PROCESS INSTRUMENTATION 90 to lOO%-it is generally not desirable to sensitize the instrument directly to this component but to effect an indirect measurement by sensitizing the instrument to the total impurities. This procedure is obviously more suitable when the sample is not too complex and consists of only 2 t o 4 infrared absorbing components.
such standardization periods. Standardization can be carried out manually or automatically a t the user’s option. When applications require extremely high sensitivity, such sensitivity is attained almost invariably a t the expense of stability of operation. Many proved applications require standardization as infrequently as once a day or once a week. Under these conditions of stability the error will be somewhat less than il% of full-scale span. When standardization must be carried out more often than once an hour, such an application should be considered borderline for the stability of the instrument. The manufacturer can now predict the stability for a given application by noting the electrical sensitivity of the instrument for the required full-scale concentration span.
Multistream Analysis Often several separate streams from as many separate unit processes are desired monitored on the same recorder. I n this case, it is assumed that all the streams are reasonably identical in major constituents; this is normally the case. All streams can be connected through solenoid operated three-way valves allowing a continuous sample flow in each line to a common manifold which in turn leads to the instrument. Each stream can then be selected sequentially by a multipoint recorder a t intervals of 3 to 5 minutee giving almost continuous analysis of each stream and thus reducing the time lag between analyses.
Conclusion The use of these instruments will increase as knowledge of their potentialities and applications becomes more widespread. Infrared methods have long been said to provide a “fingerprint” of the molecule. Continuous infrared analyzers provide the process operator with up to the minute knowledge of the concentrations of pertinent compounds entering or leaving a unit process or operation. This information should ensure higher operating efficiency, higher purity of product, and consequent lower operating cost.
Precision The continuous infrared analyzers commercially available are capable of providing analyses with error no greater than 5 1 % full-scale span. Accuracy, of course, is a function of many factors as applied to instruments of this type. dmong these factors are the frequency of standardizing the instrument on a sample of known concentration and the amount of drift occurring between
RECEIVED for review September 7, 19.53,
QCCEPTED
March 6, 1954.
Infrared Gas Analyzer for Butane Splitter Control R. 1. MARTIN AND
B. W. THOMAS
Humble Oil and Refining
Co., Boyfown,
rex.
The plant application of a nondispersive type infrared gas analyzer for continuously monitoring the isobutane losses from three parallel operating butane splitter columns is discussed. The analyzer has been in plant operation for a period of 4 years during which time a marked improvement in the fractionator operation has resulted. Facilities are discussed for completely automatic sample handling, switching, and restandardization of the analyzer. Data are presented on the accuracy of the analyzer and the product improvement obtained through its utilization.
A
S EARLY as 1944 nondispersive infrared instruments were
in use for continuous measurement of butadiene in butylenes and ethylbenzene in styrene (8). Selective and nonselective instruments have been compared, and limiting ranges for detectability of carbon monoxide, carbon dioxide, hydrocyanic acid, and xater vapor have been given (3). Means for sensitizing both positive- and negative-type filtering systems for analysis of single components in complex mixtures have been described (b, 7 ) . By placement oi sapphire and quartz filters in opposite beams of the two-beam optical system an analyzer has been sensitized for measurement of carbon dioxide in flue gas during regeneration of a normal butylene dehydrogenation catalyst ( 5 ) . Operational theory and several applications of analyzers employing acoustic detection methods have been described by Smith (6). Continuous measurement of o-ethyltoluene in admixture with Ihe para and meta isomers has been made on a process stream by means of a sodium chloride prism instruJuly 1954.
ment operating a t 13.6 microns ( 4 ) . Use of infrared instruments for continuous control of a distillation tower has been discussed by Berger (1). The present paper is concerned with a continuation and extension of several applications that have been described previously. Among the many complex operations in modern petroleum processing is the segregation of isobutane through a series of conventional fractionation processes. The final fractionator of such a system, commonly termed a butane splitter, serves to make the separation between isobutane and n-butane in the presence of small quantities of isopentane and propane. When feed stock to the butane splitter varies in composition, control of the operation by conventional instrumentation (temperature, pressure, and flow aided by laboratory analyzed spot samples) results in rather wide variations in efficiency of isobutane recovery. The application of a plant stream continuous analyzer which has resulted in a substantial improvement in control of
INDUSTRIAL AND ENGINEERING CHEMISTRY
1393
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
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Description of Analyzer The instrument utilized in this application is a double beam nondispersive type infrared gas analyzer manufactured by t'he Process Controls Division, Baird Associates. It is equipped with a separate preamplifier, volt'age regulator, strip chart recorder, and
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Figure 1.
