be "crossed." This may be done by mixing some external stream with the original feed stream in one region such that the resulting composition is in another region for further processing. However, the external stream must be completely regenerated, and mass-balance observed. For example, it is not possible to break a homogeneous binary azeotrope simply by adding one of the products to cross the azeotropic composition.
The composition of many azeotropes varies with the system pressure (Horsley, Azeotropic Data-III, American Chemical Society, Washington, 1983 and Gmehling et al., Azeotropic Data, VCH Publishers, Deerfield Beach, Florida, 1994). This effect can be exploited to separate azeotropic mixtures by so-called pressure-swing distillation if at some pressure the azeotrope simply disappears, as for example does the ethanol-water azeotrope at pressures below 11.5 kPa. However, pressure sensitivity can still be exploited if the azeotropic composition and related distillation boundary change sufficiently over a moderate change in total system pressure. A composition in one region under one set of conditions, could be in a different region under a different set of conditions. A two-column sequence for separating a binary maximum-boiling azeotrope is shown in Fig. 13-66 for a system in which the azeotropic composition at pressure P1 is richer in component B than the azeotropic composition at pressure P2. The first column, operating at pressure P1, is fed a mixture of fresh feed and recycle stream from the second column such that the overall composition lies on the A side of the azeotropic composition at P1. Pure component A is recovered as distillate and a mixture near the azeotropic composition is produced as bottoms. The pressure of this bottoms stream is changed to P2 and fed to the second column. This feed is on the B side of the azeotropic composition at P2. Pure component B is now recovered as the distillate and the azeotropic bottoms composition is recycled to the first column. An analogous flowsheet can be used for separating binary-homogeneous minimum-boiling azeotropes. In this case the pure components are recovered as bot-
toms in both columns and the distillate from each column is recycled to the other column.
For pressure-swing distillation to be practical, the azeotropic composition must vary at least 5 percent, (preferably 10 percent or more) over a moderate pressure range (not more than ten atmospheres between the two pressures). With a very large pressure range, refrigeration may be required for condensation of the low-pressure distillate or an impractically high reboiler temperature may result in the high-pressure column. The smaller the variation of azeotrope composition over the pressure range, the larger will be the recycle streams between the two columns. In particular, for minimum-boiling azeo-tropes, the pressure-swing distillation approach requires high energy usage and high capital costs (large-diameter columns) because both recycled azeotropic compositions must be taken overhead. Often one lobe of an azeotropic VLE diagram is pinched regardless of pressure; and, therefore, one of the columns will require a large number of stages to produce the corresponding pure-component product.
General information on pressure-swing distillation can be found in Van Winkle (D(st(llat(on, McGraw-Hill, New York, 1967), Wankat (Equ(l(brium-Staged Separations, Elsevier, New York, 1988), and Knapp and Doherty [Ind. Eng. Chem. Res., 31, 346 (1992)]. Only a relatively small fraction of azeotropes are sufficiently pressure sensitive for a pressure-swing process to be economical. Some applications include the minimum-boiling azeotrope tetrahydrofuran-water [Tan-abe et al., U.S. Patent 4,093,633 (1978)], and maximum-boiling azeo-tropes of hydrogen chloride-water and formic acid-water (Horsley, Azeotrop(c Data-III, American Chemical Society, Washington, 1983). Since separatrices also move with pressure-sensitive azeotropes, the pressure-swing principle can also be used for overcoming distillation boundaries in multicomponent azeotropic mixtures.
Exploitation of Boundary Curvature A second approach to boundary crossing exploits boundary curvature in order to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms are on the same residue curve in one distillation region, while the feed (which is not required to lie on the column-composition profile) is in another distillation region. In order for such a column to meet material-balance constraints (i.e., bottom, distillate, feed on a straight line), the feed must be located in a region where the boundary is concave.
