Ind. Eng. Chem. Process Des. Dev. 1965, 24, 121-128
-
0.004
System 1 A (I 3 2 0
0.003- 0
0.002
-
0.001
-
I!
I,
4 5
. Ave. abs. dev.=l2% Total pts=59
0
0.001 d,,/d,
0.002
0.003
0.004
= 0.05 C,(1+2.316~)N;P,BN;P13(d,
/dT)075
Figure 4. Experimental d3*/dIvs. predicted d 3 2 / dby ~ eq 10 with C, = 0.63.
was found that the dispersity is not a function of the drop size (the data are not shown), and the mean dispersity for the pure systems was 0.5264 while that for the contaminated systems was 0.4021 (see Soong, 1982, for the details). As a result, it can be concluded that the addition of surfactant decreases the range of droplet size distribution and therefore produces more uniform droplets than those produced for pure systems. Acknowledgment
This material is based upon work supported by the National Science Foundation under Grant CPE-800666. Nomenclature
C = concentration of surfactant in the solution, kmol/m3 C, = correction factor in eq 10, dimensionless d = droplet diameter, m d32 = Sauter mean droplet diameter, m d I = impeller diameter, m dT = tank diameter, m H = height of liquid in the vessel, m KT = constant in eq 4 , dimensionless N = impeller stirring speed, rps
121
NFr = impeller Froude number, defined by p@d?/Apdg NRe = impeller Reynolds number, defined by d:Np/h N,, = impeller Weber number, defined by IPd:pc/o P = power dissipated by impeller, (N m)/s q = amount of surfactant per unit area of interface in excess of that present in the bulk solution, kmol/m2 R = gas constant, 8314 (N m)/kmol K T = temperature, K ii2 = average difference in the mean square of the velocity across drop diameter, m2/s2 Greek Letters E = rate of energy dissipation per unit mass of fluid, W/kg 7 = Kolmogoroff s length scale, m = viscosity, Ns/m2 Y = kinematic viscosity, - . m2/s , p = density, kg/m3 u = interfacial tension, N/m 4 = the fraction of the dispersed phase, dimensionless Subscript
c = continuous phase L i t e r a t u r e Cited Adamson, A. W. “Physical Chemistry of Surfaces”, 2nd ed.; Wiley: New York. 1967; pp 78-81. Bates, R. L.; Fondy, P. L.; Corpstein, R. R. Ind. Eng. Chem. Process Des. Dev. 1963, 2, 310. Coulaloglou, C. A.; Tavlarides, L. L. AIChE J. 1978, 22, 289. Hartland, S . Trans. Instn. Chem. Eng. 1968, 46, T275. Hinze, J. 0. AIChE J. 1955, 7 , 289. Hinre, J. 0. “Turbulence”, 2nd ed.; McGraw-Hill: New York, 1975; pp 223, 399. Hodgson, T. D.; Lee, J. C. J. Colloid Interface Sci. 1969, 30, 94. Hodgson, T. D.; Woods, D. R. J. ColloM Interface Sci. 1989, 30, 429. Hong, P. 0. D.Eng. Dissertation, Cleveland State University, Cleveland, OH, 1982. Hong, P. 0.; Lee, J. M. I n d . Eng. Chem. Process Des. Dev. 1983, 22, 130. Hong, P. 0.; Lee, J. M. submitted for publication, 1984. Hsu, G. C.; Klntner, R. C. J. Chem. Eng. Data 1989, 14, 67. Skelland, A. H. P.; Lee, J. M. Ind. Eng. Chem. Process D e s . Dev. 1978, 17, 473. Skelland, A. H. P.; Lee, J. M. AIChE J. 1981, 27, 99. Soong, Y. M.S. Thesis, Cleveland State University, Cleveland, OH, 1962. Sovova, H. Chem. Eng. Sci. 1981, 36, 1567. Vermeulen, T.; Williams, G. M.; Langlois, G. E. Chem. Eng. Pmg. 1955, 57, 85F.
Received for review June 21, 1983 Accepted March 9, 1984
Solvent Effects in Supercritical Extraction of Coal Nicholas P. Vasllakos,’ Joseph M. Dobbs, and Anthony S. Parlsl Deparlment of Chemical Engineering, The Universiv of Texas, Austin, Texas 78712
The specific physical and chemical characteristics of supercritical solvents and solvent mixtures that can affect the yield and the properties of coal extracts were experimentally investigated. Strong nonideal interactions, such as polar bonding and hydrogen transfer, as well as synergistic interactions in multicomponent solvent mixtures, were shown to produce large deviations from the simple, densitydriven supercritical solubility. These interactions can be manipulated to optimize supercritical coal extraction by reducing the severity of the extraction conditions (pressure in particular). Physical and chemical changes occurring in the coal structure during supercritical solvent extraction were also examined.
