Article pubs.acs.org/EF
Moisture Adsorption Properties of Dried Lignite Changfu You,* Haiming Wang, and Kai Zhang Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: Moisture adsorption properties of dried lignite were investigated experimentally and mathematically. Three kinds of lignite (Hailaer, Huolinhe, and Indonesia) with different water contents were tested with a machine that maintained a constant temperature and relative humidity. The obtained experimental adsorption data were applied to the Guggenheim, Anderson, and de Boer (GAB), modified Guggenheim, Anderson, and de Boer (MGAB), modified Oswin (MO), modified Henderson (MH), and modified Freundlich (MF) isotherm equations to test their applicability to dried lignite. The order of best fit of adsorption across the entire temperature and relative humidity range was MF > MGAB > GAB > MH > MO. On the basis of the experimental data, the MF equation was modified to consider the effect of the initial moisture content of dried lignite on the equilibrium water. The critical temperature and relative humidity values were obtained with the modified MF equation, which was then used to determine the moisture adsorption occurrence for the dried lignite with a specific moisture content under different environmental conditions. Shanghai and Guangzhou were the target-user locations of the dried lignite. The critical water content values were obtained to avoid the moisture adsorption. These values provided the theoretical guidelines for the target control of the lignite drying process and storage conditions.
1. INTRODUCTION Lignite is commonly used throughout the world because of its abundance, easy access, and low mining cost. However, its high water content results in high transportation costs and low combustion efficiency.1 Therefore, industrial applications of lignite are interested in the drying process.2,3 The water content in the dried lignite product is affecting not only the lignite activity, such as the spontaneous combustion,4−6 but also the moisture adsorption properties because of its rich porous structure, which decrease the economic efficiency of drying technologies. These safety and economic concerns continue to pose an engineering challenge to the treatment of lignite water content for drying technology applications. Therefore, research on the moisture adsorption properties of dried lignite is required to provide the theoretical guidelines for the target control of the lignite drying process and its storage conditions. Moisture adsorption in food preservation has previously been investigated.7 In this domain, the extent of water sorption or desorption for a food product depends upon the vapor pressure of water in the food sample and its surroundings. This is similar to the situation of a lignite sample. The equilibrium moisture content (EMC) is the moisture content (MC) at which the vapor pressure of water in a food sample equals that of its surroundings. The relationship between EMC and the corresponding relative humidity (RH) at a constant temperature yields the moisture sorption isotherm. For a given material, the EMC increases with RH but decreases with an increase in the temperature. Many researchers are interested in the measurement and modeling of the sorption of food materials because of its industrial value. Comprehensive reviews on sorption behavior of foods have been published.8,9 Various mathematical models have been developed to describe the sorption isotherms. Some models were theoretically derived on the basis of thermodynamic concepts, while others were extended © 2012 American Chemical Society
or modified forms of these models. Commonly used mathematical models include Guggenheim, Anderson, and de Boer (GAB),10−13 modified Guggenheim, Anderson, and de Boer (MGAB),14 modified Oswin (MO),14,15 modified Henderson (MH),14,16,17 and modified Freundlich (MF) equations.14,18,19 GAB and MGAB are the most popular models for the preservation of dehydrated food. GAB provides the monolayer MC value and is considered to be the most useful equation for determining the optimum moisture conditions for storage stability, especially for dehydrated food.20 GAB, MH, and MO reportedly fit plantain or banana EMC data over 10−70% of the RH range.21 The applicability of these models to lignite should be tested for the following reasons: previous application has focused extensively on food preservation; there are differences in the physical properties of food and lignite, such as porous distribution and surface functional groups; and finally, these models do not consider the effect of the initial MC on the moisture adsorption properties of a product. Three kinds of lignite (Hailaer, Huolinhe, and Indonesia) with different water contents were experimentally investigated with a machine that maintained a constant temperature and RH to clarify the moisture adsorption properties of dried lignite. The obtained experimental adsorption data were applied to the GAB, MGAB, MO, MH, and MF isotherm equations to test their applicability to dried lignite. Shanghai and Guangzhou were the targetuser locations of the dried lignite. The critical water content values were obtained to avoid moisture adsorption. These values provided the theoretical guidelines for the target control of the drying processes and storage conditions. Received: October 16, 2012 Revised: November 26, 2012 Published: November 27, 2012 177
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Table 1. Ultimate and Proximate Analyses of Raw Lignite ultimate analysis (%)
proximate analysis (%)
samples
Cd
Hd
Od
Nd
Sd
volatile matterd
fixed carbond
moisturear
ashd
Hailaer lignite Huolinhe lignite Indonesia lignite
50.97 55.45 72.44
2.98 3.83 4.45
13.88 15.25 21.38
0.82 0.93 0.44
0.62 0.57 0.36
35.72 38.46 44.67
33.55 37.57 54.4
28.87 31.93 62.95
30.73 23.97 0.93
2. EXPERIMENTAL SECTION
Table 2. Performance Parameters of the Constant Temperature and RH Oven temperature range (°C)
RH range (%)
temperature fluctuation/ uniformity (°C)
RH deviation (%)
0−150
30−98
±0.5/±2
±2
2.1. Sample Preparation. Two Chinese representative lignites (Hailaer and Huolinhe) and one Indonesia lignite were used in the experiments. A sieving machine was used to obtain samples with different particle sizes. The ultimate and proximate analyses of the raw lignite are listed in Table 1. 2.2. Experimental System. The performance parameters of the constant temperature and RH oven used in the experiments were listed in Table 2. A thin layer of the sample was uniformly distributed in a reactor that was then weighted with a balance to an accuracy of 0.1 mg. The investigation was divided into two parts. First, the effects of different particle sizes of samples on EMC were experimentally tested. Second, 80 parallel experiments of upgrading samples of lignite with particle sizes
Table 3. Moisture Adsorption Experiment Conditions temperature (°C) RH (%) particle size (mm)
20 30 2.8−3.2
30 50 7−10
40 70 about 20
50 90 about 40
Figure 1. MC of the upgrading products of Huolinhe lignite with different particle sizes.
