In vertical pneumatic conveying the important research points concern the acceleration region, the gas and solids friction factor (Rautiainen et al., 1999) and the definition of flow choking conditions. Recent examples of studies on the characterization of the acceleration region are the publications of Dzido et al. (2002) and Namkung and Cho (2002), although the works of Yang and co-authors are still a basic reference (Yang and Keairns, 1976). Among the recommended correlations for the calculation of the friction factor are those of Yang (1973), Yang (1974) and Yang (1978). For the definition of the choking conditions, again the work of Yang (1975) is frequently recommended in many recent publications, like for example Raczek and Palica (1997) and Xu et al. (2001).
A previous paper of Barbosa and Pinho (2004) presented a correlation for the gas solid friction factor for steady state vertical pneumatic conveying of cork stoppers, while in the present paper the acceleration length of the cork stoppers, in the transient portion of the vertical conveying pipe, is accounted for.