Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1996-04-08
2001-11-13
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S652000, C423S659000
Reexamination Certificate
active
06315973
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is a cyclic process for operating equilibrium controlled reactions in a system which comprises a plurality of reactors operated in a predetermined timed sequence wherein the heating and cooling requirements in a moving reaction mass transfer zone within each reactor are provided by indirect heat exchange with a fluid capable of phase change at temperatures maintained in each reactor during sorpreaction, depressurization, purging and pressurization steps.
BACKGROUND OF THE INVENTION
The chemical industry performs numerous equilibrium controlled reactions to manufacture a wide range of chemical raw materials, intermediates and products. Product yield obtained in such equilibrium controlled reactions is typically limited by the thermodynamic equilibrium of the reaction. Therefore, such reactions are typically operated at an elevated temperature for endothermic reactions or at a reduced temperature for exothermic reactions in order to shift equilibrium toward the product direction. Thus, the chemical industry has been searching for improved processes for operating equilibrium controlled reactions at reduced temperatures for endothermic reactions wherein product yield is not substantially diminished due to unfavorable thermodynamic equilibrium constants.
Representative equilibrium controlled reactions include methane and hydrocarbon steam reforming reactions which are used to manufacture hydrogen or synthesis gas, the water gas shift reaction for converting CO to CO
2
, as well as the reverse water gas shift reaction for converting CO
2
to CO. Some of these reactions are typically carried out at relatively high temperatures to shift the equilibrium toward the product direction as well as to obtain relatively faster reaction kinetics. Significant efforts have been described in the literature to improve reaction kinetics by identifying new catalysts and by controlling process operating conditions. Additionally, the concept of removing a product from a reaction zone to increase product conversion is well known.
Representative processes for operating equilibrium controlled reactions include an article by Vaporciyan and Kadlec (AlChE Journal, Vol. 33, No. 8, August 1987) which discloses a unit operation comprising a rapid pressure swing cycle in a catalytic-adsorbent bed to effect both continuous gas-phase reaction and separation. The hybrid device combines features of a pressure swing adsorber with those of a flow-forced catalytic reactor.
Westerterp and coworkers (Hydrocarbon Processing) p. 69 (November 1988) disclose two process schemes for improving conversion of hydrogen and carbon monoxide to methanol. The first embodiment employs a Gas-Solid-Solid Trickle Flow Reactor (GSSTFR) wherein a solid adsorbent is trickled through a packed bed reactor to remove methanol from the reaction zone which results in increased production of methanol. The adsorbent saturated with methanol is collected on a continuous basis using multiple storage tanks wherein the methanol is desorbed by reducing the pressure. The second embodiment employs a Reactor System with Interstage Product Removal (RSIPR) wherein methanol is synthesized in several stages and removed utilizing a liquid solvent. High conversion of methanol per pass is achieved in a series of adiabatic or isothermal fixed bed reactors. Product is selectively removed in absorbers situated between the respective reactor stages.
Prior art processes for conducting simultaneous reaction and adsorption steps have not achieved commercial success because product flow rates do not remain sufficiently constant and the desired products are present in unacceptably low concentrations with respect to the undesired reaction products, unreacted feedstock and purge fluids. Industry is searching for a process for operating equilibrium controlled reactions which can be operated in continuous mode at reduced reaction temperatures wherein a reaction product can be produced in substantially pure form at high conversion, under relatively constant flow rate and at feedstock pressure.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a continuous process for operating equilibrium controlled reactions which overcomes problems associated with prior art processes wherein product flow rates do not remain relatively constant and the desired product is present in unacceptably low concentrations with respect to other reaction products, unreacted feedstock as well as the purge and rinse fluids used to desorb the desired product from the adsorbent residing in the reactor.
Applicants have overcome such problems by developing a cyclic process, operated under isothermal conditions, which utilizes a plurality of reactors operated in a predetermined timed sequence wherein the heating and cooling requirements in a moving reaction mass transfer zone within each reactor are provided by indirect heat exchange with a fluid capable of phase change at temperatures maintained in each reactor during sorpreaction, depressurization, purging and pressurization steps. The following five steps of the general embodiment are performed in each reactor during a process cycle.
The first step of the process comprises reacting a feedstock at a first pressure in a first reactor containing an admixture of an adsorbent and a catalyst suitable for conducting the equilibrium controlled reaction under reaction conditions sufficient to convert the feedstock into a more adsorbable product which is selectively adsorbed by the adsorbent and a less adsorbable product and withdrawing a stream which is enriched in the less adsorbable product and depleted in the more adsorbable product as well as unreacted feedstock.
The second step comprises countercurrently depressurizing the first reactor to a second pressure by withdrawing a mixture comprising unreacted feedstock, a portion of the less adsorbable product and a portion of the more adsorbable product.
The third step comprises countercurrently purging the first reactor at the second pressure with a weakly adsorbing purge fluid with respect to the adsorbent to desorb the more adsorbable product from the adsorbent and withdrawing a mixture comprising unreacted feedstock, a portion of the more adsorbable product and a portion of the less adsorbable product.
The fourth step comprises countercurrently purging the first reactor at the second pressure with the less adsorbable product to desorb the weakly adsorbing purge fluid and withdrawing a mixture comprising the weakly adsorbing purge fluid, a portion of the more adsorbable product and a portion of the less adsorbable product.
The fifth step comprises countercurrently pressurizing the first reactor from the second pressure to the first pressure with the less adsorbable product prior to commencing another process cycle within the first reactor.
The general embodiment can be readily adapted to utilize the following additional step following the first step and prior to the second step wherein the first reactor is countercurrently purged at the first pressure with a weakly adsorbing purge fluid and a mixture comprising unreacted feedstock, a portion of the more adsorbable product and a portion of the less adsorbable product is withdrawn from the first reactor at the first pressure. Optionally, this mixture comprising the unreacted feedstock, the more adsorbable product and the less adsorbable product can be separated to form a stream comprising unreacted feedstock and the unreacted feedstock can be recycled for use as feedstock in the first step of the process.
Applicants' process can be readily adapted to perform a variety of additional steps in order to further separate the process streams by conventional methods such as distillation to yield higher purity products or a source process fluids which may be recycled. For example, the stream of the first step which is enriched in the less adsorbable product and depleted in the more adsorbable product as well as unreacted feedstock can be separated to form a stream comprising the less adsorbable produ
Brzozowski Jeffrey Richard
Carvill Brian Thomas
Gaffney Thomas Richard
Hufton Jeffrey Raymond
Mayorga Steven Gerard
Air Products and Chemicals Inc.
Gourley Keith D.
Hendrickson Stuart L.
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