as noted above , the molecular sieves that can be used to sorb sulfur species ( e . g ., cos and h 2 s ) from liquid hydrocarbon streams form a known class of compounds with many members , any one of which can be used in the present invention . the molecular sieves of the crystalline aluminosilicate zeolite type , both naturally occurring and synthetic compositions , are presently preferred because of their commercial availability . the crystalline synthetic zeolites having an average pore diameter of about 5 angstroms are most preferred . the methods of using such materials to sweeten liquid hydrocarbon streams are well known to those skilled in the art . generally , the method of choice involves flowing the hydrocarbon stream , under pressure , through a packed bed or column of the molecular sieves under conditions ( e . g ., flow rate ) sufficient for sulfur compounds to be removed by the molecular sieves . other methods , such as a batch process , could be used but they tend to be less cost effective for treating large volumes of sour hydrocarbon . the following experimental field trial is conducted in a hydrocarbon treatment plant where the sour propane stream alone is 7 , 600 barrels per day ( bpd ). the generally preferred continuous type of operation illustrated in fig1 - 6 is chosen to sweeten the propane stream using a commercial synthetic zeolite , molecular sieve type rk - 29 from union carbide corporation , houston , tex . according to union carbide sales literature , rk - 29 molecular sieves are a type a crystalline zeolite alkali metal aluminosilicate material having : ( c ) a heat capacity of 0 . 23 british thermal units ( btu ) per pound at 10 ° f ., ( d ) a heat adsorption of 1800 maximum btu per pound of water , ( f ) a particle size and shape of a 1 / 16 inch pellet . the sulfur species in the propane is primarily carbonyl sulfide ( cos ), with trace amounts of hydrogen . the sour propane stream flows through two columns connected in parallel and packed with rk - 29 molecular sieves . each of column a and b is loaded with the following sequence of materials : the column outage measurement , from top of final layer of 1 / 4 - inch hold down balls to face of top manway flange , is 7 . 6 feet . the distance from top of the 1 / 4 - inch hold down balls to distributor is 31 / 2 to 4 feet . the sour propane is then sweetened by flow through column a , and then through column b when the molecular sieves in column a are loaded with sulfur compounds . at inlet conditions of 600 ppm ( v ) cos , 80 ° f . and 7 , 600 bpd , the predicted cos breakthrough time to 2 ppm ( v ) is about 16 hours . these are understood to be the worst case conditions which can be reasonably expected for the inlet propane . the cos adsorption capacity is consistent with predicted performance . each bed provides a 20 - hour breakthrough time to 2 ppm ( v ) cos at average inlet conditions of 320 ppm ( v ) cos , 72 ° f . and 7 , 600 bpd . column a is regenerated in several steps : first , the flow of sour propane is diverted from column a to column b . column a is then drained by heating the bed to a temperature above the boiling point of propane for a time sufficient to vaporize a substantial portion of the liquid propane in the column ; the vapor pressure of the vaporized propane forces liquid propane from the column and into column b through line 126 , per fig2 . column a is purged by heating the bed with unit 138 . the heated gaseous effluent continues to be withdrawn and passes through line 130 , manifold unit 132 and lines 136 / 137 where it is returned back to the front end of the hydrocarbon treatment plant and recombined with hydrocarbon feed , per fig3 . next , the molecular sieves in column a are regenerated by flowing hot residue gas ( 135 ° f . ; 380 psig ; 16 mmscfd ) through the column for 3 to 4 . 5 hours and passing the to the flares , per fig4 . the molecular sieves are then cooled by flowing residue gas or sweetened propane through column a and the effluent is discharged to the flare , per fig5 . column a is then refilled with sweetened propane and the effluent is passed to the front end of the hydrocarbon recovery plant for further recovery of the propane vapors which form as the liquid propane cools the molecular sieves to a temperature below the boiling point of propane , at operating pressure , so that the column can be put back onto adsorption mode , per fig6 . in this novel process , a substantial amount of sponged propane is removed and recovered . cos breakthrough during the propane purge step occurs simultaneously with a precipitous drop in one propane concentration in the effluent gas , signalling the removal of virtually all the propane from the bed of molecular sieves . column a performs as expected in sweetening the propane when the feed stream was diverted from column b back to column a . column b is then regenerated in a similar manner and with similar results .