GB2037611A

GB2037611A - Gas scrubbing tower

Info

Publication number: GB2037611A
Application number: GB8005150A
Authority: GB
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1977-11-17
Filing date: 1978-11-16
Publication date: 1980-07-16
Also published as: JPS5478364A; FR2415479A1; DK151456C; IT7851907A0; ATA820278A; AT372620B; FR2415479B1; GB2037611B; BE872042A; JPS62144733A; LU80532A1; BR7807501A; ES475153A1; DK151456B; GB2008432A; CA1120393A; JPS6336816B2; SE7811844L; NL7811366A; CH633726A5

Abstract

A gas scrubbing tower 3 has a sump region 4 and, thereabove, a gas inlet 22 and at least two washing stages. The lowermost washing stage comprises a supported layer of packing elements 31 and a second washing stage 35 is without packing elements. First and second lines 33 and 133a circulate clarified wash liquid from the sump region to the layer of packing elements and to the second washing stage, respectively. The sump 4 comprises a sludge separator, having a downwardly conical member 134 in its upper region to supply used washing liquid to an inlet tube 42 extending to the central region of the separator, a clarified liquid outlet pipe 33 extending from radially around the tube in the central region, and a sludge outlet 13. In treating off-gases, a liquid of pH below 4 is suitably used in each washing stage to avoid caking and the sludge from sump 4 is sprayed 19 into the gases in an evaporative cooler 1 upstream of the tower 3. <IMAGE>

Description

GB 2037611 A 1

SPECIFICATION

Gas scrubbing tower The invention relates to a gas scrubbing tower usable in a plant for purifying waste gases from industrial furnaces, especially from waste incinera- tion plants which scrubbing tower has several superimposed stages, in which acid, neutral and basic harmful substances (pollutants) in gaseous form or in the form of a mist present in the waste gases are removed after prior withdrawal of thermal energy and after separation of at least a substantial portion or all of the solid pollutants by means of preliminary treatments Gaseous pollutants as well as the remaining solid pollutants and pollutant mists (aerosols), present in the gas are removed in the scrubbing tower by means of wash liquid which is then cycled through a sludge separator (or thicken- er), from which a suspension of pollutants can be withdrawn.

Preferably, the thermal energy is withdrawn from the waste gases prior to their introduction in the gas purification plant in an indirect heat exchanger, in particular a steam generator being indirectly heated by the waste gases, before these enter the scrubbing tower.

Plants of the above type for carrying out the purification of waste gases from industrial furnaces are described in Austrian Patent 225,163 and in German Auslegeschrift 2,408,223, both of Gottfried Bischoff Bau Kompl Gasreinigungs und Wasseruck- kuhlanlagen KG, Essen, Germany.

Waste gases from waste incineration plants often have a high content of hydrogen chloride, stemming in particular from the combustion of polyvinyl chloride wastes, and, in addition, in particular a high content of SO 2 Moreover, their content of primary dust is usually high.

German Auslegeschrift 2,431,130 of Walther und Cie AG, Cologne, Germany describes a scrubber which operates in principle with alkaline wash liquids, especially those containing sodium ions and ammonium ions, whereby concentrated salt solu- tions are obtained in the scrubber and these are recycled in the gas purification plant.

In a further known plant according to U S Patent Specification 3,929,963 of S L Taub (E l Du Pont de

Nemours and Company) the solid pollutants are removed from the waste-gas as completely as possible, by which means the need for a sludge separator can be avoided, whereupon the waste gases, which have been substantially free from solid pollutants, are then subjected to scrubbing by means of a wash solution, the p H value of which in the scrubber or in the return line from the scrubber to the evaporative cooler can be so adjusted, by adding salt-forming chemicals, that a water-soluble or water-insoluble salt is formed with the gaseous acids or with other, basic pollutants taken up by the wash liquid.

Gas scrubbing towers containing several packing layers which extend each transversely to the perpen- dicular tower axis, and which layers are spacedly superimposed one above she other, in which towers the waste gas is introduced beneath the lowermost packing layer and the purified gas exits at or near the top end of the tower, have been described in numerous publications, for example in U S Patent 2,523,441 to Paul E McKamy issued 26 Sept 1950, French Patent 1,469,230 to Metallgesellschaft, A G.

published 10 February 1969, U S Patent 3,768,234 to Leslie C Hardison, issued 30 Oct 1973, German Offenlegungsschrift 2643211 to Gewerkschaft Keramchemie, publ 6 April 1978.

Each packing layer consists of a large number of packing elements "Hedgehog" type bodies as illus- trated in Figure 3 of a special printing of "Probleme der nass-mechanischen Abscheidung feiner Par- tikeln aus Gasstrbmen" taken from "Chemische Rundschau" 20 ( 1975) No 18 published by Chemie- Verlag Vogt-Schild AG, Solothurn, Switzerland are preferred.

