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Drying pyrethrum


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Bai, Chi Sun 1994 , 'Drying pyrethrum', Coursework Master thesis, University of Tasmania.

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Drying technology is being more and more used in chemical industry, food industry,
processing agricultural products, and fibber industry. In the majority of processing
industries, drying is carried out for one or more reasons:
1)To offer ease dealing in further processing. For example; drying sweet
chrysanthemum in order to increase sweet agents. It can be instead of the cane sugar.
The sweet taste is nearly 200 times of that of the cane sugar. Generally, after picking
up the fresh leaves of the sweet chrysanthemum, it is necessary to send them to
drying equipment at the temperature about 80 C° ---100 C°. With the help of the
drying their fresh leaves, their volume will be reduced to one of seventh or one of
eighth of fresh leaves volume. Then it is through chemical processing them to get
sweet ingredients. Finally, these sweet ingredients are dried at the temperature nearly
140 C° to get the final white products.
2) To supply the final product with satisfactory moisture, such as tea, tobacco. The
moisture of fresh tea leaves is about 70%. After drying the final moisture of tea is
about 6 %.
3) To preserve the products during storage. For example, drying rice, corn, and other
agricultural products are to impede the enzyme to oxidation of these agricultural
products, because the final lower moisture of the dried agricultural can limit the
enzyme to grow.
4) To avoid the presence of the moisture which may lead to corrosion as in drying
some metal products.
The drying process includes moisture migration and evaporation. The heat and mass
transfer are always key controls for any drying processes in designing the dryer. The
drying energy efficiency and drying cost should be considered firstly. Currently high density industrial drying technology is developing quickly based on a deeper
understanding of the drying process and the character of wet objects.
The purpose of the paper is to analyse the drying process, including heat and mass
transfer, moisture movement and moisture evaporation, and the air flow situation in
the drying chamber through discussing the practical drying pyrethrum.
A moisture moving model in capillaries is set up to study the moisture movement
inside the pyrethrum, which will influence the drying rate because the moisture in the
wet pyrethrum exists mainly in the capillaries.
A mathematical model of Heat and Mass transfer has been developed to explaining
the drying principle and to design a suitable dryer for drying pyrethrum. Psychrometric
chart and other charts are introduced to understand the change of warm air conditions
during drying pyrethrum.
The drying curve predicts the moisture changes in the wet materials and the
characteristics of the dryer.
Analysis of the air flow patterns in the air duct leads to improvement in the air
velocity distribution in the drying chamber.
After studying the whole drying theory, the forced convection multiple continuous
dryer is recommended for drying pyrethrum. In this dryer, the heat energy necessary
for moisture evaporation is supplied through the convection of warm air. The vapour
is carried away by the warm air. The continuous drying will result in higher drying
efficiency. This dryer offers some outstanding features as follows:
(1) This dryer offers a larger output for a given floor area.
(2) The drying conditions can be easily controlled.
(3) Higher heat efficiency can be obtained in this dryer due to the continuous drying
(4) Automatic loading and discharging save some labour costs.
(5) Compared with other dryers, lower operation costs and lower manufacture costs
are obtained in selecting this dryer. High density drying should produce the top quality dried products at the lowest cost.
In drying plants or agricultural products, the top drying quality has some specification
as follows.
(1) The final moisture contents of the dried products should be in equilibrium with
that of the air in which it is to be stored so that further changes in moisture are small.
This conditions is called the equilibrium moisture content. For most organic plants,
the EMC is in the ranges 9-15% of oven dry weight.
(2) The oxidation of ingredients should be lower than 15% by weight.
(3) The broken volume should be lower than 15% by the total volume.
(4) Odour and colour should be maintained. For example drying tobacco, the colour
should be the golden yellow. For drying some vegetables, the natural green colour
should be kept during the drying process.
It is hoped that this study may lead to improved drying technology in drying Chinese
tea, tobacco, vegetables, mushrooms, agricultural products and food.

Item Type: Thesis - Coursework Master
Authors/Creators:Bai, Chi Sun
Keywords: Pyrethrum (Plant), Tea
Copyright Holders: The Author
Copyright Information:

Copyright 1994 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Tech.)--University of Tasmania, 1994. Includes bibliographical references (p. 149-150). Cover title

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