In Part 1 of this file, I noted that it is necessary by desire and, in many instances, by law to separate and recover final product, dust and other potential solid emission products from the exhaust airstream. Various technologies are available to perform this separation, each having its own intended application, benefits and limitations. In this column, I’ll look at the most common systems available to separate solid particulate from main gas or exhaust streams.
Gravity and Centrifugal Separators. Some mechanical systems such as drop- out boxes (settling chambers) or impingement separators rely on reducing the gas stream velocity and promoting gravity separation. In cyclones, multiclones and certain other devices, centrifugal forces are used to increase the efficiency of these separators.
These systems have excellent high temperature performance but achieve relatively poor separation efficiencies (95 percent is the rule of thumb) with finer products. They would be used therefore as a primary separator and require secondary and perhaps tertiary separation for many applications. The systems can be wet or dry, but most commonly they are dry collectors. They are inexpensive and can be mounted almost anywhere.
Fabric Collectors. The second set of separators is filters, or fabric collectors. Numerous fabrics are available, each of which demonstrates different strengths and weaknesses. For many years, the resistance to temperature and length of bag life were limiting factors in using fabric filtration in drying applications. The availability of robust high temperature fabrics is now growing. As these products become more available and affordable, they are being used more frequently in drying applications.
Currently, media are available that can operate up to 1,000oF (538oC). Obviously, as the temperature limit increases, so does the cost. Carefully selecting the media can reduce the investment significantly. Note that moisture and other gas contaminants such as acids and other corrosives are a significant concern in the efficient operation of fabric dust collectors. If the operation is approaching the dewpoint of the gas stream or the contaminant, one must pay close attention to system design and fabric selection.
Other solid media such as ceramics, sintered metals and other proprietary materials can operate at temperatures up to 2,192oF (1,200oC). These are slowly finding their way into aggressive industries, being retarded by their expense rather than their benefits.
Fabric filtration systems operate at very high efficiencies, obtaining performance above 99.99 percent in many applications. Fabric collectors are moderately expensive, the major cost being for the fabric. The more benefit the fabric offers, the higher the cost. They can take up considerable floor areas but can be mounted strategically.
Scrubbers. Wet systems frequently are referred to as scrubbers; adding to the confusion is the fact that this term also is used to describe the removal of chemical compounds such as acids from gas streams. Scrubber technologies include venturi, vane and packed-bed systems. Scrubbers can operate at high temperatures and with exceptionally good separation efficiencies. For obvious process reasons, scrubbers are not suited to high solids loadings.
The most significant operational restriction in using scrubbers is the state of the recovered material -- the effluent. In applications where the feed is slurry, this is of little consequence. In others, it can be an operational nightmare.
Scrubbing systems are relatively inexpensive but demonstrate higher operational costs. They can be installed in a relatively small floor area but require good drainage and retainability.
Electrostatic Precipitators (ESPs). Capable of providing high separation efficiency, electrostatic precipitators provide a charge on the entrained particles (ionization), which then are attracted to elements within the collection device. ESPs can operate at elevated temperatures and, before the advent of high temperature filtration fabrics, were the preferred technology in these applications.
ESPs are not usually installed as a primary separation device, and they are not ideally suited to high solids loadings. They are relatively expensive and require a comparatively large floor area for installation.
Final ThoughtsRecovery and collection systems for dryers are numerous. Some installations do not require any sort of separation system while others rely entirely on these systems to recover the dried product. Many designs will require the installation of several separation devices in series. Others will use a single recovery system or two in parallel.
Ultimately, the selection of the separation system will depend on the suitability of the equipment to the operating condition. Solids loading, particle size distribution, temperature, moisture content, compounds, the desired final form of the recovered product and the required level of emission all should be considered when selecting a separation system.