Results of mathematical modelling of liquid flow in the tank with a screw impeller operating in the system equipped with a diffuser in a laminar regime are presented. A two-dimensional model describing liquid flow in the tank constructed in the Department of Process Equipment, LTU, was used to determine velocity fields. Results obtained were compared with experimental data given by Seichter [Coll. Czech. Chem. Commun. 46 (1981) 2032-2042]. Satisfactory agreement of the experimental and model values was obtained which confirmed that the model could be used in practical calculations. An optimisation criterion of the tank-impeller system regarding mixing time was proposed and screw impellers were optimised. (c) 2004 Elsevier B.V. All rights reserved.