CA2054707C - Fluidized bed reactor using capped dual-sided contact units and method for use - Google Patents
Fluidized bed reactor using capped dual-sided contact units and method for use Download PDFInfo
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Abstract
A fluidized bed gas-solids contact reactor (9), which utilizes at least one capped concentric dual-sided riser-downcomer unit (10) having a central inner riser passageway (12) and a concentric outer downcomer passageway (14). The riser-downcomer unit (10) is usually located above the fluidized bed (11), and is adapted for directing the downflowing solids back into the fluidized bed for recycle. The reactor unit inner and outer passageway surfaces include a heat exchange panel having inner (13) and outer (15) channels each containing a circulating liquid. Particulate solids from the dense phase fluidized bed are continuously circulated in dilute phase through the riser-downcomer unit passageways by a reactant gas, such as air, to exchange heat with the liquid. The particulate solids can be a catalyst or a fuel material such as coal.
Description
s' 2 . pC.°T1U~90/02492 F~trzoaz~r~ sE~ ~~c~o~ ~rs=~c c~~~n o~~,-szo~D
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This invention pertains to a fluidized bed gas-solids contact reactor utilizing a dual-sided contactor unit having integr al concentric walls adapted for heat exchange with a liquid therein.
Tt pertains partieularly,to such a reactor using a capped dual-sided concentric riser-downcomer unit cowtaining..a liquid and located in a module, and which reactor is principally useful for comnusting fluidized particulate fuels such as coal to heat the lia_uid and generate saturated liquid or steam.
The use of fluidized beds has been recognized as an advantageous way of reacting gases and solids such as for generating heat, far example by use with heat exchanger tubes in boilers for generating pressurized steam from feed water passing in heat exchange relation with hot combustion gases from the fluidized bed of fuel. The fluidized bed utilizes a particulate carbonaceous fuel such as coal, and is fluadized by passing air upwardly through the bed to provide the Combustion reaction. Advantages of such fluidized bed combustion.
systerr,s include inereased,heat transfer rates, increase in ccrroustion efficiency, and reduction in boiler size.
.2.
One known form of a gas-solids contactor is disclosed by U.S.
latent No. 3,820,738 to Zenz, which disc'_oses a folded transfe_ line reactor for circulating particulate materials for fluidized catalytic cracking (FCC) units. However, such Bolded riser-dow ncomer configurations apparently have not been previously used for fluidized bed combustors for particulate fuels such as coal for ge-crating saturated vapors such as steam. Other types of fluidized bed combustors include U.S. Patent No. 3,914,235 to Highley which discloses a fluidized bed combustion apparatus utilizing internally circulating beds each surrounded by a heat exchange jacket. Also, U.S: Patent No. 4,240,377 to Johnson discloses'a fluidized bed boiler utilizing circulating solids, and U.S. Patent No. 4,539,939 to Johnson discloses fluidized bed combustion tubular boiler apparatus for combusting coal with limestone to generate steam.
These known large fluidized bed combustion boiler systems have been found to be undesirably complicated and it is difficult to control the particulate solids flow, and such systems also are quite expensive. Such disadvantages of the prior art reactors and combustion systems have now been advantageously overcome by the present invention, which provides an improved fluidized bed reactor apparatus and method which utilizes at least one central riser and concentric downcomer unit having dual sided heat exchange configurations and located above a fluidized bed for exchanging heat with a liquid in a reactor module.
f1'O 90/13772 PCT/i.J590/02492 1:, , 2~~4'~~7 SU1~TMARY ~F TNVENTI(~HT
The present invention provides a gas-solids contact reactor having at leapt one circulating dilute phase solids loop provided by a central capped riser and concentric downcomer unit located above a dense phase fluidized bed, and includes a method for using same.
The riser-downcomer unit has concentric inner and outer walls which each provide a cavity for containing a liquid for use with either exothermic or endothermic type reactions with gas and particulate solids passing through the unit. For exothermic reactions, the reactions between the gas and solids such as a combustion reaction heats the liquid. For endothermic reactions, the heated gas can be used either for treating the particulate solids, or alternatively the heated solids can be used for producing a reaction in the gas.
The invention also includes the gas-solids contact reactor used in an improved fluidized bed system and method for combusting particulate fuel solids to generate heated liquids or vapors, and which operates at relatively low combustion temperatures and provides high heat transfer efficiency to the liquids or vapors.
The invention utilizes a circulating loop of dilute phase particulate solids entrained upwardly from the dense phase.fluidized bed. The circulating solids loop is provided in at least ,one dual-sided concentric riser-downcomer unit adapted for handling dilute phase fluidized gas-solids therein, the unit having a capped inner riser passageway and a concentric outer downcomer passageway, so w s to provide a continuous folded passageway for continuous .flow and reaction of particulate solids and gases passing,through the unit. In the folded. passageways, the cross-sectional area is sized and flow ve~.ocities are controlled along with the particle temperature and residence time,:so that the gas and particles are substantially completely reacted during passage through the downcomer,passageway portion of the riser-downcomer unit above the fluidized bed.
The capped central riser and concentric outer downcomer passages of each.unit are each fprmed by two concentric tubes sealed together ~t each end, so as vto provide two inner and two. outer, walls each defining an intervening cavity or compartment therebetween to provide dual heat exchange panels, which are liquid filled. The particles are continually entrained upwardly through the central rise=
passageway by an upflowing secondary gas stream injected therein.
Heat transfer occurs predominantly by convection and radiation between the flowing gas-solids and the exposed walls of the riser-downcomer unit and the liquid contained in the dual compartments therein. The downcomer exit is configured for effective separation of the entraining gas frorn the downflowing particulate solids above the fluidized bed, so that the solids are effectively returned to the bed for recirculat.ion through the riser-downcomer unit. Such recirculation of particulate solids back to the fluidized bed may be effectively facilitated by a cylindrical skirt located radially outwardly from the <iowncomer passageway exit,~with the skirt having its lower portion immersed in the fluidized bed.
