aerodynamic separation and fractional drying of alfalfa leaves and stems - a review and new concept

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©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

DRYING TECHNOLOGY

Vol. 21, No. 9, pp. 1669–1698, 2003

Aerodynamic Separation and Fractional

Drying of Alfalfa Leaves and Stems—A Review

and New Concept

Edwin A. Arinze,1,* Greg J. Schoenau,1 Shahab Sokhansanj,2

and Phani Adapa1

1Department of Mechanical Engineering,

College of Engineering, University of Saskatchewan,

Saskatoon, Canada2Bioenergy Feedstock Development Program,

Environmental Sciences Division,

Oak Ridge National Laboratory, Oak Ridge,

Tennessee, USA

ABSTRACT

This article examines the state-of-the art on aerodynamic separation

and drying of leaves and stems. Relevant aerodynamic and drying

characteristics of alfalfa leaves and stems, important in the design

and functional performance evaluation of appropriate drying

*Correspondence: Edwin A. Arinze, Department of Mechanical Engineering,

College of Engineering, University of Saskatchewan, 57 Campus Drive,

Saskatoon, SK S7N 5A9 Canada; E-mail: [email protected].

1669

DOI: 10.1081/DRT-120025503 0737-3937 (Print); 1532-2300 (Online)

Copyright & 2003 by Marcel Dekker, Inc. www.dekker.com

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and separation systems, are presented. General features and design

parameters of rotary drum dryers are discussed. A new efficient

approach to combined drying and separation in a rotary drum dryer

is described in which fresh or pre-wilted alfalfa mixture is dried at a

moderate temperature, and in the same operation the dry leaf fraction

is aerodynamically separated from the stem fraction. Preliminary test

data obtained from the dryer indicated that the separated product

stream had comparatively high leaf purity, confirming the feasibility

of the new approach.

Key Words: Alfalfa chops: leaves and stems; High quality pellets

and cubes; Value-added products; Combined aerodynamic separa-

tion and drying; Rotary drum dryers; New concept.

1. INTRODUCTION

In Canada and the United States of America, growing, manufactur-ing and exporting forage products (alfalfa pellets and cubes, andcompressed hay bales and bagged chops) to Japan and other PacificRim countries is an important commercial industry. In this majorrevenue generating processed-forage industry, alfalfa pellets or cubesare produced from dehydrated fresh alfalfa chops or sun-cured (partiallyfield dried) chops. Alfalfa (Medicago sativa) is composed of about 50%leaves and 50% stems by weight. Whole alfalfa plant contains 18–22%protein while the leaves are 26–30% protein and the stems only 10–12%.Because of the high nutrients content, the demand for alfalfa leaf incommercial production of both human and animal nutritional foodand feed supplements, pharmaceuticals, and other value-added productssuch as enzymes, carotenoids and soluble and particulate proteinconcentrates, is increasing.[1,2] Alfalfa stems are high in fiber, and thisis being commercially exploited for value-added products including paperand hardboard manufacture, bio-energy production, and ethanol andlactic acid production.[1,2]

High-temperature rotary drum dryers are commonly used to removeexcess moisture from fresh chopped alfalfa at 70–80% moisture content,wet basis (w.b.) or from pre-wilted (sun-cured) alfalfa down to about10% moisture before undergoing other processes in the production ofpellets and cubes and value-added products. In these high-temperature(400–1000�C) rotary drum-drying systems, the pneumatic drying andconveying do not adequately control the heat distribution and residencetime of the particles resulting in under-dried stems and over-dried leaves.

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Alfalfa leaves dry much faster than stems,[3,38] and any attempt to dryboth leaves and stems completely usually results in over-drying of leaves.The over-drying leads to loss of nutritious leaf fractions, charring anddiscoloration, smoke and undesirable odor, and reduction in the overallquality of the final product. To minimize these problems and increase theutilization of whole alfalfa, the chops should be dried at low-moderatetemperatures and be separated into leaves and stems as required inproducing pellets and cubes and other value-added products.

The commercial value of dehydrated alfalfa is commonly measuredin terms of protein, essential amino acids, carotene (vitamin A), andxanthophyll contents. Several process factors influence the nutritionalvalue of the components, especially the moisture content and temperatureof the forage material before and during drying, and the length of timethe material stays in the dryer. The chemical changes are greatest whenthe product becomes too dry and its temperature rises. This occurswhen the moisture content of the product falls below 12%w.b.Generally, forage should not be dried to less than 10%w.b. moisture ifheat damage is to be avoided. Carotene and xanthophyll have a similarchemical structure and susceptibility to oxidation and losses during hightemperature drying.

Several methods of separating leaves from stems have been investi-gated. Scalping material on shaking screens is the most common methodof separating denser material from lighter material.[4] While separation oflow moisture alfalfa chops (less than 20%w.b. moisture content) into leafand stem fractions is feasible, this method was found to be unacceptablefor high moisture products because of clogging of the sizing holes.Aerodynamic separation has been successfully achieved with vertical,perforated rotating drum or horizontal air stream separators.[5–7] Theseseparators are not used for drying at the same time, and they haverelatively low capacities of about 0.3–3 t/h compared to normal through-puts of over 16 t/h in industrial production of alfalfa pellets and cubes.[39]

The separators utilize the differences in aerodynamic characteristics ofthe components of a mixture to separate the components (leavesand stems).

