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Clathrate phase equilibria for the water–phenol–methane system
Fluid Phase Equilibria 146 Ž1998. 339–349
Clathrate phase equilibria for the water–phenol–methane system Seong-Pil Kang, Huen Lee
)
Department of Chemical Engineering, Korea AdÕanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, South Korea Received 26 May 1997; accepted 9 January 1998
Abstract Clathrate phase equilibria for the ternary water–phenol–methane system were studied. This system consisted of two hosts and one guest. Four-phase equilibrium boundary for this ternary system was measured over the temperature and pressure ranges of 283.2 K and 37.0 bar to 323.6 K and 275.0 bar. Isobaric equilibrium compositions were also measured at 50.0 bar and several temperature ranges. Several types of three-phase and four-phase diagrams were obtained, which were dependent on the feed concentration of methane. Separation of phenol from aqueous solution was depicted in a diagram on the basis of methane-free concentration. A schematic pressure–temperature diagram for the ternary and three corresponding binary systems was presented. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Phase equilibria; Chemical equilibria; Clathrate
1. Introduction Clathrates are a type of inclusion compound. Inclusion compounds generally consist of two molecular components spatially arranged so that one component physically entraps the other. The molecular component that becomes the entrapping framework is called the host, whereas the entrapped molecular component is called the guest. Inclusion compounds can be divided into three types according to their host–guest geometrical arrangements; layer-, cage-, and channel-types. The clathrate is a cage-type inclusion compound and occurs when the host molecules form hydrogenbonded polyhedral cages. Each cage is capable of enclosing a guest molecule. A cage and its accommodated molecules are taken as a unit cell. Three-dimensional clathrate structures can be generally determined by the X-ray diffraction method.
)
Corresponding author.
0378-3812r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 1 2 Ž 9 8 . 0 0 2 1 4 - 3
340
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
Clathrates are usually subdivided into those formed in aqueous and non-aqueous systems, depending on the host species. Clathrate structures involving an aqueous host are usually called gas hydrates or clathrate hydrates. The gas-phase guest molecules generally form non-aqueous clathrates with host molecules of phenolic compounds such as hydroquinone, phenol, and simple substituted phenols. Two distinct types of cages are formed in phenolic clathrates and are capable of including suitably sized guest molecules w1x. In the case of the methane inclusion, one and four molecules of methane can be encaged in small and large cavities, respectively. A variety of researches on gas hydrates related to natural gases have been widely carried out since it was recognized to cause the plugging problem in natural gas pipelines which contain methane, ethane, propane, and other light hydrocarbons. Three types of gas hydrate structure have been identified; structures I, II, and H. The small-sized gas molecules such as argon, krypton, and nitrogen can be accommodated in the cavities of structure II and the large-sized gas molecules such as methane, xenon, and carbon dioxide in the cavities of structure I. The structure H hydrates need relatively high molecular weight hydrocarbons such as methyl cyclohexane and adamantane w2x. The inhibition effect of non-hydrate forming agents such as methanol, ethylene glycol, and electrolytes on the hydrate dissociation pressures have been extensively investigated by many workers w3–5x. The experimental phase behavior of the non-aqueous clathrates has been rarely reported in the literature. Allison and Barrer w6x measured the dissociation pressures and heats of formation of the clathrates formed from phenol and several gases such as krypton, xenon, methane, ethylene, ethane, and carbon dioxide. Trofimov and Kazankin w7x reported that p-cresol forms clathrates with guest ˚ They also measured the dissociation pressures of p-cresol gases of molecular size smaller than 5.1 A. clathrates produced with suitably sized guest molecules. The comprehensive study on clathrate phase behavior of the ternary water–phenol–carbon dioxide system containing two hosts and one guest has been first attempted in our previous work w8x. It should be particularly noted in this ternary system that the carbon dioxide-rich liquid phase forms at the specified dissociation temperature and pressure ranges. This paper deals with the complex phase behavior for the ternary water–phenol–methane system where the methane can only exist as a gas phase at the whole temperature and pressure ranges. The binary phenol–methane system was studied in our previous work w9x to examine the dissociation pressures on the three-phase, phenol-rich liquid–phenol clathrate–vapor, boundary. Isobaric equilibrium compositions were also measured along several temperatures at 50.0 bar.