The gas-solids ,contactor unit and fluidized bed are enclosed within a casing or enclosure to form a module, which incorporates a plenum and flow distribution grid located below the fluidized bed for uniformly distributing the primary has flow upwardly through the shallow fluidized bed. The capped riser-downcomer unit embodies the concept of particle separation by impingement, and usually utilizes an increased velocity i.n the riser passageway as compared to the downcomer passageway. The cross-sectional area of the annular shaped downcomer passageway usually exceeds that of the central riser passageway by an area ratio in the range of 1.5:1 to 5:1, thus providing for reduced velocity and increased particle residence time in the downcomer passageway for effecaive complete reaction of the particles with the gas therein. The superficial gas velocity in the central riser passageway must be sufficient to_ entrain particles upwardly from the fluidized bed, and is usually 4.5-7.62 m/sec (15-25 ft/sec.), while the superficial gas velocity in the larger area outer downcomer passageway is.usual:Ly reduced to 1.52-4.57 m/sec (5-15 ft/sec).
More generally the upward superficial gas velocity in the central riser passageway should exceed the terminal or free fall velocity of the largest particle desired to be conveyed vertically upward, while the downwardly flowing superficial gas velocity in the outer downcomer passageway could in this extreme be only a function of the terminal or free fall velocity of the smallest size particle being circulated.
The total cross-sectional area of the fluidized bed should exceed that of the downcomer outermost tubular wall by an area ratio of 1.5:1 to 3:1. The configuration of each capped riser-downcomer unit will depend on its desired performance. The unit height and diameter are determined by the desired contact or residence time and throughput for the particulate solids, with the ratio of height to outer diameter being at least about 8:1 and usually not exceeding about 20:1. For example, riser height or overall length of the riser and downcomer passages may be based on complete combustion of average 500 micron size coal particles. The downcomer exit can be maintained above the fluidized bed upper level by a vertical distance equal to 0.75-5 times the radial width of the annular downcomer passageway, or the downcomer exit can be preferably submerged within the bed. The rate of circulating solids flowing through the riser-downcomer passages exceeds the feed rate of fresh solids to the fluidized bed by a recycle ratio of at least about 2:1, and usually not exceeding about 10:1 ratio. The riser-downcomer recirculating loop reduces bed height, greatly improves heat transfer by dual-sided exposure to high velocity solids, and reduces particle entrainment by lowering the velocity of the particles during their return to the shallow fluidized bed. The fluidized bed unit is designed for ease of fabrication, installation, cleaning and maintenance. A preferred design for a larger capacity reactor system can consist of multiple modules each containing a capped riser-downcomer unit arranged in parallel configuration in an assembly.
This invention also provides a method for contacting gas-solids in a reactor having a dense phase fluidized bed provided below a dilute phase capped riser-downcomer unit. This invention is useful for reacting a gas with particulate solids having a wide size range of 0.001 micron to 1.27 cm (0.50 inch). The invention can be used for burning coke deposits from catalyst particles as in the regeneration of petroleum cracking catalysts, for cooling or heating gases undergoing reactions in the presence of a conveyed solids or catalyst particles as in roasting of mineral ores, chlorination of rutile, or production of acrylonitrile or oxychlorination of hydrocarbons, or for combustion of particulate solids such as coal to generate heated liquids such as saturated steam. Temperature rise across the folded flow passageways of the contactor unit can be as small as 5.6°C (10°F) or as great at 1000°C
(1800°F) depending upon the process used. The invention is particularly useful for combusting particulate fuels such as coal, coke, and oil shales together with a particulate sorbent material such as limestone in a fluidized bed below a dual-sided riser-downcomer unit having a central riser passageway and a concentric outer downcomer passageway in a module.
This invention advantageously provides a compact and efficient gas-solids reactor or system and method for contacting gas and particulate solids either to exothermically heat a circulating liquid, or for endothermically heating the gas and solids by the circulating liquid. It particularly provides an improved method for combusting particulate fuels such as coal to heat a liquid to generate saturated vapors, such as for heating pressurized water to generate saturated steam. To provide desired high percentage turndowns for the reactor or system, one or more capped riser-downcomer units can be utilized and the turndown percentage adjusted by feedrate variation to each riser-downcomer unit.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be further described with reference to the following drawings, in which:
Fig. 1 shows a gas-solids contact reactor module having a capped central riser passageway surrounded by a concentric downcomer passageway provided above a fluidized bed within a housinq;
Fig. 2a and 2b are schematic views of two alternative configurations _7_ for the lower portion of the downcomer passageway relative to the fluidized bed:
Fig. 3 shows a partial perspective view of a grid device for flow distribution of gas upwardly into the fluidized bed of the reactor; and Fig. 4 shows a plan view of multiple reactor modules each containing a qas-solids contact unit accordinq to the invention.
DESCRIPTION OF INVENTION
A fluidized bed gas-solids reactor module including a single capped dual-sided circulating solids contactor unit is generally shown in Figure 1. The reactor module 9 consists of a shallow dense phase fluidized bed 11 and a central dilute phase capped riser-downcomer unit 10 having a central riser passageway 12 and a concentric outer downcomer passageway 14, which are centrally located above the fluidized bed 11 in an enclosure or vessel 20.
An inner compartment 13 containing a liquid is provided between passageways 12 and 14, and an outer compartment 15 also containing a liquid is provided surrounding downcomer passageway 14. The enclosure or vessel 20 can be made cylindrical or rectangular-shaped.
