Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 1

1.2 Vacuum Freeze Drying VFD Process VFD is a process where the wet product is first frozen to a solid phase and subsequently dried by sublimation of the ice layer to the vapor phase wi

CHAPTER 1 INTRODUCTION

With increasing trend towards new and better quality dried products and the added new

requirement of natural resource conservation and sustainability, the development of

appropriate drying technologies has become increasingly important

Removal of water can be achieved in a number of ways Proper dehydration of highly

heat sensitive products requires a controlled supply of heat for vaporization or

sublimation and concurrent removal of the water vapor generated The structure and

properties of the material to be dried are not altered much by the freeze-drying process

This technique has been widely applied industrially to the drying of biological

materials, pharmaceuticals, and foodstuffs (Liapis and Bruttini, 2007) The main

disadvantages of this technique are its high fixed and operational costs The

atmospheric freeze drying techniques is an outcome of recent efforts over the last two

decades to respond to this challenge It combines the advantages of both freeze-drying

(high product quality) and convective drying (low process costs), along with some of

their limitations as well

1.2 Vacuum Freeze Drying (VFD) Process

VFD is a process where the wet product is first frozen to a solid phase and

subsequently dried by sublimation of the ice layer to the vapor phase without passing

through the liquid phase at atmospheric pressure, by exposing it to a very low partial

pressure of water vapor, while supplying heat of sublimation Heat must be supplied by

conduction or convention carefully to avoid melting of ice

Chapter 1 Introduction

Figure 1.1 Phase diagram of water

The phase diagram of the freeze drying process is shown in Figure1.1 (Ratti, 2008)

Water can exist in three different states: solid, liquid or gas (vapor) depending on the

pressure and temperature Point T in Figure1.1 represents the triple point of water (at

0.01oC and 0.612 kPa) where the three phases coexist, and point C is the critical point

of water (374oC and 22060 kPa) The basic idea of freeze drying is to make use of

sublimation phenomena and it starts at temperatures lower than 0.01oC, and vapor

pressures below 0.612 kPa For freeze drying, it will follow the path from A to point D

i.e the product should be first frozen by decreasing its temperature, then the water

vapor pressure should be lowered below the pressure corresponding to the triple point,

and finally some heat should be supplied to help the ice to convert into by sublimation

Usually freeze drying works at higher vacuum (pressure levels range from 0.000013 to

0.612

101.330

100 0.01

Pressure (kPa)

Temperature (oC)

T

A

D

C

Fusion

Vapor

Sublimation

Evaporation

0.0013 bar) The driving force for the migration of sublimated vapor towards the

surface of the product depends on the difference between partial pressure of water

vapor around the product surface (ps) and vacuum pressure inside the drying chamber,

as shown in Figure 1.2

Gc

pc

pc/

p/

s

G

p s

Figure 1.2 Schematic diagram of vacuum freeze drying

Freeze-drying is an attractive process for drying foods since it retains in the dried form

material properties such as shape, appearance, taste, color, flavor, texture, and

biological activity, and also permits fast rehydration in cold or hot water In addition,

low content of micro-organisms, long shelf life, accurate dosing and low final moisture

content are also some of the distinguishing advantages of the freeze drying However,

high capital and energy costs which stem from the need to generate and maintain very

low temperatures and very high levels of vacuum, make it an expensive process

Moreover, the operation is a lengthy one due to the low thermal conductivity in a

rarefied atmosphere and the small difference in exchange potential at low pressures To

minimize the drawbacks noted above, efforts have been under way to develop an

Chapter 1 Introduction

atmospheric freeze drying process as a promising alternative cost-effective method

The main advantages of this process are simplification of the equipment due to the

absence of a vacuum chamber and ancillary equipment and significant reduction in

energy costs

1.3 Atmospheric Freeze Drying (AFD)

Meryman (1959) first showed in the laboratory the possibility of freeze-drying

products at atmospheric pressure In a series of experiments, he showed that the

diffusion of water vapor from the drying boundary through the dried shell occurs by

vapor pressure gradient, rather than by the absolute pressure on the system Hence, it is

possible to freeze-dry at atmospheric pressure This process is accomplished by

circulating cold dry air below -6oC to -10oC over a frozen product to improve the heat

and mass transfer from the frozen material at near atmospheric pressure The only

absolute requirement is that the partial pressure of water vapor in the drying medium

be kept low enough to provide a mass transfer driving force for water vapor transfer

from the frozen sample The temperature must be low enough to maintain frozen

integrity of product and should maximize the vapor pressure of products Heldman and

Hohner (1974) analyzed the kinetics of freeze drying with a simple one-dimensional

model under atmospheric freeze drying presented schematically as shown in Figure 1.3

