312
Ind. Eng.
Chem. Process Des. Dev., Vol. 18, No. 2, 1979
is uniquely defined as the intersection of the line of complete carbon conversion with the line of sensible heat equal to that required to heat and dry the coal. At 25 atm pressure and 700 O F blast temperature, this point is a t a steam to oxygen ratio of slightly above unity, which is the operating point of current experimental slagging reactors. Optimal operation of the dry ash reactor requires detailed consideration of the chemical and transport rate processes. The maximum temperature in the reactor is uniquely determined by the steam to oxygen feed ratio for a given coal, and the reactor should be operated for maximum efficiency a t the steam to oxygen ratio giving a maximum temperature just below the ash melting temperature. The optimal carbon to oxygen ratio at this maximum temperature must be determined from the mathematical model. Literature Cited Elgin, D. C., Perks, H. R., "Results of Trials of American Coals in Lurgi Pressure-GasificationPlant at Westfield, Scotland", Sixth Synthetic Pipeline Gas Symposium, p 247, 1974. Ellman, R. C., Johnson, B. C., "Slagging Fixed-Bed Gasification at the Grand Forks Energy Research Center", presented at the Eighth Synthetic Pipeline Gas Symposium, Chicago, Oct 18-20, 1976. Ellman, R. C., Johnson, 8. C., Schobert, H. H., Paulson, L. E., Fegley. M. M., "Current Status of Studies in Slagging Fixed-Bed Gasification at the Grand Forks Energy Research Center", presented at the 1977 Lignite Symposium, Grand Forks, N.D., May 18-19, 1977.
Hebden, D., Lacey, J. A,, Horsler, A. G., "Further Experiments With A Siagging Pressure Gasifier", Gas Council Research Communicatbn Gc 112 (Nov 1964). Hollings, H., Hopton, G. U., Spivey, E., "Lurgi High Pressure Gasification", BIOS Final Report No. 521 (1948). Hottel, H. C., Howard, J. B., "New Energy Technology", pp 103-105, MIT Press, Cambridge, Mass., 1971. Hougen, 0. A., Watson, K. M., Ragatz, R. A,, "Chemical Process Principles", Wiley, New York, N.Y., 1954. IGT, "Presentation of a Coal Conversion System Technical Data Book", Project 8964, Final Report by IGT for ERDA (April 1976). Johnson, C. A., Buschow, H. F., Carlsmith, L. E., "Gasification of Coal", FIAT Final Report No. 983 (1947). Kelly, B. T., Taylor, R., Chem. Phys. Carbon, 10, 1 (1973). Lacey, J. A., Adv. Chem. Ser., No. 69, 31 (1967). Lawov, N. V., Kwobov, V. V., Filippova, V. I., "The Thermodynamics of Gasficatm and Gas-Synthesis Reactions", Pergamon Press, Macmillan Co., New York, N.Y., 1963. Lowry, H. H., "Chemistry of Coal Utilization", Supplementary Volume, p 913, Wiley, New York, N.Y., 1963. Moe, J. M., "SNG From Coal Via the Lurgi Gasification Process", IGT Symposium Papers, "Clean Fuels From Coal", p 91. Sept 1973. Papic, M. M., Can. J . Chem. Eng., 5 4 , 413 (1976). Parent, J. D., Katz, S., IGT Res. Bull., 2, (Jan 1948). Woodmansee, D. E., Palmer, P. M., "Gasification of Highly Caking Coal in the GEGAS Pressurized Gas Producer", presented at the 173rd National Meeting of the American Chemical Society, New Orleans, La., March 1977. Preprinted by Fuels Division. Wright, C. C., Barclay, K. M., Mitchel, R. F.. Ind. Eng. Chem., 40, 592 (1948). Yoon, H., Wei, J., Denn, M. M., AIChEJ., 24, 885 (1978).
Received for review May 22, 1978 Accepted October 27, 1978 This work was supported by the Electric Power Research Institute.
