Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 143-147
143
111. Coal Utilization J. A. Quazi AIChE National Meeting, Houston, Texas, April 1979 (Continued from March 1980 issue)
Engineering Kinetics of Short Residence Time Coal Liquefaction Processes Rich,ard K. Traeger Depa,rtment 4740, Sandia Laboratories, Albuquerque, New Mexico 87785
Conversion of coal to liquid products occurs rapidly at temperatures over 350 OC and consequently can be significant in prseheaters or short residence time reactors. The extent of conversion can have an effect on the operation of preheaters or effectiveness of subsequent reactors. To obtain process information, Illinois No. 6 coal in SRC I1 heavy distillate was reacted at 13.8 MPa, temperatures of 400, 425, and 450 O C , and at slurry space velocities of 3200-96 000 kg/h-m3. Product compositions and viscosities were measured. High concentrations of preasphaltenes occur in early reactions resulting in a high viscosity product. Subsequent reactions to asphaltenes and oils are less rapid.
Table I
Introduction The increasing need for liquid fuels coupled with the limited domestic crude supply has emphasized the need for coal liquids technology. However, reduction of product costs and operational problems require a better understanding of the processes. The coal-solvent-hydrogen slurry preheater technology is one of the areas of major concern because the preheater operation itself may be difficult to control and changes in the preheater product can affect all subsequent reactor operations. The preheater is a pre-reactor wherein the coal goes through initial dissolution and reaction involving significant chemical and physical property changes. This paper reviews the results of work studying processes related to preheaters (or short residence time reactors) using a continuous bench scale reactor under isothlermal operation.
Conditions Used in These Studies design: helical upflow reactor temperature: isothermal at 400, 425, and 4 5 0 " C pressure: 13 MPa H, (2000 psi) slurry composition: 33% Ill. No. 6 coal in 67% SRC/II heavy distillate hydrogen rate: 6.2 std m 3 of H,/kg of coal (200 MSCF/ton)
Background In all coal liquefaction processes, coal is mixed with a recycle solvent and fed with hydrogen into a preheater to bring the slurry to temperature for subsequent liquefaction. Nominal preheater conditions are shown in Table I. Coal dissolution and reaction will occur in the preheater. A simplistic concept of changes occurring is shown in Table 11. The coal swells in the solvent, thermal energy breaks bonds forming free radicals, and hydrogen transfer in the coal molecules stabilizes the radicals. On continued heating, more thermal cleavage occurs and the free radicals can be satisfied by combining with other coal entities, with solvent or with hydrogen transferred from the solvent. Continued reaction of the fragments then yields many products, commonly classed by solubilities. Definitions used for data presented here are as follows: preasphaltenes, soluble in tetrahydrofuran and insoluble in benzene; asphaltenes, soluble in benzene and insoluble in pentane;
oils, soluble in pentane. Extended reaction results in gas production. The swelling-disintegration region is of specific concern in preheater or short residence time reactions since the products become viscous and the solvent may become immobilized leading to plugging, coking, and unstable operations. Also, physical and chemical changes in this stream could have significant effects on the effectiveness of subsequent catalytic liquefaction reactors. Experimental Section Details of the equipment (Curlee and Hawn, 1978), experiments (Traeger and Curlee, 1978), and analytical techniques (Thomas and Noles, 1978) are given elsewhere and will only be summarized here. Materials. The coal, Illinois No. 6-Burning Star Mine, was obtained from Hydrocarbon Research, IncjTrenton, NJ, screened to -45 mesh, and blended with SRC I1 heavy distillate in a homogenizer. The SRC I1 heavy distillate,
0196-4321/80/ 1219-0143$01 .OO/O
Preheater Conditions design: standard resid preheaters; helical o r serpentine, direct fired outlet temperatures: 375-460 "C (700-850 O F ) pressures: 100-270 atm (1500-4000 psi) slurry compositions: 30-40% coal hydrogen rates: bleed-40 MSCF/ton of coal (bleed-1.3 std m3/kg of coal)
1980 American Chemical Society
144 Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980 Table 11. Coal Liquefaction Processesa Occurring in Preheaters
I. preheater region physical slurry temperature, " c 11. physical processes coal changes Whitehurst classification hydrogen transfer processes
initial
middle
final
50-250
250-300
270-400
400t
none
swelling dissolution coal molecular rearrangement
disintegration H-transfer solvent t o coal
reaction hydrogenation gas t o liquid
decreasing
low steady state slow increase low
111. products formed preasphaltenes asphaltenes oils
IV. slurry viscosity
high increasing slowly net solvent loss high
low
a Table I1 is a descriptive model of processes occuring during heating of a coal-solvent system as that used in this study. Whitehurst (1978).
