Two-column relay simulated moving-bed process for gas-phase separations Rui P.P.L. Ribeiro, Isabel A.A.C. Esteves, José P.B. Mota ⇑ LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal article info Article history: Received 22 December 2016 Received in revised form 17 March 2017 Accepted 19 March 2017 Available online 21 March 2017 Keywords: Simulated moving bed Gas separation Two-column chromatography Relay SMB abstract A new two-column, relay, simulated moving-bed (2-column R-SMB) process has been designed and implemented experimentally for the first time for gas-phase separation. Contrary to classical SMB config- urations, in a R-SMB process the stream exiting a column is never partially withdrawn from the system, i.e., it is not divided into an outlet stream and a stream that is redirected to another column; instead, the stream exiting a column is handled in a relay mode: at any instant of the cycle the stream is either fully collected as product/waste or completely diverted to another column. In the present work the R-SMB concept is applied to the simplest case of separation of an isothermal, binary trace gas mixture. The sep- aration of carbon dioxide (CO 2 )/methane (CH 4 ) mixtures using nitrogen (N 2 ) as carrier gas is evaluated as a proof-of-concept. The process is designed and optimized by model-based computer simulation and the obtained results are validated experimentally at laboratory-scale using a newly designed two-column SMB unit. The optimized 2-column R-SMB process produces extract and raffinate products both with 99%-purity (in an eluent free basis). The 2-column R-SMB performance is compared with the classical four-zone SMB and VARICOL-type configurations, and it is shown that at low feed throughput the process performances are coincident. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction The simulated moving bed (SMB) is a robust and well- established adsorption-based separation process that emerged in the 1960s. The SMB process was developed as a practical solution to overcome several drawbacks of the original true moving-bed (TMB) concept. Whereas in the TMB the solid flows counter- currently to the fluid, in the SMB this movement is simulated by means of a cyclic modification of the position of the inlet/outlet nodes. The classic SMB configuration consists of a train of N iden- tical columns connected in series to build a closed loop. During SMB operation, the ports for fluid input and withdrawal are moved by one column in the direction of fluid flow, at fixed intervals. This periodic movement of the inlets and outlets simulates the counter- current contact between the solid adsorbent and the fluid [1–3]. More recently, a wide variety of novel SMB operating schemes with improved performance have been designed; their perfor- mance could be enhanced due to the extra degrees of freedom. The possibility of using asynchronous port switching [4–7], flow- rate modulation [8–11] and concentration modulation [12,13] has been evaluated. Furthermore, other configurations with reduced number of sections have been tested [14,15]. Other config- urations such as steady state recycling chromatography [16] and single-column SMB analogs [17–20] have been considered to replace batch chromatographic applications. The alternative schemes have extra degrees of freedom over the classical SMB technology that can improve the separation efficiency and reduce the number of columns. This promotes the use of more economic setups, which use less adsorbent and operate with a lower pressure drop. Over the years, the SMB technology has been mainly employed in liquid-phase separations and recently a great deal of attention has been given to biotechnology, pharmaceutical, and fine- chemistry applications [21,22]. Additionally, the SMB technology is being increasingly considered as an option for gas-phase separa- tions. Several studies have been published regarding the separation of volatile inhalation anesthetic enantiomers [23–25], xylene iso- mers [26], linear/nonlinear paraffins [27], and olefins/parafins [28–30]. In this work we combine two previously developed alternative SMB schemes and apply them to gas-phase separations. To be more specific, the two-column schemes previously developed by our group for liquid-phase separations [31–35] are combined with the relay SMB (R-SMB) concept [36]. The two-column schemes http://dx.doi.org/10.1016/j.seppur.2017.03.037 1383-5866/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: pmota@fct.unl.pt (J.P.B. Mota). Separation and Purification Technology 182 (2017) 19–28 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur