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The drying of alpeorujo, a waste product of the olive oil mill industry
Journal of Food Engineering 41 (1999) 229±234
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The drying of alpeorujo, a waste product of the olive oil mill industry R. Arjona *, A. Garcõa, P. Ollero Chemical and Environmental Engineering Department, University of Seville, Camino de los Descubrimientos s/n, 41092 Seville, Spain
Abstract Alpeorujo is a waste product of the olive oil mill industry that still has a signi®cant oil content. Before extracting the remaining oil with hexane, the moisture content of the wet waste product has to be reduced from approximately 65% to about 8%. To develop standards for dryer design and operation, an extensive study was carried out at laboratory scale. A drying tunnel was built to calculate drying curves, volatile emissions, ignition temperatures, and solids degradation at high temperatures while drying under several dierent operating conditions. The results of this experimental work allowed us to develop a useful drying model for designing new dryers and for assessing the behaviour of existing ones. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Drying; Olive cake; Alpeorujo; Olive oil
1. Introduction Up until the nineties, the olive oil production process was based on the so-called ``three-phase system'', which produced three streams: pure olive oil, a watery liquid called alpechõn and a solid cake called orujo or olive cake. The alpechõn contained soluble organic matter and ®ne solids and was extremely hazardous to the environment because of its high biochemical oxygen demand (BOD). The olive cake had a variable oil content of around 3% that could be economically recovered in oil extracting plants after moisture content had been reduced to 8%. To eliminate the alpechõn, a new process technology with only two euent streams (olive oil and alpeorujo) has emerged. The wet solid stream or alpeorujo contains all the substances that in the three-phase system were contained in the alpechõn and in the olive cake. The characteristics of alpeorujo are obviously very dierent from those of olive cake. It is a thick sludge that contains pieces of pit and pulp of the olive fruit as well as vegetation water. It has a moisture content of around 65%, while olive cake only had around 40±45%. This greater moisture, together with the sugars and ®ne solids that in the three-phase system were contained in
*
Corresponding author.
the vegetation water give the alpeorujo sludge a doughy consistency and makes transport, storage, and handling dicult ± it cannot be piled and must be kept in large ponds. In any case, the main problems associated with the processing of alpeorujo during oil extraction occur in the dryers. These dryers were designed for three-phase olive cake, made up of loose particles of pit and pulp with a homogeneous moisture distribution that can easily be piled up and fed through rotary dryers. On the other hand, the high moisture content of alpeorujo demands much more energy, and the sugars present in it make it sticky and dicult to dry. Alpeorujo tends to stick to the dryerÕs walls, particularly to the initial part of the trommel where the gases are hot, obstructing the gas stream and increasing ®re risk. To develop suitable standards for alpeorujo dryer design and operation, the drying process was studied at laboratory scale and the drying rate was determined with respect to operating conditions (temperature and air velocity) and agglomerate size. The volatile release and the production of distilled liquids during drying were also determined. This is the ®rst step to propose a model of alpeorujo drying similar to those applied to other solids (Blasi, 1997; Melaanen & Grùnli, 1997; Melaanen, 1996; Nasrallah & Perre, 1988; Saastamoinen & Richard, 1996; Saastamoinen, 1995; Saastamoinen & Impola, 1995), which is useful to analyse the process that takes place in the rotary dryers.
0260-8774/99/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 0 4 - 1
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R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
Fig. 1. Drying tunnel.
