WO1986003187A1

WO1986003187A1 - A process and apparatus for catalytic oxidation of sulfur dioxide

Info

Publication number: WO1986003187A1
Application number: PCT/FI1984/000089
Authority: WO
Inventors: Kauko Juhani Varis; Jouko Juhani JÄRVI
Original assignee: Kemira Oy
Priority date: 1983-06-03
Filing date: 1984-11-28
Publication date: 1986-06-05
Also published as: FI69046B; SE460470B; FI831997L; FI69046C; SE8702239L; FI831997A0; SE8702239D0

Abstract

Process and apparatus for catalytic oxidation of sulfur dioxide in several steps (1, 1') by directing hot, sulfur dioxide bearing gases through several successive solid catalyst beds (5) and by cooling (6) the gases between the steps. In order to prevent the catalyst bed (5) from becoming clogged, the hot, sulfur dioxide bearing gases are directed upwards from below through the topmost catalyst bed (5), and the dust which has accumulated on the floor (11) of the chamber (1) is removed.

Description

A process and apparatus for catalytic oxidation of sulfur dioxide

The present invention relates to a process for catalytic oxidation of sulfur dioxide in several steps by directing hot, sulfur dioxide bearing gases through several successive solid catalyst beds and by cooling the gases between the steps. Furthermore, this invention relates to an apparatus for carrying out the above-mentioned process, an apparatus which has at least one chamber equipped with an inlet and an outlet, the chamber having one or several horizontal grates, successive in the flow direction of the sulfur bearing gases, and on top of each grate a solid catalyst bed made up of particles, there being in each interval between the catalyst beds a device for cooling the gases between the catalyst beds. The cooling of gas between the catalyst beds can be acco pl-shed by heat transfer means interior or exterior to the reactor or by blowing cold raw gas or air into the gases.

The oxidation reactors commonly used in the sulfuric acid industry are large, partly brickwork steel plate cylinders having several superposed solid catalyst beds, successive in the flow direction of the gas, usually fitted on top of 3-5 horizontal grates, the catalyst beds usually being made up of cylindrical catalyst pieces which may have holes or protrusions. The active ingredient used in these catalyst pieces is generally alkali vanadium pyrosulfate. Since the reaction of sulfur dioxide with oxygen is exothermal and the reactor is adiabatic, it is necessary to cool the gas between the beds, there being at a point after each bed heat transfer means or cold-gas blowing for intermediate cooling of the gases heated up in the contact reactions, before they are directed into the subsequent bed. In practice the sulfur dioxide bearing gases are directed through the catalyst beds downwards from above. The greatest disadvantage of these prior known methods and apparatuses is that impurities, such as dust and oxides and sulfates of iron, carried along with the gas clog the catalyst beds in the course of time, the first catalyst bed being at greatest risk.

A second disadvantage is that, when external heat transfer means are used, there is a considerable change in the flow direction at the inlet and the outlet of the catalyst chamber and this change for its part causes a pressure difference in the gas space in the vicinity of the catalyst bed, especially if the chamber is disadvantageously designed aerodynamiσally and the openings are close to the catalyst layer. The pressure difference caused by one opening may be 150-200 Pa. In the case of modern catalysts, the pressure loss of the first layer, when new, is 500-1000 Pa. Thus it is possible that channels form in the catalyst bed, especially in the vicinity of such openings, and thereby the catalyst overheats in spots as the gas flows unevenly through the catalyst bed, especially when concentrated sulfur dioxide bearing gases are being treated. As a result, the utilization of the catalyst is in places ineffective and, in addition, the activity of vanadium catalysts gradually drops at high temperatures.

From DE Patent 138 695 it is known to direct sulfur dioxide bearing gases upwards from below through several successive platinum-based catalyst layers. However, this invention includes no means for separating the dusts, nor does the apparatus according to US Patent 3,997,655, even though in this latter invention the catalyst beds are fitted in separate chambers with intermediate cooling. Fluid bed reactors have also been experimented with for industrial oxidation of sulfur dioxide. They have at least one disadvantage, the rapid consumption of the catalyst.

The object of the present invention is to eliminate or reduce the above-mentioned disadvantages and to provide a process and apparatus for catalytic oxidation of sulfur dioxide, wherein the clogging of the catalyst bed by dust and its overheating in places because of channel formation have been avoided or substantially reduced.

The main characteristics of the invention are given in the accompanying claims.

