Reactivity of Coal and Char. 2. In Oxygen-Nitrogen Atmosphere S. Dutta and C. Y. Wen' Department of Chemical Engineering, West Virginia University, Morganto wn, West Virginia 26506
Rates of oxidation of the same coal and char samples used in Part 1 were investigated in an atmosphere of oxygen and nitrogen. The rates of char-O2 reaction were measured as a function of conversion in (a) nitrogen-air mixtures containing 0 . 2 ~ 2 %O2 at the temperature range 834-1 106 OC and (b) in air at the temperature range 424-576 OC. Rates in the conditions of category (a) are found to be controlled by gas film diffusion, whereas rates observed with the conditions of category (b) are mostly chemical reaction controlled. Rate data in the chemical-reaction-controlled region are analyzed by taking into consideration the probable changes in pore characteristics of the chars during reaction. The activation energy for the char-01 reaction is found to be 31 kcal/mol. The results from this study are compared with those from our previous investigation on reactions of the same chars with CO2.
Introduction In Part 1, the results of the investigation on the reactivities of six coal and char samples in a COS atmosphere were reported. These samples included two bituminous coals, Pittsburgh HVab and Illinois HVC, and four chars produced from these coals in pilot plant experiments using different gasification schemes, namely, the Synthane Process, Hydrane Process, and Hygas Process. This paper presents the results of an investigation on the oxidation rates of the same coals and chars in an oxygen atmosphere. Reaction rates for the oxidation of several forms of carbon have been determined in the past by various investigators ( T u e t al., 1934; Parker and Hottel, 1936; Walker et al., 1959; Golovina and Khaustovich, 1962; Walls and Strickland-Constable, 1964; Field e t al., 1967; Field, 1970; Jenkins et al., 1973; Sergeant and Smith, 1973; Nandi et al., 1975). Most of these investigators have confined their measurements of rates a t relatively high temperatures (650-2200 "C) where oxidation occurs predominantly by the diffusionlimited step a t the outer surface of carbon particles. Since the conditions used by these investigators do not allow the internal particle surfaces t o participate in the reactions, any difference in reactivity due to the pore size of the char may not be evident. A lowering of temperature in a gasifier may decrease the oxidation rate to a level which would favor the reaction to occur throughout the interior surfaces of the carbonaceous particle. Therefore, in the present study, reaction rates were measured a t low temperatures, so that pore diffusion resistance is negligible and the reaction proceeds under chemical reaction control. Reaction rates have also been measured a t higher temperatures but a t lower oxygen concentrations, so that the temperature rise within the solid particles could be neglected. Relative reactivities of the coal/char samples a t higher temperatures could thus be compared with the relative reactivities of the samples at lower temperatures. Jenkins et al. (1973) determined the reactivities of a number of heat-treated coals in air a t low temperature (500 "C) and made the following conclusions. The lower-rank coal chars are more reactive than those prepared from high-rank coals. As the heat-treatment temperature increases, there is a decrease in reactivity. The amount of macro and transitional porosity in a char has a marked influence on reactivity. The present investigation will determine whether the relative reactivities of coals and chars observed in a COz atmosphere are similar to those which occur in an atmosphere of oxygen and nitrogen. The effects, if any, of gaseous environment and the reaction mechanism involved on the relative
reactivities would be examined in this study. Pore surface areas of the chars were found to vary during reaction in a COn atmosphere. The degree and nature of pore surface area variation seem to depend on the char sample, and affect the rate of conversion. The relationship between rate of reaction and conversion for the char-02 reaction will be compared with those for the char-COz reaction.
