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D. D. FRIEL Engineering Department, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
Continuous Process Control Unique instruments, based on electrochemical and photometric principles, provide unusual reliability for process control l
O N L Y a few years ago, all new process analyzers were based on existing laboratory techniques. Today, the demands for continuous analyses are such that new analysis techniques are frequently applied first in the plants and are adapted subsequently for laboratory analytical control. At least two of the instruments described here have followed this new pattern. These are an electrolytic analyzer for analysis of water in liquids and a logarithmic photoelectric analyzer. Gas chromatography-the other area discussed-is an exception; both laboratory and process instrumentation were developed simultaneously. The timelag between development of laboratory and process analyzers has been virtually eliminated. The instruments described are rugged and dependable, being used around the clock to control chemical processes. Because these instruments are designed to be virtually maintenance-free, operating and plant personnel have developed confidence in their use. This acceptance has resulted from a long-term evolutionary process of selecting sound, operational principles, such as nullbalance and coulometric techniques, and
from careful study and gradual elimination of all factors which could adversely affect the performance of the instruments. Analysis for Water in Hydrocarbons
liquid
Electrochemical analyzers based on coulometric principles promise to provide a major breakthrough for several important analyses. The electrolytic oxygen analyzer reported in a companion article in this issue represents a major advance. Another reported previously is the electrolytic moisture analyzer (left, below), developed by F. A. Keidel (7) of this laboratory, and now sold commercially by Beckman Instruments Co., Consolidated Electrodynamics Corp., and Manufacturers Engineering and Equipment Corp. This moisture analyzer, like the oxygen analyzer, is a coulometric instrument and virtually a primary standard. I n operation as a gas analyzer, the gas passes through a fine capillary tube containing platinum wire electrodes. Any moisture is quantitatively absorbed in a meta- or orthophosphoric acid system and then is quantitatively electrolyzed by current passage between GAS
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alternate turns of wire. Essentially every water molecule coming into this special capillary passes out as molecular hydrogen and oxygen. For a given flow rate through the cell, Faraday's law quantitatively relates water content with the measured current flow. About 30 inches of this capillary tube coiled inside a pipe nipple constitute the electrolytic analysis cell. As the reader is probably acquainted with this instrument as a gas analyzer (7), further elaboration on its construction or operation is not made here. More important is a description of how this principle and sensing element can be applied for the determination of water in liquid organic systems. Determination of moisture in liquid samples is accomplished by first removing water quantitatively from the liquid sample with nitrogen sparging carried out continuously in a miniature, packed stripping column (see diagram below). The organic liquid to be analyzed is metered into the top of the stripping column at a constant known rate, on the order of a few cubic centimeters per minute, and discharged to waste from the bottom of the column. Dry nitrogen from a compressed-gas cylinder rises through the column, quantitatively strip-
CONTAININQ WATER
ELECTROLYTIC WATER
METERING
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Electrolytic moisture analyzer utilizes Faraday's law to relate water content with measured current flow
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In determination of water in liquid samples, nitrogen sparging i s used to remove water in a packed stripping column. Moisture is then measured by the electrolytic moisture analyzer
INDUSTRIAL AND ENGINEERING CHEMISTRY
PROCESS CONTROL ping water from the sample, and carrying it to an electrolytic analyzer cell where current flow is a measure of the water in the sample. A full-scale range of 1 p.p.m. can be readily obtained. Instruments of this type have been used with a full-scale range as high as several hundred parts per million. I n one version of this instrument, shown below, the liquid-sample inlet
curately controlled. It is important only to control the liquid flow and to make an accurate measurement of the current through the analysis cell. Gas flows between 25 and 100 cc. per minute are commonly employed. Such flows are high enough to strip water from the liquid quantitatively and yet low enough to ensure quantitative analysis for the removed water in the electrolytic analysis cell. The stripping-column length is selected to ensure complete removal of moisture from the sample. With liquids such as benzene and toluene, water can be quantitatively removed with a column only 3 inches in length. As a general rule, equally efficient stripping can be expected from many other liquids in which water is only slightly solublethat is, soluble up to about 100 p.p.m. At a sample flow rate of 1 gram per minute, the indicated electrolysis current is 179 pa. per p.p.m. by weight as predicted by Faraday's law. In-Process Chromatographic Analysis
Electrolytic analyzer for water A. B. C. D. E. F. G.
Analysis cell Power supply Dryer-cell current Water content Dryer cell Stripping column Gas flowmeter
H.
