Hydrodynamics and mass transfer in domestic drum-type fabric washing machines

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HYDRODYNAMICS AND MASS TRANSFER

IN DOMESTIC DRUM-TYPE

FABRIC WASHING MACHINES

L.D.M. van den Brekel

TR diss

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HYDRODYNAMICS AND MASS TRANSFER IN DOMESTIC

DRUM-TYPE FABRIC WASHING MACHINES

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GRAFISCHE VERZORGING Luc Bouwman,

Ruud van Drunen, Petra van Everdingen

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HYDRODYNAMICS AND MASS TRANSFER

IN DOMESTIC DRUM-TYPE

FABRIC WASHING MACHINES

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus, prof. dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen op donderdag 2 april 1987 te 14.00 uur

door

LUCAS DOMINICUS MARIA VAN DEN BREKEL

geboren te Tilburg scheikundig ingenieur

TR diss

1533

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Dit proefschrift is goedgekeurd door de promotor PROF. IR. E.J. DE JONG

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Stellingen behorende bij het proefschrift van L.D.M, van den Brekel

1. In een trommelwasmachine resulteert een verhoging van de vloeistofsnel heden door de textielstukken veelal in een versnelde aanvoer van wasactieve componenten en in een hogere uitspoelsnelheid van het vuile wassop. Hoewel de relatie tussen stofoverdracht en waswerking nog niet volledig opgehelderd is, ligt het voor de hand dat een verhoging van bovengenoemde transportsnelheden uiteindelijk zal leiden tot een sneller en beter wasproces.

2. De recent opgelaaide discussie over de optimale g-factor in trommelwas-machines wordt soms ernstig vertroebeld doordat de elkaar bestrijdende onderzoekers hun resultaten funderen op een veelheid van mogelijke testmethodieken en -mechanismen die van belang kunnen zijn voor de waswerking, zoals 'mechanical action' (MA), maximale valsnelheid, slij tage en fysische transportsnelheden.

3. In hun kritiek op de MA-methode van Szaraz, argumenteren Krü^mann en Hloch dat de botsing van het textiel met de trommelwand van geringe betekenis is voor het waseffect. Deze bewering is in tegenspraak met hun hierop volgende publikatie, waarin de maximale botsingssnelheid als uitgangspunt dient voor het berekenen van de optimale g-factor in de machine als functie van waterniveau en wasbelading.

H. Krü^mann, H.G. Hloch, Reiniger und Wascher 3 6 ( 1 9 8 3 ) 7, 30

H. Krü^mann, H.G. Hloch, G. Jellinek, Tenside Detergents 21 (1984) 2, 80

4. De rigoreuze behandeling van textiel bij het op de hand wassen wordt als zeer effectief beschouwd voor het wasresultaat. De bijdrage van de overeenkomstige textieldeformatie in een trommelwasmachine wordt echter veelvuldig overschat.

5. De bestudering van de invloed van het wasgoedgedrag op de hydrodynamica in de wasmachine en in de poreuze textielbelading zelf zou dramatisch vereenvoudigd worden als we de kleren van de keizer konden wassen.

"Groot Sprookjesboek"

6. Bij het berekenen van stromingspatronen in systemen van uiteenlopende geometrie, verdient een technologische benadering (zoals bij het dispersiemodel en bij het tanks-in-serle-model) in eerste instantie de voorkeur vanwege de fysische begrijpelijkheid en mathematische eenvoud. Pas als deze methoden te kort schieten, moet teruggevallen worden op de

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7. Bij de analyse van experimentele resultaten hebben de meeste onderzoe kers, wellicht onbewust, de neiging om gegevens zodanig te presenteren dat de praktische toepasbaarheid van het gebodene prevaleert boven de wetenschappelijke exactheid.

8. Met het natuurlijke functioneren van katalytisch actieve eiwitten in gedachten, is de term 'biologisch' voor enzymatische wasmiddelen niet

(bio)logisch.

9. Het veronderstelde vruchtbaarheidsverhogende effect van melatonine bij vrouwelijke schapen, dient bij de mogelijke toepassing van dit hormoon bij de bestrijding van 'jet-lag' mede in overweging genomen te worden.

A.M. Symons, J. Arendt, T. Poulton and J. English, Abstracts First Conference Britisch Society for Chronobiology, Cardiff, 1985.

J. Arendt, M. Aldhous and V. Marks, Britisch Medical Journal, 292 (1986) 1170.

10. Sport die zich handhaaft dankzij belastinggelden is onsportief.

11. Het verzinnen van en discussiëren over mogelijke stellingen is vele malen leuker dan het uiteindelijke resultaat.

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ACKNOWLEDGEMENTS

I would like to thank the Unilever Research Laboratory for the financial, technical and scientific support. I would particularly like to thank Dr. Ir. P.W. Appel for his helpful advice and constructive criticism through the course of this work.

The assistance given by my promotor Prof. Ir. E.J. de Jong and his enthousiasm had a very stimulating influence on the progress of the project.

Thanks are due to the chemical engineering students who carried out the majority of the reported experiments and considerably contributed to the interpretation of the results.

Finally, I would like to express a word of thanks to all employees of the Laboratory for Process Equipment for their valuable help and skilful assistance in the realization of this work.

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C O N T E N T S

Page

Summary and conclusions 1 Samenvatting en conclusies 3

1. SCOPE OF PRESENT STUDY 5

2. THE WASHING PROCESS 7 2.1 INTRODUCTION 7 2.2 HISTORY OF THE WASHING PROCESS 8

2.3 WASHING POWDER FORMULATIONS 14

2.4 FUTURE DEVELOPMENTS 16

3. REACTOR MODEL OF A DRUM-TYPE WASHING MACHINE 18

3.1 INTRODUCTION 18 3.2 MODEL OF THE WASHING MACHINE IN THE ABSENCE OF A LOAD 19

3.3 EXPERIMENTAL 22 3.4 RESULTS 24

3.4.1 Flow characteristics and mixing behaviour in

the reactor model 24 3.4.2 Model parameters of investigated drum machines 32

3.5 THE INFLUENCE OF A WASH LOAD AND ITS POROUS STRUCTURE 37

3.6 CONCLUSIONS 44

4. TRANSPORT PHENOMENA IN RELATION TO THE TEXTILE STRUCTURE:

SMALL-SCALE EXPERIMENTS 46 4.1 HYDRODYNAMICS IN PACKED BEDS OF TEXTILE 46

4.1.1 Introduction 46 4.1.2 Flow through fibrous media 46

4.1.3 Experimental 52 4.1.4 Results and discussion 54

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C O N T E N T S ( c o n t i n u e d )

