Process Technology: An Introduction - Haan A.B. 2015
11 Particle removal from gases
11.7 Dry-impingement separators
Baffle bundles, also known as impingement separators, are widely employed in industry for removal of droplets and particles from gas flows. In plate separators, rows of baffles can be arranged in series to increase the overall efficiency of impingement. In this manner, the rather modest collection efficiencies possible for a single row can result in industrially practical collection efficiencies for a multiple-row unit. A series of parallel impact plates is commonly referred to as a wave-plate separator. Multiple body contacting is also used in fibrous filters, which are especially important in modern particle collection technology when highly efficient collection in the finest particle size range is required. Of all collectors, these have the widest spectrum of application and thus a large share of the market. With regard to application, design, and operation, filtration devices for particle collection can be classified as deep-bed filters or surface filters.
11.7.1 Deep-bed filtration
Deep-bed filters consist of a relatively loose fiber mat with a pore fraction often > 99 %. Here the term filter denotes a packing consisting of wires or fibers which obstructs the cross section of a gas flow. The filter can be arranged either horizontally or vertically and can consist of uniformly or randomly arranged wires or fibers. Particle collection takes place in the interior of the layer where the dust accumulates. As the gas flows through the fibrous layer, it must flow past a large number of cylindrical elements. Hereby the particles in the gas continue their path, until they strike a wire or fiber element by inertial or flow-line interception. Brownian diffusion contributes mainly for submicrometer particles. An important concept for the collecting efficiency of a filter is its overall on-flow area. This follows from the total length of all the fibers from which the filter is made up. Based on the filter cross-sectional area A, the overall relative on-flow area is given by
(11.16)
where L is the total wire length of the filter, D is the wire diameter, G the mass/filter volume, H the thickness of the packing, and ρF the density of the wire material. The fact that the fibers are not at right angles to the direction of flow is accounted for through the correction factor f. With this relative on-flow area concept it is possible to transfer the collecting efficiency of a single wire to the wire filter. For a single layer of wire the collection efficiency with relative on-flow area p1 and single wire collection efficiency ηF(0) becomes
(11.17)
Extension to n consecutive layers provides the collection efficiency of a filter as the product of the single layer probabilities:
(11.18)
with
(11.19)
With eq. (11.18) and a known value of the relative on-flow area p of a wire or a fiber filter, its collection efficiency can be calculated from that of a single cylinder. The number of layers in the filter is not explicitly required, and the indicator n of the collection efficiency can henceforth be omitted.
For the collection of liquid droplets, fogs, and mists, in-depth fiber bed filters made out of horizontal pads of knitted metal wire are used (Fig. 11.16). Collection from the up-flowing gas is mainly by inertial interception. Thus efficiency will be low at low superficial velocities and for fine particles. For collection of fine mist particles the use of randomly oriented fiber beds is preferred. Fine particle removal by filtration through a bed or granular solids has appeal because of corrosion and temperature resistance. Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates. Important parameters for the collection efficiency in granular beds are bed thickness, gravel size, and air velocity.
Fig. 11.16: Schematic of a wire-mesh separator.
11.7.2 Surface filters
Surface filters are used widely because of their excellent collection performance, even for the finest particles. With pore-volume fractions in the range of 70-90 %, separation of particles from the carrying gas is not a sieving or simple filtration process, since the filter fabric pore size is much larger than the particles collected. Until a small dust layer is formed on the filter surface, the collecting efficiency is quite low. The dust bed (filter cake) is the true filter medium and is highly efficient. Because the drop in pressure increases as the cake grows, these filters must be cleaned periodically.
The three most important basic surface-filter designs are round bag, envelope, and cartridge. Envelope filters are not as widely used as round bag filters because of their low dust-handling capacity. They must be discarded or cleaned when the pressure drop becomes too large. The cloth or bag dust filter is the oldest and often the most reliable of the many methods for removing dusts from an air stream. Among their advantages are high (often 99+ %) collection efficiency, moderate pressure drop, and the recovery of the dust in a dry reusable form. The maintenance for bag replacement, their large size, and their incompatibility with liquid particle-laden gases are the main disadvantages. Bag filters may be woven or felted, an envelope supported with an internal wire cage. The classical design is a multichamber filter with shaker cleaning. Dirty gas is fed into the bags from below, flows through the filter medium, and is directed to the clean gas duct at the top of the device. Particle collection takes place on the outside of the bags. For cleaning the gas supply is cut off, and the dust is removed by shaking or rapping the bag support. Newer systems use reverse flow cleaning (Fig. 11.17), where a compressed air pulse is given to dislodges the dust from the outside of the bag. No bag fabric can withstand truly high temperatures.
Fig. 11.17: Multicompartment bag filter.
In cartridge filters the filter medium is folded to pack more filtration area into a unit volume. They are frequently employed for the purification of gases from particles. Filter candles consist of ceramic or glass sintered elements and are resistant to heat and chemicals. Fig. 11.18 shows the construction of such a separator. The gas to be cleaned flows through the porous stone material from the exterior to the interior. The particles are collected on the outside surface and need to be removed frequently.
11.7.3 Lamellar plate separators
Lamellar plate separators are often used because of their simple construction and low pressure drop. These devices consist of parallel channels with multiple deflections and retaining grooves (Fig. 11.19). The droplet-laden gas stream undergoes many changes of direction in the channels. When the gas is diverted, entrained particles that are sufficiently large will continue their motion until they strike the collecting surface. On the other hand, very small particles behave almost as gas molecules and follow the flow lines of the gas. Particles of intermediate size can only partially follow the curvatures of the gas flow. Depending on which flow line they are starting from, these droplets will hit or miss the impact body. Hence only a partial separation is possible for these particles. It follows that for impingement separators, the collection efficiency varies from zero for sufficiently small particles to 1 (=100 % collection) for large particles. The plates can have various profiles. Like all inertial collectors, lamellar separators become more efficient as flow velocity increases. On account of their simple and compact construction, wave plate separators are relatively cost effective and space saving.
Fig. 11.18: Filter candle separator.
Fig. 11.19: Operating principle of a wave-plate separator.