I
N. F. CHAMBERLAIN, B. W. THOMAS, J. B. BEAUGH, and P. B. LAND Humble Oil and Refining Co., Baytown, Tex.
Dielectric Constant Continuous Analyzers in Petroleum Refining
THE
differences in dielectric constant of the various compounds present in petroleum refinery process streams may be used to indicate continuously the composition or changes in composition of such streams. Dielectric constant has been used by Gulf Oil to detect interface movement between products flowing in a pipeline ( 7 ) , by Shell to monitor stream purity in the production of acetone (2), and by Humble to measure toluene concentrations in hydroformer distillates (3). The commercial availability of sensitive capacity measuring instruments of the continuous recording type has made it attractive to investigate further the applications of dielectric constant measurements to the continuous monitoring of hydrocarbon streams. Three new applications described in this article are continuous recording of the total aromatic content of the charge to a sulfur dioxide extraction unit; continuous recording of the oil content of the wax stream from a solvent dewaxing unit; and continuous monitoring (recorded) of the improvement in viscosity index which is effected at a lubricating oil solvent extraction unit. These instruments have operated satisfactorily since early 1954 and have proved useful to the plant operating staff. Same Basic Principle and Basic Instrument Used in All Three Applications
The relationship between dielectric constant and hydrocarbon mixture composition can be determined roughly for
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the lower boiling materials from Table I. For the same boiling range, aromatics and olefins have dielectric constants which are significantly higher than those for paraffins and naphthenes. Consequently, this property has been most useful for measuring changes in the aromatic content of mixtures of aromatics, paraffins, and naphthenes, or for measuring changes in properties related to aromatic content. Because the dielectric constant of a complex plant stream is usually affected by changes in components other than the key component to be measured, it is impractical to determine absolute calibration data for the dielectric constant analyzers in the applications tried by the authors; instead, the analyzers are employed as fast-acting continuous monitors to indicate constancy of stream composition and the appearance of changes or upsets. A practical and reasonable degree of absolute accuracy is achieved by resetting the pen as the need is indicated by laboratory tests of the measured streams. Dielectric constant measurements are affected not only by the gross composition of the sample, but also by the presence of traces of dissolved or entrained materials which are electrically conductive or magnetic, by the presence of more than one phase and by changes in temperature. In general, liquid samples fed to the capacity cells should be free from ions or metal particles, should exhibit only one phase while inside the cell, and should be maintained a t constant temperature. These conditions may re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
quire filtering and drying of the sample prior to entry into the cell, and usually require a reasonably constant pressure and flow rate. Temperature compensation may be employed in place of temperature control when the temperature fluctuations are not LOO rapid or too great. Sometimes it is convenient to employ temperature control to reduce the sample temperature fluctuations to a few degrees, and to employ temperature compensation to eliminate the effects of these reduced fluctuations. I n the case of hydrocarbon streams, minute changes in the concentration of such things as dissolved water, phenol, or methyl ethyl ketone will cause noticeable changes in the dielectric constant of the stream. The concentrations of such components must be reduced to essentially zero, or must be held constant at low values. The usual techniques of drying, distillation, or saturation under constant conditions can be used where applicable. Fortunately, a number of plant streams in petroleum refineries conform to the above requirements to a sufficient degree to permit the use of dielectric constant instruments on them with little or no special sample preparation. The commercial instrument which has been used in all three applications is the Foxboro Capacity Dynalog, Model 9550. This instrument provides the recorder and the measuring circuits necessary for recording changes of 5 micromicrofarads in capacity. The capacity cells and the sampling system used in these applications were designed and constructed by Humble and have
been described by Thomas and o t h m
(4.
Two minor modifications to the Fox-
bom instrument were dcairable tc impmve its ugtulness. First, the ShaR of the balance circuit padder capacitoi waa extended thmugh the front panel
that- the ZM point of the recorder could be readily adjusted from the front. This is a must if a reasonable degree of absolute accuracy is to be realiEed on plant streams. The instruments can bc purchased with this feature pmvided. Secondly, an extra tip jack wan installed in one of the intermediate electmmc circuits and some extra holea were drilled in the chapsis to facilitate alignment. When properly aligned, the Foxbom capacitance instruments have operated for well over 1 year with no maintenance being required. Maintenance of the sampling s y s t e m is mquired periodically bccause of mechanical difficulties which a r k in the normal cotme of plant operation.
