Bleaching and Purifying Fats and Oils (eBook)

Theory and Practice

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2009 | 2. Auflage
505 Seiten
Elsevier Science (Verlag)
978-0-12-804350-9 (ISBN)

Bleaching and Purifying Fats and Oils

Since the original publication of this book in 1992, the bleaching process has continued to attract the attention of researchers and the edible-oil industry. In this 2nd edition, the reader is directed to more modern techniques of analysis such as flame-atomic adsorption, graphite furnace atomic adsorption, and atomic emission spectrometry involving direct current plasma (DCP) and inductively coupled plasma (ICP). It also discusses the Freundlich Equation and reports on high-temperature water extraction, high- temperature oxidative aqueous regeneration, and extraction with supercritical CO2. Finally, various degumming methods improved over the past several decades are discussed Second edition features the progress in the bleaching and purifying of fats and oils since the mid-1990s Includes extensive details on the adsorptive purification of an oil prior to subsequent steps in the process, including refining and deodorization Offers practical considerations for choosing membranes, filtration equipment, and other key economic consideratons

Chapter 2

Adsorption

H.B.W. Patterson

Physical Adsorption and Chemisorption

Adsorption is a phenomenon wherein the local concentration of a substance at the surface of a solid or liquid becomes greater than the concentration throughout the bulk. We thus have the well-known phenomenon of gas molecules concentrating on charcoal or pigments passing from solution in oil to deposit on clay. By contrast, absorption relates to the uniform penetration and dispersal of one item into another. For example, hydrogen rapidly penetrates and dissolves in palladium, light of a particular wavelength is taken up by a layer of liquid, and gases are selectively removed from an air stream by droplets of a scrubbing liquid.

Thus, solute molecules which reduce surface tension will concentrate at an interface between a solid and a solution, and tend to be adsorbed on the solid. If, as happens in some cases, the solute increases surface tension, the concentration at the interface is diminished; this is known as negative adsorption. Water has a high surface tension; most solutes reduce this, so they are easily taken up by an adsorbent. Alcohol has a substantially lower surface tension than water, so most solutes are less able to lower it appreciably. Hence, they are not as readily adsorbed from an alcoholic solution. This thermodynamic aspect of adsorption is treated in physical chemistry textbooks under the heading “Gibbs adsorption equation.”

When it comes to a question of the adsorption of gases on a solid, two types of forces are operating. Weak forces attracting the gas molecules to the solid surface are seen to be of the same kind as those holding the molecules together when the gas is in the liquid state. Notably, heats of adsorption and heats of condensation are broadly similar and relatively low. These weak forces are referred to as van der Waals forces, and promote a physical adsorption. They are mostly evident at low and moderately low temperatures. Not surprisingly, gases which are most easily adsorbed are also most easily liquefied. In other words, the volumes of gases adsorbed by a set weight of charcoal in moderate conditions such as 15°C and 1 atm are in the same order as their boiling points, with much more chlorine, hydrogen sulfide, or ammonia being adsorbed than carbon monoxide or oxygen. This weak physical adsorption is reversible at the same temperature simply by lowering the pressure.

A second type of adsorption depends on the forces of chemical attraction between the surface and the surrounding gases or solute molecules in a surrounding liquid. This is called chemisorption. It is less common than physical adsorption; when it occurs, the heat of adsorption may be ten or more times greater. It is most evident at moderate temperatures, and to reverse it, a considerable rise in temperature is needed. If a mixture of hydrogen sulfide and oxygen passes over charcoal at room temperature, the hydrogen sulfide is adsorbed much more strongly. This is because the sulfur end of the H2S molecule is more negative than the hydrogen. That is, the molecule is polar, whereas the oxygen molecule is symmetrical and nonpolar. If, however, the temperature is markedly lowered, van der Waals forces come into play, and even nonpolar molecules are adsorbed. Oxygen is strongly adsorbed at 196°C. Furthermore, likely, molecules of a gas such as hydrogen may dissociate to atoms when adsorbed on the surface of metals such as nickel and copper. As is well-known, this condition facilitates their attack on the carbon-carbon double bonds in a surrounding liquid such as an unsaturated oil.

As early as 1908, two mechanisms of adsorption operated together, one being electrochemical (chemisorption) and the other physical adsorption (Glasstone & Lewis, 1963). Seemingly, once chemisorption created a unimolecular layer on the surface available for that effect, the weaker van der Waals forces may make some further addition, depending in part on the concentration of pigment remaining in the solution.

