مواد تبييض الزيوت المعدنية والنباتية

INTRODUCTION

by

TECHNOLAB EL-BAHAA GROUP

GENERAL.DR

BAHAA BADR

CHEMICAL ADVISOR

In edible oil refining, either by chemical or physical process, the bleaching treatment is a critical step and the conditions for bleaching step depend on the type of crude oil and its quality.

The main task of bleaching is the removal of color pigments and the decomposition of hydroperoxides

This increases the shelf life and aesthetic value of the product
For bleaching process, bleaching clays are used.

Neutral clays, activated earths,
synthetic silicates and carbon black are the basic kinds of materials used in edible oil
bleaching

Bleaching earths mainly consist of bentonites or montmorillonites, and these are finelly crystalline silicate of aluminium with various amounts of alkali metals and transition metals Compared to naturally active clays, activated bleaching earths indicate higher activity.

So acid treatment is applied to neutral clays to enhance sorptive properties.

The
activation of montmorillonite is usually done with mineral acid (hydrochloric acid or
sulphuric acid) and the specific surface area (BET) is increased by this treatment.
The amount of acid and conditions depend on the raw clays used and the maximum activity depends also on the type of oil 1)

The principal goal of the use of bleaching clays is the elimination of a series of
contaminants that can be divided into three groups.

That are primary, secondary and
tertiary.

The primary contaminants are oil soluble components; proteins, sterols tocopherols, hydrocarbons and natural pigments.

The secondary contaminants are organic compounds; free fatty acids, peroxides, ketones, aldehydes

The tertiary contaminants are chemical compounds; solvents, biocides, soaps, heavy metals (as Fe and Cu),
phosphoric and citric acid and others

Processing crude oils to finished edible food products is obtained by several
factors

Consumer preferences

Sensory quality and stability characteristic of oils2

Preparation of hardened products from vegetable oils, such as margarines (Mounts

In Turkey, Tonsil has been used as commercially activated bleaching earth.

Tonsil is
produced from natural bentonites by activation.

Bentonitic earths do not evaluated enough for using in vegetable oil refining in the
domestic market.

Also, acid activated zeolite was used in the bleaching of sunflower oil to see its bleaching efficiency and compared with activated clay samples

The greatest advances in the development and industrialization of vegetable oil processes have taken place within the
last 100 years.

Product diversity between edible and inedible
vegetable oil based products places a tremendous level of pressure on every refinery to run at peak efficiency.

If you have
been in this industry long enough you understand one main
fact about vegetable oil refining—reaching peak efficiency is
not easy.

Although the overall chemical processes have been honed
throughout the years, every plant is unique and demands its
own optimal processing conditions.

Vegetable oil processing
includes four basic unit operations—degumming, refining,
bleaching, and deodorization.

Every unit operation in the
refining process has its own complexity, and unique importance in achieving finished product quality.

This article focuses
on the bleaching stage of the process.

The bleaching process is the critical unit operation within
the refining process responsible for the removal of the remaining soaps, phosphorus (P), trace metals and pigments (chlorophyll, carotenoids), and most of the high molecular weight
oxidative products.

In order to achieve this there are many
variables to consider if you want to have the most economical process:

■ Type and quality of your degummed and refined oil

■ Characteristics of the sorbent employed

■ Processing conditions

■ Type of and layout of the process equipment employed

The objective of this article is to give you information on
the interaction of most of these variables in the bleaching
process, and how they affect the quality of the oil positively
and negatively, so you can find the optimal set of conditions that give you your desired quality in the most economical way

The Basics of Bleaching

“Bleaching is the physical and chemical interaction of a
sorbent with an oil or fat to improve its quality.”

Bleaching is an intermediate step in the overall refining
process.

To understand its impact on oil quality we need to
see what happens in the processes before and after bleaching ).

We also need to define what quality is
and how the interactions of the different processing variables affect the quality parameters.
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Bleaching Mechanisms

During the bleaching process adsorption is occurring via many
different mechanisms through various physical and chemical interactions, most of them improving the quality of the
oil, but some of them deteriorating it.

These mechanisms
include
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Absorption:

mechanism by which the sorbent locks onto a
contaminant.

This can occur three different ways:

■ Physically through surface attraction involving Van Der
Waals forces

■ Chemically by chemical or ionic bonds to the surface of
the clay

■ By molecular sieves which trap contaminants under pressure inside the pores of the clay during filtration

Absorption:

mechanism by which the intra-granular pores
are filled with some fluid—mainly oil—and in turn what ever
contaminants came along with it.

Oil retention has a negative impact on the cost of running the process.

As dosage
increases more oil is lost when disposing the spent clay.

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Filtration:

mechanism of trapping or physically removing
suspended contaminants.

Minor contaminants adsorbed to
the clay particles are subsequently removed.

