TECHNICAL REVIEW Liquid Fuels and Chemical Feedstocks from Coal by Supercritical Gas Extraction Narendra Gangoli and George Thodos‘ Department Of Chemical Engineering. NorthwesternUoivemity,Evanston, illinois 602001
Narendra Gangoli is a doctoral candidate in the Department of Chemical Engineering, Northwestern University. He received his B.Tech (Honors) from Osmania University, India, in 1970, and his M.S. from Northwestern Uniuersity in 1971. He has eo-authored four papers in the water pollution area. His research interests are in enuironmental sciences, thermodynamics, and synthetic fuels. Mr. Gangoli is a member of the American Institute of Chemical Engineers, Sigma X i , and Tau Beta Pi; he will be joining the Environmental Control Section of Exxon Research and Engineering Company, Florham Park, N.J.
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George Thodos, B.S. and M.S. i n Chemical Engineering, Illinois Institute of Technology, 1938-1939, Ph.D., Uniuersity of Wisconsin, 1943, has been associated with Northwestern Uniuersity since 1947 after spending nearly six years in the petroleum industry, primarily with the Phillips Petroleum Company at Bartlesuille, Okla. He was named Department Chairman, 1960-1964, and W . P. Murphy Professor, 1963. His research interests deal with mass transfer in packed and fluidized beds. He is a leading proponent for the development of methods for the correlation and prediction of physical and thermodynamic properties of gases and liquids, as well as reaction kinetics, heat transfer, and transport phenomena. He has published extensiuely i n these disciplines (225 papers) and has directed research activities of 35 doctoral and more than 100 M.S. recipients. Dr. Thodos was a member of the State of Illinois Examining Committee for Professional Engineering Registration, 1955-1960; Advisory Board of Industrial and Engineering Chemistry, 1959-1962; Eualuation Panel for NSF Fellowships by the National Academy of Sciences, 1959-1962; and Who’s Who in America, 1963. He received the University of Wisconsin Distinguished Citation Award, 1970. He was the AIChE Fellow, 1975, and Recipient of the Distinguished Appointment Award AUA (Argonne Universities Association), 1975-1976. Dr. Thodos has lectured extensively and has consulted widely with the Pure Oil Company, Office of Naval Research, and the Chicago Bridge and Iron Company. He is a member of American Chemical Society, American Institute of Chemical Engineers, Sigma Xi, Tau Beta Pi, Phi Lambda Upsilon, Pi Mu Epsilon, and Alpha Chi Sigma. He holds seven patents, five assigned to the Phillips Petroleum Company and two to the Pure Oil Company.
The recent energy crisis has made it mandatory to explore alternate sources of energy other than crude oil and has led to the institution of conservation measures and better utilization of fuel resources. Major efforts have been initiated to find satisfactory solutions to ease the existing worldwide shortage of motor fuel and also to develop sources of feedstock for the petrochemical industry. Despite the fact that the sources of petroleum are dwindling rapidly, the reserves of alternate fossil fuels are available in abundance. The availability of crude oil and the readily accessible technology associated with its exploration and processing on one hand and the corresponding lack of economic incentives on the other has been responsible for the neglect of developing processes for the utilization of alternate resources. The supercritical gas extraction of coal is suggested as a potential viable method for the extraction of liquid fuel constituents already present in coal. This method of extraction is based on the volatility enhancement of these heavy constituents in the presence of compressed solvent gases at supercritical conditions. Theoretical arguments and basic experimental evidence for this method of extraction are advanced.
Over the past 50 years, the petroleum industry has made giant technological advances to meet the energy demands which are being imposed by transportation, household heating, power generation, and the wide range of petrochemicals that eventually find their way in the manufacture of synthetic rubber, plastics, and the myriad of other commodities that, in one way or other, trace themselves back to their source, crude oil. The present excessive demands placed on the availability of the limited resources of petroleum crude have accentuated the current energy problems. The existence of these problems requires ihe institution of conservation measures aimed at the optimum utilization of not only petroleum but also of other types of fuel. Coal presents a fossil fuel whose full potential has not yet been exploited. Current practice utilizes coal exclusively as a fuel for power generating facilities. Coal reserves, being relatively large in magnitude when compared to crude oil, oil shale, and tar sands, offer means for fulfilling the energy demands until the middle of the twenty-first century. These demands obviously impose an unavoidable dependence on coal as a major source for energy and chemicals. Coal contains 25-35% hydrogen-bonded compounds that could be potential sources of gaseous and liquid fuels. These compounds could be used as fuel substitutes or chemical feedstocks which should alleviate the energy crisis and pave a way for the optimum utilization of coal. Liquid hydrocarbons have been produced from coal for nearly two centuries. During World War 11, the Germans met the major portion of their fuel demand by hydrogenating coal for the production of liquid fuels. Currently, processes for the conversion of coal to liquid fuels are being developed in the United States. Many of these processes involve the hydrogenation of coal in the presence of a liquid solvent. Squires (38) discusses at length the various by-products and a wide range of chemicals that can be derived from coal. Thus, coal offers a viable alternative to the petroleum derived feedstocks. One of the most promising methods for the recovery of hydrocarbons and related compounds involves the extraction of coal using gases under supercritical state conditions. This method of recovery constitutes a variation of solvent extraction and was pioneered by the National Coal Board in Britain ( 2 , 1 7 , 4 6 , 4 7 , 4 8 )The . gas extraction technique is based on the solvent capability of compressed gases at temperature and pressure conditions which permit the solvation of organic constituents present in coal. The solvent effect of compressed gases was recognized as far back as 1897 by Hannay and Hogarth (19).Enhanced solvent properties of dense polar gases, particularly in the supercritical region, were demonstrated by Hagenbach (18) at the turn of the century. Frank (10) reports the pronounced solvent capability of water in the supercritical state. Weale ( 4 5 ) has presented an excellent discussion of solubility of solids in compressed nonpolar and polar gases. A fluid can exist as a liquid below its critical temperature;
