Exothermic process

Details Exothermic process Exothermic process

2000-03-22

2001-09-18

Dentz, Bernard (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S523000, C562S476000, C562S545000, C585S654000

Reexamination Certificate

active

06291686

ABSTRACT:

This invention relates to an exothermic process and in particular to an oxidation process. In such reactions a feedstock stream is contacted with an oxidising agent, often in the presence of a catalyst for the reaction, to a give a products stream. As heat is usually evolved during the reaction, cooling means are often provided to effect control of the reaction and/or to cool the products stream.
One method that has been employed for analogous endothermic processes involves passing a reactants stream through a heated body, which may comprise a bed of material exhibiting catalytic activity for the desired reaction. In such a process, the heated body, hereinafter termed a heat transfer bed, provides the heat required for the endothermic reaction and it is periodically necessary to reheat the bed.
Such endothermic reactions may be effected using a plurality of static beds which are cycled, by switching the flows of reactants etc., through reaction and regeneration stages. Alternatively, and preferably, a rotatable heat transfer bed is employed which is rotated through a regeneration zone, then through a reaction zone, and then returned to the regeneration zone.
U.S. Pat. No. 2,704,741 describes a reactor for that type of process wherein a catalyst bed is disposed in compartments in a rotatable annular vessel: the vessel is disposed between, and sealed against, static outer and inner vessels which are divided into compartments. Provision is made for supply of reactants and regeneration gas to appropriate compartments and for receipt of products and spent regeneration gas from other compartments. The reactants flow radially through the annular vessel between the appropriate compartments of the inner and outer vessels.
U.S. Pat. No. A4,418,046 describes a similar arrangement wherein, instead of the catalyst bed being disposed in separate compartments in the rotating body, the catalyst bed is in the form of a honeycomb structure so that the honeycomb walls serve to separate adjacent flow passages: in this reference, depending on the honeycomb configuration, the flow can be radial or axial.
EP-A-0193511 also describes a similar arrangement wherein a honeycomb catalyst bed is employed with the honeycomb having cells providing axial passages and the flow of reactants etc. is axial.
In GB-A-837707 the dehydrogenation of a hydrocarbon feed is described using a chromia-containing catalyst. It is indicated that during an oxidative regeneration stage, part of the chromia is oxidised from the trivalent state Cr
2
O
3
to the hexavalent state CrO
3
. Upon contact of this oxidised catalyst with the feedstock, part of the feedstock is oxidised with the chromia reverting back to the trivalent state, and the heat evolved is used to supply part of the heat required for the enotherrmc dehydrogenation reaction.
We have realised that this principle may advantageously be combined with the aforesaid rortating bed principle and applied to exothermic processes such as oxidation reactions. Where the reaction is catalytic, normally the oxidising agent is a fluid which is mixed with the feedstock prior to contact with the catalyst, or while the feedstock is in contact with the catalyst. However in the present invention, the oxidising agent is in the solid state in the form of a material having upper and lower oxidation states: in a regeneration stage the material, which may also serve as a catalyst for the reaction, is oxidised to the higher oxidation state while in the reaction stage the material is reduced to its lower oxidation state. By providing the material as a rotating bed, various advantages accrue. Thus high heat and mass transfer coeffcients can be achieved, the bed can have a high heat capacity, the amount of catalyst required can be minimised and attrition of the catalyst is minimised. Plug flow operation can be achieved and the associated plant requirements are small.
Accordingly the present invention provides a process for performing an exothermic oxidation reaction using a moving bed containing a solid convertible material that can be oxidised from a lower oxidation state to a higher oxidation state, said moving bed being in the form of a rotatable member having a multiplicity of through flow passages, the walls of which are formed from, or are coated with, or which passages contain, said convertible material, said process comprising:
a) continuously rotating said rotatable member about its axis, whereby each flow passage is moved through a succession of zones including an oxidative regeneration zone, a reaction zone, and then returned to the oxidative regeneration zone;
b) passing an oxidising fluid stream through said oxidative regeneration zone, whereby the convertible material that is in said oxidative regeneration zone is oxidised from said lower oxidation state to said higher oxidation state;
c) passing feedstock through said reaction zone, whereby said feedstock is reacted to give a products stream with the concurrent reduction of the convertible material to its lower oxidation state.
The invention is particularly suited to selective oxidation processes such as the selective oxidation of propene to propene oxide, n-butane to maleic anhydride, isobutene to methacrylic acid or methacrolein, propane to propene, n-butane to n-butene or butadiene, and methane coupling to give ethylene and acetylenes, as by the selection of a suitable convertible material the oxidation can be made more selective, thus reducing the amount of byproducts formed.
Suitable convertible materials include elements exhibiting variable valency, e.g. elements of Groups IIIa. IVa, Va, VIa, VIIa, VIII, Ib, and IIb of the Periodic Table and oxides of such elements. As mentioned previously, the convertible material may be selected to have catalytic activity for the desired reaction. As described in the literature, convertible materials acting as catalysts for the above reactions are often selected from molybdenum- and/or vanadium-containing acids, e.g. heteropolyacids, or salts thereof the acid, or salt, may also contain phosphorus, or is used in admixture with phosphorus compounds. The convertible material may form the sole solid material of the bed, or it may be supported on a suitable substrate.
For any given reaction, the convertible material should be chosen so that the desired reaction reduces the convertible material from its higher oxidation state to the lower oxidation state and the regeneration fluid, and regeneration conditions, selected so that the convertible material is oxidised from the lower oxidation state produced by the aforesaid reaction to the higher oxidation state required for the reaction. It is preferred to select the convertible material such that the regeneration can be effected with air.
Both the regeneration and reaction stages usually produce heat often it is undesired that the products stream carries away all the heat produced as this could involve the production of a products stream at a temperature well above that of the feedstock. It is therefore preferred that the cycle also includes a cooling stage wherein a cooling fluid is passed through the bed containing the convertible material to remove heat produced in said oxidation reactions. Although in some cases the regeneration stage may be effected at a lower temperature than that of the desired reaction, in which case it may be possible to remove the heat as sensible heat in the spent regeneration fluid, generally the regeneration stage will be effected at a temperature above the desired reaction temperature and so a cooling stage is preferably effected before the reaction stage. Where air, or a fluid of higher oxidising power than air, is used as the regeneration fluid, the cooling may be effected by passing air through the bed after the convertible material has been fully oxidised to its higher oxidation state in the regeneration stage. Altematively an inert cooling fluid may be employed. This is desirable where contamination of the reaction products with an oxidising gas, such as air, is undesirable and/or where the presence of such an

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