The apparatus converts ethylene in a dilute ethylene stream that may be derived from an FCC product to heavier hydrocarbons. The oligomerization reactor is in communication between a primary absorber column and a secondary absorber column in an FCC product recovery section. The oligomerization catalyst may have a low silica base with a Group VIIIB metal and operate at low pressure without excessive deactivation. The catalyst is resistant to feed impurities such as hydrogen sulfide, carbon oxides, hydrogen and ammonia. At least 40 wt-% of the ethylene in the dilute ethylene stream can be converted to heavier hydrocarbons.