Filtration cross-flow/tangential

Cross-flow (tangential) filtration is finding increased interest and application in winemaking and juice/concentrate production. The process uses membranes of various porosities, depending on application. 

Cross-flow filtration (CFF) also known as tangential flow filtration is not of recent origin. It began with the development of reverse osmosis (RO) more than three decades ago. Industrial RO processes include desalting of sea water and brackish water, and recovery and purification of some fermentation products. The cross-flow membrane filtration technique was next applied to the concentration and fractionation of macromolecules commonly recognized as ultrafiltration (UF) in the late 1960 s. Major UF applications include electrocoat paint recovery, enzyme and protein recovery and pyrogen removal. 

Isolation of low- and high-molecular weight DON, colloids, and particulate present in the natural waters can be obtained by tangential cross-flow (ultra)filtration, dialysis, and field-flow fractionation techniques [31,72-78], These organic matter fractions separated in freshwater or seawater samples can be analyzed by the same techniques used for DON. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

All of the membrane processes utilize an engineering design known as "cross-flow" or "tangential flow" filtration. in this mechanism, the bulk solution flows over and parallel to the membrane surface, and because the system is pressurized, water is forced through the membrane. The turbulent flow of the bulk solution across the surface minimizes the accumulati.on of particulate matter on the membrane and facilitates the continuous operation of the system. 

In cross-flow filtration, feed water passes tangentially over the membrane surface rather than perpendicularly to it. Water and some dissolved solids pass through the membrane while the majority of dissolved solids and some water do not pass through the membrane. Hence, cross-flow filtration has one influent stream but yields two effluent streams. This is shown is Figure 2.5. 

Cross-flow helps to minimize fouling or scaling of the RO membrane. In an effort to keep the membrane surface free of solids that may accumulate and foul or scale the membrane, tangential flow across the membrane surface aids in keeping the surface clean by scouring the surface minimum flow rates across the membrane surface are required to effectively scour the surface. See Chapter 9.5 for more details about cross-flow filtration and RO system flow rates. 

The difference between conventional dead-end filtration and cross-flow filtration is the configuration of the system. For large-scale operations, only cross-flow filtration will be used. The membranes for miocrofiltration as well as ultrafiltration are commonly utilized in a variety of filtration devices. There are three basic types of tangential flow filtration devices plate and frame, hollow fiber, and spiral wound membranes. 

Cross-flow filtration is also referred to as tangential flow filtration or microfiltration, but all three terms refer to a process by which membranes are used to separate components in a liquid solution (or suspension) on the basis of their size. The development of robust membranes in polymeric and ceramic materials has provided a powerful new technology for bioseparations, which is already widespread in the process industries as well as for water treatment processes. 

Filtration can be used to separate cells or cell debris, concentrate cells or protein solution, and remove or exchange salts. Two broad categories of filtration are conventional dead-end filtration, where fluid flow is normal to the plane of the membrane, and cross-flow filtration (or tangential flow filtration), where the fluid flows parallel to the membrane surface. 

In cross-flow flltration, the wastewater flows under pressure at a fairly high velocity tangentially or across the filter medium. A thin layer of solids form on the surface of the medium, but the high liquid velocity keeps the layer from building up. At the same time, the liquid permeates the membrane producing a clear filtrate. Filter media may be ceramic, metal (e.g., sintered stainless steel or porous alumina), or a polymer membrane (cellulose acetate, polyamide, and polyacrylonitrile) with pores small enough to exclude most suspended particles. Examples of cross filtration are microfiltration with pore sizes ranging from 0.1 to 5 pm and ultrafiltration with pore sizes from 1 pm down to about 0,001 pm. 

Let us examine the TFF technique and a typical process regime. Tangential Flow Filtration is the general term used to describe filtration where cross flow parallel to the filter surface is used to enhance filtration rate. This is in contrast to dead ended filtration where the fluid path is solely through the filter. If the membrane used to make the separation is microporous (0.2-0.1+5 urn pore size), the technique is more specifically called mdcroporous tangential flow filtration. Ultrafiltration, a subset of Tangential Flow Filtration, employs a finer, anisotropic membrane able to retain macromolecules, albumin for instance. We will emphasize the use of ultrafiltration membranes in this discussion. 

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