CH638404A5 - PROCESS FOR EXTRACTING SULFUROUS GAS CONTAINED IN A GAS STREAM. - Google Patents

Description

The present invention relates to a process for extracting S02 to carry out the desulfurization of combustion gases, this process making it possible to best use a make-up water during the control of the sludge concentration to obtain washing and purification. effective of SO2.

According to a known method for removing sulfur dioxide (S02) from combustion gases, purification is carried out by means of sludge, limestone (CaC03) or calcined limestone products, quicklime and lime. extinct. The current installation, however, requires a significant amount of makeup water for the process, which therefore increases the water requirements of the entire installation. As water of suitable quality is often only available to the facility in limited quantities, it is important that the treatment plant uses only a minimum of high quality make-up water or of reused water.

Make-up water is necessary in sulfur gas purification systems to replace water lost mainly in two areas: 1) water lost by evaporation, by washing and the drop in temperature of the flue gases passing through the purifier, and 2) the water lost with the elimination of the solid products from the waste consisting of a slurry of unreacted reagents, calcium sulphite hydrates and calcium sulphate hydrates which are eliminated of the installation. We can therefore reduce as much as possible the total make-up water necessary for the installation by reducing these water losses.

It has now been discovered that when controlling the recycling water of a water elimination system and when selectively using streams of more or less concentrated solids from the absorption plant , the concentration of the washing mud can be controlled and the need for additional water is less.

The method according to the invention is defined by claim 1. The method allows the operation of the condensers and the primary absorption parts in a completely open loop, while recycled, contaminated water is avoided and the use of additional condenser wash water is maximized.

The elimination of the recycling water in these parts reduces the encrustation, the formation of tartar and the chance crystallizations which delay the continuous operation of the SO2 extraction installation. This also allows the use of less expensive materials which are necessary to avoid attack by dissolved chemicals such as chloride.

The use of recycling water from the water elimination installation in the second loop, especially in the washing part, allows the operation of the total installation in a closed loop so that all available water is used in the process instead of being eliminated.

The advantages and characteristics of the process of the invention will also emerge from the description which follows, given by way of example and with reference to the appended drawings, in which:

fig. 1 is a simplified schematic view of the installation for implementing the method, and FIG. 2 is a schematic view, in partial section, of a multi-stage washing and absorption tower of the installation.

In fig. 1, an installation 10 comprises a multi-stage washing and absorption tower 12, which will be described in more detail with reference to FIG. 2 and which comprises a washer 14 and an absorber 16.

The arrows 18a, b and c respectively designate the gas stream sent to the washer 14, the gas stream leaving the washer 14 to the absorber 16 and the SO 2 evaporated at the top of the absorber 16.

Other primary elements of the installation include an absorption tank 20, a washing tank 22, a water eliminator 24, a separator-absorber 26, pumps 28, 30 and 32,

each of them having a water seal pump inlet designated respectively by 34a, b and c for pumps 32, 30 and 28.

The primary liquid-sludge pipes of the installation are: the pipe 36 from the absorption tank 20 to the pump 28; the conduit 38 comprising the primary absorption supply conduit from the pump 28 to the absorber 16; the supply duct 40 of the secondary absorber coming from the absorption tank 20 towards the pump 30; the secondary absorption supply duct 42 coming from the pump 30 to the absorber 16; the conduit 44 comprising a branching of the conduit 42 from the supply of the secondary absorber to the separator-absorber 26; the conduit 46 from the absorber 16 to the absorption tank 20; the condenser washing water conduit 48 for the multistage washing and absorption tower 12; the duct 50 for supplying reagent to the absorption tank 20; the outlet conduit 52 from the separator-absorber 26 to the absorption tank 20; the outlet duct 54 of the separator-absorber 26; the overflow pipe 56 from the absorption tank 20 to the washing tank

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22; the conduit 58 from the washing tank 22 to the pump 32; the conduit 60 comprising the washer supply conduit from the pump 32 to the washer 14; the conduit 62 for the return of the washer from the washer 14 to the washing tank 22; the conduit 64 serving for the evacuation of the washing tank 22 towards the water eliminator 24; the conduit 66 from the water eliminator 24 to the washing tank 22; the conduit 68 serving as a descent conduit for the water eliminator coming from the water eliminator 24.

Referring now to FIG. 2, it is observed that the multi-stage washing and absorption tower 12 comprises a vertical enclosure 70 provided with an inlet 72 for combustion gases adjacent to the lower end and an outlet 73 for evaporated gas at the upper end . Below the inlet 72 of the combustion gas is provided a washing bowl, or tank 22 equipped with a bowl agitation mechanism indicated generally by the arrow 76.

