GB887295A - Fractionation control - Google Patents

Abstract

887,295. Automatic control systems. PHILLIPS PETROLEUM CO. March 4, 1960 [March 4, 1959; May 4, 1959], No. 7718/60. Class 38 (4). A fluid mixture to be separated is introduced into fractionation column 10 through conduit 11 at a predetermined rate, steam or other heating medium being introduced through conduit 14, whilst a kettle product stream is withdrawn through conduit 17 at a rate regulated by liquid level controller 18. Vapours are withdrawn from the top of the column through conduit 21 which communicates with accumulator 22 through condenser 23, condensed liquid being removed from the accumulator by pump 25 which returns a portion of this liquid to the column through conduit 26. The remainder of the liquid from the accumulator is directed through conduit 27 at a rate regulated by liquid level controller 28. Valve 41 is regulated to maintain the computed mass flow of liquid leaving the top tray (internal reflux) constant, so that, when the computed internal reflux increases or decreases, the valve is set to reduce or increase flow through conduit 26 (external reflux). The internal reflux is computed as a function of the external reflux and of the difference in temperature at the top tray and the external reflux. The two temperature measurements are made by thermocouples 45, 46, connected in opposition to amplifier 47 whose output is applied to poteniometer 48. The slider of the potentiometer is set to provide a voltage, representing the temperature-differential multiplied by a predetermined constant, to a servomotor 49 adjusting the slider of potentiometer 50. A flow-measuring instrument 35 provides an electrical output signal, representing the external reflux, across the end-terminals of potentiometer 50 whose slider is connected through resistor 51 to summing amplifier 52. The output signal from the instrument 35 is also applied to amplifier 52 through resistor 53. The amplifier 52 therefore provides an output signal which represents the internal reflux and which is applied to controller 40 for valve 41. In another arrangement, the external reflux is measured by a differential pressure transmitter the output of which is transmitted to a square root force bridge for establishing a pressure directly proportional to the external reflux. The signal so derived is multiplied by a signal comprising a constant term added to a term proportional to the temperature-differential, Fig. 5 (not shown). In a further modification, means are provided for maintaining the ratio of the internal reflux to the feed rate constant at a preselected value, Fig. 6 (not shown). A signal representing the composition of the feed components at a preselected region within the column may be used further to control the external reflux.