CHEMICAL PROCESS ENGINEERING Design and Economics

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CHEMICAL PROCESS ENGINEERING Design and Economics

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Chemical engineers develop, design, and operate processes that are vital to
our society. Hardigg* states: “I consider engineering to be understandable by the
general public by speaking about the four great ideas of engineering: structures,
machines, networks, and processes.” Processes are what distinguish chemical from
other engineering disciplines. Nevertheless, designing chemical plants requires
contributions from other branches of engineering. Before taking process design,
students’ thinking has been compartmentalized into several distinct subjects. Now,
they must be trained to think more globally than before. This is not an easy transition.
One of my students said that process design is a new way of thinking for him.
I have found it informative to read employment ads to keep abreast of skills required
of process engineers. An ad from General Dynamics* in San Diego, CA,
states, “We are interested in chemical engineers with plant operations and/or process
engineering experience because they develop the total process perspective and
problem-solving skill we need.”
The book is designed mostly for a senior course in process design. It could
be used for entry-level process engineers in industry or for a refresher course. The
book could also be used before learning to use process simulation software. Before
enrolling in process design, the student must have some knowledge of chemical
engineering prerequisites: mass and energy balances, thermodynamics, transport phenomena, separator design, and reactor design. I encourage students to refer to
their textbooks during their process design, but there is need for a single source,
covering the essentials of these subjects. One reason for a single source is the
turnover in instructors and texts. Besides, it is difficult to teach a course using several
texts, even if the students are familiar with the texts. Another objective of a
process design course is to fill the holes in their education. This book contains
many examples. In many cases, the examples are familiar to the student. Sources
of process-design case studies are: the American Institute of Chemical Engineers
(AIChE) student contest problems; the Department of Chemical Engineering,
Washington University, at St. Louis, Missouri; and my own experience.
I am fortunate to have worked with skilled engineers during my beginning
years in chemical engineering. From them I learned to design, troubleshoot, and
construct equipment. This experience gave me an appreciation of the mechanical
details of equipment. Calculating equipment size is only the beginning. The next
step is translating design calculations into equipment selection. For this task, process
engineers must know what type and size of equipment are available. At the
process design stage, the mechanical details should be considered. An example is
seals, which impacts on safety. I have not attempted to include discussion of all
possible equipment in my text. If I had, I would still be writing.
The book emphasizes approximate shortcut calculations needed for a preliminary
design. For most of the calculations, a pocket calculator and mathematics
software, such as Polymath, is sufficient. When the design reaches the final stages,
requiring more exact designs, then process simulators must be used. Approximate,
quick calculations have their use in industry for preparing proposals, for checking
more exact calculations, and for sizing some equipment before completing the
process design. In many example problems, the calculated size is rounded off to
the next highest standard size. To reduce the completion time, the approach used is
to purchase immediately equipment that has a long delivery time, such as pumps
and compressors. Once the purchase has been made the rest of the process design
is locked into the size of this equipment. Although any size equipment – within
reason – could be built, it is less costly to select a standard size, which varies from
manufacturer to manufacturer. Using approximate calculations is also an excellent
way of introducing students to process design before they get bogged down in
more complex calculations.
Units are always a problem for chemical engineers. It is unfortunate that the
US has not converted completely from English units to SI (Systeme International)
units. Many books have adopted SI units. Most equipment catalogs use English
units. Companies having overseas operations and customers must use SI units.
Thus, engineers must be fluent in both sets of units. It could be disastrous not to be
fluent. I therefore decided to use both systems. In most cases, the book contains
units in both systems, side-by-side. The appendix contains a discussion of SI units
with a table of conversion factors