CHEMICAL ENGINEERING TRANSACTIONS VOL. 78, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-76-1; ISSN 2283-9216 Modelling of the Spray Coating Process with Biodegradable Polymer Solution for Production of Controlled-Release Fertiliser Trung H. Nguyena,*, An M. N. Tranb, Nghi Tranc, Dung V. Trinha aFaculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU-HCM, Vietnam bFaculty of Chemical Engineering, Industrial University of Ho Chi Minh City, Vietnam cResearch and Development Division, Petrovietnam Fertilizer and Chemicals Company, Vietnam 1581011@hcmut.edu.vn The spray coating process is an important process of the coating particle technology for the production of controlled-release fertilisers (CRFs). Its modelling would be helpful in the production process such as increase material usage efficiency, save production costs, and reduce dispersal waste produced that will cause environmental pollution. This article presents the results of an experimental study to establish an empirical model of the spray coating process with a biodegradable polymer solution. The shape and size of the spray area, droplet size distribution (DSD) and Sauter mean diameter (SMD) were determined by using high-speed camera 1000 frames per second (fps), laser transmitter and the image processing toolbox of MATLAB software. The results showed that the spray area had a cone shape, its mean width, corresponding to a spray distance of 300 mm, was 451 mm at a pressure of 0.18 MPa, viscosity of 50 mPa.s, spray flow of 1.5 mL/min and the droplet size distributions were noted 0.153 to 1.449 mm and droplet SMD was 0.876 mm. The model also showed that droplet SMD was calculated by four dimensionless numbers, will be applied to calculate, optimize and simulate coating particle technology. 1. Introduction The enhanced efficiency fertilisers (EEFs) is important in modern crop production, which can help to save time, production costs, reduce losses and environmental impact (Timilsena et al., 2015). Controlled-release fertilisers (CRFs) is one of the EEFs and was used widely in agriculture (Trenkel, 2010). CRFs which have a coating layer to control the release of nutrients. They are usually produced using the coating particle technology, that avoids the changing the current production technology (Timilsena et al., 2015), reduces production and investment costs (Trenkel, 2010). The coating particle technology is applied to form a coating layer which can be classified into 4 types: dry coating, wet coating, melt coating and encapsulation. Among them, the wet coating process is applied the most in industrial production (Saleh and Guigon, 2006). The mechanism of the wet coating process includes spraying and wetting; recrystallization and coated particle (Naz and Sulaiman, 2016). The spray coating process plays an important role in the wet coating process, which decides contact ability of droplets with particles, evaporation of the coating solution and crystallization to form a layer. Determining the mechanism and parameters of the spray coating process will help to adjust the coating process, orient the quality and properties of products. Many experimental and theoretical models of the spray process have been researched and published. Based on the report of Reitz and Bracco (1986), Lefebvre and Mcdonell (2017) described that drop formation mechanism was classified into four regimes of breakup and shown as Table 1. Teunou and Poncelet (2002) expanded and described the model of the spray process based on the equation that had been built by Nukiyama and Tanasawa (1939), then Müller and Kleinebudde (2007) also developed that model. It showed that droplet SMD (d32) depends on air velocity, surface tension, liquid density, liquid viscosity and air-to-liquid volume ratio. Varga et al. (2003) presented the mathematical equation for calculating DOI: 10.3303/CET2078016 Paper Received: 12/04/2019; Revised: 24/08/2019; Accepted: 25/09/2019 Please cite this article as: Nguyen T.H., Tran A.M.N., Tran N., Trinh D.V., 2020, Modelling of the Spray Coating Process with Biodegradable Polymer Solution for Production of Controlled-Release Fertiliser, Chemical Engineering Transactions, 78, 91-96 DOI:10.3303/CET2078016 91 the droplet SMD. According to this equation, Aliseda et al. (2008) established and expanded the model. The dimensionless numbers such as Reynolds (Re), Weber (We) and Ohnesorge (Oh) number had been presented in it. Similarly, Yushchenko and Lapin (2011) developed a mathematical