Holdup measurement in a gas-liquid ejector for a sodium chloride-air system

Transcription

1 Indian Journal of Chemical Technology Vol. 13, March 2006, pp Holdup measurement in a gas-liquid ejector for a sodium chloride-air system P T Raghuram* & T R Das Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 0l2, India ptr@chemeng.iisc.ernet.in Received 20 December 2004; revised received 16 November 2005; accepted 10 December 2005 Holdup measurements have been carried out experimentally using gamma ray attenuation method in a gas liquid ejector for a sodium chloride-air system. The measured values are compared with the theoretically predicted values based on feed calculations. Holdup as a function of the liquid flowrate and also its distance from the nozzle of the ejector has been reported in this paper. The results obtained for this non-reactive system are also compared with those of air-water system. Keywords: Holdup measurement, Ejector, Radiological method, Gamma ray attenuation, Non-reactive system IPC Code: F04B47/00 Ejector is one of the contactors, which offer relatively high interfacial area and adjustable time of contact between gas and liquid. This co-current flow device has liquid as the continuous phase and gas as the dispersed phase. It is a device which utilises the high energy and momentum of a primary fluid to entrain the secondary fluid. In recent years attempts are made to utilise the ejector principle for reactors and reaction systems studied by many including Ogawa et al. 1 and Raghuram et al. 2 The interfacial area in the ejector is larger than that in the usual contactors such as spray columns, plate columns and the agitated vessels 3. The interfacial area is a function of both the bubble size and the holdup in the ejector. The ejector essentially consists of a small nozzle and a suction tube. Liquid is pumped through the nozzle and gas is sucked through, thus generating a dispersion in the mixing section of the ejector. As the dispersion moves down, the degree of back mixing falls slowly and at the same time, the bubble size increases due to coalescence and attains a steady value by the time it reaches the end of the diverging section of the ejector. Thus, the bubble size remains same in the outlet if any extension tube is attached. The bubble size is predominantly established by the air holdup, thus enhancing the interfacial area. As the dispersion moves down, the air holdup also increases 4 in the diverging section, which helps in the formation of stable bigger bubble 5 and thus interfacial area becomes a function of the holdup in the ejector. Experimental Procedure The systematic diagram of gas-liquid ejector 6 fabricated of borosil glass is shown in Fig. 1. It includes a 200 liters HDPE tank with stainless steel baffles, containing sodium chloride solution of 2 percent concentration on weight basis. The solution is pumped through a stainless steel centrifugal pump (3 H.P.-18 meters head) with proper control valves, rotameter etc to the ejector, which is mounted above the tank by means of a platform (not shown in the figure). A secondary line, through which air enters the ejector is also monitored by means of manometer for air flowrate. Air is taken from a compressor with suitable filters, by-pass line etc. Sodium chloride-air system is considered for the studies on holdup measurement in this downward ejector. At the nozzle of the ejector, the pressure energy is converted into kinetic energy thereby reducing the pressure to the negative gauge value. This enables air to enter in and gets dispersed into sodium chloride solution turbulently. During this Fig. 1 Experimental set up of the gas liquid ejector.

