Applied Research on the Resource Recovery of Formaldehyde in

Sep 18, 2014 - Systematic experimental research investigated the recovery of formaldehyde in ... equilibrium data of formaldehyde solution only stops ...
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Applied Research on the Resource Recovery of Formaldehyde in Pesticide Wastewater Lei Li,* Yaodong Liu, Kaihong Jin, Baorong Wang, and Zhibing Zhang Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: Applied research on the technology of resource recovery of formaldehyde in pesticide wastewater through compression distillation was accomplished intensively in this study. Vapor−liquid equilibrium data of the low-molar fraction formaldehyde aqueous solution were measured accurately in experiments. The results were in high accordance with the theoretical values through data association. It filled in a gap in data at the zone of low formaldehyde molar fraction (particularly below 0.006) and also provided theoretical basis for the distillation column design for recycling formaldehyde. Systematic experimental research investigated the recovery of formaldehyde in pesticide wastewater by compression distillation. The distillation column was operated under a top pressure of 0.2 to 0.4 MPa and with feed formaldehyde molar fraction range of approximately 0.006 to 0.03. The formaldehyde molar fractions at the top and bottom of column were 0.15 and 0.00072, respectively. Therefore, concentrated formaldehyde solution can be obtained for recycling, whereas the formaldehyde composition of residual wastewater reached the standard of biochemical treatment. Therefore, compression distillation was a feasible approach to the treatment of wastewater with low-molar fraction formaldehyde.

1. INTRODUCTION Formaldehyde is a widely used, important chemical in synthesis areas such as the production of urea-formaldehyde resin, wood processing, paper industry, textile processing, and pharmaceuticals. This compound is also essential in synthesis of pesticides including glyphosate, bronopol, and phorate.1−3 Formaldehyde is highly reactive in aqueous solution, and large amounts of wastewater containing it will be produced in the industrial production process of the pesticides. Formaldehyde in wastewater is difficult to deal with by traditional biochemical methods; microorganisms will be inhibited or killed in highly concentrated formaldehyde. If a large amount of untreated wastewater is directly released into the environment, environmental pollution and wastage of formaldehyde resources ensue.4 Therefore, recycling formaldehyde in the wastewater is reasonable and necessary. Many investigations were done to remove formaldehyde in the wastewater in the previous decades. Three typical methods of treating formaldehyde wastewater exist. First is a chemical method referring to oxidation process, such as Fenton oxidation,5 wet oxidation,6 and photocatalyze oxidation.7 Second is biodegradation,8 and last is a physical treatment, such as adsorption9 and vapor stripping. The former two have made the most conversion of formaldehyde into other material, whereas the latter cannot produce high concentrated formaldehyde. In the current study, distillation was responsible for recovering and concentrating formaldehyde. Three kinds of © 2014 American Chemical Society

mutual conversion in formaldehyde aqueous solution are generally accepted. First, formaldehyde molecules dissolved in water evaporate and turn to gas molecules, whereas formaldehyde gas molecules could also dissolve in water. This is the mutual transformation between nondissolved formaldehyde and dissolved formaldehyde.10 Second, chemical equilibrium exists between formaldehyde and methylene glycol, and they could simultaneously exist in a solution. Formaldehyde reacts with polar water to form methylene glycol; in contrast, methylene glycol turns to formaldehyde by dehydration.11 Lastly, methylene glycol molecules could automatically polymerize to polyformaldehyde by extracting water molecules,12 which is reversible. Therefore, formaldehyde aqueous solution is a complex system with both mass transfer and chemical reaction. Separating formaldehyde from its aqueous solution by atmospheric distillation is impossible because the relative volatility of formaldehyde−water is close to 1.13,14 In contrast, compressed distillation has been reported to be effective in the concentration problem of formaldehyde wastewater.15 In this process, the reaction equilibrium and rate are changed and it contributes to increase the relative volatility of formaldehyde−water. However, the study aimed to treat high-molar fraction formaldehyde instead of dilute formaldehyde solution. Extractive distillation and catalytic Received: May 27, 2014 Accepted: September 5, 2014 Published: September 18, 2014 3539

