| dc.description.abstract | Pyrethrum is the fourth largest export commodity, for
Kenya and one which today earns the country £5 ,000,000 annually.
It is grown at high. altitudes in Nyanza, Central and Rift Valley
Provinces, picked and dried on the farm, and transported to
Nakuru for extraction and export. Prior to World WarI, Dalmatia
and Japan were respectively the leading producers of pyrethrum
flowers However,over the last thirty years, Kenyahas
outstripped the rest of the world in production and today she
produces two-thirds of the global crop.
Over the years, pyrethrum growers in Kenya have developed
a variety of natural and forced convective driers which have
presented a number of operational problems: excessive use of
heat on flowers, contamination of flowers, fires, improper
distribution of air, and drier maintenance. For a start, a
survey of different types of driers used in more developed countraes
is done in an attempt to identify those which have potential
for use in drying pyrethrum. It is concluded that only the
deep-bed, spouted-bed, fluidised-bed and the solar drier are
worthy of further investigations .
Literature survey on drying and thennal degradation of
pyrethrins is done in order to gain deeper insight into past
research activities in these two fie Ids • This survey has formed
the basis for formulating theoretical models of drying and
pyrethrins loss and subsequent results compared with experimental
data.
Experimental work was carried out on the two related
aspects of drying, namely, moisture removal and thermal degradation
of pyrethrins Prior to conducting major experiments on moisture
renoval, it was found necessary to carry out preliminary work
on the establishment of methods of determination of moisture
content of pyrethrum flowers in general, and equilibrium moisture
content in particular. Results obtained from moisture removal
experiments show that the rate of drying is heavily dependent
on air temperature and to a lesser extent on initial flower
moisture content, air mass velocity and the type of clone being
dried.
Three distinct but equivalent methods f drying pyrethrins
control samples were established. Experimental work on pyrethrum
Clones 4331, M/22/246, Ma/65/99 and 1708 showed that pyrethrins
loss increases with air temperature and drying time. Furthernore,
it was shown that when subjected to the same drying conditions,
all clones tested exhibit the same relative loss of pyrethrins.
However,the absolute loss in terms of quantities of pyrethrins
lost is especially high for high yield clones such as Ma/65/99.
It was also shown that the rate of thermal degradation of
Pyrethrin is the same as that for Pyrethrin II, for all clones
tested. Analysis of test data on pyrethrins loss showed that
it was a first-order reaction, implying that the mechanism of
this loss is diffusion and evaporation rather than by some complex
chemical reaction. Laws governing the loss of pyrethrins were
developed and a drying air temperature of 800Cis recommended
on grounds of 10\'1 loss of pyrethrins and reasonable drying time.
On moisture removal, a model which treats the pyrethrum
flower as a sphere through which moisture diffuses radially
outwards followed by surface evaporation is presented. Good
agreement is obtained between the resulting theoretical equations
and experimental data.
Design and development work on large-scale pyrethrum
driers is be the natural follow-up of the present work, bearing
in mind the following specifications: low pyrethrins losses;
drying tine of 6 hours; initial drier cost of Sh. 8,000/=; drier
life-time of 10 years; drier capacity of 1,000 kg of wet flowers;
no electricity and a drying cost of 10 cents per kilogram of
dried flowers | en |