2 RAGHURAM & DAS: HOLDUP MEASUREMENT IN A GAS-LIQUID EJECTOR 145 process, the bubbles grow and the holdup also varies correspondingly. Radiation attenuation method The radiation attenuation method for holdup measurement is based on the absorption of gamma rays from a radioactive source which can be measured and related to void volume fraction. The method is a powerful tool when applied to fundamental two phase flow studies, since density and the related velocity of the two phases can be determined readily. The method is essentially simple; however the results obtained may be in slight error, unless care is taken in the experimental technique. Theory The density of the substance is related to the attenuation of mono-energetic gamma ray by the equation I = I o e ( μ x )...(1) The linear coefficient of absorption in turn, is related to the density of the absorber by the expression μ = (N/a)ρσ...(2) When the gamma ray is perpendicular to the layer of the two phase system, the equation for the holdup by Petrick et al. 7 is ε = ln(v/v f )/ln(v e /V f )...(3) from which the air holdup, ε for the system can be found. Set-up for radiation technique The set-up used for the measurement of holdup is diagrammatically shown in Fig. 2. Gamma rays from 60 mci Thulium 170 radioisotope source are directed through the test section of the ejector, where holdup, epsilon is to be measured. The unabsorbed rays are received in the photo-multiplier tube and a signal is produced. This signal is amplified and transmitted to the counter. The gamma ray assembly is mounted on a wooden platform, which encircles the ejector column. The platform can be moved up and down through a pulley arrangement (not shown in figure) and can be fixed at any location where holdup can be measured. Fig. 2 Experimental set-up of the radiation attenuation assembly with the gamma ray source. Energy spectrum is scanned by a multichannel analyser and the highest peak is chosen for the experimental setting. Gamma ray quanta falling on the sodium iodide crystal are converted to electrical voltage pulses by the scintillation head of the instrument. The pulses pertaining to the highest peak, which has been set for the experimental investigation, are counted. The number of counts per given period of time are directly proportional to the gamma ray intensity detected by the scintillation head. Holdup measurement The spectrometer is calibrated initially by testing the 'counter' with respect to the 'preset time' for a known intervals of time. The unit is continuously 'switched on' for the selected voltage (700 V) for about 2-3 h, so that the background noise levels are eliminated. The gamma source is loaded into the lead chamber housing and the 'counts' V e are noted for the empty ejector, (ε=1). The ejector is completely filled with sodium chloride solution (ε=0) by temporarily fixing a rubber bung at the outlet and the 'counts' V f are noted for this. Then the actual sodium chlorideair dispersion is generated by pumping known amount of solution and air into the ejector. Reading for this experimental 'counts' V is noted.

3 146 INDIAN J. CHEM. TECHNOL., MARCH 2006 In the same way, by moving the wooden carriage for different positions in the diverging section of the ejector, all these above steps are repeated and the holdup is estimated. The spectrometer is calibrated for each measurement at a particular location of the ejector and then the readings are taken. The experiments are done for 100 s, the period of time for each measurement. Results and Discussion Radiation attenuation technique has been used to measure the holdup of the system from a distance of 16 cm away from the nozzle and upto the end of diverging section, since the dispersion is stable and the bubble growth is stabilized and is defined in this range. Reasons for this has been explained by Bhutada and Pangarkar 8 that depending upon the pressure profiles and mass ratio, various regimes of two phase flow have been observed in the diverging section of the ejector. The flow regimes being coaxial and homogeneous bubbly flow are observed in the ejector. Hence, up to a distance of 16 cm coaxial flow is observed and then the bubbly flow is observed in the ejector. Measurements are made at each cm interval of the ejector. Four different liquid flow rates 75.8, 94.7, and m 3 /s with corresponding air flowrate being 65.1, 68.4, 72.6 and m 3 /s have been tried. The experimental results obtained are shown in Figs 3-6. The range of superficial air velocity available for a particular system has been rather limited and could not be varied more than that as the stability of the dispersion is affected beyond this. In other words, the location where the dispersion takes place, shifts downward, as the air flowrate is increased for a given liquid flow rate. At any particular location, it is observed that as Fig. 4 Effect of holdup at different distance from the nozzle for different systems with air flow rate, and liquid flow rate, m 3 /s. Fig. 5 Effect of holdup at different distance from the nozzle for different systems with air flow rate, and liquid flow rate, m 3 /s Fig. 3 Effect of holdup at different distance from the nozzle for different systems with air flow rate, and liquid flow rate, m 3 /s. Fig. 6 Effect of holdup at different distance from the nozzle for different systems with air flow rate, and liquid flow rate, m 3 /s

