HYDROGEN PRODUCTION PROCESS

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processes for producing hydrogen. In particular, the present invention relates to processes for producing hydrogen through steam reforming.

2. Discussion of the Background

Hydrogen production is typically performed through catalytic steam reforming. The general reaction for catalytic steam reforming is as follows:

$C_{x} H_{y} O_{z} + (x - z) H_{2} O \overset{catalyst}{\to} x CO + (x - z + \frac{y}{z}) H_{2}$

Steam reforming is an endothermic reaction carried out either in heat exchange reactors, or by other means where substantial heat may be transferred to the reacting fluid, such as in the case of autothermal reforming, where a portion of the feedstock is combusted inside the reactor to provide heat for steam reforming either subsequently or in the same location as the combustion. If hydrocarbon or alcohol feedstocks enriched in compounds with two or more carbon atoms per molecule (C2+ hydrocarbons) are used for hydrogen generation, the risk of catalyst deactivation by carbon deposition is greatly increased. In order to minimize the risk of carbon deposition, existing hydrogen production processes employ at least one adiabatic catalytic reactor prior to the heated reactor. These adiabatic reactors are referred to as pre-reformers.

In existing hydrogen production processes employing pre-reformers, the hydrocarbon feedstock is mixed with 1 to 5% hydrogen by volume, then is subjected to a hydrodesulphurization (HDS) pre-treatment step to remove sulphur. The feedstock hydrocarbons are then mixed with superheated steam in a ratio determined by the average molecular weight of the feedstock molecules. Natural gas or other feedstocks where the average carbon number is less than two are processed with a molar steam to carbon ratio between 3:1 and 5:1. Higher molecular weight feedstocks are often processed with steam to carbon ratios as much as twice as high. These high steam flowrates are used to suppress carbon formation, and enhance the steam reforming reaction. High steam to carbon ratios disadvantageously increase energy usage in the hydrogen production process.

Because the reaction rates for steam reforming are low at the pre-reforming feed temperatures of 400° C. to 500° C., pre-reforming catalysts are prepared with very high metal loadings, above 10% by weight, and high metal surface areas. These high metal surface areas present several disadvantages. First, they are subject to rapid sintering and reduction of activity if feedstock temperature is not controlled very closely. Second, they present substantial safety risk due to their pyrophoric reaction with oxygen, especially when nickel metal is used, thus necessitating great care in handling the catalysts during reduction and subsequent operation. Further, even at the elevated steam to carbon ratios employed in existing hydrogen production processes using pre-reformers for C2+ feedstocks, deactivation by carbon deposition remains a problem. Typically, volatile alkali or alkali-silicate promoters are added to suppress carbon deposition, as described by Twigg, et al. Catalyst Handbook: 2^ndEdition, Manson Publishing, Ltd. 1996, pp. 250-253, the contents of which are hereby incorporated by reference. These promoters are very effective, but disadvantageously reduce catalyst reaction rate, necessitating larger pre-reforming reactors. Further, the promoters tend to volatilize and subsequently deposit on downstream catalysts and equipment. This causes deactivation of downstream catalysts and potential corrosion damage to equipment, both of which may lead to serious operation problems such as hot banding of reformer tubes, carbon deposition and eventual tube failure. Further, the protective effects of the alkali promoters are lost after they are volatilized, such that eventual catalyst failure is assured in existing pre-reformers. Upon failure, the highly-reactive catalyst must be safely removed from the reactor and replaced.

There is a need for an improved hydrogen production method that can process feedstocks containing 20% or more of molecules having at least two carbon atoms each without being deactivated by carbon deposition.

SUMMARY OF THE INVENTION

The present invention provides a catalytic steam reforming process for hydrogen production in which carbon deposition is reduced by adding more than 5 vol % hydrogen with the hydrocarbon feedstock gas.

The present invention provides an improved hydrogen production method, for processing feedstocks containing 20% or more of molecules having at least two carbon atoms each, which is not deactivated by carbon deposition.

It is a further object of the present invention to reduce steam requirements below five moles of steam for every carbon atom bound in a hydrocarbon or oxygenated hydrocarbon molecule in the feedstock, particularly for those feedstocks having average carbon numbers of 2 or more.

It is a further object of the present invention to provide a catalyst especially well-suited for use in the hydrogen production method of the present invention.

