WO2008010206A1

WO2008010206A1 - Apparatus and method for the removal of gaseous pollutants from an upwardly flowing gas stream

Info

Publication number: WO2008010206A1
Application number: PCT/IL2007/000876
Authority: WO
Inventors: Vladimir Burkat; Vladimir Schyogolev; David Pegaz
Original assignee: E.E.R. Environmental Energy Resources (Israel) Ltd.
Priority date: 2006-07-17
Filing date: 2007-07-12
Publication date: 2008-01-24
Also published as: CN101489650A; EP2040821A1; AU2007274651A1; CA2658067A1; IL176899A0

Abstract

The present invention is a method and apparatus for the removal of gaseous pollutants from a gas stream. The method comprises injecting an adsorbent in a countercurrent direction into a turbulent region of the gas stream. The apparatus comprises a vertical adsorption column having a convergent inlet cone, throat and a divergent outlet cone. A vortex generator is mounted in the throat coaxially with the gas flow. An unobstructed annular area exists around the vortex generator. In this way a region of turbulence of the gas stream is created surrounding the central longitudinal axis of the apparatus and a laminar flow of gas is created along the walls of the apparatus.

Description

APPARATUS AND METHOD FOR THE REMOVAL OF GASEOUS POLLUTANTS FROMAN UPWARDLYFLOWING GAS STREAM

Field of the Invention

The present invention relates to the field of gas cleaning. More particularly, the invention relates to the removal of gaseous pollutants from an upwardly flowing gas stream by contact with adsorbent material.

Background of the Invention

The present invention relates to an apparatus and method for the removal of gaseous pollutants from a gas stream by contact with particulate or powdery solid materials, which are capable of adsorbing the respective contaminants .

Although the invention is suitable for cleaning any type of gas, the principal applications of the invention are for the cleaning of gases in the chemical, metallurgical and waste treatment industries, in order to reduce harmful emissions to the atmosphere.

The so-called dry-scrubbing of a gas stream, i.e. a cleaning system that works by contact of the gas stream with particulate absorbents and does not include any liquid, has been widely used to remove gaseous contaminants. Dry scrubbing, which involves entraining the adsorbent material in a gas stream, is carried out in a variety of different types of apparatus. The entrained adsorbent material, together with the impurities adsorbed thereby, is subsequently separated from the gas stream by suitable means, for example, a bag filter.

One known dry-scrubbing method is disclosed in US 3,780,497 wherein alumina particles are introduced at the base of an adsorption column through which a gas stream containing fluorine flows upwardly. The hase of an adsorption column is located just above a convergent-divergent venturi means which is adapted to establish turbulence in the adsorption column.

Such a method suffers from inefficient gas-solid contact due to the relatively short contact time between the alumina particles with the gas stream. Consequently the alumina particles need to be recycled, e.g. up to 20 times, and reintroduced into the column in order to achieve high scrubbing efficiency. Additionally, breakdown or reduction in alumina particle size is noticeable as a result of the high recycle rate, further reducing scrubbing efficiency.

Another method for removing contaminants from gases is described in Published International Patent Application WO 96/15846, in which fluorine containing substances such as hydrogen fluoride are separated from a gas in a countercurrent adsorption process in two stages. In the first stage, a gas is treated with aluminum oxide that has been partly spent. Aluminum oxide is separated from the gas and used for aluminum production. The gas is then supplied to the second adsorption stage and treated with essentially unspent aluminum oxide, whereupon aluminum oxide is separated downstream from the second stage and transferred to the first stage, while the gas is discharged into the surrounding atmosphere.

This adsorption method suffers from inefficient mass transfer, due to the direct flow of solid and gas phases. Also, the adsorption method of WO 96/15846 is based on at least two stages which increase the energy consumption needed for feeding recycled aluminum oxide.

SU 146 43 37 discloses a method for removal of hydrogen fluoride and resin from an electrolytic aluminum production waste gas by introducing a suspension while increasing and then reducing the gas velocity. While the gas velocity is increased by a factor of 1.5-15, the hydraulic resistance within the adsorption zone is increased due the added compression of the waste gas, thereby requiring an additional energy influx to overcome the hydraulic resistance. While the velocity of the gas is reduced, the sedimentation of coarse alumina particles generally results, requiring the removal thereof from the adsorption zone.

