libro - encapsulados y alimentos en polvo.pdf

8/11/2019 LIBRO - Encapsulados y alimentos en Polvo.pdf

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ENPO: dk2963_c004 2005/1/6 22:36 page 130 #58

130 Enrique Ortega-Rivas

(a) (b)

Feed

Screw

Roll

Product

Screw

RollFeed

Product

Figure 33 Equipment used for high-pressure agglomeration: (a) compacting roller press,(b) briquetting roller press.

shaping, extrusion has been extensively used in grain process engineering to obtain an

array of products from diverse cereals, principally ready-to-eat breakfast cereals.

C. Mixing

The unit operation in which two or more materials are interspersed in space with one

another is one of the oldest and yet one of the least understood of the unit operations

of process engineering. Mixing is used in the food industry with the main objective ofreducing differences in properties such as concentration, color, texture, taste, and so on,

between different parts of a system. Since the components being mixed can exist in any of

the three states of matter, a number of mixing possibilities arise. The mixing cases involving

a fluid, for example, liquidliquid and solidliquid, are most frequently encountered, so

they have been extensively studied. Despite the importance of the mixing of particulate

materials in many processing areas, fundamental work of real value for either designers

or users of solids mixing equipment is still relatively sparse. It is through studies in very

specific fields, such as powder technology and multiphase flow, that important advances in

the understanding of mixing of solids and pastes have been made.

Mixing is more difficult to define and evaluate with powders and particulates than it

is with fluids, but some quantitative measures of dry solids mixing may aid in evaluating

mixer performance. In actual practice, however, the proof of a mixer is in the properties

of the mixed material it produces. A significant proportion of research efforts in the foodindustry is directed toward the development of new and novel mixing devices for food

materials. These devices may be effective for many applications since they deliver a mixed

product with the required blending characteristics. Due to the complex properties of food

systems, which can themselves vary during mixing, it is extremely difficult to generalize

or standardize mixing operation for wider applications of mixing devices, either novel or

traditional. Developments in mathematical modeling of food-mixing processes are scarce

and there are no established procedures for process design and scale-up. As a result, it is


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Handling and Processing of Food Powders and Particulates 131

virtually impossible to devise relationships between mixing and quality (Niranjan, 1995),

especially for blending of food powders. With reference to solid foods, Niranjan and de

Alwis (1993) mentioned as characteristic features of food mixing, the fragile and differently

sized nature of food products, as well as the segregating tendency of blended food systems

on discharge. Thesecharacteristics, along with someothers likecohesivenessand stickiness,

makes food particulate mixing a complicated operation.

1. Mixing Mechanisms

Three mechanisms have been recognized in solids mixing: convection, diffusion, and shear.

In any particular process one or more of these three basic mechanisms may be responsible

for the course of the operation. In convective mixing masses or group of particles transfer

from one location to another, in diffusion mixing individual particles are distributed over

a surface developed within the mixture, whereas in shear mixing groups of particles are

mixed through the formation of slipping planes that develop within the mass of the mixture.

Shear mixing is sometimes considered as part of a convective mechanism. Pure diffusion,

when feasible, is highlyeffective, producingvery intimate mixtures at thelevel of individual

particles but at an exceedingly slow rate. Pure convection, on the other hand, is much morerapid but tends to be less effective, leading to a final mixture that may still exhibit poor

mixing characteristics on a fine scale. These features of diffusion and convective mixing

mechanisms suggest that an effective operation may be achieved by a combination of both,

in order to take advantage of the speed of convection and the effectiveness of diffusion.

Compared with fluid mixing, in which diffusion can be normally regarded as being

spontaneous, particulate systems will only mix as a result of mechanical agitation provided

by shaking, tumbling, vibration, or any other mechanical mean. Mechanical agitation will

provide conditions for the particles to change their relative positions either collectively or

individually. The movement of particles during a mixing operation, however, can also result

in another mechanism, which may retard, or even reverse, the mixing process and is known

as segregation. When particles differing in physical properties, particularly size and/or

density are mixed, mixing is accompanied by a tendency to unmix. Thus, in any mixing

operation, mixing and demixing may occur concurrently and the intimacy of the resulting

mix depends on the predominance of the former mechanism over the latter. Apart from the

properties already mentioned, surface properties, flow characteristics, friability, moisture

content, and the tendency to cluster or agglomerate, may also in fluence the tendency to

segregate. The closer the ingredients are in size, shape, and density, the easier the mixing

operation and the more the intimacy of the final mix. Once the mixing and demixing

mechanisms reach a state of equilibrium, the condition of the final mix is determined and

further mixing will not produce a better result.

