WO2008031535A1

WO2008031535A1 - Process for the production of sulphuric acid

Info

Publication number: WO2008031535A1
Application number: PCT/EP2007/007822
Authority: WO
Inventors: Sven Ivar Hommeltoft; Jens H. Hyldtoft
Original assignee: Haldor Topsøe A/S
Priority date: 2006-09-14
Filing date: 2007-09-07
Publication date: 2008-03-20

Abstract

Process for the separation of sulphur oxides from off-gases comprising the steps of: (a) passing off-gas through at least one moving bed of sulphur oxide absorbent having catalytic activity in the oxidation of sulphur oxides to sulphur trioxide and comprising supported on a carrier a mixture selected from the group consisting of vanadium and sulphates of one or more alkali metals, vanadium and pyrosulphates of one or more alkali metals, vanadium in addition to sulphates and pyrosulphates of one or more alkali metals; (b) converting the sulphur oxide to sulphur trioxide and absorbing sulphur trioxide on the absorbent; (c) regenerating the sulphur trioxide loaded absorbent by passing the absorbent through at least one pneumatic transporter prior to entering the at least one moving bed of step (a) and withdrawing a stream of stripping gas from the at least one pneumatic transporter; (d) removing desorbed sulphur trioxide in the stripping gas of step (c) by formation of sulphuric acid and/or oleum in a sulphuric acid condenser and withdrawing sulphuric acid and/or oleum product from the process.

Description

PROCESS FOR THE PRODUCTION OF SULPHURIC ACID

FIELD OF THE INVENTION

The present invention relates to a process for the removal of sulphur oxides from off-gases, more particularly for the removal of sulphur oxides from off-gases containing less than 5000 ppmv SO₂ such as off-gases from power plants. The process involves the catalytic oxidation of SO₂ by treating the off-gas with a sorbent containing vanadium (V) and py- rosulfate-sulphate mixture (s) on a porous carrier. The invention relates also to a moving bed apparatus specifically designed for use in the process.

BACKGROUND OF THE INVENTION

Process gases containing sulphur dioxides, hereinafter referred as off-gases, vary significantly in their SO₂ con- tent. Strong off-gases such as exhaust gases from metallurgical operations and from the combustion of hydrogen containing fuels contain sulphur dioxide in concentrations of 10% to 30% vol. The so-called thin off-gases, such as smoke gas (flue gas) from power plants, have on the other hand a low content of SO₂, often below 5000 ppmv. Conventional off- gas treatments, particularly of thin off-gases have normally involved the use of lime scrubbing in which the SO₂ is converted to gypsum, which is subsequently disposed of. This consumes lime and produces a low value waste product.

It would be desirable to convert the SO_x in the off-gases to marketable sulphuric acid, and therefore it has been pro- posed to use sulphuric acid catalyst acting both as catalyst and absorbent for the removal of SO₂ under the formation of pyrosulphate. However, for various reasons technologies based on this principle have so far had limited application.

US Patent Nos. 3,615,196 and 3,989,798 disclose a process for the conversion of SO₂ in which the gas is a catalyst comprising potassium oxide and vanadium pentoxide catalyst in an absorption zone, then continuously passing the catalyst containing absorbed SO₂ into contact with an oxygen containing gas in a conversion and desorption zone and recovering desorbed SO₃, whereby vanadium oxide functions as the catalyst and as the absorbent.

European Patent EP 0,514,941 describes a process for the reduction of SO₂ from off-gases by using a solid sulphur oxide absorbent which is regenerable by air and from which absorbed sulphur dioxides are desorbed as sulphur trioxide during regeneration. The desorbed sulphur trioxide is removed from the regeneration air by hydration to sulphuric acid followed by condensing the sulphuric acid in a sulphuric acid condenser. The process involves the use of six fixed absorber/desorber beds that are switched between ab- sorption and desorption modes and which require the inexpedient use of a large number of hot operating valves. The proper operation of these valves and consequently the proper operation of the fixed bed absorber/desorber is a rather difficult task, which significantly impairs the economy of the process and in worst cases renders the fixed bed absorbers completely inoperative. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a process which is more cost-effective than prior art processes.

