High concentration and high value organics are recovered, and the equipment cost is high.
Olefin monomer and helium have been recovered from the flushing gas of polyolefin unit in industry. In the field of environmental protection, recover hydrocarbons from gas stations; CFC is recovered from the production and use of refrigeration equipment, aerosol and foamed plastics, and vinyl chloride monomer is recovered from PVC processing. This technology is very promising. With the emergence of new high-efficiency membranes and the reduction of system cost, it will become an important recycling means.
1.3.2 elimination technology: mainly including thermal oxidation, catalytic combustion and integrated technology
22.214.171.124 thermal oxidation
The thermal oxidation system is a flame oxidizer, which eliminates organics through combustion. Its operating temperature is as high as 700 ℃ - 1000 ℃. This inevitably has a high fuel cost. In order to reduce the fuel cost, it is necessary to recover the heat in the exhaust gas leaving the oxidizer. There are two ways to recover heat, the traditional wall heat exchange and the new unsteady heat storage and heat exchange technology.
The intermediate wall thermal oxidation uses a row of tube or plate type intermediate wall heat exchanger to capture the heat of purification exhaust gas. It can recover 40% - 70% of the heat energy, and use the recovered heat to preheat the organic waste gas entering the oxidation system. The preheated exhaust gas is purified by reaching the oxidation temperature through the flame. The disadvantage of the intermediate wall heat exchange is that the heat recovery efficiency is not high.
Regenerative thermal oxidation (RTO) adopts a new unsteady heat transfer mode to recover heat. The main principle is: the organic waste gas and the purified exhaust gas are circulated alternately, and the heat is captured to the limit by continuously changing the flow direction for many times. The heat storage system provides very high heat recovery.
In a certain cycle, the organic waste gas containing VOC enters the RTO system and first enters the fire-resistant heat storage bed 1 (the bed has been heated by the purified gas of the previous cycle). The waste gas absorbs heat energy from the bed 1 to raise the temperature, and then enters the oxidation chamber; VOC is oxidized into CO2 and H2O in the oxidation chamber, and the waste gas is purified; The oxidized high-temperature purified gas leaves the combustion chamber and enters another cold heat storage bed 2, which absorbs heat from the purified exhaust gas and stores it (used to preheat the organic exhaust gas entering the system in the next cycle) and reduce the temperature of the purified exhaust gas. After a certain period of time, the gas flow direction is reversed and the organic waste gas enters the system from bed 2. This cycle continuously absorbs and releases heat, and the heat storage bed as a heat sink is also constantly changed in the operation mode of inlet and outlet, resulting in efficient heat recovery. The heat recovery rate can be as high as 95%, and the VOC elimination rate can be as high as 99%.
126.96.36.199 catalytic combustion
Catalytic combustion is a method similar to thermal oxidation to treat VOC. It uses platinum, palladium and other precious metal catalysts and transition metal oxide catalysts to purify organic matter instead of flame. The operating temperature is half lower than that of thermal oxidation, usually 250 ℃ - 500 ℃. As the temperature decreases, standard materials are allowed to replace expensive special materials, which greatly reduces the equipment cost and operation cost. Similar to thermal oxidation, the system can still be divided into two types of heat recovery methods: wall type and regenerative type.
The wall type catalytic combustion is to set a heat exchanger behind the catalytic bed. The heat exchanger not only reduces the exhaust gas temperature, but also preheats the organic waste gas containing VOC. Its heat recovery can reach 60% - 75%. Such oxidizers have long been used in industrial processes.
Regenerative catalytic combustion (RCO) is a new catalytic technology. It has the characteristics of RTO high-efficiency energy recovery and the advantages of low-temperature operation and energy efficiency of catalytic reaction. The catalyst is placed on the top of the heat storage material to achieve * purification, and its heat recovery rate is as high as 95% - 98%.
The key to the performance of RCO system is to use special catalyst, precious metal or transition metal catalyst impregnated on saddle or honeycomb ceramics, which allows oxidation to occur at half the temperature of RTO system, which not only reduces fuel consumption, but also reduces equipment cost.
At present, some countries have begun to use RCO technology for the elimination and treatment of organic waste gas, and many RTO equipment have begun to be transformed into RCO, which can reduce the operating cost by 33% - 75% and increase the exhaust gas flow by 20% - 40%.