INTRODUCTION
1.3 DIFFERENT TYPES OF DRYERS AND DRYING METHODS
1.3.3 Mechanical dryers
These dryers are becoming increasingly important because of their high ability to control. The various parameters, such as temperature, drying rate and moisture rate, can easily be controlled in these dryers. There are various types of mechanical dryers, as shown in Fig. 1.6. Based on their capacity and drying methods, some of them are commercialized, and a list of commercial mechanical dryers are heated air dryers and fluidized bed dryers. The heated air dryers are of three types: fixed batch type dryer, re-circulating batch type dryer, and continuous flow dryer.
On the other hand, fluidized bed dryers are the straight type of fluidized bed dryers and rotary type of fluidized bed dryers.
1.3.3.1 Heated air drying
These are drying methods that can be achieved at any time of day or night and can minimize labour costs. As compared to the sun dryer process, these dryers can be used for large scale mass production with reduced time. Uniform grain drying can be achieved by regulating the flow of air. Air heated dryers include batch dryers and continuous flow dryers. The batch dryers are further classified as fixed bed batch dryers and re-circulating batch dryers. In a fixed bed dryer, the wet granules are patched over the perforated bed and drying is done by supplying the hot air produced by burning kerosene or rice husk. The required drying temperature in this type of dryer is kept 10-15°C above the ambient temperature. In such dryers, products are totally exhaled to the drying air under constant drying conditions, i.e., at constant air temperature. In this dryer, the concept of a thin layer arises as a bed of thickness less than 20 cm is better suited for drying.
Figure 1.6: Types of air heated dryers [24]
On the one hand, this type of dryer is simple and affordable, and on the other hand, this dryer is labour and cost-intensive. Besides these, the wide variation of moisture gradient along the vertical direction is one of the major disadvantages of this type of dryer. The re-circulating type of dryer is compact in which grains pass through both drying and tempering sections alternately, as shown in Fig. 1.6. Because of the simultaneous action of these two, the moisture gradient in the grains reduces. However, the non-uniformity of drying is the major disadvantage associated with this type of dryer. In a continuous flow dryer, paddy granules are poured at the top and continuously flow across the dryer. Heated air is blown through the paddy as it moves down the bed. It takes 15 to 30 minutes to move across the dryers. This type of dryer is advantageous over the batch type of dryer in terms of drying uniformity and lower operating cost. Nevertheless, the investment cost is higher than those of the batch type of dryer.
Sometimes baffles are used to diverge the flow of paddy granules leading to an increase in bed pressure drop.
1.3.3.2 Fluidized bed drying (FBD)
Fluidized bed drying is a relatively new drying technique that has become very popular in recent years. In a fluidized bed, direct interaction between the solid particle and hot air or gas is possible. In a fluidized bed dryer, high-pressure hot air passes through the distributor plate, where it comes into contact with solid particles and dries them. Unlike batch-type dryers, the concept of the thin layer does not come into play as the bed is in a movable condition. There are different types of fluidized bed dryers, such as straight type, rotating type, etc. These dryers are faster, take less time, and improve particle consistency than any other conventional dryers.
Because of these, there are numerous advantages inherited with these types of dryers, such as high drying rate due to the good interaction between hot air and solid particles, automatic
operation, high rate of heat as well as mass transfer, better control of temperature and other essential parameters, need less space for installation and lower maintenance costs due to lack of rotating components. A typical fluidized bed dryer is shown in Fig. 1.7.
Figure 1.7: Fluidized bed dryer 1.4 AIM OF THE PRESENT STUDY
Although there are many advantages to fluidized bed dryers, the major shortcoming associated with the straight type of atmospheric bubbling fluidized bed dryers is the high velocity at which the solids particles are transported along the height, and elutriation of the bed leads to a shorter residence time, which affects the quality of the product as well as the thermal efficiency of the dryers. On the other hand, it is difficult to fluidize particles with lower velocity in a rotating fluidized bed due to the action of centrifugal force apart from the drag and gravitational force.
Therefore, there is a need for alternatives to fluidized bed dryers to reduce these discrepancies.
A variable cross-sectional riser of a fluidized bed can minimize these discrepancies, as reported in the literature, in which the superficial velocity gradually decreases with the height of the bed due to an increase in cross-sectional area along the height. Because of the gradual increase in the area of the cross-section along the height of the bed from bottom to top, the velocity is relatively high at the bottom, ensuring the concentration of large particles is relatively low at the top and preventing entrainment of small particles. Similarly, due to the higher velocity at the bottom, the fluidization of coarser particles takes place easily. As a result of this behaviour, the heat transfer characteristics are expected to improve and thereby expect the improvement of drying characteristics, as drying phenomena are inevitably characterized by the heat transfer characteristics in a fluidized bed dryer.
The present investigation aims to design and develop an efficient bubbling fluidized bed riser and to study the hydrodynamics, heat transfer and drying characteristics. In this study, five different atmospheric bubbling fluidized bed dryers with varying cone angles ranging from 0°
to 20° (with a change in 5°) are proposed numerically to adjudge the proper cone angle for efficient drying. Based on numerical analysis, the feasible risers will be selected, and a laboratory-scale study will be carried out to find out the best performing dryer using sand particles as bed inventory. Performance of the selected dryers will be carried out, and validation of the experimental results will be done with simulation. Furthermore, the performance of the dryers will also be carried out to assess the drying characteristics of paddy particles. The objectives of the present study will be presented in the subsequent chapter.