Analysis of Wetting Behavior and Solidification Process of Molten Urea

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Analysis of Wetting Behavior and Solidification Process of Molten Urea on a Superhydrophobic Surface and its Application in Large Granular Urea Production Xiaoqian Deng, Rongjie Xu, Chao Yang, Ji Li, Yuanlong Wang, Pan Wu, Changjun Liu, Houfang Lu, and Wei Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b03078 • Publication Date (Web): 27 Jul 2019 Downloaded from pubs.acs.org on August 2, 2019

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ACS Sustainable Chemistry & Engineering

(1): Xiaoqian Deng

E-mail: [email protected]

(2): Rongjie Xu


(3): Chao Yang


(4): Ji Li


(5): Yuanlong Wang


(6): Pan Wu


(7): Changjun Liu


(8): Houfang Lu


(9): Wei Jiang


Address of all authors: No.24 South Section 1, Yihuan Road Chengdu, Sichuan, PR China, 610065

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Analysis of Wetting Behavior and Solidification Process of Molten Urea on a Superhydrophobic Surface and its Application in Large Granular Urea Production Xiaoqian Deng, Rongjie Xu, Chao Yang, Ji Li, Yuanlong Wang, Pan Wu, Changjun Liu, Houfang Lu, Wei Jiang Low-Carbon Technology and Chemical Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu, 610065, PR China

KEYWORDS: Urea melt, Superhydrophobic surface, Granulation, Wetting behavior, Solidification.

ABSTRACT:

The

wetting

behavior

of

polar

high-temperature

melt

on

superhydrophobic surfaces is rarely studied, although water wetting process under normal temperature has been widely investigated. In this work, molten urea was considered as the typical polar melt substance, and its wetting behavior and solidifying process on a Polytetrafluoroethylene (PTFE) -coated superhydrophobic stainless steel surface (PSSSS) was investigated. The results confirm the super-repellency of PSSSS on molten urea droplets with a static angle of over 155°and a rolling angle of 3±1°,

 Corresponding author. Tel.: +86-28-85990133; fax: +86-28-85460556 E-mail address: [email protected] 2 ACS Paragon Plus Environment

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which is consistent with the Cassie-Baxter state. Such a super-urea-melt-phobic state is ascribed to the high roughness of the PTFE-coated surface and high cohesive energy density difference between the urea and PTFE. The solidification process of the urea melt on PSSSS occurred from the outside to inside in 44 s at 18 °C to form a compact urea granule of large size and high mechanical strength. This occurrence provides a feasible and promising granulation strategy to produce qualified large urea granules using a green, simple, and cost-effective process.



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INTRODUCTION Superhydrophobic surface is a natural functional surface, which can repel water with a static contact angle of over 150°and a rolling-off angle of under 10°1-3. This attractive property provides the superhydrophobic surface some exceptional characteristics, such as self-cleaning, anti-icing, drag-reduction, anti-fogging, and anti-fouling4-8, which exhibit promising application prospects in industrial production and daily life. However, the current studies on superhydrophobic surfaces mainly focus on the wetting behavior of water and oil at room temperature. Its application scope is limited in aqueous systems, such as pure water, oil/water mixture, water solution, and water slurry6, 9-13, although the preparation of superhydrophobic surface is increasingly cost-effective, green, and simple. Only a few non-aqueous systems, such as glycerol and ethanol, have been investigated to determine their wettability on superhydrophobic surfaces 9, 14. These results suggest a possible and interesting assumption that polar liquids with high surface tension could be repelled by superhydrophobic surfaces with low surface energy. However, no cases with respect to the wetting behavior of the molten substance, which is solid at room temperature but liquid at high temperature, on superhydrophobic surface have been reported so far. Thus, if these molten substances with high surface tension under high temperature exhibit similar wettability on a superhydrophobic surface, the research and application range of the superhydrophobic surface can be effectively broadened, and more industrial processes can benefit from this functional material.


