Mujumdar - Current Status of Global R&D in Drying
9.1. INTRODUCTION
Given the diversity of the 50,000 or so materials that are dried commercially at vari-
ous scales for a variety of reasons, the wide range of drying times required, the diverse
physical forms of the materials dried and the multitude of quality constraints required
to be met for the dried products, it is not surprising that literally hundreds of dryer
types are in use today. Drying is a highly energy-intensive operation that also deter-
mines the quality of most products that are dried. It combines transport phenomena
with material science. Since fossil fuels are commonly used as the energy source for
dryers, drying also has important environmental implications. It is estimated that from
9-25% of the national industrial energy consumption is attributed to thermal dehydra-
tion in developed countries. This wide range is a result of the fact that different industri-
al sectors have widely varying demands as far as drying is concerned, ranging from a
high of 35% in papermaking to just over 3% in the chemical process industries. Also, the
thermal efficiencies of dryers in use today range from a low of around 20% to a high of
80%; the latter is difficult to achieve and is most common for indirect dryers which have
so far limited industrial applications today.
It is interesting to note that about one ton equivalent of oil is consumed for, on aver-
age, six tons of water removed by thermal dehydration. Since it has already been shown
that there is a positive correlation between the amount of water removed in industrial
processing and the GDP (or standard of living) of the country it is clear that efforts must
be made to improve the drying processes as the large developing economies of the
world will soon make accelerating demands on fossil fuels to provide the energy for dry-
ing in various industrial sectors.
Drying is crucial to food preservation and agricultural processing. Thus, it is also at
the heart of the nexus of food, water and energy. Combustion of fossil fuels leads to
emissions of greenhouse gases (carbon dioxide, in particular) as well as noxious gases
and odors. It is obvious that the environmental impact of drying must deserve attention
of all governments and granting agencies in the immediate future. It is hoped that legis-
lative requirements will be placed on thermal performance of industrial dryers in com-
ing decade so dryer vendors and users will pay special attention to energy consumption
just as consumers today look at energy consumption ratings of domestic appliances like
TV, clothes dryers and washers. This will encourage R&D required to enhance dryer per-
formance and discourage non-optimal operation of existing dryers. Carbon footprints of
dryers per unit of water removed should be placed on boiler-plate so that the buyers are
aware of the performance of the units they wish to install.
This chapter summarizes attempts, principally by the author, in the past thirty eight
years to promote and encourage drying R&D across industrial sectoral as well as geopo-
litical boundaries and the results of these attempts. It is impossible to cover all aspects
of the subject, however. Over 80 percent of the scientific, technological and engineering
literature in the field of drying has appeared in the past two decades. There has been
rapid growth in technical literature on drying during 1980- 1996 when it seems to have
peaked. Fortunately, the publication rate has not subsided but reached a plateau. This
implies that much is now known about various dryers and this knowledge has entered