Catalytic Alkane Dehydrogenation (ExxonMobil)

CATEGORY: ALKANE DEHYDROGENATION
Catalytic Alkane Dehydrogenation (ExxonMobil)
United States Patent Application 20160318828
Washburn; Seth M. ;   et al.   November 3, 2016
Applicant: ExxonMobil Chemical Patents Inc.
Abstract
The invention relates to catalytic alkane dehydrogenation, to olefin produced by catalytic alkane dehydrogenation, and to processes, compositions, process configurations, equipment, and systems useful for carrying out catalytic alkane dehydrogenation. The catalytic alkane dehydrogenation is carried out in a substantially-isothermal reaction zone, which includes at least one active material having catalytic alkane dehydrogenation activity.
FIELD OF THE INVENTION
[0002] The invention relates to catalytic alkane dehydrogenation, and more particularly to olefin produced by catalytic alkane dehydrogenation. The invention encompasses processes, compositions, process configurations, equipment, and systems useful for carrying out catalytic alkane dehydrogenation. The catalytic alkane dehydrogenation is carried out in a substantially-isothermal reaction zone, which includes at least one active material having catalytic alkane dehydrogenation activity.
BACKGROUND OF THE INVENTION
[0003] Olefin, also called alkene, is a class of unsaturated hydrocarbon compounds containing at least one pair of carbon atoms, the carbon atoms of the pair being directly linked by a double bond. Since the double-bonded carbons allow the compounds to be reacted with a wide variety of other compounds to produce a wide array of useful products, olefin compounds are of considerable commercial importance.
[0004] Olefin is produced commercially by a variety of processes, e.g., steam cracking and fluid catalytic cracking of hydrocarbons. Since these processes produce olefin having a range of molecular weights, and typically also other (non-olefin) products, technologies such as catalytic dehydrogenation technologies have been developed to yield a particular olefin-an olefin having a particular number of carbon atoms. Catalytic hydrogenation involves catalytically reacting (i.e., dehydrogenating) a saturated hydrocarbon compound such as alkane to produce the desired olefin. For example, catalytic dehydrogenation technologies include those which catalytically react propane to produce propene.
[0005] Jesper et al., Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides, Chem. Rev., 2014, 114 (20), pp 10613-10653, describe a number of commercial dehydrogenation processes. Such processes include the Catofin Process (Lummus), Oleflex Process (UOP), Steam Active Reforming (STAR) Procss (Uhde), Fluidized Bed Dehydrogenation (FBD) Process (Snamprogetti and Yarsintez) and Linde-BASF Isothermal Fixed Bed Process.
[0006] Some of these are fixed-bed processes. For example, Catofin, which is based on the Houdry Catadiene process, for isobutane dehydrogenation to isobutene, dehydrogenates propane in 5-8 parallel adiabatic fixed bed reactors containing a chromia-alumina catalyst. Process conditions include a temperature of approximately 575.degree. C. and a pressure between 0.2 and 0.5 bar. Another fixed-bed process, STAR, operates at a pressure of 6 to 9 bar and a temperature of between 500.degree. C. to 600.degree. C. Steam is added to the alkane feed to reduce alkane partial pressure, resulting in less coke formation. The feed is conducted to a first reactor, which contains a catalyst comprising Pt--Sn supported on a (basic) zinc-aluminate. A calcium/magnesium-aluminate binder is used to stabilize the catalyst in the presence of the steam. The gas mixture exiting the first reactor is cooled prior to being introduced into the second reactor (called an oxyreactor), where an oxygen-steam mixture is used to selectively combust part of the hydrogen formed during the dehydrogenation. Combusting the hydrogen shifts equilibrium toward higher olefin yields. Like STAR, The Linde-BASF Dehydrogenation Process is a fixed bed reactor process, using an alkane feed diluted with steam. The catalyst comprises Pt--Sn supported on ZrO.sub.2. The process is carried out isothermally at a temperature of 590.degree. C. Isothermal conditions are maintained by externally heating the reactor.
