WO2006042986A1

WO2006042986A1 - Method for producing a syngas having a h2/co ratio less than 2.5

Info

Publication number: WO2006042986A1
Application number: PCT/FR2005/050818
Authority: WO
Inventors: Paul Wentink
Original assignee: L'air Liquide Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2004-10-20
Filing date: 2005-10-06
Publication date: 2006-04-27
Also published as: FR2876683A1; FR2876683B1

Abstract

The invention concerns a method for producing a syngas having a H2/CO ratio less than 2.5 by endothermic catalytic oxidation of hydrocarbons in the presence of CO2 and water vapour, which consists in generating a raw syngas comprising at least hydrogen, carbon monoxide, carbon dioxide, water vapour, and also in smaller amounts methane and nitrogen (3), and wherein said cooled raw syngas (5) with its condensed water eliminated (7) is subjected to a separation into: a gas stream containing the carbon dioxide and the methane (31); a gas stream of syngas having a H2/CO ratio less than 2.5 (34); a gas stream containing essentially hydrogen (35); reincorporating the gas stream containing carbon dioxide and methane (33) for feeding the reformer.

Description

PROCESS FOR PRODUCING A SYNTHESIS GAS HAVING A H ₂ / CO RATIO LESS THAN 2.5.

The present invention relates to a novel process for the production of synthesis gas having a hydrogen to carbon monoxide (H ₂ / CO) ratio of less than 2.5 by endothermic catalytic oxidation of hydrocarbons in the presence of CO ₂ and steam. water as components providing oxygen.

The conversion of the gaseous hydrocarbon compounds into a synthesis gas mainly comprising H ₂ and CO is generally carried out by partial oxidation using an oxidizing gas and / or reaction with water vapor or CO ₂ . When it is desired to directly generate a synthesis gas with a ratio

H ₂ / CO less than 2.5, it is necessary to use a synthesis gas generation technology that relies on oxidation with oxygen; in this case the ratio H ₂ / CO will naturally be low. However, such technology only applies to high capacity units and is not suitable for low productions. In the case of small capacity units, the synthesis gas is produced by reaction with steam and

CO ₂ .

Steam hydrocarbon reforming is a catalytic process. The composition of the gas obtained at the end of the reforming depends on various parameters among which the proportions in different constituents of the gaseous mixture feeding the reforming stage, the pressure and the temperature will be mentioned. The product obtained usually contains hydrogen, oxides of carbon (monoxide and dioxide), methane, steam and various gases present at the start of the reaction and inert under the reaction conditions used, such as nitrogen.

The main reactions involved in catalytic steam reforming are the following: they give rise to equilibria: CH ₄ + H ₂ O = CO + 3H ₂ (I)

CH ₄ + CO ₂ = 2CO + 2H ₂ (II)

CO + H ₂ O = CO ₂ + H ₂ (III)

The synthesis gas directly from the reforming, called synthesis gas crude in the following description, can be processed to produce pure hydrogen, pure carbon monoxide or H ₂ / CO mixtures. A mixture of H ₂ / CO is used in a large number of processes among which the synthesis of methanol, or the Fisher-process

Tropsch for the synthesis of carbon-containing compounds such as long chain hydrocarbons. For these different applications, but also for other applications not mentioned here, which use synthesis gas as starting material, the H ₂ / CO ratio is a key parameter. It is therefore essential, during the production of synthesis gas, to control this ratio in the final product. The solution usually adopted to solve this problem consists in: removing the CO ₂ contained in the raw synthesis gas generated by the steam reforming, by washing with amines to recycle it to reforming, thus increasing the level of CO ₂ present during this reforming and favoring the production of CO, which leads to the reduction of the H ₂ / CO ratio in the reforming product; - Remove excess hydrogen by passage on a membrane that allows to recover hydrogen at low pressure and low purity. In order to be able to value it, it will be necessary to recompress this hydrogen before purifying it by adsorption by pressure modulation (PSA).

Furthermore, increasing stresses with respect to the inert gases present in the synthesis gas have led to steam reforming at operating conditions for which the steam / carbon (or steam / carbon (S / C) ratio in English language) is higher than required by the energy optimum, in order to limit the presence of unreacted methane in the gas resulting from reforming. Nitrogen, which is the other main inert present after the CO ₂ removal step, can not be eliminated other than by the use of a cold box. The option to use this cold box is retained only when a customer wants CO as an additional product.