Various investigators approached the bleaching effect of activated clays as being partly an ion-exchange mechanism (Anon., Süd-Chemie). In the bleaching and purifying of fats and oils with adsorbents such as clays, carbon and special silica, van der Waals and chemical forces can play a part. This depends on the adsorbent, the nature of the minor component intended to be removed (the adsorbate), and the conditions of their contact. This brief exposition of the nature of adsorption should make clear that it is a complex phenomenon.

Adsorption Efficiency and Variation

Chemical valence-type attraction falls off rapidly with distance, so it is likely to be responsible for only one layer of adsorbed molecules, whereas the van der Waals forces, though themselves weak, are considered capable of being exerted from the first layer to attract a further layer and so on, to several layers, especially if the temperature is fairly low and pressure rather high. In 1916, Langmuir advanced a mathematical relation which related the extent of the adsorption of a gas to the pressure for any constant temperature (the Langmuir isotherm). Assuming that the layer of adsorbed molecules does not exceed one molecule in thickness, an equilibrium is reached for any particular pressure between molecules evaporating from the surface (related to how much is already covered) and the molecules condensing onto it (related to the unoccupied space left available). At low pressures (lots of unoccupied space), the amount adsorbed per unit mass of adsorbent closely follows the pressure exerted. At the other extreme, as the surface becomes almost completely covered by a unimolecular layer, the amount adsorbed per unit mass approaches a constant limiting value at any temperature as pressure increases. At low temperatures (well below 0°C), this limit is approached gradually; at normal and higher temperatures (above 0°C), this limit is approached quite quickly. This unimolecular layer is a necessary condition for the fulfillment of the Langmuir isotherm. Many instances were found where it is followed closely; where deviations exist, one reason could be the intrusion of the weak van der Waals forces.

In the coverage of the surface which is intermediate between the two extremes just described, an empirical mathematical relationship provides a useful description for practical purposes of the relationship between the extent of adsorption per unit mass of adsorbent and the pressure of surrounding gas molecules or the concentration of solute molecules in a solvent. This is known as the Freundlich isotherm because, although not its inventor, Erwin Freundlich used it frequently to interpret his findings on the adsorption of solutes from solutions. This expression also applies to adsorption from colloidal dispersion. Although different textbooks vary slightly in their terminology, a popular expression of the Freundlich isotherm is

where x = amount of substance adsorbed (adsorbate)

m = amount of adsorbent

c = concentration of substance remaining unabsorbed

K = a constant relating to the general capacity of that adsorbent for that adsorbate

n = a constant relating to the manner in which the efficiency of adsorption changes as it progresses from a higher to lower color. Some authors write the value as 1/n, but regardless of the notation, the numerical value of the index ranges, in effect, between approximately 0.1 and 1.0.

It follows that

x/m = log K + n log C

The implications of this relationship for practical purposes were clearly illustrated (Baldwin, 1949; Hassler & Hagberg, 1939) as shown in Fig. 2.1. Plotted on a log-log scale, the isotherms (within the limits for the test sequences) come close to forming a straight line, as was expected. By convention, the slope of any line is n and the intercept is K. The concentration of pigment is measured in Lovibond-color units. The value of the equation is independent of the particular color measurement chosen, as long as the results are additive.

Fig. 2.1 Bleaching of refined cottonseed oil (25Y-5,5R).

The following implications may be drawn:

1. Where K is larger, less of that adsorbent is needed to achieve the lower color in question. This means the adsorbent tolerates a larger amount of pigment being taken up in lowering the final color (residual unadsorbed pigment). Thus, for a final color of 1.0 in Fig. 2.1, the carbon–earth mixture is rather more efficient than the Fuller’s earth, but both are more efficient than carbon.

2. The index n relates to the rate of change in adsorption efficiency as bleaching progresses. Carbon tolerates a high load of pigment if the final residual color need not be low, but its efficiency drops rapidly as a low final color is sought. This suggests that a modest amount of carbon in a mixture with earth will effectively lower the color as bleaching commences, after which the earth is even better placed to continue the process and reach the required lower level. This means K for the mixture is raised because the whole isotherm is lifted by the early removal of color by the carbon...

Erscheint lt. Verlag	30.3.2009
Sprache	englisch
Themenwelt	Technik ► Lebensmitteltechnologie
ISBN-10	0-12-804350-4 / 0128043504
ISBN-13	978-0-12-804350-9 / 9780128043509

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