More advanced scrubbing tower installations comprise at least one of the flue gas washing towers of the type described by Fattinger, Schmitz and Schneidein Publication No 107 "Technikder Abgasreinigung" ("Technology of Waste Gas Purifi- cation") of the "Tagung Lufthygiene 1976 " ("Conference on Air Hygiene 1976 ") of 3rd December 1976 published by Verein zur Fbrderung der Wasser und Lufthygiene (VFWL) of Huttenstrasse 36,8006 Zurich, Switzerland (see Figures 1 to 4 and 10 therein).

A particularly advantageous tower of this type having a combination of a packing layer and an X-slot-separator is described in Figure 10 of the last-cited publication.

These known towers suffer from a number of drawbacks which increase the cost of operation of the plants and increase the consumption costs for chemicals to an economically highly undesirable extent.

Thus, in all of these known plants, there is a sump at the bottom of the tower in which wash liquor charged with solid particles and/or scum collects, and often this wash liquor is then discharged into a settling tank and may be circulated from there to a sludge filter and back into the same settling tank.

Supernatant liquor is then drawn off from the top sidewall of the settling tank and recirculated into one or several wash stages in the scrubbing tower.

The total amount of water taken up by the stream of waste gas in the scrubber firstly results in a considerable increase in the volume of gas to be purified and thus in a substantial, expensive increase in the size of the scrubbing tower and, secondly, in the formation of an undesirably intense plume of condensed water vapour at the outlet of the chimney from which the purified waste gases are released into the surrounding air.

Whilst the main object of the scrubbing tower is, in particular, to remove gaseous pollutants such as HCI, H Br, H 2 F 2, C 12, Br 2 and SO 2 and also sulphuric acid mists from the waste gas, a further important function of the scrubber is to cool the gases, whereby initially gaseous tar-like substances and also salt-like substances can be condensed and then precipitated.

However, this treatment frequently causes precipi- 2 GB 2037611 A tates on the walls of the various washing stages in the scrubber and also in the sludge separator, which precipitates form crusts or caking on these walls and in the outlets from the scrubber and impair the functioning of the entire washing apparatus to an increasing degree the longer the apparatus is in operation These incrustations or caking consist, for example, of gypsum, lime, metal salts and other solids.

According to the present invention there is pro- vided a gas scrubbing tower having a sump region and at least two washing stages above the sump region, the lowermost washing stage comprising at least one layer of packing elements extending in a horizontal plane across the cross-section of the tower and a second washing stage being without packing elements, which tower comprises ( 1) a gas entry means for introducing waste gas to be scrubbed into the tower, which entry means are located between the sump region and the lowermost washing stage.

( 2) gas exit means for the discharge of scrubbed gas from the uppermost washing stage, ( 3) a first circulating line comprising pump means, and spray nozzles in the lowermostwashing stage for circulating wash liquid from the sump region to irrigate the layer of packing elements in the lowermost washing stage, ( 4) a second circulating line for conducting wash liquid to the second washing stage, ( 5) a sludge separator, constituting the sump region of the tower, and comprising a settling vessel having an upper portion, a central portion and a base portion, the upper portion comprising an upper inner partition wall which tapers conically inwardly and downwardly to define a central orifice; an inlet tube for sludge-carrying liquid from the lowermost washing stage and extending from the central orifice to the central portion of the settling vessel; a first outlet tube for removal of liquid of reduced sludge content extending from the central portion of the settling vessel, above the lower end of the inlet tube radially toward the outside, which outlet tube forms part of the first circulating line; and a second outlet for removal of liquid charged with sediment from the base portion of the settling vessel; and ( 6) conduit means for introducing water into the scrubbing tower.

Preferably, such water is introduced into the settling vessel of the sludge separator, and/or it is introduced into the washing stages of the tower, especially for irrigating drip catchers provided therein.

Withdrawal of wash liquor from the central portion of the settling vessel about the inlet tube and above the lower end of the latter which is preferably equipped with a downwardly open conical outlet wall, enables a particularly clean phase of the liquor to be obtained and withdrawal may be effected more rapidly at that point than if the wash liquor is withdrawn in the conventional manner from the upper peripheral zone of the settling tank The largely or completely sludge-free water can then be recycled into the spray nozzles, provided in the first circulating line, in a particularly clean state, thus avoiding clogging of the fine spray nozzle openings, and augmenting the scrubbing effect of the wash liquor recirculated into this stage.

Preferably, a deflector is mounted at the lower open end of the inlet for liquid, of the sludge separator, and opposes the flow of liquid through that inlet tube.

The sludge separator also preferably comprises a conical funnel which widens downwardly and is connected with the lower end of the inlet tube.

