Mineral oils: processes and products – Chemical conversion of hydrocarbons – Reforming
Reexamination Certificate
2002-01-31
2004-11-09
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Reforming
C208S139000, C208S146000, C208S153000, C208S171000, C208S173000, C208S165000, C585S654000, C585S921000, C585S920000, C422S139000, C422S211000, C422S216000, C422S219000
Reexamination Certificate
active
06814857
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of fluid-particle contact and more specifically to a method for operation of the moving beds of radial or horizontal flow fluid-solid contacting devices. More specifically, this invention is related to a method for the contacting of a hot fluid stream with particulate material in a particle bed from which particles are continuously or periodically added and withdrawn.
BACKGROUND OF THE INVENTION
A wide variety of processes use radial or horizontal flow reactors to effect the contact of a compact bed of particulate matter with a fluid and in particular a gaseous stream. These processes include hydrocarbon conversion, adsorption, and exhaust or flue gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, systems have been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluid material will constantly contact a compact bed of particulate material having a required degree of effectiveness. A moving bed system has the advantage of maintaining production while the catalyst is removed or replaced. Typical examples and arrangements for such systems can be found in U.S. Pat. Nos. 3,647,680; 3,692,496; and 3,706,536; the contents of each of which are hereby incorporated by reference. A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field of hydrocarbon conversion reactions. Many hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles as a compact column under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150, and another such process is the dehydrocyclodimerization of aliphatic hydrocarbons.
Another well-known hydrocarbon conversion process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reactors. Typically, the catalyst particles enter the top of a first reactor, flow downwardly as a compact column under gravity flow, and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor as a compact column under gravity flow. After passing through the second reactor, the catalyst particles may pass through additional reactors before collection and transportation to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that accumulate on the catalyst in the reaction zone.
In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reactors. The reforming reaction is typically endothermic so the reactant stream is heated before each reactor to supply the necessary heat for the reaction. The reactants flow through each reactor in a substantially horizontal direction through the bed of catalyst. The catalyst particles in each reactor are typically retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on the design and operation of reactors with a horizontal flow of reactants. Catalyst pinning occurs when the frictional forces between catalyst particles and the outlet screen that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward. The frictional forces occur when the horizontal flow of vapor passes through the catalyst bed and the outlet screen. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and, as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Pinning between the outlet screen and the outer surface occurs when the frictional forces between catalyst particles that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward, thereby trapping catalyst particles against pinned catalyst particles. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from an inlet screen by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and an outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the horizontal flow of vapor creates a void between the inner screen and the inner catalyst boundary. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The trapping of catalyst particles within a reactor bed that is designed to move continuously causes some catalyst particles to remain in the bed for a longer time than other catalyst particles that still move freely through the bed. As the trapped catalyst particles deactivate and thereby become less effective at promoting the desired catalytic reactions, the reactor bed as a whole exhibits a performance decline, which imposes a direct loss in the production of the desired product. In addition, the production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems, can lead to plugging in fine screen surfaces, contributes to greater erosion of the process equipment, and in the case of expensive catalysts imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116, which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. Nos. 4,135,886; 4,141,690; 4,250,018; and 4,567,023, the contents of each of which are incorporated herein by reference. To avoid process conditions that lead to pinning, it has been the practice for many years to operate reactors of continuous and semi-continuous moving bed designs by maintaining the flow of vapor through the bed of catalyst at a rate that is below the rate that will pin catalyst when the bed is stagnant. This rate is referred to herein as the stagnant bed pinning flow rate.
As explained in further detail in the detailed
Gu Weikai
Sechrist Paul A.
Arnold, Jr James
Griffin Walter D.
Moore Michael A.
Tolomei John G.
UOP LLC
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