These and other objects of the present invention, individually or in combinations thereof, have been satisfied by the discovery of a hydrogen production process comprising:

combining a first feedstream and a second feedstream to produce, in a pre-reforming reactor, a first product stream comprising CH₄and H₂O, wherein

- the first feedstream comprises a mixture of H₂and at least one selected from the group consisting of hydrocarbons having two or more carbon atoms and alcohols having two or more carbon atoms, wherein the mixture has a hydrogen stoichiometric ratio (λ) of at least 0.1, and the second feedstream comprises steam;

feeding the first product stream into a reforming reactor; and

reacting the first product stream in the reforming reactor to produce a second product stream comprising CO and H₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood by reference to the figures.

FIG. 1 is a schematic diagram of a hydrogen production process

FIG. 2 is a propane conversion versus time plot for catalysts according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a hydrogen production process comprising combining a first feedstream and a second feedstream to produce, in a pre-reforming reactor, a first product stream comprising CH₄and H₂O. The first feedstream comprises a mixture of H₂and at least one member selected from the group consisting of hydrocarbons having two or more carbon atoms and alcohols having two or more carbon atoms, the mixture having a hydrogen stoichiometric ratio (λ) of at least 0.1. The first product stream is fed into a reforming reactor and reacted in the reforming reactor to produce a second product stream comprising CO and H₂.

It is noted that within the context of the present invention the designations “first feedstream” and “second feedstream” are used to distinguish the two feedstreams, but does not designate their order of addition into the pre-reforming reactor. Hence the “first feedstream” can be introduced into the pre-reforming reactor either before, after or simultaneously with the “second feedstream” or the first and second feedstreams can be combined prior to introduction into the pre-reforming reactor as a single combined feed.

The process of the present invention will be further described with reference to the Figures. FIG. 1 shows a typical hydrogen production process 1, in which feedstock gas 2 (the first feedstream of the present invention) is co-fed to pre-reforming reactor 3 with steam 4 (the second feedstream of the present invention). The product from the pre-reforming reactor (first product stream of the present invention) is subsequently fed to the reforming reactor 5, with or without the additional feed 6. Additional feed 6 may include further steam injection, air or other non-air oxidant injection, mixtures of desulphurized gases such as hydrocarbons, carbon oxides, inerts, etc., for instance as is practiced in reforming for the production of synthesis gas. The product from the reformer (second product stream of the present invention) then exits the reforming reactor 5 to the balance of the plant. Optionally, the feedstock gas 2 can be treated in a desulphurizing process 7. If the feedgas possesses bound organic sulphur compounds such as mercaptans, thiols, etc. then the desulphurizing process 7 usually includes a hydrogenation step, and a hydrogen-containing gas 8 is added to the feedgas to facilitate sulphur removal. The generalized hydrogen production process 1 illustrated in FIG. 1 can be used to describe both the prior art hydrogen production process for C2+ feedstocks and the present invention. The pre-reforming reactor 3 is operated at temperatures from 350° C. to 600° C. inlet temperature both in the present invention and in the prior art. For best freedom from sulphur poisoning, the pre-reformer of the present invention may be operated at temperatures of 400° C. or higher. For best freedom from coke formation in the preceding heat exchange steps, the reactor may be operated at or below 550° C. Operating pressures for pre-reforming reactors in the prior art and the present invention may be between atmospheric pressure and 50 MPa. Common industrial embodiments are operated between essentially atmospheric pressure and 5 MPa. These typical operating conditions are recited in Twigg, supra, and Rostrup-Nielsen, J. R., Catalytic Steam Reforming, Reprint from Catalysis-Science and Technology, Springer-Verlag, 1984, hereby incorporated by reference, among other sources.

In the present invention, the amount of hydrogen contained in the feedstock gas 2 is increased compared to prior-art processes. The proportion of hydrogen employed in the feedstock 2 can be best described by a hydrogenation stoichiometric ratio, or hydrogen stoichiometric ratio, or hydrogen stoichiometry (hereafter “hydrogen stoichiometry”), λ, for the hydrogenation reaction of a given hydrocarbon to methane. To define λ, it is convenient to represent the average chemical composition of the feedstock molecules in a feed mixture containing hydrocarbons and oxygenated species as C_xH_yO_z, where

$x = \sum_{i} n_{i} x_{i}$

$y = \sum_{i} n_{i} y_{i}$

$z = \sum_{i} n_{i} z_{i}$

and

i is the number of molecular species in the mixture;

n_iis the mole fraction of the i-th molecular species in the mixture; and

x_i, y_iand z_iare the mole fractions of C, H and O, respectively, in the i-th molecular species.

