Patent Application
20070260072
This invention relates to improved process technology pertaining to oxidation of sulfur dioxide to sulfur trioxide, and to improving operations in which oxidation of sulfur dioxide to sulfur trioxide is involved.
The oxidation of sulfur dioxide to sulfur trioxide using oxygen or air and a suitable catalyst such as vanadium pentoxide is well known. Such an oxidation step is typically included in the contact process for producing sulfuric acid. Also, passing a gaseous stream containing sulfur dioxide, air and some sulfur trioxide through a bed of a vanadium-containing sulfuric acid catalyst such as preferably used in the practice of this invention maintained at about 824-1100° F. (ca. 440-593° C.) to oxidize sulfur dioxide to sulfur trioxide has been carried out heretofore. Further, it is known that sulfur can be oxidized into sulfur dioxide using a suitable oxidant such as air (auto ignition 261° C.) or oxygen (at less than 260° C.). However, the conversion of sulfur dioxide into sulfur trioxide requires a temperature activated catalyst such as a vanadium-containing catalyst, e.g., vanadium pentoxide or the like.
In Latimer and Hildebrand, The Reference Book of Inorganic Chemistry, revised edition, copyrighted in 1940 by The Macmillan Company, New York, it is stated on page 246 with reference to the contact process that “One of the factors in the successful operation of the process is the removal from the sulfur dioxide of all impurities which may ‘poison’ the catalyst and render it inactive. It is particularly important to remove traces of solid sulfur, selenium dioxide, mercury, and compounds of phosphorus and arsenic.”
In one of its embodiments this invention provides an improved process in which a gaseous stream comprised of sulfur dioxide, sulfur trioxide, and oxygen and/or air is passed through and in contact with a bed of a vanadium-containing catalyst such as a vanadium oxide catalyst (typically vanadium pentoxide), and preferably a bed of a mixture of complex inorganic salts (oxosulfato vanadates) containing sodium, potassium and vanadium salts on crystalline silica support, or a catalyst including silica as a support within a salt mixture comprised of potassium and/or cesium sulfates, and vanadium sulfates coated on the solid silica support, that oxidizes sulfur dioxide to sulfur trioxide and that releases therefrom a product gaseous stream comprised of sulfur trioxide. In this embodiment the improvement comprises having molten sulfur come into contact with the catalyst and maintaining the catalyst bed at one or more temperatures at which (i) sulfur coming into contact with the catalyst is vaporized before the gaseous product(s) formed therefrom are released from a downstream end portion of the catalyst bed and (ii) the gaseous stream released from the downstream end of said bed has an enriched content of sulfur trioxide.
The temperatures of the catalyst bed bring about (i) and (ii) above differ from each other to some extent. In order to oxidize the sulfur dioxide to sulfur trioxide as in (ii) above the vanadium-containing catalyst bed should be at one or more temperatures in the range of about 450 to about 700° C., and preferably in the range of about 450 to about 600° C. However to vaporize sulfur as in (i) above, one or more temperatures in the range of about 300 to about 450° C. are sufficient although one or more temperatures in the range of about 300 to about 700° C. can be used. Thus in conducting the above embodiment of this invention:
One of the features of the above embodiment of this invention when alternative a) is employed is that because of the high temperature(s) at which the catalyst bed is operated, the sulfur is vaporized as it comes into contact with the catalyst bed. This enables the vapors to be subjected to oxidation as they pass through the catalyst bed so that the gaseous stream released from the downstream end of catalyst bed has an enriched content of sulfur trioxide. In addition, at the high temperature(s) at which the catalyst bed is operated, sulfur vaporizes to such an extent that unduly rapid formation and buildup of sulfur coatings or deposits on the catalyst surfaces does not occur. Thus the catalytic activity of the catalyst in the bed is not adversely affected.
As will be seen hereinafter, the improved process technology of this invention can be effectively utilized for various purposes wherein sulfur trioxide is put to use.
Another embodiment of this invention is an improvement in a process in which a first gaseous stream comprised of sulfur dioxide, sulfur trioxide and oxygen and/or air is passed into a bed of a vanadium-containing catalyst that oxidizes sulfur dioxide to sulfur trioxide and that releases therefrom a product gaseous stream comprised of sulfur trioxide. In this embodiment the improvement comprises oxidizing sulfur with air, oxygen and/or sulfur trioxide (preferably with a gaseous stream which contains (i) at least sulfur trioxide and air or oxygen, or (ii) sulfur trioxide, air and added oxygen) to form a second gaseous stream enriched in sulfur dioxide and introducing at least a portion of the second gaseous stream into the first gaseous stream to form a mixed gaseous stream, and passing the mixed gaseous stream into an upstream portion of the above catalyst bed maintained at one or more temperatures in the range of about 450 to about 700° C., and preferably in the range of about 450 to about 600° C. This results in the formation of a product stream emanating from a downstream portion of the catalyst bed that is enriched in sulfur trioxide. The amount of sulfur trioxide in the product stream tends to be greater than could be predicted from the oxidation of the total amount of sulfur dioxide in the mixed gaseous stream. The oxidation of sulfur in this embodiment of the invention is carried out in a separate reactor which feeds its effluent stream as a side stream into the first gaseous stream. This reactor is not an inline reactor. In conducting this embodiment of the invention the first gaseous stream need not contain sulfur trioxide if sulfur trioxide is used in the oxidation of the sulfur in such separate reactor and if an excess amount of sulfur trioxide is fed into the separate reactor so that the feed to the first gaseous stream from the separate reactor contains some residual sulfur trioxide. However, it is preferred that the first gaseous stream and the feed to the first gaseous stream from the separate reactor both contain sulfur trioxide as this tends to further increase the amount of sulfur trioxide emanating from the vanadium-containing catalyst bed over and above that which could be predicted from the sum of (A) the amount of sulfur trioxide formed by direct mole-for-mole oxidation of sulfur dioxide to sulfur trioxide and (B) the total amount of sulfur trioxide present in the first gaseous stream and in the feed to the first gaseous stream from the separate reactor, assuming all such sulfur trioxide passed through the catalyst bed unchanged.