[0007] Other processes utilize a fluidized bed. For example, Oleflex utilizes Pt--Sn-based catalyst in a fluidized bed operating at pressures between 1 and 3 bar and a temperature of 525.degree. C. to 705.degree. C. Polymer-grade propylene from the reaction product by recovering C.sub.3 hydrocarbon in a de-ethanizer, and then separating propylene from unreacted propane in a splitter. Another fluidized-bed process, FBD, incorporates the use of a fluid catalytic cracking reactor system. The alkane feed flows through a staged fluidized bed reactor, contacting the alkane with heated CrO.sub.x/Al.sub.2O.sub.3 catalyst, promoted with an alkali metal. The alkane dehydrogenation is carried out at a pressure of 1.1 to 1.5 bar and a temperature of 550.degree. C. to 600.degree. C. Carbon deposits formed on the catalyst during the dehydrogenation, resulting in deactivated catalyst. The deactivated catalyst is transported to a regenerator connected to the reactor to combust the carbon deposits, reactivating the catalyst for reuse. Fuel gas is added to the regenerator to provide sufficient heat to carry out regeneration. The regenerated catalyst is returned to the fluidized bed reactor at a temperature of around 560.degree. C., which is sufficient to carry out the desired dehydrogenation reaction.
[0008] Since alkane catalytic dehydrogenation is an equilibrium reaction, process conditions affect the amount of olefin that can be produced. An example of such a reaction is the dehydrogenation of propane to produce propene:
C.sub.3H.sub.8.revreaction.C.sub.3H.sub.6+H.sub.2(.DELTA.H.sup.0.sub.298- =124.3 kJ mol.sup.-1)
[0009] The above reaction shows that heat (124.3 kJ) is required to dehydrogenate propane, which means that the reaction is an endothermic reaction. Propylene is the desired product of the reaction, with hydrogen being an undesirable byproduct--the presence of hydrogen in the reaction product results in converting propylene back to propane. Typically, dehydrogenation of C.sub.2-C.sub.4 alkane to produce C.sub.2-C.sub.4 olefin requires a reaction temperature in the range of 550.degree. C. to 750.degree. C., with the conversion of the alkane being about 50% at about 1 bar.
[0010] According to Le Chatelier's principle, higher conversions of alkane to olefin can be obtained by increasing the reaction temperature and/or reducing the hydrogen gas (molecular hydrogen) partial pressure. However, raising the temperature can result in additional undesirable byproducts being produced through one or more undesired hydrogenolysis, cracking and isomerization side reactions, e.g., increased feed cracking to methane, etc.
[0011] Molecular hydrogen partial pressure can be lessened by removing hydrogen molecules produced by the dehydrogenation reaction through selective hydrogen combustion (SHC). One way to do this is disclosed in de Graaf et al., Two-Step Catalytic Oxidative Dehydrogenation of Propane: An Alternative Route to Propene, Vol. 9, No. 4, 397-403, 2005, Organic Process Research & Development. This reference discloses selectively combusting molecular hydrogen produced during the dehydrogenation reaction using oxygen removed from a solid oxygen carrier (a "SOC"; also called an oxygen storage material). The SOC is mixed with the dehydrogenation catalyst. During the combustion, oxygen from the SOC reacts with the molecular hydrogen from the dehydrogenation to produce water.
[0012] SOCs are typically porous, having a pore size which is large enough to admit molecular hydrogen, but small enough to exclude the relatively large alkane and olefin molecules. At the start of the dehydrogenation, the SOC is in a state identified as "SO.sub.xC", indicating that oxygen is available for removal from the SOC. During the dehydrogenation, molecular hydrogen produced by that reaction enters the SOC's pores, where it combusts with the SOC's available oxygen. Since molecular hydrogen produced in the dehydrogenation reaction can more readily migrate into the SOC than can alkane and olefin, equilibrium of the dehydrogenation reaction shifts toward increased olefin production and away from olefin hydrogenation, alkane oxidation, and olefin oxidation. Once the oxygen available in the SOC for combustion is depleted, the SOC will be in a reduced state identified conceptually as "SO.sub.x-1C". The alkane feed is then stopped, the SOC is re-oxidized from SO.sub.x-1C to an oxidized state that is conceptually identified as "SO.sub.xC," and the process is repeated. The reduction and oxidation of the SOC are conceptually exemplified by the following equations:
H.sub.2+SO.sub.xC.fwdarw.H.sub.2O+SO.sub.x-1C (Reduction)
O.sub.2+2SO.sub.x-1C.fwdarw.2SO.sub.xC (Oxidation)
[0013] Although the de Graaf alkane dehydrogenation process is efficient to the extent that heat for the dehydrogenation reaction is supplied from the highly exothermic combustion of molecular hydrogen, excessive heat is produced in the reactor. At the same time, selectivity toward olefin production remains undesirably low.
