Fluidized bed reactor

In a fluidized bed reactor having a reactor vessel, two detector vessels are mounted in the reactor vessel near the inside surface of the reactor vessel in the fluidized bed and in the gas outlet region. Each detector vessel contains larger detecting particles and passes the gas in the reactor vessel. Pressure drop across the detecting particles is measured. The ratio of the two pressure drops represents the ratio of the fluid velocity and minimum fluidizing velocity under the operating conditions and can be used as control factor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a fluidized bed reactor, and more 
specifically to a fluidized bed reactor which is suitable to treat 
particles or gas which change properties by the reaction, and which 
includes a reactor vessel which is vertically divided into three regions 
by spaced distributor plates, a gas inlet conduit through the lower 
region, a particles inlet which introduces particles to be treated into 
the middle region, a product gas outlet communicated with the upper 
region, and an overflow which discharges reacted particles from the middle 
region. 
2. Description of the Prior Art 
Recently, after successful results obtained by fluid catalytic cracking 
methods in the petroleum industry, fluidized bed reactors are being 
broadly utilized in the chemical and metallurgical industries as catalytic 
reaction, calcination, drying and particle transportation apparatus. The 
fluidized bed reactor is a reaction device which forms a fluidized bed 
formed by the reaction particles by blowing fluidizing gas into a fixed 
bed of the reaction particles. 
Generally, transformation from a fixed bed to a fluidized bed is 
illustrated in FIG. 1. The fluid velocity is illustrated by the 
logarithmic scale in FIG. 1. In FIG. 1, range (a) is a fixed bed and the 
second range (b) is a fluidized bed. In the fixed bed, pressure drop in 
the bed increases as fluid velocity is increased, i.e. as fluid flow rate 
increases. That is, particles float in fluid flow by fluid resistance 
applied to the particles against gravitational force. At a fluid velocity, 
the pressure drop is constant. The floating condition of the particles is 
called fluidized bed. In this specification, the critical fluid velocity 
corresponding to the transition from the fixed bed to the fluidized bed is 
called minimum fluidizing velocity Umf, which varies depending upon the 
properties of the particles, e.g. diameter, specific gravity and 
sphericity of particles. 
Fluidized bed reactors utilize the above mentioned characteristics of a 
fluidized bed. It is desired to maintain good fluidization, i.e. movement 
of the particles is excellent and entrainment of the particles is less. To 
obtain such fluidization condition, it is necessary to control the fluid 
velocity U of gas which has passed through the fluidizing bed. As the 
properties of the fluid and reacting particles change by the reaction 
process, the minimum fluidizing velocity Umf is also changed. Thus, the 
velocity Umf must be detected to obtain proper control. Thus, it is 
desirable to monitor the fluid velocity U and the minimum fluidizing 
velocity Umf continuously all through the operation, and also it is 
desirable to obtain the function f (U, Umf) in a form which can be 
utilized to operate the fluidized bed. 
Methods to detect the velocities U and Umf which have been proposed are as 
follows: 
(1) Means to assume the velocity Umf, by the sampling method. 
Property of particles under reaction, e.g. diameter, density and sphericity 
of particles, and property of fluid under reaction, e.g. viscosity and 
density, are detected by sampling and analysis, and the velocity Umf is 
assumed. The sampling and analysis necessitate relatively long time so 
that it is difficult to obtain continuous data to be used as operation 
control. When the properties are not detected, accurate assumed value of 
the velocity Umf cannot be obtained. Experimental formulae and theoretical 
formulae to assume the velocity Umf are not accurate enough, especially at 
a high temperature range. 
(2) Means to assume the fluid velocity U by the fluid velocity measuring 
method. 
The fluid velocity is measured directly or indirectly outside the reactor. 
Indirect measurement includes the disadvantage that the vaporizable 
liquids content must be added afterwards. Reliability of the measurements 
is very low, as, many factor, e.g. pressure, temperature, particle 
entrainment, influence the fluid velocity under operation. 
(3) Method to assume fluid velocity U by the gas quantity measuring method. 
When gas is obtained as a product of the reactor, the produced gas is 
guided outside the reactor and the gas quantity is measured. From the gas 
quantity, minimum fluidizing velocity Umf is assumed. Normally, steam and 
vapor are condensed before the measurement. Thus, it is not easy to assume 
true fluid flow condition from such measurement of dry gas. 
