Selective catalytic reduction of nitrogen oxides with ammonia in forced unsteady state reactors - Case based reasoning and mathematical model simulation reasoning
Botar-Jid, Claudiu Cristian (2007)
Väitöskirja
2007
Acta Universitatis LappeenrantaensisURN:ISSN:1456-4491
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-214-469-0
https://urn.fi/URN:ISBN:978-952-214-469-0
Tiivistelmä
The application of forced unsteady-state reactors in case of selective catalytic
reduction of nitrogen oxides (NOx) with ammonia (NH3) is sustained by the fact that
favorable temperature and composition distributions which cannot be achieved in any
steady-state regime can be obtained by means of unsteady-state operations.
In a normal way of operation the low exothermicity of the selective catalytic
reduction (SCR) reaction (usually carried out in the range of 280-350°C) is not enough
to maintain by itself the chemical reaction. A normal mode of operation usually requires
supply of supplementary heat increasing in this way the overall process operation cost.
Through forced unsteady-state operation, the main advantage that can be obtained when
exothermic reactions take place is the possibility of trapping, beside the ammonia, the moving heat wave inside the catalytic bed. The unsteady state-operation enables the
exploitation of the thermal storage capacity of the catalyticbed. The catalytic bed acts as
a regenerative heat exchanger allowing auto-thermal behaviour when the adiabatic
temperature rise is low.
Finding the optimum reactor configuration, employing the most suitable operation
model and identifying the reactor behavior are highly important steps in order to
configure a proper device for industrial applications.
The Reverse Flow Reactor (RFR) - a forced unsteady state reactor - corresponds
to the above mentioned characteristics and may be employed as an efficient device for the
treatment of dilute pollutant mixtures. As a main disadvantage, beside its advantages, the
RFR presents the 'wash out' phenomena. This phenomenon represents emissions of
unconverted reactants at every switch of the flow direction. As a consequence our
attention was focused on finding an alternative reactor configuration for RFR which is
not affected by the incontrollable emissions of unconverted reactants. In this respect the
Reactor Network (RN) was investigated. Its configuration consists of several reactors
connected in a closed sequence, simulating a moving bed by changing the reactants
feeding position. In the RN the flow direction is maintained in the same way ensuring
uniformcatalyst exploitation and in the same time the 'wash out' phenomena is
annulated.
The simulated moving bed (SMB) can operate in transient mode giving practically
constant exit concentration and high conversion levels.
The main advantage of the reactor network operation is emphasizedby the
possibility to obtain auto-thermal behavior with nearly uniformcatalyst utilization.
However, the reactor network presents only a small range of switching times which allow
to reach and to maintain an ignited state. Even so a proper study of the complex behavior
of the RN may give the necessary information to overcome all the difficulties that can
appear in the RN operation.
The unsteady-state reactors complexity arises from the fact that these reactor
types are characterized by short contact times and complex interaction between heat and
mass transportphenomena. Such complex interactions can give rise to a remarkable complex dynamic behavior characterized by a set of spatial-temporal patterns, chaotic
changes in concentration and traveling waves of heat or chemical reactivity.
The main efforts of the current research studies concern the improvement of
contact modalities between reactants, the possibility of thermal wave storage inside the
reactor and the improvement of the kinetic activity of the catalyst used. Paying attention
to the above mentioned aspects is important when higher activity even at low feeding
temperatures and low emissions of unconverted reactants are the main operation
concerns. Also, the prediction of the reactor pseudo or steady-state performance
(regarding the conversion, selectivity and thermal behavior) and the dynamicreactor
response during exploitation are important aspects in finding the optimal control strategy
for the forced unsteady state catalytic tubular reactors.
The design of an adapted reactor requires knowledge about the influence of its
operating conditions on the overall process performance and a precise evaluation of the
operating parameters rage for which a sustained dynamic behavior is obtained. An apriori
estimation of the system parameters result in diminution of the computational efforts.
Usually the convergence of unsteady state reactor systems requires integration over
hundreds of cycles depending on the initial guess of the parameter values.
The investigation of various operation models and thermal transfer strategies give
reliable means to obtain recuperative and regenerative devices which are capable to
maintain an auto-thermal behavior in case of low exothermic reactions.
In the present research work a gradual analysis of the SCR of NOx with ammonia
process in forced unsteady-state reactors was realized. The investigation covers the
presentationof the general problematic related to the effect of noxious emissions in the
environment, the analysis of the suitable catalysts types for the process, the mathematical
analysis approach for modeling and finding the system solutions and the experimental
investigation of the device found to be more suitable for the present process.
