Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 2

2.1 Freeze Drying Under Vacuum or Atmospheric Pressure Vacuum freeze-drying is a well known process for highly heat-sensitive materials.. 2006 investigated the physical properties of va

CHAPTER 2 LITERATURE REVIEW

The first part of review explains atmospheric freeze drying as a modifications

technique of the vacuum freeze drying process and covers relevant information on

modeling and experimental investigations This chapter also contains some basic

information on vibrating bed dryers along with a discussion of the vortex tube and

multimode heat transfer, which are applied in this research to develop a new integrated

atmospheric freeze drying system

2.1 Freeze Drying Under Vacuum or Atmospheric Pressure

Vacuum freeze-drying is a well known process for highly heat-sensitive materials

This method is used as a benchmark of product quality as it often gives the best quality

dried products For example, Marques et al (2006) investigated the physical properties

of vacuum freeze drying of tropical fruits and showed that this process gives high

quality products They also proved that vacuum freeze-dried (VFD) foods have high

porosity and low apparent density VFD also conserves color, flavor, and taste and

provides rapid rehydration

The main disadvantages of the freeze drying technique are its high fixed and operating

costs as demonstrated by Matteo et al (2003) The latter are due to the series of

energy-intensive operations involved in the process: freezing of the fresh product,

heating of the frozen foods at low temperature to induce sublimation, condensation of

water vapor and mechanical energy needed to maintain high vacuum Moreover,

vacuum operations are mainly carried out batchwise, which represent an additional

cost together with requirements of the apparatus operated under vacuum

Liapis et al (2007) showed that the limiting step of the traditional freeze-drying

process under vacuum is the transfer of heat to the product due to the decrease in

thermal conductivity with decreasing the pressure of the freeze-drying chamber

Efforts have been made to improve the vacuum freeze drying method but none has

given economically satisfactory results as far as industrial applications are concerned

In particular, as disclosed in U.S Patent specification No 3,319,344, an attempt has

been made to fluidize the product using vibration to be freeze-dried under vacuum in

order to improve the heat and mass exchanges, but the major drawbacks resulting from

operating under vacuum are not overcome in this design

2.2 Atmospheric Freeze Drying – Fixed Bed Dryer

The early workers in atmospheric freeze drying, namely the works of Meryman (1959),

Lewin and Matelas (1962) and Woodward (1963), reported varying degrees of success

when fixed beds of desiccants were employed to freeze dry foods and other biological

materials in the absence of vacuum The potential for atmospheric freeze-drying was

demonstrated by Meryman (1959) He showed that the drying rate of a material

undergoing freeze drying is a function of ice temperature and the vapor pressure

gradient between the site of water vapor formation and the drying media, rather than

the total pressure in the drying chamber He invented process using either a fixed bed

water vapor adsorbent adjacent to the frozen product or a condenser in a stream of cold

air

However, drying periods were observed to be very long In order to reduce drying time,

other researchers focused attention on reducing product dimensions and on utilizing

fluidized beds through modeling and experimental investigations

2.2.1 Experimental studies of atmospheric freeze drying-fluidized bed dryer

A number of researchers have performed experimental studies on atmospheric

freeze-drying using a fluidized bed to investigate the freeze-drying performance on different size and

shape of products Quality parameters of the dried products have also been investigated

Malecki et al (1969) carried out atmospheric fluidized-bed freeze drying of apple juice

and egg white Their work supports the conclusion of Dunoyer and Larpusse (1961)

and Woodward (1963) that the drying rate in atmosphere can reach that under vacuum

if the particle size is sufficiently small However, Malecki et al (1969) found that for

apple particles the bed had to be at -34oC to prevent sticking and, at this temperature,

only 1 percent ice was sublimated per hour, which is extremely low and hence not

attractive from practical standpoint

Boeh-Ocansey,O (1985) conducted experiments to investigate the drying kinetics at

different product thicknesses, chamber temperature (-5oC, -10oC and -15oC) and using

different adsorbents on carrot slices (1.8 x 33, 3.4 x 27, 4.8 x 26 and 5.8 x 22 mm)

They carried out their experiments in a drying chamber consisting of a vertical

cylindrical column 10 cm in diameter and 100 cm high at atmospheric pressure They

obtained higher freeze drying rates with particles of activated alumina (0.4 mm

average diameter) than with activated carbon Their results showed that a higher drying

temperature (-5oC) is preferable to increase the drying rate They compared their

results with conventional vacuum drying and found that product thickness is more

sensitive in drying kinetics to a great extent in AFD then in VFD

An apparatus and a technique for spray freeze aqueous solution at very low

temperatures (-90oC) and for subsequent dehydration of the resulting frozen particles

in a stream of cold, desiccated air was developed by Mumenthaler and Leuenberger

(1991).They investigated the influence of various operating variables on the drying

kinetics as well as the quality of the products They found a dry, stable and intact cake

of same shape and size as the original frozen mass, with sufficient strength to prevent

cracking and powdering or collapse They also observed uniform color and rapid

solubility upon reconstitution in water; increased inner surface area and good crystal

structure of the active substance They also reported an enhanced heat and mass

transfer between the circulating drying medium and the frozen sample

Alves-Filho et al (1998) used a fluidized bed as the first-stage freeze-dryer at

atmospheric pressure in a two-stage heat-pump system without adsorbent Adjusting

the heat pump dryer components to keep the air temperature below the drying

product’s freezing point controlled drying condition in the first stage fluid bed dryer

