ADSORPTION

 

The activated carbon ( or any other solid adsorbent) adsorption process is a simple method, but with a highly efficient  retention capacity and a possible economic payback, for the removal and recovery of solvent vapours contained in a gas stream.

 

The standard adsorbing medium is activated carbon which is obtained from a mineral or vegetable raw material and then heat treated in order to obtain a porous material with a very large activated surface. The qualities of an activated vegetable carbon are usually much better than those of an activated mineral carbon.

 

The quality of an activated carbon depends mainly on its activated surface, which is measured in B.E.T. (acronym consisting of the initials of the names of the three scientists who carried out this analysis method), expressed in square meters per gram of carbon.

 

For polluted air treatment an activated carbon with more than 1000 m2/g of B.E.T. should normally be used and Omniatex uses a very good carbon  with at least 1100 square meters of activated surface per gram.

 

Adsorption is a mass transfer process because the solvent passes from the air onto the carbon pores. The solvent molecules are attracted onto the surface of the activated

ITALY: 50,000 m3/h-Plant to recover solvent from the production of protective film

carbon by forces called Van der Waals forces after the name of the scientist who investigated this physical phenomenon.

 

Adsorption is also an exothermic process due to the fact that the solvent vapours being in the air condense on the activated surface of the carbon where they are retained in the carbon pores in liquid state.

 

The progressive formation of several layers of liquid solvent on the inner part of the carbon pores gradually reduces the power of the Van der Waals forces until  an equilibrium is reached and no more solvent can be caught by the pores.

 

Besides its activated surface, the retention efficiency of a carbon is given by:

 

·      Chemical and physical properties of the solvent

·      Chemical and physical properties of the activated carbon

·      Distribution and size of pores in the activated carbon

·      Air velocity through the carbon

·      Carbon bed depth

·      Relative humidity of the air stream

·      Air temperature

·      Solvent concentration in the air stream

·      Pressure at which the adsorption process is carried out

 

FRANCE : 120,000 m3/h-Plant with 25,000 kg/day of solvent recovered from note pads

The solvent retention capacity of the activated carbon at equilibrium and at a given temperature and pressure is shown in a special diagram called ISOTHERM, in which the adsorption capacity expressed in percentage of solvent retained by a given type of carbon is plotted for each solvent concentration (or partial  pressure). When reaching the isotherm curve, the same quantity of solvent entering the carbon bed comes out of it, just because equilibrium has been reached, i.e. the maximum retention capacity of a specific carbon referring  to a specific solvent.

 

Since the permissible emission limits are usually much lower than the emission of solvent concentrations entering the plant, the adsorption capacity expressed with the equilibrium isotherm must be used only partially. Therefore another isotherm should be used, which is called "working" or work equilibrium isotherm,  but with a much lower retention capacity.

 

The adsorption process in the carbon bed continues through several stages until equilibrium is reached. At the beginning the solvent is completely held by the carbon and the air stream leaving the carbon bed is completely solvent free. The solvent fills the carbon pores little by little, thus reducing the action of the Van der Walls forces and the solvent mass gradually transfers in  the carbon bed until some solvent traces are present in the air stream coming out of the adsorber after passig over the activated carbon bed. This step is called BREAKTHROUGH of the carbon bed. Now at this stage of adsorption the carbon bed is filled with solvent in different ways. The part of the carbon bed that was in contact with the incoming solvent-laden air stream has reached the condition of the equilibium isotherm and the carbon is totally saturated. The part of the carbon bed that was in contact with the outgoing air stream is partially saturated since the Breakthrough started. The middle  part of the carbon bed may vary between  the two above mentioned conditions.

 

 

When the Breakthrough condition is reached, also according to the preset emission limit, the  carbon should be regenerated, if possible, in order to restore its initial adsorption condition or it should be replaced with new carbon.

 

If the carbon can be regenerated, carbon regeneration is carried out using regenerating media that can loosen  the bonds  between the liquid solvent and the activated carbon surface. The standard medium is usually heat, that can be used either directly or indirectly.

