Page 26 - The engineering of chemical reactions

10 Introduction

T A B L E - 2
l
Continued
PCBs
dioxin
Specialty chemicals
ethanol
ibuprofen
aspirin
insulin
Food Chapters 9 and 12
HFCS
Wheaties
Cheerios
Materials Chapters 9 and 12
microcircuits
iron
nickel
TiOz
photographic film
catalysts Chapter 7
fibers
crystals

MODELING

Students will also discover another difference between this course and the “Principles”
courses taken previously. In this course we are interested mainly in simple approximations
to complex processes. In more “fundamental” courses rigor is essential so that we can
deal with any situation accurately. However, chemical reactors are so complex that we
cannot begin to solve the relevant mass, energy, and momentum balance equations exactly,
even on the largest supercomputers. Instead, we frequently need “back-of-the-envelope”
estimates of reactor performance. Chemical engineers usually earn their living on these
quick feasibility calculations. We need to know the details of thermodynamics and heat and
mass transfer, for example, but we will usually assume that all properties (heat capacity,
thermal conductivity, viscosity, diffusivity, etc.) are constants for a given calculation. All
gases will be assumed to be ideal mixtures of ideal gases, and all liquids will be ideal
solutions at constant density.
We will attempt to keep the mathematical details as brief as possible so that we will
not lose sight of the principles of the design and operation of chemical reactors. The student
will certainly see more applied mathematics here than in any other undergraduate course
except Process Control. However, we will try to indicate clearly where we are going so
students can see that the mathematical models developed here are essential for describing
the application at hand.
Further, we want to be able to work problems with numerical solutions. This will
require simplifying assumptions wherever possible so that the equations we need to solve
are not too messy. This will require that fluids are at constant density so that we can use
concentrations in moles/volume. This is a good approximation for liquid solutions but not
for gases, where a reaction produces a changing number of moles, and temperatures and