Shortcut Distillation Method

Multicomponent Distillation Shortcut Methods for Stage and Reflux Requirements... 

MULTICOMPONENT DISTILLATION SHORTCUT METHODS FOR STAGE AND REFLUX REQUIREMENTS... 

Truly multicomponent solutions based on continuous distillation shortcut methods have been proposed for batch distillation. The Fenske, Underwood, and Gilliland equations or correlations are commonly used in conjunction with each other to solve continuous distillation problems as described in Section 12.3. Diwekar and Madhavan (1991) describe how these techniques may be modified for the design of batch distillation columns for variable and constant reflux cases. 

J. Cerda and A.W. Westerberg. Shortcut methods for complex distillation columns. 1. Minimum reflux. Ind. Eng. Chem. Process Des. Dev., 20 546-557, 1981. 

The first objective of a shortcut method is selection of two key components and then setting all conditions around these components. Call the light key component LK and the heavy key HK. The fractionation objective is to separate these two key components per a given specification. The distribution (distillate product analysis and bottoms product analysis) of a multicomponent system is also accomplished in this proposed shortcut fractionator program, Hdist. 

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... 

For single separation duty, Al-Tuwaim and Luyben (1991) proposed a shortcut method to design and operate multicomponent batch distillation columns. Their method, however, required a great number of simulations, which must be computationally very expensive, before they could arrive at an optimum design and find an optimum reflux ratio. Further details are in Chapter 7. 

In the first step, the design alternatives are automatically generated using a superstructure. In the second step, the design alternatives are evaluated using the rectification body method (RBM) [5], a nonideal thermodynamic based shortcut method. Unlike simulation studies or MINLP optimization, shortcut methods allow fast evaluation of the single separation task without detailed specification of the distillation column. 

The available standard shortcut methods and distillation design procedures are not applicable to complex columns. However, they are for simple columns. For complex mixtures, e.g. crude oil, the standard shortcut methods are not accurate for designing the crude oil distillation column. However, these methods are used to get a starting estimation for designing the column. 

If the pressure is fixed, there are 5 degrees of freedom. A mass balance allows specily the flow of distillate (D) (benzene) and intermediate product (P) (toluene) to a narrow interval. Therefore, although the kriging metamodel depends on 5 variables two of them can be almost fixed a priori, which increase the robustness of the model. The simulator (Hysys.Plant) forced us to specify the liquid (L) and vapor (V) streams that are withdraw from second column and returned to the first one. As remaining specification we chose the reflux ratio (RR). Initial values for these last three degrees of freedom, a reasonable interval of values as well as the number of trays of each column the feed and products tray positions, can be estimated using a shortcut method [6]... 

EXAMPLE MULTICOMPONENT DISTILLATION WITH SHORTCUT METHODS... 

Multicomponent distillation with shortcut methods. Chapter 6, p. 75. 

Approximations or boundaries of process variables can often be calculated by means of shortcut methods. Like heuristics, such methods allow to identify unsuitable alternatives based on a minimum of information. The rectification body method RBM), for example, allows to compute the minimum energy demand of distillation processes [13]. 

Bausa, J., von Watzdorf, R., Marquardt, W. Shortcut methods for nonideal multicomponent distillation 1. simple columns. AIChE Journal 44, 2181-2198 (1998)... 

The first step is to assume a set of temperatures Tj and vapor flows Vj. The temperatures can be obtained by linear interpolation between the condenser and reboiler temperatures, determined by dew point and bubble point calculations of products estimated by shortcut methods or on the basis of past experience with similar columns. The vapor rates are estimated from the specified distillate and reflux rates. Constant vapor rates are assumed above and below the feed. The... 

The graphical-based shortcut methods for binary batch distillation may be applied to multicomponent distillation only when the separation is between two key components to produce one distillate product and the residue. In this case the calculations may be approximated by lumping the other components with either of the key components and treating the system as a pseudo-binary. 

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