CH504224A - Method and apparatus for removing SO2 from a gas stream containing it - Google Patents

Description

  

  
 



  Method and apparatus for removing SO2 from a gas stream containing it
 The present invention relates to a method for removing SO from a gas stream containing SO3 mixed with SO3 and having a temperature between 660 C and 4270 C and to an apparatus for carrying out this method.



   Many industrial gases such as foundry gases, waste gases from many chemical plants, flue gases from furnaces burning coal or an oil fraction, such as those used in a power plant, waste gases from sulfur furnaces such as those used in plants for the production of sulfuric acid, etc., contain sulfur dioxide. In addition, these gases often contain small amounts of suspended particulate solids and other types of impurities such as sulfur trioxide. The flue gases given off by coal combustion furnaces, for example, entrain a particulate solid matter, called fly ash, which may also contain sulfur trioxide in the absorbed state.

  Others of these 502-containing gases may have little or no particulate solids, but significant amounts of SO 3. Thus, for example, the waste gases from the sulfur burners used for the production of sulfuric acid have various contents of SO3, which is usually present in the form of a mist or fine droplets. In general it can be said that when SO2 is a combustion product in the waste gases, SOS is also present. For example the combustion of fuels containing sulfur at a flame temperature above 13710 C, often results in the production of about two parts by weight of SO, for 98 parts by weight of SO2.



   The presence of SO, in gas streams containing SO2 has posed problems for the recovery of
S02. The SO, present in the residual gases tends to react with the potassium sulphite present in the absorbent solution to give potassium sulphate which, in general, separates by crystallization with the potassium pyrosullite, when it is separated using subsequent crystallization as a separation step. Since it can be a sulfur dioxide partial pressure lowering material and consumes potassium sulfite, the potassium sulfate is advantageously removed. The need to have recourse to such an additional separation stage adversely affects the economics of the process.



   The presence of particulate solids suspended in S00 containing ga streams also has disadvantages. Fly ash, for example, is an extremely abrasive material, and if not removed it can present great difficulties due to erosion of fittings, vessels, etc., throughout the installation, and has a tendency to separate in the installation in the form of a solid mass which poses a serious sealing problem.



     Also the presence of most of the SO3 contained in these residual gases is due to its adsorption on the surface of the fly ash particles, and therefore, it is likely to remain in the gases, if the ash itself is not removed. not removed.



   Further problems arise if the particulate solids are present in the gases supplied to the chemical adsorption stage, because the solids are entrained in the absorbent solution, and, if they are not subsequently removed, they have been removed. tendency to build up as the absorbent solution is regenerated and recycled, thus slowly and continuously reducing the SO absorbent properties of the solution.



   The present invention aims to provide a method and an apparatus for easily overcoming the above-mentioned difficulties.



   To this end, the process, object of the present invention, is characterized in that the gas stream is treated to remove SOs therefrom, the gas stream thus treated is brought into contact with an aqueous absorbent solution of a metal sulfite for the purpose of adsorbing
SOI from the gas stream and to produce a spent absorbent effluent containing the corresponding metal bisulfite, which the spent absorbent effluent is treated to produce SO, and regenerated sulfite, and recycle regenerated sulfite to said contacting.



   The apparatus for carrying out this method, according to the present invention, is characterized in that it comprises a container having walls and defining a purification zone, an absorption zone and a stop zone for the water. fog arranged successively; a fluid permeable target disposed in a substantially horizontal fashion having a discontinuous surface for retaining solid substances entrained by a gas and placed in the scrub zone;

   a liquid spray device located below the target for directing a spray of scrubber generally upwards to contact the underside of the target and wets it to remove gas from the substances solids and components soluble in the liquid, a device directing the flow of gas placed below the device for spraying the liquid and for directing gas in competing relationship towards the target with the sprayed liquid, a device for absorption of SO2 in the absorption zone which is in gas flow relation with the absorption zone for absorbing SO2 from the gas, a gas entrainer arranged in relation to receive the flow of gas gas with absorption zone,

   said drive device comprising a generally horizontal woven web-shaped contact material which is inert to reactants and which is placed over the absorption zone, and a spray device for spraying a solution against the gas inlet and outlet sides of the contact material, this spray device being arranged so as to reach substantially the entire contact surface of the material on its two sides, and a device providing a sufficient flow rate of the solution - to maintain the contact material wet, the thickness of the contact material not being sufficient to cause excessive pressure drop of the gas through the material.



   Other advantages of the invention will emerge from the description which will follow, given with reference to the appended drawings, and giving by way of explanation several embodiments of the invention.



   On these drawings:
 fig. 1 is a block diagram of an installation intended to recover SO2;
 fig. 2 is a schematic view on a larger scale of the absorption column 10 shown in FIG. 1;
 fig. 3 is a sectional view of an assembly for removing solid particulate or liquid-soluble impurities from a gas stream;
 fig. 4 is an elevation of a support plate of a column packing;
 figs. 5 to 11 show three advantageous constructions of a cap or cover, and
 fig. 12 represents a block diagram of an installation intended to recover SO2 and having a modified decomposition zone.



   Referring now to Figs. 1 and 2, a gas stream containing sulfur dioxide is introduced into a reactor 10 having, for example, an internal diameter of 76.2 cl, constructed of type 304 stainless steel, and having any suitable packing 9, for example a filling of lead or a synthetic resin. The gas passes through a line 12 having an internal diameter of 30.48 cm. The reactor is typically 6.4 meters long along line B-B.



   The quantities and flow rates indicated in the present application for gases and liquids are based on a reactor having these dimensions, that is to say an internal diameter of 76.2 cm and a cross-sectional area of approximately 0.455 m2, and certain distances between the components placed in the reactor. It is obvious to those skilled in the art that the quantities and flow rates of the gases and liquids used vary depending on the size of the reactor used and the distances between the components used in the reactor; however, they can be considered to be equivalent. In addition, although the invention is described in the present application using potassium as the metal, it is also applicable to cesium or rubidium.



   The concentration of sulfur dioxide in such gases is generally between about 0.001 and less than 5 mole percent, and frequently it is less than 0.5 mole percent (about 1 percent by weight). For example, a modern power plant with a capacity of 1,350,000 kw burns around 15,000 tons of coal per day. Many coals contain about 3.5 percent sulfur by weight or even more. The release of sulfur dioxide from a plant of this size using such coal would then amount to about 1000 tonnes per day, although the concentration of sulfur dioxide in the flue gases may be very low, for example. example of the order of 0.2 to 0.3 mole percent, depending on the sulfur content of the coal.

  Large amounts, for example greater than about 75 percent by weight, sulfur dioxide can be removed from these gases.



   The concentration of SO, varies depending on the gas source and even the temperature of the flame used during treatment, among other factors.



  It is usually greater than 0.01 mole percent of the gas, and is normally between 0.0001 and 0.01 mole percent, and usually most, for example more than about 70 percent, of the soot. contained in the gas is present as SOS adsorbed on the surface of solid particles.



   The gas stream contained in line 12 is generally at a temperature of up to 4270 C, and often between 660 and 3160 C. For example, suitable gas streams suitable for use in the present process include gases. flue gas from power plants burning coal or fuel oil, which typically have a temperature between 1210, or 1490 C, and 1820 C or 2040 C, waste gases from acid production plants sulfuric acid which typically has a temperature of between 660 and 930 C, and waste gases from foundries which can reach a temperature of up to approximately 4270 C.

  However, at temperatures above 2040 C it may be desirable to cool the gas to a temperature between 1070 and 1490 C, for example by cooling with ambient air, in order to avoid adversely affecting the reactor, the packing. reactor or pre-purifier. If the gases are at a temperature below 660 ° C., the gases contained in line 12 will be heated to promote the removal of water from the solution contained in reactor 10. The flue gases generally have a relative humidity. up to about 10 percent, usually between 1 and 7 percent. A fan 28 shown in FIG. 1 draws the purified gas from the reactor and can be arranged, if desired, to force the gas through the reactor.



  The purified gases coming from the reactor 10 via a chimney 14 and the inlet gases contained in the pipe 12 can pass in a countercurrent heat transmission relationship through a heat exchanger (not shown), if desired.



   Sulfur dioxide is absorbed from the gas stream in a zone 30 of reactor 10 by absorption by reaction with an aqueous solution of potassium sulfite to produce an aqueous solution of potassium bisulfite, and the purified gases exit through stack 14. As indicated above, although the installation given by way of illustration uses potassium sulphate, it is also possible to have recourse to the corresponding sulphites, of cesium and of rubidium.



   The gases, for example the flue gases of the installation given by way of illustration, introduced through the pipe 12 pass under a cap or cover 13 which is arranged above the opening of the pipe 12 opening into the reactor 10, and which extends into the reactor 10 for a distance sufficient to prevent water or solution from passing past the opening of the line 12. Advantageously prevents obstruction of the line 12 by components such as fly ash collected from the outlet. from the gases contained in zone 20. The cap 13 is inclined slightly upwards to force the liquid to flow towards the wall of the reactor 10 and around the opening of the pipe 12 which is flush with the wall of the reactor. reactor 10.

  This inclination of the cap 13 is generally of the order of about 5 to 60 degrees, preferably 10 to 45 degrees, for example 30 degrees, and the cap 13 has a diameter which is preferably at least equal to the diameter of the liner. opening of the pipe 12, for example an internal diameter of 30.48 cl. The greater part extending upwards of the cap located in zone 20 is a sufficient distance from a nozzle 25 to avoid destroying the spray pattern, but is close enough to avoid excessive vaporization of the mist from the spray pattern. spraying, and drops falling from the target, and this distance is generally between 12.7 and 177.8 cm, advantageously between 25.4 and 76.2 cm, for example 38.1 cm in the arrangements described in present request.

