Environmental friendliness is the most important requirement in the modern chemical industry, which receives special attention at the present time. Quite a number of solutions for achieving the required environmental performance indicators have been proposed so far. But can all of them be used optimally, and is their cost justified?
One of the most efficient solutions in this area is regenerative catalytic oxidizer (RCO). The use of the RCO allows smoothing concentration in homogeneities of harmful impurities in the flow purified.
Russian state institutions started working on implementing the technological regulation system based on the principles of the best available techniques (BAT) as early as 2014. Similarly, by means of legal and economic regulation, this mechanism has been adjusted and operates effectively in foreign countries (Europe, the USA, etc.).
The attention of the Government of the Russian Federation to the environmental situation in the country has been growing consistently and steadily. This is evidenced by the fact that 2017 was declared the Year of Ecology in Russia. The "dirtiest" Russian industrial enterprises should comply with the BAT principles as early as 2019, while the legal system intends to control all industrial enterprises by 2025.
In view of this, production process engineers currently strive to minimise waste generation. The employed methods include recycling (return to the production process) of substances, release of contaminants followed by their deactivation, and the like.
The catalytic method holds a special place in deactivation, and, along with such methods as thermal oxidation, incineration, absorption/adsorption, etc., has a number of advantages related to energy efficiency. Catalytic purification of toxic compounds is based on a number of primary chemical reactions aimed at forming substances being harmless for both the environment and people. The reactions can be conventionally divided into the following groups:
- Oxidation reactions:
Volatile organic compounds (VOC) + О2→СО2 + Н2О
CxHy + O2→CO2 + H2O
H2S + ½О2→Sel + Н2О
2 NH3 + 3/2О2→N2 + 3Н2О
СО + ½O2→CO2
- Reduction reactions:
NOx + NH3→N2 + Н2О
- Decomposition reactions:
N2O→N2 + ½O2
This method of catalytic purification is most efficient if the content of contaminants, for example, VOC is 0.8-15.0 mg/m3. Meanwhile, gases containing toxic impurities in the amount of 0.8-5.0 mg/m3 should be purified preferably using the regenerative catalytic oxidizer (RCO), while gases with a higher calorific value with VOC content from 5.0 to 15.0 mg/m3 are suitable for the regenerative thermal oxidizer (RTO). The use of adsorption or other methods that allow concentrating impurities is optimal for the processing of gases with concentrations below 0.8 mg/m3.
An important aspect of using the catalytic method of purification is variation of the content of harmful impurities in the flow purified. Not infrequently, it is related to the operation mode of the contamination source. When the VOC content is low, the heat generated by the reaction will be insufficient for running the catalytic process in autothermal operation conditions, while at high concentrations, hot spots in the catalytic bed that exceed the permissible catalyst and reactor material operation temperatures are possible. All these concentration inhomogeneities are smoothed by using the RCO.
Fig. 1. Regenerative catalytic oxidizer
The method is based on periodic changes to the direction of the purified gas flow . Thus, the heat generated during a chemical reaction is stored in the catalytic bed. Once the direction of the flow is changed, the heat that has been generated during catalytic decontamination is used to heat the inlet gas subject to purification. The periodicity of switching varies from 5 to 100 minutes and depends on the concentration of impurities and volumetric flow rates. By maintaining the required temperature in the catalyst bed at 300-700 °С (depending on the concentration of substances to be decontaminated and the switching mode), the method provides highly-efficient purification from various harmful or toxic substances with different contaminant content in the flow. The primary advantage of this method is its energy efficiency. RCO allows saving energy costs by 3-5 times as compared to the conventional RTO.
Field of application and area of practical implementation. Currently, RCO is used to purify gases from volatile organic contaminants (methanol, phenol, styrene, hydrocyanic acid, butanol, acrylic acid, formaldehyde, benzene, acetone, etc.). More than 100 units with 0.5-30.0 thous. m3/h purified gas capacity are operated in the USA on its basis. Catalysts manufactured by Katalizator JSC, such as aluminium copper-chromium ICT-12-8 and aluminium manganese ICT-12-40, are used as the base catalysts for these units. Unlike catalysts that include precious metals, catalysts provided by Katalizator JSC have a number of advantages:
- a wide temperature operating range up to 1,000 оС;
- recoverability; and
- they contain no precious metals.
In Russia, the regenerative catalytic oxidizer was successfully applied to purify gas discharges from the mixture of nitrogen oxides (NOx). Catalytic purification is based on the process of selective reduction of nitrogen oxides by ammonia on solid catalyst surface. The purification process is quite efficient at variable concentrations of nitrogen oxides. The degree of purification of gas flow from nitrogen oxides reached 99%. Such unit having a capacity of 10.0-12.0 thous. m3/h was operated at an enterprise manufacturing high-energy substances to purify production flue gases after absorption columns.
As of today, the RCO has high potential for solving industrial ecology problems. Its applicability is not limited to purification from VOC and nitrogen oxides, it can be successfully applied to purify gas emissions including low concentration gases from ammonia, sulphur dioxide, organic impurities, including chlorine-bearing ones. The RCO can be employed at ferrous and non-ferrous metallurgy, machinery, petrochemical, chemical (fertilizers and explosives manufacturing, plastics processing, etc.), food and other industrial enterprises.
- S.S. Nesterenko, E.V. Ovchinnikova, V.A. Chumachenko. Catalytic Neutralization of Gas Emissions Containing Volatile Organic Compounds in a Wide range of Concentrations [Journal] // Catalysis in Industry, No4 - 2014 - pages 45-54
- Yu. Sh. Martos, A. S. Noskov, V. A. Chumachenko. Catalytic Neutralisation of Production Flue Gases [Book].-Novosibirsk: Science. Siberian branch, 1991, 224 pages
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