Systems and methods for improving feed catalyst contacting in downflow reactors

A downflow reactor, e.g. a downer reactor or system, includes an outer wall defining an interior reactor space. An elongated plug is within the outer wall having a first end and a second end, defining a longitudinal axis between the first and second ends. A distribution baffle positioned at a vertical position between the first end and the second end of the elongated plug configured and adapted to direct hot down flowing catalyst towards a feedstock spray.

BACKGROUND

The present disclosure relates to the efficiency of mixing a catalyst and feedstock together, specifically to mixing a hot catalyst with a hydrocarbon feedstock in a downflow reactor.

2. Description of Related Art

Fluid Catalytic Cracking (FCC) is a commonly-used process in oil refineries that produces high yields of gasoline and liquefied petroleum gas (LPG), propylene and other products which serve as feedstock for the petrochemical industry, which are in a high demand in the United States, and throughout the world. Despite the long existence of the FCC process, techniques are continually sought for improving product conversion and yield selectivity of high value products.

The conversion a FCC feedstock to product occurs in a reactor system where hot catalyst is contacted with an injected hydrocarbon feed. This reaction can be carried out in an upward flowing riser-reactor system or in a downward flowing downflow reactor (DFR) or downer system where the catalyst and vapors are flowing in the direction of gravity. The downward flowing reaction systems is amenable to high propylene and high conversion to light olefin product because it operates at high severity in terms of temperature and Cat/Oil for a short contact time between the catalyst and feed.

In the DFR section of high-severity-FCC (HS-FCC™) systems, a discrete number of feed nozzles are used to inject a feedstock, e.g. a hydrocarbon feed, into downward flowing hot catalyst from a regenerator. The coverage of the spray from the feed nozzles tends to be limited, leaving areas around the internal periphery of the DFR where the hot catalyst bypasses the feed spray, as shown by the green color on the periphery of the reactor wall inFIG. 7A. The DFR generally includes a center plug which causes the downflowing hot catalyst to form into an annular flow pattern in the feed zone. When it ends, the hot catalyst tends to bypass the feed spray in the center of the feed zone, as shown by the central green portion inFIG. 7A. The net effect of catalyst bypassing is retarding the rapid vaporization of the feed. Considering that the total vapor residence time in the DFR ranges from 0.5 to 1 second, it is important that the feed be completely vaporized within 0.2 seconds. Any portion of the feed not vaporized in the feed zone could remain partially vaporized at the end of the DFR representing loss in feed conversion to desired products.

In the fully developed regime of the DFR, the hot catalyst is traveling slightly faster than the gas feed and the catalyst profile across the DFR can result in a core annular flow with higher concentration of hot catalyst flowing along the wall as shown inFIG. 7A. The central flow of the catalyst as well as the peripheral bypass acts to reduce the effective mixing of catalyst and feedstock leading to poor DFR performance, reduced conversion of feedstock to valuable product such as LPG and light olefins and increased thermal cracking as compared to the desired catalytic cracking.

In CN109666503A, a downer and catalytic conversion method is described. The downer reactor (1) includes an inner wall of the main reaction section (16) having an annular baffle (14) coaxial with the main reaction section (16). See CN109666503A, Abstract. CN109666503A, however, does not address catalyst and vapor mixing inefficiencies in the feed zone caused by catalyst bypassing and the large mixing volume in the feed zone. Moreover, CN109666503 does not include a plug, nor is the baffle positioned optimized in relation to other components of the system.

The conventional techniques have been considered satisfactory for their intended purpose. However, there is an ever present need for improved systems and methods which can better convert hydrocarbon feedstocks into products such as LPG and/or olefins. This disclosure provides a solution for this need.

SUMMARY

A downflow reactor includes an outer wall defining an interior reactor space. An elongated plug is within the outer wall having a first end and a second end, defining a longitudinal axis between the first and second ends. A distribution baffle positioned at a vertical position between the first end and the second end of the elongated plug configured and adapted to direct hot down flowing catalyst towards a feedstock spray.

In some embodiments, the distribution baffle includes a converging section and a constant-area section. The converging section can converge from an upstream end of the distribution baffle towards a downstream end of the distribution baffle. An upstream end of the distribution baffle can be mounted to an interior surface of the outer wall. The distribution baffle can be integrally formed with the outer wall. The distribution baffle can be formed from at least one of a refractory material or a metallic material. The first end of the elongated plug can have a first diameter and the second end of the elongated plug can have a second diameter. The second diameter can be greater than the first diameter and the second end can be positioned downstream from the first end.