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Urea is a typical polar melt substance with a high surface tension of 55.3 mN/m at over 132.8 °C. In the industry, the urea product is usually expected to be fabricated as large spherical granules because of its high compressive strength, perfect appearance, low hygroscopicity, and slow fertilizer efficiency loss rate when compared to small particle products15-16. Currently, two processes are mainly employed for urea granulation, including the urea prilling tower process and fluid-bed granulation17. However, it is difficult to obtain large granular urea products with diameter of greater than 4 mm18 using the former. Moreover, with the latter, it is difficult to obtain products of uniform size without using a screening operation. Besides, a common inevitable problem in these two processes is the dust pollution generated during the granulation process, which results in environmental and health risks. The reported values of dust generated during the prilling tower and fluid-bed processes are respectively 30–60 kg/h and 6–18 kg/h without using an extra dedusting equipment19. Owing to severe environmental protection regulations, the allowable emission of dust generated during urea production in China have reduced from 150 to 30 mg/m3 in 201720. It is environmentally crucial and commercially significant to effectively eliminate the dust risk in urea granulation operations while achieving qualified large granular urea products21-22. The inevitable generation of urea dust in the current industrial granulation process is attributed to the small urea droplets ejected from the prilling sprayer in high tower, unstuck urea spray drops on seed during fluidization, and crashing and breaking of urea droplets as well as solidified particles during their motion and transportation. The 5 ACS Paragon Plus Environment


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traditional solutions usually employ a dedusting equipment, such as a wet cleaning equipment, cyclone separator, and a bag filter23. With the increasingly strict demands with respect to environmental protection, the energy consumption and equipment expense of these traditional dedusting processes have increased considerably. Devising a simple, green, and cost-effective alternative granulation process without the generation of urea dust is a fundamental necessity for urea production. Such an effective purpose can be realized using the Rotoform granulation systems24 developed by the IPCO International Ltd. In this process, the urea melt is dropped on a surface of motion belt to solidify and form pastilles, and the product obtained is collected. However, due to the high surface tension of urea melt, the urea granule product obtained from this process is hemispherical and adheres on the belt surface. The collection of this adhered hemispherical urea granules still results in the generation of some urea dust. The low sphericity of urea particles also causes bad granule fluidity and high moisture-absorbency. A dust-proof urea granulation process can be developed by introducing the possible super-repellency effect of urea melt on superhydrophobic surface into this belt granulation process. The granulation of aqueous system on superhydrophobic surface has already been achieved and exhibits compliance with the above-mentioned requirements. Molecular sieve slurry and some salt solution can be shaped into perfect spheres using the simple rolling-spheronization granulation process on a superhydrophobic Cu mesh without detectable adhesion, dusting, and breaking. The


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obtained particles possess satisfactory mechanical strength, excellent sphericity, and desired uniformity12. These advantages can fulfil the demands of urea granulation reformation. This water-based granulation technology can be transplanted on superhydrophobic surfaces for conducting the urea melt granulation process. However, the possibility of developing a melt granulation process based on this strategy must be systematically investigated, because the wetting behavior and solidification process of molten urea on superhydrophobic surface are not investigated, and the stability of superhydrophobic surface under such high temperature conditions need to be examined. In this research, a superhydrophobic surface with high thermostability is developed. The wetting and motion behavior of molten urea on the superhydrophobic surface are investigated to confirm the super-repellency conjecture of urea melt on such surfaces and determine the wetting model of the urea melt. The solidifying process of molten urea on the superhydrophobic metal plane is observed, and the corresponding solidification model is established. The feasibility of preparing large urea granules on superhydrophobic surface is discussed based on the obtained models. EXPERIMENTAL PROCEDURE Materials. The 316 stainless steel (06Cr17Ni12Mo2) was purchased from Shanghai Shengzhuo Materials Co. Ltd., China. The polytetrafluoroethylene (PTFE) nanoparticles (approximately 300 nm) were purchased from Shanghai Titan Science and Technology Co.Ltd. The adhesive employed is Evo-Stik Serious Glue from Bostik Co. Ltd. The polyurethane (PU) sponge (100 kg/m3) was obtained from Dotaisp Sponge 7 ACS Paragon Plus Environment