In the various embodiments of this invention the catalyst used is preferably a fixed bed of a vanadium-containing catalyst that oxidizes sulfur dioxide to sulfur trioxide and that releases therefrom a product gaseous stream comprised of sulfur trioxide.
Other embodiments, features and advantages of this invention will be still further apparent from the ensuing description, appended claims and accompanying drawings.
Unless otherwise specified, like numerals represent like parts among
While every attempt is made herein to make things as clear as the English language will permit, a couple of terms are discussed here so as to avoid such misinterpretations as can presently be foreseen. Thus, the term “vanadium-containing catalyst” standing alone means a catalyst which may or may not be at least in part on a surface of a catalyst support and which catalyst in either such case (i) contains vanadium in one or more chemical forms which need not be metallic vanadium itself, (ii) when at one or more temperatures in the range of about 450 to about 700° C. can, in the presence of air, oxygen, or air enriched in oxygen, cause all or at least a portion of a quantity of sulfur dioxide in contact therewith to be converted (i.e., transformed, or changed into) to sulfur trioxide, and (iii) when in the form of a bed thereof having at least a portion of such bed at one or more temperatures in the range of about 450 to about 700° C. and through at least a portion of such bed there exists a gaseous flow comprised of sulfur dioxide, will allow release from the bed of a product gaseous stream comprised of sulfur trioxide.
Hereinafter there is reference made to a description of certain catalysts in a product bulletin of Monsanto Enviro-Chem which are useful in the practice of the present invention. In such brochure it is indicated that their catalysts are properly referred to as “vanadium-containing catalysts” since they are not vanadium pentoxide catalysts. Vanadium pentoxide catalysts are also useful in the practice of the present invention. Therefore, as the immediately preceding paragraph is believed to make manifestly clear, as used herein the term “vanadium-containing catalysts” standing alone is not to be construed as limited to only the materials referred to in the product bulletin of Monsanto Enviro-Chem. Rather the term as used herein, and unless otherwise specified, is used as a generic term which is not only inclusive of the materials referred to in the product bulletin of Monsanto Enviro-Chem but in addition to other vanadium-containing catalysts that will work in the manner described herein under the conditions and with the materials described herein, such as, for example, a supported or unsupported vanadium pentoxide catalyst.
The term “vaporized” with reference to sulfur such as in terms such as “sulfur is vaporized” or “the vaporized sulfur” or the like does not mean that the vapors must be composed exclusively of elemental sulfur in vaporized form. Rather the vapors are composed of whatever is produced when molten sulfur approaches and/or contacts the hot surfaces of the vanadium-containing catalyst bed which is at one or more temperatures in the range of about 450 to about 700° C. in the presence of a gaseous flow comprised of sulfur dioxide, sulfur trioxide, and air, oxygen, or air enriched in oxygen.
The term “vanadium-containing catalyst that oxidizes sulfur dioxide to sulfur trioxide” does not mean that in the practice of this invention, the catalyst only serves the function of oxidizing sulfur dioxide to sulfur trioxide. Because of the complexity of the gaseous mixtures in contact with the catalyst, other reactions referred to in the body of this document may occur. Thus, the qualification that the catalyst “oxidizes sulfur dioxide to sulfur trioxide” is a descriptor to identify one function that the catalyst must be able to perform.
The term “total moles of oxygen per total moles of sulfur” (in phrases such as “total moles of oxygen per total moles of sulfur present in the gaseous stream to which the sulfur is added” and “total moles of oxygen per total moles of sulfur present in the gaseous stream prior to the addition thereto of molten sulfur”) refers to the molar ratio of (a) the total moles of elemental oxygen, oxygen in sulfur dioxide, and oxygen in sulfur trioxide to (b) the total moles of elemental sulfur, sulfur in sulfur dioxide, and sulfur in sulfur trioxide, wherein the components of (a) and of (b) are those present in the system being referred to.