1 MOLTEN SALT BATH
MOYNO PUMP REGULATOR
1 2 3 O R 4 PREHEATERS
i GAS
HIGH PRESSURE LIQUID METERING PUMP
-
1
MIST SCRUBBER
LIQUID PRODUCT
t
DRAIN
Figure 1. Schematic diagram of bench scale, continuous flow reactor used for short residence time coal liquefaction shdies.
a product from liquefaction of Illinois No. 6 coal, was obtained from the Ft. Lewis SRC I1 plant. The distillate has a boiling range of 240-560 " C (470-1050 O F ) , H/C atomic ratio of 1.02, and a specific gravity of 1.08. Equipment. The four-section, continuous reactor is shown schematically in Figure 1 and operational capabilities given in Table 111. The unit is operated in a "once through" mode, samples being withdrawn about 0.5 h after steady temperature, flow, and product viscosities are reached. The first two individual sections are each 3.05 m lengths and the last two are each 6.10 m lengths of 0.122 in. i.d. tubing. All sections were operated at the same temperature (400, 425, or 450 OC) during a given run. Effective lengths were estimated by including the length in the first reactor where the calculated stream temperature exceeded 95% of the operating temperature, full length of subsequent reactor sections and the lengths of heated interstage
Table 111. Continuous Reactor Operating Capabilities number of stages: temperature: pressures: slurry feed rates: volume per stage:
reactor tube length: liquid space velocity: gas flow rates:
4 in series 250 t o 4 7 5 "C-each of the four stages heated independently to 27 MPa at 450 " C 0.3 to 3.6 kg/h 1.4 x to 6.2 x m 3 for 5.2 mm i.d. tubing 4.5 x t o 23 x m 3 for 3.10 m m i.d. tubing 0.6 t o 6 m 6 4 0 t o 8 x l o 4 kg/h-m3 0.1 t o 8.5 std m3/h at 27 MPa
tubing. Calculations (Traeger and Curlee, 1978) indicate slug flow conditions exist and residence times are on the order of 20-60 s for the first stage, 50-150 s for stages one and two, 9 5 3 2 0 s for the first three stages, and 150-500 s for all four stages. There is no experimental confirmation
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2. 1980 SPACE TIME I H M 3 /Kg)
SPACE TIME (H-M3/Kg) 06x10 7
-.
