Decembcr, 1924
INDUSTRIAL A N D ENGILVEERINGCHEMISTRY
sidered, but the apparatus had to be arranged so that it would deal efficiently with gas containing tarry matter and naphthalene, which would clog many types of packing. I n fact, a good deal has come to be expected of this liquid purification process in addition to the simple removal of hydrogen sulfide, and it is usually demanded that the apparatus be a gas cooler, a naphthalene washer, a tar extractor, a cyanide washer, and a sulfur remover all in one. Three principal types of packing have been ued-viz., coke, wooden hurdles, and spiral tile. Coke (0.5 to 1 inch size), if carefully screened, gives good results with gas free from suspended matter and is the cheapest of the three. However, it is much more subject to trouble on account of stoppage with dirty gas. Both wooden hurdles and spiral tile are very satisfactory from the standpoint of efficiency and have given excellent results under a variety of conditions. One plant, recently put into operation, has steel turnings as packing material and is giving very satisfactory results.
EFFICIESCY OF PROCESS In all the liquid purification plants thus far installed, the process operates to remove 85 to 90 per cent of the hydrogen sulfide, the remainder being taken out in the oxide purifiers. In this way the latter are used as catch boxes and the investment in them is not lobt. The insta1lat)ion of a liquid purification plant has the effect of increasing the capacity of the oxide boxes two and a half times and makes it unnecessary to renew the oxide except after intervals of many months. It often happens that the amount of purifying capacity is the limiting factor in the growth of a plant on any particular site. In such cases, with dry purification an increase of capacity can only be obtained by building a new plant on another site, while with liquid purification the capacity of a plant on a given sito can be more than doubled. With such an installation the system is able to deal with great fluctuations in the hydrogen sulfide content of the gas. The liquid purification process absorbs such fluctuations, since it is able to treat gas of 1000 grains hydrogen sulfide per 100 cubic feet just as readily and even more efficiently than gas of 100 grains per 100 cubic feet. Thus, the catch boxes are only required to deal with comparatively small amounts and minor fluctuations. The process can also be adapted for complete removal of hydrogen sulfide, and developments are now under way which are expected to make possible the recovery of the sulfur in the hydrogen sulfide.
Bureau of Standards Annual Report Scientific investigations and tests resulting in large savings to the Government and to American industry through improvement in processes and the fixing of uniform standards are featured in the annual report of George K. Burgess, director of the Bureau of Standards. Investigations made with orifice meters for measuring gas, corrosion of underground pipes, and tests conducted covering impact stresses in highway bridges, braking systems for automobiles, and other studies, have resulted in the application of improved methods in engineering practices that are of direct and substantial savings to the industrial public. Other contributions to the public interest are the successful development of methods of reducing the loss in the baking of Japan ware, the assistance rendered the optical glass industry in the United States, the progress made in the better utilization of cotton linl ers and other cotton wastes, and the development of a method for reclamation of gasoline from dry-cleaning processes. An increase of more than one hundred and twenty five times its initial volume has taken place in the testing work of the Bureau of Standards during the twenty-three years of its existence. During the year just closed 135,852 tests were conducted by all divisions of the Bureau, as compared with 115,729 in 1923. Most of the tests were executed for other branches of the Government, but a great deal of testing was also done for commercial firms and individuals. Unfortunately, some of this latter work had to be refused because the demand exceeded the facilities. The inability to meet this demand is unfortunate.
1239
Quantitative Analysis of Mists and Fogs, Especially Acid Mists' By Harold C. Weber MASSACHUSETTS INSTITUTE OF TSCHNOLOGY, CAMBRIDGE, MASS.
