Hydrodynamics of Gas-Liquid Reactors: Normal Operation and Upset Conditions

By Barry Azzopardi, Donglin Zhao, Y. Yan and 3 more

The design of chemical reactors and their safety are as critical to the success of a chemical process as the actual chemistry taking place within the reactor. This book provides a comprehensive overview of the practical aspects of multiphase reactor design and operation with an emphasis on safety and clean technology. It considers not only standard operation conditions, but also the problems of runaway reaction conditions and protection against ensuing over-pressure.

Hydrodynamics of Multiphase Reactors addresses both practical and theoretical aspects of this topic. Initial chapters discuss various different types of gas/liquid reactors from a practical viewpoint, and later chapters focus on the modelling of multiphase systems and computational methods for reactor design and problem solving. The material is written by experts in their specific fields and will include chapters on the following topics: Multiphase flow, Bubble columns, Sparged stirred vessels, Macroscale modelling, Microscale modelling, Runaway conditions, Behaviour of vessel contents, Choked flow, Measurement techniques.

• Language: English
• Publisher: Wiley
• Release date: May 12, 2011
• ISBN: 9781119971405

Book preview

List of Figures

1.1 Chart showing structure of the book and the interrelation of the chapters

2.1 Typical arrangements for bubble columns. (a) Standard bubble column; (b) gas-lift type with internal down-comer; and (c) gas-lift type with external down-comer

2.2 Flow pattern map for bubble columns [1]

2.3 Effect of gas distributor on void fraction/gas superficial velocity relationship

2.4 Effect of pressure on void fraction/gas superficial velocity relationship. Air-distilled water

2.5 Effect of viscosity on the void fraction/gas superficial velocity relationship. Pressure = 5 bar

2.6 Effect of additives on the void fraction/gas superficial velocity relationship

2.7 Effect of liquid circulation velocity on void fraction

2.8 Bubbles sizes predicted by the equation of Gaddis and Vogelpohl [13] for air-water for a 0.16% open area – effect of gas flow rate and hole diameter

2.9 Bubbles sizes predicted by the equation of Gaddis and Vogelpohl [13] for air-viscous liquid (dynamic viscosity = 0.55 Pa s) for a 0.16% open area – effect of gas flow rate and hole diameter

2.10 Sequence of stills from high-speed video illustrating the coalescence of two spherical cap bubbles. Liquid = [EMIM][EtSO4]. Gas superficial velocity = 3.6 mm/s. Note the intermediate sized bubbles, in the wake of the upper spherical cap bubble, that are displaced by the arrival of the second spherical cap bubble and which eventually reposition themselves in the wake of the combined spherical cap bubble

2.11 Shapes of bubbles

2.12 Example of the shapes of real bubbles and their motion as illustrated by multiple images

2.13 Drag law Equation 2.21 (——) and force balance Equation 2.19 (------)

2.14 Bubble velocity dependence on bubble size and type of motion.

2.15 Velocities calculated by different methods – liquid viscosity = 0.05 Pa s

2.16 Effect of liquid viscosity on the bubble velocity calculated using the method of Lin et al. [8]

2.17 Drift flux method to identify void fraction and gas superficial velocity at the homogeneous/heterogeneous transition

2.18 Scale of bubble aggregation for air-water in a 150 mm diameter column at different pressures

2.19 Effect of gas superficial velocity and bubble size on void fraction in the homogeneous region

2.20 Gas superficial velocity dependence of void fraction distilled water-air on a 127 mm column with a distributor of 0.2% open area and 0.5 mm diameter holes

2.21 Comparison of predictions of drift flux method with experimental data

2.22 Comparison of predictions of model of Krishna et al. [46] with air-water data

2.23 Comparison of predictions of models of Krishna et al. [46] with viscous liquid data

2.24 Comparison of predictions of correlations (see Riberio and Lage [47]) with air-water data

2.25 Comparison of predictions of correlations (see Riberio and Lage [47]) with viscous liquid data

2.26 Information flow for gas lift systems

2.27 The effect of gas superficial velocity on void fraction in the riser and on liquid circulation, expressed as liquid superficial velocity. Column height and diameter were selected as 5 and 0.2 m, respectively. Bubble size assumed to be 3 mm. Fluids were air and water at atmospheric pressure

2.28 Axial variation of cross-sectionally averaged void fraction

2.29 Radial void fraction profiles measured by Mudde and Saito [53]. Also shown are predictions of modified equation of Wu et al. [51]

2.30 Radial profiles of liquid axial velocity measured by Mudde and Saito [53] for both a standard bubble column and a column with recirculation. Also shown are standard column data transposed by liquid superficial velocity and predictions of the equation suggested by Wu and Al-Dahhan [52]

