Adsorption Properties of Modified Soda Lime for Carbon Dioxide

Article pubs.acs.org/IECR

Adsorption Properties of Modified Soda Lime for Carbon Dioxide Removal within the Closed Environment of a Coal Mine Refuge Chamber Wen-mei Gai,†,‡ Yun-feng Deng,‡ and Yan Du*,† †

School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China Chinese Academy of Governance, Beijing 100089, China

‡

S Supporting Information *

ABSTRACT: In coal mine safety, providing and maintaining a safe closed environment and reducing the casualties of coal mine disasters are a longterm challenges. A novel and modified soda lime with higher adsorption rate and capacity was developed through analyzing the reaction mechanism and adsorption factors. The best combinations of soda lime with different mass fraction is additives 6%, H2O 12%, and NAOH 6%, while the best working modes is 25 W fan power, 8 cm thickness, and S-5 soda lime according to the multi-index orthogonal experiment. The order of influence for CO2 adsorption rate of different compositions (mass fraction) is NaOH > H2O > Al2O3, and the order of influence in different working modes is fan power > thickness > the type of soda lime. Through the optimization design, the novel soda lime is easy to form without binding together in the process of adsorption for CO2 and the dust outflow reduced by about 50%. The adsorption rate still remain at a high level in a high temperature and high humidity environment (humidity 70% and temperature 30 °C). It was demonstrated that the adsorption capacity increased by 28%, the adsorption rate increased by 32.2%, and the power consumption reduced by 33.3%.

1. INTRODUCTION In recent years, there have been a large amount of casualties in coal mine accidents. Avoiding these incidents and reducing casualties has become the priority of coal mine safety. The refuge chamber, a temporary shelter for miners waiting to be rescued, could provide a safe confined space for trapped miners.1,2 However, CO2 will accumulate in this space with miners’ breathing, thus this would endanger the miners’ health, or even their lives, while the storage of energy and CO2 adsorbent is limited in the refuge chamber. Thus, improving CO2 adsorption properties and reducing energy consumption seem to twin goals in the field of refuge chambers.3−5 There are a variety of new techniques capable of removing CO2, which have already been tested successfully, such as solid amine, membrane separation, biological capture, activated carbon, chemical adsorbent such as KO2, LiOH, soda lime, and others.6−9 Researchers have found that optimizing composition and adding additives to modify the solid amine can improve the adsorption properties and reduce the cost; however, these methods cannot mitigate the condition where the volume of the adsorption bed is too large.10−16 Membrane separation and biological capture are new techniques and research hot spots, which need specific reaction conditions and they react more slowly.17−20 Adsorption studies for CO2 also focus on activated carbon21,22 and other new methods such as carbon fiber and © XXXX American Chemical Society

potassium adsorbent, which can realize the removal of low concentration CO2, while requiring higher production costs or auxiliary power.23−25 Alkali metal oxides (KO2, LiOH, and soda lime) have been widely used in adsorption for CO2 in small- and medium-sized confined spaces in the case of electricity shortages or lacking space for a large adsorption bed.26−30 LiOH can ensure that the static adsorption rate reaches 75.6% and 80.8% in 24 h, but it produces dust and is hard to mold and to realize large-scale promotion of products due to its higher cost.19,31 KO2 was used by developing oxygen candles, which can achieve the supply of oxygen and the removal of CO2; however, the reaction process is too fast and hard to control, not suitable for long-term CO2 adsorption.32,33 In past decades, soda lime was widely applied in medical use and refuge chambers for CO2 adsorption with advantages such as its lower cost, technological maturity, ease of preparation, high efficiency, stability in nature, and little secondary pollution. The Refuge One Air purifier was developed by RANA-Medical Company, while the electricity driven CO2 filter developed by Australia Mine ARC System and gas driven CO2 air purifier Received: June 16, 2016 Revised: September 8, 2016 Accepted: September 16, 2016

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DOI: 10.1021/acs.iecr.6b02314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Principle diagram of the adsorption capacity measurement device.

