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Efficiency €œComparison of tanks of pre-flotation using computational fluid dynamics (cfd) simulation

OVERVIEW
  • The volumes of oily water (water + oil) that are produced during petroleum production have been increasing.
  • Engineers are concerned on what one should do with this amount of water.
  • Flotation process is a method for oil/water separation widely used in wastewater treatment.
  • Removal of free and dispersed oil in water.
  • Evaluate alternative technologies or modify existent technologies.
OBJECTIVES
  • Simulate a flotation tank for the treatment of produced water.
  • Obtain a geometric configuration that improves the separation of the original tank simulation.
  • The original tank has a distributor of oily-water in the bottom of the tank, a water collector below the mixture distributor and oil collector at the top of the tank.
  • Evaluated three different configurations:
    1. Tank with internal devices (original configuration)
    2. Tank without internals
    3. Tank with chicanes.
METHODOLOGY
  • A study of the influence of the use of source points was performed.
    1. Very simple "two-dimensional" rectangular domain.
  • In the first test, a coarse mesh was used applying a mass and momentum source.
  • In the second test, the small fluid injection face was fully reproduced with a fine mesh.
    1. Quantities at the "injection point" region behave as "space averages" of the real injection case.
    2. Conservation of mass and momentum are respected.
  • No physical phenomena was lost by the application of the source points.
  • Based on this study all the distributor/collector piping were replaced in the simulation by arrays of source points.
  • Tank internal distributor and collector.
  • Oil collector was not included due to the Lagrangian boundary condition (at the tank top).
RESULTS

Not all the collection holes are used over the collectors piping (bad holes distribution simulation).

The distribution of velocity along the distributor holes was nearly uniform while, in the collector, the flow rate distribution was not evenly distributed.

The tank with chicanes the oily water is barred by chicanes and all droplets are collected in two firsts compartments.

In the tank without internals, the oily water is injected and "short-cut" to outlet.

The use of chicanes reduces the amount of recirculation inside the tank increasing the average residence time.

CONCLUSIONS

For the tank with distributor and collectors, the collector of water works inefficiently (uneven flow rate distribution).

The efficiency of the tank with chicanes was considerably higher.

The chicanes breaks the recirculation zones and increase the average residence time.