Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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F-2792
MULTI-PHASE COUNTERCURRENT REACTOR PROCESS AND APPARATUS
This invention relates to catalytic reactor operation
wherein a liquid phase is treated with a gaseous reactant. In
particular it relates to a technique for contacting multi-phase
reactants in a fixed porous catalyst bed under continuous
countercurrent conditions, including methods and apparatus for
controlling frothing in the reactor.
Chemical reactions between liquid and gaseous reactants
present difficulties in obtaining intimate contact between phases.
Such reactions are further complicated when the desired reaction is
catalytic, and requires contact of both fluid phases with a solid
catalyst. Numerous multi-phase reactor systems have been developed
wherein a fixed porous bed of solid catalyst is retained in a
reactor. Typically, fixed bed reactors have been arranged with the
diverse phases being passed cocurrently over the catalyst, for
instance as shown in U.S. Patents No. 4,126,539 (Derr et al),
4,235,847 (Scott), 4,283,271 (Garwood et al), and 4,396, 538 (Chen
et al). In the petroleum refining industry, multi-phase catalytic
reactor systems have been employed for dewaxing, hydrogenation,
desulfurizing, hydrocracking, isomerization and other treatments of
liquid feedstocks, especially distillates, lubricants, heavy oil
fractions, residuum, etc,. Other known techniques for contacting
liquid-gas mixtures with solid catalysts include slurry catalyst,
ebullated bed and countercurrent systems, such as disclosed in U. S.
Patents 2,717,202, 3,186,935, 4,221,653, and 4,269,805. While prior
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reactor systems are satisfactory for certain needs, efficient
multi-phase contact has been difficult to achieve for many fixed bed
applications.
It would be beneficial to have a multi-phase reactor
operating under controlled flow conditions to maintain a gas-liquid
froth, which could minimize flow maldistribution patterns and
provide optimum volumetric proportions of upwardly moving gas
dispersed in a downwardly gravitating liquid phase.
Accordingly, the present invention provides a
countercurrent continuous catalytic reactor for treating a liquid
phase with a gaseous reactant characterized by an enclosed reactor
shell containing a fixed porous bed of solid catalyst, an upper
liquid inlet for introducing a stream of liquid substantially above
the catalyst bed for downward gravity flow of liquid through the
bed, a lower liquid outlet for withdrawing treated liquid from the
reactor shell, a gas inlet disposed below the catalyst bed for
introducing a gaseous reactant stream under pressure for
countercurrently contacting downwardly flowing liquid in a mixed
phase reaction zone, whereby gaseous reactant is dispersed through
the liquid phase in intimate contact with the solid catalyst, an
upper gas outlet for withdrawing gas from the reactor shell above
the catalyst bed, a level detector disposed in a froth zone above
the catalyst bed for detecting level of a mixed gas-liquid froth and
generating a signal representative of froth level, and a fluid
handling control operatively connected to the level detector and
responsive to the level signal to control fluid stream flow for
regulating froth level above the catalyst bed thereby permitting
disengagement of dispersed gas from the liquid phase of the froth
and preventing excessive liquid entrainment in the gas outlet.
In another embodiment, the present invention provides a
process for catalytic hydrocracking dewaxirg or desulfurizing of
heavy petroleum feedstock in a multi-phase catalytic reactor
containing a fixed porous bed of solid particles characterized by
using the above-described reactor.
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Figure l is a simplified process diagram showing a vertical
reactor with fixed catalyst bed, major flow streams, and control
elements;
Figure 2 is a segmented vertical cross-section view of a
reactor showing stripping and distributor sections;
Figure is a countercurrent catalyst bed, according to the
present invention, shown in segmented vertical cross section aligned
with the optional top section of Figure 2; and
Figure 4 is a process diagram for an integrated heavy oil
lo repining process.
