Effect of cohesion on pneumatic conveying characteristics of powders

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 1, February 1994, pp Effect of cohesion on pneumatic conveying characteristics of powders M K Desai"* & A G McLeanh "Department of Chemical Engineering, The University of Queensland, Brisbane, Qld 4072, Australia bdepartment of Mechanical Engineering, The University of Wollongong, Wollongong, NSW 2522, Australia Received 22 March 1993; accepted 7 September 1993 Various bench tests to assess cohesion are described. Several bench tests were performed to evaluate the significance of cohesion effects in determining a powder's suitability for pneumatic conveying. To assess the effect of cohesion, arch length was evaluated in li purpose built tester. The effect of deaeration, bed height, particle size and ambient relative humidity on the arch length were also observed. In regard to bench test development, various powder properties were evaluated. The cohesion properties so measured are then correlated" with reported pneumatic conveying powder flow behaviour. Pneumatic conveying is not fully understood in regard to the conveyability of a powder on the basis of properties determined from bench tests. The important properties of powders governing pneumatic conveying characteristics include particle size and distribution, particle shape and structure, bulk density, permeability, deaeration, fluidization, cohesiveness, explosibility, moisture absorbency, toxicity, angle of repose, etc. This large number of powder variables causes pneumatic conveying to be an extremely complex phenomenon. To partially overcome this complexity, improved knowledge of the interaction between individual powder properties and conveying characteristics is sought. This improved knowledge should provide an insight as to whether a powder can or cannot be conveyed by pneumatic means, the appropriate mode of conveying, whether the flow will be stable or otherwise and whether the powder will exhibit any flow peculiarities. Furthermore, bench tests should indicate differences in flow behaviour of seemingly similar powders and provide an insight as to which powders will exhibit the most adverse conveying characteristics and explain unusual flow behaviour. The same knowledge should provide a greater understanding of the fundamental powder properties influencing pneumatic conveying. Obviously, it would' be desirable if this information was obtained using bench tests. Such bench tests should be convenient and straightforward to use. To this end, the significance of cohesion in governing powder pneumatic conveying characteristics is examined. *Author to whom correspondence should be addressed t') To assist description of the powders examined standard powder properties are also presented and briefly discussed. Actual details of this examination are now described. Basis for Conveying Mode Selection At present, the selection of the optimum mode of conveying for fine dry powders is based on a hierarchical assessment basis. In particular the first. prediction is obtained by plotting the powder's particle density and assumed mean particle size (here taken to be the powder's D (3,2) particle size) characteristics on Geldart's fluidization diagram (Fig. 1). He classifies powders into four groups according to fluidization behaviour. Group Band D powders have high gas permeability and low gas retention capacity. Group A powders have the opposite properties. Group C powders are cohesive with pool' fluidization properties. c: E Q. lj Q. Ii 1000 ~ ~ Mean particle size, j.lm Fig. I-Geldart's classification of powders

2 2 INDIAN J. ENG. MATER. SCI., FEBRUARY 1994 Alternatively for greater relevance to pneumatic conveying the Dixon slugging diagram should be used. However, for the fly ash under study most plot near or just below the AC boundary on both the Geldart and Dixon diagrams. This that some additional basis for assessment suggests be used. This additional assessment is obtained by plotting the powder characteristics on the phase diagram proposed by Mainwaring and Reed). In this more accurate predictiun, which utilizes the synergenic effects of the individual powder properties, knowledge of the powder's fluidization and deaeration characteristics are used. By use of these properties mean particle size and distribution, particle density, particle shape, capillary tortisity and fluid particle interaction account. effects are taken into These predictions and characterizations, however, neglect the direct effect of a powder's cohesiveness. Namely, it is expected that as a powder's cohesiveness increases. the powder will exhibit increasing tendency to exhibit flow pressure fluctuations and flow instabilities. In the extreme the latter may become flow blockages. These significant practical implications suggest that a powder's cohesiveness be used as an additional criterion on which to select a powder's optimal mode of conveying. In particular powders exhibiting low cohesion can be successfully conveyed in