2. Materials and methods The laboratory dryer (Fig. 1) is a 2.5 m long, 0.15 m square tunnel with a refractory lining. The sample hangs from a stem connected to a balance that is placed over the tunnel. The sample weight is continuously recorded by a computer at scanning times chosen by the operator. The drying air is introduced into the tunnel with a blower and its ¯ow is measured by a turbine ¯owmeter and controlled by a butter¯y valve. The air is heated by four adjustable electrical resistances, and the air temperature in the tunnel is measured by two thermocouples, one placed near the sample (the drying air temperature decrease due to heat losses between the measuring position and the sample point is less than 0.3°C when the air temperature is 350°C) and the other at the tunnel inlet. An additional thermocouple may be inserted into the sample core to measure its internal temperature. The alpeorujo sample is placed in a basket made of galvanized steel mesh with an 18 mm opening that does not obstruct the air±solid contact. The baskets are cylinders with 18.0, 28.6, 34.4, and 40.1 mm diameters and 72.0 mm high. The alpeorujo sample is put into the basket immediately before the start of the drying experiment to avoid sample alterations caused by ambient air±solid interaction. With this experimental facility, it is possible to reproduce the operating conditions existing in industrial trommels used to dry alpeorujo, i.e. air ¯ows between 1000 and 10 000 kg hÿ1 mÿ2 and temperatures up to 600°C. 3. Drying experiments To analyse the drying process (drying rate vs. moisture content) under dierent operating conditions, a factorial design of experiments was carried out with air temperature, drying air velocity, and sample geometry as variables. The six temperature levels (50°C, 100°C, 200°C, 250°C, 300°C, 350°C), two velocity levels (2, 3 m/ s) and four sample sizes used in this factorial design cover the wide range of conditions actually present in industrial alpeorujo dryers. Two additional sets of experiments were carried out. The ®rst experiments were conducted with high moisture samples obtained adding water till the moisture was as high as 76%. The target of these experiences was to
determine if a constant drying rate period exists at these high moisture contents. The other set of additional experiments had the objective of studying sample core behaviour during the drying process and was carried out by inserting a thermocouple into the sample core to measure its internal temperature. The alpeorujo sample used in each experiment was previously analysed to ascertain its initial moisture, volatile and dry solid contents. The volatile emissions during drying were found by measuring the dierence between initial and ®nal volatile contents, except in those cases in which the sample was burnt. 4. Results of drying experiments Before presenting the drying curves, it is of interest to show some general observations extracted from the experiments: · The surface of the alpeorujo darkens in a few seconds. · Samples dried at temperatures of 100°C or above, harden during drying. · Samples dried at temperatures of 100°C or above shrink during drying. Shrinkage is greater at higher temperatures. · The samples dried at 250°C or above are pyrolysed and produce liquid compounds that ¯ow away from the sample falling on the inner surface of the tunnel. · The samples with a diameter of 28.6 mm or greater burn at a gas temperature of 300°C, but only when the weight loss is greater than the mass of water initially in the sample. Combustion begins in internal pores and never near the surface. At gas temperatures of 350°C or above, all the samples burn, regardless of their size. Three types of plots are used in this paper to show the experimental results: drying curves (apparent moisture content on a dry basis versus drying time), drying rate curves (drying rate expressed as apparent moisture loss per unit mass of dry solid per unit time versus apparent moisture content on a wet basis), and temperature curves (temperature of the solid versus apparent moisture content on a wet basis). The term ``apparent moisture content'' is employed instead of ``moisture content'' because it comprises both water and volatile release. Fig. 2 shows the drying curves for the four experiments conducted at 250°C with 18.0 and 28.6 mm2 diameter samples (P and G in the legend) and air velocities of 2 and 3 m/s (2 and 3 in the legend). As could be expected, the small samples dried at high drying air velocity need less time to achieve a speci®ed moisture content than large samples dried at low drying air velocity. These curves are typical curves, but the ®nal apparent moisture content is less than zero due to the
R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
Fig. 2. Experiments run at 250°C.
emissions of volatile matter that had been weighted as moisture. Fig. 3 shows the drying rate curves for an 18.0 mm diameter sample and 2 m/s air velocity. The temperature ranges from 50°C to 350°C. These curves were obtained by numerically dierentiating the moisture content vs. time data. They clearly show that the drying process is not controlled by a single mechanism throughout. There exists a warm-up period in each case but there is no constant-rate period. This last observation was explicitly proven by carrying out some drying experiments using samples to which water had been added to obtain a 76% (w.b.) moisture content. No constant-rate period was found even at this high humidity. Two falling-rate periods can also be seen in each curve. The 350°C curve presents an apparent increasing-rate period due to a quick weight loss from sample burning. As mentioned before, negative moisture contents are due to pyrolysing processes. The sample temperature-moisture content curves were obtained by inserting a thermocouple in the sample core, taking care not to alter the weight measurement. Fig. 4 shows the temperature and the drying rate curves for a drying experiment carried out at 250°C. Four dierent periods can be distinguished: an initial increasing-temperature period that matches the warm-up period of the drying rate curve, a constant-temperature period at 100°C that coincides approximately with the ®rst falling-rate period, another rising-temperature period that ends when the weight loss equalizes the initial moisture content, and a ®nal period in which the temperature of the sample slowly draws closer to the air temperature.
231
Fig. 4. Temperature curve for sample dried at 250°C.
Fig. 5. Volatile emissions.