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Operation of Butane Splitters
three butane splitters for a period of 4 years is described. This improvement in control has been effected largely through elimination of time lag previously required for laboratory analytical results on product streams.
I
circuits are mounted on a panel board in the main control room of the unit. The analyzer, cycle timer, and sample handling system are located in a separate G X 8 foot house located about 75 feet from the control room. bIounting the analyzer in the control house was not advisable from the standpoint of space limitations and to keep from piping hydrocarbons into the house. All equipment located in the analyzer house is of explosionproof design suitable for Class I, Group D, Division 1, hazardous gas service.
Description of Application This analyzer application involves three butane splitter columns operating essentially in parallel service. The charge stock to the fractionators contains a variable mixture of 30 to 45% isobutane, 50 to G5% n-butane, 1 to 5% propane, and about 1%isopentane. Isobutane and propane are taken overhead and n-butane and isopentane are withdrawn as bottoms. Most economical operation is obt,ained when isobutane loss to bottoms is maintained at a value between 1 and 57, of the bottoms product. Concentrations of isobutane greater than this represents excessive loss of isobutane while lower concentrations lead to excessive contamination of the overhead product. I n view of this criterion, the analyzer was installed to monit,or bot'toms product streams and hence, to serve as a guide for efficient operation of the splitter columns. A single nondispersive type infrared gas analyzer was sensitized and inshlled to serve all three columns. The analyzer is equipped alternately t o monitor and record the bottoms quality for each of the three columns and periodically t o analyze a separate reference sample as a check on the accuracy of the analyzer. Effects of drift in the analyzer have been minimized by the incorporation of an automatic st,andardiaation system. The standardizer unit automat,ically readjusts the zero position of the analyzer each hour on the basis of a preset value for the laboratory-analpxed reference sample. Operation of the analyzer, sample s\\-itching, and restandardization are completely automatic. The three columns involved in this application are similar but, not identical in design, piping, and instrumentation. Two of the columns are 45-plate distillation units and the third column containE 40 plates. Feed stock is currently charged a t either the 27th or 28th plate from the bottom of the towers, each of which is equipped with a steam reboiler. Operation of the fractionators is based on material balance, with overhead and charge
1394
Figure 2. B. C.
F. I.
Bolometers Compensator cell
Optical System S.
Sample cell Source T. tight trimmers M. Mirrors W. Sodium chloride windows
.1'
Filter cell Interference cell
The optical system of the analyzer is shown in Figure 2. The cell windows are of sodium chloride and all cells are suitable for use a t or near atmospheric pressure only. Optical path lengths in the sample, interference, and filter cells are 5 , 2, and 2 ems., respectively. Compositions of sensitizing gases in each of the cells are as follows: filter cell 92.5y0 isobutane 7.!y0 n-butane; compensator cell G2.1% n-butane and 37.9% 190pentane; and the interference cell lOOyo nitrogen All cells are filled to a pressure of 760 mm. absolute pressure a t the operating temperature to minimize leakage difficulties. The gaseous composition of each cell m-as detprmined empirically The trimmers were set after sensitization to minimize the effects of line voltage variations. With the above conditions, full-scale range of the analyzer is 0 to 20y0 isobutane with a 0.5-ohm transfer resietance in the bolometer bridge. This represents a change in the temperature of a 1000-ohm resistance-type sensing element sufficient to provide a change of 1ohfi in total resistance The analyzer case is thermostated to a constant temperature of
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 46, No. 7
