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Experimental optimization of a spray tower for ammonia removal
Atmospheric Pollution Research 9 (2018) 783–790
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Original Article
Experimental optimization of a spray tower for ammonia removal a
b
a,∗
T a
Mohammad Javad Jafari , Amir Hossein Matin , Alireza Rahmati , Mansour Rezazadeh Azari , Leila Omidic, Seyed Saeed Hosseinid, Davood Panahie a
Department of Occupational Health Engineering, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Occupational Health Engineering, School of Health, Safety and Environment, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Occupational Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran d Department of Electrical Engineering, Tehran-North Branch, Islamic Azad University, Tehran, Iran e Department of Occupational Health Engineering, Abadan School of Medical Sciences, Abadan, Iran b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Spray nozzle Spray tower Operational pressure Ammonia
Spray tower scrubbers ordinarily have low air resistance and gas removal efficiency. Although packed-bed wet scrubbers are efficient in gaseous contaminants treatment, a significant limitation of packed-bed wet scrubbers is that they have high pressure drop and primary costs. Appropriate features of the nozzle play an important role in system cost, efficiency, low operational costs, and optimization of spray towers. Operating pressure, nozzles size, and number of nozzles could increase mass transfer and removal efficiency and decrease investment and save operational costs. The objective of the present study was to develop a spray tower through optimization of the design and operating parameters for removal ammonia emissions from the air streams. Spray tower design parameters included nozzle type, number of stages of spray nozzle, and operating parameters such as operating pressure and inlet NH3 concentration. Among the studied parameters, only increasing ammonia concentration was in inverse proportion to the spray tower efficiency. The spray tower was optimized as equipped with an 8010SS spray nozzle with three stages working together, spraying 0.01% H2SO4 scrubbing liquid counter-current to the air stream with operating pressure of 12 bars and inlet NH3 concentration of 24.1 ppm. The highest removal efficiency was 97.92% at an 8010SS spray nozzle with three stages working together, H2SO4 solution, pressure 12 bars and inlet ammonia concentration of 24.1 ppm. The results of this study demonstrated that caustic spray tower could be a very effective technology for removal of NH3 from air stream.
1. Introduction
et al., 2009). According to USEPA, exposure to ammonia may increase in the future and there is a need to identify sources of human exposure to implement control measures (ATSDR, 2004; EPA, 1995). The mitigation of NH3 emissions in air stream is an important issue for protection of human health and the environment (Hadlocon et al., 2014b; USEPA, 2002). Some researchers used biological treatment as an effective and economical tool for the biotreatment of waste gas streams with low concentrations in large amounts of air. High water-holding capacity, good airflow characteristics, high pH buffer capacity, and good mechanical properties are some attributes of a good bioreactor medium (Chung et al., 2005; Leson and Winer, 1991). The initial medium was made from soils, however, their tendencies to short-circuit and clog are some major drawbacks of soil beds (Leson and Winer, 1991). Chung et al. (2005) used biotrickling filters by biological activated carbon for removal of high concentration of NH3 and coexistent H2S. Their
Ammonia (NH3) is a colorless and very irritant gas, which is emitted from animal husbandry and some industrial processes. Numerous sources have been considered as ammonia emitters such as the fertilizer industry, coke manufacture, fossil fuel combustion, livestock management, and refrigeration methods. Livestock waste management and fertilizer production are responsible for emitting 90% of total ammonia (EPA, 1995). Ammonia may have some impacts concerning environmental and human health (Hadlocon et al., 2014a). The main risk of ammonia in high concentrations is explosion. United States Environmental Protection Agency (USEPA) has classified ammonia in national priorities list. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Emergency Planning and Community Right-to-Know Act (EPCRA) establish requirements for reporting any releases of NH3 exceeding 100 lbs (USEPA, 2009; Zhao
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail addresses: [email protected] (M.J. Jafari), [email protected] (A.H. Matin), [email protected] (A. Rahmati), [email protected] (M.R. Azari), [email protected] (L. Omidi), [email protected] (S.S. Hosseini), [email protected] (D. Panahi). https://doi.org/10.1016/j.apr.2018.01.014 Received 8 September 2017; Received in revised form 17 December 2017; Accepted 19 January 2018 Available online 09 February 2018 1309-1042/ © 2018 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
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findings indicated that physical adsorption of ammonia gas by granular activated carbon was responsible for the first 28 days of experiment, and then biodegradation by inoculated microorganisms was responsible for other days of the experiment. The critical ammonia loading was 4.2 g-N/m3/h, and the maximal loading was 16.2 g-N/m3/h. Low molecular weight gases, highly soluble compounds, and simple chemical structures are attractive targets for removal by biotrickling method. Wet scrubbers have been prevalently used for air pollution process with significantly reduced risks for human health (Lee et al., 2008). Spray towers as one of the popular wet scrubbers that are widely used in the air pollution control and treatment technology such as CO2 removing, desulfuration, and denitrification have been used to eliminate air pollution before releasing into the atmosphere (Chen, 2004; Li and Zhao, 2012). Design parameters of spray towers can affect the air pollutants removal efficiency (Yincheng et al., 2011; Zhang, 2005). Many researchers have investigated the effects of design parameters on the increases of overall removal efficiency of spray towers (Codolo and Bizzo, 2013; Hadlocon et al., 2014a; Kuntz and Aroonwilas, 2009; Manuzon et al., 2007; Sharma and Mehta, 1970). Researchers studied the effects of droplet size and multi-stage spray nozzles on spray tower efficiency (Hadlocon et al., 2014a; Koller et al., 2011). The results of a study by Kuntz and Aroonwilas (2009) also indicated the influences of the number and droplet size on CO2 removal efficiency. In another study, researchers have shown the effects of dynamic behavior on liquid droplets and gas mass transfer operation in spray scrubbers. Among the different design parameters such as liquid to gas ratio (L/G), spray nozzles, droplet size distributions, arrangement of nozzles, gas flow rate, and liquid pressure that affect the towers performance (Bozorgi et al., 2006; Kuntz and Aroonwilas, 2009), spray nozzle, operating pressure, and velocity are more important (Zhang, 2005). Nozzle type, nozzle position, droplet size, and liquid to gas ratio are other effective parameters related to increasing the removal efficiency in spray towers. Appropriate spray nozzle can significantly lead to lower liquid rate and energy consumption (Ebert and Büttner, 1996). Liquid droplet size, liquid flow rate, operating pressure, atomization positions, and liquid distribution can be obtained by selecting appropriate nozzles and are also impressed by the specification of spray nozzles such as the type of nozzle, orifice diameter, and nominal cone angle. The most impressive zone for the mass transfer is observed to be in the proximity of the spray nozzle (Daisey et al., 2003; Javed et al., 2006, 2010; Yeh and Rochelle, 2003; Zhang, 2005). The operating pressure of scrubbing liquid is an important parameter, because it directly affects the droplet size, liquid distribution, and liquid flow rate. The droplet size is mainly affected by the nozzle types, which supply a proper balance of liquid flow rate and operating pressure. Increase in the pressure may enhance the ratio of liquid flow rate to droplet size and this leads to higher efficiency of spray tower by using all types of nozzles (Hadlocon et al., 2014a). Liquid pressure consequently is affected by the number of droplets and droplet velocity. Removal efficiency depends on these factors (Codolo and Bizzo, 2013). Then nozzle velocity has a very important role on removal efficiency (Bozorgi et al., 2006; Ebert and Büttner, 1996). Nozzle plays an important role in the system cost, efficiency, low liquid consumption, and optimization of the spray tower (Ebert and Büttner, 1996; Lee et al., 2008; Yincheng et al., 2011). The findings of some studies have shown that operating pressure, nozzle size, and multi-stage spray nozzle could increase mass transfer and removal efficiency and decrease and save operational costs. The intense contact between the scrubbing liquid and the polluted gas can be used to optimize the performance of spray towers (Bandyopadhyay and Biswas, 2008; Codolo and Bizzo, 2013; Koller et al., 2011; Kuntz and Aroonwilas, 2009; Yeh and Rochelle, 2003). According to results of some studies, caustic spray scrubber can cause high ammonia removal (Hadlocon et al., 2014a; Hahne et al., 2005; Manuzon et al., 2007). Although the number of nozzles affects removal efficiency of a spray tower, some disadvantages have been reported by some authors when
using multiple nozzles. Using multiple nozzles could lead to increasing air pressure drop and more liquid rate compared with using single spray nozzle (Ebert and Büttner, 1996; Koller et al., 2011). There are also some drawbacks related to increasing plugging and maintenance cost. Although large nozzle has low practical operating pressure loss (Ebert and Büttner, 1996), the larger nozzle size can lead to more liquid consumption, large droplet size, less contact between the scrubbing liquid and the polluted gas in the surface area, and low collection efficiency (Codolo and Bizzo, 2013; Koller et al., 2011; Kuntz and Aroonwilas, 2009). The spray nozzle can provide a large droplet surface area in a given liquid volume, causing more contact with gas and droplets and improving absorptive capacity of the scrubbing liquid. Small size spray nozzle is more disposed to plug if scrubbing liquid contains suspend aerosol. In this case, expensive and more frequent maintenance is needed. Small size spray nozzles produce larger liquid surface area, but have higher pressure loss and if the velocity of gas increases from recommended design level, scrubbing liquid may leave the scrubber requiring demister (ACGIH, 2013; Codolo and Bizzo, 2013; Ebert and Büttner, 1996; Keshavarz et al., 2008; Kim and Kim, 1997; Koller et al., 2011; Kuntz and Aroonwilas, 2009). High pressure spray nozzle leads to uniform liquid distribution, an increase in contact surface, better collection efficiency, as well as in the short retention time available. More pressure drop is expected when nozzle pressure is increased leading to an increase in the liquid volumetric flow rate and more liquid consumption (Ebert and Büttner, 1996; Lee et al., 2008). The present study conducted in 2015 aimed to optimize an experimental spray tower for NH3 removal with a constant airflow. The specific objectives of this research were: (1) to optimize some design parameters, including the selection of the best spray nozzle size and operating liquid pressure, different spray nozzle stages, and improving collection efficiency; (2) to evaluate the effects of operating parameters on removal performance (inlet NH3 concentrations and the number of spray nozzle stages); (3) to quantify the performance of optimized spray tower for exhaust air stream with both low and high ammonia concentrations and compare removal efficiency with both scrubber liquids including caustic scrubbing solution and water. 2. Material and method 2.1. Ammonia removal In a spray tower, NH3 stream reacts with scrubber liquid (water or dilute acid) droplets. Spray nozzles are used to generate droplets. The greater the surface area for chemical absorption, the higher the collection efficiency can be achieved. Although the majority of large droplets move down against the airflow, some smaller droplets can enter the fan through air flowing up. Scrubbing liquid is collected in a tank and recirculated. The equilibrium reactions for NH3 solubility in water and caustic scrubbing liquids are (Melse and Ogink, 2005; Swartz et al., 1999).
