Hydrogen is recognised as a high purity premium product. Andy Brown describes some of its many roles
There is significant, and understandable, focus today on the potential use of hydrogen as a substitute for natural gas for heating and electricity generation. Its principal advantages are seen as its high calorific value and the “carbon-free” nature of its combustion products (simplistically, water). To access these two assets, significant efforts are being made to produce hydrogen cost effectively in bulk, and to manage, or engineer a way out of, some of the downsides of simply burning it. These would include:
And yet hydrogen has been produced since 1650, when Théodore de Mayerne first poured dilute sulfuric acid on iron to produce a gas of “inflammable air”. It was not until 1783 when Jaques Charles made a hydrogen balloon large enough to carry him and a colleague over a distance of 36 km at a height of up to 550 m that it was appreciated that hydrogen had other uses. However, three subsequent discoveries really opened up the possibilities to realise its chemical potential. These were hydrogenation (1897), the Haber process to make ammonia (1910), and hydrocracking (1920).
Today, hydrogen is recognised as a premium product, and is produced with purities in the order of 99.999%. It has a role, often far beyond the obvious; and this short article describes some of them.
Hydrogen is consumed in refineries in a variety of hydro-desulfurisation (HDS) and hydrocracking operations. HDS is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products, such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils. Hydrocracking is a process which takes heavier refinery products and cracks the large molecules into smaller ones (distillate such as diesel or petrol) in the presence of hydrogen and a catalyst.
UK refineries together produce 156,563 Nm3/h of hydrogen1, which equates to over 100,000 t/y. This is likely to increase as recent legislation to ban the use of high-sulfur residual oil in ships (“Bunker C”)2 affects the market into which these heavy residues have traditionally been sold, and as the balance between diesel and petrol for transport use readjusts.
The Haber-Bosch process is the main industrial procedure for the production of ammonia today, and involves the direct combination of hydrogen and nitrogen under pressure and temperature in the presence of a metal catalyst. Ammonia (NH3) is used to produce ammonium nitrate, a fertiliser, and is part of many household cleaning products. Next to oil refineries, ammonia is currently the largest application of hydrogen.
The process, in simple terms, requires nitrogen and hydrogen, mixed in a 1:3 ratio, to be placed under pressure and temperature in a vessel containing a catalyst. The most popular catalysts are based on iron promoted with K2O, CaO, SiO2, and Al2O3. The reactions typically take place at 15–25 MPa (150–250 bar) and between 400–500°C. The mixed gases are usually passed over four catalyst beds, with cooling between each pass so as to maintain a reasonable equilibrium constant for the reactions. On each pass only about 15% of the gas is converted to ammonia: the liquid ammonia is stripped out and the unreacted gases recycled via a compressor. In modern plants, overall conversion rates in excess of 97% can be achieved.
Hydrogen for ammonia plants is normally produced using steam methane reforming (SMR) technology (Figure 1), and the projects get bigger and bigger as advantage of scale is taken. The largest single-train ammonia plant in the world is thought to be that located at Al-Jubail, Saudi Arabia. It produces
1,300 t/d, and is owned by the Al-Jubail Fertilizer Company.
Hydrogen is used commercially to extract tungsten from its ore (wolframite, scheelite, and ferberite). The same concept can be used to produce copper from tenorite and paramelaconite (copper oxide, CuO).
Direct reduction of iron (DRI) using hydrogen is an idea that has yet to reach large-scale application, the advantage being that the blast furnace gas (BFG) is comprised mostly of water vapour and nitrogen with only a small amount of CO2. The Luleå steel plant in Sweden, which is operated by SSAB, intends to build a DRI pilot plant using a process called Hybrit. If the pilot is successful, it is hoped that work to scale up to a demonstration capacity of 500,000 t/y would begin in 2025 with completion planned for 2035.3
Hydrogen can theoretically be used as a reducing agent to produce silver, gold and platinum, but is not employed commercially.
The large-scale production of hydrochloric acid (HCl) is almost always integrated with the industrial scale production of other chemicals as a pseudo byproduct. However, pure chlorine gas can be directly combined with hydrogen to produce hydrogen chloride directly in the presence of UV light. This is a highly exothermic reaction and rarely used commercially to produce HCl.
Hydrogen is used to turn unsaturated fats to saturated oils and fats. Food industries, for instance, use hydrogen to make hydrogenated vegetable oils such as margarine and butter.
Hydrogenation of saturated oils and fats is a batch process which takes place in a heated tank (see Figure 2). The oil feed (eg sunflower seed or olive oil) is pumped into a heated pressure vessel and a vacuum is applied to inhibit oxidation as the heating is applied. The temperature is raised to 140-250°C and the mixture is stirred to ensure an even temperature. Nickel catalyst solids, mixed with a small amount of oil, are then pumped in, followed by hydrogen gas, which brings the pressure to 2.7–4 barg. The hydrogenation reaction is exothermic, so the external heating is removed and cooling applied, vigorous stirring ensuring the temperature remains in the 70-80°C range. After 40–60 minutes the hydrogenated oil mixture is pumped out as a slurry and the catalyst solids removed in filters. Cooling to room temperature allows the hydrogenated oil to solidify.
Atomic hydrogen welding (AHW) is an arc welding process that uses an arc between two metal tungsten electrodes in a shielding atmosphere of hydrogen, and can be used to weld refractory metals and tungsten.
