769
V O L U M E 21, NO. 7, J U L Y 1 9 4 9 Table IV. Sample
Source
56 57 58
B B
B
62 63
C C
Water in Freon 12
P.P.M.
Sample
Source
P.P.M. 8.8 10.5 9.0 9.4
hr.
5 8
59 60 61
B' B' B'
hr.
1.8 1.8 1.7 1 8
3.9 3.5 3.9 3.8
2 ,3 2 4
64 65
J J
1.1 1.5
Ax.
1.3
2 5
10.7 9.2 9.4 AT.,
74 73
G G
24 4
AV.
9.8
313
26.9
H H
76 77
standard samples, where about 400 grams of refrigerant were used. These values are considered highly satisfactory and the routine is in order when such results can be obtained. Standard samples can also be made by filling a sample cylinder with water vapor under controlled conditions, say a t 32" F.. and then charging a controlled weight of bone-dry refrigerant into the same chamber. There is a possible second \Yay of checking. The refrigerant from cylinder A (Table 111) was examined a t a time when the routine was under control; no such control existed when cylinder B was analyzed. If the average for the three samples is not about the same as the original, the routine is probably out of control. This method is rather indirect and there is more work involved than in the other method. If the absorption train and the air supply can be checked as indicated and if a sample thoroughly dried can be run without getting more than 1 p.p.m., the technique is under control.
19.5 18.2
ACKVOW'LEDGMENT
18.9
25 7
Table V. Anal! ses of Dry Samples Sample S o .
H!O. I'.P.I\I.
78 79 80 81
-- 00 .. 33 0.3 0.7 AV.
0.3
bone dry. A very convenient drjer can be made of stainless steel, so that the distillation may be carried out a t ternperature. If the analyst can find consistently less than 1 p,p.m. of water, his procedure may be assumed to be all right. In this laboratory, a standard sample is often prepared in a refrigeration unit containing a 0.25-hp. compressor and an adsorbent type of drier. I n Table \- may be found analyses of such
The writer wishes to express his appreciation of the assistance in this work rendered by associates in the Carrier Corporation. Especially should attention be called to P. F. Mens, ~ h took o the brunt of the discouragement in the first work; to Michael Kin, who did such a painstaking job in refrigerant sampling; and to Anna llathill, who has depicted so forcefully in the dranings the apparatus that was used. LITERATURE CITED
(1) Benning, A . F., Ebert, -1.- 1 . v and Irwin, C. F., R e f r i g . E W . , 557 166-70 (1948). (2) ~ ~ p,,iI b i d , , 42, ~ 316-18 ~ (1941). ~ , (3) Kinetic Chemicals, Inc., Tech. Paper 8 (1931). (4) lveaver, E. R . , and Riley, Ralph, ASAL. CHEM.,20, 216-29 (1948).
m-.
RECEIVED
Ju,s 23, 1948.
DESIGN OF AN ULTRAVIOLET ANALYZER GILBERT KIVENSON, J. J. OSMAR, AND E. W. JONES' Mellon Znstitute of Industrial Research, Pittsburgh, Pa.
A design is presented for a double-beam ultraviolet colorimeter employing interrupted radiation and a tuned amplifier. Drift and noise are comparatively low, and over-all stability-and sensitivity are good.
I
S A study of methods of instrumentation for GR-S polymeri-
zation plants, the utilization of an ultraviolet colorimeter for continuous analysis and control of the styrene and butadiene streams seemed an attractive possibility. The ultraviolet spectrophotometer was being successfully used for the analysis of styrene mixtures in the laboratory and some attempts had been made to adapt it to continuous flow operation. In addition, such techniques as light chopping and the use of tuned amplifiers seemed t o offer a solution to some of the electronic problems encountered in the ordinary spectrophotometer. An instrument using these features was therefore designed and built. The experimental model was tested with typical plant mixtures and its properties were evaluated. This paper describes the features and performance of the instrument as compared to earlier designs and presents the experimental test results. 1
Present address, 3114 Iowa St., Pittsburgh, Pa.
