Simultaneous Removal of Fly Ash and SO, from Gas Streams by a Shaft-Filter-Sorber Raymond L. Zahradnik, Josephat Anyigbo, Richard A. Steinberg, and Herbert L. Toor Chemical Engineering Dept., Carnegie-Mellon University, Pittsburgh, Pa.
A shaft-filter-sorber has been shown to be an effective device for the simultaneous removal of SO2and fly ash from gas streams. A shaft-filter-sorber consists of a bed of slowly falling sorbent pebbles through which a gas stream to be cleaned passes horizontally. A number of tests that use alkalized alumina as sorbent have shown that complete SO2 removal and YY+ filtration efficiency are possible at gas mass velocities of 150 lb./hr. fL2 and dust loadings in excess of 3 grains per s.c.f. The SO2removal is predictable on the basis of static bed experiments, and a design curve based on this prediction is presented as a means of projecting removal efficiencies at higher gas velocities. H
7
3 i I i
T
he simultaneous removal of both dust and gaseous pollutants (SO2,SO,) from flue gas may be accomplished in many ways. Conceptually, such dual-purpose schemes could combine the steps of filtration with chemical reaction or sorption of the gaseous contaminants by hot solids. One specific way in which this combination may be effected is by use of a shaft-filter-sorber. Basically, a shaft filter consists of a bed of slowly falling pebbles through which the gas stream to be filtered passes horizontally. Sulfur dioxide is removed by the pebble bed which consists of a material that picks up the SO2, and any one of many solid sorbents can be used. This paper presents the results of experiments using alkalized alumina as the sorbent material. Previous experiments have shown that the presence of fly ash in the inlet gas stream does not reduce the rate of SO2 removal by alkalized alumina, nor does it have any other apparent harmful effect on the sorbent (Zahradnik, Toor, et a/., 1968). Experiments reported in this paper show that previously established rate models can be modified to describe the performance of a shaft-filter-sorber. Also, a number of experiments are reported which illustrate that high filtration efficiency and SO, removal capacity may be obtained in such a unit. Sliuft-Filter-Sorber Equipment
A Hoffman blower was used to move an air stream through a gate valve into the system. Two chromalox heaters (20 kW, 9 kW) were used to heat the gas stream to sorption temperature. After the air flowed through an orifice meter, SO2 was added from an SOzcylinder. A precalibrated screw feeder added fly ash from a storage bin. The shaft filter used is a square cross-section shaft, total length 8 ft.; the top 7 ft. serve as the storage area, the remaining 1 ft. is the actual filtering section. The cross-sec-
i
-1
1
I YSXN
W
I
I
D I S C H A R G E BIN
m
Figure 1. Shaft filter, front view
tional area is 1 ft.2 The gas flows cross currently and the inlet and outlet sections are louvered to obtain a uniform distribution of flow. The shaft walls are carbon steel plates, lia-in. thick. A detailed drawing of the filter is illustrated in Figure 1. The shaft was charged with filtering material at the beginning of a run, and the pebbles were removed through the lower part of the shaft by a precalibrated natural frequency vibrating feeder. This type of feeder provided a uniform solids flow rate independent of the static head, a very desirable characteristic. The SO2concentration in the inlet and outlet gas was determined by standard iodometric titration. A vacuum pump was used to provide uniform pressure in the sampling lines. The pressure drop across the filter was measured by a U-tube manometer with mercury as manometric fluid. SO2 Sorption b y u Mocing Bed
The sorbent material used in the experiments was alkalized alumina, provided in various forms by the U S . Bureau of Mines. To verify that this material behaves similarly to that observed by the Bureau and to establish specific reaction-rate parameters, the following experiment was performed. SevVolume 4, Number 8, August 1970 663
I
I
0
2
I
3
5
4
6
9 1 0
8
7
T I M E (HRS)
Figure 2. SOz removal cs. time for fixed and moving bed Gas Bow rate, 21.6 s.c.f.m.; inlet Sol, 1.33z; solids rate, 53 Ib./hr.