It is another object of the present invention to provide a process that avoids the technical problems associated with the use of hot operating valves.

We have now found that by transporting the solid catalyst absorbent around in the process rather than the process gas itself, a process is obtained which overcomes the above technical problems.

Accordingly, we provide a process for the separation of sulphur oxides from off-gases comprising the steps of:

(a) passing off-gas through at least one moving bed of sulphur oxide absorbent having catalytic activity in the oxidation of sulphur dioxide to sulphur trioxide and comprising supported on a carrier a mixture selected from the group consisting of vanadium and sulphates of one or more alkali metals, vanadium and pyrosulphates of one or more alkali metals, vanadium in addition to sulphates and pyro- sulphates of one or more alkali metals;

(b) converting the sulphur dioxide to sulphur trioxide and absorbing sulphur trioxide on the absorbent;

(c) regenerating the sulphur trioxide loaded absorbent by passing the absorbent through at least one pneumatic trans- porter prior to entering the at least one moving bed of step (a) and withdrawing a stream of stripping gas from the at least one pneumatic transporter;

(d) removing desorbed sulphur trioxide in the stripping gas of step (c) by formation of sulphuric acid and/or oleum in a sulphuric acid condenser and withdrawing sulphuric acid and/or oleum product from the process.

The solid catalyst absorbent is thus transported in the process as it is moved from a first moving bed absorber, then lifted in a pneumatic transporter up to a level above the level of solid catalyst absorbent in said first moving bed before being returned to the first moving bed.

In a preferred embodiment, the absorption in step (a) is conducted at a temperature between 300⁰C and 400⁰C. We have found that this range of temperatures gives the best results, since at lower temperatures the catalytic absorbent is inactive and at higher temperatures the SO₃ emissions from the absorbent are unacceptably high for flue gas emis- sion to the stack.

Preferably, the regeneration step (c) comprises passing the absorbent through at least one pneumatic transporter and subsequently passing the absorbent through at least one moving bed prior to entering the at least one moving bed of step (a) and withdrawing a stream of stripping gas from the least one pneumatic transporter. Thus, after lifting the absorbent in a pneumatic transporter up to a level above the level of solid catalyst absorbent in said first moving bed, the absorbent is passed through a second moving bed absorber before being returned to the first moving bed. The provision of the second moving bed absorber, which normally is much smaller than the first moving bed, enables the de- sorption of any remaining SO₃ in the solid catalyst absorbent coming from the riser as well as the cooling of the particles prior to entering said first moving bed. The second moving bed absorber acts therefore as an efficient heat exchanger, in which the solid particles are cooled by heat exchange with a suitable heat exchanging medium such as air.

The provision of the pneumatic transporter, which preferably is a riser, enables the recirculation of the solid catalyst absorbent and at the same time enables the heating and regeneration of the solid catalyst absorbent during its transport. In order to decompose the pyrosulfate and to regain the original absorption capacity of the catalytic absorbent (sulphur oxide absorbent) which normally lies in the range 15 - 20% wt SO₃, the desorption step in the riser is preferably conducted in the temperature interval 450 - 650⁰C. The riser may be provided at its top with a disengagement chamber, such as a cyclone in order to retain eventual solid catalyst absorbent. From the riser a stripping gas containing SO₃ is withdrawn. It would be understood that the stripping gas stream is withdrawn from the riser and/or the second moving bed and that both streams may be joined to form a single combined stream. Stripping gas is normally withdrawn from a disengagement chamber adapted at the top of the riser.

In a preferred embodiment the solid catalyst absorbent particles are transported in the riser in fluid-bed motion, whereby the regeneration of the solid absorbent becomes more effective due to the high heat transfer coefficient achievable during the fluid bed operation. Preferably, SO₃- depleted gas from the sulphuric acid withdrawal operation is used as fluidized medium in the riser. The provision of the riser in the form of a fluid bed enables also the riser to be heated directly and limits the need of using a fluid- ising medium such as air at high temperatures, e.g. 950⁰C. In addition, the flow of fluidising medium and thereby the size of the riser can be decreased. On the other hand the demand for residence time may increase and accordingly the fluid bed is advantageously operated at residence times of about 1 min.

