Process Plant Filtration - Why It Should Not Be Overlooked

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By Richard Farnish, The Wolfson Centre for Bulk Solids Handling Technology

In the overall perception of investment priorities in process plant, filtration systems often are the poor relatives to the value adding aspects of the plant operations.

Local dust extraction and ventilation is an obvious exception to this statement due to legislative requirements relating to Health and Safety in the workplace.

However, filtration units that serve to capture high concentrations of particles (such as on pneumatic conveying reception vessels) do have the immediate and direct benefit of such legislative requirements.

Thus in instances where ‘value engineering’ exercises are implemented for new build or plant uprating projects it can be the case that the filter systems are early victims.

Major problem securing CAPEX

A major problem for attempts at securing adequate CAPEX to support the installation of a well-designed and efficient filtration system is that, quite clearly, such systems are not ‘value adding’ and such a nominal payback period for expenditure cannot be easily arrived at – making appropriate budget allocation justifications very difficult.

The end result for many end users is that systems are installed which can have direct impacts on the plant energy consumption, process stability and in-plant dust emissions.

Function of a filter – basics

Returning to basics for a moment, the function of a filter is clearly to capture particles, hold a given volume of material (before a peak pressure drop is arrived at) and finally to release the captured material.

These are clearly obvious requirements, and yet in some instances one or all of these operational aspects can become compromised by changes in; the bulk solids being handled, transfer rate (increase), the quality of filter elements and/or cleaning system.

Taking these requirements individually, changes in the nature of the bulk solids with which the filter is expected to work are not unusual over the lifetime of many units. Changes in product lines or material suppliers are a common feature for many plants.

Such shifts in bulk solid types are usually represented by shifts (sometimes major) in the particle size distributions – the most serious manifestation of which is increased fines content.

A corresponding upgrading of the filter media specification sometimes does not follow – with the result that the filter loading burden increases dramatically when in service. Usually this increase in fines loading would be apparent as a reduced time to reach peak pressure drop across the filter.

Filter cleaning by shaker mechanism

In the event that the filter cleaning is by shaker mechanism (i.e. integral to a batch transfer process step), it may be the case that towards the end of the transfer the back pressure through the system will increase – which can manifest as line stalling (in very marginally configured pipelines) or over pressure of the filter housing (potentially over stressing sealing gaskets and creating fugitive dust). For shaker systems, the filter media will rely on surface capture of particles and release by agitation.

For reverse jet cleaning systems, the particle capture does not rely on bed development on the filter face, but instead a progressive capture mechanism occurs whereby particles are captured within the open fibre matrix of the media and progressively saturate the voids within, following which particle accumulation develops on the face of the filter.

Thus the pressure drop development characteristic is of a low initial progression during in-depth capture which then transitions to a higher rate of pressure drop development as particles begin to a bed on the face of the filter.

If the process is progressed without cleaning, ultimately the rate of pressure drop decreases due a lack of gas flow as the filter blocks (a situation that would not form part of the standard operating procedure for most types of filter).

Typical filter loading characteristics – showing the transition from in-depth to surface filtration

Clearly the rate at which the filter develops these pressure drop characteristics will be function of the operating conditions and bulk solid properties. Fine material will tend to lodge deeper into the fibre lay and as the filter progressively loses its functional surface area due to particle accumulation, so the lodgement force will increase in response to higher face velocities.

Reverse jet filters

Reverse jet filters are commonly (but not exclusively) applied to continuous processes, and as such are subject to continual cleaning cycles – with the control of the cycles being triggered by either a timer or measurement of pressure drop cross the filter housing.

Taking these two control methods into account for a scenario whereby the process or change in bulk solids has changed the function of the filter (from when it was first commissioned).

In the case of the use of a timer, the issue may now be that the filter is loading with particles more quickly and that peak pressure drop is arrived at before the timed pulse if actioned. This accelerated loading can lead to deeper embedment of particles and a progressive deterioration in the service life of the filter (towards irreversible blinding).

If the filter is cleaned on a peak pressure drop basis, an increase in loading rate into/onto the filter will result in a higher frequency of pulsing – and in this respect the system can be considered to have almost autonomous operation.

An increased frequency of cleaning may not be a major concern in the early stages of filter deterioration, but a concern can be that the frequency of pulse delivery may begin to approach or exceed the ability of the external air reservoir to recover full pressure between pulses (i.e. insufficient time).

The result of reducing pressure availability will be a weakening pulse (and air induction if a venturi nozzle is installed) –hence cleaning efficiency reduces and the rate of filter deterioration speeds up.

Life cycle of a filter

It should be noted that over the life cycle of a filter the pressure drop is initially dominated by the in-depth and surface particle capture, but over time it is the irretrievably lodged particles within the filter media that dominate pressure drop.

Process Plant Filtration System: Scanning electron microscope (SEM) image of a wet laid fibre filtration membrane — Scanning electron microscope (SEM) image of a wet laid fibre filtration membrane

If the filter becomes sufficiently blocked and pressure/air progression through filter limited, an over pressure of the ‘clean’ side of the filter can result that can overload gaskets and dust seals.

Such a break though of particles can be readily mobilised through the process hall with the finer particle content remaining airborne for substantial periods of time and hence transporting widely (even beyond the immediate process).

Although this article is clearly only dealing very lightly with engineering and physics of filter operation, it is hoped that the casual reader can gain an appreciation for piece of equipment that is often largely ‘out of sight’ and often ‘out of mind’ – until problems start of course!