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Separation of Glycol Ethers and Similar LCST-Type Hydrogen-Bonding Organics from Aqueous Solution Using Distillation or Liquid-Liquid Extraction Timothy C. Frank,* Felipe A. Donate, Andrei S. Merenov, Grant A. Von Wald, Barbara J. Alstad, Christian W. Green, and Thomas C. Thyne The Dow Chemical Company, Engineering & Process Sciences Laboratory, 1319 Building, Michigan Operations, Midland, Michigan, 48667 ∞ Infinite-dilution relative volatilities (Ri,water ) were measured at 50 °C and 80 °C using a Rayleigh distillation apparatus for dilute solutions of propylene glycol n-butyl ether (PnB), propylene glycol n-propyl ether (PnP), and dipropylene glycol n-butyl ether (DPnB) dissolved in water and brine (3 wt % NaCl). The data were ∞ ), Henry’s Law constants, partial molar analyzed to determine infinite-dilution activity coefficients (γi,aqueous enthalpies of mixing, and Setschenow constants. Partition ratios (K values) for extraction of PnP also were measured using 14 hydrophobic organic extraction solvents, including alcohols, ketones, ethers, chlorinated hydrocarbons, aromatics, and aliphatics. An interpretation of molecular interactions in solution is given based on the analysis of activity coefficients, as a function of temperature and salt concentration. General rules are proposed for the class of hydrogen-bonding organic compounds characterized by the presence of a lower critical solution temperature (LCST) in the organic + water phase diagram. The value of Ri,water is likely to increase as the temperature increases for stripping volatile LCST-type hydrogen-bonding organics from dilute aqueous solution, provided the pure-component vapor pressure relative to water (pSAT /pSAT i water) also increases or stays approximately the same. For extraction of LCST-type compounds from aqueous solution, K is likely to increase as the temperature increases, provided that the mutual solubility between phases is low.
Introduction Glycol ethers are etheric alcohols that have the general structure R′-O-[CH2-CH(R)-O]n-H, where R′ is an alkyl or aryl group and the molecule contains up to n ) 3 oxyalkylene repeating units. Compounds with more than three repeating units generally are called polyglycols. For the ethylene glycol ethers, R is H; for propylene glycol ethers, R is CH3. The common naming convention refers to monoethylene, diethylene, and triethylene glycol alkyl (or aryl) ethers (or the analogous propylene glycol ethers), where the prefixes mono-, di-, and tri- reflect the value of n. The alkyl (or aryl) designation in the name refers to the R′ group. Typical properties are summarized in Tables 1 and 2 for selected glycol ethers. Additional information is available elsewhere.1-4 For most glycol ethers, the glycol ether + water binary mixture exhibits a lower critical solution temperature (LCST) below which the mixture is completely miscible (Table 2). Above the LCST, the mixture is only partially miscible and the solubility of the glycol ether in the aqueous phase decreases as the temperature increases. This inverse solubility behavior corresponds to the lower temperature range of the two-liquidphase envelope on a temperature-composition diagram. Figure 1 shows such a diagram for ethylene glycol n-butyl ether + water to illustrate these general features. Diagrams for other glycol ether + water mixtures are available elsewhere.3 Mutual solubility and the LCST will vary, depending on the specific glycol ether (as given in Table 2). The value of the LCST reflects the relative hydrophilic/hydrophobic character of the organic compound.3 A higher LCST indicates a more-hydrophilic character. Some of the more-hydrophilic glycol ethers, including ethylene glycol n-propyl ether and propylene glycol * To whom correspondence should be addressed. Tel.: 989-6364310. Fax: 989-636-4616. E-mail address:
[email protected].
