WO2003000401A1

WO2003000401A1 - Improved fouling tolerant fixed bed reactor

Info

Publication number: WO2003000401A1
Application number: PCT/US2002/016119
Authority: WO
Inventors: Ramesh Gupta; Salvatore Joseph Rossetti
Original assignee: Exxonmobil Research And Engineering Company
Priority date: 2001-06-21
Filing date: 2002-05-21
Publication date: 2003-01-03
Also published as: JP4350507B2; JP2004535289A; EP1412076A1; EP1412076A4

Abstract

The invention is directed to a fixed bed reactor (51) having one or more bypass devices for extending the operating life of the reactor. The bypass devices may be placed within a fixed catalyst bed (56) to partition the bed (56) into a first top bed (58) and a second virtual bed (59). One embodiment of the bypass device allows a fluid feedstock to bypass the first top bed (58) of the fixed catalyst bed (56) as it fouls and enters the second virtual bed (59) under conditions that promote depositing any foulants contained in the bypass flow on the top surface of the second virtual bed (59) rather than in the interstices of the second virtual bed (59). Another embodiment relates to a multi-tier bypass that allows a fluid feedstock to bypass successive layers of a fixed bed as they foul. The invention also relates to an improved fixed bed reactor comprising the inventive bypass device and a method for extending the operating life of a fixed bed reactor that employs the inventive bypass device.

Description

IMPROVED FOULING TOLERANT FIXED BED REACTOR

FIELD OF THE INVENTION

[0001] The present invention relates generally to fixed bed reactors equipped with one or more bypass devices for extending the operating life of the fixed bed reactors. One aspect of the invention relates to a reactor having a fixed bed and a bypass device securely positioned within said bed. The bypass device partitions the fixed bed into at least a first (or top) layer (or bed) and a second (or bottom) layer (or bed). The bypass device allows an increasing amount of a fluid feedstock to the bed to bypass the first layer as it fouls and be distributed to the second unfouled layer.

[0002] One aspect of the present invention relates to a fixed bed reactor having a bypass device securely positioned in a single fixed bed of the reactor to create a virtual second bed within the single fixed bed for optimum utilization of the bed. Yet another embodiment of the present invention employs a multi-tier bypass device that partitions a single fixed bed into a plurality of successive layers (or beds) and allows a reactor feedstock to bypass successive layers of the fixed bed as they foul. The present invention also relates to bypass devices that are suitable for use with fixed bed reactors and to methods for using the inventive bypass devices for extending the operating life of fixed bed reactors.

BACKGROUND OF THE INVENTION

[0003] In the operation of fixed bed reactors, the top of the fixed bed often becomes fouled or plugged by the deposition of fouling materials (also referred to as particulates, particulate impurities, or foulants) contained in the fluid feedstocks flowing into the fixed bed. Examples of fouling materials include organometallic compounds, polymeric materials, carbonaceous materials, organic particulates and inorganic particulates. The plugging of the fixed bed results in increased pressure drop that may necessitate shutdowns, throughput reduction, and time consuming repairs and maintenance.

[0004] To overcome this problem, many bypass devices and methods have been devised. Some require equipping each reactor with more than one fixed bed and completely bypassing a fouled fixed bed. Examples of such methods are described in U. S. Patent Nos. 3,509,043 and 4,313,908. One shortcoming of such methods is that they require an auxiliary bypassable fixed bed. Thus, the above methods do not readily apply to single fixed bed reactors.

[0005] Other methods involve the use of trash baskets. For example, U. S. Patent Nos. 3,992,282 and 3,888,633, describe a fixed catalyst bed reactor having a hollow trash basket made from a screen mesh material that extends into the fixed catalyst bed. Particulate impurities are removed from a fluid stream flowing into the fixed catalyst bed by the hollow trash basket.

[0006] While the trash baskets remove some fouling materials contained in the fluid feedstocks, they generally have only a small effect in minimizing pressure drop buildup due to fouling. This is partially because fouling materials plug the trash basket walls within a short period of time. Thus, the flow passage of the fluid feedstocks is occluded and the pressure drop begins to rise, though at a somewhat slower rate than if the trash baskets were not used. Generally, it is desirable to keep fixed bed reactors on stream without significant pressure drop buildup for a long time, veiy often for several years. Thus, the methods involving trash baskets do not provide adequate protection against pressure drop buildup, and other methods are needed to further extend the operating life of fixed bed reactors. Other problems are associated with existing methods for extending the operating life of fixed bed reactors.

SUMMARY OF THE INVENTION

[0007] One aspect of the present invention relates to a reactor for treating a feedstock flowing therethiough. The reactor comprises at least one fixed bed for beating the feedstock and at least one bypass device. One embodiment of the bypass device comprises a cage positioned within the fixed bed. The cage has a top wall, side walls and a substantially open or wholly open bottom end. The cage partitions the fixed bed into a first top bed and a second viitual bed. The bypass device also comprises a bypass tube that is in fluid communication with the cage. The bypass tube is disposed within the cage and protrudes from the cage above the top surface of the fixed bed for bypassing an increasing amount of the feedstock around the first top bed as it fouls. The bypass tube is sized to regulate the bypass flow through the bypass device. The bypass tube is preferably sized to provide sufficient pressure drop to prevent any significant bypass flow when the top layer of the bed is not fouled. This provides better utilization of the top bed. When the top layer fouls, bypass flow is directed through the bypass tube into the cage and out from the cage through its perforations or its open bottom end or a combination thereof on the top surface of the second viitual bed. The cage has a substantially larger cross-section than the bypass tube to obtain an effective reduction of the velocity of the bypass flow as it exits the cage.

