ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT produces relative changes between different spectral regions. Since the Tri-Non analyzer photometer system operates a t a true radiation null, the proportional intensity changes affect' all paths equally. Hoiuever, if the radiat,ion intensities of pat,hs P I ,P2,and Ps experience relat,ive changes, small zero shifts result. This effect can be reduced if the nulling path which is chopped in phase with PI if filled with a mixture of int,erfering components in the approximate ratio that' they appear in the compensator cell. Then, the wave length distribution of t,he sum of the radiations transmitted through paths PI and P, more closely approximates t,he radiation transmitted by path P2.
1
Literature Cited (1) Hasegawa, I., and S h a r d , 12. G., PTOC. A m . Petroleum Inst., 28 (111) (1948). (2) Koppius, 0. G., AmZ. Chew?.,2 3 , 5 5 4 (1951). (3) Smith, V. N., Insfrunze?zis, 26, 421 (1953).
Conclusion
A method of sensitizing a particular infrared nondispersive
nitor J. K. WALKER, A. P. GIFFORD,
analyzer for a particular gas analysis has been presented; this method is generally applicable to nondispersive analyzers. This procedure separates the effect of filtering and compensation and allows optimum cell fillings for a particular problem to be approached. The illustration given shows that for a problem with typical spectral interference the energy actually used is a small portion of the initial full beam signal.
RECEIVED for review Septeniber 7 , 1953.
ACCEPTEDJanuftry 2 5 , 1934,
ss AND
R. H. NELSON
Consolidafed Engineering Corp., Pasadena, Calif.
a small,
portable mass spectrometer i s described that i s a conventional 180" type unique in compactness and simplicity. The instrument i s usable for batch analyses of light gas mixtures but i s primarily designed for the continuous monitoring of one or more specific components in u process gas stream. A description i s presented of a flexible building-block concept for designing process monitoring systems for given requirements. System components include an automatic programming device for monitoring numerous gases in one or more streams alternately on a preset cycle. A record of the concentrations can be presented directly in terms of mole per cent. Examples are given of analyses of known multicomponent gas mixtures and applications of mass spectrometer monitoring systems. Automatic, periodic standardization of the mass spectrometer can be made against normal stream composition or stored samples of the pure gases. The final step of closing the control loop on the basis of product analysis i s also shown to be within reach.
SMALL, portable,
inexpensive mass spectrometer is nom available for the continuous monitoring of stream compoeition or laboratory batch analyses of light gas mixtures. This instrument is an outgrowth of several forerunners in the field of monitor mass spectrometers including a large scale model developed in 1949 (3,4),t'he helium leak detector, and several models developed for use by the AEC ( 2 ) . This instrument is in effect a coinpromise between various design features of the first two instruments mentioned. Since a comparison of these instruments is covered in a report by Slarr et al. (4),the scope of this paper is limited t o an instrument description, performance characterist'ics, and applications of the instrument both as a laboratory bat'ch analyzer and a continuous stream recorder as adapted t o an acetylene plant. By monitoring vital component's of t,he lean cracked gas for maximum production versus operating parameters, the final step of closing the control loop in the automat'ic plant is shown to he within reach in the near futurc. lnotrument Description The compactness and four-wheeled portability of the instrument is shown in Figure 1. T h e instrunient is provided with a floor lock and requires only 110-volt power and cooling rvater for operation. The small 40 X 24 X 38 inch cabinet can be placed in an out-of-the-way, on-the-site location in a plant for remote recording a t a master control panel.
