CARS Spectroscopy of Ammonia in the Laboratory and a Commercial Scale Ammonia Oxidation Plant

37. W. B. DeLauder and P. Wahl, Biochem. Biophys. Res. Commun. 42, 397 (1971).

38. A. G. Szabo and D. M. Rayner, J. Am. Chem. Soc. 102, 554 (1980). 39. I. Munro, I. Pecht, and L. Stryer, Proc. Natl. Acad. Sci. U.S.A. 76,

56 (1979). 40. R. GuiUard and J. H. Ryther, Can. J. Microbiol. 8, 229 (1962). 41. M. R. Droop, Brit. Phycol. Bull. 3, 295 (1967). 42. M. J. Kronman and L. G. Holmes; Photochem. Photobiol. 14, 113

(1971). 43. A. Grinvald and I. Z. Steinberg, Biochemica et Biophysica Acta

427, 663 (1976). 44. S. S. Lehrer, Biochem. Biophys. Res. Commun. 29, 767 (1967). 45. F. W. J. Teale, Biochem. J. 76, 381 (1960). 46. S. Udenfriend, Fluorescence Assay in Biology and Medicine (Ac-

ademic Press, New York, 1962), Vol. 1; ibid., Vol. II (1969).

47. A. L. Lehninger, Biochemistry (Worth Publishers, New York, 1977), 2nd ed.

48. F. W. J. Teale and G. Weber, Biochem. J. 65, 476 (1957). 49. S. V. Konev, Fluorescence and Phosphorescence of Proteins and

Nucleic Acids (Plenum Press, New York, 1967). 50. D. Duggan and S. Udenfriend, J. Biol. Chem. 178, 53 (1949). 51. H. Edelhoch, V. Frattali, and R. F. Steiner, J. Biol. Chem. 240,

122 (1965). 52. L. J. Andrew and L. S. Forster, Photochem. Photobiol. 11, 353

(1974). 53. C. Conti and L. S. Forster, Biochem. Biophys. Res. Commun. 57,

1287 (1974). 54. J. R. Lokowicz and G. Weber, Biochemistry 12, 4171 (1973).

CARS Spectroscopy of Ammonia in the Laboratory and a Commercial Scale Ammonia Oxidation Plant*

W I L L I A M A. E N G L A N D t and A N S A R ALI$ Engineering Sciences Division, Building 551, Harwell Laboratory, Didcot, Oxfordshire, 0X11 ORA, U.K.

CARS spectra of NH3 have been obtained from a commercial catalytic ammonia oxidation plant operating at ~ 10 atmospheres (absolute) and 250°C. A purpose-designed rnggedized CARS spectrometer--incorpo- rating a novel dye laser, remote control, and fiber optic signal transfer-- is described. A new and simple technique for CARS concentration analysis using polarized beams gave a precision of 5.9% for NH3 from single- shot spectra under laboratory conditions. Unfortunately, severe beam steering problems were encountered in the full-scale plant; this prevented a quantitative analysis of the spectra obtained. The origins of this problem together with suggestions for overcoming or reducing its effect are given.

Index Headings: CARS; Ammonia oxidation.

INTRODUCTION

CARS (Coherent Anti-Stokes Raman Spectroscopy) is a laser-based spectroscopic technique which can measure the temperatures and concentrations of gaseous components. Although CARS has been extensively used to study combustion processes, such as production internal combustion engines 1 and commercial jet engines, 2 it has also been applied to chemical catalytic reactors, 3,4,~ and coal conversion technology, s

CARS offers several advantages for the study of dy- namic chemical processes involving significant mass transfer:

1. Because CARS uses laser beams, in situ concentrations or temperatures may be measured without the use of mechanical sampling probes, which frequently

Received 14 June 1988. * ©Copyright 1988, United Kingdom Atomic Energy Authority. Per-

mission to publish granted by UKAEA. Research supported by the U.K. Department of Trade and Industry.

t Author to whom correspondence should be sent. Current address: Geochemistry Branch, BP Research Centre, Chertsey Road, Sunbury- on-Thames, Middlesex, U.K. Current address: Smiths Industries, London, U.K.

disturb the systems under study and therefore create systematic uncertainties in the measurements. No sample preparation or other manipulations are necessary.