  Any liquid flowing along the wall of the reactor 10 or reaching the upper surface of the cap 13 is thus directed away from the inlet opening and along the wall of the reactor 10, without passing directly opposite from the opening of admission.



   Figs. 5 to 11 show three advantageous constructions of a cap or cover. In fig. 5, the cap 13 'extends outwardly and upwardly into the reactor 10 at an angle of approximately 300. As shown in fig. 6, the cap 13 'is in the form of a half-tube with troughs 15' which extend along the edges to the wall of the reactor 10. The cap 13 "of Fig. 7 also extends. obliquely upwards in the reactor 10 towards the outside of the inlet opening
 12 'of line 12. Cap 13 "is cut obliquely as indicated by surface 15" to disperse inlet gas from line 12 over part of the diameter of reactor 10.

  Hat
 13 "can be, for example, stamped in a flat sheet and have no troughs; along the edges, although they can be used if desired. The 13" cap is particularly suitable for a gas stream having high solids content. Figs. 9 and 10 show a cap 13 "'which is formed by a pipe extending across the diameter of the reactor 10. A tapered slot 15"' (see fig. 11) disperses the inlet gas from the pipe 12 over it. the diameter of the reactor 10. Troughs 15 "" attached to the cap 13 "'collect the liquid and the solids to convey them to the wall of the reactor 10.

  Referring to fig. 2, the hat shown is constructed in accordance with that of FIGS. 5 and 7, and extends towards the reactor 10 over a distance of 30.48cl. The cap is disposed 38.1 cm below the spray nozzles 25, and the cap troughs are approximately 2.5 cm deep. When using the bonnet, the inlet opening cannot be blocked; however, in the absence of the cap, with a flue gas containing 23 g per cubic meter of solids, the inlet opening obstructed after approximately 2 to 4 hours.



   The reactor 10, although it is advantageously shown in the form of a single vessel, comprises three zones which, if desired, can be constituted by separate assemblies. These zones are a pre-purification zone 20, an absorption or reaction zone 30, and a zone 40 for stopping the mist or the entrained humidity.



   The residual gases entering the reactor 10 first pass through the pre-purification zone 20 where particulate solid components, for example fly ash, and water-soluble components, for example SOi, hydrocarbons including methane, ethane, propane, etc. contained in the gas are selectively removed from the gas containing SO2, the SO2 content. gas being in turn selectively removed from the gas in zone 30. Since a large part of the SO content of the gas is usually adsorbed on the surface of the solids contained in the gas, it is usually removed on removal of solid particles.

  Solid substances, eg dust, in gases are generally unreacted materials, eg fly ash produced by the installation of chemicals or non-combustible components of the fuel.

 

   The chemical composition of fly ash will naturally vary with the particular fuel burned, but usually it includes a large portion of silica, alumina, and iron, with other metal oxides such as manganese and vanadium oxides frequently being present in low amounts. quantities. Other particulate suspended solids may be present in the waste gases, and include, for example, particulate hollow carbon spheres which are found in particular in the waste gases from oil burners and which, like fly ash, contain quantities significant amounts of SOi adsorbed.

  The particle size of the suspended solids in the waste gases can be between about 0.5 and 50 microns, but can be much larger, however the predominant number of particles is up to about 10 microns.



   For example, fly ash from a flue gas produced by a plant burning coal and using a turbine feed coal grinding process has the following particle size distribution in the flue gas:
 Weight percent of the whole
Fly ash size in microns
 0-10 86
 10-20 5
 20-40 4
 40-60 5 and those from a coal-burning plant using a cyclone feed coal grinding process have the following particle size distribution in the flue gas:
 Weight percent of the whole
Fly ash size in microns
 0-10 45
 11-20 23
 21-45 20
 45- 12
 The solids content of the gases can be between 0.0023 and 138 g per meter oube of gas, for example for flue gases, it is generally between 2.3 and 23 g per cubic meter.

  If the solids content of the gas is very high, for example generally more than 2.3, for example 2.3 to 23 g of solids per cubic meter of flue gas from a power plant , which usually contains from 2.76 to 9.2 g of solids per cubic meter, and in particular if it comprises large particles whose largest dimension is for example greater than 1.59 mm, it may be desirable to pre-filter gas, for example in an electric precipitator or a cyclone-type separator (see fig. 3) before introducing the gas into line 12. Generally speaking, 80 to 90 percent of the gases can be removed. particulate solids by pre-filtration with a cyclone type separator, and a larger amount with an electric precipitator which, however, is more expensive than the separator.



   Although the interposition of devices such as electric prezipitators and centrifuges, for example dry-running cyclones, between the source of the residual gases and the chemical absorption zone, makes it possible to first remove most of the solid impurities from larger dimension in order to allow the substantially complete elimination of these solid substances before bringing the gases into contact with the absorbent solution, it has hitherto been necessary to use a certain number of these devices in series or in parallel.

  This translates into high operating or investment costs. An advantage of pre-epu
 rator lies in its ability to substantially remove even minimal amounts, for example from 0.023 up to 2.3 g per cubic meter of gas from particulate solids which are not removed by an electric separator or precipitator and which if they are not were not removed would adversely affect the system; furthermore, solids are removed efficiently and with minimal investment.



   The scrub water is introduced through a line 24 into the lower part of the reactor 10, and is discharged upwards in the form of a fine spray through a nozzle 25 having an orifice generally sufficient to supply 0 , 0175 liters per minute per cubic meter per minute of gas, and producing a spray following an arc generally between 10 and 125, for example 750, having droplets whose size is generally between 200 and 800 microns approximately, advantageously for removing particles up to about 60 microns in size. The temperature of the scrubbing liquid, for example water, is generally between 100 and 490 C, preferably between 210 and 320 C.

  It is also possible to use several nozzles as shown in FIG. 2. At a distance generally between 10.2 and 45.7 cm, for example approximately 23 cm above the nozzle 25, is placed an assembly of a substantially horizontal target, permeable to the fluid, for example to a gas or a liquid, having a continuous small surface area. It is advantageously constituted by tubular or column-shaped elements forming a tray support 15 in the form of a network, preferably made of expanded metal, for example stainless steel.

  A filler 22 is supported by the carrier 15 and consists of about three layers of non-porous filler elements arranged at random, for example Raschig rings, Intalox or Berl fillers, preferably. of ceramic composition, the individual elements having a long dimension generally between 6.35 and 88.9 mm, for example 38.1 mm and forming a filling column with a height generally between 38.1 and 139.7 mu, and preferably between 76.2 and 101.6mm, generally weighing 0.48 to 1.12 kg / dm1 and generally having 50 to 90 percent free space.



   As shown in the drawings, the spray nozzle 25 is placed close enough to the holder 15 to provide a continuous spray of water at a rate sufficient to overcome gravity contact and to wet the surface of the target in a manner. generally a speed of between 0.6 and 6 meters / second approximately, for example 1.5 meters / second.



  The water reaches, touches and wets the column support, and the layer of filler material retained by the support, and falls back as large droplets to entrain particulate solids and SO, which are removed from the gas stream. The water spray entrains the larger solid particles, causing them to separate from the gas stream, while partially dissolving the water-soluble SO from the gas stream, and as the residual gas passes through
 through the material of the target, substantially all, by
 example a proportion of at least 90 percent, smaller remaining solids and S09 in the gas strike wetted areas of the material
 of the target,

   and adhere to it, forming droplets which
 enlarge until they overcome the force of gravity and adhesion to the surface of the material and fall or are washed away from the target material by spraying water. Generally, more than 95 percent by weight, eg, more than 99 percent of particulate solids can be removed from the gas. The removal of SO, is particularly advantageous, since in this system it produces potassium sulphate which adversely affects the recovery of SO, for example by consuming the potassium sulphite which would otherwise be consumed by 502.



   The residual gases entering the column via the pipe 12 rise through the support 15 and the filling 22. The gas is advantageously contacted with the liquid by directing the gas and the liquid simultaneously towards a target permeable to the fluid. , for example gas or liquid; the gas is directed toward the target at a sufficient velocity, at the instant it hits the target, to enter and pass through the latter in the presence of a liquid which is directed toward the target at a sufficient velocity, at instant when it comes in contact with the target, to snuff it and insufficient to cross the target to any significant extent.

  Generally, the speed of the gas towards the target is sufficient for a predominant amount, for example at least 60 percent by volume, preferably 90 to 100 percent by volume, to enter and pass through the target. . In general, the superficial speed of the gas, at the instant when it comes into contact with the target, is between 0.6 and 4.5 meters / second, and preferably between 2.1 and 3.6 meters / second, for example it is 3 meters / second.



   The surface velocity of the liquid, at the instant when it comes into contact with the target, is generally between 0.6 and 4.5 meters / second, and preferably between 1.5 and 3 meters / second, for example it is 2.4 meters / second. The amount of liquid used is sufficient to effect the transfer of the particulate and soluble solid components in a liquid from the gas to the liquid upon contact with the target and this depends on the amount of the gas components drawing particulate and soluble solids in the gas. liquid; in general, the quantity of liquid is between 0.00175 and 0.175 liters / minute, for example between 0.07 and 0.0875 liters / minute per cubic meter of gas containing from 1.15 to 69 g / m3 of particulate solid components, and it is sufficient to dissolve the soluble gas components in a liquid.