PROCESS INSTRUMENTATION 130" F. During the summer months it is necessary to provide forced air circulation around the outside of the analyzer case to increase the heat loss sufficiently to allow the thermostat to function properly. A permanently connected ammeter on the a.c. supply to the thermostatic heater control circuit is useful in detecting difficultyin the temperature control unit. Sampling points on the three columns range from 100 to 200 feet from the analyzer house. Liquid sample withdrawn a t 110 pounds per square inch from the process stream is immediately vaporized in a 12-inch length of steam jacketed 3/4-inch pipe. The vaporizer is mounted vertically above the line to minimize liquid holdup. The gasified sample thus obtained is then expanded through a reducing regulator to a pressure of about 5 pounds per square inch gage, and passed through steam heated '/c-inch copper tubing to the analyzer house. Each sample stream is continuously vented a t a rate of about 200 cc. per minute a t the analyzer room in order to provide fresh samples with minimum time lag. A schematic layout of the sampling system is shown in Figure 3. A small portion of each sample stream and a reference are alternately passed through the analyzer. Switching of samples is all accomplished automatically by means of an electric cycle timer and solenoid operated valves. Difficulty with valve leakage is minimized by maintaining pressure differentials across the closed valves at 1 to 2 pounds per square inch gage. A separate drum is provided to upp ply reference sample to the analyzer. The reference drum is filled with liquid sample from the operating unit through a separate 1/2-inch line. The vapor pressure of the sample is sufficient to force the reference through the analyzer. Reference sample is withdrawn from the drum as liquid through a dip pipe and immediately vaporized. Each time the reference drum is refilled, a sample is withdrawn and submitted in duplicate for laboratory analysis. The results of this analysis are used to set the standardization unit. The sample switching cycle is set to analyze each stream, including the reference for a period of 15 minutes each hour. Signal lamps in the control room and analyzer house indicate the source of sample being analyzed at any time. All sample flow rates are measured by means of indicating rotameters. One of the most troublesome difficulties associated with infrared continuous analyzers is in obtaining long term stability or freedom from drift. These difficulties have been overcome in this application by use of an automatic restandardization system supplied by the manufacturer. This system, as mentioned previously, allows the instrument to correct its own zero position a t regular intervals. Standardization can be accom-
Figure 3.
July 1954
Sampling System
T I M E . HRS
Figure
5.
lsobutane Analyzer Record
plished only with the analyzer on the reference sample. The cycle is initiated by means of a separate switch in the major cycle timer which in turn initiates a minor cycle timer as part of the standardizing network. The minor cycle timer switches the measuring network to compare the analyzer output with the voltage supplied by a separate standardization Helipot potentiometer. An additional drive motor which has also been energized by the action of the minor cycle timer rebalances the zero position of the bridge to a null against the standardization Helipot which may be manually adjusted to cause the analyzer to balance a t any selected and preset value. Upon conclusion of the standardization cycle the minor cycle timer stops itself in position to begin the next cycle. The recorder pen drive motor is de-energized during the operation of the minor cycle timer. Timing of the major and minor cycles is set to allow a short scanning period on the reference sample at the conclusion of the standardization process which allows the recorder to step to the newly standardized position. Comparison of the instrument reading on reference sample before and after standardization affords a measurement of the drift in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1395
ENGINEERING. DESIGN. A N D PROCESS DEVELOPMENT system during the previous 45 minutes of operation. I n addit,ion, the small step on the record caused hy the drift correction can be used as a key to correlating the chart record with the various samples. Details of the measuring and standardizing circuits are shown in Figure 4. A typical chart as obtained on the recorder is shown in Figure 5.
Accuracy of Measurement The range for n-hich the analyzer is calibrated is 0 to 207, isobutane. Sensitization has resulted in suppressing the srnsithat, of isobutanc. tivity of the analyzer to isopentane to about With this degree of suppression and the relatively small quantit'y of isopentane preeent, a concentration change of f1.0% isopentane result's in less than 0.2% error in isobutane. Comparison of the analyzer results with laboratory spectroscopically analyzed spot sampled indicates satisfactory agreement (usuallvr-ithin 0.2 to 0.3% isobutane). During periods of rapid changes in t'he toxers comparisons are difficult to obtain because of uneven h e lags arid nonrepresentativr samples.
