Page 232 - Fluid mechanics, heat transfer, and mass transfer

210 TWO-PHASE FLOW SYSTEMS

➢ Otherbulkpropertiesthatmustbeconsideredduring & With moving bed flow, impermeable fine solids such
design include explosivity and toxicity. Materials as cement and fly ash can be effectively conveyed.
that may contain residual hydrocarbons, such as & Because of the lower velocities involved in dense
newly reacted polyethylene or polypropylene pow- phase conveying, pipeline wear and damage to the
ders,mayhavetobeconveyedusingnitrogentolimit solids due to attrition are reduced. Gas and energy
exposure to oxygen. requirements are lower with decreased operating
➢ Static electricity may be a source of ignition for costs. Long conveying distances are possible.
materials prone to explosion, in which case the . What is choking velocity in pneumatic conveying?
charge must be either dissipated by proper ground- & The gas velocity at which particles that are conveyed
ing or neutralized.
upward within a vertical pipeline approach their free
➢ Materials that are toxic or need strict containment fall velocity.
may require a vacuum system, in which any leak & Partial settling out of solids from a flowing gas stream
will be into the line, rather than out to the
and other instabilities may develop below certain
environment.
linear velocities of the gas called choking velocities.
➢ Double walled pipelines under positive pressure
& Normal pneumatic transport of solids accordingly is
have been used to convey materials such as con-
conducted above such a calculated rate by a factor of
taminated soils.
2 or more because the best correlations are not that
➢ Hygroscopic materials exposed to humid air often accurate.
become more difficult to handle. & Above choking velocities, the process is called dilute
. What is saltation velocity and how is it used in design- phase transport, and below, dense phase transport.
ing pneumatic conveying systems? & Transition from dense phase flow to dilute phase flow
& Saltation velocity is defined as the actual gas velocity characterizes choking point.
(in a horizontal pipe run) at which the particles of a . Classify gas–solids transport in horizontal piping.
homogeneous solid flow will start to fall out of thegas
& The flow of gases and solids in horizontal pipe is
stream.
usually classified as either dilute phase or dense
& In designing, the saltation velocity is used as a basis phase flow.
for choosing the design gas velocity in a pneumatic
& For dilute phase flow, achieved at low solids load-
conveying system.
ings, high volumes, and high gas velocities, the solids
& Usually, the saltation gas velocity is multiplied by a
may be fully suspended and fairly uniformly dis-
factor that depends on the nature of the solids to persed over the pipe cross section, particularly for
arrive at a design gas velocity. low-density or small particle solids. The gas stream
& For example, the saltation velocity factor for fine carries the materials as discrete particles by means of
particles may be of the order of 2.5 while the factor lift and drag forces acting on the individual particles.
could be as high as 5 for coarse particles such as soya & At lower gas velocities, the conveying process takes
beans. place with a certain proportion of the solids moving
. What is dilute phase conveying? What are its charac- through the upper part of the pipe cross section
teristic features? together with a highly loaded stream in the lower
& Dilute phase conveying takes place when particles part of the cross section. Depending on the charac-
are conveyed at a gas/air velocity that is greater than teristics of the solids, gas velocity, solids flow rate,
saltation and choking velocities. Particles are freely and other factors such as pipe size and roughness, the
suspended in the gas stream. flow patterns in the dense phase mode can vary from
being unstable to stable or an intermediate stable/
& Adverse effects of dilute phase conveying include
unstable regime.
pipeline wear and attrition among the particles due to
the high velocities involved. & The stable regime involves smooth flow while the
unstable regime is characterized by violent pressure
. What is dense phase conveying? What are its
surges as the moving layer breaks up.
characteristics?
& With higher loadings and lower gas velocities, the
& In dense phase conveying, velocities involved are
flow patterns in the dense phase mode can vary
below saltation velocity and plug or piston flow or
from conditions in which the particles may pack the
moving bed flow occurs.
pipe and move as slugs to situations in which the
& With plug flow, coarse and permeable bulk solids
particles may settle to the bottom of the pipe, forming
such as pellets or beans can be reliably conveyed.