Introduction
Supercritical gas extraction is particularly suitable for the recovery of the liquids formed when coal is heated to above 400 OC. These liquids are normally two involatile to distill at this temperature. If the temperature is increased, they polymerize to form heavier and larger molecular species and evolve as gases and liquids. Only a relatively small amount of the coal distills as tar from the 0196-4305/85/1124-0121$01.50/0
decomposing material. Supercritical gas extraction affords a means of recovering these liquids as they are formed while avoiding the undesirable decomposition reactions Gangoli and Thodos, 1977). Supercritical fluids have a density of about 30% of that of a normal fluid, which is high enough la provide for good solvent capability, but also low enough for high diffusivity and rapid mass transfer throughout the complex coal matrix. 0 1984 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
The use of supercritical fluids for the recovery of hydrocarbons and related compounds from coal was pioneered by the National Coal Board (NCB) in Britain. Over a number of years, NCB has investigated the direct extraction of coals using light aromatic solvents (mostly toluene) under supercritical conditions. In a recent NCB report, Whitehead (1980) summarized the experimental data on supercritical extraction of coal, obtained both on bench-scale units and on a 5 kg/h continuous pilot plant operated in Britain by the Coal Research Establishment. According to these data, the residence time of coal under supercritical conditions was the variable which most influenced the extract yield. The influence of pressure and temperature on the yield was also substantial, and an extract yield of 35% by weight of the dry, ash-free coal could be obtained using toluene at 420 "C and 4000 psi pressure. It was also determined that it was not essential for the extracting fluid to be above its critical temperature to be effective. For a given operating pressure, the advantages of operating in a supercritical state were associated with the lower density and viscosity of the fluid compared with a subcritical fluid. In another coal-related effort in the United States, Kerr-McGee Corporation has developed a new solid-liquid separation technique utilizing the unique solvent capabilities of supercritical fluids (Adams et al., 1979). The process, called Critical Solvent Deashing (CSD), is used to separate mineral matter and unreacted coal from coal liquids. A two-stage CSD pilot plant (integrated into an SRC process unit) has been operated for the past four years at Wilsonville, AL. Despite the strong industrial interest and the extensive research activity in the field of supercritical extraction, much remains to be learned in the application of supercritical fluids to coal processing. An important feature of supercritical coal extraction that has received little attention in the literature concerns the specific characteristics (physical and/or chemical) of the supercritical solvent and solvent mixtures that can affect the yield and the quality of the coal extracts. Paul and Wise (1971) in their excellent monograph on gas extraction used the semiquantitative approach of Rowlinson and Richardson (1959) to show the strong dependence of supercritical solubility on the cross-virial coefficient BIZof the solvent-substrate gas phase. They also discussed empirical correlations for calculating these coefficients in relatively simple cases of nonpolar molecules. The existing B,, correlations, however, are most likely to prove completely unsatisfactory for polar solvents, or for solvents that are chemically (as well as physically) involved in the mechanism of coal extraction. Polar or hydrogen bonding solvents, for example, may exert a stronger dissociating or depolymerizing action on coal during the thermal fragmentation stage than nonpolar solvents, thus increasing the yield of extractable coal material. The present paper discusses some results from the first phase of our experimental investigation into solvent and chemical-reaction effects in supercritical coal extraction. The second phase currently under way is concerned with the combination of supercritical extraction and specific chemical treatment of the coal (such as catalytic depolymerization, alkylation, hydrogen-donor activity, etc.) to increase the yield of coal extracts and decrease the severity of the extraction conditions.
Experimental Section All supercritical extraction experiments were carried out in batch mode, in a l-L (free volume: 0.910 L), 316SS AE MagneDrive autoclave equipped with a digital temperature
controller/indicator and a digital pressure transducer/indicator in the 0-10000 psi range (Autoclave Engineers, Model DPS-0201). An Illinois No. 6 bituminous coal, 100 X 200 mesh size, was used throughout this investigation. The coal was dried under high vacuum at 110 "C for 24 h before every run. In each run a certain amount of solvent, corresponding to the desired supercritical density, was first measured into the autoclave. Then two fine-mesh baskets containing a total of 20 g of the dried and sized coal were suspended at the top of the autoclave, so that no actual contact between the liquid solvent and the coal samples was possible. We took this precaution to eliminate any ambiguity in the results, where a significant fraction of the coal would be soluble under supercritical conditions but insoluble when the system was brought back to ambient conditions, thus precipitating on the extracted coal and causing the extraction yields (which are based on the weight loss of the raw coal) to appear low (Blessing and Ross, 1978). In our system the material dissolved under supercritical conditions is carried through and recovered outside the baskets after the experiment, whether or not this material is still soluble. This is due to the very small volume of free space inside the baskets relative to the free volume of the autoclave (which is the volume occupied by the extractcontaining solvent under supercritical conditions),and also due to the placement of the baskets at the top of the autoclave. After sealing and purging, the reactor was heated for about 1.5 h to reach extraction temperature (400 "C). Following 2 h of extraction, the system was cooled to room temperature (2 h). Yield was defined as the weight loss of the vacuum dried (24 h, 110 "C, torr), char-containing baskets expressed as a percentage of the raw, dry-coal weight. The precision of the weight percent extraction values reported in this paper was normally within f O . l wt % (absolute). This precision was confirmed by comparing the extraction results for the two baskets in each experiment. Solvent recoveries after extraction ranged between 96 and loo%, the losses most probably occurring by evaporation during the opening and discharging of the autoclave at the end of a run. Decreasing the particle size and the amount of coal sample (for a given solvent density), and increasing the extraction time (from 2 to 6 h) had no effect on extraction yields. Therefore, the reported results reflect a condition of equilibrium extraction with no kinetic or mass transfer limitations. The extracted coal samples, as well as the supercritical extracts, were analyzed for elemental composition, heating value, and pyridine solubility.