Maintenance and Safety Routine maintenance of the analyzer consists primarily of daily observation for signs of difficuky and refilling of the reference sample drum as required (usually every 3 weeks). Spot samples from each of the columns are sent to the laboratory for analysis each shift. These samples are compared with the analj-zer as a furt,her check on accuracy. This maintenance of the analyzer is considered a function of the instrument man assigned to the unit who, by training and experience, is familiar v i t h the entire operation of the analyzer as well as the trouble spots to be anticipated. The routine maintenance required by the analyzer is approximately 15 man-hours per month. The service factor is estimated to be 96%. The most troublesome maintenance items are failure of the thermostatic temperature control, moisture in the Eample, and leakage of the solenoid valves. As mentioned previously. all equipment, in the analyzer house is of explosionproof design. A vent,ed manometer is attached t o the analyzer sample line as a safety to prevent overpressuring the cells. Precautions are also taken to prevcnt liquid from accidentally entering the analyzer. An electronic relay is installed t o det,ect,the presence of liquid in a trap placed in the sample
line. I n the event liquid is detected at this point a solenoid valve closes in the sample line.
Advantages of Continuous Analyzer Prior to the installation of the continuous analyzer the operation of each of the columns was adjusted on the basis of laborutory analyzed spot samples. The data available a t the units lagged 6 to 8 hours behind actual process conditions because of t,he time required to transport and analyze the samples. As a result, over-all operation of the units x a s erratic and resulted in substantial loss of isobutane through t8hetax-er bottoms streanis as shown in Figure 1. Installation of the continuous analyzer resulted in reducing the time lag to 5 minutes IThich allowed greatly improved control of the operation as also shown in Figure 1. The additional isobutane recovery resulting from this improvement in operations has been estimated t,o be worth $10,000 per year on this particular application. No credit is taken for t'he reduction in the number of laboratory analyses brought about through the use of the continuous analyzer. The possibility of utilizing the analyzer to control automatically the process operations is not considered feasible because of the technical complications involved. T o do so vould require the installation of two additional analyzers (one for each column) as intermittent automatic controls cannot be made to operate satisfactorily. This would also require elimination of the drift compensating automatic standardizing system vhich would reduce t'he analyzer accuracy. And last, it is believed that lit& addit,ional improvement in process operations would be obtained.
Literature Citad ( I ) Ijerger, D. E.. Inatruments, 26,872 (1953). ( 2 ) Devine, ,J. ll.,Ihid.. 24, 1296 (1951). (3) Fastic, W. G., and Pfund, A. H., J . O p t . SOC.Amer., 37, 702 (1947). (4) Herscher, L. W., and Wright, N., I b i d . . 43, 980 (1953). ( 5 ) Kratochvil. K. V.,and Berger, D. E.. Proc. Instrument Soc. Am., 7, 25 (1952). , I n s t r u m e n t $ , 26, 421 (1953). ( 7 ) Thomas, 13. W., Petroleum Refiner, 30, 81 (February 1951). (8) Wright, S . . and Herscher, L. TV., J . O p t . SOC.Amer., 36, 195 (1946). RECEIVED f o r r e r i e m September 7, 1953.
Sensitizing ELLIOT H. WOODHULL,
ACCEPTEDMay 3 , 1934.
lyzer E. H.
SIEGLER, AND HAROLD SOBCOV
The Perkin-Elmer Corp., Norwulk, Conn.
The operating principle of a specific nondispersive infrared-type instrument, the Tri-Non analyzer, i s described briefly. A method which permits this instrument to be calibrated in per cent concentration of one component in a multicomponent gas stream i s described. This procedure i s general and can be applied to nondispersive analyzers using either selective or total radiation-type detectors. Data taken for a typical four component stream are used to illustrate the method.
A
LTHOUGH nondispersive infrared analyzers havc been in plant
use for a number of years, the methods of sensitizing these instruments t o the gas of interest and of maliing them insensitive to the background components have seldom been described, partly as a result of their trial and error nature ( 1 , 2 ) . This paper presents a general systematic method for sensitizing a nondis-
1396
persive analyzer for a multicomponent stream analysis in which there is spectral overlap with the gas of interest. The method chosen enables one to determine t,he effcctJivcness of filtering and compensation and permits a rapid and rational determination of a sat'isfactory mixture for filling the filter cell. Also, the approach indicates the order in which an iterative
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