4.2 MASS-TRANSFER MECHANISMS IN TEXTILE PACKAGES FOR

NON-ADSORBING COMPONENTS 62 4.2.1 Introduction 62

4.2.2 Mass-transfer model of a textile package 62

4.2.2.1 Material balances 62 4.2.2.2 Mass-transfer resistances 64 4.2.2.3 Adapted model equations 68 4.2.2.4 Analysis of the flow structure in

textile 69 4.2.3 Computer simulations of the mass-transfer

model 72 4.2.3.1 Description of the programme 72

4.2.3.2 Simulation results 73

4.2.4 Experimental 75 4.2.5 Results and discussion 76

4.2.5.1 Pulse-response experiments 76

4.2.5.2 Rinsing experiments 82

4.2.6 Conclusions 85

4.3 TRANSIENT RESPONSE OF TEXTILE BEDS TO ADSORBING

TRACERS 86 4.3.1 Introduction 86

4.3.2 Kinetic studies in packed beds by

chromatographic measurements 87 4.3.3 Evaluation of packed-bed experiments 90

4.4 THE INFLUENCE OF TEXTILE DEFORMATION ON MASS

TRANSFER AND DETERGENCY 93 4.4.1 Introduction 93 4.4.2 The deformation cell 95

MASS-TRANSFER PROCESSES IN DRUM-TYPE WASHING MACHINES

CONTAINING A WASH LOAD 99

5.1 INTRODUCTION AND LITERATURE REVIEW 99

5.2 SOLUTE TRANSFER FOR NON-ADSORBING COMPONENTS AND

ITS RELATION TO DETERGENCY 102 5.2.1 Reactor model extension in the presence of a

wash load 102 5.2.2 Experimental 105

5.2.2.1 Washing process conditions 105

5.2.2.2 Plug experiments 106 5.2.2.3 Macro-deformation versus transport

times and detergency 106 5.2.3 Results and discussion 107

5.2.3.1 Washing process conditions 107 5.2.3.2 Influence of effective tracer

diffusivities 118 5.2.3.3 Plug experiments 123 5.2.3.4 Macro-deformation 129 5.2.3.5 Soil removal in relation to deformation 131

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5.2.4 Criteria for mixing-time determination 133

5.2.5 Computer simulations 135 5.2.5.1 Development of the programme 135

5.2.5.2 Simulation results 136 5.2.6 Implications for the washing process 141

5.2.7 Conclusions 144

5.3 RADIOTRACER STUDY OF WASH LOAD MOVEMENTS USING A

GAMMA CAMERA 145 5.3.1 Introduction 145

5.3.2 Experimental 146 5.3.3 Results and discussion 147

5.4 THE INFLUENCE OF SURFACTANT ADSORPTION TO THE

WASHING PROCESS 154 5.4.1 Introduction and previous work 154

5.4.2 Experimental 155 5.4.3 Adsorption/desorption kinetics and equilibria 155

5.4.4 Rinsability of actives 160

6. WASHING MACHINE DESIGN 166

6.1 CONVENTIONAL SYSTEMS 166

6.2 INNOVATED SYSTEMS: SPRAY WASHING? 170

6.2.1 Introduction 170

6.2.2 Theoretical 172 6.2.2.1 Transfer rates 111

6.2.2.2 Oily soil removal 173

6.2.3 Experimental 175 6.2.4 Results and implications 177

6.2.4.1 Product delivery 177 6.2A.2 Rinsing efficiency 180 6.2.4.3 Washing performance 182 6.2.5 Conclusions and recommendations 186

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Appendices 188 3.1 Technical details of washing machines 189

4.1 Peclet numbers 190 4.2 Material balances 191 4.3 Computer programme (packed beds) 193

4.4 Specification of sodium alkylbenzene sulphonate (LAS) 196

5.1 Qualitative wash load behaviour 196

5.2 Fabric characteristics 198 5.3 Mathematical description of textile movements in the

inner drum 198 5.4 Output computer programme (washing machine) 199

5.5 Simulated versus experimental concentration profiles 201 5.6 Parameter sensitivity of the mass-transfer model 203 5.7 The amount of water absorbed by various fabrics 204 5.8 Experimental and simulated delivery times (KC1) 204 5.9 Parameter sensitivity of simulated delivery times (1) 205 5.10 Parameter sensitivity of simulated delivery times (2) 205 5.11 Experimental and simulated delivery times (sodium

oleate and blue dextran) 206 5.12 Output computer programme (washing machine) 206

6.1 Technical details of drum-machines of various scale 208

Notation 209 References 213

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Summary and conclusions

Nowadays, drum-type washing machines for fabric cleaning are used in almost every European household. The on-going developments in textile industry and consumer habits (in relation to convenience and fabric care) set high demands on washing powders as well as on washing machines. Therefore, the interactions between washing products and machines (particularly at lower temperatures) have gained an increas ing attention of both washing machine and detergent manufacturers, to ultimately arrive at an optimal washing performance. To support this target, in this work a chemical engineering approach has been applied to elucidate the rate-determining factors in the washing process by investigating hydrodynamics and mass-transfer mechanisms in drum-type washing machines.

A model has been developed describing the machine (in the absence of a wash load) as two ideally mixed tank reactors (corresponding to the respective volumes of water in inner drum and annulus between inner and outer drum), connected by a recycle flow. This circulation flow rate was quantified by means of a tracer-response technique. The results indicated that the exchange of wash liquor between annulus and inner drum is not rate-determining in the delivery of dissolved components to the textile surface.