The term absorbent is used to define a solid sorbent, which is contacted with the gas containing sulphur oxides (SO_x) , and wherein the mass transfer occurs by transport of sulphur oxides from the gas to the solid as in the first moving bed or from solid to gas as in the second moving bed of the regeneration step.

The term "at least one moving bed" as used herein implies that the first moving bed of step (a) may be divided in a number of separated moving beds. Preferably step (a) is conducted in a single moving bed of sulphur dioxide absor- bent. Similarly, in the regeneration step (c) the moving bed may be divided in a number of separated moving beds. In another embodiment, instead of the at least one moving bed in the regeneration step (c) , staged fluid bed may be used before the absorbent enters the at least one moving bed of step (a) . In step (d) the removal of desorbed sulphur trioxide in the stripping gas of step (c) may be effected by contacting the gas with sulphuric acid under the formation of oleum if the sulphuric acid used for the absorbtion is anhydrous or if the sulphuric acid contains water by hydration of the desorbed sulphur trioxide to sulphuric acid, which is condensed into the liquid phase and withdrawn from the process as sulphuric acid product.

The sulphuric acid condenser for removing sulphur trioxide contained in the stripping gas from the riser and second moving bed may be provided as a sulphuric acid/oleum wash- tower in which the SO₃ is absorbed and recovered either as sulphuric acid or as oleum. SO₃-depleted gas from this sul- phuric acid condenser is then used as fluidized medium in the riser.

The sulphuric acid condenser for removing sulphur trioxide contained in the stripping gas from the riser and second moving bed is preferably provided with a plurality of tubes being externally cooled by air flowing counter-currently and in indirect heat exchange with the air inside the tubes. In order to further improve process economy it is advantageous that at least part of the air used for cooling the catalyst in the second moving bed comes from the sulphuric acid condenser. Hence, by the invention at least part of the cooling air leaving the sulphuric acid condenser is used as heat exchanging medium during regeneration of the solid catalyst absorbent.

In a preferred embodiment the solid catalyst absorbent comprises vanadium (V) and sulphates of alkali metals sup- ported on a porous carrier. Vanadium may for instance be present in the form of vanadium pyrosulphate. Preferably, the solid catalyst absorbent, which is for instance provided in the form of spherical pellets, comprises vanadium dissolved in a mixture of sulphates and pyrosulphates of alkali metals supported on silica selected from the group consisting of silica gel, precipitated silica and fumed silica. A preferred support is silica gel. We have found that contrary to the results obtained with conventional silica carriers used in fixed beds, such as diatomaceous earth, the provision of the above silica types as carrier enables that the particles of solid catalyst possess sufficient physical strength to be transported around in the process.

The process according to the invention is particularly suitable for thin off-gases as those from power plants having SO₂ concentrations below 5000 ppmv.

The use of a moving bed conveys the technical challenge that pressure drop tends to be high, thereby forcing a construction where the cross sectional area has to be large as means for controlling the pressure drop. Large cross sectional areas imply, however, the use of wide moving beds, which are difficult to operate. It is therefore desirable to be able to provide configurations which overcome this problem.

We have also found that by applying a cross-flow in which the gas flow and the moving bed flow are perpendicular to one-another it is possible to operate the moving bed within an acceptable pressure drop window with a sharp absorption profile as the process gas passes through and without the need of resorting to wide moving beds. Hence, by the invention the gas in contact with the solid catalyst absorbent is preferably passed in cross-flow with respect to the traveling direction of the solid absorbent in the moving bed of step (a) and/or regeneration step (c) . The gas in contact with the solid may be off-gas containing sulphur dioxide which is passed in the first moving bed or it may be the gas used as heat exchanging medium in the second moving bed of regeneration step (c) , for instance air.

The invention encompasses also a moving bed apparatus specifically designed to carry out the process. Accordingly, we provide a moving bed apparatus comprising a housing and at least one inner and at least one outer cylindrical wall arranged within said housing, the space between said inner and outer wall defining an annular cavity wherein the solid absorbent particles travel in downward direction, in which said inner wall and outer wall are provided with a number of openings for the passage of gas in cross-flow direction with respect to said solid absorbent travelling within said annular cavity, and in which the gas after passing through said cavity is withdrawn from the moving bed apparatus by passage within the at least one inner wall in upward direc- tion.

