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Water-free
washing biodiesel applied for the purification of biodiesel
batch(es) is efficacious, though not necessarily efficient, for use
in every operation of biodiesel production. The water-free washing
processes always use some form of washer beads, which may be either
Magnesium
Silicate, Zeolites, Ion-exchange resins or blend of Zeolite and
Magnesium Silicate. The most common form of water-free washing
biodiesel has been the batch washing operation. The implementation
of this approach has been quite varied and each such process has been effected with
various attendant complexities:
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In typical application of the water-free washing biodiesel, the
biodiesel transesterification reactor effluent is mixed with
washer beads, and left for a period of time deemed to long
enough to have adsorbed the transesterification reaction
byproducts;
- In another approach actually
adding the Magnesium Silicate as powder into the transesterification
reactor has been the method of adoption.
- In another yet adoption of the
process overcoming the concerns about fines filtration process,
ion-exchange resins of
beads and
amorphous materials are used to extract the reaction residues.
However, every one of these adoption of the waterfree washing
biodiesel, had the issues of separating the beads or powder from the
bio-diesel after the purification: The mixing of the biodiesel and the
adsorbents, whether powder or
ion-exchange resin, requires subsequent filtration of the adsorbents, though the
powder adsorbents requires more intense filtration than the
ion-exchange resins.
In recognition of the tedium of the filtration of fines from the
biodiesel, however, a much better
approach that has evolved has been the use of
adsorbent pellets
consisting of a matrix within which is supported the adsorbents, and
as such enables a more rapid separation of the biodiesel and the
adsorbents to which reaction
residues are adsorbed, thereby evolving a more effective process for
accomplishing batch water-free washing operations.
Yet, given the varied
implementations of the waterfree washing biodiesel process,
effective evaluation of a design of such separator requires a good
understanding of the underlying engineering sciences, common to the
implementations.
A
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structured and comprehensive
analysis of a pellet-based batch water-free washing biodiesel
separator becomes an objective.
Factors Impacting
Design Rationale
Several factors impact the design of a Packed
Bed Batch Water-Free Washing Biodiesel Separator. The over-riding of
these, of course, is the performance of the adsorbents, and as such the design rationale of
a Packed bed Batch Water-free Washing Biodiesel Separator must be
based on criteria that includes the factors impacting adsorbents performance.
Ordinarily, the agitation
of a solid within a liquid accelerates the accomplishment of the
object for which the mixing is done. So in principle the separator
design would have the pellets and the biodiesel mixture in
the Separator equipment, agitated for a time long enough to
accomplish the purification. The agitation process evidently will
cause a form of convective flow around the pellets even not a
directed
flow. During the purification period, the
mixture of biodiesel and reaction residues as well as the byproducts should flow through
the porous matrix of the pellets, thereby gain access to the
adsorbents, resulting in the residues getting adsorbed into the adsorbents
particles, and the biodiesel fuel flowing out of the matrix. In this
respect, one factor of consideration is the ease with which the biodiesel stream flows through
the bed, the faster the stream flows through the bed the more
intense the convective mixing of the fluid and relative ease of
diffusion of byproducts to the adsorbents. The rate of flow through, however, depends on the porosity of the bed, which
in turn depends on the size
of the adsorbent pellets, because the average size of interstitial spaces
increase with increasing adsorbent effective diameter. Therefore the diameter of the
adsorbents is a factor for consideration in the selection of
adsorbents.
Besides bed porosity, an operational factor also impacts
the productivity of the Separator, which is the rapidity of
re-bedding of the separator. After each bed has been exhausted from
use the separator equipment has to be re-bedded. The re-bedding
process requires the
reopening of the Separator, removing of the "wet" and impurities-laden adsorbents, repacking of the Separator with fresh adsorbents
and then the resetting for operations. Evidently then, an effective
criteria must aim to minimize the downtime. A container for the
packing of the pellets therefore is an essential design feature for
this separator; moreover, such container should be such that it
offers no resistance to the flow of the biodiesel fluid through its
wall into the bed of adsorbent-pellets.
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The Design of Separator
A configuration of a Separator adopted for the
purposes of such analysis has this description: The equipment is
essentially a cylinder; The body-cylinder is capped at both ends
with spherical cap at the top and a flat cap at the bottom but both with flanges that are fastened to flanges on the
cylinder; At the bottom cap is a pipe connected to a reversible pump
that is connected to the raw biodiesel storage tank and a purified
biodiesel storage tank; On the
top cap is a short cylindrical base serving as seat for a low speed
motor to which is connected a shaft that passes through the top cap
and is affixed to a flat-plate with circular cutaways at the
circumference; Inside the cylinder at some depth
from the top is placed a mesh container holding a bed of pellets. The bed height is
such that there are enough wash powders to purify biodiesel filling
the equipment with the bed inside. Further when the top cap closes,
the inside plate seats on the pellets container such as interlock
with the container, and the motor begins to turn the inside plate
tends to subduct under the container effectively lifting the
container a very short distance above the bottom cap.
Performing Water-free Washing
Bio-diesel
The Separator is opened by flip opening
the top cap. The bed container with the pellets bed is
inserted into the separator and settled. The cap is then flip-closed and screw-bolted locked and tightened.
Then the raw bio-diesel
feed
of a volume such as will completely cover up the bed in the
container is
pumped with the feed-pump into the separator from the feed tank. The
feed inlet valve at the bottom of the separator is then closed and
the motor on the top cap started. The bed container now begins to
rotate very slowly and gently. This is the left to continue until
the predetermined time for complete separation.
After the set time for
separation, the motor is turned off, and the now purified bio-diesel
is pumped out onto the purified-bio-diesel storage tank. The
separator is now opened and the bed Container is taken out and
re-bedded for the next batch water-free washing bio-diesel process
operation.
Designing a packed bed batch water-free washing
biodiesel separator therefore can be effectively implemented as
described above, however, this separator should be noted as only
applicable to Batch Production Process. Moreover, empirical
determination of the adsorption time must be made and used to time
the length of time the biodiesel fluid and the adsorbent pellets are
mixed in order to accomplish satisfactory purification of the
reactor effluent. |
