Device and method for holding catalyst in a radial flow reactor

A device for holding catalyst in a radial flow reactor includes a plurality of catalyst containers each of which is a segment of a cylinder divided in the axial plane thereof. Each catalyst container has a cross section of a size which enables the container to be carried in and out of the radial fow reactor through an opening formed in an upper or lower portion of the radial flow reactor, and includes a screen provided on a liquid inlet side and a screen provided on a liquid outlet side. The catalyst containers are assembled together to form a cylindrical catalyst bed in the radial flow reactor. The catalyst containers attains a uniform thickness of a catalyst bed in a radial direction between its inlet side and outlet side over the entire height of the cylindrical catalyst bed. One or more catalyst containers requiring checking or repair only can be taken out of the reactor.

BACKGROUND OF THE INVENTION
 This invention relates to a device and method for holding catalyst in a
 radial flow reactor used as a reforming reactor in a petroleum refinery,
 an ammonium synthesis apparatus in an ammonium production plant or the
 like.
 As a reforming reactor in a petroleum refinery plant or an ammonium
 synthesis apparatus in an ammonium production plant, a radial flow reactor
 is a suitable reactor for its high efficiency of contact between fluid and
 granular catalyst. A radial flow reactor is, as is well known, a reactor
 in which the materials being processed flow radially inward through a
 catalyst bed and into a chamber communicating with an outlet conduit and
 this catalyst bed is formed in a generally vertically erected cylindrical
 configuration. For building a radial flow reactor, therefore, it is
 necessary to form a vertically erected cylindrical catalyst bed. For
 forming such cylindrical catalyst bed, there are two conventional methods
 as will be described below.
 One of the conventional methods of forming a cylindrical catalyst bed
 relies on employment of a center pipe screen a and scallop screens b as
 shown in FIG. 10.
 As the scallop screens b, slitted plates of a relatively small thickness
 are used for necessity of forming a large number of slits c and for
 facility of processing and these screens b are formed in a scallop shape
 for preventing deformation or collapse. Since the scallop screens b are
 not strong enough to stand pressure of catalyst filled in the catalyst bed
 by themselves, these scallop screens b are disposed along the inner wall
 of a reactor d.
 The center pipe screen a is erected in the center of the reactor d in a
 self-supporting manner. Since the center pipe screen a which is subject to
 a strong catalyst pressure is made of wire netting or a perforated plate
 and has not sufficient strength to stand the catalyst pressure, a
 perforated pipe e of a large thickness is provided inside of the center
 pipe screen a for reinforcing it. These scallop screens b and the center
 pipe screen a are installed independently from each other by separate
 installation work and upon completion of the respective screens a and b,
 catalyst is filled in an annulus formed between the center pipe screen a
 and the scallop screens b and, as a result, a vertically erected
 cylindrical catalyst bed is formed. In using the radial flow reactor,
 fluid is generally supplied from an inlet f located in the upper portion
 of the reactor d. The fluid then enters the cylindrical catalyst bed from
 the scallop screens b for a predetermined catalytic reaction and then is
 collected in the center pipe screen a and led to an outlet provided in the
 lower portion of the reactor d.
 Conversely, fluid may be introduced from the center pipe screen a and
 collected from the scallop screen b and led to the outside of the reactor.
 Likewise, the inlet for fluid may be provided in the lower portion of the
 reactor and the outlet in the upper portion of the reactor.
 The other method for forming a cylindrical catalyst bed in a radial flow
 reactor employs, as shown in FIG. 11, an inner cylindrical screen g and an
 outer cylindrical screen h.
 The inner cylindrical screen g is of a similar construction to the center
 pipe screen a of FIG. 10 and and is installed in substantially the same
 way as the center pipe screen a.
 The outer cylindrical screen h is, as is different from the scallop screens
 b of FIG. 10, installed in a self-supporting manner and, for this purpose,
 has a reinforced cylindrical construction. A cylindrical catalyst bed is
 formed by filling catalyst in an annulus formed between the outer and
 inner cylindrical screens h and g.
 These conventional radial flow reactors have, however, several problems
 which have remained unsolved to date.
 First, for obtaining a catalytic reaction of a high efficiency in a radial
 flow reactor, time of contact of fluid with catalyst, i.e., distance of
 passage of fluid through catalyst, needs to be uniform. For this purpose,
 the catalyst bed needs to have a uniform thickness in radial direction
 throughout its entire height, i.e., a uniform radius in all cross sections
 of the cylinrical configuration.
