I n d . Eng. Chem. Res. 1988,27, 213-219
Acknowledgment Exxon Research and Engineering is gratefully acknowledged for their support of this study. Registry No. SOz, 7446-09-5.
Literature Cited Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1981,27,226-234. Borgwardt, R. H.; Bruce, K. R. USEPA (IERL), 1984. Borgwardt, R. H.; Harvey, R. D. Environ. Sci. Technol. 6, 1972, 350-360. Chrostowski, J. W.; Georgakis, C. ACS Symp. Ser. 1978,65,1. DeLucia, D. E. “The Cyclic Use of Limestone to Capture COz”. Master’s Thesis, Massachusetts Institute of Technology, Cambridge, 1985. Fee, D. C. Wilson, W. I.; Myles, K. M. “The Applicability of the ANL Hydration Process to Enhance to Calcium Utilization of Three Lowellville Limestone Sorbent Product Streams upon Being Recycled Back through the BABCOCK & WILCOX AFBC”. Argonne National Laboratory, May 1982.
273
Floess, J. K. “The Effect of Calcium on the Gasification Reactions of Carbon”. Doctoral Diss., Massachusetts Institute of Technology, Cambridge, 1985. Glasson, D. R.; O’Neill, P. Proc. 6th Int. Coni. Thermal Anal. 1980b, 1, 517-522. Hartman, M.: Pata, J.: Coughlin, R. W. Ind. Enp. Chem. Process Des. Dev. i978,i7,411-419. McClellan. G. H. Hunter. S. R.: Scheib. R. M. “X-Rav and Electron Microscope Studies of Calcined and Sulfated Limestones”. In The Reaction Parameters of Lime; Special Technical Publication No. 472; ASTM: New York, 1970; pp 32-66. Pigford, R. L.; Sliger, G. Ind. Eng. Chem. Process Des. Dev. 1973, 12, 85-91. Tanaka, H.; Ohshima, S.; Ichiba, S.; Negita, H. Thermochim. Acta 1981,48,137-146. Van Houte, G.; Maon, J. CL.; Dumont, P. H.; Delmon, B. J . Air Pollut. Control Assoc. 1978,28, 1030-1033. Van Houte, G.; Rodrigue, L.; Genet, M.; Delmon, B. Enuiron. Sci. Technol. 1981,15, 327-332. Received for review January 21, 1987 Revised manuscript received September 9, 1981 Accepted September 22, 1987
An in Situ, Multibeam, Spectrophotometric, Transient Analysis Method for Multiple Species in Chemical Reactors Yung C. Lin, Robert J. Adler,* and Robert V. Edwards Chemical Engineering Department, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106
A spectrophotometric method is reported for measuring simultaneously the transient concentrations of several species at several locations in a chemical reactor. Locally averaged composition is measured noninvasively along each of several light beams. The method employs multiple light beams, optical multiplexing, fiber optics, several filter frequencies, time-shared detection, and computerized fast data acquisition. In a version constructed for testing, dual filter frequencies are employed to follow the concentrations of two absorbing species simultaneously along each of eight light beams. Concentration transients as short as 0.2 s are handled. An IBM personal computer stores 600 readings/s (2 wavelengths, 8 light beams, and 30 samples/s for each wavelength along each light beam, plus 120 auxilliary readings). Tests confirm that the method is robust, noninvasive, and inexpensive. Minor modifications can extend the number of species monitored, and trade-offs are possible between the number of positions monitored and speed: We recently desired to monitor the transient composition of a reacting mixture with several stoichiometric degrees of freedom, where the composition transients due to mixing and reaction were on the order of a few tenths of a second. The reaction was to take place in a volume of a few liters, and it was also desired to observe simultaneously spatial composition variations due to incomplete mixing. Spectrophotometry is attractive because it is noninvasive and in principle can satisfy all of our needs, but a survey of commercially available spectrophotometers revealed no instrument capable of handling several species simultaneously with the desired speed of response. In addition, non could probe-or could be readily modified to probe-several locations simultaneously. Conventional spectrophotometers have the capability of analyzing for multiple components, but since they mechanically scan a range of wavelengths, their speed of response is typically 1-5 min. Spectrophotometers of this type are not suitable for reactive mixtures. Conventional stopped-flowspectrophotometershave fast transient capability down to the millisecond range, but they probe only one wavelength along one beam path and 0888-588518812627-0213$01.50/0
