GB2088350A

GB2088350A - Producing a Pressurized Oxide of Sulfur

Info

Publication number: GB2088350A
Application number: GB8135737A
Authority: GB
Original assignee: Chemithon Corp
Current assignee: Chemithon Corp
Priority date: 1980-11-28
Filing date: 1981-11-26
Publication date: 1982-06-09
Also published as: JPS57111210A; FR2495125A1; BR8107701A; JPH0359001B2; GB2088350B; MX156581A; AR226384A1; CA1178025A; IT8168393A0; IT1201033B; FR2495125B1; DE3146798A1

Abstract

A gaseous mixture comprising an oxide of sulfur and air, is produced by introducing into a sulfur burning zone molten sulfur and pressurized air having a pressure greater than 30 psig, at least some of the pressurized air having been preheated to a temperature above the ignition temperature of the sulphur and/or the pressurized air having been dried, burning the sulphur and the pressurized air to produce a pressurized gaseous mixture of sulfur dioxide and air, and controlling the temperature at which the pressurized gaseous mixture leaves the burning zone.

Description

SPECIFICATION Method for Producing a Pressurized Oxide of Sulfur Background of the Invention The present invention relates generally to methods of producing an oxide of sulfur, such as sulfur dioxide or sulfure trioxide, and more particularly to a method for producing a pressurized oxide of sulfur employing a sulfur burner.

Typically, sulfur dioxide is formed by reacting molten sulfur with a quantity of air containing oxygen in excess of that required to stoichiometrically react with the sulfur, thereby to produce a reaction mixture comprising sulfure dioxide and air. Because this reaction is exothermic, it is performed in a sulfur-burning zone which is thermally insulated because of the high temperatures produced by the exothermic reaction.

It is desirable that all the sulfur be combusted before it leaves the sulfur-burning zone. Typically, this is accomplished by maximizing the surface area of the molten sulfur exposed to the oxidizing air. Techniques for maximizing the surface area of the molten sulfur include dripping the molten sulfur over a brick checkerwork or atomizing the molten sulfur (e.g. atomizing with pressurized air).

The reaction mixture leaving the sulfur-burning zone comprises sulfur dioxide and air. The concentration of sulfure dioxide reflects the amount of combustion which has occurred in the sulfur-burning zone. Therefore, the greater the concentration of sulfur dioxide in the reaction mixture, the higher the temperature of the mixture as it leaves the sulfur-burning zone.

Sulfur dioxide may be an end material in itself or it may be converted to sulfur trioxide in a catalytic converter wherein the sulfur dioxide is reacted with additional or carryover air. There is an optimum temperature at which sulfur dioxide may be converted to sulfur trioxide in a catalytic converting zone, and, if the reaction mixture from the sulfur burner is at a temperature exceeding this optimum conversion temperature, the reaction mixture must be cooled before it enters the catalytic converting zone.Usually, the reaction mixture from the sulfur burner contains sufficient excess air to provide the oxygen necessary to form sulfur trioxide out of the sulfur dioxide, but, if sufficient oxygen is not contained in the reaction mixture from the sulfur-burning zone, additional air will have to be added to this reaction mixture at or before the beginning of the catalytic converting step for producing sulfur trioxide.

Pressurized air may be employed in the sulfurburning and catalytic converting steps, and this has a number of advantages. It increases the intensity of combustion, and this allows the combustion of a given quantity of sulfur to occur in a shorter time, thereby reducing the period of time the reaction ingredients are required to spend in the sulfur-burning zone. This, in turn, allows a reduction in the size of the sulfur-burning zone through which the reaction ingredients flow while they react.

Pressurized air also causes an increase in conversion efficiency in the catalytic converting zone which conventionally employs three or four stages to complete the conversion of the sulfur dioxide to sulfure trioxide. The use of pressurized air also reduces the size requirements for the entire processing plant.

However, the use of the pressurized air also requires an increase in the strength of the various components of the plant, including the conduits downstream of the sulfur-burning zone and which connect that zone with the catalytic converting zone. These components are usually made of stainless steel which has the corrosion resistant properties required when highly reactive materials such as hot sulfur dioxide flow through the conduits and the like.

As previously noted, the use of pressurized air at the sulfur-burning zone increases the intensity of combustion which in turn can increase the temperature of the reaction mixture leaving the sulfur-burning zone. This temperature can be so high as to heat the stainless steel components to a temperature at which the stainless steel loses its strength. As noted above, high strength properties are particularly necessary when pressurized air is employed in the reaction process.

Sulfur trioxide is employed, among other uses, for conditioning fly ash particles from flue gas and as a reactant in a process for producing sulfonates which in turn are used for making detergents.

When used in making sulfonates, the sulfur trioxide must be cooled to a temperature well below the temperature thereof when it leaves the catalytic converting zone. When the sulfur trioxide is cooled to a temperature suitable for use as a reactant in a sulfonating process, it is possible that, if the reaction mixture containing the sulfur tioxide also contains wate vapor, oleum may form due to a reaction between water vapor which precipitates during cooling and the sulfur trioxide. The formation of oleum is undesirable if the sulfur trioxide-containing mixture is to be used in a sulfonating operation. Accordingly, it is desirable to remove moisture from the air which is used in oxidizing the sulfur to sulfur dioxide and to sulfur trioxide, before the air is introduced into the sulfur-burning zone.Moisture removal is also desirable in most cases where the temperature of the sulfur trioxide-containing mixture is cooled to a temperature below its dew point because this causes the formation of oleum with its undesirable highly corrosive properties.