Figure 2. Principle diagram of the adsorption rate measurement device.

developed by the Strata Company are both filled with soda lime as an adsorbent.34−36 In this paper, we are committed to developing a novel and modified soda lime with higher adsorption rate and low rate of dust outflow by analyzing the reaction mechanism and adsorption factors to determine the adsorption properties. A large number of experiments (including adsorption breakthrough curves, orthogonal experiments, and human survival experiments) were implemented to verify its adsorption performance and working mode, realize energy savings, and improve the adsorption efficiency to protect the confined space of this living environment.

NaHCO3 + Ca(OH)2 → CaCO3 + H 2O + NaOH·

The process of CO2 dissolving in H2O is slower in reaction and is the key step of the whole reaction, while NaOH can be used repeatedly to accelerate the reaction as catalyst. As a result, the concentration of H2O and NaOH in soda lime play an important role for CO2 adsorption performance. 2.2. Adsorption Factors. It is gas−solid phase chemical reactions with soda lime and CO2, and the average adsorption performance is associated with air velocity, bed porosity, and concentration of adsorption properties learned by gas mass conservation equation in adsorption bed only considering the flow and mass transfer on the axial direction of air flow.39 Therefore, soda lime adsorption performance is mainly affected by the physical and chemical properties and working conditions, including the scale of particles, porosity, surface area, purifier fan power and thickness of adsorption bed. Due to exothermic reaction and involvement of H2O, the reaction rate and reaction equilibrium may also be affected by temperature and humidity within the experimental environment.

2. ADSORPTION FACTORS 2.1. Adsorption Mechanism. Soda lime is a mixture of Ca(OH)2, H2O, and NaOH, and the adsorption properties for CO2 are associated with the proportion of compositions. Currently the mass fraction of NaOH is more than 4%, while its moisture content is within 10−20%.37,38 The CO2 adsorption reaction equation is shown as follows:

3. EXPERIMENTAL SECTION 3.1. Experimental Environment. The adsorption capacity experiment of modified soda lime was carried in the absorption tower designed by our group, and the test principle is shown in Figure 1. Adsorption rate experiment was carried in the mine simulation refuge chamber, while the test principle is shown in Figure 2. The chamber was monitored by KJ70 coal mine safety production monitoring system and an automatically

NaOH

Ca(OH)2 + CO2 ⎯⎯⎯⎯⎯⎯→ CaCO3 + H 2O

(1)

Above reaction can be promoted with water catalyzed by alkali, and the reaction process can be divided into three processes: CO2 + H 2O → CO2 (aq)

(2)

CO2 (aq) + NaOH → NaHCO3·

(3)

(4)

B


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and recorded the adsorption time tm. Thus, the adsorption capacity M can be expressed as follows:

temperature and humidity control system through GD7 mine multiparameter sensors (including O2, CO2, CO, temperature, and humidity) with good air tightness and internal effective volume of 8 m3(without the equipment). We designed a CO2 adsorption bed, power provided by the explosion-proof axial flow fan air circulation with its working principle shown in Figure 3.

M=

Q∫

tm

0

(C in − Cout)dt m1

(5)

To test the adsorption capacity of S-5 soda lime and compare it with two kinds of common medical soda lime of equal cost in the market. The experiment was carried out in the same way as before while keeping other conditions unchanged. 3.3.2. Multi-index Orthogonal Experiment for CO 2 Adsorption of Different Composition. The MOE was selected in this study to help analyze the performance (in terms of adsorption rate) of soda limes for CO2 adsorption of different compositions and determine the level of influence of each factor on soda lime in different working modes. Table 1 Table 1. Influencing Factors and Level Values for Soda Limes of Different Composition factors

Figure 3. Working principle diagram of the adsorption bed.