are therefore unsuitable for multicomponent mixtures and multiposition analysis. The type of spectrophotometer which came closest to meeting our needs is based on array diodes. An example is the Hewlett-Packard Model 8450 spectrophotometer. I t uses 400 diodes in parallel to measure the spectrum virtually instantaneously (0.1 s/spectrum). This philosophy can in principle handle multicomponent mixtures with rapidly changing composition. However, data processing and storage requirements limit the sampling rate to one analysis per a few seconds. Further, modification for multiposition measurements is impractical. We concluded that we had to construct a special type of spectrophotometer to do composition analysis of a few-component mixture with transient times on the order of a few tenths of a second where multiple positions are to be observed simultaneously. We describe here a spectrophotometric method developed in our laboratories to meet the above needs. It fills the gap between the conventional stopped-flowtype (very fast speed of response-but limited to a single component and position) and the diode array type (many components-but limited to about 1-s response speed and
0 1988 American Chemical Society
274 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
DUAL BEhH LIGHT SOURCE
BELHSPLIITER-MIRROR ASSEMBLY
RElClOR CELL
I
Figure 1. Overview
a single position). The new spectrophotometric method has been given the acronym STAMPS (Spectrophotometric Transient Analysis Method for Multiple Positions and Species). It employs multiple light beams, optical multiplexing, fiber optics, several wavelengths, time-shared detection, and computerized fast data acquisition. STAMPS is applicable to batch, semibatch, or continuous-flow reactors, can probe simultaneously several light beam paths in reactor volumes as large as a few liters, and handle phenomena with response times as short as 0.2. Many methods of time-shared light detection can he used in STAMPS. Perhaps the simplest is to split the transmitted light with beam splitters and mirrors into several beams, each of which is analyzed for a different wavelength by a fdter followed by a photodiode. The most sophisticated method is to split the transmitted light with a stationary grating into 512 or 1024 beams, each focused on one of the elements of a linear photodiode array (LPDA). If only a few wavelengths are recorded in the LPDA method, the array can be scanned within 1ms, fast enough for our needs. Commercial LPDA detectors and data acquisition units are available from several suppliers, e.g., Tracor Northern or Princeton Instruments. The estimated cost with capabilities to meet our requirements is on the order of $30000. Since only a few wavelengths had to be monitored, we adopted the more economical light detection method based on beam splitters and filters described above. In the embodiment described here, two wavelengths and eight light beams are used to measure the concentrations of two absorbing species simultaneously at eight positions at a rate of 30 times/s wavelength/beam. The eight light beams, each passing through a different location in a 3-L cell (15.8-cm light path) enable composition variations in position to be monitored. Measurements of the transient absorbance of two reaction products from an azo dye coupling reaction (Boume et al. 1981,1985) are performed to check the practicality and other characteristics of STAMPS. Transient data are currently being obtained in a laboratory batch reactor study of reactive mixing. We plan in a subsequent paper to report the new data and show its usefulness in modeling reactive mixing. Principle of Measurement An overview of STAMPS is presented in Figure 1. A specific embodiment will be described in greater detail in a separate section. Light from an incandescent light source is divided into several parallel subbeams by beam splitters and mirrors. These subbeams pass through various locations of a sample cell (reactor) with transparent walls. The light intensity
is attenuated by absorption along each subbeam in accordance with Beer's law. Each species absorb its characteristic wavelength. The subbeams leaving the reactor pass through collimating lenses and are collected by optical fibers. An optical multiplexer samples each subbeam cyclically and combines the samples into a single flashing beam which is sent to a multiwavelength analysis section. There the output from the optical multiplexer is split by beam splitters and mirrors into as many beams as there are species to be analyzed. Each of these split, flashing beams pass through a filter and fall on a separate photodiode. The band-pass frequencies of the filters are chosen to match the absorption wavelengths of the species to be analyzed. A reference beam from the same light source bypasses the reador and falls on an additional photodiode. The photodiode voltages are processed and stored in the host computer by associated electronics. The photodiode signals are converted later to absorbances and concentrations. STAMPS measures the average concentration along each light beam, i.e., the average concentration over the volume illuminated by each beam, assuming the beam is sufficiently narrow that concentration does not vary over the beam cross section and is only a function of position along the beam.
