Exploring the Steady Operation of a Continuous Pilot Plant for the Di‐Nitration Reaction

Continuous‐flow synthesis of the selective herbicide pendimethalin was demonstrated in both a laboratory‐scale and a pilot‐scale reactor using only concentrated nitric acid as nitrating agent. The di‐nitration reaction follows second‐order kinetics where the reaction is first order with respect to both reactant and nitric acid. The pinched‐tube reactor was chosen for pilot‐scale reactor fabrication due to its excellent mixing and mass transfer characteristics compared to a straight‐tube reactor. The estimated mass transfer coefficient showed similar nature in the laboratory‐scale and the pilot‐scale pinched‐tube reactor, ensuring similar performance at the pilot scale. Di‐nitration in continuous flow, inline quenching, extraction, and phase separation are some of the salient features of the developed pilot plant. The importance of the start‐up time for achieving steady state in the flow system at the large scale is highlighted.     

Mixing in curved tubes

The mixing efficiency in a curved tube is a complex function of the Reynolds number, Schmidt number, curvature ratio and tube pitch, therefore, the relative effectiveness of a helical tube is quite complicated and challenging over a straight tube. A state of art review on mixing of two miscible liquids in curved tubes revealed that the mixing in coils of circular cross section has not been reported in the literature. In the present work a computational fluid dynamics study is performed in curved tubes of circular cross-section of finite pitch under laminar flow conditions to examine the scalar mixing of two miscible fluids using scalar transport technique. In the present study the phenomenon of mixing by convection and diffusion of two flow streams with inlet scalar concentrations of zero and unity in the two halves of a tube perpendicular to the plane of curvature has been reported. The mixing efficiency has been quantified with concentration distributions and unmixedness coefficient at different cross-sections and process conditions (Reynolds number, Schmidt number and curvature ratio) in the straight and curved tube of circular cross-section. The result shows that, in curved tube, for higher Schmidt number fluids, mixing is considerably improved at moderately low Reynolds numbers (Re∼10), but is not affected for Reynolds number of the order of 0.1. It is also reported that mixing in the curved tube is higher at low values of curvature ratio as compared to the higher curvature ratio.     

Investigation of Flow Patterns in Continuous‐flow Stirred Vessels by Laser Doppler Velocimetry

The flow structure of a continuous‐flow reactor stirred by a Rushton turbine was investigated by laser Doppler velocimetry for two different mean residence time‐mixing time ratios. Velocity measurements were obtained for two inlet locations, corresponding to the incoming liquid stream being fed co‐currently or counter‐currently to the flow discharged by the turbine. In all investigated configurations and for all operating conditions, it was found that the flow disruption caused by the incoming liquid stream was observable mainly in the first vessel quarter, which followed the feed‐tube plane. From comparison of the velocities encountered in the various planes in the continuous‐flow reactor to the velocities of the batch reactor, it was also concluded that it may be possible to intensify the usage of the turbine‐stirred vessel by decreasing the characteristic times ratio, without considerable flow by‐pass and/or short‐circuiting problems.     

Liquid‐phase axial dispersion of turbulent gas–liquid co‐current flow through screen‐type static mixers

This article discusses the characteristics of turbulent gas–liquid flow through tubular reactors/contactors equipped with screen‐type static mixers from a macromixing perspective. The effect of changing the reactor configuration, and the operating conditions, were investigated by using four different screen geometries of varying mesh numbers. Residence time distribution experiments were conducted in the turbulent regime (4500 < Re < 29,000). Using a deconvolution technique, the RTD function was extracted to quantify the axial/longitudinal liquid‐phase dispersion coefficient. The findings highlight that axial dispersion increases with an increasing flow rate and/or gas‐phase volume fraction. However, regardless of the number and geometry of the mixing elements, reactor configuration, and/or operating conditions, the recorded liquid‐phase axial dispersion coefficients in the presence of screens was lower than that for an empty pipe. Furthermore, the geometry of the screen was found to directly affect the axial dispersion coefficient in the reactor. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1390–1403, 2017     

Mass transfer at the confining wall of helically coiled circular tubes with gas–liquid flow and fluidized beds

