294
Ind. Eng. Chem. Process Des. Dev. 1980, 19, 294-300
Table VI. Comparison of t h e Use of Experimental vs. Predicted Compositions for Enthalpy and Viscosity Predictions
pro pert y liquid enthalpy vapor enthalpy viscosity at 100 " F (light) viscosity at 2 10 F (light) viscosity at 100 F (heavy) viscosity at 210 " F (heavy)
av deva
no. of fractions
exptl compn
pred compn
435 292 38
2.4 Btuilb 3.7 Btullb 2.9%
2.7 Btu/lb 3 . 5 Btullb 2.8%
38
5.7%
5.2%
10
8.1%
8.5%
10
3.5%
3.5%
a Absolute deviation = predicted property - experimental property. Deviation, % = [(predicted property - experimental property)/experimental property] x 100. av = (l/N) z 1 deviation 1 . iv'' = number of data points.
API Research Project 44, Selected Values of Properties of Hydrocarbons and h a t e a Compounds' , Taoles of Physical and Thermodynamic Properties of Hydrocarbons. A and M Press, College Station, Texas (extant 1978). A S M.E. Research Cornminee on Lubrication, 'Viscosity and Density of Over 40 Luoricating Fluids of Known Composition at Pressures to 150,000 psia and Temperatures to 425 OF ', Report No. 1. 1953. Boelhower. C.. Waterman, H. 1.. J . Inst. Pel., 40. 116 (1954). Hili. J B.. Coats, H. B.. Ind. Eng. Chem.. 20, 641 (1928). huang. P. K.. Ph.D Thesis, Department of Chemical Engineering, The Pennsylvanla State University, University Park, Pa., 1977. hdang. P. K.. Daubert, T. E., Ind. Eng. Chern. Process Des. Dev , 13, 359 (1974). Kurtz. S.S IJr.. King. R. W., Stout. W. J.. Peterkin, M. E., Anal. Chern , 30, 1225 (1958). Kurtz. S. S , Jr., Ward, A. L.. J . Franklin Inst.. 222. 563 (1936). K d z , S. S., Jr.. Ward. A. L., J . Franklin Inst.. 224, 583. 697 (1937). Lenoir. ". M.. Hipkin. H. G., J . Chern. Eng. Data. 18, 195 (1973). Private communications, Witco Chem. Co., 1973. Private communications. Pennzoil Co., 1975. Private communications, Indbstrial Co., 1977. Riazi, M. R.. Ph.D. Thesis. Department of Chemical Engineering, The Pennsylvania State University, University Park, Pa.. 1979. Van Nes, K , Van Westen, H. A.. "Aspects of the Constbtion of Mineral Oils". Elsever PJOiiShing Co.. InC.. New Yorrc, 1951. Waterman. H I , Boehower. C , Cornelissen, J.. Correht;on Between physical Constants and Chemical StrLcture", Elsevier Publishing Co.. Inc.. New York, 1958.
Received f o r rerieu July 23, 19'79 . k e e p r e d December 7 , 1979
sitions do not materially affect the results. Literature Cited American Petroleum Institute (API) Research Project 42, "Properties of Hydrocarbons of High Molecular Weight", American Petroleum Institute (1962).
The Department of Refining of the American Petroleum Institute provided major financial support of this research.
Kinetics of Oxydesulfurization of Upper Freeport Coal D. Slagle, Y. T. Shah,' and J.
B. Josh1
Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 1526 1
The kinetics of the oxidation of pyritic sulfur, organic sulfur, and carbon for the Upper Freeport coal are investigated. Experiments were conducted in a semi-batch manner. The effects of batch time (0-2400 s),temperature (150-210 "C),partial pressure of oxygen (0.69-3.44 MPa), and total pressure (3.44-6.88 MPa) were studied. Two alternate mechanisms have been proposed for the oxidation of pyritic sulfur. In one mechanism the fine pyrite particles are assumed to be uniformly distributed in coal particles and the continuous reaction model was found to hoM where the rate of reaction is second order with respect to pyritic sulfur. In the other mechanism, the pyrite particles are assumed to exist free from coal and the shrinking core model was found to hold where the rate of reaction is controlled by diffusion through ash. Both the carbon oxidation and organic sulfur reactions are zero order with respect to carbon and organic sulfur, respectively. The activation energies for all three reactions agree closely with those reported in the literature.