PRELIMINARY CONSIDERATIONS
The composition (6) of a typical styrene-butadiene feed stream is given in Table I. It was desired to determine styrene continuously in this stream with as high an accuracy as possible. A number of nondispersive ultraviolet instruments have been built on single- and double-beam principles (8, 4, 5 ) .
Table I. Component Styrene 1.3-Butadiene 2-Butene Vinylcyclohexene 1-Butene 12-Butadiene
Styrene-Butadiene Feed Stream Component 70b y Weight
?& by Weight 29.3
66.1 1.9 0.8 0.7
0.4
Ethylbenzene Isopropylbenzene Methylacetylene Propylene Pentenes
0.3 0.2 0.1 0.1 0.1
ANALYTICAL CHEMISTRY
770 The circuit by Hanson ( 2 ) utilizes a mercury arc source, it 935 phototube, and direct current amplifier. The arrangement of Klotz and Dole employs a mercury germicidal lamp as a source and a one-tube amplifier. Previous experiments made by the authors with steady radiation instruments indicated, however, that an interrupted light-alternating current amplifier system would be more suitable for the long operation periods necessary in plant work. Flicker photometers are described by Dobson ( 1 ) and others. In these instruments the radiation is alternately pasred through a known material (or adjustable light stop) and through the material to be measured. The preqent instrument was drsigned on a similar principle. THEORY
.Iproblem occurring oil consideration of the system was thdl The extinction coefficient of styrene
of absorption cell thickness.
(for a 1-cm. light path at a wave length of 300 millimicrons) i,i approsimately 1000; this indicates that a cell thickness of 0.02 mm. is necessary to give a transmittance of only 1%. A much smaller spacing would be needed to bring the transmittance to an easily measured value. Such cells would be not only difficult to construct but troublesome to maintain in plant operation 100,
I
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1
280-
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40% 20%
2
a (r I-
$40
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W V
I Ill
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Figure 2.
40
$0
do
7 0 0
PER CENT STYRENE
Effect of Band Position
The curves of Figure 1 were coriibiried with the emissiori curveof a hydrogen source, the transmittance curve of a Corning No. 986 filter, and the spectral response curve of an RCA 935 photocell by determining the product a t each wave length. The resultant curves for the styrene-chloroform system are set fortb in Figure 3. The areas under each were determined and plotted against the concentration to give the calibration curve, A in Figure 4, which has adherence to Beer’s law. If the emitted light is confined to a 5 mp band centered at 310 m p the theoretical calibration curve obtained is B in Figure 4 The apparent extinction coefficients for the broad and narrow hand widths are 0.15 and 0.40, respectively.
’
g20LLL-J
DESIGN OF PRESENT 1iYSTRU.MENT
Perspective views of the instrument are given inFigures 5 and A -
200 0
250
300
350
400
450
Radiation is provided by a Beckman hydrogen discharge tube. The light is divided and focused back on an interrupter disk t,hatchops the radiation 15 t’imesa second. The beams are sent alternately through two cells, one containing a standard and the other t,he flowing sample. The transmitted light is then alternately refocused by a quartz lens in each beam onto a 935 phototube. -4 Corning red purple Cores filter (So. 986) is used directly berme the phototube. The front surface spherical mirrors (Perkin Elmer 30-cm. focal length) are employed 18 degrees off-asis. I n rhih position the image of the source can be distorted to a thin horizont,al line parallel to t,he knife-edge surfaces of the interrupter disk. This effect permits a very sharp cutting of the light beams :ind considerably reduces the time of overlap between the two twams. The resulting signal (after phase discrimination) is a squart. w a w .
