Waste Gas Treatment

Chemical treatment of exhaust gases

The methods of cleaning exhaust gas are very diverse, differing in equipment structure as well as in cleaning technology. Waste cleaning methods are selected according to the volume and composition of waste. In addition, it is also selected on the basis of economic-technical analysis comparison. To achieve high efficiency of exhaust gas treatment, it is necessary to use a combination of many different filtration methods and equipment. In order to protect the environment, one of the ways to “treat exhaust gas” quite effectively and thoroughly to contribute to environmental protection is to “treat exhaust gas by chemical method. The principle of “chemical method” in exhaust gas treatment

Perform chemical interactions to convert toxic gases into less toxic products thanks to special catalysts. Implemented in reactors.

The role of chemical methods in exhaust gas treatment

Helps to increase chemical reactions in exhaust gas treatment.

Specifically, there are two problems with this treatment: Converting C, CO, and COV to non-toxic CO2 by oxidation reaction, that is, burning in the presence of O2. Converting  NOx  to O2  and N2, is the opposite reduction reaction to the above reaction. These two processes must run concurrently. Therefore, it is necessary to find an allowable “range” to adjust the oxygen concentration so that both processes can work together, and to find an appropriate catalyst. Both require a lot of time investment and a lot of modern specialized equipment. Therefore, in Vietnam it is very difficult to do, this field is almost left open.

In the ferrous metallurgy industry, aluminum is produced by electrolysis of the flue gas, which usually contains CO, and a catalyst containing 0.3% Pt carried on aluminum oxide is often used. Organic compounds are the main components in the exhaust gas of plastic factories, antibiotics, phthalic aldehydes, paint lines, etc. The catalysts commonly used here are Pt, Pd and Rh. Pt and Pd are often carried on Al2O3  to oxidize alkanes. Pd-based catalysts with high activity in CO oxidation reaction cost less than Pt. Pt has the highest activity for the oxidation of propane at 5000C, while Pd is a better catalyst for the oxidation of ethane, methane and olefins. Rh has the lowest activity in the oxidation of propane, but when combined with Pt, it forms the best catalytic material for the oxidation and reduction of NOx.

The chemical method is the process by which gases are adsorbed due to a chemical reaction with the adsorbent, the intermolecular forces in this case are stronger than in physical adsorption. Therefore, the amount of heat released is greater, and  more energy is required.

Besides, we also use chemical substances such as:  Water,  Basic solutions: KOH, NaOH, Na2CO3 , K2CO3 , Ca(OH)2…, CaCO3… to absorb gas by liquid (chemical)  and is the process converts the gas molecules to be treated into the liquid phase by dissolving chemicals when they come into contact with each other.

The above processes make up the process of chemical treatment of exhaust gases.

Methods of treating exhaust gases by chemical methods

Thus, the treatment of waste gas by chemical method is actually a treatment method that applies chemical absorption and chemical adsorption to treatment equipment.

Exhaust gas treatment by absorption method

Absorption of exhaust gas by chemical method is a method that uses liquids and solids as raw materials to absorb toxic gases from the production process of the factory. This is the process of transferring harmful gases to be treated into the liquid phase. Thanks to the dissolution process, they come into contact with each other. A chemical reaction occurs during absorption.

Exhaust gas treatment technique by absorption process is also widely used to dehumidify the air, harmful gases and odors in the exhaust gas. It helps to recover all kinds of vapors, clean gases mixed in the air or exhaust gases.

Image of exhaust gas treatment mechanism by absorption tower

The mechanism of the adsorption treatment technique consists of 3 steps

Step 1: Diffusion of gaseous pollutant molecules in the flue gas mass to the surface of the absorbing solution
Step 2: The penetration and dissolution of the gas into the surface of the absorbent solution.
Step 3: Diffusion of dissolved gas on the separator surface deep into the absorbing liquid

During the absorption process, the gaseous pollutants in the exhaust gas are trapped on the surface of the solid material and cleaned. The trapped harmful gases are called adsorbents.

The activities of dehumidifying the air, removing harmful gases and odors in the exhaust gas, and recovering valuable vapors and gases are called absorption processes.

Commonly used adsorbents

Water (H2O)
Basic solutions: KOH, NaOH, Na2CO3, K2CO3, Ca(OH)2, CaCO3,…
Monoethanolamine (OHCH2CH2NH2), diethanolamine (R2 – NH), tritanolamine (R3– NH).

The application of exhaust gas treatment by chemical absorption method to:

Treatment of pollutant emissions.
The absorption method can handle exhaust gas with a large emission flow.
Absorption methods are often used to treat SOx, HCl, H2S, HF, Cl2, NOx, acetone, etc.
Recycling substances are recovered or transferred to another production stage.