Effect of Moisture Content and Feed Rate on Size Reduction of Pearl Millet

fect of Moisture Content and Feed Rate on Size Reduction of Pearl Millet"，"SubTitle":""，"Author":""，"Summary":"S. Balasubramanian1， R. Sharma1 and S. R. Vijay Kumar2 1. Central Institute of Post Harvest Engineering and Technology， Ludhiana 141004， Punjab， India 2. College of Agricultural Engineering， UAS"，"Introduction":""，"Columns":"Research Papers"，"Volume":""，"Content":"

S. Balasubramanian1， R. Sharma1 and S. R. Vijay Kumar2

1. Central Institute of Post Harvest Engineering and Technology， Ludhiana 141004， Punjab， India

2. College of Agricultural Engineering， UAS， Raichur 584102， Karnataka， India

Received: March 29， 2011 / Published: July 20， 2011.

Abstract: Pearl millet at various moisture content (6.2， 9.4 and 12.3%， d.b.) and feed rates (3， 6 and 9 kg/h) was ground using hammer mill and its physical properties viz. arithmetic mean diameter， geometric mean diameter， thousand grain weight， aspect ratio， specific surface area， surface area and bulk density were studied. Sieve analysis results showed that the increase in moisture content produced more medium sized particles with decreased percent weight retained in pan. Bond’s work index， Kick’s constant and average particle size were increased with the decrease in total surface area at higher moisture levels. The highest energy (2.34 KWh/kg) was consumed for 12.3% moisture content. Various grinding characteristics were significantly affected by moisture content and feed rate either individually or in combination and correlated in terms of Bond’s work index， Kick’s constant， total surface area， average particle size， effectiveness of milling and bulk density. Milling loss was found to be higher at lower moisture level and decreased with the increase of moisture content as well as feed rate. The loose and compact bulk density was ranged between 46.8-199.5 kg m-3and 53.5-254.1 kg m-3， respectively among the entire sieve fractions. Water absorption capacity increased with the decrease in particle size.

Key words: Pearl millet， grinding， size reduction， particle size， sieve analysis， hammer mill， moisture content.

1. Introduction

Millets are the species of small food grains and pearl millets (Pennisetum glaucum) are grouped in millets. It is a store house of protein and phytochemicals. India is the largest producer of millet(1.06 MT) in the world [1]. Its grains are mostly used as animal feed and less popularly as human food. Size reduction is primarily used for conservation of energy and densification process [2]. Grinding of agricultural materials is mostly used to increase surface area， pore size of the material and number of contact points in the compaction process [3]. In India， dry and wet methods are adapted for grinding food grains. Hammer mills are relatively cheaper， easy to operate and suitable for wide range of particles. Energy consumption of grinding biomass depends on initial particle size， moisture content， material properties， feed rate and machine variables etc. [4].

Several investigators [4-11] have used the hammer and attrition mill for grinding of different agricultural materials and studied for particle size analysis. The specific energy requirement of hammer mill for grinding of coastal Bermuda grass was found in relation to moisture content and feed rate [12]. Experiments were conducted on wheat straw， corn and sorghum residues in a hammer mill maintaining at constant (15.8 m/s) peripheral speed [13]. Studies on cryogenic grinding of cumin seeds and cloves at different conditions and its influence on volatile oil content and its components， particle size distribution， volume mean diameter and specific energy consumption was reported [14， 15]. Therefore， the present study was carried out to ascertain the effect of moisture content and feed rate during the grinding process of pearl millet using hammer mill and its physical properties.

2. Materials and Methods

2.1 Materials Preparation

Pearl millet was procured from the local market and cleaned manually to remove the adhered foreign matter and immature， if any. The initial moisture content of the pearl millet was determined [16]. The desired moisture content (6.2， 9.4 and 12.3%， d.b.) was made and mixed thoroughly to ensure uniform moisture distribution. Sample was packed in low density polyethylene bags and kept at 5 °C for 24 h in refrigerator. Before experimentation， sample was allowed to attain the room temperature [15].

2.2 Physical Properties

Physical properties viz.， arithmetic mean diameter， geometric mean diameter， thousand grain weight， aspect ratio， specific surface area， surface area and bulk density of pearl millet were determined [17-19].

2.3 Grinding and Differential Sieve Analysis

A commercial hammer mill (Bells India Instrumentation， New Delhi) fitted with a single-phase motor (3 HP， 3900 rpm) was performed. About 250 g sample was ground at different feed rates (3， 6 and 9 kg/h). A woven-wire cloth sieves set (US standard sieves) of aperture size (2.032-0.075 mm) was used for particle size analysis. A digital balance having a least count of 0.001 g was used. If the weight on the smallest sieve containing any material changes ≤ 0.2% of the total sample weight， then the sieving was considered complete. The material collected on each sieve was accurately weighed and packed separately in zip lock polythene bags for further analysis. The milling loss(%)， moisture content and bulk density (loose and packed) of the ground material was determined.

2.4 Gravimetric， Functional and Grinding Characteristics

Sample was taken in a tarred measuring cylinder and filled to a known volume for loose bulk density， while for packed bulk density; the sample was tapped for 100 times. The weight and volume were recorded three times. The bulk density (loose and packed) was determined for each sieve fraction [20]. The surface area of the particles was measured as the ratio of surface area of the final product after grinding to that of the raw material. The calculated parameters are given below Eq. (1):

From the volume and diameter of single grain， the surface area was calculated [21] as follows:

(4)

Based on the mass fractions， the average final particle size (D2) was calculated using the following relationship:

Average (5)

The grinding energy per unit weight (E) was calculated from the wattage of hammer mill， grinding time and feed rate. Also， based on the particle sizes(initial and final)， energy required to grind a unit mass， Bond’s work index (Wi) and Kick’s constant (KK) were calculated [22].

where， D1 and D2 is the diameter of the product and feed at 80% passes from sieve; ρ is the bulk density of particle.

