Theory of Deodorization

April 7, 2018 | Author: Lam Desmond | Category: Distillation, Steam, Vegetable Oil, Temperature, Heat
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Theory all about deodorization...

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Deodorisation

Sadanand Patel HBTI-K

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Refining (Physical/Chemical) What is Deodorization? Effect of Deodorization Chemistry of Odour Sources of Odour Principles Laws Process Variables Process Steps Operation(Batch, Semi-continuous and Continuous) Equipment Effect of Deodorization on Oil Quality Losses Newer Developments Utilities Consumption Deodoriser safety

Layout of Presentation

Refining (Physical/Chemical) OILSEEDS Mechanical Pressing Solvent Extraction

Oil Extraction

Deoiled Meal

Crude Oïl Gums

Water degumming Soapstock

Chemical

Alkali Neutralisation

LECITHIN

Physical

WDG Oil

Acid Gums

Bleaching

Spent bleaching earth

Deodorization

Deodorizer Distillate

Refined Oil

Acid degumming

Bleaching Physical deacidification Deodorization

What is Deodorization?  This is usually the last stage of the refining process of edible oils  Quality defining process in the refining of oils to eliminate the FFA and odoriferous substances.  Today, the process is still commonly named ‘deodorisation’, but the objectives have become much broader than just the removal of offflavours.

 High Temperature, High Vacuum, Live Steam Distillation Process  Distillation is a physical and not a chemical process and does not change the molecular structure of the components

Desirable Actual deodorisation by removal of different off-flavours

Stripping of volatile components such as:  FFA (in the case of physical refining),  contaminants (pesticides, light PAH etc.)

Thermal destruction of pigments (socalled heat bleaching)

Its Effects Undesirable Some unwanted side-reactions  cis-trans-isomerization,  polymerization,  conjugation, and so on

Removal of valuable minor components  tocopherols,  sterols etc. If the oil is not properly pretreated, the oil can become darker during deodorization. This phenomenon is also known as ‘‘color fixation’’

Chemistry of Odour

Hydroperoxides

Fats & Oils

Factors

Primary Oxidation Products

Aldehydes, Ketones, Diene and triene

Decomposition

Secondary Oxidation Products

Acids, Epoxides, Dimers, Oxirane rings

Oxidation

Tertiary Oxidation Products

Sources of Odour  Most vegetable oil retains characteristics undesirable flavors & odors that obtain during processing.  Peroxides (primary oxidation products), Hydrocarbons, Aldehydes, Ketones (secondary oxidation products), Tocopherols, Sterols, FFA are the main odoriferous and volatile compounds need to remove.  As a total, odoriferous components generally add up to not more than 200 ppm.  Bleaching imparts “Earthy” flavors whereas hydrogenation adds on odor and flavors that can be described as undesirable.

Underlying Principle Difference in volatilities Aldehyde/Ketone > FFAs > Squalene > Tocopherols > Sterols > Sterols esters > TAGs

The degree of separation of a simple binary mixture is expressed in terms of the relative volatility of the two components. The theoretical separation ratio,α, is defined as

The vapour pressure for a given constituent is a function of the temperature and increases with increasing temperature (ClausiusClapeyron's equation)

Laws Dalton's Law describes the vapour pressure of a mixture as the sum of the partial pressures of its single components

The partial pressure of the single components can be calculated following Raoult's law:

Whitman’s two film theory: At any instant the rate of transfer of volatile components from the oil into the steam bubble is proportional to the difference in pressure inside the bubble and actual pressure in the vessel and interfacial area of the steam bubble.

Stripping Steam

Time

Pressure

Temperature

Process Variables

Deodorization Temperature • Temperature has its influence on deodorization because the vapour pressure is directly proportional to it. • T α Rate of removal of Volatile Substances

• An increase of nx17K decreases deodorization time by the factor 2n • Increase in temperature also means that stripping steam can be reduced. This is important in reducing entrainment losses

• Short chain fatty acid deodorized at low temp and hydrogenated oil deodorized at higher temp because of higher FFA content and distinctive odor it imparts. • Heat Bleach Effect is higher at higher temperature:

Higher Temperature is avoided because:  Insufficient thermal stability of oils  Economy of the process (high energy consumption)

Deodorization Pressure • If the pressure is decreased, the temperature required also decreases because a lower vapour pressure is then sufficient to ensure evaporation. • Reduced pressure helps to protect the oil from oxidation because oxygen from the air is reduced almost to zero.

• The amount of distillation steam required is also directly proportional to the pressure. If the pressure is halved steam consumption also halves.

Higher P means more steam. More steam means more possibility for entrainment. And more stripping steam will increase hydrolysis. This will increase oil losses and lower quality

Deodorization Time • The time necessary to separate a certain amount of undesired components from the oil depends mainly on the speed with which the necessary amount of stripping steam can be introduced

• Deodorization time also increases with the height of the oil in the deodorizer, i.e., with the thickness of the oil layer that the stripping stream has to penetrate.

• Because all side reactions are also time dependent, deodorization time is kept as low as possible. In Europe, longer deodorization time and lower temperatures are preferred. In the U.S., the reverse is true

Stripping Gas A Carrier to the Undesired Components

• External pressure of the gas adds to the Vapour pressure of volatiles in deodorizer and speed up their removal. • Stripping agents with the lowest possible molecular weights are selected. In most cases, steam is the best solution, but sometimes nitrogen is preferred.