Introduction The sulfur in coal occurs in three forms: pyritic, organic, or sulfate. Pyrites, classified as compounds with the formula FeS, (where the standard value of x is 2), accounts for the bulk of the sulfur in Eastern coals. Organic sulfur is a broad classification containing any sulfur which is chemically bound to the actual coal matrix. Organic sulfur is the dominant sulfur form in Western coals. Sulfates constitute less than a few percent of the total sulfur in most coals. The direct burning of coal causes the production of noxious sulfur oxides (S0,'s). Presently, control of sulfur oxide emissions is achieved mainly by either stack gas scrubbing or physical coal cleaning techniques. The former process is both expensive and energy intensive. The latter, although relatively inexpensive and simple to operate, is less effective. In fact, depending on the sulfur composition of the feed coal, a plant burning physically pre-cleaned coal 0196-4305/80/1119-0294$01.00/0
may also have to employ flue gas scrubbing in order to meet environmental standards (Trindade et al., 1974). A possible alternative to these processes is chemical coal cleaning, i.e., removal of the sulfur by means of a chemical reaction before burning the coal. There are presently six major chemical coal cleaning methods being developed (Oder et al., 1977). One of the promising processes is the oxydesulfurization process (Friedman and Warzinski, 1977; Friedman et al., 1977). In this process, the sulfur is removed by oxidizing coal in the presence of water. The process is operated at pressures between 1.6 and 10 MPa and temperatures between 150 and 220 "C. Normally air is used as the gas phase. The purpose of this paper is to report a kinetic study for the D.O.E. oxydesulfurization process. Kinetic rate expressions for the inorganic and organic sulfur removal reactions and carbon oxidation reaction for Upper Freeport coal are presented.
0 1980 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980 ,WET
Table I.
T E S T METER
Coal Particle Size Distribution
% of particles 0.2 0.3 0.9 18.0 58.4 22.2
I
pd*
SUPPLY
STIRRED
295
between (mesh size) ___ 40 40 60 60 100 200 100 325 200 -_. 325
between (Pm) -__ > 370 2 50 37 0 149 250 74 149 44 74 -44 ~
Table 11. Analysis of t h e Upper Freeport Coal (Moisture and Ash Free Basis)
AUTOCLAVE
~
component total sulfur pyritic sulfur organic sulfur sulfate carbon hydrogen
SUPPLY
wt % 2.0 1.19 0.76 0.07
88.1 4.7
Figure 1. Schematic diagram of the experimental apparatus.
Chemistry The oxydesulfurization process includes several reactions such as pyritic and organic sulfur oxidations, oxidation of carbon, and hydroperoxide formation at benzylic positions. A brief discussion of each of these reactions is given below. The primary steps in the pyritic sulfur oxidation are (McKay and Halpern, 1958) FeS2 + 2 0 2 FeS04 + S (1) ZFeS, 70, + 2Hz0 2Fe2+ + 4s042- + 4H+ (2)
+
2FeSz + 15/202 + H20 FeSz
-
-
2Fe3+ + 4So4*- + 2H+
+ i5/402 + 2 H 2 0 1/2Fe203+ 4H+ + 2S0422s + 3 0 2 + 2H20 4H+ + 2SO:-
-
(3) (4)
(5) Vracar and Vucurovic (1970) have found that the rate of reaction given by eq 5 is very slow and the reactions 1 and 5 are unlikely ito occur. Friedman and Warzinski (1977) have confirmed the above observations. The mechanism of the aqueous oxidation of C is complex. Most of the oxygen which reacts with the organic matrix remains attached as hydroperoxides and their decomposition products. Since “organic sulfur” is a broad classification for any sulfur atom which is bonded to the organic matrix of the coal, it is difficult to outline the possible reaction mechanisms for the organic sulfur reactions. Compounds which fall under the heading of organic sulfur include mercaptans, sulfides, disulfides, thiophenes, thiols, and thiopyrones (Meyers, 1977). Due to the nonhomogeneity of coals it is possible that any given sample may contain one or more of