Effects of Capillary Condensation on the Caking of Bulk Sucrose

ARTICLE IN PRESS

Journal of Food Engineering xxx (2005) xxx–xxx www.elsevier.com/locate/jfoodeng

Eﬀects of capillary condensation on the caking of bulk sucrose

S.W. Billings, J.E. Bronlund, A.H.J. Paterson *

Institute of Technology and Engineering, Massey University, P.O. Box 11-222, Palmerston North, New Zealand

Received 31 January 2005; accepted 2 August 2005

Abstract

Caking in sucrose is a major problem for the sugar industry, especially during transportation. Liquid bridging in sucrose is the first step towards caking. This paper demonstrates, by measuring the strength of the liquid bridges and then the strength of the subsequently formed solid bridges under different water activity conditions that the critical water activity for initiation of caking is about 0.8. The identification of this point allows the conditions under which bulk sucrose is stored and transported to be specified to prevent the occurrence of water activities going above this critical level to eliminate caking. 2005 Elsevier Ltd. All rights reserved.

Keywords: Capillary condensation; Kelvin radius; Sucrose; Caking

1. Introduction the contact points of sugar crystals in a packed bed. In these capillaries, the pressure is lower than the surrounding Within the sucrose industry, caking of bulk sugar often atmosphere, which results in the saturated vapour pressure occurs in bulk sucrose during transport and storage. In above the liquid surface being less than the vapour pressure environments where there is a high daily temporal shift, of the bulk air at the same temperature. If the pressure is or where the temperature of the sucrose being placed into lowered enough to cause the vapour pressure to be greater storage is warmer than the ambient stored temperature, than the saturated partial pressure above the capillary sur- temperature gradients exist and subsequently create areas face, then condensation will occur, even at humidity levels of higher relative humidity (Bagster, 1970). In the areas below total saturation in the bulk air. of higher relative humidity, liquid bridges can form and This condensation leads to surface dissolution and upon a change in humidity, the liquid bridges can re-crys- liquid bridging between the sugar particles. When the mois- tallise to form solid bridges. The strength of the solid ture is removed (due to an eﬀect such as a temperature bridge can be strong enough so that sometimes the forces change) a solid bridge is formed between the particles. encountered during the transportation process are not enough to break them (Ludlow & Aukland, 1990). 2. Kelvin radius This paper investigates the mechanism by which these liquid bridges form. The vapor pressure required for condensation to occur The pores formed between adjacent particles allow cap- is related to the size of the capillary, the Kelvin radius illary condensation to occur at a critical water activity. (rk) [m], the wetting angle (h) and surface tension (r). These Capillary condensation can be described as the process are all related by the Kelvin equation (Adamson, 1963). by which surface tension eﬀects cause the direct condensa- P 2r cos hV 0 tion of moisture in the pores or ‘‘capillaries’’, formed by v rkRðT þ273:15Þ Aw ¼ ¼ e ð1Þ P w

* Corresponding author. Tel.: +64 6 350 5241; fax: +64 6 350 5604. where Aw is water activity, Pv is the water vapor pressure of E-mail address: [email protected] (A.H.J. Paterson). the material [Pa], Pw is the vapor pressure of pure water

2 S.W. Billings et al. / Journal of Food Engineering xxx (2005) xxx–xxx

m) 30 µ

15 Capillary Radius (

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100)

Fig. 1. Capillary radius versus water activity at 20 C.

