Optimization of Hot-Air and Microwave Combined Drying Technical Parameters for Extruded Cotton Stalks Based on Response Surface Methodology

Abstract

Drying is an important process of cotton-stalk reconstituted material. The aim of this study was to find the best drying process of extruded cotton stalks by using three drying methods and applying single-factor experiments combined with response surface methodology (RSM). For experimental design, the central composite design approach was used. The hot-air temperature, moisture content at the conversion point, and microwave power were selected as influencing factors; the drying rate and energy consumption per unit precipitation were selected as experimental indexes. The regression equation between each experimental factor and the performance index was established, and the optimization calculation was carried out. The experimental results showed that the optimum drying parameters were as follows: hot-air temperature 95 °C, moisture content at the conversion point 57%, and microwave power 700 W. With these experimental conditions, it was verified that the drying rate was 4.14 kg (100 kg min)⁻¹ and the energy consumption per unit precipitation was 70.89 MJ kg⁻¹, which were 106.7% and 10.4% lower than that of hot-air drying and microwave drying, respectively. The research results will provide a theoretical and technical basis for the large-scale drying process of cotton stalks and the design of drying equipment.

1. Introduction

Cotton is one of the important cash crops in China, and cotton straw (cotton stalk for short) is the by-product of a cotton harvest. China’s cotton stalk resources are very rich, and according to information released by the National Bureau of Statistics, China’s cotton output was 5.977 million tons in 2022. The quantity of cotton and cotton stalks is calculated as 1:5; therefore, the output of cotton stalks in 2022 was close to 30 million tons [1]. In recent years, people have increased the research and utilization of cotton stalks, which is widely used in industry, agriculture, animal husbandry, and energy industry [2,3,4]. Cotton stalk is a kind of excellent long bast fiber, which is rich in cellulose, lignin, and polypentose, etc. [5,6]. Cotton stalks have a high degree of lignification. The mechanical and physicochemical properties of cotton stalks are close to those of wood. Therefore, the wood-based panel made from cotton stalks is an ideal substitute for wood [7,8]. Making full use of cotton stalk resources for the production of wood-based panels can not only alleviate the current situation of wood shortage in China, but also provide a new way to utilize cotton stalks [9]. At present, cotton-stalk fiberboard and cotton-stalk particleboard have been put into production and application. Cotton-stalk reconstituted material is a new type of wood-based panel developed in recent years. In order to facilitate the storage and process the reconsolidation of cotton stalks, it is necessary to dry the cotton stalks to a lower moisture content [10].

Traditional natural drying takes a long time, which will have a certain impact on the performance of materials and is not conducive to large-scale and industrial production [11,12]. In the slow drying stage of hot-air drying, due to the influence of mass and heat transfer inside the material, efficiency is low, drying time is long, and the quality of the material may change accordingly [13]. Microwave drying of the material creates a pressure gradient inside, which drives the internal water to spread outwards. Therefore, the moisture inside the material is dried by microwave, which has a low energy consumption and a short drying time [14]. Studies have shown drying time decreased considerably with a decrease in the sample amount of crushed cotton stalks by using the microwave drying technique [15]. Studies have also shown that in the drying process of materials, when the moisture content is less than 20%, the effect of microwave drying is better than that of hot-air drying [16]. In order to save on the cost of drying, many studies suggest that microwave power can be applied in the drying process of materials in the slow drying stage or in the lower moisture content stage [17,18]. Compared with traditional drying, combined drying could offer several advantages such as better drying efficiency, higher energy efficiency, a more efficient moisture removal process, better product quality, and enhanced retention of nutritional/bioactive compounds [19,20].