,
1.9r10
,
31:1Oe4
44\10
loo---
1
450'C
----1I
31x10
44x10
I T --r---
I
II
I
/---
80
19~10.~
06x10
145
t
425' C
c
60
40
I
20
0
1x10
'
2x10
3x10
4x10
5x10'
6x10
'
7x10
,E
'
0
1x10
2x10
of flow regimes or residence times. Chemical Analyses. After sampling the product, an aliquot is taken of the whole slurry product for measurements of viscosity at 70 "C using a Brookfield viscometer. The rest of the sample is sealed in a glass jar, chilled, and stored in a refrigerai or. Subsequent analyses include Soxhlet benzene extraction into solubles and insolubles. The insolubles are further extracted with tetrohydrofuran and the solubles further extracted with pentane. Sulfur, elemental composition, and gas analyses are also performed. Results and Discussion The individual data are detailed in other reports (Traeger and Curlee, 1978; Traeger and Bickel, 1979) and will not be tabulated here. Changes in product distribution and viscosity are shown as a function of space time in units of h-ft3/lb of slurry or h-m3/kg of slurry. Knowledge of flow regimes and actual residence times of each of the three phases is too limited to justify use of a real time scale. Current estimates suggest that the liquid maximum residence times are on the order of 40G600 s; thus initial data points would have residence times on the order of 30-50 s for the liquid phase. Residence times for the gas phase are much shorter and times for the solid phase are unknown. Conversion. Conversion of the organic portion of coal to T H F and benzene soluble products is shown in Figures 2 and 3. Benzene-soluble products include the majority of distillables, and high conversion to benzene soluble products is generally desirable. As shown in Figure 2, conversion to T H F solubles occurs rapidly (estimated at less than one minute) at 400 to 450 "C; i.e., the swelling--disintegration process in Table I is very rapid. These results agree with those of Neavel (1976), who found that 90% (MAF basis) of a vitrinite rich sample was converted to pyridine-soluble product in 2 min a t 400 "C. Plette et al. (1977) found 70% (MAF basis) West Kentucky coal became pyridine soluble after 60 s at 425 "C, and 88% after 60 s at 450 "C. The rate of conversion at these short times will depend on the solvent quality (Whitehurst, 1978) and the nature of the coal. Conversion to benzene-soluble products occurs more slowly but significant reaction can occur in preheaters. If short residence time product should be benzene soluble to facilitate subsequtmt processing, the data in Figure 3 suggest that exit temperatures over 425 "C are probably desirable. Liquid Product Compositions. Changes in preasphaltene, asphaltene and oil contents are shown in Figure
3x10
4x10
'
5r10
'
-J bxlO
'
7110
SPACE TIME ( h It' Ib SLURRY)
SPICE TIME ( h i t ' Ib SLURRY)
Figure 2. Fraction of MAP coal converted to THF soluble products
'
Figure 3. Fraction of MAF coal converted to benzene soluble products. SPACE TIME (ti M B /Kg! 06~10-~
19x10
r-'
31x10
,
425'C
45phc
4004254500C
,
0
Y - r
44x10
1 -
~
,
0
ASPHALTENFS
8
20
I ' 0
,450% ,425G C
4000
c
I
I' J 1x10.'
2x10"
3x10''
4x10"
5x10.'
6x10.'
7~10.'
SPACE TIME (h.ft3,'1b slurry)
Figure 4. Preasphaltene, asphaltene, and oil fractions of the whole liquid product.
4. The preasphaltene concentration goes through a maximum early in the reaction. Cronauer et al. (1978) noted a preasphaltene maximum at space times of less than 600 s at 400-450 "C. The maximum shifts to longer times as temperature decreases. The preasphaltene data also suggest that temperatures exceeding 425 "C are desirable if a benzene-soluble product is desired for the next stage of the process. The asphaltene concentration change is relatively small and is independent of temperature within the precision of these data. The fraction of pentane-soluble oil in the whole liquid product remains constant or decreases slightly during initial reaction. This solvent deficiency has frequently been noted and attributed to entrapment in the coal (Cronauer et al., 1978), polymerization (Lewis et al., 1978), or solution in the preasphaltene/asphaltene fraction (Thomas, 1978). In any case, the solvent is low during maximum preasphaltene concentration, which would suggest a high viscosity, difficult processing material. As the temperature increases from 400 to 450 "C, the oil fraction increases at both short and long times. Recent work (Thomas, 1978) has shown that the most significant decrease for the coal solvent system occurs in the 300-350 "C range. Figure 5 shows that 3&50% of the sulfur in the slurry is removed early in the reaction; the higher the temperature, the more the sulfur is lost. Kleinpeter and Burke (1978) noted similar losses in short residence time reac-
146
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980 SPACE TIME I H M3/Kg) 20,
,
06:10-4
0
1x10
'
2x10
19:10-4
'
3x10
'
,
4110
,
31:lO
'
5x10
'
6x10
4410
'
7x10 '
SPACE TIME Ih It' Ibs SLURRY)
Figure 5. Change in sulfur content of the whole liquid product during initial coal liquefaction processes.
