The
importance of adsorption in practical applications in industry and
environmental protection cannot be underestimated. Adsorption plays a major
role in separation of mixtures on laboratory and industrial scale and is the
first stage in all heterogeneous catalytic processes (1,2).
Adsorption processes are typically conducted in tubular adsorption columns
packed with porous materials and separation is achieved due to preferential
enrichment of components on the solid/fluid interface.

Because
of the importance of the adsorption process, innumerable research is performed
on this subject both by modelling and experimental approaches. The
investigation and modeling of adsorption processes in trickle phase conditions
however is as good as absent in literature. However, many heterogeneous
catalytic processes are conducted in trickle phase conditions, in presence of a
solid catalyst and a mixed gas/liquid feed and product stream (9-12).  Adsorption plays a predominant role in this
process, as it is the first step in the catalytic conversion of the molecules
in the feed mixture. Typically in the petrochemical sector, catalytic processes
are conducted in trickle phase conditions since these feeds are characterized
by a mixture of components with a wide range of boiling points. In
hydroconversion reactors for example, a paraffin feed is converted in the
presence of H₂ gas at high temperatures. In these conditions, the
vapor phase is enriched with the more volatile components, while the liquid
phase is enriched with the heavy fraction. It is already recognized that the
presence of both a vapor and liquid phase has an effect on the performance of
the hydroconversion process (13-15). It is often assumed that a liquid film, in
thermodynamical equilibrium with the vapor phase,
surrounds the solid phase particles. Molecules from this liquid film directly
enter the pores of the catalyst before their catalytic conversion. Operating
conditions (pressure, temperature) directly affect the composition of this
liquid film through the vapor-liquid phase equilibrium and as a result have an
impact on catalytic activity and selectivity.

Apart
from the reaction kinetics, it thus is interesting to study the effect of
trickle phase conditions on the adsorption process and to verify how
operational conditions can influence separation efficiency. As mentioned before,
with the separation of petrochemical fractions the occurrence of trickle phase
conditions is likely to occur due to the wide range of boiling temperatures in
the feed. From our previous work on the adsorption of paraffinic mixtures it
follows that adsorption behavior is strongly dependent on aggregation state
(16-18).  In vapor phase conditions (high temperature,
low pressure), n-paraffins could be
selectively adsorbed and efficiently separated on a packed bed of mesoporous silica-alumina according to differences in chain
length. Longer n-paraffins are
preferentially adsorbed and retained for a longer time in the adsorption column
as compared to the shorter paraffins. In liquid phase conditions (high
pressure, lower temperature), this adsorptive selectivity disappears and the
short and long chain paraffins co-elute. The effect of the presence of trickle
phase conditions on the separation of paraffinic mixtures was not yet studied
in detail; adequate models are completely lacking.

In this paper, a model is developed
that allows simulation of the adsorption process in trickle phase conditions of
non-dissolved species. This model is able to simulate phase transitions due to
changing compositions during breakthrough and shifts in vapor-liquid equilibrium.
The model allows investigating the effect of trickle phase conditions on
competitive adsorption behavior. It is then used to determine (1) how
operational conditions influence the selectivity of catalytic processes through
the effect of changes in vapor-liquid equilibrium on competitive adsorption and
it is also used (2) to study the effect of trickle phase conditions on
adsorption processes and verify if the presence of trickle phase conditions may
contribute to more efficient separations and if so, determine the optimal
operational conditions. 

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0in;margin-left:0in;margin-bottom:.0001pt;text-align:justify;line-height:200%">The model is used for simulation of
species migration in a packed bed tubular column for a mixed vapor/liquid feed.
In figure 1 a schematic representation is given of the different phenomena that
occur and need to be accounted for in the description of adsorption in a
trickle bed adsorption column. Since adsorption of the mixed vapor/liquid feed
results in a changing local composition inside the column -which may result in
shift to full vapor and liquid conditions- the model needs to be able to model
both single and mixed phase conditions and transitions between these states. In
case of presence of both a liquid and vapor phase it is generally assumed that
the solid adsorbent particles are wetted with a liquid film which on its turn
is surrounded by the bulk vapor phase. As shown in figure 1, the model needs to
describe equilibria at the different levels in the system:  (1) the adsorbed phase is in equilibrium with
the liquid phase, as described by the adsorption isotherm, while (2) the liquid
phase in turn is in equilibrium with the vapor phase, as described by vapor
liquid equilibrium. A schematic representation of the algorithm minor-fareast;mso-ansi-language:EN-US'>for solving model for multiphase
adsorption process is given in figure 2.

