Relationship between refractive indices and other ... - ACS Publications

Jan 1, 1987 - Relationship between refractive indices and other properties of coal hydrogenation distillates. Curt M. White, Mildred B. Perry, Charles...
0 downloads 0 Views 945KB Size
Download PDF
Energy & Fuels 1987,1,99-105

99

Relationship between Refractive Indices and Other Properties of Coal Hydrogenation Distillates? Curt M. White,* Mildred B. Perry, Charles E. Schmidt, and Louise J. Douglas Division of Coal Science, Pittsburgh Energy Technology Center, Pittsburgh, Pennsylvania 15236 Received August 12, 1986. Revised Manuscript Received September 29, 1986

The development of new correlations that enable prediction of the physical, chemical, and thermodynamic properties of coal liquids based on simple and modest characterization is important to researchers in coal liquefaction. The present investigation identifies mathematical relationships among refractive index, mid-boiling point, molecular weight, density, and aromaticity of coal-derived distillates. Mathematical equations are obtained that describe relationships among these properties. When compared with those presently in the literature, they appear to be more reliable when used for coal liquids. Two equations are developed that allow the estimation of refractive index of a coal liquid on the basis of measurement of the mid-boiling temperature, molecular weight, and density. One equation is used for coal products having molecular weights less than 200 and another for those having molecular weights 200 or greater. Equations are developed that are useful for the estimation of the molecular weight of coal-derived distillates. An equation is developed that allows estimation of aromaticity on the basis of determination of the atomic hydrogen to carbon ratio and refractive index. This investigation shows that reliable correlations can be developed that fit the measured properties of narrow-boiling-rangedistillates of coal liquefaction products from various processes and feed coals.

Introduction Information concerning the fundamental physical, chemical, and thermodynamic properties of coal liquids is needed to properly design and operate coal liquefaction plants. A large effort is aimed at determining or estimating such properties for narrow-boiling-range distillates derived from coal liquids and for individual pure compounds representative of those in coal liquefaction products. A goal of this research is to construct a synthetic fuels data base of property measurements for well-defined coal liquids from a variety of liquefaction processes and feed coals and subsequently to develop correlations that will allow estimation of their physical, chemical, and thermodynamic properties. Estimation of unmeasured or difficult-to-measure properties is possible by using a variety of approaches. One approach is to derive predictive equations based on measured inspection properties such as density ( d ) , mid-boiling point (Tb), number-average molecular weight (M,) (henceforth referred to as molecular weight), and refractive index ( n ) . Even though many correlations are derived empirically, they are often useful, particularly for developing on-line measurement techniques used for process control. As an example, refractive index can be precisely and accurately measured in a continuous mode. The relationships between refractive indices and other physical and chemical properties of petroleum distillation fractions have been thoroughly discussed and documented;l+ thus, correlations relating refractive index with other properties may be valuable for process control. The relationship between physical properties and constitution of petroleum fractions has been the subject of considerable research. Some classical analytical methods for the determination of the percent aromatic, naphthenic, and paraffinic carbons in viscous petroleum fractions are

* Author to whom correspondence should be addressed. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the U.S.Department of Energy.

based on the refractive index and the viscosity-gravity constant of the f r a ~ t i o n .Additionally, ~ Huang formulated correlations to predict the refractive index of petroleum fractions by using mid-boiling point, molecular weight, and density data.avg Huntjens and van Krevelen observed a similar relationship between the carbon content and refractive index of various vitrinites.1° McCartney and Teichmuller have established excellent correlations between reflectance and both the volatile matter of coal'l and the ratio of atomic hydrogen to carbon in coal.12 Until now, these relationships have not been described for coal hydrogenation products. This paper will illustrate the relationships between refractive index and mid-boiling point, density, molecular weight, aromaticity, and atomic hydrogen to carbon ratio for distillates of coal hydrogenation products. The interrelationship of each of these properties will be discussed relative to an H-Coal product and to three coal liquid products from the Wilsonville Advanced Coal Liquefaction Test Facility. In particular, relationships are developed that allow the estimation of refractive index based on measurement of mid-boiling point, molecular weight, and density. Relationships are developed for distillates of coal liquefaction products that estimate the molecular weight (1)Hill, J. B.;Henderson, L. M.; Ferris, S. W. Ind. Eng. Chem. 1927, 19, 128-130. (2)Kurtz, S. S.,Jr.; Ward, A. L. J. Franklin Inst. 1936,222,563-592. (3)Kurtz, S.S.,Jr.; Ward, A. L. J. Franklin Inst. 1937,224,583-601. (4)Kurtz, S.S., Jr.; Headington, C. E. Ind. Eng. Chem., Anal. Ed. 1937,9,21-25. (5)Gooding, R. M.; Adams, N. G.; Rall, H. T. Ind. Eng. Chem., Anal. Ed. 1946,18,2-13. ( 6 ) Kurtz, S. S., Jr.; Mills, I. W.; Martin, C. C.; Harvey, W. T.; Lipkin, M. R. Anal. Chem. 1947,19,175-182. (7) Kurtz, S. S., Jr.; King, R. W.; Stout, W. J.; Partikian, D. G.; Skrabek, E. A. Anal. Chem. 1956,28,1928-1936. (8) Huang, P. K. Ph.D. Thesis, Department of Chemical Engineering, The Pennsylvania State University, University Park, PA, 1977. (9)Riazi, M. R.; Daubert, T. E. Ind. Eng. Chem. Process Des. Deu. 1980,19,289-294. (10)Huntjens, F.J.; van Krevelen, D. W. Fuel 1954,33, 88-103. (11)McCartney, J. T.; Teichmuller, M. Fuel 1972,51,64-68. (12)McCartney,J. T.;Ergun, S. Bull.-U.S. Bur. Mines 1967,No. 641, 1-49.

