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Energy & Fuels 1991, 5, 138-140
Intraparticle diffusion limitations are apparently only present a t adsorption rates that are faster than the maximum rate measurable in these experiments.
Conclusions 1. A site energy distribution model can explain oxygen chemisorption data obtained with a microporous carbon. 2. A fraction of the sites on the carbon surface apparently have a larger barrier (up to possibly 130 kJ/mol) to oxygen chemisorption. It is proposed that these highbarrier sites may be the reactive sites in gasification. On a reaction coordinate diagram, surface complexes that have a high-energy adsorption transition state will have sufficient energy to drive the reaction through the energy well, corresponding to a stable surface-oxygen complex, to completion. On the other hand, complexes having a lowenergy transition state for adsorption will have insufficient energy to pass through this energy well and will consequently remain on the surface as surface oxides. This hypothesis is in agreement with other known aspects of the low-temperature reaction kinetics of oxygen with
microporous carbons: (1)an activation energy of 125-160 ~ J ~(3) the stakJ/mol; (2) reaction order near ~ n i t y ; and bility of surface oxides a t reaction temperatures in the absence of oxygen. In conclusion, it is possible that the rate-controlling step for the low-temperature reaction of oxygen with microporous carbons is oxygen adsorption at high-energy sites. 3. The oxygen content of microporous carbons increases dramatically with carbon conversion. These secondary oxides are a function of carbon conversion alone and are probably due to active site formation during gasification. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Department of Chemical Engineering at the University of Illinois at Chicago for support of this research. Registry No. 02, 7782-44-7; C, 7440-44-0. (12) Suuberg, E. M.; Wojtowicz, M.; Calo, J. M. Twenty-Second S y m posium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; p 79.
A Method for Decreasing Baseline Noise in Thermogravimetric Measurements J. K. Floess,*J J. Chomiak,i A. F. Sarofim, and J. P. Longwell Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 1, 1990. Revised Manuscript Received August 6, 1990
A simple modification to the flow system of a Cahn Model 113 thermogravimetric analyzer (TGA) resulted in an order of magnitude improvement in the baseline noise of the instrument for a broad range of operating conditions. A t typical operating conditions of 900 K and gas flow rate of 180 cm3(STP)/min, baseline noise levels were approximately 0.5 pg. This low noise level was obtained by placing a critical (choked) flow orifice a t the gas outlet, which isolated the gas in the TGA from ambient aerodynamic disturbances and provided a stable flowing gas environment. Apparently, local atmospheric pressure fluctuations caused by aerodynamic disturbances are largely responsible for most of the baseline noise in a flowing gas TGA.
Introduction Recently, it was found that a substantial decrease in baseline noise of a Cahn Model 113 thermogravimetric analyzer (TGA) could be obtained by a simple modification of the gas flow system. In this approach, the gas exiting the TGA is vented across a critical (choked) flow orifice to vacuum, which effectively isolates the gas flow in the instrument from ambient aerodynamic disturbances. Typical baseline traces with and without this flow modification are shown in Figure 1. Noise levels are reduced by over an order of magnitude from *0.7 to f0.05 pg with the critical flow orifice a t the gas outlet. A t elevated temperatures, the improvement in the baseline signal with a critical flow orifice at the gas exit is even more dramatic, although noise levels are higher
than a t room temperature. At 1220 K, baseline noise is approximately k0.5 pg, whereas without the exit orifice, noise levels are f10 pg (Figure 2). Shown in Figure 3 is a typical TG run made a t 900 K. With the exit orifice in place, baseline noise was found to not significantly depend on the gas flow rate, flow direction (upflow or downflow), or the design of the hangdown wire and sample pan. Noise levels were about the same when the sample pan was larger or smaller or if different hangdown-wire diameters or designs were used. (Most of the experiments were made with a 0.1-mm Nichrome hangdown wire and a flat 0.8-cm platinum sample pan.) Tests were made with gas flow rates of 0-500 cm3 (at 1atm; 21 "C)/min. Nominal conditions, though, were downward flow a t a rate of 200 cm3/min.
'Present address: Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60680. Institute of Aeronautics, Warsaw, Poland.
Experimental Section The flow system is shown in Figure 4. Cylinder gases were passed through metering valves (A) at conditions of critical flow (Le., at an upstream to downstream pressure ratio greater than
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0 1991 American Chemical Society
Energy & Fuels, Vol. 5, NO.1, 1991 139
Method for Decreasing Baseline Noise in TGA
Figure 1. TGA signal with (top) and without (bottom) the exit critical flow orifice. Gas flow rate 200 cm3(STP)/min; temperature 298 K.
Figure 3. T G run at 900 K (sample size 122 pg of carbon; gas flow rate 180 cm3(STF') (air)/min),
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Figure 4. TGA flow system schematic.
Figure 2. TGA signal with (top) and without (bottom) the exit critical flow orifice. Temperature 1220 K; gas flow rate 180 cmS(STP)/min. 2l). Flow rates were measured with mass flow meters located upstream of the metering valves. Selection of either the reactant or inert gas flow to the TGA was made via a four-way ball valve (B).The gas entered the TGA a t the top of the 1-in. furnace tube and exited at the bottom. Upon exiting the furnace tube, the gas flow was throttled across another high-quality metering valve to vacuum maintained by a rotary vacuum pump. The pressure in the TGA was monitored with a water manometer, which also served as a safety relief. (1) McCabe, W.; Smith, J. C. Unit Operations in Chemical Engineering; McGraw-Hill:New York, 1966.