2. Experimental section 2.1. Apparatus and sample analysis A schematic diagram of the experimental apparatus used in this work is found in our previous work w8x. The apparatus was built to measure the clathrate dissociation pressures and analyze the equilibrium compositions of liquid phases that coexist with the clathrate phase. The apparatus consisted of two major parts; equilibrium dissociation pressure and temperature measurement section and equilibrium concentration measurement section. Two types of equilibrium cells were separately used to investigate the complete phase behavior, because the system showed complicated phase behavior depending on methane compositions. An equilibrium cell is made of 316 stainless steel and
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
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its internal volume is about 50 cm3. Two thermally reinforced sight glasses were equipped at the front and back of the cell and they allowed visual observation of phase transitions. Another equilibrium cell is a variable-volume cylinder with a freely movable piston inside. This cell was used to investigate the phase equilibria of the mixtures containing solid, clathrate, and liquid phases without vapor phase. The variable-volume equilibrium cell mainly consists of two sections. One is the sample chamber at the front side where phase transitions occur along the temperature and pressure changes and the other is the pressure-generating chamber at the rear side by the movable piston. Since it is difficult to measure the system pressure directly at a specific equilibrium condition that the vapor phase cannot exist and the sample solution is dangerous to pressure gauge tube, an indirect method is applied by measuring the pressure of compressed gases in the pressure-generating chamber. According to preparatory tests, it was confirmed that the pressure difference between two chambers divided by a piston becomes less than 0.2 bar. The cell contents were agitated by a magnetic spin bar that was coupled with an immersion magnet placed under the cell in the bath. The bath contained about 30 l of a liquid mixture of ethylene glycol and water, which was controlled by an externally circulating refrigeratorrheater. The actual operating temperature in the cell was maintained with the PID temperature controller Ž Jeio Tech, MC-31. with "0.1 K accuracy and was measured by a K-type thermocouple probe with a digital thermometer ŽCole-Parmer, 8535-26. of which the resolution is "0.1 K. The thermometer is calibrated with the ERTCO ASTM mercury thermometer. ŽEver Ready Thermometer. . A Heise Bourdon tube pressure gauge Ž CMM 104957, 0–600 bar range. having the maximum error of "0.1 bar in the full-scale range is used to measure the cell pressure in the system. For the measurement of liquid compositions at a given equilibrium condition, a sampling valve ŽRheodyne, 7413. having a sampling loop of about 0.5 m l was installed and connected to a gas chromatograph ŽHewlett-Packard, 5890A. on-line through a high-pressure metering pump Ž Milton Roy, 2396-31.. The gas chromatograph used a thermal conductivity detector Ž TCD. and a 30 m by 0.25 mm i.d., 0.25 m m film thickness fused silica capillary column Ž a-DEX 120e, Supelco. for analysis of the samples. The operation conditions of gas chromatograph for analysis are carrier gas flow rate of 25 mlrmin, column temperature programmed from 333.2 to 423.2 K for 3 min with an increasing rate of 40 Krmin, injection temperature of 443.2 K, and detector temperature of 473.2 K. A vapor sampling valve which has a calibrated loop volume of 24.36 m l was used to perform the calibration for methane. After the cell was pressurized with methane at a fixed pressure, the methane in the loop was injected to the gas chromatograph to obtain the curve for moles vs. peak areas. The calibration curve was fitted to a second-order polynomial. Calibration curves for the liquid composition analysis were constructed by injecting known amounts of pure components with liquid sampling valve. All the curves were fitted to a linear correlation for the phenol and water, which showed a minor deviation from linearity. 2.2. Experimental procedure An amount of approximately 35 cm3 of the solution mixture was initially charged into the evacuated equilibrium cell. The pressure was first adjusted to a desired pressure with methane. The cell temperature was kept constant as a temperature just above that at which the clathrates formed. Clathrate nucleation was then induced by agitation of the magnetic spin bar with immersed magnet in bath. When the clathrates were formed and the system pressure reached a steady state, the cell
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external heater was used to increase the system temperature at a rate of 1 K to 2 K per hour to a condition where the clathrate phase was dissociated and in coexistence with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to remove the hysteresis phenomenon. When the amount of minute crystals and system temperature were kept constant at least for 8 to 10 h after the system pressure was stabilized, the pressure was considered as a clathrate dissociation pressure at the specified temperature. It should be noted that the best condition for the measurement of the equilibrium compositions for the ternary methane–water–phenol system could be obtained when the amount of clathrates was relatively small in the solution. It became thus important to select adequately the initial sample composition of water and phenol. The nucleation and dissociation process for the analysis of composition is nearly similar to that of the measurement of dissociation pressures. In the present experiments, both the dissociation of clathrate phase and the elevation of the system temperature caused the meaningful increase of system pressure. The total increase of pressure, however, was just slight enough to be constantly maintained by use of the metering valve. Each of the liquid phase was analyzed three times through the gas chromatograph. While the sample was analyzed, the pump was set to minimum flow rate to prevent the circulated samples from being pressurized by pumping. The reported equilibrium compositions of the individual phase were taken as the average values. Each composition was reproduced within a mole fraction of 0.0003. 2.3. Materials The methane gas with a minimum purity of 99.99% used in this study was obtained from Scientific Gas Products and the phenol supplied by Sigma-Aldrich Chemical had a purity of 99 mol%. The distilled water of HPLC grade supplied by Sigma-Aldrich Chemical was used. All chemicals were used as received without further purification.
3. Results and discussion 3.1. Pressure–temperature and isobaric temperature-composition behaÕior of clathrate phase equilibria Methane hydrate and phenol clathrate equilibrium dissociation pressure data of the binary water– methane, phenol–methane, and ternary water–phenol–methane systems are presented in Table 1 and shown in Fig. 1. The validity of the experimental apparatus and procedure applied in this work were successfully confirmed in our previous works w8,10,11x. The hydrate dissociation pressures of methane were measured and compared with the available data in the literature w12–15x. It was found that the results were in good agreement. As expected, the binary hydrate and clathrate equilibrium dissociation pressures in both the binary and ternary mixtures monotonously increased with temperature. For the water–phenol–methane system, as shown in Fig. 1, the four-phase, water-rich liquid, phenol-rich liquid, phenol–clathrate, and vapor Ž L w –L p –C–V. dissociation pressures were measured at the range of 283.2 K to 323.6 K. It is interesting to note that the L w –L p –C–V equilibrium temperatures approached those of the water-rich liquid–hydrate–vapor Ž L w –H–V. boundary. The
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
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Table 1 Equilibrium dissociation pressures of hydrate and phenol clathrate for water–methane, phenol–methane, and water– phenol–methane systems System
Phase type
T rK
Prbar
Water–methane
L w –H–V
Phenol–methane
L p –C–V
Water–phenol–methane
L w –L p –C–V
274.5 284.8 288.5 293.4 312.4 314.3 314.4 315.8 317.4 319.6 321.3 323.7 324.9 325.4 283.2 283.5 284.9 285.4 285.8 286.9 290.7 293.5 298.1 323.6
29.4 85.7 133.0 252.2 88.8 106.9 107.3 124.1 144.9 180.5 215.0 272.5 311.6 326.8 37.0 38.9 45.8 47.9 50.0 57.8 90.3 122.7 192.8 275.0
equilibrium dissociation temperatures of the water–deuterium oxide–carbon dioxide system increased as the mass % of deuterium oxide contained in water increased w10x. The equilibrium concentration of phenol in water is a mole fraction of about 0.015 at 293.2 K. It seems that a small amount of phenol dissolves in water and drives the equilibrium temperatures increased, which was a similar phenomenon to the case of the water–deuterium oxide system. However, as can be shown later, this result is due to the presence of a monotectic point for the ternary system. To examine the phase behavior of the water–phenol–methane system along the temperatures and compositions, the system pressure was set to 50.0 bar. Isobaric three- and four-phase equilibrium results for the water–phenol–methane system at 50.0 bar are presented in Table 2. The ternary equilibrium compositions at 50.0 bar and temperatures of 279.2, 281.5, 286.0, and 297.2 K are illustrated in triangular diagrams in Figs. 2–5, respectively. Methane molecules can occupy both the large and small cavities in the hydrate structure I when the mole ratio of methane to water is determined to be an ideal value of 8r46 w16x. Therefore, the maximum composition of methane in the hydrate phase became 0.148 in mole fraction. In the phenol clathrate phase, the maximum composition of methane is a value of 0.294 in the mole fraction because the mole ratio of methane to phenol in the unit clathrate structure is known as 5r12 w17x. The maximum composition can be attained when all the cavities are filled with methane molecules.
