REACTORS: IDEALIZED CHEMICAL REACTORS - Biomass as a Sustainable Energy Source for the Future - page 176

In the first case, the entire reactor is full of unpopped corn, so the residence time of the
corn is determined by the volume flow rate of unpopped corn (1 L

min −1 ). In the
second case, the entire reactor is full of popped corn, so the residence time of the corn
is determined by the volume flow rate of popped corn (28 L min −1 ). Although the

production is the same in the three cases, the residence time is different.
The residence time t represents the time that molecules (or particles) spend in the
reactor, whereas the space time

is the time necessary to process a volume V of reac-
tants . For constant density systems (all liquids and constant density gases), the mean
residence time of flowing material in the reactor t is the same as

τ

. Thus,t= =φ V 0 .

However, as exemplified in the popcorn reactor, for systems with changing density,
t depends on

τ τ =V

ε A .

6.6 DEVIATIONS FROM PLUG FLOW AND PERFECT MIXING

So far, two flow patterns have been considered, ideal plug flow and ideallymixed flow.
These flow patterns represent two extreme and opposite situations that are hardly
encountered in reality. However, these two approaches are the favorites of engineers
due to their simplicity. When is plug flow or perfectly mixed flow behavior a good
description of the flow behavior in a real reactor? The answer to that question can
be obtained either theoretically or experimentally and is determined by three factors:

1. Residence time distribution (RTD) of the material. Deviation from plug flow or
perfectly mixed flow can be caused by channeling of the fluid, recycling of the
fluid, or creation of stagnant regions in the vessel.
2. State of aggregation of the flowing stream. A microfluid is a fluid in which the
individual molecules are free tomove and intermix. Gases and ordinary, not very
viscous liquids belong to this group. A macrofluid is a fluid in which the
molecules are kept grouped together in aggregates or packets. Non-coalescing
droplets, solid particles, and very viscous liquids belong to this group.
3. Earliness and lateness of mixing of material in the vessel. The fluid elements of
a single flowing stream can mix with each other either early or late in their flow
through the vessel.

Often, one or more of these factors can be ignored. Much depends on reaction time
mixing time and average residence time. In some cases, it is even a good approximation
to ignore all three factors and to assume a pure plug flow or perfectly mixed reactor.

6.6.1 Residence Time Distribution (RTD)

Consider the CSTR described in Section 6.3. Although it is perfectly mixed, not all the
molecules that enter into the reactor spend the same time in it. There is a chance that a
molecule leaves the reactor as soon as it enters, while another molecule could stay in