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Development of Small Scale Equipment for Depulpping Locust Bean Seeds


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1110306-8484 IJET-IJENS @ December 2011 IJENS I J E N S

Development of Small Scale Equipment for

Depulpping Locust Bean Seeds

J. O. Olaoye

Agricultural and Biosystems Engineering Department University of Ilorin, P. M. B. 1515, Ilorin, 240003, Nigeria.

jolanoye@unilorin.edu.ng +2348035812797

Abstract-- This study focused on the development of small scale equipment for depulpping of locust bean seeds. Processing of African locust bean seed starts with the pretreatment of the harvested fruit before the seed can be converted into its numerous derivatives. Depulpping of locust bean seed is a crucial pretreatment operation, preceding fermentation of the seed. This operation is tedious, time consuming and energy sapping for women and children that are involved in the processing of locust bean. Small Scale equipment for depulpping of African locust bean seed was designed, constructed and tested. Techno-economic status of the women in the rural areas who are directly involved in the processing of locust bean and its derivatives was taken into consideration. The depulpping machine comprises of a vertical cylindrical tank, cylindrical sieve and a vertical rotating shaft which carries both the paddles and brushes. The vertical shaft was mounted at the central axis of the depulpping unit. The machine has a capacity to depulp 10 kg of locust bean seed during a unit batch operation. Five levels of soaking time corresponding to five levels of locust bean moisture contents and five levels of shaft speeds were tested. Test results indicated that the depulpping efficiency varied between 64 and 98 %. The seed membrane damage and seed loss were less than 5 and 9.2% respectively at 45 minutes soaking time and at 350 rpm depulpping shaft speed. The maximum power requirement was 2.25 kW at a shaft speed of 550 rpm. The operating conditions of shaft speed at 350 rpm, 45 minutes soaking time indicated higher depulpping efficiency, lower seed membrane damage and seed loss during depulpping operation. Result of process performance showed that the final depulpping process compared favourantly with that of traditional method.

Index Term-- Depulpping, Locust Bean, Soaking Time, Fermentation


Depulpping of locust bean is an essential and required unit operation when processing the seeds to its various derivatives and products. African locust bean (Parkia biglobosa) is very popular in Africa. The locust bean long pod contains small beans and sweet edible pulp, the chaff is used as animal feed and the pulp is a source of chocolate substitute. “Iru” or dawadawa is a typical example of fermented food obtained from the small beans. According to UNU [1] Iru is one of the traditional fermented condiments used to flavor soups and stews in Nigeria.

The locally woven basket or perforated calabash is used to depulp locust bean locally. decorticated locust beans are placed in the basket and submerged in a gentle flowing river,

stream or pond. The mixture of seed and pulp is stirred with the hands to push out slurry through the pore space while the basket is vigorously agitated within a fixed location in a flowing water medium. The pulp is filtered into the water and the seeds retained inside the basket or calabash. This operation is labour intensive and time consuming. This operation is compared to the washing process in scooped melon seeds. Oloko and Agbetoye [2] found out that the traditional method of washing melon consumes about 65 % of the total energy required for the processing of melon seeds. The traditional method of depulpping locust bean seeds requires large volume of water. The ease of depulpping operation is a function of availability of still running stream. The harvesting time and the processing period correspond to the off season of relative abundant supply of required water. Therefore, a depulpping machine that will reduce high dependency on large volume of water is desirable.


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Extraction of the mixture of melon seeds and slurry from melon pod precedes the melon washing operation that separates the melon seeds from its slurry. In locust bean depulpping operation, decortications of the locust bean pod precede depulpping process of locust bean. Locust bean seed is enclosed within the yellowish pulp. The unique feature of locust bean explains why a typical melon washing machine cannot be used to depulp locust bean seeds from the pulp.

This research was set out to establish possible method of separation of locust bean seed from its pulp and to improve processing procedure, market value and quality of the derived products from the locust bean seeds. The overall objective of the present work is to design, construct and evaluate the performance of simple and compact equipment for depulpping of locust bean seeds.


Equipment Description

The depulpping machine consists of a cylindrical head with a feeding hopper, a cylindrical sieve, a vertical rotating shaft with paddles and brushes, series of paddles fixed along the length of the shaft at the two opposite ends, a pair of two adjoining paddles carries the brush, concave outlet, cover, handle, sieve control stud, wheels and power transmission elements. (Figs. 1 and 2).

