Gas Hold Research Articles

Biological treatment of mixed industrial wastewaters in a fluidised bed reactor

A study has been carried out on the operating parameters that influence the biodegradation of petroleum and brewery wastewaters, with a low-density biomass support. The biodegradation rate of a mixture of two wastes was compared with that of the separate wastes. A low aspect ratio reactor was employed, and this made it possible to operate at low superficial gas and liquid velocities. The gas distributor used created a fluid flow pattern similar to that of a draft tube, which enhanced axial mixing. At a particles loading of 12% (v/v), the optimum superficial gas velocity was 2.5 cm/s for the mixture. The interstice structure of the biomass-support particles, improved microbial attachment due to the resulting large surface area. There was a low biomass concentration when petroleum wastewater was treated alone, however, for a mixture of petroleum and brewery wastewaters, an increase in the concentration was observed. There was a higher gas hold up in the mixture than in the petroleum wastewater, but lower than in the brewery wastewater. An improved biodegradation was achieved when a mixture of brewery and petroleum wastewaters was treated, and this gave an indication that nutrient deficient wastes can be treated together with phosphate and nitrate rich food industry wastewaters.

Improvement of mass transfer characteristics and productivities of inclined tubular photobioreactors by installation of internal static mixers.

The feasibility of improving mass transfer characteristics of inclined tubular photobioreactors by installation of static mixers was investigated. The mass transfer characteristics of the tubular photobioreactor varied depending on the type (shape) and the number of static mixers. The volumetric oxygen transfer coefficient ( k(L)a) and gas hold up of the photobioreactor with internal static mixers were significantly higher than those of the photobioreactor without static mixers. The k(L)a and gas hold up increased with the number of static mixers but the mixing time became longer due to restricted liquid flow through the static mixers. By installing the static mixers, the liquid flow changed from plug flow to turbulent mixing so that cells were moved between the surface and bottom of the photobioreactor. In outdoor culture of Chlorella sorokiniana, the photobioreactor with static mixers gave higher biomass productivities irrespective of the standing biomass concentration and solar radiation. The effectiveness of the static mixers (average percentage increase in the productivities of the photobioreactor with static mixers over the productivities obtained without static mixers) was higher at higher standing biomass concentrations and on cloudy days (solar radiation below 6 MJ m(-2) day(-1)).

Effect of a new high porosity packing on hydrodynamics of bubble columns

The purpose of this work was to study the effect of a new high porosity packing (solid fraction corresponds to 0.5%) on the hydrodynamics of two bubble columns of different diameters (15 and 20 cm) operating with co-current up-flow of gas and liquid. This specific packing is a stainless steel welded grid with a mesh size of 12.5 mm. Two types of gas sparger were used. Gas hold up, gas hold up profiles bubble size, slip velocity and liquid axial dispersion coefficients were determined with and without packing. The results obtained show that this type of packing has a considerable effect: it delays regime transition and can maintain a homogeneous regime over a large range of superficial gas velocities. Liquid axial mixing was also greatly affected by the presence of this packing, tending toward plug flow.

A structured catalytic bubble column reactor: hydrodynamics and mixing studies

This paper reports the results of a comprehensive experimental study of the hydrodynamics and mixing in two bubble column reactors of 0.1 and 0.24m in diameter with KATAPAK-S® as packing material. Total gas hold up and axial dispersion coefficients were measured in the structured bubble columns and the values were compared with experimental results obtained in the same work with empty bubble columns. The results reveal that the gas hold up in structured bubble columns is practically the same as in empty bubble columns when compared at the same superficial gas velocity based on open area available for gas–liquid dispersion. The presence of the structured elements in the bubble column reactor reduces the liquid phase backmixing by one order of magnitude.

Fluid dynamics and mixing in three-phase coal and oil residue hydrogenation sieve cascade reactors

