ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT
Application o ichromator ispersion Analyzer ABRAHAM SAVITZKY AND DONALD R. BRESKY The Perkin-Elmer Corp.,
Norwalk, Conn.
The Bichromator dispersion analyzer i s the result of the desire to transfer current efficient laboratory infrared analyses to the continuous monitoring of process streams. Using the familiar base-line density technique, the instrument measures the ratio of transmission of flowing samples at 2 wave lengths, XI and XZ. The sample wave length, XI,is normally chosen at an absorption maximum. The reference wave length, Xz, can be selected to eliminate interference by other components in the stream which may have overlapping absorption at the sample wave length. An optical null balance is maintained by means of a servo operated wedge. A number of applications to both gas and liquid problems are described. Curves are developed showing sensitivity to the component of interest in the presence of background interference. The problem of sampling liquids in very thin cells i s discussed,
I
N RECEiYT years there has grown up a great body of application of infrared spectrometers to quantitative analysis (1-4). Infrared procedures occupy a considerable segment of the operations in many of the control laboratories which serve the process plants. Current commercially available spectrometers-either the single- or double-beam recording instruments or single-beam point reading instruments employing a densitometer-are designed to meet the needs of the wide variety of problems encountered in such work. Control laboratory operations are necessarily on an intermittent basis. Even if reserved for a single analysis, batch sampling, lag, and the need for an operator are ever present problems. For many modern processes neither a sampling lag nor the possibility of erroneous information obtained by intermittent sampling can be tolerated. Laboratory spectrometers do not readily lend themselves to continuous operation for many reasons (6). The explosion hazard is a basic one. The necessity for intermittent standardization, the size, and unnecessary complexity are others. There is a need for an instrument designed for continuous operation on a single analysis and not subject to the difficulties mentioned which operates on the principle of a current control laboratory technique. The instrument manufacturer could then utilize directly information obtained from control laboratory instruments in current use on the analysis to provide a continuous sampling instrument preset for the particular problem. The purpose of this paper is to describe briefly a new instrument, Ihe Bichromator (trade-mark Perkin-Elmer Corp.) analyzer, which meets this need. I t is essentially a single-beam instrument using the dispersion principle but analyzing a t two wave lengths simultaneously to give an automatic continuous measurement of a ratio according to the widely used base-line density technique (6, 8, 9). This method of taking the ratio of transmissions a t a sample wave length and at a reference wave length produces an indication of sample concentration relatively independent of cell window fogging, source emission variations, 3s well as changes in the optical path which can adversely affect the operation of double-beam type analyzers. Use of this single-beam method not only simplifies the instrument optically but also allows compensation for variable interference owing to a second component which also absorbs at the sample wave length. This compensation is
1382
achieved by selecting the reference wave length so that the absorptivity of the interferent there is equal to that a t the sample wave length; absorption being exponentially dependent on concentration, the ratio of transmissions then remains a function of the sample concentration only. Designed for continuous in-plant operation, the Bichromator analyzer makes use of the optical null principle to eliminate the effects of gain changes in the electronic amplifiers, source deterioration] and fogging of optics. Rugged, nonadjustable, interchangeable optics are used. Flexibility in selection of spectral range of operation is provided by interchange of detector, prism, windows, and source to meet particular problems. Provisions for close temperature control, an inert gas purge, and explosionproof housing provide for operation under severe in-plant conditions.
Instrument Description In a conventional Littrow-type spectrometer, one has an entrance slit, then a parabolic mirror to produce a parallel beam of radiation which passes through a prism to a so-called Littrow mirror. The Littrow mirror reflects the radiation which then passes back through the prism once again to the parabola. A small portion of the xave length spectrum, as determined by the angle of the Littrow mirror and the dispersion of the prism, passes out the exit slit to the detector. Figure 1 is a schematic drawing of the over-all optical layout. The dis ersing unit which selects the analytical and reference wave lengtfis is a t the upper
Figure 1.
