Znd. Eng. Chem. Res. 1991,30, 1579-1582 phology of the filter produced by particles remaining on the surface.
Acknowledgment We thank the Scienceand Engineering Research Council for support, including a Research Assistantship for H. A.M.S.
Literature Cited Bertera, R.; Steven, H.; Metcalfe, M. Development Studies
of Crossflow Microfiltration. Chem. Eng. 1984 (June), 10-14. Bowen, W. R. The Economics of Membrane Separations in the Processing of Biological Materials. I m t . Chem. Eng. Symp. Ser. 1987,NO. 105,37-46. Bowen, W.R.; Turner, A. D. Electrical Separation Processes. In Solid-liquid separation; Gregory, J., Ed.; Ellis Horwood: ChiChester, England, 19W,pp 9-28. Bowen, W. R.; Kingdon, R. S.; Sabuni, H. A. M. Electrically Enhanced Separation Processes: the Basis of In Situ Intermittent Electrolytic Membrane Cleaning (IIEMC) and In Situ Electrolytic Membrane Restoration (IEMR). J. Membr. Sci. 1989, 40, 219-229. Davis, R. H.; Leighton, D. T. Shear Induced Transport of a Particle Layer Along a Porous Wall. Chem. Eng. Sci. 1987,42,275-281. Fane, A. G. Ultrafiltration: Factors Influencing Flux and Rejection. In Progress in filtration and separation; Wakeman, R. J., Ed.; Elsevier: 1986,Vol. 4,pp 101-179. Gatenholm, P.; Fell C. J. D.; Fane, A. G. Influence of the Membrane Structure on the Composition of the Deposit Layer During Processing of Microbial Suspensions. Eztended Abstracts of the International Membrane Technology Conference, G1, Sydney, Australia; School of Chemical Engineering and Industrial Chemistry, University of New South Wales: 1988.
1579
Henry, J. D.; Lawler L. F.; Luo, C. H. A. A Solid/liquid Separation Process Based on Cross-flow and Electrofiltration. AIChE J. 1977,23,851-859. Nakao, S.; Nomura, S.; Kimura, S. Transport Phenomena of the Cross-flow Microfiltration Process. Proceedings of the Fifth World Filtration Congress, Nice, France: S w i M Francaise de Filtration: 1990; Vol. 1, pp 564-570. Porter, M. C. Membrane Filtration. In Handbook of Separation Processes for Chemical Engineers; Schweitzer, P. A., Ed.; McGraw-Hill: New York, 1988,pp 2-3-2-103. Schneider, K.; Klein, W. The Concentration of Suspensions by Means of Cross-flow Microfiltration. Desalination 1982, 41, 263-275. Shibata, S. The Concentration of Molecular Hydrogen on the Platinum Cathode. Bull. Chem. SOC.Jpn. 1963,36,53-57. Suki, A,; Fane, A. G.; Fell, C. J. D. Flux Decline in Protein Ultrafiltration. J. Membr. Sci. 1984,21,269-283. Tanny, G. B. Dynamic Membranes in Ultrafiltration and Reverse Osmosis. Sep. Purif. Methods 1978,7, 183-220. Tiller, F. M.; Leu, W. Filtration. In Handbook of Chemical Engineering Calculations; Chopey, N. P., Hicks T. G., Eds.; McGrawHill: New York, 1984;p 11-6. Vogt, H. The Rate of Gas Evolution at Electrodes-1. An Estimate of the Efficiency of Gas Evolution from the Supersaturation of Electrolyte Adjacent to a Gas-evolving Electrode. Electrochim. Acta 1984,29,167-173. Wakeman, R.J.; Tarleton, E. S. Membrane Fouling Prevention in Crossflow Microfiltration by the use of Electric Fields. Chem. Eng. Sci. 1987,42,829-842. Yukawa, H.; Shimura, K.; Suda, S.; Maniwa, A. Cross-flow Electroultrafiltration for Colloidal Separation of Protein. J . Chem. Eng. Jpn. 1983,16,305-311. Received for review September 10,1990 Revised manuscript received January 26, 1991 Accepted January 29,1991
Study of Relationships between Solvent Effectiveness in Coal Tar Pitch Extractions and Solvent Solubility Parameters Carlos G. Blanco and Maria D. GuillBn* Instituto Nacional del Carbdn, CSIC, A p . 73, 33080 Oviedo, Spain
Relationships between solvent extractive ability with coal tar pitches and Hildebrand’s solubility parameter have been studied. In addition, taking into account Hansen’s solubility parameter components, relationships between these and solvent effectiveness in coal tar pitch extractions have been studied. It is proved that organic solvents with a high solubility parameter dispersive component are the best coal tar pitch solvents, providing their polarity and their ability to give rise to hydrogen bonding interactions do not exceed certain limits.
