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GENERAL RESEARCH Prediction of Fertilizer Granulation: Effect of Binder Viscosity on Random Coalescence Model G. M. Walker,* C. R. Holland, and M. N. Ahmad School of Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, Northern Ireland, U.K.
J. N. Fox and A. G. Kells Irish Fertilizer Industries Ltd., Herdman Channel Road, Belfast BT3 9AP, Northern Ireland, U.K.
The effect of binder viscosity on the granulation of NPK fertilizers has been experimentally investigated at ambient temperature. The viscosity of the binder solutions used in these trials was higher than in any previous work on fertilizer granulation and more representative of the viscosity in industrial granulation units. In general, a higher binder viscosity resulted in a higher degree of granulation. The experimental data were correlated using a random coalescence theory, and first-stage coalescence kernels were determined for variations in binder viscosity and fractional saturation. These experimental coalescence kernels were then modeled using a normalization procedure that took the effects of binder viscosity and fractional saturation into account. The modeled data fit the experimental data well over a wide range of binder viscosities and fractional saturations. Further results were gathered from a high-temperature industrial granulation system and were used to validate the normalized coalescence model. These results indicate that high-temperature industrial granulation systems can be accurately predicted from bench-scale data if the fractional saturation of the granulate and the viscosity of the binder solution are accurately known. 1. Introduction The granulation process for compound fertilizers was developed to minimize costs of transportation, baggage storage and handling. In general, higher concentrations of nutrients resulted in higher caking problems for nongranular compounds.1 In high-nitrate NPK fertilizer grades, caking in granular fertilizer is still a problem2 that can be partially alleviated by producing a granulate that has a high mean particle size and a narrow particle size distribution.3 The high-temperature granulation of compound fertilizers is therefore a crucial step in the manufacture of a high-quality fertilizer product. Granulation. Sastry and Fuerstenau4 detailed the mechanisms for granule growth (granulation), including nucleation, growth, random coalescence, pseudo-layering, and crushing and layering. Litster and Liu,5 in their studies on the granulation of fertilizers, found that coalescence is the most probable mechanism for lowtemperature fertilizer granulation using a feed with a broad particle size distribution. Coalescence (agglomeration) occurs when two or more particles adhere together with a liquid as the binding agent. In industrial processes, binding liquids include * Author to whom correspondence should be addressed. Tel.: (028) 91274172. Fax: (028) 91381753. E-mail
[email protected].
water, water-based solutions, and melts. Random coalescence occurs when the rate of agglomeration is sizeindependent. Preferential coalescence occurs when agglomeration is size-dependent. The solution-to-solid phase ratio has been identified by almost every researcher in this field as the governing factor in granulation, with a high ratio resulting in a high degree of granulation. Adetayo et al.6 identified the binder viscosity as another critical parameter, with high binder viscosity also resulting in a high degree of granulation. Previous research in fertilizer agglomeration concentrated on room-temperature systems. Furthermore, because saturated solutions of fertilizer salts were used as the binder solutions, only relatively low viscosity binders have been investigated.6,7 Industrial fertilizer granulation units operate at a viscosity well in excess of these bench-scale trials, typically 50-300 cps rather than 1-8 cps.7 Also, industrial systems operate at higher temperatures and lower moisture contents than bench-scale simulations. It is the aim of this work to investigate these factors, which have received limited attention to date, by undertaking bench-scale simulations that accurately mimic industrial operations in terms of binder viscosity and percentage saturated solution. This work also focuses on the application of current granulation theory to an industrial-scale unit operating at high temperature.
10.1021/ie000647s CCC: $20.00 © 2001 American Chemical Society Published on Web 04/07/2001
Ind. Eng. Chem. Res., Vol. 40, No. 9, 2001 2129 Table 1. Solubility and Viscosity of Fertilizer Materials solubilitya solubilitya viscosity at 100 °C at 20 °C at 20 °C (cps)
fertilizer salt ammonium nitrate8 monoammonium phosphate8 diammonium phosphate8 potassium chloride8 27:6:6 fertilizer carboxymethyl cellulose (0.05%) carboxymethyl cellulose (0.10%) carboxymethyl cellulose (0.15%) a
871.0 63.4 60 77.3 1043 -
192.4 27.2 40.8 37.2 202 -
5.50b 25.2 51.0 76.5
Solubility in g per 100 g of water. b Saturated solution.