US 5,658,544 discloses an adsorption process for increasing the mass transfer rate of contaminant removal by imparting a gas stream entering a reactor from underneath with components of velocity in substantially upward and circumferential directions and feeding particulate material, such as alumina, into the reactor in a countercurrent direction, whereby to contact the gas stream and to adsorb contaminants on the particulate material. A fine fraction of treated adsorbents is recovered, such that it is entrained by the gas stream and removed from the reactor before being passed to a baghouse.

A consequence of US 5,658,544 is that a significant number of adsorbent particles that are introduced into the reactor are also imparted with components of velocity in substantially circumferential directions and impinge the reactor walls. Such particles fail to contact the gas stream, and the scrubbing efficiency of the gas stream is therefore reduced. Also, impingement upon the metallic portions of the reactor walls by the highly abrasive adsorbents results in wall erosion. Furthermore, the adsorbent is contaminated by impurities, such as iron particles, that have been eroded from the reactor wall.

Another method of improving the mixing of the solid particles introduced into an upward flowing gas stream is described in EP 0 733. In the apparatus described in this patent application an upwardly flowing gas is introduced into a container by passing the gas through a Venturi nozzle comprising a converging inlet, throat, and relatively long diffuser (diverging outlet). Solid material which is to be mixed with the gas flowing into the container is introduced through an inlet located in the upper part of the diffuser of the Venturi nozzle. According to this application, the technical problem to be solved is how to keep the solid from sliding down the walls of the diffuser, not mixing entirely with the upwardly flowing gas stream, and eventually blocking the throat of the Venturi nozzle. The solution is to provide a local change in slope of the wall of the diffuser below the solid inlet that will redirect the solid particles sliding down the wall of the diffuser and will divert them towards the center of the throat where they will be carried upwards with the flowing gas.

US 4,535,778 (equivalent to FR 2 534 831) describes a solution to the problem of uniformly mixing a powdery substance with a flowing gas stream. The solution is to provide a number of nozzles the number depending on the diameter of the gas stream) distributed uniformly across the cross-section of the conduit through which the gas is flowing. The nozzles are directed in a direction opposite the flow direction of the gas, thereby injecting the solid material into the approaching gas stream.

Another patent application which addresses the problem of mixing solid particles with a gas stream is DE 43 40 908. The solution described in this publication is to create a highly turbulent flow over the entire cross section of the gas stream before introducing the solid particles by means of one or more nozzles.

It is an object of the present invention to provide an adsorption method and apparatus for increasing the scrubbing efficiency of a contaminant-laden gas stream. It is another object of the present invention to provide apparatus for injecting an adsorbent into a turbulent region of a contaminant-laden gas stream while generating a laminar flow of gas along the walls of the reactor through which the gas stream flows.

It is an additional object of the present invention to provide a one-stage adsorption method and apparatus.

It is an additional object of the present invention to provide an adsorption method and apparatus which overcomes the limitations of those of the prior art.

It is an additional object of the present invention to provide an adsorption method and apparatus for which a reduced quantity of adsorbents is required relative to that of the prior art.

It is yet an additional object of the present invention to provide an adsorption apparatus that is relatively simple mechanically and therefore economical to produce as well as to maintain.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

The present invention provides a reactor for the removal of gaseous pollutants from an upwardly flowing gas stream comprising: a) a vertical adsorption column having a convergent inlet cone, throat and a divergent outlet cone through which a gas stream flows at an angle ranging from 0-45 degrees relative to a vertically disposed plane- such an angular range of flow hereinafter referred to as being "upwardly flowable"-; b) an inlet tube through which adsorbent material is introduced into the divergent outlet cone of said adsorption column; c) a casing in fluid communication with said gas stream coaxially mounted in said throat, said casing defining a major gas stream path through the interior of said casing and a peripheral gas stream path in an annular gap between the casing and walls of the column, the thickness of said gap being selected so as to produce a laminar peripheral gas stream; and d) a vortex generator mountable within the interior of said casing for generating a turbulent region of said gas stream in the outlet cone downstream from the casing, wherein e) adsorbent material is introduced into said turbulent region by means of said inlet tube, the adsorbent material being dispersed throughout the gas stream via said turbulent region while being prevented from impinging the column walls by means of said laminar peripheral gas stream.

