I
L. G. GLASSER and D. J. TROY Engineering Research Laboratories, E. 1. du Pont d e Nemours & Co., Inc., Wilmington, Del.
Refractometer for Continuous Process-Stream Analysis \
Recent extending of its accuracy and usefulness makes this phase of refractometry applicable to automatic process control
M E A S U R E M E N T of refractive index is an excellent way to determine the composition of two-component liquid mixtures, but until recently available refractometers were solely for laboratory use. For in-process analyses a refractometer must be simple to build, easy to maintain, as sensitive as the best laboratory refractometer, and above all, unaffected by variations of sample temperature. A continuous automatic refractometer that meets these requirements has been developed (6) and applied successfully to in-process analyses. Most automatic refractometers employ photoelectric measurement of the angular deviation of a beam of light refracted by the material to be analyzed.
A grief study of variables showed that a practical continuous refractometer for flowing liquids could detect a refractive index change as small as 0.000004. This corresponds to &O.Ol% or less change in the composition of many solutions. As a 1 C. change in sample temperature, however, causes a refractive index change 100 times larger (0.0004), temperature effects must be completely compensated. Temperature-Compensating Sample Cell
Consideration of temperature compensation and sensitivity led to the choice of a refractometer optical system
Sample Previous Automatic Refractometers Spectrometer with hollow prism through which sample flows. No temperature compensation ; sample temperature controlled to 0.02' C. to take advantage of high sensitivity to refractive index changes (0.00001) (4) Similar optical system. Electrical temperature compensation to give sensitivity of 0.0008, independent of temperature changes (8) Temperature compensation by substituting for prism double sample cell with oblique interface to measure refractive index difference between sample and chemically similar reference liquid maintained at thermal equilibrium by circulation through heat exchanger. Limit of detection 0.0002 (10) Photoeldctric measurement of refrac- (0,3, tion angle 11-13)
I nc ide nt Bearn
described by Anderson in 1920 (7). The sample cell is a cylindrical stainless steel container with glass windows on t.wo opposite faces. Mounted inside is a second stainless steel container, also cylindrical but with oblique ends, containing a reference liquid; in the end walls of this container are windows, each making an angle of 45' with the optical path but tilted in opposite directions (Figure 1). The incident beam of light is deviated in one direction, when the refractive index of the sample exceeds that of the reference; it is deviated in the other direction, when the reverse is true; and it is undeviated if the indices are equal. The angular deviation (in radians) is twice the
out
1 Beam
-
___c
.Angular Deviation
Tube Reference Liquid/ Container
L Sample in
Figure 1. High-sensitivity cell shown (cutaway view) compensates for change of sample temperature VOL. 50, NO. 8
AUGUST 1958
1149
A
R e f e r e n c e cell
Dividing prism
Collimating lenses
Bridge and
Figure 2. The coniinuous refractometer optical system resembles a prism spectroscope
used for larger spans has only one oblique face, making a n angle of 15" with the light beam; the same refractive index difference produces an angular deviation 0.13 times that of the other cell. The thermal lag is 4 times as long ( 2 minutes), but this cell is normally used for larger spans, so its practical performance is just as satisfactory, The low-sensitivity cell was designed for good performance on samples of low transmittance. A re-entrant window at one end reduces the average light path in the sample stream to about 0.25 inch, so that process streams containing highly colored impurities can be analyzed, if a colorless reference liquid is available Both cells have been used at ;emperatures up to 70' C. and pressures up to 150 p.s.i. Operation at higher temperatures or pressures is possible with suitable precautions to prevent leaks and to minimize heat transfer to the refractometer.
Refractometer difference in refractive index. This cell compensates for change of sample temperature, if the sample and the reference liquid are always in thermal equilibrium. T o maintain this equilibrium while continuously sampling a process stream, the reference liquid must reach sample temperature very quickly after a change occurs; the thermal lag must be very short. The form of cell shown in Figure 1 was developed for this purpose. The container for the reference liquid is made as small and as low in heat capacity as the required optical aperture and mechanical considerations permit. I t is completely immersed in sample liquid and is unconnected to the rest of the cell except by the metal tubes used for filling. Moreover, sample connections are arranged so that the incoming sample liquid makes thermal contact with the metal body of the ref-
Figure 3.