equation for the spray process in fire equipment with We, Laplace number (La), the ratio of the viscosity of liquid and gas and dimensionless number characterizes the nozzle design. Naz et at. (2015) researched to determine the droplet SMD by a high-speed camera and phase Doppler anemometry (PDA) techniques. Recently, Formánek et al. (2019) used a camera system and the image analysis method to determine DSD and SMD in a liquid-liquid dispersions system. This method was also used by Mori (2019) to calculate particle size distributions (PDS). In this study, by using laser light scattering (LLS) techniques and the image analysis method (IAM), DSD, SMD and the size of the spray area were determined. These results were applied to establish an empirical model of the spray coating process, which was developed on basic models that were previously studied. This model would be more suitable for the coating particle process with biodegradable polymer solution than the other to produce controlled-release urea fertiliser (CRUF). Table 1: Classification of the breakup regimes Regime Description Predominant drop formation mechanism Criteria for transition to next regime 1 Rayleigh breakup Surface tension force WeA > 0.4 2 First wind-induced breakup Surface tension force; dynamic pressure of ambient air WeA > 1.2 + 3.4.Oh0,9 3 Second wind-induced breakup Dynamic pressure of ambient air opposed by surface tension force initially WeA > 13 4 Atomization Unknown WeA > 40.3 2. Materials and methods 2.1 Materials The materials for synthesis of polymer include: phosphated distarch phosphate (PDSP, E1412) was provided by Nam Bao Tin company, Vietnam; polyvinyl alcohol (PVA, PCT1316, 99 %) was purchased from HiMedia, India; sodium tetraborate (Na2B4O7.10H2O, 99.5 %), glycerol (C3H8O3, 99 %) were supplied by Guangdong, China; pure polyacrylic acid emulsion (PAA, 2030) was provided by Nuplex Resins company, Vietnam. 2.2 Synthesis of biodegradable mixing-polymer as coating material solution Gelatinized 15 g PDSP in 500 mL distilled water at 75 °C, stirring speed of 350 rpm, for 30 min. 0.3 g Na2B4O7.10H2O was added and kept stirring for 5 min. Then, add slowly 15 g PVA, stir for 30 min, with stirring speed of 450 rpm. The synthesized solution is mixed with polyacrylic acid (PAA) ratio of 3:7 to create a mixing-polymer solution. Next, add 20 mL glycerol and distilled water to adjust the viscosity. The mixing- polymer solution was determined density, viscosity by viscometer Prona RV2 and surface tension by CSC – DuNOUY tensiometer at different temperatures. 2.3 Experimental spray coating process and record images Diagram of the device to record the images spray coating process was shown in Figure 1a. The whole devices were placed in a closed, dark chamber to avoid light and the impact of environmental factors. Prona RA- 100RC-08R nozzle was used for spraying polymer which has a diameter of 0.8 mm, working pressure from 0.1 to 0.3 MPa. Laser transmitter that used to create laser light scattering of droplets, is SDLaser 303 (China), with a wavelength of 532 nm. A digital camera Sony RX100 Mark V (Japan) which has a recording speed of up to , fps, is used to record bright scattering images of droplets during the spray process. The camera and transmitter laser were also exchanged to record images in two dimensions. The experiment was repeated 4 times at an experimental condition and a spray distance (L). The liquid droplets were created by a pipette with a mixing-polymer solution. Their images, laser scattering images and a standard ruler were recorded with a camera speed of 500 fps to determine a scattering ratio (β). 2.4 Analyze images to determine the size of the droplets and spray area Videos with a speed of 1,000 fps, which recorded the spray coating process, are transferred to MATLAB software. They were separated into component images to analyze images. Based on bright image contrast, image processing toolbox analyzed and identified pixels with different brightness (Mathworks, 2016). The results of image analysis help to determine the size of the droplets and the spray area. 92 2.5 Establish an empirical model of the spray coating process by dimension analysis method The parameters of the spray coating process and their dimensions are presented in Table 2. The dimensionless numbers equation is built based on dimensional analysis methods and Buckingham’s pi- theorem (Hanche-Olsen, 2004). It is shown in Eq(1). Table 2: The parameters of the spray coating process i Parameters Symbol Unit Dimension 1 Droplet SMD 𝑑32 m L 2 Width of spray area 𝐵 m L 3 Nozzle diameter 𝑑𝑐 m L 4 Spray distance 𝐿 m L 5 Spray velocity 𝜔𝑙 m/s L.T -1 6 Density of air 𝜌𝑔 kg/m 3 M.L-3 7 Density of solution 𝜌𝑙 kg/m 3 M.L-3 8 Viscosity of air 𝜇𝑔 Pa.s M.L -1.T-1 9 Viscosity of solution 𝜇𝑙 Pa.s M.L -1.T-1 10 Surface tension of solution 𝜎𝑙 N/m M.T -2 11 Air flow 𝑄𝑔 m 3/s L3.T-1 𝑑32 𝑑𝑐 = 𝐴. 