Method of, and apparatus for, controlling a continuously operating distillation process

摘要

During the continuous distillation of a stream of feed-mixture in a fractionating column 1, which is heated by reboiler 10, the column being operated at a pressure above atmospheric and the rate of flow of cooling-medium through the condenser 5 being governed by a valve 26 which is kept as wide open as in consistent with the minimum permissible pressure-valve Pmin. in the column, the degree of separation of the components of the mixture is maintained at a required value, despite variations in composition and/or rate of flow of the feed-mixture, by automatic adjustment of the degree of reboiling of the bottoms fraction, the rate of withdrawal of the top and bottom products from the system through pipes 8 and 4, and the rate of reflux return. The adjustment is carried out so that the liquid-level in the bottom of the column and in the condensate-accumulator 6 are maintained within predetermined limits, while the column-pressure varies freely between Pmin. and a predetermined maximum value Pmax. The separation of isopentane as distillate from a mixture of hydrocarbons is referred to. The feed is introduced into the column through pipe 2 which is associated with heat-exchangers 2a and 2b, these being used only if a relatively cheap source of heat is available. The withdrawal of the top product through pipe 8 is adjusted by valve 12 which receives signals from a controller 13 connected to level-gauge 14, whereby the level in accumulator 6 is maintained close to a set value. The level in the bottom of the column is kept substantially constant by means of a level-gauge 15, controller 16, and a valve 17 in the withdrawal pipe 4 for the bottoms fraction. The composition of the top product is measured by instrument 18 which sends a signal to controller 19 where the measured value is compared with a set value; and controller 19 adjusts valve 20 in reflux-pipe 7, whereby the composition of the top product is maintained close to the set value. The composition of the bottoms product is measured by instrument 21 which sends a signal to controller 22, and the latter adjusts the valve 24 controlling the supply of heating medium to reboiler 10. Gauge 28 measures the pressure in the column and controller 29 compares it with Pmin. As long as the column-pressure is higher than Pmin., the output signal from controller 29 is a minimum, and valve 26 on condenser 5 remains open to the maximum extent. If the column-pressure becomes lower than Pmin., controller 29 will close valve 26 to such an extent that the column-pressure again becomes approximately equal to Pmin. The column-pressure is maintained below Pmax. by controllers 22 and 30, which control valve 24 on reboiler 10. Valves 24 and 26 may alternatively be placed in the discharge pipe of the heating-medium from reboiler 10 and in the discharge pipe of cooling-medium from condenser 5, respectively. Various ways of suppressing sudden variations in the column-pressure are described. Instruments 18 and 21 may be chromatographic analysers, or may be instruments for measuring infra-red or ultra-violet absorption spectra, viscosity, refractive index, or flash point. Alternatively, the difference in temperature between two trays of the column may be measured, or the difference in vapour pressure between the product obtained and the desired product.