1 150
erence container before entering the optical path. The volume of the reference liquid is 10 ml.; that of the sample is 90 ml. This cell gives excellent compensation for temperature changes. At the usual sample flow rate, 200 ml. per minute, the temperature of the reference liquid lags a changing sample temperature by less than 0.5 minutc. The difference between the refractive indices of aqueous solutions can be measured to f0.000004, in spite of sample temperature changing at 5" C. per hour. When less sensitivity is needed, proportionately more rapid temperature changes are permissible. With this sample cell, any full-scale span of refractive index from 0.0004 to 0.0100 may be selected by proper choice of the other components of the refractometer. A low sensitivity cell
The refractometer i s of simple but robust construction
INDUSTRIAL AND ENGINEERINGCHEMISTRY
The refractometer built around the Anderson cell resembles a prism spectroscope (Figure 2 ) . Light from an illuminated slit is collimated, passes through the sample cell, and is refocused on the target, a prism-shaped mirror that divides the light between two phototubes. For compactness, mirrors and a Porro prism fold the beam in several places. The sample cell is mounted on a projection of the instrument external to the case to afford greater latitude in the size and shape of the cell; the cell may even be located a t some distance, or behind a barricade. Moreover, leaks of corrosive or flammable samples will not damage the instrument, and very hot or cold samples cannot distort the base plate or cause instability in the electronic components. The cell can be serviced without turning off power or exposing electrical circuits to the plant environment; it can easily be removed and reinstalled in the correct position without recalibration of the instrument; orientation to 1 1 ' is adequate. The refractometer is pictured in Figure 3 with the optical system withdrawn from the case to show the simple but robust construction. The deviation of the light beam refracted in the sample cell is measured by an optically null-balanced photometer that keeps the beam centered on the target. The sensitive photometric circuit recently described by Glasser (5) is used. The smallest full-scale deviation changes the intensity ratio at the phototubes by 60y0,,so photometry is not critical. Optical null-balancing provides a calibration linear in refractive index and independent of both the
REFRACTOMETER characteristics of the photometric circuit and the optical quality of the refracted image. Suspended matter in the sample, or nonuniformity of sample composition or temperature, can cause poor image quality, but null-balancing minimizes its adverse effects. The beam displacement a t the target is the product of the angular deviation and the lens-to-target distance (12 inches in this instrument). At the smallest measured refractive index difference the beam displacement is 0.0001 inch. The most satisfactory means of keeping the refracted beam centered on the target with this degree of sustained accuracy proved to be a tiltable glass “balance plate” (Figure 2), that laterally displaces the beam in proportion to the tilt. When the beam moves off the target, the amplified output of the phototube circuit drives a servomotor coupled to the balance plate through a reducing-gear train to recenter the beam. The plate rotation is restricted to f30°, so that the displacement of the beam is linear with rotation to within 1% of the maximum displacement, which is then about one quarter of the plate thickness. A voltagedividing resistor coupled to the plate transmits plate position, and thus refractive index difference, to a remote recorder. If the resistor is supplied with a constant direct voltage, available millivolt-actuated potentiometric recorders may be used. The desired fullscale span is set by choosing the proper combination of sample cell and balanceplate thickness (0.040 to 1 inch). The tiltable balance plate permits extremely precise measurement without critical construction or operation. Setting a plate 0.040 inch thick only to =!=O.Go measures beam displacement to f O . O O O 1 inch and refractive index to rt0.000004. Further improvement is easily possible but the usual inhomogeneities of flowing liquids make higher sensitivity impractical in a process refractometer. A tiltable “zeroing plate” is used to set the zero of the instrument manually by initially centering the beam on the target. This adjustment compensates for any slight lack of parallelism of the end windows bf the sample cell or for differences in refractive index between sample and standard; it also provides zero-suppression in multirange analyzers. For this last purpose the zero-adjust mechaniem includes a precision gear train and 1000-digit counter, that permits setting the angle of the zeroing plate to 1 part in 2000 over a n angular range of 2 ~ 4 5 ’ . The zero can thus be reset to any predetermined value of refractive index over a range of 10 times full scale. This feature is particularly useful for processes that blend
1.34 5 0
1.3400
1.3 3 5 0
\ 1.3300
0
20
Methanol
40
60
100
00
Concentration, wt.
