Hydrogenation of Fats and Oils (eBook)

Theory and Practice

Jerry W. King, Gary R. List (Herausgeber)

eBook Download: EPUB

2015 | 2. Auflage
384 Seiten
Elsevier Science (Verlag)
978-0-12-804349-3 (ISBN)

Infos

Hydrogenation of Fats and Oils

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As in the first edition, discussion is not confined to vegetable oils, and the hydrogenation technique is considered in detail. The "e;why"e; as well as the "e;how"e; of hydrogenation are addressed. Written for both production staff who need advice on specific problems and development personnel who seek directions, if not solutions, the book offers direct practical advice along with explanations of why changes occur as they do. The glossary of technical terms contains a more detailed explanation of some features mentioned throughout the text. Emphasizes techniques for trans fatty acid reduction or complete removal in food products Features extensive information on hydrogenation methods, isomer formation, and catalysts used Includes an extensive glossary of hydrogenation and related technical terms

Textauszug

Chapter 2

Hydrogenation Process Techniques

H.B.W. Patterson

Requirements

In the previous chapter, the nature of the hydrogenation reaction and the effect on it of the usual process variables, including type of catalyst, were described. This knowledge was adapted over the years as a more accurate and intimate understanding of the reaction was acquired to achieve a variety of requirements. The need for a product stable to atmospheric oxidation—and hence, able to maintain a desirable flavor (in some cases over long periods in arduous climatic conditions)—has come to the forefront. This is allied to a whole range of requirements for texture according to the intended use of the product. The requirements can often be met simply by hydrogenation; occasionally, the result of hydrogenation needs to be corrected or adjusted by a further modification technique—such as the removal of a small amount of high melting component by winterization, or the redistribution of a saturated group by interesterification of the hardened oil alone or in mixture with another oil—finally, the physical blending of the hardened oil with other soft and hardened oils to achieve a required texture over a temperature range is as old as the process itself.

In this chapter are described the conditions commonly employed to obtain various textures as closely as possible by means of hydrogenation alone and always consistent with flavor stability. These textures range from the near liquid at ambient temperatures to the firm, brittle solid. For several products, a compromise has to be devised between factors pulling in opposing directions; in these instances, the compromise may mean that for the first part of the hydrogenation one condition is used; then, before completion, an abrupt change is made, such as deliberately raising the temperature. Such a maneuver is not illogical since by the time the reaction has reached the half-way stage, we may be hydrogenating much changed material for which different conditions may not merely be tolerable but positively advantageous.

Under process conditions, one is justifiable to say something about batch and continuous operations. Traditionally, hydrogenation-process techniques were evolved on a basis of batch operation. For continuous operation, at least two kinds of consideration are involved: firstly, the possibility of readily obtaining the reaction conditions required to obtain an optimal technical specification for the product; in practice, this amounts to saying “Can the conditions of a continuous process be made sufficiently selective?”—if indeed that is an important consideration for the reaction in question. Secondly, the possibility of the production program for a particular product being sufficiently constant to afford economical uninterrupted operations of sufficient length from reasonably consistent feedstocks. A changeover from one product to another should not give rise to an amount of intermediate product which is embarrassing (i.e., the holdup must be kept to a minimum). If these conditions can be met, then, like other operators of continuous as against batch processes, the hardener is well-situated to enjoy the advantages of a savings in space, services, and labor; a more consistent quality is likely to be obtainable, especially where runs on one product extend to 24 hours or more; and because the operating unit is considerably smaller than its batch counterpart, its fabrication in more costly construction material—if this is considered desirable—becomes more economically attractive. Having said this, nevertheless remains the case that the great majority in the industry operate batch plants for triglyceride hydrogenation, and little indication is shown that this situation is about to change (Albright, 1967; Coenen, 1976; Hastert, 1981; Snyder et al., 1978). The continuous hydrogenation of fatty acids where isomerization and selectivity are less important is, on the other hand, quite popular.

A thorough cleansing of the crude oil, most often by degumming (if necessary), alkali neutralization, washing, drying, and bleaching lays the best foundation for the attainment of a hydrogenated-oil specification. This is especially the case where exists the matter of the selective removal of nearly all polyunsaturates with a minimal formation of isomers or saturated fatty acids. Particular requirements when cleansing individual crude oils prior to hydrogenation are described as each oil is considered in turn in Chapter 8. A clean, dry hydrogen is the obvious complement to clean, dry oil, and this is treated in detail in Chapter 5. Happily, the situation here has improved steadily since the 1960s in the sense that although electrolytic hydrogen of very high purity was used by some operators since the earliest days of hardening, the alternatives, which were frequently appreciably cheaper, have improved greatly in quality as the technology of hydrogen production via the steam reforming of hydrocarbons has advanced.