The first outlet tube of the sludge separator preferably comprises a conduit having an intake opening surrounded by a cup-shaped casing and being positioned about the inlet tube for liquid, the conduit thus being adapted for receiving wash liquid rising along the outside of the conical funnel.

The sludge separator can further comprise an inner annular baffle underneath the upper inner partition, which baffle tapers conically inwardly and upwardly defining a central baffle orifice through which the inlet tube for liquid extends downwardly from the central orifice of the upper inner partition wall, the central baffle orifice also leaving an annular gap about the inlet tube, thereby permitting over- flow of scum-free or scum-carrying liquid through the gap and overthe upper surface of the baffle.

Preferably, the sludge separator further comprises means for maintaining liquid between two levels in the settling vessel of the sludge separator, the lower level being a short distance above the point at which the outlet tube opens into the settling vessel, and the upper level being at or a short distance below the central orifice of the conically tapered partition.

Furthermore, the sludge separator can comprise a scum outlet tube connected to the outlet for liquid charged with sediment, and a scum collecting vessel adapted for collecting scum which overflows from the gap of the central baffle orifice down the upper face of the conically tapered baffle when the level of liquid in the settling vessel is raised to its upper level.

The scrubbing tower according to the invention may be used in the purification of gases by the process described and claimed in our copending Application No 44703/78 (Serial No 2008432) from which this case is divided.

When operating the scrubbing tower according to the invention, the p H value of the liquid phase in the sludge separator and in each of the gas wash stages of the scrubber directly upstream of the separator is, as a rule, kept below 4.

Operating the scrubber in a p H range below 4 substantially or completely avoids the undesired formation of crusts or caking, mentioned hereinbefore, in the wash apparatus and in the sludge separator.

The content of acidic pollutants which has re- mained in the waste gases when introduced into the scrubbing tower should preferably be sufficiently high to afford in the scrubber a pollutants suspen- sion which has an acidic p H value, preferably below 4.

The proportion which can be evaporated, of the liquid phase of the pollutants suspension to be discharged from the sludge separator, should be at GB 2037611 A 3 least 70, but preferably at least 90, percent by weight.

Preferably, the p H value of the wash liquid in the lowermost washing stage and in the sludge separ- ator which is connected directly therebeneath, should be kept above 1 and preferably between 2 and 3 5 It is possible to save chemicals by adjusting the p H value to this range, when care is taken that the proportion of solid pollutants, precipitated from the waste gases before passing into the scrubber, is only such that the proportion of these pollutants (for example metal oxides) which remains in the waste gases suffices to keep the p H value of the wash liquid within the desired range of p H values without the addition of basic chemicals (for example sodium hydroxide solution) or with only a small addition of such chemicals.

Whilstthe p H value of the wash liquid which is cycled via the washing stages and through the sludge separator must, as a rule, be kept at a p H value of below 4, the p H value of the suspension or solution of harmful substances which is withdrawn from the sludge separator can also be adjusted to above 4.

The p H value should preferably be only so far below 4, but above 1, that the content of acidic pollutants in the waste-gas released from the plant, which content increases with increasing acidification of the circulating wash liquid during the washing process due to the resulting decrease in absorptive capacity of the said liquid for these pollutants, remains below a certain limit value which is to be maintained in the operation of the plant; this limit value, in turn, should usually be as far as possible below any minimum level which may have been legally prescribed for acidic pollutants in waste gas released into the atmosphere.

Furthermore, a slurry phase (pollutants suspen- sion) which has a low content of pollutants and is of very low viscosity can be withdrawn from sludge separator.

The wash liquid in the plant can be kept extremely clean and suspensions of pollutants are obtained which are particularly easy to pump and which permit trouble-free use of packed wash towers and X-separators in the scrubber, i e, without causing any blockages, as would be expected with the dirty slurry phases and effluents from the known pro- cesses.

In industry, the said "pollutants suspensions" are also termed "slurries" They are aqueous suspen- sions of solid pollutants, which pollutants are de- scribed more in detail below, or aqueous emulsions of liquid, water-insoluble pollutants, or aqueous mixtures which consist of such a suspension and emulsion, of relatively low viscosity and of a density which is preferably about 1 and up to about 1 3 g/ml.

If the pollutants are water-soluble, they are in solution in the aqueous phase of the "pollutants suspension" and, above the saturation point, the excess of water-soluble pollutants is in suspension.

Pollutants which are to be removed from industrial waste-gases with the aid of the scrubber according to the invention are not only those pollutants which can be collected by the conventional dry purification of flue gases by means of electrofilters and which, at the temperatures prevailing in the electrofilter, con- dense to sufficiently coarse dust or mist particles, but also oils or tar-like substances and also that portion of the metal oxides or salts, contained in the flue gas, which forms an aerosol only when the waste-gases are cooled upon issuing from the stack of the plant and thus lead to contamination of the environment, and, furthermore also harmful gases such as HCI, H 2 F 2 or SO 2, which cannot be separated off by means of an electrofilter.

