Ind. Eng. Chem. Res. 2006, 45, 5155-5164
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Phase Equilibrium in Formaldehyde Containing Multicomponent Mixtures: Experimental Results for Fluid Phase Equilibria of (Formaldehyde + (Water or Methanol) + Methylal)) and (Formaldehyde + Water + Methanol + Methylal) and Comparison with Predictions Christian Kuhnert,† Michael Albert,‡ Silke Breyer, Immanuel Hahnenstein, Hans Hasse,§ and Gerd Maurer* Lehrstuhl fu¨r Technische Thermodynamik, UniVersity of Kaiserslautern, D-67653 Kaiserslautern, Germany
Methylal (also known as dimethoxymethane and formaldehyde dimethyl acetal) is a side product of the production of polyacetal plastics from formaldehyde, which has to be removed in downstream processing. The design of the separation equipment requires a verified model for the vapor-liquid-phase equilibrium. New experimental results are presented for the vapor-liquid and liquid-liquid equilibrium of the ternary system (formaldehyde + water + methylal) and for the vapor-liquid equilibrium of the ternary system (formaldehyde + methanol + methylal) and of the quaternary system (formaldehyde + water + methanol + methylal). New experimental results and the literature data are compared with prediction results. Introduction Formaldehyde is an important chemical intermediate. Its most important use is in the production of polyacetal plastics, where commonly formaldehyde is converted to trioxane (a cyclic trimer of formaldehyde) before trioxane is polymerized to polyacetal polymers. Formaldehyde is a very reactive substance forming various reaction products with water (methylene glycol and poly(methyleneglycols)) and alcohols; for example, with methanol, it forms hemiformal and poly(oxyhemiformals). However, there are also many other reaction products such as, e.g., methylal. Continuing previous work on the thermodynamic properties (phase equilibrium, chemical reaction equilibrium, chemical reaction kinetics, and heat of vaporization) in systems of formaldehyde and water and/or methanol,1-16 on the properties of aqueous solutions of formaldehyde and trioxane,1,17 on the properties of nonreacting binary and ternary systems of methylal and water and/or methanol18 as well as on the properties of aqueous solutions of formaldehyde and butanol,19,20 the work presented here primarily deals with the vapor-liquid equilibrium of formaldehyde and methylal containing ternary mixtures (with either water or methanol), the liquid-liquid-phase equilibrium of the ternary system (formaldehyde + water + methylal), and the vapor-liquid equilibrium of the quaternary system (formaldehyde + water + methanol + methylal) at temperatures between ambient temperature and about 400 K, corresponding to pressures between 20 and 470 kPa. The new experimental results for the vapor-liquid equilibrium and the liquid-liquid equilibrium are compared to prediction results from a thermodynamic model which was developed exclusively based on experimental data for the pure components, binary subsystems (formaldehyde + water, formaldehyde + methanol, water + methylal, and methanol + water) and ternary subsystems * To whom correspondence should be addressed. Phone: +49 631 205 2410. Fax: +49 631 205 3835. E-mail:
[email protected]. † Current address: Robert Bosch GmbH, 70469 Stuttgart, Germany. ‡ Current address: Degussa AG, 67547 Worms, Germany. § Current address: Institut fu¨r Techn. Thermodynamik und Thermische Verfahrenstechnik, Universita¨t Stuttgart, 70550 Stuttgart, Germany.