[Pa] and V0 the volume that 1 mol of a saturated sucrose along with their equilibrium relative humidity values at solution occupies (61.3 · 106 m3/mol). 25 C(Greenspan, 1977). The Kelvin equation can be used to predict the Kelvin Two methods of caking strength measurement were used radius where capillary condensation will occur for a range on the basis of their ease of use and accuracy. These were a of water activities (Fig. 1). Capillaries of smaller radius multi point penetrometer developed by Bronlund (1997) than the Kelvin radius will be full of water/sugar solution. and a blow tester (Paterson, Bronlund, & Brooks, 2001). At 20 C, the maximum sucrose solubility was calcu- These methods also have the advantage that replicate mea- lated to be 66.7% by weight, and the surface tension for surements can be made on a sample, unlike methods such a saturated sucrose solution was estimated from sucrose as friability or angle of repose, where the sample is poured surface tension data as 80.2 N/m (Mathlouthi & Resier, out of the container or onto a surface. From the moisture 1995). Also, perfect particle wetting was assumed so a wet- sorption isotherms (Bakhit & Schmidt, 1993; Iglesias & ting (contact) angle of zero radians was used. Chirife, 1982; Roth, 1976), and results from the Kelvin Fig. 1 shows that the critical capillary radius increases equation, it was foreseen that at the higher water activities exponentially from a water activity of between 0.75 and the sugar might cake to the point where flow would not oc- 0.8 and this implies that significant liquid bridging will occur, negating the possibility of accurate results from tech- cur between the particles from this point onwards. The niques that were based on the samples flow. exponential change in capillary radius will then effect cak- The liquid and solid bridge strengths were measured ing, which should also increase exponentially from a water after one, two, and three days, as three days is typically activity of about 0.8 onwards. the maximum period of time that produced sugar will be An experiment to test this theory was carried out by stored prior to transportation. measuring the equilibrium caking strength at a range of humidities, to create a range of degrees of caking in sugar samples. Table 1 Saturated salt solutions and their relative humidities at 20 C 3. Caking strength measurement Saturated salt solution Relative humidity (%) Potassium hydroxide 9 Based on the observation that significant capillary Magnesium chloride 33 condensation between particles, (and hence liquid bridge Sodium bromide 59 Sodium chloride 75 formation), would occur above a relative humidity of 75– Ammonium chloride 79 80%, saturated salt solutions were prepared for use to Ammonium sulfate 81 provide constant relative humidity environments, with the Potassium bromide 84 main focus on the >75% RH region. The saturated salt Potassium chloride 87 solutions used in this work are listed in Table 1 below, Potassium nitrate 94 ARTICLE IN PRESS

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4. Liquid bridge strength experimental observed, the breakthrough point was the point where the penetrometer made a large shift in penetration depth, To measure the liquid bridge strength, sugar samples with the pins completely buried in the sample. The weight were placed in standard laboratory petri dishes, approxi- of water required for breakthrough was recorded and the mately 12 mm deep. The samples were then placed in air- breakthrough stress calculated by dividing by the total tight containers containing one of the saturated salt probe area. solutions above, and left to equilibrate for one, two and three days. The samples were then removed and the liquid 4.2. Blow tester bridge strength measured. Measurements for the blow test data were carried out by 4.1. Penetrometer resting the portable blow tester on the surface of the sample and holding it in place with a clamp stand. The blow tester Fig. 2 shows the penetrometer set up developed by was simply a 1 mm tube that was placed 3 mm above the Bronlund (1997). bed at an angle of 45 to the horizontal. A needle valve Penetrometer measurements were made by setting the was then used to control the airflow from a bottle of com- counter weight balance so that an empty container sat pressed air into the blow tester, with the flow rate increased weightless above the sample to be measured. Weight was steadily until the point where the airflow was sufficient to then added to the container by slowly adding water to begin dislodging sugar particles from the surface of the the container until the ‘‘breakthrough’’ point was sample. This flow rate was recorded as the break through flow rate. This flow rate was then plotted against the water activity that the samples were equilibrated at, for the different equilibration times. Further details of the blow test measurement apparatus and technique are given in Paterson et al. (2001).

5. Results

The weight of the water required for breakthrough was used to calculate the stress imposed at breakthrough on the bed and has been plotted against the water activity that the sample was equilibrated to for the diﬀering equilibration times in Fig. 3. Fig. 2. Penetrometer used in bridge strength testing.

140

120 ) 2 100

40 Breakthrough Stress (kN/m

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100)

1d equilibration 2d equilibration 3d equilibration

Fig. 3. Penetrometer data for liquid bridge strength test. ARTICLE IN PRESS

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) 70 2 y = 2.4936x + 9.3982 R2 = 0.9936 60 y = 0.75x + 35.984 R2 = 0.977 50

Intercept at a radius of 15.2 µm 40

30 Breakthrough Stress (kN/m 20

0 0 5 10 15 20 25 30 35 Kelvin Radius (µm)

Fig. 4. Breakthrough force versus Kelvin radius for penetrometer liquid bridging using two-day data.