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
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Fig. 1. Clathrate phase equilibria for the water–methane, phenol–methane, and water–phenol–methane systems. l, L w –H–V phase boundary for water–methane Žthis work.; ', L w –L p –C–V phase boundary for water–phenol–methane Žthis work.; `, L p –C–V phase boundary for phenol–methane w9x.
Fig. 2 illustrates that a four-phase, water-rich liquid–phenol clathrate–hydrate–vapor Ž L w –C–H– V., region and a three-phase, water-rich liquid–phenol-rich liquid–clathrate Ž L w –L p –C., region exist at 279.2 K and 50.0 bar. They are represented by two different shapes of quadrangle and triangle, respectively and can be observed by controlling the feed amount of methane. This means that L w –L p –C region requires less methane than L w –C–H–V region. Since the vapor phase cannot exist in the L w –L p –C region, the variable volume cell was used to maintain the isobaric condition and analyze the phase equilibrium. Over the temperatures of 279.2 K that is the L w –C–H–V equilibrium temperature at 50.0 bar, the hydrate phase disappeared so that it did not coexist with phenol clathrate phase in equilibrium. The hydrate phase could exist only up to 279.8 with L w and V phases. This temperature of 279.8 K was Table 2 Isobaric three- and four-phase equilibrium temperatures and compositions for water Ž1. –phenol Ž2. –methane Ž3. at 50.0 bar T rK 279.2 279.6 281.5 286.0 297.2 308.2
Phase description L w –H–C–V L w –L p –C L w –C–V L w –L p –C L w –C–V L w –L p –C L w –L p –C–V L w –L p –V L w –L p –V L p –C–V
L w phase
L p phase
x1
x2
x3
0.9853 0.9845 0.9847 0.9832 0.9825 0.9823 0.9804 0.9786 0.9763
0.0098 0.0133 0.0101 0.0141 0.0116 0.0143 0.0128 0.0156 0.0187
0.0049 0.0022 0.0052 0.0027 0.0059 0.0034 0.0068 0.0058 0.0050
x1
x2
x3
0.6489
0.3499
0.0012
0.6495
0.3487
0.0018
0.6593 0.6451 0.6784 0.6930 0.4052
0.3384 0.3004 0.2835 0.2759 0.4721
0.0023 0.0545 0.0381 0.0311 0.1227
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Fig. 2. Phase equilibria of water–phenol–methane at 279.2 K and 50.0 bar. `, this work.
the equilibrium dissociation temperature of the methane hydrate in water at 50.0 bar. It can be thus known that the three phase equilibrium region of the L w –H–V exists in the temperature range of 279.2 to 279.8 K. It have been well known that the hydrate dissociation pressure is inhibited by adding some organic compounds such as methanol and ethylene glycol. In this case, it is possible to interpret that the hydrate phase is inhibited in the temperature range of 279.2 to 279.8 K by the very small amount of phenol dissolved in the L w phase, that is 0.0098 in mole fraction. It was very
Fig. 3. Phase equilibria of water–phenol–methane at 281.5 K and 50.0 bar. `, this work.
Fig. 4. Phase equilibria of water–phenol–methane at 286.0 K and 50.0 bar. `, this work.
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S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
Fig. 5. Phase equilibria of water–phenol–methane at 297.2 K and 50.0 bar. `, this work.
difficult to detect the L w –H–V equilibrium compositions, because the phenol concentrations in the L w phase were too small so that the equilibrium compositions were beyond the calibration range. However, the existence of L w –H–V region was confirmed by several times of visual observation. At temperatures above 279.2 K, no more hydrate phase coexisted with the phenol clathrate phase in equilibrium. As temperatures increased, shown in Fig. 3, the hydrate phase disappeared and dissociated methane molecules from hydrate phase enriched the V phase. However, the phenol clathrate phase coexisted with the L p phase and even with the L w and V phases. Fig. 4 presents that the two three-phase regions appeared at 279.6 and 281.5 K approach and finally merged at 286.0 K to produce only a four-phase, L w –L p –C–V, region. As the temperature increased from 281.5 K to 286.0 K, the methane concentrations of the L p phase in the L w –L p –C region also increased. Above the temperature of 286.0 K, the phenol clathrate phase can exist only with the V and L p phases in equilibrium, not with L w phase as shown in Fig. 5. The L w –L p –C–V region was separated into two three-phase, L w –L p –V and L p –C–V, regions. The measured phenol concentrations in the L w and L p phases are illustrated in Fig. 6 on the methane-free basis at 50.0 bar. The liquid–liquid equilibrium Ž LLE. data w18x for the binary
Fig. 6. Isobaric temperature-composition diagram of water–phenol–methane on the basis of methane-free concentration at 50.0 bar. v, this work; `, LLE data of water–phenol w18x.