Experimental Section The reaction conditions maintained during these experiments fall into two categories: (a) reaction a t higher temperatures (834-1 106 "C) using nitrogen-air mixtures having low 2%); (b) reaction a t lower concentration of oxygen (0.2 temperatures (424-576 "C) using air. Chars and raw coals are used in the experiments of the first category. For experiments of the second category, the chars and coals are first devolatilized a t a furnace temperature of about 1024 "C for 1.5 min in a nitrogen atmosphere. After devolatilization, the samples are cooled to room temperature and preserved for lower temperature experiments. This sampling procedure is adopted in order to simulate the pyrolyzing conditions in the higher temperature runs of the present studies and also of the runs in COPatmosphere of the previous investigation. Thus the solids undergoing reactions with 0 2 or COz in the second stage of char gasification are likely to have almost identical physical characteristics. The same coal and char samples were devolatilized in identical conditions of temperature, heating rate, and time but in a COz atmosphere, to determine their pore characteristics and densities. The apparatus and the experimental procedure are the same as described in Part 1,except that COz is replaced by air or the desired mixture of air and N:! as the reactant gas stream. Since the reaction is highly exothermic, extreme care is taken to ensure that there is no appreciable difference between the temperature within reacting solid particles and that recorded by the thermocouple. A minimum amount ( 2 4 mg) of sample of particle size -35 60 mesh is spread uniformly on the sample pan (10 X 4 mm), so that the particles form a monolayer bed. As will be shown later, a maximum temperature difference of about 3 "C is calculated to exist between the reacting particle surface and its ambient atmosphere a t the temperature and gas compositions used in these experiments.
-
+
Results and Discussions If the reaction between char and 0 2 proceeds uniformly, Le., through the interior surfaces of the particles, the rate of disInd. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
31
30t B m '
2.0
. 5 P
1.5
.c-. 0
-Y
10
0.5
I
0
OB
04 '1.
02 IN
I
I
12 1. 6 & - N 2 MIXTURE
01
I
0
2.0
appearance of char per unit mass due to reaction with 0 2 (dx,/dt), for chemical reaction control, may be expressed as
In eq 1 the rate of reaction is assumed to be proportional to the partial pressure of 0 2 , since the majority of the experimental data show carbon-oxygen reaction to be first order or close to first order (Walker et al., 1959).dxc/dt can be obtained from the observed rate (dwldt) as follows:
where xc = (wg - w, - w)/(wo - w, - wash), wo is the initial weight of solid, and w, and w&, are the weights of volatiles and ash present in that solid, respectively. k (g/g s atm) is the rate constant for reaction occurring uniformly throughout the interior of the solid particles. The amounts of volatile matter in the chars and coals were assumed to be identical with those determined during the devolatilization of the same samples in a C02 atmosphere as previously reported (Wen and Dutta, 1975; Dutta et al., 1976). The ash contents of the solids were determined from the weight of residue remaining after the complete reactions. If the reaction rate is high and is confined to a thin layer a t the outside surfaces of reacting char particles, the rate (dxc/dt) may be expressed as (3) where Sg,, (cm2/g)is the specific external surface area of the particles and ks(g/cm2s atm) is the rate constant based on external surface area of the reacting solid. Comparing eq 1and 3, we find (4)
A. Experiments Using NZ-Air Mixtures With Low Oxygen Concentrations. Dried air and N2 were mixed in a The compogas stream to obtain a desired ratio of 0 2 to N?. sition of the mixed gas was analyzed before and after the runs in a Beckman Type GC-2A gas chromatograph using an 8 f t Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
04 1
Figure 1. Effect of oxygen concentration on rate a t 910 "C. Sample: Synthane Char No. 122.
32
I
02
-
I
I
I
06
08
10
xc
Figure 2. Rate vs. conversion curves at 910 "C with about 1%02 in Nz-air mixture, assuming reaction occurring at the external surfaces of particles: 0 , Illinois Coal No. 6; 0 , Synthane Char No. 122; A , Hydrane Char No. 49; 0 ,IGT Char No. HTl55; X, Pittsburgh HVah Coal; A ,Hydrane Char No. 150.