One conventional technique for removing moisture from air employs driers wherein the moisture is removed by absorption from the gas into water-absorbing media such as silica gel or molecular sieves. Eventually, these media absorb so much water that they lose their moistureabsorbing ability, and the media must be regenerated in order further to perform a water removing function. This regeneration usually involves blowing hot air through the watersaturated medium to remove the water therefrom and then blowing cool air therethrough to cool the dryer. This, of course, requires a capital expenditure for hot air and cool air blowers and conduits as well as an energy expenditure to operate these blowers.In addition, the cooling air is usually atmospheric air which contains moisture that is absorbed by the water-absorbing medium in the dryer during cooling of the latter, thereby reducing its subsequent drying ability.

Summary of the Invention The present invention comprises a method for producing an oxide of sulfur using pressurized air and in which all the advantages previously obtained by the use of pressurized air are retained while avoiding the drawbacks and disadvantages which have previously arisen with the use of pressurized air. The present invention also enables the removal of moisture from the air, upstream of the sulfur-burning zone, while substantially reducing the energy required to remove the moisture.

In accordance with the present invention, combustion of the sulfur in the sulfur-burning zone is essentially completed even without maximizing the surface area of the molten sulfur exposed to the pressurized air (as by atomizing or trickling over a brick checkerwork). The resulting pressurized mixture of sulfur dioxide and air may contain up to about 18 vol.% sulfur dioxide. The volume of the sulfur burner, of the sulfur dioxide coolers and of the conduits for transporting the pressurized mixture from the sulfur burner to the coolers and then to a catalytic converter is reduced to less than 50% (e.g. to one-third) of the volume required to combust the same weight amounts of sulfur and air and to handle the same weight amounts of sulfur dioxide and air, in a given time period, at atmospheric pressure.

Similarly, the volume of catalytic converting beds required to obtain essentially complete conversion (e.g., 99% or more) of sulfur dioxide to sulfur trioxide is reduced to less than 60% (e.g., to one-half) of the volume required to convert the same weight amount of sulfur trioxide, in a given time period, at atmospheric pressure. Conversion of about 99% can be obtained with only two conversion stages (with a cooling stage in between), and conversion greater than 99% is obtained with three converting stages. A pressure in excess of about 30 psig should be maintained, up to about 120 psig or to a pressure within the practical strength and thickness limits economically feasible for the equipment. 50 to 70 psig is a desirable and practical pressure range.

In accordance with one embodiment of the present invention, the pressurized air is divided into two portions before it enters the sulfur-burning zone. One of these two portions is preheated to a temperature above the ignition temperature of the sulfur, while the other portion remains unpreheated. Combustion of the sulfur is initiated by mixing the sulfur with the preheated portion of air, as the sulfur enters the sulfur-burning zone, to produce a reaction mixture containing sulfur dioxide. Then the unpreheated portion of air is introduced into the sulfur-burning zone downstream of the location therein where the preheated and the temperature to which it is The proportion of pressurized air which is preheated andshhe temperature to which it is preheated are controlled.As a result, it is possible not only to completely combust the sulfur within the sulfur-burning zone in a relative short time (thereby reducing the length of the sulfur-burning zone through which the gases must flow during the combustion process) but, also, it is possible to control the temperature of the pressurized mixture of sulfur dioxide and air leaving the sulfurburning zone so as not to affect adversely the stainless steel components of the processing system downstream of the sulfur-burning zone.

The preheated one portion of air is heated in a heat exchanger through which is flowed the hot pressurized mixture of sulfur dioxide and air leaving the sulfur-burning zone. In this heat exchanger, the sulfur dioxide is indirectly cooled while the one portion of pressurized air is indirectly preheated. Because the temperature to which the one portion of pressurized air is heated must be controlled to avoid an excessive temperature in the gases leaving the sulfurburning zone, it is not possible to completely cool the hot pressurized mixture of sulfur dioxide and air in this heat exchanger or to cool this mixture to an optimum temperature for conversion of the sulfur dioxide into sulfur trioxide; and, in such a case, an additional cooling step must be provided between this heat exchanger and the catalytic converting zone.

When a sulfur trioxide-air mixture produced by a method in accordance with the present invention is to be cooled to a temperature for use in a sulfonation operation or to any temperature below the dew point of undried air in the sulfur trioxide-containing mixture, then, in order to avoid the formation of significant amounts of oleum during such a cooling step, it is necessary to remove moisture from the pressurized air before it is introduced into the sulfur-burning zone. This is accomplished in accordance with the present invention by flowing the pressurized air through a drier which employs a chemical, such as silica gel, or molecular sieves to remove the moisture from the air. The higher the pressure of the air, the less moisture it will hold, and the precipitating moisture in the pressurized air is absorbed in the drier. Thus, pressurization facilitates drying of the air.