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(China). Urea (≥ 99% wt) was acquired from Kelong Chemical Co. Ltd. (Chengdu, China), and commercial urea granules were from Sichuan Meifeng Chemical Industry Co. Ltd. All other chemical reagents used were of analytical grade and obtained from Kelong Chemical Co. Ltd. (Chengdu, China). Preparation of polytetrafluoroethylene-coated superhydrophobic stainless steel surface (PSSSS). A facile adhesion method25-26 shown in Scheme S1 was used to prepare the superhydrophobic surface for urea melt. About 1 g glue was added into 30 g of absolute ethanol and mechanically stirred for 10 min to obtain a uniform glue dilution27. Then, this glue dilution was sprayed on a stainless steel sheet cleaned by deionized water and ethanol sequentially. After the sheet was dried at 80 °C for 5 min to evaporate ethanol, a uniform thin half-dried glue layer formed on the sheet surface. A PU sponge was used to dip and gently press the PTFE powders onto the sheet surface with the glue layer. It was ensured that sufficient PTFE powders were transferred so that they adhered well on the surface by the glue layer. Finally, this sheet with adhered PTFE particles was dried at 80 °C for 1 h in the oven to obtain the required PSSSS. Characterization. The surface morphology of PSSSS was observed using the scanning electron microscopy (SEM; JEOL, Model JSM-7500F) technique. The loading of PTFE was confirmed by the Raman scattering spectra with a Thermo Fisher DXR Microscope. The static contact angle (CA), advancing contact angle (ACA), receding contact angle (RCA), and rolling angle (RA) for the urea melt on PSSSS were measured by a CA meter (powereach, JC2001C1) at 135 °C. A high-speed camera 8 ACS Paragon Plus Environment

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(FASTCAM Mini WX100) was used to record the rolling, bouncing, and solidification behavior of molten urea on PSSSS at room temperature (18 °C). Solidification and granulation of urea melt on PSSSS. The solidification of molten urea on the cold PSSSS was observed. A total of three experiments were conducted to simulate possible granulation operations based on the solidification process. Static solidification: In this process, the urea with arbitrary mass was placed on a horizontal PSSSS. A heating panel under the PSSSS was used to heat urea to its melting point of 132.8 °C. After complete conversion to liquid state, the wettability of the urea melt on PSSSS could be determined. The heating panel was turned off and the urea melt droplet was allowed to naturally cool down and solidify. The time utilized for the complete solidification was determined, and the sizes of the urea granules were determined for calculating the sphericity. The complete melting and solidifying process is shown in detail in video of the whole melting and solidifying process of the urea on PSSSS Static molten-out granulation (SMG): This process was developed based on static solidification and achieved by replacing the melting process of solid urea with directly dropping the molten urea droplets on PSSSS. More than 50 urea granule samples with similar mass values were prepared for statically determining the distribution of particle size, mechanical strength, and sphericity. Rotational granulation (RTG): In this process, the static horizontal PSSSS panel was replaced by a rotating bowl-like PSSSS container fixed on a stirring shaft. With the 9 ACS Paragon Plus Environment


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slow rotating motion of the PSSSS bowl, the dropped urea melt rolled and solidified to form spherical granules. Rolling granulation (RLG): This process was conducted on an inclined PSSSS plane with a length of 60 cm and tilt angles of 5°, 10°, and 20°. The molten urea was dropped on the initial part of the inclined plane and rolled along the plane by propelling itself due to gravity. The solidified particles collected at the terminal was the final urea granules product. Molecular dynamics simulation: The interaction between PTFE and urea was investigated through a molecular dynamics simulation with the Forcite module, and other liquids, such as water, and n-Hexane, were used for comparison purposes28-29. The cohesive energy density (CED) and solubility parameter () were obtained according to the input parameters listed in Table 1. Table 1. Parameters for molecular dynamics simulation Initial density

Number of

Box

g/cm3

atoms

length/Å

PTFE

2.22

620

PTFE

2.22

urea

forcefield

T/℃

19.6

Compass

133

620

19.6

Compass

25

1.28

800

19.8

Compass

133

H2O

1

600

18.2

Compass

25

n-Hexane

0.66

800

20.5

Compass

25

δ was obtained using the following equation: 10 ACS Paragon Plus Environment

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(1)

𝛿 = √𝐶𝐸𝐷

The larger these two values, the greater the intermolecular force and the easier for the molecules to blend. If two substances are mixed, and the difference in their δ values ∆δ is within the scope of Eq.2, they can be considered miscible. However, if ∆δ is beyond this scope, the two substances are immiscible, and the attractive force between their molecules are negligent causing the repellency effect in consideration of the surface tension30.

