INDUXTRIAL A N D ENGINEERING CHEMISTRY
768
VOl. 19, No. 7
A Sulfate of Ammonia Plant' By F. A. Ernst and W. L. Edwards FIXEDNITROGEN RESEARCH LABORATORY, BUREAUOF SOILS, WASHINGTON, D. C.
ULFATE of ammonia is so well known to the fertilizer trade, it is believed, that it must be given serious consideration as a consumer of any further increase in synthetic ammonia production in this country. Ammonium phosphate has an advantage over the sulfate in that phosphoric acid, the ammonia carrier, is itself a plant food. It has the disadvantage, however, of being dependent upon phosphoric acid, the economical production of which in a concentrated form is still under investigation. Although the production of concentrated acid through the reduction of phosphate rock in either electrically heated or fuel-fired furnaces has been the subject of investigation for a number of years and has been worked out technically, this acid has not proved economical for fertilizer use. A sulfate of ammonia plant can be so constructed and erected as to be available without alteration for the production of ammonium phosphate from concentrated phosphoric acid and ammonia. It is with this thought in mind that the following brief description of the construction and operation of a sulfate of ammonia plant is given.
S
Methods of Production
Utilizing sulfuric acid, ammonium sulfate may be produced by either the cold or hot method. I n the first method, usually employed a t the by-product coke ovens, the gas containing 1 to 2 per cent of ammonia as it comes from the ovens is partly cooled and passed through the tar extractor to the ammonium sulfate saturator containing dilute sulfuric acid. The resulting salt is ejected from the saturator to the drain tables or settling boxes, while the gas minus its ammonia passes out through an acid trap.
gas and the saturator maintained a t 100" C. (212" F.). The excess water in this case passes off as steam. Flow Diagram
The flow for this operation is shown in Figure 1. From an acid supply the 60" BB. sulfuric acid flows by gravity through a measuring box to the tower distributor, over the checkered packing of the absorption tower, and into the saturator. Ammonia gas brought in through a main enters the snturator through the ammonia distributor, from which it bubbles up through the liquor which absorbs it. Any ammonia not absorbed passes out into the tower, where it is picked up by the descending acid spray and returned to the saturator. By means of the steam ejectors, the resulting crystalladen liquor is ejected to the trough serving the settling boxes. I n these boxes the crystals are permitted to build up while the liquor drains off to the return trough and is returned to the saturator. When sufficient crystals have collected in the settling box, th4y are raked t o the centrifugal drier, where they are wrung free of the larger part of the liquor. Final drying takes place in the rotary drier, from which the sulfate may be removed to storage. Conditions of Operation
Because the size of the crystals decreases as the acidity of the saturator is increased, it is very important to maintain this acidity a t a low concentration. I n plants using byproduct coke-oven gas the acidity is maintained at about 2.5 per cent. This is possible because this gas, as previously mentioned, is relatively'low in ammonia. It would not be practicable when utilizing 100 per cent ammonia gas, however, to drop this acidity below 4 per cent, the range
FLOW DIAGRAM FOR
SULPHATE OF AMMONIA M A N U F A C T U R E
The saturator in this case is maintained a t about 60" C. The excess water introduced with the sulfuric acid and as wash water is carried off with the large volume of gas passing through the saturator. . I n the hot method, which will be discussed here, the ammonia is introduced as practically 100 per cent ammonia (140' F.).
1
Received April 27, 1927.
being usually 4 to 7 per cent sulfuric acid. The acidity of the saturator for purposes of this paper will be considered as being 7 per cent. The reaction with heats of formation per mol from the elements, for the substance in the state indicated, is: 2NH3 (gas) f %SO4 (7%) = ( S H I ) * S O(aq ~ ) 23,780 cal. 209,680 cal. 281,000 cal.
July, 1927
INDUSTRIAL A N D ENGINEERING CHEMIXTRI'
The heat evolved, therefore, is 47,540 calories per mol sulfate. I n addition there is evolved a heat of dilution acid 77.67 per cent (60" Be.) sulfuric acid to 7 per cent 8669 calories per mol. The total heat evolved, then, 8669 = 56,209 cealories per mol of sulfate. 47,540
+
of of of is
769
so as t o build up a cake of uniform thickness. At the same time, however, rotation should be slow enough to prevent excessive loss of sulfate through the holes in the basket and also to prevent splashing beyond the confines of the centrifuge of the hot liquor. A maximum speed of 150 r. p. m.
ACID SUPPLY T A N K Figure 2
Let i t be considered that all this heat is utilized in converting water a t 20" C. to steam a t 100" C. Each gram of 538.7 (latent heat of water vaporized will consume 80 vaporization), or 618.7 calories.