  The pressure drop of the gases across the filling 22 is generally between 6.35 and 12.7 mm, for example it is 10.16 mm of water for 56.6 cubic meters per minute of gas. In addition, significant retention of the liquid in the interstices of the filling is avoided.



   The liquid used in the present invention can be any suitable liquid which is chemically inert towards the target and which does not adversely affect mass transfer, particularly when it is a transfer. As a mass of non-solid components, the liquid is preferably a liquid which also selectively dissolves and removes non-solid components.



  For example, water can be used to selectively and simultaneously remove particulate solids and SO, from a gas stream. The solids suspended in the gases hit the wet surface of both the support 15 and the filling material layer 22, and are retained by them. These solids are washed away from the fill and carrier by the downward flow of the used sewage water which has been stopped and returned through the entire target.



  The waste sewage water containing the solids removed and the sulfur trioxide absorbed flows by gravity along the funnel-shaped sides of the collector bottom 17 of column 10 and exits through the discharge line 26. used sewage water. An important factor is the positioning of a surface such as a collector bottom 17 in droplet receiving relationship with the target to receive droplets falling from the target and to remove them from the area 20 before significant evaporation of the droplets. can occur and thereby substantially prevent the return of particulate solids, eg, fly ash or liquid soluble components, eg SO3, to the target by the action of the inlet gases.

  Referring to fig. 2, the distance between the points a-a is 1.8 meters, and the surface 17 extends downward at an angle of 600 towards the outlet 26.



  Sulfur trioxide and suspended solids are thus removed from the residual gases before they come into contact with the absorbent chemical solution.



  If desired, the waste purification water which can most often have a pH of around 2 to 4, depending on the amount of sulfur trioxide contained in the residual gases, can be treated to separate and recover the solids, for example fly ash and sulfuric acid. Small volumes of water can be used to wet the target, a feature of particular interest because the amount of water can affect the overall yield of the process.

  Therefore, it is desirable to use only a sufficient amount of water to contact, wet, and fall off the surface of the target to collect particulate solids and 508. Generally speaking less than 0.0175 liters / minute of water is used, preferably 0.00175 to 0.1225 liters / minute, for example 0.0875 liters / minute per cubic meter of gas per minute.



  The advantage of using an amount less than 0.0175 liters / minute is shown in Table I below.



  It is possible to regulate any increase in the humidity of the gases passing through the pre-purifier 20, generally between approximately 8 and 10 0 / o at most, preferably at 2 2 / o at most, and the temperature drop across the pre-scrubber is generally less than 33O C, preferably less than about 270 C.



   By using the purifying apparatus shown in particular in FIG. 2, it has been found that, besides an extremely low temperature drop and a minimal increase in the relative humidity of the gas through the scrubbing zone, an assembly which is surprisingly free from the difficulties blocking or plugging unlike a pre-purification assembly using a continuous load spray water distribution which ensures a large temperature drop, for example 55o C and a large increase in relative humidity, for example about 10 percent, and which is also clogged in a short period of time, for example two hours, by solids removed from the gas stream.

  The position of the spray nozzles on the underside of the target allows for long term operation, for example up to 16 hours, without such clogging occurring. However, if desired, there can also be provided in this purifying apparatus a second spray assembly 27 placed above the target, which can be actuated periodically by an actuator 29, a pressure drop regulator, for example. Example every 8 to 16 hours for 1 to 3 minutes to direct an up and down spray on the target and thereby irrigate the filler material and allow continuous operation.



   The absorption or reaction zone 30, as shown, is advantageously designed for intimate contact of countercurrently flowing gas and liquid streams, although it may be designed for intimate flow. competitor, if desired.



  As shown, the absorption part is shown with two sieve plates arranged in a substantially horizontal manner, for example which may be of a conventional type. You can also use cap trays. The gases pass through the reactor 10 at a surface velocity sufficient to maintain the liquid on the contact plates, but which is not sufficient to force the liquid out of the reactor. Typical average surface velocities of gases through absorption portion 30 of reactor 10 are generally at least about 0.45 meters / second, and are preferably between 0.6 and 2.55 meters / second.



  The potassium sulphite solution is introduced into the reactor 10 through line 16, generally at a flow rate of between 0.0175 or 0.07 and 3.7 liters / minute, preferably between 0.35 and 1. , 4 liters / minute for each cubic meter of gas per minute, and the potassium bisulfite solution is discharged through line 18.



  Potassium sulfite, generally about 40 to 60 percent by weight, for example 50 percent of the total solution to be introduced, is supplied from line 16 via line 16 "'and 'a line 44' of the fog stop zone 40, which will be described hereinafter, to fall onto the surface of a tray 32 and to flow from the tray 32 onto a tray 34. An additional amount of the solution can be added, if desired, through a line 16 '.



  The potassium sulphite reacts with the sulphurous hydride contained in the gases passing through the sieve trays to produce an aqueous solution of potassium bisulphite which passes from the tray 34 into the collection basin formed by a baffle 36 protruding downward ( see Fig. 1) from which it is discharged through line 18. The sulfur dioxide content of the gas is significantly reduced, for example to less than 0.02 mole percent in a flue gas containing more than 0.2 mole percent approximately. Potassium bisulfite is separated, for example crystallized, and can be recovered in crystalline form as potassium pyrosulfite which is further decomposed to produce potassium sulfite and sulfur dioxide.

  Sulfur dioxide is extracted and can be either cooled or compressed to form a liquid product, or it can be sent as a gas to a sulfuric acid production plant or to a reduction furnace for processing. elemental sulfur. The potassium sulphite can be recycled to the reaction zone where an additional amount of sulfur dioxide is absorbed.

  The reactions used include:
 I. K2SO3 + SO2 + H2O # 2KHSO
 cold
 II. 2KHSO3 # K2S2O5 (c) + H2O
 hot
III. K2S2O5 # K2SO3 + SO2 (g)
 In order for Reaction I to proceed, the temperature of the solution in the absorption or reaction zone 30 must be maintained above the temperature at which sulfur dioxide is absorbed by reaction with the aqueous solution of sodium sulfite. potassium, and below the temperature at which the potassium bisulfite decomposes or at which reaction III takes place, for example below about 1100 C.



  In general, sulfur dioxide is all the more easily absorbed by the solution and reacts all the more easily with potassium sulphite the cooler the potassium sulphite solution is. However, with flue gases, the temperature of the solution is generally above about 320 C or 380 C, although room temperature is suitable. Preferably, the temperature is kept below about 880 C, for example between 49O and 820 C or 850 C since above these temperatures the reaction I begins to slow down to such an extent that the anhydride sulfur dioxide is not easily absorbed by the solution because the partial pressure of sulfur dioxide becomes too high.

  Since the purified flue gas is ultimately discharged to a stack after being processed in the mist stop zone 40, it is desirable to maintain the temperature of the purified or washed gases high enough to maintain their buoyancy, so that 'they rise in the chimney, for example above approximately 850 C. At lower temperatures, for example at 57O C, a fan can be used to suck the gases.

  In general, the temperature of the gases coming from the absorption zone 30 and entering the zone 40 for stopping the mist is between 49O and 820 C, and is for example 570 C. The temperature of the purified gas contained in line 14 and coming from the fog stop zone is generally 43O to 77O C, for example 54O C.



  but it can be increased, if desired, by introducing a hot furnace gas through line 14 '.



  Typically, the gas exiting the mist stop zone is approximately 85 to 100 percent water saturated.



   The potassium sulfite solution is also advantageously sprayed against the underside of the trays 32 and 34 by the spray elements 38.



  The latter are generally located at a distance of 10.16 to 45.72cm, for example 22.86cm from their respective plates. The speed of the liquid is sufficient to overcome gravity. As the potassium bisulfite solution is formed on the tray, the water is separated from the solution by the hot flue gases, which tends to supersaturate the solution and crystallize the potassium pyrosulphite and plug the trays. . The potassium sulfite solution from line 16 passes to the spray elements through line 16 ".

  The sprays are directed against the bottom of the trays where the hot flue gases initially come into contact with the trays, as this is where most of the water evaporation occurs, and therefore , crystallization. A sufficient amount of solution is sprayed upward against the bottom or contact surface of the trays to maintain the solids in a dissolved state in the solution on the trays, especially on the surface reached by the flue gases, and thus, it advantageously allows continuous operation of the process.



  It is desirable to keep the amount of the solution imparted to the spray elements as low as possible, and generally the amount of the solution is less than 0.0175 or 0.07 liters / minute per cubic meter of gas per minute. The flow rate of the solution which must be admitted into the spray elements 38 is all the greater as the temperature of the residual gases is higher. For example, with gases entering reactor 10 at 1490 C, a flow rate of 0.0175 to 0.035 liters / minute for one cubic meter of gas per minute is suitable.



   The potassium sulfite solution passes through the absorption portion 30 in an amount sufficient to react with the sulfur dioxide of the flue gases, i.e. to absorb sulfur dioxide and produce potassium bisulfite. . Generally, these amounts are stoichiometric and, advantageously, a solution containing about 25 percent by weight K2SO3 and 25 percent by weight KHSQ can be used.



   The potassium sulfite solution introduced to the reaction zone is preferably in the form of a recycle stream, and generally contains 30 or 40 to 75, preferably 40 or 50 to 75 percent by weight of solids of which a proportion, generally at least about 50 percent, preferably greater than about 75 percent, comprises potassium sulphite, the remainder being essentially potassium bisulphite, with optionally a little sulphate. This recycle stream is preferably a saturated solution of potassium sulfite. The potassium sulfite solution may contain a sufficient amount of an oxidation inhibitor, for example a hydroquinone, for example about 0.001 to 0.1 percent. , to inhibit the oxidation of the sulfite ion.

