RERORT
FOR
ANALYTICAL
CHEMISTS
Laboratory Instrumentation Moves into Plant Some time a g o i n f r a r e d analyzers w e r e converted f r o m l a b o r a t o r y instruments t o process analysis instruments. M o r e recently there has b e e n a m a r k e d upsurge in efforts t o design chromatographic analyzers f o r plant use. M a n y o f the problems o f process stream analysis a r e being overcome. The next step, control o f the process stream, is shaping up r a p i d l y . This month's Report f o r A n a l y t i c a l Chemists gives a bird's-eye v i e w o f the problems involved a n d the t y p e o f instrumentation being d e v e l o p e d t o meet the challenge.
A B R A H A M SAVITZKY, as Assistant N e w Product Coordinator of the Instrument Division of the Perkin-Elmer Corp. aids the division's General Manager in the field of new products. Savitzky joined Perkin-Elmer in 1950. He was chiefly concerned with the design and development of infrared instruments. Later, he became involved with feasibility studies and applications of nondispersive and dispersive infrared plant analyzers. He assumed his present position in 1956. A graduate of the New York State College for Teachers, Savitzky received his Ph.D. in physical chemistry from Columbia University in 1949. He holds memberships in a number of societies and is chairman of • the Education Committee for the Western Connecticut chapter of the AMERICAN CHEMICAL SOCIETY.
' T ' h e main purpose of chemical and -*- refining processes is t o produce end products of specific p u r i t y and a t a maximum yield. To do this, four r e quirements must be m e t : 1. The final product m u s t not be contaminated with waste materials. 2. No more t h a n the practical minim u m of the end products can be lost in the waste stream. 3. T h e reaction m u s t proceed to the completion point t h a t results in maximum conversion. 4. T h e reaction m u s t take place under proper conditions, so t h a t the desired products result a n d . t h a t no unfavorable side reactions t a k e place. " U n t i l recently, these requirements were m e t b y what m a y be called 'control b y inference.' B y inferring—most of the time correctly—that u n d e r . c e r tain controlled conditions of temperature and pressure, and with certain controlled flows, the required results would be attained. T h a t system of control was fine as far as it went. I t did not provide a very accurate measure of the changes t h a t constantly occur in the process, such as spent catalyst, unforeseen accumulation of
impurities, and changes in the composition of the raw material itself. Obviously, unless those basic wrongs are righted, the end product either fails to come u p to specification or is not p r o duced a t maximum yield. I t used to be t h a t serious defections in the abovementioned areas were not discovered until a laboratory analysis was run. T h a t took time and permitted wasteful operation of the process plant while the trouble was being traced down a n d corrected, usually not without considerable loss and expense (8)." I n recent years with the improvem'ent of instruments and techniques, this function has been taken over by the composition analyzers. These analyzers are being given increased a t t e n tion, both b y instrument engineers a n d more and more vendors who are offering specially developed equipment t o the m a r k e t . I n their basic form, stream analyzers are industrial variations of standard laboratory instruments. T h e y operate on the principle of measuring a physical or chemical effect related to the change in concentration of the compound involved. There is usually some degree of interference of other materials. These interferences often can be the determining factor in the success of a particular application. Few methods are as specific as the measurement of magnetic susceptibility for oxygen. A distinction can be made in terms of selective versus nonselective m e t h ods. T h e nonselective instruments measure such properties as refractive index, dielectric constant, a n d thermal conductivity. T h e y are useful only when they can be applied to essentially binary systems. Selective methods, on the other hand, measure some multivalued property—infrared or ultraviolet absorption, mass spectrum, a chemical reaction combined with a physical measurement, or v a p o r phase chromatography. I n order to consider the general problem of composition analysis only two measuring principles will be considered—infrared (represented b y dispersive and nondispersive analyzers) and t h e newer vapor phase chroma-
