Energy & Fuels 1991,5, 283-289
Art i c 1es Rheology of Slurries of Coal and Vacuum Bottoms Michio Ikura Energy Research Laboratories, Canada Centre of Mineral and Energy Technology, 555 Booth Street, Ottawa, Ontario, Canada K I A OGl Received March 6,1990. Revised Manuscript Received October 5, 1990
The rheology of coal-oil slurries was investigated by using high-volatile bituminous coal and a lignite with a medium crude oil and vacuum bottoms derived from conventional oil and synthetic crude. Experiments were conducted under atmospheric pressure and moderate temperatures using a Haake viscometer equipped with a Searle type sensing head. The viscosities of oils ranged from less than 50 to more than 2 million mPa-s. The shear rate was varied from 1 to 200 s-*, coal concentrations in slurry from 0 to 56.7 wt %, and temperature from 50 to 128 "C. Three coal particle size ranges were used: 63-74,125-149, and 250-293 pm. The effects of shear rate, coal concentration, particle size, and temperature on the flow behavior of coal-oil slurries are discussed, and a master curve that can describe the relative viscosity of all slurries is presented.
Introduction The production of synthetic fuels from heavy hydrocarbons such as heavy o h , bitumens, vacuum residues, and coals has been investigated by many in recent years. In coal-oil coprocessing, finely ground coal is mixed with very heavy oils, typically vacuum bottoms, and processed under high hydrogen pressure and temperature to liquefy the coal and to upgrade the vacuum bottoms. Coal concentration in coprocessing feed is normally higher than 30 wt ?' & which involves handling feedstocks having high viscosities. Considerable information on the reaction behavior of coal and oil slurries has been accumulated. However, although extensive reviews of the rheology of dispersed phase exist,'Z relatively little has been reported on the rheological properties of slurries consisting of coal and very heavy oils such as vacuum bottoms. Thus, it is important to investigate the rheological behavior of coal and vacuum bottoms slurries in order to handle the very heavy feedstocks in synthetic fuel production. Morelands investigated the viscosity of slurries comprising low-volatile bituminous coal and mineral oil having a viscosity of 17.7 mPa*sat 25 "C and reported that at less than 30 vol 9a coal concentration the viscosity followed the Einstein equation in its fluidity form, whereas higher concentrations the slurries were non-Newtonian. Also, the slurry viscosities at high coal concentrations depended on the size of viscometer spindle and rotational speed. Moreland reported a gradual decrease in viscosity with time, which was explained by the dilution of the slurry in the immediate vicinity of the spindle caused by the shearing of a suspension resulting in the migration of coal particles from the regions of high shear rates. Further, he (1) Sherman, P. Rheology of Dispersed Systems. In Industrial Rheology; Academic: New York, 1970; Chapter 3. (2) Rutgers, Ir. R. Relative V i i t y and Concentration. Rheol. Acta 1962.2. ~. .~ ,-, 306-337. - - ~- -
(3) Moreland, C. Visccsity of Suspensions of Coal in Mineral Oil. Can. J. Chem. Eng. 1963,41, 24-28.
reported that slurries of smaller particles have higher viscosities than those of larger particles. Munro et ala4used a subbituminous coal and No.4 fuel oil having a viscosity of 48.6 mPa-s at 25 "C to study the rheology of coal-oil slurries and reported results similar to those of Moreland. However, they detected no significant variation in viscosity with respect to the particle size distribution when narrowly screened coals were employed. Low and Bhattacharya5 investigated the rheology of brown coal in a coal-derived hydrogen donor solvent and reported two major factors which alter the viscosity of coal-oil slurries as a function of storage time: moisture and the abundance of micropores. The coal's moisture caused the aggregation of coal particles, and the abundance of micropores reduced the volume fraction of free carrier oil due to the penetration of oil into the micropores. Both high moisture and abundant micropores contributed significantly to the increased viscosity. They reported that both yield stress and apparent viscosity increased sharply when the coal contained more than 10% moisture. Papachristodoulou and Trass6 investigated the rheological properties of slurries of bituminuous coal. Unlike the references cited above, this investigation concentrated on the rheology of slurries of coal fines and various oils: coal particle diameters 10-40 pm (median particle size), coal concentration 0-60 wt %, and viscosities of oils 3.8-1090 cSt at 37.8 "C. The coal-oil slurries were classified Newtonian up to 30 wt % coal concentration and Bingham plastic at higher concentrations. It was reported that both viscosity and yield stress of coal-oil slurries decreased with increasing median particle size and de(4) Munro, J. M.; Lewellyn, M. M.; Crackel, P. R.; Bauer, L. G. A Characterization of the Rheological Properties of Coal-Fuel Oil Slurries., AIChE J. 1979,25(2), 355-358. (5) Low, G. S.;Bhattacharya, S. N. The Effect of Moisture on the Rheology of Brown Coal-Oil Suspensions. Can. J. Chem. Eng. 1983,61, 785-790. (6) Papachristodoulou, G.; Trase,0.Rheological Properties of Coal-Oil Mixture Fuels. Powder Technol. 1984,40,353-362.
