Effects Of Process Conditions

When hydrogenating a specific oil with a chosen catalyst, the reaction parameters are temperature, pressure, catalyst concentration, and agitation (mixing). Because each parameter influences both reaction rate and selectivity, and because all are interrelated, it is not possible to spe cifically predict what their combined effect will be in a particular instance. Therefore, each of the parameters will be discussed separately, as they affect reaction rate and as they affect selectivity.

Effects of Temperature

Hydrogenation, like other chemical reactions, is accelerated by increasing temperature. Both preferential selectivity and isomerization are also greater with increasing temperature. Since there is an increasing tendency toward hydrolysis above 400°F (204°C), processors limit their upper temperature accordingly, with 450°F (232°C) being a maximum. Since some investigators claim that nutritionally undesirable positional isomers are also formed at high temperatures, a limitation as low as 400°F (204°C) is sometimes observed.

Effects of Pressure

Again, as with most chemical reactions, increasing pressure increases reaction rate. Commercial hydrogenation of edible triglyceride oils is usually performed tinder hydrogen pressure of 7 to 50 psig (0.5-3.5 atm)—at the lower end for partial hardening, which constitutes most of the commercial hydrogenation of edible oils such as soybean, canola, and so on. Within this lower range, even modest change in pressure has a significant effect on the inverse relationship of increasing reaction rate/decreasing selectivity—both preferential and trans isomer.

Effects of Catalyst Concentration

Increasing the catalyst concentration in a given system increases reaction rate up to the point where hydrogen availability becomes the limiting parameter. It has a minor effect on selectivity. The catalyst concentration employed in commercial partial hydrogenation of edible oils is chosen so that the converter reaction time fits into the other batch cycle steps of heat exchange and filtration for catalyst removal.

Effects of Agitation

Agitation (mixing) is of great importance in determining both the rate and selectivities (preferential and transisomeric) of edible oil hydrogenation. In general, better mixing increases activity and decreases selectivity. Agitation must accomplish the following: (1) distribute heat or cooling for temperature control; (2) keep the solid catalyst suspended; (3) solubilize and maintain the solution of hydrogen in the oil.

Whereas (1) and (2) are straightforward and quite easily achieved, (3) is complex. In conventional batch-operated tank-type hydrogenation reactors (Fig. 1), hydrogen is bubbled into the liquid through a spider-type gas distributor at the bottom of the converter. While the hydrogen to be solubilized comes principally from bubbles absorbed during their passage up through the oil from the spider, it partially comes from gas in the headspace that is continually being stirred back into the oil. Mixing in such a converter as shown in Figure 1 is provided by two or more turbine blades attached to a central shaft. Individual

Turbine agitator

Heating and cooling coil

Turbine agitator

Heating and cooling coil

Baffle

Gas distributor

Baffle

Gas distributor

Figure 1. Conventional hydrogénation reactor. Source: Réf. 1.

blades may be flat/perpendicular, flat/canted, or foil shaped, depending on whether their purpose is to break the hydrogen into small bubbles, retard the flow of the bubbles upward, or create a vortex to reincorporate headspace hydrogen into the oil. Positioning of the blades on the shaft is very important. The rotation speed of the agitators, design and location of baffles on the side of the tank, and placement of heating/cooling coils also affect the flow of hydrogen bubbles passing through the oil and, consequently, the absorption of hydrogen into the oil. Mixers specially designed to better reincorporate hydrogen from the headspace into the body of the oil are gaining acceptance. They provide a more constant supply of hydrogen at the catalyst site, thus increasing the rate of reaction and also making it less variable. This is a significant aid in achieving uniform selectivity, which results in greater product uniformity. Three agitation systems designed to achieve this are depicted in Figures 2, 3, and 4.

The activity and selectivity effects of varying hydrogénation process conditions are summarized in Table 1.

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