Process Analytical Measurements T h e schematic diagram shown in Figure 1 is part of a process analytical instrument drawing, used in the chemical industry to identify the configuration and location of instruments for controlling a process. Analytical elements used in such a process control system include measurement probes (pH electrode, optical light path, GC column, etc.); analytical transmitters, the electronic or pneumatic meter portions of the analyzer; and analytical indicator-controllers, used when the analyzer is part of a closed-loop control system. Quite often the analyzers are instruments with which the analytical chemist is familiar, having worked with similar ones in the laboratory. Operational principles and the basic elements of their construction are quite similar to the corresponding laboratory instruments. There are, however, many differences—some of which are obvious, and some of which are subtle—and the purpose of this article is to explore some of these differences and try to explain how they come about. Until fairly recently, these differences were not always well understood, either by the manufacturer, or more often, by the chemist who applied the instrument. The lessons were long in coming and hard to learn. Even today, many process plants are controlled by the simple measurements of flow, pressure, temperature, and liquid level because they are reliable and easy to maintain. Plant engineers and plant maintenance personnel are reluctant to use automated composition analyzers because of reliability and maintenance factors. They prefer instead to control by the more physical methods and use laboratory analyses to supply them with the data required to change controller "setpoints," or to change target values for the measured variables. Actually, the concept of process analytical measurements covers a wide variety of instruments t h a t interact with the process in a multitude of ways, forming a spectrum of instrument types. At one end of the spect r u m are manual and automated labo-
ratory instruments located near the process or in the process control laboratory, to which samples are brought manually. These may be regarded as p a r t of the process analytical measurem e n t system, since information from these analyses is used to make adjustments to the process, or to provide input to a process control computer. Liquid chromatographs, "auto-analyze r s " and sieve-type particle size analyzers usually fall into this category. At the other end, there are the in-line instruments, whose sensors are inserted directly in the process line, such as for p H or conductivity measurements. T h e n there is an intermediate range of instruments, which are "on-line" rather than "in-line." T h e instrument and its sensor are located next to the process, but samples are removed to be run through the sensor. "On-line," in turn, has two subcategories: Samples may either be removed intermittently (batch sampling) as in the case of process gas chromatographs, or continuously, as is done for refractometers, process infrared instruments, and a number of "automatic chemists." Many instruments used in process control start in the research laboratory as purely laboratory devices and gradually gain acceptance in the process and quality control laboratories. In a few instances, these instruments evolve through further versions, which gradually come closer to the actual process line. In making the transition, the instrument manufacturer must choose between in-line and on-line. Most plant personnel, if asked, would strongly prefer in-line instrumentation because they perceive t h a t it is less complex, will require less attention, and will give answers t h a t are more nearly representative of current conditions. However, in-line measurements have their drawbacks. How does one determine if drifting has occurred, if the calibration still holds, if the sensor needs cleaning, or even if it is working? Unfortunately, this can be done only by removing the sensor, which is inherently more difficult if the device is in the process line. Often the device fails, or is suspected of failing, at a time when it cannot be conve-
1252 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
niently removed and, as a result, its o u t p u t is ignored. Once ignored, especially without "serious" consequences, it tends to be placed on standby and remains t h a t way for a long time. It is not uncommon to see an expensive piece of hardware dying a slow corrosive death because of its maintenance needs. Purchased and installed in a plant, it proves more costly to remove than ignore. In the meantime, its mere existence, sitting unused, is an embarrassment to the manufacturer and customer. Much effort and thought have gone into solving these problems, but they must be tackled again with each new instrument development. Instrument Design Considerations Many of the differences between laboratory and process instruments can be appreciated by looking at the problems an instrument designer must think about before moving a well-understood and well-accepted laboratory technology out to the process. Laboratory instruments have the design advantage t h a t a great deal more sample pretreatment can be done before the sample is introduced to the equipment. Many of the procedures routinely performed in the laboratory in preparing a sample for analysis are quite difficult to do on a continuous basis. These include simple steps like extraction, filtering, centifuging or even diluting. If these steps cannot be avoided, an on-line instrument may not be possible, or may be much more complex than desirable. Analyzers using these steps require so much operator attention in the plant t h a t they are accepted only if the need is very great. T h e laboratory instrument operates in a very safe, protected environment. Modern laboratories are air-conditioned, often humidity-controlled, and are staffed with technicians who take good care of their instruments. T h e reliable performance of those instruments is an important part of their job. By contrast, process environments are generally quite harsh, and often include extremes of temperature, humidity and corrosive conditions. (See Figure 2, and Tables I and 0003-2700/80/A351-1252$01.00/0 © 1980 American Chemical Society
