This work deals with chiral separation by Simulated Moving Bed (SMB) chromatography. A simulation package is used to predict the effect of the operating variables on the process performance. The "separation volume" methodology is used to study effect of the presence of mass transfer resistances presented by different particle sizes on operating conditions of the SMB unit with emphasis to the net flow in zone I and IV in terms of interstitial velocity ratio (gamma(1) and gamma(4)). The experimental operation of the a pilot plant SMB unit LICOSEP 12-26 (Novasep, France) is carried out for the separation of enantiomers of R,S-a-Tetralol using the chiral adsorbent CHIRALPAK AD with particle size 20 mu m. To obtain real insight to operation of the SB unit Licosep 12-26 the mathematical model accounts for all dead volumes inside of the SMB unit and time delay in shifting of injection and withdrawal points. Experimental and theoretical results based on the SMB model are compared in terms of process performance parameters and internal concentration profiles.