500
WAVELENGTH I N MILLIMICRONS
Figure 1. Cutoff Curves of Styrene-Chloroform System
To permit the use of thicker cells, an automatic dilutiiig apparatus was considered for a time, but the concentrations required (of the order of 12 mg. of sample per liter of solvent) made this type of arrangement appear impractical. A cutoff method nras then developed to permit the use of cell? with larger spacings. Figure 1 shows the cutoff curves obtained The simple input circuit o f the phototuhr i s illustrateti i r i the by scanning various styrene-chloroform mixtures (l-cnl. cell) tilock diagram of Figure 7. n-ith a Beckman spectrophotometer. Chloroform was whtituted for butadiene for convenience in handling. There appeared to be sufficient energy differences 100 to determine styrene in its various dilutions in the 28 to 31% range, particularly if the band width were limited by filters. z 00 If a relatively narrow band were to be used for 0 v) the analysis, there would be some question as to its 2 width and position. The position of the band de(n60 s termines to a large extent the shape of the calibraz tion curve. Figure 2 presents a series of curves a made in the Beckman spectrophotometer with the 2 40 40% S T Y R E N E styrene-chloroform system. The band width was w approximately 5 millimicrons and the position of the a 20% S T Y R E N E center of the band is the parameter in Figure 4. An 20 almost linear calibration curve was obtained a t 320 millimicrons but the sensitivity is approximately one half that a t 310 millimicrons. I n actual opera0 260 280 300 320 340 360 380 400 420 tion with filters a band width as narrow as this is WAVELENGTH IN M I L L I M I C R O N S difficult to achieve, so that the “optimum band Figure 3. Resultant Curves Obtained from Consideration o f position” becomes less significant. Source, Filter, Phototube. and Sample Characteristirc ~
:
V O L U M E 21, NO. 7, J U L Y 1 9 4 9
771
Coupling is accomplished through a 0.1-mfd. condenser. A Western Electric tuned amplifier (Type KS10,281) having a band pass a t 15 cycles filters out harmonics and boosts the signal to recorder voltages. At a gain of approximately 400 the maximal signal obtained (by blocking one light beam) is 4.97 millivolts. This is the total [‘working” signal available for analysis. The minimal noise level (with both beams balanced) is 0.02 millivolt. The minimal detectable concentration change woiild therefore he xhoi~t0.4%.
and adjusting the opposing concave mirror for maximal signal as indicated on an output meter or recorder. The light block is then removed and the other mirror is adjusted until the minimal signal is obtained. Fine adjustment of the two beams can be made by the use of two light trimmers mounted in the light paths. The minimal signal consists of the random noise effects (Johnson zoise, microphonics, light source variation, etc.) and the beam change-over” noise. The latter arises from the overlap of the two beams and is produced in the period when both beams are either off or on. This gives a nonrandom signal, which can, however, be eliminated by incorporating certain circuit changes.
Table 11.
Styrene in Plant Samples
Styrene in Original Sample,
NO.
%
Styrene after Dilution,
1
98.86 94.21 92.11 96.30 99.66 95.80 95.08 95.53 92.74
28. 00 28.00 28.00 2R. 00 28.00 28.00 28.00 28.00 28.00
Sainple 2
3 4 5 6
7 8 0
%
AV,
Styrene Read from Calibration Deviations from Actual DeviatioiiF Curve, Percentage from Mean % 30.0 +2.0 +1.7 30.0 f2.0 +1.7 -1.0 -1.3 27.0 27.0 -1.0 -1.3 -1.9 26.5 -1.5 +2.0 +1.7 30.0 f1.0 26.5 -1.5 f2.0 Cl.7 30.0 0.0 -0 :i 28.0 28.3 .- . .
-0
20
40 60 80 PER C E N T S T Y R E N E
I00
Figure 4. Theoretical Calibration Cline with and without Filter
.
.___ .