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Catalysis:

mechanism by which contaminants are degraded
by interaction with the surface of the clay.

For example, peroxides are catalytically polymerized and/or decomposed into
aldehydes and ketones.

With excessive heat and oxidation,
pigments can form color compounds that are difficult to
remove or said to be “fixed.”

In the event of color fixation,
red color is more difficult to remove by bleaching clays alone
and more resistant to thermal degradation leading to higher
red color after deodorization.

Quality

One definition of quality is “to reach a level of excellence.”

However, with respect to oil specifications, it is quite variable depending on the product and market we are dealing
with.

For example, P is very important to be as low as possible for deep frying oils, <0.5ppm is desired, but for salad
oil, you may have a spec of <2ppm.

Various processing conditions can affect one or more of
the oil characteristics that define quality specs.

By balancing
these conditions one against the other, you will be one step
closer to a more efficient bleaching process.

To have an efficient bleaching process you must first be
sure that you are getting the appropriate contaminants removed
in the refining process .

Good refined oil must be

■ Low in P (good if <15ppm, very good if <10ppm and excellent if < 5ppm)

■ Low in free fatty acids (FFA < 0.1% unless semi-physical
refining)

■ Low in soaps (< 50ppm unless silica is used).

Once you have this under control you can begin the
bleaching stage where:

■ Soaps are completely removed

■ P is reduced to a level below 2ppm

■ Iron is reduced to a level below 0.2ppm

■ Chlorophyll is reduced to a level below 0.05ppm

■ Peroxide is reduced to a level below 0.5

All values cited here serve as industry guidelines to meet
product specs.

Each refinery is unique, with its own production lines and product specifications.

The bleaching stage
however is the last opportunity in the refining process to
reduce many of these contaminants.

Controlling the Process

Clay dosing in bleaching operations is typically controlled by
monitoring color or chlorophyll; they are two of the easiest
parameters to measure.

Monitoring chlorophyll is currently more acceptable
than monitoring color.

Monitoring color is not a good parameter to measure how effective your process is, except where
your process and feedstock is completely stable (constant).

There is enough evidence proving that, when starting with
the same raw material, higher bleached colors may lead to
lower deodorized colors, depending on the processing conditions used ).

Additionally, bleached color does not correlate with
chlorophyll content, especially if you have different chloro
phyll content in the crude oil.

In the case of soybean oil, chlorophyll levels can range from 0.3 ppm to 14.0 ppm.

Bleaching
oil from each end of this spectrum to the same red color value
will result in different quality oil given the chlorophyll content and oxidative stability of the bleached oil.
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Understanding Interaction

There are numerous interactions between the process variables
that influence the removal of oil contaminants and control the
efficiency of the bleaching process.

One change in a given operational condition can affect many changes in the oil properties
at the same time—some good and some bad.

The following
sections will show the effect of the most common processing
variables in the most common quality variables.

Type and Nature of Sorbent Mineral

The adsorptive capacity of sorbent minerals is dependent on
its mineralogical structure and adsorptive properties including surface area, particle size distribution, porosity, and surface acidity.

There are mainly two minerals in which bleaching
clays are based on. One is calcium montmorillonite (commonly referred to as bentonite) and the other is a natural

occurring blend of attapulgite and montmorillonite (commonly referred to palygorskite).

Bentonite minerals have limited sorptive properties in
the natural state and require chemical treatment by acids to
create the surface area and porosity needed for bleaching vegetable oils.

Bleaching clays of this nature are commonly referred
to as “acid” or “acid activated” clays.
Palygorskite minerals have a natural high affinity to
adsorb oil contaminants, with exception to chlorophylls,
without any acid treatment.

The natural clay can be combined with mineral acids as well as chelating acids such as citric or phosphoric acids to improve chlorophyll activity

Activation level of the bleaching clay

There are several degrees of acid activation from completely natural, as explained above, to highly acid treated
clays.

In general, chlorophyll, bleached color, and anisidine
value, removal improves as the acidity of the clay increases
(pH lowers).

However, the interactive effect of using high acid
clays can result in increased levels of free fatty acids, and in
extreme cases increased color through the deodorizer.
Organic oils, due to the fact that they do not involve any
chemical agent in their manufacturing, demand natural clays
with high adsorptive capacities.

As mentioned, palygorskite
is naturally active and can be activated by natural acids such
as citric acid.

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Filtration

The filterability of a sorptive mineral, including clay minerals and diatomaceous earths, is dependent on the natural (or
created) porosity, the particle size distribution (PSD) of the
product, and the type of filtering media and equipment
employed.

In general, flowability of oil through a given sorbent can
be improved by increasing the particle size and decreasing the
range of the PSD and is dependent upon the interaction between
the particles in the system with respect to the PSD of the bleaching sorbent and the filter aid (if employed) and the mesh size
of the filter media.