This invention relates to a type of reactor in which the reaction occurs in a localized space filled with a material that is generally a gas phase, which may contain solid particles. By the term "reaction" is meant herein whatever phenomenon is caused or facilitated by adsorption, viz. not necessarily a chemical phenomenon, but a physical one or a combination of the two, as well. The adsorbent material which is introduced to the reactor is adapted to remove gaseous pollutants from a gas stream. The term "gaseous pollutant" refers to not only vaporized liquids, but also to volatilized solid substances.

The vortex generator preferably comprises vane elements for producing vortices in said major gas stream downstream to the vortex generator, said vane elements being inclined with respect to the longitudinal axis of the column, e.g. at an angle of between 5 to 15 degrees. The cross sectional area of the gap preferably narrows from the convergent inlet cone to the throat.

The cross sectional area of the gap, in the vicinity of the throat, ranges from 5-30% of the cross sectional area of the throat.

The present invention is also directed to a method for the removal of gaseous pollutants from an upwardly flowing gas stream comprising: a) directing an upwardly flowing gas stream through a vertical adsorption column having a convergent inlet cone, throat and a divergent outlet cone; b) allowing said gas stream to branch into a major gas stream path through the interior of a cylindrical casing coaxially mounted in said throat and into a peripheral gas stream path in an annular gap between the casing and walls of the column, said gap between the casing and walls of the column being selected so as to produce a laminar peripheral gas stream; c) allowing said major gas stream to flow through a vortex generator mounted within the interior of said casing, whereby to generate a turbulent region in the outlet cone downstream from the casing; and d) introducing adsorbent material to said turbulent region via an inlet tube, the adsorbent material being dispersed throughout the turbulent region of said gas stream while being prevented from impinging upon the column walls by means of said laminar peripheral gas stream.

The retention time of the absorbent in the turbulent region is 2 to 20 times higher than the retention time of the adsorbent in the laminar peripheral stream.

Preferably, the peripheral gas stream mixes with the turbulent region inwardly from the column walls, whereby to increase the scrubbing efficiency. Entrained adsorbent particles containing adsorbed impurities are subsequently separated from the gas stream.

Suitable flow conditions for effecting the method of the inventions are such that the gas stream has a temperature ranging from 0 to 300⁰C and a density ranging from 0.6 to 1.5 kg/m³, and flows at a velocity ranging from 0.6 to 25 m/s within the throat.

Brief Description of the Drawings

In the drawings:

Fig. 1 is a schematic drawing of the apparatus according to the present invention in elevation, showing generation of a turbulent region in a gas stream which is delivered to an adsorption column; and

Fig. 2 is a schematic drawing of the boundary layer between a laminar peripheral gas stream and an adjoining turbulent major gas stream.

Detailed Description of Preferred Embodiments

The present invention is a method and apparatus for the removal of gaseous pollutants from a gas stream by injecting in a countercurrent direction an adsorbent into a turbulent region of the gas stream while generating a laminar flow of gas along the walls of the reactor, whereby to prevent impingement of adsorbent particles onto the reactor walls.

The reactor of the present invention is illustrated in Fig. 1 and is generally designated as numeral 10. Reactor 10 comprises vertical adsorption column 8 having a convergent inlet cone 1, throat 2 and a divergent outlet cone 3, vortex generator 4 coaxially mounted in throat 2 of the adsorption column, and inlet tube 6 through which adsorbent material is introduced into outlet cone 3. Vortex generator 4 extends substantially the entire length of throat 2, extending downwardly into convergent inlet cone 1. Vortex generator 4 has an annular metallic casing 13, and comprises a plurality of vane elements 15 which are internally disposed with respect to the casing. Each vane element 15 is inclined with respect to a horizontal plane at an angle α ranging from 75-85 degrees. Vortex generator 4 is mounted in throat 2 of adsorption column 8 by means of a plurality of plates of sheet metal (not shown), e.g. 2-4 sheets, in such a way that the upward passage of the gas stream through the plates is substantially not disrupted. The ends of each sheet metal plate are welded on one side to casing 13 of vortex generator 4 and on the other side to the inner face of reactor wall 12.