As stated above, conventional measurement methods measure the fluid 
velocity U and the minimum fluidizing velocity Umf independently, and no 
reliable result can be obtained. Relation between the method of operating 
the reactor and the velocities U and Umf is as follows: The operating 
condition of the fluidized bed reactor is determined based on the 
velocities U and Umf, and the reactor is actually operated by the 
determined operating condition. Actual velocities U and Umf are measured 
under operation, and the operating condition is modified. Such operating 
method is suitable for an established process. However, when it is 
desirable to introduce a new operating condition, some means is necessary 
to judge whether factors to be controlled do or do not coincide with the 
predetermined or expected values. 
Methods of the judgement which have been proposed are as follows: 
(4) Method of measuring the pressure drop across a fluidized bed. 
The pressure drop across a fluidized bed is one of the most suitable 
measurable factors to judge the operating condition of a fluidized bed 
reactor as the pressure drop relates directly to the motion of particles. 
However, the pressure drop cannot be quantitatively related to all 
operating conditions of fluidized bed reactors. Thus, under normal 
operating conditions of a specified reactor for an established process, 
the pressure drop can be successfully utilized to assist experimental 
judgement. In transient operating conditions, e.g. starting up, or in new 
operation, monitoring the pressure drop cannot maintain a suitable 
fluidized bed condition. 
(5) Method of measuring temperature distribution in fluidized bed. 
Temperatures are measured at many points in the reactor vertically and 
horizontally, to know operating condition of the fluidized bed. Generally, 
when particle movement is strong, reactor temperature is substantially 
uniform. When any stagnant zone is produced, local temperature change 
indicates where the stagnant zone is. Uniformity of the temperature 
directly relates to the fluid velocity U, and temperature difference 
across the fluidized bed decreases as the fluid velocity is increased. 
However, it is not necessary to increase the fluid velocity more than 
needed to maintain the fluidized bed. The temperature distribution cannot 
quantitatively be related to all operating conditions of the fluidized 
bed, so that is is also used to assist in the experimental decision under 
normal operating conditions. As described in detail, conventional 
operation of fluidized bed reactors has been performed experimentally with 
assistance of pressure drop across fluidized bed and temperature 
distribution, and also inaccurate assumed values of fluid velocity U and 
minimum fluidizing velocity Umf. As the velocities U and Umf cannot 
accurately be obtained, safe side operation or erroneous operation may be 
the result. This means disturbance to the development of fluidized bed 
operating technics. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a fluidized bed reactor 
which accurately detects fluid velocity U and minimum fluidizing velocity 
Umf which are essential to maintaining suitable fluidized bed conditions. 
The present invention does not detect the velocities U and Umf 
independently, but detects the velocities as a function f (U, Umf). To 
this end, to utilize characteristics of a fluidized bed, or to utilize the 
fact that the velocity Umf differs by characteristics of particles, at a 
region of the reactor in which gas flow velocity represents the velocity 
Umf and at a region of the reactor in which reacted gas flow has passed 
through the fluidized bed, two pressure drop detectors are inserted. Each 
pressure drop detector contains particles which has larger velocity Umf 
than that of the reacting particles and the reacting gas also passes 
through the detectors. Pressure drops are detected across the detectors 
and the ratio between the pressure drops represents the function of f (U, 
Umf), as described hereinafter. 
The inventors of the present invention recognized that by utilizing the 
fact that the velocity Umf differs according to characteristics of 
particles, the function f (U, Umf) can be detected, and the function f (U, 
Umf) can be detected as U/Umf by suitably selecting particles. This is 
explained referring to FIG. 2. In FIG. 2 the fluid velocity is not shown 
by logarithmic scale. The example shown in FIG. 2 shows that pressure drop 
increases proportionally to fluid velocity in the fixed bed region. If 
FIG. 2, particles D is represented by curve (ii) and particles R which are 
smaller diameter than that of the particles D are represented by curve 
(i). The minimum fluidizing velocities of the particles D and R are shown 
as UmfD and UmfR respectively, and UmfD is larger than UmfR. Pressure drop 
when particles D and R are in fluidized bed condition is represented as 
.DELTA.PD=.DELTA.PR. Pressure drops when particles D and R have not 
reached to fluidized bed condition are represented as .DELTA.Pd and 
.DELTA.Pr respectively. Pressure drop of particles D at the velocity Umfr 
is shown as .DELTA.PdR. 
