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Impact and attrition shear breakage of enzyme granules and placebo particles-application to particle design and formulation
Powder Technology 149 (2005) 157 – 167 www.elsevier.com/locate/powtec
Impact and attrition shear breakage of enzyme granules and placebo particles-application to particle design and formulation K3re Jbrgensena, Poul Bachb, Anker D. Jensena,* a
CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark b Solid Products Development, Novozymes A/S, Smbrmosevej 11, DK-2880 Bagsværd, Denmark Received 12 June 2003; received in revised form 22 September 2004; accepted 1 November 2004 Available online 25 December 2004
Abstract The strength and breakage mechanisms of detergent enzyme granules and typical core materials used for enzyme granules (all of size 500–600 Am) have been investigated under impact and shear stress conditions to simulate the stresses experienced in a detergent factory. In an Impact Tester, single-particle experiments were performed at impact velocities of 8–25 m/s. Multiple (bulk) particle experiments were performed in an Attrition Shear Cell (ASC), where the particles were exposed to shear strains of about 1250 and normal stresses of 1–30 kPa. For coated enzyme granules the results indicate that the primary breakage mechanism after repeated impacts at 10 m/s is chipping associated with local delamination. Damages to the coating layer may expose the underlying enzyme-containing layer or core and lead to the release of enzyme-active dust. Markedly lower enzyme dust release was obtained by incorporating the enzyme into the core of the granule as compared to a layer-structured enzyme distribution. Furthermore, the results indicated that stronger enzyme granule core materials provide a better impact resistance of the final enzyme granule towards the release of enzyme-active dust. Coating layers of inorganic salts and watersoluble polymers are observed to enhance the breakage resistance of the enzyme granules tremendously. The impact and shear resistance of four different placebo enzyme granule core particles were investigated. A transition from chipping to fragmentation as the main breakage mechanism was observed at impact velocities from 8 to 20 m/s. Experiments performed with attrition shearing indicated that the extent of breakage depend on surface friction and particle sphericity as well as intraparticular forces. The results obtained in this work are of importance for the design and formulation of mechanically resistant enzyme granules. D 2004 Elsevier B.V. All rights reserved. Keywords: Formulation; Design; Enzyme granules; Impact; Shear; Breakage
1. Introduction The mechanical strength of particles and granules is important in many industries. Investigation of particle strength was initiated in the oil [1–3] and minerals [4–7] industry. In the oil industry the interest was mainly on clarifying the mechanisms concerning the undesired breakage of hydrocarbon cracking catalyst particles, while the minerals industry had the comminution process as the primary motivation for understanding particle breakdown.
* Corresponding author. Tel.: +45 4525 2841; fax: +45 4588 2258. E-mail address: [email protected] (A.D. Jensen). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.11.018
More recently also, the agro, pharmaceutical, food and detergent industries have intensified their research into the mechanical strength and resistance towards attrition and breakage of particulate products [8–13]. The motivations for this are many: Products may lose their desired properties or reduce their efficiency, e.g., flavour, taste, texture, miscibility, flowability, activity, stability, etc. as a result of undesired particle attrition or breakage. It may also be environmental, health or safety issues, which require a high degree of resistance towards mechanical deterioration of a particulate product. In the case of detergent enzyme granules the desire is to produce a product of superior enzyme activity and stability while avoiding the unintended release of enzyme-active dust
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Table 1 Breakage mechanisms related to the stress conditions of the particle/granule (after Beekman [19]) Type of stress/force
Normal direction
Tangential direction
Small force (only local damage)
attrition (by erosion or fatigue) chipping (not erosion nor fatigue) fracture/fragmentation
abrasion
Large force (widespread damage)
chipping
during handling in the enzyme and detergent factories, since the dust may be allergenic upon inhalation to the lungs [14,15]. Therefore, a greater understanding of the mechanisms of enzyme granule breakage as well as a desire to obtain a screening tool, which is capable of differentiating between the breakage resistance of different enzyme granule formulations, is of great importance in enzyme granule design. The breakage of particles and granular material depends on a range of parameters including size, shape, imposed stress, relative velocity between particle and target, but also on inherent properties of the particles/granules, e.g., hardness, Young’s modulus and fracture toughness [3]. These parameters are not all easy to measure, and further complication also arises since some parameters can only be evaluated through destructive tests. Furthermore, granules are often heterogeneous in composition, size and shape, thus providing extra challenges to testing equipment and subsequent data analysis in order to assess the mechanical strength. A review by Bemrose and Bridgwater [16] discusses a variety of single-particle as well as multipleparticle testing devices and principles. Multiple-particle tests show the closest resemblance to particle handling in process equipment. However, the results from multiple particle experiments are often closely related to the testing device and the results may be difficult to generalise. During processing of enzyme granules in a typical detergent factory, the granules experience numerous impact and shear stresses, e.g., in storage, filling/emptying, mixers and transport systems. Current standard tests to assess the mechanical strength of enzyme granules include the Heubach and Elutriation tests [11,17–19]. In the Heubach (Type III) test the granules are exposed to simultaneous shearing, compression, as well as impact forces by four steel balls which grind a sample of enzyme granules for a predefined period of time. The stresses acting on the granules in the Heubach (Type III) apparatus are not well defined. In the Elutriation test, a bed of particles is fluidised with air at a well-defined flow rate and for a specified period of time, and particles below a certain size are elutriated. In this work single-particle impact and attrition shear cell tests have been performed on commercial enzyme-containing granules and different types of placebo particles
commonly used in the formulation of enzyme granules. The resistance of placebo particles and enzyme granules towards impact and shear forces has been compared, and damage to granules and particles as well as the debris generated has been examined using light and scanning electron microscopy (SEM). Throughout this paper the terms ddamageT and dbreakageT have been used interchangeably as representing the loss of mass in the mother particle following an exposure to external stresses. Different definitions of the breakage mechanisms termed attrition, fragmentation, abrasion and chipping are applied in literature. The definitions used throughout this paper largely follow those given by Beekman [19] and are given in Appendix A and summarised in Table 1.
2. Experimental In order to simulate the stresses that enzyme granules experience in a detergent factory, two devices for testing the mechanical strength of particles and granules have been applied: The Attrition Shear Cell (ASC) (depicted in Fig. 1) and the Impact Tester (depicted in Fig. 2). To investigate the breakage mechanisms and subsequent enzyme release from the granules, light and scanning electron microscopy (SEM) is used to study the surface of the placebo particles and enzyme granules before and after the exposure to stresses as well as the debris generated. Since very small amounts of dust are released when impacting enzyme granules in the Impact Tester, gravimetrical dust analysis is not applicable. Instead, a standard laboratory enzyme activity measurement (ELISA-Enzyme Linked Immuno Sorbent Assay) is applied. In the ASC, only
Fig. 1. Schematic illustration of Attrition Shear Cell (ASC), wherein the particles are exposed to a well-defined shear stress and strain. After Neil and Bridgwater [20].
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narrow distribution of 500–600 Am is also chosen because it accounts for around 30 wt.% of the granules in a standard commercial detergent enzyme product. The sieving of the material is performed using Retsch DIN-ISO 3310/1 sieves with a diameter of 21 cm and a height of 3.5 cm. The sieve shaker used is a Retsch type EVS 1 Vebro and the material is sieved for 3 min at 50 Hz and an amplitude of 1.10 mm. Sieving is also used to classify placebo particles after impact or shear stress exposure. Following Couroyer et al. [3], the extent of breakage of placebo particles is defined as the mass of material smaller than two standard (British Standard 410) sieve mesh sizes below the initial size, i.e., a 355 Am sieve size in the present case. In this way highly fragmented particles and debris are counted in the broken fraction, while only slightly damaged mother particles are excluded. 2.1. Attrition Shear Cell
Fig. 2. Schematic illustration of Impact Tester. Designed and manufactured at the University of Surrey, UK. Figure is not to scale.