The aim of the present invention is to propose a process for producing synthesis gas having a hydrogen / carbon monoxide H ₂ / CO ratio of less than 2.5 using steam reforming with recycling to CO ₂ reforming. and methane contained in the raw synthesis gas.

Another object of the present invention is to propose a process for producing synthesis gas having a hydrogen / carbon monoxide ratio H ₂ / CO of less than 2.5 using steam reforming which operates under optimal energy conditions. that is, with a low vapor / carbon (V / C) ratio. The greater amount of unconverted methane does not detract from the purity of the synthesis gas ultimately produced by the recycling of methane. The purity of the synthesis gas produced depends solely on its nitrogen content. The invention also has the advantage of directly producing hydrogen which may have a purity greater than 98% (ie a hydrogen concentration greater than 98% by volume). It is therefore no longer necessary to obtain purified hydrogen to go through a recompression step before a PSA purification step. The recompression of the hydrogen flow product will be required only in the case where it will be desired to export high pressure hydrogen at a pressure greater than 30 bar. Another advantage of the invention lies in the fact that the process allows an improved carbon efficiency in the English language thanks to the recycling of methane to the reformer feed.

For this purpose, the invention relates to a process for the production of synthesis gas having a hydrogen / carbon monoxide (H ₂ / CO) ratio of less than 2.5 by endothermic catalytic oxidation of hydrocarbons in the presence of CO ₂ and steam. water as oxygen supplying components, wherein a raw synthesis gas is generated comprising at least hydrogen, carbon monoxide, carbon dioxide, water vapor, and also in amounts less, unreacted methane and nitrogen, and wherein said raw synthesis gas is subjected to a separation process comprising at least the following steps: a step of cooling the raw synthesis gas to about the temperature ambient, a step of removing the condensed water contained, and characterized in that it further comprises the following steps: a step of separating the mixture obtained in:

At least one gas stream containing more than 80%, preferably more than 90% of the carbon dioxide and methane present in the raw synthesis gas;

At least one gaseous stream of synthesis gas containing hydrogen and carbon monoxide in proportions such that the H ₂ / CO ratio is less than 2.5;

At least one gaseous stream containing hydrogen with a purity higher than 98%; a step of reincorporation of all or part of the gaseous flow containing carbon dioxide and methane to the stage of generation of the raw synthesis gas, as a partial substitution of at least a portion of the flows feeding said step of generating the gaseous gas; raw synthesis. The present invention therefore makes it possible to produce a synthesis gas having a hydrogen / carbon monoxide H ₂ / CO ratio of less than 2.5 by endothermic catalytic oxidation of hydrocarbons in the presence of CO ₂ and of water vapor to obtain a raw synthesis gas, containing mainly hydrogen, carbon monoxide, carbon dioxide, and water vapor, but also in smaller amounts, methane and nitrogen, the crude synthesis is then subjected to a treatment comprising a step of cooling the crude synthesis gas to about room temperature, followed by a step of removing the condensed water, the invention being characterized in that subsequently, the process allows in a single step to produce three gaseous streams containing respectively: CO ₂ and methane which are recycled to the reforming feed; CO _{2 makes it} possible to reduce the H ₂ / CO ratio and increase the carbon efficiency beyond that of a traditional process, as the main product, a synthesis gas containing hydrogen and monoxide in such proportions that the ratio H ₂ / CO is less than 2.5 and containing as main impurity nitrogen from the mixture of hydrocarbons fed to the reformer, and excess hydrogen, which is produced in the form of a flow of hydrogen at high pressure and high purity, comparable to the hydrogen obtained from a traditional PSA hydrogen purification unit.