At temperatures above 200°C, volatile release and moisture evaporation take place. In order to characterize the release of volatiles and their eect on the drying process measurements, the amount of volatiles emitted in each drying test were calculated by ®nding the difference between the volatile content of the sample before and after the test. Fig. 5 shows volatile release (%) vs. temperature curves obtained in the drying tunnel as well as in an electric furnace operating at the same temperatures for periods of 7 and 30 min. The samples that were placed in the furnace had previously been dried at 105°C. It is clearly seen that the devolatilization equilibrium at each temperature is reached after 30 min of residence time in the furnace except at high temperatures (>700°C) where only 7 min are necessary. Volatile release in the tunnel dryer seems to be exactly the same as during the 30 min residence time furnace tests. The discrepancy appearing at 300°C is due to sample pyrolysis, which produces distilled liquid products measured as volatile release in the lab-dryer while in the furnace they remain with the sample on the pan, thus causing no weight loss. At temperatures above 300°C, the samples dried in the tunnel burn, so there is no data on their volatile release. 5. Drying process analysis
Fig. 3. Drying rate at dierent temperatures.
The analysis of the drying process was carried out with respect to the major variables: drying temperature, air velocity, and sample size. Fig. 6 shows a parametric set of drying rate versus temperature plots with the apparent moisture content as the parameter for an 18.0 mm diameter sample dried
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Fig. 6. Drying rate vs. temperature.
with 2 m/s velocity air. The drying rate increases almost linearly with the temperature at any moisture content. However, at high moisture contents the drying rate is more sensitive to temperature than when moisture content is lower. This fact suggests that at low moisture contents, internal mass and heat transfer are limiting factors that increase the global resistance, while at high moisture contents this resistance is signi®cantly lower. Fig. 7 shows a parametric set of drying rate versus sample size plots for an operating temperature of 250°C with apparent moisture content as the parameter. As can be seen, the drying rate increases almost linearly with the inverse of the sample diameter. As in other mass and heat transfer processes, this result shows that the drying rate per unit mass of solid is proportional to the speci®c surface (external surface per unit sample volume). Fig. 8 shows the comparison between the maximum experimental drying rate, which corresponds to the end of the warm-up period, and the maximum theoretical drying rate. This drying rate is calculated assuming that free liquid moisture covers the whole sample surface,
which is at the wet air temperature, and by using wellestablished correlations (Jakob, 1959; Pitts & Sissom, 1997) for the heat transfer coecient. The radiant heat received by the sample from the inner walls is less than 6% of the convection heat in the experiments run at 350°C with an air velocity of 2 m/s and it is not taken into account (under this conditions the radiant heat is proportionately greater than under other conditions). Both drying rates start to dier at temperatures above 200°C. At these temperatures the drying rate is so high that part of the solid surface dries before reaching the end of the warm-up period. The in¯uence of the air velocity on the drying rate is clearly dierent at high (>100°C) and low drying temperatures. Fig. 9 shows the drying rate curves of two samples dried at 50°C but at two dierent air velocities (2 and 3 m/s). Both curves dier only in the warm-up and the ®rst falling-rate periods. This fact may be explained taking into account that the drying of surface moisture, which still exists during both drying periods, is controlled by convective heat and mass transfer between the bulk air and the solid surface. On the other hand, the rest of the drying process at these low temperatures is controlled by internal liquid and vapour diusion toward the solid external surface, a mechanism not affected by air velocity. Fig. 10 shows the drying rate curves of two samples dried at 250°C but, as before, at two dierent air velocities (2 and 3 m/s). At this high temperature, the drying rate curves dier in the course of the whole drying process except when the moisture content is very low. A possible explanation of this fact is that convective heat transfer to the solid surface plays a signi®cant role until the end of the drying process. The internal moisture ¯ows as vapour through the pores at a ¯ow rate that depends on the heat transfer rate. When the moisture content is very low, the migration of the bound water controls the drying process and the in¯uence of air velocity is negligible. 6. A qualitative drying description The previous experimental study helps to develop a qualitative drying process description for the alpeorujo
Fig. 7. Drying rate vs. sample size (250°C).
Fig. 8. Experimental drying rate vs. theoretical drying rate.
Fig. 9. Experiments run at 50°C.
