Effect of Primary Variables on A Confined Plunging Liquid Jet Reactor

Abstract

The effects of operating conditions including a novel downcomer geometry on the gas/air entrainment rate, Qa, were investigated for a local vertical confined plunging liquid jet reactor (CPLJR) as an alternative aeration process that is of interest to Kuwait and can be used in various applications, such as in wastewater treatment as an aerobic activated sludge process, fermentation, brine dispenser, and gas–liquid reactions. Operating conditions, such as various downcomer diameters (Dc = 45−145 mm), jet lengths (Lj = 200–500 mm), nozzle diameters (dn = 3.5–15 mm), and contraction angles (Ɵ =20–80°), were investigated. A newly designed downcomer with various mesh openings/pores (Dm = 0.25ʺ (6.35 mm)–1ʺ (25.4 mm)) was also investigated in the current study. The air entrainment results showed that these were the primary parameters for the measured air entrainment rate in confined systems. The highest gas entrainment rates were achieved when the ratio of the downcomer diameter (Dc) to the nozzle diameter (dn) was greater than approximately 5, as long as the liquid superficial velocity was sufficient to carry bubbles downward. Furthermore, a downcomer with mesh openings (Dm) less or equal to 0.5ʺ (12.7 mm) provided a higher entrainment rate than that of conventional downcomer (without a mesh).

Unconfined Dense Plunging Jets Used for Brine Disposal from Desalination Plants

“…To achieve a target near field dilution of 20, Equation (2) was used to predict the required diffuser length as L ≈ 225 m. Figure 1c depicts a line of dense plunging jets, similar to the outfall in Figure 1b, except that the ports are located at a fixed elevation above the surface, e.g., connected to a manifold supported by a pier. The plunging jet reactor concept has been in use for several decades, as a means of achieving high mass transfer rates by entraining gas bubbles into a liquid, at low capital and operating costs [22][23][24]. The typical objective is to contact the two phases to promote mass transfer.…”

“…The CPLJR is utilized to try to improve the mixing and mass transfer rates from the ambient air into the liquid. This can be achieved by: (1) increasing the jet penetration depth and (2) confining the bubbly jet region to increase the contact time between the gas and liquid [22,23]. Table 1 (modified from [26,27]) shows the oxygen transfer efficiencies (defined as the mass of oxygen introduced to a reactor system per kWh of energy expenditure) reported for a range of aeration technologies.…”

Confined Plunging Liquid Jets for Dilution of Brine from Desalination Plants

“…However, as For confined plunging jets, mixing with ambient water can occur both within the downcomer and outside the downcomer. The length of the eddy circulation region surrounding the jet is about 3-5Dc from the nozzle [31,32]. If the downcomer submergence is larger than this length, the jet is not able to entrain fresh water from outside (below) the downcomer and the entrainment demand is supplied by recirculated brine.…”

“…A variant of the plunging liquid jet reactor is the confined plunging jet reactor which includes a vertical confining tube, or downcomer, concentric with the jet and partially immersed in the receiving water (Figure 1e). Confined plunging jets have been theorized to provide better mass transfer in comparison to unconfined plunging jets by confining the bubbly jet region and increasing the penetration depth of the bubbles, which, in turn, leads to an increased contact time between the gas and the liquid phase [24,[29][30][31]. However, they have not been widely investigated and experimental measurements of the air entrainment and mass transfer rate have not clearly indicated better performance in comparison to unconfined jets [24,26,29].…”

Sensitivity Test of Jet Velocity and Void Fraction on the Prediction of Rise Height and Performance of a Confined Plunging Liquid Jet Reactor

“…In fact, in environmental scenarios like breaking waves, the aeration rate can increase 200 times since many air bubbles entrain into the wave, and there is a tremendous increase in the air-water interfacial area [2]. Entrainment by plunging jets is a phenomenon that is advantageous in many applications such as aerobic wastewater treatment, fermentation processes, brine dilution, oxygen dissolution, air pollution abatement, and froth flotation as it achieves gas absorption and diffusion by bringing two phases in contact [1,[3][4][5]. The mass transfer between the two phases can be significantly increased if large interfacial areas for contact are provided, i.e., the most common method is by dispersing the gas phase as fine bubbles into the liquid phase [6] However, in some instances, such as the formation of gas bubbles when molten liquids are poured into a container, gas entrainment is undesirable and must be avoided [1].…”