4.1 Freez Conc Flow Dia

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Crystallizer Separator

Freeze Concentration of Liquid Foods

U.I.C.T., August 26, 2004

Freeze Drying and Advanced Food technology S. A. Goldblith, L. Rey and W.W. Rothmayer

Most Liquid foods are very sensitive to thermal treatment. The components of foods arechemically unstable even at moderate temperatures. Product quality and behavioral

properties depend upon hygienic processing with good microbiological control at 5-50

deg. Celsius.

All aromatic compounds are naturally volatile and can be lost by evaporation.Thus, for high quality concentrates following conditions have to be fulfilled:

Low process temperature, short residence time and narrow residence timedistribution in the process apparatus.

Sterile operation

Selective dewatering, all components except water being retained in the concentrate.

Three concentration processes are technically feasible for selective dewatering:

Evaporation (if necessary with aroma recovery)

Reverse osmosis

Freeze concentration

Evaporation:

In evaporation the volatile aromas are lost with the vapor and can

be recovered in a distillation column.

However, the recovery is never above 50% and degradation of

aroma compounds also occurs at higher temperatures. Hence the originalquality is never fully restored.

Technically, however, evaporation is the best-developed processand also the most economical method of concentration.

Reverse Osmosis:

It involves the use of selectively permeable membranes for

removal of water.

The driving force is pressure difference across the

membrane.

However, membranes with acceptable permeabilities are

not very selective and hence the application for dewatering of aromacontaining foods remains restricted to a few products.

Freeze concentration:

Crystal IceSuspension

Feed


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Concentrate

Fig. 1 Schematic representation of a freeze concentration process

In freeze concentration water is first segregated from aqueous solution bycrystallization.

The concentrated liquid is then separated from ice. Fig. 1.

The process is suitable for sensitive liquid foods with volatile aromas.

Since water is removed from the solution by phase transformation from liquid tocrystal, aroma losses by evaporation can be completely avoided.

The process temperature of the order of 3 to 7 deg. C fully eliminates thermaland biochemical decomposition reactions.

Ice crystals of very high purity can be obtained by preventing local supercooling inthe crystallizer.

The loss of dissolved solids and of aromas is then completely controlled by theperfection of the ice-liquid separation.

Ease of separation and especially the capacity of ice liquid separators, increasesharply with decreasing specific area of the crystals. Crystallizer design and process

conditions in the crystallizer must be directed towards the formation of large and if

possible spherical crystals (Of all the geometrical shapes sphere has the least surfacearea).

The crystallization process

The selection of crystallizer depends on the effect of crystallizer on the total cost offreeze concentration process.

The cost includes the losses of dissolved solids and losses in quality due to aromalosses.

The restriction to be imposed on the crystallizer is the required minimum size of thecrystals. This value depends on the process for the separation of crystals and

concentrate.

Theoretical aspects of Crystallization

Water forms a eutectic mixture with dissolved solids at the eutectic temperature.Therefore, water segregates in the form of ice crystals upon cooling, leaving a

concentrated liquid phase. Graph 1 shows experimental freezing curves of a fruit juice, coffee extract and

some sugar solutions.

E.g. by lowering the temperature of apple juice containing 12.5 wt. % dissolvedsolids to an equilibrium temperature of -12.3 deg C the concentration becomes 50wt% and 86 % of the water segregates in the form of ice.


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1. Crystal Growth Rate

In continuously operating crystallizers, the mean size of the crystals is primarily

dependent on

a. dissolved solids concentration (ref. graph 2),b. mean residence time of the crystals,

c. Bulk supercooling and

d. Less dependent on the degree of turbulence in the crystal suspension.

The method of cooling strongly influences the crystal growth rate and

mean crystal size as demonstrated by graph 3.

A lesser than linear effect of residence time on the crystal size can beeffected by decreasing the bulk supercooling with increasing residence time of the

crystals at constant crystal concentration in a given crystallizer (i.e. the growth incrystal size is not a linear function of residence time in the crystallizer, instead,

lesser growth of crystals occurs with increase in residence time when bulk

supercooling is decreased).

