CLIFFORD A. HAMPEL Fansteel Metallurgical Corp., North Chicago, 111.
Tantalum Makes Some Products Feasible Tantalum, because of its immunity to acids and its excellent heat transfer properties, makes possible products of higher quality and greater uniformity at reduced cost
T A N T A L U M has been called the “most nearly perfect” material of construction. I t differs from most materials used in the construction of chemical equipment in that it is not merely corrosion-resisting, but, in the uses for which it is recommended, acidproof. I t is completely immune to most acids, even at high concentrations and
boiling temperatures. Tantalum is strong; it is an excellent heat conductor; it is not damaged by thermal shock. Because it is an elementary metal, not an alloy, tantalum always has uniform chemical properties. Tantalum has a density of 16.6, about twice that of steel, and it melts at 3000’ C. (5432’ F.) -definitely a refractory metal.
The use of tantalum as a material of construction in chemical equipment began about 1930 and quickly became the most important use of the metal. The chemical properties of tantalum are similar to those of glass. Likglass, it is immune to attack by almost all acids except hydrofluoric. Reactions or operations conducted in the laboraVOL. 48, NO. 1 1
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tory in glass equipment can be transferred to plant operations in tantalum equipment with complete assurance of freedom from corrosion, from product contamination, or from undesired side reactions. Tantalum is not attacked by such materials as nitric acid, hydrochloric acid, aqua regia, perchloric acid, chlorine, bromine, hydrobromic acid or any of the bromides, phosphoric acid free of the fluoride ion, nitric oxides, chlorine oxides, hypochlorous acid, organic acids including monochloroacetic, and hydrogen peroxide. I t is attacked, even at room temperatures, by strong alkalies, hydrofluoric acid, and free sulfur trioxide, as in fuming sulfuric acid, and it is attacked slowly by concentrated sulfuric acid a t temperatures above 150" C. However, the corrosion by hot, concentrated sulfuric acid is uniform, and a t temperatures as high as 200° C. tantalum can be used successfully with 9SY0 sulfuric acid (Table I). Table I.
Corrosion of Tantalum in
98% Sulfuric Acid Temp., O C, 21 145 175 200 250 300
Tantalum condensers for processing a corrosive and heat-sensitive aromatic
1 980
INDUSTRIAL AND ENGINEERING CHEMISTRY
Corrosion L o s s , Inch/ Year 0.0000 010000 0.0001 0.0015 0.029 0.342
Tantalum is resistant to reaction with most of the so-called liquid metals. I t is not affected by lead, bismuth, lithium, sodium, or sodium-potassium alloys a t temperatures to 900" C., by mercury a t temperatures to 700" C., nor by gallium a t temperatures to 450' C. This has attracted some attention to tantalum as a material of construction for nuclear energy power systems where liquid metals are used as heat transfer media. The use of tantalum in the chemical industry, which has increased nearly 130070 since 1939, is based primarily on the immunity of this metal to corrosion or chemical attack under temperature and pressure conditions usually found in industry. The weldability, ductility, and good strength of a metal whose physical properties are approximately those of steel permit it to be fabricated into the required forms and shapes. There are many nonmetallic materials which will withstand attack by many of the reagents against which tantalum is effective-glass, glassed steel, ceramics, impregnated graphite, rubber, plastics, and others-but each of these has physical limitations, particularly as regards heat transfer. It is as a heat transfer material that tantalum finds its greatest usefulness in the chemical industry. In fact. it is very frequentlv
SPECIAL M E T A L S - E Q U I P M E N T - P R O D U C T S used in conjunction with these other materials.
