Experimental Clathrate Hydrate Dissociation Data for Systems

Article pubs.acs.org/jced

Experimental Clathrate Hydrate Dissociation Data for Systems Comprising Refrigerant + CaCl2 Aqueous Solutions Peterson Thokozani Ngema,† Paramespri Naidoo,† Amir H. Mohammadi,*,† Dominique Richon,†,‡ and Deresh Ramjugernath*,† †

Thermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban, 4041, South Africa ‡ Department of Biotechnology and Chemical Technology, School of Science and Technology, Aalto University, Aalto, Finland ABSTRACT: Hydrate dissociation data are presented for systems of refrigerants (R134a, R410a, and R507) + water + CaCl2 at varying salt concentrations. The R134a + water + CaCl2 system was measured at salt concentrations of (0, 0.358, 0.591, and 0.756) mol·kg−1 in the temperature range of (276.2 to 283) K and a pressure range of (0.125 to 0.428) MPa. The systems composed of {R410a or R507} + water + CaCl2 were measured at salt concentrations of (0, 0.358 and 0.756) mol·kg−1 in the temperature range of (274.7 to 293) K and a pressure range of (0.179 to 1.421) MPa. The isochoric pressure-search method was used for all the hydrate measurements. The data generated can be used in the design and optimization of industrial wastewater treatment and desalination processes. The presence of CaCl2 in the aqueous solutions shifts the hydrate−liquid water−vapor (H−Lw−V) equilibrium phase boundary toward lower temperatures as salt concentration increases. The experimental data were modeled using a combination of the solid solution theory of van der Waals and Platteeuw, the Aasberg−Petersen et al. model, and the Peng−Robinson equation of state with classical mixing rules. The model provides a good correlation of the experimental hydrate dissociation data.

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water include purification and desalination.18,19 In purification processes there are a number of chemical and physical steps which are used to treat the feedstock to remove petroleum organics, heavy metals, and suspended solids.18,19 After preliminary treatment, fresh water can be produced by desalination using processes such as multistage flash distillation and reverse osmosis. As has been mentioned before, these processes are commonly used to produce fresh water from industrial wastewater and seawater.4,7−19 The common salt ions usually found in seawater and industrial wastewater are chlorides, sulfates, phosphates, and nitrates. These salts need to be removed to render the processed water useable for consumption.4,7−19 Traditional processes for potable water production are very expensive from an operational point of view. Factors which lead to the increased operational costs are scaling and damage to membranes.4,7−19 Generally, saturated sulfates are the cause of scaling and chlorides damage the membranes. If water can be recovered at ambient conditions from the concentrated brine solutions, there will be a significant improvement of the efficiency of the desalination process with regard to energy.4,7−19 Hence, desalination using gas hydrate technology has been proposed.4,9−20 The gas hydrate chemical structure involves only water and hydrate former molecules,

INTRODUCTION Interest in the application of gas hydrate or clathrate hydrate technology for the production of potable water from industrial wastewater and seawater has grown among researchers over the last few decades. As the demand for freshwater increases globally, processes for desalination are needed which are less energy intensive than current commercial processes like membrane based technologies such as reverse osmosis. Gas hydrates have been proposed for a number of different applications, for example, wastewater treatment and desalination, CO2 capture and separation, separation of close-boiling point compounds, hydrogen/methane storage, refrigeration and in the air conditioning industry, and in the food industry.1−6 Gas hydrates or clathrates are crystalline solid structures which comprise water molecules which form cage structures due to hydrogen bonding at conducive conditions of high pressure and low temperature.1,6 Guest molecules of appropriate size, for example, carbon dioxide and methane, are trapped in the cages. Gas hydrates are generally categorized into three crystalline structures with different cage sizes, shapes, and number of cavities or cages:3 structure I (sI), structure II (sII), and structure H (sH). One third of the world’s population is currently facing the problem of a shortage of fresh water.4,7−19 Because seawater is the most abundant resource on earth,4,7−19 there has been great interest in using it as a feedstock for fresh water production. Traditional treatment processes for the production of fresh © XXXX American Chemical Society

Received: August 6, 2015 Accepted: December 11, 2015

A

DOI: 10.1021/acs.jced.5b00675 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Criteria for the Selection of the Hydrate Formers28

