Hydrogenation - Process and Product Quality Control

Hydrogen is a very complicated process as each unsaturated bonds in each chain may be hydrogenated at difference rate depending on the position of the double bond. During hydrogenation of fats and oils, three important reactions occur simultaneously namely saturation of double bonds, cis-trans isomerization of double bonds and migration of double bonds to new positions in the fatty acid carbon chain. The relative rates of each of the reaction can be controlled by the reaction conditions.

            There are 2 important parameters in a hydrogenation process namely activity and selectivity. Activity usually refers to the rate of reaction which means that how fast the hydrogenation reaction can proceed. On the other hand, selectivity defines as the relative rate of hydrogenation of the more unsaturated fatty acids when compared with that of the less saturated acids. The conditions used in hydrogenation process is the balance between the process quality (Trans fat, selectivity and activity) and the production cost (cost of catalyst, the apparatus for use of high temperature and pressure). When a catalyst is chosen to hydrogenate specific oil, the reaction parameters are temperature, pressure, catalyst concentration and agitation rate. All the parameter must be properly controlled because they influence both the rate of the reaction and the trans isomer selectivity.

            Trans isomerization occurs when the reaction is carried out at high reaction temperature, low pressure, low catalyst dosage and slow agitation rate. On the other hand, high reaction rate can be achieved by using high reaction temperature, high pressure, high catalyst concentration and high agitation rate. However, increasing the catalyst concentration will increase the production cost and might not effective when the solubility of hydrogen is the limiting factor. Hence, high pressure and fast agitation rate are applied in most hydrogenation process. The process conditions required to achieve high selectivity, high activity and low trans fat is summarized in table below:

Parameter	High Selectivity	High Activity	Low Trans-fat
Pressure	Low	High	High
Temperature	High	High	Low
Catalyst dosage	High	High	High
Catalyst	-	-	Ni
Agitation rate	Low	High	High