In order to improve the shortcomings of hot-air and microwave drying, many scholars at home and abroad have studied the use of hot-air and microwave combined drying (AD + MD) technology to dry materials. Han Junhao et al. studied the drying characteristics of green asparagus combined with hot air and microwave [21]. Jiang Qifang et al. used a hot-air and microwave combined drying technology to dry Eucommia ulmoides leaves, and the results showed that the contents of three active components of Eucommia ulmoides leaves dried by this combination were significantly higher than those of other drying methods [22]. Maftoonazad et al. carried out the traditional hot-air and microwave-assisted hot-air (MW-HA) drying (HA) experiments on onion slices, and the results showed that hybrid MW-HA reduced the drying time by over 90% and increased the energy efficiency by 3.0–16.0 times as compared to HA [23]. Wiset et al. studied the effect of microwave power combined with hot air on the drying kinetics of silkworm pupae, and for the specific energy consumption, the best drying technique for silkworm pupae was using microwave power of 323W combined with hot air [24].

Three drying methods of food waste were used by Byung-Gil [25]. Compared to microwave and hot-air drying methods, the moisture content in the food waste could be effectively and inexpensively reduced by means of the combined microwave-hot-air drying process [25]. In addition, in order to obtain the best process of the combined drying technology, many scholars also use the response surface methodology (RSM) to optimize the process of the combined drying technology and obtain the optimization parameters. For example, the experiment of response surface optimization with combined drying means was used on fairy grass [26], frozen tofu [27], shiitake mushrooms [28], rice noodles [29], and green taro slices in China [30], which is also used on mushrooms [31], banana slices [32], green peppers [33], wood [34], and apples abroad [35].

At present, there is no relevant report on the drying of extruded cotton stalks by the combined drying technology. In this paper, the drying process on extruded cotton stalks, which is the key process for the production of reconstituted materials, was tested and analyzed by using three different drying methods: hot air, microwave, hot air and microwave combined. The central composite design theory in the RSM (response surface methodology) was applied to the experimental study of the combined drying process for extruded cotton stalks. Finding the optimal drying process for extruded cotton stalks will provide a theoretical basis and technical support for the continuous industrial production of cotton-stalk reconstituted material.

2. Materials and Methods

2.1. Material Preparation

The samples of cotton stalks were obtained from an experimental farmland of Northwest A&F University, China. Before the drying experiments, the cotton stalks were soaked in water for about one day to become fully softened, and the initial moisture content was 140% (dry basis). The softened cotton stalks were crushed twice with a cotton stalk fluffer. After a period of indoor natural drying, the extruded cotton stalks were cut into samples with a length of 20 cm (close to the width of the microwave drying chamber), which are the reconstituted materials required for the experiment. Before each experiment, the initial moisture was determined gravimetrically by oven drying at 103 ± 1 °C until constant weight (mass).

2.2. Experimental Set-Up

In order to achieve the industrial production of cotton-stalk reconstituted materials, the basic prerequisite is to study the rapid drying of extruded cotton stalks. BDGJ-B hot-air drying equipment (Shanghai Tongyu Teaching Instrument and Equipment Manufacturing Co., Ltd., Shanghai, China) and a KLT-ZH Microwave Drying Tunnel device (Qingdao Kelante Microwave Equipment Co., Ltd., Qingdao, China) were used to supply hot air flow and microwave energy (Figure 1).

Before the hot-air and microwave combined drying experiments for extruded cotton stalks, the experimental samples should be prepared, hot air velocity should be provided during the drying process, and the samples should be weighed and recorded. The required experimental equipment and instruments are as follows: cotton stalk fluffer (made by College of Mechanical and Electrical Engineering, Northwest A&F University, Xianyang, China), TSI9515 Handheld digital anemometer (TSI Group, Missoula, MT, USA), Precision Analytical balance, model PL202-S100, produced by Mettler Toledo Instruments Shanghai Co., Ltd., Shanghai, China; AB204-N electronic Analytical balance (precision 0.01 g, Mettler Toledo Instruments Shanghai Co., Ltd., Shanghai, China).

2.3. Experimental Methods

First, a handheld digital anemometer was used to adjust the wind velocity to a fixed velocity 3.8 m s⁻¹, and the extruded cotton stalks with an initial moisture content of 140% (dry base) and a mass of 150 g were placed in a hot-air drying oven for hot-air drying and dried to a certain moisture content; then, they were put into a microwave drying chamber for microwave drying until the moisture content of the sample reached less than 6%. After the experiment began, the quality data were recorded every 1 min, and each experiment was repeated 3 times; then, the average value was taken.

