Heat recovery from concentrated sulfuric acid

Abstract: In a process for the recovery of heat from concentrated sulfuric acid, sulfur trioxide is absorbed into hot concen­ trated sulfuric acid having a concentration greater than 98% and less than 101% and a temperature greater than 120°C. in an absorption zone (61) in a heat recovery tower (60) and the heat created by the exothermic reaction is recovered in a useful form in a heat exchanger (62).

Description: This invention relates to a process for the recovery of heat from a sulfuric acid plant. More particularly, this invention relates to a process for the recovery of the exothermic heat resulting from the absorption of sulfur trioxide into concentrated sulfuric acid. This invention also relates to a heat recovery tower which is used to recover the heat energy from concentrated sulfuric acid. DESCRIPTION OF THE PRIOR ART The process for the manufacture of sulfuric acid starts with a gas stream which contains sulfur dioxide. The sulfur dioxide is catalytically oxidized in a converter to sulfur trioxide which is removed from the gas stream in one or more absorption stages to form sulfuric acid. The oxidation of sulfur dioxide to sulfur trioxide is an exothermic reaction. To prevent the loss of this heat, steam has been generated in boilers, and low level process heat has been recovered by heating boiler feed water in economizers. After oxidation the gas stream containing the sulfur trioxide passes through absorption towers in which the gaseous sulfur trioxide is absorbed into concentrated sulfuric acid, having a typical concentration of 98%. In a modern sulfuric acid plant there are typically two absorption towers, designated as the interpass absorption tower and the final absorption tower, which are respectively located in the process upstream from and downstream from the final catalyst stage in the converter. In the sulfuric acid plants of today, the gas stream is cooled prior to entry into the absorption towers to maximize the recovery of energy from the gas stream. The absorption tower is operated at a temperature which is selected to facilitate the absorption of sulfur trioxide into the sulfuric acid, to minimize the corrosion of piping and heat exchangers that occurs at higher temperatures, and to minimize the formation of acid mist. The absorption of sulfur trioxide into sulfuric acid is a highly exothermic reaction, and large amounts of heat are lost to cooling water while maintaining the low temperatures in the typical absorption towers. The absorption tower is typically constructed such that the sulfuric acid flows downward through the tower and the enclosed packing material while the gas stream which contains the sulfur trioxide passes through the tower. The packing promotes contact between the sulfuric acid and the gas stream such that the sulfur trioxide is absorbed in the sulfuric acid. The acid drains into a pump tank, where water is added to dilute the acid to the desired strength. Both absorption and dilution are exothermic reactions, and the generated heat is removed in a heat exchanger which is typically located between the pump tank and the absorption tower inlet. The operation of the absorption tower is characterized by the concentration of the recirculated sulfuric acid, typically 98%, a low maximum acid exit temperature of approximately 120° C, and a typical acid inlet temperature of approximately 80° C. A lower acid inlet temperature would thermally shock the hot gas stream and often create an undesirable acid mist. A higher acid inlet temperature would increase the acid exit temperature and the corrosion of related piping and heat exchangers. It is thus known that the operating temperature of the absorption tower is set by and limited by considerations of the rate of corrosion of the equipment, and the undesirable formation of acid mist. The absorption tower has typically been constructed as a brick lined carbon steel tower to limit corrosion. Typically, cast iron or ductile iron pipe has been used around the absorption tower. Historically, a number of materials have been used for acid coolers. These include cast iron pipe or radiator sections, alloy C276 plate type heat exchangers, polytetrafluorethylene (PTFE) tank coils and stainless steel shell and tube heat exchangers. The cast iron coolers are limited in operating temperature to approximately 110°C by corrosion. They have poor heat transfer and occupy a large amount of space within the sulfuric acid plant. In addition, they have many mechanical joints which tend to leak and result in high maintenance requirements. Alloy C276 plate type heat exchangers can be cost effective relative to the cast iron coolers. However, this expensive alloy is limited in use to a maximum acid temperature of approximately 90° C; thus, the liquid exiting from the absorption tower at approximately 120° C is mixed with cold recycle acid before entering the heat exchanger. This reduces the thermal driving force and shows that the use of expensive alloys will not provide an easy solution to the problem of heat recovery from sulfuric acid. PTFE tank coils have been used to minimize corrosion. Small thin wall tubes, which are easily plugged, are required to obtain adequate heat transfer. The PTFE can withstand temperatures up to 200° C; however, in heat recovery applications its low mechanical strength limits the pressure of steam that can be generated. Thus, an intermediate heat transfer fluid is required for heat exchange with the hot sulfuric acid. A second heat exchanger is then required for heat exchange between the heat transfer fluid and steam; thus, this design is too expensive for use in this heat recovery application. Stainless steel heat exchangers, typically type 316 stainless steel, have been used as acid coolers. These require careful control of the acid temperature and the acid velocity in order to minimize corrosion. The more recent anodically protected stainless steel acid coolers have proven to be a reliable means of minimizing corrosion; however, practice has been to limit acid operating temperatures to less than 115° C. The equipment to provide the anodic passivation is expensive. Past practice with the aforementioned types of heat exchangers generally has been limited to rejecting the heat into cooling water or recovering heat in a low level form such as hot water for boiler feed or district heating. Efforts have been made in the past to recover the heat generated when the sulfur trioxide is absorbed into sulfuric acid. U.S. Patent 2,017,676 describes an apparatus for condensing sulfuric acid. Sulfur trioxide and sulfuric acid fumes are passed through a heat exchanger which has ceramic tubes to slowly and uniformly cool the gases from a temperature of about 350° C to about 140° C. The ceramic tube material is used in contact with the sulfuric acid to prevent corrosion; however, a metallic tube is used concentrically about each ceramic tube to prevent mechanical stress and breakage of the ceramic tubes. The cooling medium, a high boiling point oil or boiling hot water, is allowed to become heated to high temperatures such as would be present in a steam boiler. When operated in this manner, the patent states that approximately 1.5 tons of steam may be generated per ton of sulfuric acid and the sulfuric acid manufacturing costs may be reduced. British Patent 1,175,055 describes a method for the manufacture of sulfuric acid in which the gases are alternately passed through a catalyst bed to convert sulfur dioxide to sulfur trioxide and a heat exchanger/condenser in which the gases are cooled in the presence of water vapor to condense a part of the sulfur trioxide as sulfuric acid. The heat exchangers are lined with, or constructed of, materials which are resistant to corrosion by the hot, concentrated sulfuric acid such as ceramic materials and porcelain, metals such as steel coated with polytetrafluoroethylene or other corrosion resistant materials, or metals such as silicon-iron and nickel alloys. The heat created by formation of sulfuric acid and the heat released by condensation is utilized to create high pressure steam which may be utilized as a source of power. The British Patent also discusses recovery of the sulfuric acid in a more concentrated form. By employing a stoichiometric deficiency of steam during the intermediate condensations, it is possible to obtain sulfuric acid having a concentration of greater than 100%. Only in the final condensation after completion of the conversion of sulfur dioxide to sulfur trioxide is the remaining sulfur trioxide condensed in the presence of a sufficient excess of