Static mixers are defintely an option. To achieve gas liquid contacting, the velocities need to be sufficient for dispersion. Liquid velocities greater than 10 ft/sec should be adequate. A typical twisted element mixer, such as a Kenics mixer should be available for tubing in most sizes. Unless required for other heat transfer, the 20 feet of tubing can probably be reduced, rather than extended. My suggestion is to try a static mixer, analysis may be more costly than a trial. The cost should be reasonable for a static mixer.