Characterization of coal—oil mixtures by a pseudo diffusivity parameter

Characterization of coal-oil mixtures pseudo diffusivity parameter George

E. Klinzing

and James

by a

M. Ekmann*

Chemical/Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA *US Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, PA 15236, USA (Received 2 March 1982; revised 13 January 1983)

For the use of coal-oil mixtures as combustion fuels stable maintenance of the solid-liquid suspension is vital and for this reason the settling properties of these mixtures were studied with a system of ultrasonic sensors. The sedimentation theory of Smiles, which relies on unsteady state diffusion analogies, was applied successfully to the coal-oil mixtures and pseudo diffusion coefficients were determined. (Keywords:

coal; coalail

mixtures;

diffusivity

parameter)

Sedimentation can be a help or a hindrance according to the process of interest. In coal liquefaction processes, the settling of line particulate matter from the product liquid is essential. In coaloil mixtures for combustion, however, the goal is to keep the solid in a homogeneous suspension for as long as possible and many studies of the settling process have been made. Analyses range from the continuity wave theory analysis of Kynch’ to the use of the Fokker-Planck’ equation based on diffusional theory. The present analysis of settling was undertaken to understand the nature of the settling process that occurs in mixtures of line coal and oil. The linearized FokkerPlanck equation using an average diffusivity treats the flow of liquid through the settling particles with Darcy’s law of porous media, reducing the permeability concept to one of diffusivity. This equation was applied in the present work, the ability to classify coal-oil mixtures by use of the single parameter of diffusivity having considerable appeal and utility. THEORY A number of factors are of importance in any settling process. The settling of coal-oil mixtures is further complicated by chemical interactions that may occur between the various coal and oil types. The uncertain degree of chemical interaction can cause extreme difficulty in obtaining a predictive model for settling. This present study will determine the diffusivities of such coal-oil mixtures from experimental data. The basic diffusion process has been treated by Smiles’, considering the diffusion of volumetric liquid content per volume of solid, i. This variable can be written in terms of the volume of the solids and the depth of the slurry as a unit: [ = (dz/dm) - 1

(1) where z = depth of slurry; m =cumulative volume solids per unit area of the column. Following the principles of soil physics a potential function is used giving the final diffusion as: OOM-2361/83/1011534,3$3.00 @ 1983 Butterworth & Co. (Publishers)

Ltd

a4 -&J

(2)

where 4 = total potential of the liquid. The potential can be written as: (3)

4=$+h,+(r,-y)(M-m) where

I,$=interaction term between the liquid and solid h,=depth of slurry ye= specific gravity of solid y = specific gravity of liquid M = total volume of solids/unit area of column The diffusivity constant is defined as the mean value of the expression : -- K

dy

(1+ iW5 In this case the parameter K is the permeability ofthe fluid through the solids. The entire grouping depends on the interaction between the solid and the liquid. The initial condition for the basic equation is: t=O;

~=(II/,-h,)+(y,-y)(M-m)

OdmdM

and the boundary conditions for the upper surface and zero flux at z = 0 are: 4=(tiO+ho);

3~0. am’

m=O

m=M;

t>O

t>O

’

The terms $. and Ic/,,, which represent a interaction at time zero and at the interface have been assumed equal to zero by Smiles. mind, the solution can be taken from Carslaw and has been modified by Scott4 to give

liquid-solid respectively, With this in and Jaeger3

$=l-crexp(-fit)+af(2n+3)-3 n=O m ( - l)“exp( - (2n + 3)2pt)

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Characterization of coal-oil mixtures: G. E. Klinzing and J. M. Ekmann Tab/e 1 Pittsburgh

seam coal analysis As received

Moisture

(wt%)

(wt%)

Proximate analysis Moisture Volatile matter Fixed carbon Ash

1.7 36.9 52.6 8.8

NA 37.6 53.5 8.9

Ultimate analysis Hydrogen Carbon Nitrogen Sulphur Oxygen (difference) Ash

5.2 73.8 1.5 1.8 9.0 8.8

5.1 75.1 1.5 1.8 7.6 8.9

Heating value J g-r

Tab/e 2

30 806

free

31338

No. 6 fuel oil properties

Ultimate analysis c (wt%) H (wt%) N (wt%) s (wt%) Heating value (J g-l) Specific gravity at 26°C at 71°C Surface tension at 23’C Pour point (“C) Flash point (“C) Fire point (“C) TGA Order of volatile Molecular weight Vanadium (ppm)

employed in a manner so as to construct a pseudo interface. The Kynch continuity-wave theory’ and the Smiles diffusion theory analysis’ both rely on an interface observation. Neither of these theories is specifically designed for settling when no interface exists. In general, with a volume percent of spherical solids > 10% there is hindered settling with an interface present. The 30 wt% of coal in oil corresponds to =24 ~01% so that hindered settling should occur. Recent studies on crushed glass of similar sizes and shape (angular) as the coal used in this work indicated that for a 15 ~01% solids, an interface was not observed. From the signals from the ultrasonic sensors used on the coal-oil mixtures abrupt changes in concentrations were not observed in studies that took place over 150 h of settling. An abrupt change in signal should be indicative of an interface movement. This data tends to indicate lack of an interface in 30 wt% coal in oil mixtures. Using the ultrasonic sensor data a concentration wave velocity can be found as:

az

(mN m-l)