steam to insure that substantially all of the sulfur trioxide is removed from the gas stream. Both U.S. Patent 2,017,676 and British Patent 1,175,055 teach methods of recovering energy from the sulfuric acid process. However, both patents require the use of exotic materials of construction and emphasize the use of ceramics, porcelain materials, coated metals, brittle metals such as silicon-iron, and expensive nickel alloys for construction to prevent rapid corrosion and failure of the equipment. Blumrich et al U.S. patent 4,330,364 describes a process for strengthening dilute phosphoric acid using energy derived from a contact sulfuric acid plant. In one embodiment, heat is transferred from an H₂SO₄ absorber acid stream to a phosphoric acid stream without the use of any intermediate fluid. In an alternative embodiment, a pressurized water system is interposed between the sulfuric acid and phosphoric acid systems. In the latter system the pressurized water is said to be heated to 120° C by transfer of heat from 98.5% absorber acid which is cooled from 140° C to 120° C in the process. However, the reference contains no disclosure of the materials of construction to be used in the heat exchanger for transfer of heat from absorber acid to the pressurized water. Sander and Beckmann, "Concentration of Dilute Sulfuric Acid and Phosphoric Acid With Waste Heat", paper 25 of "Making the Most of Sulfuric Acid", Proceedings of the British Sulphur Corporations's Fifth International Conference, Part II: Additional Papers and Discussions; London, November 16-18, 1981, includes a flow sheet for an integrated sulfuric acid plant with a venturi reconcentrator unit which uses waste heat from a double absorption production unit. In this process, hot acid from each of the intermediate and final absorbers is passed through a heat exchanger which transfers heat to spent acid to be concentrated in the Venturi reconcentrator. In the case of the intermediate absorber, absorber acid having a strength of 98.5% is passed through the heat exchanger where it is cooled from 130° to 110° C by transfer of heat to the reconcentrator acid circulating stream. However, in the Sander system, the heat exchanger utilized contains Teflon rather than metal alloy tubes, and the recirculating acid is heated only to a temperature of 70°-80° C. In connection with the concentration of phosphoric acid, the Sander et al reference refers to the possibility of running the sulfur trioxide absorption system as high as 130° to 140° C to generate low pressure steam of 1.2 to 1.5 bar, and the use of such steam in a phosphoric acid vacuum concentrator. Sander et al. further report an actual installation of a system apparently patterned on the process of the Blumrich et al patent. In the latter system, Sander et al state that the sulfuric acid is cooled from 110° C to 90° C in the course of heating demineralized water up to 90° C. EP-A-0 130 967 claiming earlier priority (5.7.83) than the present application, but published after said priority date, discloses a process for the recovery of heat resulting from the absorption of sulfur trioxide into concentrated sulfuric acid. Sulfuric acid is manufactured by contacting in a heat recovery tower gas comprising sulfur trioxide with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating heat of absorption, the sulfuric acid being delivered to the heat recovery tower at a temperature greater than 120° C and sulfuric acid being discharged at increased concentration and at a temperature as high as 240° C. SUMMARY OF INVENTION The present invention is directed to an improvement in a process for the manufacture of sulfuric acid, the process comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid. In accordance with the improvement, a gas comprising sulfur trioxide is contacted with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, the absorption being carried out in a primary absorption zone, in a heat recovery tower to which said sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120° C, and from which said sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 150° C; the gas exiting from the primary absorption zone is contacted with sulfuric acid in a secondary absorption zone positioned above said primary absorption zone in said heat recovery tower, the inlet temperature of the sulfuric acid to the secondary absorption zone being at least 10° C cooler than the inlet temperature of the of the sulfuric acid to the primary absorption zone and the concentration of the acid entering the secondary absorption zone being at least 98%; and the heat of absorption is removed in useful form from the sulfuric acid exiting said primary absorption zone by transfer of heat to another fluid, and thereby heating said another fluid to a temperature greater than 120° C, the acid temperature being maintained at above 150° C throughout the heat exchanger. The process of the invention is implemented utilizing an apparatus for recovery of the energy of absorption of sulfur trioxide in sulfuric acid. This apparatus comprises a vessel containing an absorption zone in which sulfur trioxide is absorbed in a sulfuric acid stream, the absorption zone comprising means for promoting contact and mass transfer between a gas phase comprising sulfur trioxide and a liquid phase comprising sulfuric acid having a concentration of greater than 98% and less than 101% and a temperature of greater than 120° C. The apparatus further includes means for delivering sulfuric acid to the vessel for passage through the absorption zone, means for delivering gas comprising sulfur trioxide to the vessel for passage through the absorption zone, means for egress of sulfuric acid from the vessel after passage through the absorption zone, means for egress of the gas stream from the vessel after passage through the absorption zone, and a heat exchanger wherein the heat of absorption is recovered from the sulfuric acid in useful form by heat exchange with a third fluid. The heat exchanger comprises means for transfer of heat from sulfuric acid to the third fluid, the heat transfer means being fabricated of an alloy having a low corrosion rate when exposed to hot concentrated sulfuric acid. Such an apparatus includes a heat recovery tower having top and bottom inlets and top and bottom exits. The sulfur trioxide containing gas stream from the converter, after being cooled, enters the heat recovery tower through the bottom inlet and flows upward through the tower and the hot sulfuric acid stream enters the heat recovery tower through the top inlet and flows downward through the tower. At all points in the heat recovery tower and heat exchanger system the sulfuric acid has a concentration greater than 98% and less than 101% and a temperature greater than 120° C. The acid concentration is defined as being the weight percent of sulfuric acid. The counterflow of the gas stream and sulfuric acid maximizes the driving force for efficiently absorbing the sulfur trioxide into the sulfuric acid. Co-current flow of gas and acid can be utilized, but is less efficient. The absorption of sulfur trioxide into sulfuric acid is a process which is known to those having experience in the manufacture of sulfuric acid and will thus not be further described. This process will be referred to herein as the absorption of sulfur trioxide into sulfuric acid and the heat generated by the process will be referred to as the heat of absorption. The heat of absorption includes the heat liberated when water is added to dilute the recycled sulfuric acid, a process step which may occur within or external to the heat recovery tower. After the absorption of sulfur trioxide, the sulfuric acid stream passes through a heat exchanger wherein the heat of absorption is recovered through heat exchange with other fluids. It is desirable that the heat exchanger be fabricated from a metal to facilitate the transfer of heat from the sulfuric acid stream to other fluids. It has been discovered that by operating the heat recovery tower in a very narrow acid concentration range between 98% and 101%, and preferably between 98.5% and 100.0%, it is possible to absorb sulfur trioxide efficiently and to dramatically reduce the corrosion rate of certain alloys while operating at temperatures heretofore considered impracticable. It has been discovered that certain alloys exhibit excellent corrosion resistance in the concentration range previously defined, and at temperatures of 120° C or higher. Stainless steel alloys are generally superior to high nickel alloys. Excellent corrosion resistance has been found for certain iron/chromium alloys, iron/chromium/nickel