  The temperature of this stream is controlled so as to avoid disruption of the conditions prevailing in the reactor 10. The temperature of the recycle stream is typically between 320 and 710 C, approximately.



  In general, a sufficient quantity of the potassium sulphite solution is contacted with the residual gas contained in the reactor 10 to remove as much of the sulfur dioxide as possible, preferably more than 90 to 95 percent, and the residence time of the solution in the reactor, etc., is adjusted accordingly. The flow rate of the solution in the absorption zone 30 is normally kept sufficiently fast, and the residence time of the solution in the absorption zone sufficiently short that there is no problem due to crystallization in the absorption zone. absorption zone.



  With a low flow rate and a long residence time, absorption of sulfur dioxide occurs, however, the solution becomes very concentrated, and hence the risk of deposit of crystals from the solution and clogging of the screen trays becomes greater. The flow rate of the solution depends on the temperature of the gas, the amount of SO contained in the gas, the temperature and the concentration of the potassium sulfite solution, but it is however generally between 0 , 0175 and 3.5 liters / minute, preferably between 0.35 and 1.4 liters / minute per cubic meter of gas per minute. About 5 to 25 percent of the solution can be admitted into elements 38. Baffle 36 forms a collecting trough or catch basin for the potassium bisulfite solution.

  The baffle 36 may be inclined, for example 10 to 45 degrees, or it may be horizontal, so that any crystals contained in the potassium bisulfite solution descend towards the pipe 18. A relatively thick layer, for example of several centimeters (for example 20.3 cm) or several tens of centimeters, of the solution is maintained on the baffle 36 to avoid formation of crystals in the solution, and preferably, the residence time in the collection basin is short duration, usually around 5 to 10 minutes.



   The product of the reaction zone is preferably a saturated solution of potassium bisulfite, and therefore the concentration of the solution is advantageously maintained just below the saturation point by adding sufficient water to avoid precipitation of the bisulfite of potassium. potassium. As mentioned above, this would result in the crystallization of potassium pyrosulfite.



   The solution resulting from a reaction of an aqueous solution of potassium sulphite and a flue gas inherently contains many ingredients, as given below by way of example:
 Percent Range
 weight
 General Usual
Potassium sulphate 0 to 8 1 to 6
Potassium bisulfite 5 to 40 10 to 30
Potassium sulphite 10 to 50 20 to 45
Water the rest the rest
Sulfur dioxide is present in solution in chemically combined form, for example as potassium bisulfite, which can be considered a precursor of sulfur dioxide, and it is present or in contact with materials lowering the partial pressure of SQ, for example metal salts, such as alkali metal salts, generally potassium salts, such as potassium sulphite and potassium sulphate unchanged,

   which are produced by reaction between sulfur trioxide and potassium sulphite.



  The purity of potassium bisulfite in contact with the partial pressure lowering materials of SQ is generally less than about 60 percent by weight on a dry basis.



   The amount of solids in the solution varies with temperature, but is generally kept high enough to allow efficient recovery of sulfur dioxide. The solution generally contains about 40, or 45 to 75 weight percent, preferably about 45 or 50 to 55 or 65 weight percent solids. The amount of bisulfite and potassium sulfite in solids varies with the percentage of total solids and the temperature of the solution.

  In general, potassium bisulfite constitutes from 5 to 50 or 60 percent by weight, preferably 10 to 35 or 50 percent by weight, and potassium sulfite generally constitutes from 40 or 50 to 50 percent by weight. 95 weight percent, usually about 50 or 65 to 90 weight percent. For example, at 25o C, the saturated aqueous solution of potassium pyrosulphite and potassium sulphite contains about 5/0 of potassium pyrosulphite and 470 / o of potassium sulphite; at approximately 400 ° C., this solution contains approximately 7 <> / o of pyrosulfite and 48 oxo of sulfite; and at approximately 650 C, the solution contains approximately
 12/0 of pyrosulphite and 48/0 of sulphite.



   The purified flue gases contained in the reactor
 10 pass from reaction or absorption zone 30 to zone 40 for stopping the mist or humidity
 driven, which includes a contact area formed by
 a woven cloth or screen. The gas pressure drop at
 across zone 30 is generally 38.1 to 114.3, for example 63.5 mm of water, for 56.6 cubic meters per minute of gas. The woven fabric 42 is of a material chemically inert with respect to the components of the system, for example of 304 stainless steel, and is structurally
 and similar in appearance to steel wool: it is
 shown in fig. 2 as being supported by a
 support 43.

  The potassium sulfite solution prove
 from pipe 16 goes through pipe 16 "', and the
 pipes 44 'and 46' in nozzles 44 and 46, respectively
 vement, which are generally placed at a distance of 10.16 to 45.72cm, for example 22.86 from
 web 42, and which continuously spray the solution onto web 42 from opposite sides of the assembly,
 i.e. the upper side (emitting side of the fluid)
 and the lower side (fluid receiving side) for
 advantageously avoid clogging and to maintain
 continuous operation. Zone 40 is used to remove
 droplets of the solution from the gases leaving the
 reactor 10 to thereby limit the loss of chemical.

  The droplets can be liquid or solid forms of the reaction product, and are generally sufficiently small, for example of a size of.
 1 to 100 microns, to be able to be supported by the rate of rise of the gas flow which is normally between 0.6 and 2.1 meters / second approximately.



  It has surprisingly been found that when the mentioned contact material is a woven fabric-like structure, one can advantageously use
 a small amount of the solution. However, we can
 use a material providing a large contact surface and an easily irrigable contact surface, for example Raschig rings, but it requires larger quantities of the solution, for example about 26.25 liters / minute. The thickness of zone 40, for example 25 to 100mm, is sufficient to remove droplets from the exhaust gases, but is not large enough to cause a large pressure drop, and the flow of gas through the zone 40 is less than that at which the droplets would be entrained again from the contact zone.

  The maximum desirable pressure drop is generally between 635 and 12.7 mm water, and typically gas velocities of between 0.6 and 1.8 meters / second are suitable. In general, the solution is added to the mist stop zone in an amount sufficient to prevent clogging of the fabric and insufficient to be entrained by the outlet gases. This embodiment of the mist blocking zone is very efficient and only requires the use of a small amount of the solution. For example, flow rates generally of about 0.35 to 0.875 or 1.05, preferably about 0.525 to 0.613 or 0.7 liters / minute of solution per cubic meter of gas per minute are suitable, and are preferably shared equally between nozzles 44 and 46.

  The amount of the solution used decreases as the concentration of the solution decreases, and it is desirable to keep the flow rate as low as possible. Zone 40 also acts to remove the last traces of sulfur dioxide contained in the outlet gases. For example, the mist arresting cloth normally removes an additional 1 to 2 mole percent of all of the sulfur dioxide contained in the inlet waste gases.



   In this plant, sufficient quantities of the SO partial pressure lowering materials are separated from the potassium bisulfite to increase the partial pressure of sulfur dioxide in the potassium bisulfite. The separation of the SI partial pressure lowering materials can be accomplished by any suitable process, for example by selective extraction of potassium bisulfite from the solution or by extraction of any of the SI partial pressure lowering materials from the solution. . The separation is preferably carried out by crystallizing potassium bisulfite (which turns into potassium pyrosulfite during crystallization) from solution for further processing according to the process described below.



   The separation of sulfur dioxide from a precursor such as potassium pyrosulfite depends on its partial pressure under given conditions, for example when it is combined or not with other materials including salts such as potassium sulfate and sodium. potassium sulphites which are generally present in the solution thus obtained and which are produced by reaction of the burnt gas containing sulfur dioxide with the aqueous solution of the potassium sulphite.

  The partial pressure of sulfur dioxide in its precursor in the presence of other materials e.g. potassium salts, in the solution of the reaction product at its boiling point under normal pressure conditions, is so low, e.g. for the typical solution described in an example below, it is about 1.5 mm Hg, which cannot be released economically. On the contrary, its partial pressure, when the potassium pyrosulfite is in a relatively pure state, for example of a purity greater than about 98 percent by weight on a dry basis, is relatively high, for example about 300 mm of mercury. The purity of potassium pyrosulfite is increased generally to greater than about 65 percent by weight, preferably greater than 90 to 95 percent by weight.

  The following table gives the partial pressure of sulfur dioxide in potassium pyrosulfite for the purity indicated.



   Board
 Purity of K2S2Oa,
 weight percent Partial pressure of SQ,
 (dry basis) mm Hg
 65 5.0
 70 6.1
 75 8.5
 80 13.3
 85 21.0
 90 33.5
 95 75.0
 98,300.0
 The potassium bisulfite can advantageously be separated or recovered in the form of potassium pyrosulfite by crystallization from materials which lower the partial pressure of SO2 from the solution.

  The crystallization of potassium pyrosulfite can be carried out using suitable crystallization procedures, for example by supersaturation of the solution by heating it in vacuo, or advantageously, by cooling the aqueous solution of potassium bisulfite to a temperature at which a substantial part of the solution. pyrosulfite crystallizes, for example below about 380 or 430 C, the lower limit being determined by economic factors. For example, when a saturated solution of potassium bisulfite having a temperature of 650 C is cooled to 400 C, about 40 percent of the pyrosulfite crystallizes, while when the solution is cooled to 250 C , about 70 percent of the pyrosulfite crystallizes.



