FR2541665A1 - Process for the concentration of sulphuric acid - Google Patents

Abstract

The invention relates to a process for the concentration of sulphuric acid used in a concentration vessel 110. The vessel 110 is made of austenitic steel with a relatively high silicon content. The process is carried out with a plurality of concentration stages. Field of application: production of sulphuric acid.

Description

 The invention relates to an apparatus and a process for producing sulfuric acid, and more particularly to an apparatus made of austenitic stainless steel.

 Sulfuric acid is normally produced by burning elemental sulfur in previously dried air to produce sulfur dioxide from which sulfur trioxide is formed and absorbed in concentrated sulfuric acid where it reacts with sulfur dioxide. water to form additional sulfuric acid. There is a similar process for producing sulfuric acid from metallurgical gases, wherein a gas containing sulfur dioxide is dehydrated and the sulfur dioxide is converted to sulfur trioxide. The process then proceeds as indicated above. Both processes involve the passage of sulfur dioxide from sulfuric anhydride by oxidation followed by absorption into sulfuric acid. This is generally known as the contact process for the production of sulfuric acid.

 Some important operations on sulfuric acid during production are dehydration, absorption and cooling. The sulfuric acid forces during these operations are of the order of 93% -99.5%. The temperature of these strong acids or concentrates is generally of the order of 400C-1150C.

 Concentrated sulfuric acid is corrosive to most metals, especially in the upper part of the temperature range <> 1000C), and it is highly desirable that all elements of the production system have been sulfuric acid such as contact lathes, heat exchangers, pipes, valves, pumps, distributors and others, coming into contact with sulfuric acid, shall be made of materials resistant to corrosion. At present, such systems are made of cast iron, brick, various plastics and non-metallic materials, as well as various very expensive alloys, resistant to corrosion. These materials, however, do not give complete satisfaction. For example, an acid-proof brick can only be manufactured in particular forms, can swell on prolonged exposure to concentrated sulfuric acid and requires mortars and expensive work for its installation. In addition, the attack of the carbon steel shell disposed beneath the bricks may result in sulfation which may cause rupture of the shell and / or bricks. The metal elements used have high corrosion rates and, unless they are subject to special protection or a very large alloy, their service life is limited.

 High alloyed materials and cast irons are also limited by manufacturing suitability factors, which place limitations on the design of the facilities and result in the use of more flanges, fittings and fittings. a higher cost and more areas of risk of leakage.

 With regard to energy recovery, the temperature levels at which corrosion rates are acceptable are relatively low, making it very difficult to recover the large amounts of heat lost in sulfuric acid cooling systems. . The conventional transport of energy in a sulfuric acid plant is in the form of water vapor which, even at the lowest pressures used, is at a temperature above 1150C, ie say the highest temperature1 currently tolerable for concentrated acid.

 A technique that has developed over the past decade to reduce corrosion is the use of anodic protection with materials that can be electrochemically protected.

Sulfuric acid heat exchangers, which account for a large part of the cost of acid production systems, are currently manufactured in austenitic 300 series stainless steel (18 th century).
Cr, 8% Ni). The corrosion resistance of these steels is ensured by an oxide film and, in the absence of anodic protection, they can be used only at very low temperatures, for example of the order of 55.degree. 650C in 98% acid. Above this temperature, in the presence of turbulence, there is considerable corrosion which can completely destroy the equipment in a few months. It has been found that the electrochemical production of the oxide film by the use of a Anodic protection significantly reduces this corrosion and the current stainless steels can now be used at acid temperatures up to 72QOC-1250C,
Stainless steels are part of a series of iron based alloys, usually with at least 12% Cr to give them corrosion resistance.

The addition of nickel to the iron / chromium alloy modifies the crystal structure from a centered cubic structure to a center-faced cubic structure, and the resulting phase is called austenite. This family of materials based on Fe, Cr and Ni is austenitic stainless steels.

 Significant efforts have been made to extend the possible range of application of austenitic alloys in systems working on strong acids (> 90%) at high temperatures. Various normally useful alloying elements such as molybdenum, nickel, copper and chromium have been studied for this purpose. It is also possible to increase the silicon content, but a negligible improvement, or no improvement, in corrosion resistance in concentrated sulfuric acid is obtained, and this increase in the silicon content is not improved. Generally not useful in the industry because the silicon alloy material is much more difficult to produce and is therefore much more expensive.

 It is known that stainless steels containing 4% silicon have an acceptable corrosion resistance to 98% sulfuric acid at moderate temperatures (about 80 ° C.), although the behavior is not very good. different from normal austenitic stainless steels, which do not contain silicon. With the additional costs associated with the addition of silicon and the additional compensatory nickel addition to maintain the austenitic structure, therefore, such alloys have not been used in this application.

 Silicon-containing alloys have also been tested in media containing dilute sulfuric acid and it has been found that these alloys have unacceptable levels of corrosion. They have therefore not been used more in this area.

