Shortcut methods design

Discussions of shortcut methods have appeared many times in the Hterature (16,36), accompanied by the usual admonition to use such methods only for approximate designs or analyses. For multicomponent systems having significant nonideaUties, the shortcut methods can be grossly in error. 

More shortcut design methods and rules of thumb have been developed for fractionation than probably any other unit operation. For example the paper reprinted in Appendix 5 on development of shortcut equipment design methods contains 18 references for fractionation shortcut methods out of 37 total. Both the process and mechanical aspects of fractionation design have useful rules of thumb. Many of the mechanical design rules of thumb become included in checklists of do s and don ts. 

Shortcut equipment design methods are difficult to define. I was asked to speak at a session on Shortcut Equipment Design Methods at the 1985 ASEE Annual Conference in Atlanta, Georgia. My paper on Development of Shortcut Equipment Design Methods is reproduced in Appendix 5. My introduction doesn t attempt to define these elusive methods instead I ducked the issue and just gave a simple example. 

Shortcut Equipment Design Methods tend to become buried in design procedures, even computer-based ones. Therefore they often become part of the mix and don t stand free as separate entities. This is not bad. These methods need to be placed wherever they are useful. However, it is also good to draw them out since many everyday problems can be solved with undiluted shortcuts. 

Appendix 1 Shortcut Equipment Design Methods—Overview 391... 

Branan, C. R., Development of Shortcut Equipment Design Methods, ASEE Annual Conference Proceedings, 1985. 

Like Bruce, I am here to share the perspective of the field. Shortcut equipment design methods play a major role in process equipment installations. Virtually all process designs employ these methods and shortcut techniques are developed from many sources. 

What exactly do we mean by the term shortcut equipment design method Here is an example. Two rules are given for determining the thickness in inches of copper boiler level float balls ... 

Many of the better known shortcut equipment design methods have been derived by informed assumptions and mathematical analysis. Testing in the laboratory or field was classically used to validate these methods but computers now help by providing easy access to rigorous design calculations. 

All of these variables must be optimized simultaneously to obtain the best design. Some of the variables are continuous and some are discrete (the number of stages in each column section). Such optimizations are far from straightforward if carried out using detailed simulation. It is therefore convenient to carry out some optimization using shortcut methods before proceeding to detailed simulation where the optimization can be fine-tuned. 

All of the values of this list can be established at least approximately by rapid shortcut methods. In some instances such values may be useful as final ones, but ordinarily they are for exploratory purposes or as a starting basis for a computer design. Computer design of fractionation is an iterative process which depends for rapid convergence on good starting estimates of the principal quantities. 

Shortcut methods of design provide solutions to problems in a short time and at small expense. They must be used when data are limited or when the greater expense of a thorough method is not justifiable. In particular cases they may be employed to obtain information such as ... 

Shortcut methods occupy a prominent place in such a broad survey and limited space as this book. References to sources of more accurate design procedures are cited when available. 

El 2. Design a shortcut method for preparing the phosphate buffer in Study Problem 1. Hint You need only NaH2P04, a solution of NaOH, and a pH meter. 

Now that the minimum number of theoretical stages is determined, the minimum reflux is required at infinite theoretical stages. I will reference this reflux as RM, which is the molar ratio of the tower returned liquid L divided by the outgoing overhead product D. There have been several shortcut methods to derive this minimum RM. Probably the most popular algebraic method is that of Underwood [11,12], which has been proven over many years and which I personally applied while working in a major EPC company on my first process design job, in 1959. 

Modeling of H F contactors is in most papers based on a simple diffusion resistance in series approach. In many systems with reactive extractants (carriers) it could be of importance to take into account the kinetics of extraction and stripping reactions that can influence the overall transport rate, as discussed in refs. [30,46], A simple shortcut method for the design and simulation of two-phase HF contactors in MBSE and MBSS with the concentration dependent overall mass-transfer and distribution coefficients taking into account also reaction kinetics in L/L interfaces has been suggested [47]. 

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. 

For single separation duties, Al-Tuwaim and Luyben (1991) provided a shortcut method for simultaneous optimisation of design and operation for binary and ternary separations. Using repetitive simulation strategy they have explained in detail the interaction between design and operation with an objective to maximise a capacity factor (total amount of specified products over unit time). 

Before the 1950s, column calculations were performed by hand. Although rigorous calculation procedures were available, they were difficult to apply for all but very small columns. Shortcut methods were therefore the primary design tool. Rigorous procedures were only used for small columns or for final design checks. Inaccuracies and uncertainties in the shortcut procedures were usually accommodated by overdesign. 

New information is constantly becoming available through publication in periodicals, books, trade bulletins, government reports, university bulletins, and many other sources. Many of the publications are devoted to shortcut methods for estimating physical properties or making design calculations, while others present compilations of essential data in the form of nomographs or tables. 

Although nomographs, simplified equations, and shortcut methods are included, every effort has been made to indicate the theoretical background and assumptions for these relationships. The true value of plant design and economics for the chemical engineer is not found merely in "the ability to put... 

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