Molecular sieve selective adsorption process

Claim 2

The method of claim 1 wherein the off-gas fraction is separated from the selective adsorbent effluent in the fractional distillation zone by: separating a vapor fraction comprising desorbent and lighter hydrocarbons from the selective adsorbent effluent; partially condensing the vapor fraction into an off-gas containing from about 18 percent to about 25 percent desorbent hydrocarbon and a condensate fraction; and returning the condensate fraction to the fractional distillation zone as reflux.

Claim 3

The method of claim 1 wherein the selective adsorbent effluent comprises an adsorption cycle effluent stream and a desorption cycle effluent stream, and wherein the fractional distillation zone comprises an adsorption cycle effluent fractional distillation system and a desorption cycle effluent fractional distillation system.

Claim 4

In a molecular sieve selective adsorption process comprising an adsorption cycle and a desorption cycle operated in the vapor phase at an elevated temperature and super-atmospheric pressure employing a bed of selective adsorbent for recovering straight chain hydrocarbon from a hydrocarbon feed stock, wherein said process is operated at a temperature of from about 0*F. to about 50*F. above the dew point temperature of said feed stock, wherein a vaporized desorbent hydrocarbon is employed in the desorption cycle, and wherein the desorbent hydrocarbon is recycled from the selected adsorbent bed effluent to the desorption cycle, which process is characterized by undesired coke lay-down in the selective adsorbent bed because of cracking of feed components and which is further characterized by progressive increase in cracking of feed components as the bed temperature is increased, the improvement for decreasing the rate of coke lay-down which comprises: separating an off-gas fraction containing from about 18 percent to about 25 percent desorbent hydrocarbon from the selective adsorbent effluent in a fractional distillation zone; and operating the fractional distillation zone to recover substantially all the remaining desorbent hydrocarbon as a liquid at the desorbent hydrocarbon bubble point temperature.

Claim 5

The method of claim 5 wherein the hydrocarbon feed mixture comprises straight chain and non-straight chain hydrocarbons in the C8 - C14 carbon number range and the desorbent hydrocarbon comprises C7 straight chain hydrocarbon.

Claim 6

In a molecular sieve selective adsorption process comprising an adsorption cycle and a desorption cycle operated in the vapor phase at an elevated temperature and super-atmospheric pressure and employing a bed of selective adsorbent for recovering straight chain hydrocarbons from a hydrocarbon feed mixture, wherein said process is operated at a temperature of from about 0*F. to about 50*F. above the dew point temperature of said feed stock, wherein a vaporized desorbent hydrocarbon is employed in the desorption cycle, and wherein the desorbent hydrocarbon is recycled from the selective adsorbent bed effluent to the desorption cycle, which process is characterized by undesired coke lay-down in the selective adsorbent bed because of cracking of feed components and which is further characterized by progressive increase in cracking of feed stock components as the bed temperature is increased, the improvement for increasing the operating temperature of the process without increasing the rate of coke lay-down which comprises: separating an off-gas fraction containing from about 18 percent to about 25 percent desorbent hydrocarbon from the selective adsorbent effluent in a fractional distillation zone; and operating the fractional distillation zone to recover substantially all the remaining desorbent hydrocarbon as a liquid at the desorbent hydrocarbon bubble point temperature.

Claim 7

The method of claim 7 wherein the hydrocarbon feed mixture comprises straight chain and non-straight chain hydrocarbons in the C13 - C24 carbon number range and the desorbent hydrocarbon comprises C10 straight chain hydrocarbon.