Introduction The study of coal solubility in organic solvents has received much attention in the literature (Dryden, 1951; Van Krevelen, 1965; Angelovich et al., 1970) and still remains a subject of widespread interest (Chawla and Davis, 1989). In addition,the extraction of pitches using organic solvents is a very common procedure in fractionating schemes (Bartle, 19721, in order to characterize pitches by different techniques (Fischer et al., 1978; Borwitzky and &homburg, 1979), or for the obtention of pitch fractions (free of certain components) suitable for the manufacture of special materials (Weishauptova and Medek, 1985; Riggs, 1984). However, in spite of the evidently wide interest in the solubility of pitches, to the best of our knowledge, there is no research work on relationships between the solubility of pitch and solvent properties. Coal tar pitches are complex mixtures of polycyclic aromatic compounds, which themselves constitute PO-
lyeutectic solutions of nonelectrolyte components. The solubility of pitches in organic solvents could be explained by the Hildebrand-Scatchard theory of regular solutions (Hildebrand et al., 1970), if they are regular solutions or if they behave like them. According to this theory, there is a relationship between the logarithm of the equilibrium mole fraction of the solute in the solvent, r2, and the solubility parameters of the solute a2, and the solvent
In this equation u2 is the activity coefficient of the solute, V2its molar volume, and the fraction volume of the solvent. The solubility parameter was defined (Hildebrand et al., 1970) as the square root of the cohesive energy density tiH = [AU/Vj1I2,where AU is the energy of vaporization and V is the molar volume of the compound. Prausnitz et al. (Blanks and Prausnitz, 1964) propose that
0888-5886/91/2630-1579$02.50/00 1991 American Chemical Society
1580 Ind. Eng. Chem. Res., Vol. 30, No. 7,1991 Table I. Solubility, SA,,of the A1 Coal Tar Pitch in Different Solventr and Solubility Parameters of Them no. solvent SA* SQ fin hd 6, 1 formamide 0.0 19.22 8.4 12.8 2 hexane 14.0 4.1 7.29 7.3 0.0 14.1 2.7 14.47 7.4 6.0 3 methanol 4 acetonitrile 22.3 4.0 11.88 7.5 8.8 5 ethanol 28.2 9.5 12.71 7.7 4.3 6 diethyl ether 43.6 9.4 7.38 7.1 1.4 7 methyl acetate 44.9 0.6 9.58 7.6 3.5 49.0 3.3 9.88 7.6 5.1 8 acetone 9 carbon tetrachloride 49.2 1.6 8.61 8.7 0.0 10 fluorobenzene 57.0 1.8 9.10 7.5 4.4 59.6 2.3 8.9 11 toluene 8.8 0.7 12 1,4-dioxane 62.6 2.1 10.02 9.3 0.9 13 dimethylformamide 63.7 1.8 12.13 8.5 6.7 14 dichloromethane 64.4 1.4 9.68 8.9 3.1 15 chloroform 65.8 1.3 9.29 8.7 1.5 16 carbon disulfide 66.7 2.1 9.97 10.0 0.0 17 tetrahydrofuran 69.7 0.3 9.09 8.2 2.8 18 pyridine 72.2 0.2 10.71 9.3 4.3 19 nitrobenzene 74.5 3.2 10.02 9.8 4.2 20 quinoline 82.6 2.9 10.81 9.5 3.4 21 l-methyl-2-pyrrolidinone 83.0 0.8 11.30 8.8 6.0 Os
bh
9.3 0.0 10.9 3.0 9.5 2.5 3.7 3.4 0.0 2.5 1.0 3.6 5.5 3.0 2.8 0.3 3.9 2.9 2.0 3.7 3.5
= standard deviation of the solubility determinations in each solvent.
