LIQUID CONCENTRATION

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LIQUID CONCENTRATION

• EVAPORATION
• MEMBRANE SEPRATIONS
• FREEZE CONCENTRATION

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Vocabulary

• Concentration, dehydration, vital, evaporation ,
  membrane concentration. freeze concentration,
  reverse osmosis, ultrafiltration, fruit juices or
  purees, semiporous membrane, permeability, ice
  crystal slurry, coffee and tea extracts, volatile
  flavors and aromas, centrifugal force, droplets,
  entrained, agitation, buoyancy, gravity,

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Vocabulary

• flexibility viscosity sanitation bulk transport
  semipermeable equilibrate equilibrium migrate
  osmotic pressure feed permeate retentate
  solution solute solvent flux solubility
  polarization

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Concentration of liquid foods

• Concentration of liquid foods is a vital
  operation in many food processes. Concentration
  is deferent from dehydration,. Generally, foods
  that are concentrated remain in the liquid state,
  whereas drying produces solid or semisolid foods
  with significantly lower water content.

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Liquid Concentration Technologies

• Several technologies are available for liquid
  concentration in the food industry, with the most
  common being evaporation and membrane
  concentration. Freeze concentration is another
  technology that has been developed over the past
  few decades, although significant applications of
  freeze concentration of foods are limited.

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Evaporation Concentration

• Evaporation concentration means removal of water
  by boiling. Evaporation finds application in a
  variety of food processing operations. A primary
  application is concentration of fruit juices
  (orange juice concentrate), vegetable juices
  (tomato pastes and purees), and dairy products
  (condensed milk). Evaporation is also used to
  concentrate salt and sugars prior to refining.

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Membrane Separation Concentration

• The basis for membrane separations is the
  difference in permeability of a semiporous
  membrane to different molecular sizes. Smaller
  molecules pass through these membranes more
  easily than larger ones. Since water is one of
  the smallest molecules, concentration is easily
  accomplished using membranes with appropriate
  molecular-weight cutoffs.

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Freeze Concentration

• Water is partially frozen to produce an ice
  crystal slurry in concentrated product.
  Separation of ice crystals is then accomplished
  using some washing technique. Current
  applications of freeze concentration are limited
  to fruit juices, coffee, and tea extracts, and
  beer and wine. Freeze concentration produces a
  superior product

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Requirements for optimal evaporation

• (l) rapid rate of heat transfer.
• (2) low-temperature operation through
  application of a vacuum.
• (3) efficient vapor-liquid separation.
• (4) efficient energy use and recovery.

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Types of Evaporators

• Short tube or Calandria Evaporator.
• Long Tube Vertical Rising Film Evaporator
• Long Tube Vertical Falling Film Evaporator
• Forced Circulation Evaporator.
• Wipe Film or Agitated Thin Film Evaporator.
• Plate Evaporator.
• Centrifugal/Conical Evaporator.

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Short tube Evaporator

• A short but wide steam chest in the form of a
  shell and tube heat exchanger characterize this
  type of evaporator. Steam is fed to the inside of
  the internal tubes. Circulation is generated
  naturally. Density differences due to heating
  around the steam pipes cause the warmer fluid to
  rise and the colder fluid to sink. A vacuum
  source maintains to reduce boiling temperature.

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Long Tube Vertical Rising Film Evaporator

• A thin film of liquid food is formed on the
  inside of the long tubes, with steam providing
  heat transfer from the outside. The vaporizing
  bubbles of steam cause film of concentrate to
  rise upwards inside the tubes. Vapor and
  concentrate are separated, as they exit the top,
  in a separate chamber.

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Long Tube Vertical Falling Film Evaporator

• Using gravity to make liquid flow downwards.
  Steam condensing on the outside of the tubes
  causes evaporation of a thin film of product
  flowing down the inside of the tubes. Product and
  steam exit the bottom of the tubes together, then
  are separated.