placebo particles have been investigated due to health and safety considerations regarding the collection of generated dust and handling of the heavy weights used for stress imposure. The amount of dust generated in the ASC is assessed gravimetrically as described below. The particles and granules investigated in this work are sieved to 500–600 Am before being tested. This is done in order to get a narrow range of impact velocities in the Impact Tester and a well-defined shear flow in the ASC. The
The Attrition Shear Cell (ASC) used in this work is supplied by Ajax Equipment, UK and based on a design by Paramanathan and Bridgwater [21]. This reference should be consulted for a detailed description of the ASC. In order to avoid the formation of two stagnant blocks that move above one another and thus cause the particle shearing not to be well defined, a sample layer thickness of around six particle diameters is used. This follows the recommendations given by Roscoe [22], Ghadiri et al. [23] and Neil and Bridgwater [20]. To avoid the particles slipping against the upper (A) or lower (B) surfaces in the shear cell-referring to Fig. 1–these are equipped with radial grooves (0.5 mm wide and 0.25 mm deep). This ensures the existence of an approximately linear variation of the shear strain with height in the bed of particles [20,23]. Normal stresses of 1.0–30.5 kPa are exerted onto the bed of particles in the ASC, and a linear relationship exists between the normal stress imposed on the particles and the effective torsional load experienced by the particles during shearing. By rotating the cell, an average shear strain of C=D avgd pd Nd a/hc1250 is imparted onto the particles. D avg is the mean diameter of the shear cell (140 mm); N is the number of revolutions the ASC turns during each experiment (N=10); a is a bscouring factorQ to account for the fact that the radial grooves are slightly smaller than the particles tested (a=0.85, as given by the ASC manufacturer), and h is the sample layer/bed thickness (3 mm). The particles at the bottom of the ASC annulus receive the full shearing, while particles at the top are held in place by the upper grooves. The amount of strain exerted on the particles is kept constant by choosing N=10. Furthermore, this ensures reasonable amounts of dust/debris for all the materials tested and thus enables a comparison of the materials’ shear resistance.
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The extent of breakage in the shear cell is kept below 13% in order to minimise substantial segregation of particles and debris in the bed. Segregation may lead to unreliable results if the debris collects at the bottom of the ASC and thereby creates an undesired failure plane. Davg The strain rate, c˙ u dC dt c h px, is kept constant at ˙ 1 approx. c =12.2 s equivalent to a speed of rotation, x, of approximately 5 rpm. 2.2. Impact tester The Impact Tester used in this work is a slightly modified version of that developed by Yuregir et al. [24] (see Fig. 2). For further details on the design of the Impact Tester, please refer to Yuregir et al. [24]. In the Impact Tester depicted in Fig. 2, the particles impact on a hard and smooth quartz glass plate at a velocity between 2 and 33 m/s. The air velocity in the impact tube and thereby also the particle velocity is controlled by a vacuum pump. The pressure drop over the impact tube is measured with a digital pressure indicator, and the pressure drop is related to the particle velocity in the impact tube by using a calibration curve. The calibration curve was generated at the University of Surrey, UK, using Laser Doppler Anemometry. The impact plate (25 mm in diameter) is situated 30–40 mm from the impact tube outlet in order to make sure that no particles escape impact and that particles are removed from the plate after impact. Furthermore, the particles are not allowed to slow down and thus have a known velocity at the time of impact. 5 g of particles are fed at a rate of approximately 0.8 g/min (equivalent to approx. 4500 particles/min). Such a large number of particles are used in order to achieve statistically significant results. When testing enzyme granules in the Impact Tester the amount of enzyme released (measured using ELISA) is taken as a measure of the breakage resistance of the granules. The enzyme-active dust generated is captured on an impact filter (Whatman microfibre filter GF/C, Cat No 1822 125) situated in the bottom of the impact chamber after the 13 consecutive runs. Furthermore, the apparatus is swept thoroughly inside, covering the impact tube, chamber, and plate with a wet filter paper in order to collect all the enzyme-active dust generated. The enzyme-activity measurements on the sweeping of the apparatus showed results with large variations. No correlation was seen between the amount of enzyme-active dust collected in the sweeping procedure and the dust collected on the impact filter. Only for the impact filter, reliable enzyme dust measurements could be obtained and these will be reported in this paper. 2.3. Materials tested Sketches of the placebo particles investigated can be seen in Fig. 3a–d and the detergent enzyme granules
are depicted in Fig. 4a–c. The three experimental enzyme granule products investigated (546, 547, and 548) have essentially the same structure and composition. The difference lies in the degree of compaction of the cores during the high-shear granulation process. Experimental 546 has received the highest degree of compaction, while Experimental 548 has received the lowest degree of compaction. The protective layers encapsulating the enzyme-containing core have the same composition in all of the three experimental granule types. The placebo particles have been chosen to represent the most popular core materials or prospects as core materials for commercial fluid bed and high-shear granules. Beekman et al. [25] indicate that the core material as well as the different protective layers might influence the strength of the granule. The directly mined and crushed sodium sulphate particles represent the matrix core material for the high-shear granules and a possible material for the core of fluid bed granules. The nonpareil particle is a sugar/ starch agglomerate, which is produced in a gentle pan granulation process. The nonpareil particle is often used as the core material for commercial fluid bed coating of pharmaceuticals and enzymes [26]. The placebo-B granule has undergone the same production process as the highshear enzyme granule investigated. However, the enzyme and the outer polymer coating layer have been omitted.
Fig. 3. Drawings of the placebo particles investigated. (a) Raw salt (Na2SO4), directly mined and subsequently crushed. (b) Na2SO4 (coat: cell+SDG), sodium sulphate particle coated with a thin (~5 Am) layer of finely ground cellulose fibres, starch, dextrin and glycerol (SDG) in a high-shear mixer. (c) Placebo-B, high-shear granulated particle consisting of 100–300 Am-long cellulose fibres, binder and an inorganic salt matrix. (d) Nonpareil, pan granulated particle consisting of a sugar crystal core and fine starch particles bound together by PVA.
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Fig. 4. Structure of the enzyme-containing granules investigated. (a) Commercial B: A core composed of fibres, inorganic salt, binder, and enzyme-made by high-shear granulation. The coating layer is a water-soluble polymer. (b) Commercial A: Nonpareil core with enzyme layer and multiple inorganic salt scavenger layers sprayed on in a fluid bed. The coating layer is a water-soluble polymer. (c) Experimental 546, 547, and 548: Core of fibres, inorganic salt, and binder-made by high-shear granulation. Enzyme layer and subsequent scavenger inorganic salt layer is sprayed on in a fluid bed. The coating layer is a watersoluble polymer.
The placebo-B granule is also the core material for the experimental fluid bed granules (546, 547, and 548).
3. Results and discussion In general, the particles and granules investigated have a very complex structure, being composed of many small particles, which are held together by different types of interparticular forces. The granules of a certain type may vary in composition and mechanical strength and so a large number of granules are tested in order to get representative numbers for the breakage tendency. 3.1. Impact tester 3.1.1. Enzyme granules All experiments with enzyme granules in the Impact Tester are conducted at an impact velocity of 10 m/s and with 13 repeated impacts in order to resemble the conditions that the granules experience in detergent factories. Table 2 contains the results of impact tests on five different types of enzyme granules. The numbers in the table represent the amount of enzyme activity measured in the dust collected on the impact filter in the bottom of the impact chamber. The results are relative and for comparison between the specific enzyme granules tested only. It is Table 2 Enzyme activity measurements by ELISA of dust collected on filter in the bottom of the impact chamber Average, % of Com. B CV, % Number of experiments
Com. B
Exp. 546
Com. A
Exp. 547
Exp. 548
100
682
2195
2910
4100
41.7 16
31.0 4
86.8 7
62.7 4
36.2 4
The numbers are normalised by the Commercial B granule measurement.
noted that the results in Table 2 have very large coefficients of variance (CV) between 31 and 87%. In order to investigate the significance of the results equivalence tests with a 5% test level and a significance test (t-test) also with a 5% test level have been performed. The results are summarised in Table 3. The equivalence and significance tests reveal a significantly lower enzyme release from the Commercial B granule. However, we also conclude that the coefficients of variance for the other four types of granules (Exp. 546, Exp. 547, Exp. 548, and Com. A) are too large to establish any statistical significance between the impact resistances of these four layer-structured granules. When observing the numbers in Table 2, it should be remarked that the enzyme activity measured after 13 repeated impacts of the Commercial B granule corresponds to the release of less than 1/10 of the enzyme activity of a single granule per gram of granules tested (5000–8000 part./g). This fact can to a large extent explain the large coefficients of variance (CV) and also underlines the difficulties in determining the strength of enzyme granules. Applying larger impact velocities or more repeated impacts could possibly lead to larger amounts of enzyme-active dust released and subsequently lead to more statistically significant results. However, this would compromise the comparison to detergent factory processing conditions and possibly lead to results, which would not correlate to actual detergent
Table 3 Equivalence and significance tests on raw data from Table 2 with a 5% test level Com. B Exp. 546 Com. B Exp. 546 Com. A Exp. 547 Exp. 548
Com. A
Exp. 547
Exp. 548
N.E./S.D. N.E./S.D. N.E./S.D. N.E./S.D. N.E./N.S.D. N.E./N.S.D. N.E./S.D. N.E./N.S.D. N.E./N.S.D. N.E./N.S.D.