2. Since the CARS signal is exclusively generated where the laser beams intersect, a spatially resolved signal is obtained; this may be used, for example, to study the concentration profile of a species of interest.

3. CARS is temporally precise since the signal is only produced during the 10-ns laser pulses. This allows the statistics of turbulent fluctuations to be studied.

4. The combination of a laser-like signal beam and high temporal resolution permits interference-free operation, even in very luminous environments such as in- candescent lamps at temperatures around 3500 K. 7

The main disadvantages of CARS are its expense, complexity, and need for skilled experimentalists and theo- rists. Unless one (or more) of the advantages noted above is a key factor, some other (existing) technique may be preferable. Nevertheless, there are many areas (especially combustion) where CARS has established itself as an essential tool. The theory and practice of CARS are more comprehensively described elsewhere, s-l°

As a demonstration of the potential of CARS for the study of full-scale commercial chemical reactors, we de- cided to study an ammonia oxidation plant. The principal chemistry of this process is:

5 3 H NH3 + ~O2 (air) • NO + Pt catalyst 2 20"

Ammonia oxidation is a central part of artificial nitrogen fixation for manufacturing nitrates, which are mostly used as agricultural fertilizers. Typically, the overall process involves steam reforming of a hydrocar- bon feedstock to produce hydrogen. Ammonia from the

1412 Volume 42, Number 8, 1988 ooo~-7o2s/ss/42os-m2,2.oo/o APPLIED SPECTROSCOPY © 1988 Society for Applied Spectroscopy

Inlet Duct

CARS window Q

Approx Co~olyst Level Floor

560 i 1000 mm

FIG. 1. Vertical cross sect ion th rough a m m o n i a oxidat ion plant .

Harber process is then oxidized to NO. Further oxidation and hydrolysis yield nitric acid and, hence, nitrate.

Although ammonia-oxidation usually operates with high selectivity (~97%), an important side reaction is:

3 1 N 3 NH3 + ~02 (air) ) + ~H20.

Pt catalyst 2 2

Poor control of the catalyst operating temperature, gas mixing, or catalyst ageing can lead to increases in this undesirable side reaction. Because the side reaction is favored at higher temperatures, the chemical engineer has, amongst other factors, to correctly balance the operating temperature of the plant between the side reaction and incomplete oxidation of ammonia. Adjust- ments to the ammonia concentration in the reactant stream are used to control the temperature of the reactor: a balance must be struck between incomplete oxidation, undesirable side reactions, and rapid catalyst deactiva- tion by platinum loss.

A specific question of interest to chemical engineers is the degree to which the nominally premixed reactants (ammonia and air) are in fact homogeneous as they im- pinge on the "pack" of Pt, 10% Rh catalytic gauzes (Fig. 1). Any spatial or temporal variations in local concentrations of the reactants are expected to influence localized conversion efflciencies and, hence, the overall plant efficiency.

Under normal operating conditions the oxidant, ambient air, is compressed to 10 atmospheres, with a resul- tant temperature increase to approximately 250°C. Liq- uid ammonia is "sparged" into the air and evaporates: mixing of the reactants is then enhanced by a series of baffles. The exothermicity of ammonia oxidation main-

rains the catalyst temperature at about 800°C. In practice, the cherry-red color emitted by the catalytic gauze pack is observed to "flicker" at a characteristic frequency close to 1 Hz. It is speculated that the "flicker" may indicate nonoptimal operation; possible origins of the flicker include inhomogeneity of the inlet gas mixture and fluid mechanical instabilities. The aims of these experiments were: (1) to determine a relative (time averaged) NH~ concentration across a chord of the reactor just upstream of the catalyst, and (2) to investigate possible origins of the "flicker." A knowledge of these in situ parameters could lead to improved plant design and, potentially, significant financial benefits. CARS is ideally suited to this type of study because of the advantages outlined above, specifically: good spatial and temporal resolution and the ability to make noninvasive measurements.

The principal problem of applying CARS to production-scale ammonia oxidation was addressed by designing and building a ruggedized CARS apparatus for the "B-stream" of the UKF Ltd. plant at Runcorn, U.K. The main objective was the accurate measurement of spatial and temporal changes in NH3 concentration.

