PROCESS INSTRUMENTATION
Field Application of Infrared Analyzers R.
F. WALL, A. L. GIUSTI, J. W. FITZPATRICK,
Monsanfo Chemical
Co., Texas Cify,
AND
C. E. WOOD
rex.
A
total of 17 infrared process analyzers was necessary for continuous analytical control of the integrated system of plants comprising Monsanto’s acrylonitrile process. The procedure developed for this program of instrumentation i s outlined. Sample systems are discussed, and the techniques used in sensitization and calibration are reviewed. Prior to installation the instruments were set up and given an extended test run which considerably reduced the incidence o f trouble during the period of plant start-up. Routine maintenance procedures and current experience are reviewed.
M
ONSAXTO recently has placed in operation at Texas City, an integrated system of plants for the production of acrylonitrile by a new process. Quito early in the development of this project it became apparent that continuous analytical equipment would be essential for process control. It was determined that 17 analyzers of the infrared-type would be needed to provide the information required for proper control of plant operation. The justification in all cases was necessity rather than economics, the opinion being that these economic applications should be deferred until later when their value could be determined better. This immediate program of instrumentation required selection of instruments, engineering design of installations, sensitization of analyzers for the respective analyses, installation and calibration of the analyzers, and maintenance of the instruments after installation. A group without previous training in the field was set up to conduct this program. The experience derived is reported with emphasis on techniques of practical field application rather than on principles of instrument operation. Instruments were obtained from several manufacturers for a comparative evaluation because of the size of the over-all pro-
i.Ti
Main Process header
Condensi-filter
f
Drain
Figure 1 .
July 1954
Sample System
gram. No instrument exhibited outstanding superiority, and the selection was determined almost completely by the ability of the manufacturer to handle the project. The Baird Associates infrared process analyzers were used. The selection of analyzers preceded by several months their delivery. In the interim a test analyzer was used to detect ethylene in the off gas from a thermal cracking unit. This was a difficult problem, because the sample was wet and sooty, and the spectral interferences occurring were severe. However, a background of experience was obtained from which the engineering designs of the installations for the acrylonitrile plant were made. I t was thoroughly established that the sample system should be designed to operate effectively despite the most unfavorable case of sample conditions. The sample preparation system was considered a major part of the instrument rather than as an auxiliary of secondary importance. Following this experience, in the design of the instrument installations for the acrylonitrile plant a protective filter was included ahead of each analyzer, even on streams that were obviously clean. If the stream contained material which required removal, a separate filter for this purpose preceded the final protective one. Kecessity for sample stream drying was avoided in most instances. This was valuable, because it reduced maintenance requirements. Each instrument installation comprises the analyzers plus associated sample handling equipment housed in a small building for protection from the elements. A diagram outlining the basic sample system is presented in Figure 1. The gas handling equipment always includes a small protective filter, preceded by a major filter if the gas contains suspended material requiring removal. ure Certain instruments analyze several streams in sequence through the use of relav ooerated solenoid valves controlled by the multipoint recorder switch. Three-way solenoids are arranged so that while the sample is not flowing through the instrument it is flowing to Separator vent. As a slightly higher pressure drop exists on the path through the instrument, leakage will always be from the sample being analyzed to the vent system
INDUSTRIAL AND ENGINEERING CHEMISTRY
1387
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT rather than the reverse. I n some cases it is advisable to vent the samples t o a controlled low pressure system exhausting to a suitable process stream. Sample pressures and floivs are always controlled.
1 . Bolometer Bridge 3. Compensator Cell
Figure 2.
7. Mirror 0. Entronce o f
4. Sample Cell 5. Interference Cell 6. I.R. Source
2. Filter Cell
and greatly reduced by the use of lithium fluoride. This is particularly valuable either t o eliminate drying the sample stream or t o make the drying much less crit,ical. As to the initial cell fillings, a good guess inay save considerable time, and experience helps. The nomenclature of the cells is present,ed in Figure 2, a schematic diagram of the Baird analyzer. Interference cell is filled with a mixture of the interfering gases, with the most troublesome gas predominahg. Filter cell is filled with 100% of the pure gas for which the instrument is being sensitized. Compensator cell initially is filled with nitrogen. The fillings are revised to eliminate interferences. Carefully purified gases should be used for the cell fillings, arid in particular for the fillings of t,he filter and compensator cells, for the instrument \vi11 be sensitized for any gia present as an
Plant Stream
i
11% C, H, Q
Schematic Drawing of Baird Analyzer 7 5 1
Filt.
100 % GHq
Camp.
100%
Int.
