(117e) Design of a Continuous Foam-Bed Reactor System for CO2 Removal From Power-Plant Exhausts

With renewed emphasis on reducing the carbon footprint in view of the United Nations Climate Change Conference in Copenhagen, reducing the greenhouse gas emissions is of prime importance. The average concentration of CO₂, the prominent of greenhouse gases, in the atmosphere today is about 385 ppm, which was about 280 ppm before the industrialization started. Intergovermental Panel on Climate Change (IPCC) says that the Earth's average temperature has risen by 0.74 degrees in the period from 1906 to 2005, and that the average temperature will continue to rise.

Various technologies are being developed to tackle this problem of CO₂ emissions. One such promising method is use of foam-bed reactors. The reactor offers very large gas hold-ups and low pressure drops, which is ideal for treatment of gaseous pollutants. This work focuses on treating CO₂ by reacting it with aqueous barium-sulfide solution in a continuous foam-bed reactor. The CO₂ required for this carbonation reaction could be obtained from smoke stack furnaces or power-plant exhausts, thereby reducing air pollution. H₂S gas is produced in the reaction, but it reacts faster with amines as compared to CO₂ and thus it can be removed with relative ease.

Some data for CO₂ removal using foam-bed reactors are available (Gaikwad and Bhaskarwar, 2006, 2007; Gaikwad et al., 2007) but they pertain to semi-batch mode of operation of the reactor. When continuous operation of a reactor is necessary, the conversion per pass is likely to be small, especially at high throughput rates of the liquid being processed. The conversion in the exit stream may, however, have to be quite high for the process to be acceptable from the point of view of meeting the required production level or environmental regulations. Multiple reactors will then appear, among others, as a feasible option.

Subramanyam et al. (1999) have developed a design procedure for treatment of the effluent from Kraft paper mills using foam-bed reactor. The primary type in multi-reactor configurations is the reactors-in-series. For the present case of CO₂ removal using aqueous barium-sulfide solution (gas absorption with chemical reaction and desorption), the graphical approach used in the design of a foam-bed reactor system was similar to the one proposed by Subramanyam et al. (1999). The modified single-stage model of a foam-bed reactor (Gaikwad and Bhaskarwar, 2007) was applied to the system of N foam-bed reactors, all of equal size in series, operated continuously.

The design of the foam-bed reactor system was done for the following specifications:

i.         Effluent flow rate = 100 tons/ day

ii.       Concentration of aqueous barium-sulfide solution in the inlet stream = 0.56 k mol/m³

iii.      Desired conversion = at least 95%

iv.     Gas-flow rate = 8 x 10^-3 m³/s

v.       Concentration of carbon-dioxide gas in the inlet stream = 1 x 10^-2 k mol/m³

A design chart has been prepared in the form of figure 1. In this chart, the height of a foam-bed reactor (H) has been plotted against the number of reactors required to be connected in series in order to achieve 95% conversion of the inlet liquid stream under the specified conditions (i-v). It is seen from the chart that the number of reactors and the height of each reactor are both smaller for a higher radius of the reactor (r_c). Such design plots have been made for different reaction velocities starting from 4300 m³/k mol.s to 11000 m³/k mol.s.

Figure 1. Variation of the height of foam bed with the number of reactors in (design chart I).

 

One can choose an aspect ratio (ratio of reactor height to reactor diameter) of say about 10, and find out the combination of H and r_c which gives a reasonably small number of reactors (say not greater than 10) for a given reaction velocity. Then, we have to turn to the second design chart shown in figure 2 wherein the average liquid hold-up in the reactor has been plotted against the number of reactors in series, again required for 95% conversion under the specified conditions (i-v). Having found out H and r_c in the previous step, one can now locate the point on the chart corresponding to the average liquid hold-up existing under the actual operating conditions by suitably interpolating for the corresponding value of the parameter r_c²H, if not directly readable from curves shown. The number of foam-bed reactors actually required in series can then be directly read off from the corresponding abscissa. This number, if it is a fraction, will have to be rounded off to the nearest higher integer to ensure a conversion greater than 95%.

Figure 2. Variation of the average liquid hold-up with the number of reactors in series (design chart II).

Thus using these design charts one can obtain the dimensions and number of foam-bed reactors required for a continuous operation to remove CO₂. The design process covers broad range of reaction velocities giving a flexibility of choosing a reactant liquid other than aqueous barium-sulfide solution. Following the procedure outlined in this work, similar plots can be generated for different gas velocities, conversions, effluent flow-rates, and liquid and CO₂ concentration to obtain the required design parameters.