A Two-Species, Eight-Beam Embodiment In the embodiment built, dual filter frequencies are employed to follow the concentrations of two absorbing species simultaneously along each of eight light beams. The test cell is a cylindrical reactor with transparent walls. Because of the radial symmetry of the cell, passing the light beams through the center plane will best enable the concentration averaged over the total cell volume to be measured. Figure l provides an overview. Detailed descriptions of each major component follow. Light Source and Optics. Figure 2 shows the light source and associated optics. A 200-W mercury arc lamp (Oriel Model 6283) serves as a white-light source. A pair of plano-convex lenses generates an image of the light source a t the plane of a 600-pm spatial filter which reshapes the image and creates a "point source". An achromatic doublet following the spatial filter minimizes chromatic and spherical aberrations and coma and produces a highly collimated light beam of uniform intensity. The collimated light beams then pass through two pinholes, producing two thin (2-mm-diameter), parallel, collimated beams. Beam Splitter-Mirror Module. The two light beams are divided into eight subbeams by a beam splittermirror module; see Figure 1. These beam splitters and mirrors (Melles Griot) are mounted on an assembly which is a combination of mirror mounts, carriers, optical rails, and rack-and-pinion rods (parts from Newport Corporation). The eight outputs from this assembly are collimated beams
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 275
I
1
I
I
Figure 3. Optical multiplexer, side view.
(3-mm diameter, divergence angle 0.2 deg) parallel to one another, each with a light intensity approximately onefourth that of one of the two input beams. Reactor Cell and Illuminated Volume. The current reactor cell is a cylindrical acrylic tube (15.88-cm high, 17.78-cm o.d., and 15.88-cm i.d.; volume 3142 mL). The eight light beams, each having a diameter of 4 mm, enter and leave perpendicular to the tube. The light path for the spectrophotometric measurements is equal to the inside diameter of the reactor cell. The illuminated volume (the volume of the solution illuminated by each light beam) is 2 mL or 0.06% of the total reactor volume. The cell is surrounded by four acrylic glass sheets perpendicular to a bottom plate. Water between the cell walls and sheets helps maintain the collimation of the light beams. The water also serves to thermostat the cell by circulation through a constant temperature bath (rt0.2 “C). Light Receiver/Collimator. Sixteen receiver/collimators are used. Eight receive and focus the eight light beams emerging from the reactor cell onto eight optical fibers (400-pmcore diameter, General Fiber Optics). These optical fibers carry the light beams to the optical multiplexer. There the fibers terminate with eight additional receiver/collimators which produce collimated light beams. The light receiver/collimator designed especially for this research is more compact than commercially available gimbal devices; it measures 2.85-cm diameter by 5-cm long. The design is based on the concept of “ball in a tapered socket”, which enables two angular adjustments. The parts are a 1.0-cm-diameter lens, a black delrin lens holder (the ball), an aluminum rod that holds an optical fiber, and an anodized aluminum housing (the tapered socket). This miniature device provides three degrees of freedom-two angular and one linear-enabling convenient alignment between the light beam, lens, and optical fiber. Optical Multiplexer. Figure 3 is a schematic of the optical multiplexer, a most important component. As it spins, the light beams from the reactor cell (transmitted by eight optical fibers) are sequentially sampled, and in addition other light pulses are produced to synchronize the data acquisition. The eight receiver/collimators which terminate the eight optical fibers are mounted on a stationary disk of the optical multiplexer; each receiver/collimator is centered at a radius of 5.72 cm and spaced a t 45-deg intervals. A matching, parallel, rotating disk with an attached bowl is driven at 1800 ( f l )rpm by a shaft coupled to a synchronous motor (30 W, 1/25 hp). One light receiver is located on the rotating disk at a radius of 5.72 cm. An identical dummy receiver, located opposite, balances the rotating disk (calculated critical speed 17000 rpm). An optical fiber (400-pm core diameter, General Fiber Optics) is attached to the light receiver. The optical fiber is supported by the
