Note: Descriptions are shown in the official language in which they were submitted.
~i~65659
"MAINTAINING GAS FLOW DURING TRANSFER OF SO ID~"
Field of the Invention
This invention relates to the arts of solids flow and process
control. More particularly, it relates to control of gas flow through
zones containing solid particulate matter and control of internal pressure
of these zones while the solid matter is transferred between zones. A
specific use involves a catalyst treatment system for use in moving
bed hydrocarbon conversion processes, including catalytic reforming.
BACKGROUND OF THE INVENTION
It is believed that U.S. Patent No. 2,851,401 (Payne) is the
most relevant reference. This patent deals with the transfer of solid
particulate matter from one location to another, but contains no infor-
mation regarding maintaining a flow of gas througb said locations or
maintaining pressures in said locations and does not teach the use
of the gas conduits of the present invention. lJ.S. Patent NQ. 2,851,402
(Haddad) provides information on solids transfer utilizing teachings
of the ~ayne patent ('401).
~; 15 An important application of the present inYention involves
catalyst which is used in hydrocarbon conversion processes. U.S. Patent
No. 2,423,411 (Simpson); 2,531,365 (Simpson et al); 2,854,156 (Payne);
2,854,161 (Payne), and 2,9B57324 (Balentine) are exemplary of references
where hydrocarbon process catalysts are transported and treated.
For additional information on catalytic reforming and regenera-
tion of catalyst, which is the subject of a detailed example herein,
U.S. Patents 3,647,680 (Greenwood et al.) and 3,692,496 (Greenwood et al~ -
may be consulted.
i265~i59
~ There are many chemical processes where it is necessary to
bring into contact a gas and solid particulate matter, or solids, or
particles. Frequently, chemical reactions as well as physical phenomena
take place during such contact. In most cases, gas and solids must be
in contact for a minimum time period and the desired chemical or physical
reaction or change will not take place or will be incomplete if the
contact is for a shorter period. In some cases there is a maxi~um
contact time period, beyond which less than optimum or undesirable
results will be obtained. It is highly desirable to conduct gas/solid
contacting processes in a continuous or semi-continuous manner rather
than as a batch operation.
A contacting zone is usually maintained at some pO5i tive
pressure (above atmospheric~ of the contacting gas. Particles must be
introduced and withdrawn from the pressurized zone without losing
contacting gas to the atmosphere. It is often necessary to maintain
the internal pressure of the contacting zone at a particular value or
within a certain range. Contacting zone pressure may be higher than
that of the zone from which solids are provided to the contacting
zone. Feeding solids into a zone against a high pressure poses
numerous problems. When equipment, such as screw conveyors or star
valves is used, contact between equipment and solids degrades the solids
particles by breaking them into smaller particles and causes equipment
wear. It is difficul~ to maintain effective sealing to prevent
escape of gas from the contacting zone and equipment maintenance costs
are high. These problems are magnified when solids or gas or bGth are
at elevated temperatures. Pressure lock systems having on-off
valves through which the solids pass have been the preferred method
of feeding solids into a pressurized zone, but the valves are a high-
~L265659
maintenance item. A system using valves is briefly described
below.
U.S. Patent No. 2,851,401, cited above, discusses the problems
involved in solids transfer and teaches transfer of solids without using
mechanical equipment subject to wear or which degrades solids. However,
this patent does not deal with the various aspects of gas flow, such as
mentioned above. Also, it is often desirable to maintain a continuous
gas flow, even when the solids flow is batchwise. Use of continuous
gas flow permits better control of contact time period and usually
promotes the chemical or physical process taking place by constantly pre-
senting fresh gas to the solids. In some cases, it is highly important
to immediately contact incoming solids with fresh gas, that is, gas
which has not yet had significant contact with solids.
The present invention is useful in the practice of a variety
of processes and, in particular, in hydrocarbon conversion processes,
such as catalytic reforming, which is the subject of the detailed
example presented below. Another process in which ~he invention may be
utili7ed is the conversion of light paraffins to light olefins. This catalytic
dehydrogenation process will convert, for example, propane to propylene.