First of all, the reactor in which the fixed bed of vanadium-containing catalyst is disposed (positioned), can be in any position relative to ground level. A few non-limiting positions include for example horizontal, substantially horizontal, vertical, substantially vertical, upwardly inclined, downwardly inclined, and so on. In preferred embodiments the reactor is in an upstanding (upright) position. Also the reactor can have any shape and cross-sectional configuration that serves the purpose of enabling the process of this invention to be conducted therewith as described herein. Naturally, the reactor will need a gas inlet portion and a gas outlet portion and should be configured such that all or substantially all of the incoming gaseous stream will pass through the fixed bed catalyst that is maintained therein. Also, the reactor needs to be equipped with heating apparatus that will enable the catalyst to be heated (e.g., to one or more temperatures in the range of about 450 to about 700° C., and preferably in the range of about 450 to about 600° C.) during startup in order to cause the process to be initiated. Once initiated, the process is sufficiently exothermic as not to require addition of further heat during the course of the reaction as temperature control can be maintained by adjusting the feed rates to the reactor. Although the catalyst bed is kept at one or more temperatures in the range of about 450 to about 700° C., brief excursions outside of this range can usually br tolerated if the period of the excursion is sufficiently brief. In this connection, there can be zones in the catalyst bed at different temperatures within this range. In other words, the catalyst bed need not be at one and only one temperature throughout.
Since the reactor is continuously exposed to internal high temperature conditions during operation and since corrosive gases are being handled and produced within the reactor, it should be fabricated from suitable corrosive resistant materials. Alonized stainless steel reactors and reactors constructed of high nickel-content alloys serve as non-limiting examples of reactors made with suitable materials of construction. Two or more reactors may be used in tandem, if desired. Indeed, it is feasible to have multiple catalyst beds arranged in series with sulfur and oxygen or air feeds between each of them in order to moderate the exothermic nature of the oxidation reaction.
Various types of vanadium-containing catalysts can be used in the practice of this invention provided that the catalyst has the ability to oxidize sulfur dioxide to sulfur trioxide. For example, in addition to vanadium pentoxide, modified vanadium pentoxide catalysts such as described in U.S. Pat. Nos. 3,793,230 and 4,285,927 may be used. Also, a vanadium pentoxide catalyst can be on a suitable support so that structural integrity is maintained and so that the catalyst can otherwise withstand the high temperature(s) at which the bed is operated. Non-limiting examples of such supports include high temperature resistant ceramics, alumina, silica, silica alumina, zeolites, and similar materials.
Among preferred vanadium-containing catalysts used in the practice of this invention, are sulfuric acid catalysts such as are available from Monsanto Enviro-Chem as LP-120, LP-110, LP-220, T-210, T-516, T-11, Cs-120, Cs- 110, Cs-210, and presumably LP-1150. According to a product brochure by Monsanto Enviro-Chem concerning such sulfuric acid catalysts and obtained from their website on Apr. 13, 2004, the LP-120, LP-110, LP-220, Cs-120, and Cs-110 are available in the shape of rings, whereas T-210, T-516, T-11, and Cs-220 are available in the shape of pellets. Dimensions of the rings and pellets are given in that brochure. Reference to LP-1150 is not made in this brochure. According to this brochure, the main components of these catalysts include SiO2 (silica as a support), vanadium (V), potassium (K), and/or cesium (Cs), and various other additives. It appears from this brochure that these catalysts may be formed from a molten salt mixture of potassium/cesium sulfates and vanadium sulfates, coated on a solid silica support. Monsanto Enviro-Chem further states that because of the unique chemistry of this molten salt system, vanadium is present as a complex sulfated salt mixture and “NOT” as vanadium pentoxide (V2O5). The brochure further states that the catalyst is more correctly called a “vanadium-containing” catalyst rather than the commonly-used “vanadium pentoxide” catalyst. It further appears from these brochures that LP-120, T-210, LP-110, and T-11 catalysts are potassium promoted, whereas Cs-120, Cs-110, and Cs-210 are cesium promoted. The cesium promoted catalysts are indicated to be more expensive, but capable of operation in a catalyst bed at lower temperatures in the range of 390-410° C. According to the material safety data sheets of Monsanto Enviro-Chem pertaining to the sulfuric acid catalysts T-516, T-210, T-11, LP-120, LP-110, LP-220, and LP-1150 have the chemical name “mixture of complex inorganic salts (oxosulfato vanadates) containing sodium, potassium, and vanadium salts on crystalline silica support. The composition of these materials is indicated to be 39-45 wt % of vanadium salt complex (no CAS No. available), 24-32 wt % of crystalline silica (CAS No.14464-46-1), and 26-28 wt % of amorphous silica (CAS No. 68855-54-9). There is also an indication the crystalline silica may comprise cristobalite and quartz. Typical physical data for these catalysts as given in the MSDS are as follows:
Appearance:
The residence time of the gases within the catalyst bed should be sufficient to enable high conversions to sulfur trioxide, and thus limited residence times (up to 5-10 seconds) are generally sufficient.