The growth rate at low supercooling is directly proportional to bulk

supercooling.

At a given rate of heat withdrawal per unit mass of suspension , highsupercooling needed for high crystal growth rate can only be maintained if the

total area of crystals per unit mass of suspension is low.

Obtaining large crystals at an acceptable residence time dictates low netnucleation rate (i.e. low birth rate of viable crystals).

Combining the above 2 points, if large number of viable crystals are born, the surface

area of the crystals per unit mass of suspension will increase and hence crystal growthrate will no longer be linearly proportional to the degree of supercooling at specified

residence time and rate of heat removal.

2. Net Nucleation rate

The nucleation rate of ice in liquid foods and in sugar solutions increases with

increasing dissolved solids concentration and is proportional to square of bulksupercooling.


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Due to the heat withdrawal from the crystallizer and imperfect mixing of the

suspension, spots with supercooling below the bulk supercooling are formed.

These cold spots contribute much more to nucleation than to crystal growth.

The nucleation rate and the degree of supercooling of the cold spots can be

decreased by increasing the stirring rate in the crystallizer.

Very few nuclei formed in the cold spots survive the mixing process due totheir lower melting temperatures than the larger ones.

Upon mixing a suspension of small and large crystals, a bulk temperature will

be established between the lower melting temperature of the small crystalsand the higher melting temperature of the large crystals.

Since the bulk temperature is higher than the melting temperature of the small

crystals, these will melt.

The large crystals, on the contrary, will grow

(ref. graph 4 that shows effect of crystal diameter on melting temperatures).

The growth of larger crystals at the expense of smaller crystals is known asripening effect.

The size of melting crystals in an adiabatic crystal suspension is called

subcritical and that of the growing ones supercritical.

The melting rate of the subcritical and the growing rate of the supercritical

crystals increase with an increase in size difference between the small and

large crystals.

3. Technical Crystallizers

a. Crystallizers with direct heat removal

b. Crystallizers with indirect heat removal1. Internally cooled crystallizers

2. Externally cooled crystallizers

a. Crystallizers with direct heat removal:

In vacuum freezers for liquid foods the crystallizing solution boils at an

absolute pressure of about 2 mm Hg.Advantages of direct cooling over indirect cooling:

It eliminates cooling wall and consequently the use of expensive scraped

surface heat exchangers.

Very low energy consumption can be achieved by compressing the watervapor from 2 mm Hg to about 7 mm Hg and using the separated ice crystals as

condenser for the compressed vapor.

For large evaporation plants with vapor compression, power requirements aslow as 8 kWh per 1000 Kg separated ice are reported.

Direct cooling has been extensively studied for seawater desalination. The

V.F.V.C. (Vacuum Freezing Vapor Compression) process has not yet beenapplied for freeze concentration of liquid foods.

Disadvantages of direct cooling


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S

TR

IPPE

R 2

The advantages of low capital and energy costs have to be balanced against a

somewhat lower quality of the concentrated product compared with the quality ofconcentrate obtained from indirectly cooled crystallizers.

With the inert gases leaving the vacuum crystallizer, aromas can be lost.

Recovery of Aroma:

For most fruit juices, aroma losses can be recovered by counter-currently stripping thegas-vapor mixture leaving the condenser with the concentrated liquid leaving the ice-

concentrate separator.

If the temperature of the condensers is not too low and part of the non-condensablesleaving the first section of the aroma absorber are re-circulated through the condenser, the

aromas will not be condensed in the condenser and will only be removed from the gas-vapor mixture by the liquid in the aroma absorber.

Wet compressor

Atmospheric

Pressure

Inert

Gases


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2 mm Hg

VacuumCrystallization

Ice

Separator

Concentrated Liquid

Feed

Ice

Flowsheet of vacuum crystallizer with aroma recovery.

b. Crystallizers with Indirect heat removal.

So far, only indirectly cooled crystallizers have found application in the food industry.