X tiz-in. molecular sieve column. The gas flow rate used in all the experiments was 3.33 cm?/s. Rate versus conversion curves were obtained for Synthane Char No. 122, with several gas compositions. Figure 1 shows the rates a t x c = 0.3, at 910 "C, as a function of 0 2 concentration in the gas. The rate is thus found to be proportional to the partial pressure of oxygen at these conditions. Rate data for char-02 reaction for the six char and coal samples were obtained a t several temperatures in the range 834-1106 "C, culminating with complete conversions of the solids. Figure 2 shows typical rate-conversion curves for the six samples at a temperature of 910 "C, in a stream of Ni-air containing about 1%0 2 . In Figure 2 rates are expressed as (dxc/dt)/p02.Sr,x,assuming that the reaction takes place only on the outside surfaces of the reacting solid particles. Similar assumptions have been made by other investigators, in the same temperature range for reaction of several forms of carbon with 0 2 . S,,, of the particles can be calculated from the average particle diameter (0.0331 cm) and densities of the devolatilized chars. However, since the particles are highly porous ( e = 0.655-0.864), the reaction may not be confined to the outside surfaces of the particles alone, as is assumed above, but may occur involving a part of the internal surfaces. Figure 2 shows that there is but a small difference in reactivities between the chars and coals studied, a t the conditions of these experiments. In Figure 3, (dxcldt)max/pOL.Sgrx for all samples are plotted for against 1/T, in an Arrhenius diagram. (dx,/dt)/po$,,, the rates observed by other investigators are also included in this figure for comparison. The maximum rates appear to increase only slightly within the range of temperature, 834-1106 "C, and are almost proportional to T175, suggesting an activation energy of about 4 kcal/mol. These rates appear to correspond to the film-diffusion-control regime. In contrast to the rate-conversion curves for chemical-reaction-control regime (Figures 4-7), which will be discussed later, the rates in Figure 2 exhibit very little change with conversion until reaching about 80% after which they drop sharply to zero. Because of the film-diffusion control, rates do not show any appreciable decrease until they decrease enough to be controlled by chemical reaction resistance, due to the depletion of the reacting solid. Near the end
13%
2227
977
I
560
727
I
4 1
I
351 61
Key t o t n e Curves
m' Vandi four different
e t . a l . l1975),
coal-chars s i z e -35r80 mesh ~n a i r . This study with air, m a r s shown in F i g . 2
C f
5,6.7:
5.6
Shown i n F i g . 2 9 : C a l c b l a t e d p a t e s by
48
Parker a n d H o t t e l ( 1 9 3 6 ) , b r u ~ nc a r b o n c y l i i d e r 2.5 c r . d i i , i n air. 10: T i , D a v i s a n d i o t t e l n9341, b r w i l carbon Sphere 2 . 5 c m . d i a . , in 02-U2 M i x t u r e f l o w i n g a t 3 51 c w s . 1 1 : G o l o v i n d and KhdUStovish F 9 6 2 ) . electrode Carbon Sphere
c
E
5
40
B
32
5 -
2L
-
and Gray a n d Kimbers ( 1 9 6 7 1 ,
2
si 0
16
08
0
1
1
02
06
04
08
10
1 - x c
c 1671
i4
I
06
I
0.8 1000 / T
F i g u r e 3. (dxJdt)/po,Sg,, gations.
VS.
1.0
, W1
,
I
12
1.4
I
1.6
F i g u r e 5. Rate vs. conversion curves for oxidation of devolatilized IGT Char No. HT155 in air.
1/T plots for this and other investi64 L
6/4
-
5.6
-
4.0
-
56
-
t
L6OoC 02
04 I
-
06
08
10
xc
F i g u r e 4. Rate vs. conversion curves for oxidation of devolatilized Hydrane Char No. 49 in air.
of reaction, Le., for conversions greater than about 80%,the rate decreases as the reaction proceeds to completion. In Figure 3, the rate data for higher temperature runs of this study show significantly low values compared to the values expected under the diffusion-controlling mechanism. For particles of 0.0331-cm diameter (the average particle diameter
1
-
IC
F i g u r e 6. Rate vs. conversion curves for oxidation of devolatilized Pittsburgh HVab coal in air.