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distillation were also considered approaches, but the financial cost and removal effect limit practical industrial application. The objectives of present work are to investigate systematically the vapor−liquid (VL) equilibrium of low-molar fraction formaldehyde solution, especially molar fraction is below 0.006. Compressed distillation column is applied to treat or recycle formaldehyde from wastewater. To our best knowledge, VL equilibrium data of formaldehyde solution only stops at the zone of high molar fraction, and that at low molar fraction, area (particularly molar fraction is below 0.006) are not known and must be gathered. Optimal experimental condition can be achieved by controlling the different feed molar fraction, operating pressure, and reflux rate. Moreover, theoretical calculation data of VL equilibrium based on thermodynamic method are obtained; thus, chemical simulation of formaldehyde distillation are completed by theoretical support.

liquid-phase sampling valve were stored in the conical beaker for analysis. The distillation column used is constructed of 150 mm inner diameter and 4000 mm height steel packed with 2500 mm height Dixon ring (4 × 4 mm). Figure 2 represents the

2. MATERIAL AND EXPERIMENTAL METHODS 2.1. Experimental Materials. Analytical commercial grade chemicals, that is, formaldehyde aqueous solution was used to prepare dilute aqueous solutions with different molar fraction in the VL equilibrium measuring experiments and supplied by Nanjing Chemical Co., Ltd. with minimum mass fraction of 0.36. Analytical grade anhydrous sodium sulfite and sodium hydroxide were used in the analytical experiments and supplied by Nanjing Chemical Co., Ltd. with minimum mass fraction of 0.995. Sulfuric acid was supplied by Beijing Chemical Plant with mass fraction of 0.95−0.98. Thyme phenolphthalein indicator was provided by Shanghai Chemical Co., Ltd. with certain concentration of 1 g/L in ethanol solution. All reagents were used without further purification. The water used in the study was deionized water produced in a local laboratory. The pesticide wastewater containing different molar fraction of formaldehyde was used in the distillation experiments and provided by Jiangsu Yangnong Chemical Group Co., Ltd. The pesticide wastewater included glyphosate with mass fraction of 0.02, N-Phosphonomethyl aminodiacetic acid with mass fraction of 0.01, and formic acid with mass fraction of 0.002 besides formaldehyde and water. 2.2. Equipment and Operational Details. VL equilibrium measuring device is a steel pressure tank with a volume of 3.0 L shown in Figure 1. Formaldehyde solution was added into the tank and then was heated until the pressure inside reached determined value, with an uncertainty of ±0.04 kPa. Gas-phase sampling valve was opened and the condensate was collected when the pressure system was stable at definite pressure for 10 min. Simultaneously, samples drained from the

Figure 2. Schematic of pressure distillation column: 1, column; 2, condenser; 3, reboiler; 4, bottom effluent tank; 5, reflux tank; 6, product tank; 7 and 9, pump; 8, influent tank.

schematic of the column. The feed solution of pesticide wastewater containing formaldehyde at a specific concentration was added with a rotary pump to the bottom tank of the distillation tower. The oil bath was set to a certain temperature, with an uncertainty of ±0.5 K and heat transfer oil was circulated to provide energy to the reboiler for distillation. At the beginning, the operation was under a total reflux condition. A valve on top of the tower was designed to control the operational top pressure, with an uncertainty of ±0.04 kPa. After 2 h, operation of total reflux was stable, then liquid samples from the condenser and bottom of the tower was analyzed to measure the formaldehyde molar fraction with an uncertainty of the component mole fractions ±1.5% and better. If the results were in accordance with expected values, fractional reflux operation was performed. Feed was continuously added to the column at a flow rate of 15 L/h, with an uncertainty of ±0.01 L/h. The reflux rate at the top of the tower was rigorously controlled to maintain constant condition. Indications of pressure and temperature at the top of the tower were read after half an hour. Samples from the top and bottom of the tower were also analyzed when it reached a stable state. Variable pressure and reflux rate were adjusted in the experiments to optimize the manipulation parameters. 2.3. Determination of Formaldehyde Composition. According to the Chinese national standard GB685-93, the principles of formaldehyde composition determination are as follows: Na 2SO3 + HCHO + H 2O → CH 2(OH)− SO3Na + NaOH