A Regenerative Process for Fluidized-Bed Combustion of Coal with Lime Additives Ralph T. Yang" and Ming-Shing Shen Department of Energy and Environment, Brookhaven National Laboratory, Upton, New York 11973
A regenerative process for fluidized-bed combustion of coal with lime additives has been investigated. This process
is based on using carbon for lime regeneration from the sulfated limestone. Ten sulfation/regeneration cycles using Greer limestone have been conducted in a TG system and there was no sign of weakening of the SO2 sorption activity. Kinetic and mechanistic studies on the regeneration reaction have also been performed, which included the effects of temperature, water vapor, particle size, and catalysts such as sodium chloride.
Introduction A major advantage of fluidized-bed combustion with lime additives is its ability to burn coal cleanly and to produce economically a desulfurized hot gas. Recognition of the potential of this technology has accelerated national as well as international efforts in research and development in this area. The state of the art was reflected in the proceedings of a recent conference (1975). However, ecological and economical considerations require the regeneration of the lime additives from the sulfates. The only major process which is currently under serious consideration is the reductive decomposition scheme based on the Wheelock-Kent feed process (Wheelock and Boylan, 1971). This process is being modified and developed further for application in fluidized bed combustion by Argonne National Laboratory (Vogel et al., 1976) and *Address correspondence to R. T. Yang a t Department of Chemical Engineering, State University of New York at Buffalo, Amherst, N.Y. 14260. 0019-7882/79/1118-0312$01.00/0
Exxon Research and Engineering Co. (Hoke et al., 1976). In this paper, we report the preliminary results on a process for regeneration of CaO from the sulfated stone by reacting it with carbon. A kiln-type reactor would be well suited for this apparent solid-solid reaction. Comparisons between this process and the aforementioned one will be given toward the end of this paper. The basic chemistry of this process involves two reactions CaO + SO2 + 1/202 CaS04 (1) CaS04 + 1/2C
-
-
CaO
+ 1/2C02+ SO2
(2)
The equilibrium partial pressure of SO2, Pso,, is given as a function of temperature in Figure 1. I t is assumed that no O2or C 0 2is added or subtracted from the system. The equilibrium value for the thermal decomposition of CaS04 without carbon is also given in this figure as a comparison. Reaction 2 has been used for making sulfuric acid from calcium sulfate minerals (gypsum or anhydrite) in the cement-sulfuric acid process as early as 1918 (Hull et al., 0 1979 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979 313 T,
't
Table I. Time for 70% Sulfation of the Available Lime in Regenerated Stone Using Carbon. Sulfation at 950 " C and 4% SO, and 10%0, in N, cycle no. 1
2 3 4
5 6 7 8 9 10
time, min
__ 35 24 28 23.5 25 22 26 28.5 22.5
-
lo4, e % - '
Figure 1. Thermodynamic equilibria in the regenerative desulfurization process using carbon for regeneration. The reactions are: CaSOl + '/& CaO + SO2 + '/2COz (0); 3CaS04 + Cas 4Ca0 + 4 s 0 2 ( A ) ; and CaS04 CaO + SOz + ' / z 0 (0). 2
- -
-
1957). Some kinetic and mechanistic studies were undertaken recently by Turkdogan and Vinters (1978). In their studies, it was established that the amount of carbon is the controlling factor for determining the reaction product, i.e., CaO vs. Cas. It has also been shown that this reaction proceeds in the following two consecutive steps CaS0, + 2C Cas + 2C02 (3) Cas
--
+ 3CaS04
4Ca0
+ 4S02