alloys and nickel/chromium alloys having austenitic, ferritic or duplex structures. Stainless steels of such structure have been found especially suitable. Thirty alloys were tested at service conditions typical of the heat recovery system. It has been determined that the corrosion resistance of these alloys can be characterized in terms of the percentages of major alloying constituents. The alloys best suited for service in this heat recovery system contained iron, chromium, and nickel as the principal alloy constituents, and had compositions which gave a corrosion index (CI) greater than 39 CI>39 as defined by the following equation: CI = 0.35(Fe + Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) where: Fe = the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight percent of chromium in the alloy, Ni = the weight percent of nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy. In a conventional sulfuric acid plant, the heat of absorption of sulfur trioxide into sulfuric acid is lost to cooling towers. By use of the process and apparatus of this invention, a high percentage of this previously lost energy may be recovered and profitably used. The heat may be used, for example, to produce low to medium pressure steam for process heating or to power a turbogenerator for the generation of electricity. In a 2700 tonne per day sulfur burning sulfuric acid plant approximately 6 megawatts of additional electrical power can be produced from the heat recovered in the heat recovery tower. DESCRIPTION OF THE DRAWINGS Figure 1 is a process flow diagram of a sulfuric acid plant in which the process of this invention may be carried out after suitable modification of the heat recovery tower as discussed in the following. Figure 2 is a diagram which illustrates the corrosion rate of alloys and the degree of absorption of sulfur trioxide into sulfuric acid at a given temperature as the concentration of the sulfuric acid varies. Figure 3 is a diagram showing in greater detail the heat recovery tower used in the plant of FIG.1. Figure 4 is a graph showing the operating cycle of the heat recovery tower in relation to the operating cycle of a typical interpass absorption tower. Figure 5 is a diagram illustrating a further installation in which the process of the invention may also be carried out after modification of the heat recovery tower. Fig. 6 is a sulfuric acid temperature/concentration diagram having plotted thereon corrosion data and estimated isocorrosion curves for type 304L stainless steel; Fig. 7 is a plot comparable to Fig. 6 but for type 310S stainless steel; Corresponding reference characters indicate corresponding process equipment elements throughout the several views of the drawings. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 illustrates a process flow diagram for a sulfuric acid plant. The sulfuric acid process is well known; thus, portions of the sulfuric acid plant will not be described in detail herein. The drawing shows a sulfuric acid plant which burns sulfur to supply the gas stream containing sulfur dioxide to the sulfuric acid plant. Looking now at Figure 1, blower 12 supplies air through drying tower 14 to the sulfur burner 10 in which the sulfur is burned, to provide a gas stream containing sulfur dioxide. The sulfur dioxide laden feed gas stream exits from the sulfur burner 10 and passes through a first heat exchanger 22, before entering the converter 30. The feed gas is cooled in first heat exchanger 22 to a temperature near the desired inlet temperature to the converter. First heat exchanger 22 is used to generate steam for driving a turbogenerator 23 for the generation of electrical power, but other uses are also practical. Converter 30, a vessel for the catalytic conversion of sulfur dioxide into sulfur trioxide, typically has a plurality of catalyst beds which are divided into a first oxidation stage 32 and a second oxidation stage 34. Between any two catalyst beds there is heat exchange to remove the heat generated during the oxidation of the sulfur dioxide. These heat exchangers are not shown in Figure 1. In a typical sulfuric acid plant, between the first oxidation stage 32 and the second oxidation stage 34 the gas stream passes through an interpass absorption tower to remove the sulfur trioxide from the gas stream to provide a gas stream to the second oxidation stage 34 that is lean in sulfur trioxide. As the sulfur dioxide laden feed gas stream passes through the first oxidation stage 32, greater than 90% of the sulfur dioxide will be converted to sulfur trioxide. The oxidation reaction is a reversible reaction which approaches an equilibrium; thus, some of the sulfur trioxide must be removed from the gas stream to enable the remaining sulfur dioxide to be oxidized easily. Economizer 54 is used to cool the gas stream exiting from the first oxidation stage 32 to a temperature which is above the dew point of the gas stream. The sulfur trioxide in the gas stream is then absorbed into a sulfuric acid stream and heat is generated by the process. According to typical conventional practice, the absorption takes place in an absorption tower in which the acid temperature is maintained at a low level, to minimize corrosion of related piping and heat exchangers. However, the maintenance of the low temperature in the absorption tower makes it difficult to recover energy in an economically viable manner, that is, in a useful form. In accordance with the embodiment of Fig. 1, a heat recovery tower 60 is provided downstream of the economizer 54. The cooled sulfur trioxide laden gas stream enters the lower portion of heat recovery tower 60 and flows upward through an absorption zone in the tower, the absorption zone containing means, such as a bed of packing 61, for promoting contact and mass transfer between the gas and liquid phases. While this description is of a packed tower, it is contemplated that other gas-liquid contacting devices such as tray towers or venturi absorbers can be used. Hot, liquid sulfuric acid is sprayed from the top of the heat recovery tower 60 onto the bed of packing 61 and, as the sulfuric acid and sulfur trioxide contact one another in the absorption zone, the sulfur trioxide is absorbed into the sulfuric acid. As delivered to the countercurrent absorption zone in the heat recovery tower 60, the sulfuric acid has a concentration greater than 98% and less than 101%, preferably between about 98.5% and about 99.3%, and a temperature greater than 120° C. As discussed above, the heat of absorption of the sulfur trioxide into the sulfuric acid is released in this process. Throughout the absorption zone, and throughout the course of SO₃ absorption therein, the strength of the sulfuric acid stream is maintained between 98% and 101%, preferably between 98.5% and 100.0%, and the acid temperature is maintained at greater than 120° C. After absorbing the sulfur trioxide and being heated by the exothermic reaction, the sulfuric acid exits the absorption zone and passes through the bottom outlet of the heat recovery tower. At the exit of the absorption zone and outlet at the tower, the acid has a concentration of greater than 98%, preferably greater than 99%, optimally between about 99.0% and about 100.0% and a temperature of greater than 120° C. After leaving the absorption zone, the hot concentrated sulfuric acid stream passes through heat exchanger 62 to remove the heat of absorption of the sulfur trioxide, prior to again being circulated through the heat recovery tower. Preferably heat exchanger 62 comprises a means for indirect transfer of heat between two fluids, ordinarily a solid partition such as, for example, the tube wall of a shell and tube heat exchanger, or the plates of a plate type exchangers The absorption of the sulfur trioxide increases the concentration of the sulfuric acid; therefore, the sulfuric acid must be diluted at some point. The required water may be added within the heat recovery tower 60 or into the piping between the heat recovery tower 60 and the heat exchanger 62; however, it is preferred that the dilution water be added following cooling of the sulfuric acid in heat exchanger 62 at a point in which sufficient mixing may occur before the sulfuric acid enters the heat recovery tower 60. The dilution water can also be added as a vapor. This increases the amount of heat recovered and provides a means of upgrading atmospheric pressure steam to a useable higher pressure. Alternately, the dilution water can be added as dilute sulfuric acid. In Figure 1 the heat exchanger 62 is shown external from the heat recovery tower 60. While this is the preferred