2NaOH + H 2SO4 → Na 2SO4 + 2H 2O

Figure 1. Schematic of VL equilibrium measuring device: 1, manometer; 2, thermometer; 3, vaporcontrolling valve; 4, disinflation valve; 5, gas-phase sample valve; 6, liquid-phase sample valve.

Formaldehyde (HCHO) reacts with excessive sodium sulphite, and sodium hydroxide is formed beside α-hydroxy 3540

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formaldehyde, PsW is the partial pressure of water, φsW is the fugacity coefficient of water, xW and yw are the liquid composition of water, γW is activity coefficient of water, VLmW L and V mF are the liquid molar volume of water and formaldehyde, which were calculated by the modified Rackett equation,26 P is the total pressure, T is the temperature, φ̑ W is the vapor fugacity coefficient of water, φsF′ is the fugacity coefficient of formaldehyde, xF composition of formaldehyde, γF is activity coefficient of formaldehyde in the liquid, yF is composition of formaldehyde in vapor, and φ̑ F is the vapor fugacity coefficient of formaldehyde. The calculation procedure is described as follows. The fugacity coefficients were calculated by using the virial state equation truncated at the second term.27 The virtual saturated vapor pressure of formaldehyde can be calculated from eq 1. The Wilson model parameters of the formaldehyde−water binary system are calculated using the experimental VL equilibrium data and Newton−Raphson iteration. Then they are put into phase equilibrium eqs 2 and 3. The value of γ can be obtained from specific x value (mass fraction of liquid). Temperature is obtained by trial-error method according to the isobaric bubbling point rule. Finally, y value can be obtained when temperature is available. The correlation procedure was based on the minimization of the following objective function:

methyl sulfonate (CH2(OH)−SO3Na). The amount of NaOH generated is precisely measured by titration with calibrated sulfuric acid as indicated with thyme phenolphthalein. From the equivalent amount of sodium sulphite, the composition of formaldehyde is determined according to relation X=V×C×

0.03003 m

where V is the volume of sulfuric acid exhausted during the titration (mL), C is the concentration of standard sulfuric acid (C(1/2H2SO4) = 1.000 mol/L), m is the mass of the sample (g), the constant 0.03003 denotes that the mass of formaldehyde (g) equals to 1.00 mL standard sulfuric acid, and X the mass fraction of formaldehyde. Each sample was tested at least three times to ensure accuracy and repeatability of the experiment, and the final result was an average value of the measurements. The tolerance of utmost deviation for parallel test was no more than 0.1%. 2.3. Theoretical Calculation of VL Equilibrium by Data Correlation. The absolute reliability of our results was not proved because of lack of systematic and complete report of the equilibrium data of formaldehyde−water binary system at low molar fraction. Thus, further study of the experimental data is necessary. Fortunately, thermodynamics calculation provides another approach to get relative data. Many researchers have described model for the VL phase equilibrium of multicomponent aqueous and methanolic solutions of formaldehyde. Hahnenstein16−18 gave detailed specification of their model for the prediction and calculation of VL equilibrium of formaldehyde solution. Qiu19−23 proposed a model combining virtual saturation vapor pressure and conventional activity factor. Considering the aforementioned methods, the following model is proposed; however, chemical reactions between formaldehyde and active components are not considered. Thus, estimation of the equilibrium constant of the association reaction is not needed. The parameters of Wilson equation and virtual saturation vapor pressure of formaldehyde can be deduced based on the experimental VL equilibrium data for the binary system, with variance of experimental boiling point and calculated bubbling point temperature as the target functions. Then, the deduced binary system model factors are used to calculate the equilibrium of the formaldehyde−water binary system, and y value for the composition of the gas phase is also obtained. The accuracy of the calculation is approximate to prediction of the Brandani’s model.24 The equations and correlation are brief and short in the model and is suitable for calculations in engineering field. The equation for the virtual saturated vapor pressure of formaldehyde was represented as follows: ⎛ 4.50 11.91 ⎞ ⎟ PFs′ = PFcexp⎜4.50 + − Tr Tr 2 ⎠ ⎝