with hydrate formation leaving salts and impurities in the residual saline water.6,19 Fresh water is the product of the desalination process when the hydrate crystals are dissociated. It has been shown that refrigerants can normally form clathrate hydrates at pressures below 2 MPa.3,4,19−29 Clathrate hydrates have various applications in the chemical industry. Refrigerants can play an important role in water desalination or wastewater treatment.3,4,19−29 To develop low cost desalination processes, a low pressure hydrate medium is required.30,31 Certain refrigerants are good hydrate formers and enable formation and dissociation to occur at temperatures between (273 and 293) K at around atmospheric pressure.30,31 For convenient operation of a desalination processes, a water insoluble promoter should be added when hydrate dissociation data are below ambient conditions to shift temperatures or pressures closer to ambient conditions.28,29 Researchers4,20,21,29,32 have shown that the use of refrigerants for the formation of gas hydrates in water desalination processes has good promise when compared to traditional desalination processes. As the use of refrigerants enables hydrate formation at ambient conditions, there has been great interest in this research area. According to Eslamimanesh et al.6 and Cha and Seol19 the dissociation of the refrigerant hydrate results in pure/clean water being produced and the release of refrigerant which can be recycled for reuse to form additional hydrates. There have been previous investigations on the removal of salts from seawater. Park et al.,8 using a single stage gas hydrate process, showed that (72 to 80) % of each of the dissolved salts could be removed with the effect as follows: K+ > Na+ > Mg2+ > B3+ > Ca2+. The three-phase equilibria of the chloro(difluoro)methane (R22) hydrate forming systems in aqueous solutions containing NaCl, KCl, and MgCl2 was investigated by Chun et al.21 They measured hydrate dissociation data in a pressure range of (0.140 to 0.790) MPa and a temperature range of (273.9 to 287.8) K, for varying compositions of each of the electrolytes. It was observed that the addition of electrolytes to the aqueous solutions inhibits hydrate formation which shifts the phase dissociation data toward lower temperatures. Ngema et al.29 investigated the refrigerants R134a, R410a, and R507 in water and NaCl aqueous solutions of varying concentrations. It was observed that for the (R134a or R507 or R410a) + water systems, the addition of NaCl shifts the H− Lw−V equilibrium phase boundary toward lower temperatures with increasing salt concentration. Furthermore, it was found that these systems exhibit a quadruple point, that is, all four phases (H−Lw−LRefrigerant−V) coexist. As a continuation of the initial study of Ngema et al.,29 the influence of CaCl2 on refrigerant hydrates is studied herein. The refrigerants studied are R134a (1,1,1,2-tetrafluoroethane), R410a (0.5 mass fraction difluoromethane + 0.5 mass fraction 1,1,1,2,2-pentafluoroethane) and R507 (0.5 mass fraction 1,1,1trifluoroethane + 0.5 mass fraction 1,1,1,2,2-pentafluoroethane). These refrigerant gases are environmentally friendly (low global warming potential) and are able to form hydrates at lower pressures than the natural hydrate formers. Table 1 presents the screening criteria used for the selection for the hydrate formers. Such refrigerants have not been investigated extensively in literature, especially in the presence of salts. These fluorinated refrigerants are available commercially, and there is some hydrate literature data for them in the presence of pure water.

characteristic environmental acceptability nontoxicity nonflammability chemical stability compatibility with standard materials formation of a class II hydrate low cost availability water solubility

desirable criteria The former was approved by the Montreal Protocol as it has low ozone depletion and greenhouse effect. The former had a low acute toxicity as it was noncarcinogenic and nonmutagenic. The flash point temperature of the former was high, this was to reduce the risk of a fire starting. The former reacted slowly with chemicals. The former had a low chemical activity. Easier separation between the hydrate and salt in the wash column was established. Operating costs were reduced. The former was manufactured in commercial quantities from reliable sources. A former with low water solubility eliminated an extra step in the process to recover the former from the water.

In the present study measurements were undertaken for varying concentrations of calcium chloride (the main salt constituent in brine). Measurements for the R134a + water + CaCl2 system were undertaken at salt concentrations of (0.358, 0.591, and 0.756) mol·kg−1, while the {R410a or R507} + water + CaCl2 system was measured at salt concentrations of (0.358 and 0.756) mol·kg−1. Table 2 lists the range of conditions for Table 2. Molality, Temperature, and Pressure Ranges Investigated for Hydrate Dissociation Condition Measurements (wi = Molality of Salt in Aqueous Solution) hydrate former R410a

R507

R134a

salt no salt CaCl2 no salt CaCl2 no salt CaCl2

wi/mol·kg−1 0.358 0.756 0.358 0.756 0.358 0.591 0.756

T/K 277.5 280.8 281.1 277.7 276.9 274.7 277.1 276.2 276.4 276.2

to to to to to to to to to to

293.0 291.4 290.1 283.7 282.5 280.6 283.0 281.2 281.0 279.5

p/MPa 0.179 0.315 0.397 0.221 0.246 0.191 0.114 0.125 0.148 0.169

to to to to to to to to to to

1.421 1.341 1.273 0.873 0.834 0.777 0.428 0.392 0.367 0.355

the hydrate dissociation experiments. The experimental dissociation data were correlated using a combination of the Aasberg−Petersen et al.33 model, which describes the electrolyte aqueous system, and the hydrate phase described by the solid solution theory of van der Waals and Platteeuw.34 The Peng−Robinson35 equation of state with classical mixing rule was used to describe the aqueous/liquid and vapor phases.

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EXPERIMENTAL SECTION Materials. Table 3 lists the purities of the chemicals used in this study as well as details of the chemical suppliers. The purities and compositions of the refrigerants were checked using gas chromatography (Shimadzu 2010 gas chromatograph equipped with a thermal conductivity detector and a Porapak Q column). Ultrapure Mill-Q water (electrical conductivity: 0.0556 μS.cm at 298.15 K) was used in all the experiments. The gravimetric technique was used to prepare the CaCl2 aqueous solutions. An analytical balance (Ohaus Adventurer balance, model No. AV 114) which has a supplier stated B



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Table 3. Purities, Critical Properties, and Suppliers of Studied Chemicals chemicals R507 R134a R410a water calcium chloride

formula f

f

0.5 (CHF2CF3) + 0.50 (CH3CF3) CF3CH2F 0.5f(CH2F2) + 0.50f(CHF2CF3) H2O CaCl2

molecular weight (g mol−1)

supplier

purity* (mass fraction)

