GB2277887A - Monolithic catalysts for conversion of sulfur dioxide to sulfur trioxide - Google Patents

Abstract

Chemically and thermally stable monolithic catalysts are disclosed having alkali metal-vanadium active phases for use in the conversion of sulfur dioxide to sulfur trioxide. The alkali-vanadium catalyst comprises a foraminous ceramic support having a porous silica substrate for the active phase at its foraminal wall surfaces. An alkali metal-vanadium active catalyst is in the pores of the porous silica. Methods for producing the catalysts of the invention are described.

Description

MONOLITHIC CATALYSTS FOR CONVERSION OF SULFUR DIOXIDE TO SULFUR TRIOXIDE Backqround of the Invention Various monolithic catalysts have been proposed in the art for use in lieu of particulate catalysts in the contact process for the manufacture of sulfuric acid.

Platinum catalysts on both particulate and monolithic substrates have been suggested in the art for conversion of sulfur dioxide to sulfur trioxide.

DE-A-3910249 discloses a process for the production of a catalyst for the oxidation of sulfur dioxide gas that includes V205, a potassium salt, diatomaceous earth, and a sodium polyacrylate binder. Addition of water to this catalyst mixture allows it to be extruded to obtain a honeycomb-like form.

US-A-4539309 describes catalysts for the oxidation of sulfur dioxide to sulfur trioxide that are prepared by dissolving vanadium pentoxide in an alkali solution, acidifying the solution with sulfuric acid, mixing the acidified solution with a carrier, molding or extruding the mixture, and drying and calcining the molding or extrudate.

Working examples describe particulate extrudates having diameters of 6 mm.

Certain of the monolithic catalysts known to the prior art have been subject to thermal degradation, for example, by sintering of the active phase, at the temperatures of conversion of sulfur dioxide to sulfur trioxide. Initially highly active, they lose activity rapidly in commercial operation. Other catalysts provide a level of activity until contaminated by reaction byproducts or dusts contained in the reaction gases, but lack the chemical stability necessary for effective regeneration of the catalyst. Consequently, there has been a need in the art for monolithic catalysts which provide for high rates of conversion but are both thermally and chemically stable.

US-A-3259459 describes a process for the production of SO3 using either vanadium or platinum catalysts.

Interpass absorption is a common practice in the art, as further illustrated, for example, by the disclosures of US-A-1789460 and US-A-3142536.

US-A-3963423 discloses a high gas throughput process for the conversion of SO2 to SO3. Each pass of catalyst contains at least three catalyst trays that are arranged either horizontally or vertically beside one another. GB-A-2081239 describes a catalytic oxidation process for producing S03 from SO2 that uses a monolithic catalyst. An S02 and 03containing gas stream is passed through the monolithic catalyst at a superficial gas velocity of at least 500 actual ft./min. (2.54 actual meters/second).

US-A-3987153 describes an integrated process for the reduction of SO2 emissions from a single absorption sulfuric acid plant consisting of multi-stage oxidation of SO2. In at least the final pass, a cesium-containing particulate catalyst is loaded.

DE-A-3911889 describes a contact process for the production of sulfuric acid that uses a honeycomb catalyst arranged in one or more layers.

Sulfuric acid plants often are built with capacities of 2000 to 3000 short tons (ST)/day [1814.37 to 2721.55 metric tons/day] (as 100k H2S04). The SO2 gas composition is in the range of 10 to 11k by volume or higher. This rate of production leads to relatively large diameter (often 30 to 40 feet [9.14 to 12.19 meters] or more) reactor vessels containing catalyst loadings on the order of 30 to 50 liters/short ton (L/ST) [33.07 to 55.12 liters/metric ton] (as 100% H2SO4) or more per pass.

Current regulations on S02 emissions levels from sulfuric acid plants often require that 99.7% or more of the SO2 fed to the first pass of the reactor be converted to SO3. There is an unfilled need for a sulfuric acid process that gives high rates of H2 SO4 production, affords lower catalyst loadings in the upper passes, and at the same time, permits high levels of overall SO2 conversion that equal or exceed 99.7% in a four-pass process.

Summary of the Invention Among the several objects of the present invention, therefore, are the provision of a monolithic catalyst for the oxidation of sulfur dioxide to sulfur trioxide; the provision of such a catalyst which operates at high gas velocity with low pressure drop; the provision of such a catalyst which provides a high rate of conversion of sulfur dioxide to sulfur trioxide; the provision of such a catalyst which is resistant to active phase sintering and other forms of thermal degradation; the provision of such a catalyst which is chemically stable; and the provision of methods for the preparation of such a catalyst. It is a further object of this invention to provide a general process for high rates of conversion of sulfur dioxide to sulfur trioxide over an economical number of catalyst passes operated at higher gas velocity than conventional sulfuric acid plants.