Annular gap 5 is formed between the periphery of vortex generator 4 and reactor wall 12, and the cross sectional area of the gap narrows from convergent inlet cone 1 to throat 2. The flow conditions of a gas mixture having a temperature ranging from 0-300⁰C, a density ranging from 0.6-1.5 kg/m³ and flowing at a velocity ranging from 0.6 to 25 m/s within throat 2 are suitable for effecting the reaction by means of reactor 10 and the adsorbent material introduced through inlet tube 6. As gas stream G is upwardly supplied to reactor 10, a portion P of the gas stream corresponding to the ratio of the cross sectional area of annular gap 5 to the cross sectional area of throat 2 flows through the gap as a laminar peripheral stream, due to suitable selected flow conditions. The velocity of peripheral stream P increases as a result of passing through the gradually narrowing gap 5. The remaining portion of the gas which does not flow through annular gap 5 flows through vortex generator 4, coming in contact with vane elements 15 and producing vortices V downstream to the vortex generator 4.

The cross sectional area of gap 5, in the vicinity of the throat, ranges from 5- 30% of the cross sectional area of throat 2. When the cross sectional area of gap 5 is less than 5% of the cross sectional area of throat 2, the hydraulic resistance of peripheral stream P increases while the flow rate of peripheral stream P becomes insufficient to prevent the impingement of adsorbent particles onto reactor wall 12. Conversely, when the cross sectional area of gap 5 is greater than 30% of the cross sectional area of throat 2, momentum transport within vortices V is reduced, and therefore the contact time between the adsorbent particles and vortices V is reduced, resulting in lowered scrubbing efficiency.

Particulate or powdery adsorbent material A, e.g. activated carbon, calcium hydroxide, or alumina, which is capable of adsorbing contaminants carried by gas stream G, is gravity fed to divergent cone 3 via inlet tube 6 such that the velocity of adsorbent material A at the discharge end of the tube ranges from 0.1-5 m/s. The apparent density of adsorbent A, i.e. the ratio of its weight to the particle volume including the volume of the pores and gas inclusions on the particle surface, ranges from 0.3-2.0 g/cm³, its real density, i.e. the ratio of its weight to the particle volume excluding the volume of the pores and gas inclusions on the particle surface, ranges from 1-4 g/cm³, and the concentration of the adsorbent within the gas stream ranges from 0.100- 0.500 g/m³. In order to achieve an optimal scrubbing efficiency, the inclination of inlet tube 6 ranges from 40-75° with respect to a horizontal plane, the length of the tube between the penetration point of the reactor and the discharge point ranges from a factor of 0.2-0.5 of the reactor diameter at throat 2, the vertical distance between the discharge point from inlet tube 6 and the discharge point from vortex generator 4 ranges from 0.05-0.2Om, and the horizontal distance between reactor wall 12 and the discharge point from inlet tube 12 ranges from 1.1L, where L is the width of annular gap 5, to 0.45d, where d is the diameter of throat 2.

Adsorbent material A is discharged into vortices V and is consequently homogeneously diffused throughout, and entrained by, the gas stream by the turbulence of the vortices. By virtue of the turbulence induced by vortex generator 4, the average residence time of gas molecules of the gas stream in divergent cone 3 ranges from 0.5-1 s and the average contact time of a particle of adsorbent material with the gas molecules of the gas stream ranges from 5-15 seconds. It will be appreciated that the contact time between adsorbent particles and the gas stream is increased by approximately a factor of 10 relative to that of prior art methods whereas the residence time is approximately equal to that of prior art methods.

Despite the added circumferentially directed velocity component of the abrasive adsorbent material, as a result of being mixed in vortices V, the gas-adsorbent mixture is advantageously prevented from impinging upon reactor wall 12. As illustrated in Fig. 2, laminar peripheral stream P is directed vertically upwards along reactor wall 12 and provides resistance to the circumferential movement of the adsorbent-gas mixture. Peripheral stream P serves as a means of increasing the thickness of the boundary layer between reactor wall 12 and vortices V. The continual action of shear forces, which exist at any boundary layer due to friction, produces a velocity gradient from the boundary to a central portion of the flow. The velocity gradient along laminar boundary layer 25 between peripheral flow P and vortices V tends to slow the circumferential advancement of adsorbent particles at the boundary layer. Impingement of the adsorbent particles onto reactor wall 12 at an appreciable velocity, which would cause abrasion thereof and contamination of the particles, is thereby prevented. To maintain a laminar boundary layer in the zone adjacent to the inlet tube, the tube could be located into the fairing, ensuring the minimum perturbation of gas stream.