(4) It is known that the surface layer of the particle is first reduced rapidly to Cas and reaction 4 takes place between C a s and the remaining CaS04. Reaction 4 is the ratelimiting step in the overall reaction as shown in reaction 2. The equilibrium partial pressure of SOz in reaction 2 is, therefore, limited by the equilibrium value of reaction 4. The above thermodynamic argument has been discussed and experimentally verified by Yang et al. (1978). The equilibrium Pso, of reaction 4 is also shown in Figure 1, and this value is the thermodynamic limit of the regeneration reaction as shown in reaction 2. Reaction 4 has also been discussed by Hubble et al. (1975) as a possible means for lime regeneration. To apply reaction 2 for regeneration of CaO in fluidized bed combustion, the crucial question is on the SO2 sorption activity of the regenerated CaO, as well as on the kinetics of the cyclic systems involving reactions 1 and 2. Experiments described in this paper were designed to answer these questions. Experimental Section The rates of regeneration and sulfation were measured gravimetrically. Detailed experimental and calculation procedures and the apparatus used for the measurements have been described elsewhere (Yang et al., 1975; Yang and Steinberg, 1975,1976). A platinum sample holder was used in the experiments. The total amount of sample mixture for regeneration was about 0.1 g containing 2 CaS0,:l C (molar) in all cases. Gas was passed over the packed sample surface a t about 250 cm3/min (STP) which corresponded to a velocity of 2 cm/s. The composition of the gas mixture for sulfation was 4% SO2, 10% Oz, 2.9% H20, and the balance N2. Straight N2was used for regeneration. The gases used were of the following grades: SO2, anhydrous (99.98%); N2, prepurified (99.996%); and 02, commercial (99.5%). Nitrogen was passed through a
Supelco Carrier Gas Purifier (Supelco, Inc.) to remove residual O2 and H 2 0 to eliminate the reaction C + Oz C 0 2 during regeneration. Coconut charcoal (Fisher Scientific), 5% volatile and 1% ash, was used in all the regeneration experiments. The rate of regeneration is independent of the type of carbon used (Turkdogan and Vinters, 1978). CaS04 was obtained by sulfating Greer limestone to 80% or 65% at 900 "C. The size of coconut charcoal was 200/250 Tyler mesh. The Greer limestone was kindly supplied by Argonne National Laboratory (Argonne, Ill.) and was sieved into different sizes. A General Electric automatic powder diffractometer and a powder camera were used for X-ray diffraction analyses of the reaction products. Results and Discussion Cyclic, isothermal sulfation and regeneration reactions were tested a t 950 "C using Greer limestone. The stone was 16/20 Tyler mesh in size (850-1000 pm) which is similar to that used in fluidized-bed combustion and was calcined before the first sulfation cycle. One hundred milligrams of limestone was the starting weight. Ten cycles were run which included rate measurements of ten sulfation and nine regeneration experiments. In the first regeneration, only about half of the CaS04was regenerated to CaO. About the same amount of CaO was regenerated in all the subsequent cycles. Hence about 40 mg of CaO was sulfated in the first sulfation experiment and about 20 mg of CaO was sulfated in all the following cycles. Furthermore, in each regeneration, there was a small amount of CaS left on the surface layer, the weight of which was about 10% of the available CaO for sulfation. The amount of Cas was determined by, prior to sulfation in each cycle, oxidizing it with 20% O2 in Nzand the weight gain in this process was taken to be the O2 consumed by C a s to form CaS04. The oxidation a t 850 "C was very rapid; it took several minutes to complete the reaction. This method has been quantitatively verified by using a reagent C a s sample. T o summarize, before sulfation in the second to tenth cycles, the shelled structure of a regenerated (and oxidized) particle is (proceeding from the outermost layer): a thin layer of CaSO,, a thick layer of CaO followed by a layer of CaS04 of about the same thickness, and CaO in the core. This picture has not been experimentally verified and was rather drawn from a knowledge of the state of the art. With the above picture in mind, we can now proceed to discuss the results shown in Figures 2 and 3. The sulfation results are summarized in Figure 2 . Each experiment actually lasted for 4-5 h while only the initial 40 min are shown. The times for 70% completion of sulfation for the ten cycles are tabulated in Table I. From these results, the first eminent conclusion is that there was no tendency for the sorbent reactivity to decay after ten cycles. The thin CaS0, layer left on the regenerated sorbent did not seem to affect the initial rate in sulfation.
