Elutriation of finely divided solids

Claim 1

IN A CATALYTIC SYSTEM FOR THE CONVERSION OF HYDROCARBON VAPORS USING A CIRCULATING BODY OF FLUIDIZED ADSORBENT CATALYST WHEREIN THE ORIGINAL FRESH CATALYST CHARGED TO THE SYSTEM IS A MATERIAL OF SUBSTANTIALLY UNIFORM PARTICLE DENSITY WHICH HAS BEEN PRESIZED BY SIEVING, THE PROCESS OF MAINTAINING CATALYST ACTIVITY WHICH COMPRISES WITHDRAWING AN ALIQUOT PORTION OF THE TOTAL CIRCULATING CATALYST, PASSING THIS PORTION TOGETHER WITH AN UPFLOWIG ENTRAINING GAS STREAM INTO AN ELUTRIATION ZONE AND MAINTAINING IN THE BOTTOM PORTION THEREOF A FLUIDIZED BED OF CATALYST PARTICLES WHILE OPERATING AT A SUPERFICIAL ENTRAINING GAS VELOCITY AT LEAST 0.5 FT. PER SECOND GREATER THAN THE FREE FALLING VELOCITY OF THE LARGEST PARTICLES IN SAID PRESIZED FRESH CATALYST, PASSING SAID ENTRAINING GAS UPWARD THROUGH A RELATIVELY VERY TALL SETTLING ZONE, WHEREIN AT LEAST 90% OF THE SOLID PARTICLES ORIGINALLY ENTRAINED ABOVE SAID FLUID BED ARE SETTLED OUT AND RETURNED THERETO AS A REFLUX OF SOLID PARTICLES INCLUDING THE LARGEST PARTICLES IN SAID ENTRAINED MATERIAL, MAINTAINING A SOLIDS DENSITY IN THE UPPER PART OF SAID SETTLING ZONE WHICH DOES NOT EXCEED 0.1% OF THE APPARENT SOLIDS DENSITY OF THE BED IN SAID ELUTRIATING ZONE, THEREBY SELECTIVELY REJECTED THE LARGEST PARTICLES IN THE PORTION OF THE CATALYST FED TO THE ELUTRIATION ZONE, WITHDRAWING FROM SAID ELUTRIATION ZONE A BOTTOMS FRACTION COMPRISING PARTICLES OF LOW CATALYST ACTIVITY WHICH ARE LARGER IN WEIGHT PER UNIT DIAMETER TOGETHER WITH PARTICLES LARGER IN DIAMETER THAN THE LARGEST PARTICLES IN THE ORIGINAL PRESIZED FRESH CATALYST, DISCARDING SAID BOTTOMS FRACTION AND RETURNING THE LIGHTER FRACTION FROM THE ELUTRIATION PROCESS TO THE MAIN BODY OF SAID CIRCULATING CATALYST.