Ind. Eng. Chem. Res. 1993,32, 1509-1519
1609
Nucleation-AssistedProcess for the Removal of Fine Aerosol Particles Chin-Cheng Chen,’ Han-Kuan Shu, and Yeun-Kwei Yang Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C.
An efficient (up t o 99%) multistage process that removes a wide range of fine particles (several microns down to 0.1 pm) assisted by a nucleation method is proposed. In each stage, as a gas stream passes through a supersaturated vapor in a mixing chamber, water vapor condenses onto the particles which grow to bigger sizes in a subsequent cooling chamber and is removed in a cyclone after being accelerated through a nozzle. The desired efficiency can be assured by employing several stages in series. A single-stage bench-scale model is designed, and four types of test aerosol are used to demonstrate the cleaning efficiency. The efficiency increases with increasing ratio of steam to gas stream, wettability of particles, particle size, and degree of cooling in the cooling chamber, and decreases as particle concentration increases. Suggestions on how to achieve desired efficiencies a t less cost are also given.
Introduction The removal of dust (diameter, D > 1 pm) and fume (D < 1 pm) expelled in flue gas is of great interest in air pollution control. The latter submicron particles are particularly important because they have a long airborne lifetime and are largely responsible for reduced visibility in urban areas. Furthermore, they pose potential health hazards since they are too small to be trapped by the filtration mechanisms in a human being’s respiratory system (i.e., nasal hair, nasal mucus) and partially deposited in the lung (Guyton, 1981). While any number of mechanical processes (Meas et al., 1979) can effectively remove relatively large particles (D > 10 pm), they invariably exhibit low collection efficiencies in the range of 0.1-1 pm. Various solutions in the form of condensation-assisted wet scrubbers have been investigated and discussed in depth by authors such as Litvinov (1967),Lancaster and Strauss (1971),Davis and Truitt (1972), Calvert and Jhaveri (1974), Rich and Pantazelos (1974), and Spark et al. (1974). The high efficiency of submicron particle collection reported was attributed to two mechanisms: (1) condensation of vapor on the particles, causing their buildup and improving subsequent impaction aggregation, and (2) deposition of particles on collecting surfaces by flux forces (thermophoresis and diffusiophoresis). Recently, we have reported a process that requires simple equipment and little mechanical power in order to achieve an even higher collection efficiency for a wider range of particle size (several microns down to 7 nm) (Chen and Wu, 1992). Nucleation is the key step in the removal process. The reported model employs a modified version of Katz’s flow cloud chamber (FCC) to create a continuous supply of supersaturated vapor required for efficient nuclei growth, facilitating precipitation from the gas stream. Inside the FCC the degree of supersaturation can easily be controlled to render a steady-state profile of supersaturation. As aerosol particles pass through the FCC, they trigger the nucleation process and are dragged down due to gravity. Particles with a traveling distance of less than FCC length are removed from the stream. However, inside FCC the gas flow must be laminar in order not to destroy the supersaturation profile and disturb the settlement of particles, thus limiting the operation capacity and requiring equipment of a relatively large crosssectional area in the commercial operation.
* To whom correspondence should be addressed.
This paper outlines a modified multistage process that removes the above limitations and also achieves a high collection efficiency (up to 99 % ) for a wide range of particle sizes (several microns down to 0.1 pm). The underlying principle of operation in each stage is (1) the creation of a supersaturated vapor, (2) the condensation of vapor on particles, causing their buildup, and (3) the subsequent separation of aerosol particles from the gas stream. When necessary, several stages in series can be employed to achieve the desired efficiency. Three methods, mixing with steam, cooling, and adiabatic expansion, were used to create a supersaturated vapor, and their operation characteristics are theoretically elucidated. A single-stage bench-scale model is setup to validate the feasibility of the proposed process, employing (1) a coaxial jet tube for mixing aerosol stream with steam to create a supersaturated vapor, (2) a cooling chamber to create more supersaturated vapor required for a continuing growth of nuclei, (3) a nozzle to accelerate the aerosol stream to a higher velocity before entering a cyclone and to induce further condensation incurred by the adiabatic expansion cooling, and (4) a cyclone for an efficient collection of the grown particles. Four aerosols are tested, including Si02 and three powders collected in static electrical precipitators from power, clay incinerating, and ore sintering units in a steel plant, respectively. The effects of particle concentration, size, contact angle, and steam mixing ratio on the efficiency are examined. The measured efficiencies reach 99%,supporting the validity of the process, and are in qualitative agreement with the computed efficiency. Suggestions on how to achieve the desired efficiency at a reduced cost are also given.