There is a trade off, however, between flowability and
activity due to the fact that activity for a sorbent at a given acid
level correlates with PSD such that activity increases as particle size decreases; and vice versa.

For this reason, bleaching sorbents are offered with various PSD and activity to allow the
operator to achieve the best match with the equipment employed
for optimum balance between filterability and activity.
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Temperature

Bleached oil temperatures typically range from 90–125°C
(190–255°F).

Temperature effects oil viscosity and adsorption kinetics. Oil viscosity decreases with increasing temperature resulting in better dispersion of particles, improved clay
oil interactions, and flowability.

The ability to maintain a particle in slurry suspension however is inversely related to the
viscosity of the oil, implying that clays with higher PSDs will
take more agitation to stay in suspension.

A higher temper-
ature may give you benefits on chlorophyll removal, bleached
color, and filtration rates, but may deteriorate deodorized
color, and certainly will increase the speed of oxidation reactions
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Contact time

Contact time refers to the total time that the bleaching clay
is in contact with the oil, from slurry tank through the filter
presses.

In batch systems you may need to take into account
the filtration time in the overall calculation for contact time.

Times typically range from 15 to 45 minutes, with 20 to 30
minutes being most common.

The positive effect of increased
contact time is that it may improve bleached color and chlorophyll removal.

Adsorption generally occurs exponentially
having a diminishing point of return around 30 minutes.

Excessive times may lead to increased oxidization in the
bleached oil, resulting in darker red colors through the deodorizer (see figure 4).
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Oil moisture

Oil moisture typically ranges from 0.05% in vacuum dried
oil to 0.35% in oil coming directly from a centrifuge into
bleaching.

Depending on the vacuum, the type of bleaching
clay and the bleaching temperature, optimizing moisture will
improve your chlorophyll and P removal (see figure 5).

The optimum moisture level going into the bleacher is
typically less than 0.3%; above this range, bleaching efficiency decreases.

Moisture levels, however, need to be reduced
to levels below 0.05% before filtering to prevent decreased
flowability through the filter presses.

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Vacuum

Bleaching efficiency improves when operating pressure in
the bleacher is run between 50 to 100 mmHg.

Reduced pressure allows for a smooth water evaporation rate resulting in
increased efficiency for phospholipids, chlorophyll, and some
red pigments removal.

Reduced pressure also minimizes interaction of oil and air resulting in lower peroxide values,
anisidine values, and bleached oil colors.

Oil Refining

Neutralizing:

The Crude Oil temporarily stored in Crude Oil Tank is pumped into neutralizing tank by Oil Pump.

The oil is mixed with alkali lye in the Mixer and neutralized in Neutralizing Tank, free fatty acid has been neutralized or saponified to be Soapstock.
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Physical refining:

Physical Refining means removing gum in the oil in the process of degumming and removing the FFA in process of deodorizing by steam.

It has the following features:

1. Less oil loss and high oil refining ratio.

2. No water wasted in process of production.

3. More FFA is distilled out.

4. More suitable oil with high acid value and low gum-impurity.
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Chemical Refining:

Chemical Refining means removing FFA in a way (acid-base neutralization).

Gum and soapfoot is separated by centrifuges.

Chemical refining has following features:

1. Fine adaptability and less requirement of crude oil quality.

2. Consistent product oil.

3. Less clay is added in compared with physical refining.

Oil Refining process can remove phospholipid, FFA, pigment, off-flavor and other impurities.

It includes the following sections:

Oil Dugumming,

Oil Deacidifying,

Oil Bleaching,

Oil Deodorizing

Oil Dewaxing.
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Oil Bleaching:

High-activity clay is added into oil, mix and heat mixture of oil and clay to make the small particle of pigment absorbed on the crystal of clay.

This machine is of perfect mechanism, reliable performance and is well a configured piece of equipment.
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Oil Deodorizing:

The heat of most bleached oil is recovered by heat exchangers.

The bleached oil is heated to the process temperature by mineral oil or high pressure steam and then the oil enters into the combined deodorizer,

the deodorizer is a combined type:

the upper is the packing structure, which is used to remove odor components like FFA, the lower is plate is used for heat bleaching and making product quality more consistent.

Oil coming from the deodorizer is cooled and stored after series of heat exchange, volatile like FFA is collected and stored as by products.
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Oil Dewaxing:

It can improve oil palatability, so it plays an important role in the quality of finished oil. At the same time, dewaxing can improve oil transparency and brightness.

At present, winterizing method is very popular to dewaxing.

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مواد تبييض الزيوت المعدنية والنباتية Oil_re10

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عدد المساهمات : 7 تاريخ التسجيل : 15/11/2011

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