the phases and the scale up issues are the main limitationsof these multiphase reactors.With a diameter of 5 m and a height of 22 m, Sasol is oper-ating its slurry bubble column reactor for Fischer–Tropschsynthesis (Krishna, 2000). Laboratory scale bubble columnunits have diameters of about 25 cm (Krishna et al., 2001). Suchhuge difference between the two scales indicates that properscale-up calculations are essential for approaching industrialapplications.It is noteworthy that anticipated scale-up problems of aslurry bubble column for the Fischer–Tropsch synthesis wereamong the main reasons Shell decided to implement themulti-tubular trickle bed technology in their plant in Malaysia,for a quicker and safer route (De Swart, 1996).