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Geometric and Reynolds number effects on oregano (Lippia Berlandieri Schauer) essential oil extraction
Journal of Food Engineering 44 (2000) 127±133
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Geometric and Reynolds number eects on oregano (Lippia Berlandieri Schauer) essential oil extraction pez-Miranda a, I.R. Martõn-Dominguez b,1 J.A. Perez-Galindo a,*, J. Lo a
Depto. de Ing. Quõmica y Bioquõmica, Instituto Tecnol ogico de Durango, Felipe Pescador 1830 Ote., CP 34080 Durango, Dgo., M exico b CIMAV, Miguel de Cervantes 120 Complejo Industrial Chihuahua, 31109 Chihuahua, Chih., M exico
Abstract Eects of geometry and Reynolds number on the production of essential oil were investigated. Experiments, performed on an oregano packed bed, were designed to reproduce expected plant operating conditions. A diusive model was found to apply to oil extraction and eective diusion coecients were calculated as a function of the bed L/D ratio and Reynolds number. Eective diusivities were found to be of the same order of magnitude as those for drying of similar materials (10ÿ9 m2 /s). Statistical analysis showed that Re eects are more important, but that extraction is also aected by L/D variations. Application of this information to select the optimum operation parameters needs additional plant-speci®c, cost-related data. Ó 2000 Published by Elsevier Science Ltd. All rights reserved.
Notation A particle surface area per unit particle mass, m2 /kg mean particle surface area, m2 Ap Apb packed bed cross section area, m2 Bi Biot number eective diusion coecient, m2 /s Deff Kc convective mass transfer coecient, m/s L packed bed thickness, m L convection characteristic length, m diusion characteristic length, m Li M extracted oil mass, kg Mt total extracted oil mass, kg ms steam mass ¯ow rate, kg/s Re Reynolds number t time, s u characteristic velocity, m/s V particle volume per unit particle mass, m3 /kg Vp mean particle volume, m3 Vpb packed bed volume, m3 W packed bed oil content, kg initial packed bed oil content, kg Wi x distance, m e void fraction ls steam viscosity, kg/m s q particle density, kg/m3 qa packed bed apparent density, kg/m3 qs steam density, kg/m3 x particle mass fraction
*
Corresponding author. Tel.: +52-181-85586; fax: +52-181-84813. E-mail address: [email protected] (I.R. MartõÂn-Dominguez). 1 Tel.: +52-14-391148; fax: +52-14-391112.
Subscripts b branches f ¯owers ` leaves p mean for all particles 1 evaluated at an initial time 2 evaluated at a ®nal time
1. Introduction The most numerous class of ingredients in the ¯avoring industries are liquid ¯avors. This is due to the fact that they readily diuse into substrates (Furia & Bellanca, 1975). Essential oils, extracted from herbs and spices, are among liquid ¯avors used in industry. Rather than being considered as a ¯avor per se, the oregano essential oil is commonly used to prepare ¯avors. It is used as an ingredient in beverages, bakery goods, tooth paste, chewing gum and in the preparation of fragrances of oriental type (Ashurst, 1991). Essential oils contained in plants from the Labiatae family are characterized by its spicy, herbaceous, ¯avor note and their main constituents are thymol and cavracol. The oils are usually obtained by the so-called ``Dry-steam distillation'' method, in which steam is forced to pass through a bed of plant material causing evaporation of the oil. The mixture of steam and oil vapors is then condensed and the oil separated by decantation. Dry and saturated steam is used in most installations (Furia & Bellanca, 1975).
0260-8774/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 5 4 - 5
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J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
The amount of essential oil contained in the plant varies as a result of climatic factors, such as yearly rainfall, and time of harvest. The essential oil yield from the plant is also aected by harvesting and processing procedures (Guenther, 1972). Seasonal variations in the oil yield of various Labiatae species plants were investigated by Basker and Putievsky (1978), who found that maximum yields were obtained by harvesting at the end of the ¯owering period. Putievsky, Ravid and Dudai (1988) report that some of the Origanum vulgare oil components are aected by harvesting season while other components are not. Dierences in odor and ¯avor, among experimentally cultivated Origanum heracleoticum plants, were studied and reported by Fleisher and Sneer (1982). Furia and Bellanca (1975) presented essential oil composition and plant yield of some species of oregano-like plants, namely Origanum vulgare L. and Thymus vulgaris L. The composition of Mexican oregano, Lippia graveolens H.B.K., was reported by Pino, Rosado, Baluja and Borges (1989) and a very thorough study of the same species was presented by S anchez, Uribe, Hurtado and Matõnez (1991). The latter contains information about essential oil yields and physico-chemical properties of samples from 12 dierent locations in central Mexico. This work presents results of measurements of the rate of production and yield of essential oil extracted of oregano from north central Mexico. The main objective of the work was to generate information regarding effects of bed length to diameter ratio and distillation rate to be used in industrial plant design. 2. Materials and methods A single sample of oregano, of size large enough to provide material for all of the experiments performed, was obtained from a local trader. It was harvested and dried during the regular season and with traditional procedures. 2.1. Apparatus A schematic diagram of the apparatus used for oil extraction is shown in Fig. 1. The apparatus has a cylindrical stainless steel body (0.14 m inside diameter and 0.24 m long) with a grid at its lower end. The oregano bed is located between a movable square lower section 0:2 m width 0:2 m depth 0:35 m long) and a ®xed top lid. The union among these sections is pressure sealed by means of ``O'' rings. At the bottom of the steam producing section there is an electrical resistance heater whose power is controlled with a rheostat and used to heat and boil water. The lower assembly, formed by the cylindrical and square sections, may be moved up and down by means of a
Fig. 1. Schematic diagram of the apparatus.