314
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
100.
,
,
,
,
,
,
,
l
"
"
"
'
Table 11. Rate of the C a s + 3 CaSO, Reaction in Regeneration as a Function of Number of Cycles (950 " C )
1
SOL
cycle no.
1 1 2
/
401
w t loss in 2 h, mg (from 30 m i n t o 2.5 h ) 11.2 10.6 10.1
10.8 9.6 9.85 8.9 9.65 7.5
8.3
P ..l
40
5
I0
I5
20
25
30
35
40
C
5
IC
15
TIME, min
20
25
30
35
40
T I M E , mln
Figure 2. Cyclic sulfation of calcined Greer limestone (-16 + 20 Tyler mesh) a t 950 "C at 1atm. Gas composition: 4% SOz, 10% 0, in N,. Numerals indicate the order of cycles. ,
,
I
I
,
I
,
550' C
0
IO
550'
20
TIME min
30
0
C
IO 20 TIME, m n
30
Figure 3. Cyclic regeneration with carbon at 950 "C and 1 atm (CaS04/C = 2). Amount of CaS04 to be regenerated in each cycle was about 60 mg. Numerals indicate the order of cycles.
Also in Figure 2, the rate of the first sulfation, Le., sulfation of the virgin sample, was about half of the rate of the second to ninth cycles. This difference was not real because in the rate calculations, the amount of available CaO for the first cycle sulfation was about double that of the other cycles. The rate would have been the same had the rate been expressed as the amount of SO2 sorption per unit exterior surface (or per particle) per unit time. Before the regeneration experiments, the sulfated sample was intimately mixed with fine particles of carbon at a ratio of 2 CaS04:1 C. The mixed sample was heated up to 950 "C at 15 "C/min in a N2flow. Slight weight loss was incurred before the final temperature of 950 "C was reached. No attempt was made to suppress the reaction during the heating-up period (or time < t = 0) by using
SO2 and O2 because both oxidants can react with carbon. It should also be noted that in the first regeneration experiment, when more carbon was used, the reaction product was predominantly Cas, as shown by X-ray diffraction analysis. This result is in agreement with that of Turkdogan and Vinters (1978). Results of the nine regeneration experiments are summarized in Figure 3. The temperature profile (or history) is inserted on top of the weight loss curves. Before discussing the results, a brief review of the mechanism of reaction 2 is in order. It has been shown (Turkdogan and Vinters, 1978) that the reaction proceeds in two steps. The first step is the reduction (3) of sulfate in the outer layer by carbon into sulfide until the carbon is all burnt off. Reaction 4, which is slower than the reduction reaction, is subsequently the predominant reaction. The amount of carbon present is consequently critical in determining the products. Going back to Figure 3, the actual weight loss was plotted against time because it would be meaningless to plot the percentage conversion to CaO owing to the two-stage nature of the reaction. Each reaction proceeded for 4-5 h for nearly complete regeneration with the initial 30 min shown in Figure 3. The shape of the curves is very much conformative to the two-step mechanism; a rapid weight loss due to the reduction followed by a slow weight loss due to reaction 4. Two interesting observations were made: (1) the reduction rate increased significantly with the number of cycles and (2) the rate of the slower reaction 4 decreased slightly with the number of cycles. The reduction reaction was completed in about 5-10 min in the second cycle and about 2 to 4 min in the ninth cycle. Rates of the Cas + 3 CaSO, reaction for the nine cycles are shown in Table 11. A gradual but slight decrease in the rates is seen. The rate of reaction 4 controls the rate of regeneration of CaO. The temperature dependence is presented in Arrhenius fashion in Figure 4. It is recalled that a surface layer on the CaS0, is first reduced rapidly by carbon via reaction 3. The reduced layer then reacts with the remaining inner CaS04 to form CaO via reaction 4. Two distinct slopes on the weight-time curves were obtained in the reaction. The second slope was used to calculate the rates in Figure 4. The rates here were based on the area of reacting interface, assuming the sulfated Greer limestone particles were perfect spheres. The temperature dependence shown in Figure 4 is, however, accurate regardless of whether this assumption was made, because the same amounts of the same materials were used at each temperature. The activation energy of reaction 4 is 62.7 kcal/mol of CaO which indicates a solid-phase diffusion and/or chemical reaction rate-limiting mechanism. This overall temperature dependence also means that the rate of regeneration of CaO is increased by a factor of three per 50 "C rise in this temperature range.
