Determination of Moisture in Gases by Automatic Dew Point R. M. ILFELD, General Engineering Loboratr p r y , General Electric Co., Scheneetady, N. Y. The limitations of manually operated dew point equipment have stimulated the development of an automatic dew point recorder. The resulting instrument will record the dew point temperature of a gas over the temperature range from amhient to -90" F. The instrument comprises a meehanioal refrigerator, a heater-controlled mirror assembly, and electronic controls to observe and control the amount of dew formed. The mirror reflectance is monitored hy a photocell, and the resulting signal converted into an increase or decrease in the amount of heat supplied to the mirror. The automatic dew point recorder gives a continuous record of dew point without constant attention, and at the same time eliminates errors due to differences in operators. The measurement is made a t an equilibrium dew point temperature rather than the transient temperature often used in manual observation. The equipment is designed for normal service under industrial plant conditions.
cuit. The operator adjusts the flow of cooling gas until a faint presence of dew is observed. The temDerature is read directly
justable valve for the cooling gas supply.
Y
AUTOMATIC DEW POINT RECORDER
While the manually operated equipment described above is suitable for laboratory and sampling operation, in many industrial processes automatic recording equipment is required. The operation of the dew point recorder is shown in Figure 3.
A two-stage refrigerator, of more or less conventional design is aapsbk of cooling themirror assembly to a temperatureof -90°F. This makes possible measurement of dew points down to as
T
H E availability of sutomatic dew point equipment has made more convenient the application of the dew point method to industrial problems. This paper describes the dew point recorder and indicates same of the advantagcs of automatic over manual apparatus. PRINCIPLES OF DEW POINT MEASUREMENT
U
A characteristic of the dew point method of moisture determination is the fact that dew point measurements give absolute humidity, independent of ambient conditions. Dew point measurement is a temperature measurement. The dew point is the temperature a t which the air or test gas becomes saturated with moisture. the temnerature a t which moisture will beein to condense of onto a cooling surface. Care is needed in t,he interpretation . . . . the observed dew polnt temperature in terms ot acturtl morsture content. At temperatures above freezing, calculations of moisture contentbased onvapor pressures over water agreecloselywith experiment. Below freezing, to temperatures as low as -90" F., theory predicts that data on vapor pressure over ice should be used to calculate moisture content. Experiments show, however, that calculations based on extrapolated vapor pressures over water agree more elasely with moisture contents determined by other methods, even though water cilnnot exist a t these temperatures (l-S,6). Table I is the compromise data recommended by General Electric for interpretation of dew point temperature8 measured with its dew point recorder.
Figure 1. Diagram of Accurate Dew Point Indicator
I
~~
MANUALLY OPERATED DEW POINT APPARATUS
The dew point indicator shown in Figures 1 and 2 was built for industrial use.
sensitive indiciLtionoi the oresence of hew an the mi& surface. Test gas 16 introduced -into the mrror chamber, where the first Surhce contacted is the mirror itself. A light and a small viewer are provided to observe the mirror surface, and the temperature 1s measured by a thermocouple m a potentiometer cir-
F igure 2.
