Ind. Eng. Chem. Res. 1992,31,373-379
B = preexponential nucleation rate constant
r = solid molecular volume
u
= solidf fluid interfacial energy
Literature Cited Aiello, R.; Colella, C.; Casey, D. G.; Sand, L. B. Experimental Zeolite Crystallization in Rhyolitic Ash-Sodium Salt Systems. In Proceedings of the 5th International Conference on Zeolites; Rees, L. V. C., Ed.; Heyden: London, 1980; p 49. Culfaz, A.; Sand, L. B. Mechanism of Nucleation and Crystallization of Zeolites from Gels. In Molecular Sieves; Meier, W. M.; Uytterhoeven, J. B., Eds.; Advances in Chemistry Series 121; American Chemical Society: Washington, DC, 1973; p 140. Dai, F.-Y.; Suzuki, M.; Takahashi, H.; Saito, Y. Mechanism of Zeolite Crystallization without using Template Reagents of Organic Bases. In New Developments in Zeolite Science and Technology; Murakami, Y., Iijima, A., Ward, J. W., Ma.;Ehvier: Amsterdam, 1986; p 223. Dai, F.-Y.; Suzuki, M.; Takahashi, H.; Saito, Y. Crystallization of Pentasil Zeolite in the Absence of Organic Template. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989, p 244. den Ouden, C. J. J.; Thompson, R. W. Analysis of the Formation of Monodisperse Populations by Homogeneous Nucleation. J. Colloid Interface Sci. 1991, 143, 77. Hayhurst, D. T.; Sand, L. B. Crystallization Kinetics and Properties of Na, K Phillipsites. In Molecular Sieves-ZI; Katzer, J. R., Ed.;
373
ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977, p 219. Hu, H. C.; Lee, T. Y. Synthesis Kinetics of Zeolite A. Ind. Eng. Chem. Res. 1990,29,749. Huang, C. L.; Yu, W. C.; Lee, T. Y. Kinetics of Nucleation and Crystallization of Silicalite. Chem. Eng. Sci. 1986,41,625. Keijsper, J. J.; Post, M. F. M. Precursors in Zeolite Synthesis. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS SymDoeium Series 398 American Chemical Societv: Washington. DC. 1989; p 28. Randolph, A. D.; Larson, M. A. Theory of Particulate Processes, 2nd ed.; Academic Press: New York, 1988. Sand,. L. B. Personal communication, WPI; Oct 1984. Schwieger, W.; Bergk, K.-H.; Freude, D.; Hunger, M.; Pfeifer, H. Synthesis of Pentasil Zeolites with and without Organic Templates. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989; p 274. Thompson, R. W.; Dyer, A. Mathematical Analyses of Zeolite Crystallization. Zeolites 1985, 5, 202. Warzywoda, J.; Edelman, R. D.; Thompson, R. W. Thoughta on the Induction Time in Zeolite crystallization. Zeolites 1989,9,187. Zhdanov, S. P.; Samulevich, N. N. Nucleation and Crystal Growth of Zeolites in Crystallizing Aluminosilicate Gels. In Proceedings of the 5th International Conference on Zeolites; Rees, L. V. C., Ed.; Heyden: London, 1980; p 75.
Received for review December 17, 1990 Revised manuscript received May 15, 1991 Accepted August 13,1991
Kinetics of Sorbent Regeneration in the Copper Oxide Process for Flue Gas Cleanup+,$ Peter Harriott*i*and Joanna M. Markussen Pittsburgh Energy Technology Center, US.Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
In a regenerable process for flue gas cleanup, CuO supported on A1203reacts with SO2 and O2to form CuS04. The sorbent can be regenerated with CH4,but the reaction is slow and strongly inhibited by the SO2 produced. The kinetics of regeneration were studied using a thermal balance reactor, and the influence of the kinetics on design and operation of a reactor is discussed.
Introduction In the Fluidized-Bed Copper Oxide Process for simultaneous removal of sulfur dioxide (SO,) and nitrogen oxides (NO,), flue gas with added ammonia (NH,) is passed through a fluidized bed of sorbent particles at about 400 "C. The sorbent is l/ls-in.-diameter spheres of copperimpregnated alumina. The copper oxide (CuO) is converted to copper sulfate (CuSO,) by reaction with SO2and 02.