80
Moaruring Aromalie Conknt of Charge to Sulfur Dioxide Exhadion Unit The sulfur dioxide solvent extractiot unit extracts toluene or xylenes fmn special distillates. Operating condition Figure 1. Dieledric meter QI aromatics in naphtha at the unit depend on the MNX an( concentration of ammatics in the f e d The pknt can be scriody upxt b changes in feed composition wbicl sometimes occur without warning. Th purpoa of this instrument is to monito the feed continuously so as to inform thc operator immediately of composition changes. to the charge pump suction. During total random error of the anal* measBecause dielectric constant variea urn the ability of the instrument to the early months of operation of the with temperature, some typc of temreproduce the ammatic concentrations instrument, its reproducibility was perature contml or compensation must checked every 4 hours by automatic 10determined independently from API be provided. In this case the sampling gravity measurements. This figure insystem is simpMed considerably by the minute injection of a reference liquid. cludes the random error of the gravity Stability of the instrument has been so w of an automatic electrical temperameasurements as well as the dielectric g o d , and chedring of its absolute readture compemtor with a thermistor as ing by density measurement is so easy constant errors; therefore, the actual the sensing element. This device, s u p random ermr of the dielectric constant in this case that the reference sample plied by the manufacturer, compemtea check is uhnecegpary and has been disanalysis is somewhat lcss than this. for the dielectric constant changca due Time and circumstances did not permit to temperature changes in the sulfur continued. a more pndse evaluation of this.error. dioxide unit charge stream and sampling A typical chart fmm the aromatics system during both 8ummer and winter. When thii ermr is considered along with analyzer, taken during the time that the reproducibility of the instrument, T h e sampling system is placed in a automatic reference checks were run, the figure obtained for the total random threc-sidcd enclosure to protect it from is shown in Figure 1. The instrument is analytical ermr shows that the instruthe weather and the sample cells are calibrated to read directly in volume per ment should reliably indicate changes insulated to reduce temperature difcent total aromatics. Charge composiferences between them. No other prebetween 1.2 and 1.6% in ammatic tion was steady during the 24-hour period cautions are necessary to reduce temcontent. This ability to indicate Ar e p m n t e d by this chart. perature fluctuations. Data on the accuracy and reproducitively s m a l l changes in ammatic content Two sample ab, drilled into the is of most interest to the unit operator, bility of the analysis within the concensame stainlcss sml block, are d. traiion range normally monitored are and the limits are defined dosely enough One ccll measlllcs the capacity whereas preented in Table 11. Reproducibility for practical purpoaea. the other contains the thermistor of the Although gravity measurements do for the instrument was determined from ccmperamre compensator. The sample not measure ammatic content uniquely, the lradings obtained on the reference s t m m . taken from the plant charge sample. It repreknts the ability of the they are fast and easy to maLe and are pump dincharge, Rows continuously subject to fcwer ermm than dielectric instrument to repcat its wdinga under thmugh the sample cells in aeries, constant measurements. The gravity actual operating conditions on a sample thmugh a Row indicator, and then back us. ammatic confent cornlation was whose composition is constant. The VOl.48, NO. 11
NOV€ME4!RlW6
1991
checked by calculation and with the fluorescent indicator analysis, a method which requires considerable time but does measure aromatics uniquely. These checks indicated the correlations have toluene or less and biases of -37, +1.5y0 xylenes or less. Insufficient data are available to permit calculation of the statistical variations in these bias values, so only the maxima are reported. A minor adjustment of the recorder zero point is required each time a switch is made from one charge stock to the other. The correction required varies from less than 0.5% to more than 5% aromatics, but the change in setting is unnecessary except when the charge stock is changed. Resetting of the recorder zero is made after the aromatic content of the new charge has been de' y measurements termined from 4 P I grah'it made a t the unit. Because of the normal steadiness of the charge composition and the ease and speed with which the gravity can be determined, the pen can be reset to within &0.5% aromatics of the determined value. The inherent bias of the gravity test is thus introduced, however. The maximum total error in the continuous analysis consists of the total random error plus the maximum correlation and reset errors found, and
Figure 2. Sample system of dielectric constant oil-in-wax analyzer
Table I.
Pertinent Physical Properties of Compounds in Charge to Sulfur Dioxide Extraction Unit Boiling Range,
c.
Densitg
Compounds
Dielectric Constant (at 200 C.)
Toluene Xylenes mixturea C8 Paraffins Cg Paraffins
111
2.385
0.8669
125-150 100-125 125-150
2.40 1.95 1.98
0..872 0.709 0.722
100- 125
2.1a
0.771
c7-cs
Naphthenes
d
:"
cS-c9
Naphthenes
125-150 2.1a 100-1 25 2.5 CS Olefins 125-150 2.5 Water 100 80.37 Approximate composition of the xylenes mixture and properties of ponents are as follows : Ca Olefins
0.787 0.723 0.746 0.9982 its com-
DiVolume Compound Toluene Ethyl benzene +Xylene m-Xylene o-Xylene COAromatics
70 2.7 21.8 16.0 39.1 18.6 1.8
electric Constant 2.385 2.412 2.270 2.374 2.567 2.41
Densitg d io 0.8719 0.8718 0.8657 0.8687 0.8848 0.8759
...
...
~
Total 100.0 Average for mixture 2.402 Estimated from data for lower boiling naphthenes.
