I'm currently working in a Gas Processing design and I'm a little bit confused in one subject.

So here is the case

I have two Feed Gas Scrubbers (upstream and downstream of Compressor). The Feed Gas is normally in 100% vapor form (no liquid flashing expected).

Bottom of these scrubbers will be discharged to close drain vessel. Each of them have a control valve installed to control the liquid level in the vessel.

As I mentioned before that the Feed Gas stream normally contain no liquids, what will be the best design for the control valve, an on-off valve OR a controlling one ?

The on-off valve will be opened if the liquid level reach HLL, then emptying the liquid until it reaches LLL in 3-5 minutes.

OR

Control Valve that will maintain the level of liquid in the vessel ?

It really depends on your level controller.  If you are trying to maintain the fluid level in a narrow range and have a throttling level controller (like a CEMCO), then you want a valve that reacts to a variable signal (i.e., a control valve).  If you have a snap-acting controller then it is going to send a signal on an event (i.e., the float reaches the end of travel) and you want an on/off valve.

Many applications that would have been throttling in the past are going to on/off because of fugitive emissions concerns.  A common way to do throttling is to have a continuous bleed that is throttled as the float moves up and down, sending a lower pressure signal to the control valve.  This bleed is not a small amount of gas, so if you are using process gas for your control stream then you are putting that gas into the atmosphere.  On the other hand if you are using control air there is an energy cost to the bleed stream.

Some control valves, often called dump valves are designed for nearly dry vessels.  Any control valve would require tight shutoff.  Also you don't want a throttling valve to operate much really near the seat.  Thus the controller could include gap action that does nothing until some percentage then opens and throttles until near the seat then closes.  Several options exist.