per second fig4 a there was a reduction in gas holdup in the recycle line with the three stages especially as compared with the downcomer recycle pan 88 used alone fig6 b shows a more dramatic improvement when the gas flow was increased to 0 35 cubic feet per second at 0 56 cubic feet per second there was a further improvement fig7 shows a graph which illustrates elutriation in a column which was ten and a quarter inches in diameter and charged with a conventional catalyst water and air which were used as the three phases in the reactor in the column the height of the freeboard free liquid above the catalyst bed was 39 45 inches this graph shows that elutriation was approximately cut in half by the second stage and virtually eliminated by the third stage fig8 a 8b are the results of tests conducted on the inventive structure in a pilot plant column which is four feet in diameter that was charged with catalyst kerosene and nitrogen fig9 shows the dimensions of the four foot diameter pilot plant column used for conducting the tests recorded in fig8 a 8b the recycle pan 88 was three feet in diameter.