EXTRACTION and SUGAR INDUSTRY APPLICATIONS

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EXTRACTION and SUGAR INDUSTRY APPLICATIONS

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• EXTRACTION
• 1-LEACHING(SOLID EXTRACTION)
• a) GENERAL INFORMATION
• b) FACTORS INFLUENCING THE
  RATE OF EXTRACTION
• c) LEACHING EQUIPMENT
• 2- LIQUID-LIQUID EXTRACTION
• a) EXTRACTION PROCESS
• b) CLASSIFICATION OF
  EXTRACTION EQUIPMENT
• - STAGE-WISE EQUIPMENT FOR
  EXTRACTION
• - DIFFERENTIAL CONTACT
  EQUIPMENT FOR EXTRACTION

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EXTRACTION

• Extraction is the method of removing one
  constituent from a solid or liquid by means of a
  liquid solvent.Extraction techniques fall into
  two categories.The first is called leaching or
  solid extraction and the is second called liquid
  extraction.

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LEACHING (SOLID EXTRACTION)A-GENERAL INFORMATION

• Leaching is concerned with the extraction
  of soluble constituent from a solid by means of a
  solvent.The process may be used either for the
  production of a concentrated solution of a
  valuable solid material,or in order to remove an
  insoluble solid ,such as a pigment ,from a
  soluble material with which it is contaminated.

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B-FACTORS INFLUENCING THE RATE OF EXTRACTION

• The selection of the equipment for an
  extraction process is influenced by the factors
  which are responsible for limiting the extraction
  rate.There are four important factors to be
  considered
• Particle size The smaller the size ,the
  greater is the interfacial area between the solid
  and liquid,and therefore the higher is the rate
  of transfer of material and the smaller is the
  distance the solute must diffuse within the
  solid.

SolventThe liquid chosen should be a good
selective solvent and its viscosity should be
sufficiently low for it to circulate freely.

• TemperatureIn most cases,the solubility of the
  material which is being extracted will increase
  with temperature to give a higher rate of
  extraction.Further ,the diffusion coefficient
  will be expected to increase with rise in
  temperature and this will also improve the rate
  of extraction.
• Agitation of the fluidAgitation of the solvent
  is important because this increases the eddy
  diffusion and therefore the transfer of material
  from the surface of particles to tha bulk of the
  solution.

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LEACHING EQUIPMENT

• When the solids form an open ,permeable
  mass throughout the leaching operation ,solvent
  may be percolated through an unagitated bed of
  solids.With impermeable solids or materials that
  dissintegrate during leaching,the solids are
  dispersed into the solvent and are later
  separated from it.Both methods may be either
  batch or continuous.

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Leaching by percolation through stationary solid
beds Stationary solid-bed leaching is
done in a tank with a perforated false bottom to
support the solids and permit drainage of the
solvent.Solids are loaded into the tank,sprayed
with solvent until their solute content is
reduced to the economical minimum,and excavated.

• In some cases the rate of the solution is
  so rapid that one passage of solvent through the
  material is sufficient ,but countercurrent flow
  of solvent through a battery of tanks is more
  common.In this method, fresh solvent is fed to
  the tank containing the solid that is most nearly
  extractedit flows through the several tanks in
  series and is finally withdrawn from the tank
  that has been freshly charged.such a series of
  tanks is called an extraction battery.

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Moving-bed leaching

• In the machines that are used for this
  type of leaching, the solids are moved through
  the solvent with little or no agitation.The
  bollman extractor (figure a) contains a bucket
  elevator in a closed casing.There are
  perforations in the bottom of each bucket.At the
  top right-hand corner of the machine ,the
  buckets are loaded with flaky solids such as
  soybeans and are sprayed with appropriate amounts
  of half miscella as they travel downward.Half
  miscella is the intermediate solvent containing
  some extracted oil and some small solid
  particles.As solids and solvent flow cocurrently
  down the right-hand side of the machine ,the
  solvent extracts more oil from beans.

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• Simultaneously the fine solids are filtered
  out of the solvent, so that clean full miscella
  can be pumped from the right hand sump at the
  bottom of the casing.As the partially extracted
  beans rise through the left side of the machine
  ,a stream of pure solvent percolates
  countercurrently through them.It collects in the
  left-hand sump and is pumped to the half-miscella
  storage tank.Fully extracted beans are dumped
  from the buckets at the top of the elevator into
  a hopper from which they are removed by paddle
  conveyors.

