of Ninth Annual Offshore Technoloev Conference”. OTC 2934. oo 435-8, Offshore Technology Conf., Houston, Tex.,’1977. (14) Brooks, J. M.. Bernard, B. B.. Sackett, W. M.. in “Fate and Effects of Petroleum Hydrocarbons in Marine Organisms and Ecosystems”, D. A. Wolfe, Ed., p p 373-84, Pergamon Press, New York, N.Y., 1977. (15) McAullife, C. D., J . Phys. Chem., 70, 1267 (1966). (16) McAullife, C. D., in “Marine Pollution Monitoring”, NBS Spec. Pub]. 409, pp 121-5, GPO, Washington, D.C., 1974. . I
(17) Manheim. F. T.. Hathawav. J. C.. Uchuoi. E.. Limnol. Oceanogr.. 17, 17 (1972). (18) Brooks. J. M., Frvxell. G. A.. Reid, D. F.. Sackett. W. M.. in “Marine Biological Effects of Petroleum Hydrocarbons”, C: S. Giam, Ed., Heath, 1977, in press. L
I
Receiued for review August 30, 1977. Accepted December 28,1977. Partial support for some of this work was prouided by NSF Grants OCE76-81493 and OCE75-13299 AOI.
Absorption of SO2 in Lime and Limestone Slurry: Pressure Drop Effect on Turbulent Contact Absorber Performance Chin-yung Wen* and Chung-Shih Chang Department of Chemical Engineering, West Virginia University, Morgantown, W.Va. 26506
The data of wet scrubbing of SO2 from flue gas either by lime or limestone slurries using two turbulent contact absorbers (a bench scale and a large scale) under flooding or nonflooding conditions are analyzed. The mathematical model developed by McMichael et al. is refined. The effect of liquid holdup on the scrubbing efficiency is incorporated into this model to include the hydrodynamic factors. The TCA operating above the flooding point is simulated by considering its similarity to the fixed bed. The correlation developed is shown to represent the system closely and can be used for design and scale-up of TCA scrubbers for SO2 removal from stack gas.
Uchida et al. ( 1 ) correlated the liquid holdup for a TCA as:
APLN = 7.96 X lo2
Equation 4 shows that the liquid holdup in a TCA is independent of the gas velocity. Only the liquid velocity can affect the liquid holdup. Thus, the pressure drop across one stage of a TCA is independent of the change in the gas flow rate. The pressure drop due to friction is given as:
a p =~3.51 X lo-’ Uc’ 9 1 U ~271 The turbulent contact absorber (TCA) is noted for its high gas-liquid contacting area, long residence time, and lowpressure drop. It has been used as a scrubber for removing SO2 from flue gases by utilizing lime and limestone slurries. The scrubbing efficiencies of TCA have been reported to be quite satisfactory. A number of investigations have been conducted to clarify the mechanics of the TCA operation. Uchida et al. (1) summarized the operating conditions of the TCA and developed correlations for the liquid holdup and the pressure drop. Using these results, the mathematical model proposed by McMichael et al. (2) is refined to include greater range of operation and to incorporate the effect of liquid holdup on the scrubbing efficiency.