There are 2 classes of crystallizers with indirect heat removal:

STR

IP

PER 1

DRY COMPRESSOR

CONDENSER

Water Vapor +

aroma

Inerts +Aroma

Concentrate+ aroma


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i. Withdrawal of heat through the wall of the crystallizer (Internally cooled

crystallizers).

ii. Heat removal from the feed stream to the crystallizer externally (Externallycooled crystallizers).

1. Internally cooled crystallizersThis class can be divided into crystallizers producing almost completely solidified

suspension and a group producing pumpable slurries.

i. In crystallizers of the first group the liquid is, without stirring,contacted with a refrigerated wall, and upon complete solidification, it

is mechanically removed in solid slabs as a snow.

ii. In this way, even very dilute solutions can be concentrated in one step

to 40 wt.% or more.iii. However, since the crystals are very small, the liquid-ice separation

poses certain difficulties.

iv. Example of this type of crystallizer is the Linde-Krause crystallizer.

v. It consists of a large stainless steel drum internally cooled and rotatingin a trough of the liquid to be concentrated. The solid layer is removed

continuously by a scraper knife.vi. In another version, the liquid is sprayed onto a slowly revolving drum

or onto internally refrigerated discs, and is removed as flaked ice.

Most internally cooled crystallizers belong to the second group and producepumpable suspensions. The crystal suspension remains only a few minutes in the

crystallizer.

a. The crystal suspension remains only a few minutes in thecrystallizer. Because of the short residence time the product crystals are

small, less than 50 microns.

b. In the votator process, the crystals are produced in horizontalscraped surface heat exchangers.

c. Short residence times are also employed in the Gasquet process.

Their horizontal scraped surface tubular heat exchanger has an insidediameter of 14 inches.

d. Longer residence times, upto several hours, are taken in the

Union Carbide Process which involves the production of large crystals by

using a cascade array of crystallizers-separators.e. Struthers advocates the use of stirred but non-scraped indirectly

cooled crystallizers. Large crystals are obtained by closely controlling

temperature, driving force, turbulence and crystal concentration.f. Daubron employs vertical ammonia jacketed scraped cylindrical

vessels. The residence time of the crystals is about 3 h. The nuclei are

formed on the cooled wall, and the crystals grow in the slowly stirred bulkof the suspension. An advantage of this process is simplicity.

g. In yet another process advantage is taken of the ripening effect.

The crystals formed in the heat exchanger are recirculated through a

holding or ripening tank. A total residence time of 1.3 h gives sphericalcrystals with a size ranging from 0.4 to 2 mm while concentrating beer

from 3.2 wt.% to 6.0 wt.% alcohol.


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2. Externally cooled crystallizers

This class of crystallizers can be subdivided into three main types.i. Supercooling of the feed stream in a heat exchanger before it enters the

crystallizer.

ii. Recirculation of the whole suspension from the crystallizer to the heatexchanger.

iii. Crystallizers producing subcritical crystals in an external heat

exchanger.

a. In the first type, the feed stream is supercooled in a heat exchanger

before it enters the crystallizer. The supercooled crystal free liquid

releases its cold in the crystallizer.

Strong supercoolings upto 6 deg C appear to be feasible.

The wall of the heat exchanger is highly polished or coated

with a hydrophobic plastic to minimize the chance of primary

nucleation and crystallization in the heat exchanger. Ifcrystallization occurs in the heat exchanger it may block the liquid

path in the heat exchanger.

Using this type of crystallizer, local supercoolings in the

crystallizer can virtually be suppressed.

In 30% wt.% glucose solutions, crystals of about 0.2 mmare obtained with a mean residence time in the crystallizer of only

about 0.5h.

The liquid from the crystallizer can be recycled to the heatexchanger. The crystals are retained in the crystallizer by means of

a filter in the suction line.

b. In the second type, the whole suspension from the crystallizer isrecirculated to the heat exchanger.

Residence time in the heat exchanger is shorter than in the

crystallizer.

Thus the crystal growth occurs mainly in the crystallizer.

Lurgi (a firm) uses a scraped surface heat exchanger. The

slurry leaving the heat exchanger is gently mixed with the

suspension in the crystallizer.

Sepial uses a cascade of two internally cooled crystallizers.