Cnuntercurrent processes for contacting reactant fluids
have several advantages. In a single point gas entry system, as the
reactant gas rises upwardly from its point of introduction at the
bottom of a vertical reactor below the porous bed, it contacts a
lower concentration of reactive liquid components. At the point of
entry the reactant gas has its greatest concentration. Depletion of
the gaseous reactant upwardly will increase the relative
concentration of inerts and/or byproduct vapors. Likewise, the
liquid being treated is generally more reactive at the upper end of
the reactor system where it contacts the depleted rising gaseous
phase. Thus, the reactant concentration gradients for
countercurrent two-phase systems are opposing. In a typical
multi-phase reactor system, the average gas-liquid volume ratio in
the catalyst zone is about i to 4:1 under process conditions.
In those reactions wherein the volume of gas decreases due
to reactant depletion, the volumetric ratio or liquid to gas can
increase markedly as the liquid feedstock gravitates downwardly
through the reactor. In reactions which consume large amounts of
hydrogen it may be desirable to have multiple reaction gas feed
grids at various levels in the catalyst bed. In general, the
quantity of unreacted gas at any particular level should be adequate
to provide a mixed phase bulk density of at least 20% of the bulk
density of the liquid phase (at reaction conditions). Vapor
production, adiabatic heating or expansion can also affect the
volume.
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Advantageously, the multi-phase reactor system is operated
to achieve uniform distribution. If too little liquid flux is
maintained, the catalyst surface in the porous bed will be coated
with a liquid film; however, this trickle mode will permit excessive
channeling of the gas phase instead of the desired dispersion
characteristics of a reactant froth. Flow rates for both reactant
phases are controlled within constraints. Froth formation and
disengagement is a function of the liquid viscosity, surface tension
and composition. By detecting and controlling froth level, the
lo proper operation of the reactor can be assured.
In order to maintain a desirable uniform flow of reactant
streams through the fixed catalyst bed, adequate flow paths for
liquid and gaseous phases must be provided. In a continuous process
the ratio of reactant gas to liquid feedstock and the space velocity
of reactants relative to catalyst must be carefully considered.
Achievement of uniform vertical flow through a porous bed of solids
can be obtained if the catalyst is properly distributed and shaped.
The void volume in a reaction zone is a function of catalyst
configuration and loading technique. While a densely packed bed of
spherical solids may be employed to place a maximum amount of
catalyst in a predetermined reactor volume, the low void fraction
may interfere with fluid flow, especially where countercurrent flow
of two phases is required. Advantageously, the catalyst bed has a
high void volume, typically greater than one half of the bed. Void
fractions from 0.5 to 0.9 can be achieved using loosely packed
polylobal or cylindrical extrudates. Hollow ring-type supported
catalysts, such as Raschig rings or the like, permit liquids to flow
downwardly through the porous bed by gravity while the gas phase
reactant rises through the denser liquid, forming dispersed bubbles
which contact the wetted catalyst to enhance mass transfer and
catalytic phenomena.
Catalyst size can vary widely within the inventive concept,
depending upon process conditions and reactor structure. If a low
space velocity or long residence in the catalytic reacton zone is
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.
permissible, small catalysts having an average maximum dimension of
l to 5 mm may be employed. However, it is preferred to use larger
sizes, e.g., 0.5-2 cm or more, especially when extrudates, rings,
saddles or other contact materials are desired. Relatively small
catalyst particles may be loaded randomly to assure uniformity and
larger supported catalysts may be stacked in a geometric pattern to
achieve optîmum bed utilization.
Reactor conFiguration is an important consideration in the
design of a continuously operating system. In its simplest form, a
lo vertical cylindrical pressure vessel is provided with a catalyst
retaining device and operatively connected for countercurrent fluid
flow. A typical vertical reactor having a catalyst bed length to
effective diameter (L:D) ratio of about l:l to 20:1 is preferred. A
single bed or a stacked series of beds may be retained within the
same reactor shell. While a reactor of uniform horizontal cross
section is disclosed herein, other non-uniform configurations, such
as spherical reactors9 tapered vessels, etc., may be employed.
