JPS5914453B2 - Purification method for gas containing unsaturated hydrocarbons - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a method for removing oxygen contained as an impurity in an unsaturated hydrocarbon-containing gas.

When using a gas containing unsaturated hydrocarbons, for example, lower olefins such as ethylene, propylene, and butene, or when using each component separately, its use is limited due to the presence of oxygen as an impurity. has been done.

For example, a catalytic cracker that produces gasoline by thermally cracking light oil in the presence of a catalyst, or a fluid catalytic cracker (FC)
The by-product gas from C) contains unsaturated hydrocarbons such as ethylene, propylene, and butene, saturated hydrocarbons such as methane, ethane, and 10-mouth bread, hydrogen, nitrogen, etc. The by-product gas contains oxygen, and the olefin recovered from the gas contains oxygen, which limits its use. 15 Conventionally, such unsaturated hydrocarbon-containing gases could not be effectively used as chemical raw materials and were only used as fuel.

Recently, there has been increasing interest in the effective use of these unsaturated hydrocarbon-containing gases, and although methods have been proposed to remove oxygen present as an impurity, no fully satisfactory method has yet been found.

Prior art A method for separating or removing oxygen contained in an unsaturated hydrocarbon-containing gas is to bring the unsaturated hydrocarbon-containing gas into contact with an oxygen adsorbent to remove the oxygen contained therein. The adsorption removal method removes oxygen by adsorbing it on an adsorbent, and the 30 hydrogenation method removes oxygen by selectively hydrogenating it by bringing an unsaturated hydrocarbon-containing gas into contact with a catalyst in the presence of hydrogen. It has been known.

Examples of adsorption removal methods include a method in which ethylene for polyethylene synthesis containing oxygen and carbon monoxide is brought into contact with an alkali metal amide dissolved in liquid ammonia under low temperature and high pressure (Japanese Patent Publication No. 37-386035); A method of contacting the fluid with a copper-containing resin obtained by reducing a polymer complex compound of a polymer compound having a pyridine skeleton and a copper salt (Japanese Patent Application Laid-open No. 148684/1984)
, a method in which ethylene is brought into contact with a nickel-based catalyst activated with hydrogen at room temperature in the absence of hydrogen (JP-A-51-1492)
No. 07), a method in which an inorganic gas or saturated hydrocarbon is brought into contact with a heat-resistant oxide with a large surface area supporting iron, nickel, chromium, etc. (Japanese Patent Publication No. 55-10289)
(No. Publication), etc.

Examples of hydrogenation methods include a method in which molybdenum is brought into contact with α-alumina supported in the presence of hydrogen, and a method in which it is brought into contact with a palladium catalyst or a copper-based catalyst (Japanese Patent Publication No. 49-2
No. 5241), a method of contacting a catalyst containing a platinum group metal in the presence of hydrogen and a sulfur compound (U.S. Pat. No. 3,4
80,384).

However, the adsorption removal method requires complicated operations such as regeneration treatment of the adsorbent, and for raw materials with a high oxygen content, it is necessary to perform regeneration treatment more frequently or use a larger amount of adsorbent. Not economical.

Furthermore, the hydrogenation method in which oxygen is removed by hydrogenation is simple and industrially an excellent method for removing oxygen, but with conventional methods, it is difficult to selectively hydrogenate only oxygen, and olefin There was also a drawback that hydrogen was added at the same time. As a result of intensive research to improve these drawbacks of the hydrogenation method, the present inventors discovered a method for selectively hydrogenating only oxygen by using a silver or gold catalyst, and filed a patent application (patent application). No. 54170327).

The present inventors further proposed a catalytic cracking apparatus or F
By removing these impurities from an unsaturated hydrocarbon-containing gas containing acidic substances or sulfur compounds, such as a by-product gas obtained from CC, before hydrogenation and removal of oxygen, the above-mentioned silver or gold can be produced. The present invention was completed by discovering that the activity of the catalyst can be sustained for a long time. Summary of the invention That is, the present invention provides a method for hydrogenating and removing oxygen contained in an unsaturated hydrocarbon-containing gas by contacting it with a silver or (and) gold catalyst in the presence of hydrogen. , after removing the sulfurized compounds contained in the unsaturated hydrocarbon-containing gas in advance in a sulfur compound removal step, by contacting with silver or (and) gold catalyst in the presence of hydrogen, This is an industrially advantageous method for purifying unsaturated hydrocarbon-containing gas that can selectively remove oxygen contained in unsaturated hydrocarbons by maintaining catalyst activity over a period of time.

