I/EC
Instrumentation
Automatic Chemical Analyzers for Process Control Automatic chemical analyzers open many new areas to analytical process control by R. F. Wall, Monsanto Chemical Co.
^ ^ N E of the toughest problems of analytical process control-automatically performed wet analyses to sense composition of process streams—is gradually yielding to developments in automatic chemical analyzers. The most versatile are the general purpose instruments, colorimeters, and titrators that can be programmed to add reagents and perform analytical operations to reproduce automatically a wide variety of laboratory analytical procedures. Most operations of wet analytical chemistry have been accomplished by automated techniques and combined to provide complete analyses of process samples within a few minutes time, with a considerable advantage for control purposes over laboratory analyses. General purpose analysis instruments are complex and expensive, and simpler chemical analyzers are available for specific analyses of broad applicability. In addition, a number of chemical analyzers have been devised for particular control problems by instrument engineers inclined toward gadgetry and have been described in the literature. Chemical analyzers present a very difficult development problem in that the sample and reagents must be metered very precisely in a programmed sequence, including appropriate periods of reaction at controlled temperature in many cases, and the analytical value read from a developed color, the end point of a titration, solution conductivity, or possibly some other measured quantity. I t is understandable that most of the automatic chemical analyzers were first developed and used in control laboratories to perform numerous routine analyses. There the analyzer can be very closely observed by skilled personnel. Chemical analyzers have gradually moved into the process plant, drawn by the very high re-
turns obtained from direct analytical process control The same basic justifications apply as have led to the intensive instrumentation of petrochemical plants with process analyzers based on physical principles; substantial dollar savings, greatly increased process throughput, better quality, higher on-stream time, convenience and ease of unit control, and the intangible though very real benefits of directly sensing and indicating significant variations in stream composition for use in control.
Uses Chemical analyzers have a verybroad field of potential use, including any unit process where chemical methods must necessarily be used to determine a component of key interest, and where physical analyzers, as the gas chromatograph, infrared, others, are not applicable. Chemical analyzers thus make analytical process control a possibility for many processes and for many industries where previously impossible
•Titromatic Analyzer
Electronic Instruments Ltd.
VOL. 52, NO. 3
•
MARCH 1960
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INSTRUMENTATION no matter how well justified. Included are plants and industries processing many inorganic and organic materials, as foods, drugs, agricultural chemicals, beverages, soaps, paper, etc. There is even a considerable area of potential applicability in the highly instrumented petrochemical industry. As examples, caustic and/or acid treatments are frequently used and concentration control can yield substantial savings for large processes; and traces of many contaminants have been difficult to impossible to determine by physical means. While analytical process control emphasizes the chemical analyzer installed and operating in the plant, these instruments also have a great potential for use in control laboratories performing large numbers of routine analyses, where they can provide substantial savings in analyst's time. One of the most important uses for which these instruments were developed is clinical analyses for medical diagnostic purposes. G e n e r a l Purpose A n a l y z e r s
The Titromatic analyzer manufactured by Electronics Instruments Ltd. of England and marketed in the U.S. by Milton-Roy is a typical automatic titrator that can be adapted to a considerable number of analyses through appropriate programming. This instrument uses automatic pipets, a buret, and other glassware closely resembling conventional laboratory equipment but operated automatically from a sequential control unit to essentially reproduce in exact detail a laboratory titration. The end point can
be detected by any one of several means ; potentiometrically through use of pH electrodes, oxidation reduction through use of redox electrodes, a silver electrode for a chloride titration, etc. A photoelectric device can be used to detect a color change end point. This analyzer is very useful in the control laboratory and is also packaged for process plant installation, as shown in the accompanying photograph. Milton-Roy also market their Quantichem, an automatic color analyzer, for use in those analyses where a developed color intensity is proportional to the concentration of a desired component. This analyzer was developed for process use and the components used in construction are more typical of this type instrument. The Quantichem analyzer was apparently developed originally for water analyses; dissolved silica, dissolved oxygen, and residual chlorine, but can be applied to any process analysis based on a color proportional to the concentration of the desired component that is developed by no more than four reagents and does not require heating, filtering, extracting, or decanting. The instrument has been well designed for process use and includes some trouble shooting indicators to signal cell window fouling. Milton-Roy have recently placed on the market their Chemalizer, a simpler automatic colorimetric analyzer. Typical analyses are water hardness, residual chlorine, and phosphates. Analysis is accomplished by simply mixing sample with a single reagent and reading the developed color. A number of
The Technicon Autoanalyzer
A new system f o r continuous, automatic chemical analysis, detects trace materials down to parts per billion with an accuracy of 1°/© Technicon Controls, Inc.