The downcomer passageway exit zone 14a is configured to provide effective separation of solids from the entraining gas and for directing the downflowing solids back into the fluidized bed 11 for return to the mouth of riser passageway 12 so as to promote solids recycle WO 90/13772 PCf/US90/02492 ~~ r ~' _~ t...~
. .. ,. . ,.
through the riser-downcomer unit 10. To help assure such recycle of particulate solids to the fluidized bed, a cylindrical baffle 16 , may be provided and located radially outwardly from downcomer exi=
14a at a location intermediate the exit 14a and the enclosure 20 inner wall, with the baffle lower portion 16a being immersed within fluidized bed 11 . The radial distance or spacing between the downcomer exit 14a and baffle 16 should be 1-2 times the maximum radial width of downcomer passageway 14, so as to provide a lower gas velocity within the baffle 16 than that in downcomer passageway 14. Also if desired, a plurality of vertical serrations or slots 14b can be provided circumferentially spaced apart around the lower end cf passageway 14 outer wall to facilitate the escape of gas radially outwardly and upwardly from the solids downflowing in downcome=
passageway 14.' , Fig. 2 shows two alternative configurations for the downcomer passageway exit zone 14a relative to upper level 11a of the fluidized bed ll. In Fig. 2(a) 'the fluidized bed upper leve1.11a is located above the lower end of the downcomer exit 14a. The lower portion of outer wall 15a can be flared outwardly by an angle of 0-45° with a vertical plane, so as to reduce downward velocity and~.facilitate separation of the downflowing gas from the particulate solids. Also if desired, substantially the entire outer wall 15a can be tapered outwardly and used in combination with the cylindrical,baffle 16, as shown by Fig. 2(b). .. , The riser-downcomer unit 10 is provided. with. dual concentric compartments l3 and l5 each formed by two con_aentric cylindrical wal7a. Thus,:thP passageways.~l2 and 14 are completely_liquid-lined on all sides for heat absorption or, heating, by a liquid,- with only °. limited heat removal or introduc~,ion to the .liquid,. being provided . via the shallow fluidized bed~ll.. The,inner..compartment '13 is contained betweew the two inner.:walls, the outer compartment,l5 is provided between the two outer walls, arid both compartments 13 and 15 are both connected to a source l7 of a heating or cooling liquid.
. Saturated liquid generated-in.::the .:compartments, l~ and l5.- from,; an ~xothermic:.reoction.;in- the;:riser..and ;downcomer passageways:: is , withdrawn at upper outlet conduit 1S . . ~. Structural ,support ,for .the _g_ riser-downcomer unit 10 within enclosure 20 can be advantageously provided by the central conduit 18, in combination with three lateral stabilizing struts :L9 provided near the lowey end of the unit 10 and extending between the unit and the walls of enclosure or vessel 20.
Each module 9 is: also provided with a source of primary reactant feed gas or air 21 into a plenum 22 and flow distributor 23 for fluidizinQ the bed I1_ and has a supply of secondary reactant gas or air at 24 upwardly into the riser passageway 12 for producing continous circulation of solids through the loop. Such dual gas entry is particularly advantageous to reactions in which feed gases should not be premixed because of explosive or uncontrolled side reactions, such as for air, ammonia or ethylene in their reaction to form acrylonitrile and in similar processes involved in pyridine chemistry and the production of niacins. Bed drain 26 can provide for removal of spent catalyst, or removal of hot ash and spent limestone from the fluidized bed 11. A primary cyclone separator 28 provides gas-solids separation from exit gases. Gas is withdrawn from the cyclone at outlet 30, and particulate solids are recycled via conduit 27 back to the fluidized bed 11 for further combustion.
During operation of the reactor as a fuel combustion unit, a particulate fuel such as crushed coal and a sorbent material such as limestone are fed to each module 9 at inlet 31 through cyclone dipleg 27. These materials are preferably fed at alternative feed location 32 adjacent: to the primary cyclone gas outlet 30. Such an arrangement for contacting of the cold solids feed material with hot 538'C+ (1000'F) flue gas from the fluidized bed heats the feed solids rapidly to 482'C+ (5~00'F) temperature, before the solids enter the shallow fluidized bed 11 of the reactor via the cyclone dipleg conduit 27. The primary air introduced into the reactor plenum 22 is uniformly distributed upwardly into the fluidized bed 11 by an apertured distribution grid 2:3. The grid 23 can be made substantially flat or conical-shaped, and preferably consists of multiple pieces of inverted metal angle 33 having a plurality of holes 33a oriented substantially horizontally therein, as generally shown by Fig. 3:
Introducing the pricr~ary gas laterally into the fluidized bed serves to prevent high upward velocities of .the particles and resulting _ 10_ erosion of the lower: end portion of compartment I3. The secondary air introduced into the bottom of the riser passageway I2 is used for transporting the fluidized bed particulate material continuously upwardly in the riser passageway 12, and also for providing the gas such as oxygen necessary for the reaction or combustion of the solids such as a fuel occurring in the riser and downcomer continuous passages.
The single entry for coal and limestone feed to the fluidized bed via the primary cyclone dipleg is oriented so as to preferential) y feed the fresh solids into the fluidized bed. Also, the reactor bed drain 26 is located as far as possible away from the cyclone dipleg 27 entry, so as to minimize bypass losses of fresh particulate coal and limestone from the fluidized bed II. The rate of gas flowing upwardly through the bed, needed to fluidize the bed, is dependent on the size and density of the particles comprising the bed, and should usually be 0. 9I-6.1 m/sec (3-20 ft/sec) . The superficial gas velocity of the secondary air upwardly into the riser passageway should be sufficient to entrain the dilute phase particulate solids therein from the dense phase fluidized bed, and should usually be 4.57-9.14 m/sec (I5-~30 ft/sec). Far coal combustion the fluidized bed temperature is usually 649-871'C (1200-1600'F) and the temperature rise of burning coa:L particles in the riser and downcomer passage is 2.8-55.6'C (5-100'F), as the exothermic heat of combustion is continuously removed., At the downcomer exit 14a, the downflowing gas reverses direction and rises upwardly towards gas exit 25, while the remaining uncombusted particles are returned by momentum and bed impingement downwardlv into the fluidized bed lI for further combustion and recycle to the riser passageway 12. Both the primary and secondary air streams are preferably preheated against warmer effluent streams,such as the reactor hot effluent has at~25.
