Gas Seal for slowly turning shaft

I am working on a small waste treatment project where there is a small mixing vessel (about 5 liters – more or less 20cm diameter). The mixing process produces methane gas which shouldn’t escape the confines of the mixing vessel (odor and flammability).

Some other salient parameters (as I perceive them):

• The shaft diameter is small (about 16mm).
• the shaft rotates really slow (about 10-30 rpm).
• the pressure of the gas is low (just over 1Atm ).

I was wondering whether what type of seal would be most appropriate for this type of application. My first instinct was a lip sea with a garter spring. I know there are other types of seals (e.g. packing, mechanical seals, tandem, cartridge), but I have little to no experience with most of them.

The cost would be a primary driver in this application, so any indication about the *cost effectiveness * would be welcome. For example, I have seen some of gas seals used in the petroleum industry but the cost is prohibitive.