Sorption and Diffusion Interactions with Fly Ash of SO2 in Air, SO3 in Air, H20 in Air, S02+H20 in Air, SO3+H2O in Air Pave1 Ditl’ and Robert W. Coughlin** Department of Chemical Engineering, Lehigh University, Bethlehem, Pa. 18015
Sorption isotherms with fly ash as sorbent are determined with a quartz spring microbalance and a gas-flow apparatus for the following flowing gaseous sorbates: SO2 in dry air, SO3 in dry air, H20 in air, S02+H20 in air, SO,+H,O in air. Diffusion coefficients for mass transfer of sorbate molecules into a sample of fly ash particles are deduced from the time behavior of the sorption measurements, and equilibrium sorption isotherms are determined from the steady-state data. These measurements are conducted a t 20, 90 and 150 “C. Static sorption measurements with Ar a t 22 OC are also performed to measure the BET surface area and the porous structure of the ash. The data obtained offer an elucidation of the mechanism of electrical conductivity in layers of fly ash-especially from the standpoint of the influence of the gaseous environment on the electrical resistivity of the ash. This type of behavior is of vital importance in the practice of electrostatiyprecipitation, during which SO3, H2S04, and H20 are frequently added to flue gas to improve the efficiency of removal of fly ash. In particular, the results are discussed from the standpoint of a model (published elsewhere) which accounts for the effect on the electrical resistivity of fly ash of capillary condensation a t the contact points of the particles. H
Theory The equilibrium relationship a t a constant temperature T between amount of sorbate q sorbed per unit mass of sorbent and the sorbate concentration cg of the fluid surrounding the solid sorbent can frequently be represented by one bf the following expressions: Linear isotherm: q = a . cq
(1)
-
(2)
Freundlich isotherm: q = U F c r Langmuir isotherm: q =
Present address, Prague Technical University, CVUT,Suchbatarova 4, Prague 6, Czechoslovakia. Present address, Department of Chemical Engineering, University of Connecticut, Box U-139, Storrs, Conn. 06268.
1
cg
+ bLCg
(3)
where the constants a, U F , m, a ~and , b L are determined experimentally. Evaluation of Diffusion Coefficients from Experiments. Neglecting the small effects of the cylindrical sample holder mass transfer into the fly ash sample can be viewed as transport by diffusion into an infinite slab of effective diffusivity D. According to this model, as theoretically formulated and solved by Crank ( 5 ) ,the rate of mass transfer in a plane sheet having a pore structure independent of position x is governed by the differential equation d2c
dc dt
dq
DPy=P-+ dX
The efficiency of electrostatic precipitation is strongly influenced by the electrical conductivity of the fly ash which in turn depends upon ash composition and particle size as well as upon the composition of the flue gas, especially the content of H20 and S0:j ( I , 2 ) . In fact, it has become customary ( 3 )to add SO:>to the flue gas to “condition” the fly ash (thereby lowering its e1eStrical resistivity). Experiments and correlation of data suggest ( I , 2 , 4 ) that the transport of alkali metal ions contributes significantly to the electrical conductivity of fly ash, but that Fe also influences conductivity in a more complex way. In the presence of moisture and a conditioning agent such as SO:j, the electrical resistivity of fly ash can be lowered by several orders of magnitude as compared to the dry ash. The effect of SO3 appears to be very small without the simultaneous presence of water. Since it appears that the influence of flue gas composition on the electrical properties of the ash may be ascribed to sorption phenomena, the present work was undertaken with the aim of providing more information and better understanding regarding such phenomena. In the investigation reported here, transient gravimetric sorption measurements were carried out in steadily flowing gas streams of known and adjustable composition. Such measurements have permitted estimation of the diffusion coefficient for transport of sorbate into a layer of ash particles, provided steady-state data from which equilibrium isotherms could be constructed, and yielded indirect evidence of the absence of chemical reaction between SO3 and ash. Moreover, static volumetric measurements of sorption have also provided information as to BET surface area of the fly ash as well as evidence of capillary condensation within a sample of ash.
aL.
P
P
t
(4)
with the boundary conditions: c=c,att=O
O