N.E.: not equivalent; S.D.: significantly different; N.S.D.: not significantly different.
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factory enzyme dust activity measurements. This should be tested in future work. With the numbers from Tables 2 and 3 in mind, the enzyme dust release does indicate that a significantly better resistance towards enzyme release can be obtained by incorporating the enzyme in the core compared to a layered structure where a concentrated enzyme layer contains all the activity. Although the results are not significantly different (following the statistical analysis at the chosen level of significance), the experiments with the three experimental enzyme granule products (546, 547, and 548) do suggest that the quality of the cores, on which the enzyme is sprayed, might be important in the process of formulating low dust enzyme granules. This is noticed by the fact that the granules with the highest degree of compaction in the production process (Exp. 546) show the lowest enzyme dust release. The importance of the quality of the core material is also supported by Beekman et al. [13,19]. Damages to the enzyme granules were investigated under a SEM microscope. In Fig. 5, a typical damage to the surface of a Commercial B granule is seen. A piece of the coating (approximately 20*50*100 Am) is knocked off and the underlying enzyme-containing particle is exposed. In the case of a Commercial A type granule, a typical impact damage can be seen in Fig. 6 where the loss of an approximately 20*100*120 Am piece of the coating has resulted in the exposure of the underlying layer of inorganic salt. For Exp. 546 a typical impact damage can be seen in Fig. 7, where the enzyme-containing layer has been exposed through the loss of a piece of coating approximately 25*100*100 Am in size. No case where an entire granule had been cleaved was observed. This indicates that fragmentation of enzyme granules is extremely rare at impact velocities of 10 m/s. The damages to the enzyme granules in Figs. 5–7 all resemble those of local delamination, i.e., a small piece of coating is removed as a result of
Fig. 5. Impact damage after 13 times impact at 10 m/s to the surface of a Commercial B granule. A piece of the coating is knocked off (approx. 20*50*100 Am), thus leaving the underlying enzyme-containing particle exposed.
Fig. 6. Typical impact damage to Commercial A granule after 13 times impact at 10 m/s. A piece of the coating is knocked off (approx. 20*100*120 Am), thus exposing the underlying layer of inorganic salt.
the impact, but the coating also act as a crack barrier, thus preventing any further damage to the underlying granule. 3.1.2. Placebo particles Impact experiments are performed on placebo particles at impact velocities ranging from 8 to 25 m/s. This was done in order to establish the breakage mechanisms of the different types of particles at different impact velocities. The extent of breakage of the raw salt (sodium sulphate, Na2SO4), coated Na2SO4, placebo-B and nonpareil particles as a function of the impact velocity can be seen in Fig. 8. The impact experiments on the sodium sulphate particles (both raw salt and coated) reveal a dramatic increase in the extent of breakage at the transition from chipping at low impact velocities to particle fragmentation at higher impact velocities above 12–14 m/s (see Fig. 8). This tendency is confirmed by Fig. 9, where a large fragment (approximately 1/10 of the particle) has been knocked off a mother particle.
Fig. 7. Typical impact damage to Experimental 546 after 13 times impact at 10 m/s. A piece of the polymer coating is chipped off (approx. 25*100*100 Am) and an underlying layer of inorganic salt is exposed.
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Fig. 8. Extent of breakage of raw salt, coated Na2SO4, placebo-B, and nonpareil particles as a function of impact velocity.
Fig. 9 also indicates that the fragmentation could be caused by a large radial crack, which has propagated into the particle and led to the release of the fragment. However, the particle failure observed in Fig. 9 may also be a result of the shear cone and compression failure, which is formed beneath the contact area of the impact and produces meridian plane cracks into the particle [9,10,27]. In this case the meridian cracks do not bisect the particle. The transition from chipping, which primarily removes edges and corners of the mother particles, to fragmentation is also indicated by Fig. 10(a) and (b). Here it is observed that the debris fraction at low impact velocities only contains few fragments but instead a lot of fine debris, whereas the higher impact velocities result in a large number of fragments and only little fine dust. Coated Na2SO4 particles show the same tendency as the raw salt particles with a few large and many small fragments at low impact velocities and more large fragments at impact velocities above 12–14 m/s. The difference in the extent of breakage between the raw salt and the coated Na2SO4 particles at impact velocities above 14 m/s is most likely due to the fact that the weakest Na2SO4 particles have been broken down in the high-shear coating process. This means
Fig. 9. SEM picture of a damaged coated Na2SO4 particle impacted 10 times at 16 m/s.
Fig. 10. Dust/debris fraction from impact of raw salt: (a) 10 times 10 m/s; (b) 10 times 16 m/s.
that the particles, which have survived the granulation process, are stronger and therefore exhibit a higher breakage resistance than raw salt particles at impact velocities above 14 m/s. There may also be an effect of a bcushioningQ of the very thin (approx. 5 Am) coating layer of polymer with elastic properties on the coated Na2SO4 particles. This effect, however, is expected to be minor compared to the effect of the granulation process. In the case of the pan-granulated nonpareil particles, increasing the impact velocity from 8 to 10 m/s gives a very large increase in the extent of breakage. This indicates that the transition from chipping to fragmentation already sets in around 8 m/s. The high extent of breakage is most likely due to the fact that the very gentle pan granulation production process only produces weak binding forces between the individual starch grains and consequently generates granules of very low impact resistance. For high-shear granulated placebo-B particles, it is seen from Fig. 8 that the transition from surface damage by attrition and chipping to body fragmentation occurs around 20 m/s. At higher impact velocities the increase in the rate of
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breakage is slower than for the sodium sulphate and nonpareil particles. The decreased tendency of the placebo-B particles to break as a result of impact stresses is most likely caused by the presence of 100–300 Am long cellulose fibres within the particle. These act as reinforcement and prevent cracks from propagating into the granule. This gives superior breakage resistance at high impact velocities. 3.2. Attrition Shear Cell For the Attrition Shear Cell (ASC) experiments show an almost linear increase in the extent of breakage with the normal (or shear) stress for the different types of placebo materials examined-excluding the coated Na2SO4 particles. This can be seen in Fig. 11. The linear behaviour is seen after an initial offset, where it takes a certain amount of shear stress (and strain) to observe any measurable damage. The linearity between the extent of breakage and shear stress complies well with the theory suggested by Neil and Bridgwater [20]. 3.2.1. Raw salt When raw salt particles are sheared at low normal stresses, it is predominantly edges and corners that are worn off. A lot of dust/fine debris is generated, and the tendency to fragmentation is low (see Fig. 12). Only the weakest particles fragment and the surface abrasion is strong. At higher normal stresses the rounding of the mother particles is even more evident. Practically all edges and corners are worn off and fragmentation damages are hard to identify on the mother particles, since they too have been worn away by the surface abrasion. The amount of debris generated increases linearly with the normal stress. 3.2.2. Coated Na2SO4 In the case of the coated Na2SO4 particles it is observed that the coating layer of finely ground cellulose fibres, starch, dextrin and glycerol (~5 Am in thickness) is worn off easily even at very low normal stresses. This gives a high
Fig. 11. Extent of breakage of raw salt, coated Na2SO4, placebo-B, and nonpareil particles in the Attrition Shear Cell as a function of the normal stress.
Fig. 12. Dust/debris from raw salt particles sheared for a strain of 1270 (equivalent to 10 revolutions) with a normal stress of 6.6 kPa.
extent of breakage at low normal stresses. At larger normal stresses these types of particles do show signs of considerable fragmentation, but again the wounds in the mother particles caused by the loss of a fragment are difficult to identify, due to the fast abrasion and consequently rounding of the particles. The relatively low extent of breakage experienced by the coated Na2SO4 particles at high normal stresses is most likely due to the fact that the worn-off material acts as a blubricantQ and yield very easily. For the raw salt particles the abrasive properties of the worn off crystalline and semi-brittle Na2SO4 are higher than for the yielding cellulose fibres and ductile binder materials. This is the likely explanation for the flattening of the curve for the coated Na2SO4 particles in Fig. 11. 3.2.3. Nonpareil For the nonpareil particles the curve for the extent of breakage (Fig. 11) resembles that of the raw salt, even though the particle structure and material is very different. Here it is again confirmed that the nonpareil particles are
Fig. 13. Dust/debris from nonpareil granules sheared for a strain of 1335 (equivalent to 10 revolutions) with a normal stress of 6.6 kPa.
