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Thin layer drying of components of fresh alfalfa R.T. PATIL1, S. SOKHANSANJ1, E. A. ARINZE2 and G. SCHOENAU3 Department of Agricultural and Bioresource Engineering, University of Saskatchewan, Saskatoon, SK, Canada S7N OWO; Ahmadu Bellu University, Nigeria; and Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada S7N OWO. Received 6 January 1992; accepted 5 October 1992. Patil, R.T., Sokhansanj, S., Arinze, E.A. and Schoenau, G. 1992. Thin layer drying of components of fresh alfalfa. Can. Agric. Eng. 34: 343-346. All parts of fresh or prewilted chopped alfalfa must be dried uniformly to about 8% moisture content before processing them into pellets or cubes. Thin layer drying rate of leaves and chopped stems of alfalfa were studied experimentally. Leaves re quired 40% of the drying time of 50 mm long stems without leaves. Leaf and stem attached to each other required 20% longer drying time than stems alone. A decrease in chopped length increased the dryingrate logarithmically; 10mm stempieces had a dryingconstant similar to that of leaves. The alfalfa stems cut at 10% bloom stage resulted in a 44% reduction in drying time compared to prebloom samples. The longer time was mainly due to higher initial moisture content of prebloom alfalfa. Toutes les parties de la luzerne hachee fraiche ou prefanee devraient etre sechees uniformement jusqu'a une teneur en eau de 8% avant d'etre mises en boulettes ou en cubes. Des experiences ont ete menees afin d'etudier le sechage des feuilles et des tiges hachees de luzerne disposees en couches minces. Le sechage des feuilles a requis 40%du temps necessaire pourlestigesde 50 mmde longueur sans les feuilles. Pour les feuilles et les tiges attachees ensemble, le temps de sechage requis etait 20% plus long qu'avec les tiges seule- ment. Une diminution dans la longueur de coupe a augmente le taux de sechage de facon logarithmique; pourles tiges coupees a 10 mm de longueur, la constante de sechage a ete la meme qu'avec les feuilles. Lestiges de luzerne coupees a un stade de floraison de 10% ont resulte en une reduction du temps de sechage de 44% compara- tivementaux echantillonspreleves avant la floraison. La difference a eteprincipalement causee parle contenu en eauplus eleve dans la luzerne avant la floraison. INTRODUCTION Alfalfa cubes and pellets are produced from dry chopped alfalfa at a moisture content ranging from 8 to 10% (Sokhan sanj and Wood 1990). The chopped alfalfa is finely ground to form pellets but need not be groundfor cubes. The source of dried chop is either from freshly harvested or from field wilted (suncured) hay that is rolled into bales. The bales are unrolled and chopped before use. Both fresh and suncure chops are dried, usually in rotarydrumdryers,to the desired final moisture content. To produce a good quality and uniform product, chopped alfalfa must be dried as fast as possible in the field and as uniformly as possible in the dryer. However, the leavesdry faster than the stems, so the dried product exiting the dryer is not homogeneous in moisture content. The objective of this research was to determine experi mentally the drying rates of chopped stems and leaves of alfalfa to determine the length of chopped stem that renders equal drying time to that of leaves. The drying rate of alfalfa has been studied with emphasis on field drying by several researchers (Hill et al. 1977; Stun ners et al. 1989; Savoie et al. 1982). The study on drying characteristics of grass by Menzies and O'Callaghan (1971) showed that in the temperature range of 200 - 400°C the rate of drying was approximately constant. Below this tempera ture the drying rate was proportional to initial moisture content andat temperatures below 80°C, multiple falling rate periods were observed. Bilanski et al. (1965a, 1965b) sub jected stem and leaf segments to air temperatures ranging from 95°C to 865°C and recommended a safe drying tem perature without