Ind. Eng. Chem. Fundam. 1981, 20, 165-168
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EXPERIMENTAL TECHNIQUES
Indirect Measurement of Inclined Sedimentation for Ash in Coal Liquids at High Temperature and Pressure by X-ray Photography K. R. Valdyanathan, Joseph D. Henry, Jr., and Francls H. Verhoff Department of Chemical Engineering, West Virginia Universb, Morgentown, West Virginia 26506
An X-ray measurement technique for determining sedimentation rates in opaque liquids at high temperatures and pressure (Ondeyka et al. 1978) has been modified to permit inclined settling. This indirect X-ray measurement is desirable because the sedimentation process is not dstrubed. Settling rates of mineral matter in Solvent Reflned Coal are reported for 0 = 50, 70, and 90' to the horizontal and various doses of antisolvent. The X-ray measurement provides both the initial settling rate and the height of the compression region. The initial inclined settling rates are compared with the vertical settling rates as derived from the theory of Nakamara and Kuroda (1937).
Introduction Sedimentation phenomena are very difficult to observe in opaque liquids at high temperature and pressure by direct sampling methods. The motivation for developing the experimental technique discussed here was the problem of observing the sedimentation of mineral particles in coal-derived liquids at high temperature and pressure in an inclined settler. Often antisolvents are added to coal-derived liquids to precipitate asphaltic or preasphaltic fractions, which promote the agglomeration of the mineral matter (Verhoff et al., 1978). In addition to observing the initial settling rate it is critical to observe the buildup of the compression zone in such systems. Prior to the experiments performed here, there was a concern that the asphaltic-precipitate-mineral matter agglomerates may stick to the inclined plates. Previous investigators (Gorin et al., 1977; Burke, 1976; Huang and Fischer, 1976; Rodgers et al., 1977) have measured the settling rate of mineral matter in coal derived liquids by direct sampling, which is time consuming and expensive. Ondeyka et al. (1978) developed an indirect X-ray technique for a vertical settler which avoids the direct sampling problem and permits rapid measurement of solids settling in opaque liquids such as coal-derived liquids at high temperature and pressure. Vaidyanathan (1978) utilized the above technique to study the important variables that would affect the antisolvent-induced agglomeration and sedimentation process. The influence of mixing has been studied extensively for Soltrol-130 as antisolvent and is reported elsewhere (Vaidyanathan et al., 1980). The effect of other variables such as antisolvent type and dose, mixed antisolvents, and temperature on the initial settling rate has been discussed (Vaidyanathan et al., 1979). I t has been shown first by Boycott (1920) and later by others (Gosh and Vohra, 1971; Nakaruma and Kuroda, 1937; Zahavi and Rubin, 1975) that under the same experimental conditions, the initial settling rate can be increased by a factor of 3 to 4 using an inclined settler 0196-43 13/8111020-0165$01.25/0
compared to a vertical settler. This concept was extended to solids settling in coal derived liquids at high temperature and pressure. The following modifications were considered essential to the existing high temperature, high pressure settling chamber. (1)The support for the stainless steel settling chamber was altered to allow tilting to any desired angle of inclination. (2) The X-ray film holder must be inclined and be in phase with the settling chamber. (3) A universal connection between the bottom valve in the settler and the autoclave exit valve was needed. Experimental Section Equipment. The experiment setup for inclined settline studies is shown in Figure 1. The detailed design of the stainless steel chamber can be found elsewhere (Ondeyka in. thick aluminum ring et al., 1978). A semicucular is rivetted to the bottom of the insulation box in which the settling chamber is housed. A 3/a in. wide slot runs along the circumference of the aluminum ring. The whole assembly is then seated on a steel plate which is bolted to the base structure and tightened in its place with a wing nut. There are markings along the rim of the ring for every 10' from 0" to 90'. To tilt the settler, the wing nut is loosened, the settler is positioned in any desired angle, and the wing nut is tightened. The X-ray f i holder is rivetted to the back of the insulation box so that the holder can be tilted along with the settler. A universal connection is used between the bottom valve of the settler and the autoclave exit valve and a stainless steel flexible hose is used to connect the gas line with the valve on top of the settler. The settling chamber and X-ray source are placed in a lead-lined box and conneded through the box to a 300-mL Parr-Instrument autoclave. The autoclave can be heated to 400 "C and can withstand pressures up to 2000 psig. An electric motor mounted on top of the autoclave produces a continuous range of speeds from 0 to lo00 rpm. Details regarding the X-ray equipment and densitometer used can be found elsewhere (Ondeyka et al., 1978). Material. Solvent Refined Coal (SRC) filter feed obtained from the SRC Pilot Plant in Wilsonville, Ala., is 0 1981 American Chemical Society
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UNIVERSAL
151
0 S R C f 3 l l b l a n k , @=go'
9 OCM/HR
f
lo
0
S R C 1 3 O b l a n k , 8-70.