of present study) suspended in a stagnant gas mixture of N2 and 0 2 , the diffusional reaction rate coefficient (kdiff) for gas film diffusion would be 0.0333 g/cm2 s atm a t 1000 "C, assuming that carbon is converted only to CO on the surface of reacting char particles. The average rate observed at 1000 "C is 1.8 X g/cm2 s atm, assuming that the particles are spherical in shape. This is apparently due to the fact that the particles are not individually suspended in gas but form a layer at the bottom of the cylindrical sample pan. Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
33
'.T
If
4.8
5300c
12
1.0
I
:t 'y
-
24
I
1.6
i- I 4 O
0
02
06
OL 1
Z
B. Experiments Using Air. The rate data obtained in an atmosphere of air were taken at lower temperatures (424-576 "C). Figures 4-7 show rate conversion curves for four of the devolatilized coal and char samples a t several temperatures. These figures show that the rates increase until a conversion of 10-50% is attained depending on the sample used, before the rates start to decline. This increase of rate with conversion may be due to two reasons: (a) the temperature rise of the particles and (b) the increase in available internal surface areas of the particles. Since the particles were spread uniformly as a monolayer on the sampling pan, any temperature rise would be approximately due to the temperature rise within a single particle. This temperature rise can be calculated from the following heat balance equation:
- 2X(T,do- Tg)
a
sg,,
(5)
with the following assumptions: (a) the particles are freely suspended in a stagnant gas (air); (b) there is no temperature gradient within the particles; (c) reaction between char and 0 2 produces COn directly on the surface of the char particles, equal to 7900 cal/g thus having a maximum value of (-AH) of carbon. Although both CO and COn may form on the reacting char surfaces, this assumption is made here in order to estimate the maximum possible rise in surface temperature. These assumptions do not represent the actual process exactly; however, such simplified assumptions are made to avoid the complexity of calculation and reach an approximate answer. Using dx,/dt = 1.67 X lop3 s-l, which is the maximum rate observed for most of the experiments, the temperature rise in the particles is calculated to be about 3 OC a t an ambient temperature of 500 "C. The time required for this temperature rise can be calculated from eq 5 to be only 1 to 2 s. The maximum rates observed in these experiments appear after a few minutes a t higher temperatures and after 20-107 min, depending on the sample, a t lower temperatures. Moreover, the rates normalized to the rates a t a particular conversion show almost identical relationship with conversion irrespective of the temperature (except for Pittsburgh HVab 34
,
02
04
,
,
06
08
0
Figure 7. Rate vs. conversion curves for oxidation of devolatilized Hydrane Char No. 150 in air.
dT, dx, (-AH) dt = dt
,
?
10
08
- xc
2
0
CP'-
V
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
1
-
10
Xt
Figure 8. Rates normalized by the rates observed a t three conversion levels, for oxidation of devolatilized Hydrane Char No. 150 in air: 0 , 419 "C, x , = 0.20; 7 , 4 1 9 "c,x , = 0.40; a, 419 "C, x r = 0.70; A , 464 ' C , x r = 0.20; +, 464 "C, x , = 0.40; A,464 "C, x , = 0.70; A , 512 "C, x , = 0.20; +, 512 "C, x r = 0.40; L,512 " C , x , = 0.70.
