Near Infrared Spectrophotometric Determination of Ammonia, Carbon Dioxide, and Water At Elevated Temperatures and Pressures J. G. KOREN and A. J. ANDREATCH American Cyanamid Co., Stamford Research laboratories, Stamford, Conn.
b The vapor-liquid equilibrium concentrations of ammonia, carbon dioxide, and water were measured a t pressures from 500 to 5200 mm. of H g over a temperature range of 4 5 " to 130" C. Various liquid composi0 tions containing 10 to 28 mole 7 ammonia and 0.25 to 7.0 mole carbon dioxide were studied. A heated cell was constructed so that infrared radiation could b e reflected from a mirror inside the cell into a near infrared spectrophotometer. The partial pressure of ammonia and water could then b e measured without disturbing the equilibrium. Because of interference with ammonia, the near infrared carbon dioxide bands could not b e used. A Liston-Becker nondispersive infrared analyzer was used for the carbon dioxide analysis. Total pressure measurements were made with a capacitance pressure transducer. The sum of the partial pressures of ammonia, carbon dioxide, and water could then be checked against the total pressure. The Arrhenius plots of the partial pressure were straight lines.
T o VACUUM
);
70
M
of the vapor-liquid equilibrium concentrations of ammonia, carbon dioxide, and water a t various temperatures was required for the design of an ammonia carbamate absorber and decomposer. Although studies by Van Krevelen (4), Krishna ( I ) , and Otsuka et al. ( 2 ) are available, the data reported were insufficient' for the design of the equipment. The measurements by the above authors were made by passing a known volume of dry air or nitrogen through the liquid solutions; and absorbing the ammonia, water, and carbon dioxide in sulfuric acid, concentrated sulfuric acid, and soda lime. The ammonia was determined by titration, and the water and carbon dioxide by weighing. Ammonia, carbon dioxide, and water have absorption bands in the near infrared region which suggested that the equilibrium vapor concentrations could be measured spectrophotometrically. A heated cell was constructed so that 256
EASUREMENT
ANALYTICAL CHEMISTRY
ANALYZER
DETECTOR
I
TO
I\ - I I
1
h
FUSED SlLlCh WINDOWS
-
450 Prism
F . -
/R\
TO PRESSURE
TRANSDUC.FR
-
SPECTRO PHOTOMETER COMPARTMENT l S T A / N L E S S STEEL
Figure 1.
Schematic diagram of heated cell
the near infrared radiation could be reflected from a mirror inside the cell and back into the spectrophotometer. The partial pressure of ammonia and water could then be obtained without disturbing the equilibrium. The ammonia band a t 1.53 microns and the water vapor band a t 1.38 microns were free from spectral interference and were used for the analysis. Because of the overlapping of the ammonia band a t 1.96 microns with the carbon dioxide band a t 2.065 microns, the near infrared could not be used for the determination of the carbon dioxide. A Liston-Becker, nondispersive, infrared gas analyzer which used the carbon dioxide band at 4.2 microns was attached to the heated cell so that gas samples could be injected for carbon dioxide analysis. Various liquid compositions containing 10 to 28 mole yo ammonia and 0.25 to 7.0 mole % carbon dioxide were studied. Temperatures were varied from 40" to 130" C. Total pressures from 500 to 5200 mm. of H g were measured with a specially designed capacitance pressure transducer.