 In the radial flow reactor using the scallop screens b and the center pipe
 screen a shown in FIG. 10, however, the outer periphery of the cylindrical
 catalyst bed is defined by the shape of the scallop screens b and
 therefore this catalyst bed cannot inherently attain a uniform thickness
 in radial direction.
 A catalytic reaction in a radial flow reactor is generally performed under
 a high temperature and a high pressure. Since the scallop screens b are
 disposed along the inner wall of the reactor d, a gap tends to develop
 between adjacent scallop screens b due to thermal expansion and
 contraction of the scallop screens b occurring during running and stopping
 of the reactor. This causes a part of catalyst to enter space between the
 inner wall of the reactor d and the rear side of the scallop screens b
 through this gap formed between the adjacent scallop screens b with a
 resulting loss of efficiency in the catalyst reaction.
 Moreover, when the center pipe screen a which is fixed on the bottom pate
 of the reactor d is even slightly inclined due to an installation error,
 the center pipe screen a cannot have a concentric relation particularly in
 its upper portion with respect to the inner wall of the reactor d with the
 result that a uniform thickness in radial direction of the cylindrical
 catalyst bed cannot be obtained. Thus, it is extremely difficult in the
 radial flow reactor using the scallop screens b and the center pipe screen
 a to form a cylindrical catalyst bed having a uniform thickness in radial
 direction.
 In the radial flow reactor using the inner cylindrical screen g and the
 outer cylindrical screen h, the scallop screens b are not used and,
 therefore, the inherent lack of uniformity in thickness of the catalyst
 bed in the radial flow reactor using the scallop screens b as described
 above does not exist. Since, however, the inner cylindrical screens g and
 the outer cylindrical screen h are installed independently and separately
 from each other, there exists in this radial flow reactor the same problem
 as in the radial flow reactor using the scallop screens b that a slight
 inclination between the inner cylindrical screen g and the outer
 cylindrical screen h due to an installation error leads to lack of
 uniformity in thickness in radial direction of the cylindrical catalyst
 bed. Accordingly, it is also difficult to form a cylindrical catalyst bed
 having a uniform thickness in radial direction by this radial flow
 reactor.
 Aside from the above described problem of difficulty in obtaining a
 cylindrical catalyst bed having a uniform thickness due to the shape of
 the screen element and inclination of the cylindrical screens caused by an
 installation error, these conventional radial flow reactors have the
 problem that, in a case where a radial flow reactor is of relatively large
 dimensions, it is difficult in these reactors to attain a uniform
 thickness in radial direction of the cylindrical catalyst bed throughout
 the entire height of the vertically disposed cylindrical catalyst bed,
 even if there is no inclination between the scallop and center pipe
 screens or between the inner and outer cylindrical screens. In a radial
 flow reactor of a large size, the amount of catalyst used in the catalyst
 bed is large and pressure produced by catalyst against the cylindrical
 screens becomes larger in the lower portion of the catalyst bed. Since the
 cylindrical screens are fixed to the reactor in the top and bottom
 portions, they have a substantially constant thickness in radial direction
 in the upper and lower portion of the cylindrical catalyst bed in the
 vicinity of the top and bottom portions of the cylindrical screens. In the
 middle portion of the cylindrical catalyst bed as viewed in the direction
 of its height, however, the cylindrical screens tend to be bent outwardly
 of the catalyst bed due to pressure of the catalyst with the result that
 the thickness in the middle portion of the cylindrical catalyst bed
 becomes larger than the thickness of the cylindrical catalyst bed in the
 vicinity of the top and bottom plates of the cylindrical screens. This is
 particularly the case when a wedge wire screen which is weaker in strength
 than a perforated plate screen is used as the cylindrical screen. It is,
 therefore, difficult to attain a uniform thickness in radial direction
 over the entire height of the cylindrical catalyst bed.
 Another problem in a radial flow reactor is that, even when the thickness
 of the catalyst bed is uniform, fluid does not necessarily flow straightly
 and strictly radially in the cylindrical catalyst bed but it sometimes
 flows in a direction deviated from the radial direction depending upon the
 condition of packing of catalyst. That is, fluid tends to flow more easily
 to a portion of the catalyst bed in which the catalyst is less densely
 packed than to a portion in which the catalyst is densly packed, thus
 causing a deviated flow of fluid. The distance of passage of fluid through
 the catalyst bed therefore tends to vary depending upon the condition of
 packing of the catalyst in the cylindrical catalyst bed resulting in lack
 of uniformity in the product of the catalytic reaction.
