A Stochastic Model To Describe The Design And Operation Of Knitted Mesh Mist Eliminator
Authors: Wilcock S.E. , Davies G.A. 
Published: December 2007

Methods used to design and predict the performance of mist eliminators are deficient in their inability to predict the outlet concentration of dispersed liquid in the outlet gas stream. This is a requirement specified in many process applications especially in gas treatment prior to atmospheric dispersion. In this paper a model is proposed, based on determining the trajectory of droplets through a knitted mesh mist eliminator, from which both the outlet gas concentration and drop size distribution in this gas stream can be specified. The model predicts the expected variations in performance, a decrease in the separation efficiency with decreasing both drop size in the feed mixture and gas velocity. A limiting velocity is predicted at which the drainage of liquid from the pad decreases. This corresponds to the flooding condition. Quantitative predictions of this limiting velocity compare well with the experimental results measured for air-water systems. The model offers the prospect of optimising the pad construction to maximise the separation efficiency at a target pressure drop or designing to a maximum pressure drop. For all designs, the conditions of the outlet gas steam can be defined.

Introduction

The separation of liquid droplets from gases and vapours is a process common to many chemical engineering operations in both the production and processing of petroleum. Distillation, partial condensation of vapours, gas scrubbing and gas compression are all process where the entrainment of liquid drops in the fluid stream must be minimised. A range of separation devices are available for this duty, the selection is largely dependent on the quantity of liquid per unit volume of gas and the drop size distribution of the liquid dispersion. For relatively coarse dispersions in which mean drop,, is between 500-1000 m simple gravity settling drums are used. In gas-liquid systems the difference in the density between the two phases is usually high, typically from 400-1000 kg. m-3 the viscosity of gas phase is by contrast relatively low so that the settling velocity of drops in this range is high, Under these conditions adequate separation can normally be achieved in a simple drum providing that there is sufficient residence time of the gas in the drum. As the drop size of the dispersed phase is reduced, as for example is observed in the exhaust from gas scrubbers then gravity settling is no longer effective. Then some form of interceptor must be introduced in which a surface, as a baffle blade or louvre plate, is introduced into the gas stream and onto which the droplets are captured or retained and can then coalesce until such a time as they can settle out under gravity from the gas phase. Their effective range extends down to drop diameters of approximately 20 m. As the drop size decreases further then the dimensions of the capture surfaces must decrease if effective separation is to be achieved. Thus, for dispersions containing droplets down to approximately 2 m diameter, knitted mesh mist eliminators are used. Here the filaments making up the capture surfaces are normally in the range 0.15-0.4mm diameter (compared with vanes typically 30 × 3 mm in section).

For sub micron droplets fibre bed 'coalescers' are used in which the fibre diameters are in the range 3-0.5 m. As one progresses from gravity separators to fibre coalescers, the superficial gas velocities decrease from around 10 ms-1 to 0.05 ms-1 (depending on the process conditions T, etc.) whilst the pressure differential across the separators will increase from 100 Pa to 1.5 × 104 Pa. Therefore, it is clear that the cross section of separators normal to the flow and the power costs increase substantially as the dispersion drop size decreases. It is therefore essential to have reliable and accurate design procedures for equipment particularly when dealing with fine dispersion.

P. 735