(68b) Vacuum Swing Adsorption Cycles for Carbon Dioxide Capture from Flue and Stack Gases

There are many misconceptions about and misleading reports on the viability of pressure swing adsorption (PSA) cycles for CO2 capture from flue gas. These misconceptions and misleading reports stem largely from a 1994 International Energy Agency report [1] that evaluated CO2 separation and capture from flue gas using 13X zeolite in both pressure swing adsorption (PSA) and temperature swing adsorption (TSA) processes. They concluded that PSA and TSA are too energy intensive for use with gas and coal fired power systems. This conclusion has led others to extrapolate their findings and further conclude that adsorption systems, in general, are not applicable for CO2 separation and capture from flue gas. This disturbing misinformation has perpetuated for nearly a decade with essentially the same misleading statements being made in a 2003 IEA report [2]. However, it is strongly contended that those reports might have been factually inaccurate, that some grave errors might have been made in the evaluation of cyclic adsorption technologies for CO2 capture from flue gas, and that PSA might be very applicable to the capture of CO2 from flue gas. These claims are substantiated with evidence from works found in the literature and from some of the author's recent works.

First and foremost, a recent report by Izumi [3] from Mitsubishi Heavy Industry indicated that CO2 was being recovered from flue gas commercially using a layered PSA bed containing X and A type zeolites and activated carbon. Although, not much detail about the PSA cycle was provided, this news was exciting. The Japanese must have discovered something positive about using PSA for CO2 capture from flue gas that has been overlooked by investigators in the USA and apparently everyone else around the world!

Second, in a series of works by the Ritter and co-workers [4-7], novel PSA cycles have been explored systematically for the capture of CO2 from flue gas at high temperature using a K-promoted hydrotalcite like compound (HTlc) as the adsorbent. They revealed that a variety of vacuum swing adsorption (VSA) cycles could be used to concentrate CO2 to over 90 vol% at over 90% recovery from a feed stream containing 15 vol% CO2. The most effective cycles were based on the heavy reflux concept, a concept that has been largely missing from the PSA literature. What is really exciting about these high temperature heavy reflux PSA cycles is that they are completely applicable to ambient temperature adsorbents like zeolites and activated carbons. Moreover, the Ritter and co-workers are still working on the problem and have discovered some better heavy reflux PSA cycles for CO2 capture from flue gas that will be discussed during this presentation. They also feel that the lack of information on and therefore the lack of understanding of the heavy reflux PSA concept for concentrating the heavy component from a feed stream might be the principle reason why the IEA reports were mistaken in their assessments of PSA for CO2 capture from flue gas. To this end, the heavy reflux concept will also be discussed during this presentation

Third, some very recent work by Webley and co-workers [8] substantiated the claims made by the Ritter and co-workers over the past few years on the economic viability of PSA cycles for CO2 capture from flue gas, especially VSA cycles. They proclaimed that the information about the cost of PSA provided in the 2003 IEA report [2] was in error. Zhang et al [8] recalculated the cost of PSA to be around $67/tonne of CO2 produced compared to that in the IEA report of $97/tonne. This new cost for PSA, as verified through the IEA, compared much more favorably with amine scrubbing technology at $60/tonne. What is most exciting about this recent set of events is that the PSA cycles analyzed by the IEA were not designed for heavy component recovery, like CO2; instead, they were designed for light component recovery, like CH4 from a CH4/CO2 mixture.

As alluded to above, there are significant differences between these two kinds of PSA cycles, i.e., between the ones designed for light component enrichment and recovery and those designed for heavy component enrichment and recovery, with understanding of the later lacking, even by some of the best PSA researchers in the field. To make matters worse, the design of a multi-bed PSA process with complex cycle sequencing, like that envisioned for CO2 capture from flue gas, is a non-trivial exercise. PSA beds are always coupled together, usually contain more than one layer of adsorbent, and operate sequentially with each undergoing cycle steps such as pressurization, feed, heavy reflux, equalization, depressurization, light reflux, and repressurization. As an added complication, each cycle step is not necessarily of the same duration. These features make the number of possible cycle configurations very large; and unfortunately, design strategies on how to best configure such a complex PSA cycle is more of an art than a science. To this end, this presentation will also introduce some complex VSA cycle sequences that might be applicable for CO2 capture from flue and stack gases.

References:

1. International Energy Agency, ?Carbon Dioxide Capture from Power Stations,? 1994.

2. International Energy Agency, ?Carbon Dioxide Capture at Power Stations and Other Major Point Sources,? 2003.

3. J. Izumi, ?Adsorption technology applications to the industries in Japan,? Abstract and presentation at the 8th International Conference on Fundamentals of Adsorption, Sedona, AR, May (2004).

4. S. P. Reynolds, A. D. Ebner and J. A. Ritter, New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration,? Adsorption, 11, 531-536 (2005).

5. S. P. Reynolds, A. D. Ebner and J. A. Ritter, ?Carbon Dioxide Capture from Flue Gas by PSA at High Temperature using a K-Promoted HTlc: Effects of Mass Transfer on the Process Performance,? Environmental Progress, 25, 334-342 (2006).

6. S. P. Reynolds, A. D. Ebner, and J. A. Ritter, ?Stripping PSA Cycles for CO2 Recovery from Flue Gas at High Temperature Using a Hydrotalcite-Like Adsorbent,? Ind. Eng. Chem. Res., 45, 4278-4294 (2006).

7. S. P. Reynolds, A. Mehrotra, A. D. Ebner and J. A. Ritter, ?Heavy Reflux PSA Cycles for CO2 Recovery from Flue Gas. Part I. Performance Evaluation,? Adsorption, submitted (2007).

8. J. Zhang, P. A. Webley, and P. Xiao, ?Effect of Process Parameters on Power Requirements of Vacuum Swing Adsorption Technology for CO2 Capture from Flue Gas,? Energy Conversation and Management, in press on line (2007).