- August 01, 2000, Ralph Stettenbenz, Air Preheater Co.
Since the late 1970s, there has been a dramatic increase in pollution control applications involving silicone as part of the exhaust stream. Unfortunately, the resulting powder creates additional clean-up problems. In addition, new markets such as self-adhesive products and air bags have created an even greater demand for silicone-based surfaces. This increase in product demand has created an urgent demand for new production equipment and pollution control equipment.
Oxidizers—including catalytic, regenerative, and recuperative—are susceptible to plugging. The silicones in the process stream to the oxidizer are converted to silica in the high-temperature oxidation zone. This silica dust will coat the internals of the oxidizer and build up on the heat exchanger media or tubes. This coating reduces heat exchanger efficiency and eventually will plug the heat exchanger media. In addition, the coating also will reduce process flow and increase pressure drop and horsepower requirements.
These conditions will remain until the system is cleaned out or replaced. Even applications with low concentrations of silicones will plug most oxidizers, and an increasing number of product coating applications use higher quantities of silicones, which will cause the heat exchanger media to plug quickly.
Because the tightly packed bed media is susceptible to plugging, the use of regenerative oxidizers is severely limited. If the bed becomes coated or plugged, the cleaning requirements are extensive. Furthermore, if silica deposits are in the higher concentration ranges, the demand for cleaning will be more frequent, requiring longer downtime and possibly requiring full or partial bed replacement. Whatever the cleaning method—in place or bed replacement—the cost and equipment downtime are substantial.
Complicating matters is the fact that the design of some recuperative shell and tube heat exchanges makes them difficult to clean. Small-diameter, tightly packed tubes are prone to plugging, and with some tube designs, the configuration of the combustion chamber and the heat exchanger does not provide easy access for cleaning.
A New Solution
An innovative shell and tube recuperative heat exchanger design has been developed to combat the problem of silica buildup. With clean-out doors integrated into the oxidizer sides, this design allows direct access to both ends of each heat exchanger module and to the internal strategic areas of each heat exchanger bundle.
The modular design allows the tube configuration and turning plenums to be positioned for optimum cleaning, blowdown, and silica removal. The vertical configuration also provides space savings as compared to the more traditional horizontal, low-profile design.
Each heat exchanger module is positioned with the tubes in the horizontal position. Since the modules are placed end to end, the dirty process exhaust gas is directedthrough the tubes of all bundles, is preheated, and enters the burner.
The converted silica dust is contained in the hot oxidized gas in the combustion chamber. This gas then flows over the tubes but does not directly impinge on the tube sheet face, maintaining the lowest possible tube sheet temperatures. The lower tube sheet temperatures and the lower stress levels result in the highest usable life of the heat exchanger.
A Unique Design
The space between the tubes is designed for optimum velocity, thereby minimizing silica buildup on the tubes. This design also will resist blocking the gas flow through a portion of the heat exchanger. Furthermore, because the tubes are in a horizontal position and the hot oxidizer gas flows over the tubes in a vertical direction, the heat exchanger is cleaner for a longer period of time, and an efficient cleaning of the tubes is a simple process.
Eventually, silica dust will be deposited on the tubes, but this design allows for easy, efficient removal. Because the large access doors are strategically located on the heat exchanger casing, it can be cleaned without any internal access. The silica deposit can be removed from the tubes using an air lance blowdown and allowing the particulate to settle in the connecting plenums located under the heat exchanger.
This material then can be vacuumed through the access doors in the plenum without personnel having to enter the oxidizer and be exposed to the silica particulate.
Quick access dramatically reduces downtime. By monitoring heat recovery efficiency (preheat temperature leaving the heat exchanger or stack temperature) and the oxidizer pressure drop, cleaning times can be scheduled during production downtime.
The oxidizer burner design allows high preheat inlet temperatures and minimizes burner NOx emissions. This feature and a hot side bypass arrangement provide the flexibility of using high-efficiency heat recovery and the capability of a flexible solvent loading production operation range. In some cases, secondary heat recovery in the form of hot water, air, or oil has increased heat recovery savings.
The views and opinions expressed in Technical Reports are those of the author(s), not those of PFFC editors. Please address comments to author(s).
Ralph W. Stettenbenz, commercial manager at Air Preheater Co., Wellsville, NY, has been associated with air pollution control oxidizers for 34 years. He has a degree in mechanical engineering and has published articles on pollution control. He can be reached at 716/593-2700.