2. Experimental Section The bench-scale granulation process is similar to that used in previous research in low-temperature fertilizer granulation.6,7 Materials. The granulation experiments were undertaken in a stainless steel drum granulator, with a length of 0.50 m and a diameter of 0.25 m. The drum contained no internal flights. One end of the drum was completely sealed, while the other provided access for the charge of initial fertilizer and the injection of the binder solution. The critical speed within the drums is the speed at which material can be carried just around the drum by centrifugal action. The critical speed can be defined in terms of the Froude number () N2D/g) describing the ratio of the inertial and gravitational forces as
NFR ) 42.4D
differences in solubility between fertilizer salts as the ratio of the volume of the liquid phase to the volume of the solid phase in the granule
y)
3. Granulation Theory Solid-Liquid Phase Ratio. Smith and Nienow9 defined the solid-solution phase ratio to account for
(1 - ms)Fl
(1)
where m is the moisture content, s is the fertilizer solubility, Ff is the solid fertilizer density, and Fl is the liquid fertilizer density. For each granulation system, the fertilizer liquid and solid densities remain constant, and the liquid phase ratio is therefore a function of water content and fertilizer solubility, which itself is a function of granulation temperature. Industrial-scale fertilizer granulation usually takes place at elevated temperatures, above 100 °C. In this work, the solution phase ratio was calculated to simulate that of an industrial fertilizer granulator operating at 100-105 °C, with this value used as the initial solution-to-solid phase ratio for the pilot-scale drum granulation experiments (20 °C). To maintain the correct ratio, the moisture content in the granulator was significantly increased for the low-temperature experiments. The fractional saturation, Ssat, can be written in terms of the solution-solid phase ratio using eq 2, where p is the volume fraction of pores in the granule.6
Ssat )
-0.5
where N is the rotational speed (rpm) and D is the drum diameter (m). In practice, efficient granulation can be achieved in drums containing no internal flights at speeds of NFR ≈ 0.3-0.5. The rotational speed of the drum was maintained at 36 rpm, which approximated NFR ≈ 0.4 for this size drum.7 A fertilizer with a composition of 27:6:6 (NPK) was used for this research, with the analysis made up of ammonium nitrate, ammonium phosphate, and potassium chloride. Table 1 shows the solubilities of these components and of the compound fertilizer at 100 and 20 °C.8 The binder solution in these experiments was carboxy methyl cellulose (CMC). CMC is a technicalgrade binder that was dissolved in water to produce binder solutions of varying viscosity. The viscosities of the binder solutions at 20 °C are correlated in Table 1. The viscosity data were calculated using a Brookfield DVIII rheometer. Fertilizer Granulation Procedure. The laboratory experimental procedure was similar to those of previous researchers,6,7 with granulation taking place at room temperature in the stainless steel drum. Binder solution was added to the drum via a syringe and tube arrangement, with the initial fertilizer charge taken from an industrial-scale fertilizer granulation unit. The fertilizer granules were quenched with liquid nitrogen after a predetermined granulation time, before being sieved in standard test sieves. The effects of three process parameters on granulation were investigated, namely, the granulation time (5, 10, and 15 min), the amount of moisture added to the fertilizer (4, 6, and 8% wt/wt), and the binder viscosity (25, 50, and 75 cps).
m(1 - s)Ff
y(1 - p) p
(2)
Granulation Kinetics. The kinetics of fertilizer granulation have been described by Ennis et al.10 in terms of the viscous Stokes number, which was defined as the ratio of the relative kinetic energy between colliding particles to the viscous dissipation about the pendular bond. Adetayo et al.6 modified this original relationship for drum granulation, yielding the equation
Stv )
8FgrωR 9µ
(3)
where Fg is the granule density (kg m-3), r is the effective granule size (m), ω is the granulator speed (s-1), R is the granulator radius (m), and µ is the binder viscosity (kg m-1 s-1). A critical Stokes number must be surpassed for the rebound of colliding particles to occur10
( ) ()
Stv* ) 1 +
h 1 ln e ha
(4)
where e is the coefficient of restitution, h is the thickness of the binder layer (m), and ha is the surface asperites (m). Three granulation regimes were defined in terms of the magnitude of Stv in comparison with Stv*.
