March, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
227
EQUIVALENT LENGTHS,EXPRESSED AS ADDITIONAL DIAME~W OF
where A k = pressure drop in in. of water. 1 = length of pipe in f t . p = density of gas in lbs. per CLI. ft u = velocity of gas in f t . per sec. d = diameter of pipe in ft.
STRAIGHT PIPE 9 0 ° Elbows: 1-21’ in ........................... 3 - 6 i i . ............................ 7-10 in ............................ Globe Valves: 1-21/2 i n . . 3-Ain ............................... 7-10 in Tees. 1-4 in ............................... 90° Curves same inside diameter as pipe: Center iihe radius = diameter of pipe., Center line radius 2 to 8 diameters.,
30 40 50
........................... ..............................
Bends, elbows, valves, and other obstructions to flow increase the pressure drop. The effect is usually expressed as so many diameters of increased length of straight pipe. A table of such ~ O ~ ~ O T V S . ~ a “Chemical Engineering Notes,” Walker, Lewis, and McAdams.
45
60 75 60
..
20 IO
Chimneys Subjected to Acid Gases’ By Thomas S. Clark
,
95 NASSAU ST.,Nrtw YORK,N . Y .
A
CHIMNEY t o hanThe disposal of noxious fumes from various chemical industries carried to where has a[ways been a serious problem ooer which much thought and their diffusion in theatmosdle noxious and Phere greatly r e d ~ c e s any acid gases must be energy haoe been expended. Most often the solution of this dificulty has been the erection of a tall chimney of such proportions as to Objectionable Odor% if not designed and built, not only them. for adequate capacity and carry the gases high enough to dissipate them before they can reach draft, but to resist the the ground leoel, at which they are harmful. The simple solution The fine dust coming destructive effect of the of using an already existing brick and mortar chimney, or of haoing roasting horizontal one built by an ordinary brick mason, has generally led sooner or rotary kilns in the burning particular acid gases, dust, fumes, and temperatures, later to disintegration. The accompanying article deals with the of lime, Pyrites, sintering details of chimney construction that haoe been worked out particuProcesses, e% may also and, in- addition, to resist the dynamic wind forces IarIy to ooercome these dificulties. be disposed of or diffused that tend to fell it. Many to a marked degree by emitting the dust-carrying of these chimneys are not operated in connection with steam boilers, but are con- stream at a high altitude. nected directly with roasting kilns, furnaces, and other apOf the many gases coming from these industries, such as paratus in the process of the production of chemicals, acids, those of the sulfuric, nitric, chlorine, fluorine, lead, and arreduction of ores, the manufacture of colors, photo films, senic groups, those of the sulfur group are the most common. Those of the carbon family give little concern, as they are celluloid products, etc. The smoke streams emitted from such chimneys contain not particularly detrimental to a community nor do they acids in both liquid and gaseous form, They are often tend to disintegrate a brick stack. reputed to be a nuisance to a community. Some are supPROPERTIES OF SULFURFUNES posed $0 be detrhental to vegetable and life, but Sulfur trioxide,sulfurdioxide, and compounds of lead and this depends entirely upon the degree of concentration. arsenious oxide are noxious and objectionable. The first Plants of this nature are faced with the disposition of of these attacks to a marked degrei: brick and orthese gases, which of necessity must pass Off from their dinary mortar, concrete, and It is, therefore, very paratus’ Methods have been used to remove fume destructive to the ordinary chimney designed for use in conand flue dust from the smoke include washing the smoke nection with boilers burning coal. Sulfur dioxide streams In scrubbers>the use Of sprays and baffle chambers, gas in the pure state condense to a liquid at about 140 F. bag houses for filtration, and electrical precipitators-all At any temperature above this it remains a gas and will not more or less successful in reducing the quantity of fumes combine with water to form damp acid mist or liquid acid and dust. None have so far been successful in eliminating in any way to the action of sulfur trioxide, If all the objectionable elements before entering the chimney. present in small quantities in the smoke stream at atmosSome of these methods tend to reduce the stack temperatures. pheric pressure, the condensation point is much lower. Some contribute moisture to the gas stream, increase the Therefore, this particular gas has little or no effecton a brick acid mist, and sometimes add to the undesirable activity chimney. of the dust and fumes. At the present date it seems that the only solution for the Chimneys from 350 to nearly 600 ft.in height, discharging of the effectof sulfur dioxide is to see that the the gases a t high elevations above the surrounding country sulfur dioxide content of the smoke streamis so diluted be- , where they become diffused and diluted before reaching the fore it reaches the pound that it is harmless, This is being earth, have become Common* They may not Serve the pur- done through the use of tau chimneys, and, as a positive pose perfectly, but their continued use is evidence that the safeguard, by heating the gases by auxiliary heaters. results are not entirely unsatisfactory. I n chemical or inunlilie sulfur dioxide, sulfur trioxide in the presence of dustrial plants where the fumes are not acid, noxious, or water vapor, so in the smoke stream of the indusharmful, but yet are disagreeable in their odor, the gases are tries mentioned, even in extreme low concentrations ill easily disposed of by means of a comParativelY combine with the water vapor and form what may be called The smoke stream having no dWtructiVe ‘ontent, no pre- a fog of sulfuric acid or even liquid sulfuric acid on the walls cautions need be taken against acid action. The fumes are of the ,.,himney, Of that which passes out of the chimney, some may under certain atmospheric conditions eventually 1 Received January 29, 1923.