methyl ether, normally are considered completely miscible with water, because the LCST is >100 °C and is outside the range of normal processing temperatures. Because glycol ethers possess both hydrophilic and hydrophobic functional groups and can hydrogen bond with water, they are widely used as cosolvents in organic + water product formulations, including cleaners and water-borne automotive and industrial coatings. They also find application as industrial process solvents. Recent developments include the use of glycol ethers to facilitate selective release of intracellular proteins from bacterial fermentation broth5,6 and to extract highly hydrophilic carboxylic acids and polyhydroxy compounds from aqueous solutions.7 In many of these applications, it is necessary to recover glycol ether residues from aqueous process streams to facilitate recycle and reuse of the glycol ether and to reduce the organic content prior to biotreatment and discharge of clean water to the environment. Several solvent recovery technologies, such as those described elsewhere8,9 might be useful for this purpose, depending on the properties of the particular glycol ether. This paper evaluates two options: (i) the use of direct distillation or steam stripping of glycol ethers from dilute aqueous streams and (ii) liquid-liquid extraction of glycol ethers from an aqueous feed into a more-hydrophobic organic solvent. Relative volatility and partition ratio data are presented for selected glycol ether + water mixtures. Few data of this type have previously been reported. Based on these examples, guidelines are proposed for assessing trends in relative volatility and partition ratios, as a function of temperature for LCSTtype organic compounds in general. This paper focuses on the equilibrium driving force for these process options, a critical requirement for assessing technical feasibility. If the driving force seems favorable for a given application, then additional work will be needed to determine whether the proposed process can be an economical one. This
10.1021/ie061646o CCC: $37.00 © 2007 American Chemical Society Published on Web 05/02/2007
Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007 3775 Table 1. Physical Properties of Selected Glycol Ethersa Glycol Ether Structure R′-O-[CH2-CH(R)-O]n -H glycol ether designation EB EPent DPent DHex THex THep PnP PnB DPE DPnP DPnB TPnP TPnB a
R′ n-butyl n-pentyl n-pentyl n-hexyl n-hexyl n-heptyl n-propyl n-butyl ethyl n-propyl n-butyl n-propyl n-butyl
Liquid Viscosity (cP)
Vapor Pressure (mm Hg)
R
n
molecular weight, MW
normal boiling point (°C)
liquid density at 25 °C (g/mL)
at 25 °C
at 40 °C
at 20 °C
at 60 °C
H H H H H H CH3 CH3 CH3 CH3 CH3 CH3 CH3
1 1 2 2 3 3 1 1 2 2 2 3 3
118.2 132.2 176.3 190.3 234.3 248.3 118.2 132.2 162.3 176.3 190.3 234.3 248.4
171 188 246 259 >300 >300 149 171 197 213 230 261 274
0.898 0.892 0.940 0.931 0.963 0.956 0.883 0.875 0.927 0.916 0.907 0.935 0.927
2.9 3.6 5.8 6.2 16.5 18.6 2.4 2.8 3.4 3.9 4.9 6.3 7.0
2.0 2.4 3.8 3.1 10.1 11.4 1.7 1.8 2.3 2.4 3.3 3.9 4.4
0.66 0.15 0.0028 0.0010 0.00013 NDb 1.54 0.85 0.17 0.082 0.044 0.0036 0.0018
9.1 3.7 0.18 0.16 0.0084 NDb 19.6 10.6 3.0 1.6 0.88 0.13 0.070
Data taken from the physical property databank of The Dow Chemical Company. b No data available.
Table 2. Mutual Solubility of Selected Glycol Ether + Water Binariesa Solubility of Glycol Ether in Aqueous Phase (wt %)
Solubility of Water in Organic Phase (wt %)
glycol ether designationb
LCST for the mixture (°C)
at 20 °C
at 60 °C
at 20 °C
at 60 °C
EB EPent DPent DHex THex THep PnP PnB DPE DPnP DPnB TPnP TPnB
48 ∼0 40 12 45 29 32 -10c 78d 10 -10 2 -10c
complete 3.6 complete 2.0 complete complete complete 7.0 complete 20.2 6.5 16.5 4.6
11.8 2.3 3.4 1.0 1.4 0.42 18.5 3.2 complete 5.4 1.5 3.4 0.84
complete 28.9 complete 53.4 complete complete complete 17.0 complete 25.8 12.0 6.5 7.0
45.9 20.4 36.2 23.8 43.1 34.5 26.2 11.2 complete 7.1 7.3 3.6 6.3
a Data taken from ref 3. b Nomenclature from Table 1. c Freezing point temperature. Freezing occurs before a lower critical solution temperature (LCST) is reached. d Unpublished data, The Dow Chemical Company.
