Chemical Engineering September 2022 - 31

Facts At Your Fingertips
Gas Dispersion in Liquids
Department Editor: Scott Jenkins
I
njecting gases through a diffuser
into a liquid is an important aspect
of many operations in the chemical
process industries (CPI). Important
applications include dissolving reactant
gases into a liquid phase for further
reaction (such as in hydrogenation,
oxidation, ozonation), as well as
carbonation of beverages, stimulation
of fermentation processes (Figure 1),
aeration of wastewater for treatment,
stripping of air or oxygen from chemicals,
stripping volatile organic compounds
(VOCs) from liquid chemicals,
removal of moisture from fuels
and others. This one-page reference
provides information on key aspects
of gas diffusion in liquids, including
mass-transfer rate, agitation effects
and equipment selection.
Mass transfer
The main purpose of a sparging system
is to increase the gas-to-liquid
mass-transfer efficiency, (a ratio of the
amount of active gas component dissolved
in liquid to the amount of gas
injected). Low mass-transport efficiency
leads to an elevated gas-injection
rate. In this case, the increased
gas volume raises the cost to achieve
the desired results. The gas-to-liquid
mass-transfer efficiency is primarily
controlled by the mass-transfer resistance
of the liquid phase.
Fast and efficient mass transfer is
correlated with fine bubble propagation,
which increases the gas surface
area in contact with liquid.
Engineered porous metal or ceramic
materials create fine bubbles
acording to the requirements of the
application. Porous materials allowlarge
volumes of gas to be passed
with very high specific area. For example,
with equal volumes of gas,
1-mm bubbles would have 6.35 times
more gas-liquid contact surface area
than 6.35 mm (1/4-in.) bubbles [2].
The
gas-to-liquid
mass-transfer
rate per unit volume is calculated
using: KLa (C* - C), where KL is the
liquid-phase mass-transfer coefficient
that is dependent on the diffusivity,
liquid viscosity, temperature,
and mixing; a is the interfacial area of
gas bubbles in contact with liquid; C*
is the saturated concentration of the
gas in liquid; and C is the concentration
in bulk liquid.
By sparging small gas bubbles
with high surface-to-volume ratio
into the liquid, the interfacial area a is
increased and the gas mass-transfer
rate is improved. The mass-transfer
driving force (C* - C) also has a big
impact on the gas dissolution rate,
as the high-purity gas is used instead
of the lower-purity gas. For example,
the saturated concentration of oxygen
in water from pure oxygen is five
times higher than that from air, resulting
in a large increase in the oxygen
dissolution rate with pure oxygen.
Gas diffuser considerations
Spargers are chosen based on the
design and operating conditions of
the process The type and configuration
of the sparger used depend on
factors such as whether a process
is a continuous process or batch,
as well as the gas flowrate, tank
size, mechanical agitation, operating
pressure and temperature.
Materials of construction. Metal
spargers are used in high-temperature,
corrosive or oxidizing conditions,
whereas ceramic spargers are sufficient
for mild conditions.
Gas exit velocity. The gas exit velocity
at the sparger surface is an important
design criterion for sparger selection.
The actual gas volumetric flowrate for
exit velocity is calculated using the
pressure (P) that is the sum of tank
headspace pressure (PHeadspace), liquid
head pressure at the sparger (PLiquid),
and pressure drop across the
sparger element (ΔP). The minimum
sparger surface area is based on the
gas exit-velocity limit for the process.
The exit velocity limit is lowest for
the static sparging operation when
there is no mechanical agitation of
the liquid phase. For agitated tank
sparging and dynamic sparging,
where liquid has high forced velocity
along the sparger surface, the gas
exit-velocity limits are significantly
higher, requiring smaller spargers
for the same gas flow. The exit-veCHEMICAL
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SEPTEMBER 2022
FIGURE 1. In an example of a gas-sparging application,
a diffuser bar at the bottom of the tank releases
oxygen to stimulate a fermentation process
locity limit for agitated tank sparging
and dynamic sparging depends on
the impeller speed and liquid velocity,
respectively.
Agitation effects. Apart from using
a properly designed sparger, it is important
to focus on the mixing of gas
and liquid. In chemical process applications,
the reactor vessel is often
closed, so that the unreacted highpurity
gases, such as hydrogen or
oxygen, are not vented through the
system. In these applications, specially
designed mixing impellers are
used, depending on the operating
conditions of the reactor. Typically,
one impeller turbine is located above
the sparger to shear and disperse
the gas bubbles. Agitation at the liquid
surface may also be required to
entrain the headspace gas into the
liquid phase.
Diffuser sizing. The size of a sparger
depends largely on the superficial
gas exit velocity from the porous
sparger surface. This value is calculated
from the actual cubic feet
per minute (ACFM) per square foot
of sparger surface area (ACFM/ft2).
The ACFM is calculated at the liquid
pressure and temperature found at
the sparger (the ACFM is not based
on gas pressure)*.
n
Editor's note: Portions of the text in this column were adapted from
the following article: Air Products Inc., Gas Sparging, Chem. Eng.,
September 2012, p. 21.
*An additional reference is the following publication: Mott Corp., Gasliquid
Contacting Part Selector and Design Guide, www.mottcorp.com.
31
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Chemical Engineering September 2022

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