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
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phenol–water system at atmospheric pressure are added in Fig. 6 for comparison. Fig. 6 shows that the results exhibit a monotectic–eutectic shape. Monotectic and eutectic points are x phenol s 0.3503 at T s 286.0 K and x phenol s 0.0099 at T s 279.2 K, respectively. The x phenol in the L w and L p phases of the ternary system are nearly similar to those of the binary system. Below the monotectic temperature, the deviation of x phenol’s between both binary and ternary systems increases and reaches a maximum at eutectic temperature. At 279.2 K the phenol concentration is reduced remarkably than the binary system. Above the monotectic temperature, 286.0 K, the deviation of x phenol’s in both the L w and L p phases increases slightly. Between the eutectic and monotectic temperature, larger amount of phenol dissolved in water than the binary system and the phenol can be obtained in the clathrate state from the water-rich liquid ŽL w . . It means just feeding methane to the mixture of water and phenol can easily separate the phenol dissolved in water. This result shows that the methane helps the binary system overcome the LLE limitation by forming phenol clathrate. The inhibition behavior of phenol clathrate dissociation temperature by water is shown in two different types. At the temperature range of 286.0 to 311.6 K, which is the normal melting temperature of phenol, the dissociation temperatures of phenol clathrate are inhibited by water and at 286.0 K the dissociation temperature keep constant while the water concentration increase. It can be stated that this discontinuous inhibition effect of phenol clathrate dissociation temperature occurred due to the presence of the monotectic point at 286.0 K. 3.2. Complete pressure–temperature diagram A schematic pressure–temperature Ž P–T . diagram for the ternary water–phenol–methane system is presented in Fig. 7. The binary P–T diagrams of water–methane and phenol–methane system are
Fig. 7. Complete pressure–temperature diagram for the water–phenol–methane system.
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
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included for comparison. The phase equilibria are quite complex. Phase boundaries of the binary water–methane, phenol–methane, and water–phenol are represented as solid, dotted, and dotted dash lines, respectively. Those of the ternary water–phenol–methane system are bold solid lines. Used symbols of L w , L p , C, H, and V correspond with those in this paper. Additionally the phase of ice is designated by I. To simplify the whole diagram phase boundaries in low temperature and pressure ranges were excluded. Some previous works w19–21x dealt with the ternary system containing only one host and two guest components. In our previous work w8x, which dealt with the ternary water–phenol–carbon dioxide system, the phase behavior of the ternary system containing two hosts and one guest was first studied. In that work the overall P–T behavior of the ternary system was qualitatively described. Five quintuple points appeared in the ternary water–phenol–carbon dioxide system. On the other side, there are only three quintuple points in the ternary water–phenol–methane system. Unlike carbon dioxide, methane does not exist as liquid phase in the P–T range of this work. Thus, two quintuple points of water–phenol–carbon dioxide system in upper temperature and pressure range were excluded. Q1n , Q2n , and Q3n designated those three quintuple points. At a quintuple point five individual phases coexist. Therefore, five four-phase boundaries should be converged at a quintuple point. Fig. 7 was constructed under this idea and information. Each quadruple point of the binary phenol–methane and water–methane system was also designated by Q1 and Q2 , respectively. About the two quadruple points of Q1 and Q2 , the encountered four three-phase boundaries were well known in the literature w16x. Detailed interpretations should be referred to our previous work w8x. 4. Conclusion The pressure–temperature behavior of the ternary water–phenol–methane system was measured over the wide temperature and pressure ranges. The equilibrium dissociation pressures increased monotonously with temperatures. A variable-volume equilibrium cell was used to measure the multi-phase equilibria without vapor phase at isobaric condition. Separation of phenol from aqueous solution was demonstrated by measuring the isobaric three- and four-phase equilibrium compositions of the ternary system at 50.0 bar. A complete P–T diagram of the binary and ternary systems was constructed with the experimentally determined phase boundaries and related information. 5. List of Symbols C H Lp Lw P Q Qn Sp T V x
clathrate phase hydrate phase phenol-rich liquid phase water-rich liquid phase pressure quadruple points quintuple points solid phenol phase temperature vapor phase composition
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x
L. Mandelcorn, Chem. Rev. 59 Ž1959. 827. J.A. Ripmeester, J.S. Tse, C.I. Ratcliffe, B.M. Powell, Nature 325 Ž1987. 135. H.J. Ng, D.B. Robinson, Fluid Phase Equilibria 21 Ž1985. 145. K.Y. Song, R. Kobayashi, Fluid Phase Equilibria 47 Ž1989. 295. P. Englezos, Y.T. Ngan, J. Chem. Eng. Data 38 Ž1993. 250. S.A. Allison, R.M. Barrer, Trans. Faraday Soc. 64 Ž1968. 549. A.M. Trofimov, Y.N. Kazankin, Russ. Radiokhim. 10 Ž1968. 445. J.-H. Yoon, H. Lee, AIChE J. 43 Ž1997. 1884. S.-P. Kang, J.-H. Yoon, H. Lee, Fluid Phase Equilibria, 1997, in press. M.-K. Chun, J.-H. Yoon, H. Lee, J. Chem. Eng. Data 41 Ž1996. 1114. S.-P. Kang, H. Lee, J. Chem. Eng. Data 42 Ž1997. 467. W.M. Deaton, E.M. Frost, Jr., Gas Hydrates and their Relationship to the Operation of Natural Gas Pipelines, U.S. Bur. Mines Monograph. 8, 1946. H.O. McLeod, J.M. Campbell, J. Pet. Tech. 13 Ž1961. 590. D.R. Marshall, S. Saito, R. Kobayashi, AIChE J. 10 Ž1964. 202. J.L. de Roo, C.J. Peters, R.N. Lichtenthaler, G.A.M. Deipen, AIChE J. 29 Ž1983. 651. E.D. Sloan, Jr., Clathrate Hydrates of Natural Gases, Marcel Dekker, New York, 1990. M. von Stackelberg, A. Hoverath, Ch. Scheringer, Z. Elektrochem. 62 Ž1958. 123. J.M. Sfrensen, W. Arlt, Liquid–Liquid Equilibrium Data Collection, Vol. V, Part 1, DECHEMA, 1979, p. 358. M. von Stackelberg, H. Fruhbuss, Z. Elektrochem. 58 Ž1954. 99. ¨ J.J. Carroll, A.E. Mather, Can. J. Chem. Eng. 69 Ž1991. 1206. A. Harmens, E.D. Sloan Jr., Can. J. Chem. Eng. 68 Ž1990. 151.