+
56,209 calories per mol of sulfate are equivalent to 426 calories per gram.
426 X 454
56 209
132,or
2000
Then 6,8,7 X ~ . = b 164 gallons of water which may be evaporated per t m of sulfate produced. One ton of sulfate of ammonia is the equivalent of 515 pounds of ammonia and 1912 pounds of 60" Be. sulfuric acid. Allowing for loss of acid, it can be considered that one ton of sulfate requires one ton (2000 pounds) of 60' B6. sulfuric acid. A ton of this acid contains approximately 447 pounds or 53 gallons of water. The difference between the 164 gallons of water which may be evaporated and the 53 gallons brought in with the acid. or 111 gallons, is the quantity to be admitted to the system as wash water. Blthough this wash water should be hot as used, perhaps 80" to 90" C., and so enter the saturator a t something over 20" C., yet the recirculated mater loses some of' its heat in circulating from the saturator through the troughs, settling boxes, and centrifuge back to the saturator and enters the saturator a t something below 100" C. These two divergences from the assumed conditions can be considered as approximately balancing each other, leaving the result unchanged. If it is considered that a centrifuge when loaded contains the equivalent of 250 pounds of dry sulfate, then each wringing of a batch of sulfate may be washed with approximately 14 gallons of water. As the centrifuge operator, however, need not be skilled, but rather mechanically trained to this operation, and as the sulfate is not weighed into the centrifuge nor the wash water measured, this figure of 14 g'A 11ons serves merely as a guide. Actually a careful check of conditions within the saturator must be maintained. It is very easy to admit too much water, requiring additional acid to keep up the acidity, and thus build up the quantity of liquor beyond the capacity of the saturator. The quantity of sulfate as centrifuged would build up a cake 6 to 8 inches thick around the inner periphery of the basket. During loading the centrifuge should be rotated ~~~
has been found satisfactory while loading, after which for drying purposes this speed should be increased to 350 r. p. 111. and maintained a t that speed for approximately 5 minutes. With an acidity of the saturator of 7 per cent, the resulting sulfate crystals are very small. More rapid rotation of the basket than 350 r. p. m. packs the rake so tight as to render it difficult to remove Because of a rotation of only 350 r. p. m., however, the sulfate discharged from the centrifuge contains about 3 per cent of moisture, necessitating further diying, as by the rotary drier shown in the diagram (Figure 1) to the less than 0.5per cent allowed. Y F m m Acid .Supply Tank
fl
To Tower Distribufiny Box
M E A S U R I N G BOX Figure 3
Materials of Construction Much of the equipment for such a plant can be secured on the market. This is especially true of piping, fittings, and valves, and of the centrifuges, rotary driers, and conveying and elevating equipment. Some of the remaining equipment, the settling boxes in particular, and eyen the saturators, might also be available. It is believed, however, that there are advantages in constructing the last-mentioned equipment, as well as the tower and troughs, in place a t the plant. The lumber for such equipment should all be of a good
770
INDUSTRIAL AND ENGINEERING CHEMISTRY
grade of longleaf yellow pine free from knots. Bolt heads should not protrude above the surface of any face or siding to be covered with lead. The lead should be of the weights indicated in the descriptions of the various individual pieces of equipment.. A-oie-Sheet lead is usually designated b y weight as 30-pound lead, 8-pound lead, etc. As lead weighs approximately 60 pounds per square f o o t per inch of thickness, i t is ordinarily considered for weight designations a s weighing a pound per square foot per 1 / ~inch thickness. T h u s , 8pound lead is E / M or I / g inch thick, 2.2-pound lead is 2‘/aa or J!g inch thick, etc. There is a n exception t o this, i n t h a t ’/%-inch lead is known as 30pound lead.