In fig. 2, the washing part is indicated generally by the reference 14 and the absorption part is indicated generally by the reference 16. The washing part comprises several ramps 80 which are connected to the conduit 60 coming from the pump 32, each of the booms being equipped with several spray nozzles 82. According to a preferred embodiment, the washing part 14 is of the cyclone type, since the combustion gases entering through the inlet 72 must flow vertically and tangentially. The return duct 62 which appears in FIG. 1 constitutes the return by gravity of the treatment fluid from the ramps 80 to the washing tank 22.

Between the washing part 14 and the absorption part 16 is a gas-liquid separator indicated generally by the reference 84. This separator 84 collects the water from the condenser and the muddy fluid from the absorber, and the collected fluid is sent from the separator 84 to return to the absorption tank via the conduit 46. Above the separator 84 are several ramps 86 and 86a each connected to the conduits 38 and 42 constituting the primary and secondary supply conduits of the absorber.

Each of the booms 86 and 86a is equipped with several spray nozzles 88, and between the booms 86 and 86a is provided an arrangement 90 of a tower with a common type of packing. Above the ramp 86a are mounted a lower condenser 92 and an upper condenser 94. The washing water of the lower condenser is supplied by means of the ramp 96 which is equipped with spray outlet 98. The upper condenser 94 is also equipped with a washing water device comprising a ramp 100 comprising spray outlets 102.

The separator-absorber 26 and the water eliminator 24 which appear in FIG. 1 can take the form of hydroclones, thicknesses, centrifuges or vacuum filters.

Referring again to the drawings, it can be seen that the installation with two loops comprises a washing loop A, in which almost all the losses of evaporation water occur, and an absorption loop B (which comprises condensers 92 to 94), in which the gases pass first through the washing loop and then through the absorption loop. The reagent flows counter-current to the gas, first passing through the absorption loop. The solids are removed from the installation as follows: the solid products of the reaction between the calcium-based reagent and the sulfurous gas, as well as certain unreacted reagents, are taken from the absorption tank 20 of the absorption loop B to the washing loop A with a little water in the conduit 56 at a fairly constant concentration. The sludge is also sent to loop A via line 44, separator-absorber 26 and line 54. The solid materials then enter washing loop A where more reaction products are formed since the concentration of reactants does not who did not react decreases. The solid materials are then removed from the washer to the water eliminator 24 and to the ultimate elimination of waste.

Make-up water enters the absorption loop in the form of: 1) water entering with the reagent at 50; 2) small amounts of water at 34c and 34b on the mud pump clamps, and 3) condenser wash water at 48. Make-up water enters the wash loop under form: 1) small amounts of fresh make-up water for the slurry pump and for the agitator clamps; 2) make-up water for the washer (recycling water) at 66, which replaces most of the losses due to evaporation, and 3) water accompanying the solids removed from the absorption loop by 56 '.

When the needs of the make-up water process for the washer are satisfied by the recycling water produced by the sludge water eliminator and by the elimination of the absorption loop, optimal use is obtained of the 'water.

When the recycling water produced exceeds the washer's need for additional water, the concentration of solids in the sludge from the absorption loop must be increased. This is done by current controls on the absorber separator 26, which sends the stream heavily loaded with solids containing a solids content between 10 and 50% and directs it through the conduit 54 to be mixed with the flowing mud in the conduit 56, while the lightly charged stream of solids having a solids content of between 3 and 10% is returned to the absorption tank 20. The absorption loop then functions as an open installation eliminating from the sludge with a high solids content towards the washing loop. It is understood that the two loops taken together constitute a closed loop installation.

Under these operating conditions, when the amount of recycling water produced is less than the make-up water requirements of the washer, it is desirable to increase the water content or to decrease the solids content in the products eliminated by l 'absorber by sending the stream containing little solid matter leaving the separator 26 to mix it with the sludge removed from the absorption tank in the conduit 56'. At the same time, water can be added to the absorption loop according to the total need for additional water.

As described above, it is necessary to vary the amount of water leaving with the solids removed from the absorption loop, i.e. the concentration of sludge removed, without affecting the solids and chemical balance of the absorption loop. This is done using a solid-liquid separation device 26 and control devices available on the market. This device makes it possible to treat part of the mud from the absorption loop by supplying two streams, a stream with a high concentration of solids and a stream with a low concentration of solids. One or both of these streams can be combined with one or more conduits 54 with an appropriate amount of mud 56 from the untreated absorption loop, to produce a stream 56 'containing the desired concentration in the sludge flowing into the wash tank 22. In this way, a wide range of solids content can be obtained from the disposal stream of the absorber.