When the drier has absorbed sufficient moisture to become saturated, it must be regenerated. Regeneration is accomplished in accordance with one embodiment of the present invention by bleeding off a small fraction of previously dried, pressurized air from a companion pressurized dryer, depressurizing the bled-off air and flowing it through the depleted drier to absorb from that drier sufficient moisture to regenerate it. The method of this embodiment of the present invention avoids employing heating and cooling air to regenerate the driers, thereby eliminating two large fans and the associated conduits. In addition it does not deplete the drying capacity of the drier during regeneration.

Moreover, the energy expended in compressing the air to pressurize it prior to introduction into the heatless pressure driers is less than the energy expended in operating fans for regenerating unpressurized driers.

Other features and advantages are inherent in the method claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying diagrammatic drawing.

Brief Description of the Drawing Fig. 1 is a schematic flow diagram of an embodiment of a method in accordance with the present invention; and Fig. 2 is an elevational view, partially cut away and partially in section, of a sulfur-burning zone employed in a method in accordance with the present invention.

Detailed Description Referring initially to Fig. 1, air is compressed at 10 and then dried in one of a pair of driers 15, 16 following which the pressurized air is divided into two portions. One portion flows through a line 34 to a heat exchanger at 36 for preheating the qne portion of pressurized air, following which the preheated air is introduced into a sulfur-burning zone 39 along with sulfur and atomizing air. The other portion of pressurized air remains unheated and is introduced through a line 35 into sulfur burner 39 downstream of the location where the preheated air portion was mixed with sulfur.

The sulfur is combusted in the sulfur-burning zone to produce a first-pressurized mixture of sulfur dioxide and air which leaves the sulfur burner through a conduit 58. Downstream of the sulfur burner, the first-pressurized mixture of sulfur dioxide and air is cooled in heat exchanger 36 by the one portion of pressurized air which undergoes preheating at heat exchanger 36.

Downstream of heat exchanger 36 the first pressurized mixture of sulfur dioxide and air is further cooled at a radiant cooler 60 following which the mixture of sulfur dioxide and air is introduced into a catalytic converter 62 wherein the sulfur dioxide is converted to sulfur trioxide to produce a second pressurized mixture containing sulfur trioxide and air. This mixture is subsequently cooled at one or more cooling steps comprising a radiant cooler 70 and a cascade cooler 71 , following which the cooled sulfur trioxide is withdrawn through a conduit 75 which conducts the sulfur trioxide for use in a sulfonating process or for treating flue gas, for example.

The method and apparatus illustrated in Fig. 1 will now be described in greater detail. Air is compressed at compressor 10 to a pressure in the range 35-70 psig, preferably 50-70 psig. The pressurized air is then flowed through a water separator 11 for removing some of the moisture precipitated during the compression operation and through an oil separator 12 for removing oil mists which may be present in the pressurized air after the compressing operation. The pressurized air is then flowed through a conduit 1 3 to one of two unheated, pressurized driers 1 5, 1 6.

In the course of being dried, pressurized air from conduit 1 3 initially passes through a swing valve 14 which alternates the flow of pressurized air, every five minutes or so, back and forth between a branch line 23 leading to drier 15 and a branch line 24 leading to drier 16. Driers 1 5, 16 employ silica gel or molecular sieves as a drying medium, each being a stationary drying medium.

At the pressure under which the air is maintained when it flow through a drier 15 or 16 (50--70 psig), moisture is removed from the air sufficient to provide the pressurized air with a dewpoint below -400F (-400C).

Dried pressurized air is withdrawn from dryer 1 5 through a conduit 17 communicating with conduit 30 leading to sulfur burner 39. A portion of the dried air in conduit 1 7 (e.g., about 10 vol.%) is bled off through a line 26 communicating with a line 1 9 which leads through a conduit 18 into drier 16. The pressurized air bled off through line 26 is depressurized to atmospheric pressure before it is introduced into drier 16. The depressurized air flowing through drier 1 6 absorbs moisture from the drying medium in drier 1 6 to regenerate the latter. Moisture-laden air is drawn off from drier 16 through conduit 24 and from there through a swing valve 14 into a vent line 25.A similar arrangement is utilized to regenerate drier 1 5 with dry air removed from drier 16.

More particularly, dried. pressurized air is withdrawn from drier 1 6 through conduit 18 communicating with conduit 30 leading to sulfur burner 39. A portion of the dried air in conduit 18 is bled off from line 18 through line 26 into a line 20 communicating with conduit 1 7 from drier 1 5.

The pressurized air bled off from conduit 18 is depressurized to atmospheric pressure, before it is introduced into drier 1 5. As the depressurized air flows through drier 1 5 it absorbs the moisture from the drying medium in drier 1 5 to regenerate the latter. Moisture-laden air is withdrawn from drier 15 through conduit 23, swing valve 14 and vent line 25.

The drying and regenerating technique discussed above is employed in a preferred embodiment of the present invention. In a broader aspect of the present invention, pressurized drying can be employed without regenerating the drier with depressurized air in the manner described above. In such a case, the reduction of moisture to the extent possible with pressurized air is still available as an advantage, and a drier saturated with moisture can be regenerated in another fashion, e.g. with hot air.

Communicating with conduit 30 is a by-pass line 31 having a by-pass heater 32 for heating the pressurized air during standby operating conditions or during startup.

