JPH04310501A - Self heat formula steam reforming process - Google Patents

Abstract

PURPOSE: To provide the method for producing hydrogen and nitrogen which are crude gases for ammonia synthesis.

CONSTITUTION: In a heating contact steam reforming zone 8, steam, an oxidizer, and a fresh hydrocarbon supply gas as a major portion are reacted to obtain a 1st reformed gas which has an extremely small methane content. The rest of the fresh hydrocarbon supply gas is reacted with steam in a heat-absorbing contact steam reforming zone 18 to obtain a 2nd reformed gas which has a small methane content. The 1st reformed gas and 2nd reformed gas are put together to exchange heat directly with the product in the heat-absorbing contact steam reforming zone, heat needed there is all supplied, and this gas is recovered as a combined synthetic gas.

COPYRIGHT: (C)1992,JPO

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a process for producing ammonia using a hydrocarbon such as natural gas as a raw material, and in particular, it is possible by omitting the direct-fired heating tube type temporary reforming furnace commonly used in commercial practice. A method for producing ammonia synthesis gas, hydrogen and nitrogen, with reduced fuel gas requirements.

0002

BACKGROUND OF THE INVENTION Conventional processes for primary and secondary reforming to produce ammonia synthesis gas are well known both technically and economically. From the latter perspective, these two steps are recognized as rate-limiting factors in determining the amount of "feedstock and fuel" required per unit of ammonia produced. This is because both processes require heat from combustion of hydrocarbons due to the endothermic reaction between the hydrocarbon feedstock and steam.

[0003] A commercial primary reformer is a fuel-directly heated furnace with many thick tubes filled with nickel-containing catalyst, in which approximately 60% by volume of the fresh hydrocarbon feedstock reacts with the added steam. Converted to hydrogen and carbon dioxide. This primary reformed gas also contains unreacted steam and residual hydrocarbons (as methane). From a process perspective, this primary reformer is an endothermic catalytic steam reforming zone.

[0004] The primary reformed gas is then introduced into a secondary reformer, which is typically a refractory-lined box filled with a nickel-containing catalyst. They do not have a means of supplying. In secondary reforming, heat for the endothermic reaction between the remaining methane and steam is supplied by combustion of a portion of the primary reformed gas with air supplied from the outside. Incidentally, this air is the nitrogen source for the final ammonia synthesis gas. From a process perspective, this secondary reformer is an exothermic catalytic steam reforming zone, sometimes referred to as an autothermal reformer.

[0005] The hot crude synthesis gas from the secondary reformer is composed of hydrogen, nitrogen, and carbon monoxide, and is then further converted to hydrogen, carbon dioxide, unreacted steam, residual methane, and a small amount of inert gas. be done. Commercially, hot syngas is heat exchanged with boiler feed water to provide the heat source for the turbine steam required to compress the secondary reforming air, syngas, and refrigerant used for ammonia product recovery. .

[0006]

[Problem to be Solved by the Invention] Regardless of such usage, the heat of the secondary reformer outlet gas is used separately for primary reforming using an integrated reactor/heat exchanger device. death,
Therefore, it has long been a dream of industrial engineers to minimize the size of conventional direct-fired tube reforming furnaces. Ideally,
The heat requirement of the primary reformer is sufficiently transferred to the secondary reformer,
The furnace can be omitted if a balance can be struck between the heat availability from the exothermic reforming step and the heat requirement of the endothermic reforming step. If such a heat balance is adopted, the amount of combustion in the secondary reformer increases considerably, and therefore the amount of excess air used must be increased. As a result, excess nitrogen must be removed downstream to achieve the desired hydrogen/nitrogen ratio of the final synthesis gas.

The combined reactor/heat exchanger system proposed for this use was a high temperature heat exchanger having a number of single-pass tubes with each tube end secured to a tube sheet. Although it is considerably cheaper than a direct-fire heating tube furnace, the above-mentioned high-temperature design results in high manufacturing costs. Perhaps more importantly, due to the large excess nitrogen in the final synthesis gas due to the heat balance problems described above, uneconomically large amounts of nitrogen must be removed before or within the synthesis section of the ammonia plant. It is necessary to create a removal system.