Clathrate phase equilibria for the water–phenol–methane system Seong-Pil Kang, Huen Lee
)
Department of Chemical Engineering, Korea AdÕanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, South Korea Received 26 May 1997; accepted 9 January 1998
Abstract Clathrate phase equilibria for the ternary water–phenol–methane system were studied. This system consisted of two hosts and one guest. Four-phase equilibrium boundary for this ternary system was measured over the temperature and pressure ranges of 283.2 K and 37.0 bar to 323.6 K and 275.0 bar. Isobaric equilibrium compositions were also measured at 50.0 bar and several temperature ranges. Several types of three-phase and four-phase diagrams were obtained, which were dependent on the feed concentration of methane. Separation of phenol from aqueous solution was depicted in a diagram on the basis of methane-free concentration. A schematic pressure–temperature diagram for the ternary and three corresponding binary systems was presented. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Phase equilibria; Chemical equilibria; Clathrate
1. Introduction Clathrates are a type of inclusion compound. Inclusion compounds generally consist of two molecular components spatially arranged so that one component physically entraps the other. The molecular component that becomes the entrapping framework is called the host, whereas the entrapped molecular component is called the guest. Inclusion compounds can be divided into three types according to their host–guest geometrical arrangements; layer-, cage-, and channel-types. The clathrate is a cage-type inclusion compound and occurs when the host molecules form hydrogenbonded polyhedral cages. Each cage is capable of enclosing a guest molecule. A cage and its accommodated molecules are taken as a unit cell. Three-dimensional clathrate structures can be generally determined by the X-ray diffraction method.
)
Corresponding author.
0378-3812r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 1 2 Ž 9 8 . 0 0 2 1 4 - 3
340
S.-P. Kang, H. Lee r Fluid Phase Equilibria 146 (1998) 339–349
Clathrates are usually subdivided into those formed in aqueous and non-aqueous systems, depending on the host species. Clathrate structures involving an aqueous host are usually called gas hydrates or clathrate hydrates. The gas-phase guest molecules generally form non-aqueous clathrates with host molecules of phenolic compounds such as hydroquinone, phenol, and simple substituted phenols. Two distinct types of cages are formed in phenolic clathrates and are capable of including suitably sized guest molecules w1x. In the case of the methane inclusion, one and four molecules of methane can be encaged in small and large cavities, respectively. A variety of researches on gas hydrates related to natural gases have been widely carried out since it was recognized to cause the plugging problem in natural gas pipelines which contain methane, ethane, propane, and other light hydrocarbons. Three types of gas hydrate structure have been identified; structures I, II, and H. The small-sized gas molecules such as argon, krypton, and nitrogen can be accommodated in the cavities of structure II and the large-sized gas molecules such as methane, xenon, and carbon dioxide in the cavities of structure I. The structure H hydrates need relatively high molecular weight hydrocarbons such as methyl cyclohexane and adamantane w2x. The inhibition effect of non-hydrate forming agents such as methanol, ethylene glycol, and electrolytes on the hydrate dissociation pressures have been extensively investigated by many workers w3–5x. The experimental phase behavior of the non-aqueous clathrates has been rarely reported in the literature. Allison and Barrer w6x measured the dissociation pressures and heats of formation of the clathrates formed from phenol and several gases such as krypton, xenon, methane, ethylene, ethane, and carbon dioxide. Trofimov and Kazankin w7x reported that p-cresol forms clathrates with guest ˚ They also measured the dissociation pressures of p-cresol gases of molecular size smaller than 5.1 A. clathrates produced with suitably sized guest molecules. The comprehensive study on clathrate phase behavior of the ternary water–phenol–carbon dioxide system containing two hosts and one guest has been first attempted in our previous work w8x. It should be particularly noted in this ternary system that the carbon dioxide-rich liquid phase forms at the specified dissociation temperature and pressure ranges. This paper deals with the complex phase behavior for the ternary water–phenol–methane system where the methane can only exist as a gas phase at the whole temperature and pressure ranges. The binary phenol–methane system was studied in our previous work w9x to examine the dissociation pressures on the three-phase, phenol-rich liquid–phenol clathrate–vapor, boundary. Isobaric equilibrium compositions were also measured along several temperatures at 50.0 bar.