In another catalytic hydrocarbon conversion process, light paraffins and/or
olefins are processed to yield aromatics and hydrogen. The present inYention is
useful in regenerating the catalyst used in these processes. An example of a
process other than hydrocarbon conversion in which the present invention
may be applied is the treatment of a gas stream to remove a component
by means of contact with particulate solids, such as removal of
sulfur dioxide from a flue gas stream upon passing the flue gas
through a bed comprising a sulfur oxide acceptor such as copper-
bearing alumina spheres. However, the preferred use of the invention
is in hydrocarbon conversion processes and specifically in moving
bed catalytic reforming.
--3--
1~:65~5g
Brief Summary of the Inve tlon
The present invention comprises method and apparatus for
maintaining a substantially continuous flow of a gas upward through
a lower zone and then through an upper zone within a previously estab-
lished flow rate range while simultaneously transferring particles
downward from the upper zone to the lower zone.
The lower zone has a higher internal pressure than the upper
zone and the internal pressures are independently variable. The par-
ticles are passed through the upper and lower zones during practice
of a process for treatment of the particles or the gas. Either or
both zones may be used primarily for a gas/particle contact operation
or one 7One may be primarily used for storage and feeding purposes.
It is an object of the invention to avoid the use of
moving mechanical equipment in contact with particles.
It is also an object of the invention to avoid the use of
lS costly and complex control instrumentation to maintain gas flow.
It is a further ojbect to provide method and apparatus
for accomplishing particle transfer without substantially affecting
the internal pressures of the upper or lower zones.
In a broad embodiment, the invention is a method comprising
(a) passing the gas into the lower zone, whereupon gas passes upward
from the lower zone to a lock hopper zone through a lower particle
transfer conduit which communicates between the lower zone and the
lock hopper zone at a rate which prevents downward flow of particles
through the lower particle transfer conduit, where an upper particle
transfer conduit communicates between the lock hopper zone and the
upper zone, where a lower portion of the upper zone, the upper
particle transfer conduit, a lower portion of the lock hopper zone,
and the lower particle transfer conduit are filling with particles
~2~i5~iS9
without discontinuity, and where flow of particles downstream
through the upper particle transfer conduit into the lock hopper
is prevented when ~ the level of particles in the lock hopper
zone is at the lower end region of the upper particle transfer
conduit; (b) simultaneously with step (a), passing gas from the lock
hopper zone to the upper zone by means of an upper gas conduit which
communicates and substantially equali~es pressure between these zones;
(c) increasing the internal pressure of the lock hopper zone to a value substantially
equal to the pressure of the lower zone when the lock hopper zone is to be
emptied by stopping gas flow ~hrough the upper gas conduit and passing gas from
the lower zone to the lock hopper zone by means of a lower gas conduit
which communciates and substantially equalizes pressure between these
zones, causing particles to flow downward through the loher particle
transfer conduit into the lower zone, and causing gas to flow from the
lock hopper zone to the upper zonP by means of the upper particle
transfer conduit, at a gas rate which prevents downward flow of particles
through the upper particle transfer conduit; and, (d~ stopping gas flow
through the lower gas conduit when the level of particles in the lock
hopper zone falls to a previously determined low level point and
simultaneously establishing a flow of gas through the upper gas conduit,
causing particle flow out of the lower particle transfer conduit to
the lower zone to cease and causing particles to flow out of the upper
particle transfer conduit and into the lock hopper zone, said particle
flow continuing until the level of particles in the lock hopper zone
rises to the lower end region of the upper particle transfer conduit.