Tc/K

pc/MPa

ω

98.80 102.03 72.60 18.02 110.98

Afrox Afrox Afrox UKZN Merck

0.998 0.999 0.998 1.000 0.990

343.96a 374.18b 345.65c 647.14d

3.797a 4.057b 4.964c 22.064d

0.304e 0.326b 0.279e 0.344d

Reference 36. bReference 37. cReference 38. dReference 49. eReference 50. fMass fraction. *As stated by the supplier. Checked by GC analysis for the gases and liquids.

a

Figure 1. Schematic flow diagram for the equilibrium cell: A, isochoric equilibrium cell; B, impeller; C, neodymium magnet; D, pressure transducer; E, data acquisition unit; F, Pt-100; G, stainless steel stand; H, overhead mechanical stirrer; I, temperature programmable circulator; J, drain line; K, chilling fluid; L, cooling coil; M, cold finger; N, liquid syringe with aqueous solution; O, refrigerant gas cylinder; P, vacuum pump; Q, vacuum flask; R, mechanical jack; S, drain valve; T, inlet valve; U, loading valve; V, gas valve, W, vent valve to atmosphere; X, vacuum valve; Y, water bath; Z, stainless steel bolts; AA, mechanical shaft.

uncertainty of 0.0001 g in mass was used for the synthesis of the aqueous solutions. Equipment. Figure 1 shows a schematic diagram of the equilibrium cell which was operated accordingly to the isochoric pressure method. The equilibrium cell is constructed of 316-stainless steel and has a maximum operating pressure of 10 MPa. The volume of the equilibrium cell is approximately 38.5 cm3. The equilibrium apparatus boasts a modified stirring device which was designed to improve the stirring power and efficiency. This stirring device uses two Neodymium magnets (produced under customer specification by Supermagnete, Germany): one cylinder (10 mm outside diameter and 20 mm height) and one ring (28 mm external diameter, 17 mm internal diameter and 10 mm height). The cylindrical magnet is held by the shaft (see Figure 1) and is driven by a Heidolph RZR 2041 overhead stirrer. The ring-shaped magnet is situated around the glove finger, machined directly in the cap of the cell,

at the level of the cylindrical magnet which is inside the glove finger. This arrangement allows for very low friction, no sealing issues, and very strong magnetic coupling. The Vesconite piece fixed to the ring-shaped magnet holds two impeller blades (5 mm length, 13 mm height, 1 mm width) and two small neodymium magnets. The advantages of this new stirring device are (i) improvement of the stirring power and efficiency compared to a magnetic bar stirring device; (ii) reduction of the gas hydrates dissociation time through faster homogenization; (ii) overhead display speed is exactly the same as stirrer speed inside the equilibrium cell; (iv) direct agitation in the cell, which promotes the formation of a homogeneous aqueous solution. A platinum resistance thermometer (Pt-100) was used to measure the equilibrium temperature. The Pt-100 temperature probe was calibrated in a bath containing ethylene glycol with a C



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refrigerant absorption into the solution until stabilization of the temperature and pressure. Prior to the formation of gas hydrate, the temperature and pressure is stable outside the hydrate formation region. The system temperature was slowly decreased to facilitate the formation of gas hydrates. During this stage, the pressure decreases linearly with the system temperature. The point at which a drastic pressure drop is observed indicates the formation of gas hydrate (see Figure 2).

WIKA primary temperature probe which was connected to a WIKA CTH 6500 multimeter. The calculated uncertainty for the temperature calibration was 0.03 K. The expanded uncertainty U (0.95 level of confidence) for the reported temperature is 0.1 K. The equilibrium cell pressure was measured using a WIKA pressure transducer which has a maximum operating limit of 10 MPa. It was calibrated in the range of (0−10) MPa against a standard pressure transmitter from WIKA. The expanded uncertainty U (0.95 level of confidence) for the reported pressure is 0.005 MPa. After calibration of the temperature and pressure sensors, the vapor pressures of the refrigerants were measured as a means to verify the reliability and accuracy of the equilibrium cell. The vapor pressure data for R507, R134a, and R410a are presented in Table 4 . There is good agreement between the experimental vapor pressures and data in literature.36−38 Table 4. Measured Vapor Pressures for Studied Hydrate Formers (Refrigerants)a T/K

Δp/MPab,d

p/MPa

T/K

b

258.2 261.9 269.2 277.6 282.6 287.6 293.1 297.8 302.9 262.5 271.4 273.3 277.6 282.6 287.6 292.6 297.6 302.7

R507 0.381 0.435 0.555 0.725 0.843 0.974 1.133 1.287 1.475 R410ae 0.570 0.758 0.807 0.922 1.070 1.246 1.429 1.635 1.855

0.002 0.002 0.002 0.002 0.001 0.001 0.002 0.001 0.002

258.2 262.2 272.2 282.6 292.9 302.7

Δp/MPad

p/MPa R134a 0.162 0.193 0.286 0.411 0.569 0.763

c

0.002 0.000 0.003 0.004 0.002 0.004

Figure 2. Typical formation and dissociation hydrate curve for the R134a (1) + water (2) system (A (cooling curve), outside hydrate stability region; and B (heating curve), hydrate stability region).

