1、 ANSI/ASAE S448.2 SEP2014 Thin-Layer Drying of Agricultural Crops American Society of Agricultural and Biological Engineers ASABE is a professional and technical organization, of members worldwide, who are dedicated to advancement of engineering applicable to agricultural, food, and biological syste
2、ms. ASABE Standards are consensus documents developed and adopted by the American Society of Agricultural and Biological Engineers to meet standardization needs within the scope of the Society; principally agricultural field equipment, farmstead equipment, structures, soil and water resource managem
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9、m, revise, or withdraw each standard. Copyright American Society of Agricultural and Biological Engineers. All rights reserved. ASABE, 2950 Niles Road, St. Joseph, Ml 49085-9659, USA, phone 269-429-0300, fax 269-429-3852, hqasabe.org ANSI/ASAE S448.2 SEP2014 Copyright American Society of Agricultura
10、l and Biological Engineers 1 ANSI/ASAE S448.2 SEP2014 Revision approved September 2014 as an American National Standard Thin-Layer Drying of Agricultural Crops Developed by the ASAE Grain and Feed Processing and Storage Committee; approved by the Food and Process Engineering Institute Standards Comm
11、ittee; adopted by ASAE December 1993; reaffirmed December 1998; revised editorially and reaffirmed December 1999; revised July 2001; approved by ANSI July 2001; reaffirmed and editorially revised February 2006; reaffirmation approved by ANSI March 2006; reaffirmation extended two years January 2011;
12、 reaffirmed by ASABE December 2012; reaffirmed by ANSI January 2013; revised and revision approved by ANSI September 2014. Keywords: Crops, Drying 1 Purpose and Scope 1.1 The purpose of this Standard is to provide a unified procedure for determining and presenting the drying characteristics of grain
13、s and crops. 1.2 The drying data determined and presented according to this Standard can be used in characterizing the drying rate of a product, product drying computer simulation, performance testing of drying equipment, and product quality evaluations. 1.3 This Standard applies specifically to gra
14、ins and crops that are dried by forced air convection in a thin layer. 2 Normative References The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies unless noted. For undated references, the latest approved edit
15、ion of the referenced document (including any amendments) applies. ASAE D245, Moisture Relationships of Plant-Based Agricultural Products 3 Definitions 3.1 thin-layer: A layer of material exposed fully to an airstream during drying. The depth (thickness) of the layer should be uniform and should not
16、 exceed three layers of particles. 3.2 instantaneous moisture content, M: Moisture content of a product measured or calculated during drying, expressed as decimal dry basis. 3.3 initial moisture content, Mi: Moisture content of a product prior to the start of drying, expressed as decimal dry basis.
17、3.4 final moisture content, Mf: Moisture content of a product at the completion of drying, expressed as decimal dry basis. ANSI/ASAE S448.2 SEP2014 Copyright American Society of Agricultural and Biological Engineers 2 3.5 equilibrium moisture content, Me: Moisture content of a product in equilibrium
18、 with mean dry bulb temperature and relative humidity of the drying air, expressed as decimal dry basis. Equations given in ASAE D245 should be used whenever possible. 4 General Requirements 4.1 Start-up. Tests should be conducted after drying equipment has reached steady-state conditions. Steady st
19、ate is achieved when the approaching airstream temperature variation about the set-point is less than or equal to 1 C and approaching airstream relative humidity variation about the set point is less than or equal to 3 percentage points. 4.2 Sample. The sample shall be clean and representative in pa
20、rticle size. It shall be free from broken, cracked, weathered, and immature particles and other materials that are not inherently part of the product. The sample should preferably have its natural moisture content. In the absence of a fresh sample, the moisture content of the sample may be reconstit
21、uted either by wetting with water spray or by conditioning in a humid environment. Reconstituted samples shall be conditioned at room temperature in a sealed container for at least 24 h (Sokhansanj et al., 1984). Water should not be added directly to the sample if this causes physical or chemical ch
22、anges that significantly affect drying rate of the product. In such cases, reconstituting the sample moisture content by placing it in a humid environment is preferred. A dry fresh sample should be reconstituted only once. Frozen samples should be thawed and used only once. 4.3 Drying. Particles in