H NH3 (g ) ⇔ NH3 (eq)
(1)
K ′fKr′ + + NH3 + Heq ⇐⇒ = NH4( ′ ) eq) (K eq
(2)
Equation (1) describes the solubility of NH3 in water, where H is the Henry's law constant, estimated to be 5.33 × 101 M atm−1 at ′ ) can be derived as the re298.15 °K. The equilibrium constant (K eq ciprocal of the acid dissociation constant of NH+ 4 and has a value of 1.78 × 109 at 25 °C (Perrin, 1969). The overall solubility can be expressed in terms of the effective Henry's law constant (H*, M atm−1), as represented by the sum of the dissolved NH3 (aq) and protonated NH+ 4 (aq), where pNH3 is the partial pressure of NH3 (atm), T is the air temperature (°K), and R is the gas constant (atm M−1 K−1) (Calvert and Englund, 1984). Researchers developed a performance models for gas absorption in 784
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laboratory scale constructed from iron black was used to conduct the experiments. It consisted of five sections: an inlet, scrubber column, spraying system, outlet and instruments. The internal diameter and height of tower were 25 and 138 cm, respectively, and the vertical scrubber column enclosed spray nozzles and liquid pipes. The spray tower contained a maximum of three stages of spray nozzles, which were spaced 38 cm apart. The position of the nozzle in tower, as well as the space between nozzles, was designed to be at least 30.5 cm, which is the maximum height related to the 80 μm nozzle used in this study. The spraying system delivered a prepared solution of dilute sulfuric acid (H2SO4) into each nozzle stage from a 60 L feed Polyethylene tank through a magnetically driven pump with a rated pressure range of 0–12 bars. A pressure relief valve was used to regulate the pressure and liquid flow rate supplied to the tank. The liquid droplets of the known concentration of H2SO4 interacted with the NH3 inside the scrubber column. The entire scrubber system was installed with appropriate instrumentations to monitor pH and NH3 concentration. By installing a valve for each nozzle, the application of each nozzle was adjusted by the user. The spray tower consisted of three nozzles, three valves, a centrifugal pump, the chemical additive reservoir, and gauge pressure. The study spray tower and experimental apparatus are shown in Fig. 1. Airflow velocity in spray towers was between 200 and 300 feet per minute (fpm). The selected velocity was 250 fpm (1.27 m/ s), in which no droplet in fan housing or stack was detected. The most popular size of spray nozzles ranged from 20 to 100 μm which could provide large droplet surface area (ACGIH, 2013).
single-stage scrubber (Eq. (3)) (Calvert and Englund, 1984).
η=1−
(1 − (
6RTK Z exp ⎡ Δ uDG 3 N 2
⎣
L G
) − H )⎤ − H ⎦
G H∗ L
∗G L
(3)
where, η is the collection efficiency, H ∗, G , and L are Henry's law constant, the moles of air per unit time and unit tower cross-section (mol air s−1 m−2), and the molar flow rate of the NH3-free liquid stream per unit cross-sectional area of the tower (mol water air s−1 m−2), respectively. R , T , K G , and ZN are the universal gas constant (8.314 J mol−1 K−1), absolute air temperature (°K), the individual mass transfer coefficient of NH3 in air (moles NH3 s−1 m−2 Pa−1), and the column length for liquid-gas contact (m), respectively. Δu and D23 are the relative velocity between the droplets and the airflow (m s−1), and the Sauter mean diameter of spray droplets (m), respectively. A performance models for multi-stage scrubbers can be estimated according to Eq. (4). This model is based on the single-stage efficiency by considering the concept of penetration for control devices in series. The predicted efficiency for n-stage scrubbing can be obtained by assuming no strong interaction among liquid droplets between stages to cause variation in the scrubbing efficiency for each stage.
ηoverall = 1 − (1 − η1 − stage )n
(4)
where, ηoverall , n , and η1 − stage are the total collection efficiency of the n stages, the number of stages, and the scrubber efficiency of one stage, respectively (De Nevers, 2000; Hadlocon et al., 2014a). Multi-stage scrubbing is suggested to have higher removal efficiency for ammonia by promoting a higher liquid-to-gas ratio (Hadlocon et al., 2014a). Absorption is probably the main mechanism considered to remove gases from air stream. Molar ratio of sorbent to adsorb is an important factor (Kim and Kim, 1997).
2.4. Ammonia supply A 45 kg gas cylinder, a crown model stainless steel ammonia regulator and a flow meter were used to supply ammonia laden air. A constant inlet ammonia concentration was adjusted at desired levels in each sampling. Ammonia was injected to the tower through a flexible polyethylene tube and was blended with air prior to its' injection. Few pre-tests were carried out to identify probable problems after setting up. The inlet concentration of ammonia ranged from 24.1 to 68 part-permillion (ppm).
2.2. Nozzle selection and operating conditions The spray nozzles used for wet scrubbers can be characterized based on their droplet size, droplet concentration, dispersion, initial velocity, spray angle, and spray pattern. The total surface area is determined by the concentration of droplets inside the scrubber column. The selection of appropriate spray nozzles or atomizers is critical for the optimization of spray scrubber performance (Lefebvre, 1989). The droplet size generated by the atomizers has a significant impact on the surface area needed to enhance gas-liquid contact for the chemical absorption process. Proper spray dispersion leads to perfect mixing of the liquid with the surrounding gas, which promotes the contact. Hollow-cone nozzles generate droplets that are generally much smaller compared to a fullcone nozzle, but hollow-cone nozzles need additional flow to meet the same performance as the full-cone nozzles (Manuzon et al., 2007). In this study three full-cone orifice (BERTO Model 20SS, BERTO Model 8010SS (Danfoss), and a 2500 μm spray nozzle (made in Iran)) sizes were used in different NH3 concentrations, operating pressure and stages. Studies conducted by other researchers show that, there is much preference for using a full-cone nozzle over a hollow-cone nozzle, but the practical significance is associated with the bigger orifice size of a 2500-μm nozzle, which is beneficial to reduce nozzle clogging (Hadlocon et al., 2014a; Hahne et al., 2005; Manuzon et al., 2007). Three types of solid cone spray nozzle including 20SS, 8010SS (Berto) and 2500-μm nozzle (with three sizes of 20, 80 and 2500 μm, respectively), that constructed 27 cm diameter of the spray cone were used. Nozzles were located at three heights of 62.5, 87.5 and 112.5 cm from inlet of spray tower. The orifices of each spray nozzle were located at center of the tower.
2.5. Air sampling and ammonia measurement Ammonia concentrations were measured using an Ion Science Phocheck Tiger at the inlet and outlet ports of the tower in each sampling (Cordeiro et al., 2005; Dooly et al., 2011). The measuring range of Phocheck was 0.1–5000 ppm. The accuracy of the device was ± 1 ppm. Tedlar bags containing contamination-free air were used to calibrate the instrument response (Zarei et al., 2017). Atmospheric stock gas standards were transferred into bags and filling them with contamination-free air with the proper conditions for flow and time parameters. The Phocheck was calibrated for 1–120 ppm of ammonia in air (R2 = 0.997). Ammonia removal efficiency (% eff.) was calculated based on Eq. (5). (Jafari et al., 2014).
Efficiency (%) =
CNH 3in − CNH 3out × 100 CNH 3in
(5)
Also, the visible absorption spectrophotometry method (NIOSH 6015) was applied to measure the inlet and outlet concentrations of ammonia in air. For this purpose, 10 ml solution was added to the spectrophotometer cuvette and was set at 630 or 660 nm wavelengths. A total of 270 samples were collected. Solid sorbent tubes including sulfuric acid-treated silica gel were used for ammonia sampling. All safety aspects of working with chemicals were considered according to materials safety data sheet. The sampling pump (SKC) was calibrated with electronic calibrator (Defender 510-M). Samples were taken at an accurately known flow rate between 0.1 and 0.2 L min−1 with SKC air sampling pump. Standard laboratory equipment was applied for a total sample size of 0.1–96 L. After sampling at inlet and outlet of spray
2.3. Spray tower scrubber A spray tower equipped with a three-stage spray nozzle at 785
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Fig. 1. Experimental apparatus. −1 and flow rate of 3737.82 L min−1. The feed pump was operated at a nozzle pressure of 9–12 bars for tests. The inlet NH3 concentrations were set to 24.1, 52 and 68 ppm. All experiments were conducted in the laboratory at room air temperature of 23.5 °C–28 °C and relative humidity of 45%–53%. Experiments were more economical in this study because the interaction effects of the experimental factors were relatively tiny, compared to the main factor effects (Darake et al., 2016; Montgomery, 2017). Although designed experiments such as completely or semi factorial experiments are preferable to obtain comprehensive effects of factors and their interactions, in this study screening factors and separating their practical and optimum settings were applied for high tower performance. The summary of condition for experiments conducted in this study is listed in Table 1.
tower at the same time, the front and back sections of sulfuric acidtreated silica gel in each tube was transferred into separate 80- mL vials. To separate all of the reagents, ammonia-free deionized water was used in all laboratory analyses. The sodium hydroxide was used to adjust pH at 5–6.5 (Eller and Cassinelli, 1994). Each measurement was conducted according to NIOSH method 6015. 2.6. Air flow rate and head loss Airflow rate required for experiments was 3737.82 L min−1 based on the tower diameter and recommended ACGIH velocity (200–300 fpm) (ACGIH, 2013). The airflow rates were supplied by a variable flow rate fan (HVDLT-MK2, manufactured by UK air flow Co). A low pressure venturi (H type) was used to measure airflow rate. The measuring accuracy of venturi was ± 4–5%. A digital anemometer was used to monitor airflow during each test. A monometer (UK Type 504) with the measurement range of 0–5000 pa and with an accuracy of ± 10 pa was used to measure the head losses.