3.2. Sample Preparation. In order to guarantee the porosity and reduce dust, we determined to use dry pressing machine for modification of soda lime preparation after consulting agent processing factory, research company and referring to relevant papers and works, and the best tablet strength is 5−7 N.40 Additive should be added in preparation for bonding and stay hydrated. It does not react with alkali, and its mass fraction is commonly within 4−6%. Due to numerous advantages like being stable in performance, having a porous structure, no peculiar smell, and hardly any damage to humans, Al2O3 can be used as a soda-lime molding auxiliary additives, which can be used as aggregate to increase specific surface area, and be soluble in alkali solution.41,42 In order to improve the utilization rate, alkaline chromogenic agent was added for preparation, and the color of soda lime will change from red to white at the end of reaction, helping to decide when to replace the agents. We cooperated with Beijing Delley Soda lime Production Company and made a total of nine different combinations of soda lime with different mass fractions (NaOH, Al2O3, and H2O), through the three levels of orthogonal experiment, we number them from S-1 to S-9. 3.3. Experimental Model. 3.3.1. Adsorption Breakthrough Curve. Determination of Adsorption Capacity. The adsorption capacity is obtained by using adsorption breakthrough curve integral method, and the specific calculation formula is shown in formula 5. A test was conducted under atmospheric pressure with entry standard mixture, while CO2 volume fraction Cin is 45%. Absorbent cotton was loaded in the middle of U-shaped tube no. 7, and anhydrous calcium chloride was loaded in the second half, while the first half was loaded by tested sample with quality of m1 before the test. Test device was connected according to Figure 1. The researchers opened the valve switch and waved the gas with flow rate (Q) of 100 mL/ min through the reaction tube, taking 20 mL gas every 2 min at the end of the U tube no. 8. The CYES−II gas meter was used for detection of export CO2 volume fraction named Cout, and closed the valve when the volume fraction of Cout reached 45%

level

A Al2O3 mass fraction (w/%)

B H2O mass fraction (w/%)

C NaOH mass fraction (w/%)

1 2 3

4 6 8

9 12 15

4 5 6

summarized the influencing factors and level values selected for soda limes of different composition. The orthogonal array for the 9 experiments selected for this study, L9 (34), is shown in Table S1. The MOE considered three factors (mass fraction of NaOH, H2O, and Al2O3) and took adsorption rate as criteria to calculate, mainly depending on the compositions of soda limes in fixed molding process and under same experimental conditions. Determination of Adsorption Rate. An experiment was conducted under atmospheric pressure, while the adsorption bed was filled with 10 kg modified soda lime. The circulation fan was opened, the valve was switched on, the door was closed, and the gas was waved with a flow rate of 10 L/min with CO2 gas into the refuge chamber. Then, the cylinder was closed when the CO2 concentration reached 1%. The circulation fan was closed, and the air purifying device was opened to stabilized the CO2 concentration; then recorded the changes of CO2 concentration. Then we can calculate the average adsorption rate according to the adsorption time of CO2 concentration, which reduced from 1% to 0.3%.The CO2 adsorption rate was calculated from the experimental data using following equations:36,38 v=

0.7(V1 − V2) 100tr

(6)

3.3.3. Multi-index Orthogonal Experiments for CO2 Adsorption in Different Working Modes. The resistance of the fan was affected by thickness changes of the soda lime, and adsorption distances for CO2 through the soda lime are different; meanwhile, this also has an impact on contact time and adsorption efficiency for soda lime with CO2. The adsorption rate is different under different fan powers, and the noise decibels change. In terms of energy supply and physiological capacity of workers inside the chamber, the thickness filled in the absorption bed of modified soda lime and fan power should be considered in the process of practical C


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monitoring system was turned on, and S-5 soda lime with a thickness of 8 cm (about 10 kg) was added to the adsorption bed. The basic physiological parameters of experimental subjects were recorded before they entered the chamber. Four experimental subjects were allowed inside, and the door was closed if their data was normal. The change of CO2 concentration inside the chamber was observed by a monitoring system, the fan was turned on with rated power, and adsorption for CO2 began when the concentration increased by more than 0.8%. Then the purification fan was closed when the CO2 concentration reduced by 0.3%. Continuous measurement and records of each parameter were conducted at 1 h intervals until the end of the test. We ensured that the other test conditions were unchanged and replaced the type of soda lime. We observed the result of the four persons 1 h human survival experiment with three different types of soda limes (S-5 and two ordinary medical types) and the four persons 8 h human survival experiment with S-5 soda lime.

work, and we want to get a lower energy consumption with a higher efficiency combination of fan power and thickness of soda lime. To ensure the effectiveness and efficiency of the experiment, an orthogonal experiment was selected once again. Table 2 summarizes the influencing factors and level values selected for soda limes of different compositions. Table 2. Influencing Factors and Level Values for Soda Lime in Different Working Modes factors level

D the type of soda lime (no.)

