Binary distillation McCabe-Thiele method

The McCabe-Thiele Method is restricted in its application because it only applies to binary systems and involves the simplifying assumption of constant molar overflow. However, it is an important method to understand as it gives important conceptual insights into distillation that cannot be obtained in any other way. 

EXAMPLE 17.2 BATCH DISTILLATION OF A BINARY SYSTEM BASED ON THE MCCABE-THIELE METHOD... 

The separation of a binary mixture by distillation may be represented in two-dimensional space while n-dimensional space is required to represent the separation of a multicomponent mixture (i > 2). The graphical method proposed by McCabe and Thiele9 for the solution of problems involving binary mixtures is presented in a subsequent section. The McCabe-Thiele method makes use of an equilibrium curve which may be obtained from the boiling-point diagram."... 

Calculate, by modified McCabe-Thiele methods, residue composition and distillation time for binary batch rectification with constant reflux for a given number of ideal stages, boil-up rate, and specified average distillate composition. 

McCabe-Thiele Method - This MathCAD document creates a McCabe-Thiele Diagram for a binary distillation. Data are from Example 6-5. 

McCabe-Thiele diagrams for adsorption, 320 for binary distillation, 326, 329, 330, 332, 334 for countercurrent multistage separator, 309 for extraction, 317 for ion exchange, 322 for rectifying column, 314 McCabe-Thiele method for distillation, 310-315, 322-340... 

We now have all the material available for the graphical calculation of distillation conditions by the McCabe-Thiele method. This is one of the most used and simplest methods for the calculation of batch and continuous distillations of binary mixtures. It involves the simplifying assumptions that the molar heats of evaporation of the components and their mixtures are identical, and there are no heat losses from the column the consequence ist that the vapour and liquid flow rates, in moles per unit time, are constant throughout any section of the column, provided there is no addition or withdrawal of material. 

Introduction. In Section 11.4B the McCabe-Thiele method was used to calculate the number of theoretical steps or trays needed for a given separation of a binary mixture of A and B by rectification or fractional distillation. The main assumptions in the method are that the latent heats are equal, sensible heat differences are negligible, and constant molal overflow occurs in each section of the distillation tower. In this section we shall consider fractional distillation using enthalpy-concentration data where the molal overflow rates are not necessarily constant. The analysis will be made using enthalpy as well as material balances. 

What is often called the McCabe-Thiele method " for binary distillation calculations deploys a y-x (or y-x) diagram, say, for the more-volatile component, here designated the more-permeable component i. This furnishes substantiation that a separation can indeed be attained by the use of recycle or reflux in a multistage or cascade operation. 

The use of a y-x curve in the McCabe-Thiele method for binary distillation calculations brings up the matter of a flash-vaporization representation, in case the feedstream mixture is at saturation. An inspection of the y-x curve relative to a given feed composition shows that the equilibrium mixture varies along the curve over a range from the bubble point (where the liquid phase composition x is equal to that of the feed mixture x ) to the dew point (where the vapor composition y is equal to that of the feed mixture Xj.). Between the two is the region of flash vaporization, where the equilibrium compositions (y, x) respectively of phases V and L must satisfy the flash material balance relation F = V + L, where... 

Of significance is that multistage membrane separations for binary systems can be treated graphically, in a fashion similar to the classical McCabe-Thiele method for binary distillation. This is developed and illustrated in Chapter 4 and affords a convenient means for evaluating separation possibilities, in determining the effect of permeability, reflux... 

The concept will be introduced in the context of the McCabe-Thiele construction for the graphical analysis of binary distillation problems. This method uses y-x diagrams of the form shown in figure 6.2. Such diagrams are a graphical representation of equilibrium data, and so if the composition of the resultant vapour from an equilibrium stage is known, then the composition of the resultant liquid (in equilibrium with that vapour) can be obtained from the curve on the appropriate y-x diagram. 

FIGURE 4.14 McCabe-Thiele method for binary distillations. 

The operation of a binary distillation has two important limiting cases minimum and maximum stages for a given separation. Minimum stages occur when the column is operated with total reflux (i.e., no top product is withdrawn). This situation, when applied to the McCabe-Thiele method, uses the y = x line as the mass balance for both rectifying and stripping. The situation is depicted in Figure 12-11. 

Azeotropes in Ternary Systems. In binary systems, the McCabe-Thiele method provides a conceptual representation of the distillation process. In ternary systems, there is a method that provides a similar conceptual representation. It is called the boundary value design method (BVDM), and it is particularly useful for conceptualizing azeotropic distillation in ternary systems. This method is introduced here however, the reader seeking a more in-depth treatment of this method and all aspects of azeotropic distillation should consult the definitive reference in the field [IZ]. 

The elegance and simplicity of the McCabe-Thiele method make it tempting for the beginning student to view it as the centerpiece of binary distillation column design. There are, in fact, a number of key questions that need to be addressed before proceeding with plate calculations, setting reflux rations, and the like. 

This Excel spreadsheet uses the McCabe-Thiele method to calculate the number of theoretical stages needed for binary distillation. Binary distillation is a common unit of operation that separates two liquids (with one being more volatile... 

The graphics capabiUties of the CAD/CAM environment offer a number of opportunities for data manipulation, pattern recognition, and image creation. The direct appHcation of computer graphics to the automation of graphic solution techniques, such as a McCabe-Thiele binary distillation method, or to the preparation of data plots are obvious examples. Graphic simulation has been appHed to the optimisation of chemical process systems as a technique for energy analysis (84). 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

Calculation Methods. An often satisfactory approximation is to take the mixture in the presence of the solvent to be a pseudobinary of the keys on a solvent-free basis, and to employ the McCabe-Thiele or other binary distillation method to find tray and reflux demands. Since the relative volatility varies with concentration of the solvent, different equilibrium curves are used for above and below the feed based on average loads in those zones. Figure 13.25 is of such a construction. 

Understand the fundamentals of the McCabe-Thiele graphical method to analyze binary distillation in trayed towers. 

Combine material-balances information from the McCabe-Thiele graphical method with interphase mass-transfer considerations to determine the packed height of the rectifying and stripping sections of a continuous-contact binary distillation tower. 

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