In another embodiment, the method of the invention csmprises:
(a) continuously passing the gas into the lower zone; (b) passing gas
upward from the lower zone to a lock hopper zone through a lower particle
transfer conduit which communicates between the lower zone and the lock
hopper zone at a rate which prevents downward flow of particles through
_~_
~2~5659
the lower particle transfer conduit and simultaneously passing gas
from the lock hopper zone to the upper zone by means of an upper
gas conduit which communicates and substantially equalizes pressure
between these zones, thus permitting particles to flow from the
upper zone downward to the lock hopper zone through an upper parti-
cle transfer conduit which communicates between the upper zone and
the lock hopper zone; (c) when the level of part;cles in the lock
hopper zone rises to a previously determined high level point,
increasing the internal pressure of the lock hopper zone
by stopping gas flow through
the upper gas conduit, causing gas to flow from the lock hopper zone
to the upper zone by means of the upper particle transfer conduit at
a gas rate which prevents downward flow of particles through the
upper particle transfer conduit; (d) passing gas from the lower zone
to the lock hopper zone by means of a lower gas conduit which com-
municates and substantially e~ualizes pressure between these zones,
causing particles to flow downward through the lower particle trans-
- fer conduit into the lower zone; (e) stopping gas flow through the
lower gas conduit when the level of particles in the lock hopper
zone falls to a previously determined low level point and simul-
taneously establishing flow of gas through the upper gas conduits
thus re-establishing the gas flow configuration of step (b)l
causing particle flow out of the lower particle transfer conduit
to the lower zone to cease and causing particles to flow out of
the upper particle transfer conduit and into the lock hopper zone.
Apparatus for practice of a broad embodiment of the invention
--6--
~l26565~
comprises: (a) an upper zone containing particles, which zone is
maintained at an independently variable first pressure; (b) a lower
zone containing particles, which zone is maintained at an independently
variable second pressure higher than said first pressure, (c) a
lock hopper zone 10cated below the upper zone and above the lower zone;
(d) means for continuously supplying the gas to the lower zone; (e)
an upper particle transfer conduit which communicates between the
upper zone and the lock hopper zone~ (f) a lower particle transfer
conduit which communicates between the lock hopper zone and the lower
zone; (9) an upper gas conduit and a block valve located in said conduit,
which conduit communicates between the upper zone and the lock hopper
zone; (h) a lower gas conduit and a block valve located in said conduit,
which conduit communicates between the lock hopper zone and the lower
zon~; (i) means for generating a signal which initiates a transfer
of particles out of the upper zone and transmitting said initiation
signal; (j) means for sensing level of particles in the lock hopper
zone and transmitting a signal when said level is at a previously deter-
mined low location; and (k) means for controlling the position of said block
valves in such a manner that one of the valves is open when the other
of the valves is closed so that a flow path for said gas supplied to
the lower zone comprises either the lower particle transfer conduit and
the upper gas conduit or the upper particle transfer conduit and the
lower gas conduit, which position controlling means is responsive to
said level signal and said initiation signal such that (i) upon
receipt of said initiation signal the lower gas conduit block valYe
opens, permitting particle flow from the lock hopper zone through
the lower particle transfer conduit to the lower zone, and the upper gas
conduit block valve closes, causing gas to flow upward through the
upper particle transfer conduit at a rate which prevents downward
flow of particles through the upper particle transfer conduit, and
~6565~3
(ii) upon receipt of said level signal the lower gas conduit block
valve closes, causing gas to flow upward through the lower particle
transfer conduit at a rate which prevents downward flow of particles
through the lower particle transfer conduit, and the upper gas conduit
block valve opens, permitting particle flow from the upper zone through
the upper particle transfer conduit to the lock hopper zone.
Brief Descri~tion of the Drawings
Figure 1 is a schematic representation of an embodiment of
the invention depicting an upper zone, a lock hopper zone, and a lower
zone, where each zone is contained in a separate vesse1.
Figure 2 is a schematic representation depicting the zones of
Figure 1 in a common vessel and depicting three steps in a five-step
cycle comprising an embodiment of the invention. Figures 2A, 2B, and
2C depict, respectively, steps 1, 3 and 5 of the cycle.
Detailed Description of the Invention
For the purposes of promoting an understanding of the princi-
ples of the invention, reference will now be made to the embodiments
illustrated in ~he drawings and specific language relating to a par-
ticular exemplary process will be used to describe the same. The use
of these embodiments and this example is not intended to limit the scope
-8-
~L~iS ~;9
of the invention in any way. The drawinys depict only those components
which are necessary in describing the invention, the use of additional
required hardware being well within the purview of one skilled in the
art.