The cylindrical container holds water for depulpping process. The container is made from 1.5 mm mild steel sheet. It houses a vertical rotating shaft. Series of paddles are fixed at the opposite end and arranged serially along full length of the rotating shaft. A pair of adjoining paddles is fixed with a brush. The arrangement of the paddles, brushes on the main rotating shaft forms the depulpping stirring unit. The cylindrical container, cylindrical sieve and the depulpping stirring unit were arranged concentrically. The diameter of the outer cylinder is 500 mm and 400 mm for the inner cylinder

while each cylinder is 600 mm high. The clearance between the two cylinders (about 50 mm) was created as a channel through which the pulp slurry could be discharged out through the slurry outlet. The depulped seeds are collected inside the sieve through the clean seed discharge outlet.

Design Assumptions and Considerations

Volumetric Capacity and Cylindrical Tank and Sieve Arrangement

Volumetric capacity was determined from the dimensional layout of a cylindrical set up using the struck level method following the procedure for the determination of bin diameter in manure spreader. Level full capacity was taken as the struck level corresponding to the portion included within the cylinder. The gravimetric capacity was related to the volumetric capacity of the cylinder by using equation 1 and the storage capacity of the cylinder was calculated from equation 2.

Eqn. 1


Gv = Gravimetric Capacity

Vv = Volumetric Capacity

b = Nominal density of the product

The locust bean density is given as 1.18 g / cm3 and if the depulpping machine is designed to handle 80 kg of locust bean per unit operation the raw locust bean will occupy 6724.7 cm3.


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Fig. 1. Front Elevation of a Locust Bean Depulpping Machine Showing the Brush Arrangement

Fig. 2. Plan view of a Locust Bean Depulpping Machine Showing the Concentric Inner Cylinders

Feeding Chute

An Electric Motor as the main

Energy Source

Discharge Sprout

Main Support Frame

Feeding Chute

Brush Arrangement

Depulping Stirring Unit

Top cover of Concentric

Cylinder Assembly


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D = Diameter of the Cylinder (cm 3) H = Overall Height of the Cylinder (60 cm)

Lh = Struck Level of Cylinder while

the machine is in operation = H – 

The size of the cylindrical sieve tank determines the capacity of the depulpping machine. The height of struck level is related to the overall height of the tank as presented in Eqn. 2. The difference in height is governed by the nature of fluid flow (turbulence or laminar) during operation and status of tank if opened or closed as determined by 0.1% (H)   4 % (H) (ASAE, [9]; Lingley, [10]). For depulpping operation 2% (H) was used and D = 60 cm, was chosen for the cylindrical sieve to accommodate 80 kg of locust bean and required water to saturation. The dimensions of the cylindrical sieve unit are 60 cm and 40 cm as height and diameter, respectively. The dimensions of the outer cylindrical container were 60 cm, and 50 cm as height and diameter, respectively.

Sieve Size and Physical Property of the Locust bean Seeds

Sieve holes and clearance between rotating brushes and cylindrical sieve shell were established in relationship to the size of the seed. Seed size was determined by measuring the axial dimension of 100 randomly selected seeds using a venier caliper reading to 0.05 mm. The number of holes per m2 on the cylindrical sieve shell was evaluated based on the unit size of the seed. Experiment had shown that the average values of the major, intermediate and minor diameters of the seeds are 9.8, 7.9 and 4.6 mm respectively. The result also conformed to the findings of Ogunjimi et al., [11], Oje [12] and Oni [13]. An approximate hole of 4 mm was drilled with a punch on the cylindrical sieve shell. This size of the hole woult has no axial loading and bending moment prevent discharge of depulpped seed through the slurry outlet and about a hole was drilled per 1 cm2 of the cylindrical sieve size of 7.56 x 10 -1 m2.

The angle of repose of the undeppulded and clean seeds were determined following the method described by Oje, [12] for oil seeds. The average angle of repose at these two conditions was 30o. The hopper and the seed discharge chute was constructed at an angle of inclination of 35o to ensure free flow of the seed during both loading and unloading conditions.

