Azeotropic columns

Anhydrous hydrazine, required for propellant appHcations and some chemical syntheses, is made by breaking the hydrazine—water azeotrope with aniline. The bottom stream from the hydrate column (Fig. 4) is fed along with aniline to the azeotrope column. The overhead aniline—water vapor condenses and phase separates. The lower aniline layer returns to the column as reflux. The water layer, contaminated with a small amount of aniline and hydrazine, flows to a biological treatment pond. The bottoms from the azeotrope column consist of aniline and hydrazine. These are separated in the final hydrazine column to give an anhydrous overhead the aniline from the bottom is recycled to the azeotrope column. 

Completing the Separation Sequence. In the remainder of the separation sequence the distiUate stream leaving the azeotropic column, column 2 in Fig. 19a, must be separated into a product stream and a recycle stream so that the entire sequence is closed with respect to the entrainer. 

Ydibierschky Three-Column Sequence. If only simple columns are used, ie, no side-streams, side-rectifiers/strippers etc, then the separation sequence can be completed by adding an entrainer recovery column, column 3 in Figure 19a, to recycle the entrainer, and a preconcentrator column (column 1) to bring the feed to the azeotropic column up to the composition of the binary azeotrope. 

Levy SG, Van Dongen DB and Doherty MF (1985) Design and Synthesis of Homogeneous Azeotropic Distillation 2. Minimum Reflux Calculations for Non-ideal and Azeotropic Columns, Ind Eng Chem Fund, 24 463. 

Figure 2 shows a continuous azeotropic column using a fixed amount of entrainer which remains in the unit. Since reflux is largely supplied by feed of the emulsion near the top of the column, the entrainer from the decanter passes to a reboiler and is fed back to the tower as vapors. This gives a more nearly counter-current action of the azeotropic distilling operation, and a lesser heat input required into the viscous oil at the base of the column, usually with more or less silt in suspension while... 

If a naphtha fraction has been used in the dilution and as the en-trainer, it is left in as a diluent for reducing the viscosity in pumping the oil to the refinery. Alternatively, some or all is removed as a slip stream from the reflux going to the top of the azeotropic column. If toluene or other aromatic solvent is used, it would be so removed or removed in a separate distillation for recycle. 

Esterification, The process flow sheet (Fig. 4) outlines the process and equipment of the esterification step in the manufacture of the lower acrylic esters (methyl, ethyl, or butyl). For typical art, see References 69—74. The part of the flow sheet containing the dotted lines is appropriate only for butyl acrylate, since the lower alcohols, methanol and ethanol, are removed in the wash column. Since the butanol is not removed by a water or dilute caustic wash, it is removed in the azeotrope column as the butyl acrylate azeotrope dais material is recycled to the reactor. 

Process conditions for methyl acrylate are similar to those employed for ethyl acrylate. However, in the preparation of butyl acrylate the excess butanol is removed as the butanol—butyl acrylate azeotrope in the azeotrope column. 

Figure PlO-3 Hiils TBA synthesis process. R, reactor C C4 column Cg, Cg column AC, azeotrope column TC, TBA column (Adapted from R. E. Meyers,...

As for the synthesis of azeotropic columns, most of the developments have been based on geometric representations with residue curves (Doherty and Car-darola, 1985 Knight and Doherty, 1989 Van Dongen and Doherty, 1985). Guidelines have also been developed by Stichlmair et al. (1989), some of which Laroche et al. (1992) have questioned as general synthesis rules. Finally, Wahn-schafft et al. (1992) have developed the program SPLIT, which is based on a ruled-based system and evolutionary search method. 

A typical simple distillation plant is shown in Figure 35. In a first column 1, use is made of the water/furfural azeotrope having an atmospheric boiling point of 97.85 °C and a water content of 65 percent. Column 1 is commonly called the azeotropic column , although this is unfortunate as in a subsequent column the same azeotrope is used for the dehydration of furfural, so that the attribute azeotropic is not a unique feature of the first column. 

When the raw furfural produced by the decanter of the azeotropic column turns out to be acid, due to an insufficient number of trays between the feed inlet and the side stream withdrawal, it is customary to submit this raw furfural (the heavy phase of the decanter) to a neutralization with sodium carbonate. This, however, is problematic, the principal cause being the fact that the dielectric constant of furfural amounts to a mere 48 percent of the value for water (38.0 versus 78.5 at 25 °C). Thus, raw furfural, exhibiting a typical furfural content of 92 % by weight, will have a dielectric constant only half as great as that of water. This has far-reaching consequences. 

In this connection, it is noted that in the decanter of the azeotropic column the concentrations of acetic acid in the light aqueous phase and the heavy organic phase are almost equal. This can be seen from the respective distribution diagram shown in Figure 116 [121]. Thus, the phase separation in the decanter is unfortunately no help for obtaining a raw furfural of low acidity. 