standard fluidized blow tank moving bed mode. Whereas more cohesive powders demand the use of controlled slug techniques or the use of by-pass systems. In view of this significance it is appropriate that convenient methods based on bench tests be identified to assess a powder's cohesiveness. Identification of suitable techniques and discussion of the practical application~ is the basis of this paper. It will be shown that in the very least the suggested bench tests provide useful ranking of the cohesiveness of seemingly similar powders. Before commencing discussion of the direct significance of a powder's cohesiveness, it should be noted that both the Geldart (or more appropriate Dixon slugging diagram) and Mainwaring and, Reed]",classifications indirectly make ~ome account of a powder's cohesiveness. In particular Geldart's diagram suggests that it is usual fora' powder's cohesiveness to increase as its mean particle size decreases. Further, indirect evidence is apparent by the tendency for cohesive powders to channel and in some extreme cases to slug during both' fluidization and deaeration sand testing. In regard to the latter cohesive powders initially deaerate rapidly due to the formation of channels within the bed. Hence, it is not too uncommon for the presence of cohesion to render, standard deaeration and fluidization testing meaningless. Unfortunately, such indirect assessments of cohesion do not provide adequate information for predicting the conveyability of different powders. For instance, it is not too difficult to appreciate that various powders exhibiting the same mean particle size. and particle density can exhibit vastly different cohesive behaviour. This difference in cohesive behaviour may be due to particle shape, surface texture, surface chemistry, powder bed moisture, bed temperature, powder elastic properties and ambient relative humidity. The full assessment of the effect of cohesion and its implication regarding flow mode selection as conducted in this investigation now follows. Significance of Cohesion (i) Cohesive bulk solids-significantly, cohesive bulk solids are generally not suitable for pneumatic conveying. Obviously, if a bulk solid will not reliably flow from a hopper, pneumatic conveyance will be extremely difficult. (ii) Cohesive arching-as expected, the internal friction angle, shearing cohesiveness and tensile strength of powders govern greatly plug conveying characteristics. In particular, these parameters are significant during dense phase and super dense phase conveying. Notably, in these modes, the creation and breakage of plugs depend on powder cohesiveness. In fact, problems associated with powder handling originate from powder cohesiveness. In this study, cohesiveness is assessed by examining powder arch behaviour in conveying channels. For such channels, Jenike defines the flow factor (FF) as the ratio of the local principal stress to the principal stress at the outlet region, a/ac' Obstructions to flow are evaluated on the basis that an arch will fail if the principal stress acting on it a], exceeds the unconfined yield strength of the solids, ac' In the Jenike procedure, the evaluation of arching is made by plotting both a) and ac against the major principal consolidating stress a). In this evaluation the stress ratio, defined thus, FF=a] ac... (1) is termed the solids flow function. Hence, both the flow factor and flow function are plotted against the common principal consolidating stress axis. When a) = ac (i.e. at the intersection of the

3 DESAI & McLEAN: EFFECT 0F COHESION 3 flow factor and the flow function), the arch is critical. In this case the stress acting in this critical ardi is ucrit. This stress value establishes cal outlet dimension. the criti Since the critical minimum outlet dimension is given by: B=H(a) Ag Ul where H( a )=function of a, a =hopper half angle, and Ph =bulk density, kg/m3 It follows that at the critical condition, der's cohesive strength is given by, UC- BcritAg H(a)... (2\ the pow... (3) Eq. 3) provides a convenient assessment of a powder's unconfined cohesive strength by use of an arch cohesion tester. Alternatively a powder's cohesive strength may be measured by direct shear testing. Tests to Assess Cohesion The following bench tests can be used to assess cohesion characteristics of powders which can be divided into two categories, namely direct and indirect tests. Direct tests Hand squeeze test-this is a simple qualitative test for cohesion assessment of powders. A powder sample is squeezed by hand to observe whether the sample divides easily or remains as a lump on release. Angle of repose-this simple test provides a qualitative and partial indication of cohesion. Unfortunately, for cohesive powders, the test results are highly variable and ranking of measurements is difficult. More quantitative methods/tests sion now follow: to assess cohe Arch testing-the minimum width required to assume flow from a mass or plane flow hopper / bin provides a direct convenient measure of cohesion of powders