These gas streams are usually produced by processes requiring ventilation, including paint booths, printing, and paper mills.
They are suited to applications with low VOC concentrations but high waste stream flow rates. This is due to their high thermal energy recovery. The basic operation of an RTO consists of passing a hot gas stream over a heat sink material in one direction and recovering that heat by passing a cold gas stream through that same heat sink material in an alternate cycle. They are used to destroy air toxics and VOCs that are discharged in industrial process exhausts.
P&P RTO plants are individually designed for your application by our long-time experienced process-engineers.
Image 1 shows the principal process of a RTO plant:
1. Waste gas flows through a hot ceramic heat storage that has been heated just before. Thus heating the waste gas to approximately 90% of burning temperature.
2. The VOCs are oxidized and heat the burner chamber. If the caloric value of the VOCs is too low then a burner can be employed to provide the additional energy.
3. The now cleaned gas then flows over another ceramic heat storage thus heating the heat storage. After enough energy of the clean gas got recovered it flows straight to the stack.
4. After a certain amount of time gas valves redirect the gas flow and the cycle starts a new.
Because of the ceramic heat storage hating rates of 90%-100% can be achieved, thus dramatically reducing the amount of energy needed for the thermal oxidation.
RTO plants can have 2 to 5 chambers. Picture 1 shows a 2 chamber plant. 3 chambers are used to minimize the raw gas slip. When the gas flow direction is reversed within a 2 chamber RTO plant a small amount of raw gas gets immediately released through the stack. To avoid this a third chamber is employed. While gas flows into chamber 1 and out of chamber 2, chamber 3 is washed with air, thus pushing waste gas out of the chamber. Then the cycle continues: Chamber 1 gets washed, chamber 2 waste gas flows in and chamber 3 clean gas flows out. For even larger waste gas amounts a 5 chamber RTO plant can be build.
Another construction option is the placement of the gas fan. The fan can be placed pressure sided or suction sided. The benefit of a pressure sided placement is that the fan is not threatened by heat damage as it is not exposed to the hot clean gas. The benefit of a suction sided placement is that in the case of a leakage the ambient air is sucked into the RTO plant thus ensuring that no waste gas is released into the environment.
Another development of the RTO plant is the catalytic RTO plant: After the waste gas passed the heat storage it flows through a catalyst bed. In the catalyst bed the VOCs are oxidized at much lower temperatures, thus dramatically reducing energy demand for reaction. Because of that catalytic RTO plants have the lowest energy costs.
The heart piece of a RTO plant is the heat storage that heats or extracts energy from the gas flow. The P&P Group uses hexagonal ceramic cells instead of square cells. Image 2 explains the difference between square and hexagonal cells.
Hexagonal cells compared to square cells have more contact points for heat exchange and fewer dead spots. Trough that hexagonal cells have higher thermal recovery efficiencies. On top of that hexagonal cells can be packed even tighter so that heat storages can be built even more compact. Trough that 15% more gas channels can be deployed, which means 15% more flow rate und thus less pressure drop.
Another challenge is the gas distribution within the heat storage. Experiments showed that thermal recovery worked suboptimal because of flow effects within the heat storage. Image 3 shows the heat distribution within such a heat storage.
Image 3 shows that a large quantity of the heat is stored in the middle of the heat storage. Because of that the thermal efficiency is reduced which results into higher energy costs for the RTO plant. To avoid this a distributor base is deployed to distribute the gas more homogenously.
Image 4 shows how the distributor base ensures more homogeneous heat distribution within the heat storage. Trough that heat recovery efficiency can be increased which is beneficial for the overall plant.
Through hexagonal cells and distributor bases the P&P Group ensures that its RTO plants work more efficiently and reduce energy costs.
Heat removal in autotherm processes:
In case that the caloric value of the VOCs is higher than the required energy to oxidize them the temperature in the chambers will increase over time. To avoid thermal damage of the plant hot gas valves are installed so that the excess heat can escape right into the stack. Technical speaking this solution isn’t not optimal as energy goes to waste and it’s rather difficult to implement these hot gas valves.
To remove the heat more efficiently the P&P Group has designed a thermal salt energy recovery system
Because of the changing load within the RTO chambers a buffer media needs to be used to continuously transfer heat. The P&P thermal sat system extracts excess heat through finned tube heat exchangers and saves it in the salt tank. After that heat is continuously transferred to a steam generator. The steam can then be used within the process or for electricity generation.
Gas valves that redirect the gas flow within a RTO plant are working very hard. In one year they have to redirect the gas flow up to 500.000 times and thus they are likely to wear and break down. Because of that the P&P Group uses lifting valves (Image 6).
Through a pneumatic lift a flap is opened and closed. The flap is placed onto a sealing ring that ensures that no false air is sucked into the chamber. This simple construction is far more robust and resistant to break down thus guaranteeing a long and steady plant operation.
The P&P Group has several years of experience within the plant supplier business and is continuously improving its products. Our RTO plants provide the following benefits for our customers:
· Hexagonal ceramic cells for better heat recovery and tighter packing with lower pressure loss.
· Distributor bases to homogeneously distribute the heat through the heat storage, thus improving energy recovery.
· Thermal salt system to remove excess energy that can then be converted into steam.
· Lifting valves that are far more robust and resistant than conventional ones.
· Through that we achieve high availability for your plant in conjunction with high energy recovery and low operating costs.
We like to inform you about your specific RTO solution