It is a particular object of the invention to provide such a process which may be implemented using a reactor vessel for several stages which is of small diameter in relation to those required for a conventional sulfuric acid process having a given production capacity.

Briefly, therefore, the present invention is directed to a monolithic catalyst for the conversion of sulfur dioxide to sulfur trioxide comprising a foraminous monolithic ceramic support. In one embodiment, the support has at the foraminal wall surfaces thereof a high porosity silica substrate for an active catalyst phase. An alkali metal-vanadium active phase is in the pores of the high porosity silica.

The invention is further directed to a process for the preparation of sulfur trioxide. In the process, a gas containing sulfur dioxide and oxygen is contacted with a monolithic catalyst for the oxidation of sulfur dioxide to sulfur trioxide. The catalyst comprises a foraminous monolithic ceramic support, the support having at its foraminal wall surfaces a high surface area silica substrate for an active catalyst phase, said high surface area silica substrate having a surface area of at least about 15 m2/g, and, on the substrate, a platinum active phase. After a decline in the activity of said catalyst is incurred due to aging or exposure to the conditions of oxidation of sulfur dioxide to sulfur trioxide, the catalyst is contacted with a mineral acid to regenerate its activity, thereby producing a regenerated catalyst.Preparation of sulfur trioxide is resumed by contacting a gas containing sulfur dioxide and oxygen with the regenerated catalyst.

Other objects and features will be in part apparent and in part pointed out hereinafter.

Brief Descriotion of the Drawing Figure 1 is a flow sheet illustrating a novel process of the invention in which the catalysts of the invention can be used.

Description of the Preferred Embodiments Monolithic catalysts of this invention show improved performance characteristics over those of particulate catalysts for oxidation of S02 to S03, including higher mass and heat transfer, lower pressure drop per unit volume, and excellent physical stability. Use of the catalyst having these characteristics allows greater throughputs of S02-containing gas streams and lower capital costs per unit of SO3 manufacturing capacity. Relative to the monolithic catalysts of the prior art, the monolithic catalysts of this invention have high activity per unit volume for the oxidation of SO2 to SO3, improved thermal and, at least in some instances, chemical stability, and the capability for regeneration and reuse.

Physical stability of the monolithic catalyst is defined by retention over a period of service of a combination of features that include mechanical stability of the monolithic support, porosity of the monolithic surface, and microstructure of the supported active phase. These factors can be measured through various tests. Mechanical stability of the monolithic structure is measured through axial crushing strength or the modulus of rupture.

Retention of porosity and catalyst microstructure is assessed through a combination of tests on the monolithic catalysts for thermal and chemical stability.

Monolithic catalysts of the invention are composite materials comprising a foraminous support having cells, pores or channels through which gas may flow at high velocity and low pressure drop, a high surface area substrate and a promoter for a catalyst active phase at the foraminal wall surfaces of the support, and an active catalyst phase comprising an alkali metal vanadium composition. Preferably, the support comprises a honeycomb shape which may have various cell shapes and diameters, but whose cells or pores are large enough so that the support is permeable to a gas flowing at high velocity.On the foraminal wall surfaces is a substrate for the active phase, the substrate comprising a finely porous (often microporous) surface coating which is either an integral part of the honeycomb support, as generated in the preparation of the support, or is provided subsequently by way of a washcoat film. An integral high surface area substrate is provided at the foraminal wall surfaces where the support is produced by co-extrusion of a high surface area/high porosity silica together with a lower surface area/lower porosity silica.

Where a washcoat is used, a washcoat slurry is prepared comprising the high surface area silica in a filmforming sol comprising silica, zirconia, titania or the like. The washcoat slurry is applied to the foraminal walls of the support, then dried to produce a green washcoat. The green washcoat is calcined, causing the high surface area silica to become bound to the foraminal wall surfaces by an adhesive film produced by drying and calcination of the sol.

