Page 101 - Chemical process engineering design and economics

86 Chapter 3

pilot-plant studies. If we expect from experience that the exiting vapor and liquid
streams will approach equilibrium for a reasonable condenser length, then we can
calculate the compositions of the exit streams. Later, the heat exchanger designer,
the expert, will satisfy the equilibrium condition by designing a condenser of suf-
ficient length to approach equilibrium. Then, he will have to consider the rates of
mass and heat transfer because rate processes determines the size of all equipment.
Mass Balances

In general, for unsteady state, the component mass or mole balance for each proc-
ess unit may be stated as

rate of flow in + rate of depletion + rate of formation by reaction =
rate of flow out + rate of accumulation + rate of disappearance by reaction (3.2)

Because the system either gains or loses mass, drop either of the rate terms
for depletion or accumulation. To apply Equation 3.2 to a specific situation, the
first decision requires determining whether the process operation is steady or un-
steady state. The unsteady-state operations are:

1. startup
2. change over to a new operating conditions
3. periodic
4. disturbances
An example of the application of Equation 3.2 can be seen in Figure 3.1.
Consider the steady-state operation of the steam stripper. Steam stripping is a
common operation in waste-water treatment for removing small amounts of or-
ganic compounds from water. Nathan [4] discusses processes for removing chlo-
rinated hydrocarbons from wastewater. In this example, we will consider remov-
ing ethlyene dichloride. It is good practice to always analyze a problem by starting
with a general relationship, like Equation 3.2, and drop those terms that do not
apply or are too small to be of any significance. For steady state, drop both the
rates of depletion and accumulation terms. Because there is no chemical reaction,
drop the chemical reaction terms. Thus, Equation 3.2 reduces to

rate of flow in = rate of flow out (3.3)

To apply Equation 3.3, first begin by numbering the process steams, as
shown hi Figure 3.1. We will always designate the flow rate as m regardless of the
units employed: mass, molar, English or S.I., and we will frequently designate the
concentration variable as y regardless of its units. Also, use numerical subscripts

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