instrumentation
Gary D. Nichols PPG Industries, Inc.
Chemical Division-U.S. P.O. Box 1000 Lake Charles. La. 70602
Process Analyzer Sampling Systems In the past few years on-line process analyzers have become increasingly important in the chemical process and related industries ( I ). An excellent general discussion of process analyzers was presented in these pages recently ( 2 ) .That discussion was followed by a n overview of process infrared analyzers (3).In practice, choosing the appropriate process analyzer is only part of the problem in performing reliable process analyses. Frequently, the analyzer selection process is greatly affected by the ability to obtain a suitable sample. It is not unusual to spend as much time and money designing and building the sampling system as is spent on the analyzer. To begin this discussion, it is helpful to have working definitions of process analyzers and process analyzer sampling systems. A useful definition
mentally suitable automatic device that continuously or periodically measures one or more chemical parameters in a process stream and presents the results in a usable form.” This definition is exemplified in the infrared analyzer article cited above (3)and in an older article describing process gas chromatography ( 4 ) . A process analyzer sampling system can be characterized as, “a device or combination of devices which will transfer a sample from a process stream to a process aualyzer in such a way as to minimize maintenance and to preserve or enhance the analytical information contained in the sample.” Close attention must be paid to the design of the sampling system as well as to the selection of the analyzer because of the automatic, unattended nature of the process analyzer and the imnnrtance of the potential cost of an
incorrect analysis. A bad analysis may be economically costly in terms of material waste, customer dissatisfaction, and maintenance problems. This is exemplified by a custody transfer analyzer; the customer is paying only for the stream component of interest and not for whatever impurities may be included. Therefore, an error of 0.01% multiplied over a year and several hundred tons per day has a significant negative economic impact on either the buyer or the seller. Additionally, a bad process analysis may permit an occupational health or environmental hazard to go unchecked. A high heavy metal content in a discharge stream may cause a fish kill and invite a fine or civil penalty to the manufacturer. An incorrect oxygen analysis in a tank or vent may cause suffocation for workers in the area or lead to a catastrophic exnlnsion.
Caiibrati Selector Valve
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Figure 1. Sampling system for fluegas process analyzer. Courtesy of Teledyne Analytical Instruments 0003-2700/8110351-489A$01.OO/O @ 1981 American Chemical Sociehl
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
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SCHOTT'S modular titration systemsthe building blocks for the future.
DispensingCylinder The accuracy of a piston buret stands or falls with the precision of the glass cylinder. Nearly a century of glass making expertise enables us to offer ID measurement tolerancesas small as+0.005mm, which translates into the best available dispensing accuracy.
Your test requirements may change, but not your demands for accuracy. SCHOTT offers the most accurate titration system available, while giving the flexibility of the building block concept. Whether you need simple volumetric dispensing, fully automated titration or something in-between, SCHOTT has the system for you.
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Microprocessor Controlled Piston BureVDlspenser Push-button release of the interchanaeable buret modules, as wellas five buret capacities, provideeaseof operation, flexibility, and safetyespecially when working with hazardous titrants. Only DURANQborosilicateglass and PTFE come in contact with the titrant which provides high chemical resistance and longer life Even the most demading requirements are satisfiedby 0 001 ml our dispensed volume and readout accuracy to I
Precise data interpretation is easy due to selectable magnification of inflection points in thetitration curves. A built-in, high precision LCD pHimVmeter shows these valuesat any point during the titration process concurrently with the recording All modules are compatible with desktop calculators.
El.ctrodeS To complement our titration systems, we offer a complete line of plug-in research qualityelectrodes
Automatic End Point/ PH Stat ntmtion Systems This peripheral module has the capability
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Analyzers, like computers, will give the right answer only if they are provided with the proper information. Assuming that the analyzer is correctly installed, calibrated, and serviced, the sample constitutes the only variable input. Unlike the situation with laboratory instruments, however, constant, direct attention to the sample entering a process analyzer is not possible. A sampling system must automatically provide a sample a t the proper pH, temperature, pressure, flow rate, moisture content, size, etc. a t the proper time, and must suhstitute for the subjective judgment that a bench chemist is able to provide in the laboratory, Frequently, an unattended sampling system must operate reliably for weeks or months at a time. Process analyzer sampling systems must be designed with a t least as close attention being paid to the hardware as to the chemistry of the sample. Table I shows the functional effects of the most common sampling system devices on a sample stream. These functions are valid for most fluid streams-gases, liquids, slurries, aerosols, etc. While Table I is broadly applicable, there is admittedly room for argument in specific applications. It could he successfully argued, for example, that a sudden pressure reduction through a regulator or undersized valve could change chemical composition where condensible vapors or pres. sure-sensitive, unstable components are present. Table I should thus he tailored to the immediate application when used as a sample system design tool. In the next section each of the 10 sampling system devices presented in Table I will he considered.
