Distillation differential-pressure control

Figure 5.27. Cont d. (c) Composition control. Top product take-off and boil-up controlled by feed, (d) Packed column, differential pressure control. Eckert (1964) discusses the control of packed columns, (e) Batch distillation, reflux flow controlled based on temperature to infer composition.

Liquid oxygen analyses are customarily made for process control, product purity and to avoid hazards. Usually analytical information required for process control is not extensive. Use of modified Or sat apparatus for manual determination of the oxygen contents of various liquid samples is routine in most plants. Relatively simple thermal conductivity analyzer-controllers govern the flow of liquid air fractions under distillation where differential pressure control is not applicable. Pressure drop and inspection of liquid in a small glass flask are usually sufficient for mechanical filter cycle regulation but a continuous carbon dioxide analysis may be helpful as a check on the overall function. A method which is sufficiently precise for this use is discussed later. 

In practically all modem apparatus for laboratory distillation certain functions are now automatic. Heating is often controlled by contact thermometers, and the rate of evaporation of the charge with differential manometers. Reduced pressures are also frequently kept constant with automatic units. In fact, the present trend is towards fully automatic control of the conditions of distillation and simultaneous recording of the data measured, an object that has already been reached in many industrial installations. The ideal procedure is for the mixture to be merely introduced into the apparatus and the latter switched on it then automatically yields the required products of distillation and at the same time prepares a diagram of the top and bottom temperatures and other data that may be needed. 

Ftgura 18.9 The Foxboro differential vapor pressure (DVP) cell (From F. G. Shinskey, Distillation Control, second edition. Copyright by McGraw-HUl, Inc. Reprinted by permission.)... 

Variants of TVA are differential condensation TVA [903] and sub-ambient TVA or SATVA [910, 938]. In SATVA the condensable gases and volatile liquids are further fractionated volatile components are collected in gas cells and less volatile liquid fractions are separated and characterised by GC-MS. SATVA consists in the slow, controlled, distillation of the volatile products from the -196°C trap (where they have been collected) as it is heated up to room temperature. As they distil, MS or IR identifies the volatiles and at the same time they are collected in narrow fractions for further analysis. Continuous pressure monitoring during distillation is recorded as a SATVA curve [939]. McGill et al [940,941] have indicated that the very simple technique of trap-to-trap distillation of trace amounts of liquid-nitrogen condensable volatiles can be used for qualitative... 

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