R. Villalobos Beckman Instruments, Inc. Fullerton, Calif. 92634
Process gas chromatographs have been commercially available for almost 20 years. In that time they have become the most widely used process analyzer. A 1968 survey of U S . refineries by the American Petroleum Institute ( I ) showed that chromatographs comprised fully 24% of all stream analyzers in use. This was considerably more than oxygen analyzersthe second most numerous-and 10 times as many as the number of pH systems. As in the laboratory, gas chromatography has proved to be the most versatile method to come along in the history of process analytical instrumentation. While most analysts are familiar with laboratory gas chromatographs, few are familiar with the process versions and the considerable technology that has been developed with relation to their application to automated onstream monitoring. While the similarities between laboratory and process chromatographs would be readily apparent to most analysts, the differences are perhaps more important, though less well understood. These differences are not only with regard to the appearance of the instrument itself. but the manner in which the data
are obtained, and perhaps most important, the way in which it is used. What Is a Process Gas Chromatograph?
The'aim in installing analyzers online is to obtain the analytical results with a speed of response that is comparable to process changes. The objective is to use the information to take corrective action. Hence, a process gas chromatograph (GC) is an instrument which has been designed to meet this objective and operates continuously on-line, automatically analyzing a flowing process stream, in a cyclic and repetitive manner. In general, such an instrument is dedicated to performing a particular analysis on a single stream, or a t most, a few liquid or gas streams (multistream). Moreover, it will usually be designed to measure only one or, at most, a few components in the sample. A distinguishing characteristic of process chromatographs is that sample is transferred from the process sample point to the chromatographic column untouched by human hands. A supply of fresh sample is withdrawn continuously from the process and circulated to the sample valve, which in-
jects a small volume into the column. The sample valve, and the lines which connect it with the sample point, can be maintained hot. Therefore, the chromatograph can accept hot gaseous samples that contain large amounts of water vapor or other condensibles; samples which cannot be transported to the laboratory without drastically altering their composition. Indeed, many sampling situations which are difficult or impossible for laboratory analysis are routine for process chromatographs. A consequence of this is that the septum inlet, probably the most common component in a laboratory instrument, is unusable in a process analyzer. Instead, considerable emphasis has been placed on the development of sampling valves and column switching valves which are highly reliable. How Are Process Chromatographs Used?
The uses of chromatographs in industry are varied. Table I lists some of the principal uses of on-line chromatographs. The most frequently encountered applications are for openor closed-loop process control. In open-loop control the operator makes
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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Table I . Principal Uses of Process Gas Chromatographs Process Control---IJse information t o adjust p-ocess through open- 0 1 c I osed-Ioo p con t i o I Process S t LI d y-.-O bt a i n i n f o r m a t i u 1-1 about process to impi,ove yield or throughput. Correlate process vciriabies with product quality Process Development-Obtain i r i formation abcut process characteristics, ‘3s in pilat plants. Correlate process v a t i a b , e s with reaction products a id yield> Material B;ilance-Use information to calcuiail? niaterial balance for proqcesl~ i r i ~ t s Product Qr.12iit.y Specification Monitor--Voli’!tJr impurities in outgoing or incoriing product for conformance ; o specifications Waste D i s posa I Mo n it o r i ng---M oritot iiquid or qas effluent wastes for ioss of valuable product or for presence of to:tic ‘compounds Per son ne I ‘Saf i? t y-A rea M on it o r i n gMonitor ambient air for presence o f toxic comriounds
adjustments to the process conditions based on the results of the chromatograph. In closed-loop control the chromatograph data are converted to a continuous analog signal which is input to conventional control instrumentation to control the process automatically. Components of Process Gas Chromatographs
Basic elements of a process GC are shown in Figure 1. Analyzer (A). This contains all components of the analytical systemcolumns, sample valves, column switching valves, and detector-in a precisely thermostated oven compartment. For economy and simplicity, a single temperature zone is most frequently used. Multitemperature zone units with several columns a t different temperatures have also been used, but are less common. Carrier gas flow controls, temperature controls, valve controls, and detector electronics are also located in the analyzer. The entire unit is located “on-line”, as close to the sample point as possible. Supplies or carrier gas and other gases are also located in close proximity. The analyzer is usually housed in a walk-in analyzer shelter or house, as shown in Figure 2. The house provides weather protection for analyzer and sample conditioning components as well as for maintenance personnel. Several types of analyzers are often mounted in the same analyzer house. Sample Conditioning System (SH). This contains most or all of the components necessary to condition 984 A
