Understanding and revamping of industrial scale SMB units for p-xylene separation

One of the first applications of Simulated Moving Bed (SMB) technology was in p-xylene recovery from mixed xylenes. The three main industrial processes for p-xylene separation from mixed xylenes based on SMB technology are: UOP's Parex. IFP's Eluxyl, and Toray's Aromax. These units operate in liquid phase (T = 180°C and P = 9 bar), achieving high recovery of almost pure p-xylene with high on-stream efficiency and extended adsorbent life. In this work, the industrial scale SMB process is investigated from modeling, simulation, and optimization points of view, using experimentally measured xylene adsorption equilibrium and kinetics data on ion exchanged faujasite zeolite. The aim is to develop tools for training of SMB unit operators and choice of the best operating conditions. Useful studies for better understanding of the influence of the operating parameters, adsorbent packing, and separation requirements on unit productivity are presented. SMB unit revamping strategies and operative actions are proposed. The practical application of "separation volume" methodology in the selection of optimum operating conditions that lead to maximum p-xylene productivity with minimal desorbent consumption is described.