T
HIS paper is presented, not because it contains any new
methods of analysis, but rather to describe in detail the methods that the author has found successful for the determination of mists, especially sulfuric acid mists, in the plant. Such a paper has a place in an absorption symposium, for much of the published work on absorption is unreliable, because the methods of gas analysis involved completely ignore the necessity of analyzing for any mist that may be present. There can be no doubt that many of the irregularities in absorption data, especially where these data concern the absorption of acid gases, can be satisfactorily explained by the assumption that a mist was present during the analytical absorption. A mist or fog is made up of small particles of varying sizes. These particles may be either liquid or solid. One concept is that each individual particle is surrounded by an air film which effectively insulates it from its neighbors. According to this idea, then, any method of analysis to be effective must present some means of breaking through this air film. It is possible, however, to explain practically all the peculiarities associated with mist analysis and absorption without assuming the existence of' this protective air film. If we assume that the mist particles are so small that they do not settle out readily by gravity and that they are so large that their rate of diffusion is extremely small compared with the diffusion rate for gases, it is possible to see that extreme difficulty would be experienced in dissolving them. From this discussion it is evident that any method of analysis which proposes to analyze for mist by the mere bubbling of the gas through an absorbing medium should be looked upon with distrust. After a careful search of the whole field two fundamentally different methods of analysis were selected as possibilities. The first of these consists of passing the gas containing the mist through some type of porous membrane in which the pores are sufficiently small so that the mist particles are effectively trapped. Whether this trapping occurs by an actual scraping aside of the protecting gas film, whether it is due to the throwing out of particles as they pass through the torturous passages of the porous medium, or whether it is due to some other cause will not be discussed a t this point. The second method which presented itself was an adaptation of the principle involved in the Cottrell precipitator to a small portable apparatus for use in the plant.
POROUS MEMBRANE METHOD Several types of porous membrane apparatus were tried out, but only three of them gave satisfactory service; and of the three only two were finally retained. The first method tried was that involving the use of an asbestos mat. This mat was formed by catching a suspension of long-fibered asbestos on an ordinary 4-inch porcelain Buchner funnel. The preparation of these mats is rather difficult and the subsequent drying which must be done before the mat can be used is especially difficult. Cracks are very liable to form slightly below the asbestos surface. These cracks may be entirely invisible to the eye, but nevertheless they allow mists to pass through. The funnels themselves are clumsy to handle, and owing to Presented under the title "The Quantitative Determination of Mists and Smokes wlth Special Reference to the Determination of Sulfur Trioxide Mists."
INDUSTRIAL A N D ENG.TNEERING CHEMISTRY
1240
the irregular results produced by this type of filter, together time necessary for its preparation, this method of analysis was abandoned quite early. It is referred to here in some detail because it is used to a great extent in industry and is mentioned in several places in the literature. A second type of porous membrane which has been found exceedingly satisfactory for some work is the alundum thimble. The best results have been o b t a i n e d with this device by using a round-bottomed extraction thimble about s/d inch in diameter and about 2 inches long, into the top of which is slightly fitted a soft rubber I N" stopper carrying a glass flLTL2 PRPM BRffP inlet tube. The whole f/G, 1 thimble is then immersed in a suitable liquid for dissolving the entrained mist and a sample of gas of suitable size is forced through. The alundum thimble method has given very excellent results in the analysis of hydrochloric acid gas samples where a large amount of hydrochloric acid mist is often present and where one is interested usually in total hydrochloric acid content of the gas. I n general, the coarsest grade of alundum extraction thimble will be required in order to keep the pressure drop through the apparatus down to a suitable figure. Some objection may be raised to this method. The thimbles themselves are extremely fragile and the method of analysis involved does not