 In their main sector of use, austenitic stainless steels containing up to 5.3% of silicon are used successfully in the production of concentrated nitric acid, in particular acid of the order of 98% -100% , where the corrosion resistance of the conventional grades of non-silicon steels becomes inop- erant. In this case, increasing the silicon content helps to reduce corrosion, although no significant effect of the silicon level is noted.

 The addition of silicon to austenitic stainless steels produces a significant effect on the structure of the resulting alloy and requires changes in the contents of other alloying elements such as nickel which must be increased to maintain the austenitic nature of the alloy. 'alloy. In addition, the workability and shrinkability of the material is also compromised by an increase in the silicon content, which makes the alloy containing a lot of silicon expensive, compared to the less rich alloys. Up to the conventional level of 4% silicon, the behavioral improvement is sufficient, in some media containing nitric acid, to justify the additional cost, but it is not the same in the case of sulfuric acid concentrated.

 It has surprisingly been found that austenitic stainless steels having a relatively high silicon content have a corrosion resistance by hot concentrated sulfuric acid much greater than that of normal austenitic stainless steels such as 304 and 316 steels. 300 series, or that of grades with normal silicon content, having up to 4% Si. In addition, unlike all previously mentioned high alloyed austenitic stainless steels with higher levels of chromium, nickel or molybdenum, these stainless steels with high silicon content can also be adapted to anodic protection.

 It has further been found that concentrated sulfuric acid can be produced at temperatures much higher than those which hitherto can be achieved with acceptable corrosion.

 The invention therefore relates to an apparatus for producing sulfuric acid by the contact method, of the type comprising at least one concentrated gas-sulfuric acid contact unit and a heat exchanger for sulphuric acid, characterized in that said contact unit and / or heat exchanger are made of austenitic steel containing from 4.6% to 5.8% of silicon.

The expression "concentrated gas-sulfuric acid contact unit" designates a dehydration tower of sulfuric acid in which the water present in the air and in the sulfur dioxide used in the contact process is eliminated, and / or a sulfuric acid absorption tower in which concentrated sulfuric acid is produced by absorption of sulfuric anhydride
The dewatering towers and the absorption towers are generally equipped with acid distributors and fog suppressors Some installations using the contact process, namely the so-called "wet process" installations, do not use -dehydration
A typical contact process installation also requires, in addition to one or more dewatering and absorption towers and a heat exchanger, an acid circulation system including cisterns or pumping tanks. acid and a set of pipes and valves. In such prior systems, the pumping cistern is generally made of carbon steel and comprises an acid-resistant brick lining, intended to mitigate corrosion by hot acid. The pump is generally made of expensive alloy, resistant corrosion, the acid distributor and the set of piping and valves are made of cast iron, and the fog suppressor is made of a stainless steel frame with fiberglass elements However, it has been discovered that that such a system, made of austenitic steel with a relatively high silicon content, has a greater resistance to corrosion.

 Furthermore, since fog suppression devices are generally in the form of sieves, candles or panels, the use of a high silicon austenitic steel frame or framework allows reduce fogging and tolerate higher temperatures in the towers.

 Previous manifolds typically include thick-walled cast-iron pipe sections with bolted flange connections and screw-on end caps, pipes with holes equipped with polytetrafluoroethylene nozzles to reduce acid corrosion through holes . A distributor made of austenitic steel with a high silicon content is a fully welded variant, with or without a polytetrafluoroethylene nozzle. This allows to simplify and lighten the construction and work at higher temperatures.

The invention therefore also relates to an apparatus for producing sulfuric acid by the contact method of the type comprising
(a) at least one gas-sulfu acid contact unit
concentrated
(b) a fog suppressor disposed in
concentrated gas-sulfuric acid contact unit
(c) an acid dispenser disposed in the unit
of contact
(d) a heat exchanger for sulfuric acid
and an acid circulation system by means of
which of 15 acid is circulated to the unit
contact and heat exchanger; comprising
(e) a pumping tank
(f) an acid pump;
(g) a set of piping and valves for acid
characterized in that one or more of the elements
(a) to (g) inclusive, are formed from an austened steel
nitrate containing 4.6% to 5.8% silicon.

 Preferably, all elements (a) to (g), inclusive, consist of said austenitic steel. In a more particularly preferred form, in addition, one or more of the elements selected from the heat exchanger (d), the pumping tank (e), the pump (f) and the set of pipes and valves for acid (g), the device are equipped with anodic protection means. Preferably, all elements (d) to (g), inclusive, have anodic protection means.

 The invention also relates to particular elements of a sulfuric acid production plant, selected from the group consisting of a dewatering tower, an absorption tower, a fog suppressor associated with the dewatering tower or the absorption tower, an acid distributor of the dewatering or absorption tower, a heat exchanger, a sulfuric acid pumping tank, a sulfuric acid pump, and a set of sulfuric acid piping and valves, characterized in that each element is made of austenitic steel containing 4.6 to 5.8% of silicon.