When introducing a wash load, the developed reactor model had to be extended in order to account for the transport phenomena within the plugs of fabrics formed in the inner drum, which processes dominate the transfer rates of tracer materials in the machine. To obtain the key parameters of this third reactor (i.e. the volume of water enclosed in the wash load), it is necessary to elucidate the rela tions between machine agitation and the resulting hydrodynamics in the porous wash load. To this end, small-scale experiments have been carried out in perspex columns containing packed beds of textile, in which the local hydrodynamics and related mass-transfer mechanisms were evaluated by permeability measurements and by pulse-response and rinsing experiments. From this work, a mass-transfer model has been developed on the basis of the assumption that the process involved in the exchange of material is based on convection in the accessible zones of a fabric (the inter-yarn pores), and on diffusion in the stagnant zones (the intra-yarn pores). The model was verified using a computer programme, set up for the comparison of experimental results with simulated, theoretical results.

The transfer model, validated for textile packages, was successfully incorporated into the physical reactor model of a washing machine, describing the wash load as an additional reactor, parallel to the inner drum with a certain amount of axial and radial dispersion (see figure). The behaviour of the wash load plugs was found to be a very important aspect in this description, which behaviour could be moni tored by means of radioactively labelled fabrics.

Machine experiments, using various tracer materials, confirmed the conclusion from small-scale measurements that the flow of wash liquor between the yarns is the major process for solute transfer from the surrounding liquid to the interior of the fabrics, whereas diffusion

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into or out of the yarns becomes ony rate-determining in case of high pore velocities, i.e. for small amounts of laundry. Above findings emphasize the importance of increasing the wash liquor flow rates through assemblies of fabrics in a washing machine to obtain fast product delivery rates and an accelerated rinsing of loosened soil entities.

Reactor model of a drum-type washing machine: V;L - annulus, V2 - inner drum, V3 - wash load and Q, Q^ - recycle flow rates

High pore velocities within the fabrics have been realized in a modified machine design, the so-called spray machine, which operates at an increased drum speed while recirculating the wash liquor. Strongly increased delivery rates of dissolved components to the yarn and fibre surfaces were achieved in this machine at lower water volumes, possibly leading to additional energy savings. Finally, the rinse efficiency of both non-adsorbing and adsorbing (surface active) washing powder ingredients could be improved significantly at a reduced water and time consumption.

The relationship between transfer processes and detergency kinetics could not yet be fully clarified, due to a limited understanding of detergency mechanisms. However, it is tentatively suggested in this work that high pore velocities and the resulting transport rates as well as hydrodynamic forces on soil components are beneficial for the washing machine performance at lower temperatures.

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doekbelading te voorzien van radio-actieve bronnen.

Reactormodel van een tronunelwasmachine: V^ — annulus, V2 - binnentromrael, V3 - wasgoed en Q, Qi - recirculatiestromingen

Machine-experimenten met diverse tracers bevestigden de conclusie van de kleinschalige metingen, dat de stroming van waswater tussen de garens van een weefsel van doorslaggevend belang is voor het trans port van opgeloste stof van de omringende vloeistof naar het hart van de bundel textiel, terwijl diffusie in en uit de garens alleen snel-heidsbepalend wordt bij hogere poriesnelheden, hetgeen slechts voor komt bij kleine hoeveelheden wasgoed. Deze bevindingen benadrukken het belang van een verhoging van de stroming van waswater door het wasgoed in de machine om te komen tot hoge aanvoersnelheden van detergent en een sneller uitspoelen van losgemaakt vuil.

Hoge poriesnelheden in het weefsel werden gerealiseerd in een gemodi ficeerd machineontwerp, de sproeimachine, die werkt bij hogere toerentallen met recirculatie van het waswater. In deze machine kon een sterke verhoging van aanvoersnelheden van opgeloste stof naar het vezeloppervlak bereikt worden bij lagere waterinname van de machine, wat kan leiden tot verdere energiebesparing. Tenslotte kon de uitspoelefficiëntie van zowel niet-adsorberende als adsorberende (oppervlakte-actieve) waspoederingrediënten aanzienlijk verbeterd worden in een korte spoeltijd met laag watergebruik.

Het verband tussen stoftransportprocessen en waskinetiek kon nog niet volledig opgehelderd worden, als gevolg van een beperkte kennis van de waswerkingsmechanismen. Het werk in de sproeimachine geeft echter wel aanwijzingen dat hoge poriesnelheden in het textiel en de hieraan gekoppelde transportsnelheden en hydrodynamische krachten op vuil-deeltjes een positieve invloed kunnen hebben op de waswerking bij lagere temperaturen.

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1. SCOPE OF PRESENT STUDY

Drum-type fabric washing machines now hold more than 80% of the European market. Through the years, washing machine and detergent manufacturers have always stimulated each other in matters of structural machine design and detergent performance. To avoid any contradictory situation, to optimise the current process and to exploit new opportunities, the Unilever Research Laboratory and Delft University of Technology (Laboratory for Process Equipment) have started a joint research project to investigate the interactions between product and machine. In this way, the limitations of a machine in relation to the washing performance of a detergent are determined and so it becomes known within what constraints any modification of the product formulation is sensible. Studying product/machine interactions has become increasingly important in the last period of time, particularly at low-temperature washing (the so-called eco-programmes of a washing machine, saving energy and water). In addition, it may be relevant to advise machine manufacturers to make certain modifications so that the performance of detergents

(adapted where necesary) will become better.

Therefore, the main project objective is to establish rate-deter mining factors in the washing process by studying hydrodynamics and mass-transfer mechanisms in drum-type washing machines to arrive at optimal process conditions, improved product/machine interactions and better design criteria. In this way, a washing system will be created in which product and machine no longer form two separate components of the washing process, but in which they are mutually designed in an optimal way.

During this study, our main interest has not been fixed onto the various possible physico-chemical mechanisms of soil removal, as these are already investigated at the Unilever laboratory. Instead, in this thesis reference will be made to aspects related to physical transport mechanisms as a function of machine agitation, such as:

- product delivery, i.e. the supply of dissolved washing powder components to the soiled fabrics;

- soil removal by hydrodynamic forces on soil particles and transport of loosened soil to the suds;

- rinsing efficiency.

To elucidate the influence of mentioned processes on detergency (particularly at lower temperatures), a technological approach has been used in this work, considering the washing machine as a two-phase reactor system in which the behaviour of both the solid two-phase

(the wash goods) and the liquid phase (the suds) determine the machine performance.