[0008] The bottomless bypass device may include a substantially open or wholly open bottom end to maximize the top surface area of the second viitual bed where any bypassed foulants can deposit. [0009] Another aspect of the present invention relates to a fixed bed reactor having a multi-tier bypass device. The multi-tier bypass device comprises a cage having a plurality of successive chambers that are in fluid communication with one another. Each chamber may have a plurality of perforations for allowing any bypass flow that enters the chamber to exit the chamber and enter a clean bed layer surrounding the chamber. Each chamber, except the last chamber in the cage, may also have a fluid communication device for allowing any bypass flow that enters a chamber that is surrounded with a fouled layer to pass into the next chamber. This process is repeated until the bypass flow enters the last chamber and exits from that chamber into the last unfouled layer of the bed through side and/or bottom perforations. Preferably, the last chamber of the cage may have a substantially open or wholly open bottom end to create a virtual second bed inside the fixed bed.

[0010] The multi-tier bypass device also may include a bypass tube in fluid communication with at least one chamber of the cage. The bypass tube may protrude from the cage above the fixed bed for bypassing an increasing amount of the feedstock around a fouled layer of the fixed bed. The bypass flow will pass through the bypass tube into a chamber of the cage and out from that chamber through the chamber perforations into a clean bed layer. The multi-tier bypass device effectively partitions a single bed in multiple layers corresponding generally to the number of chambers in the cage.

[0011 ] Yet another aspect of the invention is directed to a method for extending the operating life of a fixed bed reactor. The method comprises partitioning the fixed bed into at least two successive layers or beds, introducing a hydrocarbon feedstock into the fixed bed and as each successive layer fouls, bypassing an increasing amount of the feedstock to the next layer of the fixed bed that is not fouled. The method may employ one or more of the inventive bottomless and/or multi-tier bypass devices.

[0012] It has been unexpectedly discovered that the use of one or more of the inventive bypass devices on fixed bed reactors reduces significantly the pressure drop buildup of the fixed bed reactors. Methods, and systems that employ bypass devices having cages with fully open bottom ends are preferred, however, mechanical or other reactor specific constraints may prevent the bottom end of a cage to be fully open. Also, wire meshes or grids that are nearly fully or substantially open to the flow may also be used at the bottom of the cage as long as they do not obstruct the top surface area of the viitual bed to any significant extent.

[0013] A single larger cage or several smaller cages may be used to create a second viitual bed and maximize the top surface area of this viitual bed. Also, some of the cages may have one or more chambers and one or more bypass tubes. Many other variations, may be used to create a second virtual bed and maximize its top surface area that is available for foulant deposition.

[0014] These and other embodiments of the present invention will become better understood with reference to the following detailed description considered in conjunction with the accompanying drawings described below.

BRIEF DESCRIPTION OF THE FIGURES

[0015] Figure 1 shows a fouling tolerant fixed catalyst bed reactor having a single layer bypass device, according to one embodiment of the invention. [0016] Figure 2 shows a fouling tolerant fixed catalyst bed reactor having a plurality of bottomless bypass devices, according to another embodiment of the invention.

[0017] Figure 3 shows a fouling tolerant fixed catalyst bed reactor having a plurality of bottomless bypass devices, according to another embodiment of the invention.

[0018] Figure 4 shows a fouling tolerant fixed catalyst bed reactor having two bottomless bypass devices, according to yet another embodiment of the invention.

[0019] Figure 5 shows pressure drop buildup data obtained in a laboratory scale reactor.

[0020] Figure 6 shows a fouling tolerant fixed catalyst bed reactor having a multi-tier bypass device, according to another embodiment of the invention.

[0021] Figure 7 shows a multi-tier bypass device according to one embodiment of the invention.

[0022] Figure 8 shows a multi-tier bypass device according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] An embodiment of the present invention finds particular applicability in increasing the cycle life (or operating life) of fixed catalyst bed reactors such as hydroprocessing (or hydrotreating) reactors. Hydroprocessing reactors may process hydrocarbons by carrying out any one of a multitude of reactions. The invention is not limited to fixed catalyst bed reactors, but it can also be applied to any fixed bed reactors, or other fixed bed equipment such as contactors and filters.

[0024] For example, fixed catalyst bed reactors may be used for the conversion or treatment of hydrocarbon or chemical feedstocks in the presence of a vapor phase, such as hydrogen containing treat gas. More specific examples of reactors that can be used with the present invention include reactors used for hydrocon version of heavy petroleum feedstocks to lower boiling point products, the hydrocracking of distillate boiling range feedstocks, and hydrotreating of various petroleum feedstocks, such as light hydrocarbons, naphtha and distillate boiling range streams. This invention is applicable to reactors having one or more catalyst beds, however, it is particularly useful for reactors having only one fixed catalyst bed, because it allows bypassing a fouled catalyst layer or multiple fouled catalyst layers within a single fixed catalyst bed.

[0025] For example, the inventive bypass device can be particularly beneficial in preventing the fouling of a fixed catalyst bed used for contacting a stream of hydrocarbon feedstock with a conventional reforming or hydro- processing catalyst. One embodiment of the bypass device allows the feedstock to bypass the upper portion or top layer of the catalyst bed once fouling occurs, enabling bed operation for substantially longer periods of time as compared to miming without the bypass apparatus. Existing or new reactors can be equipped readily with one or more bypass devices to enable them to run for longer periods of time.

[0026] One embodiment of the present invention provides a bypass device for bypassing a single layer of a catalyst bed when that layer fouls, the bypass device (also referred to as "single layer bypass") comprising: a first elongated hollow member or cage having a plurality of perforations or openings and a second elongated hollow member (also referred to as a "bypass tube") generally disposed within the cage and protruding above the top of the cage. The cage can be partially or fully embedded in the bed such that the section of the cage having openings therein may discharge and distribute the bypassed hydrocarbon feed to an elevation within the bed below a top fouling layer of the bed. The cage may be closed at the top except for where the first hollow elongated member extends therethrough. However, depending upon the application, the entire cage member may have openings therein, including in the top wall, side walls, and bottom wall. Preferably, the cage may have a substantially open bottom end or a fully open bottom end. For example, Figure 1 shows a cage having a top wall, side walls and a bottom wall with openings in the bottom wall and the lower portion of the side walls. Figures 2, 3, and 4 show bottomless bypass devices having no bottom wall.