O/O
Figure 4. Refractive index of water-methanol mix’tures shows large departure from linearity
two components in several different compositions. Applicability to In-Process Analysis The refractometer is normally considered for any liquid analysis where the concentration of the component sought exceeds O.l%, if the stream approximates a binary mixture. At very high concentrations instruments measuring density or differential vapor pressure, though less sensitive, may sometimes
give the required precision more economically. The sensitivity of the refractometer for many aqueous solutions and organic mixtures is excellent (Table I); 30 p.p.m. is usually detectable. The refractometer is less specific and sometimes less sensitive than infrared analyzers, but unlike them, it can be applied to the analysis of aqueous solutions. The analysis of methanol-water mixtures illustrates a possible pitfall in applying refractometers : sensitivity may
Figure 5. A weatherproof refractometer assembly i s satisfactory in exposed outdoor locations wiih no control of sample temperature VOL. 50, NO. 8
AUGUST 1958
1 15 1
be much higher or lower than indicated by rough estimates. Linear interpolation with available data indicates a minimum possible span of 10% methanol in water. Figure 4 shows how great is the departure from linearity (7); in the high-methanol range, the performance is 12 times as good as estimated, but near 50% methanol, a refractometer is not applicable. Nevertheless, the handbook values of refractive indices are useful for initial cautious estimates of applicability. For many materials, such as liquefied gases, which are liquid only at extremes of temperature or pressure, published values of refractive index are not available. The Lorentz-Lorenz equation ( 9 ) (n’ - l ) / ( n 2
+ 2)
=
R,p/M
can then predict refractive index, n, and thus refractometer applicability, from the density, p, and molecular weight, M, of the substance and its molar refraction, R, a property depending on its constituent atoms. A nomograph relating (nz - l ) / ( n z 2) to n facilitates the computation. Occasionally, the choice of the reference liquid presents problems. A specimen of the sample of precisely known composition, either the nominal or the midscale value, is preferred. The temperature coefficient of refractive index is then the same for sample and reference, and false concentration readings cannot result from equal changes of sample and reference temperature. When only moderate sensitivity is required, however, the reference material may have a composition substantially different from the sample. The reference liquid must be com-
+
Table I. Sensitivity of Refractometer Is Excellent for Many Aqueous Solutions and Organic Mixtures
Minimum Full-scale
Analysis Water In acetic acid In ethanol In methanol Ethanol In water I n benzene
Methanol in water Ethylene glycol in water Propylene glycol i n water Glycerol in water Acetone in water Benzene I n ethanol In cyclohexane Cyclohexane in benzene Trichlorofluoromethane in dichlorodifluoromethane Sodium chloride in water Ammonium sulfate in water
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pletely stable for weeks or months; a stagnant sample of the process stream may not meet this requirement. A substitute reference liquid then must be found to match the process stream closely in temperature coefficient of refractive index and to match it approximatelyi.e., within 0.02 unit-in refractive index itself. When the temperature coefficient is not known, a useful relation d n / d T = 1.13 ( n
-
l)y
derived from the Lorentz-Lorenz equation, can approximate it from the refractive index itself and the coefficient of cubical expansion, y . This approximation is accurate enough for screening a large number of liquids, so that only the most promising need be evaluated experimentally. The final selection is frequently a mixture, so that the composition may be adjusted for the desired temperature coefficient. The auxiliary equipment required with the refractometer includes a valve and a rotameter for setting and measuring flow and shut-off valves for isolating the instrument from the process. A filter is desirable to remove suspended solids. A tempering bath may be needed if the sample temperature fluctuates very rapidly; several feet of sample tubing coiled in a water bath is usually adequate. Sometimes the sample comes from a process vessel whose temperature can change only slowly. Here fluctuations in sample temperature are best reduced by insulating the sample lines. When the refractomerer is used at the minimum span, fluctuations in sample pressure may not exceed 2.5 p.s.i., but at larger spans this tolerance is proportionately broadened. The effect of pressure on refractive index can be conveniently estimated by a modification of the equation previously given for the temperature coefficient; thus d n / d p = -1.13 ( n - 1),8
where fl is the compressibility.