The catalyst/absorbent moves down through a reactor bed shaped like the walls of a cylinder, while the gas is passed through the bed from outside and in or from the in- side and out. The inner and outer walls may advantageously be provided in the form of a grid to enable the horizontal passage of the gas. As the catalyst bed moves down, the ab- sorption profile will move inwards in the direction of the gas flow. According to the invention, the cross flow moving bed may be arranged as several concentric cylinders with the solid absorbent particles being arranged within the an- nular cavity defined by the inner and outer walls of each cylinder.

In an alternative configuration the moving bed apparatus comprises a housing and is provided within said housing with a plurality of substantially elongated boxes within which solid absorbent particles travel in downward direction, said boxes being opened at their top and bottom and being aligned along the length of the moving bed apparatus and in parallel with respect to the traveling direction of solid absorbent particles, said boxes having walls provided with a number of openings for the passage of gas in cross- flow direction with respect to the travelling direction of the solid absorbent within said boxes and wherein the space in between consecutive boxes define a passageway for the flow of gas entering or leaving the boxes.

Thus, the gas enters the moving bed apparatus in horizontal direction, passes through the boxes at right angle with respect to the travelling direction of the solid absorbent particles passing through said boxes and leaves at the opposite end of the moving bed apparatus in horisontal direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic of a SO_x absorption process according to a preferred embodiment of the invention. Fig. 2 shows a schematic of a moving bed absorber specifically designed for carrying out the process according to the invention absorption profile obtained within said absorber.

Fig. 3 shows a top-view and side-view schematic of a moving bed absorber according to another embodiment together with the gas and solid flow patterns within the absorber.

DETAILED DESCRIPTION

In the accompanying Fig. 1 the process gas is fed to the bottom of a moving bed absorber, where it is contacted with a moving bed of solid catalyst/absorbent. The SO₂ in the gas is converted to SO₃ according to the equilibrium reaction SO₂ + ^ O₂ = SO₃ (reaction 1 in Fig. 1) , which is absorbed through formation of pyrosulphate according to M₂SO₄ + SO₃ = M₂S₂O₇ (reaction 2 in Fig. 1) . From the absorption operation the solid catalyst/absorbent is transferred to a riser, where it is heated with hot gas while being lifted up above the moving bed absorber. In the riser some of the SO3 is de- sorbed following the equilibrium reaction M₂S₂O₇ = SO3 + M₂SO₄ (reaction 3 in Fig. 1) . Since the absorbent is cata- lytically active some (approximately 20%) of the SO3 will be desorbed as SO₂. The stripping gas from the riser is passed through a heat exchanger to a sulphuric acid/oleum wash- tower in which the SO₃ is absorbed and recovered either as sulphuric acid or as oleum according to reactions H20 + SO₃ = H₂SO₄ and H₂SO₄ + SO₃ = H₂S₂O₇ (reaction 4 in Fig. 1), while the SO₂ that passes through is recycled to the process as a SO₃-depleted gas. From the riser the catalyst/absorbent is transferred to a smaller moving bed contactor (second mov- ing bed) in which the hot solid catalyst absorbent is contacted with cold air intake for the process. This operation serves several purposes. One purpose is to recover more of the absorbed SO_x. A second purpose is to cool the absorbent and recover the heat before the catalyst absorbent is returned to the main (first) moving bed absorber and a third purpose it to remove water vapour from the air intake to the process in order to keep the feed-effluent heat exchanger for the sulphuric acid absorption tower anhydrous. The water absorption is based partially on the reaction of pyrosulphate with water to form bisulphate according to M₂S₂O₇ + H₂O = 2 MHSO₄ (reaction 5 in Fig. 1) and partially on hydration of the alkali sulphates. The second moving bed has been chosen in order to achieve maximum outlet tempera- ture and thus minimum flow of the air that is heated as the pellets are cooled down. An added advantage is that desorp- tion of any remaining SO₃ becomes more effective and the cooled absorption-catalyst pellets also serve as drying medium for the air. Drying through reaction 5 requires a tem- perature well below 200⁰C to be effective and hydration as means of drying requires even lower temperatures. Normally, cooling to 100⁰C is sufficient but lower temperatures may be required, for instance 5O⁰C.

