Should laboratory exhaust be connected to waste gas treatment: How to connect fume hood exhaust with spray tower activated carbon
The fume hood addresses the removal of harmful gases from the operator's vicinity, but removal does not equate to elimination; the gas must ultimately be exhausted outdoors. The composition of the exhaust air, its concentration, and the feasibility of direct high-altitude discharge determine whether end-of-pipe exhaust treatment equipment is required. Focusing solely on ventilation without considering the end-of-pipe treatment may lead to rework during project acceptance or environmental inspections. Conversely, applying complex processes blindly when pollutants are simple may result in wasted investment. The decision on whether to install end-of-pipe treatment for laboratory exhaust should be based on the properties of the gas.
Which gases require treatment and what process to use
Acid mists that can be absorbed by water or alkaline solutions, such as hydrochloric acid mist, sulfuric acid mist, nitric acid mist, and hydrogen fluoride, are suitable for wet absorption using PP spray towers, transferring pollutants to the absorption liquid through gas-liquid contact. Alkaline gases like ammonia are treated with acidic absorption solutions. Volatile organic exhaust gases that are not soluble in water are suitable for activated carbon adsorption units, where the organic compounds are adsorbed by the pores of the carbon layer. Before entering the carbon unit, gases containing water mist, high dust, or high temperatures must be pre-cooled, de-misted, or filtered to prevent the activated carbon from becoming damp, clogged, or reducing its adsorption efficiency.
When pollutants are complex, containing both acid mists and organic compounds, a multi-stage combination of spray towers and activated carbon adsorption units is often used. Acid mists and water-soluble gases are first removed by wet absorption, and the gas is then de-misted before entering the carbon unit to treat organic compounds. The process sequence cannot be reversed.
How to integrate treatment equipment with the exhaust system
Adding end-of-pipe equipment increases system resistance. When selecting fans, the resistance of the spray tower, carbon unit, ducts, and silencer must be accounted for, ensuring sufficient pressure head, otherwise, it may result in insufficient airflow and low face velocity of the fume hood. Ducts, dampers, and the casings of treatment equipment for corrosive gases should be made of PP or PPs flame-retardant materials. The flame-retardant grade is determined based on the medium and fire protection requirements. Space for fan placement and maintenance around the treatment equipment must also be reserved.
What to clarify during construction
First, systematically review the types of experiments and common reagents used in each fume hood, listing the pollutant inventory, which serves as the basis for selecting end-of-pipe processes. Second, clarify the discharge destination and local environmental requirements to determine whether treatment is needed and the extent of treatment. Third, entrust the calculation of airflow to ventilation and environmental design, considering the number of fume hoods, simultaneous use coefficient, and treatment equipment resistance. Designing ventilation safety and exhaust compliance as an integrated system is less likely to leave shortcomings compared to designing each subsystem independently.