23、the thin layer should be exposed fully to the airstream. The airstream approaching the sample should be as uniform as possible in temperature and humidity at a given cross section parallel to the thin layer so that the air contacts sample particles uniformly. Care should be exercised to prevent disp
24、lacement of particles in the thin-layer holder during a test. This situation may arise in a vertical thin-layer in which airstream flows horizontally through the product, or in a horizontal thin-layer in which airstream flows upward through the product. The air velocity approaching the product shoul
25、d be at least 0.3 m s-1. 4.4 Measurements. Nearly continuous recording of the sample mass loss during drying is required. The corresponding records of particle temperature (surface or internal) are optional but preferred. The time interval between recordings depends on the mean dry bulb air temperat
26、ure (Byler and Brook, 1984). Higher temperatures require shorter time intervals between readings. A typical time interval between mass measurement for drying with 60 C dry-bulb air temperature is as follows: every 5 s during the first 5 min, every 1 min during the next hour, and every 15 min thereaf
27、ter. It is recommended to occasionally divert the airflow away from the sample during mass measurement to record the sample mass with no airflow. This “no airflow” mass should be determined 3 to 4 times during a drying test. Other parameters such as mean dry bulb air temperature, relative humidity,
28、and air velocity should be measured occasionally to assure consistent operation of the dryer. 4.5 Accuracy of measurements. Temperature sensors shall be accurate 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 dr
29、y basis) of the products actual moisture content. Relative humidity may be measured directly or computed from measurements of dry bulb temperature and wet bulb (or dew point) temperature; it must be accurate to within 3 percentage points. The air velocity measurement must be accurate to within 5%. 4
30、.6 Duration of experiment. In most cases the experiment should continue until the moisture ratio, defined by MR, equals 0.05 (see clause 6 Notation). Meshould be determined experimentally or numerically from equations as discussed in clause 3.5. 5 Reporting 5.1 Sample identification shall consist of
31、: type (preferably botanical name) and variety; year of production/harvest; ANSI/ASAE S448.2 SEP2014 Copyright American Society of Agricultural and Biological Engineers 3 location of harvest; harvest moisture content; storage conditions and history; sample purity; sample preparation (see clause 4.2)
32、; size and shape characteristics of the particles. 5.2 Drying conditions used shall be specified. These conditions include: sample initial and final moisture contents (decimal, dry basis); drying duration (h, min, or s); approaching air velocity (m s1); mean dry bulb temperature (C) and its variatio
33、ns; mean relative humidity (%) and its variations. All drying-condition data shall include mean and standard deviations computed for the duration of the drying test. 5.3 Drying rate data shall be reported using either of the following two forms: the numerical values of moisture content (decimal, dry
34、 basis) versus time (h, min, or s); the estimated values of parameters k and n in Pages Equation (equation 1), including standard errors of estimates (Page, 1949): ( )nktMR = exp (1) Units for k and n shall be consistent with the unit of t (h, min, or s) and shall include the standard deviation of e
35、ach. The range of equation 1 (ranges of temperature, relative humidity, and moisture content) shall be clearly stated. See clause 4.6 for the duration of experiment. If drying is terminated prior to MR = 0.05, the ending moisture content on which the model is based should be specified. 5.4 Reporting
36、 drying rate data using other forms of drying equations such as those involving diffusion theories is optional. The simple first order equation ()ktMMMMMReie= exp (2) appears to be an inadequate representation of the drying behavior of most crops especially in high temperature drying applications (d
37、ry bulb air temperature higher than 40 C for grains). 5.5 Previously determined values of k and n are reported in Table 1 for grain products. The range of test conditions is also shown. For those products for which equation 2 was found in the literature, it is assumed n equals 1. ANSI/ASAE S448.2 SE
38、P2014 Copyright American Society of Agricultural and Biological Engineers 4 Table 1 Constants k and n in the thin layer drying equation 1 for selected grains and crops Product Constants t unit Range1 Source Alfalfa (freshly chopped) k=1.7610-2 exp(-0.123Mi+4.410-2 T-5.7910-5 T2 -3.2410-3MiT) n=0.5ex
39、p(7.0510-3Mi +8.7610-3 T-1.9110-5 T2) min 40 T 250 Patil (1995) Barley k=0.0462exp(0.0154T) n=0.492+3.8410-5(T-123)2 min 50 T 150 0.215 Mi 0.41 Bruce (1985) Canola k=0.1832 n=0.572 min T = 70 Mi = 0.214 Sokhansanj et al. (1984) Corn k=exp-7.1735+1.2793 ln(1.8T+32)+0.1378v n=0.0811 ln(rh)+0.78Mi h 2.
