Applications of hydrogen in industry
Hydrogen’s use in industry can be divided into two main categories: (1) as a reactant in hydrogenation reactions and chemical processes, (2) as fuel and energy carrier. As a reactant, hydrogen is used to produce compounds with lower molecular weight, saturate compounds and crack heavy hydrocarbons into lighter hydrocarbons. In majority of these applications hydrogenation takes place to insert hydrogen atoms and saturate molecule or to cleave a molecule and remove heterogeneous atoms such as sulfur and nitrogen. Among the major uses of hydrogen in chemical industries, ammonia production accounts for almost 50%, petroleum processing accounts for 37%, and methanol production accounts for 8% [1-3].
In petroleum industry, hydrogen is used to react with hydrocarbons in hydroprocessing and hydrocracking processes. In hydroprocessing, hydrogen is used to hydro-genate sulfur and nitrogen compounds (for example from crude oil) and release them as hydrogen sulfide (H2S) and ammonia (NH3). In hydrocracking process, heavy hydrocarbons are cracked into lighter hydrocarbons to produce refined fuels with smaller molecules and higher H/C ratios .
Hydrogen is also used for production of methanol. Methanol is a feedstock for manufacturing of other chemicals and materials such as formaldehyde, plastics, plywood, paints, and textiles. In methanol production plants, hydrogen and carbon monoxide are reacted over a catalyst at a high pressure and temperature.
Other application of hydrogen in chemical and petrochemical industries include production of butyraldehyde from propylene, production of acetic acid from syngas, production of butanediol and tetrahydrofuran from maleic anhydride, production of hexamethylene diamine from adiponitrile, and production of cyclohexane from benzene.
In food industry, large amount of hydrogen is used for processing of vegetable oil and decreasing the degree of unsaturation. In this process, an increase in melting point and enhanced resistance to oxidation occur that enables preservation for a longer period of time [3, 6].
Aerospace industry is the primary consumer of fuel hydrogen. A mixture of liquid hydrogen and oxygen has been found to release the highest amount of energy per unit weight of propellant . However, the cost of hydrogen liquefaction, and difficulties associated with safely store and handling it in liquid form have kept liquid hydrogen away from other fuel applications such as in automobiles .
Fuel hydrogen is also used in fuel cells to power electrical systems. In a fuel cell, hydrogen and oxygen from air are combined and produce electricity and water.
Production of hydrogen
Industrial processes for production of hydrogen can be divided into thermal such as hydrocarbons reforming, renewable liquid and bio-oil processing, biomass, and coal gasification; electrolytic such as water splitting; photolytic such as splitting of water by sunlight through biological activity or photo-catalytic materials.
Fossil fuels such as gasoline, coal, and methane (natural gas) can be used to produce hydrogen in an industrial scale. Hydrogen from fossil fuels can be produced through three basic technologies: (1) steam reforming, (2) partial oxidation, and (3) autothermal reforming. In these technologies, carbon monoxide is produced with hydrogen as by-product which is subsequently converted into carbon dioxide (CO2) via water-gas shift reaction.
Steam reforming is the most widespread and least expensive process for hydrogen production . The most frequently used raw materials for steam reforming process are natural gas (methane) and light hydrocarbons such as propane, butane, and methanol . Steam reforming of hydrocarbons comprises two stages. In the first stage, the hydrocarbon is mixed with steam and fed in a catalytic reactor producing syngas (H2/CO gas mixture). The reaction temperature inside the catalytic reactor is achieved by addition of air to combust part of the hydrocarbon feed. In the second stage cooled syngas is fed into CO catalytic converter, where carbon monoxide is converted to carbon dioxide and hydrogen . Partial oxidation is a non-catalytic process, in which the raw material is gasified in the presence of oxygen and steam at 1300–1500 C and 3–8 MPa. The gasified raw material can be methane, biogas, and heavy oil fractions of crude oil . Autothermal reforming is a combination of steam reforming (endothermic) and partial oxidation (exothermic) reactions. Autothermal reforming has the advantages of not requiring external heat and being simpler and less expensive than steam reforming .
Pressure swing adsorption (PSA) units are used to separate mixture of various gasses such as N2, CO2, CO, H2O from hydrogen and increase the purity of final hydrogen product. The separation effect in PSA is based on differences in binding forces of gasses to adsorbent materials.
Highly volatile components with low polarity, such as hydrogen are practically non-absorbable as opposed to molecules such as N2, CO, CO2, hydrocarbons and water vapor. Consequently, these impurities can be adsorbed from a hydrogen-containing stream and high purity hydrogen is recovered . Some of the key industrial applications of PSA include gas drying, solvent vapor recovery, fractionation of air, production of hydrogen from steam-methane reformer and petroleum refinery off gases, separation of carbon dioxide and methane from landfill gas, carbon monoxide-hydrogen separation, normal isoparaffin separation, and alcohol dehydration. A PSA unit consists of absorber vessels containing the adsorbent material, tail gas drum, valve skid with interconnecting pipes, control valves and instrumentation, and a unit control system. The pressure swing adsorption process has two main steps: adsorption and regeneration . In adsorption, feed gas flows through the absorber vessels in an upward direction and impurities such as water, heavy hydrocarbons, light hydrocarbons, CO2, CO and nitrogen are selectively adsorbed on the surface of the adsorbent material. Highly pure hydrogen exits the absorber vessel at top. After a defined time, the adsorption phase of this vessel stops and regeneration starts. Another absorber takes over the task of adsorption to ensure continuous hydrogen supply. Regeneration starts at lower pressure to release the adsorbed gasses on the adsorbent material and prepare it for another cycle of adsorption.
The steps are performed in parallel in different vessels to minimize hydrogen losses and maximize the recovery rate of the PSA unit. Adsorbent materials for PSA units are selected from very porous materials due to their large specific surface areas. Typical adsorbents are activated carbon, silica gel, alumina, resin and zeolite .
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