As an example, Van Dongen [Ph.D. Thesis, University of Massachusetts, (1983)] considered the separation of a methanol-methyl acetate mixture, which forms a homogeneous azeotrope, using n-hexane as an entrainer. The separatrices for this system (Fig. 13-67a) are somewhat curved. A separation sequence that exploits this boundary curvature is shown in Fig. 13-67b. Recycled methanolmethyl acetate binary azeotrope and methanol-methyl acetate-hexane ternary azeotrope are added to the original feed F0 to produce a net-feed composition F1 for column C1 designed to lie on a line between pure methanol and the curved part of the boundary between Regions I and II. C1 is operated in the indirect mode producing the high-boiling node methanol as a bottoms product, and by mass balance, a distillate near the curved boundary. The distillate, although in Region I, becomes feed F2 to column C2 which is operated in the direct mode entirely in Region II, producing the low-boiling node ternary azeotrope as distillate and by mass balance, a methanolmethyl acetate mixture as bottoms. This bottoms mixture is on the opposite side of the methanol-methyl acetate azeotrope at the original feed F0. The bottoms from C2 is finally fed to binary distillation column C3, which produces pure methyl acetate as bottoms product and the methanol-methyl acetate azeotrope as distillate. The distillates from C2 and C3 are recycled to C1. The distillate and bottoms compositions for C2 lie on the same residue curve, and the composition profile lies entirely within Region II, even though its feed composition is in Region I.
Exploitation of boundary curvature for breaking azeotropes is very similar to exploiting pressure sensitivity from a mass-balance point of view, and suffers from the same disadvantages. Separation schemes have large recycle flows, and in the case of minimum-boiling azeotropes, the recycle streams are distillates. However, in the case of maximum-boiling azeotropes, these recycles are underflows and
the economics are improved. One such application, illustrated in Fig. 13-68, is the separation of the nitric acid-water azeotrope by adding sulfuric acid. Recycled sulfuric acid is added to a nitric acid-water mixture near the azeotropic composition to produce a net feed in Region I. The first column, operated in the direct mode, produces a nitric-acid distillate and a bottoms product, by mass balance, near the distillation boundary. In this case, sulfuric acid associates with water so strongly and the separatrix is so curved and nearly tangent to the water-sulfuric acid edge of the composition diagram that the second column operating in the indirect mode in Region II, producing sulfu-ric acid as bottoms product also produces a distillate close enough to the water specification that a third column is not required (Thiemann et al., in Ullmann's Encycloped(a of Industrial Chem(stry, Fifth Edition, Volume A17, VCH Verlagsgesellschaft mbH, Weinheim, 1991).
Exploitation of Azeotropy and Liquid-Phase Immiscibility One powerful and versatile separation approach exploits several physical phenomena simultaneously including enhanced vapor-liquid behavior, where possible, and liquid-liquid behavior to bypass difficult distillation separations. For example, the overall separation of close-boiling mixtures can be made easier by the addition of an entrainer that forms a heterogeneous minimum-boiling azeotrope with one (generally the lower-boiling) of the key components. Two-liquid-phase formation provides a means of breaking this azeotrope, thus simplifying the entrainer recovery and recycle process. Moreover,
since liquid-liquid tie lines are unaffected by distillation boundaries (and the separate liquid phases are often located in different distillation regions), liquid-liquid phase splitting is a powerful mechanism for crossing distillation boundaries. The phase separator is usually a simple decanter, but sometimes a multistage extractor is substituted. The decanter or extractor can also be replaced by some other non-VLE-based separation technique such as membrane permeation, chroma-tography, adsorption, or crystallization. In addition, sequences may include additional separation operations (distillations or other methods) for preconcentration of the feed mixture, entrainer recovery, and final-product purification.
The simplest case of combining VLE and LLE is the separation of a binary heterogeneous azeotropic mixture. One example is the dehydration of 1-butanol, a self-entraining system, in which butanol (117.7°C) and water form a minimum-boiling heterogeneous azeo-trope (93.0°C). As shown in Fig. 13-69, the fresh feed may be added to either column C1 or C2, depending on whether the feed is on the organic-rich side or the water-rich side of the azeotrope. The feed may also be added into the decanter directly if it doesn't move the overall composition of the decanter outside of the two-liquid-phase region. Column C1 produces anhydrous butanol as a bottoms product and a composition close to the butanol-water azeotrope as the distillate. After condensation, the azeotrope rapidly phase separates in the decanter. The upper layer, consisting of 78 wt % butanol, is refluxed totally to C1 for further butanol recovery. The water layer, consisting of 92 wt % water, is fed to C2. This column produces pure water as a bottoms product and, again, a composition close to the azeotrope as distillate for recycle to the decanter. Sparged steam may be used in C2, saving the cost of a reboiler. A similar flowsheet can be used for dehydration of hydrocarbons and other species that are largely immiscible with water.