4 RAGHURAM & DAS: HOLDUP MEASUREMENT IN A GAS-LIQUID EJECTOR 147 the air flow rate is increased, the holdup value increases. This is as per the expectation, since the amount of air present per unit volume will be higher. The figures also clearly indicate that as the liquid flow rate is increased, the air holdup decreases. Another interesting result obtained is that the air holdup increases as the dispersion moves down, this is due to the coalescence of bubbles and their growth. Once the bubble reaches a steady value, the air holdup also remains steady. This is seen in the figures where the slope of the line increases initially and then falls almost to asymptotic. The flowrates for this system, sodium chloride-air is kept identical to that of earlier work on air-water system 4. This gives greater advantage of comparing the ejector for its suitability to recommend it as a chemical contactor. Besides the effects of physical properties variation on holdup and bubble size can also be monitored. The slight decrease in holdup values in the present system (sodium chloride-air) compared to the earlier system of air-water, has facilitated the interfacial area enhancement. For a particular flowrate conditions the bubble size in the case of sodium chloride-air is smaller than that of the air-water system. When the density of liquid is increased, that is sodium chloride is slightly denser than pure water and the surface tension of chloride solution is lighter than water, then the Weber number 9 increases. This results in the decrease in bubble size. This is an interesting phenomenon which indicates that physical properties such as density and surface tension can be varied reasonably to control the bubble size, meaning that there is good chance for improving the interfacial area. This has helped for modelling the ejector, considering it as a series of stirred vessel reactors, which has been reported elsewhere 2. The present chosen system is also non-reactive one, and however to propose the ejector as a chemical contactor, reactive systems have to be considered. This will lead to a better understanding of the holdup in a reactive atmosphere where the gas gets depleted by way of reaction. It is expected that as the liquid flowrate is increased, the air holdup should decrease. Surprising, the observed results are in the opposite way. This may be due to the reason that stable gas-liquid dispersion could be generated in the limited region only, as has been explained by Bhutada & Pangarkar 8. Moreover the in situ holdup is controlled by the slip velocity between gas bubble and liquid inside the diverging Fig. 7 Effect of holdup for different gas-liquid ratio. diffuser of the ejector. This fact is supported by the trend of the curve in Fig. 7, where, as the liquid flowrate increases, the gas/liquid ratio decreases, meaning the higher quantum of liquid could accept less air for stable dispersion. Conclusion An experimental set-up has been constructed to measure the holdup in an ejector for sodium chlorideair system. Gamma ray attenuation technique has been employed successfully to measure it within the ejector. The experimentally measured values for this system are compared with that for air-water system. The decrease in the holdup value for sodium chloride air system compared with air-water system is as per expectation. The variation in the physical properties of the solution may help control the holdup. There is a need for a suitable model to be developed for the holdup, bubble formation and growth within the ejector, for considering the ejector as a chemical contactor. Acknowledgement Authors are grateful to Prof R. Kumar, Indian Institute of Science, Bangalore for the immense help received in discussions and for encouraging us to pursue this topic. Nomenclature I = intensity of gamma ray at x, (A/m 2 ) I o = intensity of gamma ray at source, (A/m 2 ) μ = linear coefficient of absorption, (m 1 ) x = thickness of the absorber, (m) N = Avogadro number (nuclei/kg mole) a = atomic weight (kg/kg mole) σ = microscopic absorption cross section, (m 2 /nuclei) ρ = density of absorbing medium, (kg/m 3 ) ε = holdup (-)

5 148 INDIAN J. CHEM. TECHNOL., MARCH 2006 ε = 1, when the column is devoid of liquid ε = 0, when the column is full of liquid V e = counts, when the column is empty (-) V f = counts, when the column is full of liquid (-) V = counts, for the actual two phase system (-) References 1 Ogawa S, Yamaguchi H, Tono S & Otako T, J Chem Eng Japan, 16 (1983) Raghuram P T, Mukherjee A K & Das T R, Indian Chem Eng, 34 (1992) Richardson J F & Coulson J M, Chemical Engineering, 3 (Pergamon), 1971, Raghuram P T & Das T R, Indian Chem Eng, 38 (1996) Raghuram P T & Das T R, J Chem Eng Japan, 35 (2002) Reddy Y R & Kar S, J Hydraulic Division, ASCE 94, 5 (1968) Petrick M & Swanson B S, The Review of Scientific Instruments, 29 (1958) Bhutada S R & Pangarkar V G, Chem Eng Commn, 61 (1987) Sprow F B, Chem Eng Sci, 22 (1987) 435.