40、2 T 71.1 3 rh 83 0.18 Mi 0.60 Misra and Brooker (1980) Grass (fresh ryegrass) k=0.3410-3 exp(0.02028T) n=1 s T 200 OCallaghan et al. (1971) Lentils k=0.182626+0.0043T n=0.527 h 23 T 80 5 rh 70 Tang et al. (1989) Peanut pods2(Virginia type) k=exp-0.780523-0.144026T+0.35810-2 T2 +2.13941(rh/100)+0.715
41、99Mi -0.137131T(rh/100) n=0.80 h 27 T 35 26 rh 47 0.59 Mi 0.77 Kulasiri et al. (1989) Rice, rough k=0.02958-0.4456(rh/100)+0.01215T n=0.13365+1.93653(rh/100)-1.77431(rh/100)2 +0.009468T h 32 T 51 19 rh 85 Agrawal and Singh (1977) Sorghum3 t=A ln(MR)+B ln(MR)2 A=-25.87+0.3354(1.8T+32) -0.001075(1.8T+
42、32)2 A=0.54-0.0017(1.8T+32) B=30.35exp-0.018(1.8T+32) h 26.7 T 115.5 26.7 T 71.1 71.1 T 115.5 26.5 T 115.6 Paulsen and Thompson (1973) Soybeans k=exp11.752 - 7912.7/(1.8T+492)n k=exp10.906-7357.0/(1.8T+492)n k=exp10.375-6779.3/(1.8T+492)n n=0.39642+0.002448T h Mi = 0.250 Mi = 0.299 Mi = 0.493 37.8 T
43、 104.4 Overhults et al. (1973) Sunflower (oilseed) k=5.66 10-4T1.271n=0.8281-0.004T+0.0091TMi min 27 T 93 0.26 Mi 0.33 Li et al. (1987) Walnuts k=exp-0.681+0.011(100Mi)-0.952 ln(100Mi)+0.000152(1.8T+32.2)2 n=1 h 21 T 43.2 25 rh 76 Anigbankpu et al. (1980) Wheat k=2000exp-5094/(T+273) n=1 s T 100 OCa
44、llaghan et al. (1971) 1Temperature (C); Mi(decimal d.b.); rh (%), v (m s-1)2The exponent n is the average of 14 numerical values given in the original paper. 3The constants for sorghum are not for equation 1 but are for the drying equation listed on column 2 with time as the dependent variable, as p
45、resented in Paulsen and Thompson (1973). ANSI/ASAE S448.2 SEP2014 Copyright American Society of Agricultural and Biological Engineers 5 6 Notation 6.1 The following notation is used in this Standard: A constant; B constant; d.b. dry basis; k constant; M instantaneous moisture content, decimal, dry b
46、asis; Mi initial moisture content, decimal, dry basis; Me equilibrium moisture content, decimal, dry basis; MR moisture ratio = eieMMMMn constant; rh relative humidity, %; T temperature, C; t time, s, min, h; v air velocity, m s-1. Annex A (informative) Bibliography The following documents are cited
47、 as reference sources used in development of this Standard: Agrawal, Y. C. and R. D. Singh. 1977. Thin layer drying studies for short grain rice. ASAE Paper No. 77-3531. St. Joseph, Mich.: ASAE. Anigbankpu, C. S., T. R. Rumsey, and J. F. Thompson. 1980. Thin layer drying and equilibrium moisture con
48、tent equations for Ashley walnuts. ASAE Paper No. 80-6507. St. Joseph, Mich.: ASAE. Bruce, D. M. 1985. Exposed layer barley drying: three models fitted to new data up to 150C. Journal of Agricultural Engineering Research 32(4): 337-348. Byler, R. K., and R. C. Brook. 1984. Thin layer model, temperat
49、ure and relative humidity variable. ASAE Paper No. 84-3525. St. Joseph, Mich.: ASAE. Kulasiri, D. G., D. H. Vaughn, J. S. Cundiff, and W. F. Wilcke. 1989. Thin layer drying rates of Virginia type peanuts. ASAE Paper No. 89-6600. St. Joseph, Mich.: ASAE. Li, Y., V. Morey, and M. Afinrud. 1987. Thin layer drying rates of oilseed sunflower. Transactions of the