Each module 9 can be conveniently sized for a desired capacity, such as for a combustion system burning coal and producing about 4536 kg/hr (10,000 lb/hr) of saturated steam. Each module is preferably square-shaped and arranged to be individually shop fabricated, and can be joined together with other adjacent modules.
for a particular installation. Multiples of module 9 can be used .11_ as reauired for a lai:ger capacity system, as shown by Fig. 4. Each module 9 can be separated from the adjacent module by a plenum partition wall 32a.
This invention will be further described in the following example, which should not be construed as limiting the scope of the invention.
_12_ A typical fluidi.zed bed combustion reactor module consists of a dense phase fluidize~d bed located in a vessel below a riser-downcomer unit. Crushed coal and limestone are fed into the fluidized bed through a conduit in a primary gas-solids cyclone separation unit for preheating the feed materials before they enter the shallow fluidized bed via a cyclone dipleg conduit. The riser and downcomer passages are formed by four concentric tubes and which provide an inner and an outer cavity filled with boiler feed water for heating the water to generate saturated steam. Primary air is distributed from a plenum uniformly upwardly into the fluidized bed by an apertured grid. Secondary air is injected upwardly into'the riser passageway at velocity sufficient to produce controlled velocity and particle residence time and vi.a recycle of solids to the fluidized bed provide substantially complete combustion of the coal.
Important characteristics of a typical reactor containing a fluidized bed below .a capped riser-downcomer unit for combustion of coal together with limestone are provided in Table 1 as follows:
Fluidized bec! depth, m (ft) 1.22 (4) Superficial gas velocity in fluidized bed, m/sec (ft/sec) 3.05 (10) Riser passageway height, m (ft) 6~.1 (20) Downcomer pa:~sageway height, m (ft) 5.5 (18) Cross-sectior.~al area ratio of downcomer to riser passageways 2:1 Superficial Bras velocity in riser, m/sec (ft/se;c) 6.1-9.14 (20-30) Superficial gas velocity in downcomer, m/sec (ft/sec) 3.05-4.57 (10-15) Height of downcomer exit above or below+0.46 to -0.46 fluidized bed upper level, m (ft) (+1.5 to -1.5) Fluidized bed. coal and limestone particles size, microns 200-700 W~ 90/13772 PC."T/US90/02A92 -13- w ~0~~~~7 The resulting flue gases axe passed through a cyclone separator for removal of solids which are recycled back to the fluidized bed, while ash and limestone solids are withdrawn from the bed lower portion.
Although this invention has been descxibed,braadly and in terms of preferred embodiments, it will be understood that modifications and variations can be made and that some features can be used without others all within the spirit and scope of the invention, which is defined by the following claims.
~orrTAO~ tTrrx~s .arm r~~soD Fox ~ ,5~~
$.~cx~~o~ o~ arrv~aTaoN
This invention pertains to a fluidized bed gas-solids contact reactor utilizing a dual-sided contactor unit having integr al concentric walls adapted for heat exchange with a liquid therein.
Tt pertains partieularly,to such a reactor using a capped dual-sided concentric riser-downcomer unit cowtaining..a liquid and located in a module, and which reactor is principally useful for comnusting fluidized particulate fuels such as coal to heat the lia_uid and generate saturated liquid or steam.
The use of fluidized beds has been recognized as an advantageous way of reacting gases and solids such as for generating heat, far example by use with heat exchanger tubes in boilers for generating pressurized steam from feed water passing in heat exchange relation with hot combustion gases from the fluidized bed of fuel. The fluidized bed utilizes a particulate carbonaceous fuel such as coal, and is fluadized by passing air upwardly through the bed to provide the Combustion reaction. Advantages of such fluidized bed combustion.
systerr,s include inereased,heat transfer rates, increase in ccrroustion efficiency, and reduction in boiler size.
.2.
One known form of a gas-solids contactor is disclosed by U.S.
latent No. 3,820,738 to Zenz, which disc'_oses a folded transfe_ line reactor for circulating particulate materials for fluidized catalytic cracking (FCC) units. However, such Bolded riser-dow ncomer configurations apparently have not been previously used for fluidized bed combustors for particulate fuels such as coal for ge-crating saturated vapors such as steam. Other types of fluidized bed combustors include U.S. Patent No. 3,914,235 to Highley which discloses a fluidized bed combustion apparatus utilizing internally circulating beds each surrounded by a heat exchange jacket. Also, U.S: Patent No. 4,240,377 to Johnson discloses'a fluidized bed boiler utilizing circulating solids, and U.S. Patent No. 4,539,939 to Johnson discloses fluidized bed combustion tubular boiler apparatus for combusting coal with limestone to generate steam.
These known large fluidized bed combustion boiler systems have been found to be undesirably complicated and it is difficult to control the particulate solids flow, and such systems also are quite expensive. Such disadvantages of the prior art reactors and combustion systems have now been advantageously overcome by the present invention, which provides an improved fluidized bed reactor apparatus and method which utilizes at least one central riser and concentric downcomer unit having dual sided heat exchange configurations and located above a fluidized bed for exchanging heat with a liquid in a reactor module.
f1'O 90/13772 PCT/i.J590/02492 1:, , 2~~4'~~7 SU1~TMARY ~F TNVENTI(~HT
The present invention provides a gas-solids contact reactor having at leapt one circulating dilute phase solids loop provided by a central capped riser and concentric downcomer unit located above a dense phase fluidized bed, and includes a method for using same.