R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
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tures (<200°C), however, a very short constant-rate period may exist because, as stated before, the maximum theoretical drying rate, calculated assuming liquid surface coverage at the wet-bulb temperature, coincides with the experimental maximum drying rate. 6.2. Phase II (®rst falling-rate period) Fig. 10. Experiments run at 250°C.
samples. According to this model, the drying process may have four dierent phases or periods. However, the number and the characteristics of these periods are different, depending on the drying temperature. Figs. 11 and 12 show drying curves taken as representative of low and high temperature behaviour. 6.1. Phase I (warm-up period) Initially, the entire surface is wet or saturated and the heat transferred to the solid is greater than the heat lost by evaporation. Therefore, the solid temperature rises and the drying rate grows rapidly due to higher vapour pressures. However, as the solid surface becomes unsaturated, the drying rate reaches a maximum and starts to decrease although the solid temperature is still increasing. There is no constant-rate period at the wetbulb temperature because the capillary suction does not drive enough moisture to the surface. At low tempera-
At low temperatures, the sample surface is unsaturated but free water still exists in direct contact with the drying air and there is a continuous liquid state within the porous body (funicular state (Strumillo & Kudra, 1986)). As the solid dries, the saturated portion of the surface diminishes and therefore the drying rate decreases too. At high temperatures, however, this period does not exist because it is included in phase I. 6.3. Phase III (second falling-rate period) The solid surface is now completely dry at any temperature. At low temperatures, the internal moisture diuses as liquid and vapour toward the solid surface. During this phase the surface is dry and there is no continuous liquid state within the porous body because the moisture is interspersed with gas bubbles (pendular state (Strumillo & Kudra, 1986)). At high temperatures, however, the liquid boils at approximately 100°C in the interior of the large cavities or pores formed between the pieces of pits. As the drying proceeds, the boiling front moves into the interior of the solid leaving behind a solid in the hygroscopic range. Therefore, two main zones may be distinguished (Fig. 13): a shrinking core and a partially dried shell of growing thickness. The moisture ¯ows as vapour through the interstitial spaces between the pits and pulp particles toward the solid external surface. The resistance to vapour ¯ow is very low and therefore the pressure at the boiling front is the atmospheric pressure. The moisture content of the solid in the outside shell (hygroscopic range) depends on the solid temperature that increases from 100°C at the drying front to the surface temperature.
Fig. 11. Drying process phases (50°C).
Fig. 12. Drying process phases (300°C).
Fig. 13. Drying front.
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6.4. Phase IV This phase exists only if the drying process takes place at temperatures over 200°C. The drying rate in this period decreases less than expected or it could even be approximately constant. This is because the sample surface is hot enough to lose volatile compounds that are measured as moisture loss. This qualitative description is coincident, in its major considerations, with mathematical models developed to simulate biomass drying processes. (Blasi, 1997; Melaanen & Grùnli, 1997; Nasrallah & Perre, 1988; Saastamoinen, 1995). In these models, a shrinking evaporation front is considered and the internal moisture mass transfer through the solid takes place as vapour as well as liquid. They also distinguish between high and low temperature drying behaviour. 7. Conclusions A qualitative description of the alpeorujo drying process was developed that describes the characteristics of the dierent periods that make up the whole process. In addition, a set of useful quantitative data on drying rates at dierent operating conditions was obtained. The experimental results show that a constant-rate drying period, which has traditionally been taken into account for designing alpeorujo dryers, does not exist. The operating conditions ± size, temperature, and moisture content ± at which the alpeorujo could ignite and cause a
®re in an industrial dryer were also determined. The loss of volatile matter during the drying process, which modi®es the composition of the product and may aect the quality of the oil to be extracted, was evaluated. References Blasi, C. D. (1997). Simultaneous heat, mass and momentum transfer during biomass drying. In A. V. Bridgwater & D. G. Boocock, Developments in thermochemical biomass conversion. IEA Bioenergy. Jakob. M. (1959). Heat transfer, vol. I. New York: Wiley. Melaanen, C. (1996). Numerical analysis of heat and mass transfer in drying and pyrolysis of porous media. Numerical Heat Transfer, Part A, 29, 331±355. Melaaen, C. & Grùnli, M. G. (1997). Modelling and simulation of moist wood drying and pyrolysis. In A. V. Bridgwater & D. G. Boocock, Developments in thermochemical biomass conversion. IEA Bioenergy. Nasrallah, S. B. & Perre, P. (1988). Detailed study of a model of heat and mass transfer during convective drying of porous media. International Journal of Heat Mass Transfer, 31(5), 957±967. Pitts, D. R. & Sissom, L. E. (1997). Heat transfer. New York: McGraw-Hill. Saastamoinen, J. (1995). Model for drying and pyrolysis in an updraft biomass gasi®er. In A. V. Bridgwater, Advances in thermochemical biomass conversion. Saastamoinen, J. & Impola, R. (1995). Drying of solid fuel particles in hot gases. Drying technology, 13(5±7), 1305±1315. Saastamoinen, J. & Richard, J. (1996). Simultaneous drying and pyrolysis of solid fuel particles. Combustion and Flame, 106, 288± 300. Strumillo, C. & Kudra, T. (1986). Drying: Principles, application and design. London: Gordon and Breach.
