Distillation opposite process

The opposite process of chlorine vaporization provides similar opportunities for concentration and accumulation. This can be a particularly acute hazard for chlorine consumers. The vaporization of a batch of liquid chlorine is essentially a process of differential distillation. 

If there is a reversible distillation process, then there should be also an opposite process, which may be called a process of the distilled flow mixing. 

Figure 2.15b shows a McCabe-Thiele diagram with the operation lines of the opposite process of distillation, and Fig. 2.15a illustrates a scheme of this process. 

Figure 2.15. (a) A column for the process that is opposite to the distillation process, (b) McCabe-Thiele diagram (operation hnes at infinite reflux, 1 at finite reflux, 2 at minimum reflux, 3 at reversible distillation, 4 at opposite process, 5 little circles, tray composition). 

In the case of the extractive distillation (Fig. 2.14), when increasing the entrainer flow rate oo, in the extractive section the conditions will come close to those of the top section of the opposite process. 

When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counterdiffusion. Such a process occurs in the case of the box with a movable partition, referred to in Section 10.1. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from... 

As a result of the diffusional process, there is no net overall molecular flux arising from diffusion in a binary mixture, the two components being transferred at equal and opposite rates. In the process of equimolecular counterdiffusion which occurs, for example, in a distillation column when the two components have equal molar latent heats, the diffusional velocities are the same as the velocities of the molecular species relative to the walls of the equipment or the phase boundary. 

Usually in evaporation the thick liquor is the valuable product and the vapour is condensed and discarded. There are, however, specific situations where the opposite is true. In this context, mention may be made of the fact that mineral-containing water is often evaporated to yield a solid-free product for boiler feed, for special process requirements, or for human consumption. This technique is often called water distillation, but technically it is evaporation. 

In 2002, Drioli and coworkers (304) investigated a process for obtaining protein crystals by means of membrane crystallization, which actually combines membrane distillation and crystallization techniques. The solvent evaporates at the membrane interface, migrates through the pores of the membrane, and condenses on the opposite side of the membrane. The reported preliminary results indicate interesting potentialities of this new method with respect to macromole-cular crystallization. 

Vaccum membrane distillation, such as any membrane distillation process, is a thermally driven process in which a feed solution is bought into contact with one side of a microporous membrane and a vacuum is created on the opposite side to create a driving force for transmembrane flux (Figure 19.8). The microporous membrane only acts as a support for a vapor-liquid interface, and does not affect the selectivity associated with the vapor-liquid equilibrium [76]. 

Simple extraction is often a rather inefficient process and may be modified to magnify the effect of one extraction. This may be done in either of two ways by using continuous extraction, in which the solvent is recycled through the solution by the use of a distillation-condensation cycle or by countercurrent extraction, in which the original solution and the extracting solvent are permitted to flow in opposite directions through a tube designed to promote contact between the phases. These methods will be described in turn. 

In many commercial processes such as distillation, extraction, absorption of gases in liquids, and the like, the entering and leaving streams represent two different phases that flow in opposite directions to each other, as shown in Fig. E2.22a. (The flgure could just as well be laid on its side.)... 

For oleaginous materials having a low oil content (18-20%), such as soybean and rice bran, solvent extraction is often applied for oil recovery. Hexane is widely accepted as the most effective solvent used today. Most of the extractors currently used are designed as countercurrent flow devices. The solid material flows in an opposite direction of solvent-oil miscella with an increasing oil concentration. The miscella containing around 25-30% oil after extraction is subjected to solvent distillation to recover the oil. The extracted solid material, commonly known as white flakes, is also conveyed to the desolventizing process. 

A stage in a separation process could, for instance, be a tray in a distillation column, a section in a packed absorption column, or a partial condenser or reboiler. An equilibrium stage is a contacting device—a vessel—in which two or more phases can mix thoroughly and interact with each other for a sufficient length of time until the number of moles of any component moving from one phase to another equals the number of moles of that component moving in the opposite direction, that is, until no more net mass transfer takes place between the phases. The composition of each phase is then the equilibrium composition. 

In equimolar diffusion, a binary mixture is assumed where the two components diffuse in opposite directions at equal rates. These conditions exist in binary distillation where a mole of component 1 is vaporized for each mole of component 2 that is condensed. In unimolar diffusion, one component diffuses through a second, stagnant one. This is typical of an absorption process where one component diffuses through the gas phase to the interface boundary, is absorbed by the liquid, and then diffuses to the bulk of the liquid. The other gas components are assumed to remain in the gas and the liquid components to remain in the liquid. 

Figure 6.2 shows a basic flow diagram for either an absorption or a stripping column. The process differs from distillation in that one stream enters with a contaminant and exits clean while a second enters clean and exits with the contaminant. Hence, there are two feed streams, the solute carrier and the mass-separating agent, which enter the column at opposite ends. This creates a flow pattern similar to distillation in which a gas phase flows countercurrent to a liquid phase. An absorption column is equivalent to the rectifying section of a distillation column. 

Crystallisation doesn t remove all of 4b from solution so the mother liquor contains mostly 4a with some 4b. This is clear when the solution is neutralised and the free amine 2 isolated from it by distillation. The rotation has the opposite sign to that of (.S )-2, but is smaller. Recrystallisation of the sulfate salt brings the rotation to the same value as that of (S) -2, but with the opposite sign and we have a sample of pure (R)-2. You may feel that we have laboured this very simple resolution but it is important that you understand this process before continuing not only this chapter but all this section - chapters 22-31. The amine 2 is itself an important compound as you will see in the next section. 

Equation (7.1-16) reduces to two special cases of molecalar difiiision which are customarily considered. In equimolal counterdiffusion, component A diffuses through component B, which is diffusing at the same molal rate as A relative to stetionaiy coordinates, but in die opposite direction. This process is often approximated in the distillation of a binary system. In unimolol unidirectional diffusion, only one molecalar species—component A—diffuses through component B, which is motionless relative to stationary coordinates. This type of transfer is approximated frequently in the operations of gas absorption, liquid-liquid extraction, and adsorption. 

Normally saturated PAH solutions were generated by pumping distilled water through generator columns. The results of this experiment show that identical results were obtained when the opposite situation was imposed that is, the PAH concentration in the feed solution was greater than that expected in the effluent. This could only happen through an equilibrium process. 

When reversible addition and elimination reactions are carried out under similar conditions, they follow the same mechanistic path, but in opposite directions. The principle of microscopic reversibility states that the mechanism of a reversible reaction is the same in the forward and reverse directions. The intermediates and transition structures involved in the addition process are the same as in the elimination reaction. Under these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The reversible acid-catalyzed reaction of alkenes with water is a good example. Two intermediates are involved a carbocation and a protonated alcohol. The direction of the reaction is controlled by the conditions, which can be adjusted to favor either side of the equilibrium. Addition is favored in aqueous solution, whereas elimination can be driven forward by distilling the alkene from the reaction solution. The reaction energy diagram is show in Figure 5.1. 

A packed-bed distillation column is used to adiabatically separate a mixture of methanol and water at a total pressure of 1 atm. Methanol—the more volatile of the two components—diffuses from the liquid phase toward the vapor phase, while water diffuses in the opposite direction. Assuming that the molar latent heat of vaporization is similar for the two components, this process is usually modeled as one of equimolar counterdiffusion. At a point in the column, the mass-transfer coefficient is estimated as 1.62 x 10-5 kmol/m2-s-kPa. The gas-phase methanol mole fraction at the interface is 0.707, while at the bulk of the gas it is 0.656. Estimate the methanol flux at that point. 

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