(1)

⎡ V L (P − P s ) ⎤ w ⎥ = Pyw φw̑ Pwsφws x wγw exp⎢ mw RT ⎦ ⎣

(2)

⎡ V L (P − P s ′) ⎤ F ⎥ = PyF φF̑ PFs′φFsx FγFexp⎢ mF RT ⎣ ⎦

(3)

N

F=

∑ (ti − t′i )2 i=1

(4)

Where ti and t′i are the actual and calculated boiling point. The calculated model factors for the formaldehyde−water binary system were 0.5 and 875.0.20 The bubbling point and vapor molar fraction under different pressures could be calculated by the method mentioned when the molar fraction of formaldehyde varied.

3. RESULTS AND DISCUSSION 3.1. VL Equilibrium of Formaldehyde in Low Molar Fraction. The VL equilibrium measuring device was operated by increasing pressure from 200 to 400 kPa when the initial formaldehyde molar fraction was below 0.006. Figure 3 shows that formaldehyde vapor molar fraction is determined as a function of liquid composition. Relative volatility of formaldehyde solution with low molar fraction increases dramatically and exceeds significantly one with increasing pressure,

PSF′

where is virtual saturated vapor pressure of formaldehyde, PcF is the actual saturated vapor pressure of formaldehyde, which was calculated by Antoine equation,25 Tr is the boiling point of

Figure 3. Experimental data of VL equilibrium under different pressures: red ▲, 400 kPa; pink ■, 300 kPa; blue ◆, 200 kPa; ○, 300 kPa(reported by Walker28); orange line, y = x straight line. 3541

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whereas that is near one under normal pressure.8 Therefore, the formaldehyde can be separated from dilute solution and concentrated under compressed distillation. The experimental results of the fractional pressure of vapor show minor difference between 400 and 300 kPa. This means that it is not necessary to operate under exorbitant pressure because industrial process under exorbitant pressure is more costly. VL equilibrium experimental data under 300 kPa are compared with values under 300 kPa reported by literature from Walker. 28 Apparently, those two are closely approximate when the mole fraction in the solution phase is between 0.000046 and 0.000752, which confirms the experimental measurement is accurate. Furthermore, the average value of relative volatility is 1.71 that is consistent with the value reported in the Walker’s literature (around 1.7 at 300 kPa, 130 °C).28 The series of comparison and data analyses proved the reliability of the equilibrium data of low-molar fraction formaldehyde aqueous solution obtained in our VL equilibrium device. 3.2. Comparisons of Experimental Data with Calculation by Data Correlation. As mentioned above, the bubbling point and vapor molar fraction under different pressure and variable formaldehyde feed molar fraction were measured by experiments and by theoretical calculation through data correlation. All those values are represented in Table 1, and plotted into one VL equilibrium diagram, as

Figure 4. Comparison of experimental and theoretical calculations of VL equilibrium data under 200 kPa: --- and × , vapor concentrations value, calculated and experimental results; and ●, bubbling point value, calculated and experimental results.

Table 1. Experimental Results and Calculated Data of VL Equilibrium for Formaldehyde−Water Solution under Various Pressuresa,b P/kPa

T/K

T′/K

x

y

y′

RD/%

200 200 200 200 200 300 300 300 300 300 400 400 400 400 400

391.15 389.15 389.15 388.15 387.15 403.15 403.15 401.15 400.15 399.15 413.15 412.15 412.15 410.15 409.15

392.06 389.67 389.76 388.28 387.18 404.39 404.48 402.13 401.03 399.91 413.98 412.78 409.71 410.53 409.46

