Improvements in Concentrating Sulfuric Acid M . J . Kalous PARKFIELD WORKS, STOCKTON-ON-TEES, ENGLAND
C
in a correctly designed and well-operated equipment dealing with acid free from organic matter should not exceed 0.5 per cent. The disadvantage of these processes is poor heat transfer from the hot products of combustion to the acid through the thick cast-iron walls of the pots or pipes, resulting in relatively small plant capacities. Another drawback is that the size of the pots which can be satisfactorily cast is limited, the largest having a capacity of about 20 tons sulfuric acid (100 per cent) per day. The life of the pots may be two years or less due to corrosion, effects of heating, and fractures. I n processes working under vacuum, large units can be built of brick-lined mild steel and strong acid can be obtained with fairly low acid losses. The disadvantages are the high cost of the heating medium in the form of live steam supplied from the outside, the cost of a large amount of water required to maintain a vacuum, and the low boiling point of acid under vacuum, which in most cases is not high enough to decolorize the acid. I n processes where acid is brought directly into contact with hot products of combustion and/or air, the heat transfer from the gases to the acid is good; consequently the size of equipment is relatively small for a given capacity compared with the indirectly heated systems. Large units (100 tons sulfuric acid per day or over) can be built of mild steel lined with lead and acid-resisting brick. The life of the equipment may be as long as ten years since the hot acid is in contact with high-quality acid-resisting brick. These processes have one great disadvantage, however-the formation of acid mist which is extremely difficult to deposit and which passes with the stack gases into the atmosphere, causing acid losses and a nuisance in the neighborhood. If the stack gases contain some sulfur dioxide due to decomposition of acid by organic matter, it cannot be recovered economically and must be considered as loss. It is usual to remove the acid mist in electrostatic precipitators, but the separation is incomplete and the stack gases usually retain some acid mist which causes a severe nuisance if acid is worked to high concentrations. I n practice sulfuric acid is not usually concentrated to higher than 92-93 per cent H2S04 by the process of direct contact with hot gases. There is an additional problem of maintaining electrostntic precipitators due to corrosion of electrodes and insulating seal boxes
ONCENTRATED sulfuric acid is used in large quantities for the production of such nitro compounds as picric acid, nitrobenzene, pyroxylin, etc. ; for the purification of mineral oils such as petroleum; for the manufacture of fatty acids, etc. These industries have the problem of disposing of waste products containing acid, not only for the economical recovery of as much of the acid as possible, but also to comply with regulations that prohibit the discharge of acid into sewers and of acid fumes into the atmosphere. I n England the amount of acid fume admissible into the atmosphere is limited by law to the equivalent of 1.5 grains sulfur trioxide per cubit foot of stack exit gas. I n the manufacture of nitro compounds, the spent acid is diluted to about 70-75 per cent, and it has to be denitrated and concentrated before it is suitable for re-use in the processes. Undoubtedly petroleum refining produces the greatest quantity of acid waste (sludge acid), and various methods are employed for separating the acid according to the character of the sludge. I n certain cases the treatment is live steam a t 3 to 7 atmospheres pressure, and the separated acid ranges from 48 to 60 per cent; for heavy sludge an open-tank separation method may bb preferred, and the separated acid contains perhaps 40 per cent. I n each case the weak acid is concentrated,and used again. The following figures show the magnitude of the sulfuric acid concentration industry. The total consumption of sulfuric acid in the United States in 1941 amounted to about 6,000,000 metric tons (100 per cent), distributed approximately as follows: Petroleum industry Coal products Nitration Total
800,000 tons 500 000 700:OOO
2,000,000
Generally speaking, this consumption covers the make-up for acid which has been lost in the various processes; it is evident that important economies would derive from the reduction of acid losses. CONCENTRATION O F ACID FROM NITRATION
Dilute sulfuric acid can be concentrated after denitration by the application of heat indirectly in closed vessels under atmospheric pressure (pot still, flash-film type) or under pressures substantially lower than one atmosphere (Simonson-Mantius process), or by bringing the acid in direct contact with hot gases, such as products of combustion admixed with air (Kessler type, Chemico drum type, Gaillard tower). Processes based on indirect heating have the advantage that no inert gases come in contact with the boiling strong acid, there is no formation of acid fog or acid mist, and losses
Acid Mist Formation. When hot inert gases come in contact with boiling strong acid, they become saturated with water vapor and sulfuric acid vapor. They flow in countercurrent to the weaker and cooler acid, and cause acid vapor to condense and the weaker acid to become heated. However, part of the acid vapor condenses so quickly that 38'1
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Figure I .
