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New Development of Drag Reducing Agent for Oil Pipeline (Part 2)

Drag reduction mechanism

Most of the drag reducing agents in oil phase are polymer with flow chain or long straight chain and few side chains. For example, ZODR-G350 is a high molecular polymer - σ olefin with a molecular weight of 10-10. The polymer pure agent is a rubber like solid, as a commodity, it is generally dissolved in the solution of hydrocarbons (kerosene). 10% DRA solution is very viscous viscoelastic, which is difficult to flow and can be drawn into a long filament. Polymer drag reducer can be dissolved in crude oil or oil, but not in water, and long chain curling of molecules occurs when meeting water. The results show that the drag reducing agent solution has strong Newtonian characteristics, its viscosity is as high as 3000pa · s at low shear rate, and it will not decompose below 120 ℃. Drag reduction is a special turbulence phenomenon. Drag reduction effect is a macroscopic manifestation of the influence of drag reduction on turbulent flow field, which is a pure physical action. The drag reducing agent molecules do not interact with the oil molecules and do not affect the chemical properties of the oil, but are closely related to their flow characteristics. In turbulence, the velocity of fluid particles changes randomly, forming large and small vortices. Large scale vortices absorb energy from the fluid, deform and break, and transform into small-scale vortices. Small scale vortex, also known as dissipative vortex, is weakened and subsided under the action of viscous force. Part of the energy it carries is converted into heat energy and dissipated. In the side layer near the pipe wall, the transformation is more serious due to the effect of wall shear stress and viscous force.

After the drag reducing agent is added to the pipeline, the drag reducer is dispersed in the fluid in a continuous phase. With its unique viscoelasticity, the long molecular chain extends along the flow to the natural state, and its microelements directly affect the movement of the fluid microelements. The radial force from the fluid element acts on the drag reducer element, causing it to twist and rotate. The gravitational force between the drag reducing agent molecules resists the above forces and reacts on the fluid microelement, changing the action direction and size of the fluid microelement, so that a part of the radial force is transformed into the axial force along the flow direction, thus reducing the consumption of idle work and reducing the loss of friction resistance. In laminar flow, the fluid is affected by viscous force, and there is no eddy dissipation like turbulent flow. Therefore, it is useless to add drag reducer. With the increase of Reynolds number, the drag reducing agent will show the effect of drag reduction. The larger the Reynolds number, the more obvious the drag reduction effect. When the Reynolds number is quite large and the shear stress of the fluid is enough to destroy the molecular chain structure of the drag reducer, the drag reduction effect will be reduced or even completely lost when the drag reducer is degraded. The concentration of drag reducing agent affects the thickness of the elastic bottom layer in the pipeline. The higher the concentration is, the thicker the elastic bottom layer is, the better the drag reduction effect is. Theoretically, when the elastic bottom reaches the pipe axis, the drag reduction reaches the limit, that is, the maximum drag reduction. The drag reduction effect is also related to oil viscosity, pipeline diameter, water content, pigging and other factors.