Method and device for improving the capture of sulfur dioxide from the gases of electrolysis tanks by a set of filtering modules

Abstract

Disclosed is a method and a device for capturing the SO.sub.2 present in gases by igneous electrolysis implemented on a group of at least two filtering modules traversed in parallel by the gas flow to be purified, and supplied with a powder material sorbent capable of adsorbing effluents in the gas flow by bringing the sorbent into contact with the gas flow, each filtering module having a unit for collecting the sorbent after it has been brought into contact with the gas flow, in the filtering module, and a unit for discharging the sorbent collected being contact with the gas flow to a unit for injecting same into at least one other of the filtering modules of the at least one series, except for the last filtering module of each series, and, on exiting the last filtering module or modules, the gas flow is sent to a desulfurization unit.

Claims

1. A process for capturing the SO.sub.2 present in gases to be purified, which originate from cells for an industrial production of aluminium by fused-salt electrolysis, implemented over a group of at least two filtering modules passed through in parallel by a stream of said gases to be purified, said filtering modules being fed from a storage means, at least partially in at least one series, with a sorption agent of a pulverulent material type, capable of adsorbing effluents present in said gas stream by bringing said sorption agent into contact with said gas stream, each filtering module of said group having means for collecting said sorption agent after bringing said sorption agent into contact with said gas stream in said filtering module, and having means for discharging said sorption agent collected after said bringing into contact with said gas stream towards means for injection into at least one other of said filtering modules of said at least one series, except for a final filtering module of each series, in an order of feeding of said filtering modules with sorption agent from said storage means, wherein a gas stream fraction leaving at least said final filtering module of each series is directed to a desulphurization unit, wherein a fraction of said gas stream leaving at least a first filtering module, in the order of feeding with said sorption agent said filtering modules of each series of said group, is directed to an exhaust stack without passing through a desulphurization unit.

2. The process according to claim 1, wherein said sorption agent is alumina Al.sub.2O.sub.3.

3. The process according to claim 2, wherein said desulphurization unit operates according to one at least of a dry process, an activated dry process, a semi-wet process, and a washing.

4. The process according to claim 1, wherein said desulphurization unit operates according to one at least of a dry process, an activated dry process, a semi-wet process, and a washing.

5. The process according to claim 1 wherein a part of said sorption agent discharged by at least a penultimate filtering module of at least one series of filtering modules is diverted to an outlet pipe, while another part of said discharged sorption agent feeds a final filtering module of said at least one series of filtering modules, so as to saturate said sorption agent with fluorine in said final filtering module or to cause said sorption agent to reach a level close to saturation, and to force a release of SO.sub.2, previously adsorbed on said sorption agent, into a fraction of said gas stream leaving said final filtering module and directed to said desulphurization unit.

6. The process according to claim 1, wherein collected sorption agent, after passing into at least one of said filtering modules of said group, is discharged and directed directly to said means of injection of at least one other of said filtering modules of said group, without an intermediate stage of desorption of SO.sub.2.

7. A device for capturing SO.sub.2 present in gases to be purified, which originate from cells for industrial production of aluminium by fused-salt electrolysis, implemented over a group of at least two filtering modules passed through in parallel by a stream of said gases to be purified, said filtering modules being fed, at least partially in at least one series, with a sorption agent of a pulverulent material type, capable of adsorbing effluents present in said gas stream by bringing said sorption agent into contact with said gas stream, each filtering module having means for collecting said sorption agent after bringing said sorption agent into contact with said gas stream in said filtering module, and having means for discharging said sorption agent collected after said bringing into contact with said gas stream towards means for injection into at least one other of said filtering modules of said group, the device comprising a desulphurization unit of a gas stream fraction leaving at least a final filtering module in the order of feeding said group of filtering modules with sorption agent from a storage means of said sorption agent, and wherein at least a first filtering module, in the order of feeding with sorption agent said filtering modules of each series of said group a fraction of said gas stream leaving, is directly connected to an exhaust stack.

8. The device according to claim 7, wherein an outlet pipe is located on a penultimate filtering module of a group of filtering modules in order to discharge a part of said sorption agent originating from said penultimate filtering module, so as to saturate said sorption agent with fluorine in said final filtering module or cause said final filtering module to reach a level close to saturation.