DETAILED DESCRIPTION OF THE INVENTION Unsaturated Hydrocarbon-Containing Gas The unsaturated hydrocarbon-containing gas in the present invention means a gas containing olefin, diolefin, or acetylene hydrocarbon.

Specific examples of olefinic hydrocarbons include ethylene, propylene, and butenes; examples of diolefinic hydrocarbons include propadiene, 1,4-butadiene, and
, 2-butadiene, pentadienes, hexadienes, and acetylene hydrocarbons include acetylene and methylacetylene. Components other than unsaturated hydrocarbons in the unsaturated hydrocarbon-containing gas include saturated hydrocarbons, hydrogen, nitrogen, carbon dioxide, and impurities such as carbon monoxide, oxygen, hydrogen sulfide, carbonyl sulfide, Examples include chlorine and ammonia. Specific examples of unsaturated hydrocarbon-containing gases include gases whose main component is olefins such as ethylene, propylene, and butene that contain oxygen and sulfur compounds as impurities, or lower olefins that contain oxygen and sulfur compounds as impurities. Contains various by-product gases, such as coke oven gas, oil gas, gases produced during thermal cracking or catalytic cracking of petroleum, and FCC.
The method of the present invention is particularly applicable to by-product gases such as
It is particularly suitable as a method for purifying gases containing olefinic hydrocarbons, such as gases produced by catalytic cracking of petroleum or C gases.

Sulfur Compound The sulfur compound in the present invention refers to hydrogen sulfide, carbonyl sulfide, mercaptans, thiophene, thioether, carbon disulfide, and the like.

Method for removing sulfur compounds The present invention is a method comprising removing sulfur compounds contained in an unsaturated hydrocarbon-containing gas in advance, and then contacting it with a silver or (and) gold catalyst in the presence of hydrogen. .

unsaturated hydrocarbon-containing gases, especially coke oven gases as mentioned above;
By-product gases from oil gas, petroleum catalytic cracking equipment, or FCC are usually hydrogen, paraffinic hydrocarbons such as methane, ethane, and propane, and olefinic hydrocarbons such as ethylene and propylene, as well as hydrogen sulfide and carbonyl sulfide. , carbon disulfide, sulfur compounds such as thiophene, thioether, and mercaptan, and other impurities.
In the case of by-product gas, it varies depending on the petroleum fraction of the raw material, the decomposition method, the method of recovering useful components, etc.

Various known methods can be applied to remove the sulfur compounds contained in these unsaturated hydrocarbon-containing gases. That is, removal of acidic sulfur compounds such as hydrogen sulfide is accomplished by contacting the unsaturated hydrocarbon-containing gas with an alkaline cleaning agent such as caustic or amine. Examples of caustic alkalis include sodium hydroxide and potassium hydroxide; examples of amines include monoalkanolamines such as monomethanolamine and monoethanolamine; dimethanolamine and diethanolamine;
Examples include dialkanolamines such as diisopropanolamine, trialkanolamines such as trimethanolamine and triethanolamine, monoalkylamines such as monoethylamine and monopropylamine, and dialkylamines such as diethylamine and dipropylamine. Usually, these are used after being dissolved in water or an organic solvent. The method and conditions for contacting the unsaturated hydrocarbon-containing gas with the cleaning agent may be any known method and are not particularly limited.

Most acidic substances such as hydrogen sulfide and carbon dioxide can be removed by contacting unsaturated hydrocarbon-containing gas with the above cleaning agent once or twice, but sulfur compounds such as carbonyl sulfide can be completely removed. cannot be removed. Therefore, in the case of raw materials containing neutral sulfur compounds such as carbonyl sulfide as impurities, it is necessary to further remove carbonyl sulfide. Carbonyl sulfide is usually removed after removing acidic substances, but may be carried out before removing acidic substances depending on the composition of impurities. As a method for removing carbonyl sulfide, unsaturated hydrocarbon-containing gas is
1. A method of contacting with a solid absorbent at a high temperature, for example, contacting with an absorbent in which zinc oxide is supported on silica alumina or calcium-silica alumina at a temperature of 150 to 450°C.

