Wide boiling mixture

Calculations for wide-boiling mixtures are a little more difficult to converge, especially for mixtures having very light or noncondensable components together with relatively nonvolatile components and lacking components of intermediate volatility. 

Figure 12.11 superimposes distillation lines and residue curves for the same ternary systems. Figure 12.11a shows the system n-pentane, n-hexane and n -heptane, which is a relatively wide boiling mixture. It can be observed in Figure 12.11a that there are significant differences between the paths of the distillation lines and the residue curves. By... 

This equation applies to vaporization of single components, but can be used for close boiling mixtures without too much error. Coefficients for wide boiling mixtures will be overestimated. 

Olex A version of the Sorbex process for separating olefins from paraffins in wide-boiling mixtures. It can be used for hydrocarbons in the range C6 - C20. Based on the selective adsorption of olefins in a zeolite and their subsequent recovery by displacement with a liquid at a different boiling point. Mainly used for extracting Cn - C14 olefins from the Pacol... 

The Lewis-Matheson method does not work well for wide-boiling mixtures because of the required component distribution. There is also the difficulty of deciding what is a good match at the feed stage. 

Wide-boiling mixtures and energy-coupled systems where the profiles may shift drastically from linear. 

The best-known presentations are by Tomich (32), Holland (8), and Orbach et al. (33). These vary in their choice of Newton-Raphson equations and independent variables and each may solve a different range of columns, These methods have been shown to work well for wide-boiling mixtures including refinery fractionators, absorber-stripper columns, and reboiled absorbers. 

The examples tested by Taylor et al. (80) for the efficiency homotopy were for moderate- or narrow-boiling mixtures. No wide-boiling mixtures were tested. Since the temperature profiles at the intermediate values of E yy will be flat and not broad, the homotopy may be best for the moderate- and narrow-boiling systems. Most of the mixtures were nonideal and the efficiency homotopy should lessen the effect of nonideal If-values where E yy acts as a damper on the if-values. The efficiency homotopy does not work for purity specifications because the purity will not be satisfied in solutions of early values of E yy-Vickery and Taylor (81) presented a thermodynamic homotopy where ideal If-values and enthalpies were used for the initial solution of the global Newton method and then slowly converted to the actual If-values and enthalpies using the homotopy parameter t, The homotopy functions were embedded in the If-value and enthalpy routines, freeing from having to modify the MESH equations. The If-values and enthalpies used are the homotopy functions ... 

In conclusion, the temperature profile in conventional distillation columns is the result of both phase equilibrium relations and enthalpy balances. In narrow-boiling mixtures, the phase equilibrium effect is generally more pronounced, while in wide-boiling mixtures, the enthalpy balances are more significant. The importance of the distinction between the two effects is twofold. First, different mathematical solution algorithms are better suited for each situation, as will be discussed in Chapter 13. Second, the understanding and prediction of column performance is enhanced when the two effects are recognized. Examples 7.1 and 7.2 illustrate the two cases. 

The feed stream is a wide-boiling mixture with the following composition ... 

With a single equilibrium stage and no reflux, the separation power in differential distillation is obviously limited. It is the equivalent of a batch flash operation. Consequently, practical applications would include the separation of wide-boiling mixtures, with low expectations on the purity of the products. 

The presence of an azeotrope is one indication that a mixture is not ideal, diet it has deviations from RaouiCs Jaw (ene Chapter 1 and Section 5.2 of the present chapter). Close-boiling mixtures are more likely to exhibit azeohopism than wide-boiling mixtures whan there is more than 30°C boiling point difference, it is quite unlikely that an azeotrope will be present. Thus, the combination of close-boiling and nonideality is oris dial can land to lha presence of an azeotrope. 

Figure 7.8. Algorithm for adiabatic flash calculation for wide-boiling mixtures.

The algorithm of Fig. 7.8 is very successful when (7-11) is not sensitive to Tv. This is the case for wide-boiling mixtures such as those in Example 7.2. For close-boiling mixtures (e.g., isomers), the algorithm may fail because (7-11) may become extremely sensitive to the value of Ty. In this case, it is preferable to select (fi as the tear variable and solve (7-11) iteratively for Ty. 

This is the well-known Rayleigh equation, as first applied to the separation of wide-boiling mixtures such as HCI-H2O, H2S0 -H20, and NHJ-H2O. Without reflux, yp and Xw are in equilibrium and (9-2) can be written as ... 

The main disadvantage of this technique is that in many cases (particularly with wide-boiling mixtures), its effectiveness in reducing relief discharge rate may be limited. This technique can also be expensive when column pressure exceeds 100 to 150 psig. 

Figure 15.3 illustrates the effect of liquid level in the reboiler sump on vertical thermosiphon reboiler performance. The diagram describes typical trends based on some published commercial-scale data (114, 181, 357, 372). The analysis assumes a narrow boiling range bottom product (e.g., a pure product) extension to wide-boiling mixtures is addressed following the analysis. 

While the above analysis applies to a narrow boiling range bottom product, it can readily be extended to wide-boiling mixtures. With a narrow-boiling mixture, reboiler AT is practically independent of the fraction vaporized. For this reason, the top diagram in Fig. 15.3 depicts the effect of level on both the heat transfer coefficient and the heat transfer rate. With wide-boiling mixtures, the top diagram shows only the variation in the heat transfer coefficient with level. The heat... 

Reboiler sump liquid level is best maintained at the top of the tube bundle for atmospheric and superatmospheric systems (68, 124, 150a, 181, 253, 254, 356-358). For these systems, a level lower than 40 percent of the tube length should be avoided (357,358). For vacuum columns, the level is typically about 50 percent to 70 percent of the tube length (150a, 181). It will tend to be higher for wide-boiling mixtures and lower for pure bottom products. 

In wide-boiling mixtures, this problem can be recognized by a... 

In wide-boiling mixtures, insufficient circulation can be recognized by a high temperature difference between rehoiler outlet and reboiler inlet on the process side. Alternatively, if heat transfer coefficient improves when reboiler sump level is raised or when the reboiler inlet valve is opened, insufficient circulation is implied. Very low heat transfer coefficients, excessive fouling, and low pressure drop across some sections may also suggest insufficient circulation (360). 

In wide-boiling mixtures, temperature points located at different elevations were successfully used for monitoring liquid level and checking the action of the level controller (99). Since the vapor temperatures are lower than the liquid, this technique can indicate the vapor-liquid boundary (99). This technique, however, can suffer from ge3Tsering of liquid fi t>m the bundle and may not be always successful. 

Distribution. With tray towers, liquid distribution to the reboiler is uneven. With wide-boiling mixtures, this may prompt composition pinches. Further, light components can be depleted near the inlet or the top of the bundle, causing temperature pinches. In most cases, however, distribution is not a major problem because the bundle is small and internal reboiler AT is large. 

The feed enthalpy is normally inferred from a temperature measurement of the feed leaving the preheater, and preheat is manipulated to control this temperature. This is satisfactory when the feed is a single-phase fluid, and often also with partially vaporized wide-boiling mixtures at superatmospheric pressures, but not with partially vaporized narrow-boiling mixtures. In the latter case, fractional... 

The system nitrogen/oxygen is a nearly ideal and very wide boiling mixture (relative volatility a 4) that can, in principle, very easily be fractionated in a single distillation colunm The industrial process, however, uses two distillation colurrms, see Fig. 11.1-7. 

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