Composition profiles column distillation

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

Minimum Boiling Azeotropes. AH extractive distillations of binary minimum boiling azeotropic mixtures are represented by the residue curve map and column sequence shown in Figure 6b. Typical tray-by-tray composition profiles are shown in Figure 7. 

Exploitation of Boundary Curvature A second approach to boundaiy crossing exploits boundaiy 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 hne), the feed must be located in a region where the boundary is concave. 

Extractive Distillation Design and Optimization Extractive distillation column composition profiles have a veiy characteristic... 

Why not put new lyrics to an old tune This is an excellent idea, and many have done this very thing. Rice" started w ith the Smith-Brinkley raethod" used to calculate distillation, absorption, extraction, etc., overhead and bottoms compositions, and developed distillation equations for determining the liquid composition on any tray. This together with bubble point calculations yield a column temperature profile useful for column analysis. 

Figure 11.45 shows a composition profile for the azeotropic distillation column in the process shown in Figure 11.44. This is taken from a solution presented by Robinson and Gilliland...

To a first approximation, the composition of the distillate and bottoms of a single-feed continuous distillation column lies on the same residue curve. Therefore, for systems having separatrices and multiple regions, distillation composition profiles are also constrained to lie in specific regions. The precise boundaries of these distillation regions are a function of reflux ratio, but they are closely approximated by the RCM separatrices. If a separatrix exists in a system, a corresponding distillation boundary also exists. Also, mass balance constraints require that the distillate composition, the bottoms composition, and the net feed composition plotted on an RCM for any feasible distillation be collinear and spaced in relation to distillate and bottoms flows according to the well-known lever rule. 

Figure 13.29. Composition profiles and flowsketches of two azeotropic distillation processes (adapted by King, 1980). (a) Separation of ethanol and water with benzene as entrainer. Data of the composition profiles in the first column were calculated by Robinson and Gilliland, (1950) the flowsketch is after Zdonik and Woodfield (in Chemical Engineers Handbook, McGraw-Hill, New York, 1950, p. 652). (b) Separation of n-heptane and toluene with methylethylketone entrainer which is introduced in this case at two points in the column (data calculated by Smith, 1963).

Figure 27 Liquid-phase composition profiles along the 50-mm-diameter catalytic distillation column for methyl acetate synthesis at reflux ratio of 2.0 and different reboiler duties (a) 295W (b) 873W (c) 1161W.

Fig. 4.12. Design diagrams for isopropyl acetate (IPOAc) reactive distillation column and comparison with simulation results (solid curves simulated column profile markers = stage composition for column rectifying section + = stage composition for column stripping section).

The corresponding wave patterns of the transformed concentration variable X for a reactive distillation column are shown in Fig. 5.8. Here, a single feed with pure reactant A is introduced in the middle of the column. As in the nonreactive binary case, the composition profiles consist of a single front in each column section,... 

The distillate accumulator, plate 1 (top) and the reboiler liquid composition (for benzene) profiles for case 3 are presented in Figures 3.20-3.22. Figure 3.23 presents the plate 1 liquid composition profile for case 4. Using these composition profiles Mujtaba and Macchietto (1998) made the following observations for better understanding of the role of column holdup. 

Seader and Henley (1998) considered the separation of a ternary mixture in a batch distillation column with B0 = 100 moles, xB0 = = <0.33, 0.33, 0.34> molefraction, relative volatility a= <2.0, 1.5, 1.0>, theoretical plates N = 3, reflux ratio R = 10 and vapour boilup ratio V = 110 kmol/hr. The column operation was simulated using the short-cut model of Sundaram and Evans (1993a). The results in terms of reboiler holdup (Bj), reboiler composition profile (xBI), accumulated distillate composition profile (xa), minimum number of plates (Nmin) and minimum... 

Figure 10.1 shows typical distillate composition profiles for close boiling mixtures (binary) and implications of using CBD column for such mixtures. 

Figure 10.2 shows typical distillate composition profiles for close boiling mixtures using a solvent in a CBD column. The CBD process becomes a conventional BED process with the addition of the solvent. Due to the addition of solvent, the components can be separated at high purity using a small column with low reflux ratio. See Safrit and Westerberg (1997) and Low and Sorensen (2002) for unconventional BED processes. 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

The results shown in Fig. 6.10 were generated for a 20-tray, 2-ft-diame-ter column, with a feed on tray 11 of 100 lb-mol/h at 90°F. The feed composition is 5 mol % propane, 40 mol % normal butane, 45 mol % normal pentane, and 10 mol % normal octane. Product purity specifications are 0.5 mol % butane in the bottoms and 0.5 mol pentane in the distillate. The operating pressure of the column is 73 psia, and the reflux ratio is 1.76. The temperature on tray 6 is 179.9CF and on tray 14 is 130.6°F. Figure 6.13 gives the composition profiles in the column. Note the buildup of the HHK component near the bottom of the column... 

Most distillation columns have two control degrees of freedom, once pressure and feed conditions are set. The typical control structure holds the composition profile in the column by controlling a tray temperature somewhere in the column. The other degree of freedom is then normally consumed by fixing some other variable such as the flowrate of reflux, the reflux ratio, or the heat input. 

The temperature and liquid phase composition profiles for this final case are shown in Fig. 13-42. The temperature increases from top to bottom of the column. This is normally the case in distillation columns (exceptions may occur with cold feeds or feeds with boiling points significantly lower than that of the mixture on stages above the feed stage). The composition profiles also are as expected. The components more volatile than the light key (fvbutane) are... 

FIG. 13-94 Extractive distillation column composition profile for the separation of acetone-methanol with water. 

Composition profiles for the same extractive distillation column are shown in Figure 6. The water concentration in the liquid goes through a pronounced maximum at about tray number four (see Table VI), corresponding to the minimum in the relative volatility of water with respect to ethanol. In this region the ethanol concentration in the vapor increases rapidly, changing from less than 2% at tray number three to more than 50% at tray number six. This rapid increase continues to about tray number ten where the concentration of ethanol in the vapor is about... 

Residue curve maps are being employed in an industrially useful method for the synthesis of separation schemes for azeotropic systems. To a first approximation, the compositions of the overhead and underflow of a single-feed continuous distillation column lie on the same residue curve therefore, it is not possible for the composition profile of a continuous distillation column to cross into different regions. The boundary that a distillation composition profile can never cross is called a distillation boundary. The precise location of distillation boundaries is a function of reflux ratio, but distillation boundaries closely approximate the RCM separatrices. Distillation boundaries connect azeotropic and pure component compositions just as separatrices do. If an RCM separatrix exists, a corre.sponding distillation boundary will also exist. 

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