Referring now to the one portion of pressurized air which has been preheated in heat exchanger 36, the amount of this preheated air should be at least that required to react stoichiometrically with the sulfur fed into the sulfur burner. The maximum amount of preheated air is dependent upon the temperature of the first pressurized mixture of sulfur dioxide and air leaving sulfur burner 39 through conduit 58. The temperature of this mixture should not exceed 1 500cm (81 50C) in order to avoid an adverse effect upon the strength of the stainless steel of which the equipment components downstream of sulfur burner 39 are composed.

If the temperature of this mixture is too high, it can be reduced by reducing the amount of air subjected to preheating, by reducing the temperature to which this air is preheated and by reducing the amount of sulfur which is introduced into the sulfur burner for combustion into sulfur dioxide. Generally, the amount of sulfur and the amount and temperature of the preheated air are controlled to provide the first pressurized mixture leaving sulfur burner 39 with a temperature in the range 8000--15000F (426-8150C) and a sulfur dioxide content of 3-18 vol.%.

The minimum temperature of the preheated portion of air withdrawn from heat exchanger 36 is 4750F (246 C), the ignition temperature for the sulfur. As previously noted, the other portion of pressurized air, not preheated, is introduced into sulfur burner 39 through a conduit 35. This portion of the pressurized air is at essentially the same temperature as the pressurized air in conduit 30 (e.g., 60-800F (1 6--27 OC)).

The portion of pressurized air subjected to a preheating is introduced into heat exchanger 36 through line 34 and is withdrawn from the heat exchanger through a line 37.

Communicating with the bottom of sulfur burner 39, through a conduit 55, is a startup heater 40 which is used during startup of the equipment and process, before combustion of any sulfur has occurred.

Sulfur for combustion at burner 39 is melted in a sulfur melter 41, utilizing, as a heating medium, steam introduced into sulfur melter 41 through a conduit 42. Condensate is removed from sulfur melter 41 through a line 43. The molten sulfur from sulfur melter 41 is pumped by a sulfur pump 46 through filters 44, 45 and into a line 57 communicating with sulfur burner 39. The molten sulfur in line 47 is at a temperature of minimum sulfur viscosity e.g., 2700 F (132 OC). For molten sulfur, the viscosity increases above 3000F (149"C). A preferred temperature range for the molten sulfur is 2600--3000 F (127 149 C).

To facilitate the combustion of the molten sulfur in the sulfur burner 39, it is desirable, in one embodiment of the invention, to maximize the surface area of the molten sulfur exposed to the pressurized air. This is accomplished by atomizing the molten sulfur with atomizing air obtained by withdrawing from conduit 30 through branch line 49 a portion of the pressurized air and compressing it further in a compressor 48. A line 50 leads from compressor 48 to a heater 51 for the atomizing air. The atomizing air is indirectly heated in heater 51 by steam introduced through an inlet line 52. Condensate is removed from heater 51 through a consenate line 53. Heated atomizing air is withdrawn from heater 51 through a line 54.

The pressure of the atomizing air is about 30 psi greater than that of the pressurized air fed into sulfur burner 39 through conduits 35 and 37, and that pressure is preferably about 50-70 psig (e.g., 65-70 psig). Thus, the atomizing air has a pressure up to 100 psig.

Referring now to Fig. 2, the molten sulfur and the atomizing air are fed into the top of sulfur burner 39 through a pair of coaxial conduits 47 and 54, respectively. Molten sulfur exits from conduit 47 through a nozzle 77 in which the sulfur is combined with high-pressure air from conduit 54 and is immediately atomized. The atomized globules of molten sulfur have a size in the range 20-50 microns. The mixture of atomized molten sulfur and atomizing air exits into a conical shaped portion 80 in the top of sulfur burner 39. Conical portion 80 constitutes the entry to the sulfur-burning zone.

The atomizing air is heated at heater 51 to a temperature greater than 243 OF (11 70C), the freezing temperature of sulfur, and less than 3000F, above which there is a large increase in the viscosity of molten sulfur. A preferred temperature of the atomizing air is 2700F (1 32 OC). The sulfur, upon being atomized by the atomizing air, is at a temperature less than 3000F (1490C) and preferably no less than 2600F (1270C).

As the atomized molten sulfur enters conical portion 80 at the top of sulfur burner 39, it is enveloped and mixed with preheated air entering conical portion 80 through a conduit 38 (Fig. 2) communicating with line 37 from heat exchanger 36 and concentric with atomizing air conduit 54.

As previously noted, the preheated pressurized air introduced into the sulfur burner through conduits 37, 38 has a temperature which is at least the ignition temperature of the sulfur (4750F (2460C)).

As the atomized molten sulfur is mixed with the preheated pressurized air, the minute globules of moltent sulfur are vaporized and then ignited to combust the sulfur and the oxygen in the preheated air to form sulfur dioxide. As previously noted, there is sufficient preheated pressurized air to provide enough oxygen to stoichiometrically react with the amount of sulfur being introduced into sulfur burner 39.

The unheated, pressurized portion of the pressurized air entering sulfur burner 39 through conduit 35 is typically at a temperature in the range 60-800F (1 6--27 OC) when it enters the sulfer burner. This unheated pressurized air flows from conduit 35 into a manifold 81 (fig. 2) which introduces the unheated pressurized air into the sulfur burning zone downstream of the location therein where the preheated portion of air was mixed with sulfur.

