Gas holdup and entrainment characteristics in a modified downflow bubble column with Newtonian and non-Newtonian liquid

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Abstract

Gas entrainment and holdup characteristics have been studied in a downflow bubble column with both Newtonian and non-Newtonian liquid. Aqueous CMC solutions at different concentration were used as a non-Newtonian and water as Newtonian liquid. In the present system, use of plunging liquid jet and ejector were synchronized to obtain significant entrainment and dispersion of gas into liquid. The effects of operating variables such as liquid flowrate, nozzle diameter, separator pressure, etc. on gas entrainment and holdup were investigated. Flow behaviours of non-Newtonian liquids were compared with that of Newtonian liquid. Furthermore, gas holdup data were analysed by slip velocity and drift flux model and found to be in good agreement with other reported results. The gas holdup of the present system was also correlated by some dimensionless groups of the independent parameters.

Introduction

Bubble column reactors are now being extensively used in a wide variety of chemical and biochemical processes. Particularly in hydrogenation, oxidation, fermentation, petroleum refining, coal liquefaction, etc., where the overall production rate is often controlled by the gas liquid interfacial mass transfer, the bubble column reactors are very effective. Bubble column reactors are the type of reactors, which not only provide a significant interfacial mass transfer area but also very simple in design and no mechanical agitator is required. Cocurrent downflow bubble column possesses some unique advantages over other types viz. finer and uniform bubble size, negligible coalescence of bubbles, homogenization of the phases, higher residence time of the gas bubbles and a very small amount of gas being dispersed.

Though the processing media for many processes such as sewage sludge, microbiological culture, polymer solutions, etc. are non-Newtonian in nature, a relatively little attention has been paid to study the behaviour of non-Newtonian fluids in bubble column. Li [1] has studied the bubble behaviour in non-Newtonian fluid. Schumpe and Deckwer [2] reported the gas holdup, specific interfacial areas and mass transfer coefficients of aerated CMC solutions in a cocurrent up flow bubble column. Mahalingam and Valle [3] studied two-phase flow in horizontal tubes using pseudoplastic fluids of various aqueous polymer solutions, while Chandrakar [4] discussed the gas dispersion in CMC solutions at different concentrations in a cocurrent vertically upflow bubble column with ejector type of gas distributor. However, studies on two-phase gas-non-Newtonian liquid in a cocurrent down flow bubble column are lacking.

Gas distributor often takes an important role for efficient dispersion of gas into liquid in cocurrent downflow bubble column. Apart from sparger, plunging liquid jet [5], [6] is also a good means for dispersing gas into liquid. But the Venturi or ejector types of gas–liquid distributor are more popular since their geometry ensures both high shear and a high-energy efficiency [7], [8], [9], [10]. Havelka et al. [11] discussed the effect of ejector configuration on gas suction and holdup in a loop type reactor. A good aspect of ejector type distributor is that it is very simple in design and no extra energy is required for gas dispersion as the gas phase is spontaneously sucked and dispersed by the high velocity liquid jet. So from energy point of view also it is very attractive as energy is required only for pumping liquid.

In view of the importance of gas dispersion in non-Newtonian liquids and the advantages accessible by an ejector system, the present work has been undertaken for systematic and comparative studies on gas entrainment and gas dispersion in two-phase flow of gas-Newtonian/non-Newtonian liquid system in a down flow ejector bubble column.

Section snippets

Experimental details

A schematic diagram of the experimental setup is shown in Fig. 1. It consists of an ejector assembly, E, an extended pipeline contactor, C, a gas liquid separator, SE, and other accessories like centrifugal pump (PU), pressure gauge (PG), manometers (M1–M8), control valves (V1–V5), solenoid valve (SV), rotameter (R), gas flowmeter (GM), circulating tank (T), etc. The ejector fitted with the extended pipeline contactor serves as both sparger and plunging jet type of bubble column reactor. The

Flow rates and regimes

In bubble column the different flow regimes viz. bubbly flow, churn-turbulent flow and slug flow basically depend on the gas superficial velocity for a particular column diameter. However, in the present system of cocurrent down flow as the gas is sucked by high velocity liquid jet, flow regimes primarily depend on liquid flowrate. As liquid flowrate is increased over a certain limit (which depends on physical properties and nozzle diameter), the gas bubbles coalesce to form large bubbles and

Conclusion

In the present work the hydrodynamic characteristics of two-phase gas–liquid system were experimentally investigated in a bubble column fitted with an ejector with both Newtonian and non-Newtonian liquid. It was found experimentally that the rate of gas entrainment is strongly dependent on the total height of the gas–liquid mixture in the column besides the flowrate of the motive fluid. An increase in height of the gas–liquid mixture in the column, increases the bubbles residence time also and,

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