∆𝛿 = |𝛿𝐴 − 𝛿𝐵 | > (1.3~2.1) 𝐽

1⁄ 2

· 𝑐𝑚

−2⁄ 3

(2)

Solidification simulation of molten urea on PSSSS: The solidification of urea melt droplet on PSSSS was interpreted by the finite element method (FEM) simulation31-32. The solidification and melting model of Fluent software was employed. Only a pure PTFE surface was constructed as the solid substrate surface, because only the loaded PTFE particles contact with molten urea in the real process. The rough structure constructed by the PTFE particles and entrapped air inside were neglected, because the effective heat transfer area for urea melt through PSSSS was minimal in comparison with the heat dissipation area of urea droplet with the surrounding air. The actual shape and size of a 20 mg urea droplet on PSSSS was considered as the object for calculation. The other parameters set for calculation are listed in Table 2. Table 2. Parameters for solidification & melting simulation of urea granule Cp



J/(kg·K)

W/(m·K)

Density Melting-heat kg/m3

kJ/kg

Solidifying point K 11

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Air

1006.43

0.0242

1.225

——

——

PTFE

1.05

0.255

2200

——

——

Urea-liquid

2375

0.5

1280

4162.5

405.95

Urea-solid

1548.6

0.0454

1335

4162.5

405.95

RESULTS AND DISCUSSION The SEM images of the PSSSS shown in Figure 1A confirmed the rough structure created by the aggregated nanoparticles. This non-uniform aggregation structure originated from the cumulated PTFE particles and was shaped due to the uneven compressions with sponge. This structure provided the required roughness for the superhydrophobic surface. The obtained PSSSS samples shown in Figure 1B exhibit strong Raman characteristic peaks of PTFE at 1300 and 1400 cm−1, which can be attributed to long carbon-carbon bonds, and 600 and 750 cm−1, which can attributed to the carbon-fluorine bonds. This indicates that the PSSSS was completely covered with PTFE. However, no observable peaks of stainless steel with glue (SSG) are detected. The determined static contact angle (CA) and rolling angle (RA) values of water on PSSSS

are

158.2±2.1° and

3.5±1.5° (Figure

1C),

respectively.

The

superhydrophobicity of PSSSS with low adhesion was thus confirmed. Finally, Figure 1D shows the uniformity of PSSSS prepared by the facile glue and sponge pressing procedure. It is observed from this figure that the randomly dropped water in red can consistently maintain the spherical shape anywhere.


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Figure 1. Characterization of PSSSS. A: Rough structure created by the aggregated PTFE nanoparticles; B: Raman diffuse spectra; C: Contact angle and rolling angle of water; D: Uniformity of PSSSS

The stability of superhydrophobicity of PSSSS was determined by placing the sample under natural conditions and immersing it in urea melt, as the conceived application scenario was with urea melt in air. The variations in the water CA (WCA) values are shown in Figure 2A and Figure 2B. It is observed from these figures that the PSSSS remains unchanged with a WCA of 156.9±1.8°after 400 days in air due to its excellent superhydrophobicity. After immersing the PSSSS sample into molten urea at 138±3 °C, it is discovered that its measured WCA always remains higher than 150°, although the contact angle fluctuates. Moreover, during the urea melt immersing process, no urea was transported when the sample was retrieved, which indicates the super-repellency of PSSSS to urea melt. Therefore, the prepared PSSSS can maintain



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superhydrophobicity in both air and molten urea environment and demonstrate superurea-melt-repellency.

Figure 2. Durability of PSSSS superhydrophobic property. A: in air; B: in urea melt. The wettability of urea melt on PSSSS was then determined. As shown in Figure 3A and Figure 3B, the determined CA and RA values of 5 mg urea melt on PSSSS at 140±3 °C were estimated to be 156.8±2.3°and 3.5±1.0°, respectively. The high value of CA and the low value of RA of the urea melt on PSSSS confirmed its super-ureamelt-repellency and suggested that the wetting behavior adopted the Cassie-Baxter model. The ACA of urea melt on PSSSS is estimated to be 157.4°(Figure 3C), while the RCA is 155.0°(Figure 3D). The calculated contact angle hysteresis was only 2.4°. This value confirmed a negligible dynamic adhesion and indicated that the urea melt could maintain the spherical shape while rolling on PSSSS. The urea melt was dropped on a PSSSS and the resulting behavior was recorded using a high speed camera. The serial pictures in Figure 3E confirms the bouncing and rolling of the urea melt droplet on the PSSSS. When the droplet falls on PSSSS, it bounces up immediately at 0.152 s. This bouncing is repeated successively, as the jump height declines gradually. After 14 ACS Paragon Plus Environment

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0.204 s, the droplet stops bouncing but continues to roll along the PSSSS and finally rolls away. This bouncing and rolling behavior further confirmed the super-urea-meltphobicity and low adhesion of urea melt on PSSSS. This was beneficial for the urea melt rolling-spheroidization granulation process, as the super-repellency guaranteed the spherical shape of urea melt, and the low adhesion guaranteed the rolling status.

