ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
nalyzer for onitoring ater Content Liquid Sulfur Dioxide-Gas Oil Extraction Unit F. W. KARASEK
AND
E.
C. MILLER
Chemical Engineering Division, Phillips Petroleum Co., Barflesville, O k l a .
A n infrared analyzer sensitized to the 2.7-micron absorption band of water in liquid sulfur dioxide is being used in the operation of a refinery sulfur dioxide-gas oil extraction unit. Such a continuous water content indicator i s required for maintenance of maximum extractive capacity and protection against excessive corrosion. The analyzer has a full scale span of 0.4 weight yo water with an analytical accuracy of k0.01 weight %.
C
ORROSION becomes excessive in a sulfur dioxide-gas oil extraction unit in refinery operations if the water content of the liquid sulfur dioxide exceeds 0.2 weight %. Monitoring the water content by conventional methods requires frequent and time-consuming laboratory analyses, In order to maintain maximum capacity in the extraction operation and to protect the plant against excessive corrosion, a continuous water analyacr is necessary. This instrument must be accurate, reliable, and rapid.
The use of dielectric constant and solution conductivity to indicate water content in low concentrations appears in the literature ( 3 ) . However, these methods suffer from lack of specificity in a multicomponent system. This means a lack of reliability in any instrument based on these principles. The specific nature of infrared absorption and the reliability of infrared analyzers suggests the use of such equipment for the instrumentation desired. A search of the literature does not reveal any infrared spectra of water in liquid sulfur dioxide. However, one reference indicates that the water content of refrigerants such as Freon could be determined in the range of 0 to 10 p p.m. by measuring the absorption a t the 2.67-micron watei band with a laboratory infrared spectrometer ( 1 ) . The presence of hydrocarbons interferes with the analysis in this concentration range.
Infrared Spectra
IO
I2
Figure 1.
14
I6
I 8
20 2 2 24 WAVELENGTH-MICRONS
26
28
30
32
I
Infrared Absorption Spectra of Liquid Sulfur Dioxide and Water
As a basis for development of a continuous analyzer, the existing laboratory methods applicable to the determination of water in liquid sulfur dioxide in this low concentration possess a number of disadvantages. The simplest method, which consists of the direct vaporization of a given volume of liquid sulfur dioxide and measuring the residue, cannot be considered reliable. Its accuracy is too dependent upon evaporation rates and is further complicated in our application by the presence of hydrocarbon oils in approximately the same concentration as the water. The time required for a single determination is about 30 minutes. A second method in use is the Karl Fischer (2) in which water content is determined by an iodometric titration in an anhydrous medium. Its accuracy in the 0.0 t o 0.2 weight % range has been estimated as 10 to 15%, and its use is complicated by the preparation of anhydrous solutions of iodine and the continuous control of titers that c*hangewith time. 1374
To determine the absorption bands due to water and the hydrocarbon oil present in liquid sulfur dioxide, spectra m r e obtained in the 1- to $-micron region with a Model 12C Perkin-Elmer spytrophotometer using lithium fluoride and sodium chloride prisms. Per cent transmission curves were obtained with an empty cell as reference instead of a single plate, which gives transmission values greater than 1007,.
200
220
Figure 2.
