Page 207 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 185

& Mixing of similar and varying viscosity liquids. & Controlled mixing quality. Narrow residence time
& Dispersion of immiscible liquids and gases into distributions.
polymer melts. & Plug flow behavior.
& Devolatilization. & The closed system permits hygienic product
. What are the types of applications of in-line mixers? processing.
& Mixing of low-viscosity liquids. & Semibatch processes can also be operated continu-
ously, even when volumes are low resulting in max-
& Laminar blending of high-viscosity liquids.
imum flexibility, no additional reservoir, and pro-
& Turbulent mixing of miscible liquids.
ducts that are always fresh.
& Blending, for example, blending of gasoline of dif-
& Self-cleaning, requiring short purge/changeover
ferent grades.
time.
& Blending catalyst, dye, or additive into a viscous
. What are the disadvantages of static mixer?
liquid.
& The main disadvantages of static mixers are in-
& Homogenization of gas flows with regard to concen-
creased pressure drop and fouling problems.
tration or temperature.
. What are the specific advantages of static mixers for use
& Intimate and intensive contacting of liquids with
in food processing industry?
gases for increased mass transfer.
& The specific advantages of using static mixers in food
& Dispersion of liquids into liquids (immiscible) in
processing are the following:
liquid–liquid extraction operations.
➢ Good clean-in-place characteristics.
& pH control. Neutralization of waste streams.
➢ No production losses.
& Gas–liquid reactors.
➢ No unwanted air ingress.
& Controlled reactions over a narrow residence time
➢ Small volumes, short residence times.
distribution.
➢ Low space requirements.
& Mixing of dry powders.
➢ Easy installation in existing plant.
& Enhanced heat transfer in pipes, especially while
➢ Low investment costs.
dealing with viscous liquids at low Reynolds
numbers. . What are the circumstances that favor use of in-line
blenders?
➢ Promote significant radial motion in a pipe and
thereby reduce temperature gradients across the & When contact time requirements are very small, of
pipe cross section, especially for viscous liquids, the order of 1 or 2 s.
giving enhanced heat transfer rates. . What are the essential differences between rotor–stator
& Gentle heating of temperature-sensitive products, as and in-line mixers?
well as cooling with mixing action. & Rotor–stator mixers are typically introduced into
& Solid–solid mixing, for example, food products, dry the top of the vessel and operated in batch mode.
powders, and so on. This allows moving a given mixer from vessel to
vessel.
& Mixing additives such as fragrances, flavors, ice
creams, chocolate, dyestuffs, minerals, trace ele- & Rotor–stator mixers can slow down overall mixing
ments, vitamins, emulsifiers, and preservatives, pas- process and result in off-specification batches. As
teurization and sterilization, and many other areas. vessel size and viscosity increase, self-pumping ca-
pabilities of these mixers may not be sufficient. Dead
. What are the advantages of static mixers?
zones may be created resulting in off-specification
& The absence of moving parts reduces downtime for
products due to not only insufficient mixing but also
servicing and repairs to a minimum. Low mainte-
thermal degradation of the product due to hot spots
nance costs.
developed.
& Simple construction. No mechanical seals. Requires
& In an in-line rotor–stator mixer, the rotor–stator
small space.
head is installed in a closed housing, like in a
& Lower capital and operating costs.
pump casing. These are used for continuous
& Low shear forces to prevent excessive stressing of the operation or for operation between two or more
product. vessels. More uniformity is achieved as the fluid
& Low energy requirements (by 10–100 times) com- particles flowing experience the same residence
pared to agitated vessels or extruders. characteristics.