...
a
~~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
0.8721
indicates the accuracy of the aromatic content measurement when the API gravity test is used to calibrate the instrument. If greater accuracy is desirable, the proper correction for the bias of the gravity correlation can be made periodically or a less biased test such as the fluorescent indicator analysis can be used as the calibration standard. A41thoughthe total error in the instrument analysis, even when calibration bias is eliminated, is about twice as great as the error in the fluorescent indicator analysis for single readings, the continuous record produced by the analyzer is considerably more useful and probably more reliable than spot gravity measurements or fluorescent indicator analysis tests. O n several occasions, the analyzer has detected rapid changes in charge stock composition M hich would probably have been missed by the usual periodic tests. Thus the analyzer aids in maintaining more efficient operating conditions a t all times, Measuring Oil Content of Wax from Solvent D e w a x i n g Unit Continuous monitoring of the oil content of the wax removed from motor oil stocks helps to prevent loss of the more valuable oil in the wax stream. Since the wax is highly paraffinic whereas the oil from which it is precipitated contains large proportions of naphthenes and aromatics, it is possible to correlate the dielectric constant of the wax stream with its oil content. The calibration will be different, of course, for each charge stock and probably for every run on a given charge stock. In this case, however, the oil content of the wax stream is important for only two stocks, which simplifies calibration considerably. At first the instrument was arranged to read the dielectric constants of the was and the dewaxed oil in order to determine whether useful correlations exist for both oil content of the wax and pour point of the dewaxed oil. The first correlation does exist. but there is no useful correlation between dielectric constant and pour point. Consequently, plant charge stock has been piped to the instrument in place of the dewaxed oil. This provides continuous monitoring of the charge stock composition (which should be constant for each run) and provides a reasonably constant reference material with which to check the stability of the instrument. A photograph of the sampling system for this instrument is shown in Figure 2 and a flow diagram is presented in Figure 3. Test samples from the two streams, S I and SZ are pumped continuously through flow rate indicators. F1 and Fz, through two stainless steel coils H in a constant temperature bath, through individual capacitor cells, C1 and Cz, in a
single stainless steel block partially submerged in the bath, and returned to the process streams from which they came. Temperature of the bath is maintained at 212' F. by direct steam injection, SS into water boiling at atmospheric pressure. Control of steam flow to the bath may be achieved by a temperature sensitive element in the upper portions of a water-cooled total condenser, Cq,installed above the top of the closed bath. The upper end of this condenser is open to the atmosphere in order to maintain atmospheric pressure inside the bath. Excess condensate accumulating in the bath overflows to the sewer, D. An important advantage in this system is that moderate changes in steam input to the bath do not vary the bath temperature; instead, they merely change the boiling rate of the water. Because of this fact, automatic control of the steam is not always necessary. Another advantage of this type of bath is that it can control the temperature of streams that are either hotter or colder than the bath. Alternate measurement of the two streams is accomplished by electrical switching, T1 of the recorder input, R, between separate measuring heads, 01 and 0 2 , connected to the two sample cells. Electrical switching between two cells is much faster than switching of the sample streams through a single cell, making it possible to obtain alternate measurements on the two streams every 2 to 5 minutes. Figure 4 illustrates the essentially continuous record obtained for both streams. Adjustment of the sample system and checking for malfunctioning are made more convenient if both sample cells respond the same way to the same sample. This means both cells must have the same effective capacitance (sensitivity). This is particularly desirable when the difference in chart reading is used to measure the desired property, as is the case with the viscosity index monitor. The sample cells can be adjusted. to the same capacitance by careful trial-and-error machining. A
Table 11.
simpler method would be to machine one to a higher capacitance than the other and to make the final adjustment by a suitable variable capacitor placed directly in series with the higher capacitance cell. The zero-point reading of each cell is set by a parallel variable capacitor already provided in each measuring head circuit, but the series capacitance is needed to equalize the sensitivities. Since sample cell capacity changes are small. the nonlinearity
so that a decrease in pen reading indi-
the wax stream record on the outer edge of the chart where it can be read more easily. The instrument was adjusted SQ rhat 100 minus the pen reading (direct reading when measured from the outer edge) equals the weight per cent oil in the wax for both of the charge stocks of interest. The accuracy and reproducibility of this instrument are tabulated in Table 11. The detailed discussion of the terms used in this table for the aromatics analyzer also applies to the oil-in-wax analyzer. Reproducibility of this instrument was determined from charge stock readings. Calibration of the instrument is checked against a laboratory test which uses ultraviolet spectroscopy to determine the oil content of the wax and the instrument is reset when required Maximum total error in the instrument analysis is several times as large as the error in a single laboratory determination, but the continuous record provided by the instrument more than compensates for this. The analyzer shows many large variations in oil content which never would be found by the periodic laboratory tests normally run, and permits the operator to take immediate action when necessary. I t also informs the operator of any unexpected changes in the charge stock.
Accuracy of Aromatics and Oil-in-Wax Analyzers
Approximately 95% confidence level; two standard deviations Magnitude % Aromatics 97, Oil (aromatics (oil-in-was analyzer) T y p e of Errol analyzer) Reproducibility of instrument *1.2 50.9 Total random error of analysis h1.6 f2.4 Reset error *0.5 11.0 Correlation error in test used in calibration -3 (Toluene) 1.5 (Xylenes) Maximum total error in continuous analysis - 5.0 (Toluene) f3.4 +3.5 (Xylenes) Accuracy of laboratory test fl.Oa &1.0 (10% Level)b
