14th Australasian Fluid Mechanics Conference Adelaide University, Adelaide, Australia 10-14 December 2001

A few examples of Industrial Proble

from Improved Understandin

Peter Mullinger

Department of Chemic University of Adelaide, Adelaide, Sout

Abstract Fluid flow plays an enormous, and underrated, role in industrial processes and its contribution is become more widely recognised. Topics such as flow in ducts, air distribution between burners, conveying of powders, mixing of fluids and powders, etc are becoming increasingly recognised as fluid flow problems. Many of these problems are not well understood, yet the cost of failure caused by incorrect design is very high. The author selects and describes a few industrial problems as examples where poor understanding of the fluid mechanical issues leads to poor or unpredictable performance and increased costs. Some of industry’s needs could be met by better education, that is more effective application of existing knowledge, however achieving a better understanding of even a few of the more complex problems will not be easy and offers an exciting challenge to researchers. However the potential benefits from a better understanding of the issues are very significant and will often be applicable across a wide range of industries. Introduction Fluid flow plays an enormous, and often underrated, role in most industrial processes. It is only in recent years that the importance of understanding the contribution of fluid flow to industrial manufacturing processes has become more widely recognised. Topics such as flow in ducts, air distribution between burners, conveying of powders, mixing of fluids and powders, etc are becoming increasingly recognised as fluid flow problems. As a consequence of the lack of recognition of the importance of aerodynamics and hydrodynamics, the design of many of the items affected, such as ducting, pipework, etc is usually allocated to draught-persons with little or no knowledge of fluid mechanics. It is little wonder therefore that major problems have arisen from this approach. The need for improvements in this area is driven by economic factors because the cost of failure is so high. For example, a gas fired process air heater costing US$100,000 that was installed on a North Sea oilrig in the early 1980s failed as a consequence of flame impingement within 24 hours of start-up. It was out of service for over three months necessitating flaring of the associated gas. The consequential losses were in the order of US$100,000 per day. The problem was eventually traced to poor design of the combustion air ducting. A few splitters and turning vanes costing less than $5000 solved the problem! However the solution was only determined by extensive flow modelling. I shall try to illustrate the needs of industry for improved working knowledge of fluid mechanics by examining a few examples of fluid flow induced problems from some selected industries that I have experienced during my industrial career. Some of industry’s needs could be met by better education, that is more effective application of existing knowledge, however to meet the remaining needs will require extensive research. Some of the problems are so complex that they may well defy a significantly improved understanding within the foreseeable future.

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ms that would benefit g of Fluid Flow

al Engineering h Australia, 5005 AUSTRALIA

er Generation er generation involves a wide range of fluid flow cations from “simple” airflow and gas flow issues to lex two-phase flow problems. A few examples will be

idered below.

ow Issues

t electricity is generated using fossil fuel fired steam turbine . Despite the increasing contribution of renewable energy es and nuclear power, combustion of fossil fuel will nue to provide most of our electricity for many years ahead.

bustion takes place in the following four stages with the tiveness of the fuel/air mixing determining the overall ustion rate and the heat transfer from the flame:

ng > Ignition > Chemical Reaction > Dispersal of Products

the mixing and the dispersal of the products are strongly ndent on the fluid flow patterns. The completeness of ustion and the emissions performance are critically

ndent the way that the combustion air and fuel are supplied e individual burners. For pulverised coal firing, which

unts for approximately 50% of power generation, ynamics also plays a crucial role in the fuel supply as is ssed below. Low NOx applications make special demands e distribution of the combustion air supply, which typically s to be controlled to within a tolerance of ±5% and ideally . An example of a typical low NOx burner is shown in figure s low NOx performance is dependent on the control of the w distribution between primary air, secondary air and ry air, all of which demands an excellent understanding of erodynamic flow patterns.

Figure 1 Typical aerodynamics of a low NOx burner [1]

Large boiler installations require combustion air ducting to supply air to many burners, typically more that forty and often up to seventy individual burners. The limited space available usually require the ducting to follow tightly constrained routes and usually involves several bends, with the ducting splitting several ways to feed the individual burners, figure 2. This makes prediction of the mass flow of air to individual burners very difficult and prediction of the flow distribution within the burner virtually impossible. Sometimes flow balancing dampers are installed to permit adjustment of the flows with the objective of equalising the flows at the burners. This approach is fundamentally flawed because it is virtually impossible to measure the actual airflow at the burner and the adjustment of the flow to one burner affects the flow to all the other burners. Furthermore partially closed dampers cause a major disturbance of the flow that adversely affects the air flow distribution within individual burners.