2.4. Experimental Methods

2.4.1. Single-Factor Experimental Design

In the combined drying experiment of extruded cotton stalks, hot-air temperature (60 °C, 70 °C, 80 °C, 90 °C, 100 °C), moisture content at the conversion point (20%, 30%, 40%, 50%, 60%), and microwave power (350 W, 460 W, 600 W, 700 W, 1000 W) were selected as the influencing factors. The single-factor experiment was carried out on extruded cotton stalks, the drying characteristics of which were studied when the moisture content of the dry base was below 6%.

2.4.2. Optimization Experiment of Combined Drying Process

Based on the results of the single-factor experiment, the general rotating combination design in the central composite design was used to carry out the experimental design. Hot-air temperature x₁, moisture content x₂ at the conversion point, and microwave power x₃ were used as independent variables, and drying rate Y₁ and unit precipitation energy consumption Y₂ were used as dependent variables. According to the orthogonal table, the number of central experiments is 9, with r = 1.682. The factors and level settings of the experiment are shown in

Table 1. Experimental factors and levels.

Level	Factors
	Hot Air Temperature x1 (°C)	Moisture Content at the Conversion Point x2 (%)	Microwave Power x3 (W)
Upper asterisk arm	107	63	782
Upper level	100	60	460
Zero level	90	55	580
Lower level	80	50	700
Lower asterisk arm	73	47	378

.

2.5. Measurement Indicators and Calculation Methods

2.5.1. Calculation of Moisture Content at Conversion Point

At time t, the hot air was converted into microwave to dry the extruded cotton-stalk samples. At this time, the dry-base moisture content of the samples was moisture content at the conversion point, M_t, which is defined as [36]:

$$M_t=\frac{m_t-m_0}{m_0}$$

(1)

where: m_t is mass of the dried sample at time t, g, and m₀ is absolute dry mass of the material, g.

2.5.2. Drying Rate

Drying rate (DR), which is the rate of the samples dried from the initial moisture content to the target moisture content during the whole drying process, is defined as [37]:

$$D{R}_{}=-\frac{d{M}_t}{dt}=-\frac{M_{t+\mathsf{\Delta }t}-M_t}{\mathsf{\Delta }t}$$

(2)

where: M_t+Δt is the dry basis moisture content at time t + Δt, g/g; M_t is the dry-base moisture content at time t, g/g; Δt is the drying time, min; and DR is the drying rate, g/g·min.

2.5.3. Energy Consumption per Unit Precipitation

Total drying energy consumption refers to the total energy required to dry a certain mass of material. Energy consumption per unit precipitation refers to the energy consumption required for evaporating water per unit mass during the drying process of materials [38].

When the cotton stalks are extruded for single hot-air drying, the total energy consumption Q_A of hot-air drying is defined as follows [10]:

$$Q_A=Q_1+Q_2=\frac{\pi }{4}{d}^2{c}_A{t}_{A1}\upsilon {\rho }_A\mathsf{\Delta }T+t_{A2}P$$

(3)

where: Q₁ is the hot-air energy consumption, J; Q₂ is the fan energy consumption, J; d is the diameter of the air inlet, d = 0.09 m; and C_A is the specific heat capacity of air in the drying room, J kg⁻¹ °C⁻¹; the values are shown in

Table 2. Value of heat capacity and air density at different temperatures in drying device.

Temperature T (°C)	Air Density ρ (kg·m−3)	The Heat Capacity of Air C (J·(kg·°C)−1
60	1.0250	1017
70	0.9960	1017
73	0.9876	1017
80	0.9680	1022
90	0.9420	1022
100	0.9160	1022
107	0.8999	1022

. T_A1 is the drying time of the hot-air experiment, h; v is the wind velocity of the inlet, measured directly by the anemometer, m s⁻¹; and ρ_A is the air density at different temperatures in the hot-air dryer, kg m⁻³; the values are shown in Table 2. ΔT is the difference of air temperature in the drying room during the corresponding drying time, °C; S is the cross-sectional area of the air inlet, m²; t_A2 is the working time of the fan experiment, measured directly by the stopwatch, s; and P is the fan power, 550 W.

