Effect of Bulk Density, Moisture Content, and Grain Size on Vertical Airflow Resistance of Lathyrus (Lathyrus sativus L.) Grains  

R. N. Kenghe1 , K. R. Kenghe2 , P.M. Nimkar3 , S.S. Shirkole1
1. Department of Agricultural Process Engineering, Mahatma Phule Krishi Vidyapeeth, Rahuri- 413 722, India
2. Mechanical Engineering student, BSIT&R (W), Wagholi, Pune
3. Department of Agricultural Process Engineering, Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola- 444 104, India
Author    Correspondence author
Legume Genomics and Genetics, 2013, Vol. 4, No. 2   doi: 10.5376/lgg.2013.04.0002
Received: 29 Jan., 2013    Accepted: 13 Feb., 2013    Published: 25 Feb., 2013
© 2013 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Kenghe et al., 2013, Effect of Bulk Density, Moisture Content, and Grain Size on Vertical Airflow Resistance of Lathyrus (Lathyrus sativus L.) Grains, Legume Genomics and Genetics, Vol.4, No.2 3-21 (doi: 10.5376/lgg. 2013.04.0002)

Abstract

The experiments were conducted to study the effect of various operating parameters on airflow resistance of lathyrus (legume) crop. The bulk density, moisture content, bed depth and grain size  for lathyrus (cv. NLK- 40, Pratik and Ratan) were studied at different three levels of moisture content ranged from 7.33% to 18.80%, 6.75% to 18.30% and 7.90% to 19.40% d.b., respectively. It was observed that as airflow rate increased the pressure drop also increased significantly but not proportional for all the combinations of the operating parameters studied. At any airflow rate within experimental range, increase in moisture content resulted in decrease of pressure drop for all grains. The increase in airflow resistance was linearly proportional to bed depth for loose, medium and dense packed grain beds. The increase in grain bulk density at any moisture level resulted in an increase in pressure drop at all airflow rates. Bulk density of the densely packed lathyrus grain was found to be 9.63% to 11.67% denser as compared to the loose fill conditions. The corresponding increase in airflow resistance was about 1.41 times higher than that of loose fill condition. Smaller size grain offered significantly higher (1.61 times) airflow resistance as compared to larger size grain. The pressure drop per unit bed depth was maximum for NLK-40 followed Pratik, Ratan and soybean.

Keywords
Lathyrus sativus L.; Bed depth; Bulk density; Grain size; Airflow resistance

Lathyrus (Lathyrus sativus L.) locally known as 'Khesari', 'Teora', 'Lakh' and 'Lakhdi' is a protein rich legume mainly grown in dry land areas or extreme harsh conditions of relay cropping system. It is mainly grown in India, Bangladesh, Pakistan, Nepal and Ethiopia, in areas where the irrigation facilities are very limited particularly during winter cropping. Lathyrus is grown for food, fodder, as straw feed and green manure. Lathyrus crop is grown on about 1.5 million ha in India, and its production estimate is nearly about 0.8 millon tonne of grain yield annually. Nearly two third of national acreage under lathyrus is in South-eastern Madhya Pradesh and Vidarbha region of Maharashtra, in rice based cropping system having limited irrigation facilities.

The first ever effort to systematically collect the lathyrus germplasm in India was made in 1987 when seven districts of Madhya Pradesh were surveyed jointly by Pulses Improvement Project of USAID, New Delhi; Nutritional Research Laboratory, Hyderabad; and Directorate of Agriculture and Directorate of Health, Government of Madhya Pradesh.
There appear to be few studies on the nutritional aspects of lathyrus. Rahman et al(1974) and Rotter et al (1991) gave the composition for lathyrus as energy 362.3 cal, protein 31.6%, fat 2.7%, nitrogen- free extract 51.8%, crude fibre 1.1% and ash 2.2%.
Resistance to airflow is a function of both product and air properties (Jayas et al., 1987). Shedd (1953) studied airflow resistance and presented equations and curves for a number of grains. The phenomenon of pressure drop in airflow through agricultural products has been widely investigated for various grains (Patterson et al., 1971; Matthies and Petterson, 1971; Haque et al., 1982; Grama et al., 1984; Kumar and Muir, 1986; Jayas, 1987; Giner and Denisienia, 1996; Nimkar and Chattopadhyay, 2003; Rajabipour et al., 2001 and Sacilik, 2004) and root vegetables (Neale and Messer, 1976; Abrams and Fish, 1982; Shahbazi and Rajabipour, 2008;Kasaninejad and Tabil, 2009). Relationships were compiled in the ASABE D 272.3 MAR 1996 (ASABE, 2007) standard.
Airflow resistance data is useful for designing drying and aeration systems. It is necessary to generate and provide information on airflow resistance of lathyrus to designers of drying systems for proper drying of these grains by forced draft. Earlier reported studies on airflow resistance of different agricultural grains as affected by various operating parameters were reviewed which showed that no design data on the resistance to airflow of lathyrus is available.
Therefore, the research work was planned with objective to determine pressure drop at higher airflow rates through clean grain beds of lathyrus at different levels of moisture content, bulk density, grain size and bed depth.
1 Result
The experimental results carried out for three moisture levels as indicated under section 3.5 for each lathyrus variety are shown in Table 1 through Table 2. However the results were presented only for 13.10%, 12.50% and 13.60% (d. b.) moisture content, respectively for NLK-40, Pratik and Ratan has been discussed.


