Abstract:
A method for metathesizing at least two gaseous olefins using a moving catalyst bed gas phase metathesis reactor and counter current flowing subdivided solid catalyst.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to the metathesis (disproportionation) of olefins. More particularly, it relates to a process for carrying out a metathesis reaction in a moving catalyst bed, gas phase reactor using a subdivided, solid catalyst that promotes the metathesis reaction. 
         [0003]    2. Description of the Prior Art 
         [0004]    The catalyzed metathesis of olefins was first disclosed in 1964, and, because of its versatility, has since developed into a whole new field of its own within the universe of hydrocarbon chemistry. 
         [0005]    Basically, the metathesis process utilizes a double bond displacement mechanism that involves the breaking and reformation of olefinic bonds, the type and number of bonds remaining unchanged. Starting with two different olefinic molecules, the reaction causes the displacement of double bond groups from each molecule to produce two new olefinic molecules that are not the same as the starting molecules. Displacement cleavage occurs at a double bond on each starting olefin molecule, and different olefin molecules are formed that have double bonds where the old double bonds were cleaved. For example, propylene is currently commercially produced by metathesizing 2-butene with an excess of ethylene. In this particular process, the double bonds in a molecule of 2-butene are cleaved as are the double bonds in a molecule of ethylene, and the resulting radicals reform to produce two new molecules of propylene. The process can be promoted with either homogeneous or heterogeneous catalyst systems comprised of one or more functional catalysts. 
         [0006]    The metathesis of olefins is well understood and is fully and completely disclosed in U.S. Pat. No. 6,872,862 to Bridges, Powers, and Coleman. 
         [0007]    Heretofore, metathesis reactions such as the propylene production process discussed above have been carried out using a fixed bed of catalyst through which flows the fluid (gas and/or liquid) olefin reactants, see U.S. Pat. Nos. 5,026,936 and 6,872,862. The catalyst employed in these fixed beds is a solid particle, typically pellet size, e.g., about 1/16 to ¼ inch in diameter and about 1/16 to ¼ inch in length. 
         [0008]    Metathesis reactor (reactor) cycles between catalyst regeneration operations are often dictated by the pressure drop across the reactor. For example, the pressure drop across a reactor can climb steadily over the course of 2 to 4 weeks from an initial pressure of about 2 to 10 psig to a final pressure of over 30 psig. At this point in time in the operation of the reactor, the catalyst bed is sufficiently fouled to require shutdown of the process and a catalyst regeneration operation. 
         [0009]    This pressure drop is usually caused by catalyst pellet attrition resulting in a buildup of catalyst fines in the reactor, or coke deposition on the catalyst pellets, or both. As the catalyst ages, accumulated catalyst fines in the catalyst bed increase not only the initial pressure drop across the bed, but also the rate of increase of the pressure drop over the period of time the bed is in operation. 
         [0010]    Accordingly, it is desirable to have a metathesis process that is not subject to the vagaries of catalyst attrition and coke deposition in a fixed catalyst bed. 
       SUMMARY OF THE INVENTION 
       [0011]    Pursuant to this invention a metathesis process is provided that employs a moving catalyst bed/gas phase reactant metathesis reactor, and counter current flow between the solid catalyst and the vaporous reactants in that reactor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a simplified flow sheet of a prior art metathesis process using a fixed bed of catalyst. 
           [0013]      FIG. 2  shows a flow sheet of one embodiment within the process of this invention using a moving catalyst bed reactor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    For sake of clarity and brevity, this invention will be described in respect of the metathesis of 2-butene with ethylene to form propylene, but this invention is not so limited in its scope. 
         [0015]      FIG. 1  shows a fixed bed of catalyst  1  into which flows reactant stream  2  composed of 2-butene and reactant stream  3  composed of ethylene. Catalyst bed  1  is maintained at operating conditions that favor, in the presence of the catalyst, the cleavage of double bonds in both the ethylene and 2-butene and the reformation of the resulting radicals into the desired propylene product. 
         [0016]    The reaction mixture containing unreacted ethylene and 2-butene feeds and propylene product is passed by way of line  4  to a distillation column  5  that separates ethylene  6  as overhead from the reaction mixture for recycle to bed  1 , if desired. 
         [0017]    Bottoms stream  7  of column  5  is composed primarily of 2-butene and propylene. This mixture is separated into propylene product stream  9  and separate bottoms stream  10 . Stream  10 , composed essentially of 2-butene, can also be recycled to bed  1 , if desired. 
