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DESIGN MODIFICATIONS AND PERFORMANCE EVALUATION OF A CENTRIFUGAL IMPACT PALM NUTS CRACKER

This study presented a modified centrifugal impact palm nuts cracker that will enhance cracking efficiency of the existing prototypes in traditional and small-scale oil palm mills in Southwestern Nigeria. This study focused on the effect of modified centrifugal impact blades and cracking ring at higher speed on physical properties, cultivar differences and drying period of the palm nuts. The physical properties of Dura and Tenera cultivars palm nuts were investigated prior to machine design. The modified prototype was developed and evaluated using palm nuts of three categories of geometrical mean diameters (GMD) at steady condition of 80 C and drying periods of 15, 18 and 21 hrs. The results of the physical properties such as length, breadth, thickness, mass, GMD and sphericity were in the range 25.19-35.84 (mm), 18.69-26.44 (mm), 15.0521.71 (mm), 4.60-11.57 (g), 19.10-27.58 (mm) and 0.75-0.77 (%); 26.90-33.97 (mm), 17.89-24.57 (mm), 14.32-20.44 (mm), 3.28-8.84 (g), 18.50-25.84 (mm) and 0.71-0.75 (%) for Dura and Tenera palm nuts cultivars, respectively. The optimum throughput at 15, 18 and 21 hrs drying periods were: 200 and 180, 240 and 218, 235 and 230 (kg/hr) for Dura and Tenera cultivars, respectively. The optimum cracking efficiency of 88% was obtained across all level of GMD, while the predicted value above threshold of 94% for Dura cultivar only. The damaged percentage and recovery efficiency have threshold of 8% and 6% for both cultivars, respectively. Hence, the determination of physical properties and drying condition prior to development and evaluation of this machine enhances its performance efficiently.


INTRODUCTION
The oil palm is a perennial tree crop that originated from West Africa but is now cultivated in plantations across tropical Africa, Southeast Asia and Southern America more than its origin (Mutsaers, 2019). It is botanically known as Elaeis guineensis. The major varieties are Dura, Tenera and Pisifera (Adzimah and Seckley, 2009), but the commercial and most prominent cultivars are Dura and Tenera (Morakinyo and Bamgboye, 2015). The primary products of palm tree are palm fruits, palm wine, broom and timber. However, the secondary products are palm oil, palm nut; palm kernel and shells likewise palm kernel oil and cake. A palm fruit is made of a smooth outer skin (exocarp), a fibrous pulp (mesocarp) which contains palm oil, a centrally placed palm nut with hard shell (endocarp) and the kernel in which palm kernel oil is derived. Palm kernel oil is useful in making soap, biodiesel, glycerin, margarine, candle, pomade, oil paint and polish. The shells are used as biomass to fuel boiler and brake pad composite for automobile, while palm kernel meals or cakes are utilized to produce animal feeds (Gbadamosi, 2006;Adebayo, 2004;Emeka and Olomu, 2007). Palm kernel oil processing mills are not common as oil palm fruit processing mills in Nigeria, due to inadequate technological advancement in this sector. However, exportation of palm nut and kernel into Europe and North Asia countries from Nigeria and Indonesia is highly prolific (FAO, 2002). Recently, there is an industrial revolution that necessitated for increase in the demand of palm kernel and shell in the Southwestern Nigeria. This necessitated for promulgation of government policy by placing embargo on importation of palm kernel oil. The industrialization of this section has become desirable coupled with astronomical increase in the selling price of palm kernel oil. In order to meet up with high demand of this product, there is an urgent need to modify indigenous palm nut crackers for optimum cracking efficiency and durability. Previously, some researchers reported on the development of various palm nut crackers of different cracking mechanism such as centrifugal impact, hammer or rod impact and roller crackers (Adejugbe et al., 2017;Kayo, 2006). Antia et al. 2014, investigated the optimum moisture content value and drying period that enhanced palm nut cracker efficiency. They reported that the optimum moisture content value and drying period were 2.5% (dry basis) and 18 hrs, respectively for both Dura and Tenera palm nuts cultivars at maximum cracking efficiency of 84%. Most of the palm nuts crackers are indigenous prototypes installed in the traditional and small-scale oil palm mills. They were observed to lack engineering contents, basic engineering design procedures, the selection of appropriate materials of construction, precision in machining of critical working components, applications of fits and tolerances in-between bearing housing and bearings. A typical palm nut cracker is shown in Figure  1, with a lot of leakages and undue vibration. However, for industrialization, critical modifications and optimization are still necessary in order to achieve higher efficiencies, durability and reliability. Hence this study.