Many modern large electrical generators use hydrogen gas as a rotor coolant at a pressure of around 4 bar. The advantages are:
Hydrogen is used in many manufacturing plants to check for leaks, since its environmental impact is less than that of the CClF3-based gases that were used in the past. Hydrogen can be used on its own or with other elements.
Methanol can be produced from synthesis gas (carbon monoxide and hydrogen) in a fixed bed reactor using a catalyst of alumina pellets coated with copper and zinc oxides. Methanol can also be made by the direct combination of hydrogen and carbon dioxide: this reaction has been the subject of much attention over recent years because it offers the possibility of turning atmospheric CO2 into a fossil-fuel substitute. The challenge is to make this thermodynamically efficient (ie to end up with more useful energy in the methanol than the total process energy that it takes to produce it). The majority of the work has been focussed on finding a good catalyst so that methanol can be produced in high selectivity at an efficient rate. Researchers in the US4 have discovered that a combination of palladium and copper yields the most efficient conversion using nanoparticles of the catalyst dispersed on a porous support material, used to increase the surface area of the catalyst. With a catalyst pellet the size of a walnut, the internal surface area is similar to that of a football field.
In this process, hydrogen and carbon dioxide are pumped into the sealed chamber of a reactor vessel packed with the catalyst, and the contents heated to 180–250°C. The maximum CO2-to-methanol conversion is about 24%. The unconverted carbon dioxide and hydrogen is recycled and returned to the vessel. The overall thermodynamic efficiency of the process is not stated.
Hydrogen peroxide is a routine sterilising agent used in clinics and hospitals. It is a strong oxidising agent and is particularly effective for the cleaning of wounds, cuts and other damaged tissue portions. It is also used for bleaching hair, whitening teeth and removing stains from clothing. In research, H2O2 is also used for testing anti-oxidant potential of enzymes like catalase.
Hydrogen peroxide is typically made in a multi-step, energy-intensive process that requires it to be produced in large quantities and shipped and stored in a highly concentrated form. The manufacturing process involves the catalysis of the reaction of H2 with atmospheric O2 using anthraquinone (Q) as a hydrogen carrier. The first step is hydrogenation, where palladium catalyses the reaction between hydrogen and anthraquinone to create anthrahydroquinone (H2Q). In the second step the palladium catalyst is filtered out of the solution. Next, the solution is oxidised by blowing air through the solution, forming the H2O2, and releasing the anthraquinone. Finally, the hydrogen peroxide is removed in a liquid-liquid extraction column and concentrated by vacuum distillation.
More recently, researchers from the UK and the US have developed a method of producing H2O2 on demand through a simple, one-step process, allowing dilute H2O2 to be made directly from hydrogen and oxygen in small quantities on site. This could make it more accessible to underdeveloped regions of the world, where it could be used to purify water.5 Bimetallic compounds consisting of palladium and any of six other elements can effectively catalyse the hydrogenation of oxygen to form hydrogen peroxide.
Hydrogen is the key element involved in redox reactions. It is used in the manufacture of plate glass, for instance, to prevent the formation of stannous oxide (SnO) in the float bath.
Hydrogen is used in various methods of chemical analysis. These methods include atomic absorption spectroscopy. Here the hydrogen is used as fuel to generate heat, at the same time producing the neutral atoms.
Hydrogen is one of the gases which can be used as carrier phase in gas chromatography, used to separate volatile substances
Because hydrogen is light compared to other gases, it is still used by meteorologists for high-altitude weather balloons.
Hydrogen gas is not an energy source, rather it stores and delivers energy in a usable form. Outside of combustion for heat and CHP it has an application as fuel for hydrogen fuel cells (which will be described in Uses of Hydrogen: Part 2) which may be used, for example, in trains, cars, buses, submarines, bikes and laptops.
Human progress can be mapped in terms of revolutions brought about by more efficient energy use – from the early discovery of fire, to higher intensities made possible by coal (eg steam). More recent step changes have been made possible by nuclear power and natural gas. It is very possible that the next revolution will be the hydrogen era, opening the possibility of sustainable energy into the future, with applications in the home (as described in the HyDeploy programme) and transport (to come, in Uses of Hydrogen, Part 2).
This is the ninth article in a series discussing the challenges and opportunities of the hydrogen economy, developed in partnership with IChemE’s Clean Energy Special Interest Group. To read more from the series online, visit the series hub at https://www.thechemicalengineer.com/tags/clean-energy-sig-the-
1. “Refinery Hydrogen Production Capacities by Country”, H2 Tools, https://h2tools.org/node/820
2. MEPC.1/Circ.878 “Guidance on the Development of a Ship Implementation Plan for the Consistent Implementation of the 0.50% Sulfur Limit Under MARPOL Annex VI”, International Marine Organisation, November 2018
3. “Green steel process given go ahead for testing at pilot scale”, The Chemical Engineer, February 2018
4. “Carbon dioxide-to-Methanol process Improved by Catalyst”, Penn State, Science Daily, 28 June 2018
5. “New Catalyst Makes Hydrogen Peroxide Accessible to Developing World”, Lehigh University, 25 February 2016. Project led by Graham J Hutchings, Professor of Physical Chemistry, Cardiff Catalysis Institute, Cardiff University.
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