Generally, relation between pressure drop .DELTA.P and fluid velocity U is 
represented as follows: 
EQU .DELTA.P=.alpha.U+.beta.U.sup.2 ( 1) 
in which, .alpha. and .beta. are constants based on characteristics of 
particles and gas. As to the particles D, the following relation can be 
described. 
##EQU1## 
The formula (2) shows that by detecting .DELTA.Pd/.DELTA.PdR function f 
(U, Umf) can be detected. 
By suitably selecting particles, the function f (U, Umf) can be represented 
as U/Umf or (U/Umf).sup.2 to simplify monitoring of the operation. More 
specifically, formula (2) is rewritten. 
##EQU2## 
That means, when both .beta.U/.alpha. and .beta.UmfR/.alpha. are far less 
than 1, 
##EQU3## 
Generally, .beta.U/.alpha. is represented as follows: 
##EQU4## 
in which, .phi. is spherical factor of particles, .rho.g is density of 
gas, dD is diameter of particles, .epsilon. is void fraction, .mu. is 
viscosity of gas, ReD is Reynolds number. When ReD is less than 10, 
numerator of formula (3) is about 1. Also, when the Reynolds number ReD' 
which is Reynolds number for .beta.UmfR/.alpha. is less than 10, 
denominator of formula (3) is about 1. Usually the reactor is operated 
such that the fluid velocity is more than UmfR, so that necessary 
calculation of the Reynolds number is only ReD. 
From the formula (4), when Reynolds number is ReD' is more than 600, the 
next formula obtained is: 
##EQU5## 
As described above, when particle D is selected to satisfy that Reynolds 
number is less than 10 or more than 600, the ratio .DELTA.Pd/.DELTA.PdR 
represents U/UmfR or (U/UmfR).sup.2. 
As described above, the particles R are particles to be reacted, and 
particles D are particles in the detectors, and pressure drops across the 
detectors are measured at velocities U and UmfR. From the detected values, 
the function f (U, UmfR) and also U/UmfR can be easily determined. 
The present invention will be described further referring to embodiments, 
by way of example, and the accompanying drawings, in which:

DESCRIPTION OF PREFERRED EMBODIMENTS 
One embodiment of a fluidized bed reactor, according to the present 
invention is shown in FIG. 3, wherein 1 designates a generally cylindrical 
reactor vessel which is vertically divided into three regions by spaced 
gas distributors 21 and 22. The lower region forms a wind box, the middle 
region forms a reaction chamber of fluidized bed formed by particles to be 
treated, and the upper region forms a free board. To one side of the 
reaction vessel 1, a gas inlet 3 communicates with the lower region, a 
solid inlet 4 communicates with the lower portion of the middle region, 
and from the other side of the reaction vessel, an overflow pipe 5 
communicates with upper portion of the middle region, and a gas outlet 6 
communicates with the upper region. 
Detector vessels 71 and 72, according to the present invention, are mounted 
to the inside wall of the reactor vessel 1 at the lower portion of the 
middle region and at the upper region, respectively. Each detector vessel 
71 and 72 is a cylinder which is mounted near the inside wall of the 
reactor vessel 1 within a range of 1/7 of the inside diameter of the 
vessel 1. The detector vessel 71 has wire nets 8 at the upper and lower 
end and the detector vessel 72 contacts with the gas distributor 22 at 
lower end to contain particles in the detector vessel 71 and 72. The 
particles in the detector vessels 71 and 72 are detector particles which 
have larger minimum fluidizing velocity Umf than that of particles in the 
reactor vessel 1. Each detector vessel 71 and 72 has two pressure drop 
sensing conduits 91 and 92 which communicate with upper and lower portions 
of the detector vessel 71 and 72 and are communicated outside through the 
side wall of the reactor vessel 1. 
Operation of the fluidized bed reactor shown in FIG. 3 is as follows: 
Gas 10 which contributes to fluidizing is supplied through the gas inlet 3 
into the lower region of the vessel 1. Gas flows through the gas 
distributor 21 into the middle region. Particles 11 to be treated are fed 
into the middle region through the solid inlet 4, and form a fluidized bed 
cooperating with the gas 10. In the fluidized bed, reaction of particles 
11 by high temperature and/or reaction between the particles 11 and the 
gas 10 produces gas 13 which flows upwards from upper portion of the 
fluidized bed. The produced gas 13 is separated from entrained fine 
particles by passing through the gas distributor 22 and is introduced 
outwards through the gas outlet 6 at the upper portion of the upper region 
of the vessel 1. Reacted particles 14 are discharged through the overflow 
pipe 5. 
