FR2513140A1 - PROCESS FOR THE REMOVAL OF SULFUR OXIDES FROM FUMES - Google Patents

Abstract

PROCESS FOR THE ELIMINATION OF SULFUR OXIDES FROM FUMES. THE GAS IS PUT INTO CONTACT WITH A SORBANT CHOSEN FROM SODIUM BICARBONATE, TRONA AND ACTIVE SODIUM CARBONATE AND, USING AN ALKALINE LIQUOR CONTAINING BORATE IONS SO AS TO REDUCE THE FLOW AND LOSS OF ALKALINITY, WE REGENERATE THE SORBANT EXHAUSTED WITH AN OXIDE OR HYDROXIDE OF ALKALINOTERROUS METAL.

Description

l t S 2513140

  The present invention relates to a method for absorbing

  tion of sulfur oxides from industrial fumes with a solid sorbent and regeneration of the solid sorbent

for reuse.

  In the combustion of fossil fuels and in many industrial processes, the combustion of the sulfur constituents which they contain poses a serious problem The harmful sulfur oxides produced constitute an environmental pollutant and considerable efforts have been developed in recent years to eliminate the sulfur oxides from combustion gases released into the atmosphere On

  knows several processes for the elimination of these oxides.

  For example, mention may be made of 3 US Patents 3,852,410 to Rivers et al and 3,846,535 to Fonseca. However, to the knowledge of the Applicant, all the processes of the prior art have certain drawbacks and, consequently, a

  improved process for economical disposal and-

  reliable sulfur oxides gas mixtures would

desirable and is provided here.

  In summary, the method of the invention consists in treating the fumes (or burnt gases) containing sulfur oxides (which are mainly, and which are considered for convenience below as being sulfur dioxide) with a solid sorbent chosen from carbonate

  activated sodium, sodium bicarbonate trona and their mixtures

  ges, which can remove 90% or more of sulfur dioxide

  trona is the mineral name for Na 2 C 03 Na HCO 3 2 H 20 The car-

  activated sodium bonate can be obtained from bicar.

  sodium bonate of trona or a mixture of cd & Qsubstances, by calcination at a temperature between about 70 and 200 C For a sodium bicarbonate with a characteristic particle size of about 50 micrometers a calcination time of about 10 to 30 minutes , at a temperature of about 150 C, is sufficient Clean gas is

  discharged and the resulting unreacted solids sul-

  fites and sodium sulfates and their mixtures are dissolved

  at lt> uie 't: llqueui I: iqoe suffam 1 ler t' 'l Ji' ç> ûOUL uitj'e '-

  drawing carbonic acid into bicarbonate, said liquor containing borate ions, ammonia or preferably both 9 to form soluble sodium compounds The carbonation of the resulting liquor forms a sodium precipitate containing

  bicarbonate, trona or mixtures thereof.

  The precipitate is separated from the carbonated liquor and the liquor is treated with a precipitation compound chosen from hydroxides of alkaline earth metals, oxides of alkaline earth metals and mixtures of these compounds

  to form alkali metal sulfates and sulfites

  insoluble earths, and mixtures of these compounds As suitable alkaline earth metals, caicium, barium and strontium may be mentioned After the solids have been separated,

  recycles the liquor to treat the exhausted sorbent.

  The presence of borate ions (as supplied by the acid

  boric) in the process provides various distinct advantages.

  First, it allows the use of lower flow rates

  in the regeneration section of the process Another advantage

  significant age results from the fact that borate ions do not degrade

  tooth not chemically or biologically to a significant degree

  cative, so that there is little loss of borate ions in the system, which consequently reduces the amount of material used in the process. Furthermore, since the borate ions do not act as reducing agents in the regeneration section or in the solid residue elimination zone, the sulphites and / or sulphates present are not reduced to harmful sulfur compounds such as, for example, hydrogen sulphide, which may exhibit serious problems of

health and evacuation.

  The single figure in the accompanying drawing is a representation

  schematic tion of the process of the invention.

  As indicated in the figure to which reference should be made, a smoke gas containing sulfur dioxide is introduced via line 2 into a gas-solid contact tank 4 The contact tank 4, which may have various forms (eg fixed bed, moving bed, fluidized bed, etc.) is suitably a bag collector using dust collection surfaces with a tubular type fabric filter, preloaded with an appropriate sorbent which is introduced into the contact tank 4 via line 6 As a variant, the sorbent can be introduced into the gas stream upstream of the contact tank By passing through the contact tank 4, the sulfur dioxide in the

  smoke reacts with the sodium sorbent to produce sulphide

  te and sodium sulphate leaving a smoke gas practi-

  that is completely free of sulfur dioxide and which is

  evacuated from contact tank 4 via line 8.

