Lab 8. Lab-tray dryer

Transcription

1 BAEN/CHEN-474 page 1 of 6 Student s Name: Factors influencing the drying rates in Objectives: 1.) To become familiar with the operation of a tray dryer 2.) To determine the psychrometric properties of the inlet and outlet air 3.) To measure the moisture content of the product during drying food products 4.) To determine the diffusion coefficient as function of air inlet temperature and airflow rate Procedures: 1. Measure the temperature of the air at the inlet of the tray dryer (Armfield Ltd.), using the provided psychrometer. This instrument gives two types of temperature: dry bulb, Tdb, and wet bulb, Twb. 2. Measure the air flow rate using the provided anemometer. 3. Place the products in the trays and wait 5 minutes. Lab-tray dryer 4. Measure the temperature of the air at the outlet, using the psychrometer, every 10 minutes. 5. Record the weight of the product every 10 minutes for a period of 2 hours 6. To determine the diffusion coefficient as function of temperature.

2 BAEN/CHEN-474 page 2 of 6 Initial potato thickness (in) = 0.06 Final potato thickness (in) =.043 Diameter of potato (in) 1.77 in Variable Air flow rate [ft/min] Inlet Air Temp. [ o C] Tdb [ o C] Time [min] Twb [ o C] Outlet Air Temp. [ o C] Tdb [ o C[ Twb [ o C] Weight potatoes, W [g] MC of potatoes [% w.b.] Discussion: 1.) Present a plot showing the drying rate of the product for the tray dryer data 2.) Describe the best model to fit the experimental data for the tray dryer data 3.) Determine De for the tray dryer data 4.) Determine the equilibrium moisture content using the equation for potato using the data for the tray dryer: k2 = [1] where k1 = ; k2 = ; k3 = ; k4 =1.287; k5 = ; M in [decimal db]; pvs in [kpa] 5.) Determine the air flow rate in kg/s and in m 3 /s 6.) Determine the mass transfer coefficient hd for the tray dryer data assuming that: for Pr> 0.6 [2] for 0.6<Pr <60 [3] where L = the length of tray [27.5 cm], ReL = ual/νa, Sc = νa/de, Pr = νa/αa and Nu = hl/ka (νa = µ/ρ). 7.) Discuss your results in detail

3 BAEN/CHEN-474 page 3 of 6 Tray dryer data: See Table at the end of this handout Moisture content at each time interval is calculated as: Product initial weight = 100 g Product initial moisture content = 85% w.b. Product initial moisture content in dry basis (in decimal dry basis) = 0.85/(1-0.85) = 5.67 d.b. Product dry matter, DM = 100*(1-0.85) = 15 g So, the moisture content at each time interval is calculated as: Moisture evaporated at (t = i) = DM (Mi-1-Mi) where ( i = min) and M in db For example - at 5 min of drying Water evaporated is = = 6 g 6 g = 15g(5.67-Mi), so Mi = 5.27 db, therefore MC of the product at 5 min = 5.27 db or % wb % w.b. (=(5.27/(1+5.27))*100)

4 BAEN/CHEN-474 page 4 of 6 INFORMATION ON THE TRAY DRYER heater fan fresh air exhaust air Q product n = number of trays = 4 L = tray length = 27.5 cm W = tray width = 18.3 cm d = tray depth = 1.5 cm X = space apart between trays = 3 cm EXPOSED AREA = nlw = EFFECTIVE AREA = nb W = where b = (X-d) Air flow rate = Volumetric flow rate = [kg/s] [m 3 /kg] How to determine the diffusion coefficient, De [m 2 /s or m 2 /min or m 2 /h] Transform the moisture content data to dry basis Using a non-linear regression statistic package fit the experimental data (MCdb vs. Time) using the following equation:

5 BAEN/CHEN-474 page 5 of 6 [4] The moisture diffusion equation for a flat plate is described by: [5] Where Mo = initial moisture content [d.b.]; Me equilibrium moisture content [d.b.]; a = half of the product thickness Or " Fit the experimental data to the following equation: MC db (t) Me Mo Me = 8 π 2 exp( Kt) [6] where K = π 2 De 4a 2 [7] Note: You can linearize Eqn(6) by taking the ln from both side of the equation [8] [9] where A = ln(a) and B = K = π 2 De/4a 2

6 BAEN/CHEN-474 page 6 of 6 EMC for potato slices