and maintain constant flow of sample to the analyzer (e.g., pressure reduction, filters, vaporizers, flow controls, etc.) plus sample switching or selector valves for multiple stream applications and for introducing calibration standard (B). It is usually mounted below or next to the analyzer in its own heated or unheated compartment or on a flat open-plate. Some elements of the sample conditioning system, such as vaporizer, filters (F), and pressure reducers (R), may be located a t the sample point (P) itself. I t should be emphasized that the sample system is probably the most critical part of the entire system. If the sample is not representative and properly conditioned, the entire system will fail in its objective. Hence, the sampling system must be designed as an integral part of the chromatograph and not as an afterthought. The interested reader is referred to the monograph by Houser (2). Programmer-Controller (PC). This unit contains the program timer, power supply, signal conditioning electronics, and computer interface (where applicable). I t controls all operations in the analyzer-sample injection, column switching-as well as housekeeping functions (auto zero), component gating and attenuating and data transfer to the appropriate readout channel. This unit is usually located in the control room (as much as 1,000 f t or more from the analyzer) but sometimes ilva separate room near the control room. Readout Devices (Recorders). A strip chart recorder for recording bar graph (BG) and for trend records (TR) is located in the control room.
Additionally, the system may communicate with a computer by means of priority interrupt or long-term memories (not shown). Considerations in Process Chromatograph Design
The techniques and methodology used in process GC are in a general sense the same as in laboratory GC; however, a far different emphasis is placed upon the use of various methods and accessory devices. Some methods are more widely used in process-notably the use of multiple columns and column switching valveswhile others are less frequently or seldom used-for example, capillary columns and temperature programming. Beyond that, process hardware bears little resemblance to its laboratory counterpart. Overall design and appearance are influenced by the following factors. Purpose. The system’s purpose is to obtain information to control the process. Hence, for the majority of applications, it is usually necessary to measure only one or a few components. (Exceptions are pilot plant applications, wherein it is desirable to measure all of the components to characterize the process under study.) Location. The system is usually located in a hazardous area and requires explosion proof construction to satisfy National Electrical Code requirements (3). Analyzers and sample conditioning systems are designed to meet Class I, Groups C and D, Division 1. These are locations in which hazardous concentrations of flammable gases are present under normal conditions. Programmers and recorders are
Figure 1. Basic elements of process gas chromatograph system Sample withdrawn continuously from process line, P. filtered by filter, F1, pressure reduced by regulator, R1, circulated through sample conditioner, SHI, and returned to low-pressure point, Pr. Slip stream withdrawn and circulated to sample valve in analyzer, A, which also contains columns and detector in thermostated oven. Carrier gas supply, C, controlled by Regulator R2. Calibration blend, 6,introduced by valves in conditioner, SH. Programmer, PC, controls functions in A and SH and converts signal for recording as bar graph, BR, or trend record, TR
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11. SEPTEMBER 1975
is the oven compartment which houses valves, columns, and detector. Heating is usually by forced air bath which uses air passed over an electrical heating element. Process GC's are usually limited to a maximum operating temperature of 225OC-a limitation imposed primarily by the material of construction of the sample valve. This has not been a serious limitation, since 99%of process analyses can he performed a t below 160°C. Electronics in the analyzer include the temperature controller, detector amplifier, and valve controls. The detector signal may he transmitted to the progammer a t a high level (0-1 V) to minimize line loss and noise pickUP. Sampling Valves. Sampling and column switching valves have heen the object of considerable development and refinement in recent years. Numerous types have heen used, hut the most widely used are the sliding plate (Figure 4, A), the diaphragm valve, and the spool and O-ring valve. Gases are usually metered hy an external sample loop; liquids hy the volume of a n internal hole or channel in the Figure 2. Analyzer shelter showing field mounting of chromatograph and other types of analyzers. Chromatograph is at left Counery of COMSIP-Custornlme Corrp.
