Thermal Management of Structured Adsorbents in CO 2 Capture Processes Fateme Rezaei* and Mattias Grahn Division of Chemical Technology, Luleå University of Technology, SE-97187, Luleå , Sweden ABSTRACT: In order to have an efficient adsorptive separation, structured adsorbents are expected to satisfy not only mass transfer and pressure drop requirements but also thermal management requirements. To what extent the structure of adsorbent affects the thermal behavior of the system is a question which will be addressed in this study. The primary purpose of this study was to assess the performance of alternate adsorbents through development of numerical models for prediction of their thermal behavior under a two-step pressure swing adsorption (PSA) condition. The single-step CO 2 breakthrough and temperature profiles confirmed the efficiency of structured adsorbents in managing the thermal effects evolved in the bed under nonisothermal conditions. Two-step PSA results also showed that under real cyclic processes, and especially during rapid cycling, structured adsorbents maintain their superiority and introduce themselves as potential candidates for advanced PSA units. However, the performance of a structured adsorbent is highly dependent on its dimensions and geometrical parameters describing the structures, and these parameters should be optimized for each separation. 1. INTRODUCTION Adsorption processes are commonly assumed to be isothermal. This assumption is based on the fact that thermal effects caused by heat generation inside the column impose negligible influence on adsorption dynamics. However, in practice, this assumption is no longer justified especially for high concentration feeds in which the heat of adsorption generates thermal waves in both axial and radial directions, affecting the concentration profiles. 1-3 Therefore, in most cases, the rate of adsorption may be controlled by both heat and mass transfer kinetics, and hence, the nonisothermal nature of adsorptive gas separation processes must be taken into account during process design. 4-7 In our previous study, 8 we presented a new methodology to find the optimal adsorbent structure and showed that for dilute systems, parallel channel adsorbents in the form of laminate structures exhibit substantially better performance than other structures, but for the sake of computational simplicity we confined our study to the case of isothermal operation only. However, nonisothermal effects especially at higher feed concentration are very important and can deteriorate process efficiency and deform breakthrough curves. Therefore, in order to have a comprehensive analysis of an adsorption process, it is necessary to take into consideration an efficient thermal management during cyclic adsorption processes such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). Basically, there are two different approaches for managing thermal effects in adsorptive gas separation processes, namely external and internal management. In the first approach, the thermal effect can be efficiently managed by employing an in- bed multitubular heat exchanger to extract the heat of adsorption from the column. To do so, the adsorbent material is required to exhibit high effective thermal conductivity and a high heat exchange coefficient. In this respect, the composite of expanded natural graphite with activated carbon has been already studied by several researchers. 9,10 This method is only applicable to small scale columns and the difficulty associated with its industrial application makes it impractical for real gas separation units. In the internal mode of thermal management, in order to control in situ thermal effects generated during adsorption, the focus is given to adsorbent materials. In this case, the goal is to employ the materials which are capable of storing a large amount of heat and hence mitigating thermal effects in such a way that isothermal conditions could be maintained in the column. The materials used for this purpose are normally phase change materials (PCMs) which can be mixed with active adsorbent materials. 11 Sometimes different adsorbent materials are blended to mitigate the heat generated during adsorption. Under theses conditions the adsorption heat is transferred from the “strong” adsorbent to the “weak” adsorbent resulting in enhanced equilibrium capacity. This method also possesses some limitations: the PCM could dilute the amount of active adsorbent and reduce the adsorbent loading on one hand and increase the characteristic diffusion path and, hence, the mass transfer resistance on the other hand. Another possible option to carefully manage the thermal effects is to utilize adsorbent with different geometries than pellets. The use of conventional adsorbent materials in the form of beads or granules in advanced gas separation systems is compromised when an efficient process with high performance and low energy demand is required. Recently, various structured adsorbents with enhanced adsorption characteristics such as monolithic, laminates, and foam structures have gained considerable attention as alternate candidates for traditional adsorbent particles. Over the last 20 years, there have been numerous patents on such systems including a theoretical Received: May 17, 2011 Revised: October 3, 2011 Accepted: February 24, 2012 Published: February 24, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 4025 dx.doi.org/10.1021/ie201057p | Ind. Eng. Chem. Res. 2012, 51, 4025-4034