CA1060810A - Method for the purification of waste gas containing gaseous pollutants - Google Patents
Method for the purification of waste gas containing gaseous pollutantsInfo
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- CA1060810A CA1060810A CA264,649A CA264649A CA1060810A CA 1060810 A CA1060810 A CA 1060810A CA 264649 A CA264649 A CA 264649A CA 1060810 A CA1060810 A CA 1060810A
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Abstract
METHOD FOR THE PURIFICATION OF WASTE
GAS CONTAINING GASEOUS POLLUTANTS
ABSTRACT OF THE DISCLOSURE:
In a process for the purification treatment of a waste gas containing gaseous pollutants, including the steps of up-wardly introducing the waste gas into the bottom of a tower with a multiplicity of stepped trays and at the same time introducing an activated carbon into the top of the tower, and contacting the waste gas with the activated carbon to form a fluidized bed on each of the stepped trays thereby causing the gaseous pol-lutants in the waste gas to be adsorbed on the activated carbon, the waste gas can be continuously purified by recycling the activated carbon spheres and adopting, as the stepped trays, perforated plates of a specific design.
GAS CONTAINING GASEOUS POLLUTANTS
ABSTRACT OF THE DISCLOSURE:
In a process for the purification treatment of a waste gas containing gaseous pollutants, including the steps of up-wardly introducing the waste gas into the bottom of a tower with a multiplicity of stepped trays and at the same time introducing an activated carbon into the top of the tower, and contacting the waste gas with the activated carbon to form a fluidized bed on each of the stepped trays thereby causing the gaseous pol-lutants in the waste gas to be adsorbed on the activated carbon, the waste gas can be continuously purified by recycling the activated carbon spheres and adopting, as the stepped trays, perforated plates of a specific design.
Description
FIELD OF THE INVENTION:
,The present invention relates to a method for the purification of a gas containing gaseous pollutants.
BACKGROUND OF THE INVENTION:
Various industrial processes generate waste gases containing gaseous pollutants such as organic solvents. In order to be able to recover such so:Lvents and to prevent air pollution, therefore, the waste gases must be freed of the noxious pollutants before they are released into the atmosphere.
Various methods have been devised for effecting the purification by adsorption of gases containing gaseous pollutants.
When the occasion requires, these prior methods involve the recovery of the noxious pollutants removed from the gases.
Of these prior methods, the method which makes use of the so-called fluidized-bed type adsorption system is popular, w,herein a gas to be treated and adsorbent particles, such as activated carbon, activated alumina or silica, are brought into mutual contact tc form a fluidized bed of the adsorbent particles. In the adsorption treatment of the gas by this fluidized-bed method, it is common practice to effect the gas treatment continuously by forming several fluidized beds in a multiplicity of stages within a tower. Particles from each fluidized bed can flow over a barrier or weir to fall onto the next lower bed via downcommers. Particles from the lowermost bed are then transferred to the uppermost bed via a pipe so that the particles are recycled continuously through the tower.
In the adsorption treatment of the gas by the fluidized-bed method described above, successful stabilization of the fluidized beds thus formed constitutes an essential requirement for enabling the removal of the noxious gaseous - 1- ~
8~
pollutants from the gas to be effected continuously at a high removal efficiency over long periods of service. The stability of such fluidized beds depends on the shape of the adsorbent particles used, the strength, wear resistance and other physical properties of the particles and the flow volume, flow velocity and viscosity of the gas used for fluidizing the adsorbent particles, and so on. It also depends on the extent of change in weight of the adsorbent particles being recycled. When the adsorption treatment of gas by the conventlonal fluidized-bed type technique is reviewed from this point of view, it is noted that the so-called coconut-shell activated carbon obtained from coconut husks is popularly used to form the adsorbent particles.
The activated carbon of this type is made up of particles of various, irregular shapes and this makes their transport through the adsorption apparatus substantially difficult Moreover, the adsorbent particles have poor physical properties and, for this reason, are readily pulverized as by crushing and attrition. Recyclic use of such irregularly shaped activated carbon particles, therefore, involves numerous difficulties. In the adsorption treatment of gas by the fluidized-bed method, the adsorbent particles of such shapes induce undesirable phenomena such as boiling, channeling and slugging~ when fluidized by the upward flow of the gas under treatment. They also cause similar phenomena while they are moving downwardly via the downcommers by gravity, with the result that the smooth flow of the particles inside the downcommers is impeded. This impeded flow consequently brings about a quantitative change in the weight of the adsorbent particles belng transferred for recyclic service.
With a view to precludlng these disadvantageous phenomena, the conventional techniques have attempted to lmprove the ~6~
structure of the downcommers for particle flow. It has been suggested, for example, to incorporate orifices in the bottoms of the downcommers or9 as disclosed by U.S. Patent No. 2,674,338, to have bottom plates supported on springs on the bottoms of the downcommers. These attempts at improvement of the structure of downcommers, however, effectively complicate the system itself and have the disadvantage that the activated carbon particles vary their shapes gradually with the lapse of time.
Thus, all these attempts fail to some extent to attain the preferred stabilization of the quantitative transport of adsorbent particles. Because the adsorbent particles in use are highly susceptible to pulverization and also because stabilization of the transport of these adsorbent particles is difficult to accomplish, the conventional techniques do not easily achieve the desired obejct of stabilizing the fluidized beds of the adsorbent particles.
SUMMARY 0~ THE INVENTION:
It is, therefore, a primary object of the present invention to provide a novel method and apparatus for the purification of a waste gas by the fluidized-bed principle, which method, by recyclic use of activated carbon spheres, is capable of continuously and effectively purging the gaseous pollutants from the gas.
It has now been discovered that the stabilization of the fluidized beds and the stabilization of the quantitative transport of activated carbon particles are both easily attained by using activated carbon spheres as the adsorbent particles - and also using perforated plates of a specific shape as the stePPed trays within the reaction tower.
According to one aspect of the present invention, there is provided a method for the continuous purification of a ~L~60~
waste gas containlng gaseous pollutants, which comprises:
(a) providing a tower having at least two substantially horizontal, vertically spaced per'forated plates therein arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95 % of the total surface area and a second of said portions having 5 to 20 % of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the first portion of the next lower plate; (b) continuously passing said waste gas upwardly through said tower so that the gas passes through said perforated plates and at the same time continuously and recircularly eeding activated carbon spheres downwardly through said tower so that the spheres form fluidi~ed beds on said perforated plates; and (c) continuously removing purified gas from the top of said tower.
According to another aspect of the invention there is provided a chemical process column containing at least two substantially horizontal, vertically spaced perforated plates arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95 % of the total surface area and a second of said portions having 5 to 20 % of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the flrst portion of the next lower plate.
BRIEF EXPL~ANATION OF THE DRAWINGS:
In the accompanying drawings:
Fig. 1 is a schematic explanatory diagram illustrating one example of a known adsorption apparatus for the remova] of gaseous pollutants by the fluidized-bed principle;
Fig. 2 is a plan view illustrating one preferred embodiment of one of the perforated plates used in the present invention~ which perforated plate has a weir formed on its upper surface in such a way that the plate surface is divided into a first portion accounting for 80 to 95% of the entire surface area and a second portion accouting for 5 to 20% of the entire surface area;
Fig. 3 is a plan ùiew illustrating a perforated plate similar to the plate shown in Fig. 2 but arranged in a reversed position, which perforated plate has a weir formed on its upper surface in su&h a way that the plate surface is divided into a portion accounting for 5 to 20% of the entire surface area and another portion accounting for 80 to 95% of the entire surface area;
Fig. 4 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising the perforated plates of the type of Fig. 2 and those of the type of Fig. 3 alternately disposed and vertically spaced inside a tower;
Fig. 5 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising alternative examples of perforated plates of Fig. 2 and Fig. 3 alternately disposed vertically spaced inside a tower;
Fig. 6 is a plan view illustrating one preferred embodiment of another type of the perforated plate used in the present invention, which perforated plate is divided into two -" ~.V~ 38~
portions having surface areas oE Lhe same portion as in the plate of Fig. 2;
Fig. 7 is a plan view illustrating a perforated plate similar to the plate shown in Fig. 6 but arranged in a reversed position, which perforated plate is divided into two portions having surface areas of the same portions as the plate of Fig. 3;
Fig. 8 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising the perforated plates of Fig. 6 and Fig. 7 alternately disposed and vertically spaced inside a tower;
Figo 9 is a schematic diagram illus~rating a multiplicity of horizontal stepped trays comprising alternative examples of the perforated plates of Fig. 6 and Fig. 7 alternately disposed and vertica~ly spaced inside a tower; and Fig. 10 is a schematic diagram illustrating one preferred embodiment of the present invention in which a gas containing gaseous pollutants is treated continuously for the removal of the gaseous pollutants.