EXPERIMENTAL
T h e apparatus consisted of a Spectracord 3000 spectrophotometer; a specially designed, heated cell; a modified high temperature, nondispersive, infrared gas analyzer; 10-mv. recorder; and a pressure transducer. The apparatus is schematically illustrated in Figure 1. Heated Cell. The heated cell was fabricated from 1.27-cm., Xo. 316 stainless steel. A cylinder was made 25.4 cm. in length, with a n internal diameter of 8.17 cm. with provisions for two outlets, one for the gas analyzer and the other for a safey valve. The front plate contained the entranceexit port for the infrared radiation which consisted of two fused silica windows, 3.18 cm. in diameter. Silicone rubber gaskets were used as seals between the mounting rings and the windows. The space between the windows was evacuated to reduce heat losses. The cylinder was wound with No. 22 nichrome wire and thermally insulated with magnesia. In the back plate, openings were provided for the pressure transducer, for loading and evacuating the cell, and for insertion of the thermocouple. The iron-constantan thermocouple junction was Apparatus.
placed just below the minimum level of the aqueous solution. Lead gaskets were used as seals between the end plates and the cylinder. The cell volume was approximately 1.2 liters, and was designed to operate at teniperatures up to 150' over a pressure range from 1 mm. to 8000 mm. absolute. The concave mirror was mounted on a threaded 1.59-cm. diameter shaft which screws through the back plate. The optical path length was set a t 26.3 cm. For high ammonia and water concentrations, the optical path length was reduced to 13.1 cm. Corrosion difficulties were solved by using a 4.13-em. diameter Hastalloy "C," an iron, nickel, and molybdenum alloy, concave mirror which has good reflectance in the near infrared. The focal length of the mirror was 14.6 cm. Insulated nichrome wire was wound around the entrance-esit window and a 50-watt cartridge-type heater was inserted in the mirror shaft so that heat could be applied, thereby preventing any condensation on these surfaces. Two 45' prisms, front surfaced with gold, were placed in the spectrophotometer sample compartment to reflect the infrared beam into the cell. The concave mirror refocused the beam back onto the second prism and then to the detector. The reflected image at the detector had the same dimensional characteristics as the original beam image. Nondispersive-Infrared Gas Analyzer. A Model 21, Liston-Eecker infrared gas analyzer was used for determination of the carbon dioxide. T h e analyzer is normally thermostated a t 60' C., but in this case the analyzer had t o be thermostated a t 120' C. to prevent condensation of solid ammonium carbonate on t h e walls and windows of the sample cell. A special high temperature cell, 1.91 cm. long, was designed and wrapped with nichrome heating wire. Heat losses were further reduced by cementing a mica plate, 0.0254-cm. thick, on the standard quartz window assembly. The space between the mica and quartz was then evacuated through a hole in the casting and sealed. Mica spacers, 0.0254em. thick, were also placed between the cell arid the detector to prevent overheating of the detector. Pressure Transducer. Standard pressure measuring instruments could
c.
not be used to measure the total pressure of the reaction cell because of the corrosive action of the aqueous ammonia and the high temperature required t o prevent formation of solid A capaciammonium carbonate. tance pressure transducer was constructed of No. 316 stainless steel and thermostated a t 125' C. The transducer consists of a 0.0254-cm. stainless steel diaphragm which was made the variable plate of a condenser in a crystal controlled, 10-megacycle oscillator. The device was standardized by having equal pressure on both sides of the diaphragm, then adjusting the variable tuning coil to bring the oscillator to resonance, as indicated by a maximum current on the microammeter. A change in the reaction cell pressure causes the diaphragm to move, thereby detuning the circuit. An equal pressure can now be applied to the opposite side of the diaphragm to restore the diaphragm to its original position and the circuit to resonance. An attached mercury manometer or pressure gauge indicates the total pressure in the reaction cell's atmosphere from the manometer but also serves as a very sensitive indicator for pressure equilibrium. The schematic of the transducer and electrical circuit is shown in Figure 2. Procedure. T h e concentrations of ammonia, carbon dioxide, and water in the aqueous phase to be studied were obtained from a mathematical analysis of existing data. Keighed quantities of concentrated aqueous ammonia, ammonium bicarbonate, and distilled water were used so t h a t a total of 12.5 moles was present. The ammonium bicarbonate was dissolved in the aqueous ammonia in a separatory funnel. The heated cell was evacuated and the bicarbonate solution was then drawn into the cell. When the concentrated ammonia solution did not supply
