Page 356 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 11 Utilizing Experience-Based Principles to Confirm the

Suitability of a Process Design

Experienced chemical engineers possess the skills necessary to perform detailed and accurate
calculations for the design, analysis, and operation of equipment and chemical processes. In addition,

these engineers will have formulated a number of experienced-based shortcut calculation methods and
guidelines useful for the following:
1. Checking new process designs
2. Providing equipment size and performance estimates
3. Helping troubleshoot problems with operating systems
4. Verifying that the results of computer calculations and simulations are reasonable
5. Providing reasonable initial values for input into a process simulator required to achieve
program convergence
6. Obtaining approximate costs for process units
7. Developing preliminary process layouts

These shortcut methods are forms of heuristics that are helpful to the practicing engineer. All heuristics
are, in the final analysis, fallible and sometimes difficult to justify. They are merely plausible aids or
directions toward the solution of a problem [1]. Especially for the heuristics described in this chapter, we
need to keep in mind the four characteristics of any heuristic.
1. A heuristic does not guarantee a solution.
2. It may contradict other heuristics.
3. It can reduce the time to solve a problem.
4. Its acceptance depends on the immediate context instead of on an absolute standard.

The fact that one cannot precisely follow all heuristics all the time is to be expected, as it is with any set
of technical heuristics. However, despite the limitations of heuristics, they are nevertheless valuable
guides for the process engineer.

In Chapter 6, process units and stream conditions that were identified as areas of special concern were
analyzed. These areas were highlighted in a series of informational tables. In this chapter, you will
complete the analysis of chemical processes by checking the equipment parameters and stream conditions
in the PFD for agreement with observations and experiences in similar applications.

The required information to start an analysis is provided in a series of informational tables containing
shortcut calculation techniques. In this chapter, we demonstrate the use of these resources by checking the
conditions given in the basic toluene hydrodealkylation PFD.

11.1 The Role of Experience in the Design Process

The following short narrative illustrates a situation that could be encountered early in your career as an
engineer.