Page 185 - Fluid mechanics, heat transfer, and mass transfer
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6



            MIXING


            6.1  Mixing                                  163         & The efficiency of mixing depends on the efficient use
            6.2  Mixing Equipment                        169           of energy to generate flow of the components.
                6.2.1 Baffled Vessels                     169         & Mixing reduces concentration and temperature
                6.2.2 Impellers                          172
                                                                       gradients in a mixture, thus exerting a favorable
                6.2.3 Turbine and Propeller Mixers       173
                                                                       effect on the overall rates of mass and heat transfer.
                6.2.4 Paddle Mixers                      176
                                                                     & Important aspects in the design of a mixer include the
                6.2.5 Draft Tubes                        177
                6.2.6 Other Types of Mixers              177           following:
                      6.2.6.1 Static Mixers              183           ➢ Provision of adequate input energy (for an appro-
                6.2.7 Solids Mixing                      186             priate time).
                6.2.8 Mixer Seals                        194
                                                                       ➢ Design of the mechanism for introducing the
                                                                         energy.
                                                                       ➢ Properties of the components.
                                                                   . How is mixing achieved? What are its objectives? What
                                                                    are its applications?
                                                                     & Mixing processes use mechanical means to forceful-
            6.1  MIXING
                                                                       ly circulate materials of separate phases to yield
              . What is mixing? What are its objectives and            a random distribution of the components.
                characteristics?                                     & Mixing objectives can encompass blending, gas
                & Mixing is a unit operation in which a relatively     induction, homogenization, particle reduction, emul-
                  uniform mixture is obtained from two or more com-    sification, and so on. A homogeneous mixture is
                  ponents. In some cases, mixingis requiredto promote  one in which the separate molecules being combined
                  mass transfer or chemical reaction.                  are interspersed. A heterogeneous mixture contains
                                                                       a random dispersion of materials having distinguish-
                & Typically, the objectives include creating a suspen-
                  sion of solid particles, blending miscible liquids,  able phases.
                  dispersing gases through liquids, blending or      & Mixing applications involve contacting and interfac-
                  dispersing immiscible liquids in each other, and     ing of a variety of substances, including gas–gas,
                  promoting heat transfer.                             liquid–liquid, solid–solid, gas–liquid, solid–liquid,
                & The degree of uniformity achievable varies widely. It  and gas–paste.
                  is easy to achieve virtually complete homogeneity  . What is the difference between mixing and agitation?
                  when mixing miscible liquids or mixing soluble     & Mixing refers to any operation used to change a
                  solids into a liquid, but it can be difficult to achieve  nonuniform system into a uniform one (the random
                  a homogeneous result when mixing two solids,         distribution of two or more initially separated
                  mixing two highly viscous liquids, or mixing items   phases).
                  with widely varying densities, especially if the   & Agitation implies forcing a fluid by mechanical
                  amount of one component is very small compared       means to flow in a circulatory or other pattern inside
                  to the amounts of the others.                        a vessel.




            Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
            Copyright Ó 2011 John Wiley & Sons, Inc.

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