Design Calculation of Extractive Distillation Columns

2 Possibility of the chosen spht for azeotropic mixtures is determined preliminarily during the process of calculation. Possibility of the chosen split is not determined. 

4 The solution of the task is always achieved. The solution of the task can be rmachieved, even if the chosen split and the set purity of products are feasible, for the reason of absence of convergence of iteration process. 

5 Interference of the user in the process of calculation is not required. A change in the estimated profiles of temperatures, compositions, and flow rates and other user-defined parameters of calculation process can be required to solve the task. 

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The general approach to design calculation of extractive distillation columns is similar to the approach applied for two-section columns. We use our notions about the structure of intermediate section trajectory bundles (see Sections 6.4 6.6), about possible compositions at the trays adjacent to the feed cross-section from above and below (see Section 7.2), and about possible directions of calculation... 

The task of designing of extractive distillation columns, besides calculation of section trajectories, includes a number of subtasks. These are the same subtasks as for two-section columns and additional subtasks of determination of minimum entrainer flow rate and of choice of design entrainer flow rate. Optimal designing of extractive or autoextractive distillation includes optimization by two parameters - by entrainer flow rate and by reflux number. Figure 7.14 shows influence of entrainer flow rate on section trajectories at fixed value of parameter a = LfV)mlK j (as is shown in Section 6.4 (L/y) = K j). 

The design of azeotropic or extractive distillation columns, as with con-A ventional columns, demands a knowledge of the vapor-liquid equilibrium properties of the system to be distilled. Such knowledge is obtained experimentally or calculated from other properties of the components of the system. Since the systems in azeotropic or extractive distillation processes have at least three components, direct measurement of the equilibrium properties is laborious and, therefore, expensive, so methods of calculation of these data are desirable. 

After the optimal design variables for the extractive distillation column are determined, the total TAC can be calculated with the entrainer recovery column and the recycle stream included. Additional costs in the TAC include the annualized capital cost for the entrainer recovery column, the costs associated with the cooler from B2 on the entrainer feed, the operating costs of the steam and cooling water to operate the entrainer recovery column, and the entrainer makeup cost. As an example. Figure 10.13 shows the results of the optimization runs for Case 1 with N2 and Np2 as the design variables. The y axis is the TAC of the complete flowsheet. From the flgure, N2 should be 24 and Afe should be at Stage 9. 

The results received form the optimization using inherent safety as the objective function are somewhat different compared to those calculated with an economic objective function earlier (Hurme, 1996). With the inherent safety objective function the simple distillations were favoured more than with the economic function. Exceptions are cases where the extractive distillation could improve separation very dramatically. This is because in simple distillations only one column is required per split, but in extractive distillation two columns are needed, since the solvent has to be separated too. This causes larger fluid inventory since also the extraction solvent is highly flammable. The results of the calculation are well justified by common sense, since one of the principles of inherent safety is to use simpler designs and reduce inventories to enhance safety. 

Phase-equilibrium calculations were discussed for vapor-liquid equilibria (VLB), liquid-liquid equilibria (LLE), and solid-solid equilibria (SSE). Results from VLE calculations often take the form of K-factors and relative volatilities, especially when thermodynamic calculations serve as intermediate steps in computer-aided process-design programs. In those situations, K-factors are routinely provided to subprograms that size distillation columns and gas-liquid absorbers. Similarly, the distribution coefficients computed for LLE serve as bases for sizing solvent-extraction columns moreover, liquid-liquid distribution coefficients may be helpful in screening candidate solvents for use in an extraction. 

This chapter introduces how continuous distillation columns work and serves as the lead to a series of nine chapters on distillation. The basic calculation procedures for binary distillation are developed in Chapter 4. Multicomponent distillation is introduced in Chapter 5. detailed conputer calculation procedures for these systems are developed in Chapter 6. and sinplified shortcut methods are covered in Chapter 7. More complex distillation operations such as extractive and azeotropic distillation are the subject of Chapter 8. Chapter 9 switches to batch distillation, which is commonly used for smaller systems. Detailed design procedures for both staged and packed columns are discussed in Chapter 10. Finally, Chapter 11 looks at the economics of distillation and methods to save energy (and money) in distillation systems. 

Dr. J. Madkowiak has committed to paper his 20 years experience as practitioner while operating company ENVIMAC GmbH and as researcher, publishing his results in journals and conference papers. Calculation methods of packed columns, presented in the book will increase the design accuracy of distillation, absorption and extraction which cause up to 60% of total processing costs in chemical industry. 

Liquid-liquid extraction (LLE) is an important unit operation that allows one to separate fluids based on solubility differences of solutes in different solvents. In liquid extraction, separation of liquid solution occurred as a result of contact with another insoluble liquid. If the components of the original solution are distributed differently between the two liquids, separation will result (Figure 8.1). Extraction is driven by chemical differences and it can be used in situations when distillation is impractical, such as separation of compounds with similar boiling points in which distillation is not viable, or mixtures containing temperature-sensitive components. The solution to be extracted is called the feed, and the liquid used in contacting is the solvent. The enriched solvent product is the extract and the depleted feed is called the Raffinate [1]. In the design of liquid-liquid extraction column, there are two primary calculations ... 

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