One of the preferred embodiments of this invention is to utilize a fixed bed of a vanadium-containing catalyst to oxidize sulfur dioxide into sulfur trioxide in an upstanding reactor (upright, in other words the reactor need not be perfectly vertical as it can lean or be tilted somewhat) with the incoming gaseous stream comprised of sulfur dioxide and oxygen and/or air (which stream preferably also contains sulfur trioxide) entering into the upper portion of the reactor into a headspace above the catalyst bed, and to introduce the molten sulfur above the upper end portion of the catalyst, preferably above or in an upper end portion of the headspace. In this way, the molten sulfur travels substantially downwardly toward the upper end portion of the catalyst bed and at least a portion if not substantially all of the sulfur is vaporized as it contacts and/or comes into close contact with the hot upper end portion of the catalyst. The provision of the headspace above the catalyst bed provides a zone in which at least some of the vapors produced by the vaporization of the sulfur and at least some of the incoming gaseous stream can come into contact with each other and be carried by the force of the incoming gaseous stream into the catalyst bed. Without desiring to be bound by theory, one may speculate that some oxidation of sulfur vapors may even be initiated in the lower regions of the headspace. But whatever takes place in the headspace, the end result is the release or emergence from a lower end portion of the catalyst bed of a gaseous stream enriched in sulfur trioxide with the catalyst bed itself remaining free of significant buildup or deposition of sulfur coatings on the catalytic surfaces.
In preferred embodiments of this invention wherein the incoming gaseous stream approaching the location at which the molten sulfur is fed into the gaseous stream is comprised of sulfur dioxide, sulfur trioxide and oxygen and/or air, the amount of sulfur trioxide released or emerging from the downstream end portion of the catalyst bed is higher than the amount of sulfur trioxide that would be released or that would emerge from the same downstream end portion of the same catalyst bed under the same operating conditions and with the same incoming gaseous stream in the absence of the sulfur addition. Such increased amount of sulfur trioxide released or emerging from the downstream end portion of the catalyst bed is apparently due to the occurrence of at least two reactions in the process when the incoming gaseous feed stream contains sulfur trioxide as well as sulfur dioxide and air and/or oxygen. More specifically, in the absence of sulfur addition, each mole of SO3 being fed through the oxidation catalyst is unchanged and provides one mole of SO3 in the product released or emanating from the downstream end portion of the catalyst. However, with the addition of sulfur to this incoming feed stream pursuant to this invention, theoretically each mole of SO3 upstream from the catalyst is converted in a two-step reaction sequence into 1.5 moles of SO3 in the product emanating from the catalyst. These two consecutive reactions can be expressed as follows:
2SO3+S→3SO2 (1)
3SO2+1.5O2→3SO3 (2)
Thus in addition to the conversion of the original SO2 to SO3, there is a theoretical 50% increase in SO3 formation from the sequential reactions of equations (1) and (2) above, i.e., 2 moles of SO3 becomes 3 moles of SO3. Also, both of the reactions of equations (1) and (2) can be performed in association with the catalytic oxidation of the initial SO2 in the gaseous stream to SO3—which also proceeds as in Equation (2)—without need for expensive ancillary reaction equipment. In other words, the reaction of equation (1) which can be considered as oxidation of sulfur to SO2 apparently takes place first at least to some extent, and then both the original SO2 and the newly-formed SO2 are catalytically oxidized via equation (2) to form SO3 whereby an increase in total SO3 formation occurs as compared to the same operation with the same quantities of materials except that no sulfur is fed. In fact, it is preferred to conduct these reactions in a simple reactor such as schematically depicted in
The same considerations are applicable in the embodiments of this invention wherein an improvement is provided in a process in which a first gaseous stream comprised of sulfur dioxide, sulfur trioxide and oxygen and/or air is passed through and in contact with a bed of a vanadium-containing sulfuric acid catalyst such as a vanadium-containing catalyst such as vanadium pentoxide, and preferably a bed of a mixture of complex inorganic salts (oxosulfato vanadates) containing sodium, potassium and vanadium salts on crystalline silica support, or a catalyst including silica as a support within a salt mixture comprised of potassium and/or cesium sulfates, and vanadium sulfates coated on the solid silica support, that oxidizes sulfur dioxide to sulfur trioxide and that releases therefrom a product gaseous stream comprised of sulfur trioxide. In these embodiments the improvement comprises oxidizing sulfur with air, oxygen and/or sulfur trioxide (preferably with a gaseous stream which contains (i) at least sulfur trioxide and air or oxygen, or (ii) sulfur trioxide, air and added oxygen) to form a second gaseous stream enriched in sulfur dioxide and introducing at least a portion of the second gaseous stream into the first gaseous stream to form a mixed gaseous stream, and passing the mixed gaseous stream into an upstream portion of the above catalyst bed maintained at one or more temperatures in the range of about 450 to about 700° C., and preferably in the range of about 450 to about 600° C. This results in the formation a product stream emanating from a downstream portion of the catalyst bed that is enriched in sulfur trioxide. The amount of sulfur trioxide in the product stream tends to be greater than could be predicted from the oxidation of the total amount of sulfur dioxide in the mixed gaseous stream to sulfur trioxide. The oxidation of sulfur in this embodiment of the invention is usually carried out in a separate reactor.