[0014] What is desired is an alternative process to convert alkane to desired olefin product. Processes for converting alkane to a desired olefin at high conversion of the alkane and high selectivity to the desired olefin are particularly desired.
SUMMARY OF THE INVENTION
[0015] This invention provides an alternative process to convert alkane to desired olefin product. In particular, this invention provides a catalytic process for dehydrogenating alkane to a desired olefin, with the process being carried out at high conversion of the alkane and high selectivity to the desired olefin. The catalytic dehydrogenation process is particularly desirable for converting alkanes such as C.sub.2+ alkanes into C.sub.2+ olefin, particularly C.sub.3+ alkane into C.sub.3+ olefin. In particular, the process is effective for converting ethane to ethylene, propane to propylene, and butane to butenes.
[0016] More particularly, the invention relates to a process for catalytically producing olefin from C.sub.2+ alkane. The process is carried out in a substantially isothermal reaction zone having an average temperature in the range of from 400.degree. C. to 700.degree. C. At least one active material is located in the reaction zone, the active material having catalytic alkane dehydrogenation functionality at the average temperature.
[0017] At the start of the process, flow of alkane-containing feed comprising C.sub.2+ alkane is established into the reaction zone. At least a portion of the C.sub.2+ alkane is dehydrogenated to produce a dehydrogenation product comprising olefin and molecular hydrogen. The dehydrogenating is carried out in the reaction zone in the presence of at least a portion of the active material. At least a portion of the dehydrogenation product's molecular hydrogen is combusted in the reaction zone during the catalytic dehydrogenation to produce a combustion product comprising water. The reaction zone is maintained substantially isothermal at the average temperature during the dehydrogenation and combustion. A reaction product is conducted away from the reaction zone, the reaction product comprising at least a portion of the dehydrogenation product's olefin and at last a portion of the combustion product's water.
[0018] One way the process differs from conventional processes is that a substantially isothermal temperature profile is maintained in the reaction zone during the dehydrogenation/combustion. Doing so significantly increases the process's selectivity to olefin production, and significantly decreases selectivity to undesirable products like methane and catalyst coke.
[0019] In another aspect, the invention relates to an apparatus for catalytically dehydrogenating an alkane-containing feed to produce an olefin-containing product. The apparatus comprises a reactor vessel having an interior volume, and at least one reaction zone within the reactor vessel's interior volume. The reaction zone includes at least one bed of active material comprising dehydrogenation catalyst. At least one inlet conduit in fluidic communication with the reactor vessel's interior volume is used for conveying the alkane containing feed into the reaction zone. The apparatus further comprises at least one heat transfer conduit for bringing a heat transfer fluid into indirect thermal contact with the reaction zone. The heat transfer conduit is substantially closed to flow of the heat transfer fluid into the reactor vessel's interior volume, and is substantially open to the flow of heat between the reaction zone and the heat transfer fluid. The heat transfer conduit is configured to maintain the reaction zone in a substantially isothermal temperature profile at an average temperature 400.degree. C. to 700.degree. C. during the catalytic dehydrogenation. The apparatus further includes at least one outlet conduit in fluidic communication with the reactor vessel's interior volume for conveying the olefin containing product away from reaction zone. In another aspect, the invention relates to a system for carrying out the catalytic dehydrogenation process in the apparatus.
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