charring at 250°C. Stencl (1989) conducted drying experiments at temperatures from 40°C to 120 C with the alfalfa plant treated with NH3 and NH4OH at a 5% level. He reported faster drying due to the treatment, but the drying constants and drying time were not given. MATERIALS AND METHODS Alfalfa (Medicago sativa L.) of Beaver variety grown at the experimental farm of Agriculture Canada nearSaskatoon was used for the study. The crop from first cut as well as from second cut at prebloom and 10% bloom stages was harvested manually by knifeand broughtimmediately to the laboratory. The samples were kept in a black polyethylene bag and preserved in a freezer at -20°C. To conduct drying experi ments, a quantity was taken out of the freezer and allowed to thaw for 30 min. Three samples were prepared: stems with leaves removed, leaves only, and whole plants (stems with leaves). The stem samples and whole samples were cut into 50 mm long pieces. A series of tests were alsoconducted with stems cut into pieces of 10, 20, 30, 50, and 75 mm lengths. The material was spread on the tray of a thin layer dryer at spreading density of 1.5 kg/m2. The moisture contents before the startof drying andafter drying were determined bytheair oven method (ASAE standard S358.1; ASAE 1991) at 103°C for 24 h. The thin layer drying equipment used is shownin Fig. 1. The air relative humidity and temperature were adjusted by an air conditioning unit. The final temperature adjustment was achieved by two 6 kW air heaters controlledby a solid CANADIAN AGRICULTUAL ENGINEERING Vol. 34, No. 4 October/November/December 1992 343
Transcript of alfalfa - CSBE-SCGAB · 2014. 11. 6. · tivementaux echantillonsprelevesavant la floraison.La...
Thin layer drying of components of freshalfalfa
R.T. PATIL1, S.SOKHANSANJ1, E. A. ARINZE2 and G.SCHOENAU3
Department ofAgricultural and Bioresource Engineering, UniversityofSaskatchewan, Saskatoon, SK, Canada S7N OWO;Ahmadu Bellu University, Nigeria; and Department ofMechanical Engineering, University of Saskatchewan, Saskatoon,
SK, Canada S7N OWO. Received 6 January 1992; accepted 5 October 1992.
Patil, R.T., Sokhansanj, S., Arinze, E.A. and Schoenau, G. 1992.Thin layer drying ofcomponents of fresh alfalfa. Can. Agric. Eng.34: 343-346. All parts of fresh or prewilted chopped alfalfa must bedried uniformly to about 8% moisture content before processingthem into pellets or cubes. Thin layer drying rate of leaves andchopped stems of alfalfa were studied experimentally. Leaves required40% of the drying time of 50 mm long stems without leaves.Leaf and stem attached to each other required 20% longer dryingtime than stems alone. A decrease in chopped length increased thedryingrate logarithmically; 10mm stempieceshad a dryingconstantsimilar to that of leaves. The alfalfa stems cut at 10% bloom stageresulted in a 44% reduction in drying time compared to prebloomsamples. The longertime was mainly due to higher initial moisturecontent of prebloom alfalfa.
Toutes les parties de la luzerne hachee fraiche ou prefaneedevraient etre sechees uniformement jusqu'a une teneur en eau de8% avant d'etre mises en boulettes ou en cubes. Des experiences ontete menees afin d'etudier le sechage des feuilles et des tiges hacheesde luzerne disposees en couches minces. Le sechagedes feuilles arequis 40%dutemps necessaire pourles tigesde 50 mmde longueursans les feuilles. Pour les feuilles et les tiges attachees ensemble, letemps de sechage requis etait20% plus long qu'avec lestiges seule-ment. Une diminution dans la longueur de coupe a augmente le tauxde sechage de facon logarithmique; pourles tiges coupees a 10mmde longueur, la constante de sechage a ete la meme qu'avec lesfeuilles. Lestiges de luzerne coupees a unstade defloraison de 10%ont resulte en une reduction du temps de sechage de 44% compara-tivementaux echantillonspreleves avant la floraison. La differencea eteprincipalement causee parlecontenu eneauplus eleve dans laluzerne avant la floraison.
