Abstract:
Disclosed is a third stage separator which includes two main clean gas outlets. One main clean gas outlet communicates with a power recovery unit such as an expander turbine while the second main clean gas outlet communicates with a conduit that bypasses the expander turbine. The present invention avoids use of the extra equipment, engineering and installation labor required to prevent the bypass conduit from placing a force load on the line to the power recovery unit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a novel arrangement for recovering power from a gas stream laden with solids. Specifically, the present invention relates to a third stage separator (TSS) vessel for removing catalyst fines from hot regenerator flue gas of a fluid catalytic cracking (FCC) unit followed by a power recovery unit.  
       BACKGROUND OF THE INVENTION  
       [0002]     FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.  
         [0003]     In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.  
         [0004]     The heat of combustion in the regenerator typically produces flue gas at temperatures of 718° to 760° C. (1325° to 1400° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient kinetic energy to warrant economic recovery. To recover energy from a flue gas stream, flue gas may be fed and directed into the blades of a power recovery expander turbine. The kinetic energy of the flue gas is transferred through the blades of the expander to a rotor coupled either to a regenerator air blower, to produce combustion air for the regenerator, and/or to a generator to produce electrical power. Because of the pressure drop of 138 to 207 kPa (20 to 30 psi) across the expander turbine, the flue gas discharges with a temperature drop of approximately 125° to 167° C. (225 to 300° F.). The flue gas may be run to a steam generator for further recovery.  
         [0005]     The power recovery train may include an expander turbine, a generator, an air blower, a gear reducer, and a let-down steam turbine. The expander turbine may be coupled to a main air blower shaft to power the air blower of a regenerator of the FCC unit. The expander turbine is a single stage machine. The gas to the expander turbine is accelerated over a parabolic nose cone. The pressure energy is converted to kinetic energy as the flue gas passes through the blades of the turbine. The blades of the expander turbine rotate at very high velocities necessitating measures to protect the blades from physical damage.  
         [0006]     A major distinguishing feature of an FCC process is the continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50 to 100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of catalyst with small particle diameters is the ongoing challenge to prevent this catalyst from exiting the reactor/regenerator system into effluent streams.  
         [0007]     Catalyst particles can cause erosion of expander turbine blades resulting in loss of power recovery efficiency. Moreover, even though catalyst fines; i.e., particles less than 10 μm in dimension, do not erode expander turbine blades as significantly, they still accumulate on the blades and casing. Blade accumulation can cause blade tip erosion and casing accumulation can increase the likelihood of the tip of the blade rubbing against the casing of the expander turbine which can result in high expander shaft vibration.  
         [0008]     Overall, the use of cyclone separators internal to both the reactor and regenerator has provided over 99% separation efficiency of solid catalyst. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normally sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to separate. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about 200 to 1000 mg/Nm 3 . This solids level can present difficulties related to the applicable legal emissions standards and are still high enough to risk damage to the power recovery expander turbine.  
         [0009]     A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS). The term “third” in TSS typically presumes a first stage cyclone and a second stage cyclone are used for gas-solid separation upstream of the inlet to the TSS. These cyclones are typically located in the catalyst regeneration vessel. More or less separator devices may be used upstream of the TSS. Hence, the term TSS does not require that no more nor less than two separator devices are upstream of the TSS vessel, herein. The TSS induces centripetal acceleration to a particle-laden gas, stream to force the higher-density solids to the outer edges of a spinning vortex. To be efficient, a cyclone separator for an FCC flue gas effluent will normally contain many, perhaps 100, small individual cylindrical cyclone bodies installed within a single vessel acting as a manifold. At least one tube sheet affixing the upper and/or lower ends of the cyclones act to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.  
