The present design practice for dehydration tanks and vessels is typically based on a plug flow/Stokes' law approach, which is considered as unrealistic. Instead, these tanks may be considered as deep layer settlers, where the actual separation takes place in a dispersion band located between single-phase oil and water layers. The variation of the thickness of the dispersion band with the throughput of the settler is a characteristic function of the heaviness and stability of the feed, and depends on the operating conditions. In a new design philosophy developed by Shell Research these settling characteristics are being used to establish the operating window of primary oil/water separators.
A 100 litre model settler has been built to study the settling characteristics of crude/water dispersions on a representative scale. Batch settling experiments have been carried out with crudes of various gravities to establish the relation between nominal settler capacity, crude viscosity and dispersion quality. This information was complemented with operating data of existing dehydration facilities to obtain a data-base for design.
This paper discusses the proposed design method and the results of the model settler experiments. A comparison will be made with the conventional approach and the first applications will be illustrated.
An Alternative for Stokes' Law
Present design rules for crude/water separators are based on variations of Stokes' law, which relates the maximum allowable oil flow rate to the physical properties of the crude and an arbitrarily chosen cut-off droplet diameter d (usually 200), as follows:
(1)
where Qoil is the oil flow rate, A the horizontal cross-sectional area of the separator, vs the settling velocity of a water droplet of size d, p the density difference between water and crude, and the viscosity of the crude. Stokes' law would be valid for unhindered settling, which means that it is implicitly assumed that the dispersed phase concentration is low, say not more than 5-10%. During most of its life a dehydration tank/vessel will not see such dilute dispersions. When the dispersion becomes more concentrated the droplets will start to hinder each other, resulting in a lower settling velocity. On the other hand the droplets will start to grow by coalescence, which leads to faster settling. At present it is not yet possible to predict the resulting settling process on the basis of these droplet/droplet interactions. An alternative to such a micromechanical approach is to consider the dispersion as a continuum, and to describe its behaviour on a more macroscopic basis. This is a customary approach for the design of deep layer settlers in the process industry, firmly based on the extensive investigations of Barnea and Mizrahi and their predecessors. The actual separation in a tank/vessel takes place in a two- phase zone, the dispersion band, located between an oil and a water monophase, see Fig. 1. The dispersion band is bounded at the top by the settling front, which is determined by the smallest water droplets that can just settle against the rising oil flow, and at the bottom by the coalescing front where the largest water droplets merge with the water-phase. During stationary operation the dispersion band is in equilibrium: both fronts approach each other with a velocity that just matches the superficial velocity of the incoming fresh feed, when the throughput increases, the dispersion band expands to establish a new equilibrium at a higher separation capacity: expansion means less hinder for small droplets, and more residence time for coalescence. The resulting dynamics of the dispersion band, the variation of the thickness HD as a function of (gross) throughput per unit cross-sectional area Q/A, is characteristic for each feed, and can be described by the equation:
(2)
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