CUO + so2 + 7 2 0 2
-
cuso4
(1)
The reduction of nitric oxide (NO) by NH, is catalyzed mainly by CuSO,, but CuO can also act as a catalyst. 4 N 0 + 4NH3+ O2 4Nz + 6Hz0 (2)
-
'Presentad at the AIChE Annual Meeting, Chicago, IL, Nov 11-16, 1990, paper 49c. 5 Reference in this paper to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply ita endorsement or favoring by the United States Department of Energy. Oak Ridge Associated Univerisities Faculty Research Participant. On leave from Cornel1 University, Ithaca, NY.
The partially sulfated sorbent is heated to 450-500 "C in a separate fluidized bed and is then regenerated with natural gas in a counterflow moving bed. The reaction produces 5 mol of gas per mole of methane (CH,) consumed. CUSO~ + 72CH4 CU + SO2 + 7zC02 + H2O (3) +
The regenerated sorbent is cooled and returned to the adsorber, where the copper is rapidly oxidized by oxygen in the flue gas. c u + y.20, CUO (4)
-
The kinetics of SO2removal have been studied using fEed beds and different size fluidized beds, but there is little information on the kinetics of regeneration. Early fixedbed tests by McCrea et al. (1970)showed slow regeneration with CHI at 400 "C, but nearly complete sulfur removal was obtained in 30 min at 450 "C and in less than 10 min at 500 "C. The sorbent retained about 1%S even after 1-h exposure to CHI at 500 "C. The sulfur, however, was probably bound to the alumina support and did not affect the capacity of the CuO for subsequent SOz removal. Recent tests by Yeh et al. (1987) in a microbalance reactor showed 80% reduction of CuSO, with CHI in 10 min at
0888-5885f 92 f 2631-O313$03.00 f 0 0 1992 American Chemical Society
374 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 MICROBALANCE
REGENERATION GASES
Condrnrrr LEQEND
w
c$
Ragulatlng Valve
Mlcm Valve
WatrrTrnp
@ Fwr.Woy Valve Q
Flltrr
Y P
Figure 1. Schematic of the microbalance unit.
450 "C or in 25 min at 400 "C. Their rates of reaction were
several times larger than those reported by McCrea, probably because a much larger excess of CHI was used in the microbalance tests. McCrea et al. (1971) found the rate of regeneration to be strongly dependent on temperature and relatively independent of methane concentration. In tests with 50% and 100% CH4,Yeh et ai. (1987) observed the regeneration rate to be first order with respect to methane. In commercial operation, using a large excess of methane would be impractical. If the regeneration reactor operated with 10% excess CHI, the exit gas would have only 2% CHI because of dilution by the product gases. To aid in scaleup and design studies of the regenerator, more kinetic data were needed over a wide range of methane concentrations. The effect of product gases on the rate of regeneration was also not considered in previous studies. In DOE testa of a large (1.2 m2) fluid-bed adsorber that was operated with continuous flow of solid, the spent sorbent was collected and regenerated batchwise in a static bed at 420 "C (Yeh et al., 1985). Methane flow was continued until there was no more reaction, and the regenerated sorbent had about 1 wt % S. In another DOE-sponsored study, the Fluidized-Bed Copper Oxide Process was tested in a Continuous LifeCycle Test Facility with a smaller fluidized-bedadsorber and a continuous movingbed regenerator (Plantz et al., 1986;WiUiamson et aL, 1988). The regenerator had a solids residence time of 1-2 h, depending on the flow rate, and the average temperature was about 450 OC. During the testing, it appeared that only 30-50% of the CuS04 fed to the regenerator was reduced, and incomplete regeneration decreased the rate of SOz removal in the adsorber. To get 90% SO2 removal, solid circulationrates 4-6 times the theoretical minimum had to be used. (The minimum is 1 mol of CuO/mol of SO2 in the flue gas.) High solid circulation rates increase the attrition lossee and the energy costs and may make the process too costly, so there is a strong incentive to improve the regeneration efficiency.
Incomplete regeneration in the continuous reactor could have been due to low methane concentration or to high product concentrations or a combination of factors. The exit gas had 1&20% CH4and 20-30% SO2 plus C02,HzO, and N2, which were not measured. Correcting for the estimated effect of the lower partial pressure of methane (Pch), the calculated rate constants for CuS04reduction were only about 10% of the values found from microbalance testa (Harriott, 1988). This suggested that SO2or other product gases inhibit the reaction. Therefore, a kinetic study of regeneration using a microbalance reactor was conducted.