Report Martin S. Frant Richard T. Oliver Foxboro Analytical 78 Blanchard Rd. Burlington, Mass. 0 1 8 0 3
II.) In the plant, the level of care and sophistication of the users is much lower, and the instrument is often tended by an operator whose primary concern is the process, rather than the instrument. This is especially true in the U.S., while in Europe it is more common for the laboratory to have maintenance responsibility for on-line instrumentation. To cope with this problem, the manufacturer must con sider the special problems involved in starting up and shutting down the in strument, and the need for including internal alarms and status checks to warn of instrument malfunction. A great deal of thought must also be given to the problems of drift in an untended instrument, and to reducing the need for periodic recalibration. Calibration and drift are especially difficult problems because, as noted
above, process instruments receive only limited manpower attention. It is common in running laboratory analy ses to go through a cycle that includes standardizing the instrument, making the measurements, restandardizing, making additional measurements, and then cleaning. Such procedures are not acceptable in the plant. The man ufacturer must design the instrument for low drift rates, and must include restandardizing or autostandardization procedures as part of the design. In addition, there is inherently a much greater expectation for long life in process instruments, and this must be taken into account in the design stage. Laboratory instruments are often not used full time, and even if they are, this corresponds to about 2000 hours per year. On-line process instruments are generally expected to
run 24 hours a day, 365 days a year, or about 8700 hours. (For comparison, such an instrument would have been run for as many hours as an automo bile driven 250 000 miles at an average speed of 30 mph). Further, process an alytical instruments are not expected to become obsolete, and manufactur ers can point to examples of "museum pieces" still operating today. This puts a burden on a good initial design, and an obligation to maintain spare parts long after the laboratory ver sions have vanished. Another difference is that process instruments must include a means of signal transmission, and this often be comes a complicated problem. There are questions as to the number of wires to be used (the fewer the wires the lower the installation costs), the kinds of outputs and the interfacing of
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gives an accurate result on a sample whose representativeness may in fact be unknown. Actually, except for the rare case of completely homogeneous liquid or gaseous samples, obtaining a representative sample is one of the most difficult aspects of plant analytical measurements. Even if the sample is homogeneous, the process instrument is dealing with a stream t h a t is always changing, so that the results from the process instrument not only include analytical errors that are difficult to estimate, b u t contain variations due to the changing nature of the sample. Only a small fraction of on-line process instruments contain provision for "simultaneous" removal of samples for laboratory checking, and this is obviously quite difficult for in-line measurements. Furthermore, it is often surprising to analytical chemists to find t h a t in many instances the continuous analysis is often more reproducible than the laboratory analysis using similar instrumentation. This occurs because the on-line measurement involves less sample handling and less of the con-
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tamination that can occur during transport of the sample. This is especially true of analyses such as the measurement of sodium in boiler feed water at the p p b level, and measurement of low levels of water in organic solvents by the Karl Fischer method. Many samples may be unstable, or may continue to react after removal from the process. It is very common in a lab to see samples awaiting analysis that have solidified or contain precipitates after cooling or storage. Precision and Accuracy Currently, process instruments falls into two categories: those in which accuracy is most important, and those in which precision is most important. For effluent monitoring to meet EPA requirements, for example, accuracy is extremely important. Instruments designed for this purpose tend to be relatively sophisticated devices, requiring highly skilled technical support and maintenance. As a result, effluent monitors often fail when they are moved from effluent to process control use because to ensure accuracy the manufacturers have made tradeoffs
COMMON PROBLEMS IN CHROMATOGRAPHY # 5
The Peaks Don't Separate . . . What Now? If y o u are operating the instrument and c o l u m n properly and the peaks on y o u r chromatogram aren't separated, the ob vious solution is to use a better c o l u m n . But y o u must first ask yourself " b e t t e r in what w a y ? " . This is the i m p o r t a n t question! Let's consider a few exam ples.
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The solid line in Figure A shows an in efficient c o l u m n . . . not enough theore tical plates. Better packing preparation, or better filling of the c o l u m n , could give the results indicated by the d o t t e d line. Y o u should k n o w the efficiency (number of plates per f o o t or H.E.T.P.) of each of y o u r columns so y o u have a basis for comparison. Figure Β indicates a good c o l u m n , but poor selectivity. T o improve peak sepa r a t i o n , it is necessary to use a different c o l u m n packing: possibly the same sta tionary phase but of a different concen tration.