After the hydrate formation stage, the system temperature is gradually increased. As the dissociation point is approached, the incremental step in temperature is set to 0.1 K. The time taken to achieve equilibrium for each 0.1 K incremental step in temperature is approximately 1 h. Figure 2 provides an illustration of a typical hydrate formation and disassociation cycle for the isochoric pressure search method. The gas is completely released at the point where the cooling curve (A) and heating curve (B) intersect. Finally, the system goes back to the initial conditions which are outside the hydrate formation region. Therefore, the point of intersection of the cooling and heating curves is called the gas hydrate dissociation point, where system temperature and pressure are at equilibrium.

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a

U(T) (0.95 level of confidence) = 0.1 K. U(p) (0.95 level of confidence) = 0.005 MPa. bLiterature values calculated using (eq T-1): p/kPa = exp[ln pc + (A(1 − TR) + B(1 − TR)1.5 + C(1 − TR)3.5 + D(1 − TR)4)/TR]. cLiterature values calculated using (eq T-II): p/kPa = exp[ln pc + (A(1 − TR) + B(1 − TR)1.5 + C(1 − TR)2.5 + D(1 − TR)5)/ TR]. dPressure difference calculated using (eq T-III): Δp = plit − pexpt. e Literature vapor pressure from ref 38 where pressure, p/kPa, critical pressure, pc/kPa, reduced temperature, TR/K, temperature, T/K and constants, A, B, C, and D are constants.

THERMODYNAMIC MODEL A thermodynamic model that represents the liquid (aqueous)− hydrate−vapor equilibrium condition of a system can be developed by starting with the criterion for phase equilibrium in terms of the fugacity of water in the aqueous phase, f Lw, and the hydrate phase, f wH. The water content of the gas/vapor phase2,3,20,29,31 is ignored. f wL = f wH

Experimental Method. The isochoric pressure search method1−3,26,29,39,40 was used to measure the hydrate dissociation conditions reported. Deionized water was first used to flush the equilibrium cell. Thereafter acetone was used to flush and rinse the cell. The cell was then drained and evacuated to 0.2 kPa for a period of 30 min (at an elevated temperature) to ensure that there were no traces of residue components remaining in the cell. Approximately 15 cm3 of CaCl2 aqueous solution was then charged into the cell. The equilibrium cell was lowered into the bath. Refrigerant was charged into the cell through a pressure regulating valve. The contents of the equilibrium cell were then agitated to allow

(1)

Hydrate Phase. The fugacity of water in the hydrate phase, f Hw is given by the following expression:2,3,20,29 f wH = f wMT exp

μwH − μwMT

(2) RT where denotes the fugacity of water in the hypothetical empty hydrate phase, μHw − μMT w denotes the chemical potential of water in the filled (μHw ) and empty (μMT w ) hydrate. R and T denote the universal gas constant and temperature, respectively. The solid solution theory34 can be used to calculate (μHw − 2,3,20,29 μMT w )/RT

f MT w

D



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Table 5. Constants for eqs 13, T-I, T-II, and T-III R507a R134ab CaCl2c a

A

B

C

D

E

−7.342584 −7.64850 −5.672 × 1000

1.046268 1.788344 8.037 × 10−03

1.999693 −2.633842 −3.330 × 10−01

−9.207652 −3.355961 −1.771 × 10−03

5.800 × 10−04

b

c

Reference 36. Reference 37. Reference 48.

μwH − μwMT RT

= −∑ vi′ ln(1 + i

=

molecule in a unit hydrate cell for structures I and II have been reported by Sloan and Koh3 and are available in the lit.2,3,20,29 The interactions between the hydrate former and water molecules in the cavities43 are accounted for by the Langmuir constants2,3,20,28,29,41−47 which are available for a range of temperatures and hydrate formers.3,20 The Langmuir constants for a particular temperature range are determined using an integration procedure and the Kihara potential function with a spherical core.3,20,29 Equations 8 and 93,20,29 were used to determine the model parameters for the Langmuir constants. For small cavities (pentagonal dodecahedral)

∑ Cijf j ) j

∑ ln(1 + ∑ Cijf j )−v′ i

i

j

(3)

where vi′ is the number of cavities of type i per water molecule in a unit hydrate cell, Cij denotes the Langmuir constant for the hydrate former’s interaction with each type of cavity and f j is the fugacity of hydrate former.2,3,20,29 The fugacity of water in the hypothetical empty hydrate phase is given by2,3,20,29 f wMT = pwMT φwMTexp

∫p

p

MT w

vwMT dp RT

Csmall = (4)

=

⎡ v MT(p w MT pw exp⎢ ⎢⎣

− pwMT ) ⎤ ⎥ ⎥⎦ RT

C large =

(5)

⎡ v MT(p − p MT ) ⎤ w ′ w ⎥[(1 + Csmallf V )−vsmall f wH = pwMT exp⎢ refrigerant ⎢⎣ ⎥⎦ RT (6)

where f Vrefrigerant denotes the fugacity of refrigerant hydrate former in the vapor, and “small” and “large” refer to the small and large cavities, respectively. If the dissociation pressure is greater than 2 MPa3 then the Poynting correction term should be considered in calculations. As the dissociation pressures of refrigerant hydrates are lower than 2 MPa,3,20,29 it was assumed that the fugacity of the refrigerants in the vapor phase is equal to the dissociation pressure of gas hydrate. It was also assumed that the vapor phase behaves like an ideal gas for the refrigerants.20 Therefore, f Vrefrigerant = p and the fugacity of water in hydrate phase is given by20 ′ ′ f wH = pwMT [(1 + Csmallp)−vsmall × (1 + C largep)−v large ]

