Biomedical Engineering Reference
In-Depth Information
For continuous reactor process design, the batch reactor design has been updated to
become a continuous process, which allows for better heat economization, enhanced product
purity, better recovery of excess methanol, less labour requirements for adjusting plant
parameters, and lower per unit capital costs [9]. Most large scale commercial plants use
continuous processing [11]. Continuous processes can be in two forms, using continuous
stirred tank reactors (CSTRs) in series, or using a plug-flow reactor (PFR) as shown in Figure
17. Important in the CSTR process is ensuring that inputs into each CSTR is well mixed. PFR
processes apply intense mixing inside the reactor to achieve a faster reaction time [7].
Table 15. Product information from continuous process [9]
Biodiesel Yield*
>98%
Untreated Crude Glycerine
~130kg of 75-80% glycerine/metric ton product
Fatty matter
<10 kg
Methanol stream
99.9% methanol (recycled)
Water stream (0.2% methanol)
nil
For high Free Fatty Acid Systems, an alternative to the acid catalyzed pre-treatment in
the conventional method is required. A caustic is added to the feedstock that creates soaps
which are removed using a centrifuge. This is called “caustic stripping.” [7]
The Biox Process was nearing commercialization in 2004 [7]. It utilises tetrahydrofuran
(THF) as a cosolvent to improve the miscibility of methanol in the triglyceride. Since THF
has a boiling point near methanol, both can be recovered in the same step [7]. A recently
developed process for producing biodiesel is the
McGyan Process
. The inventors propose that
this process eliminates many of the downsides of the typical batch reactor process [42]. This
process combines the alcohol and feedstock at high pressure and temperature into a
continuous fixed bed reactor. The reactor is filled with solid metal oxide catalyst. Both
transesterfication and esterification occur simultaneously in this reactor, so feedstock with
any percentage of FFA can be used [43]. After reaction, the product stream is cooled in a heat
exchanger that takes the heat to increase the temperature in the feed. The product stream is
then put through a polishing unit called the Easy Fatty Acid Removal (E-FAR) system [18].
The E-FAR contains aluminum oxide particles that remove residual FFAs. Nitrogen gas is
passed through to combust any FFAs in the particles, and this gas is recovered in order to
recycle the residue collected [18]. The catalyst was tested to be able to run for 4000 minutes
(67 hours) of continuous operation before conversion began to decrease [18]. The main
equipment required for this process is a specially designed heat exchanger, a fixed-bed
reactor, an E-FAR, rotary evaporator to recover alcohol, and distillation columns for
separation.
The supercritical methanol method has similar benefits to the Mcgyan Process, but does
not require a catalyst. The table below shows a comparison of the supercritical method to
conventional methods. Methanol (with propane co-solvent) at supercritical conditions is
mixed with the feedstock in a reactor. The supercritical methanol method can be performed as
one step (Saka Process) or two-steps (Saka and Dadan Process) [44]. The comparison to
conventional methods is depicted in Table 17. The downside of this is that the severe reaction
conditions cause iso-merization of the FAME to their trans-form, which has poor cold-flow