Bollman extractor
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• In the Rotocel extractor,illusrated in
  figure b, a horizontal basket is divided into
  walled compartments with a floor that is
  permeable to the liquid.The basket rotates slowly
  about a vertical axis.Solids are admitted to each
  compartment at the feed pointthe compartments
  then successively pass a number of solvent
  sprays, a drainage section, and a discharge point
  at which the floor of the compartment opens to
  discharge the extracted solids.The empty
  compartment moves to the feed to point to receive
  its next load of solids.To give countercurrent
  extraction, fresh solvent is fed only to the last
  compartment before the discharge point, and the
  solids in each preceeding compartment are washed
  with the effluent from the succeeding one.

Rotocel extractor
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Dispersed solid leaching

• Solids that form impermeable beds, either
  before or during leaching , are treated by
  dispersing them in the solvent by mechanical
  agitation in a tank or flow mixer.The leached
  residue is then separate from the strong solution
  by settling or filtration. Small
  quantities can be leached batchwise in this way
  in an agitated vessel with a bottom drawoff for
  settled residue.

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LIQUID-LIQUID EXTRACTION

• The separation of the components of a
  liquid mixture by treatment with a solvent in
  which one or more of the desired components is
  preferentially soluble is known as liquid-liquid
  extraction. In this operation, it is essential
  that the liquid-mixture feed and solvent are at
  least partially if not completely immiscible and,
  in essence, three stages are involved
• 1-Bringing the feed mixture and the
  solvent into intimate contact,
• 2-Seperation of the resulting two
  phases,
• 3-Removal and recovery of the solvent
  from each phase.

It is possible to combine stages 1 and 2
into a single piece of equipment such as a column
which is then operated continuously. Such an
operation is known as differential contacting.
Liquid-liquid extraction is also carried out in
stagewise equipment, the prime example being a
mixer-settler unit in which the main features are
the mixing of the two liquid phases by agitation,
following by settling in a separate vessel by
gravity.
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• Important applications of liquid-liquid
  extraction include the separation of aromatics
  from kerosene-based fuel oils to improve their
  burning qualities and the separation of aromatics
  from paraffin and naphthenic compounds to improve
  the temperature-viscosity characteristics of
  lubricating oils. It may also be used to obtain,
  for example, relatively pure compounds such as
  benzene, toluene, and xylene from catalytically
  produced reformates in the oil industry, in the
  production of anhydrous acetic acid in the
  extraction of phenol from coal tar liquors, and
  in the metallurgical and biotechnology
  industries.

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EXTRACTION PROCESSES

• All liquid-liquid extraction operations,
  may be carried out either as a batch or
  continuous process.
• In the single-stage batch process
  illustrated in the figure, the solvent and
  solution are mixed together and then allowed to
  separate into the two phases-the extract E
  containing the required solute in the added
  solvent and the raffinate R, the weaker solution
  with some associated solvent. With this simple
  arrangement mixing and seperation occur in the
  same vessel.

Fig.single-stage batch extraction
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• A continuous two-stage operation is shown
  in figure, where the mixers and separators are
  shown as separate vessels.

Fig.Multiple-contact system with fresh solvent
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CLASSIFICATION OF EXTRACTION EQUIPMENT

• Essentially there are two types of design
  by which effective multistage operation may be
  obtained
• 1-Stage-wise contactors, in which
  equipment includes a series of physical stages in
  which the phases are mixed and separated, and
• 2-Differential contactors, in which the
  phase are continuously brought into contact with
  complete phase separation only at the exits from
  the unit.
  

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STAGE-WISE EQUIPMENT FOR EXTRACTION

• The mixer settler
• In the mixer-settler, the solution and
  solvent are mixed by some form of agitator in the
  mixer, and then transferred to the settler where
  the two phases separate to give an extract and a
  raffinate. In the settler the separation is often
  gravity-controlled, and the liquid densities and
  the form of the dispersion are important
  parameters.

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Combined mixer-settler units

• Recent work has emphasised the need to
  consider the combined mixer-settler
  operation.Thus WARWICK and SCUFFHAM give details
  of a design, shown in the figure in which the two
  operations are effected in the one combined unit.
  The impeller has swept-back vanes with double
  shrouds, and the two phases meet in the draught
  tube. A baffle on the top of the agitator
  reduces air intake and a baffle on the inlet to
  the settler is important in controlling the flow
  pattern.This arrangement gives a good performance
  and is mechanically neat.