Figure 2 Typical power boiler combustion air ducting [2]

Whilst, in principle such ducting can be modelled using either physical modelling or CFD modelling [3,4] and the design modified to optimise the air distribution, the sheer complexity of the ducting means that physical modelling is extremely time consuming and hence very expensive. On the other hand the effectiveness of CFD modelling of such complex systems is still limited by computing power. Furthermore the very high cost of modelling is a major deterrent to the use of the technique by boiler designers. Many of the flow distribution problems are created by poor design of bends and splits that set up an asymmetrical flow pattern, which then persists throughout the system. The need is for a “simple” design technique that allows designers to design bends and junctions so that the flow distribution remains uniform. Pulverised Coal Conveying Pulverised coal (PF) conveying suffers from all the problems associated combustion air ducting with the additional complexity

of tpulvphasgreat One the cthe marea difficburnachieat dupalliabrasof romod

Combustion air ducting

Signpartitypicthe gravattemair fundmade Despsuchincrepulvand Indeleast Jet F Burnbehaperfocombjets undejets, largeprofi

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wo-phase flow that results from air conveying of the erised coal. Most power generation PF systems are lean e, that is the ratio of air to coal is in the order of 2:1 kg/kg or er.

of the major problems of pulverised coal conveying is that oal is not evenly distributed in the duct. Typically 80% of ass flow of the coal is concentrated in only 20% of the duct [5]. This phenomena, known as “roping”, leads to great ulties in ensuring equal distribution of coal to the different

ers, even if the airflow is equally distributed. Attempts to ve equal distribution leads to the use of complex junctions cting splits known as riffle boxes, figure 3. These are only a ative solution because they are subject to intense wear with ive coal. Little research has been undertaken on the cause ping and the phenomenon does not readily lend itself to

elling either physically or using CFD.

Figure 3 Riffle box to promote even split of pulverised coal at conveying duct junctions [6]

ificantly different velocities have been observed between the cles and the conveying air. Frompovicz [5] stated that ally the particles move at approximately half the velocity of conveying air stream no doubt as a consequence of the itational forces acting on the particles. This means that all pts to measure the mass flowrate of the coal by measuring

velocity and density of the suspended particles are amentally flawed, especially when such measurements are in vertical ducting.

ite all the research efforts lavished on clean coal technology as fluid bed combustion and coal gasification as a means of asing efficiency and improving environmental performance erised coal fired power stations are still being constructed will continue to be used well past the middle of the century. ed many of today’s existing units will be operational until at 2040, so research in this area is well worthwhile.

low

ers are, in effect, a complex system of jets and the mixing viour of these jets largely determines the combustion rmance both in terms of emissions and completeness of ustion. Whilst the performance of cylindrical co-annular

has been extensively studied and is now relatively well rstood [7] many PF burners consist of a series of rectangular figure 4. Very little work has been undertaken on such jets, ly being confined to a study of the downstream velocity les [8] yet it is known by practical experience that quite

small changes in jet geometry can have a major effect on down stream behaviour [9]. The author and his co-workers in the CRC for Clean Power from Lignite have recently commenced work in this area.

Figure 4 Typical slot type power boiler burners

Adjacent burners interact with each other as a consequence of both the interaction of the jets and the effects of transmitted and reflected radiation between the flames. This has the effect of increasing flame length and temperatures, figure 5. Again little research has been undertaken on the phenomena of the interaction of adjacent jets.

Figure 5 Effect of spacing of multiple burners on flame temperature [10]

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Figu It mcoeffdragquancoeffpartientra120 combair is

Fi

Molevelocbettefurthfigur Sincprimincreburnredureasocoal velocveloc.

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olatilising Particles

erised coal combustion involves the rapid de-volatilisation of articles in the flame. Photographs of single particles burning that the volatile combustion takes place in the wake behind article, figure 6.

re 6 Single particle combustion of pitch/water mixture [11]

ight be expected that such behaviour could affect the drag icient of the particle and in particular lead to a reduction in

. Again little work has been done in this area but there is titative evidence from the cement industry that the drag icient of burning particle is lower than for non-burning cles. In rotary kilns the fuel/air mixing is controlled by jet inment, figure 7. The coal is injected at high velocity, (20-m/s) with the primary air, a small proportion of the total ustion air, typically 8-20%. The remaining air or secondary introduced at a much lower velocity, typically 2-10 m/s.

gure 7 Principle of jet entrainment pulverised coal burners used in the cement industry[12]

s & Jenkins [13] found that increasing the primary jet ity firstly reduces the flame length as expected owing to r fuel/air mixing until it reaches a minimum length with er increases in velocity leading to a lengthening of the flame, e 8.

e fuel/air mixing continues to improve with increasing ary air velocity the only possible explanations of the asing flame length are that the particles are taking longer to or that their residence time in the early part of the flame is ced. The reduction in residence time is by far the most likely n for the longer flame and is probably caused by the burning particles continuing to travel at, or close to, their injection ity rather than being slowed by drag forces to the bulk flow ity (typically 15-25 m/s).