DETAILED DESCRIPTION OF THE INVENTION:
In Fig. 1, 1 denotes a reaction tower. A gas containing noxious gaseous pollutants to be removed is introduced into the tower 1 through a nozzle 2 in the adsorption section A. On entering the tower interior, the gas ascends vertically and comes into contact with adsorbent particles held inside the adsorption section A, causing the adsorbent particles to form fluidized beds on the stepped trays 3, 3', 3", etc. The adsorbent particles forming the fluidized beds adsorb the gaseous pollutants from the gas. The gas which has thus been freed of the noxious pollutants is released into the atmosphere via a discharge outlet 4 at the top of the tower.
The adsorbent particles on the stepped trays 3, 3', 3", Ptc., 8~0 fall through the downcommers 5, 5', 5", etc. associated with the trays and descend gradually downwardly by virtue of gravity, while slmultaneously adsorbing the gaseous pollutants from the gas. The particles then leave the adsorption section A and accumulate in the space formed on a guide plate 6 and gradually reach a regeneration section B which is located at the bottom of the reaction tower 1. On entering the regeneration section B, the adsorbent particles are heated by a heater 7, with the result that the particles are regenerated as they are forced by the heating to release the adsorbed pollutants. Subsequently, the regenerated adsorbent particles reaching the bottom 8 of the tower are transferred via a lifting pipe 9 to the top of the tower for recyclic service. In the meantime, the pollutants which have been desorbed from the adsorbent particles are forced out of the system via a nozzle 10 by means of a carrier gas introduced via a nozzle 11 disposed at the lower portion of the regeneration section B. The discharged pollutants are transferred to a desorbate treating section C composed of a condenser, decanter and the like.
In the method of the present invention, activated carbon spheres are used as the adsorbent particles. ~ecause of their spherical shape, these activated carbon spheres usually have the advantages of excellent fluidity, outstanding resistance to friction and wear and high impact strength. For the purpose of the present invention, the activated carbon spheres may be of the type which are obtained by mixing a powdery carbon or carbon precursor with a binding agent, subsequently molding the resultant mixture in the shape of spheres and activating the molded carbon spheres by an ordinary method (otherwise, referred to generally as "activated carbon spheres from coking coal"). It is, however, preferable to use the type of activated 8~6~
carbon spheres produced from a specific type of pitch as the raw material by a specific method such as disclosed in U. S.
Patent No. 3,917,806, because the activated carbon spheres of this type are excellent in terms of their spherical shape and physical properties. The superiority of this type of activated carbon spheres over various other types of activated carbon particles is easily confirmed by subjecting samples of the various types of activated carbon to a friction test, then sifting the tested sample particles through a sieve of 200 mesh ~by the Tyler standard) and comparing the weights of the corresponding sievings. To be specific3 this comparison can be accomplished by using glass containers measuring 28mm in diameter and 220mm in length9 placing 20 cm3 samples of the various types of activated carbon particles into the individual containers, rotating the containers and their contents at the rate of 36 r.p.m. for a fixed length of time, sifting the contents through a metal screen of 200 mesh and measuring the weight of the portion of each sample passing through the screen.
The results of a typical experiment performed as described above are shown in Table 1 below.
Table 1 Extent of attrition loss of particles due to friction in dry state (wt%) Length of friction test (in hours) TYPe of activated carbon 10 20 30 40 Activated carbon spheres 0 0 0.05 0.05 disclosed in ~.S. Patent No. 3,917,806 Activated carbon spheres 0.05 0.08 0.22 0.60 from coking coal Coconut-shell activated 2.3 2.9 3.3 3.5 carbon ~OtiO8~0 For the purposes of the present invention, the activated carbon s~heres preferably have a bulk density in the range of from 0.4 to 0.7 g/cm3, a particle diameter distribution range of from 0.2 to 2.0mm and an average particle diameter in the range of from 0.4 to 1.2 mm. I the particle diameter distribu-tion of these activated carbon spheres is excessively sharp, then during the actual use of the activated carbon spheres, the phenomenon known as channeling may be induced. If the particle diameter distribution of the activated carbon spheres is exces-sively broad, then the actual use of such activated carbon spheresdoes not encounter the disadvantage described above but can result in an adverse situation wherein the spheres of larger diameters and those of smaller diameters become suspended at different positions in the bed. This leads to a condition wherein only spheres of smaller diameters flow over the weirs on the trays and descend down the interior of the tower. Such a partial movement of the spheres is contrary to the requirement that the spheres should be transferred stably in a constant weight. For this reason, the activated carbon spheres preferably have a particle diame.er distribution such that the standard deviation of individual particle diameter distribution will fall in the range of fro~ 0.05 to 0.20mm.
In one embodiment of the present invention the stepped shelves or trays mentioned above are rectangular perforated plates of a type having a weir on the upper surface. As illustrated in Fig. 2 of the accompanying drawings, the surface of the tray is divided by the weir into a first portion accounting for ao to 95% of the total surface area and a second portion accounting for 5 to 20% of the total surface area. In Fig. 2, 21 denotes one such a rectangular perforated plate. The upper surface of this rectangular perforated plate ~ ~Gos~
21 is divided by a weir 24 into a first portion 22 and a second portion ~3. In Fig~ 3, 31 denotes a ~ectangular perforated plate dentical to the plate ~hown in Fi~. 2 except that the positions of the first and second portions are transposed.
The upper surface of this rectangular perforated plate 31 is divided by a weir 34 into a first portion 32 and a second portion 33. The first portions 22 of the plate of Fig. 2 and 33 of the plate of Fig. 3 are the tray portions above which fluidized beds of activated carbon spheres are formed. The second portions 23 of the plate of Fig. 2 and 32 of the plate of Fig. 3 are the portisns through which the activated carbon spheres descend to the next lower tray. The weir 24 and the weir 34 are so disposed on their respective rectangular per forated plates that the portion 22 equals the portion 33 and the portion 23 equals the portion 32 respectively in surface area. The rectangular perforated plates 21 and 31 shown respectively in Fig. 2 and Fig. 3 are level along their entire surfaces and they each have a multiplicity of perforations formed at an aperture ratio in the range of from 5 to 25%.
The entire surfaces of the rectangular perforated plates 21 and 31 may be in one level plane as shown in Figs. 4 and 8.
Otherwise, the two portions of these plates may be in different horizontal planes separated by a vertical distance of 10 to 20 mm as shown in Figs. 5 and 9. h~hen the plates are level throughout their entire surface, the perforations bored in the portions 22 and 33 preferably have a diameter in the range of from 3 to 5 mm and those bored in the portions 23 and 32 preferably have a diameter about 1.1 to 3 times that of the perforations in the portions 22 and 33. When the plates have stepped horizontal surfaces, all the perforations bored therein may have a fixed diameter falling in the approximate range of B~
from 3 to 5 mm, The heights of the weirs 24 and 34 are not specific~lly limited and are preferably in the range of from 20 to 6U mm. Moreover, it is usually necessary to make the weirs on the respective plates of equal height for the purpose of stabilizing the fluidized beds of the activated carbon spheres and also stabilizing the transportation rate of the spheres. The superficial velocity of the gas in the tower is preferably from the practical point of view, within the range of 0.5 to 2.0 m/sec. when the inside diameter of the tower is from 500 to 2,000 mm, which is the usual diameter range.