  A product made of solids is extracted from the contact tank 4 via line 10 and transferred to a tank for the spent sorbent 12 At this stage the solid product contains unused sorbent, initially introduced into the gas-solid contact tank 4, more soluble sulphite and / or sulphate (sodium) resulting from the reaction of the sorbent with sulfur dioxide in the flue gas and any particulate matter originally contained in the flue gas such as fly ash The product makes of solids is transferred to a mixing tank 16 via line 14, where it is mixed with an alkaline recycling liquor containing borate ions from

  line 18 and additional chemicals that it can

  wind understand Na 2 B 407, Na 2 C 03, Na 2 SO 4, Na Cl or different

  mixtures of these compounds In the mixing tank 16, the sul-

  soluble fite and / or sulfate that have been formed by the

  reaction of sulfur dioxide with sorbent are dissolved.

  The liquor from the mixing tank 16 is transferred through line 22 to a fly ash filter 24 which retains any fly ash present and discharged through line 26 Line 26 can lead to the reactor 64

  when only part of the sulphite or sodium sulphate

  dium from the spent sorbent tank 12 is dissolved in the mixing tank 16 The liquor without fly ash leaves the filter 24 via line 28, is introduced into the carbonation tank 30 and reacts with C 02, introduced into the tank carbonation 30 through line 32 Clean smoke gas is a convenient source of C 02 Excess Co 2 leaves carbonation tank 30 through line 34 to be discharged into the atmosphere or, if preferred, to an evacuation in the chimney of clean gas by the pipe 8 Bicarbonate ions are formed and transferred by the pipe 36 in a crystallizer 38 and converted

  in solid sodium bicarbonate, trona or their mixtures

  ges, which crystallize from the solution Carbon dioxide can also be added to the crystallizer 38 to make the crystallization of the sodium bicarbonate more complete The sodium bicarbonate crystallized in the crystallizer 38 is transferred via line 40 to the filter sodium bicarbonate 42 Sodium bicarbonate

  and / or the trona recovered from the filter 42 are trans-

  fered by line 46 to a dryer / calciner 48, where they are either dried and transferred by line 52 to the storage tank of regenerated sorbent 54, or dried and calcined to form an activated sodium carbonate which is also transferred by the line 52 to

  storage tank for regenerated sorbent 54.

  The liquid coming from the filter 42 passes through the line 44 in a carbon dioxide fractionation column 56 where it is brought into countercurrent contact with a fractionation gas (for example steam) introduced into the lower part of the fractionating column 56

  via line 58 Part of C 02, other gases not

  dissolved and any remaining fractionation gas, are discharged from the fractionation column 56 via line 60 This gas containing C 02 can be added to the carbonation tank or to the crystallizer 38 The CO-free liquor coming from the fractionation column 56 is introduced, via line 62, into a reactor 64 where it is brought into contact with an alkaline precipitation agent,

  preferably lime, introduced through line 66.

  In reactor 64, the precipitating agent, for example lime, reacts with sulfite and / or soluble sodium sulfate to produce calcium sulfate and / or

  insoluble calcium sulfite and regenerate the alkaline liquor.

  The mixture in the reactor 64 is transferred via line 68 into a spin-drying tank 70 where the insoluble calcium sulphate and / or sulphite are evacuated via line 72, the liquid coming from tank 70 being recycled as indicated above, towards the nmala-age tank

16 by line 18.

  As the drawing shows, the process comprises two stages

  fundamentals, a stage of sorptiof and a stage of regeneration-

  ration At the sorption stage: sulfur dioxide in the flue gas is brought into contact with the sorbent and

  converted into soluble; ulfate and / or subfite compounds.

