Energy & Fuels 1989, 3, 259-262 Large particles (2-3 mm), e.g., in experiment 5, enhance char formation in the particle itself. For such particles the diffusion of the product gases out of the particle needs so much time that their decomposition to char starts during the diffusion. Heavy Metals. The heavy metals are practically completely enriched in the solid products. They are more concentrated in the soot than in the char. Mechanical elutriation occurred only in some experiments where the cyclones were working far away from the optimal operating conditions, which depend on the gas flow velocity. However, Cd emission started below 600 OC and was promoted by small particle size and high turbulence in the fluidized bed. Kistler et al.5 observed the beginning of the Cd emission a t the temperature of 625 "C. Because the evaporation is mass-transfer controlled, it already occurs at a lower temperature (550 "C) in the fluidized bed. Therefore, temperatures above 600 "C are not accessible without the emission of Cd. Since Cd condenses between 400 and 500 OC, a selective removal out of the process mainstream might be feasible. Hg was not analyzed, as all authors have observed its complete emission at very low temperatures. At present no effective method exists that avoids this phenomenon without being very expensive.
259
Economics. Unfortunately, the economics of the process are not very favorable as the investment and running costs are high.6 The latter might come down if a market for the product chemicals was to exist, but today the product chemicals can only be used as liquid fuel. In addition to this, the process at the present stage would only be suitable for large waste treatment plants. Summary. The pyrolysis of sewage sludge was investigated in an oxygen-free atmosphere in a fluidized bed. The yield of high quality liquid products could be improved by use of a fluid-cracking catalyst. The temperature is the most important process parameter although the other parameters should be chosen carefully as well. In particular, blow out of the sludge out of the reaction zone prior to complete reaction must be avoided as well as the use of big particles. The emission of Cd starts already below 600 "C and is therefore the actual limit for this process. At present the economics of the pyrolysis are not favorable. A further increase in the market price of the organic liquid products can be expected to make the process economical. Registry NO.Hz, 1333-74-0; CO, 630-08-0; COZ, 124-389; CHI, 74-82-8; CzH4,74-85-1; Cz&, 74-84-0; Cd, 7440-43-9; Cr, 7440-47-3; Cu, 7440-50-8; Hg, 7439-97-6; Pb, 7439-92-1; Zn, 7440-66-6.
Rate of Intermolecular Association of Asphalt Ridge Tar Sand Bitumen after Thermal Dissociation Daniel A. Netzelh and Peggy T. Coover Western Research Institute, P.O. Box 3395, University Station, Laramie, Wyoming 82071 Received July 15, 1988. Revised Manuscript Received December 22, 1988
It is known that reversible thermal cycling implies that intermolecular interactions of the types dipole-dipole or dipole-induced dipole, rather than a chemical reaction, are responsible for the exothermic reactions in complex organic mixtures. A preliminary study was conducted to determine if the reversible intermolecular interactions of Asphalt Ridge tar sand bitumen could be followed by using nuclear magnetic resonance relaxation spectroscopy. The NMR data indicate that the intermolecular association of molecules in the bitumen to their original molecular configuration after thermal dissociation is mechanistically a pseudo-first-order process requiring nearly a week to establish an intermolecular-interaction equilibrium. The first-order rate constant was found to be 0.0139 h-' a t a mean temperature of 32 OC.
Introduction The mobility and thus the degree of association of tar sand bitumen as a function of time and temperature are not fully understood. An understanding of the molecular associative properties of bitumen in its free state may provide the information to optimize solvent extraction and in situ and above ground thermal processes. This paper is a report on the rate of molecular association after thermal dissociation for tar sand bitumen in its free state. Several types of molecular interactions are responsible for the chemical and physical properties of a complex mixture such as tar sand bitumen in its native state (native tar sand) and tar sand bitumen in its free state (extracted bitumen). These molecular interactions include the long-range van der Waals forces and the short-range valence or chemical forces. The strength of these interactions 0887-0624 J 89/2503-0259$01.50/0
between either the organic constituents or the organic constituents and the mineral matrix depends, in part, upon the nature of the dipolar interaction and intermolecular distances of the molecules. Hydrogen nuclear magnetic resonance (NMR) relaxation spectroscopy' is commonly used to investigate molecular mobility and association. With this technique, the nuclear dipolar spin-lattice (5"') and spin-spin (Tz) relaxation times are measured. The spin-lattice and spin-spin relaxation rates (the inverse of the relaxation times) can also be derived from theoretical principles. Mathematically, the relaxation rates are the product of a nuclear interaction constant, ki, and a molecular correlation time, T ~ .For random molecular motion, the correlation time is the (1) Netzel, D. A.; Miknis, F. P.Appl. Spectrosc. 1977, 31, 365-386.