2. Experimental section 2.1. Apparatus and sample analysis A schematic diagram of the experimental apparatus used in this work is found in our previous work w8x. The apparatus was built to measure the clathrate dissociation pressures and analyze the equilibrium compositions of liquid phases that coexist with the clathrate phase. The apparatus consisted of two major parts; equilibrium dissociation pressure and temperature measurement section and equilibrium concentration measurement section. Two types of equilibrium cells were separately used to investigate the complete phase behavior, because the system showed complicated phase behavior depending on methane compositions. An equilibrium cell is made of 316 stainless steel and
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its internal volume is about 50 cm3. Two thermally reinforced sight glasses were equipped at the front and back of the cell and they allowed visual observation of phase transitions. Another equilibrium cell is a variable-volume cylinder with a freely movable piston inside. This cell was used to investigate the phase equilibria of the mixtures containing solid, clathrate, and liquid phases without vapor phase. The variable-volume equilibrium cell mainly consists of two sections. One is the sample chamber at the front side where phase transitions occur along the temperature and pressure changes and the other is the pressure-generating chamber at the rear side by the movable piston. Since it is difficult to measure the system pressure directly at a specific equilibrium condition that the vapor phase cannot exist and the sample solution is dangerous to pressure gauge tube, an indirect method is applied by measuring the pressure of compressed gases in the pressure-generating chamber. According to preparatory tests, it was confirmed that the pressure difference between two chambers divided by a piston becomes less than 0.2 bar. The cell contents were agitated by a magnetic spin bar that was coupled with an immersion magnet placed under the cell in the bath. The bath contained about 30 l of a liquid mixture of ethylene glycol and water, which was controlled by an externally circulating refrigeratorrheater. The actual operating temperature in the cell was maintained with the PID temperature controller Ž Jeio Tech, MC-31. with "0.1 K accuracy and was measured by a K-type thermocouple probe with a digital thermometer ŽCole-Parmer, 8535-26. of which the resolution is "0.1 K. The thermometer is calibrated with the ERTCO ASTM mercury thermometer. ŽEver Ready Thermometer. . A Heise Bourdon tube pressure gauge Ž CMM 104957, 0–600 bar range. having the maximum error of "0.1 bar in the full-scale range is used to measure the cell pressure in the system. For the measurement of liquid compositions at a given equilibrium condition, a sampling valve ŽRheodyne, 7413. having a sampling loop of about 0.5 m l was installed and connected to a gas chromatograph ŽHewlett-Packard, 5890A. on-line through a high-pressure metering pump Ž Milton Roy, 2396-31.. The gas chromatograph used a thermal conductivity detector Ž TCD. and a 30 m by 0.25 mm i.d., 0.25 m m film thickness fused silica capillary column Ž a-DEX 120e, Supelco. for analysis of the samples. The operation conditions of gas chromatograph for analysis are carrier gas flow rate of 25 mlrmin, column temperature programmed from 333.2 to 423.2 K for 3 min with an increasing rate of 40 Krmin, injection temperature of 443.2 K, and detector temperature of 473.2 K. A vapor sampling valve which has a calibrated loop volume of 24.36 m l was used to perform the calibration for methane. After the cell was pressurized with methane at a fixed pressure, the methane in the loop was injected to the gas chromatograph to obtain the curve for moles vs. peak areas. The calibration curve was fitted to a second-order polynomial. Calibration curves for the liquid composition analysis were constructed by injecting known amounts of pure components with liquid sampling valve. All the curves were fitted to a linear correlation for the phenol and water, which showed a minor deviation from linearity. 2.2. Experimental procedure An amount of approximately 35 cm3 of the solution mixture was initially charged into the evacuated equilibrium cell. The pressure was first adjusted to a desired pressure with methane. The cell temperature was kept constant as a temperature just above that at which the clathrates formed. Clathrate nucleation was then induced by agitation of the magnetic spin bar with immersed magnet in bath. When the clathrates were formed and the system pressure reached a steady state, the cell
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external heater was used to increase the system temperature at a rate of 1 K to 2 K per hour to a condition where the clathrate phase was dissociated and in coexistence with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to remove the hysteresis phenomenon. When the amount of minute crystals and system temperature were kept constant at least for 8 to 10 h after the system pressure was stabilized, the pressure was considered as a clathrate dissociation pressure at the specified temperature. It should be noted that the best condition for the measurement of the equilibrium compositions for the ternary methane–water–phenol system could be obtained when the amount of clathrates was relatively small in the solution. It became thus important to select adequately the initial sample composition of water and phenol. The nucleation and dissociation process for the analysis of composition is nearly similar to that of the measurement of dissociation pressures. In the present experiments, both the dissociation of clathrate phase and the elevation of the system temperature caused the meaningful increase of system pressure. The total increase of pressure, however, was just slight enough to be constantly maintained by use of the metering valve. Each of the liquid phase was analyzed three times through the gas chromatograph. While the sample was analyzed, the pump was set to minimum flow rate to prevent the circulated samples from being pressurized by pumping. The reported equilibrium compositions of the individual phase were taken as the average values. Each composition was reproduced within a mole fraction of 0.0003. 2.3. Materials The methane gas with a minimum purity of 99.99% used in this study was obtained from Scientific Gas Products and the phenol supplied by Sigma-Aldrich Chemical had a purity of 99 mol%. The distilled water of HPLC grade supplied by Sigma-Aldrich Chemical was used. All chemicals were used as received without further purification.
