Solids discharge system with cooling means for pressurized fluid bed reactors

A system for cooling, depressurizing and discharging solids from a pressurized fluid bed reactor which includes a secondary fluid bed surge vessel arranged to dissipate a substantial amount of the pressure in the system and a plug flow discharge conduit leading from the secondary vessel to reduce the pressure at the outlet thereof to essentially ambient pressure.

This invention is directed to a novel system for essentially continuously 
discharging cooled particulate solids from a pressurized fluid bed reactor 
to ambient pressure. 
Many operations carried out in fluid bed reactors such a calcination and 
iron ore reduction and in systems such as pressurized fluid bed boilers 
are conducted at elevated pressure levels and require that hot solids must 
be discharged from the pressurized fluid bed reactor. The current method 
for discharging such solids is the lock-hopper system. This lock-hopper 
system is a batch type arrangement wherein a quantity of solids is first 
discharged from the fluid bed reactor into a pressurized vessel. After a 
predetermined amount of solids has entered the vessel, the solids inlet 
valve is closed off and the vessel is depressurized and then emptied. This 
cycle of pressurization, filling with the solids, depressurization and 
discharge of solids is repeated as required. The difficulties in this kind 
of operation increase as the pressure level within the reactor increases 
and with increasing hardness of the solids being discharged. 
Thus, the lock-hopper system has certain inherent shortcomings. For 
example, by definition it is a batch type operation wheras a continuous 
system would be much preferred. The system requires valves operable at 
elevated temperatures which are subject to sticking due to thermal 
expansion and seal failure under the severe operating conditions. Both the 
depressurizing valve and the vent lines are subject to severe abrasive 
conditions as the result of the rapid movement of hard solids therethrough 
under the influence of the differential pressure. Such systems are also 
subject to sequence control failures. 
A novel structure has now been provided for continuous discharge and 
cooling of solids from a pressurized fluid bed reactor, which incorporates 
a fluidized standpipe and secondary fluidized bed surge vessel as well as 
a plug flow discharge column to substantially obviate the necessity for 
high temperature valves in the discharge system. 
It is an object of this invention to provide an improved discharge system 
for a pressurized fluid bed reactor. 
It is a further object of this invention to provide a means for cooling and 
continuously discharging solids from a pressurized fluid bed reactor.

Generally speaking, the discharge and cooling system of the present 
invention is effective for pressurized fluid bed reactors operating at up 
to 150 psi and incorporates solids discharge from a main pressurized fluid 
bed reactor to a fluidized standpipe leading to a secondary fluid bed 
surge reactor or vessel at an elevation well above the fluid bed in the 
main reactor and a discharge conduit from the secondary surge reactor 
arranged to provide plug flow therein for further depressurization to 
approximately ambient pressure. 
In addition, the standpipe of the system is provided with cooling means. 
The level of the fluid bed in the pressurized main reactor is maintained 
constant by controlling the discharge of solids from the secondary surge 
reactor while the fluid bed level in the secondary surge reactor is 
maintained constant by controlling the amount of gas discharging from the 
system. 
Referring now to the drawing, the principal elements of the system 
illustrated are the main fluid bed reactor 10, the standpipe 40, the 
secondary surge reactor or vessel 50, the discharge depressurizing conduit 
71 and the scrubber system 90. Pressurized fluid bed reactor 10 has a 
metal shell 12 which may be lined with a layer of refractory material (not 
shown). The interior of reactor 10 is divided into two compartments by the 
constriction plate 14, with reaction chamber 16 above the constriction 
plate 14 and a smaller windbox 18 below the constriction plate. 
