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Water-free washing of
Biodiesel as noted is the use of adsorbents for the
removal of the
amphiphilic and ionic byproducts of the transesterification reaction
of
biodiesel production process based on the use of vegetable oils
with high Free Fatty Acids, FFA, contents.. The adsorption of the
reaction products by the adsorbents, however, depend on both the
prevailing fluid dynamical conditions as well as the intrinsic
properties of the adsorbents and the the by-products. the selection
of an adsorbent can be made based on underlying principles of
science, however, the impact of fluid dynamics still needs to be
understood for the purpose of developing efficient separators.
In this regard therefore two situations need be considered: The
state of zero convective flow dynamics, and the state of a
prevailing convective flow dynamics. Every design of separator using
waterless washing of biodiesel
in both batch and continuous
commercial operations however requires a factoring in of the effect
of fluid dynamics and therefore thorough understanding of the
underpinning sciences: Quiescent Fluid Dynamic Waterless washing and
Convective Fluid Dynamic waterless washing , as is elucidated in the
sequence listed.
However, for a more lucid
perceptual appreciation of the physics, it is recalled that the
adsorbents in their natural form including Talc and other molecular
sieves, are tiny particles and therefore embody several active ionic
sites. Yet, these particles can be convected readily within the
biodiesel fluid. So for the case of analysis, the particle is
subsumed to be stationary at a fixed location fully surrounded by
the biodiesel fluid and therefore exposed on all sides to the ionic
byproducts in the fluid.
Quiescent Fluid Dynamics Waterless Washing
This case of non-convective fluid dynamics prevalence for
waterless washing biodiesel is in essence a the default as it
allows the analysis of the dynamics without the impact of fluid
flow. Effectively it serves as the reference for analyzing the
impact of fluid flow [examined later].
Now, under conditions of non-convective fluid dynamics, the
conditions guiding the amphiphiles and other ionic byproducts is
strictly forces between the ionic charges of the adsorbents and the
byproducts, Ionic Force of Attraction. The prevailing Transport
Phenomenon on the byproducts is strictly diffusion. Under this
condition therefore a molecule of the byproducts diffuses
towards the adsorbent particle until contact is made and then the
chemistry of |
adsorption itself takes place. Effectively the
adsorption dynamics consists of the diffusion to the adsorbent
particle followed by the actual adsorption of the byproducts
molecule onto a active ionic site on the adsorbents.
Several factors play some
roles, in this seemingly simple analysis of adsorption dynamics,
worthy of elicitation. Obviously, the time required for the
byproduct molecule to diffuse to the adsorbent ion is a function of
the proximity of the ions to the adsorbent surface, hence the
relative position of the byproduct molecule to the adsorbent
particle is a factor. Less obvious is the role of
designing for
properties-specificity. Clearly the diffusivity of the byproduct
molecules is influenced by the carbon chain length. The ions count
per particle is also a factor as such ions-count determines the
effective ionic charge strength of the customary dipole
representation of particles with electrostatic charges.
While necessary to be
considered in any consideration of the adsorption dynamics of
Waterfree Washing Biodiesel, these factors are also impacted by
prevailing convective flow of the biodiesel fluid.
Convective Fluid Dynamics Waterless Washing
The case of prevailing convection of the biodiesel fluid with the
byproducts, the dynamics will be different: Clearly, with the
quiescent case as reference, the analysis begins as an impurity diffuses towards the
adsorbent particle or ionic dipole for adsorption. Then the biodiesel stream
is caused to undergo convection and flows past the adsorbents. Simultaneously
then, the biodiesel fluid
relative velocity also begins convecting the impurities away from
the active sites to which the diffusion was initiated. Accordingly,
the Ionic Force of Attraction causing the diffusion of the
amphiphiles and other ionic products is impacted by the drag on the
molecules. The relative magnitude of these two forces effectively
determines whether an impurity molecule gets adsorbed into the
attracting adsorbent particle, or gets to be convected away with the
result of ultimately getting adsorbed at another particle, perhaps.
Obviously then, for those situations in which the the drag force on
the adsorbent particle is larger than the Ionic Force of Attraction,
the impurity will get convected away.
The evaluation of the
relative strength of the two forces however, is further refined by
the fluid dynamics of the flow. Generally fluid flow around small
particles causes a very tiny region surrounding the particle that is
purely laminar and
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the diffusion of the impurities is such that the Ionic Force of
Attraction is controlling; then, of course, outside of this laminar
flow region the convective flow is controlling and all impurity
molecules are simply swept away.
In view of this impact of
convective flow on the adsorption, by inference then, only
some of the impurities at the same lateral location as an adsorbent
particle can get to be adsorbed onto the adsorbent particle immersed
in a fluid subject to convective flow, while others will be
convected a way.
Consequently, a directed
flow would have the effect of having the adsorption of the reaction
byproducts in a biodiesel fluid to occur over a range along the path
of the convective flow. Ultimately then, along the flow-path, closer
to the stream inlet, a boundary develops at which all the
adsorbents' active sites have been saturated with impurities while
farther down along the direction of flow adsorption would still be
going on. However, notwithstanding however, of the convective
transfer of the impurities being faster and farther downstream than the diffusive flow enables the saturation of the
adsorbents active sites, there is also a point farther downstream at which all the
impurities have been adsorbed from the stream, resulting in a clean
or purified biodiesel stream. These two Fronts develop invariably
because of the competitive effect of the convective and diffusive
flows that govern the point of impurity absorption along the stream
flow-path.
Hence the adsorption
dynamic of a convective flow dynamic has a set of characteristic
feature that forms from the very use of adsorbents for the
purification process. The pairwise characteristic is the Adsorption
Fronts, which consists of the Saturation Front and Clean Front.
The Saturation Front is defined as the boundary along the
direction of stream flow before which the reaction-mixture purifying
capacity of the adsorbents have been depleted, and no further
adsorption of impurities can take place. The Clean Front is defined
as the boundary along the direction of stream flow after which the
process stream has been completely purified. Both Front undoubtedly
should be traveling down along the bed with time, effectively
becoming a sort of Moving Boundary.
Intrinsically, the
effectiveness of the adsorbents
also will impact the inter-front spacing: After all, the more
adsorbent the particle the shorter will be the inter-front spacing.
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