The riser-downcomer unit has concentric inner and outer walls which each provide a cavity for containing a liquid for use with either exothermic or endothermic type reactions with gas and particulate solids passing through the unit. For exothermic reactions, the reactions between the gas and solids such as a combustion reaction heats the liquid. For endothermic reactions, the heated gas can be used either for treating the particulate solids, or alternatively the heated solids can be used for producing a reaction in the gas.
The invention also includes the gas-solids contact reactor used in an improved fluidized bed system and method for combusting particulate fuel solids to generate heated liquids or vapors, and which operates at relatively low combustion temperatures and provides high heat transfer efficiency to the liquids or vapors.
The invention utilizes a circulating loop of dilute phase particulate solids entrained upwardly from the dense phase.fluidized bed. The circulating solids loop is provided in at least ,one dual-sided concentric riser-downcomer unit adapted for handling dilute phase fluidized gas-solids therein, the unit having a capped inner riser passageway and a concentric outer downcomer passageway, so w s to provide a continuous folded passageway for continuous .flow and reaction of particulate solids and gases passing,through the unit. In the folded. passageways, the cross-sectional area is sized and flow ve~.ocities are controlled along with the particle temperature and residence time,:so that the gas and particles are substantially completely reacted during passage through the downcomer,passageway portion of the riser-downcomer unit above the fluidized bed.
The capped central riser and concentric outer downcomer passages of each.unit are each fprmed by two concentric tubes sealed together ~t each end, so as vto provide two inner and two. outer, walls each defining an intervening cavity or compartment therebetween to provide dual heat exchange panels, which are liquid filled. The particles are continually entrained upwardly through the central rise=
passageway by an upflowing secondary gas stream injected therein.
Heat transfer occurs predominantly by convection and radiation between the flowing gas-solids and the exposed walls of the riser-downcomer unit and the liquid contained in the dual compartments therein. The downcomer exit is configured for effective separation of the entraining gas frorn the downflowing particulate solids above the fluidized bed, so that the solids are effectively returned to the bed for recirculat.ion through the riser-downcomer unit. Such recirculation of particulate solids back to the fluidized bed may be effectively facilitated by a cylindrical skirt located radially outwardly from the <iowncomer passageway exit,~with the skirt having its lower portion immersed in the fluidized bed.
The gas-solids ,contactor unit and fluidized bed are enclosed within a casing or enclosure to form a module, which incorporates a plenum and flow distribution grid located below the fluidized bed for uniformly distributing the primary has flow upwardly through the shallow fluidized bed. The capped riser-downcomer unit embodies the concept of particle separation by impingement, and usually utilizes an increased velocity i.n the riser passageway as compared to the downcomer passageway. The cross-sectional area of the annular shaped downcomer passageway usually exceeds that of the central riser passageway by an area ratio in the range of 1.5:1 to 5:1, thus providing for reduced velocity and increased particle residence time in the downcomer passageway for effecaive complete reaction of the particles with the gas therein. The superficial gas velocity in the central riser passageway must be sufficient to_ entrain particles upwardly from the fluidized bed, and is usually 4.5-7.62 m/sec (15-25 ft/sec.), while the superficial gas velocity in the larger area outer downcomer passageway is.usual:Ly reduced to 1.52-4.57 m/sec (5-15 ft/sec).
More generally the upward superficial gas velocity in the central riser passageway should exceed the terminal or free fall velocity of the largest particle desired to be conveyed vertically upward, while the downwardly flowing superficial gas velocity in the outer downcomer passageway could in this extreme be only a function of the terminal or free fall velocity of the smallest size particle being circulated.
The total cross-sectional area of the fluidized bed should exceed that of the downcomer outermost tubular wall by an area ratio of 1.5:1 to 3:1. The configuration of each capped riser-downcomer unit will depend on its desired performance. The unit height and diameter are determined by the desired contact or residence time and throughput for the particulate solids, with the ratio of height to outer diameter being at least about 8:1 and usually not exceeding about 20:1. For example, riser height or overall length of the riser and downcomer passages may be based on complete combustion of average 500 micron size coal particles. The downcomer exit can be maintained above the fluidized bed upper level by a vertical distance equal to 0.75-5 times the radial width of the annular downcomer passageway, or the downcomer exit can be preferably submerged within the bed. The rate of circulating solids flowing through the riser-downcomer passages exceeds the feed rate of fresh solids to the fluidized bed by a recycle ratio of at least about 2:1, and usually not exceeding about 10:1 ratio. The riser-downcomer recirculating loop reduces bed height, greatly improves heat transfer by dual-sided exposure to high velocity solids, and reduces particle entrainment by lowering the velocity of the particles during their return to the shallow fluidized bed. The fluidized bed unit is designed for ease of fabrication, installation, cleaning and maintenance. A preferred design for a larger capacity reactor system can consist of multiple modules each containing a capped riser-downcomer unit arranged in parallel configuration in an assembly.
This invention also provides a method for contacting gas-solids in a reactor having a dense phase fluidized bed provided below a dilute phase capped riser-downcomer unit. This invention is useful for reacting a gas with particulate solids having a wide size range of 0.001 micron to 1.27 cm (0.50 inch). The invention can be used for burning coke deposits from catalyst particles as in the regeneration of petroleum cracking catalysts, for cooling or heating gases undergoing reactions in the presence of a conveyed solids or catalyst particles as in roasting of mineral ores, chlorination of rutile, or production of acrylonitrile or oxychlorination of hydrocarbons, or for combustion of particulate solids such as coal to generate heated liquids such as saturated steam. Temperature rise across the folded flow passageways of the contactor unit can be as small as 5.6°C (10°F) or as great at 1000°C
(1800°F) depending upon the process used. The invention is particularly useful for combusting particulate fuels such as coal, coke, and oil shales together with a particulate sorbent material such as limestone in a fluidized bed below a dual-sided riser-downcomer unit having a central riser passageway and a concentric outer downcomer passageway in a module.