A
usually designed to meet General Purpose Classification or by modification with air purging, Class I, Groups C and D, Division 2. The latter are locations in which hazardous concentrations of flammable gases are not normally present. Reliability. The entire system should operate continuously and without maintenance for periods of at least 4-6 months and preferably a year. Hence, a premium on simplicity, ruggedness, and dependability for all components. Automatic Operation. Provision must he included for completely automatic and repetitive operation of all analytical functions. Maintainability. When failure does occur, the design of the system must permit its rapid repair and return to service with a minimum of down-time. Troubleshooting aids must he included to aid in isolating and identifying the faulty components. U s e of Data. Provision must he included for presenting the data in a variety of ways depending upon its end use, from bar graph readout to input into a closed-loop control system to direct readout into a computer. Analyzer Construction
Figure 3. Process GC analyzer interior
A view of a typical analyzer is shown in Figure 3. The interior section
4, carrier gas and heater air flow C O ~ ~ T O I SB.: electronics in explosion proof condulets-tempsrature control, valve drivers. detector amplifier; C, analyzer oven: D. heater: E, sample valve: F, columns: G. thermal conductivity detector
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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AOAC adopts methods only after interlaboratory collaborative studies demonstrate their reliability, practicality, and reproducibility. These standardized methods are used by both regulatory agencies and the regulated industries, and by research workers in agriculture and public health.
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B
A Figure 4. Sample valves used in process chromatographs
A, sliding plate valve, IO-port: B, vaporizing valve for injecting liquids through wall of analyzer
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valve. Liquid sampling valves of this type have been used for pressures of up to 500 psig, though pressures under 200 psig are more common. Sample volumes as small as 0.5 111 are attainable, with a reproducibility of f0.25%. The choice of sample valve is a critical element in every application, since it must be compatible with the sample and operate reliably a t elevated temperatures. Valves are available in corrosion resistant materials such as Hastelloy, Monel, or in all Kel-F and Teflon construction for corrosive samples. Other special designs permit injecting the liquid sample through the wall of the analyzer from the cold outside zone to the heated interior where the sample is vaporized by the heat (Figure 4,B). This is particularly useful for liquids which have high vapor pressures (such as propane or butane) and cannot be subjected to the analyzer temperature. Process liquid sample injection techniques are in contrast to those used in the laboratory. In the latter, liquid samples are injected through a septum into a heated block where the sample is instantly vaporized and rapidly transferred to the column in plug flow fashion. The temperature of the injection block is frequently higher than the column-and usually higher than the highest boiling point of the substances being examined. In contrast, the process GC sampling valve is usually maintained a t the temperature of the column. When the captured liquid volume is injected into the stream of carrier gas, the pressure is released and the liquid vaporizes under its own vapor pressure. The mass of the valve acts as a heat sink and provides sufficient heat to cause rapid vaporization.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
This requires that the flowing liquid sample be maintained a t high enough pressure to prevent incipient boiling of the more volatile components which would form bubbles and affect reproducibility. Detectors. Although a great number of different detectors have been used in gas chromatography, only a very few meet the stringent requirements of simplicity, ruggedness, and reliability necessary for process GC use. Detectors which have been used to any significant extent and the probable frequency of their useage are as follows: 85-90'36 for the thermal conductivity detector (TCD); 10-15% for the hydrogen flame ionization detector (FID);and less than 1%for all others [gas density balance, helium ionization detector (microcross section), and flame photometric detector]. A number of other detectors, such as the catalytic combustion (filament) for trace hydrocarbons and the electrolytic Pz05 hygrometer cell for trace moisture, have also been reported. However, their use has not been significant. Programmer-Controller. Analytical and communication functions which are performed by the programmer include sample injection, column switching, component gating (select attenuator for correct scaling factor), integrator (start, stop, readout, and clear), rezero baseline (auto zero), peak picker (convert peak signal to steady-state signal), present signal to recorder, switch samples in sample conditioner, and communicate with computer (transfer signal, signal beginning of analysis, signal end of analysis, and identify stream being analyzed).