Pressure Drop Effect To investigate the pressure drop effect on scrubbing efficiency, the normal pressure drop of a TCA must be known. Uchida et al. ( 1 )studied the mechanics of the TCA operation and suggested that the total pressure drop across a TCA is: APTN = n
U
B
D
F
(1)
where APB represents the pressure drop across one stage of is the pressure drop due to the friction loss at the a TCA. PF entrance, at the exit and due to the liquid distributor as well as other internals present in the scrubber. Kit0 et al. ( 3 ) proposed that the pressure drop across one stage of a TCA is composed of the weight of the liquid holdup and the packing spheres. APB = f l p
+P L N
(5)
Since PF is a function of both gas and liquid velocity, the total pressure drop, APTN in Equation 1,is affected by both gas and liquid velocities. Under the sponsorship of EPA, a large scale ( 4 )and a bench scale ( I O ) turbulent contact scrubber-holding tank recycle system has been built to test the performance and reliability of the scrubbers. When lime or limestone slurry is used in scrubbing SO‘, some data show that the total pressure drop across the TCA is higher than the normal total pressure drop estimated by Equation 1. These data also show that the scrubbing efficiencies of the high-pressure drop runs are generally higher than those with normal pressure drop. The range of experimental conditions covered by the data is given in Table I. As seen from Equations 1 and 2,the total pressure drop is increased either by an increase in the weight of the packing spheres, the friction loss, or the liquid holdup. However, under most operating conditions, the weight of packing spheres and the friction loss are nearly constant and are not associated with the mass transfer in the TCA. But increases in total pressure drop are likely to be due to the liquid holdup which is closely related to the mass transfer behavior of the TCA. Therefore, the increase in SO2 absorption efficiency accompanied by the increase in pressure drop is probably caused by the increase in liquid holdup which in turn causes an increase in the specific gas-liquid contacting area and the residence time of the liquid in the scrubber. A modification of the absorption model of TCA (2) taking into consideration the effect of pressure drop can be written as (see Figure 1):
(2)
where the weight of the packing spheres can be written as: app =
Pp(1
-4 z p PL
(6)
(3)
0013-936X/78/0912-0703$01 .OO/O @ 1978 American Chemical Society
Volume 12, Number 6, June 1978
703
Table 1. Range of Experimental Conditions Covered by EPA/Shawnee and EPA/RTP Data n, Type of scrubber and ref
EPAlShawnee TCA (3, 4, 6-8, 17-20) EPAIRTP TCA ( 1 1- 13) EPAIShawnee TCA (6,8,22, 23) EPAIRTP TCA (21, 22)
no. of stages
3
Zp (cm) static bed height
0 Scrubbing rnedlurn
Limestone slurry
3
12.7 to 20.0 5.0 to 12.7 12.7
3
12.7
Lime
3
Limestone slurry Lime
(%)
L
(;is)
0.0099 to 0.0147 0.0106 to 0.0141 0.0099 to 0.0147 0.0135
Gas Out
1.37 to 2.744 1.94 to 3.553 1.37 to 2.744 3.15 to 3.8
PHout
nAPLN
Magneslurn concn ( ppm )
4.5 to 6.3 4.2 to 5.3
1.o to 2.3 3.0 to 24.0 1.o to 2.0 1.o to 12.5
0.0 to 15000.0 250.0 to 4000.0 300.0 to 3000.0 Less than 1000.0
APL
PSOP
(pprn)
1600.0 to 4000.0 2500.0
PHin
5.0 to 6.0 5.4
to
to
3000.0 2200.0 to 3600.0 2800.0 to 3500.0
6.2 6.0 to 9.1 6.7 to 9.5
slurry. Figure 3 shows the comparison of the calculated and experimental SOn removal efficiencies. Both the data from the large scale EPA/Shawnee TCA and the E P A B T P bench scale TCA are included in this figure. Most of the data are in agreement with the calculated values within f 5 % . Because of the large change of the pH value across the scrubber, the log mean hydrogen ion concentration a t the inlet and outlet of the TCA is used to characterize the pH of lime slurry (14) in the scrubber.
t Liquid Inlet
= Total Height of
the Transfer Region
Figure 4 shows that this model can also predict the SO:! scrubbing efficiency by lime slurry fairly well.
Liquid out Figure 1. Schematic of one-stage TCA
In Equation 6, it is assumed that the major change due to the increase in the pressure drop is in the liquid holdup of the packed section. The change in liquid holdup appears as an independent term, but actually it affects the absorption since the holdup changes the specific contacting area, a , and the residence time, Zp. Using Equation 4,the normal liquid holdup can be estimated which can be incorporated to obtain the actual liquid holdup as: APL = APT - APTN
-+ nAPLN
Flooding in a TCA Both the TCA's in the EPA/Shawnee plant (5) and in the EPA/RTP (13)have experienced flooding a t high liquid and gas velocities. The characteristics of the flooding phenomena in a TCA can be recognized by the congregation of the packing spheres under the upper supporting grid and the rapid increase of the total pressure drop with the increasing gas and liquid velocities. Uchida et al. (1 ) proposed a model to predict the flooding point. This model suggests that the flooding occurs when the
- '
[
(7)
By rearranging Equation 6, the following relation is obtained:
0
51000 m = 0.3
This relation is shown in Figure 2. The index, m , evaluated from this diagram is 0.3.
Simulation of SO2 Scrubbing with Lime and Limestone Slurries The correlations needed in the TCA model are summarized in Table 11. These correlations may be used to simulate the performance of a TCA for SO2 scrubbing with limestone 704
Environmental Science & Technology
, , o ! = c z z : l _ _ _ _ 10
5.0 A
10.0
4
P,/ nap,,
Figure 2. Liquid holdup effect on packed section of TCA ( 7 7- 13, 23)
This characteristic also makes the total pressure drop increase sharply with the increasing gas and liquid velocities.
