Tutorial 4

S Gautam 1

Tutorial 4 (Water Activity and Evaporation)

Question 1

The diagram shows the experimental setup used to determine the equilibrium

moisture content of a guava sample at different relative humidity conditions.

All experiments were conducted at a constant temperature of 30°C.

Experimental data is summarized in the table below.

Water Activity

(aw)

Equilibrium Moisture Content

(g moisture/100 g dry solids)

0.11 1.63

0.22 2.47

0.33 3.71

0.44 5.63

0.53 9.22

0.64 12.60

0.75 20.90

0.84 35.20

0.90 217.0

0.97 613.0

Calculate the monolayer moisture content of guava by fitting the experimental

data to a) BET model b) GAB model.

Desiccator

Guava Sample

Wire Mesh

Wire Mesh

Saturated Salt solution

Salt

Oven

Tutorial 4

S Gautam 2

Question 2

A climbing film evaporator is used to concentrate liquid food at 1000 kg/h

from an initial concentration of 12 wt% solids to 25 wt% solids in a single

pass operation. Feed enters at its boiling point (45°C) and the process steam

used for heating is at 135°C. Estimate the number and length of evaporator

tubes required if the internal diameter of the tube is 30 mm.

Liquid properties: density = 1000 kg/m3; viscosity = 3.0×10

-3 N-s/m

2;

specific heat = 4.2 kJ/kg.K; thermal conductivity = 0.9 W/m.K; latent heat of

vaporization = 2.3×106 J/kg

Vapour properties: density = 0.04 kg/m3; viscosity = 1×10

-5 N-s/m2

Steam side heat transfer coefficient = 8000 W/m2K