Their control strategy was based on the specific enthalpy curves developed by

Alves-Filho et al (1996) The product residence time in the first-stage dryer was selected to

reduce the moisture content to the critical values Afterwards the semi-dry product was

transferred to the second stage fluid bed to be dried at higher temperatures The

advantages of their two-stage system are that low-temperature drying reduces the

moisture content while maintaining product quality while higher-temperature drying

increases the overall heat-pump dryer capacity These authors reported excellent

quality of dried shrimp, apple pieces, carrot slices, etc., at a relatively high cost,

however

Donsi et al (2000) here demonstrated the feasibility of atmospheric freeze-drying for

shrimp and showed that the drying time is indeed an order-of-magnitude longer than

that for the vacuum process To shorten the drying time, part of the water was removed

by osmotic dehydration Freeze-drying was carried out in a fluidized bed using a

mineral adsorbent (Zeolite particles 88 μm mean diameter) or an organic adsorbent

(wheat bran) The AFD dried shrimp dehydration properties that are similar to those

obtained in conventional freeze-drying The economics of the process were not

reported

Bussmann et al (2003) invented an apparatus for drying a product using a regenerative

adsorbent which can be carried out in an energy-saving manner According to their

invention, the product is dried by bringing it into contact with adsorbent, water being

taken up from product by the adsorbent Subsequently, the adsorbent is regenerated

with superheated steam

A detailed investigation was carried out by Matteo et al (2003) of atmospheric

freeze-drying in a fluidized bed mixed with different compatible adsorbent particles They

fluidized 1 cm long potato cylinders of various diameters with various particulate

adsorbents They measured higher heat and mass transfer coefficients compared with

vacuum freeze drying due to convective heat and mass transfer, which is absent in

VFD They showed that higher freezing (-10oC) and fluidized bed temperatures (-6oC),

compatible adsorbent (bran & bentonite), reduced sample size (6mm), smaller

adsorbent particles (400 μm), higher product/adsorbent weight ratio (1/8), moderate

regeneration temperature (60oC) are conducive to enhancing the dehydration rate

However, fluidization velocity had no significant effect on the dehydration rate Their

work also revealed that the size of the product is the key parameter in atmospheric

freeze-drying Finally, they showed that there is a significant reduction of the energy

cost relative to vacuum freezes drying due to the absence of a vacuum chamber and

ancillary equipment

Stawczyk et al (2005) investigated the kinetics of atmospheric freeze-drying and

quality of apple dried cubes; all properties such as dehydration rate, shrinkage, color,

antioxidant content etc were reportedly very good They conducted experiments at

three different temperature increasing strategies; namely, constant inlet air temperature

(CT); inlet air at different temperature (DT) option and inlet air of ascending

temperature (AT) at a fixed airflow They found ascending inlet temperature condition

maintained a stable drying rate during the whole drying process to obtain an

economical AFD process and generally better quality dried product Their results

showed that AFD dried products at lower temperature (-10oC), had characteristics of

rehydration kinetics and hygroscopic properties similar to products obtained by

vacuum freeze drying They also found that AFD products are better than hot air-dried

products in terms of their anti-oxidative activity

Strommen et al (2005) carried out an experimental study of atmospheric freeze drying

of cod using a fluidized bed dryer coupled with a heat pump They found lower bulk

densities, higher rehydration, and light color, which is similar to vacuum freeze drying,

when dried at low temperature (-5oC) over a longer residence time of about 10 hours

They also noted that the typical specific moisture extraction rate (SMER) for

atmospheric freeze drying with heat pumps is in the range 4.6 to 1.5 kg of water per

kWh

Claussen et al (2005) analyzed the physical and quality parameters (color, water

content, rehydration properties and sorption isotherms) of traditional Norwegian

stockfish and compared them with atmospheric freeze dried (AFD) cod fillets

Sorption isotherms of several stockfish and AFD cod samples were measured with a

CIsorp water analyzer to determine the optimal storage conditions They found 4 and 5

times higher rehydration index for atmospheric freeze dried cod compared with

naturally dried stockfish In addition they observed the atmospheric freeze dried fish

had brighter color than that of the naturally dried cod

The influence of AFD on the physiochemical properties, quality, and functional

properties (color, water content, bulk density, rehydration properties, sorption isotherm,

specific enzyme activity, solubility, protein denaturizing) of potato was investigated by