𝑊𝑒𝑘1 . 𝐿𝑎𝑘2 . ( 𝜇𝑙 𝜇𝑔 ) 𝑘3 . ( 𝜌𝑙 𝜌𝑔 ) 𝑘4 . ( 𝐿 𝑑𝑐 ) 𝑘5 . ( 𝐵 𝑑𝑐 ) 𝑘6 . ( 𝑄𝑙 𝑄𝑔 ) 𝑘7 (1) Where: A is constant; Weber number, 𝑊𝑒 = 𝜔𝑔 2 .𝜌𝑔.𝑑𝑐 𝜎𝑙 ; Laplace number, 𝐿𝑎 = 𝜎𝑙.𝜌𝑙.𝑑𝑐 𝜇𝑙 ; spray flow, 𝑄𝑙 = 𝜔𝑔.𝜋.𝑑𝑐 2 4 ; the coefficients of equation, 𝑘𝑖 (i = 1 – 7). 3. Result and discussion 3.1 Properties of biodegradable mixing-polymer solution PDSP reacted with PVA, under the action of sodium tetraborate, creating a biodegradable polymer solution. This polymer is mixed with PAA to synthesize a mixing-polymer solution used as a coating material for CRUF (Nguyen et at., 2019). The change of solution parameters according to temperature was shown as Figure 1b. The surface tension and density of solution change very little with temperature, but its viscosity is too high and change drastically. It must be adjusted with distilled water to achieve a suitable value for the spray process. The results of adjusting the viscosity are described as Table 3 (a) (b) Figure 1: Diagram of the device to record the images (a) and change of polymer solution parameters according to temperature (b) Table 3: Results of adjusting the viscosity of the solution No. Volume PDSP – g – PVA (mL) Volume PAA (mL) Volume water (mL) Viscosity (mPa.s) Surface tension (mN/m) Density (kg/m3) 1 150 350 2.5 115.8 71.0 1,019.4 2 150 350 7.5 63.84 68.5 1,010.3 3 150 350 12.5 50.07 64.6 982.8 4 150 350 15 38.47 62.2 968.3 30 40 50 60 70 80 0 100 200 300 400 500 600 700 800 Temperature ( o C) 60 62 64 66 68 70 72 74 76 78 80 900 920 940 960 980 1,000 1,020 1,040 1,060 1,080 1,100 Viscosity (mPa.s) Surface tension (mN/m) Density (kg/m 3 ) 93 3.2 Determine the parameters of the spray process by image analysis method The result on image analysis of the ruler is shown as Figure 2a. The width of the ruler is 67 pixels, corresponding to 25.4 cm (10 inches) in fact, the conversion factor is 0.3791 pixels/mm, which helps to convert the size from pixel to mm unit. Figure 2c and e describe the result on image analysis of droplet and laser light scattering droplet. These results help to determine the scattering ratio, the ratio of the size of the droplet and scattered droplet. The mean scattering ratio with 20 samples is calculated of 1.504, an error is 0.126, standard deviation (SD) of 0.1539, which shows the repeatability and accuracy of the experimental results. (a) (b) (c) (d) (e) Figure 2: (a) An image of a ruler; (b) an image of a droplet; (c) results on image analysis of droplet; (d) an image of the laser light scattering droplet and; (e) result on image analysis of laser light scattering droplet. The real-time of a video, which was recorded by the camera at an experimental condition and a spray distance (L), was 1 s. This video contains about 1,260 frames and is read by software. About 500 frames were separated and filtered for image analysis. The experimental image of the process and laser light scattering of droplets are described as Figure 3a. The image analysis results of the droplet at the spray distance as 10, 20, 30 cm with Ql = 2 mL/min, pressure (P) of 0.18 MPa, 𝜇𝑙 = 63.84 mPa.s are shown as Figure 3b, c and d. The nozzle creates a circular spray surface, so the spray area shape is cone with two main size parameters: the spray distance and the width of the spray area. The mean width of the spray area (�̅�), error and SD at P = 0.18 MPa were calculated and given results in Table 4. (a) (b) (c) (d) Figure 3: (a) An image of the spray coating process; image analysis results of the droplet at spray distance; (b) L = 10 cm; (c) L = 20 cm (c); (d) L= 30 cm Table 4: The mean width of the spray area Exp. Experimental conditions L = 10 cm L = 20 cm L = 30 cm �̅�, 𝑚𝑚 Error SD �̅�, 𝑚𝑚 Error SD �̅�, 𝑚𝑚 Error SD 1 Q = 1.5 mL/min; μ = 63.84 mPas 165.58 15.90 18.656 298.78 24.730 34.227 439.85 15.471 34.660 2 Q = 2 mL/min; μ = 63.84 mPas 206.78 29.334 24.227 273.15 16.356 20.997 361.77 30.426 38.224 3 Q = 1.5 mL/min; μ = 50 mPas 232.43 18.975 22.996 319.25 23.977 29.537 451.15 17.627 20.846 The results in Table 4 show that when the viscosity of solution decreases the size of the spray area is widened because the spray speed increased and the ability to create the droplet is more easily. When the spray flow increases, the spray area is narrowed because the fluid rate is slower while the air flow does not change. The DSDs are obtained from the image analysis results, corresponding to different experimental conditions and spray distances, described in Figure 4. The SMD decreases as the spray distance increases because the droplets are broken up and dispersed more. As the spray flow increases, the SMD decreases which indicate that the increase of the flow will increase the resistance of fluid, so the SMD and the spray area increase. 