Table It. Summary of Equations Necessary for Simulating Performance of EPA/Shawnee TCA and EPA/RTP TCA ( 9 )
Spray section
k 2 = 0.00134
Packed section
@.8L0.4
kia = 0.0079 @.9L1.4 Rp =
A 6 B
B = exp(330 P20,) A,-' = e-1.35p H + 7.82
B = exp(330
P202)
A;' = -0.0625 pH2
+ 1.7
- 0.15
+ 0.1625 pH
6, = ~ g ~ . ~ ~ ~ / 1 0 2 1 . 6, 1 ,= exp[O.O45(pH- 4)5.27 6, I1 X (Mg/1000 - 7.2)], 6, 2 1 Z, = ZT - nZ, APL = APT - APTN napLN
In Equation 11the TCA is divided into a spray and a packed section. It is assumed that the spray section is not affected whether the TCA is operated below or above the flooding point. But for the packed section, the mass transfer coefficient obtained from a fixed bed is used instead. The mass transfer coefficient of a fixed bed is given by Epstein ( 1 6 ) as:
+
APTNgiven by Equation 1 APLN given by Equation 4
gas velocity reaches the terminal falling velocity of the wetted packing spheres. At this point all the spheres are lifted up by the gas flow. However, the upper supporting grid impedes the upward movement of the spheres. As a result, all the spheres congregate beneath the upper grid. The gas and liquid velocities at the flooding point can be estimated by this model. The total pressure drop for TCA operation above the flooding point is also assumed to be composed of the weight of the packing spheres, the pressure drop due to friction loss and the liquid holdup. The former two can be computed from Equations 3 and 5. The liquid holdup is given by (I):
x
ilpLFN= 4.0
10-17
( / . ~ , 5 ) ~ , ~ ( p , )18(2p)3,52( O. Uc)2.54( U
L ) ~ , ~ ~ (10) 84 (f)'.42( d d, /D)@ The equation shows that the liquid holdup above the flooding point is a strong function of gas and liquid velocities. X
I
I
I
I
I
/'
TCA Above Flooding Point The difference between TCA operation below and above the flooding point is due to the displacement of the packing spheres. Under normal operating condition of a TCA, i.e., below flooding point, the packed bed expands to a certain height and acts like a fluidized bed. However, for the TCA operating above the flooding point, the packing spheres are pushed up against the upper supporting grid, and the system acts like a fixed bed. The following model, modified from Equation 6, is used to simulate the TCA operation above the flooding point:
kg". = 0.014 G0.65L0.2
(12)
This correlation is based on experiments of SO2 absorption by sodium carbonate solution in a marble bed (fixed bed). In most of the runs, the pH value of the sodium carbonate solution is about 9.5. Therefore, the absorption rate of SO2 can be assumed to be totally gas film controlled (2). The gas-liquid mass transfer resistance ratio, RF, can be considered to have the same form as in Table I. The effect of SO2 partial pressure can also be estimated by Table I. T o model the TCA above the flooding point, there are three other parameters to be decided in Equation 11.They are pH factor, magnesium factor, and liquid holdup effect. The magnesium factor is correlated such that it becomes unity when the data of those runs with low magnesium concentration (less than 500 ppm) are chosen in the correlation. T o isolate liquid holdup effect, the following operating conditions are used as a reference state:
/I
c
90
40
50
60
70 80 90 So, Removal "/o EXp.
loo
Flgure 3. Comparison of calculated and observed SO2 removal efficiencies for TCA scrubbers with high-pressure drop using limestone slurry as scrubbing medium (see Table I) (5-8,7 7- 73,23)
40
I
I
50
60
I
I
70 80 SO2 Romova I %Exp.
I
90
100
Figure 4. Comparison of calculated and observed SO2 removal efficiencies for TCA scrubbers using lime as scrubbing medium (see Table I) (27-23)
Volume 12, Number 6, June 1978 705
;I I
2.51
90
A i = 2 3 3 ( - 0.0625pH2.0.1625pHt1.7)
I
C
+
1
5.0
1
5.2
5.4
I
I
5.6
5.8
I
40
6.0
40
'/
50
5% Error /
I
60
PH
Figure 5. Comparison of pH factor in packed section of TCA below flooding point, A, and above flooding point, AF (6, 8, 75, 77, 78)
I
I
I
80 90 SO, RQmOVaI"10 EXp.