The main objective of this study is to examine the possibility ofimproving product quality in the manufacture of pellets and cubes, andincreasing the utilization of alfalfa for value-added products through anew and improved method of drying and separation of alfalfa leaves andstems. Aerodynamic, and drying characteristics of alfalfa leaves andstems are presented, and different methods of aerodynamic separationwith air stream are also reviewed. A new approach is presented forcombined aerodynamic separation and drying of alfalfa leaves and

Aerodynamic Separation and Fractional Drying 1671

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stems. This is accomplished through relatively simple modifications to anexisting small industrial rotary-drum dryer (manufactured by VincentCorporation, Tampa, Florida). In industrial application of the newconcept, the objective is to have wet alfalfa chops enter the rotarydrum dryer, where a stream of mostly leaves is completely driedand separated from wetter, mostly stem product. The stem and leavefractions may be diverted to separate end uses or recombined asdesired to provide a quality-blended product. General design featuresfor rotary drum dryers, and preliminary tests conducted on the smallindustrial rotary-drum dryer based on the new concept are reported inthis presentation.

2. AERODYNAMIC AND DRYING

CHARACTERISTICS OF ALFALFA

LEAVES AND STEMS

2.1. Aerodynamic Characteristics

Separation of leaves from stems can be accomplished by usingpneumatic separators, screen shakers, or gravity tables. To utilize theseseparation methods, it is important to know the aerodynamic andphysical characteristics of both the leaves and stems. Relevant physicalcharacteristics include dimensions, weight, shape (sphericity, aspectratio), moisture content, and bulk density. Characteristics that dictateaerodynamic separation are usually described by terminal velocity anddrag coefficient. Shape and size are the major considerations in theselection and design of screening separation systems, and gravity separa-tion systems rely on the difference in weight of the particles to beseparated.[7–9,40]

The lower segment of alfalfa plant is mainly stem with no leaveswhile the upper segments contain both stems and leaves. Depending onthe stage of maturity, the moisture content of freshly harvested alfalfamay range from 70 to 80%, wet basis. In an experimental study onphysical characteristics of alfalfa stems and leaves, Patil et al.[3] foundthat the average diameter of the stem before and after drying to 8%moisture content (w.b.) was 2.40 and 1.86mm, respectively. Thus, thereis considerable shrinkage in the stems due to drying. The reductionin leaf area during a drying process was approximately 40%. The shrink-age is due primarily to the reduction in the major and minor axes ofthe leaf.

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A particle having a projected area Ap (average projected area for aleaf or stem is 1/4 of its surface area) and immersed in a flowing fluid withvelocity Vf and density �f is subjected to a drag force FD given as[7,8]

FD ¼ CDAp

�f V2f

2ð1Þ

where CD is the drag coefficient (dimensionless). All symbols are definedin the Nomenclature.

A particle of mass mp in a free fall attains a constant terminal orsuspension or critical velocity VT at which the net gravitational force Fg

equals the resisting upward aerodynamic drag force FD. When terminalvelocity is attained under steady-state conditions, the particle motion willbe upward or downward, depending on whether the particle density �p issmaller or greater than that of the fluid, respectively. Knowledge of theterminal velocities of particles would therefore define the range of fluidvelocities affecting the effective separation of the particles in the fluidstream. Consequently, terminal velocity is an important aerodynamiccharacteristic in pneumatic separation of particles.

When the fluid velocity Vf is equal to the particle terminal velocityVT, the drag coefficient can be obtained in terms of the terminal velocityas[8]

CD ¼2mpgð �p � �f Þ

V2TAp�p�f

ð2aÞ

or

VT ¼2mpgð�p � �f Þ

Ap�p�f CD

� �1=2ð2bÞ

In this study, correlated experimental data obtained for alfalfa stemsin a vertical wind tunnel[7,38] were further analyzed numerically to relatethe terminal velocity VT with stem length L and weight W. The dragcoefficient CD was also related with Reynolds number Re (based on thestem length L as the characteristic dimension).

V2T ¼ 106½ð0:00000156 þ 0:00147L� 0:0615L2

þ 0:854L3� 3:796L4

Þ

þ 0:00117L�0:851W � ð3Þ

CD ¼ 102½0:0261þ 1:782L2� 0:00238InRe� ð4Þ

The terminal velocity for 10.0–12.7mm alfalfa stems ranged from 3.9to 4.8m/s while the average terminal velocity of fresh flat leaves was


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1.2m/s.[10] Curled leaves have less surface areas and higher terminalvelocities compared to flat leaves.

2.2. Drying Characteristics

Drying, thermal, and physical characteristics of a product are impor-tant in drying process design, monitoring and control, and functionalperformance analysis as reported in Mujumdar,[11] Mujumdar andMenon,[12] Arinze et al.,[13–15] Patil et al.,[3,16] Sokhansanj et al.,[41]

Shufen et al.,[17] Mohsenin,[8] Ford and Bilanski,[18] and Bilanskiet al.[19,20] Relevant thermal characteristics include thermal conductivity,thermal diffusivity, and specific heat, all affected by product density,moisture content, and temperature. Shufen et al.[17] and Ford andBilanski[18] give experimental data and correlated relationships on ther-mal characteristics of whole alfalfa, stems and leaves at various moisturecontents, bulk densities, and temperatures.