240
260
280 300 3 20 WAVELENGTH -MICRONS
3 40
360
3 0
Infrared Absorption Spectra of Plant Stream' Liquid Sulfur Dioxide
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. T
PROCESS INSTRUMENTATION A commercial grade liquid sulfur dioxide containing approximately 0.007 weight % water was used to make blends containing additional amounts of water. Curve 1 of Figure 1 shows the spectrum ohtainecf for a blend containing 0.275 weight % ' water. Curves 2 and 3 show the spectra of 0.11 and 0.33 weight % water using a longer path cell. Curves 2 and 3 were plotted with the spectrum of the blending sulfur dioxide subtracted except in the 2.67- to 2.82-micron region. In this region the empty cell trace was used as the reference curve. This was done to separate the absorption bands due t o water from those due to sulfur dioxide and other impurities. Absorption cell window material was quartz. Inspection of Figure 1 reveals water absorption bands a t 1.41, 1.89, 2.72, and 2.79 microns. These spectra indicate that the 2.7- t o 2.8-micron water absorption doublet is the most sensitive absorption band in the spectral region transmitted by quartz. The change in transmitted energy for a specific change in water concentration is approximately 100 times as large in the 2.7- to 2.8-micron region as a t 1.89 microns for the hot-wire source used in our analyzers. Figure 2 shows the infrared spectra obtained with a sodium chloride prism for samples of liquid sulfur dioxide from the extraction unit which contain varying amounts of water. Figure 2 also shows the spectrum of a sample of C.P. sulfur dioxide which contains about 0.15 weight % water. Examination of these spectra reveals that the weak bands a t 2.3 to 2.5 microns and the strong bands at 3.4 to 3.5 microns are due to hydrocarbons present in the liquid sulfur dioxide from the extraction unit. This was verified by a mas8 spectrometer analysis. Being well separated from the water absorption at 2.7 to 2.8 microns, these hydrocarbon absorption bands will not interfere significantly with water analysis.
Figure 3.
Optical Arrangement of Infrared Analyzer
The change in transmitted energy in the 2.7- t o 2.8-micron region for a water content change from 0.0 to 0.4 weight % is about ten times the minimum change required for full-scale recorder deflection with a Phillips infrared analyzer. This analyzer has been described in a previous publication ( 2 ) .
SIX SANC
GAMPLE
LIQUID INLET
Figure 4.
Exploded View
of High Pressure Cell
The sensitizing filter used is Vycor (a 96% quartz glass) placed in the reference beam. The infrpred spectrum of Vycor is shown in Figure 5. Since Vycor has a strong absorption band centered a t 2.74 microns, the reference beam will thus be insensitive t o variations in the intensity of the 2.7- t o 2.8-micron water absorption bands while the intensity of the sensitive beam will follow these variations. The transmission characteristics of the two analyzer beams must be alike in the 3.4- to 3.5-micron region in order that the analyzer not be sensitive to changes in the hydrocarbon content of the sulfur dioxide. Transmission characteristics of quartz and Vycor are quite different a t wave lengths greater than 3.3 microns. These differences are greatly reduced by means of a thin film of polyethylene across both beams. The transmission characteristics of such a film of polyethylene appear in Figure 5.
"1
BO
Optical Arrangement
Figure 3 is a schematic representation of the optical arrangement. Standard analyzer components are used except for a special sample cell and solid filters. Concave mirrors are positioned so that the two radiation beams cross ahead of the bolometer receiver elements. The sample cell is placed a t the intersection of the two radiation beams. This arrangement provides essentially identical paths through the cell for both beams, minimizing the effects of any change in the transmission characteristics of the cell. The window area is also reduced to a minimum for ease of handling the high pressure liquid sample. The sample cell used is shown in some detail in Figure 4. It is of the conventional amalgamated lead spacer design and has been hydrostatically tested to 475 pounds per square inch. The analyzer is being operated with the sample pressure in excess of 150 pounds per square inch in order t o maintain the necessary liquid phase sample of sulfur dioxide. The high vapor pressure of sulfur dioxide requires this pressure since the temperature within the explosionproof analyzer housing is considerably above the outside ambient temperature. July 1954
3
Figure 5.