a
n-decane/SRC=O.25, 0=90'
0
n d e c a ~ e / S R C = O . Z 5 ,9=?0'
GASTANK
ir:TCCibVE I
YV
Figure 1. High pressure, temperature inclined sedimentation ap-
I
paratus.
used in this work. The SRC contains three parts of recycle solvent for each part of Monterey coal. Procedure. The settling chamber is positioned at any desired angle of inclination and secured. The bottom valve of the settler is connected to the autoclave exit valve using the universal connection and the top valve of the settler to the gas line using a flexible hose. The sedimentation chamber is then pressurized with nitrogen and heated to operating temperature. During this process, the autoclave is charged with about 10 mL of coal-derived liquid and brought to the operating temperature. Anti-solvent such as n-decane is added unheated to the autoclave and mixed initially at 550 rpm for 2 min, followed by 8 min of mixing at 250 rpm since sequential mixing was found to be better than constant, single-speed mixing (Vaidyanathan et al., 1980). The content of the autoclave is then transferred to the settler by creating a pressure difference between the cell and the autoclave. The sedimentation process is then monitored by X-ray photography. Pictures were usually taken at intervals of 3,5,10, 15, 20, 30,45, and 60 min. The interface heights are then located as a function of time using a densitometer and plotted. The slope of the constant-rate period of interface height vs. times gives the initial settling rate and is reported in cm/h.
Results and Discussion Experiments were conducted at 400 "F with and without antisolvent addition to Solvent Refined Coal. The inclined settler for the experiments was positioned at 50,70, or 90° from the horizontal. The antisolvent used was n-decane with anti-solvent to oil ratios of 0.0,0.1, and 0.25 by volume. The influence of initial height was also measured in these experiments. From the X-ray photographs it was apparent that the agglomerates, which are formed and transferred to the inclined settler, do not stick to the upward facing inclined plate but they accumulate in the bottom of the chamber. This suggests that inclined settling can be successfully employed for antisolvent-induced agglomeration and sedimentation processes. The data as obtained from the X-ray photographs is a series of interface heights as a function of time. Usually the interface heights were obtained from the X-ray photograph visually, but occasionally the densitometer was used. The interface height is plotted as a function of time in Figure 2 for four selected experiments. The settling rate is then determined by measuring the slope of the height vs. time curve initially. These settling curves definitely
0
40
20
60
TIME,MIN-
Figure 2. Interface height as a function of time for both vertical and inclined settling with and without antisolvent at 400 O C .
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13 1 ) S R C A N D N DECANE 70
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TEMPERATURE=400*F P R E S S U R E = 1 5 0 PSlG M I X I N G SPEED=550+250
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16
18
Figure 3. Influence of initial height on inclined settling of ash in coal liquids.
illustrate the increased settling rate to be obtained by the addition of the antisolvent, n-decane, and by the change of angle from the vertical (90" from the horizontal) to an angle of 70" from the horizontal. It should be noted that the major error in determining the sedimentation rate involves the estimation of the initial slope (see Ondeyka et al., 1978). In contrast to vertical settling, the initial rate of inclined sedimentation depends upon the initial height of the suspension in the batch sedimentation chamber. Figure
Ind. Eng. Chem. Fundam., Vol. 20, No. 2, 1981
167
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TEMPERATURE=400'F PRESSURE- 1 5 0 PSIG M I X I N G SPEED=550-+250 MIXING TIME=2+8
INITIAL HEIGHT=lO CM
N-K MODEL PREOICTION M O D I F I E D N-K MODEL F O R S/O-0.25 M O D I F I E D N-K MODEL F O R S / O = O . l O M O D I F I E D N-K MODEL F O R S/O-O.OO
S/O = 0 . 2 5 s/o=o.10
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Figure 4. Comparison of experimental data with inclined sedimentation theory.
