Absorption of
Hydrochloric Acid F. L. HUNTER Fansteel Metallurgical Corporation, North Chicago, Ill.
of the temperature of the cooling water. Gas within the tube is cooled to approximately the same temperature as the acid, so that vapor carried by the gas is in equilibrium with the cool acid. Acid is separated from the gas as soon as absorption is complete, and removed from the system; while the gas, passing on to the tailing tower, contacts incoming fresh water, The main absorption chamber in this equipment is a tube made of pure tantalum, a metal which is completely inert to the action of hydrogen chloride a t any temperature up to 630" F. Not only is tantalum corrosion-proof, but, being a metal, it has all the advantage of high thermal conductivity, and with hydrochloric acid has heat-transfer characteristics which are remarkably good, even in comparison with other metals. Because tantalum is a valuable metal which must be used economically if equipment containing it is to be commercially practical, attempts to design tantalum absorption equipment along conventional lines were nullified by excessive cost. I n fact, the effort to conserve the use of tantalum led t o the design described herein, resulting in the development of a unit available a t a cost comparable with and in many cases less than the installation cost of a conventional system.
SIX-IXCH ABSORPTION COLUMN Capacity, 3000 pounds of 20' Baume acid per hour
T
Absorption S y s t e m
HE production of concentrated hydrochloric acid (35 to 40 per cent) from rich hydrogen chloride gas can be a
simple, trouble-free process when the following conditions are met: (1) removal of the heat of reaction or solution at the instant it is generated and at the exact locality of its generation; (2) intimate contact of the gas and the absorbing liquid; (3) cooling of the acid produced to a temperature below the boiling point for the particular concentration; and (4) removal of the cooled strong acid from contact with hot gas as fast as it is produced. The absorption system described in this article fulfills all these conditions. Instantaneous local removal of heat is accomplished b y making the absorption chamber of a thin metal tube, jacketed so that cooling water can be circulated around it. Intimate contact between the gas and liquid is secured by a distributor inside the metal tube. A thin film of acid falls without restriction down the inner surface of the tube and its temperature is maintained close to that of the cooling water in the jacket. Concurrent flow of gas and liquid plus a separate water-jacketed extension of the absorber tube, in combination with a gas outlet at the bottom of the absorption chamber but above the cooling extension, accomplishes rapid cooling of the acid to within a few degrees
The flow sheet (Figure 1) illustrates the system, which consists of an absorption column, a tailing scrubber tower, and accessory piping and controls which are shown in solid lines. The water supply line and valves and the cooling water drain, shown in dotted lines, are usually furnished by the user. The absorption column illustrated, known as Type A, is intended for use with gas containing at least 95 per cent of hydrogen chloride. The Type E absorber, for use with more dilute gas, is described later. Absorption is complete in one pass of the gas and liquid thiough the absorption column and scrubber. The gas is introduced at the nozzle in the top bonnet. Liquid to make the acid is introduced at the nozzle in the side of the top bonnet. Both gas and liquid flow concurrently down the inside of the tantalum tube, and absorption takes place in the falling film of water on the inner surface of the tube. There is packing in the scrubber tower, but none in the absorption chamber. With 100 per cent gas, in theory at least and in one case in actual practice, all of the absorption takes place in the absorption column and none in the scrubber. Where the scrubber tower functions continuously, the load is divided between the absorption column and the scrubber tower, so that each is utilized to full efficiency. The rated load is that at which the acid coming through the scrubber tower has a maximum concentration of 18 per cent. All heat developed in the scrubber tower is brought by the acid into the absorption column and is there removed by the cooling water. It is only when noncondensable components are in the gas that the scrubber tower is required to do any work. Aided by the notched distributor ring at the top of the tantalum tube, the hydrochloric acid forms a film on the inside of
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Gas with Tantalum Equipment I
I
\
I UPPER BONNE
, a
COOLING WATER r------\ I‘ \
\
’
7--1
4 II 1
COOLING WATER DRAIN
I I
J
HYDROMETER WELL
---__
LOWER BONNET
?ACID
DELI VERY LINE ACID T R A P
FIGURE 1. FLOW DIAGRAM OF TANTALUM ABSORPTION SYSTEM Type A. Gas and “make” water concurrently flow inside t h e water-cooled tantalum absorption tube. Exhaust gas is removed when the absorption cycle is complete and the product acid cooled t o a temperature well below the fuming point. Tailing gas 1s piped t o the tower where any remaining hydrogen chloride is absorbed by incoming “make” water.