The total energy consumption Q_M required for the microwave drying experiment is calculated as follows:

$$Q_M=t_M{P}_M$$

(4)

where: t_M is the microwave drying time, s and P_M is microwave input power, W.

The total energy consumption Q_A+M required for the hot-air and microwave combined drying experiment is calculated as follows:

$$Q_{A+M}=Q_A+Q_M$$

(5)

When drying extruded cotton stalks with three different drying methods of hot air, microwave, hot-air and microwave combined, the total energy consumption per unit precipitation is defined as [39]:

$$n=\frac{Q_E(1-M_f)\times {10}^{-6}}{m_i(M_i-M_f)}$$

(6)

where: n is the total energy consumption per unit precipitation, MJ kg⁻¹; Q_E is the total energy consumption required by the experiment, KJ; M_f is the final moisture content of drying (dry base), kg/kg; M_i is the initial moisture content of drying (dry base), kg/kg; and m_i is the initial dry mass, kg.

3. Results and Discussion

3.1. Single-Factor Experiment

3.1.1. Hot-Air Temperature

With the hot-air speed 3.8 m s⁻¹ and the loading capacity 150 g, the drying curve and drying rate curve of the cotton straw under different hot-air temperatures are shown in Figure 2.

Figure 2a shows that with an increase in the hot-air temperature, the drying curve of extruded cotton stalks becomes steeper; that is, the greater the hot-air temperature, the shorter the drying time required. From Figure 2b, it can be seen that the acceleration process of the hot-air drying process for cotton stalks is not obvious, and the water-loss process is mainly concentrated in the slow drying stage. Moreover, the drying rate during the entire drying stage was greatly affected by wind temperature; the higher the hot-air temperature, the greater the drying rate. But the distance between the two curves at 90 °C and 100 °C varied less than that of the two curves at other temperature ranges.

It was indicated that the drying rate increased slowly after the temperature rose to a certain extent, which was consistent with the research results of Wang Haopeng et al. [40] on the hot-air drying of seed cotton. Some studies showed that [10] cotton stalks should not be dried at excessively high temperatures, which can easily reduce the mechanical strength of the cotton stalks and make the fibers brittle, thereby affecting the performance of the final reconstituted board. Excessive temperature also led to a significant decrease in the shrinkage rate of straw, and the surface of extruded cotton stalks was prone to hardening and is prone to fracture in subsequent processing. Low temperature hot-air drying (≤80 °C) led to a very small drying rate and a very low drying efficiency.

3.1.2. Moisture Content at the Conversion Point

The hot-air speed and loading capacity were 3.8 m s⁻¹ and 150 g, respectively, the initial moisture content of the extruded cotton stalks was about 140% (dry base), and the hot-air temperature and microwave power were 80 °C and 700 W, respectively. Under these conditions, the drying curve and the drying rate curve of extruded cotton stalks for different moisture content at the conversion point are shown in Figure 3.

Figure 3 shows that the extruded cotton stalks with an initial moisture content of about 140% was first dried by hot air at 80 °C. When it was dried to the certain moisture content (moisture content at the conversion point), it was then converted to microwave drying equipment of 700 W for microwave drying, and the dry-base moisture content was about 6%. From Figure 3, it can be seen that the drying time of hot-air and microwave combined drying was significantly shorter than that of the single hot-air drying. The drying rate of the microwave drying stage was significantly higher than that of hot-air drying stage.

When drying with hot air at 80 °C alone, it took more than 60 min to reach the required moisture content, while the time required for the hot-air and microwave combined drying was significantly shortened, and the drying time was directly related to the moisture content at the conversion point. When the moisture content at the conversion point was 85%, 60%, and 40%, the drying time was about 26 min, 34 min, and 42 min, respectively. The results showed that the earlier the microwave drying, the shorter the total drying time, which was consistent with the combined drying of garlic slices and dried beets by Figiel et al. [38,41]. However, it was found that the excessive microwave drying time can cause uneven heating of cotton stalks, which could easily lead to local cooking of cotton stalks and affect their quality and mechanical properties. Therefore, the intervention of microwave drying should also be considered comprehensively, so that the drying time can be shortened and the processing performance of cotton stalks can be ensured.