This invention advantageously provides a compact and efficient gas-solids reactor or system and method for contacting gas and particulate solids either to exothermically heat a circulating liquid, or for endothermically heating the gas and solids by the circulating liquid. It particularly provides an improved method for combusting particulate fuels such as coal to heat a liquid to generate saturated vapors, such as for heating pressurized water to generate saturated steam. To provide desired high percentage turndowns for the reactor or system, one or more capped riser-downcomer units can be utilized and the turndown percentage adjusted by feedrate variation to each riser-downcomer unit.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be further described with reference to the following drawings, in which:
Fig. 1 shows a gas-solids contact reactor module having a capped central riser passageway surrounded by a concentric downcomer passageway provided above a fluidized bed within a housinq;
Fig. 2a and 2b are schematic views of two alternative configurations _7_ for the lower portion of the downcomer passageway relative to the fluidized bed:
Fig. 3 shows a partial perspective view of a grid device for flow distribution of gas upwardly into the fluidized bed of the reactor; and Fig. 4 shows a plan view of multiple reactor modules each containing a qas-solids contact unit accordinq to the invention.
DESCRIPTION OF INVENTION
A fluidized bed gas-solids reactor module including a single capped dual-sided circulating solids contactor unit is generally shown in Figure 1. The reactor module 9 consists of a shallow dense phase fluidized bed 11 and a central dilute phase capped riser-downcomer unit 10 having a central riser passageway 12 and a concentric outer downcomer passageway 14, which are centrally located above the fluidized bed 11 in an enclosure or vessel 20.
An inner compartment 13 containing a liquid is provided between passageways 12 and 14, and an outer compartment 15 also containing a liquid is provided surrounding downcomer passageway 14. The enclosure or vessel 20 can be made cylindrical or rectangular-shaped.
The downcomer passageway exit zone 14a is configured to provide effective separation of solids from the entraining gas and for directing the downflowing solids back into the fluidized bed 11 for return to the mouth of riser passageway 12 so as to promote solids recycle WO 90/13772 PCf/US90/02492 ~~ r ~' _~ t...~
. .. ,. . ,.
through the riser-downcomer unit 10. To help assure such recycle of particulate solids to the fluidized bed, a cylindrical baffle 16 , may be provided and located radially outwardly from downcomer exi=
14a at a location intermediate the exit 14a and the enclosure 20 inner wall, with the baffle lower portion 16a being immersed within fluidized bed 11 . The radial distance or spacing between the downcomer exit 14a and baffle 16 should be 1-2 times the maximum radial width of downcomer passageway 14, so as to provide a lower gas velocity within the baffle 16 than that in downcomer passageway 14. Also if desired, a plurality of vertical serrations or slots 14b can be provided circumferentially spaced apart around the lower end cf passageway 14 outer wall to facilitate the escape of gas radially outwardly and upwardly from the solids downflowing in downcome=
passageway 14.' , Fig. 2 shows two alternative configurations for the downcomer passageway exit zone 14a relative to upper level 11a of the fluidized bed ll. In Fig. 2(a) 'the fluidized bed upper leve1.11a is located above the lower end of the downcomer exit 14a. The lower portion of outer wall 15a can be flared outwardly by an angle of 0-45° with a vertical plane, so as to reduce downward velocity and~.facilitate separation of the downflowing gas from the particulate solids. Also if desired, substantially the entire outer wall 15a can be tapered outwardly and used in combination with the cylindrical,baffle 16, as shown by Fig. 2(b). .. , The riser-downcomer unit 10 is provided. with. dual concentric compartments l3 and l5 each formed by two con_aentric cylindrical wal7a. Thus,:thP passageways.~l2 and 14 are completely_liquid-lined on all sides for heat absorption or, heating, by a liquid,- with only °. limited heat removal or introduc~,ion to the .liquid,. being provided . via the shallow fluidized bed~ll.. The,inner..compartment '13 is contained betweew the two inner.:walls, the outer compartment,l5 is provided between the two outer walls, arid both compartments 13 and 15 are both connected to a source l7 of a heating or cooling liquid.
. Saturated liquid generated-in.::the .:compartments, l~ and l5.- from,; an ~xothermic:.reoction.;in- the;:riser..and ;downcomer passageways:: is , withdrawn at upper outlet conduit 1S . . ~. Structural ,support ,for .the _g_ riser-downcomer unit 10 within enclosure 20 can be advantageously provided by the central conduit 18, in combination with three lateral stabilizing struts :L9 provided near the lowey end of the unit 10 and extending between the unit and the walls of enclosure or vessel 20.
Each module 9 is: also provided with a source of primary reactant feed gas or air 21 into a plenum 22 and flow distributor 23 for fluidizinQ the bed I1_ and has a supply of secondary reactant gas or air at 24 upwardly into the riser passageway 12 for producing continous circulation of solids through the loop. Such dual gas entry is particularly advantageous to reactions in which feed gases should not be premixed because of explosive or uncontrolled side reactions, such as for air, ammonia or ethylene in their reaction to form acrylonitrile and in similar processes involved in pyridine chemistry and the production of niacins. Bed drain 26 can provide for removal of spent catalyst, or removal of hot ash and spent limestone from the fluidized bed 11. A primary cyclone separator 28 provides gas-solids separation from exit gases. Gas is withdrawn from the cyclone at outlet 30, and particulate solids are recycled via conduit 27 back to the fluidized bed 11 for further combustion.
