thin-layer drying of grains and crops

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ASAE S448 DEC98

Thin-Layer Drying of Grains and Crops

Developed by the ASAE Grain and Feed Processing and StorageCommittee; approved by the Food and Process Engineering InstituteStandards Committee; adopted by ASAE December 1993; reaffirmed forone year December 1998.

1 Purpose and scope1.1 The purpose of this Standard is to provide a unified procedure fordetermining and presenting the drying characteristics of grains andcrops.

1.2 The drying data determined and presented according to thisStandard can be used in characterizing the drying rate of product,product drying computer simulation, performance testing of dryingequipment, and product quality evaluations.

1.3 This Standard applies specifically to grains and crops that are driedby forced air convection.

2 Normative referencesThe following standard contains provisions which, through reference in

this text, constitutes provisions of this Standard. At the time ofpublication, the editions indicated were valid. All standards are subject torevision, and parties to agreements based on this Standard areencouraged to investigate the possibility of applying the most recentedition of the standard indicated below. Standards organizations maintainregisters of currently valid standards.

ASAE D245.4 DEC93, Moisture Relationships of Grains

3 Definitions3.1 thin-layer: A layer of material exposed fully to an airstream duringdrying. The depth (thickness) of the layer should be uniform and shouldnot exceed three layers of particles.

3.2 initial moisture content, Mi: Moisture content of a product prior tothe start of drying, expressed as a decimal on a dry basis.

3.3 final moisture content, Mf: Moisture content of a product at thecompletion of drying, expressed as a decimal or a dry basis.

3.4 equilibrium moisture content, Me: Moisture content of a product inequilibrium with mean dry bulb temperature and relative humidity of thedrying air, expressed as a decimal on a dry basis. Equations given inASAE D245 should be used whenever possible.

4 General requirements4.1 Start-up. Tests should be conducted after drying equipment hasreached steady-state conditions. Steady state is achieved when theapproaching airstream temperature variation about the set-point is lessthan or equal to 1 C and approaching airstream relative humidityvariation about the set point is less than or equal to 3 percentage points.

4.2 Sample. The sample shall be clean and representative in particlesize. It shall be free from broken, cracked, weathered, and immatureparticles and other materials that are not inherently part of the product.The sample should preferably have its natural moisture content. In theabsence of a fresh sample, the moisture content of the sample may bereconstituted either by wetting with water spray or by conditioning in ahumid environment. Reconstituted samples shall be conditioned at roomtemperature in a sealed container for at least 24 h. Water should not beadded directly to the sample if this causes physical or chemical changesthat significantly affect drying rate of the product. In such cases,reconstituting the sample by placing it in a humid environment is

preferred. A dried sample should be reconstituted only once. Frozensamples should be thawed and used only once.

4.3 Drying. Particles in the thin layer should be exposed fully to theairstream. The airstream approaching the sample should be as uniformas possible in temperature and humidity at a given cross section parallel

to the thin layer so that the air contacts sample particles uniformly. Careshould be exercised to prevent displacement of particles in the thin-layerholder during a test. This situation may arise in a vertical thin-layer inwhich airstream flows horizontally through the product, or in a horizontalthin-layer in which airstream flows upward through the product. The airvelocity approaching the product should be at least 0.3 m/s.

4.4 Measurements. Nearly continuous recording of the sample massloss during drying is required. The corresponding records of particletemperature (surface or internal) are optional but preferred. The timeinterval between recordings depends on the mean dry bulb airtemperature. Higher temperatures require shorter time intervals betweenreadings. A typical time interval for drying with 60 C dry-bulb airtemperature is as follows: every 5 s during the first 5 min, every 1 minduring the next hour, and every 15 min thereafter. It is recommended todivert the airflow away from the sample occasionally to record the sample

mass with no airflow. This no airflow mass should be determined 3 to4 times during a drying test. Other parameters such as mean dry bulb airtemperature, relative humidity, and air velocity should be measuredoccasionally to assure consistent operation of the dryer.

4.5 Accuracy of measurements. Temperature sensors shall beaccurate to 1 C. Mass shall be measured with an accuracy of 0.2%of sample mass so that the calculated moisture content is within 0.002(decimal dry basis) of the products actual moisture content. Relativehumidity may be measured directly or computed from measurements ofdry bulb temperature and wet bulb (or dew point) temperature; it shall beaccurate to within 3 percentage points. The air velocity measurementshall be accurate within 5% .

4.6 Duration of experiment. In most cases the experiment shouldcontinue until the moisture ratio, MR, defined in clause 6 equals 0.05. Me

should be determined experimentally or numerically from equation in 3.4.

5 Reporting5.1 Sample identification shall consist of

type (preferably botanical name) and variety;

year;

location of harvest;

harvest moisture content;

storage conditions and history;

sample purity;

sample preparation (see 4.2).

5.2 Drying conditions used shall be specified. These conditions include: initial and final moisture contents (decimal, dry basis);

drying time (h, min or s);

approaching air velocity (m/s);

mean dry bulb temperature (C);

relative humidity (%).

All air-condition data shall include mean and standard deviationscomputed for the duration of the drying test.

5.3 Drying rate data reported shall consist of either of the following two

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forms:

the numerical values of moisture content versus time;

the estimated values of k and n including standard errors ofestimates in Pages equation

MRMMe

MiMeexp ktn (1)

The units for parameters kand nshall be consistent with the unit of t(h,min, or s) and shall include the standard deviation of each. The range ofequation 1 (ranges of temperature, relative humidity, and moisture

content) shall be clearly stated. See 3.6 for the duration of experiment.If drying is terminated prior to MR equals 0.05, the ending moisturecontent on which the model is based should be specified.

