Optimization of low-temperature drying of laying-hen manure using response surface methodology

Figures & data

Figure 1. Schematic of manure drying system.

Table 1. Properties of raw manure samples.

	Bird age, week
Manure properties	55	56	57	58
TKN, g·kg^−1	56.53 ± 1.95	58.32 ± 1.84	58.51 ± 2.00	57.03 ± 1.78
Organic-N, g·kg^−1	38.48 ± 2.20	38.75 ± 2.16	39.33 ± 2.13	37.44 ± 2.05
NH4-N, g·kg^−1	18.05 ± 1.21	19.57 ± 1.53	19.18 ± 1.46	17.66 ± 1.42
Moisture, %	76.4 ± 1.2	73.8 ± 3.5	74.5 ± 2.8	75.8 ± 2.7
pH	8.2 ± 0.2	8.1 ± 0.3	8.4 ± 0.1	8.1 ± 0.2
Ash, %	27.2 ± 0.8	28.4 ± 1.3	25.9 ± 2.2	25.5 ± 2.5
Volatiles, %	72.8 ± 4.0	71.4 ± 3.5	72.5 ± 3.6	73.7 ± 2.1

Notes. TKN refers to total nitrogen concentration.

Figure 2. Experimental dryer unit. 1. fan for force convection; 2. humidifier; 3. electric heater; 4. Humidity & Temperature Sensor and proportional controller; 5. manure sample on perforated trays.

Table 2. Levels of experimental factors and coded values.

Coded		Levels
	Hot air temperature $x_1$/°C	Air velocity $x_2$/m·s^−1	Manure layer thickness $x_3$/mm
−1.682	15	0.60	60
−1	19	0.84	76
0	25	1.20	100
1	31	1.56	124
1.682	35	1.80	140

Table 3. Uncertainties of the parameters during drying of manure.

Parameter	Uncertainty
Temperature of temperature sensor	±0.1 (°C)
Humidity of humidity sensor	±2.5 (%)
Air velocity	±0.1 (m/s)
Manure weight	±0.1(g)
Uncertainty in the reading of fan’s energy consumption	±0.05 (kW·h)
Uncertainty in the water loss measurement	±0.141(g)
Uncertainty of nitrogen loss rate	±0.224 (%)

Table 4. Central composite rotatable design for three factors and results of responses values.

Run No.	Actual values (coded values)			Experimental values of responses			Drying time /h	Manure weight
	$x_1$	$x_2$	$x_3$	Y1 /kW·h·kg^−1	Y2 /g·h^−1	Y3 /%		Initial /g	End /g
1	25 (0)	1.20 (0)	100 (0)	2.78	25.03	32.7	27	982.4	306.6
2	25 (0)	1.20 (0)	100 (0)	2.96	24.07	31.9	28	970.7	296.0
3	25 (0)	1.20 (0)	100 (0)	2.86	28.09	33.4	25	984.3	282.1
4	25 (0)	1.20 (0)	100 (0)	3.39	27.68	30.8	25	985.5	293.5
5	31 (1)	0.84 (−1)	124 (1)	2.59	27.08	31.5	30	1233.8	421.4
6	25 (0)	1.80 (1.682)	100 (0)	3.63	29.51	29.5	23	950.1	271.4
7	19 (−1)	1.56 (1)	124 (1)	4.94	17.25	27.4	47	1200.0	389.3
8	19 (−1)	1.56 (1)	76 (−1)	4.66	18.53	24.9	27	710.4	210.1
9	19 (−1)	0.84 (−1)	76 (−1)	3.83	18.77	26.9	29	726.5	182.2
10	19 (−1)	0.84 (−1)	124 (1)	4.21	16.02	28.5	50	1190.9	389.9
11	25 (0)	1.20 (0)	60 (−1.682)	2.27	28.59	28.3	15	600.7	171.9
12	31 (1)	0.84 (−1)	76 (−1)	2.42	29.31	30.3	19	750.1	193.2
13	31 (1)	1.56 (1)	124 (1)	3.17	28.93	30.8	27	1200.5	419.4
14	25 (0)	1.20 (0)	100 (0)	3.09	24.04	29.7	28	983.2	310.1
15	35 (1.682)	1.20 (0)	100 (0)	2.11	36.58	28.3	18	957.8	299.4
16	25 (0)	1.20 (0)	140 (1.682)	3.84	25.66	35.8	37	1390.3	440.9
17	31 (1)	1.56 (1)	76 (−1)	2.61	30.05	29.5	17	740.4	229.6
18	15 (−1.682)	1.20 (0)	100 (0)	4.37	16.07	25.7	44	980.7	273.6
19	25 (0)	0.60 (−1.682)	100 (0)	2.51	21.36	33.1	32	996.3	312.8
20	25 (0)	1.20 (0)	100 (0)	2.69	27.82	32.2	25	997.8	302.3

Figure 3. (a) Drying curves at different hot air temperature with the manure layer thickness of 100 mm and the air velocity of 1.2 m/s. (b) Drying curves at different air velocity with the manure layer thickness of 100 mm and the hot air temperature of 25°C. (c) Drying curves at different manure layer thickness with the hot air temperature of 25°C and the air velocity of 1.2 m/s.

Table 5. ANOVA for the regression response surface model.

Models of Responses	F value	Flf	R^2
$\begin{array}{rl}& Y_1=2.95-0.78x_1+0.31x_2+0.30x_3-0.099x_1{x}_2+0.009x_1{x}_3\\ & +0.036X_2{X}_3+0.19{X_1}^2+0.13{X_2}^2+0.12{X_3}^2\end{array}$	9.84	3.16	0.899
$\begin{array}{rl}& Y_2=26.50+6.29x_1+1.60x_2+3.61x_3+0.56x_1{x}_2+1.02x_1{x}_3\\ & +0.31x_2{x}_3-0.54{x_1}^2-1.52{x_2}^2-3.49{x_3}^2\end{array}$	7.13	1.42	0.865
$\begin{array}{rl}& Y_3=31.84+1.35x_1-0.77x_2+1.42x_3+0.18x_1{x}_2-0.22x_1{x}_3\\ & +0.15x_2{x}_3-1.95{x_1}^2-0.45{x_2}^2-0.18{x_3}^2\end{array}$	5.27	1.89	0.826

Figure 4. The effects of (a) hot air temperature and air velocity, (b) hot air temperature and manure layer thickness, (c) manure layer thickness and air velocity on response surface plots of fan’s energy consumption.

Figure 5. The effects of (a) hot air temperature and air velocity, (b) hot air temperature and manure layer thickness, (c) manure layer thickness and air velocity on response surface plots on dehydration rate.

Figure 6. The effects of (a) hot air temperature and air velocity, (b) hot air temperature and manure layer thickness, (c) manure layer thickness and air velocity on response surface plots on nitrogen loss rate.

Table 6. Validation results of optimization scheme.

Response variables	Experimental group			Mean value
	1	2	3
Energy consumption of fans/kW·h·kg^−1	2.25	2.12	2.37	2.24
Dehydration rate/g·h^−1	36.05	35.90	36.31	36.08
Nitrogen loss rate/%	26.0	24.7	26.6	25.7