of 1 butene of 66 following the fixed bed the effluent flowed to a fractionation tower to separate the isobutylene and isobutane from the 2 butene and n butane the tower following the fixed bed consisted of 94 theoretical stages with a reflux ratio of 4 5 the simulated reactor was a fixed bed with 6 7 1 butene selectivity to butane the reboiler and condenser duties were equivalent to those of cd deib tower 1a in comparative example 1 since they are set by the reflux ratio the process resulted in a 66 conversion of 1 butene comparative example 2 the results are shown on table 3 this represents the performance of the prior art fixed bed plus fractionation system a total of 90 3 of the normal butenes 1 and 2 butenes are recovered in the bottoms product the above process was repeated with the exception that a side draw was removed from the fractionation tower at stage 18 in an amount of 10 000 example 2a or 20 000 lb hr examples 2b 2c the results are shown on table 3 a side draw of 10 000 lb hr resulted in an increase in the amount of n butene recovery including 2 butene in the bottoms from 90 33 to.