 As previously mentioned, the term "austenitic steel" as used herein refers to a steel containing Fe, Ni and Cr in such proportions that the steel is in the austenitic state. More particularly, the austenitic steel used in the practice of the invention comprises Fe, Cr, Ni and Si. It should also be noted that the austenitic steel used in the present invention may comprise, as a beam, other elements such as, for example, Mn for improving the austenitic stabilization, and other elements such as alloying agents, without impairing the utility of the invention.

 It is not necessary that all of each particular element be made of the austenitic steel used in the present invention. It should be noted, however, that it is highly desirable that all parts of the element in contact with sulfuric acid, in particular with concentrated and hot acid, in the form of liquid or vapor, be produced in this austenitic steel. Therefore, according to the invention, the elements consist, in whole or in part, of this austenitic steel.

 The austenitic steel used in the practice of the invention preferably contains 5.0% to 5.6% Si.

More preferably, the steel has a composition of 17.5% Ni, 17.5% Cr, 5.3% Si, less than 0.015%
C, the remainder being substantially Fe.

 As mentioned briefly above, it has now been discovered that concentrated sulfuric acid can be produced at temperatures much higher than those used up to now, with acceptable corrosion rates. Thus, absorption can be practiced at a temperature of the order of 120-1800C, compared to the range of 60-1200C in which the conventional operations are carried out, without excessive corrosion.

 Therefore, the invention also relates to a method of producing contact type sulfuric acid containing the steps of passing air, sulfur dioxide, sulfur trioxide, or mixtures thereof. in one or more concentrated gas-sulfuric acid contact units A, said unit being optionally equipped with a mist suppressor B and an acid distributor C, to pass concentrated sulfuric acid into a heat exchanger B; and circulating the sulfuric acid to said heat exchanger contact unit by a circulation system comprising a pumping tank E, an acid pump F, and a set G of pipes and valves, the method being characterized in that one or more of the elements A to G, inclusive, consist wholly or in part of austenitic steel containing 4.6% to 5.8% silicon.

 The invention also relates to a process as defined above, in which one or more, and preferably all, of said elements selected from the heat exchanger (B), the reservoir (E) of Fcpage, the pump (F) acid and the assembly (G) piping and valves are subject to anodic protection.

 According to a preferred characteristic, the temperature of the circulating sulfuric acid entering into the chain-ring exchanger is of the order of 1200.degree. C.-1800.degree. C., and preferably of the order of 1500.degree.

 It can thus be readily seen that an installation and a process according to the invention can be carried out at conventional temperatures, which results in a significant reduction of corrosion, or can be implemented at high temperatures. , which has the effect of improving energy recovery with acceptable corrosion. The advantage offered by one or the other of these two options is even greater if anodic protection is provided.

 The invention also relates to an apparatus and a method for concentrating sulfuric acid.

 Sulfuric acid is normally concentrated by removal of water by boiling. One method consists in directly applying, by heating oil, heat to a concentration tank in the form of a cast iron boiler containing the acid and equipped with a stirrer made of silicon cast iron. The acid is introduced into the boiler from a packed column and when concentrated, it flows overflow through acid coolers to storage or pumping tanks. The feedstock is usually 70% residual or contaminated acid, which is concentrated to about 96%. The normal working temperature is, for example, 290BC for the acid, which may correspond to the boiler of the order of 3500 and more, depending on the heating mode.

The main disadvantage of such a concentrator device is corrosion and cracking of the boiler. Anode protection may be applied, if necessary, to extend the life of the boiler.

 Another system used to concentrate the acid comprises a concentration vessel in the form of a lead-coated mild steel balloon having an interior brick lining. The concentration is obtained by countercurrent contact with a hot gas, a temperature of 600-6750C, applied on and in the acid. The acid charge is generally 70% acid and leaves the flask as a 93% acid at a temperature of 2300C. The hot acid produced is generally used for the preheating of the feed acid, by means of a tantalum heat exchanger. The concentration flask generally gives satisfaction, but is expensive and subject to erosion / corrosion of the packing. of bricks. it generally lacks efficiency and can cause problems with a contaminated gaseous effluent.

 Another method of concentrating sulfuric acid is to boil by introducing a heating element, usually containing astral boiler tubes, into the concentration vessel used to boil the liquid. It appeared that sulfuric acid generally attacked tantalum at a temperature exceeding 1900C. Therefore, the sulfuric acid must be concentrated to a temperature such that the surface temperature of the tantalum tubes is below 1900C, and this temperature limitation requires a significant reduction of the pressure in the evaporator. Due to the reduced pressure established in the evaporator, a complex vacuum device is used to condense the water released from the acid by boiling. The vacuum device usually comprises a steam injector for raising the water sufficiently. pressure to condense the water vapor. The investment and steaming costs of the vacuum system and the tantalum heating tubes are high, particularly in plants used to produce high concentration acid, for example 96% sulfuric acid.