3.1.3. Microwave Power

When the hot-air speed was 3.8 m s⁻¹ and the loading capacity was 150 g, the drying curve and drying rate curve of the extruded cotton under different microwave power are shown in Figure 4.

From Figure 4, it can be seen that the microwave drying curve of extruded cotton stalks is similar to that of hot-air drying. The entire microwave drying process showed a decreasing trend in drying rate as the drying time progresses. With an increase in microwave power, the drying time was shortened. To reduce the dry basis moisture content of extruded cotton stalks less than 6%, the drying times were 27 min and 22 min at a microwave power of 350 W and 460 W, respectively, and the drying times were 16 min and 14 min at that of 700 W and 1000 W, respectively. It could be seen that the higher the microwave power, the shorter the drying time it took to reach the required moisture content.

The initial moisture content of the materials in each group was basically the same; thus, the difference in the required drying time was mainly due to the different microwave power. It could also be considered that as the microwave power increased, the microwave power density distributed per unit mass of material increased, leading to faster heat generation of the material, thereby forming a larger moisture gradient with the surrounding hot air. The greater the vapor pressure formed inside the material, the greater its outward driving force, enhancing the diffusion of internal moisture to the surface, which promotes more moisture to be evaporated, and the drying time is shortened. Through experiments, it was found that although increasing the microwave power could shorten the drying time, excessive microwave power could easily lead to cooking the cotton stalks. Therefore, when using microwave drying, an appropriate microwave power should be selected.

3.2. Response Surface Experimental Results and Analysis

Statistical analysis software Designer-Expert Version 8.0.6 and Central Composite Design were used to design a total of 20 groups of response surface tests with three factors and five levels, with Y₁ and Y₂ as response values. The test scheme and results are shown in

Table 3. Experimental design and results of response surface method.

Number	Factors			Average Drying Rate Y1 (kg (kg·100·min)−1	Energy Consumption per Unit Precipitation Y2 (MJ kg−1)
	X 1 (°C)	X2 (%)	X3 (W)
1	−1	−1	−1	3.0057	92.7671
2	1	−1	−1	3.5016	85.4825
3	−1	1	−1	3.0409	94.3448
4	1	1	−1	3.7410	82.0764
5	−1	−1	1	3.4210	79.6454
6	1	−1	1	3.4210	84.3873
7	−1	1	1	3.5494	78.7479
8	1	1	1	4.3697	69.3243
9	−1.682	0	0	2.9213	92.2064
10	1.682	0	0	3.9846	76.6567
11	0	−1.682	0	3.2098	87.3424
12	0	1.682	0	3.3548	88.0132
13	0	0	−1.682	3.4098	87.8184
14	0	0	1.682	4.1767	69.5542
15	0	0	0	3.727	78.3253
16	0	0	0	3.6879	79.7258
17	0	0	0	3.6749	79.7985
18	0	0	0	3.9543	74.7966
19	0	0	0	3.7956	77.9613
20	0	0	0	3.9097	75.0987

Note: X₁, X₂, X₃, respectively, represent the experimental code values of AD, MC, MD.

:

A ternary quadratic regression equation between the response face value and the coded value X of the experimental factor was obtained:

$$\begin{array}{l}{Y}_1=3.79+0.28X_1+0.12X_2+0.2X_3+0.13X_1{X}_2-0.047X_1{X}_3\\ +0.1X_2{X}_3-0.12{X_1}^2-0.18X_2{}^2+0.003409X_3{}^2\end{array}\left[R^2=0.9331\right]$$

(7)

$$\begin{array}{l}{Y}_2=77.64-3.69X_1-1.22X_2-5.37X_3-2.39X_1{X}_2+1.86X_1{X}_3\\ -1.77X_2{X}_3+2.28{X_1}^2+3.43X_2{}^2+0.25X_3{}^2\end{array}\left[R^2=0.9353\right]$$

(8)

3.2.1. Regression Model Equation Analysis of Variance

The variances of the two regression Equations (7) and (8) are shown in

Table 4. Variance analysis of regression equation for average drying rate.