1086
Portable Dew Point Indicator
V O L U M E 23, N O , 8, A U G U S T 1 9 5 1
88
86 84 82
80 78 76 74 72 70 68
66 64 62 60 58
56 54 52 50 48 46 44 42 40 38 30 34 32 30 28 26 24 22 20 18
2.02 1.90 1.79 1.68 1.58 1.49 1.40 1.31 1.23 1.15 1.08 1.01
32.4 4.46 30.5 4.18 28.7 3.92 26.9 3.68 25.3 3.46 23.9 3.22 22 5 3.02 21.0 2.84 19.7 , 2.65 18.4 2.47 17.3 2.31 16.2 2.16 0.95 15.2 2.02 0.89 14.2 1.88 0.83 13.3 1.75 0.777 12.5 1.63 0.725 11.6 1.51 0.677 10.9 1.40 0.632 10.1 1.30 0.589 9.5 1.21 0.549 8.81 1.12 0.511 8.20 1 . 0 4 0.475 7.62 0.966 0.442 7.08 0.894 0.410 6.58 0.827 0.381 6.12 0.765 0.354 5.68 0,707 0.328 5.26 0.653 0.304 4.88 0.602 0.280 4.50 0.553 0.269 4.15 0.511 0.240 3.84 0,472 0.221 3.55 0.434 0.204 3.28 0.398 0.189 3.02 0,367 0.174 2.79 0.337
1087
-6 -8 -10 -12 -14
9.0612 0,982 0,113 0.0558 0.896 0.102 0.0508 0.815 0.093 0,0462 0.742 0,084 0,0420 0,674 0.076
-16 -18 -20 -22 -24 -26 -28 -30 -32 -34 -86 -38 -40 -42 -44 -46 -48 -50 -52 -54 -56 -58
0.0381 0.0346 0.0314 0.0284 0.0257 0.0232 0.0209
-65 -70 -75 -80 -85 -90
-95 -100
0.007Y 0,0070 0.0063 0.0056 0.0050
0.090 0.080
0.0170 0,0153 0.0137 0.0123 0.0110 0.0098 0.0088
'
0.0685
0.0619 0.505 0,0558 0.455 0.0503 0.410 0,0452 0.372 0.0407 0.336 0,0364 0,303 0,0328 0.272 0.0294 0.245 0,0264 0.220 0.0235 0.197 0.0210 0.177 0,0188 0.157 0.0167 0.141 0.0149 0.137 0,0132 0.112 0,0117 0.101 0,0104
0.0189
-50
0.610 0.555
0,0092
0,0082 0,0044 0,071 0,0072 0.0038 0.063 0,0063 0,0034 0.064 0,0056 0.0025 0,040 0,0041 0,0018 0.029 0,0029 0.0013 0.021 0,0021 0.0009 0,014 0,0015 0.0007 0 011 o.ooio o.ooo6 0.008 o.ooo7 0,0003 0,005 0,0005 0,0002 0,003 0,0003
Vapor pressures in atmospheres &t various dew point temwratures % by ~ o l u m eby 100.
mn be obtained by dividing the Y ~ U B Sfor
I The dew point recorder measures dew point under essentially equilibrium conditions, independent of operator's skill and timing. Measuiwments with the recorder are usually considered more reliablet:han measurements made with the indiestar. PRACTICAL FEATURES O F DEW POINT RECORDER
As t he apparatus is designed for industrial applications,~the eouionbent must be well constructed far plant use. The automaticclew point recorder in its present form is shown in Figure 4. Theteinperatura recarder fills the top panel, the electronic amplifie,, an d heater controls occupy the second panel, and the gm chamb'er, phototube, and heater mirror assembly take the third panel, The two-stage mechanical refrigerator is contained in the ialf of tho apparatus. lower' For industrial use an equipment should be designed to operwi,thout frequent servicinx. This recorder requires service Only ice a d w , to change the recorder charts and to oheck the adjustsnent of the elcct,ronic circuits. The formation of foreign vapors on the surface of the mirrcr is inhibited by automatic hourly clearing. Every hour the heater is automatically turned onfull,raising the temperature of the mirror to drive off any candensed vapors which might tend to obscure the dew point. The aecuracyof thedewpointrecorderisapproximately i2°F. down to dew point temperatures of -20" F. and approxi-
._
A N A L Y T I C A L CHEMISTRY
l0ea mately +5'F. from -20' to -% F. The I' instrument is inscnsitive to changes in flow rates over a reasonable range. The temperature recorder is supplied with high and low alarm eontacts to provide either an alarm signal or a control signal. AITLICATIONS O F AUTOiWATlC DEW POINT EQUIPMENT
First in importance of the gases, in which moisture can be determined conveniently by the dew point recorder, is process air, air which has been dried for use in B chemical or other industrial process. Large quantities of nitrogen, hydrogen, and oxygen are used also in industrial processes and can be conveniently handled by a dew point recorder. Carbon monosidt-, carbon dioxide, and methane may also be used in quantities in processes where moisture content can be a critical process factor. .\loist.urc in inert gases such as argon or neon is of interest when these gases are used far filling electronic tubes. There are some factors which limit the application of the dew point method of moisture determination, however. None of the standard instruments is designed to withstand corrosive gases, which could dissolve in the layer of moisture on the surface of the mirror to form acids whioh attack the mirror. Although smnll quantities of corrosive substances can be tolerated as impurities, moisture in such gases as hydrogen chloride and hydrogen sulfide cannot be determined using the dew point equipment described. Pressure is another limiting factor on the automatic dew point apparatus; at the present time the instrument is npt designed for high pressure operation. The most important limitation is the presence of condensables other than water in the test gas. Heavy hydrocarbons, lubricating oils, ammonia, or other contaminants in the test gas may oondense on the mirror and obscure the dew point, causing the automatic dew point observation system to miss the dew point temperature and give a grossly inaccurate reading. Butane and heavier hydroortrbons in natural gas must be held to low concentration if the natural gas is to be monitored for moisture content by the dew point method. Ran ammonia from an improperly adjusted dissociator e m cause erroneous dew point resdings when moisture determination is attempted in t h e nitrogen or hydrogen streams.
