Catalytic partial oxidation with improved ignition system

Details Catalytic partial oxidation with improved ignition system Catalytic partial oxidation with improved ignition system

2000-04-07

2001-12-11

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

With preliminary reaction to form hydrogen or a carbon oxide

Gaseous oxygen utilized in the preliminary reaction

C518S700000, C518S702000, C585S734000, C208S133000, C208S134000, C252S373000

Reexamination Certificate

active

06329434

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an H
2
/O
2
ignition system for igniting a catalytic partial oxidation (CPO) catalyst bed, preferably a CPO bed utilized for the production of synthesis gas.
BACKGROUND OF THE INVENTION
Catalytic partial oxidation (CPO) is a well known process utilized to produce synthesis gas from methane and oxygen. The CPO process seeks to eliminate gas phase partial oxidation reactions and utilizes a highly active Group VIII metal catalyst at high rate or low dwell time such that such reactions do not occur. Such processes require a special ignition means to heat the catalyst bed to a temperature at which ignition occurs, without heating the catalyst bed at such a rapid rate that the bed itself would be destroyed by thermal stresses. Conventional light-off procedures, such as a preheating torch or burner, utilized in other syngas generation systems are not practical for the CPO process. The reactor is too small to accommodate such equipment, and its design prevents the functioning of a torch. Therefore, what is needed in the art is a reactor lighting process which is capable of igniting the CPO catalyst bed without thermally stressing the bed.
SUMMARY OF THE INVENTION
The instant invention is directed to a catalytic partial oxidation (CPO) process with improved ignition comprising:
(a) igniting an ignition feed comprising hydrogen, diluent and oxygen in a catalytic partial oxidation catalyst bed wherein said ignition feed has a predetermined adiabatic reaction temperature sufficient to cause said catalyst bed to ignite in a manner which prevents said catalyst bed from undergoing thermal shock,
(b) modifying said ignition feed following said ignition of said catalyst bed to obtain a reaction feed comprising oxygen and hydrocarbon-reactant in a molar ratio capable of producing partial oxidation products in said catalyst bed under partial oxidation conditions, wherein said modification of said ignition feed is conducted to accomplish a predetermined heatup rate of said catalyst bed, and wherein the amount of diluent present during said modification is sufficient to control the adiabatic reaction temperature.
BRIEF DESCRIPTION OF THE FIGURES
The FIGURE depicts a time/temperature flow rate graph illustrating the controlled variation in the temperature of a monolith heated by the combustion of gas mixtures which are varied gradually from low temperature start up mixtures to higher temperature syngas forming mixtures. The “X” axis is time, in hours:minutes. The left “Y” axis is temperature in degrees centigrade. The right “Y” axis is flow rates in units of standard cubic feet per minute (SCFM). Curve A is the flow rate for 10% H2 in N2, which has already been started at a time before the 16:35 beginning of this plot. Curve D is the temperature measured at a location downstream of the monolith. At a time of about 16:36, the oxygen (Curve B) is introduced. Curve C is methane added at 16:40. At a time around 16:52, the remaining H2/N2 is removed.
DETAILED DESCRIPTION OF THE INVENTION
CPO systems require a special means to light-off the catalyst bed due to the size constraints of such systems. Additionally, explosive mixtures can form and components within the reactor can be destroyed if the temperature is not closely controlled. Applicants have addressed all of these problems by utilizing an oxygen and hydrogen feed, with a diluent, to both ignite the catalyst bed of a CPO system, and control heatup in a manner that will not cause thermal stresses to the system. The instant light-off process affords several benefits. It requires no modification to existing CPO systems. It allows for easy control of the heat up rate of the system, thereby reducing thermal shock, and it allows for a smooth switching to the CPO product generating feed (herein called the reaction feed).
The modification of the ignition feed to form a reaction feed described in step (b) of the instant process may be conducted in a number of ways. Each way should be readily determinable by the skilled artisan depending on the starting composition of the ignition feed and the constraints of the CPO system utilized. The goal is to have controlled heat up of the system. The term reaction feed refers to the steady-state process feed that is reacted to produce CPO products. The term hydrocarbon-reactant is used herein to refer to the hydrocarbon-containing portion of the reaction feed.
Typically, the hydrogen will either be replaced or reduced. The diluent, depending on what it is may likewise be replaced or reduced. Oxygen may likewise, be increased. For example, in an ignition feed where hydrocarbon, e.g., methane, is used as the diluent in a sufficient quantity to form CPO products following ignition of the catalyst bed, the hydrogen would be reduced and the methane and oxygen would remain to form the reaction feed. If additional hydrocarbon and/or oxygen were required for the reaction feed, it would be added following ignition of the catalyst bed. If the diluent is other than hydrocarbon, the hydrogen and diluent would be replaced with sufficient hydrocarbon and additional oxygen, if needed, to form the reaction feed. It is not necessary to remove all of the hydrogen and diluent to run the instant process as some of the examples show. All that is necessary is that sufficient diluent be present during the modification of the ignition feed to achieve controlled heatup of the catalyst bed and that the ignition feed be modified to form a reaction feed capable of producing CPO products under CPO conditions. The transition to the reaction feed will be conducted in such a manner that the heatup of the system is controlled and does not thermally stress the system.
As used herein, thermal stress on the system occurs when any component in the system (e.g., insulation, monolith, etc.) becomes damaged due to excessive heatup rate. The point at which such thermal stress will occur is typically available from vendors of the monolith, etc.
In one embodiment, a stream comprising hydrogen and a separate stream comprising oxygen are fed to the reactor. Alternatively, the hydrogen and oxygen streams can be premixed to obtain a homogeneous stream. In either case, the oxygen and hydrogen will be a homogeneous mixture upon entering the catalyst bed. Preferably, the feeds will enter the reactor separately and be mixed prior to entering the catalyst bed. It is preferable to precondition the catalyst bed by flowing the stream comprising hydrogen over the catalyst bed prior to beginning the flow of the stream comprising oxygen, the combination of which two streams with diluent create the ignition feed. Typically, the streams will be preheated to a temperature of about 25 to about 600° C., preferably about 50 to about 300° C. The flow of oxygen results in instant ignition of the hydrogen on the catalyst bed. The ratio of H
2
:O
2
in the ignition feed is not limiting, but it will typically be >2. Preferably, the ratio of H
2
:O
2
in the ignition feed during ignition will be about 0.5 to about 20. Preferably, an equivalence ratio (&PHgr;), of >1, taking into consideration all combustibles in the ignition feed, will be utilized. Equivalence ratio is defined as the fuel/oxidant ratio in use, divided by the fuel/oxidant ratio at full combustion (i.e. perfect stoichiometry for CO
2
and H
2
O formation).
While the present application is directed toward catalytic partial oxidation, in which &PHgr;>1, it will be appreciated that one skilled in the art could adapt these methods to catalytic combustion systems that operate at fuel-lean equivalence ratios (&PHgr;<1).
The streams comprise hydrogen and oxygen, respectively, and each may additionally contain diluent. Diluents may be selected from multi-atomic gases, mono-atomic gases, or mixtures thereof. Multi-atomic gases are preferable.
The multi-atomic gases which are utilizable herein include, but are not limited to hydrogen, nitrogen, steam, methane, carbon monoxide, carbon dioxide, and higher hydrocarbons, e.g., ethane, propane, butane, etc., alcohols,

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