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weak, since already at low normal stresses a high degree of fragmentation and large amounts of fine dust is observed (see Fig. 13). For breakage of nonpareil particles at higher normal stresses the fragments in the dust/debris fraction are observed to be smaller than at lower normal stresses. This is probably again a result of the weak nonpareil particles, where fragments easily break down further. The nonpareil particles are almost spherical to begin with. This has most likely helped to reduce the extent of breakage at low normal stresses since the particles have simply rolled over one another and thus only suffered minor damage. At larger normal stresses fragmentation starts to occur, which accelerates the extent of breakage since the particles are now no longer spherical and thus break easier upon shearing. 3.2.4. Placebo-B As for impact experiments the placebo-B particles show the lowest extent of breakage of the placebo materials tested except at the highest normal stress of 30.5 kPa. The debris fraction mainly contains relatively large fragments of sodium sulphate particles connected by cellulose fibres. After shear under low normal stress the mother particles are surprisingly rich on edges, corners, and surface cavities and actual fragmentation by the granules is very rare. This indicates that the dominating breakage mechanism can be categorised as surface abrasion with severe damages. At higher normal stresses a larger fraction of the particles are fragmented as indicated by Fig. 14. Matrix salt particles and loose cellulose fibres can be identified in the debris/dust section. Irregular mother particles indicate that the cellulose fibres connect and reinforce the entire particle. Therefore, it is difficult to remove material on one side of the particle without it affecting the entire particle. The observed mechanism of breakage is defined
Fig. 14. Dust/debris from Placebo-B particles sheared for a strain of 1265 (equivalent to 10 revolutions) with a normal stress of 22.7 kPa.
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as lying somewhere between surface abrasion and fragmentation.
4. Conclusions Experiments with an Impact Tester and an Attrition Shear Cell have shown that when detergent enzyme granules are exposed to repeated impact stresses at impact velocities of 10 m/s, the release of enzyme-active dust is mainly caused by the release of small chips of coating associated with local delamination, and thus the exposure of the underlying enzyme layer or enzyme-containing core. There are indications that reduced enzyme release can be achieved by incorporating the enzyme into the core of the granule. Impact experiments with sodium sulphate crystal particles reveal a clear transition from chipping to fragmentation as the main breakage mechanism at impact velocities between 12 and 14 m/s. For granulated particles a high extent of breakage was observed even at low impact velocities (~8 m/s) for the gently produced nonpareil particles, while Placebo-B particles do not show a significant increase in the extent of breakage until an impact velocity of about 20 m/s is achieved. The fibres in the Placebo-B particle act as crack stoppers and thereby reinforce the particle. ASC experiments with placebo materials show that the surface topology of the particles tested as well as the abrasion properties of the worn-off material has a strong effect on the shear breakage. Therefore, attention should be paid when shearing layer-structured particles. The relationship between the ability of the enzyme granule core materials to withstand shear and impact stresses and the release of enzyme-active dust from granules produced using that specific core material is not straightforward: Factors such as surface friction and the ability of coating layers to improve the breakage resistance of enzyme granules tremendously makes a correlation between shear and impact resistance of enzyme granules and core materials difficult to achieve. However, the results do show that the applied experimental techniques may be valuable tests in the formulation and design of particles with the desired strength properties. Nomenclature a bscouring factorQ in the Attrition Shear Cell (ASC) D avg average diameter of the ASC (m) ˙c strain rate in the ASC (s 1) C average shear strain experienced by the particles in the ASC h height of bed of particles in the ASC (m) N the number of revolutions the ASC performs in each experiment x speed of rotation of the ASC (rpm) t time (s)
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Acknowledgements This project was performed in collaboration with Novozymes A/S, who also financially supported the project. The CHEC (Combustion and Harmful Emission Control) Research Centre is financially supported by the Technical University of Denmark, Elsam A/S and Eltra, Energi E2 A/S and Elkraft System, the Danish Technical Research Council, the Danish and Nordic Energy Research Programmes, EU Energy Research Programmes and industrial partners.
Appendix A This section contains the definitions of the breakage mechanisms used throughout this paper. The definitions largely follow those given by Beekman [19]. Chipping: When the particles and uncoated granules are exposed to large tangential forces, chipping may occur, i.e., the surface of the particles is damaged and some material is chipped off. Chipping is caused by the generation and propagation of lateral cracks, which form as a result of the relaxation of elastic deformation at the contact point, where the stress is imposed [28]. The term chipping is also used in this work for damage caused by small normal forces, which are not a result of erosion nor fatigue. Fracture/fragmentation: When large forces (typically impact forces) are applied to the particles and uncoated granules head-on, it may lead to fracture/fragmentation, i.e., breakage of the granule into large pieces (fragments). These actions often lead to exposure of the core and are thus considered the most severe kind of damage. Fracture or fragmentation is largely caused by radial cracks, which form into the body of the particle [28]. However, also meridian plane cracks may form as a result of shear cone and compression failure beneath the contact area of the impact site [9,10,27]. Abrasion: The small tangential forces lead to polishing and rounding of the particles and uncoated granules. This may also lead to loss of granule coating. Attrition: Small normal forces (typically impact forces) may lead to two kinds of damage to the particles and granules: – Fatigue: After several impacts, small cracks formed deep inside the particle or granule may have propagated and will eventually lead to severe breakage, where the particle/granule loses mass at a continuously increasing rate. – Erosion is defined as damage to a specific layer in the coated granule, i.e., the granule is peeled, for example, by losing large pieces of the coating layer. Erosion is only a valid concept for granules with a layered structure and it does not penetrate the granule. This type of breakage can be seen as a severe type of coating delamination.
Local delamination of coating: Chips of the granule coating may be lost as a result of local delamination caused by stresses in the granule coating [8]. This is typically revealed by loss of small pieces of coating, and the breakage typically stops at a layer–layer interface.
References [1] J.E. Gwyn, On the particle size distribution function and the attrition of cracking catalysts, AIChE J. 15 (1969) 35 – 39. [2] C. Couroyer, M. Ghadiri, Attrition of catalyst carrier beads—a relationship between single particle properties and bulk behaviour, The International Conference on Powder and Bulk Solids Handling, Organised by IMechE, 13–15 June, London, UK, 2000. [3] C. Couroyer, M. Ghadiri, P. Laval, N. Brunard, F. Kolenda, Methodology for investigating the mechanical strength of reforming catalyst beads, Oil Gas Sci. Technol.–Rev. IFP 55 (2000) 67 – 85. [4] R. Schuhmann, Energy input and size distribution in comminution, Trans. AIME 217 (1960) 21 – 26. [5] N. Arbiter, C.C. Harris, G.A. Stamboltzis, Single fracture of brittle spheres, Trans. AIME 244 (1969) 118 – 133. [6] K. Schfnert, Role of fracture physics in understanding comminution phenomena, Trans. AIME 252 (1972) 21 – 26. [7] K. Schfnert, The influence of particle bed configurations and confinements on particle breakage, Int. J. Miner. Process. 44–45 (1996) 1 – 16. [8] L.X. Liu, D. Golchert, N.W. Page, D. Page, J.D. Litster, Strength and attrition resistance of agglomerates and particulate coatings, Powder Technol. 130 (2003) 415 – 420. [9] A.D. Salman, J. Fu, D.A. Gorham, M.J. Hounslow, Impact breakage of fertiliser granules, Powder Technol. 130 (2003) 359 – 367. [10] A.D. Salman, C.A. Biggs, J. Fu, I. Angyal, M. Szabo´, M.J. Hounslow, An experimental investigation of particle fragmentation using single particle impact studies, Powder Technol. 128 (2002) 36 – 46. [11] R. Pitchumani, G.M.H. Meesters, B. Scarlett, Breakage behaviour of enzyme granules in a repeated impact test, Powder Technol. 130 (2003) 421 – 427. [12] A. Samini, M. Ghadiri, R. Boerefijn, A. Groot, R. Kohlus, Effect of structural characteristics on impact breakage of agglomerates, Powder Technol. 130 (2003) 428 – 435. [13] W.J. Beekman, G.M.H. Meesters, T. Becker, A. Gaertner, M. Gebert, B. Scarlett, Failure mechanism determination for industrial granules using a repeated compression test, Powder Technol. 130 (2003) 367 – 376. [14] M.L.H. Flindt, Pulmonary disease due to inhalation of derivatives of Bacillus subtilis containing proteolytic enzymes, Lancet 1 (1969) 1177 – 1181. [15] M. Vanhanen, Exposure, sensitization and allergy to industrial enzymes, People and work research reports 46, Finnish Institute of Occupational Health, Department of Pulmology, Helsinki University Central Hospital, Helsinki, Finland, 2001, ISBN 951-802-453-7. [16] C.R. Bemrose, J. Bridgwater, A review of attrition and attrition test methods, Powder Technol. 49 (1987) 97 – 126. [17] Australia’s National Industrial Chemicals Notification and Assessment Scheme (NICNAS), http://www.nicnas.gov.au/publications/CAR/ PEC/PEC2/PEC2c.pdf, May 2003. [18] Heubach GmbH, http://www.heubachinternational.de, May 2003. [19] W.J. Beekman, Measurement of the mechanical strength of granules and agglomerates, PhD thesis, Technische Universiteit Delft, The Netherlands (2000). [20] A.U. Neil, J. Bridgwater, Attrition of particulate solids under shear, Powder Technol. 80 (1994) 207 – 219. [21] B.K. Paramanathan, J. Bridgwater, Attrition of solids–I, cell development, Chem. Eng. Sci. 38 (1983) 197 – 206.