The reforming of hydrocarbon feedstocks, such as a naphtha
fraction derived from petroleum, utilizing a platinum group metal-alumina
catalyst, is a process well known in the art. Briefly, a naphtha
feedstock is admixed with hydrogen and contacted with the catalyst in
a reaction zone, at reforming conditions of temperature and pressure to
cause at least a portion of the naphtha feedstock to be upgraded to
products of improved octane value. After a period of time in use,
the catalyst used in the process must be regenerated, that is, it must be
treated to restore it to a satisfactory level of activity and stability
for catalyzing the reforming reactions. Regeneration consists of several
different processing steps. One of the steps involves contacting the
catalyst with a reduc;ng gas comprising hydrogen in order to accomplish
a reduction reaction. The above-cited U.S. Patents, 3,647,680 (Greenwood
et al.) and 3,692,496 (Greenwood et al.) may be consulted for background
information on reforming and catalyst regeneration.
In many modern catalytic reform;ng processes, catalyst is
moved continuously or semi-continuously through a regeneration vessel,
or through a series of regeneratlon vessels~ in which the various steps
involved in a regeneration cycle are performed. Due to the well-known
difficulties involved in transferring solids from location to location,
mentioned above, true continuous processing is difficult to achieve.
The catalytic regenerdtion process of the above-mentioned Greenwood patents
uses a semi-continuous movement of catalysts at certain points and
continuous movement at other points of the regeneration vessel, or ves-
sels. By semi-continuous movement is meant the repeated transfer of
_9_
l.X65659
a relatively small amount of catalyst at closely spaced points in time.
For exa~ple, one batch of catalyst may be transferred out of a vessel
each two minutes. If the inventory in that vessel is sufficiently
large, the movement approximates continuous transfer of catalyst.
Thisprinciple is used in the present invention. It is not necessary
to provide further information on regeneration processes, as such is
easily available in numerous sources, such as the above-mentioned
Greenwood patents, and is not required for an understanding of the
invention.
Following is a description of the embodiment of the invention
depicted in Figure 1, using language specific to the above-discussed
reforming process. Catalyst particles are accumulated in the bottom
portion of vessel 10, or upper zone 10, entering from above, as shown
by the arrow. In this zone, the portion of the catalyst regeneration
lS cycle known as reduction takes place. Gas, comprising hydrogen, at
a high temperature, is contacted with the catalyst particles in upper
zone lO in order to accomplish reduction.
It is very important that an uninterrupted flow of gas through
the reducing zone be maintained. Should the flow be interrupted for any
interval of time, reduction of the catalyst will not be properly accomplished~
with the result that its ability to catalyze reforming reactions is
severely impaired. Also, if the flow of reducing gas is sufficiently
high so that the catalyst is fluidized or partially fluidized, the
catalyst will be subject to physical damage.
After catalyst is reduced in upper zone 10, it is transferred
to lower zone 12, which serves as a retention volume for catalyst flowing
through the regeneration apparatus, and also serves an isolation function,
while feeding catalyst to pneumatic conveying means ~or transporting
ttle catalyst to a reforming reactor. Lower zone 12 is at a higher
--10--
:,
5659
pressure than upper zone 10. For example, the upper zone could be maintained at
a nominal pressure of 5psig (3~.5 kPag) and permitted to vary within a range of
2 to 8psig (13.8 to 55.2 kPag) while the lower zone nominal pressure could be
35psig (241.3 kPag), with a normal range of 30 to 40 psig (206.9 to 275.8 kPag).Thus, the differential pressure between the upper and lower zone might range
between 22 and 38 psi (151.7 to 262 kPa). However, this invention may be used
when the pressure differential between zones is much greater or much less. It may
be in a range between 0.1 psi and 100 psi to 200 psi (0.7 and 6~9.5 to 1379 kPa) or more.