the above compounds. Friedman et al. (1977) have provided some insight into the chemistry of organic sulfur removal. They found that although the treatment of coal with compressed air and steam resulted in a 25% decrease in its organic sulfur content, identical treatment of dibenzothiophene (an organic sulfur compoumd) produced no reaction. Experimental Section Experiments were conducted to establish rate expressions for the major reaction systems of the oxidative desulfurization process. The independent system parameters investigated include batch time, temperature, oxygen, partial pressure, and total pressure. Apparatus. All experiments were conducted in a 2-L stirred autoclave manufactured by the Parr Instrument Co. of Moline, Ill. Due to the corrosive nature of the reaction products (sulfuric acid), a Pyrex liner was used to protect the reaction vessel, Temperature control was
provided by the Thermoelectric 400 proportional controller. In order to keep the partial pressure of oxygen constant during an experiment, the reactor was operated in a semi-batch manner (i.e., gas flowing but liquid stationary). A schematic diagram of the experimental apparatus is given in Figure 1. Materials. The coal samples used in this study were from the upper seam of the Upper Freeport mine in Kittaning, Pa. All the samples were from a single larger sample and can therefore be assumed to be identical in basic composition. Over 98% of the crushed coal sample passed through a U S . Standards 100 mesh screen (see Table I). The analysis of the coal is given in Table 11. The water used in this study was deionized in a column manufactured by Continentia1 Deionized Water Services. Cylinder nitrogen and oxygen were supplied by Air Products Inc. These gases are classified as “Extra Dry” and therefore have a minimum purity of 99.6%. Procedure. The reactor, as mentioned earlier, was operated in a semi-batch manner. The important steps of this procedure are briefly outlined below. The details are given by Slagle (1978). Once the rotameters were calibrated for the nitrogen and oxygen flow rates under the reaction conditions, the following procedure was used. (a) The reactor was charged with feed slurry and pressurized with nitrogen. The nitrogen flow rate on the rotameter was checked. (b) The reactor was heated to experimental temperature by setting the temperature controller to experimental temperature. (c) The reaction was started by opening the valve to the oxygen supply. The flow rate of oxygen was in large excess as compared to the theoretical requirements of the oxidation reactions. The residence time of the gas phase (oxygen and nitrogen) was in the range of 2-5 s and even if the gas phase is assumed to be completely back-mixed the desired partial pressure of oxygen was obtained within 20 s. (d) At the end of the reaction time, the oxygen flow was shut off and the BPR simultaneously opened to release the pressure from the reactor. After depressurizing, the reactor was cooled and the coal sample was removed and filtered in a vacuum funnel fitted with a fritted glass filtration disk. After washing with approximately 2 L of deionized water the sample was dried to a constant weight at 100 “C. The experimental data were taken in the temperature range of 150 to 210 “C, total pressure range of 3.44-6.88 MPa, oxygen partial pressure range of 0.69-3.44 MPa, coal concentration of 0.025 to 0.22 kg/L of HzO, and for the batch times up to 2400 s. All of the data were taken a t
296
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
~
-
-
6o m t 1 5“C , 0 Po21.60 .MPa
190 210
+
i
SYMBOL 1 . ‘C -
0
c
h 0
l i
3
y1
210 190 150
1.41 1.24
/
pO2 MPa
124 141
160
1200
600
0
REACTION T I M E
1800
2400
1st
Figure 3. Second-order kinetic plot for pyrite reaction-continuous reaction model. 200 0
600
1200 1800 REACTION TIME 1 %
2400
Figure 2. Percent pyritic sulfur removed vs. time.