2.8. Data analysis The collected data were analyzed, using general descriptive statistical analysis. To study the effects of the parameters, three replicate runs were performed for each test. The data were analyzed with SPSS 16.0 software using analysis of variance (ANOVA), t-test for paired comparisons. Linear regression analysis was applied to fit the model.
2.7. Scrubbing liquid flow rate and liquid pressure The scrubbing solution was supplied using a CDLF2-13-M multistage stainless steel pump. Full fresh caustic scrubbing liquid including 0.01 M of H2SO4 was supplied through a 60-L poly ethylene reservoir for all 486 samples. Acidity of the caustic scrubbing solution was monitored through measuring of its' pH. For this purpose, a pH controller and transmitter (model Sartorius Basic Meter PB-11) with a range of 2–16 pH and an accuracy of 0.01 PH was applied. Liquid samples were also brought to the laboratory for secondary pH measurement to ensure accuracy. Liquid pressure and flow rate were consistently monitored. Liquid flow rate was monitored using a polysulfone flow meter with 2% F.S accuracy. The maximum capacity of the pump was 20 L min−1. The optimum volume of injected liquid was based on the exhaust gas flow rate defined in literature (ACGIH, 2013; Keshavarz et al., 2008). The gas-to-liquid ratio (G/L) was 747.56 based on recommendations for spray tower (Jafari et al., 2014). The liquid pressure was measured with a gauge pressure (TG model) that was installed after pump in the pressure range of 0–16 bars. The spray tower was operated at a constant air velocity of 1.27 m s
3. Results In total 486 tests in three series, including three spray nozzle sizes installed in three stages with three operating pressures and three Table 1 Condition for experiments.
786
Variables
Values and types
Inlet NH3 concentration (ppm) Number of spray stages Nozzle size (micron) Liquid pressure (bar) Types of scrubbing liquid L/G ratio
24.1, 52, 68 1, 2, 3 20, 80, 2500 9, 10, 12 Caustic, Water 0.001
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concentration ranges were carried out with fresh water as the scrubbing liquid. The same tests (486 tests) were carried out with 0.01 M solution of H2SO4 as scrubbing liquid. Ammonia average concentration in the inlet of tower was 24.1, 52 and 68 ppm. Among 270 tests using 8010SS spray nozzle installed in three stages with fresh scrubbing solution, high removal efficiency of ammonia gas was achieved. The best results in the same conditions were related to concentrations with 0.01 M of H2SO4 as the caustic scrubbing liquid. The small size of spray nozzles (8010SS) had higher efficiency. This was true for both types of scrubbing liquid. The results from 54 tests carried out with a 20SS spray nozzle at one stage revealed that the maximum removal efficiency of ammonia gas scrubbed with water and 0.01 M solution of H2SO4 were 86.72 and 93.77%, respectively. The minimum removal efficiencies for these cases were 51.47 and 80.14%, respectively. The maximum removal efficiencies for 54 tests conducted with a 20SS spray nozzle applied in two stages and two scrubbing solutions including water and 0.01 mol H2SO4 were 87.55, 94.20%, respectively. The minimum efficiencies for the same cases were 54.11 and 83.38%, respectively. The maximum removal efficiencies for 54 tests carried out with a 20SS spray nozzle applied in three stages using water and caustic scrubbing solutionswere 88.38%, and 95.85%, respectively. The minimum removal efficiencies for the same cases were 57.2% and 87.8%, respectively. See Table 2 for more details. The maximum removal efficiencies of 54 tests carried out using 8010SS spray nozzle at one stage, applying water and caustic scrubbing solutions were 89.21 and 96.26%, respectively. The minimum removal efficiencies for these cases were 61.17 and 80.88%, respectively. The maximum and minimum values of 54 efficiency tests (27 with fresh water and 27 with 0.01 M H2SO4), using 8010SS spray nozzle with two stages working together, were 90, 61.47%, and 97, 86.47%, respectively. According to the results, the maximum and minimum values of 54 efficiency tests for 8010SS spray nozzles applied at three stages working together (27 scrubbed with water and 27 with 0.01 M H2SO4) were 96.26, 78.38% and 97.9, 90.58%, respectively. See Table 3 for more details. As shown in Table 4, Fifty-four tests were done with a 2500-μm spray nozzle with one stage. The maximum and minimum removal efficiencies were 68.78, 4.4% and 78, 62.05%, respectively in the same concentration scrubbed with two liquids. When a 2500-μm spray nozzle applied at two stages working together were used, maximum and minimum efficiencies were 71.78, 20.58% and 82.57, 64.26%, respectively in the same concentrations with both scrubbing liquids. When a 2500-μm spray nozzle with three stages working together were used, maximum and minimum values of efficiency were 76.76, 25.3% and 85.47, 70.88%, respectively with each scrubbing liquid. The results revealed that, when 8010SS spray nozzles were applied at three stages working together, the highest removal efficiency of 97.9% was obtained in concentration of 24.1 and 12 bars 0.01 M H2SO4
Table 3 The removal efficiency of spray tower with applying 8010SS nozzle using water and caustic scrubbing solutions. Concentration (ppm)
24.1
52.0
68.0
24.1
52.0
68.0
Liquid pressure (bar)
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
67.63 76.76 86.72 56.53 62.88 70.76 51.47 60.29 65.44
81.74 90.45 93.77 81.34 86.10 88.65 80.14 83.00 86.76
72.61 75.1 87.55 57.00 63.46 72.70 54.11 61.74 70.44
90.00 93.30 94.20 86.50 87.70 90.40 83.38 86.90 88.90
77.60 79.66 88.38 59.23 65.79 76.34 57.20 63.52 75.30
92.10 94.20 95.85 89.42 91.50 93.26 87.80 89.70 92.64
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
71.78 78.00 89.21 59.61 66.15 75.20 61.17 64.26 70.58
83.40 90.45 96.26 83.65 87.50 90.38 80.88 85.30 88.82
79.25 84.23 90.00 65.38 69.03 75.00 61.47 66.47 71.17
90.87 95.43 97.00 88.46 90.00 93.00 86.47 88.88 92.05
83.00 86.72 96.26 80.76 86.15 94.60 78.38 85.88 88.38
95.00 96.68 97.90 92.00 95.30 96.10 90.58 92.64 95.58
Table 4 The removal efficiency of spray tower with applying a 2500-μm nozzle using water and caustic scrubbing solution. Concentration (ppm)
24.1
52.0
68.0
Liquid pressure (bar)
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
38.17 54.35 68.87 6.10 15.20 19.23 4.40 8.50 15.88
66.80 73.44 78.00 63.46 69.00 72.50 62.05 64.70 67.94
48.54 57.67 71.78 28.65 33.00 38.46 20.58 22.50 24.41
69.70 75.51 82.57 66.34 67.70 77.69 64.26 65.44 71.61
69.30 71.78 76.76 36.53 40.76 46.92 25.30 27.94 31.32
74.27 79.25 85.47 73.07 77.88 80.38 70.88 72.00 74.55
pressure. The lowest efficiency (4.4%) was obtained for a 2500 μm spray nozzle, pressure 9 bars and average concentration of 68 ppm. The results from NIOSH method 6015 with 8010SS spray nozzle applied in one stage in Fig. 2, indicated that the maximum and minimum values of efficiency in average concentrations of 24.1, 52 and 68 ppm with 0.01 M H2SO4 and 12 bars operating pressure were 96.26% and 88.82%. Similar experimental tests using NIOSH method 6015 with 8010SS spray nozzles applied at two stages working together in Fig. 3, revealed that the maximum and minimum removal efficiency in three average concentrations with 0.01 M H2SO4 and 12 bars operating pressure were 97.04% and 92.04%. Using NIOSH method 6015, with 8010SS nozzles applied at threestage spray nozzle and 0.01 M H2SO4 in Fig. 4, the highest and lowest values of efficiency were obtained at 97.94 and 95.58%, respectively. The one-way ANOVA test indicated that the enhancement of operational pressure increased removal efficiency but was not significant (P-value = 0.1). The result of statistical test showed the enhancement of H2SO4 solution pressure increased ammonia removal efficiency significantly (P-value = 0.05). The model of maximum removal efficiency, using an 8010SS spray nozzle with three stages working together and fresh water, when water pressure increased from 9 to 12 bar, in 24.1, 52, 68 ppm being exponential, linear and logarithmic, respectively. The model of maximum removal efficiency, using 8010SS spray nozzles applied at three stages working together and H2SO4 solution, in 24.1 and 52 ppm was logarithmic and in 68 ppm was exponential. According to analytical tests, increasing the spray nozzle stage from one to three in all types of spray nozzles and using water as the scrubbing liquid caused the removal efficiency to be very close to significant level or borderline (P-value = 0.06) while the efficiency tests with the same parameters (increase of the nozzle stage from one to
Table 2 The removal efficiency of spray tower with applying 20SS nozzle using water and caustic scrubbing solutions. Concentration (ppm)
Liquid pressure (bar)
787
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Fig. 2. The removal efficiency of spray tower with using 8010SS nozzle in one stage applying caustic scrubbing solutions and NIOSH method 6015.