• Since the surface area of the oil is increased by bubble formation, the mass transfer and consequently the distillation rate is increased.

• The height of the oil in the vessel also influences the consumption of steam because the steam has to work against the hydrostatic pressure to be able to penetrate the oil before being sucked off or distilling off from its surface.

where S is the total moles of steam, V, is the initial molar concentration of the volatile component in the oil, and V, is the final molar concentration of the volatile component in the oil. When the initial FFA content is low, as in the case of a classical deodorization, (Va- Vo) becomes so small that the equation can be simplified as follows:

The amount of gas required for deodorization is directly proportional to the amount of oil and the absolute pressure in the deodorizer and inversely proportional to the vapour pressure of the pure volatile component at the process temperature and the overall vaporization efficiency E.

On the surface of the steam bubbles, the oil/steam contact must be maximized; there the sparge gas has its lowest specific volume, equivalent to the highest relative surface area. Athanassiadis (1991) postulated that 300,000 m3 was a minimum contact area of oil/steam for every kilogram of gas injected to ensure good deodorization efficiency.

Process Optimization To achieve a proper deodorization, an optimal deodorizing temperature, operating pressure, and amount of stripping gas are required. These are determined not only by the type and state of the oil (chemically or physically refined), but also by the deodorizer design. Today, most deodorizers used in the refining of soft oils operate at temperatures between 230 and 26OoC, a pressure of 3 mbar or even lower, with a stripping steam consumption of ~10 kg/ton of processed oil.

Refining

chemical

physical

Temperature

230 – 250 °C

240 – 265 °C

Pressure

3 mbar

2 mbar

Sparge steam

5 – 15 kg/t

7 – 20 kg/t

Time

45 – 90 min

45 – 90 min

Final FFA

0.03 %

0.05 %

Trans-increase

0.5 – 1.5 %

0.7 – 2 %

Tocopherol retention

70 – 90 %

60 – 80 %

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Unit operations of deodorization & Processing Steps

Bleached Oil

Degassing

Heating Up

Deodorization

Cooling

Polishing

Deodorized Oil



Deaeration



Heating



Stripping



Thermal action



Cooling



Polishing

De-aeration  To avoid excessive oxidation and hence risk of polymerization.  Soft oils dissolve readily between 4 and 10% of their own volume of air and other gases at ordinary temperature.  Solubility increase with increasing in temperature. The relation between solubility (S) and temperature (t) can be expressed as:

with S (%, v/v), at normal pressure and t in °Celsius

The bleached oil is sprayed into a tank under reduced pressure. The lower the pressure applied, the lower the residual oxygen in the oil. Usually the oil is heated to at least 80°C and sprayed into a tank, which is kept at a pressure of 40 mbar. Some refiners even use the low pressure of the deodorizer or add some sparge steam in the spraying tank to improve deaeration.

Heating

Heating of the oil is usually accomplished in two stages.  First stage, Oil is heated counter-currently in an oil-oil heat exchanger (economizer), with the finished oil leaving the deodorizer.  Second stage, Oil is heated under reduced pressure to deodorizing temperature with a high temperature source such as high pressure steam, heat transfer fluids. Saturated steam of -50 bar is required to heat the oil to 240-260°C.

where O is the amount of oil (kg), T1 and T2 are the incoming and final temperature of the oil (°C), C is the average specific (Heating energy required for a deodorizing system)

heat capacity of vegetable oils [typically 2.2-2.4 kJ/(kg. °C)], fL is the heat loss factor from radiation typically 1.05-1.15) and fR, is the heat recovery factor [ 1 - (%heat recovery/100)].

Stripping It is Conducted at a temperature between 230 and 260 oC, at a pressure between 2 and 4 mbar and under injection of 0.5-2% sparging steam. From a thermodynamic point of view, the stripping agent takes over the part of the total pressure equal to its partial pressure. As a consequence, the vapour-liquid equilibrium is reached at a lower molar fraction, xi, resulting in the removal of significantly more volatile substances only.  Nitrogen - inert and non-condensable gas - lower losses (no hydrolysis) and higher distillate quality - more powerful vacuum system required - profitability is very uncertain  Steam (Superheated) - most ‘evident’ choice - Boost up the vacuum when condensed - But support hydrolysis

The sparge gas is introduced into the oil through special steam distributors. These can be sparge coils with very fine holes (between 0.5 and 2.5 mm) or even sintered metal pipes

Thermal action

The oil is then held in a retention section for a certain amount of time for thermal treatment – known as heat bleaching – that deals with undesirable pigments and ensures the stability of the final product. The length of time the oil is kept in the retention section depends heavily on the desired product specifications

Cooling Finally the oil is cooled in two stages. First in the economizer, and then to the specified final temperature. It then undergoes polish filtration and is transferred to subsequent processes, storage or packaging

Oil Polishing Oil polishing is done to remove any fine particles of soaps , metallic salts rusts , filler aids, polymerized oil or other solid impurities. Horizontal plate filters have long been used as polishing filters of choice for deodorization.

Processing Options Batch Processing

Semi-Continuous Processing Continuous Processing - Vertical Deodorizer

- Horizontal Deodorizer • Thin Film Technology • Packed Column Technology • Dual Temperature Technology

 Batch deodorization is especially suitable for small capacities (
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