AU427390B2 - Production of reducing gas - Google Patents

Description

2 7 7 00171 The present invention relates to the production of reducing gas. Specifically, it relates to a process for the production of hot reducing gas of high strength by the cat-

alytic steam reforming of hydrocarbons employing two catalyst

stages. A considerable interest has recently been shown by the -metallurgical industry in the development of processes

which would enable hot, high strength reducing gas to be produced in an efficient economical fashion. The minimum strength of such gases are contemplated to be no less than

88 mcle percent of H 2 CO and preferably the H 2 CO content should be at least 93 mole percent or higher. The delfvery temperature of the reducing gas should be at least 17500

and preferably within a range from about 18000 F to about 21000F. The reducing gases should subsequently be employed

in various direct reduction processes of metallic ores, e.g. iron ore, and also as a partial replacement for coke in blast

furnace operations, thereby minimizing coke consumption and increasing furnace capacity. It is well known in the art that gases containing hydrogen and carbon monoxide can be ;roduced by the catalytic steam reforming of hydrocarbons such as methane (natural gas)

and higher hydrocarbons, by the contacting of hydrocar- bons and steam over a suitable steam reforming catalyst,

a nickel-containing catalyst. For instance, in the

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case of methane the reaction proceeds according to the fol- lowing equation:- CH4 H2 0 CO0+ 3H12(i The stoichiometric requirement of moles of steam per carbon atom in the feed, what is known in the art as the steam/car- bon ratio, is 1.0 according to the above equation. However, other reactions can occur within the reforming zone such as the carbon monoxide disproportionation reaction: CO C0 2 +C (2) and the cracking reaction: 0114 C +2H 2 (3) The above reactions and both result in the deposition of carbon on the catalyst, which in turn causes deactivation of' catalyst and in extreme cases Plugging of the catalyst bed. In order to avoid such detrimental carbon deposition it is the general practice to employ'a consider- able excess of steam, and commercial steam to carbon ratios lie usually in range between about 3:1 and about 10:1. Due to the inherent presence of large amounts of excess steam in the product gas at such high steam-to-carbon. ratios, it is impossible to produce a hot, high strength re- ducing gas directly by means of steam reforming. The efflu- ent from the steam reforming zone would necessarily have to be treated' in at least two stages to remove the excess water, usually by means of .a cooling step to condense the steam followed by a heating step to raise the temperature of the uncondensed gas. The use of large volumes of excess steam in the reformer effluent require large and expensive equip- ment within the overall process. Furthermore, the utility requirements necessary to supply the large quantities of

3- 4 27 39 0 excess steen to the reformZ oPeUaC the required tempera- ture and pressure, to partially condense the reformer effluent and to reheat the uncondensed gas, are so large that such a process is- rendered uneconomical.

In the prior art, it has been suggested that a

reducing gas can be prepared directly by reforming of hydro-

carbon feed, in the presence of a conventional reforming

catalyst provided that a considerable amount of hydrogen is

added to the hydrocarbon feed to the reforming zone in order

to suppress the formation of carbon on catalyst and to act

as an aid in achieving the desired reducing gas strength.

It is also known in the prior art, U.S. Patent No. 3,119,667, that if steam reforming of hydrocarbons is carried out .in the presence of a nickel catalyst containing added active a&lkal'ine metal compound, the steam to carbon ratio requirement for trouble-free operations can be lowered to a great degree. However, the alkalized catalyst is unstable at very high reforming temperatures due to the volatility of -the alkaline components included in the catalyst, which -results in fouling 'of equipment and short catalyst life. In zacordance with the present invention there is provided a process for the production of high strength re- ducing gas having a minimum content of no less than 88 mole per cent of H2 +CO which comprises passing a hydrocarbon feed and steam through a first bed of reforming catalyst contain-

ing at least 0.5 weight per cent of an added active alkaline

metal compound under steam reforming conditions including :firsot bed outlet temperatures of at least 1225F., pressures