jack, this also produces the required O ring sealing pressure. The whole set-up is insulated with a 0.015 m layer of aluminum lined ®berglass. Steam produced in the lower part of the apparatus passes through the oregano bed, evaporating and carrying the desired oregano oil, and is then directed towards the condenser, located on the side of the main apparatus body. Following condensation, the mixture is decanted to separate oil and water. The volume of condensed water was measured with a graduated tube, while the mass of oil was measured by dierential weight of previously tared ¯asks where the separated oil was deposited. A Chlyo model JL-180 analytical balance of 180.0 g capacity and a readability of 0.1 mg was used for weighing. Other measured variables included the pressure and temperature of the steam below the bed and of the oil± steam mixture above the bed. Pressures were measured by means of dierential manometers, while glass thermometers were used for temperatures. They were used to evaluate steam and oil physical properties. Water level in the lower section was measured by means of a sight glass and used to evaluate when more water was needed. 2.2. Calculations Oregano plants were received from a local trader, after harvesting and drying by traditional procedures. The sample size was large enough to provide material for all of the experiments to be performed. It contained all plant material as usually marketed, which consists of branches, leaves and ¯owers. To calculate geometric characteristics branches were modeled as cylinders, ¯owers as spheres and leaves as thin cylinders. With these models, measurement of the average thickness of a sample of leaves of known weight, with the aid of leaves' density, permitted the calculation of a mean leave diameter and, in a similar manner,
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
branches' mean diameter measurements were used to obtain mean branch length. Since ¯owers are approximately ovoid, an equivalent ¯ower diameter was obtained by averaging the measurements of the ¯owers small and large diameters. Measurement of the various particles mean density and characteristic length was enough to provide information to determine particle surface area and volume and this information, aided by measurement of the particle mass fractions and the bed apparent density, was used to relate individual particle characteristics to bed averages. Branch diameter and leaf thickness were measured with a micrometer and ¯ower minor and major diameters with a vernier caliper. Densities of the dierent constituents were determined by means of a standard pycnometer, and the bed apparent density was obtained by repeated direct measurements of the weight of bed material poured into a ®xed volume. The particle mass fractions were obtained by weighing several samples of material and separating and weighing the three constituents. Whitaker's (1972) formulation was deemed appropriate to analyze experimental results on the basis of packed bed theory and, to make steam distillation rates dimensionless, the Reynolds number was calculated as follows: qs u L ; ls ms eqs ; u Apb 6Vp e ; L Ap 1ÿe Re
1 2 3
where the average particle area, volume and bed void fractions were calculated from the following equations: Ap qa x` A` xb Ab xf Af Vpb ;
4
Vp qa x` V` xb Vb xf Vf Vpb ;
5
e 1 ÿ qa x` V` xb Vb xf Vf :
6
This formulation, together with the heat±mass transfer analogy, was also used to evaluate convective mass transfer coecients. The calculated values were then used with the diusion coecient to evaluate the mass transfer Biot number Bi
Kc Li : Deff
7
Values of the Biot number are an aid to evaluate if the mass ¯ow in the system is dominated by diusion or convection. According to Okos, Ganesan, Rakesh and Weitnauer (1992) diusion dominates if the Biot number values are 10 or larger. The diusion characteristic
129
length was calculated by a simple mass fraction average of the individual particleÕs characteristic lengths: the leafÕs half thickness and the branches and ¯owers radii. The numerical evaluation of diusion coecients was based on the solution of the diusion equation and boundary conditions, which assuming zero external oil concentration leads to the following equation (Crank, 1975): 2 4Li ln W1 =W2 Deff : 8 p2 t2 ÿ t1 Examination of Eq. (8) shows that an eective diusivity, Deff , may be calculated from the slope of the curve ln W =Wi vs. time, for the straight portion of the curve. This may be accomplished by any of the methods which have been proposed for thermal conductivity measurements by the line heat source method (Perez & L opez, 1980; Murakami & Okos, 1988; Wang & Hayakawa, 1993). 2.3. Experimental procedure With the whole apparatus assembled power input was applied by adjusting the rheostat at maximum level. When boiling started, the rheostat was adjusted to set a voltage corresponding to a desired distillation rate, and the cylindrical section disengaged to permit placing of the oregano bed. In order to reproduce expected conditions in a producing facility as close as possible, the oregano plant material was not handled in any special manner. The bed was formed by pouring sucient plant material above the grid to cover the height required to ful®ll the prescribed length to diameter ratio of the current experimental run. Replacing the bed section in the apparatus was the next operation, followed by opening the condenser cooling water feed. Zero time was that at which the ®rst drops of the condensed water±essential oil mixture were observed. At intervals of 15 min the condensate receiving ¯ask was retired to eect mixture separation and water volume and oil weight measurements. A new ¯ask was used for the next period measurements and the procedure was repeated until no oil could be observed in the condensed mixture. The ®nal step of each experimental run was to empty the bed and allow steam to ¯ow in the system for cleaning purposes. Three bed lengths (0:5D; 1:0D and 1:5D) and three power levels were investigated. The latter were accomplished by setting resistance input voltages of 50, 70 and 90 V. Three replicates were made for each measurement. 3. Results and discussion Essential oil yield was calculated as a fraction of the bed plant mass for each experiment and, for the whole
