Low temperature process for the recovery of ethane from thermal hydrocracking vent gases

Claim 1

A process for separating ethane from a gaseous feed comprising a mixture of hydrogen, alkanes, aromatics and alkyl aromatics comprising: (a) feeding the gaseous feed to a first condenser to condense the aromatics and alkyl aromatics in the feed; (b) feeding the product of (a) to a first phase separator to obtain an overhead vapor product and a bottom liquid product which contains substantially all of the aromatics present in the feed; (c) feeding the overhead vapor product successively to a second and a third condenser to condense the ethane contained therein; (d) feeding the product of (c) to a second phase separator to obtain an overhead vapor product and a bottom liquid product comprising primarily ethane; (e) feeding the overhead vapor product of (d) to a fourth condenser to condense the methane contained therein; (f) feeding the product of (e) to a third phase separator to obtain an overhead vapor product comprising primarily hydrogen and a bottom liquid product comprising primarily methane; (g) feeding the overhead vapor product of (f) successively to the fourth, third, second, and first condensers and combining the bottom vapor product from the first phase separator of (b) with the overhead vapor product of (f) leaving the second condenser to obtain a product stream having ambient temperature and pressure and comprising the hydrogen and the aromatics present in the feed; (h) feeding the bottom liquid product from the third phase separator in (f) to the fourth, third, second, and first condensers in succession to obtain a fuel gas stream comprising the methane present in the feed and having ambient temperature and pressure; (i) feeding the bottom liquid product from the second phase separator of (d) to the third condenser to heat the liquid to a gas-liquid mixture; (j) feeding the gas-liquid mixture of (i) to a fourth phase separator to obtain an overhead vapor product comprising primarily methane and a bottom liquid product comprising primarily ethane; (k) throttling the liquid product of (i) and feeding the throttled product of the third condenser to heat the liquid product to form a gas-liquid mixture; (l) feeding the gas-liquid mixture of (k) to a first ethane flash drum to obtain an overhead vapor product comprising methane and ethane and a bottom liquid product which is substantially pure ethane; (m) feeding the overhead vapor product of (l) to the third condenser to condense the ethane; (n) feeding the product of (m) to a second ethane flash drum to obtain an overhead vapor product containing primarily methane and a bottom liquid product containing primarily ethane; (o) combining the bottom liquid products of steps (m) and (n) to form a liquid ethane-containing stream and feeding the ethane stream to the second and first condensers in succession to heat the ethane stream to ambient temperature and pressure; and (p) combining the overhead vapor product from step (n) with the bottom product of (g) and feeding the combined product to the second and first condensers in succession.