Although the equipment was plant-specific, all its features are generic to similar applications, in particular, the concentration measurement procedure for NH3, the novel tunable dye laser and the approach to windowing large-scale plant. Before use at UKF, the apparatus underwent an extensive calibration and testing program under laboratory conditions.

EXPERIMENTAL Since the ammonia oxidation plant was located in a

large, unheated industrial building (Fig. 2), with many openings to allow the passage of pipework, etc., large environmental temperature swings were expected. In ad- dition, the ammonia oxidation reactor generates consid- erable amounts of heat around and below the catalyst (Fig. 1). The ambient noise level was very high since the steam-driven air compressors were close to the reactor. The reactor was surrounded by ~i metal grill floor supported by girders, which proved reasonably stable in practice. Since the area around the reactor was desig- nated a UK "Zone 2" area, non-"spark-proofed" (intrinsically safe) equipment could not be used without the plant manager's authority. A feasibility study showed that it was impracticable to spark-proof the high-pow- ered lasers and electronics associated with the CARS apparatus. Operation of the laser equipment was therefore strictly limited to short periods by prior arrangement with the plant management and safety personnel.

CARS Spectrometer: Overal l Considerat ions. In es- sence, CARS relies on crossing two powerful beams of (usually) visible light from a laser at a "pump" frequency 0~1 together with a third beam at a lower frequency o~s, known as the Stokes beam. If a molecule is present in the sample with a Raman transition at a frequency equal to (~01 - ws), nonlinear interactions at the mutual crossing point of the lasers will generate a fourth "signal" beam at a frequency (2wl - ws). If, as was the case in this study, ~0~ is from a broad-band laser, - 150 cm -1 of a molecular spectrum may be recorded from the collected radiation at ~0a~. This process is illustrated in Fig. 3.

APPLIED SPECTROSCOPY 1413

Fro. 2. Ammonia oxidation plant, showing CARS apparatus.

In order to carry out experiments in the environment described above, the CARS spectrometer designed for laboratory experiments 11 was modified for use around the ammonia oxidation plant. The resulting optical system is shown in Fig. 4, the crucial design criteria being the presence of a line of sight across the reactor and the use of a mutual beam crossing angle that is as large as possible, to obtain reasonable spatial resolution. 12

The maximum convenient wl beam separation using 2-in.-diameter optics and conventional mounts was 15 mm center-to-center; the beams were expanded to ~12 mm diameter by the Gallilean telescope, T. The Stokes beam was similarly expanded to minimize the volume of the CARS interaction volume. The "field" and "recol- limating" lenses, L1 and L2, had nominal focal lengths of 1300 mm. In order to allow a reasonable traverse of the CARS interaction volume across the reactor, we studied a 1.5-m chord of the ammonia oxidation reactor, rather than a diameter. With the optical windows positioned on stand-off flanges, a traverse of ~ 500 mm was possible.

To change the position of the CARS interaction volume within the reactor, we rigidly mounted the entire top section of the apparatus, including the lasers, CARS

optics, and receiving optics, on a moveable horizontal framework. This framework could be precisely translated along three parallel linear bearings rolling on a lower structure resting on a set of vibration isolated legs. The advantage of this type of approach, first used by Eck- breth e t al . 2 to study a jet engine, but in a vertical con- figuration, is that the CARS system does not have to be realigned between measurement stations, provided that the framework is carefully designed to maintain relative alignment of the laser sources and the fiber optic signal capture system. 13

To provide some degree of thermal and environmental protection, we fitted an insulated fiberglass cover to the lasers and CARS optics. The only optics which were ther- mostatically regulated were the Nd:YAG laser output- coupling etalon, and the frequency doubling crystal associated with the Nd:YAG laser.

Dye Laser. A novel dye laser arrangement was adopted which allowed rapid and reliable retuning of the Stokes beam in order to search for various molecular species, at a minimum cost and complexity. The system works well for Stokes wavelengths from approximately 634 nm to 668 nm.

1414 Volume 42, Number 8, 1988

CARS

(COHERENT ANTI-STOKES RAMAN SPECTROSCOPY )

5~o I 1000mm

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Schematic illustration of CARS process.