Dust is an issue in coal fired power plants, which is part of the market that this moving bed absorber targets. Therefore dust deserves particular attention. In a fixed bed operation dust has to be removed in a filter before the gas reaches the catalyst bed. By the present invention, how- ever, it is possible to accept that some dust collects on the catalyst in the moving bed provided that it is shaken loose in the riser and may be collected in a cyclone as illustrated in Fig. 1.

Fig. 2 shows a schematic of a moving bed absorber having a housing (not shown) in which the gas flow enters and is subsequently passed in cross-flow with respect to the solid absorbent flow. The space in between the outer and inner wall of the cylindrical member defines a passageway for the solid absorbent, which moves downwardly by the effect of gravity. Openings in the outer and inner wall provide passageways for the gas as it travels at right angle with respect to the traveling direction of the solid absorbent. It is understood that the outer and inner wall of the cylinder may be grids with a mesh that allows the gas to pass through but not the solid catalyst/absorbent particles. After passing through the inner wall the gas travels in upward direction through the central section of the moving bed absorber and leaves at the top as a cleaned process gas free of SO_x. As the solid catalyst bed moves down the ab- sorption profile will move inwards in the direction of the gas flow as depicted in the figure. A sharp absorption profile is obtained meaning that SO₂ in the gas is rapidly converted to SO₃, which in turn is rapidly absorbed until the absorbent capacity is reached at a given depth of the mov- ing bed. At this point the SO₃ moves a little deeper into the bed in the direction of the gas flow. Since the catalyst/absorbent pellets move downward the part of the bed that is further down has been in contact with process gas for a longer period of time; consequently, the gas passes further into the bed before it reaches the absorbtion profile, where the pellets still have capacity for absorbtion of SO₃. The absorbtion profile is shown as a dotted line on Fig 2.

In Fig. 3 a top-view of an alternative embodiment of a mov- ing bed apparatus is shown. The apparatus contains within a housing a plurality of parallel arranged boxes, which are aligned along the length of the moving bed apparatus. The boxes enables the passage of solid absorbent (hatched area) from top to bottom in the moving bed apparatus as well as the passage of gas in cross-flow direction as shown by the arrows. These boxes are preferably formed as grids with a mesh that allows the gas to pass through but not the solid catalyst/absorbent particles. Gas enters to the apparatus at one end in substantially horisontal direction before be- ing forced to pass in cross-flow through the boxes. After having passed the particles in the boxes, the gas leaves the apparatus at the other end in substantially horisontal direction as shown by the arrows. The side view of the embodiment shows the flow of solid absorbent particles (hatched area) within the boxes as they travel in vertical direction from top to bottom. The gas enters and leaves the apparatus in horizontal direction as depicted by the arrows .

Example :

100.000 Nm³/hr process gas containing 2000 ppmv SO₂ is fed to the main (first) moving bed absorber at 400°C. The catalyst absorbent comprises vanadium pyrosulphate and alkali pyrosulphate supported on silica gel shaped as 3 mm spheres. The active absorption capacity is about 17 wt% SO₃ uptake and the pellet density 1000 kg/m³. The cross sec- tional area of the moving bed is about 80 m² corresponding to a diameter of about 10 m.

From the absorption step in the main (first) moving bed ab- sorber the catalyst is transferred to the riser, where it enters at 400⁰C and is heated up to 600⁰C. After passing the riser where the catalyst is lifted about 20 m, the catalyst pellets are hot and may contain additional SO₃. They are transferred to a moving bed cooler (second moving bed) in which they are contacted in counter flow by a stream of cold air. The catalyst pellets are cooled from 600⁰C to about 100⁰C, while the air is heated from 50⁰C to about 550 °C. After SO₃ desorption and cooling the catalyst/absorption pellets are returned to the first absorp- tion bed.

