(257d) Microchannel Fouling Mitigation: Flow Distribution and Wall Shear Effects
AIChE Spring Meeting and Global Congress on Process Safety
Thursday, April 27, 2006 - 8:40am to 9:00am
Microchannel process technology offers the potential to intensify a wide range of chemical reaction and heat exchange applications. However, claims by microchannel practitioners are often met with skepticism from industry, and this commonly includes concerns about the plugging or fouling of the thousands of small channels inside each microchannel devices. While this is a legitimate worry, recent long duration experiments with microchannel vaporizers show that two interrelated strategies mitigate the risk of plugging: high wall shear within the microchannel and good flow distribution.
Minimizing the heat transfer distance inside heat exchangers has long been a strategy for improving exchanger performance. However, each increment of size decrease can increase the likelihood of fouling. Heat exchangers using microchannel process technology are characterized by parallel arrays of channels, with typical dimensions in the 0.010-inch to 0.200-inch range; therefore they come with an increasing concern that scale and particles may block individual channels, thereby hurting both heat exchange and pressure drop performance.
One problematic application for fouling is boiling apparatuses. In partial or full boiling services, scale commonly forms on the heat exchange surfaces, resulting in performance degradation. Scale forms on surfaces in contact with water as a result of the precipitation of normally soluble solids that become insoluble as temperature increases. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate. An entire class of water treatment chemicals, known as scale inhibitors, attempts to prevent this buildup by keeping solids in solution. Despite their use, scale is a constant concern for boiler operators and makes the option of adopting intensifying microchannel devices worrisome.
The proposed presentation will focus on the methodologies and results from long duration microchannel vaporizer experiments, where fouling tests were run both with and without good flow distribution. In runs ranging from 1,000 to 9,000 hours for devices operated at ambient and high pressure (20 atm), no pressure drop increases were observed for the devices with good flow distribution even when the feed water was intentionally doped with high levels of dissolved solids. This absence of fouling within the individual microchannels was verified by post operational autopsies and attributed to high wall shear. Some fouling was noted in the headers and footers, but they were sufficiently large so as to not affect pressure drop or heat transfer performance.