-
era1 hundred pounds of alkalized alumina were charged into the shaft-filter-sorber. The unit was then operated with zero flow of solids ( i , e , , as a horizontal-flow packed bed, to determine the sorption rate and capacity of the material). Under the following conditions: temperature, 400' F. ; inlet gas flow, 21.6 s.c.f.m.; and inlet SO2 mole fraction, 0.0133. The percentage of SOz removal decreased to zero in about 6 hr. Figure 2 shows the time behavior of the SO2 removal capacity during this period. Six hours after the start of the experiment, when the SO2 sorption rate had decreased essentially to zero, the natural frequency vibrating feeder was turned on and a solids flow rate of 53 lb./hr. was established. Under these conditions, a steady-state operation was obtained in less than 2 hr., with a 99% SOz removal efficiency. The time behavior during this period is indicated by the experimental points plotted in Figure 2. Figure 3 shows similar results for a run with a solids flow rate of 39.6 Ib.lhr. The time behavior during a fixed-bed period has been reported (Baasel and Stevens, 1961; Lempel in Bienstock, Field, eb af., 1967). Let the coordinate x represent distance into the bed and 8 the time from the start of the experiment. If we assume that the rate of sorption may be represented as proportional to the product of the concentration by weight of SO2 in the gas with the unreacted fraction of solid, the gas phase material balance becomes :
-PREDICTED
"e a w
"--FIXED
1
2
3
4
5
6
7
TIME
8
9
1
0
1
1
1
The sorbent-phase material balance is expressed :
Z
lHRS1
Figure 3. SOz removed OS. time for fixed and moving bed runs Gas flow, 21.6 s.c.f.m.;inlet SO%,1.45x;solids rate, 39.6 Ib./hr.
Equations 1 and 2 are based on chemical reaction as ratelimiting over the full course of solid conversion. The boundary conditions are: y(o,e) =
P(X,O) =
o
(3)
where y o is the inlet gas concentration. Equations 1 , 2 , and 3 are in a form for which a solution has been presented (Baasel and Stevens, 1961). The solution is: I
I
where
I
/
3
I
TIME
IHRSJ
Figure 4. Semilog plot of fixed bed data 664 Environmental Science & Technology
To verify that Equation 4 does give a valid representation of the fixed-bed run, it may be noted that it predicts that a plot of ln[y/(yo - y ) ] us. e, for a given bed thickness, I, should be linear, with slope A and vertical intercept -ln(eBz - 1). Figure 4 is such a plot and is seen to be linear. From the slope and intercept, the values BI = 5.13 and A = 1.35 hr.-I are obtained. With the given conditions, this represents a K value of 12 hr.-I and a sorption capacity of 26 g. SO%per 100 g. alkalized alumina. The K value is below the value 29 hr.-I reported by the Bureau of Mines for their fixed-bed work at 626' F. This discrepancy may be due to the condition of the material used. However, it is representative of the data, as seen by Figure 4, and may be used to predict the moving bed behavior. During a steady period of moving bed operation, it has been
shown that Equation 2 must be replaced by the following (Zahradnik, Toor, et al., 1968, Bueno, 1968): (5) where the coordinate z is the height of the bed, measured downward from the top of the cross-flow section. If a new variable, t , is defined t = zps/Gs,then Equation 5 transforms to Equation 2 with t replacing 8. Again, Equation 4 gives the SO2concentration at any t and x. The concentration of interest, however, is the average outlet concentration, J , which may be obtained by definition 1
f
‘
v =
-
‘
T J O
v(1.t) dt
hPs
T = -
Gs
This average assumes uniform gas flow through the bed. If the substitution of Equation 4 into Equation 6 and the indicated integration are carried out, the average outlet concentration becomes : (7) Equation 7 may be used to predict the SO2removal efficiency under steady-state operating conditions. During a transient period, however, such as that encountered between the sixth and eighth hours of the experimental run shown in Figure 2 , neither Equation 4 nor 7 represents the outlet composition. Since during this time, sorbent loading is changing both as a function of time, 8, and position, f , the material balance equation must be written as follows:
Fortunately, a change of variables can convert this equation into a form similar to that already encountered. Define a new variable, 7, by the equation q = 8 - t (7 is merely a translation of the time axis by an amount t ) . This reduces Equation 8 to: (9) Suppose now the bed is started from a p = 0 value (this is boundary condition 4 and corresponds to the experimental condition at the sixth hour). Equation 9 is valid only for 7 2 0-Le., for 0 2 t. This simply states that Equation 9 applies only for experimental times longer than the transit time it takes for the new sorbent to make its way to a point of interest in the bed. For shorter times, no sorption can occur. Thus Equations 1 and 9 still have as their solution Equation 4 with 8 replaced by t. However, y(8), the average outlet concentration, is now given by the equation:
y
Yo =
I 7 [Ley(t,l)df + 1
gives a good fit to the data. The failure of the data to follow the “corner” at 7.1 hr. is due to the idealized model used for the solids flow distribution and illustrates nature’s reluctance to conform to “step change” assumptions. Similar behavior is seen in Figure 3 for the run at reduced solids flow rate. Nonetheless, the overall fit is good and indicates that the behavior of moving-bed sorbers can be predicted from staticbed models. Their combined performance as sorbers and filters is discussed in the next section. Simultaneous Fly Ash and SO2 Remooal
Experiments have been carried out in which both SO2 and fly ash were added to the inlet stream to the shaft-filter-sorber. The fly ash was screened to - 325 mesh and was added by the described screw feeder assembly. Fly ash loadings about 2 to 3 grains per standard ft.$ were achieved with this unit. Table I summarizes the best results obtained to date-best in the sense that the experiments ran smoothly and uniformly for the durations indicated. Table I1 presents the size distribution of the fly ash used as determined by sedimentation pipet. The per cent of SOz removal was determined in the usual way with use of iodometric titration. The filtration efficiency reported is an overall figure, obtained by passing the entire outlet stream through a 1-11 Cuno filter. The Cuno filter cartridge was weighed before and after the experiments and the weight gain was assumed to represent the amount of soLid fly ash not collected by the shaft-filter-sorber. The pressure drop across the filter measured in both runs was 2.0 in HzO, and it remained constant throughout the experiment. A pressure drop of 1.5 in H 2 0 was predicted
Table I. Summary of Simultaneous Removal Runs“ Expl. parameter
Run 6
Run 7
Gas flow rate, s.c.f.m. Nominal space velocity, hr.-l Sorbent rate, lb./hr. Temperature, O F. Inlet SO2mole fraction Outlet sorbent loading, g./100 g. Fly ash loading grains per ft. Duration of experiment, hr. Filtration efficiency SOz removal, Pressure drop in H20 Calculated Observed
31 .O 3500.0 10.5 400.0 0,006 19.0 2.82 3.5 99.3 100.0
31.2 3500.0 19.6 400.0 0.006 10.0 3.1 4.0 98.7 100.0
a
1.5 2.0
1.51 2.0
Sorbent was 1/win. alkalized alumina beads,
Table 11. Size Distribution of Fly Ash Used Particle radius, 1 Less than
1 dt]
which has as its solution:
(10) A plot of Equation 8 for the experiment is included in Figure 2 using the value 1.1 hr . for T , corresponding to a solids flow rate of 53 lb./hr. For a major portion of the transient period and for the ultimate steady-state removal, Equation 10
52.4 41.6 35.8 30.4 27.0 19.8 16.0 13.7
100.0 75.5 50.0 40.0 23.6 17.0 13.9 11.0 Volume 4, Number 8, August 1970 665
SO?removal. In terms of R,Equation 7 is rewritten: Y - 1 [In (eBtK BlR
YO
by use of the Ergun equation to estimate the bed friction factor. Thus, the presence of the fly ash, under the stated conditions, has only a slight effect on the overall pressure drop across the unit. With the rate and capacitance parameters evaluated earlier, Equation 7 predicted a 99+% SO2 removal efficiency, in agreement with the observed results. The high filtration efficiency indicates that sorbent-pebble filters can be used to remove fly ash while simultaneously removing gaseous SOn. Discussion Results described in this paper have shown that the shaftfilter-sorber can be successfully implemented to remove SOz and fly ash from gas streams. Moreover, the behavior of such units may be predicted if suitable sorption rate models are available. The predictive Equation 7 , derived for the rate equation given as the r.h.s. of Equation 1, describes adequately the SO, removal in all the runs made thus far on the experimental unit. This rate expression has been doubted, and other reaction models postulating alternate controlling steps have been proposed (Equation 5). However, these expressions offer no immediate advantage over the one used in this paper, although if other rate expressions are found to describe the sorption process better, the techniques developed here may be used to derive a relationship similar to Equation 7 for the improved rate expression (Schoenberg, private communication). The results reported in Table I show that complete gas cleaning is technically possible with use of a shaft-filter-sorber. However, if this unit is used commercially, higher gas flow rates and lower sorbent flow rates are required. To give some idea of the relationship between these quantities, a quantity R is defined as:
R = -AT Bl
R represents the inlet flow of SO2 divided by the inlet flow of SO, sorption capacity:
R
inlet flow of SO2 = v inlet flow of SOz sorption capacity
Thus 1/R - 1 represents the excess amount of sorbent above the stoichiometric amount required to achieve a given per cent 666 Environmental Science & Technology
+ eB1 - 1) - BI]
Figure 5 presents a plot of this equation in which the reciprocal R value required to achieve 90% SO, removal is plotted us. the gas mass velocity, for three bed thicknesses. From an overall design viewpoint, operation will probably be carried out on that portion of the curves beyond which small increases in gas mass velocity require large increases in R-l. Finally, it should be noted that Equation 11 provides a means for determining the effect of individual design variables on shaft-filter-sorber performance. For example, the effect of bed height, h, on performance could be predicted from Equation 11 in terms of its relationship to R. The experimental points represented in Table I correspond to gas mass velocities of 150 lb./hr. ft., and reciprocal R values of 1.38 and 3.44. From Figure 5 it is seen that these sorbent rates are in excess of those required to achieve 90% SOz removal. In fact, the sorbent rate could be lowered to 7 lb./hr. and the unit should still achieve 90% SO, removal. Conversely, at the higher sorbent feed rate, the unit could accept a gas mass velocity of 600 lb./hr.ft.z and still maintain 90% SO, removal. These figures better represent the commercial feasibility of such a unit and provide targets for future experiments. At higher flow rates, the pressure drop would, of course, be higher than the 2 in. of water observed in the present experiments. However, this effect can be computed from the standard correlations for packed beds (Ergun, 1952), and this factor would have to be considered in determining optimum operating conditions. Another factor to be considered in the operation of units such as a shaft-filter-sorber is the attrition loss of the solid sorbent material. No information was generated on attrition loss in the experiments performed; it is reasonable to conjecture that such loss would be less than that encountered in units where solids contact is more violent, e . g . , entrained or fluidized beds. Acknowledgment This work was sponsored by the US. Bureau of Mines Grant Coal-3. The Bureau also provided the several hundred pounds of alkalized alumina and fly ash used in the experiments as well as considerable advice and consultation, all of which are gratefully acknowledged. Nomenclature A
=
B
= =
G,,G,
h K I R
= = = =
t x y
=
Z
=
17
=
= =
0
=
p
=
ps
=
7
=
parameter grouping, Equation 5 parameter grouping, Equation 5 mass velocities of gas and sorbent, lb./hr. ft.? height of active portion of sorbent bed, ft. reaction rate constant, hr.-l bed thickness, ft. ratio of SOpinlet rate to SOZremoval capacity alternate coordinate, hr. distance coordinate into bed, ft. SO, concentration in gas, by weight distance coordinate relating to bed height, ft. translated time scale time, hr. SO2 concentration on absorbent, by weight solids bulk density, 58 1b.ift.s value o f t at z = h
Literature Cited Baasel, W. D., W. F. Stevens, “Kinetics of a typical gas-solid reaction.” Znd. Eng. Chem. 53, 485-88 (1961). Bienstock, D., J. H. Field, J. G. Myers, “Process Development in Removing Sulfur Dioxide from Hot Flue Gases: 1. Bench Scale Experimentation.” U S . Bureau of Mines RI 5735 (1961). Bienstock, D., J. H. Field, J. G. Myers, “Process Development in Removing Sulfur Dioxide from Hot Flue Gases: 3. Pilot Plant Study of the Alkalized Alumina System for SO?Removal.” U S . Bureau of Mines RI 7021 (1967).
Bueno, Carlos, “Construction of a Shaft-Filter-Sorber for the Study of Simultaneous SO2 and Dust Removal from Gas Streams.” Masters Thesis, Chemical Engineering Dept., Carnegie-Mellon University (April, 1968). Ergun, S., “Fluid flow through packed columns.” Chem. Eng. Progr. 48, 89 (1952). Schoenberg, T. Avco Corp. Private communication. Zahradnik, R. L., H. L. Toor, Carlos Bueno, “Effect of Dust Level on SO2 Sorption in a Shaft-Filter-Sorber.” Paper 68-184, APCA National Meeting, St. Paul, Minnesota (1968). Receivedfor review June 6, 1969. Accepted March 17, 1970.