The Depulpping Stirring Unit and rotating Paddles

The depulpping stirring unit was to provide effective means of removal of locust bean pulp from the seed. This operation is achieved through combination of cutting, abrasion, and rubbing actions. The paddle creates the cutting effect on the pulp by impact and the clearance between the cylindrical sieve shell and the attached brushes on the paddles creates the desired abrasion and rubbing actions for the depulpping operation. (Fig. 2)

The solid rotating shaft has no axial loading and bending moment and Eqn. 3 was used to calculate the shaft diameter. The solid shaft is subjected to little or no axial loading and the maximum bending moment, Mb = 0. The maximum torsional

moment was calculated using standard procedures (Hall et al., [14]). Estimate of all the loads on the shaft as shown in Fig. 2 was calculated and Mt = 115502 N/m2.

Eqn. 3


d = Diameter of shaft (mm)

Ss = Allowable stress for shaft (for mild steel shaft, Ss = 40 N/m2 and

kt = 1.0, (Hall et al., [14]

Kt = Combined shock and fatigue factor applied to torsional moment Mt = Maximum torsional moment,

115502 N/m2

T =

From Eqn. 3 the diameter of the shaft was 24.5 mm. Therefore, a shaft diameter of 30 mm was selected. This was determined based on the overall length of the shaft and the maximum height of the cylindrical sieve shell in relation to the volumetric capacity of the depulpping machine. Three pairs of depulpping brushes were used. These brushes were attached to the main shaft through the paddles (Fig. 2). Three adjoining paddles made of mild steel carry a depulpping brush. The dimensions of each paddle are 15 mm x 30 mm x 190 mm.

The clearance between the rotating paddles and the cylindrical sieve shell was set at 12.2 mm. This clearance is sufficient to create the required surface for effective depulpping action. Circular holes were created at 1.5 holes per cm2 of the size of the hole and the adjoining distance between two holes was 4.5 mm. Each hole was created on the cylindrical sieve shell. The size of the holes and its spatial distribution is crucial in screening off the seed from been discharged with the pulp slurry during the depulpping operation.

Belt and Pulley Design


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must be operated within a fluid medium in an enclosure. The diameter of a pulley for the driven shaft is calculated using the equation for the peripheral speed of the belt as shown in Eqn. 4 (Kurmi and Gupta, [15]).

Eqn. 4


d1 = Pulley diameter of the

electric motor (mm)

N1 = Speed of the electric

motor (rpm)

D2 = Pulley diameter of the

stirring unit (mm)

N2 = Speed of rotating the

stirring unit (rpm)

From Eqn. 4 the pulley diameter 250 mm was selected for the depulpping stirring unit. The length of the belt was determined by using Eqn. 5. The shaft to shaft centre for both the electric motor and the stirring unit shaft was chosen to be 42 cm. The minimum obtainable distance between the radius of the outer cylindrical container for depulpping process and the distance of the central axis of the electric motor to its base influenced the choice of this parameter.

Eqn. 5

Eqn. 6

A flat belt with total length of 134 cm was recommended to drive the stirring unit.

Fabrication Processes

The construction processes were carried out in the fabrication workshop, Department and Biosystems Engineering, University of Ilorin, Ilorin, Nigeria. The basic manufacturing processes which include cutting, primary shaping and joining processes were undertaken.

Cylindrical Tanks Arranged Concentrically

The outer cylindrical container and cylindrical sieve were arranged in a concentric form. The two cylinders were made from 1.5 mm galvanized mild steel sheet. The outer cylinder was marked out consisting of 500 mm x 600 mm dimensions and 400 mm x 600 mm dimensions for the inner cylinder sieve. A hole of 4 mm was marked per 1 cm2 to cover the entire cylinder sieve of 7650 cm2. The tow concentric cylinder tanks were welded to the base plate, 500 mm.

Concentric Cylinder Base Assembly

The base of two cylindrical tanks consists of a metal plate with two holes 25 mm and 320 mm, diameters. The centre of each holes are located at 25 mm and 250 mm from one of the edge of the base plate (Figures 1 and 2). The 25 mm hole serves as the slurry – draining outlet. Slurry – draining outlet pipe, 25 mm diameter, 220 mm long and 3 mm thickness was welded to the 25 mm hole at the base plate. The pipe is made up of a Galvanized lead pipe. The end of the pipe is fitted with a cork which serves as an opening for the discharge of locust bean slurry after the cleaning operation (Fig. 2). A composite unit of conical and cylindrical components which serve as a clean seed discharge outlet was welded to the 320 mm hole on the concentric cylinder assembly base. The composite unit was made of 3.0 mm galvanized mild steel. The conical section is welded directly to the base of the cylinder assembly. The dimensions of the conical section are 1700 mm height, 320 mm and 180 mm as the upper diameters and lower diameter, respectively. A cylindrical pipe of 180 mm diameter and 50 cm long was welded to the lower portion of the conical section. A threaded cap was fitted into the end of the cylindrical section of the composite unit as shown in Fig. 1. The cap is only opened at the end of each batch process operation of the depulpping action for collection of clean seeds.

