Depending on the range of gas quality this can be a standard application for thermal mass flow. For large flare lines you may need multi-point sensor to get the accuracy you need. Try FCI, Kurz, ABB etc.
At the risk of mentioning a specific product, there are stepper motor driven diaphragm dosing pumps on the market with integrated flow measurement (automated drawdown) and 4-20mA or Profibus feedback of the current delivery rate
see
http://www.alldos.com/pdf/209M-EN-15.820019.pdf
There are a number of acoustic data collection and leak detection products out there. One of the bestis described at: http://www.radcom-usa.com/soundsens.htm
dfunk,
For small capacity rechlorination applications one of the most common problems is pump failure due to degassing. While many of the small pump manufacturers can supply automatic degassing designs you should look at the double diaphragm design offered by Alldos. This head is specifically...
You want novel alternatives, consider supercritical fluid extraction for direct separation of target compound groups. Batch processing is a realistic possibility (eg. coffee decafination with CO2) if the product value is high enough and the batch size reasonable for pressure vessel sizing.