[0027] Positioning of the bypass device within the fixed catalyst bed may vaiy. Preferably, however, the top wall of the cage may be positioned sufficiently lower than the top surface of the catalyst bed and may also be perforated in order to allow full utilization of the top catalyst layer for foulant deposition.

[0028] The bypass tube is in fluid communication with the perforated cage and extends above the top of the perforated cage. The bypass device may be positioned within the fixed catalyst bed so that the top of the bypass tube extends above the top surface of the fixed catalyst bed. As the top layer of the catalyst bed fouls because of impurities in the feedstock, and thus loses its permeability to flow, an increasing amount of feedstock bypasses the top layer through the bypass tube into the cage and exits from the cage through the cage perforations and/or an open bottom end of the cage to a lower unfouled or less fouled layer of the catalyst bed.

[0029] A bypass tube provides a pressure drop or flow resistance that is sufficiently higher than the pressure drop across a clean top layer of the bed but lower than the pressure drop across a fouled bed, and preferably a top fouled layer of the bed. Thus, the feedstock may generally go through the bypass tube only when a top layer of the bed fouls. When the top layer is not fouled the feedstock may flow through the bed without any significant bypass flow.

[0030] The cage is sized to provide a desired exit velocity for the bypass flow into the unfouled layer of the bed. Although the exact size of the cage and the bypass tube may vary, the cage will generally have a substantially larger cross- section than the bypass tube. The precise ratio of the area of the cross-section of the cage over the area of the cross-section of the bypass tube may vaiy and can be readily determined by a person skilled in this art who has read and understood the description of the present invention in order to achieve the desired functions of the inventive bypass device. Typically, this ratio may range from about 1.1 to about 20, preferably from about 1.5 to about 16, more preferably from about 2 to about 10 and most preferably from about 3 to about 6.

[0031] More than one bypass tubes may be used per each cage (not shown). For example, in one embodiment, a single bypass bottomless device may be used having a plurality of bypass tubes disposed within a single large cage. The cage may preferably cover substantially the whole area of the fixed bed from one side wall of the reactor to the other.

[0032] In yet another embodiment of the invention the cage may include a top wall, side walls, and an open bottom end as shown in Figures 2, 3, and 4. Prefer- ably, the cage may have perforations on the top and side walls. The bypass tube may protrude through the top wall of the cage with an upper open end terminating above the top wall of the cage and a lower open end terminating within the cage. The bypass tube provides an effective overall pressure drop to minimize bypassing when the top catalyst layer of the catalyst bed is not fouled and divert the feedstock flow to bypass the top layer of the catalyst bed and enter the cage when the top layer is fouled.

[0033] Preferably, one or more bottomless bypass devices may be placed in a single fixed bed to effectively separate the bed into a first top bed (or top bed) and a second lower bed (also referred to as the second virtual bed). The first top bed is generally the portion of the bed that is above the top walls of the bottomless bypass devices. The second viitual bed is generally the portion of the bed that is below the bypass devices, more specifically the portion of the bed below the open bottom ends of the bottomless bypass devices. The portion of the catalyst bed between the side walls of two consecutive bypass devices and between the bypass devices and the walls of the reactor may also be available for foulant deposition, however it is preferred to minimize this area. The cage may also include a deflector plate generally located near the lower open end of the bypass tube. The deflector plate reduces the exit velocity of the bypass flow and better distributes the bypass flow to the top surface of the viitual second bed. The geometry of the bottomless cage and the bypass tube may vaiy. A preferred geometry minimizes the top wall of the cage while maximizes the area of the open bottom end of the cage. Such a design provides better utilization of the top bed before it fouls and also provides a maximum surface area for the foulants to deposit on the top of the second virtual bed.

[0034] Yet another embodiment of the present invention relates to a multi-tier bypass device for bypassing successive layers of a fixed catalyst bed as they foul. The multi-tier bypass device may comprise a perforated cage having at least one internal plate that paititions the cage into at least two compartments or chambers, an upper chamber and a lower chamber. The internal plate may include means for allowing fluid communication between the two chambers of the perforated cage. The fluid communication means may be, for example, an opening, an orifice, a tube, a spring-loaded valve, a rupture disk or any other device that allows fluid communication between the two chambers, preferably an opening, an orific or a tube. The pressure drop across a communication device connecting a first chamber to a second chamber is designed to be sufficiently higher than the pressure drop across an unfouled catalyst layer corresponding to the first cage chamber (first catalyst layer) but lower than the pressure drop across a fouled first catalyst layer. Thus, any bypassed feedstock that enters a first cage chamber may generally exit from the side perforations of the first cage chamber into an unfouled first catalyst layer, but as the catalyst layer around the first cage chamber becomes fouled an increasing amount of the bypassed feedstock may then enter the second cage chamber through the communication device.

[0035] For example, a first communication means positioned on a first internal plate may generally provide an effective pressure drop that forces the bypassed flow that enters the first chamber to exit through perforations located on the side walls of the first chamber to a first unfouled layer of the catalyst bed corresponding to the first chamber of the cage, below the top layer of the bed. As this first layer becomes increasingly fouled the pressure drop across the side perforations becomes greater than the pressure drop across the first communication means and the bypassed feedstock flow is diverted to the second cage chamber. [0036] The multi-tier bypass device also comprises a bypass tube. The bypass tube may protrude through the first or top chamber of the cage and may extend above the top wall of the cage. The bypass tube provides a pressure drop or flow resistance that is sufficiently higher than the pressure drop across a clean bed but lower than the pressure drop across a fouled bed. Thus the feedstock may generally go through the bypass tube only when the top layer of the bed fouls. When the top layer is not fouled the feedstock may flow through the bed without any significant bypass flow.