         [0010]     Proper design of the gas delivery equipment is essential to protecting the power recovery system, particularly the blades of the expander. Cold wall piping. comprises a refractory lining on the inside of a metal pipe to insulate the pipe from the hot gas carried therein to minimize thermal expansion. Cold wall piping is not typically specified between the TSS vessel and the expander turbine inlet due to concerns of spalling refractory lining entering the expander turbine and damaging the blades. Hot wall piping, which may be made of stainless steel, without refractory lining thermally expands over five times as much as cold wall piping. The large thermal expansion associated with hot wall piping systems results in significantly higher piping loads that must be accommodated in the design of the piping components and equipment. Invariably, this leads to added cost for support and installation. Additionally, the rotor of the turbo expander turbine may not be allowed to exceed a maximum velocity or the blades could detach from the rotor.  
         [0011]     TSS vessels typically only have one main clean gas outlet in communication with the multiple main clean gas outlets of respective cyclones in the TSS vessel as shown in U.S. Pat. No. 5,690,709 and U.S. Pat. No. 5,779,746. GB 2 077 631A shows two clean gas outlets in the top hemispherical head of the TSS vessel. This reference discloses that the clean gas outlets may be connected to a power recovery turbine.  
       SUMMARY OF THE INVENTION  
       [0012]     The power recovery unit, which is usually an expander turbine, for recovering energy from a hot, pressurized gas stream provides extra power to other equipment when needed such as an air blower shaft or an electrical generator, or both. If the power recovery unit produces more energy than is required by the other equipment, the machine may act as a generator and feed power into the refinery power grid. Feeding power into the refinery power grid acts as a braking mechanism and provides some over-speed protection. Given an electrical breaker disconnect from the power grid, a fast acting over-speed valve and bypass conduit or line around the power recovery unit may be required to rapidly divert flue gas around the expander turbine to limit the rotational velocity of the expander turbine. Additionally, diverting a portion of the flue gas around the expander turbine through the bypass conduit may be necessary to control the pressure in the upstream catalyst regenerator. However, as the bypass valve opens, the flow of hot flue gas would cause the hot wall piping to rapidly heat up and thermally expand. The resultant pipe expansion would impose a great deal of force loading and rotational moment on the expander turbine inlet line. The loading and moment on the expander turbine inlet must be relatively small to ensure that the housing of the expander turbine does not deform which could promote the blades to brush with the inner surface of the casing. Additional equipment, engineering design and construction installation labor, would be required to ensure that expansion of the bypass conduit does not translate to a load on the expander turbine inlet line that is in excess of the nozzle loading limits.  
         [0013]     The present invention is a system for separating particulate solids from a contaminated gas stream and recovering energy from the contaminated gas stream, typically a hot flue gas stream from a catalyst regeneration vessel. A TSS vessel has a main inlet for receiving gas laden with solids. A plurality of cyclones in the TSS vessel separates the solids from the gas. A solids outlet from the TSS vessel dispenses solids from the TSS vessel and two main clean gas outlets remove clean gas from the TSS vessel. A TSS vessel may have a tube sheet that separates the inlet to the TSS vessel from the outlet from the TSS vessel. In an embodiment, the two main clean gas outlets extend from the TSS vessel below the tube sheet. A first main clean gas outlet from the TSS vessel delivers clean gas to a power recovery unit. A second main clean gas outlet from the TSS vessel is transported through a bypass conduit that bypasses the power recovery unit and mixes with the effluent clean gas from the power recovery unit.  
         [0014]     If the actual flowing volume of the clean gas in the main clean gas conduit exceeds a level at which the power recovery unit is rated, a valve in the bypass clean gas conduit is opened to a proportional degree, so a portion of the clean gas being directed to the power recovery unit can be re-directed to bypass the power recovery unit, and maintain proper pressure control of the FCC regenerator and avoid mechanical damage to the power recovery expander. The bypass clean gas conduit is anchored on the TSS vessel instead of on the main clean gas conduit to the power recovery unit, so sudden exposure of the bypass clean gas conduit to hot gases and its concomitant rapid thermal expansion will not suddenly impose a load or moment on the power recovery unit beyond allowance. Hence, equipment, engineering, and installation labor necessary for neutralizing such effects are not necessary. Moreover, because the bypass clean gas conduit does not join with a conduit to the power recovery unit, the bypass clean gas conduit may be lined with insulating refractory to minimize thermal expansion thereof without fear that spalling refractory will damage the power recovery unit.  