E thickness of soda lime (cm)

F fan power (W)

1 2 3

S-5 S-6 S-7

6 8 10

20 25 30

The orthogonal array for the nine experiments selected for this study, L9 (34), is shown in Table S2. The MOE considers three factors (fan power, type, and thickness of soda lime) and takes adsorption rate as the criteria to calculate, ensuring that other experiment conditions are unchanged. 3.3.4. Single-Index Experiments with Performance of Modified Soda Lime in Different Temperature and Humidity. A large amount of experiments data show that the refuge chamber actually operates differently with change of temperature and humidity because of changes in respiration. When the temperature and humidity rise, common soda lime may appear to be bonding which will lead to a larger fan resistance, increasing amount of energy consumption, and slower adsorption efficiency and reaction rate. Therefore, adsorption properties of modified soda lime need to be determined under different temperature and humidity conditions; this experiment mainly analyzed the influence of environmental changes for S-5 soda lime with the best adsorption performance. According to the coal mine safety system recomentation of emergency management interim provisions for the internal temperature and humidity in emergency safety facilities from China’s work safety bureau, factors were selected in this experiment with five initial humidities (40%, 50%, 60%, 70%, and 80%) and five starting temperatures (24, 26, 28, 30, and 32 °C), while thickness of soda lime was selected with the best properties at 8 cm and 30 W of fan power; the determination method of adsorption rate experiment is the same as mentioned before. 3.3.5. Human Survival Experiment. The optimum composition and loading thickness of modified soda lime, running fan power of adsorption bed have been determined by the basic experiments mentioned above. To test the adsorption rate of S5 soda lime and compare with two kinds of common medical soda limes of equal cost in the market, we designed a four person 1 h human survival experiment. To determine service regulations of modified soda lime in applications for energy saving and environmental protection, a four persons 8 h human survival experiment was designed to obtain statistics of conditions during the survival period and to determine the rate of dust overflow at the same time. Mineral water and compressed biscuits enough for four people for 8 h were reserved in the refuge chamber. The oxygen cylinder and soda lime reserves were checked, and instruments were monitored. Under room temperature (24 °C), the GD 7 type sensor was opened, the purification fan power was adjusted to 30 W, and the fan was closed temporarily. The KJ70

4. RESULTS AND DISSCUSSION 4.1. Adsorption Capacity Analysis. Figure 4 shows breakthrough curves of modified soda lime from NO.S-1 to S-9,

Figure 4. Adsorption breakthrough curve of modified soda lime from nos. S-1 to S-9.

and the adsorption capacity for CO2 of each soda lime can be obtained by integration. We can calculate soda lime adsorption capacity with the volume of CO2 in one unit of mass, shown in Table 3. The adsorption capacity for CO2 of the S-5 soda lime with the same quality is the largest (Table 3). Therefore, the Table 3. Soda Lime Adsorption Capacity with the Volume of CO2 in Unit Mass

D

soda lime no.

mass of soda lime m1/g

CO2 volume L/kg soda lime

S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 O-1 O-2

4.7558 5.2180 4.4345 4.8940 5.0619 5.5215 5.1428 4.9826 5.2049 5.2860 5.1246

165.84 98.45 185.59 155.46 200.96 153.57 177.48 162.35 153.24 156.47 149.38 DOI: 10.1021/acs.iecr.6b02314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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different mass fraction of additives have different porosities and can be used as aggregate to increase the specific surface area, but its influence is weaker than that of concentration of H2O and NaOH. The adsorption rate order of different types of soda lime is S5 > S-6 > S-4 > S-7 > S-9 > S-8 > S-3 > S-1 > S-2, known from the analysis of results in Figure 6, while the adsorption capacity order of different types of soda lime is S-5 > S-3 > S-7 > S-1 > S-4 > S-6 > S-9 > S-2 as shown in Table 3. Adsorption properties depends on the two aspects of adsorption rate and adsorption capacity. We should choose three types of soda-lime (S-5, S-6, S-7) combinations to join the L9 (34) standard orthogonal experiment in different working modes, to balance the best conditions of two kinds of influencing factors. Figure 7 shows adsorption curve of modified soda lime in different combinations from W-1 to W-9, calculated in Table

dosage of S-5 soda lime is the smallest with the same number of rescue subjects. The adsorption capacity analysis of soda lime (no. O-1 and O-2) was obtained using the adsorption breakthrough curve integral method, and the specific calculation formula is shown in Figure 5 compared with S-5. The adsorption capacity for

Figure 5. Adsorption breakthrough curve of modified soda lime for nos. O-1 and O-2 compared with no. S-5.

