1 Background and Introduction
Drag reducers, also referred to as drag reducing agents (DRA) or flow improvers, are materials that minimise frictional pressure drop in a pipeline or conduit during fluid flow. Pressure loss is reduced by lowering the degree of turbulent motion in the flow. DRA provides more flow with the same energy or lower pressure drop for the same fluid velocity in pipelines. The resistance experienced by flowing fluid in contact with the pipe wall causes frictional pressure decrease or drag.
The frictional pressures experienced in laminar flow cannot be modified unless the fluid’s physical characteristics are altered. Most drag-reducing agents function without changing the fluid’s material characteristics, making them only useful in a turbulent flow [1]. The fluid molecules move randomly in a turbulent flow regime, wasting most of their energy as eddy currents and other indiscriminate mobility. The drag reduction agent works by interacting with the turbulence of the moving stream through the polymer structure.
Since they lower pumping power and enhance pipeline capacity, drag reduction in pipelines is a subject of significant practical engineering importance. DRAs are utilised in various engineering systems, including regional cooling and heating, oil transportation and distribution pipelines, etc. In 1979, it was first used commercially in the 1.2 m diameter Trans-Alaskan Pipeline, which saw a 50% reduction in drag and increased the pipeline’s capacity from 1.45 to 2.1 million barrels per day [2]. Consequently, the necessity for two pumping units, which were expected to meet the previously specified capacity increase, was eliminated. Since then, the DRAs have been employed in numerous petroleum product pipeline projects[3].
As a result, employing DRAs offers the following functions:
- Pipeline capacity expansion (throughput)
- Significant pumping energy savings
- Pressure reduction, as well as pipe thickness and pressure spike reductions
- Decrease in pipe diameter and the numbers or size of pumping facilities during the design stage.
Pipeline drag reduction technology consists primarily of viscous drag reduction, rib drag reduction, bionic drag reduction, and wall vibrating drag reduction. The drag-reducing technology falls under the viscous drag reduction [4]. Compared to other techniques, the drag reducer through agents technique does not require the pipeline to be improved, and it is less expensive; as a result, it is frequently employed in oil pipeline transmission. Agents having a high molecular weight or a surfactant that can decrease turbulence in pipes might reduce drag. With the inclusion of DRA, the drag force is reduced, allowing oil to be transported at lower pressures [5], resulting in low pumping energy required and reduced cost. Additionally, the entire pipeline would be safer since it would be operated at a lower pressure as the DRA was included.
2 Mechanisms of Drag Reduction
According to studies on drag-reduction mechanisms, the interplay among macrostructures and elastic macromolecules in turbulent flow causes this phenomenon [6]. In turbulent pipe flow, the region near the wall is constituted of a viscous sublayer, and a buffer layer plays an essential role in minimising drag.
The most important issue with drag reducer efficiency is polymer chain breakdown caused by shear stresses in a turbulent flow. Shear-induced degradation is particularly prone to ultrahigh-molecular-weight polymers [7]. Branched polymeric compounds and natural materials with semirigid structures are more susceptible than linear-chain agents [8].
Shear degradation is believed to be associated with the process of chain elongation. When the shear rate reaches a critical level, the chain degrades, and drag reduction quickly diminishes. The decrease of friction drag and heat transmission in turbulent flows of drag-reducing substances is incompletely understood [9]. These events are thought to be significantly linked to elastic fluid characteristics. However, not all viscoelastic fluids are drag-reducing, implying that viscoelasticity and drag reduction are most likely unrelated phenomena.
Since flow creates an analytic anisotropic fluid-structure and related characteristics, it is claimed that flow familiarisation is the primary source of reduction phenomena rather than fluid elasticity, which is a secondary cause. Laminar heat transfer amplification, on the other hand, could be a result of increased fluid elasticity.
2.1 Damping of Eddies Transmission
The propagation of eddies can be damped by the viscoelastic characteristics of fluids, which is one of the processes of drag reduction. In Maxwell fluids with viscoelastic features, the transmission process of an isolated eddy was explored, and formulas explaining such occurrences were derived [10]. The study found that eddy transmission was dramatically reduced when the fluids’ viscoelastic characteristics increased.
2.2 Interpolymer Complexes
Interpolymer complexes mediated by hydrogen bonding have proven effective drag reducers. When compared to their non-associating polymeric antecedents, the drag reduction levels in such polymer systems improve by a proportion of two to six. Their shear stability has also been demonstrated to be enhanced [11].
The soluble hydrocarbon polymers with a small polar group fraction can analyse the interplay effect and drag reduction. According to research observations, interpolymer affiliations appear to reduce the dilute solution drag reduction capabilities of single connecting polymers containing similar polar groups.