Materials
The materials for this study were Dura and Tenera palm nuts. The Dura cultivar palm nuts were obtained from oil palm mill at Eleweeran, Ede Road, Ile-Ife, Osun state, Nigeria with initial moisture content of about 23.5% (wet basis) while Tenera cultivar palm nuts of initial moisture content of about 22.8% were obtained from oil palm mill at Elere Adubi, Itori, Ogun State, Nigeria. The palm nuts of each cultivar were sorted and graded into three categories of axial dimensions: small, medium and large sizes. From the population of individual sizes were selected 100 pieces of palm nuts randomly to form eighteen samples in triplicates to be dried in three distinctive periods of 15, 18 and 21 hrs. Each category of small, medium and large sizes was kept inside labeled black polyethylene bag to determine physical properties using standard procedures according to their respective cultivar (Morakinyo and Bamgboye, 2015). The corresponding axial dimensions such as: length, breadth and thickness, geometric mean diameter, sphericity of each cultivar were measured using digital caliper while their corresponding mass was determined using electronic digital weighing balance, model: Scout Pro SPU 401 with standard error of 0.001 mm. The average values of the geometrical mean diameters of both cultivars were calculated to categorize their sizes (18.80 mm, 22.46 mm and 26.72 mm) to reduce the experimental runs. The data obtained were used as machine design parameters for the prototype modifications. All materials of construction were procured from Owode Onirin, Iron market, Ikorodu, Lagos. The exiting force convection biomass-fired cabinet drier of three tray compartments was used for the drying of eighteen samples at steady temperature of 80 o C to the final moisture content ranges from 3-5% (wet basis) at retention periods of 15, 18 and 21 hrs for Dura and Tenera cultivars consecutively. The drying operation was carried out for each drying period sequentially by loading 100 pieces of each sample of the geometrical mean diameter category in three replicate inside tray. However, for each drying period, six experimental runs were observed concurrently which summed up to eighteen experimental runs. The dried samples were kept inside desiccators for cooling and then cracked using modified centrifugal impact palm nuts cracker. The machine characteristics evaluated were throughput, the cracking efficiency, damaged percentage and the recovery efficiency for optimum purpose.

Mechanical Properties
The breaking force along longitudinal and axial directions of palm nut previously reported by Davies (2012) was adopted as 2.83 and 11.00 kN, respectively.

Machine Design Considerations
The following factors were considered while designing the new prototype. The average geometrical mean diameter of the two palm nuts cultivars was used to determine the clearance between the beater and the cracking ring. However to eliminate high rate of abrasion of the cracking ring, high carbon steel material was hardened and tempered to the hardness value of 65 HRC. To achieve optimum impact force for effective cracking, high speed electric motor of 2850 rpm with the propelling impact of 3 ph was adopted. Four beaters impeller type was designed to ensure adequate and uniform cracking of the nuts. The beaters were arranged at quadrant pattern in equal inter-distance to each other. The driven shaft and bearing housings were machined using appropriate fits and tolerances. Balancing of impeller was achieved by grinding and re-weighing the individual beater as centrifugal arm on a digital weighing scale until equal weight was achieved to eliminate throwing force. After

2.3.1
The philosophy of cracking principle for modification A palm nut falling vertically under a gravitational force through a hopper into a cracking mechanism exercising the potential energy was considered. This palm nut was assumed to fall on the surface of a rotating impeller blade at a horizontal level under a centrifugal force. It is assumed that this palm nut travelled at a distance equivalent to the cracking ring radius (r). Its initial motion under the influence of propelling force of the impeller resulted to a centrifugal force, impacting kinetic energy on the dried palm nut blowing it against the inner diameter of a circular cracking ring (hard core) at a constant velocity resulting into rupture of the dried palm nut shell instantaneously. The optimization of cracking efficiency was conceptualized by increasing the impeller speed (2850 rpm) and impeller blades width (140 mm). This arrangement enables the four impeller blades to release their kinetic energies for cracking at least once and sequentially along their circular motion at varying angle impact before dried palm nut escape through discharge chute. The clearance between the tip of impeller blade and inner diameter of the cracking ring was determined using maximum geometrical mean diameter of palm nut. However, to avoid abrasion between two nuts, this value was doubled to arrive at equal clearance of 60 mm as shown in Figure 2. For Tenera of 8.84 g = 0.00884 kg, the centrifugal force generated = 25.62 kN. These centrifugal forces calculated from above were generated from the proposed speed of the electric motor and average mass of Dura and Tenera palm nuts cultivars. These values were found to be higher than the longitudinal and axial breaking forces of palm nut previously reported by Davies (2012) as 2.83 and 11.00 kN, respectively; hence the design calculated values were adequate for effective cracking of both Dura and Tenera palm nuts cultivars.