Referring to the preferred embodiment depicted in
The makeup of the recycled gaseous stream will vary somewhat depending on whether air, oxygen, or air enriched with oxygen is fed to the gaseous mixture upstream from the place where the sulfur from 16 is introduced. If pure oxygen is fed, the recycled gaseous stream will contain unreacted gases and in preferred embodiments, will also contain some SO3. If air or air enriched with oxygen is used, the recycled gaseous stream will contain nitrogen as well as other unreacted gases and in preferred embodiments, will also contain some SO3. In either case where SO3 is present, the proportion of SO3 in the recycled gaseous stream typically will be less than about 10 percent by volume.
In the gaseous stream heading toward the place where the sulfur from 16 is introduced, the ratios of SO2 to SO3 can also vary. Typically this ratio on a molar basis will be in the range of about 15:1 to about 25:1, and preferably in the range of about 22:1 to about 24:1. The amount of sulfur fed from 16 should be at least about 0.002 mole per mole of SO2 in the gaseous mixture to which the sulfur is added, and typically will be in the range of approximately 0.005 to 0.020 mole, and preferably in the range of approximately 0.015 to 0.018 mole, of sulfur per mole of SO2 in such gaseous mixture. Greater amounts of sulfur can be used, but ordinarily will serve no useful purpose. Also with one or more reactors having an operating temperature limit of about 1200° F. (ca. 649° C.) there should be at most about 2.0 moles of molecular oxygen (O2) per mole of sulfur as SO2 and elemental sulfur present in the gaseous stream entering the catalyst bed. Typically with reactors having such operating temperature limit, this gaseous stream will contain in the range of approximately 1.25 to 1.75 moles of molecular oxygen per mole of sulfur as SO2 and elemental sulfur present in such stream. With reactors having higher operating temperature limits, more than about 2.0 moles of molecular oxygen per mole of sulfur as SO2 and elemental sulfur can be present in the gaseous stream entering the catalyst bed. The flow rates of oxygen, SO2-SO3 gaseous mixture (or SO2 when an SO2-SO3 gaseous mixture is employed), and sulfur are measured and monitored using flowmeters. The amount of SO3 is calculated based upon the distillation temperature relative to the mole fraction SO3 at a specific distillate temperature.
Molten sulfur can be introduced to the gaseous mixture in different ways. For example the gaseous mixture can be passed under pressure through the molten sulfur or the molten sulfur can be sprayed into the gaseous mixture. Preferably the molten sulfur is initially conveyed by pumping the molten sulfur into downwardly flowing gaseous stream approaching the reactor so that both the force of the stream and gravity cause the sulfur to proceed downwardly toward and/or onto the upper end portion of the catalyst bed where the sulfur is vaporized. However introduced, the molten sulfur will almost instantaneously react with SO3 in the incoming stream to form, in situ, additional SO2, and thus at that point reduce or eliminate the SO3 content in the stream.
In plant installations involving exothermic reactions, the chemical processing equipment is typically operated below a projected thermal maximum which includes a selected margin below the maximum rating for the equipment being used. It follows therefore that in order to operate safely, operational limitations are typically equipment-related thermal limitations due to the exotherms of these oxidations. For example, a given plant for use in the practice of this invention may be thermally rated for operation at 650° C. (1200° F.) or 750° C. (1382° F.).
Thus the temperatures used in the process can vary depending upon the particular equipment used and the rates and volumes at which the materials are being processed provided of course that the sulfur fed to the gaseous stream is fully vaporized. Generally speaking, the temperatures of the zone in which the molten sulfur reacts with the SO3 in the incoming stream are typically within the range of about 232 to about 600° C. and preferably in the range of about 450 to about 600° C. for incoming gaseous feeds in the range of about 2500 to about 10,000 kg/hour, provided the equipment can be safely operated under these conditions. As noted above, the temperatures in the oxidation catalyst zone are typically in the range of about 450 to about 700° C. and preferably are in the range of about 450 to about 600° C. When a supported or unsupported vanadium-containing catalyst is used, the temperature(s) in the catalyst bed should not reach the temperature at which the unsupported catalyst melts or loses its ability to perform its role as a catalyst or at which the supported catalyst melts, loses its ability to perform its role as a catalyst, undergoes material degradation or suffers loss of structural integrity. Departures from the foregoing ranges are permissible and are within the scope of this invention provided that the departures do not materially interfere with the process and do not constitute hazardous operating conditions in relation to the processing equipment being used.
Operating pressures of at least 10 psig, and preferably at least 100 psig are preferred for converting SO2 to SO3. Maximum pressures are preferably about 150 psig, but can be higher if desired.
As noted above, it is particularly preferred to conduct both the reduction of SO3 by sulfur to form SO2 (which is also oxidation of sulfur to SO2) and the subsequent catalytic oxidation of SO2 into SO3 in reactor 25. In such case, reactor 25 comprises mixing sections in which vaporized sulfur reacts with SO3 to enrich, in situ, the SO2 concentration, and a catalytic section in which the oxidation of SO2 to SO3 takes place.