Experimental Procedure The sorbent was 1/16-in. spheres of y-alumina impregnated with 6.4 wt % Cu, manufactured by UOP, Inc. (UOP SOX-3). A batch (15 g) of sulfated sorbent to be used in regeneration testing was prepared in a fixed-bed reactor by the following procedure. The CuO sorbent was exposed to two cycles of SO2 sorption and CHI regeneration followed by a finalsulfation step. The sulfation steps were conducted at 400 "C with simulated flue gas containing 4900 ppm SOz, 3.7% Oz, and a balance of N2. Regeneration was performed at 450 "C with pure methane. The final SO2 uptake was 1.03 mmol/g, which is in good agreement with the value of 1.01 mmol/g expected for complete conversion of the CuO to CuS04. The regeneration tests were carried out in a Cahn Electrobalance (Model 1OOO). A schematic of the microbalance system is shown in Figure 1. Gases were blended from certified cylinders to generate the desired regeneration gas mixture. Methane concentration ranged from 5 to 30%, with a few testa conducted with 100% CH4. During regeneration testa with CH4-S02-N2 mixtures so2 concentration was varied from 5 to 30%. A few testa included COz at a concentration of 20% to study its potential inhibiting effect on sorbent regeneration. Gas flow rates to the microbalance reactor tube were maintained at about lo00 cm3/min(STP). The temperature of the 9
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 375
0.05 3
IO0
IO 56 CH4
450°C, 100% CH4
0.031 0
1
I
IO
I
I
20
1
I
Figure 3. Effect of methane concentration on the regeneration rate of cuso4. 1
30
TIME, min
Figure 2. Regeneration of CuS04with CH4and N2.
reactor was measured with a thermocouple placed 0.5 cm below the sample basket. Regeneration kinetics were studied at reactor temperatures of 450,480, and 510 "C. Samples of sulfated sorbent weighing approximately 50 mg were spread over a layer of quartz wool placed in a quartz sample basket. The basket was positioned inside the quartz reactor tube in the furnace and suspended on a wire connected to the microbalance. The sample was then heated to the reaction temperature using pure NP The gas stream entered the microbalance at the base of the reactor tube and passed through a section containing ceramic beads, which were put in the tube to reduce the amount of dead gas volume. Steady-state temperatures were reached in about 1-1.5 h. After the heabup period, a regeneration gas mixture was introduced into the reactor by switching the air-actuated four-way valves shown in Figure 1. The flow rates of the two gas streams were matched to minimize shock buoyancy forces that can cause inaccuracies in the sample weight measurements during the initial time period. The sulfated sorbent was exposed to either a CH4-N2 gas mixture or a CH4S02-N2gas mixture until no further change in weight was observed, which typically occurred within 45 min. After the regeneration had reached completion in the tests using CH4-SO2-N2gas mixtures, the SO2 gas was shut off and the N2gas flow was increased. Thus, the sample was then exposed to CH4-N2 gas mixtures to determine if the sorbent could be further reduced. The reaction system typically reached steady-state conditions within an additional 30 min. Upon completion of the regeneration, the sample was cooled to room temperature using N2.gas. From the microbalance weight loss curves, fractional conversion data as a function of time were obtained.