2 A method of hydrolyzing carbonyl sulfide by contacting it with water at room temperature or high temperature, for example, hydrolysis in the presence of alumina, silica, bauxite, etc. Hydrolysis at high temperature in the presence of chromia alumina or copper chromia alumina do,
121℃ in the presence of a catalyst containing platinum sulfide supported on alumina
hydrolyzing at a temperature above, hydrolyzing in the presence of a polar solvent such as N-methylpyrrolidone, and contacting with a three-liquid absorbent, such as contacting with xylene, contacting with morpholine, or containing piperazine. Examples include contacting with a solvent.

A particularly desirable method for removing carbonyl sulfide is
Hydrogen sulfide is produced by hydrolyzing carbonyl sulfide by contacting activated alumina, which has a porous structure mainly composed of aluminum oxide and silicon oxide, with a gas containing unsaturated hydrocarbons in the presence of water at temperatures ranging from room temperature to 200°C. Convert it to
Furthermore, the hydrogen sulfide formed is washed with an aqueous solution of caustic or amine, or with a solid base such as zinc oxide,
The method of removal is to use iron oxide or a base such as solid caustic alkali.

The method of the present invention uses such a method to remove oxygen after removing neutral sulfur compounds and acidic substances. In this case, it is desirable that the content of sulfur compounds be small, and the amount It is generally necessary that the sulfur content be 20 ppm or less (by weight, the same applies hereinafter), preferably 10 ppm or less, particularly preferably 1 ppm or less.

Oxygen Removal The unsaturated hydrocarbon-containing gas from which sulfur compounds have been removed by the above method is brought into contact with a silver or/and gold catalyst in the presence of hydrogen to selectively remove oxygen.

Silver or gold, or a mixture of both, is used as the catalyst, with silver being particularly preferred. Although these catalysts may be used in the form of metals, they are preferably used as supported catalysts in which the metals or metal compounds are supported on a carrier by some method. Silver compounds include silver nitrate,
Silver sulfate, silver carbonate, silver nitrite, silver phosphate, silver chloride, silver iodide, silver bromide, silver chlorate, silver perchlorate, silver thiosulfate, silver sulfide, silver cyanide, silver oxide, silver hydroxide , silver oxalate, silver acetate, and various silver complex compounds.
Among them, silver nitrate is preferred.

Examples of gold compounds include chlorauric acid salts such as ferrous chloride, ferric chloride, chloroauric acid, gold cyanide, gold hydroxide, sodium chloroaurate, potassium chloroaurate, and ammonium chloroaurate. Among them, chloroauric acid and chloroauric acid salts are preferable. Examples of the carrier include alumina, silica-alumina, silica, zeolite, diatomaceous earth, zirconia, titania, magnesia, and activated carbon, but alumina, silica-alumina, and silica are preferable.

The amount of the metal or its compound supported on the carrier varies depending on the type of metal and carrier and cannot be unconditionally defined, but it is usually 0.01 to 50% by weight of the metal, preferably 0.01 to 50% by weight.
It is 1 to 20% by weight.

This catalyst may be used in the reaction as it is, but it is usually desirable to activate it by contacting it with hydrogen gas or hydrogen-containing gas or by treating it with a well-known reducing agent such as hydrazine or formalin before use. When contact activation is performed in a gas stream, the contact temperature is 200°C or higher, especially 200°C or higher.
400°C is desirable. The present invention removes oxygen by bringing unsaturated hydrocarbon-containing gas from which sulfur compounds have been removed into contact with the catalyst described above in the presence of hydrogen.
The amount of hydrogen present need only be greater than the theoretical value required to completely hydrogenate the oxygen contained in the gas, and therefore depends on the amount of oxygen contained in the gas.

However, it is usually desirable to use an excess of hydrogen, which does not significantly accelerate the hydrogenation of unsaturated hydrocarbons. For example, the content of oxygen in the gas by-produced during the cracking of the petroleum fraction is at most 0.5% by volume, while the content of hydrogen is 5-15% by volume. but,
If this by-product gas is treated by the method of the present invention, almost no hydrogenation of olefin will be observed, and oxygen can be almost completely removed by hydrogenation. The reaction temperature for removing oxygen is 0 to 400°C, preferably room temperature to 300°C, and more preferably 50 to 200°C.

