AXIAL-DISPERSION IN GASES FLOWING THROUGH A PACKED-BED AT ELEVATED PRESSURES

Axial dispersion in upward gas flow is investigated by pulse and displ acement experiments in a vertical, packed column with different concen trations of the tracer and at pressures up to 1.5 MPa. The responses t o the introduced pulse and step changes are measured at two locations and the extent of axial dispersion, represented by the Bodenstein numb er, is determined by curve fitting in the time domain. The performed e xperiments demonstrate that the residence time distribution is conside rably affected by density differences between the tracer and carrier g as, particularly at elevated pressures. Obtained Bodenstein numbers fo r step changes from nitrogen to a helium/nitrogen mixture and vice ver sa differ by as much as a factor ten, depending on the helium concentr ation and column pressure. The difference in axial dispersion may be a scribed to gravitation-driven instabilities as due to vertical density gradients in the case of a heavy gas displaced by a light gas: densit y gradients in the step changes from a light to heavy gas evidently in hibit axial dispersion. The presented observations are of major import ance for the description of flow behaviour of gases in packed bed reac tors where density gradients exist due to temperature and concentratio n gradients, particularly because many processes operate at elevated p ressures.