FREQUENTLY ASKED
QUESTIONS
A: RTO is an acronym for Regenerative Thermal Oxidizer.
A: Thermal oxidation is a means often used to control emissions of Volatile Organic Compounds (VOCs) from process industries. Oxidation of the VOCs occurs when sufficiently high temperature (greater than 1,400°F.), adequate residence time, and turbulence (the 3 T's) are present to break the molecular bonds of any hydrocarbon to ultimately convert the hydrocarbon to carbon dioxide and water. With rare exception, thermal oxidation produces no harmful byproducts nor is there any hazardous waste to treat or dispose of. Though thermal oxidation is a simple process capable of very high VOC destruction efficiency, the fuel consumption to heat the VOC laden process can be severe.
A
Regenerative Thermal Oxidizer (RTO) builds on the thermal oxidation
operating principals but at greatly enhanced fuel efficiency.
An RTO consists of two or more heat exchangers connected by a common
combustion chamber or zone. The heat exchangers consist of
beds filled with media which will allow air to pass while serving as
a mass to transfer and store heat. The media material
selection, size, and shape can vary greatly and substantially impact
the design and utility efficiency of the RTO.
The flow sequence is that the VOC laden air stream enters the first heat exchange bed where the stream passes directly through the media and is preheated en route to the combustion chamber where a burner adds any supplemental heat necessary to reach and maintain optimum combustion temperature and complete the oxidization process.
The flow through the head exchange beds is reversed at regular intervals to retain the heat of combustion within the RTO. Depending on the application, up to 95% of the heat combustion can be recovered and recycled representing a huge fuel savings advantage relative to conventional thermal oxidization.
In comparison to both thermal and catalytic oxidation, RTOs have the advantage of the VOC application flexibility and destruction of a thermal oxidizer with better fuel efficiency than a catalytic oxidizer without the risk of poisoning or fouling expensive catalyst.
A: Entrained volume is the space within an RTO that contains process air that has not yet reached a temperature to destroy VOCs.
A: A puff is what occurs on a 2-bed RTO when the entrained volume is exhausted to atmosphere as the flow within the RTO is reversed.
Q: What are the differences in functionality between a 2-bed and a 3-bed RTO?
A: A 2-Bed RTO reverses the process flow between the heat exchange beds to regenerate heat. A 3-Bed RTO rotates the process flow through the heat exchange beds to regenerate heat. 2-bed RTOs are susceptible to vacuum spikes and at 95% to 98% VOC destruction, a reduced efficiency relative to a 3-Bed RTO. A 3-Bed RTO avoids "puffs" by purging the off-line bed before queuing it up for the next exhaust cycle to achieve 99%+ destruction efficiency.
Q: What is thermal efficiency?
A: Thermal Efficiency (TE) is the measure of the heat recovery of an oxidizer. The formula for TE is as follows:
100% - (average exhaust°F. - inlet°F.) ÷ (combustion°F. - inlet°F.) = TE
examples:
typical recuperative thermal oxidizer
100% - (880°F. - 100°F.) ÷ (1,400°F.-100°F.) = 40% TE
typical recuperative catalytic oxidizer
100% - (400°F. - 100°F.) ÷ (600°F.-100°F.) = 40% TE
typical KRONUS™ or TRITON™ RTO
100% - (175°F. - 100°F.) ÷ (1,600°F.-100°F.) = 95% TE
The figure in the above examples that ultimately has a correlation¹ to fuel consumption is the oxidizer delta temperature (∆T), average exhaust°F. - inlet°F. ∆T is the heat that must be replaced by fuel. The ∆T of a typical recuperative thermal oxidizer is 780°F., a typical recuperative catalytic oxidizer is 300°F. and a typical KRONUS™ or TRITON™ RTO is 75°F. In other words, a typical recuperative catalytic oxidizer requires 4 times more energy than a KRONUS™ or TRITON™ RTO, a typical recuperative thermal oxidizer requires 10.4 times more energy than a KRONUS™ or TRITON™ RTO.
1. For purposes of this demonstration fuel contribution of any process VOCs are not taken into account.
A: A poppet valve consists of a disk on a shaft that covers an orifice. This oversimplified description does not do justice to Alliance Corporation poppet valves, however, which are refined well beyond the valves of the competition. Alliance Corporation poppet valves are robust 2-way valves. The valve seats are machined from solid steel. The valve disk is sandwiched between a machined solid steel hub with a bolted and welded end cap. The disk assembly is threaded on to solid stainless steel shaft and locked at three points. The shafts are supported and housed in a purged pipe with self aligning guide bearings at top and bottom. Alliance Corporation poppet valves are automated by specially built actuators with extended pneumatic cushions to reliably slow down the disk to ensure gentle and quiet contact with the seat. The actuators are fitted with silencers to suppress pneumatic noise. Alliance Corporation poppet valves are built to endure decades of continuous use with no appreciable wear.