Figure 3. The wetting behavior of urea melt on PSSSS including. A: static contact angle (CA); B: rolling angle (RA); C: advancing contact angle (ACA); D: receding contact angle (RCA); E: bouncing and rolling behavior.

It is important to analyze the occurrence mechanism of the super-repellency status of urea melt on PSSSS. As known, the low intermolecular force and high roughness are the key factors in generating superhydrophobicity33-35. Therefore, the intermolecular



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force between the urea and PTFE was calculated to identify the possible reason for this super-urea-melt-repellency effect. The amorphous molecular model, as shown in Figure S1, was constructed for calculating CED and δ values of PTFE and the liquids, including urea, water, and nHexane. From the results listed in Table 1, it is observed that the CED values of pure water, urea, n-Hexane, and PTFE are respectively 27.6108, 15.7108, 1.77108, and 1.21108 indicating a decreasing order. This sequence is in agreement with their respective polarity sequences (in terms of the greatest to the least) indicating that the CED value is available to characterize the polarity of different substances. The CED values of water and urea are observed to be much higher than that of PTFE; however, the CED value of n-Hexane was close to that of PTFE. Table 3. Simulation results of CED and δ of different substances Cohesive energy density (CED) J·m-3

Solubility parameter (δ) J1/2·cm-3/2

PTFE-133 °C

1.13108

10.6

PTFE-25 °C

1.21108

11.0

urea

15.7108

39.7

H2O

27.6108

52.6

n-Hexane

1.77108

13.3

∆δ-urea

∆δ-H2O

∆δ-n-Hexane

29.1

41.6

2.3

∆δ-urea, ∆δ-H2O, ∆δ-n-Hexane: the calculated values corresponding to the solubility parameters of urea, H2O, and n-Hexane compared to PTFE according to Eq.2.


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The solubility parameter ∆δ values of PTFE, urea, water, and n-Hexane were calculated next. It can be observed from the table 3 that ∆δ-urea and ∆δ-H2O are calculated to be 29.1 and 41.6, respectively, which are much higher than 2.1 J1/2·cm-3/2 according to Eq.2, suggesting the strong repellency of PTFE to water and urea. However, the value of ∆δ-n-Hexane is only 2.3 suggesting the high attraction of PTFE towards n-Hexane. This result deduced from the simulated parameters was observed to agree well with the observed occurrence demonstrated in Figure 4A. The CA value of urea and water on a smooth PTFE surface with intrinsic wettability was determined to be 109.1±0.7°and 119.7±1°, while that of n-Hexane was estimated at only 7.4±0.5°. This result also confirmed that the solubility parameter could be effectively applied for qualitatively determining the miscibility of polymers and small molecule substances, although it was originally considered only for polymers30.

Figure 4. The superwettability of different liquids. A: urea, water and n-Hexane on a smooth PTFE plate and on PSSSS; B: rough structure of PSSSS surface; C: possible contact area of PSSSS; D: ideal stacking mode of PTFE particles on PSSSS. 17 ACS Paragon Plus Environment


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As shown in Figure 4A, the apparent CA values of urea, water, and n-Hexane on PSSSS are determined to be 156.8°±2.3°, 158.2°±2.1°, and 0°±0°, respectively. These values were determined based on the Cassie-Baxter model defined as follows36: 𝑐𝑜𝑠𝜃𝐶𝐵 = 𝑓𝑆 𝑐𝑜𝑠𝜃𝑌 − 𝑓𝑉

(3)

Where θCB is the apparent contact angle; θY is the Young’s theoretical contact angle, which is experimentally determined; fS and fV are the fractions of solid and vapor (air) contacting the urea melt, and obeying 𝑓𝑉 = 1 − 𝑓𝑆

(4)

The value of fS in Eq.4 is determined in two steps. Firstly, the projected area of the elevated parts shown in the SEM of PSSSS (Figure 4B) that are highlighted in brilliant blue in Figure 4C is measured. The area fraction of these protruded PTFE aggregations was calculated to be 0.231. This value is termed f SA, which is an average of the statistical results of several different SEM images. This calculation was performed with the assumption that the aggregation comprises uniform PTFE nanoparticles with a radius of r, and the PTFE particles are closely arranged in a hexagonal closest packed or a face-centered packed form (Figure 4D). The particle occupied area ratio (φ) of these two forms are obtained using Eq.5 and Eq.6 as follows: 3𝜋𝑟 2