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228
settle to the ground in the form of sulfuric mist or dew, but in any properly constructed plant the amount is so small as to cause no trouble. It is a fact that the temperatures at which an acid gas will become an acid liquid depend largely upon the concentration of water vapor and acid gases in the smoke stream. The greater the concentration of sulfur trioxide and water vapor the higher the temperature at which the condensation will take place. As long as they remain a gas, or, in other words, as long as the sulfur trioxide is kept a t a temperature over 400" F., they have little effect upon hardburned impervious brick or so-called commercial acidproof mortar. Some authorities give the condensing point of the sulfur trioxide under the above conditions as low as 275 O F. The best practice is to maintain a temperature of the smoke stream of 400" F. or over. It will be noticed that these temperatures are above the boiling point of water.
PROPERTIESOF OTHER NOXIOUS FUMES
FIG. TYPICAL
ACIDG A S E S PENDENT
CHCMNEY HANDLINQ DESIQNED WITR I N D E AND UNDER-
GROUND
FLUE
The fumes of chlorine and nitrous oxide under certain conditions attack common brick and mortar, concrete, unvitrified tile, steel, and the common metals. The effect on these materials, particularly in the presence of moisture and low temperatures, is practically the same as that of sulfur trioxide. As tructure to stand up against them should follow the same general design and use of materials as one built to resist the action of the sulfur acids. The disposition of the chlorine and nitrous oxide fumes by emitting them at high altitudes is common practice and is quite satisfactory. Here, too, if the products of combustion carrying these two gases have a low temperature, auxiliary furnaces fired a t the foot of the stack are employed to raise the temDerature. imDetus or ..~ , give velocity to the smoke stream, decrease its density, and cause it to rise to consid-
Vol. 15, No. 3
erable heights above the top of the chimney. The diffusion in the atmosphere is thus more completely accomplished.