According to the present invention, the sulfur dioxide bearing gases to be oxidized are directed, at least in the first step, below the grate supporting the catalyst bed and are removed from above the catalyst bed, so that the gases flow through the catalyst bed upwards from below. Separated dust is removed periodically from the gases directed below the catalyst bed, from the space below the catalyst bed...

The free space above the catalyst bed has preferably a volume 1.3 - 6 times the volume of the catalyst bed and the gas outlet is preferably at a distance at least the thickness of the catalyst bed from the surface of the catalyst bed.

Thus channel formation in the catalyst bed and local overheating of the bed are considerably reduced.

Furthermore, the solid and possibly liquid impurities carried along in the gas do not automatically pass into the catalyst bed as they have until now, but they accumulate on the floor of the chamber below the catalyst bed (from where they can be removed) , since the flow of the gas slows down considerably upon its entering the chamber, a factor which promotes the separation of the impurities from the gas. For this reason the intervals between catalyst screening and replacement can be considerably lengthened.

Channel formation in the catalyst bed can be further reduced by fitting a .flow disperser in front of the gas inlet, in its vicinity. The structure of the disperser depends on the curved parts of the inlet pipe, on the shape of the mouthpiece between the inlet pipe and the reactor chamber, and on the proportion of the inner diameter of the inlet pipe to that of the reactor chamber. The optimum structure in each given case can be sought by model experiments, for example. The flow disperser decreases the uneven pressure otherwise produced under the ejector effect on the lower surface of the catalyst bed and promotes the separation of non-gaseous impurities and their settling on the chamber floor.

Modern large-sized catalyst particles (pellets) in the catalyst beds at the beginning end of sulfur dioxide contact reactors do not come into motion under normal flow and with the arrangement described above, although the gas flows, in a manner deviating from the conventional, upwards from below through the catalyst bed. However, this can be further ensured by fitting on top of the catalyst bed a rather small-mesh metal net or ceramic pieces which are heavy and/or adhere to each other.

Any dust accumulating in the catalyst bed can be removed by directing reaction gases or air momentarily in the reverse direction through the catalyst bed, for example during a shutdown. The reverse gas flow detaches dust which has possibly been caught in the catalyst bed, and the dust falls onto the floor of the chamber.

The dust can be easily removed from the floor of the chamber through an opening hatch fitted in the chamber wall below the grate. It is clear that any solid impurities carried along by the gas are much easier to remove from the floor of the reactor chamber below the catalyst bed than from the catalyst itself, as in prior known methods. In the alternative according to the invention, any dust accumulating on the floor of the chamber can be removed either mechanically or by suction even while the process is in progress, or at least during a short-time shutdown.

One additional advantage gained through the process according to the invention is that, when any of the conventional chambers of the later reaction steps is placed below the first step according to the invention, the floor between these steps need not be thermally insulated by .brickwork, since the inlet temperatures of the gases into the catalyst beds are approximately the same.

The invention is described below in graater detail with reference to the accompanying drawings, in which

Figure 1 depicts a vertical section of a conventional contact reactor, and

Figure 2 depicts a vertical section of a preferred embodiment of the invention.

The contact reactor depicted in Figure 1, known per se, has three superposed chambers 1, 1' and 1", which are separated from each other by intermediate floors 11. The sulfur dioxide bearing gases are introduced into the upper part of the topmost, or the first, chamber 1 via the inlet 2 and are directed in the chamber 1 through the solid catalyst bed 5 on top of a horizontal grate 4 downwards from above into the free space between the grate 4 and the floor 11, and from there further via the outlet 3 into the intermediate cooler 6 and from there on, via the inlet 2 ' , into the subsequent chamber 1' , located below the chamber 1. Ultimately the gases are removed via the outlet 3" in the lower part of the lowest chamber 1".

In this prior known solution, all the dust and liquid impurities carried along in the sulfur dioxide bearing gases end up in the catalyst bed, and most of them remain in the bed, causing clogging. Owing to the relatively small pressure loss of the beds at the beginning end, the location of the outlets 3 and 3' furthermore causes a significant pressure difference below the grate in the space in the vicinity of each outlet. Likewise, the location of the inlet 2' and also of the inlet 2, if the first step is not topmost, causes a significant pressure difference at the inlet above the bed in the space in the vicinity. As a result, channels are easily formed in the vicinity of the openings, whereupon the catalyst bed may be locally overheated and its use is not sufficiently effective.