Experimental Results Effect of Methane Concentration. The weight loss curves using CH4-N2gas mixtures generally showed nearly constant slope up to about 40% conversion, and then a decreasing rate was observed. In some runs, the rate decreased steadily from the beginning of the test, and in a few cases, the curves had a slight increase in slope near the start. Some typical results are shown in Figure 2, where the logarithmic scale for the fraction unreacted is used to permit easier comparison with firsborder behavior. Note that x represents the fractional conversion of CuS04 to Cu via eq 3. The data for up to 80% conversion can be fitted fairly well by a first-order plot, which would be a straight line
in Figure 2. At conversions greater than 80%, the reaction rate is less than expected for a first-order process, but it is difficult to determine accurate rate data at high conversions. The total weight loss is about 4.4 mg, and the continuing slight loas of water during the test plus baseline shifts which sometimes occur during gas switching introduce uncertanties of about 0.1 mg or *2% conversion. The crmsover of some conversion curves, as shown for two runs in Figure 2, is probably due to experimental error. Effective firsborder rate constants (k,) were calculated from the integrated form of the first-order rate equation r = &/dt = kl(1 - X ) (5)
For thisseries of runs,the effective firsborder rate constant based on the time to reach 80% conversion was taken as a measure of the reaction rate. The first-order rate constants for regeneration with CH4-N2 mixtures are plotted as a function of CHI concentration in Figure 3. The reaction rate increases with only a fractional power of the methane concentration. The data for 480 "C show a lower apparent reaction order at high concentration, which is consistent with LangmuirHinshelwood kinetics (Fogler, 1986). The data are fitted fairly well by eq 6, where K1is an adsorption equilibrium
constant and k2 is the product of an adsorption equilibrium constant and the reaction probability for adsorbed species. The best fits of the data gave denominator constants of 7.3 at 510 "C, 3.9 at 480 "C, and 5.0 at 450 "C, but since there were not enough data points to be sure of the trend with temperature, a value of 5 atm-l was chosen to fit all the data This gave the following values for the numerator rate constank T = 450 "C, k2 = 1.54 min-' atm-l; T = 480 "C, k2 = 3.02 min-' atm-'; T = 510 "C, k2 = 4.86 min-' atm-l. The apparent activation energy is 24 kcal/mol between 450 and 480 OC and 18 kcal/mol between 480 and 510 "C. The decrease in activation energy is probably due to pore diffusion effects, since the estimated effectiveness factor at 510 "C is 0.9. To show more clearly the decrease in reaction rate during the last stage of regeneration, data taken during sorbent preparation in the fEed-bed reactor were used. In the second regeneration with 100% CHI, the SO2 concentration in the exit gas was greater than 5000 ppm (off scale) for 15 min and then decreased to 0 over the next 2.5 h. Assuming complete conversion at the end of the test, the fraction of unconverted CuS04 was obtained by
376 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 I
.on
'CI
I.Or-----7
R
0.9
Microbalance, 4 5 0 T 50% CH4, 10%SO2
I
1
I
I
I
\ ~ 5 0
\03
A30
is r C H 4 I 480.C A 450.C
0.61 Para?
,
0
z
0'50
45OoC, 100% CH4 45OOC.
9
0.1
0.2
0.3
0.4
0.5
Pso,, atm
Figure 6. Apparent equilibrium conversion.
a
10-3
0
I
I
I
I
I
I
20
40
60
80
100
120
140
TIME, mln
Figure 4. Regeneration of CuS04at 450 O C .
-
30 30
26 28 I
0
30 20
I
20
Run28 I
I
40
1
-
I
60
TIME, mln
Figure 5. Regeneration of CuS04at 450 O C with CH4-S02-N2gas mixtures.
backward integration of the SO2curve. Figure 4 shows the fixed-bed results along with data for similar runs in the microbalance. The time to reach 90% conversion in the fixed bed was about 70% greater than in the microbalance, which is probably due to the higher SO2concentration in the fixed bed. The slope of the plot decreases with time, but from 95 to 99.8% conversion, the reaction appears to follow first-order kinetics. The rate constant for the last stage of conversion is 0.030 min-', which is 10-fold lower than the rate constant for 040% conversion. If 95-100% regeneration is required, more tests are needed to show the effects of temperature and gas concentration in this region. However, 80-90% conversion may suffice for a practical design, which would make data at higher conversions unnecessary. Inhibiting Effects of SO2and C02. When 10-30% SO2was added to the CHI-N2 mixture, the reduction reaction was nearly first order to CuS04 up to 50-70% conversion, but then the rate rapidly decreased, and the reaction appeared to approach an equilibrium limit. One run with 10% SO2is shown in Figure 4, and typical results