In certain embodiments, the downflow reactor includes at least one feed nozzle positioned on a perimeter of the outer wall configured to spray a feedstock into the interior reactor space. The second end of the elongated plug can terminate at a vertical position below an outlet of the at least one feed nozzle configured and adapted to minimize hot segregated catalyst flow along the elongated plug. The at least one feed nozzle can define a respective central injection axis. The respective central injection axis can intersect an exterior surface of the second end of the elongated plug.

The downflow reactor can include a mixing baffle positioned at a vertical position below a terminal end of the second end of the elongated plug. The outer wall can define an upstream cylindrical portion and a diverging section downstream from the upstream cylindrical portion. The mixing baffle can be positioned at least partially within the diverging section of the outer wall. The mixing baffle can be integrally formed with the outer wall. The mixing baffle can be formed from at least one of a refractory material or a metallic material. An upstream end of the mixing baffle can be mounted to an interior surface of the diverging section of the outer wall. The downflow reactor can include at least one distributor (e.g. nozzle, conduit, pipe, or the like) positioned within the mixing baffle and/or in abutment with the mixing baffle. The at least one distributor can be configured and adapted to supply at least one of steam, vapor, gas or hydrocarbon feed into the downflow reactor. The mixing baffle can be frustoconical and converges in a downstream direction toward a central axis of the outer wall.

The downflow reactor can include at least one lower baffle positioned at a vertical position below the diverging section of the outer wall. The at least one lower baffle can be frustoconical and converges in a downstream direction toward a central axis of the outer wall. The at least one lower baffle can be integrally formed with the outer wall. The at least one lower baffle can be formed from at least one of a refractory material or a metallic material. The downflow reactor can include at least one distributor (e.g. nozzle, conduit, pipe, or the like) positioned within the lower baffle and/or in abutment with the mixing baffle. The at least one distributor can be configured and adapted to supply at least one of steam, vapor, gas or hydrocarbon feed into the downflow reactor. The at least one lower baffle can be a plurality of lower baffles positioned at different vertical positions along the outer wall, each at a vertical position below the diverging section of the outer wall.

In some embodiments, the elongated plug defines an exterior surface where the exterior surface includes at least one projection extending therefrom. The at least one projection can be a series of helical vanes. The elongated plug can define a fluid path through a hollow portion of the plug. The fluid path can begin at a first end of the plug and terminates at a second end of the plug for injection of steam and/or hydrocarbon feedstock into the interior reactor space. The first end of the elongated plug can have a first diameter and the second end of the elongated plug can have a second diameter. The second diameter is greater than the first diameter and wherein the second end is positioned downstream from the first end, wherein the at least one projection extends from the exterior surface on the second end.

DETAILED DESCRIPTION

Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, an illustrative view of an embodiment of a downflow reactor is shown inFIG. 1and is designated generally by reference character100. Other embodiments of systems in accordance with the disclosure, or aspects thereof, are provided inFIGS. 2-11, as will be described. Embodiments of the downflow reactor described herein can be used to (i) provide enhanced mixing intensity between a catalyst and a hydrocarbon feed, (ii) improve conversion and selectivity to more valuable products such as olefins, (iii) eliminate/minimize hot catalyst bypassing of feed in the feed zone, thus increasing the effective catalyst to oil ratio, and (iv) ensure catalyst and vapor are at approximately identical temperatures leaving the downflow reactor. Embodiments of the present disclosure provide improved mixing of hot catalyst with a hydrocarbon feedstock in downflow reactor100to maximize hot catalyst utilization to enhance feed vaporization and conversion.

As shown inFIGS. 1 and 5, downflow reactor100(e.g. a downer reactor or system) includes an outer wall102defining an interior reactor space104. Outer wall102is generally cylindrical and defines a central axis Y. Outer wall102is lined with a refractory material103to protect the outer wall102from thermal degradation and erosion. An elongated plug106is within the outer wall102having a first end108and a second end110, defining a longitudinal axis A between the first and second ends. The second end110is positioned downstream from the first end108. The first end108of the elongated plug106has a first diameter D1and the second end110of the elongated plug106has a second diameter D2. The elongated plug106is mounted to outer wall via mounting arms137extending from the first diameter D1portion. The second diameter D2is greater than the first diameter D1. The start of the feed zone corresponds to a vertical position141where the catalyst meets a distribution baffle112, described in more detail below. The feed zone generally ends at the intersection of diverging section134and a downstream cylindrical section135(at vertical position Z). The second diameter D2is designed to optimize hydrocarbon spray penetration into the downwardly flowing catalyst and to have a good balance between catalyst velocity and spray momentum. The larger diameter D2relative to diameter D1reduces the mixing volume in the feed zone as well as maintains an annular catalyst flow to the feed nozzles114.