 In a radial flow reactor, it becomes necessary after running of the reactor
 for a certain period of time to check a screen portion of the reactor and
 repair it if necessary. For such checking and repair of the screen
 portion, all catalyst packed in the cylindrical catalyst bed must be
 removed out of the reactor regardless of whether the radial flow reactor
 is the reactor of the type shown in FIG. 10 or the one shown in FIG. 11.
 After checking and repairing the screen portion, the catalyst must be
 filled in the cylindrical catalyst bed again. Repair of the screen portion
 is usually made outside of the radial flow reactor. In a case of a
 chemical plant, such regular check of the radial flow reactor is performed
 by closing all plant temporarily and so only a short period of time is
 allowed before running of the plant is resumed. A lot of man power
 therefore is required for completing such check and repair in such a short
 closing period of the plant. Thus, removal and refilling of all catalyst
 packed in the cylindrical catalyst bed not only requires a tremendous
 labor and cost but impair expensive catalyst and deteriorate the quality
 of the catalyst through the removal and refilling processes thereby
 adversely affecting the efficiency of the catalytic reaction.
 Aside from the requirement for checking all screen portion of the reactor,
 there is a case where not the entire screen but only a part of the screen
 needs to be checked or repaired. Even in this case, all catalyst must be
 removed for checking or repairing the part of the screen in the
 conventional radial flow reactors.
 For the purpose of facilitating removal of catalyst from a radial flow
 reactor, U.S. Pat. No. 3,758,279 discloses a radial flow reactor in which
 a catalyst bed is composed of a plurality of concentrically positioned
 cartridges each of which is closed by a bottom member and an upper member
 and screens made of apertured plates. In this radial flow reactor, each of
 the cartridges is independently removable from the reactor and catalyst
 can be replaced in either of the cartridges as desired.
 This prior art radial flow reactor using the concentric cartridge type
 catalyst beds does facilitate removal of catalyst from the reactor. Even
 in this reactor, however, one entire annular cartrdige must be taken out
 of the reactor and all catalyst in the annular cartridge must be removed
 even in a case where only a part of the screen of the cartridge needs to
 be checked or repaired. Moreover, the prior art cartridge type reactor
 does not solve in any way the above described problems of the radial flow
 reactor that it is difficult to attain a uniform thickness in radial
 direction over the entire height of the catalyst bed due to pressure of
 catalyst and that the distance of passage of fluid through the catalyst
 bed tends to vary due to deviation of the flow of fluid from the radial
 direction depending upon the condition of packing of the catalyst.
 Further, in a case where the size of the radial flow reactor is very
 large, the weight of the annular cartridge containing catalyst therein is
 huge and it requires a crane of a tremendous power and hence it is not
 very realistic to use the prior art cartridge type reactor when the size
 of the radial flow reactor is very large.
 It is, therefore, an object of the invention to provide a device for
 holding catalyst in a radial flow reactor which is capable of forming a
 cylindrical catalyst bed having a uniform thickness in radial direction
 over the entire height of the catalyst bed.
 It is another object of the invention to provide a device for holding
 catalyst in a radial flow reactor which is capable of preventing deviation
 of flow of fluid in the catalyst bed and thereby attaining uniform
 distance of passage of fluid through the catalyst bed.
 It is another object of the invention to provide a device for holding
 catalyst in a radial flow reactor which enables checking or repairing of
 only a part of screen portion of the radial flow reactor without removing
 all catalyst in the entire catalyst bed.
 SUMMARY OF THE INVENTION
 For achieving the above described objects of the invention, the device for
 holding catalyst in a radial flow reactor comprises comprising a plurality
 of catalyst containers each of which is a segment of a cylinder divided in
 the axial plane thereof, has a cross section of a size which enables the
 container to be carried in and out of the radial fow reactor through an
 opening formed in an upper or lower portion of the radial flow reactor,
 and comprises a first screen provided on a liquid inlet side and a second
 screen provided on a liquid outlet side, said plurality of catalyst
 containers being assembled together to form a cylindrical catalyst bed in
 the radial flow reactor.
