GB1419908A - Means and method for controlling an alkylation unit to determine a desired feed isobutane flow rate - Google Patents

Abstract

1419908 Alkylating isoparaffins TEXACO DEVELOPMENT CORP 6 Feb 1974 [19 March 1972] 05559/74 Heading C5E [Also in Division G3] In an alkylation unit of the kind in which olefin and isoparaffin (e.g. isobutane) are contacted in the presence of acid to provide an acid-hydrocarbon mixture which is separated in a settler to provide a hydrocarbon product including alkylate, and a portion of the separated acid is discharged while the remainder is re-cycled with fresh acid, the isobutane flow rate and the one acid flow rate of either the fresh or the discharge acid is automatically controlled by a system in which signals corresponding to trial values of isobutane flow rate are used together with signals corresponding to certain operating parameters to calculate the effect of said values on the operation of the unit and hence on the profit, and those values of isobutane and acid flow rate which maximise the profit are selected as desired values for the control system. In the embodiment isobutene introduced at the bottom of Fig. 1 and olefin via line 3, are contacted with the acid at 4, and settled at 12, the hydrocarbon product being taken off in line 14; some of the acid is discharged via 21 and fresh acid is introduced via 17. The level of the acid/hydrocarbon interface in settler 12 8 is automatically controlled by loop 175, 174, 176 the latter controlling the inflow of fresh acid. The following other parameters are measured directly: flow rate of hydrocarbon output at 24, composition of the output by a chromatograph 30, flow rate of the discharge acid at 63, the density of the hydrocarbon, the recycle acid and the fresh acid at 70, 70A and 70B respectively, composition of the olefin by a chromatograph 82, composition of the isobutane by chromatograph 82A, flow rates of the recycle acid at 90, the olefin at 93, and the isobutane at 171. From these measurements the following parameters are calculated by the devices indicated: percentage volume of alkylate in the hydrocarbon output at 38, the acid consumption at 28, the hydrocarbon content of the recycle acid at 72, the volume fraction of olefins and of propylene in the olefin supply, at 83, the isobutane content in the supply at 83A, the existing "space velocity" of the olefin at 91, the existing octane quality at 115, and correcting factors to be applied to the acid consumption for changes in olefin space velocity at 135 and for changes in percentage isobutane in the hydrocarbon product at 160. In addition the economic value of octane rating, the acid consumption and the isobutene is known. "Control means" 170 is an analogue computer which carries out the calculation of the invention and produces the required control signals. It automatically cycles through a range of trial values, controlled by clock pulses, and the result of each change on the profile is compared with that in a store. If the new value is higher it replaces that already in the store so that at the end of each cycle the store contains the maximum value, and the flow rates associated with this are transferred to an output store for use while the computer cycle repeats. All the computing devices are shown in diagrammatic form and the associated formulµ are given.