(Solid + Liquid) Phase Equilibrium in the Aqueous Ternary System

Jun 19, 2015 - In this study, the solubility of the ternary system (NaH2PO4 + CO(NH2)2 + H2O) at (308.15, 328.15, and 348.15) K were determined, using...
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(Solid + Liquid) Phase Equilibrium in the Aqueous Ternary System NaH2PO4 + CO(NH2)2 + H2O at (308.15, 328.15, and 348.15) K Yang Xiao,† Jun-lin Su,*,‡ and Haitao Cao† †

College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, P. R. China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, P. R. China

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ABSTRACT: In this study, the solubility of the ternary system (NaH2PO4 + CO(NH2)2 + H2O) at (308.15, 328.15, and 348.15) K were determined, using isothermal solution saturation. The equilibrium solid phases were analyzed by the Schreinemaker’s method of wet residues and verified by X-ray diffraction (XRD). According to the experimental data, the phase diagrams were plotted and the crystallization areas were determined. The phase equilibrium at different temperatures were compared and discussed. This research provides an accurate answer for whether crystalline hydrate and urea phosphate exist in this system or not. All results can offer fundamental data for crystallization processes and further theoretical studies.

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INTRODUCTION Salt-water phase equilibrium and the phase diagram, an important predicting tool to use for describing the thermodynamic behavior of the crystallization and separation, play a very important guiding role for the relevant process conditions.1,2 Urea, an important organic nitrogen fertilizer,3 is easy to form double salts with other chemical compounds such as salt and acid. It has been found that the combination of urea and phosphoric acid could form a urea phosphate (CO(NH2)2· H3PO4).4 Authors of reference5 investigate the Ca(H2PO4)2 + CO(NH2)2 + H2O system and discover the new compound Ca(H2PO4)2·CO(NH2)2. Authors of ref 6 present the research on whether NaH2PO4 can react with CO(NH2)2 to form a double salt at 298.15 K. However, the data provided is far from enough, so an extensive study at other temperatures needs to be done. This paper could help fill in the blanks of data in this study aspect and provide an accurate answer for whether urea phosphate in this system or not. New experimental data given in this study is useful for researchers and engineers deal with the problem of obtaining and storage of fertilizers containing urea and phosphates. Additionally, the experiment shows the sodium dihydrogen phosphate crystalline hydrate changes with temperature.

measurement. The oscillator was made in Donglian Electronic & Technology Development Co. Ltd., Beijing, China. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer was employed for XRD characterizations. Experimental Method. The method of isothermal solution saturation7−9 was employed to determine the solubility of the ternary system. The famous Schreinemaker’s method of wet residues10−13 was employed to analyze the composition of the equilibrium solid phase indirectly. Because the traditional Schreinemaker’s method may have errors in some cases, the equilibrium solid phase was also tested by XRD to verify the crystalloid composition. In this pre-experiment, the liquid phase of the samples was analyzed at every 2 h in this pre-experiment. It demonstrated that the equilibrium was reached when the analytical results kept constant. It was shown that the phase equilibrium was reached in 10 h. According to a fixed ratio and making sure that one of the components was in excess, the experimental components were added into a series of conical flasks (250 mL) gradually, and the sealed flask was placed into the constant temperature bath oscillator. The oscillator vibrated continuously at the three specific temperatures: 308.15 K, 328.15 K, and 348.15 K (uncertainty, ± 0.3 K). After equilibrium, the oscillation was stopped and the system was allowed to stand for 2 h to make sure that all the suspended crystals settled. The liquid phase and wet residues were transferred to a 250 mL volumetric flask, respectively. Finally, these samples were quantitatively analyzed by chemical methods.

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METHODOLOGY Apparatus and Materials. Sodium dihydrogen phosphate (NaH2PO4, ≥ 0.990 by mass fraction) and urea (CO(NH2)2, ≥ 0.990 by mass fraction) were purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Doubly deionized water (electrical conductivity ≤1 × 10−4 S·m−1) was used in the work. A HZS-HA type constant temperature water bath oscillator with a precision of 0.3 K was employed for phase equilibrium © XXXX American Chemical Society