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Concentrator w i t h Tube-Type Superheater
acid droplets are formed, minute in one aspect, but of enormous size in comparison with the gas and water molecules among which they are suspended. As their movement is slow, they have little chance to meet water molecules (separated by many more gas molecules), to be absorbed, and to form large drops of weaker acid which could be easily separated. I n the pot still process there is no inert gas in contact with boiling acid; acid molecules can easily meet water molecules, acid vapor can condense in the form of weaker acid, and no acid mist forms in dephlegmators attached to the pot stills. If heat could be transmitted directly to the acid by a hot gas in which the formation of acid mist is impossible, the advantages of both the pot still and the direct heating by gas processes would be achieved without any of the disadvantages. A suitable gas for this purpose is superheated steam'. It can easily be brought to a high temperature and, on parting with its superheat, can transmit heat efficiently to the acid by direct contact, while the formation of acid mist is not possible since no inert gas is present. Figure 1 illustrates one arrangement to apply the new process. Steam is circulated by a steam booster between the dephlegmator, superheaters, and concentrator. In the superheaters the steam is heated to a temperature between 600" and 700" C. and the superheated steam is brought into direct contact with the acid in the concentrator; it thus gives up a large amount of its superheat and causes the acid to boil. 1 Kalous, M. J., Brit. Patent 556322 (May 9, 1941), U. 9. Patent Application 378,470 (filed Feb. 11. 1941); South African Patent 325, Iran Patent 801, Egyptian Patent 143 (1941): Canadian Patent 479,627, Australian Patent 114,755(1942).
The steam and acid vapors leaving the concentrator flow countercurrent to the weak acid in narrow channels, the acid vapors condense, and the weak acid is heated and concentrated to a certain extent without the formation of acid mist, as there are no inert gases. In the dephlegmator the condensing of acid vapors and the heating of weak acid continues, and the vapor leaving the top of the dephlegmator consists entirely of slightly superheated steam free of acid vapors, the partial pressure of sulfuric acid up to 140" C. being practically nil and free of acid mist. Any entrained acid droplets are removed in a droplet separator filled with ring packing. The vapor leaving the separator consists of steam only, and this is passed back to the superheater by the steam booster. An amount of steam corresponding to the dilution water evaporated from the weak acid can be released to atmosphere through a vent fitted with a nonreturn valve, or be condensed in a suitable condenser. The concentrated sulfuric acid flows to a receiver-cooler. The superheaters may be heated by products of combustion of gas, oil, or solid fuel; air required for combustion can be preheated by waste gases leaving the low-temperature superheater. Steam and air connections are provided for purging the equipment and for starting up. A by-pass line for hot steam facilitates the adjustment of steam temperature in the steam booster in order to prevent steam condensation and corrosion of the booster. Superheated steam in direct contact with dark colored, boiling, strong sulfuric acid exerts a decolorizing effect on the acid which is an additional advantage of the process.
Heat Requirement. It is important to know the degree of steam superheat required for a given acid concentration. The temperature of superheated steam in equilibrium with boiling sulfuric acid of, say, 95 per cent HgS04, can be calculated as follows: Let t
=i
X
290'
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April, 1943
= C. =
temperature of steam evaporated water from acid boiling point of 95 per cent acid
The vapors in equilibrium with boiling 95 per cent acid contain about 55 per cent H?SOd and 45 per cent HsO by weight, or 18.3 per cent H2SOd and 81.7 per cent HzO by volume. For each kg. of superheated steam the total water vapor over the acid = 1 x kg. and evapd. HzS04 1'22 ( ' 4X
1.22 (1
-
=
+
(1
55 + X) 2 = 1.22 (1 + Z) kg,
$; x = 0.069 kg. evapd. HIO
+ x) = 1.3 kg. evapd. HzSOl
heat of evapn. of HsO (300' C.) = 348 Cal./kg, heat of evapn. of HzSOc (300" C.) = 122 Cal./kg. One kg. of steam (specific heat 0.5) must supply 0.069 X 348 = 24.0 Cal. 1.3 X 122 = 158.5 Cal. = 182.5 Cal. Total ( t - 290)0.5 = 182.5 steam temperature t = 655" C.
Figure 2 .
The temperatures of steam calculated for equilibrium with boiling acids of various strengths are: Acid Strength, % ' 96 95 94 93 92
Approx. Steam Temp., 780 655 555 485 420
'C .
