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.