TEMPERATURE OF STACKGASES The most important thing in handling acid gases in a chimney is to maintain a high internal temperature. This often nullifies the detrimental effect of the gases on the masonry. Furthermore, the higher the temperatures of the emitted gases a t the top of the stack, the higher the fumes and h e dust will ascend-consequently, the greater their diffusion before reaching the ground. This is a most important fact to the management of smelters and chemical plants, especially where it has sulfur dioxide to contend with. A wet or damp-acid smoke stream in contact with ordinary mortars made of cement, lime, and sand, or sand and cement, and certain commercial mortars which do not contain cement and lime, produces a swelling and puffing of both the bed and cross joints accompanied by a tremendous pressure. The swelling amounts a t times to from 25 to 30 per cent. A chemical change takes place a t first on the surface. The mortar becomes soft and of the consistency of mud. As time goes on this softening and swelling works entirely through the walls, causinb the brickwork to bulge and crack. Steel bands are useless, even on the outside, for the masonry will bulge between the bands and in time the bands will give way. If the brick is not hard and impervious, the exposed portion becomes soft and flakes off. This process continues until the whole brick is changed into a soft mass. Cases have been observed where the swelling of the joints is quite uniform in the circumference of the chimney and irregular bulging of the structure is hardly discernible. The disintegration takes the form of vertical cracks, which usually appear first at the top where the walls are thinnest and in time work downward to the base. The vertical cracks are due to the swelling of the joints, causing circumferential strains as the diameter tends to increase. T h e s e strains are greater than the strength of the masonry. It is f u r t h e r observed that the cracks increase more rapidly and become larger on the prevailing windward or weather side. This is to be expected, for on that side the rain and snow are driven more frequently and more 2-DBTAIL OF HEADAND PROT?.$CTING C A P . forcibly against the FIG. NOTEEXPANSION SPACB AT T O P O F LININO surface and into the interior of the initial small cracks. The water enhances the disintegration of the acid-soaked joint. Once the joints are soaked with the acid, the swelling continues as long as they can take up any moisture, which by capillary attraction continues to spread through large areas. Even if the acid fumes are not wet, certain of them will attack these mortars and destroy the cement or any binder that contains an element which will combine with the acid fumes, turning the joint into a weak sandy mass. Bricks
IhlDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1923
fied brick, very low in lime and laid up in acid-proof m o r t a r 4 e., a mortar made to resist the particular kind of acid in the smoke stream. The thinnest possible joint is imperative. The bricks should be thinly butted or dipped and struck tightly I into place. Many commercial acid-proof mortars are resistant to certain acids as long CAREIX BURNING FUELOIL as the acid gases are dry and of a Acid action has been comparatively high observed from the smoke temperature. These stream resulting from the are o f t e n c o m burhing of certain fuel oils posed of mixtures of under boilers. This is par- pure clay, silica ticularly in evidence where sand or silax, kaothe sulfur content of the oil lin, asbestos fiber, is high and steam sprays are china clay, graphite ground FIG.3a-sBCTION THROUGH FLUEO P E N I N G AT AA used. In these installations, products, especially in connection gypsum, etc. A with economizers resulting common binder is silicate of soda. These mixtures are in low flue temperatures not always acid-proof and often break up under the action and when the chimneys are of moisture. They soften, swell, and disintegrate under a high, the protection of the wet acid. So the mortar must not only be acid-proof but upper portion should have be moisture-proof. Only sand of almost pure silica should the attention of a designing be used. engineer. It all depends PROTECTION OF THE CHIMNEY TOP upon the sulfur content of The top of the chimney should be protected with a cap, the oil and the flue temperacovering both the lining and the main walls, and made of tures. material not affected by the particular kind of acid under conSmoke from many fuel oils sideration. Ample room should be allowed for the lining to has no effect on the brick lining designed for coal-burn- expand upward and outward. ing steam boilers. Cases Furthermore, the cap should have been known, however, be so designed that no dust, where incomplete combus- fumes, or moisture can find tion or improper atomiza- their way down between the tion of the fuel oil caused a main walls and the lining. considerable deposit of un- It will be noted that with consumed matter to collect this design the lining has within the chimney. This room to expand upward deposit has been known to without lifting the cap. catch fire, resulting in in- The air space is protected. te r n a 1 temperatures over (Fig. 2 ) With certain acid 2000" F. Where such an conditions the cap may be incident is liable to occur the made of lead. On the other chimney should be so de- hand, some acids will affect necessarily disinsigned to safely resist the lead-not tegrate it, but cause it to temperatures. buckle. With other acid conditions a cap of Monel CHIMNEY DESIGN metal has been used with I n designing a chimney for success. The choice of maacid duty it is necessary to terial is dependent entirely protect the main walls per- upon the nature of the acid. The gases coming from fectly by an independent lining for the full height of the top of a chimney are the structure, with an ample often swirled by the wind air space between it and down the outside for disthe main walls. An air space tances varying from 25 to of not less than 3 or 4 in. 100 ft. For that reason the at any point is recommended. same acid-proof mortar used I n fact, the design is s chim- in the lining should be used ney within a chimney. on the outside joints of the (Fig. 1) The independent upper portion Of the main F I G . 4-DETAIL OF S U P P O R T I S G CORinner lining must be built of Since this surface BEL SHOWING PROTECTING -4PRON SPACE: FOR LINING impervious ,practically vitri- is exposed to the weather, it AND EXPANSION not vitrified and impervious share the same fate. The effect on concrete is a rapid disintegration of the whole mass, owing to the breaking u p of the cement content, and the acid action on certain stones and sand particles.
