Page 294 - Modeling of Chemical Kinetics and Reactor Design

264 Modeling of Chemical Kinetics and Reactor Design

• Input: Includes the amount of reactant entering the system δV in
δt by flow plus the amount of reactant formed by reaction in δV
during δt.
• Output: Includes the amount of reactant leaving δV in δt by flow
plus the amount of reactant destroyed by reaction in δV during δt.
• Accumulation: Is the amount of reactant in δV at (t + δt) minus
the amount of reactant in δV at t.
• Ideal reactors: Refers to the assumed mixing patterns in the reactor.
• Perfect mixing: Describes the contents as so well mixed that both
composition and temperature are uniform throughout the system.
The term “perfect mixing” is unambiguous as it refers to instan-
taneous and complete mixing on the molecular scale. Realistically,
no reactor can attain this ideal behavior just as no tubular reactor
can achieve a true piston (plug) flow situation. However, it is
possible to design reactors that closely approach these conditions.
Fluid mixing, residence time distribution, and micro and macro
scale are reviewed in Chapters 7 and 8.

IDEAL ISOTHERMAL REACTORS

BATCH REACTORS

A well-mixed batch reactor has no input or output of mass. The
amounts of individual components may change due to reaction, but
not because of flow into or out of the system. A typical batch reactor
is shown in Figure 5-3.
Consider a well-mixed batch reactor with a key reactant A, during
time t to time t + δt, where δt is very small. For a well-mixed batch
system, assume the following:
• Perfect mixing
• An isothermal operation
• Fluid density is constant
Rearranging the mass balance in Equation 5-1, gives

ACCUMULATION
INPUT OUTPUT
= + Concentration of A
No flow No flow
changes with time
+
Disappearance of A
by reaction