differentiate between mist or fog and actual gas. In this procedure both are obtained a t once. The alundum thimble method will be found especially unsatisfactory for the analysis of sulfuric acid mist in burner gases where a comparatively large amount of sulfur dioxide is present. Experience has shown that a slight oxidation of the sulfur dioxide to sulfur trioxide takes place on passing the burner gases through an alundum thimble type of apparatus. Even though this oxidation is comparatively slight and affects the amount of sulfur dioxide present very little, nevertheless it does introduce a serious error in the determination of sulfuric acid mist because of the comparatively small amount of mist that is usually present. The third method involving the use of porous membranes is no doubt the most satisfactory method so far outlined, es-
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of ordinary filter papers. Circles of Whatmans 11-cm., smooth finish, No. 4 filter paper were found excellent for this purpose in plant practice. In order to make an air-tight joint the edges of the paper must be lightly paraffined. Several circles, either two or three depending on the characteristics of the mist, are then placed between the two funnels and the whole lightly clamped together by means of thumb nuts. The make-up of this membrane together with its clamping rig is shown in Fig. 1. If the edges of the paper are then rapidly passed through a Iayer of hot gas such aa arises from a Bunsen burner or an electric hot plate, the paraffin will melt slightly and form an absolutely air-tight bond between the filter papers and the two funnels. I n plant work the tightness of these filters was tested by placing a vacuum of about 6 inches of mercury on them and noting whether or not it dropped an appreciable amount in 30 seconds. Two such filter units were ordinarily used in series in sulfuric acid mist analysis work. There is no reaBon why more than two could not have been used had it been necessary to do so. These frames were mounted in a small cabinet containing a I/s horsepower motor directly connected to a small centrifugal pump and having inserted in one of its lines a suitable flowmeter for measuring the quantity of gas passed. The sample to be analyzed was then sucked through the whole apparatus and the filter paper membranes were afterwards carried back to the laboratory for analysis. I n the analytical work the clamping frames were taken apart and the filter papers dropped into beakers containing a small quantity of water. Disintegration of the filter paper mats is readily accomplished by means of a stirring rod and the slurry of paper and water so formed is titrated. When this apparatus was used for the analysis of burner gases it was found advantageous to sweep out the paper mats with a current of clean air in order to remove from them any adsorbed sulfur dioxide which would give erroneous results during titration. I n order to make doubly sure that no adsorbed sulfur dioxide was retained by the mats, a blank titration with iodine was made. In most cases the correction due to adsorbed sulfur dioxide was negligibly small. I n order to test the apparatus for oxidation effects a, mixture of moist air and pure sulfur dioxide gas from a cylinder was passed through a series of filter paper mats for several hours. At the end of this time the mats were taken out and analyzed for sulfate by first titrating them with sodium hydroxide and then correcting for any adsorbed sulfur dioxide by means of iodine titration. I n no case was any sulfate Q
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pecially for the analysis of sulfuric acid mists. The apparatus consists essentially of a clamping frame holding two ordinary 2.5-inch glass funnels, the edges of which have been ground to a practically air-tight fit, using dusted carborundum. Between the two edges of the funnels is placed a mat composed
Vol. 16, No. 12
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Decembw, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
then passed through :I Tyndallometer composed of a dark box containing a 500-watt concentrated filament type lamp fitted with a lens system so that a narrow pencil of intense light wad projected across the box. The effluent from the filter-paper train was passed across this beam of light exactly a t right angles to it. In no case was any Tyndall beam effect noticeable until the breakdown of the mats occurred, which breakdown only took place after running for periods of 1.5to 2 hours and at a gas rate of about 0.3 to 0.4 cubic foot per minute. This gives a capacity far in excess of that necessary for analytical work. Filters of this same type mere also tested for the removal of tobacco smoke and were found fully as effective as for the removal of sulfuric acid mist.