the stirring speed of 16.6 r/s. Chemical analysis of the coal samples was provided by the Coal Analysis Laboratory of the U.S. Bureau of Mines in Pittsburgh, Pa. Each sample was analyzed for sulfur forms, carbon, hydrogen, and heating value. For this purpose, standard ASTM methods were employed. Results a n d Discussion For the process design, it is important to obtain the kinetic expressions for the important reactions occurring during the oxydesulfurization process. Three reactions considered in this study are (a) pyritic sulfur reaction, (b) carbon oxidation reaction, and (c) organic sulfur reaction. Results from the kinetic analysis of each of these reactions are discussed separately. Several experiments were performed to evaluate the effects of stirring rate, concentration of coal in the slurry, and the initial heating period (in the presence of nitrogen) on the conversions of three reactions mentioned above. At a temperature of 190 “C and an oxygen partial pressure of 1.6 MPa (total pressure 6.88 MPa) experiments were performed at two stirrer speeds, namely, 13.3and 16.6 r/s. The conversions for all three reactions were found to be essentially the same, implying that the mass and/or heat transfer effects were negligible at the stirrer speed of 16.6 r/s. At 190 “C, oxygen partial pressure of 1.6 MPa (total pressure of 6.88 MPa) experiments were performed at coal concentrations of 0.025,0.047, and 0.1 kg/L of water. The conversions for all three reactions for these three concentrations were found to be essentially the same. Since most other experiments were performed at the coal concentration of 0.1 kg/L of water, based on the above data at this concentration, the oxygen mass transfer is not a controlling factor on the overall kinetic process. Experiments were conducted to determine if any reaction was occurring during the time in which the reactor was being heated to experimental temperature. The data from these runs indicated that the reactions during this period were insignificant and can be neglected. Pyritic S u l f u r Reaction. Since the majority of the sulfur (61% of the total sulfur) in Upper Freeport coal is pyritic, the establishment of a kinetic model for the pyritic sulfur reaction was the major objective of this study. Figure 2 illustrates the conversions for the pyritic sulfur reaction at temperatures 150, 190, and 210 “C and oxygen
t I
20
\
,
2 0
21
-+
2 2
2 3
24
~103
Figure 4. Arrhenius plot for pyrite reaction-continuous model.
reaction
partial pressure of 1.6 MPa (total pressure 6.88 MPa). It was thought desirable to correlate the data by considering two alternate mechanisms. It is known that in a sample of powdered coal, pyrite can exist as free particles or fine pyrite particles may be embedded inside the coal particles. In the latter case, since the concentration of pyrite is very small, an order of magnitude calculation indicated that the intraparticle diffusional resistance is negligibly small. As a result the oxidation of pyritic sulfur can be assumed to follow a continuous reaction model (Levenspiel, 1972). The experimental data were found to correlate well by the second-order rate equation which implies that
where k2P is the intrinsic kinetic rate constant whose value depends upon both temperature and partial pressure of oxygen. An integration of eq 6 gives
(7) The validity of eq 7 is illustrated in Figure 3. The rate constants at different temperatures were calculated from the regression analysis of these data. An Arrhenius plot based on the rate constants from Figure 3 gives an activation energy of 46.5 X lo6 J/kmol for the pyritic reaction (see Figure 4). This value compares favorably well with those reported by McKay and
Ind. Eng. Chem. Process Des. Dev., Vol. 19,
No. 2, 1980 297
100
ao
0 w Y
z w
TOTAL PRESSURE=6 88 MPa SYMBOL l,% po2, MPa
-1 5 00 + 190
a LL 3
C
a
2
t
210
160 141 124
40 1200
600
a
2400
1800
REACTION T I M E [ S I
D. >
L
Figure 6. Shrinking core model-pyrite TEMP
20
reaction.
190'C
TOTAL PRESSURE = 6 88 MPa
0 p -688MPa
A ,"2-1 6 MPa
%-
0 p
-0564MPa
02-
0 600
1200
1800
2400
REACTION T I M E 1 %
Figure 5. Percent pyritic sulfur removed vs. time (effect of pop).