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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A timer mechanism is required to repeatably perform these functions. Early programmers used mechanical cam timers with 1 cam per function. Many forms of mechanical timers have been and are available: rotating plastic wheels with photocell actuation, turntables with adjustable pins on the periphery, etc. Present trends, however, are to solid-state electronic digital devices. The overall cycle time is selected by setting rotary switches which read directly in seconds or by patching in on a matrix board. Two sets of similar switches are associated with each function which is to be programmed. One sets the time “on,” and the other the time “off”. A decoding circuit decodes the pulses from the timer and activates the circuit a t the time selected. Programmers with microprocessors are also available. Timing for all functions is entered into microprocessor memory, which is then used to actuate each function a t the proper time. Scaling, or calibration, for each measured component is accomplished with a dedicated component attenuator. When a measured component elutes, the corresponding attenuator is activated or “gated” in by the timer, and the proper scaling factor is applied to that component. Numerous other functions are located in the programmer. The “auto zero” and “peak picker” are two which have no equivalent in the laboratory chromatography, but are absolutely essential in a process GC. Auto Zero. This circuit rezeros the baseline a t selected times during the analysis, to compensate for short- and long-term detector drift or changes in baseline due to column switching
transients. Like the other functions, it is programmable. Peak Picker and Short-Term Memory. These circuits provide the capability of converting the transient peak signal to a continuous signal which can be stored in a long-term memory or transferred to a computer. The sequence of events is shown in Figure 5 . A simple locking circuit (analogous to a diode) holds the peak maxima developed for a fixed period (e.g., 5 sec) after the component gate closes. As the gate closes, a transfer command is sent out, and the signal is transferred and stored in a long-term memory. The short-term memory is then cleared and is ready for a subsequent component peak. Hence, only one short-term memory circuit is required regardless of the number of peaks transferred. This method is widely used to communicate data to on-line process control computers via “priority interrupt”.
a. Chromatogram
Data Presentatlon Peak height is most commonly used as a measure of component concentrations. When sample size, temperature, carrier gas flow rate, and other conditions are held constant, this relationship is accurate. Moreover, if sample size is such as to avoid column overloading and obtain symmetrical peaks, the relationship is linear. The simplest method of recording the peak height is the “bar graph” in which the component is recorded with the recorder chart stopped. The attenuator provides the correct scaling factor to give the desired full-scale range (e.g., 0lo%, 0-5 ppm, etc.). The relation of the bar graph to the chromatogram is shown in Figure 6.
i - .”. - b. Select Peak ~
Presentation
c. Bar Graph
‘i iI
8
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.
.-..
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d. Trend Figure 6. Modes of data presentation Chromatogram (a) and select peak presentation (b) are used in manual operation only. Presentation during automatic operation is bar graph (c) andtor trend record (d)
Detector
I tl
I
&-
time
t2
t3
td
Figure 5. Peak picker operation Sequence of events for transferring peak value to long-term memory or to computer by priority interrupt. t i , component gate comes ON and actuates component attenuator and peak picker which follows detector signal and holds peak value. f2,component gate turns OFF and Initiates transfer of signal to long-term memory or to computer. f3, transfer of signal completed (13 - t2 = 5 sec). t4, after additional 1-sec delay, peak picker output resets to zero
99OA
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11. SEPTEMBER 1975
Not all components in the chromatogram are recorded: only those which are “gated” by the programmer. At all other times the input to the recorder is simply shorted, and the recorder reads “zero,” thus eliminating unmeasured peaks and baseline transients from the chart record. If the recorder chart is advanced continuously during bar graph presen-
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1
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interface Hardware
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Figure 7. Computer-controlled process chromatograph system-block
diagram
Dedicated minicomputer controls up to 32 chromatographs and performs all data acquisition and reduction functions. Conventional programmers are eliminated
tation, a time record of the gating for the measured components is obtained. This record is sometimes called an “elution time check” (Figure 6, b) and is useful as a maintenance check to detect shifting of the peaks within the fixed time gates. The transient peak signal can be converted to a continuous analog signal suitable for closed-loop control. A peak picker is used in conjection with a long-term memory device which holds the signal from the peak picker until it is updated with a new value the subsequent cycle. A record of the long-term memory is called a “trend” record, shown in Figure 6, d. Such a signal can also be input to a conventional controller for use in closed-loop control. Computer interfacing
The number of process GC’s which are tied into on-line computers has been increasing rapidly during the last decade. This requirement is now so common that for a number of years, process GC’s have been available with standard options to make the interface. The two most common methods of interfacing chromatographs to computers are long-term memory and pri994 A
* ANALYTICAL CHEMISTRY, VOL.