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J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
set of experimental measurements, the resulting average yield was 3:72 0:34%. The observed variation may be explained by the method of bed ®lling, which caused the 3 bed apparent density to be 70:49 6:2 kg=m . Since the mean bed apparent density was used to calculate the bed oil mass, it may be assumed that it is the main cause for the measured essential oil content variation. Measured yields are higher than those of similar crops reported in technical literature by S anchez et al. (1991) for samples of Oregano, Lippia graveolens H.B.K., obtained from the central region of Mexico, Origanum var. Viride and Virens (Guenther, 1972), and for Thymus Vulgaris (Furia & Bellanca, 1975). Results of physical measurements required to calculate the average particleÕs area and volume, Ap and Vp , and the bed void fraction, e, are given in Table 1. There are shown the particleÕs sample weight, number, density, measured and calculated dimensions and measured mass fraction and the calculated values Ap , Vp and e. These were the basis for the Reynolds number calculations. The density of branches compares well with that of wood, which varies from 500 to 1000 kg/m3 for softwoods and hardwoods, respectively and the calculated void fraction is on the high side of values for industrial packing materials, which are between 0.55 and 0.96 (Fair, Steinmeyer, Penney & Brink, 1973). The aforementioned voltage levels produced steam± water mass ¯ow rates of 0.135, 0.317 and 0.517 g/s, which are equivalent to distillation rates of 8:77 10ÿ3 , 2:06 10ÿ2 and 3:32 10ÿ2 kg/m2 -s and Reynolds numbers of 5.18, 12.18 and 19.66, respectively. These values were selected on the basis of GuentherÕs information about commercial operations (Guenther, 1972). His data on essential oil extraction from plants from the Labiatae family encompasses a range of the distillation rate from 0.00328 to 0.04167 kg/s m2 . Fig. 2 presents the amount of oil extracted (normalized with respect to total extracted oil) for the three employed L/D ratios as a function of time. Each box in
the ®gure corresponds to one of the three investigated Reynolds numbers. An overall evaluation of the data shows that the eect of raising the Reynolds number is to increase the rate of oil extraction, as shown by the increasing slopes of the curves from the upper to the lower boxes. It is also noticeable that larger Reynolds numbers lead to lower L/D ratio eects. This is in evidence by the increasing smaller separation among the curves in the upper, medium and lower boxes. A closer look at the results shows that at any time the average fraction of extracted oil grows with the Reynolds number. At 15 min it varies from around 0.5 for the lowest Re shown in the upper box of the ®gure to around 0.8 for the highest Re shown in the lower box. At 30 min the dierences are smaller (from 0.8 to 0.95) but still exist. Finally, at 45 min the extraction is practically ®nished for the highest Re, while this takes 15 and 30 more minutes for the middle and lower Re, respectively. These ®ndings are supported by the results of the analysis of variance presented in Table 2. The high calculated values of the F ratio show that there is a statistically signi®cant dierence of eects of both independent variables. Re variations are more important, since their F values are larger. Fig. 3 shows the normalized packed bed oil content as a function of time for the three investigated Reynolds numbers. Total extracted oil was used for normalization. Each box presents results for one of the three employed L/D ratios. Since there is a constant slope in the middle region of the curves, an eective diusion coecient may be obtained from the calculated slopes. Dierences in slopes for the ®rst 15 min may be explained by noting that in this time there are heating requirements and diusion of water into plant tissues. At the other end, during the last 15 min of the experiments, the oil content dropped to zero and cannot be shown in the graphs.
Table 1 Particle and packed bed characteristics Leaves
Branches
Flowers
Sample weight, g Number of particles q
1.2343 99 392.0
2.1097 92 870.9
0.402 120 171.7
Measured dimension, mm Calculated dimension, mm A V
Thickness, 0:483 0:1 Diameter, 9.11 11.68 2:551 10ÿ3
Diameter, 0:842 0:19 Length, 44.5 5.63 1:174 10ÿ3
Diameter, 0:458 0:09 ± 16.39 1:251 10ÿ2
x e L Li
0:9125 0:0172
0:0776 0:0174 0.82 0.00615 0.000255
0:0099 0:00075
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
131
Fig. 2. Oil extraction as a function of time.
Fig. 3. Normalized bed oil content as a function of time.