Figure.Mixer-settler
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• The segmented mixer-settler.In segmented
  mixer-settler specially designed KnitMesh pads
  are used to speed up the rate of coalescence. The
  centrally situated mixer is designed to give the
  required hold up, and the mixer is pumped at the
  required rate to the settler which is formed in
  segments around the mixer, each fed by individual
  pipework.

Figure.Segmented mixer-settler
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• Kuhni have recently developed a
  mixer-settler column which is a series of
  mixer-settlers in the form of a column. The unit
  consists of a number of stages installed one on
  the top of another, each hydraulically separated,
  and each with a mixing and settling zone as shown
  in the figure.

Fig.Kühni mixer-settles column
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Baffle-plate columns

• These are simple cylindrical columns provided
  with baffles to direct the flow of the dispersed
  phase, as shown in the figure. The efficiency of
  each plate is very low, though since the baffles
  can be positioned very close together at 75-150
  mm, it is possible to obtain several theoretical
  stages in a reasonable height.

Figure.Baffle-plate column
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The Scheibel column

• One of the problems with perforated plate
  and indeed packed columns is that redispersion of
  the liquids after each stage is very poor. To
  overcome this, SCHEIBEL and KARR introduced a
  unit, shown in the figure, in which a series of
  agitators is mounted on a central rotating shaft.
  Between the agitators is fitted a wire mesh
  section which successfully breaks up any
  emulsions.

Figure.Scheibel column
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DIFFERENTIAL CONTACT EQUIPMENT FOR
EXTRACTION

• Spray columns
• Two methods of operating spray columns
  are shown in next figure. Either the light or
  heavy phase may be dispersed. In the former case
  (a) the light phase enters from a distributor at
  the bottom of the column and the droplets rise
  through the heavier phase, finally coalescing to
  form a liquid-liquid interface at the top of the
  tower. Alternatively the heavier phase may be
  dispersed, in which case interface is held at the
  bottom of the tower as shown in (b). Although
  spray towers are simple in construction, they are
  inefficient because considerable recirculation of
  the continuous phase takes place. As a result
  true countercurrent flow is not maintained and up
  to 6 m may be required for the height of one
  theoretical stage.

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Figure.Spray towers
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• Packed columns
• The packing increasing the interfacial
  area, and considerably increases mass transfer
  rates compared with those obtained with spray
  columns because of the continuous coalescence and
  break-up of the drops. Packed columns are
  unsuitable for use with dirty liquids,
  suspensions, or high viscosity liquids. They have
  proved to be satisfactory in the petroleum
  industry.

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• Rotary annular columns and rotary
  disc-columns
• With these columns mechanical energy
  is provided to form the dispersed phase. The
  equipment is particularly suitable for
  installations where a moderate number of stages
  is required, and where the throughput is
  considerable. A well dispersed system is obtained
  with this arrangement. The figure shows a rotary
  annular column.

Figure.Rotary annular column
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• Pulsed columns
• In order to prevent coalescence of the
  dispersed drops, VAN DUCK and others have devised
  methods of providing the whole of the continuous
  phase with a pulsed motion. This may be done,
  either by some mechanical device, or by the
  introduction of compressed air.
• The pulsation markedly improves
  performance of packed columns. There are
  advantages in using gauze-type packings since the
  pulsation operation often breaks ceramic
  rings.Pulsed packed columns have been used in the
  nuclear industry.

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• Centrifugal extraction
• If separation is difficult in a
  mixer-settler unit, a centrifugal extractor may
  be used in which the mixing and the separation
  stages are contained in the same unit which
  operates as a differential contactor.
• In the Podbielniak contactor,the heavy
  phase is driven outwards by centrifugal force and
  the light phase is displaced inwards. Referring
  to the next figure, the heavy phases enters at D,
  passes to J and is driven out at B. The light
  phase enters at A and is displaced inwards
  towards to shaft and leaves at C. The two liquids
  intermix in zone E where they are flowing
  countercurrently through the perforated
  concentric elements are separated in the spaces
  between. In zones F and G the perforated
  elements are surfaces on which the small droplets
  of entrained liquid can coalesce, the large drops
  then being driven out by centrifugal force.

