(127c) Carbon Dioxide Removal Using a Foam-Bed Reactor

Carbon dioxide is one of the most
important greenhouse gases. The Global Warming Theory predicts that increased
amounts of carbon dioxide in the atmosphere tend to enhance the greenhouse
effect and thus contribute to global warming. Around 24,000 million tons of carbon-dioxide
gas are released per year worldwide, equivalent to about 6,500 million tons of
carbon. As per the data collected by United Nations in 2002, around 5,872
million tons of carbon-dioxide gas (24.3% of total carbon-dioxide emission) are
released in the US, 3,682 million tons (15.3%) in the European Union, and 1,220
million tons (5.1%) in India. Removal of carbon-dioxide gas from large-scale
gaseous emissions as from thermal power plants, etc. is a real challenge.

Foam-bed reactor offers a novel
method of removal of carbon-dioxide gas. These reactors can be used to treat
large volumes of gases because they offer very large gas hold-ups (more than
90%) and very less amount of liquid is required to treat these large volumes of
gases. Removal of carbon dioxide by treating it with aqueous barium-sulfide
solution has been investigated here, both experimentally as well as
theoretically, in a semi-batch foam-bed reactor. This carbonation reaction can
be carried out using carbon-dioxide gas obtained from smoke stack furnaces or
power-plant exhausts, thereby reducing the air pollution. The hydrogen-sulfide
gas produced in the reaction reacts faster with amines as compared to carbon-dioxide
gas and thus it can be removed with a relative ease. It can also be converted
into sodium hydrosulfide by reacting it with caustic solution (possibly in
another foam-bed reactor), or converted into elemental sulfur in a Claus
sulfur-recovery unit. Alternatively, the hydrogen-sulfide gas can be split to
produce the hydrogen gas. These end products would have a good market value too.

Experimental
data have been generated and analyzed to assess the role of the reverse
diffusional flux of the desorbed gas (hydrogen sulfide) in the actual
performance of the foam-bed reactor. The experiments are carried out using lean
carbon-dioxide gas. The variables studied are height of foam bed, initial
concentration of barium sulfide in aqueous solution, gas-flow rate,
concentration of carbon dioxide in mixture with nitrogen (diluent gas), volume
of the barium-sulfide solution charged into the reactor, and the surfactant
concentration in the aqueous solution.

For the case of simultaneous
gas absorption, reaction, and desorption in a foam-bed reactor, no fully-coupled
generalized model is available in the literature. A new mathematical model
describing gas absorption accompanied by a chemical reaction, and generation
and desorption of a non-reactive volatile product has therefore been developed
here. This model incorporates the gas-phase and surface resistances, which were
altogether excluded in most of the previous models of foam-bed reactors. Two
disparate models have been proposed to describe the absorption, reaction and
desorption processes in the two sections that the reactor has been divided
into, namely the storage section and the foam section. The material balance
equations for the storage section may then be written over the liquid in the
storage section to determine the dynamic performance of the reactor. The
simultaneous gas absorption, reaction, and desorption in the foam section of
the reactor has been simulated by the analysis of a single foam film surrounded
by limited gas pockets. Likewise, the storage-section analysis has been
performed by simulating the absorption-reaction-desorption process in surface-liquid
elements within the framework of the penetration theory. The performance of the
entire reactor under a variety of operating conditions has been simulated
obtaining the transient concentration of liquid-phase species B. The results of
simulation have been compared with the experimental data obtained for BaS - CO₂
(absorption) - H₂S (desorption) reaction system in a foam-bed
reactor under a variety of operating conditions.

The results indicate that the
conversion in the reactor increases with an increase in the initial concentrations
of barium sulfide in aqueous solution and of carbon dioxide in the gas mixture,
and with gas-flow rate. The conversion decreases with an increase in the volume
of the solution charged into the reactor.

Interestingly,
the effects of foam height and surfactant concentration on conversion reveal
the importance of reverse diffusional flux of desorbing hydrogen-sulfide gas.
As the foam height was increased from 0.1 to 0.4 m, the conversion increased
due to the increase in the interfacial area available for mass transfer as well
as the larger time of contact. At a foam height of 0.4 m, maximum conversion
was obtained beyond which the conversion decreased as the reverse diffusional
flux of desorbing hydrogen-sulfide gas overwhelmed the advantage of larger
interfacial areas and contact times.

The optimum conversion is obtained
at a surfactant concentration of 1000 ppm. The CMC value of non-ionic
surfactant Triton X-100 is less than 1000 ppm, and it reduces the surface
tension of the solution to a value less than 0.035 N/m. The reason for the
reduced conversion of barium sulfide, for the concentration of surfactant of
500 ppm, lies in the fact that the small number of surfactant molecules
adsorbed at the gas-liquid interface result in high initial diffusional flux of
CO₂ into the liquid-phase. This in turn results in higher reaction
rates and consequently increased reverse diffusional flux of the product gas, H₂S,
at later times. The overall diffusional flux of CO₂ is reduced by
the bulk flow induced by desorption of H₂S, and lower conversions
result. On the other hand, when the surfactant concentration is made as high as
10000 ppm, the number of surfactant molecules embedded in the film-gas
interface is much higher resulting in a tightly packed multilayer with vary
little free interfacial area available for the diffusion of CO₂. The
multilayer of surfactant molecules also offers a much greater diffusional
resistance. Both these factors contribute to the reduced fluxes of CO₂
into the foam, and hence the conversions of barium sulfide in the reactor are
lowered.

The variation of
two main parameters, viz. the height of foam bed and concentration of
surfactant, reveals the important role of desorption of hydrogen-sulfide gas in
governing the observed performance of a foam-bed reactor. The model predicted
the experimental data to within an accuracy of 10%.

Keywords:  Modeling
and simulation; Foam bed; Absorption; Desorption; Gas-phase and surface
resistances; Penetration theory

*Author
to whom all correspondence should be addressed. Phone:
091-11-26596161

Fax: 091-11-26581120. e-mail: anbhaskarwar@gmail.com
 

Homepages: www.ashoknbhaskarwar.freeservers.com

 
and http://ashoknbhaskarwar.tripod.com
  

⁺
Graduate student. e-mail: chr02017@ccsun50.iitd.ernet.in