It is desirable that combustion of the sulfur with oxygen to form sulfur dioxide be completed within the sulfur burner, which is lined with insulation 82. Completion therein is facilitated by introducing the air into the sulfur burner in a pressurized condition. Pressurized air causes a more intense combustion thereby reducing the length of the sulfur-burning zone through which the reacting gases must pass before combustion is completed. Typically, utilizing the pressure and preheated temperature conditions noted above, combustion can be completed in about 2.1 seconds.

When sulfur is combusted in the presence of pressurized air, the completion of combustion is so enhanced as to provide essentially complete combustion within the sulfur burner without maximizing the surface area of the molten sulfur exposed to the combusting air. Thus although in the illustrated embodiment the surface area of the molten sulfur is maximized by atomizing, in a broader aspect of the invention, both atomizing nozzle 77 and atomizing air conduit 54 can be eliminated and essentially complete combustion of the sulfur in sulfur burner 39 will still be obtained.

If the combustion of sulfur were not completed within the sulfur burner 39, sulfur burning would continue in conduit 58 downstream of sulfur burner 39, an occurrence which would be undesirable.

Completion of combustion within the sulfur burner is also enhanced by preheating that portion of the air which is initially mixed with the molten sulfur at the entry 80 to the sulfur-burning zone.

The unheated portion of pressurized air is introduced into the sulfur burner downstream of the location where the preheated pressurized air ignites the sulfur so as to enable combustion to proceed to a substantial extent before the unheated pressurized air imparts to the gaseous reaction mixture a diluting and cooling effect.

Maintaining the air introduced through conduit 35 in an unpreheated condition helps to control, to a temperature no greater than 1 5000F (81 60C), the pressurized mixture leaving sulfur burner 39 through conduit 58. As previously noted, this temperature limitation in conduit 58 is necessary in order to avoid an adverse effect upon the strength of the stainless steel components of the apparatus located downstream of sulfur burner 39.

The preheated portion of pressurized air introduced into the sulfur burner through conduits 37, 38 causes an immediate ignition of the molten sulfur when atomized globules of the latter enter the sulfur burner at conical entry portion.80. Moreover, because there is no contact between the preheated air and the molten sulfur until the latter has entered the sulfurburning zone at entry 80, premature ignition of the molten sulfur, upstream of the sulfur burner, is avoided. The molten sulfur is not subjected at all to the heating influence of the preheated pressurized air until the sulfur has entered the sulfur-burning zone and has left the conduits which transport it there.It would be undesirable to subject the molten sulfur to the heating influence of the preheated air while the sulfur was still in the conduit conducting it to the sulfurheating zone because to do so could heat the sulfur above the maximum temperature at which its viscosity is a minimum (3000F(1490C)). Had the sulfur been subjected to the heating influence of the preheated air while the sulfur was still in conduit 47 or 38 (Fig. 2), the increased viscosity would have interferred with the flow of the sulfur through these conduits and with the atomization of the molten sulfur.

As previously noted, the ratio of preheated to unpreheated air introduced into the sulfur burner is controlled to maintain the pressurized gaseous mixture leaving the sulfur burner at a temperature below 15000F (8150C).This ratio can be controlled with a valve (not shown) located at the junction of conduit 30 and branch lines 34, 35.

The technique described above, wherein the pressurized air is introduced into the sulfur burner as preheated and unpreheated portions, is one embodiment of the present invention. In another embodiment, no portion of the pressurized air is preheated, and the pressurization is relied upon to promote completion of combustion in the sulfurburning zone.

The first pressurized mixture of sulfur dioxide and air is cooled in two steps between sulfur burner 39 and catalytic converter 62. One of these cooling steps is at heat exchanger 36 wherein cooling is accomplished through indirect cooling by the preheat air as the latter undergoes heating, and the second cooling step is at radiant cooler 60. The temperature of the pressurized gaseous mixture in conduit 58 upstream of heat exchanger 36 is less than 1 5000F (81 60C), and the temperature of this gaseous mixture is cooled, in steps, down to about 8000F (4270C) when it enters catalytic converter 62. Typically, the temperature of the gaseous mixture in conduit 58 is about 1450"F, the temperature in conduit 59 between heat exchanger 36 and radiant cooler 60 is about 1300 F (7040C), and the temperature in line 61 between radiant cooler 60 and catalytic converter 62 is about 8000F (4270C).

the pressurized gaseous mixture in conduit 58 cannot be entirely indirectly cooled at heat exchanger 36, to the desired catalytic converting temperature of about 8000F (4270C), because to do so would cause an increase in the temperature of the preheated pressurized air entering sulfur burner 39 in turn causing an increase to a temperature above 1 5000F (81 60C) of the gaseous mixture exiting the sulfur burner through conduit 58, which is undesirable.