I
I
I
P I G . &-CHIMNEY
WITH SECTIONAL
LINING FOR HANDLING ACIDGASES
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is most necessary that the mortar be weatherproof. Common building lime should be eliminated from every part of the structure. I n some cases, where the temperature of the acid smoke stream is continualIy high and the acids not very active, the same brick and mortar may be used, but a sectional lining must be constructed instead of an independent lining. (Fig. 3) This form of construction is less expensive. The corbels built out at intervals from the main walls and supporting the lining should have the inner joints pointed with acid-proof mortar. On the top of each corbel an apron of an acid-proof material should be set ’in such a manner that the lower lip projects down over the top of the section of lining below. The air space is then protected. I n addition to this the upper 12 in. or so of the air space under each corbel should be packed with flexible material not affected by the particular acid encountered. (Fig. 4) Where lightning rods are installed on acid chimneys, the upper 50 ft. or more of the complete rod should be covered with an armor to protect the copper from effects of the acid. Lead covering is in most cases effective. All chimneys handling acid gases should be equipped with an outside ladder, the upper portion of which should be covered with lead or an acid-resisting material. CAREOF CHIMNEYS DURING SHUTDOWNS Chimneys that have been in almost continual service for years without showing any effect from the smoke stream
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have been observed to develop defects, particularly in the upper portions, after they have been shut down,for a protracted period. Although the conditions of temperature, dilution, acid mixture, etc., may be such as not to cause damage while the chimney is in operation, yet an accumulation of dust on the inner walls, which is deliquescent by virtue of its acid content, may tend to do damage when the chimney is not in operation. The weather-rain, fog, snow, or a heavy humid atmosphere-furnishes the necessary water within the chimney to convert the previously inert dust with an acid content into a liquid acid which immediately becomes active. It is wise, therefore, when the chimney is shut down for a period, to cover the entire opening at the top with a temporary weatherproof lid. This can be made in sections of light wood and is easily placed and removed. Lugs protected by an acidproof armor should be built into the head to which the sections of the temporary lid may be fastened. A satisfactory arrangement in designing a plant in which acid fumes are to be carried off is to plan the boiler house so that the gases from the boilers and the acid fumes from the apparatus can be put in the same chimney. The boiler gases not only keep the temperatures up, but they dilute the smoke stream containing acid gases. No hard and fast rules can be laid down which will apply to every case where chimneys handle acid gases. The problem of design and materials used can be solved only by an intimate knowledge of the nature and effect of the particular gases, fumes, or dust to be disposed of.
Accurate Bubble Meters for Very Small Rates of Gas Flow’ By Tyler Fuwa and G. A. Shattuck MASSACHUSETTS INSTITUTB ofl TECHNOLOGY, CAMBRIDGE, MASS.
OR THE measurement of gas flow in the laboratory, the ordinary capillary tube flowmeter is the moPt satisfactory instrument where from 200 cc. to 50 liters or more of gas are flowing per minute.2 For rates of flow below 200 cc. per min., however, the capillary tube flowmeter is not, in general, satisfactory. It therefore seemed worth while to investigate the possibilities of standardizing some type of bubble meter that could be depended upon to give reproducible results. Such a type of meter has several great advantages-it is very easy to set up, has practically no lower limit at which it cease3 to measure with good accuracy, and a single calibration curve may be used for any gas which does not react with the liquid in which the bubbles are formed.
gas through bottles C and D, thus stabilizing the flow through B, which would otherwise vary considerably, since an airline
DESCRIPTION OF APPARATUS Plate 1 shows the apparatus as set up for the flow measurements. Bottles A and B contain the meter liquid, the flow through B being regulated by the pinchcock and screwclamp HI, the object being t o divert the greater part of the 1 Received August 11, 1922. Published as Contribution No. 00 from the Research Laboratory of Applied Chemistry, Massachusetts Institute of Technology. 2 For a n excellent discussion of capillary tube flowmeters, see “GaT Flow Meters for Small Rates of Flow,” by A . F. Benton, THISJ O U R N A L , 11 (1919), 623.
PI,ATED DIAGRAM OF BUBBLEM E T E S