Gas-flow regulator Gas inlet J. Liquid outlet K. Float valve I . Sample inlet M. Sample pump N. Gas outlet
1.
is located below the liauid-sample pump. Liquid metered by this pump trickles down the stripping column to a trap and float-valve arrangement. Purging gas used to remove water passes through a flow regulator, and enters the bottom of the stripping column just above the trap. Gas moving up the column strips water from the liquid hydrocarbon, and water is measured by the electrolytic analysis cell ( 7 ) above the stripping column. An electrolytic drying cell removes the last traces of moisture from the stripping gas before it enters the column. As the reader may expect, the dryer cell is essentially a duplicate of the electrolytic analysis cell; in fact, it makes a particularly inexpensive, maintenance-free dryer for this type of application. Stripping gases commonly used are dried air and bottled nitrogen. Although this particular model has a flow regulator and rotameter for the stripping gas, for accurate analysis it is unnecessary that the gas flow be ac-
Because of the potential application of gas chromatography to process stream analysis, this laboratory has undertaken programs to extend the sensitivity of chromatographic analysis to the theoretical limit. A variety of different dptectors can be used in Chromatography, but one of the most versatile is the thermal-conductivity detector brcause of its nearly uniform sensitivity to all sample components. Ry examining each factor contributing to the sensitivity and noise of thermistors, unusually high sensitivity has been obtained. A plant-type chromatographic analyzer incorporating improved thermistor circuits (right, above) operates reliably under plant conditions with a fullscale thermistor bridge sensitivity of 50 pv. This is equivalent for many gases to full-scale response for 100 p.p.m. of individual components, or on the order of 10 p.p.m. full scale under extremely favorable conditions. This sensitivity was achieved by eliminating all the external sources of noise in the thermistors. Noise was principally due to flow variations, vibrations, ambient temperature changes, and fluctuations in bridge current. Noise caused by flow variations and mechanical vibration was eliminated by wrapping two fine metal screens around the thermistor bead (see diagram at right). One screen was sufficient to eliminate the effect of gas flow, but the second screen was necessary to eliminate the effects of mechanical vibration. It was demonstrated that thr thermistors employed are inherently insensitive to shock, but mechanical
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Plant-type chromatographic analyzer utilizes improved thermistor circuits
vibrations of the thermistor mount and assembly cause convective-type motion of the gas immediately surrounding the thermistor bead, giving rise to short-term noise. The use of the second screen completely eliminated such shock effects. The inner screen was mounted with a clearance of only 0.008 inch from the thermistor. Clearance between the first and second screen is on the order of 0.015 inch.
2 N
,Thermistor
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Glass-to-Metal
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Two fine metal screens were used around the thermistor b e a d in the chromatographic analyzer to eliminate noise from all sources VOL. 52, NO. 6
JUNE 1960
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Semi-tranrporcnt mirror
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Subtriction
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Logarithmic
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Logarithmic photoelectric analyzer can be used for films as well as liquid and gaseous samples
It is of interest that the voltage-current characteristics of the thermistor in helium are not measurably affected by the presence of these closely spaced screens. Neither is the sensitivity to gas-composition changes. The hole size of the screens can be selected so that the width of a typical chromatographic peak eluted in 10 seconds is increased only on the order of 10%. Improvements have been made in the bridge circuit to compensate for all major differences between the measuring and reference thermistor, including their cold resistance, temperature coefficient, dissipation constant, and time constant. Further details on the circuitry will be covered in future publications by Kieselbach of this laboratory (2). Photoelectric Analyzer
Another instrument used in a number of applications is a new logarithmic photoelectric analyzer which operates in the near infrared, the visible, and ultraviolet region of the spectrum. I t has proved particularly rugged, dependablr, and nearly maintenance free. This analyzer is generally applicable to the measurement of any substance that absorbs in the phototube region. Such materials include sulfur dioxide, nitrogen dioxide, the halogens, and many organics, such as benzene derivatives, ketones, and chlorinated hydrocarbons. Concentration is recorded linearly over
a very wide range of absorption with sample transmittance from 100 to 0.01%. A flexible mechanical arrangement permits analysis for rapidly moving films as well as liquid and gaseous samples. Response can be adjusted for full electrical response as short as 0.001 second for special applications. The analyzer has three basic parts, shown schematically (above) : the light source, the sample space, and photometer. The light source is typically an S-4 mercury arc. Normally, all of the source radiation passes through the sample space before any optical filtering. In the photometer, a beam splitter divides the radiation into two fra