In Chapter two some background information of the washing process is presented. Chapter three starts with the reactor modelling of a drum machine. In Chapter four, attention is focussed on the transport phenomena inside textile structures, measured in small-scale appar atus, the results of which are incorporated into the reactor model in Chapter five. Finally in Chapter six the results obtained in previous

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chapters are used to derive design parameters for both conventional and innovated washing machines.

In the majority of the washing machine experiments in this thesis, we have been using an AEG and a Zanussi machine, which does, however, not imply that this work has been carried out in cooperation with mentioned machine manufacturers.

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2. THE WASHING PROCESS

2.1 INTRODUCTION

Dealing with domestic fabric washing machines, attention is mainly focussed on drum-type machines and particularly on front loaders. This can be clearly understood from Fig. 2.1, showing the penetra tion of washing machines in Europe. The overall penetration is 77%, which covers about 100 million machines of which the greater part consists of drum-type machines.

Portugal 60 80 penetration (°/«)

Development work undertaken for drum-type machines has been relatively short. During this period, the detergents industry has had a stimulating influence on machine manufacturers, and vice versa. An example here is the development of synthetic detergents (no longer only soap-based, see Chapter 2.3), which has influenced machine manufacture. These new products caused a somewhat more rapid wear of the galvanised drums so that machine manufacturers considered to switch to other materials. The well-known soap-and soda-based detergents appeared to attack aluminium but did not do so at all in regard to zinc. For this reason, machines had to be developed which would be suitable for both products, which led to the introduction of stainless steel drums. There is also an example of developments in the field of machine manufacture that influenced the formulation of detergents. The traditional high-foaming detergents no longer performed well in

machines fitted with mechanical agitation because overfoaming occurred to a considerable extent. This triggered the development of low-foaming detergents. Also today, machine and product are not always fully geared to one another.

As mentioned in Chapter 1, the interest of detergent manufacturers in washing machine matters originates from their aim to optimise product/machine interactions and to determine within what constraints any modification of the product formulation is sensible. Some additional examples of machines placing constrains on current products are the following (Fig. 2.2):

Fig. 2.1

Penetration of fabric washing machines in Europe in 1981 (hatched, drum machines; dotted, other machines)

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dispen -sabil it v

mechanical loss

a) The dispensability of the detergent is greatly influenced by the rate of powder wetting and by the possibly resulting gelation of the powder. The geometry of the dispenser and the spray of water necessary to dispense the powder are important factors here, which may affect product delivery in the machine.

b) The mechanical loss of washing powder (i.e. the amount of powder lost by irreversible entrapment in the drain) also reduces net product delivery. The construction of the drain influences in what way this delivery can be improved by manipulating the particle size distribution of the powder granules to prevent them from sedimenting quickly.

c) Washing powders contain components protecting machines against corrosion. In addition, it was already pointed out that traditional powders were no longer suitable for mechanised processes because of strong foam formation.

In the following paragraphs attention will be paid to some historical aspects of the machine washing process, the current detergent formu lations as well as to certain developments we may expect in these fields in the coming years.

2.2 HISTORY OF THE WASHING PROCESS

Body hygiene and the cleaning of clothing have always played an important role through the ages (1). Murals in Pompeii show the washing habits of the Romans in former days. They had discovered that a mechanical treatment and an increase in temperature made the wash-goods cleaner and more rapidly so. In those days, feet and washwash-goods were cleaned at the same time by stamping with bare feet on the wash-goods soaked in urine. From this time, washing habits did not change a lot for ages. In numerous countries, wash goods were cleaned by stamping on it or by beating it on stones. Primitive equipment was also used and in the Netherlands, the washing board and brush have become relics of the past in the meantime.

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However, actual mechanisation of the wash process was undertaken some centuries ago and in 1766 Jacob Schaefer from Regensburg invented the first washing machine. The major parts were a wooden tub and a manually-operated 4-paddle agitator. Its principle is depicted in Fig. 2.3 showing a machine from the early twenties. This mechanis ation was, in fact, an imitation of manual work: moving the wash goods through the water and rubbing the articles. These 4-paddle machines have been used until the second World War.

Fig. 2.3

The first generation of washing machines

(Miele, 1921)

In 1901, the Velo factories in the Netherlands started with the manu facture of wooden tub washing machines fitted with a 4-paddle device driven by a bicycle chain. Velo's slogan in advertisements was 'A child can do the wash', and that in several minutes. The slogan is still used today although most people do not know its origin.

Around the turn of the century, the first drum-type washing machines appeared on the market. There were two versions (Fig. 2.4).

a) One version had an oval-shaped wooden barrel, which was filled with hot suds and wash goods. The barrel was slowly rotated, making the wash goods drop by gravity continuously. In this way, a good washing performance was achieved. Friction between the articles was slight so that'only little fabric wear occurred. The machines had a cloth/liquor ratio of 1:5, which- means that 5 1 of

Fig. 2.4 Early drum-type washing machines: a. wooden, electrically driven; b. metal, manually operated

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water was used for 1 kilogram of wash goods. For the 4-paddle type machine, this ratio was 1:15 to 1:20 and it is therefore very surprising that the new drum-type machines (low in water and energy consumption) obtained only a modest share of the market.

b) The other version already very much resembled the present machines. A perforated drum was rotated in a barrel and the wash goods were lifted and dropped back into the suds with the help of ribs or baffles in the machine.

The next development was the agitator washing machine (Fig. 2.5), mostly fitted with a wooden drum (later metal) with an alternating agitator driven by an electric motor. The cloth/liquor ratio was about 1 kilogram of wash goods per 15 1 of water.

With the advent of the pulsator machine, especially that manufactured by the Hoover Co., washing machines became really popular. This machine had a rapidly rotating ribbed disc (the pulsator) in its walls. It was

the fifties (see A possible of this small that it could one of its marketed in Fig. 2.6). disadvantage machine was

Against this was

Fig. 2.5

Agitator washing machine driven by an electromotor

could wash only 1.5 kilograms of wash goods. the very short washing time of 4 min.