The high steam temperatures for concentrations over 95 per cent H&04 could be achieved in superheaters working on the regenerative principle of direct heat exchange on refractory materials (Figure 2). The quantity of heat required for heating weak acid, evaporating water, and dissociating water from the acid is constant; the fuel consumption must, therefore, be similar for all processes, provided the stack gases leave the equipment a t equal temperatures. I n the new process, however, combustion gases do not come in contact with the acid; they need not, then, be particularly clean but can be obtained by solid fuel firing. Water is used in all processes for cooling the concentrated acid in receivers, and its consumption is identical in all cases. Power consumption will be similar to that of the direct heating by gas processes. CONCENTRATION OF ACID FROM OIL PURIFICATION
The usual oil refining sludge contains a large amount of oil which must be separated. This is achieved by diluting with water and agitating with steam and air; alternatively, the sludge and water, properly proportioned, may be pumped
Concentrator w i t h Regenerator Superheater
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into a vessel in which the mixture is treated with steam at 3 to 7 atmospheres pressure, the acid oil and sludge acid being discharged continuously. In some cases the sludge acid, after the separation of acid oil and tar, is subjected to heat treatment under pressure for the purpose of eliminating hydrocarbons. Despite this treatment, the separated acid is often contaminated with carbonaceous matter which, a t the high temperature during concentration, tends to decompose some of the sulfuric acid and liberate sulfur dioxide. As the separated acid may contain up to 1.26 per cent hydrocarbons and these can destroy (in the extreme case) up t o nineteen times their weight of sulfuric acid, it is possible for acid losses to rise as high as 24 per cent. In processes operating by direct contact of acid with hot inert gases, when concentrating from 40 to 95 per cent H2SO4,the stack gases contain about 80 moles HzO to 276 moles air plus combustion gases per 1000 grams H2SO4 (100 per cent). I n the superheated steam process 80 moles HzO are evaporated from the weak acid and leave the dephlegmat or. iissuming the maximum acid decomposition through action of hydrocarbons to be 24 per cent, the liberated sulfur dioxide would be 2.45 moles and the stack gases, even with this considerable decomposition of acid, would contain only
+ +
2762'46 80 loo 2.45
=
0.69% SO, by vol.
which a t such low concentration could not be used for manufacturing sulfuric acid and mould be lost. Using superheated steam, however, the conditions are different. Steam leaving the concentration equipment and containing 2'45 80
loo= 2.97% SO2 by vol. + 2.45
can be condensed in a reflux cooler placed on top of a separating column (Figure 2 ) . The temperature a t the bottom of the column is maintained near 100' C., and that in the reflux cooler close to 20" C., so that effective separation of sulfur dioxide from water is achieved. Water at nearly 100" C. will leave the column practically free of sulfur dioxide and concentrated sulfur dioxide gas will leave the cooler a t approximately 20" C.; it is available a t little more than the cost of the cooling mater if needed for sulfuric acid manu-
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facture. Losses by mist formation and decomposition of acid by action of oil can be reduced to negligible figures in the superheated steam process; only those remain which are connected with the separation of sludge acid from acid oils and tar. Design and Construction. Steam can be superheated on the principle of continuous heat exchange as indicated in Figure 1,perhaps preferably with a superheater in two sections. The low-temperature heat exchanger in which the temperature is raised t o 480" C. can be of mild steel, tube-and-shell construction. No condensation of vapors will take place in this temperature range, and steel is not affected by dry steam. The high-temperature exchanger is made of special steel in U-tube form, steam passing through the tubes with temperature increase from 450" to a maximum of 700" C. The tubes are heated by combustion products at, say, 900" C., so that the temperature of the wall will not exceed 800" C., which is not excessive for stainless steel. The loosely hanging tubes can expand individually without causing trouble through unequal expansion. An alternative means of superheating steam 1% ould be to use two heat regenerators (Figure 2), similar in design t o those widely employed for blast furnaces (hot blast stoves). While one regenerator is being heated by passing hot products of combustion through it, the other is being cooled by passing the circulated steam through; the steam becomes superheated a t the same time. The flow through the regenerators is reversed a t regular intervals so as to maintain the heat stored in refractory material inside the regenerators. The concentrator and dephlegmator can be of mild steel construction, lined with asbestos, lead, and acid-resisting brick. The steam booster can be made of steel for hot runs, or alternatively of acid-proof iron or homogeneously leadlined steel for runs a t about 100" C. The cooler for steam which contains sulfur dioxide can be made of lead. Estimated Cost. A large unit using the new method costs slightly less, as compared with direct heating by gas methods or with vacuum methods, and considerably less than an equivalent number of pot still units. Estimated operating costs are similar to those of direct heating by gas methods; the new method has the advantage of producing stronger acid with negligible acid losses. As compared with vacuum methods, the operating costs of the new method are lower on account of fuel being much cheaper than the live steam supplied from outside.
Courtesy, Texas Gulf Sulphur Company
The United States H a s the World's Largest Deposits of Sulfur, the Raw Material f o r Sulfuric Acid
