Hydrogen: Manufacture and Uses

• Hydrogen is the most widespread element in the universe.
• Hydrogen is only the ninth most common element on Earth, making up 1% by weight in the lithosphere, biosphere, and atmosphere.
• Hydrogen is primarily found in water, hydrates, biomass, and fossilized raw materials.
• Hydrogen has been mainly used as a chemical raw material and industrial chemical for commercial purposes.
• Interest in hydrogen as a nearly inexhaustible secondary energy source has grown since the 1973/74 oil crisis.
• Hydrogen has a high energy density per unit mass (121 kJ/g) compared to methane (50.3 kJ/g) along with high environmental compatibility, being non-poisonous, and easy to transport and store.

Hydrogen Manufacture

• Fossil raw materials (natural gas, oil, coal) account for over 90% of H2 production.
• Water is also a raw material source.
• H2 is a byproduct in refineries, petrochemical plants, coking plants, and the chemical industry such as chloralkali-electrolysis.

Petrochemical Processes and Coal Gasification

• The most industrially important and cheapest hydrogen production process is the catalytic steam reforming process, where steam reacts with natural gas (methane) or light crude oil fractions (propane, butane, naphtha with boiling points ≤ 200°C).
• The hydrogen produced comes partly from the steam and partly from the hydrocarbons, with methane deriving 1/3 from water and 2/3 from methane.
• The reaction CH4 + H2O → 3 H2 + CO has ΔH = 205 kJ/mol.
• 77% of worldwide hydrogen production is from natural gas/crude oil fractions.
• 18% of worldwide hydrogen production is from coal.
• 4% of worldwide hydrogen production is from water electrolysis.
• 1% of worldwide hydrogen production is from other sources.
• Coal and coke gasification increasingly produce hydrogen in countries with cheap coal, and before WWII, 90% of hydrogen was produced this way.
• Reaction: 3 C + O2 + H2O → H2 + 3 CO
• Over half the hydrogen produced is used in ammonia fertilizer production in modern integrated ammonia plants.

Electrolysis Of Water

• Electrolysis of water is very energy intensive with an efficiency of 20-25%, including electricity production.
• Electrolysis is used where particularly pure hydrogen is needed, like in food technology (margarine production) or for small users.
• Electrolysis cells consist of two electrodes separated by an asbestos diaphragm impermeable to gases with 20-30% potassium hydroxide dissolved in the electrolyte to increase conductivity.
• Electrolysis is carried out at 80-85°C.
• The theoretical decomposition potential is 1.24 V, but 1.9-2.3 V is used in practice due to overvoltage effects.
• Oxygen is produced at the anode, and hydrogen at the cathode.
• Specific energy consumption is about 4.5 to 5.45 kWh per m³ of hydrogen and 0.5 m³ of oxygen produced.
• Industrial cells are mainly bipolar, with individual plate cells connected back-to-back and coupled in blocks following the filter press principle.
• Electrolysis under pressure can reduce energy consumption by 20%.
• Recent developments include porous electrodes, high-temperature steam electrolysis, and the SPE-process (solid polymer electrolyte).
• Heavy water, D2O, can be produced by water electrolysis through enrichment in the electrolyte.

Other Manufacturing Processes for Hydrogen

• Direct thermal dissociation of water (H2O → H2 + 0.5 O2, ΔH = 285 kJ/mol) is industrially impractical, requiring temperatures over 2000°C.
• Multistep thermochemical cyclic processes can be carried out at lower temperatures with the splitting of water is assisted by an auxiliary agent.
• An example from the "Iron-Chlorine-Family":
  • 6 FeCl2 + 8 H2O → 2 Fe3O4 + 12 HCl + 2 H2
  • 2 Fe3O4 + 12 HCl + 3 Cl2 → 6 FeCl3 + 6 H2O + O2
  • 6 FeCl3 → 6 FeCl2 + 3 Cl2
  • Overall: 2 H2O → 2 H2 + O2

Production of Hydrogen as a By-Product

• H2 is produced as a byproduct in refineries and photochemical companies, particularly from reforming, aromatization, and production of olefins from saturated hydrocarbons.
• H2 is also produced from chloralkali-electrolysis: 2 NaCl + 2 H2O → H2 + Cl2 + 2 NaOH

Hydrogen Applications

• In the USA in 1996, hydrogen consumption totaled 79 x 10^9 m³, including ammonia production (40.3%), methanol reforming (10.0%), refinery processes (42.9%), food (fats and oils) (0.3%), metal refining (0.2%), electronics industry (0.1%), and other uses (6.1%).
• H2 demand in refineries is increasing due to the processing of heavier crude and future oil shale, oil sands and coal oils (compensation of the H/C ratio).
• H2 is transported as a gas in cylinders or pipelines, as a liquid in pressurized cryogenic containers, or possibly as a solid in the form of hydrides like Titanium Iron hydride (TiFeH1.9) or magnesium nickel hydride (MgNiH4.2).