A general theory of segregation, regardless of the particular circumstances in which

the operation takes place, has not yet been offered to explain the segregation phenomena in

particulate systems. In any blending operation the mixing and demixing mechanisms will

be acting simultaneously. The participation of each of these two sets of mechanisms will be

dictated by the environment and by the tendency of each component to segregate out of thesystem. Since these two mentioned sets of mechanisms will be acting against each other,

an equilibrium level will be obtained as the final state of the mixture.

2. Degree of Mixing

Over the years many workers have attempted to establish criteria for the completeness and

degree of mixture. In order to accomplish this, very frequent sampling of the mix is usually

required and, tending to be statistical in nature, such an exercise is often of more interest


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to mathematicians than to process engineers. Thus, in practical mixing applications, an

ideal mixture may be regarded as the one produced at minimum cost and which satis fies

the product specifications at the point of use.

Food mixing is a complicated task not easily described by mathematical modeling.

Mixture quality results from several complex mechanisms operating in parallel, which are

hard to follow and fit to a particular model. Dankwertz (1952) has de fined the scale and

intensity of segregation as the quantities necessary to characterize a mixture. The scale of

segregation is a description of unmixed components, while the intensity of segregation is

a measure of the standard deviation of composition from the mean, taken over all points

in the mixture. In practice it is difficult to determine these parameters, since they require

concentration data from a large number of points within the system. They provide, however,

a sound theoretical basis for assessing mixture quality. Taking into account the complexity

of components and interactions in food solids mixing, it would be rather difficult to define

a unique criterion to assess mixture quality. A mixing endpoint or optimum mixing time

can also be considered a very relative definition due to the segregating tendency of food

powder mixing. The degree of uniformity of a mixed product may be measured by analysis

of a number of spot samples. Food powder mixers act on two or more separate materials tointermingle them. Once a material is randomly distributed through another, mixing may be

considered to be complete. Based on these concepts, the well-known statistical parameters,

mean and standard deviation of component concentration, can be used to characterize the

state of a mixture. If spot samples are taken at random from a mixture and analyzed, the

standard deviation of the analyses s, about the average value of the fraction of a speci fic

powderxis estimated by the following relation:

s =N

i=1(xi x)2N 1 (42)

wherexiis every measured value of fraction of one powder andNis the number of samples.

The standard deviation value on its own may be meaningless, unless it can be checked

against limiting values of either complete segregation s0, or complete randomization sr.

The minimum standard deviation attainable with any mixture is sr and it represents the best

possible mixture. Furthermore, if a mixture is stochastically ordered, sr would equal zero.

Based on these limiting values of standard deviations, Lacey (1954) defined a mixing index

M1as follows:

M1=s20 s2s20 s2r

(43)

The numerator in Equation (43) would be an indicator of how much mixing has

occurred, while the denominator would show how much mixing can occur. In practice,

however, the values ofs, even for a very poor mixture, lie much closer to sr than to s0. Poole

et al. (1964), suggested an alternative mixing index, that is:

M2=s

sr(44)

Equation (44) clearly indicates that for efficient mixing or increasing randomization

M2 would approach unity. The values ofs0 and s can be determined theoretically. These

values would be dependent on the number of components and their size distributions.

Simple expressions can be derived for two-componentsystems, while for a binary multisized


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particulate mixture Poole et al. (1964) demonstrated that:

s2r=pq

w/(q(fawa)p +p(fawa)q)

(45)

wherepandq are the proportions by weight of components within a total sample weight w

andfais the size fraction of one component of average weight wain a particle size range. For

a given component in a multicomponent and multisized particulate system, Stange (1963)

presented an expression forsr, as follows:

s2r=p2

w

1 p

p

2p

fawa

p+ q

fawa

q+ r

fawa

r+

(46)

Equations (43) and (46) can be used to calculate mixing indices defined by

Equation (42). Another suggestion for the characterization of the degree of homogeneity

in mixing of powders, has been reviewed by Boss (1986), with the degree of mixing M3defined as:

M3= 1 s

s0(47)

Some other mixing indices have been reviewed by Fan and Wang (1975).

McCabe et al. (1992), presented the following relationship to evaluate mixing timet

for solids blending:

t= 1k

ln1 1/n1 1/M2

(48)

wherekis a constant andn is the number of particles in a spot sample. Equation (48) can

be used to calculate the time required for any required degree of mixing, provided k is

known and the segregating forces are not active. Mixing times should not be very long due

to the unavoidable segregation nature of most food solids mixtures. Instead of improving

efficiency, long mixing times often result in poor blending characteristics. A graph of the

degree of mixing versus time is recommended to select the proper mixing time quantitat-

ively. Most cases of mixing of powders will attain maximum degree of homogeneity in less

than 15 min when the proper type of machine and working capacity have been chosen.