The gaseous mixture of sulfur dioxide and air enters catalytic converter 62 through line 61. In the catalytic converter, the sulfur dioxide reacts with oxygen in the air of that mixture to produce sulfur trioxide. Conversion from the sulfur dioxide to sulfur trioxide occurs in each of the three stages, 63, 64, 65 of catalytic converter 62. However, in a method in accordance with the present invention conversion is about 99% complete after second stage 64 and over 99% complete after third stage 65. Between first conversion stage 63 and second conversion stage 64 the gaseous mixture passes through a first cooling line 66, and between second conversion stage 64 and third conversion stage 65 the gaseous mixture passes through a second cooling line 67.The function of cooling lines 66, 67 is to cool the mixture as the sulfur dioxide therein undergoes conversion to sulfur trioxide, thereby to maintain the temperature of the mixture at essentially the same temperature as when it was introduced into first conversion stage 63, or lower.

The gaseous mixture leaving catalytic converter 62 through conduit 68 comprises sulfur trioxide and pressurized air at a temperature of about 800OF (4270C) or lower. This second pressurized mixture is subjected to cooling in a radiant cooler 70 to lower the temperature of the second pressurized mixture to about 350 6000F (177-3160C).The gaseous mixture leaving radiant cooler 70 may be further cooled at a cascade cooler 71 to which cooling water is supplied through an inlet line 72 and from which water is withdrawn through a line 73.

The cooled gaseous mixture of sulfur trioxide and air is removed from the cascade cooler through a conduit 75, and the temperature of the gaseous mixture in conduit 75 is typically in the range 85-1 200F (29---49 The sulfur trioxide concentration in conduit 75 is about 3 1 8 vol.%. Because the pressurized air was dried to a dewpoint below -400F (-400C), cooling of the mixture in conduit 75 to a temperature below 1 200F does not precipitate enough water to form significant amounts of oleum in conduit 75, an occurrence which would be undesirable if the gaseous mixture containing the sulfur trioxide was subsequently to be used in sulfonating operation or where the highly corrosive properties of oleum may be objectionable.A significant amount of oleum is an amount which produces an undesirable or adverse effect.

If the gaseous mixture containing the sulfur trioxide is to be used for flue gas treatment, cooling in cascade cooler 71 may be eliminated.

In a typical operation employing the method illustrated in Fig. 1 , the following amounts of material are handled at pressures of about 55 60 psig.

sulfur into sulfur 225 Ibs/hr burner (108 kg/hr.) air into sulfur 2887 Ibs/hr.

burner (1311 kg/hr.) SO2 plus air into 3112 Ibs/hr.

catalytic converter (1413 kg/hr.) SO3 plus air out of 3112 Ibs/hr.

catalytic converter (1413 kg/hr.) The sulfur burner has interior dimensions, below entry 80, as follows: diameter 21 inches (53 cm) and height 77 inches (196 cm). The conduit from sulfur burner 39 has a diameter of 4" (10 cm).

The catalytic bed in each of the three stages of the catalytic converter has a diameter of 23 inches (58 cm) and a height of 21 inches (53 cm).

The catalyst is vanadium pentoxide.

The embodiment of the present invention illustrated in Fig. 1 employs three stages of conversion from SO2 to S03 with a first cooling step at 66 between stages 63 and 64 and a second cooling step at 67 between stages 64 and 65. However, when the gas is pressurized in accordance with the present invention, conversion is so complete in the first two stages (e.g., about 99%), that the conversion operating procedures normally employed downstream of second stage 64 may be dispensed with.

Thus, if the SO3 is intended for use in fly ash treatment, where an SO, conversion from SO2 of about 99% is adequate in the treating gas, a third stage of conversion (such as stage 65) is not needed.

If the S03 is intended for use in a sulfonation operation where an S03 conversion greater than 99% is desirable, the third stage is employed to increase the conversion from about 99% (e.g., 98.8%) at the outlet of second stage 64 to greater than 99% from the outlet of third stage 65.

However, if third conversion stage 65 is employed, the gas from second stage 64 need not be cooled at 67 before introduction into third stage 65. This is because most of the 99% conversion occurring in the first two stages occurs at first conversion stage 63, with the conversion occurring at second conversion stage 64 being relatively so small that it does not generate an increase in temperature sufficient to warrant cooling between stages 64 and 65.

Cooling is required between stages 63 and 64, at 66.

Preferably, the gas is subjected to mixing between stages 64 and 65 (as well as between stages 63 and 64), and this can be performed within converter 62 by running the gas through a layer of pebbles between conversion stages, a conventional procedure in converting processes.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims

1. A method of producing a gaseous mixture comprising an oxide of sulfur and air, which method comprises introducing into a sulfur burning zone molten sulfur and pressurized air having a pressure greater than approximately 30 psig, at least some of the pressurized air having been preheated to a temperature above the ignition temperature of the sulfur and/or the pressurized air having been dried, burning the sulfur and the pressurized air to produce a first pressurized gaseous mixture of sulfur dioxide and air, combustion of the sulfur being essentially complete, withdrawing the first pressurized gasesous mixture from the sulfur buring zone and controlling the temperature at which the first pressurized gaseous mixture leaves the sulfur burning zone.

2. A method according to Claim 1 , wherein the temperature at which the first pressurized gaseous mixture leaves the sulfur burning zone is maintained at less than 81 50C by controlling the quantity and/or the temperature of the pressurized air introduced into the sulfur burning zone and/or by controlling the quantity and/or the temperature of the molten sulfur introduced into the sulfur burning zone.