tography. B u t a n instrument b y itself is not useful in a process application. I t is also necessary to consider the equally i m p o r t a n t factors of sampling and its effect on the response time of the system, t h e use of these instruments in closed loop control, and finally the m a t t e r of installation. Announcements m a d e in t h e past year of new, low-cost infrared spectrometers, which almost certainly will find their way into the control laboratory, indicate the growing interest in infrared spectroscopy. As the control l a b oratory becomes more familiar with infrared methods, in i m p o r t a n t situations requests will almost certainly come t o provide a continuous record of the infrared absorption of the desired component. T h e laboratory infrared spectrometers are not, of course, designed t o be used under process conditions. There are no provisions for continuous sampling, or for explosion-proofing. T h e y will not operate unattended for the long periods required of a process instrument. An instrument which has been designed particularly for t h e process function (2, 6) differs from t h e conventional laboratory infrared spectrometer in three respects. First, it is enclosed in an explosion-proof case. Second, it has no provision for wave length scan under operating conditions. Two Littrow mirrors are used in place of a conventional single mirror, which in laboratory spectrometers 4 s rotated to scan the spectrum. One mirror selects a wave length a t which t h e sample absorbs, a n d thus provides the measurement information. T h e other Littrow mirror is set to a wave length a t which no sample absorption takes place, a n d is used as a reference to compensate for changes within t h e optical system as well as changes in interfering materials in the sample. Third, a separate chamber, effectively external t o the main optical system, is provided for the sample cell, which is designed for continuous flow and rapid exchange of sample. T h e beams falling on the two Littrow mirrors are actually superimposed a t the sample cell, so t h a t VOL. 30, NO. 3, MARCH 1958
•
17 A
REPORT FOR ANALYTICAL CHEMISTS
Mark
The
of Superior Craftsmanship in
OSCILLATOR PREAMPLIFIER (ON DETECTOR)
DISTILLATION GLASSWARE Plus Economy and Service A Compact Fractionation Unit for distilling small quantities i n a vacuum
65)6
6530 Distilling Head,Swinging Funnel Type. Reflux ratio regulated on a t i m e basis. Therefore, independent of throughput except at small values.
At Your Service In addition to M a n ufacturing standa r d items, Ace specializes in Custom-fabricating Glassware to your specifications.
6537 • stilling Head with Automatic reflux control.
Distilling Head for variable reflux ratio. C o n s t r u c t e d so t h a t satisfactory operation can b e m a i n t a i n e d under any pressure condition.
Figure 1. Nondispersive type infrared analyzer L
Ù I
deterioration or particles in the sample affect both beams equally and do not change the measure of value. Actually, the dispersive type of infrared analyzer, although it is most closely related to the laboratory instrument, is a relative newcomer to the analytical picture. For many years there have been available variations on the nondispersive analyzer. The nondispersive analyzer is somewhat like a filter photometer, using gases or vapors of a desired component as filters. There are two categories of these instruments available—one using a broad band black body detector such as thermopile or bolometer; the other a narrow band detector employing the gas of interest as the sensitizing absorber in a condensing microphone. Figure 1 is an example of the latter type U,5,7). The specificity of the infrared method allows an analysis to be made directly on relatively complex sample streams, while compensation against changes in the other components in the stream is possible. A detailed knowledge of the stream composition and range of variation of components is required in order to sensitize the analyzers properly. An example of the problems and pitfalls in the application of these analyzers to specific situations arose sev-
J I
I DEMODUL
H.
PREAMPLIFIER
eral years ago in connection with controlling the end point of the hydrogénation step in the hardening of fats for use in margarine. The particular manufacturer had recently installed a modern hydrogénation unit. There remained, however, the crucial problem: just when to stop hydrogénation. It was being done by following the reaction on a laboratory refractometer using spot samples. When the refractive index reached some value (different for each batch of raw material), the hydrogénation was stopped and a sample sent to the lab. Aside from the fact that there were no commercial process refractometers available at the time, it is important to note that the refractive index was not the determining criterion as to completion of the process. Examination of the spectrum of the original and final materials showed a nice strong band around 10 microns. Natural oils apparently always have a cis-configuration around the double bonds of the unsaturated fatty acids. During early stages of the hydrogénation procedure there is, in addition tohydrogénation of the double bond, considerable isomerization to the transconfiguration which gives rise to this bond. The take-up of hydrogen and change in refractive index parallel the· increase and intensity of this band-
Above items,plus many others, are listed in Ace Catalog " 5 0 " . Ask Dep't D G - F for free copy.