The gas 10 flowing through fluidized bed 12 also flows through the detector 
vessel 71, through the wire nets 8 to produce a pressure drop .DELTA.PdR 
across the contained detector particles 15 which form a fixed bed in the 
vessel 71 as shown in FIG. 3. Also, the produced gas 13 flows through the 
detector vessel 72 and produces another pressure drop .DELTA.Pd across the 
detector particles 15 in the vessel 72 which contains also a fixed bed. 
The pressure drops are detected through the sensing conduits 91 and 92. As 
described before, the ratio between the pressure drops across the detector 
vessels 71 and 72 is .DELTA.Pd/.DELTA.PdR and is a function of fluid 
velocity U and minimum fluidizing velocity Umf, and the ratio represents 
U/Umf or (U/Umf).sup.2 by suitably selecting the detecting particles 15, 
as described before. Thus, normal control means, 2, easily regulates th 
flow rate of the gas 10 and feed quantity of the particles 11 to be 
treated, to maintain the desired fluidized condition. 
The detector vessels 71 and 72, according to the present invention, 
accurately detect the ratio .DELTA.Pd/.DELTA.PdR, which represents the 
function f (U, Umf) which can be U/Umf or (U/Umf).sup.2, by suitably 
selecting the detecting particles 15. Also, as the detector vessel 71 is 
within a range of 1/7 of the inside diameter of the reactor vessel from 
the inside wall of the reactor vessel, the pressure drop at the minimum 
fluidizing velocity Umf can be accurately detected. It is known that in 
fluidized bed reactors, the region of the velocity Umf is produced near 
the inside wall of the reactor. This will be explained referring to FIG. 4 
which shows the fluidized condition in the reactor vessel 1. In FIG. 4, 
thick arrows show particles flow and slender arrows show gas flow. Near 
the inside wall surface of the vessel 1, some particles move downwards and 
few bubbles 121 flow upwards. The gas velocity flowing along the inside 
wall surface is about the velocity Umf, despite mean fluid velocity U. The 
inventors of the present invention used a model of the reactor vessel 1 of 
250 mm inside diameter. The detector vessel 71 was a tube 100 mm long 
having an inner diameter of d mm made of stainless steel. The detector 
vessel was mounted in the model reactor vessel at 50 mm upwards from the 
gas distributor 21. When diameter d was more than 65 mm, the pressure drop 
.DELTA.Pd was fractuated irregularly and was affected by gas bubbles. When 
d was 50 mm, .DELTA.Pd was intermittently unstable. When d was about 35 
mm, pressure drop .DELTA.Pd was constant and shows stable gas flow through 
the detector vessel 71. The experiment shows that when the detector vessel 
71 is arranged near the center of the reactor vessel 1, gas bubbles 121 
affect the pressure drop across the detector vessel, and that inside 
diameter of the detector vessel 71 is determined by inside diameter of the 
reactor vessel. The ratio of diameter of the detector vessel and the 
reactor vessel is preferably 1:7.14. The detector vessel 71 is to be 
mounted near the inside surface of the reactor vessel 1 within a range of 
1/7 of the inside diameter of the reactor vessel. As the upper detector 
vessel 72 is in the upper stage of the reactor vessel 1 near the fluidized 
bed surface, gas composition and temperature in the detector vessel 72 are 
fairly close with those in the detector vessel 71. 
The fluidized bed reactor shown in FIG. 3 was utilized as a gas producer of 
coal. In this case, the gas 10 was carbon dioxide gas at 950.degree. C., 
the particles 11 to be treated coal, and the reacted particles 14 ash 
consisting mainly of carbon. Table 1 shows the specification. 