The preceding chapter has shown that acids arriving in the side stream of the azeotropic column of a furfural plant end up in the raw furfural, thus requiring a respective neutralization. Inasmuch, however, as preventing a disease is better than healing it, it is decidedly preferable to prevent the acids from getting up there in the first place. 

The mass balance of an azeotropic column for eleven ROSENLEW reactors of 2.5 m diameter processing bagasse is shown in Figure 126. The following comments are made ... 

Figure 10.3c is a schematic of a possible process for the separation of such a system. The fresh feed is mixed with the azeotrope-forming entrainer and is then fed to the azeotropic column, where pure component B is taken as column bottoms and the azeotrope AE is taken as overhead. The next step is to separate AE, an azeotrope that cannot be separated by simple distillation. A workable process involves liquid-liquid extraction where a solvent S is used to extract the entrainer, leaving a product that is virtually pure A. The solvent and entrainer are then separated by conventional distillation, the entrainer is recycled and mixed with fresh feed and makeup entrainer, and the solvent is recycled to the liquid-liquid extractor where some makeup solvent may be added. 

A typical flowsheet for the separation is shown in Eigure 10.4c. The fresh feed is combined with the entrainer and then fed to the azeotropic column, where the two azeotropes are separated. Each azeotrope product is sent to a liquid-liquid extraction column to dissolve the entrainer in some suitable solvent. The combined entrainer-solvent solution from both extractors is sent to a distillation column to separate the entrainer and solvent. These are recycled to the azeotropic column and extractors with makeup added to each, as needed. 

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. 

Almost pure component A is taken as azeotropic column bottoms. The purity of A is determined by the relative amount of entrainer added. Too little entrainer will not remove sufficient amounts of B in the overhead, thus allowing part of it to flow down with the bottoms. Too much entrainer may result in part of it contaminating the bottoms. 

The entrainer-rich liquid from the receiver is returned to the azeotropic column as reflux, and the liquid phase rich in component B is sent to a stripper column, where any amounts of A and E and some of B are removed in the overhead and combined with the azeotropic column overhead. The stripper bottoms is primarily component B. 

At steady state, and aside from inevitable small losses, almost all of component A in the fresh feed is recovered as azeotropic column bottoms and B as stripper column bottoms. The entrainer, once introduced into the system, circulates within it indefinitely, and any losses are replaced by fresh makeup. The reflux rate and composition are controlled to maintain the ternary azeotrope composition in the overhead. The reflux composition is controlled by the inventory of the entrainer in the receiver and to some degree by the receiver temperature. 

As is radicated, column 2, the azeotropic column, is performing the split between the BC azeotrope and A. Also, as shown in Pig. 7.8-2, the volatile solvent is condensed to provide reflux Tor both columns 2 and 3, with the latter column performing the split between B and C. Hence, essentially 100% of the solvent C is volatilized during recycle through columns 2 and 3. 

In such cases, Fig. 7.8-3 represents a useful process flowsheet. Under these circumstances, column 2 performs the BC/AC split (i.e., it is used to remove the BC azeotrope). Column 3 is used to perform the A/C split. 

The table below provides information about azeotropes for 808 selected binary systems. Compounds are listed in the modified HUl order, with carbon-containing compounds following those compounds not containing carbon. In columns 1 and 2 are the molecular formulas of components 1 and 2 written in the HiU convention. In column 3 the names of the components are given, either a systematic lUPAC name or a name in ubiquitous use. Columns 4, 5, and 6 contain the azeotropic coordinates of the mixtures temperature T az, pressure Paz< and vapor-phase composition The explanation of the type of azeotrope (column 7) is given by the following codes ... 

Let us consider azeotropic distillation first. If a homogeneous azeotrope is produced, no changes in the normal apparatus for countercurrent distillation are required. When a heterogeneous azeotrope results the whole azeotrope is not returned to the column as reflux, but only the phase rich in additive. For this purpose an azeotropic column head is employed, as shown in Fig. 232, which allows either the heavy or the light phase to be used as reflux. The reflux is preferably returned to the column at some distance below the head. In continuous operation, if a honu eneous azeotrope is formed, the additive is mixed with the feed if the azeotrope is heterogeneous it is... 

Azeotropic column head with magnetic reflux division and phase separation... 

This technique was successfully applied in one of the experiences reported (203), and in one other case the author experienced. In the former case (203), absorption oil was injected to prevent excessive product loss at the higher temperature. In the latter case, which occurred in an azeotrope column, the additional product loss due to the higher top temperature was negligible. Others (61) also advocated this technique. 

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