Fig. 2.6 Pulsator washing machine (1951)

The model and price were a good reason to buy such a machine instead of the large wooden machines. The popularity of this type of machine increased rapidly because they could be hired (at a small fee) so that housewives could get used to working with this new machine. The machines were small and light so that transport was no

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problem either. Carrier-tricycles with Hoover machines have been a familiar city scene for a long time.

However, other manufacturers had established that Hoover's patent did not concern the pulsator itself but its position. This was the reason why many other designs with a pulsator on the bottom appeared on the market. As a result of increased competition, prices dropped further.

Seen this development, one would expect that the more expensive agitator and recently developed drum-type machines would become unmarketable. The reverse occurred. The housewife wanted a machine offering more possibilities

and the first step in the technical direction was the installation of electric heating in the drums, whereby temperature and wash time could be previously set by means of a thermally-controlled time-switch (Fig. 2.7).

This no doubt meant the start of developments ultimately resulting in fully-automated washing and rinse programmes of the drum-type machines as we know them today.

Table 2.1 summarizes the major parameters of the washing machines discussed.

The agitator machine is Fig. 2.7 Drum-type washing machine (1956) still used in more than 90%

of all US households

despite the fact that the automated drum-type machine was developed there. This machine was a failure from the very beginning because most houses in the US have central hot water supply systems so that there was no need to buy machines with built-in heating systems. The higher cloth/liquor ratio of the agitator machine is no problem either in view of the relatively lower price of detergents, water and electricity.

The washing performance of the pulsator machine with a ribbed pulsator in one of the walls or on the bottom was moderate, which is possibly attributable to the short washing time of 4 min. Such a short time is necessary to reduce textile wear. Practically all Japanese households have pulsator machines in which the wash goods are washed cold. The short washing time and the low wash temperature may be the reason why the Japanese wash so very often.

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Table 2.1 Characteristics of some washing machines Parameter Wash per formance Textile wear Water con sumption (1/kg wash goods) 4-paddle machine moderate high 15-20 agitator machine • good low 15 pulsator machine moderate high 20 drum-type very good low 6

Energy very high consumption (hot fill)

high (cheap energy in US) very low (cold-wash in Japan) low (depends on temperature)

The driving force for the developments sketched during the past century till around 1960 has always been the mechanisation of the washing process and to a lesser extent the automisation of the process. During the last twenty years, however, changes in both detergents as well as machines can be attributed to very different developments, which are summarized below.

o In western Europe, the wash loads consisted in 1960 (see Fig. 2.8) for 85% of cotton articles and 15% woollens. In 1970, the use of cotton and woollen articles decreases with the introduction of synthetics and polyester/cotton articles. In 1980, the quantity of pure synthetics declines whereas cotton articles show again an upward trend.

100-50

Fig. 2.8

Changes in the wash load composition i n Western Europe and the percentage of white a r t i c l e s

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o In 1960, more than 80% of the wash goods consisted of whites (Fig. 2.8) whereas in 1980 this percentage was under 20. These changes greatly affect wash programmes and the temperature of the suds. Synthetics, for instance, cannot be washed at too high temperatures.

o The washing habits of the housewife include the wash frequency which obviously corresponds to the degree of soiling of the textiles. In Europe, it has been the custom for a very long time to wash once a week (on Mondays). In the U.S. and Japan, washing is carried out much more frequently. The Japanese are the winners here for they sometimes wash once or twice a day, which means a daily water consumption of some 400 1.

o Washing conditions may be very divergent in different areas of a country. One aspect here is the water hardness. The negative influence of calcium and magnesium ions can be eliminated by adding polyphosphates to the detergent (see Chapter 2.3). In connection with environmental legislation, some countries have set limits to the use of phosphate, which resulted in the formulation of washing powders with phosphate replacers (e.g. zeolites).

o Energy savings are also an important aspect in regard to washing as there is a growing energy consciousness. In a wash cycle at 90°C, about 3 kWh is consumed (see Fig. 2.9), of which 12% is required for pumping, agitation and spinning and 85% for heating while of the latter percentage some 30% is required to heat machine parts. For a 40°C wash, only 1 kWh is used. However, we should realise that these trends to use lower temperatures have a pronounced negative effect on washing performance. It is therefore clear that all these developments make high demands on washing products as well as on machines. Consumption(kWh) 3 i 40 D i s t r i b u t i o n ' / . pumping agitation spinning l o s s e s 60 90 Washtemperature CC water 51 machine 31 [ 85 laundry 3

Fig. 2.9 Energy consumption in the washing process

Summarizing, we can conclude that there have been (and will be) many factors influencing product innovation and that a very important aspect here is the product/machine interaction.

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2.3 WASHING POWDER FORMULATIONS

A domestic detergent (1,2,3) frequently encounters dirts of widely different chemical structure and polarity such as solid soils (clay, dust), (liquid) fatty soils, (sebum, lubricating oil), stains (dyes, tea, wine, fruit, juice) and biopolymers (egg, blood). Moreover, certain fibers are easier to wash than others. The hardness of the fiber surface, which varies not only with the basic fiber but also with the surface finish, affects both soilability and soil removal. In addition, the hydrophobicity of the fiber surface also influences washability, and the more hydrophobic fibers (e.g. polyester) show greater soil retention. Soils can penetrate into the fiber. The interior of the hydrophilic cotton fiber (a cellulose fiber orig inating from cotton seed linters) is hollow, and soils may collect there. Polyester fibers are solid, but if polyester is washed above its glass-transition temperature it becomes relatively fluid and soils can penetrate into the fiber where they are nearly impossible to remove.

From above considerations, it will be clear that surfactant containing formulations have to cope with a versatility of different washing demands and conditions. The way in which a surfactant functions can be partially understood if we consider the structure of the molecule. They consist of a long fat-soluble chain (lyophilic) bonded to a water-soluble group at the other end (hydrophilic). In solution, the molecule may ionize in such a way that the hydrophilic group carries a negative charge. Anionic detergents, including natural soaps, are of this type. If the charge is positive the detergent is of the cationic type while the nonionic type does not ionize at all. A fourth class, known as ampholytic, has a charge which varies with the pH of the solution. For convenience the detergents may be summarized below (Table 2.2).

Table 2.2 Characterization of various surfactants.