ority interrupt (short-term memory). A third method is the computer-controlled chromatograph system, which eliminates the programmer entirely. Long-Term Memory. The longterm memory and its operation were described above. During the analysis the information is stored in the longterm memories. At the end of the analysis cycle, a contact closure signals the computer that the analysis is complete and that the information is available. The computer then scans the signal in each long-term memory, digitizes it, and stores it in its own memory bank. A disadvantage of this approach is that a separate long-term memory is required for each component. From a cost standpoint, it is less attractive than the priority interrupt approach and is now infrequently used. Priority Interrupt. As computers have increased in speed and capacity, the priority interrupt method has become the preferred approach. In this method the longlterm memories are eliminated altogether and replaced by a single short-term memory. Sequence of events associated with the transfer of the information to the computer is identical to the peak picker function shown in Figure 5 . 47, NO. 11, SEPTEMBER 1975
As previously described, the shortterm memory stores the scaled analog value for each component as it elutes from the column. As the component gate closes, a contact closure, or “come read” signal, signals the computer that a value is stored in memory. Duration of the closure is usually about 50 msec, but varies with the individual computer. Upon receipt of this signal, the computer interrupts its routine and “services” the interrupt. It scans the short-term memory output, digitizes it, and stores it in its own memory. All this occurs during the few seconds that the value is held by the shortterm memory. The short-term memory is then available for the next component to be measured. The obvious advantage of this approach is that only one memory is required regardless of the number of components which are input to the computer.
Computer-Controlled Chromatograph System
In this system a dedicated minicomputer replaces all the programmer controllers for an array of process chromatographs. The computer per-
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forms all functions associated with control of the analyzer units and all analytical data handling, namely: Control of analyzer sample inject and column switching valves Control of multiple stream and calibration sample valves in the sample conditioning modules Monitoring of detector output, integration of peak areas, component identification, and data reduction Automatic baseline correction for zero drift Separation of incompletely resolved peaks or of trace components riding on the tail of major components Automatic system monitoring and alarm for off-limits data and malfunctions Data presentation and transfer for control room operators and maintenance personnel Transfer of reduced analytical data in digital form to central supervisory computer. A system of this type consists of three major hardware segments (Figure 7): Field mounted equipment-consists of analyzer and associated sample conditioning modules Interface hardware-Provides a central terminal for all detector signals, as well as all analyzers and sample system electrical connections. Also includes provision for isolating the analyzer and sample conditioner from the computer during start-up and maintenance and for permitting manual control of all analytical functions during start-up and maintenance Computer a n d communications hardware-Consists of dedicated computer and associated teletypes for data logging, operator communications, program loading, and all other input functions. It may also include provision for analog output (pneumatic, voltage, current) to conventional control instruments. A distinctive feature of this system is that the computer samples the output of each detector at up to 10 times each second, integrates the area under each peak, identifies the component, applies a response factor and computes the composition of the sample. This may be done by internal normalization or by comparison with calibration standards. Systems of this type, like their laboratory counterparts, are able to correct for incomplete resolutions between adjacent peaks and for minor peaks which ride on the “tail” of a preceding major peak. This is a decided advantage over the conventional process chromatograph which cannot correct for incomplete resolution. Data output is by means of a teletypewriter located in the cbntrol room for maximum data accessibility to the 996A
ANALYTICAL CHEMISTRY, VOL. 47,
operators. The printouts may include a complete analysis or for selected components only, as required by the operator. A second teletypewriter is used for communicating with the minicomputer. Changes in operation or status of analyzer-such as removing from service for maintenance or reprogramming of events when columns are changed-are communicated by this means. The most significant advantage of this type of system is the greater accuracy which is potentially available. A second advantage is the more complete data which can be obtained by the computer. Availability. Complete computercontrolled process GC systems are currently available from most manufacturers of process analyzers. The software package is tailored to the specific needs of the user. Large systems of this type with as many as 32 chromatographs are most frequently purchased for new plants or expansions where formerly no analyzers existed. More and more, however, such systems are being added to existing plants with numbers of analyzers already in service. Existing programmers may be replaced with the interface hardware described above. Alternativelv, they may be left in service, with t:;e computer performing only a data acquisitioning and reduction function and the programmed analytical functions remaining under the control of the chromatograph programmer.
Column Design As in the laboratory chromatograph, the objective in designing the column is to separate all the components of interest in the minimum of time possible. Hence, good column design practices are dependent upon a thorough understanding of chromatographic column theory as developed during the last 20 years. However, the requirements of process chromatographs impose somewhat different constraints on the design of the columns. Some principles which must be observed are: All components in the sample must be quantitatively accounted for and removed from the column system each cycle. Components which are left on the column will either accumulate and change the characteristics of the column, or will elute during a subsequent cycle and interfere with a measurement. The column separation should be designed not only for the normal composition, but also for the upset condition. The chromatograph must continue to provide reliable data even when abnormal conditions prevail.