inner side of the bowl and passes through the centerline of the shaft where it terminates a t a collimator. This collimator and the light receiver are similar to the receiver/collimators described previously except that the position of the lens with respect to the optical fiber is fixed. In the collimator, the direction of the light beam is reversed (outward into the air rather than being received from the air). The light leaving the end of the shaft through this collimator passes into the next component, a dual-wavelength analysis section. Pulses for synchronizing the data acquisition are produced by seven 0.08-cm-wide slits and one 0.16-cm-wide slit located along the edge of the rotating disk at 45-deg intervals. A stationary light coupler (not shown) located at the rim of the rotating disk generates synchronizing and indexing pulses induced by the periodic passage of the slits between the light source and receiver of the light coupler. Dual-Wavelength Analysis Section. After it emerges from the optical multiplexer, the light beam enters the dual-wavelength analysis section, where it is divided into two beams by a beam splitter. Each of the two subbeams passes through a filter and is then detected by a separate photodiode. A reference beam from the same light source bypasses the reactor cell and falls on a third photodiode. See Figure 1. The beam splitter is a cube (2.54 cm by 2.54 cm each face, Melles Griot Model 03BSCOO9) which consists of a pair of identical right-angle prisms with hypotenuse faces cemented together. It transmits 45% and reflects 50% of the incident light. The selection of the filter frequencies is made by minimizing the effects of noise according to a least-squares criterion. Consideration is also given to maximizing the light beam intensities according to the emission spectrum of the light source. The three photodiodes (UDT-555D, United Detector Technology) which convert light to voltage are silicon photodiode/ current mode operational amplifier combinations. Each has an active area of 1cm2. The responsivity at 850 nm is 0.5 A/W, and the operational light range is from to W/cm2. The selection of the light filters depends largely on the absorption characteristics of the species being monitored. For the azo coupling reactions (Bourne et al., 1981,1985), two interference filters, 435 and 577.5 nm (Oriel Models 5645 and 5647, respectively, diameter 2.54 cm), each with a narrow band-pass of 9 nm and a transmittance of 50%, were used.
Data Acquisition An IBM personal computer with 640K RAM (random access memory) is used as the host computer for the data acquisition unit. A data acquisition board (DT2801-A, Data Translation Company) is incorporated to enable analog-to-digital conversions. The board has a 16-channel (single ended, 8 if differential input), 12-bit A/D converter with an accuracy of f l LSB (least significant bit). Its on-board microprocessor acts as the interface between the board and the host IBM PC. An internal clock enables sampling rates ranging from 2.5 ps to 81.9 ms, in increments of 1.25 ps. User-supplied external clock (adopted for this experiment) can also be connected to the board’s external clock port to synchronize experiments. An IBM PCapos parallel port is used as an interface between the host computer and the equipment to control the experiment. Additional electronics process and condition the signals before they are sampled by the data acquisition board. Figure 4 provides a block diagram of the data acquisition system. There are three analog signals from the photodiodes; one is the reference beam and two are the light beams at two
276 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
repeated sequences and the number of revolutions skipped between sampled revolutions.
wavelengths, 435 and 577.5 nm, respectively. These latter two signal are integrated for 1.2 ms, while the reference signal is amplified 5 times. In total, six channels (differential inputs) were used; two for temperature measurement (thermistors, upper and lower chambers), three for light intensity measurement (photodiodes), and one for impeller speed measurement (optical tachometer). Each channel has its own associated signal conditioning circuitry with independently adjustable zero and span. As the eight slits (one 0.16-cm wide, seven 0.08-cm wide) on the rotating disk of the optical multiplexer pass periodically through the gap of the light coupler, 240-Hz synchronizing pulses (30 revolutions/s, 8 pulses/revolution) are generated. The wider slit (index slit) also sends extra signals used for numbering the light beams in the optical multiplexer and as an index for counting the number of revolutions of the rotating disk. After processing by phase lock circuitry, three outputs are generated-clock pulses, integrator reset pulses, and index pulses. The clock pulses, used as an external clock source, are sent to the external clock port of the data acquisition board, the integrator reset signals are sent to integration circuitry, and the index pulses are sent to the host computer (IBM PC) through the parallel port (denoted as LPT2). The parallel port (LPT2) serves as the 1 / 0 port between the IBM PC and the equipment. Inputs to LPT2 include index pulses mentioned above and a lock status signal which indicates operating status of the phase lock circuitry. Outputs from