Since the operating conditions of the installation such as the load factor and the sulfur dioxide content vary constantly, the speed of production of solid materials in the absorption circuit and the quantity of water to enter the circuit also change. To allow the absorber to react according to the requirements of the water and solids in the mud process, a control signal is used. This signal relates to the flow of the mass of S02 in the absorption tower, to the density of the sludge from the absorber or to one or more parameters which vary according to the changing flow of the mass of S02 to the tower d absorption at the concentration necessary for removing sludge from the absorption loop to the washing tank. The signal is sent to a control device (not shown) which maintains the concentration of solids removed to the washing tank at the desired level.

More specifically, the operation of the installation according to the invention can be stated as follows.

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The hot gas stream containing the sulfurous gas is washed in the washer 14 of a combustion gas purification apparatus by means of a first aqueous slurry containing alkaline solids. Purification is carried out by sending the combustion gas vertically in the vicinity of several spray nozzles 82 which send the first aqueous slurry containing alkaline materials into the vertically flowing gas.

After having been washed in the washer 14, the combustion gas flows vertically, passing through an absorber 16 in which the washed gas stream is purified by means of a second aqueous mud containing alkaline materials. Purification is carried out by sending the second aqueous mud containing alkaline materials to several booms 86 and 86a equipped with several spray nozzles 88. The booms 86 and 86a are connected to conduits 38 and 42 which direct the aqueous mud containing materials alkaline solids from a storage tank 20. The tank 20 is separated from any stream of solid alkaline materials from the washing storage tank 22 which directs the first aqueous slurry containing solids to the ramps 80 in the washer .

The content of alkaline solids in the sewage sludge directed to the absorber 16 is maintained at a level determined beforehand with respect to the sulfur dioxide content of the gas stream, by adding alkaline solids and water according to the requirements in the storage tank 20 and according to the monitoring carried out by the solid-liquid separation device 26. The device 26 separates part of the absorption loop into two streams, a stream with a high solids content and a stream with a low solids content. The low solids stream contains 3 to 10% solids and is returned to the storage tank 20 through line 52. The method also includes maintaining the alkaline solids content in the wash mud directed to the ramps 80 at a level determined beforehand relative to the sulfur dioxide content of the combustion gas, by selectively removing from the sewage sludge in the tank 20, parts of the sludge having more alkaline solid contents or lower than the level determined beforehand, and directing the extracted sewage sludge to the washing storage tank 22 through the conduits 56 and 56 ′ which raise or lower the content of alkaline solids in the washing sludge. In addition, monitoring of the alkaline solids content in the washing mud is provided by the bottom flow of the solid-liquid separation device 26 and by the operation of the water eliminator 24 which returns the extracted liquid to the storage tank 22 via the conduit 66.

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Claims (6)

638 404 1. Process for extracting sulfur dioxide contained in a gas stream by means of a purification installation comprising two loops, this process comprising the following stages: a) washing, in a washer, the gas stream by means of a first aqueous washing mud containing an alkaline material; b) sending the product leaving the washer to a storage tank; c) purification, in a sulfurous gas absorber, of the washed gas stream, by means of a second aqueous purification mud containing an alkaline material, and d) monitoring of the recirculated water and selective use of concentration streams higher or lower solids from a separator: 1) by separating the aqueous mud from the sulfurous gas absorber in a liquid-solid concentrator, in which the mud concentrated in solid products flows down and the low solids slurry drains up; 2) by sending the slurry portion with a high solids concentration to the storage tank; 3) continuously removing the water from the wash mud contained in the storage tank, and 4) removing the solids from the water removal step while returning the water to the storage tank . 2. Method according to claim 1, characterized in that feed water and sludge concentrated in solid matter are introduced into the sewage sludge and into the washing mud to avoid the formation of sulphate compounds. calcium dissolved in sewage sludge. 2 3. Method according to one of the preceding claims, characterized in that a part of the washing mud is removed and water is extracted from the removed mud and returned to the washing mud. 4. Method according to claim 1, characterized in that the alkaline solids in the concentrated sludge introduced into the sewage sludge are maintained at a value between 30 and 40% and the solids in the sludge removed from the sludge d purification are maintained at a value between 5 and 15%. 5. Method according to claim 1, characterized in that the amount of water added to each of the sewage and washing sludge is adjusted by removing, from the sewage sludge, a portion of sludge having a high content of materials or by returning a portion of sludge having a low alkaline solids content to the sewage sludge so as to reduce the amount of water to be added to the sewage sludge. 6. Method according to claim 5, characterized in that the washing of the gas stream occurs in a closed tank, in which the gas penetrates into the bottom and exits at the top and in which the washing mud is sprayed into the gas stream near the bottom of the tank and the sewage sludge is sprayed into the gas stream near the top of the tank, and each sprayed sludge is collected separately.