Chapter 9 OTHER SEPARATION PROCESSES

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Principles of Operation

• In order to form crystals, the liquid phase needs
  to be cooled below the freezing point or
  concentrated above the solubility point. To
  freeze ice in foods, the product must be frozen
  below the freezing point, which is determined by
  the concentration of small molecular-weight
  solutes (salts, sugars). Smaller molecules, like
  salts and simple sugars, have significant effect
  on the freezing point of water.

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• In sugars and salts crystallization , the
  concentration of these components must exceed
  some equilibrium value. The solubility (or
  saturation) concentration represents the
  equilibrium between solute molecules in the
  liquid state and those in the crystalline state.
  This equilibrium often depends on temperature,
  and for many systems, the solubility
  concentration increases with increasing
  temperature.

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Distillation

• Separating components with different
  volatilities. Distillation is used for separation
  of volatile components like flavors. In
  processing of orange juice, the vapors coming off
  an evaporator contain a significant amount of
  volatile flavors and aromas. These are separated
  from the condensed vapors in a distillation
  column.
• The products from distillation are a stream rich
  in the volatile components (distillate) and
  another stream that has low volatile
  concentration (residue or bottoms).

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Principles of Operation

• When a material containing components with
  different vaporization pressures is heated, the
  more-volatile components vaporize more readily
  and can be separated from the less volatile
  components. In simple batch distillation, a
  liquid mixture is heated to boiling, and the
  vapor formed is separated and condensed to
  produce a product high in the volatile components
  (distillate). The liquid remaining (residue) is
  low in these volatile components.

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Industrial distillation operations

• The feed enters a distillation column, where it
  is heated as it flows continuously through the
  system. The vapors rise to the top of the column
  and are removed, while the residual liquid falls
  to the bottom. As the liquid and vapor streams
  pass each other in the distillation column,
  equilibrium concentrations in both vapor and
  liquid phases are approached. These equilibrium
  concentrations are based on the relative
  volatilities of the components. Flow of vapors
  goes up and flow of liquid goes down in the
  column.

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Packing tray columns

• Distillation columns are either filled with
  packing material to enhance contact between vapor
  and liquid phases or have distinct trays that
  allow intimate contact between phases. In tray
  columns, flow of liquid is down at one side of
  the tray, whereas flow of vapor is upwards
  through the tray Trays are designed to allow
  optimal flow of both vapor and liquid phases but
  still allow sufficient mixing of the two phases,
  so that equilibration of volatile components
  occurs between phases.

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Tray columns

• In tray columns, flow of liquid is down at one
  side of the tray, whereas flow of vapor is
  upwards through the tray. Trays are designed to
  allow optimal flow of both vapor and liquid
  phases but still allow sufficient mixing of the
  two phases, so that equilibration of volatile
  components occurs between phases.

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Packing columns

• In columns filled with packing material, flow in
  and around the packing material allows intimate
  contact between vapor and liquid phases. Packing
  material is often shaped like a saddle or a ring
  and made of either ceramics, plastics or metals.
  Packing materials range in size from 15 to 90 mm,
  and are typically randomly oriented within the
  column to allow uniform flow of both phases.

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Absorption/Stripping

• Removal of small amounts of impurities in a vapor
  phase by selectively absorbing or dissolving a
  component into a liquid is called absorption. It
  is commonly used, for example, to remove ammonia
  or hydrogen sulfide from air.
• The reverse process, removal of impurities from a
  liquid by selective absorption into a gas stream,
  is called stripping. In the food industry, steam
  stripping is used for deodorization of vegetable
  oils.

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• In oil processing, undesired flavors and aromas
  are caused by the presence of small amounts of
  free fatty acids and other organic material
  (aldehydes, ketones). Many applications of
  vegetable oils in the food and cosmetic
  industries require that these oils be free from
  aromas characteristic of their origin. basically,
  a steam stripping process is used to selectively
  remove them from the oil

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• Steam stripping is based on the differences in
  concentration between free fatty acids and the
  main triglycerides of vegetable oils. That is,
  free fatty acids have some finite solubility in
  steam, whereas triglycerides are essentially
  insoluble. Thus, when steam contacts the
  vegetable oil, free fatty acids (and other
  volatiles) are selectively removed, leaving
  essentially pure triglycerides in the liquid oil.
  

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Principles of Operation

• The driving force for either absorption or
  stripping is the difference in concentration of
  the impurity compound in the liquid and gas
  phases. In absorption, the impurity migrates to
  the liquid phase until equilibrium is attained,
  whereas in stripping, the impurity migrates to
  the vapor phase to reach equilibrium. In
  stripping, differences in vapor pressure between
  components provide the driving force for
  selective removal of volatile components (fatty
  acids, etc.). In deodorization of vegetable oils,
  steam stripping is accomplished under vacuum
  conditions and elevated temperatures.

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Batch Deodorization Process

• The oil is filled into the stripping column and
  steam introduced at the bottom of the vessel. As
  the bubbles of steam rise in the column, fatty
  acids are removed from the oil and absorbed into
  the steam. At the top of the column is a
  separator, followed by a condenser that cools the
  steam to condense the fatty acids, and an exhaust
  to the vacuum system. Distillate, which contains
  the free fatty acid volatiles, is removed from
  the vapor condensate, and the deodorized oil is
  removed from the bottom of the chamber.