K. Jørgensen et al. / Powder Technology 149 (2005) 157–167 [22] K.H. Roscoe, The influence of strains in soil mechanics, Ge´otechnique 20 (1970) 129 – 170. [23] M. Ghadiri, Z. Ning, S.J. Kenter, E. Puik, Attrition of granular solids in a shear cell, Chem. Eng. Sci. 55 (2000) 5445 – 5456. [24] K.R. Yuregir, M. Ghadiri, R. Clift, Observations on impact attrition of granular solids, Powder Technol. 49 (1986) 53 – 57. [25] W.J. Beekman, G.M.H. Meesters, B. Scarlett, T. Becker, Measurement of granule attrition and fatigue in a vibrating box, Part. Part. Syst. Char. 19 (2002) 5 – 11.
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[26] B. Scarlett, W.J. Beekman, G.M.H. Meesters, R. Pitchumani, Particles—their strengths and weaknesses, Key Eng. Mater. 230–232 (2002) 203 – 212. [27] A.D. Salman, G.K. Reynolds, M.J. Hounslow, Particle impact breakage in particulate processing, KONA 21 (2003) 88 – 99. [28] M. Ghadiri, Z. Zhang, Impact attrition of particulate solids. Part 1: A theoretical model of chipping, Chem. Eng. Sci. 57 (2002) 3659 – 3669.
Impact and attrition shear breakage of enzyme granules and placebo particles-application to particle design and formulation K3re Jbrgensena, Poul Bachb, Anker D. Jensena,* a
CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark b Solid Products Development, Novozymes A/S, Smbrmosevej 11, DK-2880 Bagsværd, Denmark Received 12 June 2003; received in revised form 22 September 2004; accepted 1 November 2004 Available online 25 December 2004
Abstract The strength and breakage mechanisms of detergent enzyme granules and typical core materials used for enzyme granules (all of size 500–600 Am) have been investigated under impact and shear stress conditions to simulate the stresses experienced in a detergent factory. In an Impact Tester, single-particle experiments were performed at impact velocities of 8–25 m/s. Multiple (bulk) particle experiments were performed in an Attrition Shear Cell (ASC), where the particles were exposed to shear strains of about 1250 and normal stresses of 1–30 kPa. For coated enzyme granules the results indicate that the primary breakage mechanism after repeated impacts at 10 m/s is chipping associated with local delamination. Damages to the coating layer may expose the underlying enzyme-containing layer or core and lead to the release of enzyme-active dust. Markedly lower enzyme dust release was obtained by incorporating the enzyme into the core of the granule as compared to a layer-structured enzyme distribution. Furthermore, the results indicated that stronger enzyme granule core materials provide a better impact resistance of the final enzyme granule towards the release of enzyme-active dust. Coating layers of inorganic salts and watersoluble polymers are observed to enhance the breakage resistance of the enzyme granules tremendously. The impact and shear resistance of four different placebo enzyme granule core particles were investigated. A transition from chipping to fragmentation as the main breakage mechanism was observed at impact velocities from 8 to 20 m/s. Experiments performed with attrition shearing indicated that the extent of breakage depend on surface friction and particle sphericity as well as intraparticular forces. The results obtained in this work are of importance for the design and formulation of mechanically resistant enzyme granules. D 2004 Elsevier B.V. All rights reserved. Keywords: Formulation; Design; Enzyme granules; Impact; Shear; Breakage
1. Introduction The mechanical strength of particles and granules is important in many industries. Investigation of particle strength was initiated in the oil [1–3] and minerals [4–7] industry. In the oil industry the interest was mainly on clarifying the mechanisms concerning the undesired breakage of hydrocarbon cracking catalyst particles, while the minerals industry had the comminution process as the primary motivation for understanding particle breakdown.
* Corresponding author. Tel.: +45 4525 2841; fax: +45 4588 2258. E-mail address: [email protected] (A.D. Jensen). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.11.018
More recently also, the agro, pharmaceutical, food and detergent industries have intensified their research into the mechanical strength and resistance towards attrition and breakage of particulate products [8–13]. The motivations for this are many: Products may lose their desired properties or reduce their efficiency, e.g., flavour, taste, texture, miscibility, flowability, activity, stability, etc. as a result of undesired particle attrition or breakage. It may also be environmental, health or safety issues, which require a high degree of resistance towards mechanical deterioration of a particulate product. In the case of detergent enzyme granules the desire is to produce a product of superior enzyme activity and stability while avoiding the unintended release of enzyme-active dust
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Table 1 Breakage mechanisms related to the stress conditions of the particle/granule (after Beekman [19]) Type of stress/force
Normal direction
Tangential direction
Small force (only local damage)
attrition (by erosion or fatigue) chipping (not erosion nor fatigue) fracture/fragmentation
abrasion
Large force (widespread damage)
chipping
during handling in the enzyme and detergent factories, since the dust may be allergenic upon inhalation to the lungs [14,15]. Therefore, a greater understanding of the mechanisms of enzyme granule breakage as well as a desire to obtain a screening tool, which is capable of differentiating between the breakage resistance of different enzyme granule formulations, is of great importance in enzyme granule design. The breakage of particles and granular material depends on a range of parameters including size, shape, imposed stress, relative velocity between particle and target, but also on inherent properties of the particles/granules, e.g., hardness, Young’s modulus and fracture toughness [3]. These parameters are not all easy to measure, and further complication also arises since some parameters can only be evaluated through destructive tests. Furthermore, granules are often heterogeneous in composition, size and shape, thus providing extra challenges to testing equipment and subsequent data analysis in order to assess the mechanical strength. A review by Bemrose and Bridgwater [16] discusses a variety of single-particle as well as multipleparticle testing devices and principles. Multiple-particle tests show the closest resemblance to particle handling in process equipment. However, the results from multiple particle experiments are often closely related to the testing device and the results may be difficult to generalise. During processing of enzyme granules in a typical detergent factory, the granules experience numerous impact and shear stresses, e.g., in storage, filling/emptying, mixers and transport systems. Current standard tests to assess the mechanical strength of enzyme granules include the Heubach and Elutriation tests [11,17–19]. In the Heubach (Type III) test the granules are exposed to simultaneous shearing, compression, as well as impact forces by four steel balls which grind a sample of enzyme granules for a predefined period of time. The stresses acting on the granules in the Heubach (Type III) apparatus are not well defined. In the Elutriation test, a bed of particles is fluidised with air at a well-defined flow rate and for a specified period of time, and particles below a certain size are elutriated. In this work single-particle impact and attrition shear cell tests have been performed on commercial enzyme-containing granules and different types of placebo particles
commonly used in the formulation of enzyme granules. The resistance of placebo particles and enzyme granules towards impact and shear forces has been compared, and damage to granules and particles as well as the debris generated has been examined using light and scanning electron microscopy (SEM). Throughout this paper the terms ddamageT and dbreakageT have been used interchangeably as representing the loss of mass in the mother particle following an exposure to external stresses. Different definitions of the breakage mechanisms termed attrition, fragmentation, abrasion and chipping are applied in literature. The definitions used throughout this paper largely follow those given by Beekman [19] and are given in Appendix A and summarised in Table 1.