the solubility parameters can be considered as composed of a polar part T and a nonpolar part A, connected by the relation S2 = X2 + T ~ Likewise . Hansen proposes (Hansen, 1967)a three-component solubility parameter 62 = ad2 + 6,2 + Sh2, based on dispersive, d, polar, p, and hydrogen bonding interactions, h. In a previous paper (GuillBn et al. 1991),an effectiveness scale of organic solvents in the extraction of coal tar pitches was established. Also it was found that the most effective coal tar pitch solvents can be polar or unpolar compounds with very different solubility parameters. For this reason it will be worthwhile finding out which of the solvent properties determine its extractive ability with coal tar pitches and/or with other mixtures of similar nature. In this context, the purpose of this paper is to contribute to the study of the influence of the solubility parameter of the solvents used, on the solubility of the coal tar pitch.
Experimental Section The solubility of the A1 coal tar pitch, SA1,in several solvents is taken from a previous paper, in which the experimental procedure used is reported in detail (GuillBn et al., 1991). The pitch solubility in each solvent is taken to be the extraction yield, extraction conditions being the same for all solvents, and it is defined as S = lOo(B - O / B , where B is the weight of the coal tar pitch and I is the weight of the insoluble material. In order to avoid the influence of possible experimentalerrors on this study, only those solvents for which the extraction experiment was carried out from two to eight times were taken into account. The Hildebrand solubility parameters (6H1)and the Hansen solubility parameter components (6&, bpi, bhl) of the solvents used, were taken from the literature (Barton, 1983). Table I shows the solvents used in this study, together with the Hildebrand solubility parameters and the Hansen solubility parameter components. Also Table I shows the solubility S A 1 of the A I coal tar pitch in each solvent and the standard deviations s of these determinations. Results and Discussion Coal tar pitches are made up of an isotropic part soluble in quinoline and another anisotropic part supposedly insoluble in quinoline and this is described as carbon black like. The quinoline soluble fraction is composed of a
volatile fraction (more than 50% of the whole pitch) and another nonvolatile fraction. The volatile fraction is made up of polycyclic aromatic hydrocarbons, partially hydrogenated polycyclic aromatic hydrocarbons, peri- and cata-condensed benzoderivatives of acenaphthene, fluorene, dibenzothiophene, dibenzofuran, carbazol, and pyridine, as well as methylated derivatives of all the groups of compounds just mentioned. The same types of compounds constitute the nonvolatile fraction. The volatile part of the extracted fractions obtained by using several solvents of a similar extractive ability had the same qualitative composition, and they differed basically in the relative concentration of the components. In a solution, when coal tar pitch is the solute, the solution complexity is great, and for that reason it is very difficult, if not impossible, to study this equilibrium state in a nonempirical way. Equation 1 for regular solutions can be written as V2412 In x2 = In a2 - -622
RT
+ 2-6261 V2d12 RT
-v2°t6,2
RT
(2)
Taking as starting point eq 2 for the equilibrium state of one pitch in several solvents, a2 and S2 are constants, and the mole fraction x 2 of the solute in equilibrium state with the solvent, and for that reason pitch solubility,SAl, could be considered as being dependent on the solubility parameter 8H1of the solvent, assuming the molar volume V2 of the solute and the volume fraction 41 of the solvent to be constants. For this, the fitting of the S A 1 and 6~~ data by a polynomial regression of second degree was tested. The equation obtained S A 1 = -56.9 + 21.46~1- 1.06Hl2 (3) predicts the highest pitch solubility for solvents with 6H1 = 10.8 ~ m - (this ~ / value ~ is calculated by dSAl/dbH1 = 0 in eq 3),and this value agrees with the 6~~ value for quinoline and it is very close to the 6H1 value for 1methyl-2-pyrrolidinone, the two best coal tar pitch solvents. However eq 3 does not show a high predictive value, since the correlation coefficient between experimental data and SA1 data calculated by this equation is 0.6466. This could be due to the fact that the A1 coal tar pitch Solubility in several solvents can be governed predominantly by a particular kind of interaction. Therefore a study of the relationships between each of the different Hansen’s sol-
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1581 100
101
Figure 1. Points correspond to the experimental solubility of the A1 coal tar pitch, SAl, veratu the dispersive component of the soluThe curve line corresponds bility parameter of the solvent ueed, to eq 4.
