Vacuum Drying

    Vacuum Drying

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      Feature Report
      ChemiCal engineering www.Chemengonline.Com april 2015
      In BrIef
      Vacuum drying can be a useful tool for solid products that are heat-sensitive. Here are
      some guidelines for the selection and use of various types of vacuum dryers
      Drying is an essential unit operation
      in a variety of chemical process in-
      dustries (CPI) sectors. Food, phar-
      maceuticals, chemicals, plastics,
      timber, paper and other industries use dry-
      ing equipment to eliminate moisture during
      product processing. Most dryers are classi-
      fied as direct dryers, where hot air (at near
      atmospheric pressure) is used to supply the
      heat to evaporate water or other solvents
      from the product. Another important dryer
      category, vacuum dryers, involves the use of
      a reduced-pressure atmosphere to surround
      the product.
      Drying is among the most energy-intensive
      unit operations, due to the high latent heat of
      vaporization of water and the inherent ineffi-
      ciency of using hot air as the (most common)
      drying medium. Depending on the specific
      product attributes required, different indus-
      try sectors require different types of drying
      technology. Drying high-value products that
      are likely to be heat-sensitive, such as food,
      pharmaceuticals and biological products,
      demands special attention. When dried by
      convection at higher temperatures, these
      heat-sensitive products degrade, change
      color and appearance and have lower vita-
      min or nutrient content. Vacuum dryers offer
      an alternate path. This article discusses the
      operation and selection of vacuum dryers,
      and provides examples of applications in
      which vacuum drying is used.
      Drying principles
      Drying involves two distinct drying periods,
      known as the constant drying period and the
      falling drying period (Figure 1). Drying occurs
      when liquid is vaporized by supplying heat to
      the wet feedstock. The liquid removed by the
      drying process could be either free moisture
      (unbound) or bound within the structure of
      the solid. The unbound moisture, normally
      present as a liquid film on the surface of a
      solid particle, is easily evaporated, while the
      bound moisture could be found within the
      solid material, trapped in the microstruc-
      ture of the solid. In this case, the moisture
      must travel to the surface to be evaporated.
      When a solid product is subjected to drying,
      removal of unbound and bound moisture de-
      pend on the rates at which these two pro-
      cesses proceed. Removal of unbound mois-
      ture depends on external conditions of air or
      gas temperature, flow, humidity, area of ex-
      posed surface and pressure. The movement
      of bound moisture depends on the nature
      of the product being dried and the extent of
      moisture within the product.
      Unbound moisture normally is removed
      by evaporation and vaporization. Raising the
      temperature facilitates the evaporation and air
      draws the moisture away. If the product being
      dried is heat-sensitive, then the temperature at
      which evaporation occurs (at the boiling point
      of water or other solvent) can be reduced by
      lowering the pressure with a vacuum.
      Vacuum-drying advantages
      Vacuum drying is a viable technology that
      has been used successfully for many years
      in the pharmaceutical, food, plastics and
      textile industries, among others in t
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      نشانی: بزرگراه فتح، جنب پمپ بنزین ، خیابان فتح ۲۵،پلاک ۱۵