[Handbook of Powder Technology] Granulation Volume 11 || Chapter 14 Detergent granulation

CHAPTER 14

Detergent Granulation Renee Boerefijn,* Prasanna-Rao Dontula and Reinhard Kohlus

Unflever R&D Vlaardingen, P.O. Box 114, 3130 AC Vlaardingen, The Netherlands

Contents 1. Introduction 673 2. Detergent powder ingredients 675 3. Detergent powder properties 677

3.1. In-use properties 679 3.2. Detergent powder handling 681 3.3. Stability 682

4. Granulation technologies 682 4.1. Base powder 683 4.2. Adjuncts 687

5. Granules for tabletting 687 6. Structure of detergent powders 688

6.1. Phases in a detergent granule 690 6.2. Granule design 692

6.2.1. Maximising liquid content 692 6.2.2. Retaining porosity 693 6.2.3. Example of structure effects on powder properties: granule dissolution 694

6.3. Techniques to measure granule structure 695 6.3.1. Scanning electron microscopy (SEM) 695 6.3.2. X-ray tomography 696

6.4. Quantification of particle structure 698 6.4.1. Amount 698 6.4.2. Sizes 698

7. Future directions 700 Acknowledgements 700 References 700

1. INTRODUCTION

The powder detergent market has a worldwide volume of about 14 million tons per annum, and about 50% of this global volume has traditionally been supplied by three large companies: Procter & Gamble, Unilever and Henkel, with furthermore Lion and Kao as strong regional contenders in Asia. It comes therefore as no surprise that three standard texts on powder detergents exist, each strongly rooted

*Corresponding author. E-mail: [email protected]

Granulation Edited by A.D. Salman, M.J. Hounslow and J. P. K. Seville ~'~ 2007 Elsevier B.V. All riahts reserved

674 R. Boerefijn et aL

in one of these companies [1-3]. The sheer volume of detergent powders and, as a consequence of an equally sizeable R&D effort, the conception and realisation of largely similar technologies by the main players of the industry with all kinds of inevitable complexities make it a suitable case for this handbook.

Granulation gradually became of interest to the detergent market in the 1960s in response to environmental pressure, and especially due to the following:

�9 need to reduce chemicals usage, �9 advent of heat-sensitive, weight-effective materials, �9 strict air pollution regulation, �9 pressure to reduce packaging (move towards higher bulk density powders), �9 drive to lower water consumption and �9 drive to lower energy consumption.

with respect to spray-drying. Only when Kao introduced true compact Attack powders in 1987, did the larger companies follow swiftly and in force. Compact powders became synonymous with high quality and high efficacy, specifically suited to markets with high machine penetration, although here too the reduced dissolution propensity of densified powders manifested itself. This lasted until the mid-1990s, when the advent of tablets under the convenience cloak broke the fresh dominance of compact powders, and this high-tech segment is now more and more taken over by liquids, often in capsules. High-shear granulation yielded products with bulk densities that were 50% higher than that of spray-dried powders. Consumer habits did not adjust to this very quickly, and as an intermediate position, several technology combinations and new technologies were developed, such as fluidised-bed granulation.

As powder detergents are produced in large bulk volumes to serve markets requiring up to several million tons per year, each company has developed a specific technology base and its own terminology (Table 1). Typically, the powder detergent manufacture process requires drying, mixing and densification, though not necessarily in this order.

Table 1. Reference texts and terms

Configuration (order of increasing density) Unilever [1] P&G [2] Henkel [3]

Spray-drier- Mixer

Mixer- (Mixer-) Drier

Mixer- Drier- Compactor

Tower- Post-Tower (TPT)

Non-tower route, Dry mixing Fluidised-bed granulation Granulation/ agglomeration

Granulation, tabletting

Post-tower densification

Dry neutralisation

Compaction, pressure agglomeration

Premix process

Non-tower agglomeration

Compound process

Detergent Granulation 675

This applies specifically to powders containing surfactants and builders, i.e. base powders, which make up typically between 30 and 90 wt% of the product. In addition to this, there may be granular or spray-on admix components, e.g. enzyme, perfume and bleach.

In contrast to other industries employing granulation, the amount of binder or liquid phase is not negligible in detergent manufacture. All components are func- tional and are typically added for superior final application and not for a special purpose in the granulation process. One tries to incorporate as much liquid as possible in the granule, rather than trying to just bind the solids together with the least amount of liquid possible, making optimum use of two features of the ingredients: (i) the carrying capacity of the available solids, and (ii) the precursor state of liquid and solid ingredients to generate the desired components in situ. Furthermore, the "liquid-to-solid ratio" is a governing parameter of the granulation process and determines granulometry and granulation kinetics. This key differ- ence between detergents granulation and conventional granulation invokes a different perspective on the role of the phase volume ratios that are a crucial part of the granule structure. The final section of this chapter will therefore mainly focus on the structure of detergent granules. Sections preceding this will address detergent powder ingredients, properties and granulation technologies. Given the large extent of available literature, e.g. in the above-mentioned texts and the plethora of patents, this chapter will mainly focus on recent additions.

2. DETERGENT POWDER INGREDIENTS

The wash process is a complex kinetic process and typically consists of the following steps that occur either sequentially or in parallel: water treatment, soaking and/or swelling of the clothes in the detergent solution, physical and/or chemical dirt removal, stabilisation of the removed soils in the wash solution and the rinsing of soils and chemicals from the wash load. In addition, modern detergents also include ingredients to deliver benefits such as softness and fresh- ness. Given the numerous wash conditions, e.g. water hardness, water temperature, soak time, agitation level, product dosage, soil type and level and amount of clothes, laundry detergent formulations are complex and contain several different types of ingredients. These are typically classified into: surface- active agents (or surfactants), builders and other additives, such as bleach systems, enzymes, polymers, foam regulators and fragrances. Detailed discussion of the various ingredients and their performance aspects can be found in various specialised books [1-4]. This section briefly summarises some of the common ingredients in detergent powders and their properties of relevance to the manufacture of free-flowing detergent powders via granulation.

676 R. Boerefijn et aL

The main ingredients of fabric cleaning detergents are surfactants. The surfactants used in fabric cleaning detergents are mainly anionic and non-ionic surfactants and their mixtures. A small amount of cationic surfactants may also be included. The main anionic surfactant used is the sodium salt of linear alkyl- benzene sulphonate (LAS). Other anionic surfactants include soaps (salts of fatty acids), primary alkyl sulphates (PAS), ~-olefin sulphonates (AOS), alkyl ether sulphates (AES) and methyl ester sulphonates (MES). LAS acid is highly stable and upon neutralisation with an alkali source, such as an aqueous solution of caustic soda or sodium carbonate, will form a waxy solid. In spray-drying, LAS may be neutralised prior to slurry-making (neutralised LAS paste), or in the slurry itself. The spray-drying slurry is kept pumpable and stable against separation by manipulating the rheology of the mixtures of various phases of surfactant and water in the slurry with extra electrolytes, hydrotropes and water. In the so-called "dry neutralisation", LAS acid is combined with an alkali source in high-shear mixers. LAS acid may be partly or fully neutralised in a loop reactor prior to introduction into the high-shear mixer [5]. This ensures that LAS acid is com- pletely neutralised in the detergent powder and leads to improved powder quality because any residual acid can discolour the powder, degrade the perfume lead- ing to bad odour or even make the granules and powder less resilient. Neutralised LAS, i.e. the sodium salt, is a waxy, hygroscopic solid phase, which at relatively high phase volumes may cause granules to lose their strength under compression and shear. Strengthening the surfactant phase in such situations by intimate mixing with finely divided solids or using other surfactants and polymers is rec- ommended [6]. The principal non-ionic surfactants used in detergents are fatty alcohol ethoxylates with, typically, less than 10 ethoxylate groups. These surfactants, though solid at temperatures below 20~ have a pour point in the range of 25-40~ above which they transform into a liquid with viscosity less than 100 mPa s. Such liquid-like surfactants are often retained in granules by mixing them with anionic surfactants or granulating them with finely divided solids.