2.31 Radial void fraction profile from 46 bar steam-water experiments: gas superficial velocity = 0.09 m/s; liquid superficial velocity = 0.01 m/s. Column diameter = 194 mm

2.32 Proposed circulation models for bubble columns: (a) vortex street [2]; (b) counter rotating vortices [50]; and (c) more complex pattern with moving vortices [51]

2.33 Bubble size distribution measured with wire mesh sensor tomography: 46 bar steam-water experiments; liquid superficial velocity = 0.01 m/s. Column diameter = 194 mm

2.34 Dependence of specific interfacial area on bubble size and void fraction

2.35 Effect of proportions of large and small bubbles on specific interfacial area

2.36 Effect of superficial gas velocity and system pressure on specific interfacial area calculated using the void fraction method of Krishna et al. [46]

2.37 Mechanism of drop formation from single and multiple bubbles

2.38 Typical wave plate arrangements

2.39 Wire mesh pads for demisting: (a) one layer; and (b) multiple layers. Data from Feord et al.

2.40 Particle size distribution curve and fractional separation efficiency curve

2.41 Effect of gas velocity and wave plate geometry on grade efficiency for wave plate mist eliminators. Plain wave plates (closed symbols) and data from a unit with drainage channels or hooks (open symbols)

2.42 Correlation of pressure drop with drop size whose collection efficiency is 50%

2.43 Comparison of grade efficiencies predicted by the simple equation of Burkholz [64] and by computational fluid dynamics with experimental data from a plain wave plate

2.44 Comparison of predicted and measured efficiencies for wire mesh pad mist eliminators

2.45 Pressure drop across mesh pad mist eliminator showing increase in pressure drop associated with flooding

2.46 Cumulative drop size distributions before and after mesh pad mist eliminator showing coalescing effect

3.1 Circulation pattern of a radially pumping impeller

3.2 Different types of impellers

3.3 Flow map of a gassed stirred tank equipped with a Rushton turbine

3.4 Gas cavities behind the blades

3.5 Cavity formation at increasing gas loading

3.6 Schematic diagrams of different types of spargers [2]. (a) Multiple ring sparger; (b) spider sparger; and (c) radial pipe sparger

3.7 Different cavities behind a Rushton blade

3.8 (a) Chemineer CD-6; and (b)

3.9 Lightnin A310 (Courtesy of http://www.lightninmixers.com/)

3.10 Chemineer HE-3

3.11 (a) Prochem Maxflo T; (b) Prochem Maxflo W′; and (c) Lightnin A315 (Courtesy of http://www.lightninmixers.com/)

3.12 Lightnin A340 (Courtesy of http://www.lightninmixers.com/)

3.13 Power consumption (normalised by the ungassed power) as a function of the dimensionless gas flow rate. Pitched blade impeller. Impeller/tank diameter D/T = 0.4, impeller positioned D above base of tank, sparger/impeller separation = 0.8 D

3.14 Dimensionless power versus Reynolds number for single-phase flow in a standard stirred tank (w denotes the blade height) [30]

3.15 Gas fraction in a 200 l stirred tank. Full line, Calderbank [31] correlation, Equation 3.10; symbols, experimental data taken from Laakkonen et al. [32]

3.16 Bubble colliding with an eddy

3.17 Mass transfer across the gas-liquid interface

4.1 Dry patch

4.2 Effect of wetting angle on minimum film Reynolds number for dry patch formation. Thick lines, Equation 4.5; and thin lines, alternative version by Coulon [7]

4.3 Velocities within a liquid film flowing down a vertical flat plate compared with values predicted by the model of Nusselt [6]

4.4 Spatial development of waves on a flat plate

4.5 Spatial variation of film thickness for three different liquids. In all cases the liquid flow rate was 0.28 kg/ms

4.6 Axial variation of film thickness from cases in Figure 4.5

4.7 Variation of disturbance wave frequency in a 38 mm diameter pipe; air-water [23]

4.8 Wave velocity for film flow on a flat plate (width 150 mm) [16] and 38 mm diameter pipe [23]

4.9 Effect of gas and liquid flow rates on wave velocity; air-water in a 38 mm diameter tube [23]

4.10 Measured increase in specific interfacial area of falling films on vertical flat plates. Open symbols, Portalski and Clegg [24]; and closed symbols, Sun [25]

4.11 Comparison of equation of Henstock–Hanratty [30] and modified equation for enhancement of mass transfer by gas motion with experimental data

