Chapter 2 evaporation | Môn Food technology - Trường Đại học Quốc tế, Đại học Quốc gia Thành phố Hồ Chí Minh

lOMoAR cPSD| 58562220 Lê Ngọc Liễu Office: A1-706 Email: lnlieu@hcmiu.edu.vn lOMoAR cPSD| 58562220 Evaporation Diluted liquid Concentrated liquid food products Objectives
Provide microbiological stability
Assist in reducing transportation and storage costs
What is the difference between evaporation and drying/distillation? 2 lOMoAR cPSD| 58562220
The vapor released in the evaporation chamber contains energy that may
be used for other industrial purposes. lOMoAR cPSD| 58562220
Such vapor has a temperature lower than that of steam, so its latent heat
of condensation is greater; for this reason, it is very important to take
advantage of this latent heat.
There are different methods for using this energy: vapor recompression and multiple effect. 4 lOMoAR cPSD| 58562220
Compress all vapors leaving the evaporator Accomplished
mechanically, using a compressor
driven by an electric motor, a steam turbine, or a gas engine Effective in reducing energy
demands: equivalent to 15 effects
Noisy to operate due to the use of large compressors lOMoAR cPSD| 58562220 Use a steam jet booster (highpressure steam) to
recompress major part of the exit vapors Through recompression,
the pressure and temperature of
exit vapors are increased Applied
to single-effect evaporators or to
the first effect of multiple-effect evaporators
Better suited to vacuum operation
Cheaper and easier to maintain 6 lOMoAR cPSD| 58562220 lOMoAR cPSD| 58562220 ! " " # $ 8 lOMoAR cPSD| 58562220 ! " " lOMoAR cPSD| 58562220 % 10 lOMoAR cPSD| 58562220 & ! lOMoAR cPSD| 58562220 " 12 lOMoAR cPSD| 58562220 Tube: 1-2 m long lOMoAR cPSD| 58562220 14 lOMoAR cPSD| 58562220 # lOMoAR cPSD| 58562220 # $ lOMoAR cPSD| 58562220 % lOMoAR cPSD| 58562220
For very viscous fluid foods, feed is
spread on the inside of the cylindrical
heating surface by wiper blades High rate of heat transfer
The cylindrical configuration results in
low heat-transfer area per unit volume of the product
High-pressure steam is used as the
heating medium to obtain high wall temperatures for reasonable evaporation rates
High capital and maintenance costs Low processing capacity & Heat sensitivity: low heating temperature, or short residence time, or
both Fouling: caused by solid in the feed gradual lOMoAR cPSD| 58562220
reduction in overall heattransfer coefficient eventually require shutdown
and cleaning production downtime + additional maintenance labor
Foaming: common. Reduce foaming by special design of feed inlet,
vapor/liquid separation area, reducing the boiling intensity, addition of antifoam
Solid: cause tube plugging, fouling, viscosity increase
Viscosity: affect heat-transfer coefficient
Distillate-to-concentrate ratio: affect wall wetting
Distillate vapor velocity: affect pressure drop
Heat transfer medium: hot oil or steam
Required materials of construction: affect cost and determine the thermal conductivity Type of evaporator Clean
High capacity Solids or Fouling Foamy Temperature Viscous Crystals sensitive Batch x x lOMoAR cPSD| 58562220 Horizontal tube x Vertical short tube x Vertical long tube x x Rising/falling film x x x Falling film x x x x x Forced circulation x x x x x Agitated thin film x x x x x x x ’ ( ’ ) ’ ’