Page 259 - Modeling of Chemical Kinetics and Reactor Design

Industrial and Laboratory Reactors 229

to operate isothermally. The exchange of heat between the reaction
mixture and the medium must be so intense so as to heat the reaction
mixture instantaneously and eliminate the effect of the reaction heat.
The heat transfer medium must flow through the jacket in excess to
keep its temperature unaffected by the reaction heat.
Plug flow is an idealized flow of fluids where all particles in a given
cross-section have identical velocity and direction of motion. During
plug flow, particles of different age do not mingle and there is no
backmixing. All particles that enter the reactor at the same time must
leave simultaneously. The essential features of the plug flow reactor
require that there be no longitudinal mixing of fluid elements as they
move through the reactor, and that all fluid elements take the same
length of time to move from the reactor inlet to the outlet. The plug
flow can be described as a piston flow model. This is because the
reaction occurring within differentially thin slugs of fluid, fill the entire
cross-section of the tube and are separated from one another by
hypothetical pistons that prevent axial mixing. These plugs of material
move as units through the reactor, with the assumption that the velocity
profile is flat as the fluid traverses the tube diameter. Each plug of
fluid is assumed to be uniform in temperature, composition, and
pressure and thus can be assumed that radial mixing is infinitely rapid.
The tubular plug flow reactor is relatively easy to maintain with
no moving parts, and it usually produces the highest conversion per
reactor volume of any of the flow reactors. Other advantages are:

• High throughput.
• Little or no backmixing.
• Close temperature control.

The disadvantages are:

• Expensive instrumentation.
• High operating cost (maintenance, cleaning).
• Nonuniform heat flux of the radiant section of furnace.

The principal disadvantage of the tubular reactor is the difficulty
in controlling the temperature within the reactor. This often results in
hot spots especially when the reaction is exothermic. The tubular
reactor can be in the form of one long tube or one of a number of
shorter reactors arranged in a tube bank (Figure 4-7).