ACE G L A S S I N C O R P O R A T E D
^gg^VINELAND • f i t J*)'
^ 0
NEW JERSEY
Midwestern Division
LOUISVILLE, K Y . - B o x 9 9 6
S-pœcùxiiùtà to Ivtduat'uf and ^eaeaica Circle No. 18 A on Readers' Service Card, page 83 A
18 A
•
ANALYTICAL CHEMISTRY
Figure 2. Process. chromatograph analyzer
REPORT FOR ANALYTICAL CHEMISTS T h e higher melting point t r a n s - m a terial definitely contributes t o t h e hardening of the fat. Unfortunately, in some stage in the hardening process, the r a t e of decrease of double bonds wins out over t h e r a t e of increase of trans-material, a n d the curve levels out a n d t h e n decreases with further hydrogénation, t h e critical point in margarine production is a t t h e least sensitive portion of this curve. This eliminated one possible m e t h o d of analysis. Further examination showed a change a t 15 microns which, although small, increases uniformly with h y d r o génation. T h e question then remaining was—what value is to be used for the end p o i n t ? Obviously, w h a t was needed was a determination of "quality of m a r g a r i n e " vs. some physical p r o p e r t y which they were now measuring and a comparison of t h a t physical p r o p e r t y with the infrared d a t a . Although no criterion for a "good m a r garine" was found, it was possible to correlate infrared readings with other physical d a t a with most interesting results. N o t only was there a scattering of points with the dispersive analyzer against refractive index, melting point, iodine value, dilatometer readings a n d penetrometer readings, b u t no two of these criteria gave any b e t t e r correlation. A p p a r e n t l y it just takes a n experienced eye, a "feel" for the blending a n d the final criterion—did it b a k e a good cake? T h e problem remained unsolved.
Infrared Leak Detectors Use of infrared analyzers for leak detection is a unique application in which the selection of an instrument is determined b y the requirement for rapid response coupled with high sensitivity. M a j o r automobile manufacturers r e quired such a n i n s t r u m e n t for assembly line testing of the new air suspension systems to be offered on 1958 model cars. Various normally acceptable methods of leak detection were tried and judged unsatisfactory. During these trials, engineers of P-E's I n s t r u m e n t Division were called in. T h e solution developed b y t h e m was a modified nondispersive infrared analyzer employing nitrous oxide in t h e air spring as the trace material. P r o t o type instruments worked successfully and the first such analyzers to be used b y the automobile industry were placed in operation a t several manufacturing plants early last fall. T h e analyzers will show within 3 seconds nitrous oxide concentrations as small as 1 p.p.m. High 'selectivity
FR£E
TO ANALYTICAL CHEMISTS
ANALYTlCAl
!
HLTER PAPER
SAMPLED
y Figure 3. Process chromatograph column with dome removed. Not shown are the programmer (see cover) and recorder makes the detector practically insensitive to the usual stray gases and atmospheric contaminants present in highproduction plants. Simplicity of o p eration is one of t h e m a n y advantages of the instrument over p a s t systems; no high vacuum nor capillary leaks are used. Nontechnical personnel can readily operate and move t h e selfcontained detector. By manually moving a probe over the nitrous oxide pressurized system, the operator is given b o t h visual and audible indication of a leak. Variations in loudspeaker pitch and meter reading lead to the location of t h e leak point. Gas Chromatography Process Analyzers Infrared analyzers are characterized b y the rapidity with which changes in sample composition can be followed. B y giving u p rapidity of analysis, it is possible to follow the changes in more t h a n one component b y employing the technique of gas chromatogr a p h y . T h e meteoric rise of this technique is accented by the amazing fact t h a t process chromatography analyzers were introduced just two years after the first commercial laboratory instrum e n t became available in 1955. T h e power of this analytical technique lies in t h e fact t h a t complex mixtures are first separated into their individual components, and then these
T o a c q u a i n t y o u w i t h S&S Analytical Filter Papers, w e invite analytical chemists to mail t h e coupon b e l o w for a free sampler m a d e u p of several grades. As a n a n a l y t i c a l c h e m i s t , y o u k n o w t h a t precise chemical analysis requires filter p a p e r of t h e p r o p e r quality a n d density. F o r those w h o d e m a n d only t h e finest working tools, S&S Filter P a p e r s offer these a d v a n t a g e s : 1. S t a n d a r d i z a t i o n , for consistent accuracy — complete physical uniformity. 2 . S e l e c t i o n , for m o r e precise analysis — a w i d e r a n g e of t y p e s from w h i c h to choose. M a k e y o u r o w n tests! Send for your S&S Filter P a p e r Samplerl
S&S ULTRA FILTERS S&S dual-purpose Ultra filters — smooth-surfaced membranes possessing extremely uniform microstructure—are ideal for filtration of colloids, proteins, and micro-organisms, as well as dialysis and osmosis. Mail coupon below for free S&S Ultra Filter Catalog.