A gas inlet conduit 22 is provided for supplying fluidizing gases to the 
windbox 18. Gas from windbox 18 passing through the constriction plate 14 
fluidizes a body of particulate solids 19 which is located in reaction 
chamber 16 and rests on the constriction plate 14. Exhaust gases from the 
reaction chamber 16 leave the reactor through exhaust gas duct 26. Fuel is 
injected into the fluidized bed 19 through line 28 while solids may be 
introduced into the fluidized bed through solids inlet 32. Air is supplied 
to line 22 by the compressor 21. Solids are withdrawn from the fluidized 
bed 19 through discharge conduit 34 which has an emergency or maintenance 
valve 36 therein. The solids discharge conduit 34 is connected to the 
cooling standpipe system 40 at the bottom of the standpipe element 58. The 
standpipe 58 extends from below the level of the fluid bed 19 in the main 
reactor 10 to an elevation well above the top surface of fluid bed 19 
where it is connected to a secondary surge vessel 50 at the bottom 
thereof. The standpipe air conduit 46 is connected to line 22 to provide 
air to a series of air inlets 47, 48 and 49 in the standpipe. In practice, 
it is preferred that the standpipe 58 be of tapered configuration, being 
of relatively narrow diameter near the bottom thereof, and substantially 
greater in diameter at the top, proximate the junction with the surge 
vessel 50. The standpipe 58 is surrounded over a substantial portion of 
its length by a water jacket 59 in cooling relation to standpipe 58. Water 
is supplied to water jacket 59 by conduit 61, and, after circulation 
through the water jacket 59, is removed through discharge conduit 73 and 
is received in discharge trough 74, for withdrawal from the system. If 
desired, cooling coils may be employed instead of water jacket 59 for 
cooling the standpipe 58. 
The secondary reactor or surge vessel 50 has a metal shell 52 defining a 
reaction chamber 54. At the lower end thereof, secondary reactor shell 52 
has a conical portion 53 which narrows to join with the standpipe 58 at 
the upper extremity of the later. Fluidized bed 56, composed of 
particulate solids, is present in the reactor chamber 54. The off-gases 
from reaction chamber 54 are conducted to the cyclone 64 through conduit 
63. Cyclone 64 effects a separation between the gases and entrained solids 
and returns the solids to fluidized bed 56 through downcomer 66 which 
terminates within fluidized bed 56. The gases from cyclone 64 are 
forwarded to the scrubber system 90 through conduit 67. 
The solids discharge and depressurizing conduit 71 communicates with 
reaction chamber 54 of the secondary reactor 50, having an inclined 
portion 69 connected to the main vertical portion 71. A control valve 76 
is positioned in the vertical portion 71 of the solids discharge conduit. 
The scrubber system 90 comprises a venturi scrubber 94 and a scrubber 96 of 
the impingement tray type. A water inlet conduit 93 is in communication 
with the venturi scrubber 94. The exhaust gases from the scrubber 96 leave 
through an exhaust conduit 98 which is controlled by a valve 86. The 
liquids and the solids trapped thereby leave scrubber 96 through the lower 
outlet conduit 101 controlled by valve 112. 
The control arrangements for the system of the present invention will now 
be described. The level of the fluidized bed 19 in the main reactor 10 is 
controlled by means of valve 76 which regulates the discharge of solids 
from the secondary fluidized bed vessel 50 through solids discharge 
conduit 71. The control system for valve 76 comprises an upper probe 41 
and a lower probe 42 which detect pressures within the reaction chamber 16 
above bed 19 and within bed 19 itself. These pneumatic pressure readings 
are forwarded by transmitter 38 through electrical or pneumatic conduit 77 
to the level-recorder-controller 44. The level-recorder-controller 44 
responds to the differential pressure detected by the pressure probes 41 
and 42. 
When the differential pressure detected by probes 41 and 42 is greater than 
a predetermined level, this indicates that the bed 19 is higher than 
desired and level-recorder-controller 44 will actuate valve 76 to increase 
flow until the desired bed level is attained. This circuit operates in an 
opposite fashion when the bed level of fluidized bed 19 is too low 
wherupon valve 76 is actuated to decrease flow until the bed level is 
restored to that desired. 
As for the bed level of the fluid bed surge vessel 50, the electrically or 
pneumatically controlled valve 86 responds to a circuit quite similar to 
that described above having an upper probe 79 and a lower probe 81 
together with transmitter 82 and a level-recorder-controller 83. The 
electrical or pneumatic control valve 86 regulates the gas discharge from 
the scrubber system 90; that is, an increase in the air discharge rate 
from valve 86 permits an increase in the amount of solids entering the 
surge vessel 50, while a decrease in air discharge rate throttles down the 
quantity of solids entering the reaction chamber 54. 
A third control system entirely similar to those just described is provided 
for the scrubber system 90 to regulate the discharge of the scrubber 
effluent through an electrically or pneumatically controlled valve 112. 
This control system has upper probe 103 and lower probe 104 located in 
unit 96 with transmitter 106 and level-recorder-controller 111 located in 
circuit 108 which communicates with the valve 112. This is a common system 
used for scrubbers operating in pressurized systems. 