The present process integration work employs two modified versions of the Bergius three-phase hydrogenation technology including the application of sieve cascade reactors. The first process operates at medium pressures (7– 20 MPa and 470°C) for hydroprocessing of oil refinery vacuum and or atmospheric residue to lighter products, and the second for synthetic fuel production by the direct thermal and catalytic coal liquefaction at higher operating pressures ( 30 MPa and 470°C). Technical problems for the design of large-scale multistage reactors are analyzed including the simplified demonstration for complete integration with existing petroleum refineries. Fluid dynamics, in one-stage bubble columns, are presented including the theoretical prediction of Newtonian non-settling slurry circulatory flow, and the liquid slurry and gas dispersion mixing efficiency of the sieve cascade reactor. Scale-up methodologies are proposed for columns up to 4 m in diameter, and up to 50 m in height, based on measurements and operating experience with larger three-phase petrochemical reactors, and the existing cold flow mathematical models in slurry bubble columns. The basic geometrical parameters of the oil residue or coal hydrogenation multistage reactor are discussed, and the influence analyzed of the sieve tray geometry and stage number at high pressures and temperatures on the fluid dynamics, mixing, gas hold up and interfacial area. Evidence is established that the installation of suitably designed sieve trays, operated above the minimum required liquid and gas superficial velocity, can eliminate deficiencies in the existing gas distribution designs, and therefore once again emphasize the cost effectiveness and increased process safety in the application of sieve cascade reactors.

Continuous hydrogen production by immobilized Enterobacter cloacae IIT-BT 08 using lignocellulosic materials as solid matrices

A process was developed for continuous hydrogen production using immobilized Enterobacter cloacae IIT-BT 08 on environmentally friendly lignocellulosic solid matrices. Among the three lignocellulosic carriers used, SM-C (coir) was found to be the best in terms of cell retention (0.44 g dry cell/g dry carrier), packing density (100 g/liter reactor volume), cell loading (44 g dry cell/liter reactor volume) and hydrogen production rate (62 mmol/liter h). Gas hold-up was a major problem in tubular bioreactor with immobilized cells. The tapered and rhomboid bioreactors gave better performance in terms of both the rate of hydrogen production and the gas hold up. The gas hold-up was reduced by 67% using the rhomboid bioreactor as compared to tubular one. The maximum hydrogen production rate achieved was 75.6 mmol/liter h at a dilution rate of 0.93 h −1 and recirculation ratio of 6.4. The substrate conversion efficiency was increased by 15% at these conditions as compared to the system with no recycling. There was no substrate inhibition for hydrogen production up to 1.0% glucose concentration. The μ max and k s for immobilized cells were 1.25 h −1 and 9.31 g/liter respectively. The maximum rate of hydrogen production was found to be 2.1 times higher than that of batch system. The process was found more efficient as compared to other continuous hydrogen production processes using packed bed reactor.

Anaerobic treatment of low concentration waste water in an inverse fluidized bed reactor

Low concentration synthetic and municipal wastewaters were treated at HRT as short as 3 and 0.6 h respectively in an anaerobic inverse fluidized bed. Both bioreactors showed gas hold up due to the liquid downflow pattern of the prototype. The bioreactor operated at 3 h had a removal efficiency of 83%, specific activity of 4.5 kg CODremoved/kg IVS (d and the gas hold up varied from 23 to 55%. The reactor treating municipal wastewater had a removal efficiency of 44% when operating at 0.6 h, the specific activity was 4.2 kg CODremoved/kg IVS (d and no biogas was detected apparently because an important fraction was dissolved in the liquid phase. The biomass concentration was 13.8 and 1.1 kg IVS/m3 for synthetic and municipal wastewater and the SEM microphotographs showed a bacterial diversity for the first run and only cocci cells for the second run. The system does not remove suspended solids, so a polishing postreatment to improve water quality has to be implemented.

Classification and characterization of hydrodynamic and transport behaviors of three-phase reactors

According to axial profile of solid concentration, the cocurrent upward threephase reactors with liquid as continuous phase can be classified into three types (a) gassparged slurry reactors, (b) threephase bubble columns, and (c) threephase fluidized beds Comparative study shows that the gas hold up, bubble characteristics and mass transfer are significantly dependent on the type of threephase reactors Three types of reactors exhibit the different hydrodynamic and transport behaviors with particle size, solid concentration and gas holdup The structural analysis of the axial solid distribution indicates the bubble and bubble wake dynamics are the key factors to the hydrodynamic and transport behaviors of three-phase reactors.

Gas phase hold up in gas-liquid cocurrent upflow packed bed reactors at low Reynold's number

Experimental measurements of gas phase hold up in two phase gas–liquid cocurrent upflow packed bed reactors have been made at very low gas ((NRe)g < 55) as well as liquid ((NRe)l < 8) velocities using air–water systems with two different types of packing materials. The gas hold up values thus obtained are correlated in terms of Reg, Rel and ɛ, the voidage.

