S. Wurster, D. Kampa, J. Meyer, T. Müller, B.J. Mullins, G. Kasper

Chemical Engineering Science, 2015, 132, 72-80

Liquid entrainment from coalescence filter media - i.e. flow induced “blow-off” of previously deposited oil - in the form of droplets is poorly understood, for one because the generated spectrum is very wide and difficult to characterize with temporal and size resolution, especially for very large drops which carry most of the mass. Such filters operate at much lower flow rates than classical demisters, often in vertical orientation, with much finer geometries, and gravity plays no direct role for entrainment. We present a novel combination of four measurement techniques used to capture the entrained oil drop spectrum from filters during operation in the size range of 0.01–2400 mm. The diameter range below 10 μm combines two established real-time methods including an electrical mobility particle spectrometer (EMPS; < 1 μm) and an optical particle counter (OPC; 0.3 to10 μm). The diameter range above 170 μm is covered by a newly developed “large drop detection system” (LDDS) based on single particle light scattering. OPC and LDDS continuously count and classify all drops originating from the entire filter coupon with a time resolution of 1 min. The EMPS was operated intermittently, following brief switches from aerosol to clear air flow. Drops in the size range between OPC and LDDS were collected and sized by an off-line method.
This measuring system was applied to two representative types of glass microfiber media operated with oil mist generated from compressor oil, in order to characterize time resolved drop formation rates and spectra in the range of nanometers to millimeters. Wettable and non-wettable filter media were found to show similar entrainment characteristics, each with multi-modal drop spectra having two pronounced peaks in the ranges of 1–2 μm and 200–300 μm, respectively. During steady-state operation both modes were generated quasi-continually, the large drops at the rate of a few drops per hour and cm² of filter surface, the micron size drops 10³–10⁴ times more frequently. Available in direct evidence suggests the same underlying entrainment mechanism for both types of fibrous media, namely the break-up of air bubbles formed periodically on the oil that drains on the downstream filter face. Direct detachment (blow-off) of large drops is unlikely at the prevailing operating conditions.