2.6.3
Bearing selection In assumptions of these parameters where: number of rows of balls (i), Number of balls per row (Z) = 10, Diameter of balls (D) = 10 mm, Radial load factor (f0) = 12.3, Nominal angle of contact (α) = 20 0 . The basic static radial load rating for roller bearing (Co) was calculated as shown in Equation 9.

Determination of impeller speed, v-belt pitch length and type.
The impeller speed was obtained from v-belt drive ratio as depicted in Equation 10.

Description of Modified Centrifugal Palm Nuts Cracker
The modified centrifugal palm nut cracking machine consists of centrifugal impeller, impeller shaft, cracking ring, cracking ring housing, roller bearing and bearing housing, hopper and discharge chute, pulleys, V-belt, support frame and high speed electric motor as shown in Figures 4 and 5. i.
Centrifugal impeller It was made of high-carbon steel material; consist of a bushing and four equal weights of impeller as shown in Figure 5. The bushing was of Ɵ 60 ×Ɵ 30 ×140 mm, while Impeller blades were of rectangular shape of 75 × 140 ×15 mm. Both were machined to accurate shape and hardened and tempered to the hardness value of 65 HRC.

ii.
Impeller shaft This was made of medium-carbon steel of Ɵ30 ×320 mm as shown in Figures 4 and 5. It was stepped turned at both sides to accommodate roller bearings. Appropriate fits and tolerances were observed in the assembly of the shaft, impeller bushing, bearings and pulley. iii.
Cracking rings The cracking rings were made out of leave springs, forged into two half-rings of Ɵ 290 mm × 180 mm width, having thickness of 15 mm with perforated holes for mushroom fasteners fittings at four quadrant locations in each half -ring. They were screwed to the cylindrical housing. Beneath these rings and ring housing, a discharged opening of 70 ×70 mm hole was created to discharge cracked shell and kernel out of the cracking rings. iv.
Cracking rings housing was made of low-carbon steel material of a cylindrical shape as shown in Figures 4 and 5. It has diameter of 300 mm, width of 150 mm and thickness of 5 mm. There are two outer cover plates made of low-carbon steel of Ɵ 300 × 5 mm thickness with a center hole which accommodate impeller shaft. At the upper part, 150 × 150 mm hole was made to position hopper.

v. Hopper
The hopper was made of low-carbon steel material of 2.5 mm thickness of a trapezoidal shape; through which the materials were fed into the cracking unit. It has dimensions of 300 × 150 × 150 (mm) equivalent to a volume of 0.010125 m 3 using Equation 10. vi.
The frame This was made of angle iron of 60×60×4 (mm) having a vertical height, breadth and length of 410 × 270× 620 (mm), for the positioning of cracking ring housing, bearings and electric motor.

Performance Evaluation of the Machine
The throughput of the machine was evaluated by weighing 100 pieces of palm nuts and feeding them into t*he machine through the hopper. The cracker was operated at a constant speed of 2850 rpm, as one of the modifications factor in this prototype, higher than the speed of electric motor previously reported ( Koya, 2006, ; Antia et al., 2014). The cracking period was recorded for each cracking operation. This procedure was replicated for each sample from three trays for the corresponding size and cultivar. The average machine throughput (Mt) was calculated using Equation 11.