  Since potassium sulfite is more soluble than potassium bisulfite, substantially pure pyrosulfite crystals can be obtained, for example with a purity of greater than about 95 percent by weight.



   Potassium pyrosulfite crystals can be separated, for example, by separation techniques such as centrifugation and filtration, and they can be heated to decomposition temperature and atmospheric pressure, these temperatures are generally above 380 C and sufficient to decompose potassium pyrosulfite, for example above 110o and up to about 3160 C, but preferably below the temperature at which significant amounts of potassium sulfate are formed, for example 2040 C, and preferably above at 1490 C under essentially anhydrous conditions, to liberate sulfur dioxide and convert potassium pyrosulfite into potassium sulfite which is suitable for reuse.

  This decomposition process eliminates the need to evaporate large amounts of water to remove sulfur dioxide when anhydrous conditions are desirable. The hot potassium sulphite, produced after decomposition of the crystals, is combined with the filtrate resulting from the separation of potassium pyrosulphite and is recycled to the reaction zone.



   Referring again to Figs. 1 and 2, the potassium bisulfite solution is removed from reactor 10 via line 18 to introduce it into an intermediate tank 48, from which it is pumped (for example after a residence time of only 5 to 20 minutes in the tank intermediate) in a rapid manner to avoid significant cooling and crystallization and clogging in the tank. It is pumped in a pipe 52 by a pump 54 to deliver it to the zone 50 of crystallization and cooling by expansion. Line 18 is associated with a line 18 'intended to break the vacuum. Solution pumped in line 52 can pass, if desired, through filter 56 to remove solids from the solution.



   Zone 50 comprises an expansion chamber 58 in which the solution can be advantageously cooled by evaporation of water from the solution, and a crystallization tank 60. Expansion chamber 58 is maintained under a vacuum which corresponds to generally at an absolute pressure of 0.035 to 0.28 bar, usually 0.057 to 0.105 bar, by water vapor ejectors 62 and 64. These ejectors can create a vacuum corresponding to an absolute pressure of about 0.035 bar . As shown, the ejectors 62 and 64 generate a vacuum in the chamber 58 through a line 66. The pressure in the chamber 58 is regulated by adding air through a line 68 to adjust the temperature of the chamber. solution.

  The pressure adjustment in turn determines the boiling point and the temperature drop, and hence the temperature of the solution contained in the expansion chamber and the amount of water evaporated from the solution. A heat exchanger 70 located in line 66 condenses the water evaporated from the solution contained in chamber 58. The condensate passes through line 72 and is used to dissolve the potassium pyrosulfite crystals. The water vapor coming from the ejectors 62 and 64 is also condensed and passes through pipes 74 and 76 to be used later in the installation.



   The solution contained in chamber 58 is normally cooled to a temperature sufficient to crystallize i.e. to remove from the solution a sufficient amount of potassium pyrosulfite to compensate for the amount of sulfur dioxide absorbed from the solution. gas stream contained in line 12 and thereby maintaining a correct balance of solids in the solution for recycling. Potassium bisulfite crystallizes in the form of potassium pyrosulfite. The amount of potassium pyrosulfite crystals. removed depends on the transformation of potassium sulphite into bisulphite in the purification zone, and generally 3 to 5 kg of potassium pyrosulphite crystals are removed for each kilogram of sulfur dioxide absorbed from the waste gases .

  For example, if the conversion of sulfite to bisulfite in the aqueous solution passing through reactor 10 is about 50 s / 0, about 7 kg of crystals are removed to provide a particularly suitable solution to be reused in the part of. absorption, while with a transformation of 20 a / e this quantity increases to about 15 kg of crystals per kilogram of sulfur dioxide, and with a theoretical transformation of 1000 / o the quantity is about 3 to 3.5 kg per kilogram of sulfur dioxide.

  The temperature drop occurring in chamber 58 is determined by adjusting the inlet temperature of the solution, by the pressure in chamber 58, the length of time the solution dwells in chamber 58 (for example, generally about 15 minutes to 2 hours), by the recycling report, etc.

  Since the temperature of the inlet solution is generally between 270 and 930 C, usually between 540 and 710 C for the treatment of gas from a power plant, and usually between 270 and 430 C for the treatment. of a gas from a sulfuric acid production plant, and given that the solution boils at approximately 400 C, at an absolute pressure of 0.07 bar and at 290 C at an absolute pressure of 0.035 bar, Temperature drops generally from 6 to 390 or 500 C, preferably from 17 to 280 or 390 C, are usually necessary to crystallize the desired amount of potassium pyrosulfite.

  The quantity of water removed in the expansion cooler from the installation is advantageously used to keep the water of the installation in equilibrium by correlation between the quantity of water removed by the expansion cooler and the quantity of water removed by the gas stream in the reactor. Although water can be removed in the expansion cooler, it requires an input of energy, while if it is removed in the reactor, the energy used is that which is normally available.

  Thus, a particularly interesting advantage of the pre-purifier 20 lies in carrying out the pre-purification with a liquid, for example water, while at the same time avoiding a large drop in the temperature of the gas and a large increase in gas temperature. relative humidity of gas; thereby providing the gas in the absorption zone at an optimum temperature and relative humidity to evaporate water from the plant in an inexpensive and efficient manner. In the case where a gas stream having a low temperature and a high relative humidity and having a low water absorption capacity is used, the installation has a great flexibility, since the excess water can be removed by the expansion cooler.



   The efficiency of the expansion cooler in removing water also makes it possible to advantageously regulate the filtration conditions. The filtration system used may be a simple rotary filter, and advantageous control of solution temperature and residence time can be achieved by adjusting crystal size to improve filter performance with higher filtration rates. to provide crystals of higher purity, for example 99 percent. It also desursatures without priming and substantially avoids any problem posed by clogging.



   The cooled, or slurry, solution passes from the expansion chamber 58 into a crystallization tank 60 through a line 78. The residence time of the solution in the crystallization tank 60 is sufficient to produce crystals of the correct size by filtration. for example crystals of size greater than 0.047 mm and ranging up to about 22 mm. Generally, a volume residence time of about 5 minutes to about 2 hours or more, preferably about 10 to 45 minutes, is suitable for producing crystals of the desired size. Crystals in which about 80 percent have a size between 2 and 0.25 mm can be obtained using a residence time of 45 minutes. The pH of the solution is generally adjusted between about 6.6 and 7.4.

  The pH can be regulated by adjusting the specific gravity of the reaction solution leaving the reactor, for example by using a device 69 for adjusting the density in the line 71. A recycle line 80 and a pump 82 are provided to control the temperature. and the residence time of the solution. The bottom of the expansion chamber 58 is inclined, and the pipe 78 extends to the bottom of the reservoir 60 to prevent clogging by the crystals formed in the chamber 58 and in the reservoir 60. A mist stopper cloth 84, disposed in the upper part of the expansion chamber 58, is constructed and operates in a manner analogous to zone 40, in order to eliminate the droplets entrained by the gas passing through the pipe 66.



   The suspension of the potassium pyrosulfite crystals formed in the crystallization tank 60 is pumped by the pump 86 in the line 88 to feed it into a vacuum filter comprising a rotating drum generally designated 90. The filter comprises a rotating drum 92 and a tank 94. The slurry is introduced into the tank 94 by the line 88 and is sucked by a vacuum applied inside the drum 92 through the filtering surface of the drum by a vacuum pump 96. The filtered solution is sucked from the filter drum 92 and passes through a pipe 98 into a reservoir 100 reserved for the filtrate where it is collected and from where it is subsequently pumped into a pipe 102 by a pump 104 to deliver it to a reservoir d diet 106.



   The potassium pyrosulfite crystals retained on the surface of the filter drum 92 are removed by a scraper 108 and transferred to a dissolving tank 110. The filtration rate of the filter is kept as high as possible, while still allowing a high performance. filtrate or clear solution from the filter in line 98. Filtration rates are usually obtained from 4410 to 1960, preferably 7350 to 14700 kg of crystals per hour per square meter of filter contact area. The higher the filtration speed, the smaller the required contact surface of the filter. The size of the filter cloth used can be important in maintaining the filtrate in a clear state.

  For example, if crystals pass through the wire, the pH of the solution or filtrate decreases, for example to a pH of about 6.3, and thus it adversely affects the recycle stream entering the absorption of. So as a result of the increase in the amount of bisulfite. The vacuum applied to the filter drum by the pump 96 is maintained so that the vacuum applied to the drum 92 is less than the vacuum applied to the chamber 58, for example at a higher pressure. Applying a lower pressure to this point of the plant would further cool the solution to crystallize an additional quantity of potassium pyrosulfite inside the drum 92, which would clog the plant.

  The filter used on the drum 92 is preferably a monofilament fabric, for example of nylon, which is chemically inert to the solution and to the suspension, which has mesh openings of a dimension generally between 10 and 50 or 60 microns, preferably between 20 and 40 microns approximately. The crystals can be advantageously washed by spray 112, preferably using a side stream of the 50 percent solution which is fed to the purification column on the surface of the filter to further improve their purification.

  Since the 50 percent potassium bisulfite solution is saturated, the crystals are not substantially dissolved and the purity of the crystals is increased by displacement washing of the mother liquor. Also, if desired, a diffusion or backflow of water vapor 114 can be applied to the surface of the filter near the scraper blade 108 to clean the surface of the filter by removing any crystals which have not been removed by the filter. blade 108.