LPT2 include a clock gate signal which turns on or off the clock pulses and two simultaneous trigger signals-one (reaction trigger) activates the air cylinder to rupture the stretched rubber membrane while the other (data acquisition trigger) starts the data acquisition. See Test Experiments and Results section for a description of the reactor cell and the membrane. Since the optical multiplexer is running at 1800 rpm, one revolution of the rotating disk takes 33 ms. During this period, 16 light intensity measurements (8 light beams, 2 wavelengths) are sampled together with 1 reference light intensity measurement, 2 temperature measurements, and 1 impeller speed measurement sampled in the beginning of the revolution. Because the data acquisition software was written in IBM-compiled BASIC, the RAM available for storing sampled data is limited to about 40 kbyte, enough for about 500 multiplexer revolutions. To obtain the best resolution of transient phenomena, software was developed to distribute the sampling over an extended time. The sampling operation consists of several periods, each containing an integral number of repeated sequences. Each sequence consists of a multiplexer rotation during which all of the signals are sampled, followed by any number (1, 2, 3, etc.) of multiplexer rotations without sampling. Software permits five different sampling periods, each with its own specifications of the number of
Data Reduction The photodiode signals, P, represent optical power. First the optical power is converted into absorbance, and then absorbance is converted into concentration. Conversion of Photodiode Signals to Absorbances. Three sets of photodiode data are sampled and recorded simultaneouslyby the data acquisition unit for each of the eight light beams passing through the reactor cell: one for the reference light beam and two for the light intensities at two wavelengths, 435 nm (denoted as 1)and 577.5 nm (denoted as 2), respectively. The data are first converted to absorbances at the above two wavelengths before further calculations. For each light beam passing through the reactor cell with solution containing absorbing species, the absorbance at any wavelength is expressed, according to the definition of absorbance, as
PLO A, E log PI
(1)
where A, = absorbance at wavelength i, i = 1, 2, over light path b; PI = optical power of the light beam passing through the solution; and P: = optical power of the same light beam passing through the solvent free of the absorbing species. Equation 1can be rearranged to include the optical beam power of the reference light beam as A,
PI0 PR PR log - = log - - log 5 Pl PI Pl
(2)
where PR is the optical beam power of the reference light beam bypassing the reactor cell. The photodiode signal is proportional to the light power after dark current compensation
D, = KIPl
(3)
where D, is the photodiode signal and K, is a constant which is a function of wavelength, photodiode responsivity, feedback resistances of the associated operational amplifiers, and integration time constant of the integration circuitry. Substituting eq 3 in eq 2 yields, for wavelength of 435 nm,
Similarly for wavelength of 577.5 nm,
If there is any fluctuation from the light source, it will appear simultaneously in the reference photodiode signal and the signals at both wavelengths, so fluctuations will cancel. Equations 4 and 5 are valid for each of the eight light beams passing through the reactor cell and are the working formulas for converting the recorded photodiode signals to absorbance data. Conversion of Absorbance to Concentrations. The absorbance data are further converted to the concentrations of the species being monitored by applying
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 277 PIERCING NEEDLE ( 9 , 1 SHOUN) R C R Y L I C SOURRE TRNK
10, STRINLESS STEEL T O P R N O OTTO^ PLRTES
9 SUOUN)
RCRYLlC CVLIHOERS
i . o . ~ i s . 9cn
~ ~ 1 c u i 1 7 cn .6
Figure 5. Spectrophotometric teat cell.
e
where is a 2 X 1concentration array, b is the length of the light path, E is a 2 X 2 extinction coefficient matrix which consists of four eztinction coefficients (two species, two wavelengths), and A is a 2 X 1absorbance array with elements Al and AP. The four extinction coefficients are determined prior to the experiments by filling the reactor cell with one absorbing species at various concentrations and measuring the absorbances at the two wavelengths. The concentrations obtained are the average concentrations over the volume illuminated by one light beam. Eight light beams pass through the reactor cell, each illuminating a different location in the cell. Summing over the concentrations from different locations yields volume-averaged concentrations in the reactor cell:
e, = -8 cc,j 1
8
j=1
(7)
and (8)
e,,
where = volume-averagedconcentrations of species 1and 2, respectively, and C,, CG = concentrations of 1and 2, respectively, calculated from eq 6 for light beam j .