Table 1 Effect of moisture content on pressure drop of lathyrus at different moisture content



Table 2 Mean pressure drops (Pa/m) at various airflows, bulk densities and bed depths of lathyrus at different moisture content


1.1 Effect of bulk density
The results obtained as experimental pressure drop for loose, medium and dense packed NLK-40 grain beds at grain moisture content 7.33%, 13.10% and 18.80% d.b. are as shown in Table 2. Packing the grain beds at all three moisture levels resulted in increased bulk density of grain about 4.81% to 5.06% and 9.63% to 10.12% for medium and dense packed beds, respectively, whereas, the decrease in bulk porosity value was about 8.71% to 10.86% and 17.45% to 21.76%, respectively, when compared with loose fill condition. At moisture level of 13.10% d.b. and airflow rate of 0.1687 m3·s-1·m-2, 0.5524 m3·s-1·m-2 and 1.0169 m3·s-1·m-2, the increased bulk density from 805 kg/m3 to 895 kg/m3 in packed beds offered pressure drops of 771 Pa/m, 4564 Pa/m and 11677 Pa/m, respectively, as against 583 Pa/m, 2787 Pa/m and 7615 Pa/m in loosely filled beds.
The results obtained as experimental pressure drop for loose, medium and dense packed Pratik grain beds at grain moisture content 6.75%, 12.50% and 18.30% d.b. are as shown in Table 2. At all three moisture levels the medium and densely packed Pratik grain beds showed increase in bulk density values of about 4.84% to 5.26% and 9.69% to 10.52%, respectively, whereas, the corresponding decrease in bulk porosity values were about 8.01% to 8.51% and 16.05% to 17.02%, when compared with loose fill condition. At moisture level of 12.50% d.b. and airflow rate of 0.1708, 0.5518 m3·s-1·m-2 and 1.0794 m3·s-1·m-2, the increased bulk density from 795 kg/m3 to 875 kg/m3 in packed beds offered pressure drops of 698.9 Pa/m, 3727 Pa/m and 11202 Pa/m, respectively as against 516.6 Pa/m, 2517 Pa/m and 7713 Pa/min loosely filled beds.
The experimental values of pressure drop for loose, medium and dense packed Ratan grain beds at grain moisture content 7.90%, 13.60% and 19.40% d.b. are as shown in Table 2. For grain beds of medium and dense packing at all the three moisture levels resulted in increased bulk density of about 4.93% to 5.29% and 9.87% to 10.59%, respectively whereas, the corresponding decrease in bulk porosity value was about 7.73% to 8.92% and 17.10% to 17.87%, respectively, when compared with loose fill condition. At moisture level of 13.60% d.b. and airflow rate of 0.1748 m3·s-1·m-2, 0.6224 m3·s-1·m-2 and 1.0193 m3·s-1·m-2, the increased bulk density from 770 to 850 kg/m3 in packed beds offered pressure drops of 587.6 Pa/m, 4102 Pa/m and 9475 Pa/m, respectively as against 425.4 Pa/m, 2861 Pa/m and 6761 Pa/min loosely filled beds.
1.2 Effect of bed depth and airflow
In order to study the effect of bed depth and airflow on pressure drop, only loose fill grain beds of NLK-40, Pratik and Ratan at their middle moisture contents of 13.10%, 12.50% and 13.60% d.b. were considered. In order to interpret the results regarding the effect of bed depth and airflow on pressure drop, three representative airflow rates were considered which represented the low, medium and high range of airflows under the study.
The results obtained regarding the airflow resistance of lathyrus as affected by bed depth and airflow rate is as given Table 2. It was observed that pressure drop was found linearly increased with the increase in grain bed depth and it was disproportionate to airflow for lathyrus grains.
It was observed that for NLK-40 as the bed depth increased the pressure drop also increased. For 200 mm bed depth the airflow resistance values were 517.9 Pa, 2589 Pa and 7311 Pa at airflow rates 0.1687 m3·s-1·m-2, 0.5524 m3·s-1·m-2 and 1.0169 m3·s-1·m-2, respectively, while the corresponding values of pressure drop for the same airflow rates were 563.6 Pa, 2695 Pa and 7463 Pa at a depth of 400 mm and 583 mm, 2787 Pa and 7615 Pa, respectively at a depth of 600 mm.
In case of Pratik it was observed that as the bed depth and airflow rate increased the pressure drop also increased. For 200 mm bed depth the airflow resistance values were 455.8 Pa, 2309 Pa and 7323 Pa at airflow rates 0.1708 m3·s-1·m-2, 0.5518 m3·s-1·m-2 and 1.0794 m3·s-1·m-2, respectively, while the corresponding values of pressure drop for the same airflow rates were 486.8 Pa, 2415 Pa and 7520 Pa at a depth of 400 mm and 516.6 Pa, 2517 Pa and 7713 Pa, respectively at a depth of 600 mm..