𝜑𝐻𝐶𝑃 = 6√3𝑟 2 = 0.907 𝜑𝐹𝐶𝐶 =

2𝜋𝑟 2 8𝑟 2

= 0.785

(5) (6)


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The two obtained values 0.907 and 0.785 can be considered the maximum and minimum values of φ, and the actual φ is in between these two values. Therefore, the actual contact area fraction fS is obtained using Eq.7. 𝑓𝑆−𝐻𝐶𝑃 = 0. 907𝑓𝑆𝐴 = 0.2095

𝑜𝑟 𝑓𝑆−𝐹𝐶𝐶 = 0.785𝑓𝑆𝐴 = 0.1813

(7)

The apparent CA of melt urea is calculated based on the Cassie-Baxter model using Eq.8 as follows: 𝜃𝐶𝐵 = 149.2° ± 0.3°~151.4° ± 0.3°

(8)

Similarly, the calculated apparent CA of water on PSSSS is 153.4°±0.4°~ 155.3°±0.4°. These two values obtained using the Cassie-Baxter model were marginally different from the determined CA values of urea and water on PSSSS 156.8°±2.3°and 158.2°±2.1°, respectively. This deviation was attributed to the noncompact accumulation of nonuniform PTFE particles and their imperfect sphericity in reality, which leads to higher roughness and higher CA than in ideal simulation. This fact reaffirmed that the wetting behavior of urea melt on PSSSS conformed to the Cassie-Baxter model. The solidification of urea melt on PSSSS was recorded and is shown in Figure 5A. After dropped onto the PSSSS at 18 °C, the urea melted at 145 °C, cooled down rapidly, and underwent solidification after reaching its melting point. In 0.65 s, a vague solid skin can be observed, as the transparent drop started to turn turbid. After 2.65 s, a white solid shell is detected, and the entire drop turns translucent with white flocculation. After 4.65 s, the drop further turns opaque, and finally becomes a complete white solid 19 ACS Paragon Plus Environment


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ball in 8.15 s. The solidification of urea melt occurred from the outside to inside and consumed only approximately 8 s. This conformed a typcial dropwise freezing process by air, which is analogous to the occurrence in prilling tower. The solidificaion of urea melt was simulated, and this simulation is shown in Figure 5B. From this figure, it was confirmed that the solidification occurred from the outside to inside, as the shell in blue is observed to turn increasingly thicker at room temperature. The sensible heat of urea melt was released in 2.8 s with the surface temperature attaining the room condition, and the internal temperature was already close to the freezing point of urea melt (132.8 °C). At 8 s, the thickness of the solidified layer was estimated to be 0.44 mm accounting for approximately 27.3% thickness with a long diameter of 3.245 mm as the base. At this time, the semi-solidified urea granule could be considered as a solid with considerable mechanical strength and removed from PSSSS for further cooling. However, the complete solidification of the urea droplet actually ended at 44 s. Before this terminal time, a liquid core beyond the melt point always existed inside, although its volume gradually decreased over time. The simulated solidifying process conducted at the PSSSS temperature of 90 ℃ is shown in Figure S2. These results suggested that the heat transfer of the urea melt during solidifying process mostly relied on the thermal conductivity through the solidified shell and not the small contact area between the PSSSS and urea. The main resistance was caused by the solidified urea shell.


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Figure 5. Solidification of urea melt on PSSSS at room temperature (18°C). A: by experiment; B: by simulation.

This deduction was confirmed by Figure 6, which shows the liquid phase fraction (fL) of the urea melt droplet against time, when the temperature of PSSSS is 18 °C and 90 °C. The decreasing tendency of the two plots almost overlapped at the early stage, although the total time utilized for the complete solidification of urea droplet at 90 °C is 54 s and that at 18 °C is 44 s. During the early stage, the fL value rapidly decreases to 0.5 in 6–8 s indicating that the granule can be removed from PSSSS for further operation with sufficient mechanical strength. In the successive stage, due to the greater tempeature differences in urea granules, the freezing rate at 18 °C is observed to be relatively faster than the one at 90 °C.