Optical Schematic Diagram of Bichromator Analyzer Optical System
Sample cell is placed in the space mnrked "slit image"
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46,No. 7
PROCESS INSTRUMENTATION
Figure 2.
left. This Bichromator analyzer dispersion unit differs from the usual Littrow spectrometers on which it is based in having two Littrow mirrors, one above the other, set a t different angles. The angles of these mirrors determine which two wave lengths will fall on the exit slit and be seen by the detector. The two selected wave lengths while separated in space a t the Littrow position are superimposed a t the slits and the detector. The mirrors external to the dispersing and detecting unit form images of the split Littrow mirror a t the mask and a t the chopper and images of the source in the sample space and on the entrance slit. The chopper is so slotted t h a t radiation reaches first one Littrow mirror and then the other on alternate half cycles of the 13-cycle chopping frequency. Thus, the alternating detector output is a measure of the difference in energies a t the sample wave length and a t the reference wave length. Difference in absorption of radiation between the sample wave length and the reference wave length appears as a fluctuation in the energy reaching the detector that is changed to an electrical signal, amplified, and fed to a servoo erated attenuator or optical wedge situated a t Cutaway Drawing of Bichromator Analyzer t i e mask position. The attenuator is always driven to restore the balance of the energies a t the two wave lengths so that the detector output is zero. The position of the attenuator is a measure of the energy absorbed a t the sample wave length relative to the reference and is therefore a measure of the amount of sample present. The attenuator position is transmitted by a separate electrical circuit to the recorder.
10 CM. CELL
L
I
3.5 3.25p
Figure 3.
100
Single-beam, l o w Resolution Absorption Spectra of Ethylene and Methane
-
A s = 3.3p A R = 3.1p
90-
ao ..
20 CM. C E L L 100 901
70.a 6050-
2
P L U S ETHYLENE:
8 40-
'"if;
30-
20.0
2 A L l IN
1'
A
"1 '1
As METHANE
2 0 CM. CELL
70
-"
-
I
,,ETHYLENEN E
10.1
PERCENT METHANE
Figure 4. Calibration Curves for Analysis of 0 to 5% Methane in Ethylene Plus Nitrogen Mixtures
-
O 10
3.3p
A, = 3 . l p
90%
- 10
July 1954
Since this is an optical null system, changes in gain of amplifiers, source intensity, and window fogging have little or no effect on the instrument reading. The sample cell is a t the slit image position. The chopper is deliberately placed between the source and the sample cell in order to make the instrument independent of sample temperature. The entire installation meets Class One, Group D specifications of the National Electric Code. The electronics assembly is mounted in its own explosion-proof case separate from the measuring system. Figure 2 is an artist's cutaway of the complete instrument in its explosionproof case. The dispersing unit, at the left, is separately housed and mounted on kinematic supports. The wave lengths may be set either on the instrument by using i t as a single-beam spectrometer or by projecting a monochromatic beam from a single-beam laboratory spectrometer to the entrance slit of the small unit. I n the latter case, the Littrow mirror is rotated by hand to give maximum output from the detector. The two wave length settings can then be checked by scanning the laboratory monochromator in wave length. The Littrow mirrors are securely clamped in position. The instrument, except for the sample cell compartment, is separately enclosed, with a clean atmosphere. This is especially
-
Y
2 00
. 30
I
40
50
60
70
00
90
PERCENT G A S
Figure
5.
Effect of Ethylene Interference on Methane Calibration
INDUSTRIAL AND ENGINEERING CHEMISTRY
1383
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
20 CM CELL I S O - PENTANE
ANE
GAS PRESSURE CM. H G
8.Sp
9.5p
Figure 6.
70
t
Figure 7. Calibration Curve for 0 to 10% lsobutane Compensated for Butane but Not for lsopentane Interference
I
I
Low Resolution, Single-beam Spectra of Isobutane, n-Butane, and lsopentane
A R 2 = 9.5j.L
,/lSO-BL ITANE
IO
20
30
2 0 C M CELL
40
50
-/
/ /‘So-PENTANE , 1
60
70
80 d
GAS PRESSURE CM. H G
\B u TA NE
Figure 8. lsobutane (0 to 10%) Compensated for lsopentane and Butane Interference
important since a sample can have the same effect anywhere in the optical path. The sample cell ( a 20-cm. cell is shonnj is essentially outside the instrument proper, readily accessible without disturbing the main compartment, and still totally enclosed. The sample cell chamber is large enough to allow for heaters or thermal lagging around the cell if necessary. The attenuator can be seen just behind the mask. It operates in the reference half or the mask. The chopper assembly and ~ o u r c eare a t the right. The source is a small, platinum wirewound heater element enclosed in a stainless steel shell. The scale of the dispersing unit is approximately half that of laboratory monochromators such as the Perkin-Elmer Model 83-focal length of parabola, 13.3-cm. aperture f/4.1. 411 optical elements are prefocused and cemented to their mounts to avoid the necessity for optical alignment in the field. Simplification over the laboratory-type instrument is also ohobtained by using fixed slits etched in a thin copper sheet. A selection of various slit widths is available to meet the demands of the analytical problem.