Suitable supports or substrates for the preparation of monolithic catalysts of this invention are preferably ceramic thin-walled honeycomb structures with low surface areas. Representative methods for the manufacture of such materials are given in US-A-3790654 and US-A-4364888, the disclosures of which are expressly incorporated herein by reference. Materials suitable for such foraminous supports include alumina, cordierite (orthorhombic magnesium aluminum metasilicate; Mg2Al403(SiO3)5), mullite (3Al203*SiO2), mullite-aluminum titanate, and titania. A preferred material for use in this invention is mullite, in particular, Corning Celcor Code 9494. (The word Celcor is a trade mark).Nominal cell densities of mullite honeycomb substrate include 9, 16, 25, 50, 100, 200, 300, and 400 cells per square inch (cpsi) [about 1, about 2, about 4, about 8, about 15, about 31, about 46, and about 62 cells per square centimeter]. For this invention, the preferred cell density is between 100 and 400 cpsi (about 15 and about 62 cells per square centimeter) of a cross section taken transverse to the direction of flow through the catalyst.

Most preferably, the cell density is between 100 and 300 cpsi (about 15 and about 47 cells per square centimeter) with square cells, although the choice may vary with the specific application. The permeability of the foraminous support is such that the pressure drop of a gas containing sulfur dioxide, oxygen and nitrogen flowing at a velocity of 600 standard feet per minute (3.05 standard meters per second) through a monolithic catalyst comprising such support is not greater than about 8 inches water per lineal foot (about 65.38 millibar per lineal meter) in the direction of flow. Typically the foraminous void fraction of the monolith is in the range of between 0.25 and 0.75.

An alternative honeycomb monolithic support is provided by combining a high porosity oxide with a low surface area oxide to produce a composite material that possesses the permeability necessary for gas flow, the fine porosity needed for effective catalyst activity and the mechanical strength conferred by the low surface area oxide.

Typical materials for the preparation of these silica composite honeycombs include a low density, high porosity silica powder with below 20 microns average particle size; a low surface area silica with particles having a particle size between 20 and 74 microns such as SUPERSIL silica from Pennsylvania Glass Sands Co.; and a silicone resin such as Dow Corning Resin Q6-2230. A plasticized mixture (or "dough") suitable for extrusion is made through the addition of an aqueous phase comprising water and a lower alcohol such as, for example, isopropyl alcohol. Further details on the composite monolithic supports of this type are given in US-A-4631267, US-A-4631268, and US-A-4637995, the disclosures of which are expressly incorporated herein by reference.The preferred material for the monolithic catalysts of this invention is silica extruded in nominally 100 to 300 cpsi (about 15 to about 47 cells per square centimeter) with square cells. These composite silica supports have total pore volumes from 0.25 to 0.50 mL/g with surface areas from 15 to 50 m2/g. Higher pore volumes (0.50 to 0.75 mL/g) can be obtained, but the resulting silica monolithic supports lack adequate mechanical strength. As reported in US-A-4631267, mechanical strength is adequate where the modulus of rupture is greater than 500 pounds per square inch (psi) [35.15 kg/cm2]. The high porosity silica component of the silica composite monolithic support can be selected from several silica powders with high surface areas (100 to 500 m2/g) or silicas with low surface areas (below 10 m2/g) but high pore volumes such as diatomaceous earths.

These silica honeycombs can be used with platinum active phases. Table I provides a listing of representative composite silica honeycombs prepared by the above referenced methods.

TABLE I Honeycomb Codea LBU-500/1000 LBU-1000 LFC-1000 LFD-1000 HOT-1000G HOT-1000S Honeycomb Number 4269771A 4269771B 4344421C 4344421D 3740999A 3998931B Total Intrusion Volume, b mL/g 0.303 0.309 0.375 0.419 0.390 0.361 Total Pore Area, b m/g 22.2 22.8 16.9 40.1 36.1 20.6 Median Pore Diameter, b mm 0.370 0.387 0.564 0.633 0.420 0.524 Average Pore Diameter, b mm 0.0548 0.0541 0.0887 0.0418 0.0432 0.0699 Bulk Density, b g/mL 1.38 1.44 1.28 1.25 1.26 1.26 Water Pore Volume, c cc/g 0.29 0.29 0.30 0.35 0.33 0.34 aThese honeycombs all have square cells with about 200 cells per square inch.

bDetermined through mercury intrusion porosimetry using a Micrometrics Autopore 9220-II.

cDetermined through modification of ASTM Method C127-84.

Honeycombs coded LFC-1000 and LFD-1000 were prepared using 10 and 20% diatomaceous earth, respectively.

Composite silica honeycombs exhibit a range of surface area at the foraminal walls of the support. Where a high surface area/high porosity silica is used in the preparation of the honeycomb, the surface area may range from 100 to 400 m2 per gram of the mono-lith, with a pore volume of 0.5 to 2 cc/g. However, where diatomaceous earth is used, the surface area may range as low as 2 m2/g, with a porosity in the range of 1 cc/g.

