Continuous Analysis of Reactive Organics by Selective Combustion William B. Innes Purad Inc., Upland, Calif. 91786
rn The selective combustion method for analysis of hydrocarbons involving thermocatalytic oxidation effects in a vanadia-alumina catalyst bed can be used for rapid continuous analysis of a wide range of organics providing the ratio of catalyst volume to flow rate is low, and proper steps are taken to prevent condensation and sorption effects in the inlet system. Under such conditions, response increases and selectivity decreases with temperature up to 400°C on a variety of organics. Response passes through a maximum with flow rate, the flow rate maximum decreasing with ease of oxidation. Reversible deactivation from aromatics can be neglected above 35OoC, but is significant at lower temperatures. Idle exhaust derived from leaded fuels does not noticeably deactivate the catalyst over prolonged periods under 400°C. Liquids containing organics including exhaust condensate can be analyzed after evaporation in dry air. A substantial fraction of the total response to vehicle exhaust is associated with the condensible fraction. Responses at optimum conditions (365"C, 1.0 SCFH, 0.013-cc catalyst) on gasoline vehicle sources would be expected to be essentially additive, linear, and give good correspondence with photochemical reactivity. Actual comparisons of smog effects vs. instrument responses are needed to prove the correspondence.
S
elective response to photochemical smog-forming organics designated by us as reactive organics is believed essential for effective evaluation of exhaust control systems since there is wide variation in exhaust composition from various systems, as well as in the smog-producing properties of individual components. The objective of the current work was to develop a continuous reactive organic instrument having fast response and high sensitivity with negligible inlet system losses. A discrete sample version has previously been described and commercialized (Innes, 1969). Earlier work (Innes, 1970) indicated that the primary factor in achieving fast response on a continuous basis by the selective combustion method was use of a small bore reactor. Work
Experimental
A flow diagram of the system used as well as some detail on a reactor element is shown in Figure 1, and a sampling probe is shown in Figure 2 . Atmospheric air normally flows through the instrument by suction. It passes through a heated line and then into a preheater and reactor system contained in an aluminum block. The removable reactor housed in l/s-in. stainless steel tubing, contains high-surface-area vanadiaalumina catalyst and a thermocouple temperature sensor. The gas exits from the reactor into a silica gel trap, a rotameter, and passes out through a diaphragm pump. If measurement of other combustibles, primarily CO, is wanted, a second larger vanadia-alumina catalyst bed followed by a platinum catalyst reactor detector can be used, but this option is best considered separately. In testing gas samples, the sample leg of the probe shown in Figure 2 is inserted in the gas source, and the steady-state response caused by catalyst bed temperature rise due to heat of combustion gives a measure of reactive organic content as illustrated in Figure 3. The procedure used for testing liquid samples involves injecting measured amounts of the liquid with a 0-10 p1 Hamilton syringe into a gallon glass bottle of dry air containing aluminum rivets, capping off with an aluminum foil-covered stopper, shaking to effect mixing, and then testing the vapor
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20 GA. SYRINGE N E E D L E INLET PORT
EXHAUST PIPE
PROTECTIVE TUBE CONTAINING GLASS W3OL
L ACCESS SCREW
IFERENCE iERMOCOUPLE INPUT- 1
1
reported herein involves 0.1-in. bore reactors and is primarily concerned with effect of operating parameters, such as flow rate, temperature, and air/sample ratio on response, linearity, and additivity; possible decrease in response with usage due to poisoning from coking and lead deposition; interferences; vehicle exhaust testing; response to individual organics vs. their smog effects in comparison with other instrumentation ; and theoretical considerations.
5IbNAl
Figure 2. Sample probe
REACTOR THERMOCOUPLE INPUT
PPM HEXANE
THERZlOCOUPLE JUNCTION
100
-
CATALYST
0 ALUNINUhl BLOCK
GRANULAR ALUhllhUhl
Figure 1. Flow diagram showing reactor element 710 Environmental Science & Technology
0
I
I
10
20
I
30
I
40 50 SECONDS
60
70
Figure 3. Illustrative response curve for idle exhaust
sample as described above. In the case of high-boiling liquidse.g., organics in water-the bottle is warmed to minimize adsorption on the bottle walls. The vapor carbon content of organics is calculated from the known carbon content of the liquid, the volume of the bottle, and volume injected assuming vaporization to a perfect gas. In the case of fuels, it is necessary to estimate the C/H ratio to make the computation, but the uncertainty is less than 5 %. In the case of exhaust water condensate derived from combustion, response for organics in the water phase vs. that in the gas is computed from data on measurement of both and assumption of a stoichiometric amount of water vapor in exhaust gas ( 1 6 z less about 2 % for water content of the saturated gas at condensation temperature). That is: Total R.O. in condensate _ _ _ Total R.O. in “dry” gas
1
14 (Response for condensate)* Response for “dry” gas
[
*Response for condensate
[ R.O.