Operation Characteristics of the Process In each stage, separation is accomplished in three steps: (1) onset of heterogeneous nucleation on particles, (2) buildup of particle size by the condensation of vapor, and (3) separation of aerosol particles from the gas stream. The essence of the former two steps is the creation and sustenance of a supersaturated vapor, and the last step is the efficient separation of aerosol particles from the gas stream. The separation of particles can be accomplished by various mechanical processes (Meas, 1979), and the choice of the process affects the amount of steam and mechanical power required in order to achieve a desired collection efficiency. The requirements of the former two steps are fulfilled by three methods, mixing, cooling, and adiabatic expansion, whose operation characteristics are elucidated in the following.
0888-5885/93/2632-1509$04.00/00 1993 American Chemical Society
1510 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 4.0
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0-19
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From t o p line: To = 20, 30, 40, 50, 60, 70, 80°C
1.0
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4.0
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Figure 1. Variation in maximum contact angle as a function of particle radius (nucleation rate = 1). From top to bottom, each curve was obtained at an S and T of 2.768/318.5,2.799/314.5,2.745/311.1, 2.49U306.6,2.2/310,1.8/310,1.6/310,1.4/310,1.2/310,and 1.1/310 K,respectively.
Figure 2. Variation in supersaturation, obtained after mixing 100 "C saturated steam with 1 mol of air saturated with vapor but before condensation occurs, as a function of mixing ratio (mole ratio). From top to bottom, each curve was obtained at an initial air temperature, To,of 20 to 80 O C , respectively.
A. Heterogenous Nucleation. The condensation of vapor on particles initiates the buildup of particles and is the key step to the separation. The probability for particles to induce condensation can be evaluated from heterogeneous nucleation theory. Assuming that the particle is spherical, the number of critical embryos created on its surface per unit time (the nucleation rate) can be written as (Fletcher, 1958)
efficiency to (1) flux forces (thermophoresis and diffusiophoresis) and (2) the condensation of vapor and subsequently improved impaction collection. Yet, the major factor governingthe high efficiency is controvertible, and various conclusions are reached from various types of scrubbers and different operating conditions. In our process, the contribution due to the improved impaction collectionis identified through the investigation of the particle growth process, and the other contribution is then deduced; the optimized operation condition is found by means of process simulation. In the simulation, the supersaturation and the amount of condensate attainable are calculated, employing Fletcher's theory of heterogeneous nucleation in conjunction with the conversation of energy and mass, and the following reference state and approximations are used: (1)the reference state is 0 "C water, (2) the gas mixture is composed primarily of air, saturated with vapor and behaving as an idea gas, (3) the gas stream is well mixed with steam and nucleation occurs only after completely mixing, and (4) the latent heat of condensation is evenly released in the entire system, and droplets and gas are isothermal. For naomoles of air saturated with vapor at TO, after it is mixed with n, moles of 100 "C saturated steam, the resultant temperature before condensation can be iteratively solved for from the energy conservation equation,
J = 4aR:K exp(-AG*/kT) (1) where AG* is the critical Gibbs-energy change accompanying the formation of a critical embryo on the particle surface and K is a kinetic frequency. If the growth of the embryos is a result of direct condensation of water molecules from vapor phase, K is written as (Uhlmann and Chalmers, 1965)
K = v,A,*N,
(2) where vEis the collision frequency of vapor molecules onto embryo per unit area, A,* is the area of the liquid-vapor interface between the embryo and vapor phase, and N , is the number of adsorbed water molecules on the particle surface per unit area. Basically, the nucleation rate increases with the supersaturation and the size and wetting property of particles (Hirth and Pond, 1963),and the supersaturation required to initiate the condensation of vapor on particles can be evaluated from eq 1. Figure 1 illustrates the limit of the contact angle as a function of the particle radius for several values of S at unity nucleation rate. Particles can only induce a nucleation rate of less than unity if their contact angle is larger than this limit; however, increasing the supersaturation relieves this restriction on the contact angle. Note that care has to be taken when using Fletcher's theory of heterogeneous nucleation, i.e., eq 1, to predict the critical supersaturation. The theory has been shown not to be adequate for many materials (Liu et al., 1984; Porstendorfer et al., 1985; Chen et al., 1993). B. Mixing. As a 100 "C saturated steam is mixed with a gas stream less than 100 "C, the partial pressure of vapor may increase to a level higher than the equilibrium vapor pressure, resulting in the creation of a supersaturated vapor which could subsequently condense onto particles. The addition of steam into flue gas to enhance the collection efficiency of particles has been previously reported in the operation of various types of scrubbers (Litvinov, 1967; Brock, 1962; Davis and Truitt, 1971,1972; Lancaster and Struss, 1971;Prakash and Murray, 1973)and plate columns (Rozen and Kostin, 1967). Most attribute the higher
