For example, gas-liquid mixers as disclosed in U.S. Pat. Nos. 5,916,491 and 5,925,290 may be modified to introduce butane at any desired location(s) in the liquid stream, for example, inside the draft tube of the mixers. [0040]FIG. 5 is a partially schematic side view of a gas-liquid mixer 50 including an alkane injection system 40 in accordance with an embodiment of the present invention.