Halpern [1958, (55.9 >( lo6 f 84 X lo6 J/kmol], Vracar and Vucurovic [1970, 51.2 X lo6 J/kmol] and Sareem et al. [1977, 58.8 x IO6J/kmol]. The intercept of this plot yields a frequency factor (A,) of 1.14 X lo4m3/kmol at an oxygen partial pressure of 1.6 MPa and a total pressure of 6.89 MPa. The following equation holds kzP = 1.14 X lo4 exp(-46.5 X 106/RT) ( 8) The rate of pyrite oxidation was found to be independent of the total pressure, at a constant partial pressure of oxygen, in the range between 3.44 and 6.88 MPa. The effect of partial pressure of oxygen on the extent of sulfur removal is shown in Figure 5. It can be seen from Figure 5 that the rate of pyrite oxidation is rapid so as to get more than 80% conversion in about 300 s. As a result it was thought desirable not to correlate these data in view of the accuracy of analysis. In an alternate model the pyrite particles were assumed to exist as separate particles. The experimental data were analyzed on the basis, of the shrinking core model. Levenspiel(l972) has reported the pertinent details regarding the shrinking core model. It has been discussed elsewhere that the resistance provided by the diffusion through fluid film surrounding the particle is negligible. The other cases are as follows. When the rate of reaction is controlled by the diffusion through ash layer, the relationship between time and conversion is given by the equation
,
= 1 - 3(1 7
z)2/3+ 2(1 - z)
(9)
where
The equations, when the rate is controlled by the chemical react,ion a t the core surface, are (11)
where
0 2.0
2.1
+. 2.2
23
2.4
103
Figure 7. Arrhenius plot for pyrite reaction-shrinking
core model.
In order to determine the rate-controlling step in the overall reaction the experimental data were plotted on the basis of eq 9 and 11. At all t,he three temperatures, eq 9 gave the best fit. The value of r was obtained from the slopes (Figure 6). The value of diffusivity can be calculated from the equation
Thus, for instance, at 150 "C the value of r was found to be 2220 s. It was assumed that the size distribution of the pyrite particles was the same as that of coal particles and R1 is 4.2 X los2 mm. The saturation concentration of oxygen, CAL,at a temperature of 150 "C and at an oxygen partial pressure of 1.6 MPa is 1.5 X kmol/m3 (Pray et al., 1952). The value of b can be obtained from eq 4. The molar density of pyrite is 26.9 kmol/m3. Substitution of the above values in eq 13 gives De equal to 0.9 X m?/s, which compares favorably with 1.1 x m2/s reported by Wheelock et al. (1978) for pure pyrite particles. The effect of temperature on the values of diffusivity is shown in Figure 7. From the Arrhenius type of equation the slope of the straight line (In D, vs. obtained can be used to calculate the activation energy, E . In the present case E was found to be 33 X lo6 J/kmol, which is in close agreement with the value 33.5 X lo6 J/kmol reported by Wheelock et al. (1978). From the above discussion it can be seen that the experimental data can be correlated by the continuous reaction model as well as the shrinking core model. Further
l/n
298
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980 ~
90
~~
0
TOTAL PRESSURE 1 6 88 MPa
0
A
0
0
k = 000258
0.7 L Y i
VI
k = 00165
Y a LE )9
0.6
\
0.5
-
-
-
75
;
-
1
SYMBOL
t,'C
pO2.MPa
0
210
1.24
; ::; :.
k z 00425
I
,
I
TOTAL PRESSURE.6 89 MPa
10
20
30
40
0 REACTION TIME I M l N S l
600
1200 1800 REACTION TIME IS1
2400
Figure 8. Percent carbon remaining vs. time.