In order to remain within flammability and/or (dust) explosivity limits, e.g. should fast rotating parts generate a spark, sizeable amounts of inert material need to be incorporated during high-shear granulation of high organic material (liquid, solid, acid) containing compositions.

The other components in a fabric cleaning detergent include:

�9 So-called "builders" whose role is to augment the surfactants in the wash. In addition to deactivation of calcium and magnesium ions in the water and thus preventing them from interacting with the surfactants and soils, builders may also provide alkalinity and also help to keep detached soil from redepositing onto the fabric. Common builders include sodium tripolyphosphate (STPP), various forms of zeolite, sodium silicate, nitrilotriacetic acid (NTA), layered sili- cates and sodium citrate.


�9 Sodium carbonate and sodium silicate whose role is to provide the necessary alkalinity and buffer capacity to maintain the pH at the desired value during the wash.

�9 Electrolytes including sodium carbonate and sodium sulphate that provide the necessary ionic strength.

�9 Organic non-surfactant additives are present in small amounts to serve one or more specific functions, such as to reduce redeposition of soil from the wash onto the fabric (e.g. sodium carboxymethylcellulose (SCMC)), to sequester heavy metal ions (e.g. phosphonates and sodium citrate), to reduce corrosion of machine parts and to maintain fabric whiteness (e.g. fluorescers and various blueing agents). Bleaching agents - such as sodium perborate and sodium percarbonate - and their activators - such as tetraacetyl ethylenediamine (TAED) - are widely used in Europe to provide effective bleaching at low water temperatures. Detergents for machine application may also include foam regulators to either augment the foam or reduce it depending on the application. Enzymes are also increasingly being used in detergents and specifically target proteinaceous stains (proteases), starches (amylases) and fatty esters and triglycerides (lipases).

�9 Fillers such as sodium chloride, clays and calcite. �9 Fragrances.

Table 2 lists some common solid ingredients in detergent powders and their properties, as collated from various handbooks. Here, RH refers to relative hu- midity at ambient pressure (closely related to water activity, aw), and LCC refers to the liquid carrying capacity, i.e. the maximum liquid-to-solid ratio using a standard liquid such as linseed oil, 3EO non-ionic surfactant or dibutyl phthalate [7]. The LCC of various solids is an important parameter for detergents granulation. The various hydrates indicate the ability of these solids to bind water and thus make them unavailable for carrying surfactants in the powder that can lead to poor powder properties.

The combination of various surfactants with these ingredients to formulate detergents for fabric cleaning is treated by various authors, e.g. Ho [8] and Smulders [3].

3. DETERGENT POWDER PROPERTIES

Three types of powder properties are relevant, those which affect

�9 product performance in relation to consumer habits; �9 product handle-ability in relation to manufacture, storage and transport; and �9 physical stability in relation to climate.

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Some properties have multiple relevancies; for example, the bulk density of a product is an important conversion factor for use in carton packaging machines, which typically are not weight but volume controlled, whereas the packs are sold on weight. At the same time, bulk density has an important impact on the per- ceived quality of a product; high density is typically associated with premium quality. Table 3 lists properties, measurements and indications of their relevance, as will be discussed in following sections.

3.1. In-use properties

Handwash consumers, still ~75-80% of the global market, demand rapid dissolution of the product, typically within 0.5-1.5min. Also in markets with high automatic washing machine penetration, robust dissolution behaviour of detergents is growing increasingly important as environmental awareness and regulation drive washing temperatures and water consumption down. Especially relevant for users of front-loader automatic machines is the dispensing behaviour, which is a complex interaction between the kinetics of dissolution and hydrody- namics of the dispenser drawer, as shown in Fig. 1. In most dispensers, water between 10~ and ambient temperature emerges in the form of narrow streams of liquid onto the powder and along the edges of the dispenser drawer. Some water, drawn in by the action of capillary forces, penetrates into the loose powder bed and displaces the air within. The rest flows either over or around the powder in streams and into the drum of the washing machine (Fig. 1).

Several processes then occur simultaneously: granule dissolution/disintegration, surfactant swelling, viscous phase formation and dissolution, electrolyte hydration and dissolution, granule agglomeration on account of the greater "sticking potential" conferred by partial granule dissolution and the convective transport of granules. If all of the above proceed as desired (to be defined), the powder is dispersed and dispensed into the drum within 30-60 s. Powders that dispense quickly, i.e. in 15-20s or less, dispense in spurts during which (relatively) dry portions of the powder bed break away, are lifted up by water and dispensed. If the powder does not dispense as desired, dispenser residues result. Dispenser residues are chiefly of two types. The first is a soggy, often slimy, paste of partially dissolved granules, surfactant and water. The second is a hard lump, progressively less wet from the outside to the inside of the lump (but dry compared with the first type) in which the individual granules do not appear to have dissolved much. Figure 1 also shows the forces acting on particles inside the powder bed and on the surface. The greatest force acting in the dispenser is that of buoyancy: on the powder bed with air trapped in it as a whole or on each particle. However, it acts only when water has penetrated the powder bed. The impact of the water jet on the powder bed and its subsequent transmission into

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STREAM OF WATER FROM ANOTHER JET

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Granule Weight

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Forces on an internal granule Forces on a surface or near-the-surface granule

Fig. 1. Schematic of water flow in and around the powder bed in a dispenser.

the bed is not shown. Interparticle forces are also not shown. EP0451894 [6] gives an example of a well-dispensing detergent.

Dispensing behaviour may be measured by mimicking the dispensing process itself and measuring the remaining residue after a given time of dispensing from a standardised commercial dispenser. The chief parameters are geometry of the dispenser, flow rate and temperature of the dispensing water and dispensing time.

