This plant is a part of the Naval Powder Factory located at Indian Head, Maryland. It was built in 1918 by the du Pont Company at a cost of about three-quarters of a million dollars and has a rated capacity of 70 tons 100 per cent sulphuric acid per twenty-four hours. It is of the Grillo-Schroder contact type, with the -usual platinum on magnesium sulphate contact mass; burns sulphur to sulphur dioxide (SO2), converts the SO2 to sulphur trioxide (SO3), absorbs the SO3 in sulphuric acid (H2SO4), and delivers this as fuming acid or oleum of a strength which may be expressed either as 50 per cent free (dissolved) SO3, 90.82 per cent total SO3 or 111.25 per cent (100 per cent H2SO4).
It is in constant operation twenty-four hours a day and has been since it was started in December, 1918, except about eleven months in the fiscal year 1924-25 and one week in 1929. At present, the yearly production amounts to about four and one quarter million pounds of acid as 100 per cent H2SO4. Each pound of smokeless powder requires more than two pounds of sulphuric acid.
This plant operates on the same basic principle common to all plants of this type, but in modernization and scientific application of certain principles as developed and installed here, it appears to stand out above other acid plants. It is these special developments which will be discussed in the order in which apparatus has been installed.
In 1922 in conjunction with the Bureau of Standards, the first sulphur dioxide recorder in this country was developed and installed. This recorder operates on the thermal-conductivity principle for gas analysis. This method is based on the difference in the ability of the various gases to conduct heat. If a wire is surrounded by a gas which is contained in a chamber, the walls of which are maintained at a constant temperature, and is then connected to a source of constant electromotive force, the temperature of the wire will rise until a state of equilibrium is reached. The continuous dissipation of thermal energy is then equal to the electrical energy supplied to the wire. The dissipation of energy is accomplished by radiation, by conductivity through the connection to the ends of the wire, by currents of gas circulating in the atmosphere surrounding the wire (thermal convection) and by conductance through the gas. Some heat is also carried away by the current of gas, in case there is a flow past the wire. It is difficult to ascribe to each the exact share of total energy dissipated, however, it is known that when the temperature of the wire does not exceed 400° C., radiation can account for only a small portion, and that when the distance between the walls of the chamber and the wire does not exceed 5 mm., convection is also a small factor. It is obvious, therefore, that by carefully arranging the conditions, all loss of heat except by conductance through the gas surrounding the wire can be reduced to a small proportion of the whole. Under such conditions it is evident that the equilibrium temperature attained by the wire will depend mainly upon the gas which surrounds the wire to conduct heat. By the comparison of the resistance of two similar wires surrounded respectively by a reference gas and a mixture of two gases of which the qualitative composition is known, the quantitative composition of the mixture can be rapidly and accurately determined after an empirical calibration has been made.
The comparison is made by a form of the Wheatstone bridge in which the two wires to be compared are connected in series as two adjacent arms of the bridge. The other two “ratio” arms of the bridge are variable resistances which adjust themselves to bring the bridge to a balance.
The recorder as installed consists of a Leeds and Northrup Company’s four-point resistance recorder, a Bureau of Standards thermal-conductivity cell, an Edison battery, and a purifying unit developed here. It has been in constant operation since 1926 and keeps before the plant a continuous record of the percentage of SO2 in the entrance gas. In all this time no repairs have been necessary and no care required other than the usual weekly oiling, greasing, and battery charging. Today finds it operating perfectly and in a large measure replacing the well- known Reich starch-iodine test, a volumetric SO2 determination requiring both special training for the operator and the use of expensive iodine and potassium iodide. With this continuous record before the operator, he is able to feed to the converter a gas of uniform SO2 content, which in turn keeps an even load on the converter and insures a maximum conversion of SO2 to SO3.
In 1928 a method was developed and apparatus installed for determining the strength of fuming acid or oleum by conductivity measurements. Some conductivity measurements had been made in 1920, but these results did not appear to warrant further work; however, because of the ever present need for such a method, the problem was again taken up in 1928—and this time was carried to a successful completion. Before this, all the oleum tanks forming a part of the absorbing system had to be sampled daily and samples sent to the laboratory for analysis. The sampling of oleum is both dangerous and difficult, as this acid freezes at such a high temperature (109.3 per cent H2SO4 freezes at 95°F.) that sample lines are always filled with frozen acid and have to be thawed. Often the sample bottle breaks and more often the sample freezes in the bottle. The analysis is difficult and none too reliable and costs about $1.00 for each one made.
The conductivity of an electrolyte is not measured directly but is determined from a measurement of the resistance of the solution between two electrodes immersed in it. The reciprocal of the measured resistance is the conductance of the solution. The resistance of the solution is commonly measured by an alternating current Wheatstone bridge method employed by Kohlrausch. There are available Landolt-Bornstein tables for conductance vs. concentration of pure H2SO4 in distilled water. Leeds and Northrup of Philadelphia, Pennsylvania, checked some of these points and found that the relation between H2SO4 and conductivity over a range of 96-99 per cent shows a very appreciable variation in conductance for a given change in acid strength, and they developed cells and a recorder for this determination. But with oleum, i.e. acid above 100 per cent H2SO4, they found a sharp minimum in the curve and discontinued further work. It was at this point that experiments were made here to develop a conductivity method for oleum analysis.