Coal at higher temperatures). In Figure 8 the rates normalized to the rates a t three conversion levels are shown against conversions for three temperatures, for Hydrane Char No. 150. It can thus be concluded that particle temperature rise should have but little effect on the observed rate conversion characteristics, in the studied temperature range. Ignoring the particle temperature rise, and assuming that the initial increase in reaction rate with conversion is solely due to an increase in the available pore surface area of the particles, rate eq 1 may be modified as dx ,
dt = akppo2 (1 - x , )
a is a function of conversion and represents the relative available pore surface area defined in a similar manner as in Part 1. The effective diffusivities of the particles (De)may also change with conversion. However, this change would not affect the rate in the chemical-reaction-control region. As Figure 8 indicates (and also as is found in similar curves for the other samples except for Pittsburgh HVab coal a t higher temperatures), a can be considered independent of temperature in the studied temperature range. Therefore a can also be expressed as: u = (dx,/dt)/(dx,/dt)i(l
- x,)
(7)
where (dx,/dt)i is the rate observed a t zero conversion. In Figure 9 (dx,/dt)/(dx,/dt)i(l - x,) are plotted against conversion for the six samples. In this figure the values of a as calculated for the char-COZ reaction in Part 1 are also shown for comparison. I t is interesting to note that increases in available surface area with conversion during char-02 reaction are much more than those observed in char-CO2 reactions. However, the way the values of a change with conversion in COz reactions is found to follow almost the same patterns in char-02 reactions. Thus Hydrane Char No. 150, Pittsburgh
10
c
560
527
460
496
417
Ut I
I
L
I
/+2
-1
--
7 0 L
5 6
0 0
1
I
I
0.2
0.4
06
,
0.8
1
1.0
XC
Figure 9. (dx,/dt)/(dxJdt)i(l - xC) vs. conversion ( x , ) for devolatized coals and chars: -,for reaction in 0 2 ; - - -,for reaction in CO2; 1,devolatilized Hydrane Char No. 150; 2, devolatilized Pittsburgh HVab Coal; 3, devolatilized IGT Char No. HT155; 4, devolatilized Synthane Char No. 122; 5, devolatilized Hydrane Char No. 49; 6, devolatilized lllinois Coal No. 6.
,,
lafl 1.20
HVab coal, and IGT Char No. HT155 show maximum increases in a , whereas the Hydrane Char No. 49 shows the minimum. In Figure 10, the Arrhenius plot is displayed with the maximum rates observed. Plots with maximum rates are made in order to compare the results of this study with those of Nandi et al. (19754,who also plotted the maximum rates in a similar manner for chars and coke in about the same temperature range of this study. They made these observations with chars ( - 3 5 80 mesh), obtained from three coals, subbituminous or lower rank, and a MVB coal, by pyrolyzing them to a temperature of 900 "C and holding them at that temperature for 2 h. Studies of Nandi et al. show that reactivity decreases with the increase in the rank of coal. According to the present study, the char reactivity depends not only on the nature of the parent coal but also on the process through which it is produced. Thus devolatilized Hydrane Char No. 49 has very low reactivity compared to its source coal Illinois No. 6, devolatilized under similar conditions. Devolatilized Synthane Char No. 122 and devolatilized IGT Char No. HT155 also obtained from Illinois No. 6 coal, show intermediate reactivities. Such a difference in reactivity between chars from the same source coal has also been observed in the Arrhenius plots made with the rates observed at some particular conversion level (Figure 11) and with the following average rates:
1000/ T ,
I35
1.40
1.45
OK-'
Figure 10. Arrhenius diagram with the maximum observed rates in air: -, this study: 0,devolatilized Illinois Coal No. 6; 0 , devolatilized Synthane Char No. 122; A , devolatilized Hydrane Char No. 49; 0 , devolatilized IGT Char No. HT155; X, devolatilized Pittsburgh HVab Coal; A, devolatilized Hydrane Char No. 150; - - -,study of Nandi et al. (1975): 1, MVB coal char; 2, sibbituminous coal char; 3,4, low subbituminous coal chars.
+
s
I30
125
560
521
496
468
441
I
I
I
I
TEMP.