[0037] In yet another embodiment of the invention the cage may include a top wall, side walls, a bottom wall, a plurality of perforations on at least a portion of the side walls, and at least one internal plate dividing the interior of the cage into at least two chambers, an upper chamber and a lower chamber. The internal plate may also include a fluid communication device for allowing fluid communication between the upper and the lower chamber. The bypass tube may protrude through the top wall of the cage with an upper open end terminating within the upper chamber. The bypass tube provides an effective overall pressure drop to minimize bypassing when the catalyst bed is not fouled and diverts the feedstock flow to bypass a top layer of the catalyst bed and enter the upper chamber when the top layer is fouled. The fluid communication device provides an effective pressure drop to prevent the bypass flow that has entered the upper chamber from entering the lower chamber when a first catalyst layer corresponding to the upper chamber is not fouled. It also allows the bypass flow that has entered the upper chamber to enter the lower chamber through the fluid communication device when the first catalyst layer is fouled.

[0038] In yet another embodiment of the present invention the bypass tube may protrude through the top wall of the cage with an upper open end terminating above the top wall of the cage and a lower open end teiminating within the lower chamber. The bypass tube may have an intermediate opening located within the upper chamber for discharging the bypass flow into the upper chamber when the catalyst layer around the upper chamber is not fouled. Figures 6, 7 and 8 show multi-tier bypass devices.

[0039] Preferably, the cage of the multi-tier bypass device will have a geometry that minimizes the top wall of the cage and maximizes the open bottom end of the cage such as the inverted cup geometry shown in the embodiment of Figure 3.

[0040] Refening now to Figure 1, there is illustiated a fouling tolerant reactor 6 having a fixed catalyst bed 5 and a single layer bypass device 10 embedded in the catalyst bed 5, according to one embodiment of the invention. Shown is one bypass device, however, the invention may comprise a plurality of bypass devices spaced over the catalyst bed. Each bypass device 10 may extend into the catalyst bed to different bed depths.

[0041] The bypass device 10 comprises an elongated hollow member 2 (also referred to as a cage member or cage) having a top wall 10a, side walls 10b, a bottom wall 10c, and a plurality of perforations 9 disposed generally near a lower end or section 4 of the cage 2. However, positioning of the perforations may vary. For example, all cage walls may have perforations. The bypass device 10 further comprises another elongated hollow member 1 (also refened to as bypass tube) disposed within cage 2 and protruding from the top wall 10a of cage 2 above the catalyst bed 5. The bypass tube 1 extends above the catalyst bed 5. The cage 2 has an upper enclosed portion (top wall and upper portion of the side walls) 3 and a lower perforated portion (bottom wall and lower portion of side walls) 4. Optionally, the bypass tube 1 may have a cap 7 over its top end or portion that extends above the catalyst bed 5. An optional layer of inert material 8 may be disposed within the catalyst bed 5 around the perforated section 4 of the cage 2.

[0042] The elongated hollow members 1 and 2 may be tubular members with the elongated hollow member 1 positioned or disposed within the elongated hollow member 2 in a concentric configuration as shown in Figure 1. However, it should be understood that the elongated hollow members 1 and 2 can have other geometric shapes and relative configurations. Preferably, however, the cage member (cage) 2 may have a substantially larger cross section than the bypass member 1 (bypass tube). Also preferably, the cage may have an open end, or bottom as shown in the embodiment of Figures 2 and 3.

[0043] In operation, the bypass tube 1 may receive a portion of the feedstock and direct it into cage 2 where it is discharged through the perforations 9 of the cage 2 into a lower layer of the catalyst bed 5 that is not fouled. The top wall of the bypass device in Figure 1 is flush with the top surface of the bed. However, positioning of the bypass device inside the bed may vary. Preferably, the bypass apparatus 10 may be inserted into the catalyst bed 5 such that cage 2 is buried into the bed. However, the positioning and dimensions of the bypass device may vaiy. For example, the bypass may be buried within the catalyst bed such that the bottom of the cage is contained within the catalyst bed and the bypass flow is distributed to a layer of the bed located beneath a top layer of the bed where substantial fouling occurs. Typically, fixed catalyst bed reactors that can benefit from the deployment of the inventive bypass devices include hydro- processing and reforming reactors used in petroleum refining. However, any fixed bed using a packing of solids for contacting, filtering or reacting a feed may benefit from using the present invention bypass devices. For typical, commercial scale hydroprocessing and reforming reactors the top layer may extend from about a few inches up to about 5 feet (150 cm) from the bed's top surface. Thus, the bypass device may be designed to bypass the flow to a catalyst layer underneath the top fouling layer.

[0044] In the embodiment shown in Figure 1, the second elongated member extends through the first hollow elongated cage and terminates substantially at the portion of the cage having perforations therein. However, other configurations are within the scope of the invention. For example, the second elongated member may stop short of the perforations, or extend to an area within the portion of the cage having perforations. The bottom of the cage may be enclosed and only the side walls may have perforations in the lower portion of the cage. In one embodiment, the cage is fully buried in a catalyst bed below the surface of the bed, and the entire cage length has openings therein. In a catalyst bed where only a top layer becomes fouled, bypassed feedstock may be directed just below the fouled top layer.

[0045] The cage perforations 9 may be made by a variety of methods including constructing a portion of the cage from a mesh type material. The area of the cage having openings therein may vaiy. For example, only the side walls may have perforations, or other areas of the cage such as the top and bottom walls may likewise have perforations therein. Alternatively, all walls of the cage may be perforated. Also, the size of the cage perforations may vaiy. For example, in one embodiment of the invention the perforations may be sufficiently large to allow any small quantity of the particulates that are entrained in the bypassed flow to exit the cage and get distributed into the bed. Alternatively, the cage perforations 9 may be sufficiently small so that any bypassed foulant particulates will be retained within the cage. Preferably, however, the cage perforations are sized to retain larger size particulates and allow smaller size particulates to exit the cage. Generally bypass foulant particulates are small particles contained in the hydrocarbon feed that are bypassed through the bypass tube and which contiibute to fouling of the catalyst bed. Typically, the cage openings may range in size of from about 1/8 inches (0.31 cm) to about 1/2 inches (1.25 cm) wide holes or slits. The area around the cage openings may be packed with solids that are bigger in size than the catalyst particles to prevent migration of the catalyst paiticles into the cage through the perforations.

