Automatic Recording of Carbon Dioxide by Conductometry Method f o r Determination of Rates of Combustion ROBERT DANIELS GOODWIN' Research and Development Department, Socony-Vacuum Laboratories, Paulsboro. N. J. The method described for determining burning rates and total carbon content of carbonaceous materials is relatively economical of routine operating time. Carbon dioxide evolved in combustion experiments is continuously absorbed by a solution of barium hydroxide in a conductivity cell. The total amount of carbon dioxide evolved is measured conductonietrically, and the conductivity is recorded continuously and automatically after suitable modification of the electrical signal.
A
SUMBER of techniques are suitable for the continuous
Ilrated by introducing successive k n o w quantities of carbon dioxide into a stream of air bubbling through the barium hydroxide reagent in the cell. (The air was freed of organic vapors and of carbon dioxide by means of a copper oxide furnace and an Ascarite absorption tube.) The information oht.ained in this calibration Tvas: response t'ime of the cell under these conditions, conductivity of the cell as a function of amount of carbon dioxide added, and recordei, drflrction as a function of cell conductivity.
determination of the concentration of carbon dioxide in a ga.3 stream under specific conditions. Thus in sufficient concentration antl in sl)sence of interfering gases, carbon dioxide may be nionitored by the fully automatic method of infrared absorption ( i ) . Chemical methods for continuous indication frequently have employed the change of electrical conductivity of a stream oi' dilute caustic effected by absorption of carbon dioxide from tiir. gas streani. 3Iethotls of this type eniployiug an alternating current, g;ilvanometer for the conti,ol of mechanical potentiometer recordcrs have been dwxibetl (1, 5). -1method for oliserving roiiductQnietrically the course of evolution of a finite amount 01' carbon dioxide by absorption in a fixed amount of caustic recently has been published ( 2 ) . The method described here is similar. In addition, however, there is given a description of a simple vacuum-tube converter to be employed in conjunction with a conventional direct-voltage potentiometer recorder. .li)plication of the permanent record thereby obtained to the determination of rates of combustion is illustrated. The method described for measuring rates of combustion is currently employed for studies of the regenerability characteristics of "rpent" catalysts which contain carbon or coke. It was designed t o provide a record of the accumulated total amount 01' carbon dioxide in an effluent gas stream as a function of time. .I minimum of attendance and operating time was desirable, as inight Iw achieved by automatic, continuous recording of the results I)y clectrical methods. For converting accumulated carbon dioxide into an electrical signal, alisorption in harium hydroxide is advantageous, because t lie relatively large change of conductivity obtainable with this rragent will minimize both the precision required of the associated elwtronic coinponent,s antl the disturliing effect of changes in trmpcmture of the conducting solution. A disadvantage jvhich has Iwen reported is that the conductance of barium hj-droxitle solutioii does not respond rapicll!. to the absorption of carbon dioxide (n). Pievertheless, liarium hydroxide was employed in r . the cell design and conditions employed, this work. l ~ c a u ~with the changc oi conductivity was complete in approximately 1 minute follo\ving the introduction of carbon dioxide. The atlvantage.%oi the greater conductance sensitivity of this reagrnt, therefore. were utilized. Under the conditions employed, foaming of the rmgent was not a prohlem (a). Becauw t h e electrical signal derived from the conductivity cell is not suit.al>lefor operating a conventional recorder (Brown potentiometer, Brown Instrument Co., Philadelphia, Pa.) a simple elrcrronic circuit n-as assemhled for the required conver-
API'4KATUS
Conductivity Cell. The g1a.s conductivity cell is of the dimensions shown by the front and side elrvational sections of Figure 1. Structural details resulted from the prerequisites that there be rapid circulation of every part of the solution, that the electrically conducting path be unperturbed by bubbles or surface level, that it be possible to titrate directly into the cell, and that it be possible to imniei se the cell in a thermostatic bath.
li 30ARSE FRIT
Figure 1. Glass Conductivity Cell for Absorption of Carbon Dioxide from Gas Stream Dimensions in millimeters
The removable, platinized electrodes are about 1 cm. square. The cell holds about 50 ml. of solution and produces rapid circulation of the liquid with air rates of from 50 to 200 nil. (h-.T.P.) per minute through the coarse gas dispersion frit (Corning Glass Works, Catalog Item No. 39533). The amount of water lost in saturating an initially dry air stream of 120 ml. (N.T.P.) per minute amounts t o only 0.14 ml. of water per hour. A small
siiiii.
The assenildetl components of the apparatus, consisting of al~sorption cell. electronic converter, and recorder, were cali1 Present address, Forrestal Research Center, Princeton P r i n c e t o n , X. J.