3 contains the initial settling rate as a function of suspension height for all of the experiments performed. It can be seen that the influence of height becomes more significant as the amount of antisolvent is increased. It is suspected that the solids slide down the inclined plate better with the additional antisolvent. There have been several theories derived to quantify inclined batch sedimentation. Two of these theories will be compared with the data. Nakamura and Kuroda (1937) used the conservation of mass and the geometry of the settling system to derive the following formula. They said that the settling rate in an inclined tube would be equal to the settling rate in the vertical tube multiplied by the total upward facing area in the inclined tube (that at the top of the tube plus that under the downward facing plate) divided by the area across the top of the tube. vi,, = u,,, (1 + Ho cos B/b) where Uhc = inclined settling rate, U,,, = vertical settling rate, Ho = vertical height of the slurry (Figure l), b = distance between the top and bottom inclined plates (Figure l),and 6 = angle of the settler from the horizontal. The equation does not always fit the data and hence modifications of this equation and other derivations have been presented. One simple empirical modification of Ghosh and Bohra (1971) was also used to correlate the data obtained from the settling of the ash in coal liquids. u,,c/u,,,= 1 + F(H0 cos 6 / b ) where F = correction factor. Several other possible models could be presented and used, but that is not the goal of this paper. Figure 4 contains the data plotted as the enhancement factor (ratio of inclined to vertical settline rate) vs. the parameter suggested by the model of Nakamura and Kuroda, i.e., Ho cos 6lb. It can be seen that the actual en-
11
0
0.1
0.2 ANTI-SOLVENT/OIL
0.3
R-rio-
Figure 5. Influence of antisolvent dose for settling at three different angles.
hancement factor is significantly less than that predicted by the N-K model and in fact the correction factor must take a value of 0.238 to 0.848 in order for the model to fit the data. The data for the solvent to oil ratio of 0.25 yields higher correction factors; Le., they more nearly approach the N-K theory. This may result because the particles produced by the antisolvent addition more readily slide down the lower inclined plate. Also, the antisolventa may reduce the charge on the settling particles and thus permit a more concentrated slurry to form on the lower plate. One of the important factors in the utilization of inclined plates in antisolvent removal of ash from coal liquids is the influence of antisolvent dose on the sedimentation rate for different angles of inclination. Figure 5 gives a presentation of this dependency for a constant initial slurry height of 10 cm. On a semilog plot the influence of antisolvent dose appears to be similar independent of the angle of inclination. Thus the ratio of improvement in settling rate for any given addition of antisolvent will be about the same independent of the angle of inclination. Conclusions The X-ray technique of measurement of sedimentation in opaque liquids at high temperatures and pressures has been successfully modified to permit batch inclined settling. The ash in coal liquids a t 400 O F (with or without antisolvents) settles more rapidly in an inclined chamber than in a vertical chamber and the solids do not stick to the sides. All of the experimental inclined settling rates fall below that predicted by the theory Nakaruma and Korada. The high antisolvent to oil ratio experiments yield settling rates which most nearly agree with this N-K theory. Antisolvent dose influences the inclined settling rate similarly for all angles of sedimentation. Finally, the combination of antisolvent and inclined settling can in-
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Ind. Eng. Chem. Fundam. 1981, 20, 168-171
crease the sedimentation rate by a factor of 7.
Ondeyka, J. G.;Henry, J. D.;Verhoff, F. H. Ind. fng.Chem. Fondem. 1978,
Literature Cited
Rodgers. 8. R.; Katz, S.; Westmorelena, P. R. "Supporting Research and Deveboment on Setmations Technoloav: -. Final Rewrt” ORNL/TM 5843. Oct 1977. Vaklyanathn, K. R. Ph.D. Dism‘tatbn, West Virginia Unhrerslty, June 1978. Valdyanathan, K. R.; Henry, J. D.;V e h f f , F. H. Sep. Scl. Techno/. 1979,
Boycott, A. E. Natwe (London) 1920, 104,532.