2. Experimental In order to simulate the stresses that enzyme granules experience in a detergent factory, two devices for testing the mechanical strength of particles and granules have been applied: The Attrition Shear Cell (ASC) (depicted in Fig. 1) and the Impact Tester (depicted in Fig. 2). To investigate the breakage mechanisms and subsequent enzyme release from the granules, light and scanning electron microscopy (SEM) is used to study the surface of the placebo particles and enzyme granules before and after the exposure to stresses as well as the debris generated. Since very small amounts of dust are released when impacting enzyme granules in the Impact Tester, gravimetrical dust analysis is not applicable. Instead, a standard laboratory enzyme activity measurement (ELISA-Enzyme Linked Immuno Sorbent Assay) is applied. In the ASC, only
Fig. 1. Schematic illustration of Attrition Shear Cell (ASC), wherein the particles are exposed to a well-defined shear stress and strain. After Neil and Bridgwater [20].
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narrow distribution of 500–600 Am is also chosen because it accounts for around 30 wt.% of the granules in a standard commercial detergent enzyme product. The sieving of the material is performed using Retsch DIN-ISO 3310/1 sieves with a diameter of 21 cm and a height of 3.5 cm. The sieve shaker used is a Retsch type EVS 1 Vebro and the material is sieved for 3 min at 50 Hz and an amplitude of 1.10 mm. Sieving is also used to classify placebo particles after impact or shear stress exposure. Following Couroyer et al. [3], the extent of breakage of placebo particles is defined as the mass of material smaller than two standard (British Standard 410) sieve mesh sizes below the initial size, i.e., a 355 Am sieve size in the present case. In this way highly fragmented particles and debris are counted in the broken fraction, while only slightly damaged mother particles are excluded. 2.1. Attrition Shear Cell
Fig. 2. Schematic illustration of Impact Tester. Designed and manufactured at the University of Surrey, UK. Figure is not to scale.
placebo particles have been investigated due to health and safety considerations regarding the collection of generated dust and handling of the heavy weights used for stress imposure. The amount of dust generated in the ASC is assessed gravimetrically as described below. The particles and granules investigated in this work are sieved to 500–600 Am before being tested. This is done in order to get a narrow range of impact velocities in the Impact Tester and a well-defined shear flow in the ASC. The
The Attrition Shear Cell (ASC) used in this work is supplied by Ajax Equipment, UK and based on a design by Paramanathan and Bridgwater [21]. This reference should be consulted for a detailed description of the ASC. In order to avoid the formation of two stagnant blocks that move above one another and thus cause the particle shearing not to be well defined, a sample layer thickness of around six particle diameters is used. This follows the recommendations given by Roscoe [22], Ghadiri et al. [23] and Neil and Bridgwater [20]. To avoid the particles slipping against the upper (A) or lower (B) surfaces in the shear cell-referring to Fig. 1–these are equipped with radial grooves (0.5 mm wide and 0.25 mm deep). This ensures the existence of an approximately linear variation of the shear strain with height in the bed of particles [20,23]. Normal stresses of 1.0–30.5 kPa are exerted onto the bed of particles in the ASC, and a linear relationship exists between the normal stress imposed on the particles and the effective torsional load experienced by the particles during shearing. By rotating the cell, an average shear strain of C=D avgd pd Nd a/hc1250 is imparted onto the particles. D avg is the mean diameter of the shear cell (140 mm); N is the number of revolutions the ASC turns during each experiment (N=10); a is a bscouring factorQ to account for the fact that the radial grooves are slightly smaller than the particles tested (a=0.85, as given by the ASC manufacturer), and h is the sample layer/bed thickness (3 mm). The particles at the bottom of the ASC annulus receive the full shearing, while particles at the top are held in place by the upper grooves. The amount of strain exerted on the particles is kept constant by choosing N=10. Furthermore, this ensures reasonable amounts of dust/debris for all the materials tested and thus enables a comparison of the materials’ shear resistance.
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The extent of breakage in the shear cell is kept below 13% in order to minimise substantial segregation of particles and debris in the bed. Segregation may lead to unreliable results if the debris collects at the bottom of the ASC and thereby creates an undesired failure plane. Davg The strain rate, c˙ u dC dt c h px, is kept constant at ˙ 1 approx. c =12.2 s equivalent to a speed of rotation, x, of approximately 5 rpm. 2.2. Impact tester The Impact Tester used in this work is a slightly modified version of that developed by Yuregir et al. [24] (see Fig. 2). For further details on the design of the Impact Tester, please refer to Yuregir et al. [24]. In the Impact Tester depicted in Fig. 2, the particles impact on a hard and smooth quartz glass plate at a velocity between 2 and 33 m/s. The air velocity in the impact tube and thereby also the particle velocity is controlled by a vacuum pump. The pressure drop over the impact tube is measured with a digital pressure indicator, and the pressure drop is related to the particle velocity in the impact tube by using a calibration curve. The calibration curve was generated at the University of Surrey, UK, using Laser Doppler Anemometry. The impact plate (25 mm in diameter) is situated 30–40 mm from the impact tube outlet in order to make sure that no particles escape impact and that particles are removed from the plate after impact. Furthermore, the particles are not allowed to slow down and thus have a known velocity at the time of impact. 5 g of particles are fed at a rate of approximately 0.8 g/min (equivalent to approx. 4500 particles/min). Such a large number of particles are used in order to achieve statistically significant results. When testing enzyme granules in the Impact Tester the amount of enzyme released (measured using ELISA) is taken as a measure of the breakage resistance of the granules. The enzyme-active dust generated is captured on an impact filter (Whatman microfibre filter GF/C, Cat No 1822 125) situated in the bottom of the impact chamber after the 13 consecutive runs. Furthermore, the apparatus is swept thoroughly inside, covering the impact tube, chamber, and plate with a wet filter paper in order to collect all the enzyme-active dust generated. The enzyme-activity measurements on the sweeping of the apparatus showed results with large variations. No correlation was seen between the amount of enzyme-active dust collected in the sweeping procedure and the dust collected on the impact filter. Only for the impact filter, reliable enzyme dust measurements could be obtained and these will be reported in this paper. 2.3. Materials tested Sketches of the placebo particles investigated can be seen in Fig. 3a–d and the detergent enzyme granules
are depicted in Fig. 4a–c. The three experimental enzyme granule products investigated (546, 547, and 548) have essentially the same structure and composition. The difference lies in the degree of compaction of the cores during the high-shear granulation process. Experimental 546 has received the highest degree of compaction, while Experimental 548 has received the lowest degree of compaction. The protective layers encapsulating the enzyme-containing core have the same composition in all of the three experimental granule types. The placebo particles have been chosen to represent the most popular core materials or prospects as core materials for commercial fluid bed and high-shear granules. Beekman et al. [25] indicate that the core material as well as the different protective layers might influence the strength of the granule. The directly mined and crushed sodium sulphate particles represent the matrix core material for the high-shear granules and a possible material for the core of fluid bed granules. The nonpareil particle is a sugar/ starch agglomerate, which is produced in a gentle pan granulation process. The nonpareil particle is often used as the core material for commercial fluid bed coating of pharmaceuticals and enzymes [26]. The placebo-B granule has undergone the same production process as the highshear enzyme granule investigated. However, the enzyme and the outer polymer coating layer have been omitted.
Fig. 3. Drawings of the placebo particles investigated. (a) Raw salt (Na2SO4), directly mined and subsequently crushed. (b) Na2SO4 (coat: cell+SDG), sodium sulphate particle coated with a thin (~5 Am) layer of finely ground cellulose fibres, starch, dextrin and glycerol (SDG) in a high-shear mixer. (c) Placebo-B, high-shear granulated particle consisting of 100–300 Am-long cellulose fibres, binder and an inorganic salt matrix. (d) Nonpareil, pan granulated particle consisting of a sugar crystal core and fine starch particles bound together by PVA.
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Fig. 4. Structure of the enzyme-containing granules investigated. (a) Commercial B: A core composed of fibres, inorganic salt, binder, and enzyme-made by high-shear granulation. The coating layer is a water-soluble polymer. (b) Commercial A: Nonpareil core with enzyme layer and multiple inorganic salt scavenger layers sprayed on in a fluid bed. The coating layer is a water-soluble polymer. (c) Experimental 546, 547, and 548: Core of fibres, inorganic salt, and binder-made by high-shear granulation. Enzyme layer and subsequent scavenger inorganic salt layer is sprayed on in a fluid bed. The coating layer is a watersoluble polymer.