Results and Discussion 1. Nonpolar Solvent Effects. To determine the effect of the physical parameters of the supercritical solvent on coal extraction, coal samples were extracted with a homologous series of n-paraffins (from pentane to dodecane) at 400 "C and at a constant solvent density of 2.75 mol/L. The extraction temperature was the same in the entire paraffinic series to ensure the same level of thermal depolymerization in coal during the action of the supercritical solvent. The value of 400 "C was chosen to be above the critical temperature of all the paraffins (T,= 386 "C for dodecane), and also to exceed the temperature marking the onset of significant pyrolytic activity in coal (350 "C). In addition, as the analysis of Rowlinson and Richardson (1959) suggests, the molar density of the supercritical solvent was kept constant in the homologous series, so that the individual solvent effects could be evaluated relative
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 123
24t\
Table I. Supercritical Extraction of Coal with a Homologous Series of n -Paraffins at 400 "C and at a (Constant) Solvent Density of 2.75 mol/L solvent pentane hexane heptane octane nonane decane undecane dodecane
wt % extr
18.4 19.3 19.7 20.7 21.2 22.8 23.2 23.6
extr press, psig 2032 1635 1441 1393 1290 1380 1562 1768
to this common baseline. The choice of 2.75 mol/L as the value for the solvent density was based on practical considerations: the solvent density has to be sufficiently high to affect significant coal extraction, but also low enough to allow for a reasonable amount of liquid solvent to be placed inside the 1-L autoclave in the beginning of each run (2.75 mol/L corresponds to 0.29 L of liquid pentane but also to 0.57 L of liquid dodecane). Results on weight percent extraction and final extraction pressure (which is generated by the supercritical solvent itself) are given in Table I. The critical parameters of the paraffinic solvents are listed in Table 11. The results indicate that extraction yield increases with increasing molecular weight of the solvent, dodecane being the best supercritical solvent under the given experimental conditions. On the other hand, extraction pressure displays an interesting behavior, decreasing with increasing molecular weight of the solvent (at constant temperature and molar density), then passing through a minimum (for nonane), and finally increasing with increasing molecular weight. Based on their experimental results on high-pressure gas chromatography, Giddings et al. (1968) suggested that the solvent power of a supercritical medium is directly related to its solubility parameter, 6. Using the van der Waals equation, they developed ther following correlation
#-j 2000 R E ACTOR PRESSURE, 1600
20 -
[AI
- 1400
19I8 2 6
30
SOLU B I L I T Y
3 4
42
38
P A R A M E T E R ( co1",~m'21
Figure 1. Supercritical extraction of coal with a homologous series of n-paraffins a t 400 "C and a t a (constant) solvent density of 2.75 mol/L.
correlation, and that regardless of their structural differences, all compounds largely perform in accordance with their solvent capabilities. This is a rather arbitrary statement, taking into account the fact that only one polar solvent (methanol) was included in their plot of coal extraction vs. solubility parameter of the solvent. Using Giddings' correlation, solubility-parameter values for the homologous series of n-paraffins were calculated at the experimental solvent density of 2.75 mol/L. The results are given in the last column of Table 11. Weight percent extraction is plotted against solvent solubility parameter is Figure 1. As can be seen from the figure, the plot is highly linear (correlation coefficient 0.987), yielding the following correlation (wt % extraction) = 3.056 + 11.2 Table I11 includes the results on supercritical coal extraction at 400 "C and three different solvent densities, for another nonpolar hydrocarbon solvent, namely toluene. Solubility-parameter values calculated from Giddings' correlation are given in the last column of Table 111. Linear regression on the toluene runs yields the following correlation (correlation coefficient 0.986) (wt % extraction) = 3.966
where P, is the critical pressure of the solvent in atmospheres, pr is its reduced density = p/pcrit, and pI is the reduced density of liquids, taken to be about 2.66. It is important to note here that the above correlation is likely to prove satisfactory for nonpolar solvents, but its value in predicting solvent capabilities when special solvent effects are present (polar clustering, hydrogen bonding, etc.) is very much in doubt. Blessing and Ross (1978), based on a limited number of supercritical extraction experiments on coal and lignite, concluded that the extraction yield is an almost linear function of the solubility parameter of the solvent as calculated by the Giddings
1200
I
22
+ 10.9
(3)
A measure of the accuracy of the experimental results is the predicted value for the extraction yield at the limit 6 0 (vacuum pyrolysis of coal), which should be the same for all solvents. Equations 2 (n-paraffins) and 3 (toluene) predict extraction limits very close to each other (11.2 and 10.9 w t % , respectively). However, contrary to the conclusion of Blessing and Ross, the extraction yield depends also on the solvent functionality. By comparing eq 2 and 3 it can be seen that toluene is a better solvent in supercritical coal extraction than straight-chain aliphatic hydrocarbons, for the same value of the Giddings solubility parameter. As we will show later, the effect of functionality is even stronger for polar solvents.
-
Table 11. Physical Solvent Parameters for the Homologous Series of n -Paraffins from Pentane to Dodecane solvent pentane hexane heptane octane nonane decane undecane dodecane
crit temp, OC 196.6 234.2 267.1 296 321 344.4 365.2 386
crit press., psi 489 439 397 364 331 306 282 263
crit density, mol/L 3.28 2.70 2.32 2.03 1.80 1.62 1.47 1.35
a
(x 10-4)" 0.608 0.821 1.064 1.320 1.619 1.929 2.271 2.639
b," L/mol 0.100 0.121 0.142 0.163 0.188 0.211 0.237 0.262
extr press.,b psig 1680.2 1531 1425.9 1383.3 1492.5 1755.3 2363.6 3502.1
solub param: ~al'/~/cm~/~ 2.27 2.61 2.90 3.17 3.42 3.65 3.85 4.05
"Constants of the Redlich-Kwong equation of state; units for a: (L2psi K1/z)/mol. "Calculated from the Redlich-Kwong equation at the supercritical extraction conditions: 400 "C, 2.75 mol/L. Calculated from Giddings' correlation a t the supercritical extraction conditions: p = 2.75 mol/L.