         [0018]    It is in bed  1  that attrited catalyst fines and/or coke can collect and drive the pressure drop across bed  1  (from inlets  2  and  3  to outlet  4 ) up to a level that requires the metathesis process to be terminated, and the catalyst in bed  1  regenerated or replaced. 
         [0019]      FIG. 2  shows one flow scheme within this invention. In this Figure a moving catalyst bed reactor  20  receives by way of conduit  22  a gaseous mixture of ethylene and 2-butene reactants, and by way of conduit  23  subdivided, solid metathesis promoting catalyst. 
         [0020]    In this embodiment, reactor  20  has upper and lower opposed ends  24  and  25 , respectively. Vaporous reactants  22  enter at or near bottom end  25 , and, by force of a pressure gradient across the height of reactor  20 , flow upwardly inside reactor  25 , as shown by arrow  26 , toward top end  24 . Solid catalyst particles enter at or near upper end  24 , and, by force of gravity, flow downwardly, as shown by arrow  27 , into counter current flow contact and mixing with rising reactants  22 , thereby promoting the desired metathesis reaction. In this particular embodiment, reactor  20  is maintained at operating conditions that favor the conversion of one mole of ethylene and one mole of 2-butene to two moles of propylene. 
         [0021]    The mixture of propylene product and unreacted ethylene and 2-butene is removed by way of line  30 , and passed elsewhere for further processing to separate the propylene product from the unreacted ethylene and 2-butene. The thus recovered unused reactants can be returned by way of line  22  to reactor  20  as feed therefore. For example, the reaction mixture in line  30  can be processed in columns  5  and  8  of  FIG. 1 , to recover the desired propylene product and the recycle feed reactants. 
         [0022]    Solid catalyst particles that reach the bottom of reactor  20  are collected in a conventional solid/gas separator  31  so that essentially only solid catalyst passes from the interior of the lower end of reactor  20  into a mechanical catalyst conveyor  32  that conveys the degassed solid catalyst to a conventional regeneration unit  33 . In unit  33 , coke can be air burned in a conventional manner, and thereby removed from the catalyst particles. The regenerated catalyst is then gravity fed into line  34 , through catalyst lock out valve  35 . Valve  35  is normally maintained at least partly open for the passage of catalyst there through. From valve  35  the catalyst passes back into reactor  20  by way of conduit  23  to promote additional metathesis reaction. 
         [0023]    The process of  FIG. 2  addresses the problems of catalyst attrition and/or catalyst coking causing unacceptable pressure drops across the metathesis reactor, thereby allowing reactor  20  to operate continuously, and a substantially longer time, even years longer, between reactor shutdowns. 
         [0024]    The process of this invention also allows for almost infinite flexibility for varying the make-up of the reactant/catalyst mixture that is to be subjected to metathesis conditions in reactor  20 . For example, a 2-butene reactant stream may not be wholly 2-butene. It may contain minor amounts of 1-butene, and the amount of 1-butene contained in a reactant stream can vary over time. Reactant stream compositions change over time of operation, e.g., the 1-butene content in a 2-butene stream can vary. By the process of this invention, the amount of ethylene and/or catalyst mixed with 2-butene feed component can be changed to accommodate the varying amount of 1-butene present in that component feed. For example, if the 2-butene reactant contains varying amounts of 1-butene, and one of the catalyst components has olefin isomerization functionality (i.e., magnesium oxide), the magnesium oxide level in the catalyst passed to reactor  20  can be increased in conduit  34  by way of line  36 , in any amount desired to isomerize at least part of the increasing 1-butene content in the feed. Similarly, if the 1-butene content decreases, a matching decrease in magnesium oxide content can, with this invention, easily be affected by removal of catalyst from unit  33  in a conventional manner well known in the art. Thus, by this invention superior flexibility in operation is possible since the catalyst composition can be tailored to meet varying compositions of the reactant mix  22 , and thereby carry out a more efficient process. 
         [0025]    The two or more reactants that form mix  22  can vary widely so long as they are olefins, with alpha or internal un-saturation. Generally, they can be monoolefins having from 2 to 8 carbon atoms per molecule (C2 to C8 olefins). 
         [0026]    Suitable metathesis promoting catalysts include at least one of halides, oxides and/or carbonyls of molybdenum, tungsten, rhenium, and/or magnesium carried on a support, preferably an oxide support such as silica, alumina, titania, zirconia and mixtures thereof. Activating agents can also be included in the catalyst make-up. Such agents can include organo-metallic compounds such as tetra methyl tin; oxides such as alkaline earth metal oxides, alumina, silica, and mixtures thereof. 