  At the stage or in the re-generation section 13 sulfur species is definitively eliminated from the process under the form of an insoluble sulfur compound and the sorbent is re-absorbed for its re-use in the sorption stage. The sorbent is preferably a sodium carbonate obtained by calcination of a cowpose containing sodiumq such as sf diumle trona bicarbonate or a mixture of these compounds, at a temperature of between envison 70 and 2000 C. It has been found that although the sodium carbonate which has been produced by crystallization directly at from a solution does not act as an effective sorbent in the process of the invention the sodium carbonate produced by calcination of sodium bicarbonate or trona

  an excellent sorbent and is easily obtained by calcina-

  tion of the precipitated sodium bicarbonate produced in the crystallizer 38. To remove the soluble sulfites and / or sulfates from the system, a hydroxide precipitation agent is used

  of alkaline earth metal of an alkali metal oxide

  earthy, or a mixture of these compounds Thus, for example, that the process can use an oxide or hydroxide of calcium barium or strontium, or mixtures of these compounds The preferred alkaline earth metal is calcium

  As noted above, having regard to the description

  based on the appended drawing, the process uses before-

  a column of carbon dioxide fractionation

  bone The fractionation column, which can be any gas-liquid contact system against the current, is used to eliminate from the process the excess of C 02, which would otherwise be precipitated in the form of calcium carbonate in the tank 64, thereby increasing the consumption of lime in the process. The fractionated C 02 gas can comprise water vapor or a gaseous medium containing oxygen,

such as for example air.

  As indicated above, the process of the invention uses borate ions in the liquor in the mixing tank 16 The ultimate source of alkalinity in the process is provided by the precipitating agent (here called "lime"

  for convenience) added to reactor 64.

  However, without the use of a certain medium for trans-

  the alkalinity of the solid phase (lime) to the liquid phase, the alkalinity of the solution would be quickly depleted during the carbonation operation Consequently for

  a given circulation speed in the system, the production

  sodium bicarbonate in the carbonation tank

  would be very reduced This would require a pumping speed

  ge or increased circulation in the system at the point of making the process non-economically feasible The borate Ions are used to effect the transfer of alkalinity from lime to the liquid phase and can thus be considered as an alkalinity support "This support d alkalinity has an acid form (boric acid) and a base form (borate ion), and is in the base form when it leaves the reactor 64 The clear liquid which is separated from the ash filter 24 and used to dissolve the solids of the

  gas-solid contact tank from contact tank 4.

  is pumped to the carbonation tank 30 where the alkalinity of the liquid phase of the support is now exchanged

  against the "2 inlté of the liquid phase with bicarbonate.

  The alkalinity of the bicarbonate liquid phase is then converted into the alkalinity of the solid phase of sodium bicarbonate in the crystallizer. The alkalinity support in

  the clear liquid from crystallizer 38 is now

  nant in an acid form, that is to say boric acid.

  Upon entering reactor 64, boic acid is again

  calf in contact with the alkalinity of the solid phase provided by the lime converted to the basic form and the cycle is repeated. As preferred alkalinity supports, mention may be made of boric acid and, when the activated sodium carbonate is the sorbent ammonia It is also clear that the alkalinity support can be added in its acid or base form. example that the borate ion can be added in the form of boric acid (acid form) or sodium tetraborate (base form), while ammonia can be added

  in the form of sulphate or ammonium chloride or simi -

  milk (acid form), or gas-ammonia (base form) Avanta-

  gustingly, both boric acid and ammonia are -

  used to minimize the possibility of precipitation of

  boric acid in the crystallizer.

  The limits for borate and ammonia can be determined from the following considerations. As alkalinity carriers, it is desirable to maximize their concentration. The solubility of boric acid limits the

  amount of borate that can circulate The solubility of acid

  of boric acid decreases when the temperature decreases The coldest part of the circulation section is the crystallizer effluent In the crystallizer effluent almost all of the borate in the solution exists in the form of unionized boric acid, H 3 B 3 O or B (OH) 3 7 The solubility of boric acid depends on the composition of the solution # as well as on the temperature The composition of the solution is determined both by site-specific factors (for example the quantities of H Cl or NOX eliminated from the flue gas) and by the operating conditions It is therefore impossible to fix the maximum borate concentration which can be used without knowing these factors The order of magnitude of the solubility of borate can be obtained from the data of solubility reported by Linke (Linke, W F. Solubilities: Inorganic and MetalOrganic Compounds KZ volume 11.4 th edition, American Chemical Society, W Vashington,

D.C 1965)

  Concentration, g / l O Og of H 20 Moles of Temperature, H 3 BO 3 / kg Na Cl KC 1 Na 2 SO 4 H 3 BO 03 (saturated) of H 20

O O O 5.43 0.88

O O O 7.19 1.16

36.8 O O 8.2 1.33

33.2 O 11.9 9.6 1.55

O 41.0 O 11.6 1.88

O O 53 13.1 2.12

  The ammonia concentration is limited by the vapor pressure of the ammonia It is higher at high temperature and p H high An upper limit is the vapor pressure of the total solution (water, ammonia and C 02) of the solution, equal to 5 absolute atmospheres (41.3 103 Pa) These general considerations apply everywhere in the system

  for both borate and ammonia.