for higher SO2 concentrations are given in Figure 5. For these tests, the SO2was turned off when the reaction rate was almost zero, and the N2 flow wae increased to keep Pch constant. The sharp increase in reaction rate is proof that an appreciable amount of CuS04or other reducible sulfur compound still remained on the solid even though the reaction had essentially stopped. Although the calculated equilibrium constant for reaction 3 is very large (K = 1014at 500 "C), the thermodynamic data are for t h h u l k solid phases. There may be a less favorable free energy change for the reduction of CuS04present as a monolayer on A1203 or present as very finely dispersed particles. The apparent equilibrium conversion ranged from 90 to 65%, decreasing with increasing Ps,, as shown in Figure 6 in which the fractional conversion reached at pseudoequilibrium is plotted as a function of SO2 partial pressure. If this is a true equilibrium limit for the surface reaction, high methane concentration should increase the conversion. However, Figure 6 shows no consistent effect of PcH,, and the results are about the same at 450 and 480 OC. Adding 20% COzgave a slightly lower final conversion for one run but not for a second run. There were no tests with added water vapor, and PH was very low for all the tests. Since COZ and HzO are pr$ucts of the reaction aa well aa SOz, the equilibrium conversion should have been sensitive to Pco, and PHp as well as to P Perhape the decrease in rate to a h a t zero is a kinetic or a pseudoequilibrium rather than a true equilibrium limitation. No matter what causes the apparent equilibrium, it should not limit the conversion achievable in a counterflow regenerator, since the solid leaving the regenerator would be contacted by 100% CH4. This should permit complete regeneration given a sufficient residence time and enough excess methane. However, near the top of the reactor, there might be a region where high CuSOl conversion and moderate PSO,make the rate quite small. For a parallel flow regenerator, the reaction would probably stop at about 70-80% conversion, since the gas would have 20-30% SO2 at that point and a low PCH,. The effect of SO2 on the early stages of reduction can be shown by comparing rate constants for runs with SOz CH, with thw for similar runs without SO2or by noting the change in rate following a sudden change in gas composition for a particular run. Figure 7 shows two runs in which the gas composition was changed at 30 or 40% conversion. Stopping the SO2flow resulted in an almost Sfold increase in reaction rate when the SOzconcentration was only 11%.When 19% COPwas present at the start of a run,removing the C02caused only a 30% increase in the reaction rate. Only a few testa were made with C02 in the feed gas, and most of the work was directed to
skt
+
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 377
Run %CH4 %SOz %CO2 33 II It 0 34 29 0 19
IO
to
Run 34
20
0.011 3
30
1
1
I
+
A
A 20 0
I
I I I
l 0.021 3
+
A 30 CU SO3 I
'
I I '
I
I
1
1
1
1
1
1
100
IO % CH4
Figure 8. First-order rate constants for regeneration of CuSOl at 480 "C.
studying the inhibiting effect of SO2. The rate constants were generally calculated from the time to reach 60% conversion. Figures 8 and 9 show the rate constants for CuS04regeneration at 480 and 450 "C, respectively. The dashed lines are for reduction with no SO2in the feed gas and are based on the data in Figure 3. Sulfur dioxide has a large effect on the reaction rate, and the decrease in rate is more pronounced at low methane concentrations. This is consistent with a mechanism of competitive adsorption of reactant and products on the surface, which leads to the following first-order rate constant expression.
kl =
k2PCH,
+ KlpCH,
+ K2pS0z + K f l C O z
I
, I ,
100
Figure 9. First-order rate constants for regeneration of CuSO, at 450 "C.
1
K so2 e 5 IO
o
' '
I
IO
X CH4
Figure 7. Effects of SO2 and C 0 2 on regeneration at 450 "C.
