Today, most manufacturers in the United States are required to be compliant with regulations like the National Emission Standard for Hazardous Air Pollutants (NESHAP) or Maximum Achievable Control Technology (MACT) guidelines for their specific industry. Similar regulations are being adopted and enforced worldwide. Many of the exhaust gas emissions from industries include Volatile Organic Compounds (VOCs) as well as Hazardous Air Pollutants (HAPs). When left untreated, these emissions degrade in the presence of sunlight and contribute to low-lying ozone or smog.
Emissions are burned or oxidised at a prescribed temperature for a minimum time period, with some turbulence resulting in an extremely high percentage of the pollutants converted to carbon dioxide and water vapour. Not long ago, an oxidiser sales pitch would begin with the proud claim, “Our oxidisers will convert your harmful VOCs into harmless carbon dioxide and water vapour.” But with all the recent attention given to global climate change from greenhouse gases, one may start to wonder about the future of a class of equipment that generates carbon dioxide and nitrous oxidises as a desired end-product. The objectives have changed from simply destroying target emissions to also include the minimisation of these harmful by-products of combustion. Process emissions from batch or continuous operations can vary greatly in volume and composition. Depending on the process conditions, destruction requirements and energy demands at a given facility – flares, vapour combustors, thermal oxidisers and catalytic systems have been applied for emission destruction.
Historically, manufacturers have employed industrial catalysts to promote a chemical reaction that can generate products from various reactants. Even with advances in other oxidation technologies, there are still numerous applications where environmental catalysts are the preferred solution to emission removal and for good reason. For example, carbon monoxide, aromatic compounds and alkenes are often easily removed by passing the exhaust gas emissions over a heated catalyst. This catalyst will continue to perform at a high level of removal efficiency for many years with minimal operational issues.
Catalyst life is theoretically unlimited, but in actual practice, deactivation typically occurs in three to eight years. Various forms of sintering can occur at high temperature, in which the catalyst particles, and catalyst substrate that the metals are deposited onto and tend to agglomerate into larger crystals with a resulting decrease in activity. The precious metal catalyst for emission removal represents the largest component cost for a catalytic oxidiser. Depending on the rate of deactivation, this component could be replaced numerous times during the life of the system.
Enter the regenerative thermal oxidiser
The regenerative thermal oxidiser (RTO) is an abatement technology widely used on industrial air pollution control applications because of its ability to re-use up to 97% of the thermal energy from combustion to preheat incoming, untreated pollutants.
In operation, the solvent laden air (SLA) enters into one of the RTO energy recovery chambers where the high temperature, ceramic heat transfer media preheats the emissions prior to introduction into the oxidation chamber. As the SLA passes up through the bed, its temperature rapidly increases. After the chemical oxidation purification reaction occurs, the hot, clean, outgoing gas heats the outlet energy recovery bed. In order to maintain optimum heat recovery efficiency of the beds, the SLA flow direction is switched at regular intervals by the automatic diverter valves on demand from the programmable logic control system. This periodic shift provides a uniform temperature distribution throughout the entire oxidiser. With sufficient concentration of hydrocarbons in the process air stream, the heat energy content of the emissions themselves will self-sustain the oxidation process and no additional heat energy will be required.
New features enable RTO to handle challenging conditions, such as, emission spikes by incorporating a hot gas bypass (HGB). The HGB prevents the unit from overheating and damaging the insulation or ceramic heat recovery media. Since oxidisers are designed around the emission type, concentrations, temperature and airflow, they must be built for worst-case scenario conditions. The RTOs can be built with a larger unit size than catalytic systems, resulting in lower overall capital cost for control equipment.
The primary benefit of RTOs is the high heat recovery, which correlates to low operational costs. The high thermal efficiency of the RTO results in the stack outlet temperatures not much higher than the oxidiser inlet temperatures when the emission concentrations are low. This can result in the potential for dew point corrosion concerns if there are acidic gases being handled. However, this corrosion potential can be mitigated by using special alloys of construction, or by preheating the process gas into the RTO, so that the acid gas dew point temperature is never reached. Since most RTOs do not have catalyst incorporated into the design, there is a significant saving in maintenance and replacement parts.
RTOs have a distinct advantage over catalytic systems as the auxiliary fuel usage is lower under all low process exhaust concentrations. This has a direct correlation on greenhouse gas emissions from the auxiliary fuel combustion process. RTO technology enhancements are increasing their applicability on process emissions as new RTO designs allow them to handle more concentrated streams, destruction rate efficiencies (DRE) can reach 99%+ and remain relatively consistent, and there are fewer consumable components in an RTO.
Case in point
PTA is a white powder substance used extensively in the production of plastics and polyester products, such as, fibres or films. The material is produced in a pressurised reactor where acetic acid and xylene are combined chemically in the presence of catalyst. The majority of the PTA emissions come from this reactor, although there are small emissions sources from these facilities that could also emit VOCs and/or HAPs. Gas-phase byproducts of the PTA reaction are CO2, CO, water vapour, methyl acetate, unreacted xylene, acetaldehyde and small concentrations of methyl bromide. These gas phase emissions are routed to a pressurised absorber where they are scrubbed with water to reduce emissions. The remaining air emissions are then routed to an abatement device.
Most of the recent PTA expansions have occurred in the Middle East, India and Asia where these plants are in close proximity to the plastic products made from the material. However, the low cost for carbon-based precursors will likely bring more of this PTA manufacturing back to the United States, where manufacturers can also take advantage of the new air pollution control technologies and techniques.
No two technologies are the same
Employing the proper abatement system for a given application can mean the difference between compliance and non-compliance, which in many cases translates to chemical production and a shut down. Even under compliance, the improper application of an oxidiser technology can dramatically impact operating expenses and drive down profits. It is important to note that very few processes are identical and therefore, no one technology choice can be applied to all applications. To maximise ROI, plants should consult with a professional, as each application is unique.