70
100
Figure 7. Comparison of predicted and observed S O p removal efficiencies for flooded TCA using limestone slurry as scrubbing medium
(5-8,78-20)
G
= 0.0147 g-mol/cm2-s
L
(13)
= 2.75 g/cm2 s
(14)
2 p =~ 12.7 cm
(15) Once the effect of liquid holdup is fixed, the TCA performance data a t the normal pressure drop under the reference operating condition are selected for the estimation of p H factor from Equation 11.Figure 5 shows the pH factor in the packed section of a flooded TCA. Although the value of the pH factor is different from the corresponding p H factor of a TCA operating below flooding point, the pH factor for a TCA operating below the flooding point can be applied to that operating above the flooding point by multiplying a correction factor. Furthermore, the magnesium factor correlated for the TCA below the flooding point can be directly applied to the flooding model of Equation 11. I
r
I
I
I
1
I l l l l
1 EPAiRTP TCAwlth Llme A
i
EPAiShawnee TCA with LImQStOlQ
Slurry
1
d? 0
i
5.0 1
1.0
5.0
10.0
20.0
- ARinZp A
PLmR/nZm
Figure 6. Effect of liquid holdup on performance of TCA's above flooding point (6, 8, 79-22) 706
Environmental Science & Technology
The liquid holdup in a TCA operating above the flooding point increases very rapidly with an increase in the gas and liquid velocities (see Equation 10). Therefore, the effect of the liquid holdup on mass transfer rate should be properly considered to extend the TCA model to flooding conditions. The liquid holdup correction term of Equation 6 can be considered as the effect of the liquid holdup change per unit height of the packed section. Thus, by including the liquid holdup effect for a TCA operating above the flooding point, the scrubber model can be expressed as follows:
+ kg'. (&) 1 + RF
n2,
(M L F N R ~ ~(16) Z~~ gL'nzp
)m'
where ULFNR represents the pressure drop due to the normal liquid holdup of a flooded TCA a t the reference operating condition and can be calculated by Equation 10. The value of m' can be evaluated using the same technique as in the analysis of the pressure drop effect. The correlation is shown in Figure 6. The value of m' is estimated to be approximately 0.66. A comparison of the calculated and observed SO2 removal efficiency is shown in Figure 7 . Applying a correction on the slurry pH based on Equation 9, this model can also be used to simulate the scrubbing of SO2 with lime slurries. Figure 8 shows the comparison of the calculated and experimental SO2 scrubbing efficiencies. The calculated values deviate from experimental values in most of the cases within f 5 % for both lime and limestone slurry.
Conclusions In this study, the TCA performance data under highpressure drop operation are examined, and the results of hydrodynamic analysis are used to relate the pressure drop and the scrubbing efficiency. Changes in pressure drops can be attributed t o changes in liquid holdup of the scrubber. Both the specific gas-liquid contacting area and the liquid residence time are affected by the change of the liquid holdup. Correlations based on the analysis of the TCA pressure drop are required to calculate the liquid holdup. Flooding conditions are seldom discussed in the literature.
100
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’
UL = superficial liquid velocity, cm/s
/
/
ZT = total height of the transfer region, cm 2, = total height of the spray section, cm 2, = static height of the packed section, cm Z ~ R = reference height of the packed section in the flooded TCA, cm
80
Greek Letters = voidage of static bed p p = density of packing spheres, g/cm3 p~ = density of slurry, g/cm3 p~ = viscosity of the slurry (usually at 50 “C in limehimestone SO2 scrubbers), g/cm s APB = pressure drop across one stage, cm H20 APF = pressure drop due to friction, cm H2O APL = pressure drop due to liquid holdup, cm H2O APT = total pressure drop, cm HzO APTN = total normal pressure drop, cm HzO APLN = pressure drop due to normal liquid holdup, cm H2O APLFN = pressure drop due to normal liquid holdup in flooded TCA, cm H20 APLFNR= pressure drop due to reference liquid holdup in flooded TCA, cm H20 LIP, = pressure drop due to the weight of packing, cm H2O 6, = magnesium factor in spray section 6, = magnesium factor in packed section t
1
40
50
60
70 SO,
80
90
loo
Removal “io Exp.