The furnace tube was attached to the balance housing with a 1-in. water-cooled Cajon Ultra-torr fitting. The sample pan was connected to the balance arm with a 0.1-mm Nichrome wire. A flow restrictor was positioned around the hangdown wire just above the gas inlet to minimize mixing of reactant gas with the large volume of gas in the balance housing. An internal glass baffle (supplied by the TGA manufacturer) was also positioned inside the furnace tube. A separate gas inlet on the balance housing was used to purge the balance chamber prior to a run. Operation of the flow system was not too difficult after some initial practice. In our operation, control was maintained manually; however, it would not be difficult to automate the control system. The procedure was as follows. After the sample was loaded in the pan and the furnace reassembled, the entire system was purged to remove oxygen from the system. Next the gas outlet vent valve was closed so that the gas exited across the metering valve to vacuum. Some initial experimentation was necessary to properly set the outlet valve to match the input flow rate; however, after a few tries, it was possible to maintain almost constant pressure in the TGA for periods of hours. (The use of a mass flow meter a t the gas outlet would probably have been helpful, but after some operator experience, it was possible to obtain adequate flow control without it.) Input above or below the exit flow rate results in an increase or decrease of pressure in the TGA. Typically it was possible to match inlet and outlet flows such that pressure variations in the TGA were several inches of water per hour. (Allowing the pressure to increase was preferred in runs
140 Energy & Fuels, Vol. 5, No. I, 1991 with downward flow so as to prevent gas in the balance housing from mixing with the flowing gas.) Good control was achievable in part because of the large capacitance of the system. Flow-rate adjustments to maintain constant pressure in the system were made on the supply side so as to not affect the gas flow rate past the sample pan. (For upflow, flow adjustments were made by using the outlet metering valve.) The flow rate across a critical flow orifice is directly proportional to the upstream pressure and will depend on the molecular weight of the gas and the ratio of the specific heats of the gas. The flow rate is given by where
and where g / A is the mass flow rate per unit area, P is the upstream pressure, R is the gas constant, M is the molecular weight, T i s the upstream temperature, y is the ratio of specific heats, and c1 is an orifice coefficient. Since the flow rate depends on the gas composition, switching gas streams in a run will change the flow rate through the exit orifice. This in turn will usually change the rate a t which the pressure increases or decreases in the TGA unless the flow rates for both input streams are correctly matched to the exit flow rate.
Discussion In laminar flow, the gas velocity through the furnace tube is related to the pressure drop:’ U =K A P (3) where u is the gas velocity, K a proportionality factor, and AP the pressure drop. This relationship may be differentiated and divided by (3) to give -du= - d ( M ) (4) U A P This equation shows that relative changes in the gas velocity are proportional to the relative variations in gas pressure drop. To minimize du, it is necessary to minimize d(AP) or to maximize AP. Variations in AP can be caused by aerodynamic disturbances in the laboratory, and although such pressure fluctuations are small (typically several pascals), these fluctuations can be a significant fraction of the pressure drop of the gas flowing unrestricted through the furnace tube.
Floess et al. The apparent sample weight is sensitive to the gas velocity because of drag forces on the hangdown wire and the pan. (Calculations show that the contribution of the hangdown wire to drag is actually greater than that of the sample pan.) Experiments showed a linear dependence of the apparent weight on gas velocity for flow rates up to -200 cm3/min. The change in apparent weight with gas flow rate was 0.3 pg/(cm3/min) for the hangdown wire and sample pan assembly used in these experiments. Therefore, baseline noise of 1pg corresponds to flow-rate fluctuations of about 3 cm3/min or, at a nominal gas flow rate of 200 cm3/min, 1.5%. As a result, a 1-2% variation in the pressure drop of the gas flowing through the furnace is sufficient to cause the observed baseline noise levels. Furthermore, if the pressure drop for gas flow is small, as would be the case if the exiting flow is not restricted, pressure fluctuations caused by ambient aerodynamic disturbances can be responsible for the noise in the TGA signal. For example, if the gas is simply vented to atmosphere, the gas pressure drop in passing through the furnace tube is on the order of tens of pascals. A few percent change in this pressure drop corresonds to pressure fluctuations of several pascals or less, which may easily be caused by pressure fluctuations in a ventilated laboratory. This analysis suggests that resticting the flow of gas from the furnace tube should decrease baseline noise and that it is not necessary to use critical flow. However, critical flow guarantees that downstream disturbances do not propagate upstream of the orifice and therefore provides the greatest degree of isolation from ambient aerodynamic disturbances.
Conclusions Local atmospheric pressure fluctuations caused by aerodynamic disturbances are apparently responsible for most of the baseline noise in a flowing gas TGA. The use of a critical flow orifice at the gas outlet effectively isolates the gas in a TGA from ambient disturbances and provides a stable flowing gas environment over a broad range of operating conditions. Modifications of the hangdown wire or sample pan are not necessary to decrease baseline noise. Acknowledgment. Support for thii work by the b o n Research and Engineering Co. is gratefully acknowledged.
