Often convective dryers are operated with a hot gas stream which is directly or indirectly heated. Usually the heating medium is air. But inert gases, exhaust gases or superheated steam are also applied. Superheated steam as the heating medium has energetic advantages. By condensation of the used steam and the evaporated liquids from the wet solid a high rate of energy recovery of approx. 90% is possible . On the other hand higher temperature ranges have to be applied resulting in higher thermal stress for the product.
The main operating parameters in convective dryers are the flowrate and inlet temperature of the hot gas stream. To improve the energy efficiency it is recommended that the air flowrate be reduced to a minimum and to increase the inlet temperature to the allowed maximum.
At partial load, many convective dryers are operated at excessive air flow rates. Therefore, optimization of operating parameters for the entire throughput range is recommended with regard to sufficient gas velocities in dryer and gas/solid separators for example, cyclones.
To enable operation at the energy optimum, convective dryers should be equipped with temperature measurement of inlet and outlet air, as well as with flow control. In addition, an online measurement of exhaust air humidity enables closer approach to the optimum operation point .
Often, spray dryers and fluidized-bed dryers are characterized by only little flexibility in changing the air flow rate without affecting particle characteristics within the dryer. Consequently, the air inlet temperature may be the only variable that can be controlled .
A measure to achieve significant energy savings is the recirculation of a fraction of the exhaust gas stream back to the inlet air to the heater. However, due to the increased moisture content of the hot air entering the dryer, higher temperatures are required to achieve the same drying rate. In addition, the investment costs for the ductwork and the control of the air recirculation is often high.
An additional measure to improve the energy efficiency of a convective dryer is to recover heat from the waste gas by means of heat exchangers. Usually the heat is used to preheat the inlet air (as illustrated in Figure 7.22) or the wet product feed, although it can also be used for other process streams.
In general, a major limitation for heat recovery from convective dryers is fouling of heat exchanger by particles. Therefore, cyclones or filters must be used for particle removal from the gas stream before heat recovery. However, these additional installations reduce the overall profitability of this measure. Furthermore, cleaning in place (CIP) strategies can be applied to frequently clean the heat exchanger surfaces to ensure best possible heat transfer. For avoiding fouling and blocking in the off-gas system scrubbers are often used so that a heat recovery is not possible.
Usually for heat recovery, the temperature of the hot gas stream is reduced without condensing any vapor, as convective dryers are often run with off-gas temperatures far above the dew point. An additional potential for heat recovery can be achieved by cooling the exhaust gas below its dew point.
Figure 7.22 Convective dryer with waste gas heat recovery.
7.7 Crystallization | 253
Was this article helpful?