Mixing of Gas Liquid Systems

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GBH Enterprises, Ltd.

Process Engineering Guide: GBHE-PEG-MIX-705

Mixing of Gas Liquid Systems Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Mixing of Gas Liquid Systems CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 FLUID INTERFACE AND REACTION EFFECTS 3 5 EQUIPMENT SELECTION 3 6 DESIGN OF BUBBLE COLUMNS 8

6.1 Recommendations 8 6.2 Design Procedure 8 6.3 Mass Transfer and Voidage 9 6.4 Heat Transfer 10 6.5 Mixing Patterns 10 7 DESIGN OF GAS-LIQUID STIRRED VESSELS

(Re > 104 ONLY) 11 7.1 Recommendations 11 7.2 Design Procedure 13 7.3 Mass Transfer 15



8 DESIGN OF GAS-LIQUID STATIC MIXERS 17 8.1 Recommendations 17 8.2 Design Procedure 17 9 NOMENCLATURE 19 TABLE 1 COMPARISON OF DIFFERENT GAS-LIQUID

CONTACTING DEVICES 4

FIGURES 1 BUBBLE COLUMN 4 2 STIRRED VESSEL 5 3 JET-MIXED VESSEL 5 4 STATIC MIXER 6 5 JET EJECTORS 6 6 THIN FILM 6 7 THIN FILM 7 8 DOUGH MIXER 7 9 IN-LINE DYNAMIC MIXER 7 10 MULTI-PIPE GAS SPARGER 8 11 VESSEL GEOMETRY 12 12 AGITATORS 12



13 POWER CURVES: COALESCING SYSTEM 16 14 POWER CURVES: NON-COALESCING SYSTEM 16 15 FLOW MAP FOR VERTICAL UPWARD FLOW 20 16 FLOW MAP FOR VERTICAL DOWNWARD FLOW 20 17 FRICTION FACTOR DATA FOR KENICS AND SULZER

MIXERS 21 18 PRESSURE DROP CORRELATION 22 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 23



0 INTRODUCTION/PURPOSE This Process Engineering Guide is one in a series of Mixing Guides and has been prepared for GBH Enterprises. 1 SCOPE This Process Engineering Guide concentrates on processes in which a gas is to be dispersed in a liquid with a required bubble size or interface area and mass transfer capacity. Sometimes these are a compromise between high transfer rates and rapid gas disengagement. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS No specific definitions apply to this Guide. 4 FLUID INTERFACE AND REACTION EFFECTS A crude distinction can be made between systems. A closed bottle containing the two phases when shaken for 5 seconds then held still will show if the system is 'coalescing' (bubbles disengage quickly, within 2 seconds) or 'non-coalescing' (with very small bubbles and perhaps a foaming problem). This is less useful if the test conditions are remote from the process conditions. The GBHE Mixing and Agitation Manual shows that the situation is more complex, and with the same equipment and flows, bubble sizes can vary by an order of magnitude, gas volume fractions by a factor of 5, and hence interface area per unit volume ('a') by two orders of magnitude between different reacting systems. If gas-liquid contacting results in a chemical reaction (apart from a very slow one) the GBHE Mixing and Agitation Manual should be consulted. Foam prevention is also covered in the GBHE Mixing and Agitation Manual.



5 EQUIPMENT SELECTION Equipment selection is based on liquid viscosity, transfer intensity required and preferred flow pattern (batch, plug flow or back-mixed). Performance is greatly dependent on physical properties, and concentrations of solutes and surface activity, reactions or small particles near the gas-liquid interface. These are complex effects so design is best done by thoughtful scale-up from experiments at relevant conditions. Table 1 shows the comparison of different gas-liquid contacting devices. TABLE 1 COMPARISON OF DIFFERENT GAS-LIQUID CONTACTING

DEVICES

These are orders of magnitude for comparison, not design figures. They are based on air and water physical properties. Other considerations such as heat transfer, materials of construction, suspended solids, hazardous liquids or gases (requiring low inventory) shall obviously be taken into account. Figures 1 to 6 show the selected equipment for Low Viscosity Liquid (approximately 0.05 Pas or 50 cp).