Whereas, in case of Ratan similar trend was followed as other two varieties. For 200 mm bed depth the airflow resistance values were 364.6 Pa, 2674 Pa and 6442 Pa at airflow rates 0.1748 m3·s-1·m-2, 0.6224 m3·s-1·m-2 and 1.0193 m3·s-1·m-2, respectively, while the corresponding values of pressure drop for the same airflow rates were 395.2 Pa, 2750 Pa and 6609 Pa at a depth of 400 mm and 425.4 Pa, 2861 Pa and 6761 Pa, respectively at a depth of 600 mm.
1.3 Effect of moisture content
The results indicated in Table 1 shows that for all grain samples the pressure drop decreased with increase in the moisture content. For loosely filled beds of NLK-40 at moisture content between 7.33 to 18.80% d.b. and at the representative airflow   rate of 0.5546 m3·s-1·m-2, 0.5524 m3·s-1·m-2 and 0.5511 m3·s-1·m-2, it was found that with increase in moisture content the pressure drops were decreased from 2853 Pa/m to 2721 Pa/m.
In case of Pratik beds in the moisture range of 6.75% to 19.40% d.b. and at the airflow rate of 0.5547 m3·s-1·m-2, 0.5518 m3·s-1·m-2 and 0.5563 m3·s-1·m-2, with increase in moisture content the pressure drops were decreased from 2598 Pa/m to 2441 Pa/m. Whereas, for Ratan beds in the moisture range of 7.90% to 19.40% d.b. and at the airflow rate of 0.5587 m3·s-1·m-2, 0.5575 m3·s-1·m-2 and 0.5597 m3·s-1·m-2 with increase in moisture content the pressure drops were decreased from 2436 Pa/m to 2304 Pa/m, respectively.
1.4 Effect of grain size
The effect of grain size on pressure drop for, small, medium and large sized lathyrus grain viz., NLK-40, Pratik and Ratan were studied at initial moisture content of 7.33%, 6.75% and 7.90% d.b., respectively at loose filled condition. The grain sizes and bulk densities of the samples were 4.33 mm, 4.96 mm and 5.08 mm. and 830 kg/m, 825 kg/m and 810 kg/m, respectively for large, medium and small sized grains.
The results indicated that the pressure drop decreased with increase in the grain size. For small size grain at the airflow rates of 0.1694 m3·s-1·m-2, 0.5546 m3·s-1·m-2and 0.8180 m3·s-1·m-2offered pressure drop of 614.3 Pa/m, 2853 Pa/m and 5427 Pa/m. In case of medium size grain at the airflow rate of 0.1717 m3·s-1·m-2 0.4902 m3·s-1·m-2 and 0.9500 m3·s-1·m-2 offered pressure drop of 547.0 Pa/m, 1990 Pa/m and 6447 Pa/m. Whereas, for large size grain at the airflow rates of 0.1751 m3·s-1·m-2, 0.6237 m3·s-1·m-2 and 0.9541 m3·s-1·m-2offered pressure drop of 455.8 Pa/m, 2952 Pa/m and 6219 Pa/m. At an average representative airflow rates of 0.5140 m3·s-1·m-2, 0.5373 m3·s-1·m-2 and 0.5843 m3·s-1·m-2 offered pressure drop of 2643 Pa/m, 2264 Pa/m and 1642 Pa/m, respectively, with the corresponding increase in sizes. It is evident from the above findings that medium and small size grain offered 1.31 and 1.61 times higher pressure drop values than that of large sized grain.
2 Discussion
2.1 Effect of bulk density
The results indicated that with increase in bulk density at any moisture content the pressure drop through grain bed was increased. Denser packing resulted in reduction in bulk porosity and therefore, increased in bulk density which contributed to increase pressure drop in lathyrus beds. The results showed that the increase in bulk density have resulted in increased resistance of lathyrus to airflow. The variation in resistance was dependent on the magnitude of airflow rate and the sample composition. However, the increase in pressure drop could not be explained solely by the change in bulk density alone; the orientation of particles also played the major role in increasing pressure drop.
The results obtained for the effect of grain bulk density on pressure drop are consistent with the earlier findings. Nimkar and Chattopadhyay (2002a, 2002b reported that with increase in bulk density the pressure drop was found to be increased with corresponding increase in moisture content of green gram and pigeonpea grains; Yang and Williams (1990) reported increase in bulk density causes increase in pressure drop through sorghum grains; Molenda et al (2005) reported relationship between grain density and pressure drop.
2.2 Effect of bed depth
It was observed that pressure drop was found linearly increased with the increase in grain bed depth and it was disproportionate to airflow for lathyrus grains. In general averaging the pressure drop of all three varieties it was noted that at a depth of 200 mm, two times increase in airflow rate increased the pressure drop by ratio of 2.20, while the ratio was 2.27 for 400 mm and 2.24 for 600 mm bed depth. However, when the bed depth was doubled from 200 mm to 400 mm, the pressure drop values were found to be increased by 1.10 times, 1.03 times and 1.00 times.