and other bulk solids. This test' can be used to compare seemingly similar cohesive powders. Direct shear tests-cohesion can be assessed from the instantaneous yield loci obtained from shear tests. There are two basic types of shear tests, the Jenike Shear Tester and the Annular Shear Tester. Other testers include Biaxial and Triaxial testers. These testers as the name suggests impose two or three orthogonal stresss components onto the sample. Simple tests like the Uniaxial Compression Tester can also be used for relative measurements. One advantage of the information gained from shear tests is that, in particular, that from the Jenike Shear Tester, can be used for the geometric design of blow tanks. Tensile testing-this is a good direct fundamental powder cohesive property which can be assessed from various tensile strength testers depending on the direction of pull with respect to th~ applied consolidation force. In particular, in split cell testers, the sample is subject to tensile forces perpendicular to the direction of consolidation. In comparison, other testers exert tensile forces on the powder in the same direction as that of the applied consolidation force. Cohesion tester- The Ajax Cohesion Tester2 can measure cohesive strength of powders in different states of consolidation. In this tester, a torque i~ applied through a floating head submerged in the sample. Measurement is done by a counter attached to a spring system through a torque pulley and floating head. Indirect tests Compressibility constant (b)-one of the indirect assessments of a powder's cohesion is the slope of bulk density versus consolidation stress variation plotted on a log-log basis. This slope is referred to as the powder's compressibility constant (b). It is suggested that high value of b indicates significant cohesion. Flow function-cohesion is indicated by the intercept formed by the powder flow function and the unconfined yield stress axis with the flow function determined from the instantaneous yield loci. It should be noted that the flow function, also called the failure function, can be used to characterize the flowability of powders3 since it is a property of the bulk material and its degree of compaction. It is generally known that an increasing flow function slope indicates significant sensitivity of powder strength with consolidation. Therefore, large flow function slope suggests the powder will exhibit unstable flow behaviour due to the action of any consolidation during flow. This tendency, to exhibit unstable flow, can be considered as an effect similar to cohesion. Obviously, this tendency for unstable flow behaviour is paramount in regard to suitability for conveyance by dense phase pneumatic conveying. Hence, consideration of this tendency as a quasi cohesion effect is an appropriate practical technique.

4 4 INDIAN J. ENG. MATER. sei., FEBRUARY 1994 Hausner ratio-this is a simple and rapid test for ewaluation of a powder's cohesion for seemingly similar powders based on bulk density measurements. This ratio is simply defined as the ratio between the dense and loosely packed bulk density states. Wall friction-based on the simple assumption that cohesive powders exhibit comparably large wall friction angles, it is suggested that wall friction properties evaluated from an aerated piston tester3 or shear testing be used to assess the extent of cohesion. Obviously, wall friction like cohesion acts to reduce the interparticle spacing during flow. This decreasing interparticle spacing mars flow. Hence, measurement of wall friction is one of the important parameters affecting indirectly cohesion and this reduces a powder's flowability. Measurement of wall friction is also useful for assessing the expected overall system pressure drop. General test requirements The bench cohesion test should require low operator skill and be convenient to use. Furthermore, the test should provide an accurate quantitative assessment of cohesion. The test should also reveal consistent trends with changing parameters. In addition it would be beneficial, if the test provides a fundamental insight into a powder's cohesive properties. Consideration should also be given to test robustness and test procedure. Approximate and first indicators of the extent of cohesion can be evaluated from bench type tests including the arch and drained angle of repose tests. Unfortunately, ranking and identification of similar cohesive powders is difficult from these crude bench type tests. Fortunately, mean particle size, Hausner ratio and tensile tester bench tests provide clear ranking and are also convenient to use. Experimental Procedure Standard powder properties-particle solid den~ sity was measured by use of a Beckman air-comparison pycnometer. Powder bulk density was measured in a Jenike Compressibility Tester. The particle size distributions of the various powders tested were measured in a Malvern particle sizer. Arch Tester-To obtain a convenient and rapid assessment of a powder's cohesiveness a purpose built