In the acidic environment encountered by catalysts for the oxidation of sulfur dioxide, silica-based supports are chemically stable. The silica honeycombs above are representative of a preferred type of support for active phases for the oxidation of SO2. As noted in US-A-4631267, these all-silica monolithic supports are attractive alternatives to washcoated honeycombs where in a high dustand particulate-containing gas stream, the washcoat can become removed from the underlying monolithic support.

In order to make use of the improved performance monolithic catalysts of this invention, the active phase must be added in the appropriate form and amount and then activated for use through a proper procedure. It has been found that deposition of the active phase on the monolithic support and substrate produces catalysts that exhibit excellent thermal and, in some cases, chemical stability compared to known sulfuric acid catalysts, and also provide the reaction engineering advantages of monolithic catalysts.

It is possible to prepare alkali-vanadiumcontaining monolithic catalysts from the silica honeycomb substrates described above and represented by the examples in Table I. Silica honeycomb composite materials represented by honeycomb codes LFC-1000 and LFD-1000 that contain 10 and 20%, respectively, of a diatomaceous earth are particularly suitable for preparation of alkali-vanadium-containing monolithic catalysts of this invention.

The preferred alkali-vanadium catalysts consist of one of the novel silica monolithic composite supports representated in Table I with an active phase that is applied to the silica monolith using solution impregnation of soluble salts of potassium, cesium, and vanadium followed by drying and calcination at 400 to 5000C in an oxygencontaining atmosphere, preferably containing S03. These monolithic materials may be used in a catalytically effective manner for the oxidation of SO2, particularly as low temperature "caps" upstream from a conventional particulate catalyst bed. Low temperature "cap" operation has been described by H. Jensen-Holm and T.D. King ("Oxidation of Sulphur Dioxide - New Catalyst Types," presented at the Sulphur 88 conference in Vienna, Austria, November 8, 1988, pages 75-84.) and H. Jensen-Holm and 0.

Rud-Bendixen ("Industrial Experience with the Topsoe VK5S Sulphuric Acid Catalyst and the WSA-2 Process," Sulphur 1990 Preprints, The British Sulphur Corporation Ltd., 1990, pp.

291-310) for a cesium-containing particulate catalyst. The monolithic alkali-vanadium catalysts described here are effective at gas velocities higher than those possible with particulate catalysts (i.e. > 120 SLFM [ > 0.61 standard linear meters per second]) and at low temperatures (380 to 4100cm.

Monolithic catalyst usage will depend upon the gas temperature, gas composition, and the flow rate.

The alkali-vanadium active phase under reaction conditions consists of molten mixture of sulfate salts dispersed evenly over the silica monolith support. The alkali ions are selected from a group consisting of sodium, potassium, rubidium, and cesium, and most preferably, mixtures of potassium and cesium salts including all potassium and all-cesium mixtures. Cesium-containing catalysts offer advantages in increased low temperature activity for the mixed alkali-vanadium active phase over catalysts containing nearly all alkali as potassium. Cesium containing catalysts are described in US-A-1,941,426, US-A3,789,019, US-A-3,987,153, US-A-4,193,894, US-A-4,431,573, US-A-4,539,309, US-A-4,680,281, and US-A-4,766,104, and in SU-A-1,202,610 and SU-A-1,347,972.The active phase is loaded onto the silica support by impregnation of a homogeneous solution of alkali and vanadium ions at any pH value that results in a solution of these ions. The alkali/vanadium atomic ratio is adjusted in the range of 2:1 to 6:1, preferably between 2.7 and 4.0:1, more preferably 3.0 to 3.6. The optimum ratio is between 3.3 and 3.6:1.

The optimum loading of the alkali-vanadium active phase on the silica monolithic support is adjusted in a range given by an a parameter (H. Livbjerg, K. F. Jensen, and J.

Villadsen, Journal of Catalysis, 45, 216-230 (1976)) defined here as follows: a = WVVP), where WV = (grams of vanadium calculated as V2O5)/(gram of silica monolith) pV = (grams of V2O5)/(cc of liquid molten salt) Vp = pore volume of the silica monolith in cc/(gram of silica monolith) Livbjerg, et al. note that for alkali-vanadium molten sulfate salt mixtures, the value of PV is about 0.30 g-V2O5/cc liquid using either uniform liquid film or dispersed plug models. An acceptable range of a values for use with the catalysts of this invention includes 0.10 to 0.40 but is preferably in the range from 0.10 to 0.30.

