Page 96 - The engineering of chemical reactions

80 Reaction Rates, the Batch Reactor, and the Real World

continuous reactors. Because of higher reactant costs and the fact that these reactors must
frequently be operated day and night, the costs of running continuous processes are much
higher than batch processes.
Batch reactors are frequently operated by chemists who are responsible for obtaining
the (boring) preliminary kinetic data on a process. Chemical engineers get involved when
a continuous process is being considered because chemists do not understand anything
beyond batch reactors. Steady-state continuous reactors are the subject of the next chapter.
SUMMARY

In this chapter we have defined some of the quantities we will need in considering chemical
reactors. All these concepts have been developed in previous courses that most of you
have taken, and none is particularly complicated. For students who may not have had these
courses or who have forgotten this material, the development here should be adequate for
our uses.
Then we switched topics completely to consider the chemical reactors that have
always dominated the chemical engineering industries. These are extremely complicated
and appear to have little relation to the simple batch reactors that you have seen previously.
In the rest of this book we will apply these ideas to increasingly complex situations,
so that by the last chapter you should have seen all the ideas necessary to deal with these
reactions and reactors. More important, these ideas should permit you be able to understand
the even more complex reactions and reactors that you will have deal with to develop new
processes for future technologies.

2.1 [This is one of several “descriptive” problems (no numbers) that will be assigned throughout
the course. You should be able to work these problems by just thinking about them, referring
to your old texts, and discussing them with classmates.]

Surfactants are fairly large molecules with molecular weights of 100-200 amu that make oil
particles soluble in water.
(a) What is the structure of these molecules and what is the structure of the oil+surfactant+water
solution?
(b) Your great-grandmother made soap by cooking animal fat with wood ashes and water in
a large pot open to the air for several days and then casting the product into bars. What
reactions was she carrying out in her multiphase batch reactor?
(c) In the early twentieth century companies such as Procter and Gamble began replacing the
reactants by olefins, NaOH, and phosphates to scale up grandma’s process, to reduce raw
material costs and to attain better quality control so they could sell many specialty products
at high prices. Where does each of these raw materials come from?
(d) Carboxylic acids make too much foam, branched alkyl groups are not attacked by bacteria,
and the phosphate builders in soaps are nutrients in lakes, so in the mid-twentieth century
soaps have been largely replaced by detergents. What are detergents?
(e) Biodegradable detergents are made by reacting ol-olefins with alkaline sulfates. The cz-
olefins can be made by polymerizing olefins, or forming large olefins from smaller ones
over a catalyst. Sketch how the successive reactions of ethylene with small a-olefins should
produce exclusively a-olefins.