L17 I
OC
x,=l
x,=o
d(dx,ldt)ldx,
Reactivities observed in the predominantly chemical-reaction control regime show no relationship with the total pore surface areas of the particles measured by the BET nitrogen adsorption method. However, as Figure 1 2 shows, the reactivity is found to increase with the increase in oxygen content of the original char or coal sample. In this figure, the average reactivities of the samples a t 500 "C are compared. The reactivity of Pittsburgh HVab coal appears to be comparatively low. This is partly due to the reason that Pittsburgh coal is a high swelling coal having a swelling index of 8.5. These particles agglomerate to form a large single lump of char during the initial stage of pyrolysis. For this lump of char, reaction is apparently influenced by the resistance due to pore diffusion. This possibility is also indicated in the Arrhenius diagram (in Figure 10) for this char.
zt la: 1
c
I 120
I 125
I
I
135
I30 lOOOlT ,
I
I 1 LO
145
OK-'
Figure 11. Arrhenius diagram with rates observed a t 3090 conversion level. Symbols: same as in Figure 10.
From Figure 10 the activation energy for char-0s reaction is found to be 31 kcallmol. Almost the same value of activation energy is obtained from the Arrhenius diagram plotted with the rates observed at a particular conversion level, as is shown in Figure 11. Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
35
L .o
c
external specific surface areas of the particles. An average value of 0.35 g/cm3 is assumed for density of the char particles used by Nandi et al. in these calculations. If the curves a t higher temperatures can be extrapolated to the temperature region of the present study, it shows that most of these chars are more reactive than the carbons studied by the previous investigators.
X
0
2
8
6
4
4
Wt'I.
In
10
Coal1 Char
Figure 12. Oxygen contents of coals and chars vs. average rates of oxidation in air a t 500 "C.
The minimum proposity (e) of the devolatilized samples is 0.655. If effective diffusivity (De)is assumed to be equal to D X t2 (Wakao and Smith, 1962), the minimum effective diffusivity in the solid layer of the particles would be
DeO= D
X c2 = 0.806 X
(0.655)*= 0.346 cm2/s
D is the diffusivity of the C02-Nz pair a t 500 "C. In the above calculation of De0 only the bulk diffusion in the macropores is considered. If the diffusion occurs in micro and transitional pores of the solid particles, predominantly by the Knudsen flow, the effective diffusivity De0 can be approximated from the following equation (Satterfield, 1970)
-
€2
De, = 19 400 -C
M
(8)
TrnSgPp
assuming straight round pores. For IGT Char No. HT155, for example, which has the maximum nitrogen surface area among the samples studied (see Table IV of Part l),Deo = 0.0164 cm2/s at 500 "C for C 0 2 ,assuming 7, = 2. This value of DeOis of the same order of magnitude as that determined experimentally for some metallurgical coke by Turkdogan et al. (1970). The maximum reaction rate (dx,/dt) obtained a t the temperature range 424-576 "C is about 1.67 X lop3s-'. For IGT Char No. 155, for example, the maximum appears at x, = 0.4 and when a 4. Therefore, a t 576 "C
- 82.05 x 849 x 1.67 X 0.21 X 0.6 X 4
= 231 mol/cm3
(9)
Under these conditions, the Thiele modulus, 40,becomes --
This indicates that there should be negligible resistance due to pore diffusion in the reacting particles below 576 "C. However, as is indicated in Part 1,the effective diffusivities for Hydrane Char No. 150 and the Pittsburgh HVab coal char may be significantly lower than the value calculated above. Therefore, for these two chars the effects of pore diffusion may be appreciable at temperatures above around 500 "C. In order to compare the data obtained by other investigators, reaction rates are expressed as (dxJdt)/pO2.Sgerin Figure 3. The values of (dx,/dt),,,/po, in the temperature range 424-576 "C of the present study and also those of Nandi et al. (in the temperature range 350-560 "C) are divided by the 36
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1 , 1977