however, above this temperature the fluid will exist in the gaseous state regardless of its pressure. At temperatures below the critical, a gas can be liquefied by increasing the pressure; however, at temperatures exceeding the critical temperature the fluid cannot be liquefied and is referred to as a "supercritical gas". The enhanced solvent effects of such a supercritical gas are of primary concern in this particular process of coal extraction. The gas extraction technique exploits the capability of a compressed supercritical gas to enhance the volatility of the organic compounds present in coal. Paul and Wise (31)point out that this extraction technique is similar to both solvent extraction and distillation and could be considered as a combination of the two processes. The actual conditions under which this phenomenon occurs are such that it is difficult to differentiate between these two processes since they occur simultaneously. Vaporization of a constituent into a carrier gas is equivalent to distillation, leaching, or desorption of a constituent from a substrate, and constitutes solvent extraction. Basically both of these objectives could be accomplished with the same extracting solvent medium. The ability of a supercritical gas to increase the volatility of heavy molecules from coal is of considerable practical importance. Under appropriate conditions, the volatility of a solid could be enhanced as much as 10 000 times. Ewald (8) reports the gas phase concentration of p-iodochlorobenzene as g/L in the presence of ethylene a t 15 "C and low pressures. Upon increasing the pressure of ethylene to 10 000 kN/m2 (100 atm), the concentration of p-iodochlorobenzene approaches 50 g/L. This enhancement in volatility is attributed to the fact that ethylene exists slightly above its critical state ( t , = 9.9 "C; P , = 50.50 atm). This solubility phenomenon is more a function of pressure than the chemical affinity between the solvent and the constituents to be extracted. In the proximity of its critical temperature, the gaseous solvent has a density approaching that of a liquid. In general, it is imperative that the critical temperature of the gaseous solvent be closer to the extraction temperature as it can be shown from theoretical arguments that the equilibrium content of the heavy molecules and also their volatility is greatest under such conditions (31).Alternatively, for any desired extraction temperature, this requirement will dictate the choice of a suitable gas extractant. Supercritical gas extraction is particularly suitable for the recovery of liquids formed when coal is heated to above 400 "C. The liquids thus formed are not sufficiently volatile to distill at this temperature. If the temperature is increased these heavy molecules polymerize to form heavier and larger molecular species and evolve as gases and liquids. Only a small amount of the coal distills as tar from the decomposing material. Supercritical gas extraction offers a means of recovering these liquids as they are formed and thereby avoids the undesirable decomposition reactions. Since the extraction takes place at temperatures below those at which the volatile matter Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
209
is evolved, by destructive distillation, extensive thermal degradation of coal is avoided. The gases which can be used as solvents should possess critical temperatures near the decomposition temperature of coal. Common solvents such as coal tar petroleum naphtha fractions could be used as extractants. Chemical Constitution of Coal Coal has been widely accepted as a polymeric mixture composed of a large number of units basically of a similar type but varying widely in molecular weights and structure. Coals are chiefly composed of condensed high molecular weight aromatic rings. About 70% of all carbon atoms are in the aromatic rings, but only about 23% of the hydrogen atoms are attached to aromatic carbon atoms. These compounds have molecular weights of the order of 10 000. Oxygen, sulfur, and nitrogen are combined in chemical functional groups such as CN, S, and SH etc, which occur as inOH, CO, COOH, “2, tegral parts of the original molecule ( 1 6 ) . Given (15) presents a hypothetical molecular structure for bituminous vitrinite coal possessing 82% carbon as shown in Figure 1. The model suggested by Given is based on a 9,10dihydroanthracene module. Dryden (6) recommended a dihydrophenanthrene module as being more compatible with the absence of methylene bridges reported by Brown et al. (4). Contrary to the classic view that coal is a predominantly aromatic solid, Chakrabarty and Berkowitz ( 5 ) have recently proposed that the carbon skeleton appears to be largely made up of nonaromatic structures; Le., these structures are modified bridged polymantanes. However, Ghosh et al. (11) and Aczel et al. ( 1 ) are of the opinion that the polymantane structure of coal is not tenable. Specifically, Aczel et al. ( I ) , based on their experimental studies under donor liquefaction conditions, conclude that the three-dimensional polymantane structure, if present, is not stable under conditions which convert coal to liquids and that there may not be significant amounts of the admantanes (first member of the polymantane series) type structure in coal. Several structures have been proposed, some of them more complex in nature than that proposed by Given ( 1 5 ) and others involving various types of structural arrangements. However, Given’s structure is widely accepted as a reasonable working model for practical purposes. Vahrman et al. (30, 32, 37, 42, 43) in their several investigations have established that molecules with molecular weights of less than 1000 exist in bituminous coal in quantities larger than so far believed to be present. These molecules have been classified into two categories. The first group, known as bituminous, are asphaltines possessing high oxygen content, most of which are present in the phenolic form and the rest in the ether and quinone linkages. The skeletal structure is aromatic but partly hydrogenated, and it is alkyl, mainly methyl substituted. Small amounts of sulfur and nitrogen are present, but it is not certain to what extent they form an integral part of the molecular structure. The first group of 210
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
I
50
100
150
200
250
,
,
,
,
I
300
Ternperofure, ’C
Figure 2. Critical state border curve for the ethane-n-heptane system.