Sampling System Hardware Tubing and Piping. The simplest sample system for process analyzers consists of nothing more than one or more lengths of tubing or piping to-
are best for sample cut-off and choice gether with the proper fittings such as of flow direction. Three-way, fourbends, connectors, reducers, and way, and five-wayvalves are commerunions. Piping and tubing are distincially available for alternately seguished from one another mainly in lecting one of several streams by sinthat pipe is joined by threaded or gle-valve operation. Electric and welded couplings directly on the pipe. Pipe is also thicker-walled than tubing pneumatic valve operators are available for remote automatic valve actuaand hence more difficult to work with. tion or where the valve may be in an Pipe is usually more permanent than inconvenient location. Needle valves, tubing. Tubing is connected by fricespecially under-sized needle valves, tion-compression fittings. Tubing is are used for pressure and flow reducfrequently permanent but is much tion. easier to change if a sampling system Pressure Regulators. Pressure modification becomes necessary. Pipe regulators are essential on gas samand tubing are the primary means of sample transport. With proper design, pling systems and may sometimes he necessary with liquid sampling sysheating and cooling of the sample tems. A pressure regulator ensures through pipe and tube walls and presthat the sample pressure will stay at a sure drops along the transport system are only incidental. Proper selection of preset value, as long as the process line pressure, which may vary, repipe and tube materials and insulamains above that value. This is importion, refrigeration, and heat-tracing tant, for instance, when several promaterials can help overcome or can cess units feed the process line being enhance pressure and temperature efsampled. Pressure in the process line fects in sample transport. may drop when some hut not all of the Valves. Though piping and tubing process units suspend operations; the alone are sufficient for sample transanalysis is still necessary to support port, valves are placed a t strategic points in the sample system for conve- the remaining operating units. Pressure regulation using a regulanience. The simplest sample system tor should not he confused with presshould have a minimum of two valves sure reduction using a needle valve. A of comparable size to the tubing. One valve should be a t the sample point on valve will only reduce pressure. A regulator will reduce pressure to a sethe process line and the other a t the lected value and maintain that presend of the sample line next to the prosure regardless of fluctuations above cess analyzer. The former allows serthat pressure. Pressure regulation is vicing and maintenance to the sampling system and analyzer without dis- important, for example, with photometric analyzers on gas streams where turbing the process. The latter pera pressure increase may show up erromits servicing of the analyzer without neously as a concentration increase. the need to disturb the valve a t the Pressure reduction and regulation is process line. This is particularly imalso important in protecting fragile, portant if the sample point and anawetted analyzer parts; glass sample lyzer are several hundred feet apart cells may be limited to a few atmoand perhaps separated by several levspheres pressure, in contrast to a proels of stairs. Additional valves should cess line, which may be pressurized to be used to isolate sample system deseveral hundred psi. Pressure regulavices such as filters, pumps, dryers, tion on bottled, compressed gases is and other items likely to require familiar to most analytical chemists. cleaning or replacement. Ball valves ANALYTICAL CHEMISTRY. VOL. 53. NO. 3. MARCH 1981
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x-ray Fhrorescence 0 Vacuum System and Sealed Detector 0 W r e d Reading-no calculations 0 Rapid-30 second analysis 0 Automatic Calibration-minimal operator involvement 0 Microprocessor Controlled-reliable results 0 Dual Position Sample Holder-quick and convenient
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Pressure regulators for process samples are likewise available commercially for many process applications. Flow Controllers. Flow control of samples to process analyzers is as important as pressure control. Fortunately, flow control is simpler than pressure regulation. Flow controllers, or rotameters, basically consist of a needle valve followed by a float in a transparent vertical tube. The valve and tube are fastened and mounted in the same body. The position of the float in the tapered, graduated tube gives an indication of the flow rate. The float attains a level in the tube dependent on the velocity and density of the fluid. Flow meters are usually calibrated for air or water a t a reference temperature and pressure. Therefore, a density correction must be made for their use with other samples. Flow meters are available in a variety of ranges, as determined by the tube graduations and the density of the float. It is important to remove particulate matter from the sample, because particles can cause the float to lodge in the tube. Additionally, the flow meter should be downstream from the pressure regulator because of the dependency of density, and hence flow rate, on sample pressure. Temperature