[0008] Very recently, US Patent No. 2,57
The general type of open-ended bayonet tubular reactor/heat exchanger unit shown in US Pat. No. 9,843 is a simple design compared to single-pass heat exchangers. Therefore, it has been considered for use in primary reformers. In known designs for ammonia synthesis gas production using bayonet tubes open at one end, the heat balance problems mentioned above prevent the elimination of traditional heated tube reformers. There is.

It is therefore an object of the present invention to convert the heat from exothermic catalytic reforming into endothermic reforming in the production of ammonia synthesis gas under the condition that the total heat for conversion is supplied from the exothermic reforming step. It is used in the process.

0010

According to the present invention, unlike conventional practices, ammonia synthesis gas is
It is produced by introducing a bulk of the fresh hydrocarbon along with steam and oxidizer into an exothermic catalytic steam reforming zone and withdrawing a first reformed gas therefrom. The remaining small portion of fresh hydrocarbons is reacted with steam in an endothermic catalytic steam reforming zone from which a second reformate gas is withdrawn, after which it is combined with the first reformate gas. The resulting combined gas is then indirectly heat exchanged with the reactants in an endothermic catalytic steam reforming zone to release heat and subsequently recovered as crude ammonia synthesis gas.

[0011] The exothermic catalytic steam reforming zone consists of 22 to 7
It is operated adiabatically at a pressure of 0 bar and conveniently can be implemented in known secondary reformer configurations. Although the name quadratic is a misnomer for the process of the present invention. Preferably, 55-85% by volume of the fresh hydrocarbon feedstock is introduced into the exothermic catalytic steam reforming zone together with steam and oxidizer, although hereinafter these gases are combined into the first This will be referred to as mixed supply gas. Preferably, the first mixed feed gas steam and hydrocarbon are first combined and preheated to a temperature of 450° to 650°C. If air is selected as the oxidizing agent, the steam:C1 ratio of the first mixed feed gas is preferably between 1.5 and 3.5. If oxygen-enriched air is selected as the oxidizing agent, it is preferred that oxygen accounts for 25 to 40% by volume (dry basis) of the oxidizing agent, and the steam of the first mixed feed gas: C1
The ratio is preferably between 2.5 and 3.5. Oxygen for air enrichment can be supplied by medium-scale refrigeration plants, membrane separation units, or pressure swing adsorption units. The choice between using air or oxygen-enriched air is primarily an economic question, considering the size and cost of the oxygen plant, the cost of utilities, and the cost of the ammonia plant's energy system and overall production equipment. Governed by the degree of integration with the utility system. Whichever you choose,
Preferably, the oxidizing agent is preheated to 480° to 720°C before being introduced into the exothermic catalytic steam reforming zone.

Like the secondary reformer, the exothermic catalytic steam reforming zone operates in an autothermal manner. However, unlike conventional systems, the majority of the total reforming requirements occur in this zone, and the autothermal steam reforming conditions are preferably 90
selected to produce a first reformate gas containing hydrogen, carbon oxides, nitrogen, and less than 1.0% by volume (dry basis) residual hydrocarbons, i.e. methane, at temperatures between 0° and 1100°C; .

[0013] The endothermic catalytic steam reforming zone also includes 22
It operates at a pressure of ~70 bar, but is heated by the first reformate gas through the walls of the catalyst tube, as described below. This zone is a vertical reactor/heat exchanger integrated device.
Preferably, it is embodied in a device with a bayonet tube filled with catalyst, the outlet of the gas being at its lower end. The remaining small portion of the fresh hydrocarbon feed gas along with the steam, hereinafter referred to as the second mixed feed gas, is also 45
It is preheated to a temperature between 0° and 650°C and then introduced into an endothermic catalytic steam reforming zone, typically between 825° and 102°C.
The reaction is carried out at a temperature of 5° C. to produce a second reformed gas containing hydrogen, carbon oxides, and less than 4.0% by volume (dry basis) residual hydrocarbons, ie, methane. Preferably, the steam:C1 ratio of the second mixed feed gas is between 4.0 and 5.0.

