Multicomponent distillation plate

Yaws [124] et al. provide an estimating technique for recovery of each component in the distillate and bottoms from multicomponent distillation using short-cut equations and involving the specification of the recovery of each component in the distillate, the recovery of the heavy key component in the bottoms, and the relative volatility of the light key component. The results compare very well with plate-to-plate calculations. Figure 8-46, for a wide range of recoveries of 0.05 to 99.93% in the distillate. 

Unequal Molal Flow, 63 Ponchon-Savarit Method, 63 Example 8-21 Ponchon Unequal Molal Overflow, 65 Multicomponent Distillation, 68 Minimum Reflux Ratio — Infinite Plates, 68 Example 8-22 Multicomponent Distillation by Yaw s Method, 70 Algebraic Plate-to-Plate Method,... 

Continuous multicomponent distillation simulation is illustrated by the simulation example MCSTILL, where the parametric runs facility of MADONNA provides a valuable means of assessing the effect of each parameter on the final steady state. It is thus possible to rapidly obtain the optimum steady state settings for total plate number, feed plate number and column reflux ratio via a simple use of sliders. 

This relation enables the composition of the vapour to be calculated for any desired value of x, if a is known. For separation to be achieved, a must not equal 1 and, considering the more volatile component, as a increases above unity, y increases and the separation becomes much easier. Equation 11.14 is useful in the calculation of plate enrichment and finds wide application in multicomponent distillation. 

Multicomponent distillation with rigorous plate-to-plate methods. Chapter 6, p. 80. Packed bed adsorption. Chapter 6, p. 82. 

In multicomponent distillation, there are three or more components in the products, and specifying the concentrations of one component in each does not fully characterize these products. However, if the concentrations of two out of three or three out of four components are specified for the distillate and bottoms products, it is generally impossible to meet these specifications exactly. An increase in reflux ratio or number of plates would increase the sharpness of the separation, and the desired concentration of one component in each product could be achieved, but it would be a coincidence if the other concentrations exactly matched those specified beforehand. The designer generally chooses two components whose concentrations or fractional recoveries in the distillate and bottoms products are a good index of the separation achieved. After these components are identified, they are called key components. Since the keys must differ in volatility, the more volatile, identified by subscript L, is called the light key, and the less volatile, identified by subscript H, is called the heavy key. 

MINIMUM REFLUX RATIO. The minimum reflux ratio for a multicomponent distillation has the same significance as for binary distillation at this reflux ratio, the desired separation is just barely possible, but an infinite number of plates is required. The minimum reflux ratio is a guide in choosing a reasonable reflux ratio for an operating column and in estimating the number of plates needed for a given separation at certain values of the reflux ratio. 

NUMBER OF IDEAL PLATES AT OPERATING REFLUX. Although the precise calculation of the number of plates in multicomponent distillation is best accomplished by computer, a simple empirical method due to Gilliland is much used for preliminary estimates. The correlation requires knowledge only of the minimum number of plates at total reflux and the minimum reflux ratio. The correlation is given in Fig. 19,5 and is self-explanatory. An alternate method devised by Erbar and Maddox is especially useful when the feed temperature is between the bubble point and dew point. 

Gas processing Lean oil still Column did not achieve required separation because of insufficient refliK induced ly an undersized reflux orifice plate. Problem was difficult to diagnose because of unusual behavior of top temperature. In some multicomponent distillations, normal top and bottom temperature can be observed even if adequate separation is not achieved. 

The previous chapters served as an introduction to multicomponent distillation. Matrix methods are efficient, but they still require a fair amount of time even on a fast conputer. In addition, they are simulation methods and require a known number of stages and a specified feed plate location. Fairly rapid approximate methods are required for preliminary economic estimates, for recycle calculations where the distillation is only a small portion of the entire system, for calculations for control systems, and as a first estimate for more detailed simulation calculations. 

For binary distillation only one pinch point occurs where the number of steps become infinite, and this is usually at the feed tray. For multicomponent distillation two pinch points or zones of constant composition occur one in the section above the feed plate and another below the feed tray. The rigorous plate-by-plate stepwise procedure to calculate is trial and error and can be extremely tedious for hand calculations. 

Erbar, R. C., Maddox, R. N. (1962). Minimum Reflux Rate for Multicomponent Distillation Systems by Rigorous Plate Calculations. Can. J. Chem. Eng., 2,25-30. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Most distillation systems ia commercial columns have Murphree plate efficiencies of 70% or higher. Lower efficiencies are found under system conditions of a high slope of the equiHbrium curve (Fig. lb), of high Hquid viscosity, and of large molecules having characteristically low diffusion coefficients. FiaaHy, most experimental efficiencies have been for biaary systems where by definition the efficiency of one component is equal to that of the other component. For multicomponent systems it is possible for each component to have a different efficiency. Practice has been to use a pseudo-biaary approach involving the two key components. However, a theory for multicomponent efficiency prediction has been developed (66,67) and is amenable to computational analysis. 

Although this problem is one of multicomponent batch distillation, the product remains of constant composition so that normal methods can be used for plate-to-plate calculations at a point value of the varying reflux ratio. 

The component balance equations for reflux drum, column enriching section, feed plate, column stripping section and the column base and reboiler are very similar to those derived previously for the binary distillation example CONSTILL, but are now expressed in multicomponent terms as described in Section 3.3.3.4. 

In this chapter consideration is given to the theory of the process, methods of distillation and calculation of the number of stages required for both binary and multicomponent systems, and discussion on design methods is included for plate and packed columns incorporating a variety of column internals. 

For single separation duty, Diwekar et al. (1989) considered the multiperiod optimisation problem and for each individual mixture selected the column size (number of plates) and the optimal amounts of each fraction by maximising a profit function, with a predefined conventional reflux policy. For multicomponent mixtures, both single and multiple product options were considered. The authors used a simple model with the assumptions of equimolal overflow, constant relative volatility and negligible column holdup, then applied an extended shortcut method commonly used for continuous distillation and based on the assumption that the batch distillation column can be considered as a continuous column with changing feed (see Type II model in Chapter 4). In other words, the bottom product of one time step forms the feed of the next time step. The pseudo-continuous distillation model thus obtained was then solved using a modified Fenske-Underwood-Gilliland method (see Type II model in Chapter 4) with no plate-to-plate calculations. The... 

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