         [0015]     Accordingly, an object of the present invention is to provide a TSS vessel with a first main clean gas outlet that feeds a power recovery unit and a second main clean gas outlet that feeds a bypass conduit that bypasses the power recovery unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic view of the system of the present invention.  
         [0017]      FIG. 2  is a schematic view of a TSS vessel of the present invention.  
         [0018]      FIG. 3  is a schematic view of an alternative embodiment of a TSS vessel of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The present invention applies to the purification of a broad range of solid-contaminated gas streams, and especially those containing dust particles in the 1 to 20 μm range. A number of commercial gas purification operations meet this description, including the treatment of effluent streams of solid catalyst fluidized bed processes, coal fired heaters, and power plants. Several well-known refinery operations rely on fluidized bed technology, such as a preferred embodiment of the process for converting methanol to light olefins, as described in U.S. Pat. No. 6,137,022, using a solid catalyst composition. Another area of particular interest lies in the purification of FCC effluent streams that contain entrained catalyst particles resulting from attrition, erosion, and/or abrasion under process conditions within the reactor.  
         [0020]     As mentioned, fluid catalytic cracking (FCC) is a well-known oil refinery operation relied upon in most cases for gasoline production. Process variables typically include a cracking reaction temperature of 400° to 600° C. and a catalyst regeneration temperature of 500° to 900° C. Both the cracking and regeneration occur at an absolute pressure below 5 atmospheres.  FIG. 1  shows a typical FCC process unit of the prior art, where a heavy hydrocarbon feed or raw oil in a line  12  is contacted with a newly regenerated catalyst entering from a regenerated catalyst standpipe  14 . This contacting may occur in a narrow reactor conduit  16 , known as a reactor riser, extending upwardly to the bottom of a reactor vessel  10 . The contacting of feed and catalyst is fluidized by gas from a fluidizing line  8 . Heat from the catalyst vaporizes the oil, and the oil is thereafter cracked in the presence of the catalyst as both are transferred up the reactor conduit  16  into the reactor vessel  10  itself, operating at a pressure somewhat lower than that of the reactor conduit  16 . The cracked light hydrocarbon products are thereafter separated from the catalyst using a first stage internal reactor cyclone  18  and a second stage internal reactor cyclone (not shown) and exit the reactor vessel  10  through a line  22  to subsequent fractionation operations. More or less cyclones may be used in the reactor vessel  10 . At this point, some inevitable side reactions occurring in the reactor conduit  16  have left detrimental coke deposits on the catalyst that lower catalyst activity. The catalyst is therefore referred to as being spent (or at least partially spent) and requires regeneration for further use. Spent catalyst, after separation from the hydrocarbon product, falls into a stripping section  24  where steam is injected through a nozzle  26  to purge any residual hydrocarbon vapor. After the stripping operation, the spent catalyst is fed to a catalyst regeneration vessel  30  through a spent catalyst standpipe  32 .  