Controllers. Temperature control is not generally as important as pressure and flow control because most process analyzers are less susceptible to errors attributable to temperature than to pressure or flow. Nonetheless, there are sufficient process samples that need to be heated or cooled to justify a brief discussion of the problem. These cases generally involve either keeping a condensible liquid vaporized by heating, or dropping out a condensible vapor by cooling. In simple cases, the materials of construction, especially the tubing, can be chosen for their heat conduction or thermal insulation characteristics. Constant sample temperature is often more important than precise knowledge of the temperature. Constant temperature can be accomplished by heating the sample system or the wetted parts of the analyzer to a temperature above the warmest expected temperature. This is done with electric heating coils or steam tracing. (Steam is nearly always available in process plants, especially where electricity constitutes a safety hazard.) Samples can he cooled with cooling tower water or refrigerated brine in jacketed heat exchangers (again, usually available in process plants) or by increasing the length of the sample line (at the expense of longer sample lag time). Where cooling is used to knock out condensible vapors, a collection pot is necessary to protect the
analyzer. The collection pot should be considered a separate device from temperature control for esgineering and design purposes. Filters. Process analyzer samples frequently contain particulate matter: scale from boiler feed and condensate, tars from organic distillation columns and reactors, catalyst particles from fluidized bed reactors, or salt crystals from both organic and inorganic processes. The presence of particulate matter in the process analyzer sample is undesirable both from the standpoint of maintenance and of analytical reliability. Particulates in samples can also cause noise in photometers and shorts in electrochemical devices. Filters are the primary means for removing particulates. Filters are commercially available in a variety of materials, pore sizes, and bulk capacities sufficient to handle a considerable variety of samples. Mechanical strength of the filter cartridge is important when the pressure drop across the filter may become great. A large filter with a capacity of several liters helps reduce maintenance of cleaning and changing, but also greatly increases the lag time from sample point to analyzer. Most sample system devices, but especially filters, can be installed in parallel to allow one side to be used while the other is being serviced. This is accomplished via a three-way valve a t both ends of the parallel segment. Another particulate removal device is the bypass filter, which takes only a side-stream of a sample for filtration and transport to the analyzer. Cyclone separators are
useful for removing particulates by centrifugal action. A side-stream of the sample is necessary to sweep the centrifugated matter out of the cyclone. Sample Dryers. Moisture is prevalent to some extent in most chemical manufacturing processes. The moisture content in the various process streams varies from a few parts per million to “phase water’’ in nonpolar streams. The permissible or required levels of moisture in the process stream of course depend upon the next use of the stream. Apart from the process stream, however, it may be desirable or necessary to remove moisture from the sample going to the process analyzer. Moisture in the sample may interfere with the analysis by, for instance, masking or shifting certain absorption bands necessary for photometric analysis. Or moisture may introduce maintenance problems associated with premature chromatographic column failure. Corrosion of wetted analyzer parts from wet acid gases can also be troublesome. Water vapor in gases and very low moisture levels in liquids can be removed by passing the sample over a dessicant. This practice is useful only when the moisture level is low and therefore will require only infrequent replacement or regeneration of the dessicant. Engineering and design considerations for canistertype dessicant-filled dryers are almost identical to the considerations for cartridge-type filters. An alternative to dessicant dryers is the water-permeable polyperfluorosulfonic acid membrane in a tube-and-
ANALYTICAL CHEMISTRY, VOL. 53. NO. 3, MARCH 1981
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of earlier Mettler systems. It's a completely new design that takes advantage of state-of-theart microprocessor technology to increase the"smartness" of thermal analysis whileuecreasing the physical size of the hardware.Actually, the TA3000 takes up only about one-fourth the space of other systems on the market. Unlike typical thermal analysis systems of the past decade, the TA3000 modules were designed as an integrated system. The TClO processor, the heart of the system, accepts and evaluates datahignals from the DSC, TG and TMA modules and plots the results in final form. Simple pushbuttonoperation. Thermal analysis has never been this easy. Standard programs are stored in a permanent memory. You simply push the correct button to start the analysis. From then on, it's all automatic. User-modified programs, numbered for instant recall, can be linked to any other program analysis.