4.12 Variation of the thickness of the film on a rotating disc – water; feed rate = 10 ml/s

4.13 Ratio of acceleration due centrifugal force to that of gravity

4.14 Flow rate–rotation rate plot showing regions of occurrence of different types of waves [33]

4.15 Effect of inverse Eckman number on angle of spiral waves [33]

4.16 Effect of liquid flow rate and rotation rate on mass transfer in rotating disc reactors [36]

4.17 Flow pattern map for gas-liquid flow in coils

4.18 Dependence of entrained fraction in coiled pipe on gas and liquid flow rates

4.19 Accuracy of Chisholm's algebraic equation of Lockhart–Martinelli correlation tested against experimental data for coils

4.20 Type of results produced by a model of a Nylon-6,6 reactor by Guidici et al. [44]

4.21 Honeycomb monolith reactor

4.22 Cellular ceramic monoliths [45]

4.23 Metal monolith [46]

4.24 Elongated bubble flow in a vertical microchannel [52]

4.25 VOF computations of bubble evolution and coalescence in a micro-channel of 0.2 mm diameter

4.26 Computational domain and initial/boundary conditions

4.27 Time evolution of bubble shapes at Re = 100

4.28 The velocity field at different cross-section (t = 2 ms, Re = 100)

5.1 Representative control volume out of a bubbly flow for which the average equations of the Euler–Euler approach are derived

5.2 Gas fraction (left) and liquid velocity (right) for gas superficial velocity = 0.02 m/s [8]

5.3 Predicted Sauter mean diameter contour from the Sγ model

5.4 Comparison of the simulated voidage radial distribution with the corresponding experimental data

5.5 Comparison of the simulated axial velocity profile with the corresponding experimental data

5.6 Comparison of the simulated Sauter mean diameter radial distribution with the corresponding experimental data

5.7 Comparison of the simulated Sauter mean diameter obtained from different moments (γ) with the corresponding experimental data

5.8 Comparison of the simulated interfacial area density with the corresponding experimental data. Here, S0 and S2 models were used to simulate the interfacial area density transport

5.9 Comparison of the simulated interfacial area density with the corresponding experimental data. Different drainage modes were studied

5.10 Comparison of the predicted interfacial area density with the corresponding experimental data. Different drainage modes were studied

5.11 Comparison of the simulated Sauter mean diameter radial distribution with the corresponding experimental data. (a) Case 5, (b) Case 6

5.12 Comparison of the simulated and experimental profiles of the interfacial area density. (a) Case 1, (b) Case 5, (c) Case 6

5.13 60° of a four-impeller, six-blade gassed stirred tank

5.14 Comparison of gas fractions using different bubble sizes [21]

5.15 Comparison of gas fractions using different drag force closures [21]

5.16 Gas pockets formed behind the impeller blades

6.1 Micro-, meso and macroscale numerical modelling

6.2 D2Q9 model

6.3 D3Q15 model

6.4 The flow field set-up

6.5 Layout of regularly spaced lattices and curved wall boundary

6.6 Comparison between LBM and experimental simulations (left, obtained by LBM; right, by experiment)

6.7 Distributions of local Nusselt numbers for Re = 200, Pr = 0.5, θ [−π/2, π/2]

6.8 Period-averaged Nusselt number along the cylinder surface at Re = 200, Pr = 0.5

6.9 The initial velocity field

6.10 Phase distribution of three vortices merging

6.11 Interface distributions and corresponding vorticity contours

6.12 The effect of gravity on finger patterns

6.13 Final finger patterns: (a) Bo = 2.79; (b) Bo = 5.58; (c) Bo = 8.37; and (d) Bo = 11.16

6.14 The domain of rising bubbles coalescence

6.15 Coalescence of two rising bubbles in liquid (ρl /ρg = 50, ηl /ηg = 50, Mo = 1 × 10−5, Eö = 10)

6.16 Time evolution of bubble shapes and velocity vectors at section y = Ly /2 of coalescence of two rising bubbles in liquid (ρl /ρg = 1000, ηl /ηg = 50, Mo = 1, Eö = 15)

6.17 Computational domain of a droplet on partial wetting surface

6.18 Snapshots of droplet spreading on a uniform hydrophilic surface, θw = π/4

6.19 Velocity distribution on the cross-section of x = Lx /2, θw = π/2, t = 0.006 s

6.20 Snapshots of a droplet spreading on a uniform moderate surface, θw = π/2

6.21 Snapshots of droplet spreading and its break-up on a heterogeneous surface

7.1 Division of recorded incidents according to the type of reaction and to the sector of industry

7.2 Schematic of: (a) pressure relief valve (Courtesy of Safety Systems UK Ltd.) and (b) bursting disc

7.3 Pressure and temperature behaviour for different systems

8.1 Reactor level swell processes at fast and slow vapour or gas production rates

8.2 Time evolution of the vertical distribution of a cross-sectionally averaged void fraction during degassing of CO2 from water in a 35 l vessel. These results show the effect of the vent size on the behaviour. Orifice area/vessel volume: closed symbols, 0.0068 m−1; open symbols, 0.0022 m−1.