 The invention therefore relates to an apparatus and a method for concentrating sulfuric acid in a concentrator, designed to reduce the maintenance due to the corrosion problem and to allow to obtain working temperatures higher than those used until now that higher strength acid can be produced. In its widest aspect, the invention relates to an apparatus for concentrating sulfuric acid from a force of 85 90 g, preferably 90% acid, of the type comprising a concentration tank, the apparatus characterized in that the concentration vessel is wholly or partly formed of an austenitic steel containing 4.6 e at 5.8% silicon. The expression "concentration vessel" refers to a boiler, a balloon, a vacuum evaporator vessel, or other element in which sulfuric acid, placed in the vessel, is heated directly or indirectly, by a direct flame, a heating coil or a jacketed steam pipe, or by an electric heating element, a heat exchanger or by means of a thermosiphon loop.

 It should be noted that the apparatus according to the invention can also be used in acid concentration plants in which the filler acid is less than 85%, usually 70%, acid it is then concentrated beforehand to a force of 85%, preferably 90%, before coming into contact with the austenitic steel of the concentration vessel.

 The sulfuric acid charge is generally preheated in acid concentration devices.

Preferably, the austenitic steel contains 5.0% Si to 5.6% Si. More preferably, the austenitic steel has a composition of 17.5% Ni, 17.5% Cr, 3% Si, less than 0.015% C, the remainder being substantially Fe.

 The apparatus may optionally be equipped with anodic protection means and air extraction means.

 The concentration apparatus according to the invention can also be applied to the destructive heating of organic matter contained in organic sulfur-containing sulfuric acid residual liquids, where corrosion of the concentration vessel is a problem as described above.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and on which
FIG. 1 is a diagram of an installation for producing sulfuric acid by the contact method as known in the prior art, but modified according to the invention; ;
FIG. 2 is a diagrammatic vertical section of a heat exchanger used in the production of sulfuric acid according to the prior art, but modified according to the invention,
FIG. 3 is a diagrammatic vertical section of a heat exchanger internally equipped with an anodic protection device known from the prior art, but as modified in accordance with the invention.
FIG. 4 is a transverse section along the line 4-4 of FIG.
FIG. 5 is a vertical section of a conventional dewatering tower used in the sulfuric acid production plant of FIG. 1
- Figure 6 is a vertical section of the dehydration tower of Figure 5, modified according to the invention; .

FIG. 7 is a vertical section of a conventional absorption tower used in the sulfuric acid production plant of FIG. 1
Figure 8 is a vertical section of the absorption tower of Figure 7, modified in accordance with the invention; and
Figure 9 is a schematic view, partly in section, of a portion of a typical sulfuric acid concentration equipment known in the prior art, and modified according to the invention.

The apparatus shown in FIG. 1 comprises, as concentrated gas-sulfuric acid contact units, three turns, namely a dewatering tower, an intermediate absorption tower and a final absorption tower. Each of these towers has a gas inlet
Located at the bottom and a kind of gas B located at the top.

Each tower has an acid inlet P at the top, and an acid outlet Q at the bottom. Each tower is made of carbon steel and lined
C acid resistant bricks. The upper part of each tower contains a cast iron acid distributor D above which is disposed a fog suppressor E consisting of glass fibers arranged in a frame or frame of stainless steel. Each tower is loaded with a ceramic lining. F carried by a ceramic support
S and through which a gas or air and the acid flow through percolation to establish between them a complete and intimate contact The figure also shows three heat exchangers G for sulfuric acid, stainless steel with anodic protection, in which heat is transferred to cooling water
The figure also shows an acid circulating system comprising pumping tanks or tanks, circulation pumps J and a set of pipes and valves.

 The 10O, 11Q and 12Q acid outlets open into the 10H, 11H and 12H pumping tanks, respectively, made of acid-resistant brick-coated carbon steel. Pumps 10J, 11J and 12J are made of expensive alloy, resistant to corrosion and cast iron and they circulate the acid tanks 10H, 11H and 12H to the towers 10, he and 12, through 10G heat exchangers , 11G and 12G and the cast iron, 10K, 11K and 12K piping and valves1 assembly. The pipe and valve assembly is also traversed by addition water streams L making it possible to satisfy the water demands for the acid produced, and it also comprises acid transfer lines M.

 In operation, the process gas circulation system of the apparatus works normally.

The air enters the dewatering tower 10A through the inlet 10A and is dried by contact with concentrated hot sulfuric acid, circulating against the current, which enters the tower through the inlet 10P and is distributed through the packing 10F by the dispenser 1OD. The dry air exits the tower through exit lOB and sulfuric acid through outlet 100. The mist entrained in the dry air is removed by the fog suppressor 10E. Sulfur is burned with dry air in a sulfur burner (not shown) to produce sulfur dioxide. The process gas, which is now a mixture of air and sulfur dioxide, is then passed to a converter. catalytic converter (not shown) where most of the sulfur dioxide is converted to sulfur trioxide. The process gas, charged with unprocessed SO3 and SO2, enters the intermediate absorption tower 11 through the inlet 11A.

The SO 3 is removed by absorption of this intermediate gaseous stream by contact, countercurrently, in the lining 11F with a concentrated sulfuric acid stream which enters the tower through the 11K assembly and the 11P inlet and which is distributed therein. turn by the 11D distributor.