Head of the Concentric Cylinder Assembly

The head of the concentric cylindrical assembly consists of a 1580 mm x 30 mm wall and a chute of 500 mm diameter plate made from 3.5 mm galvanized mild steel. The plate head holds the inlet and feeding chute in place as shown in Figures 1, 2, and 3. The inlet and feeding chute are made from 1.5 mm galvanized mild steel and its overall height is 350 mm. The head is split into two sections and fastened together by bolt and nut. This creates and access into the interior part of the concentric cylindrical assembly and the depulpping stirring unit to ensure ease of maintenance.

Support Components


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Depulping Stirring Unit

A 30 mm x 1200 mm mild steel shaft was cut and turned on a lathe machine to serve as the main shaft that carries the paddles, brushes and pulley (Figs. 1 and 2). The brushes were arranged along the vertical main stirring shaft. The clearance between the perforated concentric inner cylinder and the brush was set to ensure appropriate depulpping action.

Operation of the Depulping Machine

The main function of the depulpping machine is to remove, clean and set apart the seed of the locust bean fruit from the yellowish pulp. The machine is to ensure that the thin layer testa of the seed is retained. The hopper serves as the feeding point for intake of decorticated locust bean fruit and water. Depulpping operation takes place using water as a medium of separation. The depulpping action is activated as soon as the depulpping stirring unit is set in to rotating motion via an ac motor. The electric motor drives the depulpping stirring unit through belt and pulley arrangement.

Vigorous rotation of the depulpping stirring unit induces the removal, cleaning and detachment of locust bean seed from the yellowish locust bean pulp. The rigorous rotation of the stirring unit is reduced as soon as the yellow slurry is formed inside the depulpping chamber. Clean seeds with enclosed testa are discharged through the discharged outlet of the depulpping machine while the pulp slurry is drained out through the slurry – drain pipe.

Performance Testing

The depulpping machine was assembled after its various components were fabricated and evaluated for operation performance and depulpping process performance. The photograph of the fabricated locust bean depulpping machine is as shown in Fig. 3.

The depulpping machine was operated at no load at three different operating speeds of the stirring unit. The shaft is fitted with five different sizes of pulley diameters 128, 157, 200, 282, and 470 mm to generate five levels of the operating speed of 550, 450, 350, 250 and 150 rpm respectively. The electric motor was connected directly to the stirring shaft through a flat belt. A 1.5 Hp electric motor, ac (Model VIKING JONCOD, Type YL 90L – 4) was used. This was undertaken to ascertain the durability of the machine components. A Geilgy Tachometer was used to determine the stirring shaft speed. The performance of the machine at no load was investigated for about an hour for each of the combination of the operating conditions.

Process performance of the machine was undertaken to test process performance of depulpping efficiency, percentage seed loss, recovery efficiency, germination count and seed with membrane were evaluated. These were investigated

under five operating speeds (550, 450, 350, 250 and 150 rpm) and five soaking time (15, 30, 45, 60, and 75 min) on the process performance of the five moisture content of locust bean seed. The investigation was carried out in a split – split unit design with operating speed as the main unit, soaking time as the sub unit factors with three replicates. The process performance was evaluated on the basis of the following indices:

Depulpping Efficiency (De),


Mcs = Mass of cleaned (A clean

seed is consider to have more than ¾ of the seed surface exposed and devoid of locust bean pulp)

Mui = Mass of material collected at

seed outlet of the depulpping machine discharge outlet

Percentage Seed Loss (Sl),


Mds = Mass of seed damage


1110306-8484 IJET-IJENS @ December 2011 IJENS I J E N S

Mui = Mass of material collected at seed outlet of the

depulpping machine discharge outlet

Recovery Efficiency (Re),


Mswc = Mass of seed without


Mui = Mass of material collected at

seed outlet of the depulpping machine discharge outlet

Membrane Detachment Efficiency (Me), Me = 1 - Re


Re = Recovery Efficiency

Germination Count was undertaken to test the viability of the depulded locust bean. About fifteen seed samples were collected and distributed in three seed per petri dish containing soaked cotton wool in water. The petri dish and its content is to create appropriate conditions for seed growth and the seed samples were left for three weeks for observation of the germinated seed.


