Processes with Entrainer

The addition of an external substance (called entrainer e) is a veiy effective means for fractionating azeotropic mixtures by distillation. In the multicomponent mixture generated by the addition of the entrainer the azeotrope is circumvented by distillation. This principle is explained in Rg. 11.3-4 at the example of a binary mixture a-b having a minimum azeotrope. 

The process shown in Fig. 11.3-4 consists cf three distillation colitmns and two recycles. A material balance aroimd coluttms C-2 and C-3 reveals that the fraction 1)1 is split into the fraction 62 (pitre high boiler b) and the recycle fraction 63.  

the states of fractions t), B2, and 53 have to lie on a straight line (dashed line in the concentration diagram of Fig. 11.3-4). 

An important prerequisite of the process shown in Fig. 11.3-4 is that the amount of fraction D2, which has to be fractionated further in column C-3 into the recycle fractions D2 and 53, is small. This condition requires that the distance between points D2 and M2 in the concentration space is as large as possible. Hence, the boundary distillation line has to be strongly curved and the position of mixing point M2 has to be in the middle of the concentration space (Stichlmair and Fair 1998). 

The process of Fig. 11.3-4 can also be applied to mixtures with maximum azeotropes. Here, a top side down version of the flow sheet has to be used. 

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For processes with relatively low compression rates such that the air entrained between the pellets is not readily pushed back out of the hopper, solid bed breakup will eliminate a pathway back to the hopper. In this case the entrained air will discharge with the extrudate and often create defects in the product. This type of problem is presented in Section 10.2.2. 

The addition of small quantities of cosolvents, also known as modifiers or entrainers, can enhance the solubility characteristic further. Even though in earlier years attention was focused primarily on single-processing fluids such as CO2 and extractions as the primary mode of application, in recent years emphasis has been shifting to binary and multicomponent fluids and processes with a greater degree of complexity, which can include either physical or chemical transformations. Some modifiers with their relevant properties are listed in Table 3.2. 

Figure 13.27. Separation of the azeotropic mixture of acetonitrile and water which contains approximately 69 mol % or 79.3 wt % of acetonitrile. (Pratt, Countercurrent Separation Processes, Elsevier, New York, 1967, pp. 194, 497). (a) A dual pressure process with the first column at 100 Torr and the second at 760 Torr. (b) Process employing trichlorethylene as entrainer which carries over the water in a ternary azeotrope that in turn separates into two phases upon condensation.

Figures 4.7 through 4.9 are provided for hydrate limits to isenthalpic Joule-Thomson expansions, such as that which occurs when a gas with entrained free water droplets flows through a valve. A similar set of charts could in principle be determined for hydrate limits to isentropic (AS = 0) expansions such as would occur when a gas flows through a perfect turboexpander of a modern gas processing plant. To date, however, no such charts have been generated.

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

Hence, the entrainer enhances the water removal, ensuring simultaneously a recycle of alcohol to the reaction zone. As a consequence, the reaction rate can increase substantially. The comparison with a process without entrainer operating as pseudoabsorber shows that the catalyst loading can be reduced up to 50%. 

Meanwhile, Saso) II went on stream (1980) using the entrained-bed process with a capacity of approximately 2 million t/a and Sasol III with the same capacity is under construction and its production startHip is scheduled for 1982. By then, roughly 40% of the motor fuels consumed in South Africa will be produced by Fischer-Tropsch synthesis [2,12]. 

Entrainment may be defined as the carryover of ejected particles, while selective entrainment of finer or less dense particles is often referred to as elutriation. In most industrial processes, neither entrainment nor elutriation are desirable, which is in sharp contrast to this particular application. Consequently, there is very little research aimed specifically at enhancing the selective removal of less dense material from fluidised beds. Most research on entrainment is based on dimensional analysis applied to experimental data either with no or very limited consideration of the underlying physics Predictions made from these correlations are limited to very simple geometries. They may vary widely even for reactor airangements close to the experimental conditions they are based on, and are often completely unreliable when conditions are markedly different. In several intemal studies they have been found inadequate for entrainment and elutriation predictions in the fluidised bed system under investigation. The problem is too complex to be adequately represented by a small number of ordinary equations that would simply require substitution of a few parameters to obtain the rales of entrainment of the different particle size ftactions. 

There are two primary sources of particulates which may be carried out of a combustion process with the exhaust gases. One is entrainment and carryover of incoming raw materials, and the other is the production of particles as a result of the combustion process. 

In this process the entrainer is added as redux to the azeotropic column. The ternary azeotrope is taken as column overhead and condensed, whereupon it separates in the receiver into two liquid phases, one rich in the entrainer and the other rich in component B. Compositions below the curve in the ternary diagram (Figure 10.5) form two coexisting liquid phases, and those above the curve form one liquid phase. The tie lines connect compositions in the two liquid phases at equilibrium with each other. 

Low-level, low-acid, low-salt wastes are neutralized if necessary and concentrated in a simple flash or vaporlevel waste concentrates and water sufficiently decontaminated for return to process. With simple wire-mesh entrainment separators, decontamination factors of several thousand are easily obtained. 

Liquid-liquid extraction can be used to recover one of the components to be separated, as well as the entrainer to be recycled. A typical process is the separation of cyclohexane/benzene, with nbp s at 80.8 °C and 80.1 °C and minimum-boiling azeotrope at 77.6 °C (Seader and Henley, 1998). If acetone is used as entrainer (nbp 56.2 °C), then an azeotrope appears between acetone and cyclohexane (nbp 53.1 °C). RCM shows a distillation boundary between the components to be separated (Fig. 9.32). To simplify the process, consider an initial azeotropic mixture. After mixing with entrainer the feed f, is separated in two products, benzene in bottoms and acetone-cyclohexane azeotrope in top. From the last mixture cyclohexane can be extracted with water. Finally acetone and water are separated by simple distillation. 

The solvent dewaxing process involves addition of solvent during the chilling of the waxy feed. The chilling process induces crystallization of the wax and the solvent is added to maintain the fluidity. The wax crystals are filtered by rotating drum filters, resulting in a lube oil filtrate and a slack wax with entrained solvent and oil. In both fractions, the solvent is recovered by successive vaporization and distillation. 

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