The water which is to be added to the scrubber according to the invention as a replacement for the amounts of water removed from the plant together with the purified waste gases can be fresh water or waste water and can therefore be fed-in at various points into the scrubber, either as fresh water, or as waste water, or as an aqueous suspension of solid pollutants and/or as an aqueous emulsion of liquid pollutants of the type mentioned further above.

The separation of the pollutants suspension form the wash liquid takes place in a separating means which in this specification is termed a "sludge separator" for reasons of brevity It replaces the apparatus termed a "Thickener" in German Au- slegeschrift 2,408,222.

If the waste gas enters the scrubbing tower at too high a temperature (above 1700 and at most 2000 C), then too much water will be evaporated from the wash liquid in the tower, and become entrained in the waste gas and removed from the plant, and the concentration of pollutants in the pollutants suspen- sion will be too high and the wash liquid will therefore be too dirty, which results in an inadequate purifying effect when this liquid is recycled through the scrubber The wash liquid can even become supersaturated with salts, so that salt crystallisation in the scrubber can in some cases result in choking up the latter.

Preferably, the waste gases to be purified are introduced into the scrubbing tower at a tempera- ture in the range from above 1200 to 1700 C.

The preferred mode of operating the scrubbing tower according to the invention in practice de- mands that the p H value of the liquid in the sludge separator and in the gas washing stage or stages of the scrubber directly upstream of this separator remain below 4, i e that no basic neutralising agents, or only very small amounts of basic neutra- lising agents (e g sodium hydroxide solution) be added to the wash liquid or (e g lime water) to the pollutants suspension, whilst in known processes, for example that of German Auslegeschrift 2,431,130 of Walther und Cie AG, Cologne, Germany, the acid components of the waste gases, in particular SO 2, must be fully neutralised to the corresponding, preferably water-soluble, salts by chemical reaction with an alkaline solution In this known process, the p H value of the liquid in the work cycle should be between 4 0 and 7 8, but in practice below a p H of 6 5 only when the proportions of acid components are very small, while as a rule it should be between 6 5 and 7 5, i e substantial amounts of basic substances are required However, in the process according to the invention, operation at a p H value above 7 would 4 GB 2037611 A easily result in precipitates in the scrubber plant to such a degree that the entire plant would be blocked.

The scrubbing tower of the present invention is preferably operated with a highly acid wash liquid having a p H value of below 2, in the sludge separator and in each washing stage directly upstream thereof, thus saving costs for alkaline chemicals.

Only if the HCI content and SO 2 content of the waste gases to be purified is particularly high (above 2 g/Nm 3 of HCI+SO 2) is it advisable partially to neutralise the concentrates of pollutants which can be recycled continuously from the scrubberto an injection point into the waste-gas flow upstream of the scrubbing tower, to a p H value of 2 to 4 before they enter the latter The recycled pollutants suspen- sion can even be somewhat alkaline, but only to such an extent that it is subsequently rendered acid again (p H value less than 4) in the scrubber, due to the content of acid pollutants in the waste gases.

A combination of two measures in the scrubbing tower according to the invention, namely, maintain- ing the wash liquid at a p H value below 4, and holding the vaporisable proportion of the liquid phase of the pollutants suspension recycled into the waste gas stream at least at 70, but preferably at 90 or more, per cent by weight, is preferred in its operation The installations of the scrubber can be constructed of synthetic plastic material, in a mod- ern manner.

A scrubbing tower charged in the first washing stage with a single packing layer is advantageously used in the scrubber, when the velocity of the gas flowing through the scrubber is greaterthan one m/sec The packing layer preferably consists of the above-mentioned hedgehog packing units (see Figure 7 of the abovementioned description of the

Verein fur Wasser und Lufthygiene, lVFWLl, Zurich).

The free cross-sectional area of the scrubbing tower through which the waste gas flows is advantageous- ly so chosen that the gas speed is more than one m/sec.

Furthermore, the scrubber advantageously con- tains a wet-mechanical mist and dust (aerosol) separator having a gas resistance of 5 to 60, and preferably of 10 to 30 mbars The aerosol separator used is preferably a X-separator, which is also described in the said publication by the VFWL, Zurich (Figures 2 and 3).

Preferably, more than 50 % by volume of the circulating liquid, which flows through the first gas washing stage downstream, in the direction of gas flow, from the mechanical dust remover, is fed through a settling container of the sludge separator (thickener), but optionally from 70 to 100 % of the said liquid are fed through the container, the sojourn time of the liquid in the settling container of the sludge separator being, depending on the size of the latter, preferably between 1 and 8 minutes, but optionally from 3 to 5 minutes.