M A I L THIS COUPON
φ
TODAY
CARL SCHLEICHER & SCHUELL CO.
Dept. AC-3, Keene, New Hampshire
Send me your F R E E
Π S&S Filtçr Paper Sampler Ο S&S Ultra Filter Catalog NameCompany. Address CHty_
-State-
Circle Nos. 19 A-1, 19 A-2 on Readers' Service Card, page 13 A VOL. 3 0 , N O . 3 , MARCH 1 9 5 8 • 19 A
REPORT FOR ANALYTICAL CHEMISTS
ι
Dependable NALGON
ISOBUTANE " 2
FRACTOGRAM
The versatility of N A L G O N if Plastic Tubing 11 p r o v e s its *| worth on everypoint ! J,
ν
X
/
ff
Available i n a11 s i z e s
fS J$*
^"' ψ from Y%" ID # through 2" ID.
ί
Figure 4. Method of presenting peak heights (top). Typical analysis using process chromatograph (bottom)
hi d ιΜ u il d.u LlMdiLlkd.ua U Mil il il il il il il
components are measured by relatively simple techniques, as binary mixtures in a carrier gas. Figured schematically illustrates the instrument and the method. Both gas and liquid chromatography depend upon the same fundamental principle—-the passage of a mixture of components through a column, packed with a suitable adsorption or partition material, will result in a separation of the components by virtue of their dif fering affinities for the column sub strate. By impelling a sample stream through the column, the compounds having the weakest affinity for the sub strate will appear at the column exit in advance of those compounds having a greater affinity for the substrate. Carrier gas is supplied to the system from an external gas cylinder equipped with a pressure regulator. Helium is normally used because of its high thermal conductivity and inertness. Nitrogen or dry air may be used where high sensitivity is not required. The carrier gas flows through a sec ond pressure regulator to the reference side of the thermal conductivity de tector and from there to the sample in jection system. The sample is then carried into the column, where the sep aration of components occurs. As each component arrives at the column exit, it flows through the sensing side of the thermal conductivity detector. The difference in thermal conductivity be tween the sensing and reference sides,
which are part of a bridge circuit, pro duces an unbalanced voltage which is measured by a standard recorder. The time required after injection for a given component to appear at the column exit is called retention time. By main taining a constant flow rate, the re tention time for a given component can be reproduced exactly. A typical set of the components in packages for plant environments con sists of the common column and re corder of the laboratory instrument plus a programmer to provide automa tion of manipulation normally per formed by the laboratory operation (Figure 3). Quantitative analysis of up to four components is possible, al though others may be present, provided they do not interfere with components of interest. The dynamic range, or ratio of the full scale spans of the largest to smallest concentration, can be greater than 1000 to 1. The ulti mate range is, of course, dependent upon the problem and on the elution order of components since peak height measurement is used. The data are presented in the form of a bar-graph
A ^\VlV¥l PRODUCT
Λ
TIME (SEC.) !/4"l.D. SAMPLE LI NE ( TWO 8" St MPLE CELl '«O.C 520
WEΠSAMPLE CELL
lR. — RADIATION "
Ask your dealer for catalog F-957
the
RESPONSE
O)
/! /
NALGE CO. Inc.