Received: March 28, 2015 Accepted: June 11, 2015

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DOI: 10.1021/acs.jced.5b00293 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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As indicated in Figure 2 and 3, alongside the curve ET, we connects the composition points of wet residue phase with the saturated liquid phase points and then extends, the point of intersection is approximately the solid phase component for CO(NH2)2. The same method is applied to determine the equilibrium solid phase component of TF, and the result turns out to be NaH2PO4·2H2O. Similarly, the equilibrium solid phases of A and B at 328.15 K are CO(NH2)2 and NaH2PO4· H2O, respectively; and the equilibrium solid phases of A and B at 348.15 K are CO(NH2)2 and NaH2PO4, respectively. As shown in Figure 4, the equilibrium solid phase of the invariant point C is detected by XRD and verified to be the mixed crystallization region of NaH2PO4·H2O and CO(NH2)2. Consequently, the system belongs to a simple eutectic type and neither double salt nor solid solution is formed at the investigation temperature. Figure 2 shows that the area of EWF represents is unsaturated region at 308.15 K; AET is crystallization region of CO(NH2)2, whereas TFB denotes crystallization region of NaH2PO4·2H2O. Zone ATB represents the mixed crystallization zone of NaH2PO4·2H2O + CO(NH2)2. In Figures 3 and 5, this diagram further illustrates that the temperature can influence the equilibrium of the ternary system. Increasing the temperature from 308.15 K to 348.15 K, the unsaturated region becomes larger apparently, and the invariant point moves upward from point T to C and to S, which illustrates that salting out effect of CO(NH2)2 to NaH2PO4 increases more significantly, and the crystallization region of NaH2PO4·2H2O transforms into NaH2PO4·H2O and the crystallization region of NaH2PO4·H2O transforms into NaH2PO4 with temperature. The differences in the CO(NH2)2 and NaH2PO4 solubility between the pure aqueous solutions and invariant points are presented in Figure 6. The solubility of CO(NH2)2 and NaH2PO4 increases with temperature rise. Differences between the CO(NH2)2 solubility in the aqueous solutions and at invariant points are (20.38, 28.58, and 32.04) % at (308.15, 328.15, and 348.15) K, respectively, which confirms the saltingout effect of NaH2PO4 on CO(NH2)2. Differences between the NaH2PO4 solubility in the aqueous solutions and at invariant points are (22.62, 30.70, and 34.01) % at (308.15, 328.15and 348.15) K, respectively, which illustrates that CO(NH2)2 has a salting-out effect on NaH2PO4 and the effect is stronger at higher temperatures.

More details of the experimental method and the procedure of the preparation, collection, and transfer of samples were depicted in the previous studies.6,7 Analytical Method. The P2O5 concentration was analyzed by the quinoline phosphomolybdate gravimetric method,14 and the average relative deviation of the determination was less than 0.3 %. The urea concentration was analyzed by titration method after distillation process,15 and the mean relative error of the determination was less than 0.4 %. Each experimental data was obtained from the average value of three parallel determinations. The equilibrium solid phase was verified by XRD characterizations.

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RESULTS AND DISCUSSION In Figure 1, the experimental data is compared with literature data,16−18 and it is found that the experimental data is in good

Figure 1. Solubility for NaH2PO4 or CO(NH2)2 in pure water at (308.15, 328.15, and 348.15) K: ○, literary solubility of CO(NH2)2 in water;16,17 ●, experimental solubility of CO(NH2)2 in water; △, literary solubility of NaH2PO4 in water;18 ■, experimental solubility of NaH2PO4 in water.

agreement with the literature values, which demonstrates that experimental methods and devices are feasible in this study. The phase equilibria experimental data is shown in Table 1. The ion concentration values are measured in mass fraction (100w). According to the data listed in Table 1, the ternary phase diagram is plotted in Figure 2 and the phase diagrams at other temperatures are similar to that in Figure 2 and are given in Figure 3. In Figure 2, A, B, and W represent solid CO(NH2)2, solid NaH2PO4·2H2O and H2O, respectively. Point T, an invariant point at 308.15 K, denotes the cosaturated solution of NaH2PO4·2H2O and CO(NH2)2. E represents the solubility of CO(NH2)2 in water at 308.15 K. F denotes the solubility of NaH2PO4 that saturated in water at 308.15 K. The saturated liquid line ETF consists of two branches. Branch ET corresponds to the saturated CO(NH2)2 solution and visualizes changes of the CO(NH2)2 concentration with the concentration of NaH2PO4 increasing in the equilibrium solution. Branch TF corresponds to the saturated NaH2PO4 solution and indicates changes of the NaH2PO4 concentration with the concentration of CO(NH2)2 increasing in the equilibrium solution.

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CONCLUSIONS The phase equilibrium of the ternary system CO(NH2)2 + NaH2PO4 + H2O at (308.15, 328.15, and 348.15) K was investigated. The data of solubility was obtained. According to the solubility data measured, the phase diagram was plotted, the solid phase which was in equilibrium with the saturated solution was analyzed, and crystalline areas of both solid phases were determined. In the phase diagrams, there are in all two crystallization areas, one invariant point, and two univariant curves. The system belongs to a simple eutectic type and double salt is not formed at the investigation temperature. Increasing the temperature from 308.15 K to 348.15 K, the crystallization region of NaH2PO4·2H2O transforms into NaH2PO4·H2O and the crystallization region of NaH2PO4· H2O transforms into NaH2PO4. Urea has a strong salting-out effect on NaH2PO4 and the salting-out effect is stronger at higher temperatures. All results can offer fundamental data B

DOI: 10.1021/acs.jced.5b00293 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Mass Fraction Solubility of the Ternary NaH2PO4 + CO(NH2)2 + H2O System at Temperature = (308.15, 328.15, and 348.15) K and Pressure = 0.1 MPaa composition of liquid phase, 100wb no.