arrangement, it is recognized that the heat exchanger 62 may be located within the heat recovery tower 60. In heat exchanger 62 the heat of absorption released in the process is removed by heat transfer to a third fluid. Preferably, the cooling fluid is water and the heat of absorption is recovered by the generation of low to medium pressure steam, for example, steam having an absolute pressure between approximately 150 and 2000 kPa and normally between approximately 300 and 1200 kPa. Where liquid water is fed as the cooling fluid to the acid cooler, all or part of the water may be converted to steam. The steam produced by the heat recovery system is approximately 0.5 tonnes per tonne of acid produced when liquid water is used for acid dilution. This steam may be used within the manufacturing complex surrounding the sulfuric acid plant or to generate electricity. It is a common practice to remove low and/or medium pressure steam from a turbogenerator for process use. The removal of this steam reduces the electrical output of the turbogenerator. The steam generated in the neat exchanger 62 may be used to reduce the amount of steam normally removed from the turbogenerator or to eliminate this removal of steam altogether. If additional steam is available, it may be injected into the turbogenerator 23. The cessation of removal of steam from the turbogenerator increases its electrical output and the injection of additional steam will also increase the turbogenerator's electrical output. In a 2700 tonne per day sulfur burning sulfuric acid plant, approximately 6 megawatts of additional electrical power can be produced as a result of the use of the steam generated in the heat exchanger 62. Alternatively, electrical power can be generated by using heat exchanger 62 as the boiler for vaporizing an organic liquid for use in driving a generator or otherwise producing work in an organic Rankine cycle. The higher temperatures available through the use of the heat recovery tower have now made such usage economically feasible. In this way the heat of absorption of sulfur trioxide into sulfuric acid may be removed from the sulfuric acid process in a useful form; that is, in a form which may be utilized to produce a benefit either through use in a process or through the generation of electricity. This is in contrast to the current typical loss of this heat through removal by cooling water and release of the heat to the atmosphere in cooling towers. Introduction of this process increases high grade energy recovery efficiency to 90-95%. Following passage of the gas stream through the heat recovery tower, the gas stream exits from the top of the heat recovery tower 60 and passes to the absorption zone of an interpass absorption tower 64 where absorption of any sulfur trioxide remaining in the gas stream takes place. Although shown, as comprising a separate tower in Fig. 1, the interpass absorber according to the invention comprises a separate stage positioned within the same housing as the heat recovery zone. In a typical sulfuric acid plant in which all of the sulfur trioxide is absorbed into a sulfuric acid stream in the interpass absorption tower 64, it is necessary to remove the heat of absorption. Thus, an acid cooler 66 is provided for the sulfuric acid recirculating through the interpass absorption tower 64. However, in a sulfuric acid plant utilizing the current invention, most of the sulfur trioxide is absorbed into sulfuric acid in the heat recovery tower 60; thus, only a small portion of the sulfur trioxide remains to be absorbed in the interpass absorption tower 64. Only a small temperature rise occurs within the interpass absorption tower 64. In the circumstances this small heat load may be removed elsewhere in the system; thus, the acid cooler 66 is usually unnecessary and may be eliminated. The dotted lines used to show the acid cooler 66 in Figure 1 indicate that the acid cooler 66 has been removed from the process. As a result of the high temperature operation of the heat recovery tower, the gas stream exiting the heat recovery tower is relatively hot, and is in contact with hot acid. This in turn results in stripping of sulfuric acid from the acid stream into the gas stream. On passage through interpass absorption tower 64, the gas stream is cooled, typically to a temperature in the range of between about 75° C and about 120° C, preferably between about 75° C and about 100° C, by contact with the acid stream circulating through the interpass tower. At the gas exit from the interpass tower absorption zone, the acid temperature is below about 120° C, preferably below about 100° C, and at least about 10° C lower than the temperature of the acid at the inlet of the heat recovery tower. In a countercurrent interpass tower, the acid preferably enters at a temperature of about 75° to about 100° C. Where an interpass tower is used, it is preferred that the acid temperature throughout the interpass tower be lower than the acid temperature at the inlet of the heat recovery tower. As the gas stream cools during its passage through the interpass tower, sulfuric acid vapor condenses and is either absorbed by the interpass tower acid stream or collected by a mist eliminator positioned in or at the exit of the interpass tower. Such lowering of the dew point of the gas stream helps minimize condensation of acid and corrosion of downstream ducts and equipment. According to the invention the interstage absorption is accomplished essentially in a single absorption zone within heat recovery tower 60. In this instance, the gas leaving the tower is preferably cooled before return to the converter. The dew point may be lowered by passage of the gas stream through a direct contact cooler, such as a packed tower, countercurrent to sulfuric acid flowing at a relatively low flow rate, i.e., so that the temperature of the acid leaving the contact cooler approaches the temperature of the acid at the inlet of the heat recovery tower. Thus, contact with the acid is effective in reducing the dew point of the gas (and absorbing residual sulfur trioxide), but the acid is heated to a temperature high enough so that it may be blended into the acid entering the heat recovery absorption zone without significantly reducing the temperature at which energy is recovered from the heat recovery system. The relatively small temperature difference between gas and liquid throughout the cooling zone minimizes acid mist formation. Such a packed tower cooler is constructed as a separate stage within the same housing that contains the heat recovery absorption zone. The temperature of the cooling acid that is in contact with the gas leaving the cooling zone is at least 10° C lower than the acid temperature at the inlet of the heat recovery absorption zone. The cooling of the gas is carried out in a secondary absorption zone positioned above the primary absorption zone in a heat recovery tower. Secondary absorption acid is fed at a relatively low temperature and volume to the inlet of the upper zone, the heated acid leaving the secondary absorption zone is mixed with a relatively large primary absorption acid stream, and sulfur trioxide is absorbed in the combined acid stream in the primary absorption zone. Preferably, the weight basis flow rate of primary absorption acid stream is at least about four times, preferably between about four and about twenty times, that of the secondary absorption acid stream. Whatever approach is taken, lowering of the gas dew point is important for protecting carbon steel heat exchangers conventionally located downstream of the converter to which the gas stream is returned. The remainder of the sulfuric acid process shown in Figure 1 is well known. The sulfur trioxide depleted gas stream is returned to the second oxidation stage 34 of the converter 30 to complete oxidation of the remaining sulfur dioxide. This final passage through an oxidation stage will complete the conversion of sulfur dioxide to sulfur trioxide. The gas stream exits from the converter 30, passes through an economizer 68 for cooling, and passes through the final absorption tower 70 in which the sulfur trioxide in the gas stream is absorbed into sulfuric acid. Following absorption of the sulfur trioxide, the gas stream is released to the atmosphere. Referring to Figure 2, it can be seen that at constant temperature, both the corrosion rate of certain alloys and the degree of sulfur trioxide absorption decrease rapidly as the sulfuric acid concentration increases. It has been determined for this invention that there is a narrow window of operation in which the corrosiveness of the sulfuric