The absorbed S03 exits through the outlet 110 and reacts with water injected through a conduit L into the tank 11H to give sulfuric acid. The gas flows through the outlet 11B, the SO 3 being substantially completely in the acid. The gaseous effluent from the intermediate absorber then passes into a second catalytic converter (not shown) where the SO 2 present is converted to SO 3 almost entirely. The gaseous effluent from the second converter then enters the final absorption tower 12 where the last residues of SO 3 are absorbed by the acid circulating in the tower 12 and are reacted with the water injected into the tank 12H by a 12L pipe to give sulfuric acid.The gas is finally discharged to the atmosphere by means of a chimney (not shown).

 As previously mentioned, the forces and levels of the acid in the apparatus are regulated in the H tanks using the water addition points L and the transfer lines M to ensure concentratids and suitable levels. acid permitting good absorption and dehydration.

 In the dehydration operation, the conventional practice is to use dehydration acid strengths of between 93 g and 98%, at inlet temperatures of about 500 ° C. for 93% acid and amounting to 800 ° C. for the 98% acid, the temperature depends on the vapor pressure of the acid. The maximum temperatures of the acid leaving the dehydration tower are stable by the corrosive power of the acid on the equipment, on the one hand, and by the need to have an acid current sufficient for an interaction suitable gas-sulfuric acid concentrate, on the other hand. Typical maximum acid temperatures are between 700 ° C for acid at 93 ° C and 90 ° C for 98% acid. Under these conditions established for the dehydration tower, the life of the equipment is generally of the order of 5 years. The force of the acid produced from the pumping reservoir 10H through the product line is generally the same as that of the dehydration acid, ie 93% -98%.

 The absorption towers use 97.5% -99.5% acid when the total vapor pressure on the acid is the lowest. The temperatures of the acid are between 50 and 850C in the case of the acid entering the towers and rise to 1200C in the case of the acid leaving the towers. Irrigation conditions in the towers normally limit the temperature rise in the absorption devices to 350C or less, particularly in the intermediate absorber.

Under these conditions, corrosion of the cast iron is moderate and acceptable equipment life can be achieved.

 Piping and valve assemblies, distributors and acid coolers / heat exchangers are all sensitive to turbulence and acid velocity. As a result, speeds are normally set to less than 1.4 m / s. In addition, all the points having high speeds or turbulences such as valves, orifices, pump impulse members and the like are normally made of more expensive materials such as a fluorocarbon polymer, for example of the TEFLON type, higher alloys such as alloy "Hastalloy C, or the alloy allows, or a ceramic, all these elements being complex and significantly raising the cost of assembly.

FIG. 1 also shows an overall diagram of a modified acid production plant according to the invention, in which the dehydration tower 10, the absorption towers 11 and 12, the D acid distributors , fog suppressors E, heat exchangers G, pumping tanks H, circulation pumps J and the assembly
A pipes and valves are made of austenitic stainless steel relatively rich in silicon, having a composition of 17.5% Ni, 17.5% Cr, 5.3% Si, less than 0.015% C, the rest consisting of Fe.In this preferred embodiment ,. The use of this relatively high silicon austenitic steel for the pipe and valve assembly allows the use of smaller diameter pipes because of the greater corrosion resistance. In addition, the heat exchangers or coolers G are not equipped with anodic protection means, because the operation at conventional temperatures, described above, makes it possible to dispense with anodic protection and thus results in an additional simplification of the installation .

Alternatively, the corrosion resistance of austenitic steel allows an increase in working temperature to 1300C, which exceeds the limits of existing piping, valves or other equipment with acceptable corrosion rates.

The following Table I shows the corrosive effect of. various forces of sulfuric acid, at various tempera- tures, on conventional materials and on alloy
A611 which constitutes the preferred material to be used in the implementation of the invention. The materials are not subject to anodic protection.

 Other particular austenitic steels for use in the invention are 17.97 Cr, 17.15% Ni, 5.09% Si, 0.74% Mn, O. , Q13% C, the remainder being substantially Fe (from BOHLER, VIENNA, AUSTRIA), and 17.5% Cru 17.3% Ni, 5.55 g Si, 0.74% of * in , 0.013% of C and the remainder of Fe.

TABLE I

<tb><SEP> Speed <SEP> of <SEP> Corrosion <SEP> (mm / yr)
<tb><SEP> in <SEP> (A) <SEP> of <SEP><SEP> sulfuric acid <SEP> to <SEP> 98.2 <SEP>%
<tb> Temperature <SEP> (C) <SEP> A611 * <SEP> 316L ** <SEP> Cast Iron
<tb><SEP> 63 <SEP> - <SEP> - <SEP> 1.0
<tb><SEP> 100 <SEP> - <SEP>! <SEP> - <SEP> 1.5-1.75
<tb><SEP> 105 <SEP> 0.021 <SEP> 1 <SEP> 1.6
<tb><SEP> 115 <SEP> 0.11 <SEP> 2.52
<tb><SEP> 125 <SEP> 0.1 <SEP> 3.67
<tb><SEP> 135 <SEP> 0.8 <SEP> - <SEP>
<tb><SEP> 145 <SEP><SEP> 1.6 <SEP> - <SEP>
<tb><SEP> in <SEP> (B) <SEP> of <SEP><SEP> sulfuric acid <SEP> to <SEP> 93.5 <SEP><SEP>%<SEP>
<tb><SEP> 85 <SEP> 0.25 <SEP> 4.07
<Tb>
* 17.5% Ni, 17.5 t Cr, 5.3% Si, less than
0.015% of C and the remainder substantially of Fe.

