Patent ID: 7721557

Claim:
A method for removing propane from a storage tank comprising: (i) providing a mobile system for propane recovery comprising, in fluid communication, a liquid trap, a first vapor line for permitting passage of vapors from said storage tank to said liquid trap, a two-stage compressor which receives vapors from said liquid trap, one or more liquid condensing tanks downstream from said compressor, a second vapor line for passing effluent from said compressor to said one or more liquid condensing tanks, and a refrigeration unit for cooling said second vapor line at a point downstream from said compressor and upstream from said one or more liquid condensing tanks, the system being mounted on a mobile platform; (ii) establishing fluid communication through a said first vapor line between said mobile system and said storage tank; (iii) draining any liquefied propane from said storage tank into a receiving tank to achieve an internal volume of liquid in said storage tank of less than about 3%; (iv) drawing a stream of propane vapors from said storage tank through said liquid trap, to remove any liquefied propane entrained in said stream, and into said compressor which pressurizes said stream of propane vapors; (v) cooling said stream of pressurized propane vapors with said refrigeration unit; (vi) bubbling said cooled and pressurized stream of propane vapors through liquefied propane in said one or more liquid condensing tanks to condense said propane vapors into a liquid state, about 60% to about 85% of the internal volume of said liquid condensing tanks comprising liquefied propane; and (vii) pumping said liquefied propane from said one or more liquid condensing tanks, optionally through a meter, into said receiving tank; wherein said refrigeration unit employs liquefied propane drawn from said one or more liquid condensing tanks as a refrigerant and operates by forcing said liquefied propane refrigerant through a thermal expansion valve and an evaporator to convert said liquefied propane refrigerant into cooled propane refrigerant vapors which pass through cooling coils in the refrigeration unit and are thereafter recycled by combining with propane vapors drawn from said storage tank at a point in the first vapor line at or upstream from said liquid trap; and wherein said system is capable of reducing the pressure in a storage tank having a capacity of about 30,000 gallons and an initial pressure of about 110 psi at about 80° F. to a substantially evacuated state and recovering substantially all of the propane in about 5 hours or less under continuous operation.