When do you need low shear?
The answer to this question is simply where high shear leads to unwanted effects. There are several areas where fluid shearing is undesirable.
Fluid flows where emulsification can take place
If emulsification of the fluids is not the goal of the process, then it is often a negative side effect of fluid mixture handling and transport. The main challenges with emulsified fluids during transportation are increased viscosity and the need for further separation.
When two fluids are mixed, the viscosity of the resulting emulsion is often higher than the viscosity of either fluid. This is the result of droplet crowding or structural viscosity. The increased viscosity leads to higher pressure losses during the pipe flow. Increased pressure losses result in less efficient process plant operation. To overcome the effect of increased viscosity higher pumping pressure is required. The consequence of this is higher energy consumption and, hence, operational expenses for running the fluid handling process.
Separation of emulsified fluids might be a challenging task. The separation process has two aspects: the rate, at which separation takes place; and the qualities of the separated phases, i.e. how pure the fluids are after the separation. High separation rate and low impurity content of separated phases is a desired target for an effective separation equipment.
Highly emulsified fluids may require extensive investments in the separation facilities in order to reach the required specifications of separated phases. Utilization of demulsifying chemicals and heat treatment might be necessary in case of dealing with emulsions.
Typical fluids that experience these issues are water and oil. These two substances are often found together in industrial processes. The water-oil emulsions cause major challenges in crude oil production and refining plants and various other industries. Another example of fluids that cause emulsions that need to be separated are biodiesel and glycerol, found during biofuel production processes.
In order to reduce the emulsification of the flowing fluids, low shear equipment is recommended. Low shear valves and pumps should be utilized during the process design of the plant where multiphase flows are present and emulsification is possible. If the process plant has encountered challenges with emulsion formation, then retrofitting the existing equipment with low shear alternatives might be the simplest and the most cost efficient solution.
Irreversible viscosity loss of the fluid
Within the wide range of non-Newtonian fluids, some substances experience irreversible loss of viscosity at high shear rates. A typical example would be polyacrylamide-in-water solution. Polyacrylamide is a type of polymer widely used in polymer flooding, which is an enhanced oil recovery method. Polymers increase the viscosity of the injected water in order to effectively displace the reservoir oil. The challenge of this technology is that during fluid injection into the reservoir it often experiences high shear rates. Polymer solution tends to degrade under the presence of high shear forces, which lead to loss of the solutions viscosity, and hence, diminish the effect of the oil displacement. More detail of this method can be found in Polymer injection.
The detailed analysis of fluid properties is necessary to evaluate the fluid behavior with respect to shear rates. Rheology tests are necessary for these fluids in order to determine the threshold at which the fluid starts to degrade. With this knowledge, it is important to choose proper low shear equipment that does not exceed the shear threshold during operation and handling of the fluids.
Transport of shear-thickening fluids
Apparent viscosity of shear-thickening fluids increases with increasing shear rates. This type of behavior is most commonly observed in concentrated suspensions. The shear-thickening mechanism can be explained through the interaction of particles present in the suspension during the flow. At rest, liquid fills the void space of the suspension. At low flow velocities, liquid lubricates the motion of the particles, hence, minimizing the particle interaction. At high flow velocities, mixture expands slightly so that void space becomes greater than the amount of available liquid. Thus the particle direct interaction rate increases, and thereby the friction. In other words, high shear rates cause more friction and increase substance’s apparent viscosity. Certain fluids may even behave like solids while experiencing sudden high shear rates.
Shear-thickening behavior is observed for substances such as thick suspensions and pastes of kaolin, TiO2, corn flour in water, etc. It is important to have control of the fluid shearing regime while transporting such fluids.
It is always important to know the properties of the transported fluids. Consideration of shear effects on the fluid might be crucial for an efficient process operation. Low shear solutions are recommended where benefits of low shear operation are clear.