Referring to Figure l, a countercurrent continuous
catalytic reactor system is shown for treating a liquid phase with a
gaseous reactant. An enclosed reactor shell lO contains a fixed
porous bed 12 of solid catalyst. Upper liquid inlet 14 is provided
for introducing a stream of liquid substantially above the catalyst
bed for downward gravity flow through the bed toward lower liquid
outlet 16 for withdrawing treated liquid from the reactor shell.
Gas inlet 20 is disposed below the catalyst bed for introducing a
gaseous reactant stream under pressure for countercurrently
contacting downwardly flowing liquid in a mixed phase reaction zone,
whereby gaseous reactant is dispersed through the liquid phase in
intimate contact with the solid catalyst. After passing through a
reactant disengagement zone 22 above the catalyst bed 12 through
upper gas outlet 24, gas is withdrawn From the reactor shell lO.
Level detector 30 is disposed in a froth zone between
interface levels Hl and H2 above the catalyst bed for detecting
the level of a mixed gas-liquid froth and generating a signal
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representative of the froth level. Fluid handling control is
provided by control module ~27 responsive to the level signal to
control fluid stream flow for regulating froth level above the
catalyst bed, thereby permitting disengagement of dispersed gas from
the liquid phase of the froth and preventing excessive liquid
entrainment in the gas outlet. The fluid handling control may
include liquid outlet valve 34 for withdrawing treated liquid at
increased rate with increased froth level. Liquid and gas reactant
feed rates may be controlled proportionally or as otherwise
predetermined by setting control module 32 to operate liquid feed
valve 36 and/or gas feed valve 38. Inlet flow control may be
employed to vary the interface or froth level, as well as reactant
proportions.
A more detailed depiction of a catalytic reactor system,
lS for use with the present invention is shown in Figs. 2 and 3, both
of which are segments of a vertical reactor. Fig. 2 is a top
segment of a reactor 110 having upper liquid feed inlet 114 for
introducing the charged feed to an optional stripping section 120.
In a typical application, such as lube oil dewaxing or mild
desulfurization, the preheated liquid feed is introduced into the
upper part oF a stripping section, where the feed is stripped of
volatile components, dissolved and entrained contaminants by
countercurrent stripping with the product gas stream rising from the
catalyst bed. The gas stream is removed from the reactor through
top gas outlet 125, which may have a back pressure regulator 130
responsive to pressure of the enclosed reactor. Additional
stripping gas may be introduced via inlet 118 into the lower part of
the stripping section, if desired. The quantity of stripping gas
should be sufficient to saturate the entering liquid feed with the
gas plus enough additional gas to achieve the desired removal of
contaminants such as entrained water, H2S, water vapor, and the
like. The temperature and pressure in the stripping zone
approximate the conditions desired in the catalyst bed. It is
understood that the feed stripping feature may not be required in
all instances.
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F-2792 7
The liquid feed flowing down from the stripper (or directly
from the feed source if a stripper is not employed) passes to a
horizontal distributor tray 130 which spreads the liquid across the
catalyst bed. The distributor tray is provided with gas risers 132
to allow rising gas to pass through the tray and proceed upwardly.
A typical distributor tray has a spaced array of perforations or
orifices 135 sized to allow even liquid stream flow from the upper
surface of the horizontal tray 130 into the disengagement zone
below, further depicted in Fig. 3.