0 1989 American Chemical Society
260 Energy & Fuels, Vol. 3, No. 2, 1989
length of time a molecule remains in any given position before a collision causes it to change its state of motion. The molecular correlation time depends on the size and symmetry of the molecule. Rigid molecules of less than axial symmetry can have several correlation times that correspond to the preferred directions of motion. However, for large and unsymmetrical molecules with no preferred direction of motion, it may be a good approximation to assume a single correlation time.2 The molecular rotational correlation time, 76, for a nonviscous molecular system consisting of small spherical molecules tumbling isotropically is related to the intramolecular dipolar spin-lattice relaxation time, Tl, as given by eq 1: where R1 is the intramolecular dipolar spin-lattice
relaxation rate, h is Planck's constant divided by 2a, 7 H is the magnetogyric ratio for the 'H nucleus, and rHHis the H-H intranuclear distance. It can be shown that the correlation time is directly related to the molecular weight and viscosity and indirectly related to temperature and density. The derivation of the relaxation rate, R1, as a function of these molecular parameters is presented in the Thesis by Netze14 and the references cited therein. The relaxation rate is defined by where M is the molecular weight, 17 is the viscosity, p is the density, T i s the temperature, and k, is the proportionality constant. Miknis and Netze15 conducted a preliminary study on the hydrogen spin-lattice relaxation time as a function of temperature for a native tar sand bitumen and the extracted bitumen. The temperature dependence of T1 for the two bitumen samples resembles the temperature dependence of T1 in many polymer systems. The minimum in each of the curves occurs a t about 330 K, which corresponds to an upper limit for the correlation time of 3 X s and a lower limit for the activation energy of 13.4 kJ mol-l. Using both hydrogen spin-lattice and spin-spin relaxation time measurements, Sobol et al.6concluded that the extracted Athabasca tar sand bitumen is composed of three phases: solidlike rigid, solidlike mobile, and semiliquid. These phases can be regarded as different degrees of dipolar, aromatic a-bonding, and hydrogen-bonding interactions resulting in rigid and semimobile molecules through association. Schwager et al.' studied the association of Athabasca asphaltenes in various solvents and found that a solvent of high polarity, such as pyridine, was required to cause significant variation in the observed molecular weight as a function of concentration. Asphaltenes from Athabasca bitumen were also studied by Speight and Moschopedis.8 They found that the molecular weight varied considerably and that it is dependent upon the nature of the solvent (2) Noggle, J. H.; Schirmer, R. E. The Nuclear Overhauser Effect; Academic Press: New York, 1971; p 27. (3) Farrar, T.C.; Becker, E. D. Pulse and Fourier Transform NMR; Academic Press: New York, 1971. (4) Netzel, D. A. Ph.D. Dissertation, Northwestern University, 1975. (5) Miknis, F. P.; Netzel, D. A. Magnetic Resonance in Colloid and Interface Science; Resing, H. A., Wade, C. G., Eds.; ACS Symposium Series 34; American Chemical Society: Washington, DC, 1976; Chapter 16, pp 182-188. (6) Sobol, W. T.;Schreiner,L. J.; Miljkovic, L.; Marcondes-Helene,M. E.; Reeves, L. W.; Pintar, M. M. Fuel 1985, 64, 583-590. (7) Schwager, I.; Lee, W. C.; Yen, T. F. Anal. Chem. 1977, 49, 2363-2365. (8) Speight, J. G.; Moschopedis, S. E. Fuel 1977, 56, 344-345.