Source	Sum of Squares	Freedom	Mean Squares	F Value	Significant Level p
X1	1.060	1	1.060	55.930	<0.0001 **
X2	0.190	1	0.190	9.830	0.0106 *
X3	0.560	1	0.560	29.470	0.0003 **
X1X2	0.130	1	0.130	6.920	0.0251 *
X1X3	0.018	1	0.018	0.930	0.3573
X2X3	0.081	1	0.081	4.250	0.0462 *
X12	0.200	1	0.200	10.380	0.0092 **
X22	0.450	1	0.450	23.860	0.0006 **
X32	0.000176	1	0.000176	0.009263
Model	2.640	9	0.290	15.490	<0.0001 **
Residual	0.190	10	0.019	—	—
Lack of fit	0.120	5	0.024	1.750	0.2772
Pure error	0.069	5	0.014	—	—
Cor. Total	2.830	9	—	—	—

Note: ** refer to be highly significant, p < 0.01; * refer to be significant, p < 0.05.

and

Table 5. Variance analysis of regression equation for unit energy consumption of dehydration.

Source	Sum of Squares	Freedom	Mean Squares	F Value	Significant Level p
X1	185.90	1	185.90	29.15	0.0003 **
X2	20.33	1	20.33	3.19	0.1045
X3	393.23	1	393.23	61.75	<0.0001 **
X1X2	45.84	1	45.84	7.19	0.0231 *
X1X3	27.64	1	27.64	4.33	0.044 *
X2X3	24.96	1	24.96	3.91	0.0761
X12	74.88	1	74.88	11.74	0.0065 **
X 22	169.27	1	169.27	26.54	0.0004 **
X 32	0.89	1	0.89	0.14	0.7169
Model	922.95	9	102.55	16.08	<0.0001 **
Residual	63.78	10	6.38	—	—
Lack of fit	39.65	5	7.93	1.64	0.2994
Pure error	24.13	5	4.83	—	—
Cor. Total	986.73	9		—	—

Note: ** refer to be highly significant, p < 0.01; * refer to be significant, p < 0.05.

. According to analysis of variance Table 4, the order of influence intensity with the three factors on the response face value Y₁ was as follows: hot air temperature > microwave power > moisture content at the transition point. The F-value of model Equation (7) was F₁ = 15.49, p < 0.0001, indicating that the regression equation was extremely significant. The F-value of the lack of fit term was F₂ = 1.75, p = 0.2772 > 0.05, and the lack of fit was not significant, which indicated that the regression equation had good fitting performance. In this regression model, X₁ and X₃ were extremely significant (p < 0.01), while X₂, X₁X₂, X₂X₃, X₁² and X₂² were significant (p < 0.05). After eliminating non-significant terms, the regression model equation obtained was as follows:

$$Y_1=3.79+0.28X_1+0.12X_2+0.2X_3+0.13X_1{X}_2+0.1X_2{X}_3-0.12{X_1}^2-0.18X_2{}^2$$

(9)

According to the analysis of variance Table 5, the order of influence intensity with three factors on the response face value Y₂ was as follows: microwave power > hot-air temperature > moisture content at the conversion point. The F-value of model Equation (8) was F₁ = 16.08, p < 0.0001, indicating that the regression equation was extremely significant; The F-value of the lack of fit term was F₂ = 1.64, p = 0.2992 > 0.05, and the lack of fit was not significant, indicating that the regression equation had good fitting performance. In this regression model, X₁, X₃, X₁₂, and X₂₂ were extremely significant (p < 0.01), while X₁X₂ and X₁X₃ were significant (p < 0.05). After eliminating non-significant terms, the regression model equation obtained was as follows:

$$Y_2=77.64-3.69X_1-5.37X_3-2.39X_1{X}_2+1.86X_1{X}_3+2.28X_1{}^{{}_2}+3.43X_2{}^2$$

(10)

