In general, they found that the addition of the LR step resulted in the best process performance, the addition of the REC step made the process performance slightly worse and required an additional column, and the addition of the F+R step made the process performance slightly better and did not require an additional column. The best cycle based on overall performance was a 5-bed 5-step stripping PSA cycle with LR, HR from countercurrent depressurization (CnD), and F+R (CO2 purity of 98.8%, CO2 recovery of 100.0% and feed throughput of 5.8 L STP/hr/kg) [4]. The next best cycle was a 5-bed 5-step stripping PSA cycle with LR, HR from LR purge, and F+R (CO2 purity of 96.6%, CO2 recovery of 71.3% and feed throughput of 57.6 L STP/hr/kg) [4]. These improved performances were caused mainly by the counterintuitive use of a very small purge to feed ratio (0.02) for the former cycle and a larger one (0.50) for the latter cycle. The former cycle also was good at producing CO2 at high purities and recoveries but at lower feed throughputs, and the latter cycle was good at producing CO2 at high purities and feed throughputs but at lower CO2 recoveries. The best performance of a 4-bed 4-step stripping PSA cycle with HR from CnD (with and without REC or F+R steps) was disappointing because of low CO2 recoveries (CO2 purity of 99.2%, CO2 recovery of 15.2% and feed throughput of 72.0 L STP/hr/kg) [4]. This last result was also counterintuitive and revealed that the CO2 recoveries of this cycle would always be much lower than the corresponding cycles with a LR step, no matter the process conditions, and that the LR step was very important to the performance of these HR cycles for this particular application.
In general, they also found that the HR step is as interesting as it is flexible when it comes to cycle configuration. In a HR only cycle, the HR must come from the CnD effluent, with some fraction taken as reflux and the remaining fraction taken as heavy product. This constitutes one possible cycle configuration for a HR only cycle. In contrast, for a DR cycle, there are seven possible cycle configurations, because all the HR can come from the CnD, LR or both steps. This is why these DR cycles are so interesting. The seven possible cycles are comprised of the following configurations.
1) All the CnD effluent can be used as HR, with the heavy product obtained from all the LR effluent.
2) All the LR effluent can be used as HR, with the heavy product obtained from all the CnD effluent.
3) All the CnD effluent can be used as HR, with some fraction of the LR effluent also used as HR, with the remaining fraction taken as heavy product.
4) All the LR effluent can be used as HR, with some fraction of the CnD effluent also used as HR, with the remaining fraction taken as heavy product.
5) All the CnD effluent can be taken as heavy product, with some fraction of the LR effluent also taken as heavy product, with the remaining fraction taken as HR.
6) All the LR effluent can be taken as heavy product, with some fraction of the CnD effluent also taken as heavy product, with the remaining fraction taken as HR.
7) Some fraction of both the CnD and LR effluents can be used as HR, with the remaining fraction of both effluents taken as heavy product, with the two fractions not necessarily being equal, especially since the flow rates of the CnD and LR effluent streams tend to be vastly different.
Moreover, in every one of these HR cycle configurations, an option exists for where to put the light gas effluent produced during the heavy reflux step. It can be taken as light product or it can be recycled back into the process. There are two ways to recycle the light gas effluent back into the process. The first way is simply to blend this light gas effluent with the feed during the feed step. This method does not add another step to the cycle and is referred to as the F+R step. The second way is to feed this light gas effluent to the heavy end of a bed just after the feed step. This method does add another step to the cycle and hence bed, and is referred to as the REC step.
Only a fraction of these cycle configurations has been studied so far [2-4]. The objective this presentation is to introduce these relatively new ways to operate the HR step and to provide an overview of the results obtained from them. In all cases, the system of interest is the concentration of CO2 from a stack gas effluent at high temperature using K-promoted HTlc. The goal is to determine if these different configurations can improve the process performance even beyond that already achieved.
1. S. P. Reynolds, A. D. Ebner and J. A. Ritter, New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration,” Adsorption, 11, 531-536 (2005).
2. 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).
3. 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).
4. 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).