Netzel and Coover
as well as the temperatureg a t which the determinations were performed. The molecular weight of the asphaltenes measured a t a high temperature was lower than the molecular weight at ambient temperature and, therefore, assumed to be the molecular weight of individual particles. Hydrogen-bonding interactions that occur between the asphaltenes and resin entities of Athabasca bitumen were studied by Moschopedis and Speight.lo They showed that hydrogen bonding occurs readily between these fractions and postulated that the asphaltenes are peptized by the resins. There is little reported research on the rate of the intermolecular interactions in fossil fuel materials. The rates of intermolecular association of several molten asphalt materials were studied by Ensley," who found that the molecular association rate obeyed second-order kinetics with rate constants ranging from 0.14 to 1.04 L/(mol h) and activation energies from 33.5 to 58.6 kJ mol-'. Because of the low activation energy, the intermolecular interactions were considered to be dipole-dipole.
Experimental Section The native tar sand was obtained from the Asphalt Ridge site a t the Uinta County quarry, Vernal, UT. The tar sand had a bitumen content of -13.6% by weight. The free bitumen was obtained by using a standard Soxhlet extraction technique for recovering organic materials from mineral matrices. The native tar sand was weighed into a glass extraction thimble and extracted with 700 mL of toluene until the solvent leaving the thimble was colorless. The fine mineral matter was filtered from the extraction solvent containing the organic material. The toluene was then removed by rotary evaporation. The remaining native tar sand was extracted with 700 mL of pyridine for 24 h. The bulk of the pyridine was removed by rotary evaporation. Traces of pyridine were removed by two azeotropic distillations with -50 mL of toluene. All bitumen fractions were combined. An IBM PC-20 spectrometer with a fixed magnetic field of 0.47 T and a frequency field of 20 MHz for hydrogens was used. The temperature of the magnet and probe assembly was held constant at 40 i 0.01 "C. A 7.5" variable-temperature solid/liquid probe was used, but it was modified with a 1-cm Teflon spacer in the bottom of the probe assembly. This spacer was added to ensure that the sample was within the 20-mm coil height when it was placed in a 7.5-mm flat-bottom NMR tube. The spectrometer was operated in the phase-sensitive detector mode. The number of transients to ensure good signal-to-noise ratio was 25, and the delay time between pulse sequences was set at 5 s. The inversion-recovery (Ar-T-7/2) technique and an iterative, nonlinear, three-parameter curve fit were used to calculate the spin-lattice relaxation times, T1, by
M , = M,(1 - Be-'/**)
(3)
where, M , is the magnetization intensity a t a pulse delay time of T , T is the pulse delay time between A and ~ / pulses, 2 6 is the flip angle, and M , is the magnetization intensity a t infinite T (single 90' pulse). I t should be noted that with this technique, the observed magnetization is due to all hydrogen types present in the sample. To validate the instrumental and operational conditions used to obtain T1 values, several different types of materials were studied.12 The TI value of water (deionized) at 40 "C has recently been reported by Schmidt et al.13 using a PC-20 spectrometer. The value they reported was 3.84 f 0.02 s, which compares fa(9) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227-232. (10) Moschopedis, S. E.; Speight, J. G. Fuel 1976,55, 187-192. (11) Ensley, E. K. J . Colloid and Interface Sci. 1975, 53, 452-460. (12) Netzel, D. A.; Coover, P. T. "An NMR (Nuclear Magnetic Resonance) Investigation of the Chemical Association and Molecular Dynamics in Asphalt Ridge Tar Sand Ore and Bitumen', US.Department of Energy Report DOE/FE/60177-2452; NTIS: Springfiled, VA, 1987. (13) Schmidt, E. J.; Velasco, K. K.; Nur, A. M. J. Appl. Phys. 1986, 9, 2788-2197.
Energy & Fuels, Vol. 3, No. 2, 1989 261
Association of Tar Sand Bitumen
Table I. 'H Spin-Lattice Relaxation Times after Quenching of Asphalt Ridge Tar Sand Bitumen Heated to 180 O C for 1 h tar sand time after bitumen spin-lattice quenching, h relaxation time," ms 0.00 61.5 f 0.3*
loooh i
L
a
C
\
\
=I,, loo
100
\
\
,
\
200
,
300
Pulse Delay Time ( T ) ,msec
64.6 64.7 64.1 63.2 63.0 62.4 61.5 62.3 62.3 61.4 61.3 60.8
0.18 1.50 18.50 26.00 42.70 50.60 66.85 78.27 90.85 102.85 115.18 187.18 a
f 0.2 f 0.4
f 0.3 f 0.4 f 0.3 f 0.4 f 0.2 f 0.3 f 0.4 f 0.3 f 0.5 f 0.4
Measured at 40 "C. *Before heating to 180 "C.
