Circulation loop for carrying out two-stage reactions

A method of controlling the solids circulation between a downflow reactor and an entrained bed reactor is disclosed wherein at least some of the solids are transferred from the downflow reactor to a crossflow fluidized bed through a first seal leg, wherein the crossflow fluidized bed has a baffle separating the crossflow fluidized bed into two zones. Then the solids are transferred from the crossflow fluidized bed to the entrained bed reactor, and the solids are transferred from the entrained bed reactor to the downflow reactor through a second seal leg. The solids circulation rate is controlled by adjusting the rate of fluidizing gas entering the crossflow fluidized bed.

BACKGROUND OF THE INVENTION 
The present invention involves a method of controlling the solids 
circulation between a downflow reactor and an entrained bed reactor. 
In view of recent increases in the price of crude oil, researchers have 
been searching for alternative sources of energy and hydrocarbons. Much 
research has focused on recovering the hydrocarbons from 
hydrocarbon-containing solids such as shale, tar sand or coal by heating 
or pyrolysis to boil off or liquefy the hydrocarbons trapped in the solid 
or by reacting the solid with steam, for example, to convert components of 
solid carbonaceous material into more readily usable gaseous and liquid 
hydrocarbons. Other known processes involve combustion of the solid 
carbonaceous materials with an oxygen-containing gas to generate heat. 
Such processes conventionally employ a treatment zone, e.g., a reaction 
vessel, in which the solid is heated or reacted. 
In a typical coal gasification process, coal is contacted with steam and an 
oxygen-containing gas to produce a gaseous product. 
When air is used as the oxygen-containing gas, the gaseous product contains 
high levels of nitrogen, which reduces the BTU content of the gaseous 
product. Some processes have used pure oxygen instead of air, in order to 
avoid having nitrogen in the gaseous product. This does eliminate the 
nitrogen from the product but it requires a source of pure oxygen, some 
oxygen plants are almost as large as the coal gasification plant they are 
supplying. Thus, one was faced with the alternatives of either produccing 
a gaseous product diluted with nitrogen or finding a source of pure oxygen 
for their process. 
Another solution to the nitrogen dilution problem is disclosed in U.S. Pat. 
No. 4,157,245. In one embodiment of the invention disclosed in that 
patent, a solid heat-transfer material, such as sand, is introduced into 
an upper portion of a reaction vessel and coal is introduced into a lower 
portion of the vessel. The physical characteristics of the heat-transfer 
material and the coal differ such that a superficial velocity of a fluid 
flowing upwardly through the vessel is greater than the minimum fluidizing 
velocity of the heat-transfer material and the terminal velocity of the 
coal, but is less than the terminal velocity of the heat-transfer 
material. A substantially countercurrent vertical flow of the two solids 
is maintained in the vessel without substantial top-to-bottom backmixing 
by passing steam upwardly through the vessel at a rate sufficient to 
fluidize the heat-transfer material and entrain the coal whereby the 
heat-transfer material substantially flows downwardly in a fluidized state 
through the vessel and the coal substantially flows upwardly in an 
entrained state through the vessel. The steam reacts with the coal to form 
a hot char and a gaseous product. The heat-transfer material acts as a 
source of heat for the reaction between the steam and the coal. Cooled 
heat-transfer material is removed from a lower end of the vessel and the 
hot char and the gaseous product are removed from an upper end of the 
vessel. The gaseous product is then separated from the hot char by regular 
separation techniques. 
In one method, the heat-transfer material can be heated by introducing it 
into an upper portion of a combustion zone, introducing the hot char into 
a lower portion of the zone, and contacting the heat-transfer material 
with the hot char while maintaining substantially countercurrent plug flow 
of the two solids by passing air upwardly through the combustion zone at a 
rate sufficient to fluidize the heat-transfer material and entrain the 
char. The heat-transfer material substantially flows downwardly through 
the combustion zone in a fluidized state and is heated while the char 
substantially flows upwardly through the combustion zone in an entrained 
state and is combusted. 
The process in U.S. Pat. No. 4,157,245 is based in part on the discovery 
that in the typical coal gasification process, there are two separate 
reactions occurring in the same vessel: (1) an endothermic reaction 
between the coal and steam which produces the gaseous product, and (2) an 
exothermic reaction between the coal and the oxygen-containing gas which 
produces the heat necessary for the first reaction. The process of U.S. 
Pat. No. 4,157,245 separates these two reactions in two separate vessels 
and transfers the heat generated by the second reaction to the site of the 
first reaction via a heat-transfer material. 
A major advantage of this process is that air can be used as the oxidizing 
gas without causing the resulting gaseous product to be diluted with 
nitrogen. 
A major disadvantage of this process is that the sand rates must be 
carefully balanced in various sections of the circulation loop, otherwise 
the system breaks down. 