This article not subject to U.S.Copyright. Published 1987 by the American Chemical Society

100 Energy & Fuels, Vol. 1, No. 1, 1987 f r o m mid-boiling p o i n t , Tb,and specific gravity, S. A n e q u a t i o n is also presented that relates molecular weight t o refractive index, density, and mid-boiling point. The accuracy of these equations is compared and c o n t r a s t e d w i t h estimation methods currently available. Lastly, the relationships between aromaticity, fa, and atomic hydrogen to c a r b o n r a t i o and refractive i n d e x are mathematically expressed. These simple relationships are shown to be useful for e s t i m a t i o n of aromaticity.

Experimental Section The H-Coal liquid was produced a t the H-Coal Pilot Plant a t Catlettsburg, KY, while processing Illinois No. 6 coal in the synfuel mode of 0perati0n.l~ A blend of “light oil” and “heavy oil” products (k1.5) was distilled by Chevron into narrow-boilingrange di~ti1lates.l~Specifically, 28 K boiling-range fractions were obtained over the boiling range from 478 to 728 K; the initial boiling point (IBP) to 478 K fraction and the 728 K vacuum bottoms were also obtained. Wilsonville Thermal Hydrocracker and Catalytic Hydrotreater products were formed while processing a bituminous coal (Burning Star Mine, Illinois No. 6) during run 245 on Dec 14, 1983, at the Wilsonville Advanced Coal Liquefaction Test Facility, located in Wilsonville,AL. These materials were further distilled at PETC by using a Hyper-Cal precision fractionating apparatus. In the case of the Thermal Hydrocracker product, 28 K boiling-range fractions were obtained over the boiling range from 533 t o 783 K; the IBP to 533 K fraction and the 783 K vacuum bottoms were also obtained. In the case of the Catalytic Hydrotreater product, 28 K fractions were obtained over the boiling range from 505 to 672 K; the IBP to 505 K fraction and the 672 K vacuum bottoms were also obtained. A sample from the Wilsonville Thermal Hydrocracker product formed while processing a subbituminous coal (Clovis Point) during run 246 on May 30,1984, was similarly distilled. For this Thermal Hydrocracker product, 28 K boiling-range fractions were obtained over the boiling range from 561 to 727 K; the IBP to 561 K fraction and the 727 K vacuum bottoms were also obtained. The refractive indices a t 293 K, elemental analysis, water content, specific gravity a t 289 K, density a t 293 K, and number-average molecular weights of each distillate were measured by Huffman Laboratories, Inc. (Wheat Ridge, CO). The refractive index was determined at 293 K with a Bausch and Lomb Abbe-3L refractometer according t o ASTM Method D-1218.15 The refractive index of l-bromonaphthalene was measured before the refractive index of each distillate. Each refractive index measurement of l-bromonaphthalene was within f0.0005 units of the actual value. The mid-boiling point of each distillation fraction was taken as that temperature a t which 50% of the material had distilled as determined by ASTM Method D-2887-73.16 The determinations of carbon and hydrogen were performed as described by Houde and Champy.” Nitrogen was measured by using the method of Merz.ls Oxygen was determined directly by pyrolyzing a weighed sample in nitrogen a t a controlled rate. The pyrolysis products were passed over carbon a t 1393 K to convert all the oxygen to carbon monoxide (CO). After being scrubbed to remove potential interferences, the CO was oxidized (13) Ashland Synthetic Fuels, Incorporated, H-Coal Pilot Plant; final report on U S . DOE Contract No. DE-AC05-76ET10143,March 1984. Available from the National Technical Information Center, U.S. Department of Commerce, Springfield, VA. (14). Sullivan, R. F. ‘Refining and Upgrading of Synfuels from Coal and Oil Shales by Advanced Catalytic Processes”; monthly report for February 1983, US. DOE Contract No. DE-AC22-76ET10532(formerly Contract No. EF-76-C-01-2315).Available from the National Technical Information Center, U S . Department of Commerce, Springfield, VA. (15) ASTM Method D-1218, ”Standard Test Method for Refractive Index and Refractive Dispersion of Hydrocarbon Liquids”; Annual Book of ASTM Standards, Part 23, Petroleum Products and Lubricants;

ASTM Philadelphia, 1982. (16) ASTM Method D-2887-73, “Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography”; Annual Book of ASTM Standards, Petroleum PToducts, Lubricants, and Fossil Fuels; ASTM Philadelphia, 1983; Vol. 05.02, p 791. (17) Houde, M.; Champy, J. Microchem. J. 1979, 24, 300-309. (18) Merz, W. Fresenius’ 2. Anal. Chem. 1968,237, 272-279.