Test Experiments and Results Test experiments were performed to confirm the anticipated operating characteristics of STAMPS. These tests included (1)steady-state absorbance measurements in nonreactive systems with stable concentrations of absorbing species and (2) transient absorbance measurements in reactive mixing systems with rapidly changing composition. Azo coupling reactions between a-naphthol and diazotized sulfanic acid were adopted for the experiments. Homogeneous, dilute solutions of monoazo dyestuff and disazo dyestuff (reaction products) were prepared for use in steady-state absorbance measurements. Transient absorbance measurements were performed to follow the production of these two dystuffs from reactants. In this section, the test cell and the am coupling reactions are f i t discussed in greater detail, and the the steady-state and transient test experiments and results are described. Test Cell. The test cell assembly is similar to the batch reactor for dilatometric experiments (Palepu, 1985). Figure 5 shows the schematic diagram of the test cell. The cell (17.78-cm 0.d. and 15.88-cm id.) consists of two equal chambers. A thin, highly stretched, rubber membrane can be placed between the two cell cylinders to keep their contents separate until the membrane is burst. The ratio of height to diameter of the cell is 1.0. The cell is sealed between a thick stainless steel top disk and a stainless steel bottom plate to which the cell is affixed. Both cell chambers have four baffles each, attached to the walls at 90-deg intervals. Agitation is provided by a magnetically driven six-blade impeller of standard dimensions (5-cm diameter).
The speed of rotation of the impeller is monitored by an optical encoder tachometer affmed to the motor shaft. The temperature of the fluid in the reactor is monitored and measured by thermistors, one in the lower chamber and one in the upper chamber. The rubber membrane was not used in the steady-state measurements. In the transient measurements, the mixing-reaction process was initiated by puncturing the highly stretched rubber membrane separating the reactants in the upper and lower chambers (a-naphthol and diazotized sulfanilic acid) with four needles (0.16-cm 0.d.). The needles are located in the upper chamber at 90-deg intervals, immediately behind the baffles and near the walls. They are driven downward 0.6 cm by a crossbar-air cylinder-solenoid valve mechanism controlled by the data acquisition unit. Reactions. The azo coupling reactions between a-naphthol (A) and diazotized sulfanilic acid (B) consist of a primary reaction which produces monoazo dyestuff (R) and a secondary reaction which forms disazo dyestuff (S): ki
A+B-R
(94
These two second-order, irreversible reactions take place in buffered (pH lo), aqueous, dilute solution. At pH 10 and T = 25 OC, the rate constants for the primary and secondary reactions are k1 = 7.3 X lo6 and kz = 3.5 X lo3 L/(mol.s), respectively (Bourne et al., 1981). The most recent values reported are k , = 1.2 X lo' (T= 20 "C)and kz = 1.71 X lo3 L/(mol.s) (Bourne et al., 1985). Solutions of monoazo dyestuff were prepared from the reaction of stoichiometric amounts of A and B (1:l ratio) mixed very rapidly (kl >> kz). Solutions of disazo dyestuff were formed by reacting 1 mol of A with 2 mol of B. Aqueous solutions of A (a-naphthol) were prepared from its crystals. Solutions of B (diazotized sulfanilic acid) were obtained by reacting sulfanilic acid with NaNOz and HC1 in the conventional way (Zollinger, 1961). The reactants A and B are both colorless in aqueous solutions. The two azo dyestuffs have substantially overlapping absorption wavelength. A t pH 10, the monoazo dyestuff is red and the maximum absorption occurs at 515 nm; the disazo dyestuff is purple and the maximum absorption occurs at 482 and 549 nm (Bourne et al., 1981). Newer values for the disazo dyestuff have been reported at 475 and 562 (Bourne et al., 1985). Steady-State Absorbance Measurements. The cell was assembled without the stretched rubber membrane. Deionized water (buffered at pH 10) was fed into and out of the cell from a feed tank containing 6000 mL of deionized water with 0.01 M sodium carbonate (Na2C03) and bicarbonate (NaHC03). Water from a constant-temperature bath circulated through the tank surrounding the cell, and the fluid temperature in the cell was monitored and adjusted to 25 f 0.5 "C.Offset signals (dark currents) from the three photodiodes were recorded by blocking the light beams to the photodiodes. Reference and blank (zero concentration signals, eight light beams) photodiode signals were also recorded. Twenty milliliters of 3 X M unbuffered monoazo dyestuff solution was introduced into the feed tank. Fluid was allowed to recirculate from the feed tank to the cell for 15 min to ensure homogeneity of the solution in the cell. The photodiode signals (eight light beams, two wavelengths) together with the reference signal were recorded. Offset signals were also recorded prior to the actual signals to compensate for the base-line drifting.
278 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
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AVERAGE OF 8 BEAMS V A R I A T I O N S AMONG E BEAMS
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.