With continued reference toFIGS. 1 and 5, the catalyst flowing along the elongated plug106around diameter D1is spread out towards the feed nozzles114when it impacts the portion of plug with diameter D2. In some embodiments, the ratio of D1to D2is 0.3 to 1. The ratio of D2to an inner diameter of outer wall102ranges from 0.1-0.4. The ratio of D1to D2defines the catalyst velocity flowing downwards at the same time it sets the penetration distance for the spray from the feed nozzles114. The increased length L of plug106, as compared with traditional plugs, ensures that catalyst leaving the feed zone has already contacted the feed from feed injectors even if some catalyst flows along the wall of the plug106. In this way, the plug106does more than maintain annular flow, it helps optimize the mixing volume and promote catalyst oil mixing. The plug106design can take various shapes with or without vanes or extensions with the purpose of enhancing mixing of hot catalyst and feed and reducing catalyst tendency to flow along the outer surface of the plug106.

With continued reference toFIG. 1, feed nozzles114are positioned on a perimeter of the outer wall102configured to spray a hydrocarbon (oil) feedstock into the interior reactor space104. Traditional plugs generally terminate above or at the same vertical position as feed nozzles114. On the other hand, the second end110of the elongated plug106terminates at a vertical position below an outlet115of the at least one feed nozzle114resulting in minimized hot segregated catalyst flow along the elongated plug106and making the plug106an integral part of the mixing zone design. Each feed nozzle114defines a respective central injection axis C. The respective central injection axis C intersects an exterior surface128of the second end110of the elongated plug106. In certain embodiments, plug106ends at a vertical position equal to minus 0.8 to 1.5 times D2below the projected feed spray intersection.

As shown inFIG. 11, another embodiment of a plug406is shown with a fluid path405through a hollow portion of the plug406. The plug406is similar to the plug106except that plug406has fluid path405entering at a first end408and distributor407at a second end410to allow for injection of steam and/or hydrocarbon feedstock into an interior reactor space (similar to interior reactor space104). Plug406can similarly be used within reactor100. Fluid path405is in fluid communication with a feedstock or steam source from outside of reactor100, which is piped in through a pipe/conduit, or the like. Distributor407is shown with a series of outlets where steam and/or hydrocarbon feedstock is injected into the feed zone of reactor100(shown schematically by downwardly directed arrows419).

As shown inFIGS. 1 and 2, a distribution baffle112positioned at a vertical position along central axis Y between the first end108and the second end110of the elongated plug106configured and adapted to direct hot down-flowing catalyst, schematically indicated by a series of downwardly pointing arrows116, towards a feedstock spray being introduced at feed nozzles114. The hot catalyst inlet temperature varies from 1260° to 1330° F. The distribution baffle112is a protrusion extending inwardly toward central axis Y from outer wall102and includes a converging section118and a constant-area skirt section120. Distribution baffle112is metallic, e.g. made from stainless steel, or the like, and is similarly coated in a refractory material103. The converging section118converges from an upstream end122of the distribution baffle112towards a downstream end124of the distribution baffle112. The upstream end122of the distribution baffle112is mounted to an interior surface126of the outer wall102. The distribution baffle112acts to direct hot down flowing catalyst towards the feed spray from nozzles114resulting in increased contact with the feed and thereby increased feed vaporization. The feedstock inlet temperature at nozzles114depends on feed type, but generally varies from 400° to 600° F. While distribution baffle112is described as having a converging section and constant area skirt section, it is contemplated that baffle112can have a variety of suitable shapes, while still achieving the directing objectives described above.

