Chemical Engineering November 2014 - 58

Solids Processing
Bulk solids:
Operating Direct-Contact
Heat Exchangers
Such units provide advantages over heated or watercooled
screws, fluidized beds and rotating drums for
heating and cooling bulk solids
Greg Mehos
Jenike & Johanson, Inc.
B
ulk solids are frequently
used as reactants, catalysts
or products in chemical process
industries (CPI) operations.
Often, they must be heated or
cooled so that they can be handled
safely or processed at a desired temperature.
Traditionally, heat transfer
to or from powders is carried
out using heated or water-cooled
screws, fluidized beds or rotating
drums. However, a more economical
means of heating or cooling bulk
solids may be to use a direct-contact,
bulk-solids heat exchanger.
These devices are designed as
hoppers, bins or silos that have been
modified to allow for the injection of
air or another gas. Heat transfer
takes place between the powder and
gas when the two streams are fed
at different temperatures. The cooling
or heating gas can be injected
from the bottom of the bulk-solids
heat exchanger, such that it passes
upward through the moving bed of
solids (countercurrent), or it can be
injected through the walls of the
vessel so that it flows perpendicular
to the solids flow (cross-current).
Countercurrent and cross-flow designs
are shown in Figures 1 and 2.
Direct bulk-solids heat exchangers
offer a number of advantages
over other types of heating or cooling
equipment. Because there are
no moving parts, capital costs and
maintenance expenses are low.
They are ideally suited for heatsensitive
materials because using
this approach, the temperature of
the solids changes gradually. Direct
heat exchangers can also be used to
remove trace volatile compounds,
or when a humid gas is injected, to
provide the residence time necesa
A
Az
B
Cp
dP/dz
g
G
h
H
H(')
L
m
S
T
T
V
x,
z


'
1
'
'
Subscripts
g
min
o
TABLE 1. NomENcLATurE
Surface area per unit volume of bed
Cross-sectional area (side)
Cross-sectional area (plan)
Outlet size
Heat capacity
Pressure gradient
Gravitational constant
Mass flowrate of gas
Heat transfer coefficient
Height of moving bed in contact with gas
Function given by Jenike [2]
Length of cooler
Constant equal to 1 for round outlets, 0 for slotted outlets
Mass flow of solids
Local temperature
Average temperature
Cooler volume
Spatial coordinates
Effective angle of internal friction
Void fraction
Hopper angle (from vertical)
Major consolidation stress
Arch stress
Angle of wall friction
Gas
Minimum
Outlet
sary to condition a powder.
This article discusses the design
criteria for a bulk solids cooler - a
direct-contact heat exchanger that
is used to reduce the temperature
58 ChemiCal engineering www.Chemengonline.Com noVemBer 2014
s
x
z
Solids
Side
Plan
of a bulk material. The same procedure
can be used to design a solids
heater except that, of course, a hot
gas is used to raise the temperature
of the bulk material.
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Chemical Engineering November 2014

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