The waste water used to replace water removed from the plant together with the waste gases is preferably slag-quenching water from a waste incineration plant This waste water can be fed into the wash liquid cycle but is preferably fed into the sludge separator.

Finally, a heat exchanger can be inserted in the wash liquid cycle of the scrubber; this heat exchan- ger cools the wash liquid and releases the heat withdrawn therefrom, preferably in a heat pump system, via a second heat exchanger to the conven- tional auxiliary air, which is thereby heated up and is then mixed in to dilute the purified waste gases in the stack of the plant.

It is also possible to meter into the pollutants suspension which is to be recirculated into the waste gas flow upstream of the scrubber from the sludge separator and which may have been partially neut- ralised as mentioned above, a bindef and/or a chemical constituent which binds the salts present therein in the solid phase of the pollutants and also at the same time reduces their solubility in rain water, this constituent being, for example, a silicate, such as waterglass.

Further details of the invention can be seen from the following description in conjunction with the accompanying drawings, in which Figure 1 shows schematically, as an example, a waste-gas purification plant comprising a first embo- diment of the scrubbing tower of the invention, Figure 2 shows schematically a preferred practical arrangement of the embodiment of the scrubbing tower in the plant shown in Figure 1, in a side view, and Figure 3 shows a schematic representation of a longitudinal section through another, preferred embodiment of the sludge separator in the scrub- bing tower according to Figures 1 and 2.

Throughout the description with reference to the drawings "waste gas" is referred to as "off-gas".

The plant shown in Figure 1 comprises an evapor- ative cooler 1, the outer walls of which are sur- rounded by an insulating jacket 17 In the interior of the evaporative cooler 1 there is provided a cylindric- al partition 18, which separates a mixing and reactor space 100 from an outer annular chamber 101 which surrounds this space An off-gas feed line 11 for feeding the off-gas to be purified into the plant opens into this outer annular chamber 101 The cylindrical partition 18 has openings at its upper end which establish free communication between the mixing chamber 100 and the outertoroidal chamber 101 An atomizer nozzle 19, through which liquid can be sprayed into the mixing chamber 100, is provided at the upper end of the latter An off-gas line 12 leads from the lower region of the mixing chamber 100 into a cyclone dust remover 2 The walls of the off-gas line 12 and the outerwalls of the cyclone dust remover 2 are surrounded by a heating jacket 20, in which coils of semi-cylindrical tubes 23 are provided for heating the cyclone dust remover and the off-gas line 12 An outlet line 15 is located at the lower end of the mixing chamber 100, outlet lines 14 are located at the lower end of the annular chamber 101 and an outlet line 24 is located at the lower end of the cyclone dust remover 2; precipitates from the off- gas can be run off, in solid or liquid concentrated form, through these lines from the evaporative cooler and from the cyclone dust remover 2 into a dust collection vessel 9.

Running off is preferably effected batch-wise and - GB 2037611 A 5 is controlled via valves 71,72 and 74, which are provided in the lines 14,15 and 24 respectively The semi-cylindrical tubes 23 are heated by means of superheated steam, which is introduced via a steam line 231 Water which condenses in the semi- cylindrical tubes is run off continously via a conde- nsate line 232 into a condensed water collector 233.

A gas transfer line 22 leads from the upper region of the cyclone dust remover 2 into the central region of a wash tower 3 of a scrubber.

The central region of the tower 3 is filled with a layer 31 of packing units, which rests on a transverse grate 34 An injection nozzle 131, which is used to spray the packing layer 31 with wash liquid from a liquid circulating line 33, is located above the packing layer 31 in the wash tower 3 An aerosol separator 35, preferably a X-separator, is placed above the layer 31 in the upper part of the wash tower 3 and the slotted wall 135 of the aerosol separator 35 is sprayed via a spray nozzle 133 with wash liquid which is fed to the nozzle 133 from the circulating line 33 via the branch line 133 a The aerosol separator 35 is in free communication, on the one hand, with the interior of the wash tower 3 and, on the other hand, through the slotted wall 135, with a mist collector 36 A spray nozzle 136, into which a fresh water feed line 90 provided with a shut-off valve 91 opens, is arranged in front of the mist collector 36 The fresh water sprayed in via the spray nozzle 136 rinses the walls of the mist collector 36 and collects in a collection vessel 92 from which it flows via the line 192 into the wash tower 3 above the packing layer 31 A gas line 32 for the purified off-gas leads from the mist collector 36 via a fan 5 into a gas outlet line 52, which opens into a stack 6 A sludge separator 4, the construction of which is described further below in more detail in connection with Figure 3, is located at the lower end of the wash tower 3 Wash liquid is pumped from the sludge separator 4 by means of a pump 81 through the circulating line 33 to the nozzles 131 and 133.

