On-Line Drop Size Distribution Measurement of Oil-Water Dispersion Using a Par-Tec M300 laser Backscatter Instrument
Authors: EL-Hamouz A.M. , Stewart A.C. 
Published: December 2007

The scanning laser microscope (Par-tec) has been used to measure drop size distributions of oil-water mixtures in both batch and online operations and the performance of the instrument has been assessed. Measurements have been made to determine the influence of the shear produced by pipes and fittings on the dispersion characteristics of oil-water mixtures. The influence of oil composition has also been investigated.

The Par-tec has been found to give reproducible mean chord data for oil-in-water and water-in-oil dispersions in both a batch mixing process and on-line at various dispersed phase concentrations and for both clear and optically dense oil systems. However, there were some important limitations noted. Firstly, the drop size measured by the instrument is not a drop diameter as measured by most other particle sizing instruments but a drop chord length; secondly, the drop size was found to vary with the focal length of the instrument.

Changes in drop size due to changes in process conditions can easily detected using the Par-tec instrument. In this work the effect of horizontal pipe length and number of bends was investigated. It was found that the drop size increased with increasing horizontal pipe length due to drop coalescence. The drop size was also found to increase as the number of bends in the pipe was increased in a 1.5" I.D. U-pipe fitting, indicating that the energy dissipated in the system is not enough to outweigh the effect of coalescence due to the pipe length.

The sequence of pipe fitting was found to influence the nature of dispersion. The value of the mean chord at the exit of a needle valve followed by a small U-pipe fitting was not the same as the mean chord for the U-pipe followed by the needle valve. The main advantages of this instrument are its flexibility (it can be easily inserted into process streams in most pipes and vessels) and its ability to obtain data in two phase systems with high dispersed phase concentrations and/or an optically dense continuous phase.

Introduction

The detailed design of equipment for the removal of water from oil (or indeed any kind of liquid-liquid separation) requires knowledge of the dispersion properties such as the droplet size distribution, dispersed phase concentration, phase continuity and the interfacial tension of the system. An understanding of the nature of heterogeneous flow through pipes and fittings would enable more accurate predictions of conditions at a separator inlet to be made and hence the equipment could be designed in a much more scientific way. However in order to make significant improvements in performance, it is necessary not only to have information about the drop size distribution at the separator inlet but also to determine the drop size profile along vessels of different design. At present this data is usually produced from simulations such as those produced by computational fluid dynamics (CFD) software. While CFD is a very useful tool in the study of separation processes, it is limited in its ability to simulate multi-phase flow, and it is important that data produced is validated by experiment.

The scanning laser microscope (Par-tec) is a relatively new particle/droplet sizing instrument which works on the measurement of reflected light, thus it is not subject to the same restrictions of turbidity as other laser techniques which require a transmitted light signal.

This instrument offers a potential means of direct drop size measurement in the field. This would not only provide important data for equipment design but would also be an invaluable tool for field trails. In fact similar instruments have already been used during the offshore trial of novel separation equipment, Schmoll and Cowie.

The major aims of this work were:

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