Distillation operating methods

The first three of these are solely X T.E-based approaches, involving a series of simple distillation operations and recycles. The final approach also relies on distillation (X T.E), but also exploits another physical phenomena, liqnid-hqnid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-18. 

The dominance of distillation-based methods for the separation of liquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the liquid-phase composition of a simple single-stage batch stillpot as a function of time. Residue curves also approximate the liquid 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). 

To quantify the quality of liquid irrigation, the correlation uses the distribution quality rating index. Typical indexes are 10 to 70 percent for most standard commercial distributors, 75 to 90 percent for intermediate-quality distributors, and over 90 percent for high-performance distributors. Moore and Rukovena present a method for calculating a distribution-quality rating index from distributor geometry. Their method is described in detail in their paper as well as in Kister s book (.Distillation Operation, McGraw-Hill, New York, 1990). 

Many industrial users of batch distillation (Chen, 1998 Greaves, 2003) find it difficult to implement the optimum reflux ratio profiles, obtained using rigorous mathematical methods, in their pilot plants. This is due to the fact that most models for batch distillation available in the literature treat the reflux ratio as a continuous variable (either constant or variable) while most pilot plants use an on-off type (switch between total reflux and total distillate operation) reflux ratio controller. In Greaves et al. 2001) a relationship between the continuous reflux ratio used in a model and the discrete reflux ratio used in the pilot plant is developed. This allows easy comparison between the model and the plant on a common basis. 

As indicated in Fig. 11-7, the optimum reflux ratio occurs at the point where the sum of fixed charges and operating costs is a minimum. As a rough approximation, the optimum reflux mho usually falls in the range of 1.1 to 1.3 times the minimum reflux ratio. The following example illustrates the general method for determining the optimum reflux ratio in distillation operations. 

Other Operating Methods and Optimization A useful control method for difficult industrial or laboratory distillations is cycling operation. The most common form of cycling control is to operate the column at total reflux until steady state is established, take off the complete distillate for a short time, and then return to total reflux. An alternative scheme is to interrupt vapor flow to the column periodically by the use of a solenoid-operated butterfly valve in the vapor line from the pot. In both cases, the equations necessary to describe the system are complex, as shown by Schrodt et al. [Chem. Eng. Sci, 22, 759 (1967)]. The most reliable method for establishing the cycle relationships is by experimental trial on an operating column. Several investigators have also proposed that batch distillation be programmed to attain time optimization by proper variation of the reflux ratio. A comprehensive discussion was first presented by Coward [Chem. Eng. Sci, 22, 503 (1967)] and reviewed and updated by Kim and Diwekar [Rev. Chem. Eng., 17, 111 (2001)]. 

From exhaustive application of alternative simple distillation operators to all possible separations for this four-component system, or from application of ranked-list-based separations synthesis methods, it is easily shown that there are five different separation train structures for this four-component problem. Each can be generated systematically or since this pattern of solutions is already well known, each can be written down immediately or design heuristics can be used to generate one or more of the structures expected to be most suitable. After each structure is synthesized, its performance can be analyzed and evaluated with a flowsheet simulator. 

Continuous distillation, or fractionation, is a multistage, countercurrent distillation operation. For a binary solution, with certain exceptions, it is ordinarily possible by this method to separate the solution into its components, recovering each in any state of purity desired. 

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