⎛d⎞ c exp⎜ ⎟ ⎝T ⎠ T

(9)

where the units of T are in K and for C it is the reciprocal of MPa. By fitting the thermodynamic model to the experimental hydrate dissociation data3,20,29 the optimum values of the parameters a to d, are determined. Saline Aqueous Phase. The solubility of gas in aqueous electrolyte solutions is calculated using the Aasberg−Peterson et al.33 model. The aqueous phase is regarded as a salt-free mixture and the Peng−Robinson35 equation of state (EOS) used to describe it. The Debye−Hückel electrostatic term, which depends on the ionic strength of the solution and hence on the electrolyte concentration, is used to correct the fugacity coefficient which is calculated using the EOS. The correction is also dependent on the type of the electrolyte and is given by an adjustable parameter which is temperature and composition dependent. The fugacity in the aqueous phase that contains the electrolytes is given by28,29,33,48

An expression for the fugacity of water in the hydrate phase is obtained by substitution of eq 5 into eq 2:20

′ V )−v large ] + (1 + C largefrefrigerant

(8)

and for large cavities (tetrakaidecahedra (sI) and hexakaidecahedra (sII))

where pMT w is the vapor pressure of the empty hydrate lattice, φMT is the correction for the deviation of the saturated vapor of w pure lattice from ideal behavior, and vMT w is the partial molar volume of water in the empty hydrate.20 Equation 4 contains two assumptions: (1) The molar volume is independent of pressure and is equal to the partial molar volume of the hydrate. (2) pMT w is negligible (in the order of 10−3 MPa) and therefore φMT w = 1. Consequently, eq 4 is simplified to2,3,20,29 f wMT

⎛b⎞ a exp⎜ ⎟ ⎝T ⎠ T

f wL = x wϕwLp

(10)

where the fugacity coefficient of water in the aqueous phase is given by29,33,48 ln ϕwL = ln ϕEOS + ln γ EL

(11)

where γEL accounts for the effect of the electrolytes. Electrostatic interactions are accounted for using the following expression29,33,48 ln γ EL =

(7)

Model Parameters. By equating the fugacity of water in the hydrate phase to that of pure ice at the three-phase line,3 the vapor pressure of the empty hydrate lattice, pMT w , can be calculated. The vapor pressures of the empty hydrate structure and the values of number of cavities, v′i of type i per water

2A′h isM mF(B′I 0.5) B′3

(12)

where his denotes the interaction coefficient (between the dissolved salt and a nonelectrolytic component). The coefficient is dependent on temperature, composition, and ionic strength (I). Mm is the salt-free mixture molecular weight determined as a molar average. Tohidi et al.48 provide a E



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correlation for his for a number of water−electrolyte and gas− electrolyte systems. h

A + BT + Cx + Dx 2 + ETx 1000

(13)

The values for the parameters in eq 13 are listed in Table 5. The parameters A′ and B′, as well as the function F in eq 12 are given by the following equations:29,33,48 ⎛ d 0.5 ⎞ m ⎟ A′ = 1.327757 × 105⎜ 0.5 ( T ε ⎝ m ) ⎠

(14)

⎛ d 0.5 ⎞ m ⎟ B′ = 6.359696⎜ 0.5 ⎝ (εmT ) ⎠

(15)

F(B′I 0.5) = 1 + B′I 0.5 −

1 − ln(1 + B′I 0.5) 1 + B′I 0.5

Figure 4. Comparison between the experimental hydrate dissociation data and the model result of Eslamimanesh et al.20 for the R410a (1) + water (2) system: ◊, measured; •, literature;23 ―, model.

(16)

where dm is the density of the salt-free mixture (assumed to be equal to the density of water), and εm is the salt-free mixture dielectric constant which for a mixture of gases and water is given by29,33,48 εm = x NεN (17)

experimental data measured in this study and literature in the low pressure range of (0.277 to 0.377) MPa. The linearity of the experimental data (this study) in terms of the ln p versus T plot, as well as the good correlation of the data using the model, implies that there is probably experimental error associated with the literature data in the (0.277 to 0.377) MPa pressure range. The experimental data are reported in Table 6 and plotted in Figure 4.

where xN and εN are salt-free mole fraction and the dielectric constant of water, respectively.28,29,33,48

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RESULTS AND DISCUSSION The vapor pressures of {R410a or R134a or R507} were measured to check the calibration and also confirm the experimental procedure for the newly constructed equilibrium cell. Experimental vapor pressures of refrigerants were compared against the available data in literature36−38 as shown in Figure 3. The data are measured in the temperature

Table 6. Experimental Data for the Dissociation Conditions of Gas Hydrate for the R410a (1) + Water (2) + CaCl2 (3) System at Various Salt Molalitiesa R410a (1) + water (2)

R410a (1) + water (2) + 0.358 mol·kg−1 CaCl2 (3)

R410a (1) + water (2) + 0.756 mol·kg−1 CaCl2 (3)

T/K

p/MPa

T/K

p/MPa

T/K

p/MPa

293.0 291.3 290.3 289.0 287.8 286.0 284.6 283.1 280.3 277.5

1.421 1.185 1.034 0.868 0.741 0.582 0.484 0.396 0.257 0.179

291.4 290.4 289.2 287.8 286.3 285.4 283.9 282.2 280.8

1.341 1.168 0.967 0.803 0.659 0.581 0.485 0.389 0.315

290.1 290.0 289.2 288.4 287.3 286.2 284.6 283.2 281.1

1.273 1.266 1.132 1.016 0.884 0.775 0.636 0.525 0.397

a U(T) (0.95 level of confidence) = 0.1 K. U(p) (0.95 level of confidence) = 0.005 MPa.