ranging from atmospheric pressure to 250 psig and steam to

carbon ratios less than 1.5:1 to produce a partially reformed

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2-,7 PU 71 gas from said first bed through a second bed of reforming catalyst, said catalyst in its fresh state being substan- tially free from added active alkaline m~etal compounds, under steam reforming conditions including second bed outlet temperatures of at least 17.50 0 F. One advantage of the invention is that by the use of an alkaline reforming catalyst in the first stage, the formation and deposition of carbon on the catalyst is avoided, which otherwise would occur at the aforementioned elevated reforming temperatures and low steam to carbon ratios,. Such formation of carbon is mainly due to the disproportionation of carbon monoxide to carbon dioxide and carbon. Another advantage of the two-stage catalyst system employed in the process of this invention is that due to the appreciable conversion of methane with steam into hydrogen and carbon monoxide in the first contacting step, the concentration and partial pressure of methane in the second stage are relative- ly low. Such conditions are unfavorable both from a thermody- namic and kinetic standpoint for the deposition of carbon due to cracking of methane, which otherwise could occur at the high temperatures employed in the second contacting step. A high strength reducing gas is defined as a gas containing at-least 88 mole percent of hydrogen and carbon monoxide on a wet basis. The reforming zone can be of an~y design or configu- ration for carrying out the desired reaction. For instance each contacting step-can be carried out in a separate chamber Within the reforming zone. Advantageously, each reforming zone is comprised of a vertical tubular reactor positioned within a furnace and heated by hot products of combustion.

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2 700 i71 ting in parallel are usually provided within a single furnace.

Each tubular reactor is then charged with both types of

catalyst in such a manner that one catalyst bed is directly

superimposed on the other.

The feed to the reforming zone can be normally

gaseous or normally liquid hydrocarbons or mixtures there-

of. Furthermore, the process of this invention is applicable

to a feed containing aliphatic and aromatic hydrocarbons

including acyclic and alicyclic paraffinic and olefinic

organic components. Highly unsaturated hydrocarbons such

as acetylenes, diolefins, etc. can also be included in the

feed. Preferably the feed should be a hydrocarbon stream

having an average molecular weight of about 60 or below or

having an average of about 4 carbon atoms and less per mole

of feed. It is to be understood that the aforementioned

preferred composition of the fead can be had with an

infinite number of combinations of individual hydrocarbon

components, i.e. the feed can be made up of a mixture of

hydrocarbon components, the individual components having 4

or more carbon atoms as well as 4 or less hydrocarbons per

molecule. Most preferably the feed should be one of which

a substantial portion is methane, e.g. natural gas and

methane-rich cracked refinery gases are excellent feed

sources

In such cases, where the hydrocarbon feed contains

appreciable amounts of sulfur, it is usually recommended to

subject the feed to a desulfurization step prior to its

introduction into the reforming zone. Recommended maximum

amounts of sulfur in the feed which can be tolerated in the

reforming zone without causing deterioration of catalyst

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27 700/7' million activity and selectivity are about 10 parts per should be on a weight basis and advantageously said amounts 1 part less than 5 parts per million, typically less than per million. The amount of steam fed to the reforming zone moles depends on the hydrocarbon feed rate. The number of of steam charged to the reform~ing zone per at .omof carbon charged, hereinafter called the steam to carbon ratio, should not exceed 1.5:1 and preferably it should be main- tained below 1.4:1. The lower operable limit of steam to carbon ratio is generally slightly above 1:1 in the case

of single pass operations. However, this limit can be

lowered to a considerable degree when a hydrogen-rich gas, such as for instance a spent reducing gas from a subsequent the reduction zone is recycled to the reforming zone. In latter case, the spent gas is usually subjected to one or more treatment stages to at least partially remove water and carbon dioxide from said gas prior to its recycling to the reforming zone. The steam and hydrocarbon feed are first con- tacted in the presence of a steam reforming catalyst con- taining an added active alkaline metal compound. The term alkaline metal compound as used herein is intended to in- clude the &lkali metal compounds and alkaline earth metal compounds. Of these the alkali metal compounds, e.g., compounds of sodium, lithium and potassium are preferred. Thie active catalytic component for the steam reforming reaction occurring in the first stage is usually a member of the iron group of the Periodic Table, such as nickel, cobalt, etc., and is included in the catalyst- in amounts 427390O

27 700/71 exceeding about 4 weight percent, calculated as the oxide.