Figure 2. EMC variation against RH for lignite-upgrading products with different MCs. 178
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Figure 3. EMC of the lignite-upgrading products against RH at different temperatures.
Table 4. EMC Modelsa
Table 6. Modified Equilibrium Water Content Constants constants
MCe = f (Hr , T )
equation GAB
MCe =
abcHr (1 − cHr)(1 − cHr + bcHr)
MGAB
( Tc )Hr
ab MCe =
(
MH
⎛ ln(1 − Hr) ⎞1/ c MCe = ⎜ ⎟ ⎝ − a(T + b) ⎠
MF
MCe = aT + b(Hr)c
Table 5. EMC Constants constants GAB MGAB MO MH MF
−3.81 × 10 −2.13 × 101 1.52 × 101 2.49 × 10−4 −4.02 × 10−2
c
d
R2
Huolinhe lignite Hailaer lignite Indonesia lignite
−0.068 −0.060 −0.105
30.745 29.345 44.623
0.196 0.270 0.144
0.642 0.572 0.571
0.965 0.949 0.956
3. RESULTS The MC at which vapor pressure of water present in the material equals that of its surroundings is referred to as the EMC. Figure 1 shows the variations of MC over time for the Huolinhe lignite-upgrading products with MC = 0 and 15.31% under conditions of 20 °C and 70% RH. The results showed that a smaller particle size decreased the time for two products with different initial MCs to approach moisture equilibrium. The EMCs of the different particle sizes for each product were almost identical. The EMCs were all approximately 9% for the completely dried upgrading products. The EMCs were all approximately 14% for the upgrading products with initial MCs of 15.31%. That was because the particle size affected the masstransfer resistance but had little influence on the water vapor adsorption and internal water evaporation of the lignite-upgrading products. Figure 2 shows EMC variations against RH for Huolinhe and Indonesia lignites with different MCs at ambient temperatures of 20 and 50 °C. The completely dried upgrading products had the same low EMC for both lignite types when at the same temperature and RH. The EMCs of the different samples increased with an increasing initial MC. This was because both the internal water evaporation and the water vapor adsorption, which were included in the MC change process, determined the ultimate steady state. Water vapor adsorption mainly occurred in the
a MCe, EMC (%, dry basis); a, b, and c, unknown values to be estimated; T, temperature (°C); and Hr, RH (%).
b 4
b
( Tc )Hr)
⎛ Hr ⎞c MCe = (a + bT )⎜ ⎟ ⎝ 1 − Hr ⎠
a
a
(1 − bHr) 1 − bHr + b
MO
equation
samples
R2
c −3
3.60 × 10 −4.51 × 10−1 −3.17 × 10−2 −2.23 × 102 2.27 × 101
−2
−9.12 × 10 4.14 × 102 2.73 × 10−1 2.05 × 100 3.13 × 10−1
0.944 0.977 0.851 0.914 0.989
ranging from 2.8 to 3.2 mm were conducted. These experiments were conducted under variations of the following conditions: MC, temperature, and RH. At least two consecutive weighting of the same results were used to obtain the sample weight and to ensure experimental accuracy. Table 3 lists the selected conditions of the experiments. 179
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Figure 4. Comparison of experimental and predicted values for EMC.
totally dried samples, whereas water evaporation was dominant in the raw lignite. Figure 3 shows EMC variations against RH for Huolinhe and Indonesia lignites with the same MC at ambient temperatures of 20, 30, 40, and 50 °C. EMC increased as the ambient RH increased for both lignite types at the same ambient temperature and MC. This was because the higher water vapor content in the environment enhanced the water vapor adsorption of the samples and eventually led to an EMC increase. The EMC decreased with an increasing ambient temperature for both lignite types at the same ambient RH and MC. The water evaporation rate of the samples was faster in a higher ambient temperature and, therefore, resulted in an EMC decrease.