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0in;margin-left:0in;margin-bottom:.0001pt;text-align:justify;line-height:200%">Figure 1: schematically
representation of the different phenomenon?s occurring in breakthrough
experiment with feed in trickle phase conditions.

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0in;margin-left:0in;margin-bottom:.0001pt;text-align:justify;line-height:200%"> minor-fareast;mso-ansi-language:EN-US'>Figure 2: Algorithm for solving model
for multiphase adsorption process with definition of density for velocity
calculation and definition of Langmuir adsorption parameters.

minor-fareast;mso-ansi-language:EN-US'>From simulation results it is observed
that adsorption selectivity is determined both by the intrinsic adsorption
affinity and the volatility of the compound and their relative size will
determine which compound will be retained longer in the adsorption bed.
Vapor-liquid equilibrium has an effect on adsorption selectivity due to
enrichment of the less volatile compound in the liquid phase, from which the
adsorption occurs in trickle phase conditions.

Two case studies are considered; (1)
adsorption of paraffinic compounds on silica alumina adsorbent and (2)
adsorption of a polar and apolar compound on an apolar adsorbent. "Times New Roman";mso-fareast-theme-font:minor-fareast;mso-ansi-language:EN-US'>In
the first case the intrinsic adsorption affinity and volatility of the compound
cause higher adsorption selectivity for the less volatile compound, a vapor
front is formed which is followed by a vapor-liquid front (fig. 3).

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Figure 3: molar vapor fraction
profile inside the column in function of time, for a binary i-C8/n-C20 feed in
vapor-liquid equilibrium (265°C and 2.5 bar).

minor-fareast;mso-ansi-language:EN-US'>In these conditions, the presence of
vapor-liquid equilibrium contributes to the adsorption selectivity in a way
that the heavy compound is increasingly adsorbed compared to the volatile
compound. Conditions that cause a shift in vapor-liquid equilibrium to the
vapor phase a higher temperature and lower pressure) will cause increased
enrichment of the heavy compound in the liquid phase and thus, increase
selectivity (fig. 4).

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minor-fareast;mso-ansi-language:EN-US'>Figure 4: Comparison of breakthrough
curves from adsorption processes conducted at different temperatures (upper)
and different pressures (lower) in trickle phase conditions for case 1.

minor-fareast;mso-ansi-language:EN-US'>This phenomenon can be interesting for
catalytic processes where the selectivity is of importance. For the catalytic
cracking process it is desirable that preferentially the less volatile, long
chain paraffins of the feed are cracked to the more valuable middle distillate
fraction while the conversion of this fraction is minimized. Operation of the
hydroconversion process at low pressure, high temperature and high H2/wax ratio
of the feed will cause preferential adsorption and subsequently conversion of
the long chain paraffins, as was experimentally observed by different studies.
For separation processes the presence of the heavy compound in the vapor front
poses a problem, which becomes increasingly stringent in conditions that cause
high vapor fractions.

minor-fareast;mso-ansi-language:EN-US'>In the second case the volatile compound
has a higher affinity for the adsorption sites which dominates the effect of
higher enrichment of the less volatile compound in the liquid phase. This
causes the volatile compound to be retained longer in the adsorption column and
this results in a liquid front (fig 5).

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minor-fareast;mso-ansi-language:EN-US'>

Figure 5: molar vapor fraction
profile inside the column in function of time, for a binary polar/nonpolar feed
in vapor-liquid equilibrium at 250°C and 2 bar.

minor-fareast;mso-ansi-language:EN-US'> In these conditions, the presence of
vapor-liquid equilibrium decreases the adsorption selectivity and is further
decreased by shifts in vapor-liquid equilibrium to vapor phase (fig 6). This
also allows to alter selectivity of catalytic
processes. For separation processes full liquid conditions will cause higher selectivity
and the presence of vapor liquid equilibrium is undesirable.

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minor-fareast;mso-ansi-language:EN-US'>Figure 6: Comparison of breakthrough
curves from adsorption processes conducted at different temperatures (upper)
and different pressures (lower) in trickle phase conditions for case 2.