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Forced Circulation Evaporator

• Fluid is pumped from the main evaporator chamber
  through an external steam chest. Vapor-liquid
  separation occurs in the main chamber, Dilute
  feed is added to the recirculation loop, and sent
  through the steam chest
• Since external pumping is used to maintain fluid
  flow, excellent heat transfer can be obtained,
  But, recirculation of the fluid through the steam
  chest causes long residence times

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Wipe Film or Agitated Thin Film Evaporator

• Very viscous foods are difficult to evaporate
  efficiently using any of the previously discussed
  evaporators. Products such as thick fruit or
  vegetable purees, or even highly concentrated
  sugar syrups, can be efficiently evaporated when
  a thin film at the heat transfer surface is
  continuously agitated or wiped to prevent
  buildup.

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Plate Evaporator

• A series of metal plates and frames forms the
  heat exchange surface both product and steam are
  directed in alternate gaps. Evaporation can take
  place within the plate and frame system, or
  evaporation can be suppressed by maintaining
  sufficient pressure and allowing evaporation to
  occur as the heated product flashes into a lower
  pressure chamber.

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Evaporator Configurations

• Single Effect Evaporation
• Multiple Effect Evaporation.
• Thermal Vapor Recompression.
• Mechanical Vapor Recompression.

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Single Effect Evaporation

• The simplest mode of evaporation is to use a
  single stage, where steam is fed into the steam
  chest, concentrate and vapor are removed, and the
  vapor is condensed into hot water.
• However, the vapors produced are still steam, and
  thus can be used to provide the heat for
  evaporation in a subsequent stage. Therefore,
  steam can be used many times to provide
  evaporation in a series of operations.

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multiple-effect evaporation

• In a two-stage evaporator, the vapors produced by
  evaporation of water in the first stage are fed
  into the steam chest of the second stage to
  provide further evaporation. Since there is no
  driving force. Thus, operating pressure in the
  second stage must be reduced to lower the boiling
  temperature

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Thermal Vapor Recompression

• The quality of the vapors produced during
  evaporation can be recompressed. One alternative
  is to use fresh steam to enhance the value of a
  portion of the vapors. This combined steam is
  then fed into the steam chest. High pressure
  steam is passed through a nozzle (or ejector)
  before entering the evaporator chamber. As the
  fresh steam passes through the nozzle.

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Mechanical Vapor Recompression

• Mechanical compression can be used to improve the
  quality of vapors. The vapors from a single stage
  are compressed to higher pressure in a mechanical
  compressor and then reused as steam in the steam
  chest . Reuse of compressed vapors makes up most
  of the steam addition. Only a small portion of
  fresh steam is needed to account for inevitable
  energy losses. Steam economies can be obtained.

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MEMBRANE SEPRATIONS

• Operation Principles
• Reverse Osmosis.
• Concentration polarization.
• Ultrafitration.

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MEMBRANE SEPRATIONS

• Membranes allow only certain molecules to pass
  through, effectively separating water molecules
  from other food constituents,
• Classification of membrane separations is based
  primarily on molecular size. reverse osmosis/
  ultra/micro filtration.
• No vapor-liquid interface to cause the loss of
  volatile flavors and aromas
• Membranes tend to foul

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Operation Principles

• Separations in semipermeable membrane systems is
  based on forcing some of the molecules in the
  system through the membrane while retaining
  others on the feed side while larger molecules
  remain on the feed side (retentate).

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difference between reverse osmosis and
ultrafiltration

• The difference between reverse osmosis and
  ultrafiltration or microfiltration is the size of
  molecules that can pass through the membrane.
  Reverse--osmosis membranes allow only the
  smallest molecules (Water, some salts, and
  volatile compounds) to pass through, whereas
  ultrafiltration and microfiltration limit only
  the largest molecules (i.e., proteins, starches,
  gums, etc.) and allow all smaller molecules to
  pass through.

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MEMBRANE SYSTEMS

• Membrane Materials
• Cellulose Acetate.
• Polymer membranes.
• Composite or Ceramic Membranes.
• Membrane Module Design
• Plate and frame.
• Spiral Wound.
• Tubular.
• Hollow Fiber.

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Osmotic Pressure

• A salt solution and pure water are separated with
  a semipermeable membrane. Water migrates from the
  pure water into the saltwater. As this
  equilibrium is attained, the pressures on the two
  sides of the membrane are unequal, The difference
  in pressure between the two sides is the osmotic
  pressure.