NO. 11,
SEPTEMBER 1975
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Figure 8. Example of use of stripper column Analysis of light hydrocarbons in gasoline. +Pentane and heavier backflushed to vent during 5-min cycle. A, Cpentane; B, cis-butene-2; C, trans-butene-2; D, Cbutylene; E, +butane; F, Cbutane; G, backpurge; H,propane
Columns must be protected from components which are irreversibly or too strongly absorbed for the same reasons given in the first principle above. The system should be as simple as possible. Adjustment and maintenance, when necessary, should be easily and conveniently performed. Emphasis on Valves and Column Switching. Because of these design constraints, process column technology has developed along lines which are in contrast to laboratory practice. A greater emphasis has been placed on isothermal, multicolumn techniques. On the other hand, programmed temperatwe has not been widely used, although commercial units are available. Numerous other techniques such as pyrolysis, derivatization, reaction chromatography, etc., have not found even limited application in on-line process analyzers. With the emphasis on multicolumn methods, a great variety of schemes using valves to switch columns have been developed. While there are literally hundreds of possible column configurations, most are built up from only a few basic configurations. These configurations and how they are used are described in the following sections. Stripper (Precut or Backflush t o Vent). This is the most widely used. It consists of two columns, a stripper (or precut) and an analysis column in series, with provision for backflushing the stripper to a separate vent and providing carrier gas to the analysis column while the stripper is being backflushed. The stripper column makes a partial separation and is then backflushed, rejecting the unwanted components. The remaining components
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are further separated on the analysis column while the stripper is being backflushed. A typical analysis is shown in Figure 8 of a full-range gasoline containing components as high as (212.
Moisture, a contaminant often found in hydrocarbon samples, can be conveniently rejected with this configuration. In general, use of the stripper is good housekeeping practice; it ensures that unknown heavy components are completely removed from the system each cycle and will not appear unexpectedly during later cycles to interfere with a key measurement. Back Flush to Measure. This is similar to the foregoing configurations, with the exception that the analytical column is backflushed to the detector permitting measurement of all components remaining in the column. A frequent use is to measure the total of material heavier than a particular carbon number, e.g., pentanes and heavier. Dual Column. A valve is used to switch components so that components eluted from a first column are passed either into a second column or directly into the detector. A typical separation is shown in Figure 9. Heart Cut (Cutter Column). This arrangement is used most frequently for trace analysis with the hydrogen flame ionization detector (FID). It is particularly useful in measuring a trace component which elutes on the “tail” of a major component. Two columns are so arranged that narrow cuts of effluent from the first (cutter) column can be taken into the second (analysis) column, the bulk of the sample being discarded through a separate vent. A trace component riding on the interfering tail is thus separated from the tail on the second column. Complete separation can be achieved with this arrangement in much less time than could be attained with a single column. This arrangement is essential when a sensitivity of 500 ppm full scale or less is required. A typical separation with a heart cut system is shown in Figure 10. High-speed Chromatography. Closed-loop control of processes with short response times may require analysis cycle times as short as 15-90 sec. Such rapid analyses may often be achieved with conventional packed columns (2 mm i.d.) as shown in Figure 11, particularly if only one or two components in the sample are to be determined. Multicomponent analyses in short cycle times may be attained with micropacked columns (0.0300.040 in. 0.d.) packed with ultra fine diameter substrate and special lowvolume detectors ( 4 ) . Special low-volume valves are also required to obtain optimum performance. 998A
Figure 9. Example of dual-column analysis Unresolved pair Cbutene-I-butene is diverted to second column for further separation. A, Lbutene (5%); B, 1-butene (5%); C, cis-2-butene (5%); D, trans-2-bvtene (5%): E, +butane (5%); F, Cbutane (5%); 0. propane (10%): H, dual column; K, single column
Flgure 10. Example of heart cut analysis Trace toluene impurity in benzene. Toluene trace from first column is cut into second column for separation from interfering benzene tail. A, toluene (20 ppm): B, benzene tail; C, cut for toluene