vent solubility parameter components and A1 coal tar pitch solubility, SAl,was carried out. Equation 2 was taken as the model in all cases, assuming that the different solubility parameter components are independent parameters. J3quations 4 4 were obtained through the fitting of SA1 to 6dl (or to dPl, or to 6hl) data by a second-degree polynomial regression. SA1
-262.8
SA1 =
+ 57.O6d1 - 2.36dt
60.1 + 5.2bPl- 0.76,t
SA^ = 43.3 + 9.66h1 - 1.26hl2
(4) (5)
(6)
These equations do not have a high predictive value; however from them it is possible to obtain some information about the influence of solvent solubility parameter components on the A1 coal tar pitch solubility. Equation 4 is represented by the curye line in Figure 1. From Figure 1 it can be observed that, for the solvents studied here, solvent effectiveness (and for that reason pitch solubility, sA1) has an almost h e a l dependence on 6dl. That is, regardless of the other components of the solubility parameter, the greater the dispersive component of the solubility parameter b d l , the greater the solvent effectiveness with coal tar pitches. In fact, the lineal relationship between SA1 and bdl data through the equation SA, = -98.9 + 17.96d1 haa a correlation coefficient of 0.6656, a value of an order similar to the correlation coefficient of eq 4. However eq 4 predicts the highest pitch solubility for solvents with 6dl = 12.3 cal1/2 cm-312 (value for what asAl/d6dl = O), establishing a limit to this lineal dependence. On the other hand, this value could be considered as being very high, because, from among the 120 most common organic solvents for which bdl is available (Barton, 1983), none of them reaches such a high value. Points under the curve line in Figure 1 belong basically to two types of solvents: solvents with null polarity and null ability to give rise to hydrogen bonding interactions, like hexane, carbon tetrachloride, and carbon disulfide (solvent numbers 2,9, and 16), or solvents with very high values of JP1 and/or bhl, like formamide, methanol, acetonitrile, and ethanol (solvent numbers 1, 3, 4, and 5). Points over the curve line in Figure 1 belong to solvents with bPl and/or 6 h l values higher than zero and smaller than 8.8 and 9.3 dl/* respectively. From what has j u t been said it could be concluded that bdl of the solvent
is the most closely related parameter to its effectiveness in the extraction of coal tar pitches, and the other components of the solubility parameter 6,1 and $1 can increase or decrease the extractive ability detemined by 6 d p Likewise it may be concluded that polarity and the ability to give rise to hydrogen bonding interactions of the solvent contribute to increasing the solvent effectiveness with pitches whenever these tendencies do not exceed certain values of or 6hl. = 0 (or Equations 5 and 6 show that solvents with 6hl = 0) ~ m -can ~ /extract ~ considerable amounts of the A1 coal tar pitch, revealing again that these solvent parameters are not the ones most closely related to its effectivenessin coal tar pitch extractions. Also according to eqs 5 and 6, solvents with 6 1 3.4 (and bhl = 3.9) call/' cm-3/2(Values for which dS~l/dd,l = 0 and dSA1/dqh1 = 0) should be solvents whose polarity and ability to give rise to hydrogen bonding interactions contribute greatly to solvent effectiveness. Again these values are very close to the and 6h1 for quinoline. On the other hand, the minimum possible value of (or hl)is 0 cm-3/2,and in accordance with eqs 5 and 6, the effectiveness of a solvent with = 0 (or 6h1 = 0) in the extraction of the A1 coal tar pitch, regardless of the other components of the solubility parameter, is of the same order as the effectiveness of a solvent with = 7.4 (or 6hl = 8.0) cm-3/2,respectively. This fact could be taken to mean that the polarity (or the ability to give rise hydrogen bonding interactions) contributes to increasing solvent effectiveness only when 6 (or bhl) values range from zero to -7.4 (or 8.0) cm-#j2,respectively. Solvents with dP1 (or 6hl) values higher than those will not contribute positively to increasing the extractive ability of solvents, which is determined basically by its dispersive component 6dl.