1400
The instrunlent is designed t o be one of a series of building blocks necessary for a closed loop automatic plant control With this principle in mind the instrunlent is compos group of easily removed, replaceable units as illustrated in Figure 2. K i t h all of the side panels removed, the angle iron frame and track containing the magnet and analyzer assembly can be seen on the right' half of t,he case. The electronic cont,rols are locat,ed in the extended drawer-type chassis, which can bc operated either in place or removed from the instrument. The vacuum pumps are located on floating trays directly beneath the dravver. The standard unit includes a continuous gas sampler, automatic scanning circuit, and cyclic scan mechanism for repeated scanning of a single mass peak. The sampler requires only that the sample be clean, dr?, and a t atmospheric pressure. -111 coniponents of the design were chosen to withstand maximum conditions of temperature, humidity, and vibrat,ion. Accessory units include the batch inlet system and recorder contained in the top cabinet'. Space is provided above the pumps for a stream programmer and automatic peak selector. These units xi11 permit scanning of units of G selected peaks of the spectrum in one or more gas streams a t discrete time intervals.
Performance ~
~
~
This mass spectiometer by necessity of its reduced sme saciifices some of the iesolution previously enjo\red in certain laboi atory-type instruments. The resolution as implied in the name of the ionization chamber, Diatron-20, provides complete scparation of adjacent peaks a t mass 20 as shown in Figure 3. S o signifi-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46, No. 1
~
PROCESS INSTRUMENTATION cant loss of resolution is noticed up to mass 40 and by techniques similar t,o ultraviolet and infrared absorption methods the usable niass range may be extended to mass 78 or 80. The Sensitivity of the instrument is comparable to that of most mass spectrometerswith thresholds of detection in the region of 5 to 50 parts per million. Manual attenuation control permits reducing the sensitivity in discrete multiples from 1 to 1000 to provide a wide dynamic range of detectable concentration levels. Additional sensitivity regulation is provided by an adjustable filament current control operable in the range of 5 to60 microamperes. Thepattern stability of n-butane, as indicated by the usual mass 43388 ratio, was shown to vary &3% of the rat.io with a sensitivity variation of &5% over a period of 15 days without temperature control. The interference phenomena experienced in binary mixtures of helium and nitrogen was -0.5yo of the measured gas concentration in 18 cases measured over a period of 3 daysI with a maximum of 2.2% for mixtures of methane and butane. For continuous monitoring applications, the basic instrument is supplied with a capillary leak which draws the gaseous sample from a return line a t atmospheric pressure. The sample forepressure is maintained by a mechanical pump, the exhaud pressure by a combination regenerative charcoal trap, oil d i f f u s i o n pump, and forepump in series. I n addition to automatic scanning of the complete spectrum, a cyclic scanning mechanism allows continuous recording of a portion of a n y s i n g l e peak. This device is designed to scan and rescan t h e peak partially to eliminate variables that may affect inktrument focus during Figure 1 . Portable Mass unattended periods. Spectrometer The record of a period of cyclic scan is shown in Figure 4. The mass 40 peak of argon in the atmosphere is shown nhere full chart scale on the record indicates 1%. S e x t a cyclic scan of this peak is shown for a fast paper speed, then a wider cycle scan, and finally a slow, '/Z-inch-per-minute record speed for around-the-clock monitoring. Figure 5 is a record showing the cyclic scan mechanism incorporated in an automatic peak selector. A unit of six potenti-
I
4 4 h3
Figure 3.
July 1954
30
Figure 2.
Photograph Showing Accessible Units of Mass Spectrometer
ometers is designed to select significant spectral peaks in any desired order for recording a t variable time intervals. Each peak is prefocused and preattenuated, scanned for 45 seconds with a dead time before repeating the six-peak program. These units may be installed in multiples of six, incorporating the features of the previous models for automatic standardization or stream programming.
Applications A compilationof test analyses run by usual laboratory procedures is shown in Table I. Comparisons are made between synthetic and computed gas mixtures of a three-component blend of hydrogen, methane, and butane; a five-component gas mixture of hydrogen, methane, ethane, and butane; and a blend of neon, carbon monoxide, carbon dioside, nitrogen, and methane. These particular gases were so chosen to represent both the noncondensable and hydrocarbon-type rniytures.