The flow becomes unstable inward to laminar boundary layer 25, i.e. in a radial direction toward the longitudinal axis 17 of adsorption column 8 (Fig. 1), and finally forms a turbulent boundary layer T, at which the gas moves in a random, uncontrolled fashion. Peripheral stream P mixes with the adsorbent-gas vortices V at turbulent boundary layer T, and an increased contact time of up to 15 seconds between adsorbent particles and the gas stream may be realized.

With such a contact time between the adsorbent particles and the gas stream, the scrubbing efficiency, or percentage of contaminants that is adsorbed, is also increased relative to the prior art. When employing the apparatus and method according to the present invention, a dry scrubbing efficiency of 95-99% has been attained, while prior art reactors have achieved a maximum dry scrubbing efficiency of only 70-90%. The dry scrubbing efficiency is a function of gas stream speed and the amount of adsorbent supplied, so as to produce a corresponding number of contact surfaces between gas and solid phases. The velocity of periphery stream P (Fig. 1) between vortex generator 4 and reactor wall 12, however, has an insignificant influence on the scrubbing efficiency. Generally, only one cycle of adsorbent introduction into the adsorption column is required; however, the number of adsorbent introduction cycles may be as much as 5-8, depending on the specific surface area of the adsorbent material. Since impingement upon the reactor wall by the adsorbent material is precluded, thereby preventing contamination of the adsorbent material, the latter may be advantageously reused in a plurality of adsorbent introduction cycles.

The entrained adsorbent particles containing adsorbed impurities are subsequently separated from the gas stream by suitable means, such as a bag filter.

The aforementioned adsorbing method is suitable for many different applications, including the cleaning of gases, particularly exhaust gases, in the chemical, metallurgical and waste treatment industries.

One suitable application is a recycling system for a waste-converting apparatus, which is disclosed in co-pending published International Patent Application WO 03/069227 by the same Applicant. In this system, residues collected from a post-processing means are re-introduced into the apparatus such that the residues are exposed to the high temperature zone thereof. A significant portion of dangerous emissions, including heavy metals, are disposed of by producing solidified vitrified slag. The added efficiency of recycled system may be as much as 5-10%, depending on the amount of adsorbed impurities that is entrained in the gas stream, the rate of processing waste, and on the amount of residues that are re-introduced to the apparatus.

The post-processing means comprises a suitable gas cleaning system and a suitable stack operatively connected in series to a processing chamber. In various embodiments the post-processing means additionally comprises an afterburner, energy utilization means, a combustion products cooling system, a waste water treatment system operatively connected to the gas cleaning system, or a combination thereof.

For example, the gas cleaning system may comprise a "dry" gas cleaning system, and may thus include a semi-dry scrubber, into which is fed a suspension of Ca(OH)2 in water for binding the acid gases. Water is subsequently evaporated fully, and thus only gases, products Ca(OH)2, CaCl2, CaSO₄, Ca3(PO4)2, in powder form, and other dust (which did not precipitate in the boiler) exit the scrubber.

The reactor of the present invention may be deployed downstream to the scrubber, wherein a mixture of powders of Ca(O H)2 and powdered activated carbon (PAC) are fed. These powdered adsorbents have very large specific surface values (typically carbon >750 m²/g; Ca(OH)2>30m²/g), and the Ca(OH)2 may adsorb the remaining acid gases, while the PAC adsorbs dioxins and components containing heavy metals. A bag filter receives the discharge from the reactor, and residues, including Ca(OH)2, active carbon, dioxins, oxides, salts, and products of reaction (CaCb, CaSθ4, Ca3(Pθ4)2 and other substances), are precipitated. Essentially, gas carrying dust, which includes toxic components such as dioxins, heavy metals and their oxides and salts, is filtered through the layer of dust precipitated in the bags. The toxic components are adsorbed and thereby precipitate out of the carrier gas. The clean gas obtained after filtration is directed to an exhauster and then to a stack for expulsion into the atmosphere. Alternatively, clean gas may be collected, e.g. hot clean gas may be used to generate electricity.

