Adsorption Technologies for VOC/solvent polluant using active carbons: How does it work?

Adsorption is an heterogeneous process in which gas molecules are retained on a solid that prefers specific compounds to others and thus removes them from effluent streams. When the surface has adsorbed as much as it can, the adsorbed content is desorbed as part of the regeneration of the adsorbent. When the contaminants are desorbed, they are usually at a higher concentration and can either be recovered or disposed.

The adsorptive properties of charcoal were first observed near the end of the 18th century. It was discovered that charcoal was capable of decolourizing certain liquids. This discovery led to the first industrial use of charcoal in an English sugar refinery in 1794.

Today, activated carbon is used in a wide range of industrial applications, including gas and air cleaning involving reusable substance recovery applications. Heightened environmental awareness and the enactment of strict emissions guidelines have led to the development of new applications, most notably in the area of air pollutant removal. Activated carbon is also being used to an increasing extent in the treatment of water, including drinking water, groundwater, service water and waste water. Its primary role in this context is to adsorb dissolved organic impurities and to eliminate substances affecting odour, taste and colour in halogen hydrocarbons and other organic pollutants.

The selection of the most suitable type of activated carbon for a specific application depends on the physical and chemical properties of the substances to be adsorbed. Aside from this material data, other process related factors play a role in the adsorption process. Although carbon adsorption has been established as an effective technique for VOC capture and?recovery?in lower concentration ranges, significant limitations still exist (cost of direct VOC recovery after adsorption is very high due to the processes and possibility of producing secondary hazardous waste).

In an interesting work published on “Journal of Cleaner Production” by Zhang et al. (https://lnkd.in/eRHVuqN), the ability of a specific active carbon regarding adsorption for four types of VOCs (ethylbenzene, n-heptane, dichloromethane, ethyl acetate) emitted from an industrial paint factory was assessed in order to comprehensively verify the preferential activity and removal mechanisms. Different VOC adsorption configurations and energies adsorbed onto activated carbon?were investigated in detail with the method of quantum chemistry calculations, revealing the complex formation with surface molecules of active carbon. As expected, all obtained adsorption energies are negative therefore spontaneous, and C-Cl bond is the more easily to broke (fundamental for have a feasible desorption stage).

The performance of the active carbon is strongly influenced by its surface chemistry (surface functional groups) and texture (micro, meso and macropores). The nature and concentration of surface functional groups can be modified by suitable thermal or chemical post-treatments of the carbon. Oxidation in the gas or liquid phase can be used to increase the concentration of oxygen groups, while heating under inert atmosphere may be used to selectively remove some of them.

Fixed-bed dynamic behavior is usually described by effluent concentration as a function of operating time or throughput volume. The concentration profile of a typical breakthrough curve for a single adsorbate in a fixed-bed column, where the mass transfer zone is the surface of the bed where sorption takes place.

Specific application is the emission treatments of methane at low concentration in distributed surface wells. The majority of gassy mines have been located in hilly areas, and distributed surface wells are widely used to recover the methane for mining safety and commercial purposes. However, a large amount of low concentration methane from distributed wells cannot be directly transported due to its high explosion risk, and have to be emitted into the atmosphere, resulting in air pollution and wasting energy.

Conventional pressure swing adsorption (PSA) methods are mainly employed to increase the methane concentration for safe transportation. However, they typically adopt large-scale factory mode, which is not effective due to the lack of industrial land for distributed wells. Interesting study by Hu et al. (Industrial Engineering Chemistry Research, https://lnkd.in/emxAf7C)?propose an efficient approach to address this issue by a distributed PSA purification system and pressurization. After being pressurized in the pressurization unit, the methane parameters can meet the safety requirements of the long distance transportation.

PSA is a technology able to separate gases or vapours in a waste gas mixture and simultaneously regenerate the adsorbent. It consists of four steps:

• Step 1: pressure is built up by the gas streaming into the adsorber;
• Step 2: adsorption at high pressure and hence the production of pure compounds;
• Step 3: depressurisation;
• Step 4: purging at low pressure or under vacuum;

This four-step process causes a separation of compounds according to the strength with which they bond to the adsorbent. With downstream treatment facilities, this technique improves the ability of waste gas mixtures to be recovered and reused.

Typical adsorbents:

• Granular activated carbon (GAC);
• Zeolites;
• Macroporous polymer particles;
• Silica gel;
• Sodium-aluminium silicates;

Another application very specific is the Mercury (Hg) treatment with active carbon in gaseous stream. Mercury compounds?are neurotoxins and in human body, they accumulate in the liver, kidney, brain and blood. The exposure takes place via inhalation,ingestion and dermal absorption, with various mercury species (elemental, inorganic and organic compounds). Activated?carbons?due to their high specific area, pore volume and wide pore size distribution, have been extensively utilized for Hg removal via physical and chemical sorption.

In physisorption, Hg vapor adsorb on carbon’s surface, which reflects a weak bond and is very sensitive to temperature changes. The process is exothermic and the adsorption capacity decreases with temperature increase. By contrast, chemisorption lead to the formation of stronger chemical bond. One typical example is active carbon impregnated with sulfur. Detailed review regarding several abatement technology for mercury in natural gas streams was published by Chalkidis et al. (https://lnkd.in/entrCh3), investigating also absorption and catalytic conversion of Hg.

Another key application is the VOC emissions from production and distribution of oil make up is a relatively small proportion of overall global VOC emissions, but the local effects can be significant given for example the large volumes of gas displaced in marine loading of tankers, the presence of benzene?and other harmful substances.

Vapour emissions control systems have already been installed at many ports and terminals to recover NMVOCs (Non Methane Volatile Organic Compounds) from loading of both crude oil and products such as naphta and gasoline. Interesting paper of Johnson et al. (https://lnkd.in/encVarr) explains how VOC can be adsorbed on a bed of activated-carbons with subsequent regeneration by creating a vacuum. This system is combined with either upstream or downstream absorption of the VOCs in crude oil.

Dr. Matteo Compagnoni is an Industrial Chemist and Sales Manager focusing on Air & Water Pollution Treatment and Heterogeneous Catalyst applications.

Ph.D. in Industrial Chemistry at University of Milan (Italy), Master degree in Industrial Chemistry and Bachelor degree in Environmental Chemistry.