Note: Descriptions are shown in the official language in which they were submitted.
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D 1333 STAGED FLOW DISTRI~UTION GRID ASSEMBLY AND METHOD
FOR EBULLATED BED REACTOR
.
BACKGROUND OF INYENTION
This invention pertains to an improved flow distribution
grid plate assembly and method used for providing uniform
upward fluid flow distribution in ebullated bed catalytic
reactors. It pertains particularly to a staged flow distri-
bution method and grid plate assembly having an upper primary
grid plate and a lower secondary grid plate located below the
primary grid plate, each plate ~ontaining multiple vertical
flow tubes.
In ebullated catalyst bed reactors operated at elevated
temperature and pressure conditions, flow maldistribution
problems sometimes exist below the distribution grid plate
and in the catalyst bed. Such flow maldistribution is
usually due to abnormal operating conditions such as plugging
of openings in the grid plate by coke, or to excessive coke
deposits on the catalyst particles in the bed. If such
plugging of openings in the grid plate occurs, non-uniform
flow distribution and bed ebullation results, which is very
undesirable. The riser flow tubes and slotted tail pipes as
now used in reactor grid plates usually perform adequately in
distributing the recycle and feed liquid streams and hydrogen
gas into the ebullated catalyst bed. However, the presently
used grid plate arrangement has been found to be inadequate
for handling severe flow maldistribution in the plenum of the
reactor, for it can only moderately improve the flow
distribution existing below the plate, but cannot alleviate
"spouts" and major operational upset conditions which lead to
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an uneven depth of hydrogen in the plenum chamber, which can
cause a greater length of the tail pipe slot to be exposed
with a corresponding increase in hydrogen flow to those
particular riser tubes. Such maldistribution flow conditions
in a reactor plenum can be more or less constant depending on
the manner in which the feed streams and recycle streams are
introduced into the plenum Also, flow maldistribution could
possibly occur as a sloshing effect where the liquid level in
the plenum below the distribution grid is constantly tilting
from one direction to another~
The use in such ebullated bed catalytic reactors of con-
ventional cylindrical riser flow tubes covered by
cylindrical-shaped bubble caps is disclosed by U. S. Patent
No. 3,1979286 to Farkas et al; U. ~. 3,197,288 to Johanson,
and U. S. 3,475,134 to Weber et al. However, it has been
found that inadequate distribution if the gas and liquid
flows are usually provided by these reactor designs.
Accordingly, improvements in flow distribution in
ebullated bed catalytic reactors have been sought. An
improved staged grid plate configuration has now been deve-
loped which more effectively redistributes the gas and liquid
flows below the primary grid plate whenever flow
maldistribution problems exist below the grid, so as to pro-
vide more uniform ebullation of the catalyst bed in the
reactor. To aid in maintaining a "smooth" liquid level in
the plenum and consequently a reasonably equal length of slot
exposure on each riser tube for gas flow into each tube, a
secondary grid plate is provided below the primary grid.
This secondary grid is similar to the single flow
distribution grid presently used in ebullated bed reactors,
however, the secondary grid does not have caps over the riser
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tubes on the upper side of the plate, but uses only slotted
tubes attached to a plate which extends to near the inner
walls of the reactor.
SUMMARY O~ INYEN~ION
This inven~ion proYides a staged distribution grid plate
assembly and method used ~or improved flow distribution
upwardly into an ebullated catalyst bed of a reactor vessel, in
which a lower secondary flow distribution grid feeds gas and
liquid flow upwardly to a upper primary flow distribution
grid, and thence into the ebullated bed of the reactor. The
flow tubes provided in the lower secondary grid plate are
uniformly spaced and are relatively larger in diameter and
have greater total cross-sectional area than flow tubes in
the upper primary distribution grid, so that the grea~er and
controlling pressure differential ocours across the upper
primary grid plate to provide a more uni~orm flow
distribution upwardly into the ebullated bed. This staged
grid plate arrangement or assembly enables the upper primary
grid to operate more effectively, so that the ebullated
catalyst bed above the prinary grid plate will have a more
uniform distribution of gas and liquid flowing upwardly
therethrough across the entire cross-sectional area of the reactor.
In the staged grid plate assembly of the present invention,
the upper primary grid plate can be supported from either the
react~r lower head or ~rom th~ reactor L~ wall, and the lower
secondar~ grid plate is usually supported from the upper
primary grid, such as by multiple spacer rods extending
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between the two grids. Alternatively, the secondary gri d
plate can be separately supported from the reactor lower head
or wall, or it can be structurally integrated with the
primary upper grid plate so as to help withstand the total
differential pressure across the grid assembly caused by-the
~d fluid flow through the grids. Also, the lower
secondary grid plate can be attached integrally to the primary
upper plate by an extension of the primary flow tubes below
the upper plate and rlgidly attached to the lower secondary
plate, so that the ca ~ yst bed weight and total pressure diEferential
across koth plates in the grid plate assembly is carried by the assembly.