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The drying of alpeorujo, a waste product of the olive oil mill industry R. Arjona *, A. Garcõa, P. Ollero Chemical and Environmental Engineering Department, University of Seville, Camino de los Descubrimientos s/n, 41092 Seville, Spain
Abstract Alpeorujo is a waste product of the olive oil mill industry that still has a signi®cant oil content. Before extracting the remaining oil with hexane, the moisture content of the wet waste product has to be reduced from approximately 65% to about 8%. To develop standards for dryer design and operation, an extensive study was carried out at laboratory scale. A drying tunnel was built to calculate drying curves, volatile emissions, ignition temperatures, and solids degradation at high temperatures while drying under several dierent operating conditions. The results of this experimental work allowed us to develop a useful drying model for designing new dryers and for assessing the behaviour of existing ones. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Drying; Olive cake; Alpeorujo; Olive oil
1. Introduction Up until the nineties, the olive oil production process was based on the so-called ``three-phase system'', which produced three streams: pure olive oil, a watery liquid called alpechõn and a solid cake called orujo or olive cake. The alpechõn contained soluble organic matter and ®ne solids and was extremely hazardous to the environment because of its high biochemical oxygen demand (BOD). The olive cake had a variable oil content of around 3% that could be economically recovered in oil extracting plants after moisture content had been reduced to 8%. To eliminate the alpechõn, a new process technology with only two euent streams (olive oil and alpeorujo) has emerged. The wet solid stream or alpeorujo contains all the substances that in the three-phase system were contained in the alpechõn and in the olive cake. The characteristics of alpeorujo are obviously very dierent from those of olive cake. It is a thick sludge that contains pieces of pit and pulp of the olive fruit as well as vegetation water. It has a moisture content of around 65%, while olive cake only had around 40±45%. This greater moisture, together with the sugars and ®ne solids that in the three-phase system were contained in
*
Corresponding author.
the vegetation water give the alpeorujo sludge a doughy consistency and makes transport, storage, and handling dicult ± it cannot be piled and must be kept in large ponds. In any case, the main problems associated with the processing of alpeorujo during oil extraction occur in the dryers. These dryers were designed for three-phase olive cake, made up of loose particles of pit and pulp with a homogeneous moisture distribution that can easily be piled up and fed through rotary dryers. On the other hand, the high moisture content of alpeorujo demands much more energy, and the sugars present in it make it sticky and dicult to dry. Alpeorujo tends to stick to the dryerÕs walls, particularly to the initial part of the trommel where the gases are hot, obstructing the gas stream and increasing ®re risk. To develop suitable standards for alpeorujo dryer design and operation, the drying process was studied at laboratory scale and the drying rate was determined with respect to operating conditions (temperature and air velocity) and agglomerate size. The volatile release and the production of distilled liquids during drying were also determined. This is the ®rst step to propose a model of alpeorujo drying similar to those applied to other solids (Blasi, 1997; Melaanen & Grùnli, 1997; Melaanen, 1996; Nasrallah & Perre, 1988; Saastamoinen & Richard, 1996; Saastamoinen, 1995; Saastamoinen & Impola, 1995), which is useful to analyse the process that takes place in the rotary dryers.
0260-8774/99/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 0 4 - 1
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R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
Fig. 1. Drying tunnel.