(formaldehyde + water + methanol and water + methanol + methylal). The comparison is extended also to the experimental results of isobaric (p ) 101 kPa) vapor-liquid equilibrium investigations by Shan et al.,21 i.e., to the only experimental data available in the open literature for the phase equilibrium of such systems. Experimental Section Apparatus. A thin-film evaporation technique was used for measuring the vapor-liquid equilibrium. The thin-film evaporator consists of a rotating coil (made up of glass-fiber reinforced Teflon) inside a double-walled housing. For temperatures below about 350 K (i.e., experiments around and below atmospheric pressure), that housing was made of glass, whereas, for the investigations at higher temperatures (i.e., at pressures above atmospheric pressure), it was replaced by a similar housing made of stainless steel. The equipment was used in previous work and described before (cf. the work of Hasse3 and Albert et al.11). The experimental procedure applied for investigating the liquidliquid-phase equilibrium was also described before (cf. the work of Albert12 and Albert et al.18). Therefore, no details are repeated here. The methods for determining the composition of the coexisting phases were also described before (cf. the work of Hasse3, Albert,12 and Albert et al.18). Therefore, only the type of analyzing procedure and the uncertainty of the experimental results are given. The formaldehyde concentration in the coexisting phases was determined by wet chemistry applying the sodium sulfite method.22 The estimated relative uncertainty of the experimental results for the stoichiometric mole fraction of formaldehyde in an analyzed liquid mixture is (2% but always larger than the estimated absolute uncertainty of about 0.5 mol %. The concentrations of methylal and methanol were determined by gas chromatography applying the “internal standard” procedure. Dioxane was used as the internal standard. Each gas chromatographic analysis was at least repeated twice. The relative deviation of the arithmetic mean of these results from the single results was less than 2%. The uncertainty of the experimental results for the stoichiometric mole fractions of methylal and methanol is estimated to be smaller than (4%
10.1021/ie060131u CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006
(relative uncertainty) and (1 mol % (absolute uncertainty) whichever is larger. The water concentration was determined from a mass balancesonly in a few tests was it also checked by Karl Fischer titration. These tests confirmed that the mass balance gives the water concentration (of the predominantly water-rich samples) with a relative uncertainty of better than (1%. The temperature was measured by calibrated platinum resistance thermometers with an uncertainty of less than (0.1 K. The pressure was measured by calibrated pressure transducers with a maximum uncertainty of (0.5 kPa. Materials and Sample Preparation. The liquid feed mixtures were prepared by gravimetric analysis using aqueous stock solutions of formaldehyde and pure liquids methylal, water, and methanol. Aqueous stock solutions of formaldehyde were prepared by boiling paraformaldehyde in deionized water at a high pH. The turbid solution was filtered, and the stoichiometric formaldehyde concentration was determined by wet chemistry (cf. above). The stock solutions were additionally analyzed by gas chromatographysbut no significant impurities were detected. Paraformaldehyde (puriss.) was from Merck GmbH, Darmstadt, Germany, and used as supplied. Water was deionized and distilled twice. Methylal and methanol were purchased in the highest available quality (Methylal: Fluka No. 47875, g99% by gas chromatography (GC). Methanol: Merck No. 106012, g99.5% by GC) and used as supplied. Discussion of Phase Behavior. A discussion of the vaporliquid-phase behavior of the ternary systems is essential for an easy understanding of the experimental results. Therefore, the phase behavior of the ternary formaldehyde containing systems is discussed before the experimental results are presented. System (Formaldehyde + Water + Methanol). The binary system (formaldehyde + water) reveals a maximum pressure azeotropic behavior. The composition at the azeotropic point is in the vicinity of pure water. Therefore, at moderate and medium concentrations of formaldehyde, water is the more volatile component of that binary system. Raising the temperature results in a shift of the azeotropic point in the direction of water until the azeotropic behavior disappears at higher temperatures. Formaldehyde precipitates from the liquid phase (as mixtures of poly(oxymethylene) glycols) at high formaldehyde concentrations. Solid formaldehyde oligomers (mixtures of poly(oxymethylene) hemiformals) also precipitate at high formaldehyde concentrations from the liquid phase of the binary system (formaldehyde + methanol). That binary system also reveals an azeotropic behavior, but with a minimum pressure (at a given temperature). The azeotropic point is expected at rather high formaldehyde concentrations, but it has not been observed experimentally as it lies beyond the precipitation limit of formaldehyde oligomers. Therefore, methanol is the more volatile component of the binary system (formaldehyde + methanol). As is well-known, the binary system (water + methanol) shows no azeotropic behavior. Figure 1 shows for about 360 K in a qualitative triangular prism the vapor-liquid equilibrium region of the ternary system (formaldehyde + water + methanol). The pressure increases from bottom to top in that prism. At low pressures, the vaporliquid equilibrium region stretches from the (water + formaldehyde) side and at higher pressures from the (water + methanol) side, respectively, to the (formaldehyde + methanol) side. The vapor-liquid equilibrium is dominated by chemical reactions between formaldehyde (on one side) and water as well as methanol (on the other side)scf. the section on modeling. Two vapor-liquid equilibrium regions appear in a very small pressure range (between the vapor pressure of pure water and
Figure 1. Scheme of the vapor-liquid equilibrium of the ternary system (formaldehyde + water + methanol) at about 360 K.