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the tube, and is continuously cooled by water flowing in the opposite direction outside the tube. This cooling water is kept in a swirling motion by means of tangential nozzles in the jacket. The heated cooling water is removed through several outlets at the top of the column (only one shown in flow sheet). A number of water inlets are provided throughout the length of the column, so that the highest velocity of cooling water can be maintained at locations where the greatest amount of heat is generated. The water jacket surrounding the column is divided into two sections. The longer upper section is known as the absorption chamber, while the lower section is the acid cooler, or temperature conditioning chamber. A baffle in the water jacket separates these two sections, so that different rates of flow of cooling water can be maintained in the two sections.
U P P E R 6ONNET
GAS INLET I
’
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Inside the tantalum tube, a t a point corresponding to the baffle in the water jacket, is a second notched distributor ring which catches the acid descending from the absorption chamber and redistributes it over the wall of the temperature conditioning chamber, so that any channeling which may have occurred in the upper section will be avoided in the cooling section. Just above is the upper end of the gas separator tube, usually constructed of Haveg, which extends upward through the conditioning chamber slightly into the absorption chamber. Above this tube is a tantalum “umbrella” which prevents acid from falling into the separator tube, yet permits free passage of gas. By this method the third condition mentioned above is met. Absorption is stopped at a predetermined location, and the acid is permitted to cool without further contact with the gas. Product acid flows by gravity from the bottom bonnet through the traD and into the acid delivery line. R Pyrex hidrometer well, permitting continuous observation of the acid strength, is a useful aid in controlling the operation. The temperature of the product acid a t the hydrometer well is usually 4” or 5 ” F. above that of the cooling water. Tailing gas frsm the absorption column passes through the packed scrubber tower, where it comes into contact with fresh water, absorbing any remaining hydrogen chloride in the gas. The water from the scrubber, as shown in the drawing, is the “make” water supply for the system. Noncondensable gaseous material is vented from the top of the scrubber. Manufacture of strong water-white acid is no problem. Strength of the acid is controlled by the “make” water-regulating valve. The gas intake valve is usually kept wide open. Acid of 24” Baume (39.41 per cent hydrochloric acid a t 15” C., 69” F.) can be made with cooling water as high as 75” F. The system contains nothing which will discolor or contaminate the acid, so that the product acid is as pure as the gas and water from which it is made. To produce waterwhite acid, the iron content in the “make” water should not exceed 10 parts per million. Properly installed and normally maintained, the system does not leak gas or liquid. All joints are gasketed with corrosion-proof materials, and when the gas vent line from the scrubber is extended through the roof of the building, any obnoxious fumes, such as chlorine, can be carried away effectively. When correctly installed, the system may be operated safely in a room with the windows closed.
Effect of Diluting Gases
TAILING GAS OUTLET (TO SCRUBBER)
FOR DILUTE GAS FIGURE 2. ABSORBER
Type E. Similar to Type A, except that it includes a perforated gas distributor tube which brings fresh as into contact with the liquid during the entlre absorption stage and removea noncondensa%le gas and water vapor from contact with the acid during the cooling stage.
The equipment described is suitable for use with 100 per cent hydrogen chloride at full load rating, regardless of the concentration of acid produced. When there is more than 2 per cent of noncondensable material in the gas, problems arise which necessitate modifications in the construction of the absorption chamber and are intensified as the strength of product acid is increased. As long as the concentration of acid is limited to 20’ Baum6, gas containing as high as 20 per cent of noncondensable material can be absorbedwith the Type A column. When it is desired to produce stronger acid from gas containing 80 t o 95 per cent of hydrogen chloride, the Type E absorber is recommended. The essential feature of this unit, illustrated in Figure 2, is a gas distributor tube inside the tantalum absorption tube. The
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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gas distributor tube, which is connected at the upper end to the gas line, contains two series of perforations, the first occupying about 4 feet in the upper or absorption portion of the chamber. The supply of “make” water enters the absorber in the same manner as in the Type A equipment. The upper perforations are so made that the gas coming through them strikes the tantalum absorption tube a t an acute angle, and is deflected t o set up a whirling motion in the gas stream. By this means the gas is kept in turbulence, the time of contact is prolonged and vapor generated in the upper part of the tube is condensed in the lower portion instead of being carried over into the tower. The tube is plugged at the end of the first perforated section, but continues through the absorber and connects with the tailing gas line. The portion of the tube between the two groups of holes acts as a “core buster.” A t a point, just above the redistributor ring is a second group of holes through which noncondensable gas can pass out of the absorption chamber. The solid portion between the gas outlet and inlet ports serves to confine the gas in a relatively small space, thus maintaining velocity and turbulence. Not only is absorption promoted by this means, but the gas is cooled to a much greater degree than would be possible without it. The absorber is, of course, subject to and conforms to Henry’s law. The solubility of hydrogen chloride in water is exceptionally high. As soon as the concentration of the solution exceeds 20 per cent the vapor pressure becomes a factor, and the higher the concentration, the higher the vapor pressure becomes. For any particular