2. Materials and methods The laboratory dryer (Fig. 1) is a 2.5 m long, 0.15 m square tunnel with a refractory lining. The sample hangs from a stem connected to a balance that is placed over the tunnel. The sample weight is continuously recorded by a computer at scanning times chosen by the operator. The drying air is introduced into the tunnel with a blower and its ¯ow is measured by a turbine ¯owmeter and controlled by a butter¯y valve. The air is heated by four adjustable electrical resistances, and the air temperature in the tunnel is measured by two thermocouples, one placed near the sample (the drying air temperature decrease due to heat losses between the measuring position and the sample point is less than 0.3°C when the air temperature is 350°C) and the other at the tunnel inlet. An additional thermocouple may be inserted into the sample core to measure its internal temperature. The alpeorujo sample is placed in a basket made of galvanized steel mesh with an 18 mm opening that does not obstruct the air±solid contact. The baskets are cylinders with 18.0, 28.6, 34.4, and 40.1 mm diameters and 72.0 mm high. The alpeorujo sample is put into the basket immediately before the start of the drying experiment to avoid sample alterations caused by ambient air±solid interaction. With this experimental facility, it is possible to reproduce the operating conditions existing in industrial trommels used to dry alpeorujo, i.e. air ¯ows between 1000 and 10 000 kg hÿ1 mÿ2 and temperatures up to 600°C. 3. Drying experiments To analyse the drying process (drying rate vs. moisture content) under dierent operating conditions, a factorial design of experiments was carried out with air temperature, drying air velocity, and sample geometry as variables. The six temperature levels (50°C, 100°C, 200°C, 250°C, 300°C, 350°C), two velocity levels (2, 3 m/ s) and four sample sizes used in this factorial design cover the wide range of conditions actually present in industrial alpeorujo dryers. Two additional sets of experiments were carried out. The ®rst experiments were conducted with high moisture samples obtained adding water till the moisture was as high as 76%. The target of these experiences was to
determine if a constant drying rate period exists at these high moisture contents. The other set of additional experiments had the objective of studying sample core behaviour during the drying process and was carried out by inserting a thermocouple into the sample core to measure its internal temperature. The alpeorujo sample used in each experiment was previously analysed to ascertain its initial moisture, volatile and dry solid contents. The volatile emissions during drying were found by measuring the dierence between initial and ®nal volatile contents, except in those cases in which the sample was burnt. 4. Results of drying experiments Before presenting the drying curves, it is of interest to show some general observations extracted from the experiments: · The surface of the alpeorujo darkens in a few seconds. · Samples dried at temperatures of 100°C or above, harden during drying. · Samples dried at temperatures of 100°C or above shrink during drying. Shrinkage is greater at higher temperatures. · The samples dried at 250°C or above are pyrolysed and produce liquid compounds that ¯ow away from the sample falling on the inner surface of the tunnel. · The samples with a diameter of 28.6 mm or greater burn at a gas temperature of 300°C, but only when the weight loss is greater than the mass of water initially in the sample. Combustion begins in internal pores and never near the surface. At gas temperatures of 350°C or above, all the samples burn, regardless of their size. Three types of plots are used in this paper to show the experimental results: drying curves (apparent moisture content on a dry basis versus drying time), drying rate curves (drying rate expressed as apparent moisture loss per unit mass of dry solid per unit time versus apparent moisture content on a wet basis), and temperature curves (temperature of the solid versus apparent moisture content on a wet basis). The term ``apparent moisture content'' is employed instead of ``moisture content'' because it comprises both water and volatile release. Fig. 2 shows the drying curves for the four experiments conducted at 250°C with 18.0 and 28.6 mm2 diameter samples (P and G in the legend) and air velocities of 2 and 3 m/s (2 and 3 in the legend). As could be expected, the small samples dried at high drying air velocity need less time to achieve a speci®ed moisture content than large samples dried at low drying air velocity. These curves are typical curves, but the ®nal apparent moisture content is less than zero due to the
R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
Fig. 2. Experiments run at 250°C.
emissions of volatile matter that had been weighted as moisture. Fig. 3 shows the drying rate curves for an 18.0 mm diameter sample and 2 m/s air velocity. The temperature ranges from 50°C to 350°C. These curves were obtained by numerically dierentiating the moisture content vs. time data. They clearly show that the drying process is not controlled by a single mechanism throughout. There exists a warm-up period in each case but there is no constant-rate period. This last observation was explicitly proven by carrying out some drying experiments using samples to which water had been added to obtain a 76% (w.b.) moisture content. No constant-rate period was found even at this high humidity. Two falling-rate periods can also be seen in each curve. The 350°C curve presents an apparent increasing-rate period due to a quick weight loss from sample burning. As mentioned before, negative moisture contents are due to pyrolysing processes. The sample temperature-moisture content curves were obtained by inserting a thermocouple in the sample core, taking care not to alter the weight measurement. Fig. 4 shows the temperature and the drying rate curves for a drying experiment carried out at 250°C. Four dierent periods can be distinguished: an initial increasing-temperature period that matches the warm-up period of the drying rate curve, a constant-temperature period at 100°C that coincides approximately with the ®rst falling-rate period, another rising-temperature period that ends when the weight loss equalizes the initial moisture content, and a ®nal period in which the temperature of the sample slowly draws closer to the air temperature.
231
Fig. 4. Temperature curve for sample dried at 250°C.
Fig. 5. Volatile emissions.
