An inherent result of thermal drying is a relatively hot, humid exhaust gas stream. Depending on the type of dryer employed, this stream may contain final product, dust or myriad other potential solid emissions products. It is necessary by desire and, in many instances, by law to separate and recover these products from the exhaust airstream. Various technologies are available to perform this separation, each having its own intended application, benefits and limitations.
Excluding the complexities involved in meeting the emission requirements, the two biggest factors in selecting the best technology to use for this separation are:
- The ability of the equipment to handle the relatively high temperatures associated with drying.
- The materials handling of the recovered material.
This file will discuss solid particulate separation and recovery techniques, and the available equipment types for thermal drying systems.
EmissionsEmissions from a drying system may take on the form of solid particulate or evolved gases. These may be organic, odorous, toxic and/or reactive, and they may be considered to pose other health and environmental risks.
Due to the diversity in drying equipment, the respective loading of this particulate varies considerably. For example, in a flash drying system, the entire final product is entrained in the airstream, and the separation system would need to operate with extremely high loadings. Conversely, a system drying vegetable matter or a preformed pellet, for example, would have a far less severe carryover. As a result, the design of these systems will depend on the nature of the solid particulate, the quantity of material to be separated (solids loading), and the stipulated allowable emission limit.
TemperatureThe temperature of the gas stream will limit the use of several technologies. Once again, due to the diverse nature of drying, these temperatures may vary considerably from one application to another. And, although efficiently operating systems usually will have the exhaust temperature near 250oF (120oC), the separation and recovery system must be designed to cope with startup and shutdown conditions. During these conditions, the temperatures may be significantly higher than during normal operation.
Other thermal processes, calcination for example, will have very high exhaust temperatures. There are methods to reduce the exhaust gas temperature to allow the use of recognized separation systems. These include recuperation (cooling of the gas by transferring the energy to another stream), the introduction of makeup cooling air and the introduction of water sprays to provide cooling.
Products of RecoverySeparation methods are either dry or wet. In dry separation systems, the recovered product will need to be handled, by some mechanical method, to the next process. The fact that the process is drying indicates inherently that the desired final form is dry.
Wet separation systems coalesce or capture the particles in a liquid, usually water. The recovered product therefore forms a slurry or sludge residue that requires reprocessing or disposal.
VOCs and AcidsWorth special mention is the inclusion of volatile organic compounds (VOCs) and acids in the gas stream. Treatment of unwanted VOCs usually involves combustion to render an inert byproduct. This significantly raises the temperature of the exhaust gas, and the oxidizing equipment either is positioned prior to release to atmosphere or may require alternate equipment to cool the gas and capture solid particulate. Recuperation systems are well suited to both these installations.
Acids such as chlorine, sulfur or other halogen-based compounds are either treated by reaction (adding metered quantities of hydrated lime or sodium bicarbonate, for example) or condensation. Obviously, condensation is well suited to wet collection systems where the pH of the effluent can be managed. However, acids must only condense out in the desired design location. At higher concentrations of acid in the exhaust stream, dry collectors must be insulated to prevent condensation within the unit, which will ultimately destroy it.
Next, I’ll look at the most common systems available to separate solid particulate from main gas or exhaust streams.