naoHa(T) + nvoH,(T)= na,,Ha(To)+ nv,,Hv(lOO"C) (3) and the resultant vapor pressure P, and supersaturation S are
where ys is the mole fraction of vapor content in air and Haand HEare the enthalpy of air and steam, respectively. Figure 2 shows that the degree of supersaturation increases with mixing ratio, reaches a maximum, and then decreases (or levels off in the case of high initial temperature) when air stream is mixed with a 100 "C saturated steam, where the mixing ratio is the mole ratio of steam to air and the basis is 1 mol of air. Note that the peak supersaturation decreases with increasing initial temperature and the peak occurs at higher mixing ratio as summarized in Figure 3. Also illustrated are the corresponding mixing ratio and temperature which increase with increasing initial temperature. For comparison, the critical supersaturation required for a unity homogeneous
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1511 1.21
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'
'
'
,
'
,
,
* 3.5
20 30 40 50 60 70 80 90 T e m p e r a t u r e To ("C)
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Figure 3. Summary of variations in peak supersaturation, S, and the corresponding mixing temperature, 2'-, and mixing ratio, Rmix, as functions of initial temperature, TO,of air stream when mixing with 100 "C saturated steam under the condition that peak supersaturation occurs and no condensation occurs yet. The dashed line represents the supersaturation required for homogeneous nucleation (nucleation rate = 1) at the corresponding 5"- for air stream initially at TO.
nucleation rate at the corresponding mixing temperature is plotted as a function of the initial temperature. Any operation resulting in a peak supersaturation over the critical supersaturation should be avoided to minimize the depletion of supersaturated vapor due to the homogeneous nucleation. After condensation and at equilibrium, the resultant temperature P,vapor pressure PF,, and amount of condensate n, are iteratively solved for from the energy conservation equation, eq 6, and the Kelvin equation, eq 7, (Adamson, 1982),
naSa(T*)+ (nvo- n,)H,(P) + n P w ( P )= na,$fa(To)+ nv,$fv(lOO"C) (6)
$0.05
,
,
0.03
E ao'oo20
30
40 50 60 70 80 Temperature To ("C)
90°*oo
Figure 4. Variations in peak effect and the corresponding amount of a gas stream of condensate as functions of initial temperature, TO, when mixing with 100 O C saturated steam under the condition that peak effect occurs and an equilibrium state is reached. 12.5 rlOO
1.0 I
0.020 50 .- 30 40 50 60 70 80 9 0
tt:
T e m p e r a t u r e To ('C)
Figure 6. Summary of variations in the corresponding supersaturation, S, at both temperatures before and after condensation, Le., T- and TWnd, and mixing ratio R- asfunctions of initial temperature TOof air steam when mixing with 100 "C saturated steam under the condition that peak effects occurs. 30 I
PF, = P v ( P ) K e= Pt(nvo- nw)/(nao+ nvo- n,) ( 7 4 with
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-20 0,
and
where Ke is a factor to correct the vapor pressure due to curvature, V , is the volume of particle, Npis the number of particle per unit volume, m, is the molecular weight of water, and p, is the density of water. The resultant amount of condensate attainable in an operation increases with mixing ratio, reaches a maximum, and then decreases (or levels off in the case of high initial gas temperature) as it does in the case of supersaturation shown in Figure 2, but the peaks shift to higher ratios, indicating that a maximum amount of condensate is obtained not at the mixing ratio producing peak supersaturation but at a higher ratio. When the condensing "effect" of steam, defined as the amount of condensate divided by the total amount of vapor, is plotted as a function of mixing ratio, peaks are also observed and the trend is similar to that shown in Figure 2. The peak effect decreases and occurs at higher mixing ratio as the initial temperature of gas stream TOincreases. Figure 4 summarizes the decreases in the peak effect and the corresponding amount of condensate as functions of TO, and Figure 5 illustrates the resultant variations in the corresponding mixing ratio, supersaturation, and temperature both before and after condensation; both figures form a basis for the selection of mixing ratio and for the determination of the mixture properties.