124
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
Table 111. Supercritical Extraction of Coal with Various Solvents at 400 “C solvent toluene toluene toluene acetone acetone methanol methanol ethylene glycol water water
wt % extr
36.0 27.6 20.3 4.0 7.3 20.6 8.4 24.9 34.0 29.5
extr press.: psig 5140 1440 1260 2180 3480‘ 3610 1246 5264c 3580 3070
solv density, mol/L 6.75 4.0 2.75 4.0 5.5 6.75 1.75 7.0 8.25 7.08
1
solub paramb 6.46 3.83 2.63 2.69 3.70 3.31 0.86 5.40 3.17 2.72
Experimental value. Calculated from Giddings’ correlation. Significant decomposition was observed.
It has also been suggested (Fong et al., 1981) that extraction yields may be affected by molecular size and that the longest dimension of each solvent molecule should be considered when comparing the coal extraction efficiencies of various supercritical solvents. The implication here is that penetration of the micropore structure of the coal can be achieved more easily by the smaller molecules. Our results on the homologous series of n-paraffins, where dodecane, the longest molecule, proved to be the most effective extraction solvent, certainly do not support this suggestion. Mass transfer limitations do not appear to be significant in supercritical coal extraction, at least under the given experimental conditions. The behavior of the extraction pressure at constant temperature and molar density in the homologous series of n-paraffins is worth further consideration from the standpoint of optimizing the extraction conditions. Our results, for example, indicate that n-nonane is a superior solvent to n-pentane in supercritical coal extraction, not only in terms of the higher extraction yield, but also in terms of the considerably lower extraction pressure. The Redlich-Kwong equation of state a p = - -R T (4) V - b T1J2Q(Q+ b ) where
Q = molar volume = 1/p a=
(p
= molar density)
0.4278R2T>5
b=
pc O.0867RT ,
pc was used to examine the extraction-pressure trends in the homologous series of n-paraffins, under the given experimental conditions: T = 400 “Cand p = 2.75 mol/L or V = l / p = 0.3636 L/mol. Critical parameters for the nparaffins, as well as calculated values for u and b, and estimated (eq 4) values for the extraction pressure are given in Table 11. These data show that, despite the considerable differences between estimated and observed values of the extraction pressure, especially for the higher paraffins, the Redlich-Kwong equation is able to predict the pressure minimum at almost the same solubility-parameter value as the experimental one (Figure 2). The structure of the Redlich-Kwong equation, and the strong dependence of the pressure on the ( V - b)-I term in particular, suggests an interesting path for optimizing extraction conditions. In Table IV we list estimated extraction pressures and solubility-parameter values for the
Redlich-Kwong Equation
3000 PRESSURE, PSIG 2600
2200
1400
2 2
26
3 4
3 0
SOLUE I LI T Y
PARAMETER
38
4 2
“Lkm?’2)
Figure 2. Estimated and experimentally observed extraction pressures a t 400 OC and a t a (constant) solvent density of 2.75 mol/L. Table IV. Extraction Pressures Calculated from the Redlich-Kwong Equation of State at 400 “C and Two Different Solvent Densities p = 2 mol/L p = 3 mol/L solvent pentane hexane heptane octane nonane decane undecane dodecane
extr press., psig 1236.2 1106.3 978.8 863.1 771.8 705.9 690.8 727.1
solub paramn 1.65 1.90 2.11 2.31 2.49 2.65 2.80 2.95
extr press., psig 1849.3 1713.3 1650.7 1687.4 1974.5 2538.0 3757.3 6239.7
solub parama 2.48 2.85 3.16 3.46 3.73 3.98 4.2 4.42
Calculated from Giddings’ correlation.
Table V. Supercritical Extraction of Coal with n -Paraffins at 400 “C and at a (Constant) Solvent Density of 1.75 mol/L solvent octane decane undecane dodecane
wt % extr
18.7 19.9 20.3 21.1
extr press., psig 905 726 642 544
homologous series of n-paraffins, in supercritical coal extraction at 400 O C and two other molar densities. As can be seen from the table, by decreasing the molar density, the pressure minimum shifts toward the heavier hydrocarbons, which still possess the higher solubility-parameter values. Thus, by proper selection of the experimental solvent density (- 1.8 mol/L in our case), dodecane could become an excellent supercritical solvent for coal, both in terms of the highest extraction yield in the hydrocarbon series and also in terms of the lowest generated extraction pressure. To test this hypothesis, a series of supercritical coal extractions was carried out with selected n-paraffins at 400 OC and at a (constant) solvent density of 1.75 mol/L. The experimental results are listed in Table V. As predicted, dodecane at this density became the hydrocarbon solvent that affected the highest coal extraction yield at the lowest pressure, thus supporting the validity and the usefulness of the above approach. The same solvent density considerations can, of course, be applied to any other homologous series of nonpolar compounds (e.g., aromatic hydrocarbons). 2. Polar Effects in Supercritical Coal Extraction. We have conducted an extensive experimental investigation on the effect of increased solvent polarity on coal extraction yields. Table I11 gives the results of some preliminary supercritical extraction experiments at 400 O C with simple polar compounds, such as methanol, ethylene glycol, acetone, and water. Methanol gives a slightly lower extraction yield (20.6% vs. 21.3%) than estimated from the Giddings correlation
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 125 Table VI. Supercritical Extraction of Coal with n -Paraffins and the CorresDonding n -Alcohols at 400 OC solvent methane methanol ethane ethanol propane 1-propanol n-hexane 1-hexanol n-octane 1-octanol
wt % extr
(13.8)' 8.4 (15.3)' 17.5 (16.4)c 28.5 19.3 37.8 20.7 38.5
Experimental value. Calculated from eq 2.
