My guess would be that HDT effluent vapor is almost pure hydrogen at startup. Since H2 is a low mol wt gas, it is likely your compressor is not designed to operate on pure H2. If you did, it wont develop the required exit pressure (even at max speed) and you would also have high discharge temp. So higher mol wt N2 is used and the compressor is operated on close to full recycle until vapor effluent from the HDT reactor has more hydrocarbons and H2S in it.
Question is not clear. What is an alternative - air, natural gas of hydrogen? You should clarify your idea.
First start-up is risky as one is not confident that equipment operates properly. Especially such expensive and dangerous equipment as compressor. It is better to risk in inert atmosphere instead of highly explosive, right?