Experiments were conducted to investigate the effect of various dynamic and geometric parameters on mass transfer coefficients in two-phase helically coiled flow systems. Computation of mass transfer coefficients was facilitated by the measurement of limiting current at the electrodes fixed flush with the inner surface of the tube wall. Two flow systems were chosen: a two-phase liquid solid fluidized bed and a two-phase gas–liquid up flow. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide was used as the liquid phase. In the fluidized bed, glass spheres and sand of different sizes were employed as fluidizing solids. In two-phase flow system nitrogen was employed as inert gas. The pressure drop in the presence of fluidizing solids in helical coils was found to increase with increase in the pitch of the coil and was maximum for straight tube. The mass transfer coefficients were found to increase with increase in liquid velocity. The mass transfer coefficients in case of gas–liquid flow were found to be independent of liquid velocity and the pitch of the coil, and were largely influenced by gas velocity only. The data were correlated using j_D factor, Helical number, Froude number and Stanton number.     

Studies on the hydrodynamic behavior and mass transfer in a down‐flow jet loop reactor with a coaxial draft tube

The effects of certain pertinent parameters such as gas and liquid flow rates and nozzle position on the behavior of a down‐flow jet loop reactor (DJR) have been studied. The mean residence times of gas and liquid phases and the gas holdup within the reactor have been measured. In addition, the overall volumetric mass transfer coefficient, and the influence of the gas flow rate and the position of the nozzle inside the draft tube on the latter has been determined. Correlations have been presented for the gas holdup and k_La which take into account the length of the draft tube and the nozzle immersion height. The k_La values obtained at different power per unit volume (P/V) values in the DJR used in the present study compare favorably with data presented for stirred tanks and bubble columns in the literature. The liquid residence time distribution (RTD) within the reactor has been studied by tracer analysis for various operating conditions and nozzle immersion height and the results are indicative of the high mixing intensities that can be obtained in such reactions. © 2001 Society of Chemical Industry     

Process intensification of gas–liquid downflow and upflow packed beds by a new low‐shear rotating reactor concept

In the present work, a new low‐shear rotating reactor concept was introduced for process intensification of heterogeneous catalytic reactions in cocurrent gas–liquid downflow and upflow packed‐bed reactors. To properly assess potential advantages of this new reactor concept, exhaustive hydrodynamic experiments were carried out using embedded low‐intrusive wire mesh sensors. The effect of the rotational velocity on liquid flow patterns in the bed cross‐section, liquid saturation, pressure drop, and regime transition was investigated. Furthermore, liquid residence time and Péclet number estimated by a stimulus‐response technique and a macro‐mixing model were presented and discussed with respect to the prevailing flow patterns. The results revealed that the column rotation induces different flow patterns in the cross‐section of the packed bed operating in a concurrent downflow or upflow mode. Moreover, the new reactor concept exhibits a more flexible adjustment of pressure drop, liquid saturation, liquid residence time, and back‐mixing at constant flow rates. © 2016 American Institute of Chemical Engineers AIChE J, 63: 283–294, 2017     

气液两相流中顺列管束涡街特性实验研究

对受气液两相流横向冲刷的顺列管束旋涡脱离特性进行了实验研究。实验中通过测量顺列管束所受升力的频谱特性来确定涡街的脱离频率。实验中来流雷诺数的范围为 1.6× 10 4～ 8× 10 4,截面含气率的范围为 0～ 0 .30 ,横向管间距比 T/ D=1.5、 2 .0、 3.0 ,纵向管间距比 P/ D=2 .0、 3.0、 4.0。实验结果揭示了顺列管束涡街特性和斯特罗哈数随含气率、管间距、两相雷诺数的变化规律     