D
t h a t affect simplicity or ruggedness. Precision is the most important property of in-plant analytical equip ment, since the absolute value of the numbers is less important than their use to reflect the condition of the pro cess. A plant manager may be perfect ly willing to take the output of an in s t r u m e n t and establish an empirical correlation with yield, for example, without concern for the true signifi cance of the instrument output. This can be frustrating to purists. Thus, platinum redox ("ORP") electrodes are often used to control cyanide oxi dation or the reduction of hexavalent chromium, although it is well known t h a t these species are electrochemically irreversible at the O R P electrode. T h e level of cyanide or chromium can be correlated, under the particular process conditions used, to an ob served millivolt output, and this is all t h a t is really needed to control the chemical destruction of these materi als. In this case, redox is all that is re quired. For compliance, on the other hand, instruments using referee meth ods, such as colorimetry, must be used.
Choice of Technologies Although it is possible to compile a long list of analytical technologies that have been used for process analytical instrumentation, a modestly critical look at t h a t list shows t h a t most have been used only occasionally or were not commercially successful. Of those t h a t are successful, most date back to the 1950s or earlier. If one looks at the dates of introduction of more recent innovations, there is typically a five-
to ten-year gap between laboratory ac ceptance and significant plant usage. This appears to be due less to a lack of awareness of new technology than to the inherent conservatism of the field. Not only are the process instrument manufacturers conservative, but the process users are even more so, be cause their risk is greater and they may have had their fingers burned in the past. Among the older technologies t h a t are still used in process instrumenta tion and t h a t are virtually unchanged in the last 30 years are turbidity, refractometry, and photometry. In fact, there are a few older techniques, such as conductivity and differential vapor-pressure measurements, t h a t have virtually disappeared from the laboratory and are now found only in process in-line use. The only new tech nology to have gained rapid industrial acceptance has been process gas chro matography, which was unique be cause it met a need, primarily in the organic chemical and refining indus tries, for which no other suitable in strumentation was available. T h e need was so great t h a t a high level of main tenance by experienced technicians was an acceptable burden. Many new technologies, such as those described in the accompanying INSTRUMENTA TION article (lasers, microprocessors, multiple internal reflection cells) have been slow to gain acceptance. As might be predicted, some of the tech niques t h a t have been most exciting in the laboratory, such as high pressure liquid chromatography and ion chro matography, have thus far gained only limited use.
The solid line in Figure C shows the re sult if the c o l u m n packing is incompati ble w i t h the sample. For example, the analysis of hydrocarbons on a m e t h y l sil icone such as SP-2100 or SE-30 w o u l d give the chromatogram indicated by the d o t t e d line, while methanol and ethanol on the same c o l u m n might produce the solid curve. Figure D shows what can happen if y o u r | c o l u m n is t o t a l l y inappropriate for the sample. A c o l u m n designed f o r the analysis of acidic compounds (such as SP-1 2OO/H3PO4) could t o t a l l y subtract out amines and ruin the c o l u m n . There are many more things that can go wrong in c o l u m n selection. Write for our FREE Bulletin 723 which gives y o u some helpful guidelines. Better y e t , w h y | not request our list of F R E E Bulletins. These bulletins, in a d d i t i o n to discussing | c o l u m n selection, also discuss many other problems in GC analyses.
S U P E L C O , INC. SUPELCO PARK. BELLEFONTE, PA 16823 TELE: 814 359-2732 TWX: 510-670-3600
Martin S. Frant received his Ph.D. in organic chemistry from Case Western Reserve University. Presently he is an analytical consultant in the Research Dept. of the Foxboro Co., and serves as associate technical director of Fox boro Analytical, a division of the com pany. His research activities have in cluded work on organomercurials, fuel cells, electroplating, ion-selective electrodes, and new sampling con cepts for infrared spectrometry.
CIRCLE 206 ON READER SERVICE CARD 1262 A ·
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER
1980
Richard T. Oliver is a product line manager for Foxboro Analytical. He did his undergraduate work at Cen tral Connecticut State College, and pursued graduate studies at Northeastern University and Iowa State University. An active member of the Pittsburgh Conference Com mittee since 1960, he was its general chairman in 1968.