For the bed as a whole, examination of slope variation shows that the largest diusion coecients, proportional to the slopes, are those of the shorter bed and that they decrease with L/D ratio. As expected, the eect
of increasing Reynolds numbers is to increment diusion coecients, although this eect seems to be smaller for the higher L/D ratios. Table 3 shows the results of an analysis of variance performed to evaluate dierences among values of the calculated slopes as aected by the Reynolds number and the L/D ratio. As seen there, the resulting large F values con®rm that there are statistically signi®cant dierences of the eects of both independent variables and their interaction. The oregano oil extraction may be modeled by assuming that steam condenses on plant material and that the energy of condensation is then used to evaporate the oil. The produced oil vapor diuses through plant tissues and leaves by convection to the main steam ¯ow. If
Table 2 Analysis of variance of extracted oil Source of variance
DF
SS
MS
F
Re L/D Interaction Residual
2 2 4 18
0.4902 0.1512 0.0216 0.0387
0.2451 0.07562 0.0054 0.00215
114.01 35.17 2.51
Total
26
0.7017
0.02699
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J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
Table 3 Analysis of variance of slopes Source of variance
DF
SS
MS
F
Re L=D Interaction Residual
2 2 4 18
13.066 11.060 3.087 0.943
6.533 5.5299 0.7717 0.0524
124.7 105.6 14.7
Total
26
28.156
1.0829
this model is correct, diusion would be the main resistance to oil ¯ow. There are two facts supporting that for the packed bed as a whole oil extraction may be modeled as diusive process. In the ®rst place there is the constant slope of the curves in the middle region of the semi-logarithmic presentation. Although this does not prove that mass transfer control is diusive, calculations of the Biot number result in values from 1275.49 to 1956.44. These values are much higher than 10 and, therefore, diusion controls the process and the calculated diusivities may be used as an aid in design. The resulting values of eective diusivities for the packed bed are shown in Fig. 4 as a function of Reynolds number for the three investigated L/D ratios. The values of diusivities are similar to those obtained when drying wheat at 79.5°C, 2:8 10ÿ10 m2 /s (Gekas, 1992), and alfalfa stems, 2:6 10ÿ10 to 2:6 10ÿ9 m2 /s (Okos et al., 1992). From the standpoint of extraction rate, which increases with diusivity, the lower L/D ratios should be favored. However, oil is commonly produced in batch extractions so that production in the manufacturing facility is also aected by bed preparation and discharge operations. These would be fewer for the higher L/D ratios and are speci®c to a given facility. Therefore, a clear-cut decision about the optimum L/D ratio needs additional information related to operation of the speci®c manufacturing facility.
Fig. 4. Packed bed eective diusion coecients.
As mentioned before, the use of high Reynolds numbers also leads to higher extraction rates, this is shown by the higher values of diusivities as the Reynolds number increases. The fact that the range of variation of diusivities is smaller as L/D increases proves that the Reynolds number eect is less important as L/D increases. As with the L/D ratio, the selection of an optimum Reynolds number requires additional data. In this case, the use of higher values of Reynolds number to raise production in a given manufacturing facility, would be accomplished through higher steam consumption.
4. Conclusions Essential oil yield of oregano, Lippia berlandieri Schauer, was obtained by experiments designed to reproduce expected operating conditions in a manufacturing facility. Results showed that, on the basis of its oil yield, this species is superior to other similar species. Analysis of oil extraction from the packed bed showed that bed behavior may be modeled as a diusive process and eective diusion coecients were calculated. These coecients increase with increasing Reynolds numbers and decrease with increasing L/D bed ratios. According to statistical analysis the extraction rate is aected by both variables, but Re eects are more important. This information, although essential to de®ne optimum operating values, must be complemented by the use of additional plant-speci®c, cost-related operation parameters.
References Ashurst, P. R. (1991). Food ¯avorings. New York: Van Nostrand Reinhold. Basker, D., & Putievsky, (1978). Seasonal variation in the yields of herb and essential oil in some Labiatae species. Journal of Horticultural Science, 53 (3), 179±183. Crank, J. (1975). The mathematics of diusion. Oxford: Oxford University Press. Fair, J. R., Steinmeyer, M. A., Penney, W. R., & Brink, J. A. (1973). Liquid±gas systems. In R. H. Perry, C. H. Chilton, Chemical engineers' handbook (5th ed.). New York: McGraw-Hill. Fleisher, A., & Sneer, N. (1982). Oregano spices and Origanum chemotypes. Journal of the Science of Food and Agriculture, 33 (5), 441±446. Furia, T. E., & Bellanca, N. (1975). Fenaroli's handbook of ¯avor ingredients. Boca Raton, FL: CRC Press. Gekas, V. (1992). Transport phenomena of foods and biological material. Boca Raton, FL: CRC Press. Guenther, E. (1972). The essential oils. Malabar, FL: Krieger. Murakami, E. G., & Okos, M. R. (1988). Measurement and prediction of thermal properties of foods. In P. R. Singh, A. G. Medina, Food properties and computer-aided engineering of food processing systems. Boston, MA: Kluwer Academic Publishers.
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133 Okos, M. R., Ganesan, N., Rakesh, S., & Weitnauer, A. C. (1992). Food dehydration. In H. Heldeman, B. Daryl, Handbook of food engineering. New York: Marcel Dekker. Perez, J. A., & L opez, G. (1980). Sustituci on de arenas termicas por aditivos o disipadores de calor en instalaciones subterr aneas de cables de alta tensi on. MEXICON 80. Mexico, D.F. Pino, J., Rosado, A., Baluja, R., & Borges, P. (1989). Analysis of the essential oil of Mexican oregano (Lippia graveolens HBK). N ahrung, 33 (3), 289±295. Putievsky, E., Ravid, U., & Dudai, N. (1988). Phenological and seasonal in¯uences on essential oil of a cultivated clone of Origanum vulgare L. Journal of the Science of Food and Agriculture, 43 (3), 225±228.