CFD analysis of two‐phase turbulent flow in internal airlift reactors

The hydrodynamic performance of three internal airlift reactor configurations was studied by the Eulerian–Eulerian k–ε model for a two‐phase turbulent flow. Comparative evaluation of different drag and lift force coefficient models in terms of liquid velocity in the riser and downcomer and gas holdup in the riser was highlighted. Drag correlations as a function of Eötvös number performed better results in comparison to the drag expressions related to Reynolds number. However, the drag correlation as a function of both Reynolds and Eötvös numbers fitted well with experimental results for the riser gas holdup and downcomer liquid velocity in configurations I and II. Positive lift coefficients increase the liquid velocity and decrease the riser gas holdup, while opposite results were obtained for negative values. By studying the effects of bubble size and their shape, the smaller bubbles provide a lower liquid velocity and a gas holdup. The effects of bubble‐induced turbulence and other non‐drag closure models such as turbulent dispersion and added mass forces were analysed. The gas velocity and gas holdup distributions, liquid velocity in the riser and downcomer, vectors of velocity magnitude and streamlines for liquid phase, the dynamics of gas holdup distribution and turbulent viscosity at different superficial gas velocities for different reactor configurations were computed. The effects of various geometrical parameters such as the draft tube clearance and the ratio of the riser to the downcomer cross‐sectional area on liquid velocities in the riser and the downcomer, the gas velocity and the gas holdup were explored. © 2011 Canadian Society for Chemical Engineering     

The effect of the draft tube geometry on mixing in a reactor with an internal circulation loop – A CFD simulation

The main topic of this paper is to describe the effect of geometrical parameters on mixing time in a reactor with an internal circulation loop. The design of the draft tube inside the reactor is an important geometric parameter and has a big influence on two phase hydrodynamics as well as on mass transfer in the reactor. In the first section, the validation of the selected mathematical model is carried out. The results of experimental measurements (liquid velocity and gas hold-up) obtained on the laboratory scale reactor are compared with the CFD simulations. The CFD simulation (bubbly flow and mass transfer models) was made using COMSOL Multiphysics 3.5a. The results of the numerical simulation are in good agreement with the experimental data. In the second section, the study of mixing in the reactor is described with the new geometry of the draft tube. The standard experimental techniques for testing mixing processes are quite problematic because common tracers (soluble salts) have significant influence on two phase hydrodynamics inside the reactor. Though, an alternative nontrivial method had to be used. The split of the draft tube into two or three section has a significant effect on mixing (mass transfer) in the reactor.     

Oxygen Transfer in a Baffled Rectangular Air‐Lift Loop Reactor

The objective of this study is to characterize mass transfer in a rectangular air‐lift loop reactor in two‐phase flow. In a previous work, it has been shown that the reactor presents a complex gas flow pattern. Therefore, first, the global mass transfer volumetric coefficient k_La was measured in two‐phase flow, by three methods (two based on the liquid phase mass balance, one based on the gas phase mass balance). Then, second, a localized analysis was implemented in order to obtain more information about the phenomena governing the gas phase flow.     

Numerical Investigation of Flow Patterns and Concentration Profiles in Y‐Mixers

The fluid flow patterns and associated concentration fields in Y‐mixers are investigated using lattice Boltzmann method‐based models. The focus lies on the impact of the mixing angle on the flow and concentration fields, with the mixing angle varying between acute (θ = 10°) and obtuse (θ = 130°) angles. Residence time distributions are determined to study the effect of the angles on the mixing and velocity patterns, in particular, different flow regimes, i.e., stratified laminar, vortex, and engulfment flow. The results from the simulations are validated with literature data and found to be in good agreement. Maximum mixing occurs in the 100° obtuse‐angle Y‐mixer, attributed to the extensive engulfment of flows in the mixing channel.     

Hydrodynamic considerations on optimal design of a three‐phase airlift bioreactor with high solids loading

The hydrodynamic study of a three‐phase airlift (TPAL) bioreactor with an enlarged gas–liquid dual separator was carried out. Different lengths and diameters of the draft tube were tested to show how the design of the separator zone affects the hydrodynamic performance of the TPAL reactor. Ca‐alginate beads with entrapped yeast biomass at different loadings (0, 7, 14 and 21% v/v) were used in order to mimic the solid phase of conventional high cell density systems, such as those with cells immobilized on carriers or flocculating cells. Important information on multiphase flow and distribution of gas and solid phases in the internal‐loop airlift reactor (ALR) with high solids loading was obtained, which can be used for suggesting optimal hydrodynamic conditions in a TPAL bioreactor with high solids loading. It is finally suggested that the ALR with a dual separator and a downcomer to riser cross‐sectional area ratio (A_D/A_R) ranging from 1.2 to 2.0 can be successfully applied to batch/continuous high cell density systems, where the uniform distribution of solid phase, its efficient separation of particles from the liquid phase, and an improved residence time of air bubbles inside the reactor are desirable. Copyright © 2003 Society of Chemical Industry     

INVESTIGATIONS INTO PUMPING CHARACTERISTICS OF AXIAL FLOW IMPELLERS IN AN INTERNAL LOOP REACTOR FOR ANIMAL CELL CULTURE

This investigation was intended to aid in the selection of impeller design and operating conditions, which would exhibit minimum turbulence in an internal loop reactor for axial flow velocities up to 18 cm/s.