Cniversity,
263
ANALYTICAL CHEMISTRY
264
amount of nonionic detergent is added to the cell to improve the gas-liquid contact by a reduction in bubble size (one drop of 1% solution of Tween-20, Atlas Powder Co., Wilmington, Del.). Electronic Converter. Figure 2 is the circuit diagram of the converter] whose purpose is t o convert an alternating current, determined by the absorption cell conductivity, into a direct voltage for input to the recorder. The converter is supplied with 115-volt, 60-cycle per second alternating current power from a constant voltage and isolation transformer (Sola Electric Co., Chicago, Ill.). An alternating electromotive force of 6.3 volts is applied to the conductivity cell. Because the impedance of the cell (about 1000 ohms) is large relative to that of the remaining cell circuit (about 10 ohms), the cell current is proportional to cell conductivity. By use of the input transformer, T-20A05 (Thordarson Electric Co., Chicago, Ill,), a maximum of approximately 1 volt is available to drive the grid of the left triode of the 6SN7 cathode-follower circuit. Rectification is obtained if the grid signal and plate voltage are in phase. The right triode of the tube provides a relatively stable, zero reference level for the direct voltage, output signal. Wire-wound resistors were employed in the cathode and plate circuits.
1
CONDUCTIVITY CELL
\
h +
112 microequivalents of carbon dioxide, as indicated thereon. The absorption cell contained 800 microequivalents of barium hydroxide of concentration 0.016 N . The average interval of 3 minutes required for each addition corresponds to a concentration of 0.4 mole % of carbon dioxide in the air stream. It was found that the recorder deflection became steady within 2 minutes following each addition of carbon dioxide. As approximately 1 minute of this is required for sweeping the carbon dioxide from the acid bubbler as mentioned above, it follows that the conductivity cell response t.ime is of the order of 1 minute under these conditions. Conductivity us. Carbon Dioxide Absorbed. Figure 4 gives the measured conductivities of the absorption cell plotted as a function of the differences between the initial amounts of barium hydroxide, X,and the amounts, m, of carbon dioxide added. These amounts are expressed in niicroequivalents and refer to a solution volume of 50 ml. It is seen that the conductivity, u: is a linear function of (hl - m ) , u = u0
+ a ( N - m)
(1)
where the slope, a, is a constant for sufficiently dilute solutions but the intercept, 00, depends upon the residual conductivity of the solution. That portion of uo due to impurities in the barium hydroxide will depend upon the initial concentration of reagent placed in the cell; uol therefore, is t o be regarded as an empirical
J
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W
63V
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I
4
T-20
5 > I
P
Figure 2. Vacuum Tube Converter Circuit Resistances in ohms; megohms designated by M . in microfarads
Capacitances
In operation the recorder deflection is first set arbitrarily at about full scale, for the initial amount of barium hydroxide in the cell, by means of potentiometer R-2. The cell is then temporal ily disconnected and the open-circuit zero-point of the recorder is set by means of potentiometer R-1. Alternatively it d l be possible to set the recorder a t zero for that particular value of conductivity shown by experimentation to correspond to the equivalence point between carbon dioxide and barium hydrolide, so that the record mill be exactly proportional to the amount of carbon dioxide absorbed. This refinement of technique, which would be valuable for routine operation] is not applied in the following descriptions of method.
MINUTES
Figure 3. Recorder Trace during Successive Additions of Carbon Dioxide to Conductivity Cell
CALIBRATION METHOD
Response of the assembly of conductivity cell, electronic converter, and recorder was tested by introducing measured amounts of carbon dioxide to the air stream entering the cell. The recorder deflection as plotted against time was observed; then the actual cell conductivity corresponding to each steady recorder deflection was measured with a General Radio impedance bridge. The conditions of the test were similar to those employed for burning rate studies: an air flow rate of 120 ml. (N.T.P.) per minute and cell concentrations of aqueous barium hydroxide of 0.008 and of 0.004 gram-mole per liter. The carbon dioxide was generated by dropping standardized sodium carbonate (0.056 X ) from a IO-ml. buret into approximately 18 N sulfuric acid contained in a small gas bubbler. As the free gas space in the bubbler was 40 ml., the time required to sweep out 95% of the carbon dioxide generated would be 1.0 minute if the gases were continuously mixed in this space. CALIBRATION RESULTS
Cell Response Time. Figure 3 is a reproduction of the recorder trace obtained by introducing successively five equal portions of
I
a
I 0
400
200 (M-m
),
EQUIVALENTS. IO
600 6
Figure 4. Conductivity of .4bsorption Cell us. Difference between Initial .4mount of Barium Hydroxide and Amount of Carbon Dioxide Added to Cell
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
265 total amount of oxidation a t any given instant may be expressed as a fraction, x, of the final total, and may be obtained from the recorder chart alone. If the final total amount of carbon dioxide obtained from the sample when combustion is complete is designated by rn, and the corresponding final, constant recorder deflection by S I , then from Equation 3,
SO
60
x
z
0
40
=
m/mf = ( S M- Sm)/(S.v- SI)
(4)
that is, the fraction reacted a t any instant is the ratio of the recorder displacement up t o that instant, (SII - Sm),to the final recorder displacement, ( S u - S j ) .