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S anchez, N. A., Uribe, C. J., Hurtado, J. B., & Martõnez, M. A. (1991). Evaluaci on ®sicoquõmica del aceite esencial de oregano de poblaciones naturales localizadas en la zona norte del estado de Jalisco. In G. R. Melendez, S. A. Ortega, R. Pe~ na, Estado actual del conocimiento sobre el or egano en M exico. Bermejillo, Dgo.: URUZA-UACH. Wang, J., & Hayakawa, K. (1993). Maximum slope method for evaluating thermal conductivity probe data. Journal of Food Science, 58 (6), 1340±1345. Whitaker, S. (1972). Forced convection heat transfer correlations for ¯ow in pipes, past ¯at plates single cylinders,single spheres, and for ¯ow in packed beds and tube bundles. AIChE Journal, 18 (2), 361± 371.
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Geometric and Reynolds number eects on oregano (Lippia Berlandieri Schauer) essential oil extraction pez-Miranda a, I.R. Martõn-Dominguez b,1 J.A. Perez-Galindo a,*, J. Lo a
Depto. de Ing. Quõmica y Bioquõmica, Instituto Tecnol ogico de Durango, Felipe Pescador 1830 Ote., CP 34080 Durango, Dgo., M exico b CIMAV, Miguel de Cervantes 120 Complejo Industrial Chihuahua, 31109 Chihuahua, Chih., M exico
Abstract Eects of geometry and Reynolds number on the production of essential oil were investigated. Experiments, performed on an oregano packed bed, were designed to reproduce expected plant operating conditions. A diusive model was found to apply to oil extraction and eective diusion coecients were calculated as a function of the bed L/D ratio and Reynolds number. Eective diusivities were found to be of the same order of magnitude as those for drying of similar materials (10ÿ9 m2 /s). Statistical analysis showed that Re eects are more important, but that extraction is also aected by L/D variations. Application of this information to select the optimum operation parameters needs additional plant-speci®c, cost-related data. Ó 2000 Published by Elsevier Science Ltd. All rights reserved.
Notation A particle surface area per unit particle mass, m2 /kg mean particle surface area, m2 Ap Apb packed bed cross section area, m2 Bi Biot number eective diusion coecient, m2 /s Deff Kc convective mass transfer coecient, m/s L packed bed thickness, m L convection characteristic length, m diusion characteristic length, m Li M extracted oil mass, kg Mt total extracted oil mass, kg ms steam mass ¯ow rate, kg/s Re Reynolds number t time, s u characteristic velocity, m/s V particle volume per unit particle mass, m3 /kg Vp mean particle volume, m3 Vpb packed bed volume, m3 W packed bed oil content, kg initial packed bed oil content, kg Wi x distance, m e void fraction ls steam viscosity, kg/m s q particle density, kg/m3 qa packed bed apparent density, kg/m3 qs steam density, kg/m3 x particle mass fraction
*
Corresponding author. Tel.: +52-181-85586; fax: +52-181-84813. E-mail address: [email protected] (I.R. MartõÂn-Dominguez). 1 Tel.: +52-14-391148; fax: +52-14-391112.
Subscripts b branches f ¯owers ` leaves p mean for all particles 1 evaluated at an initial time 2 evaluated at a ®nal time
1. Introduction The most numerous class of ingredients in the ¯avoring industries are liquid ¯avors. This is due to the fact that they readily diuse into substrates (Furia & Bellanca, 1975). Essential oils, extracted from herbs and spices, are among liquid ¯avors used in industry. Rather than being considered as a ¯avor per se, the oregano essential oil is commonly used to prepare ¯avors. It is used as an ingredient in beverages, bakery goods, tooth paste, chewing gum and in the preparation of fragrances of oriental type (Ashurst, 1991). Essential oils contained in plants from the Labiatae family are characterized by its spicy, herbaceous, ¯avor note and their main constituents are thymol and cavracol. The oils are usually obtained by the so-called ``Dry-steam distillation'' method, in which steam is forced to pass through a bed of plant material causing evaporation of the oil. The mixture of steam and oil vapors is then condensed and the oil separated by decantation. Dry and saturated steam is used in most installations (Furia & Bellanca, 1975).