8.3 Time evolution of the vertical distribution of a cross-sectionally averaged void fraction during boiling of R12 in a 35 l vessel. Void fractions measured using multiple capacitance probes [9]

8.4 Difference between boiling and reacting systems in the transient behaviour of vessel average void fraction.

8.5 Difference between boiling and reacting systems in the transient behaviour of vessel pressure.

8.6 Effect of vent aperture diameter on the maximum temperature and pressure and of reaction mass retained in vessel, reaction of methanol and acetic anhydride.

8.7 Components of heat flow calculated for the runaway reaction of methanol and acetic anhydride

8.8 Time evolution of the vertical distribution of cross-sectionally averaged void fraction during degassing of CO2 from liquids of different viscosities in a 35 l vessel. Information obtained from multiple differential pressure measurements. Closed symbols, viscosity = 0.001 Pa s; open symbols, viscosity = 0.02 Pa s.

8.9 Effect of liquid viscosity of percentage of charge retained in vessel. Orifice area/vessel volume = 0.0037 m−1.

8.10 Temporal variation of void fraction for water without and with surfactant; vessel, 2190 l.

8.11 Effect of surfactant on maximum pressure reached during venting, esterification of acetic anhydride with methanol.

8.12 Variation of void fraction up the vessel. (a) Time varying case; (b) pseudo-steady state, Co = 1.0; and (c) pseudo-steady state, Co = 1.5

8.13 Typical pressure histories during transients

8.14 Effect of overpressure number on relative vent area for different models

8.15 Effect of volume on heat loss rate

8.16 Time history of pressure for hydrolysis of acetic anhydride.

8.17 Time history of temperature for hydrolysis of acetic anhydride

8.18 Variation of self-heat rate with temperature for hydrolysis of acetic anhydride.

8.19 Abstraction of information from temperature and pressure time histories

8.20 Comparison between measured vertical variation of void fraction and predicition of the DIERS method.

8.21 Capability of models to predict the single-/two-phase venting boundary.

8.22 Comparison between measurements of time variation of vessel pressure and predictions of SAFIRE

8.A.1 Geometry for level swell calculation

9.1 Critical flow – basic concept

9.2 Typical manifold arrangement and generalised schematic of geometry where multiple chokes might occur

9.3 Measured and calculated pressure wave propagation velocity in air-water mixtures at 1.7 bar

9.4 Measured slip ratio [10]

9.5 Pressure profile along pipework consisting of two pipes of different diameter joined at a sudden expansion and illustrating the steep pressure gradient at the two positions of choking [15]

9.6 Geometry for development of double-choke criterion

9.7 Flow diagram for calculation of two chokes in series

9.8 Performance of DEM based calculations for a double choke [15]

9.9 Effect of length to diameter ratio of vent line on flow reduction factor

10.1 Pressure sensors in ungassed and gas bubble column

10.2 Path of light beam (top); bubble signal from probe (bottom)

10.3 Piercing a bubble at the side

10.4 Four point probe and signals

10.5 Photograph of a bubble pierced by a four-point probe

10.6 Wire mesh sensor [5])

10.7 Development of the void fraction in a bubble column over time measured with a wire mesh sensor. Each pair of images corresponds to one gas flow rate. The left hand of the pair is the integral view whilst the right-hand view is the information across a diameter

10.8 Photograph of bubbles at g 50%

10.9 Photograph of bubbles at g 0.5% [7]

10.10 Bubbles distribution at g 0.5% [7]

10.11 LDA set-up in back-scatter mode

10.12 Axial velocity measured by LDA [8]

10.13 Axial velocity at low and high gas fraction measured by LDA [8]

10.14 PIV results of a bubbly flow [13]

10.15 PIV/PTV results of a bubbly flow at g 1% [14]

10.16 ECT electrodes and guards

10.17 Radial void fraction profiles as measured with ERT [19]

10.18 Electromagnetic radiation: high energy, short wavelength γ and X-rays

10.19 Attenuation of γ or X-rays by a slab

10.20 Set-up used by Kumar et al. [20]

10.21 Radial gas fraction profiles using g tomography [20]

10.22 Three-source X-ray set-up

10.23 The electronbeam X-ray scanner [23]

10.24 CARPT set-up

10.25 Principle of PEPT

10.26 Amplitude of contributions at different frequencies showing effect of switching on a pump

10.27 Size distribution of bubbles

10.28 Result of check trails using stream of bubbles from an orifice

10.29 Measured concentration using a probe with a finite response time for various values of kl aτ: A, 1.5; B, 0.5, and C, 1.5. D is the true liquid concentration

