Distillation behavior

The furfural-cyclohexane phase diagram (Fig. 146) shows that you can have mixtures that exhibit nonideal behavior, without having to form an azeotrope. In sum, without the phase diagram in front of you, you shouldn t take the distillation behavior of any liquid mixture for granted. 

The equilibrium distillation behavior of the model fuels is adequately covered in the fuel oil discussion. However, the case for the rapid droplet vaporization, which was not clearly seen for fuel oils, is more amenable to analysis for a binary system. The surface gradients are given by the following relationship,... 

Figure 13. Phase diagram jor a dilute solution with an azeotrope. Diagram can be used qualitatively to predict the distillation behavior of T (solute) in Li (solvent).

Since deuterium occurs naturally in the form of HD rather than D2, the pilot plant was designed to separate the system H2-HD. However, all the experimental work leading to the design of the pilot plant was carried out with the H2-D2 system, for D2 is readily available and HD is not. The use of the H2-D2 system in the experimental work rather than the H2-HD system is readily justified. Since the two pairs have very similar physical properties, they should have similar distillation behavior. As an index for comparison, the Drickamer and Bradford [1] correlation predicts the two systems will have the same overall plate efficiency. The Connell [7] correlation predicts a plate efficiency for the HD system only slightly higher than that for the D2 system. 

We now describe a procedure where we begin with distillation and add reaction to it, first with an equilibrium reaction and then a reaction with a finite forward rate constant (in other words, we will explore the effect of reaction kinetics on distillation behavior). A conunon example of distillation-reaction is esterification, such as Eastman Kodak s process for methyl acetate ... 

This is an important eqnation in spite of some simplifying assumptions, for it defines the effect of chemical reaction with a finite rate on the distillation behavior of the system. We now use this equation to calculate the RCMs. 

Mixtures with low relative volatility or which exhibit azeotropic behavior. The most common means of dealing with the separation of low-relative-volatility and azeotropic mixtures is to use extractive or azeotropic distillation. These processes are considered in detail later. Crystallization and liquid-liquid extraction also can be used. 

Distillation of Mixtures Which Exhibit Azeotropic Behavior or Have Low Relative Volatility... 

To evaluate the real behavior of fuels in relation to the segregation effect, the octane numbers of the fuel components can be determined as a function of their distillation intervals In this manner, new characteristics have been defined, the most well-known being the delta R 100 (A7 100) and the Distribution Octane Number (DON). Either term is sometimes called the Front-End Octane Number . 

The density, distillation curve, viscosity, and behavior at low temperature make up the essential characteristics of diesei fuel necessary for satisfactory operation of the engine. 

Monofluorophosphoric Acid. Monofluorophosphoric acid (1) is a colorless, nonvolatile, viscous Hquid having practically no odor. On cooling it does not crystallize but sets to a rigid glass at —78°C. It has a density of = 1.818 g/mL. Little decomposition occurs up to 185°C under vacuum but it caimot be distilled. An aqueous solution shows the normal behavior of a dibasic acid the first neutralization point in 0.05 N solution is at pH 3.5 and the second at pH 8.5. Conductance measurements, however, indicate H2PO2F behaves as a monobasic acid in aqueous solution (59). The... 

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

Extractive agent modifies Hquid-phase behavior (activity coefficients) of key components RCM must be of appropriate form for extractive distillation to work. 

Table 3 contains strategic separations to be considered for crossing distillation boundaries. Many of these can be eliminated after examining the pertinent physical properties and equiUbrium behavior (see Table 4) and referring to the general separation considerations. The results are summarized in Table 5. 

In the lightening of petroleum hydrocarbon oil, esters of mercaptocarboxyhc acids can modify radical behavior during the distillation step (58). Thioesters of dialkanol and trialkanolamine have been found to be effective multihinctional antiwear additives for lubricants and fuels (59). Alkanolamine salts of dithiodipropionic acid [1119-62-6] are available as water-soluble extreme pressure additives in lubricants (60). 

The increase in fuel viscosity with temperature decrease is shown for several fuels in Figure 9. The departure from linearity as temperatures approach the pour point illustrates the non-Newtonian behavior created by wax matrices. The freezing point appears before the curves depart from linearity. It is apparent that the low temperature properties of fuel are closely related to its distillation range as well as to hydrocarbon composition. Wide-cut fuels have lower viscosities and freezing points than kerosenes, whereas heavier fuels used in ground turbines exhibit much higher viscosities and freezing points. 

If the molecular species in the liquid tend to form complexes, the system will have negative deviations and activity coefficients less than unity, eg, the system chloroform—ethyl acetate. In a2eotropic and extractive distillation (see Distillation, azeotropic and extractive) and in Hquid-Hquid extraction, nonideal Hquid behavior is used to enhance component separation (see Extraction, liquid—liquid). An extensive discussion on the selection of nonideal addition agents is available (17). 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

Historically azeotropic distillation processes were developed on an individual basis using experimentation to guide the design. The use of residue curve maps as a vehicle to explain the behavior of entire sequences of heterogeneous azeotropic distillation columns as weU as the individual columns that make up the sequence provides a unifying framework for design. This process can be appHed rapidly, and produces an exceUent starting point for detailed simulations and experiments. 

Residue Curve Maps. Residue curve maps are useful for representing the infinite reflux behavior of continuous distillation columns and for getting quick estimates of the feasibiHty of carrying out a desired separation. In a heterogeneous simple distillation process, a multicomponent partially miscible Hquid mixture is vaporized ia a stiH and the vapor that is boiled off is treated as being ia phase equiHbrium with all the coexistiag Hquid phases. 

Several enhanceci distihation-based separation techniques have been developed for close-boihng or low-relative-volatihty systems, and for systems exhibiting azeotropic behavior. All of these special techniques are ultimately based on the same differences in the vapor and liquid compositions as ordinaiy distillation, but, in addition, they rely on some additional mechanism to further modify the vapor-hquid... 

Azeotropic distillation andpre.s.sure-swing distillation. Methods that cause or exploit azeotrope formation or behavior to alter the boiling characteristics and separability of the mixture. 

Extractive distillation and. salt distillation. Methods that primarily modify liquid-phase behavior to alter the relative volatility of the components of the mixture. 

Introduction The term azeotropic distillation has been apphed to a broad class of fractional distillation-based separation techniques in that specific azeotropic behavior is exploited to effect a separation. The agent that causes the specific azeotropic behavior, often called the entrainer, may already be present in the feed mixture (a self-entraining mixture) or may be an added mass-separation agent. Azeotropic distillation techniques are used throughout the petro-... 

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