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

Hydrogenation - Introduction

In edible oils and fats industry, hydrogenation is used to produce a more oxidative stable product and change liquid oil into a semi solid or solid fat with the desired melting characteristics. Hydrogenation is a chemical reaction in which hydrogen is reacts with the double bonds found in triglycerides to form saturated bond. It usually performed batch wise in a pressurized stirred reactor with nickel as catalyst. In most cases, the oil is not fully hydrogenated as fully hydrogenated oil is too hard for some applications. Hence, partially hydrogenation is applied and this gives high trans fatty acid in the hydrogenated oil. It has been proven that trans fatty acid have negative effect on human health. As a result, future approach of hydrogenation is in the reduction of trans-formation.

First of all, feed oil is pumped into a buffer tank. This buffer tank is a multi function tank where it provide heating, degassing, drying and as a buffer tank. The feed oil is preheated by the out going hydrogenated oil followed by steam to 160 ^oC. After the measuring tank reached the pre-set amount and temperature, the oil will be drained to the hydrogenation reactor. At the same time, nickel catalyst in oil slurries is pumped into the reactor.

After the filling process completed, vacuum is applied to the reactor before the pressurized hydrogen is pumped in. This is to remove moisture and oxygen in the air which may cause hazard when mixed with hydrogen. The hydrogenation reaction proceeds as follow:

R₁CH=CHR₂ + H₂ à R₁CH₂-CH2R₂                                          

IV for the hydrogenated oil is checked from time to time. When the required IV is obtained, the hydrogenated oil is pumped to a drop tank.

This drop tank act as a buffer tank to hold the hydrogenated oil before it is used for heat recovery with the feed. After heat exchange, the hydrogenated oil is sent to another buffer tank to cool down to temperature below 100 ^oC prior sent to filtration process. This is to protect the cloth in the pulse tube filter. The filtered oil is sent to final polishing filter before pumped to storage tank. The schematic representation of hydrogenation process is shown below:

 

Fractionation - Process Quality Control

 Process and Product Quality Control

            In palm oil fractionation, olein fraction is the desired product and it is important to increase the olein yield, at the same time maintaining the oil quality. Important parameters for palm olein characterization are IV, cloud point and cold test. Normally, palm oil fractionation can give a yield up to 80 %. On the other hand, the desired product in PKO fractionation is the stearin fraction which can be used as CBS. Hence, the objective in PKO fractionation is to maximize the stearin yield. Important parameters for stearin are SFC (especially 30^oc), SMP and IV. A good quality stearin will have a steep SFC profile.

alm Oil Fractionation

High final crystallization temperature will produce larger and better crystal which is easier to filter. However, this will give low stearin yield and olein with low IV. At the same time, the stearin IV will be low and olein yield is high. On the other hand, low final crystallization temperature will produce lot of smaller crystal which is weaker. In this case, the olein produced is high IV with low yield. The stearin will be high IV and high yield.

Agitation rate is another important parameter in a crystallization process. This is because crystallization is an exothermic process and oil is a rather poor heat conductor. If heat produced during crystallization process can not be removed effectively, it will melt some of the crystal causing uneven crystal formation. This will lead to difficult filtration. Hence, agitation is required to give intimate mixing of the oil for good heat transfer. On the other hand, high agitation rate tent to break the crystal causing secondary crystallization. Second crystallization will lead to uneven crystal formation. As a result, high agitation rate is used in cooling step and lower agitation rate is used in the crystal formation state.

Palm Kernel Oil Fractionation

In PKO fractionation, the important parameters that affect the product quality and yield are final slurries solid fats content, static crystallizer temperature and holding time. High final slurries solid fat content indicate that there are a lot of nucleus initiated and this will give a smaller crystal in cake formation steps. If the cake formed in static crystallizer is too soft, there are 2 possible reasons, namely temperature too high or holding time too short in static crystallizer. Soft cake will lead to low stearin yield and low stearin and kolein IV. On the other hand, hard cake will give higher stearin yield with high stearin and olein IV.

Fractionation - Process description

4.3.1 General Process flow

First, the feed oil will be preheated before entering the crystallizer to destroy all crystal memories. The oil will be cooled in the crystallizer according to the preset cooling profile depending on the feed oil quality and product quality required. For palm kernel oil fractionation, the crystallization section consists of two steps; dynamic crystallization and static crystallization. Dynamic crystallization process is performed in a crystallizer to initiate the nucleation while the static crystallization is performed in a statoliaser for cake forming. On the other hand, palm oil fractionation only has one crystallizer which is dynamic crystallizer. After crystallization, the slurries will be sent to membrane filter press to separate the olein (liquid fraction) and stearin (solid fraction). The schematic representation of the process is shown in below