0.00038 0.00056 0.01176 0.02035 0.03574 0.00030 0.00050 0.02037 0.03577 0.06665 0.00030 0.00327 0.00433 0.02911 0.03570

0.00095 0.00130 0.03435 0.05485 0.08354 0.00100 0.00160 0.06626 0.09095 0.14890 0.00110 0.01572 0.02233 0.09911 0.10407

0.00094 0.00128 0.03393 0.04772 0.08254 0.00098 0.00156 0.06487 0.08897 0.14577 0.00108 0.01547 0.02197 0.09762 0.10246

1.05 1.54 1.22 1.30 1.20 2.00 2.50 2.09 2.18 2.10 1.82 1.59 1.61 1.50 1.55

Figure 5. Comparison of experimental and theoretical calculations of VL equilibrium data under 300 kPa: --- and × , vapor concentrations value, calculated and experimental results; and ●, bubbling point value, calculated and experimental results.

a

T is the actual boiling point, T′ is the calculated boiling point, and x is molar fractions of vapor components, y and y′ are calculated and experimental molar fractions of liquid components, respectively, and RD is relative deviation between calculated and experimental results. b Standard uncertainties u are u(T) = ± 0.5 K, u(P) = ± 0.04 kPa, u(x) = ± 0.0001, u(y) = ± 0.0001. Figure 6. Comparison of experimental and theoretical calculations of VL equilibrium data under 400 kPa: --- and × , vapor concentrations value, calculated and experimental results; and ●, bubbling point value, calculated and experimental results.

shown in Figures 4−6. Clearly, the experimental results and calculated values of the vapor composition are approximate, with the maximum relative deviation of 2.50%, minimum relative deviation of 1.05%, and average relative deviation of 1.67%, as demonstrated in Table 1. Those verified further the accuracy and reliability of the VL equilibrium data for the formaldehyde aqueous solution with low molar fraction obtained by increasing pressure. Therefore, those data can be imported to chemical engineering simulation software for distillation calculation, such as Pro/II and Aspen Plus, where no

relative data can be gained for the formaldehyde aqueous solution at a low molar fraction and under high pressure. To some extent, it fills the blank of VL equilibrium data of formaldehyde solution in the low-molar fraction area (particularly molar fraction is below 0.006), thereby providing 3542

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fraction at the bottom of tower drops initially then increases again once the bottom part is full. 3.4. Comparison and Discussion of Formaldehyde Distillation by PRO-II Simulation. The VL equilibrium data for the low molar fraction formaldehyde solution are required in using chemical engineering software such as PRO-II for the simulation design of fractional distillation tower. Given the fact that no data is available in the original data bank of PRO-II, the obtained VL equilibrium data for the low molar fraction formaldehyde aqueous solution in this study can be imported and updated to the data bank through subprogramming. The data can be regulated in the simulative software, thus simulation calculation will be accomplished. The results of formaldehyde distillation by simulation with or without VL equilibrium data obtained in this study and actual experiments are all listed in Table 3. The results calculated in the simulation distillation with imported gas−liquid equilibrium data into PRO-II agreed more with the experimental results than those calculated directly from the PRO-II original database. The reliability and practicality of the VL equilibrium data of low molar fraction of formaldehyde in aqueous solution measured by experiments is confirmed further. The theoretical reference and data support for the industrial design of high-pressure fractional distillation tower for formaldehyde are also provided. The simulation also shows that 30 theoretical trays are needed and the optimal feed location is the tenth tray under the pressure of 300 kPa and with reflux rate of 12.8 L/h. Accordingly, the experiment used 24 theoretical trays inside the distillation column, and the actual feed tray is the 18th tray. All the results could instruct industrial process for recycling formaldehyde in pesticide wastewater. Moreover, the fractional distillation performance can be improved by increasing the theoretical plate number. The calculation results of energy and cost have shown that there is enough economic value for the recycle process. The price of recycled formaldehyde exceeds the expense of energy consumption and labor cost when the original formaldehyde molar fraction in wastewater is above 0.03. Thus, this method is practical in industrial manufacture. The recycling technique for formaldehyde using compressed distillation has been adopted by some chemical factories, and relative workshops are under construction.