For the silica monolithic supports used here, an a value around 0.30 is preferred.

Sources of vanadium useful in the preparation of an alkali vanadium catalyst include, for example, vanadyl sulfate, ammonium vanadate, and alkali metal vanadates.

In preparation of the monolithic catalyst, the high porosity silica substrate at the foraminal wall surfaces of the monolithic support is impregnated with a solution containing such a vanadium compound. Preferably, the solution also contains potassium or cesium ions, and anions selected from among sulfate, carbonate, hydroxide, nitrite and nitrate. The impregnated support is thereafter dried, thereby depositing an activable catalyst mixture in the micropores of the silica substrate. Activation of the dried, impregnated alkali-vanadium salt mixture on the silica monolithic support is accomplished through a thermal treatment in a range of 400 to 6000C, preferably at approximately 500'C, in an O2-containing atmosphere preferably containing S03.

Fig. 1 illustrates a process for the oxidation of sulfur dioxide to sulfur trioxide using the catalyst of the invention. A mixture of S02 and 02 enters a contact converter 11 comprising three preliminary catalyst beds 13, 15 and 17. Oxidation of S02 to S03 in catalyst bed 13 results in the generation of a substantial amount of heat which is removed by passing the gas exiting stage 13 through an external heat exchanger 19, typically a waste heat boiler. Gas exiting exchanger 19 is returned to converter 11 and passed through catalyst bed 15 where further oxidation of SO2 to S03 takes place.Gas leaving bed 15 is removed from the converter, passed through another heat exchanger 21, and thence through an interpass absorption tower 23 wherein the 503 contained in the gas is absorbed in sulfuric acid. Gas leaving interpass tower 23 is returned to converter 11 and passed through catalyst bed 17 for further oxidation of S02 to S03. The gas leaving tower 23 is reheated to the optimum initial temperature for third pass conversion before it is introduced into catalyst stage 17.

The pressure drop and catalytic efficiency of the monolithic catalysts of the invention are such that such catalysts may be used in each of catalyst beds 13, 15, and 17. Moreover, because of the favorable relationship between conversion rates and pressure drop, use of the catalyst of the invention in these beds allows the gas velocity through stages 13, 15 and 17, to be substantially higher, and thus the diameter of converter 11 to be substantially smaller, than would be the case if a conventional particulate alkali vanadium catalyst were used in these beds.

Gas leaving heat exchanger 25 is passed to a conventional fourth stage converter 27 containing a catalyst bed 29 comprising a particulate vanadium catalyst, preferably a Cs-V catalyst. In the fourth pass, residual S02 is converted to SO3, after which the gas is passed through another heat exchanger 31, and final absorption tower 33.

Because a particulate catalyst is used, the catalyst bed of converter 27 has a diameter substantially greater than that of catalyst beds 13, 15 and 17, and the flow velocity through converter 27 is substantially lower than the velocity through the beds of converter 11.

In accordance with the process of the invention, a sulfur dioxide containing gas, having an SO2 content or between about 7% and about 13%, preferably about 8% to about 12%, is introduced into first preliminary monolithic catalyst stage 13, and then passed in series through further preliminary stages 15 and 17. Preferably, all three stages operate under adiabatic conditions. The gas leaving each preliminary stage is cooled, as described above, to maintain a favorable thermodynamic equilibrium in the immediately succeeding stage.

The gas enters first stage 13 (at point A) at a temperature not substantially higher than that required for calculated temperature rise to the thermodynamic equilibrium. Preferably, the gas at point A is between 420"C and 45C0C. The converted gas stream exiting stage 13 at point B, containing sulfur dioxide, oxygen, and sulfur trioxide, is passed through heat exchanger 19 and cooled to the desired inlet temperature to the second stage, preferably 460"C to 500"C (point C).

Further conversion of SO2 to SO3 occurs in the second stage, but the rate of conversion in the second stage is significantly lower than in the first stage.

Consequently, the gas stream exiting the second stage at point D often may not reach the calculated adiabatic equilibrium conversion of sulfur dioxide.

Gas leaving the second stage at point D then passes through economizer 21, where the gas is cooled to a temperature above the dew point of the gas stream. Sulfur dioxide in the gas stream is then absorbed into a sulfuric acid stream in interpass absorption tower 23.