         [0027]    Pursuant to this invention the catalyst or catalyst combinations employed can vary widely in its subdivided form. The solid particle range for the catalyst mixture can vary from powdered catalyst of from about 0.1 inch up to right cylindrical pellets of catalyst having lengths up to about 1 inch and diameters up to about one-half inch, and any combination of particle sized in between so long as they can be made to flow counter currently with the gaseous feed reactants rising in reactor  20 . 
         [0028]    The operating conditions maintained in reactor  20  can vary widely, but will generally be a temperature of from about 300 to about 800 degrees Fahrenheit (F), and a pressure of from about 200 to about 600 psig. The pressure differential maintained over the length of reactor  20  can vary widely depending on the particle size make-up and distribution, but will in all cases be a differential within reactor  20  that is sufficient to maintain essentially continuous flow of reactant feed through the reactor against the counter currently flowing solid catalyst load. For example, the pressure differential can vary from about 600 psig at the feed inlet end of the reactor and about 200 psig at the outlet end of the reactor, or any differential within that 200 to 600 psig pressure range. 
         [0029]    Reactor  20  can be a conventional counter current flow reactor known in the art. In the operation of counter current flow reactors pursuant to this invention, solid, particulate catalyst particles as defined above are made to flow into, and mix with, counter flowing pressurized feed mixture  22 . This mixing process causes intimate contact of reactants and catalyst in reactor  20 . Metathesis occurs while the mixture of reactants and catalyst pass by one another in the interior of reactor  20 , and, at the same time, are subjected to metathesis favoring operating conditions. 
         [0030]    For example, in a moving bed reactor  20  system such as that shown in  FIG. 2 , the catalyst particles gravity flow downwardly inside the open interior of the reactor relative to the reactor wall, and typically maintain their positions relative to one another as they flow downwardly. Plug flow of both the catalyst and the reactants through the reactor is readily achievable and desirable. 
         [0031]    With the moving bed system of this invention catalyst can be withdrawn from the process either continuously or intermittently or any desired combination thereof. Thus, catalyst can be regenerated outside the system, replaced, and/or reintroduced into the system at will. 
         [0032]    Catalyst can, for example, be removed from the reactor by gravity into a conventional standpipe (not shown) that is located below the point where the reactants are introduced into the lower part of reactor  20 . The standpipe can contain a typical interlock valve system (not shown) that cycles between open and closed positions with a side vent that isolates the catalyst removed from the reactor while the reactor is in operation. Once the catalyst is below the isolating valve system, the catalyst is moved downwardly into a receiving hopper (not shown) that employs a mechanical conveyor or flowing motive gas system, represented by line  32 , to transport the spent catalyst upward to regenerator  33 . After catalyst fines removal, if necessary, the regenerated catalyst is moved by gravity into an interlock system represented by valve  35  for reintroduction into the upper part of the reactor for reuse in the process. 
         [0033]    As stated above, the reaction conditions can vary widely depending on the particular reactants and catalyst system used, so the physical configuration and orientation of the reactor can also vary widely. However, reactor  20 , as shown in the embodiment of  FIG. 2  will generally have a vertical height of from about 1 to about 100 feet thereby providing a reactant  22  residence time of from about 10 milliseconds to about 10 minutes inside the reactor itself in contact with the catalyst. 
       EXAMPLE 
       [0034]    A gaseous mixture of about 16 weight percent (wt %) 1-butene and about 84 wt % 2-butene together with a molar excess ethylene is passed into the bottom of reactor  20  at a pressure of about 350 psig. The pressure at the top of reactor  20  is about 330 psig so that the vaporous reactant mixture rises toward the top of reactor  20 . Catalyst pellets composed of tungsten oxide and magnesium oxide, and about one-half inch long and about one-eighth inch in diameter are employed in conduit  23 . The counter current flowing mixture of reactants and catalyst inside reactor  20  is maintained at a temperature of about 600 F Reactor  20  is operated at a reactant feed flow rate that provides a residence time for the reactants in the reactor of about 10 minutes. 
         [0035]    A mixture of propylene, unreacted ethylene, unreacted 2-butene, 1-butene, and propylene is recovered overhead from the reactor, and the propylene separated there from as a product of the process.