   The potassium pyrosulfite crystals can be advantageously heated in the presence of water to promote their decomposition to produce sulfur dioxide at relatively low temperatures. The water used can be the residual water of the mother liquor contained in the crystals or added water; it can be in any suitable form, for example in liquid or vapor form; it is used in amounts sufficient to promote the decomposition of potassium pyrosulphite to produce sulfur dioxide.

 

  These amounts are at least 0.01 weight percent, generally 1 to 99 weight percent, and preferably about 20 to 75 weight percent based on potassium pyrosulfite and water. With amounts of water of up to about 20 percent by weight, wet crystals are obtained, and with amounts of up to 40 percent by weight, for example, a solution can be obtained. However, when in solution, potassium pyrosulfite is in the bisulfite form.



   The crystals passing through the reservoir 110 are dissolved in a sufficient quantity of water to facilitate the treatment. Generally, enough water is added to form at least one slurry which is pumpable, for example which contains about 60 to 70 percent solids, although if desired a sufficient amount can be added. of water to form a solution. The ratio of solids to water in dissolving tank 110 is advantageously controlled by density using density regulator 103.

  In general, a quantity of between 35 and 55%, preferably between 40 and 50 / o approximately, of solid substances is contained in the solution which is discharged from the dissolution tank 110 by the pump 118 in the pipe 116. L water is introduced into dissolving tank 110 as a condensate from cooler 70 through line 72 and lines 74 and 76 from steam ejectors 62 and 64, respectively. Fresh water can be added via line 120, if desired.



   Potassium pyrosulfite, in the presence or in contact with water, is heated to temperatures sufficient to produce sulfur dioxide, and at atmospheric pressure, temperatures generally between 380 and 1070 can be used. C, preferably between 660 and 1070 C.



   The decomposition of potassium pyrosulphite in contact with water can be carried out at atmospheric pressure or at a higher pressure, for example at an absolute pressure between 0 and 21 bars, generally between 1.05 and 10, 5 bar, although atmospheric pressure can advantageously be used.

  In a variant of this aspect, it is possible to have recourse to a pressure greater than atmospheric pressure when the potassium pyrosulfite is in aqueous solution in order to increase the concentration of the solution at higher temperatures, for example above 110 ° and up to Approximately 1910 C, which results in an increase in the partial pressure of sulfur dioxide in potassium pyrosulfite and an improvement in the production of sulfur dioxide. In general, in this connection, water is used in an amount of about 5 to 50 or 70 percent by weight.

  For example, if the potassium bisulfite solution is heated to an absolute pressure of about 7 bar and a temperature of about 1770 C, the solubility of the pyrosulfite crystals is much greater, for example 75 percent by weight approximately based on crystals and water.

  This higher concentration provides a much higher partial pressure of sulfur dioxide than lower concentrations, and the percentage of conversion of potassium pyrosulfite to potassium sulfite is greater.
 The use of water to improve the decomposition of potassium pyrosulfite is preferable to the dry process, since it avoids the need to use inert materials and the costs of handling these materials, and it allows sulfur dioxide to be produced at much lower temperatures and with a higher conversion rate of potassium pyrosulfite (bisulfite) into sulfur dioxide.



   Since a small amount of sulfur trioxide is usually present in the waste gas containing sulfur dioxide, a small amount of potassium sulfate is formed which is periodically removed. Further, the oxygen present in the waste gases may react with the potassium sulfite to produce an additional amount of potassium sulfate, so that it may be desirable to add an oxidation inhibitor, for example substances. organic, such as hydroquinone, etc., to potassium sulphite solution. In general, potassium sulphate crystallizes with pyrosulphite and can be separated by periodic redissolution of pyrosulphite crystals which are more soluble in water than potassium sulphate.



  The redissolved pyrosulfite crystals can be recycled to the reaction zone after removing the solid potassium sulfate. Potassium sulfate is advantageous as a constituent of fertilizer. If desired, this process can be used to produce potassium sulfate, omitting the inhibitor and increasing the amount of potassium sulfate produced.



   The potassium bisulfite solution contained in line 16 is passed through a purification zone, generally designated by 125, where the solution is heated to a temperature of for example between 1100 and 1490 ° C., which is sufficient to decompose potassium bisulfite and to produce sulfur dioxide and potassium sulfite solution. The decomposition is advantageously carried out in a purification operation with multiple effects, in order to increase the overall transformation of potassium bisulfite into potassium sulfite and to reduce the energy requirements, that is to say of water vapor, of the installation. As shown, purification zone 125 includes two rectification columns 122 and 122 ', although three or more can be used if desired.



  The solution introduced into column 122 through line 116 is heated by steam in a reboiler 138 to a temperature sufficient to decompose the potassium bisulfite contained in the solution, in order to produce an overhead gas stream comprising the Sulfur dioxide and water vapor.



     This overhead stream is discharged through line 124 and is transferred to a reboiler 138 'of the second rectification column 122' to provide the energy required for the decomposition of an additional quantity of potassium bisulfite. The transformation of potassium bisulphite into potassium sulphite in any particular rectification column is limited, since as soon as the partial pressure of sulfur dioxide in the vapors contained in the column is equal to the partial pressure of sulfur dioxide from the solution, an additional amount of sulfur dioxide cannot be removed from the solution, regardless of the energy applied to the column.

  When using more than one column, fresh water vapor is produced in each column by removing water from the solution which is saturated with an additional amount of sulfur dioxide, so that decomposition is obtained. additional potassium bisulfite and better conversion. In addition, the plant's energy or water vapor requirements are reduced by recovering energy from the vapors produced in each rectification column by using these vapors to produce heat for the next column. . After reboiler 138 '.

 

  wherein the water vapor contained in line 124 is largely a condensed product, this product passes through line 126 into a separator 128. The condensed water is discharged from the bottom of the separator 128 through a line 130, and sulfur dioxide is discharged as a gaseous product through line 131. The partially decomposed potassium bisulfite solution is discharged from column 122 through line 134, and is delivered by pump 136 into column 122 'where an additional quantity of bisulfite potassium is broken down. The overhead vapors are evacuated from the column 122 'by a pipe 124' and through a heat exchanger, that is to say the condenser-cooler 126 'to be introduced into the reservoir 128'.



  The condensed water vapor is discharged from the swparator tank 128 'through a pipe 130' and the sulfur dioxide is discharged as a gaseous product through a pipe 131 '. The bottoms, which now comprise a solution rich in potassium sulphite, are transferred by a pump 126 'into a line 134' to be introduced into a reservoir 106 for supplying the absorption zone. A portion of the bottoms contained in line 134 passes through reboiler 138 to maintain a correct temperature, for example generally between 1100 and 1490 C, in rectification column 122. The potassium sulphite solution collected in the tank 106 is pumped by a pump 140 in a pipe 142 to supply the inlet pipe 16 of the reactor 10.

  The water separated in lines 130 and 130 'passes into dissolving tank 110 to dissolve the potassium pyrosulfite crystals.



   The first separation column is generally operated at an elevated temperature and pressure sufficient to decompose the potassium bisulfite, for example at an absolute pressure of 14 bar and at a temperature of 3160 C, but preferably below the temperature at which large quantities of potassium sulphate are formed, for example at 2040 C, and below absolute pressures of about 10.5 bar.



  Typically, the first separator 122 operates at a temperature between 1350 and 1770 C, or 2040 C and at an absolute pressure between 2.8 and 5.6 bar, preferably between 2.8 and 4, 2 bars. These conditions are chosen to keep the water vapor requirement low, that is, the amount of water vapor required to produce the desired amount of sulfur dioxide. In general, it is desirable to keep the water vapor requirements between approximately 2 and 6, preferably between 2 and 4 kg of water vapor per kilogram of sulfur dioxide.



  Each of the successive columns operates at an absolute pressure of about 1.4 bar below the previous column, and at corresponding decomposition temperatures. For example, the second separator column in a two-column system generally operates at an absolute pressure between 1.4 and 4.2 bar, preferably between 1.4 and 2.8 bar, and at a temperature between 1210 and 1490 C, preferably between 1270 and 1380 C.



   When it is desirable to remove potassium sulfate, which is an undesirable byproduct of the process, from potassium bisulfite, part of the solution in line 116 can be diverted to pass through line 144 where a centrifuge. 146, or if desired, a filter, separates the potassium sulphate crystals to pass them to a discharge line 147. If a suspension of potassium bisulphite is used as the feed to the rectification column 122, the Bypass and separation of potassium sulphate can be carried out in line 134 'or line 142.



   The nature of the installation and its operation are such that they are not adversely affected by the cyclical nature of the operations of the installation, the temperature of the gases to be treated (for example a drop in temperature of the gases from power plants during night), or the sulfur or solid content of the gas, and thus providing great flexibility. Thus, it can purify any gas containing SI, SQ, or particulate solids, or any other type of similar component into any type gas which is compatible with the installation and which is not adversely affected by it, or which does not adversely affect said installation.



   The embodiment shown in FIG. 12 is identical to that of FIG. 1 with the exception of the purification zone designated generally by 125 ', in which the potassium bisulfite is decomposed in the presence of an inert gas. In the installation of fig. 12, the potassium bisulfite solution contained in line 116 'passes into the purification zone 125' where the solution is heated to a temperature of, for example, between 110o and 1490 C, which is sufficient to decompose the potassium bisulfite and producing sulfur trioxide and a solution of potassium sulfite.

  The decomposition is advantageously carried out during a multi-effect rectification operation in order to increase the overall conversion of the potassium bisulfite to potassium sulfite, and to reduce the energy requirements, that is to say of the steam. water, installation. As shown, purification zone 125 'includes a rectification column 122 ", although two or more columns can be used if desired.