sirable that the spectra be run Kith resolution comparable t o that of the Bichromator unit. From examination of these spcctra, one can predict the sensitivity of the instrument for the sample, the sample thickness required, the probable effect of interfering components, and the instrument operating settings. The inst,rument settings include the sample and reference wave lengths, slit widths, preferred Kindov materials, and attenuator taper. The attenuators used in this set of problems handled transmission changes of 0 to 10%; 0 to 40%, and 0 to 100% fullscale, all with tapers linear in transmission. Where t’heanalysis is now being done in the control laboratory, much of this informat’ion is already available. Methane in Ethylene. The first example illustrates an cstreme caw of the interference of ethylene in the analysis of methane-nit,rogen mixtures. The absorption spectra arc s11oc.n in Figure 3. I n equivalent concentrations, ethylene is a considerably stronger absorber than methane. At the absorption peak of t,he methane band, ethylene transmits 61.2% of the radiation. The reference wave length, X R , is chosen wliert: methane absorbs considerably less than ast the peak, and \There ethylene again absorbs 61.2y0 of the radiat,ion. The best dispersing prism and Kindow material for this x a r e length region is calcium fluoride. The calibration curve for the analysis of 0 to 5% methane in a 20-em. cell is shown in Figure 1. .4 40% attenuator was uscd giving the noise level a t the upper right for a period of l / g hour between marks. The wave lengths viere set on a monochromator and then touched up to give no interference with 20% ethylene in the cell. For low concentrations of methane, the reading for lOy0 et,hylene is different from those for 50 and 90% ethylene. At the higher concentrations of methane, there is little difference between 10 and 50% ethylene, but 90% ethylene affects the curve.
t 0 v)
‘D t v)
Application
4
iLT
Four rather general problems have been selected to illustrate the application of the instrument to the analysis of both gaseous and liquid streams. The first requirement, in any case. is a set of representative spectra, preferably of the pure materials. I n general, it is de-
1384
6
7
8
9 IO WAVELENGTH
-
ll 12 MICRONS
13
14
15
Figure 9. Double-beam Spectra of Commercial Unhydrogenated and Hydrogenated Vegetable Oils
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. T
PROCESS INSTRUMENTATION Figure 5 further illustrates the effect of ethylene on the instrument reading. The effect of ethylene on the 1 % methane is primarily due to the pressure broadening of the methane band, which is different for ethylene than for nitrogen. The lower curve, ethylene with no methane present, illustrates that the relative transmission of the ethylene a t the sample snd reference wave lengths does not remain constant. This apparent deviation from Beer's law is probably due to the rather wide slits employed so that the radiations a t the two wave lengths were not monochromatic. In a practical problem where the range of interfering concentrations is defined the reference wave length can be reset to minimize this effect. Isobutane in Butane and Isopentane. This problem occurs in the measurement of the streams from a deisobutanizer column. For the purposes of this discussion the problem has been generalized for intermediate concentrations of isobutane in wide fluctuations of butane and isopentane. The spectra of the three materials obtained on a single-beam spectrometer are shown in Figure 6. The slits of the spectrometer were opened to yield the same spectral slit width as the Bichromator analyzer in this region. The desired analytical band of isobutane is a t 8.5 microns. Here, there is very little interference by butane but considerable overlap from an isopentane band. In this case, it is not possible to use for reference a position on the long wavelength side of this band having the same transmission as a t 8.5 microns, since this would produce extreme interference by n-butane. The instrument was first set up for mixtures of isobutane and n-butane only, using XRI a t 9.5 microns. There was only slight compensation for isopentane overlap here. With isopentane mixtures the reference wave length was shifted to A82 where there is complete compensation a t 53% isopentane but some extra overlap by n-butane.