The rectangular perforated plates 21 and 31 are alternately vertically disposed to form a multiplicity of stepped trays inside a tower as shown in Fig. 4 or Fig. 5 and, thus form an adsorption section into which the gas is intro-duced for purification. In Fig. 4, 41 denotes a tower. Inside this tower 41, rectangular perforated plates 45 with surfaces divided by a weir 44 into a first portion 42 and a second portion 43, and rectangular perforated plates 49 with surfaces divided by a weir 48 into a first portion 47 and a second portion 46,are alternately disposed to form a multiplicity of stepped trays. In Fig. 5, 51 denotes a tower. Inside this tower 51 are rectangular perforated plates 55 having surfaces divided by a weir 54 into a first portion 52 and a second portion 53. The surface portion 53 is located in a level plane 10 to 20 mm below the surface portion 52. Every alternate tray in tower 51 is a rectangular perforated plate 59 divided by a weir 58 into a first portion 56 and a second portion 57 located in a level plane 10 to 20 mm below the portion 56. The vertical distance by which two ad~acent trays are separated is generally expected to be approximately the sum of the height of the weir plus 60 mm. To effect the purification of gas in ~608~
the tower described above, one has only to feed the gas - upwardly,into the tower from the bottom and cause the introduced gas to come into counter-current contact with activated carbon spheres being introduced downwardly from the top, so that the activated carbon spheres are caused by the force of the flow of gas to form fluidize~ beds on the stepped trays.
The activated carbon spheres which have thus formed the fluidized beds on tray portions 42 and 47 are horizontally transferred in the direction of portions 43 and 4~, and then descend by gravity through the perforations in the portions 43 and 46 of the rectangu-lar perforated plates 45 and 49 in the tower of Fig. 4 (or the portions 53 and 57 of the rectangular perforated plates 55 and 59 in the tower of Fig. 5) as they adsorb tne gaseous pollutants of the gas under treatment. After they have passed through the adsorption section formed by the multiplicity of stepped trays, they are regenerated in the desorption and regeneration section.
The regenerated activated carbon spheres are recycled, being again introduced downwardly into the tower from the top. Before the activated carbon spheres forming the fluidized beds descend from one tray to another, the individual activated carbon spheres on the stepped trays move horizontally on the respective trays.
To be more specific, the activated carbon spheres which have fallen from the narrower portion (hereinafter referred to as "portion II") of a given tray onto the wide portion (hereinafter referred to as"portion I") of the next lower tray move to the por-tion II of the lower tray as they form in situ a fluidized bed.
The activated carbon spheres fed cnto portion 42 of the rectangular perforated plate 45 in the tower of Fig. 4, for example, are horizontally transferred in the direction of the portion 43 by the upward stream of the gas and gravitational ~L0~81~
attraction and then,descend through the perforations distributed in the portion 43. ~ecause th~ perforated plates used in the prese~t invention as st~pped trays are rectangular in shape and they have their upper surfaces divided each by a weir into two portions, the invention enjoys the advantages enumerated in (1) through (4) below.
(l)' The perforated plates are simple in their structure and therefor can be easily produced. For this reason, the plates are useful for a gas-treating apparatus of large type.
,The present invention relates to a method for the purification of a gas containing gaseous pollutants.
BACKGROUND OF THE INVENTION:
Various industrial processes generate waste gases containing gaseous pollutants such as organic solvents. In order to be able to recover such so:Lvents and to prevent air pollution, therefore, the waste gases must be freed of the noxious pollutants before they are released into the atmosphere.
Various methods have been devised for effecting the purification by adsorption of gases containing gaseous pollutants.
When the occasion requires, these prior methods involve the recovery of the noxious pollutants removed from the gases.
Of these prior methods, the method which makes use of the so-called fluidized-bed type adsorption system is popular, w,herein a gas to be treated and adsorbent particles, such as activated carbon, activated alumina or silica, are brought into mutual contact tc form a fluidized bed of the adsorbent particles. In the adsorption treatment of the gas by this fluidized-bed method, it is common practice to effect the gas treatment continuously by forming several fluidized beds in a multiplicity of stages within a tower. Particles from each fluidized bed can flow over a barrier or weir to fall onto the next lower bed via downcommers. Particles from the lowermost bed are then transferred to the uppermost bed via a pipe so that the particles are recycled continuously through the tower.
In the adsorption treatment of the gas by the fluidized-bed method described above, successful stabilization of the fluidized beds thus formed constitutes an essential requirement for enabling the removal of the noxious gaseous - 1- ~
8~
pollutants from the gas to be effected continuously at a high removal efficiency over long periods of service. The stability of such fluidized beds depends on the shape of the adsorbent particles used, the strength, wear resistance and other physical properties of the particles and the flow volume, flow velocity and viscosity of the gas used for fluidizing the adsorbent particles, and so on. It also depends on the extent of change in weight of the adsorbent particles being recycled. When the adsorption treatment of gas by the conventlonal fluidized-bed type technique is reviewed from this point of view, it is noted that the so-called coconut-shell activated carbon obtained from coconut husks is popularly used to form the adsorbent particles.
The activated carbon of this type is made up of particles of various, irregular shapes and this makes their transport through the adsorption apparatus substantially difficult Moreover, the adsorbent particles have poor physical properties and, for this reason, are readily pulverized as by crushing and attrition. Recyclic use of such irregularly shaped activated carbon particles, therefore, involves numerous difficulties. In the adsorption treatment of gas by the fluidized-bed method, the adsorbent particles of such shapes induce undesirable phenomena such as boiling, channeling and slugging~ when fluidized by the upward flow of the gas under treatment. They also cause similar phenomena while they are moving downwardly via the downcommers by gravity, with the result that the smooth flow of the particles inside the downcommers is impeded. This impeded flow consequently brings about a quantitative change in the weight of the adsorbent particles belng transferred for recyclic service.
With a view to precludlng these disadvantageous phenomena, the conventional techniques have attempted to lmprove the ~6~
structure of the downcommers for particle flow. It has been suggested, for example, to incorporate orifices in the bottoms of the downcommers or9 as disclosed by U.S. Patent No. 2,674,338, to have bottom plates supported on springs on the bottoms of the downcommers. These attempts at improvement of the structure of downcommers, however, effectively complicate the system itself and have the disadvantage that the activated carbon particles vary their shapes gradually with the lapse of time.
Thus, all these attempts fail to some extent to attain the preferred stabilization of the quantitative transport of adsorbent particles. Because the adsorbent particles in use are highly susceptible to pulverization and also because stabilization of the transport of these adsorbent particles is difficult to accomplish, the conventional techniques do not easily achieve the desired obejct of stabilizing the fluidized beds of the adsorbent particles.
SUMMARY 0~ THE INVENTION:
It is, therefore, a primary object of the present invention to provide a novel method and apparatus for the purification of a waste gas by the fluidized-bed principle, which method, by recyclic use of activated carbon spheres, is capable of continuously and effectively purging the gaseous pollutants from the gas.
It has now been discovered that the stabilization of the fluidized beds and the stabilization of the quantitative transport of activated carbon particles are both easily attained by using activated carbon spheres as the adsorbent particles - and also using perforated plates of a specific shape as the stePPed trays within the reaction tower.
According to one aspect of the present invention, there is provided a method for the continuous purification of a ~L~60~
waste gas containlng gaseous pollutants, which comprises:
(a) providing a tower having at least two substantially horizontal, vertically spaced per'forated plates therein arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95 % of the total surface area and a second of said portions having 5 to 20 % of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the first portion of the next lower plate; (b) continuously passing said waste gas upwardly through said tower so that the gas passes through said perforated plates and at the same time continuously and recircularly eeding activated carbon spheres downwardly through said tower so that the spheres form fluidi~ed beds on said perforated plates; and (c) continuously removing purified gas from the top of said tower.
