PROX
Encyclopedia
PROX is an acronym for PReferential OXidation, and refers to the preferential oxidation of a gas on a catalyst.
The catalyser preferentially oxidises carbon monoxide
(CO) using a heterogeneous catalyst placed upon a ceramic support. Catalysing agents include metals such as platinum
, platinum/iron, platinum/ruthenium, gold nanoparticles as well as novel copper oxide/ceramic conglomerate catalysts.
This reaction is a considerable subject area of research for fuel cell
design. From the reaction of carbon monoxide with steam, carbon dioxide is evolved.
The presence of CO impedes the reaction of H2/O2 and reduces the fuel cell efficiency by the absorption of CO onto the platinum anode. The PROX process allows for the reaction of CO via the water gas shift reaction
(WGS), reducing CO concentration from approximately 0.5-1.5% in the feed gas to less than 10 ppm.
However, a selectivity of 100% is impossible. The selectivity, S, is used in reference to the auxiliary reaction, which is the oxidation of hydrogen.
The disadvantage of this technology is its very strong exothermic
nature, coupled with a very narrow optimal operation temperature window, and is best operated between 353 and 450 kelvin
s, yielding a steam loss of around one percent. Effective cooling is therefore required. In order to minimise steam generation, excessive dilution with nitrogen is used. Additionally the reaction is interrupted with an intermediary cooler before proceeding to a second stage.
In the first reaction an excess of oxygen is provided, at around a factor of two, and about 90% of the CO is transformed. In the second step a substantially higher oxygen excess is used, at approximately a factor of 4, which is then processed with the remaining CO, in order to reduce the CO concentration to less than 10 ppm. To also avoid excess CO-fraction loading, the transient operation of a CO adsorber may be important.
The instrumentation and process control complexity requirements are relatively high. The advantage of this technique over selective methanation
is the higher space velocity, which reduces the required reactors size. For the case of strong temperature rises, the feed of air can simply be broken.
The technical origins for CO-PrOx lies in the synthesis of ammonia (Haber process
). Ammonia synthesis also has a strict requirement of CO-free syngas
, as CO is a strong catalyser poison for the usual catalysts used in this process.
The catalyser preferentially oxidises carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO) using a heterogeneous catalyst placed upon a ceramic support. Catalysing agents include metals such as platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, platinum/iron, platinum/ruthenium, gold nanoparticles as well as novel copper oxide/ceramic conglomerate catalysts.
This reaction is a considerable subject area of research for fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
design. From the reaction of carbon monoxide with steam, carbon dioxide is evolved.
The presence of CO impedes the reaction of H2/O2 and reduces the fuel cell efficiency by the absorption of CO onto the platinum anode. The PROX process allows for the reaction of CO via the water gas shift reaction
Water gas shift reaction
The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...
(WGS), reducing CO concentration from approximately 0.5-1.5% in the feed gas to less than 10 ppm.
- 2CO + O2 → 2CO2
However, a selectivity of 100% is impossible. The selectivity, S, is used in reference to the auxiliary reaction, which is the oxidation of hydrogen.
- 2H2 + O2 → 2H2O
The disadvantage of this technology is its very strong exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
nature, coupled with a very narrow optimal operation temperature window, and is best operated between 353 and 450 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s, yielding a steam loss of around one percent. Effective cooling is therefore required. In order to minimise steam generation, excessive dilution with nitrogen is used. Additionally the reaction is interrupted with an intermediary cooler before proceeding to a second stage.
In the first reaction an excess of oxygen is provided, at around a factor of two, and about 90% of the CO is transformed. In the second step a substantially higher oxygen excess is used, at approximately a factor of 4, which is then processed with the remaining CO, in order to reduce the CO concentration to less than 10 ppm. To also avoid excess CO-fraction loading, the transient operation of a CO adsorber may be important.
The instrumentation and process control complexity requirements are relatively high. The advantage of this technique over selective methanation
Methanation
Methanation is a physical-chemical process to generate Methane from a mixture of various gases out of biomass fermentation or thermo-chemical gasification. The main components are carbon monoxide and hydrogen...
is the higher space velocity, which reduces the required reactors size. For the case of strong temperature rises, the feed of air can simply be broken.
The technical origins for CO-PrOx lies in the synthesis of ammonia (Haber process
Haber process
The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron or ruthenium catalyst, which is used to industrially produce ammonia....
). Ammonia synthesis also has a strict requirement of CO-free syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
, as CO is a strong catalyser poison for the usual catalysts used in this process.