INTRODUCTION
Alfalfa cubes and pellets are produced from dry choppedalfalfa at a moisture content ranging from 8 to 10% (Sokhansanj and Wood 1990). The chopped alfalfa is finely groundto form pellets but neednot be groundfor cubes.The sourceof dried chop is either from freshly harvested or from fieldwilted (suncured) hay that is rolled into bales. The bales areunrolled and chopped before use. Both fresh and suncurechops are dried, usually in rotarydrumdryers, to the desiredfinal moisture content.
To produce a good quality and uniform product, choppedalfalfa must be dried as fast as possible in the field and asuniformly as possible in the dryer. However, the leaves dryfaster than the stems, so the dried product exiting the dryer isnot homogeneous in moisture content.
The objective of this research was to determine experimentally the drying rates of chopped stems and leaves ofalfalfa to determine the length of chopped stem that rendersequal drying time to that of leaves.
The drying rate of alfalfa has been studied with emphasison field drying by several researchers (Hill et al. 1977; Stunners et al. 1989; Savoie et al. 1982). The study on dryingcharacteristics of grass by Menzies and O'Callaghan (1971)showed that in thetemperature range of 200- 400°C therateof drying was approximately constant. Below this temperature the drying rate was proportional to initial moisturecontent andat temperatures below 80°C, multiple falling rateperiods were observed. Bilanski et al. (1965a, 1965b) subjected stem and leaf segments to air temperatures rangingfrom 95°C to 865°C and recommended a safe drying temperature without charring at250°C. Stencl (1989) conducteddrying experiments attemperatures from 40°C to 120 Cwiththe alfalfa plant treated withNH3 and NH4OH at a 5% level.Hereported faster drying dueto the treatment, but thedryingconstants and drying time were not given.
MATERIALS AND METHODS
Alfalfa(Medicago sativa L.) of Beavervarietygrown at theexperimental farm ofAgriculture Canada nearSaskatoon wasused for the study. The crop from first cut as well as fromsecond cutat prebloom and 10% bloom stages was harvestedmanually by knifeandbroughtimmediately to the laboratory.The samples were kept in a black polyethylene bag andpreserved in a freezer at -20°C. To conduct drying experiments, a quantity was taken out of thefreezer andallowed tothaw for 30 min. Three samples were prepared: stems withleaves removed, leaves only, and whole plants (stems withleaves). The stem samples and whole samples were cut into50mm long pieces. Aseries oftestswere alsoconducted withstems cut into pieces of 10, 20, 30, 50, and 75 mm lengths.The material was spread on the tray of a thin layer dryer atspreading density of1.5 kg/m2. The moisture contents beforethestartofdrying andafterdrying were determined bytheairoven method (ASAE standard S358.1; ASAE 1991) at103°Cfor 24 h.
The thin layer drying equipment used is shownin Fig. 1.The air relative humidity and temperature were adjusted byan air conditioning unit. The final temperature adjustmentwas achieved by two 6 kW air heaters controlledby a solid
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October/November/December 1992
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^Aminco unit v DamperFlow meter
Thermocouples
Thin layer of material
Fig. 1. Schematic of the thin-layer dryer used in theexperiment (Not to scale).
state temperature controller. The air velocity approaching thethin layer was maintained at about 0.3 m/s. The air flow ratewas controlled manually by an air damper in the loop beforethe air conditioning unit and the flow rate was monitored withan averaging pitot tube and a bellow type differential pressure transducer. The masses, the drying air temperature, andrelative humidity were recorded by a data logger connectedto a computer. The masses were converted to instantaneousmoisture content values. Type T thermocouples were used tomonitor the air dry bulb temperature.
Drying was continued until the difference between twoconsecutive hourly mass readings was less than 10 mg. Atthat time, sample moisture content was considered to be inequilibrium with drying air. Drying experiments were conducted in duplication.