[0046] The tube in cage design of the present invention (tube-cage bypass) offers many advantages over prior art bypass devices. For example, the "tube- cage" bypass maintains the integrity of the catalyst paiticles because it allows for generally lower exit velocities of the bypassed flow into the catalyst bed. High exit velocities would generally erode the bed or cause it to slump, increase its pressure drop, and deteriorate the overall reactor performance. In the present inventive bypass device, the cage has a substantially larger cross-section than the bypass tube allowing an effective reduction of the exit velocity of the bypass flow. Other advantages exist. For instance, the larger cross-section cage allows a higher surface area for depositing any foulants found in the bypass flow. Also, more bypass flow can be directed through the bypass tubes at higher velocities thus allowing to reduce the cross-section of the tubes and better utilize the top surface of the bed before it fouls.

[0047] The reactor 6 may be operated by introducing a feedstock 11, such as hydrocarbons, to be reacted in the catalyst bed 5 along with any suitable treat gas and chemical, as needed, such as hydrogen. The feedstock 11 can be a liquid, gas, or a mixture thereof. The reactor 6 may be operated at any suitable process conditions. Such conditions are known in the art and are generally not modified by use of the inventive bypass apparatus. The feedstock 11 may undergo any desired chemical reactions as it moves through the catalyst bed. At the beginning, when the catalyst bed 5 is clean and no foulants or only a few foulants have been deposited at the bed top, a majority of the flow may go through the catalyst bed 5 instead of the bypass apparatus 10. This is because the bypass tube 1 is sized to have a higher pressure drop relative to the clean bed, and thus the flow takes the path of least resistance through the unfouled catalyst bed 5. Generally the bypass tube 1 may be sized to provide a pressure drop of from about 2 to about 100 times, preferably of from about 5 to about 80 times, and more preferably of fiom about 10 to about 50 times the pressure drop of the fouling top layer prior to fouling. During operation, as the bed top fouls the resistance to flow through the bed increases causing an increasing fraction of the flow to bypass the top of the bed through the bypass apparatus 10.

[0048] For example, the pressure drop through a clean, (unfouled) top four feet layer of a catalyst bed may typically be 2 psi in a typical commercial scale, hydroprocessing reactor. For such a reactor, depending upon the operation, the bypass tubes 1 may be sized to have a flow resistance of about 5 to about 200 psi, preferably from about 10 to 160 psi, and more preferably from about 20 to about 100 psi, with total feedstock flow in the tubes 1. By employing one or more bypass devices, the pressure drop through the top four feet section of the bed may generally not exceed about 50 psi for an extended period of time. If the inventive bypass devices 10 are not used, the pressure drop can be significantly higher than 50 psi upon fouling which may necessitate a reactor shutdown or throughput reduction.

[0049] The inventive bypass apparatus may be constructed from any material compatible with the operating conditions of the reactor. For example, suitable materials may include metals such as carbon steel and stainless steel, ceramic materials, and other composite materials such as carbon fiber reinforced materials. [0050] The bypass tube 1, through which the feedstock is bypassed, may be of any diameter or width depending upon the amount and rate of bypass flow to the unfouled layer of the catalyst bed and the desired pressure drop. Such diameters may easily be determined by the skilled artisan. For example, typically, the diameter of the bypass tube 1 may range from about 0.25 inches (0.625 cm) to about 12 inches (30 cm), more preferably from about 0.5 inches (1.25 cm) to about 6 inches (15 cm), and most preferably from about 0.5 inches (1.25 cm) to about 3 inches (7.5 cm). The cage 2, likewise, may be of any diameter, but is generally of a substantially greater diameter or cross-section than the bypass tube 1, in order to allow for sufficiently low exit velocities of the bypassed flow into the bed to prevent excessive disturbance of the bed. For example, the cage diameter may range from about 3 inches (7.5 cm) to about 20 inches (50 cm), more preferably from about 4 inches (10 cm) to about 12 inches (30 cm), and most preferably from about 4 inches to about 10 inches.

[0051] One or more bypass devices may be utilized. The number of bypass devices utilized generally may depend upon the size of the reactor and the flow rate of the feedstock in the reactor. The design and number of the bypass devices is such that the bypass devices may offer higher resistance to flow than the clean beds, and less flow resistance than a fouled bed. When determining the number and location of the bypass devices, the skilled artisan may take into consideration, inter alia, localized velocities, residence times, and temperature distribution. The number and location of the bypass devices for a given reactor may be chosen to maintain the overall performance of the unit.

[0052] Section 4 of the cage 2 distributes the bypassed feedstock into the catalyst beds. The area su ounding the cage perforations 9 may include a layer of packing material 8 having a size that assists in the distiibution of the bypassed feedstock through the catalyst bed. Use of packing material is optional. The packing material 8 may allow any paiticulates flowing into the bypass appaiatus 10 to be dispersed upon exiting the cage perforations 9. Suitable packing material 8 may be any inert material such as alumina balls typically used to support catalyst particles in a fixed bed. The packing material 8 could also be any other material or even catalyst particles, provided that the catalyst paiticles are of gi'eater size than the perforations 9. Thus, catalyst particles, if chosen as a packing material 8, may preferably be of an appropriate size to distribute the feedstock being bypassed. Typically, the particles may range in size from about 0.25 inches (0.625 cm), to about 4 inches (10 cm). In addition to alumina balls, several other packing materials such as those typically used in packed towers may also be used.

[0053] In a preferred embodiment of the invention, the bypass tube 1 may have a device or cap 7 at the top to facilitate separation of paiticulates from the bypassed hydrocarbon feed, as shown in Figure 1. The downward moving hydrocarbon feed from the reactor inlet is forced to change its direction by the cap 7 so that the feed can move upward and then enter the bypass apparatus 10. While the flow direction of the feed is changed by the cap 7, the inertia of the paiticulates prevent these paiticulates from changing their flow direction. These paiticulates separate out and accumulate at the bed top. Thus, the cap 7 may remove a significant number of paiticulates, and minimize fouling in the interior bed sections. The separation cap 7 (or separator) may generally remove the larger size paiticulates. Depending upon the sizes of the incoming particulates, some of the veiy small particulates may not get separated by the separation cap 7. Often, these particulates may be so small in size that they may pass through the catalyst bed 5 without plugging it. Inert packing 8 that suirounds the cage perforations 9 may help disperse these small size paiticulates in the layer of the inert material and further minimize pressure drop buildup. Other separation devices could also be used. Examples of suitable separation devices may include small centrifugal separators or cyclones mounted on the top of each bypass tube 1.