ITALY: 220,000 m3/h-Plant to recover solvent in the production of flexible packages . The solvent is re-used both for inks and glues

 

 

 

 

Heat supplied by steam is the cheapest regenerating medium used for activated carbon regeneration. The heat evaporates the solvent in the inner side of the pores, empties them almost completely and leaves a free activated surface of the pore. The same effect can be achieved with less efficiency by a hot gas stream that lowers the partial pressure of the solvent. Usually the efficiency of the carbon regeneration by means of  steam, hot gas or vacuum can be assumed as follows:

 

 

REGENERATING MEDIUM, fed for 20 min

Efficiency %

Direct steam at 100 °C

98

Hot gas at 130 °C

45

Vacuum   25 mm Hg at  20 °C

25

Indirect heating at 100 °C

15

Ó Carbon Adsorption Handbook, Cheremisinoff / Ellerbusch - Ann Arbor Science

 

The regeneration efficiency is mainly given by the following factors:

 

·       chemical and physical properties of solvent

·       chemical and physical properties of activated carbon

·       regeneration temperature

·       regeneration time

·       mechanical action of the regenerating medium inside the pores

 

 

The adsorption process efficiency is strongly affected by the presence of compounds in the air stream to be treated  that cannot be removed from the pores of the activated carbon, e.g. dust, dirt, high boilers, various chemicals, acids and polymerization agents.

 

 

 

When these compounds are entrained along the carbon bed, the adsorption  process cannot be run with an acceptable efficiency and the exhausted carbon must be changed; this should be usually done a few months, max. 2-3 years, after bringing the plant into operation. 

 

Extreme attention should be devoted to this factor because the cost for  replacing the carbon is a very high operating cost for the final customer. If the polluted air entering the plant is fairly free from other compounds in addition to   the compound to be removed and recovered without problems, the physical life of the carbon will usually be 8 to 10 years.

 

BELGIUM: 14,000 m3/h-Plant to recover anhydrous acetone from gumming

 

In 1989 Omniatex prepared a mathematical model showing the differentiated adsorption process of a given type of solvent in the bed of a given type of activated carbon for CNR (National Research Committee). Every  simulation should be made entering the chemical and physical properties both of the carbon and of the  solvent to be adsorbed first and then, possibly, be regenerated. The same model shows clearly the advance  of the adsorbing phase in the activated carbon bed as well as of the possible regeneration and gives the  possibility of comparing different regenerating media such as steam, vacuum and hot gas.

 

After reaching the saturation of the adsorbing mass, that can be possibly detected by means of an analyzer placed on the adsorber chimneys,  you go over to the regenerating phase of the activated carbons which is made by sending steam in countercurrent  to the stream of air to be purified. 

 

The steam deadsorbes the solvents from the activated carbons so that a mixture of steam and solvent comes out of the adsorber and flows into two exchangers installed in series, the first one of which will  condense the vapours and the second one will cool the condensate.

 

 

The special steam regenerating system employed by Omniatex SpA allows a limited reduction to  powder of the carbon and the use of the same activated carbon placed in the plant when it was started  for at least 8 years without having to sieve  or compensate the carbon lost in order to remedy the efficiency loss, as it usually occurs in other types of plants.

 

 

The liquid water-solvent mixture flows then in a decanter where two phases become stratified, which are :

 

·       Upper phase (organic), almost only solvent mixture that can be re-used as it is ;

·       Lower phase (aqueous) containing some solvent traces.

              

 

The aqueous phase from the separator is  stored  in a special tank and is sent to the treatment plant later on.

 

The organic phase from the separator is  stored  in a special tank and is sent to another main tank later on.

 

In case of solvents that are mutually miscible with water, the plant includes also one or more distillation columns, the purpose of which is to make the distilled water re-usable and the solvent anhydrous to such an extent that it can be re-used for glue  reticulation or for the production of chips.

 

PolAND : 40,000 m3/h-Plant to recover gasoline from 

a military establishment 

 

 

The various working steps are remotely preset and controlled by means of a central control panel, located in a safe area,  with a microprocessor and a PC Touchscreen allowing automatic operation both with timers and with signals from the emission analysing system.

 

The plant was designed taking into special consideration the ATEX Standards (CEI 31 and 64/2) and following ones, thus reducing the danger centers of I Level, and consequently reducing the hazard of the plant itself.