.--
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n
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O- 7
-6
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-4
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Figure 6. Variation in degree of cooling, required for heterogeneous nucleation (nucleation rate = 1) on particle at an initial temperature 2'0 of 90 O C , as a function of particle size. From top to bottom, each line was obtained at a contact angle 0 of 90" to Oo, respectively.
Thus, if no condensation on particles occurs during the mixing operation, any subsequent improvement in collection efficiency would be attributed to the flux forces occurring later on in the cooling operation. On the other hand, once condensation occurs, to achieve more efficient use of steam in the mixing operation, mixing should be operated employing a gas stream of an inlet temperature as low as possible and a mixing ratio at which peak effect occurs. C. Cooling. As a humid gas stream is cooled, its vapor becomes saturated or even supersaturated due to the decrease of equilibrium vapor pressure. Making the following approximations: (1) condensation occurs only on particles, (2) the condensate is evenly distributed in each particle, and (3) droplets and gas are isothermal, then for a saturated gas mixture containing 1 mol of dry
1512 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993
gas, as it is cooled from an initial temperature TOto a temperature T a t a pressure of 1atm, the supersaturation attainable before condensation is S = F,(To)/F,(T) (8) The resultant supersaturation increases with decreasing final temperature and increasing initial temperature until condensation occurs. The condensation occurs at a supersaturation high enough to induce heterogeneous nucleation. The degree of cooling required to achieve such supersaturation increases with increasing contact angle and decreases with increasing initial temperature as well as particle size (Figure 6). It is interesting to note that for well-wetted particles/surfaces a few degrees of cooling will bring about the condensation; for example, for a particle of a contact angle 30' and a radius larger than 0.01 pm, 5 "C of cooling is enough to induce condensation on it. Furthermore, to avoid condensation on walls, particles should possess a contact angle smaller than the walls, and the temperature difference between the gas stream and walls should be kept below a level determined by the wetting property of walls. Once condensation occurs, subsequent cooling brings more vapor to condense instead of increasing supersaturation. Given the condensation and size of particles, the amount of condensate n, is iteratively solved for from the Kelvin equation (eq 7) and the equation
n, = P v ( T o ) / ( -l P,(To))- KeF,(T)/(l - K,F,(T)) (9) The resultant amount of condensate increases with increasing initial vapor content and decreasing final temperature. The final volume of a droplet exiting the cooling chamber is equal to the sum of its volume and the condensate on it. D. Adiabatic Expansion. When a humid gas adiabatically expands along the axis of a nozzle, the equilibrium vapor pressure decreases with decreasing temperature more rapidly than the expansion pressure, down to or below the saturation pressure. Further expansion may lead to condensation (Hill, 1966; Wegener and Mack, 1958; Wegener and Pouring, 1964). The variation of temperature, partial pressure of vapor, gas velocity, and amount of condensate along the axis can be evaluated from the equations of mass, momentum, and energy and the equation of state in conjunction with nucleation theory and the rate equation of droplet growth (Kotake and Glass, 1981; Pirumov and Roslyakov, 1981). Assuming one-dimensional flow and temperature-independent heat capacities, the following equations are obtained, (10)
142 pdx
1dA 1do A dx udx
(13)
where P, u , p , p1, and T are pressure, axial flow velocity,
1.5
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0.2
0.4
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Figure 7. Variation of supersaturation alongthe axis of nozzle during an adiabatic expansion of a gas stream saturated with vapor through a nozzle for three nozzles of the same inlet, throat, and outlet radius equal to 0.1,0.038, and 0.041 m, respectively (inlet temperature = 80 "C, inlet pressure = 1.21 atm, particle concentration = l@/cma, particle radius = 0.1 wm, mass flow rate = 1kg/s). Curves 1,2, and 3 are obtained for the nozzle of a length 0.35, 0.43, and 0.68 m, respectively.