extr press.: wig 1246 1719 1652 1635 1598 1393 1273
solv density, mol/L 2.75 1.75 2.75 2.14 2.75 2.32 2.75 2.54 2.75 2.68
solub param* 0.86 0.86 1.33 1.33 1.70 1.70 2.61 2.61 3.17 3.17
* Calculated from Giddings' correlation.
and eq 2. From a practical standpoint, however, methanol is a far inferior supercritical solvent to aliphatic hydrocarbons, because it gives lower extraction yields at much higher extraction pressures (20.6% at 3610 psi compared with 21.2% at 1290 psi for n-nonane). An extraction yield lower than that predicted for an aliphatic hydrocarbon of the same solubility parameter value (eq 2) is also observed for ethylene glycol (24.9% vs. 27.7%). The predictive value of eq 2 when coupled with the Giddings correlation breaks down completely for a polar solvent such as acetone, which shows anomalously low coal extraction yields at 400 "C. These low yields can only in part be attributed to acetone decompostion that was experimentally observed at 400 "C. Water, on the other hand, displays large positive deviations from the estimated extraction values. For example, at 400 "C and a supercritical density of 8.25 mol/L the coal extraction yield is 34.0 wt % compared with 20.9 w t % , which is the value calculated from eq 1and 2. The results in Table I11 seem to indicate that water is an even better supercritical solvent than toluene for coal extraction, effecting high extraction yields at moderately high pressures (34 w t % at T, = 1.04 and P, = 1.12). An interesting feature of the supercritical water runs is that the extract-containing aqueous phase, when brought back to ambient conditions, is an almost clear solution possessing a pungent odor and containing a minimal amount of precipitate, and in any case it is completely different from the black coal solutions obtained in supercritical toluene runs. The increased coal extraction in the presence of supercritical water may be attributed to its remarkable ability mentioned in the literature (Modell et al., 1978) to prevent the recondensation (charring) of partially pyrolyzed macromolecules, thus enhancing their conversion to lower molecular weight products. Following this preliminary study, a more systematic evaluation of polar effects in supercritical coal extraction was undertaken. Raw-coal samples were extracted with a series of n-alcohols (from methanol to 1-octanol) at 400 "C, and the extraction yields were compared with those of the corresponding n-paraffins, at the same value of the Giddings solubility parameter. The solvent density for all the paraffinic solvents was kept constant a t 2.75 mol/L, but the density of each alcohol was properly adjusted so that its solubility parameter was the same with the solubility parameter of the n-paraffii having the same number of carbon atoms. The results of these experiments are given in Table VI. For the lower alcohols methanol and ethanol the extraction levels were close to those projected by eq 2 for methane and ethane, respectively. Methanol and ethanol seem to perform merely according to their solvent capa-
REACTOR PRESSURE. PSIG [ A I
0
20
40 MOLE %
60
80
IO0
TOLUENE
Figure 3. Supercritical extraction of coal with toluene-methanol mixtures at 400 "C and at a (constant) solvent density of 6.75 mol/L.
bilities, and no significant polar effect is observed. For propanol and the higher alcohols, however, extraction yields much higher than the corresponding paraffin yields were obtained, thus revealing in this case a strong positive dependence of supercritical coal extraction on the solvent functionality. Hexanol, for example, proved to be a far superior supercritical solvent to hexane, at the same value of Giddings solubility parameter, promoting almost twice as high an extraction yield as hexane at a slightly lower pressure. Similar results for alcohols up to C4have been reported recently by Jezko et al. (1982). The difference between the extraction levels for the homologous series of n-alcohols and those for the paraffiiic series (serving as baseline) can be used as a measure of the polar solvent/coal interactions in supercritical extraction. It is interesting to note that most of this difference occurs in the transition from ethanol to 1-propanol in the homologous series, and while it increases somewhat up to 1-hexanol, it appears to level off for higher alcohols (compare the differences in extraction yields between hexanol and hexane, and octanol and octane). This suggests the presence of a polarity-induced, reactive component of supercritical coal extraction, of nearly constant magnitude in the alcohol series. The possibility, of course, exists that alcohol dehydration at the high temperature of extraction (400 "C) would yield water, which could actually be the supercritical solvent responsible for the high extraction yields. However, even the maximum molar density of water that could be obtained from the complete decomposition of the alcohols (-2.7 mol/L) is still too low to account for the high extraction levels that were observed (compare extraction levels and molar densities for the water runs in Table 111). The transfer of a-hydrogen from propanol or higher alcohols to coal that leads to selective disruption of the coal macromolecule and to increased extraction is well documented in the coal liquefaction literature (Ross and Blessing, 1979). This reactive interaction may quite likely be the one responsible for the substantially increased extraction yields. More experimental work, however, must be carried out before final conclusions can be drawn. 3. Synergistic Effects of Solvent Mixtures. Possible nonideal effects arising from polar/nonpolar combinations were studied in supercritical extraction of coal at 400 "C and constant total molar density for a series of toluene/ methanol mixtures. Extraction yields and generated pressures are depicted in Figures 3 and 4. Figure 3 for the toluene/methanol mixtures reveals a very interesting feature of the extraction curve, which passes through a maximum at a composition of approximately 70 mol % toluene and then descends slowly to the pure toluene value. This extraction maximum, which is higher than the extraction yield of either solvent alone (synergistic effect), is of even greater importance, because