94 Conversely, when the viscosity of solution decreases, the SMD is larger and the spray area is widened, due to the resistance of the fluid decreases in the nozzles. (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 4: Distribution of droplet size corresponding to spray distances at experimental conditions as Table 4 with experiment No. 1 (a, d, g), No. 2 (b, e, h) and No. 3 (c, f, i) 3.3 Determine the model of the spray coating process The coefficients of Eq(1) are determined based on the experimental results by using the least-squares regression in Microsoft Excel. The model of the spray coating process is obtained and shown as Eq(2): 𝑑32 𝑑𝑐 = 1.62. 𝑊𝑒−0.00565 . 𝐿𝑎0.8714. ( 𝐿 𝑑𝑐 ) −0.2022 . ( 𝐵 𝑑𝑐 ) −0.2839 (2) The Eq(2) show that the ratio of density, viscosity, flow of liquid and gas do not affect the droplet SMD. Evaluating the calculated from Eq(2) ((𝑑32)𝑐𝑎𝑙.) and the experimental results ((𝑑32)𝑒𝑥𝑝.) are shown in Figure 5. Figure 5: Evaluating the calculated and experimental results Figure 5 shows that the calculated size of the droplets and experimental SMD are equivalent to each other. The result of evaluating the data compatibility by Fisher distribution also noted the coefficient of determination 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 Diameter, mm D is tr ib u ti o n c u rv e 0 10 20 30 40 50 60 70 80 90 100 L = 10 cm C u m u la ti v e c u rv e SMD = 1.039 mm 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 C u m u la ti v e c u rv e Diameter, mm L = 10 cm SMD = 0.939 mm 0 10 20 30 40 50 60 70 80 90 100 D is tr ib u ti o n c u rv e 0 2 4 6 8 10 12 14 16 18 20 0 3 6 9 12 15 18 21 24 27 30 C u m u la ti v e c u rv e D is tr ib u ti o n c u rv e Diameter, mm SMD = 1.455 mm L = 10 cm 0 10 20 30 40 50 60 70 80 90 100 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 Diameter, mm D is tr ib u ti o n c u rv e SMD = 0.805 mm L = 20 cm 0 10 20 30 40 50 60 70 80 90 100 C u m u la ti v e c u rv e 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 Diameter, mm D is tr ib u ti o n c u rv e SMD = 0.835 mm L = 20 cm 0 10 20 30 40 50 60 70 80 90 100 C u m u la ti v e c u rv e 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 Diameter, mm D is tr ib u ti o n c u rv e SMD = 1.093 mm L = 20 cm 0 10 20 30 40 50 60 70 80 90 100 C u m u la ti v e c u rv e 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 Diameter, mm D is tr ib u ti o n c u rv e L = 30 cm SMD = 0.643 mm 0 10 20 30 40 50 60 70 80 90 100 C u m u la ti v e c u rv e 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 L = 30 cm D is tr ib u ti o n c u rv e SMD = 0.693 mm 0 10 20 30 40 50 60 70 80 90 100 Diameter, mm C u m u la ti v e c u rv e 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 3 6 9 12 15 18 21 24 27 30 D is tr ib u ti o n c u rv e 0 10 20 30 40 50 60 70 80 90 100 Diameter, mm C u m u la ti v e c u rv e SMD = 0.876 mm L = 30 cm 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 (d 3 2 ) e x p . (m m ) (d 32 ) cal. (mm) 95 R2 is 0.968, F = 33.6 > F0.95,4,5 = 4.757, so they fit. This model can be used to calculate the actual spray coating process and application for similar processes. 4. Conclusions The results of the study show that the authors selected and determined the method for synthesizing biodegradable mixing-polymer from PVA, PDSP and PAA. This polymer can be used as a coating material to apply in coating particle technology for production of CRUF, which is low cost, environmentally friendly. The parameters of the spray coating process such as the size of the spray area, DSD and SMD were determined by high-speed camera, combining image processing toolbox of MATLAB software. Those results help to 2.5 establish and complete the empirical model of the spray coating process. This is the basis to calculate and control it. 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