For this purpose we measured power consumption, liquid flow velocity and mixing time with two marine propellers (pitch ratio tan alpha =1 and 0.57, respectively) and with flat-blade fan-type impellers (blade angles 10 to 90°).

The present results showed that at flow velocities between 7 and 14 cm/s a reduction in the pitch ratio (tan alpha) of the marine propeller from 1 to 0.57 increased the mechanical flow efficiency.

In addition, the fan-type impeller with a blade angle of 20° displayed flow characteristics comparable to the marine propellers; therefore, owing to its simple design, the fan-type impeller offers a practical substitute for the marine propellers.

In cell culture, draft-tubes used in impeller-driven reactors (internal loop reactors) offer two important features:

1. They simplify the design and scale-up process over that of open-blade impellers (flow patterns are more uniform);

2. They provide a mechanical support for bubblte-free membrane oxygenators;

Furthermore, on the assumption that loop reactors display lower bulk turbulence than open blade impellers for similar power consumption, this investigation was intended to aid in the selection of impeller design and operating conditions which would exhibit maximum pumping efficiency (with minimum mixing).

For this purpose, using a pH tracer method, we determined mechanical flow efficiency and dimensionless mixing time, which served as parameters for pumping efficiency to establish the following specific design characteristics:

1. Bulk mixing decreases relative to bulk flow with increasing bulk flow (mixing therefore conforms to the flow-in-pipe model) for two marine propellers (pitch tan alpha = 1 and 0.57) and for fan-type impellers (variable pitch) up to a blade angle of 30°;

2. Liquid velocity increases directly proportional to impeller speed up to 17 cm/s for (above) propellers, as well as fan-type impellers with blade angles of less than 30°;

3. The mechanical efficiency of axial flow (liquid velocity/power input) increases by decreasing the impeller pitch of marine impellers from tan alpha = 1 to tan alpha = 0.57, in the range of 7-14 cm/s;

4. A fan-type impeller (variable pitch) with a pitch angle of 20° displayed flow velocity and mechanical efficiency values, which lay between those of the (above) two marine propellers.

Fan-type impellers are considerably simpler in construction and are simpler to modify than marine propellers; for this reason this impeller type serves as a useful investigational tool. In addition we have found that at an impeller of 20° this type of impeller may be used to substitute marine propellers with a pitch ratio between 1 and 0.57.

We also suggest that square pitch marine propellers are not the most suitable for reactors in which mechanical efficiency of axial flow is critical, such as those for “shear sensitive” animal and plant cells.     

Die Modellierung von Stofftransportprozessen in Mehrphasenströmungen – am Beispiel des Strahlzonen‐Schlaufenreaktors

The efficiency and selectivity of chemical reactions are influenced by the mixing characteristics of the reactor. Existing models often assume homogeneous mixing on micro scale to calculate the reaction yield. However, neglecting the local hydrodynamic phenomena causes a discrepancy between model calculation and experimental data especially considering mass transfer limited reactions. In two‐phase flows different mass transfer phenomena have to be considered: the diffusion in the gas‐liquid boundary layer and diffusion in the Batchelor layer. The aim of the paper is to describe the mass transfer affecting mechanisms in multi‐phase flows and to discuss the first results of the investigation of local mass transfer phenomena in a two phase flow driven jet‐zone loop reactor.     