Figure 9. Percent organic sulfur remaining vs. time.
insight in the reaction mechanism can be obtained by studying the effect of particle size. For the case of the continuous reaction model there will not be any effect of particle size. In the case of the shrinking core model, for the same extent of sulfur oxidation, the reaction time is proportional to the square of the particle size. In order to confirm the reaction mechanism, systematic experimentation is planned to study the effect of particle size. Carbon Oxidation Reaction. Large carbon losses resulting during the chemical cleaning of the coal could make the oxydesulfurization process economically unattractive. Therefore a kinetic study of the carbon oxidation reaction in this system is of great importance. Carbon losses were found to be insignificant at 150 "C but at temperatures of 190 and 210 "C the losses were found to be significant. A plot of weight percentage carbon in the coal vs. time is shown in Figure 8. From this plot it can be seen that, at each temperature, there is a sharp change in the rate of oxidation. The decrease in reaction rate is probably due to the decrease in the number of easily oxidizable carbon atoms. In both regions the reaction demonstrates zero-order kinetics. This is consistent with the model for solid carbon oxidation reported by Lowry (1963) for temperatures less than 800 "C. Based on the zero-order rate constants, from Figure 8, an Arrhenius plot was constructed. In the initial period of high rate of oxidation, the activation energy was found to be 85.5 X lo6 J/kmol. In the later period of low rate of oxidation the activation energy is 380 X lo6 J/kmol. The value in the initial period is well within the range of values given by Lowry (1963) (73.5 x lo6 to 184.8 x IO6 J/kmol) and reasonably close to that reported by Sareen et al. (1977) (74 X lo6 J/kmol). The zero-order rate constants are given by the following equations: (i) initial fast rate of oxidation KOc = 1.469 X lo7 exp(45.5 X 106/RT) (14) (ii) slow rate of oxidation KOc = 2.88 X exp(-380 X 106/RT) (15)
in the range of operating conditions covered in this work, the loss in weight of coal during an experiment was very small. The details pertaining to the loss of coal are reported by Slagle (1978). Organic Sulfur Reaction. Because of the wide variability in the data pertaining to the organic sulfur reaction it is difficult to establish an accurate kinetic model for this reaction. The variation in these data is probably a result of the inherently low organic sulfur content (0.76% weight of coal) of Upper Freeport coal, which is compounded by the fact that there is, as yet, no reliable chemical analysis for organic sulfur compounds. In fact, the organic sulfur contents reported in this work were found by subtracting the percentage sulfates and pyritic sulfur from the total sulfur content of the coal sample. Figure 9 illustrates the degree of organic sulfur removal as a function of reaction time for temperatures between 150 and 210 "C. These data indicate that organic sulfur removal is insignificant below 190 "C. Based primarily on statistical optimization of these data a zero-order model is postulated. The rate constants (k,") shown in Figure 9 represent linear regression slopes based on this model. An Arrhenius plot was also constructed for this system and is shown in Figure 10. This plot yields an activation energy of 78.9 X lo6 J/kmol and a frequency factor of 2.1 X lo4 kg of S/kg of coal-s. Therefore the expression for the rate constant can be written as
Further, preliminary data indicates, a t a particular temperature, the rate to be proportional to the partial pressure of oxygen in the range 1.72 to 3.44 MPa. It should be noted that although the rates of the carbon oxidation reaction were significant at higher temperatures,
ko" = 2.1 x lo4 exp(-78.9 X 106/RT) (kg of S/kg of coal-s) (16)
In addition to the kinetic models, the data also illustrate several interesting trends from the process development point of view for Upper Freeport coal. Figure 11 demonstrates the effect of temperature on the removal of total sulfur. According to Meyers (1977) an Eastern coal containing 0.8% or less total sulfur meets the federal pollution standards. This sulfur content, which corresponds to a 60% sulfur removal in Figure 11 can be attained rapidly for every temperature studied, especially for the run at 210 "C where the reaction time required is of the order of a few minutes. Figure 12 illustrates the typical degree of removal of hydrogen with respect to reaction time. The removal of
Ind. Eng. Chem. Process Des. Dev., Vol, 19, No. 2, 1980 299 -5 0
TOTAL PRESSURE
30
-
6.89MPa
-6.C
20
n Y
0 > ? I
2:
W Y
::
Y i z
-7
> I
c
at 10
TOTAL PRESSURE=G 89MPa -8.C
0
~
I
I
1
I
600
1200
1800
2400
REACTION TIME IS1 ~~
21
23
22 1 T X 1000
24
Figure 12. Percent hydrogen removed vs. time. 20.0
Figure 10. Arrhenius plot for organic sulfur reaction.
t , "C 210 0 WMBOL
0
190
po ,MPa
1.24 141
SO 'OTAL
16.0
v) c
6C
9
12.0
J
w
2 I
a w
0' 5 a 3 LL
c?