=
+
z
=
Response to mixture air of condensate in glass container
I[
Flow Rate. Response generally decreases with flow rate for less reactive organics in the range 0.75-1.5 SCFH as shown by the data in Table 11. However, the effect is small or in the opposite direction for reactive organics. Linearity, Additivity, and Air/Sample Ratio. From a theoretical standpoint as discussed below, results like those for discrete sample testing (Innes, 1970) would be expected to be substantially linear and additive, if oxygen is well in excess of combustibles. This seems to be the case as illustrated by the data in Table 111. Changing air/sample ratio is simply one way of changing the level of reactive organics, and results are predictable on the basis of response proportional to overall reactive organic concentration providing there is a substantial excess of oxygen. With lesser amounts of oxygen, response may increase less than linearly with reacthe organic content, and reproducibility may be affected as discussed below. Possible Decrease in Response with Usage. SHORTTERM EFFECTS.It is well-known that oxidation intermediates are
Volume of glass container Volume of condensate assuming vaporization
1
total reactive organics based on instrument response
Data and Discussion
Effect of Operating Parameters. Major parameters that can affect results include: nature of sampling system; ratio of combustion-supporting gas (normally air) to sample gas; total flow rate; reactor temperature; nature of catalyst; nature of probe; reactor geometry and design particularly reactor bore diameter; and catalyst temperature. Their effects are complex and this work is limited to a 0.013-cc, 0.25cm diam bed of high-surface-area vanadia-alumina catalyst ahead of the temperature sensor. Reactor Temperature. The effect of temperature on steadystate response at indicated conditions is shown in Figure 4 and Table I for various organics and for a standard mixture. In general, the more reactive organics have a lower temperature coefficient so that more selective response is observed at low temperatures.
Table I. Response in “C/ C vs. Temperature for Miscellaneous Components Temp, “C 300 350 365 400 9 . 5 11 15 Phenol 6.0 Ethanol Methanol Acetone Formaldehyde 5.1 7.0 7.7 9 4 Benzaldehyde 1.4 7.0 9.7 2.3 3.9 Acetic acid 1.1 8.5 i 1.5 Auto exhausta 4,5 + 1,2 8.5 i 1.5 Gasoline9 2.1 i0.5 3.0 3.8 7.3 Standard mixb 2,1 a Based on numerous tests with 10 fuels. Most exhaust from Idle Mode. See Table 11.
Table 11. Illustrative Effect of Flow Rate on Response
zcarbon Organic Compound nature
after air dilution
Flow rate,
SCFH
0.5 1. 0 1.5 Response in “C/z C 0,27 0.43 0.39 n-Butane 0.77 1.65 2.50 2.30 Propylene 0.10 5.2 5.0 4.6 Standard mixa 0.12 a 0.65 % n-butane, 0.65 % 1-butene, 0.65 % propylene, 25 2; CO, and
73 % nitrogen.
Table 111. Linearity and Additivity Av
Flow rate, SCFH Catalyst temp. “C 329 343 357 399
V
.. 1
01
300
350
400
DEGREES CENTIGRADE
additivity Av linearity 1.5 0.5 1.5 Theoretical response,“ 96 90’ looc 1ooc looc 1ooc 95“ 95c
+
“Theoretical response = V I R I V Z R Zwhere V , = volume of ith component, and R , is the response per unit volume of ith component. Values ranged from 0.75 to 0.95 for the nine organics tested; temperaturerise up to 8 “ C . Equal to 100 within reproducibility of test (5 %) for various mixtures of butene, butane, propene, and toluene up to at least 8°C temperature rise. Block temperature between 625-750°F.
Figure 4. Response YS. temperature for hydrocarbons Volume 6 , Number 8, August 1972 711
Table IV. Reversible Deactivation From Heavy Dosage HC Exposurea Cataly2t Time required for response recovery, min. Toluene6 Xylenec Butened temp, C 330 3 40