A vessel denoted lock hopper 11 is used in effecting the
transfer of catalyst from zone 10 to ~one 12. Catalyst passes frôm
zone 10 to lock hopper 11 through upper particle transfer conduit 15,
which sealably extends through a nozzle on the top of lock hopper 11
to project into lock hopper zone 11. Catalyst passes from lock hopper
11 to lower zone 12 via lower particle transfer conduit 165 which sealably
extends into lower zone 12. As will be shown below, the extension of
conduit 16 into lower zone 12 is not required; while a minimum length
of conduit is required, it may be outside the vessels. The extension
of conduit 15 into lock hopper 11 is not necessary when means for monitoring
particle level at a high location in lock hopper zone 11 is provided, but is
required when no high level instrumentation is provided. Such high
level instrumentation is not shown in Figure 1, since it is not necessary
to the embodiment depicted therein, but will be described below.
A comrnon prior art procedure is to locate valves in conduit
15 and 16, between the three vessels, so that lock hopper 11 can be
~5 alternately filled with catalyst from upper zone 10 with the valve in
conduit 16 closed, and then discharged to lower zone 12 while the valve
in conduit 15 is in a closed position. However, as mentioned above, it
is highly desirable to avoid the use of moving equipment, including
valves, in the transfer paths of catalyst particles.
~X~S~j59
Reducing gas enters lower zone 12 through conduit 20. Valve
?1 regulates the quantity of gas flowing into lower zone 12; this flow
rate may be varied independently of the invention by means for controlling
the pressure of lower zone 12 (not shown). For example7 the pressure of
lower zone 12 might be varied, within a previously established range,
in response to signals from the above-mentioned pneumatic conveying means.
Gas may flow from lower zone 12 to upper zone 10 via one of two
alternate paths, where the lock hopper zone is a part of each path. One
gas flow path comprises conduit 16, lock hopper 11, and upper gas conduit
13. The other flow path comprises lower gas conduit 14, lock hopper 11,
and conduit 15. Since in the first mentioned path, catalyst occupies the
lower portion of vessel 10 and the gas enters above the catalyst level,
it is necessary in upper zone 10 to provide means for conveying gas
downward and distributing it, so that contact between gas and catalyst
takes place. This is accomplished by cylindrical baffle 30, which is
smaller in diameter than upper vessel 10 and disposed in a concentric
manner inside it to form an annular space. The top of the annular space
is closed to gas flow by means of an annular horizontal plate. The open
center area of the annular plate permits flow of catalyst and gas.
20 - Gas entering the annular space from conduit 13 must therefore flow
downward to the bottom of cylindrical baffle 30 and make a 180 turn to flow
upward through the catalyst.
The internal pressure of upper zone 10 is independently
controlled by means not shown on the drawing. For example, upper zone
10 might be connected, by means of a conduit, to another vessel used in the
catalytic reforming process, so that the upper ~one pressure depends upon
and varies with the pressure in that vessel.
Low level switch 17 is provided at lock hopper 11 to sense
when catalyst level in the lock hopper zone is at a previously
determined low level and transmit a signal to contrsller 22. Controller
-12-
~:65~59
22 adjusts the positions of valves 18 and 19, which are on-off valves
in this embodiment of the invention. Controller 22 also includes a
timer, which generates or causes to be generated a cycle initiation
signal at a frequency determined by adjusting the timer. The cycle
initiation signal causes valves 18 and 19 to move to the start of a
particle transfer cycle, as will be explained below.
The following description is presented w;th reference to
both Figures 1 and 2. The above description relating to Figure 1
also applies to Figure 2. It can be seen that the same reference
numbers used in Figure 1 also appear on Figure 2 where appropriate.
Certain items have been omitted from Figure 2 for drawing convenience,
such as controller 22 and valve 21, but it is to be understood that
these items are required for the operation of the embodiment of
Figure 2. In Figure 2, which depicts a preferred arrangement,
the three zones of Figure 1 are located in a single vessel rather
than separate vessels. In Figure 1, lower gas conduit 14 communicates
between lower zone 12 and lock hopper 11 and upper gas conduit 13
communicates between lock hopper 11 and upper zone 10. In Figure 2,
the gas conduits are shown with a common portion 26. Thus, in Figure
2, lower conduit 14 includes a portion labeled reference number 26 and
upper conduit 13 includes a portion labeled reference number 26.