With continued reference toFIGS. 1 and 2, the converging distribution baffle112together with the enlarged and lengthened plug106minimize the reactor100volume and intensifies mixing of hot catalyst and feed, further resulting in increasing the speed of feed vaporization as the feed is rapidly vaporized (within 0.2 seconds) and diffused into the catalyst pores where reaction takes place. Rapid catalyst oil mixing and the resulting near isothermal conditions provide minimal hot catalyst bypassing the feedstock, which increases the effective utilization of catalyst and hydrocarbon feedstock. The number, type, shape, length and angle of the distribution baffle112can be optimized depending on the application.

As shown inFIGS. 1 and 3, the downflow reactor100includes a mixing baffle130positioned at a vertical position below a terminal end109of the second end110of the elongated plug106. The outer wall102defines an upstream cylindrical portion132and a diverging section134downstream from the upstream cylindrical portion132. Diverging section134is generally defined between vertical positions X and Z. The mixing baffle130is a protrusion positioned at least partially within the diverging section134that extends inwardly from the outer wall102. Mixing baffle130is metallic, e.g. is made from stainless steel, or the like, and is similarly coated in a refractory material103. An upstream end136of the mixing baffle130is mounted to interior surface126of the diverging section134of the outer wall102. The mixing baffle130is frustoconical and converges in a downstream direction toward central axis Y of the outer wall102. It is contemplated, however, that other embodiments of mixing baffle130can have a variety of other shapes. In addition to mixing in the feed zone, the mixing baffle130aids in moving catalyst flowing along the wall which might otherwise bypass the feedstock vapor to contact the feedstock vapor, resulting in reduced thermal cracking and increased catalytic cracking. The type, shape, location length and angle of the mixing baffle130can be optimized depending on the application.

With continued reference toFIGS. 1-3, the increased length and diameter of the plug106, as compared with traditional plugs, and the inclusion of converging distribution baffle112and mixing baffle130, acts to eliminates/minimize hot catalyst bypassing of feed in the feed zone. and redirects hot catalyst flowing along the reactor100wall below the feed zone into the main gas vapor flow path for additional catalytic cracking. These elements also ensure that catalyst and vapor are at approximately identical temperatures leaving the downer reactor100thus, ensuring that the effective catalyst to oil ratio is optimized. The outlet temperature for downer reactor100varies from 1080° to 1200° F. The propylene yield depends on feedstock and operating conditions and can vary from 16 wt % to above 20 wt %.

With reference now toFIGS. 1 and 4, the downflow reactor100includes a plurality of lower baffles138, e.g. lower mixing baffles, positioned at a vertical position below the diverging section134of the outer wall102. Each lower baffle138is frustoconical and converges in a downstream direction toward central axis Y of the outer wall102. Each lower baffle138is metallic, e.g. made from stainless steel, or the like, and is similarly coated in a refractory material103. Lower baffles138are strategically positioned at different vertical positions along the outer wall102, each at a vertical position below the diverging section134of the outer wall102. Each baffle138is a protrusion that extends inwardly from outer wall102toward central axis Y. While a plurality of lower baffles138are shown, it is contemplated that a single lower baffle138may be used. The natural tendency of two phase vapor-catalyst flowing downwards within reactor100under gravity is for the catalyst to migrate towards the outer wall102and flow downwards along the wall. This segregation represents loss in catalyst and hydrocarbon contacting which lowers the potential for hydrocarbon to contact the catalyst to convert to lighter valuable product. The mixing baffles, mixing baffle130and lower baffles138, are used to re-direct catalyst flowing along the wall towards the center of the reactor100to promote additional catalyst vapor contacting and increase conversion. It also reduces the amount of hot catalyst bypassing and this improves the effective catalyst to oil ratio. Additionally, while mixing baffles130and lower baffles138are shown frustoconically, it is contemplated that, in some embodiments, a variety of baffle shapes can be used to provide similar redirection, reduced bypass, and/or increased mixing between catalyst and feedstock vapors. The number, type, shape, length, position and angle of the lower baffle138can be optimized depending on the application. Mixing baffle130and lower baffles138(also mixing baffles) are incorporated to increase the mixing intensity between the hot catalyst and hydrocarbon feedstock to enhance rapid feed vaporization and conversion into valuable products.

As shown inFIGS. 1 and 6, in certain embodiments, an elongated plug206includes at least one projection240extending therefrom. Otherwise, elongated plug206is the same as elongated plug106. The projections240extend from an exterior surface228of the second end210of elongated plug206. In the embodiment ofFIG. 6, projections240are shown as a series of helical vanes circumferentially spaced apart around the cylindrical perimeter of the elongated plug106. However, it is contemplated that a variety of suitable projections could be used. With reference now toFIG. 10, another embodiment of a downflow reactor300is shown.