the pressure at the azeotropic point of the binary system (formaldehyde + water)). These regions are not shown in Figure 1. Furthermore, the regions where formaldehyde oligomers precipitate have been omitted, as they are of no interest here. System (Formaldehyde + Methanol + Methylal). Replacing water by methylal results in a simpler phase behavior as methylal behaves as an inert component, i.e., it does not react with formaldehyde. Methylal boils at a lower temperature than methanol, but at a higher temperature than formaldehyde. The binary system (methanol + methylal) reveals azeotropic behavior with a minimum boiling point temperature. There is no information available on the vapor-liquid equilibrium of the (from an industrial point of view unimportant) binary system (formaldehyde + methylal). Therefore, the discussion is limited here to mixtures which contain only small amounts of methylal, i.e., to methanol-rich solutions around the saturation pressure of pure methanol. Figure 2 shows the ternary vapor-liquid behavior of that ternary system for a temperature of around 323 K. At 323 K, the vapor pressure of the pure components amounts to about 1030 kPa (formaldehyde), 130 kPa (methylal), and 55 kPa (methanol). At pressures below the vapor pressure of methanol, the vapor-liquid two-phase region starts from the binary system (formaldehyde + methanol), whereas at pressures above the vapor pressure of methanol it starts from the binary system (methanol + methylal). In those regions, the concentration of formaldehyde is always higher in the liquid phase when compared to the vapor phase. System (Formaldehyde + Water + Methylal). Replacing methanol by water again results in a more complicated phase diagram, as the binary system (water + methylal) reveals a liquid-liquid phase split at low temperatures and a three-phase liquid + liquid + vapor equilibrium, but no azeotropic behavior. As long as the discussion of the phase behavior at constant temperature is limited to pressures below the three-phase L1L2Vline of the binary system (water + methylal)swhich is close to the rather high vapor pressure of pure methylalsthe isothermal phase diagram of the ternary system is still rather simple. It is shown in Figure 3 predominantly at high water
Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 5157
(formaldehyde + water). These regions as well as the regions where formaldehyde oligomers precipitate are not shown in Figure 3. Experimental Results
Figure 2. Scheme of the vapor-liquid equilibrium of the ternary system (formaldehyde + methanol + methylal) at about 320 K.
Figure 3. Scheme of the vapor-liquid equilibrium of the ternary system (formaldehyde + water + methylal) at about 330 K.
and low formaldehyde concentrations for a temperature around 330 K. At pressures below the vapor pressure of water, the vapor-liquid region starts from the binary system (formaldehyde + water), whereas it starts from the binary system (water + methylal) at higher pressures. It stretches into the direction of the corner which represents formaldehyde in the triangular diagram. At low temperatures, there is a liquid-liquid phase region which starts in the binary system (water + methylal) and extends into the triangular concentration diagram. Similar to the system (formaldehyde + water + methanol), there are two small vapor-liquid equilibrium regions in the very small pressure range between the vapor pressure of pure water and the pressure at the azeotropic point of the binary system
Vapor-Liquid Equilibrium. The experimental investigations aim to provide a database for evaluating a thermodynamic model for the prediction of vapor-liquid equilibrium in multicomponent aqueous and/or methanolic mixtures of formaldehyde. System (Formaldehyde + Methanol + Methylal). The vapor-liquid equilibrium of the ternary system (formaldehyde + methanol + methylal) was investigated at 313 K (between about 33 and 66 kPa) and at 323 K (between 30 and 83 kPa) for methanol-rich liquid phases. These pressures are around the vapor pressure of methanol (e.g., at 323 K, psMe/kPa ) 55), and the features of that phase equilibrium are shown in Figure 2. The stoichiometric molar concentrations of formaldehyde and methylal in the liquid phase were between 13 and 60% (formaldehyde) and up to 24% (methylal). The stoichiometric molar concentration of methanol in the liquid phase was between about 30 and 60%. Methylal (in most experiments) or