Law, Mujumdar - Energy Savings in Drying of FVF
7.1. INTRODUCTION
Thermal drying operations are found in almost all industrial sectors and are known,
according to various estimates, to consume 10-25% of the national industrial energy in
the developed world. As energy cost is increasing in recent years, operating cost of ener-
gy intensive operations such as thermal drying is escalating as well; whilst, greenhouse
gas emissions are correlated with energy consumption. With the global emerging econ-
omies rapidly industrializing, the energy consumed for thermal drying and the resulting
adverse environmental impact of the greenhouse gas emissions will inevitably increase
over time. This gives pressure to the energy intensive industry to look for energy effi-
cient technology and to find ways to reduce energy consumption. Since drying is an
energy intensive operation, energy consumption of an industrial dryer is huge and this
in turn makes the unit operation costly.
The most effective solution to this growing problem is to develop and utilize highly
energy-efficient drying technologies that will reduce net energy consumption and miti-
gate the environmental impact. Conventional industrial dryers usually operate at 30-70%
efficiency levels. Hence, industrial drying is definitely an area that is desperate for ener-
gy efficient technology.
7.2. WHY CONVENTIONAL DRYERS HAVE LOW ENERGY EFFI-
CIENCY
Generally, conventional dryer are operated at low energy efficiency due to poor de-
sign and lack of understanding of drying fundamental and science. Moreover, these
dryers were designed when energy was abundant and low in cost. To date, the scenario
has changed tremendously where energy cost is a major concern not only from the point
of view of economy but also environmental impact. The inefficiency of conventional
dryers is due to the factors given in Figure 7. 1.
7.2.1. Low contacting efficiency
Most conventional dryers are convective direct dryer where drying medium makes
contact directly with drying material for heat and mass transfer. However, drying ma-
terial itself is a resistance to the flow of the drying medium and hence, drying medium
tends to bypass the drying material through the empty space between the drying cham-
ber and the drying material. This results in low contacting efficiency between the drying
medium and the materials.
In this regard, fluidized bed drying has been recommended to improve the contact-
ing efficiency. As the bed of solids is fluidized in the fluidizing gas stream, the gas makes
contact with the bed of solids at a larger degree to enhance heat and mass transfer be-
tween the two phases. With reference to this, spouted bed, pulsating bed, agitated bed
also have the advantage of improving the contacting efficiency between drying medium
and drying material. Readers are to refer to the Handbook of Industrial Drying, Guide to
Industrial Drying and other chapters in this book for more details about the respective
types of dryer.