The drying rates of alfalfa and other forage materials, such as grass,have been studied in various temperature ranges.[3,13–16,19–21] Generally,the form of the thin-layer drying rate equation varies with drying gastemperature and product moisture content. Two or three distinct dryingperiods are usually observed, depending on the temperature and moisturecontent: constant rate period, and first and second falling rate periods, orfirst, second, and third drying stages.[12,21]

At high drying gas temperatures (above 200�C) typical in rotarydrum dryers, the drying rate of forages is constant, similar to evaporationfrom a free water surface. Thus,

�dM

dt¼ k or M ¼ M0 � kt ð5Þ

For ryegrass, Menzies and O’Callaghm[21] related the drying rateconstant to drying gas temperature Tf by

k ¼ �1:84þ 0:25Tf ð6Þ

where the drying constant k is in min�1 and temperature Tf is in�C.

Bilanski et al.[19,20] experimentally investigated high-temperaturedrying of alfalfa leaves and stems in thin layers. The experimentaldrying results indicating free moisture ratio vs. time are shown in Fig. 1for various drying temperatures. For each drying temperature level

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(95–865�C) linear drying equations similar to Eq. (5) above were devel-oped in terms of free moisture ratio MR and time t in the form:

MR ¼M �Me

M0 �Me

¼ K0 � kt ð7Þ

where the drying constant k is in min�1 and time t in min.In this study, values of the drying constant k and constant K0 as a

function of the drying temperature Tf were obtained from the high-tem-perature drying experimental data (Fig. 1) and Table-Curve regressionanalysis as follows:

k ¼ �0:408þ 0:003Tf R2¼ 0:99 for alfalfa stem ð8aÞ

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Time, minutes

Fre

e m

ois

ture

ratio, M

95oC

205oC

315oC

425oC

535oC

645oC

755oC

865oC

Figure 1a. Free moisture ratio vs. time at various temperature levels for alfalfa

stems.[19]


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k ¼ �2:076þ 0:016Tf R2¼ 0:96 for alfalfa leaf ð8bÞ

K0 ¼ 0:986� 482:6=T2f R2

¼ 0:97 for alfalfa stem ð9aÞ

K0 ¼ 0:965� 0:0002Tf R2¼ 0:95 for alfalfa leaf ð9bÞ

It is more convenient to use a single relationship combining dryinggas temperature and time to predict the moisture ratio MR at any instantduring a thin-layer drying process. Using again the high-temperaturedrying data (Fig. 1) and Table-Curve regression analysis, in this studythe following moisture ratio equations were obtained for alfalfa stems

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Time, minutes

Fre

e m

ois

ture

ra

tio

, M

95oC

205oC

315oC

425oC

535oC

755oC

645oC

Figure 1b. Free moisture ratio vs. time at various temperature levels for alfalfa

stems.[20]

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and leaves:

MR ¼ �0:457þ0:12

tþ257:4

Tf

�13945

T2f

R2¼ 0:91 ð10Þ

for alfalfa stem and 95�C � Tf� 865�C

MR ¼ �0:368þ0:157

t 0:5þ76:35

Tf

þ1353

T2f

R2¼ 0:91 ð11Þ

for alfalfa leaf and 95�C � Tf� 755�CCalculated data from Eqs. (8)–(11) and the experimental data (Fig. 1)

show clearly that at drying temperatures above 200�C, alfalfa leaves dryover five times faster than stems when exposed in thin layers. While ittakes about 0.61 and 0.16min to dry fresh alfalfa leaves to 10% finalmoisture content at 200 and 400�C drying gas temperatures, respectively,the corresponding drying times for fresh alfalfa stems to 10% final mois-ture content are 3.17min and 0.93min, respectively.

At moderate to low drying gas temperatures (below 200�C), thedrying rate of alfalfa and grass were found to be directly proportionalto the moisture content in excess of the equilibrium moisture content.[3,21]

Thus,

�dM

dt¼ kðM �MeÞ or

M �Me

M0 �Me

¼ e�ktð12Þ

For ryegrass, the drying constant k (min�1) and equilibrium moisturecontent Me were related to drying air temperature Tf in the range80–200�C and absolute humidity of the air Xf as follows

[21]:

k ¼ 0:02expð0:02Tf Þ ð13Þ

Me ¼33000Xf

T2f � 0:22

ð14Þ

Below 80�C drying air temperature, Menzies and O’Callaghan[21]

found that for grass there were up to three distinct falling rate periodsof drying with different drying constants and moisture content ranges.Patil et al.[3,16] used Eq. (12) and experimental data to characterize thethin-layer drying of fresh alfalfa, stems, leaves, and whole plant for var-ious lengths of cut at 60–80�C and 5–30% humidity for the drying air.The low-temperature (60�C�Tf � 80�C) experimental data were furtheranalyzed in this study to obtain the following relationship for the dryingconstant k (min�1) and equilibrium moisture content as a function of