I-
4
4c
80
I
i
VI 3
PRESSURE = 6 . 8 8 M P a
#.
Y
4.0
2c SYMBOL -
0
P
t,"C 210
po2,MPa -
1.24 1.41 1.60
190 150
0
0.0 0
TOTAL PRESSURE= 6.89 MPa
0
I 600
I
I
I
I
600
1200
1600
2400
REACTION TIME S I
I
I
I
1200
1800
2400
REACTION T I M E IS1
Figure 11. Percent sulfur removed vs. time.
hydrogen is, of course, harmful for the subsequent usage of coal for the prod.uctions of liquid and gaseous fuels. Figure 13 illustrates the percentage loss in heating value of coal with respect to reaction time. These data, as expected, indicate that the loss in heating value is directly related to the removal of carbon and hydrogen. This loss is insignificant for thle experiments at 150 O C and moderate for the 190 "C experiments; however, at 210 "C the heating value losses become significant. It is generally known that the removal of organic sulfur by the method used here is very difficult. Furthermore, as shown in Figure 14, the data for removal efficiency for organic sulfur obtained in this study appears to be related to the heating value loss of the coal. This plot thus indicates that high organic sulfur removal by this process may be detrimental to the subsequent usage of the coal.
Figure 13. Percent heating value lost vs. time.
Conclusions As a result of this study the following conclusions are made. (1)For temperatures between 150 and 210 "C the coal samples were treated to within federal pollution standards in less than 2400 s of reaction time. The degree of sulfur removal was found to be independent of total pressure at a constant partial pressure of oxygen. (2) The pyritic sulfur reaction is rapid and exhibits both a temperature and oxygen partial pressure dependence. The experimental data fit the continuous reaction model with second-order rate expression, having an activation energy of 46.5 X lo6 J/kmol. The data also fit the shrinking core model where the overall rate is controlled by the diffusion through ash layer. (3) Loss of heating value due to carbon oxidation during the desulfurization process could pose a serious problem. The kinetic mechanisms of the carbon reaction can be
300
Ind. Eng. Chem. Process Des. Dev. 1980, 19, 300-305
Nomenclature A,, = Arrhenius frequency factor b = stoichiometric coefficient (eq 4) c, = concentration of pyrite in coal, kmol/m3 C A L = saturation concentration of oxygen, kmol/m3 c,, = initial concentration of pyrite in coal, kmol/m3 D, = diffusivity through ash layer, mz/s E = activation energy, J/kmol kzP = second-order rate constant for pyrite reaction, m3/ (kmol-s) k$ = zero-order rate constant for carbon reaction, kg of carbon/(kg of coal-s) ko0 = zero-order rate constant for organic sulfur reaction, kg of S/(kg of coal-min) PO,= partial pressure of oxygen R = gas constant, J/(kmol K) R1 = average radius of the particles, m T = temperature, K t = reaction time, s n = fractional conversion T = time required for complete conversion, s pB = molar concentration of pyrite, kmol/m3 L i t e r a t u r e Cited
25
n 20
E a
? 2
15
a
u
::
10
u?
0 210.C
A 19OY
5
0
0
5
15
10
20
% H E A T I N G VALUE LOST
F i g u r e 14.