Figure 6 shows an installation of the dew point recorder in a refrigerator manufacturing plant. The compressors and evaporators are dried with warm air from whioh the moisture has been removed chemically. The system is similar to the air-control system described in the previous paragraph, except that an alarm and manual switching are used. In an airoraft plant a dew point recorder is used to record the amount of moisture in a dissociated ammonia stream. The equipment gives a continuous indication and record of the moisture content. At one of the water works stations in Philadelphia, water purification is sided by a h e stream of ozone bubbles in the purifying tank. The mane is produced in an electrical discharge appmatus, using air which has been carefully dried by both mechanical rofriger-etion and chemical drying action. The dryness of this air is continuously monitored by the dew point recorder; an itlarrn aignal announce8 the exhaustion of one set of chemical dryom, so t,hat,the operators can switoh to a new set. In t,he General Electric manufaet,uring plant wherc nitrogen-
5
=AIR
INLET
WASHERS
VALVES SAMPLING REGENERATlON LY DRY
dld
OUT
Fi,qire 5. Use of Dew Point Recordler to Control Dryness of Air Siipply
SPECIFIC USES OF DEW POINT RECORDER
Figure 5 shows the use of the dew point recorder to control the a y n e s s of an sir supply. The air is brought through filters and washers i n t o a chemical drvine tower. where .~~~~~~~~~~~ m o i s t u r e I's r e m o v e d . The dry air leaving We tower is further filtered and is ready to be used in the process. The dew paint recorder samples a vory small amount of the air
the mokture content 6 the air output increases. When the moisture content has increased t o t h e maximum allowable, a contact on the dew Doint recorder actuates an alirm and a control systom. T h e c o n t r o l s y s t e m switches the motor-ooerated 3-way valves to brihg the second dryer on stream and puts the first dryer on a reg~neration cycle.
Figure 6. Dew Point Recorder Installed in Refrigerator Manufacturing P l a n t to Monitor Dryness of Air Used to Dry Kefrigeratoi Components
V O L U M E 23, NO. 8, A U G U S T 1 9 5 1 filled cable is manufactured, the dryness of the nitrogen is monitored by a de\y point reccrder. Other uses for the equipment have been found in manufacturing operations. ACKNOWLEDGMENT
Included in the group active in the development and design of the automatic dew point equipment are Herbert Robinson, R. C. Leever, and S. S. Stack of the General Engineering Laboratory, General Electric Co. The suggestions of E. R. TT7eaver of the National Bureau of Standards have been helpful in preparing the h a 1 manuscript of this paper.
1089 BIBLIOGRAPHY
(1) Awberry and Griffiths, Proc. Phys. SOC.London, 47, 684 (1935). (2) Ewell, A. D., Refinery Eng., 27, 131 (1934). (3) International Critical Tables, Vol. 111, p. 211, Xew York, 11rGraw-Hill Book Co. (vapor pressures over water at 1 atmos-
phere).
(4) Thornthwaite and Owen, Mo. Weather Rev.,68, 315-19 (1940); Z;.S.Patents2,240,082 (April 29, 1941),2,268,785(Jan.6, 1942).
( 5 ) Zimmerman, 0. T., and Lavine, Irvin, “Psychrometric Tables
and Charts,” Dover, K. H., Industrial Research Service, 1945. RECEIVED September 5 , 1950. Presented before the Division of Analytical Chemistry a t the 118th Meeting of the AMERICANCHEMICALSOCIETY. Chicago, Ill.