When the pyrosulphate on the catalyst is decomposed, 80% forming SO₃ and 20% forming SO₂, the stripping gas leaving the desorption step conducted in the riser and pellet cooler (second moving bed) , hot dry gas containing about 2.3 mole% SO₃ and 0.57 % SO₂ leaves the pellet cooler at about 550-580⁰C. This stripping gas is cooled in a heat exchanger to a temperature sufficiently low to allow absorption of SO₃ by wash with cold sulphuric acid in a wash tower. The sulphuric acid is recycled through a heat exchanger in order to remove the heat of absorption, in particular if the desired product is oleum, which requires a lower absorption temperature. The process enables a production rate of about 850 kg/hr sulphuric acid.

The SO₃ depleted gas stream leaving at the top of the wash tower contains 5700 ppmv SO₂. This gas is heated to about 550⁰C by heat exchange with the hot gas from the desorption stage, i.e. from the riser and second moving bed and is then recycled to the riser after passing it through a heater where the gas is heated to about 950°C. Part of the SO₃ depleted gas stream is recycled to the power plant as preheated air for combustion.

Claims

1. Process for the separation of sulphur oxides from off-gases comprising the steps of: (a) passing off-gas through at least one moving bed of sulphur oxide absorbent having catalytic activity in the oxidation of sulphur oxides to sulphur trioxide and comprising supported on a carrier a mixture selected from the group consisting of vanadium and sulphates of one or more alkali metals, vanadium and pyrosulphates of one or more alkali metals, vanadium in addition to sulphates and pyrosulphates of one or more alkali metals;

(b) converting the sulphur oxide to sulphur trioxide and absorbing sulphur trioxide on the absorbent; (c) regenerating the sulphur trioxide loaded absorbent by passing the absorbent through at least one pneumatic transporter prior to entering the at least one moving bed of step (a) and withdrawing a stream of stripping gas from the at least one pneumatic transporter; (d) removing desorbed sulphur trioxide in the stripping gas of step (c) by formation of sulphuric acid and/or oleum in a sulphuric acid condenser and withdrawing sulphuric acid and/or oleum product from the process.

2. Process according to claim 1, wherein the regeneration step (c) comprises passing the absorbent through at least one pneumatic transporter and subsequently passing the absorbent through at least one moving bed prior to entering the at least one moving bed of step (a) .

3. Process according to claims 1 or 2, wherein the pneumatic transporter is a fluid bed riser.

4. Process according to any preceding claim, wherein the solid catalyst absorbent comprises vanadium and a mixture of sulphate and pyrosulphate supported on silica selected from the group consisting of silica gel, precipitated silica and fumed silica.

5. Process according to any preceding claim, wherein the sulphuric acid condenser is a sulphuric acid/oleum wash- tower in which the SO₃ is absorbed and recovered either as sulphuric acid or as oleum.

6. Process according to any of claims 1 to 4, wherein the sulphuric acid condenser for removing sulphur trioxide contained in the stripping gas is provided with a plurality of tubes being externally cooled by air flowing counter- currently and in indirect heat exchange with the air inside the tubes.

7. Process according to claim 6, wherein at least part of the cooling air leaving the sulphuric acid condenser is used as heat exchanging medium during regeneration of the solid catalyst absorbent.

8. Process according to any of claims 1 to 5, wherein Sθ₃~depleted gas from the sulphuric acid condenser is used as fluidized medium in the riser.

9. Process according to any preceding claim, wherein the gas in contact with the solid catalyst absorbent passes in cross-flow with respect to the travelling direction of the solid catalyst absorbent in the moving bed of step (a) and/or regeneration step (c) .

10. Moving bed absorber specifically designed for carrying out the process of claim 9 comprising a housing and at least one inner and at least one outer cylindrical wall ar- ranged within said housing, the space between said inner and outer wall defining an annular cavity wherein the solid absorbent particles travel in downward direction, in which said inner wall and outer wall are provided with a number of openings for the passage of gas in cross-flow direction with respect to said solid absorbent travelling within said annular cavity, and in which the gas after passing through said cavity is withdrawn from the moving bed apparatus by passage within the at least one inner wall in upward direction.