White et al. to C02 and passed into a C 0 2 coulometer for quantification. A Fisher Model 470 sulfur analyzer was used for sulfur analysis. The sample was combusted in oxygen at 1573 K, and the SOzproduced was titrated with iodine. Karl Fischer water determinations were performed as described by Mitchell and Smith.lS Sodium tartrate dihydrate was used for standardization of the system. Huffman Laboratories states the precision of the Karl Fischer water determinations to be within &0.1% (absolute) of the measured value for samples having less than 1%water. The relative accuracy of the Karl Fischer water determinations performed by Huffman Laboratories is &lo% for the sodium tartrate dihydrate standard. The specific gravity and density of samples were measured by using a pycnometer that had been previously standardized with boiled deionized water at the same temperature used for sample determinations. The number-average molecular weight of each pseudocomponent was measured by vapor pressure osmometry (VPO) using a Knauer cryoscope. The measurement of number-average molecular weight by VPO was similar for each set of distillates and is described in detail for the H-Coal distillates. The measurements were made in either methylene chloride a t 303 K or pyridine a t 353 K. Specifically, the H-Coal distillates boiling from 478 to 616 K (400 to 650 O F ) were measured in methylene chloride, and the H-Coal distillates boiling above 616 K (650 OF) were measured in pyridine. The VPO determinations were made as four-point measurements over sample concentrations ranging from 2.50 to 24.10 mg/mL. The number-average molecular weight at zero concentration was estimated by a linear extrapolation. Specifically, a plot of sample concentration (C) vs. the change in temperature divided by sample concentration ( A t / C ) was constructed for the four points and extrapolated to zero by using a linear least-squares regression. The same procedure was used to measure the average molecular weight of a sample of high-purity benzil, analyzed just before the sample. The plot of C vs. A t / C for benzil was extrapolated to zero benzil concentration and multiplied by the molecular weight of b e n d This arithmetic product is divided by the A t / C a t zero sample concentration to provide the number-average molecular weight of the sample at zero concentration. When this technique is applied to standards it has an accuracy of &5% of the true value. The average molecular weights of the H-Coal distillates were also measured by freezing point depression and low-voltage, high-resolution mass spectrometry to confirm the results obtained by VPO. The cryoscopic molecular weights were obtained by using a Knauer cryoscopic apparatus with freshly distilled nitrobenzene. The samples were weighed to the nearest microgram t o provide concentrations of approximately 0.05 M (calculated by using the molecular weight found by VPO). Multiple measurements of molecular weight were made for each H-Coal distillate. The ‘H NMR spectra of the H-Coal and Wilsonville distillates were obtained on a Varian XL-100-15 spectrometer equipped with a deuterium lock. The samples were dissolved in CD,C12 (99.9% D) containing a trace of tetramethylsilane, filtered through glass wool, and placed in 5-mm-0.d. NMR tubes. The lH NMFt spectra were recorded at 303 K by using a spectral width of 1200 Hz and an accumulation time of 3.4 s following a 30° pulse. Twenty-five transients were accumulated with a pulse delay of 10 s. The free induction decay was exponentially weighted, employing a negative time constant of 1 s. The NMR data were used to calculate the aromaticity (fa) of the samples by using the Brown and Ladner treatment.20 Statistical treatment of the resulting analytical data was. performed on a VAX-11/780 computer employing the SAS statistical package available from the SAS Institute, Inc., Cary, NC.

Results and Discussion The mid-boiling point (Tb, K), specific gravity at 289 K ( S ) , ratio of a t o m i c hydrogen t o carbon (H/C) o n a d r y basis, and the aromaticity (fa) of the H-Coal a n d Wilsonville distillates are contained in Table I. T a b l e I1 lists the average values, standard deviations, a n d n u m b e r of de(19) Mitchell, J., Jr.; Smith, D. M. Aquametry, 2nd ed., Part 111; Wiley: New York, 1980; Vol. 5, pp 107-136. (20) Brown, J. K.; Ladner, W. R. Fuel 1960, 39, 87-95.

Energy &Fuels, Vol. I, No. 1 , 1987 101

Coal Hydrogenation Distillates I

I

I

I

1

1

I

I

I

1 778

-

711

-

644

-

578

-

511

-

b'

r

I

1

I

I

bo

'

I

I

I

I

I

I

I

I

I

I

1.54 1.58 1.62 REFRACTIVE INDEX (n)

I

I

I

I.66

I

= t L

1.10

1.02

0.94 I

1.54 I .58 I .62 REFRACTIVE INDEX (n)

K

I

0.70 -

r

A

I

I

I

I

I

I

I.66

I

W l t l o n i l l l ~C a l a l y I I ~HydiOlia.1er d1s1111aI.~ 245 0 = Wll~onrlll. Thermal Hydrocrickel dlillllilc% 245 0

~

i

WIIsonrlll. 1h.m"

Hydimracker d1s1111a1~s. 246

0 = H.CoaI d l s l l l l ~ l ~ s

0 = n.Coal d111111al.n

t

H/C=1.2310 ~ 0 . 5 3

-

W

E- 0.30 d'

i.bo

'

1

I

I

1.54 1.58 1.62 REFRACTIVE INDEX (n)

I

I

I.66 REFRACTIVE INDEX (n)