Reactor300is the same as reactor100except that instead of having baffles112,130and138formed mostly by a metallic projection welded to outer wall102, reactor300includes baffles312,330and338that are mostly formed by refractory material303integral with the other refractory material303along the inner diameter surface of the outer wall302(or a section thereof). Within the refractory baffles312,330and338are respective metallic supports331. The metallic supports331are welded to wall302and can be continuous circumferential supports or can be spaced apart circumferentially. The cross-sectional profile for each support can mirror that of its respective baffle, e.g. for baffle330, the metallic support331forms a triangular projection. However, it is contemplated that a variety of suitable support projections could be used.

With continued reference toFIG. 10, in some embodiments, baffles330and338can also include staged steam injection or hydrocarbon feed injection capability through distributors314, e.g. baffle nozzles, conduits, pipes, or the like. The distributors314are configured and adapted to supply at least one of steam, vapor, gas or hydrocarbon feed into the downflow reactor. Distributors314are positioned circumferentially about outer wall304and can alternate circumferentially with metallic supports331. While distributors314are shown in lower baffle338, it is contemplated that distributors314can readily be integrated within and/or in abutment with baffles112,312,130,330or138. Refractory baffles312,330and338have similar functions and advantages as baffles112,130and138as described above. Distribution baffle312is similar to baffle112in that it too has a converging section318, similar to converging section118and a constant-area skirt section320, similar to skirt section120. Additional baffles338can be distributed along the length of the reactor300.

As shown by the comparison ofFIGS. 7A-7B, reactor devices constructed in accordance with embodiments of the present disclosure, e.g. reactor100or300, provide improved catalyst flow distribution. InFIG. 7Aa color CFD model of the axial catalyst distribution for a traditional DFR is shown. InFIG. 7B, a color CFD model of the axial catalyst distribution for a DFR constructed in accordance with the present disclosure, e.g. reactor100or300, is shown. Without the elongated plug106and distribution baffle112, catalyst (shown mostly in green) has the tendency to converge into a central column almost immediately after the termination of the plug. In contrast, with elongated plug106and distribution baffle112, catalyst is more uniformly distributed across the cross-section resulting in improved catalytic cracking of the feedstock (shown in blue).

As shown by the comparison ofFIGS. 8A-8B, reactor devices constructed in accordance with embodiments of the present disclosure, e.g. reactor100or300, provide improved axial temperature distribution. InFIG. 8Aa color CFD model of the temperature distribution for a traditional DFR is shown. InFIG. 8B, a color CFD model of the axial temperature distribution for a DFR constructed in accordance with the present disclosure, e.g. reactor100or300, is shown. With the elongated plug106and distribution baffle112, the almost even yellow ofFIG. 8Bshows less-hot catalyst bypassing along reactor100, as compared toFIG. 8A, which has columns of orange (hotter) catalyst in the center and along the periphery. Thus, the effective catalyst to oil mixing is enhanced in the reactor shown inFIG. 8B.

As shown by the comparison ofFIGS. 9A-9B, reactor devices constructed in accordance with embodiments of the present disclosure, e.g. reactor100or300, provide improved droplet vaporization time along the reactor. InFIG. 9Aa color CFD model of feedstock droplet vaporization time along the reactor for a traditional DFR is shown. InFIG. 9B, a color CFD model of feedstock droplet vaporization time along the reactor for a DFR constructed in accordance with the present disclosure, e.g. reactor100or300, is shown. The effectiveness of the reactor in accordance with the present disclosure is manifested inFIG. 9B, which shows the droplets (represented in blue) are all essentially vaporized before they reach mixing baffle130, e.g. within 0.2 seconds, which leaves more time for reacting during the overall residence time of 0.5 to 1 seconds. On the other hand, droplets inFIG. 9Aare shown in a variety of colors, some of which indicate a longer residence time as indicated by the scale on the left. This indicates that the effective mixing in the embodiments of the present disclosure, leads to faster feed vaporization than in traditional DFRs. The improved conversion results in more valuable products such as olefins.

The methods and systems of the present disclosure, as described above and shown in the drawings, provide for downflow reactors with more even catalyst distribution, more even temperature distribution and faster feed vaporization, which ensures that the effective catalyst to oil (hydrocarbon) ratio is optimized. While the systems and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.