The placebo-B granule is also the core material for the experimental fluid bed granules (546, 547, and 548).
3. Results and discussion In general, the particles and granules investigated have a very complex structure, being composed of many small particles, which are held together by different types of interparticular forces. The granules of a certain type may vary in composition and mechanical strength and so a large number of granules are tested in order to get representative numbers for the breakage tendency. 3.1. Impact tester 3.1.1. Enzyme granules All experiments with enzyme granules in the Impact Tester are conducted at an impact velocity of 10 m/s and with 13 repeated impacts in order to resemble the conditions that the granules experience in detergent factories. Table 2 contains the results of impact tests on five different types of enzyme granules. The numbers in the table represent the amount of enzyme activity measured in the dust collected on the impact filter in the bottom of the impact chamber. The results are relative and for comparison between the specific enzyme granules tested only. It is Table 2 Enzyme activity measurements by ELISA of dust collected on filter in the bottom of the impact chamber Average, % of Com. B CV, % Number of experiments
Com. B
Exp. 546
Com. A
Exp. 547
Exp. 548
100
682
2195
2910
4100
41.7 16
31.0 4
86.8 7
62.7 4
36.2 4
The numbers are normalised by the Commercial B granule measurement.
noted that the results in Table 2 have very large coefficients of variance (CV) between 31 and 87%. In order to investigate the significance of the results equivalence tests with a 5% test level and a significance test (t-test) also with a 5% test level have been performed. The results are summarised in Table 3. The equivalence and significance tests reveal a significantly lower enzyme release from the Commercial B granule. However, we also conclude that the coefficients of variance for the other four types of granules (Exp. 546, Exp. 547, Exp. 548, and Com. A) are too large to establish any statistical significance between the impact resistances of these four layer-structured granules. When observing the numbers in Table 2, it should be remarked that the enzyme activity measured after 13 repeated impacts of the Commercial B granule corresponds to the release of less than 1/10 of the enzyme activity of a single granule per gram of granules tested (5000–8000 part./g). This fact can to a large extent explain the large coefficients of variance (CV) and also underlines the difficulties in determining the strength of enzyme granules. Applying larger impact velocities or more repeated impacts could possibly lead to larger amounts of enzyme-active dust released and subsequently lead to more statistically significant results. However, this would compromise the comparison to detergent factory processing conditions and possibly lead to results, which would not correlate to actual detergent
Table 3 Equivalence and significance tests on raw data from Table 2 with a 5% test level Com. B Exp. 546 Com. B Exp. 546 Com. A Exp. 547 Exp. 548
Com. A
Exp. 547
Exp. 548
N.E./S.D. N.E./S.D. N.E./S.D. N.E./S.D. N.E./N.S.D. N.E./N.S.D. N.E./S.D. N.E./N.S.D. N.E./N.S.D. N.E./N.S.D.
N.E.: not equivalent; S.D.: significantly different; N.S.D.: not significantly different.
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factory enzyme dust activity measurements. This should be tested in future work. With the numbers from Tables 2 and 3 in mind, the enzyme dust release does indicate that a significantly better resistance towards enzyme release can be obtained by incorporating the enzyme in the core compared to a layered structure where a concentrated enzyme layer contains all the activity. Although the results are not significantly different (following the statistical analysis at the chosen level of significance), the experiments with the three experimental enzyme granule products (546, 547, and 548) do suggest that the quality of the cores, on which the enzyme is sprayed, might be important in the process of formulating low dust enzyme granules. This is noticed by the fact that the granules with the highest degree of compaction in the production process (Exp. 546) show the lowest enzyme dust release. The importance of the quality of the core material is also supported by Beekman et al. [13,19]. Damages to the enzyme granules were investigated under a SEM microscope. In Fig. 5, a typical damage to the surface of a Commercial B granule is seen. A piece of the coating (approximately 20*50*100 Am) is knocked off and the underlying enzyme-containing particle is exposed. In the case of a Commercial A type granule, a typical impact damage can be seen in Fig. 6 where the loss of an approximately 20*100*120 Am piece of the coating has resulted in the exposure of the underlying layer of inorganic salt. For Exp. 546 a typical impact damage can be seen in Fig. 7, where the enzyme-containing layer has been exposed through the loss of a piece of coating approximately 25*100*100 Am in size. No case where an entire granule had been cleaved was observed. This indicates that fragmentation of enzyme granules is extremely rare at impact velocities of 10 m/s. The damages to the enzyme granules in Figs. 5–7 all resemble those of local delamination, i.e., a small piece of coating is removed as a result of
Fig. 5. Impact damage after 13 times impact at 10 m/s to the surface of a Commercial B granule. A piece of the coating is knocked off (approx. 20*50*100 Am), thus leaving the underlying enzyme-containing particle exposed.
Fig. 6. Typical impact damage to Commercial A granule after 13 times impact at 10 m/s. A piece of the coating is knocked off (approx. 20*100*120 Am), thus exposing the underlying layer of inorganic salt.
the impact, but the coating also act as a crack barrier, thus preventing any further damage to the underlying granule. 3.1.2. Placebo particles Impact experiments are performed on placebo particles at impact velocities ranging from 8 to 25 m/s. This was done in order to establish the breakage mechanisms of the different types of particles at different impact velocities. The extent of breakage of the raw salt (sodium sulphate, Na2SO4), coated Na2SO4, placebo-B and nonpareil particles as a function of the impact velocity can be seen in Fig. 8. The impact experiments on the sodium sulphate particles (both raw salt and coated) reveal a dramatic increase in the extent of breakage at the transition from chipping at low impact velocities to particle fragmentation at higher impact velocities above 12–14 m/s (see Fig. 8). This tendency is confirmed by Fig. 9, where a large fragment (approximately 1/10 of the particle) has been knocked off a mother particle.
Fig. 7. Typical impact damage to Experimental 546 after 13 times impact at 10 m/s. A piece of the polymer coating is chipped off (approx. 25*100*100 Am) and an underlying layer of inorganic salt is exposed.
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Fig. 8. Extent of breakage of raw salt, coated Na2SO4, placebo-B, and nonpareil particles as a function of impact velocity.
Fig. 9 also indicates that the fragmentation could be caused by a large radial crack, which has propagated into the particle and led to the release of the fragment. However, the particle failure observed in Fig. 9 may also be a result of the shear cone and compression failure, which is formed beneath the contact area of the impact and produces meridian plane cracks into the particle [9,10,27]. In this case the meridian cracks do not bisect the particle. The transition from chipping, which primarily removes edges and corners of the mother particles, to fragmentation is also indicated by Fig. 10(a) and (b). Here it is observed that the debris fraction at low impact velocities only contains few fragments but instead a lot of fine debris, whereas the higher impact velocities result in a large number of fragments and only little fine dust. Coated Na2SO4 particles show the same tendency as the raw salt particles with a few large and many small fragments at low impact velocities and more large fragments at impact velocities above 12–14 m/s. The difference in the extent of breakage between the raw salt and the coated Na2SO4 particles at impact velocities above 14 m/s is most likely due to the fact that the weakest Na2SO4 particles have been broken down in the high-shear coating process. This means
Fig. 9. SEM picture of a damaged coated Na2SO4 particle impacted 10 times at 16 m/s.