3. A method according to Claim 2, wherein the temperature at which the first pressurized gaseous mixture leaves the sulfur burning zone is maintained at between 426 OC and 81 5 or.

4. A method according to any one of the preceding claims, wherein the pressurized air comprises first and second portions, of which only the first portion is preheated, the second portion being introduced into the sulfur burning zone downstream of the first portion.

5. A method according to Claim 4, wherein the first preheated portion comprises at least the stoichiometrie amount of oxygen required to convert all of the molten sulfur to sulfur dioxide and has a temperature in the range of from 4260Cto 81 50C and the temperature at which the first pressurized gaseous mixture leaves the sulfur burning zone is maintained at less than 81 50C by controlling the ratio of preheated to unpreheated air.

6. A method according to any one of the preceding claims, wherein at least some of the pressurized air is preheated, which method further comprises preventing heating of the molten sulfur by the preheated air prior to the entry of the molten sulfur into the sulfur burning zone.

7. A method according to any one of the preceding claims, wherein the pressurized air has a pressure in the range of from 50 to 70 psig.

8. A method according to any one of the preceding claims, wherein the pressurized air is predried and has a dew point of less than -400C.

9. A method according to any one of the preceding claims, wherein the pressurized air is predried by passing the unheated pressurized air through a drying medium.

10. A method according to Claim 9, which further comprises regenerating the drying medium.

11.A method according to Claim 10, wherein the drying medium is regenerated by passing heated air and cooling air successively therethrough.

12. A method according to Claim 10, wherein the drying medium is regenerated by depressurizing a portion of the pressurized air which has been dried by means of the drying medium and passing the portion of dried depressurized air through the drying medium.

1 3. A method according to Claim 12, wherein the drying is effected in first and second zones and the drying medium in the first zone is regenerated by depressurizing a portion of the pressurized air which has been dried by means of the drying medium in the second zone and passing the portion of dried depressurized air through the drying medium in the first zone, the two zones being arranged in series.

14. A method according to any one of the preceding claims, wherein the surface area of the molten sulfur entering the sulfur burning zone is maximized.

1 5. A method according to Claim 14, wherein the volume of the sulfur burning zone is less than about 50% of the volume which would be required to burn the same weights of sulfur and air if the air were at atmospheric pressure.

1 6. A method according to Claim 15, wherein the volume of the sulfur burning zone is about one third of the volume which would be required to burn the same weights of sulfur and air if the pressurized air were at atmospheric pressure.

17. A method according to any one of the preceding claims, wherein the first pressurized gaseous mixture comprises between 3 and 1 8 vol.% of sulfur dioxide.

18. A method according to any one of the preceding claims, wherein the first pressurized gaseous mixture is cooled subsequent to leaving the sulfur burning zone.

19. A method according to Claim 18, wherein at least some of the pressurized air is preheated by heat exchange with the first pressurized gaseous mixture which has left the sulfur burning zone.

20. A method according to Claim 19, wherein the first pressurized gaseous mixture is further cooled to approximately 427 OC.

21. A method according to any one of the preceding claims, which further comprises subjecting the first pressurized gaseous mixture to a catalytic conversion reaction thereby producing a second pressurized gaseous mixture comprising sulfurtrioxide and air.

22. A method according to Claims 18 and 21, wherein the first pressurized gaseous mixture is transported via first conduit means from sulfur burning zone to a cooling zone, cooled in the cooling zone and transported via second conduit means from the cooling zone to a zone in which the catalytic conversion reaction is carried out, the total volume of the first and second conduit means and the cooling zone being less than about 50% of the volume which would be required to transport and cool the same weights of sulfur and air at atmospheric pressure.

23. A method according to Claim 22, wherein the total volume of the first and second conduit means and the cooling zone is about one third of the volume which would be required to transport and cool the same weights of sulfur dioxide and air at atmospheric pressure.

24. A method according to any one of Claims 21 to 23, wherein the catalytic conversion reaction is carried out in catalytic conversion beds arranged in a plurality of converting stages.

25. A method according to Claim 24, wherein the reaction mixture is cooled between at least the first and second stages.

26. A method according to Claim 24 or 25, wherein the total volume of the catalytic conversion beds is less than 60% of the volume which would be required to carry out the reaction on the same weights of sulfur dioxide and air at atmospheric pressure.

27. A method according to any one of Claims 24 to 26, wherein the catalytic conversion reaction is carried out in two converting stages, the extent of conversion being at least about 99%.

28. A method according to Claim 27, which further comprises using the product of the reaction in a fly ash treatment.

29. A method according to any one of Claims 24 to 26, wherein the catalytic conversion reaction is carried out in three converting stages without cooling the reaction mixture between the second and third stages, the extent of conversion being greater than 99%.

30. A method according to any one of Claims 21 to 29, which further comprises cooling the second pressurized gaseous mixture.

31. A method according to Claim 30, wherein the second pressurized gaseous mixture is cooled to a temperature of less than 490C.