The R134a + water binary test system was also measured and the results compared to those of literature.22,23,26−29 Table 7 and Figure 5 present corresponding experimental data for the R134a + water test system. These two binary systems were studied to verify the reliability of the equilibrium cell and the isochoric pressure-search method which was used to obtain gas hydrate dissociation data. The hydrate measurements were performed at a stirrer speed of 600 rpm. The use of a new stirring device in the equilibrium cell enabled a significant reduction in the gas hydrate dissociation time through faster homogenization. It was found that the total time was reduced from approximately 54 h to 20 h at a stirrer speed of 600 rpm when compared previously apparatus used.28,29 It was found that the stirring device successfully

Figure 3. Vapor pressure curves of the studied refrigerants: □, R134a; ◊, R507; Δ, R410a; , literature.36−38

range of (258.2 to 302.9) K. The data presented are in the form of a ln P versus (1/T) plot and show a linear trend, consistent with the thermodynamic relationship. The hydrate dissociation data for the R410a + water binary test system agree well with that in the literature23 in the pressure range of (0.465 and 1.421) MPa. As observed in Figure 4, the literature23 data have some scatter at a pressure of 1.246 MPa, and there is significant deviation between F



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Table 7. Experimental Data for the Dissociation Conditions of Gas Hydrate for the R134a (1) + Water (2) + CaCl2 (3) System at Various Salt Molalitiesa T/K

p/MPa

R134a (1) + Water (2) 283.0 0.428 282.8 0.400 282.4 0.368 282.2 0.350 281.6 0.308 281.0 0.269 280.5 0.236 279.2 0.180 278.4 0.150 277.1 0.114 R134a (1) + Water (2) + 0.756 mol·kg−1 CaCl2 (3) 279.5 0.355 279.1 0.334 278.2 0.272 277.7 0.237 277.2 0.214 276.2 0.169

T/K

p/MPa

R134a (1) + Water (2) + 0.358 mol·kg−1 CaCl2 (3) 281.2 0.392 280.7 0.346 280.2 0.306 279.7 0.274 279.0 0.229 277.8 0.179 277.0 0.152 276.2 0.125

T/K

Table 8. Experimental Data for the Dissociation Conditions of Gas Hydrate for the R507 (1) + Water (2) + CaCl2 (3) System at Various Salt Molalitiesa

p/MPa

T/K

p/MPa

R507 (1) + Water (2)

R134a (1) + Water (2) + 0.591 mol·kg−1 CaCl2 (3) 280.1 0.367 279.6 0.334 279.2 0.304 278.6 0.255 277.9 0.215 277.1 0.179 276.4 0.148

283.7 283.2 282.5 281.6 281.0 280.1 279.3 278.1 277.7

0.873 0.740 0.611 0.504 0.444 0.370 0.297 0.241 0.221

T/K

p/MPa

R507 (1) + Water (2) + 0.358 mol·kg−1 CaCl2 (3) 282.5 0.834 281.9 0.737 281.5 0.657 280.8 0.568 280.1 0.487 279.4 0.428 278.9 0.388 278.4 0.342 277.7 0.295 276.9 0.253 276.9 0.246

T/K

p/MPa

R507 (1) + Water (2) + 0.756 mol·kg−1 CaCl2 (3) 280.6 0.777 280.2 0.701 279.5 0.579 278.5 0.496 277.8 0.414 277.0 0.344 276.2 0.300 275.3 0.235 274.7 0.191

a U(T) (0.95 level of confidence) = 0.1 K. U(p) (0.95 level of confidence) = 0.005 MPa.

a

U(T) (0.95 level of confidence) = 0.1 K. U(p) (0.95 level of confidence) = 0.005 MPa.

Figure 6. Experimental and calculated hydrate dissociation pressures for the R134a (1) + water (2) + CaCl2 (3) system: ◊, no salt; ○, 0.358 mol·kg−1; △, 0.591 mol·kg−1; □, 0.758 mol·kg−1; ―, model; ---, quadruple point line.

Figure 5. Hydrate dissociation conditions for the R134a (1) + water (2) system: ●, experimental data at 400 rpm; ▲, experimental data at 600 rpm; ○, literature22 > 273 K; ■, literature22 < 273 K; ◊, literature.23

improved the stirring power and efficiency compared to the magnetic bar stirring device used previously.28,29 Gas hydrate dissociation data for the {R134a or R507 or R410a} + water systems are presented in Tables 6 to 8 and illustrated in Figures 6 to 8. In the proposed desalination process, it is expected that when the gas hydrate dissociates, pure/clean water is produced. However, it is possible for some interstitial/surface salt to be present on the hydrate. When the refrigerant is finally released, it can be recycled for reuse.3,4,6−9,21 The dissociation temperatures for the {R134a or R507 or R410a} + water + NaCl systems measured previously29 are

Figure 7. Experimental and calculated hydrate dissociation pressures for the R507 (1) + water (2) + CaCl2 (3) system: ◊, no salt; ○, 0.358 mol·kg−1; △, 0.756 mol·kg−1; ―, model; − − , quadruple point line. G



