Stages in distillation

Figure 2.2 Idea] and nonideal stages in distillation systems, (a), (f>) Thermosiphon reboiler arrangements (c). id) fractionation tray arrangements (ej, (j) inter-condenaor arrangements. Arrangements (6), (d), and (/) are ideal stages. Arrangements (a), (c), and (e) an nonideal stages.

The procedure is similar to the McCabe-Thiele construction for determining the number of stages in distillation Section 11.5.2. The slope of the operating line is the ratio of the final raffinate to fresh solvent flow-rates. 

The minimum number of stages in distillation is at total reflux when L= V, the composition of the passing streams are equal, and the heat addition per pound of distillate is infinite. For the simple configuration in Fig. 11.3a, because there is no... 

In all the previous discussions on theoretical trays or stages in distillation, we assumed that the vapor leaving a tray was in equilibrium with the liquid leaving. However, if the time of contact and the degree of mixing on the tray is insufficient, the streams will not be in equilibrium. As a result the efficiency of the stage or tray is not 100%. This means that we must use more actual trays for a given separation than the theoretical number of trays determined by calculation. The discussions in this section apply to both absorption and distillation tray towers. 

The double line in each cell denotes the membrane proper and corresponds to a plate or stage in distillation, the latter as represented by a single horizontal line in Figure 4.1. It may be observed as a distinguishing feature that the phase designated L is introduced immediately below the membrane, on the high-pressure or reject side of the membrane and, in this case, is a gaseous phase, as is phase V, whereas in distillation practice, the liquid phase L is introduced at or onto the plate. 

As in distillation, sidestreams can be introduced or sidestream products can be withdrawn at different points or stages. In distillation practice at least, it is preferable that the sidestreams introduced be approximately of the same composition as at the point or stage introduced. Again by analogy, the membrane section above the feed location may be referred to as the rectifying section, and the section below as the stripping section or vice versa. The former is the accepted convention in distillation practice, where the more-volatile component(s) move toward the top of the column, the less-volatile toward the bottom. 

The concept of the super-fractionation separation stage in distillation is a key concept in process ojjerations and chemical engineering. Yet I have never seen this idea discussed in any... 

A stage in distillation column may be defined as a unit of equipment in which two dissimilar phases are brought into intimate contact with each other and then are separated. The contact allows for the diffusing components of the feed mixture to redistribute themselves between the phases. It is said that the resultant two phases have approached equilibrium. 

FIGURE 8.4 Data from Table 8.1 plotted as an y-x plot for the more volatile species (which is in this case is acetone, which boils at 56.15°C, compared to 100.0°C for water). The data points from the table are shown, and a simple interpolation curve is added. The 45° line represents the condition ya Xa- This line is customarily shown because the equilibrium curve and this line interact in the McCabe-Thiele method of calculation of equilibrium stages in distillation. The plot is aU for 1 atm pressure (as are most such plots). The temperatures from Table 8.1 are not shown. 

Figure 8.5 Vapor vs. liquid mole fractions (xy) phase diagram for methanol (a)-water (b) binary mixtures at a constant pressure of 1 atm. (a) xy diagram with 45° line indicated (b) stages in distillation at total reflux (c) stages in a five tray distillation column with operating lines shown. Stages in distillation are illustrated on the right of (b) and (c).

Other definitions of HTU and NTU can be based on other forms of the overall mass transfer coefficients. The use of transfer units is a rough parallel with the use of stages in distillation or the term theoretical plates in chromatography. As such, it is a historical genuflection by the more recent absorption analyses in the direction of the older equilibrium stage separation analysis. 

As with distillation, no attempt should be made to carry out any optimization of liquid flow rate, temperature, or pressure at this stage in the design. The separation in absorption is sometimes enhanced by adding a component to the liquid which reacts with the solute. 

In the next stage in the recovery and refining of tar acids, water and pitch ate removed from the cmde tar acids in a continuous-vacuum still heated by superheated steam or circulating hot oil. The aqueous phenol overhead distillate is recycled, the stream of once-mn tar acids is refined, and the phenoHc pitch bottoms are burned. 

Fractional vacuum distillation is the method used to separate terpene mixtures into their components. The terpene chemist usually has in the laboratory a range of columns with differing numbers of theoretical stages. Experimental distillation in the laboratory is useful in providing data for manufacturing plants that produce commercial quantities of terpene products. 

Basic distillation involves appHcation of heat to a Hquid mixture, vapori2ation of part of the mixture, and removal of the heat from the vapori2ed portion. The resultant condensed Hquid, the distillate, is richer in the more volatile components and the residual unvapori2ed bottoms are richer in the less volatile components. Most commercial distillations involve some form of multiple staging in order to obtain a greater enrichment than is possible by a single vapori2ation and condensation. 

The results of the analyses for all the various elements commonly encountered in distillation processes are summarized in Table 13-5. Details of the analyses are given by Smith (Design of Equilibrium Stage Processes, McGraw-Hul, New York, 1967) and in a somewhat different form by Henley and Seader (op. cit.). 

A knowledge of the reflux ratio (obtained from the specified distillate and top vapor rates) permits the calculation of (f d) from which ( i/d) is obtained, etc. Equation (13-50) is applied to each stage in succession until the ratio 2/d in the overflow from the stage above the feed stage is obtained. The calculations are then switched to the stripping section. 

Use of HETP Data for Absorber Design Distillation design methods (see Sec. 13) normally involve determination of the number of theoretical equihbrium stages or plates N. Thus, when packed towers are employed in distillation appRcations, it is common practice to rate the efficiency of tower packings in terms of the height of packing equivalent to one theoretical plate (HETP). 

When chemical equilibrium is achieved qiiickly throughout the liquid phase (or can be assumed to exist), the problem becomes one of properly defining the physical and chemical equilibria for the system. It sometimes is possible to design a plate-type absorber by assuming chemical-equilibrium relationships in conjunction with a stage efficiency factor as is done in distillation calculations. Rivas and Prausnitz [Am. Tn.st. Chem. Eng. J., 25, 975 (1979)] have presented an excellent discussion and example of the correct procedures to be followed for systems involving chemical equihbria. 

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