associated with membrane separations) can strongly impact the thermodynamic
efficiency of the whole process.
Such a situation can be illustrated by means of two consolidated applications of
zeolites in adsorption processes. The choice of the first one (air separation) is directly
driven by economic reasons (balance between capital and operational expenditures).
In the case of the second one (xylene separation), the intrinsic difficulty of the
separation reduces the problematic making evident the choice for adsorption. In the
case of the market of air separation, the adsorption-based technologies are in
competition with the more widespread cryogenic distillation technology. Since the
operational expenditures are directly linked to the final product purity, when intermediate values of purity are required, adsorption becomes well suited. Nevertheless,
when higher capacities are expected, the capital expenditures of adsorption-based
technologies rise linearly when increasing the oxygen production implying an
important penalty for their applications.
As an extreme case, the separation of xylenes is entirely driven by the difficulty
associated with separate compounds with very close boiling points (<1
C between
some of the isomers) by distillation. In such conditions, the implementation of
complex adsorption-based technologies allows reaching high-quality separations.
As an example, notwithstanding the difficulty of the separation, in the case of the
ELUXYL© process commercialized by AXENS (implementing a specialty zeolite
furnished by ARKEMA), the production of almost pure (99.7%) p-xylene is combined with recoveries higher than 95%.
Irrespective of the economic or energetic context, the competitiveness of
adsorption-based technologies goes through the development of technologies and
adsorbents able to fulfill high standards. From the point of view of the adsorbent, a
complicated optimum must be found between the different properties of the separation agent. Evidently, the design of an efficient process is not always associated with
a unique combination of adsorbent and process scheme, but several competing
optimal combinations can be found. The flexibility provided by the available
commercial zeolites (as well as other conventional adsorbents) has lead in the last
decades to a huge number of patents dealing with gas separations [9]. As a consequence a large number of industrial processes involving the use of zeolites is
nowadays available, while future innovations are expected based on the mentioned
adsorbent-process flexibility of choice.
Table 2 provides a non-exhaustive list of the more important adsorption-based
processes involving the use of zeolites that can be commercially found in bulk
configuration. Separation processes can be divided in two main types: the separations taking place in a bulk configuration and those in a purification configuration. In
the last case, when compared to other conventional separation techniques such
distillation, adsorption presents the advantage of being able to selectively remove
a given compound from a mixture.
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J. Pérez-Pellitero and G. D. Pirngruber
efficiency of the whole process.
Such a situation can be illustrated by means of two consolidated applications of
zeolites in adsorption processes. The choice of the first one (air separation) is directly
driven by economic reasons (balance between capital and operational expenditures).
In the case of the second one (xylene separation), the intrinsic difficulty of the
separation reduces the problematic making evident the choice for adsorption. In the
case of the market of air separation, the adsorption-based technologies are in
competition with the more widespread cryogenic distillation technology. Since the
operational expenditures are directly linked to the final product purity, when intermediate values of purity are required, adsorption becomes well suited. Nevertheless,
when higher capacities are expected, the capital expenditures of adsorption-based
technologies rise linearly when increasing the oxygen production implying an
important penalty for their applications.
As an extreme case, the separation of xylenes is entirely driven by the difficulty
associated with separate compounds with very close boiling points (<1
C between
some of the isomers) by distillation. In such conditions, the implementation of
complex adsorption-based technologies allows reaching high-quality separations.
As an example, notwithstanding the difficulty of the separation, in the case of the
ELUXYL© process commercialized by AXENS (implementing a specialty zeolite
furnished by ARKEMA), the production of almost pure (99.7%) p-xylene is combined with recoveries higher than 95%.
Irrespective of the economic or energetic context, the competitiveness of
adsorption-based technologies goes through the development of technologies and
adsorbents able to fulfill high standards. From the point of view of the adsorbent, a
complicated optimum must be found between the different properties of the separation agent. Evidently, the design of an efficient process is not always associated with
a unique combination of adsorbent and process scheme, but several competing
optimal combinations can be found. The flexibility provided by the available
commercial zeolites (as well as other conventional adsorbents) has lead in the last
decades to a huge number of patents dealing with gas separations [9]. As a consequence a large number of industrial processes involving the use of zeolites is
nowadays available, while future innovations are expected based on the mentioned
adsorbent-process flexibility of choice.
Table 2 provides a non-exhaustive list of the more important adsorption-based
processes involving the use of zeolites that can be commercially found in bulk
configuration. Separation processes can be divided in two main types: the separations taking place in a bulk configuration and those in a purification configuration. In
the last case, when compared to other conventional separation techniques such
distillation, adsorption presents the advantage of being able to selectively remove
a given compound from a mixture.
202
J. Pérez-Pellitero and G. D. Pirngruber