Since the advantageous interpolymer associations construct larger structures of higher apparent molecular mass, interpolymer complexes formed by one polymer with cationic groups and another polymer with anionic groups can overcome this limitation and improve dilute solution drag reduction activity. The latter associations may improve the polymers’ resilience to degradation in turbulent flows.
2.3 Polymer Degradation
Industrial activities such as long-distance liquid transportation and oil well operations might advantage from minimising the drag in a turbulent flow. Still, the challenge of polymer degradation makes it difficult to achieve this.
The polymer decomposed more in a poor solvent at low Reynolds numbers, but the reverse effect was seen at high Reynolds numbers. The dimensionless concentration c(η) was shown to rise with molecular weight and polymer concentration as expressed by the critical Reynolds number.
Poly (saccharide) guar gum is employed in aquatic environments to reduce turbulent drag. It dramatically decreases friction drag in a turbulent flow [12].
2.4 Drag Reduction in Two-Phase Flow
A horizontal pipe was used to investigate the drag-reducing characteristics (PAM) in a two-phase water/airflow [13]. The polymer’s characteristics were examined in single-phase water flow, and the result was found to complement the pressure drop reduction. Positive effects in two-phase flow were discovered to depend on the flowing liquid Reynolds number. As a result, the drag decrease was negative or minimal in the fluids that changed the density while moving in the vertical direction. The drag reduction seems to originate in the liquid slug rather than the layer beneath the bubble in a two-phase flow. The drag-reducing agents appear to have little effect on the flow regime. Drag reducers have been effective corrosion inhibitors in the multiphase flow because of their ability to make smoother the flow profile close to the wall [14].
2.5 Drag Reduction in Gas Flow
Drag reducing agents are also effective in the gas flow as well. Ammonia gas (NH3) is often utilised in natural gas for storage or pipeline transit at pressures more than 5.5 MPa (800 psi). The temperature and pressure need to be adjusted so that ammonia should not convert into the liquid phase. The energy required to compress or pump the combination of natural gas with ammonia is less than that energy required to compress or pump a comparable volume of natural gas alone. The pumping process through long pipelines is further assisted by the refrigerant or cooling effect of ammonia, which lowers the temperature of the gas being conveyed when more than 4% by volume of ammonia is involved [15].
2.6 Microfibrils
A specified amount of chosen organo-polymeric microfibrils can be added to a liquid to decrease friction loss while maintaining optimal stability [16]. To be processed into microfibrils, polymeric materials must be insoluble yet extremely dispersible in a particular solvent.
A solid organic polymer in the form of microfibrils has an average diameter between 100-1000 Å, length in the range 1-500 μm, and length to diameter ratio (aspect ratio) of 10- 1,000,000.
2.7 Drag Reduction with Surfactant
Certain cationic surfactant solutions containing counter ions are widely recognised for reducing drag in a turbulent flow. The existence of rod-like micelles generated by single surfactant molecules over a specific concentration characterises drag-reducing surfactant solutions [1]. The temperature and electrolyte content significantly impact the essential micelle concentration. Shear viscosity assessments of drag-reducing surfactant solutions demonstrate that at shear rates above a critical value, the viscosity increases abruptly, resulting in the formation of a shear-induced state. The micelles coalesce into larger structures and are completely aligned in the flow direction.
3 Drag Reducer Agents
3.1 Ultra-High-Molecular Weight Polymers (UHMWP)
A non-agglomerating suspension of UHMWP in water with tiny quantities of surfactant can be introduced into the flowing stream hydrocarbons for improving flow characteristics and reducing the energy required to carry out the pumping process [17]. Polymerisation produces precisely split UHMWP, which is subsequently cryoground below the glass transition temperature. This way, it acts as a drag reducer by absorbing the vibrations generated by turbulence and shifting the flow towards the laminar.
3.2 Copolymers of α-Olefins
Various -olefin copolymers are employed as drag reducers. Concentrates can be formulated by polymer precipitation from a hydrocarbon solution with an isopropyl alcohol [18]. In flowing hydrocarbon streams, the resultant slurry concentration dissolves quickly. A non-agglomerating, non-aqueous solution may be made by covering poly-olefins with a fatty acid wax as a fragmentation agent and scattering it in a lengthier chain of alcohol [19].
3.3 Latex Drag Reducers
An emulsion-based polymerisation technique is used to produce latex drag reducers, which are distributed in a continuous phase and may be changed to increase the polymer’s hydrocarbon solubility. 2-Ethylhexyl methacrylate is the preferred monomer. Traditional emulsion polymerisation methods are used for the polymerisation [1]. The emulsion polymerisation produces the initial latex composition consisting of colloidal particle dispersion. Water and surfactant make up the continuous phase. Additional surfactants and organic solvents can be added to the latex to modify or formulate it, and viscosity characteristics can be changed to get the required properties. Conventional or umbilical delivery techniques can be utilised to introduce a drag reducer into the pipeline.