Efficiency of organic sulfur removal.
divided into two regions. In both regions the order with respect to carbon was found to be zero. The carbon oxidation reaction demonstrates a strong temperature dependence with the activation energies of 85.5 X lo6 and 380 X lo6 J/kmol in the two regions, respectively. (4) Due to scatter in the experimental data, only a tentative kinetic model was developed for the organic sulfur reaction. The data were found to strongly support a zero-order kinetic model, and an activation energy of 78.9 X lo6 J/kmol was obtained. Acknowledgment The financial support of Pittsburgh Energy Research Center (D.O.E. Contract EY-76-S-02-4163) is gratefully acknowledged. The help of Mr. Harry Ritz and the coal analysis group a t the Bureau of Mines is also gratefully acknowledged. The constructive comments and the idea for Figure 14 by Dr. John Ruether are also gratefully acknowledged.
Friedman, S., Warzinski, R. P., Eng. Power, 98, 361 (1977). Friedman, S.,Lacount, R. B., Warzinski, R. P., "Proceedings of the National Meeting of the Division of Fuel Chemistry", New Orleans, Mar 20-25, 1977. Levenspiei, O., "Chemical Reaction Engineering", 2nd ed, Wiley, New York, 1972. Lowry, H. H., Ed., "Chemistry of Coal Utilization, Supplementary Volume", "Thermcdynamics and Kinetlcs of Combustkm of S o l i Fuels, by M. V. Thring and R. H. Essenhigh", Chapter 17, Wiley, New York, 1963. McKay, D. R., Halpern, J., Trans. Mefall. SOC. AIM€, 212, 301 (1958). Meyers, R. A., "Coal Desulfurization", pp 2-3, Marcel Dekker, New York, 1977. M e r , R. R., Kuhpadltharorn, L., Lee, A. D., Ekholm, E. L., Min. Congr. J., 63, 42 (1977). Pray, H. A,, Schweickert, C. E., Minnich, B. H., Ind. Eng. Chem., 44, 1146 (1952). Sareen, S. S.,Gilberti, R. A., Irminger, P. F., Petrovic, L. J., AIChESymp. Ser., 73, 183 (1977). Slagle. D.. M.S. Thesis, University of Pittsburgh, 1978. Trindade, S.C., Howard, J. B., Kolm, H. H., Powers, G. J., Fuel, 53, 181 (1974). Vracar, R., Vucurovic, D., Rudarstvo I Metalurgija. (1970). Weisz, P. B., Goodwin, R. D.. J. Cafal., 2, 397 (1963). Wheelock, T. D., Gteev, R. T., Markuszewdki, R.. Fisher, R. W., "Advanced Development of Fine Coal DesuMrization and Recovery Technologl", Annual Technical Rogress Report to US. Department of Energy under Contract No. W-7405-eng-82, March 1978.
Received for review August 14, 1978 Accepted November 21, 1979
The Solubility of Oxygen in Brines from 0 to 300 "C Stephen D. Cramer Avondale Research Center, Bureau of Mines, U.S. Department of the Interior, Avondale, Maryland 20782
The solubility of oxygen in water, in sDdium chloride brines, and in two geothermal brines typical of the Imperial Valley, California, was determined for temperatures from 0 to 300 OC and for brine concentrations up to 5.69 rn in dissolved salts. Measurements were made in a high-pressure, stirred autoclave by the technique of gas extraction. The solubility, expressed in terms of the Henry's law constant k , was described with a standard deviation of 6 % or less by the empirical equation, In k = a , a , / T + a,/P a 3 / p a q / P ,where Tis the absolute temperature in K and the coefficients a, through a, depend on the concentration of dissolved salts in the brine. A minimum in the solubility occurred in the temperature range 60 to 100 OC. The salting-out coefficient for sodium chloride brines varied by a factor of 2 over the temperature range 0 to 300 OC with a minimum at 155 OC. The effect of temperature on solubility in the two geothermal brines was different from that in the sodium chloride brines in the mid- and high-temperature ranges.
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Introduction Many natural and industrial processes occur in aqueous environments that involve or are affected by dissolved oxygen. Such processes range from biological and synthesis This article
not subject to US.
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reactions to the corrosion and oxidation of materials. The most widely encountered environments in which such processes occur are brine solutions containing halide salts, predominantly sodium chloride. In addition, certain
Copyright. Published 1980 by the American Chemical Society