More specifically, the present invention comprises a
staged grid plate assembly for providing uniform flow
distribution of a gas/liquid mixture upwardly into an
ebullated bed of a ~reactor, said grid plate assembly
comprising an upper primary gr~d plate supported w~thin the
reactor near the lower end of the reactor, said primary grid
plate containing multiple flow pr~y distribution tubes exte~ng
substantially vertically through said plate, said primary
tubes being uniformly sized and spaced in the plate, a cap
covering the upper end of each tube in said primary grid,
said cap being rigidly attached to and spaced outwardly from
the tube upper end above the grid plate, so as to permit flow
of fluid upwardly through the tubes and then outwardly from
under the lower edges of the cap into the ebullated bed; and
a lower secondary grid plate located below and spaced from
said primary grid plate, said secondary grid plate containing
multiple flow secon~ distribution tubes passing subs~ntially
Yertically through the secondary plate, said secondary tubes
having uniform diameter and spacing, whereby fluid passes
upwardly through the flow tubes in the secondary grid, and
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then upwardly through the flow tubes in the primary grid and
outwardly from under the lower edges of the caps to provide
uniform fluid flow in the reactor ebullated bed.
In another aspect of the invention, it comprises a method
for uniformly distributing gas and liquid flow upwardly into
an ebullated bed reactor, wherein the method comprises
introducing gas and liquid flow streams into a plenum located
at the lower end of a reactor below a flow distribution grid;
passing said gas and liquid flow upwardly from said plenum
through multiple tubular flow passages located in a secondary
grid plate into an interim zone located above the secondary
grid plate and below a primary grid plate; remixing and
redistributing the gas and liquid flow in said interim zone
above said lower grid plate; and passing the remixed gas and
liquid upwardly through multiple tubular flow passages
located in said upper primary grid plate, then under bubble
caps each located over said multiple upper flow passages and
uniformly into an ebullated catalyst bed of the reactor.
~RIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a partial vertical sectional view through
the lower portion of a reactor vessel containing a staged
grid plate assembly havintl multiple riser tubes therein in
accordance with the invention.
FIG. 2 shows a portion of the primary upper grid plate
containing multiple riser flow tubes each covered by a single
bubble cap and containing a ball check.
FIG. 3 shows a partial vertical sectional view of an
alternative staged grid plate assembly in which both grid
plates are supported from the reactor lower head.
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FIG. 4 shows a partial sectional view of an alternative
grid plate assembly in which the staged grid plates~ are
structurally integrated into a single unit supported from the
reactor wall.
DESCRIPTION OF INVENTION
In liquid phase catalytic reactors for contacting
liquids, gases and par iculate solids, ~t is very important
for achieving comple~e and effective catalytic reactions that
the upflowing liquid and gas mixture be uniformly distributed
across the entire horizontal cross-section of the reactor vessel~ so
as to maintain the bed of partioulate solids or catalyst in a
uniformly expanded condition with random motion of the
catalys~. For certain reactions, such as the catalytic
hydrogenation oF heavy oils or coal-oil slurries or the
hydrocracking o~ heavy hydrocarbon feedstreams at elevated
temperature and pressure conditions, such as at 500-1000F
temperature and 500-5000 psig pressure, to produce lower-
boiling liquid fractions, flow maldistribution through the
reactor flow distributor or grid plate assembly can cause
relatively inactive zones in the bed where the catalyst is
not in uniform random motion. This condition leads to the
undesired formation of agglomerates of catalyst particles by
coking of the hot oil or slurry. The desired uniform flow
distribution upwardly through the grid plate into the
ebullated catalyst bed can be impaired either by restrictions
occurring in the riser tubes due to coking, or by catalyst
particles in the tubes. The pr2sent invention provides an
effective solution to these flow maldistribution problems in
the ebullated catalyst bed.
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The flow distributor or grid plate assembly must also
function to prevent catalyst particles from draining down-
wardly back through the distributor whenever the reactor is
shutdown, while most of the liquid contained within the
catalyst bed is drained down to below the bed. If catalyst
is allowed to drain bark through the grid plate flow distri-
butor, it can plug the flow passages therein and interfere
with operations so that re-ebullating the catalyst bed be-
comes very difficult because the flow passages are at least
partly restricted. Furthermore, such restrictions can pro-
duce undesired flow maldistribution in the catalyst bed. To
prevent such backflow of catalyst, a ball check valve is
usually pr~vided in each riser tube.
