Heat integration dryers

Evolving Dryer Design to Improve Heat Integration... 

The mechanical integrity focus of this section covers stationary existing chemical processing plant equipment and piping. Equipment includes storage tanks, pressure vessels, dryers, heat exchangers, reactors, incinerators, columns, filters, knock-out pots, and so forth. As previously stated, this section assumes the equipment is designed and fabricated to... 

Systems with the rotary cascade dryer with integral burner give a much higher than either of the other two systems in all cases (by about 13% on average), because of the utilisation of the undersize biomass which results in additional heat export. 

The minimum CoE obtained is 8.36 c/kWh, for a system incorporating an RFSG with unenriched air and a rotary cascade dryer with integral burner, drying the biomass to 10% moisture content. The system exports 3.1 MW of electricity and 5.2 MW,i, of heat. The minimum CoE obtained for systems with the UGETC is 8.93 c/kWh, with the same dryer and final moisture content. The system exports 2.8 MW of electricity and 5.5 MW,h of heat. 

Typical heat-to-power ratios are 1.5 for systems with a band conveyor dryer, and 2.0 for systems with a rotary cascade dryer with integral burner. 

It was noted earlier that dryers are quite difierent in character from both distillation and evaporation. However, heat is still taken in at a high temperature to be rejected in the dryer exhaust. The appropriate placement principle as applied to distillation columns and evaporators also applies to dryers. The plus/minus principle from Chap. 12 provides a general tool that can be used to understand the integration of dryers in the overall process context. If the designer has the freedom to manipulate drying temperature and gas flow rates, then these can be changed in accordance with the plus/minus principle in order to reduce overall utility costs. 

Although some integration has taken place where the appliance or boilermaker has assumed responsibility for the combustion system, overall, specialist manufacturers of combustion equipment who have developed products for each application such as boilers, furnaces, kilns and dryers, etc. serve the market. The burner makers have manufactured products which provide a packaged solution to the combustion requirement, looking after not just the burners and controls but also the fuel supply system, which may involve pumping, heating of the fuel, filtration and other peripheral equipment and functions. 

A modem tray dryer consists of a well-insulated cabinet with integral fans and trays which are stacked on racks, or loaded on to trucks which are pushed into the dryer. Tray areas are 0.3-1 m2 with a depth of material of 10-100 mm, depending on the particle size of the product. Air velocities of 1-10 m/s are used and, in order to conserve heat,... 

The principles of fluidisation, discussed in Chapter 6, are applied in this type of dryer, shown typically in Figure 16.25. Heated air, or hot gas from a burner, is passed by way of a plenum chamber and a diffuser plate, fitted with suitable nozzles to prevent any back-flow of solids, into the fluidised bed of material, from which it passes to a dust separator. Wet material is fed continuously into the bed through a rotary valve, and this mixes immediately with the dry charge. Dry material overflows through a downcomer to an integral after-cooler. An alternative design of this type of dryer is one in which a thin bed is used. 

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