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Biodiesel fuel production
processes development has been receiving
consumer-level
attention as well as larger scale community level and corporate
level attention. A direct map of the process implementation in
correspondence with the various steps of the experimental
procedures, had a CSTR
as the designated Transesterification Reactor. The design, however,
of the reactor is not so simple. The design is
rather complicated by the almost instantaneous separation of the
glycerine from the bio-diesel fuel or
solution of
bio-diesel and soap depending on whether the
vegetable oil
contained free fatty acids. The design rationale of the reactor
as such must necessarily factor all this into the design-product.
The Separation Process is also
very strongly dependent on the manner of operation of the process,
such as whether the possibility of the reactor being fed with excess
alcohol is also taken into consideration, as is excess hydroxide. Of
course, the operating procedure can be regimented with operator
training and implementation of extensive real-time control system;
hence, this aspect of the consideration may not be critical though
worthy of consideration.
The Reactor Design Rationale
In any event the design of the
reactor as a CSTR
was rather for ease of mapping from the batch reactors that are
being used at the consumer-level of production to a larger scale
design of continuous flow operation. The designation of the CSTR
design for the reactor, stipulates necessarily as an implicit
specification that the separation of the glycerine and the
bio-diesel or of soap solution not be allowed to occur in the
reactor. Based on this specification then the reactor design must
have the reactants to be pumped in from the top of the reactor
and the reaction-products must be pumped out of the reactor from the bottom. Further
the residence time of any reaction fluid particle must also be the
same as the reaction time for the completion of the transesterification
reaction. If the residence time and the reaction time do not match
then one of two situations will develop: The separation will occur
within the reactor, and The reaction will be incomplete at the point
of exiting the reactor - only to keep reacting in the transport
pipe. The latter case is better though not desired, while the former
clearly violates the specification. The matching of the residence
and reaction times very strongly impacts the height of the reactor.
Of course this tight restriction invariably impacts the diameter of
the reactor, given that the reactor must be of such volume as to
support the production volume as per the design specification; and
where the diameter is unacceptably large then the
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volume must be allocated between as many other reactors as
necessary. The height of the reactor, however, is not directly
determined by a direct measurement of the residence time of flow
from the top to the bottom, followed by the usual volume
calculations. The residence time is impacted by the design of the
impeller of the stirrer that is deployed to ensure that the reaction
fluid is well mixed - the assumed state of mixedness of the reaction
fluid.
Impeller selection, or even design,
therefore is of impact in the performance of the reactor as per the
specification. Clearly the impeller is required to accomplish to
objectives: Direct the flow of the fluid particles towards the
bottom of the reactor vessel, and Keep the fluid particle well-mixed
as it flows down towards the outlet. The first specification is
readily accomplished by the presence of a bias in the impeller
blades orientation. The blades must have a curving twist that is in
the downwards direction and turn in the direction that allows this
twist of the blades to push the fluid down. The second specification
is somewhat more demanding to satisfy; for this specification, the
impeller blades must also be such as to force the fluid particles to
adopt flow-paths that flows into each others paths, thereby causing
thorough mixing as the entire fluid flows down the reactor vessel.
Obviously the consequence of this impeller form is the nonlinear
flow of a fluid particle from the top inlets of the reactor to the
bottom outlet of the reactor. So the actual height of the reactor
then is given by that height of reactor, speed of impeller, and
fluid dynamics impact of the impeller design that in combination
gives a residence time that matches the reaction time.
Reaction Time Assessment
Much reference has been made of
the reaction time, a precise statement of the reaction time is that
time determined in course of the reaction-engineering analysis of
the reaction when the transesterification reaction achieved a desired
extent of completion. Effectively the reaction time must be
determined under conditions absent of any form of transport effects
of heat, mass and fluid flow; and such condition normally would be
equivalent to batch reaction analysis.
Residence time Assessment
The determination of the
residence time of course is a crucial component of the design of the
reactor: The residence time for a given reactor will vary with
impeller design; and as such the selection process may be tedious as
it may have to be performed several times. Alternatively for a given
impeller, the flow rates of the reactant-feeds and the outlet stream
may be treated as design parameters so as to determine an
efficacious operating condition in |
which the reaction and residence
times match within acceptable bounds. This alternative approach,
however, is much preferred as the
dimensions of the reactor can then be keep within the bounds of the
accepted rule of thumb.
The requirement of matching
the residence time and reaction time, to ensure the prevalence of
the production-specified extent of reaction is better enabled if the
transesterification reaction is planned to be conducted with
feed-oil that is
characterized with properties-specificity through the process
design.
The design rationale developed
here is clearly consistent with the operation of the reactor
as a CSTR as per asserted in the analysis of the
bio-diesel production
process; there are in fact other designs that do not quite match
the textbook CSTR concept that are equally viable but are not
presented. Ultimately the reactor design engineer will take the
intrinsic specification imposed by the reaction to develop an
efficacious reactor. |