Figure 1. 'H spin-lattice magnetization decay of Asphalt Ridge bitumen as a function of the pulse spacing 50.6 h after quenching. vorably with our value of 3.98 f 0.01 s reported in ref 12. The Asphalt Ridge bitumen sample was placed in an NMR tube and heated to 180 "C in a fluidized sand bath14for 1h. The sample was removed and quenched in water at 21 "C. The sample was then placed in the NMR spectrometer and allowed to equilibrate at 40 "C before the relaxation time was measured. The sample was removed and stored at ambient temperature of 24 "C until the next measurement, in which the sample was again heated to 40 "C before the relaxation measurement was taken.
Results and Discussion A tar sand bitumen is a heterogeneous mixture of organic molecules that, because of association, exists in various phases in which molecules exhibit different degrees of mobility. Molecules in these various dynamic states of motion have different nuclear relaxation times that, after excitation, give rise to nonexponential decay of the total nuclear magnetization. In favorable cases, the nonexponential decay of the total nuclear magnetization can be resolved into a sum of exponential decays where each exponential decay represents molecules within a given mobility range. Spin-spin relaxation time measurements have shown that Athabasca tar sand bitumen is composed of a semiliquid phase (T2= 550 ps), a solidlike mobile phase (T2= 105 ks), and a solidlike rigid phase (T2= 15 ps). The spin-lattice relaxation time for the semiliquid phase was found to be 97 A typical spin-lattice magnetization decay for the hydrogens in an Asphalt Ridge bitumen after quenching is shown in Figure 1. All magnetization decays were found to be exponential over the range of pulse delays used in the experiment. Because of the relatively long pulse delays used, only one dynamic state of motion was observed. That is, the magnetization decays attributed to the solidlike phases have long disappeared because of their faster relaxation rates. Table I lists the spin-lattice relaxation times measured at various times after quenching. The spin-lattice relaxation times measured for the Asphalt Ridge bitumen (-62 ms) are of the same magnitude as the relaxation times for the Athabasca bitumen (97 ms): suggesting, therefore, that the molecular motion being observed is due to the semiliquid phase. The difference in relaxation values can be attributed to differences in chemical composition, temperature, degree of association, and paramagnetic effects. (14) Miknis, F. P.; Turner, T. F.; Berdan, G. L.; Conn, P. J. Energy Fuels 1987, 1, 477-483.
-":
"'"!
i s 15.0 0
20
40
60
80
100
120
140
160
180
~
0
l i m e After Qusnrhing, hr
Figure 2. 'H spin-lattice relaxation rate of Asphalt Ridge bitumen as a function of time after quenching. In this experiment, it is not the absolute relaxation value but the relative changes that are important because the chemical composition, temperature, and the concentration of paramagnetic species remain constant. It can be assumed that at 180 "C the tar sand bitumen is partially dissociated relative to ambient temperature, that is, a reduction of the van der Waals forces and/or hydrogen bonding occurs? The dissociated state of the bitumen is "frozen" by quenching the system to room temperature. In this state, the spin-lattice relaxation rate for the relatively nonassociated molecules would be slow because of the short correlation time. However, with time, the diffusion of the small molecules causes them to reorientate themselves and reassociate into large, slowly tumbling molecules. The spin-lattice relaxation rate increases with time because the correlation time for these associated molecules increases. The spin-lattice relaxation rate as a function of time for the tar sand bitumen is shown in Figure 2. The solid line in Figure 2 represents the best statistical fit of the data for a first-order rate equation of the form (4)
With integration and rearrangement this becomes Rtl = Rol + (R", - Rol)(l - e-ki)
(5)
In these equations, Ril, Rol, and Rmlare the spin-lattice relaxation rates a t time t, zero time, and infinite time (initial value before heating), respectively, t is the time in
Energy & Fuels 1989, 3, 262-267
262
hours; and k is the first-order rate constant. The square of the correlation coefficient, r2,for the data is 0.965. The calculated values for Rol and Rmlare 15.45 and 16.50 s-l, respectively. Rmlrepresents the spin-lattice relaxation rate for a system in which the intermolecular interactions are at equilibrium. The first-order rate constant for the molecular association is 0.0139 h-l at a mean temperature of 32 “C. The only interpretation attached to the parameters in eq 5 is that the “driving force for a ~ s o c i a t i o n ”is~the ~ remaining nonassociated molecules and that the association asymptotically approaches the value of Rmlas t m. That is, mechanistically, the association of molecules is a pseudo-first-order rate process in which the amount of associated molecules exceeds the amount of nonassociated molecules.
