Hydrogen purification

Details Hydrogen purification Hydrogen purification

2000-06-15

2002-06-11

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Carbon monoxide component

C252S373000, C422S172000, C422S177000, C422S198000, C422S228000, C423S248000, C423S437200, C429S010000

Reexamination Certificate

active

06403049

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a method of purifying hydrogen-rich gas streams which contain carbon monoxide and more particularly but not exclusively relates to a method of removing carbon monoxide from reformate gas mixtures.
Hydrogen is one of the most important industrial gases. It is used, for example, in ammonia synthesis, methanol synthesis, chemical hydrogenations, metal manufacture, glass processing and fuel cells. In most of these applications, the hydrogen has to be virtually free of any reactive contaminants.
Solid polymer fuel cells use hydrogen as a fuel. These fuel cells can be operated with a pure hydrogen stream or a hydrogen-rich stream. In vehicular applications, in order to avoid problems associated with hydrogen storage on board a vehicle, it can be preferable to produce the hydrogen on board the vehicle from conventional fuels. For example, a hydrogen-rich stream can be produced by reforming a conventional fuel with water and/or air in a fuel reformer. Such a hydrogen-rich stream typically contains hydrogen, CO, CO
2
, N
2
, H
2
O and traces of hydrocarbons or alcohols, (depending on the fuel).
As stated above, fuel reformers often produce small amounts of carbon monoxide (typically 0.5 to 5 mol %). Carbon monoxide is a severe poison to the fuel cell anode catalyst. Extremely small amounts of carbon monoxide have a detrimental effect on the cell voltage and reduce the fuel cell's power output. Hence, the carbon monoxide concentration needs to be attenuated to very low levels (preferably below 10 ppm) for the hydrogen-rich reformate to be suitable as fuel cell feed.
Fuel cell systems incorporating a fuel reformer require a carbon monoxide clean-up system which can reliably remove virtually all of the carbon monoxide. For vehicular applications, there are the additional requirements that such a system:
works under varying flow conditions (through-put and carbon monoxide concentration);
is compact;
is economically feasible;
introduces only a small pressure drop; and
has a minimal start-up time.
Catalytic carbon monoxide removal options from a hydrogen-rich stream include:
reduction of carbon monoxide with water vapour to carbon dioxide and hydrogen (water-gas shift);
selective methanation of carbon monoxide to methane; and
selective oxidation of carbon monoxide to carbon dioxide.
As currently practised, none of the above three options is ideal on its own for attenuating the carbon monoxide to the desired concentration below 10 ppm.
Water-gas Shift Reaction
The water-gas shift reaction would appear to be the most favourable because it produces hydrogen, which is fuel for the fuel cell. However, for the hydrogen-rich streams generated in fuel reformers it is not possible to reduce the carbon monoxide concentration to levels acceptable for the fuel cell by the water-gas shift reaction. This results from the fact that the water-gas shift reaction is an equilibrium reaction (also called a reversible reaction).
CO+H
2
O(g)⇄CO
2
+H
2
−41.2 kJ/mole CO
The equilibrium constant for this reaction is a function of the concentrations of the reactants and products as well as of the temperature. Whether the equilibrium conversion will be reached depends on the size of the catalyst bed, the catalyst activity, and the reactor operating conditions. The reaction rate of the water-gas shift reaction can generally be
Rate
⁢
 
⁢
 
⁢
CO
=
A
·
EXP
⁡
(
-
E
a
RT
)
·
[
CO
]
⁡
[
H
2
]
·
(
K
eq
-
[
H
2
]
⁡
[
CO
2
]
[
CO
]
⁡
[
H
2
⁢
O
]
)
Equation
⁢
 
