Process Technology: An Introduction - Haan A.B. 2015

11 Particle removal from gases
11.1 Introduction

In many industrial processes, solid (dust) or liquid (droplets, mist) particles are at some stage generated and entrained by a carrier gas. As a rule, the particles must be separated from the transporting gas for a variety of process reasons. Examples are the recovery of a valuable product from the gas stream or the separation of solid and gas at the end of a pneumatic conveying line. Other important reasons are the purification of gaseous feeds and environmental protection. A single-stage separator can be schematically drawn, as in Fig. 11.1. As the efficiency of separation is very often particle-sizedependent, the feed flow is separated into a coarse fraction containing the separated particles and the cleaned gas that still contains some residual fine material. In order to meet the required limits set by process demands or environmental regulations collection efficiencies greater than 99.99 % are often necessary. This illustrates the high importance of highly efficient and well-designed gas-particle separation equipment. One difficulty is that the size distribution of the particles to be collected generally extends over a wide range, from < 0.1 µm to > 100 µm.

Fig. 11.1: Schematic of a gas-particle separator.

11.1.1 Separation mechanisms

The removal of particles (liquids, solids, or mixtures) from a gas stream requires their deposition or attachment to a surface. The surface may be continuous, such as the wall of a cyclone or the collecting plates of an electrostatic precipitator, or it may be discontinuous, such as spray droplets in a scrubbing tower and fibers of a filter. Once deposited on a surface, the collected particles must be removed at intervals without appreciable re-entrainment in the gas stream. Fig. 11.2 illustrates that there are four major mechanisms used to separate particles from gas steams:

· (1) gravitation;

· (2) centrifugal;

· (3) electrostatic;

· (4) interception.

Fig. 11.2: Overview of most important gas-particle separation mechanisms and equipment.

Although this list is not all-inclusive, it represents the mechanisms for the majority of gas-solid separation process equipment currently in operation. The first category includes simple gravity-settling chambers, where the particles are fed to a large box-like vessel and allowed to settle under their own weight. Gravity sedimentation is efficient only for large (dp > 50 µm) particles. The second includes cyclones and swirl separators, in which the applied centrifugal forces extend the applicability of sedimentation to considerably smaller particle sizes. The third encompasses process equipment where an electrical charge is used to separate the solids from the gas. While their use is not widespread in the chemical industry, electrostatic precipitators find wide application in the power industry. Electrostatic forces are the strongest forces available and are able to separate even very fine particles loosely defined as < 2-3 3 µm. The last category encompasses a variety of dust collectors such as filters and wet scrubbers, where interception of the particles by a medium (filter bag, plates, droplets) is used to separate the particles. These mechanisms are effective for the separation of particles down to 2-3 µm.

All of these collectors separate particles from gas by a common principle. Forces acting on the particles cause them to enter regions from which the gas cannot transport them away. These regions may be the inner wall of a cyclone, the droplet surface in a scrubber, the fiber or gain surface in a filter, or the collecting electrode in a precipitator.