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
315
TEMPERATURE,'C ~
30.
t-
950'c
1 ,2.9 X S T E A M 1
9
1
\
1
2 2 -
IO-$
70
I
I
7 5
80
8 5
-
lI/TlX104,K~1
Figure 4. Rate of the reaction 3CaS04 + C a s 4Ca0 + SSOZ; 80% sulfated Greer l i e (16/20 mesh) and coconut charcoal (200/250 mesh) were the starting reactants.
3% N a C l
/
--r
3 2 - 900'C 28
--
-
60
10 4 0
1
0
y
20
1 I /
40 60
I
I
,
I
I
I
I
l
l
1
,
80 100 120 140 160 180 200 2 2 0 2 4 0 TIME, min.
Figure 6. Rate of reaction 3CaS04 + C a s OC, with powdered reagent grade samples.
-
4Ca0
+ 4 SO2 a t 900
The effects of NaCl on regeneration are shown in Figures 5 and 6. Figure 5 shows that 3% NaCl (of CaSO,, by weight) catalyzes both reactions 3 and 4 at 900 "C. Figure 6 shows that at 900 "C, NaCl first catalyzes reaction 3 but very intriguingly retards the reaction after about 35 70 regeneration. However, the results in Figure 6 were produced with analytical reagent grade powdered chemicals. As will be shown later, the retarding effect does not take place in the systems containing coal ash and limestone. Figure 7 shows a comparison of regeneration with and without 2.9% steam. Steam increases the reduction rate
but it is not clear at the present time whether it affects the rate of the second stage reaction 4. In the regeneration scheme, the size of the sulfated stone has to be suitable for fluidized-bed operation while the size of carbon can be small. Figure 8 shows the integrated rates of the two stages (reactions 3 and 4 of the reactions between a partially sulfated Greer lime and coconut charcoal. The integrated rate was based on the total weight loss divided by the total time for each reaction. As shown in this figure, reaction 3 depends strongly on the size of the sulfated stone while reaction 4 has only a very slight dependence on the size. This is understandable because reaction 3 requires close contact between CaS04 and C whereas reaction 4 does not, and the finer sizes of CaSO, provide greater contacting areas. By unsophisticated testing methods, the regenerated lime particles appeared to be as strong (if not more) as the starting material. The strength of the sample depends on the amount of sulfate remaining. The residual sulfate as well as other impurities seemed to strengthen the sample. The strength is important in resisting attrition in the combustor. A final note is that since the fly ash from fluidized-bed combustion contains a great amount of carbon; about 5-4070 from the Argonne and Exxon pilot combustors, fly ash appears to be an ideal candidate as the carbon source for regeneration of the sorbent. A bench scale kiln-type regenerator is being constructed for further testing of this process.
316
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
Acknowledgment We gratefully acknowledge the many helpful discussions and the guidance provided by Dr. Andrej Macek of ERDA. Discussions with Drs. Paul T. Cunningham, Irving Johnson, Albert A. Jonke, William M. Swift, and Gerhard J. Vogel of Argonne National Laboratory are also acknowledged. Excellent technical assistance was provided by Messrs. Otto F. Kammerer, Jacob Pruzansky, and Robert Smol of the Brookhaven Staff. Literature Cited Hoke, R. C. et al., "Studies of the Fluidized-Bed Coal Combustion Process", Annual Report, Exxon RBD Co., Linden, N.J., EPA-600/7-76-011 (1976). Hubble, B. R., Siegel, S.,Cunningham, P. T., APCA J . , 25, 1256 (1975). Hull, W. Q., Schon, F., Zirngibl, Ind. Eng. Chem., 49, 1204 (1957).
