Doherty-Malone method

The methodology for flowsheet synthesis presented in this book combines the Hierarchical Methodology of Douglas with a knowledge-based approach, proposed by Bamicki and Fair (1990, 1992). Since distillation is the main separation method, the reader should be familiar with modem design concepts described in specialised books as written by Kister (1992), and more recently by Stichlmair Fair (1999) and Doherty Malone (2001). For a more detailed treatment of separation techniques, we recommend the book of Seader and Henley (1998). 

The feasibility of separations of nonideal mixtures, as well as the screening of mass-separation agents for breaking azeotropes can be rationalized by means of thermodynamic methods based on residue curve maps. The treatment was extended processes with simultaneous chemical reaction. Two comprehensive books have been published recently by Stichlmair and Frey [10], as well as by Doherty and Malone [11]. 

Equation-Based Design Methods Exact design equations have been developed for mixtures with constant relative volatility. Minimum stages can be computed with the Fenske equation, minimum reflux from the Underwood equation, and the total number of stages in each section of the column from either the Smoker equation (Trans. Am. Inst. Chem. Eng., 34, 165 (1938) the derivation of the equation is shown, and its use is illustrated by Smith, op. cit.), or Underwoods method. A detailed treatment of these approaches is given in Doherty and Malone (op. cit., chap. 3). Equation-based methods have also been developed for nonconstant relative volatility mixtures (including nonideal and azeotropic mixtures) by Julka and Doherty [Chem. Eng. Set., 45,1801 (1990) Chem. Eng. Sci., 48,1367 (1993)], and Fidkowski et al. [AIChE /., 37, 1761 (1991)]. Also see Doherty and Malone (op. cit., chap. 4). 

More efficient is a generic tree representation of the separations based on tasks. The sequencing can be formulated as a structural optimisation problem where standard techniques based on Mixed Integer Linear Programming (MILP) apply. The tasks consist of simple distillation columns, as well as of hybrids for complex column arrangements, modelled by appropriate shortcut or semi-rigorous methods. Details can be found in Doherty and Malone (2001). 

Rooks, R.E., Malone, M.F. and Doherty, M.F. (1996) A geometric design method for side-stream distillation columns. Industrial Engineering Chemistry Research, 35, 3653-3664. 

Most chemical processes are dominated by the need to separate multicomponent chemical mixtures. In general, a number of separation steps must be employed, where each step separates between two components of the feed to that step. During process design, separation methods must be selected and sequenced for these steps. This chapter discusses some of the techniques for the synthesis of separation trains. More detailed treatments are given by Douglas (1995), Bamicki and Siirola (1997), and Doherty and Malone (2001). 

Initially, the homotopy parameter, t, is set to 0 and all values of Xj are set to 0 except for one, which is set to 1.0. Then t is gradually and systematically increased until a value of 1.0 is obtained. With each increase, the temperature and mole fractions are computed. If the resulting composition at f = 1.0 is not a pure component, it is an azeotrope. By starting from each pure component, all azeotropes are computed. The method of Fidkowski, Malone, and Doherty is included in many of the process simulation programs. Eckert and Kubicek (1997) extended the method of Fidkowski, Malone, and Doherty to the estimation of heterogeneous multi-component azeotropes. 

Only rigorous simulations are used in this book. The book by Doherty and Malone is highly recommended for a detailed coverage of approximate methods for conceptual steady-state design of distillation systems. 

This method is particularly helpful to find fuUy or partially optimized solutions for RD design variables (Malone and Doherty, 2000). The objective function for the RD problem is commonly composed of two basic terms annual operating cost (e.g. consumption of raw materials, steam and cooling water) and the annualized investment i.e. column, internals, reboiler and condenser). The constraints are formed from the MESH equations on each tray, material balances at the top and bottom of the column, kinetic and thermodynamic relationships and logical relationships between process variables and the number of trays. 

Flowsheets for processes are sometimes generated without following the hierarchy of properties described previously. As an example, Siirola [20] proposed a reactive-distiUation solution to make methyl acetate. Unit operations that combine the property differences present abrupt departures from common methodologies. With the advent of various pieces of equipment, such as differential side-stream feed reactors (i.e., semicontinuously fed batch reactors), continuous evaporator-reactors (e.g., wiped-film evaporators), and reactive distillation columns, one can consider these unit operations in the development of conceptual designs. As an example, Doherty and Malone [21] have presented systematic methods for reactive distillation design. 

To build a structural matrix, it is necessary to obtain information about compositions and boiling temperatures of all the pure components and azeotropes. It is possible to get this information in various reference books on azeotropy (Gmehling et al., 1994a, 1994b) and/or by calculation using the known models of phase equilibrium. In Fidkowski, Malone, Doherty (1993), there is a general algorithm based on the method of homotopy that allows all azeotropes of n-component mixture to be found simultaneously. 

Rooks, R. E., Malone, M. F., Doherty, M. F. (1996). Geometric Design Method for Side-Stream Distillation Columns. Ind. Eng. Chem. Res, 35,3653-64. 

Gadewar, S.B., Doherty, M.F., Malone, M.F. A systematic method for reaction invariants and mole balances for complex chemistries. Comput. Chem. Eng. 25, 1199-1217 (2001)... 

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