3.2. Detergent powder handling

Granules require special care in handling, and as the technology grew more or less organically from post-tower operations, which include spray-drying as an early unit operation, to separate systems, the layout of granulation plants is often determined by existing systems and buildings. Through various handling steps, such as belt conveying, belt-belt and belt-hopper-belt transfer, screw feeding, etc., size reductions of up to 30% may occur. Hoppers are often emptied by belts running underneath at speeds up to 1 m s -1, and normal loads may be considerable. Granules may experience tens of impacts at up to 10 m s -~, shear at normal loads in excess of 30 kPa, rates above 100 Hz and compression at loads

682 R. Boerefijn et al.

- # Location Exit of the granulation process Sieve unit Temporary base powder storage Admix collector belt

~',~'::~;~,~,~,,J 5 Drum mixer and sieve unit - ~ a s s _ ~ o w hopper, feeding packing units

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Fig. 2. Typical post-process handling plant layout (bars indicate transfer belts).

above 50 kPa. Screw feeding and pneumatic conveying [14] may result in size reductions of up to 25%, each accompanied by large amounts of fines generated. This is why often bucket elevators are preferred for vertical transport.

Figure 2 depicts a typical handling system, starting from the exit of the base powder production process, passing through a bucket elevator and a sieve unit via transfer belts to storage hoppers and finally through a loss-in-weight feeder onto an admix collector belt. Then the powder may be transferred via a second bucket elevator into a drum mixer that includes a perfume spray, through a final quality sieve (admix components are commonly not sieved before mixing) and then into a mass-flow hopper feeding storage bins or packing units.

3.3. Stability

Typically, highly soluble materials such as detergent powders also exhibit hygroscopicity, and "powdering" or dry-layering (e.g. with zeolite)is common practice to prevent caking. Layering may take place at any stage after the formation of initial granules. A tight control over the zeolite dosage is required to prevent dustiness and lack of flowability while preserving caking protection.

4. GRANULATION TECHNOLOGIES

Extensive layout diagrams and specific operating parameters for most of the processes described below may be found in Ref. [1].


4.1. Base powder

We recall that base powder commonly contains surfactant and builder, and consti- tutes 30-90% wt of the total product. It is commonly made via the routes indicated in Table 1. As surfactant often forms a soft or waxy solid phase within the granules, granule strength has to be obtained by an efficient construction of a solid network throughout the granule. This requires micromixing of liquids and solids, and is commonly performed in high-shear mixers. Perhaps counter-intuitively, while mixing is on-going, granule growth has to be delayed as much as possible in order to maximise the liquid load [15].

As it arose out of post-tower densification, after elimination of the spray-dried powder, the granulation process used in the detergent industry is commonly termed the "non-tower process". Typical layouts are as shown in Fig. 3, and comprise a high-shear mixer, followed by another moderate to high shear mixer and then usually followed by a conditioning step (cooling, drying), e.g. in a fluidised bed. For non-tower granulation [16-18], equipment of choice commonly comprises a L6dige Recycler (CB-type) and Ploughshare (KM-type). Appel [19] lists a number of equipment manufacturers commonly found in the industry.

In the process depicted in Fig. 3, the anionic feed can be partly or fully neutralised. The second stage (ploughshare) serves mainly for densification, and distribution of the layering agent. It can also be replaced by a recycler unit. Liquids can be pumped or sprayed in. Typical residence times in the recycler are of the order of tens of seconds, whereas in the ploughshare it may be above 1 min. Residence time in the fluidised bed may amount to 30 min. For plant

�9 ~ I

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ploughshare

hot hot cool air air air

Fig. 3. Typical layout of a non-tower detergent granulation process.


flexibility and better control of product quality, in the early days of non- tower granulation, spray-dried base powders were used as carrier materials. Nowadays, admixtures of non-tower and spray-dried base powders may be used to achieve the same. Conversion kinetics of the surfactant precursor neutralisation depend largely on surface renewal, which occurs in the first mixer at high tip speeds, generating a crumbly dough of up to 20vo1% porosity. In the second mixer, this dough-like material is densified and spheronised and the resulting granules have at most 10 vol% porosity. Throughputs of several tens of tons per hour are common.

Only recently have satisfactory scaling rules for high-shear granulation of LAS granules been published [20]: tip speed and apparent viscosity, which may be grouped in the typical Ennis and Tardos' critical Stokes number to constitute the balance between break-up and sticking force [21] as well as the volumetric liquid- to-solid ratio are indicated to be the essential parameters. This analysis has a limited scope to systems employing highly viscous binders and fine carrier solids, as is the case with LAS and zeolites. It clearly shows how closely the process passes by the wet-mass region in the Litster map of deformation vs. saturation [22] at which the entire hold-up turns into a single paste.

If spray nozzles are fitted in the fluidised bed depicted in Fig. 3, a fluidised-bed granulation system arises. This may be used to advantage to obtain a better control over the particle size distribution and the bulk density in the intermediate range between spray-drying and non-tower granulation [23, 24]. A typical layout of this system is shown in Fig. 4. The surface area of the fluidised bed is typically 10-40m 2 and residence times of the order of tens of minutes are common. Equipment of choice includes those supplied by Ventilex and Niro. The fluidised bed is commonly operated in plug flow mode by suitable choice of distributor plate (gill orientation). The premixer before the fluidised bed can be run either in batch or continuous mode. Throughputs can be as above or much lower, e.g. several tons per hour in the semi-batch mode. Two-phase nozzles are typically used here.

Fig. 4. Typical layout of a fluidised-bed granulation process.


Fluidised-bed granulation is a self-limiting growth process. The operating airflow yields a superficial gas velocity in the fluidised bed, which corresponds to the minimum fluidisation velocity to be calculated using the Ergun equation [25], of the largest granules; those larger will settle and be unavailable for futher growth. At the same time, the elutriation or terminal velocity sets the limit on the smallest particles or granules; any smaller will be blown out. The elutriation velocity can be calculated using drag correlations [26]. The premixer, commonly a LSdige recycler or ploughshare, is used to extend the particle size range to smaller, normally not fluidisable particle sizes, owing to elutriation and/or co- hesivity, which exhibit high liquid carrying capacity.

Extensive research has resulted in the quantification of the dominant controls for stable operation of fluidised-bed granulation to prevent wet-quenching [27], and to prevent granulation in the case of a coating process [28], as depicted in Fig. 5. The flux or Akkermans number expresses the balance of the binder spray- flux and the solids recirculation rate through the spray-zone.

Aspraypp(Us - Umf) Akk - F N - log 10 (1)

qb

0.9 # Qb(FN=2) kg/hr

0.8

.~ 0.7

0.6 . . . .

$ 0.5

,T 0.4

t~ 0.3

0.2

. FN < 3.5 0.1 # ' Granulation

0 ~ " ~ ~

0.7

0 0.2 0.4 0.6 FN > 3.5 Coating 1 1.2

Superficial Gas Velocity (m s -1)

Fig. 5. Typical granulation regime map for fluidised-bed operation.


The Akkermans number is also a useful tool for scale-up of fluidised-bed granulation systems [29]. Furthermore, the unique relation between the Akkermans number and the growth rate constant used in population balance modelling allows a priori determination of the growth rate constant [30]. Adequate description of granulation kinetics, in addition to reliable sensor technology, is the main chal- lenge for online control [31,32], which can be in part alleviated with this approach.

Fluidised-bed granulation is an intrinsically robust process with moderate shear, which allows for more controlled structure formation of granules. If the binder solidification can be boosted by chemical reaction and a fine crystal dis- persion within it, strong and porous granules may arise as shown in Fig. 6, which allow a granule to break away from surface limited, slow shrinking core dissolution behaviour [33]. This is described further in Section 6.2.