In the present invention, two grid plates are provided supported
in a reactor vessel in ~eries flow relationship so that a relatively
more unifonm flow distribution upwardly into the ebullated catalyst bed
above the upper primary gr~d plate ~s thereby ~ch~eved. It
is thus a basic feature of the present invention that both
grid plates contain multiple flow tubes having uniform size
and spacing, with only the tubes in the upper grid plate
having caps covering the upper ends of the tubes. The flow
tubes used in the secondary grid plate should be uniformly
spaced and have relatively larger diameter and total cross-
sectional area than the flow tubes in the upper primary grid
plate. The secondary flow tubes do not necessarily need to
be cylindrical in shape but can be square, rectangular) or
triangular in cross-seotional shape or practically any
configuration can be employe~. However, the combination of
tube effective diameter and number of tubes should provide
the desired uniform flow and differential pressure across the
secondary grid, which should be between about 0.10 and 0.90
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times the different7al pressure across the upper primary
grid. Also, the length/diameter ratio for the secondary
tubes should be at least about 1.0, and usually need not
exceed aboùt 5Ø
In operation, the gas liquid mixture which passes through
the multiple flow tubes ~in the lower secondary grid is
redis~ributed in the horizontal space between the two grid
plates. Thus, the flow of gas/liquid mixture flowing through
the multiple flow tubes in the upper primary grid into the
ebullated bed will be more uniform than when only a single
grid plate is used.
As generally shcwn Ln FI~. 1, reactor vessel 10 contaLns primary
upper grid plate 12 which is rigidly supported therein
usually at its outer edges by a cylindrical shaped support
skirt 13 connected to the reaotQr lower head 14, and is
sealed to the s1de wall ~n the lower portion of the reactor,
so as to provlde a plenum space 15 be1~ a lower secondary grid plate
30. The feedstream to the reactor enters through conduit ll
and the flow is deflected radially outwardly by stationary
baffle lla. The upper primary grid plate 12 serves to support catalyst bed
25 and contains multiple riser flow tubes 16. As shown in
greater detail in Fl~. 2, each prima ~ riser tube 16 has at least one
opening or slot 17 at its upper end and is covered by a cap
18, which is rigi~ly attached to the upper end of tube 16 by
suitable fastening means 19~ such as a threaded bolt an~ nut.
The lcwer end of cap 18 is spaced outwardly from tube 16 to provide for
uniform flow of fluid upwardly through the tubes 16 and slot
17 of the grid plate 12 and into the bed 25 of catalyst
particles.
As shown in FIG. 2, the lo~er edge of ~he cap 18 is pre-
ferably provi~bd with numer~us notches 18a to provide for the loca-
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lized exit flow Df gas and promote the formation of small ga;
bubbles in bed 25. The notches are intended to let the gas
emerge from under the caps 18 ~ ~11 discrete bubbles instead
of larye globs of gas, and the notch widths should usually be
5-10 times the catalyst effective particle diameter. The
notches located around the bottom of the caps can be used
with indiYidual caps of any shape, such ~ a cylindrical or
tapered shape. Also to prevent backflow of catalyst from the
bed 25 to plenum 15 below the grid plate following reactor
shutdown or loss of recycle liquid flow, a ball check 20 is
usually provided and is prefera~ly located in the upper end
of each riser tube 16, as shown in FI&. 2. The ball check
20 mates with seat 22 provided within the upper end of the
riser tube 16 to prevent any backf!ow of catalyst from the
bed 25 ~o the plenum 15 below the distributor plate 12. To
facilitate the entry of gas suoh as hydrogen into the lower
end of the riser tube 16, openings such as holes 23 or
vertical slots 24 are provided in the tube below the grid
plate 12.
Located below the primary grid plate 12 is a secondary
grid plate 30, containing multiple parallel secc~ndary flc~w tu~es 32
each having ope ~ gs such as holes 33 or vertical slot 34 in
the lower end thereof. The secondary grid plate 30 is spaced
below and usually supported from the upper grid plate 12,
such as by multiple rods 36 ~ith each rod having a spacer
tube 37 located around the rod for maintaining the desired
space 35 between the upper and lower grid plates, as shown in
FIG. 1. The secondary grid 30 can be extended to contact
support skirt 13, or pre~era~ly can have a small annular
spa~e 38 therebetween and be provided with a circumferential
skirt 40 extending downwardly from plate 30. The lower end of
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skirt 40 should extend to substantially the same level as the
lower ends of secondary flow tubes 32. Also, the skirt 40 is
provided with openings such as holes 39 or slots 41, which
are similar to holes 33 or slots 34 in the secondary flow
tubes 32. Furthermore, the flow area of the annular space 38
should not exceed about lO percent of the total flow area for
the openings in the secondary grid plate, i.e. that provided
both by multiple flow tubes 3Z and annular flow space 38.