  The following examples are given by way of illustration

  preferred embodiments of the invention.

EXAMPLE I

  A flue gas containing 317 kg mole / hour SO 2 was treated with 344 kg mole / hour of activated sodium carbonate and reacted with 90% of the sulfur dioxide in the flue gas The resulting solids were collected in a bag chamber The solids of the bag collector were dissolved using 5 110 1 / minute of a recycling liquor containing 2.6 moles of borate, 6.5 moles of sodium and other dissolved species, such as chlorides, sulfites, sulfates, calcium carbonates, etc. Sodium carbonate and borate were also dissolved in the liquor, as auxiliary products, at the rate of 12.7 kg mole / hour and 2.9 kgmole / hour, respectively The liquor was then carbonated with 344 kg mole / hour of C 02 coming from a combination of clean smoke gas and CO 2 recycled from other parts of the process, in the carbonation tank and the crystallizer The carbonated liquor was cooled to 35 C in the crystallizer to precipitate 689 kg mole / hour of e sodium bicarbonate The sodium bicarbonate solids were separated from the liquor dried and calcined at 149 ° C. so as to form a carbonate of

  activated sodium, which has been recycled to the collector -

  bags to treat The flue gas The separated liquor was passed through a C 02 fractionation column to remove 40.8 kgmole / hour of carbon dioxide from the liquor We are dealing with the liquor leaving the CO fractionation column with 299 kg mole / hour of lime in a reactor, to 'r: iiter the sulfite and / or calcium sulfate solids These Colides were separated from the suspension leaving the reactor and' 1 S cons Iituait the residual product We a rzcyc la luiqueur separates> as noted

  above to dissolve solids from the bag collector.

  The following examples II and TII carried out according to the pro-

  general yield of the example Isii it is not the presence or in the absence of an alkalinity support as indicated in the following table 2 shows that the circulation speed

  would be increased at least 1000 times (if it were technical-

  mentable) without aloalinity support Molality, moles / kg of H 20 Speed of Example Am io iac Borate circulation I 0.0 236 5 ol 10

II 1.3 1.3 5 110

III O O 5 o 110,000 and more

Claims (11)

- CLAIMS -   1. Process for the removal of sulfur oxide from industrial fumes containing this compound, characterized in that it comprises the steps consisting in: (a) bringing said gas into contact with a solid sorbent chosen from sodium carbonate sodium bicarbonate, trona and mixtures of these compounds in one   enough to react with practical-   all of the sulfur oxide present in the gas, so as to form unreacted sorbent solids, sodium sulfite, sodium sulfate and their mixtures, and fumes practically free of l sulfur oxide; (b) removing the resulting fumes from the process,   dissolve said solids in an alkaline liquor   holding borate ions to form soluble sodium compounds; (c) carbonating the alkaline sodium liquor resulting from step (b) and cooling to a temperature sufficient to form crystals of sodium bicarbonate or trona; (d) separating the sodium bicarbonate or trona crystals from the liquor from stage (c) and recycling said crystals to stage (a) or heating the crystals for a time and at a temperature sufficient to form activated sodium carbonate , and recycle the activated sodium carbonate to stage (a), and   (e) removing carbon dioxide from the cooled liquor   die of stage (c), add a precipitation agent selected from hydroxides and oxides of alkaline earth metals and their mixtures, to the resulting liquor for the   make alkaline and form insoluble solids   of alkaline earth sulfates, alkaline earth sulfites and mixtures thereof, separate   said solids and recycle the resulting alkaline liquor te towards stage (b).   2 Method according to claim l, characterized in that the liquor containing borate ions further contains dissolved ammonia.   3. Method according to claim l, characterized in that said sorbent comprises trona. 4. Method according to claim l, characterized in that   that said sorbent comprises sodium bicarbonate.   5. Method according to claim l, characterized in that   that said sorbent comprises activated sodium carbonate.   6 Method according to claim l, characterized in that   that the precipitation agent comprises calcium oxide.   7. Method according to claim l, characterized in that the precipitation agent comprises calcium hydroxide. 8 Method according to claim l, characterized in that boric acid is used to provide borate ions. 9. Method according to claim l, characterized in that an activated sodium carbonate is used with supports   alkalinity of boric acid and ammonia.   10. Process according to claim 1, characterized in that sodium tetraborate is used to provide the borate ions.   11. Method according to claim l, stage c, characterized in that C 02 is added to the crystallizer for   make the crystallization of sodium bicarbonate complete.