Gas Mixing

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Process Engineering Guide: GBHE-PEG-MIX-702

Gas Mixing 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: Gas Mixing CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 RECOMMENDATIONS FOR GAS MIXING:

PLUG FLOW 2 5 RECOMMENDATIONS FOR GAS MIXING:

BACKMIXED INITIAL ZONE 4 6 BIBLIOGRAPHY 6



FIGURES 1 RESULTS OBTAINED USING A SULZER SMV MIXER 3 2 RECIRCULATION ZONE : CONFINED JET 4 3 RECIRCULATION ZONE : BLUFF BODY 5 4 STREAMLINES OF RECIRCULATION EDDY IN SWIRLING

JET, S = 1.57 5 5 SCHEME OF SWIRL BURNER WITH AXIAL AND

TANGENTIAL AIR ENTRIES 6 6 COMPARISON OF THE RADIAL DISTRIBUTIONS OF VELOCITY

IN THE VORTEX REGION FOR JETS WITH AND WITHOUT DIVERGENT EXTENSIONS 7

7 BACKMIXED 'PRE-REACTION' ZONE 7 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 8



0 INTRODUCTION/PURPOSE This Guide is one in a series of Mixing Guides and has been produced for GBH Enterprises. 1 SCOPE This guide covers the mixing of gases. If a backmixed 'pre-reaction' zone is not required, refer to Clause 4. Where a backmixed 'pre-reaction' zone is required, refer to Clause 5. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS No specific definitions apply to this Guide. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 RECOMMENDATIONS FOR GAS MIXING: PLUG FLOW Jet flow mixers and static mixers are recommended for gas mixing. The design procedures presented for miscible liquids in GBHE-PEG-MIX-701, should be followed using the appropriate gas physical properties. Empty pipes may be used in the turbulent region: for gases of the same density, a length of about 100 pipe diameters is required for good mixing. For gases with a density ratio > 2, up to 1000 pipe diameters may be required because of the formation of stable layers.



Static mixers can achieve good mixing in less than five pipe diameters regardless of the density difference. Figure 1 shows the results obtained using a Sulzer SMV mixer. Unless the densities are similar and the flow rates constant, a minimum operating velocity, UG, is recommended. For the Sulzer SMV mixer, this is obtained if the modified Froude number exceeds 20 [Ref 1 : Germain et al., 1982]:

. The values of f and DH are obtained from the manufacturers' data;



FIGURE 1 RESULTS OBTAINED USING A SULZER SMV MIXER



5 RECOMMENDATIONS FOR GAS MIXING: BACKMIXED INITIAL ZONE Such cases are covered in the combustion literature, e.g. in Beer, J M and Chigier, N A, ”Combustion Aerodynamics”, Krieger, 1983 (reprint of 1972 edition). In this reference design of jet mixers for the particular requirements of combustion, namely rapid mixing and stable flame, are discussed, and information provided for entrainment rate into the jet, flow patterns, recirculation vortex position, effects of jet swirl, etc. A recirculation zone is often used to stabilized the flame (see Figure 2). Hot gases, entrained at the downstream end, return to pre-heat the incoming fresh gas and supply chemically active species to it. This enables velocities much higher than the flame propagation velocity to be used without lift-off. Recirculation zones are provided by:

(a) Confining the jet and starving the entrainment. FIGURE 2 RECIRCULATION ZONE : CONFINED JET



(b) Using the wake behind a bluff body (see Figure 3).

FIGURE 3 RECIRCULATION ZONE : BLUFF BODY



(c) Swirling the jet, with swirl number S > 0.6 (see Figure 4).

FIGURE 4 STREAMLINES OF RECIRCULATION EDDY IN SWIRLING JET, S = 1.57



This can be evaluated from velocity profiles either assumed, or evaluated from computational fluid dynamics (CFD) analysis. Swirl can be produced by using tangential inlets, helical guide vanes in the flow, or by mechanically rotated vanes or walls (see Figure 5). The degree of swirl is increased by higher swirl numbers and by the addition of a divergent exit duct (see Figure 6). FIGURE 5 SCHEME OF SWIRL BURNER WITH AXIAL AND TANGENTIAL

AIR ENTRIES



6 BIBLIOGRAPHY Reference Authors, Title and Source [1] Germain, E & Wetter, R. Mélange Statique de gaz, Informations

Chimie No. 232. Dec 1982, p 135 (referred to in Clause 4).



FIGURE 6 COMPARISON OF THE RADIAL DISTRIBUTIONS OF VELOCITY IN THE VORTEX REGION FOR JETS WITH AND WITHOUT DIVERGENT EXTENSIONS

A European plant operator reactor achieves a backmixed 'pre-reaction' zone by jet entrainment (see Figure 7).



FIGURE 7 BACKMIXED 'PRE-REACTION' ZONE

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBH ENTERPRISES ENGINEERING GUIDES Glossary of Engineering Terms (referred to in Clause 3) GBHE-PEG-MIX-701 Mixing of Miscible Liquids

(referred to in Clause 4) OTHER GBHE DOCUMENTS “A Design Procedure for Droplet Production in Static Mixers for Newtonian and Power Law Fluid” (referred to in Clause 4).