The reaction pressure is normal pressure to 50 k9/Cdl, preferably 5 to 3
It is 0 kg/d. Further, the reaction is usually carried out in a gas phase flow system, but a batch system may also be used. When the reaction is carried out, for example, in a gas phase or a flow system, the ratio of the mixed gas of unsaturated hydrocarbon-containing gas and hydrogen to the catalyst is determined by the gas hourly space velocity (GHSV).
), usually 100 to 100.0001 preferably 500 to
It is 10.000.

Effect The method for purifying unsaturated hydrocarbon-containing gas of the present invention is a method in which sulfur compounds are removed in advance, and then oxygen is selectively hydrogenated using a silver or/and gold catalyst. There is almost no loss due to hydrogenation, and only the contained oxygen can be selectively removed, and by removing sulfur compounds harmful to the catalyst in advance, the activity of the silver or gold catalyst can be maintained for a long time. Since it lasts long, it is extremely advantageous industrially.

In particular, carbonyl sulfide is hydrolyzed with an activated alumina catalyst in the presence of water, hydrogen sulfide and carbon dioxide are adsorbed and removed by contact with a solid base, such as zinc oxide, and then treated with a silver or/and gold catalyst in the presence of hydrogen. The contacting method is a method for purifying unsaturated hydrocarbon-containing gases that has simple steps, consumes little energy, and is industrially extremely advantageous.

Example of a process for purifying unsaturated hydrocarbon-containing gas Next, an example of a process for carrying out the method of the present invention on an industrial scale will be explained using a schematic process diagram.

In FIG. 1, unsaturated hydrocarbon-containing gas obtained from a coke oven, petroleum catalytic cracker, etc.
Supplied to washing tower 1. In the first washing tower 1, a mixture of a 24% by weight aqueous solution of diisopropanolamine supplied through a pipe 14 and an amine solution recycled through a pipe 15 is supplied to the first washing tower 1.
Supplied from 3. The unsaturated hydrocarbon-containing gas from which sulfur compounds have been removed by contacting the diisopropanolamine aqueous solution in the first washing tower 1 is discharged from the pipe 12 and then
It is supplied to the washing tower 2. Pipe 15 from the bottom of the first washing tower 1
A portion of the diisopropanolamine aqueous solution that has absorbed the sulfur compounds discharged from the pipe is recycled, and the remainder is sent to an amine solution purifier (not shown) through pipe 16 to separate and remove the sulfur compounds, and then returned to pipe 14. is supplied to the first washing tower 1 from The second washing tower 2 is supplied with an aqueous solution of sodium hydroxide from a pipe 23 through a pipe 22 . Second
The unsaturated hydrocarbon-containing gas from which most acidic sulfur compounds and other acidic substances have been removed by contacting with an aqueous solution of sodium hydroxide in the washing tower 2 is discharged from a pipe 21 to remove remaining carbonyl sulfide. Then, the alumina is supplied to an alumina packed column 3 filled with activated alumina. The unsaturated hydrocarbon-containing gas supplied to the alumina packed column 3 is brought into contact with activated alumina in the presence of water at a temperature of room temperature to 200°C, particularly at a temperature of 100 to 200°C, so that carbonyl sulfide is converted into hydrogen sulfide. and is hydrolyzed to carbon dioxide gas. In this case, the amount of water to be present depends on the amount of carbonyl sulfide contained in the gas, and if the content of carbonyl sulfide is small, the water saturated in the gas can be used to sufficiently hydrolyze the carbonyl sulfide. There is no need to add water. Next, the unsaturated hydrocarbon-containing gas discharged from the alumina packed tower 3 through the pipe 32 is treated with a base, such as an alkaline solution such as an aqueous caustic alkali solution or an amine solution, or with zinc oxide or oxide, in order to adsorb and remove hydrogen sulfide. The base is supplied to a base packed tower 4 filled with a solid base such as iron or solid caustic alkali. The unsaturated hydrocarbon-containing gas that does not contain sulfur compounds discharged from the base-packed tower 4 is then supplied to a reaction tower 5 packed with a silver or/and gold catalyst to remove oxygen. The purified gas discharged from the reaction tower is then sent to an olefin recovery device (not shown) to separate and recover olefins, or used as is. FIG. 2 is a modification of FIG. 1, in which the unsaturated hydrocarbon-containing gas is immediately supplied to the alumina packed column 3 after removing acidic sulfur compounds in the first washing tower 1, where it is sulfurized in the presence of water. Decomposes carbonyl.