Stage I
Stv , Stv* noninertial regime
Intermediate stage Stv ≈ Stv* inertial regime Stage II
Stv . Stv* coating regime6
In the first noninertial regime, there is a very high probability of successful collisions between granules, and granulation can be described by random coalescence. As the granulation process continues, the presence of large particles narrows the size distribution. This is caused by collisions between fine and large particles,
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which removes the small particles from the distribution. This inertial regime is characterized by low moisture content and rapid attainment of an equilibrium size distribution. In the coating regime, granules can grow significantly larger than the initial size distribution, and the size of the distribution is broadened. In this regime, not all collisions are successful, and growth is by preferential coalescence with larger particles, i.e., the size distribution of the granulator will be composed of oversize and undersize.6 Growth of a size-independent kernel was studied previously by Adetayo et al.,6 who showed that the total granule number and mean granule size vary as
N ) exp(-k1t/2) N0
(5)
r ) exp(-k1t/6) r0
(6)
A size-independent kernel is suitable for describing the first stage of granulation or the noninertial regime where the probability of successful coalescence following a collision is independent of particle size. The granulation size distribution data from this experimental program were modeled using the normalized k1t1 granulation procedure postulated by Adetayo et al.,6 where k1t1 is the normalized extent of granulation and is given by
( )
k1t1 ) k1t1 + m ln
µ j Fg
µFjg
Ssat
(7)
where t1 is the granulation time in first stage, Fjg is j is granule density of reference material (kg m-3), and µ the binder viscosity of the reference material (kg m-1 s-1). The reference material in this case is diammonium phosphate (DAP). The calculation of the constant, m, has been well documented by previous researchers in fertilizer drum granulation.6 This procedure uses the knowledge of the first-stage extent of granulation k1t1 for DAP materials6 to predict the corresponding extent of granulation k1t1 for other fertilizer materials of known viscosity and density. 4. Results and Discussion Effect of Solution-Solid Phase Ratio on Granulation. The effect of the solution-solid phase ratio on the extent of granulation is illustrated in Figure 1 as a plot of particle size (mm) versus accumulative percentage for moisture contents of 4, 6, and 8% (wt/wt) for a constant granulation time and viscosity of 5 min and 75 cps, respectively. The results indicate that the extent of granulation, i.e., the increase in the granulate particle size distribution, is dependent on the moisture content and, thus, the solution-solid phase ratio. These results agree with the findings of many previous researchers in that increased moisture content results in increased fertilizer granulation.6,7 From these initial results, we can reasonably conclude that the CMC solution acts in much the same way as the soluble fertilizer salts in the binding of fertilizer granules. Furthermore, the results for granulation under all experimental conditions used in this work (see Table 2) indicate that fertilizer granulation proceeds in the first noninertial stage of granulation. This phenomenon
Figure 1. Granulate particle size versus cumulative percentage weight for variations in percentage (%) binder solution (temperature ) 15 °C, time ) 5 min, viscosity ) 75 cps).
