Fractionation process for a process for oligomerising light olefins

Abstract

The invention relates to a process for oligomerising light olefins in which the effluent from the oligomerisation section is passed to a prefractionator that leads to at least one head fraction containing a mixture of liquefied petroleum gas and light gasoline and a bottom fraction containing a mixture of heavy gasoline and middle distillate, the said head fraction being passed to a debutaniser that leads to at least one liquefied petroleum gas cut and a light gasoline cut, the said bottom fraction and at least part of the said light gasoline cut being passed to a separator enabling at least a gaseous fraction, a gasoline fraction and a gasoil fraction to be obtained.

1 . Process for the oligomerisation of light olefins in which the effluent from the oligomerisation section is passed to a prefractionator, which leads to at least one head fraction containing a mixture of liquefied petroleum gas and light gasoline and a bottom fraction containing a mixture of heavy gasoline and middle distillate, the said head fraction being passed to a debutaniser that leads to at least one liquefied petroleum gas cut and a light gasoline cut, the said bottom fraction and at least part of the said light gasoline cut being passed to a separator enabling at least a gaseous fraction, a gasoline fraction and a gasoil fraction to be obtained. 
     
     
         2 . Process according to  claim 1 , in which the said head fraction contains a liquefied petroleum gas (LPG)/light gasoline mixture, containing less than 15% by weight with respect to the hydrocarbon feedstock containing at least 8 carbon atoms, and the said bottom fraction containing a heavy gasoline/middle distillate mixture containing less than 15% by weight with respect to the hydrocarbon feedstock containing at most 5 carbon atoms. 
     
     
         3 . Process according to  claim 1 , in which the said prefractionator contains between 5 and 20 theoretical trays, operates at a pressure between 0.1 and 2 MPa, the feedstock being supplied to a tray situated at the head of the column, the head vapours being withdrawn at a temperature between 100° and 150° C. and the bottom liquid being withdrawn at a temperature between 170° C. and 220° C. 
     
     
         4 . Process according to  claim 1 , in which the said liquefied petroleum gas cut contains hydrocarbons containing 4 or fewer carbon atoms and the said light gasoline cut contains hydrocarbons containing at least 5 carbon atoms. 
     
     
         5 . Process according to  claim 1 , in which the said debutaniser contains between 15 and 35 theoretical trays and operates at the same pressure as the prefractionator, the feedstock being fed between the theoretical trays 8 and 25, the trays being numbered starting from the head of the column, the head vapours are removed, condensed and subcooled to a temperature between 35° and 55° C. in an air-cooled condenser, and the bottom liquid is removed at a temperature between 100° C. and 130° C. 
     
     
         6 . Process according to  claim 1 , in which the said separator contains between 20 and 40 theoretical trays and operates at a lower pressure than the pressure of the said debutaniser, namely between 0.01 MPa and 0.6 MPa. 
     
     
         7 . Process according to  claim 1 , in which the said bottom fraction obtained from the prefractionator is fed between the trays 5 and 15 of the said separator, the said bottom fraction obtained from the debutaniser is fed between the theoretical trays 2 and 12 of the said separator, the trays being numbered starting from the head of the column, the head vapours being removed, condensed and subcooled to a temperature between 40° C. and 80° C. in an air-cooled condenser and the bottom liquid being removed at a temperature between 200° C. and 240° C. 
     
     
         8 . Process according to  claim 1 , in which the said bottom fraction and the said bottom cut supply the said separator at two different levels. 
     
     
         9 . Process according to  claim 1 , in which the said bottom cut is in part recycled to the prefractionation column. 
     
     
         10 . Process according to  claim 9 , in which the said bottom cut is preferably introduced to several trays above the reboiler. 
     
     
         11 . Process according to  claim 9 , in which the degree of recycling of the said bottom cut is between 5% by weight and 20% by weight. 
     
     
         12 . Process according to  claim 1 , in which the bottom effluent from the said separator is used for a heat exchange with the reboiler of the said debutaniser. 
     
     
         13 . Process according to  claim 1 , in which the effluent obtained from the last reactor is cooled before its pressure is released and it is introduced to the prefractionator, which enables heat to be recovered and the reboiler of the said debutaniser to be reheated.