Span,
Wt.
70
0-0.40 0-1.07 0-0.79
0-0.59 47-50 0-0.26 0-1.78 0-0.42 0-0.37 0-0.33 0-0.58 0-0.31 0-0.75 0-0.40 0-0.36 0-0.41 0-0.28
Typical Uses of Refractometer The convenience with which span and zero may be reset makes the refractometer particularly suitable for controlling blending operations, where two liquids must be mixed in several different ratios. An example is the blending of Freon fluorinated hydrocarbon aerosol propellants. Trichlorofluoromethane(F11) and dichlorodifluoromethane (F-12) are desired in a variety of mixtures, such as 70-30, 60-40, and 50-50, with a close control of composition. The 50-50 mixture is used as a reference liquid, and the full-scale span is set at 10% by weight. By adjusting the zeroing plate to predetermined settings, any desired composition can be
INDUSTRIAL AND ENGINEERING CHEMISTRY
made to indicate midscale, and the composition can be accurately measured to =tO.l70. Refractometers perform this analysis satisfactorily in exposed outdoor locations with no control of sample temperature. Figure 5 shows the weatherproof assembly of refractometer and sampling system used for this service ; the recorder is remotely located. Table I shows typical analyses the refractometer can make, although it has not yet been used for all of them. An application that is especially useful in chemical processing is the measurement of total dissolved organic compounds in water. The minimum fullscale span is approximately the same for a number of organic compounds in water, particularly for larger molecules. Frequently the refractometer span is identical for a group of chemically similar compounds-for example, aqueous solutions of sugars and related substances for which the minimum refractometer span is 0.28% solids; aqueous solutions of many other groups of compounds behave in this way. As a result, several of these refractometers are being used to measure total dissolved organics in water. The refractometer can monitor a concentrating process such as an evaporation, or, in combination with a more specific type of analyzer measuring the concentration of a single reactant or product, can continuously indicate the degree of conversion or the yield. literature Cited
(1) Anderson, J. S., Trans. O$t. Sac., London 22, 156-60 (1920). (2) Campbell, D. N., Fellows, C. G., Spracklen, S. B., Hwang, C. F., IND. ENG. CHEM. 46, 1409-12 (1 054) \ - - - ’/.
( 3 ) Claesson, S., Arkiv. Kemi Mineral. Geol. 23A, NO. 1, 1-133 (1946). (4) Forrest, J. W., Straat, H. W., Shurkus, A. A,, Control Ene. - 2,. No. 11, 103-5 (1955). (5) Glasser, L. G., J. Ofit. SOC. Am. 45, 556-63 (1955). (6) Glasser, L. G., U. S. Patent 2,612,814 (1952). (7) International Critical Tables, vol. VII, p. 66, McGraw-Hill, New York, 1930. (8) Johnsen, S. E. J., Schnelle, P. D., Rev. Sci.Znstr. 24,26-35 (1953). (9) Lange, N. A,, “Handbook of Chemistry,” 8th ed., p. 1421, Handbook Publishers, Sandusky, Ohio, 1952. (10) Miller, E. C., .Crawford, F. W., Simmons, B. J., Anal. Chem. 24, 1087-90 (1952). (11) Thomas, G. E., O’Konski, C. T., Hurd, C. D., Ibzd., 25, 1221- 3 (1950). (12) Trenner, N. R., Warren, C. W., Jones, S. L., Ibzd., 2 5 , 1685-90 (1953). (13) Zaukelies, D., Frost, A. A., Ibid., 21, 743-5 (1949).
RECEIVED for review May 2, 1957 ACCEPTED February 27, 1958 Instrument Society of America, New York, N. Y., September 1956.
