Pulsed-tracer experiments conducted for horizontal gas flow through a horizontal doer of rectangular cross section (height = 12.7 mm) containing powder in the coherent-expanded vibrated-bed state have been reported (Benge and Squires, 1995; Squires and Benge, 1995). Axial gas dispersion in the duct can be treated by a simple model : the dispersion is a "spreading" superimposed on plug flow, with no gas bypassing and no lagging gas indicative of dead zones. At a given weight hourly space velocity, a large variation in axial Peclet number (Pe(ax)) can be achieved by varying length of duct and gas superficial velocity in proportion, with no substantial effect ripen gas-solid contacting efficiency. Herein, we report data for the methanol-to-olefin reaction conducted irt three ducts at lengths affording Pe(ax)-values from similar to 2 to similar to 20. The new data display a trend in light-olefin yield vs. Pe(ax) similar to that reported earlier for three fluid-bed reactors at diameters of 10, 102 and 600 mm.