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Factors Influencing Osmotic Pressure

• Type of solutes (smaller molecules or larger
  molecules)
• Concentration.
• Salts and sugars influenced osmotic pressure
  mainly.

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Osmotic Pressure of Dilute Solution

• Csolute concentration
• Mwmolecular weight of solute
• Rgas constant

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Reverse Osmosis

• To cause an increase in concentration of the salt
  solution , the pressure of the salt must be
  raised above the osmotic pressure. When the
  applied pressure on the salt side exceeds the
  osmotic pressure, water molecules begin to flow
  from the saltwater into the pure water. This is
  called reverse osmosis.

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reverse osmosis process

• Feed under high pressure, exceeding the osmotic
  pressure of the feed, contacts the membrane.
  Material that passes through it is the permeate,
  while material that does not pass through the
  membrane, is retentate. Since membranes are not
  perfectly selective, they allow some smaller
  solute molecules to pass through the permeate is
  not pure water

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Solvent Flux in Reverse Osmotic Processing

• Kwmembrane permeability factor
• ?Ppressure differential across the membrane
• ?p difference in osmotic pressure between feed
  and permeate

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Mass Flux of Solute

• Nsmass flux of solute through membrane
• Ks membrane permeability coefficient
• Cf Cpsolute concentration in feed and permeate
  respectively

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Definition of solute rejection parameter

• A solute rejection parameter, R, is defined as
  the ratio of the amount of solute that passes
  through the membrane divided by the initial feed
  concentration.

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Concentration Polarization

• Molecules that do not get through the membrane
  accumulate on the feed side. A boundary layer is
  built up at the membrane surface due to this
  solute rejection. Concentrations of factors 1.2
  to 2 higher than the initial feed concentration
  can be developed in this polarization layer

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Negative Influences of Concentration Polarization

• The pressure driving force is reduced, so solvent
  flux is reduced In addition, solute flux is
  increased.
• Concentration buildup often leads to severe
  fouling on the membrane surface. When the
  concentration in this polarization layer exceeds
  the solubility concentration of the salt it
  precipitates and forms a more solid layer. This
  layer has reduced permeability.

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Techniques Reducing Polarization

• The feed should be as clear of insoluble solids
  as possible. Citrus juice concentration by
  reverse osmosis requires an initial filtration
  step to remove the pulp.
• Techniques that result in higher flow velocities
  across the membrane "sweep" away the
  concentration polarization layer and maximize
  permeate flux.
• Reduced concentrations in the feed also result in
  reduced polarization layer.

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Factors Influencing Flux in Reverse Osmosis

• 1.Transmembrane pressure (?P)
• 2.Type of feed material (concentration molecular
  weight of solute)
• 3.Temperature (Higher temperature gives lower
  viscosity and reduces concentration polarization)
• 4.Feed concentration
• 5.Feed flow rate (polarization layers)

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Ultrafiltration

• Ultrafiltration use higher permeability membranes
  allowing small molecules to pass through and
  retain larger molecules.
• Larger molecules are retained and dissolved
  sugars and salts pass through.
• In the dairy industry, ultrafiltration is used to
  concentrate milk or whey, allowing everything but
  the proteins to pass through.

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MEMBRANE SYSTEMS

• Membrane Materials
• Cellulose Acetate/Polymer membranes/ Composite or
  Ceramic Membranes.
• Membrane Module Design
• Plate and frame/Spiral Wound/Tubular/ Hollow
  Fiber.

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Cellulose Acetate

• The membranes provide high permeate flux and good
  salt rejection in reverse osmosis. However,
  cellulose acetate breaks down at high
  temperatures, is pH sensitive (pH 5 to 6), and is
  broken down by Cl- ions. Since chlorine cleaners
  and sanitizers are commonly used in the food
  industry, the sensitivity of cellulose acetate
  membranes to chlorine has caused significant
  problems.