The unsaturated hydrocarbon-containing gas from which carbonyl sulfide has been decomposed in the alumina packed tower 3 is discharged through a pipe 32 and supplied to the second cleaning tower 2. The unsaturated hydrocarbon-containing gas from which acidic substances such as hydrogen sulfide have been removed in the second cleaning tower 2 is discharged from the pipe 21 and supplied to the reaction tower 5, where oxygen is removed.
In addition, when the amount of sulfur compounds in the unsaturated hydrocarbon-containing gas is small, in FIG. It is also possible to omit the treatment in the base packed column 4, and also in FIG. 2, the treatment in the first cleaning column 1 or the treatment in the alumina packed column 3 and the second cleaning column 2.

Experimental Results Next, the method for purifying unsaturated hydrocarbon-containing gas by the method of the present invention will be explained in detail with reference to specific examples and schematic process diagrams shown in the drawings. It is not limited to the illustration.

Example 1 Experiments Nos. 1 to 7 used raw material gases in which the residual amount of hydrogen sulfide was varied by changing the contact time of an ethylene-containing gas containing hydrogen sulfide and an aqueous solution of potassium hydroxide.
The relationship between the residual amount of hydrogen sulfide and the persistence rate of catalyst activity was determined.

Preparation of silver catalyst Commercially available alumina (specific surface area: 189 I/9) 1009 was added to an aqueous solution of 7.99 silver nitrate dissolved in 150 ml of water, left to stand for 3 hours, and the solution was heated to evaporate.
It was dried for 6 hours.

The catalyst was then calcined in air at 500° C. for 12 hours to obtain a catalyst carrying 5% by weight of silver. Removal of sulfur compounds Ethylene 14 (:Ff) (volume %, same below), Hydrogen 1
An unsaturated hydrocarbon-containing gas consisting of 4% carbon monoxide, 0.9% carbon monoxide, 1500 ppm oxygen (by volume), 385 ppm hydrogen sulfide (by weight as sulfur) and the remainder nitrogen was mixed with a 40% by weight aqueous solution of potassium hydroxide. It was brought into contact and washed.

By changing the contact time of the unsaturated hydrocarbon-containing gas with the aqueous solution of potassium hydroxide during this cleaning, unsaturated hydrocarbon-containing gases having different hydrogen sulfide contents were prepared. Removal of oxygen Next, silver catalyst 6 prepared as above
CC was charged into a reaction tube and activated by flowing hydrogen at 300° C. for 1 hour.

Next, the unsaturated hydrocarbon-containing gases having different hydrogen sulfide contents were heated at 100° C. and normal pressure at a gas hourly space velocity of GHS2. Remove oxygen by flowing under OOO/H/ conditions,
The gas at the outlet of the reaction tube was analyzed. The results are shown in Table 1.
As is clear from the above results, the presence of hydrogen sulfide causes a rapid decrease in the activity of the silver catalyst, but it can be seen that by removing hydrogen sulfide, the catalytic activity can be maintained for a long time. In particular, the hydrogen sulfide content is 1pp as sulfur.
It can be seen that the catalyst activity can be maintained stably for a long time by setting the amount to be less than m. Example 2 Unsaturated hydrocarbon-containing gas separated from a fluid catalytic cracker (FCC), which produces gasoline fraction by cracking petroleum fraction in the presence of a catalyst, is processed under the following conditions by the process shown in Figure 1. It was purified by

The results are shown in Table 2. For comparison, Table 2 shows the results for gases in which sulfur compounds were not removed. Conditions for refining treatment Composition of raw material Ganu (% is volume %, Prn is weight Ppm as sulfur) Hydrogen (2.1%), Methane (31.1%)
Ethylene (12.0%), Ethane (17.40t)) Propylene (8.9%), Propane (3.301)) Nitrogen (11.0 (fl)), Carbon Monoxide (2.