The settling container 41 of the sludge separator 4, which is filled by wash liquid contains a conical baffle 44 which is tapered upwards and towards the centre.

The wash liquid, from the nozzle 131, which trickles downwards in the tower 3 collects on the base formed by upper partition 134, which is inclined conically downwards towards the centre of the wash tower 3, and flows from there through an inlet tube 42, which extends through the opening 244 in the centre of the baffle 44 downwards into the settling container 41 A scum outlet 43 leads from the annular space 144, which is located above the baffle 44 and below the partition 134, via a shut-off valve 87 downwards and joins with a slurry line 13 which leads from the lower end of the settling container 41 of the sludge separator 4 downward via a non-return valve 86.

After joining with the line 43, the line 13 leads via a circulation pump 82, and via a sieve or filter 213 to the upper end of the evaporative cooler 1, where it is connected to the injection nozzle 19 The injection nozzle 19 is designed as a two-component nozzle, and air or steam can be blown in as the atomising medium via a line 89 and a shut-off valve 88.

In Figures 2 and 3, the parts corresponding to the elements of the apparatus in Figure 1 are characte- rised by the same reference numbers as in Figure 1.

In the embodiment according to Figure 2, the evaporative cooler 1 isdesigned as a cylindrical container The feed line 11 for the off-gases to be purified in this case opens into the upper end of the evaporative cooler 1, whilst a number of nozzles 19 in a lower position are fed by the branch line 13 a, which branches off from the slurry line 13, via a ring line 113 a, and a number of nozzles 19 in a higher position are fed bythe branch line 13 b, which branches off from the line 13, via a ring line 113 b.

In this embodiment, the nozzles 19 direct the liquid spray cone upwards, that is to say towards the off-gas flowing in through line 11.

Ten cyclone dust removers 2 are arranged around the lower region of the evaporative cooler 1, and the off-gas which leaves the evaporative cooler 1 in its lower region is fed into these cyclone dust removers via ten lines 12 The off-gas, from which the dust has been removed dry in the ten cyclone dust removers 2, passes via a loop line 112 into the gas transfer line 12, throttling of the stream of gas in line 12 being prevented by the fact that the cross-sectional area of the ring line 112 gradually widens towards the last-mentioned line 12 An outlet line 15 for agglom- erates of pollutants, which is provided with a shut-off valve 72, is located at the lower end of the evaporative cooler 1, in the same manner as in the embodiment according to Figure 1.

The lower ends of the cyclone dust removers 2 are connected to outlet lines 114 which, together with the line 15, end in a collection vessel 115 which can be emptied by means of a line 116 provided with a shut-off valve 70.

The off-gas transfer line 22 is connected in the same way as in the plant according to Figure 1 to a wash tower 3 of the scrubber, which is equipped in the same way as that in Figure 1.

The preferred embodiment of the sludge separator 4 shown in Figure 3 comprises a settling container 41 and a liquid inlet tube 42, which extends, through the central opening 244 in a baffle 44 which is tapered conically towards the top and extends towards the centre, into the settling container 41 and is con- nected, by means of its upper end, to the central orifice 234 of the base formed by partition 134 which base slopes conically inwardly and downwardly.

At its lower end, the tube 42 carries an outlet funnel 142, which widens conically downwardly and outwardly and is open at its base; in the interior of funnel 142 a deflector 45 is fastened, vertically below the end of the tube 42, by means of struts 46.

A vent tube 47 is fitted centrally in the feed tube 42 by means of struts with the upper open end of this tube ending above the partition 134, and the lower open end thereof a short distance above the deflec- tor 45.

The inlet orifice 48 of an outlet connecting branch 49, to which the recirculating line 33 for the wash liquid is connected outside the sludge separator 4, is located in the central portion of the container 41 in the lowerwall of an inletcasing 148, a short distance 6 GB 2037611 A above the outletfunnel 142.

The baffle 44 projects into the lower open end of the wash tower 3 which is fitted on the upper region, surrounding the orifice 244, of the separator 4, and which encloses the annular space 144 A two- position level controller 75 is connected to the sludge separator 4 and, of the two measuring heads or probes 76 and 77 of the controller, the lower measuring head 76 responds when the liquid in the container 41 has fallen to level N 1, whilst the upper probe 77 responds when the liquid in the container 41 has risen to the upper limiting level designated N 2.

The procedure for operation of the sludge separ- ator 4 is that wash water first flows from the wash tower 3 through the feed line 42 into the container 41 and fills the latter, with the pump 81 inactive and with the valve 86 closed, until the upper level N 2 is reached The pump 81 is then switched on and the valve 86 is opened.