N,
50% C H ,,
h general procedure for servicing, sensitizing, and tcsting the analyzers evolved during the preparation of these instruments for field installation. This same systematic approach modified to fit the particular circumstance is useful in servicing instruments which are returned t o the laboratory for one reason or another. Servicing and Sensitizing Inspection and reconditioning procedure is as follows: 1. Inspect’ visually for obvious damage: correct as required. 2. Inspect electrical equipment for correctness of viiring and grounds. Check the operation of the blower, thermost,at, and voltage regulator. Check the general operation of the instrument by removing the sample cell and inserting a card or pencil in t’heoptical path to obtain a pen deflection. 3. Inspect the cell block for leakage using a halogen leak detector.
Span 0 . 5 1
WAVE NUMBERS IN
3000
4000
2500
IO00
ILOO
Figure 4.
1300
80 UI
0 Q c
2
60
4 .x
5 ul Y
40
a
20
0 2
3
4
5 b 7 WAVE LENGTH IN MICRONS
Figure 3.
1388
Composite Spectrum
LIF
a.
3” c e l l
-15
CMI 1500
co,
0 -/o
I n order to sensitize the instrument obtain the composite infrared spectrum of the sample stream components, and from its examination determine the optical filters, if any, t o be used. While the calcium fluoride opt,ics of the instrument transmit t o about 9 microns, in many cases it is advantageous to limit the transmission by the use of either quartz or lit,hium fluoiide as infrared filters, usually as window on the sample cell. Water interference is almost completely reduced by the use of quartz
5000 100,
10%
8
Per Cent Gas in Nitrogen
impurity. While impurities in t,he interference cell gases ehould be avoided, this cell intercepts both beams and impurities lierc usually are not so detrimental as they are in the filter and compensator cells since, each of these intercept only one beam. Successive revisions of the cell fillings are made folloi~inginspection of intcrference curves prepared by running ‘1 I200 1100 IO1 I belies of samples of each iriteifering gas in nitrogen covering the concentratioir limits expected in the sample. Over the desired range of operation these curve:: should be independent of concentration, and although t,hey do not necessarily need to exhibit zero interference, thif is desirable. Usually trends can be seen quite rapidly, permitting a satisfactory sensitization after a fev trials. Where complex interferences occur logic is of little value, and a great, deal of work may be required t o obtain a satisfactory filling. I n practice, the sensitizat,ion procedure is largely a matter of trial and error. Data pertaining to t’he sensitization I of t,he a n a l y z e r f o r e t h y l e n e i n cracked gas are offered as an example of this system. Figure 3 presents the
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. 7
PROCESS INSTRUMENTATION
50
I
Filt. 100% C, H,
50
COmp. 20 % C, H,
1
e
11%
C,H,
Filt. 100% C, H, Comp. 25% C,H,
15% C 3 H g
8 0 % N,
60% N,
Int. 100% C,H,
7 0 % C3H,
Int.
10 %e C3H, 2 0 % C H, Spon 0.25
20
10
-15
’I
n
30
b 10%
co, CaH8
* -I I-
Figure 5.
Figure 6.
Per Cent Gas in Nitrogen
composite spectra of this stream with the transmission curves of calcium fluoride, lithium fluoride, and quartz superimposed. An examination of this figure indicates that the use of lithium fluoride as an infrared filter will reduce appreciably the interference to ethylene of water vapor and of other stream components in the 5- to 9-micron region. A lithium fluoride filter is therefore applied. The initial cell fillings and results obtained are indicated in Figure 4. A subsequent filling and the final filling are presented in Figures 5 and 6, respectively. The instrument is calibrated best on the actual sample stream by reference analyses. This is a complex problem. The solution offered is one of many Crystal Window and quite possibly not the best. m \
Per Cent Gas in Nitrogen
sensitize and place the instruments in operation a t the time the plant was started.
Modification The analyzers were supplied with undemountable cells that were considered undesirable in view of the possibility of cell contamination by some of the streams analyzed. Accordingly new cells were designed to replace these. Their construction is
&
Pipe Thread D r i l l On Tonamt
Cell Mounting Plate
/
Test Run After the instruments for the acrylonitrile plant had been received, checked, and satisfactory sensitizations obtained, each analyzer was run for several days, with occasional sensitivity checks made. The instrument was then turned off, and restarted after a day or two for another run of scveral days. This was repeated as time permitted. The trial run of 2 weeks or longer, including one or more cycles of heating and cooling, disclosed cell leakage and instrument troubles that o t h e r w i s e m i g h t h a v e developed shortly after the instrument was installed in the plant. Following satisfactory completion of the sensitization and test runs, the cell blocks were removed pending installation of the analyzer cases in the plant. The subsequent experience when the instruments were initially placed in operation in the plant indicated that this rather extensive preliminary procedure paid off in reduced troubles during the critical period of plant start-up when most of the difficulties weie associated with the rather complicated sample systems, and the instruments themselves were rather trouble-free. It would have been impossible to July 1954
Tubing End Block
Figure 7.
