Reactive extraction of lactic acid in a packed column

Reactive extraction of lactic acid was performed continuously in a packed column. The 0.6 M trioctylamine (TOA)/l-chlorobutane system was used as an extradant. The initial concentration of lactic acid was 10 wt% based on fermentation results. Raschig rings (5 and 7 mm diameter) were used to measure hydrodynamic data. Disperse phase holdup was nearly constant at V_d<0.8V_d,f.It can be seen that the flooding data obtained from this study were consistent with the literature. NTU and HTU were calculated. NTU varied from 1 to 2 and HTU from 96 cm to 44 cm with variation of V_d. The overall mass transfer coefficients of the continuous phase were nearly constant to 8.98x 10^-5 mol/cm²s with variation of V_d.     

Comparison of normal phase operation and phase reversal studies in a pulsed sieve plate extraction column

The hydrodynamic characteristics of a pulsed sieve plate extraction column (PSPEC) was studied experimentally using two different liquid phase systems, namely water/kerosene and 30%TBP (tributyl phosphate) in NPH (normal paraffin hydrocarbon)/0.3 M HNO₃. The aqueous phase as the dispersed phase and the organic phase as the continuous phase (phase reversal) and vice versa (normal phase operation) studies in a pulsed sieve plate extraction column 0.076 m in diameter and 1 m height are presented in this paper. The hydrodynamic properties like drop size and holdup are characterized as a function of various operating parameters namely pulse velocity, dispersed phase and continuous phase velocity and duty cycle of pulsing. Flooding in the column was also investigated for the changes involving flow ratio of continuous phase to that of the dispersed phase for both insufficient and excessive pulsing. It has been observed that phase reversal mode of operation is not efficient as compared to normal phase operation for the PSPEC.     

Falling film and flooding phenomena in small diameter vertical tubes: The influence of liquid properties

The effect of liquid properties on flooding in small diameter vertical tubes is studied for various liquids with the aim to contribute to the interpretation of flooding mechanisms in such geometries. Data on free falling film characteristics for the various liquids used are acquired using a photographic technique from which mean layer thickness and its statistical quantities are calculated. The experimental data confirm previous observations that in almost all cases the dominant mechanism is wave growth and upward dragging by the gas and that consequently the onset of flooding is strongly affected by the liquid film structure. The results also confirm the influence of the liquid properties on the interfacial wave evolution and film characteristics. New correlations based on dimensionless groups for the prediction of flooding in narrow passages are proposed and found to be in good agreement with available data.     

Sugaring-out: A novel phase separation and extraction system

In this study, we made a novel observation that by introducing a monomeric sugar or a disaccharide into an acetonitrile-water solution, the acetonitrile (ACN) can be separated from water to form a new phase. The two-phase formation triggered by sugar addition was visualized with Sudan I. The ability of different sugars to form an ACN-water two-phase system and the effect of glucose and xylose concentration on the phase separation were studied. The distribution of syringic acid, furfural, para-coumaric acid, ferulic acid and 5-hydroxymethyl furfural in the upper ACN phase and lower water phase was examined. The lower concentration limit for the two-phase formation for glucose and xylose at 1 °C was 15 and 25 g/L, respectively. At higher temperatures, the concentration needed for phase separation increased. Addition of polysaccharides (starch and dextran) did not result in phase separation. The distribution coefficient of the five organic compounds in the ACN-water two-phase system was in the range of 1.7-8.9 when the corresponding sugar concentration was 15-50 g/L. The phase ratio of the five organic compounds in the two-phase system was in the range of 0.1-0.5. The new two-phase system may find applications in the separation of chemicals having different solubility in water and in ACN.     