Flgure 8. Comparison of predicted and observed SO2 removal efficiencies for flooded TCA using lime as scrubbing medium (27, 22)
Due to the sudden change of the internal conditions of a TCA, the flooding point is usually avoided in the normal operation of a TCA. A fixed bed model is utilized to simulate the performance of the packed section present in a flooded TCA. The effect of liquid holdup was considered in the TCA model which is then extended to include the flooded TCA. This model may be used to compare the performance of the TCA below and above flooding point.
Nomenclature a = specific interfacial area available to mass transfer, cm-l A, = pH factor for spray section A, = pH factor for packed section A f = pH factor for packed section of a flooded TCA B = SO2 partial pressure factor d = equivalent diameter of the hole on the grid, cm D = diameter of the column, cm d, = diameter of packing, cm f = free opening fraction of supporting grid G = molar gas flow rate based on cross-sectional area of the scrubber (based on inlet condition), gmol/cm2 s k$ = gas side mass transfer coefficient for the spray section, g mol/cm3 atm s k f a = gas side mass transfer coefficient for the packed section, g mol/cm3 atm s k:a = gas side mass transfer coefficient for the packed section of a flooded TCA, g mol/cm3 atm s L = liquid flow rate based on cross-sectional area of the scrubber, g/cm2 s Mg = magnesium concentration, ppm n = number of stages in a TCA PT = total pressure, atm PFo2 = inlet partial pressure of SO2 in the bulk gas phase, atm P& = outlet partial pressure of SO2 in the bulk gas phase, atm R = ratio of the gas to liquid film mass transfer resistance R,, R, = value of R in the spray and packed section, respectively RF = value of R in the packed section of a flooded TCA UC = superficial gas velocity, cm/s
Literature Cited ( 1 ) Uchida, S., Chang, C. S., Wen, C. Y., Can. J . Chem. Eng., 55,392 (Aug. 1977). (2) McMichael, W. J., Fan, L. S., Wen, C. Y., Ind. Eng. Chem. Process Des. Dev., 15,459 (Jul. 1976). (3) Kito, M., Monma, T., Kayama, Y., Sugiyama, S., Kagaku Kogaku Rombunshu, in press. (4) Epstein, M., “EPA Alkali Scrubbing Test Facility: Advanced Program”, progress rep. prepared by Bechtel for EPA, Sept. 1976. (5) Epstein, M., ibid., Sept. 1975. (6) Epstein, M., ibid., progress rep. for Feb. 1-28, 1976 (Mar. 26, 1976). (7) Epstein, M., ibid., progress rep. for March 1-31, 1976 (Apr. 30, 1976). (8) Epstein, M., ibid., progress rep. for April 1-30, 1976 (May 28, 1976). (9) Chang, C. S., thesis, West Virginia University, Morgantown, W.Va., 1977. (10) Borgwardt, R., “Limestone Scrubbing of SO2 a t EPA Pilot Plant”, Rep. No. 1,Aug. 1972. (11) Borgwardt, R., ibid., Rep. No. 14 (Jan. 1974). (12) Borgwardt, R., ibid., Rep. No. 16 (June 1974). (13) Borgwardt, R., ibid., Rep. No. 21 (June 1975). (14) Fan, L. S., dissertation, West Virginia University, Morgantown, W.Va., 1975. (15) Epstein, M., “EPA Alkali Scrubbing Test Facility: Summary of Testing Through October 1974”, June 1975. (16) Epstein, M., “EPA Alkali Scrubbing Test Facility: Sodium Carbonate and Limestone Test Results”, Aug. 1973. (17) Epstein, M., ibid., progress rep. for Nov. 1-Dec. 31,1975 (Jan. 23, 1976). (18) Epstein, M., ibid., progress rep. for January 1-31,1976 (Feb. 27, 1976). (19) Epstein, M., ibid., progress rep. for May 1-31, 1976 (June 28, 1976). (20) Epstein, M., ibid., progress rep. for June 1-30, 1976 (Jul. 28, 1976). (21) Epstein, M., ibid., progress rep. for Aug. 1-31, 1976 (Oct. 6, 1976). (22) Epstein, M., ibid., progress rep. for July 1-31, 1976 (Aug. 28, 1976). (23) Epstein, M., et al., private communication, Bechtel Corp., San Francisco, Calif., Factorial Run Summary, Jul. 28, 1976.
Received for review February 22, 1977. Accepted J a n u a r y 3, 1978. Work supported b y t h e Environmental Protection Agency u n d e r grant number R800781-03-3.
Volume 12, Number 6, June 1978 707