Figure 1. Linear least-squaresregression lines illustrating the relationships between refractive index and (a) number-averagemolecular weight, (b) mid-boiling point, (c) density, (d) aromaticity, and (e) atomic hydrogen to carbon ratio for 28 K boiling range distillates from H-Coal and Wilsonville: H-Coal(@);Wilsonville Thermal Hydrocracker product from run 245 (0); Wilsonville Catalytic Hydrotreater The f a and H/C values adjacent to the regression product from run 245 (A); Wilsonville Thermal Hydrocracker product from run 246 (0). lines are the f a and H/C values of the original coal liquids that the distillates were derived from.

terminations of the molecular weight (M,,) determined by VPO, density as g/mL at 293 K ( d ) , and refractive index (n)at 293 K for the H-Coal and Wilsonville distillates. The values given in Table I1 provide an estimate of the precision of the measurements. The grand average absolute percent relative standard deviation of molecular weight, density, and refractive index values in Table I11 are l . l 6 % , 0.12%,and 0.02%, respectively. Linear relationships between some of these properties and the refractive index are illustrated in Figure 1. The f a and atomic hydrogen to carbon ratio values adjacent to the regression lines in Figure 1 are the values for the original coal liquids from which the distillates were derived. For each of the four sets of distillates, the lines illustrated in Figure 1 were

obtained by linear least-squares regression. The resulting slopes, intercepts, and correlation coefficients are given in Table 111. Although some of the relationships illustrated in Figure 1 are well documented for petroleum products, to our knowledge, they!ave not been reported previously for coal liquefaction products. Researchers at the former Bartlesville Energy Research Center (BERC) have reported values for the refractive indices of distillate fractions from H-Coa121 and the (21)Holmes, S. A.; Woodward, P. W.; Sturm, G. P., Jr.; Vogh, J. W.; Dooley,J. E. "Characterization of Coal Liquids Derived From The H-Coal Process"; BERC/RI-76/10,1976. Available from the National Technical Information Center, US. Department of Commerce, Springfield, VA.

102 Energy & Fuels, Vol. 1, No. 1, 1987

White et al.

Table I. Boiling Range and Average Values’ of the Mid-Boiling Point (Tb), Specific Gravity at 289 K (S), Atomic Hydrogen to Carbon Ratio (H/C), and Aromaticity (f.) of the H-Coal and Wilsonville Distillates

s

H/C

fa

0.9472 0.9499 0.9545 0.9820 0.9901 1.0220 1.0691 1.0464

1.37 1.35 1.36 1.23 1.28 1.20 1.08 1.10 1.07

0.49 0.49 0.48 0.56 0.54 0.58 0.65 0.64 0.65

Wilsonville (245)Thermal Hydrocracker 1.40 533-561 (500-550) 547.05 (525.02) 0.9632 561-589 (550-600) 574.85 (575.06) 0.9717 1.40 589-616 (600-650) 599.85 (620.06) 0.9847 1.39 1.35 616-644 (650-700) 625.95 (667.04) 0.9991 644-672 (700-750) 663.75 (735.08) 1.0264 1.28 672-700 (750-800) 699.25 (798.98) 1.0468 1.23 700-728 (800-850) 722.55 (840.92) 1.0661 1.19 728-755 (850-900) 737.55 (867.92) 1.0820 1.14 755-783 (900-950) 762.55 (912.92) 1.10

0.43 0.44 0.44 0.48 0.51 0.54 0.57 0.59 0.59

Wilsonville (245)Catalytic Hydrotreater 1.59 (450-500) 518.15 (473.00) 0.9069 (500-550) 543.75 (519.08) 0.9242 1.56 (550-600) 569.85 (566.06) 0.9404 1.55 (600-650) 595.95 (613.04) 0.9530 1.49 (650-700) 619.25 (654.98) 0.9683 1.48 (700-750) 650.95 (712.04) 0.9792 1.43

0.31 0.32 0.35 0.36 0.39 0.39

Wilsonville (246)Thermal Hydrocracker 1.38 561-589 (550-600) 571.45 (568.94) 0.9719 589-616 (600-650) 594.85 (611.06) 0.9781 1.36 616-644 (650-700) 622.05 (660.02) 0.9873 1.33 644-672 (700-750) 647.05 (705.02) 1.0150 1.24 672-700 (750-800) 665.75 (738.68) 1.0257 1.24 700-728 (800-850) 693.15 (788.00) 1.0442 1.23

0.46 0.46 0.49 0.54 0.54 0.56

boiling range, K

(OF)

478-505 (400-450) 505-533 (450-500) 533-561 (500-550) 561-589 (550-600) 589-616 (600-650) 616-644 (650-700) 644-672 (700-750) 672-700 (750-800) 700-728 (800-850)

505-533 533-561 561-589 589-616 616-644 644-672

Tb, K

(OF)

H-Coal 493.75 (429.08) 513.55 (464.72) 541.32 (514.70) 567.05 (561.02) 598.15 (617.00) 623.15 (662.00) 646.63 (704.26) 666.75 (740.48) 694.25 (789.98)

‘These are average values based on multiple measurement of each property. In some cases, the unrounded values are presented because these were the values used in the regressions.