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Figure 6. Measured vs actual concentration, monoazo dyestuff (R).
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Figure 8. Transient absorbance at 435 nm; reactant concentration = 1.2 x M, Re = 10000.
1
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Figure 7. Measured vs actual concentration, diazo dyestuff (SI.
Another 20 mL of the 3 x M solution was further introduced into the feed tank, and the same procedure was repeated. Nine 20-mL additions were mated sequentially to the system (feed tank and reactor), and nine sets of photodiode signals were recorded. The monoazo dystuff concentration in the cell was calculated each time an additional 20 mL of solution was introduced in the system. Each set of signals contains the photodiode signals (offset and actual) for the eight light beams at two mentioned wavelengths. The same procedure was repeated with 2 x M diazo dyestuff solution. The signals were further converted to absorbances, and plots of absorbance vs concentration for the eight light beams at two wavelengths were obtained. Least-squares lines fitted to the data of each plot result in a slope equal to the product of the extinction coefficient and its respective light path. The calculated extinction coefficients were within 1% of the values obtained independently by using a commercial spectrophotometer (Varian Instrument, Cary 219) and were within 10% of those reported by Bourne et al. (1981) and about 20% of these of Bourne et al. (1985). Concentrations were calculated from the measured absorbances and the above extinction coefficients. The resulting measured concentrations are plotted in Figures 6 and 7 vs actual concentrations. The points represent the average of the concentrations from the eight light beams. The maximum deviations of individual beams are indicated by error bars. Transient Absorbance Measurements. The same cell assembly was used for following the transient absorbance of the azo coupling reactions. The cell was assembled with the stretched rubber membrane separating the upper and lower chambers. The upper chamber contained B (diazotized sulfanilic acid), while the lower chamber contained A (a-naphthol). The feed concentration of A and B was 1.2 X low5M. With this fed concentration the homogeneous reaction half time (50% conversion) was 22
ms. Solution B remained in its acidic form with a pH value of approximately 2.5, and solution A was buffered at pH 10 by adding 0.02 M sodium carbonate (Na2CO3)and bicarbonate (NaHCO,). Before initiation of reaction, the temperature of the two chambers was equalized and adjusted to match the ambient temperature which was maintained at a constant 25 "C. The impeller speed was set and controlled at 234 rpm (impeller Reynolds number 10000). The mixing time is on the order of 30 s (Dickey and Fenic, 1976), and the mixing half time is about 4 s (assuming exponential decay). Offset and blank photodiode signals were recorded. Reaction was initiated by rupturing the membrane with the aid of the four needles in the upper chamber. Sampling of all sensors (thermistors, photodiodes, and tachometer) was started simultaneously. Upon the completion of reactions and sampling, raw data from thermistors, photodiodes, and tachometer were stored in the RAM memory of the host computer (IBM PC). The raw data were then transferred to a floppy disk for further analysis. The photodiode signals were later converted to transient absorbances. For each measurement, eight sets of transient data, each from one light beam, were obtained. Figures 8 and 9 show typical results. Repeated runs gave reproducible results within 2.5% of the end absorbances.
Summary STAMPS (a Spectrophotometric Transient Analysis Method for Multiple Positions and Species) has been reported. I t employs multiple light beams, fiber optics, several filter frequencies, time-shared detection, and computerized fast data acquisition. STAMPS is applicable to batch, semibatch, and continuous-flowreactors, to reactor
Ind. Eng. Chem. Res. 1988, 27, 279-283
volumes as large as a few liters, to situations where several positions are to be monitored simultaneously, and to experiments with transient times from about two-tenths of a second to several hours. It is capable of measuring transient, volume-averaged composition and observing spatial composition variations. In the embodiment demonstrated, dual-wavelengthand time-shared detection permit the concentrations of two absorbing species to be followed simultaneously 30 times/s. Eight light beams, each passing through different locations of a 3-L cell (15.8-cm light path), enable the transient volume-averaged concentrations as well as composition variations to be monitored. An IBM personal computer records 600 readings/s (2 wavelengths, 8 light beams, 30 samples/s, plus 30 reference light intensities, 60 thermistor temperatures, and 30 impeller speeds). The operating characteristics of the current embodiment have been confirmed by two types of measurements. Steady-state absorbance measurements using various concentrations of azo dyestuffs (products of the azo coupling reactions) confirm that the method is accurate to within 2%. Transient absorbance measurements following the progress of the azo coupling reactions while mixing occurs confirm that the method is robust, fast in response time, and accurate to within 2.5% in measuring light absorption. Modifications of the current embodiment appear feasible for monitoring four species simultaneously. This extension requires expansion of the dual-wavelengthanalysis section of four wavelengths. The current data acquisition system is capable of sampling and processing the two extra light signals. Also, trade-offs are possible between the number of positions monitored and speed. For example, if only two positions are monitored rather than eight, the speed of response can be 4 times faster, so transient phenomena on the order of five-hundredths of a second can be handled. This change is accomplished by shifting the assembly of beam splitters and mirrors to the interfacce between the reactor cell and the receiver/collimators at the receiving end of the optical fibers.