I
28
Mass Spectrometer Model 2 1-6 10 Spectrum of Typical
18
16
CI,C2,CB, and Cq
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
15
1'
iz
Mixture
1401
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table 1.
Analyses of Synthetic Test Mixtures with Mass Spectrometer Model 2 1-610 Analyses, Mole 7% Blend 1 BynComthetic puted
Hz
Ne CO COa
sz
C H4
13.6
20.7 13.8
31.1 20.8
.. ..
CtH6
CsHs
n-CaHlo (1
13 21 13 32 20
..
Blend 2 SynComthetic puted 10.0 9 6a
3 5 8 0
..
8
30'0
.. .. 30.0 20.0
..
10.0
.. ..
Blend 3 SynComthetic puted 31.0 30 8
..
..
10:4
3i:9
28'6
29'3
acetylenes were available a t the time, API spectra were used to compute the relative amounts of these contaminants A spectrum of one of the above analyses is shown in Figure 7. Note the usable iesolution in the region of the four carbon ions mass to charge ( m l e ) 50, 51, 52. Total peak height of the various mole ions were used for the computation s ithout regard for interfering spectra.
..
29.8 30.1 90.0 10.1
Table II.
Mass Spectrometer Analyses of Pure Product Stream Vulff Process Co. .4nalyses, Mole % ~bIass Spectrometer Model _21-610 21-103 21-610 95,7 95.7 96 2 0 3 0 3 0 3 1,s 1 8 1 4 0 3 0 1 0 3
B y difference.
An excellent indust'rial application was afforded the instrument in the Wulff Process Co. of Los Angeles ( 1 ) where pilot plant stream monitoring applicat'ions were made. The Wulff process plant is designed for the production of high purity acet'ylene and/or ethylene by cracking of natural gas or pure hydrocarbons. Since the Wulff process is typical of the short contact time, high throughput plants in operation in the chemical and pet,rochemical plants of today, it is mandatory that the process be monitored and controlled automatically. Since t,he present plant design incorporates automatic principles of flow, temperature, and pressure control, the only remaining critical parameters are that of product purity and product,ion efficiency. Both of these variables can be controlled by continuously monitoring gas stream composition by mass spectrometry. The monitor mass spectrometer was put on stream in the plant as shown in Figure 6. The instrument is located bet,ween the control panel and a compressor in front of the high temperature cracking furnace. Sineteen sampling points of interest were connected to a common manifold leading to the inst'rument' to study the following plant operations:
1 2 0 9 0.1 0.1
1 0 1 0
1 0 0 0
0 2 0.1
1
4 2 1
f
i l n esaniple of the true value of continuous monitoring of a product st'ream is shown in Figure 8. The 26 peak of acetylene is monit,ored in the cracked gas coming direct81yfrom the unit as the temperature of the furnace is varied. From right to left the temperature of operation was slowly increased. The rise in the cyclically scaiined 26 peak indicates the acetylene content of the cracked gas. Each vertical division on t,lie chart represenh approximately 1 minute and the peak height, is proportional to per cent acetylene. Acetylene production was increased approximately 40% in a period of 15 minutes with a temperature rise of 5' C. Further increase in t'emperature indiAcetylene
1. Cracked gas composition versus temperature 2. Fuel gas composition 3. Acetylene or ethylene content of recycle gas 4. Diacetylene scrubber efficiency 5 . Acetylene or ethylene absorber efficiency 6. Solvent recovery and/or losses T. Final product purity
TzQ
44
16
Cracking
28
Go%
40
26
27
---.-
Purity of the final product and composition of the cracked gas were of utmost concein. Table I1 shows two analyses of the pure gas made on the monitor mass spectrometer as compared to the laboratory instrument. Since no calibrations for the substituted
Figure 5. Automatic Peak Selector Sequence
6"/min
tA r g o n In A i r
Figure
1402
4.