The active medium in the transversely pumped laser was a saturated solution of DCM in ethanol. This dye has the unusual property that its fluorescence spectrum does not significantly overlap its absorption bands. 14 Normally DCM cannot thus be tuned by adjustment of its concentration in a broad-band dye laser. Without our laser system, DCM produced a Stokes beam with a wavelength suitable for methane spectroscopy, but not for ammonia.

However, by the insertion of various red Schott optical absorption filters into the oscillator cavity, it was possible to shift the Stokes frequency over a useful range of 30 nm (see Table I). Thus one may study a wide variety of molecules by simply interchanging the glass filters. Since the filters are not critically aligned with the dye laser's main optical axis, this is easily achieved. As well as of- fering economy and simplicity, glass filters may be wedged and tilted so that potential interferences from etalon modes of the filter are avoided.

Remote Control and Data Acquisition. Because of the hostile environment around the ammonia reactor, the CARS spectrometer was designed for remote control, with the more delicate components, specifically the mul- tichannel spectrograph and computer, being located some 15 m away in the plant's control room. Figure 5 illustrates the control and data transfer arrangements employed. The connections between the control room and the laser equipment were routed through a flexible steel umbilical. The umbilical was used to protect both the many elec- trical cables and the delicate optical fibers, used to transfer the CARS signal to the spectrograph.

B = Bea Stop ~ U Window F = F i l te r L~

Mirror B M = F

T = L3 Telescope

F o = Fibre Optic

* = Remote Control

CARS opt ics

L a s e r s i

M: , I

Fro. 4. Plan of CARS apparatus and ammonia oxidation plant.

Remote mechanical adjustments were effected by six integral dc motorized micrometers (Oriel & Ardel Ki- nematic) as follows:

1. Final CARS cross-over alignment (M3, Fig. 4), X and Y. 2. Fiber optic coupler (F °, Fig. 4), X and Y. 3. X/2 plate rotator (X/2, Fig. 4), R. 4. YAG doubling crystal tilter, R.

All other adjustments were made manually during initial alignment and were subsequently kept fixed.

A PDP 11/23 + with a 30 MByte Winchester disc was used to acquire data and to control the rotation of the X/2 plate. It also controlled a shutter in the ~1 beam for background subtractions. The intensified diode array (Tracor Northern 1710) used a special Harwell-designed interface to enable direct memory access (DMA) data transfer in real time to the PDP minicomputer.

Concentration Analysis. A k/2 plate was used to rotate the plane of polarization of the Stokes beam with respect to that of the pump beam. For vibration modes with zero or near-zero depolarization (often associated with totally symmetrical modes of vibration), such as the vl(al) mode of ammonia, the resonant CARS signal may be effectively


TABLE I. Effect of various glass filters on the Stokes laser wavelength.

Relative nonresonant

Maximum Raman shift CARS Schott filter Stokes (pump @ signal

(4 ram) wavelength 532 nm) from air Molecule

None 635.5 nm 3061 cm -1 1.0 (CH4 A1) RG 630 638.7 3140 0.51 RG 645 650.7 3429 0.74 (NH3 AI) 2 × RG 645 656.1 3555 0.56 RG 665 662.9 3712 0.45 (H20 A1) 2 x RG 665 667.4 3813 0.20

cancelled by using orthogonal pump and Stokes laser beam polarizations. This permits the generation of an in situ dye laser reference signal similar to that obtained with the use of an Ar cell, but with the crucial advantage for this experiment, that it could be obtained in situ from the chemical reactor. It would not, of course, have been convenient to periodically fill the chemical plant with argon to meet the requirements of CARS.

It was also possible to partially cancel the resonant CARS signal by using appropriate relative beam polarization (so that the resonant and nonresonant signals were of comparable intensity); the interference between the signals then permits a concentration analysis of the spectra obtained. This is believed to be a novel extension of polarization schemes which have been derived to cancel or reduce the nonresonant background contribu- tion. 154~ Some additional details are given in the results section.