EXPERIMENTAL RESULTS
\$.it11 a 27 ~ 7 8styrene-chlorofornl iiiisture in both cells as the zero condition, a calibration curve was made by varying the material composition in the analytical cell. The curve in Figure 8 covering the 25 to 31% styrene range is typical and agrees with The chopper is driven by an 1800 r.p.m. Bodine synchroiious that obtained from integration of the cutoff curves. With the niotor geared down by one half to give the 15-cycle chopping frequency. average sensitivity over this range established by t,he calibration With the sample in both beams and the filter in place, t,he total curve, several long-period runs were made. I n some of these energy incident on the phototube is of the order of microruns styrene was placed arross bot’h beams, in others the cells watt. The power consumed by the source is 19 watts. Greater remained empty. Tn one run styreue was added to OIIP side illumination intensity could easily be obtaintd, hut thr over-all sensitivity is sufficient for this analysis. only and the beams were balanced by partially blocking off the The ahsorption cells were of a special pressure constructiou ( 3 ) . empty cell. The maximal drift per 24 hours as observed did not The t~ellsare made of quartz plates separated by a spacer 1.5 exceed 14,7y0 of full scale, corrrspontfing (F’inurr I)) t o x styrene mm. thick. The cell spacing used is approximately the smallest concentration change of 1.6%. value that would be practical for plant, use. Although thicker cells would permit the analysis to be performed, the minimal diTwo general testing methods were used. 111 the first, $ainples mrnsion governs because of the decreasing effect of impurity conof styrene drawn from plant streanis over a riumber of months wntration changes with thinner cells. were analyzed by freezing point methods and diluted with chloroThe method of halancing consists of filling both cells with a repfnrm to R rnrnmon styrene content. With pure styrene in thr resentativi, swmplr of t h e matrrial. hlnrking nne h a m rompletrly, comparison cell, these saniples were :tnalyzed in the instrument itnd the ,/’ readings referred to the original raliro 30 CYCLE TUNED bration curvr. The results arcs given W4PLIFIER RECORDER in Table 11. These tests were made under t i d y rigorous conditions, as the use of pure styrene in one beam instead of ii
$1.970
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0
q'.g60
; 5
;6
7 :
A
9 ;
o;
31
PER CENT S T Y R E N E
Figure 8.
Calibration Curve for 27.0% Styrene in Comparison Cell
styrene content of blended styrene-butadiene samples from the feed stream of a GR-S reactor unit. Pure styrene was again used as the comparison material. Table Ill presents typical results. The deviations in this case were somewhat less, and more r e p resentative of average instrumental performance as based on the stability studies.
Figure 9. Instrument Stability Run
773
V O L U M E 21, NO. 7, J U L Y 1 9 4 9 ACKNOWLEDGMENTS
Table 111. Plant Sample
Styrene in Styrene-Butadiene Plant Samples Styrene Determined by Plant Material Balance,
70
NO.
1 2 3 4
29.2 29.3 29.3 29.5
f
*
f f
0.5 0.5 0.5 0.5
Styrene Found in Ultraviolet Instrument,
Percentage Deviation +0.6 -1.3
70
29.8 28.0 29.9 29.7
+0.6
The authors would like to express their appreciation of the help given them by Ralph Steinback, Albert Roth, and Myrtle Rider of this laboratory. They are also indebted to E. E. Stahly and 0. W. Burke for administrative guidance and encouragement, and to Howard Cary of the Applied Physics Corporation for helpful suggestions.
+0.2 Av.
=to,7
faces and the precise matching of absorption cells would reduce the unwanted signal to a minimum.
A synchronized commutating switch, which might increase the ratio of signal to background is being considered for incorporation into another model of the analyzer. I t would be mounted on the chopping motor shaft and control the output so that the changeover signal could be more readily eliminated.
LITERATURE CITED
(1) Dobson, G . M . B., Proc. Roy. Soc., ( L o n d o n ) ,104,248 (1923). (2) Hanson, V. F., IND.E N G . C H E HANAL. ., ED., 13,119 (1941). (3) Kivenson, G., Roth, A., and Rider, M. A , , J. Am. Optical SOC.,
in press.