0260-8774/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 5 4 - 5
128
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
The amount of essential oil contained in the plant varies as a result of climatic factors, such as yearly rainfall, and time of harvest. The essential oil yield from the plant is also aected by harvesting and processing procedures (Guenther, 1972). Seasonal variations in the oil yield of various Labiatae species plants were investigated by Basker and Putievsky (1978), who found that maximum yields were obtained by harvesting at the end of the ¯owering period. Putievsky, Ravid and Dudai (1988) report that some of the Origanum vulgare oil components are aected by harvesting season while other components are not. Dierences in odor and ¯avor, among experimentally cultivated Origanum heracleoticum plants, were studied and reported by Fleisher and Sneer (1982). Furia and Bellanca (1975) presented essential oil composition and plant yield of some species of oregano-like plants, namely Origanum vulgare L. and Thymus vulgaris L. The composition of Mexican oregano, Lippia graveolens H.B.K., was reported by Pino, Rosado, Baluja and Borges (1989) and a very thorough study of the same species was presented by S anchez, Uribe, Hurtado and Matõnez (1991). The latter contains information about essential oil yields and physico-chemical properties of samples from 12 dierent locations in central Mexico. This work presents results of measurements of the rate of production and yield of essential oil extracted of oregano from north central Mexico. The main objective of the work was to generate information regarding effects of bed length to diameter ratio and distillation rate to be used in industrial plant design. 2. Materials and methods A single sample of oregano, of size large enough to provide material for all of the experiments performed, was obtained from a local trader. It was harvested and dried during the regular season and with traditional procedures. 2.1. Apparatus A schematic diagram of the apparatus used for oil extraction is shown in Fig. 1. The apparatus has a cylindrical stainless steel body (0.14 m inside diameter and 0.24 m long) with a grid at its lower end. The oregano bed is located between a movable square lower section 0:2 m width 0:2 m depth 0:35 m long) and a ®xed top lid. The union among these sections is pressure sealed by means of ``O'' rings. At the bottom of the steam producing section there is an electrical resistance heater whose power is controlled with a rheostat and used to heat and boil water. The lower assembly, formed by the cylindrical and square sections, may be moved up and down by means of a
Fig. 1. Schematic diagram of the apparatus.
jack, this also produces the required O ring sealing pressure. The whole set-up is insulated with a 0.015 m layer of aluminum lined ®berglass. Steam produced in the lower part of the apparatus passes through the oregano bed, evaporating and carrying the desired oregano oil, and is then directed towards the condenser, located on the side of the main apparatus body. Following condensation, the mixture is decanted to separate oil and water. The volume of condensed water was measured with a graduated tube, while the mass of oil was measured by dierential weight of previously tared ¯asks where the separated oil was deposited. A Chlyo model JL-180 analytical balance of 180.0 g capacity and a readability of 0.1 mg was used for weighing. Other measured variables included the pressure and temperature of the steam below the bed and of the oil± steam mixture above the bed. Pressures were measured by means of dierential manometers, while glass thermometers were used for temperatures. They were used to evaluate steam and oil physical properties. Water level in the lower section was measured by means of a sight glass and used to evaluate when more water was needed. 2.2. Calculations Oregano plants were received from a local trader, after harvesting and drying by traditional procedures. The sample size was large enough to provide material for all of the experiments to be performed. It contained all plant material as usually marketed, which consists of branches, leaves and ¯owers. To calculate geometric characteristics branches were modeled as cylinders, ¯owers as spheres and leaves as thin cylinders. With these models, measurement of the average thickness of a sample of leaves of known weight, with the aid of leaves' density, permitted the calculation of a mean leave diameter and, in a similar manner,
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
branches' mean diameter measurements were used to obtain mean branch length. Since ¯owers are approximately ovoid, an equivalent ¯ower diameter was obtained by averaging the measurements of the ¯owers small and large diameters. Measurement of the various particles mean density and characteristic length was enough to provide information to determine particle surface area and volume and this information, aided by measurement of the particle mass fractions and the bed apparent density, was used to relate individual particle characteristics to bed averages. Branch diameter and leaf thickness were measured with a micrometer and ¯ower minor and major diameters with a vernier caliper. Densities of the dierent constituents were determined by means of a standard pycnometer, and the bed apparent density was obtained by repeated direct measurements of the weight of bed material poured into a ®xed volume. The particle mass fractions were obtained by weighing several samples of material and separating and weighing the three constituents. Whitaker's (1972) formulation was deemed appropriate to analyze experimental results on the basis of packed bed theory and, to make steam distillation rates dimensionless, the Reynolds number was calculated as follows: qs u L ; ls ms eqs ; u Apb 6Vp e ; L Ap 1ÿe Re
1 2 3
where the average particle area, volume and bed void fractions were calculated from the following equations: Ap qa x` A` xb Ab xf Af Vpb ;
4
Vp qa x` V` xb Vb xf Vf Vpb ;
5
e 1 ÿ qa x` V` xb Vb xf Vf :
6
This formulation, together with the heat±mass transfer analogy, was also used to evaluate convective mass transfer coecients. The calculated values were then used with the diusion coecient to evaluate the mass transfer Biot number Bi
Kc Li : Deff
7
Values of the Biot number are an aid to evaluate if the mass ¯ow in the system is dominated by diusion or convection. According to Okos, Ganesan, Rakesh and Weitnauer (1992) diusion dominates if the Biot number values are 10 or larger. The diusion characteristic