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Continuous Deodorization Process

• Steam rises upwards through a column as the
  liquid oil migrates to the bottom. To provide
  adequate contact area between the vapor and
  liquid in either absorption or stripping (1) the
  vapor can be broken into small bubbles within a
  continuous liquid (tray column) (2) the liquid
  stream can be broken into multiple thin films
  that flow across the continuous vapor phase
  packed columns) or (3) the liquid can be
  dispersed into small droplets within the vapor
  phase (spray contactors).

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Extraction / Leaching

• One component contained in a solid raw material
  is extracted by dissolving it in a liquid
  solvent.
• Brewing of coffee or tea from ground coffee beans
  or tea leaves is a form of leaching. Here, the
  soluble components (organic acids, volatile
  flavors and aromas, etc.) contained in coffee
  grounds or tea leaves are extracted by passing
  hot water over the solid material. The components
  in the grounds or leaves that are soluble in hot
  water diffuse into the hot water, based on the
  difference in concentration.

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• the water-soluble compounds in ground sugarcane
  or beet are extracted by soaking them in hot
  water prior to sugar refining. The dilute aqueous
  solution after leaching contains the soluble
  sugars, as well as all other water-soluble
  components. To refine the sugar from this dilute
  juice, the nonsugar components must be removed
  and the juice concentrated.

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extraction of vegetable oils by organic solvents

• Organic solvents such as hexane, acetone, or
  alcohol come into contact with ground or flaked
  seeds to extract the oils contained within.
  Pretreatment of the raw material (flaking or
  grinding) is necessary in this case in order to
  break the cell walls and free the oil entrapped
  within. Sometimes the residual oil in a press
  cake after mechanical expression is removed by
  solvent extraction. Residual solvents in both the
  extracted phase and the remaining solid phase
  must be removed by some evaporation technique.

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Supercritical Fluid Extraction

• By raising the pressure and temperature of CO2
  gas above its critical point (T 31.2"C P
  7.38 Mpa), it becomes a supercritical fluid.
  Under these conditions, CO2 has densities near
  that of a liquid but still maintains gas like
  qualities of penetration. This makes it an
  excellent solvent for certain types of compounds.
  In addition, removal of CO2 after extraction is
  easily accomplished by simply lowering pressure
  and causing the CO2 to convert to the gaseous
  phase. supercritical fluid extraction using CO2
  has found commercial application in
  decaffeination of coffee beans.

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Principles of Operation

• The basic principle underlying solid-liquid
  extraction is that there is a concentration
  difference between the solid and liquid that
  causes molecules to diffuse from one to the
  other. Actually, diffusion of solute within the
  solids may not be the only mechanism involved in
  leaching. Washing of solutes off the solid
  surface, displacement of extract from inter
  particle pores, and solubilization (or
  reation-induced creation of soluble solutes from
  insoluble precursors) may also occur during
  solid--liquid extraction.

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• In the case of sugar refining, the concentration
  of sucrose in the sugarcane is fairly small (only
  a few percent). But since the initial
  concentration of sugar in the hot water is zero,
  there is a concentration driving force for the
  sugar to diffuse into the hot water. This mass
  transfer continues until the concentrations are
  identical in the solid and liquid phases.

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pretreat

• In order to ensure efficient and rapid removal of
  desired compounds in food raw materials, it is
  often necessary to pretreat the material in some
  way. For example, nuts and seeds are ground or
  flaked to ensure rupture of cell walls and allow
  efficient mass transfer of desired compounds into
  the extraction liquid. Extraction of coffee from
  ground beans and tea from tea leaves is best
  accomplished after drying and grinding the raw
  materials. In addition to rupturing cell walls,
  drying and grinding provide a large surface area
  for extraction to take place.

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Batch processes

• Solvent is allowed to come into contact with the
  prepared solids, and batch time is determined by
  rates of diffusion of the soluble components out
  of the solids. Once a batch of solids has been
  extracted, the vessel is emptied, cleaned, and
  refilled with a new charge of solids. A battery
  of batch extractors is shown in Figure 9.20.

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Continuous leaching processes

• percolation extractors.
• screw-conveyor extractors.

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percolation extractors

• In this case, the flakes are filled into discrete
  hoppers, which move along the extractor. Solvent
  is fed to the top of each bed and collected as it
  drains through at the bottom. Solvent is fed
  countercurrent to the movement of the bed of
  flakes. A moving conveyor also can be used to
  transport solids along the length of the
  extractor, with solvent fed countercurrent in a
  similar fashion.

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screw-conveyor extractors

• In the sugar-refining industry, screw conveyors
  are often used to transport the sliced sugarcane
  or beet for contact with hot water. In these
  systems, the screw conveyor provides a moving bed
  of cossettes for countercurrent extraction, as
  shown in Figure 9.22. Having the cossette move up
  a slope allows the hot water to drain at the
  bottom.

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