Article

The Langmuir constant parameters (c and d in Table 9) were obtained using eq 9 in the absence of salt. The constants were Table 9. Regressed Langmuir Constants Parameters for the Refrigerant Gas Hydrate Systems in the Presence of CaCl2 c/K·MPa−1

hydrate former

−3

5.70 × 10 4.75 × 10−3 4.50 × 10−4

R134a R410a R507 a

d/K

AADa

4908.71 5969.68 6233.08

5.30 0.79 0.83

Absolute average deviation (AAD) is calculated as AAD(%) =

100 N

N

∑ i

|pical − piexp | piexp

thereafter regressed by adjusting the parameters in eq 13 using hydrate dissociation data in the presence of CaCl2 aqueous solutions. The Langmuir constant parameters for the 50 wt % mixture of gases (R410a and R507) were calculated using the assumption that they are pure gases. The objective function (OF) listed below was used to calculate all adjustable parameters for the models:

Figure 8. Experimental and calculated hydrate dissociation pressures for the R410a (1) + water (2) + CaCl2 (3) system: ◊, no salt; ○, 0.358 mol·kg−1; △, 0.756 mol·kg−1; ―, model; ---, quadruple point line.

lower than those for the {R134a or R507 or R410a} + water + CaCl2 systems. This indicates that NaCl has a stronger inhibition effect than CaCl2 on hydrate formation. Therefore, the order of inhibition strength of {R134a or R507 or R410a} between two metal chlorides is Na > Ca. The {R507 or R410a or R134a} + water + CaCl2 systems exhibit quadruple points where four phases (H−Lw−Lrefrigerant−V) coexist. Previous studies of Ngema et al.29 pointed out that a water insoluble promoter (cyclopentane or cyclohexane) is required for the {R507 or R134a} + water + NaCl systems, in order for hydrate formation to occur around ambient conditions. In this study, a water insoluble promoter is required for the {R507 or R134a} + water + CaCl2 systems because their dissociation temperatures are lower than ambient temperature. Furthermore, it is pointed out that the R410a + water + CaCl2 system is the most suitable for desalination and wastewater treatment processes because its dissociation temperatures are closer to ambient conditions. It should be noted that in the present study only thermodynamic conditions for the process were determined. Thermodynamic conditions are however not the only item that should be considered when assessing the feasibility of desalination using clathrate hydrates. Other factors to take into consideration, which are potential limitations for the process are, for example, nucleation/growth processes and interstitial salt solution between/on the surface of hydrate crystallites. Experimental results show that the addition of CaCl2 causes the phase equilibrium boundary to shift significantly to low-dissociation temperatures as salt molality increases. However, CaCl2 shows a lower inhibition effect on R410a + water + CaCl2 system compared to {R507 or R134a} + water + CaCl2 systems. The experimental data were modeled using a combination of the Aasberg−Petersen et al.33 model (for electrolyte aqueous systems) with the solid solution theory of van der Waals and Platteeuw34 (for the hydrate phase) and the Peng−Robinson35 equation of state with classical mixing rule used for the aqueous/liquid and vapor phases, as mentioned earlier. The model for the electrolyte system assumes that no ions are present in the vapor phase and CaCl2 does not enter the hydrate phase. The solubility of refrigerants in the CaCl2 aqueous solutions is very low and the formation and dissociation pressures are also low. Consequently the effect of the solubility in aqueous solution is negligible.

OF =

100 N

N

∑ i

|pical − piexp | piexp

(18)

where N denotes the number of data points, subscript i denotes the ith calculated or experimental hydrate dissociation point and the superscripts “cal” and “exp” refer to calculated and experimental hydrate dissociation points, respectively. The Langmuir constants and absolute average deviation (AAD) of the R134a + water system (listed in Table 9) are in fair agreement with that of Eslamimanesh et al.20 Previous studies20,22,26 show that R134a forms type sII hydrates, with large cavities. R134a, R410a, and R507 are large molecules and consequently cannot enter the small cavities of their relevant gas hydrate structures. Therefore, eq 9 was used to calculate the Langmuir constants for structure type sII (large cavities). The model combination used in this study provides a satisfactory agreement with experimental hydrate dissociation data.

■

CONCLUSIONS Experimental hydrate dissociation data for systems consisting of {R410a, R507, or R134a} + water in the presence of CaCl2 were measured at varying concentrations of the salt. The R134a + water + CaCl2 system was measured at salt molalities of (0.358, 0.591, and 0.756) mol·kg−1, while the {R410a or R507} + water + CaCl2 systems were measured at salt molalities of (0.358 and 0.756) mol·kg−1. Quadruple points at which the four phases (H−Lw−LRefrigerant−V) coexist were determined for all systems measured. The isochoric pressure-search method1−3,26,29,39,40 was used for the hydrate dissociation measurements. Hydrate dissociation data obtained indicate that R507 and R134a are not suitable for application in desalination processes at conditions close to ambient. In this case, a water insoluble promoter (cyclopentane or cyclohexane) could be used, which when added to the {R507 or R134a} + water + CaCl2 system would shift the H−Lw−V equilibrium phase boundary closer to ambient conditions. The presence of CaCl2 salt in the aqueous solutions exhibits a thermodynamic inhibition effect on the refrigerant gas hydrates, in which the H−Lw−V equilibrium phase boundary is shifted to low H



