EXPERIMENT NO. 4-5 (revised 2011)
ROTARY DRYER
OBJECTIVE:
The objective of this experiment is to operate the rotary dryer in the South Dakota School of Mines and Technology laboratory and to determine its performance in drying a wet solid with a heated air stream of given inlet temperature and humidity.
NOTATION:
A - drying area (length2)
D - diameter (length)
G - mass velocity (mass/time length2 of dryer cross section)
h - convective heat transfer coefficient (energy/[length2 time temperature])
mv - rate of evaporation (mass/time)
qt - rate of heat transfer (energy/time)
T - temperature of the gas
Ti - temperature at the solid-gas interface
V - volume of the dryer (length3)
Va - volumetric heat transfer coefficient (energy/length3 time temperature)
i - latent heat at Ti (energy/mass)
Tlm - average temperature difference, taken as the logrithmic mean of wet-bulb depressions at inlet and outlet of dryer
THEORY:
The drying of a wet solid involves simultaneous transfer of heat into the solid and transfer of the more volatile material, the solvent, from the solid by evaporation. Usually the vapors from the volatile material are transferred into a carrier gas stream and so are removed from the system. In a rotary dryer the heat may be provided by heated surfaces in contact with the wet solid or by heated air or some other gas which contacts the wet solid. In the latter instance the heated air or gas is also the carrier gas to remove the evaporated solvent. The rate of drying, mv, depends on the temperature of the heat source, T, the temperature of the solid-gas interface, Ti, the surface area of the solid, A, the heat of vaporization of the solvent i at temperature Ti, and the heat transfer coefficient, h,
mv = h(T-Ti)A (ref. 1, eq. 25-12).
i
Solving for the individual heat transfer coefficient, equation (25-12) becomes,
h = mv i
A (T-Ti) .
Estimation of A for a solid of definite known dimensions is easy and leads to an easily determined value of "h". For a granular solid, the area may be estimated from the particle size and shape, (ref.1) and a value of "h" can then be computed (1,3).
For a rotary dryer the volumetric heat transfer coefficient, Va, is
0.67
Va = 0.5G (ref. 1, p. 733).
D
The rate of heat transfer, qt, is therefore
0.67
qt = 0.5 G V Tlm .
D
EQUIPMENT:
The rotary dryer in the South Dakota School of Mines and Technology laboratory is 16 inches in diameter and 10 feet long. Hot air is provided by a fan and steam-heated unit heater. The heated air flows co-currently with the solid being dried. A feed hopper is provided for feeding wet solids. Thermocouples are provided for measuring temperatures of the air and solids at both the inlet and the outlet.
The rotary dryer is also equipped with a cyclone filter and a Torit bag filter at the end of the dryer. This system removes any airborne dust particles from the air stream, as well as providing an induced draft on the system. Because of the last feature it is important to leave the bag filter on while running the experiment.
PROCEDURE:
Safety: Hard hats and safety glasses must be worn.
Become familiar with the dryer by on-the-spot inspection, checking to see that it is in proper working order. Check the instrumentation, thermocouples, and recorder to see that they are also in proper condition.
Temperature, Humidity, and Dew Point readings for the rotary dryer can be found at the following web address: http://omega-thx1.sdsmt.edu/ Click ‘Read Sensor’ to obtain numerical values of Temp, Humidity, and Dew Point. Channel 1 is the front of the dryer and Channel 2 is the back of the dryer. ‘Chart’ will show a plot of the Humidity and Temperature over time.
Before starting the dryer, inject water onto the 2 thermocouples with cloth around them until they are damp. Have instructor/TA open steam header. When starting steam flow, have the steam bypass the dryer and go to the condensate drain. When ready to start the dryer, slowly open the steam inlet and outlet valves of the dryer. Then, turn on the fan, tumbler, and dust collector to start the dryer.
Prepare a wet solid feed for use in this experiment. Dry the solid by passing it through the dryer. Run the dryer long enough at constant air flow and constant solid feed rate to establish steady state operating conditions before taking data for the drying run itself. The drying run should be at least 30 minutes.
REPORTING:
1. Take measurements so that you can determine moisture content of the feed and of the product, rate of flow of solids through the dryer, area of the solid, and drying rate.
2. Determine the moisture content of the incoming and outgoing air. Use the humidity chart in the Appendix.
3. Estimate the residence time of the solid in the dryer as well as the air flow rate through the dryer. The latter may have to be estimated from a material balance, or by using a pitot tube traverse of air leaving the Torit filter.
4. Determine the heat transfer coefficient from the equation given in the Theory section.
5. Calculate the volumetric heat transfer coefficient.
REFERENCES:
1. McCabe, Smith and Harriott, "Unit Operations of Chemical Engineering", 4th edition, pp. 709-743 and 749-765, McGraw-Hill Book Co., 1985.
2. Walker, Lewis, McAdams, and Gilliland, "Principles of Chemical Engineering", 3rd edition, pp. 251-254, McGraw-Hill Book Co., 1937.
FOR ADDITIONAL INFORMATION:
1. Treybal, "Mass Transfer Operations", 3rd edition, pp. 686-694, McGraw-Hill Book Co., 1980.
2. Brown & Associates, "Unit Operations", pp. 17-22, John Wiley & Co., 1950.