improved scrubber collection was related to increasing operating pressure for all spray nozzles due to generating tiny droplets, which increased the surface area for chemical reaction and the liquid flow rate and so subsequent increase in L/G ratio. In this study a spray tower equipped with a three-stage spray nozzle at laboratory scale was used, to measure ammonia in inlet and outlet with Phocheck Tiger. Two hundred and seventy samples were separately collected with solid sorbent tubes and subsequently were analyzed with spectrophotometry and the results were compared, using Phocheck with NIOSH 6015 (Eller and Cassinelli, 1994). Hadlocon et al. (2014a) showed that the maximum spray scrubber removal efficiency was observed using the PJ40 nozzle with values of 70.2% ± 3.7%, 59.6% ± 2.5%, and 26.1% ± 0% at operating pressures of 0.62, 0.48, and 0.21 MPa, respectively. According to Manuzon et al. (2007) the collection efficiency of the PJ20 spray nozzle was 22% ± 1%, 29% ± 1%, and 35% ± 1% at operating pressures of 207, 414, and 620 KPa, respectively. The comparison of results of the present study with those of Hadlocon and Manuzon study showed that the removal efficiency of ammonia using 8010SS nozzle in 12 bars pressure and 24.1 ppm concentration with both water and sulfuric acid scrubbing liquid were higher than that obtained in Hadlocon and Manuzon study, which might be due to higher operating pressure leading to more number of droplets and high droplet velocity, more surface area and L/ G ratio. According to Hadlocon et al. (2014a) study scrubber efficiencies ranged from 90% to 34% when the NH3 concentration varied from 10 to 400 ppm, respectively. The results of a study by Manuzon et al. (2007) indicated ammonia scrubber efficiency decreased from 60% ± 1%–27% ± 2% as the inlet NH3 concentration was enhanced
three in all type of spray nozzles), using 0.01 M H2SO4 as the scrubbing liquid cause the removal efficiency to increase significantly (Pvalue = 0.02). One-way ANOVA test has shown that increases in the orifice size of the spray nozzles in the tower yield a significant decrease (P-value < 0.001) in the removal efficiency of ammonia gas with both types of scrubbing liquids. The main mechanism to remove gases from air stream is the absorption, and molar ratio of sorbent to adsorb in this procedure is important (Kim and Kim, 1997). Based on one-way ANOVA test, with increase of ammonia concentration in the airflow, the removal efficiency of spray tower with both types of scrubbing liquids decreased significantly (P-value < 0.001). According to the Bland Altman graph, it can be concluded that there is a correlation between removal efficiency of NIOSH method 6015 and Phochek Tiger efficiency in 8010SS nozzles. Fig. 5 shows the details. 4. Discussion In this study, with the objective of increasing NH3 removal efficiency and optimizing spray tower design parameters, different spray nozzles, operating pressures, number of nozzle stages, ammonia concentrations, and two types of scrubbing liquid were used. Considering our validation tests and data, new spray nozzle, operating pressure, dilute H2SO4 solution produced a considerable performance and it was less complicated and costly. The performance of the PJ40 nozzle used by Hadlocon et al. (2014a) and PJ20 nozzle used by Manuzon et al. (2007) for treating air with a low inlet concentration of NH3 was compared to those of the new nozzles (8010SS and 20SS) in this study. According to the findings,
Fig. 3. The removal efficiency of spray tower using 8010SS nozzle in two stages applying caustic scrubbing solutions and NIOSH method 6015.
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Fig. 4. The removal efficiency of spray tower using 8010SS nozzle in three stages applying caustic scrubbing solutions and NIOSH method 6015.
atomization positions and liquid distribution can be obtained by selecting appropriate spray nozzle. Spray nozzle plays an important role in system cost, efficiency, low consumption liquid and droplet size, and operating pressure affects liquid velocity and more contact liquid with pollutants, which subsequently these parameters together have a very important effect on spray tower removal efficiency. Generally, further effort is recommended to upgrade the spray nozzle and to optimize its performance for achieving more absorption for other gases and vapors having low solubility and low Henry constants. A new spray nozzle for occupational NH3 of control and treatment was developed and validated in this study. It is apparent that the 8010SS spray nozzle in this study has more effect on optimization spray tower, highest removal efficiency and low consumption scrubbing liquid. The presented spray nozzle could increase NH3 removal efficiency with low Henry's law constant, lower operational costs compared to other spray nozzles and compared to other studies and appropriate for other pollutants that have similar chemical properties to ammonia. However, further efforts should focus on decreasing pressure drop for spray nozzles and clogging. It is also recommended that this spray nozzle should further be developed for higher ammonia concentrations. Fig. 5. Comparing the removal efficiencies measured by Niosh 6015 and Phochek in 80 μm nozzles.
Conflicts of interest
from 5 to 100 ppm. The difference between our collection efficiency of that with Hadlocon & Manuzon efficiency probably is related to the low range of our average concentrations. The results of performance evaluation of a spray tower in scrubbing H2S from air indicated that the maximum removal efficiency of H2S was 70.53 ± 1.54% at 1800 L min−1 of air flow rate and H2S concentration of 15 ppm. Also, the findings indicated that increases in input flow rate and input concentration lead to decrease in the removal efficiency of the spray tower. Using chemical scrubbing treatment can increase the removal efficiency of the spray tower (Jafari et al., 2015). According to Hadlocon et al. (2014a) a single-stage scrubbing efficiency was 57.4% ( ± 2.8%). Collection increased to about 89% ( ± 2%) when the spray-stages were increased to three. Comparison between the results of the present study with those of Hadlocon revealed higher removal efficiency of this work. According to Hadlocon et al. (2014a), the best removal efficiency with PJ20 and AAP01 spray nozzle were 53.9% and 49.1%, respectively. The results of the present study showed higher removal efficiency, compared to those obtained by Hadlocon, which might be due to constant scrubbing liquid flow rate and a 2500-μm nozzle and smallest droplets and more surface area that create 20SS contrast of PJ20 spray nozzle. Among different spray tower design parameters, spray nozzle and operating pressure play important roles in tower efficiency and optimization. Scrubbing droplet size, liquid flow rate, operating pressure,
The authors declared no conflict of interest. Acknowledgments This study was supported by a grant from Shahid Beheshti University of Medical Sciences (registered No. 13683). The authors wish to greatly appreciate them for their effort. References ACGIH, 2013. Industrial Ventilation: a Manual of Recommended Practice for Design (The Design Manual). ATSDR, 2004. Public Health Statement Ammonia. Agency for Toxic Substances and Disease Registry. Bandyopadhyay, A., Biswas, M.N., 2008. Critical flow atomizer in SO 2 spray scrubbing. Chem. Eng. J. 139, 29–41. Bozorgi, Y., Keshavarz, P., Taheri, M., Fathikaljahi, J., 2006. Simulation of a spray scrubber performance with Eulerian/Lagrangian approach in the aerosol removing process. J. Hazard Mater. 137, 509–517. Calvert, S., Englund, H.M., 1984. Handbook of Air Pollution Technology. Chen, W.-H., 2004. Atmospheric ammonia scavenging mechanisms around a liquid droplet in convective flow. Atmos. Environ. 38, 1107–1116. Chung, Y.-C., Lin, Y.-Y., Tseng, C.-P., 2005. Removal of high concentration of NH 3 and coexistent H 2 S by biological activated carbon (BAC) biotrickling filter. Bioresour. Technol. 96, 1812–1820. Codolo, M.C., Bizzo, W.A., 2013. Experimental study of the SO 2 removal efficiency and volumetric mass transfer coefficients in a pilot-scale multi-nozzle spray tower. Int. J. Heat Mass Tran. 66, 80–89. Cordeiro, M., Tinôco, I., Vigoderis, R., Oliveira, P., Gates, R.S., Xin, H., 2005. Ammonia
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M.J. Jafari et al. concentration evaluation in deep-bedded and concrete floor housing systems for grow-finish swine in Brazil. In: Livestock Environment VII, 18-20 May 2005. American Society of Agricultural and Biological Engineers, Beijing, China, pp. 485. Daisey, J.M., Angell, W.J., Apte, M.G., 2003. Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information. Indoor Air 13, 53–64. Darake, S., Hatamipour, M., Rahimi, A., Hamzeloui, P., 2016. SO 2 removal by seawater in a spray tower: experimental study and mathematical modeling. Chem. Eng. Res. Des. 109, 180–189. De Nevers, N., 2000. Air Pollution Control Engineering, second ed. McGraw-Hill, New York, N.Y. Dooly, G., Manap, H., O'Keeffe, S., Lewis, E., 2011. In-situ monitoring of ammonia gas using an optical fibre based approach. In: Journal of Physics: Conference Series. IOP Publishing, 012058. Ebert, F., Büttner, H., 1996. Recent investigations with nozzle scrubbers. Powder Technol. 86, 31–36. Eller, P.M., Cassinelli, M.E., 1994. NIOSH Manual of Analytical Methods. Diane Publishing. EPA, 1995. Control and Pollution Prevention Options for Ammonia Emissions. (New York:Research Triangle Park). Hadlocon, L.J.S., Manuzon, R.B., Zhao, L., 2014a. Optimization of ammonia absorption using acid spray wet scrubbers. Trans. ASABE 57, 647–659. Hadlocon, L.J.S., Zhao, L., Manuzon, R.B., Elbatawi, I.E., 2014b. An acid spray scrubber for recovery of ammonia emissions from a deep-pit swine facility. Trans. ASABE 57, 949–960. Hahne, J., K.K., Munack, A., Vorlop, K.D., 2005. In: Harms, H.H., M.F. (Eds.), Bio-engineering, Environmental Engineering. VDMA Landtechnik, Germany. Jafari, M.J., Nourian, S., Zendehdel, R., Massoudinejad, M.R., Sarbakhsh, P., Rahmati, A.R., Mofidi, A.A., 2015. The performance of a spray tower in scrubbing H2S from air. Safety Promot. Injury Prevent. 2, 321–328. Jafari, M.J., Omidi, L., Azari, M.R., Massoudi Nejad, M., Namdari, M., 2014. Raschig rings versus PVC as a packed tower media in scrubbing ammonia from air. Iran. J. Energy Environ. 5, 270–276. Javed, K., Mahmud, T., Purba, E., 2006. Enhancement of mass transfer in a spray tower using swirling gas flow. Chem. Eng. Res. Des. 84, 465–477. Javed, K., Mahmud, T., Purba, E., 2010. The CO 2 capture performance of a high-intensity vortex spray scrubber. Chem. Eng. J. 162, 448–456. Keshavarz, P., Bozorgi, Y., Fathikalajahi, J., Taheri, M., 2008. Prediction of the spray scrubbers' performance in the gaseous and particulate scrubbing processes. Chem. Eng. J. 140, 22–31. Kim, J., Kim, D., 1997. Effect of scrubber operating parameters on droplet size, droplet velocity and dust suppressibility. Environ. Educ. Res. 2, 279–286.
