Packed Bed flooding.pdf - Youngstown State University's Personal ...

14-90 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION
Equation (14-182) says that the backcalculated NG is 2:
NG =−ln(1 − 0.86) = 2
For diffusing gases of similar molecular weight, the properties that control
heat transfer follow the same rules as those that control mass transfer. As a
result, the NH3 scrubbing and gas cooling processes achieve similar approaches
to equilibrium.
For an entry temperature of 120°C and an adiabatic saturation temperature
of 70°C, the expected outlet temperature would be
70 + (1 − 0.86)(120 − 70) = 77°C
This looks like a powerful concept, but its value is limited due to uncertainty
in estimating hGa. Both h G and a are difficult to estimate due to
dependence on power dissipation as discussed below. The primary
value of the N G concept is in estimating an expected change from baseline
data as in the comparison of Example 19 with Example 20.
Example 20: A Contactor That Is Twice as Long, No Bypassing
If we double the length of the pipeline contactor, double the effective
contact area, and double the number of transfer units to 4, what do we expect
for performance?
For NG = 4,
E = 1 − e−4 = 0.982
The NH3 in the exit gas would be expected to drop to
(1 − 0.982)(1000) = 18 ppm
and the expected outlet temperature would be
70 + (1 − 0.982)(120 − 70) = 70.9°C
If we double the length again, we increase the number of transfer units to 8 and
achieve an approach of
E = 1 − e −8 = 0.9997
The outlet temperature would be
70 + (1 − 0.9997)(120 − 70) = 70.015°C
Similarly the NH3 in the exit gas would be
(1 − 0.9997)(1000) = 0.3 ppm
Note that this approximates the exit condition of Example 17.
Transfer Coefficient—Impact of Droplet Size The transfer
coefficients increase as the size of droplets decreases. This is so
because the transfer process is easier if it only has to move mass or
heat a shorter distance (i.e., as the bubble or droplet gets smaller).
In the limiting case of quiescent small bubbles or droplets, the
transfer coefficients vary inversely with average bubble or droplet
diameter. For example, in heat transfer from a droplet interface to a
gas, the minimum value is
hG,min = heat transfer coefficient from interface to gas = 2kG/D
(14-183)
where kG = gas thermal conductivity and
D = droplet diameter.
IMPORTANCE OF TURBULENCE
The designer usually has control over the size of a droplet. As discussed
below, several of the correlations show that droplet diameter
varies with turbulent energy dissipation. For example, Eqs. (14-190)
and (14-201) suggest that in droplet systems
D ∝ {1/(gas velocity)] 1.2
and hence from Eq. (14-178)
a ∝ 1/D ∝ (gas velocity) 1.2 (14-184)
However, just looking at the impact of velocity on droplet size underestimates
the velocity impact because turbulence gives higher transfer
than Eq. (14-183) predicts. Transfer coefficients increase as the mixing
adjacent to the surface increases. This mixing depends on the
energy dissipated into the phases. To a first approximation this transfer
from droplets increases with local power dissipation raised to the
0.2 power.
h G,turbulent ∝ (power dissipated) 0.2
and since power dissipation per unit volume increases with (velocity) 3 ,
h G,turbulent ∝ (velocity) 0.6 (14-185)
The combined effect on interfacial area and on the transfer coefficient
is that the effective transfer increases greatly with gas velocity. From
Eqs. (14-178) and (14-185)
h Ga turbulent ∝ (velocity) 1.8 (14-186)
For quenching operations, this means that even though residence
time is cut as gas velocity goes up, the effective approach to equilibrium
increases. Since the volume for a given length of pipe falls with
(velocity) −1 , the expected number of transfer units NG in a given length
of pipe increases with (velocity) 0.8 .
See Example 21.
NG,turbulent ∝ (hGaturbulent)(volume) ∝ (velocity) 0.8 (14-187)
EXAMPLES OF CONTACTORS
High-Velocity Pipeline Contactors High-velocity cocurrent
flow can give more power input than any other approach. This is
critical when extremely high rates of reaction quenching are
needed.
Example 21: Doubling the Velocity in a Horizontal Pipeline
Contactor—Impact on Effective Heat Transfer Velocity in pipeline
quench systems often exceeds 62 m/s (200 ft/s). Note that this is far above the
flooding velocity in distillation packing, distillation trays, or gas-sparged reactors.
There are few data available to validate performance even though liquid
injection into high-velocity gas streams has been historically used in quenching
reactor effluent systems. However, the designer knows the directional impact of
parameters as given by Eq. (14-187).
For example, if a 10-ft length of pipe gives a 90 percent approach to equilibrium
in a quench operation, Eq. (14-182) says that the backcalculated NG is
2.303:
NG −ln(1 − 0.9) =2.303
Equation (14-187) says if we double velocity but retain the same length, we
would expect an increase of NG to 4.0.
NG = 2.303(2) 0.8 = 4
and
E = 1 − e−4 = 0.982
Restated, the approach to equilibrium rises from 90 percent to greater than 98
percent even though the contact time is cut in half.
Vertical Reverse Jet Contactor A surprisingly effective
modification of the liquid injection quench concept is to inject the
liquid countercurrent upward into a gas flowing downward, with
the gas velocity at some multiple of the flooding velocity defined
by Eq. (14-203). The reverse jet contactor can be envisioned as an
upside-down distillation tray. For large gas volumes, multiple
injection nozzles are used. One advantage of this configuration is
that it minimizes the chance of liquid or gas bypassing. Another
advantage is that it operates in the froth region which generates
greater area per unit volume than the higher-velocity cocurrent
pipeline quench.
The concept was first outlined in U.S. Patent 3,803,805 (1974) and
was amplified in U.S. Patent 6,339,169 (2002). The 1974 patent presents
data which clarify that the key power input is from the gas stream.
A more recent article discusses use of the reverse jet in refinery offgas
scrubbing for removal of both SO 2 and small particles [Hydrocarbon