Batch reactors are part of the production process in most chemical and petroleum companies. They generally have vents, which are opened and closed during production steps such as heating, cooling, mixing, filling, and emptying. According to most government regulations, it is essential to control the gases released from these vents. The emissions released are relatively low flow inert streams with high VOC concentration.
This article discusses the problem faced by a company operating many reactors that needed venting for mixing, filling, and depressurizing. The process flow contained all light hydrocarbons and some halogenated hydrocarbons with a flow rate of below 50 SCFM. Diluting the process vent with fresh air is one oxidation strategy for this type of process stream. This strategy uses a traditional oxidizer system to supply oxygen for combustion and thereby reducing the Lower Explosive Limit (LEL) to below 50% as per the National Fire Protection Association and FM Global guidelines.
However, in this case, fresh air is required in high volumes to attain the necessary LEL condition due to the high BTU content of the process vent. Although this strategy is sometimes acceptable, it significantly increases operating expenses and raises safety concerns.
For this application, Anguil employed a safer strategy (Figure 1), by which the process stream is maintained inert rather than adding fresh air to dilute it. The process vent is passed directly through a burner port of a multi-stage Direct Fired Thermal Oxidizer (DFTO), enabling the combustion system to carry out oxidation using the high BTU content as fuel. After brining the DFTO to operating temperature with natural gas, the inert process gases are sent to the burner.
Figure 1. Anguil employed a safer strategy to control reactor vent emissions.
During normal system operation, the pollution control device (Figure 2) will be fueled by the VOC laden process vent. During periods of low energy content or process flow, a supplemental fuel (natural gas) is directed to the burner in order to keep the operating temperature in the oxidizer combustion chamber in the range of 760°C-870°C. At these temperatures, even one second residence time is adequate to achieve a destruction efficiency of 99%+. The necessary oxygen level is ensured by placing an oxygen meter in the exhaust stack for total destruction of the VOCs.
Figure 2. Anguil’s pollution control device.
A soft refractory lining inside the oxidizer allows the system to be started and stopped without causing refractory failure, which is a problem with other designs. This enables shutting down the oxidizer during process downtime without impacting the longevity of the equipment. For inert gases free from sulfur compounds or halogens, the hot, purified, gas will be directed to a heat exchanger for energy recovery or vented off from the combustion chamber to atmosphere.
However, in this case, the process vent consists of halogenated compounds and therefore the hot exhaust gases from the DFTO are sent to a hastelloy quench to cool down prior to reaching a packed bed scrubber. The recirculation inside the scrubber with a caustic solution eliminates HCl and HBr. An induced draft fan then directs the scrubbed gases to an exhaust stack. The use of an induced draft fan with halogenated streams prevents corrosive gases from releasing into the atmosphere.
Any leakage of acid gases will negatively affect the equipment longevity and personnel. These scrubbing systems ensure the removal of 99%+ acid gases before discharging into the atmosphere. In addition, these acid gases could corrode the oxidizer shell behind the insulation when the metal temperature is less than the acid dew point. This corrosion risk is prevented by designing the oxidizer with an external shroud that maintains the carbon steel shell temperature above the acid gas dew point.
With a complete control system, the Anguil System is able to communicate with distributed control systems and modems for diagnostics and remote monitoring. These controls provide a variety of operating conditions such as system heat-up and automatic purge. The Anguil System is also equipped with magnetic driven pumps and scrubber controls for automatic operation.
This information has been sourced, reviewed and adapted from materials provided by Anguil Environmental Systems.
For more information on this source, please visit Anguil Environmental Systems.