Residues collected frσπf the^~pσst-processing -means- are very-toxic. However, since such residues are hygroscopic (especially the CaCk portion thereof), they may absorb water from the water vapor that is generated along with other combustion products, and thus may have a sludge-type consistency. Accordingly, tubes which are used for transporting the residues through the gas cleaning system may be optionally heated to enable the residues to dry.

A residue recycling system may be employed for recycling the residues that are typically produced in the gas cleaning system. Accordingly, the residue recycling system preferably comprises a suitable reservoir for the temporary storage and accumulation of the residues which were precipitated by the gas cleaning system, or alternatively, which originated from the post-processing means.

While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without exceeding the scope of the claims.

Claims

1. An apparatus for the removal of gaseous pollutants from an upwardly flowing gas stream, comprising: a) a vertical adsorption column having a convergent inlet cone, throat and a divergent outlet cone through which a gas stream is upwardly flowable; and b) an inlet tube through which adsorbent material is introduced into the divergent outlet cone of said adsorption column, characterized in that said apparatus further comprises: c) a casing coaxially mounted in said throat and in fluid communication with said gas stream, said casing defining a major gas stream path through the interior of said casing and a peripheral gas stream path in an annular gap between the casing and walls of the column, the thickness of said gap being selected so as to produce a laminar peripheral gas stream; and d) a vortex generator mountable within the interior of said casing for generating a turbulent region of said gas stream in the outlet cone downstream from said casing, e) wherein adsorbent material is introducible into said turbulent region by means of said inlet tube, the adsorbent material being dispersed throughout the gas stream via said turbulent region while being prevented from impinging the column walls by means of said laminar peripheral gas stream.

2. The apparatus according to claim 1, wherein the vortex generator comprises vane elements for producing vortices in the major gas stream downstream to the vortex generator, said vane elements being inclined with respect to the longitudinal axis of the column.

3. The apparatus according to claim 2, wherein the vane elements are inclined with respect to the longitudinal axis of the column at an angle of between 5 to 15 degrees.

4. The apparatus according to claim 1, wherein the cross sectional area of the gap narrows from the convergent inlet cone to the throat.

5. The apparatus according to claim 1, wherein the cross sectional area of the gap, in the vicinity of the throat, ranges from 5 to 30% of the cross sectional area of the throat.

6. A method for the removal of gaseous pollutants from an upwardly flowing gas stream comprising: a) directing an upwardly flowing gas stream through a vertical adsorption column having a convergent inlet cone, a throat and a divergent outlet cone; b) allowing said gas stream to branch into a major gas stream path through the interior of a cylindrical casing coaxially mounted in said throat and into a peripheral gas stream path in an annular gap between the casing and walls of said column, said gap between the casing and walls of said column being selected so as to produce a laminar peripheral gas stream; c) allowing said major gas stream to flow through a vortex generator mounted within the interior of said casing, whereby to generate a turbulent region in said outlet cone downstream from said casing; and d) introducing adsorbent material to said turbulent region via an inlet tube, said adsorbent material being dispersed throughout said turbulent region of said gas stream while being prevented from impinging upon said column walls by means of said laminar peripheral gas stream.

7. The method according to claim 6, wherein the retention time of the absorbent in the turbulent region is 2 to 20 times higher than the retention time of the adsorbent in the laminar peripheral stream.

8. The method according to claim 6, wherein the peripheral gas stream mixes with the turbulent region inwardly from the column walls, whereby to increase the scrubbing efficiency.

9. The method according to claim 6, wherein entrained adsorbent particles containing adsorbed impurities are subsequently separated from the gas stream.

10. The method according to claim 6, wherein the gas stream has a temperature ranging from 0 to 300⁰C and a density ranging from 0.6 to 1.5 kg/m³, and flows at a velocity ranging from 0.6 to 25 m/s within the throat.

11. Apparatus for the removal of gaseous pollutants from an upwardly flowing gas stream, substantially as described and illustrated.

12. Method for the removal of gaseous pollutants from an upwardly flowing gas stream, substantially as described and illustrated.