TWO EXCHANGERS. Each of these vertically hung exchangers contains 1350 tubes; they replace 180 feet of 6-inch cascade cooler piping
Sulfuric acid corrosion problems solved by heat exchangers with Teflon tubes Du Pont claims to have a solution to the corrosion problems that plague sulfuric acid makers. From Wilmington, Del., the word is, "switch to Du Pont heat exchangers with tubes of Teflon/' These exchangers combine the corrosion resistance of Teflon fluorocarbon resin with the efficiency of shell-and-tube design to bring a lower-cost cooling systems to sulfuric acid plants, Du Pont says. Last year 28,383,304 short tons of sulfuric acid were made in the U.S. and it is likely that if acid makers were polled, corrosion would be named as their single biggest problem. Moreover, such a poll would also likely show near unanimity in dislike of present cooling systems— the most vulnerable target for sulfuric acid corrosion. Perhaps their dislike would be emphasized with, "We don't like the mist, the fog, the acid leaks, costly maintenance, and the down time." Conventional coolers (cascade cooler pipes) are made of cast iron. As such, they have been and are a thorn in the flesh. Why? The ease with which they fail, because of corrosion, results in costly maintenancean annual expense so characteristic of the industry that it can be predicted before ground breaking ceremonies are held at a new plant site. If Du Pont's solution to this problem is accepted it might well be the start of a multimillion-dollar per year business, in an industry growing at a rate of about 5% annually. The company's acid-resistant units are single-
pass, parallel flow exchangers. The shells are made of 304 stainless steel or carbon steel with or without linings of Teflon or corrosion-resistant coating. The tube bundles are made of straight tubes which are joined together in integral honeycomb tube sheets. Saving money in the sulfuric acid industry depends primarily on the difference in initial investment that may come about by using the new smaller, more compact heat exchangers, says William H. Darnell, research and engineering manager of Du Pont's heat transfer products division. For example, at Du Pont's contact sulfuric acid plant in Watson, Ala., two 6-foot, vertically placed exchangers have been installed to replace 180 feet of 6-inch (o.d.) cascade cooler pipes. Today, Mr. Darnell explains, it costs about $2.6 million to build a 1000 ton-per-day sulfuric acid plant. The difference in cost between installing a Du Pont cooling system and a conventional one is $122,600 and $150,500 for a 10-p.s.i. pressure drop system and a 20-p.s.i. pressure drop system, respectively. If the Teflon unit is used, the savings may run from 7.8 to 9.5%, according to Mr. Darnell. The initial installation savings are realized in part because the new units can be mounted on structural steel, thus eliminating the need for large concrete basins and pits for water collection—enclosed by fences— which are common to conventional
units now in service around the world. In addition, with the shell-and-tube unit, cooling water can be used in succession through the absorber and drying towers. This common use of cooling water, in series, reduces the investment needed in cooling water facilities. On the other hand, he adds, replacing old units with new ones from Du Pont involves a different degree of savings because an initial investment has already been made. Thus, putting in the Teflon heat exchangers as "plant change" items would probably cut the savings by a factor of three. However, rejuvenation is not going to be overlooked, because environmental problems associated with sulfuric acid plants and tougher legislation against sulfur dioxide pollution will cause it to figure significantly in market considerations, Mr. Darnell says. Besides savings which result from the use of less space and less water, Du Pont maintains that its system results in lower maintenance costs because the new units can be repaired in line without stopping production. Because of increased space available with the change to small, compact units, increased capacity is also possible. A 6-foot exchanger, for example, containing about 2550 small tubes, has a higher area-to-volume ratio and thus about twice the heat transfer area of a conventional unit. The exchangers can also be used as dilution coolers. The auxiliary equipment—for the Teflon units, primarily filters—is cheaper than concrete pits, basins, fence enclosures, and the like associated with conventional coolers now in service. As for overseas markets, Mr. Darnell says, "We think the potential abroad will be on the same order of magnitude as in the U.S." He goes on to note that sulfuric acid is closely tied to man's needs, and that its growth will therefore always be worldwide. To date, however, the heat transfer products division at Du Pont has not been swamped with requests for the Teflon heat exchangers—from domestic or foreign sulfuric acid producers. And, for that matter, not even from the company's own plants has there been a demand for new tower acid coolers. One possible explanation for this seeming lack of enthusiasm is that producers feel that constant maintenance is cheaper, in the long run, than shutting down to effect a complete changeover in cooling systems. APRIL 7, 1969 C&EN 47