The fall in color during hydrogenation has always been a welcome bonus. Inevitably, if earlier process steps such as earth bleaching have lightened the color, the change due to hydrogenation will be less dramatic. Provided catalyst poisons are adequately removed, management must evaluate how heavy (in color) a bleach is really necessary if subsequent hydrogenation will itself remove much color. Obtaining the optimal effect with the greatest overall economy is the important consideration. Obviously, oils which were physically refined (i.e., free-fatty-acid-stripped by steam under reduced pressure) instead of alkali-refined are perfectly suitable for hydrogenation, since the removal of gummy material from the crude oil—normally entailing adsorptive cleansing by contact with acid-activated earth—which precedes the high-temperature steam stripping greatly reduces catalyst poisons. Again, as alkali refining has become more efficient via the use of centrifuges, this step alone, combined with washing and drying, may suffice as a pretreatment prior to hardening in a good number of cases. Do not forget that after hydrogenation a post-treatment stage follows. Hence, several ways to achieve an overall cost-savings present themselves; just how far one can take them is a matter for trial in the plant.

Batch Hydrogenation—Dead-End and Circulating

As stated previously, this is the traditional technique used in the fat-hardening industry, although several designs for continuous operation were put forward and a few operated within a decade of Normann’s patent (Albright, 1967; Coenen, 1976; Hastert, 1981; Leuteritz, 1969; Snyder et al., 1978; Swern, 1964). The heart of the batch process is naturally the actual gassing time which often accounts for from one-third to one-half of the total cycle time. During this period, the temperature is certainly controlled, and may be deliberately varied according to a previously established pattern. The same is substantially true for pressure, at least in the case of so-called “dead-end” systems, that is, where hydrogen is simply fed to an internally agitated system and more can enter only as some is taken into combination by the oil. The alternative is the circulating system where the bulk of the hydrogen is passed through the oil several times, a proportion being absorbed each time; this is considered later. If pressure is to be varied, control is exercised via the gas-inlet valve situated on the gas line from the high-pressure gas store which itself is maintained at perhaps 7–10 atm for autoclaves operating up to 5 atm. Only a few plants have the facility for switching to an auxiliary high-pressure store (say, 20 atm) when their autoclaves (dead-end) are equipped to work at 10 atm. In the case of fatty-acid hardening, the normal operating pressure is likely to be in the 20–30 atm range in any case. Outside of the reaction time, the operations of filling, catalyst addition (which may be simultaneous or immediately following), preheating, final cooling, and then filtering have to be completed. In many designs, certain of these were divorced from the autoclave itself so that one may employ it for a higher proportion of its time on its essential function. Hence, the preheating may be largely accomplished as the oil enters the autoclave, through some form of heat exchange. Similarly, on the completion of hydrogenation, the oil batch may be dropped in its entirety to a receiving tank (drop tank).

Such a maneuver releases the autoclave as rapidly as possible for refilling. The cool, unhardened oil on its way to refill the autoclave may be passed through a coil in the drop tank to accomplish the preheating mentioned earlier. In some cases, the proposal was even made to discharge the hot, hardened oil from the autoclave via a heat exchanger to a filter while cool, unhardened oil passes through the other side of the heat exchanger to a second autoclave. As energy grows more costly, means of saving it will be studied more closely, and at the same time higher capital expenditure to ensure economy in its use becomes worthwhile. At the same time, remember that when numerous hydrogenation tasks are required of the plant, cycle times will vary, and therefore one should allow some flexibility in successive movements. One must therefore compare capital expenditure with energy saved and increased autoclave utilization in a realistic way; much depends on the complexity of the work program. One must provide an autoclave with cooling...

Produkt-Details

• Erscheint lt. Verlag: 9.12.2015
• Sprache: englisch
• Themenwelt: Technik ► Lebensmitteltechnologie
• ISBN-10: 0-12-804349-0 / 0128043490
• ISBN-13: 978-0-12-804349-3 / 9780128043493