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LIQUID CONCENTRATION EVAPORATION MEMBRANE SEPRATIONS FREEZE

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LIQUID CONCENTRATION EVAPORATION MEMBRANE SEPRATIONS FREEZE CONCENTRATION Vocabulary Concentration, dehydration, vital, evaporation , membrane concentration. freeze ... – PowerPoint PPT presentation

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Title: LIQUID CONCENTRATION EVAPORATION MEMBRANE SEPRATIONS FREEZE


1
LIQUID CONCENTRATION
  • EVAPORATION
  • MEMBRANE SEPRATIONS
  • FREEZE CONCENTRATION

2
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,

3
Vocabulary
  • flexibility viscosity sanitation bulk transport
    semipermeable equilibrate equilibrium migrate
    osmotic pressure feed permeate retentate
    solution solute solvent flux solubility
    polarization

4
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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8
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.

9
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.

10
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

11
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.

12
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.

13
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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21
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.

26
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.

35
MEMBRANE SEPRATIONS
  • Operation Principles
  • Reverse Osmosis.
  • Concentration polarization.
  • Ultrafitration.

36
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

37
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).

38
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.

39
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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42
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.

43
Factors Influencing Osmotic Pressure
  • Type of solutes (smaller molecules or larger
    molecules)
  • Concentration.
  • Salts and sugars influenced osmotic pressure
    mainly.

44
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

49
Solvent Flux in Reverse Osmotic Processing
  • Kwmembrane permeability factor
  • ?Ppressure differential across the membrane
  • ?p difference in osmotic pressure between feed
    and permeate

50
Mass Flux of Solute
  • Nsmass flux of solute through membrane
  • Ks membrane permeability coefficient
  • Cf Cpsolute concentration in feed and permeate
    respectively

51
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

54
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.

55
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.

56
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.

62
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.

63
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

64
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.

65
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.

73
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.

74
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

75
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

76
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.

77
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.

78
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.

86
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

87
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.
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