3.2.2. Interaction Analysis of Model Significant Factors

According to the analysis of variance Table 4 and Table 5, it can be seen that hot-air temperature and moisture content at the conversion point X₁X₂, moisture content at the conversion point, and microwave power X₂X₃ had significant interactions on the response value Y₁ (drying rate). Moreover, hot-air temperature and moisture content at conversion point X₁X₂ and hot-air temperature and microwave power X₁X₃ had significant interaction on the response value Y₂ (energy consumption per unit precipitation). The relationship diagram between significant factors X_i (i = 1, 2, 3) and Y_j (j = 1, 2) was drawn by Designer Expert version 8.0.6, as shown in Figure 5.

As shown in Figure 5a, with fixed microwave power, the drying rate increased with an increase in hot-air temperature, and the increasing trend slowed down when the temperature exceeded 95 °C. The drying rate firstly increased and then decreased with an increase in moisture content at the conversion point. It also can be seen that the influence of hot-air temperature on the drying rate was relatively obvious, but the curve between the moisture content at the conversion point and the drying rate was relatively smooth.

As shown in Figure 5b, with a fixed hot-air temperature, the drying rate increased with an increase in microwave power, and the effect of microwave power on the drying rate was linear; thus, the effect of microwave power on the drying rate was more significant.

As can be seen in Figure 5a,b, compared with microwave power, a change in hot-air temperature has a more significant impact on the drying rate. According to the analysis, during the hot-air drying process, the drying time is obviously shortened and the drying rate increases with an increase in the hot-air temperature. In summary, during the entire hot-air and microwave combined drying process, the hot-air temperature directly determines the combined drying time, and the variation of the combined drying time is directly related to the drying rate.

As shown in Figure 6a, with the fixed microwave power, the energy consumption per unit precipitation first decreased and then increased with an increase in the hot-air temperature; also, first it decreased and then increased with an increase in moisture content at the conversion point. It also can be seen that the hot-air temperature curve was steep, which shows that the hot-air temperature has a strong influence on the energy consumption per unit precipitation.

As shown in Figure 6b, with the fixed moisture content at the conversion point, the energy consumption per unit precipitation first decreased and then increased with an increase in the hot-air temperature, and decreased with an increase in microwave power. It also could be seen that microwave power had a significant impact on the energy consumption per unit of precipitation. As the microwave power decreased, the drying time increased, and the drying energy consumption increased.

As can be seen in Figure 6a,b, compared with the hot-air temperature, a change in the microwave power has a more obvious influence on the energy consumption per unit precipitation. It is mainly because the drying time and energy consumption decrease with an increase in power during microwave drying. In the whole drying process, the drying energy consumption is affected by microwave power.

3.3. The Optimal Process of Hot-Air and Microwave Combined Drying

The response surface Figure 4 and Figure 5 show that the drying rate and energy consumption per unit precipitation increased with an increase in hot-air temperature and microwave power. Therefore, multi-objective optimization of regression model Equations (9) and (10) was carried out to find suitable drying process parameters. The objective function is as follows:

$$\{\begin{array}{l}{Y}_1\to Y_{1\max }\\ Y_2\to Y_{2\min }\end{array}$$

(11)

Constraints:

$$\{\begin{array}{c}-1.682\le X_i\le +1.682\left(i=1,2,3\right)\\ Y_j\ge 0\left(j=1,2\right)\end{array}$$

(12)

The software Designer Expert version 8.0.6 was used to optimize the objective function, and the optimal factor level of the combined drying for extruded cotton stalks was obtained. By converting each factor level into actual values, the optimal process parameters for hot-air and microwave combined drying were obtained as follows: hot-air temperature 95.45 °C, moisture content at conversion point 57.36%, and microwave power 700 W. With the optimal drying process conditions, a drying rate of 4.243 kg (100 kg·min)⁻¹ and an energy consumption per unit precipitation of 70.7623 MJ kg⁻¹ were obtained for the combined drying of extruded cotton stalks. Considering the operability in actual production, the results were rounded to obtain the final process parameters as follows: hot-air temperature 95 °C, moisture content at conversion point 57%, and microwave power 700 W. The drying rate and unit precipitation energy consumption were 4.136 kg (100 kg·min)⁻¹ and 70.885 MJ kg⁻¹, respectively. Therefore, the drying rate and energy consumption per unit precipitation of extruded cotton stalks were 4.136 kg (100 kg·min)⁻¹ and 70.885 MJ kg⁻¹, respectively.