Extraction in microreactors: Intensification by adding an inert gas phase

The influence of an inert gas phase on liquid extraction using a microstructured device is analyzed. The gas phase establishes a modified flow pattern. The performance of the gas-liquid-liquid flow is compared to that of a segmented two phase flow, as regards mass transport as well as separation. The extraction of vanillin dissolved in water with toluene was chosen as an example and experiments at different residence times were conducted by varying the total volumetric flow rate. μ-PIV measurements were performed to reveal the influence of the inert gas phase on recirculation within the liquid slugs. Addition of the gas leads to an increase in mass transfer at flow velocities above 0.08 m/s. However, no difference can be noted at lower flow velocities and longer residence times, respectively. The two liquid phases were separated within the microstructured device by using a capillary separator. Purity was always higher than 96%. For two phase segmented flow, the toluene phase was pure, whereas the water phase was free of toluene rests when applying the inert gas phase. Thus, the inert gas phase can be used to enhance mass transfer under certain circumstances and to tune the separation behavior of a capillary separator.     

CFD modeling of slurry bubble column reactors for Fisher-Tropsch synthesis

Industrial bubble column reactors for Fischer-Tropsch (FT) synthesis include complex hydrodynamic, chemical and thermal interaction of three material phases: a population of gas bubbles of different sizes, a liquid phase and solid catalyst particles suspended in the liquid. In this paper, a CFD model of FT reactors has been developed, including variable gas bubble size, effects of the catalyst present in the liquid phase and chemical reactions, with the objective of predicting quantitative reactor performance information useful for design purposes. The model is based on a Eulerian multifluid formulation and includes two phases: liquid-catalyst slurry and syngas bubbles. The bubble size distribution is predicted using a Population Balance (PB) model. Experimentally observed strong influence of the catalyst particles concentration on the bubble size distribution is taken into account by including a catalyst particle induced modification of the turbulent dissipation rate in the liquid. A simple scaling modification to the dissipation rate is proposed to model this influence in the PB model. Additional mass conservation equations are introduced for chemical species associated with the gas and liquid phases. Heterogeneous and homogeneous reaction rates representing simplified FT synthesis are taken from the literature and incorporated in the model.Hydrodynamic effects have been validated against experimental results for laboratory scale bubble columns, including the influence of catalyst particles. Good agreement was observed on bubble size distribution and gas holdup for bubble columns operating in the bubble and churn turbulence regimes. Finally, the complete model including chemical species transport was applied to an industrial scale bubble column. Resulting hydrocarbon production rates were compared to predictions made by previously published one-dimensional semi-empirical models. As confirmed by the comparisons with available data, the modeling methodology proposed in this work represents the physics of FT reactors consistently, since the influence of chemical reactions, catalyst particles, bubble coalescence and breakup on the key bubble-fluid drag force and interfacial area effects are accounted for. However, heat transfer effects have not yet been considered. Inclusion of heat transfer should be the final step in the creation of a comprehensive FT CFD simulation methodology. A significant conclusion from the modeling results is that a highly localized FT reaction rate appears next to the gas injection region when the syngas flow rate is low. As the FT reaction is exothermal, it may lead to a highly concentrated heat release in the liquid. From the design perspective, the introduction of appropriate heat removal devices may be required.     

Shape oscillations and path transition of bubbles rising in a model bubble column

We investigate experimentally the occurrence of shape oscillations accompanied by path transition of periodically produced air bubbles rising in water. Within the period of bubble formation, the induced velocity is measured to examine bubble-liquid and bubble-bubble interactions. The flow is produced in a small-scale bubble column with square-shaped cross section. A capillary aerator produces bubbles of size 3.4 mm at a frequency of 5 Hz. Measuring techniques employed are high-speed imaging to capture bubble shape oscillations and path geometry, and laser-Doppler anemometry (LDA) to measure the velocity in the liquid near the rising bubbles. The experimentally obtained bubble shape data are expanded in Legendre polynomials. The results show the occurrence of oscillations by the periodicity of the expansion coefficients in space. Significant shape oscillations accompanied by path transition are observed as the second-mode oscillation frequency converges to the frequency of the initial shape oscillations. The mean velocity field in the water obtained by LDA agrees well with potential theory. An analysis of the decay of the induced flow shows that there is no interaction between the flow fields of two succeeding 3.4 mm bubbles in the rectilinear path when the bubble production frequency is lower than 7.4 Hz.     