According to another aspect of the invention there is provided a chemical process column containing at least two substantially horizontal, vertically spaced perforated plates arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95 % of the total surface area and a second of said portions having 5 to 20 % of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the flrst portion of the next lower plate.
BRIEF EXPL~ANATION OF THE DRAWINGS:
In the accompanying drawings:
Fig. 1 is a schematic explanatory diagram illustrating one example of a known adsorption apparatus for the remova] of gaseous pollutants by the fluidized-bed principle;
Fig. 2 is a plan view illustrating one preferred embodiment of one of the perforated plates used in the present invention~ which perforated plate has a weir formed on its upper surface in such a way that the plate surface is divided into a first portion accounting for 80 to 95% of the entire surface area and a second portion accouting for 5 to 20% of the entire surface area;
Fig. 3 is a plan ùiew illustrating a perforated plate similar to the plate shown in Fig. 2 but arranged in a reversed position, which perforated plate has a weir formed on its upper surface in su&h a way that the plate surface is divided into a portion accounting for 5 to 20% of the entire surface area and another portion accounting for 80 to 95% of the entire surface area;
Fig. 4 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising the perforated plates of the type of Fig. 2 and those of the type of Fig. 3 alternately disposed and vertically spaced inside a tower;
Fig. 5 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising alternative examples of perforated plates of Fig. 2 and Fig. 3 alternately disposed vertically spaced inside a tower;
Fig. 6 is a plan view illustrating one preferred embodiment of another type of the perforated plate used in the present invention, which perforated plate is divided into two -" ~.V~ 38~
portions having surface areas oE Lhe same portion as in the plate of Fig. 2;
Fig. 7 is a plan view illustrating a perforated plate similar to the plate shown in Fig. 6 but arranged in a reversed position, which perforated plate is divided into two portions having surface areas of the same portions as the plate of Fig. 3;
Fig. 8 is a schematic diagram illustrating a multiplicity of horizontal stepped trays comprising the perforated plates of Fig. 6 and Fig. 7 alternately disposed and vertically spaced inside a tower;
Figo 9 is a schematic diagram illus~rating a multiplicity of horizontal stepped trays comprising alternative examples of the perforated plates of Fig. 6 and Fig. 7 alternately disposed and vertica~ly spaced inside a tower; and Fig. 10 is a schematic diagram illustrating one preferred embodiment of the present invention in which a gas containing gaseous pollutants is treated continuously for the removal of the gaseous pollutants.
DETAILED DESCRIPTION OF THE INVENTION:
In Fig. 1, 1 denotes a reaction tower. A gas containing noxious gaseous pollutants to be removed is introduced into the tower 1 through a nozzle 2 in the adsorption section A. On entering the tower interior, the gas ascends vertically and comes into contact with adsorbent particles held inside the adsorption section A, causing the adsorbent particles to form fluidized beds on the stepped trays 3, 3', 3", etc. The adsorbent particles forming the fluidized beds adsorb the gaseous pollutants from the gas. The gas which has thus been freed of the noxious pollutants is released into the atmosphere via a discharge outlet 4 at the top of the tower.
The adsorbent particles on the stepped trays 3, 3', 3", Ptc., 8~0 fall through the downcommers 5, 5', 5", etc. associated with the trays and descend gradually downwardly by virtue of gravity, while slmultaneously adsorbing the gaseous pollutants from the gas. The particles then leave the adsorption section A and accumulate in the space formed on a guide plate 6 and gradually reach a regeneration section B which is located at the bottom of the reaction tower 1. On entering the regeneration section B, the adsorbent particles are heated by a heater 7, with the result that the particles are regenerated as they are forced by the heating to release the adsorbed pollutants. Subsequently, the regenerated adsorbent particles reaching the bottom 8 of the tower are transferred via a lifting pipe 9 to the top of the tower for recyclic service. In the meantime, the pollutants which have been desorbed from the adsorbent particles are forced out of the system via a nozzle 10 by means of a carrier gas introduced via a nozzle 11 disposed at the lower portion of the regeneration section B. The discharged pollutants are transferred to a desorbate treating section C composed of a condenser, decanter and the like.
In the method of the present invention, activated carbon spheres are used as the adsorbent particles. ~ecause of their spherical shape, these activated carbon spheres usually have the advantages of excellent fluidity, outstanding resistance to friction and wear and high impact strength. For the purpose of the present invention, the activated carbon spheres may be of the type which are obtained by mixing a powdery carbon or carbon precursor with a binding agent, subsequently molding the resultant mixture in the shape of spheres and activating the molded carbon spheres by an ordinary method (otherwise, referred to generally as "activated carbon spheres from coking coal"). It is, however, preferable to use the type of activated 8~6~
carbon spheres produced from a specific type of pitch as the raw material by a specific method such as disclosed in U. S.
Patent No. 3,917,806, because the activated carbon spheres of this type are excellent in terms of their spherical shape and physical properties. The superiority of this type of activated carbon spheres over various other types of activated carbon particles is easily confirmed by subjecting samples of the various types of activated carbon to a friction test, then sifting the tested sample particles through a sieve of 200 mesh ~by the Tyler standard) and comparing the weights of the corresponding sievings. To be specific3 this comparison can be accomplished by using glass containers measuring 28mm in diameter and 220mm in length9 placing 20 cm3 samples of the various types of activated carbon particles into the individual containers, rotating the containers and their contents at the rate of 36 r.p.m. for a fixed length of time, sifting the contents through a metal screen of 200 mesh and measuring the weight of the portion of each sample passing through the screen.
The results of a typical experiment performed as described above are shown in Table 1 below.
Table 1 Extent of attrition loss of particles due to friction in dry state (wt%) Length of friction test (in hours) TYPe of activated carbon 10 20 30 40 Activated carbon spheres 0 0 0.05 0.05 disclosed in ~.S. Patent No. 3,917,806 Activated carbon spheres 0.05 0.08 0.22 0.60 from coking coal Coconut-shell activated 2.3 2.9 3.3 3.5 carbon ~OtiO8~0 For the purposes of the present invention, the activated carbon s~heres preferably have a bulk density in the range of from 0.4 to 0.7 g/cm3, a particle diameter distribution range of from 0.2 to 2.0mm and an average particle diameter in the range of from 0.4 to 1.2 mm. I the particle diameter distribu-tion of these activated carbon spheres is excessively sharp, then during the actual use of the activated carbon spheres, the phenomenon known as channeling may be induced. If the particle diameter distribution of the activated carbon spheres is exces-sively broad, then the actual use of such activated carbon spheresdoes not encounter the disadvantage described above but can result in an adverse situation wherein the spheres of larger diameters and those of smaller diameters become suspended at different positions in the bed. This leads to a condition wherein only spheres of smaller diameters flow over the weirs on the trays and descend down the interior of the tower. Such a partial movement of the spheres is contrary to the requirement that the spheres should be transferred stably in a constant weight. For this reason, the activated carbon spheres preferably have a particle diame.er distribution such that the standard deviation of individual particle diameter distribution will fall in the range of fro~ 0.05 to 0.20mm.
In one embodiment of the present invention the stepped shelves or trays mentioned above are rectangular perforated plates of a type having a weir on the upper surface. As illustrated in Fig. 2 of the accompanying drawings, the surface of the tray is divided by the weir into a first portion accounting for ao to 95% of the total surface area and a second portion accounting for 5 to 20% of the total surface area. In Fig. 2, 21 denotes one such a rectangular perforated plate. The upper surface of this rectangular perforated plate ~ ~Gos~
21 is divided by a weir 24 into a first portion 22 and a second portion ~3. In Fig~ 3, 31 denotes a ~ectangular perforated plate dentical to the plate ~hown in Fi~. 2 except that the positions of the first and second portions are transposed.