Claussen et al (2007) Their results showed that atmospheric freeze drying is a gentle

drying process than spray or vacuum freeze drying The solubility measurement gave

better results for AFD potato protein samples at pH between 3.5 and 5, while the

lowest value was obtained for spray dried samples over the whole pH range Moreover,

both enthalpy measurements and sorption isotherms indicate reduced protein

denaturizing of AFD samples, while specific enzyme activity was at same level for all

dried samples

Drying kinetics, sorption properties, shrinkage, and freezing point depression were

determined by Claussen et al (2007) in atmospheric freeze drying (AFD) of pieces of

apple, turnip cabbage, and cod They observed that drying at -5oC resulted in a greater

shrinkage than drying at –10oC Claussen et al (2007) also carried out measurement of

the physical properties of atmospheric freeze-dried cod and turnip cabbage True

density, apparent density and pore size distribution were measured using helium

pycnometry, geopycnometry and light microscopy They concluded that thawing

during drying and product shrinkage affects the drying rate and the diffusion of water

leading to poor product quality of the end product

2.2.2 Modeling of atmospheric freeze drying

Heldman et al (1974) proposed a simple mathematical model and validated it with

their results experimentally They concluded that the rate of drying, as expected, is

higher for smaller particles and by increasing the surface mass transfer coefficient

Since the drying kinetics for AFD are determined by internal resistance to heat and

mass transfer Their conclusion about effect of external mass transfer coefficient is

surprising

Boeh-Ocansey et al (1983, 1985) reported measurements of kinetics of ice sublimation

in vacuum and in a fluidized bed drying system under atmospheric condition They

showed that for ice sublimation the recommended partial vapor pressure, temperature

of drying chamber and relative humidity of air are: 4.58 mm Hg, approximately 0oC

and below 20 ppm, respectively Since there is freezing point depression with soluble

components in drying material is 0oC really the optimum temperature

A further kinetics study of ice sublimation in a fluidized-bed dryer operating under

atmospheric conditions was reported by Boeh-Ocansey and Wachet (1986)

Mathematical expressions were formulated to account for mass variation and

dimensions of ice samples during sublimation They investigated the influence of

chamber temperature and air flow rate on sublimation and showed that sublimation of

ice was obtained at drying chamber temperatures greater than 0oC (7.6oC maximum)

They also noted that there exists a correlation ice sublimation temperature and the

temperature of the fluidized bed Ice sublimation temperature can be predicted

accurately for a given fluidized-bed temperature Finally they showed that the kinetics

of ice sublimation was regulated by the expression: M/M0 = (1 – t/tT)2 where M and t

represent mass of product and time, respectively Subscript o and T stand for initial

and final time, respectively

Wolf and Gilbert (1990b) proposed a model for atmospheric freeze-drying in a

fluidized bed of a particulate adsorbent (starch) incorporated in different mass ratios

Their model was based on uniformly retreating ice front (URIF) They found spherical

regeneration temperature of adsorbent (50oC) are most advantageous for drying They

also concluded that a mass fraction between the mass of water sublimated and the mass

of adsorbent used (mw / ma ), of the order of 0.10, or even 0.05 at higher temperatures,

seems satisfactory They validated their model with experiments with potato

parallelepipeds of different thicknesses (2, 3 & 5 mm)

Joseph et al (1996) presented two sets of nonlinear coupled heat and mass transfer

models to describe the absorption and desorption process Model I describes the

temperature and moisture distribution in a porous medium with a moving evaporation

front They reported that in the two phase system with moving boundary condition, the

rate of movement of the evaporation front decreased with deepening of the evaporation

front in the porous body They showed that the higher the value of nondimensional

vaporization parameter γ, the slower is the movement of the evaporation front The

temperature decreased and the moisture content increased as the nondimensional

vaporization parameter γ increased In model II they described a set of simultaneous

heat and mass transfer equations describing moisture adsorption during the steeping of

barely kernels

A study of freeze drying, by immersion in an adsorbent medium both at atmospheric

pressure and under vacuum was carried out by Lombrana and Villaran (1996) They

used spherical moistened particles of commercial cereal food paste as a drying product

They employed zeolite as adsorbent particles (0.63g/cm3 density, diameter 0.7 mm)

with adsorbent to product mass ratio of 10:1 in a fluidized bed dryer They evaluated

the effect of pressure and temperature on the drying kinetics through a model by

considering a uniformly retreating ice front in spherical geometry Their model

calculated the pressure and temperature at the sublimations front in terms of the

without adsorbent, were also investigated to analyze the possibility elimination of

adsorbent when vacuum was employed They found that values of total time and

shrinkage varied from 400 to 390 min and from 0.567 to 0.573, respectively, they

concluded that adsorbent usage is recommended Also they proposed some operational

strategies which can reduce the process duration without damage to product quality

First step starts at atmospheric pressure and a temperature of -10oC followed by a

second step at where low pressure with temperature in the range 0oC to 15oC and a

third step at atmospheric pressure with temperature below 15oC

Lombrana and Villaran (1997) developed a mathematical model for AFD in a fluid bed

dryer They evaluated the effect of pressure and temperature on the drying kinetics

through a model by considering a uniformly retreating ice front in spherical geometry

Their model calculated the pressure and temperature in the sublimations front in terms

of the product moisture Good agreement was found between predicted and

experimental results