Page 360 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Step 3—Reevaluate/Rethink: The original assumptions that velocity and temperature had no effect on
the heat transfer coefficient have been rejected. Improved assumptions for future predictions are as
follows.
1. The temperature effect on viscosity must be evaluated.
2. The effects of temperature on C , ρ, and k are negligible.
p
3. Pipe diameter has a small effect on h (all other things being equal).
4. Results are limited to the range where the Sieder-Tate equation is valid.

With these assumptions, the values for water at 21°C are substituted into Equation (11.2). This creates a
useful heuristic for evaluating the heat transfer coefficients for water.

0.8
2
h′(W/m °C) = 125u′ /μ′ 0.47 for u′(m/s), μ′ (kg/m/s)
Although it takes longer to obtain a solution when you start to apply the PAR process, the development of

the heuristic and the addition of a more in-depth understanding of the factors that are important offer
substantial long-term advantages.

There are hundreds of heuristics covering areas in chemical engineering—some general, and others
specific to a given application, process, or material. In the next section, we have gathered a number of
these rules that you can use to make predictions to start the PAR analysis.

11.2 Presentation of Tables of Technical Heuristics and Guidelines

We provide a number of these guidelines for you in this section. The information given is limited to
operations most frequently encountered in this text. Most of the information was extracted from a
collection presented in Walas [3]. In addition, this excellent reference also includes additional guidelines
for the following equipment:
1. Conveyors for particulate solids
2. Cooling towers
3. Crystallization from solution
4. Disintegration
5. Drying of solids
6. Evaporators

7. Size separation of particles

The heuristics or rules are contained in a number of tables and apply to operating conditions that are most
often encountered. The information provided is used in Example 11.2 and should be used to work

problems at the end of the chapter and to check information on any PFD.

Example 11.2

Refer to the information given in Chapter 1 for the toluene hydrodealkylation process, namely, Figure 1.7
and Tables 1.5 and 1.7. Using the information provided in the tables in this chapter, estimate the size of
the equipment and other operating parameters for the following units:
a. V-102