A11 joints or seams should be burned and not soldered. All parts of lead to be burned, as well as the burning strips, should be scraped free of all foreign material and oxide to a silvery white appea.rance immediately prior to burning. Holes should not be macle in the lead sheets for holding such sheets in place, either temporarily or permanently. If a temporary means of holding the lead in place during construction is required, straps should be burned to the face of the lining and the sheets hung by these. These straps may later be cut off. Equipment
The equipment described here is for a plant having a capacity of 60 ‘tons of sulfate of ammonia per 24-hour day. ACID-SUPPLY Tam-The acid-supply tank may be as shown in Figure 2. With the dimensions as indicated, the tank will hold sufficient acid for an 8-hour shift run. The lumber should be of southern longleaf yellow pine of “square edge and sound” grade. The posts are mortised into the sill and cap timbers and the frame is reinforced with 3/4-inch tie rods as shown. The floor planking is 3 inches, while the siding is 2 inches thick. The box is
Vol. 19, No. 7
lined with 16-pound lead and covered with 12-pound lead carried by iron pipe to which it is strapped as shown. A vent through the cover and an acid overflow should be provided. Acid flow from the tank is regulated by means of a hard lead plug and seat controlled by the rod and winged nut arrangement as shown. The acid level “tell tale”.may be placed a t the tank or a t some other point of vantage to the operator. The float is a sphere of IO-pound lead, 12 inches in diameter. This will float approximately half submerged in the acid. MEASURING Box-Because of its shape, the measuring box (Figure 3) is generally called “piano box.” This is simply a small box constructed of 12-pound lead, with a partition or diaphragm containing R series of holes one above the other and ranging in diameter from ‘/8 to 1 inch, increasing in increments of I/g inch on the diameter. By watching the flow of the acid through this diaphragm, while adjusting the tank plug with the winged nuts, a rather fine regulation of flow may be secured. For a flow of 21/2tons per hour necessary for the production of 60 tons of sulfate, the piano box should run “3l/3 holes.” ABSORPTIONTOWER-In erecting the absorption tower (Figure 4) care must be taken to place the splash brick 01 tower packing as shown t o avoid “salting up.” The walls of the tower are builtof acid-proof brickwith a mortar of silicate soda with a filler. This mortar is mixed in the approximate proportions of 200 pounds of the filler known by various trade names as acid-proof cement to every 700 pounds of silicate of soda. A small amount of powdered barytes should be added to each batch, preferably by the mason as used, to accelerate the setting up of the mortar. The quantity of barytes varies acco-ding to the consistency of the mortar
n m n
SECTlOWS OF ABSORPTION TOWER
Figure 4
I S D C S T R I A L A S D ESGI,l-EERI,VG CHEMISTRY
July, 1927
IfA
771
SECTION-AA
ELEVATION AND SECTIONS OF SATURATOR
Figure 5
SECTION
OF T R O U G H
Figure b
40 CENTRIFUGAL O R l E R
Figure 7
but, to give an idea of the relative proportions, it might be regarded as a handful per pailful of mortar. The brick should be so laid up as to avoid through courses. To accomplish this, “splits” may be used, but these should be kept a t a minimum. The bricks should be dipped into the mortar, which should be smooth and of the consistency of pancake batter, and no mortar should be placed with a trowel. I n laying, the bricks should be hammered into place in order to get the interior of the wall tight regardless of irregularities in the wall faces. Cut bricks should not be used. The brickwork laic1 up each day in mortar should be thoroughly washed down with 60” €36. sulfuric acid a t the close of that day’s work. All arches should be built around a form. Keep holes, three on each of two sides and two on each of the other two sides, are to be provided a t the base of the tower as shown. Leadwork may be flanged to the brickwork RR shown. The purpose of the distributing box, supported by the nnlls of the toner a9 indicated in the diagram, is to distribute the acid over the whole area of the toner. The acid in!ets to the towar from this distributing box shauld be luted t o prevent e-(-apeof vapor fumes to the building. These fumes will then be carried out through the take-off to the atmosphere. f 3 . 4 ~ IToR-The ~ ~ lumber for the construction of the saturator (Figure 5 ) should be southern longleaf yellow pine of “select structural material” grade. The inside is lined with 1/4-inch asbestos lumber and with 24-pound lead as shown. 111 joints should be cut to fit and the bevels on bearing timbers should be so cut that the 4 by 10-inch lining planking has full bearing across the whole beveled face. All bolts should be placed with care and countersunk so that the heads of the bolts are either flush or slightly below
772
I N D U S T R l A L A N D ENGINEERING CHEMISTRY
Vol. 19, No. 7
I S D G S T R I 9 L d S D ESGIYEERING CHE.VISTRY
July, 1927
the face of the timbers on the inside walls of the tank. The ‘/(-inch asbestos lumber is to be placed in sheets as large as possible with tight joints and secured to the m-alls of the absorber with large-head roofing nails. The lead lining should be so placed that there will be as few joints as possible. The cover of this saturator may be either of cast iron or of wood, lead-covered. The ammonia distributor may be of 30-pound lead with a wrought-iron pipe collar to reenforce the point of flange. The ejector should be of Duriron or equivalent material. The steam nozzle should fit snugly through the floor of the absorber and be caulked tight with lead wool. The steam line to this ejector should be of lead pipe to a point above the liquor level of the saturator. As shown in the sketch, there is provided OIL one side a “level box,” which is open and by means of which the level of the liquor in the saturator may be noted. This open box also provides an entrance to the saturator of the acid and of the return liquor. TROUGHS AND SETTLING BoxEs-Both the troughs and settling boxes may be constructed of wood, lead-lined, with the lead protected by acid tile as shown in Figure 6. This tile may be held in place by lead laps as indicated, but need not be liquor-tight. Its purpose is to protect the lead against mechanical wear by movement of the sharp sulfate crystals and the raking of the crystals with either a wood or metal hoe. The lining may be of 16-pound lead, using 12-pound lead laps to hold the tile in place. The outlet gates for the settling box and the chutes connecting the box with the centrifugal driers may be of copper.
