KR960003232B1 - Auto thermal steam reforming process - Google Patents

Abstract

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Description

Self heating steam reforming method

1 shows an embodiment of an apparatus for practicing the method of the present invention.

* Explanation of symbols for main parts of the drawings

3: supply / discharge heat exchange 8: reformer

13 reformer exchanger 14 secondary reformed gas

The present invention relates to the production of ammonia from hydrocarbons, such as natural gas, and specifically to the production of ammonia synthesis gas, hydrogen and nitrogen while using less fuel gas by not using commercially available heating tube primary reforming furnaces. It's about doing.

Known steps of primary and secondary reforming to produce ammonia synthesis gas are well known technically and economically. From the latter point of view, these stages are the controlling factors in determining the “raw and fuel” requirements for each unit of ammonia produced, since these two stages require heat from the combustion of hydrocarbons for the endothermic reaction between steam and hydrocarbon feedstocks. It is recognized as

Commercial primary reformers are fuel heating furnaces in the form of large tubes filled with nickel-containing catalysts, where they are converted to hydrogen and carbon monoxide with steam added to about 60% by volume of a pure hydrocarbon feedstock. This primary reforming gas additionally contains unreacted steam and the remainder of the hydrocarbon feedstock as methane. From the process point of view, the primary reformer is an endothermic catalytic steam reforming zone.

The primary reforming gas is an internal refractory vessel, usually filled with a nickel containing catalyst, which is fed to a secondary reformer without an external heat supply. In secondary reforming, the heat for the endothermic reaction of residual methane and steam is supplied by combustion of a portion of the primary reforming gas with external feed air, which is a nitrogen source for the final ammonia synthesis gas. From a process point of view, the secondary reformer is an exothermic catalytic steam reforming zone, sometimes referred to as an autothermal reformer.

Raw hot synthesis gas from the secondary reformer consists of hydrogen, nitrogen, followed by additional hydrogen, carbon dioxide, unreacted steam, residual methane and carbon monoxide to be converted to a small amount of inert gas. In general, hot syngas is heat exchanged with boiler feed water to generate the turbine steam required for the compression plant for the refrigerants used for secondary reformer air, syngas and ammonia product recovery.

In spite of these uses, the practitioners continue to make efforts to reduce the size of existing heating and reforming furnaces by using the off-gas heat from the secondary reforming unit for an alternative to primary reforming through the use of reactors / heat exchangers. come. Ideally, the furnace is removable if it can transfer a sufficient amount of primary reforming to the secondary reforming unit to balance the heat demand of the endothermic reforming stage with the heat availability of the exothermic reforming stage. This thermal balance actually requires a significant amount of combustion in the secondary reformer, because excess air is used to remove excess nitrogen downstream to achieve the desired hydrogen / nitrogen ratio in the final synthesis gas.

The reactor / exchanger proposed for this installation was a high temperature heat exchanger with single-pass tubes each end fixed to a tube plate. These high temperature designs can save a significant amount of expense compared to heating tube furnaces, but with high manufacturing costs. On the other hand, more importantly, the significant excess of nitrogen in the final synthesis gas due to the above pointed out heat balance problem necessitates the entailment of an inexpensive large nitrogen exclusion system before or within the synthesis section of the ammonia plant.

The open bayonet tube reactor / exchanger, which is the general form shown in US Pat. No. 2,579,843, is considered as a primary reformer because of its simpler design than a single-pass exchanger. In a known design for ammonia syngas production using open bayonet tubes, the heat balance problem prevented the removal of existing heating tube reforming simulations.

Accordingly, it is an object of the present invention to produce ammonia synthesis gas using the heat of exothermic catalytic reforming in the endothermic reforming stage, under conditions where the total conversion heat is supplied in the exothermic reforming stage.

According to the present invention and unlike known methods, ammonia synthesis gas is produced by introducing a major portion of the fresh hydrocarbon into the exothermic catalytic steam reforming zone together with steam and oxidant and recovering the primary reforming gas therefrom. The remaining small amount of hydrocarbon is reacted with steam in the endothermic catalytic steam reforming zone, and the secondary reformed gas is recovered here and immediately mixed with the primary reformed gas. The resulting mixed gas then carries an endothermic catalytic steam reforming zone with indirect heat exchange with the reactants, where they release heat and are subsequently recovered as raw ammonia syngas.

The exothermic catalytic steam reformer zone is operated adiabatically at a pressure of 22-70 bar, although this name is incorrectly called in the process of the present invention, it can be conveniently implemented in existing arrangements of secondary reformers. Preferably, 55-85% by volume of the fresh hydrocarbon feedstock is introduced into the exothermic reforming zone with steam and oxidant (hereinafter together referred to as primary mixed feedstock). The steam and hydrocarbon components of the primary mixed feed are preferably first mixed and then preheated to a temperature between 450 and 650 ° C. When air is selected as the oxidant, the ratio of steam to C ₁ of the primary mixed feed is preferably between 1.5 and 3.5. When selecting a large amount of oxygen containing air as the oxidant, the oxygen preferably constitutes 25-40% by volume (dry basis) of the oxidant, and the steam ratio to C ₁ of the primary mixed raw material is preferably between 2.5 and 3.5. Oxygen for improving air quality may be supplied by a low temperature membrane of moderate size, or by a pressure swing absorption unit. The choice between the use of air or a large amount of oxygen-containing air is primarily an economic matter, depending on the size and cost of the oxygen unit, the cost of the installation and the degree of integration of the ammonia plant energy system with the operating system of the entire production plant. Either option, the oxidant should preferably be preheated at 480 ° C. to 720 ° C. before being introduced into the exothermic catalytic steam reforming zone.

