What are the effects of drag reducing agents(DRA)?

Drag reducers are high polymer compounds that have the effect of reducing resistance. When it is added to the fluid, the flow rate can be increased and the energy consumption can be reduced.

The chemicals used to reduce resistance to fluid flow are called drag reducing agents, or DRA. Drag reducing agent is widely used in crude oil and refined oil pipeline transportation. It is an important means to improve pipeline circulation capacity and reduce energy consumption in specific areas. The frictional resistance of the fluid restricts the flow of the fluid in the pipeline, resulting in the decrease of the pipeline transport and the increase of energy consumption. However, the polymer drag reduction method is to inject a small amount of polymer into the fluid to reduce the flow resistance in the turbulent state.

From the perspective of the structure of oil phase drag reducer, most of them are polymer with flow chain or long straight chain with few side chains. For example, CDR102 is a polymer poly -σ olefin with molecular weight of 10 ~ 10. The pure agent of this polymer is a rubbery solid. As a commodity, it is generally dissolved in a solution of hydrocarbons (kerosene). The 10% drag reducer solution is a very viscous viscous elastomer, which is difficult to flow and can be drawn into very long silk. Polymer drag reducer can be soluble in crude oil or oil products, but not soluble in water, water molecules long chain crimp. The solution of drag reducer has strong Newtonian properties, and the viscosity is up to 3000Pa·S at low shear rate. It will not decompose below 120℃ and is relatively stable. Drag reduction is a special turbulent phenomenon. It is a purely physical effect, which is the macroscopic expression of the influence of drag reduction on turbulent field. The drag reducer molecules do not interact with the oil molecules, nor do they affect the chemical properties of the oil, but are closely related to its flow characteristics. In turbulence, the velocity of fluid particles changes randomly, forming large and small vortexes. The large-scale vortexes absorb energy from the fluid, deform and break, and transform into small-scale vortexes. Small-scale vortex, also called dissipative vortex, is weakened and subsides under the action of viscous force. Some of the energy it carries is dissipated as heat. In the layer near the wall, the transformation is more serious because of the shear stress and viscous force.

After the drag reducer is added to the pipeline, the drag reducer is dispersed in the fluid in a continuous phase. By its own unique viscoelasticity, the long chain of molecules extends to the natural flow, and its micro elements directly affect the movement of the fluid micro elements. The radial force from the fluid element acts on the drag reducer element, causing it to distort and rotate. The gravitational force between the drag reducer molecules resists the above forces acting on the fluid elements, changing the direction and magnitude of the action of the fluid elements, so that part of the radial force is transformed into the axial force downstream, thus reducing the consumption of useless work and reducing the loss of friction resistance on the macro level.

In laminar flows, where the fluid is subjected to viscous forces and there is no eddy dissipation as in turbulence, it is futile to add drag reducers. With the increase of Reynolds number into turbulence, the drag reduction agent shows the drag reduction effect. The larger the Reynolds number is, the more obvious the drag reduction effect is. When the Reynolds number is quite large and the fluid shear stress is enough to destroy the molecular chain structure of the drag reducer, the drag reducer degrades, and the drag reduction effect decreases or even completely loses its drag reduction effect. The added concentration of drag reducer affects the thickness of elastic bottom layer formed in the pipeline. The greater the concentration, the thicker the elastic bottom layer, and the better the drag reduction effect. Theoretically, when the elastic bottom reaches the pipe shaft, the drag reduction reaches the limit, that is, the maximum drag reduction. The drag reduction effect is also related to oil viscosity, pipe diameter, water content, pigging and other factors.

The use of drag reducer as a short - time emergency measure has great advantages. But for the pipeline that needs long-term increase transportation, the economic benefit is not obvious because of the need for a large amount of drag reducer, and the injection device of drag reducer is added to the oil transportation system, which increases the operation amount and failure rate of the whole system, which is not conducive to daily operation and management. Therefore, the technology of drag reducing agent should be given priority and should not be applied blindly. It can be foreseen that in the near future, as a new transportation technology, the injection drag reducer technology will create greater economic and social benefits for the pipeline industry in China.