9. The device according to claim 7, which is deprived of any means of desorption of SO.sub.2 between said means for discharging at least one of said filtering modules of said group and said injection means of a next filtering module in an order of feeding said group of filtering modules with sorption agent from said storage means.

Description

(1) The present invention is now described in more detail with reference to examples which are purely illustrative and in no way limitative of the scope of the invention, and the following illustrations in which:

(2) FIG. 1 is a diagrammatic representation of a gas treatment centre according to a first embodiment example of the invention.

(3) FIG. 2 is a representation similar to that of FIG. 1 in which both downstream filtering modules are linked to the desulphurization system, rather than just one.

(4) FIG. 3 is a representation similar to those of FIGS. 1 and 2 in which the gas treatment centre is composed of two cascades of filtering modules.

(5) FIG. 4 is a representation similar to that of FIG. 1 for which the flow of sorption agent discharged from the penultimate module is not entirely directed to the final module, a part being able to be diverted to the outlet pipe, the final module not receiving any fresh sorption agent.

(6) As shown in FIG. 1, the invention relates to a gas treatment centre 1 comprising n filtering modules numbered 2.sub.1 to 2.sub.n.

(7) The gas stream to be treated is distributed by a pipe 3 in parallel with the reactors 8 of the n filtering modules. The fractions of the gas stream originating from the modules n leave via pipes 20.sub.1 to 20.sub.n and join at a stack 22 of the gas treatment centre 1. The fractions of the stream conveyed by the pipes 20.sub.i are loaded with an SO.sub.2 concentration which increases with i; for example the gas stream conveyed by 20.sub.n is more loaded with SO.sub.2 than the flow conveyed by 20.sub.n-1 which is itself more loaded than the flow conveyed by 20.sub.n-2, etc.

(8) The reactor 8 of the first module 2.sub.1 or upstream module is fed only with fresh alumina up to its injection means 16, via a transport pipe 14 fed by a storage means 4 of fresh alumina. After leaving the 1.sup.st filtering module 2.sub.1 via collection 10 and discharge 11 means of the module 2.sub.1, the alumina is discharged into a pipe 15 which joins an injection means 16 of the reactor 8 of the second module 2.sub.2, which can also be fed with fresh alumina via the pipe 14, in a smaller quantity than the upstream module 2.sub.1, and so on up to the final module 2.sub.nl. The sorption agent, which is alumina, is discharged, into the outlet of the final module 2.sub.n or downstream module to an outlet pipe 13.

(9) The first module 2.sub.1 feeds the second module 2.sub.2 with sorption agent so that the alumina injected into the module 2.sub.1 to be brought into contact there with the gas stream fraction which passes through this module 2.sub.1, then filtered and collected in this module 2.sub.1, originates from the module 2.sub.1 being already lightly loaded with fluorine and flows into the module 2.sub.2. The module 2.sub.2 is fed with sorption agent partially enriched with pollutant by the module 2.sub.1 but also with fresh sorption agent, of the fresh alumina type via the pipe 14. The proportion of alumina originating from the module 2.sub.1 and that originating from the pipe 14 depends on the requirements of the installation; this proportion can be adjusted differently for each module by appropriate means, each module 2.sub.i remaining connected to the pipe 14; according to the principle of the invention, the flow of fresh alumina conveyed via the pipe 14 may be zero most of the time for the majority of the filtering modules, but can still be modified in specific cases.

(10) After passing into the module 2.sub.2, this alumina is discharged via the discharge pipe 15 towards the next module and so on. The alumina discharged by the final module 2.sub.n into the transport pipe 13 has therefore passed through all of the modules, through which it is successively enriched with fluorine. The module 2.sub.n is the final module passed through by the alumina. The gas stream fraction leaving the final module 2.sub.n via the pipe 20.sub.n is directed to the desulphurization unit 21. On leaving the desulphurization unit 21, the gas stream treated (i.e. Q/n in the configuration of FIG. 1 with Q being the total flow of fumes treated in the centre 1) joins the remaining flow, i.e. (n1)*Q/n at the stack 22, at which all the pipes 20.sub.1 to 20.sub.n-1 discharging the fractions of the gas stream passing through the filtering modules 2.sub.1 to 2.sub.n-1 end directly, without passing through the desulphurization unit 21.

