Edible and Essential Oils | Chemical Technology - Chemical Engineering PDF Download

31a.1 Introduction 

Oils are organic molecule of carbon, hydrogen, oxygen and sometimes nitrogen and sulfur. It is composed of long chain fatty acids and esters (glyceride ester) as well as derivative of glycerine, long chain fatty alcohol, sulfate and sulfonates. Oils like caster oil, linseed oil are non edible and oils like ground nut oil, coconut oil are edible .

Oil is used in producing vanaspati ghee, soaps and detergents, cosmetics, medicines, polymers, paints and varnishes and in many other applications.

31a.2 Vegetable 

oil extraction Raw material: Seeds of oilseed plant

Process flow sheet: Illustrated in Figure.

Figure 31.1 Flow sheet of vegetable oil extraction

31a.3 Functional role of various processes (Figure 31.1):
 

(a) Cleaner and dehulling:

• Mechanical cleaning is done to remove stones and other undesirable material.
• Dehulling remove hulls, dry outer covering of seed.

(b) Cracking rolls:

• Crushing rolls crush the oil seeds and gets flacked seeds.

(c) Digester:

• 100 parts of flaked seeds are thoroughly mixed with 5-10 parts of water by rotating blades.
• Softening by means of heat and moisture is done here.
• Steam is added for heating purpose.
• Acid is formed by hydrolysis of ester.
• The seeds get swollen up.

(d) Expeller:

• The swollen seeds are crushed under great pressure.
• Cells of seeds get ruptured and oil is released.
• The tapering shape ensures more application of pressure on the seeds 
• The oil depleted cake is either sent for solvent extraction or used as animal feed. · Oil is sent for purification.

(e) Mix tank:

• The extracted oil is treated with alkali like NaOH or Na₂CO₃ to remove fatty acids.
• It also removes heavy metals, which can  start oxidation of oil.

(f) Centrifuge:

• Acids are separated in centrifuge separator.
• These separated acids are used as foots for soap manufacturing.

(g) Rotary filter:

• The clear oil is treated with some bleaching agent like”Fullers Earth Carbon” in filter aid before rotary filter.
• Rotary drum filter is used for removal of seed particles which may be present.
• This finished oil is produced.
• From this process around 1-2% oil content remains in the meal. 

(h) Extractor:

• The flaked seeds or oil extracted seeds from crushing rolls are fed on a moving bed.
• Oil depleted cake is also added in extractor.
• The solvent extracts oil from the seeds.
• The wet meal (the left seed part) is collected in the middle and is sent for solvent removal.

(i) Solvent removal:

• Steam is used to extract solvent carried by the oil seeds.
• The rotating blades ensure better exposure to steam.
• The solvent free meal is sent to dryer and then used as animal feed.
• Solvent is collected from top and is recycled after cooling it in heat exchanger.

(j) Flash film evaporator: 

• The solvent is preheated by steam.
• Oil is concentrated here by evaporating the more volatile component (hexane).
• The hexane is recycled back to extractor after heat recovery.
• The use of flash evaporator reduces the cost of vacuum stripping column.

(k) Vacuum stripping column:

• The oil from flash evaporator is fed here.
• The stripping is done by steam i.e. steam carries away the hexane.
• And it is under vacuum for the reason that the boiling point of hexane and oil are close to each other.
• Jet ejector is used here to generate vacuum.
• The hexane is recycled again.
• Oil produced here is either directly used or sent for purification.  

31b.1 Hydrogenation of oil

Hydrogenation process is used to remove double bonds and to make fats and oil saturated. Hydrogenation also raises its melting point and improves its resistance to rancid oxidation. The most common end product of hydrogenation is Vanaspati ghee. Other products include vegetable ghee, hardened industrial oils and partially hydrogenated liquid oil.  

Chemical Reactions:  (* indicates activated catalytic state) 

Ni* catalyst

(a) R₁(C=C)_xR₂  +  (x-y)H₂ → R₁(C=C)yR₂

(b) Nickel catalyst preparation:

                  190˚C

Ni(HCOO)₂.2H₂O  →Ni* + 2CO₂ + H₂ + 2H₂O

This produces a finely divided catalyst which is preferred for well-stirred hydrogenation reactors.

(c) Nickel catalyst preparation (reduced Ni on inert catalyst support)

Ni(OH₂) + H₂ →Ni* + H₂O

NiCO₃ + H2→Ni* + H₂O + CO₂

Nickel salts are precipitated on inert porous carrier such as kieselguhr or diatomaceous earth and reduced at high temperature in a hydrogen atmosphere.

(d) Nickel catalyst preparation

2Al.Ni + 6NaOH → Ni* + 2NaAlO₃ + 3H₂

Alloy                                             Raney nickel

Produces spongy, high-surface area catalyst when the sodium aluminate is washed with water.

Raw material: Oils which is to be saturated and pure hydrogen.

Process flow sheet: Illustrated in Figure.

Figure 31.2 Flow sheet of hydrogenation of vegetable oils

(a) Hydrogenator: 

• Oil is fed to the hydrogenator.
• Hydrogen, and steam is introduced in it.
• The catalyst oil slurry of concentration 5 to 15 kg per ton of oil is also put.
• The reaction is slightly exothermic so steam is sometimes turned off.

(b) Deodorizer:

• The hydrogenated oil is now fed to vacuum steam deodorizer.
• It operates in continuous basis.

(c) Finishing

• Oil colour is removed by treating with fuller’s earth or carbon.
• It is then filtered, mixed with some vitamins.
• Now the finished oil is sent for packaging and storage. 

31b.3 Technical Questions

1. Discuss the detailed process related to  the gas liquid hydrogenation reaction? 

Ans: For gas liquid reaction in hydrogenation

H₂ (g) + Oil (l) →Fat (l)

A series of rate process and 8 kinetic reaction exist as shown in fig below:

For many hetrogenous catalytic reactions, the rate behavior is characterized by:

• Surface reaction and adsorption controls the  reaction rate at 100˚C.
• Rates increases as H2 pressure increases (at the max 7 atm gage)
• At 180˚C, mass transfer for solution of H2 and diffusion to and from catalyst are rate controlling steps.

2. What is Rancidity?

Ans: The moisture and oxygen cause deterioration of fats. The combine action of both moisture and oxygen causes rancidity which is characterised by bad smell, development of red colour, increase in acidity and increase in viscosity. During frying, cooking, baking etc, oil undergo hydrolytic and polymerization reaction. These reactions are undesirable in edible oils. Prolonged exposure to air yield undesirable resinous products causing rancidity.

3. What is winterising?

Ans: When oils are cooled slowly to a lower temperature, then less soluble glycerides rich in fatty acid crystallizes out. These crystals can be separated by centrifuge or by filter. This separation of solid fats by chilling is called 
winterising.

4. What is saponification value?

Ans: Alkaline hydrolysis of fats and oils is  known as saponification. This forms the basis of soap making.

Saponification value indicates the average molecular weight of a fat or oil. This may also be defined as number of mg of caustic potash required to neutralize the fatty acid obtained by complete hydrolysis of 1gm of oil or fats.

5. What is acid value?

Ans: It indicates the proportion of free fatty acid present in oil or fats. Acid value is defined as number of mg of caustic potash required to neutralize the acid in 1gm of the sample. Generally acid value for most of samples lies within 0.5. 

6. What is Iodine value?

Ans: Iodine value is a measure of extent of unsaturated fatty acid present in fats. It is defined as number of grams of iodine that combines with 100gms of 
oil and fats. 
The iodine value of non-drying oil is less than 90, of semi-dryng oil is between 90 to 140.