Drying of Solids

Topics: Time, Rate, Pressure Pages: 14 (3916 words) Published: August 25, 2012
Department of Chemical Engineering
University of San Carlos – Technological Center
Nasipit, Talamban, Cebu City

ChE 512L
Chemical Engineering Laboratory 2

(Vacuum Tray Dryer)

A Final Report Submitted to
Engr. Luis K. Cabatingan
Instructor, ChE 512L

Madeja, Rayam John D.

July 31, 2012
1. Introduction
Drying generally means the removal of relatively small amounts of liquid from materials. It is also defined as the transfer of liquid from a wet solid into an unsaturated gas phase. The goal of most drying operations is not only to separate a volatile liquid, but also to produce a dry solid of a desirable size, shape, porosity, density, texture, color and flavour. Drying processes can be classified as batch, where the material is inserted into the drying equipment and drying proceeds for a given period of time, or as continuous, where the material is continuously added to the dryer and dried material is continuously removed (Geankoplis, 2003).

In this experiment, ethanol-wetted sawdust is dried using a vacuum tray dryer. This type of dryer is an example of an indirectly heated batch dryer (Geankoplis, 2003). In indirect-heat dryers, heat is transferred mostly by conduction, but heat transfer by radiation is significant when conducting surface temperatures exceed 150°C (Brown, 1973).

There are two ways in describing the mechanism of drying of the ethanol-wetted sawdust. First is through the basis of ethanol content versus time and the other is on the basis of its rate of drying versus the free ethanol content. The free ethanol content is the ethanol content that can be removed by drying under the given percent relative humidity. Equilibrium content is found in the ethanol content vs. time plot which is the lowest obtainable ethanol content of the material even when it is dried for a long period of time. On the other hand, critical ethanol content, constant and falling rate periods can be derived from the drying rate curve. Constant drying rate is the horizontal line found at the beginning of the plot. The decrease of the rate as the free ethanol content decreases corresponds to the falling rate drying period. The free ethanol content that marks the end of the constant drying rate period is what is called as the critical ethanol content.

As in other transfer processes, such as mass transfer, the process of drying of materials must also be approached from the viewpoint of equilibrium relationships together with the rate relationship. For many solid materials, it will contain significant ethanol content when the drying rate has dropped to zero. The ethanol remains no matter how long drying continue as long as the drying gas conditions do not change. It is in equilibrium with the vapor contained in the drying medium and is called the equilibrium ethanol content. The equilibrium ethanol content varies greatly with the type of material and the conditions of the drying process. In the drying process, there are two mechanisms at which the process is experiencing. First, in the constant rate drying period, the surface of the solid is initially very wet and a continuous film of ethanol exists on the drying surface. This ethanol is entirely unbound ethanol and it acts as if the solid were not present. This period continues only as long as the ethanol is supplied to the surface as fast as it is evaporated (Geankoplis, 2003). Secondly, the shape of the drying curve during the falling rate period is as difficult to predict as the critical ethanol content. The shape will depend primarily upon the structure of the solid being dried and also upon the drying rate during the constant-rate period and on the critical ethanol content. The ethanol content that separates the constant-rate and the falling-rate periods is the critical ethanol content. At this point, there is insufficient ethanol in the surface to maintain a continuous film of liquid. The entire surface is no longer wetted, and the...
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