-
(15)Tahiri, M.; Sliepcevich, C. M.; Mallinson, R. G. Energy Fuels 1988,2, 93-100.
To a first approximation, eq 2 may be used to explain the data for the extracted tar sand bitumen. At a constant temperature, the increase in the relaxation rate with time suggests that the molecular weight as well as the kinematic increases after quenching. However, any viscosity (TIP) increase in viscosity is caused by a decrease in the mobility of molecules that results from an increase in the molecular weight of these molecules in the semiliquid phase. It is assumed that the density does not change significantly with reassociation of the molecules. Thus, reassociation of the bitumen molecules begins after quenching and continues for nearly a week before the intermolecular-interaction equilibrium is reestablished. Acknowledgment. We express thanks and appreciation to the U.S. Department of Energy for funding of this work under Cooperative Agreement No. DE-FC2183FE60177 and to Turner for the treatment of the kinetic data.
Methods for Quantifying JFTOT Heater Tube Deposits Produced from Jet Fuels Robert E. Morris*>+and Robert N. Hazlett* Chemistry Division, Navy Technology Center for Safety and Survivability, Code 6180, U S . Naval Research Laboratory, Washington, D.C. 20375-5000, and Hughes Associates, 2730 University Boulevard West, Wheaton, Maryland 20902 Received August 29, 1988. Revised Manuscript Received November 18, 1988
One measure of the thermal stability of aviation fuels is the quantity of deposits formed on heated metal surfaces. In accelerated stability tests conducted in accordance with the JFTOT procedure (ASTM D3241), the rating methods currently employed involve either visual comparisons or measurements of reflected light by the tube deposit rater (TDR), both of which are sensitive to deposit color and surface texture. In this study, deposits formed on stainless-steel JFTOT heater tubes have been examined by the TDR, a gravimetric carbon combustion method, and two new nondestructive techniques for determining deposit volumes based on measruements of dielectric strength and optical interference. Measurements of total carbon content by combustion were used as a reference. It was found that the dielectric and interference methods correlated well with the combustion analyses and each other, while the total TDR often yielded misleading results.
Introduction
The thermal oxidation of liquid fuels is often accompanied by the formation of insoluble reaction products, either as suspended particulate or as a gum that adheres to heated surfaces. Modern aircraft engine designs and aerodynamic heating of wing surfaces place severe thermal stress on the fuel, increasing the likelihood of the formation of insoluble deposits. Aircraft fuel system deposits can be responsible for a variety of problems including decreased efficiency of engine heat exchangers, seizing of fuel control valves and injector fouling. It is known that thermally initiated fuel degradation is accelerated by the presence of oxygen through autoxidative processes involving free-radical chain reactions. The jet ‘U.S. Naval Research Laboratory. Hughes Associates. f
0887-0624/89/2503-0262$01.50/0
fuel thermal oxidation tester (JFTOT) is widely used to characterize the thermal oxidation stability of a fuel. In the JFTOT, the fuel is stressed under conditions of high oxygen availability and slowly increasing temperatures. The quantities of insoluble products formed under these conditions constitute a measure of the deposit-forming characteristics of the fuel. In accordance with standard ASTM D3241 test procedures,’ the formation of adherent insolubles on the heated tube is characterized by visual comparison with color standards. The highly subjective nature of the visual method of rating heater tube deposits was revealed in a round-robin effort conducted by the Coordinating Research Council.2 The poor precision of visual ratings from unusual and highly colored deposits (1) ASTM Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure). In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1976;Part 25, ASTM D3241-74.
0 1989 American Chemical Society
