Carbon dioxide removal using a foam-bed reactor Amit A. Gaikwad+, Gaurav Wadhwa, Ashok N. Bhaskarwar* Department of Chemical Engineering Indian Institute of Technology, Delhi Hauz Khas, New Delhi 110 016 INDIA Abstract 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 sulfurrecovery 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:
[email protected] Homepages: www.ashoknbhaskarwar.freeservers.com and http://ashoknbhaskarwar.tripod.com + Graduate student. e-mail:
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