compounds is classified by Vahrman et al. (42) as O-compounds while the second group is referred to as H-compounds. These H-compounds comprise of a range of hydrocarbons, both aliphatic and aromatic, together with smaller amounts of aromatics containing heterocyclic oxygen and aromatic ether linked compounds. The 0- and H-compounds are invariably found together in all the extracts and low temperature tars, but their total amounts and relative proportions vary considerably depending on their mode of preparation and the type of original coal ( 4 2 ) . Blayden et al. ( 3 )found that interatomic distances in coal are approximately two to three times greater in one dimension than the other. This result indicates a highly planar polymeric molecule in a layered structure with considerable pore volume and surface area. The porosity of various coals ranges from 8 to 20%. The internal capillary structure of coal is of considerable importance in solvent extraction. Solvent vapors have to penetrate these ultrafine capillaries during a chemical reaction or absorption process and will be influenced by the restrictions imposed by the narrow channels in a coal. The size of pores in the ultrafine structure ranges from a few angstroms to 100 A. In addition, there exist large capillaries and cracks which constitute from 20 to 50% of the total internal free volume in bituminous coal. Critical State Behavior of Simple a n d Complex Mixtures The vapor pressure behavior of a pure substance relates to the coexistence of a saturated vapor and a corresponding saturated liquid if the temperature of the system is kept below the critical temperature of the substance. The coexistence of both phases will persist as long as the temperature of the system does not exceed the critical temperature of the substance and the pressure is below the critical pressure. Therefore, it follows that a gas can be liquefied by increasing the pressure, if the temperature is maintained below the critical temperature of the gas. On the other hand, if the temperature of the system is maintained above the critical temperature of the gas, no liquefaction can ever occur for a pure substance, regardless of the pressure. The phase behavior of pure substances in the fluid state has been the basis of behavior of binary and complex mixtures (23).However, with the advent and development of experimental techniques, critical border curves were established to define for a system, the region of homogeneous fluid in which no phase separations occur. This behavior is illustrated in Figure 2 showing the border curve of various mixtures of the ethane-n -heptane system as determined experimentally by Kay ( 2 4 ) .In Figure 2 line AE is the vapor pressure curve of
Methane
Methane
Figure 3. Critical temperature and critical pressure behavior for the ternary system methane-ethane-n-butane.
ethane and point E is the critical point. Line BH has the same significance for pure heptane. The envelope E H represents the critical point locus for all possible mixtures of ethanen-heptane. The critical state behavior presented in Figure 2 for the ethane-n-heptane system is typical of binary systems exhibiting normal behavior. For abnormal mixtures consisting of polar and nonpolar components, the shape of the critical locus can change and quite often reverses in curvature to produce a locus exhibiting critical pressures for mixtures that are always below the critical points of the pure components. For example, the critical locus of the benzene-ethanol system (36)exhibits a continuous but opposite in curvature behavior to that presented in Figure 2. For ternary systems, the critical state behavior can be expressed by surfaces which, depending on the system, can become rather involved. For the methane-ethane-butane system Forman and Thodos (9)based on their experimental measurements and the data reported in the literature presented the critical temperature and critical pressure behavior for this system on triangular coordinates as shown in Figure 3. This figure serves to illustrate that methane, the lightest component of this ternary system which also happens to possess the lowest critical temperature and critical pressure of this system ( T , = 191 K, P, = 4.88 MN/m2) represents the lowest point on this critical surface. Therefore, pure ethane or pure n-butane, or mixtures of these two components, when diluted with methane will produce in the limit critical values approaching those of methane. For systems containing n components, an nth-degree surface is needed to properly describe the critical state behavior of such a system. However, complex mixtures may be assumed to consist of two components, the lightest component of the mixture representing the light component of a binary mixture and the remaining components taken collectively to represent the heavy component of the system. The background surrounding the critical state behavior of multicomponent mixtures offers an approach capable of vaporizing a component of low volatility, if the temperature of the system is in the vicinity of the critical point of the light component and if the pressure of the system is kept above the critical pressure. Depending on the nature of the components the critical state behavior of such systems can be simple or complex. For example, in the case of the methane-n-heptane binary system, the critical temperature vs. composition and the critical pressure vs. composition relationships are continuous over the entire composition range. This normal behavior results from the fact that the critical temperature of the light component, methane ( t , = -82.2 O C ) is higher than the freezing temperature of the heavier component, n-heptane