(More particularly 17.5% + 0.5% Cr, 17.5%
+ 0.5% Ni, 5.3% + 0.3% Si, less than 0.015%
of C, the remainder being substantially FeB
after VEW, BOHLER, VIENNA AUSTRIA)
** Classic austenitic stainless steel with
Composition 18% Cr, 10% Ni, 3 Mo, <0.2%
Si, <0.03% C, the rest being Fe.

 The corrosion studies with 70% sulfuric acid show that at 500C, the alloy A611 has a high corrosion rate which is also higher than that of the 316L alloy under the same test conditions.

 The above results show quite unexpectedly the very favorable resistance to corrosion by concentrated sulfuric acid of the alloy A611 compared to the cast iron and compared to the alloy 316L. This is different from the results obtained in less concentrated acid, with alloys A611 and 316L.

 Table II shows the comparative corrosion effect of sulfuric acid of various forces, at various temperatures, on the alloy A611, during polarized corrosion tests, that is, tests with protection. simulated anode, conducted at a stable potential for long periods of time. The corrosion rates depend on the potential applied during the test. The results obtained correspond to potential values within a reasonable working range for anodic protection, that is to say -100 mV to +200mV.

TABLE II

<tb> Strength <SEP> of <SEP> acid <SEP> Speed <SEP> of <SEP> corro
<tb> sulfuric <SEP> (%) <SEP> Temperature <SEP> (C) <SEP><SEP> (Sm | an) <SEP>
<tb><SEP> 98.5 <SEP> 100 <SEP> 19.25
<tb><SEP> 98.5 <SEP> 274 <SEP> 142.5
<tb><SEP> 98.2 <SEP> 115 <SEP> 6.5
<tb><SEP> 93.5 <SEP> 65 <SEP> 1.25
<tb><SEP> 93.5 <SEP> 75 <SEP> 5.0
<tb><SEP> 93.5 <SEP> 85 <SEP> 28.75
<tb><SEP> 93.5 <SEP> 95 <SEP> 22.75
<tb><SEP> 93.5 <SEP> 200 <SEP> 1125.0
<Tb>
Table II shows the favorable effect of the anodic protection of the alloy iN611 when the latter is to be used in contact with the concentrated sulfuric acid.

 Thus, a preferred method of implementing the invention, having advantages leading to improved energy recovery, will now be described with reference to FIG.

 The elements of the plant consist of silicon-rich austenitic stainless steel having a composition of 17.5% NiB 17.5% Cr, 5.3% Si, <0.015% C, the rest being As previously described, Fe, as previously stated, the heat exchangers 11G, 12G, the pump tanks 11H, 12H and all the pipes and valves between the towers 11, 12 and the heat exchangers 11G and 12G, respectively, present anodic protection means (not shown). In addition, because of the difficulty of providing anodic protection on the absorption towers 11 and 12 and because of the high temperatures prevailing at the lower part of these towers, the lower parts and interior are lined with bricks.

 In this embodiment, dehydration is performed in the dehydration tower to achieve appropriate dehydration at temperatures in the upper portion of the previously indicated range. The dehydration operation is carried out by means of concentrated acid, having a force of the order of 98%, in order to allow the use of higher temperatures which are more suitable for energy recovery, i.e. say 80 GC at 1100C.

 In the cooler zones of the intermediate and final absorption devices, namely the upper part of the towers 11 and 12, the distributors 11D and 12D, the fog suppressors 11E and 12E and the pipes between the heat exchangers 11G, 12G and the turns 11, 12, respectively, the temperature of the acid is of the order of 1300C, which is close to the maximum temperature that can be used with austenitic steel rich in silicon, without anodic protection.

 In the warmer zones of the absorption devices, ie the zones where the elements are subjected to anodic protection, as previously described, namely heat exchangers 11G, 12G, pumping 11, 12H, the pipes between the towers 11, t2, and the heat exchangers 11G, 12G, the temperatures of the acid are of the order of 1600C-1700C.

 The direct result of the operations carried out in accordance with this form of the process of the invention, at the indicated temperatures, is the increase of the energy level in the acid circuit at values compatible with an energy recovery of interest. The overall energy efficiency of this sulfuric acid facility can be raised from a typical level of 60-65% to a level of about 90%.