Fig. 10. Dust/debris fraction from impact of raw salt: (a) 10 times 10 m/s; (b) 10 times 16 m/s.
that the particles, which have survived the granulation process, are stronger and therefore exhibit a higher breakage resistance than raw salt particles at impact velocities above 14 m/s. There may also be an effect of a bcushioningQ of the very thin (approx. 5 Am) coating layer of polymer with elastic properties on the coated Na2SO4 particles. This effect, however, is expected to be minor compared to the effect of the granulation process. In the case of the pan-granulated nonpareil particles, increasing the impact velocity from 8 to 10 m/s gives a very large increase in the extent of breakage. This indicates that the transition from chipping to fragmentation already sets in around 8 m/s. The high extent of breakage is most likely due to the fact that the very gentle pan granulation production process only produces weak binding forces between the individual starch grains and consequently generates granules of very low impact resistance. For high-shear granulated placebo-B particles, it is seen from Fig. 8 that the transition from surface damage by attrition and chipping to body fragmentation occurs around 20 m/s. At higher impact velocities the increase in the rate of
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breakage is slower than for the sodium sulphate and nonpareil particles. The decreased tendency of the placebo-B particles to break as a result of impact stresses is most likely caused by the presence of 100–300 Am long cellulose fibres within the particle. These act as reinforcement and prevent cracks from propagating into the granule. This gives superior breakage resistance at high impact velocities. 3.2. Attrition Shear Cell For the Attrition Shear Cell (ASC) experiments show an almost linear increase in the extent of breakage with the normal (or shear) stress for the different types of placebo materials examined-excluding the coated Na2SO4 particles. This can be seen in Fig. 11. The linear behaviour is seen after an initial offset, where it takes a certain amount of shear stress (and strain) to observe any measurable damage. The linearity between the extent of breakage and shear stress complies well with the theory suggested by Neil and Bridgwater [20]. 3.2.1. Raw salt When raw salt particles are sheared at low normal stresses, it is predominantly edges and corners that are worn off. A lot of dust/fine debris is generated, and the tendency to fragmentation is low (see Fig. 12). Only the weakest particles fragment and the surface abrasion is strong. At higher normal stresses the rounding of the mother particles is even more evident. Practically all edges and corners are worn off and fragmentation damages are hard to identify on the mother particles, since they too have been worn away by the surface abrasion. The amount of debris generated increases linearly with the normal stress. 3.2.2. Coated Na2SO4 In the case of the coated Na2SO4 particles it is observed that the coating layer of finely ground cellulose fibres, starch, dextrin and glycerol (~5 Am in thickness) is worn off easily even at very low normal stresses. This gives a high
Fig. 11. Extent of breakage of raw salt, coated Na2SO4, placebo-B, and nonpareil particles in the Attrition Shear Cell as a function of the normal stress.
Fig. 12. Dust/debris from raw salt particles sheared for a strain of 1270 (equivalent to 10 revolutions) with a normal stress of 6.6 kPa.
extent of breakage at low normal stresses. At larger normal stresses these types of particles do show signs of considerable fragmentation, but again the wounds in the mother particles caused by the loss of a fragment are difficult to identify, due to the fast abrasion and consequently rounding of the particles. The relatively low extent of breakage experienced by the coated Na2SO4 particles at high normal stresses is most likely due to the fact that the worn-off material acts as a blubricantQ and yield very easily. For the raw salt particles the abrasive properties of the worn off crystalline and semi-brittle Na2SO4 are higher than for the yielding cellulose fibres and ductile binder materials. This is the likely explanation for the flattening of the curve for the coated Na2SO4 particles in Fig. 11. 3.2.3. Nonpareil For the nonpareil particles the curve for the extent of breakage (Fig. 11) resembles that of the raw salt, even though the particle structure and material is very different. Here it is again confirmed that the nonpareil particles are
Fig. 13. Dust/debris from nonpareil granules sheared for a strain of 1335 (equivalent to 10 revolutions) with a normal stress of 6.6 kPa.
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weak, since already at low normal stresses a high degree of fragmentation and large amounts of fine dust is observed (see Fig. 13). For breakage of nonpareil particles at higher normal stresses the fragments in the dust/debris fraction are observed to be smaller than at lower normal stresses. This is probably again a result of the weak nonpareil particles, where fragments easily break down further. The nonpareil particles are almost spherical to begin with. This has most likely helped to reduce the extent of breakage at low normal stresses since the particles have simply rolled over one another and thus only suffered minor damage. At larger normal stresses fragmentation starts to occur, which accelerates the extent of breakage since the particles are now no longer spherical and thus break easier upon shearing. 3.2.4. Placebo-B As for impact experiments the placebo-B particles show the lowest extent of breakage of the placebo materials tested except at the highest normal stress of 30.5 kPa. The debris fraction mainly contains relatively large fragments of sodium sulphate particles connected by cellulose fibres. After shear under low normal stress the mother particles are surprisingly rich on edges, corners, and surface cavities and actual fragmentation by the granules is very rare. This indicates that the dominating breakage mechanism can be categorised as surface abrasion with severe damages. At higher normal stresses a larger fraction of the particles are fragmented as indicated by Fig. 14. Matrix salt particles and loose cellulose fibres can be identified in the debris/dust section. Irregular mother particles indicate that the cellulose fibres connect and reinforce the entire particle. Therefore, it is difficult to remove material on one side of the particle without it affecting the entire particle. The observed mechanism of breakage is defined
Fig. 14. Dust/debris from Placebo-B particles sheared for a strain of 1265 (equivalent to 10 revolutions) with a normal stress of 22.7 kPa.
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as lying somewhere between surface abrasion and fragmentation.
4. Conclusions Experiments with an Impact Tester and an Attrition Shear Cell have shown that when detergent enzyme granules are exposed to repeated impact stresses at impact velocities of 10 m/s, the release of enzyme-active dust is mainly caused by the release of small chips of coating associated with local delamination, and thus the exposure of the underlying enzyme layer or enzyme-containing core. There are indications that reduced enzyme release can be achieved by incorporating the enzyme into the core of the granule. Impact experiments with sodium sulphate crystal particles reveal a clear transition from chipping to fragmentation as the main breakage mechanism at impact velocities between 12 and 14 m/s. For granulated particles a high extent of breakage was observed even at low impact velocities (~8 m/s) for the gently produced nonpareil particles, while Placebo-B particles do not show a significant increase in the extent of breakage until an impact velocity of about 20 m/s is achieved. The fibres in the Placebo-B particle act as crack stoppers and thereby reinforce the particle. ASC experiments with placebo materials show that the surface topology of the particles tested as well as the abrasion properties of the worn-off material has a strong effect on the shear breakage. Therefore, attention should be paid when shearing layer-structured particles. The relationship between the ability of the enzyme granule core materials to withstand shear and impact stresses and the release of enzyme-active dust from granules produced using that specific core material is not straightforward: Factors such as surface friction and the ability of coating layers to improve the breakage resistance of enzyme granules tremendously makes a correlation between shear and impact resistance of enzyme granules and core materials difficult to achieve. However, the results do show that the applied experimental techniques may be valuable tests in the formulation and design of particles with the desired strength properties. Nomenclature a bscouring factorQ in the Attrition Shear Cell (ASC) D avg average diameter of the ASC (m) ˙c strain rate in the ASC (s 1) C average shear strain experienced by the particles in the ASC h height of bed of particles in the ASC (m) N the number of revolutions the ASC performs in each experiment x speed of rotation of the ASC (rpm) t time (s)
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Acknowledgements This project was performed in collaboration with Novozymes A/S, who also financially supported the project. The CHEC (Combustion and Harmful Emission Control) Research Centre is financially supported by the Technical University of Denmark, Elsam A/S and Eltra, Energi E2 A/S and Elkraft System, the Danish Technical Research Council, the Danish and Nordic Energy Research Programmes, EU Energy Research Programmes and industrial partners.
Appendix A This section contains the definitions of the breakage mechanisms used throughout this paper. The definitions largely follow those given by Beekman [19]. Chipping: When the particles and uncoated granules are exposed to large tangential forces, chipping may occur, i.e., the surface of the particles is damaged and some material is chipped off. Chipping is caused by the generation and propagation of lateral cracks, which form as a result of the relaxation of elastic deformation at the contact point, where the stress is imposed [28]. The term chipping is also used in this work for damage caused by small normal forces, which are not a result of erosion nor fatigue. Fracture/fragmentation: When large forces (typically impact forces) are applied to the particles and uncoated granules head-on, it may lead to fracture/fragmentation, i.e., breakage of the granule into large pieces (fragments). These actions often lead to exposure of the core and are thus considered the most severe kind of damage. Fracture or fragmentation is largely caused by radial cracks, which form into the body of the particle [28]. However, also meridian plane cracks may form as a result of shear cone and compression failure beneath the contact area of the impact site [9,10,27]. Abrasion: The small tangential forces lead to polishing and rounding of the particles and uncoated granules. This may also lead to loss of granule coating. Attrition: Small normal forces (typically impact forces) may lead to two kinds of damage to the particles and granules: – Fatigue: After several impacts, small cracks formed deep inside the particle or granule may have propagated and will eventually lead to severe breakage, where the particle/granule loses mass at a continuously increasing rate. – Erosion is defined as damage to a specific layer in the coated granule, i.e., the granule is peeled, for example, by losing large pieces of the coating layer. Erosion is only a valid concept for granules with a layered structure and it does not penetrate the granule. This type of breakage can be seen as a severe type of coating delamination.
Local delamination of coating: Chips of the granule coating may be lost as a result of local delamination caused by stresses in the granule coating [8]. This is typically revealed by loss of small pieces of coating, and the breakage typically stops at a layer–layer interface.
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