Figure 8 Effect of primary quantity and air/fuel injection velocity on flame length in a simple pulverised coal fired cement kiln [13] Minerals Processing Minerals processing encompasses a wide range of applications where solids are processed as suspensions in either air or gases or suspended in liquids. These processes include grinding & classification processes, powder mixing & blending, calcination, flotation processes, sedimentation, etc. The effectiveness and energy consumption of many of these processes is dependent on fluid mechanical issues. Grinding and Classification Grinding is used to reduce the size of hard materials such as bauxite, limestone, quartz, metal ores, etc. It is normally undertaken in water or air/gas swept mills and can be open or closed circuit, figure 9.

Figure 9 Open and closed circuit milling systems

With closed circuit mills the ground material is conveyed to a classifier where the oversize is separated from the product and returned to the mill for re-grinding. Classifiers are complex devices and may be either stationary or rotary, figure 10. Separation is achieved using a combination of the inertial, gravitational and drag forces on the particle.

1 Ou 2 Advane 3 ovcolle

Grinding mill

Feed Product

Open circuit milling system

Product

Clasprodconsallowonlythan

Closed circuit milling system

Grinding mill

Feed

Oversize return

1 F

2 D 3 O 4 O 5 O

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Figure 10a Typical stationary classifier [14]

ter cone

justable guide s,

ersize ction cone

4 Product outlet, 5 Vane adjuster 6 oversize outlet

Figure 10b Typical rotary classifier [15]

sifier performance controls the size distribution of the final uct and its efficiency has a large influence on the energy umption of the grinding circuit. Ideally a classifier would only the specified particle size to pass through and return

the oversize to the mill. In practice the cut is much wider this and often a significant proportion of oversize passes the

an 6 Counter vanes 7 Feed 8 Air slide 9 Collector 10 Product outlet

istribution disc

versize cone

versize outlet

uter Cone

classifier as product unless the cut is set unduly fine in which case excessive material is returned to the mill for re-grinding, increasing the power consumption and reducing the capacity of the milling circuit. With the power consumption of industrial milling circuits typically in the range of 1-5 MW with some mills up to 20MW the potential financial returns on improved classifier performance are significant. Little fundamental work has been undertaken on this topic, most of the knowledge arising from trial and error development undertaken by the commercial manufacturers. This is probably because the complexity of the process makes it difficult to study from a fundamental perspective and also difficult to model effectively. Without doubt physical modelling of classifiers is very difficult owing to the difficulty of scaling and maintaining dynamic similarity while the effectiveness of CFD modelling is still limited by computer performance for such a complex system. Powder Mixing & Blending Certain processes, such as cement manufacture require the blending of naturally variable materials in multi-component mixtures to close tolerances. Owing to the difficulty of blending powders this was often achieved in the past by grinding wet and mixing the resulting slurry. This method is no longer acceptable owing to the huge energy consumption involved in evaporating the water so the raw materials are now ground dry and blended in large concrete silos, figure 11.

Figure 11 Typical continuous blending silo [16]

Combasethorohom H = Whe

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pressed air is blown in though jets or distributor plates in the to agitate the powder with the objective of mixing it ughly. The blending performance is defied by the

ogenising effect:

Sin/Sout

re: Sin is the standard deviation of variability of the incoming feed Sout is the standard deviation of the blended material

pical target for the homogenising effect is 8 to 10. However effective blending is rarely achieved for example, a recent of the performance of a large blending silo indicating that it an homogenising effect of only 1.6-2.1 [17]. As for ifiers, modelling the performance of blending silos is mely challenging but would have significant benefits, both rms of product quality and, for the cement industry, in ficantly reduced kiln energy consumption.

ination

ination involves the use of heat to drive off gas or water to uce a stable oxide such as CaO (lime) or Al2O3 (alumina). It ually undertaken in rotary kilns, fixed beds, fluid beds or calciners. The choice of equipment depends on the raw rial and product type, particle size, fuel available, etc. All esses involve combustion and its related fluid flow and ng problems whilst both fluid beds and flash calciners lve devolatilising particles suspended in a hot gas stream. In case of oil fired units both the fuel and product are latilising simultaneously!

nsive modelling of both combustion and fluid flow has dy been undertaken on these units using both acid/alkali and modelling, figure 12. [18] However investigations into the uct flow and residence time are very limited and, to date, latilisation has not been accounted for at all.

mical Manufacture

t chemical processes involve the transport and mixing of ants, followed by the separation of products. In many cases eactants and products are in a similar physical state ie all s, all liquids, or all solids but often the systems are ogeneous, ie gases and vapours reacting with solids, liquids ing with solids, or gases reacting with solids. Again latilising particles often play a major role in the reaction and processes taking place in vessels.