Koller, M., Wappel, D., Trofaier, N., Gronald, G., 2011. Test results of CO2 spray scrubbing with Monoethanolamine. Energy Proced. 4, 1777–1782. Kuntz, J., Aroonwilas, A., 2009. Mass-transfer efficiency of a spray column for CO2 capture by MEA. Energy Proced. 1, 205–209. Lee, B.-K., Jung, K.-R., Park, S.-H., 2008. Development and application of a novel swirl cyclone scrubber—(1) Experimental. J. Aerosol Sci. 39, 1079–1088. Lefebvre, A.H., 1989. Atomization and Sprays. Hemisphere Pub. Corp., New York 1989. Leson, G., Winer, A.M., 1991. Biofiltration: an innovative air pollution control technology for VOC emissions. J. Air Waste Manag. Assoc. 41, 1045–1054. Li, L., Zhao, B., 2012. Numerical simulation of gas and liquid flow within a vortex spray tower. Energy Proced. 16, 1072–1077. Manuzon, R.B., Zhao, L., Keener, H.M., Darr, M.J., 2007. A prototype acid spray scrubber for absorbing ammonia emissions from exhaust fans of animal buildings. Trans. ASABE 50, 1395–1407. Melse, R.W., Ogink, N., 2005. Air scrubbing techniques for ammonia and odor reduction at livestock operations: review of on-farm research in The Netherlands. Trans. ASAE 48, 2303–2313. Montgomery, D.C., 2017. Design and Analysis of Experiments. John Wiley & Sons. Perrin, D.D., 1969. Dissociation contants of inorganic acids and bases in aqueous solution. Pure Appl. Chem. 20, 133–236. Sharma, M., Mehta, K., 1970. Mass transfer in spray towers. Chem. Eng. 15, 1440–1444. Swartz, E., Shi, Q., Davidovits, P., Jayne, J., Worsnop, D., Kolb, C., 1999. Uptake of gasphase ammonia. 2. Uptake by sulfuric acid surfaces. J. Phys. Chem. 103, 8824–8833. USEPA, 2002. Air Pollution Control Technology. Fact Sheet. Spray-Chamber/SprayTower Wet Scrubber. USEPA, 2009. Emergency Planning and Community Right-to- Know Act. Yeh, N.K., Rochelle, G.T., 2003. Liquid-phase mass transfer in spray contactors. AIChE J. 49, 2363–2373. Yincheng, G., Zhenqi, N., Wenyi, L., 2011. Comparison of removal efficiencies of carbon dioxide between aqueous ammonia and NaOH solution in a fine spray column. Energy Proced. 4, 512–518. Zarei, F., Rezazadeh Azari, M., Salehpour, S., Khodakarim, S., Omidi, L., Tavakol, E., 2017. Respiratory effects of simultaneous exposure to respirable crystalline silica dust, formaldehyde, and triethylamine of a group of foundry workers. J. Res. Health Sci. 17. Zhang, Y., 2005. Indoor Air Quality Engineering. CRC Press, Boca Raton, FL 2005. Zhao, L., Manuzon, R., Darr, M., Keener, H., Heber, A.J., Ni, J.-Q., 2009. Ammonia emissions from a commercial poultry manure composting facility. In: Livestock Environment VIII, 31 August–4 September 2008. American Society of Agricultural and Biological Engineers, Iguassu Falls, Brazil, pp. 6.
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Original Article
Experimental optimization of a spray tower for ammonia removal a
b
a,∗
T a
Mohammad Javad Jafari , Amir Hossein Matin , Alireza Rahmati , Mansour Rezazadeh Azari , Leila Omidic, Seyed Saeed Hosseinid, Davood Panahie a
Department of Occupational Health Engineering, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Occupational Health Engineering, School of Health, Safety and Environment, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Occupational Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran d Department of Electrical Engineering, Tehran-North Branch, Islamic Azad University, Tehran, Iran e Department of Occupational Health Engineering, Abadan School of Medical Sciences, Abadan, Iran b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Spray nozzle Spray tower Operational pressure Ammonia
Spray tower scrubbers ordinarily have low air resistance and gas removal efficiency. Although packed-bed wet scrubbers are efficient in gaseous contaminants treatment, a significant limitation of packed-bed wet scrubbers is that they have high pressure drop and primary costs. Appropriate features of the nozzle play an important role in system cost, efficiency, low operational costs, and optimization of spray towers. Operating pressure, nozzles size, and number of nozzles could increase mass transfer and removal efficiency and decrease investment and save operational costs. The objective of the present study was to develop a spray tower through optimization of the design and operating parameters for removal ammonia emissions from the air streams. Spray tower design parameters included nozzle type, number of stages of spray nozzle, and operating parameters such as operating pressure and inlet NH3 concentration. Among the studied parameters, only increasing ammonia concentration was in inverse proportion to the spray tower efficiency. The spray tower was optimized as equipped with an 8010SS spray nozzle with three stages working together, spraying 0.01% H2SO4 scrubbing liquid counter-current to the air stream with operating pressure of 12 bars and inlet NH3 concentration of 24.1 ppm. The highest removal efficiency was 97.92% at an 8010SS spray nozzle with three stages working together, H2SO4 solution, pressure 12 bars and inlet ammonia concentration of 24.1 ppm. The results of this study demonstrated that caustic spray tower could be a very effective technology for removal of NH3 from air stream.
1. Introduction
et al., 2009). According to USEPA, exposure to ammonia may increase in the future and there is a need to identify sources of human exposure to implement control measures (ATSDR, 2004; EPA, 1995). The mitigation of NH3 emissions in air stream is an important issue for protection of human health and the environment (Hadlocon et al., 2014b; USEPA, 2002). Some researchers used biological treatment as an effective and economical tool for the biotreatment of waste gas streams with low concentrations in large amounts of air. High water-holding capacity, good airflow characteristics, high pH buffer capacity, and good mechanical properties are some attributes of a good bioreactor medium (Chung et al., 2005; Leson and Winer, 1991). The initial medium was made from soils, however, their tendencies to short-circuit and clog are some major drawbacks of soil beds (Leson and Winer, 1991). Chung et al. (2005) used biotrickling filters by biological activated carbon for removal of high concentration of NH3 and coexistent H2S. Their
Ammonia (NH3) is a colorless and very irritant gas, which is emitted from animal husbandry and some industrial processes. Numerous sources have been considered as ammonia emitters such as the fertilizer industry, coke manufacture, fossil fuel combustion, livestock management, and refrigeration methods. Livestock waste management and fertilizer production are responsible for emitting 90% of total ammonia (EPA, 1995). Ammonia may have some impacts concerning environmental and human health (Hadlocon et al., 2014a). The main risk of ammonia in high concentrations is explosion. United States Environmental Protection Agency (USEPA) has classified ammonia in national priorities list. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Emergency Planning and Community Right-to-Know Act (EPCRA) establish requirements for reporting any releases of NH3 exceeding 100 lbs (USEPA, 2009; Zhao
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail addresses: [email protected] (M.J. Jafari), [email protected] (A.H. Matin), [email protected] (A. Rahmati), [email protected] (M.R. Azari), [email protected] (L. Omidi), [email protected] (S.S. Hosseini), [email protected] (D. Panahi). https://doi.org/10.1016/j.apr.2018.01.014 Received 8 September 2017; Received in revised form 17 December 2017; Accepted 19 January 2018 Available online 09 February 2018 1309-1042/ © 2018 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
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findings indicated that physical adsorption of ammonia gas by granular activated carbon was responsible for the first 28 days of experiment, and then biodegradation by inoculated microorganisms was responsible for other days of the experiment. The critical ammonia loading was 4.2 g-N/m3/h, and the maximal loading was 16.2 g-N/m3/h. Low molecular weight gases, highly soluble compounds, and simple chemical structures are attractive targets for removal by biotrickling method. Wet scrubbers have been prevalently used for air pollution process with significantly reduced risks for human health (Lee et al., 2008). Spray towers as one of the popular wet scrubbers that are widely used in the air pollution control and treatment technology such as CO2 removing, desulfuration, and denitrification have been used to eliminate air pollution before releasing into the atmosphere (Chen, 2004; Li and Zhao, 2012). Design parameters of spray towers can affect the air pollutants removal efficiency (Yincheng et al., 2011; Zhang, 2005). Many researchers have investigated the effects of design parameters on the increases of overall removal efficiency of spray towers (Codolo and Bizzo, 2013; Hadlocon et al., 2014a; Kuntz and Aroonwilas, 2009; Manuzon et al., 2007; Sharma and Mehta, 1970). Researchers studied the effects of droplet size and multi-stage spray nozzles on spray tower efficiency (Hadlocon et al., 2014a; Koller et al., 2011). The results of a study by Kuntz and Aroonwilas (2009) also indicated the influences of the number and droplet size on CO2 removal efficiency. In another study, researchers have shown the effects of dynamic behavior on liquid droplets and gas mass transfer operation in spray scrubbers. Among the different design parameters such as liquid to gas ratio (L/G), spray nozzles, droplet size distributions, arrangement of nozzles, gas flow rate, and liquid pressure that affect the towers performance (Bozorgi et al., 2006; Kuntz and Aroonwilas, 2009), spray nozzle, operating pressure, and velocity are more important (Zhang, 2005). Nozzle type, nozzle position, droplet size, and liquid to gas ratio are other effective parameters related to increasing the removal efficiency in spray towers. Appropriate spray nozzle can significantly lead to lower liquid rate and energy consumption (Ebert and Büttner, 1996). Liquid droplet size, liquid flow