The sludge separator 4 is now operated con- tinuously The rate of discharge (volume per unit time) of the wash liquid from the central region, which has a low content of pollutants, of the container 41 via the line 33 and the rate of discharge of the pollutants suspension enriched with settling slurry, i e slurry having a density of more than 1, through the line 13 added to each other are some- what greater than the rate at which the wash liquid flows into the container 41 through the line 42.

As a result thereof and of the evaporation, which takes place in the wash tower 3, of a portion of the wash water, the level of liquid in the container falls from the upper level N 2 to the lower level N 1, the evaporated portion being removed by the off-gases.

When this level has been reached, the probe 76 of the level controller 75 responds and opens the valve 91, through which fresh water passes through the line 90 into the wash tower 3, and a non-return valve 79 in a waste water feed line 78 through which waste water now flows directly through the orifice 244 into the container 41 Advantageously, the waste water used is slag-quenching water from a refuse incinera- tion plant.

The level of liquid now rises again in the container 41 until it has reached the upper level N 2, whereupon the probe 77 responds and the level controller 75 closes the valves 79 and 91 again.

When the level of liquid rises up to level N 2, a layer of scum (density less than 1) which has collected on the surface of the liquid is pushed out upwards from the orifice 244 and runs downwards on the upper side of the conical partition 44 and through the scum outlet 43 to combine with the settling slurry in the line 13 The combined suspension of pollutants formed from the scum and the settling slurry is now pumped up by means of the pump 81 into the nozzles 19 of the evaporative cooler 1.

Preferably, the cross-sectional areas of flow of the lines 42 and 33 are so designed that, when the valve 86 in the main outlet 13 for settling slurry is closed, the amount of wash liquid which flows into the container 41 through line 42 is the same as that which flows out of the said container via line 33 The removal of pollutants suspension via line 13, and from time to time via line 43, as well as the evaporation from the wash tower 3, which both reduce the amount of liquid flowing in through line 42, thus causes the level of liquid to drop from level N 2 to level N 1 and this drop can thus be influenced primarily by means of the valve 86.

Whilst in the case of a known separator (German Auslegeschrift 2,408,222) only a part of the pollu- tants, specifically the settling slurry constituent, is conveyed into the evaporative cooler and the scum constituent is pumped back into the wash liquid, in the sludge separator shown in Figure 3 both the settling slurry and the scum are separated off and back-circulation into the wash liquid is restricted to a minimum (suspension of particles of pollutants having a density of 1).

If a particularly large amount of scum is obtained, it is also possible to provide, in the annular space 144, a special rinsing water line (not shown), from which water is sprayed onto the outer surface of the partition 44 and rinses the scum deposited thereon into the line 43.

The settling of settling slurry from the liquid which flows through the feed line 42 into the container 41 is particularly promoted firstly by the fact that this stream of liquid is broken and divided at the deflector 45 and secondly by the fact that the liquid which then flows down on the inner wall of the outlet funnel 142 is further slowed down by the downward- lywidening circumference of the funnel 142.

The throughput rate through the container 41 is very high and in practical operation of the separator 4 about eighttimes the capacity of the container are circulated per hour; this is in contrast to previously known plants, in which only a small stream of liquid flows into a settling container of large volume and provision is made for a correspondingly small outflow of liquid, so that each circulation of the contents of a container having an identical purifying action requires a ten times greater volume of the container, and about one or one and a half hours are as a rule required for each circulation.

As can be seen from the figures of the drawing, the slurry obtained in the gas scrubber is sprayed, optionally after adding milk or lime, into a reactor (evaporative cooler) and dried by coming into con- tact with the flue gases which are at a temperature above 2000 C and which leave a boiler in which they had to give up a large proportion of theirthermal energy The finely sprayed slurry binds the major portion of the dust content of the crude gases and, as a result of this binding, the gas contains only a small amount of dust at the outlet of the downstream cyclone 2 The circulating liquid in the downstream scrubber remains relatively pure because adjust- mentto an optimum temperature in reactor 1 and corresponding control of the composition of the wash liquid make it possible to separate off the slurry virtually completely in the sedimentation apparatus (sludge separator) 4.

When the plant has been in operation for several months, no troublesome incrustations or caking in the wash system or baking on of dust in the dry pre-purification (cyclone 2) are observed.

When the flue gases from a municipal refuse - GB 2037 611 A 7 incineration plant were purified, the following advantageous results were obtained, without an expensive electrofilter being required:

Content of pollutants in the purified off-gas to be released into the atmosphere:

Dust content reduced to below 50 mg/Nm 3 (mea- sured after gas cooling) Free hydrochloric acid less than 5 mg/Nm 3 Total chloride content (as CI-) less than 15 mg/ Nm 3 SO 2 less than 100 mg/Nm 3 Nitrogen oxides less than 100 ppm.

Furthermore, the following advantages were obtained:

Utilisation of the temperature of the flue gas (after the boiler) for drying the slurries obtained in the gas scrubber and for removing the slag-quenching water from the incineration plant.