The upper surface of this rectangular perforated plate 31 is divided by a weir 34 into a first portion 32 and a second portion 33. The first portions 22 of the plate of Fig. 2 and 33 of the plate of Fig. 3 are the tray portions above which fluidized beds of activated carbon spheres are formed. The second portions 23 of the plate of Fig. 2 and 32 of the plate of Fig. 3 are the portisns through which the activated carbon spheres descend to the next lower tray. The weir 24 and the weir 34 are so disposed on their respective rectangular per forated plates that the portion 22 equals the portion 33 and the portion 23 equals the portion 32 respectively in surface area. The rectangular perforated plates 21 and 31 shown respectively in Fig. 2 and Fig. 3 are level along their entire surfaces and they each have a multiplicity of perforations formed at an aperture ratio in the range of from 5 to 25%.
The entire surfaces of the rectangular perforated plates 21 and 31 may be in one level plane as shown in Figs. 4 and 8.
Otherwise, the two portions of these plates may be in different horizontal planes separated by a vertical distance of 10 to 20 mm as shown in Figs. 5 and 9. h~hen the plates are level throughout their entire surface, the perforations bored in the portions 22 and 33 preferably have a diameter in the range of from 3 to 5 mm and those bored in the portions 23 and 32 preferably have a diameter about 1.1 to 3 times that of the perforations in the portions 22 and 33. When the plates have stepped horizontal surfaces, all the perforations bored therein may have a fixed diameter falling in the approximate range of B~
from 3 to 5 mm, The heights of the weirs 24 and 34 are not specific~lly limited and are preferably in the range of from 20 to 6U mm. Moreover, it is usually necessary to make the weirs on the respective plates of equal height for the purpose of stabilizing the fluidized beds of the activated carbon spheres and also stabilizing the transportation rate of the spheres. The superficial velocity of the gas in the tower is preferably from the practical point of view, within the range of 0.5 to 2.0 m/sec. when the inside diameter of the tower is from 500 to 2,000 mm, which is the usual diameter range.
The rectangular perforated plates 21 and 31 are alternately vertically disposed to form a multiplicity of stepped trays inside a tower as shown in Fig. 4 or Fig. 5 and, thus form an adsorption section into which the gas is intro-duced for purification. In Fig. 4, 41 denotes a tower. Inside this tower 41, rectangular perforated plates 45 with surfaces divided by a weir 44 into a first portion 42 and a second portion 43, and rectangular perforated plates 49 with surfaces divided by a weir 48 into a first portion 47 and a second portion 46,are alternately disposed to form a multiplicity of stepped trays. In Fig. 5, 51 denotes a tower. Inside this tower 51 are rectangular perforated plates 55 having surfaces divided by a weir 54 into a first portion 52 and a second portion 53. The surface portion 53 is located in a level plane 10 to 20 mm below the surface portion 52. Every alternate tray in tower 51 is a rectangular perforated plate 59 divided by a weir 58 into a first portion 56 and a second portion 57 located in a level plane 10 to 20 mm below the portion 56. The vertical distance by which two ad~acent trays are separated is generally expected to be approximately the sum of the height of the weir plus 60 mm. To effect the purification of gas in ~608~
the tower described above, one has only to feed the gas - upwardly,into the tower from the bottom and cause the introduced gas to come into counter-current contact with activated carbon spheres being introduced downwardly from the top, so that the activated carbon spheres are caused by the force of the flow of gas to form fluidize~ beds on the stepped trays.
The activated carbon spheres which have thus formed the fluidized beds on tray portions 42 and 47 are horizontally transferred in the direction of portions 43 and 4~, and then descend by gravity through the perforations in the portions 43 and 46 of the rectangu-lar perforated plates 45 and 49 in the tower of Fig. 4 (or the portions 53 and 57 of the rectangular perforated plates 55 and 59 in the tower of Fig. 5) as they adsorb tne gaseous pollutants of the gas under treatment. After they have passed through the adsorption section formed by the multiplicity of stepped trays, they are regenerated in the desorption and regeneration section.
The regenerated activated carbon spheres are recycled, being again introduced downwardly into the tower from the top. Before the activated carbon spheres forming the fluidized beds descend from one tray to another, the individual activated carbon spheres on the stepped trays move horizontally on the respective trays.
To be more specific, the activated carbon spheres which have fallen from the narrower portion (hereinafter referred to as "portion II") of a given tray onto the wide portion (hereinafter referred to as"portion I") of the next lower tray move to the por-tion II of the lower tray as they form in situ a fluidized bed.
The activated carbon spheres fed cnto portion 42 of the rectangular perforated plate 45 in the tower of Fig. 4, for example, are horizontally transferred in the direction of the portion 43 by the upward stream of the gas and gravitational ~L0~81~
attraction and then,descend through the perforations distributed in the portion 43. ~ecause th~ perforated plates used in the prese~t invention as st~pped trays are rectangular in shape and they have their upper surfaces divided each by a weir into two portions, the invention enjoys the advantages enumerated in (1) through (4) below.
(l)' The perforated plates are simple in their structure and therefor can be easily produced. For this reason, the plates are useful for a gas-treating apparatus of large type.
(2) All the particles of activated carbon spheres move horizontally on the respective stepped trays. This means that the stabilization of the fluidized bed on each tray is not disturbed by the movement of the activated carbon spheres suspended on that tray.
(3~ The direction in'which the horizontal transfer of individual activated carbon spheres occurs on the stepped trays alternates from tray to tray and the heights of the fluidized beds on the respective stepped trays are equal due to the weirs being of equal height. These facts make it possible to have the activated carbon spneres achieve steady transfer and uniform contact with the gas under treatment.
(4) Each of the stepped trays is divided into a portion for permitting downward flow of activated carbon spheres ~portion II) and a portion for forminq a fluidized bed of t,h~
spheres (portion I) and the area ratio of these two portions is constant. Therefore, by fixing the total aperture area in the portion I at a value falling in the range of from 4 to 20 times the total aperture area in the portion II, the weight of the spheres to be transferred can be stabilized with minimal deviation.
The perforated plates to be used in the present ~L06~)8~3 invention are not necessarily ~ectangular in shape in order to satisfy the advantages of (1) through (4) described above.
They may be of a circular shape as shown in Fig. 6 and Fig. 7, for example. When a large scale system is used, such as a tower having an inside diameter exceeding 1,500 mm, in the invention for some special reason, a slight inclination of the stepped trays, of not more than 2 degrees, may possibly aid in the horizontal movement of activated carbon spheres on the stepped trays. If the inside diameter of the tower is small, however, such an inclination may conversely result in an increased variation in the volume of activated carbon spheres transferred. The desirability of such an inclination, there-fore, should be evaluated with due respect to the inside diameter of the tower.
In view of the fact that activated carbon spheres are used as the adsorbent particles and perforated plates of a specific shape are used as stepped trays as described above, the present invention enables the fluidized beds formed on the stepped trays to be stabilized to a height equalling the height of the weirs disposed on the stepped trays and, furthermore, permits the variation in the weight of activated carbon spheres being transferred to be limited within +10% by weight without resorting to any auxiliary device. Thus, the invention, in most cases, enables the purification of the waste gas to be carried out continuously for a long period of time, e.g. more than 200 hours, with the efficiency or removal of the gaseous pollutants kept at a high level (usually far exceeding 80%).
The gas which has been purified can be released into the atmos-phere without further treatment from the top portion of the tower. The present invention also serves the purpose of simplifying the system itself, because it obviates the necessity ~0~
of providing the stepped trays with downcommers as in conventional techniques.
The present invention wi:Ll now be described more specifically below with reference to the following Examples.
It should be noted that the present invention is not limited in any way to these Examples.