10.30 Response time measurement of oxygen probe

10.31 Example of the gas fraction distribution in a gassed, stirred tank (operated in the loading regime) [36]

10.32 Response of probe in mixing experiment

10.33 Gas flow through the reactor showing that the gas volume fraction as well as the bubble size distribution needs to be constant during the determination of kla

10.34 kl a values from measurements analysed by a first- and second-order model [38]

10.35 Examples of electrode arrangement for flush mounted type: (a) Coney [40]; (b) Geraci et al. [41]; and (c) Brown [42] (Data from Brown, D.J., Non-equilibrium annular flow, DPhil Thesis, University of Oxford (1978).)

10.36 Calibration curves for conductance film thickness measurement: (a) flush mounted electrodes [type (b) in Figure 10.35]; and (b) wire pair

10.37 Schematic diagram for fluorescence technique showing arrangement for in situ calibration and possible additional measurements for mass transfer or temperature measurements discussed in the next section

10.38 Experimentally obtained calibration data. Line is calculated from Equation 10.43

10.39 Effect of concentration and film thickness on fluorescence response showing the effect of self quenching at higher concentrations

10.40 Fluorescence and phosphorescence spectra for biacetyl [55]

10.41 Temperature profiles determined using the luminescence technique [55]

10.Q.1 Optical probe piercing a spherical bubble

10.Q.2 High energy beam radiating through a gas-liquid flow

10.Q.3 Two different arrangements of the phases at the same volume fraction. Left: the gas-liquid flow consists of two separate, vertical layers; the layer on the left is liquid, the right layer is gas. Right: alternating gas-liquid flow, where the dark areas denote liquid, light areas indicate gas

List of Tables

Table 2.1 Dependence of exponent of Richardson and Zaki [31] on Reynolds number

Table 2.Q.1 Properties of the fluids employed at process conditions

Table 4.1 Constants [1] for Equation 4.28

Table 5.1 Drag coefficients for a single rising bubble in water by Wang [4]

Table 5.2 Case definition and parameters used

Table 7.1 Causes of runaways

Table 8.1 Examples of experiments involving chemical reactions employed in runaway/venting studies

Table 8.2 Properties of the reactants and reaction product for the esterification of acetic anhydride by methanol or its hydrolysis by water

Table 8.3 Effect of fill level and presence of surfactant on major parameters during the runaway hydrolysis of acetic anhydride in a 2500 l vessel

Table 8.4 Characteristics of codes used in level swell/venting calculations

Table 8.Q.1 Contents of reactor after all chemicals added

Table 8.Q.2 Data for Question 4

Table 10.1 Comparison of gas fraction

Table 10.Q.1 Solutions to Question 10.1

Preface

One way of identifying areas of importance in Chemical Engineering is to look at the subject groups or working parties of national or transnational professional bodies. Amongst the Working Parties of the European Federation of Chemical Engineers, there are two that are relevant to the present text, one on Chemical Reaction Engineering and the other on Multiphase Flow. Although each has its own programme of events, they come together once in a while to air matters of common interest. Multiphase flow, the simultaneous flow of more than one phase, has applications in: electrical power generation – nuclear and fossil fired; oil and natural gas production and refining; distillation and absorption and in heat transfer. This has produced a body of knowledge that can be drawn on for the benefit of the design of chemical reactors.

The relative efforts in the two facets of reactor design have been very succinctly illustrated in Figure A A, by Professor Octave Levenspiel [Levenspiel, O. (1999) Chemical Reaction Engineering, Ind. Eng. Chem. Res. 38, 4140–4143]. This text aims at strengthening the weaker link of the chain shown.

Figure A Reprinted with permission from Industrial & Engineering Chemistry Research, 38, 11, Chemical Reaction Engineering, Levenspiel © 1999, Americal Chemical Society

The preparation of this present work brought together a team with complimentary skills. All are firmly based in multiphase flows and they research into chemical reactor applications in addition to other industrial applications. The work presented here is an expression of the realization that there is the need for more and more complex modeling methods. There are also requirements to consider not just mixing but also separation of gas and liquid and to look into the upset conditions, which might lead to accidents, as well as a steady state. All of these have found a place in this book from the point of view of multiphase flows, in a combination of well proven engineering concepts and modern developments, both numerically and experimentally.

Nomenclature