Experimental study of drop size distributions at high phase ratio in liquid-liquid dispersions

An experimental investigation has been carried out in order to analyse the drop size distributions of a liquid-liquid dispersion in a stirred vessel at high phase ratio. Two liquid-liquid systems have been investigated: one at low and one at high coalescence rate. A sampling technique has been developed in order to measure the drop size distributions in the mixer with the help of a laser granulometer. A statistical approach has been attempted to derive the most probable drop size distribution in the mixer and the results have been compared with the experimental primary distributions. The comparison suggests that the energy dissipation cannot be considered as uniformly distributed in the mixer. The mean diameter of the distribution has been correlated to the global mechanical input power and to the volume phase fraction (phase ratio) for both systems in the frame of the classical Hinze-Kolmogorov theory. The results show that for each volume fraction studied, the mean diameter of the dispersion is a decreasing power law of the Weber number with an exponent equal to −0.6 at low phase ratio. However, it appears that for both systems studied this exponent is a decreasing function of the phase ratio. This result reveals the existence of a more complex breakup mechanism with high phase ratio than that assumed in the theory which has to be discriminated from dampening effect of the dispersed phase upon the turbulent energy of the bulk phase. The study of the secondary distributions mean diameter seems to be in good agreement with the numerical predictions of Stone (Annu. Rev. Fluid Mech. 26 (1994) 65). The ratio between the mean diameter of the primary distribution to the satellite drop mean diameter is a growing function of the viscosity ratio.     

Simulating a multiphase reactor with continuous phase transition

A novel multiphase fixed-bed reactor with continuous phase transition for benzene hydrogenation to cyclohexane has been presented by Cheng et al. (A.I.Ch.E. J. 47 (2001a) 1185; Chem. Eng. Sci. 56 (2001b) 6025; 57 (2002) 3407; A.I.Ch.E. J. 49 (2003) 225) which makes use of evaporation of volatile components to remove the reaction heat. This paper investigates issues related to the numerical simulation of the model for this novel fixed-bed reactor. The reactor is supposed to be divided into two parts subject to the phase transition point on the bed level, that is, the liquid-contacted zone at the lower part and the gas-contacted zone at the upper part. A steady-state mathematical model for the reactor is developed. All external mass and heat transfer resistances can be neglected in the case investigated, as well as temperature differences in the particles. The effects of feed inlet temperature, inlet benzene concentration, gas flow rate and liquid flow rate on the reactor performance are theoretically studied. Reliability of the model is verified by bench-scale experiments. The results show that dryout phenomena will occur in the reactor both on the bed-scale and on the pellet-scale under certain conditions.     

Characterization of flooding in a wirz extraction column

The flooding behaviour of a 150-mm diameter Wirz-II extraction column has been investigated for six different liquid-liquid systems. Normally, flooding occurred by phase inversion but when the density difference was low could also occur by entrainment of dispersed-phase drops at the continuous-phase exit. Empirical expressions for flooding velocities and holdup at flooding are derived in terms of the operating variables, column geometry and physical properties of the liquid systems.     

Hydrodynamic stability of a bubble column with a bottom-mounted point air source

Bubble column is widely used in both industrial and environmental applications. In this study, we examine the flow dynamics and stability of a bubble column driven by a point air source centrally mounted at the bottom using Phase Doppler anemometry (PDA). The model cylindrical bubble column had an inner diameter of 152 mm and was filled with the liquid to about 1 m height, above the point air source, which was made of a 30-mm diameter perforated air stone. The bubble diameters were within the range of 400–1300 μm. A customized setup was developed for accurate PDA measurements of the two phases, and detailed turbulent characteristics of the liquid phase velocity, bubble diameter, bubble velocity and the slip velocity were collected throughout the column. The comprehensiveness of the data set enabled a close examination of the hydrodynamic stability inside the column. Measurements were taken at three different air rates, namely 0.13, 0.25 and 0.38 L/min (corresponding to average gas volume fractions of 0.0065, 0.0138 and 0.0197, respectively). The results illustrated a large-scale coherent liquid circulation pattern inside the column. The circulation pattern in the upper column was relatively steady, while the pattern in the lower column was strongly unsteady with the probability density functions (pdf) for both the liquid and bubble velocities showing distinct twin peaks. An analysis based on the determination of the bubble drag forces and transversal lift forces is performed by decomposing the twin-peaked pdfs into two separated Gaussian distributions, one for the upward flow due to the bubble rises and the other for the downward flow due to circulation. Through the decomposition, a stability criterion can then be established by choosing the local bubble size as the representative length scale for the turbulent eddies inside the column. The analysis with the criterion illustrates why a steady circulation pattern was achieved in the upper column, and at the same time shows that the instability at the bottom column was induced by the low frequency meandering of the bubble swarm.     