Cracking efficiency
The cracking efficiency is the ratio of the number of cracked nuts to the total number of nuts fed into the cracker expressed in percentage. This was determined using the mathematical expression reported by Gbabo et al. (2013) depicted as Equation (12).
where, C EFF is the Cracking efficiency (%), NC = the number of the cracked; NT is the initial total number of the nuts

2.2.3
Damaged percentage The damaged percentage is the ratio of the number of broken nuts to the total number of nuts fed into the cracker expressed in percentage. This was determined using the expression reported by Gbabo where, DDP is the damaged percentage (%), NB = the number of the broken nuts NT is the initial total number of the nuts

Recovery efficiency
The recovery efficiency is the ratio of the un-cracked nuts to the total number of the nuts before cracking. This is expressed in percentage as reported by Gbabo et al. (2013). This was determined using Equation (14).
where, REFF is the recovery efficiency (%), NUN = the number of unbroken shelled nuts NT is the total number of nuts

Data Analysis
The experimental data of the physical properties were analyzed using descriptive and inferential statistics to determine means and standard deviation. However, experimental design for the performance characterizations of the modified centrifugal palm nut cracker was generated using Response Surface Method with Multilevel-Categoric Factors of Design-Expert 11.1.0 (Stat-Ease, Inc., 2019). Table 1 showed the 3 Categoric factors of 2×3×3 factorial design as cultivar, geometrical mean diameter (size) and drying period, respectively. Eighteen experimental runs were generated and corresponding four responses (cracking efficiency, throughput, and damage percentage and recovery efficiency) were obtained during each run. The optimum performance characterizations of this machine were established. Tables 3 -4, showed the results of the physical properties of palm nuts such as length, breadth, thickness, mass, geometrical mean diameter and sphericity ranges of both palm nuts cultivars at their respective initial moisture contents.

3.2
Results of performance evaluation of the machine The performance characterizations of the machine evaluated were the responses of the experimental design which were: throughput, the cracking efficiency, damaged percentage and the recovery efficiency. The results of the performance evaluation are shown in Table 5.

The throughput
It was observed that increased in the weight of the palm nut increased the throughput and likewise the optimum throughput occurred at 18 hrs of drying period. This observation could be validated by considering Figures 3-5

472
of Dura and Tenera palm nuts cultivars increase relatively to the increase in their geometric mean diameters. More importantly, throughputs of the Dura palm nuts cultivar were found to be higher at geometrical mean diameter and drying period of 26.72 mm and 18 hrs, respectively. This same trend was reported by Antia et al. (2014). Considering Table 6 of ANOVA, all categorical factors such as cultivar, geometrical mean diameter and drying period influenced significantly the throughput of the modified centrifugal palm nuts cracker. In Table 6, the p-values was less than 0.0500, likewise the Model F-value of 15.35 implies that the model was significant. Furthermore, since the predicted R² of 0.6958 obtained was reasonably agreed with the Adjusted R² of 0.8084 and their differences was less than 0.2, hence categorical factors A, B, C were significant model terms for optimizing throughput value. The adequate signal to noise ratio of 13.681 was also obtained which was highly desirable since the value was greater than 4. Considered  Figure  9, the predicted throughput for Dura and Tenera was 235 and 215 (kg/hr), respectively. However, design point above predicted value for Dura and Tenera cultivars was 255 and 205 (kg/hr), respectively. The variability may be due to their shell thickness as reported by Ebunilo and Ojariafe, (2014) and Koya, 2006. They reported the cracked shell thickness of the fresh palm nuts of Dura and Tenera as 7.0 and 6.0 (mm) respectively. They also reported that the average cracked shell thickness after drying operations at 180 o C and 170 o C of Dura and Tenera cultivars palm nuts were 4.5 mm and 3.3 mm, respectively. This shows that Tenera cultivar shell has tendency of higher moisture migration that Dura cultivar even at almost the same drying temperature and retention time. This was the reason why Tenera cultivar has higher optimum throughput and cracking efficiency at 18 hrs drying period while Dura cultivar occurred at 21 hrs drying period. In summary, as the drying period increased from 15 -21hrs, the throughput for both cultivars increased from 140 to 255 kg/hr.