Run 39
30
A 30 CUSO3
O
TIME. min
8 A
% so2
8
(7)
The data do not have a close fit to eq 7, since the largest measured reduction in rate come8 between 0 and 10% SO2, and a further increase in SO2 content to 30% does not cause as large a change in rate. However, to study possible regenerator designs and operating conditions, the data were fitted to eq 7 using K1 = 5.0 atm-', the value previously found for CH4-N2 mixtures. A t 450 "C, the best fit was obtained with K2 = 15 atm-', and at 480 "C,the value of K2 was 17 atm-'. This difference is not significant, and a value of 16 atm-' was used for both temperatures. In-
Table I. Kinetic Tests with run temp, "C % SO2 7 510 0 12 510 0 6 480 0 480 8 0 9 480 0 480 10 0 11 480 0 19 0 480 31b 0 480 450 5 0 22 460 0 20b 450 0 21b 450 0 14 480 10.5 15 480 19.7 16 480 29.3 17 20.1 480 30.2 18 480 19.9 480 30 480 31 31.6 32 480 5.2 480 35 30 480 36 10.1 480 39" 29 450 20 30.2 21 450 10.8 450 23 29.8 24 450 30.2 25 450 29.8 450 30 26 450 27 20 450 28 20 450 10 29 450 10.9 33 450 346 0 37c 450 29.2
CH,-S02-N2 Mixtures % CHI kl, min-' kl*(pred)/kl 30.4 0.56 1.05 10.1 0.34 0.96 30 0.92 0.39 30.2 0.41 0.88 10.2 0.19 1.07 10.3 0.175 1.16 100 0.56 0.89 10.7 0.18 1.16 0.115 5.0 1.04 0.14 30.2 1.33 100 0.28 0.92 10.2 0.12 0.87 10.2 0.105 1.0 29.7 0.14 1.53 0.093 30.7 1.74 0.083 29 1.47 20 1.27 0.090 10.6 0.075 0.67 49.4 0.26 0.86 0.025 5 0.95 0.22 28.9 1.20 10.5 0.71 0.070 0.20 28.3 1.05 0.062 10.6 0.70 10.4 0.025 1.0 0.031 10.3 1.59 10.5 0.027 0.96 0.018 5.3 0.75 19.9 0.91 0.050 29.7 0.064 0.99 29.2 0.087 0.92 30 0.086 0.95 48.5 0.91 0.165 11.3 1.26 0.042 28.6 1.17 0.105 1.25 10.6 0.018
"Run 39 also had 19.4% C02. bRun 34 also had 19.4% COS. 'Run 37 also had 19.6% C02.
cluding an approximate value for K3,the final equations are:
The experimental values of kl are given in Table I, along
+
378 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
with the ratio of k l * / k l , where kl* is the predicted rate constant. The average deviation in the ratio of these values is 16% at 450 "C and 33% at 480 "C. The effect of water vapor has not been determined, and since both SOzand COzinhibit the reaction, it is likely that HzOhas some effect. The gas streams were not humidified during the microbalance tests, and the amount of water vapor formed in the testa was negligible. If water has a large effect, it should be evident when comparing the overall conversion in a moving-bed or a fixed-bed reactor with that predicted from the microbalance tests or from eqs 8 and 9. Effect of Copper Oxide. In the Fluidized-BedCopper Oxide Process, the sorbent is only partially sulfated in the adsorber, and the effect of the CuO on the regeneration process is not clear. One test (run 38) with unsulfated sorbent exposed to 10% CH4 and 30% SO2 at 450 "C showed a rapid weight gain, indicating that nearly complete conversion to CuS03 took place before much reduction occurred. CUO so2 cuso3 (10)
+
-
The weight gain was followed by a slow loss of weight, presumably from the reaction with CHI. CuS03 + f/4CH4 --+ CU + SO2 + f/4CO2 + l/zHzO
(11)
The reduction reaction was only half as fast as the reduction of CuS04at the same conditions, and it appeared to reach a pseudoequilibrium after 40 min with less than 50% conversion of the CuS03. Shutting off the SOz flow started the reaction again, but the rate was still lower than that for CuS04 reduction. In four other tests, unsulfated sorbent was exposed in sequence to CHI Nz,Oz + Nz, SOz Nz, and CHI SOz + N2so that the reactions could be studied separately. The reduction of CuO with 10% CHI was approximately first order, and the rate constants were about 0.13 min-' at 450 "C and 0.23 m i d at 480 "C. When the reduced samples were exposed to 10% Oz, the rate of oxidation was 10 times faster than the rate of reduction. However, the weight loss on reduction and the weight loss on reduction and the weight gain on reoxidation were only 50-70% of the values expected for CuO, indicating that CuzO may have been present as well.