0887-0624/91/2505-0283$02.50/0 Published 1991 by the American Chemical Society
Zkura
284 Energy & Fuels, Vol. 5, No. 2, 1991 Table I. Analyses of Coals Coronach (Saskatchewan Lingan (Nova lignite) Scotia bituminous) as as received drv basis received drv basis proximate analysis, wt%
moisture ash volatile fixed carbon ultimate analysis (moisture-free basis), w t % carbon hydrogen sulfur nitrogen ash oxygen (by diff)
13.5 16.1 34.6 35.8
0.0 18.6 40.0 41.3
56.20 3.75 0.81 0.71 18.60 19.93
2.46 6.15 35.00 56.40
0.00 6.31 35.90 57.80
79.70 5.22 1.63 1.93 6.31 5.21
creasing slope of the particle size distribution. Ozum et a17 investigated the viscosities of coalanthracene oil and coal-bitumen slurries up to 50 wt % coal at atmospheric pressure and moderate temperatures. The viscosities were 450 CPat 20 "C for anthracene oil and 40000 CP at 20 "C for bitumen. It was reported that coal-anthracene oil slurries became pseudoplastic at higher coal concentrations whereas coal-bitumen slurries remained Newtonian. As evident in the above, the rheological behavior of coal-oil slurry depends on shear rate, coal concentration, particle size, coal particle distribution, coal moisture, oil type, and physicochemical interaction between coal and oil. These are, in essence, the manifestation of the hydrodynamic interaction between particles, particle orientation in slurrying oil, collision between particles, mutual exclusion of particles, doublet and higher order agglomerate formation, and physical interference between particles at very high particle concentrations.8 These are in addition to the apparent increase in coal concentration due to the penetration of oil into micropores of coal. It is also evident that little is reported on the behavior of slurries of coal and vacuum bottoms which are solid at room temperature. This paper describes the rheological behavior of coal-oil slurries using high-viscosity oils for the eventual utilization of data in synfuels production through coprocewing. The effect of shear rates, coal rank, concentration, particle size, and temperature on the flow behavior of coal-oil slurries is discussed. A master curve that can describe the relative viscosity of slurries is presented. Experimental Section Materials. A Coronach (Saskatchewan) lignite and a Lingan Harbour (Nova Scotia) bituminous coal were used to examine possible physicochemical interaction between oils and coals. The bituminous coal was dry, whereas the as-received lignite was wet. It was air-dried for 1-2 days, then crushed to approximately 1 mm in diameter, followed by further grinding in a Brinkmann grinding mill. Coal was sieved into three fractions: 50-60 mesh (297-250 pm), 100-120 mesh (149-125 pm), and 200-230 mesh (74-63 pm). The three fractions were stored in argon until mixed with oils. The analyses of the coals are given in Table I. (7) Ozum, B.; Stefurak, G. R.; du Plessis, M. P. Viscosity of Coal-Bitumen and Coal-Anthracene Oil Slurries. Presented at the 33rd Annual Technical Meeting, The Petroleum Society of CIM, Calgary, Alberta, Canada, June 6-9, 1982. (8) Thomas, D. G.Transport Characteristics of Suspension: VII. A Nota on the Viecosity of Newtonian Suspensions of Uniform Spherical Particles. J. Colloid Sci. 1965, 20, 267-277.
Table 11. Analyses of Vacuum Bottoms Fosterton
Athabasca elemental analysis, wt YO carbon hydrogen oxygen nitrogen sulfur 525 "C + fraction pentane insolubles toluene insolubles
Blend 24 IPPL
82.60 9.69