  The solution introduced into column 122 "through line 116 'is heated by steam in a heat exchanger 150' to a temperature generally below the boiling point at the operating pressure of the column, sufficient to decompose the potassium bisulfite in the solution, and to produce potassium sulfite and sulfur dioxide, and it is introduced into column 122 "at this temperature. Although the bisulfite solution has been decomposed, sulfur dioxide cannot be released until a driving force is supplied.

  Consequently, an inert gas such as nitrogen, methane, argon, helium, etc. is introduced into column 122 "through line 130" to entrain SO, out of solution and preferably saturate the inert gas. with SQ; when the partial pressure of sulfur dioxide in the vapors in the column is equal to the partial pressure of sulfur dioxide in the solution, an additional amount of sulfur dioxide cannot be removed from the solution regardless of the energy applied to the column.

  When using several columns, the energy or water vapor requirements of the installation are reduced by recovering the energy from the vapors produced in each rectification column by using these vapors to produce the heat intended for the next column. The inert gas containing SO, is discharged through a line 124 ", a heat exchanger 126" in which SQ is condensed and transferred to the separator 128 ". This condensed sulfur dioxide (containing the water removed from the column 122") is discharged from the bottom of separator 128 "through line 138", and the inert gas is discharged through line 132 ". A blower 130" provided in line 132 "discharges inert gas into column 122".

 

  The partially decomposed potassium bisulfite solution is discharged from column 122 "by a pipe 134" and is discharged by a pump 136 "into the reservoir 106 '. Temperatures between 1100 and 1490 C and absolute pressures up to 'at about 14 bar are typical for rectification column 122 ".



   The solution in line 116 'is generally heated to as high a temperature as possible without producing potassium sulfate and without boiling the solution. The heat increases the partial pressure of SO from the solution to the boiling point of the solution to provide a greater pressure difference between SQ of the solution and that of the inert gas, so that the inert gas can sweep away
SI of the solution as it passes through column 122 ".

  In general, temperatures between approximately 1100 and 3160 C, but preferably below 2040 C, temperatures above which sulfate can form, and absolute pressures between 0 and 25 bars approximately, preferably between 1.05 and 10.5 bar, can be used in the decomposition zone. The inert gas passes through the column at a speed sufficient to drive SOI out of solution, but insufficient to force the solution out of the column, for example it is between about 0.45 and 6 meters per second, preferably between 0, 6 and 2.4 meters per second.

  Since most of the water is condensed in the column under its operating conditions, only a small amount of water vapor is removed with the inert gases, allowing the regenerated potassium sulfite solution to be reused. .



   The SO recovery system, described above and with reference to Figs. 1 and 2 was used to treat flue gas from a power plant burning coal or coal. To estimate the temperature drop of the inlet flue gases after passing through the pneumatic scrubber, several tests were performed at various ratios of scrub water to flue gas. During each test, flue gases were introduced into the pre-scrubber at a flow rate of 56.6 cubic meters per minute and at a temperature of about 1490 C. The temperature and humidity of the gases were recorded at. exit from the top of the target. The results are shown in Table I below.



   Example I
 The following table, Table II, gives a particular example of this process under the conditions and with the parameters mentioned in the description of FIGS. 1 and 2 and using the installations shown in the drawings, the table giving particular compositions of the streams for the various flow lines and the prevailing temperatures. The flue gases have the typical composition in mole percent: 0.3 / o sulfur dioxide, 3.4 / o oxygen, 6.0 0 / o water vapor, 14.2 0 / o d carbon dioxide, 76.1 / o nitrogen, and 0.0003O / o sulfur trioxide. The production of potassium sulphate is substantially zero.

  The vacuum type expansion cooler operates at an absolute pressure of 0.07 bar, and at 400 C, water is discharged from the expansion cooler at a flow rate of approximately 63.5 liters per hour, and the pH of the solution contained in the crystallization apparatus is about 7.0 to 7.2.



   Example 2
 The same procedure was followed as that used in Example 1, except that an aqueous solution of cesium sulfite was used in place of potassium sulfite to produce cesium bisulfite and to recover SQ.



   Example 3
 The procedure used in Example 1 was followed, except that an aqueous solution of rubidium sulfite was used in place of potassium sulfite to produce rubidium bisulfite and to recover SOO.



   Fig. 3 is a sectional view of one type of apparatus of the present invention as described with reference to Figs. 1 and 2, but which is intended for independent use in separating particulate solids and liquid soluble components from a gas stream. It also shows the passage of gas, for the removal of large particles, through an electric precipitator 200 and the effect of the precursor 10 is used on the part of the particulate solids having a size generally rendering it. more difficult to separate.

  Referring to fig. 3, there is shown a reaction vessel 100 comprising a cylindrical middle part 101 having a circular inlet 105 reserved for gases provided in the lower quarter of the middle part and a liquid spraying means comprising a pipe introduced into an orifice 106 and s' extending laterally in the middle part. Said container has a conical lower part 102 having an outlet 26 for discharging the liquid and particulate solids, and an upper part 103 in the form of an inverted cone having an outlet 104 reserved for gases for discharging the purified gas.

  A circular support plate 15 having several orifices 107 (see fig. 4) is mounted axially in an area 20 above the pipe 24, and is secured to the inner wall 108 of the middle part 101 and supports a filling column. 22 consisting of three layers of Raschig rings of ceramic material. The fill column and support plate define a gas permeable target. The inlet 105 communicates with a gas inlet line 112 having a cap-shaped end 13 extending upwardly into the area 20 below the line 24, and which communicates with the target 110.

  Spray nozzles 25 are preferably evenly spaced along the internal portion 109 of the conduit 24 and are mounted to spray a continuous stream of liquid, e.g. water, toward the target 110, so as to spray a continuous stream of liquid, eg, water. to reach it.



   Table I
 Water flow in Moles of H20 Humidity
 Inlet temperature spray nozzles per mole Flue gas outlet temperature
Burnt gas test, OC liters / minute of dry burnt gas, OC at the outlet,

   0 / o
 1 I49 0.0 0.060 149 6
 2 149 0.38 0.075 124 8
 3,149 0.78 0.085 102 12
 4,149 1.14 0.105 82 20
 5,149 ¯ 0.135 53 100 Table II
 Blade Evacuation Load 108
 Gas Burned In Burned Gas Absorber Scrape Absorber Filtrate
Process Running in line 12 absorber 30 in line 14 in line 16 in line 18 crystals in line 102
SO2 kg / min 0.292 0.292 0.292 6.817 7.081 1.129 5.951
K2S2O5 kg / mn 6.01 6.926 1.927 4.99
K2SO3 kg / mn 8.26 7.611 0.04 7.575
K2SO4 kg / mn 0.803 0.803 0.122 0.68
SO3 moles% 0.0003 0
H2O kg / mn X X + 2.27 16.33 13.88 0.0 13.88
Solid substances kg / min 15,037 15,332 2,091 13,245
Total....

   kg / min 49.9 51.9 30.541 29.21 2.091 27.125
Burnt gas m / min 56.9 43.9
Fly ash g / m 0.69 (1) 0
Liters / minute 21.2 19.7 18.9
Specific gravity 1.44 1.48 1.43
Temperature C 149 124 52 66 57 40 40 pH 7.2 7.1
Relative humidity 6% 8% (a) (1) 3.45 g / m in an electric precipitator and 0.69 g outside the electric precipitator.

 

  (2) On leaving absorption zone 30, the relative humidity is 83%.



      Table II (continued)
 Column load Head product Product Bottom product Water admitted
 rectification. rectification columns, rectification columns, rectification column, in the pre-purifier,
Process Current line 116 lines 124 and 124 'lines 131 and 131' line 134 'line 24
SO2 kg / min 1.129 0.263 0.263 0.867
K2S2O5 kg / mn 1,927 1.02
K2SO3 kg / min 0.004 0.685
K2SO4 kg / mn 0.122 0.122
SO3 moles%
H2O kg / min 2.99 0.952 0.02 2.041 0.376
Solid substances kg / min 2,091 1,828
Total kg / min 5.085 1.215 0.283 3.869 0.376
Burnt gas m / min
Fly ash g / m 2.84 3.79
Liters / minute 3.7 1.42 1.0
Specific gravity 1.37 30
Temperature C 49 107 pH 92 125
Relative humidity

 

Claims (1)