(11) FIG. 2 represents an embodiment variant in which the desulphurization unit 21 treats the fractions of the gas stream originating from several downstream modules instead of just the final module 2.sub.n. Here, the desulphurization unit 21 treats the fractions of the stream leaving the final module 2.sub.n and the penultimate module 2.sub.n-1. The successive feeding with alumina of all of the modules remains unchanged relative to the solution represented in FIG. 1. The desulphurization unit 21, by contrast, treats a larger gas stream, corresponding to 2*Q/n and this flow joins the flow (n2)*Q/n at the stack 22, since the pipes 20.sub.1 to 20.sub.n-2 of all the modules except the last two are directly connected to this stack 22. This embodiment variant can have a larger number of modules connected to one or more desulphurization unit(s) 21.

(12) The number of modules feeding the desulphurization unit 21 is advantageously less than n/2 in order to retain the economic benefit of the invention. The number of desulphurization units 21 can be increased depending on the flow to be treated.

(13) FIG. 3 represents another embodiment variant, close to that shown in FIG. 2, and in which the gas treatment centre 1 is a set of several cascades, two for example, constituted by 2 filtering modules each in series. The second module 2.sub.2 or 2.sub.n of each cascade (2.sub.1, 2.sub.2) or (2.sub.n-1, 2.sub.n) is linked via the corresponding pipe 20.sub.2 or 20.sub.n to the desulphurization unit 21, i.e. the fractions of the gas stream leaving the modules 2.sub.2 and 2.sub.n are treated by the desulphurization system 21. The first module, or upstream module 2.sub.1 or 2.sub.n-1 of each cascade is fed only with fresh alumina via the transport pipe 14, from the storage means 4, and the second module 2.sub.2 or 2.sub.n of each cascade is fed with both fresh alumina via the pipe 14 and with alumina collected in and originating from the first module 2.sub.1 or 2.sub.n-1, the alumina originating from each downstream module 2.sub.2 or 2.sub.n being discharged via the pipe 13.

(14) The discharge pipes 20.sub.1 and 20.sub.n-1 of the fractions of the gas stream passing through the upstream modules 2.sub.1 and 2.sub.n-1 of the cascades are linked directly to the stack 22 without passing through the desulphurization unit 21. No desulphurization unit is provided for treating the sorption agent loaded with effluents between the discharge means 11 of an upstream module and the injection means 16 of the adjacent downstream module.

(15) This embodiment variant can have a greater number of cascades and of modules in series per cascade, for example 3, 4 or 5 filtering modules per cascade, the final module(s) of each cascade being linked to one or more desulphurization unit(s) such as 21.

(16) FIG. 4 shows an embodiment variant close to that of FIG. 1 and in which the flow of alumina discharged by the penultimate module 2.sub.n-1 is not all injected into the final module 2.sub.n. A part of this flow is diverted directly into the outlet pipe 13. A redirection unit 23 inserted between the alumina outlet of the module 2.sub.n-1 and the alumina inlet of the module 2.sub.n enables this diversion. Such a box 23 can also be generally applied to all the modules linked to the desulphurization unit(s) 21 with a proportion of outflow to be established according to the requirements of the installation case by case. As explained above, the concentration of SO.sub.2 in the fraction of the gas stream passing through the downstream module 2.sub.n is increased by the forced release of the SO.sub.2 due to the preferential adsorption of the HF by the smaller quantity of alumina loaded with effluents injected into the downstream module 2.sub.n. Therefore, in FIG. 4, the pipe 20.sub.n discharging the fraction of the gas stream originating from the downstream filtering module 2.sub.n is the only one to join the desulphurization unit 21, the other pipes 20.sub.1, 20.sub.2 and 20.sub.n-1 conveying all the fractions of the gas stream originating from the other modules 2.sub.1, 2.sub.2 and 2.sub.n-1 directly to the stack 22.

(17) According to the invention, the desulphurization system can be, for example, based on a dry process, an activated dry process in which the adsorbing agent has been previously moistened, a semi-wet process with a limited injection of liquid, a wet process of the seawater-washing type, or any other washing.

(18) According to the invention, it is then not necessary to desorb the SO.sub.2 from the alumina after the alumina is brought into contact with the gas stream to be cleaned in each reactor, but the alumina is directly sent into the next reactor, without an intermediate stage of cleaning the SO.sub.2, i.e. without desorption of the SO.sub.2. Thus, the gas treatment centre 1 is simplified, in that it has no means of desorption of the SO.sub.2 from the alumina between the reactors 8.