In operation of the dual grid plate assembly, the
gas/liq3id mixture fed into pl~n 15 forms a gas space 15a bel~ lower
sec~ndary grid plate 30 and abave liqu~d level 15b. me gas an~ liquid
mixture in plenum 15 passes upwardly ~hrough multiple flow
tubes 32 and annular space 38 into space 35 between the upper
and lower grid plates. In space 35, the gas/liquid mixture
is redistr~buted generatly horizontally and the gas portion
rises to form gas space 35a above the liquid level 35b. The
liquid level 35b is controlled by the vertical location of
slots 24 in the lower ends of riser tubes 16 and by the flow
rate ~hrough the grid plate.
It is thus an advantage of the present invention that the
lower secondary grid plate prov1des for the lateral
redistribution of fluid flow below the upper primary grid
plate and thereby tends to correct any flow maldistribution
.below the primary grid plate, which may be caused by flow
maldistribution problems on the underside of the grid.
Reactor bed ebullation wfll be generally uniform unless some
riser tubes become plugged by coke formation, etc.
In an alternative embodiment of the present invention, as
shown in FIG. 3~ ~oth the upper primary grid plate 12 and the
lower secondary grid plate 30 can be separately supported
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from the reactor lower head 14 by means of an outer
cylindrical support skirt 13 for upper grid 12 and an inner
cylindrical skirt 45 for supporting the lower grid plate 30.
For this grid plate configuration, the support rods 36 and
spacer tubes 3~ used for the FIG. 1 embodiment are not
needed. Also, the multiple flow tubes 32 m grid plate 30
are provided with multiple openings 33 or slots 34 to faci-
litate the entry of gas such as hydrogen into the flow tubes,
similarly as for the flow tubes 16 in the upper grid plate
12. If desired9 dual nozzles 11 for the rea~r f~stream ~ be
provided into plenum 15.
It is another important feature of the present invention
that the two grid plates can be structurally integrated, ~o
that the total pressure differential across the grid plate
assembly due to upward fluid flow therethrough and the cata-
lyst bed weight is carried by the assembly of bothgridplates.
As shown in FIG. 49 the riser tubes 42 for the primary upper
grid plate 12 are ex~ended downwardly and rigidly attached
such as by welding to the lower secondary grid plate 30.
Openings such as holes 43 or slots 44 are provided in the
lower end of each tube 42 as before, to provide for entry of
gas such as hydrogen into the flow tube. Also, the upper
primary grid plate 12 can be suitably supported from the
reactcr inner wall by a continuous ring 26 welded to the
wall. The upper grid plate 12 is attached to ring 26 by
multiple fastener bolts 28 and nuts 29. In this grid plate
assembly, the periphery of lower grid plate 30 can be
extended to near the inner wall of reactor 10 so as to pro-
vide a small annular space 46 therebetween, and be provided
with a peripheral depending skirt 48 and holes 49 similarly
as previously described for the FIG. 1 embodiment.
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The utility and effectiveness of the staged flow distri-
bution grid assembly is illustrated by the ~ollowing specific
example, which should not be construed as l~miting the scope
of the invention.
EXAMPLE
In an ebullated bed catalytic hydrogenation reactor for petrole~n
feedstoc3c material, a du~l grid assembly provided near the reac~r
er end has the foll~wing characteristics and diIrensior~
Reactor Temperature, F 750-850
Reactor Pressure, psig 1000-3009
Reactor Ins~de Diameter9 ft 12
Vertical Spacing Between Primary and
Secondary Grid Plates, in. 16
Primary Grid Flow Tube Diameter, in. 1.30
Bubble Cap Diameter, in. 3
Primary Flow Tube Extension
Below Primary Grid Plate, in. 9
Flow Area of Primary Grid Tubes, in. Z.16
Secondary Grid Flow Tube Diameter, in. 4
Secondary Flow Tube Extension Below
Grid Plate, in. 5
Fl ow Area of Secondary Grid Tubes, in. 12.7
Pressure Differential Across Upper Primary
Grid, ~si 5-8
Pressure Differential Across Lower Secondary
Grid, psi 1-3
The catalyst ebullation pattern in the reactor is uniform
o~er a wide range of liquid and gas flow rates from the
plenum upwardly into the reactor bed.
12
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Although this invention has been described broadly and in
terms of certain preferred embodiments thereof, it will be
understood that modifications and variations to the apparatus
can be made and that some elements can be used without others
all within the spirit and ~cope of the invention, which is
defined by the following claims.
