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More Update Post; 08_18_2008
Production of
biodiesel from vegetable oils with high concentration of Free
Fatty acids, FFAs, includes a
process step of separating the reaction-byproducts from biodiesel in
the reactor effluent stream. This separation step has been often
effected by one of two process:
Water washing
Biodiesel and Water-free
washing Bio-diesel. The water-free washing process as often performed
entails the direct mixing of transesterification reaction products and adsorbent
particles or of
Bio-diesel
Washing Pellets. The biodiesel fluid containing the adsorbent
particles subsequently
requires filtration, that is both tedious and time consuming.
Alternatively, Packed bed
water-free washing has also been adopted as a method that effects
automatic separation of the adsorbents. A
packed bed waterfree washing biodiesel separator, in a nutshell,
has a fixed bed of adsorbent beads within the separator vessel, and has the
biodiesel fluid flowing through it, until the adsorbents beads are
exhausted of active adsorbent sites. The design of a Packed
Bed Water-Free Washing Biodiesel Separator can be quite varied,
given the several factors that must be factored into the design.
Hence, effective adoption of Packed
Bed washing Biodiesel for the purification of the reactor-effluent
stream entails a good understanding of the underlying engineering
sciences,
often ignored features that are examined.
Factors Impacting
Design Rationale
Several factors impact the design of a Packed
Bed Continuous Water-Free Washing Biodiesel Separator. The over-riding of
these, of course, is the performance of the adsorbents as all the
others hinge on this one factor, and as such the design rationale of
a Packed bed Water-free Washing Biodiesel Separator must
include in the criteria developed, the factors impacting
performance. One of the
factors is the ease with which the biodiesel stream flows through
the bed, the faster the stream flows through the bed the higher the
productivity, provided that the purification of the biodiesel also
occurs simultaneously. The rate of flow through, however, is
dependent on the porosity of the bed, which is dependent on the size
of the adsorbent, because the average size of interstitial spaces
increase with increasing adsorbent effective diameter. Moreover, a
bed that allows the stream to flow through relatively easily will
demand a pump with a power that may not have a very large discharge
pressure; obviously the smaller the interstitial spaces of the
adsorbents the higher the pressure drop per unit length and as such
the Pump must have large power to overcome the pressure drop as to
pump the fluid through the bed. Therefore the diameter of the
adsorbents is a factor for consideration in the selection of
adsorbents. |
Besides the
design factor of 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 is found to be also tedious to some extent; requiring 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, a downtime
required for the re-bedding of the Packed Bed Water-free washing
Biodiesel Separator is a significant factor in the selection of such
separators, and the smaller the downtime the higher should be the
productivity of the specific Separator.
Waterfree washing biodiesel
separators, except for
batch separators, characterized by a relative directed velocity
have a set of characteristic feature that forms from the very use of
adsorbents for the purification process. These features, however,
particularly of importance in the operation of packed bed water-free
washing bio-diesel is the
Adsorption Fronts: Saturation Front and Clean Front;
as
elicited in the elicitation of the adsorption dynamics of the
adsorbents used in
water-free washing biodiesel separators designs. This set of
characteristics also strongly impact the design of the separators.
This feature obtains because at a given point point of the
flow path, within these Fronts the adsorbents are never actually exhausted before
some impurities are convected farther downstream along the flow-path. The distance between these
two fronts is related to the ratio of the convective flowrate
and the diffusive flowrate. The effectiveness of an adsorbents
bed is determined by the proximity of these two fronts, with the
most effective bed being the case in which the bed supports
coincident Fronts and the worst
would be of distance 12feet or longer. The choice of 12 feet is a
heuristic number based on the experience that most industrial
separators, except for distillation columns are seldom that long.
The two fronts in this class
of separator will travel along the separator because for a given
constant ratio of the convective to diffusive flowrates
resulting in a fixed separating length, as the Saturation Fronts
moving forward along the direction of flow, the Clean Front will
similarly travel further away from the Saturation Front, until the Clean
Front
reaches the exit-side boundary of the bed, at which time the washing
of the biodiesel is no longer accomplished, and the effluent
biodiesel fluid will have impurities.
Yet certain design
specification for this separator impacts the dynamics of these
adsorption Fronts: The Inter-fronts distance - the magnitude of the
spacing between the two adsorption fronts,
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however, is controlled in this class of separators by the flowrate of the biodiesel fluid: The lower
the flowrate,
the
lower will be the ratio of the convective to diffusive
flowrate ratios, and hence the shorter the inter-fronts
spacing will be and vice versa.
The designer of this class of
separator therefore has the leverage of sizing the equipment such
that the flowrate is low enough to provide production volume
requirement before the inevitable regeneration and rebedding of the
separator that obtain as the Clean Front reaches the equipment
effluent-exit point. Moreover, the designer has the
design requirement of keeping these two fronts always within the
inlet and outlet bounds of the separator, and hence the most
effective separator becomes the one that accomplishes this goal
essentially.
Separator Design
Rationale
All considered then, the concept design for a Packed Bed Water-free
Washing Biodiesel separator, generally consists essentially of a
cylinder capped at both ends with spherical cap with flanges that
are fastened to flanges on the cylinder; At both the top and bottom
caps are short flanged pipe; Two Bed-Plates, each of which consists
of two perforated plates sandwiching a mesh of size smaller than the
size of the adsorbent that would be used as the washing-bed - one of
the these Bed-Plates is slide down inside the main-body cylinder to
provide support for the adsorbent particles and the other is then
slide-down on top of the adsorbent particles. The lower of the Two
Bed-Plates usually comes to rest at a level that is flush with the
lower flange of the main-body cylinder.
In order to load the bed, the top
cap of the Separator is slid open after the removal of all the bolts
but one that is left to serve as a hinge. One Bed-Plate is slid-in
as explained above. The adsorbents, resins,
washing pellets (or pelletized adsorbents), as the case may be,
are then poured into the Separator up to a pre-determined height:
The height is calculated as to leave sufficient distance between the
bed and the Separator top flange level, to allow for bed-expansion
that occurs with the "wetting" of the bed when the effluent
biodiesel mixture is pumped into the separator. Then the other
Bed-Plate is slid on top of the bed, and the top cap slid back in
place and fastened.
A design that meets all
these requirements as a minimum then would function as productivity
as would be expected of a separator, as is the object to be meet. |