acid to certain alloys at high temperature is greatly reduced while the absorption of sulfur trioxide into sulfuric acid is maintained at a sufficient level to remove the sulfur trioxide from the gas stream passing through the heat recovery tower. Referring now to Figure 3, the heat recovery tower 60 and its associated piping, including the heat exchanger 62 and pump 63 are shown. The sulfur trioxide laden gas stream, leaving the first oxidation stage 32 of converter 30, enters the heat recovery tower 60 through bottom inlet 82. The gas stream passes upward through the absorption zone comprising the bed of packing 61 in which the gas contacts the sulfuric acid stream and the sulfur trioxide is absorbed into the sulfuric acid. The sulfur trioxide depleted gas stream exits from the heat recovery tower 60 through mist eliminator 89 and top outlet 88. The sulfuric acid enters the heat recovery tower 60 through top inlet 84 and is sprayed through a plurality of acid distributors 85 onto the upper surface of the bed of packing 61. The sulfuric acid flows downward through the bed of packing 61 in which it comes in contact with the rising sulfur trioxide laden gas stream and the sulfur trioxide is absorbed into the sulfuric acid. The absorption of the sulfur trioxide into the sulfuric acid is an exothermic process. The hot sulfuric acid enters the heat recovery tower 60 at a temperature greater than 120° C and, after absorbing the sulfur trioxide and being heated by the exothermic reaction, the sulfuric acid exits from the heat recovery tower 60 at a temperature as high as about 250° C. The sulfuric acid exits from the heat recovery tower through bottom outlet 86 and is pumped by pump 63 through heat exchanger 62 to remove, by transfer to a third fluid, the heat generated by the absorption of the sulfur trioxide prior to again being circulated through tower 60. The acid stream has a concentration of greater than 98% and less than 101% and a temperature of greater than 120° C, more preferably greater than 125° C throughout the course of heat transfer to the another fluid. It is also preferred that the third fluid be heated to a temperature of at least 120° C, more preferably greater than 125° C, in its passage through exchanger 62. Corrosion of the heat exchanger is minimized if the acid concentration is at least 99% throughout the course of heat transfer. Following passage through heat exchanger 62, a portion of the sulfuric acid is removed as product through the acid pipe 95. In addition to the rise in the temperature of the sulfuric acid, the absorption of the sulfur trioxide increases the concentration of the sulfuric acid; therefore, the sulfuric acid must be diluted. The sulfuric acid may be diluted by the addition of dilute sulfuric acid or water in the liquid or vapor state; and the terms water or dilution water will be used to refer to the diluent. The required water for dilution of the acid is shown being added in line through pipe 90 upstream of the absorption zone. It is preferred that the dilution water be added, as shown, in the piping 91 between the heat exchanger 62 and the entry of the sulfuric acid into the heat recovery tower 60 through top inlet 84. However, the addition of the dilution water at this point is not required for this invention. The dilution water may be added prior to passage of the sulfuric acid through the heat exchanger 62 or may be added to the sulfuric acid within the heat recovery tower 60, either upstream, downstream or within the absorption zone comprising packing 61. The preferred location for the addition of dilution water, represented by pipe 90, allows the alloy pump and heat exchanger to operate at the highest sulfuric acid concentration, which gives the lowest rate of corrosion, at any given temperature within the operating range. It is particularly preferred that the acid strength be maintained above 99% throughout the alloy heat exchanger. Acid strength downstream of the dilution point is monitored by a conductivity probe. Conveniently, the rate of dilution water addition is controlled by throttling a feed control valve in response to a feedback controller that in turn operates in response to the conductivity probe downstream of the addition point. For the purposes of this invention it has been determined that the operating conditions for the primary absorption zone comprise sulfuric acid temperatures greater than 120° C and concentrations greater than 98% and less than 101%, more preferably 98.5% to 100.0%. Operation of the heat recovery tower under these conditions provides the reduced sulfuric acid corrosiveness to certain alloys necessary for operation of the equipment for long periods of time, and a high degree of absorption of sulfur trioxide into the sulfuric acid. A reduced level of sulfur trioxide absorption would reduce the amount of energy recovered in useful form. To provide good absorption efficiency, the temperature of the acid at the outlet of the tower, and at the inlet of the acid cooler, is preferably not greater than about 250° C. Within that limit, however, the acid temperature may be varied rather widely, depending on the manner in which the absorption heat is ultimately to be utilized. Thus, for example, the energy may be used to generate low pressure steam for concentration of phosphoric acid in an integrated sulfuric acid/phosphoric acid plant, in which case the temperature of the acid should be above 120° C through the heat exchanger, and consequently somewhat higher than that at both the inlet and exit of the heat recovery tower. To provide a working margin to compensate for line pressure drop, phosphoric acid evaporator steam chest fouling, and the like, the acid temperature throughout the heat exchanger is maintained above 150° C. Temperatures significantly above 150° C are generally unnecessary in an integrated sulfuric acid/phosphoric acid plant. However, it has been found that the process of the invention is capable of operating with substantially higher acid temperatures, both at the inlet and exit of the tower and throughout the heat exchanger. Operation at such higher temperatures makes possible the generation of medium pressure steam for such applications as the operation of a turbogenerator for the production of electricity. In such instance, it is preferred that the acid temperature throughout the heat exchanger be above 175° C, and even more preferably above 200° C. To provide such temperatures throughout the heat exchanger, it is, of course, necessary to maintain even higher temperatures at both the exit and inlet of the heat recovery tower. The upper acid concentration limit of 101% is established on the basis of achieving a high degree of absorption in the heat recovery tower while operating at conventional acid plant pressures which are near atmospheric. It has been determined that certain materials are particularly suitable for construction of the heat recovery tower 60, the heat exchanger 62, the pump 63 and the other equipment associated with them. In particular certain alloys have been found to be preferable for their corrosion resistance at the conditions under which the heat recovery tower, heat exchanger and pump will be operated. Similarly, such alloys can be utilized as materials of construction for other equipment or components exposed to sulfuric acid at the abovedescribed concentrations and temperatures, for example, in piping, ductwork, heat exchangers or other equipment located downstream of the heat recovery tower. These alloys exhibit minimal, industrially acceptable, corrosion at acid concentrations of 98-101% and temperatures above 120° C. It has been determined that the corrosion resistance of these alloys can be characterized in terms of the percentages of the major alloying constituents. Provided that the acid concentration is maintained within the ranges specified hereinabove, it has been determined that the rates of corrosion are surprisingly low for a substantial range of iron/chromium, nickel/chromium, and iron/chromium/nickel alloys, even at highly elevated temperatures substantially above 120° C. As indicated above, the alloys used for heat exchanger tubes, and other equipment elements exposed to the hot concentrated heat recovery tower absorber acid, should be constituted of either a ferrous alloy containing chromium, an iron-nickel alloy containing chromium, or a nickel alloy containing chromium. In each instance, the alloy should have an austenitic, ferritic, or duplex structure. Generally suitable stainless steel (ferrous) and iron-nickel alloys may contain between about 16% and about 30% by weight chromium, up to about 