acid concentration above 20 per cent, there is a critical temperature a t which the partial pressure of vapor equals the partial pressure of the gas. It is therefore necessary only to keep the acid below its critical temperature in order to secure the concentration desired a t atmospheric pressure. The presence of noncondensable components in the gas complicates matters, so that partial pressures of hydrogen chloride and hydrochloric acid or water vapor in the gas phase must be considered. Absorption will proceed rapidly a t atmospheric pressure and below the critical temperature as long as the resultant partial pressure of hydrogen chloride in the gas phase exceeds the partial pressure of hydrochloric acid vapor a t the surface of the liquid. This excess of partial pressures is called the “driving potential.” Another complication is the film of noncondensable gas stripped of its hydrogen chloride which accumulates a t the liquid surface. Rapid or continuous disturbance of this film or barrier is necessary to obtain absorption in the short period of time in which the gas inside the tantalum absorber tube can remain in contact with the liquid. It is to obtain these results that the gas distributor tube is used in the equipment when the amount of noncondensables exceeds 5 per cent. The upper bonnet construction in the Type E absorber, as shown in Figure 2, contains two nozzles, the smaller of which delivers “make” water from the scrubber, while the larger nozzle supports and centers the gas distributor tube. The inside diameter of the nozzle is made larger than the outside diameter of the gas distributor tube proper in order to permit observation of the flow of liquid over the distributor ringin the top of the tantalum absorption tube. The top row of holes or ports in the gas distributor tube is located opposite the bottom of the liquid distributor ring. By this means absorption is delayed until the liquid has reached a cooling surface, thus avoiding the generation of steam or water vapor. This feature is important, for if considerable amounts of water vapor are carried along in the presence of noncondensable gas, they tend to condense in the lower portion of the absorption chamber and heat the acid to an undesirably high temperature. Also, such vapors add to the volume of gas passing into the tailing tower and carry along additional hydrogen chloride. The Type E absorber, however, has been proved to be most effective in decreasing the load on the tailing tower and operating with stability when producing acid a t 22‘ to 24’
TANTALUM ABSORPTIONTUBE 6 inches in inside diameter 6 feet 6 inches in over-all height. Ribs in the tube 6dd strength and aid in the distribution of acid over the surface of the tube.
Baume (35.21 to 39.41 per cent) from gas containing as high as 20 per cent of noncondensable material. With 100 per cent hydrogen chloride, the use of the gas distributor tube is not essential; neither is it a detriment. As long as the gas remains 100 per cent, absorption is complete within the absorber column. The tailing scrubber tower comes into operation only during periods when air or other noncondensable gas is introduced, as a t the beginning of operation, or in case of accidental introduction of air with the “makeJ’water. The presence of even 1 or 2 per cent of noncondensable gas in the hydrogen chloride puts restrictions on the equipment, unless the Type E absorber column is used. The continual passage of the noncondensable gas into the air a t the bottom of the absorber causes a diffusion of hydrogen chloride and hydrochloric acid vapor into the “air plug.” This, of course, does not occur with 100 per cent hydrogen chloride gas. The presence of noncondensable gas causes a continual flow of gas and acid vapor from the absorption column to the tailing scrubber tower. This flow produces a condition different from that encountered when the noncondensable component is intermittent. When the flow is intermittent, the packed tower does not have an opportunity to reach a temperature balance, and considerable amounts of heat can be absorbed by the packing and “make” water before the boiling point is attained. With a steady flow of noncondensables, the tower reaches equilibrium, and the capacity of the system is fixed by the amount of heat generated in the tower. When the
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system is producing acid of a concentration lower than 20.5” Baume (32.38 per cent), the tower can be permitted to boil without fear of losing an excess of hydrogen chloride through the vapors, The reason for this is that the concentration of the tower acid is below the constant-boiling mixture, or 20 per cent; therefore a t temperatures up to 221” F. (105” C.), water instead of acid is evolved on boiling. When the same system is operated to produce acid stronger than 20.5” Baume a t the same load, the concentration of acid produced in the tower tends to become stronger than 20 per cent. A 3-inch Type A absorption column (without a gas distributor tube) working in conjunction with a suitable tailing scrubber tower has a capacity of absorbing 300 pounds of hydrogen chloride per hour in the form of 95 per cent gas, when producing acid a t 20” Baume (31.45 per cent). If, under the same conditions, the system is required to produce 22” Baume acid, the capacity decreases to approximately 200 pounds of hydrogen chloride per hour. Addition of the distributor tube restores the capacity rating to 300 pounds per hour a t 22’ Baume acid. As the percentage of noncondensable gas is increased beyond 5, the absorption capacity of the system drops off sharply until 8 per cent has been reached. Beyond 8 per cent, there is no appreciable change up to 20 per cent. The equipment described above, even using the Type E absorption column with the distributor tube, has a capacity of approximately 220 pounds of hydrogen chloride per hour when producing 20” Baume acid from 80 to 92 per cent hydrogen chloride gas. When 22” Baume acid is required, the capacity decreases to somewhere between 190 and 200 pounds of hydrogen chloride per hour. The same proportionate change in capacity holds true for systems of other ratings. For gas diluted with more than 20 per cent of noncondensable material, neither the Type A nor Type E system is recommended. Another tantalum absorption system is being developed for use with dilute gas, and will be the subject of a later paper.