correlates with previous researchers who found that the (second) inertial regime of granulation does not occur in the granulation of fertilizer with a low moisture content and that a critical saturation must be reached before the second stage of granulation proceeds.6,7 The second stage of granulation relies on plastic deformation of colliding granules at the point of contact to achieve successful coalescence.6 Effect of Time on Granulation. The effect of time on granulation is illustrated in Figure 2 as a plot of particle size (mm) versus accumulated percentage for granulation times of 5, 10, and 15 min and a constant binder content and viscosity of 6% and 75 cps, respectively. The results appear to indicate that the extent of granulation, i.e., the increase in the granulate particle size distribution, is dependent on granulation time; however, the rate of increase in granulation decreases with time, as would be expected with the Ennis granulation model.10 Effect of Binder Viscosity on Granulation. The effect of binder viscosity on the extent of granulation is illustrated in Figure 3 as a plot of particle size (mm) versus accumulative percentage for binder viscosities of 25, 50, and 75 cps and a constant granulation time and binder moisture content of 5 min and 6%, respectively. The results indicate that the extent of granulation, i.e., the increase in the granulate particle size distribution, is dependent on the binder viscosity. These results are in agreement with results from previous researchers in that increased binder viscosity results in increased fertilizer granulation.6 The previous researchers, however, used fertilizer-saturated solution binders, which have a relatively low viscosity of between 3 and 8 cps and a limited range of viscosity. The results depicted in Figure 3 show that this trend in increased granulation with increased viscosity occurs over an extended range of viscosity. Effect of Binder Viscosity on Coalescence Kernel. The data taken from the granulation experiments were correlated with the coalescence kernel theory for the first 5 min of the granulation using eq 6. The results from this correlation are illustrated in Figure 4 as a plot of the first-stage coalescence kernel (k1) versus binder viscosity with variation in binder content. The results indicate that the coalescence kernel, and therefore the extent of granulation, is a function of both binder viscosity and binder moisture content, with the coalescence kernel increasing with increasing binder viscosity and binder content. The increase in coalescence kernel with viscosity for
Ind. Eng. Chem. Res., Vol. 40, No. 9, 2001 2131 Table 2. Granulation Data from Bench-Scale and Industrial-Scale Trials run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 industrial unit industrial unit industrial unit
cmc (%)
moisture (%)
moisturea (%)
yb
time (min)
Ssatc (%)
viscosity (cps)
dexpt
k1t1d
Stokes no.e
kt normalf
k1g
dpredictedh (mm)
0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1 0.1 0.1 0.15 0.15 0.15 0.15 0.15
4 6 8 4 6 8 6 4 8 4 6 8 4 8 6 6 4 -
3.96 5.94 7.92 3.96 5.94 7.92 5.88 3.92 7.84 3.92 5.88 7.84 3.88 7.76 5.82 5.82 3.88 2.20 1.90 2.00
0.069 0.105 0.143 0.069 0.105 0.143 0.104 0.069 0.141 0.069 0.104 0.141 0.068 0.139 0.103 0.103 0.068 -
5 5 5 10 10 10 5 5 5 10 10 10 5 5 5 10 10 1 1 1
0.075 0.114 0.154 0.075 0.114 0.154 0.113 0.074 0.153 0.074 0.113 0.153 0.073 0.151 0.112 0.112 0.073 0.075 0.075 0.075
25 25 25 25 25 25 50 50 50 50 50 50 75 75 75 75 75 25 25 25
1.50 1.78 1.94 1.36 1.75 2.35 1.96 1.53 2.80 1.51 2.14 2.40 2.00 3.20 2.30 2.55 1.90 4.20 3.50 3.80
3.77 4.81 5.31 3.17 4.69 6.47 4.67 3.19 6.81 3.10 5.20 5.88 3.70 6.52 4.54 5.15 3.39 3.88 3.67 3.85
14.17 16.85 18.32 12.82 16.52 22.19 9.26 7.23 13.22 7.12 10.11 11.33 6.30 10.07 7.24 8.03 5.98 242 242 242
3.32 5.06 6.84 3.32 5.06 6.84 5.32 3.50 7.19 3.50 5.32 7.19 3.58 7.36 5.44 5.44 3.58 3.65 3.65 3.65
0.75 0.96 1.06 0.32 0.47 0.65 0.93 0.64 1.36 0.31 0.52 0.59 0.74 0.91 0.91 0.91 0.34 0.3 3.67 3.85
1.39 1.86 2.50 1.39 1.86 2.50 2.18 1.61 2.98 1.61 2.18 2.98 1.96 2.68 2.68 2.68 1.96 4.04 3.49 3.68
a Moisture available for granulation. b Solution-solid phase ratio, eq 1. c Fractional saturation, eq 2. p in eq 2 ) 0.45. d Calculated using eq 6. e Calculated using eq 3. Stokes number for industrial unit published rather than details on drum design. f Calculated using eq 7. Constant m in eq 7 calculated as 4.0 in these systems. g Calculated using eq 6. h Calculated from eq 6 using predicted k1t1.
Figure 2. Granulate particle size versus cumulative percentage weight for variations in granulation time (temperature ) 15 °C, viscosity ) 75 cps, binder solution ) 6%).