⁢
A
where,
A=reaction rate constant
Ea=reaction activation energy
R=gas constant
T=absolute temperature
K
eq
=
[
H
2
]
eq
⁡
[
CO
2
]
eq
[
CO
]
eq
⁡
[
H
2
⁢
O
]
eq
FIG. 4
of the accompanying drawings shows the dry CO percentages from a water-gas shift reaction as a function of temperature and water inlet concentration. The dry feed here is 58% H
2
, 21% CO
2
, 19.5% N
2
and 1.5% CO. This figure demonstrates that at low temperatures and increasing H
2
O levels in the feed, the CO concentration in the product is predicted to be quite low based on thermodynamics. In practice, however, these low levels will not be reached because the reaction rate becomes extremely slow at low temperatures as it approaches equilibrium. Equation A illustrates that the reaction rate is exponentially dependent on the temperature. The last term in Equation A shows that if the reaction mixture approaches equilibrium concentrations, the reaction rate approaches zero. Hence, practical water-gas shift reactors usually can only bring the CO concentration down to 0.5-1.0 mol % dry. Depending on the reformer technology, and thus on the amount of CO in the reformate product, one or more water-gas shift reactors (or reactor stages) could be used to reduce the CO to levels suitable for further removal by selective oxidation and/or methanation.
Selective Methanation
Two methanation reactions are possible:
CO+3H
2
→CH
4
+H
2
O(g)−206.2 kJ/mole CO
CO
2
+4H
2
→CH
4
+2H
2
O(g)−163.1 kJ/mole CO
2
Selective methanation of carbon monoxide is only an option when the carbon monoxide concentration in the feed stream to the methanation reactor is sufficiently low (4000 to 100 ppm) so that hydrogen losses are minimised. The methanation process has the major disadvantage that for each molecule of carbon monoxide removed, three molecules of hydrogen are lost. Thus, if the initial carbon monoxide concentrations are high then the hydrogen loss will be significant. Moreover, although selective methanation is an exothermic reaction, if the carbon monoxide concentration in the feed stream to the methanator is low the temperature rise in the methanation is not too great. If carbon dioxide is present in the feed stream, it is important that a selective methanation reactor is operated at a temperature low enough to minimise carbon dioxide methanation to methane and water as a side reaction. For these reasons, a methanation reactor either needs to be cooled or requires a carefully controlled feed inlet temperature.
Selective Oxidation
Two oxidation reactions are possible:
CO+½O
2
→CO
2
−283.1 kJ/mole CO
H
2
+½O
2
→H
2
O(g)−242.0 kJ/mole H
2
As shown above, carbon monoxide can be removed from a hydrogen-rich stream by selective oxidation. This selectivity is relatively easy to achieve at high carbon monoxide concentrations. However, at lower carbon monoxide concentrations where hydrogen is present in a large excess, the hydrogen becomes more competitive and the high selectivity for carbon monoxide is lost. Also, the amount of oxygen injected into the system is important as any excess will tend to react with hydrogen. Thus, if carbon monoxide selectivity decreases and oxygen reacts with hydrogen there will not be enough oxygen left to remove all the carbon monoxide. If extra oxygen is used, hydrogen loss can be significant. Moreover, the levels of carbon monoxide produced by a fuel reformer may vary with time. Thus, the amount of oxygen required to remove the carbon monoxide will vary. Furthermore, as the oxidation reaction is highly exothermic, changes in the amount of oxygen will lead to temperature variations in the catalyst bed. The selectivity of the oxidation reaction is highly temperature dependent and an increase in the amount of oxygen added could result in an increase in the final carbon monoxide level. In summary, it is easy to achieve a reduction in carbon monoxide by selective oxidation from 0.1 to 10 vol % to levels between 350 to 750 ppm with little loss of hydrogen. However, in practice it is very difficult to reach very low carbon monoxide concentrations by selective oxidation alone, without excessive loss of hydrogen.
Description of the Related Art
U.S. Pat. No. 5,271,916 describes the selective removal of carbon monoxide from a hydrogen-rich gas stream by oxidation in a multi-stage system. This system includes

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