Forced Convection dryer

CL-352

Chemical Engineering Lab-III

Submitted by:

Prashant Shekhar

12110066

Forced convection dryer

1. Aim:

To plot the drying curve and study different stages drying by the method of forced convection.

2. Apparatus and materials required:

Sprouted seeds, tray, digital ammeter, digital voltmeter, heater, dryer, digital balance.

3. Theory:

Drying has been defined as the process whereby moisture is vaporized from a material and is

swept away from the surface, sometimes under vacuum, but normally by means of carrier gas

which passes through or over the material. Other methods include vacuum drying1, where heat

is supplied by conduction or radiation (or microwaves), while the vapor thus produced is

removed by the vacuum system. Another indirect technique is drum drying where a heated

surface is used to provide the energy, and aspirators draw the vapor outside the room. Some

more methods are freeze drying, supercritical drying, natural air drying, etc.

There are two basic mechanisms involved in the drying process; the migration of moisture

from the interior of an individual grain to the surface, and the evaporation of moisture from

the surface to the surrounding air.

Common applications can be seen in food industry where drying is used to inhibit microbial

development and quality decay. Non-food products include drying of wood, paper and washing

powder.

1 Reference: http://en.wikipedia.org/wiki/Drying

4. Procedure:

Soaked seeds are weighed and kept in the dryer on a metal plate

Temperature of the dryer is set between 50-60 oC

After 15 minutes, metal tray is removed and weighed again.

It is again kept for drying, in the forced convection dryer

Next reading is taken at 15 minutes and above mentioned steps are repeated for 9 readings

Then rate of change of weight is plotted against time and different stages in drying is

studied

5. Observations:

Temperature = 60oC

Table 1: observed values of weight with regular intervals of time

Obs.No. Time (min) Weight (gm)

1 0 151.72

2 15 149.69

3 30 147.13

4 45 144.74

5 60 142.58

6 75 139.52

7 90 136.67

8 105 134.61

9 120 132.69

10 135 131.57

Table 2: Calculated values

Rate of change of

weight (w/t) (g/s)

Time (t)(s)

0.14 15

0.17 30

0.16 45

0.14 60

0.20 75

0.19 90

0.14 105

0.13 120

0.07 135

Fig 1: Drying curve

6. Sample calculation for reading 4:

Change in weight (w) = 144.74 142.58 = 2.16 g

Time (t) = 15 min

Rate of change of weight = w/t = 0.144 g/s

0

0.05

0.1

0.15

0.2

0.25

0 20 40 60 80 100 120 140 160 180

Rat

e o

f ch

ange

of

we

igh

t

time

(w/t) vs. t

7. Results and conclusions:

Observing the graph, we can study the three stages in drying that is the increase in the rate of

drying, then a constant rate (approx.) and finally a decreasing trend. Initially the rate increases

because the free moisture is loosely bounded to the surface and thus easily can be evaporated.

The rate therefore increases with the time. Gradually the moisture at surface diminishes, now

the moisture from inside the seed travels up to the surface and gets evaporated. But this rate of

transfer from inside to outer surface is very slow due to various factors, major being the

resistance provided from the seed itself. Also, since the moisture inside the seeds is less

compared to the outer surface, the driving force is weak. Thus the rate, mostly remains

constant. Finally there is little or no moisture left in the seeds, hence the rate of drying

drastically falls. These are the main three stages of drying.

8. Precautions:

The temperature of the hot air must be remained constant. The variation in temperature will

affect the drying rate. For this a well calibrated digital thermometer must be used to

maintain the temperature.

RPM of the dryer should be kept constant as increase or decrease in the rpm will increase

or decrease the drying rate.

Horizontal level of the weigh balance should be checked. The bubble should be in center

before taking any reading.

Always put the seed tray in center of the balance otherwise it may show varying results.

Thus more sophisticated apparatus should be employed to avoid this error as measurement

of weight plays a crucial part in studying the drying processes.

Care should be taken while handling the seed tray, as dropping a single seed can affect the

weight and eventually add up to the errors.