During operation of the reactor as a fuel combustion unit, a particulate fuel such as crushed coal and a sorbent material such as limestone are fed to each module 9 at inlet 31 through cyclone dipleg 27. These materials are preferably fed at alternative feed location 32 adjacent: to the primary cyclone gas outlet 30. Such an arrangement for contacting of the cold solids feed material with hot 538'C+ (1000'F) flue gas from the fluidized bed heats the feed solids rapidly to 482'C+ (5~00'F) temperature, before the solids enter the shallow fluidized bed 11 of the reactor via the cyclone dipleg conduit 27. The primary air introduced into the reactor plenum 22 is uniformly distributed upwardly into the fluidized bed 11 by an apertured distribution grid 2:3. The grid 23 can be made substantially flat or conical-shaped, and preferably consists of multiple pieces of inverted metal angle 33 having a plurality of holes 33a oriented substantially horizontally therein, as generally shown by Fig. 3:
Introducing the pricr~ary gas laterally into the fluidized bed serves to prevent high upward velocities of .the particles and resulting _ 10_ erosion of the lower: end portion of compartment I3. The secondary air introduced into the bottom of the riser passageway I2 is used for transporting the fluidized bed particulate material continuously upwardly in the riser passageway 12, and also for providing the gas such as oxygen necessary for the reaction or combustion of the solids such as a fuel occurring in the riser and downcomer continuous passages.
The single entry for coal and limestone feed to the fluidized bed via the primary cyclone dipleg is oriented so as to preferential) y feed the fresh solids into the fluidized bed. Also, the reactor bed drain 26 is located as far as possible away from the cyclone dipleg 27 entry, so as to minimize bypass losses of fresh particulate coal and limestone from the fluidized bed II. The rate of gas flowing upwardly through the bed, needed to fluidize the bed, is dependent on the size and density of the particles comprising the bed, and should usually be 0. 9I-6.1 m/sec (3-20 ft/sec) . The superficial gas velocity of the secondary air upwardly into the riser passageway should be sufficient to entrain the dilute phase particulate solids therein from the dense phase fluidized bed, and should usually be 4.57-9.14 m/sec (I5-~30 ft/sec). Far coal combustion the fluidized bed temperature is usually 649-871'C (1200-1600'F) and the temperature rise of burning coa:L particles in the riser and downcomer passage is 2.8-55.6'C (5-100'F), as the exothermic heat of combustion is continuously removed., At the downcomer exit 14a, the downflowing gas reverses direction and rises upwardly towards gas exit 25, while the remaining uncombusted particles are returned by momentum and bed impingement downwardlv into the fluidized bed lI for further combustion and recycle to the riser passageway 12. Both the primary and secondary air streams are preferably preheated against warmer effluent streams,such as the reactor hot effluent has at~25.
Each module 9 can be conveniently sized for a desired capacity, such as for a combustion system burning coal and producing about 4536 kg/hr (10,000 lb/hr) of saturated steam. Each module is preferably square-shaped and arranged to be individually shop fabricated, and can be joined together with other adjacent modules.
for a particular installation. Multiples of module 9 can be used .11_ as reauired for a lai:ger capacity system, as shown by Fig. 4. Each module 9 can be separated from the adjacent module by a plenum partition wall 32a.
This invention will be further described in the following example, which should not be construed as limiting the scope of the invention.
_12_ A typical fluidi.zed bed combustion reactor module consists of a dense phase fluidize~d bed located in a vessel below a riser-downcomer unit. Crushed coal and limestone are fed into the fluidized bed through a conduit in a primary gas-solids cyclone separation unit for preheating the feed materials before they enter the shallow fluidized bed via a cyclone dipleg conduit. The riser and downcomer passages are formed by four concentric tubes and which provide an inner and an outer cavity filled with boiler feed water for heating the water to generate saturated steam. Primary air is distributed from a plenum uniformly upwardly into the fluidized bed by an apertured grid. Secondary air is injected upwardly into'the riser passageway at velocity sufficient to produce controlled velocity and particle residence time and vi.a recycle of solids to the fluidized bed provide substantially complete combustion of the coal.
Important characteristics of a typical reactor containing a fluidized bed below .a capped riser-downcomer unit for combustion of coal together with limestone are provided in Table 1 as follows:
Fluidized bec! depth, m (ft) 1.22 (4) Superficial gas velocity in fluidized bed, m/sec (ft/sec) 3.05 (10) Riser passageway height, m (ft) 6~.1 (20) Downcomer pa:~sageway height, m (ft) 5.5 (18) Cross-sectior.~al area ratio of downcomer to riser passageways 2:1 Superficial Bras velocity in riser, m/sec (ft/se;c) 6.1-9.14 (20-30) Superficial gas velocity in downcomer, m/sec (ft/sec) 3.05-4.57 (10-15) Height of downcomer exit above or below+0.46 to -0.46 fluidized bed upper level, m (ft) (+1.5 to -1.5) Fluidized bed. coal and limestone particles size, microns 200-700 W~ 90/13772 PC."T/US90/02A92 -13- w ~0~~~~7 The resulting flue gases axe passed through a cyclone separator for removal of solids which are recycled back to the fluidized bed, while ash and limestone solids are withdrawn from the bed lower portion.
Although this invention has been descxibed,braadly and in terms of preferred embodiments, it will be understood that modifications and variations can be made and that some features can be used without others all within the spirit and scope of the invention, which is defined by the following claims.