Char-Oil-Energy Development (COED) processz but made no mention of the relationship between refractive index and other properties of the distillates. Each coal-derived liquid studied atBERC and those studied here showed an approximately linear relationship between boiling point (either cut temperature or mid-boiling point) and refractive index, but the slope and intercept of the line describing the linear relationship are unique for each sample studied. Huang@and Riazi and Daubedg have developed mathematical equations that describe the relationship among these properties for petroleum fractions. From their correlations, the refractive indices of petroleum distillates (M, S 200) are predicted with an average absolute deviation of approximately 0.5%,and the refractive indices of petroleum distillates (M, 2 200) are predicted with an average absolute deviation of 0.2%. From the Riazi and Daubert relationships, the refractive indices of the distillates from the four coal liquefaction products were predicted and are presented in Table IV (supplementary material) with the absolute percent deviation from the experimental values. The refractive indices in Table IV were calculated by using number-average molecular weights determined by vapor pressure osmometry (VPO). (22)Sturm, G. P.,Jr.; Woodward, P. W.; Vogh, J. W.; Holmes, S. A.; Dooley, J. E. “Analyzing Syncrude from Western Kentucky Coal”; BERC/RI-75/12,1975. Available from the National Technical Information Center, US. Department of Commerce, Springfield, VA.

The average percent absolute deviations from the Riazi and Daubert relationships were 0.67 for distillates with M, < 200 and 0.80 for distillates with M, I200. The average absolute percent deviation from the Riazi and Daubertg equations is less for petroleum-derived distillates than for coal-derived distillates. This is not surprising, since the equations were developed for petroleum products, not for coal liquefaction products. To increase the accuracy of the predicted refractive indices, the experimentally determined refractive indices, mid-boiling point (in degrees Rankine, OR), density, and molecular weight from the H-Coal and three sets of Wilsonville distillates were subjected to multiple linear least-squares regression analysis. Equations 1 and 2 were developed to estimate refractive indices. The refractive for M ,

< 200, n

= 0.1234Tb0.5045(Mn/d)4,1814 (1)

for M , I200, n = 0.0909Tb0~5761(Mn/d)-0~2182 (2) indices of the H-Coal and Wilsonville distillates were calculated by using eq 1and 2 (see Table IV, supplementary material). The average absolute deviation between calculated and experimental refractive indices from eq 1 was 0.24%; from eq 2, it was 0.47%. Thus, for the coalderived distillates examined, eq 1and 2 provide estimates of refractive index that are in better agreement with the experimentalresults than those obtained by using the Riazi and Daubert equations. It should be noted that these equations can be rearranged and solved for Tb,M,, or d. This is important because these other parameters may be significantly harder to measure than refractive index. The molecular weight of a coal liquid, or any fuel, is an important property that impacts other physical, chemical, and thermodynamic properties. Consequently, many investigators have developed correlations, based on easily measured inspection properties, that allow the estimation of molecular weight. In fact, three equations, those of Brule et al.,23Gray et al.,24and Wilson,25have been developed specifically to estimate the molecular weight of coal-derived materials. These methods correlate molecular weight with boiling point and/or specific gravity. Because substantially more inspection property data are available herein, several new relationships that allow the estimation of molecular weight have been developed. Equation 3

M , = (4.059 x 10*)Tb2~5497S-1.4002

(3)

relates molecular weight to mid-boiling point (in degrees Rankine) and to specific gravity at 289 K, as previous equations have done. Equation 3 was formulated by multiple linear regression of the appropriate inspection property data from the H-Coal and Wilsonville distillates, and estimates the molecular weights of these distillates with an average absolute deviation of 1.8%. For the correlations presented herein to be useful, they must be based on data of high precision and accuracy. The precision of some of the measurements used are presented in Table 11. The accuracy of the measurements can not in most cases be addressed because a true value needed for comparison with the experimental value is not available. Nevertheless, by measuring the same property by a variety of techniques, it is possible to mutually confirm (23)Brd6, M.R.:Lin, C. T.;Lee, L. L.; Starling,K. E. AIChE J. 1982, 28,616-625. (24) Gray, J. A.; Brady, C. J.; Cunningham, J. R.; Freeman, J. R.; Wilson, G. M.Znd. Eng. Chem. Process Des. Deu. 1983,22, 410-424. (25)W b n , G. M.Foundations of Computer-Aided Chemical Process Design: Mah. R. S.H.. Seider. W. D... Eds.:. Engineering Foundation: New YO&, isai;VOI. 11, i p . 31-51.

Energy &Fuels, Vol. 1, No. 1, 1987 103

Coal Hydrogenation Distillates

Table 11. Average Values," Standard Deviations and Number of Determinations of the Number-Average Molecular Weight ( M , ) As Determined by VPO, Density at 293 K ( a ) ,and the Refractive Index at 293 K (n)of the H-Coal and Wilsonville Distillates boiling range, K (OF) fin std dev meas std dev meas il std dev meas

a

H-Coal 0.9421 0.9460 0.9506 0.9810 0.9870 1.0181 1.0634 1.0693

478-505 505-533 533-561 561-589 589-616 616-644 644-672 672-700 700-728

(400-450) (450-500) (500-550) (550-600) (600-650) (650-700) (700-750) (750-800) (800-850)

143.8 160.9 176.3 195.6 213.1 233.7 249.6 262.1 295.2

2.7 3.0 4.2

533-561 561-589 589-616 616-644 644-672 672-700 700-728 728-755 755-783

(500-550) (550-600) (600-650) (650-700) (700-750) (750-800) (800-850) (850-900) (900-950)