 According to the device of the invention, each catalyst container has a
 uniform thickness of a catalyst bed between the liquid inlet side and the
 liquid outlet side and, accordingly, this uniform thickness is not
 affected at all, even if a slight inclination of the catalyst container
 has arisen in placing it on the container seat.
 Fluid supplied from an inlet provided in the upper or lower portion of the
 radial flow reactor enters the catalyst containers from the first screen
 provided on the liquid inlet side, passes through the catalyst bed of the
 uniform thickness substantially in a radial direction and led to an outlet
 of the reactor through the second screen provided on the liquid outlet
 side.
 Since influence of thermal expansion and contraction due to repetitious
 starting and stopping of running of the reactor is absorbed by each
 catalyst container, the uniformity of thickness of the catalyst bed inside
 the container is hardly affected by such thermal expansion and
 contraction. Even if the thickness of the catalyst bed in the container
 after thermal expansion differs from the thickness of the catalyst bed at
 the initial state of running due to difference in area between the first
 screen and the second screen of the container, the manner of difference in
 the thickness of the catalyst bed is uniform because these containers have
 the same construction and, therefore, the thickness of the catalyst layer
 after thermal expansion is still uniform throughout all catalyst
 containers. Thus, according to the invention, a cylindrical catalyst bed
 of a uniform thickness can be maintained in the catalytic reactor in all
 cases.
 Since the respective catalyst containers are completely sealed from their
 adjacent catalyst containers by their side plates, there hardly occurs
 deviation or bypassing in the flow of fluid. Fluid flows in a regular
 radial flow within each catalyst container restricted by the side plates
 from the first screen to the second screen whereby a uniform, and
 therefore efficient, catalytic reaction can be achieved.
 The respective catalyst containers are assembled to form a cylindrical
 catalyst bed as a whole in the radial flow reactor. This assembled
 cylindrical catalyst bed which has an integral construction constitutes a
 strong and rugged structure which can sufficiently stand the running
 conditions under high temperature and high pressure. Particularly, since
 the cylidrical catalyst bed which was formed in a simple annular
 configuration in the conventional radial flow reactors and also in the
 radial flow reactor disclosed in U.S. Pat. No. 3,758,279 is now composed
 of several segements of the catayst containers in the present invention,
 the catalyst bed of the invention is by far superior in its strength to
 the catalyst beds of the conventional and prior art radial flow reactors.
 The method for holding catalyst in a radial flow reactor comprises filling
 catalyst in each of a plurality of catalyst containers each of which is a
 segment of a cylinder divided in the axial plane thereof and has a cross
 section which enables the container to be carried in and out of the radial
 flow reactor through an opening formed in an upper or lower portion of the
 radial flow reactor, and comprises a first screen provided on a liquid
 inlet side and a second screen provided on a liquid outlet side; carrying
 the catalyst containers having been filled with catalyst into the radial
 flow reactor through the opening thereof and assembling the catalyst
 containers to form a cylindrical catalyst bed; and taking, when necessary,
 only one or more of the catalyst containers which require checking or
 repair out of the radial flow reactor through the opening of the reactor
 for checking or repair, retaining the rest of the catalyst containers
 which do not require checking or repair in the radial flow reactor.
 According to the method of the invention, the troublesome work of removing
 all catalyst from the radial flow reactor for checking or repairing a
 screen portion is obviated and only one or more of the catalyst containers
 which are considered to require checking or repairing need to be taken out
 of the radial flow reactor. Checking of the screen portion per se can be
 carried out without removing catalyst from the catalyst container as
 different from the conventional method according to which catalyst must be
 removed for checking a screen portion. When repair of a screen in a
 catalyst container is necessary, catalyst of the catalyst container only
 is removed, repair is made and catalyst is refilled in the catalyst
 container. This saves labor and time for the repair of the screen as
 compared with the conventional method. Such checking or repair will be
 further facilitated and resumption of running of a plant will be expedited
 if one prepares for a spare catalyst container before carrying out the
 checking or repair and replace a catalyst container requiring checking or
 repair by the spare catalyst container.
 In filling catalyst initially in the catalyst containers of the invention
 before starting running of the radial flow reactor, a relatively small
 amount of catalyst is filled in the catalyst container as compared with
 the convnetional method in which catalyst is filled in the entire annulus
 for a cylindrical catalyst bed. This is beneficial for catalyst life
 because this method of filling a small amount of catalyst is less likely
 to damage catalyst than in the conventional method in which catalyst is
 filled in bulk with the result that a part of catalyst is in a damaged
 condition already when it has been filled in the catalyst bed.