analysis, the emperature or the measured signal (such as heat flow or dimensional change) is displayed on the processor. Finally, a digital result will ++tear.The system will also deliver a hard-copy, stepby-step record, consisting of pertinent digital values as well as the measured curve. Moderately priced. For about $25,000, you can get a basic TA3000 system with built-in data reduction, along with the DSC20 module that measures up to 600'C and the printer/plotter. Low-temperatureDSC, TG, and TMA can be readily added. Easily set up for demonstratlon. Because of the compactness of the system, it's simple to set up and demonstrate in your lab. Contact us for a demonstration or a brochure. Mettler Instrument Corporation, Box 71, Hightstown, NJ 08520. (609) 448-3000.
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Locations in which flammablegases or vapors are M ma be presenl in ihe air in quanti explosive or ignltible mixtures
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Chemistry and Chemical Engineering in the People’s Republic
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An overview of the current state of chemistry in China a s seen through the eyes of 12 leading American chemists
Example8 Butadiene. hydrogen, propylene oxide
Ethylene, hydrcgen sulfide,telrahydrofuran
A Trip Report of the U.S. Delegation in Pure and Applied Chemistry
Acetorm, benzene, gasoline. methanol Flour, grain, combustible plarrtics dusts
John D. Baldeschwieler. Editor Foreword by Glenn T. Seaborg
Code.’’ mpyrlght 1977
Fully lltustratea wlfh photographs. charts, and diagrams, this report offers a clear, impartial. detailed look at chemistry as it is practiced in the People’sRepublic 01 China today. In addition to covering such varied areas as NMR spectroscopy, lasers. oil production. port facilities. catalytic cracking units. ammonia plants, oil shale. pollution, computers, chromatography, and much more, it ais0 gives interesting insights into the minds and working methods of chemists at every level of aicomplishment. 4 Vital book forthose interested in this Pxpanding world and for those who may seek market or research information in :his recently opened area, 2ONTENTS 3001s 01 Chemical Research and kvelopment in China * The institutional Structure of Chemical Research and Ievelopment in China * Chemistry and :hemica1 Engineering in the Context of ?ationai Science Policy * Chemistry and -hemid Engineering as Elements of kience Education in China Basic Research p the Subdisciplines of Chemistry and in .hemica1 Engineering Status of Research p Key Areas of Technology * Organization of .hemicai Research and Development in had National Mission Areas Infrastructure Discussion * Recommendations
shell exchanger. A wet gaseous or liquid sample is passed on the tube (inside) side of the membrane. A dry gas stream on the shell (outside) side of the memhrane sweeps the water vapor away. These exchangers are available commercially in a variety of designs and sizes. Entrained moisture in gaseous samples can he removed mechanically by commercially available traps or coalescers. Coalescers are also useful for removing small amounts of phase water from nonpolar liquid samples. Strippers. Occasionally it is necessary to remove dissolved gases from a liquid in order to obtain a simpler sample for analysis. Either the liquid or the gas may be the object of the analysis. The separation can he accomplished in a stripping column. The liquid sample is introduced a t the top of the column and a “clean” gas is run from the bottom of the column counter-current to the liquid. The stripped liquid is removed from the bottom, and the gas mixture containing the stripped gases or volatile components is taken from the top. Stripping is a continuous operation. Either the liquid or gaseous column effluent, or both, may he analyzed for desired components. Stripping is related to distillation; however, the “moving force” in stripping is the “clean” stripping gas rather than heat. The column is packed with a permanent material such as glass rings or beads, ceramic saddles, or even metal particles, to increase the gas-liquid contact area. A stripper is relatively maintenance-free except for washing or cleaning the packing occasionally. Even this can be minimized
