Engineering, 2012, 4, 339-348
http://dx.doi.org/10.4236/eng.2012.47044 Published Online July 2012 (http://www.SciRP.org/journal/eng)
Hydrodynamics of the Pilot Scale Wet Scrubber with
Restricted Outlet Absorbent Flow Rate
Samwel Victor Manyele
Department of Chemical and Mining Engineering, College of Engineering and Technology,
University of Dar es Salaam, Dar es Salaam, Tanzania
Email: smanyele@udsm.ac.tz
Received February 24, 2012; revised April 20, 2012; accepted April 30, 2012
ABSTRACT
This paper presents the hydrodynamics of the wet scrubber coupled to a pilot CFB incineration facility. The scrubber
was operated using tap water as a scrubbing liquid. The outlet liquid flow rate, Q
o
, and accumulation rate, Q
a
, strongly
depend on the inlet liquid flow rate, Q
in
, with different profiles. At higher Q
in
values, Q
o
stabilizes, leading to higher Q
a
and finally flooding. The values of Q
a
were higher than Q
o
except for Q
in
ranging between 0.53 and 0.72 L/s (safe oper-
ating range) in which Q
a
Q
o
and Q
a
= Q
in
/2. The outlet-to-inlet liquid flow rate ratio, Q
o
/Q
in
decreased for Q
in
> 0.53
L/s. The increase in the accumulation-to-inlet liquid flow rate ratio, Q
a
/Q
in,
at higher Q
in
indicates a change in flow re-
gime towards flooding, accompanied by an abrupt increase in the height of accumulating liquid, H
a
. The difference be-
tween Q
a
/Q
in
and Q
o
/Q
in
(denoted as, ΔQ
ao
/Q
in
), shows a minimum close to zero in the safe operating range. The gas
flow rate towards the wet scrubber had slight effect on Q
o
and Q
a
when Q
in
was maintained constant. The ratio Q
o
/Q
in
decreased slightly with H
a
/H
t
irrespective of gas velocity. Changing the liquid-to-gas ratio, L/G and Q
in
strongly affects
the maximum and minimum values of Q
o
/Q
in
and Q
a
/Q
in
.
Keywords: Wet Scrubber; Liquid Accumulation Rate; Liquid Accumulation Level; Outlet Liquid Flow Rate;
Liquid-to-Gas Ratio; Valve Opening Position; Incineration
1. Introduction
Wet scrubbers are wet collection devices for fumes, mists
and suspended dusts used in air pollution control systems
[1]. Other air pollution control devices for dusts control
include electrostatic precipitators [2], cyclones [3], bag
houses [2,4-6], etc. Wet scrubbers collect particles by
direct contact with a liquid (usually water or lime water).
Wet scrubbers have been used for air pollution control in
incineration and waste-to-energy processes [7]. There are
multitudes of scrubber designs grouped according to the
liquid contacting mechanism used. In addition, scrubbers
can be broadly classified as low-, moderate-, or high-
energy units depending on how the liquid is introduced
into the scrubber [8]. Energy requirements in wet scrub-
bers can be expressed as the pressure drop across the
scrubber or by the level of contacting power, expressed
as kWh/m
3
gas treated [9]. The most common units of
measurement for contacting power are kWh/1000m
3
of
gas, while the liquid circulation rates are given in liters
per m
3
of gas [10-12].
During wet scrubber operation, the liquid inlet flow
rate corresponding to the existing gas flow rate is deter-
mined from mass balance equations. However, the outlet
liquid flow rate is normally less than the inlet liquid flow
rate due to some of the liquid accumulating in the bottom
of the scrubber. The advantages of the liquid accumula-
tion include sealing effect provided which prevents the
flue gas from short-circuiting to the scrubbing solution
tank and also as a cooling agent for the incoming hot
gases. However, the critical disadvantage of liquid accu-
mulation is the possibility of flooding, which occurs
when the liquid level in wet scrubber becomes exces-
sively high leading to overflow of the liquid towards the
gas inlet pipe and thus interfering with the combustion
process.
This paper investigates the hydrodynamics of the wet
scrubber based on inlet and outlet liquid flow rates, accu-
mulation rate for the liquid in the wet scrubber bottom
(expressed as the accumulation rate and accumulation
height), gas flow rate, liquid outlet control valve opening
position, liquid-to-gas ratio, etc.
2. Literature Review
Wet scrubbers rely on a liquid spray to remove gaseous
emissions, with particulate control as a secondary fun-
ction. The major types of wet scrubbers are venturi scru-
bbers, jet (fume) scrubbers, and spray towers or cham-
bers. Venturi scrubber’s consume large quantities of
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