TABLE 1 
______________________________________ 
Particles to Taiheiyo Coal 
be treated density 1.4 g/cm.sup.3 
grain size 12-14 mesh 
Supply gas CO.sub.2 gas 
Detector Spherical alumina 
particle (Al.sub.2 O.sub.3 &gt; 99.9%) 
density 3.96 g/cm.sup.3 
grain size 1.8 mm .phi. 
height of layer 
100 mm 
______________________________________ 
In this case, the ratio .DELTA.Pd/.DELTA.PdR represents U/UmfR. At first, 
range of U/UmfR provides proper fluidized bed condition. Supply quantity 
of coal was 7.0 kg/hr., and flow rate of the CO.sub.2 gas was varied. The 
result is shown in Table 2. 
TABLE 2 
______________________________________ 
CO.sub.2 gas flow rate (Nm.sup.3 /hr) 
6.2 8.5 3.6 
Gas inlet temperature (.degree.C.) 
1,060 1,060 1,060 
Middle stage temperature (.degree.C.) 
910 930 905 
Produced gas quantity 
(Nm.sup.3 /hr) 13.8 18.7 7.9 
PdR (mm Aq) 31 33 30 
Pd (mm Aq) 93 135 50 
U/UmfR 3.0 4.1 1.7 
Remarks good too much lack of 
fluidizing 
entrain- fluidizing 
ment 
______________________________________ 
When the CO.sub.2 gas flow rate was increased to 8.5 Nm.sup.3 /hr., the 
fluid velocity U was too large, and too much entrainment of fines was 
realized so that it was not suitable for operation as a gas producer of 
coal. When CO.sub.2 gas flow rate was decreased to 3.6 Nm.sup.3 /hr, the 
fluid velocity U was too small, and sufficient fluidized bed was not 
produced, so that it was not a proper operation. Thus, a U/Umf value which 
provides a good fluidized bed condition was between 1.8-4.0. Sufficient 
operations were performed within this range and the results are shown in 
Table 3. 
TABLE 3 
______________________________________ 
Solid product ash 51% 
carbon 49% 
density 0.68 g/cm.sup.3 
Product gas CO.sub.2 10.6 Vol % 
CO 76.0 Vol % 
H.sub.2 6.6 Vol % 
CH.sub.4 5.8 Vol % 
C.sub.2 H.sub.4 
1.0 Vol % 
______________________________________ 
In the example shown, characteristics of particles and gas substantially 
change by reaction so that characteristics of coal and supply gas cannot 
be used to assume the velocities U and UmfR. For example, density of coal 
is 1.4 g/cm.sup.3, and the solid product discharged from the reactor is 
only 0.68 g/cm.sup.3 which is 51% less than raw coal. The gas supplied is 
CO.sub.2 gas, and the produced gas consists mainly of CO, CO.sub.2, 
H.sub.2 and CH.sub.4. Gas flow rate changes from 1.3 to 1.8 Nm.sup.3 /kg 
of coal. Thus, operation of the reactor must correspond to the changing 
conditions in the reactor. Accordingly, only the detector vessels, 
according to the present invention, can be effectively used, to detect 
ever-changing operating conditions. 
FIG. 5 shows a fluidized bed reactor which is a second embodiment of the 
present invention. In FIG. 5 the same reference numerals with the 
embodiment shown in FIG. 3 indicates similar parts or portions and will 
not be explained further. The difference between FIG. 5 and FIG. 3 is 
that, the produced gas outlet 6 is opened at the top of the reactor vessel 
1, and that the top opening is filled with detecting particles 15 on a 
wire net 81 to form the detector vessel 72. As before, the sensing 
conduits 92 communicate with the detector vessel 72 to measure pressure 
drop across the layer of the detecting particles. In the embodiment shown, 
fluid velocity U is increased in the gas outlet 6 compared with that of 
gas in the body of the reactor vessel 1, however, this can be converted 
easily. The operation and advantages of the reactor shown in FIG. 5 are 
similar with those of the reactor shown in FIG. 3. 
The embodiments shown in FIGS. 3 and 5 relate to single stage fluidized bed 
reactors. The present invention can clearly be applied to multi-stage 
fluidized bed reactors. 
It will be appreciated that the fluidized bed reactor according to the 
present invention directly detects the fluid velocity U and the minimum 
fluidizing velocity Umf under operation as a function f (U, Umf) or in 
simpler form U/Umf or (U/Umf).sup.2 by suitably selecting detecting 
particles in the detector vessels. Thus, very accurate operation and 
control of the fluidized bed can be obtained in response to variations of 
reacting conditions in the reactor. Consequently, the fluidized bed 
reactor is operated steadily whether the operating conditions are changed 
or not.