Example 1:
A number of rectangular perforated plates were each fabricated by joining a rectangular perforated plate measuring 20cm x lOcm and containing perforations 5mm in diameter at an aperture ratio of 17.9%, and a rectangular perforated plate measuring 20cm x 90cm and containing perforations 4 mm in diameter at an aperture ratio of 17.9% along their respective 20 cm sides and placing a weir in the form of a flat plate 20 mm in height along the joint-so that the respective portions or zones had an aperture area ratio of 1 . 9. The zones containing the perforations 5 mm in diameter formed zones II
for permitting downward flow of activated carbon spheres. A
~ box-type fluidized bed test apparatus was made by disposing four of such trays in such a way that the horizontal direction of the movement of activated carbon spheres would alternate as the particles descended from tray to tray. Activated carbon spheres were fed downwardly into the uppermost tray at a rate of 40 kg/hour and dry air was introduced upwardly below the lowermost tray at a superficial tower velocity of 1 m/sec. to fluidize the spheres. The activated carbon spheres were of the type having an average particle diameter of 0.7 mm and a particle diameter distribution range of 0.2 mm to 2.0 mm.
During a total of two hours of continued operation, the weight of activated carbon spheres which flowed out of the tower in two minutes (corresponding to average retention time of ~o~
activated carbon spheres per tray in the apparatus of the present case) was measured at a total of ten randomly selected points of time. The ten values thus obtained averaged 1.33 kg and the difference between the lar~est and smallest of the ten values was 0.12 kg.
For the purpose of comparison, a number of rectan-gular perforated plates were each fabricated by joining a square plate measuring 20 cm x 20 cm and containing perfor-ations 5 mm in diameter at an aperture ratio of 24%, and a rectangular plate measuring 20 cm x 80 cm and containing perforations 4 mm in diameter at an aperture ratio of 17.9%
along their respective 20-cm sides and placing a weir in the form of a flat plate 20 mm in height along the joint so that the respective zones had an aperture area ratio of 1 : 3. The square zones II permitted downward flow of activated carbon spheres. A fluidized bed test apparatus was made by disposing such trays in a total of four steps in the same way as above. By using this apparatus, the experiment described above was repeated under the same conditions. The average of the ~0 values per tray was 1.28 kg and the difference between the largest and smallest of the values was 0.31 kg. The operation was further continued, without alteration, and the weight of activated carbon spheres which flowed out of the tower over a period of eight minutes ~corresponding to average retention time of activated carbon spheres per tower in the apparatus of this case) was measured three times at intervals of 20 minutes. The values were 5.8 kg, 5.4 kg and 6.7 kg, indicating that the rate of transport of the spheres was not stable.
To adapt the above test apparatus for the present invention, about half of the perforations contained in the zones II, permitting downward flow of spheres, in all the perforated ~0~
plates were closed with adhesive tape. The same operation was repeated; The amount of activated carbon spheres which flowed out of the tower over a fixed period of two minutes was measured four times during a period of 30 minutes. In this case, the difference between the largest and smallest o~ the values per tray was 0.15 kg. In the continued operation, the amount of spheres which flowed out over a fixed period of eight minutes was measured three times at interyals of 30 minutes. The values per tower were 5.3 kg, 5.4 kg and 5.1 kg, indicating that the closure of half of ~he perforations served to stabilize the rate of transport of spheres.
By following the procedure described above, the flow amount of spheres for the average retention time (per tray) and the flow amount of spheres for the average retention time (per tower) were measured for various aperture area ratios. The results were as shown in Table 2 below.
Table - 2 . . _ Aperture area ratio Flow amount during Flow amount during between zone for retention time per retention time per 20downward flow and tray tower zone for fluidized __ __ bed Average Difference Average Differenc .. . __ __ . _ .
1/3 1.28 kg 0.31 6.0 kg 1.3 1/4 1.24 kg 0.27 5.~ kg 0.9 1/6 _ _ 5.3 kg 0.3 1/10 1.33 kg 0.12 _ _ 1/10 1.33 kg 0.18 5.7 kg 0.2 1~12 1.25 kg 0.15 5.7 kg 0.9 1/12 _ _ 5.4 kg 0.3 1/18 1.31 kg 0.20 _ _ 1/20 1.28 kg 0.15 5.3 kg 0.3 1/24 _ _ 5.0 kg 1.2 From the above results, it was concluded that the rate of transport of activated carbon spheres could be stabilized to within 10~ by weight where the aperture area of the zone I was in ~he range of from 4 to 20 times the aperture area of the zone II.
Example 2:
(1) In the test apparatus of Example 1 which had an aperture area ratio of 9 : 1, an experiment was performed with four superficial-tower gas velocities of 0.6 m/sec, 0.8 m/sec, 1.0 m/sec and 1.2 m/sec and three recirculation rates of 20 kg/hr, 40 kg/hr and 50 kg/hr to determine changes in the pressure drop across the entire tower. It was found that under all the conditions, the pressure drop remained constant at a value of 40 mm of water. Under all the conditions, the variation in the water level in the manometer was very slight, on the order of about 5 mm.
(2) To permit sampling o spheres from each of the stepped trays of this test apparatus, the zones of the plates permitting downward flow of spheres were disposed at levels 20 mm lower than those of the other zones supporting the fluidized beds and were each provided with a sampling port. Colored spheres prepared by spraying activated carbon spheres with a white paint were fed for a moment into the tower. Then, samples from the various trays were examined to determined the time-course change of the density of colored spheres in the samples. In all the trays, the intervals from the time the colored spheres were introduced to the time the density of colored spheres in the samples reached its peak were invariably in the range of from 90 to 100 seconds. This means that the average speed of movement of spheres in the horizontal direction was equal for all trays and, therefore, the fluidized beds of spheres were so stable as to 8~L~
have equal average retention times.
It was further observe~ that the downward flow of spheres was extremely stable where the zones of the plates for downward flow of spheres (zone II) were at levels lower than those zones supportiihg the fluidized beds (zone I).
Example 3:
Vinyl chloride monomer (VCM) was removed and recovered from an exhaust gas containing the VCM in accordance with the method of the present invention b~ recirculating activated carbon spheres measuring 0.8 mm in average diameter, with a gas-treating apparatus as shown in Fig. lO.
In Fig. lO, 101 denotes the gas-treating apparatus having a fluidized-bed adsorptlon section (D) consisting of the six stepped trays formed by alternately disposing the rectangular perforated plates as shown in Fig. 2 and Fig. 3. The zone II
on each tray is at level lower than the zone I as shown in Fig. 5.
The aperture area ratio of the zone I to the zone II
is 9 : l. Each of the rectangular perforated plates, which is made from a stainless steel, measures 450 mm x 450 mm and contains perforations 3.5 mm in diameter at an aperture ratio of 17.5%. The weir on each surface of the rectangular perforated plates is 40 mm in height. The distance between zone I and zone II is 15 mm. The desorbing section (E) in the apparatus 101 is connected witn the adsorption section (D) by means of flange 102 and is in the so-called shell-and-tube type structure consisting of a preheating zone and a desorbing zone. The desorbing section (E) is in the form of circular tower having an inner diameter of 690 mm and a height of about 3,000 mm. The adsorption section (D) is in the form of rectangular tower having a height of 2,000 mm. Further, in Fig. 10, 103 stands for a preheating jacket and 104 for a decanter.
~:)608~L~
The operation conditions and the results in this Example were as shown in Table 3 below.
Example 4:
VCM was removed and recovered from the exhaust gas by the same procedure as in Example 3, with the same apparatus as used in Example 3 except for using the circular perforated plates as shown Fig. 6 and Fig. 7 in place of Lhe rectangular perforated plates.
Each of the circular perforated plates, which is made from a carbon-steel, measures S00 mm in diameter and contains perforations 3.5 mm in diameter at an aperture ratio of 17.5%.
The aperture area ratio of the zone I to the zone II is 9 : 1.
The weir on each surface of the circular perforated plates is 40 mm in height. The zone II on each tray is at level lower than the zone I as shown in Fig. 9. The distance between zone I
and zone II is 15 mm. The adsorption section (D) is in the form of circular tower having a height of 2,000mm.
The operation conditions and the results in this Example were as shown in Table 3 below.