Generally, the aforementioned component is present in

amounts ranging from about 10 to about 50 weight percent but

even higher concentrations can be used with advantage. The

amount of added active alkaline metal compound (calculated

as the metal) in the catalyst should be at least 0.5 weight

percent and preferably said amount should be between about

2 and about 20 weight percent. The catalyst is also com- prised of a support and may also contain a binder. Such supports and binders are well known'in the art and need not

be described here in detail. Examples of catalysts espec- ially suited for the first stage of the process of the invention are those disclosed in U.S. Patent No. 3,119,667 or the commercial catalyst 46-1 available from Imperial, Chemical Industries, and which contains about 7 weight per-

cent of potassium calculated as the oxide. It is to be understood that the term "added active alkaline metal compound", is employed in the same manner as in the aforementioned United States Patent No. 3,119,667 with respect to the various methods of preparation of the catalyst as well as to the compositions of the catalyst supports and binders. Specifically, since the supports and binders can contain substantial amounts of various alkaline metal compounds in combination with other components which are capable of neutralizing said alkaline metal compounds the added alkaline metal component should be added in an amount sufficient to compensate for any loss due to such neutralization and still have the net requisite amount of active uncombined alkaline compound to achieve reduction of steam requirements. 4.

2770 0 7' The effluent from the aforementioned first'con- tacting step is subsequently contacted with an active, con- ventional steam reforming catalyst exhibiting good heat resistance at high reforming temperatures, at temper- 0 atures of about 1750 F and above. Again, the active cat- alytic component of the catalyst is usually a member of the iron group of the Periodic Table, nickel'or cobalt. Advantageously, the content of the active component is -larger in this catalyst than is the case with the first stage catalyst. In a fresh state, the catalyst employed in the second contacting step should contain at most only small amounts of added active alkaline metal components and most preferably the catalyst should be substantially free thereof. Examples of commercially available catalysts particularly suited for the second contacting steps are 54-2 and 57-1 (Imperial Chemical Industries) and G 56-B (Girdler). As in case of the first stage catalyst, the support and binder (if used) can contain substantial amounts of various alkaline metal compounds in a combined or neu- tralized state. The relative amounts of first stage and second stage catalysts can vary over a rather wide range. Gener- ally, the volumetric ratio of the first stage catalyst to the total catalyst inventory lies in a range from about 0.2:1 to about The actual ratio employed depends to some degree upon the inlet temperature of the first stagp, the outlet temperature of the second stage and the temperature profile throughout the reforming zone. Suffi- cient amounts of first stage catalyst should be charged to the reforming zone such that the portion of the total

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27TC'O71 reforming reaction which occurs at temperatures above 12250F, preferably inl the range of 1225 0F to 1500 0F be conducted in the presence of said first stage catalyst. within the afore- mentioned temperature range there is a narrower temperature region in which carbon is otherwise 4qposited when opera- tions are carried out at the low steam to carbon ratios of the invention in the presence of a conventional alkaline- free reforming catalyst. Owing to the volatility at very high temperatures of the alkaline metal components the temperature of'the first stage catalyst bed should not exceed 1600OF and pre- ferably the maximum temperature should be no higher than 0 About 1500 F. Preheating of the reactant~s renders attainment and maintenance of suitable temperatures in the reforming zone easier. Generally, the hydrocarbon feed is preheated 0 to between about 600 F and about 1200 0 F. This is of some importance because if the feedstock contacts the catalyst at excessively low temperatures, an otherwise adequate steam to carbon ratio will not prevent carbon formation at or near the entrance to the catalyst bed. Since catalytic steam reforming is endothermic and there are practical limits to the amounts of heat which can be added to maintain suit- able elevated temperatures in the reforming zone, it is preferred to preheat the feedstock to as high a temperature as is consistent with avoiding pyrolysis or other heat de- terioration of the feestock. The preheat can be carried out by indirect heat exchange either prior to the introduction of the feed to the reforming zone, or, where applicable, prior to the introduction of the feed to a desulfurization I11 4 2 73 z one. The outlet temperature of the second stage should be substantially consistent with the temperature requirement of the product gas at the inlet to a subsequent contacting zone, a reducing zone.. Advantageously, the outlet temperature is maintained. at' least at 1750 0F and preferably said temperature should range between about 1800 0 F and 0 about 2100 F. The pressures employed in the process of the-in- vention range from substantially atmospheric pressure to about 250 psig. The actual pressure to be used in the process is mainly dictated by the pressure of the subsequent reduction zone measured at the spent reducing gas outlet and taking into account all pressure drops occurrin'g within said reduction zone and in other necessary appurtenant equipment such as valves, piping, etc.. Preferred pressures in the steam reforming zone range from about 10 psig to about 150 psig. The space velocities based on the combined cat- alyst inventories of the first and second stage should be at least 400 standard cubic feet of methane equivalent in the hydrocarbon ffeed per hour per cubic foot of catalyst and advantageously the space velocity should range between about 500 atnd about 2000. zr even hIghsr; The following examples are offered as a better understanding of the present invention and are not to be construed as unnecessarily limiting thereto. Example 1 The bottom portion of a pilot plant tubular re- forming reactor having an ID of 2.79 inches was charged