191.0 207.5 220.4 239.6 267.3 303.8 343.2 333.7 371.2

1.0 2.0 2.2 2.6 8.7 3.1 4.1 11.8 9.8

505-533 533-561 561-589 589-616 616-644 644-672

(450-500) (500-550) (550-600) (600-650) (650-700) (700-750)

175.2 193.8 211.4 233.6 254.2 279.2

1.8 3.5 0.5 3.2 2.7 3.8

Wilsonville (245) Catalytic Hydrotreater 5 0.9052 5 0.9215 5 0.9381 5 0.9512 5 0.9666 5 0.9768

561-589 (550-600) 589-616 (600-650) 616-644 (650-700) 644-672 (700-750) 672-700 (750-800) 700-728 (800-850)

201.8 223.6 244.2 257.2 263.4 311.6

1.3 3.4 1.6 3.7 4.7 2.4

2.1

7.6 4.4 7.0 5.2 6.1

1.52092 1.52983 1.53447 1.56103 1.56470 1.58742 1.634 70 1.636 20

0.000 12 0.000 15 0.000 06 0.000 06 0.000 80 0.000 68 0.000 00 0.000 20

6 3 3 3 3 6 3 3

2 4 2 2 4 2 2 4

1.528 13 1.53893 1.54823 1.560 90 1.586 45 1.60390 1.62077 1.63485 1.65157

0.000 06 0.000 23 0.000 12 0.000 20 0.000 16 0.000 70 0.000 15 0.000 73 0.000 42

3 6 3 3 6 3 3 6 3

2 2 2 2 2 2

1.49730 1.507 17 1.51767 1.52557 1.53663 1.54653

0.000 00 0.000 06 0.000 15 0.000 12 0.000 15 0.000 06

3 3 3 3 3 3

Wilsonville (246) Thermal Hydrocracker 5 0.9701 2 2 5 0.9777 5 0.9867 2 5 1.0138 2 5 1.0123 2 5 1.0418 2

1.54233 1.55620 1.56163 1.59253 1.602 07

0.000 32 0.000 26 0.00006 0.000 31 0.000 40

3 3 3 3 3

10 30 10 LO 10 10 10 10 10

Wilsonville (245) Thermal 5 0.9656 10 0.9745 5 0.9909 5 1.0031 10 1.0308 5 1.0522 5 1.0713 10 1.0913 5

0.000 08

0.00006

Hydrocracker 0.000 33

0.001 13

0.00401

"These are average values based on multiple measurements. In some cases, the unrounded values are presented because these are the values used in the regressions. The molecular weight values should be rounded to three significant figures when used for purposes other than regressions.

Table 111. Slope, y Intercept, and Correlation Coefficient of Linear Least-Squares Regression Lines Shown in Figure 1 slope y intercept correln coeff H-Coal Thermal Hydrocracker (245) Catalytic Hydrotreater (245) Thermal Hydrocracker (246) H-Coal Thermal Hydrocracker (245) Catalytic Hydrotreater (245) Thermal Hydrocracker (246)

n vs. Tb,K (Dependent Variable) 1339 f 157 -1523 1709 f 69 -2051 2674 f 65 -3486 1466 f 205 -1683 n vs. d (Dependent Variable) 1.126 f 0.029 1.166 f 0.020 1.478 f 0.056 0.778 f 0.083

f 246 f 110 f 98 f 323

-0.775 f 0.045 -0.817 f 0.032 -1.306 f 0.085 -0.231 f 0.130

n vs. H/C (Dependent Variable) H-Coal Thermal Hydrocracker (245) Catalytic Hydrotreater (245) Thermal Hydrocracker (246) H-Coal Thermal Hydrocracker (245) Catalytic Hydrotreater (245) Thermal Hydrocracker (246) H-Coal Thermal Hydrocracker (245) Catalytic Hydrotreater (245) Thermal Hydrocracker (246)

-2.495 -21600 -3.193 -2.587

f 0.183 f 0.103 f 0.351 f 0.251

0.961 0.994 0.999 0.972

*

5.166 0.288 5.399 f 0.164 6.376 f 0.535 5.374 f 0.395

n vs. M, (Dependent Variable) 913 f 101 -1229 f 158 -2033 f 111 1455 f 70 -2986 f 110 2110 f 72 -1233 f 324 937 f 206 n vs. fa (Dependent Variable) 1.429 i 0.110 1.414 f 0.070 1.754 f 0.187 1.445 f 0.240

-1.692 -1.731 -2.317 -1.772

f

0.173

f 0.110 f f

0.284 0.337

0.998 0.999 0.997 0.984 0.984 0.995 0.977 0.986 0.965 0.992 0.998 0.934 0.983 0.992 0.978 0.961

104 Energy &Fuels, Vol. 1, No. 1, 1987

White et al.

