|Muhammad Ilham Rizaldi||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Arif Rahman||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Deendarlianto||Department of Mechanical and Industrial Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia|
|Nining Betawati Prihantini||Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Nasruddin||Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
Microbubbles are known for their many applications. Recently there has been new findings regarding the growth of susceptible microalgae through microbubble aeration. There are three methods used to generate microbubbles for this microalgae strain. Unfortunately, for some methods, the cost of generating microbubbles is still high. However, fluidic oscillators can be used to produce microbubbles at a reasonable cost. There are two types of fluid oscillators: single loop and double loop. This study determined the bubble size produced with these oscillators. Bubble size data was recorded using a high-speed camera at air flow rates of 6 LPM, 9 LPM, 12 LPM, and 15 LPM, and utilized 10 µm microporous shafts as the diffuser. The data were processed using ImageJ software. The results showed that the size of the bubble using a single loop fluid oscillator was smaller than that of the double loop fluid oscillator. The smallest bubble size was obtained in a single loop fluid oscillator with an airflow of 6 LPM.
Fluid oscillator; Microbubble; Photobioreactor
The issues related to increased energy demand, environmental pollution and depletion of fossil fuels are considered very urgent: renewable and alternative fuels must replace fossil fuels while maintaining fresh air and ensuring energy security (Pham et al., 2018). Biodiesel is an alternative fuel that contains long chain fatty acids known as mono alkyl esters. It is predominantly a renewable, clean-burning fuel that is environmentally friendly, nontoxic, and free from harmful sulphur (Hidayat et al., 2018). The application of microalgae to biodiesel production has great potential: it has gained attention because it can produce oil in the cells of its body. The oil content in microalgae ranges from 20–50% and microalgae can exceed 80% of the weight of dry biomass (Rahman et al., 2019).
A lot of microalgae biomass can be cultivated by using photobioreactors. Using this method, microalgae conduct photosynthesis as they would in their natural habitat. The process of photosynthesis requires light and carbon dioxide as energy sources for the growth of microalgae. During the process of photosynthesis in photobioreactors, microalgae absorbs the content of carbon dioxide, which dissolves in the medium such as Bold Basal Medium (BBM), Medium Cyanobacteria TAPS (CT) and NPK (Nitrogen Phospor Potassium) medium. A good medium for the transfer process between carbon dioxide and microalgae requires microbubbles.
A microbubble isdefined as a bubble with a diameter ofless than one millimeter (50–200 µm) (Juwana et al., 2019;Deendarlianto et al., 2015). Microbubbles have advantages across many applications due to their bubble size. For example, they have been used in wastewater treatment (Rehman et al., 2015;Budhijanto et al.), biomolecular separation (Lye et al., 2001) and microorganism aeration (Hanotu et al., 2016). Their small size yields advantages, such as higher surface to volume ratio, which provide higher mass transfer rates. Another advantage of microbubbles is slow rise velocity, which allows more substances to dissolve in the medium due to its residence time (Zheng et al., 2018).
Microbubbles have unique characteristics, such as high gas dissolution, low rising velocity, and high interfacial area (Deendarlianto et al., 2015). Recently, there has been a special case regarding the growth of susceptible microalgae strains using microbubbles. One method of generating microbubbles uses pumped water; however, this cannot be used to breed micro algae because it creates circulation. As a result, the strain would experience high shear stress due to the pumping action and would eventually decease. The only way to develop microbubbles without creating circulation involves pumping air through sparger in photobioreactor, and there are three methods of achieving this. The most common method uses compressed air, which flows through a specifically designed nozzle to generate small bubbles based on the cavitation principle. The second method uses ultrasonic sound waves to oscillate a needle tip following air coming through the water chamber, thus creating a continuous stream of tiny bubbles (Makuta et al., 2005). Unfortunately, both methods require high energy densities; this makes the operational cost of a photobioreactor quite high (Zimmerman et al., 2008). One possible low-cost method involves microbubble generation by oscillating the airflow using mechanical vibration. The bubbles generated will break off at a size that is close to the diameter of the hemisphericalcap.
A fluidic oscillator is a no moving part jet actuator that is able to oscillate airflow because of its special geometry. It has a low cost because it is easy to manufacture using the CNC (Computer Numerical Control) machining process and does not need frequent maintenance. There have been many reviews about the characteristics of fluidic oscillators. The most common types of fluidic oscillators are the double loop fluid oscillator and the single loop fluid oscillator, created by Warren and Spyropoulos (Warren, 1964;Spyropoulos, 1964). Recently, Tesar has made a modified model using both types that can achieve an oscillation frequency up to ~200 Hz (Tesa? et al., 2013;Zimmerman et al., 2011). However, no reviews have comprehensively studied bubble generation through fluidic oscillators using a microporous sparger as a diffuser. This study will investigate the bubble size produced by fluidic oscillators effectively.
In conclusion, airflow affects the characteristics of bubble size: a higher airflow results in more bubbles with larger diameters and the uneven distribution of bubble sizes formed along the microporous sparger. This phenomenon occurred due to the conjunction of bubbles that had been formed earlier. Otherwise, the bubbles formed would be smaller and more evenly distributed. Single loop fluidic oscillators can generate smaller bubbles than double loop fluidic oscillators based on this research data.
The authors would like to thank the Ministry of Research, Technology and Higher Education of Republik Indonesia (KEMENRISTEK-DIKTI RI) for funding this research for the Masters program toward a Doctorate for Superior Bachelor (PMDSU) 2019 with contract number NKB-1862/UN2.R3.1/HKP.05.00/2019.
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