In the process according to the invention, at least one process step, that is to say the step of separate removal of the CO ₂ from the raw gas, is eliminated with respect to the solution known to those skilled in the art. , which allows a considerable simplification of apparatus and a significant decrease in costs. The process according to the invention is particularly advantageous when the synthesis gas is generated by steam reforming (the stresses related to the presence of excessive amounts of inert gas in the synthesis gas produced are in fact lower when the processes are operated at higher temperatures. 'oxygen). It is therefore particularly interesting since the synthesis gas production plant will have a low production capacity, typically 5,000 to 40,000 Nm ³ / h, since in this case this production will be provided by steam reforming of hydrocarbons . Steam reforming produces a mixture of H ₂ / CO / CO ₂ and CH ₄ having a composition that is a function of that of the feed mixture, and a function of the reaction equilibria involved during reforming. According to a first variant of the process according to the invention, the separation process is advantageously a pressure swing adsorption process (PSA separation process) ("Pressure Swing Adsorption"). After the cooling and water removal steps, the raw synthesis gas is thus sent to a PSA type unit making it possible to obtain at least the three main gas flows described above.

Preferably, the PSA unit comprises one or more adsorbers in parallel, each adsorber comprises at least two beds, a lower bed and an upper bed, the order of the beds being as follows according to the sense circulation of the raw synthesis gas in the adsorber: lower bed, then upper bed.

Most of the hydrogen and the CO pass through in the form of high pressure products, at controlled pressure, the lower adsorbent bed which retains at least 80%, preferably at least 90% of the CO ₂ and methane contained in crude synthesis gas, as well as a lower proportion of CO and hydrogen (this adsorbent belongs to the family of zeolites and / or activated carbons); a controlled flow passes through the upper bed where nitrogen and CO are retained, allowing the production of purified hydrogen, in such quantities that the required H ₂ / CO ratio is obtained. After the production step, the bed follows the traditional regeneration cycle during which a CO / CO ₂ / CH ₄ / N ₂ mixture is recovered. This stream is recycled to the top of the process where it is used as a carbon rich feed mixture for reforming.

Advantageously, in order to stabilize the synthesis gas composition produced over time, the composition of the synthesis gas leaving (from the separation step) is stabilized by adding to said stream of synthesis gas leaving hydrogen from the gas flow leaving the upper bed during the production step.

Depending on the different pressure cycles, the separation process PSA makes it possible successively to obtain the gas stream containing H ₂ , and the gas stream containing the synthesis gas during the PSA cycle production step, followed during the step of regeneration of the adsorbent by a flow N ₂ / CO (whose recovery in the form of separated stream is optional), and the flow containing CO ₂ and methane.

Other characteristics and advantages of the invention will appear on reading the description which follows. Embodiments and embodiments of the invention are given by way of non-limiting examples, illustrated by the accompanying drawings in which:

FIG. 1 is a diagram of a method according to the prior art, FIG. 2 is a diagram of a method according to the invention,

- Figure 3 illustrates a separation principle used. FIG. 1 illustrates a process according to the prior art in which a gaseous mixture of base hydrocarbons (1) is treated in a unit for preparing a synthesis gas by reforming methane with steam (2) to provide a raw synthesis gas (3) containing essentially CO, H ₂ , CO ₂ , H ₂ O, but also CH ₄ and N ₂ . This raw synthesis gas (3) is introduced into a cooler (4) in which it is cooled to a temperature of the order of room temperature. The cooled raw gas (5) is then introduced into a water separator (6) in which the condensed water is removed.

The gas (7) is treated by a CO ₂ removal unit (8) which delivers a gas (9) freed from CO ₂ ; the recovered CO ₂ (10) is compressed via the compressor (11) to produce a compressed CO ₂ (12) at a pressure compatible with reforming. The gas (12) is then returned to the reforming to increase the CO content and thus reduce the ratio H ₂ / CO reforming. The gas (9) is treated by passage over a membrane (13) which removes excess hydrogen in a gas stream (14) at low pressure and produces synthesis gas (15) having the H ₂ / CO ratio less than 2.5. In order to value the hydrogen contained in the gas stream (14), it is re¬ compressed via the compressor (16), the gas (17) thus compressed is then purified by passing through a PSA unit (18) which delivers purified hydrogen (19) and a waste (20) which is returned as fuel to reforming.