It can be seen from Fig. 3 that the two- and three-day The graph also indicates that the onset of significant li- liquid bridge strengths are very similar. The liquid bridge quid bridge formation occurs between relative humidities strength for the sample equilibrated for one day is consid- of 75% and 80 %, which gives a value for the breakthrough erably less than that for the two and three-day samples stress of 42 kN/m2. The values for the relative humidity indicating that at least two days was required to reach a agree with the area shown in Fig. 1 where capillary conden- steady caking strength in the sample beds. It should be sation causes an exponential rise in the size of the capillar- noted that liquid bridging was expected to take place very ies being filled, explaining the occurrence of liquid bridges quickly at a particular local region in the bed and it is the between the crystals at the observed relative humidities. dynamics of moisture transfer through the bed that is Using the two-day equilibration time data, a graph of responsible for the differences noted here between days the observed liquid bridge strength was then plotted one and two. against the Kelvin radius, as shown in Fig. 4.

10 Air Flow rate (L/min)

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100)

1d equilibration 2d equilibration 3d equilibration

Fig. 5. Air ﬂow rate required for blowing a channel in the powder bed for liquid bridge strength test. ARTICLE IN PRESS

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10 y = 0.5351x - 3.2426 R2 = 0.9829 8

y = 0.2922x + 0.3613 2 µ Air Flow rate (L/min) R = 0.9523 Intercept occurs at a radius of 14.8 m 4

0 0 5 10 15 20 25 30 35 Kelvin Radius (µm)

Fig. 6. Air ﬂow rate required for blowing a channel in the powder bed with liquid bridging versus Kelvin radius for blow tester using two-day data.

It can be seen in Fig. 4 that the data can be interpreted bridges, of significant strength, began to form. Again, there as two linear relationships. The intersection of the two lines are two strong linear relationships plotted, with the size of can be interpreted as being the point at which the two the capillaries at the intersection being calculated as mechanisms of adsorption change over. Below this point 14.8 lm. This corresponds to a water activity of 0.76, there is very little significant caking, while above the inter- which again is in the range where the capillary condensa- section, caking is significant and the Kelvin equation shows tion size begins to increase exponentially. that capillary condensation builds up rapidly from this point. By solving the equations simultaneously, it was pos- 6. Solid bridge strength experimental sible to calculate the Kelvin radius at the intersection of the two lines, 15.2 lm. The corresponding water activity was Solid bridge formation was defined as the strength that then calculated from the Kelvin equation. the liquid bridges gained after being allowed to dry and 2r cos hV 0 crystallise out. After the liquid bridge equilibration times lnðAWÞ of one to three days, the samples were left exposed to the rk ¼ ð2Þ RðT þ 273:15Þ ambient conditions for five days that would typically be A radius of 15.2 lm corresponds to a water activity of encountered by sugar after the transportation process. 0.77, which is in the range where the capillary radius in- The shape of the graph (Fig. 7) is similar to that of the creases exponentially (from Fig. 1), and the point where isotherm for the moisture content of sugar. This would be the liquid bridge strength began to increase significantly. expected, as the size and concentration of the liquid bridge As with the penetrometer data, the blow test data in formed would increase, as the amount of water available Fig. 5 shows that the difference in strength between the for the bridges to form increased. The graph shows that be- two- and three-day equilibration time is small when com- low a water activity of 0.75 there was no significant solid pared to the differences between the one and two day equil- bridge formation between particles and that the sugar ibration time data and that at least two days were required should remain free flowing. At a water activity of greater to achieve a steady caking strength. than about 0.8, the strength of the solid bridges increases The graph shows that the strength of the liquid bridges exponentially, indicating the onset of capillary condensa- starts to become exponential at a water activity of 0.75. tion between crystals. This agrees with the Kelvin equation Above values of 94% relative humidity it was difficult to predictions, which indicated that significant capillary con- measure liquid bridge strength experimentally, as liquifica- densation would occur between the crystals at a water tion at the surface of the sample begins before the sample activity of between 0.75 and 0.8. has had a chance to equilibrate. The blow test data (Fig. 8) is similar to the penetrometer As with the penetrometer data, the liquid bridge data. The graph shows the increase of strength with water strength was plotted against the Kelvin radius (Fig. 6)in activity at a level above 0.8, with significantly strong order to observe the capillary radius at which liquid bridges being formed after only one day. As with the liquid ARTICLE IN PRESS

