Nanobubbles

Nanobubbles( NBs) are nanoscopic gaseous structures having a diameter of less than two hundred nanometers that can exist in liquids or interact or attach to submerged surfaces. They are of great interest for industrial and medical real-world applications in that they offer longevity and stability in water while providing high gas transfer rates. 1 They undergo Brownian motion within a fluid which is the random dispersal of particles suspended in a medium. Unlike macro or micro bubbles that will rise and coalesce, this pattern comprises random movement by the bubbles in all directions within a fluid and vessel. This enables them to continuously stimulate physical, biological, and chemical interactions between the bubble surfaces and components within the fluid through contact, electrostatic interaction, or penetration. 2 It is this capability that has been shown to induce the removal of fouling mineral sites on surfaces and cause free-floating fouling agents to not bind. 3, 4 This motion also provides for the volumetric presentation of the bubbles throughout a fluid stream and due to their size a much greater surface area than microbubbles. These capabilities were evident at a study performed to alleviate pitting caused by sulfate-reducing bacteria at the Three Mile Island Nuclear Power Station where NB presentation reduced and eliminated surface foul. 5 Furthermore, NBs have been used to clean stainless-steel surfaces of foulants, 6 which are commonly found in many fluids-borne industrial processes. Nanobubbles are visible in water through reflection by a 532 nm green laser as seen in Figure 1 and through Brownian motion and light scatter using a NanoSight 300 device by Malvern Panalytical( Figure 2) for particle size and tracking.

In recent years, there has been ever-growing interest in how nanobubbles influence heat exchanger function and fouling. This is a logical extension from the prior research performed that indicates micro and / or macro bubbles will have a foul-inhibiting effect within heat exchangers, 7 improve heat transfer, 8 and disrupt biofilms. 9 Unfortunately, the presentation of larger bubbles into a flow stream can create conditions that are deleterious or non-productive within a system. Bubbles will coalesce within a horizontal flow stream thus reducing or eliminating the potential for lower or side pipe surface interaction. Furthermore, air pockets can occur within a high point or pipe bend which can influence fluid flow, creating back pressure. Dependent on system design, if these bubbles are introduced prior to a pump, they could interfere with pump function or create cavitations potentially causing damage. Although the presentation of air micro or macro bubbles may be offered as a dynamic system purge, it will not provide for the benign, systemic benefits that NBs may offer.

As previously mentioned, fouling is an insidious condition that begins immediately after a newly cleaned system is brought back online. Therefore, it is imperative that an anti-fouling technology be utilized that would operate during heat exchanger function, and NBs may hold promise. There are numerous studies that indicate that NBs are capable of foul remediation through their interaction with crystalline fouling structures 10 and the inactivation of microbes within biofilms. 11 Although NBs may have foul

remediating or preventative capabilities( Figures 4 and 5), they may also maintain or improve heat transfer through varying means such as the creation of wake zones, 12 disrupting insulating foul formations and increasing the thermal conductivity of a fluid. 13 It has been shown that NBs can allow for more efficient heat transfer through the disruption of boundary layers and the creation of micro-convections within the fluid. When presented into a fluid, NBs have led to improved heat transfer rates when compared to a fluid without nanobubbles. 14

Figure 3: Seawater pipe with NB infusion.( Courtesy of I2 Air Fluid Innovation, Inc.)

Figure 4: Seawater pipe without NB infusion.( Courtesy of I2 Air Fluid Innovation, Inc.)

For small-scale scientific investigation or low-volume applications, the method

of nanobubble creation is determined by the bubble size desired. To generate nonspecific high-volume bulk fluid nanobubbles for use in large commercial applications, various methods have been utilized. 15 These may include liquid flow generators that use membranes, static mixing, swirling liquid flow, shearing, 16 and pressurized dissolution to create nanobubble emulsions of significant density. These techniques for nanobubble creation require water flow manipulation or excessive energy use. Alternating magnetic fields, 17 vapor infusion, 18 as shown in Figure 5, and other benign methods can create dense emulsions of NBs continuously, without excessive expenditure of energy or water. Regardless of the method chosen, the unique characteristics of NBs present themselves as a potential means to reduce the impact of fouling.

The Important Role of Heat Exchangers on Net Zero Goals

The ever-growing global concern about excessive expression of GHG into the atmosphere, particularly from industry, has begun a revolution whereby corporations are becoming environmentally aware. Due to external forces such as public acceptance and financial pressure, companies utilize tools, such as their environmental, social, and governance( ESG) score to identify the potential areas of improvement. The relevance of ESG reporting has grown as investors and other corporate stakeholders desire to invest in or work with companies that wish to undertake the current environmental, economic, public health, and social justice crises. Additionally, ESG reporting provides a roadmap offering guidance to the public and consumers by which to measure the company’ s environmental and social stance. Consumers are now driven by how and where a product is manufactured and that process’ s impact on the environment. An unbiased source available to those concerned about registered ESG ratings is the Carbon Disclosure Project( CDP), which is a non-governmental organization that focuses on environmental factors.