Figure 4 depicts the high-shear mixer, used to pregranulate a portion of the binder with the fine solid carrier to extend the carrying capacity, as a separate entity. The Schugi Flexomix is an example of a fluidised bed with integrated high- shear impeller, as can be used to produce detergent base powders [34].

Some less common process routes for base powder production exist as well:

�9 the Unilever VRV process [35-37], which employs a flash-drier with a thick rotor shaft and short blades with small wall clearance to produce granules containing well in excess of 50 wt% anionic surfactant (Fig. 7) and

�9 the Henkel Megaperls extrusion process, which employs a cooled twin screw extruder to mould a mixture of spray-dried base powder and other liquids and solids into highly spherical and uniform particles [3, 38, 39].

+,a

~D

0

0 ct~ 3

Particle Size (arb units)

Fig. 6. Schematic influence of granule mesostructure on granule dissolution time.


air

FQ ~ U m m m U ~!;UiJ;lm ml

' I J__l- LAS acid air ~

P R O D U C T

Fig. 7. Layout of the VRV process capable of manufacturing high surfactant-containing granules.

4.2. Adjuncts

Adjuncts are commonly defined as granules containing high levels of minor ingredients, which may individually be added at levels between 0.2 and 20 wt%. Notable examples are enzymes, anti-redeposition polymers and bleach. In order to maintain good control over the bulk density of the final mixture, a simple mixing rule may be employed if the granule size distributions of the individual components are reasonably similar (which in the absence of cohesion is a prerequisite to avoid segregation):

1 x/ BDmix- ~ ~ with ~ x / - 1 (2)

n n

It appears that the non-tower process in its essentials, i.e. a high-shear mixer and a fluidised bed, is the new standard not only for base powders, but also for adjuncts, such as

�9 TAED bleach precursor [40], which is often provided with an acid coating for stability;

�9 silicone antifoam [41], which is processed anhydrously with a starch carrier; �9 builder granules [42], which are typically bound with a surfactant or polymer; �9 perfume granules [43], which are typically encapsulated; and �9 enzyme granulation [44, 45], which may contain cellulosic fibres or film-forming

polymers for increased resilience and solubility [46, 47].

5. GRANULES FOR TABLETTING

Designing granules consisting of a mixture of materials with a complex mechanical response, including elasto-viscoplastic, for incorporation of tablets of a few centimetres in size is well beyond the scope of most of the available literature,


which typically addresses small pharmaceutical pills made of virtually pure, highly elastic substances [48], with exception of the work of Adams and co-workers [49, 50]. Existing techniques for quantification of compaction behaviour are still useful, as summarised by Celik [51].

Providing a unit dose for laundry applications requires compacting between 30 and 100g of powder into 1 or 2 tablets, resulting in a considerable size tablet, typically ,~2 cm in height and ~4 cm in diameter, which affects both solubility and strength. Functionality of the tablet relies on a suitable trade-off between the two. Commonly a brick-and-mortar system is employed, with the mortar providing for the integrity and bricks for rapid dissolution. Henkel and P&G rely mainly on swelling cellulosic polymers respectively inside and around the tablets [52-54], Unilever to some extent on phosphates [55]. Tablet strength is commonly ex- pressed as diametral fracture strength (DFS), a so-called "Brazilian test" for tablets. Tablets of powder mixtures depend in a complex way on the constituent properties, as quantified by Van Veen [56].

DFS may be related to a composite yield strength (CYS) as follows:

CYS = a - b DFS (3)

where

1 _ - n ~ xj (4) CYS ~0, i

with ~ x / = 1 and ~o,/the Kawakita yield strength as determined by bulk com- n

pression of single component beds [57]: bed compression tests using a mould of similar diameter to the rotary press and plotting stress P vs. strain ~ allows for the determination of ~o from

In P - c ( s + In(~~ (5) \ ( Z / '

Repeating this measurement at different starting bed heights, plotting ~:o as a function of initial bed height and extrapolation to the abscissa yields ~:o,/. Param- eter a is proportional to the maximum compaction force and b to the compaction speed. Knowing the formulation and the target DFS for a tablet and ~o,/of the remaining components, the target ~o,/of a new granule to be incorporated may now be specified. Evidently, design rules of a granule for a specified strength are next in order as part of granule structure formation.

6. S T R U C T U R E O F D E T E R G E N T P O W D E R S

A detergent granule consists of three major components: the primary particles (solid), the detergent (liquid or soft solid) and porosity (gas). The amount, size


Table 4. Relation between basic powder properties and structure

Property Relation to structure

Bulk density Attrition Compressibility Bleeding Solubility

Dispensing

Intra- and infra-granular porosity Shape (asperities) Phase volume ratios Liquid retention in micro-/mesopore structure Shrinking core vs. disintegration, viscous phase formation

(can be suppressed by ionic strength or hydrotropes), water ingress

Drag and buoyancy (size, density) vs. phase formation and dissolution

and distribution of these three phases determine the granule structure. The granule structure is generated by the process route and conditions and is a free handle to optimise product properties (Table 4), within the limitations imposed by the formulation.

The term "structure" is widely used but not well defined and therefore needs further specification for technical use. The structure of a system is related to the manner in which the system is internally built up from its basic components. As agglomerates are multiple component systems, the structure of granules or agglomerates will be defined as "the spatial arrangement of its basic components" [58]. Typically, a structure definition is combined with length scale information such as macro-, meso- and microstructure. In the case of particulate systems, this would be the powder bed structure, the granule structure and the structure of the basic components itself, e.g. crystal structure of primary particles.

The quantification of structure has several aspects as depicted in Fig. 8: the amounts of various components, their sizes and the manner of their assembly, in particulate systems, the amounts of the basic building blocks are the most important variables that define the internal spatial arrangement (or granule structure). The granule porosity is of special importance because it is not predetermined by the formulation, but a parameter affected if not controlled by the formation process. At the next level of detail, the size of the spatial phases formed is of interest. And last but not least, the distribution of the phases through the system defines the homogeneity of the structure and its composite behaviour. All these measures just quantify the structure of an isotropic system. The granule shape or its outer morphology, as well as radial gradients, is not taken into account here. Therefore, one would additionally use shape descriptors, which are well known [61], and radial distribution functions, which give the radial depend- ence of the concentrations of the various phases.


Type/Scale

Macro Powder bed / Tablet

M e s o

Granule

Micro Raw Materials /

molecular level

Amount

BD/ Bed porosity

Phase volume / Particle porosity

Formulation

Size

Particle Size Distribution

Chord length / Covariance function

Raw material characteristics (e.g. PSD solid)

Distribution

Pore size" tablet / powder bed

Covariance function / distance distribution / radial distribution function

Spacings, crystal types

Fig. 8. Definition and overview of granule structure parameters [59, 60].

6.1. Phases in a detergent granule

A detergent base granule is chemically composed of inorganic salts, surfactants and some water. The behaviours of these groups of components are distinctly different and do not necessarily mix. The salts are typically solids, the surfactants are liquid-like or soft solids. A detergent granule therefore has at least two well-defined separate phases: a solid phase and a liquid phase. The liquid phase, typically consisting of surfactants and water, binds the solids during the granulation process; thus it is often termed the "binder phase". Besides these two distinct phases, entrapped air or porosity forms the third phase in a detergent granule.