arch tester (Fig. 2) was developed. Here, the actual cohesive strength of the powder is assessed by utilizing the theory of failure of powders bridges as developed by Jenike3 5. The strength so predicted was compared to that measured using a Jenike Direct Shear Tester. The details of the latter tester and operating procedure for the same are provided by Arnold et al6 In particular, Eq. (3) reveals that there isa strong relation between the critical arch dimension displayed by a bulk solid and the cohesive strength possessed by the same. The Arch Tester consists of a squat perspex silo. The slotted outlet is operated by a chain drive mechanism. The novel feature of the tester is that the powder remains undisturbed as the opening span increases. In comparison, conventional outlet slide gates disturb the powder during opening. The maximum outlet opening size of the slot is 100 mm. The height of the squat perspex silo is 500 mm and side length and width are 250 mm. A scale is affixed to read the arch length. The silo can be filled by raining the powder through a 3 mm or 4 mm aperture sieve. A set deaeration time was allowed before opening the outlet~ After discharge the powder was collected in a bin located below the tester. In the latter part of the testing, filling of the silo was effected by a conveyor belt arrangement. Since the ratio of the silo width to maximum outlet opening span is f.5:1, plane strain conditions are assumed to apply during gravity discharge. Hence, the tester can be used to measure the plane flow outlet span necessary to attain reliable gravity discharge. In plane strain, the channel formed within the powder itself may be referred as rough walls. This span is commonly known as the critical rough wall plane flow arch dimension3 For very cohesive excessive.powders arching prevents realistic measurement of this arching dimension. At present, the tester can be used to measure the arching dimension of powders displaying critical slot arch dimensions less thim 100 mm. DUST COUER PERSPEH OTRTlNG SCALE SIGHT GlRSS..J...J I L.. DISCHRRGE BIN Fig. 2-Arch Tester SilO HRNOlE

5 DESAI & McLEAN: EFFECT OF COHESION 5 Powder Fly ash A Fly ash B Fly ash C Fly ash D Fly ash E Fly ash F Cement Table I-Powder properties Mean particle Solid density, size ds(j'!-tm kg/m l H IU IH Bulk density, kg/ml Compressibility constant, b ;:, c 40 :i FLYASH +++I c I) IJ ~ '0''C' ~ _'A' "/ M r I., '8' I.ỊỊI AB CD "0 ox Ct E 40?: 'i ox m I r~ M V ':r= Major consolidation atress, kp. a Fig. 5-Bulk density versus major consolidation stress ""rtlcle alz., Jlm M ox Q E ~ c "0 :J m Fig. 3-Particle size distribution versus 0;;, undersize of fly ~ ash A, B, C and D _ 'F' -ca.err ash 'E, F, G and cement ze distribution versus % undersize of fly fly ASH "" 60 ~...c ;:, _'E' ~ _'G' 1600 FG E "OO~ ca.t:nt 800 o Major consolidation streas, kp. Fig. 6-Bulk density versus major consolidation stress Re.sults The observed powder properties are summarized in Table 1. Figs 3 and 4 depict the particle size distributions of the powders tested. Figs 5 and 6 depict the variation of bulk density with major consolidation stress for the fly ash and cement tested. Table 2 summarizes the observed critical arch dimension with maximum deaeration time as measured in the Arch Tester. In addition, this table

6 m 6 friction, Jenike's Walker's of rom 0./0< Slope degree ' kpa Experimental Internal Effective d; < friction, internal f' 64 a method min. Critical angle method Oeaeration arch INDIAN length offf time J. ENG. MATER incl. filling SCI., FEBRUARY 1994 Table 2-Cohesive Arch Tester results and comparison represents a comparison of the predicted arch di mension assuming rough wall boundaries to apply in the Jenike analysis in particular Eq. (2). In this latter evaluation, the flow functions declared in Table 3 were used. Figs 7, 8 and 9 depict the observed arch length variation with deaeration time and ambient relative humidity for fly ash A, C and cement, respectively. Fig. 9 depicts the variation of bed height versus arch length for cement, whereas Fig. 10 represents the variation of mean particle size versus arch length for the fly ash and cement tested in the arch tester. The observed pow~er flow functions are summarized in Table 3, whereas loose poured bulk density and Hausner ratio are shown in Table 4. Discussion Cohesive arching-from the arch tester results, the ranking of powders tested is clearly, from, worst to best, fly ash E, C, cement, fly ash F, D and B. This ranking does not correlate with that predicted by Jenike and Walker methods. The arch tester may give the lower bound on the critical arch opening, whereas the Jenike method the upper bound. The arch length for fly ash A was tested on two days having different relative humidity, namely