(tf = -90.6 "C). However, for systems where this condition is reversed, abnormal behavior arises for their critical state. In this context, for the methane-n-decane binary system, the critical temperature of methane ( t , = -82.2 O C ) is lower than the freezing temperature of n-decane (tf = -29.7 O C ) and therefore for compositions exceeding nl = 0.915, corresponding to temperatures lower than -29.7 O C , n-decane goes into the solid phase thereby causing a discontinuity in the critical state of this system as shown in Figure 4. It is therefore conceivable that a heavy hydrocarbon such as n-decane, tb= 174 "C (345 O F ) can exist in a homogeneous fluid state in the presence of methane, t b = -161.5 "C (-259 O F ) if the temperature of the system is kept above the freezing point of this heavy component, tf = -29.7 "C (-21.5 O F ) . The critical state behavior of the methane-n-decane system has been studied experimentally by Reamer et al. (33).Their results for the m'ethane-rich section of this system are presented in Figure 4. Upon reference to this figure, it follows that for a mixture containing 0.85 mole fraction methane and 0.15 mole fraction n-decane a t 148 O C (298 O F ) , and 30.4 MN/m2 (4410 psia), n-decane will not exist at these conditions as a liquid, but instead it will be maintained in a state of equilibrium between the two-phase region and the homogeneous fluid region of this binary system. If the pressure of the system is increased above P, = 30.4 MN/m2 (4410 psia), then the methane-n-decane mixture becomes supercritical with respect to pressure and therefore this mixture at these conditions ( t , = 148 "C, P, > 30.4 MN/m2) exists solely in the homogeneous fluid region. I t is this critical-state behavior that makes the supercritical gas extraction of coal a reality and the background associated with it becomes the key issue for the removal of high boiling constituents existing within the structural constitution of coal. State of the Art The background surrounding the behavior of pure substances and mixtures associated with them accounts for the well established fact that a gas can be liquefied by increasing the pressure, if the temperature is maintained below the critical temperature of the gas. This assumes no significant retrograde behavior for mixtures. For mixtures associated with retrograde phenomena, the cricondentherm of the mixture (the temperature of critical condensation) constitutes the temperature above which no liquid can exist. For such a case, a liquid phase can exist between the critical point of the mixture and its corresponding cricondentherm provided the pressure conditions are proper. If the temperature is above Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
211
Methane-n-Decane Binary
I
C..-.^_
a,,,sr,,
\
maximum pressure (cricondenbar)
maximum
t
e
ie,
L
a ,BiiY
R C O N I , H H . R H O I d T , B U S ~ q e , o nWNLacCy d
-100
0
Ind Enq Chem ~ , 1 5 2 6 1 1 9 4 2 1
31
02
04
03 n
05
06
07
Temperature 08
09
Memole Mole Fr0~11on
Figure 4. Critical state behavior of the methane-n-decane binary system.
the critical temperature of the gas, no liquefaction can ever occur regardless of the pressure. The phase behavior associated with a multicomponent system of a specified composition may be conveniently represented by a pressure vs. temperature diagram as shown in Figure 5 . The bubble point and the dew point lines corresponding to this composition, with increasing temperature and pressure, approach each other and converge a t the critical point of this mixture. The area enclosed by this border curve represents a two-phase region in which both liquid and vapor are present in equilibrium. This two-phase region separates the single-phase region outside of this border curve represented by the mixture existing in its dense state as liquid and its corresponding dilute state as vapor. The bubble-point curve, the dew point curve, and all the constant quality lines (constant liquid volume percent) meet a t the critical point. In contrast to the behavior of a single component, the critical point of mixtures is not necessarily tne point of highest pressure and temperature a t which vapor can co-exist. Instead, the point associated with the highest temperature on the dewpoint line, higher than the critical temperature is termed as the “cricondentherm” (critical condensation temperature). Similarly in the case of many mixtures, the bubble-point line passes through a point of maximum pressure, higher than the critical pressure, is termed as the “cricondenbar” (critical condensation pressure). The shaded areas represent regions of “retrograde condensation”. The term retrograde denotes phase changes in which the direction of the temperature and pressure change causing phase change is opposite to the normal behavior of mixtures a t low pressures. When a single-phase supercritical fluid a t point A is expanded isothermally, condensation will commence upon crossing point e, the dew point of the mixture to enter the two-phase region. Upon further expansion, condensation continues until point g is reached. Further decrease in pressure beyond point g, causes the process to reverse itself with vaporization commencing and continuing until point b is reached on the dew point line. At this point, all of the liquid formed from points e to g is vaporized and the mixture exists a t its lower dew point. The condensation occurring from e to g is termed as “isothermal retrograde condensation”. Similarly, the fluid at A may be cooled to point c a t which a bubble of the vapor will appear and vaporization continues until point f is reached. Further decrease in temperature from f to d results in normal condensation. The vaporization from c to f is termed 212
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
-
Figure 5. Vapor-liquid behavior of a multicomponent mixture in the region of the critical point.