279
Acknowledgment Much needed financial support was provided by the National Science Foundation (NSF CPE 83-17142) and the Petroleum Research Fund, administered by the American Chemical Society. Nomenclature 4 = a-naphthol; see eq 9a A = 2 X 1 absorbance array; see eq 6 AI, A , = absorbances at wavelengths 435 nm (1)and 577.5 nm (2), respectively, over light path b, dimensionless b = length of light path, cm @ = diazotized sulfanilic acid; see eq 9a C =,2 X 1 concentration array; see eq 6 C1, C, = volume-averaged concentrations of species 1and 2, respectively, mol/L C1., C, = concentration of species 1 and 2, respectively, for iight beam j , mol/L D,, D? = photodiode signals for Piand P:, respectively, V E = 2 X 2 extinction coefficient matrix; see eq 6 k,,kz = second-order rate constants of the azo coupling reactions, eq 9a and 9b, L/(mol-s) P,, P,O = optical beam power of the light beam at wavelength i passing through a solution and a solvent free of absorbing species, respectively, W = optical beam power of the reference light beam bypassing the reactor cell, W R, S = monoazo and disazo dyestuffs, respectively; see eq 9a and 9b Literature Cited Bourne, J. R.; Hilber, C.; Tovstiga, G. Chem. Eng. Commun. 1985 37,293. Bourne, J. R.; Kozicki, F.; Rys, P. Chem. Eng. Sei. 1981, 36, 1643. Dickey, D. S.;Fenic, J. G. Chem. Eng. 1976, 83(1), 139. Palepu, P. T. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1985. Zollinger,H. (Translated by Nursten, H.) Azo and Diazo Chemistry; Interscience: New York, 1961; Chapter 1.
Received for review December 3, 1986 Revised manuscript received August 7, 1987 Accepted September 4, 1987
Iron/Manganese Oxide Catalysts for Fischer-Tropsch Synthesis. 4. Activity and Selectivity Reiner Malessa and M a n f r e d B a e r n s * Lehrstuhl fur Technische Chemie, Ruhr- Universitat Bochum, 0-4630 Bochum, West Germany
Activity and selectivity of iron/manganese oxide Fischer-Tropsch (FT) catalysts of various compositions were studied. The catalysts were reduced at 300 or 400 “C. FT experiments were performed = 11 bar, pco = in a tubular catalytic fixed bed reactor for approximately 200 h of operation (Ptot 2.75 bar, pH2 = 5.5 bar, pN2= 2.75 bar; T = 225 “C 75 h 270 O C ) . Catalysts of pure iron or low manganese content reduced a t 400 “C exhibited a high initial activity which decreased, however, rather rapidly with time. Catalysts reduced at 300 “C were less active; their activity was, however, only slightly affected by time. Small amounts of manganese (3-15 w t %) enhanced the formation of olefins and lowered simultaneously the isomerization activity. Selectivity for oxygenated hydrocarbons was small but highest for catalysts reduced a t low temperature.
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Iron-based Fischer-Tropsch catalysts containing “more than 50% manganese the remainder being iron” (Kolbel and Tillmetz, 1976), or “about equal parts of iron and manganese” (Bussemeier et al., 1976) have been proposed for high olefin selectivity. In contrast to these results, van Dijk et al. (1982) did not find any changes in product
formation when adding manganese oxide to an iron catalyst; their work was, however, done at atmospheric pressure, while the former authors operated the catalysts close to industrial conditions. To elucidate these controversial results, the catalytic performances of iron/manganese oxides having different manganese contents varying from
0888-5885/88/2627-0279$01.50/0 0 1988 American Chemical Society
