Understanding The Membrane Filtration Spectrum

Written by Critical Process Filtration | Nov 25, 2024 8:47:33 PM

Membrane Filtration

In the membrane filtration world, there are many filters and a wide array of filter configurations. These different filters have unique properties and specific applications. Many systems use multiple filter types, working harmoniously to produce a desired outcome. This article will summarize the different filter types and configurations, briefly describing how they work, where they may be applied, and how they may work together.

Membrane filters are generally separated into four categories - Microfiltration, Ultrafiltration, Nanofiltration, and Reverse Osmosis.

Microfiltration and Ultrafiltration membranes work by size exclusion. Fluid passes through the membrane, with contaminants larger than the membrane pores prevented from passing. In some cases, retention can be enhanced by surface modifications that will absorb certain particles onto the membrane surface, even if they are smaller than the pores.

Reverse Osmosis membranes are semi-permeable membranes that can separate dissolved solids (such as salts or metal ions) from water. Under static conditions, pure water will flow across the membrane to dilute the contaminated side. The driving force for this is called osmotic pressure. By applying pressure on the contaminated side, this process is reversed, and purified water will flow through the membrane, leaving the contaminants behind.

Filter configurations include Flat Sheets, Pleated Membrane Cartridges, Spiral Wound Cartridges, and Hollow Fibers.

Filter Configurations

Flat Sheet

Flat sheet membranes can be cut into any number of configurations - discs and rectangles are common. They also generally require some type of filter holder, although they are often sealed into a self-contained, disposable holder. They are used in laboratory and small-volume applications

Pleated Membrane Cartridges

Pleated membrane cartridges have a membrane that is protected by porous support layers so they can be pleated and sealed in a cartridge - similar to the air filter in your automobile. These can be stacked into longer devices and finished with a variety of end fittings to adapt to industry-standard housings.

They can also be sealed in a self-contained capsule that does not require a separate housing.

Flat sheet and pleated membrane cartridges are typically used in microporous filtration applications, but also find some use with nanofiltration membranes (virus retention, etc.). These devices are used in a Normal Flow manner, where all feed fluid flows through the membrane and contaminants are collected on the surface or trapped in the body of the membrane by absorption.

Spiral Wound Cartridges

Spiral Wound membrane cartridges are made with alternating layers of feed spacer, membrane, and permeate carrier, rolled up around a central core. The cartridge is constructed so the feed spacer is open on both ends, and the permeate carrier is sealed at the ends and opens to the central tube which carries the permeate out of the cartridge. The rolled-cartridge is then overwrapped with tape to maintain its shape. For high-pressure applications, the cartridge can be further wrapped with resin-reinforced fiberglass.

In operation, the feed fluid enters from one end of the spiral and flows across the membrane through the feed spacer. The filtered fluid passes into the permeate spacer and flows into the central collection tube. Concentrated feed flows out the other end of the spiral to carry away contaminants that might foul the membrane. The central collection tube is isolated from the feed spacer to dispense the purified fluid.

This type of filtration is called Tangential Flow and is used primarily in ultrafiltration, nanofiltration, and reverse osmosis applications. There are some specialty applications for high-fouling fluids where it can also be used for microfiltration membranes.

Hollow Fibers

Hollow Fiber modules are just what they sound like. A bundle of hollow fiber membranes is potted into a shell, and cut to expose the ends. The shell may be fitted with openings to allow fluid in or out, depending on the intended flow path. There also may be a central tube running through the shell. Hollow fiber modules are mostly ultrafiltration and nanofiltration but can be used for reverse osmosis or microfiltration as well. Flow through the fibers can be either outside-to-inside or inside-to-outside depending on the application.

Inside-to-outside is usually run in a Tangential Flow mode, where the feed flows through the fiber, with filtered permeate collected from the shell outside the fibers, and the concentrate flows out the other end. For these types of applications, there could be a central tube inside the shell to collect the permeate, or it may be collected through openings in the shell.

Outside-to-Inside is usually a Normal Flow mode, where the feed enters the shell, either through side fittings or a central tube. Filtered fluid flows into the fiber, and is collected from one or both ends.

Our Placement on the Filter Spectrum

Critical Process Filtration produces a diverse line of microporous filtration products, using a wide range of membrane filter materials (PES, Nylon, PVDF, PTFE) in flat sheet, cartridge, and capsule configurations. As stand-alone filters, they find use in an equally wide range of applications - pharmaceutical sterilization, food and beverage clarification, electronics particle removal, industrial filtration, and more.

In large-scale filtration systems (centralized water purification, municipal water treatment, industrial boiler feed, etc.), they find use as prefilters to prevent fouling of reverse osmosis and ultrafiltration membranes, prolonging filter life and increasing the time required between system cleanings.

If you have an application that may benefit from one of our microporous filter products, contact customerservice@criticalprocess.com so we can help you determine the right filter for your application.

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