Phase volumes have the largest impact on the granule properties. This is, for example, the well-known effects of the granule porosity on dissolution and bulk density, or that of the liquid-to-solid ratio (L/S) and granulation index on the granulation process [20]. The granulation index is defined as the ratio of L/S and the LCC of the solids. In granulation science, this has been captured in the so- called capillary state of the granule. The different types of granule structures are schematically depicted in Fig. 9 and can be described as

(a) solids that are just bound together by some binder (pendular state); (b) well-bound solids with interconnected porosity (funicular state);


( ( (d)

Fig. 9. Granules in varying capillary state as defined by Rumpf [62]: (a) pendular state, (b) funicular state, (c) capillary state and (d) droplet state.

P I F -

a) Dense granule b) Porous granule c) Agglomerate d) High porosity

Fig. 11). Different types of detergent granules containing surfactant [58, 60]: (a) dense granule, (b) porous granule, (c) agglomerate and (d) high porosity.

(c) liquid-filled solid assembly bound by capillary forces at the boundary (capillary state) and

(d) a droplet with some solids inclusions and no porosity (droplet state).

All these types can be found in detergent granules. Figure 10 depicts generalised structures as described above. Examples of

cross-sections of detergent granules are shown below the four schematic structures in the figure. The dense system depicted in Fig. 10(a)is typical for a high- shear mixer granulation process, e.g. European non-tower detergent powder (Section 4.1). Almost no porosity is found and the coarse solids are not densely packed. Figure 10(b) shows a sodium LAS adjunct manufactured via dry neutralisation and containing a lot of porosity generated by carbon dioxide released during the neutralisation process. Figure 10(c) shows an agglomerate of primaries. The primaries may either be pre-granulated material or relatively coarse raw material solids. Here the porosity has become the predominantly continuous phase rather than the solids or the binder phase. Binding of the primaries is the main issue in this type of structure. The given example is a granule bound by a


melting-type binder and produced in a fluidised bed [63]. The last type of granule structure depicted (Fig. 10(d)) is one where the porosity is entrapped by a shell formed by bridging particles, rather than porosity being an interstitial space between attached primary particles. This requires some "blowing action" as often found in non-disperse systems such as polymer foams, or products manufactured by the reactive foaming process such as bakery products produced using sodium or ammonium bicarbonate, or citric acid [11, 34, 64]. Here binding between primaries is crucial to retain the high amount of porosity and still form a mechanically strong granule. The low bulk density of the fluidised-bed granule based on sodium sulphate generated in situ is an example of such a granule that shows a high amount of porosity and rapid dissolution [33]. Looking at the variety of granulation processes on offer, it is clear that the granule structure can be varied even further. Figure 10 also schematically depicts the variation in porosity in granules produced via different processes.

The properties of a granule are a direct consequence of the granule structure and the characteristic of the used raw materials. Hence an optimisation process of granule properties needs a systematic approach based on an understanding of granule structure formation.

6.2. Granule design

6.2.1. Maximising liquid content

Design of a granulated powder typically starts with a formulation. This formulation determines the mass fraction of the powder ingredients. The so-called process aids may be used if cost and formulation space and regulations permit. One would run through the following steps and decision points when faced with the task of designing a manufacturing process.

The amounts of liquid and solid components are given when a formulation is specified. The volume fraction of each component can be calculated using the densities of the components. The volumes of the liquid and solid phases then follow by summing the volumes of all liquid component and solid components, respectively.

The next question to be answered is "How to create a dry granular structure with the given amount of solid particles to accommodate the required amount of liquids?" Being the first dimension of the structure space, the amount axis is fixed; the other two dimensions are the free parameters. This means that the size and distribution of the phases need to be adjusted to design the granule. The most natural way to create a dry liquid-solid system is that of a liquid-filled particle packing wherein the solids are densely packed and touch each other to form a disperse but percolating solid network- a skeleton. The free room between the solid particles can then be filled with liquid without changing the spatial


Fig. 11. An example of a brick-and-mortar structure.

Fig. 12. Sequential packing of primary structures.

arrangement of the solids. Such a structure would appear solid-like because the mechanical properties are governed by the percolating solid network. We call this a brick-and-mortar system (Fig. 11). The phase volumes here are determined by the packing behaviour of the solids, which can be roughly predicted by particle packing theory, e.g. using the Kerner equation.

Filling the porosity of the packing only partially enables higher liquid contents. This has its limit in the binding capacity of the liquid, at least when the liquid is the binding material. Higher amounts of liquid can be realised by distributing the solids and liquids in a designed way. The brick-and-mortar system shown in Fig. 11 is a random homogeneous distribution of the solids and liquid. A sequential packing of granules from the first process that results in brick-and-mortar primaries is a straightforward route to obtain a structure with a higher liquid content or higher liquid-to-solid ratio (Fig. 12).

6. 2. 2. Retaining porosity

The air content or porosity can be approached in a manner similar to that described for the liquid content. However, the desired level of porosity ('s is not a specified formulation component, but is determined by the desired physical


90% - - - rel. porosity change b

80% - - - ~ 5% . /

~-m .... 10% . , ~ m r

..... 20 % j / ' -

_-- ............. :.~,~:..~ .............. 30% ~ , ' ~

/ 100%

0.00 0.10 0.20 0.30 0.40

"r ' 7 0 % - ,.__,

6 0 % - o')

5 0 % -

o 4 0 % -

ra 3 0 % -

,...| 2 0 %

0.50

Par t i c l e p o r o s i t y [-]

Fig. 13. The influence of particle porosity on bulk density of a granular system (cf. equation (6)).

properties, especially bulk density (BD) and speed of dissolution. The bulk density can be calculated as

BD - Psolids(1 - - ,%ed)(1 - - ,~granule) (6)

The bed porosity (Sbed) depends on particle shape and particle size distribution and cohesiveness of the powder. For a normal detergent powder, the bed porosity may be initially approximated to be 0.5. If the bed porosity remains constant and only the granule porosity varies, then the bulk density variation is as depicted in Fig. 13. This figure should be read as in the following example: suppose we have a powder with granules of 20% porosity, and we increase the porosity by 100%, then the bulk density is reduced by 25%.

6.2.3. Example of structure effects on powder properties: granule dissolution

Granule dissolution speed is primarily determined by the granule size and its distribution. One would think that it is really the surface area of the powder that determines this dissolution speed. However, surface roughness and asperities are dissolved away quickly, so that it is really the granule size that determines the kinetics of the dissolution process. The speed of dissolution may also be viewed as the time required for an amount of material per disperse element to get into solution. This can again be altered by the granule porosity; the higher the granule porosity, the smaller the relative volume to dissolve per granule and hence, the quicker the dissolution.