Because of severe space limitations in the control room, a 156-ram-focal-length Jarrell-Ash 82-477 F3.8 spectrograph was used with a 1800 g/mm 550-nm blaze grating fitted, instead of the 1.5-m spectrograph used for laboratory calibrations. Due to its size, the spectrograph could be fitted into a standard electronic rack system with the detector unit and other miscellaneous electronics. No loss of experimental accuracy or precision was incurred by the low resolution of the spectrograph, since the "k '/''' method of concentration analysis 17 is largely unaffected by resolution.

Reactor Windows. The insertion of windows in a large production plant must be carried out with great care. The consequence of a window failure would be severe from both safety and financial standpoints.

Because the pressure vessel immediately downstream of the catalyst is double-walled (to permit water cooling) (Fig. 1), it was not possible to insert windows at this point; therefore the study of products of the reaction was excluded. The window locations were therefore chosen as close as practicable upstream of the catalyst, namely, in the single-walled region of the pressure vessel.

Figure 6 shows the detailed design which was used for the optical windows. The complete assembly was designed to bolt onto a standard industrial flange, the car- responding halves of which had been previously welded onto the main pressure vessel. A gate value between the pressure vessel and the flanges allowed the windows to be both connected during normal plant operation and to be removed for cleaning if required.

I n s t r u m e n t

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Remote control and data acquisition system.

Figure 6 shows how the 75-mm-diameter "Spectrasil B" silica window was sealed by two high-temperature "Kalrez" O-rings. Gaskets of the same material were used to cushion the silica window from localized stressing. A 5 ° tilt of the silica window from the optical axis prevents back reflections from the uncoated window from dam- aging the CARS front end optics. A purge point is provided in order to maintain window cleanliness, and elec- trical trace heating to 50°C was used to prevent water condensation.

Graphite gaskets were used to ensure a good seal between the window assembly and the reactor flanges (these were superior to PTFE gaskets which contain plasticiz- ers, which were found to cause misting of the windows. The design temperature and pressure of the window assembly were 260°C and 9.7 atm. During operation it was found necessary to heat the windows electrically to elim- inate moisture on the windows.

Miscellaneous. Due to the high ambient noise level, which prevented normal spoken communication, and to the necessity of allowing conversation with the control room, an intercom system was essential. This consisted of a military tank commander's helmet (supplied by Ra- cal plc) with built-in headphones and a neck-mounted noise-cancelling microphone. The effects of adjustments to the CARS setup could then be monitored by a col- league in the control room, wearing a headphone set and microphone. The equipment and procedure were most effective, especially the "noise-cancellation" feature of the headware; the experiment would have been impossible without it.

RESULTS '

Laboratory Calibration. Before moving the equipment to the chemical reactor, we carried out laboratory cali-


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brations using a 1.5-m-focal-length high-resolution spectrograph. CARS spectra were recorded from accurately analyzed mixtures of NH3 in N2 at 250°C and 9 atmospheres pressure.

Figure 7 shows a CARS spectrum of 10% NH3 with all laser polarizations parallel. The very high intensity of the resonant NH~ CARS ~ignal completely overwhelms the nonresonant background signal. This makes the es- timation of concentration from the CARS spectra, by using the nonresonant signal as an "internal standard," essentially impossible. Therefore, it was essential to reduce the strength of the resonant signal in a controlled way. This was accomplished by partially cancelling the resonant signal by rotation of the pump laser polarizations by a X/2 plate described in the experimental section. In practice, a X/2 plate rotal;ion 4 ° away from the position

J / \ 512

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FIG. 7. CARS spec t rum of 10% ammonia in ni t rogen (all beam polarizations parallel).

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of maximum cancellation was found to be convenient, and was adopted for all calibrations.

Figure 8 shows representative CARS spectra obtained from 10%, 6%, and 3% NH~ in N2 at 250°C and 9 atmospheres pressure. The substantial reduction in the resonant NH3 signal in relation to the nonresonant background is clearly shown, with the interference of the two signals evident. (The sharp feature is the resonant NH3 signal, and the much broader "hump" is the nonresonant signal, which also mirrors the dye laser spectral profile.)

In situ nonresonant background signals were obtained by total cancellation of the resonant signal with the X/2 plate. The PDP data collection computer was used to generate the sequence of X/2 movements and o~ 1 beam shutter operations, listed in Table II.