2? 7OO^ Industries catalyst with 17,097 gms. of Imperial Chemical rings which 54-2 in the shape of 5/8" x 5/8" Raschig total reactor length. occupied 14 feet 4 3/4 inches of the 46-1 (Imperial Chemical A charge of 12,675 gms. of catalyst to O10feet of Industries) occupying a space corresponding oh top of the 54-2 cat- the reactor was charged directly of 5/8" alyst bed. The 46-i1 catalyst was also in the shape

x 5/8" Raschig rings. The volume of the total catalyst and the volumetric inventory amounted to 1.07 cubic feet to the total cat- ratio of first stage alkalized catalyst

alyst inventory was 0.38. The reactor was positioned

having an inside Within an electric resistance furnace said furnace being diameter of 23 inches, the walls of

resistance heated. natural The hydrocarbon feed was a desulfurized

gas having the following composition: Component Mole.% I 0.2

'002o 0.7 C04 95.6 C0H 2.9

C3 H6 o.4 CH6 0.1

C5 H1 2 0.1 Total 100.0

The steam and the hydrocarbon feed were separately to 9140 F in a measured and subsequently mixed, preheated the top of the tubular gas fired furnace and introduced to psig pressure (at reactor which was maintained under 58

the reactor outlet). The steam rate was 1538 gm. moles/hr 1 14 427 3

27 7l0 71 and the hydrocarbon feed rate was 1192 gm. moles/hour

resulting in a steam to carbon ratio of 1.23 and a methane

equivalent space velocity of 970. The following temperatures

were maintained in the reforming zone: at the inlet 9730F, at the catalyst interface 1450 F and at the outlet 1810 F.

Under the aforementioned conditions the overall

methane equivalent conversion was 97.8 mol percent. The effluents from the first and second stages had the follow-

ing compositions (on a nitrogen-free basis):

Component First Stage Second Stage Mole percent Mole percent

H2 51.0 70.5 CH4 14.6 CO 11.3 22.4 CO2 5.0 1.2

C2 H6 0.2 17.9 5.4 Total 100.0 100.0 The length of the run was 10 hours, during which time there was no indication of any carbon deposition. Example 2 The run described in this Example was carried out in the same equipment as the one of Example 1. The reactor contained at the top thereof 10 feet of first stage catalyst (Imperial Chemical Industries 46-1) weighing 11,958 gms.

The bottom bed consisted of 14 feet 3-1/4 inches of Girdler

G -56 B catalyst in the shape of 5/8" x 3/8" Raschig rings

weighing 17,549 gms. The volume of the total catalyst inventory was 1.01 cubic feet. Reactor pressure at the out- let was maintained at 21 psig. The feed used in this example j> i 427390

277 0i' 71 was a natural gas supplemented with about 25 percent of a

cracked gas oil containing considerable quantities of

unsaturated hydrocarbons. The mixed feed stock had the

following composition: CompDnent Mole Percent

N 1.4

C02 o.6 H2 4.9

CH4 77.8 C2H2 0.2

C24 5.3 C2 H6 2.6

C3 4 0.1

CA1 0.9 CH 8 5.4 C4 H6 0.2 C4 H8 0.1

C4 Hio 0.1

C6 0.1

C548 0.1 CSH 1 2 0.1

C6 H6 0.2 After desulfurization, the feed was mixed with

steam, preheated to 848 F and introduced to the top of the reactor at a hydrocarbon feed rate of 563 gm. moles per hour and a steam rate of 767 gm. moles per hour. These rates corresponded to a steam to carbon ratio of 1.2 and a methane equivalent space velocity of 545 standard cubic

feet/hr/cubic feet of catalyst. The reactor temperatures

were maintained at 980 F at the inlet, at 1447 F at the 4 Z'I 3 v 0

92- 70C (:71 catalyst Interface and at 18140?F at the outlet. Samples of effluents having the following composition (on a nitrogen- free basis) were withdrawn from the catalyst interface and *the bottom of the reactor: First Stage -Second Stage Component Mole Percent Mole Percent H2 52.2 70.7 CH4 13.7 0.3 Co 1.1.5 24.1 CO2 6.4 1.6 02H6 0.7 C3 H8 0.3 H2 0 1523.3 Total 100.0 100.0 The total methane equivalent conversion amounted to 98.7%. There were no indication of any carbon laydown on the catalyst during the course of the experiment.

