LIQUID CONCENTRATION

LIQUID CONCENTRATION

• EVAPORATION • MEMBRANE SEPRATIONS• FREEZE CONCENTRATION

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.

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.

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. 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.

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

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.

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.

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.

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.

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

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.

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.

Evaporator Configurations

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

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.

• 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.

Multiple-effect evaporation

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.

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.

MEMBRANE SEPRATIONS

• Operation Principles

• Reverse Osmosis.

• Concentration polarization.

• Ultrafiltration.

Reverse osmosis is a process to separate solute and solvent

components in the solution.

Although the solvent is usually water, it is not necessarily

restricted to water.

The pore radius of the membrane is less than 1 nm.

While solvent water molecules, whose radius is about one tenth of

1 nm, can pass through the membrane freely, electrolyte solutes, such

as sodium chloride and organic solutes that contain more than one

hydrophilic functional group in the molecule (sucrose, for example),

can not pass though the membrane. These solutes are either rejected

from the membrane surface, or they are more strongly attracted to the

solvent water phase to the membrane surface.

The preferential sorption of water molecules at the solvent-

membrane interface, which is caused by the interaction force working

between the membrane-solvent-solute, is therefore responsible for the

separation.

Polymeric materials such as cellulose acetate and aromatic

polyamide are typically used for the preparation of reverse osmosis

membranes.

When a membrane is placed between pure water and an aqueous

sodium chloride solution, water flows from the chamber filled with

pure water to that filled with the sodium chloride solution, whereas

sodium chloride does not flow (Figure 1a). As water flows into the

sodium chloride solution chamber, the water level of the solution

increases until the flow of pure water stops (Figure 1b) at the steady

state.

The difference between the water level of the sodium chloride solution

and that of pure water at the steady state, when converted to hydrostatic

pressure, is called osmotic pressure.

When a pressure higher than the osmotic pressure is applied to the

sodium chloride solution, the flow of pure water is reversed: the flow

from the sodium chloride solution to the pure water begins to occur.

There is no flow of sodium chloride through the membrane. As a result,

pure water can be obtained from the sodium chloride solution. The above

separation process is called reverse osmosis.

The most successful application of the reverse osmosis process is in

the production of drinking water from seawater.

This process is known as seawater desalination and is currently

producing millions of gallons of potable water daily in the Middle East.

Fishing boats, ocean liners, and submarines also carry reverse

osmosis units to obtain potable water from the sea.

Ultrafiltration is a process based on the same principle as that of

reverse osmosis. The main difference between reverse osmosis and

ultrafiltration is that ultrafiltration membranes have larger pore sizes

than reverse osmosis membranes, ranging from 1 to 100 nm.

Ultrafiltration membranes are used for the separation and

concentration of macromolecules and colloidal particles.

Osmotic pressures of macromolecules are much smaller than those

of small solute molecules, and therefore operating pressures applied

in the ultrafiltration process are usually much lower than those

applied in the reverse osmosis process.

Membranes having pore sizes between those for reverse osmosis

and ultrafiltration membranes are sometimes called nanofiltration

membranes. The size of the solute molecules that are separated from

water, and the range of operating pressures, are also between those

for reverse osmosis and ultrafiltration.

Ultrafiltration membranes are prepared from polymeric materials

such as polysulfone, polyethersulfone, polyacrylonitrile, and

cellulosic polymers.

Inorganic materials such as alumina can also be used for

ultrafiltration membranes.

Typical applications of ultrafiltration processes are the treatment of

electroplating rinse water, the treatment of cheese whey, and the

treatment of waste water from the pulp and paper industry.

The pore sizes of microfiltration membranes are even larger than

those of ultrafiltration membranes and range from 0.1 µm (100 nm) to

several µm. The sizes of the particles separated by microfiltration

membranes are therefore even larger than those separated by

ultrafiltration membranes.

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

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).

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.

MEMBRANE SYSTEMS

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

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.

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.

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

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.

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.

MEMBRANE SYSTEMS

• Membrane Materials

• Cellulose Acetate/Polymer membranes/ Composite or Ceramic Membranes.

• Membrane Module Design

• Plate and frame/Spiral Wound/Tubular/ Hollow Fiber.

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.

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

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.

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

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.

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.

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.

Problem

• How to obtain high quality food product in evaporation concentration.

• How to lower the cost in liquid concentration operation.

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

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.