Like the secondary reforming unit, the exothermic catalytic steam reforming zone operates autothermally, but most of the total reforming amount is run in this zone, unlike existing systems, and the autothermal steam reforming conditions are preferably between 900 ° C and 1100 ° C. Is selected to produce hydrogen, carbon monoxide, nitrogen-containing primary reforming gases and 1% by volume (dry basis) residual hydrocarbons, ie methanol.

The endothermic catalytic steam reforming zone is also operated at a pressure between 22 and 70 bar but is heated through the catalyst tube wall with the primary reforming gas described below. This zone is preferably realized in a vertical reactor / heat exchanger having a gas passage at the lower end of the tube and having a bayonet tube filled with catalyst. Along with steam, the remaining minor fraction of the fresh hydrocarbon feedstock (hereinafter abbreviated as secondary blend feedstock) is also preheated at temperatures between 450 ° C and 650 ° C and then introduced into the endothermic catalytic steam reforming zone, usually 825 ° C to 1025 ° C. Reaction at temperatures between produces a secondary reforming gas containing less than 4.0 volume% of residual hydrocarbons, ie methane, on a hydrogen, carbon monoxide and dry basis. Preferably, the ratio of steam to C ₁ of the secondary mixed raw material is between 4.0 and 5.0.

In order to provide the total heat required for the endothermic reforming zone, the primary and secondary reforming gases are mixed and then cooled by indirect heat exchange with the secondary mixed feed in the endothermic catalytic steam reforming zone, where it is recovered as raw ammonia synthesis gas. do.

Since the raw ammonia synthesis gas is usually recovered at a temperature between 565 ° C and 735 ° C, the sensible heat in this gas is preferably recovered by indirect heat exchange with the fresh hydrocarbon raw material, whereby the hydrocarbon raw material is preheated.

The fresh hydrocarbon feed of line 1, preferably saturated with water, is mixed with the steam of line 2 and preheated in feed / discharge heat exchanger 3. The main portion of the fresh hydrocarbon feedstock in line 4 is mixed with additional steam and oxidant introduced in lines 5 and 6 respectively to form the primary mixed feed introduced into the exothermic catalytic steam reformer 8 via line 7, where the reaction This produces a primary reformed gas that is recovered via line 9.

A fractional portion of the fresh feed of line 10 is mixed with additional steam from line 11 to produce a secondary mixed feed introduced through line 12 into the catalyst tube wall of reformer-exchanger 13 which constitutes an endothermic catalytic steam reforming zone. The catalyst is supported in the open tube by a screen which is not shown. Secondary reformed gas 14 recovered from the bottom of the catalyst tube is mixed with the primary reformed gas of line 9. The resulting mixed gas is cooled in the catalyst tube by indirect heat exchange with the secondary mixed raw material and is recovered as raw ammonia syngas via line 15 from outside the tube of the reactor-exchanger 13. This raw ammonia syngas is then further cooled in feed / discharge heat exchanger 3, recovered via line 16 for further heat recovery and processing, and processed by known steps for the production of ammonia.

The following table shows examples of relevant operating conditions and stream compositions in alternative designs using air or massive oxygen containing air as the oxidant in the exothermic catalytic steam reformer 8.

TABLE 1

TABLE 2

Claims (6)

a) a primary mixed feed comprising a major part of the steam, fresh hydrocarbon feedstock, and an oxidant selected from the group consisting of air and a large amount of oxygen-containing air, is introduced into the exothermic catalytic steam reforming zone, from which 1.0 volume on a dry basis Recovering the primary reforming gas containing less than% residual hydrocarbons; b) introducing a secondary mixed feed containing a small amount of the remainder of the steam and fresh hydrocarbon feedstock into the endothermic catalytic steam reforming zone and recovering therefrom a secondary reformed gas containing less than 4.0% by volume residual hydrocarbons on a dry basis; ; c) obtaining all the heat required in the endothermic catalytic steam reforming zone by combining the primary and secondary reforming gases and cooling the mixed gas by indirect heat exchange with the secondary mixed feed in the endothermic catalytic steam reforming zone; d) recovering the resulting cold mixed gas as a raw ammonia syngas, wherein the ammonia syngas is produced from a fresh hydrocarbon feedstock. The process of claim 1 wherein the major portion of the fresh hydrocarbon feedstock is 55-85% by volume of the fresh hydrocarbon feedstock. The method of claim 2 wherein the fresh hydrocarbon feedstock is preheated to a temperature between 450 ° and 650 ° C. 4. The process of claim 3 wherein the fresh hydrocarbon feedstock is preheated by indirect heat exchange with the feed ammonia synthesis gas. The method according to claim 1 or 3, wherein the oxidizing agent is air preheated to a temperature of 480 ° to 720 ° C, the ratio of steam to C ₁ of the primary mixed raw material is 1.5 to 3.5, and that of steam to C ₁ of the secondary mixed raw material. The ratio is between 4.0 and 5.0. The method according to claim 1 or 3, wherein the oxidant is a large amount of oxygen-containing air preheated to a temperature between 480 ° and 720 °, the ratio of steam to C ₁ of the primary mixed raw material is 2.5 to 4.5, and C of the secondary mixed raw material. The ratio of steam to ₁ is 4.0 to 5.0.

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