2%) carbon dioxide (2.0%), oxygen (0.15%) hydrogen sulfide (13180ppm) carbonyl sulfide (21ppm)
), Others (remainder) Conditional cleaning liquid for the first cleaning tower Diisopropanolamine aqueous solution (24% by weight) Cleaning temperature 4
5℃ Conditional cleaning liquid for second washing tower 7 degree porter sodium hydroxide aqueous solution cleaning temperature 45℃ Conditional packing for alumina packed column Activated alumina contact temperature 11'C Conditional packing for base packed column Zinc oxide contact temperature 11'C Reaction tower conditions Catalyst Silver catalyst used in Example 1 Contact temperature 11'C Contact pressure 10k9/? GHSV2, OOOV/H/V Example 3 An unsaturated hydrocarbon-containing gas with a low content of sulfur compounds and using silver catalysts with different supported amounts of silver was transferred to the first washing tower 1 and the second washing tower in the process shown in FIG. Purification was carried out in a step in which washing tower 2 was omitted, and the results are shown in Table 3.

For comparison, Table 3 shows the results for gases in which sulfur compounds were not removed. Silver catalyst preparation water 150ml
Commercially available alumina 1009 (specific surface area: 195 I/fl) was added to an aqueous solution in which 3.2 g of silver nitrate was dissolved, and after being left for 3 hours, the solution was heated to evaporate, and then dried at 110° C. for 16 hours.

Next, it was calcined in air at 500° C. for 12 hours to obtain a silver catalyst carrying 2% by weight of silver. When using this silver catalyst, it was packed into a reaction column and activated by flowing hydrogen at 300° C. for 1 hour. Conditions for refining treatment Composition of raw material gas (% and Ppm of oxygen are based on volume, Ppm of sulfur compounds are weight Ppm as sulfur) Hydrogen (8.
90t)), methane (27.80!)) ethylene (10
．． 5%), ethane (14.3%), propylene (17.1
%), propane (5.1%), nitrogen (12.4%), carbon monoxide (2.0%), carbon dioxide (0.3%), oxygen (50%),
ppm) hydrogen sulfide (1ppm), carbonyl sulfide (3p
pm) Others (remainder) Conditional packing material for alumina packed column Activated alumina contact temperature 110°C Conditional packing material for base packed column Contact temperature for solid sodium hydroxide 110'C Conditional catalyst for reaction column Silver catalyst reaction temperature prepared as above 150° C. Reaction pressure 10 kg/D GHSV4, OOOV/H/V Example 4 An unsaturated hydrocarbon-containing gas was purified using a gold catalyst instead of a silver catalyst.

Preparation of gold catalyst Alumina 1009, which was used in preparing the silver catalyst in Example 1, was added to an aqueous solution of 0.869 chloroauric acid dissolved in 170 ml of water.
After adding and leaving for 12 hours, remove the solution and wash with water.
It was dried at 10'C for 16 hours.

Then, it was fired in air at 50°C for 12 hours to form gold.
．． A catalyst having a loading of 5% by weight was obtained. Removal of oxygen Fill the reaction tube with 6CC of gold catalyst prepared as above,
Activation was performed by flowing hydrogen at 300°C for 1 hour.

Next, the unsaturated hydrocarbon-containing gases used in Example 1 with different contents of sulfur compounds were heated at 2000C to GHSV2,
Oxygen was removed by flowing under the conditions of OOOV/H/, and the results are shown in Table 4. As is clear from Table 4, due to the presence of hydrogen sulfide, the gold catalyst, like the silver catalyst,
Although the activity decreases rapidly, it is found that by removing hydrogen sulfide, the catalyst activity can be maintained for a long time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic process diagram when the method of the present invention is carried out on an industrial scale, and FIG. 2 is a schematic process diagram of a modification thereof. 1...First washing tower, 2...Second washing tower, 3...Alumina packed tower, 4...Base packed tower, 5... ...Reaction tower.

Claims (1)

[Claims] 1 In a method of hydrogenating and removing oxygen in an unsaturated hydrocarbon-containing gas by contacting it with a silver or (and) gold catalyst in the presence of hydrogen, A method for purifying an unsaturated hydrocarbon-containing gas, which comprises removing sulfur compounds contained in the gas in a sulfur compound removal step, and then contacting the gas with a silver or/and gold catalyst in the presence of hydrogen.