Although various configurations for reactor 25 can be used,
In a typical plant installation employing a supported vanadium-containing catalyst and incoming gaseous stream containing SO2 and a small amount of SO3, the approximate heat-limited flowrates around reactor 25 are 20,000 lb/hr (9072 kg/hr) of SO2/SO3 gaseous mixture containing in the range of about 0.010 to about 0.042 mole of SO3 per mole of SO2, 175 lb/hr (79 kg/hr) of sulfur, and 560 lb/hr (254 kg/hr) of oxygen. In an operation of this scale, reactor 25A having a total height of about 10 feet, and a diameter of 3 feet with a 4-foot high headspace or dead volume 60 and a 6-foot high catalyst bed packed with supported vanadium-containing catalyst having an average particle size of about 0.25-0.50 inch, has been found very suitable. Reactor 25A is preferably constructed of alonized stainless steel.
Initially the reactor is heated by an indirect gas-fired furnace and the temperature is easily maintained from the resulting exotherms of the oxidation reactions.
Distillation column 35 is used to separate the recycle mixture of SO2/SO3 and inerts from the product SO3. As with the reactor, it is preferred to run the column under 100 psig although higher pressures can be used, within normal high-pressure equipment limitations.
Conversions of SO2 to SO3 of at least about 99.8% are achievable by proper conduct of this invention.
It will be readily apparent that the flow diagram of
In
Referring more specifically to
The schematic flow diagram of
The following Examples demonstrate the efficacy of the process of this invention in producing high yields of sulfur trioxide from gaseous streams containing both SO2 and some SO3. In particular, these Examples involve tests demonstrating that the temperature depend equilibrium which liberates SO3 (from thermally cracking H2SO4) can be used along with sulfur to generate additional SO2. These Examples are presented for purposes of illustration and not limitation with respect to the generic scope of this invention.
A 1-inch by 24-inch quartz furnace tube was filled with ¼-inch ceramic Berl saddles containing 2.02 g of sulfur (preloaded). The materials were placed inside a furnace operated at 450° C. and the exit vent was fitted with a trap containing 18 g of NaOH as an 11.7 wt % aqueous solution in a gas absorption bottle. Sulfuric acid (106.35 g) was pumped into the furnace tube during a period of about 2.5 hours with observation of steam reflux which included traces of vaporized sulfur. The exit gas was trapped as sodium sulfite (Na2SO3) and analyzed using excess iodine and then back-titrated with sodium thiosulfate. The SO2 yield was 49.33%.
The procedure of Example 1 was repeated using 2.16 g of sulfur (preloaded) and 24.72 g of 96% H2SO4, which were placed inside the furnace (operated at 450° C.) and the exit vent was fitted with a trap comprised of a 250 mL gas absorption bottle containing 152.05 g of 21.1 wt % aqueous NaOH. Sulfuric acid was pumped into the furnace tube for a period of about 0.3 hour with observation of steam reflux which included traces of vaporized sulfur. The exit gas was trapped with sodium sulfite (Na2SO3) and analyzed using excess iodine and then back-titrated with sodium thiosulfate. The SO2 yield was 50.8%.
The procedure of Example 1 was repeated except that 4.5 g, 140 mmols of sulfur were preloaded into the furnace tube and reacted with 81 mL (149.04 g, 1.52 mols) of H2SO4 in a 1-inch by 18-inch pipe heated to 452-464° C. inside a furnace. The exit gas was trapped using aqueous NaOH and analyzed as in Example 1. The total SO2 yield was 86%.
A study was carried out in order to determine whether sulfur could be oxidized by SO3 at temperatures in the range of about 700-1100° F. (ca. 370-593° C.) and whether the other compounds in the sulfur would also be oxidized or form solids and plug a vanadium oxide catalyst bed in the reactor. Accordingly, a continuous, laboratory scale, sulfur oxidation unit was constructed. This unit is schematically depicted in
In
The exit gases were passed through an empty flask to knock-out any entrained liquids and then were collected in a series of two scrubbers. In the first run, the first scrubber contained water (800 gms) and the second scrubber contained a 25 wt % caustic solution (840 gms). The caustic scrubber was maintained at 40° C. to prevent Na2SO3 precipitation and pluggage of the dip-tube. Accurate analysis of the scrubber contents for SO2 and SO3 was difficult with this approach. After the first experimental run, the materials in these scrubbers were changed to improve the analysis. Following this change, the first scrubber contained a bromine/water mixture (25 Ogms/380.2 gms) while the second scrubber contained only water (800 gms). The first scrubber oxidized the SO2 to SO3 by the following reaction:
SO2+2H2O+Br2→H2SO4+2HBr
The second scrubber trapped the SO3 and HBr exiting from the first scrubber. Samples of each scrubber were collected and analyzed for wt % bromide and wt % acid. During each experimental run, the unit was operated for 35-60 minutes. Conditions spanning a wide range of sulfur:SO3 molar ratios (2.4:1 to 0.25:1) were investigated.