Tower 23 may be operated at a low acid temperature to minimize corrosion to piping and heat exchangers. Alternatively, the interpass absorption may be operated at high temperature under the conditions described in McAlister et al. U.S. patents 4,576,813 and 4,670,242 for recovery of the heat of absorption. In the processes of these patents, the acid entering absorption tower 23 has a temperature of at least 1200C and a strength of at least 98.5%. Absorption acid discharged from tower 23 has a temperature of at least 140"C and a strength of at least 99%. The discharge acid is cooled in a heat exchanger by transfer of heat to another fluid, thereby heating the other fluid to a temperature of at least 1200C, preferably greater than 1400C.Advantageously, steam may be generated in the heat exchanger at a pressure of, for example, 55 psig (4.81 bar) or higher. By maintaining the acid strength throughout the cooler at > 99%, various conventional iron/chromium and iron/ chromium/nickel alloys may be used for construction of the heat exchanger.

Gas exiting tower 23 is returned to converter 11 at point F at the desired inlet temperature for the third pass, preferably between 4500C and 475"C. In stage 17, the reaction approaches the thermodynamic equilibrium.

The converted gas is cooled in heat exchanger 25 to a temperature which is above its dew point, preferably 360"C to 415"C. The gas stream exiting heat exchanger 25 (at point H) is introduced into the second reactor vessel 27 containing fourth stage particulate catalyst bed 29. As in the case of the first pass, the preferred temperature of the gas entering the third and fourth passes is not substantially higher than that required for adiabatic temperature rise to the calculated thermodynamic equilibrium or near thereto.

The gas stream leaving the fourth pass (point I) is cooled in heat exchanger 31 and then passes into a final absorption tower 33.

Under the conditions described above, the monolithic catalyst activity in the first three stages is high enough to afford high reaction rates at high gas velocities, so that the thermodynamic equilibrium is reached or closely approached in both the first and third stages. However, the gas temperatures are kept low enough that a favourable equilibrium is preserved for the reversible reaction: SO2 + 2 = S 3' and maximum conversions are achieved.

To achieve a total conversion of 99.7% of the gas entering the first preliminary catalyst stage, and/or to achieve an SO2 emission level of not greater than about 350 ppm in the tail gas exiting the process, a particulate vanadium catalyst containing cesium is required for the final catalyst stage. By use of a particulate Cs-V catalyst, fourth stage reaction can proceed to the thermodynamic equilibrium with a low inlet gas temperature in the aforesaid range of 3600C to 4150C. Thus, equilibrium is reached at a low temperature, which favours a higher conversion of SO2 to SO3. A suitable catalyst is described in US-A-4,193,894. Preferably, the mole ratio of Cs to V in the particulate catalyst is at least 0.75.

Catalyst loadings for the final stage are essentially the same as those used in a conventional contact acid plant.

Because a particulate catalyst is used, the final stage is operated at conventional gas velocities. As a consequence, the vessel housing the fourth stage catalyst bed is generally of a diameter comparable to that of the final contact stage of a conventional sulfuric acid plant having the same productive capacity.

For the desired ultimate conversion of 99.7W, it has been found necessary that interpass absorption be conducted between the second and third stages (2:2) system.

The process can be operated if the interpass absorber is located between the third and fourth stages, but the highest overall SO2 conversion is achieved with a 2:2 rather than a 3:1 system.

In a further method of the invention for the manufacture of sulfur trioxide, sulfur dioxide is reacted with oxygen in converter 11 as described above in connection with Fig. 1. After a period of operation, the activity of the catalyst will typically decline, due to the combined effect of aging and exposure to the temperature, erosion and corrosion conditions prevailing during the SO2 oxidation process. At that point, the catalyst may be temporarily taken out of service and treated with a mineral acid to produce a regenerated catalyst of renewed high activity.

Because of the chemical stability of the catalyst of the invention, the catalyst may be subjected to vigorous treatment which has the effect of restoring its activity, not causing damage to it. Thereafter the regenerated catalyst may be placed back in service and oxidation of sulfur dioxide to sulfur trioxide resumed.

The following examples illustrate the invention.

Test Reaction Methods for Examples Comparative reactor evaluation methods were used to quantify the differences in performance between the monolithic catalysts of this invention and those existing in the prior art. Two of these methods are particularly useful for comparative studies of the ability of various materials to oxidize catalytically sulfur dioxide: (1) the thermal catalyst aging tester (TCAT) reactor and (2) the activity tester reactor.

