Maximum utilization of energy in isoparaffin recycle in alkylation

Abstract

In a fractionation system wherein a stream of heat-containing fluid is taken from a zone effecting at least a partial control of at least one operating condition in said zone by withdrawing said fluid at a rate responsive to and correlated with a change in said condition and utilizing the heat in said withdrawn fluid. In one application, in operation of an HF alkylation of isoparaffin with olefin, in which a mixed paraffins feed is fractionated in a first fractionator to obtain isoparaffin therefrom, an alkylate product is fractionated in a second fractionator to recover isoparaffin as a side draw from the second fractionator the temperature in the upper section of the second fractionator is controlled by removing more or less isoparaffin vapor therefrom, determining the change in amount of isoparaffin being removed and applying this measurement to adjust the amount of isoparaffin fed to the mixed paraffins feed fractionator reboiler section, increasing isoparaffin to said section when the preselected temperature in the overhead outlet of the alkylate product fractionator attempts to increase (and vice versa) and causing adjustment in the amount of steam needed to supply the remainder of heat required to reboil the mixed paraffins fractionation zone. In one embodiment the isoparaffin vapor removed from the alkylate product fractionator is passed through a cooling zone, condensed therein, and the flow of the produced condensate, while being passed to the alkylation, is detected and is representative of the attempted temperature change in the alkylate product fractionator and is employed to adjust flow of the isoparaffin vapor to the isoparaffin vapor-heated reboiler in the mixed paraffin fractionation zone; the operation providing for suitable adjustment of steam to the steam reboiler to said mixed paraffins fractionation zone. In another modification, the amount of steam is further controlled with respect to the amount of total heat needed in the reboiled section of the mixed paraffins fractionation zone in manner correlated with the composition of paraffins removed in a side draw from said mixed paraffins fractionation zone.