Multicomponent boiling

J. G. Collier, Multicomponent Boiling and Condensation, in Two-Phase Flow and Heat Transfer in the Power and Process Industries, pp. 520-557, Hemisphere, Washington, DC, and McGraw-Hill, New York, 1981. 

Extensive reviews of multicomponent boiling are given by Shock [126] and Collier and Tliome [3]. 

The implication is that correlations similar to those used for pool boiling can be used for multicomponent boiling at low-quality or subcooled conditions. For the annular flow region, predictions based on the models described in the preceding section should give reasonable results based on mean physical properties of the mixture. 

Figure 5.11 Information-Flow Diagram for Multicomponent Boiling.

In considering the effect of mass transfer on the boiling of a multicomponent mixture, both the boiling mechanism and the driving force for transport must be examined (17—20). Moreover, the process is strongly influenced by the effects of convective flow on the boundary layer. In Reference 20 both effects have been taken into consideration to obtain a general correlation based on mechanistic reasoning that fits all available data within 15%. 

The number of columns ia a multicomponent train can be reduced from the N — 1 relationship if side-stream draw-offs are used for some of the component cuts. The feasibiUty of multicomponent separation by such draw-offs depends on side-stream purity requirements, feed compositions, and equihbrium relationships. In most cases, side-stream draw-off distillations are economically feasible only if component specifications for the side-stream are not tight. If a single component is to be recovered ia an essentially pure state from a mixture containing both lower and higher boiling components, a... 

Residue Curve Maps. Residue curve maps are useful for representing the infinite reflux behavior of continuous distillation columns and for getting quick estimates of the feasibiHty of carrying out a desired separation. In a heterogeneous simple distillation process, a multicomponent partially miscible Hquid mixture is vaporized ia a stiH and the vapor that is boiled off is treated as being ia phase equiHbrium with all the coexistiag Hquid phases. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Product separation for main fractionators is also often called black oil separation. Main fractionators are typically used for such operations as preflash separation, atmospheric crude, gas oil crude, vacuum preflash crude, vacuum crude, visbreaking, coking, and fluid catalytic cracking. In all these services the object is to recover clean, boiling range components from a black multicomponent mixture. But main fractionators are also used in hydrocracker downstream processing. This operation has a clean feed. Nevertheless, whenever you hear the term black oil, understand that what is really meant is main fractionator processing. 

Torres-Marchal [110] and [111] present a detailed graphical solution for multicomponent ternary systems that can be useful to establish the important parameters prior to undertaking a more rigorous solution with a computer program. This technique can be used for azeotropic mixtures, close-boiling mixtures and similar situations. 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

This example is based on the model description of Sec. 3.3.4, and involves a multicomponent, semi-batch system, with both heating and boiling periods. The compositions and boiling point temperatures will change with time. The water phase will accumulate in the boiler. The system simulated is based on a mixture of n-octane and n-decane, which for simplicity will be assumed to be ideal but which has been simulated using detailed activity coefficient relations by Prenosil (1976). 

When the condensation process is not exactly isothermal but the temperature change is small such as where there is a significant change in pressure, or where a narrow boiling range multicomponent mixture is being condensed the logarithmic temperature difference can still be used but the temperature correction factor will be needed for multipass condensers. The appropriate terminal temperatures should be used in the calculation. 

For multicomponent systems with boiling range greater than 80°C, a single adiabatic flash calculation to 80 to 90 percent of the inlet pressure P0 yields the two-phase specific volume oI at pressure P1 and co is calculated from (Nazario and Leung, Sizing Pressure Relief Valves in Flashing and Two-Phase Service An Alternative Procedure, J. Loss Prev. Process lnd. 5(5), pp. 263-269, 1992)... 

As CH-acids in the MCRs with aldehydes and aminoazoles, other classes of organic compounds were used as well. Cyanoacetic acid derivatives, acetoyl(aroyl) acetonitriles, ketosulfones, acetophenones, and other reagents were successfully introduced into these three-component heterocyclizations. For example, synthesis of pyrazolo[3,4-b]pyridine-5-carbonitriles 40 was carried out as the multicomponent treatment of 5-aminopyrazole, aldehyde, and benzoylacetonitriles solvent-free by fusion either in ammonium acetate at 120°C or in boiling ethanol with EtsN (Scheme 17) [69]. The second approach gave the worst results from the viewpoint of yields and purity of the target compounds. 

Significant contribution in the studying of the MCRs based on aminoazoles, aldehydes, and Meldrum s acid was made by Lipson and co-authors in their publications [ 114—119]. It was established that in some cases these multicomponent treatments can yield positional isomers. For example, refluxing of 3-amino-l,2,4-triazole with aldehydes and Meldrum s acid gave only triazolopyrimidinones 82 [114] (Scheme 36). On the other hand, MCRs involving 3-amino-5-methylthio-1,2,4-triazoles in boiling DMF yielded solely 5-pyrimidinones 83, while the... 

Because of the limitations of the Margules equation—especially in predicting multicomponent VLE data—the Wilson, NRTL, and LEMF (16) equations are employed in this study. The experimental data on the systems presented in Table I were used in this work. These are the only systems for which both binary and ternary data could be found in the literature. As a matter of fact, uncertainties do exist about the accuracy of the two HgC systems. The maximum boiling... 

In view of the complicated reaction kinetics of multicomponent systems, it was not clear whether or not the diffusional effects would also affect the relative rate of conversion of feed molecules in a mixture. To answer this question we studied the hydrocracking of three multicomponent systems. The first was a C5-C8 mixture, a C5 360° C boiling range midcontinent reformate which contained 12.5 wt % n-paraffins including 4.2% n-pentane, 4.3% n-hexane, 2.9% n-heptane, l.l%n-octane, and <1% C9+ n-paraffins, with the remainder isoparaffins and aromatics. The reaction was carried out at 400 psig, 2 H2/HC, 2 LHSV, and 800°F. Secondly, a Cg-Cie mixture... 

The maximum boiling point is that temperature corresponding to a definite composition of a Iwo-coinponenl or multicomponent system al which the boiling point of the system is a maximum. At this temperature the liquid and vapor have the same composition and the solution distills completely without change in temperature. Binary liquid systems that show negative deviations from Raoult s law have maximum boiling points. See Raoult s I xiw and Van t Hoff I,aw. 

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