Burke, F. P. “Assessment of Anti-Solvent Requirements for Wilsonville SRC Power Plant: Batch Studies”, CONOCO coal Development Company Memorandum Rh4-14224,Nov 1976. Gorln, E.; Kulik, C. J.; Lebowltz, H. E. Ind. Eng. Chem. process ~ s mV. . 1977, 16,95. Gosh,0.;Vohra, 0.K. Indian Chem. Eng. 1971, 13, 32. Huang, H.; Flscher, J. “Laboratory Studies for Separation of Solvents from Synthdl Gross Product”, Argonne National Laboratory Report ANL 76-9
77. 217.
14(2),107. Vaidyanathan, K. R.; Vehoff, F. H.; Henry, J. D. A I M J. 1980, 26, 83. V e h f f , F. H.; Lui, S. S.; Henry, J. D.,Jr. Can. J. Chem. fng.1978, 56,
504. ZahaVl,
E.;Rubin. E. M .Eng. Chem. Process Des. Dev. 1975, 74, 34.
Received for review September 14, 1979 Accepted December 5, 1980
(1976). Nakamura, H.; Kuroda, Kk. KeJo J. Med. 1937, 7(8), 256.
COMMUNICATIONS Capture of Aerosols on a Sphere in the Presence of Weak Electrostatic Forces The aerosol filtration efficiency due to weak electrostatic attraction between a dust particle and a uniformly dtstributed surface charge on a spherical granule is computed by calculating the dust particle trajectory starting from the nodal point. A comparison of the order of magnitude of different collecting mechanisms such as diffusion, interception, and gravity with weak electrostatic forces is also presented.
Introduction A common occurrence during the operation of granular bed filters (fixed, fluidized, or moving beds) is the electrostatic charging of the dielectric granules due to the mutual friction between the granules and between the granules and the container wall. This phenomenon, called triboelectrification, results in a higher collection efficiency of aerosol (dust) particles in a granular bed filter than that which is predicted taking into account only the classical mechanisms of inertia, diffusion, interception, and gravity (Tardos et al., 1979; Gutfinger and Tardos, 1979; Figueroa and Licht, 1978; Balusubramanian et al., 1978). In a recent paper (Tardos and Pfeffer, 1979) a theoretical model was developed to compute the collection efficiency of a small charged dust particle moving past a charged single sphere in an infinite fluid or a charged spherical granule in a granular bed filter using the “limiting tranjectory computation” method (Nielsen and Hill, 1976; Gutfinger and Tardos, 1979). In addition to electrical effects, different mechanisms including interception, inertia, and gravity were considered to contribute to the deposition of the small particles on the collecting sphere. It was shown that the single particle collection efficiency of the sphere, E , can be determined from the computation of the trajectory of each dust particle described by the dimensionless equation
Here d2gi/dT2is the dust particle acceleration, d z / d T is the dimensionless particle velocity, ii is the gas velocity, St is the Stokes number, Ga is the Galileo number, Rp is the interception parameter, e‘g is a unit vector in the direction of the gravitational force, and u’, is the dimensionless “velocity” due to electrostatic forces defined by
where a is the collector radius, mp is the mass of t$e dust particle, Uois the undisturbed flwd velocity, and F, is the pertinent electrical force. Different expressions for ii, are obtained for different types of electrical forces. For the simplest (but unrealistic) case of a point interaction (collector and dust particle are both considered to be point charges and attract each other), u’, becomes
(3) where R is a dimensionless radial coordinate rla, & is a unit vector in the radial direction, and K , is a positive dimensionless electrical constant (Nielsen and Hill, 1979) defined by
K, =
CQcQp
- 2CrpQcQ,
247r2tfrpa2pU,
3Wu0
(4)
If the collector is dielectric and has a uniformly distributed surface charge, ii, becomes (Tardos and Pfeffer, 1979) (5)
The solution of eq 1 with ii, given by either eq 3 or 5 requires a complicated numerical procedure. This note presents a simple solution for the practically important case of a uniformly distributed surface charge on the granule when the electric constant Kc is small ( K , < 0.1). Furthermore, a comparison of the order of magnitude of different mechanisms as they affect particle collection by a sphere is presented.
0196-4313/81/1020-0168$01.25/00 1981 American Chemical Societv