I by filtering the particulates from the sample-filters are easier to service than strippers. As with other sample system devices, a large stripper requires less maintenance and increases separation efficiency, hut simultaneously increases sample lag time. Strippers are usually fabricated from pipe and pipe-and-tube fittings. The packing is purchased commercially from laboratory supply sources. Pumps. Process streams are usually a t some positive pressure and therefore flow unassisted from the sample point through the sample system and the process analyzer and on to a sample return or sewering loeation. Nonetheless, pumps are frequently used in process analyzer sampling systems. Ambient air monitors require a pump to draw in air and force the sample through the analyzer. The pump may be placed upstream from the analyzer and exert a positive pressure on the unanalyzed sample or downstream and exert a vacuum on the unanalyzed sample. The latter configuration is advantageous in that the pulsing action of a peristaltic pump is not felt by the analyzer. The different devices in a sampling system each have an associated pressure drop due to fluid friction, turbulence, etc. Therefore, the combined pressure drops of the devices in the sampling system may exceed the pregsure drop between the sample point and sample disposal point. In this case, a pump would he necessary as a booster. Sample pumps are available in peristaltic, centrifugal, and diaphragm types. The peristaltic and diaphragm pumps minimize mechanical contact with the sample. These two types are
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 3. MARCH 1981
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essential for viscous samples or thick slurries. Centrifugal pumps do not have the possibly undesirable pulsing action. Centrifugal pumps are available in corrosion-resistant exotic metals as well as in a variety of hard synthetic materials. Peristaltic and diaphragm pumps are available with oiland solvent-resistant wetted parts. Integration of a pump into a sampling system should not he approached casually. While there are many extremely reliable pumps for sale commercially, a good general philosophy in designing a sampling system is to minimize maintenance by minimizing moving parts. Most pumps are electrically operated and therefore must not be used where an explosion hazard is likely. Pneumatic and fluid-operated pumps are available for limited appliZations, as are explosion-proof electric pumps. Solids Samplers. This discussion ias been limited to fluids sampling, However, solids occasionally require rampling. There is a “mechanical iand”availah1e for this purpose. It is iseful when it is impossible or inconrenient to place a process analyzer in I solids stream. This situation can he llustrated in the case of a process in‘rared moisture analyzer. This is a relectance-type analyzer that can be >lacedabove a conveyer belt, say, lownstream from a forced-air or conrection dryer. However, the analyzer vould not be appropriate for a forcediir conveyance system such as in a :rain elevator or similar plant; the anilyzer would not respond well to a dry ample dispersed in a gas and would ikely suffer maintenance problems rom the continuous abrasive action of he particles. But a “mechanical land,” fabricated from rugged parts, an periodically “grab” a sample from he process stream, transport it a few nches to the analyzer and return it to the stream when the analysis is complete. This solids sampler eliminates both the abrasive effect of the flying iolid particles and the spectral noise h e to the inhomogeneous, dustlike xocess stream.