Conclusions There are no simple relationships between coal tar pitch extraction yields and the Hildebrand solubility parameters of the solvents used. In spite of this,the equation obtained from the fitting of SA1 to aB1 data through a polynomial regression predicts the highest pitch solubility in solventa with aH1 = 10.8 cal1l2cm"l2 in accordance with the value of quinoline and l-methyl-2-pyrrolidinone, the two best coal tar pitch solvents. It has also been found that the solvent dispersive component of the solubility parameter is the parameter that is most influential in solvent effectiveness in pitch extractions. Polarity and the ability to give rise to hydrogen bonding interactions contribute to increasing solvent extractive ability, whenever these tendencies do not exceed certain values of (-7.4 ~ m - ~and / ~of) 6 h l (-8.0 cm"l2). Also the highest pitch solubility is predicted in solvents with dPl = 3.4 and 6hl = 3.9 cal1/2 cm-312,values very close to the quinoline ones. In general from this study it can be concluded that the greater the dispersive component of the solubility parameter of the solvent used, the greater the coal tar pitch extraction yield, provided that the polar and/or hydrogen bonding components of its solubility parameter do not exceed the above mentioned values. Acknowledgment This work was supported by the DGICYT, Project No. PB88-002. R e d s t m NO. 1, 75-12-7; 2, 110-54-3; 3,67-66-1; 4,76-06-8; 6, 64-17-5; 6,60-29-7; 7,79-20-9; 8,6744-1; 9,56-23-5; 10,462-06-6; 11,108-883; 12,123-91-1; 13,6&12-2; 14,7549-2; 16,67464; 16,
Ind. Eng. Chem. Res. 1991,30, 1582-1588
1582
75-15-0;17, 109-99-9;18, 110-86-1; 19,98-95-3;20,91-22-5;21, 872-50-4.
Literature Cited Angelovich, J. M.; Pastor, G. R.; Silver, H. F. Solvents Used in the Conversion of Coal. Ind. Eng. Chem. Process Des. Dev. 1970,9 (l),m. Bade, K. D. The Structure and Composition of Coal Tar and Pitch. Rev. Pure Appl. Chem. 1972,22,79. Barton, A. F. M. Handbook of Solubility Parameters and other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983. Blanks, R. F.; Prauenitz, J. M. Thermodynamics of Polymer Solubility in Polar and Nonpolar Systems. Ind. Eng. Chem. Fundam. 1964,3, 1. Borwitzky, H.;Schomburg, G. Separation and Identification of Polynuclear Aromatic Compounds in Coal Tar by Using Glass Capillary Chromatography Including Combined Gas Chromatography-Mass Spectrometry. J. Chromatogr. 1979,170,99. Chawla, B.; Davis, B. H. Effect of Temperature and Solvent on Coal Extraction under Mild Conditions. Fuel Process Technol. 1989, 23, 133. Dryden, I. G. C. Action of Solvents on Coals at Lower Temperatures II-Mechanism of Extraction of Coals by Specific Solvents and the Significance of Quantitative Measurements. Fuel 1951,30,39, 145.