 Referring now to Figure 2 which shows schematically a typical heat exchanger 100 of the prior art, of the type currently used industrially for the production of sulfuric acid. Heat transmission equipment, such as an envelope, and tubular heat exchangers, are used to cool sulfuric acid, equipment in which, generally speaking, the corrosive acid passes around the at least one tube. heat exchange, while the water passes inside the tubes to cool the fluid flowing on the outer surface of these tubes, i.e. at the location where the corrosive fluid is on the shell side of the heat exchanger.

 The heat exchanger 100 comprises an outer casing 120 divided into a water supply box 14, a water outlet box 16 and a cooling section 18, the three parts being separated by tubular plates 20 22. Heat exchange tubes 24 are arranged between tubular plates to allow the flow of water between them. The envelope, the tube plates and the tubes are commonly made of austenitic steel of normal qualities which, in the absence of anodic protection, corrodes at a rate which is much too fast in the presence of concen trated and hot sulfuric acid. The box 14 of water inlet and the box 16 output are made of carbon steel. Only two tubes 24 are shown in FIG. 2, but in practice the number of tubes 24 may exceed 1000 units, these tubes being tightly packed together with small gaps (generally 10.35 to 2.7 mm). ) between them. The cooling water enters the water supply box 14 through an inlet 26, flows through the tubes 24 and leaves the water outlet box 16 through an outlet 28. The hot acid enters the cooling section 18 through an acid inlet 30 and exits through an outlet 32 of acid. Conventional baffles 34 are provided for following a sinuous path through the acid in the cooling section 18 to provide maximum cooling.

 In operation, when the heat exchanger does not include anodic protection means, the tube wall temperatures are generally of the order of 30 ° C in the case of 93% acid and 550C in the case of 98% acid.

Reference can again be made to FIG. 2 which represents a heat exchanger according to the invention, in which the casing 20, the tubular plates 20, 22, the tubes 24 and the baffles 34 are made of relatively rich austenitic stainless steel. silicon, having a composition of 17.5% Ni, 17.5% Cr, 5.3%
If, <0.015 of C, the remainder substantially consists of Fe. In operation, this embodiment of the invention, when not equipped with anodic protection means, has wall temperatures of the tubes. order of 800C in the case of 93% acid and 1300C in the 98% case-acid.

 At temperatures at which corrosion rates become too high to permit the use of a conventional nitrated ausic stainless steel heat exchanger, anodic protection is applied.

 Figures 3 and 4 show a conventional device for anodic protection. The numerical references and materials indicated previously for the heat exchanger of FIG. 2, that is to say without anodic protection, apply and are used hereinafter in FIGS. 3 and 4. FIGS. and 4 also show an elongate cathode 36, generally 9 m or longer in length, which is introduced into the heat exchanger 100 from one end of the heat exchanger 100. The cathode 36 comprises a central alloy core 38 relatively acid resistant, commercially available under the name "Hastelloy C276", surrounded by an insulating sleeve 40 of polytetrafluoroethylene perforated with numerous holes 42 to allow the acid passing through the cooling section to enter. contact with the metal core 38 of the cathode. The sheath 40 prevents the cathode core 38 from being grounded by contact with the metal parts of the heat exchanger and avoids transpassivity of the baffles and tubular plates in the immediate vicinity of the cathode. The cathode 36 is supplied with current from the negative terminal 44 of a DC power supply 46 whose positive terminal 48 is connected directly to the envelope 120.

The supply source 46 is controlled by an automatic controller 50 which is itself controlled by the potential obtained from a reference electrode 52.

 In operation, such conventional heat exchangers equipped with anodic protection can withstand tube wall temperatures of the order of 700C in the case of 96% acid and 1000C in the case of 98% acid .

 Referring again to Figures 3 and 4 which also show a heat exchanger equipped with anodic protection means according to the invention, the heat exchanger in which the casing 120, the tubular plates 20, 22, the tubes 24 and the baffles 34 are made of austenitic stainless steel having a composition of 17.5% Ni, 17.5% Cr, 5.3% Si, <0.015 t of C, the remainder being substantially Fe.

 In operation, this embodiment has desiccated wall temperatures of the order of 120C in the case of 93% acid and 1800C in the case of 98% acid. Thus, the use of this silicon-rich steel for the manufacture of heat exchangers with anodic protection extends in the temperature range in which such sulfuric acid coolers can operate and allows an increase in the recovery of the energy from the hot acid.

 In another embodiment of the invention, the cathode may also be made of this austenitic steel rich in silicon, for certain applications.

 FIG. 5 represents a conventional source of dehydration produced and implemented so as to form part of the sulfuric acid production plant previously described with reference to FIG.

 FIG. 6 shows the dehydration tower 10 of FIG. 5, modified in accordance with the invention according to which the tower 10, the distributor 10D, the mechanical anti-fog screen 10E and the support 10S are made of austenitic steel whose composition 17.5% of Ni, 17.5% of Cr, 5.3% of Si, <0.015% of C, the remainder substantially consisting of Fe, the lower part of the tower having no antacid brick coating.