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 3. MARCH I981
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In the foregoing discussion, some ispects of overall sample system de,ign considerations were touched ipon. Particularly, it was mentioned hat the lag or delay time between ampling and analysis should he kept IS low as possible. For tubing and iipe, it is possible to use the flow rate, nternal cross section of the tubing ind the length of the sample line to alculate the approximate lag time. If ‘ilters, regulators, or strippers are part of the sampling system, this calcula-
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 3, MARCH 1981
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Flgure 2. Sampling system for oils in water process analyzer. L = infrared light source: R = reference cell; S D = detector: A = amplifier; DVM = digital voltmeter. Courtesy of Horiba Instruments, Inc. tion is more difficult, even when the volumes of the additional devices are known. It is seldom necessary to know the precise lag time, though. Selecting the proper materials of construction is also important. Table 11 is a short chemical resistance guide covering representative organic and inorganic substances. That some materials such as PVC and even stainless steel have limitations is obvious. However, it would appear that Teflon is universally applicable. But even Teflon has the limitation that it is permeable to oxygen and that Teflon tube fittings tend toslowly loosen or "creep," creating system leaks. It is not unreasonable to construct a Sample system from several different materials, depending upon the stream concentration, temperature, pressure, etc., at different points in the system. Over-specifying exotic chemical-resistant materials such as Teflon, Hastelloy C and titanium can result in an unnecessarily expensive sample system. Multistream sampling and provision fur the introduction of zeroing and calibration standards can usually he facilitated by adding multiway and blocking valves to the sampling system. Some analyzers already are fitted with zero and calibration input ports interna I I y . Sample point selection is easily overlooked. Strictly speaking, the sample point is part of the process plant and not part of the sampling system. But the selection of the sample point is the responsibility of the person who engineers the process analyzer system. A sample point should never he on the bottom of a pipe; even the cleanest process streams have an accumulation of rust, tar or other for. eign matter on the bottom of the process line. This foreign matter can wreak havoc with a sampling system 498A
or analyzer. The sample point should also provide a statistically valid Sample in accord with the objective of the analysis. If laminar flow in a process stream is assumed, the fastest flowing part of the fluid is in the middle. Frequently, though, an eductor tube in the middle of a process line for the purpose of taking an analyzer sample is not possible. Proper sample return is even more neglected than sample point selection. A sample likely to contain a hazardous material such as PCB, vinyl chloride, or cyanide cannot be directly sewered or released into the atmosphere. These samoles must either be out hack into the process at some point or be piped to a designated process stream disposal area. Some samples, such as metal plating solutions and specialty chemical streams, must he recovered or recycled for economic as well as for safety and environmental reasons Process analyzers and their associated sampling systems should be chosen or designed in accordance with the National Electrical Code for hazardous locations. The locations are designated by class, group and division, respectively. Examples of class locations and chemicals in different groups are given in'Table 111. Definitions of the divisions are more involved and depend, to some extent, on the class. There are three classes, two divisions and seven groups. Some combinations of the three designators are mutually exclusive, as is obvious from Table 111. It is seldom necessary to design a sampling system with no indication of where tu start. Most process analyzer vendors have published applications notes for a variety of applications for which they sell analyzers. These are usually available without charge or obligation. For special applications problems, some analyzer vendors have a
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
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special applications laboratory equipped for designing and testing sample systems. There may be a charge for this service, especially if there is little chance that there will be additional markets for the system after the prototype has been sold. Vendor services do not, however, preelude the necessity for the chemist working with the analyzer to understand the principles of the process analyzer sample system. Outside assistance can save much time and money, but the analytical chemist is generally the person who must either make the analyzer system work or recommend an alternative. Examples ofProcess Analyzer SamPi1ng Systems High fuel costs have made combustion control in furnaces, boilers, and incinerators a particularly attractive tarzet for imDrovement via DrOCeSS analyzers. The technology i f the control scheme and the economic justification are described by Kane (5). Combustion control involves monitoring of CO or combustibles and O2in the flue gas. The analyzer technology is well developed, but presenting a good sample to the analyzers from a hot, wet, dirty, and perhaps corrosive stack gas stream is a challenge to Sample system design and engineering. One sample system design is presented in Figure 1.The pump is the prime mover of the sample since flue gas in a stack or chimney is near atmospheric pressure. The strainer ensures a clean water supply to the sample system. The water mist in the spray nozzle device cools the sample. The mixer downstream from the pump provides for sample scrubbing by intimate gas-liquid contact. The separator removes the phase liquid. The filter removes the last particulate matter and entrained moisture. The clean
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hidebook for ndustrial Chemical Technologists ind Technicians >y the Writing Team for the Chemical Technician Curriculum Project
?dited by Robert L. Pecsok, +eject Director, Kenneth Chapnan and Wade H. Ponder, 4ssociate Project Directors ‘The material presented is excellent . . of considerable use to commer:ial and industrial laboratories as a eference. It is written simply and is m y to comprehend.” :ood Technology (1978) November, I05 ipecific topics covered in 16 c h a p ers include: ‘irst aid, good practices in the chemcal laboratory, personal protective !quipment, fire safety, toxic chemcals, radiation and electrical hazirds, compressed gases, laboratory iotebooks, aids for calculations, use ind interpretation of data, drawings ind diagrams, laboratory tools and !quipment, specifications testing, ind chemical literature and the linary. !15 pages (19751 Paperback $10.00 LC 75-22497 ISBN 0-8412-0578-i Ode, Imn: SIS Drpt. Box 15
American Chemical Sociny 1155 SirlcenlhSI., N.W. Washington, D.C. 20036 or CALL TO11 FREE 800-424-6747 and use your c d i t card.