Conclusions Reactivity of a char in chemical reaction control regime for char-02 reactions is found to depend more on the degree of gasification of the char than on its parent coal. This is in contrast to the observations in char-C02 reaction studies, where the reactivity of a char was found to depend more on its coal seam than on the degree of gasification of the char. Reactivity of a char therefore appears to be determined not only by its own physical characteristics but also by the reaction it undergoes and by the gaseous environments. Changes of available pore surface areas with conversion due to char-02 reaction are much greater compared to those due to char-CO2 reaction. Due to such drastic modification of pore characteristics during char-02 reaction, the reactivities of the chars in an oxygen atmosphere show less dependence on their parent coals. The reactivities are apparently determined to a greater extent by the degree an individual char changes its pore characteristics during the reaction. The activation energy of char-02 reaction is obtained as 31 kcal/mol. There is but little difference between the reactivities of different chars/coals at higher temperatures, when the reaction is controlled by the rate of gas-film diffusion. Nomenclature available pore surface area ratio, as defined in eq 6 CSO = initial concentration of solid reactant, mol/cm3 c p = specific heat of char, cal/g K D = molecular diffusivity, cm2/s D e = effective diffusivity in the reacting solid particles; Dee, a t zero conversion, cm2/s do = diameter of particles, cm -AH = heat of reaction, cal/g. k , k , = reaction rate constants as defined by eq 1and 6, respectively, g/g s atm k' = reaction rate constant as defined by eq 9, mol/cm3 s k , = reaction rate constant as defined by eq 3, g/cm2 s atm kdiff = diffusional reaction rate coefficient, g/cm2 s atm M = molecular weight of the diffusing gas P O , = partial pressure of oxygen, atm R = gas constant, atm cm3/mol K ro = radius of particles, cm S, = specific surface area of particles determined by Nz adsorption method, cm2/g S,,, = specific external surface area of particles, cm2/g T = temperature, K T,, T, = temperatures of the ambient atmosphere and at the surface of reacting particles respectively, K t = time, s w = weight of solid,g x, = fraction conversion of solid a =
Greek Letters t = porosity of solid particles X = thermal conductivity of ambient gas, cal/cm s K pp = particle density, g/cm3 A, = a factor that takes account of the tortuousity and varying cross section of individual pores in eq 8 40 = Thiele modulus defined by eq 10 Literature Cited Dutta, S..Wen, C. Y.. Belt, R. J., preceding article (Part l ) , lnd. Eng. Chem., Process Des. Dev., (1976). Field, M. A,, Combust. Flame, 14, 237 (1970). Field, M. A,, Gill, D. W., Morgan, B. E., Hawksley. P. G. W., "Combustion of Pulverised Coal", The British Coal Utilization Research Association, 1967.
Golovina, E. S.,Khaustovich, G. P., "Eighth lnternation Symposium on Combustion", p 784, 1962. Jenkins, R. G.. Nandi. S. P., Walker, P. L., Jr., Fuel, 52, 288 (1973). Nandi, S. P., Lo, R., Fischer, J., Prep. Pap. Natl. Meet. Div. FueiChem., Am. Chem. Soc., 88 (1975). Parker, A. S.,Hottel, ti. C., ind. Eng. Chem., 28, 1334 (1936). Satterfied. C. N., "Mass Transfer in Heterogeneous Catalysis", M.I.T. Press, p 41, 1970. Sergeant, G. D., Smith, I. W., Fuel, 52, 52 (1973). Tu. C. M., Davis, H., Hottel, H. C., ind. Eng. Chem., 26, 749 (1934). Turkdogan, E. T., Olson, R. G., Vinters, J. V., Carbon, 8, 545 (1970). Walls, J. R.. Strickland-Constable, R. F., Carbon, 1, 333 (1964).
Walker, P. L., Rusinko, F.,Austin, L. G., Adv. Catai., 11, 133 (1959). Wakao, N., Smith, J. M., Chem. Eng. Sci., 17, 825 (1962). Wen, C. Y., Dutta, S.,"Reaction Rates of Coals and Chars with Carbon Dioxide", report submitted to the Energy Research and Development Administration, Morgantown, W.Va.. 1975.
Receioed for reuiew October 31, 1975 Accepted J u l y 8, 1976 T h e work was supported by grants f r o m t h e U n i t e d States Energy Research and Development Administration.