includes an evaluation of mass-transfer resistance, because this affects the selection and sizing of appropriate process equipment, and an assessment of energy consumption for the overall process scheme. Materials and Methods Chemicals for Rayleigh distillation experiments were purchased from Sigma-Aldrich and include the following: 1-butoxy-2-propanol (commonly called propylene glycol n-butyl ether, or PnB) (99.0+ wt %); 1-propoxy-2-propanol (commonly called propylene glycol n-propyl ether, or PnP) (98.5+ wt %); 1-(2-butoxy-1-methylethoxy)-2-propanol (commonly called dipropylene glycol n-butyl ether, or DPnB) (98.5+ wt %); and reagent-grade sodium chloride (99+ wt %). For liquid-liquid extraction experiments, PnP (98.5+ wt %) was obtained from Dow. Reagent grade 1-octanol, methyl isobutyl ketone (MIBK), toluene, dichloromethane, dichlorobenzene, cyclohexane, and tetrahydrofuran (THF) were purchased from Fisher Scientific. Diisobutyl ketone (DIBK), n-butyl ether, 2-pentanone, 2-nonanone, and methyl t-butyl ether (MTBE) were purchased from Sigma-Aldrich. 2-Ethylhexanol was purchased from Eastman Chemical. Isopar M (which is a trademarked ExxonMobil product) and Aromatic 200 were obtained as samples from ExxonMobil. Isopar M is a mixture of aliphatic hydrocarbons (normal boiling point ) 225-254 °C). Aromatic 200 is a mixture of aromatic hydrocarbons (normal boiling point ) 237-280 °C). For all experiments, a U.S. Filter
Figure 1. Temperature-composition diagram for ethylene glycol n-butyl ether + water (data taken from ref 3).
double-deionized water station fitted with a 0.2-µm filter provided water. Rayleigh Distillation Apparatus and Procedures. The relative volatilities of selected glycol ethers dissolved in dilute aqueous solution were measured using a Rayleigh distillation apparatus (Figure 2). Methods similar to this one, as well as alternative methods, have been described in the literature.10,11 Experiments were performed by distilling a small amount of
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Figure 2. Schematic of the Rayleigh distillation apparatus.
material (∼1%) out of a 500-mL batch without reflux. A slow boil-up rate of ∼0.5 mL/min was maintained, so that the dynamic experiment closely approached an equilibrium stage. The relative volatility of the organic, with respect to water, was then calculated from the relationship
[
ln 1 + Ri,water )
[
ln 1 +
]
W DY i WBXi
WD(1 - Yi) WB(1 - Xi)
]
(1)
where WD and WB are the weights of distillate and bottoms, Yi is the weight fraction of organic in the distillate, and Xi is the weight fraction of organic in the bottoms. Equation 1 is derived from the Rayleigh differential distillation equation for a dilute binary mixture with constant relative volatility;12 the result expressed here is given in terms of distillate and bottoms masses and component weight fractions. All surfaces of the still were jacketed and kept at a uniform temperature above the dew point, to eliminate condensation inside the vessel. An agitator was used to mix the pot contents to achieve uniform composition and temperature in the liquid phase. Vapor was collected via an overhead-vapor side draw and condensed at -1 °C. After an experiment, the overheads condenser and associated lines were carefully washed to recover all material. The weight and concentration values for the distillate were adjusted to include a minor amount of material recovered from a dry ice trap upstream of the vacuum pump. The material balance accountability for glycol ether in these experiments was 91% or better. Uncertainties in the results obtained using eq 1 have been estimated from analysis of potential experimental errors, and these are reported with the results. Repeated calibration of the apparatus using the isopropanol + water system at atmospheric pressure gave experimental results within (12% of a standard value for isopropanol relative volatility, with respect to water at infinite dilution. The standard value, equal to R∞ ) 24.6 ( 1.5 at 100 °C and 1 atm, is based on the analysis of infinitedilution activity coefficients for isopropanol in water, given by Bergmann and Eckert.13 Samples were analyzed using a Agilent model 6890 gas chromatograph fitted with a flame ionization detector, a 6890 Series Autosampler, and a split injector (10:1 split ratio). The GC column was a 25 m × 0.53 mm ID Chrompack CP-Wax Volatile Amines column (2 µm film thickness). The temperature program was as follows: 50 °C (hold for 2 min), 15 °C/min to 110 °C (no hold), and 20 °C/min to 200 °C (hold for 1.5 min). The injector temperature was 200 °C, and the detector temper-