Cracking Efficiency
It also observed that as the geometrical mean diameter increased which correlated to the increase in weight of Dura and Tenera palm nuts cultivars, the cracking efficiency at each drying period almost remain constant, but Dura palm nut cultivar was found to be higher at 18 hrs of drying period than Tenera. In Table 7, the P-values were less than 0.0500 while Model F-value of 9.93, both indicated that the model was significant. In this case only drying period among all categorical factors has significant model term. The Predicted R² of 0.3802 was reasonably agreed with the Adjusted R² of 0.5122; since their difference was less than 0.2. However, only 0.18% chance that an F-value could occur due to signal to noise ratio of 5.797, which indicated adequate signal occurred within all categorical factors. In Figure 10, the optimum cracking efficiencies at drying period of 15 hrs for both cultivars were almost the same of an average value of 74%. However, in Dura cultivar, the design points above predicted values were on increased relative to GMD of palm nuts samples, hence the cracking efficiency increased from 74% to 80%. But contrary to that in Tenera cultivar, in Figure 10 of drying temperature of 15 hrs, the design point below predicted value of the cracking efficiency decreased relative to geometrical mean diameters from 26.22 to 18.80 (mm). Considering Figure 11, as drying temperature increased to 18 hrs, the optimum cracking efficiencies were almost at the threshold value of 88% in all three levels of GMD of both cultivars. However, the cracking efficiencies design points above predicted were almost equal to that of the optimum value across all the GMD of Dura cultivar palm nuts. In contrary to this observation, in Tenera cultivar, the design points above predicted value of cracking efficiency increased to 94% from design point of 88%, most especially at the GMD of 26.22 mm. In Figure 12, as drying temperature increased to 21 hrs, the optimum cracking efficiency decreased to 87% across the cultivars and GMD, while design points above predicted values increased to 94% in Dura cultivar only but contrary to this observation, in Tenera, design points below predicted values occurred and it decreased as the GMD of palm nuts decreased. The variability in their cracking efficiencies at equal drying temperature may be due to difference in their physical and mechanical properties regards to mass and thickness of the palm nut shell which ultimately influenced differences in their cracking force requirement as reported by Gbadamosi, (2006) and Ebunilo and Ojariafe, (2014).
In Figure 13, it was observed that Tenera has optimum cracking efficiency of 95% occurred at 18 hrs of drying period, while that of Dura cultivar occurred at 21 hrs. Furthermore, in Figure 13, the optimum cracking efficiency increased linearly from 75-90% in both cultivars relatively to the drying period from 15 hrs to 18 hrs, but decreased to 87% and 85% for Dura and Tenera cultivars, respectively. These observations were earlier reported by Ndukwu

The Damage percentage
The damage percentage was almost constant irrespective of varying in the experimental factors in both Dura and Tenera palm nuts cultivars. For example in Table 8, there were no Pvalues, likewise no significant model terms among the categorical factors. The Predicted R² was negative, which implies that the overall mean may be a better predictor of response than the current model. This was the reason why in Figure 17, a horizontal line indicating a uniform damaged percentage value of 8% that cut across both cultivars. These could be validated by considering Figures 14-16, where there was no significant difference in their damaged percentage across the GMD and drying temperature. However, design points above predicted values were on increased to a value of 19% in Dura cultivar at GMD of 26.72 mm during constant drying period of 15 hrs, while that of Tenera cultivar was at15% on both GMD of 18.80 and 22.46 mm. Conversely, in Figures 14 -16, the design points were below predicted values except in Tenera cultivar.

IV.
CONCLUSIONS The determination of the physical properties has enhanced appropriate configuration of clearance between the tip of centrifugal impeller to the inner diameter to achieve optimum throughput, cracking efficiency, minimum damaged and recovery percentages. The optimum throughput increased relatively to geometrical mean diameters (GMD) of both cultivars. At 15, 18 and 21 hrs drying period, the optimum throughputs were 200 and 180, 240 and 218, 235 and 230 (kg/hr) for Dura and Tenera cultivars, respectively. The optimum cracking efficiency of 88% was obtained across all level of GMD, while predicted value above this threshold was 94% for Dura cultivar only, which were found higher than values obtained by previous researchers reported. Among all categorical factors, only throughput and cracking efficiency influenced optimization model terms to be significant at level of P> 0.0500, while others such as damaged percentage and recovery efficiency were insignificant factors. Damaged percentage and recovery efficiency have threshold of 8% and 6%, respectively. Hence, the determination of physical properties and inferential statistical analysis, before embarking on development of the modified centrifugal palm nuts cracker improved performance characteristics and reduced damaged percentages.