FRO'* C V W C I'R 4
dl
19.8
Figure 7. Liquefaction kinetic model by Gronauer et al. (1978).
~
0.80
2
N
8
L -
LL
2
B
t /
060-
PREASPHALTENE/EXPERIMENTAL
\ \ ASPHALTENEIEXPERIMENTAL
I 0 0
*\
-
--a ?--o---
1x10'
- - - P R E ~ P ~ ~ ~ ~ ~ -~ , -C -A _L C_V L I T I O N
- - - - - _- -T-:-o
-
00
2x10'
3x10'
4x10'
5x10'
6110'
7110'
SPACE T I M E (ht It' Ib SLURRY1
Figure 6. Viscosity of the whole liquid product measured at 70 "C. Viscosity values exceeding 100 P could not be measured with available equipment.
tions. The major loss is expected to be sulfur removal from the mineral matter. Physical Properties-Viscosity. Knowledge of the viscosity level is important in process considerations for short residence time reaction products, and time or space location of the peak is important for the design of preheaters. Past work (Scheller et al., 1977; Thomas and Granoff, 1978) has shown dissolved preasphaltenes increase solution viscosities; consequently a viscosity maximum should be noted at the preasphaltene peak. Whole liquid product viscosities, measured at 70 "C (Figure 61, indicate that high viscosities occur at short reaction times paralleling the preasphaltene maxima. The viscosity decrease is rapid at 450 " C compared to that at 400 "C. Unfortunately, the capability does not yet exist to calculate the viscosity at the temperature, pressure, and shear rates in a continuous reactor. Kinetics The ability to predict flow regimes, slurry properties, thermal effects, and chemical changes is important to design short residence time reactors as preheaters. However, coal liquefaction processes pose the following problems. (1)The process stream contains changing solid, liquid, and gas contents with complex chemical reactions and products. Residence times of the gas, liquid, and solid phases are not adequately defined. (2) The flow regime is not known and rate-controlling mechanisms are not defined. Slug flow conditions are believed to occur in the
40
80
REACTOR LENGTH (M)
Figure 8. Experimentally determined product distributions at 425 "C compared to those predicted by the model.
bench scale reactor used to generate data presented in this report. (3) Heats of reactions, known to occur, are not defined. (4) The chemistry and physics of conversion are not understood. The initial approach (Bickel, 1978) was to use a reasonably simple model developed by Cronauer et al. (1978) using data from a continuous, stirred autoclave system. Cronauer used the same coal and product extraction scheme as described in this paper. The reaction model and equations are shown in Figure 7. The product distribution for a 425 "C, isothermal operation was calculated using Cronauer's constants and is shown in Figure 8. Figure 8 also shows data generated in the bench scale reactor superimposed on Cronauer's predictions. The results are in good agreement at long times. This is to be expected since the Cronauer reactions had long residence times. The major differences is in the early time behavior of preasphaltenes. Bench scale reactor data show a higher preasphaltene content early in the reaction than predicted by Cronauer's model. The peak noted in preasphaltenes indicates that a higher order reaction is needed to represent the preasphaltene concentrations. Also a better description of an "effective" reaction sequence is needed to develop general kinetic expressions. Summary Illinois No. 6 coal has been liquefied in SRC I1 heavy distillate using a continuous bench scale reactor. Dissolution of the coal is rapid, resulting in high preasphaltene contents, high product viscosities, and possible solvent losses early in the reaction. This early reaction stage could
Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 147-151
lead to operational problems in preheaters or short residence time reactors. At long reaction times (> 3 min), chemical changes occur less rapidly. Kinetics developed by Cronauer et al. (1978) can be used to describse these long time reactions with this coal and these product definitions. Further study is needed to define changes occurring at early reaction times. L i t e r a t u r e Cited Bickel, T. C., unpublished data, Sandia Laboratories, Oct 1978. Cronauer, R. G., Roberto, R. G., Shah, Y. T., Proceedings of the EPRI Contractors' Conference on Coal Liquefaction, Electric Power Research Institute/Palo Alto, CA, p 4-1, May 1978. Curlee, R. M., Hawn, D. N., SAND78-0837, Sandia LaboratorieslAlbuquerque, NM, July 1978. Kleinpeter, J. A., Burke, F. P., Proceedings of EPRI Contractors' Conference on Coal Liquefaction, ElecWic Power Research InstiRutelPalo Alto, Ca, p 11-1 May 1978. Lewis, H. E., Weber, W. H.. Usnick, G. B., Hollenack, W. R., Bhir, H. O., Baykin, R. G., Quarterly Technical Progress Report for July-Sept 1978, Catalytic Inc.lWilsonville, Ala. (in process). Neavel, R. C., Fuel, 55(3), 237 (1976). Plett. E. G., Alkidas, A. C., Flogers, F. E., Summerfield, M., Fuel, 58(3), 241 (1977).
147
Scheller, J. E., Farnum, B. W., Sondreal, E. A,, Am. Chem. SOC. Div. fuel Chem. Prepr., 22(6) 33 (1977). Thomas, M. G., private communication, Nov 1978. Thomas, M. G.. Granoff, B., Fuel, 57(2), 122 (1978). Thomas, M. G., Noles, G. T., SAND78-0088, Sandia Laboratwies/Albuquerque, NM, April 1978. Traeger, R . K., Bickel, T. C., Curlee, R . M., Annual Report for October 1977 to September 1978, SAND79-0150, Sandi Laboratories/Aibuquerque,NM. May 1979. Traeaer, R. K.. Curlee, R. M., SAND78-1124, Sandi LaboratorieslAlbuquerque, NM, June 1978. Traeger, R. K.,Curlee, R. M., SAND78-1872, S a d i LaboratorieslAlbuquerque, NM . ...., oct - -. 1978 . .. - . Whitehurst, D. D., Proceedings of EPRI Contractors' Conference on Coal Liquefaction, Electric Power Research InstitutdPalo Alto, Ca, p 1-1, May 1978.
Received for review May 4, 1979 Accepted January 7, 1980 This work was supported by the United States Department of Energy (DOE) under Conract Number DE-AC04-76DP00789, This paper was presented at the American Institute of Chemical Engineers National Meeting in Houston, T X , Apr 2, 1978.
IV. 53rd Colloid and Surface Science Symposium Rolla, Missouri, June 1979
Formation of Ultrafine Fe,O, Aerosols from a Flame Supported Reaction Pierre! G. Vergnon" and Habib Batis Landoulsi Laboraltoire de Catalyse AppliquBe et CinBtique HBtBrogBne de I'UniversitB Claude Bernard (Lyon I) associ6 au C.N.R.S. (L.A. No. 23 1). 139622 Villeurbanne Cedex, France
Ferric oxide is prepared in an oxhydric flame from FeCI, vapor. Gas phase reaction, nonequilibrium conditions, and the temperature distribution in the flame allow one to obtain nonporous oxide particles under different sizes (10 to 200 nm) and crystalline forms (yor a).A formation mechanism of oxide particles in the flame is proposed. A variation of the lattice parameters is observed in the case of smallest particles. The relative intensity of superstructure lines observed in y-ferric oxide depends on the particle size and it can be correlated to the magnetic properties characterized by the magnetization at infinite field.
Introduction Ultrafine powders (size
