(148c) Operation of Simulated Moving Bed in Presence of Adsorbent Aging
ABSTRACT Adsorbent/Catalyst deactivation or ageing problems, considering either loss of equilibrium capacity as increase of the mass transfer resistances, has been the subject of several studies throughout the last years, either in the reactive as for the separation engineering fields. Adsorbents ageing is quite common in reactive/separation processes, is from common wisdom that the deactivation processes are divided into four general groups(1): Poisoning, caused by irreversible adsorption of some species in the adsorbent active sites, as lead, sulfur or carbon monoxide; Losses of catalytic material sometimes by vapor transport of the active metal. An example is the formation of volatile nickel carbonyls when a supported nickel catalyst is exposed to carbon monoxide, also in the case liquid phase if the catalyst is not stable in the presence of the solution; Sintering is usual processes running at high temperatures. Metal atoms or small crystallites may migrate and form larger aggregates, resulting on the loss of active surface area; Coke formation, witch takes place in various and different hydrocarbon reactive/separation processes. Nevertheless when it accounts to the Simulated Moving Bed (SMB) technology, the work published on this matter is almost inexistent, and the small amount of information remains in the experience of this units operation being in some cases almost empirical. A productive SMB unit depends on the calculation of optimal operating parameters obtained from the so-called separation region, a feasible region that represents all of a range of points that will provide the require separation performance, usually with a triangular shape(2). The optimal unit performance is obtained when working near to one of the vertices of this triangle/triangular shape figure, and therefore a slight oscillation in the operating parameters will lead to the violation of one or more of the separation conditions ruining all the operation. This slight movement of the operating point can the result of the adsorbent ageing problem, even if the operating parameters (internal fluxes, columns arrangements etc.), are kept at constant values as the ones calculated for the initial unit design. A way to overcome this problem is to operate the unit at the centre of the separation triangle, but away from the maximum possible for the unit productivity. The idea of working near to the maximum operation point (near of one of the vertices), and adjust this location with the adsorbent deactivation, trying to keep it at the same relative distance, it is quite attractive. Therefore, this study has as principal objectives the study of the adsorbent ageing influence on the performance parameters of a SMB unit as the presentation of some operative strategies to overcome this problem. Those adsorbent ageing correctional operative strategies leads to the extension of the sieve life maintaining the same performance parameters (productivity, recuperation and outlet streams purities), as the ones established with the initial SMB unit design project. A second approach is to calculate how much can be lost in terms of certain performance parameters to keep the others as required as the compensation of the adsorbent ageing difficulty.
(1) Bartholomew, C. H. Catalyst deactivation, Chem. Eng., 12, 1984 96-112. (2) Minceva, M.; A. E. Rodrigues Two-Level optimization of an existing SMB for p-xylene separation Computers and Chemical Engineering 29, 2005 2215-2228.