129
length was calculated by a simple mass fraction average of the individual particleÕs characteristic lengths: the leafÕs half thickness and the branches and ¯owers radii. The numerical evaluation of diusion coecients was based on the solution of the diusion equation and boundary conditions, which assuming zero external oil concentration leads to the following equation (Crank, 1975): 2 4Li ln W1 =W2 Deff : 8 p2 t2 ÿ t1 Examination of Eq. (8) shows that an eective diusivity, Deff , may be calculated from the slope of the curve ln W =Wi vs. time, for the straight portion of the curve. This may be accomplished by any of the methods which have been proposed for thermal conductivity measurements by the line heat source method (Perez & L opez, 1980; Murakami & Okos, 1988; Wang & Hayakawa, 1993). 2.3. Experimental procedure With the whole apparatus assembled power input was applied by adjusting the rheostat at maximum level. When boiling started, the rheostat was adjusted to set a voltage corresponding to a desired distillation rate, and the cylindrical section disengaged to permit placing of the oregano bed. In order to reproduce expected conditions in a producing facility as close as possible, the oregano plant material was not handled in any special manner. The bed was formed by pouring sucient plant material above the grid to cover the height required to ful®ll the prescribed length to diameter ratio of the current experimental run. Replacing the bed section in the apparatus was the next operation, followed by opening the condenser cooling water feed. Zero time was that at which the ®rst drops of the condensed water±essential oil mixture were observed. At intervals of 15 min the condensate receiving ¯ask was retired to eect mixture separation and water volume and oil weight measurements. A new ¯ask was used for the next period measurements and the procedure was repeated until no oil could be observed in the condensed mixture. The ®nal step of each experimental run was to empty the bed and allow steam to ¯ow in the system for cleaning purposes. Three bed lengths (0:5D; 1:0D and 1:5D) and three power levels were investigated. The latter were accomplished by setting resistance input voltages of 50, 70 and 90 V. Three replicates were made for each measurement. 3. Results and discussion Essential oil yield was calculated as a fraction of the bed plant mass for each experiment and, for the whole
130
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
set of experimental measurements, the resulting average yield was 3:72 0:34%. The observed variation may be explained by the method of bed ®lling, which caused the 3 bed apparent density to be 70:49 6:2 kg=m . Since the mean bed apparent density was used to calculate the bed oil mass, it may be assumed that it is the main cause for the measured essential oil content variation. Measured yields are higher than those of similar crops reported in technical literature by S anchez et al. (1991) for samples of Oregano, Lippia graveolens H.B.K., obtained from the central region of Mexico, Origanum var. Viride and Virens (Guenther, 1972), and for Thymus Vulgaris (Furia & Bellanca, 1975). Results of physical measurements required to calculate the average particleÕs area and volume, Ap and Vp , and the bed void fraction, e, are given in Table 1. There are shown the particleÕs sample weight, number, density, measured and calculated dimensions and measured mass fraction and the calculated values Ap , Vp and e. These were the basis for the Reynolds number calculations. The density of branches compares well with that of wood, which varies from 500 to 1000 kg/m3 for softwoods and hardwoods, respectively and the calculated void fraction is on the high side of values for industrial packing materials, which are between 0.55 and 0.96 (Fair, Steinmeyer, Penney & Brink, 1973). The aforementioned voltage levels produced steam± water mass ¯ow rates of 0.135, 0.317 and 0.517 g/s, which are equivalent to distillation rates of 8:77 10ÿ3 , 2:06 10ÿ2 and 3:32 10ÿ2 kg/m2 -s and Reynolds numbers of 5.18, 12.18 and 19.66, respectively. These values were selected on the basis of GuentherÕs information about commercial operations (Guenther, 1972). His data on essential oil extraction from plants from the Labiatae family encompasses a range of the distillation rate from 0.00328 to 0.04167 kg/s m2 . Fig. 2 presents the amount of oil extracted (normalized with respect to total extracted oil) for the three employed L/D ratios as a function of time. Each box in
the ®gure corresponds to one of the three investigated Reynolds numbers. An overall evaluation of the data shows that the eect of raising the Reynolds number is to increase the rate of oil extraction, as shown by the increasing slopes of the curves from the upper to the lower boxes. It is also noticeable that larger Reynolds numbers lead to lower L/D ratio eects. This is in evidence by the increasing smaller separation among the curves in the upper, medium and lower boxes. A closer look at the results shows that at any time the average fraction of extracted oil grows with the Reynolds number. At 15 min it varies from around 0.5 for the lowest Re shown in the upper box of the ®gure to around 0.8 for the highest Re shown in the lower box. At 30 min the dierences are smaller (from 0.8 to 0.95) but still exist. Finally, at 45 min the extraction is practically ®nished for the highest Re, while this takes 15 and 30 more minutes for the middle and lower Re, respectively. These ®ndings are supported by the results of the analysis of variance presented in Table 2. The high calculated values of the F ratio show that there is a statistically signi®cant dierence of eects of both independent variables. Re variations are more important, since their F values are larger. Fig. 3 shows the normalized packed bed oil content as a function of time for the three investigated Reynolds numbers. Total extracted oil was used for normalization. Each box presents results for one of the three employed L/D ratios. Since there is a constant slope in the middle region of the curves, an eective diusion coecient may be obtained from the calculated slopes. Dierences in slopes for the ®rst 15 min may be explained by noting that in this time there are heating requirements and diusion of water into plant tissues. At the other end, during the last 15 min of the experiments, the oil content dropped to zero and cannot be shown in the graphs.
