Chemistry Reference
In-Depth Information
There are two main types of continuous reactor: the continuously stirred tank reactor
(CSTR) and the plug flow reactor (PFR):
1. CSTRs are essentially stirred batch vessels with flows in and out. They exhibit very
poor control of residence time, the residence time distribution (RTD) being essen-
tially an exponential decay, meaning that much of the material will spend too long in
the reactor, leading to undesired side reactions. However, some of the material will
not spend enough time in the reactor, as it is able to 'shoot through' from inlet to
outlet, and will therefore not reach the desired conversion. This is usually addressed
by oversizing the reactor.
2. The solution to the problem of poor control of residence time is to use a PFR. In its
simplest incarnation, the PFR is simply a tube through which the reactants flow at a
high enough velocity to achieve turbulence, and with it a flat velocity profile; that is,
the material flows as a 'plug'. Each element of fluid entering the reactor can be said
to experience the same 'processing history', and the conversion in any one element
of fluid is the same as that in any other. These reactors can therefore be designed for
the exact volume required. However, an important limitation of PFRs is that a high
velocity must be maintained. This is a problem for 'long' reactions, as maintaining
a high velocity for a long period of time necessarily results in an extremely long,
narrow reactor, engendering a variety of practical problems, particularly control,
footprint and pumping. The OBR is the solution to this.
It should be noted that there are other possible solutions to these problems:
PFRs can be fitted with various inserts to facilitate earlier onset of turbulence, thereby
reducing the velocity required and shortening the reactor. However, this does not entirely
solve the problem, as there is still a linear dependence of operation on velocity.
CSTRs can be arranged in series to give an approximation to plug flow, but this tends to be
an expensive solution, as each new CSTR incurs the same costs. The increasing cost must
be balanced against the increasing quality of the plug flow with each additional tank.
OBRs can perform relatively long reactions continuously, without requiring impracti-
cally long, narrow reactor designs. This is because unlike in conventional PFRs the
mixing is decoupled from the net flow velocity, as it is achieved by oscillating fluid
through orifice plate baffles, each acting as a small CSTR and thereby achieving plug
flow, but in a much more integrated and inexpensive manner, which produces extremely
high degrees of plug flow. The quality of plug flow can be expressed as the number of
perfect CSTRs in series. In practice, a value of 10 is adequate for any reaction, but this is
easily exceeded, and values
>
100 have been achieved.
A typical OBR configuration can be seen in Figure 5.1. As for most PFR designs,
changes to the operation, such as staged addition and integration of different reaction
stages, can readily be achieved. Furthermore, there are a range of features that cannot
necessarily be achieved by other forms of PFR: they can accommodate different forms
of mixing (liquid-liquid, liquid-gas etc.) where necessary, and suspension of catalyst
particles.
The improved control of mixing opens up the possibility of operation in the absence
of a solvent, as the OBR has been demonstrated to operate well as a two-phase mixing
device [1].
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