T A B L E II. Sequence used to acquire CARS spectra."

1. 25 shots, resonant signal par t ia l ly cancelled. 2. 25 shots, resonant signal total ly cancelled. 3. 25 shots, resonant signal par t ia l ly cancelled. 4. 25 shots, r esonan t signal total ly cancelled. 5. 50 shots, ambien t background, w, blocked.

"CARS signal = [(1) + (3) - (5)]/[(2) + (4) - (5)].


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3'0

2"5

2'0

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I I I I 2.5 5-0 7.5 10'0

Ammonia Concentrat ion { mole %)

F[c,. 9. Dependence of k '~ on ammonia concentration.

The shapes of the spectra were used to determine NH~ concentrations using the k '/' method2 ~ k '/~ can be shown to be linearly related to concentration over a wide range:

k '/, o~ ( I~/ I~) '/~ (1)

where I~ and I~ are the CARS intensities at the peak and an arbitrary "background" position (chosen to be mainly sensitive to the nonresonant signal). In this study I, and I~ were 100 channels = 26 cm -~ apart; Fig. 9 shows k V~ plotted against the independently established concentrations of NH~ for the spectra shown in Fig. 8. The excellent linearity and repeatability of the plot demon- strate the validity of this technique for using CARS spectra to measure ammonia concentrations.

As discussed in the experimental section, lack of space demanded that a compact spectrograph be used for the experiments at the chemical plant; its dispersion was 0.073 nm per channel.

The k '/' concentration analysis algorithm is insensitive to changes in dispersion (provided that a reasonable dis- tinction between I, and Is is possible). This was demonstrated by a series of laboratory tests of the precision of repeated CARS NH~ concentration analyses using a 2-m-long cell fitted with 10% NH~ at 25°C and 2.5 atm to simulate the chemical reactor, along with the low dispersion spectrograph.

Five hundred repeated firings of the laser, operating at 10 Hz, were used to collect 500 single-shot NH~ CARS spectra, and a value of k '~' was collected from each one. Plots of successive values of k '/' are shown in Fig. 10a; the standard deviation of this set of data was given by:

a._~ = 0.059[k'/~]~v,- (2)

i.e., the precision of the method was 5.9 % relative to its mean value [k'/']~v .

The procedure was repeated in an identical fashion,

except that 10 spectra were integrated on the detector before data transfer to the computer, i.e., one spectrum was collected every second. In this case the precision was 2.6 % relative to [k v~]~v. A sequential series of k '~ is shown in Fig. 10b.

These data define the potential which CARS offers for detecting r e l a t i v e (as opposed to absolute) concentration

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fluctuations in a turbulent system such as a chemical reactor. The response time/precision is clearly excellent and could be further enhanced by the use of higher- repetition-rate lasers.

Results from the Chemical Reactor. The CARS equipment described above wa..~ disassembled into its main components and moved to the chemical reactor by road. On arrival at the production plant, the following operations were carried out:

1. Transport to the amraonia oxidation plant using cranes.

2. Connection of power artd water to laser. 3. Re-alignment of lasers. 4. Alignment of CARS optics. 5. Beam steering through reactor windows. 6. Alignment of fiber optic/receiving optics.

7. Deployment of umbilical containing control cables. 8. Connection of electronics and PDP computer.

These complex operations were achieved within 3 days from off-loading of the equipment, with the use of two people. Much time was saved by moving the CARS Optics (Fig. 4) in pre-aligned units, and by carefully designing sufficient degrees of mechanical freedom into the apparatus. These features allowed optical alignment with the reactor windows, without the movement of the equipment from its initial resting position (as deposited by nontechnical personnel operating the crane).

When the alignment was complete, a series of single- shot CARS spectra were recorded from the center of the ammonia oxidation plant while it was operating at full production with an inlet pressure and temperature of 9 atm and 224°C, respectively. All laser polarizations were parallel.


TABLE III. Calculation of the maximum possible refractive index excursions due to temperature or concentration variations.