as “isobaric retrograde vaporization”. Both types of retrograde phenomena occur only within the shaded regions, as shown in the figure. This retrograde behavior is not necessarily restricted to binary systems, but applies as well to a multicomponent system. Another unusual phenomenon unique to binary mixtures is the “barotropic effect”, which is observed in systems with restricted solubility of gases. When such mixtures are compressed isothermally, the gaseous phase can become denser than the liquid phase in equilibrium and thus sink to the bottom, thereby causing inversion of phases. This phenomenon was first observed in 1907 by Kamerlingh-Onnes and Keesom (20) for the helium-hydrogen system. Since then, this peculiar effect has been observed for several systems such as water-carbon dioxide, water-argon, ammonia-nitrogen, ammonia-argon, and others. The petroleum industry recognized the fact that the influence of pressure was a significant variable in the phase behavior of light and heavy hydrocarbons as far back as 1932 (26). In this connection, the condensation of a water-white hydrocarbon oil accompanying production of natural gas from the Big Lake Field, Texas, was explained on the basis that the vapor pressure of this liquid oil increased beyond its normal expectancy due to the presence and pressure of the natural gas in contact with it (23). These unexpected states of behavior constitute for mixtures the complex background surrounding the principles associated with retrograde condensation and supercritical gas extraction phenomena. Therefore, the present gas extraction scheme is based on the ability of a compressed gas existing in the vicinity of its critical temperature, but a t a pressure above its corresponding critical pressure to increase the volatility of high boiling substances with which it is in contact. Under favorable conditions, this enhancement in volatility of solid p-iodochlorobenzenein the presence of ethylene a t 15 “C and atmospheric pressure is so low that the concentration of this compound in the gas phase is only about 0.01 g/L. However, upon increasing the pressure of ethylene to around 10 MN/m2 (100 atm), this compound in the gas phase approaches a concentration of 50 g/L. The critical temperature of ethylene is 282 K (9 O C ) . This effect is reversible with the solid precipitating out upon pressure reduction (8). Although there is no limit placed on the pressure prescribed for the gas extractant, except that it must be above its corresponding critical pressure, the requirement for maximum effectiveness places the condition that the temperature of the system be near the critical temperature of this gas extractant.
Temperatures significantly higher than the critical temperature of the gas extractant do not necessarily favor this method of separation unless substantially high pressures are involved. This consequence follows from the fact that the solvent power of the compressed gas is greater the higher the density. Since the greatest density increase occurs a t the critical pressure of the critical isotherm, the needed high densities favoring extraction can be realized in the vicinity of the critical temperature of the extracting gas, but at supercritical pressure conditions. The involvement of temperatures exceeding the critical temperature of the extracting gas can introduce at these elevated pressures complex side phenomena recognized as gasgas phase equilibria. These phenomena were first predicted by van der Waals ( 4 4 ) ,who stipulated that if one component of a binary system is much more volatile than the other, the introduction of gas-gas phase equilibria a t temperatures exceeding the critical temperatures of the pure components becomes likely. Later in 1907, Kammerlingh-Onnes and Keesom (20) made a similar prediction and noted that this phase separation should be described as the immiscibility of two gas phases. However, it was not until 1940 that the first observation of this kind of behavior was noted by Krichevskii on the Na-NH3 system (25). Gas-gas phase equilibria were also predicted from theoretical arguments for binary mixtures by Temkin (39)using the van der Waals equation of state and deduced that gas-gas phase separation of components should be expected if
> 0.42b2
(1)
a1 < 0.053~2
(2)
bl
basic principles to develop a relationship for solids dissolving in compressed gases. The virial equation of state of a pure gas may be expressed as
PV _- 1 + -B+ - +C. (4) RT v v2 where B , C, . . . are the second, third. , . virial coefficients. For a binary mixture of gases, eq 4 may be written as PV BM C M -=1+-+-+.. RT v v2 where
+ X22B22 (6) CM = X13Clll + 3X12X&112 + 3X1X22C122 + X ~ ' C ~ Z Z B M = Xl2Bi1+ 2XIXzB12
(7) in the case where the gas solutions are dilute, one could assume the approximation that X 1 = 1,X 2 = 0 , then eq 6 and 7 may be written as
The chemical potential of a pure solid may be expressed as P Z b ) = P2*
+ pv,
+ RT In (RTc2O)
~ 2 = * ~ 2 '