ature was 250 °C. The helium carrier gas flow rate was 4.6 mL/min. The detector-gas flow rates were 450.0 mL/min for air, 40.0 mL/min for hydrogen, and 45.0 mL/min for makeup helium. Calibration was done using external standard methods with solutions of known concentration. Gas chromatography (GC) data were recorded as the average of two injections. The overall analytical precision was (5%. Method and Apparatus for Extraction of Glycol Ethers from Water. Partition ratios were measured using a 100-mL jacketed-glass vessel to mix, equilibrate, and settle the two liquid phases. Approximately equal weights of the test solvent and an aqueous solution, containing ∼10 wt % PnP, were used. The total weight typically was 90-100 g. The apparatus contained an agitator for good mixing of the two liquids. A temperaturecontrolled recirculating bath with 50% aqueous propylene glycol was used to control the jacket temperature. The vessel vent was connected to a cooled run-back vent condenser maintained at 2 °C to reduce any vapor losses to a negligible level. In a typical experiment, the liquids were mixed for at least 5 min at the preset temperature, using fairly intense mixing, and then allowed to separate by gravity at the operating temperature. Measurements were performed over a range of temperatures, typically from 25 or 30 °C up to 95 °C. Samples of each clarified phase were collected at the operating temperature using a syringe. The material balance accountability for glycol ether in these experiments was 98% or better. Estimated experimental uncertainties are reported with the results. Samples of the clarified aqueous phase and the clarified organic phase were analyzed for PnP and extraction solvent, using an Agilent model 6890 Series gas chromatograph that was equipped with dual capillary inlets, a thermal conductivity detector (TCD), and an HP-7683 auto-injector. The GC column was a 30 m × 0.25 mm inner diameter (ID) Zebron ZB-1 (100% polydimethylsiloxane) capillary column from Phenomenex (film thickness of 0.25 µm). The injector and detector temperatures were 250 °C. For most extraction solvents, the oven temperature program was as follows: 50 °C (hold for 2 min), 20 °C/min to 220 °C (no hold). Minor adjustments were made for some solvents, to optimize the chromatogram. Gas flow rates also were varied somewhat. Typical values were as follows: column pressure, 20 psig helium; split flow, 400 mL/min; split ratio, 200:1; and detector reference flow, 20 mL/min. THF was used as a diluent, internal standard, and reference solvent. Standards containing the test extraction solvent, PnP, and water were prepared to mimic typical composition ranges for both organic and aqueous samples. GC data were recorded as the average of two injections. The overall analytical precision was (5%. Results and Discussion Direct Steam Stripping of Glycol Ethers. Steam stripping may be used to remove dissolved organics from aqueous streams provided the relative volatility of the dissolved organic, with respect to water, is sufficiently large.14-18 The separation is easier if the organic forms a heterogeneous azeotrope with water; that is, if the organic is only partially miscible with water, such that the overheads stream from the stripping tower separates on condensation into an organic layer and an aqueous layer. In a typical process (Figure 3), the aqueous layer is sent back to the stripper feed, and the organic layer is decanted and further processed if needed for recycle back to the main operation. Detailed discussions of these types of process schemes are available elsewhere.15,16,18 Many high-boiling hydrophobic organics with limited solubility in water can be steam stripped from dilute aqueous solutions,
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Figure 3. Schematic of a typical process for stripping organics that form heterogeneous azeotropes with water, showing heat exchange between the incoming feed and the bottoms stream.