Typical Cyclic Scan Record of Mass Spectrometer Model 2 1-610
Figure
6. Monitor Mass Spectrometer on Stream in Plant
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46,No. 1
PROCESS INSTRUMENTATION or ethylene production versus tarring as measured by an increase in the substituted acetylene compounds. Production versus temperature is being studied by completely scanning the spectra to establish material balance studies. Once these programs are established to reveal the most information, an automatic programmer will be 17 I5 I3 52 43 39 36 27 24 made to monitor the streams of interest. An example of the desired composite Figure 7. Mass Spectrometer Analysis Spectrum of Wulff Process Acetylene Produd Gas data is represented by Figure 10, which is a graphic representation of the cracked gas composition versus temperature on the basis of the propane charged. As the temk E l a p r c d time 15 mi".+ Temperatures perature is increased in the furnace the propane is first cracked A-K)3O0C to methane, folloved by hydrogen, then propylene, ethane, B-10450c 40% ethylene, and finally acetylene a t the higher temperatures. , Temperatures t C ~ H I Prodwtieli Temperaiures It is possible to select a temperature of operation to obtain approximately any desired ratio of ethylene to acetylene along the lower curve below which acetylene content of the gas is indicated and the ethylene, methane, hydrogen, carbon monoxide, and carbon dioxide above. 1"51101*
I"
Figure 8. Automatic Record of Mass 26 Peak as Temperature Is Varied in Product Stream Wulff Process pilot plant for acetylene
Figure 9. Acetylene Content of Off Gas Versus Dimethylformamide Flow Rate
cates a drop in acetylene production as a result of the formation of substitution products. Furnace burning cycles are indicated by the periodic dip in the magnitude of the 26 peak. The cycle is repeated a t 1-minute intervals. Figure 9 is an example of a record obtained on the monitor mass spectrometer while continuously analyzing the overhead gas from the acetylene absorber tower. Here the 27 and 26 mass peaks were repeatedly scanned as the flow rate of the dimethylformamide absorber was increased from 5 gallons per minute a t the left to 8 gallons per minute on the right. The 27 peak is directly proportional to the ethylene content of the overhead gas while the 26 peak varies with the acetylene content above the threshold shown a t the right. Ethylene is likewise absorbed from the cracked gas a t a flow rate of approximately 7.5 gallons per minute, thereby decreasing the purity of the final product. Other similar tests have been run on full-scale twin furnaces and a definite relation between furnace efficiencies and reactor temperatures established. This difference was used as a tool to measure the sample dead time, which was established to be a t most 1 cycle. Procedures being used a t present on this application involve periodic scanning of several critical peaks to establish acetylene
July 1954
Conclusion The large number of component gases monitored and the automatic scanning features enable the plant operator to determine both the major components of interest and periodically check for stream contamination from unknown sources. The ultimate conversion of spectra to composition is shown to be possible but is probably of only secondary interest, since the variations in the spectra alone can be related directly to stream control parameters and utilized to maintain optimum operating conditions.
c oo
..
z z >>
Temperature
Figure 10. Cracking Gas Composition Versus Temperature Based on propane charged at Wulff Process Co., using mass spectrometer Model 2 1-61 0
The low cost, simplicity of design and maintenance, and versatility of application of the mass spectrometer may point the way toward continuous process control based on the all-important factor of final product quality. literature Cited and Coberly, C. W., IND.EXG.CHEM.,45, 2596 (1953). ( 2 ) Sier, A. 0. C., et al., Anal. Chem., 20, 188 (1948). (3) Robinson, C. F., Washburn, H. W.,Berry, C. E., and Perkins, G. D., Instruments, 24, 221 (1951). (4) Starr, C. E., Jr., Johnson, F. B., Purdy, K. AI., Charlet, E. bl., and Lanneau, K. P., Esso Laboratories, Standard Oil Derelop-
(1) Bixler, G. H.,
ment Co., Baton Rouge, La., in press. RECEIVED for review September 7, 1953.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDMarch 23, 1954.
1403