Table 1 Particle and packed bed characteristics Leaves
Branches
Flowers
Sample weight, g Number of particles q
1.2343 99 392.0
2.1097 92 870.9
0.402 120 171.7
Measured dimension, mm Calculated dimension, mm A V
Thickness, 0:483 0:1 Diameter, 9.11 11.68 2:551 10ÿ3
Diameter, 0:842 0:19 Length, 44.5 5.63 1:174 10ÿ3
Diameter, 0:458 0:09 ± 16.39 1:251 10ÿ2
x e L Li
0:9125 0:0172
0:0776 0:0174 0.82 0.00615 0.000255
0:0099 0:00075
J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
131
Fig. 2. Oil extraction as a function of time.
Fig. 3. Normalized bed oil content as a function of time.
For the bed as a whole, examination of slope variation shows that the largest diusion coecients, proportional to the slopes, are those of the shorter bed and that they decrease with L/D ratio. As expected, the eect
of increasing Reynolds numbers is to increment diusion coecients, although this eect seems to be smaller for the higher L/D ratios. Table 3 shows the results of an analysis of variance performed to evaluate dierences among values of the calculated slopes as aected by the Reynolds number and the L/D ratio. As seen there, the resulting large F values con®rm that there are statistically signi®cant dierences of the eects of both independent variables and their interaction. The oregano oil extraction may be modeled by assuming that steam condenses on plant material and that the energy of condensation is then used to evaporate the oil. The produced oil vapor diuses through plant tissues and leaves by convection to the main steam ¯ow. If
Table 2 Analysis of variance of extracted oil Source of variance
DF
SS
MS
F
Re L/D Interaction Residual
2 2 4 18
0.4902 0.1512 0.0216 0.0387
0.2451 0.07562 0.0054 0.00215
114.01 35.17 2.51
Total
26
0.7017
0.02699
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J.A. Perez-Galindo et al. / Journal of Food Engineering 44 (2000) 127±133
Table 3 Analysis of variance of slopes Source of variance
DF
SS
MS
F
Re L=D Interaction Residual
2 2 4 18
13.066 11.060 3.087 0.943
6.533 5.5299 0.7717 0.0524
124.7 105.6 14.7
Total
26
28.156
1.0829
this model is correct, diusion would be the main resistance to oil ¯ow. There are two facts supporting that for the packed bed as a whole oil extraction may be modeled as diusive process. In the ®rst place there is the constant slope of the curves in the middle region of the semi-logarithmic presentation. Although this does not prove that mass transfer control is diusive, calculations of the Biot number result in values from 1275.49 to 1956.44. These values are much higher than 10 and, therefore, diusion controls the process and the calculated diusivities may be used as an aid in design. The resulting values of eective diusivities for the packed bed are shown in Fig. 4 as a function of Reynolds number for the three investigated L/D ratios. The values of diusivities are similar to those obtained when drying wheat at 79.5°C, 2:8 10ÿ10 m2 /s (Gekas, 1992), and alfalfa stems, 2:6 10ÿ10 to 2:6 10ÿ9 m2 /s (Okos et al., 1992). From the standpoint of extraction rate, which increases with diusivity, the lower L/D ratios should be favored. However, oil is commonly produced in batch extractions so that production in the manufacturing facility is also aected by bed preparation and discharge operations. These would be fewer for the higher L/D ratios and are speci®c to a given facility. Therefore, a clear-cut decision about the optimum L/D ratio needs additional information related to operation of the speci®c manufacturing facility.
Fig. 4. Packed bed eective diusion coecients.
As mentioned before, the use of high Reynolds numbers also leads to higher extraction rates, this is shown by the higher values of diusivities as the Reynolds number increases. The fact that the range of variation of diusivities is smaller as L/D increases proves that the Reynolds number eect is less important as L/D increases. As with the L/D ratio, the selection of an optimum Reynolds number requires additional data. In this case, the use of higher values of Reynolds number to raise production in a given manufacturing facility, would be accomplished through higher steam consumption.
4. Conclusions Essential oil yield of oregano, Lippia berlandieri Schauer, was obtained by experiments designed to reproduce expected operating conditions in a manufacturing facility. Results showed that, on the basis of its oil yield, this species is superior to other similar species. Analysis of oil extraction from the packed bed showed that bed behavior may be modeled as a diusive process and eective diusion coecients were calculated. These coecients increase with increasing Reynolds numbers and decrease with increasing L/D bed ratios. According to statistical analysis the extraction rate is aected by both variables, but Re eects are more important. This information, although essential to de®ne optimum operating values, must be complemented by the use of additional plant-speci®c, cost-related operation parameters.
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S anchez, N. A., Uribe, C. J., Hurtado, J. B., & Martõnez, M. A. (1991). Evaluaci on ®sicoquõmica del aceite esencial de oregano de poblaciones naturales localizadas en la zona norte del estado de Jalisco. In G. R. Melendez, S. A. Ortega, R. Pe~ na, Estado actual del conocimiento sobre el or egano en M exico. Bermejillo, Dgo.: URUZA-UACH. Wang, J., & Hayakawa, K. (1993). Maximum slope method for evaluating thermal conductivity probe data. Journal of Food Science, 58 (6), 1340±1345. Whitaker, S. (1972). Forced convection heat transfer correlations for ¯ow in pipes, past ¯at plates single cylinders,single spheres, and for ¯ow in packed beds and tube bundles. AIChE Journal, 18 (2), 361± 371.