1 cample 3 The experiment was conducted with the same hydro- carbon feed over the same catalyst as in Excample The reactor outlet pressure was increased to 58 psig. The steam rate and the hydrocarbon feed rates were maintained at 1290 and 1053 gin. moles/hour respectively, which corres- ponded to a steam to carbon ratio of 1.04 and a methane equivalent space velocity of 1019. The reactants were preheated to 8870 and introduced to the reformer reactor which was maintained at 98-5 0F at the inlet, at 14510? at 0 the catalyst interface and at 1815 F at the outlet. A total methane equivalent conversion of 95.7 mole percent was obtained and gas samples collected after the first and second stages had the following compositions (on a nitrogen-

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2-1 "700 free basis): -First Stage Second Stage Component Mole Percent Mole Percent

2 45.9 70.8 CH18.5 1.2 CO 114. 251.3 Co2 o.6

C2 H6 1.2-

C3 f8 17.1 2.1 Total 100.0 100;0 Ther~e was no indication of any carbon deposition during the-course of this experiment-.which lasted.6 houres.

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Claims (4)

27 70 071 The claims defining the invention are as follows:

1. A process for the production of high strength re-

ducing gas having a minimuam content of no less than 88 mole

per cent of H2 +CO which comprises passing a h ydrocarbon

feed and steam through a first bed of reforming catalyst

containing at least 0-5 weight per cent of an added active alkaline metal compound under steam reforming conditions in-

cluding first bed outlet temperatures of at least 12250?.,

pressures ranging from atmospheric pressure to 250 psig and

steam to carbon ratios less than 1.5:l to produce a parti-

ally reformed gas, and passing said partially reformed gas

from said first bed through a second bed of reforming

catalyst, said catalyst in its fresh state being substan- tially free from added active alkaline metal compounds,

under steam reforming conditions including second bed out- let temperatures of at least 17500F.

2. A process according to claim 1, wherein the

alkaline metal compound is an alkali metal.

3. A process according to claim 1. or 2, wherein

the first bed outlet temperature is maintained in the 0 range from 1225 F. to 15000F.

4- A process according to any one of the preced-

ing claims, wherein the volumetric ratio of first stage

catalyst to the total catalyst inventory is between 0.2:1 and 0.8:1.

A process accring to any one of the preceding claims, a wherein the first and the second beds are contained within tubular reactor, one bed being directly superimposed on the

7 70 0 71 other. 6. A process according to any one of the preceding claims, wherein the catalyst of the first stage contains be- tween-a-beo 2 and &aet20 weight per cent of the added active alkaline metal compound. 7. A process according to any one of the preceding claims, wherein the steam reforming conditions include a pressure between -eabo 10 psig and abeewt 150 psig. 8. A process according to any one of the preceding claims, wherein the space velocity,through the beds of cata- lyst is at least 500 standard cubic feet of hydrocarbons per hour per cubic foot of total catalyst inventory. 9. A process according to any one of the preceding claims, wherein the steam to carbon ratio is maintained below-sbeMt 1.4:1. A process according to any one of the preceding claims, in which the hydrocarbon feed has an average mole- cular weight not exceeding ebtout 0. 11. A process according to claim 10, in which a sub- stantial portion of the hydrocarbon feed is :methane. 12. A process according to any one of claims 1 to 9, wherein the hydrocarbon feed is a normally liquid hydro- carbon stream. 13. A process according to any one of claims 1 to 9, wherein the hydrocarbon feed is a normally gaseous hydro- carbon stream. *4- A process for 'the production of 'high strength re- ducing gas substantially as herein described with reference to the Examples. A reducing gas when produced by the process accord- ing to any one of the preceding claims.

DATED this 2 7700 "71 THIRTEENTH day Of APRIL, 1971 PULLMAN~ INCORPORATED

Patent Attorneys for the Applicant SPRUSON &FERGUSON

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