128
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
Table VII. Proximate a n d Ultimate Analyses of Coal Samples Extracted with Supercritical Solvents at 400 OC pentane; octane; dodecane; toluene; methanol; 2.75; 2.75; 2.75; 4.0; toluene; 6.75; acetone; raw coal 18.43%" 20.70% 23.56% 27.6% 6.75; 36.0% 20.6% 4.0; 4.0% proximate analysis,b % 42.3 21.9 24.0 25.4 21.6 25.3 24.6 23.1 volatiles 58.0 57.5 60.4 fixed carbon 43.6 59.4 58.0 55.4 53.4 14.1 18.7 20.4 21.3 17.9 ash 18.0 19.2 16.5 ultimate analysis,* % carbon hydrogen nitrogen sulfur oxygen (by difference) heating value, Btu/lb
62.9 4.7 1.1 4.4 26.9 11415
62.2 3.5 1.1 4.1 29.1
62.8 3.5 1.1 4.1 28.5
11560
11150
67.8 3.7 1.2 4.2 23.1
61.8 3.1 1.1 4.1 29.9
11510
10980
10805
C/H ratio
1.11
1.48
1.50
1.53
1.66
% sulfur reduction
0
6.8
6.8
4.5
6.8
a
Solvent; supercritical density; weight percent extraction.
,
63.6 3.1 1.1 3.9 28.3
56.9 3.6 1.1 3.0 35.4 11690
1.71 11.4
water; 7.08; 29.5% 18.1 64.0 17.9
53.2 3.7 0.8 3.4 38.9
55.6 3.2 1.0 3.4 36.8
12280
1.32
11510
1.20
31.8
1.45
22.7
22.7
On a dry basis.
I
1
t
EXTRACTION
2 2 0 c REACTOR
0
0 3
20
60
40
MOLE
'la
80
2: 3
I00
3co
350
E X - R A CT IDN TEbl P E E A T U R E
TOLUENE
433
C'I
Figure 5. Supercritical extraction of coal with n-hexane (T, = 234 "C) a t a (constant) solvent density of 5.0 mol/L and at various
Figure 4. Supercritical extraction of coal with toluene-acetone mixtures at 400 "C and at a thermal (constant) solvent density of 4.0 mol/L.
temperatures.
it is attained at a pressure much lower than the pressure generated in pure toluene extraction under similar conditions (3700 psi as compared with 5140 psi). Fong et al. (1981) observed a similar maximum-extraction effect in supercritical extraction of coal with toluene/methanol mixtures at 360 "C and at a constant pressure of 2000 psi, but their results are of limited practical value because of the constant-pressure condition that was employed (rather than constant solvent density). Toluene/acetone mixtures, on the other hand, display almost linear dependence of both the extraction yield and the extraction pressure on molar composition. No synergistic effect is present under the given experimental conditions, so that pure toluene becomes a far superior supercritical solvent to pure acetone or any toluene/acetone mixture, effecting much higher coal extraction at a considerably lower extraction pressure. 4. Effect of Temperature. To determine the effect of temperature on supercritical coal extraction, a series of experimental runs were carried out with supercritical hexane at a constant solvent density of 5.0 mol/L and at various temperatures in the range of 250-400 "C. Results on weight percent extraction and final extraction pressure are depicted in Figure 5. These results indicate that extraction yield increases continuously with temperature, the higher yields obtained at temperatures being far apart from the critical temperature of the solvent. Paul and Wise (1971), however, in their classical monograph on gas extraction argued (on the basis of simple thermodynamic relationships) that the solvent power of a supercritical fluid should be at a maximum at temperatures close to its critical temperature, where it is least ideal. This conclusion constituted the
basis of the early British work (Whitehead, 1980) on supercritical extraction of coal with toluene at temperatures around 350 "C (critical temperature of toluene = 321 "C). Our results, which confirm some earlier results of Blessing and Ross (1978) and Fong et al. (1981),show that simple thermal depolymerization effects, and not solvent effects, are primarily responsible for the increased coal solubility in this case. Higher temperatures result in higher pyrolysis yields, the supercritical solvent acting simply as a dense medium for the distribution of the soluble coal fragments. 5. Physical and Chemical Changes in Coal Structure during Supercritical Extraction. Table VI1 gives proximate and ultimate analyses and heat content values for selected coal samples that were extracted with pure supercritical solvents under a variety of experimental conditions. The analysis of the raw coal that was used in all these experimental runs is also included. The data show a drastic (more than 60% in most cases) reduction in the volatiles content and a consequent increase in the fixed-carbon content of the coal after supercritical extraction. The reduction in volatiles is accompanied by a significant increase in the C/H atomic ratio of the treated coal, thus indicating a progressive extraction of hydrogen-rich fractions from the coal matrix. It is interesting to note, however, that even at high extraction levels, the extracted coal retains most of its fuel value, as indicated by a moderate C/H ratio and a high heat-content value. For example, coal extracted with supercritical toluene at 400 "C and at a solvent density of 6.75 mol/L retains 94.7% of the specific heating value of the raw coal, despite a 36% extraction loss and a corresponding 61.7% loss of volatiles.
