Advances in VOC/HAP control technology  

Since the Clean Air Act (CAA) was enacted in 1970, and amended in 1977, and again in 1990, the Environmental Protection Agency (EPA) has continued to tighten the limits on emissions of Hazardous Air Pollutants (HAP) and Volatile Organic Compounds (VOC).  Fortunately, the development of new air pollution control technologies has kept pace with changing EPA regulations. 

05/19/2003


By Rodney Pennington, PE


Since the Clean Air Act (CAA) was enacted in 1970, and amended in 1977, and again in 1990, the Environmental Protection Agency (EPA) has continued to tighten the limits on emissions of hazardous air pollutants (HAP) and volatile organic compounds (VOC).

Fortunately, the development of new air pollution control technologies has kept pace with changing EPA regulations.each industry or specific facility and the correct integration into the process.

Thermal Oxidation

Every thermal oxidizer works on the principle of converting VOCs and HAPs into carbon dioxide and water.in use 30 years ago, incorporate heat recovery systems.  Their thermal energy recovery (TER) varies by as much as 90 percent depending on the type of oxidation system.

 

In 1969, flares and common afterburners were the predominate oxidation systems.kept at given temperature.  Many processes require a combustion air blower that adds to the quantity of gases being treated and increases the fuel cost.

 

A well-designed common afterburner is refractory lined and has an internal configuration allowing temperature stabilization at 1400°F to 1600°F.

 

Recuperative Oxidizers

Common afterburner with single-pass heat recovery

This design adds a thin metal, tube-type (or plate) heat recovery section to the afterburner.

 

These types of units are hindered by corrosion, material buildup, and thermal stresses that limit the operating temperature to 1400°F or, occasionally, 1500°F.failure.

 

These types of oxidation systems typically required large amounts of fuel.RTOs), with their high TER efficiency and potential capital equipment paybacks of less than six months, moved to the forefront. 

 

Today, the RTO is the technology of choice for most oxidation applications.

 

It was only in the late-1990s and early 2000s that the RTO/RCO units effectively addressed and corrected air distribution problems with the integral valve design.

 

Regenerative thermal oxidation (RTO)

This evolution of the RTO from inception to today’s design brought a multitude of changes.


Auxiliary features such as variable energy recovery (VER), chamber flushing, fuel injection, valve sealing, bake-out, idle mode, and recirculation have enhanced and extended the flexibility of the RTO.

Heat exchange media
Industrial regenerative heat recovery was born in the early 1980s with glass furnaces.m the hot furnace exhaust flow.  That heat then was reused to preheat air flowing into the furnace.  A minimum of two chambers with flow-control dampers were required to alternate flow. 

One chamber functioned in a recovery mode and the other in a preheat mode.en modes also was relatively long, and the TER low.

Cycle time, surface area, and mass per unit of volume are key to optimizing the TER of the regenerative device.

The RTO of the early 1970s used a random ceramic saddle media, named after its configuration.ly oxidize VOCs.  This high level of thermal efficiency provided preheat temperatures within the ceramic heat recovery bed that promoted auto ignition and proved the effectiveness of flameless thermal oxidation.

By the early 1980s, increased depths of random saddle media in the RTO were achieving TER efficiencies as great as 95 percent.

For several years, various sizes and shapes of random packing media were used in an effort to reduce the pressure drop, but had little or no improvement.

The structured, or monolithic, ceramic media provides an ideal heat recovery media for RTOs.urface area per unit of volume and the greater the potential thermal energy recovery.  To optimize performance of the structured media, much shorter cycle times and uniform air distribution are essential.

The conventional RTO design, with individual inlet and outlet flow control valves, large individual heat recovery chambers, and linear configuration, is unable to optimize the performance of the structured media.

Configuration
The original RTO, with its horizontal airflow design, contained a central oxidation zone with symmetrical individual heat recovery chambers around the periphery.to development of a vertical airflow configuration in the early 1980s.  This marked a significant change in the configuration of RTO design.

The vertical airflow design with multiple recovery chambers underneath a common purification chamber eliminated the need for hot face retainers, reducing the capital cost of the system.

In the late 1980s, designers minimized the problem by moving the manifolds underneath the heat recovery chamber.tional resistance to the unit.  This helped even out the flow and improved distribution.  Although this design reduced the cold face plenum volume (which is the chamber between the inlet/outlet valve and the heat recovery bed) and associated chamber flushing volume, air distribution still was poorin the larger individual heat recovery chambers.

At about the same time, the two-chamber vertical flow design was introduced.

Integral horizontal shaft poppet valve
It was in the early 2000s that RTO/RCO design started to incorporate the integral horizontal shaft poppet valve and cold face plenum specifically designed for the operation of the RTO unit.

 


 

 

The original RTO used a clearance-seated butterfly valve on the inlet and outlet of each heat recovery chamber to control or alternate airflow from preheat to recovery mode.

Thermal expansion problems with the clearance‑seated valve, and associated leaking, led to use of the step‑seated butterfly valve.  These valves became the standard in all RTO/RCO units except two‑chamber systems, which require the faster-acting poppet valve.  Both types of valves have an oscillating make-or-break seat contact approximately every 1-minute to 2 minutes.  A typical, old‑fashioned three‑chamber RTO with nine flow‑control valves will have more than 3 million make‑or‑break seat contacts in a given year.

Test reports show that most RTOs with a proven operating history destroy nearly 100 percent of entering VOCs when the affect of flow‑control valve leakage and chamber flushing bypass are controlled.  Reliable, consistent valve seating is essential for optimal RTO performance.  In addition, a faster acting flow‑control mechanism is needed to optimize use of structured media, and uniform air distribution is essential.

The integral horizontal shaft poppet valve design, located beneath the heat recovery chambers, promotes uniform air distribution into and out of the heat recovery chambers with a single valve and actuator.

The integral horizontal shaft poppet valve is designed into the RTO inlet, outlet, and bottom cold face plenums for maximum performance.quick, easy maintenance without completely removing or replacing the valve.  The integral horizontal shaft poppet design also includes a walk-in, quick-acting, easy‑access door to both sides of the seats for inspection, cleaning, and maintenance.

Today’s RTO / RCOs have optimized the benefits of structured media, simplified flow control, and compacted the configuration into a smaller prepackaged unit, greatly improving on the original design. Thirty years of design improvements have resulted in RTO / RCOs that offer a smaller, more compact unit for a lower capital investment; a 30‑percent to 50‑percent reduction in energy consumption; increased destruction efficiency; and shop‑assembled modules or units with a field‑installation time of three or fewer days.  Together, these improvements make it possible for industry to remain in regulatory compliance while continuing to turn a profit.

Rodney Pennington, P.E., is National Sales and Marketing Manager for Western Pneumatics Environmental located at their Orlando, Florida branch; phone 407-822-9203; email rodneyp@westernp.com





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