Figure 2 illustrates the method according to the invention. Unlike the processes of the prior art, as illustrated by the method of FIG. 1, the gas (7) is treated by a separation process (30) leading to three products:

a gas (31) comprising mainly CO ₂ and methane which is recompressed via the compressor (32) at a pressure compatible with the reforming to provide a compressed gas (33). The gas (33) is then returned to the reforming to increase the CO content and thus reduce the H ₂ / CO ratio, the methane present in the gas (33) partially substituting for the gas (1),

a gas (34) essentially comprising a mixture H ₂ and CO having the ratio H ₂ / CO required less than 2.5. This ratio being obtained by the separation of excess hydrogen, a gas (35) containing the hydrogen thus separated in the form of high purity hydrogen, suitable for sale. Figure 3 illustrates the principle of separation used in the implementation of the invention. The gaseous mixture (7) is sent under pressure to a first bed of adsorbent (40) called "lower bed" where are retained mainly CO ₂ and methane and less than the other constituents while a gas stream under pressure containing most of the CO, hydrogen and nitrogen passes through this first bed to reach the upper bed (41). A flow (35) controlled flow of hydrogen through the second bed to produce 98% pure hydrogen in an amount such that the gas mixture (34) recovered at the junction between the two beds has the ratio H ₂ / CO required. During the regeneration step of the adsorbent, the stream (31) is recovered at low pressure at the lower bed. During the regeneration step, a low pressure stream containing nitrogen and CO is recovered following the stream (34); it may optionally, in the context of the invention, be treated independently of the flow (34).

The following tables present the material balance obtained during the production of synthesis gas having an H ₂ / CO ratio of less than 2.5. Table 1 presents a material balance obtained by implementing the method known to those skilled in the art according to FIG. 1, in which the synthesis gas is generated by steam reforming of natural gas, the CO elimination step. ₂ employs an amine wash, and wherein the hydrogen is separated by membrane.

Table 2 presents a material balance obtained by implementing the process of the invention according to FIGS. 2 and 3, in which the synthesis gas is generated by steam reforming of natural gas, and the raw synthesis gas obtained is treated in a PSA according to the invention, in substitution of CO ₂ removal and removal of excess hydrogen per membrane.

Table 1

Table 2

Table 3 below lists the performance improvements observed when substituting the solution of the invention for the traditional solution.

Table 3

The implementation of the method of the invention has the following characteristics compared with the conventional solution:

- a reduction in natural gas consumption, - an overall higher energy consumption, but with an overall reduction in operating costs of approximately EUR 1, 2 million / year,

- a total investment overall lower; indeed, the costs of compression are comparable, while the removal of the CO ₂ removal unit offsets the cost of the new separation unit. This allows to consider that the removal of the membrane, as well as that of the PSA H ₂ purification unit are investment gains.

In this case, it can be considered that, reduced to the whole of the unit, the invention allows a cost reduction of 5% at the level of the investment, and a reduction of the operating costs of 8%, considering that the The cost of energy accounts for 70% of these operating costs.

Claims

A process for the production of synthesis gas having a hydrogen to carbon monoxide (H ₂ / CO) ratio of less than 2.5 by endothermic catalytic oxidation of hydrocarbons in the presence of CO ₂ and water vapor as supplying components oxygen, in which a raw synthesis gas comprising at least hydrogen, carbon monoxide, carbon dioxide, water vapor, and also less amounts of methane and nitrogen, and wherein said crude synthesis gas is subjected to a separation process comprising at least the following steps: a step of cooling the crude synthesis gas to about room temperature, a step of removing the condensed water contained, and characterized in that it further comprises the following steps: a step of separating the mixture obtained in:

At least one gaseous stream of synthesis gas produced containing hydrogen and carbon monoxide in proportions such that the H ₂ / CO ratio is less than 2.5;

At least one gaseous stream containing hydrogen with a purity higher than 98%; a step of reincorporation of all or part of the gas stream containing carbon dioxide and methane to the step of generating the raw synthesis gas, as a partial substitution of at least a portion of the flows feeding said step of generating the gas of raw synthesis.

2. Method according to claim 1, characterized in that the step of separation of the raw synthesis gas uses a PSA unit.

3. Method according to claim 2, characterized in that each adsorber of the PSA unit comprises at least two beds, a lower bed and an upper bed, the order of the beds being as follows. the flow direction of the raw synthesis gas in the adsorber: lower bed, then upper bed.

4. Method according to one of claims 2 or 3 characterized in that the composition of the outgoing synthesis gas, resulting from the separation step, is stabilized by adding to said outgoing synthesis gas stream, hydrogen from the gas stream containing hydrogen from said separation.