the validity of the measurements. Table V (supplementary material) compares the average molecular weight measurements made on the H-Coal distillates by vapor pressure osmometry, freezing point depression, and low-voltage high-resolution mass spectrometry. In all cases, the results compare favorably. In the case of the Wilsonville distillates, the average molecular weight of each distillate was measured by VPO and at least one other method. In a11 cases, the results confirmed one another. To be useful, predictive equations must be applicable not only to the data set from which they were derived but also to other existing data. Similar data have been published by Gray et al. on 19 distillates derived from an SRC-I1 product.24 When eq 3 is applied to the SRC-I1 data, the average absolute deviation between the predicted and experimental molecular weight is 5.2%. The average absolute deviation between calculated and experimental molecular weights for eq 3 averaged over the H-Coal, Wilsonville, and SRC-I1 distillates was 3.2%. When the data published by Gray et are added to the data set presented here, and multiple linear regression of all the data is performed, eq 4 results. The mid-boiling point in

M , = (1.766 X 1 0 - 6 ) b~2.6667s-1.7696

(4)

this equation and in eq 5 is in degrees Rankine. The average absolute percent deviation between calculated and experimental molecular weights is 3.0% from eq 4 (see Table VI, available as supplementary material). Because the relationship between refractive index, mid-boiling point (in degrees Rankine, OR), density (at 293 K and 1atm), and molecular weight shown in eq 1 and 2 led to extremely accurate estimates of refractive indices, the regression of these parameters to estimate the molecular weight was performed. The resulting eq 5 was developed. Equation 5 estimates the molecular weights

M , = (9.502 x 1 0 - 7 ) ~ 1 . 9 ~ 6b)(-2.6291 q~

(5)

of the H-Coal and Wilsonville distillates with an average absolute deviation of 2.0%. The accuracy of eq 5 is slightly less than that of eq 3 with respect to the H-Coal and Wilsonville data. These three new equations for estimating molecular weight (eq 3-5) and those of Riazi and Daubed: Brul6 et al.,23Gray et al.,24and Wilson25have been evaluated to determine which equations give the most accurate estimations of molecular weight. The equations were evaluated with respect to the inspection property data from the H-Coal and Wilsonville distillates and (where possible) the 19 SRC-I1 distillates described by Gray et al.24 When data from Gray et al.24were employed, where multiple determinations of molecular weight were presented, the average value was used. The experimentally determined molecular weights, those estimated by the above procedures, and absolute percent deviation are given in Table VI (available as supplementary material). In general, the equations developed here (eq 3-5) are more accurate in estimating the molecular weights of the distillates studied herein than other predictive techniques available in the literature. The equation of Brul6 et al.23 gives the least accurate predictions. Density, aromaticity, atomic hydrogen to carbon ratio, mid-boiling point, and molecular weight are linearly related to refractive index for each of the four sets of distillates investigated. These linear correlations indicate that the distillates may be composed of homologous series (or mixtures of various homologous series) of compounds. This observation is thoroughly supported by low-voltage, high-resolution mass spectral analysis of the samples.26

Within any single set of distillates, the rate of change of the properties (aromaticity, mid-boiling point, etc.) with respect to refractive index is approximately uniform over the entire distillation range. The slopes of the lines of aromaticity, and the atomic hydrogen to carbon ratio as a function of refractive index are approximately the same for each of the four sets of distillates investigated, while the slopes of the lines of mid-boiling point, density, and molecular weight as a function of refractive index are different. The slopes and intercepts of the linear leastsquares regression lines shown in Figure 1 and presented in Table I11 are linearly related to the aromaticity and atomic hydrogen to carbon ratio of the sample from which the distillates were derived (except with respect to the relationship between refractive index and density). Thus, by measuring the hydrogen to carbon ratio and/or the aromaticity of the wide-boiling-range coal liquid, it is possible to predict the slope and intercept of the lines described in Figure 1. The aromaticity, fa, of fuels is a frequently sought property that can be difficult and time consuming to determine. Development of means to estimate aromaticity from easly determined parameters is useful. Multiple linear regression of the aromaticities (lH NMR), atomic hydrogen to carbon ratios, and refractive indices from the four sets of distillates led to eq 6. The aromaticities fa

= 5.243(H/C)-2.6810(n)-3.6964

(6)

determined by using the Brown and Ladner20procedure and those calculated by using eq 6 along with their absolute deviation and absolute percent deviation, are given in Table VI1 (available as supplementary material). The average absolute deviation between the aromaticities determined by 'H NMR spectroscopy and those calculated by using eq 6 is 0.037 f a units, and the average absolute percent deviation is 3.69%. It should be noted that a simple linear correlation between f a and the atomic hydrogen to carbon ratio allows estimation of the f a values of these distillates to within the uncertainty of eq 6. lH NMR was employed for determination of aromaticity because it is much more rapid than 13CNMR and because it requires much less sample and can be reliably performed on samples of lower solubility. Further, with I3C NMR there can be problems with relaxation effects. The validity of the Brown and Ladner technique for estimation of sample aromaticity has been demonstrated by Retcofsky et aLZ7 Comparison of the properties of the distillates from the same liquefaction experiment but from different process streams (the Thermal Hydrocracker and Catalytic Hydrotreater from Wilsonville run 245) is enlightening. The Thermal Hydrocracker product is deashed and subjected to catalytic hydrogenation in the Catalytic Hydrotreater. Thus, comparison of the properties of these streams allows the determination of the effect of increased hydrogen content. A priori, increased hydrogen content often leads to decreased aromaticity. Thus, at any given refractive index, the hydrotreated product has a higher mid-boiling point, a higher hydrogen-to-carbon ratio, a higher molecular weight, a lower aromaticity, and a lower density than the product before hydrotreatment. This is because for a hydrotreated product to have the same refractive index as an aromatic product, the hydrotreated product must (26) Schmidt, C. E.; Sprecher, R.; Bath, B. Low-Voltage, High-Resolution Mass-Spectrometric Methods for Fuel Analysis: Application to H-Coal Distillates, in preparation. (27) Retcofsky, H. L.; Schweighardt, F. K.; Hough, M. Anal. Chem. 1977,49,585-588.