The majority of the ice crystals are formed in the first crystallizer,

and the crystals are grown to the desired dimensions in the second

stage.c. The third type of crystallizer produces subcritical crystals in an

external heat exchanger (ref. fig. 3).

Crystal free liquid is partly recirculated from thecrystallizer to the heat exchanger.

The heat flux through the wall of the scraped heatexchanger is taken very high which results in a very strong

nucleation.


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The residence time of the crystals in the heat exchanger is

only a few seconds. The extremely small crystals produced in theheat exchanger are fed continuously to the crystallizer, where the

residence time is at least half an hour.

Due to the long residence time in the crystallizer the bulk

supercooling is small, amounting to less than 0.02 deg C. The larger crystals grow at the expense of the melting of

the small crystals. The heat of melting of the small feed crystals is

consumed by the growing large crystals.

An increase in the nucleation rate in the heat exchanger

causes an increase of the mean diameter of the product crystals.

At a residence time of about 2h, spherical crystals of about1mm are grown from 40 wt% sucrose solution.

4. Ice-Concentrate Separators

1. Presses2. Centrifuges3. Wash Columns

4. Combination of presses and wash columns

The cost of freeze concentration is strongly affected by the fractional loss ofdissolved solids with the ice. The separation, or least partial separation, can be

performed in presses, filtering centrifuges, wash columns, or a combination of two of

these devices.In all separators

the capacity is inversely proportional to the viscosity of the liquid

the capacity is directly proportional to the square of the meandiameter of the crystals.

Presses:

Hydraulic piston presses and screw presses are used in the separation process.

The loss of dissolved solids is determined by the amount of liquid that

remains occluded in the compressed ice cake.

At pressures upto 10 kg/cm2, about 0.65 kg remains occluded per kg ice.

At pressures above 100 Kg/cm2 and with long compression times, the amount

of occluded liquid can be reduced to about 0.1 kg/kg ice.

Because of these high losses, presses can only be used to increase the icecontent of the slurry before final complete separation of ice and liquid is

effected.

Centrifuges:

The dissolved solids losses in centrifuges are lesser than those in presses.

The filter cake in the centrifuge can be washed with water or crystal melt. The

wash water will of course dilute the concentrate.


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Depending on the crystal size and liquid viscosity the loss can be as high as 10%

of the solids in the feed.

Volatile aromas are lost when the liquid is spun off from the filter cake. Theliquid is thereby brought into intimate contact with excess of air.

Wash Columns:

Wash columns facilitate perfect separation of ice and liquid with out any dilution (ref. fig.

4).

Wash columns are completely closed and operate without a gas headspace.

Hence, aroma losses are almost negligible.

In continuously operating wash columns the crystal suspension is fed in at oneend of the column.

The concentrated liquid is removed from the column through a filter near the

same end by expressing the slurry.

The compact bed of crystals is forced to move towards the other end of the

column where the crystals are removed. In the column the crystals are washed countercurrently with the wash liquid

obtained by melting part of the purified crystals leaving the column.

The depth of the region between the part of the bed with still undiluted liquid and

the part of the bed with purified crystals is only a few centimeters.

In case of stable displacement of a concentrated liquid the concentration of

dissolved solids in the melt of the ice crystals leaving the columns is generally

lesser than 10 ppm.

Combination of press and wash column (ref. fig. 5):

It is the most economic process.

The concentrated slurry leaving the crystallizer is partly separated in a press.

The ice cake still containing about 40 wt% occluded concentrate is then

dispersed in the feed to the system.

The diluted ice suspension is completely separated in the wash column.

The liquid separated from the ice in the wash column is fed to the crystallizer.


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5. Conclusions:

a. Food liquids can be concentrated economically up to 40-50

wt% dissolved solids in one stage.b. The maximum obtainable concentration is controlled by the

viscosity of the concentrated product at freezing

temperature.c. Higher concentration requires an extra freezing step and

thus the cost of concentrate increases.

d. The superior quality of freeze concentrated foods has to bepaid for by higher dewatering costs, being about a factor of

2 higher than the dewatering costs of evaporation with

aroma recovery.

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4.1 Freez Conc Flow Dia