Point, azeotropic

These are azeotropic points where the azeotropes occur. In other words, azeotropic systems give rise to VLE plots where the equilibrium curves crosses the diagonals. Both plots are however, obtained from homogenous azeotropic systems. An azeotrope that contains one liquid phase in contact with vapor is called a homogenous azeotrope. A homogenous azeotrope carmot be separated by conventional distillation. However, vacuum distillation may be used as the lower pressures can shift the azeotropic point. Alternatively, an additional substance may added to shift the azeotropic point to a more favorable position. When this additional component appears in appreciable amounts at the top of the column, the operation is referred to as an azeotropic distillation. When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation. 

During the investigation of the principles governing the process of copolymerization of AN with ISP in DMSO at 30 °C in the presence of ammonium persulfate, it was established that the anisotropic type of copolymerization is characteristic for this pair of monomers. The azeotropic point, as it is seen from Fig. 1 corresponds to a content of 60% of monomeric units of ISP in the monomer mixture. 

If a binary system forms an azeotrope, the activity coefficients can be calculated from a knowledge of the composition of the azeotrope and the azeotropic temperature. At the azeotropic point the composition of the liquid and vapour are the same, so from equation 8.31 ... 

We would like to find a solvent that breaks the azeotrope between acetone-chloroform (or moves the azeotrope point sufficiently to one side to allow separation by distillation) so that high purity acetone and chloroform can be recovered by extractive distillation. The solvent should be more selective to chloroform than acetone. The solvent, acetone and chloroform must form a totally miscible liquid. The solvent must not form azeotrope with either acetone or chloroform. The solvent should be easy to recover and recycle. The solvent should have favorable EH S properties. 

Very little data exist on the separation efficiency of multilayer diffusion and capillary condensation. Asaeda and Du (1986) used a thin modified alumina membrane to separate alcohol/water gaseous mixtures at high relative pressures (near 1). The azeotropic point could be bypassed for water/ethanol and water/isopropanol mixture by employing eapillary condensation as a separation mechanism at a temperature of 70°C. By deereasing the pore size to the microporous range (pore diameter < 2 nm by plugging the pores with hydroxides), the separation faetors were inereased to above 60 (Asaeda and... 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

Depending on monomer combination, azeotropic points were found... 

Advantageously, the monomer feed is adjusted to the azeotropic point of the pair of monomers, so that the polymer has the same composition as the monomer. Azeotropic points are shown in Table 10.3. 

Actually, most SAN types have a composition near the azeotropic point, just for the reasons explained above. 

Figure 2.2-1. Two phase equilibria three cases were the composition of the two phase are equal, a Pure component boiling point, b Azeotropic point, c Critical point.

In such a process an additive or solvent of low volatility is introduced in the separation of mixtures of low relative volatilities or for concentrating a mixture beyond the azeotropic point. From an extractive distillation tower, the overhead is a finished product and the bottoms is an extract which is separated down the line into a product and the additive for recycle. The key property of the additive is that it enhance the relative volatilities of the substances to be separated. From a practical point of view, the additive should be stable, of low cost, require moderate reboiler temperatures particularly for mixtures subject to polymerization or thermal degradation, effective in low to moderate concentrations, and easily recoverable from the extract. Some common additives have boiling points 50-100°C higher than those of the products. 

The vapor curve KLMNP gives the composition of the vapor as a function of the temperature T, and the liquid curve KKMSP gives the composition of die liquid as a function of die temperature. These two curves have a common point M. The state represented by M is that in which the two states, vapor and liquid, have the same composition xaB on die mole fraction scale. Because of die special properties associated with systems in this state, the Point M is called an azeotropic point and the system is said to form an azeotrope. In an azeotropic system, one phase may be transformed to the other at constant temperature, pressure and composition without affecting the equilibrium state. This property justifies the name azeotropy, which means a system diat boils unchanged. 