[0054] In one embodiment of the invention shown in Figure 2, a fixed bed reactor 51 is provided comprising a fixed bed 56 and a plurality of bottomless bypass devices 50. Each bypass device 50 comprises a bypass tube 53 disposed within a bottomless cage 54. The bottomless cage 54 has a top wall 50a, and side walls 50b, but no bottom wall, that is the cage 54 has an open bottom end 57. The side walls 50b of the bottomless cage 54 have perforations for discharging at least some of the bypass flow in the area of the bed that is between two consecutive bypass devices 50, and between the bypass devices 50 and the side walls 51b of the reactor 51. Preferably, a plurality of bottomless bypass devices 50 may be embedded in a single catalyst bed 56 to create a virtual second bed 59 within the single catalyst bed 56. A first top bed 58 is generally defined by the catalyst bed that is above the top walls 50a of the bypass devices 50, while a second viitual bed 59 is created below the open bottom end 57 of the bypass devices 50. The area of the catalyst bed between the bypass devices and between the bypass devices and the walls 51b of the reactor 51 is also generally available for foulant deposition. However it is preferred to minimize this area so that the bypass flow exits the cages 54 through their open bottom ends 57. A deflector plate 60 is preferably located at the vicinity of the lower open end 53b of the bypass tube 53 to reduce the exit velocity of the bypass flow and better distribute it to the top surface of the virtual second bed 59. This configuration promotes deposition of any foulants contained in the bypass flow on the top surface of the second virtual bed 59 rather than into the interstices of the second viitual bed 59. Optionally, a cap 61 may be placed over the top end 53a of the bypass tube 53 as in the embodiment of Figure 1. [0055] The geometry of the bottomless cages 54 may vaiy. For example, the cage may be a frustion cone, a frustion pyramid or a cylinder. One alternative design is shown in Figure 3 whereas each bottomless cage 64 has the shape of an inverted conical cup. This design maximizes the surface area of the open bottom end 67 of the cage 64 and thus the top smface area of the second viitual bed 69 that is available for foulant deposition. Preferably, the cage 64 has perforated top and side walls, 64a and 64b respectively. The amount, size and positioning of the perforations may vary. The embodiments of Figures 2, 3, and 4 provide a virtual second bed within a single fixed bed for the foulants to deposit. Any foulants contained in the bypass flow may deposit at the top surface of the second virtual bed instead of depositing in the bed interstices. Deflector plates 70 and caps 71 may also be used as in the embodiment shown in Figures 2, 3, and 4.

[0056] Figure 4 shows yet another embodiment of the present invention comprising a reactor 81 comprising a fixed bed 86 and two bottomless bypass devices 80. Each bypass device 80 comprises a bypass tube 83 securely positioned within a bottomless cage 84. The bottomless cage 84 has a top wall 80a, side walls 80b, and a wholly open bottom end 87. The side walls 80b and the top wall are perforated. The cages 84 cover substantially the whole area of the fixed bed leaving veiy little space between the cages and between each cage and the side walls 8 lb of the reactor 81. Thus, substantially all of the bypass flow exits the cages 84 through their open bottom ends 87. The bypass devices 80 also include a deflector plate 90 and a cap 91 as in the embodiment of Figures 2, 3, and 4.

[0057] It has been unexpectedly discovered that lower pressure buildup occurs with a bottomless cage bypass device. Without wishing to limit the invention in anyway it is theorized that the bottomless cage promotes foulant deposition on the surface of the bed rather than in the interstitial space in between the catalyst particles. Figure 5, shows pressure drop buildup data obtained in a laboratory fixed catalyst bed reactor. The reactor was made to foul by contaminating a gas feed with finely crushed walnut shells. The average particle size of the crushed walnut shells was about 250 micrometers, in order to simulate the typical size of foulants found in commercial feedstocks. The reactor did not contain any bypass devices. Figure 4 shows that the pressure drop increases rapidly in the beginning as the foulant particles fill the interstices near the bed top. Later as the catalyst bed interstices near the bed top were filled up and the paiticles began to deposit above the bed, the rate of pressure drop build-up reduced dramatically.

[0058] Unlike the bypass device shown in Figure 1, a bottomless bypass device has no bottom wall. Examples of bottomless bypass devices are shown in Figures 2 and 3. Employing a plurality of bottomless bypass devices allows the creation of a viitual second bed within a single bed reactor. The bottomless bypass devices allow depositing the foulants contained in the bypass flow to deposit above the top surface of the virtual bed rather than in the bed interstices. Generally, the pressure drop buildup obtained with a bottomless bypass device may be about an order of magnitude slower, and thus a much longer reactor run length may be achieved.

[0059] The length of a bottomless bypass device may vary, however preferably it is less than the full length of the bed as shown in Figures 2 and 3. Typically, the length of a bottomless bypass device may be from a few inches to about 5-8 feet.