Class Type Structure

I Anionic II Cationic soap alkylsulphate alkylsulphonate substituted ammonium halide RCOO-Na ROSC^-Na R-S03-Na R4 = N-Cl III Nonionic IV Ampholytic alcohol ethoxylates

alkyl araino propionic acid

R-(OCH2CH2)n-OH

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Heavy-duty or all-purpose powder detergents represent the bulk of the synthetic detergents and their formulation is a complex chemical issue. The main constituents of heavy-duty powders can be described as follows: 1) Surfactant 2) Compound phosphates 3) Silicates 4) Bleaching system 5) Foam regulator/depressor 6) Carboxy-methyl cellulose 7) Enzymes

8) Fluorescer and perfume.

1) Surfactant: The enhanced cleaning effect of a powder is caused primarily by the presence of the various surfactants described earlier, which act by altering interfacial effects at the various phase boundaries within the system (wetting of soil and substrate, soil removal and suspension). Mixtures of anionic and nonionic detergents are commonly used in todays products.

2) Compound phosphates: Sodium tripolyphosphate (STP) is the most widely used and most effective builder in heavy-duty fabric washing compositions. Builders remove hardness ions from the wash liquor and this prevent them from interacting with the surfactant and fatty soil components. Such interactions reduce detersive effectiveness. Phosphates are strong sequestrants for calcium and magnesium and provide additional suspending action for soils.

3) Silicates: Sodium silicates function primarily as alkalies in the wash liquor. In addition, they act as anticorrosive agents and prevent deterioration of washing machines, specifically metal pump parts.

4) Bleaching system: Sodium perborate is commonly present in significant amounts in European laundry detergents. At high washing temperatures, sodium perborate (producing hydrogen peroxide in the suds) effectively bleaches chemical stains such as wine, fruit, juice etc. By addition of tetra-acetyl ethylene diamine (TAED) to the system (i.e. a precursor which produces peracetic acid by reaction with perborate) effective bleaching has been achieved at lower temperatures (30-60°C).

5) Foam depressor: As copious foam volumes interfere with the efficiency of the washing process and may result in overfoaming of the machine, anti-foams are incorporated in the detergent formulation.

6) CMC: Addition of sodium carboxymethyl cellulose is extremely effective in the prevention of soil redeposition onto the washed articles.

7) Enzymes: Proteolytic enzymes interact with certain proteinaceous stains and contribute to overall washing performance.

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8) Fluorescers/perfumes: The sensory package of a detergent formulation includes fluorescers and perfumes. Fluorescent whitening agents absorb ultraviolet radiation and subsequently emit some of the radiation energy in the blue part of the visible spectrum. As a result, they confer enhanced whiteness to the appearance of washed articles.

To incorporate above components into a washing powder, the spray-drying process has been developed. When processing these powders, the synthetic base is usually in the form of a paste, to which are added the various other ingredients in a mixing tank. The wet slurry is well mixed and then forced through a series of fine jets at the top of a tower to meet an upward current of hot air. The wet slurry has turned into fine granules by the time it reaches the floor of the tower and is carried away for final processing and packaging. Engineering and tower design are among the important factors determining the final character of the granular powder, but physical factors are also relevant. These include the concentration of the slurry, the proportion of the various ingredients and the content of occluded air. The latter parameter has an important effect on the bulk density of the beady end product, which density may be increased by deaerating the slurry immediately before spray-drying.

2.4 FUTURE DEVELOPMENTS

In paragraph 2.2 it has been shown that many washing machine manufac turers did tests in the past to improve the washing process. At present, their emphasis is mainly on convenience and economy and we may think here of the incorporation of "sportswear" and low-temperature programmes. However, such programmes adversely affect bleaching action and washing performance. Consequently, much innovative effort is required to develop new bleaching systems and enzyme applications for washing powders to be used at lower temperatures. Moreover, machine manufacturers have the technical know-how to build in appliances taking over certain detergent functions such as water softening and bleaching. These machine adaptions might ask for modified washing powder formulations.

Finally, there are developments in the direction of automatic dosing in an integrated machine-product system only requiring setting of degree of soiling and water hardness, after which the machine doses the required amount of (liquid) detergent. Such an automatically controled machine might be realized by constructing special sensors. Product development for these systems should go hand in hand with the monitoring of developments in machine construction, research into product/machine interactions and with the modelling of drum-type washing machines.

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References Chapter 2

1) Wasmachines, wasmiddelen en waseffect, Modern huishouden 3, 1970, 8-17.

2) Kirk-Othmer, Encyclopedia of Che mical Technology, 3rd. Ed. , Vol. 11 (1978) 393.

3) Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 5 (1979) 77.

4) L. Chalmers, Domestic and industrial chemical specialities, 1955, Leonard Hill, London.

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3. REACTOR MODEL OF A DRUM-TYPE WASHING MACHINE

3.1 INTRODUCTION

Cleaning of dirty textile is always performed under some form of mechanical agitation which promotes soil removal. A literature review (1) shows that an enormous amount of research has been done with regard to the influence of agitation on detergency. From all this work it was concluded that the exact influence of agitation is badly understood and at best empirically correlated under specific conditions with flow characteristics (shear stress) and deformation phenomena (rubbing, folding). The only conclusion these research efforts had in common was that the degree of agitation and the therefrom resulting mass-transfer rates are governing mechanisms in the overall soil removal kinetics. So it may be possible that, although a washing powder can give very acceptable results when physico-chemical aspects are considered in small-scale experiments, the overall soil-removal rate can be reduced to a large extent by physical transport limitations in a washing machine.

In a washing machine, a number of processes takes place more or less consecutively (Fig. 3.1). These are: disintegration/dissolution of detergent powder, penetration into fabrics, soil removal/suspension, rinsing, spinning and drying. The major part of these processes are governed by physical transport phenomena, such as the hydrodynamics in the system and the related mass-transfer rates. A shortening of the duration of the washing process is only possible by accelerating the slowest step in the mentioned series of transfer resistances. In addition, an improvement of the machine performance may well be obtained by a fast supply of detergent components (e.g. to avoid calcium-fatty acid deposits) or by an increase of hydrodynamic forces on soil particles. product/ water/ machine -interactions dismteg dissolut sedimen ation on ation

ïïlf

/

mixing, ' transport to inner drum formation / reactions spinning Fig. 3.1 Consecutive processes in a drum-type washing machine determined by product/ water/ machine interactions

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Therefore, we have to characterize and quantify the hydrodynamics and physical transport mechanisms in drum-type washing machines instead of defining empirical correlations. The washing machine has to be considered from a technological point of view, viz. as a chemical reactor which has to be modelled using chemical-engineering principles.

