Optimization of density and durability of pellets using the response surface methodology in ultrasonic vibration-assisted pelleting of corn stover

Li, Wentao; Li, Chenxi; Zhang, Bo; Zhou, Guixia; Liu, Dan

doi:10.1590/1678-992X-2022-0243

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Agricultural Engineering • Sci. agric. (Piracicaba, Braz.) 80 • 2023 • https://doi.org/10.1590/1678-992X-2022-0243 copy

Optimization of density and durability of pellets using the response surface methodology in ultrasonic vibration-assisted pelleting of corn stover

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

Top-quality pellets can significantly increase density and durability of agricultural residues, reducing logistic costs. However, these pellets depend on numerous parameters, including feedstock properties and production conditions. To ensure high-quality pellets, a single-factor experiment and the response surface methodology were used to investigate the effects of particle size, moisture content, molding pressure, pelleting time, ultrasonic power, and interaction effects between variables on density and durability of pellets for ultrasonic vibration-assisted pelleting of corn stover. The response surface models between variables and response were established. The results showed that all variables affect the density and durability of pellets. An optimal condition for density and durability was obtained, and a further experiment was conducted to validate the values. The results suggested that desirability (0.999) under optimal conditions confirmed the validation of models. The optimal combination of process parameters included particle size of 1.5 mm, moisture content of 10 %, molding pressure of 379 kPa, pelleting time of 80 s, ultrasonic power of 250 W, with values of 1,381.14 kg m^–3 and 97.58 % for density and durability of pellets, respectively.

pellet properties; process parameters; prediction models; crop residues

Variables	Levels
Particle size (mm)	0~1.0	1.0~1.5	1.5~2.0	2.0~2.5	2.5~3.0
Moisture content (%)	6	8	10	12	14
Molding Pressure (kPa)	138	207	276	345	414
Pelleting time (s)	30	50	70	90	110
Ultrasonic power (W)	0	100	200	300	400

Variables	Symbols	Level
Variables	Symbols	–2		–1		0		+1	+2
Particle size (mm)	x ₁	0.5	1.0		1.5		2.0		2.5
Moisture content (%)	x ₂	6	8		10		12		14
Molding Pressure (kPa)	x ₃	276	310		345		379		414
Pelleting time (s)	x ₄	30	50		70		90		110
Ultrasonic power (W)	x ₅	100	150		200		250		300

Run	Particle size	Moisture content	Molding pressure	pelleting time	Ultrasonic power	Pellet density / y₁	Pellet durability / y₂
	mm	%	kPa	s	W	kg m^–3	%
1	2.0	12	310	50	250	1203.78	84.84
2	2.0	12	379	90	250	1212.33	90.85
3	1.0	12	379	90	150	1274.91	94.16
4	1.0	10	345	70	200	1290.16	96.40
5	2.0	12	310	90	150	1180.14	85.86
6	1.5	10	345	70	200	1269.39	96.79
7	1.5	10	414	70	200	1355.04	98.12
8	1.0	8	379	90	250	1335.88	90.10
9	1.5	6	345	70	200	1001.49	81.08
10	2.0	8	310	90	250	1095.39	88.81
11	1.0	12	379	50	250	1310.12	94.74
12	1.5	10	345	70	200	1303.60	96.42
13	2.0	12	379	50	150	1160.06	86.16
14	1.5	10	345	70	200	1303.60	95.71
15	2.5	10	345	70	200	1027.81	84.71
16	2.0	8	379	50	300	1133.67	90.17
17	2.0	8	379	50	150	1123.17	86.93
18	1.5	10	345	70	100	1256.84	87.58
19	1.0	12	310	50	150	1210.30	86.21
20	1.5	10	345	70	200	1257.36	93.85
21	1.5	10	345	110	200	1303.83	97.08
22	1.5	10	345	70	200	1267.60	94.02
23	2.0	8	379	90	150	1212.59	86.28
24	1.0	8	310	90	150	1099.11	83.29
25	1.0	8	310	50	250	1174.00	88.98
26	0.5	10	345	70	200	1198.92	86.23
27	2.0	8	310	50	150	1109.63	78.59
28	1.5	10	345	70	300	1390.10	97.33
29	1.5	14	345	70	200	1102.70	83.54
30	1.5	10	345	70	200	1280.44	94.29
31	1.5	10	276	70	200	1195.45	87.75
32	1.5	10	345	70	200	1261.10	95.97
33	1.5	10	345	70	200	1257.44	95.63
34	1.5	10	345	70	200	1259.87	96.40
35	1.0	12	310	90	250	1260.91	92.14
36	1.5	10	345	30	200	1186.00	88.18