Q: What is "heat exchange" media?
A: Heat exchange media is a ceramic material that is located at the inlet and exhaust of a combustion chamber of an RTO. The purpose of heat exchange media is to collect the heat leaving the combustion chamber and transfer the stored heat to the combustion chamber when the flow is reversed.
Q: What is the difference between "random packed" and "structured" heat exchange media?
A:
Random packed media consists of relatively small ceramic pieces of a
repetitive shape such as saddles, bow ties, dog bones, etc.
Random packed has the advantage of being relatively inexpensive and
easy to handle in bulk. Random packed media has the
disadvantage of high pressure drop due to turbulent flow, limited
thermal efficiency and erosion. Random packed media is also
more susceptible to plugging from particulates and condensables.
Structured media consists of monolithic blocks of straight channeled
cells. Structured media has laminar flow and therefore offers
much better performance in terms of pressure drop and thermal
efficiency. Structured media is considerably more labor
intensive to install, however, structured media does not erode and
does not require the maintenance and replenishment of random packed
media.
Q: What is "fuel-free" operation?, Q: What is "self-sustained" operation?, Q: What is "auto-thermal" operation?
A: Fuel-free, self-sustained and auto-thermal are different names for the same thing. These names describe the condition when the fuel value of the VOCs being destroyed is equal to or greater than the heat loss of the RTO such that no supplemental heat from the burner is required.
A: FGI is an acronym for Fuel Gas Injection. Just as the name implies, Fuel Gas Injection is a method by which supplemental fuel, typically natural gas, is injected into the VOC contaminated process upstream of the RTO to boost the process fuel value to a "fuel-free" operation level. The advantage of Fuel Gas Injection is reduced supplemental fuel consumption and operating costs since combustion air from the burner is not needed. Also, since the burner is not needed, the NOX generation is negligible. Fuel Gas Injection should be reserved for processes where the fuel value of the VOCs being destroyed is less than that required for "fuel-free" operation.
A: Horns are a phenomenon that occurs when the the fuel value of the VOCs being destroyed is substantially greater than the heat loss of the RTO. In this condition, the fuel rich VOCs are destroyed in the heat exchange beds rather than in the combustion chamber. As such, the heat of combustion is released before reaching the combustion chamber and the temperature of the heat exchange beds can actually be greater than the temperature of the combustion chamber. The term "horn" comes from how a chart of the overall temperature profile appears while the RTO is operating in this state.
Q: What is a "hot-side bypass"?
A: A hot-side bypass is a duct with a valve that connects the combustion chamber to the exhaust stack. An RTO is a heat processor. When BTUs from the process VOCs are significantly greater than the RTO heat losses, the RTO exhaust temperature will climb. If there enough BTUs left unchecked, the exhaust temperature could climb to a point where the heat exchange media support and/or poppet valves could be damaged. A hot-side bypass is a means of diverting the excess BTUs to atmosphere to protect the RTO and reduce unnecessary pressure drop.
A: Telemetry is a means of remotely monitoring an RTO control system typically via modem over a phone line.
A: PLC is an acronym for Programmable Logic Controller. A Programmable Logic Controller is the brain of an RTO.
A: VFD is an acronym for Variable Frequency Drive. A Variable Frequency Drive is a device that controls the speed of an electric motor. Variable Frequency Drives are often used on fans that convey the VOC contaminated process air through an RTO. The advantage of a Variable frequency Drive in comparison to a motor starter is reduced electric consumption since the Variable Frequency Drive is able to deliver only the power needed to the motor which maybe significantly less than the name plate. Also, with a Variable Frequency Drive is easy fine tune the process flow or adapt an RTO to operation changes in the the process.
A: VOC is an acronym for Volatile Organic Compound. Many solvents used in industrial processes are Volatile Organic Compounds.
Q: Can an RTO treat chlorinated compounds?
A: Yes an RTO can treat chlorinated compounds, however, the chlorine atoms broken up in the VOC compound chain will combine with water and exit the RTO as hydrochloric acid. RTOs on chlorinated processes need to be constructed of hydrochloric acid resistant alloys and typically need a downstream caustic scrubber to neutralize the acid.
Q: Can an RTO treat fluorinated compounds?
A: Yes an RTO can treat fluorinated compounds, however, the fluorine atoms broken up in the VOC compound chain will combine with water and exit the RTO as hydrofluoric acid. RTOs on fluorinated processes need to be constructed of hydrofluoric acid resistant alloys and typically need a downstream caustic scrubber to neutralize the acid.
Q: Can an RTO handle cold climates?
A: Yes, an RTO can be equipped to withstand freezing ambient conditions. All control cabinets and/or junction boxes that are outdoors and contain instruments will need to be equipped with heaters and and the instrument air that is supplied to the RTO will need to be clean and free of any moisture. Alliance Corporation offers thermostatically controlled heaters for control cabinets and junction boxes as an option.