The hydrogen stoichiometry, λ, is then defined in terms of molar flow rates, F, to the hydrogen production process as follows:

$C_{x} H_{y} O_{z} + (2 x - \frac{y}{2} + z) H_{2} \overset{catalyst}{\to} x {CH}_{4} + z H_{2} O$

$λ = \frac{F_{H_{2}}}{F_{C_{x} H_{y} O_{z}} (2 x - \frac{y}{2} + z)}$

For typical hydrocarbon feedstocks, molecules containing other atoms such as nitrogen or sulphur may be present, but the concentration of these molecules is not normally high. Prior-art pre-reformers operating with 1% to 5% di-hydrogen by volume as a ratio to the hydrocarbon feed, for a nominal propane feed (C₃H₈) would possess a hydrogen stoichiometry, λ, of between 0.005 and 0.026.

In one embodiment of the present invention, the hydrogen stoichiometry, λ, is preferably at least 0.1. In another embodiment of the present invention, λ is greater than or equal to 0.2, and also less than or equal to 1.5. In another embodiment of the present invention, λ is greater than or equal to 0.25, and also less than or equal to 1. The increased amount of hydrogen in the method of the present invention may be added prior to the desulphurizing process 7 as part of hydrogen-containing gas 8, or it may be added prior to the pre-reforming reactor 3 as stream 10. Further, distributed injection of the hydrogen may be practiced through the pre-reformer 3. The relative amounts of hydrogen added at any of these locations does not limit the application of the present invention.

As noted above, when the level of added hydrogen is at very low hydrogen stoichimetry (i.e. λ is less than 0.1), the catalyst gets deactivated by carbon formation. At very high hydrogen stoichiometry (i.e. λ is greater than 1.5), the equilibrium for the subsequent steam methane reforming and water gas shift reactions is adversely affected according to Le Chatelier's principle. However, the present inventors have found that by adding molecular hydrogen (H₂) in the present range (i.e. 0.1<λ<1.5), catalyst deactivation is suppressed while also leading to rapid reaction of C2+ molecules to C1 (methane) product.

The addition of hydrogen in the amounts of the present invention advantageously promotes the hydrogenation reaction. This reaction is exothermic, whereas the steam reforming reaction is endothermic. Thus, the method of the present invention advantageously compensates for the usual drop in temperature, and thus reaction rate, observed in traditional methods. Furthermore, the method of the present invention can advantageously be employed to obtain a temperature increase in the pre-reforming reactor 3. This temperature increase promotes both the hydrogenation reaction rate and both the rate and equilibrium limits to the steam reforming reaction. Thus, the present method advantageously increases conversion of C2+ hydrocarbons and alcohols via hydrogenation while also enhancing the rate and extent of conversion possible through the steam reforming reaction in the same reactor.

The method of this invention is not catalyst specific and can be applied to various reforming catalyst normally used in the art.

In a preferred embodiment, the catalyst used in the present process is a catalyst having an active component supported on a non-reducible oxide support. Suitable active components include, but are not limited to, Pt, Rh, Ru, Ni, Co, Pd, Ir and any combination thereof. Suitable supports include, but are not limited to, TiO₂, ZrO₂, alkaline-earth metal hexaaluminates (preferably barium hexaaluminate), monoclinic zirconia or alumina. The support of the preferred catalyst has a preferred surface area of from 10 to 250 m²/g, preferably from 80 to 180 m²/g.

One preferred catalyst formulation is described in U.S. Patent Application Publication No. US 2005/0232857, the contents of which is incorporated by reference herein in its entirety, and contain as active metal at least one of Ir, Pt and Pd, on a support of monoclinic zirconia or an alkaline-earth metal hexaaluminate.

The oxide support of the preferred catalyst can optionally include one or more surface area stabilizers. Suitable surface area stabilizers include, but are not limited to, REO (La, Ce, Pr, Nd, Sm), Zr, Y, Ti, Sc and combinations thereof. Such stabilizers can be present in the support in an amount from 0 to 30% by weight, preferably from 5 to 20% by weight, based on amount of support.

The preferred catalyst formulation can be provided in any desired physical form. Preferably the supported catalyst is in a form selected from the group consisting of powders, granulates, tablets, extrudates, and washcoats on ceramic or metallic monoliths or tubular structures.

In a further embodiment of the present invention, the present process is performed with a molar ratio of steam per carbon atom in the first feedstream, S:C, that is less than 5, preferably less than 4, more preferably between 3 and 4. By using the hydrogen stoichiometry required in the present invention, the present process enables the use of a hydrocarbon feedstocks having higher molecular weight (more carbon atoms per molecule), with low steam to carbon ratios being useable.

EXAMPLES

The following Examples illustrate certain specific embodiments of the disclosed invention and should not be construed as limiting the scope of the invention as many variations are possible within the disclosed invention, as will be recognized by those skilled in the art.