Promoting disintegration or crumbling of granules by manipulating the granule structure is an alternative method to influence the dissolution speed. A granule will disintegrate when the binding elements between the primary particles are


either dissolved or broken. A bond between two primaries in a granule will dissolve before the primaries if it is readily accessible to the surrounding water and of discrete size. A granule in funicular or pendular state demonstrates this behaviour when the binder is soluble. Breaking of the bonds is typically achieved by a swelling material (disintegrant) or by an effervescent action. The dissolution time of a granule in the ideal disintegration case is determined by the time needed for the disintegration process and the time needed to dissolve the primary particles generated via disintegration.

tdissolution - - tdisintegration -4-- tdissolution of disintegration products (7)

Under the assumption that these disintegration products dissolve by a shrinking core mechanism, the dissolution of the disintegration products is solely determined by their size. This is a relatively safe assumption for the granule structures typical to detergent powders. The size of the disintegration products is likely to follow from the size of primaries already in the granule and can be measured by X-ray tomography (Section 6.3) and used to estimate the dissolution time. The time taken to dissolve a collection of granules/primaries purely dissolving via the shrinking core mechanism varies as d 2 under stagnant conditions (external mass-transfer controlled process), whereas it varies as d 118 if the same collection of granules is stirred (internal-diffusion controlled process). An example can show the order of change to be expected by disintegration. Combining the two effects of disintegration and dissolution, dissolution times of granules can be dramatically lowered. For example, a typical slow dissolution time of a 500 l~m fraction of granules would be 50 s (as measured by conductivity release indicative of 90 vol% dissolved, cf. Table 4). If the granules are composed of primaries each about 150 l~m in diameter, the dissolution time measured in the same manner would be 12 s plus the time needed for the initial disintegration or crumbling process. This is exemplified in Fig. 6 (Section 4.1).

6.3. Techniques to measure granule structure

Before a structure can be quantified, a measurement is needed to provide quantitative data for a structure analysis. Since structure is defined in this article as the spatial arrangement of the basic components, the measurement technique should give a two- or three-dimensional image of the structure in every instance. These images will then be analysed to derive the quantitative information needed to predict the granule behaviour.

6.3.1. Scanning electron microscopy (SEM)

Electron microscopy has become a standard tool for the visualisation of microstructures [66, 67]. The advantages of this technique are its high spatial resolution and good material contrast, both of which result in a good ability to


BG

Fig. 14. (Above) Scanning electron micrograph of two different granules in back scattering electron mode. (Below) Elemental mapping via EDX mode of the right-hand image [58].

distinguish and identify the phase of a granule or composite. As SEM is a two- dimensional technique, the third dimension must be chosen representatively. This is done by carefully slicing granules near their meridian plane. Figure 14 shows two example granules. The main images are made in a back scattering electron mode. This technique already gives an element-dependent contrast, which can be analysed. Elemental scans of the right-hand image are also included below these two images. Here, the concentration of a selected element is presented semi-quantitatively. These images allow for the composition of the identified phases to be qualitatively determined. The first example granule is a compact homogeneous granule with excellent mechanical properties. The second example shows a very different open structure, although the components per se are relatively homogeneously distributed.

6. 3.2. X-ray tomography

Real three-dimensional techniques have the advantage of excellent statistical ba- sis. This enables especially to check the assumption of isotropic structure. The available three-dimensional techniques are non-destructive and based on compu- ter tomography. For micro-tomography either X-ray absorption or magnetic resonance (Magnetic Resonance Imaging, MRI)is used. In the case of granules, the higher spatial resolution of the X-ray tomography, up to 1 ~m pixel -~, is advan- tageous. Figure 15 shows an image of a slice through a compact granule obtained by X-ray tomography. The right-hand part of Fig. 15 shows the image after seg-


Fig. 15. X-ray tomography slice through a granule, before and after segmentation [58].

mentation of the phases. The different phases could be separated using an al- gorithm based on the greyscale histogram of the image. The contrast between the phases is sufficient to identify and analyse the structure of agglomerates.

6.4. Quantification of particle structure

Extensive quantification of granule structure requires the use of stereological methods as described by Kohlus [58, 60]. Stereology is the field of spatial statistics and especially useful to characterise composite materials. An excellent introduction can be found in Underwood [68]. In this section, we focus on a simple quantitative description of granule structure. Granule structure has been introduced as a combination of amount, size and distribution of the constitutive phases. The various phases in a detergent granule were identified as solids, binder and porosity (air).

6. 4.1. Amount

The amounts of material that can be mixed to form a dispersed system are specified by the volume ratios of the materials and not their mass ratio. This is dictated by simple steric effects; the amount must pack together to fill the space. The feasible ratios are purely volume-based. Air and liquids cannot transfer forces without flow. Capillary forces are typically strong enough during granule formation but not during handling and storage. The achievable liquid-to-solid ratios are not changed by the inclusion of air.

6.4.2. Sizes

The size of the phases or primary structures within a granule directly affects the granule strength and dissolution behaviour. At present, extensive finite-element


studies and dissolution simulations are needed to theoretically assess these properties [69]. A much more direct approach is the comparison of the desired properties for various size fractions of the powder.

The size of an object is easily described by the diameter of the sphere of equivalent projection area; however, reality is more complex. In order to obtain detailed, quantitative size information of multiple continuous phases, one needs to somehow separate the phases. A common technique would be to use the sizes of the biggest spheres that fit inside the phase. This needs a three-dimensional data space and is thus volume-centred. The use of linear analysis is an unbiased method to generate the size distribution of a continuous phase. This technique measures basically straight point distances, and gives thereby detailed size information. Figure 16 shows a typical profile of a granule structure starting at the highest value and decaying quickly to zero. This hyperbolic trend indicates a high probability of choosing a short chord. Weighting the number distribution with the chord length results in a distribution in which the occupied area of the chord is depicted, assuming a standard thickness of a chord [68]. The curve is typically skewed to the left. This measure can be interpreted directly as the free distances between two points of a given phase.

The methods described above result in a set of distributions or functions, which allow generation of statistically similar structures. For use in property or process functions, scalar parameters are needed to avoid convolution operations. A set of key descriptors also enables an unambiguous comparison of different structures. These descriptors should capture the amounts of the phase volumes and their sizes as well as a homogeneity measure. In summary, the phase volumes of the

0.30 0.08

7 "

E=. 0.25

o O "

E o 0.20 . . {3 L

~5 0.15 o c "

0.10 l _

i _

. . 0 E 0.05

z

0.00

= Number frequency distribution, q0

- - -~ . - - Length frequency distribution, ql 7 -

E

- 0 . 0 6 ,--

\ ._o

0.04 .~

c

0.02 ~-'- t -

O 3

- " - , , , , ~ . _ . . . . . . . . . . . . . ~ ~ , - ~ ~ _ ~ i ! r . . . . . . . . . . . . . . . . . . ,!- . . . . . . . . . --i-

10 20 30 40 50 60 Chord length, I(#m)

0.00

Fig. 16. q0 and ql of the chord length distribution [58,60].


solid, binder and void phase are the descriptors on the amount axis. The sizes can be covered by mean diameter of the chord length distribution. These are generally defined as