According to the one-dimensional model, the ice interface (f) recedes to the interior as

heat of sublimation (Q) flows from the surface (s) to the interface to a temperature

gradient (Ta) represented by the dotted line curve Simultaneously, water vapor flows

through the dry layer in response to the water vapor pressure (Pva) gradient represented

by the solid line curve

Figure 1.3 Schematic diagram of atmospheric freeze drying (f- interface; s-surface; Q-heat transfer; m-mass transfer; Ta-temperature gradient; Pva-partial pressure gradient;

ps- partial pressure of water vapor around the product surface; pva- partial pressure of water in the drying chamber)

Despite the promises of low energy consumption and better quality product, certain

problems still exist in the atmospheric freeze drying process, limited to practical

implementation Conventional atmospheric freeze dryers utilize a bulky system of

mechanical heat pump system to lower temperature and a condenser to reduce

humidity of the air At least two mechanical agents are required for this operation,

which does not seem economical from the energy point of view

It also takes time to setup, de-humidify and cool the drying chamber In addition,

atmospheric freeze drying is controlled by internal resistance heat and mass transfer

due to the lower vapor diffusivity at atmospheric pressure which makes it a slower

drying process Difficult to control the structure of the frozen products, abrasion of

Ice Layer

………

………

………

………

………

………

………

Dry Layer

(f)

(s)

Q

m

Pva

x y

Cold

dry air

Chapter 1 Introduction

are the drawbacks of atmospheric fluidized-bed drying system listed by Khan-Wyler

(1987)

1.4 Vibrating Bed with Multimode Heat Input and Vortex Tube

Conventional atmospheric freeze dryers utilize a system of mechanical heat pump

system to lower temperature and in turns reduce the humidity of the air It takes time to

setup, de-humidify and cool the drying chamber To overcome the above limitations, a

vortex chiller for the purpose of cooling and de-humidification (Scheper, 1951)

coupled with multimode and intermittent heat input by conduction and radiation can be

used to speed up AFD process

The vibro-fluidized bed offers lower power needs, attrition rates and elutriations rates

than gas-fluidized beds (Rogelio et al., 2000) The vibrating state of the bed allows

higher heat and mass transfer rates due to de-agglomeration and the consequent

increase of specific area in the gas-solid contact (Gupta et al., 1980) To further

improve the drying kinetics and quality of the product as well as reducing the energy

consumption on AFD system, osmotic dehydration is a suitable option of partial

removal of water by soaking foods, mostly fruits and vegetables in hypertonic

solutions prior to start the drying operation (Alakali et al, 2006) Therefore, a

vibro-fluidized bed coupled with vortex tube and multimode heat input under atmospheric

pressure is proposed an attractive option to overcome the limitations of atmospheric

freeze drying using a fluidized bed No prior work exists on this new drying concept

1.5 Objectives

The main objective of this research is to develop new atmospheric freeze drying

system for drying of heat sensitive materials To reach this goal, a laboratory scale

atmospheric freeze drying system consisting of a vortex tube, a vibro-fluidized bed and

an adsorbent is designed to permit simultaneous or intermittent application of

conductive and radiant heat input for drying of food materials An experimental

approach is employed to carry out a systematic parametric evaluation over a range of

parameter values The sub-objectives of this project are as follows:

z To study experimentally the effect of different operating variables on the drying kinetics and quality in the vibrating bed atmospheric freeze drying process

z To develop a mathematical model to capture the drying kinetics of AFD along with the effects of affecting parameters (product thickness and carrier gas

temperature) on drying rate

• To carry out computational fluid dynamic studies of the vortex tube for the

investigation of flow behavior and heat transfer

1.6 Scope

z Design, fabricate and test a novel vibro-fluidized bed using a vortex tube to supply subzero temperature air and selective adsorbent and multimode heat input through

conduction and radiation under atmospheric pressure Effect of adding an

adsorbent to the bed of drying material is also examined

z Compare the physical quality parameters (e.g color, rehydration behavior, shape, size and microstructure) and drying characteristics of selected model

heat-Chapter 1 Introduction

sensitive materials with heat pump drying, vacuum freeze drying and atmospheric

freeze drying in a vibro-fluidized bed

z To investigate the effect of osmotic dehydration prior to AFD on the drying

kinetics and quality parameters in the vibro-fluidized AFD system

z To develop a mathematical model simulates to the AFD drying process and

compare simulation results with experiments for validation of the model

z A computational fluid dynamic study of vortex tube performance to model the

flow behavior and associated energy separation process

Appendix: Appendix A presents results of experimental and modeling studies using a

heat pump dryer These experiments were done to allow comparison with AFD results

Appendix B includes results of osmotic dehydration while Appendix C presents

experimental results showing effect of vibration on bed hydrodynamics