 Additionally, adjustment of density of filled catalyst is much easier than
 in the conventional method.
 In checking or repair of a screen portion, possibility of damage to
 catalyst is remarkablly reduced according to the method of the invention
 as compared with the conventional method for the reason described above.
 Thus, according to the method of the invention, not only a cylindrical
 catalyst bed having a uniform thickness in radial direction can be
 obtained but the deterioration of catalyst can be prevented and the
 efficiency of catalytic reaction can be improved.
 Preferred embodiments of the invention will now be described with reference
 to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS
 FIGS. 1 to 4 illustrate an embodiment of the device for holding catalyst
 according to the invention. FIG. 1 is a persepective view of the device
 schematically showing a state wherein catalyst containers are assembled
 cylindrically in a radial flow reactor.
 A radial flow reactor 1 has an inlet 2 of liquid to perform catalytic
 reaction in the upper portion thereof and an outlet 3 of liquid which has
 finished the catalytic reaction in the lower portion thereof.
 A catalyst container 4 is a box-like container in a form which is obtained
 by dividing a cylinder in the axial direction of the cylinder to plural
 segment portions having a cross section of a size which enables the
 container to be carried into or out of the reactor 1 through the liquid
 inlet 2. As shown in FIG. 2, an outer screen 5 is provided on the
 circumferential surface of the liquid inlet side (i.e., outer
 circumferential surface) and an inner screen 6 is provided on the
 circumferential surface of the liquid outlet side (i.e., inner
 circumferential surface). Side plates 7 and 8 are provided on side
 portions of the catalyst container 4 which connect the screens 5 and 6.
 The bottom portion of the catalyst container 4 is closed with a bottom
 plate 9 and the top portion of the catalyst container 4 is closed with a
 lid plate 10 which can be opened after filling catalyst in the container
 4. The respective screens 5 and 6 consist of wedge wires 12 extending in
 the vertical direction welded to horizontally extending support rods 11.
 For forming a cylindrical catalyst bed by using these catalyst containers
 4, each catalyst container 4 is filled with catalyst. After closing the
 lid plates 10, the catalyst containers 4 are carried into the catalyst
 reactor 1 through the liquid inlet 2 by, for example, hanging them with a
 crane. These containers 4 are placed at predetermined locations on a
 container seat 13 in such a manner that the outer screens 5 face outward
 and the inner screens 6 face inward. A projection may be provided in
 either of the container seat 13 and the bottom plate 9 of the container 4
 and a corresponding depression to fit with the projection in the other of
 the container seat 13 and the bottom plate 9. The adjacent containers 4
 are disposed without a gap therebetween so that the side plates 7 of the
 adjacent containers 4 are in contact with each other. By assembling the
 catalyst containers 4 in the radial flow reactor 1 in such a manner that
 the containers 4 after assembling will constitute a cylinder as a whole, a
 cylindrical catalyst bed 14 is formed. A reference character 30 designates
 spacers which hold the containers 4 with a gap between the inner wall of
 the radial flow reactor 1 and the outer surface of the containers 4.
 During running of the radial flow reactor 1, liquid supplied from the
 liquid inlet 2 of the reactor 1 enters the containers 4, as shown in the
 plan view of FIG. 4, from the screen 5 of the liquid inlet side of the
 containers 4 which are cylindrically arranged and passes through
 cylindrically arranged catalyst bed 14 formed in a uniform thickness in
 radial direction to perform catalytic reaction with the catalyst and then
 enters an inner cylindrical outlet passage 15 and is led out of the liquid
 outlet 3 of the reactor 1.
 In a case where the screens 5 and 6 need to be checked or repaired, one or
 more catalyst containers 4 which need check or repair are carried out of
 the reactor 1 by means of a crane or the like and, after conducting a
 necessary check or repair work, the catalyst containers 4 which have
 completed the check or repair work are restored to their locations in the
 reactor 1. The other catalyst containers 4 which do not require such check
 or repair are left in their original locations in the catalytic reactor 1.
 In the above described embodiment, the catalytic containers 4 are arranged
 cylindrically in one layer in the radial direction to form the cylindrical
 catalyst bed 14. Alternatively, the catalyst containers 4 may be arranged
 in two layers in the radial direction as shown in FIG. 5 by providing
 additional screens 55. The catalyst containers 4 may also be arranged in
 three or more layers in the radial direction.