The TCAT reactor is designed to test different catalyst samples under identical conditions for the oxidation of S02 at various inlet temperatures. A number of reactors, each containing a catalyst sample, operate under closely isothermal conditions. A common feed gas supply is mixed and delivered at the same volumetric flow rate to each sample by means of individual mass flow controllers. The inlet and outlet gas samples are analyzed and the S02 conversion of the inlet gas stream is determined. A set of SO2 conversions is measured at each of a series of temperatures for all samples of catalyst in their fresh states. Then the temperature is raised to 700-7500C for 24 hours. This high-temperature treatment provides a simulated accelerated aging process.The temperature is then lowered to the lowest fresh inlet temperature and the S02 conversions are again measured for all samples. The temperatures are again incremented to give the same set of initial temperatures used for the fresh sample cycle and the SO2 conversions are again measured. The most effectively thermally stabilized catalyst samples are those which show the least decline in SO2 conversions between the fresh and aged cycles at various inlet temperatures.

The second comparative reactor evaluation method uses an activity tester reactor which measures differential conversions of sulfur dioxide when a gas stream at a specified volumetric flow rate and gas composition is passed over a catalyst sample. The inlet temperature is held at between 475 to 4820C. A slip stream of the outlet gas is passed through a sulfuric acid bubbler to remove the S03, then the gas stream is sampled and analyzed for SO2, 02, and N2. A portion of the inlet gas stream is analyzed by gas chromatography for the initial S02, 02, and N2 compositions and the S02 conversions determined.

A third reactor system was used to evaluate the ability of the monolithic catalysts of this invention te operate under the heat and mass transfer conditions encountered in a full-sized sulfuric acid plant. An integral reactor system in the laboratory was used that would simulate a single bed of an adiabatic sulfuric acid converter. At various intervals down the reactor, sample tubes are located that contain thermocouples to measure the gas temperatures down the bed. Additionally, the sample tubes withdraw gas samples into a multiport rotary valve for gas chromatographic analysis. Gas is supplied to the reactor by means of mass flow controllers and may be preconverted to simulate lower pass operation. A preheater section is used to adjust the S02-containing gas stream to the desired inlet temperature to the reactor.

EXAMPLE 1 Alkali-vanadium-containing silica composite monolithic catalysts represent preferred embodiments of this invention. The use of silica composite honeycomb substrates affords much more mechanical strength than honeycomb substrates prepared from 100% of a porous silica powder such as diatomaceous earth. These silica composite monolithic catalysts are shown in this example to have excellent thermal stability toward high-temperature accelerated aging.

Two 2.6 cc samples of the LFC-1000 and LFD-1000 silica monolithic substrates given in Table I were impregnated with the sulfate salts of cesium and vanadium (IV), dried at 900C, and calcined in a 5000C S03-containing gas stream.

These samples have Cs/V = 3.34 and a values of 0.30, with the a parameter used as defined above. These samples were loaded into the TCAT reactor tubes as samples 31 (LFC-1000) and 32 (LFD-1000) along with a 2.6 cc sample of 10 to 20 mesh ( < 2.0 mm to > 850 ym) of LP-120 catalyst having a K-V formulation, designated sample 30. The TCAT reactor data are shown in Table II.

TABLE II Catalyst 30 30A 31 31A 32 32A SO2 Conversion, %, of 9 SO2, 11% 02 at Temperature, C 360 3.4 3.6 3.7 9.9 7.0 11.7 370 5.1 5.6 5.1 14.7 10.9 17.2 380 7.3 8.1 13.8 22.0 18.0 26.6 390 10.8 12.1 25.1 32.9 28.5 40.7 400 16.2 17.1 40.0 46.8 45.3 55.4 410 26.5 23.3 58.7 59.6 58.7 64.8 420 42.0 33.3 67.8 67.3 67.8 71.8 EXAMPLE 2 This example demonstrates the use of cesium as an active phase promoter for increasing S02 conversions per unit volume of catalyst. The amount of cesium added was varied according to the inlet gas temperature and S02 content of the gas stream.

A series of four alkali-vanadium monolithic catalysts was prepared according to the procedure described in Example 1. Each catalyst has an LFC-1000 silica composite honeycomb substrate, (Cs+K)/V = 3.34, and a = 0.30, but vary in the Cs-K content. The catalysts are designated K3.34 (33), Cs0.75-K22.59 (34), Cs1.50-K1.84 (35), and Cs3 34 (36) with the numbers in parentheses corresponding to the sample number. Differential conversions were measured at 125 SLFM [0.635 standard linear meters per second] for the four Cs-K-V-containing monolithic catalysts using both 10 and 8% S02 gas streams. The results are given in Table III for a constant inlet temperature of 4800C.