The first step in this model is formed by a description of the hydro-dynamic behaviour in a washing machine not containing any wash load. This description is required to determine the contribution of the hydrodynamics in the unloaded machine to the transport resistances under practical circumstances, i.e. in the presence of a wash load. In this chapter, experiments are presented to verify the proposed reactor model and to measure the relevant model parameters.

3.2 MODEL OF THE WASHING MACHINE IN THE ABSENCE OF A LOAD

A drum-type washing machine consists of two concentric cylinders of which the inner one is rotating alternately to the left and to the right with a standstill period in between. The drums are partially filled with water. Because the inner drum is provided with baffles, the liquid is lifted and leaves the drum through the holes on one side, passing the annulus, and returning through the other side of the drum. Therefore, the washing machine could be considered as a series of two ideally stirred tank reactors, in each of which the liquid is perfectly mixed, connected by a recycle flow (2). The direction of the recycle flow alternates corresponding to the direction of rotation.

If there is no washload present in the inner drum, the system can be represented by the two-reactor model given in Fig. 3.2 in which V^ corresponds to the liquor volume in the outer drum and V2 corresponds to the liquor volume in the inner drum, connected by recirculating water having a flow rate Q.

ÜJ

C O

Vl v2

'

Fig. 3.2 Reactor model of a drum-type washing machine in the absence of a wash load

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The response of the system to a tracer pulse in the annulus at t=0 is governed by the following differential equations:

v

i d T = Q <

c

2 " V

[ 3

-

1 ]

IS

"2 dt x v"l " "2'

V, TT = Q (C, - C0) [3.2]

with boundary conditions: 1) t = 0 2) t > 0

cx = c

l i 0

; c

2

- o

VxCi + V2C2 - V i C ^ Q = (VX + V2) C

= v

c

a

The solution of equation [3.1] is given by:

C2 = e (1 - exp (t/r)} [3.3]

C! - e,» (1 + V2A l exp (- t/r)) [3.4]

V V vl 2

in which r = [3.5]

The parameter r - which has the dimension of [s] - can be regarded as the characteristic response time of the washing machine in the absence of load. Evidently, this response time will be longer if the watervolume V in the machine is larger or the recirculating flow rate Q lower.

The value of the flow rate Q can be calculated from the slope of a semi-log plot of the concentration versus time if the volumes of liquid in inner and outer drum are known. This important parameter could not be established by previous investigators.

It is clear that the equations are valid only if the liquid in the machine is agitated continuously, which is not the usual situation. Therefore we have to find a correction for the real agitation times for the relevant machine rhythms.

To this end, we consider the time needed to reach a certain percentage of the equilibrium concentration after a concentration

change due to e.g. injection of a tracer. For example, tm (5%) is

the time after which the measured concentration deviates less than 5%

of the equilibrium concentration. These times are given by tm^ for a

machine operating at continuous agitation.

For a machine that has an alternating direction of rotation with periods of standstill in between, the corrected mixing time will be given approximately by:

tm0 = tm - n ts [3.6]

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The n in this equation corresponds with the number of standstill periods during mixing of the tracer material and can be calculated for a known machine rhythm by an entier function:

(t + t )

r s ( tr + Cs

[3.7]

The approximation in Eq. [3.7] is allowed as the value of n is in general considerably larger than unity (especially for short rhythms and in the presence of a load).

Substituting Eq. [3.7] into Eq. [3.6] yields

(1 - _{(t + t )'}

r s lt + t r s

[3.8]

From this equation it determined only by tm

these times ts and tr are not important.

can be

and the ratio ts/t

seen that the mixing time t„, The absolute values of

The time tm has to be considered as a corrected experimental mixing

time based on continuous agitation. In this way experiments carried out for various machine rhythms should give the same equilibration times of an injected tracer. The parameter between brackets in the second part of Eq. [3.8] (x 100%) is shortly referred to as the percentage of agitation. In most of the current washing machines, this percentage is approximately 50.

The number and the size of the holes in the inner drum can be used to estimate the recycle flow rate Q by considering respectively the total surface area available for liquid flow and the pressure drop across each hole. A relation between the latter and the liquid velocity in such a hole can be obtained from the Bernoulli equation (3) assuming that the fluid velocity before the drum perforation is negligible with respect to the fluid velocity in the holes. In that case, the velocity in a hole can be expressed by

vh c J 2 Ap/p~ [3.9]

with Ap = pressure drop across holes

c — flow coefficient, which is a combination of a contraction factor and a resistance factor

- 0.9 for smooth perforations.

As shown in Fig. 3.2, a hydrostatic pressure drop occurs across the holes of the inner drum due to the lifting of the liquid by the baffles. When this lifting height is estimated from machine observation to be about 5 cm, we may apply equation [3.9] to calculate the liquid velocity in the drum perforations:

Ap = 500 N/m2 p - 1000 kg/m3

c = 0.9

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Assuming that in the perspex model washing machine the liquid can flow through about three rows of holes (i.e. 35 holes, approximately covering the above mentioned 5 cm of the inner drum wall) with a size dft - 5.5 mm, we can now roughly estimate the magnitude of the recycle flow rate:

Q = vh • 35 • f d^2 - 0.75 1/s

Referring to the experiments mentioned in paragraph 3.4, an experimental . value is found of Q = 0.4 - 1.2 1/s, which is in good correspondence with the above calculated value.

In addition, the recycle flow rates have been determined as a function of the water intake of the machine and the revolutional speed of the inner drum. Finally, some measurements were carried out to examine the influence of a wash load on the hydrodynamics in the annulus. To do this properly, the exchange of liquid with the wash goods had to be excluded by wrapping up the wash load in big plastic bags.