Data obtained from the tests were also subjected to analysis of variance (ANOVA) and test of significance using New Duncan’s Multiple Range Tests. Results of the of the analyses carried out indicate that there were significance differences in the magnitudes of depulpping efficiency, recovery efficiency, and membrane detachment efficiency at all speeds tested . However, the effects of seed on the percentage seed loss indicate no significance difference during the depulpping operation at . The influences of soaking time indicate significance difference in the magnitude of seed membrane detachment efficiency and seed loss efficiency at and show no significance difference at the magnitude of depulpping efficiency.

Depulpping Efficiency

Depulpping of soaked decorticated locust bean was achieved at various soaking time when the machine was operated at 5 different operating speeds of the depulpping stirrer. The soaked locust bean was easily depulpped under the influence the rotating brushes against the perforated concentric cylinder. The highest depulpping was observed at depulpping efficiency of 96 % at soaking time of 45 minutes and depulpping speed of 350 rpm (Fig. 4). The increase in depulpping efficiency from speed 150 rpm to 350 rpm clearly indicated that greater energy impact was induced on the locust bean pulp. Increased speed beyond 350 rpm reduces the depulpping efficiency this implied that excessive energy impacted on the pulp causes on

due losses and damage to the bean seed as shown in Fig. 5. This observation could be responsible for increased detachment of seed membrane at higher depulpping speed above 350 rpm. The exerted energy above the depulpping speed of 350 rpm destroys the membrane and seed testa.

Fig. 4. Depulpping Efficiency against Speed of Depulpping

Percentage Seed Loss, Recovery Efficiency and Membrane Detachment Efficiency


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of 350 rpm. The results indicated that seed recovery efficiency gradually increases from 150 rpm to maximum value at 350 rpm and beyond this speed the recovery efficiency decreases. Similarly, the seed membrane detachment decreases from 150 rpm to the least value at 350 rpm and beyond this speed the detachment efficiency increases for all soaking time investigated (Figures 6 to 8). The observed characteristics displayed by the seed recovery efficiency and seed membrane detachment efficiency between 150 rpm and 350 rpm as shown in Figures 6 and 7 could be due to the insufficient energy generated by this low speed for depulpping action. These speeds may be too low to create required momentum that would lead to effective separation of the pulp from the seed without removal of the seed membrane. Whereas at higher speed between 350 rpm and 550 rpm excessive energy could be generated to cause total removal of the pulp and membrane.

Fig. 5. Percentage Seed Loss against Depulpping Speed

Fig. 6. Locust Bean Recovery Efficiency against Depulpping Speed


1110306-8484 IJET-IJENS @ December 2011 IJENS I J E N S

Fig. 8. Effects of Soaking Time on Recovery Efficiency

Germination Count

The result of the Germination count test was illustrated with scatter points and the pattern do not follow any specific curve variation. The scatter points however indicated that at soaking time of 45 minutes 4 to 5 seeds germinate. The viability test of the depulpped locust bean seed also indicated that at depulpping speed of 350 rpm 4 to 5 seeds tested were found to be viable at soaking time of 45 and 75 minutes.

Fig. 9. Effects of Deppulpping Speed on Viability of Depulpped Seed


A machine for depulpping of locust bean has been designed, fabricated and tested for preliminary performance. The highest depulpping efficiency of 98% was achieved at depulpping speed of 350 rpm and at soaking time of 45 minutes. The highest seed recovery efficiency was recorded at the soaking time of 45 minutes. All materials used for fabricating the machine were sourced locally. The machine performed satisfactorily during the period of operation.

The speeds of the operation of the depulpping machine affect the magnitude of deppulping efficiency and membrane detachment efficiency. The effect of the machine speed has no significant influence on the percentage seed loss. The soaking time has direct influence on the magnitude of seed membrane detachment efficiency.


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Pressure Drop on the Airflow Resistance of Locust Bean Seed”. Agricultural Engineering International: the CIGR Ejournal. Manuscript FP 06 010. Vol. 8. May, 2006.



Fig. 3. Photographic View of a Locust Bean Depulpping Machine Ready for  Use
Fig. 4. Depulpping Efficiency against Speed of Depulpping
Fig. 6. Locust Bean Recovery Efficiency against Depulpping Speed
Fig. 8. Effects of Soaking Time on Recovery Efficiency


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