FIGURE 1 BUBBLE COLUMN Simple, cheap, low mass transfer rate, uncontrolled flow pattern. Used for slow reactions.

FIGURE 2 STIRRED VESSEL Higher mass transfer rate, can approach back-mixed liquid.



FIGURE 3 JET-MIXED VESSEL Very sparse design data (use stirred vessel correlation for mass transfer).

FIGURE 4 STATIC MIXER Very high mass transfer intensity. Short residence time. Near plug flow. Performance linked to throughput. Use for very fast reactions, or as a predisperser

.



FIGURE 5 JET EJECTORS Gas sucked in by the liquid stream. Very high transfer intensity in throat. Short residence.

FIGURE 6 THIN FILM Packed column (see Absorption Brochure*) or spinning cone for gas-film diffusion controlled reactions.

Figures 7 to 9 show the selected equipment for High Viscosity Liquid.



FIGURE 7 THIN FILM e.g. Spinning cone, Graesser rotating disc contactor, falling film column.

FIGURE 8 DOUGH MIXER e.g. ‘Z-blade’, Winkworth, Banbury (refer to manufacturers).

FIGURE 9 IN-LINE DYNAMIC MIXER e.g. Oakes, Kinematica, Greaves (refer to manufacturers).



6 DESIGN OF BUBBLE COLUMNS If solid particles are present see GBHE Mixing and Agitation Manual. 6.1 Recommendations The vessel shall be accurately vertical (to ± 2°). Height/diameter ratio is usually between 3 and 12 (set as below). Gas should be well distributed across the base, by using a porous plate or a multiple-pipe sparger. Pressure drop through the holes should be the largest in the gas system, approximately >10 times that through the pipes, and 10 times any external pressure fluctuations. This should be optimized in large systems. FIGURE 10 MULTI-PIPE GAS SPARGER



6.2 Design Procedure (a) Choose outlet gas concentration: calculate gas and liquid throughputs, QG

and QL, from reaction requirements and mass balance, using an assumed pressure.

(b) Calculate required volume of liquid in vessel, VL, from reaction kinetics or

from small scale experiments. (c) Choose a superficial gas velocity VSG to give adequate mass transfer (see

experimental data, or below for 'pure' liquids).

(Gas superficial velocities of >0.1 m/sec may cause entrainment of liquid in the outlet gas: use baffle plate or foam breaker). Check mixing patterns (see below).

(d) Calculate bubble column diameter T from:

(e) Calculate gassed liquid height H from:

Gas voidage, εG, is obtained from the experiments, or the correlations below.

(f) Check H/T is in the range 3-12. Calculate mean pressure and correct step

(a). (g) Check heat transfer is adequate: add coils or an external exchanger and

pumped circulation loop if necessary. (h) Allow sufficient head space for volume changes and spray disengagement

(at least 0.75T).



6.3 Mass Transfer and Voidage

Where CL* is the dissolved gas concentration in the liquid in equilibrium with the gas phase, and the mean is taken across the bubble column using appropriate assumptions of mixing pattern (see 6.5). Measure KLa experimentally if possible; otherwise: (a) For aqueous systems use KLa = (0.6 εG) sec-1 (b) For others use the smallest value from:

Voidage should be measured experimentally if possible, otherwise: (c) For aqueous systems use:

(d) For others use:



6.4 Heat Transfer Heat transfer coefficient, h, for internal surfaces may be estimated from:

though this takes no account of position or orientation of surfaces. 6.5 Mixing Patterns



7 DESIGN OF GAS-LIQUID STIRRED VESSELS (Re > 104 ONLY) 7.1 Recommendations (a) The agitator impeller and gas inlet should be designed so that all the gas

enters the impeller blade tip zone and is effectively dispersed there: gas may be sparged in below the impeller, or drawn in from the surface using a special 'self-inducing' impeller (see later).