/ /
SAMPLE LINE VOL. | Ο" VOL. FLOW RATE ! _ SAMPLE CELL VOL. ! ' VOL. FLOW RATE
L-T„H— T , - - I
R O C H E S T E R
2 .
N E W
World's largest producer of Plastic Laboratory Ware! Circle No. 20 A on Readers' Service Card, page 83 A 20
A
•
ANALYTICAL CHEMISTRY
TIME
Y O R K
Figure 5.
Optical analyzer
u
98
100
102
104 106 108 TIME (SEC)
110
112
114
116
Figure 6. Response of nondispersive analyzer with low sample flow rates (top); and with higher sample flow rates (bottom)
(Figure 4, top). This is simply a convenient display of peak heights with a recorder chart advanced a uniform distance between successive component peaks. Direct calibration, by means of a sample of known composition, together with a separate range control for each component, makes it possible to read percentage concentration directly from the recorder chart. A size 1A bottle of helium carrier gas should last for about 2 months of continuous operation. A clean, dry sample, free from known column contaminants is introduced into the analyzers at a rate of about 1 liter per minute for reasonably rapid response of the sampling system. The sample flow can be as low as 50 cc. per minute under special conditions. The instrument proper—column, sampling valve, detector, regulator, and flowmeters— are in or adjacent to the explosionproof dome. The chassis below the recorder contains the timing mechanisms and electrical circuits. The recorder and programmer are mounted in the control room and can be purged. The sensing head is at the process sampling point. Figure 4, bottom, is a typical analysis, which has been done on this instrument. Since there are seven components in this process stream, and only four can be recorded, the recorder has to be made sensitive at the times when these components occur. The components which will not be recorded are irons-2-butene, isobutylene and 1-butene, and cis-2-butene. In addition, as there is a wide range of concentration, it is necessary to program the instrument to expand the ranges for the relatively weaker components. Having discussed measurement methods there still remains the problem of getting something to measure and getting it in a reasonable length of time. This is especially important where the instrument output is to be fed to a controller. A sampling system consists of sample purifiers, the lines which carry the sample to the analyzer, various niters, and valves—all of which have a finite volume. I n order for a change in concentration to be measured by the analyzer, the sample must travel through the dead volume which is represented here by the sample line. Then, and only then, can it begin to be seen by the instrument. At this point, the change begins to take place in the sample cell, and the response time of the instrument, itself, enters in. Figure 5 applies primarily to the optical analyzers; the vapor chromatography instruments are unique in that even after a change reaches the sampling valve, there is an additional wait for the particular component to
/ ALWAYS KNOW £XACUY HOW MUCH I'M DRINKIN' SAYS LIMEY . . . THE LIME GLASS MAN "Knowin' exactly how much you can drink . . . and then drinkin' only that much and no more . . . that's the real secret of this tap 'n bottle business. I learned that while I was still at the glassmakin' plant. Saved me lots of headaches, too! " O f course, you don't need a Diamond D Blue Line cylinder to measure every drop. In the picture above I'm just usin' the cylinder to point out that Doerr Diamond D Blue Line cylinders are so accurately calibrated that whenever precision counts. Diamond D is the brand to use. It's strong and sturdy, too, useful for practically any laboratory service. "By the way, it might seem crazy to you to see me usin' a Diamond D Blue Line cylinder just to measure a few ounces of sparklin' brew. But, you know, many l a b men today are buyin' expensive heat-resistant borosilicate glassware and yet they know fer sure that it will never see an open flame. Now . . . that's what I call a waste of good money. It's as crazy as a tavern owner buying heat-resistant glasses to serve beer to his patrons. "Most fellows you talk to just never realize that heat-resistant glassware was that much more expensive. You should see the look on their faces when I tell 'em that a research director out on the West Coast saved $3,659.83 in just six months by switchin' to lime glass. "The whole story is in the new Doerr Glass "FACTS" book. If you haven't received your copy yet, just ask your lab supply house for a copy or write directly to Doerr Glass Company, Vineland, New Jersey." DIAMOND D BLUE
LINE
IS A PRODUCT OF
DoerrGlassCompany V i n e l a n d , Itfe-w J e r e e y For further information, circle number 21 A on Readers' Service Card, page 83 A VOL. 30, NO. 3, MARCH 1958
•
21 A
SCIENTIFIC TESTING INSTRUMENTS MADE BY A
FUNCTIONING LABORATORY
TESTING INSTRUMENTS for the CHEMICAL INDUSTRY
W e Make Them More Efficient Because W e Use Them, Too! As the only functioning labora tory, manufacturing instruments, we harness the talents of the labo ratory and the instrument techni cians. Result: the instruments are conventional in operation, simple to calibrate, easy to maintain. They are sensitive, stable, and, in most cases, applicable to multi purpose testing requirements. And all are pre-tested in our own laboratories. Our instrument service is four fold: we design and manufacture new instruments ; we modify exist ing instruments to meet special needs; we manufacture instru ments from existing designs; we operate new instruments where impartial laboratory reports are essential. Write for our free Instrument Service Bulletins.