5.4 Reporting additional forms of drying equations such as thoseinvolving diffusion theories is optional. The simple first order equation

MRMMe

MiMeexp kt (2)

appears to be an inadequate representation of the drying behavior ofmost crops especially in high temperature drying applications (dry bulbair temperature higher than 40 C for grains).

5.5 Previously determined values of k and n are reported in table 1 forgrain products. The range of test conditions is also shown. For thoseproducts for which equation 2 was found in the literature, it is assumedn equals 1.

6 Notation6.1 The following notation is used in this Standard:

A constant;

B constant;

d.b. dry basis;

k constant;

M instantaneous moisture content, decimal dry basis;

Mi initial moisture content, decimal dry basis;

Me equilibrium moisture content, decimal dry basis;

Table 1 Constants k and n in the thin-layer drying equations 1 and 2. Equations and constants are given for other drying models (see notation in clause6).

Product Constants

t

unit Range Source

Barley k0.0462 exp(0.0154T) min 50T150 Bruce (1985)

n0.4923.84105(T123)2 0.215Mi0.41

Corn kexp(7.17351.2793 ln T0.1378v) h 2.2T71.1 Misra and Brooker (1980)

n0.0811 ln(rh)0.78 Mi 3rh83

0.18Mi0.60

Grass (fresh ryegrass) k0.34103 exp(0.02028T) s T200 OCallaghan et al. (1971)

n1

Lentils k0.1826260.0043T h 23T80 Tang et al. (1989)

n0.527 5rh70

Peanut pods (Virginia type) kexp(0.7805230.144026T0.358102T2 h 27T35 Kulasiri et al. (1989)

2.13941rh0.71599Mi0.137131T rh) 26rh47

n0.988670.019836T0.608103T21.033613 0.59Mi0.77

rh

0.6382401Mi

0.0499769T rhCanola k1.35520.00301Mi0.00751T0.5112v h 30T60 Patil and Ward (1989)

n0.50680.0015Mi0.0103T0.2440v 0.21v0.53

13.7Mi0.25

Rice, rough k0.029580.4456rh0.01215T h 32T51 Agrawal and Singh (1977)

n0.133651.93653rh1.77431rh20.009468T 19rh85

Rice, parboiled MR0.134 exp(5105kt)0.586 exp(5104t) h 17.3T40.2 Byler and Brook (1984)

0.28 exp(8103kt), kexp(3590/[T273]) 25rh51

Sorghum tA ln MRB(ln MR)2 h 26.5T71 Paulsen and Thompson (1973)

A 25.870.33540.001075T2 71T115.5

A0.0540.0017T 26.5T115.5

B30.5 exp(0.018T)

Soybeans k0.0330.0003T min 32T49 Hutchinson and Otten (1982)

n0.37440.00916T rh 34rh65

Sunflower (oilseed)k

5.66

10

4

T1.271min

27T93Li et al. (1987)

n0.82810.004T0.000091T Mi 0.26Mi0.33

Walnuts kexp(0.6810.011Mi0.952 ln(Mi) h 27T43.2 Anigbankpu et al. (1980)

0.000152(1.8T32)2 25rh76

n1

Wheat k139.3 exp(4426/[T273]) s T100 OCallaghan et al. (1971)

n1

White beans k0.04660.0104rh min 32T49 Hutchinson and Otten (1982)

n0.40020.00728rh T 34rh65

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MR moistureratioMMe /MiM;

n constant; rh relative humidity, %;

T temperature, C; t time, s, min, h; v air velocity, m/s.

Annex A(informative)

Bibliography

The following documents are cited as reference sources used indevelopment of this Standard:

Anigbankpu, C. S., T. R. Rumsey, and J. F. Thompson. 1980. Thin layerdrying and equilibrium moisture content equations for Ashley walnuts.ASAE Paper No. 80-6507.

Agrawal, Y. C. and R. D. Singh. 1977. Thin layer drying studies for shortgrain rice. ASAE Paper No. 77-3531.

Bruce, D. M. 1985. Exposed layer barley drying: three models fitted tonew data up to 150 C. J. Agric. Engng. Res. 32:337347.

Byler, R. K. and R. C. Brook. 1984. Thin layer model, temperature andrelative humidity variable. ASAE Paper No. 84-2535.

Hutchinson, D. and L. Otten. 1982. Thin layer drying of soybeans andwhite beans. ASAE/CSAE Paper No. 82-104.

Kulasiri, D. G., D. H. Vaughan, J. S. Cundiff and W. F. Wilcke. 1989. Thinlayer drying rates of Virginia type peanuts. ASAE Paper No. 89-6600.

Li, Y., V. Morey and M. Afinrud. 1987. Thin layer drying rates of oilseedsunflower. Transactions of the ASAE 30(4):11711175, 1180.

Misra, M. K. and D. B. Brooker. 1980. Thin layer drying and rewetting

equations for shelled yellow corn. Transactions of the ASAE 23(5):12541260.

OCallaghan, J. R., D. J. Menzies and P. H. Bailey. 1971. Digitalsimulation of agricultural drier performance. J. Agric. Engng. Res.16:223244.

Paulsen, M. R. and T. L. Thompson. 1973. Drying analysis of grainsorghum. Transactions of the ASAE 16(3):537540.

Patil, B. G. and G. T. Ward. 1989. Heated air drying of rapeseed.Agricultural Mechanization in Asia, Africa, and Latin America. 20(4):5258.

Tang, J., S. Sokhansanj and F. W. Sosulski. 1989. Thin-layer drying oflentil. ASAE Paper No. 89-6540.

ASAE STANDARDS 1999 583

thin-layer drying of grains and crops

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