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the drying gas temperature and relative humidity Xf (decimal) for alfalfastems (50mm long) and leaves:

k ¼ 0:051� 3:43� 1024expð � Tf Þ R2¼ 0:99 for alfalfa stem

ð15aÞ

k ¼ 0:064� 2:17� 1024expð � Tf Þ R2¼ 0:99 for alfalfa leaf

ð15bÞ

Me ¼ �0:096þ9:11

Tf

�0:0003

Xf

R2¼ 0:98 for alfalfa stem ð16aÞ

Me ¼ �0:075þ7:78

Tf

�0:0006

Xf

R2¼ 0:92 for alfalfa leaf ð16bÞ

Calculations from the above low-temperature drying Eqs. (15a) and (15b)and experimental data show that alfalfa leaves dry over two times fasterthan stems when exposed in thin layers at drying temperatures below80�C. Stem length also affects the drying rate of alfalfa stem, and thefollowing equation by Patil et al.[3] may be used to relate stem length L(mm) and drying constant k (min�1):

k ¼ 0:133L�0:48 R2¼ 0:97 ð17Þ

3. AERODYNAMIC SEPARATION SYSTEMS

FOR LEAVES AND STEMS

As stated earlier, differences in aerodynamic properties of leaves andstems can be used in separating freshly chopped or partially driedforages. Because of their shape and greater exposed surface area, leavesgenerally have a higher aerodynamic drag force than stems of the sameweight. This means that an air stream could be used to carry the leaves alonger distance or on a higher trajectory and separate them from thestems.[5,6,10] Pneumatic separation of leaf and stem fractions have beendemonstrated to be feasible for herbs and spices,[6] and alfalfa forage.[10]

Design features and functional performance of typical aerodynamicseparation systems are discussed in this section.

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3.1. Vertical Air Stream and Perforated

Rotating Drum Separators

Figure 2 shows the schematic of a commercial-type vertical air-stream separator with an oscillating sieve placed in the air stream toprevent clumping, which can be used in separation of leaves and stemsfor forages, herbs, and spices. However, the sieve causes vortices, whichresult in a continuous change in particle orientation and aerodynamicdrag force. To eliminate the orientation changes observed in a commer-cial vertical air-stream separator, an outer or inner perforated rotatingdrum under suction is used in which the particles to be separatedare located one particle thick on the perforated surface of thedrum. Disconnection of a particle from the perforated surface isachieved by increasing the gravity component of the force (mgcos�) ina direction perpendicular to the drum surface (Fig. 3) or by reducing thesuction.

Figure 2. Schematic of a vertical air stream separator. The oscillating sieve is

placed to prevent leaves from penetrating downward with clumps.[6]


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As illustrated in Fig. 3, a particle of mass m with particle angle � andmoving at a constant peripheral speed Vo on the outer surface of aperforated rotating drum of radius R, is subjected to a centrifugal force(mVo

2/R), gravitational force (mg), reaction force (N ), and approxi-mately constant aerodynamic drag force (Fa). For a free fall, the finalhorizontal distance of the particle from the center of the drum (xc) isgiven as[6]

xc ¼ R sin � þVo cos �

g½V2

o sin2 � þ 2gðy1 þRþR cos �Þ�1=2 �Vo sin �

� �ð18aÞ

cos � ¼V2

o

gR¼

Fa

mgð18bÞ

where R, Vo, and y1 are known values. Equations (18a) and (18b) can beused to provide guidelines in designing a perforated drum separator.Figure 4 shows the separation efficiency for four perforated drum and

Figure 3a. Schematic of a perforated rotating outer drum separator with suction

indicating the forces acting on a particle located on the outer surface of the

drum.[6]

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vertical air stream separators tested by Gar-Mor et al.[6] The feed ratevaried between 100 and 600 g/s (360 and 2160 kg/h). Separation efficien-cies higher than 90% can be achieved with the drum separators at lowfeed rates. The separation efficiency drops sharply at high feed rates forthe inner drum and vertical air stream separators. The best separationefficiency (�95% for the entire range of feed rate) is obtained withthe combination of outer and inner drum separators as illustrated inFig. 4. Although the power consumption is relatively low (<2kW),perforated drum separators are suitable only for separating particles atrelatively low capacity and feed rates (<600 g/s), since in operation thematerial must be handled in a layer one particle thick. Material feed ratesin processed forage plants for production of alfalfa cubes and pellets canbe as high as 16.3 t/h or 9000 g/s of fresh alfalfa.