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Figure 6. Liquid phase fraction of ure melt vs. Time After the spontaneous spheroidization and solidification of the urea melt on PSSSS were determined, a simple granulation process was developed to obtain large granular urea products. The schematics of the three possible granulation processes, including the SMG, RTG, and RLG are shown in Figure 7A. The SMG process was conducted by dropping a urea melt droplet onto a horizon PSSSS and solidifying without any extra motion, which is similar to the Rotoform process. The RTG and RLG processes introduced motion to eliminate the flattening of granules due to gravity and improve the sphericity of urea granules. All these processes were attempted and compared for identifying the most feasible technology for possible future large-scale applications in industries. The optical pictures shown in Figure 7B exhibit the outline of the urea granules with varied mass values acquired by the SMG. It can be observed that as the mass increases, the shape of the urea granule tends to be oblate with lower sphericity, although the 22 ACS Paragon Plus Environment

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particle size increases synchronously. This change ascribed to an increase in gravity, which caused the flattening of droplet. The long axis (a) and short axis (b) were measured and plotted in Figure 7C. It is observed that these two axes were almost similar at 5 mg mass and turned increasingly different with an increase in the mass values. However, the axes linearly increased against mass. Meanwhile, the calculated sphericity decreased significantly. It is important to notice that when urea mass is 5mg, the short and long axes of acquired granules are respectively 1.80±0.03 mm and 1.92±0.03 mm with a sphericity of 0.9991. The granules can thus be regarded as perfect spherical particles that fulfil the size requirements of large urea granular products. When the urea mass attains 20 mg, the granule sphericity decreases to 0.9889 with a flattening bottom. Figure 7D shows the size distribution by considering the diameters of standard spherical particles with equal volume as an equivalent diameter and counting the diameter of 50 particles each with 20 mg urea. This statistic results confirmed that the largest difference between the maximum and minimum equivalent diameters is only 0.1 mm suggesting that the particle size distribution of the obtained urea granules could be effectively controlled during the SMG process, in which the uniformity of the granules is guaranteed. However, the sphericity of the obtained urea granules with large diameters was not retained, as the urea mass increased. Introducing rolling and rotating could annihilate this flattening bottom caused due to gravity in the SMG. As shown in Figure 7E and 23 ACS Paragon Plus Environment


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Figure 7F, the urea granules obtained by the RTG and RLG processes possess a perfect sphericity of 0.99971 and 0.99969, respectively, although their surface was relatively rougher than that obtained by the SMG. This roughness ascribed to the close contacting and molding of the urea melt droplet on the rough PSSSS surface during the solidification process.

Figure 7. Urea granule product obtained by different granulation processes. A: schematic of three granulation processes; B: Urea granules obtained by SMG; C:


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Plot of mass, diameter, and sphericity of the granule by SMG; D. Size distribution of the granules by SMG; E: Urea granules obtained by RTG; F: Urea granules obtained by RLG. Another important index of a large urea granule is the mechanical strength. This was tested, and the results are shown in Figure 8. A linear increase of compressive mechanical strength of urea products against particle diameter is observed for SMG. Typically, the granules with 2.23 mm diameter attained a determined strength of 5.33 N/mm, which is significantly higher than that of commercial products of similar size (4.75 N/mm) (Figure 8A). Further, the mechanical strengths of the 20-mg-urea granules prepared by the RTG and RLG were compared with those of the granules obtained by the SMG. The results shown in Figure 8B confirmed that the mean strength of the urea granules obtained by RTG and RLG were 5.85 N/mm and 5.80 N/mm, respectively. This value was significantly lower than the one obtained by SMG (7.87 N/mm). Then the acquired urea granules with PSSSS and the commercial ones were bisected, and the cross section of these granules are shown in Figure 8C. It can be observed that the compactness sequence of the urea granules is SMG, RTG, RLG, and then the commercial urea. The visibly loose and porous internal structure of the commercial urea can be detected suggesting that the compressive strength relied on the interior compactness of the granules. The formation of this loose internal structure can ascribe to the relatively fast heat transfer during the rotating and rolling process. Thus, it can be summarized that the urea granules obtained by the SMG possessed a better strength



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and worse sphericity than the ones obtained by the RTG, RLG, and commercial products from the prilling tower.

Figure 8. Mechanical strength and appearance of urea granule product obtained. A: diameter vs. compressive strength of SMG products; B: Compressive strength obtained by various methods with same diameter; C: Cross sectional view of the granules obtained by various methods.


