Separations azeotropic

The water content of crude oils is determined by a standardized method whose procedure is to cause the water to form an azeotrope with an aromatic (generally industrial xylene). Brought to ambient temperature, this azeotrope separates into two phases water and xylene. The volume of water is then measured and compared with the total volume of treated crude. 

The entrainer recovery column takes the distillate stream, from the azeo-column and separates it into a bottoms stream of pure water, and a ternary distillate stream for recycle to column 2. The overall material balance line for column 3 is shown in Figure 19b. This sequence was one of two original continuous processes disclosed in 1915 (106). More recendy, it has been appHed to other azeotropic separations (38,107,108). 

Thus, distillation line and residue curve maps are excellent tools to evaluate feasibility of azeotropic separations, with just one exception, namely, the use of high-boiling entrainers for separation. In such cases, the equi-volatility curves discussed in this chapter are a better way of determining separation feasibility. 

However, this so far assumes that the feed to the column is fixed. Even if the overall feed to the separation system is fixed, the feed to each column can be changed by changing the amount of entrainer recycled. Such a trade-off has already been seen in Figure 12.21. As the amount of entrainer recycled is increased, this helps the azeotropic separation. This allows the reflux ratio to be decreased. However, as the entrainer recycle increases, it creates an excessive load on the overall system. The amount of entrainer recycled is therefore an important degree of freedom to be optimized. 

Many other azeotropic separations are known. Butadiene, styrene, benzene, and xylenes are examples of compounds that may be segregated from refinery streams by this means. In fact, any separation of nonaromatic from aromatic hydrocarbons lends itself to this method, but requires the selection of the proper azeotrope former and processing conditions. In bench scale operations, azeotropy has been applied up to and including the lubricating oil range. 

Another field in which azeotropic distillation is finding application is in the separation of the complex mixtures of organic acids, aldehydes, ketones, and alcohols produced by the Hydrocol process. As petroleum stocks are utilized more and more for the production of chemicals, processing of azeotropic mixtures and the use of azeotropic separations should assume increasingly greater importance. 

Although these specifications lead to only moderate tray and reflux requirements, in practice distillation with only two towers and the assistance of an azeotropic separating agent such as benzene is found more economical. Calculation of such a process is made by Robinson and Gilliland (1950, p. 313). 

Water and n-butanol in the concentration range of about 50-98.1 mol % water form two liquid phases that boil at 92.7°C at one atm. On cooling to 40°C, the hetero-azeotrope separates into phases containing 53 and 98 mol % water. 

Uses of Oldershaw columns to less conventional systems and applications were described by Fair, Reeves, and Seibert [Topical Conference on Distillation, AIChE Spring Meeting, New Orleans, p. 27 (March 10-14, 2002)]. The applications described include scale-up in the absence of good VLE, steam stripping efficiencies, individual component efficiencies in multicomponent distillation, determining component behavior in azeotropic separation, and foam testing. 

A benzene solution (5 ml) of 15 mg of the 4b-methyl-ip,2a-bis-(2-formylmethyl)-2p-formyl-4p,7a-dihydroxyperhydrophenanthrene 2p,4p-lactol 7a-acetate containing 3.4 mg of piperidine and 6.2 mg of acetic acid was heated at 60°C in slow stream of nitrogen with an azeotropic separator. After 1 h, half the solution was withdrawn, diluted with benzene, and washed with diluted aqueous hydrochloric acid and with aqueous sodium bicarbonate. The benzene extract dried over magnesium sulfate and concentrated to dryness under reduced pressure, giving 3a-acetoxy-17-formyl-16-etiocholen-lip-ol-18-one 1 ip, 18-lactol as colorless oil. 

Dichlorobenzaldehyde is reacted with methyl acetoacetate in a suitable solvent in the presence of a catalytic amount of acetic acid and piperidine. Water is azeotropically separated off during the reaction. The reaction mixture is extracted in order to remove the catalysts. The solvent is evaporated and methanol is added. The product is crystallized by cooling the solution, isolated by filtration and finally washed with methanol. 

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. 

Perform difficult zeotropic separations later, but before azeotropic separations. Examine other options, such as extractive distillation, L-L extraction, crystallization, adsorption, or molecular sieves. 

For the separation of a minimum-boiling binary azeotrope by extractive distillation, there is clearly a minimum-solvent flow rate below which the separation is impossible (due to the azeotrope). For azeotropic separations, the number of equilibrium stages is infinite at or below (S/F) i and decreases rapidly with increasing solvent feed... 

Axial flow pumps, 134, 136, 140 applicafion range, 150 Azeotrope separation, 387,388,420-426 Azeotropic distillation, 420-426 acetonitrile/water separation, 422 commercial examples, 421-424 design method, 424 ethanol/water/benzene process, 424 n-heptane/toluene/MEK process, 424 vapor-liquid equilibrium data, 421, 423, 425,426... 

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