9 Bar 9 Bar

Refractive index data 224°C 600°C

Pure ammonia 1.0019 (A) 1.0011 Air 1.0015 (B) 1.00084 (C)

Maximum possible refractive index excursions Concentration induced: (A) - (B) = 0.0004

(assuming pure air and pure ammonia as "worst case" mixing).

Thermally induced: (B) - (C) = 0.0007

(assuming ~375°C fluctuations).

A sequence of 8 successive spectra, recorded as described, is shown in Fig. 11. Two important features of these spectra are: (1) their much lower than expected absolute intensity and (2) the high variability in their relative intensities (much greater than usual for single- shot CARS).

The cause of the intensity reduction by a factor of ~ 104, and the occurrence of totally spectra-free intervals, was the reduction in optical beam quality caused by passage through the reactor. Visual examination of the ex- iting o~1 beam revealed severe disruption into a pattern of randomly shifting bright spots.

Beam disruption and beam steering are caused by the presence of refractive index gradients in the medium. The magnitude of the phenomenon depends more on the three-dimensional distribution of refractive indices along the beam path than on the maximum refractive index excursion encountered. For example, passage through a single, smoothly varying refractive index gradient is far less disruptive than the disruption caused by random, sub-centimeter scale fluctuations (such as might occur in turbulent mixing). In the limiting case of passage through an optical window, the large, but precisely controlled, refractive-index gradient does not disrupt the laser beam--although the whole beam may be redirected.

Clearly, both concentration and temperature heterogeneities contributed to beam disruption in the ammonia oxidation reactor. However, their re la t ive importance is impossible to determine, since the three-dimensional thermal and concentration fields are unknown. Table III shows that the maximum possible refractive index excursions are of a similar order of magnitude for both thermal and concentration variations. In other words, e i ther concentration or thermal fluctuations (induced by turbulence) or both caused the reduction in beam quality, but it is not possible to decide which factor was more important.

The loss of CARS signal strength is a result of (1) worsened coherence, (2) diminished beam intersection in the interaction volume, and (3) reduced CARS signal collection efficiency by the fiber optic which acts as a narrow aperture in the overall optical system.

In fact, given the exceedingly poor beam quality, it was remarkable that CARS spectra were collected at all. Figure 12 shows an averaged CARS spectrum, obtained in 50 s, which is of high enough quality to allow the nonresonant background to be discernible on the original data. Because of this reduction in signal intensity, concentration measurements were precluded, particularly

c

I J I I J 0 50 100 1 5 0 2 0 0 2 5 0

C h a n n e l

FIG. 12. CARS spectrum of ammonia from ammonia oxidation reactor (average of 500 shots).

since polarization cancellation reduced the signals to an unquantifiable level.

The problem of loss of beam quality could be overcome by using shorter optical pathlengths and/or selecting dif- ferent reactors for study. However, it has been clearly demonstrated that it is possible to use CARS in the hostile environment of a chemical production plant. The technical feasibility of CARS having been demonstrated, the possibility of using the technique for fundamental chemical engineering studies is great. Ultimately, it may even be possible to use CARS as a rapid, noninvasive spectroscopic technique for on-line monitoring of plant efficiency.

CONCLUSIONS

A rugged, remotely controlled CARS apparatus has been successfully used to record spectra from a full-sized (production) chemical reactor.

Laboratory simulation has demonstrated that very precise measurements of concentration fluctuations may be made in systems of interest to chemical engineers.

Beam disruption caused by heterogeneities in the optical quality of chemical feedstocks may cause problems via CARS signal reductions. This could be overcome by studying reactors with shorter pathlengths and/or lower heterogeneities. Schlierren techniques should be used to screen future potential chemical reactors in which CARS is to be applied, and they may be intrinsically useful in providing valuable information in their own right.

ACKNOWLEDGMENTS

We acknowledge the vision and practical assistance of the staff of UKF Fertilizers Ltd., Ince, Cheshire, U.K.--particularly the help of David Evans, Hans Fishoff, Steve Hallsworth, John Hudson, and Roger Sutcliffe. We acknowledge the particular contributions of John Harvey, who provided engineering support, and of Dr. C. J. Wright. We also acknowledge Dr. D. A. Greenhalgh for his encouragement and useful discussions.