3.2. Horizontal Air Stream Separator

For effective aerodynamic separation of a mixture of two particlesinto two fractions, a significant difference must exist in at least one of the

Figure 3b. Schematic of a perforated rotating inner drum separator indicating

the forces acting on a particle located on the inner surface of the drum.[6]


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differentiating aerodynamic characteristics: terminal velocity and dragcoefficient. Figure 5 shows the schematic of a horizontal air streamseparator developed and used for separation of a mixture of alfalfaleaves and stems with processing capacity of 2.3 tonnes of haylage perhour.[5] Separation in a horizontal air stream is achieved by introducingthe particles at appropriate direction and speed into the air stream, whichdisperses the particles laterally along the working chamber by differentialdeflection of their trajectories (Fig. 5). Higher drag coefficients for leavesfor a given mass of chops result in greater separation or displacements of

Figure 4. Comparison of separation efficiency of perforated rotating drum

separators and vertical air stream separator.[6]

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leaves and stems. Figure 6 shows a typical distribution of alfalfa fractionsfor the horizontal air stream separator. It is evident from the distributionpatterns that the characteristics of the leaf and stem particles overlap,thus precluding complete or 100% separation efficiency. Short thickstems fall close to the inlet while leaves with attached stems fall at inter-mediate points, and small leaf particles are farthest from the inlet.

Figure 5. Horizontal air stream alfalfa separator.[5]

Figure 6. Typical alfalfa distribution in a horizontal air stream separator. Feed

rate¼ 0.5 kg/s, air velocity¼ 4m/s, moisture content¼ 45%w.b.[5]


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To achieve 100% purity or separation, all the leaves and no stemsare in the leaf fraction, or the size of the leaf fraction is equal to theproportion of leaves in the original material. Table 1 shows the separa-tion efficiency and the purity at known feed rates for two alfalfa mixtureseparated at different moisture contents in the horizontal air streamseparator. From the data in Table 1, the system is effective, achieving

Table 1. Separator performance with respect to protein improvement index

(PII), separation efficiency (SE), purity, and feed rate.[5]

Haylage (1) specifications: (1) Moisture content, % (wb) 38.60

(2) Pure stem crude protein, % 13.60

(3) Pure leaves (% protein) 26.60

(4) Whole plant (% protein) 20.00

Size of leaf fraction, % 25.0 30.0 35.0 42.7

Protein content, %a 24.9 24.0 24.3 23.2

PII, % 4.9 4.0 4.3 3.2

Purity, % 86.5 80 81.6 73.6

SE, % Relative 74 61 65 48

Absolute 37 36 46 42

Feed rate, t/h Average 1.33 0.81 1.08 0.70

Range 0.75–2.30 0.53–1.1 0.82–1.20 0.64–0.75

Variance 0.60 0.73 0.08 0.05

Haylage (2) specifications: (1) Moisture content, % (wb) 63.0

(2) Pure stem crude protein, % 18.9

(3) Pure leaves (% protein) 27.1

(4) Whole plant (% protein) 23.4

Size of leaf fraction, % 20.0 30.0

Protein content, % a 24.7 24.2

PII, % 1.3 0.8

Purity, % 71 63

SE, % Relative 35 21.6

Absolute 14 13

Feed rate, t/h Average 6.2 5.6

PII¼Protein Improvement Index

¼Crude protein in the leaf fraction after separation of alfalfa sample

� crude protein in the original sample before separation.

SE¼ separation efficiency (%).

Relative SE ¼ PIIMaximumPII

� 100%.aMean of four replications.

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high leaf purity up to 86%. Air stream velocity is the most importantvariable influencing separation. Separation is improved by increasing airstream velocities. Separation also improves by decreasing the initialmoisture content of chops and feed rate.

The major setback with the horizontal air stream separator is that itis bulky, relatively low capacity, and requiring two separate conveyingsystems to remove mostly leafy fractions and mostly stemmy fractionsafter separation (Fig. 5). The system is therefore not suitable forindustrial applications in most existing processed forage plants, wherecontinuous online processing operation is required.

4. ROTARY DRUM DRYERS FOR

DRYING AND SEPARATION

Rotary drum dryers are primarily used in the industry for high-tem-perature drying of various wet products, including grain, forages, woodparticles, coal, fertilizer, sand, and other aggregates. There is a wide varia-tion in rotary drum configurations for these numerous industrial applica-tions. A rotary drum dryer can have more than one stage of operation.

4.1. Design and Operational Features

of Rotary Drum Dryers

4.1.1. General Description

A rotary drum dryer consists primarily of a sloped or horizontal rotat-ing drum (single-pass arrangement) or rotating drums (multi-pass arrange-ment as shown in Fig. 7). A gas burner is placed at the inlet end of the drumto increase the temperature of the drying gas, and a fan is used to circulatethe heated drying gas through the drum(s). A cyclone is placed at the outletend of the drum to separate dried products from the gas stream.

A rotary drum dryer may be fired directly or indirectly, andhave various interior lifting-flight (paddle) configurations on thedrum periphery (Fig. 8). The wet solid particles enter the rotatingdrum and are then continuously cascaded through the moving gasstream. Solid particles caught on the lifting flights rotate with thedrum. In concurrent flow arrangement, a particle falling from a flightis moved along the length of the drum in the gas flow direction as a resultof a drag force imparted by the gas stream. In countercurrent flow


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arrangement, the gas flow works against the movement of the particlesthrough the drum.

4.1.2. Design and Operational Considerations

Basic parameters to be considered in the design, operation and ana-lysis of a commercial rotary drum dryer-separator include:

(i) Solids feed rate (F ) and initial moisture content (Min).(ii) Drumdiameter(D),length(L),slope(a),androtationalspeed(N).