It is clear from the results shown in Table 3 that the circular perforated plates of Example 4 are substantially little inferior in performance to the rectangular perforated plates of Example 3.
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(3~ The direction in'which the horizontal transfer of individual activated carbon spheres occurs on the stepped trays alternates from tray to tray and the heights of the fluidized beds on the respective stepped trays are equal due to the weirs being of equal height. These facts make it possible to have the activated carbon spneres achieve steady transfer and uniform contact with the gas under treatment.
(4) Each of the stepped trays is divided into a portion for permitting downward flow of activated carbon spheres ~portion II) and a portion for forminq a fluidized bed of t,h~
spheres (portion I) and the area ratio of these two portions is constant. Therefore, by fixing the total aperture area in the portion I at a value falling in the range of from 4 to 20 times the total aperture area in the portion II, the weight of the spheres to be transferred can be stabilized with minimal deviation.
The perforated plates to be used in the present ~L06~)8~3 invention are not necessarily ~ectangular in shape in order to satisfy the advantages of (1) through (4) described above.
They may be of a circular shape as shown in Fig. 6 and Fig. 7, for example. When a large scale system is used, such as a tower having an inside diameter exceeding 1,500 mm, in the invention for some special reason, a slight inclination of the stepped trays, of not more than 2 degrees, may possibly aid in the horizontal movement of activated carbon spheres on the stepped trays. If the inside diameter of the tower is small, however, such an inclination may conversely result in an increased variation in the volume of activated carbon spheres transferred. The desirability of such an inclination, there-fore, should be evaluated with due respect to the inside diameter of the tower.
In view of the fact that activated carbon spheres are used as the adsorbent particles and perforated plates of a specific shape are used as stepped trays as described above, the present invention enables the fluidized beds formed on the stepped trays to be stabilized to a height equalling the height of the weirs disposed on the stepped trays and, furthermore, permits the variation in the weight of activated carbon spheres being transferred to be limited within +10% by weight without resorting to any auxiliary device. Thus, the invention, in most cases, enables the purification of the waste gas to be carried out continuously for a long period of time, e.g. more than 200 hours, with the efficiency or removal of the gaseous pollutants kept at a high level (usually far exceeding 80%).
The gas which has been purified can be released into the atmos-phere without further treatment from the top portion of the tower. The present invention also serves the purpose of simplifying the system itself, because it obviates the necessity ~0~
of providing the stepped trays with downcommers as in conventional techniques.
The present invention wi:Ll now be described more specifically below with reference to the following Examples.
It should be noted that the present invention is not limited in any way to these Examples.
Example 1:
A number of rectangular perforated plates were each fabricated by joining a rectangular perforated plate measuring 20cm x lOcm and containing perforations 5mm in diameter at an aperture ratio of 17.9%, and a rectangular perforated plate measuring 20cm x 90cm and containing perforations 4 mm in diameter at an aperture ratio of 17.9% along their respective 20 cm sides and placing a weir in the form of a flat plate 20 mm in height along the joint-so that the respective portions or zones had an aperture area ratio of 1 . 9. The zones containing the perforations 5 mm in diameter formed zones II
for permitting downward flow of activated carbon spheres. A
~ box-type fluidized bed test apparatus was made by disposing four of such trays in such a way that the horizontal direction of the movement of activated carbon spheres would alternate as the particles descended from tray to tray. Activated carbon spheres were fed downwardly into the uppermost tray at a rate of 40 kg/hour and dry air was introduced upwardly below the lowermost tray at a superficial tower velocity of 1 m/sec. to fluidize the spheres. The activated carbon spheres were of the type having an average particle diameter of 0.7 mm and a particle diameter distribution range of 0.2 mm to 2.0 mm.
During a total of two hours of continued operation, the weight of activated carbon spheres which flowed out of the tower in two minutes (corresponding to average retention time of ~o~
activated carbon spheres per tray in the apparatus of the present case) was measured at a total of ten randomly selected points of time. The ten values thus obtained averaged 1.33 kg and the difference between the lar~est and smallest of the ten values was 0.12 kg.
For the purpose of comparison, a number of rectan-gular perforated plates were each fabricated by joining a square plate measuring 20 cm x 20 cm and containing perfor-ations 5 mm in diameter at an aperture ratio of 24%, and a rectangular plate measuring 20 cm x 80 cm and containing perforations 4 mm in diameter at an aperture ratio of 17.9%
along their respective 20-cm sides and placing a weir in the form of a flat plate 20 mm in height along the joint so that the respective zones had an aperture area ratio of 1 : 3. The square zones II permitted downward flow of activated carbon spheres. A fluidized bed test apparatus was made by disposing such trays in a total of four steps in the same way as above. By using this apparatus, the experiment described above was repeated under the same conditions. The average of the ~0 values per tray was 1.28 kg and the difference between the largest and smallest of the values was 0.31 kg. The operation was further continued, without alteration, and the weight of activated carbon spheres which flowed out of the tower over a period of eight minutes ~corresponding to average retention time of activated carbon spheres per tower in the apparatus of this case) was measured three times at intervals of 20 minutes. The values were 5.8 kg, 5.4 kg and 6.7 kg, indicating that the rate of transport of the spheres was not stable.
To adapt the above test apparatus for the present invention, about half of the perforations contained in the zones II, permitting downward flow of spheres, in all the perforated ~0~
plates were closed with adhesive tape. The same operation was repeated; The amount of activated carbon spheres which flowed out of the tower over a fixed period of two minutes was measured four times during a period of 30 minutes. In this case, the difference between the largest and smallest o~ the values per tray was 0.15 kg. In the continued operation, the amount of spheres which flowed out over a fixed period of eight minutes was measured three times at interyals of 30 minutes. The values per tower were 5.3 kg, 5.4 kg and 5.1 kg, indicating that the closure of half of ~he perforations served to stabilize the rate of transport of spheres.
By following the procedure described above, the flow amount of spheres for the average retention time (per tray) and the flow amount of spheres for the average retention time (per tower) were measured for various aperture area ratios. The results were as shown in Table 2 below.
Table - 2 . . _ Aperture area ratio Flow amount during Flow amount during between zone for retention time per retention time per 20downward flow and tray tower zone for fluidized __ __ bed Average Difference Average Differenc .. . __ __ . _ .
1/3 1.28 kg 0.31 6.0 kg 1.3 1/4 1.24 kg 0.27 5.~ kg 0.9 1/6 _ _ 5.3 kg 0.3 1/10 1.33 kg 0.12 _ _ 1/10 1.33 kg 0.18 5.7 kg 0.2 1~12 1.25 kg 0.15 5.7 kg 0.9 1/12 _ _ 5.4 kg 0.3 1/18 1.31 kg 0.20 _ _ 1/20 1.28 kg 0.15 5.3 kg 0.3 1/24 _ _ 5.0 kg 1.2 From the above results, it was concluded that the rate of transport of activated carbon spheres could be stabilized to within 10~ by weight where the aperture area of the zone I was in ~he range of from 4 to 20 times the aperture area of the zone II.
Example 2:
(1) In the test apparatus of Example 1 which had an aperture area ratio of 9 : 1, an experiment was performed with four superficial-tower gas velocities of 0.6 m/sec, 0.8 m/sec, 1.0 m/sec and 1.2 m/sec and three recirculation rates of 20 kg/hr, 40 kg/hr and 50 kg/hr to determine changes in the pressure drop across the entire tower. It was found that under all the conditions, the pressure drop remained constant at a value of 40 mm of water. Under all the conditions, the variation in the water level in the manometer was very slight, on the order of about 5 mm.
(2) To permit sampling o spheres from each of the stepped trays of this test apparatus, the zones of the plates permitting downward flow of spheres were disposed at levels 20 mm lower than those of the other zones supporting the fluidized beds and were each provided with a sampling port. Colored spheres prepared by spraying activated carbon spheres with a white paint were fed for a moment into the tower. Then, samples from the various trays were examined to determined the time-course change of the density of colored spheres in the samples. In all the trays, the intervals from the time the colored spheres were introduced to the time the density of colored spheres in the samples reached its peak were invariably in the range of from 90 to 100 seconds. This means that the average speed of movement of spheres in the horizontal direction was equal for all trays and, therefore, the fluidized beds of spheres were so stable as to 8~L~
have equal average retention times.