In order to address the concern regarding the buildup of solids and potential pluggage of the vanadium-containing catalyst bed by the other components present in the sulfur, a sample of commercial plant sulfur was burned with air to determine the amount of material remaining. Sufficient material was burned to collect enough residue for analysis by ICP.
Table 1 contains a summary of the results from these four sulfur oxidation experimental runs. The amount of sulfur conveyed with nitrogen was calculated with ChemCad by assuming the nitrogen was saturated with sulfur vapors at the molten sulfur conditions. Note that the molten sulfur was held at 365-390° C. for the excess sulfur experiments (runs 1 and 2) and at 290° C. for the excess SO3 experiments (runs 3 and 4). The SO3 flowrate was varied from 1.0 to 1.8 gms/min. Even though equipment problems existed with all but one of the four experimental runs conducted, the results indicate that a substantial amount of the SO3 and sulfur react to form SO2 under suitable reaction conditions.
Throughout each experiment, the average reactor residence time was maintained at approximately 2-3 seconds. During periods where the residence time through the reactor was shorter, a blue liquid was observed to exit the reactor and collect in the knock-out flask. A sample was collected and analyzed by GC-MS. While both solution (with methylene chloride/methanol and acetonitrile) and headspace analysis revealed only SO2, a residue was observed on the GC injection port liner. Literature references (Kagramanov, et al., “Physicochemical Properties of a Sulfur-Fuming Sulfuric Acid System”, Zh. Prikl. Khim., 1987, 60(10), pp 2177-82; Weast (ed.), CRC Handbook of Chemistry and Physics, 56th Ed., Boca Raton, Fla., 1975) suggest that this blue liquid is S2O3 an intermediate in the desired reaction.
To determine the residue contained in the sulfur after oxidation, sulfur (267.5 gms) was added to a quartz tube and placed in a tube furnace. Air was fed continuously to the bottom of the tube through a quartz dip-tube. The furnace temperature was maintained at 380° C. Both oxidation and vaporization of the sulfur occurred. The vaporized sulfur was condensed and collected in a round-bottom flask. The SO3 produced from oxidation was trapped in a large scrubber. The exit gas temperature was monitored throughout the oxidation process. Over the 6-8 hr period during this experimental run, the exit gas temperature fluctuated between 270 and 340° C. The majority of the residue was produced after the molten sulfur level in the tube fell below the dip-tube exit where the air was being fed. Prior to this point in this run, most of the oxidation occurred below the surface of the molten sulfur. Since operation temperatures were well above the sulfur auto-ignition temperature (232.2° C.) the liquid surface ignited after the sulfur liquid level fell below the dip-tube exit. All subsequent oxidation occurred at or above the gas-liquid interface. A rapid increase in the exit gas temperature to a maximum value of 473° C. was also observed after surface ignition. Oxidation during these conditions was less efficient and left more residue. The residue collected during this experiment was ash-like in form and was estimated at 84 ppm based upon the initial sulfur charged. A sample of this residue was collected end analyzed by ICP. Results of this analysis are provided in Table 2. Note that the residue is primarily comprised of sulfur and components contained in quartz. Sulfur samples were also submitted for a sulfur-ash test. The residue contents (after vaporization at 300° C.) for the starting material and the vaporized sulfur collected during the oxidation experiment were determined from the ash test to be 241 and 79 ppm, respectively. After ignition at 1000° C., the residue contents for these same samples were 28 and 54 ppm, respectively.
A separate sample of commercial plant sulfur was submitted to an outside laboratory (Galbraith Laboratories, Inc.) to determine the carbon content. The commercial plant sulfur was determined to contain 0.19±0.01 wt % carbon. In order to determine whether this carbon would be sufficiently oxidized by SO3, amorphous carbon powder (5 gms) was placed in a furnace tube with approximately 3-4 inches of glass beads on top to prevent powder entrainment. Sulfur trioxide (approximately 80 gms) was fed over a 1.5-hour period through the carbon bed with the furnace temperature set at 538° C. (1000° F.). Following this feed period, the tube was allowed to cool and then was weighed to determine the carbon remaining. Approximately 1.8 gms of carbon was estimated to have been oxidized during the experiment. The remaining carbon was displaced to one side of the tube bottom which suggests that some SO3 may have by-passed the carbon after a period of time. The results of this experiment indicate that SO3 will oxidize the carbon that enters with the sulfur at the 700-1100° F. oxidation bed operating temperatures. A small amount of carbon powder escaped from the furnace tube and was trapped in the liquid SO3 collected in the condenser attached at the top of the tube. While no carbon was observed to be exiting the condenser, the SO3 puddles in the condenser became clearer after a period of approximately 10-20 minutes. From this observation, it was concluded that oxidation of the carbon could occur at lower temperatures as well.