0.047 MM. CELL
v 40
MODEL 93 E I C H RO M AT0 R A N A LY Z E R
RELATIVE % HYDROGENATION Figure 10. Predicted and Experimental Calibration Curves for Hydrogenation of Edible Oils
Figure 7 shom the calibration curve using a 4Oy0 attentuator for 10% isobutane full-scale base on RI as the reference wave length. The noise level is shown for '/z hour between marks. n-Butane interference is very low, but isopentane interference is what m s anticipated. Changing the reference to h ~ 2resulted in the calibration curves of Figure 8 for the particular compromise conditions selected. Hydrogenation of Edible Oils. The hydrogenation of edible oils is usually a batch process using a nickel catalyst under conditions of 20 to 40 pounds per square inch and 175" to 190" C. Under these conditions the hydrogen tends to add to the more unsaturated triglycerides. The desired products are transisooleic acid triglycerides. The usual starting materiale-cottonseed oil, soybean oil, or corn oil-have essentially the mme infrared spectra. A typical spectrum is the upper curve of Figure 9. The trans region in the '
July 1954
I I OP
I
II
AR
as
12
I
1
14
IS
WAVELENGTH
Figure 1 1. Double-Beam, Low Resolution Spectra of 0 - , rn-, and p-Xylenes, and Ethylbenzene in Carbon Disulfide Solution
vicinity of 10.3 microns is clear of sharp bands. Hydrogenation produces only one marked change in the spectrum-the appearance of the prominent trans band at 10.3 microns shown in the lower curve of Figure 9. The progress of hydrogenation can be followed easily and accurately. In laboratory analyses, accuracy is i l y o of the amount of the actual isooleic acid present ( 7 ) . We have run liquid phase mixtures of cottonseed oil and Crisco on both the Model 112 single-beam laboratory spectrometer and the Bichromator analyzer, using a 0.047-mm. cell under static conditions. Wesson oil was taken as 0% and Crisco as 100% hydrogenation. The wave lengths were set up with the spectrometer and used on the Bichromator analyzer without further adjustment. Figure 10 shows the calibration curves on the two instruments. The one marked Model 112 is a normalized curve predicting the performance of the Bichromator analyzer; the other is the actual Bichromator analyzer calibration. The slight difference in the two curves is attributable to the use of a somewhat wider spectral slit width in the Bichromator analyzer. The noise level is 3 ~ 0 . 5 %of full scale. A rock salt prism and cell were used for these measurements. In actual application, we would still use the rock salt prism, but barium fluoride windows on the cell are preferred in this spectral region. p-Xylene in Mixed Xylenes and Ethylbenzene. An example of a more complex liquid system than the two-component mixture just described is the analysis of p-xylene in the presence of the other xylene isomers and a common solvent, ethylbenzene. Figure 11 shows the spectra made on a double-beam spectrometer of the individual components in carbon disulfide solutions a t the maximum concentrations. A salt window was placed in the reference beam of the spectrometer. The spectrometer slits were opened to yield resolution equivalent to the Bichromator analyzer with I-mm. slits. The choice of sample wave length is obvious. o-Xylene will apparently not interfere. Both ethylbenzene and m-xylene have about the same transmission a t this wave length but note that the concentrations differ. Although the m-xylene appears to have some ortho and para impurity, it was treated as though it were pure m-xylene. The para component was treated similarly, although it has considerable meta and some ortho impurity. A reference wave length must be found at which there is no absorption by o-xylene, and both m-xylene and ethylbenzene have the same per cent absorption as a t the sample wave length. The best compromise that can be made is the choice of X R a t 11.1microns.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1385
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT PROCESS
PARA-XYLENE I N INTERFERING MIX
70
STREAM
w
r CAPILLARY -WINDOWS
41"t?t */L
SPACER
Il j
-"I
!I
I/
S A M P L E CELL
FILTE
1
-
i
I
io
20
30
40
50
60
70
80
100
90
PER CENT
Figure 12. Calibration Curves for p-Xylene in Aromatic and Carbon Disulfide Mixtures Effect on instrument of filling cell with 100% o f interferents is shown
0 Low, Ethylbenzene m-Xylene o-Xylene
%
10 35 20
V High,
%
25 40 30
With these rvave-length settings the calibration curves obtained Bichromator analyzer are shoml in Figure 12. The 0 033mm. cell was filled with each of the pure interfering components to give the points a t the lower right. Since the interfeiences could not be perfectly compensated by selection of reference wave length, the curve of p-xylene in carbon disulfide is displaced ielative to the curve of the p-xylene in the aromatic mixtures, but the net interference is small through the range of interest. The curvature of the calibration curves if due to the eqmnential nature of the absorption. These materials absorb very strongly even in the thin cell used here. For this analysis sodium chloride was used for both prism and cell windows. 011 the