Spectra of Filters for Analyzer
Figure 6 shows the transmission characteristics of the two analyzer radiation paths. In these composite spectra the thickness of the fused quartz is somewhat less than is actually present in the analyzer so that the difference in transmission between 3.5 to 4.0 microns is shown to be larger than in practice. The absorption band a t 2.7 microns in Figure 6 is due to impurities in the fused quartz. Since this absorption varies from batch to batch, it is necessary that the transmission characterie-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1375
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT The electrical sensitivity required of the analyzer is low. Therefore, the usual howl.. automatic standardization to compensate for environmental effects is not rpquired to maintain an analytical accuracy of &0.01 weight % ’ water. Manual standardization based 011 daily laboratory infrared analyses is used Automatic standardization of this analyzer would be difficult because of sample handling problems. To check the analytical performance of the & , I - 201i! I I analyzer after installation, a laboratory inflared analysis for water based on the above spectral data mas developed and used a t the 10 , refinery. This 1s a base line technique using 0 IO 20 .30 PERCENT WATER (BY WEIGHT) 20 2 5 30 3 5 40 4 5 the 2.72-micron water absorotion band. WAVELENGTH -MICRONS Figure 7. Calibration for This liquid phase infrared analyzer has been Figure 6. Transmission CharacterWater in Sulfur Dioxide in continuous operation for several months a t istics of Analyzer Beams i t a present location in a sulfur dioxide-gas oil Phillips Infrared Analvzer extraction unit a t a refinery. Refinery personnel are using the analyzer as a guide for operating the extraction unit. During this period, manual tics of the quartz be checked before use in this analyzer. Since standardization based on the laboratory analyses has been rethe sensitivity of the analyzer depends upon the ratio of the transquired only a few times. mission of the sensitive radiation path to that of the reference path in the 2.7- to 2.8-micron region, this absorption is significant.
31
1
,ii b\,
--
References
Perfor ma nce h sample of liquid sulfur dioside from the extraction unit containing some hydrocarbon was analyzed for water by the Karl Fischer method and blended with known amounts of water for calibration of the analyzer. Figure 7 is the calibration curve obtained lvith these miutures.
(1) Renning, A. F..Ana2. Chem., 19,867 (1917) (2) Kratochvil, K. V., Petroleum Engr., 25, C13-15 (July 1953). (3) Mitchell, J., and Smith, D. &I., “.lquarnetry,” pp. 1-444, London, Interscience Publishers, 1948. (4) Pleskov, T. d.,Zavodskaya Lab., 5, 1319 22 (1936). RECEIVED f o r rryiew September 7 , 1933.
ACCEPTED
l r B r e i x 20, 1954
Continuous Infrared Analyzers GLENN
E. SMITH
Process Confrols Division, Baird Associates, Inc., Cambridge 38, Mass,
The negative filtering-type continuous infrared analyzer i s described operationally for both gas and liquid operations. Details of sensitization, calibration, and sensitivity to several specific gas phase applications are discussed. Sensitivity obtainable in the measurement of water in various organic compounds i s given. The application to determine monochlorobenzene in o-dichlorobenzene, acetic acid in acetic anhydride, and toluene in benzene i s described with discussion of sample flow rates and other operation parameter.
D
URING the past several years the negative filter-type, continuous infrared analyzw has proved its versatility and practicality in the successful monitoring of a lvide variety of gas streams in industry. Kithin the past 3 years this instrument has demonstrated its suitability for many liquid phase applications, both in the laboratory and in the plant.
Description The operating principle and the design (Figure 1) except for the cell thickness, remain as described by Patterson ( 1 ) . I n general, for liquid applications either component of any binary system may he monitored in the range, zero to loo%, with a sensitivity of, a t least, =tl$?’o.Limiting factors determining the minimum amount measurable may be small energy absorption of the compound sought and/or large energy absorption of the ve-
1376
hicle. Some fortuitous cases esist where the vehicle-dichlorodifluoromethane-has so litt,le absorption that sample cell lengths of several centimeters may be utilized, thus obtaining in the system a correspondingly large amount of the minor component sought. In general, however, sample cell lengths on the order of 0.5 mm. are indicated. For tertiary and more comples systems, success is dependent upon removal of spectral interferences, as it is for gas syatems. I n liquid applications the instrument is appreciably more critical of sample conditions than for gases. It is imperative that the combinations of t’emperature, pressure, and flow rate be such that no vapor is formed in the sample cell. This dictates low temperatures, high pressuree, and high flow rates for low boiling liquids. Primarily because of the relatively high specific heats of liquids, changes in sample temperat’ure and flow rate cause comparatively large deviat’ions from t,hermal equilibrium.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