the theoretical basis for the design of a distillation column for formaldehyde recycling. 3.3. Compressed Distillation of Wastewater Containing Formaldehyde. Reflux rate and condenser pressure were altered in the fractional distillation of formaldehyde wastewater to optimize the experimental condition. The detailed results are given in Table 2. Comparison of the top and bottom Table 2. Result of Compressed Distillation for Wastewater Containing Formaldehyde under Different Conditiona,b CF

PC/kPa

R/L·h−1

CT

CB

0.00639 0.00616 0.00687 0.00616 0.00592 0.00688 0.01246 0.01893 0.02594

300 300 300 400 400 400 300 300 300

3.2 6.4 12.8 3.2 6.4 12.8 12.8 12.8 12.8

0.0127 0.0201 0.150 0.0436 0.0727 0.1417 0.1505 0.1650 0.1783

0.00144 0.00150 0.00072 0.00270 0.00198 0.00168 0.00096 0.00078 0.00084

a

CF is feed molar fraction of formaldehyde, CT and CB are top and bottom molar fraction of formaldehyde, respectively, PC is condenser pressure, and R is reflux rate. bStandard uncertainties u are u(C) = ± 0.004%, u(P) = ± 0.04 kPa, u(R) = ± 0.01 L/h.

formaldehyde composition reveals that an optimal result is obtained under the following conditions: feeding molar fraction of formaldehyde wastewater, about 0.00687; operational pressure at the top of the tower, 300 kPa; reflux rate, 12.8 L/ h; and top formaldehyde molar fraction, 0.15; bottom removed to 0.00072. The top formaldehyde molar fraction reaches 0.16, whereas the bottom formaldehyde molar fraction declines to almost zero under a pressure of 300 kPa, as shown in Figure 3. Apparently, the actual distillation effect reaches the theoretical value. However, the expected separation effect is difficult to completely achieve because all formaldehyde in solution with low molar fraction is converted into its hydrate [HO(CH2OH)]. During fractional distillation, the hydrate is dehydrated into formaldehyde that will be distilled from the liquid phase afterward. The residence time of liquid phase is very limited because of relatively small space at the bottom of the tower. Thus, most of the methylene glycol have limited time to dehydrate and removed out of tower immediately as bottom product. In the observation, the formaldehyde molar

4. CONCLUSIONS (1) The low molar fraction VL equilibrium data for aqueous solution of formaldehyde can be used in research, and in

Table 3. Comparison of Distillation by Simulation of PRO-II and Experimentsa,b simulation without VL equilibrium data CF

PT/kPa

0.00639 0.00616 0.00687 0.00616 0.00592 0.00688 0.01246 0.01893 0.02594

300 300 300 400 400 400 300 300 300

R/L·h

−1

3.2 6.4 12.8 3.2 6.4 12.8 12.8 12.8 12.8

simulation with VL equilibrium data

experimental results

CT

CB

CT

CB

CT

CB

0.0356 0.0502 0.1741 0.0756 0.0957 0.1630 0.1779 0.1930 0.2046

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

0.0130 0.0206 0.149 0.0439 0.0735 0.142 0.151 0.164 0.178

0.00132 0.00138 0.00090 0.00234 0.00186 0.00150 0.00078 0.00090 0.00108

0.0126 0.0201 0.150 0.0436 0.0727 0.142 0.151 0.165 0.178

0.00144 0.00150 0.00072 0.00270 0.00198 0.00168 0.00096 0.00078 0.00084

a

CF is feed molar fraction of formaldehyde, CT and CB are top and bottom molar fraction of formaldehyde, respectively, PT is top pressure, and R is reflux rate bStandard uncertainties u are u(C) = ± 0.004%, u(P) = ± 0.04 kPa, u(R) = ± 0.01 L/h. 3543