The solution according to the invention depicted in Figure 2 differs from the solution shown in Figure 1, known per se, in that the sulfur dioxide bearing gases are introduced below the grate 4 , at least in the chamber 1 of the first step, the gas inlet 2 being below the grate and the outlet 3 above the catalyst bed 5. Thereafter the gases can be directed via the intermediate cooler 6 into the next step, i.e. the chamber 1', in which the inlet 2 ' may be fitted above the catalyst bed 5 in the conventional manner, since during this step the gases do not contain so much dust that the dust would cause difficulties in the catalyst bed.

Most of the dust present in the sulfur dioxide bearing gases and most of the liquid impurities of the starting step separate as early as in the space between the grate 4 and the intermediate floor 11 and fall on the intermediate floor 11 and thus do not end up in the catalyst bed 5, as they do in the solution according to Figure 1.

In the side wall of the chamber 1, above the grate 4, ther is an opening equipped with an opening hatch 10 via which the dust and impurities that have fallen on the floor 11 can be removed from the chamber 1.

It is also easy to eliminate the ejector effect of the outlet 3 in the alternative depicted in Figure 2, since the free space 9 above the catalyst bed 5 in the chamber 1 can easily be made so large, preferably 1.3 - 6 times the volume of the catalyst bed 5, and the outlet 3 can be placed at such a distance that no considerable ejector effect nor channel formation in the catalyst bed appear. Furthermore, in front of the inlet 2 , at a distance from it, in the chamber 1 there is a flow disperser 7 which distributes the gas evenly over the entire surface area of the catalyst beα, thereby reducing the ejector effect below and thus also reducing channel formation.

Furthermore, ceramic pieces or a metal net 8 is fitted on the surface of the catalyst bed 5, and these pieces or this net prevents the catalyst particles from coming into a fluidized motion should the flow velocity through the catalyst bed rise too high.

In the solutions according to both Figure 1 and Figure 2, a ceramic piece layer a few tens of mm thick is normally used between the catalyst bed and the grate.

Claims

1. A process for catalytic oxidation of sulfur dioxide in several steps by directing hot, sulfur dioxide bearing gases through several successive solid catalyst beds and by cooling the gases between the steps, the hot, sulfur dioxide bearing gases being directed at least in the first step below the solid catalyst bed and removed from above it, c h a r a c t e r i z e d i n that the dust separated from the sulfur dioxide bearing gases directed under the solid catalyst bed is removed from the space below the solid catalyst bed.

2. A process according to Claim 1, c h a r a c t e r i z i n that, in order to clean the solid catalyst bed, sulfur dioxide bearing gases or some other gas, such as air, are fed through it temporarily downwards from above.

3. A process according to Claim 1 or 2, c h a r a c t e i z e d i n that the sulfur diodide bearing gases directed below the solid catalyst bed are caused to impinge on a baffle or baffles in order to disperse the gas flow and ..to distribute it evenly over the catalyst bed.

4. An apparatus for oxidation of sulfur dioxide, an apparatus which has at least one chamber (1, 1') provided with an inlet (2, 2') and an outlet (3, 3') for the gas, the chamber having one or more approximately horizontal grates (4) successively in the flow direction of the sulfur dioxide bearing gases and on top of each grate a solid catalyst bed (5) made up of particles, there being a cooling device (6) in each interval between the catalyst beds (5) , and the gas inlet (2) being located below at least the first catalyst bed (5) , c h a r a c t e r ¬ i z e d i n that the chamber (1) has below the grate (4) an opening hatch (10) for the removal of the dust which has separated from the gases and accumulated on the floor (11) of the chamber (1) .

5. An apparatus according to Claim 4, c h a r a c t e r i z e d by a flow disperser (7) fitted in front of the gas inlet (2) .

6. An apparatus according to Claim 4 or 5, c h a r a c t e r i z e d by a metal net (8) or a layer of particles which are heavier and/or adhere to each other, fitted on top of the catalyst bed.

7. An apparatus according to any of Claims 4-6, c h a r a c t e r i z e d i n that the distance of the outlet (3) of the chamber (1) from the upper surface of the catalyst bed (5) is at least equal to the average thickness of the catalyst bed.

8. An apparatus according to Claim 7, c h a r a c t e r i z e d i n that the volume of the space (9) above the catalyst bed (5) in the chamber (1) is 1.3 - 6 times the volume of the. catalyst bed (5) .