Hydrogenation Process Technologies
INTRODUCTION
Hydrogenation is a means of converting liquid oils to semisolid plastic fats suitable
for margarine, shortening, heavy-duty frying fats, and other specialty products (1).
Liquid oils containing an undesirable component can be selectively hydrogenated
to modify the component, followed by a physical separation of the undesirable
component, as in the manufacture of lightly hydrogenated, winterized soybean
salad oil (2–4). Hydrogenation consists of direct addition of hydrogen at the double
bonds in the fatty acid chains of the triacylglycerol, or oils. For hydrogenation to
take place, gaseous hydrogen, liquid oil, and nickel catalyst are placed in a specially
designed reaction vessel under controlled temperature and pressure (5).
By definition, a catalyst is a substance or a compound that alters the speed of
a chemical reaction without becoming a part of the reaction. The catalyst is not
changed in composition or chemical structure. Nickel catalyst is the most common
catalyst currently used in the hydrogenation of fats and oils. Commercially avail-
able nickel catalyst contains 22–25% active catalyst supported by a totally saturated
or completely hydrogenated fat. Other catalyst supports include alumina,
kieselghur, silica, as well as proprietary supports (5). The oil, catalyst, and gaseous
hydrogen mixture is agitated to promote the introduction of hydrogen into the oil
Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.
Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.
and to renew the oil continuously at the catalyst surface. The rate or speed at which
the reaction takes place and the type of product produced to give a particular solid
fat index (SFI) curve, iodine value, and melting point depends on the following
process variables: (1) starting temperature of the oil, (2) activity of the catalyst,
(3) concentration of the catalyst, (4) hydrogen uptake rate, (5) reaction temperature,
(6) oil quality, (7) hydrogen purity, and (8) degree of agitation. When the reaction
is complete and the end point is confirmed, the batch is cooled and filtered to
remove all the catalyst and other impurities. The oil may also be post bleached
with filter-aid and bleaching clay.
Although one may not need to be a chemist or chemical engineer to supervise
or operate the hydrogenation process, some knowledge of the chemistry of fats
and oils is helpful to explain the reaction process in the oxidation and hydrogena-
tion of oils.
The number of double bonds in a fatty ester radical significantly affects both
physical and chemical properties of the triacylglycerol. The highly unsaturated
(three double bonds) linolenic acid (18:3) is unstable to oxidation, and undesirable
odors and flavors can develop. The rate of oxidation 18:3 is 15-fold greater than that
of oleic acid (18:1).
If the linolenic ester is hydrogenated to linoleic (18:2), the relative oxidation rate
is ten-fold greater than that of 18:1. The oxidation rate of 18:1 is ten-fold greater
than that of stearic acid (18:0), and the oxidation potential is completely eliminated
if hydrogenation proceeds to 18:0 (totally saturated, or all double bonds removed).
The relative rates of 18:3 and 18:2 hydrogenation are, respectively, 40-fold and
20-fold greater than that of 18:1. As the degree of hydrogenation (or saturation)
increases, the melting point of the fat increases, as shown in Table 1.
A more stable soybean salad oil (more resistant to oxidation) is manufactured
by selectively hydrogenating the 18:3 ester content from 8% to less than 3%. To
maximize winterizing yields and winterizing performance, it is necessary to
minimize formation of 18:1 and 18:0 esters. Thus, preferential selectivity is needed
to ensure that most of the 18:3 is converted to 18:2, with little conversion of 18:2
to 18:1, and very little 18:1, converted to 18:0 (2).
Lightly hydrogenated, winterized soybean salad oil became popular in the
United States in the early 1960s (6), and all retail salad oil was of this type until
the mid-1980s, when a new Wesson Oil was introduced. This oil, processed by the
Wesson patented process, was stable and had a long shelf life without the need for
hydrogenation. All other manufacturers soon changed to RBD (refined, bleached,
deodorized) salad oil.