Figure 3. Granulate particle size versus cumulative percentage weight for variations in binder viscosity (temperature ) 15 °C, time ) 5 min, binder solution ) 6%).
all three binder contents is shown to be nonlinear, with an apparent reduction in the increase in k1 with viscosity. This phenomenon can be attributed to difficulties in attaining complete coverage of the granule surface with binder solution using high binder viscosity. The effect of granulation time on the first-stage coalescence kernel with variation in binder viscosity is illustrated in Figure 5. The data indicate that the first-
Figure 4. First-stage coalescence kernel, k1, versus binder viscosity with variations in percentage binder solution (temperature ) 15 °C, time ) 5 min).
Figure 5. First-stage coalescence kernel, k1, versus granulation time with variations in binder viscosity (temperature ) 15 °C, binder content ) 6%).
stage coalescence kernel decreases with increasing granulation time, because the granulation moves from the noninertial to the inertial regime. The data also indicate that, for the high-viscosity binder, the coalescence kernel decreases by 90% from 5 to 10 min. This decrease in coalescence kernel is also apparent with the lower-viscosity binders but to a lesser degree. Extrapolation of the data to a zero first-stage coalescence kernel would indicate that the length of time spent in the noninertial stage depends on the viscosity of the binder
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Figure 6. Extent of granulation, k1t1, versus fractional saturation with variations in binder viscosity (temperature ) 15 °C, time ) 5 min).
Figure 7. Normalized extent of granulation, k1t1, versus fractional saturation with variations in binder viscosity (temperature ) 15 °C, time ) 5 min).
solution, with shorter granulation times being found for higher-viscosity binders. It is also noted that, although the granulation time might be shorter, the extent of granulation is increased with the higher-viscosity binders. Figure 6 shows the extent of granulation, expressed as k1t1, for the three values of binder viscosity as a function of liquid content, expressed as the fractional saturation of the granule Ssat. The extent of fertilizer granulation, k1t1, increases linearly as a function of Ssat for the three binders of differing viscosity. The linearization of these data indicates that an increase in viscosity increases the slope of the plot. The data shown here for binders with relatively high viscosities correlate well with other published work in the area using lowviscosity saturated fertilizer solutions.6,7 Figure 7 and Table 2 show the results of this analysis on the data presented here in comparison with those of previous investigators. It can be seen that, after normalization (using eq 7), the 25, 50, and 75 cps data fall very neatly on the results from Kapur.11 Although the normalized extent of granulation is higher in this work than in the work of Adetayo et al.,6 there is general agreement. The slopes of the normalized k1t1 versus Ssat plots for the data presented here are virtually identical, although it is noted that the values are in slight excess of the normalized DAP data. This indicates that the experimentally determined k1t1 value is slightly lower than would be expected with this increase in viscosity. This error can be attributed to a deterioration in the distribution of the binder solution in this experimental program, which was caused by the higher-viscosity binder. From the accuracy of the plot, we can assume that
Figure 8. Predicted versus experimental granule mean particle size based on normalized extent of granulation model for benchscale and industrial-scale granulation units.
the first-stage growth model, the viscous Stokes correlation, and the normalized extent of granulation model are successful for fertilizer granulation with binders having a viscosity of up to 75 cps. Considering the high viscosity (25-75 cps) used in this work compared with that used by previous investigators (0.85-8.5 cps), the results of this normalization procedure show an excellent correlation with the theoretical granulation rate. The results from the normalized extent of granulation were then used to predict the mean exit particle size from the bench-scale granulation unit (see Table 2). The results of this procedure are illustrated in Figure 8 as a plot of experimental versus predicted mean particle size. It can be seen that there is a good correlation between the experimental data and the data predicted from the normalized extent of granulation, with r2 correlation coefficients of 0.82, 0.91, and 0.96 for binder viscosities of 25, 50, and 75 cps, respectively. Granulation Plant Trials. The results presented in the previous section validate the first-stage random model coalescence model for a bench-scale granulation apparatus at ambient temperature. Industrial-scale fertilizer granulators operate at approximately 100 °C and at much lower moisture contents than the benchscale studies. A