[0060] Refening now to Figure 6, a side cross-sectional view of a fouling tolerant reactor 620 having two, multi-tier bypass devices 621 is provided. The reactor 620 can be any of many well-known fixed bed catalyst reactors having a single or a plurality of fixed catalyst beds. Preferably, the multi-tier bypass devices may be embedded in a first catalyst bed 622a. However, it should be understood that bypass devices 621 may also be embedded in a second catalyst hed 622b, if needed. The bypass devices 621 may be embedded as shown in Figure 6 with the top wall of the cage member 624 being substantially coplanar with the top surface of the catalyst bed. Preferably, however, each bypass device 621 may be buried in the bed 622a with the top wall of the cage member 624 below the top surface of the catalyst layer. A single bypass device 621 may be used, preferably, however, a plurality of bypass devices may be used that cover substantially the whole cross section of the catalyst bed. The cage 624 of each bypass device 621 may extend the whole depth of the catalyst bed 622a or terminate at some desirable depth within the catalyst bed 622a. The cage 624 may comprise one or more internal plates 625 that partition the cage into a plurality of chambers that are in fluid communication with one another through an opening 738 (shown in Figure 7) or some other communication means such as a tube, a rupture disc, or a spring loaded valve. A bypass tube 623 is securely attached to the cage 624 and is in fluid communication with the first chamber of the cage 624. The length and diameter of the bypass tube 623 may vary to provide an effective pressure drop across the tube to prevent any significant bypass flow when the top layer of the bed is not fouled, but allow bypass flow when the top layer of the bed becomes fouled. A cap 627, or some other separation device may also be used. The multi-tier bypass device effectively may divide the catalyst bed into a plurality of successive layers. For example, when the multi-tier bypass device is buried in the catalyst bed the following layers can be discerned: a top layer generally above the top wall of the cage of the bypass device, and a plurality of catalyst layers coπesponding to the chambers of the multi-tier bypass device, e.g., a first layer generally refening to the catalyst layer around the first chamber of the cage, a second layer generally refening to the catalyst layer around the second chamber of the cage, etc.

[0061] Refening to Figure 7, one embodiment of a multi-tier bypass device 720 is provided. The multi-tier bypass device comprises a bypass tube 723 disposed within a cage 724. The cage 724 has two internal plates or walls 725 that partition the cage 724 into three chambers 724a, 724b, and 724c. The bypass tube 723 protrudes through the top wall of the cage 724 with an upper open end 736 temiinating above the top wall of the cage 724 and a lower open end 737 terminating within the first chamber 724a of the cage 724. A plate 726 may be positioned near the open end 737 of the bypass tube 723 to deflect the bypass flow as it exits the bypass tube 723. Each internal plate has an opening 738 sized to provide an effective pressure drop to divert the bypass flow from one chamber to the next chamber as the catalyst layer around a chamber becomes fouled. Thus, in operation, as the top catalyst layer becomes fouled, more of the feedstock flow will pass thiough the bypass tube 723 and will enter the first chamber 724a through the open end 737 of the bypass tube 723. The bypassed flow will exit the first chamber 724a through the side perforations 733a into a first catalyst layer positioned around the side walls of the first chamber 724a. As the first catalyst layer fouls more of the bypass flow will pass through the first opening 738 into the second chamber 724b and out thiough the side perforations of the second chamber 733b into a second catalyst layer corresponding to the second chamber 724b. Likewise, as the second catalyst layer fouls more of the bypass flow will be directed through the second opening 738 into the third chamber 724c and exit the cage 724 into the remaining catalyst bed through the side and bottom perforation 733c of the third chamber 724c. A separation cap 727 or some separation device may be used to facilitate the separation of large paiticulates 739 from the bypass hydrocarbon stream 729a. [0062] For example, for a catalyst bed having a pressure drop of about 2 psi in the top four feet layer when it is not fouled, the multi-tier bypass device 720 may preferably be designed to have a pressure drop across tube 723 of from about 20 to about 80 psi, and a pressure drop across each opening 738 of from about 10 to about 40 psi, with an overall pressure drop across the bypass device 720 of from about 40 to about 160 psi.

[0063] Refening to Figure 8, another embodiment of a multi-tier device 840 is provided comprising a bypass tube 843 disposed within a cage 844. The cage 844 has two internal walls 845 that partition the cage 844 into three chambers 844a, 844b, and 844c. The bypass tube 843 protrudes through the top wall of the cage 844 with an upper open end 846 teiminating above the top wall of the cage 844 and a lower open end 847 terminating within the lower chamber 844c of the cage. The bypass tube 843 also has two inteimediate openings 848a and 848b, one intermediate opening 848a positioned within the first upper chamber 844a and one intermediate opening 848b positioned within the second chamber 844b. The intermediate openings 848a and 848b are sized to provide an effective pressure drop to divert the bypass flow from an upper chamber to the next lower chamber through the bypass tube 843 as the catalyst layer around that upper chamber fouls. Thus, in operation, as the top catalyst layer fouls, more of the feedstock flow will pass through the first section of the bypass tube 843a and thiough the first intermediate opening 848a into the first chamber 844a of the cage 844 and exit through the side perforations 849a of the first chamber 844a into a first unfouled catalyst layer conesponding to the first chamber 844a of the cage 844. When the first catalyst layer fouls the bypass flow may then pass thiough the second section of the bypass tube 843b and the second intermediate opening 848b into the second chamber 844b of the cage 844 and exit through the side perforations 849b of the second chamber 844b into a second unfouled catalyst layer conesponding to the second chamber 844b. [0064] Likewise, when the second catalyst layer fouls the bypass flow will be directed through the last section 843c of the bypass tube 843 into the lower chamber 844c of the cage and exit the cage 844 into the remaining of the catalyst bed through the side and bottom perforations 849c of chamber 844c.

[0065] The bypass tube section 843b is designed to have an effective pressure drop to prevent most of the bypass flow to enter the second chamber 844b when the first catalyst layer corresponding to the first chamber 844a is not fouled but allow most of the bypass flow to enter the second chamber 844b when the first catalyst layer is fouled. Generally the bypass tube section 843b provides a pressure drop that is sufficiently higher than the overall pressure drop across the first intermediate opening 848a and the side perforations 849a of the first chamber 844a when the first catalyst layer is not fouled and it is sufficiently lower than the pressure drop across the same fluid path when the first catalyst layer is fouled. Likewise the bypass tube section 843c provides a pressure drop that is sufficiently higher than the overall pressure drop across the second intermediate opening 848b and the side perforations 849b of the second chamber 844 when the second catalyst layer coπesponding to the second chamber 844b is not fouled. The pressure drop of the bypass tube section 843 c is also sufficiently lower than the pressure drop across the same fluid path when the second catalyst layer is fouled.