Study of Hydrodynamics,Mixing and Gas‐Liquid Mass Transfer in a Split‐Rectangular Airlift Reactor

Global hydrodynamic characteristics, liquid mixing and gas‐liquid mass transfer for a 63 L split‐rectangular airlift reactor were studied. Correlations for gas holdup and overall liquid circulation velocity were derived for the air‐water system as a function of the specific power input; these were compared to data and correlations for reactor volumes between 4.7 L and 4600 L. A partial recirculation of small bubbles in the riser was observed when U_gr > 0.03 m/s, which was attributed to the use of a single‐orifice nozzle as the gas phase distributor. The dimensionless mixing time and the overall axial dispersion coefficient were nearly constant for the range of gas flow rates studied. However, values of K_L/d_B were greater than those reported in previous studies and this is caused by the partial recirculation of the gas phase in the riser. While scale effects remain slight, the use of a gas distributor favouring this partial recirculation seems adequate for mass transfer in split‐rectangular airlift reactors.     

A Comparison of the Mixing Characteristics in Single‐ and Two‐Phase Grid‐Generated Turbulent Flow Systems

The mixing process is studied in grid‐generated turbulent flow for single‐ and bubbly two‐phase flow systems. Concentration and mixing characteristics in the liquid phase are measured with the aid of a PLIF/PLIF arrangement. A nearly isotropic turbulent flow field is generated at the center of the vertical pipe by using a honeycomb, three grids and a contraction. In two‐phase flow experiments, air bubbles were injected into the flow from a rectangular grid, with mesh size M = 6 mm, which is placed midway between two circular grids each with a mesh size of M = 2 mm. For single‐phase flow, the normalized mean concentration cross‐stream profiles have rather similar Gaussian shapes, and the cross‐stream profiles of the normalized root‐mean‐square (RMS) values of concentration were found to be quite similar. Cross‐stream profiles of the mean concentration, for bubbly two‐phase flow, were also found to be quite similar, but they did not have the Gaussian shape of the profiles for single‐phase flow. Almost self‐similar behavior was also found for the RMS values of the concentration in two‐phase systems. The turbulent diffusion coefficient in the liquid phase was also calculated. At the center of the plume, the flow was found to have a periodic coherent structure, probably of vortex shedding character. Observations showed that the period of oscillation is higher in the case of two‐phase flow than in single‐phase flow.     

Regulating the micromixing efficiency of a novel helical tube reactor by premixing behavior optimization

In this work, a novel helical tube reactor (HTR) was constructed, including a pre‐mixer for adjusting the premixing behavior of reactants and a helical tube as a further mixing unit. The pre‐mixer was modified to optimize the premixing behavior by using two methods, named as tangential‐feeding and insertion of a helical baffle. The premixing behaviors were investigated via computational fluid dynamics (CFD) simulation. Simulation results indicated that both methods can change the fluid flow, enhance the turbulence kinetic energy, and improve the premixing performance in the pre‐mixers. Based on the results of CFD simulation, it could be predicted that the micromixing efficiency of the HTR can be regulated by these methods accordingly. Then the predicated results were confirmed experimentally by a parallel competing reaction. Furthermore, the relationship between the premixing performance increasing and the corresponding micromixing efficiency increasing of the HTR was quantitatively analyzed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2876–2887, 2017     

Volume‐of‐fluid‐based model for multiphase flow in high‐pressure trickle‐bed reactor: Optimization of numerical parameters

Aiming to understand the effect of various parameters such as liquid velocity, surface tension, and wetting phenomena, a Volume‐of‐Fluid (VOF) model was developed to simulate the multiphase flow in high‐pressure trickle‐bed reactor (TBR). As the accuracy of the simulation is largely dependent on mesh density, different mesh sizes were compared for the hydrodynamic validation of the multiphase flow model. Several model solution parameters comprising different time steps, convergence criteria and discretization schemes were examined to establish model parametric independency results. High‐order differencing schemes were found to agree better with the experimental data from the literature given that its formulation includes inherently the minimization of artificial numerical dissipation. The optimum values for the numerical solution parameters were then used to evaluate the hydrodynamic predictions at high‐pressure demonstrating the significant influence of the gas flow rate mainly on liquid holdup rather than on two‐phase pressure drop and exhibiting hysteresis in both hydrodynamic parameters. Afterwards, the VOF model was applied to evaluate successive radial planes of liquid volume fraction at different packed bed cross‐sections. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Gas holdup in a two‐phase reversed flow jet loop reactor

Experimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (E_gd) and gas holdup with circulation loop (E_g). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.