Energy & Fuels 1987,1, 105-110 have a much higher molecular weight.

Conclusions For a given set of distillate fractions, the following trends can be observed. The refractive index increases as the distillate aromaticity increases, and the refractive index increases as the atomic hydrogen to carbon ratio of the distillate decreases. A decrease in the ratio of atomic hydrogen to carbon with a concomitant increase in aromaticity, boiling point, and molecular weight is a consequence of the increasing degree of aromatization and condensation of the molecular components of the distillates. Equations 1 and 2 lead to estimates of the refractive index of the coal-derived materials studied here that are more accurate than other correlations in the literature. Similarly, eq 3-5 lead to a more accurate estimation of molecular weight of coal-derived distillates than other equations in the literature, including the correlations of Riazi and Dauberty Brul6 et al.,23Gray et al.,24and WilEquation 6 calculates aromaticity from the measured atomic hydrogen to carbon ratio and refractive index with an average absolute deviation of 0.037 f a units. It is generally thought that fa can be determined by NMR tQ within 0.03 f a units.27 Thus, the values estimated by using eq 6 are as precise as those determined by NMR. The correlations developed indicate that refractive index is an important property of coal-derived materials that is related to a variety of physical and chemical properties. Measurement of refractive index is rapid and precise, and does not require expensive instrumentation or expertise. These qualities combine to make refractive index a potentially attractive on-line monitoring measurement for coal liquefaction processes, possibly in combination with density or some other parameter. The correlations developed may also find application in quality control of analytical data. For example, if the measured refractive index, mid-boiling point, and density lead to a predicted

105

molecular weight significantly different from the measured value, it does not necessarily mean the measured value is incorrect, but it does indicate that the value should be checked and further measurements may be needed. An important aspect of this investigation has been to determine if reliable correlations could be developed that allow the estimation of properties of coal-derived materials formed by different process streams of the same process (Thermal Hydrocracker and Catalytic Hydrotreater), by different processes using bituminous coals (Wilsonville, H-Coal, and SRC-11),and by different ranks of feed coals (bituminous and subbituminous). This investigation shows that reliable correlations with refractive index can be developed that fit the observed properties of distillates from various coal liquefaction products, processes, and feed coals. Nevertheless, the work should be continued and expanded to determine the general applicability of the results. The results presented will be useful in developing other new correlations and in modifying existing correlations.

Acknowledgment. The authors are indebted to M. E. Bott and M. Farabaugh for helping to perform the calculations and to M. Hough and J. Adkins for making the NMR measurements. Special recognition is given to K. Champagne and J. Knoer for distilling the Wilsonville products into narrow-boiling-rangedistillates. The authors acknowledge helpful discussions of the manuscript with D. Finseth, R. Warzinski, G. Holder, A. Cavanaugh, and G. Gibbon. Supplementary Material Available: Table IV, the experimentally determined refractive indices a t 293 K of the 28 K boiling range distillates from H-Coal and Wilsonville along with the values predicted by both eq 1and 2 as well as the equations of Riazi and Daubert; Table V, average molecular weights determined by VPO, freezing point depression, and LVHRMS for the H-Coal distillates; Table VI, experimental molecular weights and those predicted by using a variety of methods; Table VII, experimental aromaticities and those estimated by eq 6 (8 pages). Ordering information is given on any current masthead page.

Electron Microscope Investigation of the Structures of Annealed Carbons Peter R. Buseck,* Huang Bo-Jun,f and Lindsay P. Keller Departments of Geology and Chemistry, Arizona State Uniuersity, Tempe, Arizona 85287 Receiued August 6, 1986. Reuised Manuscript Receiued Nouember 4 , 1986 High-resolution transmission electron microscope images were obtained of the layers that form when samples of coke, produced in the laboratory by pyrolysis of known organic compounds, become structurally ordered as a function of heating temperature. The images show the progressive changes that occur as amorphous material grows to form isolated layers of carbon. These then grow laterally as well as form clusters of poorly stacked layers. Increased heating changes the subparallel layers to parallel seta, initially with many discontinuities, but then to crystals having few or no such defects. By the use of images obtained with transmission electron microscopy, details of graphite crystallization can be followed in considerable detail.

Introduction Carbonaceous materials are widespread in the natural environment and are, of course, major sources of energy and fuels. During maturation over geological time such Current address: Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou Province, People’s Republic of China.

materials lose heteroatoms (such as H, N, 0, S) and become richer in carbon, finally ending in fully ordered, crystalline graphite. For convenience, following common usage, we shall refer to the carbon-rich materials collectively as carbon, recognizing that they may range from the pure element to carbon-rich organic molecules that contain minor, unspecified amounts of other elements. The

0887-0624/87/2501-OlO5$01.50/0 0 1987 American Chemical Society