Chemical Conversions. The measurement of carbon monoxide and carbon dioxide at trace levels cannot be done with the thermal conductivity detector because of its limited sensitivity. However, by methanating in the presence of hydrogen over a nickel catalyst, the oxides can be made measurable with the FID (5).This method is used most commonly for measuring low ppm levels of carbon monoxide and carbon dioxide in polymerization grade ethylene. By combining methanation with the heart cut technique, these compounds can be determined on the same system used to measure acetylene a t the same low levels. An example is shown in Figure 12. Quantitation and Calibration Quantitation methods most commonly used are comparison with
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
Figure 11. High-speed analysis with conventional packed columns 6 R X 2 mm i.d. bis(butoxyethy1)phthalate on Chromosorb P. Sample size, 50 11. A, n-butane; B, 2-butane: C, propane
Figure 12. Determination of trace COP in high-purity ethylene by methanation over nickel catalyst Heart cut column system is used to separate CO? and acetylene from interfering tail from ethylene peak. A, acetylene (10 ppm); B, ethylene tall; C, cut for acetylene: D, ethylene tail; E,carbon dioxide (5ppm): F, cut for carbon dioxide
known standards (calibration blends), comparison with laboratory analysis (grab sample), and internal normalization. Most widely employed is, of course, calibration with a known standard, in which a cylinder of calibration gas or liquid is periodically analyzed by the chromatograph. Component attenuators are then adjusted as required to give the correct reading. For this purpose, “certified” standards with guaranteed accuracies of 1%are available. Some users, prefer to have the blend analyzed by their own laboratory and to use that value instead. A further requirement of the calibration blend is that it be stable over periods of time, as long as a year or more. Reactive components or compounds which are strongly absorbed on cylinder walls add a dimension of
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Adaptable-transforms in less than one minute to manual cuvette operation; immediately converts back to flowcell mode with three thumbscrew adjustments for alignment CIRCLE 211 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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uncertainty. At trace levels, even compounds which are not reactive in the percent levels can be troublesome. Fortunately, the technology of synthetic blends is currently undergoing major development and should within a short time result in improved quality of calibration standards, particularly in the parts-per-million range. Analyzers may also be calibrated by having the laboratory analyze a sample taken manually a t the same time that the analyzer injected the sample. This method is generally less satisfactory simply because it introduces all the uncertainties associated with manual sampling-possibility of fractionation during transfer, adsorption of components on container surfaces, condensation of heavy ends, etc. The API survey (I)indicated that the highest frequency of dissatisfaction with calibration methods occurred among users of this technique. Internal normalization is of course potentially the most accurate method, but this requires integration of all components in the sample and access to an on-line computer. Hence, this method is used only when a dedicated computer is available for continuous on-line data acquisition and reduction, as in the computer-controlled chromatographic system previously described.
Figure 13. Determination of dissolved gases in transformer oil
Accuracy vs. Repeatability
A process chromatograph is capable of short-term repeatability of &1/4 to lh% of the full-scale range when in the conventional bar graph mode and using peak height measurements. This approximates a standard deviation of as small as 0.1%. In theory, this represents the limit of attainable accuracy. However, long-term effects, which include instrument drift, temperature effects on the calibration blend, and barometric effects on sample size and detector sensitivity, all conspire to degrade the repeatability which can be obtained in practice. Osborne (6) has reported that records maintained over a period of several months show a long-term repeatability of &8.2% relative at the 95%confidence level. Since the effect of these long-term variations can to a great extent be minimized through the use of on-line computer data acquisition and reduction, one can expect to see increasing use of on-line computers dedicated to process chromatographs. Applications
The vast majority of applications are in the petroleum refining and petrochemical industries and for the most part are relatively straightforward and will not be discussed. A few which are of special current interest will be described below. 1000 A
+
Dual column with stripper configuration. Column 1: 3-ft Porapak N. Column 2: 3-ft Porapak Q 4 4 Porapak N. Column 3: 6-ft Molecular Sieve 5A. Helium carrier 7OoC. l-ml oil sample is Injected into column system where dissolved gases are removed and separated. Oil backflushed from system each cycle. A, CO (X10); E, methane (X10): C, acetylene (XIO); D, ethane (XIO); E, ethylene (X10); F, COS(X10); 0, N2 ( X i ) ; H, 02 ( X l ) ; K, H2 (Xl);L, single column