Experimental determination of bubble size distributions in bubble columns: prediction of mean bubble diameter and gas hold up

Bubble diameters were measured photographically in a bubble column, which was operating in the homogeneous regime with air and aqueous isopropanol solutions. The bubble size data were determined for several values of the superficial gas velocity, and used to fit bubble size distributions. The gas hold up was measured under the same conditions and its values were calculated from the bubble size distributions, using the theory for the conservation of the bubble size distribution function. The experimental results were compared to bubble size predictions given by models of bubble formation in orifices and to results from well-known correlations for the gas hold up. It has been verified that the models for bubble formation in orifices give reasonable predictions of the mean bubble diameter, while the correlations for gas hold up lead to poor agreement with the experimental data. The indirect measurement of gas hold up from the bubble size distribution was considerably more accurate.

Preparation and characterization of chars from oil palm waste

The feasibility of preparing activated carbons from extracted oil palm fibre, an abundant oil palm waste, was studied. Preliminary tests were conducted to investigate the influences of different operating parameters, such as initial material size, inert gas flow rate, heating rate, pyrolysis temperature and hold time, on the properties of the pyrolysed chars. The pyrolysed chars were characterized by a thermogravimetric analyzer for the proximate analyses and the pyrolysis kinetic studies, and by an accelerated surface area and porosimetry system for the determination of their BET and micropore surface areas and pore size distributions. The optimum conditions for pyrolysis were found to be for 0.5–1.0mm material sizes at a final temperature of 850°C for 3.5-hour hold time, heating rate of 10°Cmin−1 and a nitrogen flow rate of 150Nmlmin−1. For these conditions, chars with a maximum BET surface area of 521m2g−1 were obtained. Experimental results showed that it was feasible to prepare chars with high BET surface areas from extracted oil palm fibre. With further activation, the pyrolysed chars would be expected to improve their adsorption capability.

Protein recovery using gas–liquid dispersions

Two separation techniques, foam separation and colloidal gas aphrons (CGAs), both of which are based on gas–liquid dispersions, are compared as potential applications for protein recovery in downstream processing. The potential advantages of each method are described and the concentration and selectivity achieved with each method, for a range of proteins is discussed. The physical basis of foam separation is the preferential adsorption of surface active species at a gas–liquid interface, with surface inactive species remaining in bulk solution. When a solution containing surface active species is sparged with gas, a foam is produced at the surface: this foam can be collected, and upon collapse contains surface active species in a concentrated form. CGAs are microbubble dispersions (bubble diameters 10–100 μm) with high gas hold ups (>50%) and relatively high stability, which are formed by stirring a surfactant solution at speeds above a critical value (typically around 5000 rpm). It is expected that when proteins are brought into contact with aphrons, protein adsorbs to the surfactant through electrostatic and/or hydrophobic forces. The aphron phase can be separated easily from the bulk solution due to its buoyancy, thus allowing separation of protein in a concentrated form.

97/02421 Experimental confirmation of the theoretical prediction of the gas hold up and circulation velocity in internal loop air lift reactor and the possibilities of scale up

97/02421 Experimental confirmation of the theoretical prediction of the gas hold up and circulation velocity in internal loop air lift reactor and the possibilities of scale up

Wet air oxidation: kinetics of reaction, carbon dioxide equilibrium and reactor design - an overview

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200 to 325°C) and pressure (up to 150 bar). In western Europe, only an handful of industrial plants are in operation. Most of them were designed by extrapolating results from pilot plants. Only a very few studies have been devoted to the scientific design of such reactors (bubble columns). This paper discusses what needs to be known for this scientific design. The usual design methods assume that the column is divided into a number of cells. Each cell is a perfectly mixed reactor connected with the contiguous cells. Reliable data and models are available for hydrodynamics (axial dispersion, gas hold up) and mass transfer. Data are also needed for the kinetics of chemical reactions and are available from numerous sources, but have not yet been generalised. The thermodynamics (fugacities and enthalpy) of the gas phase can be determined with the Peng Robinson equation of state, and the Henry law for the solubility of gases in water describes the equilibrium conditions. But additional data are needed for water solutions containing salts. This paper describes a method that allows such a determination to be made by establishing rigorous balances on a batch autoclave. At present, these data are being used to implement a computer program aimed at determining the performance of a wet air oxidation reactor depending on the various operating parameters. Some typical profiles within the reactor are presented, establishing that the pH of the solution is a very important parameter.