Automatic Measurement of Optical Rotation GABOR B. LEVY Schenley Laboratories, Inc., Lawrenceburg, Znd. A technique involving an automatic recording polarimeter has been established for studying the kinetics of reactions in whichachangeinoptical activity occurs. It lends itself to various analytical chemical applications. In some instances the presence of a catalyst or enzyme can be detected by the reaction
T
HE measurement of optical rotation affords a valuable analytical method because it is nondestructive. It is of particular potential advantage in kinetic studies, in which analyses should be performed without interference with the reaction under investigation. Although these advantages have been generally recognized, the technique is not employed frequently in research work, because of the tedium of reading values of optical rotation and time simultaneously. Furthermore, only isolated data are obtained and the precision of the readings of continually changing rotation falls significantly short of optimum. To eliminate these objections a continuous recording polarimeter was constructed ( 6 ) , which registers optical rotation within a range of 5’ mith a precision of about f0.005 O.
The advantage of automatic operation with respect to precision and economy is apparent. In additioii, it permits quantitative evaluation of reaction rates even when they change n the course of the reaction, and determination of the exact location of these changes. In nianual operation this would be possible only if an infinite number of points were determined. Therefore this automatic operation, for which the author suggests the name ‘Lrotography,J’permits new applications to analytical problems. Some of these applications are discussed below, When the course of a reaction is follow~edby a recording polarimeter, a permanent record, “a rotogram,” is obtained, which shows optical rotation on the abscissa and time on the ordinate (Figure I). (The arrangement of scale expansion causes a return to zero after a travel of every 0 . 5 O . Figure I represents the inactivation of a solution pf sodium benzylpenicillin in 0.2 JI phosphate buffer of p H 7.0. Concentration is 3.6 mg. per nil.; 15,000 units of penicillinase were added.) The zero or starting point on the ordinate is conveniently made by starting the recorder when the reactants are mixed, but the first pertinent values recorded on the abscissa are obtained only after the mixture has been placed in the instrument and ha1nnc.e is reached. By conducting the reaction over a long period of t h e , this initial region of uncertain values of rotation may be reduvd to an insignificant portion, Thus, essentially the entire course of the reaction is mapped. The evaluation of the reaction rate from the curvature of the
order. The activity or concentration of this agent can be determined by the slope of a ‘(rotogram.” The concentration of a reactant or reaction product may be determined simultaneously. De terminations show a high degree of accuracy and precision and surpass many analytical methods in simplicity.
rotogram is an easy task and changes in rate can often be detected by simple inspection. Of particular interest to analytical applications are the zero-order reactions which appear as a straight line on the rotogram. Deviation from the straight lint. can easily be detected by inspection. Because catalytic or enzymatic reactions frequently exhibit zero-order reaction rates in some ranges of concentration, the presence of such catalysts can be detected. Furthermore, in these ranges, the reaction rate, and consequently the slope of thr rotogram, are proportional to the concentration of the catalysts or enzymes. This offers a convenient method for their quantitative determination. An abrupt change in reaction order indicates a change in the reaction mechanism. In a closed system, this is usually caused by the disappearance of a reactant. The location of such a point can be determined with great precision by rotography. If the characteristics of the reaction are sufficiently known and if the reaction is specific to a reactant, the concentration of this component can be determined accurately by the change in rotation between the initial value, corresponding to the control, and the value a t the “break” in the rotogram. Optically active impurities, inhibitors, subsequent rearrangements, etc., do not interfere, as this second type of rotographic analysis is based on the determination of absolute differences in optical rotation within a certain phase of a complex reaction. To illustrate the various rotographic techniques, results obtained with the penicillin-penicillinase system are presented. INACTIVATION OF PENICILLIN
Abraham and Chain have found (1) that penicillin is inactivated rapidly a t room temperature by the action of penicillinase. The enzymatic degradation is assumed to be due to the hydrolysis of penicillin to penicilloic acid ( 2 ) . A typical curve representing the reaction is shown in Figure 1. I t is immediately apparent that the reaction is of zero order, essentially, throughout its entire course-Le., the reaction rate is independent of the penicillin concentration. According to the theory of Nichaelis and Menten ( 7 , 1 1 ) , thiP is due to the fact that thc rate-determining process is thr hydrolysis proper rather than the formation of the enzyme-