The differential conversion results in Table III show a marked improvement in S02 conversion activity as the cesium content of the alkali-vanadium catalyst is increased.

TABLE III Catalyst 33 34 35 36 Space Velocity, /hr., 91700 92900 91700 93600 10% SO2, 11% O2; 88 SO2, 13% 2 Inlet Temperature, cc 480 480 480 480 SO2 Conversion, of 10% SO2 3.87 5.70 6.91 7.45 Space Time Yield,a x106, 10% S02 1.95 2.84 3.49 3.69 502 Conversion, of 88 SO2 6.02 8.36 9.65 10.41 Space Time Yield, b x106, 8% SO2 2.45 3.36 3.92 4.14 aSpace Time Yield = t% SO2 Conversion/100)(Volumetric Flow Rate, S02)/(Space Velocity), where the Volumetric Flow Rate is 4.63 moles/hr., and the Space Time Yield is given in units of moles of SO2 converted or moles of SO3 produced.

bSpace Time Yield same as defined in footnote except the Volumetric Flow Rate is 3.71 moles/hr.

EXAMPLE 3 A set of 12 silica composite monolithic catalysts with an average volume of 34.57 cc and 2.40 cm in diameter were loaded into a tubular reactor. These samples were prepared with the same composition and procedure as those for sample 36 in Example 2. The reactor was loaded into a vacuum jacketed vessel and sealed. Gas streams containing 8 and 10% SO2 were passed through the reactor at 125 SLFM [0.635 standard linear meters per second]. The gas temperatures were monitored down the reactor in order to determine the point at which a temperature of 4250C was obtained in this closely adiabatic reactor. Inlet temperatures were varied from 380 to 4100C at 100C intervals. Based on the observed temperatures, the bed depth to the nearest inch (2.54 cm) was determined in which a temperature rise to 4250C would be observed.A summary of the bed depths is given in Table IV.

TABLE IV Inlet Bed Depth in Inches [cm] required Temperature to give 4250C for the S02 Gas Strenath C 8% 10% 380 33 [83.82] 27 [68.58] 390 19 [48.26] 21 [53.34] 400 8 [20.32] 12 [30.48] 410 3 [ 7.62] 5 [12.70] At a temperature of 4250C, a conventional bed of K V particulate sulfuric acid catalyst gives an adiabatic temperature rise to yield essentially the theoretical equilibrium conversion of the SO2 in the gas stream (75.5 and 67.5% conversion of 8 and 10% S02, respectively, gas streams). Through the use of low temperature (380-4100C) caps of cesium-containing alkali-vanadium monolithic catalyst upstream from a conventional particulate sulfuric acid catalyst bed, the overall conversion in the first pass can be increased with more heat generated for recovery.

Claims (5)

1. A monolithic catalyst for the conversion of sulfur dioxide to sulfur trioxide comprising a foraminous monolithic ceramic support, said support having at the foraminal wall surfaces thereof a porous silica substrate for an active catalyst phase, and an alkali metal-vanadium active catalyst in the pores of said porous silica.

2. A monolithic catalyst according to claim 1, wherein said support is produced by co-extrusion of a high surface area silica having a surface area of at least 100 m2/g and a low surface area silica having a surface area of less than 10 m2/g, said substrate comprising high surface area silica at the foraminal wall surfaces of said support.

3. A monolithic catalyst according to claim 1 or claim 2, wherein said alkali metal is selected from potassium and cesium and the atomic ratio of alkali metal to vanadium in the active catalyst is between 2:1 and 6:1.

4. A process for the preparation of a monolithic catalyst for the oxidation of sulfur dioxide to sulfur trioxide comprising the steps of: impregnating a foraminous composite silica substrate with a solution of vanadium compounds and alkali metal salts activable to provide a catalyst for the conversion of sulfur dioxide to sulfur trioxide, said foraminous support having been produced by co-extrusion of a microporous high surface area silica having a surface area of at least 100 m2/g and a low surface area silica having a surface area of less than 10 m2/g, said support being permeable to high velocity flow of a gas containing sulfur dioxide and oxygen; drying the impregnated support, thereby depositing an active catalyst mixture of alkali metal and vanadium compounds in the micropores of said microporous silica; contacting the surfaces of said active catalyst mixture with a stream of air containing an oxide of sulfur selected from sulfur dioxide and sulfur trioxide at temperature above 4000C to activate the catalyst.

5. A process according to claim 4, wherein said solution contains cesium and potassium ions and anions selected from sulfate, carbonate, hydroxide, nitrite and nitrate.