Internal loop reactors having three different downcomer-to-riser cross-sectional area AD/AR ratios were investigated in laboratory scale (V-1-0.031 m(3), V-2=0.063 m(3), V-3=0.126 m(3)) and in pilot scale (V-4-0.252 m(3), V-5=0.510 m(3), V-6=1.190 m(3)) regarding their mixing, average liquid velocity and mass transfer behaviour in the experimental work. The AD/AR ratio of reactors (AD/AR=3-18) was increased to extremely high value in contrast to literature, so as to observe the effect of the riser. The above-mentioned parameters as a function of the superficial gas velocity hi the riser were investigated. The gas velocity was changed in the range of 0.07-0.49 ms(-1). The effect of different risers (empty tube, tube with horizontal slits, and its combination with FixMix type static mixers) were also investigated in this work.