87.0 11 .o 0.3 1 .o 43012 0.960 0.934 32.8 -4 141 197 1 430 0.31

where

C-1

L’w =- at

(5)

c

Figure 1 can yield various values of u at different times by connecting horizontal lines between the two data curves (upper and lower measuring points which are 6 inches (152.4 mm) apart) and noting the time necessary for the concentration wave to move through this distance. An interface can then be constructed as:

z = - v,t

(6)

Using this technique for a pseudo interface the Smiles diffusion theory as given by Equation (4) produces a pseudo diffusion coefficient. Figure 2 shows a comparison between the predicted and experimental values of the settling height employing the diffusion theory analysis. This settling height ratio is directly proportional to the left

Q = cumulative volume of liquid Q, = volume of liquid at t = co CI= 32/x3 fl= &c2/4M2 Solution of Equation 4 for B can be found by numerical techniques. EXPERIMENTAL The experiments describing the coal-oil mixtures have been given in detail by Ekmann and Bienstock’. The use of ultrasonic sensors permits determination of solids concentration and the concentration wave velocity. The coal used was a Pittsburgh seam coal, 90% passing 200 US mesh (74 pm) and the oil was a No 6fuel oil. The temperature of the study was 60°C. The average particle size of the coal by microscopic analysis and averaging procedures was 32 pm. Tables 1 and 2 give the analyses of the coal and the fuel oil used. The coal-oil mixture concentration was 30 wt’?, a number of different nonionic additives being used. Figure 1 shows typical output data from the ultrasonic sensors, which were calibrated by a weight analysis. The initial settling height was held constant at 0.76 m. Analysis

As an interface cannot usually be observed in coal-oil mixtures the data from the ultrasonic sensor must be

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FUEL, 1983, Vol 62, October

g10-

I

_I

I

50 Time,

h

100

Figure I Suspended solids (wt%) versus time (h). 30% initial coal concentration in No.6 fuel oil. 0, Lower ultrasonic probe response; A, upper ultrasonic probe response

Characterization of coal-oil mixtures: G. E. Klinzing and J. M. Ekmann Table 3 Summary

of data analysis for COMsa --

Run

a he = 3 ft (91.4

Additive

Oil

None None None BASF U-14734 Cobey No. 1 ICI-16-540 BASF ES-7263 DOW XF43130 22

Gulf No. Gulf No. No. No. No. No. No.

cm); temperature,

60°C;

D [cm2 h-l low sulphur oil 6 fuel oil high sulphur oil 6 fuel oil 6 fuel oil 6 fuel oil 6 fuel oil 6 fuel oil 6 fuel oil

initial coal concentration,

2.00 7.30 2.45 14.18 3.93 -0 11.73 17.13 8.88

I

Residual sum squares 2.26 1.13 2.12 1.52 8.50

x 1O-3 x10-2 x 10-s x 1O-2 x 1O-2 -

5.11 x10-2 3.58 x lO-5 1.05 x10-2

30 wt%

low sulphur oil (Nigerian crude) and a Gulf high sulphur oil (Venezuelan crude). Various non-ionic additives were used to increase the stability of the coalloil mixtures, i.e., to decrease sedimentation rates. The system without an additive, with the lowest rate ofliquid moving through the solids, ie, the lowest diffusivity, was found to be the most stable. Table 3 shows run 1 with Gulf low sulphur oil with a diffusivity of 2 cm2 h - ’ as the most stable while the least stable system has a diffusivity over eight times greater than this value. It can also be observed from Table 3 that some additives have a destabilizing effect at higher diffusivities. One additive with good stabilizing action is ICI-16-540, which produces a diffusivity of practically zero. The present analysis affords a consistent measure of coal-oil mixture stability through the measurement of a pseudo diffusion coefficient. The dependency of the pseudo diffusion coefficient on coal type and size, oil type and chemistry, temperature concentration of solids and initial height and surfactant nature is currently being investigated. Figure 2 Comparison of experimental and calculated results for settling of coal-oil mixtures. Ho, Initial slurry height; H,, final slurry height. -, perfect correlation

REFERENCES 1

side of Equation (4). It is noteworth that this theory predicts the derived settling height over the entire settling time of 15Ck200 h. Table 3 gives a tabulation of the diffusion coefficients for the various coal-oil systems involving Pittsburgh seam coal and No.6 fuel oil a Gulf

Kynch, G. J. Trans. Faraday Sot. 1952,48, 166 Smiles, D. E. Chem. Eng. Sci. 1976,31,273 Carslaw, H. S. and Jaeger, J. C. ‘(?onduction of Heat in Solids’, 2nd ed., Oxford University Press, Oxford, 1959 Scott, K. J. Chem. Engr. Sci. 1978,33, 793 Ekmann, J. M. and Bienstock, D. 2nd Int. Symp. on COM Combustion, Danvers, MA, 1979

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