CO2 of S-5 soda-lime (200.96 L/kg) is larger than those of O-1 (156.47 L/kg) and O-2 (149.38 L/kg). We can obtain that its adsorption capacity increased 28% and 30% compared with common medical soda lime. 4.2. Multi-index Orthogonal Experiment Analysis. Figure 6 shows adsorption curves of modified soda lime from Figure 7. Adsorption curve of modified soda lime in different combinations from W-1 to W-9.

S4. The results in Table S4 suggest that the order of influence for CO2 adsorption rate with different combinations is F > E > D. Optimal levels of each factor in turn is F3E3D3, actually the optimal solution is F3E3D1 (no. W-3) according to test analysis. It is gas−solid phase chemical reactions with soda lime and CO2, and the average adsorption performance is associated with air velocity, bed porosity, and concentration of adsorption properties learned by gas mass conservation equation in adsorption bed only considering the flow and mass transfer on the axial direction of air flow. The results proved that the fan power plays a more important role in working modes than thickness and types of soda lime. The different power produced different air velocity and affect the contact time and possibility between soda lime and CO2, which is the key factor of practical reaction process. The resistance of adsorption bed was affected by thickness change of soda lime, adsorption distance for CO2 through the soda lime is different meanwhile, taking an impact on contact time and adsorption efficiency for soda-lime with CO2, but its influence is weaker than fan power, while the influence of chemical compositions (types of soda lime) is the weakest in practical application. 4.3. Single-Index Experiment Analysis. Figure 8 shows adsorption curve of modified soda-lime in different temperature from 24 to 32 °C.When the temperature is 28 °C within the chamber, CO2 adsorption effect rate is the highest, proving that increase in temperature can improve reaction rate to a certain extent. With the increase in temperature (over 28 °C), adsorption rate gradually reduced, while adsorption rate

Figure 6. Adsorption curve of modified soda lime from nos. S-1 to S-9.

nos. S-1 to S-9, calculated in Table S3. The results in Table S3 suggest that the order of influence for CO2 adsorption rate of different composition is C > B > A. Optimal levels of each factor in turn is C3B3A3, while actually the optimal solution is C3B1A3 (no. S-7) according to test analysis. On the basis of reaction mechanism, the different surface area and different composition of reactants will affect the reaction rate. The result proved that the mass fraction of H2O in the soda lime plays an important role for CO2 adsorption performance, while mass fraction of NaOH is more important as a catalyst. The process of CO2 dissolving in H2O is slower in reaction and is the key step of whole reaction, but too much water will affect the positive response. Meanwhile NaOH can be used repeatedly to accelerate the reaction as catalyst. It was demonstrated that E


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4.4. Four Persons 8 and 1 h Survival Experiments. Figure 10 compared the adsorption properties of S-5 modified

Figure 8. Adsorption curve of modified soda lime in different temperatures from 24 to 32 °C in the best working modes (25 W fan power, 8 cm thickness).

reduces by 50% when temperature reaches 30 °C, proving that the positive reaction rate reduces when the temperature is too high since the reaction is exothermic. It can also be resulted from the Figure 9, when the temperature is between 26 and 28

Figure 10. Adsorption curve of modified soda lime S-5 and two kinds of medical soda lime (named no. O-1 and O-2) upon exposure to the respiration of four people over 1 h.

soda lime with two kinds of common medical soda lime (named no. O-1 and O-2). We found that the adsorption rate of S-5 (10.8 L/min) is higher than that of O-1 (7.3 L/min) and O-2 (6.9 L/min), achieving the same adsorption result with runtime reducing by 32.2%. Figure 11 shows the adsorption curve of S-5 modified soda lime in the best working mode (25 W fan power, 8 cm

Figure 9. Adsorption curve of modified soda lime in different humilities from 40% to 85% in the best working modes (25 W fan power, 8 cm thickness).

