Reactor for temperature moderation

Details Reactor for temperature moderation Reactor for temperature moderation

2002-07-16

2004-09-14

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Hydrogen or compound thereof

Elemental hydrogen

C208S133000, C208S142000, C252S373000, C518S703000

Reexamination Certificate

active

06790431

ABSTRACT:

REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a plurality of chemical reactors arranged to moderate the intensity of a hot spot in at least one of the reactors.
BACKGROUND OF THE INVENTION
Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, a significant amount of natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane, the main component of natural gas, as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into the higher hydrocarbons.
Current industrial use of methane or natural gas as a chemical feedstock proceeds by the initial conversion of the feedstock to carbon monoxide and hydrogen by either steam reforming (the most widespread process), dry reforming, autothermal reforming, or catalytic partial oxidation. Examples of these processes are disclosed in GUNARDSON, HAROLD,
Industrial Gases in Petrochemical Processing
41-80 (1998), incorporated herein by reference. Steam reforming, dry reforming, and catalytic partial oxidation proceed according to the following reactions respectively:
CH
4
+H
2
O⇄CO+3H
2
  (1)
CH
4
+CO
2
⇄2CO+2H
2
  (2)
CH
4
+½O
2
→CO+2H
2
  (3)
Catalytic partial oxidation (CPOX) has recently attracted much attention due to significant inherent advantages, such as the fact that heat is released during the process, in contrast to the endothermic steam and dry reforming processes.
In catalytic partial oxidation, a hydrocarbon feedstock is mixed with an oxygen source, such as air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. When the feedstock comprises primarily methane (e.g., natural gas), the approximately 2:1 H
2
:CO molar ratio achieved by partial oxidation is generally more useful than the H
2
:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch synthesis. An example of a Fischer-Tropsch process is disclosed in U.S. Pat. No. 6,333,294 to Chao et al., incorporated herein by reference.
Unfortunately, the heat production during a CPOX reaction can be a double-edged sword. In a CPOX reactor, in addition to the desirable selective partial oxidation reaction, there often occurs a substantial amount of undesirable reactions such as the non-selective oxidation of methane (e.g. to products other than CO and H
2
, for example, C, CO
2
and H
2
O). Non-selective oxidation is much more exothermic than the desirable selective CPOX reaction (CH
4
+½O
2
→CO+2H
2
) and, therefore, produces much more heat. This heat produced by the desirable partial oxidation reactions and the undesirable non-selective reactions can sometimes have undesirable consequences.
DEUTSCHMANN ET AL.,
Natural Gas Conversion In Monolithic Catalysts: Interaction Of Chemical Reactions And Transport Phenomena,
6th National Gas Conversion Symposium, Girdwood, USA (2001), incorporated herein by reference, predicted that the hot spot in a monolithic syngas reactor, the catalyst surface temperature can be as high as 2000K. This hot spot can cause active phase transformation and/or sinter the catalyst, causing a loss of surface area and, consequently, a loss of catalytic activity. This loss of catalytic activity can, in turn, lead to an increase in the rate of non-selective reaction, causing faster heat liberation and an even quicker deactivation of the catalyst, thus perpetuating a spiral of deactivation. Thus, there is a desire to limit the presence of and moderate the intensity of hot spots in the reaction zone.
An example of a single pass reactor is shown schematically in FIG.
2
. It includes reactor
70
, feedstream
10
, reaction zone
60
, and product stream
30
. Feedstream
10
enters reactor
70
and reacts in reaction zone
60
to form product stream
30
. Preferably, reactor
70
is a syngas reactor. Accordingly, feedstream
10
preferably comprises an oxygen-containing gas such as air, oxygen-enriched air, or substantially pure oxygen, and a hydrocarbon-containing gas such as methane or natural gas. Feedstream
10
is preheated and passed through reaction zone
60
. Reaction zone
60
preferably comprises a supported catalyst comprising rhodium. The feedstream
10
reacts to form product stream
30
which comprises synthesis gas. It is often observed that the temperature profile in, for example, a single pass catalytic partial oxidation reactor such as the reactor of
FIG. 2
, an example of which is shown in the graph of
FIG. 6
, has a temperature spike in the front half of the reaction zone followed by a drop in temperature in the second half of the reaction zone.
As is shown in
FIG. 1
prior attempts to solve the problem of hot spots in the reaction zone have led to reactor designs, such as reactor
40
, in which feed stream
10
enters reactor zone 1 where it reacts exothermically to produce intermediate stream
20
, is redirected to reactor zone 2 where it reacts more mildly, and exits reactor zone 2 as product stream
30
. The extreme temperatures of the front of the reactor zone 1 are moderated by the lower temperature of the same flow stream at the rear of the reactor zone 2. Other examples of reactors in which the flow is folded in on itself is shown in FIG. 3 of REDENIUS ET AL.,
Millisecond Catalytic Wall Reactors: I. Radiant Burner
, AICHE Journal (May 2001) and
FIG. 1
of IOANNIDES AND VERYKIOS,
Development of a Novel Heat
-
Integrated Wall Reactor for the Partial Oxidation of Methane to Synthesis Gas
, Catalysis Today (1998), both references incorporated herein by reference in their entirety. FIG. 3 of REDENIUS shows a reactor configured such that the reactor spirals around itself.
FIG. 1
of IOANNIDES teaches a reactor in which the reactor doubles back against itself and an exothermic combustion catalyst is positioned adjacent an endothermic reforming catalyst, thus causing heat to transfer from the exothermic reaction zone to the endothermic reaction zone. With reference to
FIG. 1
, these types of configurations are disadvantageous because (1) all of the gas passing over reaction zone 1 must pass over reaction zone 2, thus making it impossible to vary the flow over both zones independently and (2) there is a gap between the two reaction zones 1 and 2 where the intermediate gas stream
20
may react non-selectively due to the absence of a catalyst to catalyze the preferred selective reactions. As noted above, the undesirable non-selective oxidation reactions produce more heat than the desired selective oxidation, thus this area of non-selective reaction will not only decrease the yield of the desired products, but also undesirably increase the amount of heat liberation.
SUMMARY OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention run chemical reactions in adjacent reactors to ameliorate and reduce the effects of the hot spots generated

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