Using a Tray Dryer to Investigate Convective and Conductive Heat Transfer

Tray drying is a convective heat and mass transfer process commonly used in the industry to separate solids from liquids. In tray drying, a stream of hot gas is passed over a damp solid to vaporize the liquid. The process requires no agitation and allows for control over temperature and other properties. The flexibility of tray drying allows it to be used in chemical, pharmaceutical, and food production prior to packaging. Although seemingly simple, the process generally requires thorough experimentation to optimize drying conditions without damaging the solid. This video will illustrate how a tray dryer works, demonstrate a typical procedure for drying trials, and discuss some applications.

To start, let's look at the operation of a tray dryer. The most basic design employs a rectangular metal frame consisting of an air inlet, ventilators, heaters, a tray compartment, and an outlet. The wet solid is loaded into a shallow tray, which is placed in the tray compartment. The ventilators and heaters force a stream of hot gas over the tray at a carefully controlled temperature and flow rate. The liquid in the tray evaporates, and is removed from the solid. Tray drying is a batch process, meaning the addition and removal of solids from the dryer are discrete steps that cannot occur simultaneously. Its advantages over other drying methods include simplicity, and flexibility of operation, and relatively low fixed-costs. Its disadvantages include high labor costs and high energy usage, although the latter can be somewhat offset by filtering and pre-forming the solid. Now that we've seen some basics of how drying machinery operates, let's look at how the separation occurs.

The separation of the liquid from the solid consists of two steps. In the first step, the gas transfers heat to the liquid through convection, causing the liquid to evaporate. The rate of heat transfer, and therefore the rate of evaporation depends on the temperature difference between the gas and the liquid and an empirically determined proportionality constant called the "convective heat transfer coefficient." In the second step, the newly vaporized liquid is transferred away from the interface through convective mass transfer to prevent recondensation. The rate of this process depends on the difference in vapor concentration between the interface and the bulk of the gas stream. The empirical proportionality constant here is the convective mass transfer coefficient. Although the coefficients can be estimated, they are unique to the solid being dried and the geometry of the tray dryer. Furthermore, the coefficients are only valid as long as the solid surface is saturated. After the surface moisture has been significantly lowered, the drying rate will decrease as the internal movement of liquid within the solid becomes the dominant mass transfer mechanism. Now that you know the principles, let's see the sample drying procedure.

This demonstration illustrates the drying of a sand-water slurry at various temperatures and air speeds. First, check the dryer for safety hazards and ensure the psychrometer and other measuring devices are ready for use. Prepare the slurry by mixing 500 grams of sand with 150 grams of water. Pour the slurry into the tray and ensure it is spread evenly. Turn on the unit, and place the tray in the drying chamber, and record the weight. Then, turn on the blower and the dryer. Set the air velocity and temperature for the trial. Each trial will last 45 minutes with measurements taken at five-minute intervals. Use a digital thermometer to measure inlet air temperature, sand temperature, and outlet air temperature. Use the psychrometer to measure the dry-bulb and wet-bulb temperatures. Finally, record the outlet airflow rate and the weight from the balance. Repeat the process for each set of settings, totaling four unique runs. When the run is complete, use psychrometric charts or similar tools to find the absolute humidity.

The evaporation rate for each trial was determined by plotting the slurry weight as a function of time. Increasing temperatures correlated positively with the conductive heat transfer rate, and thus the evaporation rate. Increasing air velocity is associated with higher rates of convective heat transfer and mass transfer, and also increases evaporation rates. There is a positive linear correlation between air temperature and evaporation rate, as well as between air flow and evaporation rate. However, the experimental correlation between the heat and mass transfer coefficients was weaker than predicted. This may be due to the relative humidity at the air-sand interface or the effect of the air velocity on the weight of the tray.

Tray drying is commonly applied in industrial applications, both in specialty chemical production and in larger scale manufacturing, where labor expenses are not too great. Tray dryers are used in the food processing industry, typically as the last manufacturing step prior to packaging. To increase evaporation rates while controlling food quality, the dryer may be outfitted with indirect drying mechanisms, such as heated trays and radio-frequency heating. The heating conditions can be altered during the drying cycle to prevent solid materials from decomposing, warping, or cracking. Tray dryers are especially useful in facilities where several different foods, prepared in varying quantities, must be dried at unique conditions of temperature and humidity. An alternate dryer design uses multi-tray trucks. In the pharmaceutical industry, trucks are used to reduce time and labor and increase product uniformity. This design uses recirculators and baffles to control airflow, avoid dead zones, and maintain consistent temperature and humidity. For sensitive chemicals, inert atmospheres are generated. Since tray drying is physically gentler than fluid-bed or rotary tumble methods, it is suitable for the drying of sticky crystalline materials such as powders and crude drugs.

You've just watched JoVE's introduction to the tray dryer. You should now understand the principles of tray drying, a process for conducting tray drying experiments, and some applications. Thanks for watching.