Example 1

Propane was used as hydrocarbon feedstock having three carbon atoms. A reactor vessel having 1.4″ ID was loaded with 10 g of catalyst having 1 wt % Ir deposited on a non-reducible oxide support comprising barium hexaaluminate, making an approximately 1 cm deep catalyst bed. The pre-reforming reaction was conducted under nearly adiabatic conditions. Two ˜2 cm deep layers of 3 mm glass beads were placed below and above the catalyst bed to provide uniform flow of reacting gas through the bed. Two thermocouples were installed just below and above the catalyst bed to measure the temperature differential across the catalyst.

The reactor was placed in a furnace and the furnace temperature was set constant at 450° C. Propane and steam flows were constant at a molar steam to carbon ratio, or S:C=3.7. Hydrogen flow was changed stepwise between hydrogen stoichiometry λ of 1.5, 1, 0.5, 0.25 and 0.13. Overall gas space velocity was approximately 35,000 l/hr. The reactor was stabilized for about 1 hour at each step before acquiring a sample of the reformate gas and switching to the next setting for H₂flow. After the first set of testing the catalyst was aged overnight under the reaction conditions with highest hydrogen flow and then the measurements were repeated the next day.

Table 1 shows conversion of C₃H₈into C₁species (CH₄and CO₂, below detectable amounts of CO were observed for all samples) and ΔT between inlet and outlet (T_in−T_out) of the catalyst (negative sign indicates temperature increase over the catalyst). Increasing negative values of ΔT indicate a high degree of the exothermic methanation reaction according to the present invention, whereas high positive values of ΔT indicate endothermic steam reforming reaction dominates the observed conversion of the propane feedstock.

TABLE 1

Hydrogen
C₃H₈

stoichiometry, λ
conversion
ΔT

1.5
95
−56

1
93
−47

0.5
91
−31

0.25
87
−20

0.13
83
−13

After aging overnight

1.5
95
−60

1
93
−48

0.5
91
−34

0.25
87
−22

0.13
83
−14

In all cases, the addition of hydrogen according to the method of the present invention achieves extensive conversion of C2+ hydrocarbons in the feedstock and exhibits exothermic reaction consistent with the desired methanation reaction.

Example 2

The catalyst of Example 1 was aged for about 1500 hrs in the steam methane reforming (SMR) reaction. The catalyst was then removed and loaded into the reactor of Example 1. The same testing procedure was used as described above. Table 2 shows the results for the second catalyst testing.

TABLE 2

Hydrogen
C₃H₈

stoichiometry, λ
conversion
ΔT

1.5
30
−21

1
23
−14

0.5
13
−1

0.25
5
10

0.13
1
12

After aging overnight

1.5
19
−11

1
16
−5

0.5
9
3

0.25
4
9

0.13
1
10

For an aged catalyst, the method of the present invention yields a surprising increase in C2+ hydrocarbon conversion with increasing hydrogen stoichiometry, λ. Furthermore, the exothermic temperature change increases with increased hydrogen stoichiometry within the inventive range.

Example 3

Ten grams of fresh FCR-69-4 catalyst obtained from Sud-Chemie Corporation was loaded into the same test vessel. The same testing procedure was used as described above, with results shown in Table 3. This catalyst has a metal loading of approximately 4 wt % Iridum on an alumina carrier promoted with a mixture of rare earth oxides, namely, CeO₂at 14-20 wt %, La₂O₃at 1-5 wt %, and Y₂O₃at 1-5 wt %, based on amount of catalyst.

TABLE 3

Hydrogenation

stoichiometry
C₃H₈

of feed, λ
conversion
ΔT

1.5
94
N/A

1
92
N/A

0.5
87
N/A

0.25
81
N/A

0.13
76
N/A

After aging overnight

1.5
88
N/A

1
86
N/A

0.5
80
N/A

0.25
73
N/A

0.13
67
N/A

Example 4

Ten grams of fresh FCR-69-1 catalyst obtained from Sud-Chemie Corporation was loaded into the same test vessel. FCR-69-1 has active metal loading of 1 wt % but is otherwise identical to the FCR-69-4 catalyst of the Example 3. The same testing procedure was used as described above testing, and the results are shown in Table 4. Even with a reduction in metal loading to ¼ of the value in Example 3, extensive conversion of feed was achieved in proportion to the hydrogen stoichiometry.

