Hydrogen Production and Purification Flashcards
(14 cards)
H2 Producers (Processes)
Catalytic Reforming
Methane Steam Reforming H2 Plant
H2 Consumers (processes) (4)
Hydrocracking
Hydrotreating (Hydrogenation)
Isomerization
Hydroprocesssing
Supplemental hydrogen requirement can be provided by two processes:
Which one uses catalysts?
- Partial oxidation of heavy hydrocarbons such as fuel oil.
- Steam reforming of light ends such as methane (primarily), ethane, or propane.
- The steam reforming process employs catalysts.
What is a main factor for the cost between the 2 processes
Which one is cheaper and most widely used in N.A?
Feedstock.
Steam reforming is cheaper and therefore most widely used in N.A
4 steps of steam reforming
- Reforming
- Shift Conversion
- Gas Purification
- Methanation
What happens in the reforming step (4)
Methane undergoes catalytic reaction at T of 1400 to 1500°F (760–816°C):
CH4 + H2O → CO + 3H2
This is endothermic and is done by passing the gas through catalyst-filled tubes in a furnace.
The catalyst is 25 to 40% nickel oxide deposited on silica. Form: hollow cylindrical rings ranging up to 3/4 in. in diameter.
.
What happens in the shift conversion step (4)
More steam is added to convert CO to equivalent amount of hydrogen
CO + H2O → CO2 + H2
This is an exothermic reaction and is conducted in a fixed-bed catalytic reactor at about 650°F (343°C). Thus Multiple catalyst beds in one reactor w/ external cooling between beds are used to prevent the temperature from getting too high, as this would adversely affect the equilibrium conversion.
The catalyst used is a mixture of chromium and iron oxide.
Gas purification Step (3)
CO2 is removed by absorption in a circulating amine or hot potassium carbonate solution.
Absorber has about 24 trays, or the equivalent amount of packing. CO2 is then sent to a still for regeneration.
Alternatively, adsorption on a solid adsorbent may be used for purification.
Methanation:
The remaining small quantities of carbon monoxide and carbon dioxide are converted to methane:
CO + 3H2 → CH4 + H2O
CO2 + 4H2 → CH4 + 2H2O
Both reactions are exothermic and are conducted in a fixed-bed catalytic reactor at temperatures of about 700 to 800°F. The catalyst has 10 to 20% nickel.
If feed concentration of CO and CO2 is more than 3%, part of the cooled exit gas is recycled to dissipate the heat of reaction.
Partial Oxidation process:
Hint: Pressure, O2, Steam, Equation, Removal.
Accomplished by burning the fuel at
high pressures (800 to 1300 psig).
The oxygen present is limited to what is required to convert the fuel oil to carbon monoxide and hydrogen.
Enough water (steam) is added to shift the carbon monoxide to hydrogen in a catalytic shift conversion step.
Ideally the partial oxidation reactions are as follows:
2CnHm + nO2 → 2nCO + mH2
2nCO + 2nH2O → 2nCO2 + 2nH2
The resulting carbon dioxide is removed by absorption in hot potassium carbonate or other solvents.
3 processes for hydrogen recovery. Describe the instances when each should be used
- Cryogenic phase separation: Bulk removal of hydrocarbons from large streams (> 570Mm3/day)
- Adsorption: For hydrogen recovery where the purity needs to be over 95 mol%
- Diffusion: Small streams of gas (55-85Mm3/day)
Cryogenic Phase Separation (4)
The gas is cooled to about -200° to -250°F at
pressures ranging from 200 to 500 psig
The resulting vapor phase is 90 mol% hydrogen and the liquid phase contains most of the methane and other hydrocarbons.
The liquid phase is expanded to about 50 psig and used to cool the feed gas. This revaporizes the liquid phase.
Carbon dioxide, hydrogen sulfide, and water vapor must be removed from the feed gas prior to chilling.
Explain Adsorption Process (3)
Hydrocarbons are adsorbed from the gas on a solid adsorbent and the hydrogen leaves the adsorber at the desired purity.
Several adsorbent vessels are used, and the feed gas flow is periodically switched from one vessel to another so that the adsorbent can be regenerated.
The adsorbed methane and other impurities are released from the adsorbent by simple pressure reduction and purging. Hence, this process is called pressure swing adsorption.
Explain Diffusion
Separates hydrogen from methane and other gases by allowing the hydrogen to permeate through a membrane composed of small synthetic hollow fibers.
The driving force for this process is the difference between the hydrogen partial pressures on each side of the membrane.
A substantial pressure drop must be taken on the hydrogen product to achieve high recoveries.