33% by weight nickel, up to about 6%, preferably up to about 4%, by weight molybdenum, and between about 35% and about 83%, preferably about 46% to about 83%, by weight iron. Other alloying elements such as copper, manganese, silicon, cobalt, tungsten, niobium, titanium, nitrogen, sulfur, phosphorus, and carbon may also be present in minor proportions. Nickel base alloys useful in the method of the invention may generally contain between about 32% and about 76% preferably about 37% to about 43%, by weight nickel, between about 1% and about 31%, preferably about 23% to about 31%, by weight chromium, between about 2% and about 46%, preferably between about 16% and about 28%, by weight iron, and up to about 28%, preferably up to about 3%, by weight molybdenum. As in the case of stainless steel type and iron/chromium/nickel type alloys, the nickel base alloys may also contain minor proportions of other alloying elements. The alloys best suited for service in this heat recovery system have compositions which give a corrosion index (CI) greater than 39, CI>39 as defined by the following equation: CI = 0.35(Fe + Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) where: Fe = the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight percent of chromium in the alloy, Ni = the weight percent of nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy. For alloys of the above-described type, industrially acceptable corrosion rates are a matter of economics. In addition to varying with general economic factors bearing on the profitability of a given plant, the acceptable rate of corrosion varies with the function of the particular piece of equipment for which an alloy is considered as a candidate. For heat exchanger tubes, however, it may be said that an acceptable rate is generally in the range of 0.05 to 0.08 mm/yr. For piping, pump tanks, distributors, or the shell of an absorption tower, slightly to moderately higher rates of corrosion may be tolerated. Based on corrosion data obtained in connection with the development of the process of the invention, it has been found that stainless steel type alloys are generally preferred to nickel alloys for use in equipment exposed to the heat recovery tower absorber acid at temperatures above 120° C. A preferred material of construction is 304L stainless steel. Another preferred material is 309 stainless steel. Especially preferred is 310 stainless steel. Other particularly useful alloys include E-Brite Alloy 26-l, type 329 stainless steel, Alloy 29-4-2, and Ferralium Alloy 255. As generally exemplified by Alloy 26-l, Alloy 255, 304L stainless steel, 309 stainless steel and 310 stainless steel, a preferred range of alloys comprises between about 16% and about 30% by weight chromium, up to about 23% by weight nickel, up to about 4% by weight molybdenum, and the balance essentially iron. Other conventional alloying elements may be present in minor proportions. While these alloys of the above-described type are preferred for construction, sometimes conventional materials of construction for the heat recovery tower will be more cost effective. At these times the heat recovery tower can be constructed of carbon steel and lined with a ceramic material to protect the carbon steel shell from the corrosive attack of the sulfuric acid. Such construction is very similar to that typically utilized for the interpass absorption tower. The function of the heat recovery tower is to contain the sulfuric acid and the sulfur trioxide laden gas stream and to provide for the contacting of these two streams such that the sulfur trioxide is absorbed into the sulfuric acid. As the absorption of the sulfur trioxide into the sulfuric acid is an exothermic reaction, the acid is heated. The sulfuric acid entering the heat recovery tower has a temperature greater than 150° C. The heat of absorption will raise this temperature, generally within the range of 10-60° C, more typically by 20-40° C. The exit temperature from the tower may be as high as about 250° C. While this is the preferred operating range for the acid temperature, it is possible to operate at higher temperatures by increasing the pressure or reducing the degree of sulfur trioxide absorption. In the preferred temperature range low to medium pressure steam, for example, steam having a pressure between approximately 150 and 2000 kPa, preferably 300-1200 kPa, may be generated. Increasing the steam pressure will require a corresponding increase in the temperature of the sulfuric acid entering the heat recovery tower. In a particularly preferred mode of operation, the tower is operated at essentially atmospheric pressure, the acid leaving the tower 60 is maintained at a temperature greater than 150° C, and steam is generated in heat exchanger 62 at a pressure of 440 kPa absolute or greater. In Figure 3 the heat exchanger 62 is shown located externally of the heat recovery tower 60. This is for numerous reasons the preferred embodiment; however, the heat exchanger may also be located within the heat recovery tower 60. Similarly, the dilution water is shown being added through pipe 90 into pipe 91 between the heat exchanger 62 and the top inlet 84 to the heat recovery tower 60. With this preferred configuration the heat exchanger 62 and pump 63 always contact acid of the highest concentration which provides the lowest corrosion rates and the highest degree of protection to the heat exchanger and pump. It is also contemplated that the dilution water may be added into pipe 92 which is located between the bottom outlet 86 of heat recovery tower 60 and the heat exchanger 62. It is also contemplated that the dilution water could be added directly to the heat recovery tower 60 or to the gas stream entering the tower. It is noted that the addition of dilution water downstream of the absorption zone, or in the zone near the acid exit point, will reduce the sulfuric acid concentration before it passes through the pump 63 and heat exchanger 62 and that this lower sulfuric acid concentration will result in a higher corrosion rate for the heat exchanger 62 and pump 63. In the system of Figure 5, boiler feed water is preheated in a heat exchanger 96 by transfer of heat from final absorption tower circulating acid, and in a heat exchanger 100 by transfer of heat from acid circulating through drying tower 14 and interpass absorption tower 64. Preheated boiler feed water is sent to a de-aerator (not shown), and a portion of the water exiting the de-aerator is returned to another heat exchanger 98 where it is further heated by transfer of heat from the product acid stream that is withdrawn from the heat recovery tower circulating loop via discharge line 95. A major portion of the boiler feed water stream leaving exchanger 98 is delivered to heat exchanger 62 where it is vaporized to produce steam that may be used for power generation or process heat. However, a side stream of the water leaving exchanger 98 is diverted to line 90 through which it is injected as makeup water into the heat recovery tower circulating loop. Depending on the volume of feed water and the particular conditions prevailing in the final absorption tower, the boiler feed water may be partially vaporized by the time it reaches pipe 90. A circulating acid pump tank 99 is provided for drying tower 14 and interpass absorption tower 64. Product acid from exchanger 98 is delivered to tank 99. Some or all of the acid returned to tower 64 may bypass exchanger 100 through line 101, thereby increasing the temperature of the acid in the pump tank and allowing the boiler feed water passing through the exchanger to be heated to a higher temperature than would otherwise be feasible. A product acid stream is removed from tank 99 through boiler feed water preheater 100. Figure 4 is a graph which shows the operating cycle of the heat recovery tower 60 in terms of the temperature and concentration of the sulfuric acid. Shown on the graph are isocorrosion lines for type 304L stainless steel. Also shown is a line representing the sulfur trioxide equilibrium between typical inlet gas and outlet acid. This equilibrium line defines the limiting conditions for the absorption of sulfur trioxide in the acid at atmospheric pressure. The operating cycle of the heat recovery tower is shown as a triangle ABC. The location of points A, B and C in the process may be found in Figure 3 and Figure 1. Point A of the operating cycle represents the conditions, the temperature and concentration of the sulfuric acid, at the bottom outlet 86 from the heat recovery tower 60. Point B represents the sulfuric acid conditions after passage through the heat