where ro is the radius of molecular volume ( u = ?'qrro3), c and u are the parameters of the Lennard-Jones potential, xis the Boltzmann constant, and T is the temperature. For the sake of brevity, it is not deemed necessary to present a detailed discussion of gas phase immiscibility. It would only suffice to point the possible occurrence of this phenomenon in the supercritical gas-phase extraction of coal and should be taken into consideration from a design point of view. Semiquantitative T r e a t m e n t of Supercritical Gas Extraction A rigorous thermodynamic treatment of the supercritical region is not a straightforward issue even when dealing with the behavior of a single component. This behavior becomes more involved when dealing with binary mixtures and approaches a complex state for mixtures consisting of components that possess a diversity of volatilities. For its rigorous treatment, the behavior of systems at supercritical conditions brings into play the involvement of an equation of state to describe the behavior of mixtures consisting of the extractant gas (solvent), and the component (solute) or components to be vaporized in this supercritical region. In addition, the chemical potentials associated with the components of the mixture must be incorporated into the treatment of this extraction process. Rowlinson and Richardson (35)utilize these
(10)
where V , is the volume of the solid phase. When the pure solid is in equilibrium with its vapor
and
where ( 1 1 , bl and u2, b2 are the van der Waals constants of the light and heavy component gases, respectively. Tsiklis and Rott (41) have reviewed at considerable length the phenomena of a phase equilibria between immiscible gaseous phases. Rott ( 3 4 )defines p the criterion for layering which may be expressed as
(5)
The chemical potential may be expressed as P2O
112
(11)
of component 2 in the gas phase
+
p2(g)IRT = - In (RTc2) RT
+ 2B22*/V + 3C23*/2V2 (12)
where
when the pure solid is in equilibrium with its gas phase in the presence of component 1 at high pressure d g ) = P2b)
(15)
and
where a = c2/czo is a measure of enhanced solubility. Substituting PIRT from eq 5 into eq 16, results in In a =
v, - 2B12 V
where V , is the molar volume of the component being extracted and B12 is the cross second virial coefficient between the gas extractant and the component being extracted. A broad rationalization of the behavior of a system a t supercritical conditions can be described by eq 17 to depict the strong influence of pressure on the enhancement of the volatility parameter, a. Also, this relationship can be applied to extend arguments favoring effective means for enhancing the solubility of heavy constituents in the supercritical fluid phase region. Thus, with increasing pressure, the molar volume V decreases to effectively increase in an exponential manner the value of a. Since the density increase is greatest along the path Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
213
0 ul
cg 50t ,-
$ 5
.CA t = 4OoC P=40MN/mz
Temperature,
OK
Figure 8. Effect of temperature on solubility of naphthalene in ethylene a t different pressures.
Critical Temperature of Solvent Gas:C
Figure 6. Solubility of phenanthrene in a number of supercritical solvent gases.
3t
-Id
Od05
O.dl0
-.-
OA15
0.620
I q-moles
v
cms
Figure 7. Enhancement of naphthalene solubility with ethylene density at 285 and 318 K.
of the critical isotherm, it necessitates the involvement of system temperatures in the vicinity of the critical temperature of the gas extractant for the maximum recovery effectiveness of a solid or a liquid in the homogeneous gas phase existing at these supercritical conditions. The importance of the critical temperature of the gas extractant is illustrated in Figure 6. This figure shows results obtained when phenanthrene a t an extraction temperature of 313 K and a pressure of 40 MN/m2 (394 atm) was contacted at these conditions with the following gases: nitrogen, methane, carbon tetrafluoride, ethylene, carbon dioxide, and ethane. These gases play a significant role in establishing their corresponding extraction capability and it is not until ethylene ( T , = 283 K), carbon dioxide ( T , = 304 K), and ethane ( T , = 305 K) are used that the concentration of phenanthrene shows a marked increase in the gas phase at the temperature of the system of 40 O C (313 K). The influence of pressure on the extractive capability of ethylene on naphthalene has been studied extensively by Tsekhanskaya et al. (40). The results of these investigations are presented in Figure 7 as concentration of naphthalene in the gas phase against volumetric concentration for 285 K and 318 K. As suggested by eq 17, these plots are linear and represent a convenient method for expressing solubility results. Additional work is presented by Tsekhanskaya et al. (40) in the literature for the ethylene-naphathalene system to show that the concentration of naphthalene in the gas-phase in214
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
creases with temperature for pressures exceeding approximately twice the critical pressure of ethylene (P,= 5.03 MN/m2). Figure 8 presents the dependence of naphthalene concentration in ethylene on temperature for constant pressures of P = 6.1, 8.1, 10.1, 12.7, and 30.4 MN/m2. The relationships of this figure indicate that the solubility of naphthalene is not enhanced significantly until pressures in excess of 10.1 MN/m2 are exceeded. The gas-phase concentration of naphthalene increases with temperature a t high pressures while a t low pressures it decreases with temperature. This is due to the temperature effect on a and cz0. The volatility term N decreases with temperature while c ~ O ,the molar concentration of the solute in its pure state, increases with temperature. Thus, at P = 30.4 MN/m2, the concentration of naphathalene increases from c = 20 g/dm3 at 285 K to c = 150 g/dm3 at 320 K. The critical temperature of ethylene is 282
