Page 466 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GNH/GUR P2: GPJ Final Pages
Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

28 Reactors in Process Engineering

reactor performance; (3) avoidance of reactor hot spots for CSTR for most reactions. These conditions are best met
highly exothermic reactions, making temperature easier for short residence times where velocity profiles in the
to control; (4) favoring lower-order reactions in parallel tubes can be maintained in the turbulent flow regime. In
reaction schemes; (5) economical operation when large an empty tube this requires high flow rates; for packed
volumes require high contact time; and (6) enhancement columns the flow rates need not be as high. Noncatalytic
of heat transfer by mixing. reactions performed in PFRs include high-pressure poly-
For the kinetics of decreasing rate with increasing merization of ethylene and naphtha conversion to ethy-
conversion (most reactions), isothermal CSTRs have lene. A gas–liquid noncatalytic PFR is used for adipinic
lower product composition than plug flow reactors. Addi- nitrile production. A gas–solid PFR is a packed-bed re-
tional disadvantages of CSTR are that larger reactor vol- actor (Section IV). An example of a noncatalytic gas–
umes are usually required, compared with other reactor solid PFR is the convertor for steel production. Catalytic
schemes, and that energy for agitation is required in the PFRs are used for sulfur dioxide combustion and ammonia
tank, increasing operating costs. synthesis.

D. Plug Flow
5. Advantages–Disadvantages
1. Description
The advantages of a PFR include (1) steady-state opera-
This reactor has continuous input and output of mate- tion, (2) minimum back mixing of product so that concen-
rial through a tube. Assumptions made for the plug flow tration remains higher than in a CSTR for normal reaction
reactor (PFR) are (1) material passes through the reac- kinetics, (3) minimum reactor volume in comparison with
tor in incremental slices (each slice is perfectly mixed CSTR (since each incremental slice of the reactor looks
radially but has no forward or backward mixing between like an individual CSTR, we can operate at an infinite
slices; each slice can be envisioned as a miniature CSTR), number of points along the rate curve), (4) application of
(2) composition and conversion vary with residence time heat transfer in only those sections of the reactor where it is
and can be correlated with reactor volume or reactor needed (allowing for temperature profiles to be generated
length, and (3) the reactor operates at steady state. down the reactor), and (5) no requirement for agitation
The PFR can be imagined as a tube, but not all tubular and baffling.
reactors respond as PFRs. The assumptions need to be The plug flow reactor is more complex than the
verified with experimental data. continuous-stirred tank alternative with regard to oper-
ating conditions. There are a few other disadvantages
associated with the PFR. For the kinetics where rate in-
2. Classification
creases with conversion (rare), an isothermal plug flow
The plug flow reactor is the second primary type of ideal reactor has lower product composition than a CSTR. For
flow reactor. It is also erroneously referred to as a tubular highly viscous reactants, problems can develop due to
reactor. high-pressure drop through the tubes and unusual flow
profiles.
3. Design Parameters
The parameters for PFRs include space time, concen- E. Fluidized Bed
tration, volumetric flow rate, and volume. This reactor
1. Description
follows an integral reaction expression identical to the
batch reactor except that space time has been substituted Fluidization occurs when a fluid is passed upward through
for reaction time. In the plug flow reactor, concentration a bed of fine solids. At low flow rates the gases or liquids
can be envisioned as having a profile down the reactor. channel around the packed bed of solids, and the bed pres-
Conversion and concentration can be directly related to sure drop changes linearly with flow rate. At higher flow
the reactor length, which in turn corresponds to reactor rates the force of the gas or liquid is sufficient to lift the
volume. bed, and a bubbling action is observed. During normal
operation of a fluidized bed the solid particles take on the
appearance of a boiling fluid. The reactor configuration is
4. Applications
usuallyaverticalcolumn.Thefluidizedsolidmaybeeither
For normal reaction kinetics the plug flow reactor is a reactant, a catalyst, or an inert. The solid may be con-
smaller than the continuous-stirred tank reactor under sim- sidered well mixed, while the fluid passing up through the
ilar conditions. This gives the PFR an advantage over bed may be either plug flow or well mixed depending on