ReFerring to Fix. 3, the bottom segment of a typical
vertical reactor shell 210 is shown in broken vertical cross section
view. A bed of catalyst particles 212 is maintained within the main
central portion of reactor 210 by a catalyst support grid 214,
having retainer 215 to prevent loss of particles from the bed. A
plurality of liquid collector pipes 217 recover treated liquid via
plenum chamber 218, outlet conduit 216 and liquid control valve
234. Flow equalizing restrictions may be ernployed with the
collector pipes to assure even withdrawal. Reaction gas is injected
into the lower part of the catalyst bed through a gas sparger-type
inlet 220. This device may employ a grid with nozzles in a known
manner to obtain substantially uniform gas bubble streams. Gas
injected into the lower part of the catalyst bed rises through the
descending column of liquid and creates a froth or foam. This froth
can have a bulk density of between 20% and 80% of the density of the
liquid feed (at reaction conditions), and normally consists
essentially of a continuous liquid phase with gas bubbles dispersed
therein. Above the catalyst bed 212, the rising gas becomes
disengaged from the two phase froth in zone 222. Interface level
controller 230 senses the density by measuring differential pressure
of the fluids between its pressure taps and regulates the flow of
liquid from valve 234 at the bottom of the reactor and/or reactant
gas inlet valve 238 to maintain the froth-to-gas interface at the
desired level H above the top of the catalyst bed.
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It should be understood that a sharp interface may not
exist, but rather a vertical gradient in bulk density can prevail in
the disengagement zone. This gradient in density can be used to
regulate flow of liquid from the bottom of the reactor. The froth
level detector may employ any oF several techniques for locating the
interface within the disengagement zone. While differential
pressure taps disposed at upper and lower points on the reactor
shell adjacent the disengagement zone are suitable, other detectors,
such as radiation gauges and the like, may be employed.
Catalyst particles should be of a size, e.g., 6mm (l/4")
that they will not easily be dislodged by the rising gas bubbles.
Catalyst holddown screens across the top and perhaps at intermediate
points in the reactor bed may be helpful in minimizing catalyst
motion. Multiple catalyst beds with redistributors between beds may
be useful in some applications. Likewise a mechanical device for
creating and maintaining the desired froth bulk density may be
usefulv Devices, such as rotating disc contactors, may be adapted
for this purpose.
The present technique is adaptable to a variety of
interphase catalytic reactions, particularly for treatment oF heavy
oils with hydrogen-containing gas at elevated temperature.
Industrial processes employing hydrogen, especially petroleum
refining, employ recycled impure gas containing lO to 50 mole % or
more of impurities, usually light hydrocarbons and nitrogen. Such
reactant gases are available and useful herein, especially for high
temperature hydrogenation or hydrogenolysis at superatmospheric
pressure.
In the refining of lubricants derived from petroleum by
fractionation oF crude, a series of catalytic reactions are employed
to severely hydrotreat, convert and remove sulfur and nitrogen
contaminants, hydrocracking and isomerizing components of the
lubricant charge stock in one or more catalytic reactors. This can
be followed by hydrodewaxing and/or hydrogenation (mild
hydrotreating) in contact with different catalysts under varying
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F-2792 - 9 -
reacton conditions. An integrated three-step lube refining process
disclosed by Garwood et al, in U.S. Patent No. 4,283,271 is
adaptable to countercurrent processing according to the present
invention. This modification is depicted schematically in Figure 4,
wherein a waxy lubricant oil petroleum fraction is introduced to
stage I (severe hydrotreating, cracking, etc.) countercurrently with
a gas stream containing hydrogen reactant Product gas from the
first stage may contain converted sulfur and/or nitrogen which can
be removed by a sorber prior to introduction to the bottom of the
Stage II hydrodewaxing reactor for countercurrent contact with
liquid from the bottom of Stage I. Following dewaxing, the gas and
liquid streams are then combined for cocurrent downflow through the
mild hydrotreating reactor in Stage III~ Treated lube oil liquid
product is recovered from a phase separator. Spent hydrogen may be
-15 recovered as off gas or purified for recycle in a known manner.
The advantages of the present invention include: (l) longer
liquid residence time in contact with the catalyst than with a
typical cocurrent downflow reactor or in an entrained up-flow
reactor; (2) countercurrent flow pattern will lessen the need for
large volumes of gas and alleviate flow maldistribution
characteristic of prior art mixed phase cocurrent tlow; (3) upward
flowing gas bubbles serve to agitate the downward moving froth and
thus facilitate intimate contact between the gas, liquid, and solid
(catalyst) phases.
