Abstract: Experimental study of flow pattern and liquid velocity distribution by the tracer technique in a twometers diameter single pass sieve half-tray using air-water system reveals that there exists a highlynonideal flowing behavior.Typical residence time distribution curves covering the whole tray are presented,from which four flow regions can be observed: a main channelling region of nonuniform flow,a slowly forward flow region in the segmental area,a small region of backward flow and a complete-mixing region next to the inlet weir.The tray models available in literature seem incapable to simulateall these complicated hydrodynamic phenomena in a large tray.An eddy diffusion model with consideration of nonuniform liquid velocity field has been developedfrom the basic partial differential equation of mass transfer by introducing a velocity distributionfunction j.The nonlinear partial differential equation is solved by the method of finite element.Acomputer program is compiled according to this method to compute numerically the concentration pro-files in the whole tray as well as the Murphree tray efficiency enhancement under various operatingconditions.Typical computed results are presented and discussed.