At temperatures above 200°C, volatile release and moisture evaporation take place. In order to characterize the release of volatiles and their eect on the drying process measurements, the amount of volatiles emitted in each drying test were calculated by ®nding the difference between the volatile content of the sample before and after the test. Fig. 5 shows volatile release (%) vs. temperature curves obtained in the drying tunnel as well as in an electric furnace operating at the same temperatures for periods of 7 and 30 min. The samples that were placed in the furnace had previously been dried at 105°C. It is clearly seen that the devolatilization equilibrium at each temperature is reached after 30 min of residence time in the furnace except at high temperatures (>700°C) where only 7 min are necessary. Volatile release in the tunnel dryer seems to be exactly the same as during the 30 min residence time furnace tests. The discrepancy appearing at 300°C is due to sample pyrolysis, which produces distilled liquid products measured as volatile release in the lab-dryer while in the furnace they remain with the sample on the pan, thus causing no weight loss. At temperatures above 300°C, the samples dried in the tunnel burn, so there is no data on their volatile release. 5. Drying process analysis
Fig. 3. Drying rate at dierent temperatures.
The analysis of the drying process was carried out with respect to the major variables: drying temperature, air velocity, and sample size. Fig. 6 shows a parametric set of drying rate versus temperature plots with the apparent moisture content as the parameter for an 18.0 mm diameter sample dried
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R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
Fig. 6. Drying rate vs. temperature.
with 2 m/s velocity air. The drying rate increases almost linearly with the temperature at any moisture content. However, at high moisture contents the drying rate is more sensitive to temperature than when moisture content is lower. This fact suggests that at low moisture contents, internal mass and heat transfer are limiting factors that increase the global resistance, while at high moisture contents this resistance is signi®cantly lower. Fig. 7 shows a parametric set of drying rate versus sample size plots for an operating temperature of 250°C with apparent moisture content as the parameter. As can be seen, the drying rate increases almost linearly with the inverse of the sample diameter. As in other mass and heat transfer processes, this result shows that the drying rate per unit mass of solid is proportional to the speci®c surface (external surface per unit sample volume). Fig. 8 shows the comparison between the maximum experimental drying rate, which corresponds to the end of the warm-up period, and the maximum theoretical drying rate. This drying rate is calculated assuming that free liquid moisture covers the whole sample surface,
which is at the wet air temperature, and by using wellestablished correlations (Jakob, 1959; Pitts & Sissom, 1997) for the heat transfer coecient. The radiant heat received by the sample from the inner walls is less than 6% of the convection heat in the experiments run at 350°C with an air velocity of 2 m/s and it is not taken into account (under this conditions the radiant heat is proportionately greater than under other conditions). Both drying rates start to dier at temperatures above 200°C. At these temperatures the drying rate is so high that part of the solid surface dries before reaching the end of the warm-up period. The in¯uence of the air velocity on the drying rate is clearly dierent at high (>100°C) and low drying temperatures. Fig. 9 shows the drying rate curves of two samples dried at 50°C but at two dierent air velocities (2 and 3 m/s). Both curves dier only in the warm-up and the ®rst falling-rate periods. This fact may be explained taking into account that the drying of surface moisture, which still exists during both drying periods, is controlled by convective heat and mass transfer between the bulk air and the solid surface. On the other hand, the rest of the drying process at these low temperatures is controlled by internal liquid and vapour diusion toward the solid external surface, a mechanism not affected by air velocity. Fig. 10 shows the drying rate curves of two samples dried at 250°C but, as before, at two dierent air velocities (2 and 3 m/s). At this high temperature, the drying rate curves dier in the course of the whole drying process except when the moisture content is very low. A possible explanation of this fact is that convective heat transfer to the solid surface plays a signi®cant role until the end of the drying process. The internal moisture ¯ows as vapour through the pores at a ¯ow rate that depends on the heat transfer rate. When the moisture content is very low, the migration of the bound water controls the drying process and the in¯uence of air velocity is negligible. 6. A qualitative drying description The previous experimental study helps to develop a qualitative drying process description for the alpeorujo
Fig. 7. Drying rate vs. sample size (250°C).
Fig. 8. Experimental drying rate vs. theoretical drying rate.
Fig. 9. Experiments run at 50°C.
R. Arjona et al. / Journal of Food Engineering 41 (1999) 229±234
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tures (<200°C), however, a very short constant-rate period may exist because, as stated before, the maximum theoretical drying rate, calculated assuming liquid surface coverage at the wet-bulb temperature, coincides with the experimental maximum drying rate. 6.2. Phase II (®rst falling-rate period) Fig. 10. Experiments run at 250°C.
