Catalytic partial oxidation — particulate catalysts

Details Catalytic partial oxidation — particulate catalysts Catalytic partial oxidation — particulate catalysts

2001-02-23

2002-09-24

Parsa, Jafar (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

C518S700000, C518S702000, C518S703000, C252S373000

Reexamination Certificate

active

06455597

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a catalytic partial oxidation process wherein a light hydrocarbon, e.g., methane, is converted to synthesis gas, carbon monoxide and hydrogen. More particularly, this invention relates to a particular particulate catalyst for the catalytic partial oxidation process.
BACKGROUND OF THE INVENTION
Catalytic partial oxidation is a known process herein a light hydrocarbon, for example, a C
1
-C
4
alkane or hydrocarbon, or more likely methane, as may be in or obtained from natural gas is converted catalytically in the presence of an oxygen containing stream to synthesis gas. The following stoichiometric equation exemplifies the reaction:
CH
4
+½
O
2
→2
H
2
+CO
The reaction is particularly attractive in gas to liquids projects wherein natural gas in converted to synthesis gas, and the synthesis gas is converted to heavy hydrocarbons, C2+, via the Fischer-Tropsch process. Because the stoichiometric reactant ratio for the Fischer-Tropsch process with non-shifting catalysts is about 2.1/1, the synthesis gas produced by catalytic partial oxidation is a particularly valuable feed for the Fischer-Tropsch process.
The catalytic partial oxidation process has been reported in a number of recently published patent applications, e.g., EP 0 576 096 A2. Nevertheless, there is a desire to improve both the yield and selectivity of the process, particularly regarding hydrogen selectivity, and thereby further the commercial prospects for the process.
SUMMARY OF THE INVENTION
The invention may be exemplified by a catalytic partial oxidation process comprising the reaction of a light hydrocarbon, e.g., a C
1
-C
4
alkyl, preferably methane, by itself, or as a component of natural gas, with oxygen in the presence of a supported, Group VIII noble or non-noble metal catalyst, the support comprising particulate solids of a particular size range.
The prior art in the area of catalytic partial oxidation does not suggest that a particular particle size range exists in which the process becomes highly efficient with regard to hydrogen selectivity. The prior art suggests that while particulates may be used din the catalytic partial oxidation process, it is preferred to use monoliths or foams as the catalyst support. The reasoning being that foams, for example, readily conduct heat because of their bi-continuous structure, whereas particles only conduct heat at narrow point contacts, and therefore, are thought to have a lower overall axial thermal conductivity than foams or monoliths.
Nevertheless, the efficiency of the process and an aspect of this invention is the adequate management of both the heat conductivity, or heat flux along the catalyst bed and the number of active catalytic sites that can be placed on the external surface of the support per unit volume of the reactor.
Due to their large porosity, catalytic support materials, such as foams and monoliths have relatively poorer axial and radial heat conductivity than smaller particulates. Also, the number of active catalytic sites that can be placed on a support is proportional to the surface to volume ratio (S/V), i.e., the external, geometric surface to volume ratio of a granular material, excluding the intra particle surface area, and therefore, smaller and smaller particles would seem to be preferred. However, very small particles do not lend themselves to good heat conductivity in the bed, heat being transferred by point contacts and by radiation along in the catalyst bed. Consequently, there is a need to balance the competing aspects of the overall axial thermal conductivity of the bed with the surface density of catalytic sites in order to achieve the necessary process efficiency.
The surface area to volume (S/V) ratio (also referred to as the geometric surface to volume ratio) of a catalytic bed, e.g., a packed bed, can be readily determined from a knowledge of the particle size and the porosity of the particulate bed. For example, the S/V ratio of a packed bed of spherical particles or particles that can be assumed to be, or described as spherical, can be described by S/V=3(1−Ø)/r
p
; where Ø is the porosity of the bed and r
p
is the particle radius. Because packed beds can contain a range of particle sizes, the particle radius can be selected as the radius of the average particle size (i.e., volumetric average particle size). For packed beds, Ø ranges from about 0.3 to about 0.5 and the preferred surface to volume ratio is about 15-230 cm
−1
. However, a more preferred surface to volume range is 18-140 cm
−1
, more preferably 18-105 cm
−1
.
Preferred particles have a diameter ranging from about 200-2000 microns, more preferably about 400-1600 microns, and still more preferably about 400-1200 microns. The particles may be spherical or other shapes which can be described or approximated by a diameter and are generally described as being granular.
Thus, the process balances surface to area ratio, as that ratio has been defined, with the overall bed (or packing) thermal conductivity which is described in D. Kunii and J. M. Smith, AlChE J.
6
(1), p. 71-78 (1960), incorporated herein by reference.
The catalyst support is generally a difficult to reduce refractory metal oxide, such as alumina, particularly alpha alumina, zirconia, titania, hafnia, silica, silica-alumina; rare earth modified refractory metal oxides, where the rare earth may be any rare earth metal, e.g., lanthanum, yttrium; alkali earth metal modified refractory metal oxides; and these materials may be generally categorized as materials having a substantially stable surface area at reaction conditions, for example, a surface area that is not substantially altered by reaction conditions, or altered in any way that affects the reaction.
Because thermal conductivity is one of the competing elements for the nature of the catalyst support, the support having better thermal conductivity can be used in the form of smaller average particle sizes. Nevertheless, the S/V ratios given above generally take this element into account and apply for all preferred supports, i.e., zirconia and alpha alumina, particularly preferred being alpha alumina, and rare earth stabilized alumina. The preferred support particles generally have a low total surface area, e.g., <20 m
2
/gm, and microporosity is not important to the process.
The catalytic metal is preferably a Group VIII noble metal, e.g., platinum, iridium, rhodium, osmium, ruthenium, although nickel may also be used as the catalytic metal. Rhodium, however, is most preferred as the catalytic metal.
The hydrocarbon feed is preferably a light alkane, e.g., C
1
-C
4
, most preferably methane or a gas containing substantial amounts of methane, e.g., natural gas.
The oxygen used in the catalytic partial oxidation process may be pure or substantially pure oxygen or an oxygen containing gas, e.g., air, or a mixture of oxygen with an inert gas. Substantially pure oxygen is preferred, and pure oxygen is still more preferred. Optionally, either the hydrocarbon feed or the oxygen stream, or both, may be mixed with steam. When steam is present, the steam to carbon ratio may be about 0 to 2.5, preferably about 0.2 to 1.5.
The ratio of hydrocarbon feed to oxygen in the reaction zone may range from about 0.45 to about 0.75 oxygen to carbon ratio, more preferably 0.45-0.55. There may be some carbon dioxide in the feed, as for example, from recycle gases or as a diluent. Generally, however, virtually no CO
2
is consumed, e.g., CO
2
conversion in the catalyst bed is less than about 10%, preferably less than about 5%. Consequently, there is essentially no synthesis gas formation via CO
2
reforming.
In an embodiment of the invention reaction temperature is achieved quickly at the inlet of the catalyst bed for best results. In a preferred embodiment, the process takes place in a thin reaction zone, e.g., at high (reaction) temperatures, preferably ≦5 particle diameters from the bed inlet, more preferably ≦3 particle diameters from the bed inlet. In t

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