1. D. A. Greenhalgh, D. R. Williams, and C. A. Baker, Proceedings of the 16th ISATA Conference (Automotive Automation, Ltd., Croyden, 1984), p. 485.

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Application of CARS Spectroscopy to the Detection of SO2

MARCUS ALDI~N* and W I L H E L M WENDT Combustion Center (M.A.) and Department of Physics (W.W.), Lund Institute of Technology, P.O. Box I18, S-221 O0 Lund, Sweden

Experiments have been performed to investigate the possibility of detecting low concentrations of SO2 using CARS spectroscopy. The experiments were also aimed at high-temperature investigations both in a heated cell and in a flame. During the cell measurements it was clearly revealed that the temperature has a dramatic influence on the shape of the CARS spectra, indicating a good potential for thermometry using SO2 and CARS spectroscopy. Index Headings: CARS; Flames; Sulphur dioxide; Spectroscopy.

INTRODUCTION

During the past two decades, the energy crisis and the growing awareness of our serious environmental situation caused by air pollution have encouraged increased research in basic combustion. The aim of this research has thus been to achieve a deeper insight into the phenomena which will ultimately increase the efficiency in industrial combustion processes at the same time that the emission of air pollutants is decreased. One of the key issues in understanding these phenomena is the use of nonintru- sive diagnostic techniques, e.g., those utilizing lasers which give high spatial and temporal resolution.

Since the first demonstration of the application of laser techniques to combustion diagnostics--about twenty years ago--several techniques have emerged. For com- prehensive reviews see, for example, Refs. 1-3. The technique which probably has the largest potential, at least in the practical sphere of measurements, is Coherent Anti-Stokes Raman Scattering, or CARS. Thanks to pi- oneering work by Taran and co-workers, the technique was introduced for studies of combustion phenomena in the early seventies. 4,5 After these first demonstrations, the technique was extended to allow it to yield both high temporal and spatial resolution measurements, through the introduction of the broad-band CARS 6 and the BOXCARS 7 techniques, respectively. During the past decade CARS has been used in numerous combustion applications, including sooty flames, s internal combus-

Received 28 March 1988; revision received 3 June 1988. * Author to whom correspondence should be sent.

tion engines, 9 and large-scale furnaces. I°,11 CARS has mostly been utilized for temperature measurements using the temperature dependence of the spectra. Since N2 is the most abundant species in air-fed combustion, this molecule has been of specific interest for thermometry. Other diatomic molecules that may be of interest are, for example, CO, 02, and H2. Three-atomic molecules that might be of interest for CARS investigations in a combustion environment are H20 and C02, which also have been the subject of detailed investigations (see, for example, Refs. 12 and 13).

In the present paper, results are presented of CARS investigations concerning the potential for the detection of SO2 molecules, and the use of its spectral shape for thermometry. There are several reasons why nonintru- sive measurements of S02 would be of major importance. First, in industrial chemical plants (for example, those producing liquid H2S04), SO2 is present in concentrations of up to 20%, and thus in situ measurements of temperature and/or concentration, preferably on-line, could be used for process control and steering purposes. The second reason why SO2 is of interest~and it may be the more important one--is the role SO2 plays as a major air pollutant. There are both direct biological effects caused by S02 and also indirect effects when S02 acts as a pre- cursor of acidic rain by its reaction with water vapor.

The conventional technique for measurements of S02 is IR absorption, where normally a gas sample is extract- ed from the unknown gas mixture with the use of a probe. Other techniques for measurement of S02 based on probe sampling are conductometric and coulometric techniques. All these probe techniques have drawback:s, in so far as they are somewhat perturbing in nature, and have temporal and spatial resolution that is not always adequate. It is, in principle, possible to make an in situ absorption S02 experiment which would yield a nonper- turbing measurement; however, the spatial and temporal resolution would still be low, and problems could arise in a flame environment because of spectral interferences from other flame species. Several of the problems ap- pearing with the conventional techniques described above

Volume 42, Number 8, 1988 0003-7028/88/4208-142152.00/0 APPLIED SPECTROSCOPY 1421 © 1988 Society for Applied Spectroscopy

Documents

CARS Spectroscopy of Ammonia in the Laboratory and a Commercial Scale Ammonia Oxidation Plant