SUMMARY OF THE INVENTION 
The present invention pertains to an improved method of controlling the 
circulation of a moving burden between a downflow reactor used for 
endothermic reactions, such as coal gasification, and a fluidized 
bed/entrained transport combination reactor used for exothermic reactions 
such as char combustion. No mechanical valves or L valves are used to 
control the circulation rate of solids or to isolate gas streams. 
In its broadest aspect, some solids are transferred from a downflow reactor 
to a crossflow fluidized bed through a first seal leg, wherein the 
crossflow fluidized bed has a baffle separating the crossflow fluidized 
bed into two zones, then the solids are transferred from the crossflow 
fluidized bed to the entrained bed reactor and the solids are transferred 
from the entrained bed reactor to the downflow reactor through a second 
seal leg. The solids circulation rate is controlled by adjusting the rate 
of fluidizing gas entering the crossflow fluidized bed. 
In one embodiment, coal is gasified in a downflow reactor, which contains 
internals, to form char and gasification products. Sand is transferred 
from the downflow reactor to a crossflow fluidized bed through the first 
seal leg, wherein the first seal leg connects the downflow reactor and the 
crossflow fluidized bed. This crossflow fluidized bed has a baffle 
separating the crossflow fluidized bed into two zones. The char and 
gasification products are passed from the downflow reactor to a first 
cyclone which separates the gasification products from the char, and a 
portion of the char flows through a third seal leg into the crossflow 
fluidized bed. Some of the char is recycled from the first cyclone to the 
downflow reactor. The sand and char are transferred from the crossflow 
fluidized bed to an entrained bed reactor, where the char is combusted 
with air to form combustion products. This air may be added to the 
entrained bed reactor in stages. The combustion products are separated 
from the sand by a second cyclone. The pressure difference between the 
second cyclone and the downflow reactor is maintained by a differential 
pressure controller. The sand is transferred from the entrained bed 
reactor to the downflow reactor through the second seal leg. The solids 
circulation rate is controlled by adjusting the rate of fluidizing gas 
entering the crossflow fluidized bed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In its broadest aspect, the present invention involves the use of a 
crossflow fluidized bed, connecting a downflow reactor and an entrained 
bed, to control the solids circulation rate in the system. 
Referring to the drawing, which is a schematic drawing of one embodiment of 
the present invention, sand and char from Crossflow Fluidizied Bed 10 pass 
through Fluidized Zone 21 into the bottom of Char Combustor 20, which is 
an entrained bed reactor. These solids move up into the jet of Lift Air 22 
which entrains them and carries them sufficiently so that an expanded 
entrained bed exists in Combustion Zone 23 of Combustor 20. The density 
difference between Combustion Zone 23 and Fluidized Zone 21 causes solids 
to continue to flow from Fluidized Zone 21 into Combustion Zone 23. 
The quantity of solids flowing through Combustor 20 is proportional to the 
flow of fluidizing gas through Crossflow Fluidized Bed 10. 
Secondary combustion air is added, via Combustion Air Inlet 24, to the sand 
and char entering Combustion Zone 23 and the char is combusted to form 
combustion products. Combustion air may be added in stages along Char 
Combustor 20 in order to stage the combustion process to minimize NO.sub.x 
formation. The combustion of char in Combustor 20 heats the sand which 
supplies the heat required for the gasification reactions. 
The heated sand and combustion products leave Char Combustor 20 and enter 
Combustor Cyclone 30, from which combustion products leave through 
Combustion Product Outlet 31 for heat recovery, and the sand enters 
Combustor Cyclone Seal Leg 32, then passes into Coal Gasifier 40, which is 
a downflow reactor. Due to the pressure difference between Cyclone 30 and 
Gasifier 40, the sand builds up to Combustor Cyclone Seal Level 33 in Seal 
Leg 32, forming an effective gas seal between Cyclone 30 and Gasifier 40. 
The pressure difference combustor Cyclone 30 and Coal Gasifier 40 is 
maintained by Differential Pressure Controller 34 which monitors pressure 
in Gasifier 40 through First Pressure Tap 41 and that in Cyclone 30 
through Second Pressure Tap 35. Controller 34 actuates Damper 51 located 
in Gasification Products Outlet 52 so as to maintain the pressure in Line 
42, which connects Gasifier 40 and Gasifier Cyclone 50, above that in 
Combustor Cyclone 30. 
Coal Gasifier 40 may have internals, such as screen cylinders, Raschig 
rings, baffle plates, etc. Packing 43 is held in place by Packing Supports 
44 and 45. The heated sand passes down through Gasifier 40, providing heat 
for endothermic reactions occurring in Gasifier 40, then the sand passes 
below Packing Support 45 into Gasifier Seal Leg 46, and then into 
Crossflow Fluidized Bed 10. 
