Food Engineering

Lesson 36. HEAT AND MASS TRANSFER IN FREEZE DEHYDRATION

Module 4. Freeze dehydration

Lesson 36

HEAT AND MASS TRANSFER IN FREEZE DEHYDRATION

36.1 Introduction

In most conventional freeze drying systems, the vapor pressure gradient necessary for sublimation is attained by maintaining the total pressure in the drying chamber at a low level of the order of 0.1 to 2.0 torr (13.5 to 270 N/m²). A condensing system is provided to remove the water vapor formed and a heating system to supply the necessary latent heat of sublimation to the frozen material. Such a system is shown schematically in the fig. 36.1

In such a system, the two main factors affecting the rate of drying are the rate of movement of water vapor from the ice surface through the porous layer of the dry material and the rate of transfer of heat to the ice front. The movement of water vapor through the porous layer of the dried material has been discussed by many workers. By considering the flow of vapor through the dried meat as being analogous to flow through a bundle of capillary tubes, with slip flow conditions prevailing the mass flow rate of gas, m, at constant temperature is given by an expression of the form:

where, dw/dt = mass flow rate of vapor through the dry layer

b = permeability of the dry layer

P_i = Vapor pressure of ice at specified temperature

P_d = Partial pressure of water vapor

l = thickness of the dry layer

In order to achieve the maximum rate of drying, the ice temperature should be as high as possible, consistent with the product quality. Unless the heat of sublimation is supplied to the ice at a sufficient rate, the ice temperature will fall and consequently will the drying rate. In a system where the heat is transferred to the ice through the dried layer only and where drying takes place from the heated surface, then the rate of heat input, dQ/dt is given by:

dQ/dt = (K_d.A.(T_D –T_i)/l …………………………………(3)

Where, K_d is the thermal conductivity of the dried layer

T_d is the temperature of the dried layer

T_i is the temperature of the ice surface

An energy balance combining equations (2) and (3), gives

- L_s. A.b. (P_i – P_d)/l = (K_d.A.(T_d –T_i)/l ……………………(4)

Where Ls is the latent heat of sublimation at T_i

Ideally, to carry out the freeze drying process correctly, all the liquid present in the food should be frozen. However, in practice this is not feasible, provided the amount of unfrozen liquid remaining small. The product quality is not seriously affected. The optimum rate of freezing for freeze drying depends largely on the nature of the product, variations in the rate of freezing affects ice crystal size and hence pore size in the dried product and so can be expected to influence the rate of drying and the characteristics of the dried product, particularly its reconstitutability. Optimum rates of freezing should be determined experimentally.

36.2 Heat and Mass Transfer

There are three methods of transferring heat to the sublimation front.

1. Heat Transfer Through The Frozen Layer: The rate of heat transfer depends on thickness and the thermal conductivity of ice layer. As drying proceeds the thickness of ice is reduced and the rate of heat transfer increases.
2. Heat Transfer Through Dried Layer : The rate of heat transfer to sublimation front depends on thickness and area of the food, the thermal conductivity of the dry layer and temperature difference between the surface of the food and ice front. At a constant cabinet pressure the temperature of ice front remains constant. The dried layer of food has a very low thermal conductivity and therefore offers high resistance to heat flow. As drying proceeds, this layer becomes thicker and the resistance increases. As in other unit operations, a reduction in size or thickness of the food and an increase in the temperature difference increase the rate of heat transfer.
3. Heating By Microwaves : Heat is generated at the ice front and the rate of heat transfer is not influenced by thermal conductivity of ice or dry food or the thickness of dry layer.

36.3 Rate of Mass Transfer

When heat reaches sublimation front it raises the temperature and water vapour pressure of ice. Vapour then moves through the dried food to a region of low vapour pressure in drying chamber. 1g of ice forms 2 m³ at vapour at 67 Pa and it is therefore necessary to remove several hundred cubic meters of vapour per second through pores in the dry food.

The Factors That Control The Water Vapour Pressure Gradient are:

· Pressure is in drying chamber.

· The temperature of the vapour condenser, both of which should be as low as possible.

· Temperature of ice at sublimation front, which should be as high as possible.

In practice the lowest economical chamber pressure is approximately 13 Pa and the lowest condenser temperature is –35^oC.

36.4 Heat and Vapour Movement