Temperature control in distillation

VAc 0 Vy, Product stream 0 Temperature control in distillation column... 

PRESSURE-COMPENSATED TEMPERATURE CONTROL IN DISTILLATION COLUMNS... 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

All syntheses discussed below used commonly available commercial materials without further purification. Manually sampled reactions were analyzed for their density at temperatures between 20-30°C. Continuously sampled reactions were analyzed at the polymerization temperature. Temperature control in the density cell was better than 0.1°C for static samples. Temperature fluctuated more for continuously sampled reactions, sometimes rising. 1-.3°C because of the exothermic reaction continuing in the density cell. Calibration of the density cell below 95°C was accomplished using values for densities for moist air(10) and degassed, distilled water(11). For measurements above 95°C a certified viscosity standard oil (Number S-200 oil from Cannon Instrument Co.) was used. 

EXAMPLE 4.2. Composition control in distillation columns is Ifequently done by controlling a temperature somewhere in the column The location of the best temperature control tray is a popular subject in the. process control literature. The ideal location for controlling distillate composition xo with reflux flow by using a tray temperature would be at the top of the column for a binary system (see Fig. 4.9a). This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from... 

This chapter reviews the application of temperature and analyzer control in distillation columns, highlights the problem areas for each technique, examines the pitfalls of composition controls and the consequences of overlooking them, and provides guidelines for good composition control practices. 

In this process, the oil is firstly hydrolyzed to give free fatty acids which are then heated at 200-240°C with a mixture of polyol and dibasic acid. Simultaneous condensation of the polyol, dibasic acid and fatty acids thereby occurs and the latter become incorporated into the polymer structure. The process may be conducted in two ways. In the first procedure, which is known as the fusion or solventless method, the reactants are heated in a simple kettle under an inert atmosphere. At the end of the heating period, inert gas is blown into the resin to remove water and unreacted materials. In the second procedure, which is known as the solvent or solution method, a small amount (generally about 5%) of a solvent, usually xylene, is added to the reactants. The mixture is heated in a reactor fitted with equipment which condenses volatile vapours, separates water and returns the organic distillate to the reactor. The solvent facilitates removal of water by azeotropic distillation and, compared to the fusion method, allows much better temperature control. In addition, the solvent reduces the viscosity of the reactants this permits more effective agitation which contributes to easier water removal and faster reaction. The solvent also continually cleans resin from the sides of the reactor and enables a more uniform product to be prepared that is free from gel particles. However, despite these advantages of the solvent method over the fusion method, the latter is widely used since it requires simpler equipment. 

It is often said that derivative action should only be used in temperature controllers. It is true that temperatures, such as those on the outlet of fired heater and on distillation column trays, will often exhibit significantly more deadtime than measurements such as flow, level and pressure. However this is not universally the case, as illustrated in Figure 3.7. Manipulating the bypass of the stream on which we wish to install a temperature controller, in this case around the tube side of the exchanger, will provide an almost immediate response. Indeed, if accurate control of temperature is a priority, this would be preferred to the alternative configuration of bypassing the shell side. 

We can adopt a similar approach to tray temperature controllers on distillation columns. They provide some control of product composition because this correlates with the bubble point of the liquid. However, changing pressure changes this relationship. Figure 5.11 shows the effect pressure has on bubble point, in this case water, but all liquids show a similar behaviour. 

The conclusion is therefore that implementing temperature control in the lower section of the column causes the distillate composition to vary considerably more when column feed rate changes - no matter what level control strategy is adopted. This can be resolved by the implementation of ratio feedforward to the remaining unused MV. 

There are two reasons for the improvement in control. The first has already been discussed the elimination of inverse response. The second is equally important the two temperature controllers in the CS7-RR structure have the same action (direct). This means that when both control loops see a positive vapor boilup disturbance, which increases both tray temperatures, the two controllers will increase both fresh feeds. This helps to maintain the delicate stoichiometric balance between the reactants that is essential for neat operation of a reactive distillation column. Because a reactive distillation column acts like a pure integrator with respect to the reactants, this similar initial response is very important for CS7-RR, where feedstreams are used as manipulated variables. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

AH glass capillary viscometers should be caUbrated carefully (21). The standard method is to determine the efflux time of distilled water at 20°C. Unfortunately, because of its low viscosity, water can be used only to standardize small capillary instmments. However, a caUbrated viscometer can be used to determine the viscosity of a higher viscosity Hquid, such as a mineral oil. This oil can then be used to caUbrate a viscometer with a larger capillary. Another method is to caUbrate directly with two or more certified standard oils differing in viscosity by a factor of approximately five. Such oils are useful for cahbrating virtually all types of viscometers. Because viscosity is temperature-dependent, particularly in the case of standard oils, temperature control must be extremely good for accurate caUbration. 

States or Australia. In some cases, pot stills, arranged in cascade, are still used. The more sophisticated plants employ one or more carbon steel or cast-iron vessels heated electrically and equipped with temperature controls for both the bulk Hquid and the vessel walls. Contact time is usually 6—10 h. However, modem pitches are vacuum-distilled, producing no secondary quinoline insolubles, to improve the rheological properties. 

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