because hydrophobic organic/water interactions yield high activity coefficients for the dissolved organic, resulting in relative volatilities that are significantly greater than unity.14,17 In most cases, these organics also form heterogeneous azeotropes with water. For a stripping process to be economical, the relative volatility must be sufficiently high such that the amount of steam required to remove a high percentage of the dissolved organic is not excessive. In principle, the minimum amount of steam required for a multistage countercurrent stripping tower corresponds to a stripping factor of ∼1.2:
S)min ) R ×
V ≈ 1.2 L min
)
(2)
where S is the stripping factor, R the relative volatility of the dissolved solute with respect to water, and V/L the ratio of steam (or vapor) to liquid within the stripping tower. Equation 2 assumes that the stripping tower has a sufficient number of masstransfer units or theoretical stages for the required separation. A stripping factor of at least 1.2 is needed, because at lower values of S, the required number of transfer units or stages generally will be too high for practical application, and at S values of 5)
(3)
where V is the vapor flow rate in the tower and F is the rate of feed entering the process, as indicated in Figure 3. Equation 3 is a short-hand calculation that is reasonably accurate for estimating the minimum steam consumption for relative volatilities greater than ∼5. It is obtained from the material balance around the stripper, assuming that the relative volatility is constant and that essentially all of the overheads condensate is returned to the stripper such that the combined feed is approximately equal to F + V. The resulting calculation overestimates the minimum steam consumption, but the error is small for R > 5. The calculation also assumes that the stripper is adiabatic and condensation of steam within the tower is negligible; that is, the feed enters the tower near its boiling point. The relationship described by eq 3 provides the basis for a set of guidelines concerning the general feasibility of steam stripping organic compounds, specifically those that form
heterogeneous azeotropes with water, from dilute aqueous solution. These rules are based on our experience with stripper design and are very similar to those given elsewhere:16,19 (1) Organics with relative volatilities greater than ∼30 are easily steam stripped, provided the stripper is sufficiently tall; that is, provided the stripper achieves a sufficient number of mass-transfer units. (2) Organics with relative volatilities on the order of 10-30 can be stripped, but energy consumption begins to be a significant factor that limits the number of economical applications. (3) For R ) 5, the minimum steam consumption is on the order of 0.3 kg steam per kilogram of incoming feed, and only a few applications will be economically feasible. Steam stripping normally is not practical for R < 5, because of very high steam consumption and high operating cost. The actual cutoff point will vary, depending on many factors affecting affordability (that is, depending on the cost of stripping per kilogram of product produced by the overall operation, relative to the final product’s market value). Equations 2 and 3 and the steam stripping guidelines listed above do not apply to stripping operations utilizing air or another inert gas; in that case, the relative volatility in eq 2 must be further multiplied by the vapor pressure of water divided by total pressure,11 reducing the effectiveness of R by that ratio. Furthermore, for very high relative volatilities on the order of 1000 or higher, it becomes difficult to operate at the minimum stripping factor, because of significant backmixing effects at such a low V/F ratio. Most glycol ethers cannot be effectively stripped from water, because they are relatively high-boiling compounds with significant hydrophilic character. Many of the lower-boiling glycol ethers, such as propylene glycol methyl ether, form homogeneous azeotropes with water. At least a few glycol ethers, however, form heterogeneous azeotropes and can be stripped from dilute aqueous solution. Table 3 summarizes our measurements of relative volatilities, with respect to water, at 50 and 80 °C for dilute aqueous solutions of PnB, PnP, and DPnB. The relative volatility is >5 for two of them (PnB and PnP). In all cases, the relative volatility increases as the temperature increases. It also increases with the addition of NaCl to the aqueous solution, as expected, because of a salting-out effect.20-22 More details of the Rayleigh distillation experiments are summarized in Tables 4-6. The activity coefficient values listed in these tables were determined from relative volatility (Table 3) and the ratio of pure-component vapor pressures (Table 7), using the relationship ∞ Ri,water
)
∞ γi,aqueous
pSAT i pSAT water
(4)
∞ where Ri,water is the relative volatility of the solute, with respect ∞ is the activity coefficient to water at infinite dilution; γi,aqueous for the solute dissolved in aqueous solution at infinite dilution; and pSAT is the pure-component vapor pressure.11,17 Applicai tion of eq 4 assumes that any deviation from infinite dilution is within experimental error and can be neglected, the activity coefficient for water in solution is unity, and the vapor phase is ideal. These are excellent approximations in this case, because glycol ether concentrations are very low (typically