comparison between the granulation parameters for the Irish Fertilizer Industries’ (IFI’s) industrial-scale granulator and the bench-scale granulator used in this experimental program is given in Table 2. The high initial viscous Stokes number is due to the industrial unit having a larger diameter drum, a high rotational speed, and a large mean inlet particle size. The fractional saturated solution in the industrial system is well within the range of the bench-scale experimental program because of the increase in solubility at higher temperatures. The binder solution in the industrial system was a mixture of ammonium nitrate and ammonium phosphate and was measured at 100 °C using a Brookfield DVII Thermosel rheometer. The moisture content of the granulate was measured using the standard Karl Fisher titration technique. The normalized extent of granulation k1t1 for the industrial unit was calculated to be 3.65. This value was then used to predict the mean exit granulate particle size in the same manner as was used in the bench-scale experiment. The results of the prediction are illustrated in Figure 8 and show an excellent correlation with the data obtained from the industrial granulation unit, with
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a regression coefficient (r2) of 0.981. These results indicate that high-temperature industrial granulation systems can be accurately predicted from bench-scale data if the fractional saturation of the granulate and the viscosity of the binder solution are accurately known. 5. Conclusions The effect of binder viscosity on the granulation of NPK fertilizers has been experimentally investigated at ambient temperature. The viscosities of the binder solutions used in these trials were higher than those of any previous work on fertilizer granulation and more representative of the viscosity in industrial granulation units. In general, a higher binder viscosity resulted in a higher degree of granulation. The experimental data were correlated using a random coalescence theory, and first-stage coalescence kernels were determined for variations in the binder viscosity and fractional saturation. These experimental coalescence kernels were then modeled using a normalization procedure, that took the effects of binder viscosity and fractional saturation into account. The normalized data showed an excellent fit to the experimental data over a wide range of binder viscosities and fractional saturations. Granulation data were obtained from an industrialscale high-temperature fertilizer granulator. These data, binder viscosity and fractional saturation, were used in the model to predict the extent of granulation in the industrial unit using the normalized coalescence model. Excellent agreement between predicted and experimental results were obtained. These results indicate that high-temperature industrial granulation systems can be predicted from bench-scale data if the fractional saturation of the granulate and the viscosity of the binder solution are accurately known. Acknowledgment This work was funded by Irish Fertilizer Industries and The Industrial Research and Technology Unit for Northern Ireland under Grant ST177.
Literature Cited (1) United Nations Industrial Development Organisation. Fertilizer Manual; Development and Transfer of Technology Series No. 13; United Nations: New York, 1999. (2) Walker, G. M.; Magee, T. R. A.; Holland, C. R.; Ahmad, M. N.; Fox, N.; Moffatt, N. A.; Kells, A. G. Caking Processes in Granular NPK Fertilizer. Ind. Eng. Chem. Res. 1998, 37 (2), 435438. (3) Walker, G. M.; Magee, T. R. A.; Holland, C. R.; Ahmad, M. N.; Fox, N.; Kells, A. G. Granular fertilizer agglomeration in accelerated caking tests. Ind. Eng. Chem. Res. 1999, 38 (10), 41004103. (4) Sastry, K. V. S.; Fuerstenau, D. W. Mechanism of agglomerate growth in green pelletization. Trans. AIME 1973, 252, 254258. (5) Litster, J. D.; Liu, L. X. Population balance modelling of fertilizer granulation. In Proceedings of the 5th International Symposium on Agglomeration. Institution of Chemical Engineers: Rugby, Warwickshire, U.K., 1989; pp 611-617. (6) Adetayo, A. A.; Litster, J. D.; Pratsinis, S. E.; Ennis, B. J. Population balance modelling of drum granulation of materials with wide size distribution. Powder Technol. 1995, 82, 37-49. (7) Walker, G. M.; Holland, C. R.; Ahmad, M. N.; Fox, N.; Kells, A. G. Drum granulation of NPK fertilizers. Powder Technol. 2000, 107, pp282-288. (8) Lange, N. A. Lange’s Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1985. (9) Smith, I. G.; Nienow, A. W. Particle growth mechanisms in fluidized bed granulation II. Chem. Eng. Sci. 1983, 38, 1233-1240. (10) Ennis, B. J.; Tardos, G. I.; Pfeffer, R. A micro level based characterization of granulation phenomena. Powder Technol. 1991, 65, 257-272. (11) Kapur, P. C. Kinetics of granulation by nonrandom coalescence mechanism. Chem. Eng. Sci. 1972, 27, 1863-1869.
Received for review July 11, 2000 Revised manuscript received February 5, 2001 Accepted February 14, 2001 IE000647S
