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Separation Friendly Produced Water Pumps

Nocente, Alessandro
Doctoral thesis
Åpne
Fulltext not available (Låst)
Permanent lenke
http://hdl.handle.net/11250/2414453
Utgivelsesdato
2016
Metadata
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Samlinger
  • Institutt for energi og prosessteknikk [4536]
Sammendrag
Produced water is the largest by-product of the oil and gas industry and its handling

represents an important challenge. The fluid is mainly constituted by water, but

its high content of sand, salt, sediments and especially the presence of dispersed

oil makes its treatment necessary before its disposal or its use. The content of

oil, in form of dispersed droplets surrounded by a continuous water phase, is the

main environmental issue and must be reduced to the lowest possible level. The

most common de-oiling treatments are gravity based, i.e. they use the difference

in density between oil and water to remove the dispersed phase. All the gravity

based systems are based on the Stokes’ Law for dispersed fluid and it is therefore

of capital importance that the droplet dimension is not reduced during the fluid

handling, so that the components of the circuit (valves and machines) do not cause

an excessive droplet breakage.

The solutions adopted today are mainly based on the use of volumetric pumps,

such as screw pumps. These machines ensure a low breakage level, but the nature

of the fluid and the working conditions significantly reduces their operating life.

Operating life would be longer if dynamic machines were used, but they introduce

a higher shear in the fluid because of their very operating principle.

Typhonix AS is developing multistage centrifugal pumps for produced water

treatment which couple the longer life of a centrifugal pump with the low droplet

breakage typical of a volumetric machine. By means of a special hydraulic design,

these pumps have also demonstrated a coalescing effect on the droplets which

translates in an increase in the efficiency of downstream removal system.

The present work is aimed to the study of the flow characteristics in the machine

passage to understand what promotes the droplets growth in order to improve

the design. A particular attention was focused on the recurring turbulent flow structures

in the machine passages, and the evaluation of the possible influence these

can have on the droplets breakage and on the coalescing promotion.

To study the internal flow a numerical model was prepared and several computer

simulations were carried out. In order to evaluate the accuracy of the simulation,

the CFD code was validated by means of comparison with experimental results. An experimental rig was designed and built at the NTNU Waterpower

laboratory and the flow velocity distribution was measured with laser techniques.

The validation of the numerical model gave satisfactory results and the model

considered appropriate for a good flow prediction. In particular recurrent turbulent

structures were found in the passage between the diffuser and the return vanes,

caused by the interaction between the flow and the machine surfaces. The influence

of these structures on the dispersed phase was simulated by means of numerical

particle tracking which showed how they have a retaining effect on the oil droplets.

The retaining effect demonstrated to have a higher effect on the smaller droplets,

increasing their residence time in the machine and therefore increasing the possibility

of coalescence. Moreover, because of the difference in density between the

dispersed and the surrounding phase, the droplet will tend to migrate in the center

of these vortexes under the action of centrifugal forces. This phenomenon means

that the vortexes not only have a retaining effect, but represent the active driving

force for the coalescing effect.
Utgiver
NTNU
Serie
Doctoral thesis at NTNU;2016:263

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