exchanger 62. Point C represents the sulfuric acid conditions as the sulfuric acid is entering the heat recovery tower through top inlet 84, after the addition of the dilution water. Looking at Figures 3 and 4, it is of interest to discuss a complete operating cycle by considering the flow of the sulfuric acid starting at point C. The sulfuric acid enters the heat recovery tower 60 at a temperature of approximately 165° C and a concentration of approximately 99%. As the sulfuric acid passes downward through the heat recovery tower and absorbs the sulfur trioxide from the gas stream passing upward through the heat recovery tower an exothermic reaction takes place. The temperature of the sulfuric acid rises and the concentration increases. At the outlet of the heat recovery tower, represented by point A, the sulfuric acid has a temperature of approximately 200° C and a concentration of approximately 100%. Following the heat recovery tower, the sulfuric acid flows through heat exchanger 62 and is cooled. Reference point B is at the exit of the heat exchanger 62. At this point the acid has been cooled from approximately 200° C to approximately 157° C and the acid concentration has remained constant. Before the sulfuric acid enters the heat recovery tower again, dilution water is added. The addition of water to the concentrated sulfuric acid reduces the concentration and causes a rise in temperature. Therefore, the triangle of the operating cycle shows that the concentration of the sulfuric acid is reduced from approximately 100% to approximately 99% and that, during this reduction, the temperature of the sulfuric acid rises from approximately 157° C to 165° C. At this point the sulfuric acid again enters the heat recovery tower and the operating cycle is repeated. It is easy to see in Figure 4 the relationship between the operating cycle of the heat recovery tower 60, which includes the heat exchanger 62, the corrosion rates for type 304L stainless steel at different temperatures and sulfuric acid strengths, the equilibrium line for the absorption of sulfur trioxide into the sulfuric acid, and the temperature of the steam that may be generated. This operating cycle is compared to the operating cycle of a typical interpass absorption tower represented by the triangle DEF. The location of points D, E, and F in the process may be found in Figure 1. Point D represents the conditions of temperature and acid concentration leaving the interpass tower. Point E represents a typical condition where the acid in the pump tank has been diluted with water and cooled by mixing with colder acid draining from the drying tower circuit. Point F represents the temperature and acid concentration leaving the acid cooler and recirculating to the interpass absorbing tower. It can be seen in Figure 4 that the heat recovery tower of this invention permits absorbing sulfur trioxide at significantly higher temperature than previously practiced, while reducing corrosion rates for type 304L stainless steel by a factor of ten or more, compared to that obtained at acid temperatures and concentrations characteristic of past practices. Significant reductions in corrosion rates were found for the other alloys which have a corrosion index (CI) greater than 39, with the extent of the reduction dependent upon the specific alloy. It may be noted that the discoveries of the present invention with respect to materials of construction have general application to the storage, transportation or handling of hot concentrated sulfuric acid streams irrespective of the particular process or other operation that may be involved. Thus, a basic method has been discovered for storing, transporting or handling sulfuric acid having a concentration greater than 98.5% and less than 101% at temperatures greater than 120° C. This method comprises containing the acid in a conduit or vessel constituted of an Fe/Cr, Ni/Cr or Fe/Cr/Ni alloy having an austenitic, ferritic, or duplex structure and having a corrosion index corresponding to the algorithm set forth hereinabove. Particularly preferred alloys have compositions falling within the ranges particularly specified hereinabove. EXAMPLE 1 Static corrosion immersion tests were carried out in hot concentrated sulfuric acid over a range of temperatures to determine the corrosion resistance of carbon steel and various alloy metals whose compositions are set forth in Table 1, Set forth in Table 2 is a compilation of the data obtained in these static immersion tests. This data provided the basis for the corrosion index set forth hereinabove. Collated in Table 3 is data at a variety of acid concentrations for several alloys, including certain alloys which exhibited favorable corrosion resistance based on the data of Table 2. Among the specific alloys for which corrosion data is presented in Table 3 are E-Brite Alloy 26-l, a ferritic stainless steel, Alloy 255 duplex stainless steel, Alloy 304L, an austenitic stainless steel, and Alloy C276, a high nickel alloy. Other alloys for which data is presented in Table 3 are 304L stainless, 310 stainless and Alloy 29-4-2, This data illustrates the very significant effect of small differences in acid concentration on the corrosion rates of these alloys. Thus, the stainless steel alloys experience as much as a 35-fold increase in the corrosion rate when the sulfuric acid concentration is decreased from 100 weight percent acid to approximately 98%. Alloy C276 shows a reduction in corrosion in the 98-99% concentration range, but the corrosion rate increases between 99% and 100%. If the Alloy C276 data is compared with any of the stainless steel data, it can easily be seen that there is a considerable advantage for stainless steel alloys when used in accordance with the teachings of this invention. At the elevated temperatures normally encountered in the heat recovery tower and heat exchanger many alloys become more passive, or resistant to corrosion. This effect can be seen to prevail for a number of alloys as illustrated by the data given in Table 3. Set forth in Fig. 6 are estimated isocorrosion curves based on the data of Table 3 for type 304L stainless steel. It may be noted that the isocorrosion curves of Fig. 6 are not straight. The data of Table 3 shows the corrosion rate increasing rapidly with temperature in the range of 250° C and exposure to 97-98% acid concentration. However, for acid concentrations between 99-100%, the temperature effect was found not to be substantial. It may be noted that the isocorrosion curves of Fig. 6 exhibit an "S" shape, resulting from an actual decrease in corrosion as a function of temperature in elevated temperature regions typically above 130° C. Set forth in Fig. 7 is a plot of corrosion data of Table 3 on a temperature vs. acid concentration diagram for Type 310S stainless. Also set forth in Fig. 7 are estimated isocorrosion curves. Based on the static corrosion tests of Example 1, the alloys tested were ranked according to their relative suitability for use as the material of construction of the tubes of a heat exchanger for recovery of the heat of absorption from 98% to 100% absorber acid at temperatures of greaterthan 120° C, in accordance with the process of the invention. In making such ranking, the corrosion data was compared for the various alloys at each of the three corners of the heat recovery tower operating diagram as shown in Fig. 4. Given the vicissitudes of startup and process upset conditions, the ranking also took into account the corrosion performance at temperatures and concentrations adjacent but somewhat outside the operating ranges. The ranking is set forth in Table 9, together with the corrosion index for each of the listed alloys as calculated from the corrosion index equation set forth hereinabove. The dotted line on Table 4 divides the alloys considered to be most suitable for implementation of the process of the invention from those which would not currently be considered for commercial application, at least not for use in the tubes of the heat exchanger in which the heat of absorption is recovered by transfer from the absorption acid to another fluid. It should be understood that the corrosion rates for most of the alloys below the dotted line are reasonably satisfactory, and in fact lower than would have been expected prior to the corrosion testing program described in Example 1. However, the alloys below the line are not currently considered commercial candidates because of the even more favorable corrosion performance of the alloys listed above the dotted line. It should further be noted that the ranking set forth in Table 4 is based entirely on