With a homemade conductivity cell and a measuring unit consisting of a half-meter slide wire, resistance box, buzzer, dry-cell battery, and a telephone, the resistance values at a standard temperature on acid of strengths ranging from 0 to 120 per cent were determined. These values when expressed in a curve using ohms resistance as ordinates and acid strengths as abscissae, showed, as was expected, a sharp minimum at about 102 per cent, so that it was impossible to tell by measurements alone which side of the minimum you were on. However, as the particular problem here concerns the analysis of oleum in the absorption system and as this acid seldom varies more in strength than 102-111 per cent, its position relative to the minimum remains fixed and its values fall on that part of the curve which shows an easy rise at 102-106 per cent, a steeper rise at 106-109 per cent and a very steep rise from 110 per cent up, so steep that a difference of 0.15 per cent in acid strength makes a difference of ten ohms in resistance. Obviously, therefore, by conductivity measurements, the strength of this oleum can be determined with an ease and accuracy never before known.
A conductivity indicator with a range of 0.5 ohms to 1,000 ohms and designed to operate on A.C. current was purchased from the Leeds and Northrup Company. This simplified the conductivity measurements and enabled us to prepare a very accurate curve. To be of the maximum plant value the cells must be mounted directly in the acid inside the tank. In conjunction with the Leeds and Northrup Company, cells for this purpose were prepared. These cells are in general arrangement similar to those sold by that company for acid analysis up to 100 per cent, but in size and style are of a special design. They operate in pairs. One is known as the standard cell and is filled with acid of a strength somewhere near the concentration of the acid to be measured, and together with the second cell called the measuring cell, is immersed in the tank of acid. Obviously when the acid is of the same strength as that of the standard cell, the ratio of the resistance will be equal to one. By employment of the standard cell, temperature compensation is entirely automatic. The difference in the readings of the two cells fixes the point on the conductivity curve.
The value of this system was immediately established. Cells were installed in all oleum tanks and connection made to the conductivity indicator. This made it possible to determine the strength of any of these tanks of acid in less than a minute, by simply throwing a switch, pressing a button, and reading the conductivity curve. Another great advantage is the application to temperature determination of the acid inside the tanks. This was previously done by mounting thermometers somewhere in the acid lines. Special thermometers were necessary; they were hard to read and none too accurate. Because the acid in the standard cell has a definite value for each temperature, tests were made over a wide range of temperatures and the values plotted. With this curve and the conductivity curve both the temperature and the strength of the acid are now read off in one operation.
The increased safety to the men in handling so dangerous a product is important; the saving in the cost of $1.00 for each analysis is important; but these mean little as compared to the value to the absorption system, for it is here that condition of temperature and strength of acid must be carefully controlled and maintained in order to secure the maximum absorption of SO3. This conductivity system as installed, acts as an eye, for mounted right into the tanks of fuming, whirling oil of hell, it remains but to press the button and see directly the way to maximum SO3 absorption and maximum yield.
In 1930 there was developed and installed a system for adding the necessary water to the absorbing system in the plant. In the absorbing system, SO3 gas passes to five absorbing towers, two connected in parallel and three in series, with each fed by its own tank of acid. The strengths of these acids are allowed to build up in all units except the last. In this the strength is carefully maintained as near to 98.3 per cent as possible, as this acid has first a minimum vapor tension of SO3 and so is able to extract the SO3 completely from the gas, and second, it has a minimum vapor tension of water and so causes no mist formation.
In the old system the water was added from time to time in the form of weak acid which was made especially for the purpose by mixing together strong acid and water. This required, however, much of the operator’s time and in addition employed a whole system of pots, coils, boxes, and pumps, all of which had to be constructed out of chemical lead. Because of the strength of the acid and the heat formed by mixing the acid with water much time and skilled labor were required to keep this system operating.
In the new system, suction is produced by by-passing a part of the circulating acid directly to the cooler, thence to the tank without first passing over the absorption tower. Into this by-pass line is connected a line from a steam chamber made out of 3-inch pipe and fed by a %-inch steam line. The suction in the by-pass line draws from this chamber steam and water which are quickly absorbed by the rapidly moving acid. A valve on this line controls the quantity of acid by-passed and therefore the amount of suction. Once the valve is set, the system remains practically automatic as steam and water continue to be drawn in at a uniform rate until the valve is changed.
In brief, the operation of the new system is distinctly simple and practically automatic. It introduces water in the system in such a way that the formation of H2SO4 mist is reduced to a minimum and the absorption of SO3 increased to the maximum. The cost of the new system is about 1 per cent of the old and has an upkeep that follows out in like proportion.
The plant has produced since it was started in 1918, over 51,000,000 pounds of acid and maintained an all time conversion of 95 per cent and a yield of over 94 per cent. In all this time there has been very little acid loss and but a slight injury to one operator. After producing such corrosive material for so long a time, the plant still stands today in excellent shape, ready, almost over night, to increase its production more than ten times. It is equipped with apparatus that makes adjustment simple, control easy, operation definite; things which make the Navy’s plant a leader of acid plants.