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450

400

) 350 2

300

250

200

150

Breakthrough Stress (kN/m 100

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100)

1d equilibration 2d equilibration 3d equilibration

Fig. 7. Penetrometer solid bridge strength data.

15 Air Flow rate (L/min) 10

0 0 0.10.20.30.40.50.60.70.80.91 Water Activity (RH/100)

1d equilibration 2d equilibration 3d equilibration

Fig. 8. Solid bridge blow test strength data. bridge experimental results, plots of solid bridge strength sponds to a water activity of 0.78, again in the region of versus the Kelvin radius were then made for both the pen- the graph in Fig. 1 where the increase becomes expo- etrometer and the blow tester. nential. Fig. 9 shows the graph for the penetrometer solid bridge Fig. 10 shows the similar treatment of the blow test data. strength versus Kelvin radius. The relationship is similar to The Kelvin radius at the intersection was calculated as that of the liquid bridge with there being two strong linear 16.3 lm, with the corresponding water activity being relationships present. The Kelvin radius at the intersection 0.78, which also lies in the region of Fig. 1 where the graph of the two ﬁt-lines was calculated as 16.2 lm which corre- is exponential. ARTICLE IN PRESS

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350

300 )

2 250 y = 21.486x - 305.31 R2 = 0.9984 200

150

100

Breakthrough Stress (kN/m y = 0.3672x + 35.774 R2 = 0.9629 50 Intercept occurs at a radius of 16.2 µm

0 0 5 10 15 20 25 30 35 Kelvin Radius (µm)

Fig. 9. Breakthrough stress versus Kelvin radius for penetrometer for solid bridges.

25 y = 2.0434x - 27.493 R2 = 0.9891 20

15 Air Flow rate (L/min) 10

y = 0.3426x + 0.2435 Intercept occurs at a radius of 16.3 µm R2 = 0.981 5

0 0 5 10 15 20 25 30 35 Kelvin Radius(µm)

Fig. 10. Air ﬂow rate required to blow a channel versus Kelvin radius for blow tester for solid bridges.

7. Summary of results

By using linear relationships between the force required Table 2 for the two techniques to disrupt the bridges formed by li- Summary of results for liquid and solid bridge experimental quid and solid bridging, a mechanistic model can be built Experiment Observed strength Maximum up to describe the caking in sugar that is exposed to an at onset of caking observed strength environment with a high relative humidity. LB penetrometer 57.78 kN/m2 116.51 kN/m2 Table 2 shows a summary of the results for the liquid SB penetrometer 90.91 kN/m2 >120 kN/m2 (LB) and solid bridge (SB) data for the penetrometer and LB blow tester 8 L/min 21 L/min the blow tester. The results in Table 2 also show that the SB blow tester 8 L/min >50 L/min ARTICLE IN PRESS

8 S.W. Billings et al. / Journal of Food Engineering xxx (2005) xxx–xxx diﬀerence in stress required to break up a solid and a liquid The results of the data calculated from the Kelvin radius bridge is large. Figs. 11 and 12 plot the liquid bridge graphs, are summarised in Table 3. It can be seen that the strength against the resulting solid bridge strength using water activity at the onset of bridge formation between par- the penetrometer and the blow tester respectively and dem- ticles is between 0.76 and 0.78. At this point, the strength onstrates the linear relationship between the two. This rein- that is provided by the bridging will not be able to tolerate forces the importance of having a way of predicting the the forces naturally encountered during the transportation potential liquid bridge strength of sugar that has come process. A reasonable estimation of the point where sucrose from the dryers or silo, so that steps can be taken to avoid will form bridges that will result in signiﬁcant caking of the the formation of solid bridges that will result from the li- sucrose is at a water activity of 0.8, which corresponds to a quid bridges. Kelvin radius for the bed of 18.1 lm.