The effect of surface tension on phase distribution of two-phase flow in a micro-T-junction

An experimental investigation has been conducted to study the effect of surface tension on phase distribution of gas–liquid two-phase ?ow through a T-junction with diameter 0.5 mm. It is found that the decrease in liquid surface tension makes the liquid taken off reduce when inlet flow pattern is slug flow, slug-annular and annular flow. These results highlight that phase distribution is remarkably influenced by surface tension in micro-T-junctions. To be specific, the surface tension contributes positively to liquid taking off. High surface tension seems to make the liquid capture more kinetic energy transported from the gas and dissipate it in form of vortexes. It is suggested that phase distribution in micro-T-junctions can be partly controlled by adjusting liquid surface tension.     

Numerical simulation of churn flow in a vertical pipe

A numerical simulation of the churn flow regime of air-water and R134a vapour-liquid mixtures by means of the volume of fluid (VOF) method is presented. The focus of the paper is on the inlet region of a vertical pipe. An axisymmetrical domain is used, reproducing the region next to the porous wall liquid injector of a typical test section for the investigation of vertical gas-liquid flows.A simplified model of the levitation process of the ring-type waves typical in churn flow is proposed. The influence of the gas Froude number on the waves amplitude is shown by means of the simplified model and used to explain the numerical results.A comparison of the numerical results with experimental wave frequency data and visualizations available in the literature is performed. The velocity field in the forming wave region and the pressure and shear stress variations along the interface are shown.Simulations have been performed at different liquid and gas superficial velocities and pipe diameters and the influence of these parameters on the gas-liquid interface is discussed.     

Effects of internals on phase holdup and backmixing in a slightlyexpanded-bed reactor with gas-liquid concurrent upflow

Five different internals were designed, and their effects on phase holdup and backmixing were investigated in a gas-liquid concurrent upflow reactor where the spherical alumina packing particles of three diameters (3.0, 4.5 and 6.0 mm) were slightly expanded under the conditions of varied superficial gas velocities (6.77×10^-2-3.61×10^-1 m·s^-1) and superficial liquid velocities (9.47×10^-4-2.17×10^-3 m·s^-1). The experimental results show that the gas holdup increases with the superficial gas velocity and particle size, opposite to the variational trend of liquid holdup. When an internal component is installed amid the upflow reactor, a higher gas holdup, a less liquid holdup and a larger Peclet number characterizing the weaker backmixing are obtained compared to those in the bed without internals under the same operating conditions. Additionally, the minimal backmixing is observed in the reactor equipped with the internals with a novel multi-step design. Finally, empirical correlations were proposed for estimating gas holdup, liquid holdup and Peclet number with the relative deviations within 11%, 12% and 25%, respectively.     

Liquid-liquid phase transition in flow systems

Liquid-liquid phase transition is occurring in many chemical engineering processes either as a desired phenomenon or as an undesired side effect. Typically, the phase split is modeled by neglecting all non-equilibrium effects. Here, a simple non-equilibrium situation is considered. Convective flow of a liquid along a decreasing temperature profile in a cooled channel is studied analytically. Three different scenarios for the transition process with two typical phase diagrams for binary mixtures are examined. For a phase diagram with critical concentration, phase segregation occurs via spinodal decomposition as a convective instability. For a cigar-shaped phase diagram the phase transformation is shown to evolve in analogy to directional solidification. Finger-like structures may be established under certain circumstances.     