°C, adsorption rate is less than 30% of the highest. However, the change is not obvious, still meets the standard of coal mine safety system construction of emergency management interim provisions of the adsorption rate. Figure 9 shows the adsorption curve of modified soda lime of different humidities from 40% to 85%. When the humidity is 50% inside the chamber, the CO2 adsorption rate is the highest, proving that water regulation can improve reaction rate to a certain extent. With the increase of humidity (over 50%), adsorption rate gradually reduced. The adsorption rate reduces by 50% when humidity reaches 80%, proving that the positive reaction rate reduces when moisture is too high. It can also be observed that when a large amount of moisture is absorbed on the surface, soda lime bonded together, affecting the adsorption rate and increasing surface resistance. It can also be noticed in Figure 8, that when humidity is between 60% and 70%, the adsorption rate is less than 50% of the highest, but the change is not obvious, and it still meets the standard of the coal mine safety system construction of emergency management interim provisions of the adsorption rate. Due to its limited space in the chamber, the ice storage of air conditioning should be as little as possible, while humidity should be controlled at less than 70%, keeping the adsorption rate at a high level.

Figure 11. Adsorption curve of modified soda lime S-5 in the best working modes (25 W fan power, 8 cm thickness) upon exposure to the respiration of four people over 8 h.

thickness) upon exposure to the respiration of four people over 8 h; 10 kg soda lime can maintain a survivable environment for four people for 504 min (8.4 h). The CO2 expiratory rate and adsorption rate were calculated and are shown in Table 4. In the 8 h survival test, the soda lime adsorption device operated 7 times, the adsorption process was relatively stable, and the average adsorption rate reached 1.34 L/min. We found that the purification fan was at work 40% of the time and that it can meet the requirements of CO2 adsorption. Intermittent operation mode (IOM) can be used with air purifier in the chamber to improve the efficiency of air purification and reservation of power. To achieve energy saving, the purifier begins to run when the CO2 concentration reaches F


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Table 4. Data of CO2 Concentration Change of Four Persons 8 h Survival Test with the S-5 Soda Lime in the Best Working Mode (25 W Fan Power, 8 cm thickness) time/min

CO2 concentration/%

CO2 concentration capacity/%

time usage/min

CO2 expiratory rate or adsorption rate/L/min

8−43 43−68 68−128 128−163 163−181 181−236 236−255 255−313 312.340 340−388 388−420 420−466 466−495 average

0.82−0.32 0.32−0.75 0.75−0.45 0.45−0.78 0.78−0.30 0.30−0.79 0.79−0.31 0.31−0.79 0.79−0.32 0.32−0.79 0.79−0.30 0.30−0.81 0.81−0.41

0.50 0.43 0.30 0.33 0.48 0.49 0.48 0.48 0.47 0.47 0.49 0.51 0.40

35 25 60 35 18 55 19 58 27 48 32 46 29

1.14 (adsorption rate) 0.34 (expiratory rate) 0.4 (adsorption rate) 0.19 (expiratory rate) 2.13 (adsorption rate) 0.18 (expiratory rate) 2.02 (adsorption rate) 0.17 (expiratory rate) 1.39 (adsorption rate) 0.2 (expiratory rate) 1.23 (adsorption rate) 0.22 (expiratory rate) 1.1 (adsorption rate) 0.22 (expiratory rate) 1.34 (adsorption rate)

adsorption rate of different compositions and working modes were designed to achieve the best combinations of modified soda lime in practical application. The results were utilized to find the optimum operating conditions and analyze the relationship between influential parameters. The order of influence for CO2 adsorption rate of different composition (mass fraction) of modified soda lime is NaOH > H2O > Al2O3, and the order of influence for CO2 adsorption rate with different working modes is fan power > thickness > type of soda lime. A single-index experiment was designed to achieve the adsorption properties of S-5 modified soda lime under different temperatures and humidity. Due to the limited space in the chamber, the ice storage of air conditioning should be as little as possible, while the humidity should be controlled at less than 70%, the temperature, at less than 30 °C, and the adsorption rate, at a high level. Through the four person 1 and 8 h survival experiment, we found that the adsorption rate of S-5 (10.8 L/ min) is higher than that of O-1 (7.3 L/min) and O-2 (6.9 L/ min), achieving the same adsorption result with a runtime reduction of 32.2%. Intermittent operation mode can be used with the air purifier in the chamber in order to improve the efficiency of air purification and reservation of power. The power consumption reduced by 33.3%, and the dust outflow reduced by about 50%. We can get a cost saving of $166 and volume occupied saving of 50 L in reservations of modified soda lime according to the level of eight people, and the time for soda lime replacement reduced by 37.5%. The results of this paper can provide technical support for CO2 adsorption technology in the refuge chamber space and other confined working spaces with need to control the concentration of carbon dioxide such as indoor museums and realize energy savings.