(4) Klota, I. M., IND.ENG.CHEM., ANAL.ED.,15,277 (1943). (5) Klotz, I. M . , and Dole, M . , I b i d . , 18,741 (1946). (6) Natl. Bur. Standards, Rept. 46-08-02(Aug. 7,1946). RECEIVEDSeptember 10, 1948. P a r t of the program sponsored by t h e Office of Rubber Reserve, Reconstruction Finance Corporation, a n d approved by t h a t office for publication.
Pneumatic Autodetector for Infrared Gas Analysis K. D. R'IILLEK' AND 11. B. RUSSELL D e p a r t m e n t of Agronomy, Cornell Unizersity, Zthaca, N . Y . The absorption of bands in the infrared by heteroatomic gas molecules provides a method for continuous analysis of gas streams. High sensitivity to minor components, such as carbon dioxide in the atmosphere, may be obtained with relatively simple apparatus in which the detecting element consists of a cell containing the component for which analysis is desired. A major difficulty in the application of the self-detection principle is the control of drift. A simple analyzer designed to minimize drift is described.
T
HE increasing exploitation of selective absorption in the infrared by various molecular species is furnishing new and sensitive methods to the analytical chemist. Several infrared gas analyzers (f-4, 8, 11) for continuous analysis of gas mixtures avoid the expensive optical components of the infrared spectrometer and are related to ordinary colorimeters in their conception and design. However, the low energy of infrared radiations requires new techniques of measurement, and the complexity of the absorption spectra requires new concepts of filtering. The most effective filter for isolation of the bands absorbed by one component of a gas mixture is a quantity of the same eomponent. A filter cell containing a component, X, will absorb only certain well defined bands characteristic of X. This absorption is in accord with Beer's law, though interactions with other components and variations in total pressure may cause marked deviations. If a beam of infrared entering such a filter has traversed a sample cell containing a gas mixture which includes X , energy in the region of the absorption bands of X will have been partially removed from the beam and the energy in these bands subsequently absorbed by the filter cell will be correspondingly diminished; the energy absorbed by t,he filter cell is then a n inverse function of the concentration of X in the mixture. Two methods of measuring the energy absorbed by the filter cell may be applied. The first method, aptly known as the negative filter method, compares the total radiant energy emerging from the filter cell with that which entered it. The second method measures the heating of the filter cell itself resulting from t,he absorption of the residual energy in the absorption bands. If the absorption bands of some ot,her component present in the sample overlap those of X, a second filter containing the interfer1
Present address, Division of Soils, Univprsity of California, Berkeley
4 , Calif.
ing component must be placed in the path to remove thc interfering bands. The usual application of the negative filter incthod is illustrated in Figure 1, A . Twin beams of radiation from a source, S , traverse a sample cell, C , containing a gas mixture which includes X. One of the emerging beams passes through a filter cell, F , filled with X. The other beam passes through a dummy cell, F ' , containing a nonabsorbing gas such as oxygen. The two beams are absorbed by temperature-sensitive elements, D , D ' , of a differential thermopile or bolometer. Overlapping bands ;f other components are removed by additional filter cells, F . The beams reaching the receiver are identical except in the region of the absorption bands of X. These bands have been partially removed from the beam which traverses the dummy cell, depending on the con0' A. centration of X in the sample, whereas absorption of these bands has been virtually completed by the filter in the other beam. Thus, the two beams represent thc energy entcring and leaving the filter B. cell, respectively. The presence of other absorbing components in the sample ti 1 affects both beams equally, Is1 I C 1 3 1 so that the two beams cancel a t the receiver except for G. the bands absorbed by X. Figure Infrared Gas The resultant thermoelectric . Analyzers signal from the receiver is A . Typical negative Alter anainversely related to the conlyzer centration of X in the B . Twin-path analyzer C. Single-path analyzer sample.
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