[0066] Yet another aspect of the invention is directed to a method for extending the operating life of a fixed bed reactor. The method comprises partitioning the fixed bed into a top layer and a bottom layer. The pressure drop across the top layer of the fixed bed increases during processing of the feedstock due to fouling. The method further comprises introducing a hydrocarbon feedstock into the fixed bed of catalytic material, and as the top layer of the fixed bed fouls, bypassing an increasing amount of the feedstock to the bottom layer of the fixed bed, using the inventive bypass device.

[0067] Yet another embodiment of the present invention relates to a method for extending the operating life of a fixed bed reactor, the method comprising providing a fixed bed reactor, and partitioning the fixed bed into a plurality of successive layers. The method further comprises introducing a feedstock into the fixed bed, and as each successive layer fouls bypassing an increasing amount of the feedstock to the next unfouled layer. In one particular embodiment the bypassing step may include positioning at least one of the inventive multi-tier bypass devices in the fixed bed.

[0068] Many modifications of the above exemplary embodiments will naturally occur to the skilled practitioner of this art without departing from the scope of the appended claims.

Claims

CLAIMS:

1. A reactor for treating a feedstock flowing therethrough, said reactor comprising,

at least one fixed bed for treating said feedstock; and

at least one bottomless bypass device;

said at least one bottomless bypass device comprising a cage positioned within said at least one fixed bed, said cage having a top wall, side walls and a substantially open bottom end, said cage partitioning said at least one fixed bed into a first top bed and a second viitual bed;

a bypass tube in fluid communication with said cage, said bypass tube protruding fiom said cage above said at least one fixed bed for bypassing an increasing amount of said feedstock around said top bed as it fouls through said bypass tube into said cage and out from said cage thiough said open bottom end on the top surface of said second viitual bed;

and wherein said cage has a substantially larger cross-section than said bypass tube.

2. The reactor of claim 1, wherein said bypass tube has an upper open end and a lower open end and said cage further comprises a deflector plate securely positioned within said cage near the lower end of said bypass tube.

3. A reactor for reacting a feedstock flowing therethrough, said reactor comprising,

at least one fixed bed for treating said feedstock; and at least one multi-tier bypass device positioned within said at least one fixed bed for bypassing at least two or more successive layers of said at least one fixed bed as they foul.

4. The reactor of claim 3, wherein said at least one multi-tier bypass device comprises:

a cage positioned within said at least one fixed bed, said cage having a plurality of successive chambers in fluid communication with one another, each chamber having a plurality of perforations; and

a bypass tube in fluid communication with at least one chamber of said cage, said bypass tube protruding from said cage above said at least one fixed bed for bypassing an increasing amount of said feedstock around a fouling layer of said fixed bed through said bypass tube into at least one chamber of said cage and out from at least one chamber of said cage through said perforations into a non-fouled layer of said fixed bed.

5. The reactor of claim 3, wherein said at least one multi-tier bypass device comprises:

a cage positioned within said at least one fixed bed, said cage member having a plurality of successive chambers in fluid communication with one another, and a plurality of perforations for discharging any bypassed feedstock to a non-fouled layer of the bed; and

a bypass tube in fluid communication with said cage.

6. The reactor of claim 3, wherein said at least one multi-tier bypass device comprises:

a cage member securely positioned within said at least one fixed bed, said cage member having at least one internal plate for dividing the cage into at least two chambers in fluid communication with one another through a fluid communication device; and

a bypass tube in fluid communication with said cage member.

7. The reactor of claim 3, wherein said multi-tier bypass device comprises:

a cage including a top wall, side walls, a bottom wall, a plurality of perforations on at least a portion of said side walls, and at least one internal plate dividing the interior of the cage into at least two chambers an upper chamber and a lower chamber, said at least one internal plate including a fluid communication device for allowing fluid communication between said at least two chambers; and

a bypass tube in fluid communication with said cage and protruding from said cage above said at least one fixed bed for bypassing an increasing amount of said feedstock around a fouled layer of said fixed bed through said bypass tube into the cage and out from the cage through said perforations into a non-fouled layer of said at least one fixed bed.

8. The reactor of claim 3, wherein said bypass tube protrudes through the top wall of the cage with an upper open end terminating above the top wall of the cage and a lower open end terminating within said upper chamber.

9. The reactor of claim 3, wherein said fluid communication device provides an effective pressure drop to prevent the bypass flow that has entered the upper chamber from entering the lower chamber when a first bed layer corresponding to the upper chamber is not fouled and allow the bypass flow that has entered the upper chamber to enter the second chamber through said fluid communication device when said bed layer is fouled.

10. The reactor of claim 3, wherein said bypass tube protrudes through the top wall of the cage with an upper end terminating above the top wall of the cage and a lower open end terminating within said lower chamber, and has an intermediate opening located within said upper chamber.

11. The reactor of claim 3, wherein said bypass tube has an effective overall pressure drop to minimize bypassing when the bed is clean and divert the feedstock flow to bypass a top layer of the bed and enter said upper chamber when said top layer is fouled.

12. A method for extending the operating life of a fixed bed reactor, the method comprising

providing a reactor comprising at least one fixed bed;

partitioning the fixed bed into at least two successive layers;

introducing a feedstock into the fixed bed and as each successive layer fouls bypassing an increasing amount of the feedstock to the next unfouled layer.

13. The method of claim 1 1 wherein said bypassing step includes positioning a multi-tier bypass device in the fixed bed, said multi-tier bypass device comprising:

a cage positioned within said at least one fixed catalyst bed, said cage having a plurality of successive chambers in fluid communication with one another, each chamber having a plurality of perforations; and

a bypass tube in fluid communication with at least one chamber of said cage, said bypass tube protruding from said cage above said at least one fixed bed for bypassing an increasing amount of said feedstock aiound a fouled layer of said fixed bed through said bypass tube into at least one chamber of said cage and out from at least one chamber of said cage through said perforations into a non-fouled layer of said fixed bed.