Transfer of catalyst particles from reducing zone 10 to
lower zone 12 without using valves, while maintaining a flow of gas
through the zones, may be accomplished with a five-step cycle. Three
of the five steps are shown in Figure 2. A s;ngle cycle results in
the transfer of one batch of particles from the upper zone to the
lower zone. Figure 2A depicts step 1 of the cycle, where the
apparatus is in a hold or ready mode. Lock hopper 11 is filled to its
-13-
~l2~;S6s9
maximum capacity with catalyst. There is an inventory of catalyst
in reducing zone 10, which catalyst remains in the zone for a time
sufficient to attain proper reduction. Conduits 15 and 16 are filled
with catalyst so that there is no discontinuity in a mass of catalyst
occupying a lower portion of reducing zone 10, upper transfer conduit
15, a lower portion of lock hopper zone 11, and lower transfer conduit
16. The inventory in upper zone 10 is replenished with catalyst from
that portion of the regeneration apparatus located above the upper
zone (not shown). Catalyst accumulation in lower zone 12 is depicted.
Gas passes from lower zone 12 to lock hopper 11 through
lower transfer conduit 16 during step 1. The differential pressure
between the lower and lock hopper zones may be in a range between 0,1 and 100 Psi
(0.7 to 689.5 kPa) or more, with the lower value usually above 5 p5i ~34.5 kPa).Downward flow of particles from lock hopper zone 11 to lower zone 12
is prevented at this time by upward flow of gas through lower transfer
conduit 16. With a high upward flow rate of gas and a relatively low
depth of catalyst above upper transfer conduit 16, the particles
in conduit 16 may be pushed upward into zone 11, causing a large increase
in gas flow and partial fluidization of catalyst in zone 11. In the
design of the apparatus, a minimum length of conduit 16 plus a minimum
depth of the particle bed immediately above it must be specified,
based on the maximum gas flow rate expected/required through conduit
16. In establishing this length plus depth above the minimum, it
is necessary to consider the minimum required flow of gas and the
pressure differential between zones. For a particular pressure
differential, the longer the conduit, the lower the gas flow. Conduit
diameter may be increased in order to increase gas flow at a given
conduit length and pressure differential.
-14-
1~65~59
Flow of catalyst from upper ~one 10 to lock hopper zone 11 does
not occur at this time (step 1) by virtue of the fact that the level
of particles in lock hopper zone 11 is at the end region of upper transfer
conduit 15; reference number 27 denotes the end region. It can be
seen that for catalyst to flow out of conduit 15 (Figure 2A), catalys~
at the end region of the conduit and oul:side of the conduit must be
displaced. A suFficient amount of force to accomplish displacement
is not available in this situation and the level never rises above the
end region.
In step 2 (not shown) of the cycle, which may be denoted the
pressurization step, valve 18 is closed and valve 19 in lower gas conduit
14 is opened. This results in the equalization of pressure between the
lock hopper zone and the lower zone; thus the lock hopper zone internal
pressure increases in this step, so that it becomes greater than the
internal pressure of the upper zone. Upon completion of pressurization
of the lock hopper zone9 step 3 of the cycle is entered.
Flgure 2B depicts the latter portion of step 3 of the cycle,
in which the catalyst level in lock hopper 20ne 11 is near its normal
low point. Step 3 is referred to as the "empty" portion of the cycle,
where the lock hopper is emptied of catalyst. Flow of solids from the
upper zone into the lock hopper zone is prevented by flow of gas
upward through upper transfer conduit 15, in the same manner as
discussed above in regard to conduit 16. The level of particles in
the lock hopper zone falls as solids flow out of conduit 16 to lower
zone 12. During t~is time, the gas entering via line 20 flows through lower
gas conduit 14 and valve 19 to enter the lock hopper zone. The
pressures of the lower zone and the lock hopper zone are substantially
the same at this time (step ~) though, of course, a small pressure
~L~ 5 ~ 5 9
difference exists, since there is flow through conduit 14.
