## 3 Safety Management Strategy
### Water Tank for Pressure-Relief-Valve Discharging  
An SCWO system is operated under high pressure. In our commercial plant, pressure relief valves are installed on the heat exchanger and the reactor to protect the vessels by venting fluids when serious overpressure occurs. In general, the discharged fluids from a pressure relief valve are recommended to blow directly into the atmosphere. However, the discharging fluid from the heat exchanger mainly contains unreacted organic matter because the feedstock has not been mixed with oxygen. The discharging fluid from the  
Figure 2: Schematic diagram of the heater in the SCWO system.  
reactor is more complicated, containing reaction products such as CO\({}_{2}\) and NH\({}_{3}\), and partially oxidized products such as H\({}_{2}\)S, olefin, phenols, pyrrole, and so on. Moreover, a considerable amount of unreacted oxygen is also discharged with the effluent. The toxic, flammable and strong-smelling effluent should not be directly discharged into the atmosphere; therefore, a tank is used to collect the discharging fluids.  
Figure 3 shows the schematic of the tank for pressure-relief-valve discharging. High-temperature fluid is ejected from the effluent eject via the multi-nozzles; meanwhile, cooling water is ejected from the spray. There is a gas outlet located at the top of the tank, and a fluid outlet at the bottom. Due to the special characteristics of the discharging fluids, some special designs are used to guarantee safety during the relief-valve discharging process.  
The effluent eject contains three pipes, and there are many nozzles on each pipe, as shown in Figure 3b. The multi-nozzle structure is used to divide the effluent into many parts and obtain a better heat exchanging effect between high-temperature effluents and cooling water. It is notable that there would be a considerable amount of oxygen contained in the discharging fluid if the pressure relief valve on the reactor opened. There is an in-tank fire risk when the discharging fluid is injected into the tank via the nozzles because the high-velocity oxygen rubs dangerously with the surface of the nozzles. To avoid the ignition of fire and the ensuing explosion in the water tank, Monel 400 (Ni-Cu alloy) is used as the component material of the effluent ejector.  
The temperature and pressure of the discharging fluids that are ejected from a pressure relief valve can be up to 600 \({}^{\circ}\)C and 30 MPa. Once a pressure relief valve opens, the pressure of the discharging fluid drastically decreases, resulting in the formation of steam. At the bottom of the tank, some water at room temperature is stored. The nozzles from the effluent ejector are not directly in contact with the stored water, to avoid water hammer when the steam dissolves in water. By this means, the solid and liquid materials in the high-temperature discharging fluid that is ejected from the nozzles are able to mix with the stored water. The gases from the discharging fluid, including soluble gases and insoluble gases and steam, are cooled down by the cooling water from the cooling water spray at the middle of the tank. Moreover, there is a coil heat exchanger located above the tank. The high-temperature gases can be totally cooled down by the heat exchanger, and all the steam can be transferred into liquid phase in the coil heat exchanger and flow back to the tank.  
Figure 3: Schematic diagram of the tank for pressure-relief-valve discharging: (**a**) is the front view, and (**b**) is the top view of the ejecting pipes of the discharging fluids.  
Insoluble gases are introduced to a biofilter system, which is a highly efficient separation process for removing organic matter in the gas phase.