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CONCLUSIONS In this research, the super-repellency of the urea melt on PSSSS was confirmed, and the wettablity behavior conformed to the Cassie-Baxter model. The determined CA and RA values of the urea melt on PSSSS attained 156.8±2.3°and 3.5±1.0°, respectively. These results confirmed that superhydrophobic surface can effectively repel polar liquids and melts. This super-urea-melt-phobicity originated from the strong immiscibility of PTFE and urea and the rough structure created by the aggregated PTFE particles. The urea melt solidified on PSSSS with an unchanging spherical shape owing to the super-urea-melt-phobicity property and finally formed a large granular urea ball particle with an adequate mechanical strength of 5.33 N/mm. The solidification process happened from the outside to inside. Therefore, the granule turned into a spherical shell and could be removed from PSSSS after 8 s, although the duration utilized for complete solidification was 44 s. The three granulation processes, including SMG, RTG, and RLG, can be developed based on the wetting and solidification of urea melt on PSSSS to prepare high-quality large granular urea products with high sphericity and mechanical strength. The 20-mg-product can attain a strength of 7.87 N/mm with SMG; however, it can achieve sphericity values of 0.99995 and 0.99969 with RTG and RLG, respectively. All the three processes exhibited promising prospects for industrial applications. In summary, PSSSS, a surperhydrophobic surface, can effectively repel polar urea melt at high temperatures, and the spontaneous spheronization and solidification of urea melt 27 ACS Paragon Plus Environment


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on PSSSS can be utilized to develop a facile dust-free large granular urea production process for establishing industrializable technologies.


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Supporting Information. Diagram of preparation of PSSSS; Amorphous molecular model of PTFE, urea, water and n-Hexane for calculating CED and δ; Solidification of urea melt on PSSSS at PSSSS temperature being 90℃ simulated by FEM; Video of the whole melting and solidifying process of urea on PSSSS AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are thankful for the financial support provided by the National Natural Science Foundation of China Project (No. 21676168 & 21476146), and Huo Hua Ku Project of Sichuan University (2019SCUH0022)



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Super-repellency and low-adhesion behavior of urea melt on superhydrophobic surface provides a simple green process for large urea spherical granulation


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Figure 1. Characterization of PSSSS. A: Rough structure created by the aggregated PTFE nanoparticles; B: Raman diffuse spectra; C: Contact angle and rolling angle of water; D: Uniformity of PSSSS

Figure 2. Durability of PSSSS superhydrophobic property. A: in air; B: in urea melt.



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Figure 3. The wetting behavior of urea melt on PSSSS including. A: static contact angle (CA); B: rolling angle (RA); C: advancing contact angle (ACA); D: receding contact angle (RCA); E: bouncing and rolling behavior.

Table 3. Simulation results of CED and δ of different substances Cohesive energy density (CED) J·m-3

Solubility parameter (δ) J1/2·cm-3/2

PTFE-133 °C

1.13108

10.6

PTFE-25 °C

1.21108

11.0

urea

15.7108

39.7

H2O

27.6108

52.6

n-Hexane

1.77108

13.3

∆δ-urea

∆δ-H2O

∆δ-n-Hexane

29.1

41.6

2.3

∆δ-urea, ∆δ-H2O, ∆δ-n-Hexane: the calculated values corresponding to the solubility parameters of urea, H2O, and n-Hexane compared to PTFE according to Eq.2.


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Figure 4. The superwettability of different liquids. A: urea, water and n-Hexane on a smooth PTFE plate and on PSSSS; B: rough structure of PSSSS surface; C: possible contact area of PSSSS; D: ideal stacking mode of PTFE particles on PSSSS.

Figure 5. Solidification of urea melt on PSSSS at room temperature (18°C). A: by experiment; B: by simulation.



Figure 6. Liquid phase fraction of ure melt vs. Time


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Figure 7. Urea granule product obtained by different granulation processes. A: schematic of three granulation processes; B: Urea granules obtained by SMG; C: Plot of mass, diameter, and sphericity of the granule by SMG; D. Size distribution of the granules by SMG; E: Urea granules obtained by RTG; F: Urea granules obtained by RLG.



Figure 8. Mechanical strength and appearance of urea granule product obtained. A: diameter vs. compressive strength of SMG products; B: Compressive strength obtained by various methods with same diameter; C: Cross sectional view of the granules obtained by various methods.


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Analysis of Wetting Behavior and Solidification Process of Molten Urea

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