7 55%.,..Ị..,.,., I 70 DESAI & MclEAN: J C. e.c RELHUM. EFFECT Ec: SO a..! c % 70"'- OF COHESION7 61% L201 30~ I I 2 373"'-.,..,.I o..."lon lime, mln o... lion lime, 1 mln Fig. 60 t:lcp. ~ 70 E.., a. Ec:.sE '. 2040, I.! 30.r. uc 9-Arch lenglh versus deaeration time for cement I o... lion lime, mln Bed height, mm Fig. 8-Arch length versus deaeration time for fly ash C Fig. lo-arch lenglh versus bed height for cement 55% and 59%, refer Fig. 7. An examination of this figure reveals that the arch length increased significantly with increasing relative humidity and the arch length increased as the deaeration time including filling was increased. This observation is consistent with that of Moleros 7 Further evidence for the increase in cohesive arch length with increasing relative humidity was clearly apparent from the arch tests conducted on fly ash C and cement, refer Figs 8 and 9, respectively. This may be due to capillary condensation of water in the interstices resulting in a component additional to the van der Waals' attractions. Thus, the environmental relative humidity strongly influences cohesive arch length and hence cohesive properties of powders during transport.. It was observed that fly ash E becomes cohesive after considerable deaeration. In fact, the cohesive arch formed was so strong that no flow occurred and with arch some 30 mm high developing across the outlet. Since, fly ash E exhibited the smallest mean particle size, the observed trend in the arch length suggested that as the mean particle size decreases, the cohesive arch

8 8 INDIAN J. ENG. MATER SCI., FEBRUARY 1994 length increases. This small particle size was consistent with the agglomeration tendency observed for this fly ash whilst filling the arch tester using sieves. Fig. 11 depicts the variation of mean particle size verlms arch length for the fly ash and cement tested. This figure reveals that as the mean particle size dso decreases, the arch length increases. This trend is consistent with that observed by Borg9; Unfortunately, testing with the arch tester indicated that the arch length was dependent on the depth of material in the tester: This trend is evident in Fig. 10 as observed for cement. Fly ash B was found to be relatively free flowing compared to the other fly ash. In comparison, fly ash C was very cohesive. In fact, the powder tends to form vertical flow channels after long deaeration times. Furthermore, this powder tended to arch across the tester outlet. In particular after considerable deaeration, measurement of the cohesive arch length was impossible due to the occurrence of cohesive arches. Unfortunately, results from the cohesive Arch Tester proved to be highly sensitive to test conditions including ambient relative humidity, filling procedure and extent of deaeration. Hence, at best the Arch Tester should only be used for indicating the cohesiveness of various powders or to indicate the relative differences in cohesiveness between seemingly similar powders. Flow function-the flow function of powders were determined in a Jenike Direct Sheat Tester at various consolidation loads. This testing revealed that fly ash E is the most cohesive and cement is least cohesive of the powders tested. The s: m C.. E I L iii 100?- oo~ other fly ash exhibited intermediate cohesive strength as revealed in Table 4. A further characteristic of the flow function is its slope. From Table 4, it is evident that fly ash E has the highest slope of Noting that an increase in the flow function slope indicates increasing cohesiveness, it is evident that fly ash E is the most cohesive. This observation is consistent with its small mean particle size. Furthermore, powders exhibiting steep flow functions have a greater strength and ability to support an arch and strong tendency to form plugs during pneumatic conveyingl0. These plugs will possess considerable strength and be hard to dislodge due to the consolidation effect caused by the action of the significant body forces present during the plug formation process. Hausner ratio-from Table 4, it is evident that the Hausner ratio is highest for fly ash C compared to other fly ash tested. The loose poured bulk density of the powders tested is the average of three observations. This value is closely followed by that for fly ash E, D, B, A, cement, fly ash F, in that order. Hence, the variation of this ratio suggests that it is a good indication of the cohesion characteristics of a powder. In particular, a higher value indicating greater cohesiveness and hence decreasing flowability. This trend is consistent with that observed by Geldart et aly who reported that the ratio of tapped and aerated bulk density is less than 1.25 for group A, from 1.25 to 1.4 for group AC and greater than 1.4 for group C powders. Furthermore, Seville12 has indicated that the Hausner ratio is an indirect measure of interparticle forces. Hamby et al.13 also reported the variation of the Hausner ratio with relative humidity for Ballotini. In particular, an increase of the Hausner ratio was observed with a decrease in particle size and an increase in relative humidity. Fig. ll-arch length versus mean particle size for the fly ash and cement tested Table 4-Loose poured bulk density and Hausner ratio Material Loose poured Hausner bulk density, ratio. Fly ash A Fly ash B FlyashC FlyashD FlyashE Fly ash F Cement kglm ].342 ].5]2 1.8]5 ] ] 1.38]

