Purification in the Gaseous Phase

Activated carbons for applications in the gaseous phase are mainly used for the purification of air, purification of process gases, recuperation of solvents, protection of the environment and as a catalyst.

The nature and the concentration of the pollutant, together with the specific conditions of the process determine the kind of carbon to be used. Generally, because the size of the vapour molecules is relatively small, microscopic carbons are required. In some applications, carbon activity is improved by impregnating it with catalysts or chemisorbates.

Adsorption in the gaseous phase is carried out by means of vapour condensation as the result of the interaction between the surface of the carbon and the vapour; this being an exothermal reaction.

The adsorption process in the gaseous phase can be described by means of special empirical equations such as the BET theory or the DUBININ equations.

The DUBININ equation relates the balanced adsorption capacity for a certain type of carbon at a determined temperature to the existing vapour concentration in the gas current that must be purified.

The most important parameters on which the good performance of activated carbons in the gaseous phase depends are:

1.-Adsorbate Concentration
Only when the vapour current to be treated is fully saturated will all the carbon pores be filled with the adsorbate. When the adsorbate concentration is at its lowest, only the smallest micropores will have the sufficient adsorption energy to adsorb it.

2.-Adsorption Temperature
Generally, as temperature rises, the adsorption capacity decreases due to:

a. Energy Content
As the temperature rises, the energy content increases, therefore, the adsorbent requires more energy to remain in a liquid state, thus directly affecting the adsorption balance.

b. Saturated Vapour Pressure
As the temperature rises, the vapour pressure increases making it more difficult to keep the adsorbent in its liquid state.

c. Adsorbate Density
As the temperature rises, the density of the adsorbent decreases.

3.- Nature of the Adsorbate
The more complex the adsorbate mixture to be treated is, the more easily it will be adsorbed.

4.- Adsorbate Boiling Point
When the adsorbate has a high boiling point, a higher degree of carbon activity is required.

5.- Relative Carbon Humidity
High relative humidity leads to a high humidity content in the carbon, thus strongly inhibiting its adsorption capacity.

Variables in a Gaseous
Phase System

Applications in the gaseous phase can be divided into several groups:

1. Purification of process gases.
Activated carbon is especially effective to adsorb impurities present at a ppm concentration. At these levels any other technique is generally more expensive. Activated carbon can adsorb almost any organic compound.

Impurities with higher boiling points, are more easily adsorbed with activated carbon. If the impurity has a low boiling point, an impregnated carbon can be used to improve the efficiency.

Some examples for this kind of application are:

• Removal of acid pollutants such as: SO2, NO2, HCL, HF, CL2
• Removal of sulfuric compounds such as: H2S metacaptanes and other present in CO2, H2, CH4, and N2 flows.
• Elimination of oil residues from lubricant oils present in compressed air or gases.
• Removal of traces of mercury vapor present in gaseous fuels.

In general, a few seconds are enough in all these applications to achieve adsorption.

2. Purifying Air
When purifying air, activated carbon is usually the most efficient method to control dangerous emissions or bad smell coming from a number of industries, mainly when the pollutant is present in a maximum concentration of hundreds of ppm.

As in the previous case, the higher the boiling point the impurity has, the more efficient the adsorption will be.

Depending on the removal efficiency required, the necessary contact time might be of a fraction of a second. The air flow oscillates between 0.05 and 0.4 m/sec; a common value is 0.25 m/sec.

3. Solvent Recovery
In many industrial processes that use organic solvents there are losses due to evaporation. The use of activated carbon systems allows the recovery of the solvent at a lower cost than that of the recovered solvent.

These are two stage systems:

1) In the first stage, the gas and solvent current goes through a carbon bed. The carbon adsorbs the vapours until it becomes saturated.

2) In the second stage, the adsorbent is removed from the operation and is subjected to a "desorption" process in which a low pressure vapour or a hot inert gas current passes through the carbon bed. Once the desorption cycle has been completed the adsorbent begins operating again. The solvent is recovered from the condensates or the inert gas applying the appropriate technique.

The use of the lowest temperature possible (less than 50°C), and a relative air humidity lower than 70% (the lower the better) are recommended for this type of application.

Contact time generally oscillates between 2 and 4 seconds, and the height of the carbon between 50 and 150 cms.

4. Others
There are other applications for activated carbon in the gaseous phase, among which the following should be mentioned:

• Industrial masks and breathers used as protection against toxic gases. - Military masks.
• As a catalyst or catalyst support, for example in the "Merox" process for sweetening gasoline.

The use of granular or pelletized activated carbons that provide a high level of hardness, and the appropriate flow behaviour is recommended together with the fall in pressure.

Carbons for the gaseous phase are characterized by the fact that they have a great number of micropores.

As mentioned before, the factors that affect the length of the mass transference zone and the fall in pressure, in the liquid phase, also apply in the case of the gaseous phase; nevertheless, there are additional factors that must be considered when designing a gaseous phase system, such as relative humidity. Consequently, it is convenient to asses the process by performing a test and also by getting in touch with your specialist.

Also visit:
[ The Mystery of Activated | Activation Process | Classification of Activated Carbon | Liquid Fase Purification | Gas Fase Purification ]