Theoretical and practical aspects of flow control in programmed-temperature gas chromatography

Behavior patterns for the temperature dependence of column head pressure and outlet volumeteric flow-rate are analyzed for some particular configurations of the chromatograph's flow control system. Quantitative prediction of this behavior, and especially its effect on gas hold up time, is evaluated. Attention is centered on the application of programmed-temperature gas chromatography (PTGC) retention simulation.

Wet air oxidation: Kinetics of reaction, carbon dioxide equilibrium and reactor design - an overview

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200 to 325°C) and pressure (up to 150 bar). In western Europe, only an handful of industrial plants are in operation. Most of them were designed by extrapolating results from pilot plants. Only a very few studies have been devoted to the scientific design of such reactors (bubble columns). This paper discusses what needs to be known for this scientific design. The usual design methods assume that the column is divided into a number of cells. Each cell is a perfectly mixed reactor connected with the contiguous cells. Reliable data and models are available for hydrodynamics (axial dispersion, gas hold up) and mass transfer. Data are also needed for the kinetics of chemical reactions and are available from numerous sources, but have not yet been generalised. The thermodynamics (fugacities and enthalpy) ofthe gas phase can be determined with the Peng Robinson equation of state, and the Henry law for the solubility of gases in water describes the equilibrium conditions. But additional data are needed for water solutions containing salts. This paper describes a method that allows such a determination to be made by establishing rigorous balances on a batch autoclave. At present, these data are being used to implement a computer program aimed at determining the performance of a wet air oxidation reactor depending on the various operating parameters. Some typical profiles within the reactor are presented, establishing that the pH of the solution is a very important parameter.

Fractional gas hold‐up in gas inducing type of mechanically agitated contactors

AbstractFractional gas hold up was measured in gas inducing type of mechanically agitated contactors (GIMAC) with single and multiple impellers. Three vessels of 0.57, 1.0 and 1.5 m i.d. were used. For the multiple impeller system, six different impeller designs were employed. The impeller speed was varied in the range of 0.30 to 15.45 s−1. The ratio of impeller diameter to tank diameter (D/T), the submergence (S) of the upper impeller, the clearance of the lower impeller from the tank bottom (C1) and the impeller spacing (C3) were varied over a wide range. The design of the lower impeller was optimized in terms of diameter (D), blade width (W), blade angle (Bø), number of blades (nb) and the blade thickness (tb). An optimum design has been proposed for the multiple impeller system. Rational correlations have been proposed.

Power consumption of agitated gas-liquid-solid systems with particular relevance to solid substrate submerged culture fermentations

The Michel and Miller (1962) equation is routinely used to estimate the effect of aeration on power consumption in submerged culture fermentation systems. However this correlation was developed for systems that do not contain solid substrates. Since many industrial fermentations use solid substrates, this study developed a modification of the Michel and Miller equation that can be used to predict power consumption when solid substrates are present. A correlation relating power to solids addition was established by adding bran and apple pomace to water, and measuring the power drawn by four separate impeller configurations.This correlation applies to solids content up to 16.7% w/w and bulk densities between 1020 to 1210 kg/m3. It was found that when aerated the power consumption decreased upon addition of solid substrate. The most likely explanation of this behaviour is that initially the gas hold up increased, decreasing power consumption. At high solid loadings, the power decrease was up to 30% (compared to 66% decrease in power caused by high aeration rates).

Numerical simulation of the drag on a swarm of bubbles

The quasi-steady rise of a bubble swarm through a quiescent liquid has been considered. The drag force on a typical bubble in the bubble cluster has been calculated by solving the complete Navier-Stokes equations in the framework of the free surface cell model. A useful correlation has been derived for the variation of drag coefficient with the bubble Reynolds number and the average gas hold up.

Effect of addition of alcohol on the design parameters of mechanically agitated three-phase reactors

The effect of the presence of isopropyl alcohol on the design parameters of gas-liquid-solid systems has been studied in a vessel with an internal diameter of 0.57 m. A pitched blade downflow turbine was used as an impeller. Isopropyl alcohol concentration was varied from 0 to 2 vol.%. Air and quartz particles of 2000 μm in size were used as the gas and solid phase, respectively. The solid loading ( X) was varied from 0 to 15 wt.%. The impeller speed and superficial gas velocity were varied in the range of 2.4 to 10 rev s −1 and 0 to 9.4 mm s −1 respectively. The variations of power number and mixing time with respect to impeller speed were found to be similar. The effect of isopropanol concentration on the power consumption, liquid phase mixing time, critical impeller speed for gas dispersion, critical impeller speed for solid suspension and fractional gas hold up have been studied.