To provide all of the heat demand for the endothermic reforming zone, the first and second mixed feed gases are combined and then indirectly combined with the second mixed feed gas in the endothermic catalytic steam reforming zone. The crude ammonia synthesis gas is extracted from it as crude ammonia synthesis gas.

[0015] Since the crude ammonia synthesis gas is typically recovered at a temperature of 565°C to 735°C, the sensible heat in the gas is preferably recovered by indirect heat exchange with fresh hydrocarbon feed gas. . At this time, the hydrocarbon will be preheated.

Now, reference is made to the drawings. Fresh hydrocarbon feed gas in line 1, preferably saturated with water, is combined with steam in line 2 and preheated in feed/effluent reactant heat exchanger 3. The majority of the fresh hydrocarbon feed gas in line 4 is combined with additional steam and oxidizer introduced in lines 5 and 6, respectively, to form a first mixed feed gas that is passed through line 7 to exothermic contact. It is introduced into a steam reformer 8 where it reacts to produce a first reformed gas, which is recovered via line 9.

A small portion of the fresh feed gas in line 10 is combined with additional steam from line 11 to form a second mixed feed gas which is passed through line 12 to the reformer constituting the endothermic catalytic steam reforming zone. The catalytic converter is introduced into the catalyst tube side of the heat exchanger 13. The catalyst is placed in a tube that is open at one end.
It is supported by a wire mesh (not shown). The second reformate gas recovered from the outlet at the bottom of the catalyst tube is combined with the first reformate gas in line 9. The combined gas is cooled by indirect heat exchange with the second mixed feed gas in the catalyst tube and is recovered from the shell side of the reformer-heat exchanger 13 via line 15 as crude ammonia synthesis gas. This crude ammonia synthesis gas is then further cooled in feed/effluent reactant heat exchanger 3 and recovered via line 16 for further heat recovery and processing in known processes for ammonia production.

Tables 1 and 2 below illustrate examples of suitable operating conditions and stream compositions for alternative designs using air or oxygen-enriched air as the oxidizing agent in the exothermic catalytic steam reformer 8. This is what is shown.

[Table 1]

[Table 2]

[Brief explanation of drawings]

FIG. 1 is a schematic diagram illustrating the method of the invention.

[Explanation of symbols]

Claims (6)

[Claims] 1. A method for producing crude ammonia synthesis gas from fresh hydrocarbon feed gas, comprising: a) steam, a majority of fresh hydrocarbon feed gas, and air and oxygen-enriched air; introducing a first mixed feed gas containing an oxidizing agent to an exothermic catalytic steam reforming zone and withdrawing therefrom a first reformed gas containing less than 1.0% by volume residual hydrocarbons on a dry basis; b) introducing a second mixed feed gas containing steam and a small portion of the remainder of fresh hydrocarbon feed gas into an endothermic catalytic steam reforming zone from which less than 4.0% by volume on a dry basis withdrawing a second reformed gas containing residual hydrocarbons; c) combining the first and second mixed feed gases and indirectly communicating the combined gas with a second mixed gas in an endothermic contact steam reforming zone; d) recovering the resulting combined and cooled gas as crude ammonia synthesis gas; A method for producing ammonia synthesis gas comprising: 2. The method of claim 1, wherein said majority of said fresh hydrocarbon feed gas is 55 to 85% by volume of fresh hydrocarbon feed gas. 3. The fresh hydrocarbon feed gas comprises 45
3. A method according to claim 2, characterized in that it is preheated to a temperature between 0[deg.] and 650[deg.]C. 4. The fresh hydrocarbon feed gas comprises:
4. The method according to claim 3, wherein the preheating is performed by indirect heat exchange with crude ammonia synthesis gas. 5. The oxidizing agent is air preheated to a temperature between 480° and 720° C., and the steam:C1 ratio of the first mixed feed gas is between 1.5 and 3.5. 4. A method as claimed in claim 1 or 3, characterized in that the steam:C1 ratio of the second mixed feed gas is between 4.0 and 5.0. 6. The oxidizing agent is oxygen-enriched air preheated to a temperature between 480° and 720° C. and the steam:C1 ratio of the first mixed feed gas is between 2.5 and 4.5. 4. A method according to claim 1, wherein the steam:C1 ratio of the second mixed feed gas is between 4.0 and 5.0.