Rwh

Feed

Reboiler vapor controller ^

Reflux drum

Vent

Liquid level controller^ /

Liquid level controller^ /

Reflux Overhead PumP product

Reflux Overhead PumP product

Reflux flow controller

Feed

Reboiler vapor controller ^

Reflux flow controller

Liquid level controller>

Steam

"sSteam trap

Bottom product

Fig. 18-1. Diagram of instrumentation for continuous rectification system.

Steam

"sSteam trap

Liquid level controller>

Bottom product

Fig. 18-1. Diagram of instrumentation for continuous rectification system.

and runs to a reflux drum accumulator, and a portion of the condensate is pumped back to the column for reflux. The excess condensate is the overhead product and is removed from the drum by the liquid level controller. The reflux rate is regulated by the reflux-flow controller which attempts to hold the temperature near the top of the column at a fixed value. The steam rate to the still is controlled by a reboiler vapor controller operating on a temperature indicator, and the unva-porized liquid is removed from the still by a liquid-level controller.

In the operation of the system illustrated in Fig. 18-1, the feed-flow, reflux-flow, and reboiler vapor controllers would be set at the desired values. If the temperature at the top control point became lower than the desired value, the reflux-flow controller would reduce the reflux rate which would increase the product withdrawal rate and raise the temperature. If the temperature were too high, the controller would take the opposite action. If the temperature at the reboiler control point becomes lower than desired, the controller will increase the steam supply to the still, resulting in a higher vapor rate. This will give better stripping of the light components and will raise the temperature at the control point. This action of the reboiler vapor controller will lower the reflux ratio by increasing the overhead product rate and will probably require the reflux-flow controller to increase the reflux rate to hold the top temperature down. In this system the reboiler vapor controller operates to give the desired temperature at the bottom control point which presumably gives the specified bottom product. The reflux-flow controller operates to give the desired temperature difference between the two control points. The system is operable because for a given number of plates the degree of separation can be varied by changing the reflux ratio. The feed rate and the temperature values that are fixed must obviously be within the fractionating capabilities of the system.

A number of alternate control systems can be employed. Thus a fixed reflux ratio could be used with the top control point regulating the feed rate to the column with the bottom control operating the same as before. In case the overhead product is difficult to liquefy, a partial condenser may be employed, and this requires a different method of control. A typical case of this type is gasoline stabilization where a small amount of C2, Cg, and C4 hydrocarbons are removed to obtain the desired volatility of the bottoms product. For this case, the system of Fig. 18-1 could be modified as shown in Fig. 18-2 by removing the overhead product through the vent line. This vent line would be equipped with a pressure controller which would adjust the vapor removal rate to maintain the desired operating pressure. The liquidlevel controller would be used to adjust the cooling water rate to the condenser instead of the product withdrawal rate. The reflux-flow controller would operate to maintain the top temperature as before, and the liquid-level controller would decrease the cooling water rate if the level became too high, thereby decreasing the rate of condensate production and increasing the overhead vapor rate.

Control Variables. One of the difficult problems in the automatic control of fractionating towers is finding an easily measured charac teristic that will ensure the desired separation. Temperature is the most commonly used factor, but it is not always a satisfactory criterion. Thus, if the product is of high purity and contains only a small amount of other constituents, these can vary several fold without significantly changing the equilibrium temperature. This is particularly serious in the separation of close boiling constituents. In the case of multi-component mixtures, temperature is not a good criterion of composition, but it can be a satisfactory indication of volatility. The problem

is particularly difficult in extractive distillation systems where the presence of the large quantity of solvent masks the effect of composition of the key components on the temperature. Temperature control is also sensitive to the column pressure.

In some of these cases, the effectiveness of temperature can be improved by proper location of the temperature control point. For example, in the case of high product purity, the temperature control point can be placed several plates from the end of the column at a point where the minor constituents have higher concentrations. This results in a larger temperature variation which makes the instrumentation easier, but it removes the direct control on the product.

Other control factors besides temperature have been used, such as density of the liquid, refractive index of the liquid, infrared or other spectrographic types of analysis, and freezing point. The factor should be one which gives an indication of product composition and which can be easily and rapidly determined by4 a relatively inexpensive, stable instrument capable of operating electronic or pneumatic equipment. The spectographic type of instruments should be very useful because they can frequently be adjusted to indicate the amount of an impurity present in extremely small amounts. Unfortunately they are somewhat delicate and need frequent adjustment, but these defects can undoubtedly be eliminated.

Rate of Approach to Equilibrium. Corrective action or other changes in the operating conditions introduce a transient condition into the system. Consider a section of a column operating at steady-state conditions. Assume that a change is made on one of the plates (such as a change in the feed composition to the feed plate), and the problem is to determine how rapidly other plates will approach their new equilibrium condition. The exact mathematical solution of the general case is very complex, but solutions based on certain approximations can be obtained. The curves given in Fig. 18-3, based on a number of stepwise integrations, should be helpful in obtaining approximate results. In this figure the ordinate, F, is the fractional approach to the new equilibrium conditions of a plate which is n plates from the one on which the change in composition was made. The value of F is defined as,

where yn = vapor leaving plate at time 9

(yrdoo = new equilibrium value of yn, i>e.f value of y» at 9 = oo t/^ = equilibrium vapor composition before change in conditions were made

The abscissa is the dimensionless ratio of liquid flow through the section to liquid holdup in the section. This group is

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