Hydrogen Peroxide

• Commercially, hydrogen peroxide is available in various concentrations.
• Aqueous solutions containing 35, 50, and 70% by weight of Hydrogen Peroxide are the most common.
• Producers of hydrogen peroxide include Degussa (now Evonik), DuPont, EKA Nobel, FMC, Kemira, Mitsubishi Gas Chemical, Oxysynthese, Solvay-Interox, and National Peroxide.

Hydrogen Peroxide Production

• Oxidation of isopropanol is of mainly only historical significance, with plants producing 15,000 t/a of hydrogen peroxide and 30,000 t/a of acetone being shut down in 1980, with only plants in former USSR states still in use.
• Electrochemical oxidation of sulfuric acid or ammonium sulfate.
• Cathodic reduction of oxygen.
• Over 95% of hydrogen peroxide is produced with the anthraquinone process.

Isopropanol Oxidation Process

• Acetone and hydrogen peroxide are produced with 80% selectivity upon multistage oxidation of isopropanol with air at 15-20 bar and 90-40°C:
  • CH3CH(OH)CH3 + O2 → CH3COCH3 + H2O2
• The degree of conversion is limited to about 30% to suppress side reactions.
• The reaction mixture is diluted with water, then acetone, unreacted isopropanol, and water are distilled off.
• A ~20% hydrogen peroxide solution is removed from the sump at around 120°C.
• Acetone is separated from the distillate, and isopropanol-water solution is fed back.
• The 20% hydrogen peroxide solution is then purified with ion exchangers and concentrated by distillation.
• Disadavantage: Method has one disadvantage: the wheight of acetone produced double that of the hydrogen peroxide amount.
• Acetone must either be used as is or reduce back.

Electrochemical Process

• The electrochemical processes involve the oxidation of an aqueous solution of sulfuric acid or ammonium sulfate at the anode to peroxodisulfuric acid or ammonium peroxodisulfate, with reduction at the cathode to produce hydrogen.
  • Sulfuric Acid: (550 to 570 g/L) (Degussa-WeiBenstein Process)
  • Sulfuric Acid & Ammonium: (260 g/L) and ammonium sulfate (210 to 220 g/L) (Lowenstein-Riedel Process)
• Adding small amounts of ammonium thiocyanate or hydrochloric acid increases the anode potential.
• The peroxo compound obtained is then hydrolyzed by peroxomonosulfate (Caro's acid).
  • H2S2O8 + H2O → H2SO5 + H2SO4
  • H2SO5 + H2O → H2SO4 + H2O2
• The hydrogen peroxide formed is then distilled off, and the sulfuric acid or sulfuric acid-ammonium sulfate solutions are recycled.
• The total yield for both processes is around 70% relative to electricity consumed.
• These processes have high plant and production costs because of the high cost of electricity.

Anthraquinone Process

• Oxidation of a 2-alkyl-anthrahydroquinone with air to the corresponding 2-alkyl-anthraquinone and hydrogen peroxide, and catalytic (back)-reduction of the 2-alkyl-anthraquinone to the 2-alkyl-anthrahydroquinone with hydrogen.
• This cyclic process produces hydrogen peroxide from hydrogen and oxygen.
• The alkyl-group substituent R on the anthraquinone differs among manufacturers, including 2-ethyl (mainly used), 2-tert-butyl, 2-tert-amyl, and 2-secamyl-anthraquinones.
• Mixtures of various alkyl anthraquinones are also utilized.
• The solvent mixture must dissolve both quinone and hydroquinone-compounds.
• The "working solution" contains a mixture of aromatic compounds such as naphthalene or trimethylbenzene.
• Polar compounds like tris-(2-ethylhexyl)-phosphate, diisobutylcarbinol, or methylcyclohexanol-acetate are used as solvents for the hydroquinone.
• The solvent mixture must have low solubility in water, low volatility, good dissolving properties, chemical stability, and low viscosity.
• In the first step, anthraquinone is hydrogenated to hydroquinone with palladium as the preferred catalyst, carried out at 40°C and pressures up to 5 bar with cooling, but with only 50% hydrogenation used in order to reduce side reactions.
• Subsequent oxidation proceeds with air at 30-80°C and pressures up to 5 bar after catalyst separation and filtration carried out in continuous or countercurrent mode, in a single or multistep method.
• Hydrogen peroxide created during the oxidation is extracted from the reaction fluid with water.
• The extraction yield is roughly 98%.
• Hydrogen peroxide solutions obtained range from 15-35% by weight and must be freed of residual organic compounds before concentration by distillation.
• Commercial hydrogen peroxide solutions contain stabilizers such as diphosphates, organic complexing agents, or tin compounds to prevent decomposition to oxygen and water.
• After separation, the working solution has to be dried and freed of byproducts using aluminum oxide in a bypass.
• The anthraquinone process is more complicated, as byproducts like 1,2,3,4-tetrahydroanthraquinone are created, mainly during hydrogenation.
• Similar behavior to anthrahydroquinones happens, but further hydrogenation results in octahydroanthrahydroquinones which are unusable.
• Byproducts like oxanthrones and anthrones can only be partially regenerated, hence their removal.