CLAIM I Process for removing SO, from a gas stream containing it mixed with SO, and having a temperature between 660 C and 4270 C, characterized in that the gas stream is treated to remove SQ therefrom. the gas stream thus treated is contacted with an aqueous absorbent solution of a metal sulfite for the purpose of absorbing SO 3 from the gas stream and producing a spent absorbent effluent containing the corresponding metal bisulfite, which is treated l Absorbent effluent spent to produce SO, and regenerated sulphite and recycled regenerated sulphite to said contacting. SUB-CLAIMS 1. Method according to claim I, characterized in that the gas stream which is treated also contains particulate solids and in that these are removed from the gas stream before bringing the latter into contact with the solution. aqueous absorbent of metal sulfite. 2. A method according to claim I, characterized in that the removal of SQ from the gas stream is effected by directing the gas stream upwardly in a scrub zone towards a fluid permeable target disposed in a substantially manner. horizontal and having a discontinuous surface, an aqueous liquid is led upwards in the purification zone towards said target simultaneously with the gas stream to touch and wet the lower surface of this target, the quantity of said liquid serving to dissolve SQ of the gas stream, the gas stream is passed through the target to produce a stream of effluent gas from this target with a reduced SO3 content, liquid containing SOn is removed from the undersurface of the target and the removed liquid containing Sub is removed from the scrub zone. 3. Method according to sub-claim 2, characterized in that the treated gas stream also contains particulate solids, and in that these are removed from the stream mixed with the SO3 which is removed in the purification area. 4. Method according to sub-claim 3, characterized in that the content of the treated gas stream in solid substances is between 0.0023 and 138 g per cubic meter of gas and in that the SQ is present in an amount of 0 , 0001 and 0.0I mole percent. 5. Method according to sub-claim 4, characterized in that the solid substances comprise fly ash, in that the gas stream is previously subjected to a pre-filtration operation to remove large quantities of the particulate solids before it. reaches the scrubbing zone, and in that the solids content of the gas stream introduced into the scrubbing zone is between 1.15 and 69 g / m3 of gas. 6. Method according to sub-claim 5, characterized in that the aqueous liquid is water, and in that this liquid is directed towards the target in an amount between 0.00175 and 0.175 liters / minute per cubic meter of gas per minute, sufficient to form, at the lower surface of the target, droplets containing solid substances, droplets which fall from the target, and which are discharged from the scrub zone before significant evaporation of the droplets can occur. 7. Method according to sub-claim 6, characterized in that the water is at a temperature between 100 and 49O C and the gas is at a temperature between 66 and 3160 C and has a relative humidity of up to 10 percent. 8. Method according to sub-claim 7, characterized in that the liquid is directed towards the target in the form of a spray by a means for spraying the liquid placed at a distance of 10.16 to 45.72 cm below the target. and which delivers liquid to the target at a rate sufficient to hit and wet the undersurface of the target and insufficient to penetrate the target in significant amounts. 9. The method of sub-claim 8, characterized in that the liquid spray is continuous and has droplets whose size is between 200 and 800 microns, the gas stream has a surface speed at the time it touches the target, which is between 1.5 and 3 meters / second, in that the gas stream is introduced into the zone of => pu- ration at a point sufficiently below the spray medium to avoid altering the spray configuration, but close enough to avoid excessive evaporation of droplets falling from below the target. 10. The method of sub-claim 9, characterized in that the gas stream is introduced into the purification zone at a point between 12.7 and 177.8 cm below the target and at a surface speed of between between 0.6 and 4.5 meters / second the instant it hits the target. 11. The method of sub-claim 10, characterized in that the treated gas stream contains between 0.001 and less than 5 mole percent SO, and is at a temperature between 1210 C and 2040 C. 12. Method according to sub-claim 11, characterized in that the target comprises a filling having 50 to 90 percent free space and formed of solid particles, the largest dimension of which is between 6.35 and 88.9. .mm. 13. The method of sub-claim 12, characterized in that the particles weigh from 0.48 to 1.12 kg per cubic decimeter and in that the filling is composed of 2 to 4 layers of these particles and has a thickness of between between 3.81 and 13.97 cm. 14. The method of sub-claim 13, characterized in that the particles have a maximum length of 25.4 to 38.1 mm and in that the filling is irrigated intermittently by means of a water jet directed towards down intermittently on the top surface of the target. 15. The method of sub-claim 2, characterized in that the metal sulphite is potassium sulphite, cesium or rubidium, the spent absorbent effluent is a solution of bisulphite and unreacted metal sulphite, this solution is treated to produce SO9 and regenerated sulfite by removing water from it in an amount sufficient to crystallize bisulfite out of solution as the corresponding pyrosulfite and to separate pyrosulfite from unreacted sulfite, thereby increasing the partial pressure of SO, in pyrosulfite, crystallized pyrosulfite is separated from its mother liquid and is heated, either in solid form or in redissolved form of bisulfite, at a temperature sufficient to decompose it and to produce SO9 and the corresponding metal sulfite. 16. The method of sub-claim 15, characterized in that the metal sulphite in the absorbent zone is potassium sulphite, in that the absorbent solution contains 40 to 65 percent by weight of solid substances including a proportion of at least 75 percent by weight is potassium sulfite, the remainder being substantially potassium bisulfite and an antioxidant to inhibit the reaction of sulfite ion with oxygen, the temperature in the absorbent zone is below 1100 C, the spent absorbent solution containing potassium bisulfite comprising between 45 and 65 percent by weight solids of which between 10 and 50 percent by weight is potassium bisulfite, and the absorbent solution is cooled to aid crystallization, from this solution, bisulfite in the form of potassium pyrosulfite. 17. Method according to sub-claim 16, characterized in that the absorbent solution is cooled below 43.30 C to crystallize pyrosulfite having a purity of at least 65 percent by weight which is then decomposed at a temperature of between between 110 and 3160 C. 18. The method of sub-claim 17, characterized in that the temperature of the spent absorbent solution is between 270 and 930C, and its pH is adjusted between 6.6 and 7.4, in that the crystallization zone is maintained at an absolute subatmospheric pressure between 0.056 and 0.105 bar, to evaporate the water and cool the solution by a temperature drop of between 170 and 390 C, and in that the duration of the solution's residence in the zone of crystallization is between 5 minutes and 2 hours approximately. 19. The method of sub-claim 18, characterized in that a sufficient quantity of water is added to the separated pyrosulfite crystals to form a pumpable mixture containing between 20 and 75 percent by weight of water, and heats the mixture by introducing the mixture into a decomposition zone having first and second stages, by heating the mixture in the first stage of the decomposition zone to a temperature between 1350 and 2040 C and an absolute pressure between 2.8 and 5.6 bars to decompose the bisulfite and produce a first gaseous product of water vapor saturated with SO, and an aqueous solution containing sulfite and bisulfite by bringing the liquid product to the second stage, by heating the second stage indirectly with the first gaseous product and maintaining it at a temperature between 1210 and 1490 C and at an absolute pressure between 1.4 and 4.2 bar to decompose the bisulfite and to produce a second gaseous product of water vapor saturated with SOO and an aqueous solution of sulphite, separating the gaseous products and recovering SQ. 20. The method of sub-claim 19, characterized in that a sufficient quantity of water is added to the separated pyrosulphite crystals to form a pumpable mixture comprising between 20 and 75 percent by weight of water, and in that the mixture is led through a decomposition zone at a temperature between 1100 C and 2040 C, where it is brought into contact in countercurrent with nitrogen, helium, methane or argon in sufficient to remove SQ from the mixture, then the SQ gas is separated from the nitrogen, helium, methane or argon by condensation and recycled to the decomposition zone. 21. The method of sub-claim 14, characterized in that the plugging of the gas inlet into the purification zone by the deposition, at this inlet, of particulate solids removed from the gas stream is prevented by introducing the gas. gas in the scrubbing zone through a duct projecting into the zone at an angle of 5 to 600 above the horizontal and terminating at a point inside the scrubbing zone which is at least 25.4 mm away from the vertical wall of the area. 22. The method of sub-claim 18, characterized in that the contacting of the stream of purified gas in the purification zone with the absorbent solution of potassium sulphite for absorbing SO, of the gas stream is carried out by directing the gas stream. flow of purified gas in a general upward direction through a substantially horizontal contact screen which supports a layer of the absorbent solution, while spraying the underside of the screen with a portion of the absorbent solution to prevent deposition of salts in and plugging of the openings of the sieve, the temperature in the absorbent zone being kept below 880 C. 23. The method of sub-claim 18, characterized in that the gas stream from which the SO has been absorbed is then conducted to a drive zone in which the gas is passed through a woven fabric and sprayed. on the gas receiving side and the gas outlet side an aqueous solution of potassium sulphite which is then used to absorb SO9 in the absorbent zone, the gas flow rate through the screen being between 0.6 and 4 , 2 meters / second and the pressure drop across the canvas being less than 12.7 mm of water. CLAIM It Apparatus for carrying out the method according to claim I, characterized in that it comprises a container having walls and defining a purification zone, an absorption zone and a mist stop zone arranged successively, a fluid permeable target disposed in a substantially horizontal fashion having a discontinuous surface for retaining solid substances entrained by a gas and placed in the scrub zone; a liquid spray device located below the target for directing a spray of scrubber generally upwards to contact the underside of the target and wets it to remove gas from the substances solids and components soluble in the liquid, a device directing the flow of gas placed below the device for spraying the liquid and for directing gas in competing relationship towards the target with the sprayed liquid, a device for absorption of SQ in the absorption zone which is in gas flow relation with the absorption zone for absorbing SQ from the gas, a gas entrainer arranged in relation to receive the flow of gas gas with absorption zone, said drive device comprising a generally horizontal woven web-shaped contact material which is inert to reactants and which is placed over the absorption zone, and a spray device for spraying a solution against the gas inlet and outlet sides of the contact material, said spray device being arranged so as to reach substantially the entire contact surface of the material on both sides, and a device providing sufficient flow rate of the solution to keep the contact material moist, the thickness of the contact material not being sufficient to cause an excessive pressure drop of the gas through the material. SUB-CLAIMS 24. Apparatus according to claim II, characterized in that the absorption zone contains an SOS absorption device comprising at least one contact plate mounted to cooperate with a gas flow means in order to introduce into the plate. a solution removing components from the gas and a spray device arranged to spray a solution against the lower surface of at least one first contact plate in an amount sufficient to keep the lower surface of the plate moist. 25. Apparatus according to sub-claim 24, characterized in that the device directing the gas flow comprises a cap extending inside the container for a distance of at least 25 mm and obliquely upwards in a direction. angle between 5 and 600 and the thickness of the contact material is between 2.5 and 10 cm.