Cooling Water The cooling water for absorption systems should receive careful consideration. Well water containing only calcium and magnesium salts can be used without treatment if its scaling teniperature is carefully determined and the flow through the absorber jacket is regulated to keep the outlet temperature well below the scaling point. Thermometer wells are provided in all outlet pipes, so that indicating apparatus may be easily installed. A high iron or sulfur content in the water may be a much more serious problem. Iron is particularly bothersome in water which is saturated with carbon dioxide and which, on release of pressure, may leave a funguslike deposit even on cool surfaces. The water space in the jackets is designed to maintain a reasonable velocity under operation a t half capacity and a fairly high velocity a t full capacity. The flow may not be uniform through the jacket, however; there is opportunity for such water to deposit scale, especially in the lower part of the jacket. Carbonate scale can be removed by flushing the jacket with inhibited hydrochloric acid, but such an operation should be done quickly and all acid drained and flushed from the jacket. Prolonged exposure will liberate hydrogen on the tantalum tube which may cause it to become brittle. Algae, fungus, and other live organisms can cause trouble. They are usually present in sluggish streams or canals, and if water from such sources is used, it should be chlorinated. A combination of filtration and chlorination is preferable to chlorination alone. Mud, sand, and slime should be kept out of all parts of the absorber system.
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By draining the jacket a t intervals through the hole in the lower flange provided for this purpose, cooling water troubles may be avoided. The Fansteel system, however, uses so little cooling water in comparison with conventional systems that the use of city or municipal water occasions no great expense. I n some localities where no other suitable water supply is available, it is practicable to circulate distilled water, treated water, or steam condensate through radiators for cooling. This scheme has the advantage over spray ponds that loss by evaporation is negligible. Several old automobile radiators equipped with motor-driven fans will provide all the cooling needed for a large-capacity system. Under no circumstances, however, should the water outlet tubes of the absorber be piped solidly into the circulating system. They should always flow into an open well, or, if evaporation must be reduced to the minimum, a well which is covered but provided with a vent to the air. This precaution is necessary to prevent the possibility of building up a pressure which might collapse the tantalum tube.
Automatic Control An automatic control of acid concentration is being developed by the company’s engineers. Observation of installed equipment has shown that as long as a small amount of noncondensable material is continually present in the gas, the tower temperature remains proportional to acid concentration regardless of the load on the system. This is understandable when it is remembered that the flow of “make” water fed to the tower remains proportional to the load for any given concentration of product acid. Thus, a temperature regulator installed in the tower vent can be used to adjust the “make” water supply to the tower. By this means, the “make” water is automatically regulated and fluctuations in the gas supply will neither overload the equipment nor produce acid which is too weak.
Operation The system is started by opening valves for both cooling and “make” water, then opening the gas valve wide. By observing the strength of the acid in the hydrometer well, the “make” water supply is adjusted to produce acid of the desired strength. The cooling water valves and cocks are adjusted to secure the proper temperatures in all parts of the water jacket and avoid exceeding the scaling point of the effluent. Indications that absorption is taking place are the steady maintenance of temperature in the water jacket and the flow of acid of desired strength through the hydrometer well. Fluctuations in acid concentration and in water temperature are indications that something is wrong. Such occurrences are usually accompanied by a rise in temperature in the scrubber tower and may be followed by escape of acid vapors from the tower vent. Unless some adjustment is wrong, such behavior is usually momentary, and is caused by a “slug” of air or very dilute gas. As soon as the flow of rich gas is established, the equipment reverts to its normal good behavior. When adjustments are once made and a steady flow of relatively constant strength gas is maintained, the system requires little or no attention beyond occasional observation of the acid strength.