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(13) Gmehling, J.; Onken, U. Vapor-liquid equilibrium data collection, aqueous-organic systems. Chem. Data Ser. 1977, 1 (1), 1−10. (14) Auerbach, F.; Barachall, H. Arb Reichagesunth. Chem. Commun. 1905, 22, 584. (15) Hallm, W.; Piret, E. The vapour pressures of pure substance. Ind. Eng. Chem. 1949, 41, 1177. (16) Hahnenstein, I.; Hasse, H.; Kreiter, C. G.; Maurer. 1H- and 13CNMR-spectroscopic study of chemical equilibria in solutions of formaldehyde in water, deuterium oxide, and methanol. Ind. Eng. Chem. Res. 1994, 33, 1022−1029. (17) Hahnenstein, I.; Albert, M.; Hasse, H.; Kreiter, C. G.; Maurer, G. NMR-Spectroscopic and densimetric studies of reaction kinetics of formaldehyde-polymer formation in water, deuterium oxide, and methanol. Ind. Eng. Chem. Res. 1995, 34, 440−450. (18) Albert, M.; Hahnenstein, I.; Hasse, H.; Maurer, G. Vapor-Liquid equilibrium of formaldehyde mixtures: new experimental data and model revision. AIChE J. 1996, 42, 1741−1752. (19) Qiu, Z.; Luo, Z.; Hu, Y. Vapor-liquid equilibrium of binary system containing methylal. J. Chem. Eng. Chin. Univ. 1994, 8, 105− 110. (20) Qiu, Z.; Luo, Z.; Hu, Y. Study on vapor liquid equilibria of complex system containing formaldehyde. J. Nanchang Univ. 1995, 17, 56−63. (21) Qiu, Z.; Luo, Z.; Hu, Y. Study on vapor liquid equilibrium of trioxane-water binary system. J. Nanchang Univ. 1996, 20, 9−14. (22) Qiu, Z.; Luo, Z.; Hu, Y. Study on vapor liquid equilibria of the methylal- formaldehyde- water ternary system. J. Nanchang Univ. 1996, 18, 43−44. (23) Qiu, Z.; Luo, Z.; Hu, Y. Vapor-Liquid Equilibria Model of the Complex Systems Containing Formaldehyde. J. Chem. Eng. Chin. Univ. 1996, 10, 225−231. (24) Brandani, V.; Di Giacomo, G.; Foscolo, P. U. Isothermal vaporliquid equilibria for the water-formaldehyde system. A predictive thermodynamic model. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 179−185. (25) Boublík, T.; Fried, V.; Hala, E. The Vapour Pressures of Pure Substances; Elsevier: Amsterdam, 1984. (26) Rackett, H. G. Equation of State for Saturated Liquids. J. Chem. Eng. Data 1970, 15, 514−517. (27) Susial, P.; R. Rios-Santana Vapor-liquid equilibrium measurements for the binary system methyl acetate+ethanol at 0.3 and 0.7 MPa. Braz. J. Chem. Eng. vol.28 No.2. (28) Walker, J. F. Formaldehyde, 2nd ed.; American Chemical Society: Washington, DC, 1985; 110−120.

industrial design, especially in the simulate calculation used in chemical engineering software. This report is the first one of formaldehyde VL equilibrium data with molar fraction concentration is below 0.006, which fills the blanks in software such as RPO-II. Those data also provides reference for the design of formaldehyde fractional distillation. (2) The absolute pressure is 300 kPa when the reflux ratio is 12.8 L/h, and formaldehyde solution with molar fraction of 0.0066 in the wastewater can be concentrated to about molar fraction of 0.151 on the top and molar fraction of 0.00072 at the bottom by compressed distillation. The technique not only recycled formaldehyde from pesticide wastewater solely for industrial process but also alleviated the environmental pollution.

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ASSOCIATED CONTENT

* Supporting Information S

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 086 + 025 + 8359-6665-810. Fax: 086 + 025 + 8359-3772. Funding

This study was supported by the Fundamental Research Funds (Grant Nos. 1118020506 and 1114020503) for the Chinese Central Universities. Notes

The authors declare no competing financial interest.

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REFERENCES

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