Patent GB-1026356-A (GB)
Title: Method of,and apparatus for controlling a continuously operating distillation process
IPC: B01D3/42
Filed: 1964-03-10
Abstract: 1,026,356. Distillation columns; control systems. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. March 10, 1964 [March 12, 1963], No. 10110/64. Heading B1B. In the continuous distillation of a variable stream of feed mixture in a fractionating column 1 which is heated by reboiler 12, the column being operated at a pressure above atmospheric, the degree of separation of the components of the mixture, supplied to the column through preheater 5, is maintained at a required value by automatic adjustment of the rate of reflux return and the degree of reboiling, the valve 35 governing the supply of heating-medium to the preheater is kept as far closed as possible while the valve 29 governing the supply of cooling-medium to the condenser 7 is kept as wide open as possible, consistent with the pressure in the column being automatically maintained substantially between a maximum value P max . and a minimum value P min.,  and in such a way that the degree of reboiling is automatically decreased if the maximum permissible load on one or more of the trays in the rectifying section of the column is exceeded and the degree of preheating of the feed is automatically increased and/or the degree of reboiling decreased if the load on one or more of the trays in the stripping section exceeds a maximum permissible value. The rate of withdrawal of the top and bottom fractions from the system is controlled by valves 14 and 19, respectively, which are controlled so as to maintain substantially constant the liquid-level in condensate-accumulator 8 and in the base of the column. Meters 20 and 23 measure the composition of the top and bottom fractions, respectively, gauge 33 measures the load on a rectifying-tray and gauge 37 measures the load on a stripping-tray. Each of these instruments is connected to a controller. Pressure-gauge 30 is connected to two controllers 31 and 32, which compare the measured pressure in the column with P min.  and p max.,  respectively. The controllers, which form part of a pneumatic, hydraulic or electronic control system, are connected to various control-valves either directly, or via selector 25, or selectors 28 and 25, each of which gives an output signal equal to the lower value of two input signals. The reflux valve 22 is controlled in accordance with signals transmitted from meter 20. In general, valve 29 is open as wide as possible, but if the pressure becomes lower than P min ., controller 31 adjusts valve 29 so far that the pressure again becomes equal to P min.  If the load on the test tray in the stripping section becomes higher than the permissible maximum, the valve 35 governing supply of heating-medium to preheater 5 is opened sufficiently to reduce the load to the desired value. Valve 26 admitting heating-medium to the reboiler 12 is controlled, via selectors 25 and 28 in accordance with signals from meter 23, pressure-gauge 30, and load-gauge 33. As long as the pressure in the column is lower than P max.,  the output signal from controller 32 is a maximum, and valve 26 is governed by the composition of the bottoms fraction. But if the pressure becomes higher than P max., the output signal from controller 32 becomes smaller, and is transmitted by selector 25 to valve 26 which is thereby closed sufficiently to reduce the pressure to approximately P max . If the load on the test tray in the rectifying section becomes larger than the permissible maximum, valve 26 is further closed. The order of the connections of controllers 24, 32 and 34 to selectors 25 and 28 can be permutated. The tray-load can be measured with a meter for the pressure-difference across one or more trays, or with a conductivity meter in the space over a tray. Various systems are described whereby large, rapid changes in the column-pressure or the load on the trays can be suppressed.