The comparison of experimental and model-predicted values was shown in Figure 4 for EMC of the three different lignite types. The results showed that the modified model appeared more suitable for predicting EMC. The error margin was within 10%. The critical temperature and RH curve revealed the relationship between the ambient temperature and RH. The critical curve was obtained if MCe = MCin in the modified EMC equation (eq 1), on which the water content did not change during storage. Figure 5 shows the critical curve of the lignite-upgrading
4. THEORETICAL ANALYSIS GAB, MGAB, MO, MH, and MF are the most commonly used isotherm equations. These equations shown in Table 4 were selected to fit the experimental data for all three lignite sorption isotherms. Table 5 lists the constants of the EMC equations for completely dried Huolinhe lignite. These were estimated by numerical regression using typical EMC models listed in Table 4. MF was most suitable for predicting EMC at different RHs and temperatures. In addition to the influence of the ambient temperature and RH, the experimental data showed that the initial MC of the lignite-upgrading product also significantly influenced EMC. Therefore, the MF model was modified as eq 1, where MCin is the initial MC of the lignite-upgrading product. The regression analysis was used to obtain the constants of the modified MF model for each lignite type. The analysis was based on the 80 parallel experiments, in which the condition variables were initial MC, temperature, and RH. The modified constants were listed in Table 6. MCe = aT + b(Hr )c + (MCin)d
Figure 5. Critical temperature and RH curve of Huolinhe lignite with MC = 15.31%.
products with MC = 15.31%. The MC increased during storage in the area above the critical curve but not in the area below the critical curve.
5. NUMERICAL RESULTS AND ANALYSIS Shanghai and Guangzhou were the target-user locations of the dried lignite for this research. In both locations, the range of MC without water vapor readsorption was determined for the ligniteupgrading products. Table 7 lists the normalized values of ambient temperatures and RHs in Shanghai and Guangzhou from
(1) 180
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Table 7. Mean Monthly Temperature and RH in Shanghai and Guangzhou month
1
2
3
4
5
6
7
8
9
10
11
12
temperature in Guangzhou (°C) RH in Guangzhou (%) temperature in Shanghai (°C) RH in Shanghai (%)
13.6 72 4.7 75
14.5 77 6 72
17.9 82 9.2 78
22.1 84 14.7 75
25.5 84 20.3 74
27.6 84 23.8 82
28.6 82 28 80
28.4 82 27.8 81
27.1 78 24.4 77
24.2 71 19.2 74
19.6 66 13.5 74
15.3 66 7.8 73
Figure 6. Critical temperature and RH curve of lignite-upgrading products with different initial MCs (the discrete points represent the different temperatures and RHs in Guangzhou and Shanghai).
GAB, MGAB, MO, MH, and MF isotherm equations to test their applicability to dried lignite. The order of best fit of adsorption across the entire temperature and RH range was MF > MGAB > GAB > MH > MO. (1) On the basis of the experimental data, the MF equation was modified to consider the effect of the initial MC of dried lignite on the equilibrium water content. (2) The critical temperature and RH values were obtained with the modified MF equation, which was used to judge the moisture adsorption occurrence for the dried lignite with specific MC under different environmental conditions. (3) Shanghai and Guangzhou were the targetuser locations of the dried lignite. The critical water content values were obtained to avoid the moisture adsorption. These values provided the theoretical guidelines for the target control of the drying processes and storage conditions.
1971 to 2000. The RH in these cities was high for all values above 65%. Therefore, dried lignite is more prone to water vapor readsorption in these two cities. The critical temperature and RH curves are shown in Figure 6 for the three lignite-upgrading products with different MCs. The dashed lines represent the critical initial MC that ensured that lignite MC did not increase. The water vapor readsorption phenomena did not occur when the initial MC of lignite was higher than the critical value. Table 8 lists the critical values of the lignite Table 8. Critical MC of Lignite without Water Adsorption samples
critical MC (%)
Hailaer lignite Huolinhe lignite Indonesia lignite
14 16 19
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ASSOCIATED CONTENT
* Supporting Information
based on the critical temperature and RH curves. The critical MC values can be used as guidelines for the design and operation of lignite drying technologies.
S
View of the constant temperature and RH oven and coal samples with different sizes as well as the repeatability of the experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.
6. CONCLUSION Moisture adsorption properties of dried lignite were investigated experimentally and mathematically. Three kinds of lignite (Hailaer, Huolinhe, and Indonesia) with different water contents were tested with a machine that maintained a constant temperature and RH. The obtained experimental adsorption data were applied to the
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-10-62785669. E-mail:
[email protected]. 181
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant 51076083). REFERENCES
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