Chromium Plating-A
773
CENTRIFUGAL DRIER-The centrifugal driers may be the 40-inch engine-driven suspended type, as shown in Figure 7. The steel cases should be lined on the inside with 8-pound sheet lead. The baskets should be of perforated copper walls with top ring and bottom of bronze, and with reenforcing hoops of copper, The bottom discharge is to be closed with a bronze cone valve with a copper sleeve around the driving shaft. The liquor outlet should be a lead pipe burned to the drier lining. Such a drier is easily obtained on the market. MISCELLANEOUS EQuIPmxT-The rotary drier might preferably be of the steam-heated air variety, such as is manufactured by a number of concerns in this country. The conveyor belting should be of 3-ply center, 4-ply edge, with ‘/*-inch rubber cover on the carrying side. Troughing idlers should be spaced about 4 feet 6 inches on centers, with return idlers spaced about 10 feet on centers and top-guide idlers spaced from 20 to 30 feet on centers. The drive pulley should be rubber-covered, while the take-up pulley may be plain with ball-and-socket protected screw. The conveyor to storage should be provided with a weightometer and an automatic self-reversing tripper for discharging the material automatically to any predetermined point in the storage. The Plant
This equipment might be assembled into the plant as pictured in Figure 8. Such a plant would be available for the production of either sulfate of ammonia or phosphate of ammonia without alteration or for both at the same time with but slight alteration.
New Aid to Industry’
By D. H. Killeffer, Associate Editor
M
UCH interest and not a little unnecessary mystery have centered lately around the electroplating of metallic chromium, a metal long known and long used in industrial alloys but only now commercially available as a pure metal. As long ago as 1854, Bunsen succeeded in electrodepositing chromium from solution, but nothing remotely approaching industrial application of this remarkably useful metal came into being until within a very few recent years. It is one thing to form an electroplate of chromium and quite another to make such a coating smooth, adherent, and commercially useful, as many failures in the past have clearly demonstrated. The utility of present chromium coatings and the methods by which they are obtained form the subject matter of this article. It is hoped that by a careful consideration of facts the mystery and unfounded rumors so widely current may be somewhat dissipated and chromium‘s true place in industry more clearly defined. Properties of Chromium
The properties of chromium which make it particularly useful to industry are its extreme hardness and resistance to abrasion, and its ability t o withstand many of the ordinary agents of corrosion, including oxygen a t high temperatures and superheated steam. Not only does the metal itself possess these properties to a remarkable degree, but a comparatively thin electroplate of it on an unresistant base metal imparts these properties to the combination to a useful extent. HARDNESS-The hardness of chromium on bloh’s mineralogical scale is stated as 9, which places it in the class of Received February 8, 1927.
emery and far above any other known metal. This figure is not necessarily absolute, as the result of a test depends largely on the method of determination used, but the fact remains that chromium is harder than iridium and the hardest of steels. I n electroplating it some variations in hardness are possible according to the conditions of plating, but it is easily possible to realize its full hardness in a plate. One hears of tests made on very thin plates of chromium, over a soft base metal, which can be broken by a file, but a sufficiently strong base supporting a reasonably thick film will show a hardness greater than many gem stones, approaching that of sapphire and ruby. WE~R AND BRASIO ION REsrsTAscE-Related to its hardness, but not necessarily inherent in it, is chromium’s resistance to wear and abrasion. Even very thin films exhibit this property to a remarkable degree and metal parts subjected to the destructive forces of sliding wear can have their lives increased many fold by protection with a chromium plate. Such parts as bearings in automotive engines, engravers’ plates, and fine gages have had their useful lives prolonged as much as four to eight times by chromium films less than a mil in thickness. CORROSION REsIsTascE-chromium is decidedly resistant to a variety of chemical reagents, but it is not a panacea for all ills. Perhaps no other field is so subject to misunderstandings and misstatements by the overenthusiastic as that of corrosion resistance. The natural optimism of some one vitally interested in the success of a particular material for this purpose not infrequently leads to such overstatements of the case and the possible user becomes so involved in halftruths of nothing more than nuisance value, that facts of