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Polymer membranes

• Polyamides provide better pH resistance than
  cellulose acetate. Polysulfones provide a good
  alternative, operate at a wide pH range (1 to
  15), and have chlorine resistance (up to 50 ppm).
  They are easy be produced with a wide range of
  pore size cutoffs. But, these membranes do not
  withstand high pressures and are used almost
  exclusively for ultra-filtration

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Composite or Ceramic Membranes

• These membranes are made from porous carbon,
  zirconium oxide, or alumina. Due to the inert
  nature of the composite materials, membranes made
  from these materials have a wide range of
  operating conditions (temperature, pH). They are
  also resistant to chlorine attack and can be
  cleaned easily.

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Membrane Module Design

• Membranes can be packaged in many ways to provide
  options for separation. The main categories
  include
• Plate-and-frame arrangement
• Spiral-wound membranes,
• Tubular membranes
• Hollow-fiber membranes.

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Spiral Wound

• Rolling up a flat membrane and spacer system into
  a spiral-wound package Feed is distributed to the
  appropriate channels at one end of the roll
  permeate passes through the membrane and makes
  its way back around the spiral to a collector
  tube at the center of the roll. Permeate then
  passes out the center, while retentate is
  collected at the opposite end.

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Tubular

• A cylindrical membrane and support system is
  housed inside a larger tube. Feed is pumped into
  the center of the tube under applied pressure
  permeate passes through the membrane system and
  is collected in the outside tube. Retentate
  passes directly through the membrane and is
  removed from the opposite side.

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Hollow Fiber

• A bundle of smaller membrane tubes (only
  millimeters in diameter) containing hundreds of
  individual tubes may be housed in a single larger
  shell. Feed is directed into the tubes at one
  end, while concentrate is removed at the other
  end. Permeate passing through the membranes is
  collected from the shell side of the housing.

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CLEANING AND SANITATION

• Mild acids and bases with nonionic surfactants,
  enzymes, and complexing agents are used to clean
  membranes
• Clean-in-place systems can be used to clean
  membrane modules, with the most rapid flow rate
  possible to induce turbulence at the membrane
  surface.

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FOOD QUALITY IN MEMBRANE OPERATIONS

• Because low temperature operation, thermal
  degradation of nutrients does not occur.
• The quality of foods processed using membrane
  systems is generally superior to that produced
  using other concentration technologies

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FREEZE CONCENTRATION

• TYPES OF FREEZE CONC. UNITS
• Ice Crystallization
• Direct Contact Freezers.
• Indirect-Contact freezers
• Separation Devices
• Mechanical Press
• Centrifugal.
• Wash Column.
• ECONOMIC DESIGN OF FREEZE CONCENTRATION

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Definition of Freeze Concentration

• A liquid food is cooled with sufficient
  agitation, ice crystals nucleate and grow, and a
  slurry of relatively pure ice crystals removed,
  The concentrate can be obtained. Separation of
  these pure ice crystals leaves a concentrated
  product.

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Advantages Disadvantage of Freeze Concentration

• High product quality due to low-temperature
  operation
• Absence of a vapor-liquid interface maintaining
  original flavors.
• Higher cost of than the other two.

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Employed on Wide Range of Products

• Fruit juices, milk products, vinegar, coffee and
  tea extracts, beer and wine, and other flavor
  products.
• Concentration of alcoholic beverages is one
  application where freeze concentration is
  superior to other techniques.

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Freezing-point Depression

• Products containing low-molecular weight
  compounds, like sugars and salts, experience a
  reduction in freezing point as product is
  concentrated.

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TYPES OF FREEZE CONCENTRATION UNITS

• Ice Crystallization
• Direct Contact Freezers
• Indirect-Contact freezers
• Separation Devices
• Mechanical Press
• Centrifugal
• Wash Column.

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Problem

• How to obtain high quality food product in
  evaporation concentration.
• How to lower the cost in liquid concentration
  operation.

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Problem

• Describe the principles of both evaporation and
  membrane concentration
• What are the differences between reverse osmosis
  and ultra-filtration.
• How to understand the membrane materials
• How to consider the membrane module design

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Problem

• Explain the principles of freeze concentration
• How to understand the operation of freeze
  concentration
• What are the advantages of freeze concentration
  and how to to obtain food in high quality
  economically.