Figure 7. Drum arrangements and material flow for a three-pass rotary drum

dryer.[27]

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(iii) Lifting-flights number (n) and profile.(iv) Gas stream direction (concurrent or countercurrent), velocity

(Vf), and inlet and outlet temperatures (Tf-in and Tf-out).(v) Drum solids holdup (H ) or the quantity of particles in the

drum at any instant during steady-state operation.(vi) Solids mean residence time (tr), and(vii) Solids outlet moisture content (Mout) and temperature (Ts).

A typical three-pass industrial-type rotary drum dryer for alfalfa hasouter, middle, and inner drums of 2.50, 1.75, and 1.00m diameter, respec-tively, with a total drying length of 36.0m. Drum speed is 8–10 rpm withairflow rate of about 1000m3/min. Inlet gas temperature ranges from500–900�C. With 72–75%w.b. initial moisture content and feed rate ofabout 16 t/h the dryer evaporates about 8–10 t of moisture per hour, anddries the product to a final moisture content of about 8–12% w.b. Outletgas temperature ranges from 110 to 125�C, and the dehydrator consumesabout 200–300m3 of natural gas or propane per tonne of dry product,or about 3250–4875 kJ/kg water evaporated.[22,23] Uniform moisturecontent of about 10%w.b. in the final product is essential. Under-driedproduct is susceptible to spoilage during storage while over-drying iscostly in terms of energy input, weight loss, product brittleness, and overallproduct quality.

Drum solids holdup and solids outlet moisture content Mout are themajor operational parameters to be derived or optimized for a given

Figure 8. Flight and material cascade movement for a rotary drum dryer.[15]


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system. The mean residence time of the particles tr and solids holdup Hare related to the operational parameters as follows[24,25]:

tr ¼H

Fð19aÞ

H ¼1

2ðnþ 1ÞhL ð19bÞ

where H is the solids holdup in m3 or kg, F the feed rate in m3/s or kg/s, hthe design holdup for the flight (known quantity), n the number of flights,and L drum length. In industrial design practice, drum holdup H isusually within the range 10 to 15% of the total drum volume.

Simultaneous heat and mass transfer processes take place duringdrying of the moving particles in the drum. Analysis of these heat andmass transfer processes require an understanding of the particle move-ments or dynamics through the drum. The particle dynamic movementsthrough the drum consist of:

(i) Forward movement of the cascading particles due to drumslope angle (�).

(ii) Forward or backward movement due to aerodynamic drag onsolid particles (Figs. 8 and 9).

(iii) Sliding or kiln motion of particles between flights in the lowerhalf of the drum (Fig. 9).

These particle dynamic movement components are usually incorpo-rated in various residence time models that have been developed based

Figure 9. Cascade motion of material in the drum of a single-pass counter-

current flow rotary drum dryer.[14]

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on basic design and operating parameters.[24,25] Although none of theresidence time models is universally acceptable, the followingSaeman[26] model is widely acceptable for design purposes:

tr ¼L

FHDNð�� aVf Þð20Þ

where FH is the drum loading factor (� for heavily loaded or overloadeddrums, and 2.0 for lightly loaded or under-loaded drums), a constant,ranging from 0.003 s/m to 0.026 s/m for different operating situations,and N the drum rotational speed (rps).

Rotary drum dryers are presently controlled both manually andautomatically. A major problem usually encountered in the control pro-cess appears to be the large fluctuations in the mass flow rate of rawmaterial into the dryer. This inevitably leads to fluctuation in the dryerexhaust temperature, and hence, to large changes in the product tempera-ture and moisture content. Excessively high furnace temperatures causescorching of the forage and significant deterioration of the productquality and nutrient value.

4.2. Modification of a Small Industrial Rotary Drum Dryer

for Both Drying and Separation

A new modified approach is explored in this study to separate theleaves from the stems while drying the leaves to some desired target moist-ure level. Since there is a natural tendency for the lighter leaves to both dryabout five times faster and be aerodynamically separated from the stems,wet chops may enter a modified rotary drum dryer, where a stream ofmostly leaves is completely dried and separated from wetter, mostly stemproduct. The wetter stem and drier leaf fractions may be diverted to sepa-rate end uses, production lines, or recombined as desired to provide ablended product. In this modified approach, over-drying, discoloration,loss of nutritious components, charring or burning of the leaves, whichusually occur when both leaves and stems are completely dried in a singledrying process at high temperatures, are avoided or minimized.

Figure 10 shows the photograph of a three-pass, small industrialrotary drum dryer (manufactured and patented by the VincentCorporation, Tampa, Florida), which is being used in this study toevaluate the drying and separation of alfalfa leaves and stems in thesame operation. The unit has the capability of separating heavier,wetter material from the stream. In normal operation, this material isblended with the wet incoming product. The original intention on this


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design was to avoid sticking of wetter material to the drum wall. For thepurpose of this study, this material is removed from the stream. Acyclone separator located at the exit from the drum collects the lighterdry material (mostly leaves).

