Abstract:
The invention relates to a method for regenerating a used solid absorbent from a desulphurization zone wherein partial combustion of a regenerating gas, regeneration of the absorbent by contacting, in a chamber, said absorbent with the gas effluents derived from the combustion, cooling the gas mixture resulting from regeneration to a temperature higher than the liquid sulphur formation temperature, and filtering the cooled mixture to separate the solid particles of regenerated absorbent from the gas fraction of said mixture is carried out. The invention also relates to an installation for implementing said method.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of combustion and more particularly of regeneration of sulphur oxide absorbents used to process, by capturing said sulphur oxides, fumes coming for example from a combustion means for combustion of a fuel containing sulphur.  
           [0002]    In general, said fumes derive from thermal power plants, industrial furnaces and boilers.  
           [0003]    The present invention can also be applied for example to effluents from various chemical or refining processes, sulphuric acid production shops, ore calcining plants, catalytic cracking plants.  
         BACKGROUND OF THE INVENTION  
         [0004]    Many installations intended for fumes desulphurization are already known.  
           [0005]    French patent FR-2,636,720 notably discloses a boiler wherein desulphurizing agents are injected into a specific chamber referred to as desulphurization chamber, intercalated between the combustion chamber and the convective-exchange heat recovery zone. The desulphurizing agents provided in this installation are preferably non-regeneratable calcic absorbents such as limes or limestones, or industrial residues with a high calcium carbonate content (sugar factory scum, paper mill scum).  
           [0006]    Various improvements have been made to this type of boilers in order to increase the output thereof while having a high efficiency as regards trapping of sulphur oxides notably.  
           [0007]    An improvement illustrated in French patent FR-2,671,855 consisted in using absorbents referred to as &lt;&lt;regeneratable&gt;&gt; absorbents, which are regenerated in a device arranged downstream from the boiler, after the final dust separator. This improvement, which keeps the advantages of installations using non regeneratable absorbents, notably as regards desulphurization, furthermore allows to limit very appreciably the amounts of used absorbent to be dumped, which is favourable to the quality of the environment. Besides, the marked decrease in the amounts of used absorbent to be eliminated allows to envisage inerting treatments and/or storage in technical burial centers at non prohibitive costs.  
           [0008]    A different embodiment, illustrated by French patent FR-2,730,424, provides regeneration at the same time as filtration of the used absorbent, in a single reactor.  
           [0009]    The methods described above use as the regeneration gas a hydrogen or hydrocarbon containing compound having a total carbon number below 10, such as hydrogen, methane, ethane, propane, isobutane and/or a mixture of said gases. Hydrogen is the most suitable regeneration gas because it does not lead to coking of the absorbent. However, supplying it to the industrial site, a refinery for example, can pose problems. In fact, hydrogen is not always available in sufficient amounts in refineries, in particular when many conversion and/or hydrotreating operations are to be carried out.  
           [0010]    Furthermore, costly specific equipments linked with the use of hydrogen during the regeneration stage are necessary for implementing the aforementioned two methods.  
           [0011]    Patent application FR-00/07,121 filed by the applicant describes the use of hydrogen sulfide for regeneration of the desulphurization absorbent before the regeneration-filtering stage. This solution may however lead to considerable corrosion problems because of the high reducing regeneration gas concentrations, and because of the high temperatures required for regeneration (between about 600° C. and about 800° C.). Under such conditions, the filtering elements used will necessarily have a limited lifetime.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention allows to overcome some of the drawbacks encountered in prior installations, notably linked with the corrosion at high temperatures of the filtering and/or regeneration elements by hydrogen sulfide.  
           [0013]    The present invention thus relates to a method of regenerating a used solid absorbent from a desulphurization zone comprising the following stages:  
           [0014]    a) partial combustion of a regeneration gas,  
           [0015]    b) regeneration of the absorbent by contacting, in a chamber, said absorbent with the gaseous effluents from stage a),  
           [0016]    c) cooling the gaseous mixture from stage b) to a temperature higher than the liquid sulphur formation temperature,  
           [0017]    d) filtering the cooled mixture so as to separate the solid particles of regenerated absorbent from the gas fraction of said mixture.  