It can be seen that the gas flow path of steps 2 and 3 is
different from that of step 1, but that there is no interruption
in gas flow caused by the transition from step 1 to step 2. Arrows
28 indicate the gas flow path in step 1 and arrows 29 depict the flow
of gas in step 3. It-may be desirable to program a slight delay in
closing of valve 18 at the start of step 2, on the order of a few
seconds or less. This would ensure that if valve 19 were to open
relatively slowly9 there would be no significant transient flow
disturbance due to valve operation.
When the level in lock hopper zone 11 falls to a previously
determined low point, step 4, depressurization, is initiated. Low
level switch 17 detects the absence of particles at said low point
as soon as particle level falls to that location and immediately
transmits a signal to controller 22. Controller 22 causes valve 19
to close and valve 18 to open, thus depressurizing lock hopper zone 11
and changing the gas flow path to the same configuration as in step 1.
Step 4 ends when the pressure in the lock hopper zone becomes substantially
equal to the pressure of the upper zone. In step 5, catalyst enters
the lock hopper zone via conduit 15. Step 5 differs from step 1 in
that lock hopper 11 is full during step 1 and there is no flow of
catalyst at all in step 1. During step 5, catalyst flows from upper
zone 10 to lock hopper zone 11 until the level rises to the end
region ofupper transfer pipe 15, thus completing the cycle and
returning to a hoid mode, represented by step 1.
This cycle of five steps is normally repeated continuously.
For example, it may take approximately 50 seconds to transfer one
batch of catalyst from upper zone 10 to lower zone 12. Controller 22
is capable of accepting a desired cycle repetition rate, which is
- -16-
3l;2 6~;6~3
usually manually entered, and sending a signal to initiate a cycle,
that is, the move~ent of valves 18 and 19 so that step 2 is entered.
A practical maximum cycle repetition rate for a 50-second cycle would
be about once per 60 seconds. The catalyst transfer rate would then be,
if the volume of the lock hopper zone between normal maximum capacity
(level at conduit 15 end region) and the low level switch were one
unit volume, one unit volume per minute. A transfer rate of half of that
maximum would require that controller 22 initiate a new cycle every two
minutes.
Controller 22 functions as means for receiving a level signal
from low level switch 17, means for controlling the positions of block
valves 18 and 19, and means for an operator to set a cycle repetition
rate. There are many different types of apparatus capable of performing
the functions of controller 22, such as process control computers and
programmable controllers. Also, these functions can be accomplished
by means of a cycle timer to provide signals to initiate a cycle and
a flip-flop control device responsive to low level switch 17 for pro-
viding signals to enter step 4.
The length of conduits 15 and 16 are quite important to the
operation of the system, as explained above. The magnitude of the
permissible pressure differential between zones is dependent primarily
on the length of the column of particles betwePn zones, for a given
diameter of transfer conduit and particle type. The length of the
column of particles between zones is defined as the length of the
transfer conduit plus the depth of the bed of particles above it in
the zone, where the lowest point of the bed of particles is at the
bottom of the conical section of the zone. If the pressure differential
is too high, the catalyst will be blown out of the transfer pipe and
up into the zone above it. In experimental work, as pressure in a
-17-
3L~ S 6~3
zone was increased, blowout was manifested by a loud noise and could
be clearly observed in the zone above the transfer pipe. If the
pressure differential between zones is too low, the gas flow rate
will be too low, resulting in poor catalyst regeneration. The column
of catalyst through which the gas flows upward may be viewed as a re-
sistance to flow; flow rate through such a resistance, or restriction,
varies with pressure drop across the restriction.
In a typical design situation, the pressure differential
across a lock hopper is known, since it is normally independently
fixed by factors having no relationship to the lock hopper systeril.
Thus the starting point in design is the given pressures of pressure
ranges in the upper zone and lower zone. The required maximum and
minimum flow rates of gas upward through the zones and the required
particle transfer rate are also known, being set by the process. The
length of the catalyst column and the diameter of the particle transfer
pipe are then considered. A balance between length and diameter
is required to achieve the desired gas rate at the same time as
the desired instantaneous flow rate of particles. Shortening the
length with other factors constant or increasing the diameter with
other factors constant will result in blowout if carried too far.
