instrumentation instruments for measuring water content

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  • Demands continue for better tools and techniques for the measurement of water

    by B. W. Thomas

    H E problem of developing better T procedures and faster tools with which to monitor and/or control water in both operational processes and final products is of growing concern in many major industries. Means for measuring and controlling pH, solids contents, and other chemical proper- ties of water associated with fouling of cooling towers and heat exchangers and pollution of industrial effluents are also of wide interest. Knowledge of water content is of prime importance in crude oil charged to distillation stills, in sulfur dioxide, phenol or methyl ethyl ketone used in counter- current liquid-phase extraction proc- esses, or in acetone, furfural, and other materials employed in extrac- tive distillation or separation methods. Strict observance must be maintained for the low level n-ater contents re- quired in marketing specifications that are applied to such petroleum products as lube oils, heating oils, liquified petroleum fuels, and such compressed gases as oxygen, nitrogen, or hydrogen. Agricultural industries are confronted u-ith the measurement of water in flour, corn meal, fertilizers, and many other commodities while water con- tent of pure compounds like acetone, methyl ethyl ketone, or dioxane is of interest to the chemical industry.

    Chemical titration and dew point are older methods

    A discovery of more than 100 years ago that iodine reacts with water in the presence of sulfur dioxide to form hy- driodic acid and sulfuric acid is the basis of a titration procedure named for the German chemist, Karl Fischer, who actually put the technique to wide usage only 20 years ago. Today Karl Fischer titrations for the de- termination of water are applied to such substances as petroleum prod- ucts, food stuffs, paper, pharmaceu- ticals, antibiotics, ethers, and alcohols. During the titration, Karl Fischer re- agent is added to solutions of weighed

    sample from a suitable buret while polarized platinum electrodes equip- ped with appropriate potentiometric circuitry regulate introduction of the reagent and automatically record the end point or sample condition for which no further water is available for reaction with the titrant. The amount of reagent used for a given sample is a direct function of the weight percent- age of water in the unknown sample. ilccuracies of less than 0.05 ml. of re- agent are not uncommon.

    Semi- and near automatic equip- ment for measuring water by the Karl Fischer method is marketed by Beck- man Instruments, Inc., Fullerton, Calif., and Fisher Scientific Co., Chi- cago, Ill. Refinements built into the Beckman Aquameter which utilizes the Karl Fischer method include high speed, good reproducibility, and ease of operation. I ts alternating current null detecting amplifier which responds to control addition rate near the end point makes possible automatic com- pletion of a titration within 1 or 2 minutes. With the Fisher titrimeter is furnished an accessory kit to extend the reagent life a year.

    Water determinations by dew point are much less widely used than the Karl Fischer test since they apply only to samples which become vapor a t atmospheric pressure. The dew point method is an accepted test for L.P.G. propane and other fuels and is con- ducted by allowing the sample gas to escape slowly and impinge on the face of a mirror whose temperature is being reduced a t a specified rate. The temperature of the mirror a t which a film of moisture first appears as the temperature is lowered is known as the dew point and is a function of water content of the liquified sample. Pro- pane having a dew point 15' below 0" F. or lower is considered free of mois- ture. Spot sample dew point equip- ment has been developed by the Bur- eau of Mines and both indicating and recording dew point instruments are available from General Electric.

    Newer techniques for analyzing and/or measuring water employ such properties as electrical conductivity, infrared absorption, thermal adsorption, dielectric constant, and others

    A World War I1 development for the determination of minute quantities of water in aviator's breathing oxygen has been extended to many other com- pressed gases, and equipment for this purpose is being manufactured by the American Instrument Co., Silver Spring, Md. Measurements can be made i n 2 to 5 minutes for water con- tents as low as 2 p.p.m. by weight with a *lo% accuracy. Theory of the measurement is based on the elec- trical resistance of an electrolytic hygroscopic film in contact with the test gas. Water contents per unit volume of unknown gas are made equal to standard or known gases by adjustment of pressure to achieve equal resistances of the hygroscopic film. A circular scale slide rule cal- culator is furnished for conversion of pressures and resistances to moisture contents. Employing a vacuum tube amplifier capable of operating 300 hours from two small batteries, the American Instrument Co. water vapor indicator is simple, portable, and sells for about $1000.

    Phillips Petroleum Co., Bartles- ville, Okla., makes use of a nondis- persion infrared analyzer for continu- ous monitoring of water in the recycle sulfur dioxide stream of a gas oil ex- traction process. Operating a t a sample pressure in excess of 150 pounds per square inch, full span of the liquid phase analyzer is 0 to 0.4 weight % mater with an accuracy of rt 0.01 weight yo. With the sample cell ex-, tending across both beams of the usual double beam infrared instrument, quartz and Vycor filters are placed in the separate beams to isolate the 2.7- micron energy for which water has a high absorptivity. Calibration of the infrared instrument is accomplished with specially blended samples for

    October 1955 I N C U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 77 A

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    Instrumentation which water contents are determined by the Karl Fischer method. Other details of this infrared analyzer de- veloped by Phillips Petroleum Co., and currently marketed by Consolidated Engineering Corp., Pasadena, Calif., include bolometer bridge detection, safe operation in explosive areas, and circular chart continuous recording, with automatic control if required.

    A water analyzer for vapor phase hydrocarbon or other fixed gas samples, based on heats of adsorption and evaporation and developed by Esso Research and Engineering Co., Linden, N.J., is soon to be marketed by Mine Safety Appliance Co., Pitts- burgh, Pa. Operating a t atmospheric pressure, full span of the recorder can be set for 0 to 100 p.p.m. of water on a weight basis with an accuracy of f 1 p.p.m. In this apparatus a portion of the moist test gas is dried; after which the wet and dry streams are caused to flow separately, past opposite junctions of a thermopile imbedded in suitable dessicant. Cooling occurs by evapora- tion of moisture into the dry stream and heating results from adsorption of water from the wet stream. This dif- ference in temperature a t the two junctions is converted to weight parts per million of water by alternate switching of the wet and dry streams to the two hot and cold junc- tions, appropriate regenerative driers and suitable electronic rectifying, am- plifying, and recording circuitry. The apparatus can be calibrated for opera- tion on a wide variety of hydrocarbon and/or fixed process gas mixtures; it is highly satisfactory for monitoring the water content of L. P. G. propane.

    Dielectric constant measurement is the basis for water monitoring in a wide variety of processes and products. Gulf Oil Co., Pittsburgh, Pa., makes use of a cylindrical capacitor within the moving stream to detect water in gasoline or crude oil pipe lines. Simi- lar arrangements may be employed to monitor water movements in crude oil desalting operations to prevent ex- cess water from entering fractionation units. E. H. Sargent and Co., Chicago Ill., manufactures and markets a di- electric constant meter known as the Oscillometer. Employing a small volume cylindrical sample cell with insulated electrodes, balance between two high frequency oscillating circuits is accomplished in the Oscillometer by adjusting a wide range calibrated con-

    denser. The Oscillometer is arranged primarily for laboratory operation; it has been applied successfully to a wide variety of mixtures including water in acetone (0 to 10%) and di- oxane (0 to 3%).

    An instrument developed by