W Y W
20
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I
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I
400
800
CONOUCfIVITY,
Figure 5.
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I
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I 1200
1
10"
Recorder Deflection US. Cell Conductivity
E 02m
9
01
-
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I! k
5
,005-
LL
0
z
P s
,002
.02
.05
,I
.2
.5
FRACTION OF I N I T I A L C A R B O N REMAINING
Figure 7. Relative Reaction Rate us. Instantaneous Carbon Concentration
20
10
30
MINUTES
Figure 6. Observed Fraction of Initial Carbon Remaining in Sample us. Time
Determination of the absolute amount of carbonaceous material in the sample requires (Equation 3) that the amount, iV, 'of reagent placed in the cell be known, as well as the value of the empirical constant, o ~ / a . For the concentrations used here, for example, the value of a, obtained from Figure 4, is u = 1.46 ohm-' equivalent-'. The corresponding value of uO/u for the case Jf = 800 microequivalents (Figure 4) is then u o / a = 168/1.46 = '115 microequivalents. APPLICATION
constant which must be determined for the conditions employed. Converter-Recorder Characteristics. Figure 5 gives the obeerved recorder deflection S , in arbitrary units, as a function of the measured conductivities. The two lines correspond t o the two different initial concentrations of reagent described for Figure 4. For each different initial concentration of reagent, before the addition of any carbon dioxide, the recorder deflection, S, was arbitrarily set near to full-scale deflection. I t is seen that the recorder deflection is proportional to the conductivity, u, S = bo where the proportionality constant, b, is set arbitrarily at the start of each experiment. COMPUTATIONS
.4n expression for the amount of carbon dioxide absorbed is obtained by combining Equations 1 and 2,
+
m = [(ads) MI [ I - (Sm/S.w)l (3) In Equation 3, S , is the recorder deflection corresponding to the amount, m, of carbon dioxide and S M is the initial deflection corresponding to the initial amount, &If, of reagent in the cell before any carbon dioxide has been absorbed-that is, for the case m = 0. I t will be observed that the variable in Equation 3 is a ratio of recorder deflections, S,/S.w, and is therefore independent of the setting of the recorder scale represented by b. It follows that in an oxidation experiment which comes to a stop because of the complete consumption of the carbonaceous reactant, the
.An example of the application of this method is given by the results presented in Figures 6 and 7 for the burning of coke froni a sample of silica-alumina cracking catalyst fines in air a t 980" F. Figure 6 gives the instantaneous fraction of initial carbon on the sample as a function of time, as computed from the relative recorder deflections, while Figure 7 gives burning rates computed from these data as the ratios of successive small increments of x and of the t,irne. Such a burning rate plot (both scales logarithmic) gives the reaction order in carbon as the slope of the plot and gives the reaction velocity constant as the intercept with the ordinate representing 100% of initial carbon remaining. The stream of air in this experiment was sufficient to maintain the oxygen concentration at a value constant within 3% of that of pure air even during the initially rapid stage of the combustion. The results given represent total carbon in the effluent gases, obtained by oxidizing carbon monoxide before admitting the gases to the conductivity cell. Studies of the carbon monoxidecarbon dioxide ratio in the combustion gases have been conducted by this method. The carbon dioxide is absorbed in a first conductivity cell; the remaining carbon monoxide is then oxidized and absorbed in a second cell. The t,wo conductivities are recorded simultaneously by a multipoint recorder. LITERATURE CITED
Brown, E. H., and Felger, 31. M.,IND..E r c . CHEW.,~ N A L . ED., 17, 283-4 (1945). (2) Hale, C. H., and Hale, hf. iX., A s a r . . CHEM.,23, 724-6 (1951). S.. IND. ENG.CHEM..ANAL.ED..6 . 293-5 11934). (4) Sobcov, H., and Hochgesang. F,P I PTOC. Am. Petroleum I n s t . , Sect. III,28,23-30 (1948). ( 5 ) Thomas, h.1. D., IND. ENG.CHEM.,ANALE D . 5, 193-8 (1933). (1)
RECEIVED f o r review August 6 ,
1952.
Accepted October 17. 1982