-
Sample Cell
I Recorder
Revised
Bolometer Wiring
INDUSTRIAL AND ENGINEERING CHEMISTRY
1389
ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT shown in Figure 7. They have gasketed rabher than cemented cell windows. These have proved satisfactory in practice. Some difficult'y was experienced with excessive BO-cycle noise in the rather long leads between the instrument and recorder. A revision of t,he bolometer wiring as shown in Figure 8 reduced the noise level. With the original wiring the bolometer out,put was floating superimposed on an a x . signal xhich was dependent on the values of bridge resistance and also upon the temperature. This may have amounted t o a few tenths of a volt, With the revised bridge wiring the output' is very close t o ground a t all temperatures. Usable instrument sensitivjty is almost entirely determined by drift. The specifications allow a 5% of scale drift with a 0.1-ohm transfer resistor which determines sensitivity. Normally inst.rument operation is within this tolerance; however, this has not proved t o be a very useful range. As the natural tendency is to read the chart to within at, least 1 division, the accuracy of the
instrument generally is assumed to correspond to this, and a detrimental evaluation of the instrument usually results from daily drift in excess of 1 division. X transfer resistor of about 0.2 ohm provides a much more practical noise and drift level, and for most applications a Satisfactory calibration curve has been obtained with a transfer resistor in this range. These analyzerq have been maintained by the plant instrument department since shortly after their installation. The routine maintenance comprises a daily zero adjustment and a weekly calibration and check of certain items of instrument operation. Trouble shooting has been concerned as much with associated equipment as with the analyzer units. Maintenance requirements have decreased steadily, and instrument accuracy has improved. From the very beginning these analyzers have provided the information needed for process control. RECEIVED for review September 7, 1953.
ACCEPTEDMay
4, 1951.
Installation of Continuous Infrared Analyzers Sample Treatment S. H. WALTERS Process Controls Division. Boird Asrociafes, fnc., Cambridge, Mass.
The use of plant stream analyzers (infrared absorption-type instruments) requires special considerations of sample treatment. These involve possible corrosiveness of sample, temperature and pressure control of sample, flow rates, and sample filtering of solid particles. Engineering considerations further involve accuracy of measurements, suitability of instrument to control of process, and monitoring of more than one stream with the same instrument. The intent of this article is to emphasize the importance of sample handling considerations and to answer the more general questions confronting the potential user of this type of instrumentation.
T
HE operating principles of continuous infrared arialyzers have
been known for upward of 20 years, and instruments have been commercially available for some 9 years. In spite of this, the widespread use of these instruments has only been evidenced during the last 3 t o 5 years. The purpose of this paper is to answer some of the quedons which confront the potential user of this type of instrumentation,
Considerations of Suitable Samples The continuous infrared analyzer is fundamentally an instrument for indicating the concentration of one component in a multicomponent sample by virtue of the unique infrared absorption exhibited by that single component. It is, therefoie, fundamental that the desired component evhibit some measure of infrared abeorption. Those molecules that do not exhibj t infrared absorption are the symmetrical elemental gases such as oxygen, hydrogen, nitrogen, halogens, and the rare gases. I n theory, a t least, all other inolecular compounds can be detected uniquelv and measured on a continuous basis even though in the sample they exist in the presence of many other infrared absorbing components Since the sample is caused t o flow continuously through some form of optical cell within the instrument, extreme piecautions
1390
are necessary to ensure that components within the sample will cause no deterioration to the optical properties of the cell. Typical cells are shown in Figure 1. Although varying xyidely in design, cells have a common property in that they include windows which must, be transparent t,o the infrared light passing through the cells. It follows, then, that. the sample stream must be free of solid particles, since, should these be present, there could result a build-up of these solids on the inner surface of the windows. Over a period of t,ime the window would become increasingly opaque t o infrared radiation and, hence, the instrument would lose its sensitivity or its calibration. With an instrument designed and calibrated for a specific gas analysis, should the sample contain any component which could condense and cause a liquid film on the inner surface of the windows, this too could easily result in extreme changes in sensitivity and calibrat,ion. The infrared absorption of a compound in liquid phase is vastly different from its absorption when in gas phase. The continuous infrared analyzer in effect counts the particular molecule for which it is sensitized. Any factor which therefore changes the number of such molecules within the sample cell will be indicated by the instrument as a change in concentration. The instrument is calibrated normally with samples of known concentration and a t fixed temperatures and pressures. It is
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 7
