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Fabrizio Sarghini » 23.Evaporators


Evaporators

Evaporation process consists in the separation of the liquid volatile fraction from the nonvolatile solid fraction.

An high percentage of water is usually present in foods, and it is removed in processing by vaporization.

Several situations may appear in the vaporization process: the foodstuff may contain flavor compounds more volatile than water, it may be so viscous that it will hardly flow, it may deposit scale on the heating surface, it may tend to produce foam, it may precipitate crystals, or it may have a relative high boiling point.

This wide variety of liquid-food characteristics requires several typologies of evaporators to be used in the food industry.

Evaporators (cont’d)

Evaporative concentration is a process used:

  1. to produce a concentrate containing the desired solids in the solution;
  2. sometimes to remove the pollutants from waste streams.

The primary objectives of liquid-food evaporation are:

  1. to reduce the weight and volume of products and thereby reduce packaging, transportation, and storage costs;
  2. to reduce water activity to enhance storage stability;
  3. to reduce energy consumption during subsequent drying.

Evaporators: steam economy


Evaporators: single effect


Evaporators: latent heat table


Evaporators


Evaporators (cont’d)

Multiple Effects Evaporators: Steam Economy

Multiple Effects Evaporators: Steam Economy


Evaporators (cont’d)


Evaporators (cont’d)

Short-Tube Evaporator
Advantages of the short-tube vertical evaporator include:

  • low head-space required
  • suitable for liquids that have a moderate tendency to scale, since the product is on the tubeside, which is accessible for cleaning
  • fairly high heat-transfer coefficients can be obtained with thin liquids (up to 5–10 cP)
  • relatively inexpensive to manufacture.

However, heat transfer depends greatly on the effect of viscosity and temperature, it is not for use with temperature-sensitive materials, and it is unsuitable for crystalline products unless agitation is provided.

Evaporators (cont’d)

Falling Film Evaporator
The primary advantages of falling-film evaporators are:

  • relatively low cost
  • large heating surface in one body
  • low product hold-up
  • small floor space requirements
  • good heat-transfer coefficients at reasonable temperature differences.

The primary disadvantages are:

  • high headroom requirements
  • generally not suited for salting or scaling materials
  • recirculation is usually required.

Typical applications for falling-film evaporators are the concentration of dairy products (such as whey, milk protein, skim milk, cream and hydrolyzed milk), sugar solutions, urea, and black liquor.


Evaporators (cont’d)

Forced-Circulation
Evaporator

The advantages of using a forced-circulation evaporator are:

  • high heat-transfer coefficients
  • positive circulation
  • reduced fouling or scaling.

The primary disadvantages of forced circulation are:

  • possibly higher cost
  • power consumption for the circulating pump
  • a longer holdup of the product within the heating zone.

Evaporators (cont’d)

Rising Film Evaporator
Advantages of the long-tube vertical evaporator are:

  • reduced floor space requirements
  • relatively high heat-transfer coefficients due to partial two-phase flow
  • ability to handle foamy liquids.

Disadvantages are:

  • high head-room requirements
  • higher pressure drop through the tubes than in a falling film evaporator
  • hydrostatic head at the bottom of the tubes may increase product temperature and cause temperature-sensitivity problems.

Evaporators (cont’d)

Rising-Falling Film Evaporator
The rising/falling-film evaporator’s advantages are:

  • relatively low residence times
  • relatively high heat transfer rates
  • relatively low cost
  • large units can be manufactured
  • low hold-up
  • small floor space requirements
  • good heat transfer over a wide range of services.

Disadvantages include:

  • high head-room requirements
  • recirculation is frequently required
  • generally unsuited for salting or severely fouling fluids.

They are best applied when handling clear fluids or foamy liquids, and when large evaporation loads are required.


Evaporators (cont’d)

Plate-Type Evaporator
Advantages of gasketed plate-and-frame evaporators are:

  • well adapted to evaporating heat-sensitive, viscous and foamy materials;
  • compact with low headroom required;
  • easily cleaned and modified.

A major disadvantage is the large gasketed area. Leakage can be avoided by selecting the proper gasket for the application and following proper assembly procedures.


Evaporators (cont’d)


Evaporators (cont’d)

Agitated thin-film evaporators
Advantages of an agitated thin-film evaporator are:

  • short residence time in the heated zone, measured in seconds to minutes
  • high heat-transfer coefficients due to the turbulence imparted by the rotor
  • plug flow with minimum back-mixing
  • ability to handle high solids concentrations and viscous materials
  • less product decomposition, resulting in higher yields
  • high recovery because of the “squeezing” of residues by the rotor.

A disadvantage is its higher cost compared to standard evaporation equipment. In addition, staging or vapor recompression for energy recovery are not practical.


Evaporators (cont’d)

ENERGY CONSERVATION AND VAPOR RECOMPRESSION

  • Thermal vapor recompression (TVR, steam jet);
  • Mechanical vapor recompression (MVR).

Evaporators (cont’d)

Mechanical vapor recompression (MVR)

Mechanical vapor recompression (MVR)


Evaporators (cont’d)

Approximately 3.5 kJ are spent in generating 1 kJ of electrical energy and the amount of energy required to evaporate 1 kg of water can be estimated from the latent heat of vaporization of water (2257.1 kJ/kg at 100°C).

The conversion of fuel into steam usually takes place at approximately 80% efficiency due to energy losses.

N=Initial effects n=added effect

N=Initial effects n=added effect


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