The physical characteristics of the material to be driedwere measured for 10% bloom second cut samples by extracting random samples of stem pieces and leaves before andafter drying. This information isessential because thechangein physical characteristics during drying exerts a high influence on the drying rates. For the stems, the diameter wasmeasured at four different points on 50 mm length using adigital caliper with an accuracyof ± 0.01 mm. For the leaves,50 leaf samples were drawn at random and their surface areaswere photocopied at 1:1 ratio. The major and minor axes andthe area of leaves were measured using an image analysissoftware package on a Macintosh computer. The number ofleaves on each 50 mm chopped length of whole plant wasalso measuredfor 50 randomly drawn pieces.
The accepted model for drying of forage is to neglect themoisturegradientwithinthe material,resultingin thefollowing expression (Menzies and O'Callaghan 1971; Shinners etal. 1987; Hill et al. 1977.) relating moisture content and time:
M-Me _ -ktMo-Me e
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(1)
where:
Mo = initial moisture content (db),
M = instantaneous moisture content at time t (db),
Me = equilibrium moisture content (db),k = drying constant (min" ), andt = elapsed time (min).
Equation 1 was fitted to drying data to determine k whichcharacterizes the rate of moisture removal from alfalfa. A
faster drying rate results in a larger value of k. To determinethe parameter k, the nonlinear least squares procedure(NLIN) in the Statistical Analysis System (SAS 1985) wasused. A paired t test statistic at 5% level was used to comparethe drying constants.
RESULTS AND DISCUSSION
On an average, each 50 mm stem contained 6 leaves; however, the lower segments of the plant, which were mainlystems, did not contain any leaves while upper segments contained more than 20 leaves. The average diameters of thestemsbefore and after drying were 2.40 and 1.86mm, respectively. The reduction in stem diameter was 23.5% and showsthat there is considerable shrinkage in the stems due to drying. The reduction in the leaf areas upon drying wasapproximately 41% from 256 mm2 to 151 mm2. This shrinkage was due to an 18.8% reduction in major axis and 26.9%reduction in minor axis of the leaf. The results indicate thatthere is rapid reduction in surface area due to shrinkageduring drying. Table I gives the range and standard deviationof the data on physical dimensions. Table II summarizes thedrying testresults of whole andcomponents of theplant.Therelative values of drying constant in comparison to the prebloom whole plant cut into 50 mm length are shown in thelast column of the table.
Table I. Physical characteristics of the alfalfacomponent parts
Characteristics Ave. Stddev. Min. Max.
No of leaves on 50 mm length stem 6 7.3 0 27
Dia. of 50 mm cut(fresh stem) (mm) 2.4 0.5 1.3 3.8
Dia. of 50 mm cut(dried stem) (mm) 1.8 0.5 0.9 3.3
Area of fresh leaf(mm2) 256.0 67.3 140.2 448.4
Major axis (fresh leaf) (mm) 25.3 3.5 18.7 33.5
Minor axis (fresh leaf) (mm) 12.9 2.2 8.6 18.6
Area ofdried leaf(mm2) 151.3 45.7 83.4 359.6
Major axis (dried leaf) (mm) 20.5 3.9 14.4 31.9
Minor axis (dried leaf) (mm) 9.5 1.7 6.9 14.7
Drying rate of components
The drying test results for components dried at 60°C and 5%relative humidity; and those at 60°C and 30% relative humidity show that in both cases the leaves dried faster than thestems and also faster than stems and leaves attached. Thisfasterdryingof leaveswas mainlydue to a larger surfaceareaand presence of a number of stomatal openings associatedwith the leaves. The stems with leaves removed dried faster
PATIL, SOKHANSANJ, ARINZE and SCHOENAU
than the stems and leaves attached. The reason for slowdrying of the latter can be explained by the fact that once theleaves on the stems are dried, the movement of water fromstems to leaves and there on to the atmosphere is restricted.This behavior was reported as early as 1933 by Jones andPalmer (1933) in their paper on field curing of hay. The stemsdried faster than the whole plant because there is a scar leftdue to the removal of the leaves. The predicted and experimental drying curves at 60°C and 5% RH shown in Fig. 2indicate that Eq. 1 represents the experimental data well.