Sodium Perborate and Sodium Carbonate Perhydrate

• Sodium perborate is produced in almost all Western industrialized countries, especially Europe where it is used in washing powders.
• The world capacity for sodium carbonate perhydrate is about 20% of the sodium perborate capacity, with plants often producing both alternately.
• About 40% of hydrogen peroxide production in Western Europe is used to produce sodium perborate and sodium carbonate perhydrate.
• Sodium perborate, or more correctly sodium peroxoborate, is NaB02(OH)2 · 3 H2O.
• Sodium perborate is produced from borax in a two-step method:
  • Na2B4O7 + 2 NaOH → 4 NaBO2 + H2O
  • NaBO2 + H2O2 + 3 H2O → NaBO2(OH)2 · 3 H2O
• The first step is the creation of sodium metaborate from borax and sodium hydroxide occurring at 90°C, filtering it used impure borax.
• The second step occurs at 25°C, with the solution cooling.
• Residual moisture (3–10%) is removed through air-drying.
• Stabilizers for the perborate such as silicates (or magnesium salts) is added to the reaction fluid.
• The mother liquor from the second step can be used more than once.
• The final substance has active oxygen, ca. 10.1-10.4% or 10.38% theoretical. The amount in the compound is changed depending on what it is used for.

Sodium Carbonate Perhydrate

• Sodium percarbonate, unlike perborate, is simply a perhydrate called Na2CO3 · 1.5H2O2. It can be produced in “dry” and “wet" processes.
• Dry process: A modern dry method sprays hydrogen peroxide over over sodium percarbonate.
• Process: Hydrogen preoxide and a sodium carbonate solution is sprayed onto a bed of sodium percarbonate, with the fines returned and the oversized particles grounded.
• The wet method: Sodium carbonate and hydrogen peroxide solution are combined and reacted at predetermined ratio.
• Precautions: percabonate is much less stable and must be stablized
• Stabillizers: Alkali silicates and phosphates. can also be coated with organic ingredients
• Oxegen conten: Ca. active oxygen (13.5%) - 15.28 theoretical.

Alkali Peroxodisulfate

• Diammonium peroxodisulfate is produced via electronic solutions (ammonium sulfate, sufuric acid)
• Reaction: (NH4)2SO4 + H2SO4 (NH4)2S2O8 + H2
• Can be made by reacting with alhali compounds:
• Reaction: (NH4)2S2O8 + 2 KHSO4 → K2S2O8 + 2 NH4HSO4 && (NH4)2S2O8 + 2 NaOH → Na2S2O8 + 2 NH3 + H2O

Sodium Peroxide

• Step 1: 2 Na + 0.5 0₂ → Na2O
• Step 2: Na2O + 0.5 0₂ → Na2O2
• How: Add soduim to rotatorary tube. Temperature 200C to 700C.
• Oxidation occurs in similar reactor at 350C

Applications

• Hydrogen Peroxide, Sodium Perborate and Sodium Carbonate Perhydrate are important in ally1 alcohol to glycerine the production of epoxy-compounds.
• Used as free radical initiators in polymerization.
• Also used as cleaning products (lauryldimethyl-amine-oxide) for dishwashers.
• Perborate and percarbonate production is a large application: Western Europe (170 of 103 t), USA/Canada (13 of 103 t), Japan (10 of 103 t).
• Bleaching of paper is also a major application: Western Europe (142 of 103 t), USA/Canada (127 of 103 t), Japan (66 of 103 t).
• Bleaching of textiles is also another primary application: Western Europe (40 of 103 t), USA/Canada (23 of 103 t), Japan (18 of 103 t). Western Europe (84 of 103 t), USA/Canada (28 of 103 t), Japan (52 of 103 t).
• Used to treat effluent.
• Percarbonate used a polymerization initiator (acrylonitrile).
• SODUIM PEROXIDE IS A SUBSTITUTE TO HYDROXIDE AND ALSO FOR OXYGEN IN PAPER.