500A
flue gas sample then goes through the oxygen analyzer. The sample is then carefully mixed with air for stoichiometric combustion in the combustibles analyzer. This sampling system illustrates the use of several devices discussed earlier: tubing, filters, valves, pumps, separators. and flow meters. In addition, the flow meters, valves, and sensor for the combustibles section are mounted in a temperaturecontrolled enclosure at 130 O F to keep all the components above the dew point of the sample. The spray nozzle may be considered a sample pretreatment device, because in addition to removing large particles and cooling the gas, the water may also dissolve some of the permanent gases, thereby changing, if only slightly, the chemical composition of the sample. In sample systems design, however, it is frequently necessary to compromise analytical exactness in order to increase analysis speed and minimize maintenance. This compromise is acceptable provided the personnel involved in the analyzer project, and especially the supervisors and operators of the production unit, understand what is being done and why. Figure 2 shows a sample system for a nondispersive infrared oil content monitor. This process analyzer system continuously monitors for the concentration of oil and grease in industrial process and waste waters. The system is based upon solvent extraction followed by infrared absorptiometry. This is another example of the sample system being incorporated into the analyzer housing and purchased simultaneously with the analyzer. Still, the sampling system is best regarded as engineered separately from the analytical technology. The sample is pumped to a strainer that removes large particles. The sample is then mixed with clean solvent a t a positive displacement pump and rotary extractor. The clean solvent stream is used as a reference standard in the analyzer as well. The water-solvent mixture is separated in the water filter. The water-free solvent phase goes to the sample cell of the analyzer and on to the solvent reclaimer. The reclaimer is a bed of activated charcoal that adsorbs the oil and grease from the solvent. The aqueous phase from the water filter goes to the sludge trap for removal of entrained solvent and sludges and emulsions. The analyzer responds to the carbon-hydrogen bond, so CCll is an appropriate solvent for recycling in the analyzer. Maintenance for this sample system involves strainer and sludge trap cleaning and replacement of the activated charcoal. Maintenance frequency depends upon how dirty the sample stream is. This sample system illustrates the use of
ANALYTICAL CHEMISTRY. VOL. 53. NO. 3. MARCH 1981
pumps, sample pretreatment devices, separators, and valves.
Summary The above two examples give some idea of the use and importance of the process analyzer sample system componenta that have been discussed in this article. A detailed discussion of these and some more complex, less common devices can be found in a monograph by Houser (6). Process analyzers are increasingly a topic of discussion among analytical chemists. T o design adequate process analyzers, it is necessary for the ana. lytical chemist to understand automatic sample preparation and the reality that process analyzers operate with much less than ideal samples. This discourse, hopefully, has shed some light on this suhject.
Acknowledgment The author is grateful to PPG Industries, Inc.. for providing the time and assistance necessary to prepare this article. PPG neitherendorses nor implies its own use of any of the prod. u c h or services referred to in this article.
References A.O. Williams and P.W. Green, Hydm. Hy carbon Process.. 60 (6). 45 (1980). (2) M.S.Frant and R.T. Oliver, Oliver. Anal. Chem., 52,1252 52,1252A A ll9Rn1 (1980). and G. LaBatti, Anal. (3) M.S.Frant ar Chem., 52.133 52.1331 A (1980). (4) R. Villalobos, Anal. Chem., 41,983 A (1)
119761 119761~ ,... . ,.
(5) L.A. Kane, Hydmcarbon Process., 60 (6),65 (1980). (6) E.A. Houser,“Principlesof Sample Handling and Sampling Systems Design for Process Analysis,” Instrument Society of America, Pittsburgh. 1972.
Gary D.Nirhofs is an advanced research chemist of the Lake Charles, La. chemic01 plant of PPC Industries, Inc. He has been working wilh process analyzers and sampling systems of PPG for four years. Nichols has a ES in chemistry from California Polyfechnic Stale Uniuersily 01 Son Luis Obispo, and an MS in analytical chemistry from the University of Georgia, where he worked fur D.M.Hercules.