32. An apparatus suitable for use in producing a gaseous mixture comprising an oxide of sulfur and air, which apparatus comprises a sulfur burner, means for producing molten sulfur, means for pressurizing air to a pressure greater than approximately 30 psig, means for preheating at least some of the pressurized air to a temperature above the ignition temperature of the sulfur and/or means for drying the pressurized air and means for introducing the molten sulfur and the at least partially preheated and/or dried pressurized air into the sulfur burner, the arrangement being such that, in use, essentially complete combustion of the sulfur occurs in the sulfur burner to produce a first pressurized gaseous mixture of sulfur dioxide and air and the temperature at which the first pressurized gaseous mixture leaves the sulfur burner can be controlled.

33. An apparatus according to Claim 32, which apparatus comprises means for dividing the pressurized air into first and second portions, means for preheating the first portion, first and second conduit means for introducing the first and second portions, respectively, into the sulfur burner, and second conduit means being arranged downstream of the first conduit means.

34. An apparatus according to Claim 33, which comprises valve means for controlling the ratio of preheated to unpreheated air introduced into the sulfur burner, thereby controlling the temperature at which the first pressurized gaseous mixture leaves the sulfur burner.

35. An apparatus according to any one of Claims 32 to 34, which comprises conduit means for introducing molten sulfur into the sulfur burner, the arrangement being such that, in use, when the molten sulfur is introduced into the sulfur burner with preheated pressurized air, the molten sulfur in the conduit means is not heated significantly by the preheated pressurized air.

36. An apparatus according to any one of Claims 32 to 35, which comprises means for drying the pressurized air prior to the introduction thereof into the sulfur burner, which drying means comprises a drying medium through which the pressurized air is passed in use.

37. An apparatus according to Claim 36, wherein the drying medium is of a type which can be regenerated by passing air therethrough.

38. An apparatus according to Claim 37, which comprises means for depressurizing a portion of pressurized air which has been dried by means of the drying medium and for passing the portion of dried depressurized air through the drying medium to regenerate the same.

39. An apparatus according to Claim 38, wherein the drying medium is disposed in first and second zones, the two zones being arranged in series and the arrangement being such that, in use, the drying medium in the first zone is regenerated by depressurizing a portion of the pressurized air which has been dried by means of the drying medium in the second zone and passing the portion of dried depressurized air through the drying medium in the first zone.

40. An apparatus according to any one of Claims 32 to 39, which comprises means for maximizing the surface area of the molten sulfur introduced into sulfur burner.

41. An apparatus according to Claim 40, wherein the means for maximizing the surface area of the molten sulfur comprises means for further pressurizing a portion of the pressurized air and mixing the further pressurized portion with the molten sulfur to produce an atomised spray.

42. An apparatus according to Claim 40 or 41, wherein the volume of the sulfur burner is less than 50% of the volume which would be required to burn the same weights of sulfur and air if the air were at atmospheric pressure.

43. An apparatus according to Claim 42, wherein the volume of the sulfur burner is about one third of the volume which would be required to burn the same weight of sulfur and air if the air were at atmospheric pressure.

44. An apparatus according to any one of Claims 32 to 43, which further comprises means for cooling the first pressurized gaseous mixture after leaving the sulfur burner.

45. An apparatus according to Claim 44, wherein the cooling means is adapted to preheat at least a part of the pressurized air and comprises a heat exchanger for heat exchange between pressurized air and the first pressurized gaseous mixture.

46. An apparatus according to Claim 45, wherein the cooling means further comprises a radiant cooler for cooling the first pressurized gaseous mixture.

47. An apparatus according to any one of Claims 32 to 46, which further comprises means for subjecting the first pressurized gaseous mixture to a catalytic conversion reaction to produce a second pressurized gaseous mixture comprising sulfur trioxide and air.

48. An apparatus according to Claims 44 to 47, which comprises first conduit means for transporting the first pressurized gaseous mixture from the sulfur burner to the cooling means and second conduit means for transporting the first pressurized gaseous mixture from the cooling means to the zone in which, in use, the catalytic conversion reaction takes place, the total volume of the first and second conduit means and the cooling means being less than about 50% of the volume which would be required for the same weights of sulfur dioxide and air at atmospheric pressure.

49. An apparatus according to Claim 48, wherein the total volume of the first and second conduit means and the cooling means is about one third of the volume which would be required for the same weights of sulfur dioxide and air at atmospheric pressure.

50. An apparatus according to any one of Claims 47 to 49, wherein the means for effecting the catalytic conversion reaction comprises catalytic conversion beds arranged in a plurality of converting stages.

51 An apparatus according to Claim 50, wherein a means for cooling the reaction mixture is provided at least between the first and second stages.

52. An apparatus according to Claim 51, wherein there are provided three converting stages, no cooling means being provided between the second and third stages.

53. An apparatus according to any one of Claims 50 to 52, wherein the total volume of the catalytic conversion beds is less than 60% of the volume which would be required to carry out the reaction on the same weights of sulfur dioxide and air at atmospheric pressure.

54. An apparatus according to any one of Claims 47 to 53, which further comprises means for cooling the second pressurized gaseous mixture.

55. An apparatus according to Claim 54, wherein the cooling means comprises a cascade cooler.

56. A method of producing a gaseous mixture comprising an oxide of sulfur and air, substantially as hereinbefore described with reference to the accompanying drawings.

57. An apparatus suitable for use in producing a gaseous mixture comprising an oxide of sulfur and air, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

58. Any novel feature or combination of features described herein.