Thermal dryers

Four of the more common types of thermal dryers used in the process industries are illustrated in Fig. 3.15. 

Coal-cleaning plants (with thermal dryers) Kraft-pulp mills Portland-cement plants Primary zinc smelters Iron and steel mill plants Primary aluminum-ore-reduction plants Primary copper smelters ... 

The SAREX process is a trailer/skid-mounted, modular system designed for on-site, in-line separation and resource recovery of oily waste sludges and contaminated soils. This process provides dewatering and thermal desorption to maximize the recovery of oil while obtaining substantial reduction in volume and contaminant concentrations. The SAREX process includes the MX-1500 centrifuge, the MX-2000 low-temperature thermal dryer, and the MX-2500 medium-temperature thermal desorber. The MX-1500 centrifuge is used to separate the solids from the... 

Goal-cleaning plants (with thermal dryers)... 

As shown in the flowsheet (Figure 3), the washed product from the second-stage centrifuge is repulped in an appropriate vehicle preparatory to the final purification stage. This vehicle might be methanol, if a molding grade type of polymer is desired as the end product, water if over-all plant economics dictate a final thermal dryer system suitable for the evaporation of water rather than flammable solvent, or aqueous hydrochloric acid, if the end use of the polymer is for fiber and film production. 

Thermal dryers are sometimes used for predrying, but are almost always used for final drying because of the limitations of mechanical dryers. After mechanical predrying, much of the remaining water is chemically bonded to the fiber and must be evaporated. This is accomplished using several types of thermal system such as heated cans, convection ovens, and radio frequency (RF) dryers. Recently, microwave dryers have been designed for drying textiles and carpet tiles [5]. 

Conduction drying is usually carried out using hot cylinders, which are rotating metallic cans that are heated using steam or special heat-transfer liqnids. When steam is used, the drying system is often referred to as steam cans, which are shown schematically in Figure 38.6. The energy requirements removed are typically aronnd 4700 kJ/kg, which is usually lower than that of other thermal dryers. 

Typical emission control systems in coal dryers are cyclone collectors and high-efficiency wet scrubbers or packed towers in series. Fabric collectors are not usually used because of fire hazard. High-efficiency cyclone, a venturi scrubber, and a demister installed on a thermal dryer can reduce PM from 4.5 to 0.1 g/m [64]. The estimated emission factors for coal drying plants expressed as kg/t of coal dried are summarized in Table 53.15. 

Screening and centrifugal dryers dewater the coarser products while screen-bowl centrifuges and sometimes thermal dryers are utilized to reduce the moisture content of the finest sizes. 

Metallurgical coal cleaning plants utilize thermal dryers—the coal is softer and more friable and thus has a finer size distribution after extraction by the mining machines. Coals for metallurgical use must be thoroughly processed and dried to meet the end user requirements. Additionally, flotation is typically utilized in these circuits due to the quantity of coal and quality of the needed end product (low ash, low sulfur) (Apian, 1993 Burchfield, 1993). 

The incoming coal must be screened, and, because feed to the tables must be dry, thermal drying of the raw feed is required at some plants. The thermal dryers, in turn, require cyclones and scrubbers for control of particulate emissions. Thermal dryers are fired with coal, oil, or gas. 

Jig-table washing plants are so named because jigs are used to clean the >0.25 in. increment and Diester tables (oscillating table-sized sluices with a flat, riffled surface, approximately 12 ft, which oscillates perpendicular to the riffles, in the direction of the flow of coal) are used to clean the <0.25 in. increment. Froth cells and/or thermal dryers may be used in conjunction with this equipment. 

Drying in the thermal dryer is achieved by a direct contact between the wet coal and currents of hot combustion gases. Various dryers marketed by different manufecturers work on the same basic principle. 

It establishes more stringent standards for emissions of particulate matter and opacity included in the existing guidelines for thermal dryers, pneumatic coal-cleaning equipment and coal-handling equipment. 

It expands the applicability of the thermal dryer standards so that they will apply to both direct contact and indirect contact thermal dryers drying all coal ranks, and to pneumatic coal-cleaning eqnipment cleaning all coal ranks. 

It establishes an emission limit sulfur dioxide and a combined emissions limit for nitrogen oxides and carom monoxide from thermal dryers. The USEPA promotes the combined standard because a decrease in emissions of one of the two pollutants often leads to an increase in emissions of the other. 

Experimentation has shown that different adsorption isotherms are occasionally obtained, for data taken during adsorption or desorption under the same eonditions. This hysteresis in the sorption isotherm (Fig. 1-36) must be considered in the design of adsorbers and thermal dryers. The hysteresis phenomena can be explained by capillary condensation. 

Fig. 11-6. Computer controlled thermal dryer for aqueously cleaned piece parts. (Photo courtesy of United Space Alli-...

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