+
+
--
CUO+ f/4CH4
CuzO + Y4CH4
+ f/2Hz0 2Cu + f/4CO2 + '/zHzO CU + '/C02
+
(12)
(13)
The addition of SOz to the reoxidized samples was very rapid, with a rate constant of 1.3 min-' at 450 "C and 30% SOz. Thus, the addition of SOz to unsulfated sorbent is 10 times as rapid as the rate of reduction by CHI, which is consistent with the initial increase in weight found in run 38. The weight gain with SOz was always less than that expected for formation of CuS03, again indicating formation of a mixture of cuprous and cupric compounds. CUO + so2 cuso3 (14) cuzo + so2 CuzSO3 (15) In the previous tests with fully sulfated sorbent, the eventual weight loss on reduction was in close agreement with that expected based on CuSO, (see reaction 31, and it is not clear why the unsulfated sorbent did not show weight changes consistent with cupric compounds. The last test of the sequence was the reaction of the sulfited samples (CuS03+ CuzS03)with gas containing 10% CHI and 30% SOz. The reduction reaction was much slower than the rate of SOz addition, and the initial rate
--
I .01
I
1
I
I
A
I O % Excess CH4
*,,
C CUO u=
0.61 v)
a cuo+so2
0
0.2
-
CuSO3
0.4 0.6 0.8 SOLIDS CONVERSION
1.0
Figure 10. Gas composition for counterflow regeneration of CuSOl + CuO via CuSO,.
corresponded to rate constants of about 0.04 m i d at 450 "C and 0.13 min-' at 480 "C. These rate constants are somewhat greater than those for the reactions of CuS04 a t the same conditions, as shown in Figures 8 and 9. However, the reaction of CH4with CuS03 + CuzS03appeared to reach an equilibrium limit at only 31340% conversion, based on removal of the SOzpreviously added. Removing SOz from the feed gas permitted the reaction to continue, but the maximum conversion was only about 70% with 10% CHI in the feed gas. The presence of CuO or CuzO in the partially sulfated solid fed to the regenerator creates two problems for regenerator design. The very rapid uptake of SOz by the copper oxides in the top of the regenerator tends to make the SOz concentration in the gas decrease as the gas passes through the top portion of the moving bed (reactions 14 and 15). Opposing this change is the reduction of CuS04 by CHI according to reaction 3. According to the data collected so far, the uptake of SOz is so much faster than the reduction reaction that the SOzconcentrationprobably goes through a maximum near the top of the bed. The maximum value depends on the moles of SO2taken up per mole of Cu in the sample since CuS03or CuzS03could be formed. The second problem is the limiting conversion when CuS03 is exposed to gas with higher CH4/SOzratios further down the bed. The data from the few runs made with unsulfated sorbent are not consistent enough to use for design calculations, but they indicate that a pinch would probably be reached, with zero or very low rate of reaction. The fraction of CuS03regenerated could be less than 0.5 even when most of the CuS04 has been reduced to Cu. In practice, the spent sorbent will probably have more CuS04 than CuO, but if the CuO converts to CuS03 instead of reacting directly with CHI, the concentration of SOz near the top of the reactor will be higher than the value in the exit gas. Gas compositions calculated for the case of 10% excess CHI and 100% solids conversion are shown in Figure 10 for a sorbent that was 67% sulfated. The gas has 43% SOznear the top of the reactor, but then drops to 34% SOzat the top of the reactor where entering CuO is rapidly converted to CuS03. For this example, about half of the solids conversion reaction is carried out
Ind. Eng. Chem. Res. 1992,31, 37+389 with an average concentration of 40% SO2and a methane concentration of 2% to 18%. The initial reaction rates will be less than those shown in Figure 9,where the highest SO2 level was 30%. A few kinetic tests need to be performed with 40-45% SO2 concentrations.
Summary Kinetic tests in a thermal balance reactor showed that the reaction of CuS04/A1203with CH, is strongly inhibited by the product gases, particularly SO2. The regeneration reaction is adversely affected by CuO in the spent solids, since this converts to CuS03before being reduced to Cu. The reactions of CuS04 and CuS03 with CHI do not go to completion when SO2 is present, but there are not enough data to clearly define the equilibrium limits. More laboratory tests are needed to develop kinetic correlations that can be used to help interpret pilot-plant data and determine the best conditions for sorbent regeneration. Acknowledgment We gratefully acknowledge Robert Navadauskas and Charles Brodd of the Pittsburgh Energy Technology Center for their efforts in performing the microbalance tests. Registry No. CaO, 1317-380;SOz,7446-09-5;NO,,11104-93-1; NHB,7664-41-7;Alto3,1344-281; CuS04,7758-98-7;CHI, 7482-8.