           [0018]    This method additionally comprises the following stages:  
           [0019]    e) cooling the gas fraction from stage d) to a temperature lower than the sulphur liquefaction temperature,  
           [0020]    f) separation of the liquid sulphur and of the gaseous effluent resulting from said cooling stage.  
           [0021]    Advantageously, the gaseous effluents from stage a) are mixed with the used absorbent prior to the regeneration stage.  
           [0022]    Advantageously, the gaseous effluents from stage a) are mixed with the used absorbent during the regeneration stage.  
           [0023]    In general, the gaseous effluent from stage f) is sent to the catalytic stages of a Claus chain.  
           [0024]    Said regeneration gas preferably comprises hydrogen sulfide.  
           [0025]    According to an embodiment, the gaseous effluents from combustion stage a) are partly cooled.  
           [0026]    The regenerated absorbent obtained after the regeneration stage and the filtering stage is sent to a storage unit.  
           [0027]    By way of example, the regenerated absorbent is mixed with a carrier gas, then sent to said desulphurization zone.  
           [0028]    Alternatively, a regeneration catalyst is mixed with the used absorbent.  
           [0029]    The invention also relates to an installation intended for regeneration of a used solid absorbent coming from a thermal desulphurization zone and comprising:  
           [0030]    means for partial combustion of a regeneration gas,  
           [0031]    regeneration means including delivery means for the gaseous effluents from said combustion means and delivery means for the used absorbent,  
           [0032]    discharge means for the regenerated absorbent and discharge means for the gaseous mixture from the regeneration stage,  
           [0033]    means for cooling said gaseous mixture to a temperature higher than the sulphur liquefaction temperature,  
           [0034]    means for filtering the cooled mixture, including discharge means for the gas fraction of said mixture and discharge means for the solid particles of regenerated absorbent.  
           [0035]    This installation can comprise:  
           [0036]    means for cooling the gas fraction coming from the filtering means to a temperature lower than the sulphur liquefaction temperature and means for separating the liquid sulphur and the gaseous effluent resulting from said cooling stage.  
           [0037]    According to a preferred embodiment, the installation can comprise, between the combustion means and the regeneration means, a means intended for partial cooling of the gaseous effluents coming from said combustion means.  
           [0038]    The cooling means can include gaseous effluent discharge means connected to an inlet of a Claus plant.  
           [0039]    The regeneration device can comprise a chamber innerly coated with a heat-resisting and non-corrodible material.  
           [0040]    This chamber can comprise a cooling means.  
           [0041]    The regeneration device comprises a stirring means allowing to suspend at least part of the solid particles of the absorbent in the gas phase.  
           [0042]    By way of example, this stirring means can consist of paddles carried by a shaft.  
           [0043]    By means of the invention, the device provided can be readily integrated in a Claus chain by simple addition of a regeneration device and of a filtering means. A certain number of equipments existing in said chain, such as the furnaces and burners of the thermal stage(s), the heat recuperators, the condensers, the catalytic conversion stages of the Claus chain can therefore be advantageously used for implementing the present method and/or installation.  
           [0044]    Furthermore, in relation to techniques of the prior art, this regeneration mode avoids having gases with high solids contents outside the regeneration device, and it prevents the fouling and clogging risks that may arise, and consequently a decrease in the exchange coefficients in the heat recovery means used.  
           [0045]    The regeneration device according to the invention is simple, robust and minimizes contacts of the regeneration gas with the metallic surfaces of the various elements that constitute the high-temperature regeneration and filtering chain. Corrosion and clogging risks are also limited.  
           [0046]    The present invention also allows, through the combined effects of a partial combustion of the regeneration gas prior to said regeneration and of the proximity of the heating means and of the regeneration zone, to limit handling and transportation of the corrosive fumes containing, for example, hydrogen sulfide. It also allows to use a regeneration gas that is readily available on the site.  
           [0047]    Besides, no recycling of the regeneration gas is necessary so that all of the physical devices linked with said recycling can be removed. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0048]    Other features and advantages of the device according to the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying drawings wherein:  
         [0049]    [0049]FIG. 1 is a diagrammatic view of an installation according to the invention,  
         [0050]    [0050]FIG. 2 illustrates an embodiment of the regeneration device described in connection with FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0051]    [0051]FIG. 1 diagrammatically shows an embodiment of the invention wherein fumes coming from a thermal generator (not shown) are treated by a regeneratable absorbent such as a magnesian absorbent as formulated for example in patent FR-2,692,813.  