In order to gain information about the forced unsteady state reactor design,
operation, important system parameters and their values, mathematical description,
mathematicalmethod for solving systems of partial differential equations and other
specific aspects, in a fast and easy way, and a case based reasoning (CBR) approach has
been used. This approach, using the experience of past similarproblems and their adapted solutions, may provide a method for gaining informations and solutions for new problems
related to the forced unsteady state reactors technology. As a consequence a CBR system
was implemented and a corresponding tool was developed.
Further on, grooving up the hypothesis of isothermal operation, the investigation
by means of numerical simulation of the feasibility of the SCR of NOx with ammonia in
the RFRand in the RN with variable feeding position was realized. The hypothesis of
non-isothermal operation was taken into account because in our opinion ifa commercial
catalyst is considered, is not possible to modify the chemical activity and its adsorptive
capacity to improve the operation butis possible to change the operation regime.
In order to identify the most suitable device for the unsteady state reduction of
NOx with ammonia, considering the perspective of recuperative and regenerative
devices, a comparative analysis of the above mentioned two devices performance was
realized.
The assumption of isothermal conditions in the beginningof the forced unsteadystate
investigation allowed the simplification of the analysis enabling to focus on the
impact of the conditions and mode of operation on the dynamic features caused by the
trapping of one reactant in the reactor, without considering the impact of thermal effect
on overall reactor performance.
The non-isothermal system approach has been investigated in order to point out
the important influence of the thermal effect on overall reactor performance, studying the
possibility of RFR and RN utilization as recuperative and regenerative devices and the
possibility of achieving a sustained auto-thermal behavior in case of lowexothermic
reaction of SCR of NOx with ammonia and low temperature gasfeeding.
Beside the influence of the thermal effect, the influence of the principal operating
parameters, as switching time, inlet flow rate and initial catalyst temperature have been
stressed. This analysis is important not only because it allows a comparison between the
two devices and optimisation of the operation, but also the switching time is the main
operating parameter. An appropriate choice of this parameter enables the fulfilment of the
process constraints.
The level of the conversions achieved, the more uniform temperature profiles, the
uniformity ofcatalyst exploitation and the much simpler mode of operation imposed the RN as a much more suitable device for SCR of NOx with ammonia, in usual operation
and also in the perspective of control strategy implementation.
Theoretical simplified models have also been proposed in order to describe the
forced unsteady state reactors performance and to estimate their internal temperature and
concentration profiles. The general idea was to extend the study of catalytic reactor
dynamics taking into account the perspectives that haven't been analyzed yet.
The experimental investigation ofRN revealed a good agreement between the
data obtained by model simulation and the ones obtained experimentally.
reduction of nitrogen oxides (NOx) with ammonia (NH3) is sustained by the fact that
favorable temperature and composition distributions which cannot be achieved in any
steady-state regime can be obtained by means of unsteady-state operations.
In a normal way of operation the low exothermicity of the selective catalytic
reduction (SCR) reaction (usually carried out in the range of 280-350°C) is not enough
to maintain by itself the chemical reaction. A normal mode of operation usually requires
supply of supplementary heat increasing in this way the overall process operation cost.
Through forced unsteady-state operation, the main advantage that can be obtained when
exothermic reactions take place is the possibility of trapping, beside the ammonia, the moving heat wave inside the catalytic bed. The unsteady state-operation enables the
exploitation of the thermal storage capacity of the catalyticbed. The catalytic bed acts as
a regenerative heat exchanger allowing auto-thermal behaviour when the adiabatic
temperature rise is low.
Finding the optimum reactor configuration, employing the most suitable operation
model and identifying the reactor behavior are highly important steps in order to
configure a proper device for industrial applications.
The Reverse Flow Reactor (RFR) - a forced unsteady state reactor - corresponds
to the above mentioned characteristics and may be employed as an efficient device for the
treatment of dilute pollutant mixtures. As a main disadvantage, beside its advantages, the
RFR presents the 'wash out' phenomena. This phenomenon represents emissions of
unconverted reactants at every switch of the flow direction. As a consequence our
attention was focused on finding an alternative reactor configuration for RFR which is
not affected by the incontrollable emissions of unconverted reactants. In this respect the
Reactor Network (RN) was investigated. Its configuration consists of several reactors
connected in a closed sequence, simulating a moving bed by changing the reactants
feeding position. In the RN the flow direction is maintained in the same way ensuring
uniformcatalyst exploitation and in the same time the 'wash out' phenomena is
annulated.
The simulated moving bed (SMB) can operate in transient mode giving practically
constant exit concentration and high conversion levels.
The main advantage of the reactor network operation is emphasizedby the
possibility to obtain auto-thermal behavior with nearly uniformcatalyst utilization.