The above discussed results revealed that for lathyrus grains, in general the resistance to airflow increased linearly with depth of grain bed since twice the grain bed depth nearly twice as much resistance; but twice the airflow rate offered more than as much resistance. Increase in pressure drops across these grain beds were proportionate with increase in bed depths, but were disproportionate with increase in airflow rates.
The trends observed were in agreement with the results reported for soybean and oats (Henderson, 1944); Nimkar and Chattopadhyay (2002a; 2002b; 2003) found the similar trend of increase in pressure drop through green gram ,pigeonpea and other pulse crop., Tabak et al (2004) reported similar trend for increase in bed depth shows increase in pressure drop through granular beds packed with cottonseeds; Nimkar and Khobragade (2006) discussed the relationship of bed depth and pressure drop in moth gram packed beds.
2.3 Effect of moisture content
The results indicated that for all grain samples the pressure drop decreased with increase in the moisture content. The reduction in pressure drops was possibly due to the decrease in bulk density and bulk porosity resulted from increase in moisture content. These results were in conformity with the results of Shedd (1953) for various grains and for biological material; Pagano et al (2000) obtained similar decrease in pressure drop with corresponding increase in moisture content of oat seed; Chung et al (2001) discussed the effect of moisture content on static pressure drop in sorghum and rough rice beds.
2.4 Effect of grain size
The results indicated that the pressure drop decreased with increase in the grain size. The difference in pressure drop observed might be due to different physical properties, such as shape and size, which in turn, determine the bulk density and bulk porosity of the bulk sample of lathyrus grains. Shedd’s (1953) data showed the same behavior for large and small grains; Masoumi and Tabil (2008) reported relationship of grain size and pressure drop for chickpea cultivars. Nimkar and Chattopadhyay (2003) reported that with increase in grain size the pressure drop was found decreased. 
3 Materials and Methods
3.1 Raw Material
The samples of lathyrus grain varieties were procured from the National Bureau of Plant Genetic Resources (NBPGR) laboratory. The small, medium and large sized grains viz., NLK-40, Pratik and Ratan were selected for further studies. The selected grain samples were used for determination of their relevant physical properties and resistance to airflow. For selecting grain size of the test samples to be used in the study, it was found by sieve analysis technique with the help of Ro-Tap sieve shaker and using BS standard set of sieves. Average of five replications has been reported.
3.2 Preparation of test sample
Test sample of NLK- 40, Pratik and Ratan having initial moisture content of 7.33, 6.75 and 7.90 % d. b., respectively were brought to the laboratory. To increases the moisture content; the samples were moistened with a calculated quantity of water and conditioned to raise its moisture content to the desired level by adopting the method suggested by Kiani et al (2008).
3.3 Physical Properties
The relevant physical properties, viz., grain size, in situ bulk density and bulk porosity of NLK-40, Pratik and Ratan were studied at different three levels of moisture content ranged from 7.33 to 18.80, 6.75 to 18.30 and 7.90 to 19.40 % d.b., respectively. Moisture content of grain sample was determined by adopting standard procedure suggested by Mohesenin (1986) Average of five replications was noted and reported as moisture content of the sample.
3.3.1 Grain size
To characterize the grain used in this study, the geometric mean diameter was considered as the size criterion. It is the cube root of product of three axes of grain. Three major principle axes of grain were measured with the help of vernier caliper (Mitutoyo, Japan) having least count of 0.02 mm. Average observations of 100 randomly selected lathyrus grain was calculated and standard deviation was also determined.
3.3.2 Bulk density
Bulk density of the grain is the ratio of its mass to bulk volume. To determine the bulk density of the grain, the method given in IS: 4333 (Part III)–1967 was used. True density was determined with toluene displacement method. Average of five replications was reported as the bulk density and true density values of grain sample.