T»re-0-Tome*er for detergenty tarts
Universal Pendulum Impact Teil*f
SERVICE WITH TRAINED MANPOWER, EXTENSIVE EQUIPMENT
How an Independent Labor atory Can Accelerate Your CHEMICAL TESTING a n d RESEARCH If your research chemists are immersed in time-consuming projects at the expense of more vital creative effort, our facil ities and services can break the bottleneck. We offer a program of testing and research as broad as the chemical industry itself . . . fully capable of assuming many of your laboratory burdens and releasing your chemists into more fruitful research ventures. Included in our services are: Evaluation of Products Applied Research Physical Tests Analysis Specification Tests Almost any problem in chemical testing, research, product design, and product development falls within the scope of our services.
UNITED STATES TESTING COMPANY, INC. Established 1880 1270 Park Avenue, Hoboken, N.J. Boston · Brownsville · Dallas · Denver · Los Angeles · Memphis Philadelphia · Providence · San Angela Far further information, circle numbers 22 A-1, 22 A
•
ANALYTICAL CHEMISTRY
· New
York
22 A-2 on Readers' Service Card, page 83 A
REPORT plute. The data appear as a spike, and the next piece of information regarding the particle component appears in the next operational cycle. Here one is essentially trading time for information. The optical analyzers give a continu ous record of the concentration of a single component. The chromatog raphy instruments give a discontinuous record up to four components. The time lag in the response of a nondispersive analyzer to a step change in concentration for a very low sample flow rate using a 100-foot sample line is due primarily to the delay in the sample reaching the instrument (Fig ure 6, A). At much higher sample flow rate, the sample arrives at the instrument in a few seconds. The change within the sample cell is so rapid that the response of the analyzer servo shows up in the slight overshoot of the ob served curve (Figure 6, B). The sample lags can become an especially difficult problem with liquids in an instrument such as the dispersive analyzer, for often very thin sample cells are used. They are just incapable of passing sample at a reasonable rate when one considers the holdup in any practical sample line. It is possible to improve the response time of an over-all system with a by pass. In the cells actually used, the bypass line is built directly into the sample cell body, so that there is mini mum delay from bypass to measure ment. Substituting the sampling valve of the vapor chromatograph for the sample cell reduces the lag time for these instruments in a similar manner. The response time of these analyzers is still the limiting problem in closing the control loop. For this reason, it is not usually desirable to control a valve directly from the composition analyzer. The analyzer is best employed in a cascade control system, where it merely resets the control point of an environ mental control ( 1 ). If analytical data are to be used for reset in a relatively stable process, the lag time of the vapor chromatograph becomes much less im portant. Literature Cited
(1) Aikman, A. R., ISA Journal 4, 363 (1947). (2) Atwood, J. G., U. S. Patent 2,679,185 (Mav 25, 1954). (3) Kelloggram, No. 3, 1955, M. W. Kellogg Co. (4) Luft, K. F., Zeits, F., Tech. Physik 5, 97 (1943). (5) Patterson, W. Α., Chem. Eng. 59, 132-6 (September 1952). (6) Savitzkv, Α., Breskv, D. R., Ind. Eng. Chem.. 46, 1382 (1954). (7) Woodhull, Ε. Η., Siegler, Ε. Η., Sobeov, H., Ind. Eng. Chem. 46, 1396 (1954).