 The structure of the screens 5 and 6 is not limited to the one shown in
 FIG. 3 but, as shown in FIG. 6, vertical reinforcing members 16 may be
 provided at proper locations in the screens 5 and 6. The wire 12 of the
 screens 5 and 6 need not necessarily be a wedge wire as shown but a wire
 of other cross section such as a square or circular cross section may be
 used as well. As the screens 5 and 6, not only a wire type screen but a
 screen selected from other types of screens made of a perforated or
 slitted plate, wire netting etc. may be used depending upon the situation
 in which the radial flow reactor is used.
 The catalyst containers 4 of the above embodiment are used for a stationary
 catalyst bed. The invention is applicable also to a radial flow reactor of
 a moving catalyst bed type. Since catalyst moves constantly from a
 catalyst inlet to a catalyst outlet, a catalyst container adapted to the
 moving catalyst bed needs to have a structure which enables such movement
 of catalyst. FIG. 7 shows an example of a catalyst container having such
 construction adapted to the moving catalyst bed. A catalyst container 24
 has, in the same manner as the catalyst container 4 of FIG. 2, an outer
 screen 25, an inner screen 26 and side plates 27, 27. The catalyst
 container 24 is formed in the bottom portion thereof with a funnel shaped
 catalyst outlet 28 and in the top portion thereof with a funnel shaped
 catalyst inlet 29. In the same manner as in the above described embodiment
 of the stationary catalyst bed type radial flow reactor, the catalyst
 containers 24 are assembled to a cylindrical catalyst bed. In the moving
 catalyst bed, catalyst gradually enters the catalyst container 24 through
 a pipe (not shown) connected to the catalyst inlet 29, moves downward in
 the catalyst container 24 and is removed outside from the catalyst outlet
 28. New catalyst is always supplied into the catalyst container 24 from
 the catalyst inlet 29.
 In the above described embodiments, the catalyst containers 4 or 24 which
 are of the same cross sectional shape are cylindrically arranged.
 Alternatively, as shown in FIG. 8, catalyst containers 34 and 35 having
 cross sectional shapes which are different from each other may be arranged
 in combination to form a cylindrical catalyst bed.
 In the above described embodiments, the catalyst containers are so
 constructed that a completely radial flow path of fluid is formed in the
 catalyst bed. The invention is not limited to this but other type of flow
 path such as an oblique flow path with respect to the radial direction of
 the cylindrical catalyst bed may be employed if it can maintain a
 substantially uniform distance of passage of fluid flow.
 In the above described embodiments, each of the catalyst containers 4 and
 24 is made of a single oblong box-like body. Alternatively, a catalyst
 container may be divided in plural sections in the longitudinal direction
 by a substantially horizontal plane or inclined plane as shown in FIGS. 9A
 or 9B. By the arrangement of FIGS. 9A or 9B, carrying of the catalyst
 container in or out of the radial flow reactor can be facilitated. In the
 above described embodiments, the catalyst containers are carried in and
 out of the reactor 1 through the upper inlet 2. The catalyst container may
 also be carried in and out of the reactor 1 through a man hole (not shown)
 formed in the lower portion of the reactor 1.
 In the above described embodiments, the catalytic container 4 or 24 has
 side plates 7, 8 or 27, 21 provided in the side portions of the container
 4 or 24. This structure is particularly preferably because this structure
 is very effective for regulating the flow of fluid from the outer screen 5
 or 25 to the inner screen 6 or 26 to a regular radial flow without
 deviation. However, it is possible, if necessary, to provide a screen in a
 part of all of one or both of the side plates 7, 7 or 27.
 Instead of forming the entire outer or circumferential surface of the
 catalyst container with a screen, only a part of the outer or inner
 circumferential surface may be formed with a screen so as to extend the
 distance of passage of fluid in the catalyst bed.
 In the above described embodiment, the inlet for fluid is provided in the
 upper portion of the reactor and the outlet for fluid in the lower portion
 of the reactor. Conversely, the inlet for fluid may be provided in the
 lower portion of the reactor and the outlet in the upper portion of the
 reactor. Also, conversely to the above described embodiment, the fluid may
 be caused to flow into the catalyst container 4 from the inner
 circumferential surface and flow out of the outer circumferential surface.