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Filtration Concept: Conventional vs Membrane Filtration

Innovation of PTFE membrane technology in 1974 by WL Gore has revolutionized industrial filtration.Introduced in India in year 1993 & has been successfully applied to industrial filtration in over 200,000 applications (20% of which are in Minerals industry).

3 Major Benefit of using PTFE membrane technology

  1. Lower emission rate (below 10 mg/Nm3 is possible with seam sealed bags)

  2. Lower Filter Drag & Pressure Drop

  3. Higher Bag life

PTFE membrane will find its application in any process baghouse to improve operations and extend bag life. In many cases switching to membrane avoids retrofitting of the entire baghouse and desired air flow can be achieved.

Conventional Filtration Process

A conventional depth filtration process relies on the formation of an internal ('primary') dust cake within the cross-section of the filtration material to filter particulate from the gas stream. The formation of this cake is sometimes referred to as seasoning; or conditioning.Sometimes a puffing phenomenon of particulate emissions occurs during each cleaning cycle as the primary cake is disturbed and rendered less effective.

During filtration, an external ('secondary') dust cake accumulates on the outside filtration surface of the media. Ideally, it is this secondary cake that is removed during a cleaning cycle. The permeability recovered during each cleaning cycle (reflected in a decrease in the pressure differential across the media) is dependent on how much of the secondary cake is released. The more cleaning energy imparted to the media, the more secondary cake is released. Over a period of time, the internal, primary cake becomes denser, eventually decreasing the media permeability. This is reflected in a rising differential pressure that can not be recovered during normal cleaning cycles.

Membrane Filtration Process

In membrane surface filtration, a laminate consisting of an ePTFE membrane and a backing substrate is employed to remove particulate matter from the gas stream. PTFE membrane is microporous so doesn’t allow even the smoke particles to go through (particle size of smoke 0.01a 1.0 Pm). This membrane is laminated to the surface of a backing material. The backing material is typically manufactured from a traditional filtration fiber and is selected based on the thermal and chemical requirements of the gas stream. The ePTFE membrane is chemically inert to most compounds and can withstand continuous operating temperatures of 260 Deg is non-stick and moisture resistant, so particles don’t form heavy cake on the surface and slide off easily during a cleaning cycle.Since no internal dust cake is ever developed, the permeability recovers to almost original levels after each cleaning cycle with very little decrease in permeability over time. This provides steady state conditions and predictable baghouse Pressure Drop.

“The membrane can be thought of as a permanent, factory applied primary dust cake.”

Membrane (left) collects all course and fine particle on surface even the smoke particles to can not pass through it.

Dust can be cleaned with lesser force in membrane, resulting less frequent cleaning and higher bag life

One should remember PTFE membrane alone is not the the hallmark of good baghouse performance, please refer our blog on (Extend life of bag filter 10 Installation & Maintenance practices). To know about membrane quality parameter refer (Membrane Quality Parameters)

There are different ways to evaluate performance of the PTFE membrane one can ask for VDI test report ( Evaluation of remaining bag life and fabric tests(MIT, VDI, Mullen, tensile) or can calculate operating Filiter Drag (Filter Drag: Cost of Filtration) to know how they’re membrane bags is performing.

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