2.5 Statistical Analysis

Analysis of variance test (ANOVA) was carried out using SPSS 13.0 software and statistical procedures were followed to examine the effect of moisture level and feed rate (P ≤ 0.05) on the grinding characteristics of pearl millet [23， 24]. The results were compared with Duncan’s multiple range test (DMRT) and Pearson’s correlation coefficients among various grinding characteristics were calculated using MS Excel (Microsoft Corp.， Redmond， WA， USA).

3. Results and Discussion

3.1 Physical Characteristics of Pearl Millet

Table 1 illustrates the thousand grain weight， arithmetic mean diameter， geometric mean diameter， sphericity， aspect ratio， surface area and bulk density were varied in the range of 412-459 g， 20.4-20.8 mm， 20.5-20.9 mm， 0.84-0.88%， 84.3-87.9， 1305-1353 mm2 and 56.4-59.2 kg m-3， respectively.

3.2 Grinding Studies

Milling loss was found to be higher at lower moisture level and decreased with the increase of moisture content as well as feed rate (Table 2). The loss at lower moisture content might be due the formation of more fine powdered material that gets easily lost in the form of dust particles during the grinding process. The loose and compact bulk density increased in the range of 122.5-151.7 and 129.1-157.2 kg m-3， respectively showed a direct relation with feed rate and indirect relation with the moisture content (Table 2).

3.3 Percent Fractions Retained

The percent fractions retained in different sieves after the differential sieve analysis is presented in Table 3. It is clear from the data that as the moisture content of the material increased， there were more percent of medium size (0.592-0.157 mm) particles produced during grinding. However， in contrary to this statement， feed rate had significant positive effect i.e. more fine particle were produced. This was attributed to higher friction produced among the feed particles due to sufficient filling of the grinding cavity with the feed during grinding process with the increased feed rate. The results were in compliance with the results for makhana at different moisture and grinding time [25]. 3.4 Gravimetric Characteristics

Tables 4 and 5 illustrate the variation of bulk density(loose and compact) as a function of moisture level and feed rate among different sieve fractions. The loose and compact bulk density was ranged between 46.8-199.5 kg m-3 and 53.5-254.1 kg m-3， respectively among the entire sieve fractions. The compact bulk density showed slight variations with the treatments as characteristics of makhana for varied particle size [25]. However， water absorption capacity increased with the increase of feed rate within the same particle range. The residual final moisture retained after the grinding process might be one of the reason for increase in water absorption capacity of ground material (Table 6).

3.6 Effect of Treatments on Various Grinding Characteristics

Table 7 describes the effect of moisture content and feed rate on various grinding characteristics viz.， Bond’s work index， Kick’s constant， average particle size and surface area. Milling performance was also studied in terms of grinding effectiveness. It was observed that the Bond’s work index and Kick’s constant which are the measure of energy uptake got increased with the moisture level. However， it shows an inverse relation with feed rate. This might be attributed to comparatively less grinding time taken to grind the unit mass with the increase of feed rate. However， with the increase of moisture level， the material becoming soggy/tougher， resulting in more consumption of energy and hence the increase in Bond’s work index and Kick’s constant. The total surface area decreased with the increase of moisture level. The average particle size increased with the moisture level but decreased with feed rate. This was due to the reason of lesser grinding effect at higher moisture level and thus less formation of fine material. The effectiveness of milling showed a slight variation for lower moisture range (6.2-9.4%， d.b) but thereafter it decreased drastically at higher moisture (12.3%， d.b). Similar types of observations were also reported for legumes and microwave dried maize grains， respectively [10， 21].

3.7 Analysis of Variance

ANOVA was carried out to examine the effect of treatments on various grinding characteristics viz.， Bond’s work index， Kick’s constant， average particle size， surface area and effectiveness of milling (Table 8). There were significant differences among all the grinding characteristics. The Kick’s constant varied significantly with the moisture levels but the feed rate showed a non-significant effect. The interaction effect of moisture level and feed rate was also found to have significant effect on all the grinding attributes， except for Kick’s constant. The correlation coefficients calculated for various grinding and gravimetric characteristics are shown in Table 9. The correlation data showed that the various grinding characteristics for pearl millet could be well correlated in terms of Bond’s work index， Kick’s constant， total surface area， average particle size， effectiveness of milling and bulk density. Thus， it is apparent that the variations in independent variables (moisture level and feed rate) either individually or in combination (interactions) significantly influenced the measured parameters.

4. Conclusions

The moisture level and feed rate affect the grinding as well as gravimetric characteristics of pearl millet significantly. More fine particles and less medium size particles (0.592-0.157 mm) were produced at lower moisture level and higher feed rate. With the increase in moisture level， the energy consumption for grinding process increased， but with increase in feed rate was declined. It appears from the statistical analysis that the variation in moisture levels as well as feed rate either individually or in combination (interaction) influences the grinding process significantly.

Acknowledgment

The authors would like to thank the National Agricultural Innovation Project (NAIP)， Indian Council of Agricultural Research (ICAR) New Delhi， India.

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