The manufacture of lightly hydrogenated, winterized soybean oil led to the
new terms ‘‘selective hydrogenation’’ and ‘‘selectivity catalyst.’’ ‘‘Selective hydro-
genation’’ technically defines the preferential conversion of 18:3 ) 18:2 relative to
18:1 > 18:0. In practical terms, this process reflects the selective removal of double
bonds via hydrogen addition such that saturated fatty acid (stearic) formation is
minimized (7).
‘‘Catalyst selectivity’’ is somewhat meaningless unless the term is defined. There
also are selective catalysts that do not meet the technical or practical definition of
hydrogen selectivity. Such catalysts are sulfur-poisoned catalyst. Sulfided nickel
catalyst produces high trans-isomers, has lower activity than conventional nickel,
exhibits longer reaction times, and is used for specialty applications (e.g., coating
fats and hard butters).
Most unsaturated bonds in vegetable oils naturally occur in the cis-form. During
partial hydrogenation, part of the cis-isomers is changed to trans-isomers. Trans-
isomers have a dramatically higher melting point (42 C) as compared with cis-
isomers (6 C). The creation of trans-isomers is desirable in margarine oil in that
a higher melting point can be achieved without developing a higher level of nutri-
tionally undesirable saturated compounds. Altering hydrogenation conditions to
produce higher (or lower) trans-isomers is termed ‘‘trans-isomer selectivity.’’
Factors influencing cis-trans-isomerization are shown in Table 2.
A typical hydrogenation converter is shown in Figure 1. The converter is the
heart of the complete hydrogenation system. Proper design and maintenance of the
hydrogen gas distributor, the agitator, and the heating cooling coils are mandatory
for optimum productivity and consistency of basestocks produced. Most conver-
ters are 30,000-pound, 40,000-pound, or 60,000-pound batch sizes with some
now as large as 90,000 pounds. The common agitator design provides approxi-
mately 100 rpm, and radial flow impellers are used. The lower impeller is posi-
tioned slightly above the hydrogen gas distributor; therefore, the diameter of the
gas distributor and the tip-to-tip dimension of the lower impeller are critical.
Originally, the middle and top impellers were of the radial flow type also. Some
converters have now been operating for many years with an axial flow impeller
at the top position. Although the lower and middle radial flow impellers are ideally
suited for gas dispersion, the top impeller pumps the oil downward, and if posi-
tioned properly, hydrogen gas in the headspace re-enters the oil. This design has
enhanced the success of dead-end hydrogenation, dramatically reducing the amountof purge or vent gas. These improvements are demonstrated in Figure 2. Other
special agitation and hydrogen distribution systems have been developed, such as
the Buss reactor, and the AGR (Advanced Gas Reactor), but these systems are
falling out of favor because the added maintenance offsets any advantages these
systems were supposed to provide.
Proper hydrogen gas distribution and agitator design is important. Stratification
of reacted and unreacted areas in the converter, as a result of improper agitation or
hydrogen distribution, will add to unpredictability in basestocks from batch to
batch. A complete semicontinuous hydrogenation plant is depicted in Figure 3.
The main features of this system are: (1) a preheating and measuring tank, (2) a
reactor or converter, (3) a drop tank, (4) a heat-recovery system, (5) steam genera-
tion via reactor cooling, and (6) single-step filtration.
By arranging all the vessels for gravity drop, very rapid turnover of batches in
the reactor results. In this manner, the reactor is used for reaction only; all heating,
cooling, and filtration is accomplished external to the reactor. For example, if the
average iodine value (IV) drop for all basestocks produced can be achieved in one
hour, then a single system can deliver 24 batches per day (a 24-hour period).
This system also demonstrates the latest technologies in heat recovery by
heat exchange of the hot oil in the drop tank with the incoming cold oil and by
steam generation for reactor cooling. The hydrogenation department becomes a
net exporter of steam, the ultimate form of energy conservation.
The system depicts improved reactor agitator design and improved automation.
The complete process is controlled by programmable logic controllers (PLCs),
giving precise in-point control that leads to extreme consistency from batch to
batch.