I have received financial support in the form of a National Science Foundation fellowship and a Haagen-Smit/Tyler fellowship. It also received financial assistance in the form of a grant from the US Environmental Protection Agency. We had some great times humming out of sync on the radio, going crazy late at night in the lab, planning equipment layouts, talking about this and that, etc.
The device consists of a continuously stirred tank reactor (CSTR) in which the growth of the aqueous aerosol is measured. Special care is taken to minimize the size deviation of the aqueous aerosol in the electrostatic classifier used to measure the reactor feed and effluent distributions. The aerosol behavior in the reactor is modeled assuming an ideal CSTR and, given the solution thermodynamics and equilibrium chemistry, the effluent distribution can be predicted using one of the proposed reaction rate mechanisms.
Boundaries bounding the regions of mass transfer limitation for 122 first-order S(IV) oxidation in a 0.1 μm aerosol particle as . as a function of particle pH. Predicted CSTR effluent distributions assuming no wall loss 170 as a function of residence time, τ = 1 h.
INTRODUCTION
A model of aerosol behavior in the reactor was used to predict the size distribution of the reactor effluent from the inlet distribution and the proposed reaction rate expression. Numerous studies have been conducted on this system, both in aerosol and bulk; however, the form of the reaction rate expression is still uncertain. While the system described here was used to measure growth of liquid MnSO4 aerosol in an SO2-containing atmosphere, this device could be used to study growth in various moist aerosol/gas systems with minor modifications, e.g. CuSO4/SO2. systems MnCl2∕SO2 or (NH4)2SO4∕NO2.
It is convenient to operate the classifier "balanced" - the mantle and excess flow are the same. The humidity in the bottom reactor is controlled by the temperature of this humidifying water bath, the temperature of the reactor and the subsequent addition of dilution air. After leaving the premix vessel, the aerosol is sent either to measuring instruments for analysis of the reactor feed, or to the reactor and then to the instruments.
The reactor feed or effluent is divided into three branches for analysis of the SO2, water content and the aerosol size distribution. The pressure of the system immediately upstream of the dew point gauge is measured using a Magnehelical pressure gauge.
CHAPTER 3
The importance of the absolute accuracy of the thermometer will be discussed in the next section. To "balance" the optical/thermal circuit in the meter, the mirror surface is heated to remove condensate. The possibility of using a saturated solution (eg the humidity above a saturated solution of NH4H2PO4 at 250 °C is 93%) was considered to create a standard of known humidity.
There are several potentiometers in the cooling control loop that can be adjusted to vary 1) the current supplied to the cooler in the sensor, 2) the gain in the control circuit, and 3) the light reduction required before the current from the cooler is turned off (i.e. the thickness of the condensate layer on the mirror interrupting the optical signal). This result was about 2∣ times greater than the standard deviation of the 911 voltages and was due to the noisier circuitry in the 880. Therefore, the exact accuracy of the thermometer is unimportant and the calculated relative humidity is not affected.
The residual or excess flow (Qm), passes out of the classifier and is used to measure the relative humidity in the instrument. The transfer function Ω is a function of flow rates, classifier geometry (rod length L, rod radius ∏, and cylinder radius r2), and central rod voltage V. The average mobility is. It is clear that the envelope and excess flow "position" the mobility band, while the sample and aerosol flows determine its shape.
Again, the envelope airflow should be selected so that the largest expected particle diameter (single charged) is collected at a voltage lower than 8000 V. The selection of envelope airflow should be based on core diameters. they respond to singly charged particles for two reasons — l) the ratio of singly to multiply charged particles is always greater than one and 2) a particle of size Dp with charge p will have mobility zp at a voltage V„ = Vi∕v. This is equivalent to "measuring" the CNC data at one set of flows and reversing it at another. A linear fit to the data for each of the three flowmeters is presented in Table 3.1.
Flow error is determined to be ± 0.8% - longer measurement times and increased care with EMC2 resulting in a 50% reduction in error compared to EMC1. These measurements were made (on different days) in order to check the reproducibility of the size distribution. Dry aerosol filter samples were also collected to determine airborne metal mass loading.