Effect of stem length
The waxy layer on the surface of the stem provides a barrierfor water movement and thus water movement is easier longitudinally. Therefore the chop lengthplays an importantrolein reducing the drying time. This reduction in drying time isespecially useful during low temperature drying when thereis no disruption in the cutin layer due to the low level ofapplied heat.
.Q
T3
O
2
60 C, 5%RH
Predicted
Leaves+Stems
Stem
Leaves
100 200
Dryingtime (min)
Fig. 2. Drying curves for parts of fresh alfalfa,
Table II. Summary of drying testsand the results of drying to 8% moisture content
300
To study the effect of stem length on drying rate, thedrying constants of stems of 10,20,30,50, and 75 mm lengthwere plotted in Fig. 3 and the following equation was fittedto the data.
k= 0.133L"048where:
L = stem length (mm), andk = drying constant (min" ).
Equation 2 shows that the drying constant reduces logarithmically with increase in stem length.
Effect of maturity on drying rate
The drying time for the second cut at 10% bloom was 143min compared to 257 min for the second cut at prebloom. Theprebloom alfalfa stems had an 85% moisture content (wb)and the 10% bloom samples had 68% moisture content. Therelative drying constant indicates about 43% saving in drying
R =0.97
c
"E
c
Q
0.0b- \ 60°C,5%RH • ExperimentalPredicted
0.04-
^v
0.03-
•
0.02"
^-^>>i>i^ •
0.01 -0 20 40 60 80
Stem length (mm)
Fig. 3. Drying constant at 60 °C versus stem length.
(2)
Material Length Harvest
stage
Growth
stage
Temp RH MCi EMC Drying
time
k
, . -K
ReL*
(mm) (°C)
60
(%) (% wb) (% wb) (min) (mm ')
Stem 50 second cut pre bloom 5 85.5 5.16 257 0.0211 1.09
Leaves _ second cut pre bloom 60 5 83.2 3.65 105 0.0451 2.35
Whole 50 second cut pre bloom 60 5 85.1 5.29 313 0.0192 1.00
Stem 50 first cut 10% bloom 60 30 78.6 5.50 505 0.0098 0.51
Leaves _ first cut 10% bloom 60 30 77.9 5.00 376 0.0113 0.59
Whole 50 first cut 10% bloom 60 30 78.2 6.00 811 0.0067 0.35
Stem 10 second cut pre bloom 60 5 85.8 3.36 110 0.0445 2.32
Stem 20 second cut pre bloom 60 5 86.2 3.51 154 0.0321 1.67
Stem 30 second cut pre bloom 60 5 85.8 4.64 215 0.0244 1.27
Stem 50 second cut pre bloom 60 5 85.5 5.16 257 0.0211 1.10
Stem 75 second cut pre bloom 60 5 85.7 5.88 352 0.0165 0.86
Stem 50 second cut 10% bloom 70 5 64.7 2.43 70 0.0511 2.66
Stem 50 second cut 10% bloom 80 5 64.4 1.42 66 0.0511 2.66
Stem 50 first cut 10% bloom 60 5 77.9 3.71 146 0.0303 1.58
Stem 50 second cut 10% bloom 60 5 68.0 2.91 143 0.0261 1.36
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time when stems were dried at 10% bloom stage. It may be,therefore, economical to harvest alfalfa at 10% bloom.