of the major fluid flow problems of interest is the flow ibution and residence time in vessels. Where solid materials present as particles in the flow then the distribution and ence time of the particles will also be of interest, and in cases of greater interest than the gas distribution and

ence time. Brown [19] has shown that the vessel geometry he inlet geometry have enormous and different influences on the gas and particle residence time. It is clear that fluid flow a controlling influence on many vessels and should be the r tool used in vessel design including the sizing, overall etry, location of inlet and outlets and sizing of inlets and ts (choice of velocity). Unfortunately this is rarely the case geometry, inlets and outlets being designed on the basis of anical convenience rather than process flow needs. Where ls have internal fittings such as trays, packing, catalyst beds,

then again fluid mechanics should play a major role in the n of the vessel.

Similarly separation processes, hydrodynamic performance of fluidised beds, residence time of gases and particles in vessels, etc. are all significantly influenced by fluid mechanics issues that should be taken into account during the design process.

Figure 12 Acid/alkali modelling of flash calciner Concluding Remarks The above examples are but a small selection of the many fluid mechanics related problems that I have experienced over the years. Better education has a role to play in providing industrial designers with greater understanding of the fluid mechanics issues that affect their work. There simply is not sufficient information available in a readily assimilated form on the topics of duct and vessel design or gas/particle interaction. For example a standard text on fluid flow for chemical engineers has a mere 16 pages on “Fluid motion in the presence of particles” [20]. Achieving a better understanding of even a few of these very complex problems will not be easy; indeed many of the problems described above offer an enormous challenge to researchers. However the potential benefits from a better understanding of the issues are very significant and will often be applicable across a wide range of industries. Hopefully this paper has raised the reader’s awareness of a few of the many industrial problems that still await serious study, enjoy the challenge!

Ack I shoopponeed I shoallowinduempl Refe [1]In [2]M

[3]M

[4]W

[5]Fr

Recirculation zone

[6]M

Flow interpreted from water/bead modelling

Acid/alkali modelling of fuel/air mixing

[7]Je

[8]Pe

[9]H [10]A [11]M

[12]M

[13]M

[14]H [15]H [16]H [17]G

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nowledgments

uld like to thank the organisers of this conference for the rtunity to present this highly personal view of industry’s s.

uld also like to thank those process plant operators who ed my to implement many of the pioneering (from an

strial viewpoint) modelling techniques that my company oyed.

rences

ternational Flame Research Foundation

odern Power Station Practice, Volume 2, p 9, Pergamon Press, Oxford 1971

ullinger, PJ, The contribution of modelling to several industrial problems, Inst E. Midland Branch Symposium: Modelling – Profitable applications to industrial processes

arren, S & Jones, C, Practical utilisation of physical modelling at reduced scale, Inst E. Midland Branch Symposium: Modelling – Profitable applications to industrial processes

ompovicz, ZJ, The New International Standard Method for Sampling Pulverised Coal, Current Developments in Solid Fuel Combustion Systems, Council of Industrial Boiler Owners, Cleveland, Ohio, May 1990.

odern Power Station Practice, Volume 2, p 31, Pergamon Press, Oxford 1971

nkins, BG, Moles, FD, Design of burners for kiln applications using modelling techniques, 5th International rotary kiln conference, London 1988

rry, JH, & Pleasance, GE, The influence of nozzle geometry on the development of a three-jet isothermal flow field, 8th Australasian Fluid Mechanics Conference, Newcastle, 1983

art, J, Private communication 2001

pak, G, University of Sheffield report 1971

atthews, KJ, & Street, PJ, Observations on the combustion of pitch water fuel, Jnl. Inst. E, Sep 1988, pp 129-133

ullinger, PJ, Rotary kiln firing: The problems and the solutions, 3rd International rotary kiln conference, Ocho Rios, Jamaica, 1987 (Published by World Cement, London 1987)

oles & Jenkins Private Communication to Rugby Cement, June 1977

elming, B, Cement Manufacture Vol 2 pp 104

elming, B, Cement Manufacture Vol 4 pp 263-268

elming, B, Cement Manufacture Vol 2 pp 146

orden, GD, Simulation of the Effect of Variability of Raw material composition on product Quality for a cement plant, Dept. of Chem. Eng., University of Adelaide, 2001

[18]Jenkins B.G. & Mullinger P.J. Optimising pre calciner design and performance using combustion process modelling. World Cement, April 1996.

[19]Brown G.J CFD Prediction of Particle and Fluid Residence

Time in Industrial Vessels, 6th World Congress of Chemical Engineering, Melbourne, Australia, 2001

[20]Holland, FA, & Bragg, R, Fluid flow for chemical engineers,

2nd Edition, 1995, Edward Arnold, London

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