gas density, liquid density, and temperature, respectively; g is the mass fraction of the condensate, mais the molecular weight of noncondensable gas, C,, and Cp2.are the gasphase constant-pressure heat capacity per unit mass before and after condensation, y is the ratio of C, to C,, Ma is the Mach number, A is the cross-sectional area of the nozzle, and npis the flow rate of particles. In our subsonic flow, it is reasonable to assume an equilibrium flow, where relative motions and relative differences of temperature between the condensate droplets and gas mixture are usually negligible (Wegener,1969). The stream properties (i.e., supersaturation, temperature, velocity, amount of condensate, etc.) along the axis can be evaluated from the set of eqs 10-14. For a given inlet condition, essentially the same outlet stream properties are obtained irrespective of the nozzle shape, and the variation of stream properties along the axis remain similar except for supersaturation and temperature. For a quickly convergent nozzle, the gas velocity increases so quickly that a supersaturated vapor does not have enough time to complete the condensation, leading to a higher peak supersaturation (Figure 7) and a lower valley temperature. A t a constant mass flow rate, the variation of stream properties along the axis depends on the particles' wettability and concentration, inlet pressure, and initial temperature. A poor wettability shifts the starting position of condensation away from the inlet, causing a higher peak supersaturation and lower valley temperature, but the outlet stream properties approach those in the case of perfectly wetted particles. A higher concentration of particles provides more condensation area, resulting in a lowering of peak supersaturation and a rising of valley temperature, and the outlet stream properties approach but do not equal those in the case of low concentration due to the smaller outlet drop size which leads to a higher vapor pressure and consequently less condensate. Furthermore, increasing the inlet temperature requires a higher inlet pressure to maintain the same mass flow rate and results in a higher outlet velocity and temperature. More outlet condensate is obtained due to the higher inlet vapor contain as shown in Figure 8. On the other hand, for a saturated gas of a given temperature, increasing the inlet pressure increases the mass flow rate, producing more condensate (Figure 91, higher outlet velocity, and lower outlet temperature than
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1513 1.28
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Inlet temperature
80
("C)
l '
0.05 2
a
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Figure 8. Variation of the inlet pressure and the amount of condensate at outlet as functions of inlet gas stream temperature during an adiabatic expansion of a gas stream saturated with vapor through a nozzle (nozzle length = 0.43 m, particle concentration = l@/cm3, particle radius = 0.1pm, contact angle = 0, mass flow rate = 1 kg/s). -1.40
C
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I
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/
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0.10 1
=(J 0.20~ 1.10
1.20 0
1.30.
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1
2
1.40 0 a
0
inlet pressure ( a t m )
Figure 9. Variation of the mass flow rate and the amount of condensate at outlet as functions of inlet gas stream pressure during an adiabatic expansion of a gas stream saturated with vapor through a nozzle (nozzlelength = 0.43 m, inlet temperature = 80 "C,particle concentration = l@/cma, particle radius = 0.1 pm, contact angle = 0).
those in the case of low inlet pressure, but at the expense of consuming more mechanical power.
Experimental Details
A. Particle Characterization. 1. Particle Size and Wettability. Four aerosols are tested, including Si02 and three powders collected from electrostatic precipitators in a steel plant, later on referred to as red, black, and white powder. Their particle sizes are measured using SEM and/or TEM (scanning and/or transmission electric microscopy), their compositions are measured by EDS (energy dispersive spectrometer), and their contact angles with water are measured by the capillary rising method (Ayala et al., 1987). Si02 is a fumed colloidal silica, 0.006-pm radius, a product of Strem Chemical Inc., USA; our TEM observation shows that the particles are 0.006 pm in radius, but most cluster to a size in the range of 0.1-0.3-pm radius; and it has a contact angle greater than 90°, because water cannot penetrate into it, Red powder is collected from an ore sintering plant and is composed of oxides (>go%) and chlorides(98%) and sulfides (