 This embodiment is implemented as part of the sulfuric acid production plant previously described with reference to FIG.

 The absorption tower of FIG. 7 is made and implemented as intermediate tower 11 (and as final tower 12) forming part of the sulfuric acid production plant described above with reference to FIG. .

 FIG. 8 represents the absorption tower of FIG. 7, modified in accordance with the invention, according to which the tower 11, the distributor 11D, the anti-mist device 11E, of the candle type, and the support 11S are made of steel stainless steel composition having 17.5% Ni, 17.5% Cr, 5.3% Si, <0.015% C, the remainder being Fe, the tower having no coating of antacid bricks.

 In a variant of the invention, in addition to the use of austenitic steel rich in silicon, the tower is coated with anti-acid bricks. This allows these towers to reach higher internal temperatures than those achieved when silicon-rich steel is used alone. The absorption towers according to the invention are used as part of the sulfuric acid production plant previously described with reference to FIG.

 Reference will now be made to FIG. 9 which, as indicated, represents a portion of a sulfuric acid concentration device, comprising a vacuum evaporation device currently used by the Applicant. The apparatus comprises a plurality of vacuum stages for removing water from the lean acid, by evaporation, to retain the desired concentrated product. The apparatus comprises, as a concentration vessel, a first evaporator 110 which receives a charge. of previously heated acid, using a conduit 112 from a preheater (not shown) The bottom of the evaporator 110 is equipped with a thermosiphon loop 114 having a first branch 116 through which the acid can flow downwards and a second branch 118 allowing the acid to rise, the lower ends of the branches being interconnected by a conduit 120. The upper ends of the two branches are connected to the bottom of the evapooator 110, the level or just below the level of the surface of the acid 122 it contains.

 The acid present in the branch 118 is heated by a tantalum heating element 124 containing tantalum tubes (not shown separately) and in which high pressure hot water vapor is injected at 126, the condensation product being evacuated in 128.

Convection currents cause a flow of acid downstream from branch 116, followed by upward flow through branch 118, beyond heating element 124 where heat is passed to the acid. The water vapor evolved by the evaporation of the acid leaves the evaporator 110 via a conduit 130 and is condensed in a direct contact condenser 132. A steam ejector 134 is used to extract the air and thereby maintain the desired vacuum in the apparatus. An anti-fog screen 136 is incorporated in the evaporator 110 to remove acid drops and fog particles.

 The sulfuric acid, which has been partially concentrated in the evaporator 110, flows by gravity through a conduit 138 to the next evaporator stage 140 which is similar in design to the evaporator 110. but who works at a lower absolute pressure.

Generally, an overpressure ejector 141 is required to raise the pressure in the final stage condenser to obtain satisfactory condensation.

The evaporator 110 and the pipework used are generally made of borosilicate glass (of the "Pyrex" type)
Qu en steel coated glass to prevent any attack by acid.

 The cold feed acid (> 85%) is generally preheated by the hot product acid (93%) using a tantalum heat transfer surface located in a glass-shell heat exchanger or of steel coated with glass. The acid produced, after having been cooled to the storage temperature in a final water-cooled heat exchanger, is directed to a reservoir or pumping vessel by means of an associated set of pipes and valves.

 Anodic protection is preferably provided for all elements in contact with the acid.

 FIG. 9 is also intended to illustrate a modified acid concentration device according to the invention, in which device the concentration tank 110 and the associated thermosiphon loop 114, the heat exchangers, the pump tank and the associated set of pipes and valves are made of austenitic steel type A611.

The process is carried out as described above with reference to the prior art, except that higher working temperatures (> 300 C) can be achieved with acceptable corrosion rates to allow the production of a stronger acid (98%). Alternatively, it is possible to remove the complex and expensive vacuum devices, and the tantalum heating elements and exchangers. Higher working temperatures reduce the need for a vacuum device and replacement of tantalum elements with alloy elements
A611 achieves acceptable corrosion rates.

 In another embodiment, a concentrator having alloy elements A611 can be used in series with one of the concentrators of the prior art to concentrate the acid at higher forces than could be achieved. so far, that is, up to 98% acid, 290-300 ° C.

 It goes without saying that many modifications can be made to the apparatus described and shown without departing from the scope of the invention.

Claims (5)

 A process for concentrating sulfuric acid from an acid having a strength of 90%, characterized in that it consists in heating the acid in a concentration tank (110) formed wholly or partly of an austenitic steel containing 4.6% to 5.8% silicon.  2. Method according to claim 1, characterized in that the concentration tank is subject to anodic protection.  3. Method according to one of claims 1 and 2, characterized in that the acid is heated under reduced pressure.  4. Method according to any one of claims 1, 2 and 3, characterized in that the austenitic steel contains 5.0% to 5.6% of silicon and in particular 17.5% of Ni, 17, 5% Cr, 5.3 Si, <0.015% C, the remainder being substantially Fe.  5. Method according to any one of claims 1 to 4, characterized in that 90% placid is produced by prior concentration of the sulfuric acid filler.