rate, operating pressure, atomization positions, and liquid distribution can be obtained by selecting appropriate nozzles and are also impressed by the specification of spray nozzles such as the type of nozzle, orifice diameter, and nominal cone angle. The most impressive zone for the mass transfer is observed to be in the proximity of the spray nozzle (Daisey et al., 2003; Javed et al., 2006, 2010; Yeh and Rochelle, 2003; Zhang, 2005). The operating pressure of scrubbing liquid is an important parameter, because it directly affects the droplet size, liquid distribution, and liquid flow rate. The droplet size is mainly affected by the nozzle types, which supply a proper balance of liquid flow rate and operating pressure. Increase in the pressure may enhance the ratio of liquid flow rate to droplet size and this leads to higher efficiency of spray tower by using all types of nozzles (Hadlocon et al., 2014a). Liquid pressure consequently is affected by the number of droplets and droplet velocity. Removal efficiency depends on these factors (Codolo and Bizzo, 2013). Then nozzle velocity has a very important role on removal efficiency (Bozorgi et al., 2006; Ebert and Büttner, 1996). Nozzle plays an important role in the system cost, efficiency, low liquid consumption, and optimization of the spray tower (Ebert and Büttner, 1996; Lee et al., 2008; Yincheng et al., 2011). The findings of some studies have shown that operating pressure, nozzle size, and multi-stage spray nozzle could increase mass transfer and removal efficiency and decrease and save operational costs. The intense contact between the scrubbing liquid and the polluted gas can be used to optimize the performance of spray towers (Bandyopadhyay and Biswas, 2008; Codolo and Bizzo, 2013; Koller et al., 2011; Kuntz and Aroonwilas, 2009; Yeh and Rochelle, 2003). According to results of some studies, caustic spray scrubber can cause high ammonia removal (Hadlocon et al., 2014a; Hahne et al., 2005; Manuzon et al., 2007). Although the number of nozzles affects removal efficiency of a spray tower, some disadvantages have been reported by some authors when
using multiple nozzles. Using multiple nozzles could lead to increasing air pressure drop and more liquid rate compared with using single spray nozzle (Ebert and Büttner, 1996; Koller et al., 2011). There are also some drawbacks related to increasing plugging and maintenance cost. Although large nozzle has low practical operating pressure loss (Ebert and Büttner, 1996), the larger nozzle size can lead to more liquid consumption, large droplet size, less contact between the scrubbing liquid and the polluted gas in the surface area, and low collection efficiency (Codolo and Bizzo, 2013; Koller et al., 2011; Kuntz and Aroonwilas, 2009). The spray nozzle can provide a large droplet surface area in a given liquid volume, causing more contact with gas and droplets and improving absorptive capacity of the scrubbing liquid. Small size spray nozzle is more disposed to plug if scrubbing liquid contains suspend aerosol. In this case, expensive and more frequent maintenance is needed. Small size spray nozzles produce larger liquid surface area, but have higher pressure loss and if the velocity of gas increases from recommended design level, scrubbing liquid may leave the scrubber requiring demister (ACGIH, 2013; Codolo and Bizzo, 2013; Ebert and Büttner, 1996; Keshavarz et al., 2008; Kim and Kim, 1997; Koller et al., 2011; Kuntz and Aroonwilas, 2009). High pressure spray nozzle leads to uniform liquid distribution, an increase in contact surface, better collection efficiency, as well as in the short retention time available. More pressure drop is expected when nozzle pressure is increased leading to an increase in the liquid volumetric flow rate and more liquid consumption (Ebert and Büttner, 1996; Lee et al., 2008). The present study conducted in 2015 aimed to optimize an experimental spray tower for NH3 removal with a constant airflow. The specific objectives of this research were: (1) to optimize some design parameters, including the selection of the best spray nozzle size and operating liquid pressure, different spray nozzle stages, and improving collection efficiency; (2) to evaluate the effects of operating parameters on removal performance (inlet NH3 concentrations and the number of spray nozzle stages); (3) to quantify the performance of optimized spray tower for exhaust air stream with both low and high ammonia concentrations and compare removal efficiency with both scrubber liquids including caustic scrubbing solution and water. 2. Material and method 2.1. Ammonia removal In a spray tower, NH3 stream reacts with scrubber liquid (water or dilute acid) droplets. Spray nozzles are used to generate droplets. The greater the surface area for chemical absorption, the higher the collection efficiency can be achieved. Although the majority of large droplets move down against the airflow, some smaller droplets can enter the fan through air flowing up. Scrubbing liquid is collected in a tank and recirculated. The equilibrium reactions for NH3 solubility in water and caustic scrubbing liquids are (Melse and Ogink, 2005; Swartz et al., 1999).
H NH3 (g ) ⇔ NH3 (eq)
(1)
K ′fKr′ + + NH3 + Heq ⇐⇒ = NH4( ′ ) eq) (K eq
(2)
Equation (1) describes the solubility of NH3 in water, where H is the Henry's law constant, estimated to be 5.33 × 101 M atm−1 at ′ ) can be derived as the re298.15 °K. The equilibrium constant (K eq ciprocal of the acid dissociation constant of NH+ 4 and has a value of 1.78 × 109 at 25 °C (Perrin, 1969). The overall solubility can be expressed in terms of the effective Henry's law constant (H*, M atm−1), as represented by the sum of the dissolved NH3 (aq) and protonated NH+ 4 (aq), where pNH3 is the partial pressure of NH3 (atm), T is the air temperature (°K), and R is the gas constant (atm M−1 K−1) (Calvert and Englund, 1984). Researchers developed a performance models for gas absorption in 784
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laboratory scale constructed from iron black was used to conduct the experiments. It consisted of five sections: an inlet, scrubber column, spraying system, outlet and instruments. The internal diameter and height of tower were 25 and 138 cm, respectively, and the vertical scrubber column enclosed spray nozzles and liquid pipes. The spray tower contained a maximum of three stages of spray nozzles, which were spaced 38 cm apart. The position of the nozzle in tower, as well as the space between nozzles, was designed to be at least 30.5 cm, which is the maximum height related to the 80 μm nozzle used in this study. The spraying system delivered a prepared solution of dilute sulfuric acid (H2SO4) into each nozzle stage from a 60 L feed Polyethylene tank through a magnetically driven pump with a rated pressure range of 0–12 bars. A pressure relief valve was used to regulate the pressure and liquid flow rate supplied to the tank. The liquid droplets of the known concentration of H2SO4 interacted with the NH3 inside the scrubber column. The entire scrubber system was installed with appropriate instrumentations to monitor pH and NH3 concentration. By installing a valve for each nozzle, the application of each nozzle was adjusted by the user. The spray tower consisted of three nozzles, three valves, a centrifugal pump, the chemical additive reservoir, and gauge pressure. The study spray tower and experimental apparatus are shown in Fig. 1. Airflow velocity in spray towers was between 200 and 300 feet per minute (fpm). The selected velocity was 250 fpm (1.27 m/ s), in which no droplet in fan housing or stack was detected. The most popular size of spray nozzles ranged from 20 to 100 μm which could provide large droplet surface area (ACGIH, 2013).
single-stage scrubber (Eq. (3)) (Calvert and Englund, 1984).
η=1−
(1 − (
6RTK Z exp ⎡ Δ uDG 3 N 2
⎣
L G
) − H )⎤ − H ⎦
G H∗ L
∗G L
(3)
where, η is the collection efficiency, H ∗, G , and L are Henry's law constant, the moles of air per unit time and unit tower cross-section (mol air s−1 m−2), and the molar flow rate of the NH3-free liquid stream per unit cross-sectional area of the tower (mol water air s−1 m−2), respectively. R , T , K G , and ZN are the universal gas constant (8.314 J mol−1 K−1), absolute air temperature (°K), the individual mass transfer coefficient of NH3 in air (moles NH3 s−1 m−2 Pa−1), and the column length for liquid-gas contact (m), respectively. Δu and D23 are the relative velocity between the droplets and the airflow (m s−1), and the Sauter mean diameter of spray droplets (m), respectively. A performance models for multi-stage scrubbers can be estimated according to Eq. (4). This model is based on the single-stage efficiency by considering the concept of penetration for control devices in series. The predicted efficiency for n-stage scrubbing can be obtained by assuming no strong interaction among liquid droplets between stages to cause variation in the scrubbing efficiency for each stage.
ηoverall = 1 − (1 − η1 − stage )n
(4)
where, ηoverall , n , and η1 − stage are the total collection efficiency of the n stages, the number of stages, and the scrubber efficiency of one stage, respectively (De Nevers, 2000; Hadlocon et al., 2014a). Multi-stage scrubbing is suggested to have higher removal efficiency for ammonia by promoting a higher liquid-to-gas ratio (Hadlocon et al., 2014a). Absorption is probably the main mechanism considered to remove gases from air stream. Molar ratio of sorbent to adsorb is an important factor (Kim and Kim, 1997).
2.4. Ammonia supply A 45 kg gas cylinder, a crown model stainless steel ammonia regulator and a flow meter were used to supply ammonia laden air. A constant inlet ammonia concentration was adjusted at desired levels in each sampling. Ammonia was injected to the tower through a flexible polyethylene tube and was blended with air prior to its' injection. Few pre-tests were carried out to identify probable problems after setting up. The inlet concentration of ammonia ranged from 24.1 to 68 part-permillion (ppm).