NaH2PO4

CO(NH2)2

composition of wet residue phase, 100wb NaH2PO4

CO(NH2)2

equilibrium solid phase

ND 69.76 68.32 70.94 74.56 73.46 73.97 71.10 71.61 39.60 8.85 7.83 6.66 5.73 4.89 3.96 3.37 2.27 ND

CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 NaH2PO4·2H2O + CO(NH2)2 NaH2PO4·2H2O + CO(NH2)2 NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O

ND 77.76 76.83 78.52 77.93 79.28 65.54 31.93 14.15 12.30 9.35 6.91 4.72 2.95 ND

CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 NaH2PO4·H2O + CO(NH2)2 NaH2PO4·H2O + CO(NH2)2 NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O

ND 87.70 87.11 86.18 87.27 88.29 85.85 84.92 69.50 29.65 10.03 9.86 7.83 6.15 5.56 4.21 3.29 ND

CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 CO(NH2)2 NaH2PO4 + CO(NH2)2 NaH2PO4 + CO(NH2)2 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4

308.15 K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

0.00 5.43 9.90 15.32 17.82 23.20 26.75 29.72 31.74 31.74 32.83 34.86 37.06 39.31 41.63 43.95 46.44 49.47 54.36

59.13 54.76 52.10 48.79 47.35 44.61 41.93 39.65 38.75 38.75 37.21 33.61 29.55 24.94 20.13 16.34 12.26 7.33 0.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.00 8.86 13.57 17.75 23.36 28.22 32.29 32.29 37.48 40.63 44.12 49.61 54.82 59.69 62.99

69.14 62.21 60.15 55.93 51.43 46.20 40.56 40.56 33.73 30.71 25.86 17.93 11.41 6.41 0.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.00 5.24 8.02 12.04 17.82 22.85 27.38 30.64 33.19 33.19 36.44 39.94 44.83 49.25 52.51 56.89 61.51 67.20

77.60 71.57 68.16 64.95 61.71 57.31 53.10 49.29 45.56 45.56 40.65 35.21 29.74 21.99 17.05 12.60 8.06 0.00

NDd 3.70 6.52 8.71 8.71 11.10 12.04 14.29 18.75 43.45 66.59 67.17 68.09 68.42 68.57 69.15 68.81 68.57 ND 328.15 K ND 5.14 7.84 8.71 10.60 10.85 25.27 53.42 66.52 68.48 71.54 72.79 73.92 74.48 ND 348.15 K ND 2.26 3.26 4.70 5.83 6.21 8.21 9.09 26.52 66.46 84.39 83.07 85.52 85.83 84.70 85.71 84.76 ND

a Standard uncertainties u(T) = 0.3 K, u(p) = 0.05 kPa, ur(NaH2PO4) = 0.01 (mass fraction), ur(CO(NH2)2) = 0.02 (mass fraction). bw, mass fraction. dND, not determined.

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DOI: 10.1021/acs.jced.5b00293 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 4. X-ray diffraction pattern of the invariant point C. Figure 2. Equilibrium phase diagram of the ternary system NaH2PO4 + CO(NH2)2 + H2O at 308.15 K: ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of CO(NH2)2; B, pure solid of NaH2PO4·2H2O; W, water; E, solubility of CO(NH2)2 in water; F, solubility of NaH2PO4 in water; T, cosaturated point of NaH2PO4·2H2O + CO(NH2)2.

Figure 5. Solubility isotherms of the ternary system NaH2PO4 + CO(NH2)2 + H2O at (308.15 328.15 and 348.15) K: ■, 308.15 K; ▲, 328.15 K; ●, 348.15 K.

Figure 3. Equilibrium phase diagram of the ternary system NaH2PO4 + CO(NH2)2 + H2O at (328.15 and 348.15) K: ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of CO(NH2)2; B, pure solid of NaH2PO4·H2O and NaH2PO4, respectively; W, water; D and G, solubility of CO(NH2)2 in water; R and M, solubility of NaH2PO4 in water; C and S, cosaturated points.

Figure 6. Comparison of the NaH2PO4 and CO(NH2)2 solubility in aqueous solutions and at cosaturated points T, C, and S. D

DOI: 10.1021/acs.jced.5b00293 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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support; these data, conducive to separation and crystallization, could be used for further theoretical researches.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.5b00293 J. Chem. Eng. Data XXXX, XXX, XXX−XXX