1241
of comparatively fine particles, the large particles having been removed by the coke box, and that some attempt should be made to cause these particles to grow in size before any further apparatus is installed for removing them. Fig. 4 shows the mist content of gases at various points in a contact plant which was operating satisfactorily as far as mist was concerned. It will be noted on Fig. 5 that the ratio of mist caught on the second mat to that caught on both is extremely high for a point just before the gases enter the coke box. Evidently, the coke box in this plant is running under such conditions that it can effectively remove a fine mist, as is shown by the decrease in mist content of the gases in passing through the box. COTTRELL PRECIPITATOR METHOD
There is one other interesting point in connection with the use oi filter-paper mats. If a series of such mats is used in plant analysis work, it will be found that each succeeding mat in the series does take up some mist, as can be found by titrating ertch one separately. In case only two mats are used (and two have been found sufficient for all the work that has been done), the ratio of the amount of mist caught on the second to the amount caught on the first and the second at various s:tmpling points in the system is not constant. This ratio offers a very satisfactory means of determining exactly how a giyen piece of plant apparatus used for mist elimination is Eunctioning. If the mist is composed almost wholly of largp-sized particles, the amount caught on the second filter will be very small in comparison with that caught on both. As the size of the mist particles decreases, however, more and more of the mist will be caught on those mats following the first. I n this way it was found that certain large spaces in a given contact sulfuric acid plant, which appeared to be absolutely ineffective as far as mist removal was concerned, were very effective in giving the particles time enough to grow in size, so that a sinall coke filter following these empty chambers could effectively remove the large particles, whereas the coke box would have been absolutely ineffective had it not been preceded by the open chambers. It is this feature which makes the filter paper method especially valuable in correcting, and analyzing mist troubles, especially in sulfuric acid plants. Fig. 2 shows a series of three groups of analyses made on a contact sulfuric acid plant where trouble was being experienced with mists. Each point on these lines represents a t least two analy-es made at approximately the same time and checking wii!iin a few per cent. Fig. 3 shows the ratio of the mist caught on the second mat to that caught on the first and the second for the same three sets of analyses. Two interesting points are brought out by Fig. 3. The particles are given a chance to grow in size as they pass from the burner to the cooler entrance. The sharp upward bend of all three groups at the end of the system points rather eonclusiwly to the fact that the mist at this point is composed
The filler paper method of analysis will be found unsatisfactory for use where plugging of the pores is likely to take place or where nitric acid vapors are to be analyzed. For this work a modified laboratory type Cottrell precipitator was constructed. Such an apparatus presents several mechanical difficulties. It must be built entirely of acid-resisting material and yet so designed that it may be operated a t low potential and with a small amount of power, thus rendering it safe to handle in the plant. All of its current-carrying parts must be of reasonably good conductivity. Fig. 6 shows the precipitation chamber which was erentually used in the analysis of nitric acid stack gases for mist. A long, slender glass tube about 3/4 inch in diameter and 18 inches long is fitted with two side arms as shown. Around the outside of this tube between the two side arms is cemented an electrode of copper foil. It was found advantageous to cut two windows through this foil in order to observe the progress of precipitation. Lead glass was most satisfactory for use in constructing this tube, and Pyrex glass because of its low conductivity was absolutely useless at the low voltages employed in this precipitator. The top of the tube is fitted with a rubber stopper and a suitable connection for making contact with the fine platinum wire which runs through the exact center of the tube. This wire, forming the high potential electrode, is fastened at its lower extremity to a glass rod bent as shown in the sketch. By adjusting this rod with
either a rotary or an up-and-down motion the platinum wire can be centered exactly in the precipitator tube. This is extremely important. If the mire is off center the field of force will tend to be concentrated on that side where the wire comes near the precipitator tube and bad channeling of the smoke or mist up the opposite side will occur. d large rubber stopper fits over the bottom of the precipitator tube and holds the centering device. A special lipless beaker completes the bottom part of the apparatus and forms a catchall for the precipitated material as it drains down the central electrode.