         [0052]    These fumes flow through line  2  into a filter  1  such as a bag filter or an electrostatic filter or any other equivalent device. The fumes freed of the used regeneratable absorbent and of the dust are discharged through the stack by means of line  3 , whereas said used regeneratable absorbent is discharged through line  4 , then sent to an intermediate storage device  5 , and transportation through connecting line  4  can be advantageously carried out under pneumatic conditions with oxygen-poor or oxygen-free gas.  
         [0053]    The absorbent is then sent to a chain comprising a furnace  6  equipped with one or more burners  7  which partially burn, under lack of oxygen, a H 2 S (hydrogen sulfide)-rich gas supplied through line  8 . The oxidizer, which may be air or an oxygen-enriched gas, is fed through line  9 . The gaeous effluents from furnace  6  contain, among other things, hydrogen and hydrogen sulfide. Their temperature generally ranges between 1000 and 1500° C. according to optimized operating conditions.  
         [0054]    These gaseous effluents are partly cooled in a cooling device  10 , an exchanger for example. This device is equipped with a temperature control means such as, for example, a bypass line  11  provided with a valve  12 , preferably cooled, which may be a simple shutter. This control means is dimensioned and/or controlled so as to maintain the temperature of the gaseous effluents at the inlet of an absorbent regeneration device  13  between about 600° C. and about 900° C., preferably between about 700° C. and about 800° C., whatever the working conditions of furnace  6  upstream.  
         [0055]    Regeneration device  13  is designed so as to prevent contact of the gaseous effluents with metallic parts at high temperatures, i.e. above 350° C. This protection of the metallic parts from the high-temperature gaseous effluents can be obtained, for example, by cooling said metallic parts by means of an inner circulation of a cold fluid and/or by insulating said parts by means of a heat-resisting ceramic material layer, refractory concrete for example.  
         [0056]    The absorbent to be regenerated and the gaseous regeneration effluents are introduced at one end of the device, and the regenerated absorbent and the gaseous mixture resulting from the regeneration stage leave the device at another end, through lines  14  and  15  respectively.  
         [0057]    Without departing from the scope of the invention, the absorbent to be regenerated coming from storage zone  5  and circulating through line  103  can be fed either through a line  27  into device  13  so as to be mixed with the gaseous effluents in this device, or through a line  28  into pipe  29 , downstream from said device  13  in relation to the direction of circulation of the hot gaseous mixture, so as to be mixed with these gaseous effluents prior to being fed into said device.  
         [0058]    The regenerated absorbent can be sent to a cooling device  16  such as, for example, a cooled screw inside which a cooling fluid flows as shown by arrows in FIG. 1, then to a storage chamber  17  through a line  26 . The gaseous mixture from regeneration device  13  is sent to a cooling means  18  through line  15  so as not to exceed a temperature ranging between 200 and 400° C., preferably 350° C., then it passes, via a connection  33 , into a filtering device  19 , advantageously of electrostatic filter type, operated at a temperature higher than the liquid sulphur formation temperature under the pressure conditions prevailing in said device  19 , so that the sulphur is in the gas phase. Cooling means  18  can be equipped with a gaseous mixture temperature control means such as, for example, a bypass line  20  provided with a valve  21  in order to maintain said temperature substantially constant at the inlet of filtering device  19 , whatever the working conditions upstream Said inlet temperature is determined so as to, on the one hand, minimize corrosion problems and, on the other hand, to prevent liquefaction of the sulphur in device  19 . The absorbent collected at the level of filtering device  19  is sent to storage chamber  17  through line  22 . The absorbent can then be mixed with a carrier gas and sent to a combustion fumes processing zone (not shown) downstream from element  1 . The gas fraction resulting from the filtering stage then passes through a line  34  into a condenser  23  which brings the temperature of this gas fraction to a value ranging between about 100° C. and about 200° C., and allows to recover elementary sulphur in liquid form. The sulphur is discharged through line  24  and the gaseous effluent cooled through line  25  is sent to the catalytic stages of a Claus chain (not shown in FIG. 1).  