Figure 15. Determination of trace water in xylenes Column, 3-ft X %e-in. Porapak-Q. Sample size, 0.5 ml liquid. Temperature, 105OC.A, water (2 ppm): E, sample inject; C, start analysis
Figure 14. Detection of vinyl chloride in ambient air Three-minute cycle time permits monitoring several sample points with a single analyzer. A, vinyl chloride (1 ppm. X2); E, propylene (100 ppm, X400): C, methyl acetylene (100 ppm): D, methyl chloride (100 ppm); E, ethane (100 ppm); F. air balance: G, start
Dissolved Gases in Transformer Oil. The power industry has long been interested in the gases present in large oil-filled transformers. Gases of interest include hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, and acetylene. A process chromatograph modified with a special sparger in the sample valve per-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
mits injecting a large volume of oil directly into the column system. A dual column with stripper configuration permits determining all these components in a single run, as shown in FIGURE [3/ A thermal conductivity detector permits detecting as little as 20-40 ppm wt/wt of each gas (30-300 volumes gas per million volumes oil). With argon carrier the sensitivity for hydrogen can be extended by a factor of 10. With an FID, as little as 0.1-0.2 ppb wt/wt (0.2 volumes gas per million volumes oil) can be detected. Vinyl Chloride in Ambient Air Monitoring. Recent OSHA regulations require the monitoring of vinyl chloride in the atmosphere in the vicinity of plants which either produce it or use it. The analysis shown in Figure 14 illustrates this separation using a FID to detect vinyl chloride with a full-scale sensitivity of as little as 0-1 ppm. A single column stripper configuration is used to reject all components heavier than CB.The design of the column is such that an interfer-
GC/MS DATA ANALYSIS SYSTEMS System/lSO System/lSO System/lSO System/lSO
Quadrupole Time of Flight Magnetic Dodecapole -
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ence-free measurement is produced regardless of the presence of any other chemical compound which is likely to be present in the atmosphere. This could include literally any other hydrocarbon found in a petrochemical plant or refinery. A short cycle time of 3 min is used to monitor 10 sample points within a total time of 30 min. The sampling system includes pumps to draw air from various points in the plant through lh-in. 0.d. lines up to 400 f t long. Trace Dissolved Water in Hydrocarbons. Dissolved water can be measured a t the parts-per-million level with a T C detector by using porous polymer substrates (Porapak, Chromosorb Century Series) which permit rapid elution of water. By using liquic: injection and large sample volumes (0.5-1 ml), water can be determined a t the 1-5 ppm level, as shown in Figure 15. Trace Hydrocarbons in Steam Condensate. Trace hydrocarbons in steam condensate return are undesirable in steam generation plants. The presence of hydrocarbons is indicative of leaks in heat exchangers or reboilers. A process chromatograph with FID can be used to monitor individual condensate return lines for hydrocarbons with sensitivities of as much as 0-5 ppm carbon. A porous polymer column separates the water from the hydrocarbons which are then backflushed from the column and measured by integration, as shown in Figure 16. This method is limited to hydrocarbons which have some volatility. Nonvolatile hydrocarbons such as polymers or organic salts require other analytical methods (e.g., catalytic combustion). The process GC method is best suited for chemical plants or refinery applications where the hydrocarbons are of known composition and volatility.
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ANALYTICAL CHEMISTRY, VOL.
Summary In the foregoing sections we have attempted to summarize the present state of the art and science of process gas chromatography-its hardware and technology-and to provide a few examples of their use. There is little doubt that process chromatographs are now indispensable instruments in the efficient operation of a refinery or petrochemical or chemical plant. Moreover, additional new applications are continuously being found in all areas of industry. Chromatographs will also continue to increase in their capability and sophistication. Future efforts will be directed at increasing both reliability and accuracy to the maximum of which chromatographs are potentially capable. The recent explosive develop.7, NO. 11, SEPTEMBER 1975
Figure 16. Determinationof trace hydrocarbons in condensate Backflush column configuration. Column, 5-ft Porapak Q, 12OoC. A, Total hydrocarbons (45 ppm); B, backflush; C, water: D, sample inject
ment in microprocessors undoubtedly will have an impact in that regard. For the analytical chemist who is instrumentation oriented, this discipline will continue to provide an opportunity for creative efforts within the context of one of the more interesting fields of analytical instrumentation.
References