3. Results and discussion 3.1. Pressure–temperature and isobaric temperature-composition behaÕior of clathrate phase equilibria Methane hydrate and phenol clathrate equilibrium dissociation pressure data of the binary water– methane, phenol–methane, and ternary water–phenol–methane systems are presented in Table 1 and shown in Fig. 1. The validity of the experimental apparatus and procedure applied in this work were successfully confirmed in our previous works w8,10,11x. The hydrate dissociation pressures of methane were measured and compared with the available data in the literature w12–15x. It was found that the results were in good agreement. As expected, the binary hydrate and clathrate equilibrium dissociation pressures in both the binary and ternary mixtures monotonously increased with temperature. For the water–phenol–methane system, as shown in Fig. 1, the four-phase, water-rich liquid, phenol-rich liquid, phenol–clathrate, and vapor Ž L w –L p –C–V. dissociation pressures were measured at the range of 283.2 K to 323.6 K. It is interesting to note that the L w –L p –C–V equilibrium temperatures approached those of the water-rich liquid–hydrate–vapor Ž L w –H–V. boundary. The
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Table 1 Equilibrium dissociation pressures of hydrate and phenol clathrate for water–methane, phenol–methane, and water– phenol–methane systems System
Phase type
T rK
Prbar
Water–methane
L w –H–V
Phenol–methane
L p –C–V
Water–phenol–methane
L w –L p –C–V
274.5 284.8 288.5 293.4 312.4 314.3 314.4 315.8 317.4 319.6 321.3 323.7 324.9 325.4 283.2 283.5 284.9 285.4 285.8 286.9 290.7 293.5 298.1 323.6
29.4 85.7 133.0 252.2 88.8 106.9 107.3 124.1 144.9 180.5 215.0 272.5 311.6 326.8 37.0 38.9 45.8 47.9 50.0 57.8 90.3 122.7 192.8 275.0
equilibrium dissociation temperatures of the water–deuterium oxide–carbon dioxide system increased as the mass % of deuterium oxide contained in water increased w10x. The equilibrium concentration of phenol in water is a mole fraction of about 0.015 at 293.2 K. It seems that a small amount of phenol dissolves in water and drives the equilibrium temperatures increased, which was a similar phenomenon to the case of the water–deuterium oxide system. However, as can be shown later, this result is due to the presence of a monotectic point for the ternary system. To examine the phase behavior of the water–phenol–methane system along the temperatures and compositions, the system pressure was set to 50.0 bar. Isobaric three- and four-phase equilibrium results for the water–phenol–methane system at 50.0 bar are presented in Table 2. The ternary equilibrium compositions at 50.0 bar and temperatures of 279.2, 281.5, 286.0, and 297.2 K are illustrated in triangular diagrams in Figs. 2–5, respectively. Methane molecules can occupy both the large and small cavities in the hydrate structure I when the mole ratio of methane to water is determined to be an ideal value of 8r46 w16x. Therefore, the maximum composition of methane in the hydrate phase became 0.148 in mole fraction. In the phenol clathrate phase, the maximum composition of methane is a value of 0.294 in the mole fraction because the mole ratio of methane to phenol in the unit clathrate structure is known as 5r12 w17x. The maximum composition can be attained when all the cavities are filled with methane molecules.
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Fig. 1. Clathrate phase equilibria for the water–methane, phenol–methane, and water–phenol–methane systems. l, L w –H–V phase boundary for water–methane Žthis work.; ', L w –L p –C–V phase boundary for water–phenol–methane Žthis work.; `, L p –C–V phase boundary for phenol–methane w9x.
Fig. 2 illustrates that a four-phase, water-rich liquid–phenol clathrate–hydrate–vapor Ž L w –C–H– V., region and a three-phase, water-rich liquid–phenol-rich liquid–clathrate Ž L w –L p –C., region exist at 279.2 K and 50.0 bar. They are represented by two different shapes of quadrangle and triangle, respectively and can be observed by controlling the feed amount of methane. This means that L w –L p –C region requires less methane than L w –C–H–V region. Since the vapor phase cannot exist in the L w –L p –C region, the variable volume cell was used to maintain the isobaric condition and analyze the phase equilibrium. Over the temperatures of 279.2 K that is the L w –C–H–V equilibrium temperature at 50.0 bar, the hydrate phase disappeared so that it did not coexist with phenol clathrate phase in equilibrium. The hydrate phase could exist only up to 279.8 with L w and V phases. This temperature of 279.8 K was Table 2 Isobaric three- and four-phase equilibrium temperatures and compositions for water Ž1. –phenol Ž2. –methane Ž3. at 50.0 bar T rK 279.2 279.6 281.5 286.0 297.2 308.2
Phase description L w –H–C–V L w –L p –C L w –C–V L w –L p –C L w –C–V L w –L p –C L w –L p –C–V L w –L p –V L w –L p –V L p –C–V
L w phase
L p phase
x1
x2
x3
0.9853 0.9845 0.9847 0.9832 0.9825 0.9823 0.9804 0.9786 0.9763
0.0098 0.0133 0.0101 0.0141 0.0116 0.0143 0.0128 0.0156 0.0187
0.0049 0.0022 0.0052 0.0027 0.0059 0.0034 0.0068 0.0058 0.0050
x1
x2
x3
0.6489
0.3499
0.0012
0.6495
0.3487
0.0018
0.6593 0.6451 0.6784 0.6930 0.4052
0.3384 0.3004 0.2835 0.2759 0.4721
0.0023 0.0545 0.0381 0.0311 0.1227
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Fig. 2. Phase equilibria of water–phenol–methane at 279.2 K and 50.0 bar. `, this work.
the equilibrium dissociation temperature of the methane hydrate in water at 50.0 bar. It can be thus known that the three phase equilibrium region of the L w –H–V exists in the temperature range of 279.2 to 279.8 K. It have been well known that the hydrate dissociation pressure is inhibited by adding some organic compounds such as methanol and ethylene glycol. In this case, it is possible to interpret that the hydrate phase is inhibited in the temperature range of 279.2 to 279.8 K by the very small amount of phenol dissolved in the L w phase, that is 0.0098 in mole fraction. It was very
Fig. 3. Phase equilibria of water–phenol–methane at 281.5 K and 50.0 bar. `, this work.