important analytical procedures are possible, and where the instrument is suitable, economy and simplicity will have considerable advantage in justification and in ease of application and maintenance. Another automatic analyzer of great versatility is the Analmatic analyzer manufactured by Baird and Tatlock, Ltd., of England and marketed in this country by the Chicago Apparatus Co. Each Analmatic analyzer is assembled for a particular analysis or type of analyses from a system of operational units that perform the required basic functions of an analytical procedure. A programming unit is assembled to control the operation of the separate functional units performing the analytical operations. Various metering devices are available for precise admission of sample and a sequence of reagents. A titration unit is available with a variety of end point detectors both potentiometric and colorimetric. The instrument may also be set up as a colorimetric analyzer. The system of functional units makes for great versatility, and some analyses of considerable complexity have been accomplished with this instrument. It is useful both in the laboratory and as a process control analyzer. Another automatic chemical analyzer that has had many successful applications is the Autoanalyzer developed by Technicon Controls, Inc. They also follow the approach of having available a system of functional units performing the basic operations of analytical chemistry. Their approach has been very ingenious and original, and their operational units use some very effective techniques well adapted to automatic operation to accomplish the basic analytical functions, rather than reproducing automatically conventional laboratory procedures. The general approach is based on a continuous flow principle, rather than batch operation that is characteristic of most automatic chemical analyzers as derived from automating conventional laboratory procedures. The key feature making this approach possible is the inclusion of air in the continuously flowing streams. Air bubbles segment the flowing liquids and prevent mix(Continued on page 58 A)
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INDUSTRIAL AND ENGINEERING CHEMISTRY
INSTRUMENTATION
for
MORE ACCURATE PROCESS CONTROL
ing so that each segment acts like a separate small analysis made at a high repetition rate. The indicated analysis is essentially continuous with an analysis time lag of a few minutes, depending on the particular analysis being performed, but in general comparable to the response time of a gas chromatograph. A wide variety of functional units is available, with a dialysis unit being one of the most interesting. This can perform separation and purification procedures, equivalent to filtration, precipitation, decanting, etc., in many cases. Technicon's analysis system is highly versatile and has proved very valuable for a great number of laboratory analyses, with clinical applications probably predominating. The Autoanalyzer is also assembled and packaged in an assembly suitable for analytical process control in many processes. Special Purpose Analyzers
• Complete corrosion resistance • Full visibility for ultimate product control • Teflon gate is only moving part • Solenoid and timer for accurate performance Construction of *Pyrex and Teflon makes this Reflux Splitter completely corrosion resistant to all liquids except Hydrofluoric Acid and hot concentrated caustics. Eliminates valves, product line and reflux line rotometers for greater economy. Available in Column and In-Line designs, as well as other materials of construction. Ask for Bulletin RS-2. *T.M., Corning Glass Works
CALL C H E M F L O W FOR TEFLON GASKETS Complete line of standard and special gaskets to solve any sealing problem. Solid Teflon and envelope type gaskets in all sizes. Regular and odd shapes in all materials.
CHEM FLOW CORP. 195 P a t e r s o n Avenue L i t t l e F a l l s , N. J .
While these general purpose chemical analyzers have the greatest versatility, there are many limited purpose analyzers for particular applications. Several free residual chlorine analyzers are on the market for water treating purposes. Consolidated Electrodynamics market their Titrilog, a coulometric analyzer for determination of trace quantities of oxidizable materials, typically H 2 S and SO 2. This is particularly useful in air pollution problems. Chemical analyzers differ considerably from physical analyzers used for some time in analytical process control, and will present new and different problems in selection, installation, and maintenance. Most have considerable glassware and complex assemblies which present serious maintenance problems from the process instrumentation standpoint Instrument craftsmen will definitely need to acquire new techniques and new know-how, with a prime requisite the ability to handle glass equipment without breaking it. Intricate and specialized equipment of this type can be best handled by a specialist, but this maintenance approach runs head-on into labor problems and it is not good practice in a continuously operating process to create a situation where one or two men
Circle No. 69 on Readers' Service Card 58 A
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become essential. Maintenance will probably be relatively high, for these instruments require reagent solutions which must be replaced at regular intervals. Chemical analyzers, particularly the more versatile ones, are relatively new to the process plant and much development toward more rugged models better suited for process installation may be expected in the near future. Setup A chemical analyzer for a process application requires a technical understanding of analytical chemistry in addition to instrumentation knowhow. While valuable assistance may be expected from the manufacturers, in practice it is essential that the user have a thorough understanding of how an instrument operates if a fully successful application is to be obtained. There is opportunity for considerable ingenuity and originality in devising analytical schemes for use with these automatic chemical analyzers. The precision of their operation may permit use of methods not successful in the analytical laboratory. For example, if a color development is time dependent the laboratory analysis may be inaccurate while the automatic analysis with its precisely controlled time may be possible. The accuracy of analyses by an automatic analyzer may exceed that obtainable from a control laboratory, for the reproducibility of manipulation obtained from these machines is considerably better than that of the best analyst. Although chemical analyzers for process use are relatively new, they open so many new areas of application to analytical process control that their rapidly increasing use appears inevitable. The potential for use and for economic returns to the user appears tremendous. They are a part of the rapidly developing control technology.
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