1242
INDUSTRIAL A N D ENGINEERING CHEMISTRY
In general, the upper side arm serves as the inlet for the sample and the lower for the exit for the mist-clear gases. Most small-scale Cottrell' precipitators require an exceedingly high voltage and a considerable power input for their successful operation. Using a Pyrex tube, 30 to 35 thousand volts were required for successful operation. With the leadglass tube, however, precipitation was practically complete when 11 to 13 hundred volts d. c. were used, if the gas velocity through the tube did not exceed 0.3 cubic foot per minute. The effluent was tested for mists by means of the same Tyndallometer as was used in the case of the filter-paper method. Drinker,2of the Harvard Medical School, has used a precipitator somewhat similar to this one for measuring the amounts of dust particles in the atmosphere in factories. He uses a Pyrex tube and an exceedingly high potential, and in his case no attempt is made to apply direct current for precipitation purposes. For the purpose for which the present apparatus was designed it was practically impossible to use the 35 or 40 thousand volts which are usually required in Cottrell precipitators, as there would have been extreme danger of electrical shock in using such an apparatus around the plant. Furthermore, it was believed that such a high voltage would tend to give a heavy corona discharge, which in turn would cause excessive oxidation of the product being analyzed, especially if this product happened to be burner gas. An ordinary Ford spark coil run from a 6-volt storage battery proved entirely successful as a source of power when lead glass was used for the precipitator tube. In fact, the precipitator would work almost as well on a 2-volt storage battery as on a 6-volt battery. Tolman3 has described a somewhat similar arrangement. In order to get successful precipitation with such low power inputs one must adjust the spark coil vibrator carefully. In general, the adjustment should be such that the coil gives a slow make and a very rapid break. This will tend to make the wave of the alternating current supplied to the precipitator verv much distorted. 7Mff aRSJ TKEL th&e loops occurring a t 79' IN7, D/A. the make being very OUTJ7Dl MRRPP&D small and those occurru m COPPLRPOIL ing a t the break very ,fl&CrXCRL PR€CiP/i7?TOR large. In this way it is f/G. 6 possible to get approximately a pulsating direct current. This type of alternating current has been found to work very much better than the ordinary form delivered by commercial transformers working with an alternating current input. The apparatus was tested f o r o x i d a t i o n effects by passing a mixture of sulfur dioxide'. air, and water vapor through the tube for several hours and then FIG.6 carefully testing the interior of the apparatus for sulfate; no sulfate was found. A serious difficulty which is usually experienced in laboratory Cottrell precipitators when used on acid mists is the breaking 9 8
J . I n d . Hyg., 6,19,62,162 (1923). J. A m . Chem. SOC.,41, 587 (1919).
Vol. 16, No. 12
down of the insulation owing to the formation of a mist layer over the nonconducting parts. This difficulty has been obviated in the case of the present precipitator by coating the upper and lower ends of the tube on the inside with a layer of paraffin wax. This tends to make whatever mist does form on these parts condense as a discontinuous film of droplets rather than as a continuous liquid film.
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Very successful results were obtained with this apparatus in the analysis of nitric acid plant stack gases. The sample is sucked through the precipitator tube. The duration of the sampling period can be accurately determined by the instant of closing and opening of the high tension circuit. After a suitable sample has been collected it is a simple matter to wash the total precipitate down the precipitator tube into the small beaker a t the bottom where it can easily be titrated.
ACKNOWLNDGME NT The author wishes to acknowledge his indebtedness to the Merrimac Chemical Company, in whose plant most of the analytical work was carried out, and especially to their research laboratory for helpful advice. He would also express his thanks to W. A. Hoops, T. B. Drew, and R. B. Abrams, of the School of Chemical Engineering Practice, for their aid in carrying out the experimental work.
.......... DISCUSSION Dr. Cottrell suggested that the high-tension discharge necessary for the operation of a small-scale Cottrell precipitator might be furnished by the inductive impulse which is set up in a circuit containing a large amount of inductance when this circuit is rapidly made and broken. He said that to do this the device might be constructed small enough so that it would be no larger than an ordinary flash light, and stated that a similar device was in operation several years ago on many of the Moore type vacuum tube electric lights. This would eliminate the necessity of using a coil with two windings such as all induction coils have and might tend to simplify the apparatus somewhat. Mr. Weber admitted that this device could be used in place of the Ford spark coil suggested, but he felt that very little advantage would be gained by substituting this device for the Ford induction coil. Mr. Mallinckrodt asked if any of these methods had ever been used by the Mellon Institute in examining the air around Pittsburgh for smoke pollution. Further discussion brought out the fact that these methods were probably not used by the Mellon Institute in this work, but that a method involving a small-sized Cottrell precipitator had been successfully used by Drinker of Harvard University for the determination of small quantities of dust in air.