Article

(15) Bradshaw, R. W.; Greathouse, J. A.; Cygan, R. T.; Simmons, B. A.; Dedrick, D. E.; Majzoub, E. H. Desalination Utilizing Gas Hydrates; LDRD Final Report; Sandia National Laboratories: Albuquerque, NM, Livermore, CA, 2008. (16) McCormack, A. J., Niblock, G. A. Build and Operate a Clathrate Desalination Pilot Plant; Water Treatment Technology Program Report No. 31; Thermal Energy Storage, Inc.: San Diego, CA, 1998. (17) Corak, D.; Barth, T.; Hoiland, S.; Skodvin, T.; Larsen, R.; Skjetne, T. Effect of subcooling and amount of hydrate former on formation of cyclopentane hydrate in brine. Desalination 2011, 278, 268−274. (18) Almadun, F.-R.; Pendashteh, A.; Abdullah, L. C.; Biak, D. R. A.; Madaeni, S. S.; Abidin, Z. Z. Review of technologies for oil and gas produced water treatment. J. Hazard. Mater. 2009, 170, 530−551. (19) Cha, J.-H.; Seol, Y. Increasing gas hydrate formation temperature for desalination of high salinity produced water with secondary guests. ACS Sustainable Chem. Eng. 2013, 1, 1218−1224. (20) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D. Thermodynamic model for predicting phase equilibria of simple clathrate hydrates of refrigerants. Chem. Eng. Sci. 2011, 66, 5439−5445. (21) Chun, M.-K.; Lee, H.; Ryu, B. J. Phase equilibria of R22 (CHClF2) hydrate systems in the presence of NaCl, KCl and MgCl2. J. Chem. Eng. Data 2000, 45, 1150−1153. (22) Liang, D.; Guo, K.; Wang, R.; Fan, S. Hydrate equilibrium data of 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1-dichloro-1-fluoroethane (HCFC-141b) and 1,1-difluoroethane (HFC-152a). Fluid Phase Equilib. 2001, 187−188, 61−70. (23) Akiya, T.; Shimazaki, T.; Oowa, M.; Matsuo, M.; Yoshida, Y. Formation conditions of clathrates between HFC alternative refrigerants and water. Int. J. Thermophys. 1999, 20, 1753−1763. (24) Seo, Y.; Tajima, H.; Yamasaki, A.; Takeya, S.; Ebinuma, T.; Kiyono, F. A new method for separating HFC-134a from gas mixtures using clathrate hydrate formation. Environ. Sci. Technol. 2004, 38, 4635−4639. (25) Maeda, K.; Katsura, Y.; Asakuma, Y.; Fukui, K. Concentration of sodium chloride in aqueous solution by chlorodifluoromethane gas hydrate. Chem. Eng. Process. 2008, 47, 2281−2286. (26) Mohammadi, A. H.; Richon, D. Pressure temperature phase diagrams of clathrate hydrates of HFC-134a, HFC-152a and HFC-32, AIChE Annu. Meet., 2010, Proceeding, Salt Lake City, UT. (27) Hashimoto, S.; Miyauchi, H.; Inoue, Y.; Ohgaki, K. Thermodynamic and Raman spectropic studies on difluoroethane (HFC-32) + water binary system. J. Chem. Eng. Data 2010, 55, 2764− 2768. (28) Petticrew, C. An investigation into the use of fluorinated hydrating agents in the desalination of industrial wastewater. MSc Thesis in Chemical Engineering, University of KwaZulu-Natal, Durban, South Africa, 2011. (29) Ngema, P. T.; Petticrew, C.; Naidoo, P.; Mohammadi, A. H.; Ramjugernath, D. Experimental measurements and thermodynamic modelling of the dissociation conditions of clathrate hydrates for (refrigerant + NaCl + water) systems. J. Chem. Eng. Data 2014, 59, 466−475. (30) Guo, K.-H.; Shu, B.-F.; Meng, Z.-X.; Zeng, L. Direct-contact gas hydrate cool storage vessel and cool storage air-conditioning system. Chin. Pat ZL95107268.4, 1995. (31) Guo, K.-H., Shu, B.-F., Yang, W.-J. Advances and applications of gas hydrate thermal energy storage technology. In Proceedings of 1st Trabzon International Energy and Environment Symposium, (Trabzon, Turkey), 1996; Vol. 1, pp 381−386. (32) Seo, Y.; Lee, H. A new hydrate-based recovery process for removing chlorinated hydrocarbons from aqueous solutions. Environ. Sci. Technol. 2001, 35, 3386−3390. (33) Aasberg-Petersen, K.; Stenby, E.; Fredenslund, A. Prediction of high pressure gas solubilities in aqueous mixtures of electrolytes. Ind. Eng. Chem. Res. 1991, 30, 2180−2185. (34) Van der Waals, J. H.; Platteeuw, J. C. Clathrate Solutions. Adv. Chem. Phys. 1959, 2, 1−57.

dissociation temperatures. The experimental dissociation data were modeled with a combination of the Aasberg−Petersen et al.33 model for electrolyte aqueous systems, the solid solution theory of van der Waals and Platteeuw34 for the hydrate phase, and the Peng−Robinson35 equation of state with classical mixing rule for the aqueous/liquid and vapor phases.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation. The authors would like to thank the NRF Focus Area Programme and the NRF Thuthuka Programme. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This paper was published in honor of Anthony R. H. Goodwin. REFERENCES

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Experimental Clathrate Hydrate Dissociation Data for Systems

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