Cyclic autothermal hydrocarbon reforming process

Compositions – Gaseous compositions – Carbon-oxide and hydrogen containing

Reexamination Certificate

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C423S418200, C423S648100

Reexamination Certificate

active

06761838

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the partial oxidation and/or reforming of hydrocarbons, and more particularly to the production of hydrogen and carbon monoxide by the partial oxidation of hydrocarbons, steam reforming of hydrocarbons or a combination of the two to achieve an auto-thermal process. Specifically, the invention relates to the use of an oxygen ion conducting ceramic in particulate form in a cyclic process, involving the reaction of oxygen in air feed with the ceramic in one step and the reaction of hydrocarbon feed, with or without steam, with the above oxygen-enriched ceramic in another step, to produce hydrogen and carbon monoxide products.
2. Description of Art
Synthesis gas and its components, hydrogen and carbon monoxide, are conventionally produced by the steam methane reforming (SMR) or by the high temperature partial oxidation of hydrocarbons with controlled amounts of air or oxygen. In the SMR process, a large amount of heat must be supplied into the reactor for sustaining the highly endothermic SMR reaction. Therefore, expensive shell-and-tube type reactors must be used to facilitate the heat exchange. The overall production rate of the SMR process is often limited by the heat transfer rate from the shell side to the tube side where the reaction is taking place. In the partial oxidation process, the overall reaction is exothermic, therefore it does not require an external heat supply. However, it does require the use of an oxidant, such as air or oxygen. Although air is less expensive and more convenient to use in partial oxidation reactions, it is less attractive than oxygen because the large quantities of nitrogen that are produced when air is used as the oxidant must be subsequently separated from the product gas prior to its use. The cost of gas separation and purification equipment required to purify the product gas adds considerably to the cost of synthesis gas production using air.
U.S. Pat. No. 5,149,516 to Han et al., discloses a process for the partial oxidation of methane over a perovskite catalyst to produce carbon monoxide and hydrogen. The process involves contacting a source of methane and a source of oxygen together with a perovskite catalyst. The perovskite acts as a catalyst in the reaction of methane and oxygen in a continuous process, i.e. the perovskite does not participate in the reaction, but promotes the reaction rate. In this process, if air is used as the oxygen source, the products (hydrogen and carbon monoxide) will be contaminated with a large amount of nitrogen.
Although oxygen is more desirable than air as an oxidant for partial oxidation reactions, its use involves certain disadvantages. The oxygen must be imported into the system, or it must be generated on site, for example, by means of a cryogenic air separation plant or other means, such as a membrane or a Pressure Swing Adsorption (PSA) system. In either alternative, using oxygen as the oxidant likewise adds considerably to the cost of the process.
More economical methods of on site production of oxygen for applications such as hydrocarbon partial oxidation reactions are continuously sought. U.S. Pat. No. 5,755,840 to Beer discloses a process for providing oxygen to a feed gas, wherein the oxygen is first absorbed from an oxygen-containing gas (e.g., air) by passing the air over an oxygen-sorbent material (e.g., a solid-state lithium cyanocobaltate) until the sorbent material is substantially saturated after which the feed gas (e.g. natural gas) is passed in contact with the sorbent material to desorb the oxygen into the feed gas. This process produces a gaseous mixture of oxygen and natural gas, which will require additional equipment and means to make hydrogen and carbon monoxide products, such as reactors, catalyst and means for heating up the mixture. The sorbent material, such as lithium cyanocobaltate can only adsorb oxygen on its surface at temperatures lower than 100° C.; the lower the temperature, the higher the amount adsorbed.
U.S. Pat. No. 5,714,091 discloses an oxygen-based hydrocarbon partial oxidation process in which the oxygen is produced on site by subjecting air to membrane separation using a membrane constructed of oxygen ion conducting ceramic material. Oxygen, which is permeable to the membrane, passes through the membrane and is made to react with hydrocarbons on the downstream side of the membrane unit. The disadvantages of this method of oxygen production are the high cost of fabrication of the membrane and the difficulty of producing membrane structures that are leak-proof.
The present invention provides a system and process for the partial oxidation of hydrocarbons, steam reforming of hydrocarbons, or a combination of the two to achieve an auto-thermal process in which oxygen is supplied into the reaction from an oxygen-containing gas using a relatively inexpensive particulate oxygen ion conducting ceramic and a simple reactor design. The inventive process is cyclic, wherein oxygen containing gas and hydrocarbon are fed into the reactor in separate steps. In one step, the oxygen ion conducting ceramic selectively reacts with molecular oxygen at high temperatures by dissociating gas phase oxygen molecules into oxygen ions at its surface and then incorporating these ions into its lattice structure by means of ion conduction through the oxygen vacancies in its lattice structure. This results in the formation of an oxygen-enriched ceramic. In another step, the oxygen-enriched ceramic reacts with hydrocarbon feed to form a product containing hydrogen and carbon monoxide. The process of this invention has several advantages, namely: (1) the separation of oxygen from oxygen-containing gas is conducted in the same vessel as that used for the partial oxidation of hydrocarbons; (2) there is no oxygen in the gas phase during the partial oxidation of hydrocarbons, providing a much safer operating environment; and (3) oxygen ion conducting ceramic in a particulate form is easier to fabricate and costs much less than one in a membrane form. In addition, the process has the advantage that the heat produced during the step in which oxygen reacts with the oxygen ion conducting ceramic can be used to increase the overall efficiency of the process by maintaining the ceramic at the desired temperature without an external heat source and it can also be used to preheat incoming feed streams, by way of heat exchange means.
SUMMARY OF THE INVENTION
According to a broad embodiment, the invention includes a process for producing hydrogen and carbon monoxide by the partial oxidation of at least one hydrocarbon comprising the steps of:
(a) contacting an oxygen ion conducting ceramic in particulate form in a reactor with an oxygen-containing gas at a temperature in the range between about 300 and 1400° C. and at a pressure in the range between about 1 and 50 bara, wherein oxygen from the oxygen-containing gas is reacted with the ceramic, thereby producing an oxygen-enriched ceramic; and
(b) contacting the oxygen-enriched ceramic in the reactor with a hydrocarbon at a temperature in the range between about 300 and 1400° C., thereby producing a product gas through the reaction between the oxygen-enriched ceramic and the hydrocarbon;
wherein step (b) is conducted at a pressure of between about 1 and 50 bara.
Another embodiment of the present invention includes a process for the continuous production of hydrogen and carbon monoxide by the partial oxidation of at least one hydrocarbon, using two reactors, comprising the steps of:
(a) in a first reactor, contacting a first oxygen ion conducting ceramic with an oxygen-containing gas at a temperature in the range between about 300 and 1400° C. and at a pressure in the range between about 1 and 50 bara, wherein oxygen from the oxygen-containing gas reacts with the first ceramic, thereby producing a first oxygen-enriched ceramic; and contacting the first oxygen-enriched ceramic with the hydrocarbon at a temperature in the range between about 300 and 1400° C., thereby producing a fir

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