Drying pyrethrum

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 process. (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.