3.4. Verification of Optimization Process for Hot-Air and Microwave Combined Drying

Using the final process parameters (hot-air temperature 95 °C, moisture content at the conversion point 57%, microwave power of 700 W) as the experimental conditions, hot air, microwave, and combined drying experiments were conducted on the extruded cotton stalks with a loading capacity of 150 g. The experimental data were processed, verified and compared, and the drying rate and energy consumption per unit precipitation with different drying methods were obtained, as shown in

Table 6. Comparison of drying rate and unit energy consumption per unit precipitation at different drying methods.

Different Drying Methods	Average Drying Rate Y1 (kg (kg·100·min)−1	Energy Consumption per Unit Precipitation Y2 (MJ kg−1)
Hot-air drying	1.73	146.58
Microwave drying	4.33	78.28
Combination drying	4.14	70.89

. From Table 6, it can be seen that microwave drying has the fastest drying rate, which means that the time of microwave drying for cotton stalks is the shortest. The combined drying of cotton stalks has the lowest energy consumption per unit precipitation, which is 106.7% and 10.4% less than that of hot air drying and microwave drying, respectively.

4. Conclusions

The drying process of hot air and microwave combined for extruded cotton stalks was investigated in this study. Based on the results of this study, the following conclusions were drawn.

• During the hot-air and microwave combined drying process, extruded cotton stalks were greatly affected by three factors: hot-air temperature, moisture content at the conversion point, and microwave power. The drying time required for extruded cotton stalks decreased with an increase in the three factors, which also had a significant impact on the energy consumption per unit precipitation of extruded cotton stalks.
• Based on the regression analysis of the experimental data, the ternary quadratic regression equations of drying rate (Y₁) and energy consumption per unit precipitation (Y₂) were obtained as follows:

$$Y_1=3.79+0.28X_1+0.12X_2+0.2X_3+0.13X_1{X}_2+0.1X_2{X}_3-0.12{X_1}^2-0.18X_2{}^2$$

$$Y_2=77.64-3.69X_1-5.37X_3-2.39X_1{X}_2+1.86X_1{X}_3+2.28X_1{}^{{}_2}+3.43X_2{}^2$$

Through the analysis of variance table, it was found that the influencing sequence of experimental factors on the two response values Y₁ and Y₂ was as follows: hot-air temperature (X₁) > microwave power (X₃) > moisture content at the conversion point (X₂), microwave power (X₃) > hot-air temperature (X₁) > moisture content at the conversion point (X₂).

3.

The optimum process parameters of hot-air and microwave combined drying for extruded cotton stalks were as follows: hot-air temperature 95 °C, moisture content at the conversion point 57%, and microwave power 700 W. With this experimental condition, through experimental verification, the drying rate and energy consumption per unit precipitation were 4.13632 kg (100 kg·min)⁻¹ and 70.88522 MJ kg⁻¹, respectively, which were close to the simulated values, proving the feasibility of the regression model.

4.

With the experimental conditions of hot-air temperature 95 °C, moisture content at the conversion point 57%, and microwave power 700 W, we compared hot-air and microwave combined drying with single drying, and the drying rate of microwave drying was the highest. The energy consumption per unit precipitation of hot-air and microwave combined drying was the lowest, and it was 106.7% and 10.4% lower than that of hot-air drying and microwave drying, respectively.

Therefore, in order to shorten the drying time and reduce the energy consumption, it is a relatively economical and effective method to use the hot-air and microwave combined drying technology to dry extruded cotton stalks.