Backmixing and Liquid Holdup in a Gas-Liquid Cocurrent Upflow Packed Column G. J. Stiegel and Y. T. Shah' Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 1526 1
An experimental investigation was carried out to determine the effects of gas and liquid flow rates and packing size on the liquid phase axial dispersion coefficent and the liquid holdup in a gas-liquid, cocurrent, upflow packed column. The experimental measurements were carried out in a 2-in. i.d. glass column using approximately '/*-in. diameter polyethylene extrudate with two different lengths. Sulfuric acid was used as a tracer and its concentration in the liquid phase was determined by measuringthe electrical conductivity of the liquid phase. The data were analyzed by the method of moments and by the evaluation of the transfer function of the axial dispersion model. The measured liquid holdup was in excellent agreement with those reported by previous investigators and was dependent upon both gas and liquid velocities. Unlike in a bubble column, the liquid Peclet number was found to be dependent on both gas and liquid flow rates. The experimental data for liquid Peclet number are compared with the previously reported data of Heilmann and Hofmann (1971) obtained under different ranges of packing size and gas and liquid velocities.
Introduction During recent years a considerable amount of literature has been published on the dynamics of a packed column. A quantitative understanding of the fluid behavior, such as the liquid holdup and the backmixing in packed bubble columns, is of considerable importance for the proper design of packed, multiphase catalytic reactors. A vast amount of work has been published on countercurrent, gas-liquid packed columns. Michell and Furzer (1972) and Chung and Wen (1968) have summarized a number of these studies. More recent articles have been published by Hoogendoorn and Lips (1965), Sater and Levenspiel (1966), Mears (1971), Co and Bibaud (1971), and Chen (1975). For cocurrent downflow systems considerably less information is available. Hochman and Effron (1969) and Charpentier (1971) studied the effect of gas and liquid flow rates on the liquid holdup and the liquid backmixing coefficient. Bischoff (1966), Mashelkar (1970), and Wen and Fan (19751, have reviewed the literature in these areas along the work published for single phase flow. Recent developments in hydroprocessing (Montagna and Shah, 1975) have created a considerable interest in understanding the dynamics of a two-phase cocurrent upflow packed column. In particular, the quantitative correlations among the liquid and gas flow rates on the liquid holdup and the liquid phase backmixing coefficient are of considerable interest. l'urpin and Huntington (1967) have studied theholdup characteristics in cocurrent, upflow packed columns. Chen et ai. (1971) recently studied the dispersion of a liquid in a single phase, upflow packed column while Eissa et al.
(1971) and Kato and Nishiwaki (1972), have studied the liquid phase dispersion in a bubble column. To date, only Hofmann (1961) and Heilmann and Hofmann (1971) have published data concerning dispersion in a gas-liquid, cocurrent, upflow packed column. Results of these studies showed that the liquid phase dispersion coefficient is dependent on both gas and liquid flow rates. Heilmann and Hofmann (1971) presented a correlation for the liquid phase Peclet number as a function of liquid and gas holdup, liquid Reynolds number, and the particle size. Their experimental data were, however, obtained for the particle sizes larger than the ones commonly encountered in catalytic hydroprocessing reactors. The purpose of this paper is to present correlations for the liquid holdup and the liquid phase backmixing coefficient as functions of gas and liquid phase Reynolds numbers and the particle size for a two phase cocurrent upflow packed column. The results are obtained for the packing size similar to the ones encountered in catalytic hydroprocessing reactors. The empirical correlations for the liquid phase Peclet number presented in this paper are based on the experimental measurements obtained using standard tracer analysis.
Experimental Section Figure 1 presents a schematic diagram of the apparatus employed in this study. The packed column was constructed using a 2-in. i.d. glass pipe with a glass tee and a reducer located a t the ends of the pipe. The reducer a t the entrance served as a transition for the fluid between the piping and the packed column and also to hold the packing support plate in place. The tee located at the exit was used to hold the retaining Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1. 1977
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