TABLE 4

Hydrogen
C₃H₈

stoichiometry, λ
conversion
ΔT

1.5
92
−66

1
81
−59

0.5
58
−34

0.25
20
0

0.13
11
8

After aging overnight

1.5
57
−55

1
54
−49

0.5
32
−17

0.25
14
2

0.13
6
8

FIG. 2 shows the relative conversion C₃H₈versus time for the rare earth oxide promoted catalyst of Example 3 and 4 versus the unpromoted catalyst of Examples 1 and 2. The rare earth oxide promoted catalyst shows a surprising advantage in deactivation rate compared to the unpromoted catalyst. However, the hydrogen stoichiometry λ for the promoted catalysts was uniformly higher than that for the unpromoted catalyst, such that the effects of increased hydrogen stoichiometry and catalyst composition can not be readily separated. In all cases, catalysts provided with hydrogen stoichiometries below the inventive range deactivated within less than two days onstream over multiple tests at different metal loadings and steam to carbon ratios. Both the promoted and unpromoted catalysts possess substantially lower active metal loadings than prior art pre-reforming catalysts. Further, neither catalyst is promoted with alkaline earth promoters with their attendant disadvantages.

Claims

1. A hydrogen production process comprising combining a first feedstream and a second feedstream to produce, in a pre-reforming reactor, a first product stream comprising CH4 and H2O, wherein the first feedstream comprises a mixture of H2 and at least one selected from the group consisting of hydrocarbons having two or more carbon atoms and alcohols having two or more carbon atoms, said mixture having a hydrogen stoichiometric ratio (λ) of at least 0.1; andthe second feedstream comprises steam;feeding the first product stream into a reforming reactor; andreacting the first product stream in the reforming reactor to produce a second product stream comprising CO and H2.
2. The hydrogen production process according to claim 1, wherein 0.2≦λ≦1.5.
3. The hydrogen production process according to claim 1, wherein 0.25≦λ≦1.
4. The hydrogen production process according to claim 1, wherein the first feedstream is produced in a desulphurizing process in which the H2 is combined with the at least one member selected from the group consisting of hydrocarbons having two or more carbon atoms and alcohols having two or more carbon atoms.
5. The hydrogen production process according to claim 1, further comprising feeding into the reforming reactor with the first product stream an additional feedstream comprising one or more members selected from the group consisting of steam, air, non-air oxidants, mixtures of desulphurized gases selected from hydrocarbons, carbon oxides, and inert gases.
6. The hydrogen production process according to claim 1, wherein, in the pre-reforming reactor, reaction is conducted using a catalyst comprising an active metal dispersed on a support, where the active metal comprises at least one metal selected from the group consisting of Pt, Rh, Ru, Ni, Co, Pd, Ir and combinations thereof.
7. The hydrogen production process according to claim 6, wherein the support is at least one member selected from the group consisting of TiO2, ZrO2, alkaline-earth metal hexaaluminates, monoclinic zirconia and alumina.
8. The hydrogen production process according to claim 6, wherein the active metal is at least one member selected from the group consisting of Ir, Pt and Pd; and the support comprises at least one member selected from the group consisting of monoclinic zirconia and an alkaline-earth metal hexaaluminate.
9. The hydrogen production process according to claim 1, wherein the molar ratio of steam to carbon atom contained in the first feedstream, S:C, is less than 5.
10. The hydrogen production process according to claim 9, wherein S:C is less than 4.
11. The hydrogen production process according to claim 10, wherein S:C is from 3 to 4.
12. The hydrogen production process according to claim 7, wherein the active metal is at least one member selected from the group consisting of Ir, Pt and Pd; and the support comprises at least one member selected from the group consisting of alumina and an alkaline-earth hexaaluminate; and the catalyst further comprises one or more rare earth oxide promoters.
13. The hydrogen production process according to claim 12, wherein the catalyst is a promoted catalyst having Ir as active metal on alumina, with the promoters being a combination of 14-20 wt % of CeO2, 1-5 wt % of La2O3, and 1-5 wt % of Y2O3.
14. The hydrogen production process according to claim 1, wherein the catalyst is in a form selected from the group consisting of powders, granulates, tablets, extrudates, and washcoats on ceramic or metallic monoliths or tubular structures.
15. The hydrogen production process according to claim 1, wherein the first feedstream is introduced into the pre-reforming reactor prior to the second feedstream being introduced.
16. The hydrogen production process according to claim 1, wherein the second feedstream is introduced into the pre-reforming reactor prior to the first feedstream being introduced.
17. The hydrogen production process according to claim 1, wherein the first feedstream and second feedstream are introduced into the pre-reforming reactor simultaneously.
18. The hydrogen production process according to claim 1, wherein the first feedstream and second feedstream are combined prior to introduction into the pre-reforming reactor.

Provisional Applications (1)

	Number	Date	Country
	60912495	Apr 2007	US

HYDROGEN PRODUCTION PROCESS

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Provisional Applications (1)