0.8%, stops when the CO2 concentration reduces to 0.3%, and then run after the CO2 concentration reaches the 0.8% ceiling again. After the experiment, the subjects were physically examined, and there were no abnormalities. According to the survey of the experimental subjects in the chamber, the observed dust outflow of the modified soda lime was smaller than those of the medical O-1 and O-2 soda lime. A layer of filter paper was added to the adsorption bed, and residual dust was collected on the purifying device and chamber ground; the amount of dust has reduced by 50% compared with medical soda lime in the experiment. The benefit of evaluation of modified soda lime and its working modes are calculated as follows: the power consumption is reduced by 33.3%, and the dust outflow is reduced by about 50% without peculiar smell in the confined chamber. We can get a cost saving of $166 and volume occupied saving of 50 L in the reservoir of modified soda lime according to the level of eight subjects, and the number of soda lime replacement reduced by 37.5%. According to China’s national standard requirements, to ensure the survival of eight people for 96 h in a refuge chamber (total volume 13 m3 including equipment, food, soda lime, etc.), we should store 276 kg of soda lime (S-5), use 2.4 kW h of fan power, of which the usage coefficient is 1.2 (considering the corresponding loss coefficient to ensure safety in practical use, which is 1.2 according to China’s standard). We can also design a kit according to the experimental data, sized appropriately for the purifier which can be installed directly.

5. CONCLUSION In this paper, we are committed to developing a novel and modified soda lime with higher adsorption rate and low rate of dust outflow by analyzing its reaction mechanism and adsorption factors to determine the adsorption activity. Adsorption properties depend on the two evaluating indicators of adsorption rate and adsorption capacity. The adsorption capacities of modified soda-lime from nos. S-1 to S-9 were calculated by adsorption breakthrough curve experiments, while S-5 was the highest with an increase in adsorption capacity of 28% and 30% compared with medical soda lime. The multiindex orthogonal design method was utilized to aid the design of the experiments and select representative parameter combinations to replace the full factorial experiments. Two L9 (34) standard orthogonal experiments for soda lime

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02314. Introduction of the multi-index orthogonal experiment, L9 (34) standard orthogonal experiment plan, and results and analysis for soda limes of different compositions and different working modes (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 13426214037. E-mail addresses: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper is partially supported by National Natural Science Foundation of China, grant nos. 91324017, 71603017, China Postdoctoral Science Foundation, grant nos. 2016M591081, and National Science and Technology Support Program of China, grant nos. 2012BAK03B05, 2012BAK20B02.

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NOMENCLATURE M = adsorption capacity (L/kg) Q = flow rate (mL/min) tm = breakthrough time of soda lime adsorption for CO2 (min) Cout = export CO2 volume fraction (%) Cin = entry CO2 volume fraction (%) m1 = equal of modified soda lime sample in breakthrough experiment (kg) v = average rate of adsorption (L/min) V1 = volume of chamber (13 m3) V2 = volume of equipment in chamber (about 5 m3) tr = adsorption time of soda lime (min) Ki = sum of adsorption rate data with the level of i R = difference between maximum and minimum values T = total sum of adsorption rate data

Abbreviations

MOE = multi-index orthogonal experiment ABC = adsorption breakthrough curve GD7 = mine multiparameter sensors KJ70 = coal mine safety production monitoring system S-1−S-9 = species of modified soda lime W-1−W-9 = combinations of modified soda lime with different working modes IOM = intermittent operation mode

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Adsorption Properties of Modified Soda Lime for Carbon Dioxide

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