the required ammonia, the difference was made by adding dry gaseous ammonia. The reaction cell was then heated electrically and the temperature was adjusted by varying the input voltage. The infrared spectra for ammonia and water were t'aken when the temperature and the total pressure reached equilibrium. After the spectra recordings were made, a gas sample was injected into a 7-ml. evacuated chamber before t'he gas analyzer, reduced to atmospheric pressure, expanded into the evacuated analyzer cell, and further expanded into an evacuated 50-ml. heated glass bulb. The analyzer was calibrated using the same procedure with known carbon dioxide standards. The partial pressure of carbon dioxide in the reaction cell could then be calculated from the measured t'otal pressure and the per cent carbon dioxide could be obtained by the analyzer. At the completion of each run a sample of the cooled aqueous phase was chemically analyzed for ammonia and carbon dioxide. The ammonia was determined by indirect acid-base titration and the carbon dioxide by either gas evolution or a modified formal titration method. DISCUSSION AND RESULTS
Effect of Increased Temperature and Pressure on Molar Absorptivity of Ammonia and Water. T h e molar absorptivity ( 6 ) of ammonia and water may vary at the high tem1)eratures and pressures because of pressure and temperature broadening of the near infrared bands. T o determine the variation in the molar absorptivity, the temperature was varied from 100' to 13OoC., and the pressure was varied from 55 to 90 p.s.i. with prepurified
10'
10 MEGACYCLE OSCILLATOR
n+ ieov mmf Z M
2.7
Figure 2.
2.8
2 9
30
I T X I O ~
0-100
Electrical schematic of pressure transducer
Figure 3. Plot of partial pressure vs. 1 / T for liquid composition containing 4.98 mole 70 carbon dioxide, and 14.50 mole 7 0 ammonia VOL. 37, NO. 2, FEBRUARY 1965
257
Comparison of Ammonia, Carbon Dioxide, and Water Analysis of Known Mixtures. T o determine the accuracy of the entire procedure, analyses were made on mixtures with known vapor pressures of ammonia,
nitrogen. The 1.38-micron water band showed that both temperature and pressure effected the molar absorptivity. A mathematical study showed that the effect could be expressed by the following equation : =
B
+ 0.0071 Xi
0.555
0.00053
X1 = (P - 75)/10 Xz = (TOK - 385)/6 =
T
= absolute temperature
c
=
pressure in mm. of H g (OK.)
molar absorptivity
The molar absorptivity of the 1.53micron ammonia band showed only a small variation with increased pressure. The molar absorptivity changed from 0.389 at 15 p.s.i. to 0.407 a t 60 to 90 p s i KO temperature effect was observed up to a temperature of 130’ C.
Table 1.
Vapor-Liquid Equilibrium Data
Total pressure mm. of Hg Obsd. Calcd.
Vapor phase
c.
Gas NHI HVO NHz
82.5 100 8
Perry’s
H-0 --._
NHo H20 NHI HzO
122.9 126.3
(1)
X22 carbon dioxide, and water. An ammonia-water composition was taken from Perry’s Chemical Engineering Handbook (3). Table I compares the data obtained by analysis with that from the Handbook. The difference between the readings was less than f 1%. A second analysis was made from a liquid composition of ammonia, carbon dioxide, and water obtained from references 3 and 4. The results are shown in Table 11. The difference between the sum of the partial pressure of the ammonia, carbon dioxide, and
where
P
+ 0.0004 XlXz - 0.0016 X i 2
- 0.0090 Xz
-
5 0 . 2 wt. % ’ 4 9 . 4 wt. 9 54 8 wt. % 45 _ _ -2 wt. 9 3050 mm.’Ef 1350 mm. of 3250 mm. of 1450 mm. of
Found
Hg Hg Hg Hg
5 0 . 6 wt. % ’ 4 9 . 8 wt. 9n 55 5 wt. % 44 .5 wt 92900 mm.’if Hg 1335 mm. of Hg 3150 mm. of Hg 1360 mm. of Hg
735
737
1363
1362
4235
4270
4510
4610
water gases and the total pressure varied between 3 and 5%. High results of this magnitude are probably due to air in the reaction cell introduced when the cell is charged. Calculations. Since the absorbance is directly proportional to the number of gas molecules in the light path, the partial pressure of ammonia and water can be calculated by use of the ideal gas law and Beer’s law, as expressed in the following equation.