- ( f o rnax qo( I ) . I p. dl~ (1/(p-q)) Lp'q -- k fLmax--qo-(~-).i-~ -~ J p V= q (8)

where qo(/) denotes the number frequency distribution of the chord length distribution and/ the chord length. The average length inside a phase would be given by/-2,1, the length-weighted mean length. Inside a granule, the elements of each phase are typically closely packed. The nearest-neighbour distance of two elements of a phase is not of great interest as it is typically close to the elements size; what is of interest is the mean free distance between two elements of a phase. The mean free distance ,t; between objects of phase/ is given by

1 - ( D i

,ti-/-1.0; r (9)

where d)i denotes the phase volume of phase i and L1,0 the arithmetic mean length. For more details see Refs [68, 70]. The volume equivalent of the mean free distance between phase i would be the volume-weighted star volume of the inverse volume to phase i. While the mean free distance mainly applies to the solid phase, the star volume approach mainly applies to the more continuous phase as binder phase and porosity. The volume-weighted star volume of an object is the volume seen from an interior point of the object averaged over all interior points and is given by [70]

--3 ~V--~ L3,0 (10)

This quantifies the distance relations inside a granule. The contact area between the different phases still remains to be characterised. The direct measure is the volume specific surface area or the surface-to-volume ratio of each phase, which is directly related to/-1.0. The radial homogeneity can be quantified by the coefficient of variance of the radial distribution function. In addition to phase volume, each phase is described by four key structure indicators: mean free distance, star volume, specific surface area and coefficient of variance for the radial distribution function.

The analysis of distribution characteristics requires stereological methods and lies beyond the scope of this chapter. It is explained in detail by Reed [70] and Kohlus [59]. The main focus there is on covariance functions that are also called one- and two-point correlation functions. They describe the spatial phase distribution for the reconstruction of physical measures of three-dimensional bodies from two-dimensional images. These techniques are not unique to detergents, but clearly benefit from the fact that generally a mixture of different elements exists within a detergent granule, enabling easy identification.


In summary, the advent of novel measurement techniques enables quantification of microstructures for the explicit relation to process conditions and configurations.

7. FUTURE DIRECTIONS

Environmental concerns continuously drive down the use of chemicals in detergents. This implies further concentration by using more targeted delivery systems and less process aids [8]. However, as long as consumer habits do not change dramatically, and a dosing unit of powder is still a scoop, granulation will be driven towards more lean-structured granules, incorporating more porosity and also capable of containing and retaining liquids with lower melting points to operate in lower temperature and low water washes [16, 66, 67].

Product microstructure holds the key to product performance and should be the focal point in product engineering. Scale up of processes [29, 63] should now proceed by the preservation of strict relations between growth (and breakage) kinetics, such as the Akkermans or Flux number, and microstructural descriptors. Though implicit parameters such as bulk density and particle size distributions may still be useful as intermediate control parameters while online product structure quantifiers are unavailable, processes should be designed, scaled up and operated to build a desired product microstructure.

ACKNOWLEDGEMENTS

The authors wish to acknowledge Unilever for permission to publish, Dr Terry Instone (formerly UR&DPS), Profs Mike Adams (UR&DPS), Joel de Coninck (Mons-Hainaut), Mojtaba Ghadiri (Leeds), Mike Hounslow (Sheffield), Hans Kui- pers (Twente), Jonathan Seville (Birmingham), their co-workers, and Paul Mort III (P&G) and Andre Groot, Roland van Pomeren, Kees Montanus, Jan Akkermans, Manske Tammes, Michel de Ruijter, Remy Verburgh, our (former) colleagues at Unilever R&D Vlaardingen, for stimulating discussions and fruitful collaborations.

REFERENCES

[1] W. Herman de Groot, I. Adami, G.F. Moretti, The Manufacture of Modern Detergent Powders, Herman de Groot Academic Publisher, Wassenaar, The Netherlands, 1995.

[2] M.Showell (Ed.), Powdered Detergents, Marcel Dekker, New York, 1998. [3] E. Smulders, Laundry Detergents, Wiley, Weinheim, Germany, 2002. [4] P. Zini, Polymeric Additives for High Performing Detergents, Technomic Pub. Co.,

Lancaster, USA, 1995. [5] E. Wilms, Process for Producing Anionic-Tenside Containing Washing and Cleaning

Agents, Henkel patent EP0641380, 1995. [6] M. Curtis, F.H. Gortemaker, B. Rezzani, High Bulk Density Granular Detergent Com-

positions and Process for Preparing Them, Unilever patent EP0451894, 1991.


[7] Testing of Carbon Blacks; Determination of the Dibutylphthalate Absorption of Carbon Blacks, DIN53601, 1978, www.din.de

[8] L.T.T. Ho, Formulating Detergents and Personal Care Products, AOCS Press, Champaign, Illinois, USA, 2000.

[9] W.D. Emery, T. Instone, R. Kohlus, Particulate Laundry Detergent Compositions Containing Nonionic Surfactant Granules, Unilever patent WO027031222, 2000.

[10] J.V. Boskamp, M.P. Houghton, C. Joyeux, C.A. Rowe, C.E.J. van Lare, G.M. Verschelling, P. Zuidguest, Detergent Compositions, Unilever patent WO9514767, 1995.

[11] M.G. McGoff, S.E. Stephans, H.H. Tantawy,C.C. Driffield, Process for Preparing a Foam Component, P&G patent US6706773, 2004.

[12] R. Lopez, Process for Preparation of Detergent Granules, Unilever patent US2004014629, 2004.

[13] R.M. Wise, Detergent Composition and Process, P&G Patent US4166039, 1979. [14] T. Taylor, World Congress Particle Technology 3, IChemE, Brighton, UK, Paper 160,

1998. [15] D.W. York, 7th Int. Symp. on Agglomeration, Albi, Volume 1, pp. 59-64, 2001. [16] P.W. Appel, P.L.J. Swinkels, Process for Preparing a High Bulk Density Granular

Detergent Composition, Unilever patent EP0367339, 1990. [17] F. Bortolotti, L. Pietrantoni, Detergent Compositions and Process for Preparing Them,

Unilever patent EP0390251, 1990. [18] P.W. Appel, L.D. van den Brekel, Process for Preparing High Bulk Density Detergent

Compositions, Unilever patent EP0420317, 1991. [19] P.W. Appel, J. Surf. Det. 3 (2000) 395-405. [20] S.L. Rough, D.I. Wilson, A.E. Bayly, D.W. York, Trans. IChemE, Part A, Chem. Eng.

Res. Des. 83 (A1) (2005) 7-23. [21] B.J. Ennis, G. Tardos, R. Pfeffer, Powder Technol. 65 (1991) 257-272. [22] J. Litster, B. Ennis, L. Liu, The Science and Engineering of Granulation Processes,

Kluwer, Dordrecht, 2004. [23] S.V. Dhalewadikar, V.R. Dhanuka, Granulation in a Fluidized Bed, Unilever patent

WO9604359, 1996. [24] V.J. Bakker, A. Kaess, Process for Preparing Granular Detergent Compositions,

Unilever patent WO0077146, 2000. [25] S. Ergun, Fluid Flow through Packed Columns, Chem. Eng. Prog. 48 (1952) 89-94. [26] R. Clift, J.R. Grace, M.E. Weber, Bubbles, Drops and Particles, Academic Press,

New York, 1978. [27] J.H.M. Akkermans, M.F. Edwards, A.T.J. Groot, C.P.M. Montanus, R.W.J. van

Pomeren, K.A.R. YQregir, Production of Detergent Granulates, Unilever patent WO9858046, 1998.