A second example of the use of liquid-liquid immiscibilities in an azeotropic-distillation sequence is the separation of the ethanol-water minimum-boiling azeotrope. For this separation, a number of entrainers have been proposed, which are usually chosen to be immiscible with water, form a ternary minimum-boiling (preferably heterogeneous) azeotrope with ethanol and water (and, therefore, usually also binary minimum-boiling azeotropes with both ethanol and water). All such systems correspond to DRD 058, although the labeling of the vertices depends on whether the entrainer is lower boiling than ethanol, intermediate boiling, or higher boiling than water. The residue curve map for the case for cyclohexane as entrainer was illustrated in Fig. 13-58c. One three-column distillation sequence is shown in Fig. 13-70. Other two-, three-, or four-column sequences have been described by Knapp and Doherty (Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Vol. 8, Wiley, New York, 1993).
Fresh aqueous ethanol feed is first preconcentrated to nearly the azeotropic composition in column C3, while producing a water bottoms product. The distillate from C3 is sent to column C1, which is refluxed with the entire organic (entrainer-rich) layer, recycled from a decanter. Mixing of these two streams is the key to this sequence as it allows the overall feed composition to cross the distillation boundary into Region II. column C1 is operated to recover pure high-boiling node ethanol as a bottoms product and to produce a distillate close to the ternary azeotrope. If the ternary azeotrope is heterogeneous (as it
is in this case), it is sent to the decanter for phase separation. If the ternary azeotrope is homogeneous (as it is in the alternative case of ethyl acetate as the entrainer) the distillate is first mixed with water before being sent to the decanter. The inventory of entrainer is adjusted to allow C1 to operate essentially between two nodes, although such practice, as discussed previously, is relatively susceptible to instabilities from minor feed or reflux perturbations. Refluxing a fraction of the water-rich decanter layer results in an additional degree of freedom to mitigate against variability in the feed composition. The remaining portion of the water layer from the decanter is stripped of residual cyclohexane in column C2, which may be operated either in the direct mode (producing low-boiling node ternary azeotrope as distillate and, by mass balance, an ethanol-water bottoms for recycle to C3 or, in the indirect mode (producing high-boiling node water as bottoms and, by mass balance, a ternary distillate near the distillation boundary. The distillate may be recycled to the decanter, the top of C2, or C2 feed.
Design and Operation of Azeotropic Distillation Columns Simulation and design of azeotropic distillation columns is a difficult computational problem, but one that is readily handled, in most cases, by widely available commercial computer process simulation packages [Glasscock and Hale, Chem. Eng., 101(11), 82 (1994)]. Most simula tors are capable of modeling the steady state and dynamic behavior of both homogeneous azeotropic distillation systems and those systems involving two-liquid phase behavior within the column, if accurate thermodynamic data and activity-coefficient or equation-of-state models are available. However, VLE and VLLE estimated or extrapolated from binary data or predicted from such methods as UNIFAC may not be able to accurately locate boundaries and predict the extent of liquid immiscibilities. Moreover, different activity-coefficient models fit to the same experimental data often give very different results for the shape of boundaries and liquid-liquid regions. Therefore the design of separation schemes relying on boundary curvature should not be attempted unless accurate, reliable experimental equilibrium data are available.
Two liquid phases can occur within a column in the distillation of heterogeneous systems. Older references, for example Robinson and Gilliland (Elements of Fractional Distillation, McGraw-Hill, New York, 1950) state that the presence of two liquid phases in a column should be avoided as much as possible because performance may be reduced. However, more recent studies indicate that problems with two-phase flow have been overstated [Herron 1267 (1988) and Harrison, Chem. Eng. Prog., 86(11), 80 (1990)]. Based on case-history data and experimental evidence, there is no reason to expect unusual capacity or pressure-drop limitations, and standard correlations for these parameters should give acceptable results. Because of the violent nature of the gas/liquid/liquid mixing on trays, trayed column efficiencies are relatively unaffected by liquid-liquid phase behavior. The falling-film nature of gas/liquid/liquid contact in packing, however, makes that situation more uncertain. Reduced efficiencies may be expected in systems where one of the keys distributes between the phases.
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