The partial pressure ( P ) will be in millimeters of mercury when R is 62.360 liters millimeter Hg(OK.)-I(gram mole)-’. Temperature T is in OK., b is cell path length in centimeters, A h is absorbance at subscript wavelength A, and is the molar absorptivity. The molar absorptivity for water was obtained from Equation 1. Experimental results are listed in Table 111. When the log of the partial pressure was plotted against the reciprocal of the absolute temperature linear curves were obtained. A typical plot is shown in Figure 3. The linear plot indicates no other compounds are formed in the liquid phase. The partial pressures from duplicate runs agreed within 10%. In almost all cases, the sums of the partial pressures were within 10% of the observed total pressures. LITERATURE CITED
Table II.
Vapor-Liquid Equilibrium Data of Ammonia, Carbon Dioxide, and Water System
c.
Partial pressure (mm. of Hg) Otsuka ( 2 ) Krishna ( I ) Exptl.
Component
100
603 105 812 1520
“I
co, - -_ Hz0 Total Obsd.
337 30 362 729
Total Obsd.
Liquid phase mole 7 0 NHI COz 14.5
14.5
258
4.98
690 178 769 1637 1690 395 35 355 790 830
...
80
Table 111.
695 130 795 1620
...
...
RECEIVED for review September 16, 1964. Accepted November 19, 1964.
Vapor-Liquid Equilibrium of Ammonia, Carbon Dioxide, and Water System 1
Temperature 0 c . I / T 103 73.5 69.2 79.2 85.6 89.6 92.3 89.2 89.1
2.889 2.922 2.839 2.789 2.758 2.737 2.761 2.762
5.0
ANALYTICAL CHEMISTRY
.
Total P mm. of Hg P mm. of Hg NH1 COz H2O Calcd. Obsd.
-
764 0 600 0 616 0
222 242 366 468 54,5 574 509 507
0 0 0 0 0 0 0 0
o
293
o o
490 415 593 728 831 948 850 847
0 0 0 0 0 0 0 0
49n
n
211
0 0 0 0
i42 443 356 774 573
433 714 602 916 840
0
179 126 282 454
0 5 5 0
o 0 0 0
0
248 455 367 571 519
0 0 0 0
891 783 1241 1650 2286 1959 1970 994 823 1612 1325 2261 1932
( 1 ) Krishna, hI. G., Ph.D. thesis, Leeds University, England, 1948. (2) Otsuka, E., Yoshimura, S., Yakabe, M., Inone, S., J . Chem. SOC.Japan 6 3 , 1214-18 (1960). (3) Perry, John Hj.; “Chemical Engineering Handbook, 3rd ed., p. 1685, McGraw-Hill, 1950. (4) Van Krevelen, D. W., Hoftixer, P. J., Huntjens, F. J., Rec. Trav. Chzm. 68, 191-216 (11949).
862 741 1074 1382 1635 1794 1614 1604 851 735 1338 1197 1853 1593
Correction Compositional Analysis of n Component Systems by X-Ray Absorption Method In this article by Robert Lefker [ANAL.CHEM.36,1877 (1964)l there are terms which are defined incorrectly in formula ( 1 ) . The correct definitions are as follows: mass absorption coefficient of nth component d = px where p = density of unknown z = thickness of unknown.
k, =
All formulations are correct if these changes in defined terms are applied.