corrosion data and does not necessarily reflect the exact current ranking when detailed economic or fabrication considerations are taken into account. However, regardless of whether the ranking incorporates design optimization considerations or is done strictly on the basis of corrosion results as in Table 4, the general correlation between a corrosion index greater than 39 and suitability for use in the heat recovery process of the invention remains consistently valid. Rank Material Type+ CI CI>39 CI<38 39>CI>38 1 E-Brite 26-1 S 43.5 x 2 29-4-2 S 43.2 x 3 Ferralium 255 S 41.5 x 4 310 S 41.6 x 5 Carpenter 7 MO S 42.7 x 6 44 LN S 41.8 x 7 Lewmet 66 N 39.4 x 8 309 S 41.5 x 9 Incoloy 800 N 40.6 x 10 430 S 40.1 x 11 Incoloy 825 N 38.6 x 12 Nitronic 60 S 39.1 x 13 304 L S 40.1 x 14 317 S 38.7 x 15 Cronifer 1815 S 38.7 x 16 316 L S 38.6 x 17 Inconel 600 N 36.3 x 18 Lewmet 55 N 35.2 x 19 Nitronic 50 S 40.8 x 20 Lewmet 44 N 38.9 x 21 Sanicro 28 S 40.5 x 22 Carpenter 20 S 37.0 x 23 904 L S 37.6 x 24 Inconel 625 N 34.5 x 25 254 SMO S 37.8 x 26 Hastelloy G N 36.4 x 27 Hastelloy C276 N 31.5 x 28 Hastelloy B2 N 18.8 x Example 2 A two stage pilot plant heat recovery tower was installed ahead of the final absorption tower in a non-interpass sulfur-burning sulfuric acid plant. The two-stage pilot plant tower was 0.34 m I.D. and contained a primary absorption zone comprising No. 2 ceramic Intalox saddles with a packed height of 1.1 m and a secondary absorption zone comprising No. 1.5 cermic Intalox saddles with a packed height of 1.3 m positioned directly above the primary absorption zone. A process gas slipstream containing 7.8 volume percent sulfur trioxide at a temperature of 232° C was fed to the bottom inlet of the two stage heat recovery tower at a volumetric rate of 4.2 Nm³ per minute. A primary absorption sulfuric acid feed stream having an average concentration of 98.8% and temperature of 181 ° C was fed to the top inlet of the primary absorption zone at a rate of 46 Kg/min. A secondary absorption sulfuric acid feed stream having an average concentration of 99.5% and temperature of 83° C was fed to the top inlet of the secondary absorption zone at a rate of 7.8 Kg/min. The following conditions occurred at steady state operation: Sulfuric acid exiting from the bottom of the secondary (upper) absorption zone at an average concentration of 99.8% was combined with the primary absorption feed stream, and the combined acid stream entered the primary (lower) absorption zone. The process gas was cooled to 89° C prior to exiting the secondary absorption zone. A sulfuric acid stream having an average concentration of 99.5% by weight and a temperature of 196° C flowed through the bottom outlet of the primary absorption zone. During passage through the two stage tower, approximately 99.8% of the sulfur trioxide content of the feed gas stream was absorbed into the sulfuric acid streams. The gas to liquid temperature difference was 6° C at the top and 14° C at the bottom of the secondary absorption zone, and the consequently gradual cooling resulted in a total mist loading of only 0.60 mg/m³, whereas a predicted mist loading of only 6.0 Mg/M³ would be produced if the hot, saturated gas from the primary absorption zone were shock cooled by passing the gas directly to an absorption tower opearated at conventional conditions. Sulfuric acid leaving the two stage tower flowed by gravity to a pump tank from whence it was pumped to a boiler where 0.80 kg/min of steam was generated at a pressure of 915 kPa. Acid left the boiler at a temperature of 178° C, 1.5 kg/min product acid was removed, 7.8 kg/min acid was cooled and delivered as the secondary absorption acid feed stream to the secondary absorption zone, and the remaining acid stream was thereafter diluted to 98.8% weight percent by inline addition of liquid water prior to recycle to the primary absorption zone of the two stage tower. The heat of dilution increased the acid temperature to 181 ° C. Corrosion probes of type 310 stainless steal were mounted in the acid line before the boiler. The indicated corrosion rate of type 310 stainless steel was less than 0.03 mm per year.

Claims: A process for the manufacture of sulfuric acid, comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, characterized by; contacting a gas comprising sulfur trioxide with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, the absorption being carried out in a primary absorption zone in a heat recovery tower, to which said sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120° C, and from which said sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 150 °C; and contacting the gas exiting from said primary absorption zone with sulfuric acid in a secondary absorption zone positioned above said primary absorption zone in said heat recovery tower, the inlet temperature of the sulfuric acid to the secondary absorption zone being at least 10° C cooler than the inlet temperature of the sulfuric acid to the primary absorption zone, and the concentration of the acid entering the secondary absorption zone being at least 98%; and removing said heat of absorption in useful form from the sulfuric acid exiting said primary absorption zone by indirect transfer of heat to another fluid, and thereby heating said another fluid to a temperature greater than 120° C, the acid temperature being maintained at above 150° C throughout the heat exchanger. A process as set forth in claim 1 wherein sulfuric acid is supplied to said secondary absorption zone in a secondary absorption acid feed stream, and the sulfuric acid exiting said secondary absorption zone is combined with a primary absorption acid feed stream to provide the sulfuric acid stream with which the sulfur trioxide-containing gas is contacted in said primary absorption zone. A process as set forth in claim 2 wherein, on a weight basis, the flow rate of said primary absorption acid feed stream is at least about four times the flow rate of said secondary absorption acid feed stream. A process of claim 2, wherein the temperature of the gas leaving the secondary absorption zone is between about 75° C and about 100° C. A process as set forth in claim 2 wherein sulfuric acid leaving said heat exchanger is divided into a plurality of streams, one of said plurality of streams comprising said secondary absorption acid feed stream and another of said plurality of streams comprising said primary absorption acid feed stream. A process as set forth in claim 5 wherein said secondary absorption acid feed stream is further cooled before being fed to said secondary absorption zone. A process as set forth in claim 5 wherein said primary absorption acid feed stream is diluted by addition of water thereto before being fed to said primary absorption zone. A process of claim 3, in which. sulfuric acid is fed to the secondary absorption zone in such an amount that the sulfuric acid exiting the secondary absorption zone is produced at a temperature such that it may be blended into the acid entering the primary absorption zone without significantly reducing the temperature thereof. A process of claim 3, in which sulfuric acid is fed to the secondary absorption zone in such an amount as to maintain a relatively small temperature difference between gas and liquid throughout said absorption zone. A process of any of the preceding claims, comprising circulating at least a portion of the cooled sulfuric acid back from the heat exchanger to the inlet of the absorption zone, whereby an acid circulating loop is established comprising said absorption zone and said heat exchanger, removing a portion of the acid exiting said circulating loop heat exchanger as a discharge stream from said circulating loop; and recovering energy from said discharge stream by passing said stream through a second heat exchanger. A process of claim 10, wherein heat is transferred from said discharge stream to said another fluid in said second heat exchanger, and thereafter said another fluid is passed through the heat exchanger in said circulating loop for removal of heat of absorption from the circulating acid. A process of claim 11, wherein said another fluid as introduced into said second heat exchanger comprises boiler feed water, said boiler feed water is preheated in said second heat exchanger, and steam is generated by vaporizing the preheated boiler feed water in said circulating loop heat exchanger.

Inventors: MCALISTER DONALD RAY, ZIEBOLD STEVEN ANTHONY, MCALISTER, DONALD RAY, ZIEBOLD, STEVEN ANTHONY
Applicants: MONSANTO CO, MONSANTO COMPANY
Application year: 1985
Main classification: C01B17
IPC codes: C22C38, C01B17, B01J19