Fig. 4. Phase equilibria of water–phenol–methane at 286.0 K and 50.0 bar. `, this work.
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Fig. 5. Phase equilibria of water–phenol–methane at 297.2 K and 50.0 bar. `, this work.
difficult to detect the L w –H–V equilibrium compositions, because the phenol concentrations in the L w phase were too small so that the equilibrium compositions were beyond the calibration range. However, the existence of L w –H–V region was confirmed by several times of visual observation. At temperatures above 279.2 K, no more hydrate phase coexisted with the phenol clathrate phase in equilibrium. As temperatures increased, shown in Fig. 3, the hydrate phase disappeared and dissociated methane molecules from hydrate phase enriched the V phase. However, the phenol clathrate phase coexisted with the L p phase and even with the L w and V phases. Fig. 4 presents that the two three-phase regions appeared at 279.6 and 281.5 K approach and finally merged at 286.0 K to produce only a four-phase, L w –L p –C–V, region. As the temperature increased from 281.5 K to 286.0 K, the methane concentrations of the L p phase in the L w –L p –C region also increased. Above the temperature of 286.0 K, the phenol clathrate phase can exist only with the V and L p phases in equilibrium, not with L w phase as shown in Fig. 5. The L w –L p –C–V region was separated into two three-phase, L w –L p –V and L p –C–V, regions. The measured phenol concentrations in the L w and L p phases are illustrated in Fig. 6 on the methane-free basis at 50.0 bar. The liquid–liquid equilibrium Ž LLE. data w18x for the binary
Fig. 6. Isobaric temperature-composition diagram of water–phenol–methane on the basis of methane-free concentration at 50.0 bar. v, this work; `, LLE data of water–phenol w18x.
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phenol–water system at atmospheric pressure are added in Fig. 6 for comparison. Fig. 6 shows that the results exhibit a monotectic–eutectic shape. Monotectic and eutectic points are x phenol s 0.3503 at T s 286.0 K and x phenol s 0.0099 at T s 279.2 K, respectively. The x phenol in the L w and L p phases of the ternary system are nearly similar to those of the binary system. Below the monotectic temperature, the deviation of x phenol’s between both binary and ternary systems increases and reaches a maximum at eutectic temperature. At 279.2 K the phenol concentration is reduced remarkably than the binary system. Above the monotectic temperature, 286.0 K, the deviation of x phenol’s in both the L w and L p phases increases slightly. Between the eutectic and monotectic temperature, larger amount of phenol dissolved in water than the binary system and the phenol can be obtained in the clathrate state from the water-rich liquid ŽL w . . It means just feeding methane to the mixture of water and phenol can easily separate the phenol dissolved in water. This result shows that the methane helps the binary system overcome the LLE limitation by forming phenol clathrate. The inhibition behavior of phenol clathrate dissociation temperature by water is shown in two different types. At the temperature range of 286.0 to 311.6 K, which is the normal melting temperature of phenol, the dissociation temperatures of phenol clathrate are inhibited by water and at 286.0 K the dissociation temperature keep constant while the water concentration increase. It can be stated that this discontinuous inhibition effect of phenol clathrate dissociation temperature occurred due to the presence of the monotectic point at 286.0 K. 3.2. Complete pressure–temperature diagram A schematic pressure–temperature Ž P–T . diagram for the ternary water–phenol–methane system is presented in Fig. 7. The binary P–T diagrams of water–methane and phenol–methane system are
Fig. 7. Complete pressure–temperature diagram for the water–phenol–methane system.
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included for comparison. The phase equilibria are quite complex. Phase boundaries of the binary water–methane, phenol–methane, and water–phenol are represented as solid, dotted, and dotted dash lines, respectively. Those of the ternary water–phenol–methane system are bold solid lines. Used symbols of L w , L p , C, H, and V correspond with those in this paper. Additionally the phase of ice is designated by I. To simplify the whole diagram phase boundaries in low temperature and pressure ranges were excluded. Some previous works w19–21x dealt with the ternary system containing only one host and two guest components. In our previous work w8x, which dealt with the ternary water–phenol–carbon dioxide system, the phase behavior of the ternary system containing two hosts and one guest was first studied. In that work the overall P–T behavior of the ternary system was qualitatively described. Five quintuple points appeared in the ternary water–phenol–carbon dioxide system. On the other side, there are only three quintuple points in the ternary water–phenol–methane system. Unlike carbon dioxide, methane does not exist as liquid phase in the P–T range of this work. Thus, two quintuple points of water–phenol–carbon dioxide system in upper temperature and pressure range were excluded. Q1n , Q2n , and Q3n designated those three quintuple points. At a quintuple point five individual phases coexist. Therefore, five four-phase boundaries should be converged at a quintuple point. Fig. 7 was constructed under this idea and information. Each quadruple point of the binary phenol–methane and water–methane system was also designated by Q1 and Q2 , respectively. About the two quadruple points of Q1 and Q2 , the encountered four three-phase boundaries were well known in the literature w16x. Detailed interpretations should be referred to our previous work w8x. 4. Conclusion The pressure–temperature behavior of the ternary water–phenol–methane system was measured over the wide temperature and pressure ranges. The equilibrium dissociation pressures increased monotonously with temperatures. A variable-volume equilibrium cell was used to measure the multi-phase equilibria without vapor phase at isobaric condition. Separation of phenol from aqueous solution was demonstrated by measuring the isobaric three- and four-phase equilibrium compositions of the ternary system at 50.0 bar. A complete P–T diagram of the binary and ternary systems was constructed with the experimentally determined phase boundaries and related information. 5. List of Symbols C H Lp Lw P Q Qn Sp T V x
clathrate phase hydrate phase phenol-rich liquid phase water-rich liquid phase pressure quadruple points quintuple points solid phenol phase temperature vapor phase composition
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