V. Sibanda

Particle separation from gases using cross-flow filtration

Abstract Most filters used in solid/gas separation are applied in the “dead-end” mode, where the aerosol particles arrive at the filter on trajectories that are approximately perpendicular to it. This study concerns the alternative method of cross-flow filtration, which is in common use for filtering liquids. In this mode, previously studied by Menard et al. [Powder Technol. 71 (1992) 263], Thomas et al. [Powder Technol. 76 (1993) 79] and Ferrer et al. [Powder Technol. 113 (200) 197], some of the flow passes through the filter, depositing particles on the surface, while some sweeps past the surface, causing shear on the deposited cake. Depending on the operating conditions, particle aggregation can occur on the surface and the resulting aggregates can be removed by the through-flow and collected in a downstream device. Laboratory scale equipment has been set up in which a cross-flow filter module is coupled to a cyclone postseparator to separate the aggregates leaving the filter. The filters used were ceramic tubes of 6 cm outside diameter, 4 cm inside diameter and variable lengths. An aerosol of limestone dust particles (mmd=5.0 μm) in ambient air enters the inside of the filter along the axis; due to the aerodynamic conditions within the filter some of the particles are deflected towards the filter and are captured. The results show that the cake is detached in the form of loose agglomerates rather than individual particles, which are easily collected by means of the cyclone. The collection efficiency shown by the cross-flow filter/cyclone combination is over 99% for 5.0 μm (mmd) particles under optimum conditions compared to ≈90% shown by the stand-alone cyclone. This separation efficiency is comparable to that of a surface filter and the pressure loss savings with this system make it an attractive gas cleaning option.

Gas cleaning at high temperatures using rigid ceramic filters

Abstract Rigid ceramic filters have emerged in the last two decades as the most promising technology for particulate removal from process gases at temperatures up to 1000°C. Granular and fibrous forms of media have been developed and both are commonly employed in the form of cylindrical ‘candles’ which are periodically cleaned by application of a reverse gas pulse. Research has focused on this cleaning process, which governs the long-term performance of the filter. The problem is 2-fold: to determine the dust cake ‘detachment stress’, which depends on the dust particle properties and cake structure, and to understand the propagation of the cleaning pulse which is applied to remove it. The results of research in these areas are summarized and experimental methods for the investigation of filter cleaning briefly described. The implications for design and further development of ceramic filters are discussed.