3. Powder Mixers

In general terms, mixers for dry solids have nothing to do with mixers involving a liquid

phase. According to the mixing mechanisms previously discussed, solids mixers can be clas-

sified into two groups: segregating mixers and nonsegregating mixers. The former operate

mainly by a diffusive mechanism while the latter practically involve a convective mech-

anism. Segregating mixers are normally nonimpeller type units, such as tumbler mixers,

whereas nonsegregating mixers may include screws, blades, and ploughs in their designs,

and examples include horizontal trough mixers and vertical screw mixers. Food powders

can also be mixed by aeration using a fluidized bed. The resulting turbulence of passingair through a bed of particulate material causes material to blend. Mixing times required in

fluidized beds are significantly lower than those required in conventional powder mixers.

Van Deemter (1985) discussed different mixing mechanisms prevailing in fluidized beds.

Tumbler mixers operate by tumbling the mass of solids inside a revolving vessel.

These vessels take various forms, such as those illustrated in Figure 34, which may be

fitted with baffles or stays to improve their performance. The shells rotate at variable speeds

having values up to 100 r/min with working capacities around 50 to 60% of the total. They


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(a) (b)

(c) (d)

Figure 34 Tumbler mixers used in food powder blending: (a) horizontal cylinder, (b) double cone,(c) V-cone, (d) Y-cone.

are manufactured using a wide variety of materials, including stainless steel. This type of

equipmentis best suited for gentle blending of powders with similar physical characteristics.

Segregation can represent a problem if particles vary, particularly in size and shape.

Horizontal trough mixers consist of a semicylindrical horizontal vessel in which one

or more rotating devices are located. For simple operations single or twin screw conveyors

are appropriate and one passage through such a system may be good enough. For more

demanding duties a ribbon mixer, like the one shown in Figure 35, may be used. A typical

design of a ribbon mixer will consist of two counteracting ribbons mounted on the same

shaft. One moves the solids slowly in one direction while the other moves it quickly in

the opposite direction. There is a resultant movement of solids in one direction, so the

equipment can be used as a continuous mixer. Some other types of ribbon mixers operate

on a batch basis. In these designs troughs may be closed, as to minimize dust hazard, or

may be jacketed to allow temperature control. Due to small clearance between the ribbon

and the trough wall, these kinds of mixers can cause particle damage and may consume

high amounts of power.

In vertical screw mixers a rotating vertical screw is located in a cylindrical or cone-

shaped vessel. The screw may be mounted centrally in the vessel or may rotate or orbit

around the central axis of the vessel near the wall. Such mixers are schematically shownin Figure 36(a) and 36(b) respectively. The latter arrangement is more effective and stag-

nant layers near the wall are eliminated. Vertical screw mixers are quick, efficient, and

particularly useful for mixing small quantities of additives into large masses of material.

4. Applications

Applications of powder mixing in food systems are diverse and varied and include

blending of grains prior to milling, blending of flours and incorporation of additives to


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Feed

Product

Figure 35 Plain view of an open ribbon mixer.

(a) (b)

Figure 36 Vertical screw mixers: (a) central screw, (b) orbiting screw.

flours, preparation of custard powders and cake mixes, blending of soup mixes, blending of

spice mixes, incorporation of additives in dried products, preparation of baby formula, etc.

D. Cyclonic Separations

Separation techniques are involved in a great number of processing industries and repres-

ent, in many cases, the everyday problem of a practicing engineer. In spite of this, the

topic is normally not covered efficiently and sufficiently in higher education curricula of

some engineering programs, mainly because its theoretical principles deal with a number

of subjects ranging from physics principles to appliedfluid mechanics. In recent years, sep-

aration techniques involving solids have been considered in the general interest of powder

and particle technology, as many of these separations involve removal of discrete particles

or droplets from a fluid stream.

Separation techniques are defined as those operations, that isolate specific ingredients

of a mixture without a chemical reaction being carried out. Several criteria have been used

to classify or categorize separation techniques. One such criteria consists in grouping them

according to the phases involved, that is, solid with liquid, solid with solid, liquid with

liquid, etc. A classification based on this criterion is shown in Table 18. Dry separationtechniques would, therefore, constitute all those cases in which the particle to be isolated

or segregated from a mixture is not wet, and would include particular examples of the

solid mixtures and gassolid mixture cases listed in Table 18. The most important dry

separation techniques in processing industries have been reviewed by Beddow (1981). In

food processing, there are important applications of dry separation techniques, such as the

removal of particles from dust-laden air in milling operations or the recovery of the dried

product in spray dehydration.

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