However, the reactor network presents only a small range of switching times which allow
to reach and to maintain an ignited state. Even so a proper study of the complex behavior
of the RN may give the necessary information to overcome all the difficulties that can
appear in the RN operation.
The unsteady-state reactors complexity arises from the fact that these reactor
types are characterized by short contact times and complex interaction between heat and
mass transportphenomena. Such complex interactions can give rise to a remarkable complex dynamic behavior characterized by a set of spatial-temporal patterns, chaotic
changes in concentration and traveling waves of heat or chemical reactivity.
The main efforts of the current research studies concern the improvement of
contact modalities between reactants, the possibility of thermal wave storage inside the
reactor and the improvement of the kinetic activity of the catalyst used. Paying attention
to the above mentioned aspects is important when higher activity even at low feeding
temperatures and low emissions of unconverted reactants are the main operation
concerns. Also, the prediction of the reactor pseudo or steady-state performance
(regarding the conversion, selectivity and thermal behavior) and the dynamicreactor
response during exploitation are important aspects in finding the optimal control strategy
for the forced unsteady state catalytic tubular reactors.
The design of an adapted reactor requires knowledge about the influence of its
operating conditions on the overall process performance and a precise evaluation of the
operating parameters rage for which a sustained dynamic behavior is obtained. An apriori
estimation of the system parameters result in diminution of the computational efforts.
Usually the convergence of unsteady state reactor systems requires integration over
hundreds of cycles depending on the initial guess of the parameter values.
The investigation of various operation models and thermal transfer strategies give
reliable means to obtain recuperative and regenerative devices which are capable to
maintain an auto-thermal behavior in case of low exothermic reactions.
In the present research work a gradual analysis of the SCR of NOx with ammonia
process in forced unsteady-state reactors was realized. The investigation covers the
presentationof the general problematic related to the effect of noxious emissions in the
environment, the analysis of the suitable catalysts types for the process, the mathematical
analysis approach for modeling and finding the system solutions and the experimental
investigation of the device found to be more suitable for the present process.
In order to gain information about the forced unsteady state reactor design,
operation, important system parameters and their values, mathematical description,
mathematicalmethod for solving systems of partial differential equations and other
specific aspects, in a fast and easy way, and a case based reasoning (CBR) approach has
been used. This approach, using the experience of past similarproblems and their adapted solutions, may provide a method for gaining informations and solutions for new problems
related to the forced unsteady state reactors technology. As a consequence a CBR system
was implemented and a corresponding tool was developed.
Further on, grooving up the hypothesis of isothermal operation, the investigation
by means of numerical simulation of the feasibility of the SCR of NOx with ammonia in
the RFRand in the RN with variable feeding position was realized. The hypothesis of
non-isothermal operation was taken into account because in our opinion ifa commercial
catalyst is considered, is not possible to modify the chemical activity and its adsorptive
capacity to improve the operation butis possible to change the operation regime.
In order to identify the most suitable device for the unsteady state reduction of
NOx with ammonia, considering the perspective of recuperative and regenerative
devices, a comparative analysis of the above mentioned two devices performance was
realized.
The assumption of isothermal conditions in the beginningof the forced unsteadystate
investigation allowed the simplification of the analysis enabling to focus on the
impact of the conditions and mode of operation on the dynamic features caused by the
trapping of one reactant in the reactor, without considering the impact of thermal effect
on overall reactor performance.
The non-isothermal system approach has been investigated in order to point out
the important influence of the thermal effect on overall reactor performance, studying the
possibility of RFR and RN utilization as recuperative and regenerative devices and the
possibility of achieving a sustained auto-thermal behavior in case of lowexothermic
reaction of SCR of NOx with ammonia and low temperature gasfeeding.
Beside the influence of the thermal effect, the influence of the principal operating
parameters, as switching time, inlet flow rate and initial catalyst temperature have been
stressed. This analysis is important not only because it allows a comparison between the
two devices and optimisation of the operation, but also the switching time is the main
operating parameter. An appropriate choice of this parameter enables the fulfilment of the
process constraints.
The level of the conversions achieved, the more uniform temperature profiles, the
uniformity ofcatalyst exploitation and the much simpler mode of operation imposed the RN as a much more suitable device for SCR of NOx with ammonia, in usual operation
and also in the perspective of control strategy implementation.
Theoretical simplified models have also been proposed in order to describe the
forced unsteady state reactors performance and to estimate their internal temperature and
concentration profiles. The general idea was to extend the study of catalytic reactor
dynamics taking into account the perspectives that haven't been analyzed yet.
The experimental investigation ofRN revealed a good agreement between the
data obtained by model simulation and the ones obtained experimentally.
Kokoelmat
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