Another feature of importance during design is the length of each
of the components which make up the total column height of particles.
Gas flow through the transfer pipe requires a significantly higher
pressure drop per unit length than the same gas flow through the
particle bed immecliately above the transfer pipe. It should also be
not~d that gas flow rate through the particle bed must always be less
than the rate which will cause fluidization of the particles. Those
skilled in the art will now appreciate the interplay of variables and how
to adjust each to obtain an appropriate design. Principles of solids
flow are known to those skilled in the art and need not be discussed
herein. For additional information on solids flow in the context of this
-~8-
~265~59
invention, U.S. Patent No. 2,851,401, mentioned above, may be
consulted, though it does not deal with gas flow. It should be
noted that common practice in design of solids flow systems is to
conduct experiments to determine flow characterist;cs of the particular
solid involved.
It can be seen that design of a system of this invention
requires careful calculations. Given the internal pressures of the
upper and lower zones, the minimum and maximum gas flow rates required
by the process, the identities of the gas and the particles, and the
required range of particle transfer rates, the system designer must
carefully choose the size of the lock hopper zone, in particular the
normal minimum and maximum volumes occupied by the particles, the
lock hopper zone bed depth above the transfer conduit, the diameter
of the transfer conduits, and the lengths of the transfer conduits.
Of course, there are other parameters to be chosen by the designer,
such as gas conduit size, but these are the most important.
It can be readily seen by reference to Figure 2 and the
flow paths indicated by the sequence of arrows 28, 29, and 32 in
Figures 2A, 2B, and 2C, that both of the alternate flow paths between
the outlet of conduit 20 and the top portion of upper zone 10 involve
substantially the same pressure drop at all times. The pressure
drop of gas flowing through catalyst in the large diameter portions
of the apparatus is small in comparison to the pressure drop of gas
flowing in the transfer conduits.
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12~565~
Thç apparatus of the lnvention may be used as a solids flow
control device for an entire process, since the flow rate of particles
from the upper zone to the lower zone can be varied, as discussed
above.
S It is necessary that the lower end of a partical transferconduit have a smaller cross-sectional area for particle flow than the
balance of the conduit; this is referred to as a restriction. For
example, in the case of a circular conduit~ the inside diameter of
the end may be tess than that of the balance of the conduit, such as
is shown in Figure 2A at reference number 27. The purpose of the res-
triction is to keep the particle transfer conduit full of particles
when the pressures of the zones between which the transfer conduit
communicates are about the same. When the pressures are not equal
and gas is flowing upward, the particles will remain in the conduit
because of proper selection of length and size of conduits 15 and 16
to set the length of the column of particles between zones and thus
prevent blowouts as previously explained. Without the restriction, par-
ticles passing through a conduit will be in dilute phase and when
a pressure dif~erential between zones is established, the conduit will
be only partially full of particles, thus defeating the invention.
In another embodiment of this invention, a high level sensor
may be used to limit the level of particles in the lock hopper zone
to a point below the end region of the upper particle transfer conduit.
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When the nigh level point is adjustable over a range, the size of each
batch transferred may be adjusted. When the lock hopper zone reaches
a high level point, the high level sensor provides a signal to controller
22 and controller 22 closes the upper gas conduit block valve, leaving
the lcwer conduit block valve also in the closed position. The gas
path between the upper and lower zones then comprises both the upper
and lower particle transfer conduits, so that particle flow in both con-
duits is prevented. Then when it is desired to start a cycle from this
hold position comprising two closed block valves, the lower gas conduit
block valve is opened to start the lock hopper empty step.
A reason for using a high level instrument instead of letting
particle level rise to the lower end region of the upper particle
transfer conduit is that, in this situation, gas flowing up the
conduit tends to agitate the particles at the lower end region.
This agitation may cause physical damage to the particles. Another
method which has been proposed to solve this problem, should it occur,
is to provide a perforated conduit portion at the lower end of the
condùit. All or a portion of the gas would then flow through the
perforations, thereby by-passing the catalyst and not causing
agitation. The catalyst level would not rise bèyond the lower
end of the perforated portion of the conduit.
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