In the operation of a system in accordance with this invention, a 
pressurized fluidized bed 19 is maintained in the reaction chamber of the 
main fluid bed reactor 10. Air from compressor 21 is supplied through line 
22 to the windbox 18 of the reactor 10 from which it passes upwardly 
through the constriction plate 14 to fluidize the solids forming bed 19, 
with the exhaust gases leaving the reaction chamber through exhaust gas 
duct 26. Fuel or other liquids, slurries, gases or gas-blown fine solids 
required in the reaction chamber 16 may be introduced through line 28, 
while coarser solid constituents required within the reactor 10 may be 
introduced through conduit 32. The solids from reactor chamber 16 are 
discharged by means of inclined discharge pipe 34 into the vertical, 
water-cooled standpipe 58, at the top of which is the secondary surge 
vessel 50. Fluidizing air is introduced into the standpipe 58 and the 
surge vessel 50 through conduit 46 and air inlets 47, 48 and 49. The 
solids that flow into the standpipe 58 through discharge pipe 34 are 
fluidized within the standpipe 58 and, reaching reactor chamber 54 of the 
surge vessel 50, form a fluidized bed therein. If required, air inlets 
(not shown) may be provided for introducing air directly into the bottom 
of surge vessel 50 from line 22 to maintain fluidization in vessel 50. 
The flow of fluidizing air to the main reactor is kept essentially constant 
to satisfy the requirements of the process being carried out in the main 
reactor. However, the air flow to the standpipe 58 and the surge vessel 50 
is regulated by the control circuit including level-recorded-controller 
83, which senses the bed level in the surge vessel chamber 54. The output 
from level-recorder-controller 83 controls the operation of valve 86 at 
the gas discharge outlet of the scrubber unit 96, either increasing or 
decreasing the amount of air discharged from the system. As indicated 
previously, an increase in the air discharge from valve 86 increases the 
amount of solids entering the surge vessel 50 via the standpipe 58, while 
a decrease in air discharge rate decreases solids flow up the standpipe 
58. 
Standpipe 58 is made as high as practical to maximize depressurization in 
this portion of the system. With some materials, a fluid density of about 
70 lbs. per cubic foot at low space rates is possible in the standpipe, 
and this represents a very substantial delivery capacity. The standpipe 
should be of tapered design to maintain about uniform space rates. The 
static pressure differential in standpipe 58 is about 0.5 psig/ft. If, for 
example, the difference in elevation between the top of the fluid bed in 
the main reactor 10 and the surge vessel 50 is 150 feet, a pressure 
decrease of 75 psig is effected in the standpipe 58 and the surge vessel 
bed 56. The decrease in pressure in the standpipe as the gases move upward 
involves a substantial increase in gas volume. If the space rate is to be 
maintained, the standpipe must be tapered to accommodate the increasing 
gas volume. Further, a substantial amount of cooling is effected in the 
standpipe and in the surge vessel bed 56 by the fluidizing air and, in 
addition, the water jacket 59 surrounding standpipe 58 also produces 
considerable cooling of the particulate solids moving through standpipe 
58. The surge vessel bed 56 is the disengaging bed wherein the fluidizing 
gas is released from contact with the particulate solids and it also 
provides an inventory of solids to assure continuous operation of the 
system despite variations in the discharge flow permitted by valve 76. 
The pressure within the surge vessel 50 under the conditions just described 
is still elevated substantially above the ambient pressure. This pressure 
has, therefore, to be dissipated in the discharge conduit or plug flow 
column 71. The solid material in the conduit 71 is not fluidized but, 
instead, forms a plug of particulate solids moving more or less as a body 
down conduit 71 and through which gas from reaction chamber 54 flows. 
These gases are depressurized as they flow through the voids in the 
particles, thus minimizing the pressure drop across the discharge valve 
76. In a working system as described above, this vertical plug flow column 
71 will be quite long; for example, 25 feet is a practical minimum length, 
to allow relatively complete defluidization of the solids. 
There has thus been disclosed a relatively simplified method for cooling 
and depressurizing solids discharged from a pressurized fluid bed reactor 
system. 
Although the present invention has been described with particular reference 
to preferred embodiments, it will be apparent to those skilled in the art 
that variations and modifications may be made without departing from the 
essential spirit and scope of the invention. It is intended to include all 
such variations and modifications.