3.3.3 Bulk porosity
It is the percentage of volume of voids in the test sample at given moisture content. It was calculated as the ratio of the difference in the true and bulk density to true density value and expressed in percentage.
3.4 Airflow resistance apparatus
The vertical airflow resistance apparatus was fabricated under strict supervision of mechanical engineer keeping in view all the possible pressure losses through pumps, joints, elbows etc. The apparatus used in the experiments is consisted of the air blow system, airflow measurement system, plenum chamber, test bin and pressure measurement system.
3.5 Experimental Procedure
In all experiments, conditioned sample was removed from the refrigerator and left at room temperature for six hours so as to equilibrate it with the ambient temperature before using it for pressure drop measurement. Moisture content of test sample was determined before and after of each trial with an oven drying method. The experiments were carried out at three densities obtained with loose, medium and dense packed grains. Firstly, test bed was filled by the loose fill method as described by Shedd (1953). Medium and dense packed bed conditions were obtained by tapping the side wall with rubber hammer which allowed the grains to settle and thereby decreased its volume till the required depth. With the known mass of grain and volume occupied by them, in-situ bulk density of test sample was determined.
At each airflow rate, the test runs in triplicate were conducted at each bulk density level. The tests were carried out starting initially from highest airflow rate and subsequently by proceeding to lower airflow rates. The air velocity was measured at a proper position in the circular air duct with digital electric anemometer (ACD, India). Relative humidity, atmospheric pressure and temperature were measured three times during each test run and were used for airflow rate calculations to standard conditions of air at temperature of 27 and pressure of 101.325 kPa. The average of three reading corrected for standard condition was taken as the designated airflow rate. During airflow resistance studies with lathyrus beds, the temperature and relative humidity conditions recorded were (24±4.5)and (40±5.5)%, respectively. During the period of experimentation the barometric pressure was recorded with a Barograph (Lawrence and Mayo, India). The relative humidity was recorded with hygrometer (Hunger Hygro, West Germany) with 1% accuracy and ambient tempe- rature was recorded with dry and wet bulb thermometer (Zeal, England) having least count of ±1. Temperature of the air flowing towards grain bed in the apparatus was recorded with dial gauge thermometer (Premium Instruments, USA) having a range of 0~50 and least count of 0.5. Static air pressure was recorded from the tap provided at a location of 30 mm below the perforated floor with U tube manometer filled with distilled water.
The experiments were carried out at three different bulk densities for each moisture levels of 7.33%, 13.10% and 18.80%; 6.75%, 12.50% and 18.30% and 7.90%, 13.60% and 19.40% d.b. for NLK-40, Pratik and Ratan, respectively for three bed depths (200 mm, 400 mm and 600 mm). The experiments were carried out at all possible airflow ranges. Fifteen airflow rates ranged from 0.0411≤V≤  1.0262 m3·s-1·m-2, seventeen airflow rates ranged from 0.0450≤V≤1.0794 m3·s-1·m-2 and nineteen airflow ranged from 0.0484≤V≤1.0193 m3·s-1·m-2 forNLK-40, Pratik and Ratan, respectively.
Each pressure difference, measured by the inclined manometer, was divided by the distance of the two taps to obtain the pressure drop per meter depth. The average of five replications was expressed as pressure drop in Pascal per meter for each airflow rate.
Authors contributions
RNK designed and conducted this experiment. KRK was in charge of experimental set up fabrication in workshop as per the design done by her. PMN was responsible for technical editing and SSS was in charge of data analysis, writing and modification of manuscript.
Acknowledgement
This experiment was conducted with financial support of Sakal India Foundation, Pune and laboratory support from Panjabrao Deshmukh Krishi Vidyapeeth, Akola (India). We also thank one anonymous editor for his critical comments and advice on this paper.
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