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  • br Introduction Wet scrubbers are important air pollution

    2018-11-05


    Introduction Wet scrubbers are important air pollution control devices for cleaning large volumes of industrial effluent gas streams and they remove particulates and dangerous gases simultaneously by capturing into the droplets or pool of liquid and dissolving or absorbing into the liquid respectively (Semrau, 1977; Frank and Nancy, 2005). Different types of dusts like explosive, flammable, cement and foundry dusts can be collected and various gaseous pollutants like calcium channel blocker mists, furnace fumes can be absorbed with the wet scrubbers. Plate column scrubbers, spray column scrubbers and bubble column scrubbers are most important devices and many other types of scrubbers developed to appropriate particular pollutant removal with greater collection efficiencies (Raj Mohan et al., 2008; Meikap et al., 1999, 2002; Bandyopadhyay and Biswas, 2006; Bangwoo et al., 2007). Plate scrubbers are being used in many chemical industries for individual and simultaneous removal of acid gases and dust particles from industrial flue gases. Plate without down comer is called as dual-flow plate. Dual-flow sieve trays which consist of punched holes are most commonly used for their simple design, economical viability, less susceptible to fouling, easy installation and maintenance (Kister, 1992; Wankat, 1988; Trambouze, 1999). Liquid and gas flow alternatively through the sieves of dual-flow plate in opposite direction which also provides self-cleaning of plate (Garcia and Fair, 2002). The dual-flow tray develops a two phase dynamic mixture from the alternative flow of two phases through the sieves (Zhang et al., 2015). The absence of down-comers and more area for bubbling enhances the capacity of the plate by 20% higher than normal sieve plates with down-comers due to this the dual-flow trays have high capacities and low pressure drops (Furzer, 2000; Domingues et al., 2010). The positive weeping of liquid from tray above is key difference between dual-flow tray and normal tray which creates high gas–liquid contact space. The satisfactory efficiencies can be achieved using dual-flow trays within the design values of flow rates but the efficiencies are small due to little contact time between gas and liquid (Trambouze, 1999). The absence of down comer in dual-flow trays increases the effective area and they can achieve more than 90% efficiencies (Walas, 1990). The operating ranges of dual-flow sieve plates are classified by Miyahara et al. (1990) as froth and transition based on gas velocities. The froth is at relatively low gas flow rates and transition is at high gas flow rates. They have also mentioned that the complete spray regime cannot be occurred in dual-flow due to simultaneous passage of gas and liquid develops bubbling and suppressed jetting at the hole. The transient jetting between bubbling and jetting at a single hole and between froth and spray at multiple holes from quantifying the phenomena of the transition from froth to spray on a sieve plate have been earlier noticed and reported in the literature (Miyahara et al., 1983; Miyahara and Takahashi, 1984). Recently, Zhang et al. (2015) have observed four main flow regimes of wetting, bubbling, froth and fluctuating on dual-flow valve trays based on Fs where Fs is gas load factor based on superficial area (, (m/s) (kg/m3)0.5) and VG is superficial gas velocity and ρG is gas density. They have also suggested that the bubbling and froth regimes are feasible operating regimes for industrial application. This paper reports the experimental investigations of clear liquid height, tray pressure drop, and fraction of holes passing gas. It also reports the comparison of the present results of clear liquid heights of dual-flow sieve plate with the values predicted from the equation that has arrived after modifying the correlation reported in the literature.
    Experimental setup and technique The experimental apparatus of three stage dual-flow sieve plate column is schematically represented in Fig. 1. Perspex glass is used to make the column of 2.6 m height with 0.152 m (6 inches) inner diameter and the column has a frusto-conical (slant angle 64.8°) gas outlet at the top section. The three plates (T1–3), gas distributor (Gd) and liquid distributor (Ld) were made of stainless steel with 3 × 10−3 m thickness and holes with 3 × 10−3 m size. The photograph of the dual-flow sieve plate is shown in Fig. 1 (SP). The plates were placed at 0.61 m (24 inches) spacing. The space between plates is a stage. The air supply to the column is facilitated by an air compressor (AB). The compressed air was stored in tank and the outlet of the tank is connected to an air rotameter (R1). The air rotameter outlet is fitted to the column gas inlet at the bottom section of the scrubber and presymptomatic screening passes through a gas distributor for uniform gas flow and the gas flows in upward direction through the sieves of the plates and comes out from gas outlet at the top of the scrubber. The liquid pumped to the column from liquid inlet at the top using 0.5 horsepower centrifugal pump (P) from a water tank (T). A rotameter (R2) was used for controlling the liquid flow rate and liquid distributor facilitates the uniform liquid flow. The liquid flows downward through the plates and finally drains out at the bottom of the column and can be collected in the water collector (S). Five pressure ports (P1P5) were made to the column to measure the pressure drop across each tray and entire column. The pressure ports were connected to U-tube manometers (M1–4) in which carbon tetrachloride (CCl4) was used as manometric liquid. Four quick acting solenoid valves (V) are fitted to the column at inlets and outlets of gas and liquid to measure the liquid hold up.