lnd. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 127 Table VIII. Proximate and Ultimate Analysis of Coal Samples Extracted with Supercritical n -Hexane at a (Constant) Solvent Density of 5.0 mol/L and at Various Temperatures raw coal 250 "C; 2.42%O 300 OC;5.11% 350 O C ; 14.72% 375 O C ; 20.08% 400 OC;26.6290 proximate analysis,b % volatiles fixed carbon ash
43.4 47.2 9.4
44.3 46.5 9.2
39.5 48.5 12.0
31.6 56.2 12.2
29.0 59.5 11.5
24.6 63.1 12.3
ultimate analysis,b % carbon hydrogen nitrogen sulfur oxygen (by difference)
70.6 4.6 1.2 4.4 19.2
68.0 4.4 1.2 4.2 22.2
68.3 4.2 1.2 4.2 22.1
61.5 3.9 1.2 3.9 29.5
69.9 3.7 1.3 3.6 21.5
66.8 3.6 1.0 3.5 25.1
heating value, Btu/lb
12200
12220
12340
C/Hratio
1.28
1.29
1.35
90 sulfur reduction
0
4.5
4.5
a
Temperature; weight percent extraction.
12200 1.32 11.4
11910
12480
1.55
1.57 18.2
20.5
On a dry basis.
Table IX. Nitrogen Surface Areas of Coal Samples Extracted with Various Supercritical Solvents at 400 "C raw coal supercritical density, mol/L weight percent extraction Nzsurface area, m2/g surface area reduction, 90
0 38.4 0
hexane 2.75 19.3 12.7 66.9
The data indicate, once again, that supercritical water may be one of the most promising solvents in supercritical coal extraction, affecting higher extraction yields at much lower heat content and H/C losses (for the extracted coal) than hydrocarbon solvents (for example, compare the results of the pure toluene and the water runs). A very interesting selective desulfurization effect is observed in supercritical extraction of coal with methanolbased solvents, and to a lesser extent with acetone and water. Ideally, if there is no special affinity of the supercritical solvent toward sulfur (or other heteroatom), the extraction of sulfur fractions from the coal matrix should be nonselective, so that the sulfur content of the extracted coal, as well as that of the extract, is in every case the same with the sulfur content of the raw coal. Our data show that, although this is true for nitrogen, sulfur undergoes strong selective extraction in the presence of methanolbased solvents. Supercritical extraction of coal with pure methanol results in a 31.8% level of selective sulfur reduction, while the other two oxygenated solvents, acetone and water, also display significant (-23 %) desulfurizing action on extracted coal samples. Chemical participation of the methanol in the coal-pyrolysis stage that enhances selectively the fragmentation of sulfur clusters in the coal molecule and their subsequent extraction in the supercritical phase is proposed to be the main cause of this effect. Table VI11 summarizes chemical analysis data for a series of coal samples extracted with supercritical n-hexane at a (constant) solvent density of 5.0 mol/L and at various temperatures. A continuous reduction in the volatile content of the coal with increasing extraction temperature is observed, while the heating value remains approximately constant in the entire temperature range (250-400 "C). Despite the significant decrease in volatiles, the C/H ratio shows little change up to 350 "C but experiences a noticeable increase between 350 and 375 "C (i.e., at the onset of strong thermal depolymerization activity in coal). Selective sulfur removal from coal also becomes important above 350 OC, although no methanol-based solvent is present. In this case, the increased density of the paraffinic
decane 2.75 22.8 3.0 92.2
solvent undecane 2.75 23.2 2.5 93.5
dod eca n e 2.75 23.6 2.4 93.8
methanol 6.75 20.6 1.6 95.8
solvent (5.0 mol/L compared with 2.75 mol/L in Table VII) maka up for the absence of polar interactions, at least as far as desulfurization is concerned. Finally, surface areas of extracted coal samples were measured by nitrogen adsorption at -196 "C to determine the extent of possible changes ocurring in the physical structure of the coal during supercritical extraction. The results are given in Table IX. A sharp decrease in the available surface area of the coal is observed in all the extracted samples, and this decrease, at least for the paraffinic series, is progressively more pronounced at higher extraction levels. Methanol-extracted coal, on the other hand, shows a higher surface area reduction than hexane-extracted coal, despite the lower extraction level. The surface-area-reduction effect may be attributed to a shift in the pore-size distribution of the coal caused by the extractive action of the supercritical solvent. The removal of coal material from the solid matrix may result in the progressive opening of larger pore spaces, thus shifting the pore-size distribution toward the macropore range, generally associated with low specific surface areas. Swelling phenomena in the coal matrix caused by strong H-bonding solvents, such as methanol, can contribute further to the closing of smaller pores and, thus, to additional collapse of the microporous network. Fong et al. (1981) proposed a different explanation based on the retention of extract in the pores. At the end of an extraction cycle, the removal of the volatile supercritical solvent from the pores could result in partial condensation of the coal extract and, thus, in extensive pore blockage. Concluding this section we would like to report some results on the pyridine solubility of raw and extracted coal samples. The pyridine solubilities of dried samples were determined by stirring 0.5 g in 50 cm3 of pyridine for 2 h at room temperature, then filtering and evaporating the solvent, and drying the solid residue under vacuum at 110 "C for 24 h. Results for the raw coal and for the pure toluene and pure methanol runs are given in Table X. The pyridine solubility of coal decreases sharply after supercritical extraction, indicating that most of the coal material that is soluble in pyridine at room temperature
128
Ind.