K. These experimental results point to the fact that the gas extraction technique is based on the solvent capability of compressed gases at temperature and pressure conditions which permit the solvation of organic constituents. This solvent effect of compressed gases was discovered by Hannay and Hogarth as early as 1897 (19). Hagenbach (18)successfully demonstrated solvent properties of dense polar gases existing at supercritical conditions. The work of Tsekhansk@yaet al. (40) for the ethylenenaphthalene system exhibits increase in the gas-phase concentration of naphthalene with density at supercritical temperatures as shown in Figure 7. Paul and Wise (31) have shown from theoretical considerations that the volatility enhancement is greatest when the temperature at which the extraction is carried out is near the critical temperature of the extracting gas. Conversely, if the extraction temperature is determined by other considerations, this relationship guides the selection of the gas to be used. Solvent Extraction of Coal The process of solvent extraction with a supercritical gas may be broken down into the following five steps: (1)penetration of the coal micropore structure by the solvent gas; (2) depolymerization of large molecular aggregates and the dissolution of the resulting products in the solvent gas; (3) breaking up of the molecular bonds betweell the molecular species to be extracted and the coal structure; (4) diffusion of the extract and the solvent gas from the coal micropore structure; (5) recovery of extract from the solvent gas as a precipitate or condensate by reduction in pressure. Most of the hydrocarbon molecules on extraction have to pass through the micropores between the sheets of molecules. This is evidenced by the fact that most of the hydrocarbons
are extracted with difficulty and the order of extraction is governed by the molecular structure and size (43).The ability of the molecules to penetrate into the micropore structure of coal is well known. This sieve effect does not yield distinct separations as in the case of inorganic molecular sieves with regular size spherical pores. The pores in coal differ widely in size and shape and are slit-like or oval rather than spherical in shape. Therefore, it is very unlikely that the penetrating molecules would fit neatly into the channels of width corresponding to their diameter. Size and shape of a solvent molecule is important in determining the ease of extraction of hydrocarbons. Thin molecules such as benzene and tetralin are relatively effective in spite of the fact that their critical diameters may exceed the size of the slit shaped micropore (43). The mechanism of gas flow through the coal structure is of considerable importance in the case of supercritical gas phase’ extraction. The mechanism of gas flow through coal could be a combination of molecular diffusion through the small pores and bulk diffusion through large pores, or permeation through the fracture system of the coal. Karn et al. (21,221 have reported investigations of gas transport through sections of solid coal. The conclusions report that these types of flow are influenced by the molecular weight and the gas, temperature, pressure, and the internal coal structure of the individual sample. The flow of gases in porous media possesses other distinct characteristics which distinguish it from the flow of liquids. Gases have a tendency to adsorb on the pore surface, and the permeabilities observed with highly absorbing gases greatly exceed those obtained with nonadsorbing gases. This excess permeability has been attributed to surface diffusion in the adsorbed phase. The complexities arising from the compressibility of gases must also be taken into consideration. Compressibility of gases leads to unsteady-state gas flow in porous media. Additional factors seem to be of considerable importance. In solvent extraction when small amounts of polar solvents, e.g. alcohols, are added to less polar ones such as paraffins and aromatic hydrocarbons, the extraction rate is increased. Polar solvents exert a stronger dissociating or depolymerizing action on coal than nonpolar solvents. For example, anhydrous glycerol was not extracted by ethylene or propane, but it was readily extracted by ammonia as might be expected from the polar nature of the ammonia molecule ( 3 1 ) .In general, polar gaseous molecules can be expected to penetrate the coal gel if: (1)the gel contains polar groups, (2) the gas molecules are sufficiently small, and (3) the gel does not possess a high order of orientation of crystallinity ( 2 7 ) . Although coal seems to possess little order from x-ray measurements ( 7 ) , in some of the higher rank coals, the presence of condensed structures might form to some extent ordered regions. On adding a polar gas to the system, competitive reactions result so that a given polar group can either bond intermolecularly with a polar group on a neighboring molecule adsorbed on the surface of the coal structure or a gas molecule. If the polar gas forms stronger bonds with any given group then the intermolecular bonds between adjacent molecules can be broken. The molecules in order to be desorbed would have to overcome the energy barrier due to the adsorption forces responsible for their retention in the pores. When polar molecules are involved, there is mutual interaction between the adsorbed molecules and the solvent gas molecules and also between the solid structure and the solvent gas. These interactions would be of considerable importance due to their influence on the adsorptive retention forces. The increased solvent effect of polar and nonpolar gases in the supercritical region is well known. Also, pressure extraction at high temperatures might open up the pore structure and thus increase the rate of diffusion of the solute and the
solvent molecules into and out of the pores and thus enhance their solvent action (32).