Conversion of all precipitated pollutants to a free-flowing ash.

Low consumption of water (less than 50 kg/1,000 Nm 3 of flue gas) Very low consumption of milk of lime (less than g of Ca(OH)2/1,000 Nm 3 of flue gas) Low expenditure of energy by virtue of the use of a X-separator for separating off the aerosols (fan differential pressure less than 360 mm of water column).

Claims

1 A gas scrubbing tower having a sump region and at leasttwo washing stages above the sump region, the lowermost washing stage comprising at least one layer of packing elements extending in a horizontal plane across the cross-section of the tower and a second washing stage being without packing elements, which tower comprises ( 1) a gas entry means for introducing waste gas to be scrubbed into the tower, which entry means are located between the sump region and the lowermost washing stage, ( 2) gas exit means forthe discharge of scrubbed gas from the uppermost washing stage, ( 3) a first circulating line comprising pump means, and spray nozzles in the lowermostwashing stage for circulating wash liquid from the sump region to irrigate the layer of packing elements in the lowermost washing stage, ( 4) a second circulating line for conducting wash liquid to the second washing stage, ( 5) a sludge separator, constituting the sump region of the tower, and comprising a settling vessel having an upper portion, a central portion and a base portion, the upper portion comprising an upper inner partition wall which tapers conically inwardly and downwardly to define a central orifice; an inlet tube for sludge-carrying liquid from the lowermost washing stage and extending from the central orifice to the central portion of the settling vessel; a first outlet tube for removal of liquid of reduced sludge content extending from the central portion of the settling vessel, above the lower end of the inlet tube radially toward the outside, which outlet tube forms part of the first circulating line; and a second outlet for removal of liquid charged with sediment from the base portion of the settling vessel; and ( 6) conduit means for introducing water into the scrubbing tower.

2 A scrubbing tower according to claim 1,which further comprises in the sludge separator a deflector mounted at the lower open end of the inlet tube and opposing the flow of liquid therethrough.

3 A scrubbing tower according to claim 2, which further comprises in the sludge separator a conical funnel which widens downwardly connected with the lower end of the inlet tube.

4 A scrubbing tower according to claim 3, wherein, in the sludge separator, the first outlettube comprises a conduit having an intake opening surrounded by a cup-shaped casing positioned about the inlet tube, the conduit thus being adapted for receiving wash liquid rising along the outside of the conical funnel.

5 A scrubbing tower according to claim 4, wherein the sludge separator further comprises an inner annular baffle underneath the upper inner partition, which baffle tapers conically inwardly and upwardly defining a central baffle orifice through which the inlet tube for liquid extends downwardly from the central orifice in the upper inner partition wall, the central baffle orifice also leaving an annular gap aboutthe inlettube thereby permitting overflow of scum-free or scum-carrying liquid through the gap and over the upper surface of said baffle.

6 A scrubbing tower according to claim 5, which further comprises means for maintaining liquid between two levels in the settling vessel of the sludge separator, the lower level being a short distance above the point at which the outlet tube opens into the settling vessel, and the upper level being at or a short distance below the central orifice of the conically tapered partition.

7 A scrubbing tower according to claim 6, wherein the sludge separator further comprises a scum outlet tube connected to the outlet for liquid charged with sediment, and a scum collecting vessel adapted for collecting scum which overflows from the gap of the central baffle orifice down the upper face of the baffle when the level of liquid in the settling vessel is raised to its upper level.

8 A gas scrubbing tower substantially as illus- trated in and described with reference to the accom- panying drawings.

9 A process for scrubbing a gas containing solid, liquid or gaseous pollutants in a scrubbing tower as claimed in any one of the preceding claims compris- ing the steps of (a) passing a flow of the gas to be scrubbed upwardly through the at least two washing stages superimposed one above the other; (b) irrigating the lowermost washing stage with a first aqueous wash liquid having a p H below 4, and (c) irrigating the second washing stage with a second aqueous wash liquid having an acidic p H above the p H of the first washing liquor; (d) collecting the washing liquorfromatleastthe lowermost washing stage in the sludge separator therebelow and withdrawing wash liquid from the central portion of the latter and recycling itto the 8 GB 2037611 A lowermost washing stage.

A process according to claim 9, wherein the p H of the second wash liquid is at most 4.

11 A process according to claim 9 or 10, wherein sodium hydroxide solution is introduced into the second wash liquid to increase the p H of the latter above that of the first wash liquid.

12 Aprocessforscrubbing a gas containing solid, liquid or gaseous pollutants in a scrubbing tower which process is substantially as described with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company Limited, Croydon Surrey, 1980.

Published bythe Patent Office,25 Southampton Buildings, London,WC 2 A l AY, from which copies may be obtained.

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