Effect of drying air temperature
The alfalfa stems of second cut were dried at 60°C, 70°C, and80°C with relative humidity lower than 5%. The dryingcurves are shown in Fig. 4. The variation of temperature ofdrying air was within 1.5°C. It was observed, thatcomparedto 60°C, thedrying was about twice as fast at 70°C requiringonly 70 min compared to 143 min at 60°C. However, therewas no appreciable increase in drying rate when temperaturewas at 80°C. Drying temperature of 70°C may be critical tothe cutin layer on the stem. Thaine (1969) showed that theremoval of cuticular waxes increased the drying rate at lowtemperature to a marked degree. More tests are needed todetermine the exact drying behavior of alfalfa stems at theseand higher temperatures.
100 200
Drying time (min)
Predicted
300
Fig. 4. Effect of air temperature on drying of alfalfastems.
SUMMARY AND CONCLUSIONS
1. It was demonstrated that at 60°C, alfalfa leaves dry 2.5times as fast as stems to reach the same moisture level of8%. It may beadvisable todryleaves andstems separatelyand later on remix them in the proportion desired forfurther processing.
2. Drying ratewas faster at shorter stem lengths. The dryingconstant, k, decreased logarithmically with the length ofstem. It is recommended that the shortest possible stemlengthbe used to achieveuniform drying.
3.Thestageofharvest hada significant effectondrying time.The alfalfa stems from 10% bloom samples dried fasterthan pre bloom samples.
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REFERENCES
ASAE. 1991. Moisture measurement-forages, S 358.1. ASAEStandards 1991, 38th ed. St. Joseph, MI: ASAE.
Bilanski, W.K., J.H.A. Lee and R.M. Halyk. 1965a. Hightemperature drying of alfalfa stems. Canadian Journal ofPlant Science 45:471-476.
Bilanski, W.K., J.H.A. Lee and R.M. Halyk. 1965b. Hightemperature drying of alfalfa leaves. CanadianAgricultural Engineering 7(1): 50-51, 56.
Hill, J.D., I.J. Ross and B.J. Barfield. 1977. The use of vaporpressure deficit to predict drying time for alfalfa hay.Transactions of the ASAE 20(2):372-374.
Jones, T.N. and L. O. Palmer. 1933. Field curing of hay asinfluenced by plant physiological reactions. II: The roleof leaves in the dehydration of hay plants. AgriculturalEngineering 14(6): 156-158.
Menzies, D.J. and J.R. O'Callaghan. 1971. The effect oftemperature on the drying rates of grass. Journal ofAgricultural Engineering Research 16(3):213-222.
SAS. 1985.SAS Users Guide: Statistics. Statistical AnalysisSystem Institute Inc., Cary, NC.
Savoie, P., C.A. Rotz, H.F. Bucholtz and R.C. Brook. 1982.Hay harvesting system losses and drying rates.Transactions of the ASAE 25 (3):581-585, 589.
Shinners K.J., R.G. Koegel and RJ. Straub. 1987. Dryingrates of macerated alfalfa mats. Transactions of theASAE30(4):909-912.
Shinners K.J.,R.G.Koegel andR.J. Straub.1989. Rapidfielddrying of forages utilizing shredded mat technology. InProceedings of the Eleventh International Congress onAgricultural Engineering, Volume 1- AgriculturalMechanization, 2063-2070. Rotterdam, Netherlands:Balkema.
Sokhansanj, S. andH.C. Wood. 1990. Engineering aspects offorage processing for pellets, cubes, chops and densebales. Advances in Feed Technology 5(l):6-24.
Stencl, J. 1989. Low temperature drying of alfalfa. InProceedings of4thInternational Conference onPhysicalProperties ofAgricultural Materials andTheir Influenceon Technological Processes, 803-807. Rostock,Germany.
Thaine, R. 1969. Theeffectof temperature onthedrying ratesof grass. Journal of Agricultural Engineering Research16(3):213-222.
PATIL, SOKHANSANJ, ARINZE and SCHOENAU