Ratings, Capacities, and Dimensions Fansteel absorption systems are made in three standard sizes, and are rated according to the diameter of the tantalum absorption tube. The ratings given are for Type A absorb-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ers, using 100 per cent hydrogen chloride gas. Capacities for Type E systems, and for Type A with 5 per cent or less of noncondensable gas, have been set a t that load a t which less than 0.5 per cent of hydrogen chloride appears in the exhaust gas when producing acid of 20.5’ Baum6. The S-inch system is rated a t 300 pounds of gas per hour, the 4-inch system a t 500 pounds, and the 6-inch system a t 1000 pounds. The influence of dilute gases upon system capacity has been discussed above. In all three standard systems, the tantalum absorption tube is 6 feet 6 inches in length. The over-all height of the absorption column from the gas intake flange in the upper bonnet to the bottom of the lower bonnet is from 8 feet 7 . 5 inches to 9 feet 7 inches. The size of the tailing scrubber tower is dependent upon the nature, concentration, and rate of flow of the gas. Where flow is intermittent or subject to surges, the scrubber is made sufficiently large. Any of the standard systems can be installed in a floor space not more than 10 feet square, with head room between 14 and 23 feet. Special absorption systems are made for capacities either snialler or larger than those of the standard systems, or for particular conditions of gas or production.
Materials of Construction All parts of the equipment which come into contact with hydrogen chloride gas or acid are of materials inert to corrosive attack. Tantalum is used in the absorption tube, the tube sheets, the distributor rings, and the “umbrella” over the gas separator tube. I n Type E absorbers, the gas distributor tube is partly of tantalum and partly of Haveg. The upper and lower bonnets of the absorption column, the gas trap and water baffle, the gas separator tube, the scrubber tower, the “make” water inlet and tailing gas lines, and the acid trap are usually constructed of Haveg, although stoneware or rubber-lined metal may be substituted when desired. The water jacket of the absorption column may be of steel, Haveg, or rubber-lined steel. Gaskets are pure gum rubber,
costs The cost of Fansteel absorption systems, depending largely upon the size of the scrubber tower required, the material
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used in it and accessory equipment, and the nature and amount of accessory equipment, ranges from $5 to $10 per pound of 100 per cent hydrogen chloride gas per hour rating. The absorption column alone without other accessories ranges from $4 to $8 per pound of gas per hour rating. The cost range, however, does not change in direct ratio to the size of the units because certain elements of cost remain relatively constant, while others vary directly with size or capacity. Installation costs are decidedly lower than for most other absorption equipment. S o foundations or elaborate supporting framework are required. Cabinets or other enclosures or separate buildings are unnecessary. Recirculating pumps, pressure pipe lines, and acid coolers are eliminated. Even an elaborate installation should not add more than $2 per pound per hour to the cost based on the ratings mentioned above Little or no maintenance is required. Periodic inspection of the entire system, painting of the external metal parts, and replacing of gaskets when necessary is about all that can be done. The only parts susceptible to breakage are the hydrometer well and any ceramic pipes which may be included in the installation. Pump maintenance is absent, since the system operates by gravity. As there are no fumes when the system is functioning properly, surrounding structures as well as the absorber supports are not subject to 1 attack. Operating costs are very low. The system is economical in water consumption and uses no power. It does not require the continual presence of a n operator, nor any attention beyond occasional inspection. Labor costs, therefore, are practically nil.
Acknowledgment The writer wishes to make grateful acknowledgment t o the following for assistance in the compilation of the information set forth in this paper: E. I. du Pont de Nemours & Co., Inc., Heyden Chemical Corporation, Hooker Electrochemical Co., Halowax Corporation, Mathieson Alkali Works, Inc., Monsanto Chemical Co., National Aniline & Chemical Co., and Victor Chekical Works. RECEIVED September 2, 1938
FOUR-INCH ABSORPTIONCOLUMN This unit, which absorbs 500 pounds of 100 per cent’hydrochloric acid gas per hour, weighs less than 300 pounds and IS installed in a space of less t h a n 60 cubic feet.