Advantages of Supercritical Extraction (1) Unique properties of the supercritical gases enable the penetration of the coal structure and permit the release of constituents which were not recoverable without thermal degradation. (2) The solvent power of a compressed gas can be continuously varied by altering its pressure. Physical properties, rather than the chemical nature of the gas extractant, are significant. I t should prove expedient to use gas mixtures as solvents, rather than pure gases. (3) Supercritical gas extracts undergo very little, if any chemical degradation during the extraction stage. The chemical structure of the extract is basically unaltered from its original structure within the coal framework preceding the extraction step. (4) The mildness of extraction is significantly beneficial in the case of heat labile compounds since this technique enables their extraction a t temperatures well below those a t which thermal degradation sets in. ( 5 ) The extraction mechanism is reversible. The extract can be separated from the solvent by a simple reduction in pressure. Under such conditions the extracting gas and the extract possess low density and viscosity, thus enabling the separation of undissolved char residue from the “coal solution” without filtration. (6) Some fractionation of the extract can be accomplished by stepwise reduction of pressure. (7) The extraction temperature can be chosen to obtain a controlled amount of breakdown of coal to yield the desired chemicals. The temperatures of extraction are determined by the type of coal and the molecular species desired. (8) Pressure extraction a t elevated temperatures helps to open up the pore structure and increase the rate of diffusion of the solvent and the solute molecules out of the coal micropore structure thereby enhancing the solvent action. (9) It has been experimentally demonstrated that a low sulfur bearing extract results from the supercritical extraction of coal ( 4 6 ) .This technique permits the utilization of high sulfur containing coal to obtain liquid extracts of low sulfur content. (10) The presence of water or moisture in coal could be exploited to advantage. Water being a polar compound would perhaps serve as an effective solvent at supercritical state conditions. (11) It is possible to combine some degree of hydrogenation in the extraction stage. (12) Chemical feedstocks, especially benzene and a variety of alkyl hydrocarbons, can be derived from the extract by reduction of molecular weight. (13) The undissolved char residue can be further processed for the production of hydrogen by its gasification with steam. This hydrogen can then be used for further processing of the extract. Comparison with Liquid Extraction (1) The gas-phase concentrations obtained in gas extraction are higher than those obtained from distillation, but are generally smaller than the concentration of the solute obtained with liquid solvent. (2) Better separation of the solvent gas from the extract and the residue resuits in lower contamination of the products and lower solvent losses. The separation of undissolved material from coal solution is easier, since gas densities and viscosities are considerably lower than those encountered in conventional solvent extraction. (3) Separation of the solvent from the extract is accomInd. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
215 r
plished easily as a result of the large differences in their volatilities and virtually complete recovery of the solvent is effected. (4) The coal extracts are richer in hydrogen (6,Wocompared to 4.9%) and have lower molecular weights (500 compared to 2000) than those obtained using anthracene oil type solvents in the absence of hydrogen gas and may therefore be readily converted to hydrocarbon oils and chemicals (46). ( 5 ) The solvent power of the gas can be varied by controlling the pressure. To vary the solvent power of liquids it is necessary to change either their temperature or to mix them with another solvent gas. (6) Lower solvent losses in the residue result when gas extraction is used compared to solvent extraction.
Applications of Gas-Phase Extraction Enhanced solvent effect of compressed gases has been applied in gas chromatography to overcome severe limitations imposed by the requirement of volatility. Giddings and coworkers (12, 13, 14, 28, 29) have successfully developed gas chromatographic techniques using supercritical gases as carrier fluids, thereby enabling determination of new useful data on the equilibrium between complex substrates and compressed gases. Zhuze and Yushkevich (50) have reported investigations on the stripping of crude oil with methane which indicate that gas-phase extraction would prove invaluable for deasphalting petroleum fractions. Zhuze (49) extended the same basic principles to the extraction of ozokerites (high-melting paraffins and other hydrocarbons with long alkyl chains) from naturally occurring mineral ores using a propane/propylene mixture as a gas extractant. Wise (48) reported application of the gas extraction to the conventional distillation of coal tar to obtain pitch. Weale (45) has discussed the dissolution of polymers in compressed gases as a general phenomenon. Gas extraction technique has been employed as a purification method for the recovery of a purified oil from waste gear oil using a gas as an extractant. This technique has also been employed for the separation of naphthalene from anthracene. Several applications have been reported elsewhere in the literature. Ethylene was used for a wide range of substrates such as paraffin oil, silicone oil, dibutyl ether, benzyl alcohol, 2-ethyl hexanol, aniline, nitrobenzene, and camphor. Quite a number of applications have been reported in the food processing industry. Ethylene was used to extract egg yolk, milk, and vegetable oils. A broad spectrum of applications of the gas-phase extraction presented reveals the tremendous potential of this mode of extraction (311. Conclusions A relatively new separation technique based on the enhancement of volatility by compressed gases has been reviewed. This mode of extraction offers a viable alternative for liquid solvent extraction when the solvent losses have to be minimized. This method also enables processing of heat-labile substances which undergo decomposition during distillation. This technique promises to have considerable potential in the energy related areas. Supercritical gas phase extraction could be successfully exploited for the extraction of coal to produce chemical feedstocks and liquid fuels and for the extraction of oil from shale. Nomenclature B,, C, = second and third virial coefficients for the i t h component B M ,CM = second and third virial coefficient for the mixture B,, = second cross virial coefficient a,, b, = van der Waals constants 216
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
c = concentration of the solute P = pressure R = universal gas constant ro = radius of molecular volume T = temperature V = volume V , = molar volume of the component being extracted X i = mole fraction of the i t h component Greek letters N = c/co = measure of volatility enhancement @ = criterion for layering t , u = Lennard-Jones potential parameters x = Boltzmann constant p = chemical potential
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Receiued for reuiew February 25,1977 Accepted May 11,1977
