Pulsed-tracer experiments conducted for horizontal gas flow through a horizontal duct 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 (Peax) can be achieved by varying length of duct and gas superficial velocity in proportion, with no substantial effect upon gas–solid contacting efficiency. Herein, we report data for the methanol-to-olefin reaction conducted in three ducts at lengths affording Peax-values from ∼ 2 to ∼ 20. The new data display a trend in light-olefin yield vs. Peax similar to that reported earlier for three fluid-bed reactors at diameters of 10, 102 and 600 mm.