         [0021]      FIG. 1  depicts a regeneration vessel  30  known as a combustor. However, other types of regeneration vessels are suitable. In the catalyst regeneration vessel  30 , a stream of air is introduced through an air distributor  28  to contact the spent catalyst, burn coke deposited thereon, and provide regenerated catalyst. The catalyst regeneration process adds a substantial amount of heat to the catalyst, providing energy to offset the endothermic cracking reactions occurring in the reactor conduit  16 . Some fresh catalyst is added in a line  36  to the base of the catalyst regeneration vessel  30  to replenish catalyst exiting the reactor vessel  10  as fines material or entrained particles. Catalyst and air flow upward together along a combustor riser  38  located within the catalyst regeneration vessel  30  and, after regeneration (i.e. coke burn), are initially separated by discharge through a disengager  40 , also within the catalyst regeneration vessel  30 . Finer separation of the regenerated catalyst and flue gas exiting the disengager  40  is achieved using a first stage separator cyclone  44  and a second stage separator cyclone  46  within the catalyst regeneration vessel  30 . More or less separator cyclones may be used in the regeneration vessel  30 . Flue gas enters the first stage separator cyclone  44  through an inlet  44   a . Catalyst separated from flue gas dispenses through a dipleg  44   b  while flue gas relatively lighter in catalyst travels through a conduit  46   a  into the second stage separator cyclone  46 . Additional catalyst separated from the flue gas in the second stage separator cyclone  46  is dispensed into the catalyst regeneration vessel  30  through a dipleg  46   b  while flue gas relatively even lighter in solids exits the second stage separator cyclone  46  through an outlet tube  46   c . Regenerated catalyst is recycled back to the reactor vessel  10  through the regenerated catalyst standpipe  14 . As a result of the coke burning, the flue gas vapors exiting at the top of the catalyst regeneration vessel  30  in a nozzle  42  contain CO, CO 2  and H 2 O, along with smaller amounts of other species. While the first stage separator cyclone  44  and the second stage separator cyclone  46  can remove the vast majority of the regenerated catalyst from the flue gas in the nozzle  42 , fine catalyst particles, resulting mostly from attrition, invariably contaminate this effluent stream. The fines-contaminated flue gas therefore typically contains about 200 to 1000 mg/Nm 3  of particulates, most of which are less than 50 microns in diameter. In view of this contamination level, and considering both environmental regulations as well as the option to recover power from the flue gas, the incentive to further purify the relatively contaminated flue gas using a TSS vessel is significant. A conduit  48  delivers the contaminated flue gas to a TSS vessel  50 .  
         [0022]     The TSS vessel  50 , containing numerous individual cyclones  51 , that may be used in the present invention is shown in  FIG. 2 . Although only four cyclones  51  are shown in  FIG. 2 , at least 10 and as many as 200 cyclones  51  are anticipated for variously sized units. The cyclones  51  and the TSS vessel  50  need not include all the details disclosed herein to utilize the present invention. The TSS vessel  50  is normally lined with a refractory material  52  to reduce erosion of the metal surfaces by the entrained catalyst particles. The fines-contaminated flue gas from the catalyst regeneration vessel  30  enters the top of the TSS vessel  50  at a main contaminated gas inlet  54  through a nozzle  53 . The main contaminated gas inlet  54  is above an upper tube sheet  56  that retains top ends  58  of each cylindrical cyclone body  62 . In an embodiment, the upper tube sheet  56  at least in part defines an inlet chamber  57 , limits communication between the inlet chamber  57  and the rest of the TSS vessel  50  and/or extends the entire cross-section of the TSS vessel  50 ; A cover  56   a  of an optional manway provides access through the upper tube sheet  56  and assists in the aforementioned functions. An optional diffuser  55  may spread out the flow of contaminated flue gas into the TSS vessel  50 . The contaminated gas stream is then distributed among cyclone contaminated gas inlets  60  and encounters one or more swirl vanes  64  proximate the inlets  60  to induce centripetal acceleration of the particle-contaminated gas. The swirl vanes  64  are structures within the cylindrical cyclone body  62  that have the characteristic of restricting the passageway through which incoming gas can flow, thereby accelerating the flowing gas stream. The swirl vanes  64  also change the direction of the contaminated gas stream to provide a helical or spiral formation of gas flow through the length of the cylindrical cyclone body  62 . This spinning motion imparted to the gas sends the higher-density solid phase toward the wall of the cylindrical cyclone body  62 . The cyclones  51 , in an embodiment, include a closed bottom end  66  of the cylindrical cyclone body  62 . In an embodiment, slots in the cylindrical cyclone body  62  allow solid particles that have been thrown near the wall of the cylindrical cyclone body  62  to fall into a solids chamber  68  between the upper tube sheet  56  and a lower tube sheet  74 . The upper tube sheet  56  and the lower tube sheet  74  limit communication between the solids chamber  68  and the rest of the TSS vessel  50 . In an embodiment, the upper tube sheet  56  and the lower tube sheet  74  define at least in part