Nitric acid is a colourless liquid at room temperature and atmospheric pressure. It is soluble in water in all proportions and there is a release of heat of solution upon dilution. This solubility has tended to shape the process methods for commercial nitric acid manufacture. It is a strong acid that almost completely ionizes when in dilute solution. It is also a powerful oxidizing agent with the ability to passivate some metals such as iron and aluminium. A compilation of many of the physical and chemical properties of nitric acid is presented in Table A.1 of Appendix A. Arguably the most important physical property of nitric acid is its azeotropic point, this influences the techniques associated with strong acid production. The constant-boiling mixture occurs at 121.9°C, for a concentration of 68.4%(wt) acid at atmospheric pressure. 

An important point is that for most uses concerned with chemical production, the acid must be concentrated above its azeotropic point to greater than 95%(wt). Conversely, the commercial manufacture of ammonium nitrate uses nitric acid below its azeotropic point in the range 50-65%(wt). If the stronger chemical grade is to be produced, additional process equipment appropriate to super-azeotropic distillation is required. 

The possibility of combining two different separation units into one, hybrid, process has not been considered in this chapter. Hybrid processes are quite novel and have only very recently been considered by industry and have, therefore, so far not made it into the standard textbooks. A hybrid process has the combined benefits of both of the component units and the benefits should theoretically outweigh the disadvantages. An example is a hybrid of a distillation column and a pervaporation unit for azeotropic separation, where the distillation unit alone is limited by the azeotropic point. Again, a lot of research is currently devoted to this type of operation and it is generally believed that it will become more widely used in the future. 

The vapor-liquid equilibrium for the nitric acid / water system at atmospheric pressure is shown in Figure 9.4. This figure shows that a concentration of 68.4 weight % nitric acid is the maximum (i.e., the azeotropic point) that can be obtained by simple distillation of the weak acid220. 

Figure 9.5 illustrates the sulfuric acid concentrations that lead to the production of higher concentrations of nitric acid. At a concentration of 67 weight % H2SO4, the azeotropic point has vanished, and 99 weight % nitric acid can be distilled. The nitric acid is the lighter phase and is extracted as vapor. These vapors are condensed overhead and a portion of the nitric acid is returned to the distillation column as reflux. The sulfuric acid and water go with the bottom liquid phase and are concentrated for reuse in the process104,220. 

The phase diagram for the partially miscible binaries water/2-ethylhexanol and water/lauric acid can be described satisfactorily by UNIQUAC with binary interaction parameters from LLE data plus the azeotropic point This procedure allows accurate prediction of the liquid-liquid split, while preserving sufficient accuracy for VLE. The interaction parameters are given in Tables 8.5 and 8.6. 

After recovering the acetonitrile the problem is breaking its azeotrope with water. VLE investigation shows that this is sensitive to the pressure change. For example, at 0.4 bar the azeotropic point is x(ACN) = 0.80 and T = 326 K, while at 6 bar this shifts to x(ACN) = 0.57 and T = 412 K. Consequently, pressure-swing distillation may be applied as indicated in a recent patent [15]. 

In the systems (I) and (III) 2-simplex consists of a sole cell, all the trajectories inside which approach SP corresponding to homopolymer Ms where rs < 1. The systems (I) and (III) topologically are equivalent, since they differ from each other only by the inversion of the monomer indexes therefore their phase portraits are of the same type, too. In the systems (II) and (IV) the azeotropic point separates the simplex into the two cells. However, the system (IV), in which both parameters r, and r2 exceed unity practically is non-realizable [20-24]. That is why the stable binary azeotropes are excluded from the consideration, and the dynamics of the copolymerization of two monomers is exhaustively characterized by only two types (I) and (II) of phase portraits. 

From the isothermal vapor-liquid equilibrium data for the ethanol(l)/toluene(2) system given in Table 1.11, calculate (a) vapor composition, assuming that the liquid phase and the vapor phase obey Raoult s and Dalton s laws, respectively, (b) the values of the infinite-dilution activity coefficients, Y and y2°°, (c) Van Laar parameters using data at the azeotropic point as well as from the infinite-dilution activity coefficients, and (d) Wilson parameters using data at the azeotropic point as well as from the infinite-dilution activity coefficients. 

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