Digital Control of In-Line Blending

Two basic solutions are proposed for the problem of automatically controlling the in-line blending of petroleum products in oil refineries. Two different methods are presented for digitally controlling the flow of many fluids in an in-line manner to produce an accurately controlled final blend. One scheme makes use of a small, inexpensive, general purpose digital computer which accepts pulse trains proportional in rate to component flow rates, and is programmed to calculate control valve settings in accordance with the percentage of each component in the final blend. The other method utilizes a series of small special purpose logic units, each of which accepts a pulse rate proportional to the rate of flow of one blend component and generates the appropriate control valve setting by comparing the component pulse rate with a pulse rate proportional to the desired final blend rate, as a function of component percentage. Both systems are completely digital throughout and achieve control by means of a total pulse comparison technique. No roundoff errors exist in the computer controlled system; hence, no inaccuracies are introduced by either control system beyond the inherent accuracy of the flow sensors employed.