3.4 Microencapsulated Polymers
Microencapsulating a monomer or a polymer technique can also create highly concentrated drag-reducing agents. Microencapsulation can be performed before, during, or after polymerising a monomer into an efficient drag-reducing polymer. A catalyst must be present if the encapsulation is performed before or during polymerisation, although little or no solvent is needed. As a result, the microcapsule undergoes bulk polymerisation. Microencapsulated drag minimisation can be introduced into a fluid stream before, during, or after removing the inert capsule or shell. There is no need for injection equipment or another specialised probe to enter the drag-reducing slurry into the liquid stream and no need for polymer grinding to form a suitable grad reducing agent [20].
3.5 Aluminum Carboxylate
Aluminium carboxylate based drag-reducing agents is non-polymeric in nature. These additives are not affected by shear, and they do not produce undesired changes in the fluid composition and emulsion of the fluid which is going to be treated, nor do they induce unwanted foaming.
Aluminium carboxylate and fatty acids are the constituents of this compound. Aluminium carboxylates are made from fatty acid aluminium salts such as octoates, oleates, naphthenates, or stearates [21]. Long-chain carboxylic acids are used to make fatty acids. Aluminium salts with a mix of short and long-chain carboxylic acids may offer the best balance of viscosity alteration and drag reduction.
3.6 Solid-Particle Suspensions
Solid-particle suspensions are used to reduce resistance by adding solid particles to a flowing stream utilising two different forms of suspensions [22], granulation and fibre. The size and composition of granular and fibre suspensions are different. Granular resembles a sphere-shaped particle, whereas fibre is a component of many materials. Based on the stated effect, the benefit of various drag reduction approaches is that suspensions utilised in moving fluids may be easily separated due to their larger size. Furthermore, research on the degradation of solid-suspension particles shows that the suspension has little effect on the moving fluid components [23]. This ease of use is unquestionably crucial in its application to the marine sector on a large scale. Sand, charcoal, asbestos, nylon and other suspensions are examples of solid suspensions.
3.7 Surfactant Solutions
Small amounts of particular surfactants, polymers, and combinations added to water can significantly reduce frictional drag [24]. The formation of a microstructure made up of long micellar associates is a crucial prerequisite for the drag reduction phenomena to exist in the solution. Cationic, anionic, nonionic, and amphoteric surfactants can all cause drag reduction; however, the bulk of research published to date has focused on the cationic surfactants [25]. Cationic surfactants have the advantage of being stable and performing well throughout a wide temperature range. The downside is that quaternary ammonium compounds are harmful to marine species and slowly biodegrade [26]. Cationic surfactants can be substituted with more ecologically friendly nonionic surfactants at relatively low temperatures.
4 Economic Evaluation of Drag Reducing Agents
The transporting companies of liquid hydrocarbon chemicals through pipelines can save money by eliminating the requirement for underused intermediate or accelerator pump stations by utilising a drag-reducing flow improver [27]. Product lines operated at a low flow rate in a predesigned pipeline due to consumption requirements or employing temporary boosters can also save transmission costs. The overall advantages are most likely to be seen in 6–8-in lines running at 67% to 92% of their specified throughput capacity. Engineers have proven significant power saving of up to 22% (through lower demand charges and reduced energy consumption) for systems that employ booster stations 85% of the transmission duration. Depending on the width of the line and power rates, overall energy cost reductions can exceed 45% when stations are only operational 70% of the time [1].
Another advantage of adopting drag reduction agents is that they may be introduced instantly or temporarily, allowing companies much flexibility in overall operations. The research discovered that the necessary DRA-injection rates for multiphase flows were four times greater than for stabilised crude oil [3]. This was linked to the multiphase system’s increased shear degradation caused by the higher degree of flow turbulence. DRAs are evaluated based on their efficacy, which is defined as:
Several factors influence DRA effectiveness, including pipe diameter, temperature, the viscosity of the fluid, and the inclusion of hydrocarbon and/or water. In conclusion, DRAs are very effective in fluid transmission operations, and the appropriate selection of them during particular applications is crucial.
5 References
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[25] J. Różański, “Flow of drag-reducing surfactant solutions in rough pipes,” Journal of Non-Newtonian Fluid Mechanics, vol. 166, no. 5-6, pp. 279-288, 2011.
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[27] C. L. GOUDY, “How flow improvers can reduce liquid line operating costs,” Pipe line industry, vol. 74, no. 6, pp. 49-51, 1991.
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