Tanfalum-Improved Processing There may not be a product on the market that could not be made without tantalum equipment, but there are products that are made to a better standard of quality and purity, to a higher degree of uniformity, and at a substantially lower cost because they are processed in tantalum equipment. I n 1935, a producer of ammonium chloride for dry battery mixes sought to improve the quality of his product in order to extend the shelf-life of batteries. The synthesis had been conducted in silver equipment, but traces of silver in the product caused a small but definite reaction in the battery cells. A shelland-tube heat exchanger containing 174 seamless tantalum tubes with an effective heating area of 187.5 square feet was adopted. With 150-pound steam this unit transmits approximately 4 million B.t.u. per hour. Metal contamination of the product was entirely eliminated by the use of tantalum. The cost of this heat exchanger in 1939 was approximately $128 per square foot of tantalum. The current cost of a similar unit, because of higher costs of material and labor-the cost of tantalum is lower than it was in 1939-would be about $176 per square foot. Thus, while the general cost index of chemical plant equipment is reported at 237% of 1939 prices, the corresponding index for this tantalum heat exchanger is 137%. Ammonium nitrate is another product that has indirectly been improved through the use of tantalum equipment. This product was once a very hygroscopic crystal that was difficult to store and pack, but a flake form of ammonium nitrate, which is marketed as a commercial fertilizer, is now produced by the Stengel process. In the Stengel process, 42 to 50% nitric acid is neutralized with anhydrous ammonia a t 400' to 460' F. Heat from the reaction drives off water introduced with the nitric acid. The acid is heated in two stages before the reaction. A type 304L stainless steel preheater heats the acid to 175' F., and tantalum heat exchangers are used to bring the temperature to 330' F. before the acid enters the reactor. There are few metallic materials, among them platinum, which could be used with nitric acid a t this temperature. The preparation of one rather valuable aromatic, used for cosmetics, soaps, etc., involves the distillation of a high boiling acidic organic material. The material is not only very corrosive but is also heat sensitive so that evapora-
tion and condensation must be accomplished very quickly. This excludes the use of glass or other nonmetallic material. The corrosive nature of the material and the danger of contaminating the product rule out the so-called corrosion-resistant alloys. The material is boiled a t 260' C. with a tantalum bayonet heater in an impregnated graphite tower, using electrically heated Aroclor (trade-mark, Monsanto Chemical Co.) as the heating medium. The wall thickness of the tantalum heaters is only 0.015 inch, so that heating efficiency is excellent and temperature can be very closely controlled. Condensation is carried out in tantalum condensers, 0.015-inch wall thickness, under a vacuum of 2 to 3 mm. of mercury. The product is made a t a rate of 400 to 500 pounds per hour, which would be difficult in nonmetallic equipment because of temperature lag and thermal shock.
Heat Transfer Equipment Design The bayonet heater is one of the most widely used types of tantalum heat transfer equipment. I t consists of a flanged tantalum tube closed a t one end, a slotted steel inner pipe to admit the steam, a tee for steam and condensate, a flange for connection to the vessel nozzle, and necessary gaskets. The bayonet heater can be inserted in a horizontal or vertical position in the vessel. I t is built with either a single tube or multiple tubes on a single flange. One of the largest currently in service consists of 19 tubes with a total heating area of 11.2 square feet and is capable of transmitting 2,500,000 B.t.u. per hour or 223,000 B.t.u. per square foot per hour. This is the equivalent of the evaporation of 30 tons of hydrochloric acid per day. Bayonet heaters have been designed to be used with steam pressures as high as 190 pounds per square inch gage; the most commonly used pressure is 150 pounds per square inch gage. The temperatures corresponding to these pressures, 384' and 366' F., respectively, result in very high temperature drops when applied to the heating or boiling of most aqueous or organic liquid systems. Coupled with the very high overall heat transfer coefficients attained by thin-walled tantalum units, the result is a most efficient heat transfer device. The well-known coil-type heater can be fabricated of tantalum for use in deep kettles or tanks. These heaters are almost always designed for use with steam a t 150 pounds per square inch and have been made in sizes as large as 77inch loop diameter and a total of 167
feet of deveioped length of tubing. In some cases coils have been mounted on a tantalum-protected flange to be attached to an opening in the bottom of a vessel, thereby making all of the tantalum tubing available for heat transfer. For use chiefly in open tanks of the type used for plating and pickling, the U or hairpin loop heater can be made of tantalum. These often are used in chrome-plating baths where their long life makes them economically attractive. Several kinds of shell-and-tube type heat exchangers are built of tantalum. The shell-and-tube exchanger can be designed to handle the corrosive material either inside the tubes. in which case the shell can be made of steel, or outside the tubes, in which case the shell can be made of suitable corrosion-resisting material that need not be a good heat transfer material. They are built of straight tubes held between two fixed tube sheets or of U-bends with the tube ends fixed in one tube sheet so that, with proper manifolding, the liquid in the tubes enters and leaves at the same end of the exchanger. This latter design is valuable for long tube exchangers where the effect of the differences in thermal expansion coefficients of tantalum and other metals becomes important. The coefficient for tantalum is one half that of steel. Shell-and-tube heat exchangers can be made in sizes from the single-tube type to those containing several hundred tubes. Plate heaters made of tantalum soon will be commercially available. Inherently lower in cost than tubular units, they also will combine the several attractive features of plate heaters, such as high surface to volume ratio, ease of handling and cleaning, and effective geometry with the high heat transfer coefficients obtained by all tantalum heaters. For heating slurry containing hydrochloric acid in a glasslined vessel it has been determined that three tantalum plate heaters each 1 X 2 feet are equivalent to 70 square feet of jacket surface. The cost of these heaters is the same as the cost of the jacket. Tantalum condensers of shell-andtube design in tapered construction are widely used. The tapered construction takes advantage of the decreasing volume as the vapors entering the larger end of the condenser are condensed and permits the economical, efficient use of tantalum as the condensing surface. Heat transfer rates of 120,000 B.t.u. per hour per square foot of watercooled surface are commonly obtained when condensing steam. RECEIVED for review- April 23, 1956 ACCEPTED September 12, 1956
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