350

300 ] 2 250

200

150

100 Solid Bridge Stress [kN/m

0 0102030405060708090 Liquid Bridge Stress [kN/m2]

Fig. 11. Plot of equivalent solid bridge strength versus the liquid bridge strength for samples equilibrated for two days at diﬀerent relative humidities as measured by the penetrometer.

10 Solid Bridge Strength (L/min)

0 0 2 4 6 8 10 12 14 Liquid Bridge Strength (L/min)

Fig. 12. Plot of equivalent solid bridge strength versus the liquid bridge strength for samples equilibrated for two days at diﬀerent relative humidities as measured by the blow tester. ARTICLE IN PRESS

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Table 3 Summary of results for Kelvin radius data Experiment Strength at onset of caking (y) Kelvin radius at Water activity at ﬁt-line intersection (lm) ﬁt-line intersection LB penetrometer 57.75 kN/m2 15.2 0.77 SB penetrometer 90.91 kN/m2 16.2 0.78 LB blow tester 5 L/min 14.8 0.76 SB blow tester 13 L/min 16.3 0.78

8. Conclusion as the help from the New Zealand Sugar staff in the com- pletion of this project. The caking strength results measured by either the penetrometer or the blow tester, show that solid bridges formed References from liquid bridges at water activity above 0.8 are two to three times stronger than the liquid bridges. They also show Adamson, A. W. (1963). Physical chemistry of surfaces. New York: John that the level of liquid bridging that is required to form cak- Wiley and Sons. ing starts at a water activity of 0.77, but a level of 0.8 can be Bagster, D. F. (1970). Cause, prevention and measurement of the caking taken as the point where this becomes significant. of refined sugar—a review (part 1). International Sugar Journal, The change in slope at this critical point of the strength 72(861), 263–267. Bakhit, R., & Schmidt, S. (1993). Sorption behavior of mechanically parameter when plotted against Kelvin radius, points to mixed and freeze-dried sucrose/casein mixtures. Journal of Food the point where capillary condensation becomes the domi- Science, 58(5), 1162–1165. nant mechanism for bridge formation. It is only after this Bronlund, J. (1997). The modelling of caking in bulk lactose. PhD Thesis, that caking becomes a problem, especially if there is the Massey University, Palmerston North. opportunity for the liquid bridges to form into solid bridges. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research the National Bureau of Standards— It is therefore concluded that for the safe transportation Section A, 81A, 89–123. and storage of bulk sucrose that conditions which lead to Iglesias, H. A., & Chirife, J. (1982). Handbook of food isotherms: Water water activities above 0.8 (critical water activity for liquid sorption parameters for food and food components. New York, USA: bridge formation) be avoided, so as to prevent liquid bridg- Academic Press Inc. ing. If liquid bridging can be prevented then subsequent so- Ludlow, D. K., & Aukland, N. R. (1990). Caking and flowability problems of sugar in a cold environment. Powder Handling and lid bridging can be prevented and the longevity of storage Processing, 2(1), 21–24. ensured. This means that sugar must be packaged at mois- Mathlouthi, M., & Resier, P. (1995). Sucrose. Properties and applications. ture contents and temperatures that are sufficiently low so Glasgow, UK: Blackie Academic and Professional. that moisture movement induced by temperature changes Paterson, A. H. J., Bronlund, J. E., & Brooks, G. F. (2001). The blow test within the bulk sucrose will not cause any part of the bulk for measuring the stickiness of powders (pp. 408–441). Conference of Food Engineering 2001, AICHE conference, Reno, Nevada, USA, 4–9 sucrose to exceed the water activity of 0.8. November. Roth, D. (1976). Amorphous icing sugar produced during crushing and Acknowledgements recrystallisation as the cause of agglomeration and procedure for its avoidance. Doctorate Thesis, University of Karlsruhe (Translated The authors would like to acknowledge the financial from German by P.M.E. Merten). support of TechNZ and New Zealand Sugar Ltd as well