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Figure.Podbielniak contactor
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• The Alfa-Laval contactor shown in the
  figure, has a vertical spindle and the rotor is
  fitted with concentric cylindrical inserts with
  helical wings forming a series of spiral
  passages. The two phases are fed into the bottom,
  the light phase being led to the periphery from
  which it flows inwards along the spiral, with the
  heavy phase flowing countercurrently. High shear
  forces are thus generated giving high extraction
  rates.

Fig.Working principle of Alfa-Laval centrifugal
extractor
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EXTRACTION IN SUGAR INDUSTRY APPLICATIONS

• Extraction is needed for sucrose extraction
  from beets and cane.
• Beets are washed and seperated from any
  remaining beet leaves before processing.

The processing starts by slicing the beets
into thin chips. The slicing is done with sharp
knives which cut a V selection slice 4 to 5 mm
thickness to increase the surface area of the
beet to make it easier to extract the sugar.
The extraction takes place in diffusers. The
two well known diffusers for sucrose extraction
are The fixed-bed or Robert diffusion battery and
Continuous diffusion batteries or Silver
continuous diffuser.
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THE FIXED-BED OR ROBERT DIFFUSION BATTERY

• This was developed primarily in the
  beet-sugar industry, but is also used for the
  extraction of tanning extracts from tanbark,
• for the extraction of certain pharmaceuticals
  from barks and seeds, and similar processes. It
  consists of a row of vessels filled with the
  material to be extracted and through which water
  flows in series.The piping is so arranged that
  the fresh water comes in contact with the most
  nearly extracted material, and the strongest
  solution leaves from contact with the fresh
  material.Since each cell is filled and discharged
  completely ,

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• one at a time, each cell in the battery
  changes its position in the cycle,and therefore
  the piping must be so arranged that water can be
  fed to any cell, and the thick liquor drawn off
  from any cell, as circumstances may dictate.The
  arrangement of valves and piping became
  standardized in the beet industry and is
  generally found an all forms of diffusion
  battery. Figure shows that is a diagrammatic
  illustration of the principle of a diffusion
  battery.For every vessel or cell there is a
  heater, because the diffusion process takes place
  more rapidly at higher temperatures.Two main
  headers are necessary .One handles water and the
  other handles solutionand for every cell there
  must be three valves.In figure shows that the
  valves that are open are shown as circles and the
  valves that are closed are shown in solid black.

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• Fig.Diagram of diffusion battery

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• Consider figure Cell 1 is nearly exhausted
  and the cell 3 has just been charged.The space
  between the cossettes in cell 3 is therefore
  filled with air.Water is introduced into cell 1
  and flows down through cell 2, and up through its
  heater.It would not be convenient to pass the
  solution down through cell 3 because of the air
  which would be entrappedand the charge is cold,
  therefore additional heating is
  desirable.Consequently, the liquid flows from the
  heater of the cell 2 through the solution line,
  down through the heater of cell 3,and up through
  cell 3. A vent at the top of this cell
  discharges air.When liquid appears at this vent,
  the valves are quickly changed to the position
  shown in figure.Liquid now flows down through
  cell 3, up through its heater, and out ti the
  process.The operation shown in figure continued
  until cell 1 is completely extracted.By this time
  another cell to the right of those shown has been
  filled, cell 1 is dumped, water is introduced to
  cell 2, and the process continued.In a diffusion
  battery for beet cossettes there may be from 10
  to 15 cells.

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CONTINUOUS DIFFUSION BATTERIES

The next figure shows the Silver Continuous
diffuser.The figure shows only three units but
actually the battery consists of 20 to 24
units arranged in two tiers, one above the
other.The battery consists essentially of a
series of closed troughs A,A,A, each provided
with a helical screw B.Cossettes are intoduced
into the battery through chute C and are carried
together with the liquid in the direction
indicated by the arrows.At the end of the first
trough is a Wheel D with inclined perforated
buckets on the inside.It is so arranged that the
screw B discharges the cossettes into this wheel,
where they are picked up by the bucketsdrained
free from juice lifted, and discharged through
chute E which takes them into the second trough
A. Here the helix carries them in the opposite
direction discharges them from this to another
wheel which in turn forwards them to another
trough A, and so on until they are exhausted
and leave the battery.
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• Silver continuous diffuserA,A,A, extraction
  trougs B,conveyor for moving cossettes C, feed
  chute D, transfer Wheel E, transfer chute for
  chips.