Located in Gasifier Seal Leg 46 are Steam and Coal Feed Nozzles 47. These 
nozzles are located below the packing so that the steam coal mixture 
issuing from the nozzles will have sufficient residence time in the moving 
bed of sand to have heated the coal through its plastic stage, thus 
avoiding the possibility of coal agglomerating in the packed region or 
sticking to the packing. The feed nozzles have to be jacketed with coolant 
to prevent sticking of coal internally in the feed nozzles. The optimum 
position for the Feed Nozzles 47 can be determined by trial and error 
methods or estimated from heat transfer calculations. Through the correct 
positioning of the nozzles it should be possible to feed coking coals. 
The coal/steam mixture issuing from Feed Nozzles 47 passes upward through 
Coal Gasifier 40, forming a fluidized bed. The Coal is first pyrolyzed, 
then gasified as it passes through Coal Gasifier 40. At high enough 
temperatures pyrolysis tars would also be cracked. The resulting char and 
other gasification products pass through Line 42 into Gasifier Cyclone 50. 
Gasification products leave Gasifier Cyclone 50 via Gasification Products 
Outlet 52, passing Damper 51. Char passes into Gasifier Cyclone Seal Leg 
53, forming Gasifier Cyclone Seal Level 54, then the char flows out of 
Seal Leg 53 into Crossflow Fluidized Bed 10. Seal leg 53 prevents the flow 
of gas between Cyclone 50 and Crossflow Fluidized Bed 10. Variable Speed 
Char Auger 55 is used to recycle some of the char to Coal Gasifier 40. 
Crossflow Fluidized Bed 10 is fluidized by gas entering from Fluidizing Gas 
Inlet 12, passing through Plenum 13 into Gas Distributor 14. Generally, 
the amount of fluidizing gas flowing into Crossflow Fluidized Bed 10 
determines the solids circulation rate of the complete system. The 
fluidizing gas would be recycled products of combustion, N.sub.2, or air. 
The fluidizing gas expands the Crossflow Fluidized Bed 10, passing through 
Free Board Area 15 over Baffle 11, into Combustion Zone 23 of Char 
Combustor 20. Baffle 11 serves to isolate what is happening at the 
fluidized zone 21 from what is happening in the rest of Crossflow 
Fluidized Bed 10. 
Because solids are being removed from Crossflow Fluidized Bed 10 by means 
of the Lift Air 22 in Char Combustor 20, char from Gasifier Cyclone Seal 
Leg 53 and sand from Gasifier Seal Leg 46 move across Crossflow Fluidized 
Bed 10 into Char Combustor 20. Generally, the Crossflow Fluidized Bed 10 
cross-sectional area would be only large enough to accommodate the 
positioning of Gasifier Seal Leg 46, Gasifier Cyclone Seal Leg 53, Char 
Combustor 20 and Baffle 11. If cooling were required for the process 
Crossflow Fluidized Bed 10 would be expanded to include heat transfer 
surface. 
This invention can be used for the gasification pyrolyzing or retorting of 
solid fuels or any process which is divided into an endothermic section 
and an exothermic section. The endothermic reactions occur in a reactor 
which has a downward moving bed of solids. The endothermic reactor can be 
either fluidized or not fluidized. Also, the endothermic reactor can 
either have internals or not have internals. 
A chief advantage of this invention is that it provides a means of smoothly 
controlling the circulation of solids between two reactors and a means of 
sealing and maintaining separate the gaseous products from the two 
reactors. This is accomplished without mechanical valves. 
It would be advantageous to recycle a portion of the char to the gasifier. 
This would be done because it is impossible to design the gasifier to 
always gasify the correct amount of char for various feed stocks. The 
recycle allows only the char needed to heat the sand heat carrier to enter 
the fluidized bed. If the auger allowed too much gas bypassing, the char 
might have to be removed from the gasifier cyclone seal leg and 
re-introduced into the gasifier through a lock hopper. The amount of char 
allowed into the combustor would be controlled by the outlet temperature 
of the char combustor. 
Another reason for recycling char is that it allows a shorter gasifier 
since less residence time is required if the char can make several passes 
through the gasifier. 
It would also be possible to use the connecting fluidized bed as the main 
char combustor taking advantage of the long solids residence time. The 
char combustor would act more as an afterburner for control of NO.sub.x by 
operating the fluidized bed fuel rich and staging the entrained bed. 
Although the above embodiment deals with the gasification of coal, this 
process can be used for the gasification of other carbonaceous materials 
such as organic char and coke products. Also, catalysts can be 
incorporated into the coal to catalyze the gasification reaction. The use 
of such catalysts as alkali metal compounds are well known in the art. 
Also, sulfur getters, such as compounds of alkaline earth metals, can also 
be incorporated into the coal in this process to remove any sulfur 
generated by the process. 
While the present invention has been described with reference to specific 
embodiments, this application is intended to cover those various changes 
and substitutions which may be made by those skilled in the art without 
departing from the spirit and scope of the appended claims.