         [0059]    Valves  30 ,  31 ,  32 ,  104  distributed in the installation described above in connection with FIG. 1 allow to control the flows of solid particles between the various elements that constitute said installation.  
         [0060]    [0060]FIG. 2 is a non limitative illustration of an embodiment of absorbent regeneration device  13  shown in FIG. 1. This device consists of a substantially cylindrical metallic chamber  101  innerly coated with a layer  101  of a heat-resisting and non corrodible material such as refractory concrete and delimiting a reaction zone  105 . Said metallic chamber can be partly or totally cooled by a water jacket  102  so as to avoid any risk of corrosion that might appear in case of local breakage of insulating refractory material layer  101 .  
         [0061]    The absorbent is conveyed through line  103  and a valve  104 , preferably cooled, is arranged on said line  103  to control and regulate the absorbent flow rate. This valve also provides atmosphere insulation between absorbent storage zone  5  arranged upstream, as shown in FIG. 1, and reaction zone  105  containing the reducing gases such as hydrogen and hydrogen sulfide reputed to be toxic and flammable.  
         [0062]    These gases, generated by partial oxidation of a gas essentially consisting of hydrogen sulfide H 2 S, flow into regeneration device  13  through line  29 , also innerly coated with refractory materials allowing to protect its metallic parts against corrosion.  
         [0063]    Valve  104  is connected to metallic chamber  100  by line  27  provided with a cooling device  106  in order to avoid risks of corrosion by the hydrogen sulfide upstream. Injection of a non-corrosive gas such as steam as a scavenging means for line  27  to prevent contact of valve  104  with the corrosive gases of the generator can be provided without departing from the scope of the invention.  
         [0064]    In the embodiment described in connection with FIG. 1, the used absorbent is thus contacted with the partly burnt regeneration gas close to the inlet of reaction zone  105 . Without departing from the scope of the invention, mixing can also be carried out upstream from said zone, for example in pipe  29  via line  28  shown in FIG. 1.  
         [0065]    The absorbent falls, under the effect of gravity, into chamber  105  and forms a layer  107 . This layer is stirred by a stirrer  108  which can be cooled by circulation of a fluid such as water.  
         [0066]    This stirrer consists of a central shaft  109  and of arms  110  which are provided, at the ends thereof, with paddles or equivalent parts  111 . These paddles  111  are intended to provide proper stirring of the solid absorbent particles and to suspend in the gas phase part of the particles of this absorbent in order to favour contacts and chemical reactions between the gas phase and the solid phase of the mixture. They are also used to cause the particles bed to progress in the chamber by means of the slight inclination of the assembly.  
         [0067]    In the sense of the present invention, it is understood that said gas phase comprises at most some grams of solid particles per cubic meter of gas, i.e. between about 1 and about 50 g, preferably between about 1 and about 10 g.  
         [0068]    The major part of the regenerated solid particles of the absorbent is extracted from the chamber under the effect of the gravity forces through line  112 . The gaseous effluents and a minor part of the regenerated solid particles of this absorbent are discharged through line  15  to cooling device  19  (FIG. 1).  
         [0069]    Devices (not shown in FIG. 2) such as baffles, cooled or protected by refractory materials, can advantageously be arranged upstream from the gas outlet through line  15  in order to minimize entrainment of the fine particles to said line  15 .  
         [0070]    The central shaft of the stirrer is supported by support means carried by the front  113  and rear  114  faces of chamber  100 . These support means can comprise, for example, bearings, a bearing cooling device  115  and expansion take-up means  116 , as well as delivery means  117  supplying the stirrer with cooling fluid.  
         [0071]    Without departing from the scope of the invention, the means allowing stirring of the solid particles in reaction zone  105  can consist of any known equivalent means allowing better contact and exchange between said solid particles and the hot gases, in particular a cooled coreless helical screw or a hearth furnace.  
         [0072]    The present invention is not limited to the examples described and it includes any variant.  
         [0073]    A regeneration catalyst can notably be mixed with the used absorbent. This catalyst can for example comprise at least one noble metal from group VIII of the periodic table, such as platinum or palladium, or a compound comprising at least one element from the rare earths group, preferably cerium or a cerium oxide.