(b) Impeller Reynolds Number (ρND2/µ) should be >104. (c) 4 flat plate wall baffles (width ~T/10) should be fitted, (with a small gap at

the wall if desired) (see Figure 11) even if dip pipes are present. (d) Impeller diameter D should be T/3 to T/4. (e) Impeller clearance from base C, should be T/4. (f) For under-sparging, the 'standard' agitator is a flat-blade disc turbine, but

better performance is obtained from the Gasfoil (see Figure 12). If a flat-blade turbine is used the 'flooded' ('large-cavity') zone (where power drops grossly as QG increases) should be avoided (see GBHE Mixing and Agitation Manual). Use 6, 12 or 18 blades.

(g) For surface aeration self-inducers can be used, (see GBHE Mixing and

Agitation Manual). Frings are often used but not recommended. Good results have been obtained from 6 and 12 blade concave blade turbines (nearer the surface than shown in Figure 11). The baffles may be removed from the surface region to enhance surface aeration. The vortex can be unstable; experimental verification is advisable (see GBHE Mixing and Agitation Manual).

(h) If H > C + 3D a second agitator should also be used, placed 2D above the

first. If H > C + 5D a third should also be used, again 2D above the second, and so on.

If these are radial flow agitators (such as disc turbine or Gasfoils) the flow shall be compartmented into one well-mixed 'cell' per agitator, with slow intercell mixing. The second, third.... agitator can preferably be axial flow (preferably up-pumping) to promote axial mixing.



(j) Axial flow or pitched blade turbines can be used, if up-pumping, as the primary disperser but supply less power and suffer from deteriorating performance as the gas rate increases. Down flow turbines are unstable with gassing and not recommended.

FIGURE 11 VESSEL GEOMETRY



FIGURE 12 AGITATORS

7.2 Design Procedure (a) By laboratory experiment, (see GBHE-PEG-RXT-810 3 - Gas-Liquid

Reactions) establish whether chemical kinetics or mass transfer determines rate. Also measure rate and εG for conditions giving desired conversion or product.

(b) Calculate required liquid volume VL to keep (VL/QL) constant if kinetics

control, or to keep







FIGURE 13 POWER CURVES : COALESCING SYSTEM



FIGURE 14 POWER CURVES : NON-COALESCING SYSTEM

8 DESIGN OF GAS-LIQUID STATIC MIXERS 8.1 Recommendations (a) Suitable where turbulent flow can be achieved

(Re L > 1000, see below). (b) Mass transfer intensity (KLa) can be very high. (c) For plug flow operate single-pass. Residence time will be short and

performance linked to throughput. Can also operate in batch loop mode, or in a continuous-flow loop, in which case performance is decoupled from throughput, and at high recycled ratios back-mixing is approached.

(d) Should be mounted for vertical flow.



8.2 Design Procedure (a) Select single pass or loop mode. Select Kenics or Sulzer SMX mixers. (b) Establish liquid and gas flow rates and value of:

required, from experiment. Assume plug flow. (c) Check flow regime from Figure 16. Avoid slug flow. Annular and mist give

high interface area but the correlations cannot be guaranteed. (d) Choose D (= mixer internal diameter: see manufacturers’ standard sizes). (e) Scale mass transfer rate from:







FIGURE 15 FLOW MAP FOR VERTICAL UPWARD FLOW

FIGURE 16 FLOW MAP FOR VERTICAL DOWNWARD FLOW



FIGURE 17 FRICTION FACTOR DATA FOR KENICS AND SULZER MIXERS



FIGURE 18 PRESSURE DROP CORRELATION Lockhart-Martinelli plot for turbulent flow in a static mixer.



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: Process Engineering Guides GBHE-PEG-RXT-810 Gas-Liquid Reactions (referred to in 7.2) Other GBHE Documents GBHE Mixing and Agitation Manual (referred to in Clause 4, Clause 6, 7.1 and 7.2).