379
Literature Cited Fogler, H. Scott. Elements of Chemical Reaction Engineering; Prentice-Hak Englewood Cliffs, NJ, 1986; p 245. Harriott, P. Kinetic Tests of CuSO, Regeneration. Memo to C. J. Drummond; Pittsburgh Energy Technology Center, Pittsburgh, PA, Sept 30,1988. McCrea, D. H.; Forney, A. J.; Myers, J. G. Recovery of Sulfur from Flue Gases Using a Copper Oxide Absorbent. JAPCA 1970,20, 819-824. McCrea, D. H.; Myers, J. G., Forney, A. J. Evaluation of Solid Absorbents for Sulfur Oxides Removal from Stack Gases. Proceedings Second International Clean Air Congress; Academic: New York, 1971;p 922. Plantz, A. R.; Drummond, C. J.; Hedges, S. W.; Gromicko, F. N. Performance of the Fluidized-Bed Copper Oxide Process in an Integrated Test Facility. Presented at the 79th Annual Meeting APCA, Minneapolis, MN; June 1986. Williamson, R. R.; Movici, J. A.; Lacrosse, T. L. “Sorbent Life-Cycle Testing Fluidized-Bed Copper Oxide Process”;Project Report for the Department of Energy; UOP Des Plaines, IL, 1988. Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined S02/N0, Removal from Flue Gas. Enuiron. Prog. 1985, 4, 223-228. Yeh, J. T.; Strakey, J. P.; Joubert, J. I. SO2Absorption and Regeneration Kinetics Employing Supported Copper Oxide. Unpublished paper; Pittsburgh Energy Technology Center, Pittsburgh, PA, 1987.
Received for review December 18, 1990 Revised manuscript received July 22, 1991 Accepted August 19,1991
Characterization of the Melt Blowing Process with Laser Doppler Velocimetry Tien T. Wu and Robert L. Shambaugh* Department of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
Laser Doppler velocimetry (LDV) was used to study the process of melt blowing. For an array of positions below the melt blowing die, LDV measurements were made on the fiber velocities in three-space. This information quantifies the existence of an expanding “cone” of fibers below the melt blowing die. Through the use of a correlation function developed in this work, one can determine the actual rate of fibers passing through a specified area below the melt blowing die. An LDV system can be calibrated to measure the mass flux of fibers a t any point in space. This mass flux information is particularly useful for rapid, on-line prediction of the basis weight of a melt blown web of fibers.
Introduction Melt Blowing. In the melt blowing process, a fine polymeric stream is extruded into a high-velocity gas stream. The force of the gas rapidly attenuates the polymer into filaments of very small diameters; see Figure 1 for a diagram of the process. Melt blown fibers typically range from 30 pm in diameter down to filaments as fine as 0.1 pm in diameter. This extreme fineness gives melt blown fibers advantages in uses such as insulation, absorbent material, and filters. An overview of the melt blowing process is given in the article by Shambaugh (1988). The performance characteristics of various melt blowing geometries are given by Kayser and Shambaugh (1990). Uyttendaele and Shambaugh (1989)and Majumdar and Shambaugh (1991)examined the gas velocity and temperature fields in melt blowing, while Majumdar and Shambaugh (1990)measured the drag coefficient between
* Author to whom correspondence should be addressed.
the gas and the filament. Uyttendaele and Shambaugh (1990)developed a mathematical model for melt blowing. Laser Measurements in Fiber Science. Since the construction of the fmt laser in 1960 (Hieftje et al., 1981), the use of lasers in science, industry, and commerce has risen exponentially. The use of lasers in the fiber industry is no exception. For example, a single laser beam, when shined upon a fiber, will produce a forward scatter pattern that is dependent on the fiber diameter. Fouda et al. (1988)showed that, as a method of off-line fiber size measurement, use of this pattern is more accurate and much faster than obtaining diameters by optical microscopy or scanning electton microscopy. This forward scatter technique can also be used to measure the diameter of moving filaments. Thus, for example, the procedure can be used to monitor the diameter of a fiber during conventional melt spinning (Hamza et al., 1980). In addition, the backscattered radiation from a moving (or stationary) filament can be used to measure refractive index (Presby, 1974;Hamza et al., 1980;Wilkes, 1982).
0888-5885/92/2631-0379$03.00/00 1992 American Chemical Society