methanol are the components with the highest vapor phase concentration. The experimental results are given in Table 1. Additionally, the experimental results for the composition of the coexisting phases are shown in Figure 4 for 323 K. System (Formaldehyde + Water + Methylal). The vaporliquid equilibrium of that ternary system was investigated for water-rich liquid phases at 323 K (at about 45 kPa), at 333 K (between 20 and about 90 kPa), at 363 K (around 150 kPa), and at 393 K (at about 215 kPa). These pressures are above the saturation pressure of water (e.g., at 333 K, psW/kPa ) 20) and well below the pressure at the three-phase L1L2V-equilibrium of the binary system (water + methylal). That three-phase pressure is very close to the saturation pressure of methyal (e.g., at 333 K, psMAL/kPa ) 181)scf. Figure 3. The stoichiometric molar formaldehyde concentration in the liquid phase was at maximum about 20% (but predominantly around 10%). The stoichiometric molar concentration of methylal in the liquid did not exceed 2%. Due to the high volatility of methylal, the vapor phase concentration of methylal was in most cases rather high (between 50 and 80 mol %). The experimental results are given in Table 2. The experimental results for 363 K are shown as typical examples also in Figure 5. System (Formaldehyde + Water + Methanol + Methylal). The vapor-liquid equilibrium of the quaternary system was investigated at 403 K and at 413 K at pressures between 305 and 476 kPa and at rather high formaldehyde concentrations (20 to 60%), very small methylal concentrations ( 1) poly(oxymethylene)hemiformal HO(CH2O)nCH3 (n > 1)
1 CH2O 1
2 H2O
3 C3H6O2
4 HO(CH2O)H
5 OH
6 CH2
7 CH3O
8 CH2OH
9 CH3OH
1 1 1 1 n-1
2
1
1
1
1
1
n-1
Table 7. UNIFAC Interaction Parameters ai,j/K group j group i
1
1 2 3 4 5 6 7 8 9
-254.5a 0.0 59.20c 28.06c 251.5c 0.0c -128.6c -128.6c
2 867.8a a3,2(T)b,d -191.8 353.5c 1318c 423.8a a8,2(T)e,a -181.0d
3
4
5
6
7
8
9
0.0 a2,3(T)b,d
189.2c 189.5c a3,2(T)b
237.7c -229.1c 237.7 -229.1c
83.36c 300.0c 83.36 300.0c 156.4c
0.0c -219.3a 0.0 -142.4a 112.8a 447.8c
238.4c a2,8(T)e,a 0.0 289.6c -137.1c 697.2c 238.4c
238.4c 289.6d 410.0d 289.6c -137.1c 697.2c 238.4c 0.0c
a2,3(T)b 28.06 251.5 0.0 0.0 -71.21d
353.5c 1318c 774.8a -181.0c -181.0c
986.5c 1164.8a 249.1c 249.1c
273.0c 16.5c 16.5c
Here, a2,3(T) ) -225.5 + 0.7205(T/K); a3,2(T) ) 1031.0 - 1.749(T/K). Albert et 114 100/(T/K); a8,2(T) ) -1018.57 + 329 900/(T/K). a
Kuhnert.16
b
Comparison between Experimental Data and Prediction Results The model described above was used to calculate the composition of the vapor phase and the phase equilibrium pressure for a liquid phase at the experimentally determined compositions and temperatures. The comparison between the new experimental results and the model predictions are given in Tables 1-4. In the investigations on the ternary system (formaldehyde + water + methylal), the stoichiometric mole fraction of formaldehyde in the liquid phase was between about 2 and 20%, whereas for methylal that concentration was lower and at maximum about 2%. As shown in Table 2 (and for 363 K also in Figure 5), the prediction results for the vapor phase concentration of formaldehyde rarely deviate from the experimental results by more than about 0.5 mol % (i.e., the estimated absolute uncertainty). A similar statement holds for the vapor phase mole fraction of methylal, although in some cases the relative deviation between an experimental result and a model prediction can be as large a 500%. Such extremely large deviations are only observed in experiments with very small methylal concentrations in the liquid phase when the estimated uncertainty of the experimental result for the methylal concentration is much larger than the methylal concentration in the liquid itself. The prediction results for the total pressure agree within a few percent with the experimental data.
c
-128.6c -128.6c al.14
d
Albert et
0.0c al.18
e
Here, a2,8(T) ) 451.64 -
The prediction results for the vapor-liquid equilibrium of the ternary system (formaldehyde + methanol + methylal) reveal similar absolute deviations as observed for the system (formaldehyde + water + methylal). However, as in the experimental work for the system with methanol the molar concentrations of formaldehyde and methylal in the liquid phase were higher (formaldehyde: 10-60%; methylal: 2-20%) than in the system with water (formaldehyde: 2-20%; methylal:
