(197a) Co2 Capture from Flue Gas Using a Modified Duplex Pressure-Swing Adsorption

Carbon dioxide is one of the greenhouse gases causing global warming. The flue gas discharged by power plants and industries contributes to more than 70% of the total CO₂ emitted into the atmosphere. Therefore, it is important to capture CO₂ from flue gases. Currently, the capture by absorption using various solvents is being practiced commercially. The energy required for the solvent regeneration is significantly high. Adsorptive and membrane separations are the potential alternatives to the existing absorption processes. Alternate to the conventional Pressure Swing Adsorption (PSA), based on the Skarstrom cycle and its variants, a new PSA called Duplex PSA has been proposed by Leavitt, (1992). He claimed that air can be fractionated into pure components and the products purity improves as the P_H/P_L approaches 1, a trend opposite to that in a conventional PSA. Diagne et al. (Diagne et al., 1994) too came up with the same process which they called it as Dual Reflux PSA. Sivakumar and Rao, (2005) presented a mechanistic view of the separation by duplex PSA. In the duplex PSA, the mass transfer zones spreads over the entire bed length and the concentration profiles are not destroyed and reformed in each cycle, unlike in the conventional PSA. The profiles are only shifted marginally depending upon the pressure ratio that is employed.

In this work, we present a modified version of duplex PSA which enhances the product purities and simulation studies on its performance for the capture of CO₂.

Duplex PSA: A schematic diagram of duplex PSA unit is shown in Fig. 1. It employs two beds and operates in a cycle consisting of four steps.

Step-1: Initially, Bed-1 is at a high pressure, P_H, and Bed-2 at a low pressure, P_L. The feed is introduced into Bed-1 somewhere along the bed, as in distillation. A part of the gas drawn from Bed-1 is recycled to Bed-2 and the rest is drawn as the raffinate product. Likewise, a part of the gas drawn from Bed-2 is recycled to Bed-1 and the rest is drawn as the extract product.

Step-2: First, the pressures in the beds are equalized by connecting either top or bottom ends of the beds and then Bed-2 pressurized to P_H with the gas that is drawn from Bed-1.

Step-3: The feed is introduced into Bed-2 and the end streams recycled as in Step-1.

Step-4:  Pressure equalization and resetting pressures as in step-2 is done.

This completes one cycle. The feed could also be introduced into the low pressure bed instead of the high pressure bed.

Modeling and Simulation: We developed a mathematical model to simulate the four steps of the duplex PSA and assessed the performance of the duplex PSA for the recovery of CO₂ from the flue gas. In the present work, we have taken flue gas as a binary mixture of CO₂ and N₂. As the amount of N₂ adsorbed inside the bed is very small compared to that of CO₂, we have considered N₂ as a non-adsorbing component. The theoretical energy required was calculated as the sum of the energies required for the recycle of the gas from the bed at P_L to the bed at pressure P_H, pressurizing a bed from P_L to P_H and evacuating a bed from P_H to P_L. The results of the simulation are given below.

Pressure equalization: Figure 2 shows the two modes of pressure equalization and pressure resetting in the original duplex PSA. It could be done either from the CO₂ rich end (mode-I) or the N₂ rich end (mode-II).

The performance of both the modes was compared. The product purities were about the same for both the modes. But, the energy requirement in mode-I was about 1.5 times more than that in mode-II. This was because of the difference in the amounts of gas to be compressed or evacuated between the mode-I and mode-II. The amount of extract recycle to be compressed was almost same for both the modes, but the amount coming out from the bed undergoing blowdown in mode-I was about 7 times more than the amount coming out in mode-II resulting in more energy requirement. So, in all the simulations reported below mode-II was employed for pressure equalization and resetting.

Effect of raffinate reflux ratio: Figure 3 shows the effect of raffinate recycle ratio, R_R, defined as the ratio of the raffinate recycled to the raffinate withdrawn, on the product purities and energy requirement at two different values of feed rate. At a low raffinate recycle ratio, R_R (0.1), the product purity was small. The purities of the products passed maxima at about R_R = 0.225. As R_R increased, the gas recirculation rate through the beds increased and therefore the energy requirement increased.

Effect of feed flow rate: Figure 4 shows the effect of feed on the product purities and the energy requirement. The purities of the products decreased monotonically with feed rate.  With increasing feed the relative amount of the extract recycle (i.e. R_E/F) that was to be compressed from P_L (0.3 atm) to P_H (1 atm) decreased. Therefore, as the feed rate increased, the energy requirement per unit mole of feed decreased.

Effect of desorption pressure: Figure 5 shows the effect of desorption pressure on the product purities and the energy requirement. The product purities increased on decreasing the desorption pressure and reached maxima at a P_L value of 0.3 atm. As shown in Fig 6, further decrease in the desorption pressure, resulted in the breakthrough of CO₂ concentration during pressure equalization and resetting. Therefore, greater amount of CO₂ came out of the bed and it was not possible to maintain profiles across bed-1, with CO₂ mole fraction close to 0 at top end and 1 at the bottom. As this stream was used for pressurization, the CO₂ got adsorbed at the end from where N₂ rich raffinate would be drawn in the next step. Because of this, the product purities dropped rather dramatically.  As the P_L decreased, the energy requirement increased because of the increase in the pressure ratio (P_H/P_L).

Modified duplex PSA: The Simulation studies on the duplex PSA revealed that the CO₂ composition in the outlet stream from Bed-2 in step 1 decreased with time. The CO₂ mole fraction in the gas drawn in the beginning of the step was much higher than the one drawn towards the end of the step.  Therefore, we modified the withdrawal of the extract. The extract was drawn first and then the remainder recycled to bed-1. Table 1 presents the comparison of

the performance of the original and modified duplex PSA. It can be seen that there is a significant improvement in purities of products on the modification of the cycle. The variation of purities of products and energy requirements with different parameters are similar that of the original duplex PSA.

  References:

1.      Leavitt, F.W., ?Duplex Adsorption Process', US Patent 5,085,674 (1992).

2.      Diagne, D., M. Goto, and T. Hirose, ?New PSA Process with Intermediate Feed Inlet Position Operated with Dual Refluxes: Application to Carbon Dioxide Removal and Enrichment', J. Chem. Eng. Japan, 27, 85 (1994).

3.      Sivakumar, S.V., and D.P. Rao, ?A mechanistic view of pressure swing adsorption processes', AIChE Annual Meeting,
Cincinnati, OH, USA (2005).