[28] M.I. Wasserman, M.W. Ridyard, S.W. Capeci, W.E. Beimesch, P.R. Mort III, Process for Coating Detergent Granules in a Fluidized Bed, P&G patent WO0078912, 2000.

[29] R. Boerefijn, M. Klaver, M.J. Hounslow, Scale-up of a fluidised bed granulation process from batch to continuous based on similarity of growth Kinetics, in: U. Arena, R. Chirone, M. Miccio, P. Salatino (Eds), Fluidization XI, Engineering Foundation, New York, 2004, pp. 427-434.

[30] R. Boerefijn, M.J. Hounslow, Chem. Eng. Sci. 60 (2005) 3879-3890. [31] P.R. Mort, S.W. Capeci, J.W. Holder, Powder Technol. 117 (2001) 173-176. [32] J.G. Reuvers, C. Heffels, W. Wammes, M. R&dle, K. Reindel, J. EttmQIler,

R.E. Nitzchmann, Process for Monitoring and Control of Industrial Granulation Proc- esses, BASF Patent EP1213051, 2001 (Priority document DE10061085, 2000).

[33] B.B. Sampaio de Menezes, L. Valli, Production of Detergent Granulates, Unilever patent WO9900475, 1999.

[34] J.H.M. Akkermans, A.T.J. Groot, C.P.M. Montanus, Process for the Production of Detergent Granules, Unilever Patent WO03016454, 2003.


[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48] [49]

[5o]

[51] [52]

[53]

[54]

[55] [56] [57] [58]

[59] [60] [61] [62]

[63]

W.D. Emery, K. Metcalfe, P.J. Tollington, Production of Anionic Surfactant Granules, Unilever patent WO9606916, 1996. W.D. Emery, K. Metcalfe, P.J. Tollington, Production of Anionic Surfactant Granules by In Situ Neutralisation, Unilever patent WO9606917, 1996. W.D. Emery, W. R,~hse, J. Jacobs, Production of Anionic Detergent Particles, Uni- lever patent WO9732002, 1997. F.-J. Carduck, H. Pawelczyk, W. R~,hse, J. Jacobs, Process for the Production of Granules of a Detergent, Henkel patent DE4007601, 1991. F.-J. Carduck, H. Pawelczyk, W. R~hse, J. Jacobs, E. Smulders, and G. Vogt, Man- ufacture of Compacted Granules for Washing Agents, Henkel patent EP0486592, 1992. A. Dovey, A.G. Sorrie, Low Gelling Detergent Compositions and a Process for Making such Compositions, P&G patent WO9403568, 1994. B. Michel, P. Descamps, G. Nachon, S.F.A. Creutz, S. Stassen, Granulation Process, Dow Corning patent WO02090476, 2002. W.D. Emery, A.T.J. Groot, T. Instone, S.D. Liem, K. Metcalfe, R.G. Smith, G.M. Verschelling, Free-Flowing Particulate Detergent Compositions, Unilever patent WO9854289, 1998. T. Nebashi, S. Yabe, F. Sai, Y. Izumi, T. Fujieda, Detergent Composition Containing Clathrate Granules of a Perfume-Clathrate Compound, Kao patent US4992198, 1991. E.K. Markussen, A.W. Schmidt, Enzyme Granulate Composition and Process for Forming Enzyme Granulates, Novo patent US4106991, 1978. P. Bach, H.K. Vilsbell, C. Sommer, A Method for Improving Particle Compositions, Novozymes patent WO2004033083, 2004. P. Bach, Method for Preparing an Enzyme Containing Granule, Novozymes patent US2005054068, 2005. M. Ghani, Protein-Containing Granules and Granule Formulations, Genencor patent WO0129170, 2001. P. York, Drug Dev. Ind. Pharm. 18 (1992) 677-721. M.J. Adams, I. Aydin, B.J. Briscoe, S.K. Sinha, J. Non-Newtonian Fluid Mech. 71 (1997) 41-57. G. Lian, Y. Xu, W. Huang, M.J. Adams, J. Non-Newtonian Fluid Mech. 100 (2001) 151-164. M. Celik, Drug Dev. Ind. Pharm. 18 (1992) 767-810. G. Blasey, D. Jung, Process for Preparing Household Detergent or Cleaner Shapes, Henkel patent US6506720, 2003. W. R~hse, B. D. Larson, C. Block, H.-J. Welling, Detergent for Use in Washing Machines or Mechanical Dishwashers, Comprises Wash-Active Agent Enclosed in Polymer, Includes Disintegrating Agent Activated by Moisture to Facilitate Opening of the Polymer Material, Henkel patent DE19949981, 2001. B.T. Ingram, S.W. Heinzman, Disintegrating Component and Detergent Composition Containing It, P&G patent WO0112767, 2001. J.V. Boskamp, Detergent Compositions, Unilever patent WO9842816, 1998. B. van Veen, PhD Thesis, Rijksuniversiteit Groningen, The Netherlands, 2003. A. Samimi, PhD Thesis, University of Surrey, 2003. R. Kohlus, Paper 248, Proc. 4th World Congress on Powder Technology, 21-25 July, Australia, 2002. R. Kohlus, PhD Thesis, TU Clausthal, 1998. R. Kohlus, Quantification of Granule Structure, Partec 2001, Nuremberg, Germany, 2001. M.N. Pons, H. Vivier, J.A. Dodds, Part. Part. Syst. Charact. 14 (1997) 272-277. H. Rumpf, W.A. Knepper (Eds.), Agglomeration Int. Sympos. Philadelphia, USA, 1961, pp. 379-418. R. Boerefijn, M. Buscan, M.J. Hounslow, Effects of non-ideal powder properties on granulation kinetics, in: M. Kwauk, J. Li, W.-C. Yang (Eds.), Fluidization X, Engi- neering Foundation, New York, 2001, pp. 629-636.


[64] A.H. Rau, Personal and Household Care Compositions, Phyzz Inc. patent US6310014, 2001.

[65] Sodium Perborates for Industrial Use - Determination of Degree of Attrition, ISO 5937, 1980.

[66] T. Kubota, H. Takaya, S. Yamaguchi, H. Yamashita, S. Takana, Detergent Particles and Method for Producing the Same, Kao patent WO9929829, 1999.

[67] T. Kubota, H. Takaya, Detergent Patricles, Kao patent WO9929830, 1999. [68] E.E. Underwood, Quantitative Stereology, Addison-Wesley Publishing Company,

Reading, MA, 1972. [69] F. Stepanek, P.B. Warren, Proc. 4th World Congress on Particle Technology, 21-25

July, Australia, Paper 247, 2002. [70] M. Reed, PhD Thesis, University of Liverpool, UK, 1997.

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[Handbook of Powder Technology] Granulation Volume 11 || Chapter 14 Detergent granulation

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