It was further observe~ that the downward flow of spheres was extremely stable where the zones of the plates for downward flow of spheres (zone II) were at levels lower than those zones supportiihg the fluidized beds (zone I).
Example 3:
Vinyl chloride monomer (VCM) was removed and recovered from an exhaust gas containing the VCM in accordance with the method of the present invention b~ recirculating activated carbon spheres measuring 0.8 mm in average diameter, with a gas-treating apparatus as shown in Fig. lO.
In Fig. lO, 101 denotes the gas-treating apparatus having a fluidized-bed adsorptlon section (D) consisting of the six stepped trays formed by alternately disposing the rectangular perforated plates as shown in Fig. 2 and Fig. 3. The zone II
on each tray is at level lower than the zone I as shown in Fig. 5.
The aperture area ratio of the zone I to the zone II
is 9 : l. Each of the rectangular perforated plates, which is made from a stainless steel, measures 450 mm x 450 mm and contains perforations 3.5 mm in diameter at an aperture ratio of 17.5%. The weir on each surface of the rectangular perforated plates is 40 mm in height. The distance between zone I and zone II is 15 mm. The desorbing section (E) in the apparatus 101 is connected witn the adsorption section (D) by means of flange 102 and is in the so-called shell-and-tube type structure consisting of a preheating zone and a desorbing zone. The desorbing section (E) is in the form of circular tower having an inner diameter of 690 mm and a height of about 3,000 mm. The adsorption section (D) is in the form of rectangular tower having a height of 2,000 mm. Further, in Fig. 10, 103 stands for a preheating jacket and 104 for a decanter.
~:)608~L~
The operation conditions and the results in this Example were as shown in Table 3 below.
Example 4:
VCM was removed and recovered from the exhaust gas by the same procedure as in Example 3, with the same apparatus as used in Example 3 except for using the circular perforated plates as shown Fig. 6 and Fig. 7 in place of Lhe rectangular perforated plates.
Each of the circular perforated plates, which is made from a carbon-steel, measures S00 mm in diameter and contains perforations 3.5 mm in diameter at an aperture ratio of 17.5%.
The aperture area ratio of the zone I to the zone II is 9 : 1.
The weir on each surface of the circular perforated plates is 40 mm in height. The zone II on each tray is at level lower than the zone I as shown in Fig. 9. The distance between zone I
and zone II is 15 mm. The adsorption section (D) is in the form of circular tower having a height of 2,000mm.
The operation conditions and the results in this Example were as shown in Table 3 below.
It is clear from the results shown in Table 3 that the circular perforated plates of Example 4 are substantially little inferior in performance to the rectangular perforated plates of Example 3.
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Claims (20)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for the continuous purification of a waste gas containing gaseous pollutants, which comprises:
(a) providing a tower having at least two substantially horizontal, vertically spaced perforated plates therein arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95% of the total surface area and a second of said portions having 5 to 20% of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the first portion of the next lower plate;
(b) continuously passing said waste gas upwardly through said tower so that the gas passes through said perforated plates and at the same time continuously and recircularly feeding activated carbon spheres downwardly through said tower so that the spheres form fluidised beds on said perforated plates;
(c) continuously removing purified gas from the top of said tower.
(a) providing a tower having at least two substantially horizontal, vertically spaced perforated plates therein arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95% of the total surface area and a second of said portions having 5 to 20% of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the first portion of the next lower plate;
(b) continuously passing said waste gas upwardly through said tower so that the gas passes through said perforated plates and at the same time continuously and recircularly feeding activated carbon spheres downwardly through said tower so that the spheres form fluidised beds on said perforated plates;
(c) continuously removing purified gas from the top of said tower.
2. The method of Claim 1, wherein the aperture area of said first portion falls in the range of from 4 to 20 times that of said second portion.
3. The method of Claim 1, wherein the apertures of each plate constitute 5 to 25% of the total plate area.
4. The method of Claim 1, wherein the height of each of said weirs is in the range of from 20 to 60mm.
5. The method of Claim 1, wherein said waste gas is introduced into said tower at a superficial tower velocity in the range of from 0.5 to 2.0 m/sec.
6. The method of Claim 1, wherein each of said perforated plates is rectangular in shape.
7. The method of Claim 1 wherein each of said perforated plates is circular in shape.
8. The method of Claim 1, wherein said second portion of each of said plates is located at the same horizontal level as said first portion.
9. The method of Claim 1, wherein said second portion of each of said plates is located at a lower horizontal level than said first portion.
10. The method of Claim 1, wherein said activated carbon spheres have a particle diameter distribution range of from 0.2 to 2.0 mm, an average particle diameter in the range of from 0.5 to 1.2 mm and a bulk density of from 0.4 to 0.7 g/cm2.
11. The method of Claim 1, wherein said activated carbon spheres are produced by fusing pitch, molding the fused pitch into spheres and subjecting the resultant pitch spheres to the treatment of infusibilization, carbonization, and activation.
12. The method of Claim 1, wherein said activated carbon spheres leaving the lowermost plate are regenerated and then recycled to the uppermost plate.
13. A chemical process column containing at least two substantially horizontal, vertically spaced perforated plates arranged so that gas flowing upwardly through said tower passes through each of said perforated plates in succession, each of said perforated plates having a single, non-circular weir provided on its upper surface dividing said surface into two portions, a first of said portions having 80 to 95% of the total surface area and a second of said portions having 5 to 20% of the total surface area, the weirs on each of said plates being of substantially the same height, and the plates being arranged in such a manner that the second portion of each plate overlies the first portion of the next lower plate.
14. The column of Claim 13, wherein the aperture area of said first portion falls in the range of from 4 to 20 times that of said second portion.
15. The column of Claim 13, wherein the aperture of each plate constitute 5 to 25% of the total plate area.
16. The column of Claim 13, wherein the height of each weir is in the range of from 20 to 60 mm.
17. The column of Claim 13, wherein each of said plates is rectangular in shape.
18. The column of Claim 13 wherein each of said plates is circular in shape.
19. The column of Claim 13, wherein said second portion is at the same horizontal level as said first portion.
20. The column of Claim 13, wherein said second portion of each plate is at a lower level than said first portion.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA264,649A CA1060810A (en) | 1976-11-02 | 1976-11-02 | Method for the purification of waste gas containing gaseous pollutants |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA264,649A CA1060810A (en) | 1976-11-02 | 1976-11-02 | Method for the purification of waste gas containing gaseous pollutants |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1060810A true CA1060810A (en) | 1979-08-21 |
Family
ID=4107176
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA264,649A Expired CA1060810A (en) | 1976-11-02 | 1976-11-02 | Method for the purification of waste gas containing gaseous pollutants |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1060810A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106268175A (en) * | 2016-09-30 | 2017-01-04 | 贵州蔚蓝环保有限公司 | A kind of uniform flow activated carbon adsorption bed |
| CN114801466A (en) * | 2022-05-05 | 2022-07-29 | 杭州添丽装饰纸有限公司 | Environment-friendly gravure printing machine equipment |
-
1976
- 1976-11-02 CA CA264,649A patent/CA1060810A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106268175A (en) * | 2016-09-30 | 2017-01-04 | 贵州蔚蓝环保有限公司 | A kind of uniform flow activated carbon adsorption bed |
| CN114801466A (en) * | 2022-05-05 | 2022-07-29 | 杭州添丽装饰纸有限公司 | Environment-friendly gravure printing machine equipment |
| CN114801466B (en) * | 2022-05-05 | 2022-12-30 | 杭州添丽装饰纸有限公司 | Environment-friendly gravure printing machine equipment |
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