To verify the above proposed model, pulse-response techniques were used, measuring electrical conductivity of a tracer (potassium chloride). It has to be noticed the reactor model gives a description of the liquid phase, not disturbed by the presence of a wash load. To account for the latter will be very complicated; the way in which this problem can, in principle, be overcome will be discussed in a later section of this chapter (paragraph 3.5).

EXPERIMENTAL

The experiments were carried out in a (down-scaled) model perspex washing machine (top loader), in a Zanussi Z925X and an AEG Lavamat Princess 803 (both front loaders). The technical details of these three machines are given in Appendix 3.1.

Pulse-response experiments were carried out using 100-200 ml of a 2M potassium chloride solution as a tracer material and the conductivity of the liquid was measured with Philips conductometers (PR9501) or Scottgerate Konductometers (CG 851). The signals of both annulus and inner drum were registered on two-pen recorders.

The distribution of the wash liquor over annulus and inner drum (in the absence of fabrics) was measured for all three machines as a function of the total water intake. These graphs allow the recycle flow rate Q to be calculated (as a function of rotation speed, liquor volume, number of holes, etc.) from the slopes of the semi-log plots of concentration in annulus and inner drum versus time. The intercepts of these lines with the Y-axis are applied to calculate the ratio V^/V^. This ratio can be compared with the value obtained from the geometrical volume distribution. In addition, the water volumes in annulus and inner drum are used to check the tracer material balances.

In all three machines, the potassium chloride solution was injected into the annulus directly or via the machine dispenser. In the latter case, a small time-lag was introduced, which could be neglected

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compared to the observed mixing times. During injection, care was taken that the tracer solution would not go directly into the inner drum, but was fastly dispersed in the annular zone instead. The concentrations at different locations in the annulus (Fig. 3.3) as well as in the inner drum were measured by means of electrical conductivity. To do this, conductivity cells were inserted into the outer drum or samples were taken from annulus and inner drum by means of a piston pump after which the flows were led through conductivity cells. The machines were

provided with Pt 100 elements to correct the conductivity signals for small temperature vari ations. It was necessary to calibrate the conductivity cells present inside the investigated machines due to induction effects of the metal parts of the machine during rotation. Fortu nately, these effects were no function of liquor level, the presence of a wash load and the speed of rotation of the inner drum.

Most of the experiments were carried out at continuous agitation (no periods of standstill) in tap water of room temperature.

All conductivity signals were, if necessary, cor rected for the dead times in the measuring tubes.

To study and visualize tl

washing machine, little polystyrene balls (p = 1017 kg/mJ) were added to the liquid in the AEG. To minimize the settling velocity of these flow followers in the machine, a sodium chloride solution (20-25 g/1) has been used instead of pure water. When the flow followers moved in front of the inner drum (between the inner drum and the front plate of the outer drum) pictures were taken with a Leica camera

(MDa 1205-264) provided with a bellows and a 50 mm lens. These pictures were used to determine the fluid velocity in the outer drum, which velocity can be related to the magnitude of the recycle flow rate Q. Also, to visualize the flow pattern in the annulus of the perspex washing machine, little circular pieces of paper were added to the liquid and video images were recorded.

23 \

\

dispenser configuration a drain conductivity meter recorder

Fig. 3.3 Perspex model washing machine with apparatus for measuring electrical conductivity

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The velocity profile in the annular space between inner drum and tub (i.e. on the bottom of the machine) have been determined qualitatively by scanning the annulus with woollen yarns. To do this a small perspex plate was inserted in the annulus (parallel to the axis) , consisting of five small metal pins (on equidistant intervals on the plate) to each of which three woollen yarns were fixed. These yarns follow the direction of flow.

RESULTS

Flow characteristics and mixing behaviour in the reactor model

To check the proposed two reactor model for an unloaded drum, pulse-response measurements were carried out in the perspex machine with continuous agitation (N = 35 RPM) and a water intake of 5-10 1. The corresponding water volumes in annulus and inner drum can be taken from Fig. 3.4.

In a first series of experiments, conductivity was measured at two different locations in the annulus with 100% agitation using only one conductivity cell (Fig. 3.3). The results of these measurements are given in Tables 3.1 and 3.2.

T 1 1 1 1 1 1 1 1 1 r

1 2 3 4 5 6 7 8 9 10 II

V

[']

Fig. 3.4 The distribution of water over annulus and inner drum

respectively as a function of total water intake

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One of the main things that draws attention is the different shape of the experimental concentration curves (Figs. 3.5 and 3.6). When the sampling point is a short distance from the injection point, a concentration maximum is found after a very short time (< 1 second) followed by a slow decrease until the equilibrium concentration is reached. The shape of the curve after the maximum concentration has been reached, is in agreement with the shape predicted by the two-reactor model.

As can be seen in Fig. 3.6, there is not a maximum present in the concentration curves measured further away from the injection point. Moreover, in this case the tracer is only detected after a short (dead) time (one to three seconds).

It will be clear that the semi-log plots of concentration versus time are not in agreement with the theory either. In most of the experiments these figures show two straight lines of different shape having an intersection after very short time (~ 3 s, see Fig. 3.7). For the case the conductivity has been measured according to configuration a, the concentration at this intersection approaches the theoretical initial annulus concentration C]^ Q , as defined in the boundary conditions of equation [3.1] and [3.2], whereas the measured initial concentration peak is much higher. The theoretical initial annulus concentration can be calculated from the injected amount of tracer, using the experimentally determined volume V^ as a function

Table 3.1 Experimental determined flow rates between inner and outer drum in a continuously rotating drum (L - 0 kg, perspex machine) V [1] 5 5 10.5 10.5 5 5 10.5 10.5 T Is] Conf 2.95 2.92 2.89 3.10 .guration Configuration 2.68 2.70 3.03 3.50

v

x

v

2 V [1] a 1.20 1.20 2.54 2.54 b 1.20 1.20 2.54 2.54 Q [1 s-1] 0.41 0.41 0.88 0.82 0.45 0.44 0.84 0.73

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Fig. 3.5 General form of the experimental concentration-time profiles for machines without load (configuration a)

Fig. 3.6 General form of the experimental concentration-time profiles for machines without load