What is the best way to control the pressure of a chemical reactor in which the feed gas is wholly absorbed and a byproduct gas is formed.
1. Modulating the gas make-up is the best method
2. Relieve valve.
3. ???
Make sure you select the right configuration of reactor for this case. Obviously you don't want to be feeding the feed gas into the headspace because you will need to vent the byproduct gas. Sounds like you need potentially more than one equilibrium stage or else you will definitely lose some of your feed gas while venting the product gas.
You haven't given us nearly enough information about the reaction for us to determine for you how best to control it.