Figure 11 shows the schematic of the small industrial rotary-drumdryer. It consists primarily of a propane gas burner, feed auger for wetmaterial mixture (leaves and stems), three concentric inner, middle, andouter drums, and two drum outlets on the outer drum for feeding heavieror wetter materials (mostly stems) onto a screw conveyor with dryeroutlet. The diameters of the inner, middle, and outer drums are 0.38m,0.57m, and 0.81m, and length 2.40m, 2.46m, and 2.74m, respectively.Each drum is provided with internal and external flights of rectangular-and channel-cross sections. The lighter materials (mostly leaves) from thedrum outlet pass through a cyclone to be separated from the gas stream(Fig. 11). A suction fan is mounted on top of the cyclone and the lightermaterials collected at the cyclone are conveyed to a cyclone exit port byanother screw conveyor. The circulating gas and very fine particles exitthrough the fan outlet.

Figure 10. Photograph of the three-pass, small industrial rotary-drum dryer

(Vincent Corporation, Tampa, Florida).

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The dryer has been instrumented to measure airflow and tempera-tures. Extensive testing of the dryer is ongoing. Operating parameters tobe explored or determined include alfalfa chops inlet moisture contentand feed rate, drying gas inlet and outlet temperatures and flow rate,chops outlet moisture content and residence time, and the proportionof leaves and stems at the dryer and cyclone exits. The influence ofthese parameters on the functional performance of the modified rotarydrum dryer is being investigated for both fresh and partially dried wetchops. For the purpose of this presentation, the functional performanceof the modified dryer is evaluated by the parameter: Leaf Purity (LPl) andStem Purity (LPs) defined, respectively, as

Leaf purity ¼ LPl¼Total mass of leaves at cyclone exit

Total mass of leaves and stems at cyclone exit

ð21aÞ

Stem purity ¼ LPs¼Total mass of stems at dryer exit

Total mass of stems and leaves at dryer exit

ð21bÞ

Harvested fresh alfalfa chops consist of a wide range of particle sizesranging from minute leaf fractions to whole triplet leaves with petriole,and from stem fragments to curved branches. Generally, chop lengthdetermines the particle contents and cross-sectional areas. In theory,

Figure 11. Schematic of the three-pass rotary drum dryer-separator.


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100% purity or perfect separation should result in the leaves in onefraction and the stems in the other. However, due to the nature of thealfalfa chop material, some leaves have stem segments attached and somestem pieces have leaf segments attached. It is therefore not possible toobtain 100% leaf purity. As stated earlier, alfalfa chops contain about50% leaves and 50% stems by weight. While high leaf purity denotes highseparation efficiency, the best purity level can be obtained with a leaffraction of less than 50% by mass.[5]

Table 2 shows leaf purity, stem purity, and chops final moisturecontent at the dryer and cyclone exits when fresh alfalfa chops at72%w.b. initial moisture content were dried with the small industrialrotary drum dryer. The measured drying gas flow rate was 0.39 kg/s,and gas inlet temperatures ranged from 400–440�C. For the drying con-ditions indicated, leaf purity varied from 69–76%. The leaf purity values(Table 2) are comparable with the figures in Table 1 obtained for thehorizontal air stream separator for alfalfa chops at 63% initial moisturecontent. The separation performance of the rotary drum was thereforesatisfactory, indicating that the new approach for combined drying andseparation of alfalfa chops in a rotary drum dryer is feasible.

The final moisture contents at the dryer exit ranged from3.3–18.6%w.b. and 3.0–7.9%w.b. at the cyclone exit for feed rates ran-ging from 41–105 kg/h. Final moisture contents increased with increasingfeed rate. In all tests, the difference between the product moisture contentat the dryer exit (mostly stems) and cyclone exit (mostly leaves) rangedfrom 1.8–10.7%w.b. This evidently showed that leaves dry much fasterthan stems under the same drying conditions, and that the difference inweight of the dried fractions makes aerodynamic separation possible.

The fluctuations in the final moisture content with feed rate (Table 2)indicate the common problem encountered in rotary drum dryers. Thealfalfa fractions were over-dried at low feed rates and under-dried at highfeed rates. The ongoing investigations on the small industrial rotary-drum dryer will address this undesirable effect. Generally, the chopsdrying time decreased with increasing gas temperature. Significantover-drying of mostly leaf fractions occurred at the cyclone exit atlower feed rates and higher temperatures.

CONCLUSIONS

To improve the quality of alfalfa pellets and cubes, and increase theutilization of alfalfa components in value-added products, a new

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Table2.

Stem

andleafpurity

andfinalmoisture

contentwhen

freshalfalfachopwith72%

w.b.initialmoisture

contentweredried

at400–440� C

,0.39kg/s

gasflow

rate,and40–105kg/h

feed

rate

withthemodified

smallindustrialrotary

drum

dryer.

Total

mass

offresh

chops

(kg)

Fresh

chops

feed

rate

(kg/h)

Drying

gases

mass

flow

rate

(kg/s)

Dryinggases

temperature

atdryer

inlet

(�C)

Dryinggases

temperature

atdryer

exit

(�C)

Total

drying

time(s)

Finalmoisture

contentofdry

chops,(%

w.b.)