LDA measurements of liquid velocities in a refractive index matched packed bubble column

LDA has been used to measure liquid velocities in a small-scale bubble column, internal diameter of 50 mm, packed with glass Raschig rings, 10 and 15 mm. A mixture of benzyl-alcohol and ethyl alcohol was index matched against the packing material. A method to separate the signals from liquid and bubbles was developed. It was found that the axial time-averaged liquid velocity was lower than that obtained in empty bubble columns, and that both the time-averaged liquid velocity and the RMS value of the liquid increased for the larger packing size.     

Hydrodynamics and mass transfer in bubble column: Influence of liquid phase surface tension

According to literature, few experiments are performed in organic solvents which are mostly used in commercial gas-liquid reactors. However, it is commonly accepted that data obtained in aqueous solution allow to predict the surface tension effects, and to model the behaviour of organic solvents. In this work, we examine the validity of this approximation.In this objective, the flows observed in two pure media having similar viscosity but different surface tension—respectively, water (reference) and cyclohexane (solvent)—are successively compared at two scales: in a bubble column and in bubble plumes.In bubble plumes, as expected, the mean bubble size is smaller in the medium having the smallest surface tension (cyclohexane), but for this medium the destabilisation of flow is observed to occur at smaller gas velocity, due to break-up and coalescence phenomena. In bubble column, these phenomena induce the bubbling transition regime at lower gas velocity, whatever the operating conditions for liquid phase: batch or continuous. Consequently, when the two media are used at similar gas superficial velocity, but in different hydrodynamic regimes, greater gas hold-up and smaller bubble diameter can be observed in water; the interfacial area is then not always higher in cyclohexane.This result differs from the behaviour observed in the literature for aqueous solutions. The analysis of bubble plumes in aqueous solutions of butanol shows that this difference is due to a fundamental difference in coalescent behaviour between pure solvents and aqueous mixtures: the surface tension effect is less important in pure liquid than in aqueous solutions, because of the specific behaviour of surfactants.It is then still difficult to predict a priori the bubbling regime or the flow characteristics for a given medium, and all the more to choose an appropriate liquid as a model for industrial solvents.     

Mesoscopic modeling of two-phase behavior and flooding phenomena in polymer electrolyte fuel cells

A key performance limitation in polymer electrolyte fuel cells (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. Liquid water covers the electrochemically active sites in the catalyst layer (CL) rendering reduced catalytic activity and blocks the available pore space in the porous CL and fibrous gas diffusion layer (GDL) resulting in hindered oxygen transport to the active reaction sites. The cathode CL and the GDL play a major role in the mass transport loss and hence in the water management of a PEFC. In this work the development of a mesoscopic modeling formalism coupled with realistic microstructural delineation is presented to study the influence of the pore structure and surface wettability on liquid water transport and interfacial dynamics in the PEFC catalyst layer and gas diffusion layer. The two-phase regime transition phenomenon in the capillary dominated transport in the CL and the influence of the mixed wetting characteristics on the flooding dynamics in the GDL are highlighted.     

Performance characteristics for particles of sand FCC and fly ash in a novel hydrocyclone

With growing industrialization in power sector and petrochemical industries water has been contaminated with particulates like sand, fly ash and FCC. In the present investigation, a new type of hydrocyclone was designed and fabricated. The system is designed in such a manner that it can operate in a wide range of variables for sand, sand-ash and sand-FCC systems. A detailed study on performance analysis of hydrocyclone, pressure drop, cut size particle diameter and particle characterization has been carried out for the design of the hydrocyclone. A mathematical model for predicting particle separation efficiency and cut size particle diameter has been developed. A correlation has been developed for percentage removal of particles and retention of particles in the hydrocyclone. An experimental finding shows 96% removal efficiency for particle with cut size diameter of , which can meet the required separation in industrial application.