2.2. Nozzle selection and operating conditions The spray nozzles used for wet scrubbers can be characterized based on their droplet size, droplet concentration, dispersion, initial velocity, spray angle, and spray pattern. The total surface area is determined by the concentration of droplets inside the scrubber column. The selection of appropriate spray nozzles or atomizers is critical for the optimization of spray scrubber performance (Lefebvre, 1989). The droplet size generated by the atomizers has a significant impact on the surface area needed to enhance gas-liquid contact for the chemical absorption process. Proper spray dispersion leads to perfect mixing of the liquid with the surrounding gas, which promotes the contact. Hollow-cone nozzles generate droplets that are generally much smaller compared to a fullcone nozzle, but hollow-cone nozzles need additional flow to meet the same performance as the full-cone nozzles (Manuzon et al., 2007). In this study three full-cone orifice (BERTO Model 20SS, BERTO Model 8010SS (Danfoss), and a 2500 μm spray nozzle (made in Iran)) sizes were used in different NH3 concentrations, operating pressure and stages. Studies conducted by other researchers show that, there is much preference for using a full-cone nozzle over a hollow-cone nozzle, but the practical significance is associated with the bigger orifice size of a 2500-μm nozzle, which is beneficial to reduce nozzle clogging (Hadlocon et al., 2014a; Hahne et al., 2005; Manuzon et al., 2007). Three types of solid cone spray nozzle including 20SS, 8010SS (Berto) and 2500-μm nozzle (with three sizes of 20, 80 and 2500 μm, respectively), that constructed 27 cm diameter of the spray cone were used. Nozzles were located at three heights of 62.5, 87.5 and 112.5 cm from inlet of spray tower. The orifices of each spray nozzle were located at center of the tower.
2.5. Air sampling and ammonia measurement Ammonia concentrations were measured using an Ion Science Phocheck Tiger at the inlet and outlet ports of the tower in each sampling (Cordeiro et al., 2005; Dooly et al., 2011). The measuring range of Phocheck was 0.1–5000 ppm. The accuracy of the device was ± 1 ppm. Tedlar bags containing contamination-free air were used to calibrate the instrument response (Zarei et al., 2017). Atmospheric stock gas standards were transferred into bags and filling them with contamination-free air with the proper conditions for flow and time parameters. The Phocheck was calibrated for 1–120 ppm of ammonia in air (R2 = 0.997). Ammonia removal efficiency (% eff.) was calculated based on Eq. (5). (Jafari et al., 2014).
Efficiency (%) =
CNH 3in − CNH 3out × 100 CNH 3in
(5)
Also, the visible absorption spectrophotometry method (NIOSH 6015) was applied to measure the inlet and outlet concentrations of ammonia in air. For this purpose, 10 ml solution was added to the spectrophotometer cuvette and was set at 630 or 660 nm wavelengths. A total of 270 samples were collected. Solid sorbent tubes including sulfuric acid-treated silica gel were used for ammonia sampling. All safety aspects of working with chemicals were considered according to materials safety data sheet. The sampling pump (SKC) was calibrated with electronic calibrator (Defender 510-M). Samples were taken at an accurately known flow rate between 0.1 and 0.2 L min−1 with SKC air sampling pump. Standard laboratory equipment was applied for a total sample size of 0.1–96 L. After sampling at inlet and outlet of spray
2.3. Spray tower scrubber A spray tower equipped with a three-stage spray nozzle at 785
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Fig. 1. Experimental apparatus. −1 and flow rate of 3737.82 L min−1. The feed pump was operated at a nozzle pressure of 9–12 bars for tests. The inlet NH3 concentrations were set to 24.1, 52 and 68 ppm. All experiments were conducted in the laboratory at room air temperature of 23.5 °C–28 °C and relative humidity of 45%–53%. Experiments were more economical in this study because the interaction effects of the experimental factors were relatively tiny, compared to the main factor effects (Darake et al., 2016; Montgomery, 2017). Although designed experiments such as completely or semi factorial experiments are preferable to obtain comprehensive effects of factors and their interactions, in this study screening factors and separating their practical and optimum settings were applied for high tower performance. The summary of condition for experiments conducted in this study is listed in Table 1.
tower at the same time, the front and back sections of sulfuric acidtreated silica gel in each tube was transferred into separate 80- mL vials. To separate all of the reagents, ammonia-free deionized water was used in all laboratory analyses. The sodium hydroxide was used to adjust pH at 5–6.5 (Eller and Cassinelli, 1994). Each measurement was conducted according to NIOSH method 6015. 2.6. Air flow rate and head loss Airflow rate required for experiments was 3737.82 L min−1 based on the tower diameter and recommended ACGIH velocity (200–300 fpm) (ACGIH, 2013). The airflow rates were supplied by a variable flow rate fan (HVDLT-MK2, manufactured by UK air flow Co). A low pressure venturi (H type) was used to measure airflow rate. The measuring accuracy of venturi was ± 4–5%. A digital anemometer was used to monitor airflow during each test. A monometer (UK Type 504) with the measurement range of 0–5000 pa and with an accuracy of ± 10 pa was used to measure the head losses.
2.8. Data analysis The collected data were analyzed, using general descriptive statistical analysis. To study the effects of the parameters, three replicate runs were performed for each test. The data were analyzed with SPSS 16.0 software using analysis of variance (ANOVA), t-test for paired comparisons. Linear regression analysis was applied to fit the model.
2.7. Scrubbing liquid flow rate and liquid pressure The scrubbing solution was supplied using a CDLF2-13-M multistage stainless steel pump. Full fresh caustic scrubbing liquid including 0.01 M of H2SO4 was supplied through a 60-L poly ethylene reservoir for all 486 samples. Acidity of the caustic scrubbing solution was monitored through measuring of its' pH. For this purpose, a pH controller and transmitter (model Sartorius Basic Meter PB-11) with a range of 2–16 pH and an accuracy of 0.01 PH was applied. Liquid samples were also brought to the laboratory for secondary pH measurement to ensure accuracy. Liquid pressure and flow rate were consistently monitored. Liquid flow rate was monitored using a polysulfone flow meter with 2% F.S accuracy. The maximum capacity of the pump was 20 L min−1. The optimum volume of injected liquid was based on the exhaust gas flow rate defined in literature (ACGIH, 2013; Keshavarz et al., 2008). The gas-to-liquid ratio (G/L) was 747.56 based on recommendations for spray tower (Jafari et al., 2014). The liquid pressure was measured with a gauge pressure (TG model) that was installed after pump in the pressure range of 0–16 bars. The spray tower was operated at a constant air velocity of 1.27 m s
3. Results In total 486 tests in three series, including three spray nozzle sizes installed in three stages with three operating pressures and three Table 1 Condition for experiments.
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Variables
Values and types
Inlet NH3 concentration (ppm) Number of spray stages Nozzle size (micron) Liquid pressure (bar) Types of scrubbing liquid L/G ratio
24.1, 52, 68 1, 2, 3 20, 80, 2500 9, 10, 12 Caustic, Water 0.001
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concentration ranges were carried out with fresh water as the scrubbing liquid. The same tests (486 tests) were carried out with 0.01 M solution of H2SO4 as scrubbing liquid. Ammonia average concentration in the inlet of tower was 24.1, 52 and 68 ppm. Among 270 tests using 8010SS spray nozzle installed in three stages with fresh scrubbing solution, high removal efficiency of ammonia gas was achieved. The best results in the same conditions were related to concentrations with 0.01 M of H2SO4 as the caustic scrubbing liquid. The small size of spray nozzles (8010SS) had higher efficiency. This was true for both types of scrubbing liquid. The results from 54 tests carried out with a 20SS spray nozzle at one stage revealed that the maximum removal efficiency of ammonia gas scrubbed with water and 0.01 M solution of H2SO4 were 86.72 and 93.77%, respectively. The minimum removal efficiencies for these cases were 51.47 and 80.14%, respectively. The maximum removal efficiencies for 54 tests conducted with a 20SS spray nozzle applied in two stages and two scrubbing solutions including water and 0.01 mol H2SO4 were 87.55, 94.20%, respectively. The minimum efficiencies for the same cases were 54.11 and 83.38%, respectively. The maximum removal efficiencies for 54 tests carried out with a 20SS spray nozzle applied in three stages using water and caustic scrubbing solutionswere 88.38%, and 95.85%, respectively. The minimum removal efficiencies for the same cases were 57.2% and 87.8%, respectively. See Table 2 for more details. The maximum removal efficiencies of 54 tests carried out using 8010SS spray nozzle at one stage, applying water and caustic scrubbing solutions were 89.21 and 96.26%, respectively. The minimum removal efficiencies for these cases were 61.17 and 80.88%, respectively. The maximum and minimum values of 54 efficiency tests (27 with fresh water and 27 with 0.01 M H2SO4), using 8010SS spray nozzle with two stages working together, were 90, 61.47%, and 97, 86.47%, respectively. According to the results, the maximum and minimum values of 54 efficiency tests for 8010SS spray nozzles applied at three stages working together (27 scrubbed with water and 27 with 0.01 M H2SO4) were 96.26, 78.38% and 97.9, 90.58%, respectively. See Table 3 for more details. As shown in Table 4, Fifty-four tests were done with a 2500-μm spray nozzle with one stage. The maximum and minimum removal efficiencies were 68.78, 4.4% and 78, 62.05%, respectively in the same concentration scrubbed with two liquids. When a 2500-μm spray nozzle applied at two stages working together were used, maximum and minimum efficiencies were 71.78, 20.58% and 82.57, 64.26%, respectively in the same concentrations with both scrubbing liquids. When a 2500-μm spray nozzle with three stages working together were used, maximum and minimum values of efficiency were 76.76, 25.3% and 85.47, 70.88%, respectively with each scrubbing liquid. The results revealed that, when 8010SS spray nozzles were applied at three stages working together, the highest removal efficiency of 97.9% was obtained in concentration of 24.1 and 12 bars 0.01 M H2SO4
