A superstructure‐based optimal synthesis of PSA cycles for post‐combustion CO₂ capture

Abstract

Recent developments have shown pressure/vacuum swing adsorption (PSA/VSA) to be a promising option to effectively capture CO 2 from flue gas streams. In most commercial PSA cycles, the weakly adsorbed component in the mixture is the desired product, and enriching the strongly adsorbed CO 2 is not a concern. On the other hand, it is necessary to concentrate CO 2 to high purity to reduce CO 2 sequestration costs and minimize safety and environmental risks. Thus, it is necessary to develop PSA processes specifically targeted to obtain pure strongly adsorbed component. A multitude of PSA/VSA cycles have been developed in the literature for CO 2 capture from feedstocks low in CO 2 concentration. However, no systematic methodology has been suggested to develop, evaluate, and optimize PSA cycles for high purity CO 2 capture. This study presents a systematic optimization-based formulation to synthesize novel PSA cycles for a given application. In particular, a novel PSA superstructure is presented to design optimal PSA cycle configurations and evaluate CO 2 capture strategies. The superstructure is rich enough to predict a number of different PSA operating steps. The bed connections in the superstructure are governed by time-dependent control variables, which can be varied to realize most PSA operating steps. An optimal sequence of operating steps is achieved through the formulation of an optimal control problem with the partial differential and algebraic equations of the PSA system and the cyclic steady state condition. Large-scale optimization capabilities have enabled us to adopt a complete discretization methodology to solve the optimal control problem as a largescale nonlinear program, using the nonlinear optimization solver IPOPT. The superstructure approach is demonstrated for case studies related to post-combustion CO 2 capture. In particular, optimal PSA cycles were synthesized, which maximize CO 2 recovery for a given purity, and minimize overall power consumption. The results show the potential of the superstructure to predict PSA cycles with up to 98% purity and recovery of CO 2 . Moreover, for recovery of around 85% and purity of over 90%, these cycles can recover CO 2 from atmospheric flue gas with a low power consumption of 465 k Wh tonne À1 CO 2 . The approach presented is, therefore, very promising and quite useful for evaluating the suitability of different adsorbents, feedstocks, and operating strategies for PSA, and assessing its usefulness for CO 2 capture.

Advances in Pressure Swing Adsorption for Gas Separation

“…For the selected cycle, all the step times, blowdown pressure, and flowrates of rinse and purge steps should be determined [25,[188][189][190][191][192]. Alternatively, it has been suggested that a general "super-cycle" can be used to estimate the optimal duration of each of the steps [193].…”

Reply to the Letter to the Editor

“…This approach can be applied to a wide variety of sorbents, as well as feed and product specifications. We also note that a similar optimization approach was recently applied to precombustion CO 2 capture in IGCC processes, 5 where the energy requirements for PSA were only 46.8 kWh/tonne CO 2 captured, an order of magnitude less.…”

“…For a 583 MW PC plant (Case 9 in Exhibit), the amount of CO 2 3 we agree that the energy requirements for PSA are high, and CO 2 compression costs still need to be considered. On the other hand, in our study 1 we have considered only a standard sorbent, Zeolite 13X. As we cited, a number of higher performing sorbents, such as K-promoted Hydrotalcite, are also being investigated (see, e.g.…”

“…In Case 3 of our study, 1 a PSA cycle is designed to achieve a specified CO 2 recovery, while minimizing power consumption per tonne of CO 2 captured. The specified 85% CO 2 recovery and corresponding power requirement of 465 kWh/tonne CO 2, is independent of the amount of overall power generated by the pulverized coal (PC) plant.…”

Rigorous Global Optimization for Dynamic Systems Subject to Inequality Path Constraints

“…Thus both the sequential and simultaneous approaches have been successfully applied to a wide range of problems, and both are in active use. Recently, for example, the optimization of periodic adsorption processes has been considered by Vetukuri et al 14 using the sequential approach and by Agarwal et al 15,16 using the simultaneous approach. Features of the two approaches can also be combined into a quasi-sequential approach.…”