the solids chamber  68 . The lower tube sheet  74  may extend the entire cross-section of the interior of the TSS vessel  50 . However, a solids outlet tube  76  allows solids to pass from the solids chamber  68 . In an embodiment, the solids outlet tube  76  extends from the TSS vessel  50  through an outlet  84  defined by a nozzle  83 . In an embodiment, the upper tube sheet  56  and/or the lower tube sheet  74  define an inverted cone to facilitate the exit of solids from the downward vertex of the conical lower tube sheet  74  at an inlet  75  to the solids outlet tube  76 . Clean gas, flowing along the centerline of the cylindrical cyclone body  62 , passes through an inlet  70  of a cyclone gas outlet tube  72 . The clean gas is then discharged via the cyclone gas outlet tube  72  below the lower tube sheet  74  into a clean gas chamber  78 . In an embodiment, the lower tube sheet  74  at least in part defines the clean gas chamber  78  and limits communication between the clean gas chamber  78  and the rest of the TSS vessel  50  and particularly the solids chamber  68 . The combined clean gas stream, representing the bulk of the flue gas fed to the TSS vessel  50 , then exits through one of a first main clean gas outlet  80  and a second main clean gas outlet  82  (shown in phantom in  FIG. 2 ) near the bottom of the TSS vessel  50 . Both main clean gas outlets  80 ,  82  may be defined by a first clean gas outlet nozzle  81  and a second clean gas outlet nozzle  83 , respectively. The first and second main clean gas outlets  80 ,  82  communicate only with the clean gas chamber  78 . In an embodiment, the first and second main clean gas outlets  80 ,  82  are below the upper and lower tube sheets  56 ,  74  and particularly below the lower tube sheet  74 . The first and second main clean gas outlet nozzles  81 ,  83  may extend from a vertical wall  86  of the TSS vessel  50 . Manways  88  to the TSS vessel  50  are covered during operation and allow access during maintenance and construction. Separated particles and a minor amount (typically less than 10 wt-% of the contaminated flue gas) of underflow gas are removed through a separate solids outlet  84  at the bottom of the TSS vessel  50 . A trash screen or grating (not shown) may be installed in the main clean gas outlets  80 ,  82  to block passage of spalling refractors.  
         [0023]     Turning back to  FIG. 1 , the clean gas exiting the first main clean gas outlet  80  travels in a power recovery inlet line  90  or conduit through a control valve  92  to a power recovery unit  94  through a power recovery inlet  93 . Clean gas outlets  80 ,  82  are shown schematically different in  FIG. 1  than in  FIG. 2  for purposes of illustration. The power recovery inlet line  90  is devoid of refractory lining. In an embodiment, the power recovery unit  94  is an expander turbine. A typical expander turbine has an outer casing  96  and a plurality of blades  98  fastened to a rotor (not shown). As the hot flue gas enters the power recovery unit  94  and accelerates over a parabolic nose cone  100 , the high velocity pressurized flue gas propels the blades  98  to turn at high velocity, turning a shaft  102 . The shaft  102  may be linked to a generator  104  through a gear box  106 . The flue gas exits the power recovery unit  94  through a power recovery outlet  99 . Although not shown, the shaft  102  may alternatively or additionally be connected to the main air blower that pumps air into the catalyst regeneration vessel  30  or other equipment on site. Power generated by the power recovery unit  94  in excess of that required to power the main air blower or other equipment is translated into electricity that feeds the power grid for the facility for which the TSS is a component or may be fed to another power grid. Although the power required to operate the main air blower or other equipment and to generate electricity in the generator  104  serves to resist excessive rotational speed of the blades  98 , other precautions must be taken to ensure proper pressure control of the catalyst regeneration vessel  30  and ensure that the expander blades  98  do not exceed a maximum speed which would cause damage to the power recovery unit  94 . Therefore, the second main clean gas outlet  82  feeds a bypass conduit  110  or line. The bypass conduit  110  passes through a control valve  112  and joins a power recovery outlet conduit  114  or line passing from the power recovery outlet  99 . A combined flue gas outlet line  116  carries the gas in the lines  110 ,  114  to the atmosphere or to further processing. The clean gas effluent from the TSS vessel  50  captures nearly 100% of particles having a dimension of greater than 10 microns and has an overall concentration of solids that meets the most stringent environmental protection regulations in the United States and internationally. A pressure indicator controller (PIC)  120  is linked to the control valves  92  on the power recovery inlet line  90  and the control valve  112  on the bypass conduit  110 . The PIC  120  will signal the control valve  92  first to control the pressure in the catalyst regeneration vessel  30  while the control valve  112  in the bypass conduit  1110  will be closed. However, if the control valve  92  is fully open to reduce the pressure in the catalyst regeneration vessel  30 , the control valve  112  in the bypass conduit  110  can be opened in an appropriate amount from the signal from the PIC  120  to ensure that the kinetic energy in the power recovery inlet line  90  will not cause the power recovery unit  94  to exceed its allowance rating.  
