Hmmm, I must have missed part of this exchange. The pressure in that tank had
better not vary (other than with temperature) as long as liquid and gas are
in equilibrium. Its a law, no variation with volume in the two phase system.

Now as to cooling beer by immersing it in liquid propane, ... there are
easier ways. Such as drinking it quickly under a strong negative differential
pressure. This  leads to vapor and aerosol formation and rapid cooling of the
residual liquid!

Frank Grunthaner