The Proceedings of the Fourth International Conference on Fluidized-Bed Combustion, sponswed by USERDA, coordinated by the Mitre Corp., McLean, Va., Dec 9-1'1. 1975. . Turkdogen, E. T., Vinters, J. V., Trans. Inst. Min. Metall., in press (1978). Vogel, G. J., et al., "A Development Program on Pressurized Fluidized-Bed Combustion", Annual Report, Argonne National Laboratory, Argonne, Ill,, ANL/ES-CEN-1016 (1976). Wheelock, T. D., Boylan, D. d., "Sulfuric Acid from Calcium Sulfate", Chemical Engineering Progress-Sulfur and SO, Developments, Technical Manual, 1971. Yang, R. T., Cunningham, P. T., Wilson, W. I., Johnson, S. A,, Adv. Chem. Ser., No. 139, 149 (1975). Yang, R. T., Steinberg. M., Carbon, 13, 411 (1975). Yang, R. T., Steinberg, M., J . Phys. Chem., 80, 965 (1976). Yang, R. T., et al., "Regenerative Process for Desulfurization of High Temperature Combustion and Fuel Gases", Quarterly Report No. 9. Brookhaven National Laboratory, Upton, N.Y., June 1978.
Receiued for review June 5 , 1978 Accepted September 29, 1978
Gas-Liquid Interfacial Areas for High Porosity Tower Packings in Concurrent Downward Flow Melvin G. Mitchell and Joseph J. Perona" Deparlment of Chemical, Metallurgical and Polymer Engineering, The University of Tennessee, Knoxville, Tennessee 379 76
Interfacial areas were measured in columns packed with 12.7-mm ceramic Intalox saddles and Raschig rings and 16-mm stainless steel Pall rings. The Danckwerts method was used with the carbon dioxide-sodium hydroxide reaction. Interfacial areas for all three packings fell in the range of 1-10 cm2/cm3of packed volume and increased with both liquid and gas velocities. Values of a / a , for all three packings were correlated with the pressuredrop-porosity function used by previous investigators.
Introduction The development of methods for the analysis and design of packed column gas-liquid reactors has created a need for the means to predict interfacial areas. The Danckwerts method for measuring interfacial areas (Richards et al., 1964) has been used by several researchers and some results are available for the more common packings. The Danckwerts method employs a reaction of known kinetics in the contactor such that a measurement of the extent of reaction can be used to calculate the interfacial area. Danckwerts and Sharma (1966) reported measurements of effective interfacial areas of Intalox saddles, Pall rings, and Raschig rings in countercurrent flow of the gas and liquid phases. Area increased with liquid velocity but was independent of gas velocity. Concurrent operation is receiving some attention for gas-liquid reactions because of its advantages of low pressure drop and unlimited flow rates. If the reaction is fast, the primary advantage of countercurrent operation of a larger average concentration difference between phases is somewhat lessened. Measurements of interfacial area with concurrent operation have been reported for 6-mm packings by G h e t t o . & al. (1973) for downward flow and Specchia et al. (1974) for upward flow, and for larger packings by Shende and Sharma (1974). Holdup and area measurements for 9.5-mm Raschig rings and larger spheres in concurrent downward flow were reported by Fukushima and Kusaka (1977). An excellent review paper on mass transfer in packed beds was presented by Charpentier (1976). Experimental Section Measurements of interfacial areas were made using the carbon dioxide-sodium hydroxide system. As explained 0019-7882/79/1118-0316$01.00/0
by Danckwerts (1970), the reaction is pseudo first order :c I1
(DAk2B0)'/' kL
BO 2A*