The results obtained in Examples 4-6 indicate that sulfur trioxide (SO3) will oxidize sulfur to form sulfur dioxide (SO2) at suitable operating temperatures used in the oxidation of SO2 to SO3. Conversion of sulfur in a 2-3 second residence time gas phase in a plug-flow reactor at 700° F. (ca. 370° C.) should be at least 90% or greater. Small amounts of the reaction intermediate S2O3 was observed to exit the reactor at shorter residence times. Sufficient residue was collected from oxidation and vaporization of the commercial plant sulfur to perform an elemental analysis. This residue was primarily comprised of un-oxidized sulfur and elements contained in the quartz tube used during the test. The commercial plant sulfur was analyzed for carbon content and determined to be 0.19±0.01 wt %. The additional experiment of Example 6 suggests that carbon oxidation will occur at 700-1100° F. (ca. 370-593° C.) operating temperatures often used in the oxidation of SO2 to SO3. Carbon oxidation was observed to occur at lower temperatures as well.
In an experimental plant scale operation using a system comparable to that of
Brief Resume
From the foregoing description of this invention it will be seen that this invention in its broadest aspects involves providing a catalyst zone which can be in a single reactor or in two or more reactors in series. One or more vanadium-containing catalysts beds are in the reactor(s), preferably as one or more fixed beds. Different vanadium-containing catalysts can be used in the vanadium-containing catalyst bed(s). At least one bed, and preferably all of the beds, in the catalyst zone contain a vanadium-containing catalyst that oxidizes sulfur dioxide to sulfur trioxide. Fed into the bed (or a first bed in a series of beds) is a first gaseous stream comprised of sulfur dioxide, sulfur trioxide, and air or oxygen or air mixed with additional oxygen. (Air mixed with additional oxygen is also known as air enriched with oxygen). Emanating from the bed (or a last bed in a series of beds) is a second gaseous stream enriched in sulfur trioxide.
A feature of one such process is that sulfur, preferably molten sulfur, is introduced into at least the first gaseous stream so that the sulfur is vaporized and carried into the catalyst bed of the reactor (or a first catalyst bed in a series of catalyst beds) so that the stream emanating from the first catalyst bed (or from the first catalyst bed in a series of catalyst beds) is enriched in sulfur trioxide. When only one catalyst bed is used, the stream emanating therefrom is the above second gaseous stream. When more than one catalyst bed is used, the stream emanating from the last catalyst bed in the series is the above second gaseous stream. When more than one catalyst bed is used, sulfur can be fed upstream from each catalyst bed so that vaporized sulfur is formed from the sulfur of each sulfur feed and such vaporized sulfur enters its downstream catalyst bed. Preferably, additional sulfur trioxide, and/or air or oxygen or air mixed with additional oxygen, is also fed upstream from each catalyst bed. Additionally, when more than one catalyst bed is used, heat removal from the stream emanating from each successive bed should be carried out because of the highly exothermic reactions taking place in the reactors.
A feature of another such process is that sulfur dioxide is generated from sulfur and sulfur trioxide, and/or air, oxygen, or air mixed with additional oxygen and the resultant sulfur dioxide-containing gaseous stream is added to the first gaseous stream referred to above. In other respects, this process embodiment is generally similar to the system described in the immediately preceding paragraph except that sulfur itself is not introduced into the first gaseous stream referred to above.
In all of the processes referred to in this resume, the make up of the feed(s) to the catalyst bed is/are preferably adjusted or controlled (or the feeds to the catalyst beds are preferably adjusted or controlled), and the amount(s) of sulfur and any additional air or oxygen or air mixed with additional oxygen added to the stream heading to the catalyst bed is/are preferably adjusted or controlled (or the feeds to the catalyst beds are preferably adjusted or controlled) to the numerical values given hereinabove so that significant increases in sulfur trioxide content in the final product stream are achieved.
Compounds referred to by chemical name or formula anywhere in this document, whether referred to in the singular or plural, are identified as they exist prior to coming into contact with another substance referred to by chemical name or chemical type (e.g., another component, a solvent, or etc.). It matters not what chemical changes, if any, take place in the resulting mixture or solution, as such changes are the natural result of bringing the specified substances together under the conditions called for pursuant to this disclosure.
Also, even though the claims may refer to substances in the present tense (e.g., “comprises”, “is”, etc.), the reference is to the substance as it exists at the time just before it is first contacted, blended or mixed with one or more other substances in accordance with the present disclosure.
Except as may be expressly otherwise indicated, the article “a” or “an” if and as used herein is not intended to limit, and should not be construed as limiting, the description or a claim to a single element to which the article refers. Rather, the article “a” or “an” if and as used herein is intended to cover one or more such elements, unless the text expressly indicates otherwise.
All documents referred to herein are incorporated herein by reference in toto as if fully set forth in this document.
This invention is susceptible to considerable variation within the spirit and scope of the appended claims. Therefore the foregoing description is not intended to limit, and should not be construed as limiting, the invention to the particular exemplifications presented hereinabove. Rather, what is intended to be covered is as set forth in the ensuing claims and the equivalents thereof permitted as a matter of law.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US04/12861 | 4/27/2004 | WO | 10/25/2006 |