Eng. Chem. Process Des. Dev. 1985, 2 4 , 128-132
Table X. Results for Raw Coal and for Pure Toluene and Methanol extr pyridine solub supercrit solvent conditions w t 5% extr of extr coal, 5% raw coal 11.2 toluene
400 “C
36.0
0.8
20.6
2.4
6.75 mol/L
methanol
400 “C 6.75 mol/L
dissolves in the supercritical solvent during the high-temperature extraction. It is also important to report here that part of the supercritical extract recovered in the toluene phase after cooling the reactor’s contents to room temperature is in the form of a precipitate, fully soluble in pyridine, and amounting to approximately 8 wt 5% on the basis of raw dry coal. Therefore, in contrast to the results of Blessing and Ross, a significant part of the coal material which is soluble in the solvent under supercritical conditions becomes insoluble when the solvent is brought back to ambient conditions. Summary The specific physical and chemical characteristics of supercritical solvents and solvent mixtures that can affect the yield and the properties of coal extracts were experimentally investigated. Strong nonideal interactions, such as polar bonding and hydrogen transfer, as well as synergistic interactions in multicomponent solvent mixtures, were shown to produce large deviations from the simple, density-driven supercritical solubility. These interactions can be manipulated to optimize supercritical coal extrac-
tion by reducing the severity of the extraction conditions (pressure in particular). Physical and chemical changes occurring in the coal structure during supercritical solvent extraction were also examined. Acknowledgment This work was supported by the United States Department of Energy under Grant No. DEFG22-81PC40801 and by the Center for Energy Studies, Austin, TX. The support of the two agencies is gratefully acknowledged. Registry No. Pentane, 109-66-0; hexane, 110-54-3;heptane, 142-82-5; octane, 111-65-9; nonane, 111-84-2; decane, 124-18-5; undecane, 1120-21-4; dodecane, 112-40-3; toluene, 108-88-3; acetone, 67-64-1; methanol, 67-56-1; ethylene glycol, 107-21-1; methane, 74-82-8; ethane, 74-84-0; ethanol, 64-17-5; propane, 74-98-6; 1-propanol, 71-23-8; 1-hexanol, 111-27-3; 1-octanol, 111-87-5; pyridine, 110-86-1.
Literature Cited Adams, R. M.; Knebel, A. H.; Rhodes, D. E. Chem. Eng. Rog. 1979, 75, 44. Blessing, J. E.; Ross, D. S. ACS Symp. Ser. 1978, No. 7 1 , 171-185. Fmg, W. S., et al. ”Experimental Observatlons on a Systematic Approach to Supercritical Extractbn of Coal”, paper presented at the 89th AIChE Natlonal Meeting, New Orleans, LA, Nov 1981. Gangdl. N.; Thodos, G. Ind. Eng. Chem. Prcd. Res. D e v . 1977, 16, 208. Giiddings, J. C., et el. Sc/enca 1968, 762, 87. Jezko, J.; Gray, D.; Kershaw, J. R. Fuel Process. Techno/. 1982, 5 , 229. Modell, M.; ReM, R. C.; Amin, S. I . U.S. Patent 4113446, 1978. Paul, P. F. M.; Wise, W. S. “The Principles of Gas Extraction”: Mllis and Boon Ltd.: London, 1971. Ross, D. S.; Blessing, J. E. Fuel 1979, 58, 433. Rowllnson, J. S.; Richardson, M. J. A&. Chem. Fhys. 1959, 2 , 85. Whitehead, J. C. “Development of a Process for the Supercritical Gas Extraction of Coel”, paper presented at the 88th AIChE National Meeting, Philadelphia, June 1980.
Received for review June 27, 1983 Revised manuscript received October 17, 1983 Accepted February 23, 1984
Optimization of a DPstiltation Column with a Direct Vapor Recampremion Heat Pump Josep A. Ferr6, France= Castells,’ and Joaquh Flores Departament de O&nica T6cnIca. facultat de Ouimica, Unlversitat de Barcebna, Tatragona, Catalunya. Spain
Heat pumps can afford important savings of energy In distillation processes. However, the lack of knowledge of the profttabllity of their applicatlon in every case restricts thek use as an alternative for saving energy in distillation cdumns. In this paper we present the results of a simulation and optknizatlon computer program we developed to analyze the economical profitability of substhit@ the conventknal reboiler and condenser of an existing distillation column by a vapor recompression heat pump. Two examples of calculatlons are presented which correspond to the separation of organic compounds of mediwn molecular welght of close boiling points. Results obtained show that the POT for the substltution of the conventional distillation scheme by an overhead vapor recompression heat pump is about two years for these kinds of separations.
Introduction Distillation is one of the most popular methods of separation in the chemical and petrochemical industries. However, this process consumes a great deal of energy *Department de Qdmica TBcnica, Facultat de Qui’mica, P h p Imperial Tarraco s/n, 43005 Tarragona, Catalunya, Spain. 0196-4305/85/ 1124-0128$01.50/0
which is later degradated to the surroundings as condensing heat. One of the most efficient ways of saving energy is through the w e of heat P ~ P inSthe distillation process. This technique is already used in Some of the more difficult separations: isobutaneln-butane (Barnwell and Morris, 1982),propane/propylene (Quadri, 1981a,b), ethane/ethylene (Menzies and Johnson, 1971), owing to the specific advantages involved (Finelt, 1979). Apart from 0 1984 American Chemical Society
