Efficient Control System for Low-Concentration Inorganic Gases from a Process Vent Stream:  Application of Surfactants in Spray and Packed Columns

Abstract

Control of low-concentration pollutants from a semiconductor process vent stream using a wet-scrubbing technique is a challenging task to meet Taiwan environmental emission standards. An efficient wet-scrubber is designed on a pilot scale and tested to control low concentration acid and base waste-gas emission. The scrubber system consisted of two columns, i.e., a fine spray column [cutoff diameter (based on volume), D v (50) ) 15.63 µm; Sauter mean diameter (SMD) ) 7.62 µm], which is especially efficient for NH 3 removal as the pH of the spraying liquid is ≈ 7 followed by a packed column with a scrubbing liquid pH ≈ 9.0 mainly for acids removal. It is observed that use of the surfactants in low concentration about 10 -4 M and 10 -7 M in the spray liquid and in the scrubbing liquid, respectively, remarkably enhances the removal efficiency of the system. A traditional packed column (without the spray column and the surfactant) showed that the removal efficiencies of NH 3 , HF, and HCl for the inlet concentration range 0.2 to 3 ppm were (n ) 5) 22.6 ( 3.4%, 43.4 ( 5.5%, and 40.4 ( 7.4%, respectively. The overall efficiencies of the proposed system (the spray column and the packed column) in the presence of the surfactant in the spray liquid and in the scrubbing liquid for these three species were found to increase significantly (n ) 5) from 60.3 ( 3.6 to 82.8 ( 6.8%, 59.1 ( 2.7 to 83.4 ( 4.2%, and 56.2 ( 7.3 to 81.0 ( 6.7%, respectively. In this work, development of charge on the gas-liquid interface due to the surfactants has been measured and discussed. It is concluded that the presence of charge on the gas-liquid interface is the responsible factor for enhancement of the removal efficiency (mass-transfer in liquid phase). The effects of the type of surfactants, their chain length, concentration in liquid, etc. on the removal efficiency are discussed. Since the pilot tests were performed under the operating conditions similar to most of the wet-scrubbers operated in semiconductors manufacturing facilities for inorganic pollutants, this study can be applied to modify the existing wet-scrubbers to enhance the removal efficiencies, especially for low-concentration pollutants.

New Correlation for Mass Transfer Characteristics of Spray Column

“…Biswas et al obtained a correlation for the pressure drop in a column. Boundyopadhyay and Biswas, , Rajmohan et al, Chein et al, and Yang and Shaw obtained the removal efficiency of absorption. Bandyopadhyay and Biswas developed the models of removal efficiency.…”

Field Enhancements of Packed-Bed Performance for Low-Concentration Acidic and Basic-Waste Gases from Semiconductor Manufacturing Process

“…The nozzle works on atomization principle, and provides mist of droplet size 7.48 m Sauter mean diameter (D s ) at air pressure (P a ) of 4 kg/cm 2 and water pressure (P w ) of 3 kg/cm 2 with water flow rate 0.15 L/min/nozzle (D s was measured using insitec, equipped with a laser beam photodetector as discussed elsewhere). 20 The nozzles were connected with airand water-supply piping, and flow meters were followed by flow-through auto proportional differential pressure regulator fixed on a trolley, which was especially homedesigned for this study ( Figure 1). The trolley also consisted of surfactant dosing systems for mist water and scrubbing liquid, reverse-osmosis water generating system, and so forth.…”

“…The results of the pilot scale study have been discussed elsewhere. 20 The results concluded that an anionic surfactant, C 12 , with concentration (M) 50 -100 times lower than its critical micelle concentration (CMC), and a cationic surfactant, C 19 , with concentration (M) 500-1000 times lower than its CMC value are adequate for water-mist and for packed bed liquid, respectively, for the purpose.…”

“…20 It was observed that the overall charge generated on droplets, which is the responsible factor for the enhancement of the performance, is the function of concentration of surfactant, as well as the droplets size range used in the experiment. In packed beds, it is reported that the surfactants exert two major effects, which modify the gas absorption process: the hydrodynamic effect and the interfacial effect.…”

Chemical Reaction Effect upon Gas–Liquid Interfacial Area in a Bubble Column Reactor

“…Different studies have analysed the interfacial area produced in gas-liquid contactors in the presence of chemical reaction in the bulk of the liquid phase [11] using chemical methods, but this kind of procedures have shown different problems caused by the interfacial turbulence produced by surface tension gradients [12][13][14]. Besides, these studies were performed in continuous regime, and then, the influence of operation time (and carbon dioxide loading) upon the hydrodynamic parameters cannot be studied.…”