Mass

ofdry

chopscollected,

(kg)

Stem

purity

atdryer

exit(%

)

Leafpurity

atcyclone

exit(%

)

Dryer

exit

Cyclone

exit

Dryer

exit

Cyclone

exit

10

41.0

0.39

422

101

1090

8.3

3.0

1.20

1.81

61

71

10

41.0

0.39

429

104

1047

3.3

1.5

1.26

1.47

60

71

10

44.1

0.39

402

76

1000

15.0

5.4

1.42

1.52

72

73

10

62.3

0.39

435

107

785

5.9

2.4

1.40

1.64

65

76

10

59.7

0.39

406

90

833

7.0

3.9

1.45

1.36

62

70

10

60.4

0.39

398

92

851

8.4

3.8

1.52

1.67

64

74

10

84.9

0.39

402

102

712

9.8

4.4

1.43

1.41

69

74

10

78.9

0.39

439

91

710

6.4

3.6

1.63

1.33

67

69

10

82.8

0.39

409

84

728

8.4

4.1

1.69

1.17

70

73

10

98.4

0.39

404

61

671

18.6

7.9

1.84

1.32

79

73

10

105.0

0.39

429

83

629

10.9

5.8

1.77

1.41

69

71


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approach has been presented. Fresh or pre-wilted alfalfa mixture is driedat low-moderate temperatures, and in the same operation the dry leaffraction is aerodynamically separated from the moist stem fraction in arotary drum dryer.

Experimental tests to investigate the new approach in drying andaerodynamic separation of alfalfa leaves and stems have been initiatedwith simple modifications on a small industrial rotary-drum dryer (byVincent Corporation). The preliminary test results are encouraging, andafter separation comparatively high leaf purity was achieved with themodified small rotary drum dryer. This indicates that the new approachis feasible in combined drying and separation of alfalfa chops in modifiedrotary drum dryers.

Aerodynamic separation of alfalfa leaves and stems in conventionalvertical air stream, perforated rotating drum, and horizontal air streamseparators was also found feasible on a small scale. Aerodynamic anddrying characteristics of alfalfa leaves and stems, which are important inthe design and functional performance evaluation of alfalfa drying andseparation systems, were presented. At high drying temperatures, leavesdry about five times faster than stems. Alfalfa leaves are lighter than stemsand are subjected tomuch higher aerodynamic drag forces in an air stream,making it possible to separate the leaves from stems in a mixture.

NOMENCLATURE

Ap Projected area of leaf or stem fraction (m2)a Constant in Eq. (20)CD Drag coefficient (dimensionless)D Drum diameter (m)F Feed rate (kg/s or m3) or force (N)Fa Aerodynamic drag force on a particle (N)FD Drag force on a particle (N)Fg Net gravitational force on a particle (N)FH Drum loading factor in Eq. (20)g Gravitational acceleration (m/s2)H Solids holdup in the drum (kg or m3)h Flight solids holdup (kg or m3)K0 Constant (intercept on MR-axis at t¼ 0 as indicated in Fig. 1)k Drying rate constant or simply drying constant (min�1 or h�1)M Particle or product moisture content at time t (% or decimal)Me Equilibrium moisture content (% or decimal)Min Initial solid particles moisture content (% or decimal)

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M0 Initial moisture content of the product (% or decimal)Mout Solid particles outlet moisture content (% or decimal)MR Free-moisture ratio at time t (dimensionless)m Mass of a particle (kg)N Normal reaction force (N) or drum rotational speed (rpm)n Number of flightsL Drum length (m), stem length (mm)LPl Leaf purity (% or decimal)LPs Stem purity (% or decimal)R Radius of perforated rotating drum (m or mm)Re Reynolds number (dimensionless)SE Separation efficiency (% or decimal)T Temperature (�C)Tf Gas or air temperature (�C)Tf-in Gas stream inlet temperature (�C)Tf-out Gas stream outlet temperature (�C)Ts Solid particle temperature (�C)t Time (s, min, or h)tr Mean residence time of particles (s or min)Va Air velocity near the drum surface (m/s)Vf Gas or fluid velocity (m/s)V0 Peripheral velocity of the rotating drum (m/s)VT Terminal or suspension velocity of a particle immersed in a

fluid (m/s)x Horizontal distance traveled by a particle in an air stream (m)xc Calculated horizontal distance traveled by a particle (m)y Particle height (m)y1 Height to bottom of drum (m)

Greek Symbols

� Drum slope angle (radians)� Particle or fluid density (kg/m3)� Particle angle (degree)�f Kinematic viscosity of the fluid (m2/s)

Subscripts

a Aire Equilibrium


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f Fluid or gasg Gravityp particle

ACKNOWLEDGMENT

Natural Sciences and Engineering Research Council (NSERC) ofCanada is gratefully acknowledged for providing funds for this ongoingresearch project through the Strategic Research Project. The authors arealso grateful to Vincent Corporation Incorporated, Tampa, Florida, forallowing us to use their small industrial rotary drum dryer for theongoing experimental tests.

REFERENCES

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aerodynamic separation and fractional drying of alfalfa leaves and stems - a review and new concept

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