Fiacher, P.; Stadelhofer, J. W.; Zander, M. Structural Investigation of Coal Tar Pitches and Coal Extracts by '8c n.m.r. Spectroscopy. Fuel 1978,57,345. Guillh, M. D.;Blanco, J.; Canga, J. S.; Blanco, C. G. Study of the Effectivenew of 27 Organic Solventa in the Extraction of Coal Tar Pitches. Energy Fuels 1991,5,188. Hansen, C. M. The Three Dimensional Solubility Parameter-Key to Paint Component Affinities. 11. Dyes, Emulsifiers, Mutual Solubility and Compatibility, and Pigments. J. Paint Technol. 1967,39,505. Hildebrand, J. H.; Wachter, J. M.; Scott, R. L. Regular and Related Solutions; Van Nostrand Reinhold: New York, 1970;Chapters 1 and 2. Riggs, D. M. Carbon Fiber from Solvent Extracted Pitch. Preprints-American Chemical Society, Division of Petroleum Chemistry (Symposia), St. Louis, MO; American Chemical Society: Washington, DC, 1984;Vol. 29, p 480. Van Krevelen, D. W. Chemical Structure and Properties of Coal. XXVIII-Coal Constitution and Solvent Extraction. Fuel 1966,44, 229. Weishauptova, Z.;Medek, J. Pitch Coke Structure and its 'Pramition into Graphite. Fuel 1985,64,999. Received for review July 3, 1990 Revised manuscript received December 5, 1990 Accepted December 24, 1990
Extraction in a Single-Stage Mixer-Settler Abu Baker S. Salem* Department of Chemical Engineering, Qatar University, Doha, P.O.Box 2713, Qatar
A new design of a singlestage mixer-settler is presented. A horizontal mixing compartment, 5.5-cm diameter and 12 cm long, leading to a similar vertical coalescence compartment reduced the settler volume to a minimum. Almost complete solute transfer occurred in the mixer for the system used, while the vertical settler, 3.5-cm diameter and 40 cm long, served merely as a flash separator. This provision may prove that only a single properly designed single stage may be enough for many extraction duties with the result of low overall investment compared to other contactors. The contactor performance was investigated by the test system toluene-acetic acid-water at 25 OC. Data analysis using a flash vaporization technique gave very good agreement with the experimental results.
Introduction Different types of equipment have been developed for liquid-liquid extraction, and the choice among them can involve many parameters. These include the stages required, flow rates, floor space available, residence times, type of solvent used, scaleup reliability, capital, and economic factors. Logsdail and Lowes (1971) have presented an excellent review on this subject. Robbins (1979) proposed a decision network based on the criterion that the least complicated contactor that will perform the extraction with low maintenance is preferred for industrial use. Mixer-settlers are usually preferred over tower extractors,with or without mechanical agitation in cases of long residence times and limited head-room space. Shaw and Long (1957) reported that mixer-settlers typically exert a high agitation intensity, Le., tip speed of 170 m/min, and employ a settler throughput of up to about 4 m3/h.m2of interface area. However, mixer-settlers are usually arranged in a horizontal pattern, therefore requiring large floor space, high inventory of liquids, independent agitation for each stage, and interstage pumping (Laddha and Degaleesan, 1976). Nevertheless,they are considered one of the most efficient *Present address: P.O. Box 5933, Helliopolis, West Cairo, Egypt.
contactors still in use for industrial applications. In the past decade, considerable effort has been expended on improving the design of mixer-settlers, and many new units have been reported in an attempt to reduce the space requirement and to increase throughput per unit volume without introducing interstage pumping. Reference can be made to the works of Treybal(1964) and others (Cheng, 1979). Horvath and Hartland (1985) introduced a mixel-settler extraction column that has the advantages of both mixer-settlers and column contactors. The column can contain a number of stages placed vertically one above the other. A typical stage contains one mixer and a settler. The stirrers of the mixers were attached to one common shaft along the axis of the column. Stage efficiencies of up to 170% have been obtained at 375 rpm agitation speed and 2.1-min residence time per stage. They reported that this was due to repeated complete coalescence and redispersion in each stage and also to the large interfacial area produced by the intensive mixing combined with low back-mixing in the column. However, Smith (1963) indicated that efficiencies greater than 100% are possible in cases where each stream leaving the stage is not of the same composition due to incomplete contact between the phases. The object of the present work was to develop a contactor that may combine the advantages of columns with
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