samples. According to this model, the drying process may have four dierent phases or periods. However, the number and the characteristics of these periods are different, depending on the drying temperature. Figs. 11 and 12 show drying curves taken as representative of low and high temperature behaviour. 6.1. Phase I (warm-up period) Initially, the entire surface is wet or saturated and the heat transferred to the solid is greater than the heat lost by evaporation. Therefore, the solid temperature rises and the drying rate grows rapidly due to higher vapour pressures. However, as the solid surface becomes unsaturated, the drying rate reaches a maximum and starts to decrease although the solid temperature is still increasing. There is no constant-rate period at the wetbulb temperature because the capillary suction does not drive enough moisture to the surface. At low tempera-
At low temperatures, the sample surface is unsaturated but free water still exists in direct contact with the drying air and there is a continuous liquid state within the porous body (funicular state (Strumillo & Kudra, 1986)). As the solid dries, the saturated portion of the surface diminishes and therefore the drying rate decreases too. At high temperatures, however, this period does not exist because it is included in phase I. 6.3. Phase III (second falling-rate period) The solid surface is now completely dry at any temperature. At low temperatures, the internal moisture diuses as liquid and vapour toward the solid surface. During this phase the surface is dry and there is no continuous liquid state within the porous body because the moisture is interspersed with gas bubbles (pendular state (Strumillo & Kudra, 1986)). At high temperatures, however, the liquid boils at approximately 100°C in the interior of the large cavities or pores formed between the pieces of pits. As the drying proceeds, the boiling front moves into the interior of the solid leaving behind a solid in the hygroscopic range. Therefore, two main zones may be distinguished (Fig. 13): a shrinking core and a partially dried shell of growing thickness. The moisture ¯ows as vapour through the interstitial spaces between the pits and pulp particles toward the solid external surface. The resistance to vapour ¯ow is very low and therefore the pressure at the boiling front is the atmospheric pressure. The moisture content of the solid in the outside shell (hygroscopic range) depends on the solid temperature that increases from 100°C at the drying front to the surface temperature.
Fig. 11. Drying process phases (50°C).
Fig. 12. Drying process phases (300°C).
Fig. 13. Drying front.
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6.4. Phase IV This phase exists only if the drying process takes place at temperatures over 200°C. The drying rate in this period decreases less than expected or it could even be approximately constant. This is because the sample surface is hot enough to lose volatile compounds that are measured as moisture loss. This qualitative description is coincident, in its major considerations, with mathematical models developed to simulate biomass drying processes. (Blasi, 1997; Melaanen & Grùnli, 1997; Nasrallah & Perre, 1988; Saastamoinen, 1995). In these models, a shrinking evaporation front is considered and the internal moisture mass transfer through the solid takes place as vapour as well as liquid. They also distinguish between high and low temperature drying behaviour. 7. Conclusions A qualitative description of the alpeorujo drying process was developed that describes the characteristics of the dierent periods that make up the whole process. In addition, a set of useful quantitative data on drying rates at dierent operating conditions was obtained. The experimental results show that a constant-rate drying period, which has traditionally been taken into account for designing alpeorujo dryers, does not exist. The operating conditions ± size, temperature, and moisture content ± at which the alpeorujo could ignite and cause a
®re in an industrial dryer were also determined. The loss of volatile matter during the drying process, which modi®es the composition of the product and may aect the quality of the oil to be extracted, was evaluated. References Blasi, C. D. (1997). Simultaneous heat, mass and momentum transfer during biomass drying. In A. V. Bridgwater & D. G. Boocock, Developments in thermochemical biomass conversion. IEA Bioenergy. Jakob. M. (1959). Heat transfer, vol. I. New York: Wiley. Melaanen, C. (1996). Numerical analysis of heat and mass transfer in drying and pyrolysis of porous media. Numerical Heat Transfer, Part A, 29, 331±355. Melaaen, C. & Grùnli, M. G. (1997). Modelling and simulation of moist wood drying and pyrolysis. In A. V. Bridgwater & D. G. Boocock, Developments in thermochemical biomass conversion. IEA Bioenergy. Nasrallah, S. B. & Perre, P. (1988). Detailed study of a model of heat and mass transfer during convective drying of porous media. International Journal of Heat Mass Transfer, 31(5), 957±967. Pitts, D. R. & Sissom, L. E. (1997). Heat transfer. New York: McGraw-Hill. Saastamoinen, J. (1995). Model for drying and pyrolysis in an updraft biomass gasi®er. In A. V. Bridgwater, Advances in thermochemical biomass conversion. Saastamoinen, J. & Impola, R. (1995). Drying of solid fuel particles in hot gases. Drying technology, 13(5±7), 1305±1315. Saastamoinen, J. & Richard, J. (1996). Simultaneous drying and pyrolysis of solid fuel particles. Combustion and Flame, 106, 288± 300. Strumillo, C. & Kudra, T. (1986). Drying: Principles, application and design. London: Gordon and Breach.