Analysis of Liquids The greatest question in the application of these instruments for the analysis of liquid streams is the sampling problem. Cont,rary to the situation in gases, where cells may vary froin 1 to 20 em. in length, liquids usually require cell thicknesses of 1 mm. and leas. In the two examples cited above, cells of 0.047and 0.033-mm. thickness were used. For the xylene analysis, a cell of 0.020 mm. rrould be preferred. The infrared sprct'roscopiet who knows horv long it takes t o force sample through these cells v-ith a 1-cc. syringe will usually claim that cont,inuous sampling is impractical under these conditions. A detailed study of the problem shows otherwise. The flow of cottonseed oil n-as studied using standard laboratory sealed cells which were 0.026 mm. thick. These are made up by sandwiching a 0.001-inch silver spacer betiveen sodium chloride windows. The opening in the spacer exposed to the beam is 1 x 2.6 cm. One of t8hesalt iyindows has two 0.030-inch diameter holes drilled a t top and bottom t o permit sample flow through the interior of the sandwich. The channel cross section doivn which the sample flows is 0,0026 sq. cm. and the cell volume exposed to the beam is 0.007 cc. A sample of cot,tonseed oil at' room temperature, viscosity 50 ccntipoise, was run through under 15 pounds per square inch pressure differential across the cell. The measured f l o was ~ 1 to 1.5 cc. per hour. This apparenily d o w rate of flop7 is actually a flow velocity through the cell of over 6 em. per minute, or 2.5 t o 4 complet,e changes in cell content per minute. Heating this particular sample to 85" C. would lower its viscosity t o 10 centipoises, increasing tha flow rate by a factor of 5 or allowing the pressure differential to drop a factor of five. The flow through a similar cell was increased to
1386
--SAMPLE TAKEOFF /4 T J B I V G
1
T O WASTE
Figure 13. Schematic Drawing of System for Rapid Sampling of Liquids in Thin Infrared Cells Spacers m a y be as thin as 0.0005 inch (0.013 mm.)
2.3 cc. per hour by shaping the inlet and outlet clianiiels of the cell for a less abrupt change in cross section in going from the 0.030-inch inlet hole to the sandwich opening. This last cell passed 35 cc. per hour of carbon tetzachloride, which has a much lower viscosity, under 4.5 pounds per square inch different,ial. The exchange of material in the cell is fast enough for the usual plant instrument time constant, but this is only for the cell itself. The volume flow of the cottonseed oil is st,ill very low when it is considered that material must be brought from the process stream, through a filter, to the cell, perhaps 25 feet, preferably in a time shorter than the time of passage through the cell. Considering 'I4-inchtubing for a 25-foot sampling run from process t o instrument', the flow must be 84 liters per hour for a 6-second sampling lag. (About 1 quart per minute.) Figure 13 shows a schematic draw-ing of a system which will give minimum sampling lag. Since thc bypass line returns to the process stream, a very high sample flow can be run past the cell. The small amount which goes t>hroughthe cell is preferably led to v-aste. The lead-in channel in the drilled rindow is shaped t o spread the sample orer the illuminated area and to minimize channeling. The cell is placed a t the intermediate slit image. Conclusions
The Bichromator a,nalyzer can bridge the gap from the control laboratory to conl~ir~uous analysis in the process plant. Application of the instrument to particular problems requires a knowledge of the spectra of the materials which make up the stream. Because of the ability to select specific Tvave length intervals, performance of the instrument is readily predicted. For a single interfering component it should always be possible to find a coinpeneating reference n-ave length. h compromise compensation IS often possible for more than one interferent. The instrument appears to be especially suit,ed to the analysis of liquid systems. h cell and sampling system have been proposed which allow rapid sanipling of liquid etreanis. Literature Cited (1) Barnes, R. B., and Gore, R. C., Bnal. Chcm.. 21, 7 (1949). fa) Gore, K. C.. I b z d . , 22, 7 (1950). ( 3 ) I b i d . , 23, 7 (1961). (4) Ibid., 24, 8 (1952). (6) FJeigl, J. J., Bell, 1'1.F.. and White. J. U.,Ihid., 19,293 (1947). (6) Herscher, L. W., and Kright, S . , d . Opt. SOC.Amer., 43, 980 (1953). ( 7 ) Jackson, F. L.. and Callen, J. E., J . -4m.Oil Ckem&s' Soc., 28, G1 (1951). ( 8 ) White, J. U., Liston, XI. D., and Simard, R. G., Anal. Chem., 21, 1166 (1949). (9) Wright, N., IND. FAT:.CRPX.,AXAL.ED., 13, 1 (1941). RECEIVEDfor review September 7 , 1953.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDDecember 11, 1953.
Vol. 46, No. 7