Source	Sum of square	Degree of freedom	Mean square	F-value	p-value
Model	275300	20	13767.05	28.42	< 0.0001^**
x₁- Particle sizes	28223.72	1	28223.72	58.26	< 0.0001^**
x₂- Moisture content	22296.43	1	22296.43	46.03	< 0.0001^**
x₃- Molding pressure	23352.55	1	23352.55	48.21	< 0.0001^**
x₄- Pelleting time	9687.37	1	9687.37	20.00	0.0004^**
x₅- Ultrasonic power	16155.99	1	16155.99	33.35	< 0.0001^**
x ₁ x ₂	885.58	1	885.58	1.83	0.1964^ns
x ₁ x ₃	1807.13	1	1807.13	3.73	0.0725^ns
x ₁ x ₄	224.25	1	224.25	0.46	0.5066^ns
x ₁ x ₅	9538.95	1	9538.95	19.69	0.0005^**
x ₂ x ₃	3160.95	1	3160.95	6.53	0.0220^*
x ₂ x ₄	1569.56	1	1569.56	3.24	0.0920^ns
x ₂ x ₅	66.91	1	66.91	0.14	0.7154^ns
x ₃ x ₄	8596.00	1	8596.00	17.74	0.0008^**
x ₃ x ₅	466.25	1	466.25	0.96	0.3421^ns
x ₄ x ₅	406.53	1	406.53	0.84	0.3741^ns
x ₁ ²	48068.58	1	48068.58	99.23	< 0.0001^**
x ₂ ²	93571.35	1	93571.35	193.16	< 0.0001^**
x ₃ ²	93.84	1	93.84	0.19	0.6661^ns
x ₄ ²	1102.62	1	1102.62	2.28	0.1522^ns
x ₅ ²	6066.52	1	6066.52	12.52	0.0030^**
Residual	7266.48	15	484.43	-	-
Lack of Fit	4242.81	6	707.14	2.10	0.1517
Pure error	3023.67	9	335.96	-	-
Total	282600	35	-	-	-

Source	Sum of square	Degree of freedom	Mean square	F-value	p-value
Model	944.51	20	47.23	20.75	< 0.0001^**
x₁- Particle sizes	32.75	1	32.75	14.39	0.0018^**
x₂- Moisture content	29.83	1	29.83	13.11	0.0025^**
x₃- Molding pressure	110.17	1	110.17	48.41	< 0.0001^**
x₄- Pelleting time	44.44	1	44.44	19.53	0.0005^**
x₅- Ultrasonic power	115.49	1	115.49	50.75	< 0.0001^**
x ₁ x ₂	12.44	1	12.44	5.47	0.0336^*
x ₁ x ₃	0.00	1	0.00	0.00	0.9954^ns
x ₁ x ₄	5.32	1	5.32	2.34	0.1472^ns
x ₁ x ₅	0.36	1	0.36	0.16	0.6964^ns
x ₂ x ₃	0.58	1	0.58	0.25	0.6211^ns
x ₂ x ₄	3.29	1	3.29	1.45	0.2478^ns
x ₂ x ₅	10.20	1	10.20	4.48	0.0514^ns
x ₃ x ₄	4.08	1	4.08	1.79	0.2006^ns
x ₃ x ₅	4.50	1	4.50	1.98	0.1799^ns
x ₄ x ₅	4.54	1	4.54	1.99	0.1783^ns
x ₁ ²	191.37	1	191.37	84.09	< 0.0001^**
x ₂ ²	335.03	1	335.03	147.21	< 0.0001^**
x ₃ ²	10.73	1	10.73	4.72	0.0463^*
x ₄ ²	13.75	1	13.75	6.04	0.0266^*
x ₅ ²	15.63	1	15.63	6.87	0.0193^*
Residual	34.14	15	2.28	-	-
Lack of Fit	23.37	6	3.89	3.25	0.0550
Pure error	10.77	9	1.20	-	-
Total	978.64	35	-	-	-

Comparison	Process variables					Pellet density	Pellet durability
Comparison	Particle size	Moisture content	Molding pressure	Pelleting time	Ultrasonic power	Pellet density	Pellet durability
	mm	%	kPa	s	W	kg m^–3	%
Predicted value	1.33	10.30	379	80.26	250	1390.10	98.07
Experimental value	1.5	10	379	80	250	1381.14	97.58

Escola Superior de Agricultura "Luiz de Queiroz" USP/ESALQ - Scientia Agricola, Av. Pádua Dias, 11, 13418-900 Piracicaba SP Brazil, Phone: +55 19 3429-4401 / 3429-4486 - Piracicaba - SP - Brazil
E-mail: scientia@usp.br

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[1] *Corresponding author <yuesetingquan@byau.edu.cn>