9 DESAI & McLEAN: EFFEcr OF COHESION 9.!!.... c -. ~ x o -. '8' '0, 'f' '''' ca.err Fig. 12-The Hausner ratio versus mean particle size for the fly ash and cement tested Fig. 12 which depicts the variation of the Hausner ratio with d50, mean particle diameter Cum) indicates this ratio increases with decreasing particle size. A closer examination of this figure suggests that the Hausner ratio is relatively high for fly ash C. This high numerical value is consistent with the higher cohesiveness exhibited by this fly ash as predicted from other bench tests. The same also suggests that fly ash C will display greater tendencies for flow fluctuations especially in dense phase systems. The exponential trend correlation for the variation in Fig. 11 is y= X-O.173 where x= d50 Effect of Cohesion Simple non-cohesive bulk solids can be pneumatically conveyed in dense phase pneumatic conveying in either plug or moving bed depending upon the powder's deaeration and permeability characteristics. Increasing cohesion results in greater instabilities during pneumatic conveying and more attention should be given to pipe details, e.g., bends, flanges, pipe internal smoothness and blow tank design. Location of aeration points also become paramount. Obviously, unstable flow and blockages are relevant when transporting powders with considerable cohesion. Notably, cohesive powders can be maintained in a fluidized state, this will act to offset to some extent, the rapid deaeration characteristics of such powders. Here aeration must be effective to reduce the difficulties associated with cohesiveness. These requirements reflect the unstable flow behaviour resulting from any flow disengagement or compaction phenomena. Generally, for higwy cohesive materials only dilute phase is possible. For such materials, reliable and consistent feeding is difficult. Such powders require application of pipe vibration, controlled 10 n particle alze, 11m 20 slug length or by-pass conveying systems for reliable transport. Hence, cohesion is an important powder property for selection of a powder's optimal pneumatic conveying mode. Such information is vital in dense, super dense phase and _to a lesserextent dilute phase pneumatic conveying. Conclusion's 1 In assessing the suitability and most suitable mode of pneumatic conveying, particle density, bulk density, compressibility and particle size are important parameters. Furthermore, knowledge of these powder properties is essential for predicting pneumatic conveying behaviour. Fortunately, these properties can be assessed by simple and convenient test procedures. In comparison, assessment of a powder's cohesion is more difficult. To' this end, more convenient and rapid test procedures are required. 2 The ratio of packed to loose poured bulk density is a relatively simple yet accurate bench test to compare cohesion of seemingly similar powders. 3 The cohesive arch tester provides a convenient comparative assessment or ranking of a powder's cohesive strength. In this tester, significant cohesiveness is indicated by the tendency of a powder to arch in the tester. 4 Various bench tests provide varying degrees of qualitative and quantitative evaluation of powder cohesiveness. Depending upon the requirements, selection of bench tests should be made. Crude qualitative tests including hand squeeze test, cohesive arch tester and angie of repose, whereas, for simple and convenient but accurate measurement, quantitative bench tests such as the tensile tester and Hausner ratio should be used. 5. The various powder properties determined from bench tests provide convenient and rapid assessment of a powder's cohesion/flowability. This. assessment is particularly useful for rapid relative flowability evaluation and ranking of different powders. The powder properties determined from bench tests yield useful information in regard to the suitability of and identification of the optimum mode of powder transport by pneumatic conveying. This same quantitative information is most useful for the prediction and design of practical pneumatic conveying systems in particular. References I Mainwaring N J & Reed A R, Bulk Solids Handling, 7 (1987) Svarovsky L, Powder testing guide (Elsevier Applied Science Publisher, UK), 1987,67-71.

10 10 INDIAN J. ENG. MATER. SCI., FEBRUARY Jeni.eA W, GmvilyFlow ofbulic Solids, Des8i M K, Flow assessment of powders -in pne&urultic conveying, Ph.D. Dissertation, University of Wollongong, Jenike A W, GmvUy. Flow of Bulk Solids, Arnold P C, McLean A G & Robens A W, Bulk Solids: StomgeandHandiing (Tunra Ltd., Newcastle), MolerusO, ChemEngCommun, 15(1982) Visser J, Powder Techno~ 58 (1989) Borg L Ter, Ger Chem Eng. 5 (1982) Konrad K, Powder Techno~ 49 (1986) Geldan D, Hamby N & Wong A C, Powder Technhl. 37 (1984) Seville J P K, Tribology in Particulilte Technology (Adam Hilger), Hamby N, Hawkins A E & Vandame D, Adv in Particulate Technology Symp., Manchester (1987).