Because minimal size adjustment of the particle size due to humidity changes was desired, the relative humidity in EMC2 was adjusted to the relative humidity in the CSTR. Since this exact requirement was not met experimentally (although the differences in humidity were typically less than 1%, see Table 5.1), the humidities had to match theoretically.
Γigure 5.5 CSTR effluent aerosol given a monodisperse feed aerosol
1987b) studied the success of various mixing rules in predicting water activity when the only information available is the water activity of the individual compost. The success of the mixing rules in predicting the water activity of this particular mixture was important in deciding how to treat the Na2SO4-MnSO4 system, since sulfate ions are highly associative in solution. 1987b) found that the ZSR (Zdanovdkii-Stokes-Robinson) and simplified versions of the Pitzer and RWR (Reilly-Wood-Robinson) methods all performed satisfactorily in predicting the water activities of the mixed-electrolyte solutions studied.
Since the Kelvin effect was included in the calculations, the parameter λ = (Pp,initial∕Dp,βhifted}3 is a function of the diameter and equation (7) must be used to obtain the value of the shifted distribution. this particular experiment It was concluded that a distribution shift when SO2 was present could be attributed to the reaction of the S(ΓV) species in the presence of manganese Since sodium sulfate was used to lower the manganese concentration in several experiments, it was necessary to to show that a shift between feed and wastewater distribution was not due to the sodium ions present in the aerosol.
Nevertheless, with the Na2SO4-MnSO4 aerosol, the possibility of growth due to the presence of sodium had to be excluded. A solid "liquefies" or becomes liquid when the relative humidity of the atmosphere is equal to the water activity of the saturated salt solution. The primary peak of wet food distribution for this experiment (see Figure 5.11) fell at approximately 0.152 μm.
An additional conclusion can be drawn from these results - establishment of S(IV)-SO2 equilibrium did not cause particle growth. Some runs (13–18) were run with a Na2SO4-MnSO4 mixture to reduce the initial manganese concentration. Results typical of those listed in Table 5.1 were consistently achieved regardless of changes and adjustments to the reactor system.
As mentioned earlier, the possibility that the growth was simply evidence of aerosol balancing by S(ΓV) was considered and rejected. Consequently, Da is a weak function of solution pH and rtjα will increase slightly in the pH range from 2 to 8. When creating the graph, a particle diameter of 0.1 μm, typical of an aerosol in a reactor experiment, was used.
Γigure 5.8 Characteristic times associated with establishing steady-state SO2 and S(ΓV) concentration profiles in and about a
The particles in the CSTR can be seen as an aqueous solution consisting of the feed manganese sulfate and the sulfuric acid, or S(VI), generated by reaction. An expression for the time rate of change of the acid concentration can be obtained from that for thermodynamic equilibrium. Cmvp = Cm«ρ,ο, (7) where Cm,q and vfiP are the manganese concentration and particle volume of the feed aerosol.
Wall loss in a CSTR depends on the physical properties of the aerosol, the size and shape of the vessel, and the mixing characteristics of the vessel. Therefore, the prediction of the CSTR effluent distribution for any inlet aerosol reduces to the calculation of the particle growth rate. Perhaps the most important calculation in the aerosol growth model is the determination of water particle activity.
The effect of the ion pairs on the species concentrations is more obvious in this case, par. The results on this point are independent of the feed distribution and the expression of the reaction rate. For each of the distributions in Figure 6.5, the total number of particles in the predicted effluent distribution was 104 particles∕cm3.
Fortunately, as can be seen in Figure 6.7a, these errors result in a imperceptible shift of the predicted effluent distribution. Therefore, neglecting the Kelvin effect in the CSTR model calculations was deemed justifiable. All other perturbations of the solution chemistry resulted in changes less severe than this case.
The Berresheim and Jaeschke (1986) rate expression used in this example is a function of hydrogen ion concentration. It is clear that in this case the choice of chemistry affected the distribution of the effluent. Also shown in each figure are the predicted reaction rates as a function of the dimensionless diameter x.
There is little correlation between the accuracy of the total number and tip diameter prediction. Some of the difference in the total number of concentrations can be attributed to the Teflon lines that carry the aerosol from the feed circuit to the reactor and back again (see Figure 2.l).