Claims (17)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A fluidized bed gas-solids reactor, comprising a vessel having a reaction chamber provided in its lower portion containing a dense phase fluidized bed of particulate solids material; means for feeding fresh particulate solids material into the fluidized bed in the reaction chamber; distributor means for introducing primary gas upwardly into the fluidized bed and means for introducing secondary gas upwardly into central riser passageway and a cyclone separator flow connected to the vessel upper end portion for outward passage of gases and entrained solids therethrough, whereby particulate solids can be fed into the fluidized bed and circulated in dilute phase through passageways of a dual-sided riser-downcomer unit in heat exchange relation with panel walls thereof and with liquid therein, and the particulate solids collected in the gas-solids cyclone separator can be recycled back to the fluidized bed;
characterized by said riser-downcomer unit having said central riser passageway flow connected to a concentric outer downcomer passageway, said unit extending substantially vertically within said vessel with the downcomer passageway exit being located near the upper level of the fluidized bed, and being configured for directing downflowing particulate solids from the downcomer passageway back to the fluidized bed, said riser-downcomer unit including dual concentric compartments each forming heat exchange panel means provided on its adjacent walls for containing a liquid.
characterized by said riser-downcomer unit having said central riser passageway flow connected to a concentric outer downcomer passageway, said unit extending substantially vertically within said vessel with the downcomer passageway exit being located near the upper level of the fluidized bed, and being configured for directing downflowing particulate solids from the downcomer passageway back to the fluidized bed, said riser-downcomer unit including dual concentric compartments each forming heat exchange panel means provided on its adjacent walls for containing a liquid.
2. A gas-solids reactor according to claim 1, wherein a cylindrical-shaped baffle is provided spaced outwardly from said downcomer passageway exit for directing the downflowing particulate solids back to the fluidized bed.
3. A gas-solids reactor according to claim 1, wherein said distributor means includes a plurality of flow conduits having openings which are oriented substantially horizontally for supplying the primary gas into said fluidized bed.
4. A gas-solids reactor according to claim 1, including means for introducing a liquid into a lower portion of said heat exchange panel means, and means provided at the upper end of the panel for withdrawing heated or cooled fluid from the panel means.
5. A gas-solids reactor according to claim 1, wherein the cross-sectional area of the downcomer passageway exceeds that of the riser passageway by a ratio in the range of 1.5:1 to 3:1.
6. A gas-solids reactor according to claim 1, wherein the ratio of height to outer diameter for the riser-downcomer unit is between 8:1 and 20:1.
7. A gas-solids reactor according to claim 1, wherein the fluidized bed upper level is maintained above the downcomer passageway exit by a distance equal to 0.75-5 times the radial width of the downcomer passageway.
8. A gas-solids reactor according to claim 1, wherein the particulate solids feed means is arranged in heat exchange relation with the gases passing outwardly through said cyclone separator, so that the feed particles can be combined with the solid particles being recycled from the cyclone back to the fluidized bed.
9. A gas-solids reactor according to claim 1 or 4, including means for solids withdrawal from the lower portion of the fluidized bed.
10. A gas-solids reactor according to claim 1, wherein multiple modules each containing a riser-downcomer unit are combined to provide a larger capacity reactor system.
11. A gas-solids reactor according to claim 10, wherein the spacing between adjacent said riser-downcomer units is 1.5-2.5 times the unit outer diameter.
12. A method for reacting a gas and particulate solids in a fluidized bed reaction system, comprising feeding particulate reactive or catalytic solids into a dense phase fluidized bed located in the lower portion of a vessel, feeding a primary gas upwardly into the fluidized bed to fluidize the bed, characterized by at least the dual-sided riser-downcomer contact unit being located above the fluidized bed, said riser-downcomer unit including dual concentric compartments each forming heat exchange panel means provided on its adjacent walls for containing a liquid, and said unit having a central riser passageway and a concentric outer downcomer passageway, and feeding a secondary gas upwardly into the riser passageway to entrain particles from the fluidized bed into the riser passageway; feeding vaporizable liquid into the lower portion of a heat exchange panel included in said dual-sided riser-downcomer unit; continuously passing a portion of the particulate solids in dilute phase upwardly from said fluidized bed through said central riser passageway and then downwardly through said concentric outer passageway back to the fluidized bed at a temperature and flow rate selected to substantially completely react the feed gases with the solids; and heating the liquids in said panel to generate a saturated liquid, and withdrawing the saturated liquid from the upper portion of the panel.
13. The gas-solids reaction method of claim 12, wherein the superficial upward gas velocity in the riser passageway is 4.57-9.14 m/sec (15-30 ft/sec), and the superficial gas velocity in the downcomer passageway is 1.52-4.57 m/sec(5-15 ft/sec).
14. The gas-solids reaction method of claim 12, wherein the recycle ratio of solids circulating through the riser-downcomer passages exceeds the fresh solids feed rate by a ratio of at least 2:1.
15. The gas-solids reaction method of claim 12, wherein solids exiting the downcomer passageway are substantially returned to the fluidized bed, and gases are passed to a cyclone gas-solids separator from which particulate solids are returned to the fluidized bed.
16. The gas-solids reaction method of claim 12 or 13, wherein the particulate solids are fuel solids and a sorbent material for sulfur removal, the primary gas is air, and the vaporizable liquid is water.
17. The gas-solids reaction method of claim 16, wherein the fuel solids are coal and the sorbent material is limestone, including the step of withdrawing unburned fuel ash and limestone from the lower portion of said fluidized bed.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/348,838 US4979265A (en) | 1988-05-06 | 1989-05-08 | Wide angle hinge with gear drive |
| US348,848 | 1989-05-09 | ||
| PCT/US1990/002492 WO1990013772A1 (en) | 1989-05-08 | 1990-05-04 | Fluidized bed reactor using capped dual-sided contact units and method for use |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2054707A1 CA2054707A1 (en) | 1990-11-10 |
| CA2054707C true CA2054707C (en) | 2000-07-11 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002054707A Expired - Lifetime CA2054707C (en) | 1989-05-08 | 1990-05-04 | Fluidized bed reactor using capped dual-sided contact units and method for use |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2054707C (en) |
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1990
- 1990-05-04 CA CA002054707A patent/CA2054707C/en not_active Expired - Lifetime
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| Publication number | Publication date |
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| CA2054707A1 (en) | 1990-11-10 |
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