Table 3 The removal efficiency of spray tower with applying 8010SS nozzle using water and caustic scrubbing solutions. Concentration (ppm)
24.1
52.0
68.0
24.1
52.0
68.0
Liquid pressure (bar)
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
67.63 76.76 86.72 56.53 62.88 70.76 51.47 60.29 65.44
81.74 90.45 93.77 81.34 86.10 88.65 80.14 83.00 86.76
72.61 75.1 87.55 57.00 63.46 72.70 54.11 61.74 70.44
90.00 93.30 94.20 86.50 87.70 90.40 83.38 86.90 88.90
77.60 79.66 88.38 59.23 65.79 76.34 57.20 63.52 75.30
92.10 94.20 95.85 89.42 91.50 93.26 87.80 89.70 92.64
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
71.78 78.00 89.21 59.61 66.15 75.20 61.17 64.26 70.58
83.40 90.45 96.26 83.65 87.50 90.38 80.88 85.30 88.82
79.25 84.23 90.00 65.38 69.03 75.00 61.47 66.47 71.17
90.87 95.43 97.00 88.46 90.00 93.00 86.47 88.88 92.05
83.00 86.72 96.26 80.76 86.15 94.60 78.38 85.88 88.38
95.00 96.68 97.90 92.00 95.30 96.10 90.58 92.64 95.58
Table 4 The removal efficiency of spray tower with applying a 2500-μm nozzle using water and caustic scrubbing solution. Concentration (ppm)
24.1
52.0
68.0
Liquid pressure (bar)
9 10 12 9 10 12 9 10 12
Efficiency (%) (one stage)
Efficiency (%) (two stages)
Efficiency (%) (three stages)
Water
Caustic
Water
Caustic
Water
Caustic
38.17 54.35 68.87 6.10 15.20 19.23 4.40 8.50 15.88
66.80 73.44 78.00 63.46 69.00 72.50 62.05 64.70 67.94
48.54 57.67 71.78 28.65 33.00 38.46 20.58 22.50 24.41
69.70 75.51 82.57 66.34 67.70 77.69 64.26 65.44 71.61
69.30 71.78 76.76 36.53 40.76 46.92 25.30 27.94 31.32
74.27 79.25 85.47 73.07 77.88 80.38 70.88 72.00 74.55
pressure. The lowest efficiency (4.4%) was obtained for a 2500 μm spray nozzle, pressure 9 bars and average concentration of 68 ppm. The results from NIOSH method 6015 with 8010SS spray nozzle applied in one stage in Fig. 2, indicated that the maximum and minimum values of efficiency in average concentrations of 24.1, 52 and 68 ppm with 0.01 M H2SO4 and 12 bars operating pressure were 96.26% and 88.82%. Similar experimental tests using NIOSH method 6015 with 8010SS spray nozzles applied at two stages working together in Fig. 3, revealed that the maximum and minimum removal efficiency in three average concentrations with 0.01 M H2SO4 and 12 bars operating pressure were 97.04% and 92.04%. Using NIOSH method 6015, with 8010SS nozzles applied at threestage spray nozzle and 0.01 M H2SO4 in Fig. 4, the highest and lowest values of efficiency were obtained at 97.94 and 95.58%, respectively. The one-way ANOVA test indicated that the enhancement of operational pressure increased removal efficiency but was not significant (P-value = 0.1). The result of statistical test showed the enhancement of H2SO4 solution pressure increased ammonia removal efficiency significantly (P-value = 0.05). The model of maximum removal efficiency, using an 8010SS spray nozzle with three stages working together and fresh water, when water pressure increased from 9 to 12 bar, in 24.1, 52, 68 ppm being exponential, linear and logarithmic, respectively. The model of maximum removal efficiency, using 8010SS spray nozzles applied at three stages working together and H2SO4 solution, in 24.1 and 52 ppm was logarithmic and in 68 ppm was exponential. According to analytical tests, increasing the spray nozzle stage from one to three in all types of spray nozzles and using water as the scrubbing liquid caused the removal efficiency to be very close to significant level or borderline (P-value = 0.06) while the efficiency tests with the same parameters (increase of the nozzle stage from one to
Table 2 The removal efficiency of spray tower with applying 20SS nozzle using water and caustic scrubbing solutions. Concentration (ppm)
Liquid pressure (bar)
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Fig. 2. The removal efficiency of spray tower with using 8010SS nozzle in one stage applying caustic scrubbing solutions and NIOSH method 6015.
improved scrubber collection was related to increasing operating pressure for all spray nozzles due to generating tiny droplets, which increased the surface area for chemical reaction and the liquid flow rate and so subsequent increase in L/G ratio. In this study a spray tower equipped with a three-stage spray nozzle at laboratory scale was used, to measure ammonia in inlet and outlet with Phocheck Tiger. Two hundred and seventy samples were separately collected with solid sorbent tubes and subsequently were analyzed with spectrophotometry and the results were compared, using Phocheck with NIOSH 6015 (Eller and Cassinelli, 1994). Hadlocon et al. (2014a) showed that the maximum spray scrubber removal efficiency was observed using the PJ40 nozzle with values of 70.2% ± 3.7%, 59.6% ± 2.5%, and 26.1% ± 0% at operating pressures of 0.62, 0.48, and 0.21 MPa, respectively. According to Manuzon et al. (2007) the collection efficiency of the PJ20 spray nozzle was 22% ± 1%, 29% ± 1%, and 35% ± 1% at operating pressures of 207, 414, and 620 KPa, respectively. The comparison of results of the present study with those of Hadlocon and Manuzon study showed that the removal efficiency of ammonia using 8010SS nozzle in 12 bars pressure and 24.1 ppm concentration with both water and sulfuric acid scrubbing liquid were higher than that obtained in Hadlocon and Manuzon study, which might be due to higher operating pressure leading to more number of droplets and high droplet velocity, more surface area and L/ G ratio. According to Hadlocon et al. (2014a) study scrubber efficiencies ranged from 90% to 34% when the NH3 concentration varied from 10 to 400 ppm, respectively. The results of a study by Manuzon et al. (2007) indicated ammonia scrubber efficiency decreased from 60% ± 1%–27% ± 2% as the inlet NH3 concentration was enhanced
three in all type of spray nozzles), using 0.01 M H2SO4 as the scrubbing liquid cause the removal efficiency to increase significantly (Pvalue = 0.02). One-way ANOVA test has shown that increases in the orifice size of the spray nozzles in the tower yield a significant decrease (P-value < 0.001) in the removal efficiency of ammonia gas with both types of scrubbing liquids. The main mechanism to remove gases from air stream is the absorption, and molar ratio of sorbent to adsorb in this procedure is important (Kim and Kim, 1997). Based on one-way ANOVA test, with increase of ammonia concentration in the airflow, the removal efficiency of spray tower with both types of scrubbing liquids decreased significantly (P-value < 0.001). According to the Bland Altman graph, it can be concluded that there is a correlation between removal efficiency of NIOSH method 6015 and Phochek Tiger efficiency in 8010SS nozzles. Fig. 5 shows the details. 4. Discussion In this study, with the objective of increasing NH3 removal efficiency and optimizing spray tower design parameters, different spray nozzles, operating pressures, number of nozzle stages, ammonia concentrations, and two types of scrubbing liquid were used. Considering our validation tests and data, new spray nozzle, operating pressure, dilute H2SO4 solution produced a considerable performance and it was less complicated and costly. The performance of the PJ40 nozzle used by Hadlocon et al. (2014a) and PJ20 nozzle used by Manuzon et al. (2007) for treating air with a low inlet concentration of NH3 was compared to those of the new nozzles (8010SS and 20SS) in this study. According to the findings,
Fig. 3. The removal efficiency of spray tower using 8010SS nozzle in two stages applying caustic scrubbing solutions and NIOSH method 6015.
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Fig. 4. The removal efficiency of spray tower using 8010SS nozzle in three stages applying caustic scrubbing solutions and NIOSH method 6015.
atomization positions and liquid distribution can be obtained by selecting appropriate spray nozzle. Spray nozzle plays an important role in system cost, efficiency, low consumption liquid and droplet size, and operating pressure affects liquid velocity and more contact liquid with pollutants, which subsequently these parameters together have a very important effect on spray tower removal efficiency. Generally, further effort is recommended to upgrade the spray nozzle and to optimize its performance for achieving more absorption for other gases and vapors having low solubility and low Henry constants. A new spray nozzle for occupational NH3 of control and treatment was developed and validated in this study. It is apparent that the 8010SS spray nozzle in this study has more effect on optimization spray tower, highest removal efficiency and low consumption scrubbing liquid. The presented spray nozzle could increase NH3 removal efficiency with low Henry's law constant, lower operational costs compared to other spray nozzles and compared to other studies and appropriate for other pollutants that have similar chemical properties to ammonia. However, further efforts should focus on decreasing pressure drop for spray nozzles and clogging. It is also recommended that this spray nozzle should further be developed for higher ammonia concentrations. Fig. 5. Comparing the removal efficiencies measured by Niosh 6015 and Phochek in 80 μm nozzles.
Conflicts of interest
from 5 to 100 ppm. The difference between our collection efficiency of that with Hadlocon & Manuzon efficiency probably is related to the low range of our average concentrations. The results of performance evaluation of a spray tower in scrubbing H2S from air indicated that the maximum removal efficiency of H2S was 70.53 ± 1.54% at 1800 L min−1 of air flow rate and H2S concentration of 15 ppm. Also, the findings indicated that increases in input flow rate and input concentration lead to decrease in the removal efficiency of the spray tower. Using chemical scrubbing treatment can increase the removal efficiency of the spray tower (Jafari et al., 2015). According to Hadlocon et al. (2014a) a single-stage scrubbing efficiency was 57.4% ( ± 2.8%). Collection increased to about 89% ( ± 2%) when the spray-stages were increased to three. Comparison between the results of the present study with those of Hadlocon revealed higher removal efficiency of this work. According to Hadlocon et al. (2014a), the best removal efficiency with PJ20 and AAP01 spray nozzle were 53.9% and 49.1%, respectively. The results of the present study showed higher removal efficiency, compared to those obtained by Hadlocon, which might be due to constant scrubbing liquid flow rate and a 2500-μm nozzle and smallest droplets and more surface area that create 20SS contrast of PJ20 spray nozzle. Among different spray tower design parameters, spray nozzle and operating pressure play important roles in tower efficiency and optimization. Scrubbing droplet size, liquid flow rate, operating pressure,
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