product is maintained at or above the minimum value then either the output signal from unit optimizing transmitter 47 or local optimizing tramsmitter 43 is utilized to modify the set point value of flow controller 11 and thus optimize the product cost however when the purity falls below the minimum value represented bu the set point value then the output signal from analyzer controller 26 is utilized to increase the rate of flow of steam to return the product to the desired minimum purity value at any time that the operating pressure exceeds the safe maximum limit time that the operating pressure exceeds the safe maximum limit represented by the set point value then the output signal from pressure controller 18 is utilized to decrease the rate of steam flow and thus reduce the operating pressure to a safe value now again turning to the apparatus shown to the right of column 1 a differential pressure transmitter 49 is connected on one side by pipe 50 to column 1 and on the other side by pipe 51 to pipe 32 near the condenser 33 the differential pressure thus sensed at the spaced apart points by the differential pressure transmitter 49 is proportional to the velocity of fluids flowing in column 1 the term 34 fluid 34 is used in relation to the flow in column 1 to mean a fluid in any phase or degree of saturation connection may be made to column 1 at other spaced apart points but the locations disclosed in fig1 are preferred locations because piping and calibration are facilitated the output signal from differential pressure transmitter 49.