Isoprene: Identified as a Forest-Type Emission to the Atmosphere Reinhold A. Rasmussen Air Pollution Section, College of Engineering Research Div., Washington State Univ., Pullman, Wash. 99163.
The hemiterpene isoprene (2-methyl-l,3-butadiene)until recently has not been considered to be present in natural plant products. Direct gas chromatographic, infrared, and mass spectrometric analyses of the air in contact with the plant foliage for five common tree species confirm previous observations (Rasmussen, 1964) of the existence of isoprene as a plant product. Preliminary data from surveying the dominant forest tree species of North America indicate that the frequency of occurrence of isoprene is similar to that of apinene although species differ. Isoprene has been resolved directly from the out-of-doors air over mango foliages in Panama at concentrations of 0.5 to 24 p.p.b. in 5 ml. of air. The biological and air chemistry fates of isoprene are discussed.
T
here exists an extensive literature describing the composition of essential oils and leaf oils for numerous plant species (Guenther, 1949; Pridham, 1967; Simonsen, 1953). In none of this literature is isoprene (2-methyl-1,3butadiene) described as a major component of plant products. Many investigators have concluded from both their own studies and literature surveys (Fisher, 1954; Sandermann, 1962; Weissmann, 1966) that isoprene itself is apparently not present in natural products. Bonner (1950) in his classic textbook of plant biochemistry states that isoprene is not a natural plant product. Recently, the occurrence of isoprene in plant material has been reported by several investigators. Rhoades (1960) and Radtke, Springer, et a/. (1963) identified isoprene as a volatile component of roasted coffee. Gil-Av and Shabatai (1963) reported isoprene to constitute 50-80 of the total unsaturated gaseous hydrocarbon in tobacco smoke. However, isoprene was not isolated from the tobacco leaf itself. The volatile hop oil constituents have been extensively studied by Buttery, McFadden, et al. (1963,1964) and isoprene was identified by capillary gas chromatography retention times, infrared structure, and mass spectra. The relative percentages of isoprene in five European hop steam distillates ranged between 0.06-0.1 %. It has not been definitely estab-
z
lished that isoprene is present in the hops on the vine. However, Hartley and Fawcett (1967) showed hops contain reasonably large quantities of 2-methylbut-3-en-2-01which could readily dehydrate to isoprene in the drying of the hops, in the steam distillation of the oil, or in the gas chromatography apparatus. Rasmussen (1964) and Rasmussen and Went (1965) observed a hemiterpene tentatively identified as isoprene emanating from oak leaves, aspen, and sweetgum foliages during in situ studies using a gas chromatograph in remote natural areas in the United States. The identification was based on relative retention time data by use of dissimilar substrates. Previously Sanadze and Dolidze (1961) in Russia had found peaks in the molecular weight regions corresponding to isoprene, butane, and propane in mass spectrometric analyses of the freeze out condensates of the air in contact with the foliages of Amorpha fructicosa (L.), the false indigo; Buxus (sp.), boxwood; and Quercus iberica (Bieb.), the iberian oak. This current paper presents evidence that it is indeed isoprene which is involved in these leaf emissions. A thorough proof of structure is presented based on direct gas chromatographic, infrared, and mass spectrometric analyses. Also this paper presents evidence that the emission of isoprene occurs under natural conditions from the intact, living foliage of numerous plant species. Experimental Apparatus. The gas chromatographic equipment used in the study was a Hewlett-Packard Model 5751-B, a Perkin Elmer Model F11, and a Beckman GC-5 with flame ionization detectors. All columns were constructed of 0.125-in.-o.d., 0.105-in.-i.d., 304 stainless steel tubing. For readout a 1 mV potentiometric recorder was used with chart speed of 30 in. per hour. Column A was 6 ft. in length, packed with 4 z Carbowax 20M on HMDS-treated Chromosorb W, acid wash, 60- to 80mesh. Column temperature was maintained at 60’ C., injector temperature was 170” C., detector temperature was 250” C. Helium flow rate was 50 d.per min. Sample sizes vaned between l and 5 d. All samples were prepressurized to inlet pressure before injection using Pressure-Lok gas syringes (Precision Sampling Corp.). Volume 4, Number 8, August 1970 667