         [0024]     The solids retrieved from the TSS vessel  50  in the solids outlet  84  can be optionally taken by a line  122  to a fourth stage separator (not shown) to further remove underflow gas from catalyst and collect the catalyst in a spent catalyst hopper and/or the underflow gas may be delivered to other types of additional processing.  
         [0025]     The configuration of the present invention permits the bypass conduit  1   10  to be a refractory lined, cold wall line connected directly at an inlet end to the second main clean gas outlet  82  on the TSS vessel  50 . The piping design from the fixed foundation TSS vessel  50  to the inlet  93  of the power recovery unit  94  becomes a very elegant design. The transient loads applied to the inlet to the power recovery unit  94  associated with intermittently bypassing hot flue gas to the bypass conduit  110  are eliminated. The bypass conduit  110  becomes a much shorter, cold wall design, lowering the overall capital cost. The first main clean gas outlet  80  is in upstream fluid communication with the power recovery inlet  93  to the power recovery unit  94  through the power recovery inlet line  90  and the control valve  92 . The second main clean gas out  82  is not in downstream communication with the power recovery unit  94  but in upstream fluid communication with the power recovery outlet conduit  114 . The power recovery inlet  93  is in downstream fluid communication with the first main clean gas outlet  80  via the power recovery inlet line  90  and the control valve  92 , and the power recovery outlet  99  is in downstream fluid communication with the second main clean gas outlet  82  via the bypass conduit  110 , the control valve  112  and the power recovery outlet conduit  114 . In other words, the power recovery inlet  93  receives at least a portion of the clean gas effluent from the first main clean gas outlet  80 , but none of the clean gas effluent from the second main clean gas outlet  82 . Moreover, the flue gas outlet line  116  receives clean gas effluent from the second main clean gas outlet  82  and clean gas effluent from the first main clean gas outlet  80  via power recovery outlet  99 . The bypass conduit  1   10  and the power recovery outlet conduit  114  join together to deliver the two effluents to the flue gas outlet line  116 .  
         [0026]      FIG. 3  shows a TSS vessel  50 ′ as shown in  FIG. 2  but with a different main clean gas outlet and solids outlet configuration. All the reference numerals in  FIG. 3  will be the same as in  FIG. 2  unless the element designated by the reference numeral in  FIG. 3  is configured differently than in  FIG. 2 .  FIG. 3  shows a second main clean gas outlet  82 ′ that extends from the bottom of the TSS vessel  50 ′ instead of the second main clean gas outlet  82  shown in phantom in  FIG. 2  in the vertical wall  86  of the TSS vessel  50 . A solids outlet tube  76 ′ extending from the lower tube sheet  74  extends through the second main clean gas outlet  82 ′ defined by a nozzle  83 ′ and then diverges from a power recovery inlet line  90 ′. This configuration provides flexibility for incorporating the TSS vessel  50 ′ into a particular flow scheme. The second main clean gas outlet  82 ′ at the bottom of the TSS vessel  50 ′ may be in upstream fluid communication either with the bypass conduit  1   10  or the power recovery inlet line  90 ′. Additionally, the configuration in  FIG. 3  may be used when only one main clean gas outlet  82 ′ extends from the TSS vessel  50 ′ which may omit the first main clean gas outlet  80  shown in  FIG. 3 .  
         [0027]     Although it is not shown in the drawings, it is also contemplated that both main clean gas outlets may extend through or be contained in the same nozzle of the TSS vessel.