrane surface clean and provide sufficient fluid contact to all sections of the membrane (feed and permeate sides) plus associated piping.
Depending on the specific configuration and the given application, the cleaning flow velocities may be greater than or less than the corresponding processing conditions. The optimal cleaning regime consists of an adequate shear rate to effectively scour the membrane surface combined with the appropriate chemistry, such that the procedure does not significantly decrease membrane life.
For tubular and spiral wound modules, this is easily achieved at flow velocities somewhat lower than the design production conditions. An exception to this would be spiral wound elements processing very high viscosity streams where the cleaning flow required is higher than the process flow, e.g. the last stage of a whey protein isolate (WPI) system. In these cases, it is often expedient to design a split-stage cleaning arrangement that allows one-half of the stage to be cleaned at a time to accommodate pump and pipe sizing.
For hollow fiber systems that generally operate at lower velocities, cleaning flows are equal to or higher than the process parameters in order to provide sufficient cleaning turbulence. Unfortunately, ideal cleaning conditions are often unavailable. This is one more reason why optimizing the chemical effectiveness plays such an important role.